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This paper presents a modelling study of the electrochemical hydrogen oxidation reaction at nickel/yttria-stabilized zirconia (Ni/YSZ) patterned anodes. An elementary kinetic reaction-diffusion model accounts for coupled heterogeneous chemistry and transport on the Ni and YSZ surfaces. Charge transfer is modeled as a spillover of adsorbates between the Ni and YSZ surfaces at the three-phase boundary (TPB). No a priori assumptions on rate-determining processes are made. Thermodynamic, kinetic, and transport parameters are compiled from various literature sources serving as a base for quantitative simulations. Seven different spillover reaction pathways of the hydrogen oxidation reaction are compared to experimental patterned anode data obtained previously by Bieberle et al. [ J. Electrochem. Soc. , 148 , A646 (2001)] under a range of operating conditions. Only one reaction pathway, based on two hydrogen spillover reactions, is able to describe consistently the complete experimental data set. A sensitivity analysis for this case allows identification of rate-determining processes. Surface concentrations close to the TPB are predicted to differ from the concentration derived from thermodynamical equilibrium by up to 2 orders of magnitude. The simulation results and the validity of the model are critically discussed. Directions for future theoretical and experimental studies for elucidating the mechanistic details of Ni/YSZ anodes are given.

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Density functional theory (DFT) simulations of the oxygen evolution reaction (OER) are considered essential for understanding the limitations of water splitting. Most DFT calculations of the OER use an acidic reaction mechanism and the standard hydrogen electrode (SHE) as reference electrode. However, experimental studies are usually carried out under alkaline conditions using the reversible hydrogen electrode (RHE) as reference electrode. The difference between the conditions in experiment and calculations is then usually taken into account by applying a pH-dependent correction factor to the latter. As, however, the OER reaction mechanisms under acidic and under alkaline conditions are quite different, it is not clear a priori whether a simple correction factor can account for this difference. We derive in this paper step by step the theory to simulate the OER based on the alkaline reaction mechanism and explain the OER process with this mechanism and the RHE as reference electrode. We compare the mechanisms for alkaline and acidic OER catalysis and highlight the roles of the RHE and the SHE. Our detailed analysis validates current OER simulations in the literature and explains the differences in OER calculations with acidic and alkaline mechanisms.