Effect of molar concentration on the crystallite structures and electrochemical properties of cobalt fluoride hydroxide for hybrid supercapacitors

Cobalt fluoride hydroxide (Co(OH)F) crystallite structures with various cobalt concentrations (20, 40, 60, and 80 mM) were grown on nickel foam (NF) substrate using a traditional hydrothermal method and potentially applied as an electrode material for battery-type supercapacitors. Physicochemical analyses showed that the optimal Co(OH)F–60/NF electrode had orthorhombic crystalline phase and comprised of several rhombohedral, octahedral, and decahedral crystallite structures. The electrochemical double-layer capacitance and impedance spectroscopy analyses confirmed that the Co(OH)F–60/NF sample had high active sites and lower equivalent series resistance, respectively. Owing to the more electroactive sites, the Co(OH)F–60/NF electrode delivered a high specific capacity (389.9 C g−1 at 1 A g−1), and remarkable long-term stability (90.02% capacity retention after 10000 cycles) compared to the other Co(OH)F/NF electrodes assessed. A hybrid supercapacitor device (HSC) was designed using the optimized Co(OH)F–60/NF as a positive electrode and activated carbon (AC) as a negative electrode, which delivered a reasonable specific capacity of 130.0 C g−1 and an outstanding stability (92.74% capacitance retention after 8000 continuous cycles). The Co(OH)F–60/NF//AC HSC device provided a maximum energy density of 28.89 Wh kg−1 at a power density of 800.0 W kg−1.