Synergetic effect of a battery-like nickel phosphide and a pseudocapacitive cobalt phosphide electrodes for enhanced energy storage

The development of high energy density supercapacitors is essential due to the increasing concerns about environmental pollution and the demand for energy storage systems. Although the commercial electrode for the supercapacitor is composed of an electric double layer capacitance (EDLC)-based carbon materials, other transition metal-based pseudocapacitive or battery-like materials have also been investigated by many researchers; the energy density value of these devices need subsequent improvement. In this study, we developed hybrid electrodes composed of metal-organic frame-derived cobalt phosphide (CoP) and electrodeposited nickel phosphide (NiP) materials on nickel foam (NF). This hybrid electrode shows a high energy density value due to the combination of a pseudocapacitive CoP with a battery-like NiP material. The NiP/CoP@NF electrode exhibits an enhanced specific capacitance of 1154.4 F g⁻¹ at 1 A g⁻¹, which are much higher than those of individual NiP@NF and CoP@NF. The asymmetric supercapacitor (ASC) exhibits an excellent energy density of 27.7 Wh kg⁻¹ at a power density of 800 W kg⁻¹ and long-term stability with capacitance retention of 86.36 % for up to 10,000 cycles, representing their remarkable energy storage performance when compared to the previously reported NiCoP-based ASCs.