Flexible Supercapacitor Matches Battery Volumetric Energy Density

12 May 2014

Researchers have developed carbon-based microscale supercapacitors that exhibit a volumetric energy density of 6.3 microwatt-hours/mm3, a stored energy comparable to that of a 4-volt-500-microampere-hour thin-film lithium battery The devices are built out of flexible fiber, making them good candidates to serve as energy storage devices that can be woven into clothing, or used to power small wearable electronics.

To continue the trend of increasingly miniaturized electronics, industry needs tiny energy storage devices with large volumetric energy densities. Supercapacitors have energy storage capacity comparable to that of batteries, but have always lagged badly in energy density by volume because they require large amounts of accessible surface area to store energy.

To improve the volumetric energy density of the new device, the researchers designed a hybrid fiber. The product's developers, engineers and scientists at Nanyang Technological University (NTU) in Singapore, Tsinghua University in China, and Case Western Reserve University claim the fiber-structured hybrid materials offer huge accessible surface areas and are highly conductive.

Their device contains acid-oxidized single-wall nanotubes, graphene oxide, and ethylenediamine. The latter promotes synthesis and dopes graphene with nitrogen. The material is pumped through a capillary column and heated for six hours.

This results in sheets of graphene self-assembled into an interconnected porous network that runs the length of the fiber. The accessible surface area is 396 sq. m/ gram of hybrid fiber for the transport and storage of charge.

High volumetric energy density results from the materials being tightly packed in the capillary column and remaining so as they are pumped out. According to the researchers, the process expanded to multiple capillary columns will enable engineers to make fibers continuously and maintain consistent quality. The researchers have made fibers as long as 50 meters and found they remain flexible with high capacitance of 300 Farad/cm3.

The researchers found three pairs of fibers arranged in series tripled the voltage while keeping the charging/discharging time the same. Also, 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. Integrating multiple pairs of fibers between two electrodes resulted in capacitance increasing linearly with the number of fibers.

Conventional rechargeable batteries have a lifetime of less than 1,000 cycles, while the fiber device lasted for 10,000 charge/discharge cycles, and the device retained about 93% of its original performance, according to the researchers.

The Ministry of Education, Singapore, the Asian Office of Aerospace Research and Development of the U.S. Air Force, and the U.S, Air Force Office of Scientific Research funded the research. The scientists reported their research in Nature Nanotechnology:

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