Reagent-and solvent-mediated Fe<inf>2</inf>O<inf>3</inf> morphologies and electrochemical mechanism of Fe<inf>2</inf>O<inf>3</inf> supercapacitors

Abstract

A solvothermal technique was used to synthesize nine different ferric oxide (Fe2O3) morphologies: rhomb (R), flower (F), hollow sphere (HS), crystal (C), elongated hexagon (EH), hexagon (H), sugar apple (SA), sand/spherical particle (SSP) and mixed particle (MP). X-ray diffraction, high-resolution transmission electron microscopy and selected area electron diffraction reveal six of the nine powders to be composed of the pure α-Fe2O3 structure, whereas the EH-Fe2O3, H-Fe2O3 and SA-Fe2O3 powders contain the mixed α-Fe2O3/Fe3O4 structure. The F-Fe2O3 powder has the highest total specific pore volume (0.059 cm3 g−1), the largest average pore size (23.983 nm), and a high specific surface area (9.82 m2 g−1), which subsequently produce the highest specific capacitance of 218.49 F g−1. X-ray photoemission spectroscopy and energy dispersive spectroscopy detect H2O and K+ adsorption on the F-Fe2O3 electrode and the reduction of Fe3+ to Fe2+ in the charged state, whereas H2O molecules and K+ ions are released from the F-Fe2O3 electrode, and Fe2+ is oxidized to Fe3+ in the discharged state. The simulated K-inserted-α-Fe2O3 structure shows an increased electron density surrounding Fe atoms, which is indicative of Fe3+ reduction during the charged state. The F-Fe2O3 film is able to retain 76.81 % of its 20th cycle value after 1,000 cycles. Four series-supercapacitor coin cells constructed from the F-Fe2O3 anode and the MnO2 cathode deliver an outstanding energy density of 10.96 Wh kg−1 and power density of 0.461 kW kg−1.