Tracking the Active Phase and Reaction Pathway of OER Mediated by MnMoO4 Microrod Electro(Pre)-catalyst

MnMoO4 is a barely explored material for electrocatalytic oxygen evolution reaction (OER) and in-situ tracking of the reactive intermediates and final active species during 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 and subsequent potential-driven anodic transformation to 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 OER with dominant MnIII content and remains highly reactive toward OER with η10 at 333 K of 239 mV. Temperature-dependent OER study provides an unimolecular reaction order for [OH]- and an anodic transfer coefficient (a) of 0.7. A low activation barrier of 9.77 k J mol-1 and a high exchange current density (j0) of 0.095 mA cm-2 validate that the improved OER activity on α/δ-MnO2 is due to fast electro-kinetics. DFT study on the (21 @#x0305;(1 ) @#x0305;6) surface of the δ-MnO2 concluded that the dissociation of the *O-H bond to form the *O is the rate-limiting for OER and the *O intermediate is stabilized by a weak O—O interaction (1.4 Å) with one 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.