Incorporating self-assembled MXene to improve electrical conductivity of novel ZIF67 derivatives induced by NH4BF4 and NH4HF2 for energy storage

Zeolitic imidazolate framework (ZIF-67) with high surface area and tunable pore size are considered as a promising active material for energy storage, but the low electrical conductivity restricts its performance. Developing ZIF67 derivatives with improved electrical conductivity is necessary to broaden its application in electrochemical fields. In this work, two strategies are applied to improve electrical conductivity and electrochemical performance of ZIF67. Novel structure directing agents of NH4BF4 and NH4HF2 are used to synthesize ZIF67 derivatives with higher theoretical capacitances and favorable layered structures. Self-assembled MXene with different amounts is further incorporated in ZIF67 derivatives to establish composites. The optimal composite electrode shows a high specific capacitance (CF) of 1410.4 F/g (775.6C/g) at 20 mV/s, while the ZIF67 derivative electrode without MXene only shows a CF value of 942.3 F/g (518.3C/g), due to the high surface area and better pore structure as well as the close combination of MXene and the layered ZIF67 derivative for the former case. An energy storage device with the optimal composite and carbon electrodes shows the maximum energy density of 32.40 Wh/kg at 375 W/kg. The Coulombic efficiency of 100% and the CF retention of 77.4% are also achieved after 10,000 cycles.