Two-step pyrolysis-hydrothermal synthesis of hierarchical nickel‑cobalt phosphate nanoflakes decorated on g-C3N4 nanosheets for high-performance hybrid supercapacitors

Abstract

Ni-Co phosphate (NCP) nanoflakes and graphitic carbon nitride (g-CN) nanosheets are combined to fabricate a composite electrochemical electrode via a two-step pyrolysis-hydrothermal process. Physicochemical characterizations reveal the proper formation of the target material with highly porous morphology and crumpled nanostructures with highly active surface sites. The electrochemical properties of the NCP/g-CN composite in a three-electrode system depict excellent pseudocapacitive performance due to the faradaic behavior and redox activities of both NCP and g-CN with a very high capacitive value of 2000 F/g (242 mAh/g) @ 1 A/g current density. The charge storage behavior is mainly dominated by the surface-controlled (capacitive) process at higher scan rates, which is mostly dominated by surface pseudocapacitance, whereas, at low scan rates both surface-controlled and diffusion-controlled processes are equally active to provide a combined battery-capacitor behavior. Due to this, a hybrid supercapacitor device, fabricated with NCP/g-CN as positive and activated carbon (AC) as a negative electrode, showed excellent energy density (50 Wh/kg) along with exceptional cyclability (∼90 % capacity retention over 5000 cycles). Additionally, the redox-rich g-CN matrix within the composite dominates the sluggish diffusion-controlled process at higher scan rates to enhance the capacitive process without compromising the charge storage kinetics, thus enhancing the power density to ∼7.0 kW/kg. This excellent energy-power performance of the NCP/g-CN//AC hybrid supercapacitor cell (HSC) is better than some of the commercial devices like lead-acid, Ni-Cd, and Ni-MH batteries, indicating potential applications in next-generation charge storage devices.