Mechanisms of Manganese Oxide Electrocatalysts Degradation\nduring Oxygen Reduction and Oxygen Evolution Reactions

Abstract

Anion-exchange membrane\nfuel cells and electrolyzers offer a unique\nopportunity of using non-noble metal electrocatalysts for catalyzing\nthe sluggish oxygen reduction and oxygen evolution reactions (ORR\nand OER). In recent years, various Mn-based oxides were identified\nas promising catalysts for both reactions. While electrocatalytic\nactivity of such oxides is well addressed, their stability is still\nto be proven. Herein, we investigate the stability of four main manganese\noxide allotropes by following their Mn dissolution rate in operando\nORR and OER conditions. Using an electrochemical on-line inductively\ncoupled plasma mass spectrometer, we uncover unexpected instability\nof this class of catalysts, with different degradation mechanisms\nidentified under OER and ORR conditions. The reason for their degradation\nis shown to be related to the production of hydrogen peroxide species\non manganese oxides during ORR. Furthermore, we discuss how limits\nin thermodynamically stable windows of each Mn oxidation state lead\nto increased dissolution during applications with high potential perturbations,\nthat is, change in load, start/stop conditions, and especially bifunctional\napplication. Therefore, we recommend clear guidelines for future development\nof platinum group metal-free electrocatalysts for affordable alkaline\nenergy conversion technologies.