High Volumetric Energy Density Asymmetric Supercapacitors Based on Well-Balanced Graphene and Graphene-MnO2 Electrodes with Densely Stacked Architectures.

The well-matched electrochemical parameters of positive and negative electrodes, such as specific capacitance, rate performance, and cycling stability, are important for obtaining high-performance asymmetric supercapacitors. Herein, a facile and cost-effective strategy is demonstrated for the fabrication of 3D densely stacked graphene (DSG) and graphene-MnO2 (G-MnO2 ) architectures as the electrode materials for asymmetric supercapacitors (ASCs) by using MnO2 -intercalated graphite oxide (GO-MnO2 ) as the precursor. DSG has a stacked graphene structure with continuous ion transport network in-between the sheets, resulting in a high volumetric capacitance of 366 F cm-3 , almost 2.5 times than that of reduced graphene oxide, as well as long cycle life (93% capacitance retention after 10 000 cycles). More importantly, almost similar electrochemical properties, such as specific capacitance, rate performance, and cycling stability, are obtained for DSG as the negative electrode and G-MnO2 as the positive electrode. As a result, the assembled ASC delivers both ultrahigh gravimetric and volumetric energy densities of 62.4 Wh kg-1 and 54.4 Wh L-1 (based on total volume of two electrodes) in 1 m Na2 SO4 aqueous electrolyte, respectively, much higher than most of previously reported ASCs in aqueous electrolytes.