High energy density asymmetric supercapacitors with a nickel oxide nanoflake cathode and a 3D reduced graphene oxide anode

Here we demonstrate a high energy density asymmetric supercapacitor with nickel oxide nanoflake arrays as the cathode and reduced graphene oxide as the anode. Nickel oxide nanoflake arrays were synthesized on a flexible carbon cloth substrate using a seed-mediated hydrothermal method. The reduced graphene oxide sheets were deposited on three-dimensional (3D) nickel foam by hydrothermal treatment of nickel foam in graphene oxide solution. The nanostructured electrodes provide a large effective surface area. The asymmetric supercapacitor device operates with a voltage of 1.7 V and achieved a remarkable areal capacitance of 248 mF cm^-2 (specific capacitance of 50 F g ^-1) at a charge/discharge current density of 1 mA cm^-2 and a maximum energy density of 39.9 W h kg^-1 (based on the total mass of active materials of 5.0 mg). Furthermore, the device showed an excellent charge/discharge cycling performance in 1.0 M KOH electrolyte at a current density of 5 mA cm^-2, with a capacitance retention of 95% after 3000 cycles.