A layered manganese oxide catalyst containing K+ cations [K/MnO x catalyst] was developed, and its ability to efficiently decompose water was demonstrated. Operando hard/tender/soft X-ray absorption fine structure (XAFS) techniques were used to investigate the function of the K/MnO x catalyst under working conditions. The Mn valency depended on the potential and the amount of K+ cation adsorption. Mn K-edge (hard X-ray) XAFS measurements for the K/MnO x catalyst suggested that the Mn in the catalyst was an Mn3+ species with an octahedral δ-MnO2 structure at a lower electrode potential, which changed reversibly to an Mn4+ species with a δ-MnO2 structure at higher potential during oxygen evolution. A similar result was obtained from operando O K-edge (soft X-ray) XAFS. The chemical state of K species was analyzed using operando K K-edge (tender X-ray) XAFS, which indicated that K+ cations were intercalated with hydrated states in the δ-MnO2 layers. These operando XAFS results demonstrated that the layered δ-MnO2 containing hydrated K+ cations functioned as efficient oxygen evolution electrocatalysts because of the presence of Mn3+ reaction sites.

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The move toward sustainable hydrogen production from water using renewable energy, a highly efficient oxygen evolution electrocatalyst, is crucial because watersplitting efficiency is restricted to the oxygen evolution capability, which is insufficient compared to the hydrogen evolution reaction. Herein, we report a new method that improves the oxygen evolution activity by integration of active nickel oxide clusters using amino acids, meaning that the amount of electrodeposited nickel oxides is increasing with maintaining the catalytic activity. This method enhances the catalytic activity because the reaction sites drastically increase in three dimensions. The detailed reaction mechanism was investigated using operando UV/vis absorption and Ni K-edge X-ray absorption spectroscopic techniques, which suggested that amino acids such as glycine, alanine, and glutamine promoted the electrodeposition of NiO 6 octahedral structure clusters. Meanwhile, the analysis of N and O K-edge X-ray absorption spectra showed that the amino acid (glycine) in the nickel electrocatalyst was present in the molecular state. Therefore, it was spectroscopically demonstrated that amino acids are bound to nickel oxide clusters accompanied by oxygen evolution activity.