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

The oxygen evolution reaction (OER) represents a bottle neck in the development of sustainable light-driven production of hydrogen from water using cheap and abundant electro-catalysts. Even though metal oxide catalysts have been used for decades, the mechanism of the OER has not been fully understood yet. We investigated Cobalt and Manganese Oxide thin films in their behavior during OER using photoelectron spectroscopy (PES) and synchrotron radiation. In an in-line system we could perform cyclic voltammetry as well as potentiostatic measurements in the anodic range under well-defined inert atmospheric conditions and afterwards transfer the samples into the UHV system for PES studies, without breaking the atmosphere. The electrochemical experiments have been interrupted at several potentials by removing the electrolyte drop and XPS as well as UPS spectra have been recorded successively. Despite the fact that the cyclic voltammetry curves (CV) of amorphous CoO_x layers - deposited potentiostatically on FTO glass – and crystalline α-Mn₂O₃ – prepared by reactive sputtering and galvanostatic deposition – differ significantly in the low potential range before the onset of the OER, the differences in the PES spectra are little. Nevertheless, we could find some shifts in the peaks that have been interpreted as a change in the oxidation state of the metal ions at the surface depending on the potential applied and associated with the appearance of new oxo-bondings.