The Microkinetic Performance Barriers of Ruthenium and Iridium Oxides During the Electrocatalytic Oxygen Evolution Reaction

Electrocatalytic water splitting is currently one of the most promising reactions to produce green hydrogen in a de-fossilized energy system.[1] The performance of PEM electrolyzers is substantially determined by the sluggish reaction kinetics of the oxygen evolution reaction (OER). Even highly active catalysts such as nanoparticulated transition metal oxides IrO2, RuO2 and their mixtures IrxRu1-xO2 exhibit overpotentials up to several hundreds of millivolts.[2] Dynamic microkinetic modeling is a powerful tool to analyze the single reactions and their interplay during OER at the electrode surface.[3] Quantitative insight into process-limiting steps on different active sites on the material surface can be used to suggest ways to improve dynamic OER operation. In this work, we present the analysis of an experimentally validated microkinetic model. It allows to study the electrocatalytic reaction mechanism including the coverage of emerging surface species (e.g. *OH, *O, *OOH and *OO) for a wide potential range including OER. We show that there is a correlation between performance and the Ir:Ru ratio, that can be explained by two different potential determining deprotonation steps. In particular, the highest equilibrium potentials of 1.44 V and 1.58 V are quantified for the production of the adsorbate species *OOH on rutile RuO2 and *OO on IrO2, respectively. During OER at a potential of >1.5 V, adsorbed oxygen *O covers >40 % of the active sites, suggesting that subsequent water adsorption is the major process limitation. The surface of the oxide mixtures IrxRu1-xO2, is found to consist of actives sites of both Ir and Ru on which the OER mechanism is processed independently and at different overpotentials. Compared to the pure oxides, the mixtures reveal reduced reaction energies of the potential determining deprotonation processes. One can conclude that incorporating Ru into IrO2 provides higher overall performance while stability is remained. Dynamic microkinetic modelling is therefore a viable method to study catalytic surface processes as well as their limitations and to make suggestions for improving the performance of electrocatalytic systems. [1] K. Kalz et al., ChemCatChem, 2017, 9, 17. [2] D. Escalera-López et al., ACS Catalysis, 2021, 11, 15, 9300. [3] J. Geppert et al., Electrochimica Acta, 2021, 380, 137902.