Operando Investigation of Mn₃O_4+δ 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 α-Fe 2 O 3 thin film photoanodes coated with manganese oxide (Mn 3 O 4+δ ) co-catalysts are prepared by a dip-coating method. The co-catalyst coating significantly enhances PEC water oxidation performance as compared with the uncoated α-Fe 2 O 3 . To understand the origin of this enhancement, in situ X-ray absorption spectroscopy is employed to monitor the valence state of Mn in the Mn 3 O 4+δ 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 ∼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 Mn 3 O 4+δ co-catalyst to OER kinetics is variable at different applied potentials.