▎ 摘　　要

Flexible energy-storage devices based on supercapacitors rely largely on the scrupulous design of flexible electrodes with both good electrochemical performance and high mechanical properties. Here, nitrogen-doped carbon nanofiber networks/reduced graphene oxide/bacterial cellulose (N-CNFs/RGO/BC) freestanding paper is first designed as a high-performance, mechanically tough, and bendable electrode for a supercapacitor. The BC is exploited as both a supporting substrate for a large mass loading of 8 mg cm(-2) and a biomass precursor for N-CNFs by pyrolysis. The one-step carbonization treatment not only fabricates the nitrogen-doped three-dimensional (3D) nanostructured carbon composite materials but also forms the reduction of the GO sheets at the same time. The fabricated paper electrode exhibits an ultrahigh areal capacitance of 2106 mF cm(-2) (263 F g(-1)) in a KOH electrolyte and 2544 mF cm(-2) (318 F g(-1)) in a H2SO4 electrolyte, exceptional cycling stability (similar to 100% retention after 20000 cycles), and excellent tensile strength (40.7 MPa). The symmetric supercapacitor shows a high areal capacitance (810 mF cm(-2) in KOH and 920 mF cm(-2) in H2SO4) and thus delivers a high energy density (0.11 mWh cm(-2) in KOH and 0.29 mWh cm(-2) in H2SO4) and a maximum power density (27 mW cm(-2) in KOH and 37.5 mW cm(-2) in H2SO4). This work shows that the new procedure is a powerful and promising way to design flexible and freestanding supercapacitor electrodes.