Compounding δ-MnO2 with modified graphene nanosheets for highly stable asymmetric supercapacitors

Abstract

MnO2 is extremely attractive in the energy storage field, but the combination with highly conductive materials to enhance the electrical conductivity is still desired. Here we synthesize the composite of δ-MnO2 and glucose modified graphene nanosheets via a facile hydrothermal process. Graphene is transformed into 3D substrate by short-range self-assembly, on which MnO2 grows densely to construct the composite with both small mesopores (4 nm) and large mesopores (9–50 nm), showing high specific capacitance and ultrastability when used as supercapacitor materials. The synthesized composite exhibits the specific capacitance of 270 F g−1 at 0.5 A g−1, still 168 F g−1 at the high current density of 15 A g−1. Importantly, the kinetics and quantitative analysis confirms the domination of capacitive contribution that enlarges with the scan speed and reaches 87.1% at 100 mV s−1, which accounts for the excellent rate capability. The composite electrode maintains 90.12% of the initial specific capacitance after 50,000 cycles at 10 A g−1. The asymmetric supercapacitor assembled with the composite cathode and graphene anode delivers the specific capacitance of 76 F g−1 at 0.5 A g−1. The high energy density of 30.1 Wh kg−1 is provided at the power density of 945 W kg−1. In addition, the supercapacitor exhibits an ultra-high stability of 15,000 cycles with 90.4% of initial capacitance remains while maintaining the Coulombic efficiency close to 100%. Thus, the proper modification of graphene greatly benefits the performance of supercapacitors.