Manganese Oxide as an Inorganic Catalyst for the Oxygen Evolution Reaction Studied by X-Ray Photoelectron and Operando Raman Spectroscopy

Manganese oxide (MnO${}_x$) is considered a promising material for the oxygen evolution reaction (OER) to replace noble metal catalysts in water splitting. The improvement of MnO${}_x$ requires mechanistic and kinetic knowledge of the four-electron transfer steps of the OER. X-ray photoelectron spectroscopy, a widely used tool to characterize the electronic structure of thin films, is used in combination with surface-enhanced Raman spectroscopy to gain a deeper knowledge of the different mixed MnO${}_x$ types and their respective change in chemical composition. Using Raman spectroscopy during electrochemical measurements, all samples were found to reveal Birnessite-type MnO${}_2$ motifs in alkaline media at an applied potential. Their activity correlates with two shifting Raman active modes, one of them being assigned to the formation of Mn${}^{III}$ species, and one to the expansion of layers of MnO${}_6$ octahedra. A special activation treatment leads independent of the starting material to a highly amorphous mixed-valence oxide, which shows the highest OER activity.