Modeling Water Reduction on 10 Mole% Gadolinia-Doped Ceria (GDC10) Porous Electrodes

Currently, the most widely considered material for solid oxide fuel and electrolysis cells is the nickel/yttria-stabilized zirconia (Ni/YSZ) cermet¹. However, issues such as low resistance to poisoning, coking, and changes in the microstructure have pushed for the use of new materials. Mixed ionic conductors² alleviate the shortcomings of Ni/YSZ and can achieve more efficient use of the electrode material, since a three-phase boundary is not required for reaction. Among these materials, doped ceria has shown promise, yet little is known regarding the reaction mechanism for water electrolysis. The advent of techniques such as ambient-pressure x-ray photoelectron spectroscopy (AP-XPS) has helped to provide a better picture of the surface, as well as the species involved in the reaction. Unfortunately, there is still controversy regarding the nature of the rate-determining step^3,4. In addition, most work has made use of electrochemical impedance spectroscope (EIS), a linear technique that cannot differentiate among rate determining scenarios. The use of nonlinear techniques such as non-linear EIS (NLEIS) has been shown to be a powerful tool that is sensitive to different reaction mechanisms⁵.

In order to elucidate these rate limiting phenomena, porous 10% doped gadolinia-doped ceria /YSZ electrochemical cells were analyzed at 750 and 800 °C under different H₂-H₂O gas environments using both EIS and NLEIS. The experimental results were then analyzed using a macro-homogeneous model⁶ involving oxygen vacancies and the chemical potential of electrons⁷. Multiple modeling scenarios were considered including one-dimensional and multidimensional transport, as well as the presence or absence of surface diffusion of hydroxyl species in the surface.  

References

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2. 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
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