A mixture of α-Ni(OH)2 and β-Ni(OH)2 nanoparticles with snow-ball like morphology was successfully synthesized by a facile gas–liquid co-deposition method. The capacitive behavior was tested by cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge/discharge measurements. A maximum specific capacitance of 2122 F/g was achieved at a current density of 250 mA/g and a potential range of 0.10–0.35 V (vs. Ag/AgCl electrode) for the Ni(OH)2 electrode. The asymmetric supercapacitor based on this Ni(OH)2 and activated carbon was fabricated. The maximum specific capacitance of 349 F/g and the maximal energy density of 66 Wh/kg were obtained at a current density of 250 mA/g with a capacitance loss of 5.4 % after 1,000 continuous cycles. These results demonstrate that gas–liquid co-deposition was an effective and facile method for synthesizing nanoparticles of nickel hydroxide with high capacitance.