Hierarchical\nFe₃O₄@Fe₂O₃ Core–Shell\nNanorod Arrays as High-Performance\nAnodes for Asymmetric Supercapacitors

Abstract

Anode materials with relatively low\ncapacitance remain a great\nchallenge for asymmetric supercapacitors (ASCs) to pursue high energy\ndensity. Hematite (α-Fe₂O₃) has attracted\nintensive attention as anode material for ASCs, because of its suitable\nreversible redox reactions in a negative potential window (from 0\nV to −1 V vs Ag/AgCl), high theoretical capacitance, rich abundance,\nand nontoxic features. Nevertheless, the Fe₂O₃ electrode cannot deliver large volumetric capacitance at a high\nrate, because of its poor electrical conductivity (∼10^–14 S/cm), resulting in low power density and low energy\ndensity. In this work, a hierarchical heterostructure comprising Fe₃O₄@Fe₂O₃ core–shell\nnanorod arrays (NRAs) is presented and investigated as the negative\nelectrode for ASCs. Consequently, the Fe₃O₄@Fe₂O₃ electrode exhibits superior supercapacitive\nperformance, compared to the bare Fe₂O₃ and\nFe₃O₄ NRAs electrodes, demonstrating large volumetric\ncapacitance (up to 1206 F/cm³ with a mass loading of 1.25\nmg/cm²), as well as good rate capability and cycling stability.\nThe hybrid electrode design is also adopted to prepare Fe₃O₄@MnO₂ core–shell NRAs as the positive\nelectrode for ASCs. Significantly, the as-assembled 2 V ASC device\ndelivered a high energy density of 0.83 mWh/cm³ at a power\ndensity of 15.6 mW/cm³. This work constitutes the first\ndemonstration of Fe₃O₄ as the conductive supports\nfor Fe₂O₃ to address the concerns about its\npoor electronic and ionic transport.