A multinational research team comprised of scientists from Nanyang Technological University (NTU) in Singapore, Tsinghua University in China, and Case Western Reserve University (CWRU) in the United States has announced the development of a fiber capacitor that can be woven into clothing to power wearable medical monitors. This supercapacitor tightly integrates fibers synthesized from graphene and carbon nanotubes, thus storing energy like a conventional thin-film lithium battery. The developers believe that this device achieves the highest volumetric energy density to date among small-sized carbon-based supercapacitors, at approximately 6.3 mA/hour per cubic millimeter.
This component also boasts the advantage of faster charging and discharging than conventional batteries. The use of a hybrid material with a fiber structure provides readily available surface area and high conductivity.
Researchers have developed a method for the continuous production of this elastic fiber, enabling it to be mass-produced for a variety of applications. They say they can currently manufacture fibers up to 50 meters long, but there are no limitations on length.
Scientists anticipate that these fiber supercapacitors could be woven into clothing to power medical electronics in the home or communication devices for soldiers in the field. The fiber could also serve as a space-saving power source, acting as an "energy-carrying wire" in medical implants.
This research was led by Yuan Chen, Professor of Chemical Engineering at NTU, in collaboration with scientists including Dingshan Yu, Kunli Goh, Hong Wang, Li Wei, and Wenchao Jiang from NTU; Qiang Zhang from Tsinghua University; and Liming Dai from Case Western Reserve University. The results of this research have been published in Nature Nanotechnology.
Liming Dai, professor of polymer science and engineering at Case Western Reserve University, explained that most supercapacitors have high power density but low energy density, meaning they can charge and boost power quickly but cannot sustain power for long periods. Conversely, batteries have high energy density but low power density, meaning they can operate for extended periods but cannot provide large amounts of energy quickly.
By tightly bonding graphene and carbon nanotube synthetic fibers, supercapacitors can achieve energy storage or energy density equivalent to that of batteries. However, due to the need for a large usable surface area for energy storage, the actual energy density is still insufficient.
This fiber is produced in a solution containing oxidizing acid, single-walled carbon nanotubes, graphene oxide, and ethylenediamine, and a nitrogen-containing graphene liquid is synthesized. This liquid is then transported through a flexible capillary column and heated for 6 hours. Graphene sheets, approximately a few atoms thick, and neatly arranged single-walled carbon nanotubes self-assemble into an interconnected porous network capable of operating the fiber.
This method provides a large accessible surface area—approximately 396 square meters per gram of hybrid fiber—that can be used for charge transfer and storage. However, these materials are tightly packed within capillary columns, thus achieving a high volumetric energy density during transport.
Using multiple capillary columns in this process allows engineers to continuously produce fibers while maintaining a certain level of quality, Chen said. Researchers have already created fibers up to 50 meters long that can maintain a high capacity of up to 300 farads per cubic centimeter. During testing, researchers also found that three sets of fibers arranged in series simultaneously increased the voltage by three times while maintaining the same charge/discharge time.
Compared to a single fiber operating at the same current density, three sets of fibers arranged in parallel can increase the output current by three times and the charging/discharging time by three times. When multiple pairs of fibers are integrated between two electrodes, the capacity for storing electrical energy (i.e., capacitance) increases linearly with the number of fibers used.
When using polyvinyl alcohol/phosphate gel as the electrolyte, a solid-state micro supercapacitor made from a set of fibers provides a volumetric density of 6.3 mW/h per cubic centimeter, equivalent to the hourly output of a 4V, 500mA thin-film lithium battery. Fiber supercapacitors can exhibit ultra-high energy density while maintaining high power density and cycle stability.
The research team is also actively testing and exploring other functional applications of this fiber, including batteries, solar cells, biofuel cells, and sensors for flexible and wearable optoelectronic systems.
