Effect of Fluorine on β-Ni(OH) ₂ for High-Performance Aqueous Hybrid Supercapacitor Electrodes

This study aims to enhance the electrochemical properties of β-Ni(OH)2 by doping it with a single, highly electronegative fluorine atom. It is anticipated that doping with an optimal fluorine ratio will result in significant improvements in both structural integrity and electrical conductivity. Fluorine incorporation can modulate the electronic structure of Ni(OH)2, enhance its intrinsic conductivity, create additional defect sites, and increase the electrochemically active sites available for charge storage. These effects collectively facilitate faster charge transfer and ion diffusion, thereby improving electrochemical performance. Fluorine is doped into β-Ni(OH)2, prepared for different concentrations using ammonium fluoride (NH4F). The electrochemical performance of F-β-Ni(OH) 2 is studied and compared with that of bare β-Ni(OH)2. To evaluate the electrochemical behavior of the prepared electrodes, cyclic voltammetry, galvanostatic charge–discharge, and electrochemical impedance spectroscopy were performed in a standard three-electrode configuration using 2 M KOH as the electrolyte. The fluorine-doped β-Ni(OH)2 electrode containing 2 wt % F exhibited a maximum specific capacity of 258 mA h g–1 at a current density of 1 A g–1, which corresponds to nearly 90% of the theoretical capacity of pristine β-Ni(OH)2. To check the practical viability of the prepared F-doped β-Ni(OH)2 material, pouch cells were fabricated. It was observed that the 2 wt % AC||PVA–KOH||F-β-Ni(OH)2 hybrid supercapacitor demonstrated the maximum specific capacitance of 180 F g–1 when tested at 0.5 A g–1, compared to 77 F g–1 by the bare AC||PVA-KOH||β-Ni(OH)2 hybrid supercapacitor HSCs at the same current density. The AC||PVA-KOH||F-β-Ni(OH)2 HSC delivered a maximum specific energy of 64.8 W h kg–1 at a specific power of 704.25 W kg–1 and a maximum specific power of 3.97 kW kg–1 to a specific energy of 44.62 W h kg–1. AC||PVA-KOH||F-β-Ni(OH)2 HSC demonstrated excellent cycling stability over 10,000 charge–discharge cycles, maintaining a Coulombic efficiency above 99%. Thus, the obtained results indicate that F doping in layered β-Ni(OH)2 significantly improves both charge storage and durability, making it a promising positive electrode for hybrid capacitor applications.