A systematic approach to achieve high energy density hybrid supercapacitors based on Ni–Co–Fe hydroxide

Recently, ternary metal compounds have received substantial attention owing to their remarkable characteristics such as multiple valence states and a high electrochemical activity. Herein, we propose and validate a nickel–cobalt–iron (NCF) hydroxide cathode material for use in high energy density hybrid supercapacitors. The molar ratio of the NCF compounds was controlled and systematically studied to improve the electrochemical performance of the proposed cathode material. A high nickel ratio can enhance the specific capacitance of the metal hydroxide, and cobalt increases the rate capability. High nickel-based NCF hydroxide exhibits a high specific capacitance of 1321 F g⁻¹ at a current density of 1 A g⁻¹ with 57.7% capacitance retention observed at 20 A g⁻¹. An aqueous HSC based on NCF hydroxide and activated carbon exhibits a high energy density (73.07 Wh kg⁻¹) at a power density of 1.07 kW kg⁻¹ with remarkable cycle stability (75.5% after 50,000 cycles).