Space confined assembly of all carbon hybrid fibers for capacitive energy storage: realizing a built to order concept for micro supercapacitors
Miniaturized portable and wearable electronics have diverse power requirements, ranging from one microwatt to several milliwatts. Fiber based micro supercapacitors are promising energy storage devices that can address these manifold power requirements. Here, we demonstrate a hydrothermal assembly method using space confinement fillers to control the formation of nitrogen doped reduced graphene oxide and multi walled carbon nanotube hybrid fibers. Consequently, the all carbon hybrid fibers have tunable geometries, while maintaining good electrical conductivity, high ion accessible surface area and mechanical strength; this allows us to address two important issues in micro supercapacitor research. First, we found a clear correlation between the geometry of the hybrid fibers and their capacitive energy storage properties. Thinner fibers (30 mm in diameter) have higher specific volumetric capacitance (281 F cm 3) superior rate capability, and better length dependent performance. In contrast, larger diameter hybrid fibers (236 mm in diameter) can achieve much higher specific length capacitance (42 mF cm 1) Second, we realized the first built to order concept for micro supercapacitors by using all carbon hybrid fibers with diversified geometry as electrodes. The device energy can cover two orders of magnitude, from <0.1 mW h to nearly 10 mW h, and the device power can be tuned in four orders of magnitude, from 0.2 mW to 2000 mW. Furthermore, multiple mechanically flexible fiber based micro supercapacitors can be integrated into complex energy storage units with wider operation voltage windows, demonstrating broad application potentials in flexible devices.