AN INVESTIGATION OF SOLID OXIDE ELECTROCHEMICAL CELL CHEMISTRY: AN OPERANDO SPECTROSCOPIC APPROACH
Eichhorn, Bryan W
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Investigations of electrochemical reactions on solid oxide based electrochemical cells under operating conditions are described in this dissertation. <italic>Operando</italic> capabilities of ambient pressure X-ray photoelectron spectroscopy are utilized to study the specially designed single-sided electrochemical cells and to extract detailed information of cell processes. Mixed-ionic-electronic-conducting materials of cerium oxide are used as electrocatalysts. Mappings of local surface potentials and length scales of the electrochemically active region on thin film CeO<sub>2-x</sub> electrocatalysts are obtained in studies of water electrolysis and hydrogen electro-oxidation reactions. Electrochemically active region is shown extend 150 - 200 μm away from current collectors. Foreign elements of silicon and carbon on the electrode are found as excellent trace markers of surface potentials and active region. The observations of transient intermediates (OH<super>-</super> and Ce<super>3+</super>) accumulation in the active region on CeO<sub>2-x</sub> electrode allow for identification of the rate-limiting charge transfer process (H<sub>2</sub>O + Ce<super>3+</super> ↔ Ce<super>4+</super> + OH<super>-</super> + H∙) in both H<sub>2</sub>O electrolysis and H<sub>2</sub> electro-oxidation reactions. The observed potential separation of the adsorbed OH<super>-</super> and incorporated O<super>2-</super> ions is interpreted by the effective double layer model and the surface potential step model, which provides insights into the gas-solid interface chemistry. From studies of carbon dioxide electrolysis and carbon monoxide electro-oxidation over CeO<sub>2-x</sub>-based solid oxide electrochemical cells, carbonate are identified as reaction intermediates in both electrochemical reactions. The steady-state concentration increase of CO<sub>3</sub><super>2-</super> during CO<sub>2</sub> electrolysis and its slight decrease during CO electro-oxidation on CeO<sub>2-x</sub> electrode suggest the charge transfer process to/from CO<sub>3</sub><super>2-</super> is a rate-limiting process. The graphitic carbon formed on the CeO<sub>2-x</sub> electrode surfaces during CO<sub>2</sub> electrolysis extends the electrochemically active region away from the Au electron source by enhancing the electronic conductivity of cerium oxide. Measurements of overpotentials at the electrode-electrolyte interface reveal very high charge transfer resistance at the interface for CO<sub>2</sub> electrolysis that dominates the cell losses in these environments. Mechanistic information extracted from investigating these solid oxide electrochemical cells provides insight into the high temperature surface chemistry on mixed-ionic-electronic-conducting ceria electrodes, and is valuable and guiding to the development of solid oxide fuel cells due to the similar chemical processes occurring in these electrochemical devices.