Abstract
Iron oxides have been considered as one of the promising pseudocapacitor electrode materials in recent years, owing to their low cost, Earth abundance, and low toxicity. In addition, the suitable working potentials of iron oxides are below 0 V (vs. Ag/AgCl), and they are thus a promising candidate as an anode material. But their limited conductivities are disadvantageous, and many reports show that the high rate capability and cycling stability of iron oxides are not good enough. In this study, we develop a successful preparation method for γ-Fe2O3/graphene composites, which involves a simple cathodic electrodeposition of γ-Fe2O3 nanocrystals into a mesoporous graphene film of high conductivity and high specific surface area (737 m2/g). The synergistic effects between the γ-Fe2O3 and graphene drastically improve the capacitive performance of γ-Fe2O3. γ-Fe2O3 nanoparticles with an average particle size of 5 nm are well-dispersed in the grapheme film. The electrochemical performance of the resulting γ-Fe2O3/graphene composite electrode is tested by cyclic voltammetry(CV) and galvanostatic charge-discharge in Na2SO3. The results show that when the γ-Fe2O3 is deposited potentiometrically at a potential of -0.6 V, the γ-Fe2O3/graphene electrode exhibits the highest specific capacitance, 223.5 F/g, within the potential window of -0.8 V to 0 V at a scan rate of 25 mV/s. The relevant cycling performance is excellent, with the specific capacitance even increasing after 1000 cycles. In addition, the increasing trend in specific capacitance is observed after 40000 cycles at the scan rate of 500 mV/s. The composite also shows an outstanding high rate capability, with a retention of 67 % in specific capacitance when operated at a high scan rate of 1000 mV/s as compared with the specific capacitance obtained at the scan rate of 25 mV/s. The specific capacitance maintains at a 91 % level when the charging/discharging rate increases from 1 to 20 A/g. At the high discharging rate of 20 A/g, the Coulombic efficiency can still maintain at 95 %. The incorporation of γ-Fe2O3 nanoparticles into the graphene film reduces the aggregation of the nanoparticles, which makes possible the well-dispersed and thus better utilized γ-Fe2O3 nanoparticle. Here, graphene provides a highly conductive network for electron transport during the charge and discharge processes.