Hierarchical bifunctional catalysts with tailored catalytic activity for high-energy rechargeable Zn-air batteries

For Zn-air batteries, much research is still needed to obtain low-cost, high-efficient, and strong durability bifunctional catalysts to promote various electrochemical reactions. In this article, MnO2/carbon-nanotubes (CNTs) hybrid catalysts anchored with a metal oxide (M-MnO2/CNTs) are developed via a facile hydrothermal process toward an efficient oxygen catalytic (evolution and reduction) reactions. M-MnO2/CNTs displays better catalytic activity than MnO2/CNTs. The overpotential of Co-MnO2/CNTs is lowered to 0.803 V, very close to that of Pt/C + IrO2 (0.801 V). Moreover, Co-MnO2/CNTs exhibits faster kinetics for oxygen evolution reaction with low Tafel slope (62 mV dec(-1)) than Pt/C + IrO2. The improved performance and stability originated from (i) the novel hierarchal structure improving the mass transport, (ii) the redistribution of electrons activating the Mn catalytic sites, (iii) the coupling effect between the metal oxide and MnO2/CNTs, and (iv) an excellent conductivity building a fast electron transfer high-way. The density functional theory results reveal that the anchored metal oxide cluster directly activates neighbor Mn sites on the surface of the hybrid catalyst, and enhances the oxygen catalytic activity by decreasing the bonding with OH and OOH. Accordingly, the maximum power density of the Co-MnO2/CNTs-based Zn-air battery reaches 342.7 mW cm(-2). The Zn-air battery also exhibits good charge-discharge stability with a low voltage gap of 0.72 V for 128 h. This work provides an effective method to achieve MnO2/CNTs materials with tailored catalytic activity by anchoring different metal oxides and demonstrates great potential in the field of high specific energy batteries for electrical vehicles, portable electronics, and wearable devices.