Electrochemical trapping of metastable Mn³⁺ ions for activation of MnO₂ oxygen evolution catalysts

Electrodeposited manganese oxide films are promising catalysts for promoting the oxygen evolution reaction (OER), especially in acidic solutions. The activity of these catalysts is known to be enhanced by the introduction of Mn³⁺ We present in situ electrochemical and X-ray absorption spectroscopic studies, which reveal that Mn³⁺ may be introduced into MnO₂ by an electrochemically induced comproportionation reaction with Mn²⁺ and that Mn³⁺ persists in OER active films. Extended X-ray absorption fine structure (EXAFS) spectra of the Mn³⁺-activated films indicate a decrease in the Mn-O coordination number, and Raman microspectroscopy reveals the presence of distorted Mn-O environments. Computational studies show that Mn³⁺ is kinetically trapped in tetrahedral sites and in a fully oxidized structure, consistent with the reduction of coordination number observed in EXAFS. Although in a reduced state, computation shows that Mn³⁺ states are stabilized relative to those of oxygen and that the highest occupied molecular orbital (HOMO) is thus dominated by oxygen states. Furthermore, the Mn³⁺(T_d) induces local strain on the oxide sublattice as observed in Raman spectra and results in a reduced gap between the HOMO and the lowest unoccupied molecular orbital (LUMO). The confluence of a reduced HOMO-LUMO gap and oxygen-based HOMO results in the facilitation of OER on the application of anodic potentials to the δ-MnO₂ polymorph incorporating Mn³⁺ ions.