Active Site Revealed for Water Oxidation on Electrochemically Induced δ-MnO₂: Role of Spinel-to-Layer Phase...

Seeking for active MnO_x material as artificial water splitting catalyst has been a long history since the discovery of PSII system in nature. To date, the highest activity MnO_x catalyst reported for oxygen evolution reaction (OER) does however not belong to common MnO₂ polymorphs (α-, β-, δ-MnO₂), but rather to nascent δ-MnO₂ layer produced in situ from spinel under electrochemical conditions with unknown active site structure. Here with the stochastic surface walking (SSW) pathway sampling method, we for the first time resolve the atomic-level mechanism of spinel-to-layer Mn₃O₄ solid phase transition in aqueous electrolyte. We show that a transient H_0.5MnO₂ phase is the precursor of transition that forms at high voltage (>1 V), and it undergoes the solid-to-solid phase transition to produce a δ-MnO₂ layer, which is accompanied by Mn dissolution, dislocation, layer-breaking, and insertion of water/cations between layers. This leads to the generation of a variety of possible defective structures. We demonstrate using first-principles calculations that a special edge site with neighboring Mn vacancy provides the best OER activity with an overpotential of 0.59 V, 0.19 V lower than that of pristine MnO₂. The high activity of such Mn sites are attributed to its special local structure: pseudocubane with one corner missing. The presence of the Mn vacancy near the active site enhances the adsorption of OH intermediate in OER. This defective cubane structure shares the common geometrical and electronic features found in the PSII system.