Green Synthesis of Three-Dimensional MnO2/Graphene Hydrogel Composites as a High-Performance Electrode Material for Supercapacitors.

Graphene hydrogels (GHs) and their composites have attracted wide attention because of the special structure of graphene assembly and exceptional electrochemical performance as electrodes for energy storage devices. Here, we report a GH with three-dimensional architecture prepared by a hydrothermal method via a self-assembled process in glucose and ammonia system as well as subsequent freeze-drying. The δ-MnO2/GH composite was then obtained by immersing GH in KMnO4 solution with a certain concentration under heat treatment. The asymmetric supercapacitor MnO2/GH//GH consisting of pseudocapacitive nanosheet-like δ-MnO2/GH as the cathode and electric double-layer capacitive GH as the anode provides high energy density of 34.7 W h/kg at a power density of 1.0 kW/kg. Importantly, it is found that the pseudocapacitive behavior of MnO2 has great effects on the rate performance of the supercapacitor, which is identified by kinetic analysis.