Investigation of Surface Oxidation Processes on Manganese Oxide Electrocatalysts Using Electrochemical Methods and Ex Situ X-ray Photoelectron Spectroscopy

Recently, we developed a promising non-precious metal catalyst with bifunctional activity for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER), two important energy conversion reactions with relevance to fuel cells and water-splitting. Having previously identified the catalyst as a manganese (III) oxide with alpha-Mn2O3 crystallinity, in this study we further characterize the catalyst surface to understand changes in the surface chemistry under different reaction conditions. We identify key electrochemical features of the catalyst using cyclic voltammetry and then probe the changes in the surface oxidation state of manganese in the potential region between the ORR and the OER using ex-situ X-ray photoelectron spectroscopy (XPS). Despite the surface sensitivity of XPS, these measurements do not reveal any differences in surface oxidation state between samples tested at ORR potentials and those examined at potentials relevant to the OER. We show, however, that electrochemical methods can discern a difference in oxidation states of the two samples upon re-insertion of the samples into an electrochemical cell after the completion of ex-situ XPS measurements. These results suggest that oxidative processes on the catalyst surface are extremely shallow, which we quantify based on the experimental results and theoretical modeling of the XPS spectra.