MnO2 decorated on carbon sphere intercalated graphene film for high-performance supercapacitor electrodes

Free-standing two-dimensional MnO2/carbon sphere/graphene (MCG) films are rationally designed and used as efficient electrode materials for supercapacitor. Carbon sphere/graphene (CG) films are firstly constructed through a simple vacuum filtration method, then MCG electrodes are prepared by simply floating CGs on KMnO4 solution at room temperature. CG films act as the reducing agents as well as freestanding substrates. Birnessite-type MnO2 in situ grow on the surface of carbon spheres and graphene nanosheets. In the unique structure, carbon spheres spaced sandwich-like porous structures can provide plenty of paths for electrolyte-ion penetration. After decorating with high performance MnO2, MCG films can deliver a gravimetric capacitance of 319.3 F g−1 and a volumetric capacitance of 277.8 F cm−3, much higher than graphene and CG films. More importantly, the electrodes also exhibits high gravimetric and volumetric energy densities of 29.4 Wh kg−1 and 25.6 Wh L−1, respectively, as well as excellent cycling stability with 94.1% of its initial capacitance after 5000 charge-discharge cycles at a current density of 5 A g−1.