Hierarchical Fe₃O₄@Fe₂O₃ Core–Shell Nanorod Arrays as High-Performance Anodes for Asymmetric Supercapacitors

Anode materials with relatively low capacitance remain a great challenge for asymmetric supercapacitors (ASCs) to pursue high energy density. Hematite (α-Fe₂O₃) has attracted intensive attention as anode material for ASCs, because of its suitable reversible redox reactions in a negative potential window (from 0 V to −1 V vs Ag/AgCl), high theoretical capacitance, rich abundance, and nontoxic features. Nevertheless, the Fe₂O₃ electrode cannot deliver large volumetric capacitance at a high rate, because of its poor electrical conductivity (∼10^–14 S/cm), resulting in low power density and low energy density. In this work, a hierarchical heterostructure comprising Fe₃O₄@Fe₂O₃ core–shell nanorod arrays (NRAs) is presented and investigated as the negative electrode for ASCs. Consequently, the Fe₃O₄@Fe₂O₃ electrode exhibits superior supercapacitive performance, compared to the bare Fe₂O₃ and Fe₃O₄ NRAs electrodes, demonstrating large volumetric capacitance (up to 1206 F/cm³ with a mass loading of 1.25 mg/cm²), as well as good rate capability and cycling stability. The hybrid electrode design is also adopted to prepare Fe₃O₄@MnO₂ core–shell NRAs as the positive electrode for ASCs. Significantly, the as-assembled 2 V ASC device delivered a high energy density of 0.83 mWh/cm³ at a power density of 15.6 mW/cm³. This work constitutes the first demonstration of Fe₃O₄ as the conductive supports for Fe₂O₃ to address the concerns about its poor electronic and ionic transport