Novel synthesis of ammonia borane fluoride induced ZIF67 derivatives using facile one-step solution process for energy storage

Battery and supercapacitor hybrids (BSH) are regarded as efficient energy storage devices due to their high energy and power densities. Metal organic frameworks (MOF) with enhanced electrical conductivity by post treatments are widely utilized as electroactive materials in BSH, but in-situ modifications are considered better ways to improve charge storage in MOF due to their lower cost in money and time. Ammonium fluoride has been intensively used as a structure directing agents (SDA) for designing electroactive materials. Based on benefits from the ammonium cation group and fluorine-based anion group, ammonium tetrafluoroborate (NH₄BF₄) is applied as the novel SDA to design a series of zeolitic imidazolate framework-67 (ZIF67) derivatives using a one-step solution process as the electroactive material of BSH in this work. The ratio of 2-methylimidazole and NH₄BF₄ has a large influence on compositions and surface properties. The largest specific capacitance (C_F) of 1593.1 F/g is obtained for the optimal ZIF67 derivative owing to the high surface area, large pore volume, and high occupation of metal hydroxide. The optimal ZIF67 derivative and reduced graphene oxide are used to assemble a BSH, which presents a maximum energy density of 15.1 Wh/kg at 857.1 W/kg. The C_F retention of 87.5% and Coulombic efficiency of 97.2% after 6000 cycles are also obtained.