Novel insights of structure evolution between ZIF and hydroxide via controlled doses of ammonium bifluoride and applications on battery supercapacitor hybrids

Metal organic framework is often post-treated to increase electrical conductivity and electroactivity for application on battery supercapacitor hybrids (BSH). An in-situ modification is previously proposed to replace post-treatments via incorporating ammonium fluoride (NH₄F) during synthesizing zeolitic imidazolate framework-67 (ZIF67) to develop derivatives with excellent energy storage ability. Inspired by structure directing ability of ammonium group, high electronegativity of fluorine-based group, and time and money-saving process of in-situ modification, a novel structure directing agent (SDA), ammonium bifluoride (NH₄HF₂), is proposed in this study to design ZIF67 derivatives via the facile one-step solution process for the first time. The amounts of NH₄HF₂ and 2-methylimidazole are varied to fabricate a series of ZIF67 derivatives, and the SDA effects on physical and electrochemical properties are investigated. The highest specific capacitance (C_F) of 1116.7 F/g and the capacity of 217.1 mAh/g are achieved for the optimal ZIF67 derivative at 20 mV/s, because of the thinnest layers with large surface area and abundant electrolyte contacts. The ZIF67 derivative and carbon electrodes are applied to fabricate a BSH, which displays a C_F retention of 84.8% and Coulombic efficiency of 96.1% after measuring the charge/discharge curves at 1.0 A/g for 10,000 cycles, as well as the maximum energy density of 7.3 Wh/kg at the power density of 714.3 W/kg.