A reduced graphene oxide/mixed-valence manganese oxide composite electrode for tailorable and surface mountable supercapacitors with high capacitance and super-long life

Developing supercapacitor electrodes with an ultra-long cycle life and a high specific capacitance is critical to the future energy storage devices. Herein, we report a scalable synthesis technology of mixed-valence manganese oxide nanoparticles anchored to reduced graphene oxide (rGO/MnO_x) as the high-performance supercapacitor electrodes. First, 2-dimensional (2D) δ-MnO₂ nanosheets are formed on a graphene oxide (GO) template, which is then in situ reduced by hydrazine vapour to mixed-valence manganese oxide nanoparticles evenly distributed on a rGO conductive network. The obtained rGO/MnO_x electrode material exhibits a high specific capacitance of 202 F g^-1 (mass loading of 2 mg cm^-2), a large areal specific capacitance of 2.5 F cm^-2 (mass loading of up to 19 mg cm^-2), and a super-long-life stability of 106% capacitance retention after 115 000 charge/discharge cycles. By using an ionic liquid electrolyte and an activated carbon anode, asymmetric supercapacitors (AScs) are also constructed and can be packaged into a high performance miniaturized energy storage component in either a tailorable or surface mountable configuration. Our ASc shows superior performance characteristics, with typical figures of merit including maximum energy densities of 47.9 W h kg^-1 at 270 W kg^-1 and 19.1 W h kg^-1 at the maximum power density of 20.8 kW kg^-1. The capacitance retention of the ASc is 96% after 80 000 charge/discharge cycles, which is the most excellent stability performance in an ionic liquid electrolyte as compared with the recently reported pseudo-supercapacitors. This technology may find vast applications in future miniaturized portable and wearable electronics.