new cathode materials for intermediate … library/research/coal...jacobson mims rieke 1 new cathode...

1Jacobson Mims Rieke

New Cathode Materials for Intermediate New Cathode Materials for Intermediate Temperature Solid Oxide Fuel CellsTemperature Solid Oxide Fuel Cells

Allan J. JacobsonTexas Center for Superconductivity, University of Houston

Charles A. MimsUniversity of Toronto

Peter RiekePacific Northwest National Laboratories

Project Manager: Dr. Lane WilsonDOE National Energy Technology Laboratory


AcknowledgementsAcknowledgements

• University of Houston– Chonglin Chen– Susan Wang– Chan Young Park– Guntae Kim – Frank Azzarello

• University of Toronto– Leszek Reimus– Peter Brodersen


ObjectivesObjectives• The specific objectives are to develop cathode materials

that meet the electrode performance targets– 1.0 W/cm2 at 0.7 V in combination

with YSZ at 700 ºC and with GDC at 600 ºC.

• The research strategy is to:– investigate both established classes of materials and new

candidates as cathodes; – determine fundamental performance parameters such as bulk

diffusion, surface reactivity and interfacial transfer; – couple these parameters to performance in single cell tests;– use model thin film structures to isolate specific features of

oxygen reactivity and transport at surfaces and interfaces


OverviewOverview

New Bulk Electrode Electrode/Materials Properties Performance Electrolyte Structure

Model Surfaces/ Electrode ModelingSystems Interfaces Polarization

Optimized electrode /electrolyte combinationsOptimized electrode /electrolyte combinations


SOFC Cathode PerformanceSOFC Cathode Performance

)(),(

),(),(

)()()(),(),(

)()()()()(

)()()(/

IIDIID

YIIVIIYV

YOIIhIISYIIOIIYV

IIOIIhIISIIOIIV

IIOIISgO

sO

OO

OtbpO

OsO

s

fDBPk

rDBPk

fTBPk

rTBPk

fexk

rexk

fadsk

rdesk

••⎯⎯ →⎯

⎯⎯ ⎯←

••

×•⎯⎯ →⎯

⎯⎯ ⎯←

••

×•⎯⎯→⎯

⎯⎯⎯←

••

⎯⎯ →⎯

⎯⎯⎯←

+++

+++

+

2

2

21 2

from Coffey et al. JECS 150, A1139 (2003)

Both surface and bulk diffusion considered


"Simple" overall cathode reaction involves

-3 phases (+ current collector) O2 (gas) + 4e- (cathode, collector) = 2 O2-(electrolyte)

-and multiple (largely unknown) elementary reaction stepsO2 (gas) O2 (cathode) 2O (cathode)

O2 - (cathode)

O2 2- (cathode)

2O - (cathode)

2O 2- (cathode) 2O 2- (electrolyte)

2O (TPB) O2 (TPB)

O2 - (TPB)

O2 2- (TPB)

2O - (TPB)

2O 2- (TPB)

rate limiting steps develop electrical/chemical gradients

red = reduction

Oxygen Activation MechanismsOxygen Activation Mechanisms


SOFC CathodesSOFC CathodesA model that does not consider surface diffusion gives the cathode resistance in terms of the diffusion coefficient (DO

*) and the surface exchange rate (kO0 )

(cm/s units)

where τ, ϕ and S are the electrode tortuosity, porosity and surface area

DO and kO can be measured by:Isotope exchange and depth profiling (IEDP)Electrical conductivity relaxation (ECR)

21

022 12

/

*)φ(τ

⎥⎥⎦

⎤

⎢⎢⎣

⎡

−=

OOOcathode

kDScFRTR

(Adler et al. J. Electrochem. Soc. (1996) 143 3554)


Experimental MethodsExperimental Methods

− Stoichiometry measurements − Conductivity and Conductivity relaxation − Isotope exchange and depth profiling (IEDP)− Photoelectron spectroscopy / Kelvin probe− AC impedance on symmetric cells− Half cell measurements− Thin film synthesis of model structures


Materials ClassesMaterials Classes• perovskite ferrites

– oxygen vacancies• La1-xSrxFeO3-x, x= 0.2, 0.3• La0.7Sr0.3Cu0.2Fe0.8O3-x

• perovskite related structures– oxygen interstitials

• La2NiO4+x• Pr2NiO4+x

• perovskite oxides with ordered A cations– 2 dimensional vacancies

• PrBaCo2O5+x• LaBaCuFeO5+x,


Previous ResultsPrevious Results

• perovskite ferrites– Stoichiometry, conductivity, measured for La1-

xSrxFeO3-x and La0.7Sr0.3Cu0.2Fe0.8O3-x

– Both share strange, sluggish stoichiometry changes with other ferrites at intermediate pO2

• perovskite related structures– Stoichiometry, conductivity, IEDP, ECR measured for

La2NiO4+x , Pr2NiO4+x

– Symmetric cells (YSZ and CGO electrolyte) ASR = 1 ohm at 600C

• perovskite oxides with ordered A cations– PrBaCo2O5+x films ECR, IEDP


20 40 60 802θ

Inte

nsity

20 40 60 802θ

Inte

nsity

20 40 60 802θ

Inte

nsity

20 40 60 802θ

Inte

nsity

Bulk Powders of Double Bulk Powders of Double PerovskitesPerovskites

Rietveld refinement of PrBaCo2O5.7a = 3.9084(1) Å b = 3.9053(1) Å c = 7.6343(2) Å.

Powders synthesized for ceramic (bulk) samples, cell electrodes and

targets for thin films

PrBaCo2O5.7

NdBaCo2O5.x


PLD Film Deposition PLD Film Deposition • (100) LaAlO3 SrTiO3 and YSZ single crystal substrates • PBCO at 300 mTorr oxygen partial pressure with a

substrate temperature of 880 °C. • LNO at 300 mTorr and 840 °C • KrF excimer laser with pulse frequency of 7 Hz. • Films 2000 to 5000 Å


Oxygen Activation on PBCO FilmsOxygen Activation on PBCO Films

0

0.2

0.4

0.6

0.8

1

-0.1

-0.05

0

0.05

0.1

0 800 1600

g(t)

Error

time (s)

ECR involves oxygen pressure jumpConductivity change results from stoichiometry change (O uptake).

Two time constants used to fit data

10-8

10-7

10-6

10-5

1.25 1.35 1.45 1.55

k1 2-10%k2 2-10%k1 10-2%k2 10-2%IEDP

k chem

(cm

s-1

)

1000/T (K-1)PrBaCo2O5.7 on STO

PBCOSTO


Insignificant interfacial transfer barriersExchange limited by surface kO,PBCO and DO, YSZ

Thin film PBCO on YSZ (100)Thin film PBCO on YSZ (100)

18O at 700 ºC, pO2 = 0.2 atm

(PrO-, CoO2-) (YO-, ZrO2

-)

fraction18O (× 5)

nominal depth / nm

fractionion signal

0 100 200 300 400 500 6000

0.2

0.4

0.6

0.8

1

PBCOYSZ

PBCOCGOYSZ


EpitaxialEpitaxial PBCO Thin FilmsPBCO Thin Films

50 100 150 200 250 300depth / nm

0

0.2

0.4

0.6

0.8

1

1.2

Inte

nsit

y

38 40 42 44 46 48 50

10-3

10-2

10-1

100

101

102

STO(200)

PBCO(004)

coun

ts /

mon

itor

2θ [deg]

Λ=50Å Λ=75Å Λ=140Å Λ=250Å PBCO(200)

*

PBCOSTO

PBCO ions

18 O fraction (x 10)


Surface Surface kkOO Values on PBCO FilmsValues on PBCO Films

10-9

10-8

10-7

10-6

10-5

0.9 1 1.1 1.2 1.3 1.4 1.5 1.6

k chem

(cm

-1 s

)

1000/T (K-1)

LaSrCo2O

6-x

PrBaCo2O

6-x

after 900 oC annealas synthesized

k1

k2IEDP

Higher surface rates than LSCOSensitivity to surfacepreparation/ history

PBCO/YSZPBCO/STO (epitaxial)

Accepted Appl. Phys. Lett. (2005)


Composition and Total Conductivity Composition and Total Conductivity of of Bulk PrBaCoBulk PrBaCo22OO5+5+δδ

5.3

5.4

5.5

5.6

5.7

5.8

200 300 400 500 600 700 800 900

0.01 atm0.02 atm 0.03 atm0.05 atm0.21 atm

5.5+

x

Temperature (oC)

200

300

400

500600700800900

1000

0.5 1 1.5 2 2.5 3 3.5

0.01 atm0.01 atm0.21 atm0.21 atm0.01 atm0.21 atm

Con

duct

ivity

(S/c

m)

1000/T (K-1)

NdBaCo2O

5.5+x

PrBaCo2O

5.5+x

127 oC


Kinetics on Bulk Materials Kinetics on Bulk Materials

10-9

10-8

10-7

10-6

10-5

10-4

10-9

10-8

10-7

10-6

10-5

10-4

1.4 1.5 1.6 1.7 1.8 1.9 2

DO (c

m2 s

ec-1

)

kO (cm

s-1) at 0.01 atm

1000/T (K-1)

NdBCO

NdBCO

PBCO

PBCO

iO = 1A cm-2


PBCO DPBCO Doo and and kkoo Results (IEDP)Results (IEDP)

0 140depth / um

0

0.10

f 18O

a

b

c

400oC – 5 minutes – polished cross section - ToFSIMS

Secondary electron image

18O image

18O profile and fit


Kinetics on Bulk Materials Kinetics on Bulk Materials

10-9

10-8

10-7

10-6

10-5

10-4

10-9

10-8

10-7

10-6

10-5

10-4

1.4 1.5 1.6 1.7 1.8 1.9 2

DO (c

m2 s

ec-1

)

kO (cm

s-1) at 0.01 atm

1000/T (K-1)

NdBCO

NdBCO

PBCO

PBCO

iO = 1A cm-2


NBCO DNBCO Doo and and kkoo Results (IEDP)Results (IEDP)

DO less than 0.05 x PBCO


TECS for PrBaCoTECS for PrBaCo22OO5+5+δδ andand NdBaCoNdBaCo22OO5+5+δδ

0.000

0.005

0.010

0.015

12

14

16

18

20

22

24

0 200 400 600 800 1000

dL/L

TEC

T (oC)

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

12

14

16

18

20

22

24

0 200 400 600 800 1000

dL/L

TEC

T (oC)

PrBaCoPrBaCo22OO5+5+δδ NdBaCoNdBaCo22OO5+5+δδ


SummarySummary

PBCO kinetic parameters are superior to previous cobaltites

Cell tests underway - initial batch of powder did not make good electrodes


Plans Plans

Cell tests (Pt anode) with PBCO cathodes

Patterned films for IEDP Ink jet printed compositional ferrite series (underway)Surface studies


Model materials to study individual stepsModel materials to study individual stepsSingle phase material

Surface and bulk

Two phase (films on single xtals)Surface, interface and bulk(s)

Patterned materials (masked films, printed patterns)

Surfaces, interface, bulk(s) and TPB Combinatorial investigations

Synthesis: Pulsed Laser Deposition and Ink Jet Printing


Surface StudiesSurface Studies

Mechanistic sequence of oxygen activationRole of adlineated sites Surface structure (including vacancies) of perovskite oxidesSurface oxygen vacancy concentration as function of pO2

WHAT WE DON’T KNOW


Surface StudiesSurface Studies

reference (Pd/PdO)V

O2

YSZ

O2

i

sample

heated stage

Photoelectron spectroscopy and work function measurements + other techniques (STM)

Probe molecules

Surface studies of electrochemically polarized materials


Work function measurementsWork function measurements

La0.7Sr0.3FeO3-δ

3.5

3.6

3.7

3.8

3.9

4.0

4.1

4.2

4.3

4.4

4.5

0.001 0.01 0.1 1 10 100pO2 (%)

Φ (e

V)


LaCrLaCr11--xxNiNixxOO33--yy XPS Under XPS Under PolarizationPolarization

0 0.5 1

0 0. 5 1

69

67

578

576

530

528

x in LaCr1-XNixO3-*

0 0.5 1.0

Bind

ing

ener

gy /

eVLa

II/ L

aI

0.8

0.9

1.0

1.1

(b)

(a)

O 1s

Cr 2p3/2

Ni 3p

La 3d 5/2

La2NiO4

La2O3

6065707580Binding energy / eV

100

150

200

250

300

Inte

nsit

y / c

ps

Ni 3p 3/2

Cr 3s

LaNiO3

LaCr0.6Ni0.4O3

(a)

(b)

(c)

(d)

1/2


0

1000

2000

3000

4000

5000

Inte

nsit

y / cp

s

La 3d3/2 5/2

II I

Ni 2p3/2

LaNiO3

LaCr0.6Ni0.4O3

(a)

(b)

(c)

(d)

Reduced to Ni2+

stoichiometry



LaNiO3

O 2p Ni 3d O 2p

La 3d La 3d0eV

~1.5eV

~0.7eV

Ni 3p

Ni 3p

O 1s

LaNiO2.5

~0.5eV EF

O 1s

Ni 3dVBVB




1200

1400

1600

1800

2000

2200

Inte

nsity

/ cps

Co 2p3/21/2

**

(a)

(b)

La0.5Sr0.5CoO3-x


2000

2500

3000

3500

Inte

nsity

/ cps

La 3d 5/2

II I

(a)

(b)

La0.5Sr0.5CoO3-x

0 0.1 0.2 0.3 0.4-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

Bin

ding

ene

rgy

shift

/ eV

Degree of reduction (as * in La0.5Sr0.5CoO3-* )

Co

SrO

La0.5Sr0.5O3-d


LaLa0.70.7SrSr0.30.3FeFe0.80.8CuCu0.20.2OO33--x x StoichiometryStoichiometry

0.00

0.03

0.06

0.09

0.12

0.15

0.18

1st cell at 850 oCRun 3_HLRun 4_LH

2nd cell at 850 oCRun 7_HLRun 8_LH

Criteria: 0.08%/min

Rel

ativ

e N

onst

oich

iom

etry

()

log pO2

(atm)

δ

10 -10 10 -8 10 -6 10 -4 10-2 10 0

0.00

0.04

0.08

0.12

0.16

0.20

0.24

0.28

1000 oC

900 oC

800 oC

Closed: high to low pO2

Open: low to high pO2

The data above 850 ºC are offset for clarity

Rel

ativ

e N

onst

oich

iom

etry

(

)δ

log pO2

(atm)

Measurements made in an electrochemical cell with Pt|YSZ|Pt as sensor and pump

10-9 10-7 10-5 10-3 10-1


Extra Slides Extra Slides

Thank You Thank You


Conductivity and StoichiometryConductivity and Stoichiometry

4.08

4.1

4.12

4.14

4.16

4.18

4.2

400 500 600 700 800 900 1000

7%- O25%- O23%- O2

Temperature (ºC)

4.05

4.07

4.09

4.11

4.13

400 500 600 700 800 900 1000

5%-O23%-O22%-O21%-O2

Temperature (ºC)40

50

60

70

80

90

100

0.5 1 1.5 2 2.5 3 3.5

Con

duct

ivity

(S/c

m)

1000/T (K-1)

closed : high to low pO2

open : low to high pO2

circle : 100 % O2

triangle : 10 % O2

diamond : 1 % O2

40

50

60

70

80

90

100

0.5 1 1.5 2 2.5 3 3.5

Con

duct

ivity

(S/c

m)

1000/T (K-1)

closed : High to low pO2


1% O2

0.1% O2

40

50

60

70

80

90

100

0.5 1 1.5 2 2.5 3 3.5

Con

duct

ivity

(S/c

m)

1000/T (K-1)

closed : High to low pO2


1% O2

0.1% O2

LNO

PNO


Electrical Conductivity RelaxationElectrical Conductivity Relaxation

• The normalized conductivity data g(t) are fit to a model that includes Dchem and kchem as variables

• The surface exchange coefficient (kex) and the self diffusion coefficient (DO) were obtained using the relationships below :

Thermodynamic factor

Self diffusion coefficient

Surface exchange coefficient

0

0.2

0.4

0.6

0.8

1

1.2

-0.04

-0.03

-0.02

-0.01

0

0.01

0.02

0.03

0.04

0 50 100 150 200g(

t)

error

Time (s)

a line : data fitdiamond : experimental

La2NiO

4+δ

at 800oC pO2 : 0.01 to 0.02 atm

OchemO ΓDD 2 ≈−

−∂∂

⋅=2O

2O ln C

ln pO21Γ

Ochemex Γk k =


Oxygen Diffusion Coefficients Oxygen Diffusion Coefficients for Lafor La22NiONiO44 and Prand Pr22NiONiO44

10-4

10-3

0.85 0.9 0.95 1 1.05 1.1

Dch

em (c

m2 /s

)

1000/T (K-1)

Pr2NiO

4+δ

La2NiO

4+δ

3-7 %3-5 %1-3 %

3-5 %2-3 %1-2 %0.5-1 %

10-8

10-7

10-6

0.85 0.9 0.95 1 1.05 1.1

05-1%1-2%2-3%3-5%

1-3%3-5%3-7%

Do c

m2 /s

1000/T(K-1)

Pr2NiO

4+x

La2NiO

4+x

10-4

10-3

0.85 0.9 0.95 1 1.05 1.1

Dch

em (c

m2 /s

)

1000/T (K-1)

Pr2NiO

4+δ

La2NiO

4+δ

3-7 %3-5 %1-3 %

3-5 %2-3 %1-2 %0.5-1 %

10-8

10-7

10-6

0.85 0.9 0.95 1 1.05 1.1

05-1%1-2%2-3%3-5%

1-3%3-5%3-7%

Do c

m2 /s

1000/T(K-1)

Pr2NiO

4+x

La2NiO

4+x


Area Resistances on YSZArea Resistances on YSZ

Ralph et al., in Solid Oxide Fuel Cells VII, PV 2001-16, p. 466, 2001.

1

10

100

600 650 700 750 800 850 900

100%25%1%

Are

a S

peci

fic R

esis

tanc

e (O

hms*

cm2 )

temperature (oC)


Symmetric cells Symmetric cells LNO|CGO|LNO and PNO|CGO|PNOLNO|CGO|LNO and PNO|CGO|PNO

-70

-60

-50

-40

-30

-20

-10

0

1020 30 40 50 60 70 80 90 100

1% O2

5% O2

21% O2

100% O2

Z''

Z'

Pr2NiO

4 /CGO/ Pr

2NiO

4

Area specific resistance for 1% pO2

Representative impedance spectra of the Pr2NiO4electrochemical cell: Pr2NiO4 /CGO/ Pr2NiO4 (T= 527 °C).


Comparisons on CGOComparisons on CGO

0.01

0.1

1

10

0.01 0.1 1

1/A

SR (1

/(ohm

s*cm

2 ))

pO2 (atm)

Dashed line = La2NiO

4+δ

Line =Pr2NiO

4+δ

526 0C

581 0C

634 0C

1/ASR=M0 (pO

2)n

n= 0.27~0.31

Ralph et al., in Solid Oxide Fuel Cells VII, PV 2001-16, p. 466, 2001.


Isotope exchange Isotope exchange -- SIMS analysisSIMS analysis

Patterned materials (printed films, patterns) Surfaces (2), interface, bulk(2) and TPB

Single phase materialSurface and bulk

Two phase (films)Surface, interface and bulk(2)

O2 O O O O O O O O OO O O O O O O O OO O O O O O O O

O O O O O O O O O

O

O O O O O O O O OO O O O O O O O OO O O O O O O O

O O O O O O O O O

O

O O O O O OO O O O O O O O OO O O O O O O O

O O O O O O

O

O2

O2

18O represented by red color

+ Functioning cathodes?


Ink Ink -- Jet printing on YSZ(100)Jet printing on YSZ(100)

TOF-SIMS image - portion of a circle (dia. 4mm, linewidth 500 micron)

CGO ions(Ce+,CeO+,Gd+,GdO+,etc.)

YSZ ions(Y+,YO+, Zr+,ZrO+, etc.)

100 micrometers


Some ConclusionsSome Conclusions• La0.7Sr0.3Cu0.2Fe0.8O3-x shows complex phase behavior

– Further structural data needed

• Pr2NiO4 is a promising material– appears to be less reactive with YSZ than La2NiO4

• PrBaCo2O5+x thin films have:– very fast surface kinetics;– good interfaces on YSZ and CGO;– bulk materials require further study.

• Anisotropic materials show two surface kinetic pathways in addition to anisotropic bulk diffusion


LaLa22NiONiO44 and Prand Pr22NiONiO44 XRD DataXRD Data

10 20 30 40 50 60 70 80

0

100

200

300

400

500

600

(310

)(3

03)

(217

)(3

01)

(118

)

(206

)

(008

)(2

13)

(107

)(2

04)

(116

)(2

11)

(200

)

(114

)(1

05)

(110

)

(103

)(0

04)

(101

)

(002

)

Inte

nsity

La2NiO4+δI4/mmma=3.8635 Åc=12.6667 Å

10 20 30 40 50 60 70 80

0

50

100

150

(002

)

(402

)(135

)(133

)(0

08)

(008

)(117

)

(311

)(024

)(2

20)

(204

)(0

06)

(206

)

(400

)

(202

)(0

20)

(115

)

(004

)

(111

)

(200

)

Inte

nsity

2 theta

Pr2NiO4+δFmmma = 5.4523 Åb = 5.3962 Åc = 12.4314 Å

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