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Flexible fiber-type solid-state supercapacitors (FSCs) are demanded for energy storage of wearable soft electronics. A flexible carbon fiber (CF) with high electrical conductivity and good energy storage ability can act as the current collector and the active material simultaneously. Improving the energy storage ability of the CF without depositing active materials is highly promising to avoid active materials from peeling off, especially under bending conditions. In this study, the CF is treated with acid mixtures containing different HNO3 to H2SO4 ratios. The roughened surface and the production of graphene quantum dots and functional groups are achieved for the acid-treated CF, especially for the CF treated with the acid mixtures. The largely enhanced specific capacitance (C L) is first obtained for the optimized CF electrode (641.08 mF/cm at 5 mV/s) treated with the acid mixture containing the HNO3 to H2SO4 ratio of 1 to 3. The FSC also shows a maximum energy density of 16.57 μW h/cm at the power density of 62.50 μW/cm, as well as a C L retention of 77% and Coulombic efficiency of 100% in 5000 times of a charge/discharge cycling process. Excellent flexibility is also attained for the FSC with no shape distortion of cyclic voltammetry curves measured under the bending angle of 180° and with 1000 times of repeated bending. This work proposes a simple acid treatment to largely enhance the energy storage ability of pure CF electrodes. The bifunctional substance with current collecting and energy storing abilities is expected to be well developed in the future for applying on numerous efficient flexible energy storage devices.

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