γ-Fe2O3/graphene nanocomposites as a stable high performance anode material for neutral aqueous supercapacitors

Abstract

γ-Fe₂O₃/graphene nanocomposites were developed as a high performance anode material for environmentally friendly neutral aqueous supercapacitors. This composite electrode, in addition to its good capacitive characteristics, exhibited outstanding rate capability and cycling stability. γ-Fe₂O₃ nanocrystals of 5 nm in size were potentiostatically deposited onto a hosting porous graphene film to form the γ-Fe₂O₃/graphene nanocomposite. The porous graphene film provided a highly conductive network to enhance the charge transport/transfer involved in the capacitance generation process and helped disperse the γ-Fe₂O₃ nanocrystals within the graphene network to promote better utilization of the γ-Fe₂O₃ nanocrystals for capacitance generation. The capacitive performances of the γ-Fe₂O₃/graphene composite electrode were investigated by cyclic voltammetry and galvanostatic charging/discharging analyses in 1 M Na₂SO₃ over a potential window of 0 to −0.8 V (vs. Ag/AgCl). A high specific capacitance of 224 F g_Fe2O3⁻¹ was achieved for the γ-Fe₂O₃/graphene composite electrode at 25 mV s⁻¹, 2.4 fold that of the plain γ-Fe₂O₃ electrode. The outstanding rate capability of the composite electrode was demonstrated with a high capacitance retention rate of 91% when the charging/discharging current density increased from 1 to 20 A g⁻¹. At the high charging/discharging current density of 20 A g⁻¹, the Coulombic efficiency remained high at 95%. The composite electrode experienced an electrochemical activation and its specific capacitance remained high above the starting specific capacitance after 40 000 cycles at 500 mV s⁻¹, exhibiting outstanding cycling stability.