Stability Design Principles of Mn-Based Oxides for Acidic Electrolytes

Abstract

Manganese-based oxides are promising, earth-abundant candidate materials in many electrochemical energy applications in aqueous environments, including supercapacitors,1 electrocatalysis2 and solar fuels.3 The use of these oxides in acidic systems, such as proton-exchange membrane fuel cells and electrolyzers,4,5 is, however, severely limited by the bulk instability of Mn oxide phases according to their Pourbaix diagrams.2 Designing active and stable Mn-based oxide catalysts in acidic electrolytes therefore requires fundamental understanding towards their aqueous stability and dissolution mechanism in acid.In this study, we systematically examined the aqueous stability of Mn-based oxides with a diverse range of chemistry, structures and oxidation states in acidic electrolytes. Decreasing the metal-oxygen covalency, e.g. by having Mn ions with lower nominal oxidation states, was found to weaken the manganese-oxygen bonds, reduce the energy penalty for the formation of Mn vacancies and subsequent ion solvation, rendering poorer stability in acidic solutions. Moreover, we identified the critical role of oxide acid-base chemistry6 in dictating the stability of Mn-based oxides, where lifting up the Fermi levels on the absolute energy scale by decreasing covalency can lead to greater thermodynamic driving force for the surface protonation of these metal oxides, which further weakens the surface metal-oxygen bonds and contributes to the instability of Mn-based oxides in acidic electrolytes. This work highlights the importance of understanding the physical origin of the aqueous stability trends for metal oxides with electronic and/or energetic descriptors, and provides guiding principles for designing oxide catalysts with enhanced stability in acidic systems.