By mass, supercapacitors might have energy storage or energy density comparable to batteries. But because they require large amounts of accessible surface area to store energy, they have always lagged badly in energy density by volume.
The fiber is produced from a solution containing acid-oxidized, single-wall nanotubes, graphene oxide, and ethylenediamine, which promotes synthesis and dopes graphene with nitrogen. It is pumped through a flexible narrow reinforced tube called a capillary column, and is heated in an oven for six hours.
Sheets of graphene, one to a few atoms thick, and aligned, single-walled carbon nanotubes self-assemble into an interconnected porous network that run the length of the fiber.
The arrangement provides huge amounts of accessible surface area -- 396 square meters per gram of hybrid fiber -- for the transport and storage of charges. But the materials are tightly packed in the capillary column and remain so as they are pumped out, resulting in the high volumetric energy density.
The process using multiple capillary columns will enable the engineers to make fibers continuously and maintain consistent quality, says Chen. The researchers have made fibers as long as 50 meters and found they remain flexible with high capacity of 300 Farad per cubic centimeter. In testing, they found that three pairs of fibers arranged in series tripled the voltage while keeping the charging/discharging time the same.
Three pairs of fibers in parallel tripled the output current and tripled the charging/discharging time, compared to a single fiber operated at the same current density.
When they integrate multiple pairs of fibers between two electrodes, the ability to store electricity, called capacitance, increased linearly according to the number of fibers used.
Using a polyvinyl alcohol/phosphoric acid gel as an electrolyte, a solid-state micro-supercapacitor made from a pair of fibers offered a volumetric density of 6.3 microwatt hours per cubic millimeter, which is comparable to that of a 4 volt, 500 microampere per hour, thin-film lithium battery.