Generating more Mn4+ ions on surface of nonstoichiometric MnO2 nanorods via microwave heating for improved oxygen electroreduction

Abstract

Catalytic activities of transition metal oxides are usually influenced by their surface states. Herein, microwave heating is exploited as a fast and economic alternative approach to conventional postsynthesis calcination to effectively tune the surface states of nonstoichiometric MnO2 nanorods. After heating with microwave for 30 min, although the crystalline phase and morphology of nonstoichiometric MnO2 nanorods are largely reserved, the content of high-valent Mn cations, i.e. Mn4+, on the surface of nonstoichiometric MnO2 is maximized, which leads to a remarkably reduced charge transfer resistance for ORR, and the corresponding catalytic activity of oxygen reduction reaction (ORR) is markedly enhanced and becomes close to that of commercial Pt/C catalyst, within the context of onset potential, diffusion-limiting current density and average electron transfer number. The present work not only enriches the way to tune the ORR activity of catalysts, but also provides a deeper insight into the influence of surface state on electrochemically catalytic activity of manganese oxides.