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TITLE HIGH TEMPERATURE REVERSIBLE SOLID OXIDE CELLS FOR GRID SCALE STORAGE OF RENEWABLE ELECTRICITY
ADVISOR: Srikanth Gopalan, MSE, ME
COMMITTEE: Soumendra Basu ME, MSE; Uday Pal, MSE, ME; Michael Gevelber, ME, MSE, SE
CHAIR: Jörg Werner, ME, MSE
ABSTRACT: Intermittency of energy availability from renewable energy resources is a long standing challenge as we transition away from fossil fuels. There is a dire need for storing energy during periods of excess renewable power generation, and the ability to utilize the stored energy during periods of peak demand. Reversible solid oxide cells (RSOCs) can be an efficient solution to this societal challenge if the device can be operated reversibly between fuel cell (SOFC) and electrolysis modes (SOEC). One of the barriers to designing an efficient RSOC device is the non-availability of a highly reversible oxygen electrode material that is satisfactory under both modes of operation, i.e. SOFC and SOEC.
Mixed ionic and electronic conducting (MIEC) rare earth nickelate materials, i.e., neodymium nickelate (NNO), lanthanum nickelate have high oxygen surface exchange coefficients and bulk diffusion coefficients which makes them eminently suitable as RSOC oxygen electrodes. Furthermore, the reactivity of the solid electrolyte, namely yttria-zirconia with rare-earth nickelates leads to insulating phases at the oxygen electrode-electrolyte interface. Thus an appropriate barrier layer between the zirconia-based electrolyte and the oxygen electrode is required. The influence of such a barrier layer on electrode polarization is explored in this research.
In this research, RSOCs featuring NNO as the oxygen electrode are operated under various fuel and oxidant compositions in both SOFC, SOEC, and mode-switched reversible conditions to characterize polarization losses. The roles of the oxygen electrode active layer and current layer thickness on electrode polarization losses are explored. Single cells with an optimized oxygen electrode architecture have been tested in reversible, and electrolysis-only conditions for 500 h to assess their long-term performance stability. Detailed analysis of the electrochemical impedance spectra using equivalent circuit modeling and distribution of relaxation time analysis, along with micro structural characterization using scanning electron microscopy, provides insights into the rate-limiting steps and degradation mechanisms of the different oxygen electrodes and barrier layer cells. Based on these studies, suggestions are made for optimum cell design and operating conditions for RSOCs.
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