Elucidating and Quantifying Parasitic Reactions on Manganese Oxide Electrodes for Acidic Oxygen Evolution Reaction Using In Situ Spectroscopic Techniques

Catalyzing the anodic oxygen evolution reaction (OER) in proton exchange membrane water electrolysis by non-precious metal catalysts represents a promising technique. Recently, manganese oxides have been recognized as one of the most popular materials in this field. However, they often suffer from structural instability caused by obvious manganese dissolution process and the resulting complicated side reactions, which are not well understood but are of great significance for the design and development of manganese-based catalysts. In this work, we have first developed a method based on UV-vis absorption spectroscopy to achieve in situ detection and distinction of different valence states of manganese ions in acidic media. Meanwhile, the interfacial solid-phase structure evolution has been systematically investigated via electron microscopy and spectroscopy techniques under ex situ/in situ conditions. We have successfully captured the parasitic reactions for manganese oxide electrode in both liquid and solid phases from open circuit potential to oxygen evolution potential. Thus, a comprehensive surface reconstruction model for manganese oxide under acidic conditions has been proposed, involving disproportion, redeposition feature, and over-oxidation behaviors. We believe that our work provides in-depth insight into the side reactions and resulting structural instability of manganese-based materials during acidic OER, which could be used for optimization of the catalyst toward oxygen electrocatalysis.