Investigating energy storage ability of ZIF67-derived perovskite fluoride via tuning ammonium fluoride amounts

Abstract

Zeolitic imidazolate framework 67 (ZIF67) is widely considered as potential active material for supercapacitors (SC) due to large surface area and tunable structures, but small electrical conductivity limits its energy storage ability. Fluoride with high electrical conductivity is reported to be beneficial on reducing charge-transfer resistance of SC. In this study, ZIF67-derived perovskite fluoride is synthesized using ammonium fluoride (NH₄F) as electroactive material of SC at the first time. Different NH₄F amounts are used to produce perovskite ZIF67-derived fluorides (ZIF67-N). The optimized ZIF67-N electrode shows specific capacitance (C_F) of 636.8 F/g at 10 mV/s, owing to small particle size and suitable F^- to 2-methylimidazole ratio for providing high electronegativity. The ZIF67 and cobalt nickel fluoride prepared using NH₄F but no 2-methylimidazole (CoNi-N) are synthesized to understand roles of fluorine and 2-methylimidazole on energy storage. The ZIF67 electrode shows much smaller C_F (1.6 F/g) than ZIF67-N electrode, owing to largely enhanced pore width of ZIF67-N even if surface area is largely reduced when NH₄F is added during synthesis. The SC comprising optimized ZIF67-N electrodes shows maximum energy density of 27.2 Wh/kg at 650.0 W/kg as well as C_F retention of 86% and Coulombic efficiency of 100% in 8000 times charge/discharge process.