Comparative study of mixed ammonium fluoride complex-derived single metal hydroxides based on cobalt and nickel for energy storage applications

Abstract

Zeolitic imidazolate framework 67 (ZIF67) have been intensively applied as efficient active materials for energy storage. Structure directing agents (SDA) play important roles on morphology and electrochemical performance of active materials. Incorporating SDA in synthesis is expected to confine growth of ZIF67 derivatives and create efficient active materials. Ammonium fluoride (NH4F) has been widely applied as an effective SDA for synthesizing active materials in electrochemical systems. Inspired by the confiner of NH4+ and the conductive agent of F−, two novel SDA of ammonium hydrogen fluoride (NH4HF2) and ammonia borane fluoride (NH4BF4) are introduced to synthesize Ni-based or Co-based ZIF67 derivatives via a one-step solution process at room temperature. Substituting 2-methylimidazole with HF2− and BF4− improves conductivity of ZIF67. Effects of NH4HF2 to NH4BF4 ratio for synthesizing ZIF67 derivatives are investigated. The main compositions of ZIF67 derivatives are cobalt and nickel hydroxides. The largest specific capacitance (CF) of 1112.4 F/g is attained for the optimized Ni-based ZIF67 derivative (NiHB) at 20 mV/s. An asymmetric supercapacitor with the NiHB and activated carbon electrodes shows a maximum energy density of 32 Wh/kg at the power density of 750 W/kg, and the CF retention of 70% and Coulombic efficiency of 90.6% after 10,000 cycles.