Sulfurization of nickel–cobalt fluoride decorating ammonia ions as efficient active material of supercapacitor

Abstract

Nickel cobalt fluoride decorating ammonia ions (NCNF) with excellent energy storage ability is proposed by incorporating ammonia fluoride during synthesizing ZIF-67 in our previous work. It is necessary to modify NCNF for achieving better energy storage ability and widen its application. In this study, sulfurization and copper decoration are firstly adopted to enhance energy storage ability of NCNF. Without Cu decoration, aggregations completely convert to separated nanoparticles at sulfurization temperature of 160°C, which also shows partial conversion of NCNF to cobalt sulfide. The highest specific capacitance (CF) of 1666.1 ​F/g (277.7 mAh/g) is achieved for optimized NCNF-derived sulfide, and untreated NCNF electrode only shows a CF value of 519.8 ​F/g (277.7 mAh/g), due to partial sulfurization of the former case with high electrocapacitive property of NCNF and high electrical conductivity of sulfide. Copper is then incorporated using one-step and two-step hydrothermal processes. However, the best performed copper-decorated NCNF-derived sulfide electrode only shows a CF value of 508.5 ​F/g (84.8 mAh/g), due to the dominated formation of copper sulfide and limited appearances of cobalt sulfide and NCNF. A symmetric energy storage device comprising optimized NCNF-derived sulfide electrodes presents a maximum energy density of 8.64 ​Wh/kg at 2.16 ​kW/kg and high capacitance retention of 125% after 13,000 cycles.