| Summary: | Flexible supercapacitors are demanded for energy storage of wearable electronics. In this paper, a simple strategy for preparing flexible carbon fibers (CFs) with good energy storage capacity using a mixed acid treatment process is reported. When the volume ratio of concentrated sulfuric acid to concentrated nitric acid is 3:1, the carbon fiber electrodes have the best electrochemical performance with a high capacitance of 27.83 F g<sup>−1</sup> at 15 mA g<sup>−1</sup> and extremely high capacitance retention of 79.9% after 500 cycles at 100 mA g<sup>−1</sup>. Furthermore, their energy density can reach 3.86 Wh kg<sup>−1</sup> with a power density of 7.5 W kg<sup>−1</sup>. Such an excellent electrochemical performance of carbon fiber electrodes is attributed to their surface rich oxygen-containing functional groups, rough surface, and a certain number of graphene quantum dots (GQDs). Importantly, the all-solid-state flexible supercapacitor performs excellent bending stability performance with a capacitance retention of almost 100% after 500 times of bending at 180°, showing good prospects and applications in the field of flexible energy storage devices.
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