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Fibre supercapacitor aims to power wearables

Posted: 15 May 2014     Print Version  Bookmark and Share

Keywords:Nanyang Technological University  supercapacitor  wearable  medical monitor  graphene 

Researchers in Nanyang Technological University (NTU) in Singapore, Tsinghua University in China and Case Western Reserve University in the USA claimed to have created a fibre supercapacitor that can be woven into clothing and power wearable medical monitors and communications. According to them, the device features an interconnected network of graphene and carbon nanotubes so tightly that it stores energy comparable to some thin-film lithium batteries.

The product's developers believe the device's volumetric energy density is the highest reported for carbon-based microscale supercapacitors to date: 6.3uW hours per cubic millimetre.

The device also maintains the advantage of charging and releasing energy much faster than a battery. The fibre-structured hybrid materials offer accessible surface areas and are highly conductive.

The researchers have developed a way to continuously produce the flexible fibre, enabling them to scale up production for a variety of uses. To date, they've made 50m long fibres, and see no limits on length.

The scientists envision the fibre supercapacitor could be woven into clothing to power medical devices for people at home, or communications devices for soldiers in the field. The fibre could also be used as a space-saving power source and serve as 'energy-carrying wires' in medical implants.

Yuan Chen, a professor of chemical engineering at NTU led the study, working with Dingshan Yu, Kunli Goh, Hong Wang, Li Wei and Wenchao Jiang at NTU; Qiang Zhang at Tsinghua; and Liming Dai at Case Western Reserve.

Dai, a professor of macromolecular science and engineering at Case Western Reserve and a co-author of the paper, explained that most supercapacitors have high power density but low energy density, which means they can charge quickly and give a boost of power, but don't last long. Conversely, batteries have high energy density and low power density, which means they can last a long time, but do not deliver a large amount of energy quickly.

By mass, supercapacitors might have comparable energy storage, or energy density, 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 fibre is produced from a solution containing acid-oxidised single-wall nanotubes, graphene oxide and ethylenediamine, which promotes synthesis and dopes graphene with nitrogen, is pumped through a flexible narrow reinforced tube called a capillary column and 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 fibre.

The arrangement provides huge amounts of accessible surface area, 396 square metres per gram of hybrid fibre, 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 fibres continuously and maintain consistent quality, Chen said.

The researchers have made fibres as long as 50m and found they remain flexible with high capacity of 300 farad per cubic centimetre. In testing, they found that three pairs of fibres arranged in series tripled the voltage while keeping the charging/discharging time the same.

Three pairs of fibres in parallel tripled the output current and tripled the charging/discharging time, compared to a single fibre operated at the same current density. When they integrate multiple pairs of fibres between two electrodes, the ability to store electricity, called capacitance, increased linearly according to the number of fibres used.

Using a polyvinyl alcohol /phosphoric acid gel as an electrolyte, a solid-state micro-supercapacitor made from a pair of fibres offered a volumetric density of 6.3uW hours per cubic millimetre, which is comparable to that of a 4V-500uA-hour thin film lithium battery.

The fibre supercapacitor demonstrated ultrahigh energy-density value, while maintaining the high power density and cycle stability.

"We have tested the fibre device for 10,000 charge/discharge cycles, and the device retains about 93 per cent of its original performance," Yu said, "while conventional rechargeable batteries have a lifetime of less than 1000 cycles."

The team also tested the device for flexible energy storage. The device was subjected to constant mechanical stress and its performance was evaluated. "The fibre supercapacitor continues to work without performance loss, even after bending hundreds of times," Yu said.

"Because they remain flexible and structurally consistent over their length, the fibres can also be woven into a crossing pattern into clothing for wearable devices in smart textiles." said Chen.

"The team is also interested in testing these fibres for multifunctional applications, including batteries, solar cells, biofuel cells, and sensors for flexible and wearable optoelectronic systems," Dai said.

- Paul Buckley
  EE Times Europe

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