Enhanced electrochemical activity of perforated graphene in nickel-oxide-based supercapacitors and fabrication of potential asymmetric supercapacitors

We synthesize a hierarchically porous electrode material with a high specific capacity composed of nickel oxide (NiO) nanosheets and perforated graphene (PG) sheets grown on a three-dimensional (3D) macroporous nickel foam via a facile hydrothermal method. Employing perforated graphene instead of non-perforated graphene greatly improves the electrochemical performance of the composite by increasing the specific surface area of the NiO/PG composite owing to the small perforations in the graphene and by improving the mechanical strength and electrical conductivity of NiO due to the graphene covering layer. The PG also provides (a) the accessibility and diffusion of ions onto NiO and PG surfaces through the perforations; (b) the electrolyte cages in the spaces below the perforations to allow ion-reversible adsorption to the inner surface of the graphene; (c) the inevitable edge defects around the holes causing reactivity with ions. After assembling an asymmetric supercapacitor coin cell composed of NiO/PG as the positive electrode, a separator, PG as the negative electrode, and a 1 M KOH electrolyte, the coin cell exhibits a high energy density of 57.8 W h kg−1 at a power density of 1030.9 W kg−1 and excellent cycling stability (82.1%) after 10 000 cycles.