Bimetallic Nickel–Cobalt Selenide (Ni–CoSe ₂ ) Nanomaterials for Asymmetric Supercapacitors

Supercapacitors have garnered significant attention as promising electrochemical energy storage devices owing to their rapid charge–discharge kinetics, high power density, low cost, and excellent operational safety. In this study, bimetallic nickel–cobalt selenide (Ni–CoSe2) was successfully synthesized via hydrothermal selenization of Ni–Co(OH)2. The as-prepared Ni–CoSe2 nanomaterials electrode exhibited a high specific capacitance of 1022 F g–1 at a current density of 1 A g–1 and retained 70% of its initial capacity even at 25 A g–1, underscoring its outstanding rate capability. The enhanced electrochemical performance is attributed to the synergistic effect of Ni and Co cations, which promote rich redox activity and fast charge transport. To further evaluate its practical application, an asymmetric supercapacitor (ASC) device was assembled using Ni–CoSe2 as the positive electrode and reduced graphene oxide (rGO) as the negative electrode. The ASC device delivered a specific capacitance of 104 F g–1 at 1 A g–1, with a maximum energy density of 37 Wh kg–1 at a power density of 3809 W kg–1. Notably, the device demonstrated excellent cycling durability, retaining 73% of its initial capacitance with a high Coulombic efficiency of 98.58% after 10,000 charge–discharge cycles at 5 A g–1. These findings highlight the potential of Ni–CoSe2 as a high-performance electrode material for next-generation energy storage devices.