Strategic design of cobalt-based bimetallic compounds using NH₄BF₄ as a structure-directing agent for enhancing energy storage ability of battery supercapacitor hybrids

Modifying morphology and composition of metal-organic frameworks (MOFs) can enhance their electrical conductivities and redox states. Structure-directing agents (SDAs) play a key role in shaping surface properties of materials. Ammonium fluoride-based complexes are useful in designing MOF derivatives with excellent energy storage capabilities. Metal species involved also affect redox reactions, which are crucial for energy storage in battery-type electrodes. In this study, cobalt-based bimetallic compounds derived from ZIF-67 are synthesized using 2-methylimidazole and the SDA of NH₄BF₄. The secondary metal species contain Cu, Al, Zn and Mn, and resulting compositions include hydroxides and hydroxide nitrates. With the optimal 2-methylimidazole to NH₄BF₄ ratio, the best-performed cobalt-based compound is related to Ni (MB21-CoNi) which demonstrates the highest specific capacitance (C_F) of 763.2 F/g at 20 mV/s, attributed to high theoretical capacities and sheets-assembled flower-like structures. A battery-supercapacitor hybrid (BSH) composed of carbon and MB21-CoNi electrodes achieves a maximum energy density of 17.2 Wh/kg at 650 W/kg. A C_F retention of 94.9% and a Coulombic efficiency of 88.9% after 10,000 cycles are attained for this BSH. This work provides a blueprint for designing novel active materials with helps of 2-methylimidazole and NH₄BF₄, and for discussing significant parameters to achieve excellent energy storage ability.