Tracking the active phase and reaction pathway of the OER mediated by an MnMoO4 microrod electro(pre)-catalyst

Abstract

MnMoO4 is a barely explored material for the electrocatalytic oxygen evolution reaction (OER) and in situ tracking of the reactive intermediates and final active species during the OER in an alkaline pH lacks a sequential study. Herein, in situ spectroscopic and ex situ microscopic studies unravel a pH-dependent [MoO4]2− dissolution from MnMoO4 with a kobs of 4.5 s−1 to form α-MnO2 followed by a subsequent potential-driven anodic transformation into δ-MnO2. The electrochemically derived δ-MnO2 delivers a fairly stable current density (15 mA cm−2) at 1.55 V (vs. RHE) for over 24 h. However, a thermally stable mixed-phase α/δ-MnO2 species evolved during the OER with dominant MnIII content and remains highly reactive towards OER with an overpotetial (η10) at 333 K of 239 mV. Temperature-dependent OER study provides a unimolecular reaction order for [OH]− and an anodic transfer coefficient (αa) of 0.7. A low activation barrier of 9.77 kJ mol−1 and a high exchange current density (j0) of 0.095 mA cm−2 prove that the improved OER activity on α/δ-MnO2 is due to fast electro-kinetics. DFT study on the (21̄1̄6) surface of the δ-MnO2 concluded that the dissociation of the *O-H bond to form the *O is the rate-limiting step for the OER and the *O intermediate is stabilized by a weak O-O interaction (1.4 Å) with lattice-oxygen before forming a hydroperoxide intermediate. Herein, in situ tracking of the reactive phases generated from the MnMoO4 pre-catalyst, detailed electro-kinetics, and the theoretical study help to unravel the OER mechanism.