Fuel Cell Anode

8/3/2019 Fuel Cell Anode

1/45

Fabrication of SOFC Electrodes byImpregnation Methods

R. J. GorteChemical & Biomolecular EngineeringUniversity of Pennsylvania

Collaborators: J. M. Vohs, M. D. Gross, S.W. Jung,and W. Wang

Support: ONR, DOE-BES (H 2 program)


2/45

Solid Oxide Fuel Cell

Solid Oxide Fuel Cell

O 2 + 4e - 2O 2-

H 2 + O 2- H 2O + 2e -

e-e-

e-e-Anode

Electrolyte

Cathode

Air

H2O

Fuel

O 2- O 2-

Porous Ni/YSZ cermet

Porous YSZ/Sr-LaMnO 3

YSZ (Y-stabilized ZrO 2)

YSZ is a conductor for O2-

but an insulator for electrons.

Operating temperature:600 to 800C


3/45

-0.2

0

0.2

0.4

0.6

0.8

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6Z', (ohm*cm 2)

Z " , ( o h

m * c m

2 )

1 kHz

4 Hz

0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8 1Current Density (A/cm 2)

V o

l t a g e

( V )

0

50

100

150

200

250

300

350

P o w e r

D e n s i

t y ( m W / c m

2 )Performance Analysis:

Open Circuit:V = Nernst Potential

= G/nF

Power output:P = V* i

Electrolyte Losses:V = i*R electrolyte Electrode Losses:

V = overpotential= i*R electrode

R electrode is composed of diffusion, surface kinetics, etc.

Electrolyte

Electrodes

R electrolyte

(R electrolyte +R electrode )


4/45

Research Goals at Penn:1) Anodes for direct utilization of hydrocarbons

Reforming leads to system complexity and to energy lossesat high temperatures:

2) High-performance cathodesLSM-based cathodes used today limit SOFC performance.

Consider: C 4H 10 + 2O 2 = 4CO + 5H 2 At 700C H (kJ/mol) G #electrons

C 4H 10 + 6 O 2 = 4CO 2 + 5H 2O -2,660 -2,810 264CO + 5H 2 + 4 O 2 = 4CO 2 + 5H 2O -2,370 -1,760 18

-11% -37% -31%


5/45

Ni-based anodes cannot be exposed to hydrocarbons:

Ni catalyzes carbon formation

This can be avoided with other metals

Ni in toluenefor 3 hrs, 700C

Ni in methanefor 3 hrs, 800C

Cu in methane

for 3 hrs, 800C

Cu in toluene

for 3 hrs, 700C


6/45

Carbon formation is in , not just on, Ni:

(M. L. Toebes, et al., Catalysis Today, 2002)

20% CO/7% H 2550 C.

Ni Particles

Note: Fe, Co, and Ru will all form catalyze carbon formation


7/45

Traditional electrode fabrication limits materials that

can be used because of high-temperature processing

YSZ Powder

Pressed NiO/YSZ powder

NiO/YSZ compositeReduce in H 2

800C

Porous Ni/YSZ cermet

in air Dense YSZDense YSZYSZ Powder

Cathodes: (Sr-doped LaMnO 3/YSZ)

Dense YSZ Dense YSZ

Screen print LSM-YSZ and fire to 1200C

1400C


8/45

High temperatures restrict choices of materials:

1) Cannot use materials that will melt at sintering temp.(Ex: CuO melts at 1235C)

2) High-temperatures can cause solid-state reactions(Ex: LaCoO 3 reacts rapidly with YSZ above 1000C.)

3) Must be able to form the correct phase(Ex: Alloys and layered structures are not possible.)


9/45

YSZ+pore formers

Electrolyte

Cathode

Anode

YSZ+pore formers

YSZ70 C, 8 MPa

Firing1400 1550 C

Lamination

Porous YSZ

Impregnation &

Calcination at 450 ~ 700 C

Electrode Fabrication by Impregnation:

Both Anodes & Cathodes formed by impregnation.


10/45

Structure near electrolyte interface:

Electrode I

electrolyte

Electrode II


11/45

Add active phase separately:

Ex: La 0.8Sr 0.2FeO 3-YSZ

40 wt% LSF-YSZ by impregnationPorous YSZ


12/45

Advantages:

1. Separate firing temperatures for YSZ and active phase.- Avoids solid-state reactions between LSF & YSZ.- Can use low-melting solids (CuO).

2. Composite is non-random structure.

Mixed powdersImpregnated

LSM

Percolation thresholdfor random media

12.612.611.710.3CTE (10 -6/K), 300 to 1073 K

55%45%35%0%LSCo Weight Fraction in YSZb) CTE of LSCo-YSZ

a) Electrical conductivity of LSM-YSZ

CTE of LSCo is 23x10 -6/K

700 C in air, compositescalcined at 1523 K.


13/45

YSZO2-

CeO 2

Cu Cu

C2H6

e-

CO 2H2O

Our past focus: Cu-ceria anodes

a) Cu for electronic conductivityb) Ceria for catalysis


14/45

Cu-ceria composites perform in real fuels!

15%CeO2 25%Cu, 24um electrolyte, laminated cell, LSF, Heavy Naph 700 oC

0

0.2

0.4

0.60.8

1

1.2

0 5 10 15 20 25 30 35 40 45

Time, hour

V o l

t a g e , V

0

0.2

0.4

0.60.8

1

1.2

P o w e r

d e n s

i t y ,

W / c m

2Voltage

Power density

Operation on Heavy Naphtha, 700C; fuel was undiluted.


15/45

0

100

200

300

400

500

0 200 400 600 800 1000 1200 1400

700 C, 30% conversion at peak power725 C, 40% conversion at peak power750 C, 44% conversion at peak power775C, 47% conversion at peak power800C, 50% conversion at peak power

P o w e r

D e n s i

t y ( m W / c m

2 )

Current Density (mA/cm2

)

700 C

725 C

750 C

775 C

800 C

0

200

400

600

800

1000

1200

0 200 400 600 800 1000 1200 1400

700 C, 1.2 ml/h decane725 C, 1.2 ml/h decane750 C, 1.2 ml/h decane775C, 1.4 ml/h decane800C, 1.7 ml/h decane

P o t e n

t i a l ( m

V )

Current(mA/cm2)

700 C725 C 750 C

775 C

800 C

Power densities can be good with optimized cells:Fuel: pure n-decane

Data from SRI InternationalCell area 6.4 cm 2


16/45

0 50 100 150 200 250 300 3500.0

0.1

0.2

0.3

0.4

800 0C, Keep at ~0.6V

H2 with 10vol% H 2Oand

50ppm H 2S100ppm H 2S200ppm H 2S300ppm450ppm

600ppm900ppm

P o w e r

d e n s

i t y ( W / c m

2 )

Time (h) -22.0 -21.5 -21.0 -20.5 -20.0-14

-13

-12

-11

-10

-9

800 0C

Ce 2O2S

log P O 2 (P O 2 in atm)

l o g

P S 2

( P S 2

i n a t m

)

CeO 1.83

Cu-ceria-YSZ electrodes stable in sulfur:

Measure performance in H 2-H 2O-H 2S mixtures

900 ppm600 ppm450 ppm300 ppm200 ppm

100 ppm

50 ppm

Sulfur poisoning occurs when Ce 2O 2S is formed.

Note: Ni-anodes are poisoned at surface monolayer, ~ 5 ppm.


17/45

Cu-based anodes have limitations:

Low catalytic activity (ceria is the catalyst)Limited to use at lower temperatures.

700C 900CB

Cu

B

Cu


18/45

-0.5

0

0.5

1

1.5

2

0 0.5 1 1.5 2 2.5 3 3.5 4

Z re cm 2

- Z

i m

c m

2

Effect of thermal treatment on cell performance:

After heating to 800C After heating to 900C

Sintering of Cu:1) Loss of anode conduction2) Increased ohmic resistance

Cu-ceria-YSZ YSZ LSM-YSZ

3% H2O-97% H

2at 700C


19/45

Solution 1: Electroplate Co onto Cu cermets

Idea: Coat thermally stable Cowith chemically stable Cu:

Co

Cu

Cu and Co do not form alloys

Cu Anode Ni Cermet

CuSO 4 + H 2S0 4Solution

DC

Cu 2+

e

e

V

Cu Anode Ni Cermet


DC

Cu 2+

e

e

Cu Anode Ni Cermet


DC

Cu Anode Ni Cermet


DCC

Cu 2+

e

e

V

Co


20/45

Key is to deposit evenly

throughout the pores; Co platingis much easier than Cu.

But Cu segregates to Co surface

XPS of 250-nm Co filmon Cu with heating

Co plated Cu stable inCH 4 at 800C for 3 h

Cu-Co Co only


21/45

Enhanced thermal stability achieved:

Ohmic Resistances of Cells at 900C

(o) Co-Cu-ceria-YSZ(5-vol% Co electroplated;13-vol% Cu)

() Cu-ceria-YSZ(18-vol% Cu)

( ) Co-ceria-YSZ(18-vol% Co)


22/45

0

0. 2

0. 4

0. 6

0. 8

1

1. 2

0 50 100 150 200 250 300 350 400 450 500

Time, hour

V o l

t a g e , V

0

0. 1

0. 2

0. 3

0. 4

0. 5

0. 6

P o w e r

d e n s

i t y , W / c m

2voltage

power denisityDry CH 4, 800C50:50 Co-Cu

Cu-Co composites stable for long times


23/45

Solution 2: Ceramic anodes (the Holy Grail?)

Problem: Oxides have either poor conductivityat low P(O 2) or poor catalytic activity

Concept: Separate the two required functions

YSZ electrolyte

Anode current collector

Porous ceramic, optimized for conductivity.

Anode functional layer

Optimized for catalytic performance.Thin to avoid need for high conductivity.

Key point:If = 10 m and R ohmic must be < 0.1 .cm 2, need only be 0.01 S/cm!


24/45

Concept works:1. Functional layer can provide high performance when

Ag used for current collection.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0 0.5 1.0 1.5 2.0 2.5

Current Density, A/cm2

V o l t a g e ,

V

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

P o w e r

D e n s i

t y , W

/ c m

2

650C700C

750C800C

LSF-YSZ cathode (300 m)

YSZ electrolyte (75 m)Anode, Ceria-YSZ (10 m)Current collector, Ag paint

10 m

H 2 (3% H 2O)

800C 750C 700C 650C

(Pd doped)


25/45

2. Ag can be replaced by La 0.3Sr 0.7TiO 3:

20 mP

or o u s Y

S Z - a

c t i v e

r e gi on

La0.3 Sr 0.7 TiO 3 current collector

Y S Z

el e

c t r ol y

t e

0

0.2

0.4

0.6

0.8

1

1.2

0 0.4 0.8 1.2 1.6

Current Density / A cm -2

V o

l t a g e

/ V

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

P o w e r

D e n s i

t y / W c m

- 2

650C

800C750C

H 2 (3% H 2O)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.4 0.8 1.2 1.6

Current Density / A cm -2

V o l

t a g e

/ V

0

0.1

0.2

0.3

0.4

0.5

0.6

P o w e r

D e n s i t y

/ W c m

- 2

CH 4 (3% H 2O)

800C

750C

700C


26/45

Cathode Research: Background

Standard Material: LSM (La 0.8Sr 0.2MnO 3)-YSZ compositeLSM is used because- it has good electronic conductivity.- low reactivity with YSZ, even at 1200C.- good long-term stability.- good CTE match with YSZ.

But,- it has poor ionic conductivity, so that performance poor below 800C.- activated by cathodic polarization (implications for electrolysis.).

LSCo ((La 0.8Sr 0.2CoO 3):- good electronic and ionic conductivity- can provide outstanding electrode performance at below 700C.

But,- electrode performance is not stable.- it reacts with YSZ, even at 700C (forms La 2Zr 2O 7).- poor CTE match with YSZ.


27/45

Performance independent of LSM precursorPerformance independent of LSM precursor

Fewest steps required by Molten SaltFewest steps required by Molten Salt

Impregnated LSM indistinguishable from screenImpregnated LSM indistinguishable from screen --printedprinted

LSM/YSZLSM/YSZ

ObservationsObservations

We need a minimum of 30 to 40 wt% of LSM.We need a minimum of 30 to 40 wt% of LSM.

a) Nano-particles ( )20 wt% in butandiol

b) Aqueous solutions ( )1.6 molar, nitrate solution

c) Molten salts ( )

ESSL, 9 (2006) A237ESSL, 9 (2006) A237 --240240

0

0.2

0.4

0.6

0.8

1

1.2


V o l

t a g e

( V )

0

50

100

150

200

250

300

350

P o w e r

D e n s i

t y ( m W / c m

2 )

0

0.2

0.4

0.6

0.8

1

1.2


V o l

t a g e

( V )

0

50

100

150

200

250

300

350

P o w e r

D e n s i

t y ( m W / c m

2 )

700700 CC

CoCo --ceriaceria |YSZ|LSM|YSZ|LSM --YSZYSZ

LSM by impregnation:


28/45

-1

-0.5

0

0.5

1

1.5

2

0 0.5 1 1.5 2 2.5 3 3.5

Z Re, .cm 2

- Z I m , . c

m 2

2 kHz

0.8 Hz

1.6 Hz

3 Hz4 Hz

Understanding LSMUnderstanding LSM --YSZ cathodes: PolarizationYSZ cathodes: Polarization activationactivation

LSM-YSZ

CeO 2 Pd- C - YSZ

700C, H 2/3%H 2O, OCV after applying current

- 850 mA/cm 2- 60 mA/cm 2- 0 mA/cm 2 - 150 mA/cm 2

Note: These changes are reversible.Note: These changes are reversible. ~ 120 minutes.~ 120 minutes.

ESSL, 7 (2004) A111ESSL, 7 (2004) A111 --A114.A114.


29/45

What we believe is happening:

YSZ

1) Dense LSM covers YSZ Gaps in LSM film caused by reduction

2) Performance limited by Gas can get to YSZ interface.oxygen diffusion.

Process driven by surface interactions between LSM & YSZ

YSZ

LSM

Electrode before activation Activated Electrode


30/45

LSM Particles on YSZ (100): Effect of LSM Particles on YSZ (100): Effect of calcinationcalcination temperaturetemperature

850C 1050C 1150C

ESSL, 9 (2006) A237ESSL, 9 (2006) A237 --240240


31/45

2 m x 2 mCalcination at 1150C Reduced in H 2 (10%H 2O) at 700C

Movement of particles is reversible:Movement of particles is reversible:

1)1) LSM is stable in 10%HLSM is stable in 10%H 22OO --90% H90% H 22 at 700at 700 C.C.

2)2) Reducing LSMReducing LSM --YSZ electrode activate it.YSZ electrode activate it.


32/45

Consequences for electrolysis (anode environment is oxidizing):Consequences for electrolysis (anode environment is oxidizing):

0 50 100 150 200 250 3000.0

0.2

0.4

0.6

0.8

1.0

1.21.4

C e l

l p o t e n t

i a l ( V )

Time (min)

1.25

1.3

1.35

1.4

1.45

-20 10 40 70 100 130 160 190

Time (min)

C e l l p

o t e n

t i a l ( V )

0.5

1

1.5

2

2.5

3

3.5

E l e c t r o

d e i m

p e d a n c e

( - c m

2 )

285285 mAmA /cm/cm 22; 700; 700 C; 85%HC; 85%H 22--15%H15%H 22OO | air| air

LSMLSM --YSZ anodeYSZ anode LSFLSF --YSZ anodeYSZ anode

JECS, in press.JECS, in press.


33/45

LSFLSF --YSZ (this cannot be made by screen printing)YSZ (this cannot be made by screen printing)

2)2) Formation of Formation of ZrZr --doped LaFeOdoped LaFeO 33only above 1200only above 1200 CC

3)3) ZrZr doping is not a deactivationdoping is not a deactivation

process.process.

-0.15

-0.1

-0.05

00 0.05 0.1 0.15 0.2 0.25

Z' (ohm.cm 2)

Z ' ' ( o h m . c

m 2 )

10%Zr-LSF+YSZ

LSF+YSZ

700700 C, symmetric cellC, symmetric cell

Observations:Observations:1)1) See no additional phases withSee no additional phases withcalcinationcalcination temperature (Verytemperature (Verydifferent from LaCoOdifferent from LaCoO 33))


34/45

Time &Time & calcinationcalcination temperature have similar effect:temperature have similar effect:

Symmetric CellsSymmetric CellsCalcineCalcine @ 850@ 850 CC

t = 0 h @700t = 0 h @700 CC

CalcineCalcine @ 850@ 850 CC

t = 2500 h @700t = 2500 h @700 CC

CalcineCalcine @ 1100@ 1100 CC

t = 0 ht = 0 h

R R = expected value for= expected value forYSZ electrolyteYSZ electrolyte

R R PP = 0.12= 0.12 cmcm 22 @ 700@ 700 CCindependent of independent of ii

R R = expected value for= expected value forYSZ electrolyteYSZ electrolyte

R R PP = 0.6 to 0.1= 0.6 to 0.1 cmcm 22 ,, depends strongly depends strongly onon ii

NoNo hysteresishysteresis ! Not like LSM.! Not like LSM.

R R = expected value for= expected value for

YSZ electrolyteYSZ electrolyte

R R PP = 2.5 to 0.1= 2.5 to 0.1 cmcm 22 ,, depends strongly depends strongly onon ii

NoNo hysteresishysteresis ..


35/45

Effect of Effect of CalcinationCalcination Temperature, Performance @ 700Temperature, Performance @ 700 CC

CalcineCalcine

@ 850@ 850 CC

t = 0 ht = 0 h

CalcineCalcine

@ 1100@ 1100 CC

0

0.1

0.2

0.3

0.3 0.4 0.5 0.6 0.7 0.8Z Re, ohm*cm2

- Z I m

, o h m

* c m

2

450 mA/cm2OCV- 450 mA/cm2

0

0.2

0.4

0.6

0.8

1

0.3 0.5 0.7 0.9 1.1 1.3 1.5

Z Re, ohm*cm2

- Z I m

, o h m * c m

2

450 mA/cm2100 mA/cm2OCV- 100 mA/cm2- 450 mA/cm2

Anode: metalAnode: metal --doped ceriadoped ceria --YSZYSZ Ag, 50Ag, 50 m YSZ electrolytem YSZ electrolyte


36/45

Proposed deactivation mechanism:

Implications:Implications:1.1. Deactivation is structural, not associated with interfacial reacDeactivation is structural, not associated with interfacial reac tionstions2.2. InterlayersInterlayers will probably not be effectivewill probably not be effective3.3. With LSM, activation of very good electrodes less importantWith LSM, activation of very good electrodes less important use sameuse same

concepts for stabilizing LSF?concepts for stabilizing LSF?

YSZYSZLSF

850C calcination 1100C

11 mm 11 mm


37/45

SummaryElectrode fabrication by impregnation allows great

flexibility in preparing anodes.

Direct utilization of hydrocarbon fuels is possible inCu-ceria based anodes.

The thermal stability of Cu-based anodes can beimproved by:

1. Co electrodeposition2. Using ceramic anodes.

High-performance LSF-YSZ cathodes may be possible.

http://www.franklinfuelcells.com/http://www.seas.upenn.edu/cbe/Fuel_Cell_index.htm


38/45

-0.1

0

0.1

0.2

0.3

0 0.1 0.2 0.3 0.4 0.5

Zre .cm 2

- Z i m

. c m

2500 Hz

LSM(850C)

-1

0

1

2

3

4

5

0 1 2 3 4 5 6 7Zre .cm 2

- Z i m

. c m

20.2 Hz

0.2 Hz

LSM(1250C)

( ) Initial spectrum.() After applying 250 mA/cm 2 for 10 minutes.

( ) 5 h after applying current.

4040 --wt% impregnated LSMwt% impregnated LSM --YSZ, fired to 850YSZ, fired to 850 C or 1250C or 1250 CCMeasurement at 700Measurement at 700 C in air at OCV.C in air at OCV. LSM/YSZ

LSM/YSZ

CalcinationCalcination temperature affects activation:temperature affects activation:

R = 0.5 cm 2 R = 6.5 cm 2

Not activated Activated by polarization

JECS, 152 (2005) A1347JECS, 152 (2005) A1347 --53.53.


39/45

BET Surface Areas (40 wt% LSM in YSZ)BET Surface Areas (40 wt% LSM in YSZ)

0.380.38 0.020.02LSM(1250LSM(1250 C)C) --YSZYSZ

2.532.53LSM(850LSM(850 C)C) --YSZYSZ

0.770.77 0.020.02Porous YSZ without LSMPorous YSZ without LSM

Surface Area (mSurface Area (m 2 2 /g) /g)

4 [ (1-) ]

=

= porosity= surface area= YSZ density

= 0.77 m 2/g = 1.6 microns = 0.38 m 2/g = 0.58 microns

dense LSM film ondense LSM film onYSZ poresYSZ pores

0.780.78 0.030.03LSM(1250LSM(1250 ooC)C) --YSZ w/ 700YSZ w/ 700 C reductionC reduction


40/45

Gas-phase pyrolysis.

Tony Dean, CO School of Mines

Control requires consideration of flow geometries and coating of surfaces.


41/45

10 15 20 25 30 35 40 45 50 55

2500e3

5000e3

7500e3

10.0e6

12.5e6

15.0e6

17.5e6

20.0e6

TIC

1

2

3

4

56

7

8

9

10

11

12 13

10 15 20 25 30 35 40 45 50 55

Time (min)

0

1

2

3

4

5

6

7

8

925 950 975 1000 1025 1050

Temperature (K)

W e i g h

t G a i n P e r c e n t a g e

( % )

Wt change of YSZ slab after 4 h in butane:

0.3 ml/s

1.0 m/s

S:C=1.5

Tar formation on a YSZ Slab:

1) Rate depends on flow rate2) Steam does not affect rate3) For CH 4, negligible tar


42/45

Time (hr)

0 5 10 15 20 25

P o w e r

d e n s

i t y ( m W / c m

2 )

0

100

200

300

400

Perform decreases as anode pores fill with tar:

Performance on un-diluted light naphtha, Saudi-Aramco:

800C

750C

700C


43/45

Most hydrocarbons behave similarly:

Time (hr)

0 5 10 15 20 25

P o w e r

d e n s

i t y

( m W / c m

2 )

0

100

200

300

400

500

Time (hr)

0 5 10 15 20 25

P o w e r

d e n s i

t y ( m W / c m

2 )

0

50

100

150

200

250

300

350

Heavy naphtha n-Decane with 20% toluene

800C

700C

800C

700C

Butane and low-sulfur diesel are also similar

P ibl S l i


44/45

Possible Solutions:1) Prevent gas-phase reactions (radical termination)2) Remove tar faster than it forms

TPO using 10% H 2O in He; tar on YSZ

YSZ

Temperature (K)

800 900 1000 1100 1200 1300

I n t e n s

i t y

( a . u . )

CeO 2 /YSZ

H 2

H 2

CO 2

CO

CO 2

CO


45/45

Support Layers can be added for strength:

Laminated cellDense YSZ

Porous YSZ

Dense YSZ

Porous YSZ

Final Product

FFC, Inc.

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