319 gurupreet

Department of Chemical Engineering Indian Institute of Technology-Delhi, New Delhi 110 016, India

Performance Studies of Copper-Iron/Ceria-Yttria Stabilized Zirconia

Anode for Electro-oxidation of Hydrogen and Methane Fuels in

Solid Oxide Fuel CellsPresented by Gurpreet Kaur

Department of Chemical EngineeringIndian Institute of Technology Delhi

International Conference on Advances in Energy ResearchDecember 10-11, 2013


Solid Oxide Fuel CellSolid oxide fuel cell is a device that converts gaseous fuels (hydrogen, natural gas) via an electro-chemical process directly into electricity. Salient Features of SOFC

SOFCs are over 60 % efficient (conversion of fuel to

electricity)

Provides environment friendly power generation

Principle of SOFC

22 2

24 2 2

24 10 2 2

2 2 2 4

4 2 8

13 4 5 26

H O H O e

CH O CO H O e

C H O CO H O e

22

22

22

4 2

8 4

26 13

O e O

O e O

O e O

Anode Side Reactions

Cathode Side Reactions

Operating Temperature: 700-1000 °C

Conventional SOFC ComponentsElectrolyte – 8 % Yttria Stabilized

Zirconia (YSZ) – a pure ionic conductorAnode – Ni provides electronic conductivity and enables electrochemical oxidation of fuel.Cathode - La0.8Sr0.2MnO3 (LSM) provides electronic conductivity and enables electrochemical reduction of O2.

1

ApplicationsStationary electrical power generation


Why Direct Hydrocarbons ?

Anode requirements for oxidation of hydrocarbons

High electro catalytic activity for oxidation of fuel Good electronic conductivity for transport of electrons from

the TPB Good ionic conductivity for transport of oxide ions to the

TPB Sufficient porosity for diffusion of fuel gases and exhaust

gases to and from the TPB

Production of hydrogen by steam reforming reactions of natural gas and higher hydrocarbons requires additional purification steps to satisfy fuel cell demands

Direct hydrocarbon solid oxide fuel cell can operate in hydrocarbon fuels without the need for pre-reforming.

2


Anodes for Direct Hydrocarbon Solid Oxide Fuel Cell Nickel/Yttria-stabilized Zirconia based Anodes1

High catalytic activity for fuel oxidation and for steam reforming of methane

Relatively inexpensive; chemically and physically compatible with YSZ electrolyte

Problems in use with dry hydrocarbons; Tends to promote carbon deposition1

1. M .L.Toebes, J.H. Bitter, A.J. Van Dillen and K.P.de Jong, Catal. Today 2000; 76, 33 – 42 .

2. R. J. Gorte, S. Park, J. M. Vohs, C. Wang, Adv Mater. 2000; 12: 1465 -69

Copper/Ceria/Yttria Stabilized Zirconia2- Alternative anode material for direct hydrocarbons

CeO2 : Mixed ionic and electronic conductor in reducing medium.

Good oxidation catalyst for hydrocarbons Poorer electronic conductor Cu: To increase the electronic conductivity, addition of Cu is necessary Cu/CeO2-YSZ anodes are stable in variety of hydrocarbons

Limited by lower performance (~ 100 mW/cm2 at 800 °C).

Literature Review

3


Objective of Research Work Fabrication of complete solid oxide fuel cell in laboratory scale using tape casting

technique of thickness of < 600 µm and anode porosity of 70 %. Additives composition is optimized to get defect free SOFC

Preparation of Cu/CeO2-YSZ and Cu-Fe/CeO2-YSZ anodes using wet impregnation method.

Characterization of prepared anodes using thermal gravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM), elemental dispersive X-ray (EDX) to investigate the thermal, structural, morphological properties and elemental analysis

Current-Voltage characterization of prepared anodes with YSZ electrolyte and LSM-YSZ cathodes in H2 and methane fuels.

Frequency response analysis of SOFC with prepared anodes to study various resistances e.g. ohmic resistance, polarization resistance.

Study the effect of temperature, bimetallic molar ratio and addition of precious metals on the performance of SOFC in H2 and methane fuels.

Investigation of carbon deposition using optical microscopy and thermal gravimetric analysis.

Longevity testing in methane fuel.

4

Gurpreet Kaur and Suddhasatwa Basu, Journal of Power Sources, 241, 783-790, 2013


Magnetic Stirring, 24 h

Binder (Polyethylene Glycol and Polyvinyl

butryl)

Electrolyte Tape Casting and Drying for 24 h

Solvent (Ethanol and MEK)+

Dispersant (oleic acid)

Stirring

Homogeneous SlurryStirring

Magnetic Stirring, 24 h

Homogeneous Slurry

Yttria-Stabilized ZirconiaPore-formers i.e. Graphite and Polystyrene) for anode only

Porous YSZ Anode Tape Casting and Drying for 24 h

Co-sintering, 1450 °C

Tape casted electrolyte layer

Porous YSZ layer on dense YSZ

electrolyte

SEM of porous YSZ

Porosity 70 vol %

SEM of porous and dense YSZ sintered at

1450 ºC

Preparation Procedure for Anode and Electrolyte Slurry for Tape Casting

SOFC Fabrication

Component Quantity

YSZ 24 gmGraphite 5 gmPolystyrene 3.8 gmEthanol (EtOH)

16 mlMethylethyl ketone (MEK) 9 mlOleic acid

1.0 mlPolyvinyl butyral (PVB) 3.8 gmPolyethylene glycol (PEG)

3 ml

Composition of anode and electrolyte for tape casting slurry

No poreformers (graphite and polystrene) added in electrolyte slurryFabrication issues

Green tape – Pin holesSintered layers – cracking, delamination etc 5


Preparation Procedure of Anode for SOFC

TGA of impregnated nitrate solution in porous YSZ

Data was collected from room temperature to 1000 BC at a rate of

10 BC/min. Zero air flow rate: 50 ml/min Calcination temperature of 400 ºC is selected to get metal oxides

(Wet Impregnation Method)Porous YSZ

Repeated impregnation to get desired loading

Calcinations at 400 ºC for 2 h

Impregnation of 1M Ce(NO3)3, 6H2O

Calcinations at 400 ºC for 2 h

Anode Cu/CeO2-YSZ and Cu-Fe/CeO2-YSZ

Impregnation of 1M Cu(NO3), 3H2O and Fe(NO3)3, 9H2O solution

6

Cu-Fe [1:1]


Synthesis Procedure of Cathode (La0.8Sr0.2MnO3)

Dissolve La(NO3)3, Sr(NO3)2, Mn(NO3)2 in stoichiometric ratio

Calcination1100 oC for 2 h

Mixing (Agate mortar)La0.8Sr0.2MnO3– 0.45 g

YSZ – 0.45 gGraphite– 0.1 g

Slurry preparationMixed powders with

glycerol

0

500

1000

1500

2000

2500

20 30 40 50 60 70 80

Inte

nsi

ty (

cps)

2θ( � )

XRD spectra of La0.8Sr0.2MnO3

All peaks corresponds to perovskite phase

Particles size of LSM ~0.3 µm

SEM of La0.8Sr0.2MnO3

La0.8Sr0.2MnO3 showed good chemical and thermal compatibility with YSZ electrolyte material

* R.J. Bell, G.J. Millar, J. Drennan, Solid State Ionics 2000; 131: 211–220.

7


Experimental set up and Procedure

CathodeLSM:YS

Z

Electrolyte

YSZ

High Temperature SOFC Furnace

PGSTAT 30, Autolab (i-V and impedance measurements)

8


X-ray Diffraction of Cu-Fe/CeO2-YSZ Anodes

Cu-Fe/CeO2-YSZ anodes were prepared for three molar ratios of Cu-Fe [1:0, 3:1 and 1:1].

Peaks at 43.3º and 44.2° corresponds to Cu and Fe and in metals are present cubic structure.

Small shift in the peaks for Cu and Fe was observed in the spectra, according to phase diagram1, some Fe can be incorporated in Cu phase at 800 °C.

1. Turchanin MA, Agraval PG, Nikolaenko IV, J Phase Equilibria 2003;24:307-19.

9

++*,o

¤

*

•

••

•

•

(a)

(b)

(c)

¤

ɵ

ɵ

0

2000

4000

6000

8000

25 35 45 55 65 75

Inte

nsit

y (a

.u)

2 Theta (Degree)

Ɣ

(a)

(b)

(c)

(d)

Ɣ Ɣ Ɣ

ƔƔ

** *

Ÿ

Ƈ

Ƈ

ο ¤¤ ¤ ¤

ο

(•) YSZ(*) Fe2O3

(o) CuFe2O4

(+) CuO(¤) Cu(ɵ) Fe

XRD patterns of (a) YSZ, (b) Cu-Fe/YSZ calcined at 300 °C (c) Cu-Fe/YSZ reduced at 800 °C

XRD patterns of (a) YSZ, (b) Fe/CeO2-YSZ, (c) Cu/CeO2-YSZ and (d) Cu-Fe/CeO2-YSZ after reduction in H2 at 800 °C

(●) YSZ (▲) Cu (♦) Fe (*) CeO2 (ο) Fe3O4 (¤) Fe2O3


Addition of Fe in Cu based anodes improves the

catalyst dispersion Better interconnection between

particles helps to improve the electrical conduction and provides more surface area for fuel oxidation reaction Particle size was observed to be 1 µm

20 wt% Cu-Fe [3:1]

Scanning electron microscopy of Cu-Fe/CeO2-YSZ anodes after reduction in H2 at 800ºC

20 wt% Cu-Fe [1:1]

20 wt% Cu-Fe [3:1]

10 wt% CeO2, 20 wt% Cu-Fe [1:0, 3:1 and 1:1]

10


Elemental dispersive analysis of Cu/CeO2-YSZ and Cu-Fe/CeO2-YSZ anodes

Presence of metals inside the pores with no significant impurity observed.

Results indicate the success of fabrication of anodes by wet impregnation method.

11


SEM of SOFC shows anode, electrolyte and cathode thickness of 90, 80 and 40 µm.

Power density of ~ 190, 260 and 330 mW/cm-2 was observed for Cu-Fe/CeO2-YSZ anodes for Cu-Fe molar ratio of 1:0, 3:1 and 1:1.

Performance increased with increase in Fe loading in Cu/CeO2-YSZ anodes

Performance of SOFC in H2 at 800°C (Cu-Fe/CeO2/YSZ anodes for Cu-Fe molar ratio of 1:0, 3:1 and 1:1, YSZ as electrolyte,

LSM/YSZ as cathode

~90 µm

80 µm

40 µm

SEM of SOFC Performance Curves

i-V (filled symbols) and power curves (open symbols) for different molar ratio of Cu-Fe

0

50

100

150

200

250

300

350

400

0

0.2

0.4

0.6

0.8

1

1.2

0 200 400 600 800

Pow

er D

ensi

ty (m

W/c

m2 )

Vol

tage

(V)

Current Density (mA/cm2)

12


0

0.1

0.2

0.3

0 0.3 0.6 0.9 1.2 1.5 1.8

-Zim

(ohm

cm

2 )

Zre (ohm cm2)

0

0.1

0.2

0.3

7 7.5 8 8.5 9 9.5 10

-Zim

(ohm

cm

2)

Zre (ohm cm2)

Calculated electrolyte resistance for 80 µm thick electrolyte is ~0.38 Ω. cm2. Less additional ohmic resistance was observed for Cu-Fe [1:1] due to better dispersion

between catalyst particles resulting better electronic conduction.Total polarization resistance decreases with increase in Fe molar ratio suggest that

prepared anodes have better electro-catalytic activity towards oxidation of H2.

Improvement in the performance of cell might also be due to incorporation of Cu and Fe ions in CeO2 lattice

Lattice parameter CeO2 calculated from XRD: 5.36Å Pure CeO2 lattice parameter- 5.41 Å

EIS of SOFC for different molar ratio of Cu-Fe of 1:0(∆), 3:1 (◊)

and 1:1 (□)

XRD of Cu-Fe/CeO2-YSZ anode after reduction in H2

13

RΩ

Rp


Optical microscopy images of Cu-Fe/CeO2-YSZ anodes for Cu-Fe molar ratio of (a) 1:0, (b) 3:1 (c) 1:1 after exposure to CH4 for 1h

(a) (b) (c)

Metal wt% in porous

CeO2/YSZ

Weight

change (%)

Cu: Fe [1:0]- (20wt%)

-0.030

Cu: Fe [3:1]- (20wt%)

-0.027

Cu: Fe [1:1]- (20wt%)

-0.021

Table - Weight changes after CH4 flow

TGA of Cu-Fe/CeO2-YSZ anode after reduction in (a) H2 and (b) H2 followed by exposure of CH4 for 1 h

No significant weight gain was observed due to carbon deposition after CH4 flow

Characterization of Cu-Fe/CeO2-YSZ anodes after exposure to CH4

at 800°C

14


0

50

100

150

0

0.2

0.4

0.6

0.8

1

1.2

0 50 100 150 200 250 300 350

Pow

er D

ensi

ty (m

W/c

m2 )

Vol

tage

(V)


Cu-Fe [1:1] Cu-Fe [3:1] Cu-Fe [1:0]

Performance of Cu-Fe/CeO2-YSZ anodes in CH4 at 800 ºC

Cu-Fe/CeO2/YSZ anodes for Cu-Fe molar ratio of 1:1 showed higher performance than 1:0 and 3:1.

Performance of all the anodes are lower in CH4 than H2 might be due to less reactive nature of CH4

in comparison to H2. OCV was observed to be less than Nernst potential (> 1.05 V ) suggesting that

complete oxidation of CH4 is not taking place. Oxidation of hydrocarbon on surface may occur in multiple steps and

equilibrium has been established between hydrocarbons and partial oxidation products.

15


Effect of addition of 1 wt% Pd on the performance of Cu-Fe/CeO2-YSZ anodes in H2 and CH4

Significant improvement in the cell performance in CH4 was observed with addition of 1 wt% of Pd

Results suggest that resistance associated with surface reactions decreases with addition of 1 wt% Pd.Anode 160 µm, Electrolyte 100 µm

Performance curves of Cu-Fe/CeO2-YSZ anodes with (□) and without (○) 1 wt% Pd

16

0

50

100

150

200

250

0

0.2

0.4

0.6

0.8

1

1.2

0 100 200 300 400 500 600

Pow

er D

ensi

ty (m

W/c

m2 )

Vol

tage

(V)


Cu-Fe [1:1]

H2

0

20

40

60

80

100

120

140

0

0.2

0.4

0.6

0.8

1

1.2

0 100 200 300 400

Pow

er D

ensi

ty (m

W/c

m2 )

Vol

tage

(V)


Cu-Fe [1:1]

CH4


0

0.1

0.2

0.3

0.4

0 0.5 1 1.5 2

-Zim

(ohm

cm

2 )

Zre (ohm cm2)

22 h

30 h

46 h0

0.05

0.1

0.15

0.2

0.3 0.4 0.5 0.6

-Zim

(ohm

cm

2 )

Zre (ohm cm2)

1 h

Long term performance of Cu-Fe/CeO2/YSZ anodes in CH4

Power density decreased from 125 mW/cm2 to 100 mW/cm2 during 46 h testing

Increase in ohmic resistance may be due to increase in particle size of catalyst particles at

800 °C during stability test. (repeated thrice). Increase in ohmic resistance and polarization resistance might be

responsible for this loss. Cu-Fe/CeO2/YSZ anode showed much better stability than Ni/YSZ

anodes in which complete performance degradation takes place within 5 h.

0

50

100

150

200

0 10 20 30 40 50

Pow

er D

ensi

ty (m

W/c

m2 )

Time (h)

Cu-Fe [1:1]

CH4, 800 °C

0.5V

17


SEM and TGA of Cu-Fe/CeO2-YSZ anodes after cell testing in

CH4 for 46 h

SEM shows catalyst particle size increased from 1.0 to 1.5 µm after cell operation at 800 °C for 46 h

TGA shows no significant weight loss suggesting that carbon ,if present, is not in significant quantity.

18


Summary

SOFC Fabrication and Electrochemical Characterization Solid oxide fuel cells was fabricated by tape casting and wet impregnation

method. Additives (pore-formers, binder and solvent) composition was optimized to

get defect free button cells.SOFC testing (i-V and EIS) was carried out for Cu/CeO2-YSZ and Cu-Fe/CeO2-

YSZ anodes with YSZ as electrolyte and LSM/YSZ as cathode.

Performance of Cu-Fe/CeO2-YSZ Anodes in H2 and Methane XRD shows the formation of Cu and Fe phase. Addition of Cu to Fe enhances

the reduction of Fe. SEM shows that better dispersion between catalyst particles achieved with

addition of Fe in Cu/CeO2-YSZ anodes Addition of Fe in Cu/CeO2-YSZ anodes showed improved performance in H2

and CH4 fuels. Electrochemical impedance spectra showed less ohmic as well as charge

transfer resistance for Cu-Fe/CeO2-YSZ anodes in comparison to Cu/CeO2-YSZ anodes.

SOFC performance increased with addition of 1 wt % Pd in Cu-Fe/CeO2-YSZ anodes.

No significant degradation in the performance observed during cell operation in CH4 suggesting that anodes are stable in comparison to conventional Ni/YSZ anodes.

19


References

20

[1] Lashtabeg, A. and Skinner, S. J. (2006) Solid oxide fuel cells-a challenge for materials chemists, Journal of materials chemistry, 16, pp. 3161-70.[2] Baker, R. T. K. (1989) Catalytic growth of carbon filaments, Carbon, 27, pp.1315-23.[3] Gorte, R. J., Vohs, J. M. (2003) Novel SOFC anodes for direct electrochemical oxidation of hydrocarbons, Journal of catalysis, 216, pp. 477-86.[4] Gorte, R. J., Park, S., Vohs, J. M. and Wang, C. (2000) Anodes for direct oxidation of dry hydrocarbons in solid oxide fuel cells, Advanced Material, 12, pp.1465-69.[5] Zhu, H., Wang, W., Ran, R., Su, C., Shi, H. and Shao, Z. (2012) Iron incorporated Ni-ZrO2 catalysts for electric power generation from methane, International Journal of Hydrogen Energy, 37, pp. 9801-9808.[6] Gordes, P., Christiansen, N., Jensen, E. J. and Villadsen, J. (1995) Synthesis of perovskite-type compounds by drip Pyrolysis, Journal of Material Science, 30, pp.1053-58.[7] Mitterdorfer, A. and Gauckler, L.G. (1998) La2Zr2O7 formation and oxygen reduction kinetics of La0.85Sr0.15MnYO3,O2(g) YSZ system, Solid State Ionics, 111, pp. 185-218.[8] Turchanin, M. A., Agraval, P. G. and Nikolaenko I. V. (2003) Thermodynamics of alloys and phase equilibria in the copper iron system, Journal of Phase Equilibria, 24, pp. 307-309.[9] Kameoka, S., Tanabe, T. and Tsai, A. P. (2005) Spinel CuFe2O4: a precursor for copper catalyst with high thermal stability and activity, Catalysis Letters, 100, pp. 89-93. [10] Lv, H., Tu, H., Zhao, B., Wu, Y. and Hu, K. (2007) Synthesis and electrochemical behavior of Ce1-xFex02-δ as a possible SOFC anode materials, Solid State Ionics, 177, pp. 3467-3472.[11] Xing, Z., Hua, W., Honggang, W., Kongzhai, L. and Xianming C. (2010) Hydrogen and syngas production from two-step steam reforming of methane over CeO2-Fe2O3 oxygen carrier, Journal of Rare Earth, 28, pp. 907-913.[12] Buccheri, M. A., Singh, A. and Hill, J. M. (2011) Anode- versus electrolyte-supported Ni-YSZ/YSZ/Pt SOFCs: Effect of cell design on OCV, performance and carbon formation for the direct utilization of dry methane, Journal of Power Sources, 196, pp. 968-976


Thank You



Nernst Potential calculation for multiple reactions

4 2 2 2

4 2 2

2 2

2

2 2

4 2 2

2 2

3 / 2 2

1/ 2

1/ 2

2

CH O CO H O

CH O CO H O

C O CO

C O CO

CO O CO

CH O C H O

Experimental

/E G nF

An observed OCV is less than Nernst potential suggesting that complete oxidation of methane is not taking place.

Multiple anode reactions may occur simultaneously with dominating contribution from one reaction.

0.85

0.9

0.95

1

1.05

1.1

1.15

850 900 950 1000 1050 1100

OC

V (V

)

Temperature (� K)

319 gurupreet

Technology

fuel oxidation

new delhi

solid oxide fuel cells

hporous ysz anode tape

ysz electrolyte problems

anode porosity

diffusion of fuel gases

yttria stabilized zirconia