Improving energy storage ability of nickel and cobalt hydroxides coupled with reduced graphene oxide via incorporating novel structure-directing agents for supercapacitors

Abstract

Zeolitic imidazolate framework 67 (ZIF67) derivatives are regarded as potential active materials for energy storage, owing to the possible enhancement on electrical conductivity and maintenance of high surface area and tunable pore structure. However, current improvements on electrical and physical properties of ZIF67 derivatives are still insufficient to achieve excellent energy storage abilities. In this study, new types of carbon material and structure-directing agents (SDA) are incorporated into the synthesis of ZIF67 derivatives to enhance electrical conductivity and modulate morphology. Commercial reduced graphene oxide (rGO), NH₄BF₄ and NH₄HF₂ are applied as the carbon material and SDAs, respectively. With the optimal rGO ratio, the rGO/ZIF67 derivative electrode exhibits a higher specific capacitance (C_F) of 1304.7 F/g (913.0 C/g) at 20 mV/s, while the ZIF67 derivative electrode without rGO shows a C_F value of 786.9 F/g (550.8 C/g). The supercapacitor composed of the optimal rGO/ZIF67 derivative and rGO electrodes presents a maximum energy density of 30.2 Wh/kg at 350 W/kg. The Coulombic efficiency of 93.0% and C_F retention of 86.5% are achieved for the device after 10,000 cycles. The novel carbon material and SDAs are expected to be applicable in more electrochemical systems to achieve better energy storage ability.