Manganese-based oxides have been used for many electrochemical energy conversion/storage technologies in alkaline/neutral electrolytes, e.g., electrocatalysis, supercapacitors, solar fuels, and rechargeable batteries. The promise of Mn-based oxides for these systems is rooted in their abundant, inexpensive, and non-toxic nature. More importantly, Mn-based oxides can possess a wide range of crystal structures and valence states, offering rich oxide chemistries, various active motifs, and tunable electronic structures. The excellent kinetics of these oxides under alkaline/neutral conditions, e.g., for electrocatalysis, have motivated the development of activity descriptors. For example, for the alkaline oxygen reduction reaction, having Mn ions with an eg orbital occupancy of slightly less than one or a nominal valence state between 3+ and 4+ has been shown to give rise to the most active Mn-based oxide catalysts. Unfortunately, the use of Mn-based oxides in acidic devices/systems, e.g., proton-exchange membrane fuel cells and electrolyzers, has been significantly hindered by the dissolution of these oxides at low pHs. Designing Mn-based oxides with enhanced stability in acidic environments requires a detailed understanding of the physical origin of their instability. In this work, we employed a library of Mn-based oxides with diverse structures and Mn oxidation states to identify oxide stability descriptors in acid based on intrinsic electronic structure and energetic parameters. Using time-dependent dissolution experiments and density functional theory calculations, we found that greater amounts and faster kinetics of oxide dissolution in acid can be correlated with decreased Mn oxidation states, which is accompanied by lowered Mn-O covalency, weakened Mn-O bonds, and reduced free energy barriers for essential protonation, vacancy formation, and ion solvation steps. Such correlations were consistent with an analogous computational evaluation across a broad chemical space involving ~1,000 Mn-based oxides, where an optimal Mn valance state of 4+ is critical to result in the lowest driving force for the dissolution of Mn-based oxides at acidic pHs. Moreover, limiting the fraction of ionic substituents in oxides and using more acidic substituents (with less favorable leaching energetics) can lead to more stable Mn-based oxides in acid. These findings can provide new insights to design durable Mn-based oxides for clean energy applications with acidic systems.

Stability design principles of manganese-based oxides for acidic electrolytes

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