High-Performance Pseudocapacitor Electrodes Based on α-Fe₂O₃/MnO₂Core–Shell Nanowire Heterostructure Arrays

A simple wet chemical technique has been employed to fabricate MnO2 nanolayer-coated α-Fe2O3/MnO2 core–shell nanowire heterostructure arrays to prepare unique pseudocapacitor electrodes. The coating of MnO2 on α-Fe2O3 nanowires is triggered by the reduction of KMnO4 solutions by the metallic (Au) film on which the polycrystalline α-Fe2O3 nanowires have been grown electrochemically. This metallic film also acts as the current collector by making direct contact with the arrays of the 1D nanoheterostructures. The as-prepared α-Fe2O3/MnO2 nanoheterostructures are found to exhibit excellent specific capacitance, high energy density, high power density, and long-term cyclic stability as compared with the bare α-Fe2O3 nanowire electrodes. The unique geometry of the 1D nanoheterostructures with high effective surface area to allow faster redox reaction kinetics, the incorporation of two highly redox active materials in the same structure, and the porous surface structures of the heterostructure to allow facile electrolyte diffusion help in the superior electrochemical performance of the α-Fe2O3/MnO2 nanoheterostructures. The maximum specific capacitance of 838 F g–1 (based on pristine MnO2) has been achieved by cyclic voltammetry at a scan rate of 2 mV s–1 in 1 M KOH aqueous solution. The hybrid α-Fe2O3/MnO2 nanocomposite electrodes also exhibit good rate capability with excellent specific energy density of 17 Wh kg–1 and specific power density of 30.6 kW kg–1 at a current density of 50 A g–1 and good long-term cycling stability (only 1.5% loss of its initial specific capacitance after 1000 cycles). These studies indicate that the α-Fe2O3/MnO2 nanoheterostructure architecture is very promising for next-generation high-performance pseudocapacitors.