Hierarchical construction of high-performance all-carbon flexible fiber supercapacitors with graphene hydrogel and nitrogen-doped graphene quantum dots

Carbon-based flexible fiber-based supercapacitors (FFSCs) are promising power sources for portable and wearable electronics, but their applications are limited by low energy density due to a lower specific capacitance of common carbon materials. We fabricated ternary all-carbon fiber electrodes (N-GQD/GH/CF) with improved electrochemical performance. In this structure, graphene hydrogel (GH) was grown on carbon fibers (CFs) to form a 3D interconnected porous network (GH/CF), and nitrogen-doped graphene quantum dots (N-GQDs) with high pseudocapacitive activity were electrodeposited into the GH/CF network. The ternary N-GQD/GH/CF hybrid electrode delivered a volumetric capacitance of 93.7 F cm−3 at 20 mA cm−3, which was around 7 times higher than that of the GH/CF, while the capacitance retention after 5000 cycles reached 87.9%, which was only a little lower than that of the GH/CF electrode (90.2%). It indicates the introduction of N-GQDs drastically improves the capacitance of the FFSCs without sacrificing the cycle stability. All-carbon asymmetric FFSCs were assembled using N-GQD/GH/CF as positive electrode and GH/CF as negative electrode. The assembled FFSCs exhibited a high energy density of 3.6 mW h cm−3 at power density of 35.6 mW cm−3 owing to the wider potential window (2 V) and the higher volumetric capacitance as well as excellent flexibility and cycling stability.