On the Structure-Function Relationship of Cobalt and Manganese Oxides as Oxygen Evolving Catalysts for Light-Driven Water Electrolysis: An In-Line Synchrotron Radiation Photoelectron Spectroscopy Study

In the effort to develop an efficient and cost effective photo-electrochemical device for water splitting driven by sunlight only, transition metal oxides are promising candidates to catalyze the oxygen evolution reaction (OER) at the anode. We used X-ray photoelectron spectroscopy (XPS) to characterize very active manganese and cobalt oxide thin films deposited on FTO substrates before and after the application of different anodic potentials, in order to investigate the bias potential dependent changes on the catalysts' surfaces. alpha-Mn2O3 undergoes a reversible partial oxidation from Mn3+ to Mn4+ under high anodic potentials, while the transition from Co2+ to Co3+ in amorphous CoOx samples occurs already at a potential well below the OER onset potential. This Co3+ state is then stable throughout the investigated potential range and no clear evidence for a Co4+ state at or above the OER onset potential could be found. We conclude that the OER reaction mechanism on the surface of these oxide films might be significantly different.