Operando Investigation of Mn3O4+delta Co-catalyst on Fe2O3 Photoanode: Manganese-Valency-Determined Enhancement at Varied Potentials

The development of efficient catalysts containing earth-abundant elements for the oxygen evolution reaction (OER) in photoelectrochemical (PEC) systems is highly desired for low-cost energy storage and conversion. In this work, mesoporous alpha-Fe2O3 thin film photoanodes coated with manganese oxide (Mn3O4+delta) co-catalysts are prepared by a dip-coating method. The co-catalyst coating significantly enhances PEC water oxidation performance as compared with the uncoated alpha-Fe2O3. To understand the origin of this enhancement, in situ X-ray absorption spectroscopy is employed to monitor the valence state of Mn in the Mn3O4+delta co-catalyst as a function of applied potential. It is found that the enhancement of the photocurrent is governed by the Mn valency, and the most prominent enhancement takes place at the valency of similar to 3.4+, which is due to the optimal e(g) electron filling in Mn cations as the electrocatalyst for OER Our investigation indicates that the contribution of Mn3O4+delta co-catalyst to OER kinetics is variable at different applied potentials.