Synergistic effect of Mn3+ and oxygen vacancy on the bifunctional oxygen electrocatalytic performance of MnOX/CNTs composites

Manganese oxides have been proved to be a promising low-cost electrocatalyst for oxygen reduction reaction (ORR) and a critical step in systems of zinc-air batteries, however, which is limited by the poor electrical conductivity. In this paper, we introduce carbon nanotubes (CNTs) as conductive supports to further prepare manganese oxide/CNTs composites with bifunctional electrocatalytic activity by the incorporation of redox and calcination two-step approach. Meanwhile, the effects of mass ratio, calcination temperature and oxidation degree on the oxygen reduction reaction (ORR) activity of the composites were studied. The electrocatalytic results showed that the MnO2/CNTs exhibited better electrocatalytic activity than that of manganese dioxide (MnO2) and CNTs when the mass ratio of CNTs to MnO2 was 60%. After calcination at 400 °C and reduction treatment of sulfur (S) to adjust the surface oxygen vacancy concentration and Mn3+ content, the ORR and oxygen evolution reaction (OER) bifunctional catalytic activity of the obtained MnOX/CNTs-400 was significantly enhanced. The ORR half-wave potential (E1/2) and bifunctional potential difference (ΔEOER-ORR) between the ORR half-wave potential and OER potential at 10 mA cm−2 were 0.77 V and 0.92 V, respectively, due to the introducing of highly conductive CNTs, increase of Mn3+ content and oxygen vacancy concentration. Furthermore, the composite also exhibits higher methanol tolerance and long-term stability than the commercial Pt/C catalyst, which provided a class of highly efficient ORR/OER bifunctional electrocatalysts with great energy potential.