XAS at the metal K-edges is a bulk-sensitive method that can be used to determine the metal oxidation state of a specific atom type with a precision of up to 0.1 oxidation-state units. This can be gained by systematic studies of metal oxidation state changes during electrocatalysis and a very well-suited method for this is hard X-ray absorption spectroscopy (XAS). To develop a strategy to improve catalytic activity and stability, a better understanding of the mechanism of water oxidation is needed. For example, the proposed reaction mechanism for OER on cobalt-based catalysts involves two proton-coupled electron transfer reactions, linked to Co II → Co III → Co IV oxidation, before onset of the rate-limiting O–O bond formation. This is also observed in the biological water oxidation reaction. The stored oxidizing equivalents are then used to oxidize water molecules. According to our current knowledge, during the catalytic splitting of H 2O into protons, electrons, and molecular oxygen, catalysts accumulate oxidizing equivalents via changes in metal oxidation state. First-row transition metal oxides are amongst the best choices as electrocatalysts for water oxidation due to their proven OER activity and potential in low-cost large-scale application. In this context, synthetic (non-fossil) fuels are of high interest, with electrocatalytic water oxidation emerging as a critical key process. The transition from fossil fuels towards renewable energy sources requires massive efforts in technological developments. Tracking of the oxidation state changes of Co ions in electrodeposited oxide films during cyclic voltammetry in neutral pH electrolyte serves as a proof of principle. We conclude that the combination of X-ray absorption spectroscopy with electrochemical techniques allows us to investigate the kinetics of redox transitions and to distinguish the catalytic current from the redox current. We demonstrate the significance of this approach as well as possible sources of data misinterpretation. The procedure to obtain time-resolved oxidation state values, using two calibration curves, is explained in detail. Here, we propose a strategy to use single-energy X-ray absorption spectroscopy for monitoring metal oxidation-state changes during OER operation with millisecond time resolution. Tracing these oxidation states under operation conditions could provide relevant information for performance optimization and development of durable catalysts, but further methodical developments are needed. ![]() The involvement of oxidation state changes of the metal in OER electrocatalysis is increasingly recognized in the literature. Transition metal oxides are promising electrocatalysts for water oxidation, i.e., the oxygen evolution reaction (OER), which is critical in electrochemical production of non-fossil fuels.
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