Novel synthesis of ZIF67 derived cobalt and nickel oxides induced by ammonium tetrafluoroborate as electroactive material of battery supercapacitor hybrids

Cobalt and nickel oxides with large theoretical capacitance, high redox activity and low-cost features are widely applied as electroactive material of battery supercapacitor hybrids (BSH) to reduce energy consumption and create renewable energy. Synthesizing cobalt and nickel oxides using zeolitic imidazolate framework 67 (ZIF67) can enhance conductivity and electroactivity and maintain surface area and porosity of ZIF67. Structure directing agent (SDA) can control morphology and improve electrochemical performance. It is attractive to apply SDA for controlling morphology of oxidized ZIF67 and achieving excellent energy storage. In this work, ammonium tetrafluoroborate (NH₄BF₄) is firstly applied as SDA to synthesize ZIF67 derivative, which is then annealed to form leaf-like cobalt and nickel oxides as electroactive material. The optimal oxidized ZIF67 derivative shows a larger specific capacitance (C_F) of 828.9 F/g than those of 519.9 and 5.0 F/g respectively for the oxidized ZIF67 and pristine ZIF67 electrodes, due to preferable leaf-like structure and (311) facet for the former case. A BSH with oxidized ZIF67 derivative positive electrode and graphene negative electrode shows maximum energy density of 10.2 Wh/kg at 357 W/kg, and C_F retention of 91% and Coulombic efficiency of 97% after 10,000 cycles. The novel SDA of NH₄BF₄ is expected to design other efficient electroactive materials for effective energy storage.