Modeling Water Reduction on Doped Ceria Thin Films

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 surface^1-2 and the species involved in the reactions^3-4. 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 rates^3,4.

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 literature^3,4, 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. 

References

1. Mueller, D. N.; Machala, M. L.; Bluhm, H; Chueh, W. C. Redox activity of surface oxygen anions in oxygen-deficient perovskite oxides during electrochemical reactions. Nature Comm. 2015, 6, 6097
2. Chueh, W.C.; McDaniel, A. H.; Grass, M. E.; Hao, Y; Jabeen, N; Liu, Z; Haile, S. M.; McCarthy, K. F.; Bluhm, H; El Gabaly, F. Highly Enhanced Concentration and Stability of Reactive Ce³⁺ on Doped CeO₂ Surface Revealed in Operando. Chem. Mater. 2012, 24, 1876-1882
3. Zhang, C; Yu, Y; Grass, M. E; Dejoie, C; Ding, W; Gaskell, K; Jabeen, N; Hong, Y. P; Shavorskiy, A; Bluhm, H; Li, W. X; Jackson, G. S; Hussain, Z; Liu, Z; Eichhom, B. W. Mechanistic Studies of Water Electrolysis and Hydrogen Electro-Oxidation on High Temperature Ceria-Based Solid Oxide Electrochemical Cells. J. Am. Chem. Soc. 2013, 135, 11572-15579
4. Feng, Z. A; El Gabaly, F; Ye, X; Shen, Z. X; Chueh, W. C. Fast vacancy-mediated oxygen ion incorporation across the ceria-gas electrochemical interface. Nature Comm. 2014, 5, 4374
5. Brisse, A; Chefold, J; Zahid, M. High Temperature Water Electrolysis in Solid Oxide Cells. International Journal of Hydrogen Energy. 2008, 33, 5375-5382
6. Nakamura, T; Yashiro, K; Kaimai, A; Otake, T; Sato, K; Kawada, T; Mizusaki, J. Determination of the Reaction Zone in Gadolinia-Doped Ceria Anode for Solid Oxide Fuel Cell. J Electrochem Soc. 2008, 155, B1244-B1250
7. Wilson, J. R; Schwartz, D. T; Adler, S. B. Nonlinear electrochemical impedance spectroscopy for solid oxide fuel cell cathode materials. Electrochimica Acta. 2006, 51, 1389-1402
8. Wilson, J.R.; Sase, M; Kawada, T; Adler, S. B. Measurement of Oxygen Exchange Kinetics on Thin-Film La_0.6Sr_0.4CoO_3-δ Using Nonlinear Electrochemical Impedance Spectroscopy. Electrochemical and Solid State Letters. 2007, 10, B81-B86