Stability Design Principles of Manganese-Based Oxides in Acid
Authors: Peng Jiayu , Giordano Livia , Davenport Timothy , Shao-Horn Yang 
Published: November 2022

The use of non-precious catalysts (e.g., transition metal oxides) in electrochemical energy technologies in acid (e.g., proton exchange membrane fuel cells and electrolyzers) has been significantly hampered by the instability of such catalysts at low pHs. While first-row late transition metal oxides based on Fe, Co, and/or Ni have been reported with comparable or higher catalytic activity for the alkaline oxygen reduction and oxygen evolution reactions than their precious-metal-based counterparts (e.g., Pt and IrO2),[1] these oxide catalysts are not stable in acid.[2] Moreover, Mn-based oxides are more acid-stable than those based on late transition metals[1] and can be potentially stabilized by reaching dissolution-deposition equilibrium at moderately acidic pHs,[3] but they still corrode and deactivate in strong acid and after long operation.[4] To tackle this challenge, extensive efforts over the past decades have been focused on combining active, yet unstable metal oxides with corrosion-resistant oxides.[5-7] Nevertheless, having a higher fraction of corrosion-resistant elements results in more acid-stable, yet less active catalysts, indicating an activity-stability trade-off.[2,5,6]

To design transition metal oxide catalysts (such as Mn-based oxides) with an optimal trade-off or bypass this limitation, it is critical to establish their stability descriptors in acid. These descriptors can offer a fundamental understanding of oxide dissolution and provide guiding principles to enhance their intrinsic stability. 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, greater amounts and faster kinetics of oxide dissolution in acid were correlated with decreased Mn oxidation states, which are accompanied by lowered Mn-O covalency, weakened Mn-O bonds, and reduced barriers for key reaction steps (such as protonation, vacancy formation, and metal ion solvation). Such design principles were shown broadly with a computational screening across a vast chemical space of ~1,000 Mn-based oxides, where an Mn oxidation state of 4+ was found to give rise to the lowest energetic driving force for the dissolution of Mn-based oxides in acid. Moreover, limiting the percentage of ionic metal substituents in oxides and using more acidic substituents were shown to stabilize Mn-based oxides in acid. These findings can provide novel insights into designing acid-stable Mn-based oxides for renewable energy storage and conversion.
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