In-situ growth of novel iron oxide/iron organic framework composites as efficient electroactive materials of battery supercapacitor hybrids

Iron oxide (Fe₂O₃) with a high theoretical capacitance, wide potential ranges and easy availability has attracted much attentions as the electrode material of battery supercapacitor hybrids (BSH). Metal–organic framework (MOF) is one of the promising potential energy storage materials due to its large surface area and controllable pore structures. In this work, novel iron oxide/iron organic framework composites (Fe₂O₃/FeMOF) are in-situ grown on the Ni foam as efficient energy storage electrodes of BSH. By tuning the solvothermal durations, the morphology changes from random sizes of sheets, regular grown vertical sheets, to flower-like structures. The d-spacing also increases for the Fe₂O₃/FeMOF synthesized using longer solvothermal durations. The optimal Fe₂O₃/FeMOF electrode synthesized using 9 h shows a high areal capacitance of 10.97 F/cm² along with an areal capacity of 5.49 F/cm² at the current density of 35 mA/cm², owing to the most regular growth of sheets, the large d-spacing, and the smallest charge-transfer resistances. A BSH composed of the Fe₂O₃/FeMOF positive electrode and a graphene negative electrode shows a maximum energy density of 0.76 mWh/cm² at 22.5 mW/cm². The excellent long-term cycling stability with the capacitance retention of 85% and Coulombic efficiency of 98% are also achieved after 10,000 charge/discharge cycles.