Phase- and Crystal Structure-Controlled Synthesis of Bi2O3, Fe2O3, and BiFeO3 Nanomaterials for Energy Storage Devices

Controlling the phase and crystal structure of nanomaterials is a challenging mission in a wet chemical method and has remarkable importance to the materials properties. Herein, we demonstrate a facile sol-gel method to synthesize Bi2O3, Fe2O3, BiFeO3, Bi36Fe2O57, secondary phase, and mixed phase of BiFeO3 (Bi25FeO40 and Bi2Fe4O9) by tailoring the parameters such as molar concentration, calcination temperature, and duration. Further, all the electrode materials were demonstrated for supercapacitor (SC) application. The pure-phase BiFeO3 nanoparticles show a highest specific capacitance of 253 F/g at a current density of 1 A/g compared to all other electrodes under a 3 M KOH electrolyte. The higher specific capacitance of BiFeO3 nanoparticles is ascribed to their higher surface area, pure ABO3 structure, and lower charge-transfer resistance. Moreover, the BiFeO3 nanoparticles were also tested under a neutral electrolyte (1 M Na2SO4) and found to have 3.7 times lower specific capacitance compared to the alkaline electrolyte (3 M KOH). The electrokinetic study of the as-synthesized active electrodes illustrates the maximum capacitive involvement to store the overall charge. The BiFeO3 nanoparticles display outstanding stability with a retention rate of 99.02% after 1100 consecutive galvanostatic charge-discharge cycles at various current densities. Moreover, a solid-state symmetric SC device (SSD) was fabricated using BiFeO3 nanoparticles. The device delivered a maximum energy density of 17.01 W h/kg at a current density of 1 A/g and a power density of 7.2 kW/kg at a current density of 10 A/g. The BiFeO3 SSD showed an excellent capacitive retention rate of 88% after 5000 cycles, suggesting that it could be a promising electrode material for practical application in energy storage devices.