| High-efficient energy storage devices have stimulated tremendous attention from researchers due to the exhaustion of non-renewable resources and their increasing damage to environment. Supercapacitors (SCs), also known as electrochemical capacitors, are considered to represent a new class of energy storage devices because they are capable of storing more energy than conventional capacitors and providing higher power than batteries. However, in order to meet the increasing energy requirements for the next-generation electronic devices, the energy density of current SCs need to be further improved without sacrificing the power density and cycling life. According to the equation of energy density E=1/2CV2, two strategies aiming at extending the voltage window (V) and increasing capacitance (C) have been proposed to achieve this target. The first one can be achieved by the construction of asymmetric supercapacitors (ASCs), which possess an expanded voltage window up to-2.2 V in aqueous electrolytes. The second one is to develop nanostructured electrode materials with high capacitance and excellent rate capability. In recent years, cathode materials with excellent capacitive performance were reported intensively. Yet, current anode materials still suffer from low capacitance to match large capacitance cathodes. Therefore, it remains a great challenge for ASCs to pursue high energy density at high power density.In order to make progress in the energy-density limit of ASCs, a new class of negative electrode materials combining high capacitance and conductivity is indispensable. In this regard, a series of negative electrode materials such as MoO3-x, V2O5, VN, and Fe2O3 have been developed for ASCs. Among these materials, Fe2O3 is considered as a promising electrode material in negative potential window (0～-1 V vs. Ag/AgCl) owing to its outstanding features such as appropriate reversible redox reactions (Fe2+/Fe3+), large theoretical capacitance, rich abundance, and non-toxicity. Nevertheless, the capacitance of Fe2O3 electrode at higher current densities is still fairly low. This is mainly due to its poor electrical conductivity (～10-14 S/cm), resulting in low power density and corresponding low energy density at high rate.In this paper, the first work is that a hierarchical heterostructure comprising Fe3O4@Fe2O3 core-shell nanorod arrays (NRAs) is presented and investigated as 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 1.25 mg/cm2 mass loading) as well as good rate capability and cycling stability. The hybrid electrode design is also adopted to prepare Fe3O4@MnO2 core-shell NRAs as 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.In order to address the matter of cycling of iron oxide, in our second work, one simple method is adopted to design and synthesize the Fe2O3@TiO2 core-shell nanorod array. After annealing in Ar, the Fe2O3 with Oxygen-Deficient are considered as a new class of material with high conductivity. In the meanwhile, due to the protection of TiO2 which is synthesized by ALD, the property of the hybrid material is stable. Consequently, the hybrid electrode can work in a large potential window (0--1.3V), which lead to a superior supercapacitive performance compared to other iron oxide electrode. The hybrid electrode exhibits large areal capacitance (930 mF/cm2 at 5 mV/s) as well as cycling stability (70% retained after 5000 cycle). This work constitutes the first demonstration of TiO2 as the protection-shell for Fe2O3 to address the concerns about its cycling stability. |
PDF Full Text Request