Bioinspired networks consisting of spongy carbon wrapped by graphene sheath for flexible transparent supercapacitors

Hierarchically ordered structures with low tortuosity, excellent mechanical flexibility, high optical transparency, and outstanding electrical conductivity are critically important in developing flexible transparent supercapacitor electrodes for innovative applications in electronics and displays. Here a CVD process is employed to fabricate leaf-skeleton inspired electrodes, which are reticulated monolithic networks consisting of carbon nanostructures serving as a 3D spongy core and graphene-based films as a protective/conductive shell. The network electrodes show optical transmittance of 85–88%, an electrical sheet resistance of ~1.8 Ω/sq, and an areal capacitance of 7.06 mF cm−2 (at 0.78 mA cm−2 in a three-electrode cell) in Na2SO4 aqueous electrolyte. Flexible transparent and symmetric supercapacitors, based on PVA/H3PO4 gel and the network electrodes, possess a stable working voltage of 1.6 V, energy and power density of 0.068 μWh cm−2 and 47.08 μW cm−2 at an optical transparency of ~80%, and no capacitance loss over 30,000 flat-bend-release cycles.