High-performance asymmetric supercapacitor based on graphene hydrogel and nanostructured MnO ₂

We have successfully fabricated an asymmetric supercapacitor with high energy and power densities using graphene hydrogel (GH) with 3D interconnected pores as the negative electrode and vertically aligned MnO ₂ nanoplates on nickel foam (MnO ₂-NF) as the positive electrode in a neutral aqueous Na ₂SO ₄ electrolyte. Because of the desirable porous structure, high specific capacitance and rate capability of GH and MnO ₂-NF, complementary potential window of the two electrodes, and the elimination of polymer binders and conducting additives, the asymmetric supercapacitor can be cycled reversibly in a wide potential window of 02.0 V and exhibits an energy density of 23.2 Wh kg ¹ with a power density of 1.0 kW kg ¹. Energy density of the asymmetric supercapacitor is significantly improved in comparison with those of symmetric supercapacitors based on GH (5.5 Wh kg ¹) and MnO ₂-NF (6.7 Wh kg ¹). Even at a high power density of 10.0 kW kg ¹, the asymmetric supercapacitor can deliver a high energy density of 14.9 Wh kg ¹. The asymmetric supercapacitor also presents stable cycling performance with 83.4% capacitance retention after 5000 cycles.