spiral.imperial.ac.uk  · Web viewStrategies for carbon and sulfur tolerant solid oxide fuel cell materials, incorporating lessons from heterogeneous catalysis . Paul . Boldrin.

Strategies for carbon and sulfur tolerant solid oxide fuel cell materials,

incorporating lessons from heterogeneous catalysis

Paul Boldrina*, Enrique Ruiz-Trejoa, Joshua Mermelsteinb, Jose Bermudez

Menendezc, Tomas Ramirez Reinad, Nigel P. Brandona

a: Department of Earth Science & Engineering, Imperial College London

b: The Boeing Company

c: Department of Chemical Engineering, Imperial College London

d: Department of Chemical and Process Engineering, University of Surrey

* Corresponding author: [email protected]

Abstract

Solid oxide fuel cells (SOFCs) are a rapidly emerging energy technology for a low

carbon world, providing high efficiency, potential to use carbonaceous fuels and

compatibility with carbon capture and storage. However, current state-of-the-art materials

have low tolerance to sulfur, a common contaminant of many fuels, and are vulnerable to

deactivation due to carbon deposition when using carbon-containing compounds. In this

review we first study the theoretical basis behind carbon and sulfur poisoning, before

examining the strategies towards carbon and sulfur tolerance used so far in the SOFC

literature. We then study the more extensive relevant heterogeneous catalysis literature for

strategies and materials which could be incorporated into carbon and sulfur tolerant fuel

cells.

Contents

1. Introduction

2. Scope of the review

3. Fundamentals of carbon poisoning

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3.1 Theoretical studies on carbon deposition in catalysts and fuel cell anodes

4. Fundamentals of sulfur poisoning

4.1 Theoretical studies on sulfur poisoning of catalysts and SOFC anodes

5. Systems approaches to carbon and sulfur tolerance

6. Materials design strategies for carbon tolerance in SOFC anodes

6.1 Ni-YSZ cermets

6.2 Alloying with noble metals

6.3 Alloying or replacement of nickel with base metals

6.4 Replacement of nickel with non-metal electronic conductors

6.5 Increasing alkalinity

6.6 Use of ceria and other oxygen storage materials

6.7 Replacement of cermets with mixed ionic-electronic conductors (MIECs)

6.7.1 Single phase MIECs

6.7.2 Addition of catalytic metal nanoparticles to MIECs

6.8 Regeneration of SOFC anodes deactivated by carbon

7. Materials design strategies for sulfur tolerance in SOFC anodes

7.1 Replacement of YSZ with ceria

7.2 All-ceramic anodes

7.3 Alloying of nickel with other metals

8. Strategies from conventional catalysis

8.1 Carbon tolerance in conventional catalysis

8.1.1 Sulfur passivation

8.1.2 Alloying and bimetallic systems

8.1.3 Promoters

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8.1.4 Regeneration of Catalysts Deactivated by Carbon Deposition

8.2 Strategies against sulfur poisoning

8.2.1 Noble metal-based catalysts

8.2.2 Alloys, bimetallic and promoters

8.2.3 Support and structural modifications

8.2.4 Regeneration of sulfur poisoned catalysts

9. Conclusions and Perspectives

9.1 Alloying of nickel

9.2 Alkaline promoters and supports

9.3 Ceria, doped ceria and oxygen storage

9.4 Preferential sulfur binding sites

9.5 Non-metal electronic conductors

9.6 Infiltration of nanoparticles

9.7 Regeneration

9.8 Theoretical and computational studies

9.9 Reflections on experimental work

10. Acknowledgements

11. References

1. Introduction to solid oxide fuel cells

Solid oxide fuel cells (SOFCs) are electrochemical devices for the direct conversion of

fuels into electricity. Because they operate by the conduction of oxide ions they are capable

of using a wide variety of fuels including hydrocarbons, syngas, biogas and ammonia, as well

as hydrogen. The oxidation of fuel takes place at the anode, which needs to be active for

electrochemical oxidation of the fuel species and possess both electronic and ionic

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conductivity. Typically anodes are made either from ceramic-metallic composites (cermets)

where each component provides one aspect of the conductivity, or from a mixed ionic-

electronic conductor (MIEC), a ceramic which provides both ionic and electronic

conductivity.

There are a number of other properties that any materials to be used in SOFC

anodes need to possess, including stability towards high temperatures and highly reducing

conditions, chemical compatibility with other materials such as electrolytes and

interconnect materials, and thermal expansion coefficients matched to the other

components during operation and manufacture. The need for these properties places a

limitation on which materials can be used, for example there are materials with high ionic

conductivity which are not stable in reducing atmospheres, or which have a large thermal

expansion mismatch compared to common electrolyte materials. As well as the direct

electrochemical oxidation of fuel species, other relevant reactions which take place in an

SOFC anode are water-gas shift, steam reforming, dry reforming, Boudouard reaction,

methanation and hydrocarbon decomposition and cracking, among others.

The development of SOFCs has reached an important phase, with rapid technological

advancement over the last decade resulting in multiple programs run by governments

and/or companies testing systems greater than 100 kW, and installed commercial products

in the low kW range combined heat and power market. An initial understanding of the

recent progress of multi-kW-scale SOFC development can be gained by studying the US

Department of Energy’s SOFC program (Solid State Conversion Alliance, SECA) which is

interested in systems of 100 kW and upwards operating on syngas from coal or natural gas.

For the period 2005 – 2007 the SOFC targets were for 1500 hour tests on fuel cell stacks

with performance degradation targets at steady state of <4%/1000 hours, while the latest

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target is for >25 kW stacks with >4 years lifetime and degradation of <0.2%/1000 hours,

with cumulative operation times of between 5,000 and 10,000 hours for this generation of

SOFCs by 2020. The financial year 2015 funding round supported Fuel Cell Energy and Versa

Power to produce a 400 kW system1. Other large projects include Mitsubishi Heavy

Industries demonstrating a 200 kW combined SOFC-gas turbine system operating on syngas

at 900 °C, with a degradation rate of 0.13%/1000 hours2, while Bloom Energy, based in

California, have a commercially-available SOFC capable of generating 100 – 200 kW aimed at

the commercial market, especially data centres, with an installed base of over 30 MW3.

Figure 1 shows a diagram of a combined cycle SOFC with integrated gas turbine.

Figure 1 – Diagram of a combined cycle SOFC system with integrated gas turbine

The focus on degradation rates clearly seen above in the large scale and commercial

programs is a reflection that one of the key issues facing SOFCs is degradation and its effect

on lifetime costs. Performance degradation can be caused by thermal gradients or thermal

cycling, oxidation cycling and long-term incompatibility of components. For SOFCs to

continue to become successful commercially, they will need to operate on carbonaceous

fuels, and be tolerant to common contaminants in those fuels. Two of the most common

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poisons are carbon and sulfur, and current anodes based on composites of nickel and

yttrium-doped zirconia (yttria-stabilised zirconia, YSZ) are not tolerant to them, resulting in

long term degradation and a need for regeneration, which has additional effects on

degradation relating to thermal and/or oxidation cycling. For this reason carbon and sulfur

tolerance is a vital area of research for the next generation of SOFCs to compete with

conventional power plants at the grid scale, and with boilers and combustion engines at

smaller scales.

The application of catalysis in fuels processing has been an important research topic

for several decades. In catalytic processes involving fuels, carbon (from the fuel itself) and

sulfur (present as contaminant) critically affect the performance of the catalyst. Under

certain conditions, this effect can be extremely important and the catalyst is deactivated

quickly, leading to unpractical and/or costly processes. For these reasons, huge research

efforts have focused on the design of catalysts resistant to carbon deposition and sulfur

poisoning. As a result of this, a vast knowledge of possible alternatives to address these

issues has been generated. This literature could provide insights into improving the carbon

and sulfur tolerance in SOFC materials.

2. Scope of the review

This review discusses all aspects of carbon and sulfur tolerance in SOFC anodes, from

mechanistic and theoretical studies to strategies for materials design. In addition, we have

studied the catalysis literature, focussing on fundamental studies and catalysts used in

reactions under conditions similar to those in an SOFC anode (e.g. steam reforming and

partial oxidation). Since, in the end, poisoning by carbon and sulfur may be inevitable, we

have also included sections on regeneration. We have chosen to put these at the end of the

relevant materials design section (e.g. regeneration of SOFCs after carbon deposition is at

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the end of the carbon tolerant SOFC section). In the final section, we have summarised the

various strategies used in catalysts and SOFCs to provide carbon and sulfur tolerance with

lessons learned from each. Certain aspects of this review have been covered in other

reviews in the last decade: Ni-based anodes in hydrocarbon fuels4, sulfur poisoning of Ni-

based anodes and catalysts5-6, anode performance in hydrocarbons7-8, catalyst deactivation

and regeneration9, steam reforming for fuel cells10, internal reforming in fuel cells11 and

sulfur tolerance in hydrogen production catalysts12.

3. Fundamentals of carbon poisoning

Deposition of carbon-containing species on metal catalysts is one of the main causes

of catalyst deactivation and is virtually inevitable in any reaction involving hydrocarbons9, 13-

16. It should be clarified that carbon and coke, although often used interchangeably, refer to

different species. Carbon refers to the product of CO disproportionation whereas coke is

produced by decomposition or condensation of hydrocarbons9, 16-17. However, for the sake of

clarity and readability, only the term carbon will be used in this work.

In reactions involving carbon-containing fuels, the principal reactions leading to

carbon deposition can be summarized as follows13:

2 CO (g) C (s) + CO2 (g)

CnHm (g) n C (s) + m/2 H2 (g)

CO (g) + H2 (g) C (s) + H2O (g)

The first reaction is the disproportionation of carbon monoxide and is commonly

known as the Boudouard reaction, after its discoverer Octave Leopold Boudouard, a French

chemist of the late 19th and early 20th century. It is exothermic at all temperatures but due

to the reduction in entropy becomes more favourable at lower temperatures. The second

reaction is the decomposition of hydrocarbons and conversely is endothermic with an

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increase in entropy, so is favoured at high temperatures. The final reaction is the reverse of

the original “water-gas reaction” used to produce “water gas” (now known as syngas) from

coke using steam. It is distinct from the water-gas shift reaction, which was originally used

to reduce (or shift) the carbon monoxide content of the water gas, so that it could be more

safely used. It has similar thermodynamics to the Boudouard reaction and so is more

favoured at lower temperatures.

Carbon deposition is strongly affected by the presence of sulfur and aromatic

compounds in the fuel13, 18. Sulfur deactivation can either promote or reduce carbon

deposition depending on the conditions19, and the ability of sulfur to potentially improve

carbon tolerance is discussed later on in section 7.1.1. The presence of aromatics in the fuel

tends to increase carbon deposition far more than would be expected from their

concentration in the fuel. This is likely because carbon deposition is thought to proceed

through a mechanism involving the formation of aromatics. Once formed, these aromatics

are less reactive than other compounds in the fuel and serve as nucleation sites for the

formation of polynuclear carbon compounds9, 13. The mechanism of carbon formation varies

with material (e.g., if it is a metal or metal oxide/sulfide)9, 16. This is important because the

effect of the structure and location of carbon on deactivation can be more relevant than the

total quantity of carbon deposited on the catalyst9, 20. In the case of metals, the rate of

carbon deposition is a function of the type of metal, the crystal size, the promoters and the

interaction between the metal and the support9, 21-29.

Formation of solid carbon is favoured thermodynamically in a large proportion of the

potential operating space of SOFCs30. Figure 2 shows the region in which carbon deposition

is favoured at different temperatures, showing that all common carbon containing fuels are

in the carbon deposition region below 1000 °C, including CO and CH3OH. This indicates that

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oxygen-containing species need to be added to make carbon deposition thermodynamically

unfavourable. Factors which increase the thermodynamic favourability of carbon deposition

include lower temperatures, higher carbon:oxygen ratios and low oxygen fluxes. In addition

to this, carbon deposition is strongly influenced both inside and outside this thermodynamic

window by kinetic factors, especially the relative rates of the forward and reverse

Boudouard and methane decomposition reactions, and the presence of aromatic and

polyaromatic compounds.

Figure 2 – Carbon deposition limit lines in the C-H-O phase diagram. Reproduced by

permission of The Electrochemical Society from J. Electrochem. Soc. 150 (7) A885-A888

(2003). Copyright 2003 The Electrochemical Society

When carbon deposition takes places on metal particles, several situations can lead

to deactivation (Figure 3)9:

Strong chemisorption as a monolayer or physical adsorption in multilayers

blocking access to metal surface sites.

Encapsulation of metal particles, deactivating them completely.

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Plugging of micro- and mesopores blocking access to the active sites inside

them.

Growth of carbon filaments (whisker carbon) that can stress and fracture the

support or push the metal particles off the support. In the case of SOFC anodes the growth

of this carbon can destroy the structure of the fuel cell.

Dissolution of carbon atoms into the metal, causing a volume expansion. This

is mainly a problem for SOFC anodes, where the metal may have a structural role and

therefore these volume changes can destroy the structure of the anode.

By blocking active sites for catalytic and electrocatalytic reactions, carbon can reduce

the performance of both catalysts and SOFCs. This type of deactivation can occur even at

low levels of carbon deposition, but is generally fully reversible by oxidation of the carbon.

Techniques for achieving this are discussed in sections 5.8 (for SOFC anodes) and 7.1.4 (for

catalysts). Structural deactivation, where carbon deposition causes structural failure, tends

to be the most serious problem caused by carbon poisoning in SOFCs. This mode of

deactivation is caused by longer term running under conditions favourable to carbon

deposition, or when using materials such as nickel which catalyse carbon deposition. In

SOFCs, because the metal component can have some structural role, failure can also occur

by dissolution of carbon into components of the anode, causing a volume expansion which

can result in “dusting”, where the anode becomes pulverised. This tends to occur when

carbon is repeatedly dissolved and removed from the anode materials.

Different types of carbon can be formed in these reactions9, 13. These types of carbon

have different reactivities and morphologies, which affect their potential for deactivation. In

addition, they can react to be transformed in a different type of carbon, thus varying during

the reaction their potential to deactivate the catalyst9, 13, 16.

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In the case of metal oxides and sulfides, the formation of carbon is the result of

cracking reactions catalysed by acid sites. The rate of carbon deposition depends on the

acidity of the catalyst and its porous structure. In this case deactivation can be caused by

chemical or physical effects. In the case of chemical deactivation, carbon can strongly

adsorb on the acidic sites while physical deactivation is the result of the pore plugging which

blocks access to some catalytic sites.

Figure 3. Different situations in which carbon deposits can lead to deactivation: a)

Carbon layers chemisorbed on metal particles (reprinted from J. Power Sources 2010, 195

(2), 649-661, with permission from Elsevier); b) encapsulation of metal particles by carbon

deposits (Reprinted with permission from J. Am. Chem. Soc. 2006, 128 (35), 11354-11355.

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Copyright (2006) American Chemical Society); c) growth of carbon nanofilaments that push

metal particles off the support (reprinted from Int. J. Hydrogen Energy 2014, 39 (24), 12586-

12596, with permission from Elsevier); and d) pore blockage by carbon deposits (reprinted

from Chem. Eng. J. 2010, 163 (3), 389-394, with permission from Elsevier).

An ideal carbon tolerant cell would be able to run on hydrocarbons without any

added oxidant and would therefore not require high temperatures, steam generators or

other extra modules which are currently used to mitigate carbon poisoning in SOFC-based

power generators.

Generally, there are two ways for suppressing (or at least minimizing) the rate of

carbon deposition: changing process conditions, such as increasing steam to carbon ratio or

increasing temperature; or developing carbon resistant materials9, 13, 15, 31-32. The rate of

deactivation is related to the balance between the rates of formation and

gasification/oxidation of the carbon, which are strongly influenced by the reaction

conditions and the catalytic activity of the materials towards the different reactions

involved9, 13, 15, 31.

In catalysis, the range of variation of the reaction conditions is often quite limited

since the conditions need to be designed to optimise the yield of the desired product rather

than protect the catalyst. In SOFCs, there is more scope to alter reaction conditions, with

compromises made to cost, power and flexibility. Since Ni is such an effective catalyst for

hydrocarbon decomposition, use of reforming to convert hydrocarbons into syngas can be

effective. This reforming can be done internally or externally. External reforming requires

the extra cost of a separate reforming unit, but has the advantage that the reformer has a

protective effect on the fuel cell. Internal reforming with steam, CO2 or O2 can be effective

due to the high activity of Ni for reforming reactions but certain conditions such as periods

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at open circuit voltage (OCV) and low oxygen:carbon ratios can result in carbon deposition

33-34. Alternatively, separate reforming layers have been investigated, but these could

complicate fabrication and would need to be composed of a carbon tolerant catalyst 35-38.

Both internal and external reforming have problems with the separation of endothermic

reforming and exothermic oxidation reactions – in external reforming there is a need for

heat exchangers while in internal reforming the proximity of exothermic and endothermic

reactions causes thermal gradients. Both types of reforming reduce the power output of the

cell.

Because of how SOFCs operate, increasing the current density also mitigates against

carbon deposition (at least in sulfur-free fuels), due to the increased flow of oxygen into the

anode side of the cell. This has the advantage of encouraging reforming reactions without

reducing the power output of the cell. Because of the protective effect of oxygen flow

across the electrolyte, SOFCs can be started up under hydrogen with the carbon-containing

fuel being switched on once the cell is already under load, if it is feasible to have a dedicated

hydrogen supply for this purpose.

Running the fuel cell at high temperature can move the conditions outside the

region where carbon deposition is thermodynamically favoured, although this does not

guarantee there will be no carbon deposition. The higher temperatures increase the cost of

components other than the anode, which need to be designed to withstand higher

temperatures, for example above 800 °C, the most suitable alloys for interconnects have

high levels of chromium, which can cause problems with formation of resistive phases39 and

cathode degradation40.The higher temperatures may also reduce the overall lifetime of the

system. Alternatively, with Ni-YSZ anodes, it has been shown that decreasing the

temperature reduces carbon deposition in a cell operating under load in humidified

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methane as it slows the methane cracking reaction more than the electrocatalytic oxidation,

although high currents and thus high oxygen fluxes into the anode were still required to

eliminate carbon deposition entirely.33

3.1 Theoretical studies on carbon deposition in catalysts and fuel cell anodes

The formation of carbon deposits in catalytic reactions involving hydrocarbons is the

consequence of the dehydrogenation of these hydrocarbons. Methane is the simplest

hydrocarbon and therefore provides the simplest model for understanding the

fundamentals of carbon deposition. The dissociation of CH4 over metal surfaces occurs in

four steps41-42:

CH4(g)a ↔ *CH3a+ *Ha

*CH3a ↔ *CH2a + *Ha

*CH2a ↔ *CHa + *Ha

*CHa ↔ *Ca + *Ha

Considering Ni as the active metal surface, the dissociation of methane can take

place on two different kinds of active sites: those associated with the planar surfaces (or

terraces) and those associated with stepped and defect sites on the metal surfaces41-42.

Considering the planar sites, theoretical studies have shown results that can be surprising at

a first view, as can be seen in Figure 441. The most stable intermediate in planar surfaces is

*CH and the last step of methane dissociation from *CH to produce carbon is an

endothermic process with high activation energy (Table 1)41-43. These data suggest that

carbon deposition should not take place on those Ni surfaces, something that contradicts

what has been widely reported experimentally. However, observing the results from the

stepped sites, the phenomenon of carbon deposition is easily explained. Stepped surfaces

are more reactive than planar, due to electronic and geometrical defects that take place in

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these low-coordinated surface geometries41-42, 44-46. As a consequence of this, the production

of carbon on stepped surfaces is exothermic and thermodynamically favoured, creating the

driving force for the formation of graphitic carbon deposits42. A similar situation occurs in

the case of other metals and alloys, as can be seen in Table 1. In all the cases, the formation

of carbon is thermodynamically more feasible on stepped than planar surfaces.

Figure 4. Thermodynamic pathway for the dissociation of methane (CH4) on planar

(111) and stepped (211) Ni surfaces. Reprinted from J. Catal. 2007, 247 (1), 20-33., with

permission from Elsevier.

The process starts with the activation of the first C–H bond in methane. As can be

seen in Figure 5, in both cases (planar and stepped surfaces), this takes place over the top of

a surface Ni atom. However, in the case of planar surfaces, the energy barrier is higher than

in the case of stepped (Table 1). This is due to the higher stability of the adsorbed CH3 on

the stepped surface, which gives rise to a stronger bond44. Similarly, the subsequent steps of

the dissociation of methane give rise to species that are more stable on stepped than on

planar surfaces. Finally, whereas CH is the most stable species in planar surfaces, C is the

most stable species on the stepped, favouring its deposition in these sites41.

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Table 1. Activation barriers (Ea) and reaction energies (∆E) of the different steps in

the dissociation of methane and adsorption energies (Eadsorption) of the different species

involved in the process reported on different metal surfaces. Stepped surfaces are shaded,

exothermic steps are in bold. All values are in kJ/mol.

CH4↔*CH3+*H *CH3↔*CH2+*H *CH2↔*CH+*H *CH↔*C+*H

Ea ∆E Ea ∆E Ea ∆E Ea ∆E

Ni (1 1 1)38 100a 54a 75 a 17 a 29 a -29 a 130 a 63 a

Ni (1 1 1)40 113.9 12.5 74.3 9.6 35.7 -25.1 131.2 53.1

Ni (2 1 1)38 84 a 42 a 88 a 8 a 42 a -33 a 88 a -29 a

Cu (1 1 1)38 188 96 138 92 113 46 205 130

Cu (1 1 1)40 181.4 83.0 141.8 67.5 101.3 30.9 213.2 115.8

Cu (2 1 1)38 138 33 134 79 184 13 176 75

Fe (1 1 1)40 98.4 -60.8 56.0 -30.9 12.5 -66.6 100.3 3.9

Co (1 1 1)40 110.0 4.8 66.6 10.6 30.9 -21.2 120.6 54.0

Cu-Ni (1 1 1)40 105.2 6.8 63.7 4.8 34.7 -34.7 132.2 49.2

Cu-Ni (2 1 1)38 n.a. 29 n.a. 63 n.a. -54 n.a. 21

Fe-Ni (1 1 1)40 120.6 -29.9 68.5 -19.3 30.9 -56.0 111.9 4.8

Co-Ni (1 1 1)40 124.5 4.8 73.3 20.3 32.8 -14.5 121.6 61.8

Rh (1 1 1)44 332 47.3 220 -42.7 26.4 -234 452 224

Rh(1 1 0)44 282 -54.8 127 -46.9 465 -51.0 207 -72.0

Rh(1 0 0)44 261 -53.6 136 -39.7 14.2 -250 284 -93.3

a - Approximate values extracted from Figure 2 in 41

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(a)

(b)

Figure 5. From left to right, initial, transition and final state for the dissociation of

methane on: a) Ni (1 1 1); and b) Ni(2 1 1). C, H and Ni atoms are represented by dark grey,

black and white colours respectively. Reprinted from Surf. Sci. 2005, 590 (2–3), 127-137,

with permission from Elsevier.

Once carbon has been deposited in the metallic sites, two different processes that

lead to carbon deposits formation can take place. Either C-C bonds can be formed and then

graphitic planes grow parallel to the planar surfaces of the Ni. Graphene is more stable on

planar surfaces because carbon atoms are organized in hexagonal structures that can lie

parallel to the Ni atoms 41-42, 45-48. Alternatively, those isolated atom carbons, once adsorbed,

can dissolve into the bulk Ni forming carbides (Figure 6). As a result of this diffusion, carbon

atoms can reach facets on the support side of the metal particle. These facets are suitable

for the eventual growth of carbon nanotubes42, 47, 49.

(a) (b)

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Figure 6. (a) Transition and (b) product states of the diffusion of one C atom from an

fcc hollow site to a sublayer octahedral site of the Ni(111) surface. Green (light) spheres

represent Ni atoms and black (dark) spheres represent C atoms. Reprinted with permission

from ACS Catalysis 2011, 1 (6), 574-582. Copyright 2012 American Chemical Society.

However, as stated by Abild-Petersen et al.44, the fact that stepped surfaces are

more active than planar ones does not mean that steps control the activity of the catalyst.

So, if the steps could be blocked, side reactions like carbon deposition would be eliminated,

while only moderately reducing the activity of the catalysts for methane processing. This can

explain the effect that the addition of Au to Ni catalysts has on coke deposition. Au

preferentially binds to low-coordinated Ni sites (like those present on steps). Consequently,

it increases the effective coordination number of adjacent Ni atoms and lowers the Ni

surface energy due to electronic interaction with gold41-42.

Another strategy for decreasing carbon deposition is to increase the reaction rate of

C-O bond formation relative to C-C bond formation. C atoms can be removed from the

surface of the catalyst by oxidation to form CO and CO249-50. Thus, if carbon diffusion and C-C

formation rates are decreased and oxidation rate increased, carbon deposition can be

avoided 49.

Following these ideas, the use of different promoters 41, 51, partial passivation 41, 45 and

alloys 41, 43, 49-50, 52 have been proposed. Table 1 shows that in all cases alloys have higher

thermodynamic barriers to carbon deposition than the metals which make them up,

meaning they are obvious targets. Two clear examples of this can be the effect of alloying Ni

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with Rh or Sn 49-50. The studies by Guo et al.49 and Nikolla et al.50 showed that when Ni is

alloyed with Rh or Sn both carbon and oxygen diffusion in the metal lattice and the C-C and

C-O bond formation are hindered, but to a different extent, as shown in Table 2 and Figure

7. Consequently, the overall carbon deposition rate was diminished. These studies have

supported their theoretical studies with experimental findings that point in the same

direction as the DFT results.

Table 2. Activation barriers (Ea), and reaction energies (∆E) of C-C bond and C-O

formation over different surfaces of Ni(1 1 1) and Ni-Rh (1 1 1). All energies are shown in eV.

C-C formation C-O formation

Ea ∆E Ea ∆E

Ni(1 1 1) -0.90 0.63 -1.75 1.18

Ni2Rh1(1 1 1) -0.69 0.75 -1.66 1.16

Rh(1 1 1) 0.19 1.34 -1.43 1.37

Figure 7. (a) DFT-calculated potential energy surfaces for C-C bond formation on Ni(1

1 1) and Sn/Ni(1 1 1). Inserts show the lowest energy pathways for the attachment of a C

atom to a carbon nucleation center (modelled as a chain of carbon atoms) on the two

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surfaces shown in the insert, (b) C-O bond formation on Ni(1 1 1) and Sn/Ni(1 1 1). Inserts

show the lowest energy pathways for the two surfaces shown in the insert. Ni is depicted as

large blue (light) atom, Sn as a large purple atom, carbon chain as a chain of small black

atoms. Reprinted from Catal. Today 2008, 136 (3–4), 243-248, with permission from

Elsevier.

While the metal particles are regarded as the main sites for carbon deposition, this is

also possible on oxide surfaces, for example both CO and CH4 will form carbon on Y2O3, YSZ

and ZrO2, with the amount of carbon decreasing in that order53-54, so clearly there is a

mechanism for carbon deposition on oxides which is controlled by the surface chemistry.

DFT studies on ceria and doped ceria show that carbon deposition should be

extremely unfavourable on a ceria surface as long as there are oxygen ions available to react

with the carbon atom, which will desorb as CO or CO255. The presence of Ni does not affect

the favourability of this process, indicating that the activity of the ceria in cermets should be

similar to the activity of pure ceria56. DFT studies on Ce2O3, show that surface vacancy

formation is as energetically unfavourable as on YSZ, indicating a low activity towards

oxidation reactions. Combined with experimental measurements showing that the ceria

surface was more active in a more reduced state, this indicates that Ce2O3 is not formed at

the surface57. In fact, DFT modelling shows that it is energetically favourable for CeO2 to

have two oxygen vacancies, providing the explanation for these results and the high

oxidation activity of ceria.

A study on BaCeO3 perovskites found that CeO2-terminated surfaces had much

stronger interactions with CH4 than BaO-terminated surfaces, although they did not link this

to carbon deposition but to methane oxidation58. Unfortunately the thermodynamically-

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favoured termination under SOFC anodes conditions is BaO, meaning that BaCeO3 should be

inactive for methane oxidation.

4. Fundamentals of sulfur poisoning

In addition to the tolerance to carbon, tolerance to sulfur is required to make an

SOFC or a catalyst flexible to fuels. The interaction of sulfur with anodes in SOFC has been

reviewed in the last few years5, 59-60. Sulfur, contained in all fuels originated from natural

sources (fossil or biogas), can be minimised but will always be present in a wide range of

concentrations, for example from 85 to 5000 ppmw for diesel61. If degradation is

unavoidable, at least a certain degree of regeneration must exist in order to guarantee long

term operation.

Several studies have addressed the influence of sulfur poisoning mainly on Ni/YSZ

anodes operating on H2/H2O 62-68. In recent years, the interest has grown to include carbon

fuels and H2S, again mainly on Ni/YSZ69-75. The reactivity of sulfur is related to the number of

electron pairs available for bonding, therefore, from the chemical point of view, toxicity

decreases in the order H2S, SO2 and SO42- 76. Other compounds of sulfur may exist in the fuels

but it is expected that in the majority of conditions occurring in an SOFC anode all sulfur

compounds are transformed into H2S77. Following the notion of chemical reactivity and

electrons available for bonding, a non-noble metal with electrons available for bonding will

be more affected by sulfur than a ceramic78.

In terms of SOFCs, H2S itself is a fuel that can be oxidised electrochemically and the

obvious choice to oxidise the sulfur is the oxygen ion that is being transported through the

electrolyte. This happens in the same way that hydrogen is oxidised but it should be noted

that three times more electrons are being used per mole in the electrochemical oxidation of

H2S.

21

H2S + 3O2- H2O + SO2 + 6e-

H2 + O2- H2O + 2e-

Examples of the use of SOFCs with H2S as a fuel have been given in the literature but

are rather limited79-86. H2S can and has been used as a fuel and it has been shown that SO2 is

the product of utilisation in a fuel cell with Pt as the catalyst85, 87 or a highly conductive and

catalytically active thiospinel86. It is not clear however, if these thiospinels could operate in

hydrogen rich fuels, are ionically conductive or are resistant to redox cycling. To facilitate

this electrochemical reaction the supply of electrons and oxygen ions must be a continuous

process and therefore, as in the case of hydrogen oxidation in a classic Ni/YSZ anode, the

triple phase boundary (TPB), i.e. the interface between Ni, YSZ and gas phase, is critical to

the performance. Anything that hinders or slows down this supply of oxygen and electrons

to the TPB will have a detrimental effect; examples of hindrance are carbon deposition or

agglomeration of the Ni phase. Similarly, anything that blocks the reaction sites for

hydrogen oxidation or internal reforming will be equally detrimental. In the case of nickel

anodes, sulfur poisoning is one of the reasons for the decreased electrochemical activity. In

what follows, the scope is more concentrated on the presence of H2S in the fuels as

pollutant rather than as fuel.

4.1 Theoretical studies on sulfur poisoning of catalysts and SOFC anodes

Considering H2S as the source of sulfur, the depletion of the anode performance

under H2S containing gas mixtures at elevated temperatures originates from H2S dissociation

leading to the adsorption of atomic sulfur (S*) on the anode surface (i.e. adsorbed on Ni

atoms when a model anode Ni/YSZ is considered)88-89. The strongly adsorbed S* species

block the active sites of the anode surface, decreasing the electrochemical oxidation

performance. Experimental studies have shown that sulfur coverage fits a Temkin isotherm

22

on nickel surfaces in catalysts, where the enthalpy of adsorption of sulfur varies linearly with

coverage90. In solid oxide fuel cell anodes, performance degradation is proportional to sulfur

coverage at constant current density91. Figure 8 shows the coverage of sulfur on Ni and

phase equilibria, highlighting that sulfur coverage is high even at low sulfur concentrations,

with a very strong dependence on temperature. Bulk sulfidation of Ni does not occur until

much higher sulfur concentrations92.

DFT calculations clearly illustrate the situation60. Figure 9 pictures a Ni based anode

built as infinite slabs with an adequate vacuum space (around 15 Å). Under these

circumstances four types of active sites can be imagined including atop, bridge, and three-

fold fcc- and hcp-hollow sites. As schematically illustrated in Figure 9 (c), the mechanism of

S* formation could be described as an interfacial reaction of adsorbed H2S* with the Ni

surface via two elementary steps of S–H bond cleavages (i.e., H2S* /HS* + H* and HS* / H* +

S*).

23

Figure 8 Chemisorption equilibria plotted in the chemical potential diagram for the

Ni–S–H system, log[p(H2S)/p(H2)] vs 1/T plot. Dotted and dashed lines for θs = 0.6 and 0.8,

respectively, are isocoverage lines calculated from the equation given in literature91.

Reproduced by permission of The Electrochemical Society from J. Electrochem. Soc. 157 (6)

B802-B813 (2010). Copyright 2010 The Electrochemical Society

The associated energy barriers for the subsequent steps of dissociation and

adsorption of H2S are presented in Table 3. For sake of comparison, Table 3 includes

analogue calculations for several noble metals. The calculated energies evidence that sulfur

adsorption is clearly a favourable process on Ni surfaces with large exothermic reaction

energies (ΔE) and low activation energies Ea. Furthermore, these computational results

suggest that replacing Ni with noble metals is not a viable solution to mitigate sulfur

poisoning since energy-wise H2S dissociation and S* adsorption also take place on noble

metal surfaces60. The adsorption energies summarized in the table also show that H2S* and

HS* bind to metallic surfaces weaker than S*. Hence in principle greater sulfur resistance

could be achieved by avoiding H2S dissociation on the anode surface, although the latter is

difficult to achieve given that a stronger S* adsorption energy involves a redistribution of

the electronic density that reduces the energy demand for H-S bond breaking.

24

Figure 9 (a) Schematic representation of a slab model with a proper vacuum space

for periodic DFT calculations. (b) Four active sites on a (111) plane. (c) Schematic energy

profile of gas-phase H2S dissociation on Ni (111) forming atomic S* and H*. “*” denotes

surface species. TS1 and TS2 are the transition states. Extracted from Energy and

Environmental Science 2011, 4 (11), 4380-4409 with permission of The Royal Society of

Chemistry

Table 3 Activation barriers (Ea) and reaction energies (ΔE) for the elementary

steps in a H2S dissociative adsorption process and adsorption energies (Eads) of sulfur species

(S*, HS* and H2S*). All values in eV.

metal Ea1a ΔE1

a Ea2b ΔE2

b EadsS* EadsHS* EadsH2S*

Pt(111)93 0.02 -0.90 0.04 -1.19 5.14 3.00 0.90

Pd(111)94 0.37 -1.25 0.04 -0.73 5.15 3.02 0.71

Rh(211) 95 0.01 -1.50 0.32 -1.50 6.0 3.69 1.00

Ni(100)88 0.29 -1.56 0.45 -1.05 5.96 3.72 0.83

Ni(111) 88 0.15 -0.98 0.11 -0.86 5.14 2.95 0.67

a - Ea1 and ΔE1 correspond to H2S* → HS* + H*

b - Ea2 and ΔE2 correspond to HS* → H* + S*

Alternatively to noble metals, alloying Ni with base metals as Cu may result in an

improved sulfur tolerance96. Indeed, DFT calculations evidenced that Cu based anode

materials display better tolerance to carbon deposition and sulfur than Ni based anodes97.

Figure 10 shows the evolution of sulfur adsorption energies with the Ni-Cu alloy

composition. It seems very clear that the alloying approach increases Ni resistance to sulfur

poisoning but the bimetallic system never reaches lower sulfur adsorption energy than

monometallic Cu.

25

A B

Figure 10 a) Supercell models of homogeneous Ni1−xCux as a function of the alloy

composition. Ni and Cu are in grey and in brown, respectively B) Comparison of the

predicted adsorption energies of atomic sulfur on Ni1−xCux(1 1 1) at PAW–GGA–DFT (•) and

GGA–DFT (ο). Adapted from J. Alloys Compd. 2007, 427 (1-2), 25-29 with permission of

Elsevier

The smaller adsorption energy exhibited by the Ni-Cu alloy can be explained in terms

of the density of states (DOS) analysis, as detailed in Norskov’s d-band theory98-99. As shown

in Figure 11, the antibonding orbitals in Ni-S are higher energy than the Cu-S antibonding

orbitals, meaning that it is easier to excite electrons into the Cu-S antibonding orbital to

break the Cu-S bond. This favourable situation has motivated a number of studies focusing

on Cu89, 97, 100,Ni-Cu alloys96, 101 and other alloys such as Ni-Sn102-103 targeting weaker sulfur

interaction with the fuel cell anode. Nevertheless, the main problem of these alternative

materials is their poor catalytic activity for the hydrogen oxidation reaction (HOR).

Simultaneously improving activity for the HOR and reducing poisoning by H2S is difficult as

both are related to the affinity of the material towards hydrogen-containing species104. In

other words, the alloys can effectively enhance the tolerance towards sulfur poisoning but

cell performance is sacrificed in turn.

26

Figure 11 (a) DOS analysis of S* on Ni(111) and Cu(111) in red and blue curves,

respectively. A circle represents the antibonding states around the Fermi level. (b) A scheme

of the energies of bonding and antibonding states corresponded to those of metal d bands.

Adapted from Energy and Environmental Science 2011, 4 (11), 4380-4409 with permission of

The Royal Society of Chemistry

According to thermodynamics, sulfur poisoning of traditional Ni based anodes is

largely unavoidable under a wide range of conditions at very low concentrations of H2S (e.g.

below 0.1 ppm H2S at 800 °C and below 10 ppm at 1000 °C under dry hydrogen)10558.

However, further DFT calculations106 have demonstrated that Ni anodes could be

regenerated through a two-step treatment: (1) addition of H2 to reduce sulfur coverage; (2)

oxidation with oxygen realising S as SO2.

Galea et al. described the sulfur removal pathways via oxidation106. They described a

two-step mechanism. In the first step sulfur concentration is reduced from 0.5 to 0.25

monolayers and in the final stage surface cleaning from 0.25 monolayers of sulfur to

complete sulfur removal is achieved (Figure 12).

27

A B

Figure 12. Energy profiles of the regeneration process via sulfur oxidation. A) Gibbs

free energy (ΔG at 800 ◦C, black line) and enthalpy (ΔH, red line) profile illustrating relative

thermodynamic energy and kinetic pathways of O2 adsorption and SO2 desorption on S–

Ni(111) surface with initial coverage θS = 0.50 ML B) Gibbs free energy (ΔG at 800 ◦C, black

line) and enthalpy (ΔH, blue line) profile illustrating relative thermodynamic energy and

kinetic pathway of O2 adsorption and SO2 desorption on S–Ni(111) surface with initial

coverage θS = 0.25 ML. Adapted from J. Catal. 2009, 263 (2), 380-389 with permission of

Elsevier.

Although this oxidative treatment is effective it has an associated drawback which is

a high likelihood for Ni oxidation. Therefore, ideally this approach could be improved if the

oxygen is supplied by oxygen ion flux through the electrolyte and interacts selectively with

sulfur. In response to this problem, YSZ could be total or partially substituted by other

ceramic phases with higher oxygen conductivity as CGO showing greater sulfur tolerance60.

The presence of a highly oxygen conductive phase in the anode permits a certain degree of

electrochemical oxidation of S* to SO2 facilitating sulfur removal. This strategy of using

mixed oxides with high oxygen mobility seems to mitigate (but not fully eliminate) sulfur

poisoning in both SOFCs and catalytic processes.

5. Systems approaches to carbon and sulfur tolerance

28

As discussed above, carbon and sulfur represent a technical challenge for SOFC

technology. Although this review focuses on anode materials design strategies, currently the

main method for mitigating carbon and sulfur poisoning is processing of the fuel externally

to the SOFC stack, so it is worthwhile briefly reviewing these aspects of SOFC-based power

generation systems which are intended to achieve carbon and sulfur tolerance. Haldor

Topsøe have been involved in gas cleaning for SOFCs for many years, and John Bøgild

Hansen has very helpfully reviewed the company’s experience in this area107.

The main strategy used in relatively clean fuel (e.g. consumer grade natural gas, LPG

etc.) is fuel reforming. This converts most of the hydrocarbons to H2 and CO. The reformer

can be provided with oxidising gas as fresh steam or as recycled anode gas. CO2 can also be

used in so-called dry reforming. The use of reformers has been demonstrated practically in a

number of systems. Reformers can add significantly to the cost of the system, with the cost

reported as being similar to the fuel cell module itself108.

For dirtier fuels, such as gasified biomass or coal, or biogas, fuel processing becomes

more complicated and hence expensive109. The feedstock may contain up to several percent

of sulfur compounds as well as other contaminants such as alkali metals, halides and

phosphorus compounds. For solid fuels, the gasification process which converts the

feedstock into a gaseous form suitable for fuel cells can produce significant amounts of

aromatic compounds, including smaller molecules such as toluene, and larger polyaromatic

compounds which can cause carbon deposition in SOFCs. All of the feedstocks mentioned

above contain methane and/or short chain hydrocarbons, which again can cause carbon

deposition. These feedstocks need several layers of treatment, from desulfurisation to

particulate filtering, although most of this is not exclusive to SOFCs, so may not impact on

the economics of the process compared to competing technologies.

29

For SOFC-based systems using these fuels, the level of desulfurisation required is

crucial to the cost and complexity of the system. For example, to reach levels below 10 ppm,

deep desulfurisation is needed, which is normally carried out at 40 bar of pressure or

higher110-111, necessitating gas compression and increasing safety issues. More recent work

has reduced sulfur below 1 ppm at 10 bar, but this pressure is still too high112.

A final intermediate case is provided by liquid transport fuels, which may be an

important market for SOFCs in future. These have largely been cleaned of contaminants

such as tars and metallic impurities, but may still contain varying levels of sulfur. In general

the level of sulfur in these fuels is being driven downwards due to legislation. Ultra-low

sulfur diesel (ULSD) standards are normally around 10 – 50 ppm, although the actual

content of sulfur may be as low as 2 – 3 ppm. For aviation fuels, the sulfur levels are up to

3000 ppm, with an average of around 600 ppm.

As discussed above, some of these levels of sulfur may be too high for Ni/YSZ

anodes, although there are examples of SOFC stacks being run on reformed ULSD without

desulfurisation. A Topsøe SOFC stack was run on steam reformed ULSD (<10 ppm S) for 1200

hours113. After an initial 150 h period of rapid degradation there was only 0.2%/1000 h

voltage degradation over the rest of the test. Delphi tested a 5-cell stack with simulated

reformate containing 2.5 ppm sulfur, and also found a rapid initial degradation followed by

stable performance114, indicating that SOFCs may be able to operate stably with ULSD

reformate without desulfurisation, albeit with a performance drop caused by sulfur

poisoning. If SOFCs can tolerate ULSD-levels of sulfur, they should be economically attractive

for truck auxiliary power units (APUs)115, and higher sulfur tolerance would allow them to be

used in aircraft APUs).

30

Crucially, both the examples above had no hydrocarbons in the reformate. Even low

levels of compounds such as ethylene are capable of causing carbon deposition on Ni, even

in thermodynamic regimes which do not encourage carbon formation116. In the first example

above, a secondary reformer was used to remove the low levels of hydrocarbons produced

by the first reformer117, while the second example, in common with most studies, used a

simulated reformate without these problematic molecules.

From this summary, several points relating to fuel processing become clear. The first

is that since producing a suitable feed gas for an SOFC from almost any starting material is

technically feasible, then the driver for carbon and sulfur tolerance in the anode itself is

almost entirely economic. The costs of reforming and desulfurisation are each of a similar

order of magnitude to the cost of the fuel cell itself, and become more important for lower

power and/or more portable systems. That being the case, it becomes clear that the key

targets for carbon and sulfur tolerance in SOFC anodes are related to either eliminating or

reducing the specifications for the reforming and desulfurisation units. So for carbon

tolerance, some important targets could be: to be able to operate directly on methane,

propane, ethanol or biogas (methane-carbon dioxide mixtures), preferably without steam

generation or off-gas recycling; or to be able to tolerate low levels of species such as ethene

and tars. Meanwhile, for sulfur tolerance, important targets are tolerance to the low sulfur

levels in ULSD or natural gas (<10 ppm), then for fuels with higher levels of sulfur, tolerance

to the levels of sulfur after hydrodesulfurisation catalysis (i.e. without deep desulfurisation

at high pressure, or ZnO or other sorbents), then finally tolerance to the levels in those fuels

themselves.

6. Materials design strategies for carbon tolerance in SOFC anodes

31

6.1 Ni-YSZ cermets

Cermet-based anodes are the most widely used anodes in SOFCs. Traditionally they

have the advantage that the best oxide ion conductors can be used, while the metal can

provide the catalytic activity and the electronic conductivity. The industry standard material

is yttria-stabilised zirconia (YSZ), so-called because the addition of yttrium ions to the

zirconia stabilises the cubic form of the material under a wide range of temperatures. The

presence of the Y3+ ions also creates oxygen vacancies, which allows oxygen ion transport,

with the maximum conductivity being with 8 mol% of yttria added (Known as 8YSZ,

(ZrO2)0.92(Y2O3)0.08). YSZ is very stable towards high temperatures and a wide range of

oxidising conditions. It is also the most widely used electrolyte, having extremely low

electronic conductivity, meaning that issues of compatibility between the anode and

electrolyte are eliminated by using YSZ in both the anode and electrolyte. The industry

standard metal is nickel. Nickel is relatively cheap and highly active towards various

reactions involving carbon, as well as being active towards electrochemical oxidation. It is

also more stable than other base metals towards high temperatures, and unreactive

towards common electrolytes such as YSZ.

Both components of Ni/YSZ have problems relating to carbon and sulfur tolerance.

Nickel easily dissolves both carbon and sulfur, leading to volume expansions which can

cause structural failure of the anode. Nickel is also an extremely good catalyst for solid

carbon formation, meaning that carbon filaments can be formed, potentially destroying the

structure of the anode and blocking gas diffusion pathways as discussed in section 3. As well

as causing failure of the cell, this propensity towards carbon formation also renders nickel a

poor catalyst for direct oxidation of hydrocarbons, meaning that high quantities of steam

need to be used for cells running on methane or higher hydrocarbons. It also makes nickel

32

susceptible to poisoning by aromatic or polyaromatic compounds which may be present in

gasified coal or biomass118-124.

The problems of YSZ relate to its inertness and consequent inability to mitigate any

of the failings of nickel. It has little activity towards electrochemical oxidation or any of the

other important catalytic reactions and possesses extremely low electronic conductivity,

meaning that once the nickel has deactivated the cell is useless. It also has no oxygen

storage capacity and no ability to absorb sulfur, either of which could help improve carbon

or sulfur tolerance. Strategies to mitigate the issues with Ni and YSZ are described schematically in

figure 13.

Figure 13. Schematics of the most common materials strategies to improve carbon

tolerance. The diagram shows a strategy and does not imply a specific mechanism.

As shown above, the propensity of Ni/YSZ anodes towards carbon deposition is

largely a function of nickel’s ability to catalyse carbon formation. Thus it is natural to look at

partially or entirely replacing the nickel. Since nickel is an exceptional electrocatalyst, many

efforts to replace this have focused on substituting some other potentially active material

for some of the nickel rather than replacing the nickel entirely. The rationale behind this is

two-fold: firstly, heteroatoms could break up large continuous nickel surfaces which are

predisposed towards carbon deposition; and secondly, to enhance the rates of reactions

which compete with carbon deposition, such as carbon oxidation and steam reforming.

33

6.2 Alloying with noble metals

The so-called noble metals (roughly the second and third row transition metals in

groups 8 – 11) may offer enhanced catalysis as well as reducing carbon deposition, and are

known from conventional catalysis to be active in very small quantities. The earliest example

was gold, which causes a reduction in carbon deposition under oxygen-methane mixtures,

at the expense of methane reforming activity. Carbon deposition was reduced by up to 8000

times with one-fifth of the nickel replaced with gold125-126. SOFCs using Au doping have been

tested in dry and humidified methane atmospheres, where they showed no carbon

deposition after 200 hours127. A stabilisation of CHx surface species leading to a reduction in

the rate of graphite formation was found to be responsible128.

Impregnation of Pd into Ni/YSZ anodes showed a marked decrease in polarisation

resistance in hydrogen, methane and ethanol, with suppression but not elimination of

carbon deposition under the carbonaceous fuels129. The same was found for impregnation of

Pd into Ni on Ce0.9Gd0.1O1.95 (CGO)130. Carbon deposition was primarily found to occur in Pd-

poor regions. Likewise impregnation of Ru, also into Ni/CGO anodes, was found to improve

stability under methane, ethane and propane under load and short periods at open circuit

voltage (OCV), with the caveat that a 25 - 40 µm CGO electrolyte was used – CGO possesses

a relatively high electronic conduction under reducing conditions, meaning that there would

be a significant oxygen flux even at OCV131. Carbon deposition was not seen, as measured

from carbon balance analysis. This study also noted one of the problems with the use of Ru,

which is its tendency towards vaporisation during synthesis.

A comparative study looking at Ru, Pt, Pd and Rh on Ni/YSZ found that Ru, Pt and Pd

suppressed carbon deposition under dry methane compared to the unpromoted material,

while Rh actually increased carbon deposition132. In addition, Ru and Pt improved the power

34

density in fuel cell tests. Rh has however been shown in other tests to reduce carbon

deposition on Ni-CGO in microreactors and give more stable performance in button cells in

humidified butane, although the butane used in this paper contained sulfur compounds, so

the improved performance may be due to improved sulfur tolerance133. A further difference

could be due to the high activity of Rh-ceria for water-gas shift compared to Rh on other

supports134.

Silver has been shown to be a good catalyst for CO oxidation, with no propensity

towards carbon deposition135. Co-doping of Ni/YSZ with Ag and Cu was found to reduce

carbon deposition by a factor of three or four relative to samples doped with Cu or Co, with

the carbon deposited being more amorphous136. Silver can also be deposited electrolessly,

and this appears to reduce carbon deposition in dry methane and ethane137. Cells produced

in this way were stable over a period of 100 h in dry methane138.

Noble metals are also used in so-called catalyst or barrier layers in anodes – where a

layer is placed between the active anode and the gas supply. This serves to reduce the

hydrocarbon content in the anode by blocking hydrocarbons from entering or water and

CO2 from leaving. If reforming catalysts are used they can also increase the reforming rate.

This is at the cost of power density, due to the increased resistance to diffusion to the

electrochemically-active layer. In the original work showing this effect, Ru supported on

CeO2 was used in a catalytic reforming layer over a Ni-YSZ anode which showed good

stability in iso-octane-air-CO2 and propane-air mixtures139. Ir-CGO has also been used

successfully140, but more recent work has shown that barrier layers made from materials

which show less reforming activity such as Ni-Cu on Zr-doped ceria141, Ni-doped ceria142,

La0.75Sr0.25Cr0.5Mn0.5O3 (LSCM)-CeO2143, partially-stabilised zirconia and zirconium-doped

ceria144 and even Ni/Al2O3145 can also give low or no carbon deposition in the Ni-based

35

anodes underneath, indicating that the main effect may be the barrier layer effect rather

than the reforming activity. A further drawback to practical use of barrier layers is their non-

conductivity, which may hinder current collection. This could be combatted by incorporation

of reforming catalysts into a mainly metallic composite146. Nevertheless, further work on

barrier layers may be informed by section 7 which discusses sulfur and carbon tolerant

catalyst materials.

6.3 Alloying or replacement of nickel with base metals

A similar rationale is behind the use of top row transition metals Co, Cu and Fe,

which also act to break up the continuous nickel sheets. In comparative studies with Ni/YSZ-

based anodes, all of these elements, while reducing the carbon deposition, also reduce the

electrocatalytic activity compared to pure Ni/YSZ147. Despite this, the benefits of reduced

carbon deposition may outweigh the reduced performance, so these systems have been

extensively studied, including using different fabrication techniques such as impregnation,

microwave irradiation148, and electroless deposition149. Impregnation was used to produce a

series of Ni-Cu alloys on a porous YSZ substrate which was also impregnated with CeO2150.

While no carbon was detected on the pure Cu anode, anodes with a Cu-Ni ratio of 9:1 and

lower displayed significant weight gain due to carbon deposition, although the deposition

seemed to be self-limiting at 4:1 and higher, and the cell structure was not destroyed.

Interestingly, a higher reduction temperature resulted in lower carbon deposition, and it

was suggested that this is caused by copper enrichment at the surface of the alloy. Cell tests

on the 4:1 Cu:Ni anode showed a large increase in performance caused by carbon

deposition improving electronic percolation. A catalytic study of Ni-Cu/YSZ+CeO2 with the

Ni, Cu and CeO2 impregnated into the YSZ also found significant carbon deposition in the

50:50 Ni:Cu sample after exposure to a 2:1 CH4:O2 mixture for 20 h at 800 °C151. The amount

36

of carbon was also not reduced by addition of Pd to the composite. A sample with a 25:75

Ni:Cu mixture in contrast showed no carbon deposition.

Electroless deposition149 produced an inhomogeneous distribution of copper, leading

to carbon deposition in copper-poor areas. When microwave irradiation was used to deposit

copper nanoparticles on a Ni-YSZ anode, the effect was similar to cells produced using

impregnation of a Ni-Cu solution, indicating that alloying during synthesis may not be

necessary to reduce carbon deposition148.

Tests using copper alone have shown very low activities compared to nickel, with

Cu/YSZ anodes showing very low OCV with a dry methane fuel, indicating that it has little

activity towards methane oxidation147, 152-153. This highlights the importance of the ceria used

in a number of the above studies, which will be discussed later.

Iron has also been tested. In a series of studies it was found that iron could reduce

carbon deposition in quantities as low as 10%, in both Ni-Fe/La0.9Sr0.1Ga0.8Mg0.2O3 (LSGM)154

and Ni-Fe/CGO155 anodes. One study compared Ni and Ni0.9Fe0.1 as supports in metal-

supported cells under humidified methane at 650 °C156. They found that while carbon was

deposited in both supports, the carbon in the Ni-Fe support was amorphous, did not retard

the rate of the methane reforming reaction in the support, and prevented carbon

deposition in the Ni-CGO anode layer (from SEM). In contrast, carbon on the Ni support was

highly graphitic, completely deactivated the reforming reaction and led to cell failure due to

carbon deposition in the anode in less than 10 h.

Cobalt, similarly to nickel, has known catalytic activity towards carbon-containing

compounds, so has been investigated in anodes. It seems promising for electro-oxidation of

CO, with alloys with Cu producing higher performance in syngas than an equivalent Ni or Cu

only cell157 and Ni alloys with Co producing higher exchange current densities in syngas than

37

in hydrogen158. Cobalt is expected to have less tendency to carbon deposition than nickel,

but in tests where nickel is entirely replaced with cobalt in a YSZ cell, carbon deposition was

still observed after 15 hours in dry methane. No performance loss was observed however,

indicating that the carbon is not poisoning the activity of the Co/YSZ cell, although it could

still eventually cause structural failure159. Under syngas, cells based on Ni-Co alloys became

completely delaminated in CO:H2 ratios above 60:40, indicating that Ni-Co alloys are still

vulnerable to carbon deposition158.

In a catalytic study, Co-Cu/YSZ+CeO2 with a 50:50 mix of Co and Cu produced by

impregnation into YSZ showed very little carbon deposition after exposure to a 2:1 CH4:O2

mixture for 20 h at 800 °C, much lower than a comparative Ni-Cu sample151. A similar study

conducted with dry butane found that the amount of carbon deposition increased with

increasing metal loading, indicating that the metal is still encouraging the formation of

carbon, despite the lack of nickel160. This carbon was amorphous, and did not cause any

short term degradation of the anode performance, although metal particles were seen

encapsulated in the carbon fibres formed, indicating that the carbon deposition would cause

long term disruption of the anode structure.

Tin is another metal which has been used to reduce the tendency of nickel to form

carbon. Tin has the advantage that it alloys easily with nickel, and the tin segregates to the

surface of the particles, meaning that a large improvement in the stability in dry methane

and isooctane while under load can be achieved with only 1% of tin with respect to nickel161.

The effect of 1% tin in reducing carbon deposition was also seen in ethanol-fuelled SOFCs162.

There has been some debate about the role and effect of tin. One study replicated

some of the testing conditions in reference 161 as well as other conditions with dry and wet

methane at different temperatures, but failed to observe improvements in carbon tolerance

38

under most conditions163. They ascribed this to their use of electrolyte-supported cells

(compared to anode-supported cells in reference 161). Further work by the group suggested

that the tin appears to cause the formation of less stable carbon species, meaning that any

carbon deposited in the electrochemical region is oxidised, but that carbon can still form

outside of this region164. TPO experiments agree that the stability is due to a reduced rate of

graphitic carbon formation rather than total elimination of carbon formation, and that

keeping the cell under load is still necessary165. Another paper suggests that hydroxyl groups

formed at the tin atoms on the surface are responsible for the effect (figure 14)166, while a

study using DFT and microreactor tests showed that the effect is due to an increase in

formation energy of carbon nucleation sites with no increase in energy for CO formation167.

One further study failed to show any improvement when using 1% tin, with increased

carbon deposition on Ni-CGO in microreactor tests on humidified butane, which they

ascribed to a low operating temperature of 600 °C133.

Figure 14 - Polarization curves and power densities of (a) Ni–CGO and (b) Sn/Ni–CGO

anode-supported single cell SOFCs operating at 650 °C with H2 and CH4, and (c) their voltage

variations measured at 650 °C in CH4 as a function of time. Reproduced from Reference 166

with permission of The Royal Society of Chemistry.

6.4 Replacement of nickel with non-metal electronic conductors

39

It is also possible to use electronically-conducting non-metals, and these should have

intrinsically less tendency towards carbon deposition. They can also have the advantage that

nanostructured catalysts, especially precious metal catalysts, can be used without the loss

of activity or function caused by alloying with base metals like copper. The carbon deposited

due to hydrocarbon cracking is conductive, and one study exploited this. Porous YSZ scaffold

was exposed to dry butane, depositing a conductive carbonaceous layer. This was then

impregnated with ceria and/or noble metals to improve the catalysis. Pd showed the best

activity out of Pd, Pt or Rh in these cells 168 (Figure 15). The cells’ performances in butane

showed a much smaller improvement through adding a catalytic metal, which was

suggested to be due to saturation of carbon on the active metal surfaces169.

Figure 15. Potentials (open symbols) and power densities (closed symbols) as a

function of current density at 973 K for H2 (diamonds), n-butane (triangles), and CH4

(circles). In (A), the cell had a C-ceria-YSZ anode; in (B), the anode also contained 1 wt% Pd.

Reproduced by permission of The Electrochemical Society from Electrochem. Solid-State

Lett. 2003, 6 (11), A240-A243. Copyright 2003, The Electrochemical Society.

A longer term test of the Pt/C-CeO2-YSZ cell in dry methane showed a 15% drop in

performance over 100 h. The impedance spectra showed an increase in the Ohmic

resistance, so the loss in performance was attributed to a loss in carbon. An earlier paper by

the same group had shown that for a Cu/C-CeO2-YSZ cell, the OCV in C4H10 settles over time

40

to a value of 0.85 V, implying that an equilibrium is reached between partial oxidation

products170. The authors also observed gradual changes in the performance under load,

implying changes in the carbonaceous layer over time. These results taken together suggest

that the carbonaceous layer will reach an equilibrium over time depending on the fuel,

oxygen flux, presence of catalytic metals and other factors.

A combined thermodynamic and experimental investigation looked at the stability of

a range of electronically-conducting carbides, borides, nitrides and silicides in humidified

hydrogen with a partial pressure of CO of either 10-1 or 10-6 at 950 °C64. Of these, only the

tungsten carbides and molybdenum carbides were stable, and then only at the higher

concentration of CO. Since carbides should have an intrinsic carbon tolerance, as well as

having been investigated in catalysis for various reforming reactions, this marks them out as

potential anode materials. Despite this, tungsten carbide has only recently been

investigated in actual anodes, in a WC-YSZ anode171. The performance with pure WC-YSZ

was poor, but could be improved several times by impregnation of a Ru-CeO2 catalyst. The

cell was stable under dry methane, with low carbon deposition which was not detrimental

to the performance, but careful balancing of the fuel utilisation is required to prevent

oxidation of the WC. A follow-up study tested fuel cells in humidified methane and

methane-hydrogen mixtures, with maximum power densities of ~80 and ~250 mW/cm2

respectively at 900 °C with a 300 µm YSZ electrolyte172. In a further study, the Ru was

replaced with Ni, and this cell showed stable performance over a week under humidified

methane at 850 °C with no evidence of carbon deposition from SEM173. Removal of the ceria

reduced the performance, but the high stability remained, indicated that it is the tungsten

carbide which is preventing carbon deposition on the nickel particles.

41

Molybdenum carbide has been tested in a proton-conducting cell with a

BaCe0.7Zr0.1Y0.2O3 − δ (BCZY) electrolyte operating on ethane174. The carbide was stable and

showed very low levels of carbon deposition both in the cell and when exposed to ethane as

a powder as assessed by thermogravimetry and XPS. Under hydrogen at 0.55 V there was

degradation of 5% over 100 h which was attributed to reduction of the carbide to metallic

molybdenum.

One study looking at molybdenum as a dopant in a Ni-YSZ anode observed extremely

good performance under humidified methane in steam reforming activity, low carbon

deposition and fuel cell tests, especially in materials which were reduced in the humidified

methane rather than in hydrogen. This was suggested to be due to the formation of highly

active molybdenum carbide, but no further investigations were conducted to test this

hypothesis125.

The ability of tungsten and molybdenum to form carbides means they may be able

to Tungsten and molybdenum have also simply been used as promoters in a similar way as

the other base metals described above. In a combined mass spectrometry-

thermogravimetric study, Mo-Cu, W-Cu and Cu doped Ni-YSZ were exposed to dry methane

at 800, 650 and 500 °C. The samples were produced so as to retain Mo and W in their

metallic state. The Mo and W doped samples showed improved tolerance to carbon

deposition, which may have been due to the formation of carbides175.

6.5 Increasing alkalinity

A third strategy involves increasing the basicity of the material, especially by using an

alkaline earth. This strategy is known to reduce carbon deposition in conventional catalysis.

All of the alkaline earths have been tested except Be, which can be highly toxic. They have

strong basicity, and this modifies the electronic state of nearby nickel to make it less active

42

for carbon deposition176. In the particular case of MgO, (Ni,Mg)O solid solutions are formed,

from which the nickel can be reversibly exsolved177. This may help when regeneration is

required.

Microreactor tests using Ni/YSZ cermets doped with small amounts of MgO, CaO and

SrO (0.2, 1 and 2% of anode mass) showed that CaO and SrO suppressed carbon deposition

even at the lowest loadings, while MgO increased the rate of carbon deposition176. A

separate study looking at the same elements plus BaO and La2O3 made similar findings, but

also noted large changes in microstructure and conductivity depending on which promoters

were used, highlighting the need to take into account all the effects of promoters178. They

found that CaO-promoted cells had the highest performance in humidified methane, due to

a combination of good steam reforming activity, high conductivity and low carbon

deposition.

The alkaline earths can also dissociatively uptake water, which is then able to oxidise

nearby carbon. First principles studies indicate that BaO is the best alkaline earth for water

adsorption58 and its effect is shown by a study on the effect of Ba on Ni/YSZ anodes fuelled

with dry propane179. Using thermogravimetic analysis (TGA) and Raman spectroscopy they

observed water incorporation (weight gain and O-H stretching modes) in the anode. This

water uptake may assist the oxidation of carbon on the Ni particle and was further

evidenced using DFT calculations179.

This type of carbon elimination process occurs preferentially at the BaO/Ni

interfaces. The catalyst works synergistically: the water splitting takes place on barium

oxide, the carbon deposition occurs on Ni sites of BaO/Ni and the subsequent steps occur

near the BaO/Ni interfaces. Figure 16 summarizes the proposed mechanism for carbon

mitigation in a BaO/Ni-YSZ composite anode of a fuel cell fed with propane. A combined

43

microreactor and fuel cell study on Ni-Cu/CGO anodes doped or not doped with Ba showed

that the Ba does not reduce the rate of carbon deposition in microreactor tests in dry

methane, but does reduce the rate of carbon deposition in fuel cell tests under load180. It

was suggested that Ba assists in oxygen transfer from the electrolyte to the metal surface,

although other explanations were not ruled out. Impregnating Ni-CGO with BaO also greatly

reduces carbon deposition in humidified CO181.

Because of the mechanisms of carbon suppression of the alkaline earths, the

nanostructure of the anode plays a large role in the results for these elements as the oxide

must be very near to the nickel without completely covering it. Incorporation of CaO by solid

state methods was found to decrease the performance of the cell176, while doping with Ba

by impregnation was found to eliminate carbon deposition while only lowering power

density by around 10%182. Experiments using vapour deposition of Ba on Ni/YSZ showed

remarkable stability in dry C3H8 with a sustained power output of 0.4 W/cm² over 100 hours

compared to complete deactivation after less than 1 hour for a cell without Ba179.

Figure 16 Proposed mechanism for water-mediated carbon removal on the anode

with BaO/Ni interfaces. Large balls in bright blue, green, red, blue-grey and purple are Ni,

Ba, O of BaO or YSZ, Zr and Y, respectively, whereas small balls in red, white and grey are O

44

from H2O, H and C, respectively. D1 is the dissociative adsorption of H2O, whereas D2 is the

dehydrogenation of hydrocarbons or the CO disproportionation reaction. TPB is the triple

phase boundary. Reprinted by permission from Macmillan Publishers Ltd: Nat. Commun. 2,

357, copyright 2011

An expansion of this technique has been to use Ba-containing proton conductors,

which have the dual ability to store water and provide some ionic and electronic

conductivity. Impregnation of yttrium-doped barium zirconate (BYZ)183 reduced carbon

deposition at the same time as improving the performance, but only if the BYZ was present

in the electrochemically active zone. This indicates that the improvement may be due to the

increased electrochemical oxidation activity. A DFT study on Ni on yttrium-doped barium

cerate (BYC) or YSZ indicated that the termination of the surface of the oxide is important –

BaO-terminated surfaces adsorb water much more strongly than ZrO2 or CeO2-terminated

surfaces, and are thus more able to oxidise carbon at the triple phase boundary58.

The water storage capacity of one material – Ni- BaZr0.4Ce0.4Y0.2O3 (BZCY) was actually

measured, and found to be four to five times higher than a selection of other anode and

catalyst materials184. These materials included Ni-BaZrO3, indicating that the water storage

capacity is not solely due to the barium, but may also have some contribution from the

proton or electron conductivity, the other elements present or, as mentioned above,

differences in the preferred surface termination. The Ni-BZCY showed much lower levels of

carbon deposited in microreactor tests at all temperatures and ethanol-steam mixtures, and

anodes based on Ni-BZCY were stable under ethanol-steam for 180 h at 750 °C, in contrast

to Ni-SDC and Ni-YSZ anodes which failed after less than 2 h.

BaZr0.1Ce0.7Y0.1Yb0.1O3-δ (BZCYYb) is a MIEC with proton conductivity, but on its own it

has poor performance due to low electronic conductivity185. When impregnated with metals

45

the performance improves markedly, and with Ni-Cu impregnated there is no carbon

deposition as measured by Raman under dry methane at OCV at 750 °C, while Ni-

impregnated cells showed no carbon deposition under humidified methane at OCV at 750

°C. Ni-BZYYb composites have also been used with ethanol as the fuel, in this case there was

significant carbon deposition but it was limited to the outside of the anode and was

amorphous in nature, indicating that these materials may also reduce the amount of

graphitic carbon when carbon deposition does occur186.

Lastly, an in situ Raman study on BaO and barium zirconates showed that, as well as

the water adsorption effect on carbon deposition, there was also a reverse Boudouard

effect in the barium zirconates, where CO2 adsorbed as CO32- ions was able to react directly

with deposited carbon at the triple phase boundary. This effect was not seen in BaO, as the

BaCO3 formed was too stable187.

In theory the alkali metals should also reduce carbon deposition, and this has been

shown for Li in reforming layers in SOFC anodes, where doping with Li or La and Li reduced

carbon deposition in the Ni-Al2O3 layer under an 11.5:1 CH4:O2 mixture. It should be noted

that in this study, Ca was more effective at reducing carbon deposition, but Li (on its own or

combined with La) also showed an improvement in the methane reforming reaction188. The

main concern with use of alkali metals, especially Li, is their volatility. One study used a Li-

ion conducting material, Li0.33La0.56TiO3, as the support rather than Al2O3, and found that this

improved long term stability189. The authors measured the lithium loss, and found that the

lithium content reduced from 4.68 to 4.63 wt% after 100 hours at 800 °C in air, compared to

0.42 to 0.20 wt% for a sample of Li-doped Al2O3. The fact that this study was carried out at

800 °C indicates that the volatility at lower temperatures may be less of a problem.

46

Other than the oxides of alkali metals and alkaline earths, there are a few other basic

oxides which have been tested. Under the conditions of an SOFC anode Mn occurs in the

form of MnO which is a basic oxide. Under wet methane at 800 °C, Ni-YSZ doped with 2 or

5% of the NiO replaced with MnO, the cells lasted less than 1 h, similar to the performance

of an undoped cell. With 10% MnO the cell showed dramatically improved performance,

with no degradation over 40 h190. Microreactor tests showed that the amount of carbon

decreased by over 150 times compared to undoped Ni-YSZ, and this was shown to be due to

a relative decrease in the rate of methane cracking compared to steam reforming.

Conversely, increasing the acidity can worsen carbon deposition. Adding 2.7%

aluminium oxide, an acidic oxide, to a Ni-YSZ anode reduced the amount of carbon

deposition in a simulated CH4-CO2 biogas mixture, due to an improvement in the dry

reforming rate191. However, when the amount of aluminium oxide was increased to 10%, the

carbon deposition was increased due to the increased acidity.

6.6 Use of ceria and other oxygen storage materials

The concept of oxygen storage materials was first used in three-way catalysts, where

a partially reducible oxide is able to supply oxide ions during periods of fuel-rich conditions,

and is reoxidised during fuel-lean conditions192. In SOFCs this may help to prevent carbon

deposition by increasing the rate of supply of oxide ions for oxidising carbon on the surface

of the anode. In reality, for SOFCs the only oxygen storage material of note is ceria and

doped ceria. Although there are potentially other oxygen storage materials relevant to

anode conditions, only one, MnO190, has been used (as discussed above), and the effect of

its oxygen storage capacity was implied rather than confirmed through experiments.

Ceria is particularly attractive due to its high ionic conductivity arising from the

creation of oxygen vacancies on its fluorite lattice when exposed to reductive atmospheres

47

becoming a mixed conductor193-195. These structural defects are known to improve the

oxygen mobility of surface and bulk oxygen of ceria resulting in an enhanced oxygen storage

capacity (OSC) which at the same time benefits the oxidation processes196. These boosted

redox features might be useful to eliminate C and S adsorbed species via oxidation and

release them as CO2 or SO2.

The oxygen vacancies population and consequently the ionic conductivity of ceria

may be enhanced using promoters196-198. In particular, acceptor-dopants (e.g., Sm2O3 or

Gd2O3) are used to substitute some cerium ions in the fluorite structure resulting in the

formation of oxide-ion vacancy site to compensate the charge-balance. Furthermore, the

well-known activity of ceria based catalysts for soot combustion in automobiles makes this

material interesting for SOFC anodes194, 199. For instance, Gd-doped ceria mixed oxide was

employed for methane and hydrogen oxidation exhibiting high current density and good

tolerance towards carbon deposition200.

Initial work used impregnation to introduce ceria into porous Ni-YSZ anodes. It was

reported by some papers to eliminate carbon deposition while using dry methane201, but

others disagree, showing deactivation over only 30 mins.152 Doped cerias can also be

impregnated. Doped cerias are oxide ion conductors, and have some electronic conductivity

under the conditions in a fuel cell anode. This has the effect of extending the triple phase

boundary region, which outweighs the fact that pure ceria is a better catalyst for direct

oxidation than doped ceria153, 202. Impregnation of samarium-doped ceria (CSO)

nanoparticles into a Ni-YSZ electrode produced a cell with stable performance under dry

methane over 1000 h, which was attributed to suppression of nickel sintering and carbon

deposition observed in separate catalytic reactions with methane-air mixtures.203

Impregnation of gadolinium-doped ceria (CGO) into anodes of nickel/scandia-stabilised

48

zirconia (Ni/ScSZ) showed relatively stable performance under humidified methane,

although carbon deposition could still be observed, and had the effect of improving the

performance initially (due to formation of conducting carbon networks), before eventually

degrading.204 It should be noted that Ni/ScSZ cells without CGO also showed relatively stable

but inferior performance, indicating that the main effect of the CGO was to improve

performance rather than reduce carbon deposition.

The ceria-zirconia system is well known in catalysis for its high oxygen storage

capacity as the seven coordinate zirconium ions serve to stabilise Ce3+. Ce0.9Zr0.1O2 was

impregnated into Ni-YSZ anodes and was found to greatly reduce carbon deposition in

methanol at OCV, and almost eliminate it under load, as measured by EDX and TPO205.

Cu/CeO2-YSZ anodes show performance in cell tests under dry methane close to that

of Ni/CeO2-YSZ, but with no carbon deposition152. Importantly, the ceria needs to be

impregnated before the Cu, showing that the catalysed step is the oxidation of

hydrocarbons on ceria using oxide ions from the electrolyte.206 The non-catalytic nature of

the copper was reinforced by a study which showed that the replacement of copper with

gold showed very little change in performance207.

These results indicate that ceria is active towards electrochemical oxidation, while

copper simply acts as a current collector, meaning that the use of ceria allows the complete

replacement of Ni with Cu. The advantage of using copper rather than nickel is that copper

does not catalyse carbon formation, but the low melting point of copper oxide (1326 °C)

means that traditional electrode fabrication methods cannot be used. The above papers use

impregnation of copper nitrate into porous YSZ substrates. To improve the conductivity of

this type of cell, Fe was added to Cu/CeO2-YSZ anodes, causing carbon deposition, which

49

initially improved the performance by improving the conductivity before causing it to

decline slowly208.

Cells based on Cu produced by impregnation may be limited by their electronic

conductivity at low Cu contents. In this case, small (<2 wt%) amounts of carbon deposited

from exposure of the anode to dry butane were found to improve performance, again

because of an increase in electronic conductivity170. The rate of carbon deposition was the

same on YSZ and Cu/YSZ-CGO, implying that Cu and CGO are not catalysing the carbon

deposition. In addition, oxidation and re-reduction returned the cell to its original

performance, suggesting that the carbon deposits caused no permanent changes in the

structure of the anode.

Further improvements to these Cu/CeO2-YSZ anodes can be made by using CSO

rather than pure ceria209. The developed system was suitable for several type of fuels and

conserves high power densities after switching from one fuel to another. Figure 17 presents

the effect of switching fuel type on the cell with the Cu-(doped ceria) composite anode at

973 K. As shown in the plot, 1-butene and ethane leads to the higher power density while

toluene generates a current sensitive current drop. Toluene as an aromatic compound

increases C formation, however they observed that the anode was self-cleaning upon

switching to n-butane. Use of a porous doped ceria interlayer can also reduce carbon

deposition with humidified methane as fuel210.

50

Figure 17. Effect of switching fuel type on the cell with the Cu-(doped ceria)

composite anode at 973 K. The power density is shown as a function of time. The fuels were:

n-butane (C4H10), toluene (C7H8), n-butane, methane (CH4), ethane (C2H6), and 1-butene

(C4H8). Reprinted from Nature 2000, 404 (6775), 265-267, with permission from Nature

Publishing Group.

Since doped cerias are oxide ion conductors in their own right, it is possible to

dispense with the YSZ altogether. Cells produced using Ni-CGO synthesised via a Pechini

method showed no carbon deposition from Raman under humidified methane at 600 °C for

50 hours, although it should be noted that the cells, which used a 20 µm CGO electrolyte,

showed extremely low OCVs, and no attempt was made to find out whether this low OCV

was due to oxygen leaks into the anode or to the non-zero electronic conductivity of CGO211.

High levels of carbon deposition were still observed under humidified propane. A Ni-CSO

anode was operated on dry methane at 600 °C for 72 hours under a current load of 300 mA

with very low levels of carbon detected by FIB-SEM and TPO post-test analysis, although

again thin CSO electrolytes were used and OCVs of ~0.9 V obtained212. Ni on Mo-doped ceria

showed less than 0.04 wt% carbon deposition after exposure to a methane-oxygen mixture

(5:2 molar ratio)213. Cells based on this material using 400 µm LSGM electrolytes showed

51

reasonable stability over 10 h under load and wet methane, although unfortunately the

amount of carbon deposition was not quantified.

There is strong evidence that CSO has high enough electronic conductivity under

hydrogen that the limiting factor is not the triple phase boundary length but the surface

area of the CSO, intimating that an optimal strategy may be to optimise the surface area of

the ceria and improving the catalysis for hydrocarbon oxidation, utilising the minimum

amount of current collecting metal necessary214. This electronic conductivity, along with the

reforming activity of CGO, has been used in the current-collecting layer, where CGO-coated

Ni was used at the top of the anode to reduce exposure of Ni to unreformed methane. The

cells were stable in dry methane at 610 °C over 1000 h, compared to cells without the CGO-

coated Ni layer which failed after <200 h215.

While work on metal-ceria composites has understandably focused on the doped

cerias with the highest ionic conductivities (CSO, CGO etc.) some work has been done on

materials with higher oxygen storage capacities. Ce0.9Zr0.1O2-based (CZO) impregnated

anodes were found by EDX to reduce carbon deposition in humidified methane compared to

CeO2-based anodes216. A larger effect was seen by replacing Ni with Cu, but unlike

replacement of CeO2 with CZO this had a large negative effect on performance for total

replacement. A partial replacement of Ni with Cu on CZO was found to be the best

compromise between carbon tolerance and performance.

A further advantage of ceria and doped ceria is that the methane cracking reaction is

extremely slow. Undoped ceria or ceria doped with varying amounts of Nb or Gd showed

between 0.07 and 0.9 monolayer coverage of carbon after 150 minutes exposure to

methane at 900 °C217, compared to 142 monolayers deposited on Ni/YSZ at the same

temperature218. The electrochemical oxidation of hydrocarbons over doped ceria is still

52

relatively low however219, with higher activities caused by the activity of either Pt current

collectors or Ni towards steam reforming. In theory, cells based on Cu and ceria could be

doped with noble metals to improve their activity, but the noble metals alloy with Cu

forming less active phases169.

6.7 Replacement of cermets with mixed ionic-electronic conductors (MIECs)

6.7.1 Single phase MIECs

At the time of the resurgence in interest in SOFCs in the late 1980s, a concurrent

area of interest was direct hydrocarbon oxidation catalysts, for removal of hydrocarbons

from car and power plant exhausts. It had been established that a number of oxides were

active towards this reaction, and they came to the attention of groups working on SOFCs,

with particular attention paid to the Perovskite family of oxides.

Perovskites are defined as a family of materials, which present the same structure as

the face-centered cubic calcium titanium oxide CaTiO3. The structure of these compounds of

general formula ABO3 may be described as a combination of the oxygen and A-site cations

that form the cubic close-packed (ccp) framework, the oxygen atoms occupy three quarters

of the sites of the cubic close packed layer and the A-site cation, the larger one, the

remaining quarter. The B-site cations occupy one fourth of the octahedral holes of the ccp

arrangement. This structure can be also viewed as the B-site cations occupying the center of

the cubic structure while A and O ions are located at the corners and half edges,

respectively. Perovskites have a high degree of structural and electronic flexibility, with

many different elements and oxidation states able to be incorporated into the structure.

The A-site cation can be a low valence rare earth, alkali or alkaline earth ion, for example La,

Na, Ca, Sr or Ba, while the B-site is a transition metal, such as Ti, Zr, Fe, Co, Ni or Cu. Both

sites are able to accept multiple different ions simultaneously, and this produces

53

possibilities for variable oxidation states220-221. In addition, if there is more than one different

element occupying the B-site, these can become ordered, and Perovskites displaying this

behaviour are known as double Perovskites (materials with only one B-site element or two

or more unordered B-site elements are single Perovskites). Their defect chemistry gives the

potential for them to exhibit MIEC properties under a wide range of partial pressures of O2

at elevated temperatures199.

Early work centred on perovskites of lanthanum with top row transition metals, for

example La0.8Sr0.2FeO3 (LSF), nowadays more familiar as a cathode material, which while it

showed better activity than Pt electrodes and no carbon deposition under dry methane, was

not stable under relevant anode overpotentials (<-0.3 V)222-223. Attention quickly focused on

substituted lanthanum chromites which were already used as interconnects in SOFCs due to

their stability in very low pO2224, despite the fact that the base material exhibited one of the

lowest activities for methane oxidation225.

While lanthanum chromites are not expected to catalyse carbon formation to the

same extent as nickel, carbon deposition has been observed. When exposed to dry methane

in a fixed bed catalytic reactor, at temperatures above 600 °C calcium-doped lanthanum

chromite was observed to catalyse methane decomposition, resulting in an average of half a

monolayer coverage of carbon, compared to 112 for Ni/YSZ under the same conditions226.

While this amount of carbon is small, it was found to have a deleterious effect on the

catalytic reactions. Addition of 3% steam to the 5% methane feed prevented this carbon

build-up. A study of various strontium and manganese-doped lanthanum chromites (LSCM)

containing varying amounts of Cr and Mn found that larger amounts of carbon deposition

for Cr-rich compounds and/or exposure to methane at higher temperatures were linked to

lower selectivities towards the total oxidation of methane relative to partial oxidation227.

54

Due to the low activity of lanthanum chromites, the immediate focus for

improvement was on the activity, rather than on further reducing carbon deposition.

Notwithstanding this, several authors did measure the tendency of doped lanthanum

chromites towards carbon deposition. One such study tested various first row transition

metal dopants to improve the activity as well as alkaline earth dopants to improve the

conductivity. All dopants produced carbon deposition of less than four monolayers at 800

°C. The exception was the Fe-doped material under conditions representing internal

reforming, which produced 69.4 monolayers of carbon228, which is similar to levels which

would be expected from a nickel-based cermet218. Materials which produced no carbon

under any conditions were the Sr and Mg double-doped material and the Co-doped

material. The two Ni-doped materials (singly doped and co-doped with calcium) surprisingly

showed no extra carbon deposition compared to most other dopants, but did produce

considerably higher conversions – between 3 and 5 times higher than any other materials

for the reactions representing partial oxidation and dry reforming, and 1 – 2 orders of

magnitude higher for the reaction representing steam reforming. In fact, although not

known at the time, it is likely that the nickel-doped samples were producing nickel metal

nanoparticles under reducing conditions, which helps explain the vastly improved

catalysis229. This is discussed further below in section 5.7.2.

Composites of LSCM with doped ceria show better activity, as well as increased

carbon deposition. In one study, carbon deposition after exposure to dry methane for 6 h at

750 °C increased from less than 0.1 wt% in pure LSCM to 1.5 wt% in 33 wt% LSCM:67 wt%

lanthanum-doped ceria230. An increase in the amount of the doped ceria improved

performance in fuel cell tests in methane, although above 50 wt% ceria the performance

dropped, probably due to lower electronic conduction. Doping lanthanum chromites with

55

ceria may also help. Iron-doped lanthanum chromite co-doped with 5% Ce showed much

lower carbon deposition at 800 °C in syngas in a microreactor, while symmetrical cells

showed less drop in performance under the same conditions and were able to be

regenerated by 24 h under load in H2231.

Other perovskite-based anode materials have been tested, for example lanthanum

aluminates232, and barium titanate233, but the most studied single perovskite other than

lanthanum chromite is strontium titanate, which is stable and when doped is a MIEC under

reducing conditions. To induce electronic conductivity, the base material can be doped with

La3+ on the A-site (known as LST)234-235 or Nb5+ on the B-site236, with the stoichiometry

controlled to produce either Ti3+, Nb4+ or oxygen deficiency or excess, meaning that this

system is compositionally very flexible. A possible hindrance to using this material is the high

temperature (>1000 °C) reduction needed to induce a suitable degree of electronic

conductivity, and the fact that this conductivity is lost under oxidation. LST does possess

very low propensity towards carbon deposition, with less than 1 wt% of carbon deposited

after 6 h under dry methane at 800 °C in a microreactor237. Composites of LST and CGO in

the same study showed increased carbon deposition, although still less than 2 wt% carbon

with 40 vol% CGO. The increase in carbon deposition was related to the greater degree of

interaction between CGO and methane, but in fuel cells the carbon was not shown to have

any detrimental effect on the performance, with a direct correlation found between

polarisation resistance in the impedance spectrum and propensity towards carbon

deposition in the microreactor tests.

More recently Sr2MMoO6 (where M is a small 2+ cation such as Mg or Ni) double

perovskites (perovskites in which the B-site cations are ordered) have also been used as

SOFC anodes. The ordering occurs as a consequence of the very different charges on the B-

56

site cations, but currently no advantage for carbon or sulfur tolerance of using a double

perovskite rather than a single perovskite related specifically to this ordering has been

suggested. Sr2MgMoO6 showed good activity for CH4 oxidation and stability under short

term testing of 15 hours238. The power density dropped under wet CH4 compared to dry CH4,

indicating that direct oxidation was the main route for CH4 conversion. Materials where the

Mg was partially or fully replaced with Mn performed worse, with a power density of 838

mW/cm² for the pure Mg sample reducing to 650 mW/cm² for the pure Mn sample.

Materials using Co or Ni rather than Mn showed similar performance decreases compared

to the Mg sample239. Co was seen to exsolve from the perovskite as Co metal, although

initial performance was similar. Co and Ni showed different catalytic behaviours – Co acted

mainly through steam reforming, with low performance in dry CH4, while Ni showed no

steam reforming activity. It is important to note that all the above studies were carried out

with Pt current collectors and a doped ceria barrier layer, which later work has suggested

could be responsible for most of the methane oxidation240. A study doping the Mo site with

Nb which did not use a barrier layer or Pt current collector agreed with this poor activity

towards methane oxidation, and suggested that similarly to other MIECs studied, the

catalysed reactions between the methane and the MIEC were likely to be limiting in pure

MIEC-based systems241. The study did find that the amount of carbon deposited was very

low, however this would be expected from a system with poor activity towards methane

conversion.

Due to the structural flexibility of perovskites, they are able to form reduced

compounds while maintaining the perovskite structure, and one promising material which

illustrates this is the A-site layered double perovskite PrBaMn2O5+δ (PBMO)242. This material is

stable across a good pO2 and temperature range, and unlike many of the perovskites

57

described above appears to have some activity towards hydrocarbon oxidation (with silver

used for current collection). Although this material was not tested for carbon deposition or

stability under operation in hydrocarbons, a calcium-doped version, PrBa0.8Ca0.2Mn2O5

(PBCMO) was, and was stable for 50 h in humidified iso-octane followed by 150 h in

humidified propane, with currents of 0.2 – 0.3 A/cm² achieved at 0.6 V at 700 °C243.

6.7.2 Addition of catalytic metal nanoparticles to MIECs

Since MIECs by definition are electronically percolating, a percolating metal phase is

not necessary, but dispersed metals can still be added to promote the catalysis. However,

these metal nanoparticles can also be prone to carbon deposition. Impregnation of metal

salts (typically nitrates) into the anode is a technique borrowed from catalysis, where it is an

extremely widely-used method for producing catalysts. Ni, Pd and Ni-Pd were added to Sr-

doped LaCrO3, with Ni-Pd showing a synergistic effect for methane oxidation in dry

methane, with little or no carbon deposition observed using a carbon balance approach

during testing at 0.5 V and 800 °C244. A short period at OCV was sufficient to completely

deactivate the electrode towards methane decomposition, while returning the cell to 0.5 V

could only recover 60% of the activity. Addition of hydrogen to the cell was necessary to

fully reactivate the cell through methanation of the carbon.

Further work on Ni-Pd and Pd nanoparticles dispersed on an LSCr-CSO anode

suggested that the reaction mechanism for the oxidation is fundamentally different

comparing Ni-Pd alloys with pure Pd particles245. The work suggested that the reaction on Pd

was close to direct electrochemical oxidation, while the reaction on Ni-Pd alloys was likely

through methane cracking followed by electrochemical oxidation of hydrogen, steam

reforming of carbon and electrochemical oxidation of the CO produced. These alloys were

found to be resistant to carbon deposition, and it was proposed that doping of Ce, Sr, La or

58

Sm into the alloy was preventing the formation of carbon fibres, as highlighted by the fact

that the Ni-Pd nanoparticles contained trace amounts of these elements246.

Improvements to the stability of Pd nanoparticles can be achieved by impregnation

of Pd-core/CeO2-shell nanoparticles, which are able to operate on dry methane without

carbon deposition and survive heat treatments in air up to 900 °C with only 9% loss in

performance compared to 40% loss in performance with impregnation of just Pd247.

These results highlight some advantages of using a MIEC combined with dispersed

metal particles compared to cermets:

although the cell can still be deactivated through carbon deposition under

certain conditions, since the metal is not load bearing, complete structural failure does not

occur and the cell can be regenerated.

in cells based on cermets, the only economically feasible method for adding

expensive elements such as Pd is via impregnation into an already formed cermet anode.

This results in segregated Pd and Ni particles, which rules out this synergistic alloying effect

and does not prevent carbon deposition in Pd poor regions130.

An interesting approach towards decoration of MIECs with catalytic nanoparticles is

exsolution, where a reducible metal is incorporated into the oxide structure during

synthesis, and exsolved forming remarkably stable nanoparticles under reducing

conditions248. A feature of these systems is that the nanoparticles can be cyclically

readsorbed and exsolved from the structure. This method has the advantage compared to

traditional impregnation that it produces stronger particle-support interactions and so less

sintering occurs and the particles are more stable. There are two main disadvantages

compared to impregnation: higher reduction temperatures and less control over the

59

composition of the particles – currently there are no reports of alloy nanoparticles

deliberately produced by exsolution.

Exsolution was first (deliberately) tested in SOFC anodes with Ru-doped LSCM249. The

Ru exsolved forming particles up to 5 nm in diameter over 50 hours under hydrogen at 800

°C, doubling the cell performance and reducing the polarisation resistance by a factor of

three. Only 15% of the Ru was found to have exsolved, and the authors suggest that this is

due to a combination of slow diffusion and energetic barriers towards removal of too much

Ru from the perovskite structure. Later studies found that the exsolved Ru particles acted to

hinder carbon deposition on the LSCM250. Aside from improving the performance of cells

running on dry ethanol, it was found in fixed bed tests that carbon deposition was

eliminated compared to around 1 wt% for the Ru-free material250. Co particles can also be

exsolved, and these showed <1% weight gain due to carbon deposition when exposed to dry

methane for four hours, compared to >100% weight gain for Co/CeO2 prepared by

impregnation251.

As mentioned previously, nickel doped into LSCM can exsolve out as nickel

nanoparticles, and depending on the particle size produced these can be resistant to carbon

deposition. Pulse reaction studies on Ni-doped LSCM indicated that essentially all the

methane was converted to carbon dioxide until oxygen stoichiometries below 2.7, where

the methane conversion continues to increase despite CO2 conversion reducing, indicating

that methane decomposition (and consequent carbon deposition) was taking place229. This

was considered to be due to the greater degree of nickel exsolution implied by lower oxygen

stoichiometries and potentially larger particle size. Co particles were also found to exsolve

from Co-doped LSCM, and it was found that exsolved Ni and Co particles have a large effect

on the methane oxidation rate with only a small increase in the rate of carbon deposition

60

compared to LSCM. Exsolved Ni showed far better carbon resistance than impregnated Ni252

(Figure 18).

While exsolved Ni and Co can still show raised levels of carbon deposition, addition

of Cu to form nano-alloys can mitigate this. Ni and Co exsolved from Ce0.8(Co,Ni)0.2VO3

showed significant amounts of carbon deposition on exposure to dry methane at 700 °C,

with 10% weight gain caused by carbon deposition for Co and 27% for Ni, compared to <1%

for the undoped material.253 However, double doping Cu and Co reduced the weight gain to

2%254. Double doping Cu and Ni did not reduce the amount of carbon deposition, probably

because the 50:50 mix of Cu and Ni used is still prone to carbon deposition as discussed in

section 5.3.

Likewise, LSC double-doped with Ni and Fe showed less carbon deposition after

exposure to syngas at 850 °C than singly-doped Ni-LSC. Singly-doped Fe-LSC showed less

carbon deposition than either, but performed worse in fuel cell tests, while Ni-Fe doubly-

doped cells performed best255. XRD and SEM analysis showed that the exsolved Ni and Fe

formed alloy particles of around 25 – 30 nm. The promising symmetrical electrode material

Pr0.4Sr0.6Co0.2Fe0.7Nb0.1O3-δ (PSCFN) forms Co-Fe nano-alloys at 900 °C under hydrogen, with

stable performance for 50 h under dry methane and 100 h under dry butane at 800 °C256.

Microreactor tests on reduced PSCFN showed considerable carbon deposition (30 wt%)

under methane at 850 °C and also high activity for methane cracking257. The stable

performance under methane could be explained by the fact that on initial exposure to

methane CO2 (and presumably water) is produced rather than hydrogen, indicating that

there are species active for methane oxidation. In addition, the carbon was able to be

oxidised at 450 °C, implying that it was dispersed and amorphous, and therefore may be

oxidised by oxygen flux under SOFC anode conditions.

61

Figure 18 - Carbon production rates averaged per pulse for LSCM, LSCMCo, LSCMFe,

LSCMNi and LSCM+Ni. Note that the latter is shown on the right axis. Reprinted with

permission from Chem. Mater. 2010, 22 (21), 5856-5865. Copyright 2010 American

Chemical Society.

The exsolution of nanoparticles could be limited by energetic barriers towards

removing B-site cations from a stoichiometric perovskite. This can be combatted by

synthesis of A-site deficient materials, which allow B-site cations to be removed much more

efficiently, allowing even metals such as iron to be exsolved from LST258. Control of the

stoichiometry in this way also allowed Ni metal and CeO2 to be exsolved from lanthanum

cerium titanate259. Exsolution of Ni from Ni and Ce double-doped LST was found to greatly

62

reduce the amount and ease of removal of deposited carbon in microreactor tests in

methane compared to Ni-doped LST260.

SrMoO3 is a MIEC stable under anode conditions, but doping with Ca allowed Mo

nanoparticles to exsolve under reducing conditions. These particles had a small beneficial

effect under hydrogen, but under methane the Ca-doped materials allowed carbon

deposition, in contrast to the undoped material which did not261. While the formation of

carbon implies a greater ability to interact with methane, strangely both the undoped and

doped materials showed very low OCVs which indicates a lack of ability to convert methane.

Nanoparticles of oxides can also be produced through exsolution-type processes.

LSCF impregnated with nickel was used as an anode, where it decomposed into strontium

cobalt iron oxide perovskite with La2NiO4 finely dispersed over the surface262. The La2NiO4

was presumed to be the electrocatalytically-active phase, and the cells exhibited good

performance in dry propane with only a few carbon whiskers observed in the SEM after 100

h of use.

6.8 Regeneration of SOFC anodes deactivated by carbon

As can be seen above, carbon deposition can occur, to varying degrees, on all

materials so far studied. In some circumstances the carbon deposition is not detrimental to

performance, or can even be positive in small amounts as it can improve the electronic

conductivity of the anode. As the amount of carbon increases in an SOFC operating over

potentially tens of thousands of hours deleterious effects such as pore blocking and risk of

structural failure will inevitably increase, so it may be desirable from time to time to remove

this carbon. Clearly it is always possible to remove carbon by heating the cell to high

temperatures in air, but a number of studies have investigated the possibilities for removing

carbon without damaging the cell.

63

Kirtley et al. studied carbon removal from Ni-YSZ using 3% H2O, 10% O2 or 11% CO2 in

nitrogen, and found that the carbon was removed fastest in H2O and slowest in CO2, with

times ranging from 10 – 125 s263. Through examining the OCV and in situ Raman, the authors

were able to identify the stages of carbon removal. First the OCV increased to -0.99

accompanied by the disappearance of carbon peaks in the Raman. This was attributed to the

formation of a CO/CO2 gas mixture. This is followed by the appearance of NiO peaks in the

Raman and an OCV reflecting the thermodynamic equilibrium of the Ni/NiO couple in the

regenerating gas. O2 leads to complete oxidation of Ni to NiO, while H2O and CO2 lead to

partial oxidation. The above study induced carbon deposition from dry methane at OCV, but

carbon induced using diesel reformate under load was able to be removed and the cell fully

regenerated using dry and wet hydrogen, albeit over a time period of 44 h264. Regeneration

via this method was not possible under conditions where the cathode had also degraded,

indicating that the carbon is removed largely by oxygen flux through the electrolyte265.

It is theoretically possible to regain performance without changing the gas mixture

by moving from an operating regime where carbon deposition is favoured to one where it is

not. Ni-YSZ cells were found to completely regain their initial performance after 24 hours

under load at 850 °C in a simulated partial oxidation reformate feed, having previously had

carbon deposited in the same gas mixture at 650 °C under OCV266.

Symmetrical cells (where the electrode material used during fabrication is the same

for anode and cathode) offer interesting theoretical potential for regeneration, given that if

carbon deposition occurs they can simply be reversed, whereupon the deposited carbon will

be exposed to air and thus oxidised. In a recent review on symmetrical electrode materials,

the authors conclude that little work has been done on regeneration of these materials after

carbon deposition267.

64

Table 4 Selected papers reporting improved carbon tolerance in SOFC anodes

through materials strategies.

Noble metals

Metal Fabrication Performance Ref Comments

Au 1.5 mol% Au

deposited onto

Ni/CGO powder,

screen-printed onto

electrolyte

Tested under CH4-H2O,

850 °C, compared to no

Au: 0.15 V higher OCV,

0.2 V higher at 500

mA/cm² (both S/C = 3/2);

no V degradation under

dry methane vs. 0.3 mV/h

degradation under S/C =

1/2 (no Au); less C

deposition (visual)

127 Microreactor and

mechanistic studies

reinforce effect of

gold125-126, 128

Pd 0 – 0.15 mg/cm² Pd

impregnated into

slurry-painted

Ni/CGO electrode

Tested in wet CH4 and

EtOH, 800 °C, OCV,

compared to no Pd: Rp

decreases by 2x in CH4

and 4x in EtOH in

loadings above 0.07

mg/cm²; C deposition still

observed (SEM, EDX)

129 Microreactor tests

suggest carbon

suppression effect132

Ru 0 – 9 wt% RuO2

mixed into Ni/CGO,

Tested in wet CH4, 600 °C,

compared to no Ru:

131 Another paper

agrees that Ru has a

65

formed into a pellet

with ~30 µm CGO

electrolyte

current density increased

by 2x at 0.4 V; also, stable

performance for 20 h, no

C deposition (from

carbon balance) (not

compared to no Ru)

beneficial effect132,

but there are few

papers on Ru due to

problems with its

oxides’ volatility. Ru

doped in ceramic

anodes may be

more feasible249-250

Ag 0.9 – 2.5 wt% Ag

impregnated into

Ni/YSZ anode

supports, 20 µm YSZ

electrolyte

Tested in dry CH4, 750 °C,

0.3 A/cm², compared to

no Ag (at 0.6 A/cm²): 0.9

and 2.5 wt% Ag failed at

12 and 81 h respectively,

1.6% Ag showed no

degradation to 100 h.

Control failed at 5 h. Very

little carbon observed by

EDX

138 A further paper by

the same group

showed similar

results for C2H6137,

while microreactor

results also show C

tolerance136. The

high mobility of Ag

at ~600 °C and

above must be

noted.

Base metals

Metal Fabrication Performance Ref Comments

Cu 0 – 100% Cu

impregnated with Ni

(Cu + Ni = 20 wt%)

Powders and cells tested

in dry CH4: at 700 °C,

powders with Cu:Ni of

150 There are many

papers on Cu with

widespread

66

and CeO2 (10 wt%)

into 400 µm porous

YSZ support with 60

µm YSZ electrolyte

9:1 or 10:0 showed no C

deposition. 4:1 gave <0.1

g C/g. In a cell at 800 °C,

performance of 4:1

improved from ~0.1

A/cm² to >0.6 A/cm² over

500 h due to C

deposition.

agreement that it

reduces C

deposition. Activity

is poor so normally

CeO2 or doped ceria

is used153.

Fe Anode supports

were prepared from

Fe2O3, NiO and CGO,

powders (Fe:Ni

0:100 – 50:50 w/w),

CGO electrolyte

Cells tested in dry CH4,

650 °C, 0.2 A/cm²: Cells

Fe:Ni up to 30:70 gave

similar power densities

(~0.3 A/cm²), 50:50 gave

<0.2 A/cm². All Fe-

containing cells were

stable over 50 h at 0.2

A/cm², Ni only cell

stopped after ~12 h. No

carbon observed on Fe:Ni

10:90 by SEM after test

155 There is some

evidence that Ni:Fe

alloys at 10% Fe are

more active for

methane

oxidation155 and

reforming156. This

level of Fe gives

stable cells with a

variety of oxide ion

conductors154

Co Ni/YSZ and Co/YSZ

were prepared by

coprecipitation then

coated on a 500 µm

Cells tested in dry CH4,

850 °C, OCV: anodic

overpotential remained

stable over 15 h in Co

159 Other papers

confirm that Co is

less vulnerable to C

deposition, but still

67

electrolyte support anode, Ni anode failed. C

deposition still observed

in Co anode by SEM.

vulnerable158, 160.

Activity for CO

oxidation looks

promising157-158

Sn 1% Sn was

impregnated into

Ni/YSZ anode-

supported cells with

a 20 µm YSZ

electrolyte.

Cells tested in dry CH4

and C8H18-air mixtures at

740 °C and 0.6 V and 0.5

V respectively. Stable

performance was

obtained in both fuels

over 6 h (CH4) or 13 h

(C8H18). Ni-only cells

completely deactivated.

161 Despite some

papers showing

little impact of tin133,

163, the bulk of

papers studying

performance and

mechanisms in

SOFCs161-162, 164-167

and catalysts (see

section 8.1.2)

suggest that the

effect of tin is real.

Several papers have

examined 1% and

5% loading, with 1%

being the best.

Non-metal conductors

Phase Fabrication Performance Ref Comments

C Porous YSZ scaffold

impregnated with

Tested in dry CH4 and

C4H10 at 700 °C: Maximum

168 Several papers in

the early 2000s

68

10 wt% CeO2 and

optionally 1 wt% Pt,

Pd or Rh. ~4 wt%

carbon is then

deposited in dry

C4H10 at 700 °C, 100

µm YSZ electrolyte.

power densities of 0.1

W/cm² in C4H10 and 0.02

W/cm² in CH4, similar to

Cu/CeO2-YSZ cells.

Performance in all fuels

was greatly increased by

adding 1% Pd. A 100 h

test of Pt/C-CeO2-YSZ in

CH4 showed large

increase in Ohmic

resistance due to loss of

carbon.

looked promising

for this technique168-

170, however there

have been no

papers since by this

group or others.

WC Porous YSZ scaffold

impregnated with

25 vol% WC, then 5

wt% CeO2 and 5 wt

% Ni

Tested in humidified CH4

at 850 °C at OCV: no

carbon observed visually

after 36 h; at 0.7 V stable

performance of 50

mW/cm² over 24 h with

no carbon observed

visually.

173 This strategy is quite

unexplored, but the

ability of WC to

protect Ni from C

deposition could be

interesting.

Increasing alkalinity

Phase Fabrication Performance Ref Comments

BaO Vapour deposition

of BaO onto an

Tested in dry C3H8 at 750

°C at 0.5 A/cm²: Cell

179 While CaO and SrO

also reduce C

69

anode-supported

NiO/YSZ cell with 15

µm YSZ electrolyte

voltage stable at 0.8 V for

over 100 h compared to

BaO-free cell which failed

after <1 h. Similar results

for wet CO and gasified

carbon. No C deposition

observed by SEM.

deposition176, 178 BaO

appears to be the

best prospect.

Microstructure

appears to be vitally

important.

BZCY Sol-gel synthesis of

NiO/BZCY

composites co-

pressed with CSO to

form an anode-

supported cell with

20 µm electrolyte

Tested in wet C2H5OH at

600 °C at 0.3 A/cm².

Voltage stable at 0.75 V

for 180 h compared to

Ni/YSZ and Ni/CSO which

failed after <2 h due to C

deposition. No carbon

detected or morphology

changes detected by SEM

after testing.

184 Numerous Ba-based

perovskites have

now been tested

including BYZ183,

BYC58 and BZCYYb185.

The efficacy of these

perovskites seems

clear, and

microreactor and

modelling studies

both confirm this

and elucidate the

mechanisms.

Ceria-based oxygen storage materials

Phase Fabrication Performance Ref Comments

CGO Ni-coated CGO by

hydrothermal

Tested in dry CH4 at 610

°C at 1.2 A/cm². Voltage

215 The benefits of ceria

and doped ceria

70

synthesis, 2 µm

layer deposited at

the top of a

conventional

Ni/CGO anode

support, 5 µm CGO

electrolyte

stable at 0.6 V for 1000 h

compared to Ni/CGO cell

without Ni-coated CGO

layer. No carbon

detected by SEM after

test or by Raman after

microreactor tests.

with regards to

carbon tolerance

are so long

established as to be

beyond doubt. Still

work is continuing

with recent studies

including use of high

OSC cerias205, 216,

microstructuring

and the

electrocatalytic

performance of

ceria itself214.

Mixed ionic-electronic conductors

Phase Fabrication Performance Ref Comments

SMMO SMMO powders by

solid state method,

slurry painted onto

250 µm LSGM

electrolyte with CLO

buffer layer. Pt, Au,

Ag and LST current

collectors used.

Cells tested in dry CH4 at

800 °C: OCVs of all cells

were stable over 100 h,

very low OCVs were

observed for Au and Ag

current collectors. At 0.5

V the cell with Pt current

collector was stable for

240 It is becoming clear

that most MIECs

appear to lack

catalytic activity

towards methane

and other

hydrocarbons. In

this paper, the

71

80 h at ~0.3 A/cm² after a

large initial drop.

activity was

dominated by the

current collector.

PBCMO PBCMO prepared by

Pechini method,

screen printed on a

250 µm LSGM

electrolyte with CLO

buffer layer.

Cells tested in wet C8H18

and C3H8 at 700 °C and

0.6 V: stable at 0.2 – 0.3

A/cm² in C8H18 for 50 h

and C3H8 for 150 h. C

deposition was not

measured.

243 This class of reduced

perovskites may

hold some promise

regarding activity

towards

hydrocarbon

oxidation, but much

more work is

needed to confirm

this.

LST+Ni

+ CeO2

LSCNT powders

made by sol-gel,

screen printed onto

a 300 µm YSZ

electrolyte, with Ni

and CeO2 exsolved

in situ

Cells tested in dry CH4 at

900 °C and 0.5 V: The cell

without CeO2 gave an

initial current of 0.4

A/cm² which declined to

0.35 A/cm² over 80 h,

while the cell with

exsolved Ni and CeO2

increased in current from

0.6 – 0.75 A/cm².

Microreactor tests

260 Exsolution of metal

nanoparticles from

perovskites has

been of intense

interest recently,

however long term

studies in operating

fuel cells are lacking.

In this study, the

performance is

increasing after 80

72

showed little carbon

deposition in the material

with Ni and CeO2.

h, perhaps

indicating that Ni is

still exsolving, a

process which is

slow in

stoichiometric

compounds259, 268

7. Materials design strategies for sulfur tolerance in SOFC anodes

There seems to be sufficient consensus in the literature that sulfur will be adsorbed

at the surface of nickel blocking the reaction sites for oxidation or reforming reactions 72, 269;

although initially an unwelcome feature, it can be used as an advantage to minimise carbon

deposition21, 71. It is also accepted that absorption is more dramatic at lower temperatures

and at higher concentration of sulfur62, 66. It is also known that two stages of sulfur poisoning

have been observed, one is the surface absorption of sulfur that blocks the reaction sites

but that can be reversed and a second one related to an in-depth formation of nickel sulfide

that changes the microstructure of nickel and is therefore irreversible71, 270. Figure 19 shows

a possible mechanism for sulfur poisoning in hydrogen and carbon fuel environments62.

There is, however, no consensus in the effect of the current densities on sulfur poisoning.

Some authors reported that increasing current densities leads to a decrease in sulfur

coverage because of its conversion to SO263, 65, 271. On the contrary, other authors have

indicated that sulfur coverage increases with current density60, 272-273. A very good agreement

with the latter view is concluded in the recent modelling work of Riegraf et al274, where the

model involves all gas and solid chemical reactions coupled with electrochemistry. When

73

operating in methane, the adsorbed sulfur supresses the reforming reaction by blocking the

catalytically active sites, and these sites become available if sufficient hydrogen is present to

unblock the sites274.

Again there is agreement that full recovery can be achieved if H2S is removed

completely from the fuel stream but there is a limit beyond which damage is irreversible.

Concentration and temperatures where recovery is possible vary from article to article but

reversibility has been reported independently. This may be related to desorption of sulfur

from the nickel surface and reaction with H2 from the clean stream. Some good examples of

this recovery are the work of Rasmussen and Hagen68, Sasaki62 and Zha63. It is also generally

accepted that the whole surface of nickel is covered and not only the TPB region270.

74

Figure 19. Possible mechanisms of degradation by sulfur poisoning. Taken from J.

Power Sources 2011, 196 (22), 9130-9140. Reprinted with permission from Elsevier.

Whatever change takes place in the anode during poisoning, it must be reversible

and provided that oxygen is migrating to the anode via the electrolyte, it is of paramount

importance that this process is not stopped and that oxidation or removal of adsorbed

sulfur is favourable. To provide sulfur tolerance, the materials and structure of the anode

should therefore be capable of adsorbing sulfur and then react with any of the gaseous

species present H2, H-C or even O2- to form SO2.

From the point of view of the materials modification, the strategies more frequently

used for the development of sulfur tolerant anodes can be summarised as follows:

1) High oxygen transport to increase sulfur oxidation (Figure 20a). In similar

conditions, ScSZ working under H2S/H2 atmospheres shows a higher tolerance to H2S than

YSZ, indicating the importance of a higher oxygen supply through the electrolyte62.

2) Incorporation of additives or partial substitution of nickel (Figure 20b).

Substituting Ni for a more sulfur tolerant metal without compromising H2 activity has been

behind much of the work on alloys89, 96. Some of the earlier work attempted copper97, 275

while the most recent use of additives has been aiming at using these as preferential sites of

sulfur incorporation276, and this is reinforced by a thermodynamics studies showing that

oxides such as BaO and CeO2 reduce the coverage of sulfur on Ni277, which is strongly linked

to performance91. Catalytic activity for hydrogen oxidation reaction and H2S dissociation

seem to follow analogous trends, maintaining the catalytic activity while simultaneously

improving sulfur tolerance difficult via this route104.

3) Use of all-ceramic anodes (Figure 20c). Perovskites are favoured as they can be

tailored on the A and B site to improve ionic conductivity, electronic conductivity, catalytic

75

activity, and resistance (e.g. Mg2+ more resistance to sulfide formation than Cr3+ or Mn2+)278.

Additionally, the reactivity of a ceramic material is expected to be smaller than that of a

metal surface78.

Figure 20. Schematics of the most common materials strategies to improve sulfur

tolerance. The diagram shows a strategy and does not imply a specific mechanism of

desulfurization.

7.1 Replacement of YSZ with ceria

A few papers have compared Ni-YSZ and Ni-CGO electrodes and, for example, Zhang

has shown that degradation in Ni-CGO is lower than in the Ni-YSZ under similar conditions of

operation279. This may not be surprising considering that sulfur can also accumulate in the

surface of the CGO forming Ce0xSy-type phases which can react with O2- to produce SO2.

Recent studies on the adsorption and removal of H2S from fuel streams by rare earth oxides

again suggest that CGO is one of the most promising anodes for operation under H2S

poisoned fuels. Elimination of the adsorbed sulfur can take place in ceria and other rare

earth oxides using a reducing, oxidising, or inert gas or even steam280-281. This may explain

the tolerance to H2S of an anode that has been infiltrated with ceria282 and the minimised

76

potential drop in anodes with lanthanides as additives62. The tolerance of ceria-based

materials to sulfur environments has been known for some years283.

Donor-dopants of ceria have been studied to a lesser extent compared to acceptor

species. Nevertheless some of them are interesting for improving sulfur tolerance, for

example Mo. This dopant is especially desirable for sulfur tolerance goals since it can trap S

forming MoS2. In this sense, Li et al. investigated the electrical properties of the Mo-doped

CeO2 (CMO) as potential anodes for SOFCs. Mo and rare-earth-co-doped Ce0.9-x RExMo0.1O2.1–

0.5x (x=0.2, 0.3) (CRMO) oxides were found to retain their fluorite-type structure under H2 at

elevated temperatures284. The same team demonstrated the remarkable stability of these

Mo-doped CeO2–anodes in wet H2 and wet CH4 mixtures285. As mentioned above Mo is a key

element to incorporate sulfur resilience. In a recent publication, Chen and co-workers

developed a sulfur-resistant SOFC anode by impregnation of Mo0.1Ce0.9O2+δ into a typical Ni-

YSZ material286. Figure 21 shows the successful performance of this material when submitted

to 50 ppm of H2S.

77

Figure 21. Sulfur tolerance test for a CMO-impregnated cell under a current density

of 0.60 A cm−2 at 750 ◦C using H2 and H2 with 50 ppm H2S as the fuel, respectively. Reprinted

from J. Power Sources 2012, 204, 40-45 with permission of Elsevier.

The system allows power densities of 440 mW cm−2 and 420 mW cm−2 using H2 with

50 ppm H2S and methane as fuel, respectively under a current density of 0.60 A cm−2 at 750

⁰C.

7.2 All-ceramic anodes

A number of all-ceramic electrodes have shown promising performance in SOFCs or

SOECs258-259, 287-288. As mentioned before it is expected that an oxide is less prone to adsorb

sulfur than a metal. The classic perovskite SrTiO3 can be doped both in the A and B site or

even have A site deficiency278. Some work has been performed on Y-doped SrTiO3 doped

with Ru and CeO2 showing a limited tolerance to H2S (up to 40 ppm) and especially

reversibility when the H2S stream is removed289. The Sr0.6La0.4TiO3/YSZ (50/50 wt %) anode

showed no degradation in the presence of up to 5000 ppm of H2S in a hydrogen fuel290 and it

has even suggested that the presence of H2S can promote the oxidation of methane291-293. In

general it seems that perovskites are stable towards operation in sulfur, with many

examples being reported, including double perovskites242, lithium-ion conducting

perovskites294.

It is generally recognised that Perovskite-based materials lack the catalytic activity of

nickel. As discussed in section 5.7.2, one method to improve the catalytic activity has been

to dope the perovskite with transition metals which then exsolve out as catalytically-active

nanoparticles on reduction. Little work has been done on the tolerance of these

nanoparticles towards sulfur, but one study showed that Fe nanoparticles ex-solved out of

Sr2Fe1.5Mo0.5O6 forms FeS under 50 ppm H2S in H2, with a decline in activity of around 20%

78

from around 0.1 to around 0.08 W/cm² at 600 °C over a period of 46 hours, followed by

stable operation for a further 200 hours295.

It has also been reported that the presence of H2S improves the performance of the

fuel cell when methane is used as the fuel for La0.4Sr0.6Ba0.1TiO3-d291, 296 but the oxidation of

H2S to SO2 does not seem to be the main reaction as suggested previously269 in SOFCs but

rather a gas reaction with methane and potentially an increase in the conductivity of the

perovskite by some as yet unclear mechanism296. Although doped SrTiO3 has been

independently shown to be stable289, 291 in H2S, the high concentrations used need to be

independently confirmed.

Barium-based perovskites have also shown promise for sulfur tolerance. For

instance, Kan et al. prepared the proton-conducting Ba3CaNb2O9 doped with Mn, Fe and Co

and checked the stability of these materials towards H2S297. They used a 5000 ppm H2S/H2

stream to evaluate whether the investigated samples can be used as electrodes in

contaminated fuels (e.g. natural gas with ppm levels of H2S). Their XRD study indicated that

the samples preserved the double-perovskite structure at 600 ⁰C for 12 h. No secondary

phase was detected due to the formation of sulfides such as MnSx, FeSx, or CoSx. The SEM

study also confirmed that the particle sizes and shapes did not change after H2S treatment.

This result suggests that their materials are physically and chemically stable in the SOFC

working environments. The same group reported enhanced stability of perovskite-type

BaZr0.1Ce0.7Y0.1M0.1O3-δ (M=Fe, Mn and Co) with a substantial chemical stability in 30 ppm H2S/

H2 at elevated temperature during 24 h297.

Another Ba-based Perovskite prepared by Yang et al. seems to be a promising

material276. In a very complete paper they reported outstanding sulfur and coking resistance

of a barium zirconate-cerate co-doped with Y and Yb (BaZr0.1Ce0.7Y0.2-xYbxO3-δ) anode. The

79

terminal voltages of the same cells (with BZCYYb and SDC as electrolyte) at 750°C were

recorded as a function of time when the fuel was contaminated with different

concentrations of H2S. The Ni-BZCYYb anodes for both cells showed no observable change in

power output as the fuel was switched from clean hydrogen to hydrogen contaminated with

10, 20, or 30 ppm H2S. XRD data corroborated the chemical stability of the designed anodes.

A study on Ni-BZCY anodes featured even higher levels of H2S (up to 1000 ppm), used EIS to

show that as well as reducing the anode polarisation losses compared to Ni-CSO, the BZCY-

based anodes showed little increase in Ohmic losses even at 200 ppm H2S, while the Ni-CSO

cell showed severe increases in Ohmic resistance at 100 ppm H2S (figure 22). Post-test EDX

showed large decreases in Ni content in the Ni-CSO anode, which were not seen in the Ni-

BZCY anode298. This indicates that these materials may be hindering restructuring of the Ni

at high sulfur levels.

80

Figure 22. I–V, I–P curves and EIS for the fuel cells with the Ni+SDC (a, c) and Ni+BZCY

(b, d) anodes operating on different fuels at 600 °C. Reproduced from Environ. Sci. Technol.,

2014, 48 (20), pp 12427–12434, copyright 2014 American Chemical Society

The role of barium in the improved tolerance may be related to the reduction of

sulfur chemisorption on nickel. Da Silva and Heck have calculated that the incorporation of

oxides, in particular BaO, reduces the sulfur chemisorption on Ni by minimizing the sulfur

chemical potential and favouring the formation of BaS. This sulfide can be reconverted to

BaO in the presence of water and additional BaO, leading to an in situ regeneration299. It was

also predicted that the addition of BaO enables the anode to tolerate 100 ppm in humidified

H2.

7.3 Alloying of nickel with other metals

The incorporation of additives or secondary phases has been known in metallurgy for

many years. The extraction of nickel (or cobalt) metal from ores involve roasting or

oxidation of the sulfide to the oxide followed by in situ reduction with CO. It should be

noted that all the key elements necessary for oxidation and elimination of sulfur used in

metallurgical processes are present in a fuel cell anode and the analysis of roasting may

provide the key to achieve tolerance to sulfur in SOFC. In roasting, the sulfide minerals are

treated with very hot air and the sulfide is converted to an oxide while sulfur is released as

sulfur dioxide; typical examples being ZnS, FeS2, PbS2 and Cu2S. Roasting is usually carried

out between 500 and 1000 °C300, the same range of operation of SOFC. Improvement of the

roasting process is achieved by adding pyrite (FeS) with the highest rest potential among

sulfide minerals, therefore acting as a cathode which accelerates the oxidation of the other

sulfides. Finally, reduction with CO leads to the formation of the metal although a few

81

metals can be obtained directly by oxidation of their sulfides since their oxide is less stable

than SO2, well known examples being: Cu, Ag and Hg.

Finally, it is worth mentioning the idea of decomposing H2S into hydrogen and sulfur,

both valuable products; several routes have been explored in the past301. The most

straightforward suggestion of relevance to SOFC is that H2S be decomposed thermally

according to:

2H2S → 2H2 + 1/4S8, ΔH = 79.5 kJ/mol

This decomposition has been performed in the presence of MoS2302 between 500-800

°C and more recently, it has been reported H2S can be decomposed in the 700-1000 °C

temperature range using the Perovskite oxide LaSr0.5Mo0.5O3303. The presence of Mo and a

possible decomposition of H2S may be behind the activity and the reported stability of

Sr2Mg1-xMnxMoO6-δ in these complex perovskites anodes238, 304. Mo-containing catalysts are

commonly used in hydrodesulfurisation processes305. The interaction of molybdenum with

sulfur can be modified with the presence of a second metal with direct consequences for

the hydrodesulfurisation properties. In particular the synergistic effects of Ni-Mo bonding

has been proved to be active towards the hydrodesulfurisation306. In contrast, the effects of

Zn, Cu, and Fe on the Mo-S interactions and hydrodesulfurisation activity are less

pronounced. The Ni-Mo and Ni-S-Mo interactions increase the electron density on Mo

making it more chemically active in two key steps for the reactions: the adsorption of S-

containing molecules and the dissociation of H. It is therefore not unthinkable that

molybdenum plays a key role in the sulfur tolerance in anodes containing this metal.

Most studies within the literature regarding the tolerance to sulfur report the effect

of sulfur poisoning on the electrochemical properties as it the most direct way of in-situ

degradation analysis. Therefore, cell voltage changes, power output and area specific

82

resistances are commonly used to describe the changes to the anode upon a modification.

Comparison between the different reports is difficult and therefore we shall provide the

overall change observed in the very same paper when the anodes is modified with the

intention to improve sulfur tolerance. Table 5 presents results from selected papers where

there has been a variation in the anode with the intention to improve the tolerance to

sulfur.

Table 5 Selected papers reporting improved sulfur tolerance in SOFC anodes through

materials strategies.

Cell Modification Sulfur tolerance and figures of merit Reference

Ni-CGO in YSZ

scaffold anode

ScSZ

electrolyte

(L

a0.6Sr0.4)0.99CoO3-

δ cathode

Ferritic (FeCr)

stainless steel

supported

High porosity

(not quantified)

Two stage degradation with area specific

resistance of 0.35 Ω cm2 at 650 °C, 0.25 A

cm-2. Full regeneration possible.

307

Low porosity

(not quantified)

Two stage degradation area specific

resistance of 0.70 Ω cm2 at 650 °C, 0.25 A

cm-2. Full regeneration possible.

Ni/8YSZ and

Ni/CGO

3YSZ

electrolyte

Ni/CGO anode Two stage poisoning. Stack voltage

decrease only to 98.7 % of initial value

upon addition of 2 ppm H2S to fuel at 850

°C, 0.225 A cm-2 after 15 h. Fuel mixture:

308

83

support

LSM cathode

43.8% H2, 6.2% H2O and 50% N2.

Ni/8YSZ anode One stage poisoning. Stack voltage

decrease to 86.5 % of initial value upon

addition of 2 ppm H2S to fuel at 850 °C,

0.319 A cm-2 after 15 h. Fuel mixture:

43.8% H2, 6.2% H2O and 50% N2.

Ni/YSZ or

Ni/CGO anodes

YSZ or CGO

electrolyte

supported

Pt cathode

Ni/CGO anode Polarization resistance of anode is 4.3 Ω

cm2 in 700 ppm H2S in H2, 200 mA/cm2 at

800 °C after 2 h. Regeneration possible.

279

Ni/YSZ anode Polarization resistance of anode is 1.2 Ω

cm2 in 700 ppm H2S in H2, 200 mA/cm2 at

800 °C after 2 h.

Ni-CGO anode

supported

CGO

electrolyte

NdB

a0.75Ca0.25Co2O5+

δ-CGO cathode

Ni/CGO+

BaCe0.9Yb0.1O3−δ

Cell voltage 0.74 V in pure H2 at 650 °C

with 640 mA/cm2 goes immediately to 0.7

V stable over 20 h upon introduction of

500 ppm H2S. Full regeneration possible.

309

Ni/CGO Cell voltage 0.63 V in pure H2 at 650 °C

with 640 mA/cm2 goes immediately to

0.61 V and decreases continuously for 6 h.

Regeneration not possible.

84

Ni1−xCox-YSZ

anodes

YSZ electrolyte

LSM cathode

Ni0.69Co0.31O-YSZ Current exchange density 0.024 A/cm2 in

pure H2 improving to 0.094 A/cm2 in 10%

H2S in CH4 after 15 h at 850 °C.

101

Ni-YSZ Current exchange density 0.018 A/cm2 in

pure H2 improving to 0.032 A/cm2 in 10%

H2S in CH4 after 15 h at 850 °C.

Ni–YSZ or

S

r1−xCexCo0.2Fe0.8

O3−δ anodes

ScSZ

electrolyte

supported

LSM cathode

S

r0.85Ce0.15Co0.2Fe0

.8O3−δ anode

Current density is 0.088 A/cm2 in H2 at 0.9

V.

Lowered to 88 % of initial value upon

addition of 20 ppm H2S at 800 °C after 500

min at constant 0.6 V.

310

Ni-YSZ Current density is 0.080 A/cm2 in H2 at 0.9

V.

Immediate drop in current. After 500 min,

current lowered to 81 % initial normalized

value in 50 ppm H2S at 800 °C at constant

0.6 V.

Ni YSZ anode

YSZ electrolyte

support

SDC/LSCF.

Ni-YSZ +

infiltrated

BaZ

r0.1Ce0.7Y0.1Yb0.1O

3−δ (BZCYYb)

Cell voltage remains above 0.72 V even

upon addition of 30 ppm H2S to pure H2, at

700°C, 0.054 A/cm2. V. slow degradation.

311

Ni-YSZ Cell voltage from 0.72 V in H2 to 0.62 V to

85

20 ppm H2S in H2 within a few minutes at

700°C, 0.054 A/cm2.

Ni-doped

zirconia anode

supported cell

Doped zirconia

LSM cathode

Ni-ScSZ anodes

ScSZ electrolyte

Two stage poisoning mechanisms, one

fast, reversible and one slow and

irreversible. Poisoning of methane

reforming.

Cell voltage 0.7 V in 2 ppm H2S, 13% H2, 58

% H2O, 29% CH4, 850 °C, 1 A/cm2 after 500

h

312

Ni-YSZ

YSZ electrolyte

Cell voltage 0.45 V in 2 ppm H2S, 13% H2,

58 % H2O, 29% CH4, 850 °C, 1 A/cm2 after

500 h.

Two stage poisoning mechanisms.

Poisoning of methane reforming.

Ni-doped

Zirconia anode

YSZ electrolyte

LSM cathode

Ni-ScSZ anode Cell voltage: 0.55 V

at 200 mA/cm2, 800 °C, 100 ppm H2S in H2

after 1000 s with stable performance.

62

Ni-YSZ anode Cell voltage: 0.18 V

200 mA/cm2, 800 °C, 20 ppm H2S in H2

after 1000 s. Null voltage after 1650 s.

Two stage poisoning: initial one is fast and

reversible, second is slow and irreversible

Ni-YSZ anode Ni-YSZ anode Voltage drop 0.12 V, 200 mA/cm2 62

86

Doped zirconia

electrolyte

LSM cathode

ScSZ electrolyte in 5 ppm H2S in H2 at 850 °C.

Ni-YSZ anode

ScSZ electrolyte

Voltage drop 0.52 V, 200 mA/cm2

in 5 ppm H2S in H2 at 850 °C.

Ni-YSZ Anode-

supported

YSZ electrolyte

LSM cathode

Ceria-modified

Ni-YSZ

Cell voltage = 0.6 V at 0.3 A/cm2 in H2 +

200 ppm H2S at 700 °C

313

Ni–YSZ anode Cell voltage = 0.4 V at 0.3 A/cm2, 700 °C in

H2 + 200 ppm H2S

Ni-

BaZ

r0.4Ce0.4Y0.2O3-δ

(BZCY) and Ni-

Sm0.2Ce0.8O1.9

(SDC) anodes

SDC electrolyte

B

a0.5Sr0.5Co0.8Fe0.2

O3-δ (BSCF) and

Sm0.5Sr0.5CoO3-δ

(SSC) cathodes

Ni-BZCY OCV = 1.01 V in pure H2

Stable 148 mW/cm2 in 100 ppm of H2S at

200 mA/cm2, 600 °C for 700 min

298

Ni-SDC OCV = 0.709 V in pure H2

From 137 mW/cm2 to 81 mW/cm2 in 100

ppm of H2S at 200 mA/cm2, 600 °C for 150

min

87

Ni–YSZ anode

supported

YSZ electrolyte

LSCF-GDC

cathode

Bimetallic

coating

Ni-Cu/Co/Fe on

anode

Peak power densities ∼1.4 W/cm2 in H2.

Decreases to 1.0 W/cm2 in 500 ppm H2S-

H2.

Enhanced dry reforming of methane.

141

No extra

coating

Peak power densities ∼1.4 W/cm2 in H2.

NiSn-YSZ

anode

supported

YSZ electrolyte

LSM-YSZ

cathode

Infiltrated NiSn

+ reformer

NiSn/Al2O3

Cell voltage decreases from 0.72 V in pure

H2 to 0.63 V on addition of 500 ppm H2S at

at 850 °C and 1.25 A/cm2. Complete

regeneration.

314

No infiltration

and without

reformer

Cell voltage decreases continuously from

0.5 V to 0.45 V in 48 h at 1.25 A/cm2, 850

°C, 200 ppm in CO2:CH4

8. Strategies from conventional catalysis

The problems faced by fuel cell anodes regarding carbon and sulfur poisoning are

similar in many ways to those faced by conventional catalysts. In fact, in one very important

respect carbon and sulfur tolerance is more challenging in conventional catalysis – there is

no equivalent of the oxygen flux through the electrolyte which occurs in SOFCs, which tends

to reduce the problems with carbon and sulfur. Because of this, it is instructive to look at

solutions for tolerant catalysts. This issue has been studied over a much longer period of

88

time, and more intensively, than for SOFCs, and many of the findings have not yet been

incorporated into SOFC research.

8.1 Carbon tolerance in conventional catalysis

Several strategies have been studied for minimizing the carbon deposition in catalyst

used in reactions involving hydrocarbons, such as steam reforming, dry reforming, partial

oxidation or water gas shift. The use of noble metals, instead of Ni, as the active phase is the

best option in terms of carbon resistance. However, similarly to SOFCs, the high cost and

low availability of noble metals mean that Ni-based catalysts are favoured, and strategies

for minimizing carbon deposition in these catalysts have been developed in the last

decades16, 315.

8.1.1 Sulfur passivation

Sulfur passivation was one of the first strategies developed to diminish carbon

deposition. The first published works, in steam and dry reforming of methane, appeared in

the mid-80s18, 316-317. The approach consists of partially passivating the active centers of Ni

catalysts with sulfur, normally using H2S18, 316-319. Lately the use of alkanethiols for the

passivation has also been proposed with promising results320-321.

Hydrogen sulfide chemisorbs on the nickel surface and blocks access to the catalytic

centres. This blockage decreases the carbon deposition rate more than the methane

reforming rate316. At complete coverage, carbon atoms cannot be dissolved into the nickel

crystal and the whisker growth mechanism is inhibited. However, the complete coverage of

the nickel surface with sulfur results in total deactivation316. Using coverage ratios of around

0.7 it is possible to diminish carbon deposition without compromising reforming activity316-

318. At this coverage ratio it is not possible to inhibit carbon formation. Nevertheless, the

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usual whisker structure is replaced by more amorphous structures, which are less

deactivating316-317, 319. Hence, at this coverage, the reaction still takes place. This is due to the

number of active nickel surface sites needed for each process. Carbon nucleation needs

larger sites, which are almost completely blocked at high coverage rates, whereas methane

reforming reactions can proceed in the smaller sites which are still available18, 316-319.

However, reforming of larger molecules, like toluene and tars, requires large active sites, so

the sulfur passivation can deactivate reforming reactions as well318. This technology, in the

case of the dry reforming of CH4, has been industrially implemented by Haldor Topsøe in the

SPARG process21, 32, 322.

Sulfur passivated catalysts have been also applied to the dehydrogenation of

isobutane, with sulfur passivation improving both the selectivity of the process and

inhibiting carbon formation323.

8.1.2 Alloying and bimetallic systems

The introduction of additional metals that can modify the ability of carbon to

assemble or to dissolve in the bulk metal of the catalyst can drastically reduce the potential

for carbon deposition. A vast number of bimetallic combinations can be found in the

literature. Focusing only on Ni catalysts, bimetallic systems like Ni-Co324-328, Ni-Fe324, Ni-Cu324-

325, 329-332, Ni-Mn324, Ni-Sn167, 325, 333-334 and Ni-NM (being NM a noble metal: Rh, Pt, Pd, Ir, Ru,

Au, Ag)9, 49, 325, 330, 335-341, have been studied, showing in some cases very promising results.

When one or more other metals are introduced into the system, different structures

can be originated depending on the metals’ properties, interactions with the support,

atmosphere, temperature, etc325, 336. A schematic representation of these structures can be

seen in Figure 23. Among these structures, interest in alloys is increasing. The use of alloys

of different metals as the active phase has been deeply developed in the last years. This is

90

due to the superior performance of alloys in terms of conversion and resistance to carbon

deposition341-342.

Figure 23. Possible structures shown by bimetallic nanoparticles: a) core-shell; b)

heterostructure; c) nano-alloy; d) segregation; e) ensembles (adapted from Catal. Today

2012, 197 (1), 190-205, with permission from Elsevier).

The interaction between two or more metals can give rise to geometric and

electronic effects, which could affect carbon deposition325. The geometric effect is the result

of the dilution of the atoms of one metal in the other. Thus, surface ensembles are reduced

in size. This can dramatically affect the catalyst performance, since many reactions depend

on the size of the ensembles, as was explained for the partial sulfur passivation325, 341. The

electronic effect is the result of the difference in electronic affinity between the metals, that

can produce an electronic density increase or a decrease in the main metal depending on

whether the secondary metal has a lower or higher electronic affinity. These modifications

in the electronic density alter phenomena such as adsorption or desorption of species

during the reaction process, affecting activity and selectivity325.

Noble metals are well known to be more resistant to carbon deposition than Ni, as

well as possessing other features such as improved catalytic activity, suppression of Ni

oxidation or sustainability in daily start-stop operations324-325, 340, 343. Among the noble metals,

the most common used in bimetallic systems with Ni is Rh. In this type of catalyst Rh atoms

91

enrich the Ni surface, forming a surface alloy Ni–Rh, rather than dissolving into Ni particles

and forming a bulk alloy. However, the formation of the alloy strongly depends on the

support used and its interaction with the metallic particles324, 340. In addition, preparation

conditions can also affect the carbon resistance of the bimetallic system. Thus, if the

catalysts are calcined in oxygen at high temperatures, metal segregation can occur, giving

rise to lower carbon resistance339. The presence of Rh increases the energy barriers of C

diffusion and C–C bond formation whereas the O diffusion and C–O bond formation are not

significantly affected. As a consequence, the global rate of carbon deposition is decreased49,

325. Similar behaviour has been found in the case of Ni-Pt systems325, 336. The presence of Pt

has been found to promote the formation of small NiO crystals, which facilitates the

reduction to Ni0 and improves Ni dispersion340. The versatility of Ni-Pt system allows the

creation of different surface structures (core shell, monolayers, alloys) that need to be

controlled to minimize carbon deposition336, 340.

Although less studied, Au and Ag have given rise to interesting results in terms of

carbon resistance5, 330. Particularly in the case of the Ni–Au alloys, it has been found that the

presence of a small amount of gold on a supported nickel catalyst can induce a significant

effect on the carbon formation process during the steam reforming of methane5. Au makes

the diffusion of the CHx species (intermediates in carbon growth) significantly difficult,

preventing carbon nucleation330. In the dry reforming of methane, the presence of gold

promotes the formation of carbonaceous species which have high reactivity with CO2, thus

facilitating gasification340.

However, the elevated cost of noble metals makes it more practical from the

industrial point of view to develop noble metal-free catalysts49, 324-332, 344. Ni-Co might be a

more affordable option. Ni-Co bimetallic catalysts show a synergetic effect that makes the

92

catalyst more active and resistant to carbon deposition than Ni and Co monometallic

catalysts324, 326-327. Ni and Co benefit from the electronic effects that appear in bimetallic

systems. They present different oxidation states depending whether they are used in

monometallic or in bimetallic catalysts, indicating an electronic transfer between Ni and Co

in the bimetallic catalyst324. This protects metal from oxidation during the reaction and

confirms the near-distance interaction between the two metal atoms, making it easier to

form Ni–Co alloy on the catalyst surface324. In addition to the synergetic effect, the

formation of various spinel-type solid solutions with the supports improves the metal-

support interaction and therefore the carbon resistance324, 327. Cu-Ni system stability has

been found to be dependent on temperature and Cu/Ni ratio 329, 331. Copper seems to

stabilize the structure of the active site on Ni surface, thus preventing sintering or loss of

nickel crystallites. Adding Cu into Ni catalyst system can fine-tune the catalytic activity, so

that carbon formation and removal can be balanced, preventing deactivation by carbon

accumulation 329-330, 332. However, an excessive load of Cu could give rise to a Cu-rich alloy

that can increase carbon deposition 331.

Sn/Ni alloys seem to be the most promising alternative, not only for their high

resistance to carbon deposition but also for the low price of Sn compared to noble metals167,

325, 333-334. Sn/Ni alloys have shown up a huge potential for improving the carbon resistance in

steam reforming processes by modifying the relative kinetics of C-O and C-C bond

formation. Once again, in this case the formation of the surface alloy is favoured over the

bulk alloy, especially at low Sn loadings167, 333. DFT studies showed that the presence of Sn,

which is mainly located at the surface of the alloy particles, imposes an important barrier to

carbon diffusion in Ni crystallites, thus hindering carbides formation and the subsequent

93

nucleation of carbon 167, 334. These theoretical results were confirmed in steam reforming of

various hydrocarbons 167, 334.

8.1.3 Promoters

The addition of different promoters can affect the interaction between the metal

and the support or the acid-basic nature of the support, which modifies its tendency to give

rise to carbon deposition 27, 29. The use of several promoters can be found in the literature,

including Li 22, Na 345-346, K 22, 29, 346-350, Mg 15, 17, 22, 346, 351-359, Ca 346, 351, 360-361, La 22, 26-27, Zr 25, 326, 335, 337,

362-369, Mn 29, 348, Ce 29, 348, 370.

Alkali (Li, Na, K) 22, 29, 345-350 or alkali earths (Mg, Ca) 17, 22, 346, 351, 360-361, are usually

introduced in catalyst formulations with the aim of accelerating carbon removal from the

catalyst surface due to their basic nature371. K2O can reduce the carbon deposition rate in

reforming processes, but occasionally it also compromises the catalytic activity 22, 29, 347-350.

This reduction of the carbon deposition is a consequence of an improvement of the

gasification rate of the carbon deposits, thus improving the stability of the catalyst.

However, the interaction of K with Ni gives rise to large NiO crystalline particles. This species

has high mobility, thus promoting aggregation of particles and decreasing the activity, while

larger Ni particles also promote carbon deposition21-24, 31. K2O has also been used as a

promoter in Ni catalysts for the water–gas shift reaction345 and in bimetallic Ni-Mo catalysts

for the dry reforming of propane 372, presenting in these cases improvements both in carbon

resistance and catalytic activity. Similar behaviour to K, although less pronounced has been

observed in the case of Na2O and CaO 346, 351, 360-361, 373-375. In the case of CaO some researchers

have suggested that while the amount of carbon deposits increases, the reactivity of the

deposits also increases, leading to a higher stability of the catalyst 351, 360, 374.

94

The use of Mg has also been shown as an interesting option for minimizing carbon

deposition, although in this case the preparation of the catalyst and the interaction of Mg

with the support play a critical role 17, 22, 345, 352, 354-356, 358-359. If Mg is used as a promoter of

Ni/Al2O3, it interacts with Ni, leading to a NiO-MgO solid solution, but when Mg is used as a

dopant of the Al2O3 support, it can react to produce MgAl2O4 spinel. In both cases, Ni

sintering is prevented and the carbon deposition is reduced whereas the catalytic activity

increases, but the effect achieved by the spinel has been shown to be quantitatively better

than that from the solid solution.

Mn is used as a promoter, especially in dry reforming of methane, to reduce carbon

deposition, both in Ni and Co based catalysts 364-365, 375-378. MnOx forms patches that partially

cover the Ni surface giving a similar effect to that from sulfur passivation 375-377. Moreover, Ni

dispersion is improved 376, 378 and the moderate basicity of the MnOx improves CO2

adsorption and increases carbon gasification rate by forming reactive carbonate species 376-

377.

Lanthanide oxides have also been thoroughly studied, with La and Ce oxides the

most promising promoters 379-381. La2O3 has been found to affect positively both activity and

resistance to carbon deposition22, 26-27. Two different effects are promoted by the presence of

La2O3. On one hand, its basicity promotes the absorption of CO2, giving rise to lanthanum

oxycarbonates 22, 26-27, 382-383. These species play a role in conserving the stability of the

catalyst, since they promote the CO2 decomposition to CO and O, which can increase the

carbon gasification rate 22, 27. On the other hand, the interaction between La and Ni forms a

mixed oxide (NiLa2O4) in the same way as happens with Al2O3. This phase prevents the

sintering of Ni particles, thus reducing carbon deposition 27, 382. It has also been found that

the performance can be improved by the addition of alkaline oxides 384 or other lanthanides

95

like Ce or Pr 383, 385. In the case of the combination of La-Ce, it has been found that after

reduction of the catalysts, particles of a mixed oxide appear on top of nickel particles. This

decoration of Ni particles reduces the ensemble of Ni similarly as in the case of sulfur

passivation, thus lowering the probability of the nucleation of carbon precursors 383.

CeO2 constitutes another interesting promoter for minimizing and even suppressing

carbon deposition 29, 348. It has been used as support as well, but the conversions were lower

probably due to the strong metal-support interaction. However the results showed that its

use as promoter is much better, giving rise to high conversions and resistance to carbon

deposition 370, 386. It should be noted that the amount of CeO2 used as a promoter should not

exceed certain value to avoid compromising the catalytic activity338, 387. The high resistance

to carbon deposition comes from the oxygen storage capacity and oxygen mobility that

ceria presents 192, 388. CeO2 can store and release reversibly a large amount of oxygen, thus

increasing its availability for gasifying the carbon deposits29, 348, 353. As discussed in section 5.6

CeO2 exhibits excellent redox properties with a Ce3+-Ce4+ equilibrium and the coexistence of

CeO2 and Ce3O4 192, 353, 388. This behaviour can influence the oxidation state of atoms on the

surface of the active metal particles (for example Rh0/Rhδ+) favouring the activation of

reacting molecules 338, 386, 389. Other features that can improve catalyst performance are that

CeO2 gives rise to a better dispersion of the active phase, enhancing the catalyst

performance and inhibiting the transition of the γ-Al2O3 used as support to the low-surface-

area α-Al2O3 at high temperatures 387. When CeO2 is used as dopant of the Al2O3, CeAlO3

species are formed. This species completely inhibits the growth of filamentous carbon,

although amorphous carbon is still deposited (Figure 24) 380, 383, 387. It has been suggested that

these species suppress the growth of this filamentous carbon by chemical blocking rather

than by gasifying them after they have been deposited in the catalyst 387.

96

Figure 24. Carbon deposition models in the steam reforming of hydrocarbons over

Ni/Al2O3 and Ni-Ce/Al2O3 catalyst: a) not doped; and b) doped with Ce (adapted from J.

Catal. 2005, 234 (2), 496-508, with permission from Elsevier).

Finally, ZrO2 is also able to enhance carbon deposition resistance 25, 335, 362, 364-366, 368.

This promoter enhances the dissociation of CO2, forming oxygen intermediates near the

contact between ZrO2 and Ni. These intermediates increase the rate of gasification of the

carbon deposits 362. In addition, ZrO2 has both basic and weak acidic sites, which improves its

resistance to carbon deposition 326, 367. However, the main interest in ZrO2 seems to be its

use in combination with CeO2, since ZrO2 enhances the oxygen storage capacity and oxygen

ion mobility of CeO2. This enhancement of CeO2 properties results in an improvement of the

resistance against carbon deposition 335, 337, 363-364, 367, 369, 390.

8.1.4 Regeneration of Catalysts Deactivated by Carbon Deposition

Under certain conditions catalyst deactivation due to carbon deposition can be

inevitable even in the most resistant catalysts. For this reason the regeneration of the

catalysts is extremely important to maximize the benefit obtained from them9, 13. Catalysts

deactivated by carbon deposition can be regenerated using different gasifying agents (in

97

order of gasification rate): oxygen, steam, carbon dioxide or hydrogen. The reactions

involved in the regeneration processes with these gasifying agents are shown below 9, 13, 391-

393.

C + O2 CO2

C + H2O H2 + CO

C + CO2 2CO

C + 2H2 CH4

As discussed above, different types of carbon can deposit on the catalyst surface13.

Thus, they will behave differently during the regeneration. The carbons formed on Ni

catalysts involved in reactions with hydrocarbons can be monoatomic carbon, polymeric

amorphous films, vermicular fibres or whiskers, nickel carbide and graphitic films9, 13. Both,

preparation of the catalyst (metal loading, calcination temperature, particle size, use of

promoters) and reaction conditions (temperature, H/C ratio, presence of carbon precursors)

can affect the type and amount of carbon deposited.

Due to the differences in reactivity between the different types of carbon deposits,

different conditions should be applied. Thus, less ordered and more reactive carbons

(monoatomic carbon or amorphous) need lower temperatures and weak gasifying agents

(about 400 °C in H2 or H2O) whereas graphitic carbon needs higher temperatures and strong

gasifying agents (700-900 °C in air)9, 392. However, as O2 is the strongest and cheapest

gasifying agent, in industry catalysts are usually regenerated in air at about 600 °C9.

Although the catalytic activity can be recovered almost completely under certain

conditions, catalysts lose activity after each recovery cycle due to different reasons13, 394. For

example, regeneration in air is a very exothermic process that can lead to hot spots. These

hot spots can lead to metal reorganization or sintering, thus deactivating the catalysts in the

98

attempt of recovering the catalytic activity lost due to carbon deposition9, 13, 392. The main

reasons for losing catalytic activity during regeneration are:

Loss of metal particles that were pulled out from the support due to the

formation of carbon filaments9

Oxidation of the metals9, 13, 26, 390, 395. Although it can be reversed by

subsequent reduction of the catalyst, it sometimes gives rise to the irreversible formation of

different structures that can be inactive.13, 396-397 398

Sintering9, 26, 393, 398-399

Damage of the support328, 400.

However, in the same way that the addition of promoters or the formation of alloys

can enhance catalytic activity and resistance against carbon deposition, the regeneration

can be positively affected by the presence of these promoters27, 361 and alloys325-326, 328, 395.

Thus, the presence of small amounts of noble metals can improve reducibility of the main

metal324-325, 340. Moreover, in some cases, after a few regeneration cycles the performance of

the catalyst can be enhanced325. These strategies for improving carbon resistance can help

to facilitate carbon removal during regeneration of the spent catalyst.

Table 6. Strategies to minimize carbon deposition in catalysts

Strategy Catalysts Process Ref.

Sulfur

passivation Ni(S) Steam reforming

of CH4

320-321

Ni(S)/Al2O3 CO2 reforming of

CH4

316

Ni(S)/MgAl2O4 CO2 reforming of 316

99

CH4

Dehydrogenation

of isobutane

323

Bimetallic

catalysts

Ni-Au/MgAl2O4 Steam reforming

of n-butane

342

Ni-Co/Al2O3 Steam reforming

of glycerol

328, 393

Ni-Co/CeO2-Al2O3 CO2 reforming of

CH4

379

Ni-Co/MgAl2O4 CO2 reforming of

CH4

324, 327

Ni-Co/MgO-ZrO2 CO2 reforming of

CH4

326

Ni-Cu/Al2O3 CO2 reforming of

CH4

331

Ni-Cu/SiO2 CO2 reforming of

CH4

329

Ni-Cu/MgO-SiO2 Steam reforming

of ethanol

358

Ni-Cu/CaO-SiO2 Steam reforming

of ethanol

358

Ni-Cu/ZnO-Al2O3 Steam reforming 332

100

of methanol

Steam reforming

of ethanol

332

Ni-Mn/MgAl2O4 CO2 reforming of

CH4

324

Ni-Mo/Al2O3 Steam reforming

of gasoline

344

Ni-Mo-K2O/Al2O3 CO2 reforming of

propane

372

Ni-Mo/Al2O3 Steam reforming

of gasoline

344

Ni-Pt/Al2O3 CO2 reforming of

CH4

336

Ni-Re/Al2O3 Steam reforming

of gasoline

344

Ni-Rh/SiO2 CO2 reforming of

CH4

339

Ni-Rh/CeO2-Al2O3 CO2 reforming of

CH4

49, 338

Ni-Rh/CeO2-ZrO2 Steam reforming

of ethanol

335

Methanation of

carbon dioxide

337

Ni-Sn/YSZ Steam reforming 167, 333-334

101

of methane

Steam reforming

of propane

167, 333-334

Steam reforming

of isooctane

167, 333-334

Co-Rh/CeO2-ZrO2 Steam reforming

of ethanol

335

Co–Rh/SiO2 Oxidative steam-

reforming of

ethanol

395

Co–Ru/SiO2 Oxidative steam-

reforming of

ethanol

395

Promoters

Ni-K2O Water gas shift 345

Ni-CaO/Al2O3 CO2 reforming of

CH4

346, 351, 360, 374

Ni/CeO2-Al2O3 CO2 reforming of

CH4

348, 353, 370, 387, 389

Steam

gasification of

polypropylene

357

Steam reforming

of propane

380

102

Oxidative

reforming of

hexadecane

383

Ni-K2O/Al2O3 CO2 reforming of

CH4

22, 346-350

Ni/La2O3-Al2O3 CO2 reforming of

CH4

22, 27

Steam reforming

of ethanol

401

Steam reforming

of propane

380

Ni/CeO2-ZrO2-Al2O3 Steam reforming

of CH4

369, 397

Ni/CeO2-La2O3-Al2O3 Oxidative

reforming of

hexadecane

383

Ni-Li2O/Al2O3 CO2 reforming of

CH4

22

Ni-MgO/Al2O3 CO2 reforming of

CH4

22, 346

351-355

Steam reforming

of ethanol

356

Steam

gasification of

357

103

polypropylene

Ni-MnO/Al2O3 CO2 reforming of

CH4

348, 365, 376-377

Ni-Na2O/Al2O3 CO2 reforming of

CH4

346

Ni/ZrO2-Al2O3 CO2 reforming of

CH4

362

Ni/ CeO2-ZrO2 Methanation of

carbon dioxide

337, 363

Autothermal

reforming of

isooctane

390

Ni-CaO/La2O3 CO2 reforming of

CH4

384

Ni-SrO/La2O3 CO2 reforming of

CH4

384

Partial oxidation

of CH4

367

Ni-MgO/SiO2 CO2 reforming of

CH4

359

Ni-MnO/SiO2 CO2 reforming of

CH4

378

Ni-ZrO2/SiO2 CO2 reforming of

CH4

378

104

Ni-K-Ca/ NaZSM-5 CO2 reforming of

CH4

361

Ni-MgO/Zeolite HY CO2 reforming of

CH4

375

Ni-MnO/Zeolite HY CO2 reforming of

CH4

375

Ni-Co/CeO2-Al2O3 CO2 reforming of

CH4

379

Ni-Co/MgAl2O4 CO2 reforming of

CH4

324, 327

Ni-Co/MgO-ZrO2 CO2 reforming of

CH4

326

Ni-Cu-MgO/SiO2 Steam reforming

of ethanol

358

Ni-Cu-CaO/SiO2 Steam reforming

of ethanol

358

Ni-K/CeO2-Al2O3 CO2 reforming of

CH4

29

Ni-Mo-K2O/Al2O3 CO2 reforming of

propane

372

Ni-Rh/CeO2-Al2O3 CO2 reforming of

CH4

49, 338

Ni-Rh/CeO2-ZrO2 Methanation of

carbon dioxide

337

105

Steam reforming

of ethanol

335

Co/CeO2-ZrO2 CO2 reforming of

CH4

364

Co-Rh/CeO2-ZrO2 Steam reforming

of ethanol

335

Pt/CeO2-Al2O3 Oxidative

reforming of

hexadecane

383

Pt/ZrO2-Al2O3 CO2 reforming of

CH4

368

Partial oxidation

of CH4

368

Pt/CeO2-ZrO2 Partial oxidation

of n-tetradecane

366

Pt/CeO2-La2O3-Al2O3 Oxidative

reforming of

hexadecane

383

Rh/CeO2-ZrO2 Partial oxidation

of n-tetradecane

366

8.2 Strategies against sulfur poisoning

Sulfur containing molecules are frequent impurities in fuels and oil-derived

feedstock. These impurities, even at very low concentrations, are responsible for

106

heterogeneous catalyst deactivation. Millions of dollars are lost in chemical and oil

industries as a result of sulfur poisoning 402 403.

Generally speaking, two different approaches have been intensively studied to face

this problem. The first consists of the sulfur removal from the fuel via hydrodesulfurization

and involves a thoughtful catalyst design to achieve high efficiencies (see section 5). In

industrial reactors, sulfur is removed to levels below 0.1 ppm by a multiple step process,

finishing with adsorbents normally based on ZnO404-405. However, a balance has to be struck

between cost, convenience and effectiveness, and significant savings can be made if higher

levels of sulfur can be tolerated. In this sense, the second strategy is to develop sulfur-

tolerant catalysts able to operate in sulfur-rich reaction mixtures78. This second approach is

in line with the aim of this review, which is to provide an overview of the current status in

carbon and sulfur tolerant systems. At this point, a brief reiteration of the fundamental basis

of sulfur poisoning given in detail in section 4 may help to understand the developed

strategies.

Sulfur poisoning takes place due to sulfidation of the active catalytic species, namely

metallic particles and/or metal oxides13. In the case of a metallic particle (Me) and

considering H2S as the source of sulfur the process could be simplified as follows:

Me0 + H2S ----- MeS + H2

At high T its effect should decrease, because sulfidation is thermodynamically

unfavored. However, its kinetics is favoured and the result can be different to the expected,

depending on the metal used (for example, with Ni ΔG is not positive even at 1000 °C).

Sulfur as a poison causes a multifold effect in the catalytic activity. Firstly, sulfur adsorption

physically blocks the catalyst active sites limiting accessibility for the reactants and reducing

the probability of reactant molecules encountering each other. Secondly, by virtue of its

107

strong chemical bond it electronically modifies the neighbour metal atoms thus modulating

their ability to adsorb and/or dissociate reactant molecules9. In addition, the catalyst surface

could be reconstructed due to the strong chemical adsorption. Finally, the presence of

strongly bonded sulfur species on the surface of the catalyst hinders the diffusion of both

product and reactant species. Figure 25 schematizes the multiple effects caused by sulfur in

a metal supported catalyst.

A

BA B

CAS S S S

MMM MM M

support

Electrons withdrawal

Site blockage

Hindered reactants encounter limited products

diffusion

Figure 25 Simplified representation of the multifold poisoning effect due to sulfur

chemisorption (M represents an active metal. A and B represent reactant molecules and C

the reaction product)

In this scenario, catalyst deactivation must be overcome and/or the poisoned

catalysts must be regenerated. It must be always kept in mind that the degree of poisoning

depends on the studied reaction, process conditions and the involved catalysts, among

other factors. Consequently, a specific catalyst and/or strategy is required for each process.

In response to these needs, intensive research has been carried out in the field of

heterogeneous catalysis in the last decades generating a wide variety of multicomponent

catalysts with different natures and different features aimed at sulfur poison mitigation.

Herein, a summary of the most conventional approaches and proposed materials are

discussed.

8.2.1 Noble metal-based catalysts

108

Nickel-based catalysts are still the most preferred materials for reforming reactions

due to their good performance, low cost, relatively simple preparation and wide availability

406-408. However, apart from the well-known Ni deactivation due to sintering and carbon

deposition, this metal is one of the most sensitive active phases towards sulfur poisoning 409-

410. The chemical equilibrium of sulfidation at 900 °C for Ni is much more favourable

compared to the values obtained for Ru, Pt, Rh or Co underlining that Ni is the most sulfur

sensitive metal among the conventional reforming active phases13.

In this sense, the use of noble metals based catalysts is a good choice from an

activity and sulfur tolerance point of view, although the cost must be considered411-412. Mono

and bimetallic Pt-based catalysts developed by Farrauto et al. were stable under continuous

operation when exposed to sulfur-containing streams in reforming reactions411. Pt/CGO was

successfully employed in the steam reforming of isooctane to produce hydrogen

demonstrating complete sulfur tolerance413. In this study the authors compared the

performance of this material with a similar Ni/CGO and a conventional Pt/Al2O3. Only Pt

supported on ceria resisted the effect of sulfur. The latter indicates that not only the active

phase matters but also the support plays a crucial role in sulfur resistance. Apparently, the

Pt atoms in the Pt/CGO are more electron-deficient than Pt atoms in Pt/Al2O3 limiting the

interaction with S species413. However, Pt tolerance towards S poisoning also depends on

the considered reaction. For example, in the WGS reaction many high performance Pt based

catalysts suffer from severe deactivation when exposed to sulfur9, 414-419. Furthermore, this

adverse effect seems to be proportional to the amount of sulfur. For example for a Pt/ZrO2

catalyst, Xue and co-workers reported that the conversion went from 44% in the absence of

H2S to 25% (50 ppm H2S) to 14 % (200 ppm H2S) and finally 12% when 1000 ppm H2S were

introduced into the reactant mixture414.

109

Not only the considered reaction, but also the nature of the noble metal influences

the sulfur tolerance capacity of the catalysts. In other words, not all the noble metals exhibit

the same sulfur resistance. For example, McCoy et al. and Azad et al. demonstrated in

different papers that Rh is remarkably less sensitive than Pd towards sulfur poisoning420-422 .

The combination of metals (Rh-Pd) enhanced the tolerance, conserving high and stable

conversion during 12 h of reaction (50 ppm of H2S were used as a sulfur source). The

unsuitability of Pd for sulfur tolerance was also shown in the work of Goud et al.25 Their

results show the deactivation of a Pd/ZrO2 catalyst on the reforming of hexadecane after a

few hours of operation. The inefficiency of Pd was also evidenced in the WGS this time using

ceria as a support and SO2 as a source of S423.

As mentioned above sulfur poisoning can be envisaged as a steric and an electronic

effect. From the electronic point of view, sulfur ligands withdraw electron density from the

metals. For instance, the differences among Rh, Pt and Pd can be explained in terms of

electronic effects. Theoretical calculations for model clusters S/M12 (M = Rh, Pt and Pd)

indicate that the tendency of a metal to lose d electrons increases in the following order: Rh

< Pt < Pd9, 103, 424. This agrees well with the relative occupancy of the d shell in the isolated

elements: Rh: d8s1< Pt d9s1<Pd d10s0. This tendency correlates with the decrease of density

of states around the Fermi level for these elements (25 % reduction for Rh, 50 % for Pt and

approximately 55 % for Pd). According to the latter, and strictly considering electronic

effects, Pd is the most vulnerable to sulfur poisoning among the three mentioned noble

species, in good agreement with the observed behaviour in reforming reactions425.

In summary, noble metals may constitute an alternative to mitigate the sulfur

poisoning effects in heterogeneous catalysts. Nevertheless, there is no guarantee that these

precious metals will completely tolerate sulfur and indeed they frequently fail depending on

110

the reaction conditions and the sulfur concentration. In addition, the nature of the noble

metal is a factor to take into account. In this sense, Rh seems to be one of the most

promising.

8.2.2 Alloys, bimetallic and promoters

Many efforts have been made aiming to improve the sulfur tolerance capacity of the

traditional Ni based catalysts for reforming reactions12. The use of promoters and bimetallic

combinations (whether alloys or not) have been a frequent strategy in recent years. For

example, Xie et al. investigated the behaviour of Ni, Rh, and Ni-Rh supported on CeO2-Al2O3

catalysts in the steam reforming of hydrocarbons, introducing sulfur into the reactant

mixture 426-427 None of the Ni-containing catalysts was stable to sulfur-laden mixtures,

although the Ni-Rh catalyst requires more time before deactivation; over 60 h on-stream.

Moreover enhanced carbon deposition due to sulfur was observed, especially for Ni-based

materials, but also noble metal combinations, for example in a commercial Pt-Rh/ZrO2

catalyst for the steam reforming of ethanol/gasolines428. A small amount of sulfur (5 ppm)

was enough to deactivate this catalyst after 22 h on stream.

Other Ni based bimetallic combinations have been tried. For example, Wang et al.

carried out screening of catalysts for liquid hydrocarbon reforming using Ni-Mo, Ni-Co and

Ni-Re supported on Al2O3 and introducing 20 ppm of sulfur in the reactant mixture429. All the

bimetallic samples exhibit superior performance to the primary monometallic Ni with Ni-

Re/Al2O3 being the most active sample. Indeed, this Ni-Re/Al2O3 sample showed an

outstanding performance maintaining hydrocarbon conversions around 90% during a 300 h

test in a sulfur-containing stream and at relatively low reforming temperatures (580 °C). A

similar positive effect due to the addition of Re in a Ni/Zeolite ZSM5 system was reported

elsewhere highlighting the ability of Re to mitigate sulfur poisoning430. In addition, Re can be

111

employed not only as a part of bimetallic systems but also as a promoter of an already

active catalyst. For example, Murata et al. developed a very active Ni/Sr/ZrO2 catalyst but

with poor sulfur tolerance431. In order to improve sulfur resistance a series of dopants were

added including Re, La, Nd, Sm, Ce, Yb, Eu and Mo. Among all the dopants only La and

especially Re enhanced sulfur tolerance. Actually the best sample in this study was

Ni-Sr/ZrO2 with 5 wt% Re, which was able to remain stable during 30 h processing a

commercial premium gasoline. It can be argued that rhenium seems to be the most

promising metal to diminish Ni sulfur poisoning with the extra benefit of enhanced catalytic

activity, although the exact mechanism ascribed (sulfur tolerant alloy formation or sacrificial

phase) varies between different studies.

Some other traditional bimetallic systems are Ni-Mo and Ni-W. As indicated in the

paper of González et al., the addition of Mo and W to Ni-based catalysts reduces

deactivation in steam reforming432. The idea is to use Mo as a sacrificial agent given its

facility to be sulfidized. In the presence of any sulfur species Mo would tend to form MoS2,

Ni atoms would not be affected and so in principle the active sites should be available.

Indeed, the electronic interaction between Ni and Mo in the Ni-Mo ensemble increases Mo

electron density easing its interaction with electronegative ligands such as S78. In other

words, Ni promotes the formation of MoS2 and in some particular applications, for example

hydrodesulfurization reactions, Ni is considered a promoter while Mo is the metal that

carries out the sulfur removal. In reforming reactions the classic paper of Bartholomew

proved that a Ni-catalyst doped with Mo was more sulfur resistant than the Ni catalyst

alone in a feed containing 10 ppm sulfur 433.

The combination of an active metal for reforming reactions such as Ni or Pt with Sn is

another widely explored alternative16, 434-437. Dumesic et al. obtained very promising results

112

in hydrogen production from biomass reforming using Ni-Sn catalysts434. In principle,

bimetallic Ni-Sn phases were designed to avoid Ni deactivation due to C deposition. As

proposed by Trimm the similar electronic structure of carbon and elements of groups IV and

V of the periodic system may favor the interaction of these metals with Ni 3d electrons

thereby reducing the chance of nickel interactions to carbon16. Further, as explained by

Rodriguez and Hrbek, the addition of tin to platinum is a good strategy to prevent sulfur

poisoning78. Tin may act as a site blocker to platinum avoiding the noble metal interaction

with sulfur and improving the stability of the reforming catalysts78. Tin and platinum form

well defined alloys that are very stable103. When compared to pure Sn and Pt, these alloys

exhibit a lower chemical reactivity towards sulfur-containing species such as SO2, H2S, S2 and

thiophene438-439. Figure 26 adapted from Rodriguez´s paper underlines the superiority of the

Pt-Sn alloy in terms of sulfur uptake compared to the monometallic systems.

Among the typical site blockers (Cu, Au, Ag, Zn, Sn) tin is the best choice to promote

sulfur tolerance of Pt based catalysts78. The electronic perturbations arising from the Pt-Sn

bond produce a system which has remarkably low reactivity towards sulfur poisoning78.

Other types of bimetallic systems and alloys involving noble metals have been

proposed aiming to gain sulfur resistance440-442. Bimetallic Pt-Pd and Pt-Ni catalysts were

significantly higher sulfur tolerant compared to the monometallic Pt based catalysts during

50 h of a stability test440. A commercial catalyst from BASF based on Pt-Rh was also tested

for the ATR of JP8442. The addition of 125 ppm of sulfur in the stream slightly deactivated the

catalysts on the first 250 h of operation. A more demanding stability test based on start-

up/shutdown operations strongly affected the catalysts’ performance with these series of

start/stop cycles the main reason for the catalysts’ deactivation.

113

0 1 2 3 4 5 6 7 8 9 100,00

0,05

0,10

0,15

0,20

0,25

0,30

0,35

0,40

Sulphur uptake 300 - 310 K

Sn/Pt (111)

Pt (111)

Polycrystalline SnTo

tal s

ulph

ur c

over

age

(ML)

SO2 exposure (L)

Figure 26. Total sulfur uptake for the adsorption of SO2 on polycrystalline Sn, Pt(111),

and a Sn/Pt(111) alloy. Adapted from Acc. Chem. Res. 1999, 32 (9), 719-728. Copyright 1999

American Chemical Society.

As mentioned in the previous section, among the noble metals Pd seems to be the

least sulfur tolerant. Nevertheless, bimetallic combinations also open up a route to improve

Pd-based catalysts sulfur resistance78. Metal-metal interactions reduce the electron donor

capacity of Pd limiting its tendency to form strong bonds with sulfur-like ligands443. In

particular, Pd-Rh, Pd-Ni and Pd-Mn may present a good catalytic behaviour and be notably

less sensitive to the presence of sulfur-containing molecules in the reactant mixtures than

pure Pd78.

Briefly, it can be concluded that most of the bimetallic systems proposed in the

literature exhibit superior performance (higher catalytic activity and enhanced carbon and

sulfur resistance) compared to their individual counterparts. Several reasons account for

the positive results obtained with the bimetallic materials: (i) a change in the number of

active sites (cooperative effects); (ii) the sacrificial role played by one of the species forming

114

the bimetallic system leaving free and available the second metal; (iii) an electronic effect

coming from the metal-metal interactions resulting in less sensitive materials towards sulfur

poisoning (the bimetallic bonding modifies the chemical reactivity of the metal towards

sulfur-containing molecules, "ligand effect")

The addition of promoters is an alternative to the bimetallic systems. Special

attention has been devoted to alkali metals in this regard. Due to their electropositive

behaviour they can easily donate electrons to sulfur ligands thus shielding the interaction

between sulfur species and the actual active phase of the catalyst. Apart from the electronic

effect, these types of promoters may act as a site blocker species, physically hindering the

arrival of sulfur to the catalytic active centre. Ferrandon and co-workers demonstrated that

the addition of potassium to a Rh/Al2O3 catalyst in gasoline steam reforming appreciably

increased sulfur tolerance444. They pointed out that sulfur adsorption on the Rh/Al2O3 was

limited due to site blockage attributed to K. In turn, they found a drawback: alkali inclusion

increased the temperature in the catalyst bed by inhibition of the endothermic steam

reforming reaction more than the partial oxidation processes. At the same time, this effect

enhanced the sulfur tolerance beyond the initial expectations when K was intended to be a

mere sorbent since the stability of sulfide species decreases with temperature.

8.2.3 Support and structural modifications

So far all the discussed approaches are focused on the metallic active phase of the

catalysts. However, similarly to SOFC anodes, conventional catalysts are composed of

metal/oxide mixtures and therefore the role of the support and its behaviour towards sulfur

poisoning should not be disregarded. Indeed, on the surface of a metal oxide sulfur can

interact with the metal oxygen sites, producing species that have different electronic

properties (i.e. sulfides and sulfates) and maybe responsible for catalyst deactivation.

115

In this sense, one of the most widely used strategies to alleviate sulfur poisoning is to

select supports with high oxygen mobility13. It is well established that oxygen mobility

mitigates the carbon deposition which can accompany sulfur poisoning445-450 and presumably

helps avoid the formation of inactive metal sulfides. As mentioned in previous sections,

ceria is one of the most desirable supports when oxygen mobility is required451-452. In this

way, a rather sulfur tolerant catalyst was developed by Xue et al. using Pt supported on

alumina impregnated with ceria and gadolinia450. In this report the Pt/CGO-alumina catalysts

were compared vs. a conventional Pt/Al2O3 sample. Only the ceria based materials resulted

in immunity to sulfur attack, with significant differences depending on the order of addition

of ceria and gadolinia. Interestingly, the sample where the ceria was impregnated first was

the most stable, which the authors ascribe to an improved Pt-CeO2 interaction. This catalyst

presented good activity in commercial-gasoline reforming with relatively high sulfur

concentration (100-500 ppm provided by thiophene). The authors argue that Pt possesses

different electronic properties when supported on bare alumina compared to the ceria-

alumina based support. Pt metallic sites in alumina are unable to resist sulfur poisoning. A

valuable point of this paper is the redox mechanism that the authors proposed for sulfur

elimination. Under steam reforming conditions, thiophene was transformed to H2S which is

released and eliminated from the cycle via reduction and re-oxidation of the ceria-doped

support450. Azad and Duran obtained also some interesting results using Rh/CeO2 based

materials420. In this work, the presence of 50 ppm of H2S “activates” the catalysts increasing

H2 yields in the steam reforming of toluene. They suggested that such positive effect could

be due to the formation of Ce2O2S which presumably promotes the activity of the supported

Rh. Actually, in this particular situation ceria is acting as a sulfur sorbent and the redox

properties of ceria are useful since the reduced oxide (Ce2O3) is more prone to trap sulfur.

116

It is well known that the redox properties of ceria can be boosted by the use of

promoters resulting in materials with enhanced oxygen storage capacity196, 198, 453.

Laosiripojana et al. investigated the steam reforming of biomass tar using Ni-Fe supported

on MgO-Al2O3, coated with CGO454. The results indicated that the formation of various Ce-O-

S phases influences the catalytic activity with the sulfates having a positive effect in the

oxygen mobility and therefore increasing the activity and the sulfides producing an activity

drop. Some other examples using Pd/CeO2 samples and CuO and Y2O3 as metal oxide

additives benefit the reforming performance. These dopants increase H2 yield due to an

increase in metal surface area available for reaction. In addition, CuO increased the stability

against sulfur poisoning due to the oxide acting as a sacrificial sulfidation site, taking the

sulfur species away from the active metal and/or the ceria support421.

Some groups proposed other types of support modifications in order to improve

sulfur tolerance. For instance, incorporation of the active metal into the crystal structure of

the oxide phase, followed by exsolution of metal particles on reduction with the aim of

stabilizing the particles and at the same time increasing metal dispersion. Smaller, more

stable particles should improve sulfur tolerance since the sintering of Ni particles leads to

larger crystallites that are more easily poisoned76, 455. For example, Ni particles can be

stabilized on hexaaluminate structures 456-458. Smith et al. prepared nickel hexaaluminate

dispersed on zirconia doped ceria catalysts obtaining rather good sulfur tolerance in the

partial oxidation of methylnaphthalene458.

Pyrochlore-like structures are also interesting to avoid sulfur poisoning. Pyrochlores

are a class of ternary metal oxides based on the fluorite structure with a cubic unit cell with

a general formula of A2B2O7. An important property of these materials is that catalytically

117

active noble metals can be substituted isomorphically on the B site to form a

crystalline catalyst457. In particular, metals like Ru, Rh and Pt can be introduced into the B

site of the pyrochlore structure because they meet ionic radius constraints and have the

required oxidation state. In this situation, the metal is included in the solid network and

somehow protected towards sulfur species. The group of Spivey has done intensive research

on this type of materials457, 459-462. For example, they found that a La/Sr/Zr/Ni-pyrochlore

loses some activity with 50 ppm of dibenzothiophene at the initial stages of the reforming

reactions. However, the deactivation was not continuous with time on stream. This suggests

that the poisoning species are adsorbed on the catalyst surface in the initial steps but are

not accumulated on the surface itself. In addition, almost complete activity was recuperated

when sulfur was removed from the stream. Similar results were obtained when Rh instead

of Ni was introduced in the pyrochlore lattice462. The pyrochlore structure, although it

experienced some deactivation, was more tolerant to sulfur compared to a reference

Rh/Al2O3 catalyst.

In a similar way to the carbon tolerance section, Table 7 summarizes the developed

catalytic formulations following a given strategy to mitigate sulfur poisoning.

Table 7. Strategies to minimize sulfur poisoning in catalysts

118

119

Strategy Catalysts Process Ref.

Bimetallic catalysts

Ni-Co/MgO-Al2O3

Partial oxidation

reforming of

isooctane

409

Ni-Co/ Al2O3

Steam reforming of

liquid

methylcyclohexane

429, 432

Ni-Mo/ Al2O3

Steam reforming of

liquid

methylcyclohexane

429

Ni-Re/ Al2O3

Steam reforming of

liquid

methylcyclohexane

429

Steam reforming of

gasoline

430

Ni-Fe/MgO-Al2O3

Partial oxidation

reforming of

isooctane

409

Ni-Rh/ CeO2-Al2O3

Steam reforming of

liquid hydrocarbons

426

Ni-Sn/MgO-Al2O3

Steam reforming of

glycerol

407

Ni-Sn/CeO2-MgO-Al2O3

Steam reforming of

glycerol

406

Rh-Pd/ Gd2O3-CeO2

Steam reforming of

toluene

422

Rh-Pd/ ZrO2-CeO2

Steam reforming of

toluene

422

8.2.4 Regeneration of sulfur poisoned catalysts

Since most of catalysts are expensive and industrial productivity in many cases

depends on the catalysts’ performance, there is a need to reactivate or regenerate them.

Although the regeneration method is rather catalyst-specific, generally, they involve thermal

treatments in oxygen, hydrogen or steam atmospheres.

As indicated by Bartholomew the toxicity of the sulfur species depends on how many

electron pairs are available for the interaction with the metals9 . In general toxicity

decreases as follows: H2S > SO2 >SO42-

etc. in the order of increased shielding by oxygen.

Therefore oxidation treatment to eliminate sulfides is an alternative to recover activity.

Ideally, the main goal of oxygen treatments is to remove all the sulfur species as SO2 (Ssolid +

O2 gas → SO2 gas) at high temperature. For example, Choudhary et al. managed complete

activity recovery of a Ni-ceria based catalyst after being exposed to 7400 ppm of thiophene

by thermal treatment at 800 °C in an O2/N2 50:50 mixture463. An inherent drawback of this

procedure is the oxidation of the active phase (Ni) during the recovery thus making

necessary a reduction step before re-running the reaction.

Apart from the active phase oxidation, this type of oxidative treatment involves

other disadvantages that limit its application and therefore it cannot be considered as a

general regeneration procedure. More precisely, the exothermicity associated with this

process may produce irreversible catalyst deactivation by thermal degradation and/or phase

transformation of the active components13. For instance, some authors reported irreversible

formation of the inactive NiAl2O4 spinel when they tried to re-activate a Ni/Al2O3 reforming

catalyst using diluted oxygen at high temperatures396. In this scenario, oxidative treatment is

only useful in some specific cases when the oxidation at high temperatures would not risk

modifying the catalyst structure.

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Alternatively to oxygen, thermal treatments in steam can be applied to re-activate

sulfur contaminated catalysts. One of the seminal works in this area was carried out by

Rostrup-Nielsen dealing with Ni-based catalysts deactivated upon H2S exposure464. This

indicated that steam can remove sulfur as hydrogen sulfide via:

Ni-S + H2O → NiO + H2S

Although H2O may produce also some oxidation of reduced Ni:

Ni + H2O → Ni-O + H2

At temperatures between 800 and 900 °C up to 90% of the sulfur can be removed

from the catalyst surface. In the same paper, the positive role of alkali promoters such as Ca

and Mg in the steam regeneration was discovered. The catalysts doped with small amounts

of calcium and magnesium were easier to re-activate. In turn, some other dopants like K or

Na did not improve the regeneration process, most likely because sulfur is converted into a

form that is retained in the catalysts in the presence of K and Na.

Complete recuperation of reforming activity for bulk Ni catalysts was found by

Hassini et al. using Ar/steam mixtures465. Regenerated catalysts were characterized by

means of XPS indicating complete sulfur removal from the catalyst surface after the steam

treatment. Nevertheless, as indicated above, Ni oxidation occurred and some oxidised Ni

species were identified by infrared spectroscopy underlining again the risk of altering the

catalysts structure when an oxidative treatment is applied.

Reducing atmospheres do not present the catalyst oxidation drawback observed

when the spent samples are treated with steam or oxygen. In this sense, this alternative is

currently viewed as the most desirable way to remove sulfur from catalysts. Typically, sulfur

is released as H2S by the direct reaction of adsorbed sulfur species and H2 (Ssolid + H2 gas → H2S

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gas). According to the thermodynamics of sulfide formation, this process is essentially

reversing the metal sulfide equilibrium formation13.

Cheekatamarla et al. observed complete regeneration of a molybdenum carbide

catalyst deactivated upon exposure to 500 ppm of benzothiophene using a sequential

thermal treatment: first one hour in He and later one hour in hydrogen at 900 °C for both

processes466. The authors claimed that the heating step in He may remove weakly adsorbed

sulfur species while for the chemisorbed species (likely forming metal sulfides) heating in

hydrogen was required.

The effectiveness of the hydrogen thermal treatment depends as expected on the

sulfur concentration used in the catalytic test. For example Hepola et al.318, 467 demonstrated

through temperature programmed hydrogenation (TPH) that complete sulfur removal from

a commercial Ni based catalysts was achieved when 500 ppm of H2S was used. On the

contrary, when the H2S concentration was increased up to 2000 ppm the hydrogen

treatment was not sufficient to eliminate all the chemisorbed sulfur. Figure 27 represents

the TPH profiles discussed in ref318

. It is clear that 2000 ppm provoked a strong adsorption of sulfur on the catalyst’s

surface making complete sulfide removal almost impossible.

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2000 ppm H2S500 ppm H2S

300 400 500 600 700 800 900 1000Temperature (oC)

H2S

con

cent

ratio

n (a

.u.)

Figure 27. TPH profiles (70% Ar/30% H2) of Ni based catalysts after exposure to 500

and 2000 ppm of H2S in N2 at 2 MPa, 900 C and 4-6 h. (reprinted from Appl. Catal. B 1997, 14

(3-4), 305-321, with permission from Elsevier)

Other reducing mixtures have been successfully employed to regenerate sulfur

poisoned catalysts. For example Arosio et al. demonstrated that CH4-reductive pulses can

partially recuperate a sulfur-contaminated Pd/Al2O3 catalyst spent in methane

combustion468. A small increase of the temperature up to 600 °C using short time pulses (2

min.) gave almost complete catalyst regeneration. Such a treatment combines extensive

sulfate decomposition with a PdO reduction/oxidation cycle. The authors claimed that the

reductive regeneration of sulfur-poisoned catalysts with CH4-containing atmospheres could

be more effective than the analogous treatment in H2, possibly due to the milder reducing

action resulting in minor formation of sulfide species on the catalyst surface468.

In summary, there are routes to regenerate sulfur-poisoned catalysts via thermal

treatments in different atmospheres. However, the success of this process depends on

several factors such as sulfur concentration, strength of sulfur interaction with the catalyst

surface, catalyst composition and its susceptibility to be affected by the recovery treatment,

123

etc. This complex situation makes necessary a careful choice of the thermal treatment and

may not ensure complete activity recuperation.

9. Conclusions and Perspectives

Solid oxide fuel cells (SOFCs) generate electricity and heat electrochemically from

hydrogen and/or carbon-based fuels. The electrodes in SOFCs need to exhibit electronic

conductivity, oxygen ion conductivity and catalytic activity. The fuel oxidation takes place at

the anode, where the deactivation by carbon and/or sulfur is one of the key challenges in

SOFC technology.

We have reviewed the approaches used in catalysis to prevent or minimise the

effects of carbon or sulfur on catalysts. Carbon and sulfur poisoning are much more

challenging in conventional catalysis since, as opposed to SOFCs, normally there is no

oxygen flux that could help to minimise their deleterious effect. Some strategies have been

shown to work in both catalysis and SOFCs, we can have confidence that the effect is real

and the basic knowledge is in place to expand or refine those strategies.

It is clear that the search for carbon and sulfur tolerance in catalysts and solid oxide

fuel cells is exemplified by the properties of one element, nickel. Its unrivalled propensity to

catalyse carbon-carbon bond formation is matched by superiority to other base metals in a

variety of other useful reactions. It is also extremely vulnerable to the electron withdrawing

effects of sulfur. The search, then, has focused on two different goals – first to mitigate

carbon deposition and sulfur poisoning in nickel-based catalysts, and second to find

catalysts which approach the activity (and cost) of nickel without vulnerability towards

sulfur poisoning or catalytic activity towards carbon formation. Several strategies to achieve

these goals have emerged in the SOFC and catalysis literature.

9.1 Alloying of nickel

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Alloying of nickel is a strategy that can have an effect on both carbon and sulfur

tolerance. Alloying can improve carbon tolerance by reducing the rate of carbon-carbon

bond formation, reducing the amount of the most destructive and deactivating graphitic

carbon, and/or increasing the rate of competing reactions, such as carbon oxidation. For

sulfur tolerance, nickel is the element most vulnerable to sulfidation, so alloying with almost

anything improves sulfur tolerance. Conversely, since nickel is an excellent catalyst for many

of the reactions in an SOFC anode, alloying may reduce the activity for these reactions.

This strategy has been implemented in a number of different ways in both catalysis

and SOFC studies. The classic example in catalysis is addition of noble metals such as Rh and

Au, and these have been used in SOFCs with some success. The use of noble metals in SOFCs

is complicated by the larger total amount of metal, meaning that proportionally more of the

expensive noble metals need to be used. For this group of elements, the developments in

MIECs and non-metal electronic conductors for SOFC anodes may allow more realistic

amounts of these metals to be used, and nanoalloys of Ni with Au, Rh or Re may be

promising for carbon and sulfur tolerance.

In both fields the issue of cost has driven a search for cheaper alternatives. For

obvious reasons the top row transition metals from Fe through to Cu have been explored

extensively. These seem to be effective in reducing the overall carbon deposition, and

decreasing the amount of graphitic carbon. In the case of these promoters, research could

switch to other issues affecting SOFCs, for example tolerance to redox cycling or

compatibility with electrolytes and other components, as their ability to mitigate carbon

deposition seems largely agreed upon.

Outside of the top row transition metals, there are some other candidates for carbon

and sulfur tolerance, the most promising of which is tin. Tin has been trialled in both

125

catalysis and SOFC anodes and appears to confer both carbon and sulfur tolerance. A

further advantage of tin is that the mechanism by which it works is reasonably well known,

meaning this is a good target for further testing in terms of long term stability and

compatibility. Another promising element is molybdenum, which is widely used in catalysis.

It has complicated chemistry, with different carbide, sulfide and oxide phases being stable

under possible regimes in an SOFC anode, meaning that further work to clarify its behaviour

is needed. However, it is potentially a promising electrocatalyst in its own right, so further

investigation may be fruitful.

9.2 Alkaline promoters and supports

It is well known in catalysis that basic oxides reduce carbon deposition by increasing

the carbon oxidation rate. This is thought to work as the basic sites act as stores for highly

reactive hydroxyl radicals. The strategy has found use in the SOFC literature, with elements

such as Ba and La looking the most promising for further investigations. The use of alkali

metals is underexplored compared to catalyst science, due to the higher mobility of these

elements, and also their potential for poisoning the catalytic reactions. The vapour

pressures of their oxides approaches that of Ni at 1000 °C (~10-10 bar) at temperatures

ranging from ~800 °C for Li2O down to ~500 °C for K2O, while the melting points of Na2O

(1132 °C) and K2O (740 °C) are also a concern. The move to intermediate temperature fuel

cells may bring at least Na and Li into play.

One interesting strategy which currently appears to be unique to the SOFC literature

is the use of basic cationic conductors such as Li+ and H+ conductors for carbon tolerance

(although the latter have recently begun to be used as catalysts for the reverse water-gas

shift reaction, the link to carbon tolerance has not been made469-470). These maintain the

basicity of the materials promoted solely with simple non-conducting alkali and alkaline

126

earth oxides but add in some extra conductivity to improve both the carbon tolerance and

electrochemical performance. In the case of Li+ conductors the Li is also stabilised so less

volatile.

One aspect which needs exploration regarding these basic promoters is their sulfur

tolerance, especially regarding their ability to mitigate carbon deposition in a sulfur-

containing gas feed.

9.3 Ceria, doped ceria and oxygen storage

It is fair to say that the discovery of the redox properties of ceria and doped cerias

has revolutionised both catalysis and SOFC science, especially given the oxide ion and

electronic conductivity of doped cerias. These materials work both by acting as a store for

oxygen which is then able to react with carbon species, and by their ability to trap sulfur

species. It has also been shown conclusively that doped cerias are both electrocatalysts and

catalysts in their own right for important reactions such as electrooxidation of hydrogen and

reforming of hydrocarbons. It is clear that doped cerias will continue to be incorporated into

the current and next generations of SOFC anodes.

With such a useful and varied class of materials there are obviously many fruitful

avenues for research. One of the most obvious is the use of the extremely high oxygen

storage capacity materials found in three-way catalysts and other catalytic systems. The

earliest of these, the Ce-Zr system, has been somewhat investigated, but it does not appear

that other ceria-based systems have been used at all in SOFCs. It is also worth noting that

although ceria-based oxygen storage materials are favoured because of their relative

structural stability on redox cycling, the use of impregnation and MIECs may allow the use of

less stable oxygen storage materials.

127

Other possible routes for further investigation include trying to improve the catalytic

activity of doped cerias. Current work in SOFCs has focused on the doped cerias with the

highest ionic conductivity, but a focus on the activity towards electrooxidation and

reforming of hydrocarbons may be useful, especially where ceria is not the only ionic

conducting species. Work to improve the sulfur storage capacity could also be important.

9.4 Preferential sulfur binding sites

Phases which preferentially bind sulfur, and thereby lower the sulfur coverage on Ni

or other active metals have been used in both SOFCs and catalysis. In catalysis species such

as Cu, Zn and Mo are known to act as sulfur sorbents in preference to Ni, while in SOFCs this

effect has been noted in ceria and Ba-containing compounds. In addition, there is literature

on sulfur sorbents for gas cleaning which may be useful405. The deposit of a barrier layer (i.e.

the first point of contact with the fuel) that protects the most electrochemically active area

of the anode (i.e. close to the electrolyte layer) is known to protect against carbon

deposition in SOFCs, but has not yet been investigated for sulfur poisoning.

9.5 Non-metal electronic conductors

Removal of the metal electronic-conducting phase solves many of worst effects of

carbon and sulfur poisoning, and there are two possible solutions for this. Non-metal

conductors in a cermet such as carbides or carbon retain the benefits of cermets, such as

the ability to independently optimise the electronic and oxide conducting phases, as well as

the disadvantages, such as having to match thermal expansion coefficients and more

complicated microstructure optimisation. MIECs lose both the advantages and

disadvantages of cermets. Both non-metal conductors and MIECs as potential solutions have

deficiencies in different areas. Non-metal conductors are generally under-researched and in

particular more work on stability is needed. MIECs are lacking in either electronic or ionic

128

conductivity, and some of the more widely used materials, such as the strontium titanates,

require high processing temperatures and are difficult to fabricate into anodes. Both

solutions are lacking in catalytic activity and will likely require a further catalytic phase.

9.6 Infiltration of nanoparticles

In catalysis, infiltration of porous structures with metal nanoparticles is a common

practice to maximise the active surface while simultaneously hindering carbon deposition by

decreasing the area of graphitic growth. In SOFCs, this approach was first used because the

low melting point of copper oxide meant that Cu-YSZ anodes could not be produced by the

conventional solid state route. Since then it has been used to add a variety of metals

(including nickel) and now has been proved that it can improve important parameters such

as the triple phase boundary length.

There are many issues to be resolved with infiltration, especially relating to long

term stability and feasibility of scaling up the process to industrial-sized anodes, but the

reason it is interesting for carbon and sulfur tolerance is that it allows much greater control

over the chemistry and structure of the electrode. The exploration of the possibilities in

SOFC anodes is only just beginning but already we have seen that the infiltration of barium

or ceria allows fine dispersion of the promoter over the surface of the material, enhancing

carbon tolerance by ensuring that any given nickel particle is close to a particle of the

promoter.

In the future we could see more complex oxygen storage materials or more

advanced catalysts incorporated into the anode structure by this method. The advances in

MIECs and possibilities of non-metal conducting phases such as carbon and carbides should

allow designed catalyst nanoparticles (whether containing nickel or not) to be added

without their effect being destroyed by alloying into the percolating metal phase.

129

9.7 Regeneration

Catalysts deactivated by carbon deposition are commonly regenerated by stopping

the process and then passing a stream of cleaning gas (hydrogen, steam, carbon dioxide or

oxygen). All of these gases are eventually present in a SOFC anode: as fuel, as a product of

oxidation, as a permeant gas, etc. The literature on regeneration of SOFC anodes is very

sparse, and modifications of the anode to aid regeneration are non-existent. Nevertheless, it

has already been shown that it is possible to remove carbon deposits from Ni-YSZ anodes by

a variety of gases, and also by oxygen flux through the electrolyte.

Ideally anodes would be designed so that they can be regenerated without use of

alternative feedstocks or extensive downtime, but failing this they need to be designed to

be regenerated at the minimum temperature for as short a time as possible, and be able to

withstand any changes which take place during regeneration. From the catalysis literature it

is probable that many promoters which prevent carbon deposition in the first place are also

effective in aiding regeneration, whereas for sulfur tolerance where sacrificial phases are

used, these might bind more strongly to sulfur, requiring harsher conditions for

regeneration.

Exsolution of nanoparticles from MIECs and symmetrical SOFCs also provide

interesting alternatives to conventional cermet anodes in terms of regeneration. While this

potential benefit has been noted in the literature on these materials there is little published

experimental work proving it.

9.8 Theoretical and computational studies

It has become clear in the last five years that theoretical and computational

chemistry is finally becoming able to provide accurate insights into the chemical behaviour

of materials and even interfaces471. Very recently accurate predictions have even been made

130

as to the structure and properties of previously unknown materials relevant to SOFCs472. A

historical problem in SOFC research (and scientific research in general) is the lack of

coordination between groups working in different fields. Groups working in fields such as

materials chemistry and detailed in situ and ex situ characterisation would surely benefit

from incorporating insights from theoretical chemistry in the future, and making sure their

work is relevant to the challenges in SOFCs.

9.9 Reflections on experimental work

Many in the literature have claimed experimental results of tolerance against carbon

and sulfur. However, caution needs to be taken in the techniques used to analyze these

results, and during the research and writing of this review we have noted some points

relating to this:

Claiming carbon tolerance by lack of performance degradation. It is certainly

true that an anode is carbon tolerant if it maintains performance over a long period of time

regardless of whether or not carbon is actually present. However, it cannot be claimed that

a lack of degradation means that there is no carbon, as it has been shown many times that

cells can operate without performance degradation for significant periods despite carbon

being deposited. Certainly a testing period of a day or a week as used in many papers is not

long enough to claim carbon tolerance in the absence of other data proving that either

there is no carbon or that the carbon has reached some kind of steady state.

Claiming carbon tolerance by lack of carbon in SEM. While SEM is clearly a

useful technique for assessing microstructure, the lack of carbon whiskers in an SEM image

is not proof of a lack of carbon. Even EDX needs careful sample preparation for accurate

quantitative analysis, for example polishing.

131

Low measured OCVs. This applies to two different systems – thin doped ceria

electrolytes and air leakage into the anode chamber. It has been shown that leaks causing

an increase in measured OCV of less than 0.02 V from 1.223 V to 1.239 V in dry methane

results in a decrease in implied water content (as calculated from the H2/O2 equilibrium) of

30% from 0.24% to 0.17%473. This is enough to result in a dramatic decrease by over half in

the amount of carbon deposition over a period of one hour. Likewise, many groups using 10

– 50 µm CGO and CSO electrolytes report OCVs below 0.9 V, which implies a substantial

oxygen flux at OCV through the electrolyte caused by the electronic conductivity of doped

ceria (or through a slightly permeable electrolyte). This oxygen flux or leakage should be

extremely effective at preventing carbon deposition and improving sulfur tolerance, but if

the aim is to study the carbon and sulfur tolerance of the electrode materials then care

should be taken to account for this.

Current collectors. The paper referred to above also showed that the

coverage of silver current collector paste can have a large effect on carbon deposition, even

completely preventing it if the electrode is completely covered473. Presumably this works in

a similar way to the barrier layer concept discussed in section 5.2. There has also been

considerable controversy over the use of platinum current collectors, with claimed high

power densities in dry methane for some MIECs being shown to be almost entirely due to

the use of platinum current collectors and doped ceria electrolytes240.

Humidification. As shown above, the exact level of humidification can have a

profound effect on carbon and sulfur tolerance. Many studies report 3% humidification

levels, which if using a bubbler, implies a water temperature of just under 25 °C (25 °C

would actually be 3.1%). This seems quite warm for a lab temperature, although the authors

of this review are based in Britain so maybe used to cooler temperatures than many. The

132

humidity level at 20 °C is 2.3%, and at 16 °C is 1.8%, care should be taken that the correct

humidity levels are being reported. It should also be borne in mind that these are 100%

relative humidity levels, whereas it is known that bubblers are not necessarily effective in

reaching 100% relative humidity474.

Interaction between carbon and sulfur. Many papers claim carbon and sulfur

tolerance, with separate experiments done to prove each using model gas feeds (e.g. one

experiment with dry methane and another with H2S in H2). However, it is well known from

both catalysis and SOFC literature that there is strong interaction between carbon and

sulfur, with each having the possibility of hindering or promoting the other under different

operating conditions. While there are undoubtedly advantages in simplifying the system by

separately studying carbon and sulfur tolerance, it does not necessarily follow that a system

which is separately carbon and sulfur tolerant will be simultaneously carbon and sulfur

tolerant. This could especially be the case where sulfur may react with promoters which are

present to reduce carbon deposition, for example with BaO. More work needs to be done

on sulfur-containing carbonaceous fuel in SOFC anodes.

In this review, we have seen the different materials solutions to carbon and sulfur

tolerance in catalysis and solid oxide fuel cells, but there is also an aspect of different

experimental techniques in the two fields. The foundational techniques of catalysis are well

established over decades, with a focus on gas phase techniques such as chemisorption

measurements and temperature-programmed reactions. Some of these have started to be

incorporated into SOFC studies, for example temperature-programmed oxidation. In SOFCs,

electrochemical impedance spectroscopy (EIS) has long been a key feature of the

investigations, and although it has not been used in catalysis, with advances in impedance

133

analysers, analysis techniques such as distribution of relaxation times (DRT) and modelling,

it is possible that EIS could start to be used in model catalysis studies.

Because of the more widespread nature of catalysis science, newer techniques have

generally been adopted in catalysis first, and later in solid oxide fuel cells. An example of this

is in situ Raman, which has been known in catalysis since the early 1990s, but is now being

used to investigate SOFCs. Other techniques such as high-resolution TEM, XPS and XRD are

following a similar path. A notable counter-example is FIB-SEM and tomography in general,

which appears to have been far more enthusiastically adopted in the SOFC community than

in catalysis.

As can be seen, the experience gained in the field of catalysis has had an increasing

influence in the research paths in SOFC and hopefully in the near future, this inspiration may

be reciprocated as catalysis can profit from the experience of SOFCs. Although electro-

catalysis at low temperatures is common, the high temperature regime is still an area of

opportunity for catalysis as it features the unique capability of supplying/extracting O2- or H+

to reactant species under an electric bias. A few examples are the synthesis of ammonia at

atmospheric pressure475, the non-Faradaic electrochemical modification of catalytic activity87

and the electrochemical reduction of CO2 and H2O476-477.

It is clear that the SOFC community has made great strides towards carbon and

sulfur tolerance over the last decade. Going forward, the strategies already implemented at

lab scale need to be incorporated into more commercially-focused devices, while at lab

scale the learnings from catalysis should be used to develop materials which are carbon and

sulfur tolerant, especially at lower temperatures. We hope that this review is able to help

with both of these.

10. Acknowledgements

134

Funding for this effort has been provided by Boeing Research & Technology

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Biographies

Paul Boldrin received a PhD in materials science from Queen Mary University of London

working on continuous hydrothermal synthesis of nanomaterial for catalysis. This was

followed by postdoctoral work in chemistry at the University of Liverpool working initially on

high throughput discovery of catalysts as a research associate and later as research

coordinator. Currently he is a postdoctoral research associate at Imperial College London

working on solid oxide fuel cells. His research interests include characterisation of the

catalytic and electrocatalytic processes occurring in solid oxide cells and membrane

reactors, and the use of nanomaterials in those devices.

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Enrique Ruiz-Trejo obtained his PhD. in Materials from Imperial College and immediately

after was appointed lecturer at Universidad Nacional Autónoma de México. He was then

awarded a Humboldt scholarship at the Max Planck Institute for Solid State Research. In

2009 he moved to Denmark as Senior Scientist at Risoe National Laboratories for

Sustainable Energy followed by a position as Research Fellow at the University of St

Andrews. Since 2012 he is Research Associate in Fuel Cells and Materials Processing at

Imperial College. His areas of interest include materials for energy applications and gas

separation membranes, the development of electrodes for fuel cells and the manufacture of

metal-ceramic composites.

Joshua Mermelstein is a fuel cell systems engineer at the Boeing Company in Huntington

Beach, CA with an expertise in solid oxide fuel cell (SOFC) and proton exchange membrane

(PEM) fuel cell systems. He is currently the lead scientist for fuel cell system development

within Boeing’s Electronic and Information Solutions Advanced Technology Programs (ATP).

Joshua is currently leading efforts as the chief engineer for Boeing’s development of a 50 kW

reversible solid oxide fuel cell (RSOFC) system used for microgrid energy storage. Joshua

also provides technical support for the development of other SOFC and PEM based fuel cell

systems throughout Boeing.

Joshua earned his Bachelor’s degree in Chemical Engineering from the University of Arizona

in 1999, Masters from the University of Southern California in 2000, and Ph.D. from Imperial

College of London in 2010 with the Department of Chemical Engineering and Fuel Cell

Research Group of the Energy Futures Lab, after working in industry as a chemical engineer

for 7 years. His research at Imperial College focused on the impact and mitigation of carbon

formation on SOFC anodes arising from biomass gasification tars through steam reforming,

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partial oxidation, and dry reforming technologies. Joshua has published over 10 publications

related to his work in this field. His career background spans 10+ years of industry

experience in chemical/process engineering, cryogenic and compressed gases, hydrogen

and fuel cell technology, fuel cell electric vehicles, plug-in hybrids and BEVs, alternative and

renewable energy for stationary power, hydrogen production, and combined heat and

power (CHP) for energy efficiency.

Jose M. Bermúdez graduated in Chemical Engineering (2008) and got a MSc in Process and

Environmental Engineering (2010) from the University of Oviedo, Spain. He obtained his PhD

in Chemical Engineering (2013) from the same university. His PhD Thesis deals with the CO2

reforming of coke oven gases to produce syngas for methanol synthesis and was developed

in the National Institute of Coal-CSIC (Spain). He worked in this research centre for more

than 5 years, where he was involved in the development of microwave-assisted processes in

the field of energy, mainly focusing on pyrolysis, gasification and catalytic heterogeneous

reactions. He gained a postdoctoral position in Imperial College London in 2014, where he is

working on the thermochemical stability of mixed ionic-electronic conductors for oxygen

transport membranes. He is also involved in the development of thermochemical processes

like supercritical water upgrading or catalytic hydrocracking of heavy oils and biomass. He

has co-authored more than 30 peer-reviewed papers and 2 patents on these topics and has

been finalist of the Best Young Researcher Award 2015 of the Spanish Group of Coal.

Tomas Ramirez Reina received his PhD in Chemistry from the University of Seville (Spain) in

2014 under the supervision of Prof. Odriozola and Dr. Ivanova. For his PhD work, he was

awarded “best PhD thesis 2014” by the Spanish Society of Catalysis (SECAT). He worked as

visiting researcher in 2011 in the Brookhaven National Laboratory (NY, USA) and in 2012 in

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the Institute of Chemical Engineering ICE-HT (Patras, Greece). In 2014 he moved to UK as

Research Associate in the Chemical Engineering Department at Imperial College London.

Currently, Dr. Reina is a lecturer in the Department of Chemical and Process Engineering in

the University of Surrey. His research interests include the development of advanced

heterogeneous catalysts for energy and sustainability. In particular, his work is focused on

clean hydrogen production, selective oxidation and hydrocarbon upgrading.

Nigel Brandon's research is focused on electrochemical devices for energy applications, with

a particular focus on fuel cells, electrolysers, and batteries. He is Director of the UK Research

Council Energy programme funded Hydrogen and Fuel Cells SUPERGEN Hub, and Co-

Director of the SUPERGEN Energy Storage Hub. He was the founding Director of the Energy

Futures Lab at Imperial College, and a founder of Ceres Power, an AIM listed fuel cell

company spun out from Imperial College in 2000. In 2014 he was appointed to the BG Chair

in Sustainable Gas and as founder Director of the Sustainable Gas Institute at Imperial

College.

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