Insight into pH-Dependent Formation of Manganese Oxide Phases in Electrodeposited Catalytic Films Probed by Soft X-Ray Absorption Spectroscopy

MnOx films electrodeposited under basic, neutral, and acidic conditions from an ionic liquid were investigated by means of X-ray absorption spectroscopy at the manganese L-2,L-3-edges and the oxygen K-edge. Such films can serve as catalysts for the water oxidation reaction. Previous studies showed that the catalytic activity could be controlled by varying the deposition parameters, which influence the formation of MnOx phases and the film composition. Herein the film compositions are investigated in detail, indicating different ratios of MnOx structural phases in the films. All films in the series predominately consist of varying proportions of three MnOx phases-Mn2O3, Mn3O4, and birnessite, while an increase of the average Mn oxidation state in the film is identified when going from basic to acidic conditions during electrodeposition. The contribution of these three phases shows a systematic dependency on the pH during electrodeposition. While no specific single MnOx phase was found to dominate the composition of samples that were previously found to show high catalytic activity, the X-ray spectroscopic results revealed the compositions of those samples prepared under close to neutral conditions to be most sensitive to changes in pH.