MnIII-enriched α-MnO2 nanowires as efficient bifunctional oxygen catalysts for rechargeable Zn-air batteries

The development of highly efficient bifunctional catalysts for OER and ORR is crucial for improving the efficiency of metal-air batteries. Herein we report a high-performance bifunctional oxygen catalyst of alpha-MnO2 nanowires synthesized using ionic liquid (IL) as the structure-directing agent. While increasing the IL addition enables to gradually transform the reaction products from beta-MnO2 nanorods to alpha-MnO2 nanowires, increasing the reaction time enables to further improve their crystallinity. For both OER and ORR activities, the same trend was found following beta-MnO2 nanorods < alpha-MnO2 nanowires with low crystallinity < alpha-MnO2 nanowires with high crystallinity, corroborating the view that the crystallographic structure is more determinative on the electrocatalytic behaviour of MnO2 than other contributors such as surface area. The superior OER and ORR activities of the alpha-MnO2 nanowires are mainly attributed to its large K+-embedded tunnel structure and high Mn-III content accompanied with ample oxygen vacancies. Liquid and solid-state rechargeable Zn-air batteries fabricated using the alpha-MnO2 nanowires demonstrate a high open-circuit potential of 1.51 V, a maximal power density of 166 mW cm(-2), superior cycling stability, as well as great potential in powering portable electronics.