MnO2 nanosheets@nitrogen-doped graphene aerogel enable high specific energy and high specific power for supercapacitor and Zn-air battery

High specific energy and specific power are required for practical energy storage and conversion devices such as supercapacitors and Zn–air batteries. In this study, an aerogel of MnO2 nanosheet@nitrogen-doped graphene (MNSs@NGA) featuring high ratio of exposed surface unit cells of MNS, high specific surface area, hierarchical pores as well as good electronic conductivity is obtained with an ultrathin and poorly-crystallized MNSs network vertically grown on the surface of the porous NGA with the Mn–O–C and Mn–N bonds. The high ratio of exposed surface unit cells in ultrathin MNSs was calculated and quantitatively reported for the first time for further understanding of high specific capacitance related to the large surface area of MNSs@NGA. This MNSs@NGA yields excellent performances, including high specific energy and high specific power, for both supercapacitors and Zn–air batteries, better than those reported by MnOx-based materials. Based on the weight ratio of MnO2, MNSs@NGA shows high specific capacitance, approaching the theoretical specific capacitance of MnO2. Related mechanisms are discussed concerning fast electronic and ionic transfer characteristics of MNSs@NGA in aqueous media for efficient energy conversion and storage.