Concentration and reaction duration effects on synthesizing ZIF67 derivatives with NH4BF4 and NH4HF2 as efficient active material of energy storage device

Zeolitic imidazolate framework-67 (ZIF67) with large surface area and tunable pore structure is intensively applied as a tailoring target for design active materials. An in-situ tailoring technique is proposed to design ZIF67 derivatives by using structure directing agents (SDA) to induce favorable morphology and composition. Ammonium fluoride is a common SDA for modifying surface properties of active materials. With the same ammonium and similar fluorine-based groups, NH4BF4 and NH4HF2 are priming to play as SDA for synthesizing ZIF67 derivatives. In this study, the synthetic process of ZIF67 derivatives is tailored by varying the precursor concentration and reaction time in systems with NH4BF4 and NH4HF2 as SDA. The precursor concentration plays significant roles in the composition of ZIF67 derivatives. The reaction duration has higher influences on the morphology. The highest specific capacitance (CF) of 1347.0 F/g at 20 mV/s is achieved for the ZIF67 derivative synthesized using low concentration and 18 h, owing to favorable cobalt and nickel hydroxide compositions and two-dimensional sheet-like structure. An energy storage device composed of the optimal ZIF67 derivative and carbon electrodes shows a maximum energy density of 11.96 Wh/kg at 650 W/kg. The excellent cycling stability with CF retention of 83.6% and Coulombic efficiency of 92.0% is obtained after 10000 cycles.