Operando Observations of a Manganese Oxide Electrocatalyst for Water Oxidation Using Hard/Tender/Soft X-ray Absorption Spectroscopy

A layered manganese oxide catalyst containing K⁺ cations [K/MnOₓ 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/MnOₓ 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/MnOₓ 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.