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Oxygen-deficient bismuth oxide (r-Bi 2 O 3 )/graphene (GN) is designed, fabricated, and demonstrated via a facile solvothermal and subsequent solution reduction method. The ultrafine network bacterial cellulose (BC) as substrate for r-Bi 2 O 3 /GN exhibits high flexibility, remarkable tensile strength (55.1 MPa), and large mass loading of 9.8 mg cm −2 . The flexible r-Bi 2 O 3 /GN/ BC anode delivers appreciable areal capacitance (6675 mF cm −2 at 1 mA cm −2 ) coupled with good rate capability (3750 mF cm −2 at 50 mA cm −2 ). In addition, oxygen vacancies have great influence on the capacitive performance of Bi 2 O 3 , delivering significantly improved capacitive values than the untreated Bi 2 O 3 flexible electrode, and ultrahigh gravimetric capacitance of 1137 F g −1 (based on the mass of r-Bi 2 O 3 ) can be obtained, achieving 83% of the theoretical value (1370 F g −1 ). Flexible asymmetric supercapacitor is fabricated with r-Bi 2 O 3 /GN/BC and Co 3 O 4 /GN/BC paper as the negative and positive electrodes, respectively. The operation voltage is expanded to 1.6 V, revealing a maximum areal energy density of 0.449 mWh cm −2 (7.74 mWh cm −3 ) and an areal power density of 40 mW cm −2 (690 mW cm −3 ). Therefore, this flexible anode with excellent electrochemical performance and high mechanical properties shows great potential in the field of flexible energy storage devices.

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 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 (263 F g) in a KOH electrolyte and 2544 mF cm (318 F g) in a HSO electrolyte, exceptional cycling stability (∼100% retention after 20000 cycles), and excellent tensile strength (40.7 MPa). The symmetric supercapacitor shows a high areal capacitance (810 mF cm in KOH and 920 mF cm in HSO) and thus delivers a high energy density (0.11 mWh cm in KOH and 0.29 mWh cm in HSO) and a maximum power density (27 mW cm in KOH and 37.5 mW cm in HSO). This work shows that the new procedure is a powerful and promising way to design flexible and freestanding supercapacitor electrodes.

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