(578d) Microkinetic Modeling of the Oxygen Evolution Reaction on Oxide Surfaces

The oxygen evolution reaction (OER) is an important anodic reaction that enables the sustainable production of H₂ from water at ambient conditions via the hydrogen evolution reaction or provides protons for cathodic CO_x reduction reactions. Extensive density functional theory (DFT) calculations have been previously carried out to determine the theoretical limiting potential (where all steps are exergonic) of the OER as a function of binding energy descriptors [1]. This work extends those analyses by feeding DFT calculated energies into a mean field, steady state microkinetic model for the OER. This model demonstrates that a simple mechanism consisting of four coupled proton electron transfers can be used to predict activity trends in relevant catalyst surfaces such as rutile and perovskite oxides.

A potential dependent barrier is assumed for each coupled proton electron transfer step [2]. The location of the peak of the volcano at OER relevant potentials agrees with thermodynamic predictions. A degree of rate control analysis and a sensitivity analysis of rate with assumed barrier are carried out. Changes in Tafel slope for a given surface have been observed experimentally for the OER. The model presented here predicts these changes in Tafel slope and demonstrates that changes in Tafel slope can be due to switches in reaction mechanism, coverage, and rate controlling step [3]. Additionally, this research outlines a method to determine the non-equilibrium, kinetic coverage of a surface under OER conditions. Generally, we find the steady state coverage corresponds to the precursor of the thermodynamically limiting step for potentials above the limiting potential.

This analysis demonstrates that qualitative trends can be predicted by a simple model and assumed proton transfer barriers. Future work in determining electrochemical barriers and their potential dependence is necessary to quantitatively predict rates and compare to experimentally measured rates. An analysis of electrochemical barriers for the OER on IrO₂ (110) is carried out to explore the assumption that all OER steps can be modeled as simple proton electron transfers. A charge extrapolation scheme is used to determine electrochemical barriers as a function of potential [4]. This analysis indicates that the barriers are low and scale with reaction energy with a slope near 0.5 as assumed in the microkinetic model. However, further work is necessary to fully quantify the electrochemical barriers for this system.

1. Man, I.C., Su, H., Calle-Vallejo, F., Hansen, H.A., Martinez, J.I., Inoglu, N.G., Kitchin, J., Jaramillo, T.F., Nørskov, J.K., Rossmeisl, J. Universality in Oxygen Evolution Electrolysis on Oxide Surfaces. ChemCatChem 3, 1159-1165 (Mar. 2011).
2. Tripkovic, V., Skulason, E., Siahrostami, S., Nørskov, J.K., Rossmeisl, J. The Oxygen Reduction Reaction Mechanism on Pt(111) from Density Functional Theory Calculations. Electrochimica Acta 55, 7975-7981 (Nov. 2010).
3. Holewinski, A., Linic, S. Elementary Mechanisms in Electrocatalysis: Revisiting the ORR Tafel Slope. J. Electrochem. Soc. 159, H864-H870 (Sept. 2012).
4. Chan, K., Nørskov, J.K. Electrochemical Barriers Made Simple. J. Phys. Chem. Lett. 6, 2663-2668 (June 2015).