11

Degradation Mechanisms in Degradation Mechanisms in Solid Oxide Electrolysis Anodes and Bond Layer:Solid Oxide Electrolysis Anodes and Bond Layer:

CrCr--Poisoning and Cation TransportPoisoning and Cation Transport

Vivek I. Sharma and Vivek I. Sharma and Bilge YildizBilge Yildiz

Department of Nuclear Science and EngineeringDepartment of Nuclear Science and EngineeringMassachusetts Institute of Technology, Cambridge, MAMassachusetts Institute of Technology, Cambridge, MA

International Workshop on High Temperature

Electrolysis Limiting

Factors, June

9-10, Karlsruhe, Germany.

Motivation:–

Enable durable, efficient, cost-competitive steam electrolysis technology.

–

Help identify degradation modes of SOECs

tested thus far.

Outline:Challenges, and research in:

degradation mechanisms in the

anode and bond layer materials in present SOECs

Post-mortem analysis of stack-cellsElectrochemical tests

Outstanding questions, and research needs

*Properietary

Information

SOECs

H2

O + 2e-

→ O2-

+ H2

CO2

+ 2e-

→ CO + O2-

O2-

→ ½ O2

+ 2e-

Cathode

Anode

Degradation rate ~40%/1000hrs is greater than in SOFCs

(<1.5%)

Air in

Air out

O2H2

H2

O

e- e-

Electrolyte

[ScSZ] Anode*Cathode

[Ni / Sc2

O3

-ZrO2 (ScSZ) Cermet]

O=

Bond Layer

[La0.8

Sr0.2

CoO3 (LSC)]

0

10

20

30

40

50

0.0 500.0 1000.0 1500.0 2000.0

Time (hours)

Cur

rent

, A

Time, hours

Stoots, O’Brien, Elangovan, Hartvigsen, Herring et al. 2008

0 200 400 600 800 1000

Time (hours)

10

15

20

Cur

rent

(A) 800oC 830oC

0 200 400 600 800 1000

Time (hours)

10

15

20

Cur

rent

(A)

0 200 400 600 800 1000

Time (hours)

10

15

20

Cur

rent

(A)

0 200 400 600 800 1000

Time (hours)

10

15

20

Cur

rent

(A) 800oC 830oC

O’Brien et al, Nuclear Technology, 2006

Components of the stack repeat unit

Interconnect (airside)–

Native scale (Cr2

O3

or AB2

O4

spinel)–

Bond (protective) layer

Flow field (not shown)Bond layerAnode: Oxygen Electrode ElectrolyteCathode: Steam/hydrogen electrodeBond layerFlow field Interconnect (H2O/H2 side)–

Native Scale –

Bond layerSeal

Interconnect

Bond (protective) layer

Bond layer

Oxygen electrode

Flow field here ↑↓

Electrolyte

Steam/H2

electrode

not shown

Composite SEM photo

Carter et al. ACS, 2008

Suspect causes of stack degradation

Oxygen bond layer–

Cr “contamination”

from interconnect–

Delamination

from the oxygen electrodeOxygen electrode–

Delamination from the electrolyteElectrolyte–

Possible aging due to Tetragonal → Monoclinic transitionSteam/hydrogen electrode–

Silica deposition from seal–

Nickel oxidation–

Mn diffusion from interconnect

Single cell tests suggest that SOEC mode has greater degradation rates than SOFC mode governing mechanisms differ.

Objective: Identify and control degradation mechanisms in SOEC anode / bond layer

E.g. Cation interdiffusion between cathode and electrolyte of SOFCs

E.g. Presence of Cr-containing species in SOFC cathode

1. Bond Layer Dissociation

Cation segregation Local variations in cation ratios at

the surface

2. Interdiffusion

of Cr-containing species from interconnects into bond layer and anode

Cr distribution across the bond layer and anode

Relationship between Cr and

local composition

(Cr,Mn)3

O4

Grosjean et al. SSI, 2006

Salvador et al. SECA report, 2006

Approach

Technique Objective

Raman SpectroscopyIdentification of secondary phases

formed

on the bond layer

Nanoprobe

Auger Electron Spectroscopy (NAES)

Electrode surface chemistry and microstructure

and its variation across the cross section at a small

scale (μm-nm)

Focused Ion Beam (FIB)Selectively choose the interface

of interest to prepare TEM samples

Energy Dispersive X-Ray Spectroscopy (EDX) / Transmission Electron

Microscopy (TEM)

High resolution

identification of the chemical composition and

secondary structures

formed

LSCAnode

LSC

ScSZ

Top View

C/S View

10cm

Oxygen electrode microstructural change

Microstructure evolved from round grains for as-prepared anode to faceted grains for used anode. Small changes evolved in the surface chemistry – not disclosed here.

Anode

LSC

Delamination and/or weak interface

3

20000 X4/18/2008

1

1.0 µm

As-prepared anode

Used anode

The weakly bound and delaminated interface could prevent Cr solid-state diffusion into the anode Cr not found on the anode surfaceThis can enable the anode stability, however, not desirable for electronically activating the anode.

Cr-contamination and dissociation of LSC bond layer

Regions or crystallites with high Cr content exist. Cr content varies locally (average 2-10%)

Drastic variations in La/Co at surface seen even at a local level and no Sr-presence was found on the surface of the bond layer.

–

The as-sintered LSC has A/B~6 consistently

SEM image of cross-section in the LSC bond layer of an

operated cell. AES spectra from the 3 points is on the right.

O Cr La Co La/CoArea 1 0.70 0.08 0.08 0.13 0.61Area 2 0.57 0.01 0.38 0.04 9.50Area 3 0.61 0.02 0.29 0.07 4.14

O Cr La Co La/CoArea 1 0.70 0.08 0.08 0.13 0.61Area 2 0.57 0.01 0.38 0.04 9.50Area 3 0.61 0.02 0.29 0.07 4.14

Cr association with LSC cations - surface

No trends in Cr distribution as a function of depth detected by the AES.

No Sr-presence was found on the surface of the bond layer across the entire

depth of LSC.

Interconnect/LSC interface

LSC Middle region

LSC/Anode interface

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3

CrCoLa

Cat

ion

frac

tion,

at-%

Spatial scale for the LSC dissociation: Site-specific chemical and structural characterization

TEM sample B,Near the interconnect

interfaceTEM sample A,Near the anode

interfaceFIB-prepared TEM

membraneAnode Bond Layer, LSC

Focused Ion Beam (FIB) to selectively choose the interfaces of interest for high resolution structural and chemical studies :–

LSC-interconnect, LSC-Anode, Anode-SSZ Electrolyte

LSC Anod

e

SEM view of a TEM membrane being prepared using FIB

TEM membrane

High resolution image of a FIB-

prepared TEM

sample

Ga-ion damage was unavoidable

Dissociation of the LSC at the nanoscale

LSC dissociation evident even at the nanoscale.

Regions rich in Cr have higher La and lower Co.

Low Sr content, even in bulk.

1.2μm

CrCo

La Sr

600nm

CrCo

La Sr

Structural dissociation of LSC bond layer: Secondary phases

Raman spectra, showing secondary phases

with low conductivity

Phase Conductivity (S/cm)

La0.6

Sr0.4

CoO3

at 8000C 1.6x103

[9]Co3

O4

at 8000C 3.9x101

[4]Cr2

O3

at 10000C 1.0x10-3

[6]LaCrO3

at 8000C 3.4x10-1

[7]

UsedReference

Dissociation of the LSC at the nanoscale

Element

Bond Layer, Atomic % (TEM/EDX)

Near the anode Near the interconnect

Cr-rich region

Co-rich region

Cr-rich region

Co-rich region

La 54.92 22.45 50.00 23.08

Sr 2.81 2.85 4.16 3.84

Co 15.49 63.63 12.50 57.60

Cr 27.03 10.20 33.33 15.38

Co/Cr 0.57 6.24 0.38 3.74

La/Cr 2.03 2.20 1.50 1.50

? La2 CrO6, Co3 O4 ? ? La2 CrO6 , LaCrO3Cr2 O3, Co3 O4 ?

Transport of Sr and Co to LSC top

Sr-rich

Co-rich

Cr diffusing into LSCSr and Co transported to the LSC top –

due to Cr, or electronic/ionic current?

41%

>95%,17%.<5%,74%

Cr

O=

e-

SSZ

LSCCoSr

Anode

LSC dissociation via long-range transport of cations?

Mechanism for the LSC dissociation with local variations in La/Co ratio, and transport of Sr

and Co several tens of microns, to the top.

LSC bond

Anode 1

Anode 2

SSZElectrolyte

Cr distribution in SOFC

SOFC [Carter et al. ACS 2008]

Cr

O= e-

SSZ

LSC

CoSr

Anode

Cation distribution in SOEC

Hypotheses: 1) Electronic or ionic current drives the Sr

and Co cations

out of the LSC

structure to the interconnect interface.2) Cr-driven thermodynamics favors the Sr-

and Co-

rich phases near the

Cr-containing interconnects.

Reference half-cell electrochemistry w/o Cr

Degradation in the absence of Cr

Anode Electrolyte Reference Electrodes

CathodeA = 0.5cm2

0.2

0.25

0.3

0.35

0.4

0.45

0 100 200 300

Time (hours)

Pote

ntia

l (V

)

T = 830oCI = 400mA/cm2

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Z_real (ohm)

Z_i

mg

(ohm

)

0.10 Hz

10 Hz

100 Hz

1000 Hz

0.10 Hz5 Hz3500 Hz

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Z_real (ohm)

Z_i

mg

(ohm

)

100 Hz1000 Hz

10000 Hz

0.10 Hz10 Hz

750 Hz

30 Hz

O2

O2

Anode Cathode

Worse degradation in anode compared to cathode.

With Au mesh contacts

Chemical changes in LSC in the reference cells

No chemical differences detectable by the AES between the thermal reference cell, electrochemical reference cell, and the regions in each electrode.

20000 X 10.0 keV

23

1.0 µm

LSC Anode SSZ

20μm

No delaminationin the microstructure

Therm

Echem

Driving force for the Cr-reaction here?Electrochemical reduction of Cr-vapor phases, CrO3 or CrO2(OH) to Cr2O3, and block active sites at the cathode, in SOFC [Hilpert et al., 1996], in competition with O2 reduction under similar cathodic polarization potentials.A non-electrochemical process, kinetically limited by the nucleation reaction between the CrO3 or CrO2(OH) and nucleation agents [Jiang et

al., 2006].

62323

33

332

CrOLaOLaCrOLaCrOLaCrO

:ONCr

CrOOCr

x

(g)(oxide)

→+→+

−−

→

LSC

La2 O3

CrO3

LSC

La2 O3La2 CrO6

Here, LaO-

and SrO-species due to surface segregates?

Bond layer degradation mechanism differ from those proposed for SOFCs?

SOFC:

Formation of an

oxide scale at the bond layer/ interconnect interface is responsible [Yang et al. JPS 2006]. Oxide scale conductance depends on the bond layer composition.

SOEC:

Dissociation of LSC, favoring the Sr-

and Co-

rich phases near the

Cr-containing interconnects.Cr-driven chemical reaction thermodynamics and kinetics in electrolytic conditions.

LSC bond

Anode 1

Anode 2

SSZElectrolyte

Cr distribution in SOFC

SOFC [Carter et al. ACS 2008]

Cr

O= e-

SSZ

LSC

CoSr

Anode

Cation distribution in SOEC

Summary

LSC bond layer dissociation at the nanoscale, local variations in secondary phases, evidenced in the La/Co ratio.

Sr and Co have transported several tens of microns, to the top.

–

Likely driven by Cr-related thermodynamics and kinetics in electrolytic conditions.

Cr distributed throughout the bond layer, in various phases, not only as Cr2O3 blocking species.

Degradation rate and/or mechanism in SOEC, with and without Cr-interconnect, differ from that in SOFC.

Future needs

A combination of applied and fundamental studies to consistently quantify the causes and effects of degradation mechanisms.

–

In situ structural, chemical, and electrochemical characterization of interfaces and bulk, to correlate the evolving state to its reasons

X-ray scattering and spectroscopy, high resolution site-specific microscopy and chemical analyses, and surface chemistry.

–

Use of model material configurations, in addition to the conventional cells, to isolate the complicated degradation mechanisms.

–

Theoretical studies to understand the degradation mechanisms at a more fundamental level, to predict durable electrode compositions.

Phase stability, cation mobility, at the atomistic scale.

2323

AcknowledgementsAcknowledgements

Idaho National Laboratory, NHI and NGNP programs for Idaho National Laboratory, NHI and NGNP programs for financially supporting our work.financially supporting our work.

CeramatecCeramatec Inc. for providing the samples for Inc. for providing the samples for characterization in this work.characterization in this work.

Drs. S. Herring (INL), S. Elangovan, J. Hartvigsen Drs. S. Herring (INL), S. Elangovan, J. Hartvigsen ((CeramatecCeramatec Inc.), and D. Carter (ANL) for technical Inc.), and D. Carter (ANL) for technical discussions and feedback.discussions and feedback.

Center for Materials Science and Engineering at MIT, and Center for Materials Science and Engineering at MIT, and Center for Nanoscale Systems at Harvard University were Center for Nanoscale Systems at Harvard University were used in part for AES, FIB, and TEM/EDX.used in part for AES, FIB, and TEM/EDX.

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