Manganese oxide-based mesoporous thin-film electrodes: manganese disproportionation reaction in alkaline media

Abstract

In this study, we explore the disproportionation reaction mechanism in alkaline media during the oxygen evolution reaction (OER) utilizing mesoporous electrodes, namely LiMn₂O₄ (m-LMO), Mn₃O₄ (m-Mn₃O₄), and Mn₂P₂O₇ (m-MnPP). The electrodes are crafted through the molten salt-assisted self-assembly (MASA) process. The procedure commences with the application of a metal salt(s)–surfactant and metal salt–pyrophosphoric acid–surfactant (where the surfactant is P123) lyotropic liquid crystalline mesophase coating over a FTO surface by employing a clear ethanol or aqueous solution of the ingredients, followed by a subsequent calcination step at 300 °C. The electrodes are characterized by spectroscopic, diffraction, imaging, and electrochemical techniques. At low electrochemical potentials, Mn(III), and at more positive potentials, Mn(VI) disproportionation reactions make these materials highly unstable in alkaline media. The aforementioned degradation processes have been investigated by examination of the electrodes both prior to and after subsequent use in electrochemical measurements in various electrolytes. We found that the degradation process is relatively slow in m-LMO, but elevated in m-Mn₃O₄ and m-MnPP electrodes. m-LMO is fully converted into the λ-MnO₂ phase upon its oxidation and more robust to decomposition; making it ultra-thin further improves its robustness. However, the m-Mn₃O₄ and m-MnPP electrodes behave similarly to each other and degrade more quickly (more pronounced in the latter), by releasing purple-colored permanganate ions into the electrolyte media. A Mn(VI) disproportionation reaction mechanism is suggested using the experimentally gathered spectroscopic, diffraction, and electrochemical data. The formation of Mn(VI) surface species and their electronegativity play vital roles in the disproportionation reaction.