Redox Processes of Manganese Oxide in Catalyzing Oxygen Evolution and Reduction: An in Situ Soft X-ray Absorption Spectroscopy Study

Manganese oxides with rich redox chemistry have been widely used in (electro)catalysis in applications of energy and environmental consequence. While they are ubiquitous in catalyzing the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), redox processes occurring on the surface of manganese oxides are poorly understood. We report valence changes at OER- and ORR-relevant voltages of a layered manganese oxide film prepared by electrodeposition. X-ray absorption spectra were collected in situ in O-2-saturated 0.1 M KOH using inverse partial fluorescence yield (IPFY) at the Mn L-3,(2)-edges and partial fluorescence yield (PFY) at the O K-edge. Overall, we found reversible yet hysteretic Mn redox and qualitatively reproducible spectral changes by Mn L3,2 IPFY XAS. Oxidation to a mixed Mn3+/4+ valence preceded the oxygen evolution at 1.65 V vs RHE, while manganese reduced below Mn3+ and contained tetrahedral Mn2+ during oxygen reduction at 0.5 V vs RHE. Analysis of the pre-edge in O K-edge XAS provided the MnO hybridization, which was highest for Mn3+ (e(g)(1)). Our study demonstrates that combined in situ experiments at the metal L- and oxygen K-edges are indispensable to identify both the active valence during catalysis and the hybridization with oxygen adsorbates, critical to the rational design of active catalysts for oxygen electrocatalysis