Cathode Contact Material Development

Michael C. Tucker*, Lei Cheng, Lutgard C. DeJonghe

Materials Sciences DivisionLawrence Berkeley National Laboratory

Berkeley, CA 94720

Pittsburgh, PAJuly 2010

Problem Statement

Bonding at 1000°C or less to avoid oxidation of steel

This is a low sintering/bonding temperature!!- poor bonding - incomplete sintering = reduced conductivity

- Loose powder CCM is acceptable if stack experiences uniform compression

BUT: cross-cell thermal gradients, warping of components, etc causes local variation - delamination and loss of electrical contact

Well-bonded CCM is desirable, but challenging

- Can we find a material that is reactive enough to bond at <1000°Cbut stable at 800°C operation?

Ni-YSZ anode

YSZ electrolyteLSCF cathode

441 stainless steel

(MnCo)3O4 coating

Contact material (CCM)

Contact Material Literature

Contact between Crofer22APu and LSFPNNL Yang et al., Journal of Power Sources 155 (2006) 246–252

LSCM

Contact between Crofer22APU and LSFIkerlan, Juelich Montero et al., Journal of Power Sources 188 (2009) 148–155

LSF

Crofer22APU/MCF/CCM/LSF

Crofer22APU/CCM/LSF

Enhancement of LSM SinteringMcCarthy et al., Journal of Power Sources 180 (2008) 294–300

Electrical resistivity of spinel-coated Crofer 22 APU/LSM-10 contact paste/spinel-coated Crofer 22 APU sandwich specimen versus time, measured in air at 800 ◦C.

Sinter at 900°C, switch between air and nitrogenenhance sintering by creation of transient defects

Observations from Literature

- 200h is enough to capture initial transient

- No standard test geometry

- No standard CCM paste application method

- Compressive load applied (bonding not typically tested)

- No consensus on “best” CCM

Approach and Results

Candidate MaterialsLa0.6Sr0.4Co0.8Fe0.2 LSCFLa0.8Sr0.2Cu0.9Fe0.1O2.5 LSCuFLa0.7Sr0.3CoO3 LSCSm0.5Sr0.5CoO3 SSCSmBa0.5Sr0.5Co2O5 SBSCGdSrCo2O5 GSCLa0.65Sr0.30MnO3 LSMLaBaCo2O5 LBCYBaCo2O5 YBCNd1.8Ce0.2CuO4 NCCLa0.8Sr0.2Co0.3Mn0.1Fe0.6O3 LSCMFLa0.98Ni0.6Fe0.4O3 LNFLa1.2Sr0.8NiO4 LSNLa0.7Sr0.3FeO3 LSFLa2Ni0.6Cu0.4O4 LNC

LSM, LNF, SSC, LSCF purchased from Praxair

All others synthesized by GNP

CCM requirements:- good bonding- high electronic conductivity - good CTE match - chemical compatibility with LSCF

and (MnCo)3O4

Approach:Select candidates from cathode literature - high conductivity- low sintering temperature

GNP Synthesis, Coarsening, XRD Phase Confirmation

10 20 30 40 50 60 70 80

In

tens

ity (a

.u.)

2θ (degrees)

LSC

SBSC

NCC

LSCMF

LSF

800°C calcination

10 20 30 40 50 60

LBC 800°C

LBC 1100°C

LBC 1000°C

LBC 900°C

Inte

nsity

(a.u

.)2θ (Degrees)

800°C: LSC, SBSC, NCC, LSCMF, LSF900°C: GSC, LSN, LSCuF1100°C: LBC, YBC

CTE

Note CTE for interconnect and cell <14ppm/K

- Matched CTE is desirable

- High CTE does not disqualify candidate material

- Thin, porous layer

600 700 800 90012

14

16

18

20

22

24

26

LNCu

441

MnCo

CTE

(ppm

/K)

Temperature (°C)

LSCF

GSC

LSN

LSMLSF

NCC

LSCMF

LSCuF

YBC

LBC

SBSC

LNF

SSC

LSC

Conductivity of Porous CCM

650 700 750 800 850 900100

150

200

250

300

350

400

LSM sintered 1300°C

LSM sintered 1000°C

Con

duct

ivity

(S/c

m)

Temperature (°C)

(70% dense)

Conductivity less than predicted by density- minimal sintering/particle necking or GB issue

iV

~1cm

Pt wires/paste

Pt mesh/paste

Sintered bar of oxide

Conductivity of Dense CCM

600 650 700 750 800 850 900 950

100

1000

LSM

LSF

NCCLSCMF

LSCuFYBCLBC

LSNLSCFGSC

SBSC

LNF

SSC

Con

duct

ivity

(S/c

m)

Temperature (°C)

LSC

- Measured for dense bars

- Conductivity of porous CCM after bonding at 900-1000°C will be lower

600 700 800 900 1000 1100 1200 1300

-20

-15

-10

-5

0

LNCu

Per

cent

Lin

ear C

hang

e (%

)

Temperature (°C)

LSCF

GSC

LSN

LSM

LSF

NCCLSCMF

LSCuF

YBC

LBCSBSCLNF

SSCLSC

Sintering Behavior

- Only a few candidates display significant sintering in the 900-1000°C range

700 800 900 1000-10

-5

0

LNCu

Per

cent

Lin

ear C

hang

e (%

)

Temperature (°C)

LSCF

GSC

LSN

LSM

LSF

NCC

LSCMF

LSCuF

YBC

LBC

SBSC

LNF

SSCLSC

- Extent-of-sintering related to strength in the CCM layer

(not necessarily related to bonding at the interface with neighbor layers)

Reactivity with Neighbor Materials

20 30 400.0

0.5

1.0

LSCF + MCO1000°C 10h

LSCF

Inte

nsity

(a.u

.)

2θ (Degrees)

MCO

20 30 400.0

0.5

1.0

NCC + MCO1000°C 10h

NCC

Inte

nsity

(a.u

.)

2θ (Degrees)

MCO

Pellets of mixed MCO/CCM and LSCF/CCM Reacted in air at: Operating conditions (800°C 120h) and Sintering conditions (1000°C 10h)

Example: No Reaction Example: Significant Reaction

Most candidates were non-reactive with MCO, but reacted with LSCF

Reaction/Diffusion Distance

-10 0 10 20 30 40 50 60 70

0

50

100

150

200

250

300

Sig

nal I

nten

sity

(a.u

.)

Distance (μm)

Nd

Sr

La

Co

Fe

Ce

Cu

LSCF NCC

LSCF

NCC

In all cases, reaction zone restricted to <40μmreaction may be acceptable for

- thick LSCF layer- electrically conductive reaction products

EDAX

Screening Summary

Incipient Sintering Point (°C)

Shrinkage at 900°C

Shrinkage at 1000°C

CTE at

800°C800°C 150h

1000°C 10h

800°C 150h

1000°C 10h

Conductivity of bulk

dense pellet 800°C (S/cm)

LSCF 637 2.7 7.6 17.3 NO NO N/A N/A 426LSCuF 820 1.1 10.1 15.5 NO NO NO NO 201LSC 677 1.1 3.3 18.7 NO NO Minor Minor 1702SSC 740 0.5 2.3 22 NO Trace NO NO 1338SBSC 708 1.6 3.4 22 NO Trace YES YES 458GSC 760 1.3 3.2 19.5 NO Trace YES YES 350LSM 784 0.7 3.3 12.8 NO NO YES YES 272LBC 770 0.7 2.3 25 NO NO Minor Minor 314YBC 689 1.7 3.8 16.8 NO YES YES YES 260NCC 657 1.5 5.5 14.5 YES YES YES YES 107LSCMF 786 0.4 2.1 17.6 NO NO N/A N/A 110LNF 932 0 1.1 13.8 NO NO YES YES 589LSN 975 0 0.1 13.5 Minor YES NO NO 352LSF 690 0.3 0.9 13.3 NO NO NO NO 133LNC 782 0.4 2.4 14.6 NO NO NO NO 11

The most promising candidates are:- LSCF: good sintering and moderate conductivity- LSCuF: very good sintering at 1000°C- LSC and SSC: extremely high conductivity, moderate sintering

Reacts with MCO?

Reacts with LSCF?

ASR Measurement

NCC, LSC, LSCF and LSCuF show low and stable ASR

LSCF cathode(MnCo)3O4 coatingPrepared at PNNL

441 stainless steel

Contact material (CCM)

441 stainless steel

screen printed CCM and LSCF(PNNL ink recipe)

No compressive load

0 50 100 150 2000.0000

0.0025

0.0050

0.0075

0.0100

0.0125

0.0150

LSCuF

LSCFPt

App

aren

t AS

R(O

hm-c

m2)

Time (h)

LSC

SSC

NCC

800°C

Thermal Cycling

1 2 3 4 50.0000.0010.0020.0030.0040.0050.0060.0070.0080.0090.0100.0110.0120.0130.0140.015

SSC

LSC

LSCuF

AS

R (O

hm-c

m2 )

Thermal Cycle

NCC

21-800°C 10°C/min

No obvious delamination despite wide range of CTE

LSC Post-Mortem

25μm

LSC CCM

Porous LSCF

MCO

441 Steel

Sr

La

LaLa

La Fe

Co

-10 0 10 20 300.00

0.25

0.50

0.75

1.00

1.25

1.50

1.75

Fe/C

o S

igna

l int

ensi

ty (a

.u.)

Distance from Interface (μm)

Minor diffusion of Fe into LSC

Good bonding at LSC/LSCF interface

SSC Post-Mortem

441MCO

LSCF

SSC

441

MCODelamination at SSC/MCO interface (during sample prep?)

Good bonding at SSC/LSCF interface

SSC Post-Mortem

441

MCO

LSCF

SSC

SrSm Co

Co

SmSm

Sr La

LaLa

Fe

Co

Mn

Mn

Co

Co

No interdiffusion detected in bulk layers

CCM Layer Uniformity

- Wet-print on both substrates and assemble- Voids created during solvent burnout?

Develop alternative processing to ensure uniform contact - Print / dry / assemble- Print / dry print / assemble

441

CCM

LSCF

441

Voids

0 500 1000 15000.0

0.2

0.4

0.6

0.8

1.0

Current Density (mA/cm2)

Cel

l Pot

entia

l (V) 800°C

750°C

850°C

0

100

200

300

400

500

600

700

Pow

er Density (m

W/cm

2)

Commercial Cell Selection

0 500 10000.0

0.2

0.4

0.6

0.8

1.0

Current Density (mA/cm2)

Cel

l Pot

entia

l (V)

800°C

0

100

200

300

400

500

600

Power D

ensity (mW

/cm2)

0 50 1000.0

0.4

0.8

800°C

Cel

l Pot

entia

l (V

)

Time (h)

350mA/cm2air/H2-H2O

- $24 each for 25 cell batch- 350-425mW/cm2 at 0.7V- 25-38%/1000h degradation

Tape-cast cells from NIMTEwww.sofc.com.cn

0.0 0.5 1.0 1.5 2.00.0

-0.1-0.2-0.3-0.4-0.5

Z" (o

hm c

m2 )

Z' (ohm cm2)

800°C 750°C850°C

0.0 0.5 1.0 1.50.0

-0.5

Z" (o

hm c

m2 )

Z' (ohm cm2)

800°C

Future Directions

- Mix LSC (high conductivity) with LSCuF (good sintering)

- Sintering aids to improve bonding and mechanical properties after firing at 1000°C

- Identify new candidates from outside the SOFC world

- Reactive sintering

- Improve uniformity of CCM printing procedure

- In-depth post-mortem analysis

Thanks to Joe Stoffa and Briggs White

This work was supported by the US DOE through project MSD-NETL-01

Contact InfoMike Tucker mctucker@LBL.GOV

Lut DeJonghe lcdejonghe@LBL.GOV