Vaporized Hydrothermal Functionalization of Carbon Fiber and Its Superior Supercapacitor Performance

Carbon fibers are promising in directly being used as a freestanding electrode in flexible and wearable devices due to their high conductivity and good mechanical performance. However, those micrometer-sized carbon fibers usually suffer from disadvantages including poor activity and a low surface area due to an inert surface, solid structure, and high diameter (7–15 μm). Traditional chemical activation methods in a liquid phase have a low efficiency and produce a vast waste solution. In this work, a vaporized hydrothermal oxidation process is proposed, in which vaporized functionalization of carbon fibers is completed during a hydrothermal process of 5 mL of aqueous solution containing 0.5 mL of nitric acid in a 100 mL autoclave. Combined with the subsequent aerobic pyrolysis process, this vaporized hydrothermal method converts smooth and inert carbon fibers into porous and activated carbon fibers, accompanied by a decrease of their diameter from 6.9 to 2.7 μm. Consequently, functionalized carbon fiber directly used as an electrode exhibits a high mass specific capacitance of 118.97 F g^–1 (vs 0.217 F g^–1 for pristine CF) whereas its symmetric supercapacitor delivers an energy density of 6.5 W h kg^–1 and a maximum power density of 5000 W kg^–1.