Mesoporous Manganese Oxides with High-Valent Mn Species and Disordered Local Structures for Efficient Oxygen Electrocatalysis

Active and nonprecious-metal bifunctional electrocatalystsforthe oxygen reduction reaction (ORR) and oxygen evolution reaction(OER) are vital components of clean energy conversion devices suchas regenerative fuel cells and rechargeable metal-air batteries.Porous manganese oxides (MnO x ) are promisingelectrocatalyst candidates because of their high surface area andthe abundance of Mn. MnO x catalysts exhibitvarious oxidation states and crystal structures, which criticallyaffect their electrocatalytic activity. These effects remain elusivemainly because the synthesis of oxidation-state-controlled porousMnO( x ) with similar structural propertiesis challenging. In this work, four different mesoporous manganeseoxides (m-MnO x ) weresynthesized and used as model catalysts to investigate the effectsof local structures and Mn valence states on the activity toward oxygenelectrocatalysis. The following activity trends were observed: m-Mn2O3 > m-MnO2 > m-MnO > m-Mn3O4 for the ORR and m-MnO2 > m-Mn2O3 > m-MnO & AP; m-Mn3O4 for the OER. These activitytrends suggest that high-valent Mn species (Mn(III) and Mn(IV)) withdisordered atomic arrangements induced by nanostructuring significantlyinfluence electrocatalysis. In situ X-ray absorption spectroscopywas used to analyze the changes in the oxidation states under theORR and OER conditions, which showed the surface phase transformationand generation of active species during electrocatalysis.

Automated summary

In this study, four different mesoporous manganese oxides were synthesized and used as model catalysts to investigate the effects of local structures and Mn valence states on the activity of oxygen electrocatalysis. The results showed that for the oxygen reduction reaction, m-Mn2O3 exhibited the highest activity, followed by m-MnO2, m-MnO, and m-Mn3O4. For the oxygen evolution reaction, m-MnO2 showed the highest activity, followed by m-Mn2O3, while m-MnO and m-Mn3O4 had lower activity. These findings suggest that high-valent Mn species and disordered atomic arrangements induced by nanostructuring significantly affect electrocatalysis.