Modeling Water Reduction on Doped Ceria Thin Films
Authors: Valdes-Espinosa Honorio , Stuve Eric M. , Adler Stuart B. 
Published: February 2020

In recent years, there has been an increasing interest in understanding electrochemical reactions on oxide surfaces. The advent of techniques such as ambient-pressure x-ray photoelectron spectroscopy (AP-XPS) has helped to provide a better understanding of aspects such as the nature of the surface1-2 and the species involved in the reactions3-4. However, a quantitative connection between macroscopic rates and the surface structure and dynamics remains elusive. Among these reactions, water electrolysis has attracted interest due to favorable kinetics when compared to low temperature electrolysis5. Doped ceria, a mixed ionic and electronic conductor, is a potential candidate for this reaction due to higher stability and more efficient use of the electrode area compared to currently available materials, such as nickel/yttria-stabilized zirconia cermet6. Recent spectroscopic studies of water reduction/oxidation on doped ceria have shed valuable new insights about this reaction, but uncertainties remain in the interpretation of these measurements (in terms of a rate-determining step) and the connection to macroscopic rates3,4.

In order to better connect spectroscopic observations to macroscopic rates, we have developed a thermodynamically self-consistent elementary kinetic model for H2/H2O exchange on the surface of ceria. This model considers both the bulk defect thermodynamics, as well as the effects of cation reconstruction/segregation at the surface and space charge effects, both of which are expected to alter the defect chemistry and concentrations of reactive intermediates.  We have used this model to explore a variety of mechanisms and rate-limiting steps proposed in the literature3,4, and to predict the expected nonlinear electrochemical rate law. This includes prediction of the nonlinear electrochemical impedance (NLEIS) response of thin film ceria electrodes, in an approach similar to our previous studies of O2 reduction kinetics on perovskite thin films8. 

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