Partial Surface Oxidation of Manganese Oxides as an Effective Treatment to Improve Their Activity in Electrochemical Oxygen Reduction Reaction

Enhancing the electrocatalytic activity of low-cost transition-metal oxides for oxygen reduction reaction (ORR) is a crucial challenge for extensive application of fuel cells. A promising approach demonstrated previously is the formation of catalysts with mixed valent metal active sites. Because catalysis happens primarily on the surface of the catalyst, we hypothesize that creating such active sites only on the surface will be an effective strategy for improving the catalytic activities. Here, we present a partial oxidation approach that grows δ-MnO₂ nanoflakes on the surface of octahedron Mn₃O₄ nanocrystals for increasing their ORR activity. The δ-MnO₂/Mn₃O₄ nanocomposite exhibits significantly improved ORR activity with a half-wave potential of 0.75 V versus reversible hydrogen electrode, which is ∼110 and ∼90 mV lower than those of the Mn₃O₄ nanocrystal and δ-MnO₂ nanoflakes in their pure forms, respectively. The electrochemical impedance spectroscopy reveals that the δ-MnO₂/Mn₃O₄ nanocomposite possesses a lower ORR charge transfer resistance than either component alone. We propose that the reason for such significant improvement in catalytic activities is due to the tuning of the position of δ-MnO₂ nanoflake d-band center by the Mn₃O₄ nanocrystal which can effectively facilitate the electron transfer between the active sites and adsorbed oxygen molecules. This work illustrates a facile pathway to improve catalytic activity of mixed valence metal oxides.