Unravelling the correlation between nickel to copper ratio of binary oxides and their superior supercapacitor performance

Specific capacitance and energy density are found increasing greatly after the incorporation of certain amount of copper (Cu) into nickel oxide (NiO) electrode. In this work, a series of binary nickel copper oxides is synthesized directly on flexible lightweight carbon fiber paper substrates as binder-free electrodes. The correlation between the Ni/Cu ratio and supercapacitor performance is revealled. The highest specific capacitance achieved is 1711 F g−1 at a higher discharge current density of 5 A g−1, which is significantly higher than the NiO sample without Cu incorporation. The success in high loading of active material growth on lightweight carbon substrate leads to a very high specific capacitance of 637 F g−1 per total weight of the active material and the substrate at 1 A g−1. The asymmetric supercapacitor full cell, composed of nickel copper oxide as positive electrode and reduced graphene oxide as negative electrode, reaches up to a very high energy density of 70 Wh kg−1. The energy density per total mass of all the active materials and substrates is fantastically high, 40 Wh kg−1, which is of great practical importance in applications. The remarkable electrochemical performance is attributed to the desirable composition and the synergistic effect between nickel and copper.