Degradation Mechanisms in Solid Oxide Electrolytic Cell (SOEC) Anodes Vivek Inder Sharma and Bilge Yildiz Department of Nuclear Science and Engineering Massachusetts Institute of Technology Fourth Information Exchange Meeting on the Nuclear Production of Hydrogen, Chicago, April 14, 2009

SOECs e- e- H2O O2- O2 H2 Air in An electrochemical device that produces H2 and other fuels using electrical energy H2O O2- O2 H2 H2O + 2e- → O2- + H2 CO2 + 2e- → CO + O2- O2- → ½ O2 + 2e- Air out Cathode Cathode [Ni-Sc2O3-ZrO2 (SSZ) Cermet] Electrolyte [SSZ] Anode Bond Layer [La0.8Sr0.2CoO3 (LSC)] Anode* Degradation rate of ~15%/1000hrs is greater than in SOFCs (<2%) [1] *Properietary Information

Research Objective: Investigation of degradation mechanisms in SOEC anodes 2. Interdiffusion of Cr-containing species from interconnects into bond layer and anode 1. Bond Layer Dissociation Cation segregation Local variations in cation ratios at catalyst surface Cr distribution across the bond layer and anode Relationship between Cr and local composition Cation interdiffusion between cathode and electrolyte of SOFCs [2] Presence of Cr-containing species in SOFC cathode [3] Cr-Mn Spinel

Approach Technique Objective Raman Spectroscopy Preliminary identification of secondary phases formed on the surface of the bond layer Nanoprobe Auger Electron Spectroscopy (NAES) Electrode surface chemistry and microstructure and its variation across the cross section at a small scale (m-nm) Focused Ion Beam (FIB) Selectively choose the interface of interest to prepare TEM samples from Energy Dispersive X-Ray Spectroscopy (EDX) Transmission Electron Microscopy (TEM) High resolution identification of the chemical composition and secondary structures formed SSZ 10cm Top View LSC Anode LSC C/S View

Structural dissociation of the bond layer: Secondary Phases Constituent Conductivity (S/cm) Co3O4 at 8000 C 39 [4] Cr2O3 (10000 C) 0.001 [5] LaCrO3 at 8000 C 0.34 [6] Air Electrode at 8000 C 159 [7] La0.6Sr0.4CoO3 at 8000 C 1600 [8] Raman spectra, showing different secondary phases with low conductivity

Cr-contamination and dissociation of LSC bond layer La/Co Area 1 0.70 0.08 0.13 0.61 Area 2 0.57 0.01 0.38 0.04 9.50 Area 3 0.02 0.29 0.07 4.14 SEM image of a region in the LSC bond layer of an operated cell. AES spectra from the 3 points is shown on the left ‘Crystallites’ having high Cr content exist. Cr content varies locally (1-8% in general, ~30% maximum found) Drastic variations in La/Co at catalyst surface seen even at a local level

Weak interface between anode and bond layer LSC Delamination and weak interface between anode and LSC found across the whole cell Anode Cr not seen in the anode The weakly bound and delaminated interface can prevent Cr from diffusing into the anode in solid state This can enable the anode to stay stable, however the weak interface is not desirable for electrically activating the anode

Cr association with LSC cations - surface LSC/Interconnect interface LSC Middle region LSC/Anode interface Regions with the higher Cr content have relatively higher Co and lower La content on the catalyst surface Nucleation agents for Cr deposition in SOFCs have been identified [9]. Cr and Co could have a similar relationship here No Sr-presence was found on the surface of the bond layer

Transport of Sr and Co to LSC surface Sr-rich 41% Co-rich Sr Cr Co e- LSC O= Anode 20%,4% resp. SSZ 8%,41% resp. Cr diffusing into LSC Sr and Co transported to the LSC surface – due to Cr degradation or electronic current

The FIB technique for preparing TEM sample Advantages Challenges Precise selection of target area Ga implantation and damage on the sample [10] Can be used for all types of materials – soft, hard or combinations Porous bond layer – tough to lift out without breaking Anode LSC 2 μm SEM view of a TEM membrane being prepared using FIB

Dissociation of the LSC at the nanoscale Cr 600nm Co Sr La Element Atomic fraction La 0.52 Sr .01 Co 0.16 Cr 0.29 LSC dissociation evident even at a nanoscale Regions rich in Cr have higher La and lower Co - in contrast to surface results by NAES Low Sr content, even in bulk

Concluding Remarks Mechanism different from those proposed for SOFCs SOFC: Formation of an oxide scale at the bond layer/interconnect interface is responsible [12]. Oxide scale conductance depends on the bond layer composition. SOEC: Bond layer dissociation, severe variations in La/Co ratio. Sr and Co have transported several tens of microns, to the top-due to Cr related degradation or electronic current Cation distribution in SOEC Cr distribution in SOFC LSC Cr Sr Co LSC bond Anode 1 Anode 2 SSZ Electrolyte e- Fundamental understanding of the degradation mechanisms in SOECs are leading to evolutionary improvements in the HTSE performance within the existing framework of materials, device design, and operating temperature (>800oC). What else to explore? … O= Anode SSZ SOFC [11]

Future Work To enable a more durable bond layer composition, we will first isolate the reason for the cation transport mechanism in LSC, using half-cells operated under controlled conditions, Thermodynamically dependent on presence of Cr? Ionic and electronic current?

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Acknowledgements Our sincere thanks to Our research sponsors DoE, Next Generation Nuclear Energy DoE, Nuclear Hydrogen Initiative Prof. Harry Tuller (MIT) - Thesis Reader D. Carter (Argonne National Laboratory) for discussions and S. Elangovan (Ceramatec Inc.) for providing us the samples Center for Materials Science and Engineering (CMSE), MIT and Center for Nanoscale Systems (CNS), Harvard for the use of characterization facilities