Fuel Cell Mats & Comps Corr

8/3/2019 Fuel Cell Mats & Comps Corr

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5982 S.M. Haile / Acta Materialia 51 (2003) 59816000

Fig. 1. Schematic of a fuel cell, comprised of an electrolyte, an anode and a cathode. The overall chemical reaction is H2 + 1/2 O2

H2O. Anode and cathode reactions given are appropriate only for oxide ion conducting electrolytes. The reactions would bemodified for electrolytes with different mobile ions, but the general principle remains unchanged.

as a barrier to gas diffusion, but will let ionsmigrate across it. Accordingly, half cell reactionsoccur at the anode and cathode, producing ionswhich can traverse the electrolyte. For example, ifthe electrolyte conducts oxide ions, oxygen will beelectro-reduced at the cathode to produce O= ionsand consume electrons, whereas oxide ions, aftermigrating across the electrolyte, will react at the

cathode with hydrogen and release electrons:

Cathode:1

2O2 2e

O2 (1)

Anode: H2 O2H2O 2e

(2)

Overall:1

2O2 H2H2O (3)

Analogous cathode and anode reactions for protonconducting electrolyte are:

Cathode: 12O2 2H+ 2eH2O (4)

Anode: H22H+ 2e (5)

The flow of ionic charge through the electrolytemust be balanced by the flow of electronic chargethrough an outside circuit, and it is this balancethat produces electrical power.

Electrolytes in which protons, hydronium ions,hydroxide ions, oxide ions, and carbonate ions aremobile are all known, and are the basis for the

many categories of fuel cells under developmenttoday. Because ion conduction is a thermally acti-vated process and its magnitude varies dramati-cally from one material to the next, the type ofelectrolyte, which may be either liquid or solid,determines the temperature at which the fuel cellis operated. State-of-the art fuel cell electrolytesare listed in Table 1, along with the mobile ionic

species, temperatures of operation and fuels typi-cally utilized. For reasons of electrode activity(which translates into higher efficiency and greaterfuel flexibility), higher temperature operation ispreferred, but for portable (intermittent) powerapplications, lower temperature operation is typi-cally favored as it enables rapid start-up and mini-mizes stresses due to thermal cycling. In addition,solid electrolyte systems obviate the need to con-tain corrosive liquids and thus solid oxide andpolymer electrolyte fuel cells are preferred by

many developers over alkali, phosphoric acid ormolten carbonate fuel cells. Nevertheless, each ofthe fuel cell types listed in Table 1 has been dem-onstrated in complete fuel cell systems, with alkaliand phosphoric being the most mature techno-logies, and polymer electrolyte membrane fuelcells the most recent.

Although substantial progress has been madeover the last several decades in each of the fuelcell technologies listed in Table 1, only thoseemploying solid electrolytes, PEMFCs and SOFCs,


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5983S.M. Haile / Acta Materialia 51 (2003) 59816000

Table 1Fuel cell types and selected features [1,2]

Type Temperature C Fuel Electrolyte Mobile ion

PEMa: polymer electrolyte 70110 H2, CH3OH Sulfonated polymers (Nafion) (H2O)nH+

membraneAFC: alkali fuel cell 100250 H2 Aqueous KOH OH

PAFC: phosphoric acid fuel cell 150250 H2 H3PO4 H+

MCFC: molten carbonate fuel cell 500700 hydrocarbons, CO (Na,K)2CO3 CO32

SOFC: solid oxide fuel cell 7001000 hydrocarbons, CO (Zr,Y)O2-d O2

a Also known as proton exchange membrane.

are the subject of the present review. Polymeric

fuel cells, in particular, have garnered a tremen-dous amount of attention as a result of their appli-cability to transportation systems, and worldwideinvestment in PEMFCs continues to grow. Alkalifuel cells, while providing extremely high powerdensities, are considered by most to be impracticalbecause of the need to remove trace CO2 from boththe fuel and oxidant streams in order to preventreaction of the electrolyte to form solid, non-con-ducting alkali carbonates. Nevertheless, some com-mercialization efforts are underway. Phosphoricacid fuel cells, the leading technology in the early

1990s, have been largely abandoned because of theinability of developers to reach high power den-sities, and thus meet competitive cost targets on aper watt output basis. Given their high tempera-tures of operation, molten carbonate and solidoxide fuel cells have their greatest applicability instationary power generation. The former sufferfrom the difficulties of containing a corrosiveliquid electrolyte. In particular, dissolution of NiOat the cathode and its precipitation in the form ofNi at the anode can result in electrical shorts across

the electrolyte, and few US developers continue topursue this technology. Research and developmentefforts in SOFCs, though perhaps not as prominentin the public eye as PEMFC R&D, continue in bothindustrial and academic laboratories across theworld. Most, but certainly not all, of those effortsare directed at stationary power applications, asSOFCs may also serve the transportation industryas auxiliary power units.

In this review we present a brief discussion ofthe features that determine fuel cell performance,

describe the state of the art in SOFC and PEM fuel

cells, and evaluate electrolyte materials that mayenable new types of fuel cells. Althoughelectrodes/electrocatalysts are equally important toadvanced fuel cells, space limitations preclude ameaningful overview of this topic. Readers arereferred to an excellent overview of the manyaspects of fuel cell science and technology by Car-rette and coworkers for additional information [3].

2. Fuel cell characteristics

The key performance measure of a fuel cell isthe voltage output as a function of electrical currentdensity drawn, or the polarization curve, Fig. 2[4,5]. The measured voltage, E, can be written as

Fig. 2. Schematic fuel cell polarization (voltage vs. currentdensity) and power density curves.


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E EeqELhacthiRhdiff (6)

where Eeq is the equilibrium (expected or Nernstianvoltage), E

Lis the loss in voltage due to leaks

across the electrolyte, hact is the activation overpot-ential due to slow electrode reactions; hiR is theoverpotential due to ohmic resistances in the cell;and hdiff is the overpotential due to mass dif-fusion limitations.

The equilibrium potential, Eeq, can, in principle,be calculated from a knowledge of the thermodyn-amics of the reaction in question. One first deter-mines the change in Gibbs free energy, G, for thereaction [for example, reaction (3)] under the givenconditions and Eeq is then given by G/nF,

where n is the number of electrons transferred inthe reaction and F is Faradays constant. For reac-tion (3), the Gibbs free energy is

G Go(T) RTlnPH2P

1/2O2

PH2O(7)

where Go(T) is the Gibbs free energy of the reac-tion for the case when all species are in their stan-dard states (1 atm, pure gases) and the pressuresin the second term refer to the actual pressures inthe fuel cell experiment. The term Go(T) is tabu-

lated for most reactions of interest (or can be calcu-lated from the formation energies of the speciesinvolved). In the case of reaction (3) it is 242kJ/mol + (45.8 J/mol K) T, for all componentsin the vapor phase [6]. This term alone is used todefine the standard potential

Eo(T) Go(T) /nF (8)

of a particular reaction. In an operational fuel cell,knowledge of the equilibrium (Nernstian) potentialrequires knowledge of the partial pressures of all

the species involved in the reaction. In most fuelcell experiments involving hydrocarbon fuels, thepartial pressures of the product gases are neithercontrolled nor measured, and cell potentials aresimply compared to the standard potential.

Under open circuit conditions (that is, no currentis drawn) the measured voltage should, in prin-ciple, be exactly the Nernstian voltage. In hightemperature cells, in which reaction kinetics at theelectrodes are fast, deviations from this voltage areattributed to gas leaks across the membrane due to

poor sealing or cracks in the electrolyte, or to par-tial electronic conductivity. These two factorsdefine EL. In low temperature cells, sluggish

reaction kinetics may prevent measurement of theequilibrium potential even under otherwise idealconditions (no electronic conductivity, no gascross-over). Slow electrode reactions give rise toactivation overpotentials at both the cathode andthe anode:

hact RT

anFlnI0

I (9)

where a, termed the transfer coefficient, is the frac-tion of the overpotential assisting the reaction, and

I0, termed the exchange current density, is the cur-rent flowing equally in the forward and reversedirections at equilibrium (zero overpotential). Aswritten, Eq. (9) accounts only for forward reactions(i.e. reduction in the case of the cathode and oxi-dation in the case of the anode), and is a simplifiedform of the more general Bulter-Volmer equation:

I I0expanFhRT

exp(1a)nFhRT

(10)which accounts for reaction rates in both direc-

tions. Alternative approximations to (10) which aremore representative at the zero current limit havebeen presented [7]. The simple form of Eq. (9) canbe further approximated by the empirical Tafelequation

hact blog(I0) blog(I) (11)

with b defined as the Tafel slope. Eq. (11) impliesthat the slow reaction kinetics of low temperaturefuel cells lead to an offset in the open circuit poten-tial, Eo, by an amount

EoEeq blogIo. (12)

In hydrogen/oxygen PEM fuel cells, voltage off-sets on the order of 400 mV are typically observed[8], but it is unclear whether the entirety of thisis due to slow electrode kinetics or in part due tohydrogen and/or oxygen diffusion through theelectrolyte (i.e., contributions to EL). In any case,for all known hydrogen/oxygen fuel cells, regard-less of electrolyte type, it is the cathode which israte limiting (hydrogen electro-oxidation is


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extremely rapid on a wide range of catalysts) andthe activation overpotential is almost entirely dueto the cathode. At high temperatures, even cathode

activation overpotentials may be absent. Whenimpure hydrogen or hydrocarbon fuels are utilized,however, anode kinetics may become rate-limitingat any temperature.

The fourth term in Eq. (6), the ohmic overpoten-tial of a cell, is simply given by IR, where R is thearea specific resistance, and includes terms notonly from the electrolyte, but also the electrodes,current collectors and lead wires in the system. Thefinal term, hdiff, the mass diffusion term, has a formthat is specific to the geometry under consider-

ation, but it is generally established by the rate ofreactants flowing to the electrolyte through theelectrodes and the rate of products flowing away.Both anode and cathode can contribute to this term.As greater and greater current is drawn, the reac-tant depletion zone or the by-product build-upbecome greater and greater, and thus mass dif-fusion overpotentials become severe at high cur-rent densities. The impact of all the terms of Eq.(6) are highlighted in Fig. 2. The power density issimply multiple of the voltage and the current den-sity, and, as also shown in the figure, reaches some

peak value at intermediate voltages (or currentdensity). In contrast, efficiency, which decreaseswith increasing overpotential, is greatest at lowcurrent densities.

In the context of the polarization curve, it is evi-dent that high power densities (and highefficiencies) result when gas diffusion and electrontransport through the electrolytes are slow, electro-catalysis at the electrodes is rapid, the conductivityof each of the components, in particular, the elec-trolyte, is high, and mass diffusion through the

porous electrodes is facile. Thus, the ideal fuel cellelectrolyte is not only highly ionically conducting,but also impermeable to gases, electronicallyresistive and chemically stable under a wide rangeof conditions. Moreover, the electrolyte must exhi-bit sufficient mechanical and chemical integrity soas not to develop cracks or pores either duringmanufacture or in the course of long-term oper-ation.

The demands on fuel cell electrodes are perhapseven more extreme than those on the electrolyte.

The ideal electrode must transport gaseous (orliquid) species, ions, and electrons; and, at thepoints where all three meet, the so-called triple-

point boundaries, the electrocatalysts must rapidlycatalyze electro-oxidation (anode) or electro-reduction (cathode). Thus, the electrodes must beporous, electronically and ionically conducting,electrochemically active, and have high surfaceareas. It is rare for a single material to fulfill allof these functions, especially at low temperatures,and consequently a composite electrode, of whichthe electrocatalyst is one component, is often util-ized. For high-temperature solid oxide fuel cells,single component electrodes are, in principle, poss-

ible because mixed conducting (O

2

and e

) cer-amics are known, and it is precisely such materialswhich facilitate electrocatalysis. For low tempera-ture proton conducting polymer systems, mixedconducting systems are virtually unknown, andslow reaction kinetics require (precious metal)catalysts. Thus, composite systems are standard.Furthermore, the electrocatalyst is typically restric-ted to a very thin layer adjacent to the electrolyte,and another layer, the gas-diffusion-layer (GDL)serves the role of transporting electrons and gasesto and from the rest of the MEA. An image of a

single-cell ceramic fuel cell and a schematic dia-gram of a polymeric MEA are shown in Fig. 3.

Additional components in a complete fuel-cellpower generator are the so-called interconnects orbipolar plates. These components serve to link upindividual fuel cells in a fuel cell stack; intercon-nect is the term used in the SOFC communitywhereas bipolar plate is that used in the PEMcommunity. In addition to providing electrical con-duction pathways, the interconnects/bipolar platesserve to keep oxidant and fuel gases separate from

one another. Thus, the ideal interconnect has highelectronic conductivity (its resistance contributesto the overall IR losses in the stack), excellentimpermeability, and chemical stability under bothoxidizing and reducing conditions. Good mechan-ical properties are also required, particularly forlow temperature systems in which pressure isapplied to maintain gas seals. For reasons of spacelimitations, these components will not be discussedin any detail. Suffice it to note that tremendousefforts are being directed towards developing inex-


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Fig. 3. Membrane-electrode-assemblies: (a) scanning electronmicrograph of a typical SOFC structure obtained in the authors

laboratory; (b) schematic of a PEM structure in which gas dif-fusion layers, are additionally incorporated. Together, the elec-trocatalyst and gas diffusion layers are often referred to as thegas diffusion electrodes.

pensive stainless steel alternatives for both thepolymer and solid oxide fuel cells, and in bothcases, difficulties arise from the tendency of chro-mium to evaporate and/or diffuse from the metalinto the electrolyte.

Overall, the fuel cell environment places severedemands on the properties of each of the fuel cellcomponents. In addition to meeting those individ-ual demands, fuel cell materials must be chemi-cally compatible with one another, such that no

detrimental reactions occur over the many tens ofthousands of hours expected for fuel cell lifetimes.Moreover, the components in high temperature fuel

cells, must exhibit thermo-mechanical compati-bility; that is, the thermal expansion coefficientsmust match, and/or the materials must be toughenough to withstand mechanical stresses due to dif-ferences in thermal expansion.

3. Solid oxide fuel cells: State-of-the-art

An excellent review of ceramic fuel cells andthe materials from which they are constructed has

been presented by Minh [9], and we only brieflysummarize here the technology status for what onecan term conventional solid oxide fuel cells.Somewhat more recent, but less comprehensive,reviews have been published by Ormerod [10] andby Singhal [11]. Todays demonstration SOFCsutilize yttria stabilized zirconia (YSZ), containingtypically 8 mol% Y, as the electrolyte; a ceramic-metal composite (cermet) comprised of Ni + YSZas the anode; and La1xSrxMnO3-d, (lanthanumstrontium manganite or LSM) as the cathode. Spe-cific anode and cathode compositions are often

omitted from publications, but typically x is 0.15to 0.25 in LSM cathodes, whereas Ni-YSZ anodesare prepared from ~50:50 wt ratio NiO + YSZ mix-tures which are subsequently reduced in situ toyield Ni metal particles dispersed in a porous YSZmatrix. Anode and cathode porosities are typically2540 vol%. The interconnect material is alkalidoped LaCrO3 (lanthanum chromite), with the spe-cific dopant (typically, Sr, Ca or Mg) and concen-tration being selected to best match the thermalexpansion of the other fuel cell components in the

geometry of interest. In addition to these fourcomponents, planar SOFCs require sealantmaterials to isolate anode and cathode chambersin a stacked configuration. The literature on SOFCsealants is limited, but these have been typicallyfabricated from various glasses or glass-ceramics.Recent advances in YSZ-based fuel cells includea move from electrolyte or cathode supportedstructures to ones in which the anode serves to sup-port a thin electrolyte and thin cathode, and the useof composite, YSZ + LSM, cathodes (YSZ:LSM =


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50:50) rather than simply LSM. These designchanges, together with improved fabrication tech-niques, have resulted in an increase in peak power

densities for laboratory planar cells operated onhydrogen from ~250 mW/cm2 @ 1000 C in 1989[12] to almost 2 W/cm2 @ 800 C today[11,13,14].

Solid oxide fuel cells have shown tremendousreliability when operated continuously. Forexample, a 100 kW system fabricated by Siemens-Westinghouse has successfully produced power forover 20,000 hrs without any measurable degra-dation in performance [15]. In addition, such fuelcells offer good fuel flexibility, allowing a variety

of hydrocarbon fuels to be utilized. However,SOFCs are still much too costly for widespreadcommercialization, they function poorly underintermittent operation and the possibilities fordirect utilization of hydrocarbon fuels has hardlybeen explored. Several of the challenges hinderingSOFC technology are a consequence of the hightemperatures required for their operation. Hightemperatures preclude the use of metals, whichtypically have lower fabrication costs thanceramics, for any of the non-electrochemicalcomponents of the fuel cell and also increase the

likelihood of cracks developing upon thermal cyc-ling. The interconnect material, lanthanum chro-mite, is particularly difficult to process because ofchromia evaporation at high temperatures, whichleads to poor densification. Thermal stress-inducedfailure at glass seals is an acute problem for planarSOFCs. Tubular SOFCs, though free of seals, arevery costly on a per power output basis becausethe tubular design inherently produces low powerdensities as a consequence of the long currentpaths, and the manufacture of tubular ceramics has

only been achieved with costly techniques such aselectrochemical vapor deposition [11]. Further-more, while SOFCs offer good fuel flexibility,allowing a variety of hydrocarbon fuels to be util-ized, the less reactive fuels must typically beinternally steam-reformed, that is, reacted withH2O in the anode chamber, to produce CO and H2which can subsequently be utilized in the electro-chemical reactions [16]. Although not anticipatedto prevent SOFC commercialization, internal steamreforming requires recirculation of water, and its

elimination simplifies fuel cell operation andreduces costs.

In recognition of these challenges, there are

presently major research effort ongoing to reducethe temperature of SOFC operation to 500600 C(although even 800 C is often referred to asreduced temperature of operation), to developanode materials which can directly electro-oxidizehydrocarbon fuels at these temperatures withoutcarbon deposition, and to develop cathodematerials which are active for oxygen electroreduc-tion at these reduced temperatures. Furtherreductions in the temperature of operation are gen-erally deemed undesirable because of concomitant

losses in electrode activity.

4. Polymer electrolyte membrane fuel cells:

State-of-the-art

Polymer electrolyte membrane (PEM) fuel cellshave been reviewed by Costamagna and Srinivasan[17,18] and readers are referred to that work for amore comprehensive discussion than can be pro-vided here. The most widely implemented electro-lyte in PEM fuel cells is Nafion manufactured by

duPont. Nafion and related polymers are comprisedof perfluorinated back-bones, which providechemical stability, and of sulfonated side-groupswhich aggregate and facilitate hydration (see dis-cussion below). It is these hydrated, acidic regionswhich allow relatively facile transport of protons,but also restrict PEMFCs to low temperatures ofoperation. As a consequence, precious metals arerequired for electrocatalysis. For hydrogen/air fuelcells, Pt nano-particles supported on carbon are uti-lized for both the anode and cathode. Proprietary

Pt-based alloys may instead be used in the anodeof fuel cells using impure hydrogen, that is,hydrogen obtained by steam reforming of hydro-carbon fuels. The reformate from such processescan contain 10 s to 100 s of ppm CO and this hasdetrimental impact on fuel cell performance as aresult of the strong adsorption of CO onto Pt. Indirect methanol fuel cells PtRu alloys (typically50:50 molar ratio) are used at the anode becauseof the ability of Ru to electrooxidize CO adsorbedonto Pt. The platinum content (or Pt loading) in


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hydrogen/air PEM fuel cells has been reduced dra-matically over the past decade from ~4 mg/cm2

(per electrode) to less than 0.4 mg/cm2, as a result

of optimal dispersion of nanoparticle Pt in the elec-trocatalyst layer of the fuel cell. In contrast toSOFCs, the current collector, which also serves asa gas diffusion layer, is a distinct but integralcomponent of the PEM membrane-electrode-assembly. Typically, highly porous carbon papertreated with a hydrophobic polymer (e.g. PTFE) isused. In the fabrication of a stack, graphite bipolarplates are placed between individual MEAs.Recent advances in perfluorosulfonated polymerbased fuel cells include reduction of the electrolyte

thickness from ~175 to ~ 25 m, an increase inthe extent of sulfonation of the polymer to increaseconductivity, control of the porosity in the electro-catalyst and gas diffusion layers, and optimizationof the MEA processing to achieve excellentadhesion between the multiple layers. These devel-opment efforts have led to increases in the powerdensities of hydrogen/air PEM fuel cells from ~100mW/cm2 in 1984 to over 1 W/cm2 in 2002 [1].

The long-term durability of PEM fuel cells ismore of a concern than that of continuously oper-ated SOFCs, although Ballard has reported 2000 h

of operation without measurable loss in PEMFCpower output. More significant at this stage thanlong-term durability is the hydration requirementof the polymer in order to maintain conductivity.First, because, in fact, hydronium ions, (H2O)nH

+,rather than bare protons are transported across themembrane, excess water must be removed from thecathode and replenished at the anode. Second,upon hydration, sulfonated polymers swell signifi-cantly and their mechanical properties can degrade.Third, operation is generally limited to ~ 90 C,

at which temperatures the precious metal anodecatalysts are susceptible to CO poisoning, and elec-trocatalysis at both electrodes is generally sluggish.Higher temperatures of operation open up thepossibility of simplifying the CO removal process,lowering Pt loadings and even the use of non-pre-cious metal catalysts. And fourth, as a consequenceof its miscibility with water, methanol easily dif-fuses across the hydrated polymer electrolyte fromthe anode to the cathode (fuel cross-over) resultingin significant efficiency losses and low power den-

sities. Indeed, to minimize cross-over, relativelythick membranes (~175 m) and low methanolconcentrations (~4% in H2O) are used in direct

methanol fuel cells [19]. Despite the drawbacksassociated with perfluorinated sulfonated poly-mers, Nafion and its close relatives continue to bethe electrolyte of choice in demonstration PEMFCsbecause of their combination of very high conduc-tivity and adequate mechanical properties.

5. Electrolytes

The most important property of a candidate elec-

trolyte material is, of course, the ionic conduc-tivity. Conductivity data of a broad range ofmaterials are summarized in Fig. 4 [2025].Material classes for electrolyte applications rangefrom ceramics, to polymers to acid salts, and themobile ion can be O2, H+, or (H2O)nH

+. Solidsfor which OH or CO3 are mobile are alsoknown, but the conductivities are not high enoughto be of technological relevance. It should be notedthat independent of the magnitude of the conduc-tivity, fuel cell design inherently leads to a prefer-ence for a specific mobile species. In general, hyd-

ronium, hydroxide and carbonate ion conductorsare unattractive because one must, by definition,recycle an otherwise inert species: H2O in the caseof hydronium and hydroxide conductors or CO2 inthe case of carbonate conductors, to maintain iontransport.

Where hydrogen is the fuel, a proton conductoroffers the benefits of generating the by-productH2O at the cathode, and thus the fuel does notbecome diluted as a function of utilization.Consequently, so long as oxygen is plentiful, cell

voltages remain high. Hydronium ion conductorsgenerally provide this same benefit, however thewater recirculation requirements noted abovedemand delicate water management. In the casewhere hydrocarbons serve as the fuel, an oxygenion conductor offers, in principle, the prospect ofdirect electro-oxidation:

CH4 4O2CO2 2H2O 8e (13)

This possibility has only recently been explored,and is discussed further below. Generally, it is


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Fig. 4. Conductivities of selected electrolyte materials. (a)high temperature conductors [2022]; and (b) low temperatureconductors [2325]. PBI is polybenzymidazole where high andlow refer to the acid content; PWA is phosphotungstic acid and

the numeral following the acronym is the number of water mol-ecules of hydration.

instead presumed that the high temperature ofoperation associated with oxygen ion conductorscan be used to facilitate internal steam reforming:

CH4 H2OCO 3H2 (14)

with CO and H2 then used in the electro-oxidationreactions. Even in this more conservative scenario,oxygen-ion conducting electrolytes are preferredbecause CO can be electro-oxidized, rather than(particularly in the case of low temperaturesystems) poisoning the anode catalyst. Ceramicproton conductors may offer an interestingcombination of benefits because of their ability totransport both protons and oxygen ions. It has been

suggested [26] that water can diffuse across theelectrolyte membrane, inducing steam reformingand even conversion of CO to CO2 through thewater-gas shift reaction:

CO H2O CO2 H2. (15)

Hydrogen generated by reactions (14) and (15) isthen electrooxidized to form protons. In this case,hydrocarbons can be directly utilized and no wateris produced at the anode, again avoiding dilutioneffects as fuel utilization increases (although CO2dilution still occurs).

5.1. Oxygen ion conductors

The classic oxygen ion conductors, stabilized(cubic) zirconia and ceria are based on the fluoritestructure, Fig. 5. In order to introduce mobile oxy-gen vacancies into the compound, and, in the caseof zirconia, stabilize the cubic structure, tri- ordivalent dopants are added to the host material.The incorporation reaction can, for a typical tri-valent dopant, M, be written as:

M2O3ZrO2

2MZr 3Oo V

O (16)

with one oxygen vacancy created for every two Matoms incorporated. For both zirconia and ceria,conductivity increases with increasing dopant con-centration up to some maximum value and thendecreases sharply. Similarly, the conductivityincreases then decreases across the rare earth seriesfrom Yb to La. For zirconia, Sc gives rise to thehighest conductivity, but Y is typically utilized for


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Fig. 5. Crystal structure of conducting oxides: (a) fluoritestructure, exhibited by stabilized zirconia and by ceria; (b) per-ovskite structure, exhibited by oxygen ion conducting LaGaO3,and by proton conducting BaZrO3.

reasons of cost. With yttrium as the dopant, theconductivity of zirconia peaks at about 8 mole %dopant concentration. In the case of ceria, Sm [27]and Gd [28] give the highest values of conduc-tivity, and optimal dopant concentrations are1020%. The strong dependence of ionic conduc-

tivity on dopant type and concentration has beenexplained in terms of the lattice distortions intro-duced by the dopant, with those that produce the

least amount of strain causing the smallest vari-ation in the potential energy landscape [29]. Over-all, the ionic conductivity of ceria is approximatelyan order of magnitude greater than that of stabil-ized zirconia for comparable doping conditions.This is a result of the larger ionic radius of Ce4+

(0.87 A in 6-fold coordination) than Zr4+ (0.72 A),which produces a more open structure throughwhich O= ions can easily migrate.

Despite its favorable ion transport properties,ceria had not, until quite recently, been considered

a realistic candidate for fuel cell applicationsbecause of its high electronic conductivity. Inparticular, under reducing conditions, CeO2becomes CeO2x, and n-type conductivityincreases with a P(O2)

1/4 dependence. From ananalysis of relevant literature data, Steele [28] hasproposed that the electrolytic domain boundary, theoxygen partial pressure at which electronic andionic conductivities are equal, can be estimated asshown in Fig. 6(a), for 10 and 20% Gd doped ceria.In principle, one expects to be well within the elec-trolytic domain of ceria for fuel cells operated

below 700 C, although even at these temperaturessome voltage loss is expected. Reported open cir-cuit potentials for doped ceria, Fig. 6(b), are lowerthan what one would expect on the basis of theelectronic conductivity of ceria and representedsimply as the multiple of the ionic transferencenumber (greater than ~0.9 [30] at 700 C and 1018

atm oxygen partial pressure) and the Nernst poten-tial. The reasons for this discrepancy are notentirely obvious, but are likely due to electrode (inparticular cathode) overpotentials, and emphasize

the importance of developing electrodes compat-ible with ceria that enable theoretical open circuitpotentials to be reached. An additional challengelies with the chemical expansion of ceria underreducing conditions and the internal stress thatresult [31]. At this stage, the significance of thisissue on the long-term viability of ceria-based fuelcells is unknown. It is noteworthy that planar cellsexperience lower stresses than tubular cells, sug-gesting that clever designs may alleviate possiblestresses.


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Fig. 6. Electrochemical properties of doped ceria [43]: (a)

Electrolytic domain boundary at which ionic conductivityequals electronic conductivity, after [28]; (b) Open circuitpotential for a hydrogen (3% H2O) // oxygen cell. The Nernstpotential is that expected for an ideal electrolyte, the dottedline is approximately that expected for an electrolyte with 10%

electronic conductivity and the red and blue curves are experi-mental data.

Oxide-ion conducting perovskites have appearedin the literature for several years, but only recentlyhave compositions with conductivities high enough

for consideration in fuel cell applications appeared.The ABO3 perovskite structure, Fig. 5, isextremely amenable to tailoring via doping on boththe A and B cation sites. A large variety and con-centration of dopants can be accommodated in awide range of host compounds. Introduction ofdivalent dopant ions, typically Sr and Mg, onto theLa and Ga sites, respectively, of lanthanum gallateproduces a material with a high concentration ofmobile oxygen vacancies and thereby high oxygenion conductivity. The conductivity of the particular

composition La0.9Sr0.1Ga0.8Mg0.2O3d was reportedalmost simultaneously by Goodenough andcoworkers [32] and by Ishihara and coworkers[21]. The transport properties of LSGM, as it isknown, are comparable to those of scandia-dopedzirconia. The conductivity is entirely ionic over anextremely wide oxygen partial pressure range attemperatures as high as 1000 C, but is not as highas that of suitably doped ceria. Thus, the conditionsunder which LSGM might be preferable to dopedceria appear limited to the temperature range of7001000 C. Moreover, lanthanum gallate suffers

from reactivity with nickel, the typical SOFCanode electrocatalyst. To address this challenge,(non-reactive) ceria buffer layers have been incor-porated between the electrolyte and the anode [33].Nevertheless, intensive research efforts to developSOFCs incorporating lanthanum gallate continue,and recent work suggests that the ionic conduc-tivity can be increased by further adjustments tothe stoichiometry, in particular, via the addition ofsmall concentrations of Ni or Co [34]. A samplingof recent achievements in fuel cell research using

ceria and LSGM electrolytes is summarized inTable 2.The ion transport properties of bismuth oxide

have received significant academic attention as aresult of a rather spectacular phase transition at~700 C which leads to an increase in conductivityby almost three orders of magnitude. In the hightemperature d phase, the compound has a cubicfluorite structure, with an extremely high (25%)oxygen vacancy content. Below the transition, thevacancies are ordered, and hence the low conduc-


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Table 2Reduced temperature solid oxide fuel cells; materials and performance at 650 C on hydrogen/air input gases, unless otherwise indi-cated

Electrolyte Thickness, m Anode Cathode Peak power density Reference

LSGMC 205 Ni-SDC SSC 240410 [35]Ce0.9Gd0.1O1.95 ~40 Ni-Ru-GDC Sm0.5Sr0.5CoO3- 770

a [36]Ce0.9Gd0.1O1.95 510 Ni-YSZ La0.6Sr0.4Co0.2Fe0.8O3- 150 [37]Ce0.9Gd0.1O1.95 150 Ni-GDC La0.6Sr0.4Co0.2Fe0.8O3- 110 [38]YSZ ~30 Ni-YSZ LSM + SDC 190 [39]YSZ ~10 Ni-YSZ La0.8Sr0.2FeO3- 400 [40]

LSGMC = La0.8Sr0.2Ga0.8Mg0.15Co0.05O3-.a measured at 600 C, an even higher value can be anticipated for 650 C.

tivity. Efforts to stabilize the high temperaturephase at low temperatures have led to the develop-ment of (Bi2O3)x(Ln2O3)1x materials (Ln = lantha-num metal) which show much less pronouncedtransition behavior, but retain overall high conduc-tivity. Just as in the case of the Zr and Ce fluoritesand the perovskites, proper matching of the dopantionic radius to the host lattice is essential. Acrossthe lanthanide series, the conductivity of(Bi2O3)0.75(Ln2O3)0.25 peaks for Ln = Er, and isonly slightly lower for Ln = Y [41,42]. The keylimitations of bismuth based compounds are their

very high electronic conductivities, and tendenciesto become reduced to bismuth metal in hydrogenor fuel containing atmospheres. To date, no fuelcells have been fabricated using this material,although there have been claims that placing a thinlayer of bismuth oxide on the cathode side of aceria fuel cell can increase open circuit potentials[43].

5.2. Ceramic proton conductors

In analogy to the defect chemistry of lanthanumgallate, proton transport in barium zirconate, bar-ium cerate and related materials is achieved by firstdoping the material with a trivalent species (suchas yttrium) on the B site so as to introduce oxygenvacancies. The dopant incorporation reaction isnormally assumed to occur as per Eq. (17) (writtenin Kroeger-Vink notation).

2CexCe OxO Gd2O32GdCe V

O (17)

2CeO2.

Subsequent exposure of the material to humidatmospheres is presumed to lead to the incorpor-ation of protons as per Eq. (18).

H2O (gas) V

O OO2OH

O. (18)

The protons introduced by this manner are gener-ally not bound to any particular oxygen ion, butare instead free to migrate from one ion to the next.This easy migration results in the high proton con-ductivity (as high as 102 1cm1 at 500 C)observed in these oxides. Much like the oxide con-ductors, proton conductivity peaks at intermediate

dopant concentrations and with suitable matchingof the dopant ionic radius to the host structure. Fur-thermore, proton transport dominates the overallelectrical transport to temperatures of approxi-mately 600 C; the proton transference number ofBaCe0.95Sm0.05O3, for example, is ~0.85 at thistemperature [44]. At higher temperatures, bothoxide ion transport and electron transport becomesignificant.

The defect chemistry of doped A2+B4+O3 perov-skites is complicated by the possibility that the tri-

valent ion may reside on both cation sites, and notonly the B4+ site as desired [45]. The consequenceof partial incorporation of the dopant onto the A2+

site is that fewer oxygen vacancies than anticipatedwill result. The effect is exacerbated by high tem-perature processing which can induce BaO evapor-ation. A second complication arises from thehighly refractive nature of the zirconate protonconductors, e.g. doped BaZrO3. In comparison tothe cerates (BaCeO3 and SrCeO3), barium zircon-ate offers high conductivity and excellent chemical


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stability against reaction with CO2. However, fab-rication of dense electrolyte membranes from thismaterial remains a significant challenge. Indeed,

the high bulk conductivity of BaZrO3 had, for sev-eral years, remained obscured as a consequence ofthe materials refractory nature, which results infine-grained samples with high total grain bound-ary resistance [46,22]. In light of the reactivity ofcerates with CO2 and the difficultly of fabricatingdense zirconate electrolytes, it is perhaps no sur-prise that few complete MEAs of proton-conduc-ing electrolytes have been constructed and charac-terized. Early experiments were focused simply onproof of principle and utilized Pt for both elec-

trodes [47]. More recently, oxides have been incor-porated into both the anode and cathode. In parti-cular, La0.8Ba0.4CoO3- has served as the cathodeand at 1000 C, power densities as high as 200mW/cm2 have been reached [48].

5.3. Polymeric proton (hydronium) conductors

The perfluorinated sulfonated polymers, used inthe most mature PEM fuel cells, have a teflon-likeback-bone with sulfonated sidegroups, and amicrostructure in which hydrophobic and hydro-

philic regions phase separate on the nanoscale, Fig.7. The latter has been shown experimentally byrecent SAXS (small angle X-ray scattering)measurements [49], and computationally, usingmolecular dynamics simulations [50]. In general,proton conductivity increases as the hydration levelof the polymer increases, with H2O:SO3 ~ 15:1 andconductivity ~101 1cm1 under operationalconditions. Materials with low equivalent weight(small y) and short distances between hydrophilicside groups (small x), Fig. 7(a), typically exhibit

high conductivity. Furthermore, relative humidityhas a greater impact on conductivity than does tem-perature, although at temperatures above ~90 C,conductivity drops dramatically due to dehy-dration. As noted above, increased water content,though beneficial for the conductivity, increasesswelling (resulting in dimensional instabilities),and at the highest sulfonation levels the polymercan become water soluble. Other challenges asso-ciated with the use of hydrated polymer electro-lytes: methanol cross-over, difficult humidification

requirements and inoperability at high tempera-tures, have also been noted above. In addition, theperfluorinated nature of the back-bone in polymers

such as Nafion results in very high cost materials(~$700/m2) [17]. Accordingly, research effortshave targeted several areas: (1) reduction of costthrough the use of entirely hydrocarbon basedpolymers, (2) reduction of humidification require-ments to yield overall system simplifications, (3)increasing thermal stability so as to enable warmtemperature operation, particularly for automotiveapplications, and (4) decreasing methanol cross-over. Improving mechanical properties under highlevels of hydration has also received some atten-

tion.A review of all the many, many approachesdescribed in the literature for addressing thesechallenges is beyond the scope of this work.Rather, we briefly highlight a few selected stra-tegies. High chemical and thermal stability aretypically features of aromatic hydrocarbon poly-mers that incorporate benzene into their structures.Thus, several groups are pursuing sulfonation ofpoly(ether ether ketone), poly(styrene) and relatedmaterials, Fig. 8, to produce high proton conduc-tivity polymers free of fluorine [51]. Poly(styrene

sulfonic acid) is, in fact, the polymer utilized inthe 1960s in various NASA and other missions.The conductivities of sulfonated hydrocarbons tendto be lower that that of perfluorinated materials,but with appropriate modifications improvementsmay be attained. Sulfonation can be carried outdirectly on the polymer back-bone, as indicated inFig. 8, or short, sulfonate-terminated side-groupscan be introduced. The latter appears to provideincreased thermal stability. To improve the waterretention (which decreases sensitivity to humidity

and enables high temperature operation) and toreduce the swelling of Nafion, others have usedinorganic additives such as complex clays, silica,and phosphotungstic acid [52]. In a somewhatrelated approach, membranes comprised of porousteflon impregnated with Nafion have been pre-pared. These retain excellent mechanical propertiesto much thinner dimensions than Nafion alone, andhas led to very high power density fuel cells [43].Methanol cross-over can also be reduced utilizingthese approaches [53].


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Fig. 7. Structure of perfluorinated sulfonated polymers (i.e. Nafion and its close relatives). (a) chemical structure; (b) nanoscale

phase separated microstructure as determined by SAXS (small angle X-ray scattering), after [49]; (c) microstructure as predicted bymolecular dynamics simulations, after [50].

In order to avoid completely the many difficult-

ies associated with water-saturated polymers,others are pursuing acid-base polymer complexes,in which a strong acid is coupled to a highly basicpolymer. The concept is similar to that of lithiumconducting polymers, in which a lithium salt isblended with a basic polymer such aspoly(ethylene oxide) [PEO]. The mobile ions,either lithium or protons, are displaced from thehost salt or acid, via attraction to the oxygen ionsin the polymer. Protons hop between basic sites ofthe polymer and/or hydrogen bond sites between

anions, depending on the acid concentration, and

do not require the migration of a host species (asis the case for proton transport in hydratedpolymers). A wide range of basic polymers, includ-ing PEO, PVA [poly(vinylalchohol)], Paam[poly(acrylamide)] PVP [poly(vinylpyrrolidone)]PEI poly(ethyleneimine), various poly(amino-silicates), and PBI [poly(benzimidazole)] havebeen examined in combination with sulfuric, phos-phoric and various halide acids [54]. The protontransfer from the acid to the polymer can be suf-ficiently extreme so as to render the polymer host


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16/20


inorganic proton conductors known, Fig. 4, hydro-gen bonded solid acids with disordered phasesshow particularly high conductivities. In general,

such compounds are comprised of oxyanions, forexample SO4, SeO4, PO4, AsO4, or even PO3H etc.,which are linked together via OH...O hydrogenbonds. At room temperature, the structures aretypically ordered and the transport properties arerather conventional. Upon slight heating, however,many in the MHXO4 and M3H(XO4) families ofsolid acids, where M = Cs, NH4, and X = S or Se,transform into a disordered structure and exhibitconductivities as high as 102 1cm1. Protontransport is facilitated by the rapid reorientation of

XO4 groups in the disordered structure, Fig. 9[59,60]. These materials are true proton conduc-tors; no water molecules are required to serve ashosts for a vehicular transport mechanism, and theelectrolyte need not be hydrated. While the protontransport properties of this class of solid acids arevery attractive for fuel cell applications [61], sev-eral challenges must be addressed before theyreach technological relevance. The most importantof these is the tendency of sulfate and selenatebased materials to become reduced under hydrogenin the presence of typical anode catalysts such as

Pt [62]. The by-product of this reduction reaction,H2S (or H2Se), is an exceptional poison for theelectrocatalyst, and even if membrane degradationis only slight, the impact on fuel cell performanceis devastating.

Other inorganic proton conductors are knownand include the zirconyl phosphates,Zr(HPO4)2x(OH)xnH2O and HUO2PO4nH2O

Fig. 9. Proton transport mechanism in a disordered acid sulfate compound. HSO4 tetrahedra undergo rapid reorientations with the

proton attached to a particular oxygen atom, left; proton transfer from one tetrahedron to the next occurs on a much slower timescale, right.

[25]. These are water-insoluble, layered com-pounds containing intercalated hydronium ions andhave reasonable room temperature conductivity,

Fig. 7(b). However, the proton transport propertiesare highly dependent on the humidity level of theatmosphere and thus for fuel cell applicationswater management remains a challenge. The com-plex (solid) acids H3PMo12O40nH2O andH3PW12O40nH2O (also known asheteropolyacids) exhibit exceptionally high con-ductivities at room temperature, ~0.17 S/cm, when29 waters of hydration are present (n = 29) [63].Upon slight heating, the compounds dehydrate andthe conductivity drops precipitously. Moreover

these materials are water soluble. As such, use ofthese materials in fuel cells implies the impossiblerequirements of retaining hydration to ensure highconductivity and removing by-product water toprevent dissolution. Although these compounds areof little value as solid state electrolytes, they mayprovide benefits with respect to rapid oxygenreduction kinetics when implemented as aqueouselectrolytes. Indeed, a peak power density of ~700mW/cm2 has been reported for a room-tempera-ture, aqueous H3PW12O40nH2O electrolyte fuelcell operating on hydrogen/oxygen at one atmos-

phere [64].

6. Electrodes and electrocatalysts

6.1. Anode electrocatalysts

Electrocatalysis of hydrogen on metals such asPt and Ni is relatively facile, and anode overpoten-


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tials are a small contribution to the overall dropin fuel cell voltage. The rate limiting step is theadsorption of hydrogen onto the metal surface, as

opposed to the subsequent reaction of that hydro-gen to yield protons and electrons. In the acidicenvironment of a sulfonated polymer, Pt particlecoarsening, particularly at the cathode (wherehydration is greatest) has been observed, but hassurprisingly little influence on cell voltage [65].Challenges in PEM anode electrocatalysis arisewhen the hydrogen fuel contains residual CO, orwhen methanol is to be directly electro-oxidized.The reactions in a DMFC are:

Anode: CH3OH H2OCO2 6H+ 6e (19)

Cathode: 1.5O2 6H+ 6e3H2O (20)

Overall: CH3OH 1.5O2CO2 2H2O (21)

The most effective catalyst for methanol electro-oxidation is PtRu, in which the two componentsare typically segregated on the nanoscale. Meth-anol is adsorbed onto Pt clusters and then frag-mented into dehydrogenation products, essentiallyCO. Electro-oxidation of CO, which would other-wise remain strongly absorbed onto Pt, is sub-sequently catalyzed by oxygen-like or hydroxyl

species absorbed onto neighboring Ru sites[66,67]. Thus, anode electrocatalysts that are effec-tive for direct methanol fuel cells, tend also to beCO tolerant.

Despite significant research efforts to developadvanced Pt-alloy catalysts, none that are moreeffective than PtRu have been identified. An alter-native approach has been to introduce smallamounts of oxygen to the anode chamber, the so-called air-bleed approach, to directly oxidize COand remove it from the Pt surface. This technique

by definition causes some loss in efficiencies, anda catalyst approach would ultimately be preferable.In order for bifunctional catalysts to be effective,the two constituents must be arranged in a specificconfiguration on the catalyst surface. However,alloys, with their random distribution of atomicspecies, are ill-suited to provide precise chemicalarrangements. In contrast, intermetallic compoundshave highly regular and thermodynamically stablesurface arrangements, and may circumvent the fun-damental limitations of alloy-based electrocata-

lysts. Few studies on such systems have been car-ried out to date, but preliminary investigations ofPtBi compounds by cyclic voltammetry are prom-

ising [68], and may open a new avenue of electro-catalyst research.

In solid oxide fuel cell research, recent reports ofdirect electro-oxidation of hydrocarbon fuels haveappeared [69,70]. Direct electrochemical oxidationrefers to the reaction of hydrogen carbon fuelsdirectly with oxygen ions, without intermediaryreaction steps with water. As noted above, thetechnological consequence is that excess waterneed not be recirculated in the anode chamber ofSOFCs [16]. While the detailed reaction steps

remain to be elucidated, it is evident that incorpor-ation of ceria into the anode cermet is essential.Moreover, minimization of the nickel content (byreplacing it with a metal such as copper) is alsoimportant in order to limit carbon deposition orcoking. It is quite possible that instead of directelectro-oxidation, water generated in situ in theanode compartment induces steam reforming of thehydrocarbon fuel. Establishing whether or not thisoccurs will be essential for understanding and ulti-mately optimizing the process.

6.2. Cathode electrocatalysts

As in the case of CO tolerant anode catalysts,the search for effective cathode catalysts for PEMfuel cells has focused on Pt alloys. Success indeveloping alternatives has been only marginal.The PtCr system may offer some slight advan-tages over Pt alone, and this has been explained interms of both the electronic structure of the alloy(d-orbital vacancy per atom) and the crystal struc-ture (mean PtPt bond distance) [43]. This latter

requirement again calls for an approach usingintermetallic compounds rather than alloys. Aneven greater departure from the hegemony of Ptalloys is to consider oxide and other compounds,either as supports for Pt or as cathode materialsdirectly. Such materials are expected to remainstable under the oxidizing conditions of the cath-ode and may offer the benefits of mixedelectronic/proton conduction. Moreover, it is well-known in the (non-electrochemical) catalysis com-munity that the choice of catalyst support has a


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profound effect on catalyst activity, and, in parti-cular, mixed conducting oxides such as ceria cansignificantly enhance reaction rates. While few

oxide materials have been considered for PEM fuelcell applications, recent studies of Pt supported onhydrous, amorphous FePOx demonstrate the valueof this approach [71].

Oxygen electroreduction on the surface ofoxides such as LSM in solid oxide fuel cells relieson the mixed valence of the B-site cation in thetransition metal perovskite. Incorporation ofdivalent dopants into lanthanum manganite underoxidizing conditions results in the creation of Mn4+

species, which, in turn, give rise to high electronic

(p-type) conductivity via Mn3+

Mn4+

electrontransfer. Furthermore, if oxygen vacancies can beretained in the perovskite, oxygen ion conductivitycan be increased. In the case of LSM, oxygen ionconductivity is very low, and the material is prefer-ably utilized in a composite cathode, in which theelectrolyte material provides high ionic conduc-tivity. For reduced-temperature solid oxide fuelcells, the search for replacements to LSM has cent-ered around related transition metal perovskites, inwhich a variable valence element resides at the

octahedral site. It has been known for some timethat lanthanum cobaltites have higher electronicconductivities that lanthanum manganites, but theyhave not been suitable for YSZ-based fuel cellsbecause they become reduced at very high tem-peratures. At reduced temperatures and significantFe incorporation, however, they show excellentpromise. In particular, La1xSrxCo1yFeyO3d(LSCF, typically x ~ 0.2, y ~ 0.8) has emerged asa viable candidate. Even more recently,Sm0.5Sr0.5CoO3 has been reported to exhibit excel-lent cathode activity and examined by severalgroups. At this stage it is unclear exactly whichfeatures of the perovskite lead to low cathode over-potentials, but the tunability of the perovskitestructure certainly bodes well for further improve-ments. As these various materials are developed,constraints regarding chemical and thermomechan-ical compatibility with the electrolyte will requireattention. In addition, it is worth noting that lantha-nide and alkaline earth containing compounds tendto form carbonates (upon reaction with CO2 in the

air stream) and this can be extremely deleteriousto fuel cell performance.

6.3. Single chamber fuel cellsA cue frombiology

A very recent development in solid oxide fuelcells is the demonstration of single chamber fuelcells [72]. Such cells, Fig. 10, utilize mixedfuel/oxidant mixtures in a single gas inlet and relyon carefully selected anode and cathode catalyststo produce well-controlled, half-cell reactions:

Anode: CH4 1

2O2 CO (22)

2H2 (chemical)

H2 O= H2O 2e

(electrochemical) (23)

CO O= CO2 (24)

2e (electrochemical)

Cathode:1

2O2 2e

O= (electrochemical) (25)

Ideally, simple chemical oxidation of the hydro-carbon, which would yield CO2 and H2O, does not

take place. Instead, partial oxidation occurs at theanode and the products of this reaction are thenconsumed electrochemically, while oxygen is con-sumed electrochemically at the cathode. Because

Fig. 10. Schematic of a single chamber fuel cell. A hydro-carbon fuel is partially oxidized at the anode producing CO andH2 and consuming O2. The resulting oxygen partial pressuregradient drives the electrochemical reactions of the fuel cell.


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complications due to sealing are eliminated, theSCFC greatly simplifies system design andenhances thermal and mechanical shock resistance,

thereby allowing rapid start up and cool down.While it may seem optimistic, at first glance, toexpect that catalysts could be sufficiently selectiveso as to yield good power densities, such chemicalprecision is routinely utilized in nature. Indeed,biofuel cells operating on aqueous glucose, that isnot separated from ambient oxygen, demonstrateessentially perfect electrode selectivity [73] andmay ultimately enable extremely compact fuel celldesigns that are ideally suited to miniature, low-power applications.

7. Conclusions

After almost a century of slow and at timesalmost sputtering progress, fuel cell research hasexploded with activity over the past decade. Theresults have been tremendous, with power densitiesincreasing by factors of two and catalyst utilizationby more than an order of magnitude. Theseachievements have resulted from the developmentof new materials (e.g. La1xSrxGa1yMgyO3doxide ion conductors) as well as new processing

techniques (e.g. electrocatalyst-layer deposition forpolymer electrolyte fuel cells). Reduction of costand system complexity remain significant chal-lenges. Current efforts in SOFC research are aimedat (1) reducing operating temperatures to 500800C to permit the use of low-cost ferritic alloys forthe interconnect component of the fuel cell stackand (2) enabling the direct utilization of hydro-carbon fuels. Achieving these goals will require thedevelopment of highly active cathode materialsand of highly selective anode materials that do not

catalyze carbon deposition. Ceramic electrolytesthat are operable at reduced temperatures are avail-able (doped ceria and LSGM) but high perform-ance single-cell fuel cells based on these materialshave yet to be demonstrated. Current efforts inPEMFC research are focused on (1) reducingmembrane cost via the use of non-fluorinated poly-mer electrolytes and (2) reducing system com-plexity via the development ofwater-free electro-lytes that do not require cumbersome hydrationparaphernalia. Such electrolytes would additionally

enable operation under warm conditions (i.e.above 100 C) and be impermeable to methanol.Additional PEMFC research is very much directed

towards the development of high activity cathodeelectrocatalysts and CO tolerant anode electrocat-atlysts, which would furthermore be well-suited todirect methanol fuel cells. Dramatic reductions inthe Pt content in PEM fuel cells have been achi-eved over the past 20 years, however completeelimination of Pt remains a goal. Successes in thesearenas of cost and complexity reduction rely oncontinued advances in materials development andfabrication routes, and are essential for realizingthe market and environmental potential of fuel

cells.

Acknowledgements

The electron micrograph shown in Fig. 3(a) wasprovided by Dr. Zongping Shao, post-doctoralscholar in the authors laboratory and funded byDARPA, Microsystems Technology Office.Additional financial support was provided by theDepartment of Energy, Office of Energy Efficiencyand Renewable Energy.

References

[1] Hirschenhofer JH, Stauffer DB, Engelman RR, Klett MG.Fuel Cell Handbook, 4th edn. Parsons Corp., for U.S.Dept. of Energy Report No. DOE/FETC-99/1076; 1998.

[2] Larmine J, Andrews D. Fuel cell systems explained.Chichester: John Wiley & Sons Ltd.; 2000.

[3] Carrette L, Friedrich KA, Stimming U. Chem PhysChem 2000;1:162.

[4] Crow DR. Principles and applications of electrochemistry.

3rd ed. London: Chapman & Hall; 1988.[5] Bockris JOM, Srinivasan S. Fuel cells: Their electro-

chemistry. New York City: McGraw-Hill; 1969.[6] Wagman DD, Evans WH, Parker VB, Schumm RH,

Halow I, Bailey SM, Churney KL, Nutall RL. J Phys

Chem Ref Data 1982;11(suppl. 2).[7] Reiss I, Goedickemeier M, Gauckler LJ. Solid State Ion-

ics 1996;90:91.[8] Hirano S, Kim J, Srinivasan S. Electrochem Acta

1997;42:1587.[9] Minh NQ. J Am Cer Soc 1993;78:563.

[10] Ormerod RM. Chem Soc Rev 2003;32:17.[11] Singhal SC. Solid State Ionics 2002;405:152153.


20/20


[12] Singhal SC, editor. Solid oxide fuel cells. Proceedings of1st International Symposium. Pennington (NJ): The Elec-trochemical Society; 1989.

[13] de Souza S, Visco SJ, DeJohnge LC. J Electrochem Soc

1997;144:L35.[14] Virkar AV, Chen J, Tanner CW, Kim JW. Solid State Ion-

ics 2000;131:189.[15] George R, Casanova AC, Veyo S. Status of Siemens West-

inghouse SOFC Program. Extended Abstracts of the 2002Fuel Cell Seminar. Washington (DC): Courtesy Associ-ates, Inc., 2002.

[16] Steele BCH. Nature 1999;400:619.[17] Costamagna P, Srinivasan S. J Power Sources,

2001;102:242.[18] Costamagna P, Srinivasan S. J Power Sources,

2001;102:253.[19] Thomas SC, Ren X, Gottesfeld S, Zelenay P. Electrochem

Acta 2002;47:374.[20] Steele BCH, Mat Sci and Eng 1992;B13:79.

[21] Ishihara T, Matsuda H, Takita Y. J Am Chem Soc

1994;116:380[22] Bohn HG, Schober T. J Am Cer Soc 2000;83:768.[23] Kreuer KD, Chem Phys Chem 2002;3:771.[24] Slade RCT, Omana MJ. Solid State Ionics 1992;58:195.[25] Alberti G, Casciola M. Solid State Ionics 2001;145:3.[26] Coors G. J Power Sources 2003;118:150.[27] Eguchi K, Setoguchi T, Inoue T, Arai H. Solid State Ion-

ics 1992;52:165.

[28] Steele BCH. Solid State Ionics 2000;129:95.[29] Mogenson M, et al. 2003 (manuscript in preparation).[30] Milliken C, Guruswamy S. J Am Cer Soc 2002;85:2479.

[31] Atkinson A, Ramos TMGM. Solid State Ionics2000;129:259.

[32] Feng M, Goodenough JB. Eur J Solid State Inorg Chem1994;31:663.

[33] Huang HQ, Wan JH, Goodenough JB. J Electrochem

Soc 2001;148:A788.[34] Ishihara T, Shibayama T, Nishiguchi H, Takita Y. J Mat

Sci 2001;36:1125.[35] Kuroda K, Hashimoto I, Adachi K, Akikusa J, Tamou Y,

Komado N, Ishihara T, Takita Y. Solid State Ionics2000;132:199.

[36] Hibino T, Hashimoto A, Asano K, Yano M, Suzuki M,Sano M. Elect Solid State Letters 2002;5:A242.

[37] Sahibzada M, Steele BCH, Barth D, Rudkin AR, Metcalfe

IS. Fuel 1999;78:639.[38] Cheng JG, Fu QX, Liu XQ, Peng DK, Meng GY. Proceed-

ings of Key Engineering Materials 2002;2242:173177.[39] Yoon SP, Han J, Nam SW, Lim TH, Oh IH, Hong SA,

Yoo YS, Lim HC. Jo. Power Sources 2002;106:160.[40] Simner SP, Bonnett JF, Canfield NF, Meinhardt KD,

Sprenkle VL, Stevenson JW. Electr Solid State Letters2002;5:A173.

[41] Sammes NM, Tompsett GA, Nafe H, Aldinger F. J EurCer So 1999;19:1801.

[42] Shuk P, Weimhofer HD, Gush U, Gopel W, GreenblattM. Solid State Ionics 1996;89:179.

[43] Wachsman ED. Solid State Ionics 2002;152153:657.[44] Iwahara H, Yajima T, Hibino T, Ushida H. J Electrochem

Soc 1993;140:1687.[45] Haile SM, Staneff G, Ryu KH. J Mat Sci 2001;36:1149.

[46] Kreuer KD. Solid State Ionics 1999;125:285.[47] Taniguchi N, Yasumoto E, Gamo T. J Electrochem Soc

1996;143:1186.[48] Iwahara H, Yajima T, Hibino T, Ushida H. J Electrochem

Soc 1993;140:1687.[49] Kreuer KD, J Membr Sci 2001;185:2.[50] Jang SS, Molinero V, Cagin, T, Merinov BV, Goddard III

WA. Solid State Ionics (submitted for publication).[51] Rikukawa M, Sanui K. Prog Polym Sci 2000;25:1463

1502.[52] Statti P, Arico AS, Baglio V, Lufrano F, Passalacqua E,

Antonucci V. Solid State Ionics 2001;145:101.[53] Yamaguchi T, Miyata F, Nakaoa SI. J Membr Sci

2003;214:283.[54] Lassegues JC. In: Colomban PH, editor. Proton conduc-

tors: Solids, membranes and gelsmaterials and devices.

Cambridge: Cambridge University Press; 1992,p. 311328.

[55] Wang JT, Savinell RF, Wainright J, Litt M, Yu H. Electro-chem Acta 1996;41:193.

[56] Wang JT, Wainright JS, Savinell RS, Litt M. J Appl Elec-trochem 1996;26:751.

[57] Savadogo O, Xing B. J New Mat Electrochem Sys

2000;3:345.[58] Kreuer KD, Fuchs A, Ise M, Spaeth M, Maier J. Electro-

chem Acta 1998;43:1281.[59] Haile SM. Mat Today 2003:24.

[60] Munch W, Kreuer KD, Traub U, Maier J. Solid State Ion-ics 1995;77:10.

[61] Haile SM, Boysen DA, Chisholm CRI, Merle RB. Nat-ure 2001;410:910.

[62] Merle RB, Chisholm CRI, Boysen DA, Haile SM.Energy & Fuels 2003;1:21.

[63] Nakamura O, Kodama T, Ogino I, Miyake Y. Chem LettChem Soc of Jap 1979:17.

[64] Giordano N, Staiti P, Hocevar S, Arico AS. Electrochim-ica Acta 1996;41:397.

[65] Wilson MS, Garzon FH, Sickafus KE, Gottessfeld S. JElectrochem Soc 1993;140:2872.

[66] Gasteiger HA, Markivic N, Ross PN, Cairns EJ. J Phys

Chem 1993;97:12020.

[67] Long JW, Stroud RM, Swider-Lyons KE, Rolison DR. JPhys Chem B 2000;104:9772.

[68] Casado-Rivera D, Gal Z, DAngelo AC, Lind C, DiSalvoFJ, Abruna HD. Chem Phys Chem (accepted).

[69] Murray EP, Tsai T, Barnett SA. Nature 1999;400:649.[70] Park S, Vohs JM, Gorte RJ. Nature 2000;404:265.[71] Bouwman PJ, Dmowski W, Swider-Lyons K. Ext

Abstracts, 204th Meeting of The Electrochemical Society.[72] Hibino T, Hashimoto A, Yano M, Suzuki M, Yoshia S,

Sano M. J Electrochem Soc 2002;149:A133.[73] Mano N, Mao F, Heller A. J Am Chem Soc

2002;124:12962.

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