Graphene and maghemite composites based supercapacitors delivering high volumetric capacitance and extraordinary cycling stability

Metal oxides (like MnO2, Fe2O3, and Co3O4, etc) based supercapacitors have disadvantages, such as low volumetric capacitance for thick-film electrodes, or short cycling life because a Faradaic process involves chemical changes of state of the reactant species. In the present work, we report that supercapacitors based on reduced graphene oxide and maghemite (γ-Fe2O3) composites (GγM) exhibit superior performance. GγM electrodes with average electrode thickness up to ∼60 μm have a high volumetric capacitance of 230 F cm−3, together with an outstanding electrode package density of 1.44 g cm−3. Particularly, the GγM electrodes have excellent cycling performance of ∼90% capacitance retention over 100,000 galvanostatic charge–discharge cycles or voltage floating at 0.9 V for 300 h. No detectable change in phase and an effective inhibition of γ-Fe2O3 refinement after cycle-life test are confirmed by X-ray diffraction and transmission electron microscopy.