A lightweight, flexible, and highly efficient energy management strategy is needed for flexible energy-storage devices to meet a rapidly growing demand. Graphene-based flexible supercapacitors are one of the most promising candidates because of their intriguing features. In this report, we describe the use of freestanding, lightweight (0.75 mg/cm(2)), ultrathin (<200 mu m), highly conductive (55 S/cm), and flexible three-dimensional (3D) graphene networks, loaded with MnO2 by electrodeposition, as the electrodes of a flexible supercapacitor. It was found that the 3D graphene networks showed an ideal supporter for active materials and permitted a large MnO2 mass loading of 9.8 mg/cm(2) (similar to 92.9% of the mass of the entire electrode), leading to a high area capacitance of 1.42 F/cm(2) at a scan rate of 2 mV/s. With a view to practical applications, we have further optimized the MnO2 content with respect to the entire electrode and achieved a maximum specific capacitance of 130 F/g. In addition, we have also explored the excellent electrochemical performance of a symmetrical supercapacitor (of weight less than 10 mg and thickness similar to 0.8 mm) consisting of a sandwich structure of two pieces of 3D graphene/MnO2 composite network separated by a membrane and encapsulated in polyethylene terephthalate (PET) membranes. This research might provide a method for flexible, lightweight, high-performance, low-cost, and environmentally friendly materials used in energy conversion and storage systems for the effective use of renewable energy.
National Natural Science Foundation of China  ; Key Technology Research and Development Program of Gansu Province, China [1011GKCA027] ; Natural Science Foundation of Gansu Province [1107RJYA280]