Abstract
Mixed conducting perovskite oxides and related structures serving as electrodes for electrochemical oxygen incorporation and evolution in solid oxide fuel and electrolysis cells, respectively, play a significant role in determining the cell efficiency and lifetime. Desired improvements in catalytic activity for rapid surface oxygen exchange, fast bulk transport (electronic and ionic), and thermo-chemo-mechanical stability of oxygen electrodes will require increased understanding of the impact of both bulk and surface chemistry on these properties. This review highlights selected work at the International Institute for Carbon-Neutral Energy Research (I2CNER), Kyushu University, set in the context of work in the broader community, aiming to characterize and understand relationships between bulk and surface composition and oxygen electrode performance. Insights into aspects of bulk point defect chemistry, electronic structure, crystal structure, and cation choice that impact carrier concentrations and mobilities, surface exchange kinetics, and chemical expansion coefficients are emerging. At the same time, an understanding of the relationship between bulk and surface chemistry is being developed that may assist design of electrodes with more robust surface chemistries, e.g., impurity tolerance or limited surface segregation. Ion scattering techniques (e.g., secondary ion mass spectrometry, SIMS, or low energy ion scattering spectroscopy, LEIS) with high surface sensitivity and increasing lateral resolution are proving useful for measuring surface exchange kinetics, diffusivity, and corresponding outer monolayer chemistry of electrodes exposed to typical operating conditions. Beyond consideration of chemical composition, the use of strain and/or a high density of active interfaces also show promise for enhancing performance.
Highlights
Solid Oxide Fuel and Electrolysis CellsFuel cells directly convert the chemical energy of fuel and oxygen to electrical power with potential for significantly higher efficiency than conventional heat engines [1,2,3,4,5]
Instabilities may arise from: decomposition, ordering, changes of thetemperature electrode phase itself if Potential it is metastable in the operating conditions; reactions between or crystal structurephases changesorofbetween the electrode phase itself if itother is metastable in the operating conditions; composite electrode electrodes and cell components; changes in surface reactions between composite electrode phases or between electrodes and other cell components; chemistry arising from intrinsic surface segregation, extrinsic surface poisoning, or a combination of changes in surface chemistry arising from intrinsic surface segregation, extrinsic surface poisoning, both; microstructural coarsening; delamination from the electrolyte in Solid Oxide Electrolysis Cells (SOECs) mode when high oxygen or a combination of both; microstructural coarsening; delamination from the electrolyte in SOEC
One outcome of the model sample, ex situ studies has been the confirmation of A-site cation termination/segregation widely observed in the solid oxide fuel cells (SOFCs)/SOEC/catalysis community using a variety of other approaches that have slightly less surface sensitivity, such as total reflection X-ray fluorescence, nanoprobe Auger spectroscopy, secondary ion mass spectrometry (SIMS), scanning electron microscopy with energy-dispersive spectroscopy, and X-ray photoelectron spectroscopy [73,101,102,103,104]
Summary
Fuel cells directly convert the chemical energy of fuel and oxygen to electrical power with potential for significantly higher efficiency than conventional heat engines [1,2,3,4,5]. The materials used for SOECs are similar to those of SOFCs; compared with SOFCs, greater tolerance for severe oxidation and tight gas sealing are required [12,13] For both types of cell, oxygen electrodes play a highly important role in determining efficiency, and superior catalytic activity, transport properties, and stability are required. In this non-exhaustive review, the role and fundamental properties required of oxygen electrode materials in SOFC and SOEC modes are explained, examples of oxygen electrode chemistries are given, and investigations into the impact of both bulk and surface chemistry (such as surface segregation) in determining performance and degradation are summarized. University, and selected work from the broader community is discussed and highlighted
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