Fabrication of a 3D MnO2/graphene hydrogel for high-performance asymmetric supercapacitors

A three-dimensional hydrogel of graphene and MnO2 nanoparticles has been prepared by a two-step synthesis. At first, nanoparticles of MnO2 were grown onto both surfaces of nanosheets of graphene oxide with the aid of benzyl alcohol in an aqueous suspension, and then the composite hydrogel of reduced graphene oxide (rGO) and MnO2 was prepared by a reduction-induced in situ self-assembly. Then, asymmetric supercapacitors with high energy and power densities were fabricated using the MnO2/rGO hydrogel as the positive electrode and a pure rGO hydrogel as the negative electrode in a neutral aqueous Na2SO4 (1.0 M) electrolyte. The asymmetric supercapacitor exhibited a high performance with an energy density of 21.2 W h kg−1 and a power density of 0.82 kW kg−1, and it can be cycled reversibly in a wide potential window of 1.6 V (0–1.6 V). The asymmetric supercapacitor also exhibited stable cycling performance with 89.6% capacitance after 1000 cycles. This work provides a new method for the preparation of a metal oxide/graphene hydrogel-based asymmetric supercapacitor free of polymer binders and conducting additives.