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 k_obs 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 Mn^III 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 (α_a) 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 (26) 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.