Oxygen Electrocatalysts Based on Ordered Mesoporous Manganese Oxides with Different Oxidation States and Crystal Phases

Abstract

Ordered mesoporous manganese oxides with different crystal structures and oxidation states (Meso-MnO2, Mn2O3, Mn3O4, and MnO) were prepared by nanocasting KIT-6 mesoporous silica. Oxygen electrocatalytic activity of the Meso-MnOx was investigated in 0.1 M KOH. Meso-MnO2 and Mn2O3 showed higher oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activities than meso-Mn3O4 and MnO. Contrary to the previous report that revealed low ORR and OER activities for β-MnO2, relatively higher oxygen electrocatalytic activity was observed with Meso-MnO2, which can be attributed to its slightly lower oxidation state below 4 (~3.9), and unique disordered (or mixed) crystal phase. Comparing the Meso-MnO and Mn3O4, the former showed better ORR activity than the latter, demonstrating the advantage of the mixed phase in Meso-MnO for facilitating oxygen electrocatalytic activity. In-situ XAS showed noticeable potential-dependent change in oxidation state for Meso-MnOx catalysts, suggesting the oxygen electrocatalytic reactions invoke in situ phase transformation of the catalyst surface.