Comparative studies of cobalt hydroxide and nickel hydroxide designed using novel metal tetrafluoroborate as active materials of battery supercapacitor hybrids

Structure directing agent (SDA) can tailor morphology of active materials with preferable surface properties. Ammonium fluoroborate has been proposed as an effective SDA to synthesize metal organic framework (MOF) derivatives in previous reports, where fluoroborate ions play as morphology regulators by providing steric hindrance. Nickel and cobalt hydroxides are intensively used as active materials in battery supercapacitor hybrids (BSHs), owing to high theoretical capacitances and durability. However, previous studies rarely investigate metal specie effects on energy storage and compare electrochemical performance of nickel and cobalt hydroxides particularly in unique synthesis environments. In this study, nickel tetrafluoroborate (Ni(BF 4 ) 2 ) and cobalt tetrafluoroborate (Co(BF 4 ) 2 ) are respectively used as nickel and cobalt sources to synthesize hydroxides as active materials of BSHs. Tetrafluoroborate ions act as SDAs to design favorable morphologies, while 2-methylimidazole is incorporated to control the pH value for facilitating precipitations. The Ni(OH) 2 and Co(OH) 2 electrodes respectively show specific capacitances (C F ) of 1118.5 and 334.3 F/g at 20 mV/s, owing to the preferable alpha-type hydroxide and larger pore volume/width for Ni(OH) 2 . A BSH consisted of a Ni(OH) 2 positive electrode shows a maximum energy density of 64.0 Wh/kg at 700.0 W/kg, and the C F retention of 116% and Coulombic efficiency of 94% after 5000 cycles. • Ni(BF 4 ) 2 and Co(BF 4 ) 2 are used as new metal sources and structure directing agent. • Ni and Co hydroxides are active materials of battery supercapacitor hybrids (BSH). • Ligand of 2-methylimidazole is used to control pH values and induce precipitation. • A Ni(OH) 2 electrode shows a higher C F value of 1118.5 F/g due to α-type hydroxide. • A maximum energy density of 64.0 Wh/kg at 700.0 W/kg is obtained for the BSH.