Degradation Mechanisms in
Solid Oxide Electrolysis Anodes and Bond Layer:
Cr-Poisoning and Cation Transport
Vivek I. Sharma and Bilge Yildiz
Department of Nuclear Science and Engineering
Massachusetts Institute of Technology, Cambridge, MA
International Workshop on High Temperature Electrolysis
Limiting Factors, June 9-10, Karlsruhe, Germany.
– Enable durable, efficient, cost-competitive steam
– Help identify degradation modes of SOECs tested
Challenges, and research in:
degradation mechanisms in the anode and bond layer
materials in present SOECs
Post-mortem analysis of stack-cells
Outstanding questions, and research needs
H2O + 2e- → O2- + H2
CO2 + 2e- → CO + O2- O= O2
O2- → ½ O2 + 2e- Anode
Anode* Bond Layer
[Ni / Sc2O3-ZrO2 [ScSZ]
(ScSZ) Cermet] [La0.8Sr0.2CoO3
Stoots, O’Brien, Elangovan, (LSC)]
20 800oC 830oC Hartvigsen, Herring et al. 2008
20 Degradation rate ~40%/1000hrs
O’Brien et al, Nuclear Technology, 2006 is greater than in SOFCs (<1.5%)
0 200 400 600 800 1000
Time (hours) 1500.0
Time, hours (hours)
Components of the stack repeat unit
Composite SEM photo
Interconnect (airside) Interconnect
– Native scale (Cr2O3 or AB2O4 spinel)
– Bond (protective) layer
Flow field (not shown) Bond (protective) layer
Flow field here ↑↓
Anode: Oxygen Electrode
Electrolyte Bond layer
Cathode: Steam/hydrogen electrode
Interconnect (H2O/H2 side)
– Native Scale
– Bond layer
Carter et al. ACS, 2008 Steam/H2 electrode
Suspect causes of stack degradation
Oxygen bond layer
– Cr “contamination” from interconnect
– Delamination from the oxygen electrode
– Delamination from the electrolyte
– Possible aging due to Tetragonal → Monoclinic transition
– 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
2. Interdiffusion of Cr-containing species
1. Bond Layer Dissociation from interconnects into bond layer and anode
Cation segregation Local variations Cr distribution Relationship
in cation ratios at across the bond between Cr and
the surface layer and anode local composition
E.g. Cation interdiffusion between E.g. Presence of Cr-containing
cathode and electrolyte of SOFCs species in SOFC cathode
Salvador et al.
Grosjean et al. SECA report,
SSI, 2006 2006
Technique Objective 10cm
Identification of secondary phases Top View
formed on the bond layer
Electrode surface chemistry and LSC
Nanoprobe Auger Electron microstructure and its variation
Spectroscopy (NAES) across the cross section at a small
Selectively choose the interface of
Focused Ion Beam (FIB)
interest to prepare TEM samples Anode
Energy Dispersive X-Ray
High resolution identification of the C/S View
Spectroscopy (EDX) /
chemical composition and
secondary structures formed
Oxygen electrode microstructural change
anode Delamination and/or
20000 X 1.0 µm
The weakly bound and delaminated
interface could prevent Cr solid-state
Microstructure evolved from round
diffusion into the anode Cr not found
grains for as-prepared anode to
on the anode surface
faceted grains for used anode. Small
This can enable the anode stability,
changes evolved in the surface
however, not desirable for electronically
chemistry – not disclosed here.
activating the anode.
Cr-contamination and dissociation of LSC bond layer
O Cr La Co La/Co
Area 1 0.70 0.08 0.08 0.13 0.61
Area 2 0.57 0.01 0.38 0.04 9.50
Area 3 0.61 0.02 0.29 0.07 4.14
SEM image of cross-section in
the LSC bond layer of an
operated cell. AES spectra from
the 3 points is on the right.
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
Cr association with LSC cations - surface
Cation fraction, at-%
0 1 2 3
Interconnect/LSC LSC Middle LSC/Anode
interface region interface
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.
Spatial scale for the LSC dissociation:
Site-specific chemical and structural characterization
TEM sample B,
TEM sample A, Near the interconnect
Near the anode interface
Bond Layer, membrane
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
TEM was unavoidable
SEM view of a TEM membrane High resolution image of a FIB-
being prepared using FIB prepared TEM sample
Dissociation of the LSC at the nanoscale
La Sr La Sr
Co Cr Co Cr
LSC dissociation evident even at the nanoscale.
Regions rich in Cr have higher La and lower Co.
Low Sr content, even in bulk.
Structural dissociation of LSC bond layer:
Raman spectra, showing secondary La0.6Sr0.4CoO3 at 8000C 1.6x103 
phases with low conductivity
Co3O4 at 8000C 3.9x101 
Cr2O3 at 10000C 1.0x10-3 
LaCrO3 at 8000C 3.4x10-1 
Dissociation of the LSC at the nanoscale
Bond Layer, Atomic % (TEM/EDX)
Element Near the anode Near the interconnect
Cr-rich Co-rich Cr-rich Co-rich
region region region 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
? La2CrO6, Co3O4 ? ? La2CrO6, LaCrO3
Cr2O3, Co3O4 ?
Transport of Sr and Co to LSC top
Sr Co e- Cr
<5%,74% >95%,17%. SSZ
Cr diffusing into LSC
Sr and Co transported to the LSC top
– due to Cr, or electronic/ionic
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.
Cation distribution in SOEC Cr distribution in SOFC
Anode Anode 2
SOFC [Carter et al. ACS 2008]
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
Chemical changes in LSC in the reference cells
LSC Anode SSZ
No delamination 2
20μm 20000 X 10.0 keV 1.0 µm
No chemical differences detectable by the AES between the thermal reference
cell, electrochemical reference cell, and the regions in each electrode.
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
A non-electrochemical process, kinetically limited by the nucleation
reaction between the CrO3 or CrO2(OH) and nucleation agents [Jiang et
Here, LaO- and SrO-species due to
Cr2O3(oxide) → CrO3(g) surface segregates?
Cr − N − Ox: La2O3 La2CrO6
CrO3 + La → LaCrO3
CrO3 + La2O3 → La2CrO6
Bond layer degradation mechanism differ from those
proposed for SOFCs?
SOFC: Formation of an oxide SOEC: Dissociation of LSC, favoring
scale at the bond layer/ the Sr- and Co- rich phases near the
interconnect interface is Cr-containing interconnects.
responsible [Yang et al. JPS 2006]. Cr-driven chemical reaction
Oxide scale conductance thermodynamics and kinetics in
depends on the bond layer electrolytic conditions.
Cation distribution Cr distribution
in SOEC in SOFC
Anode Anode 2
SOFC [Carter et al. ACS 2008]
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
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.
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.
Idaho National Laboratory, NHI and NGNP programs for
financially supporting our work.
Ceramatec Inc. for providing the samples for
characterization in this work.
Drs. S. Herring (INL), S. Elangovan, J. Hartvigsen
(Ceramatec Inc.), and D. Carter (ANL) for technical
discussions and feedback.
Center for Materials Science and Engineering at MIT, and
Center for Nanoscale Systems at Harvard University were
used in part for AES, FIB, and TEM/EDX.
References for SOEC degradation
1. O’ Brien et al, Nuclear Technology 2006, S. Elangovan et al, personal commn. 2008
2. A. Grosjean, O. Sanseau, V. Radmilovic, A. Thorel, “Reactivity and diffusion between
LSM and ZrO2 interfaces in SOFC cores by TEM analyses on FIB samples”, Solid State
Ionics 177 (2006) 1977 – 1980
3. P. Salvador, S. Wang, “Investigations of Cr contamination in SOFC cathodes using TEM”,
FY 2007 Annual Report, Office of Fossil energy Fuel Cell Program 59 – 63
4. Sakamato, Yoshinaka, Hirato and Yamaguchi, “Fabrication, Mechanical Properties, and
Electrical Conductivity of Co3O4 Ceramics”, Journal of American Ceramic Society, 80 
5. Hu, Li, Huang and Chen, “Improve the electrochemical performances of Cr2O3 anode for
lithium ion batteries”, Solid State Ionics 177 (2006) 2791 – 2799
6. Atkinson, Levy, Roche and Rudkin, “Defect properties of Ti-doped Cr2O3”, Solid State
Ionics 177 (2006) 1767 – 1770
7. Ong, Wu, Liu and Jiang, “Optimization of electrical conductivity of LaCrO3 through
doping: A combined study of molecular modeling and experiment”, Applied Physics
Letters 90, 044109 (2007)
8. Email communication with Ceramatec
9. P. Hjalmarsson, M. Soggard, A. Hagen and M. Mogensen, “Structural properties and
electrochemical performance of strontium- and nickel-substituted lanthanum cobaltite”,
Solid State Ionics 179 (2008) 636-646
10. Y. Zhen, A. Tok, F. Boey et al., “Development of Cr-tolerant cathodes of solid oxide fuel
cells”, Electrochemical and Solid State Letters 11 (3) B42-B46 (2008)
11. J. Mayer, L. A. Gianuzzi, T. Kamino and J, Michael, “TEM sample preparation and FIB-
induced damage”, MRS Bulletin 32 (May 2007) 400-407
12. Z. Yang, G. Xia, P. Singh and J. W. Stevenson, “Electrical contacts between cathodes and
metallic interconnects in solid oxide fuel cells”, J. Power Sources 155 (2006) 246-252
13. D. Carter et al, Chemical Sciences and Engineering Division, Argonne National Laboratory