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.