Knitted Supercapacitors to Power Smart Shirts

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Researchers from Drexel University in collaboration with the U.S. Naval Academy, have invented a way to embed activated carbon particles into different types of yarn to form a knitted textile that can store energy to power sensors and electronics integrated into smart clothing.

Smart fabrics, which incorporate different types of sensors into garments, have been in development for decades. However, only in recent years have we started seeing the first consumer smart garments reaching the market, including biometric smartwear that can monitor an athlete's health (like the ones made by Hexoskin, Athos, omsignal, and other companies), as well as products designed to track health and well being (such as the Somnus Sleep Shirt developed to monitor both the quality and quantity of your sleep). According to a Gartner study published in November 2014, the smart garment sector will grow from around 100,000 units sold worldwide in 2014 to a whooping 26 million units predicted for 2016.

So far, most (if not all) of the products on the market use an external "brain", a small computer that records data and communicates to a user's smartphone or other systems. These modules, although relatively small, use a solid battery which is less than ideal both from an aesthetic and functional perspective.

Creating a flexible energy storage that can be integrated into the fabric has been the goal of several research groups around the world. Last year teams from China and the United States demonstrated a fiber-like supercapacitor made from both graphene and carbon nanotubes that could be woven into clothing. At the time these fibers were said to obtain the highest volumetric energy density reported for carbon-based microscale supercapacitors (6.3 microwatt-hours per cubic millimeter, which is comparable to a 4-volt-500-microampere-hour battery). However carbon nanotubes are still expensive not to mention the fact that there is still some debate about their possible toxicity.

Trying to create a textile that can store energy without the use of exotic, expensive materials, the Drexel University team turned to activated carbon. This far less expensive material, is also nontoxic and is even used to reduce absorption of poisonous substances. According to Drexel University materials scientist Yury Gogotsi who supervised the research, using an area of about 3000 cm2 (about the size of the center back panel of a shirt) it is possible to store the equivalent energy of a 4 cm2, V coin cell battery.

In order to reach this energy density, the Drexel team used a technique called natural fiber welding or NFW. This technique was invented chemistry professor Paul C. Trulove’s research team at the U.S. Naval Academy and Hugh C. Delong of the Air Force Office of Scientific Research. It was  patented in 2009. NFW allows embedding materials, such as activated carbon, into a cellulose-based yarn made from cotton, linen, bamboo, or viscose—all of which have been electrochemically tested by Drexel’s team at the A.J. Drexel Nanomaterials Institute, and knitted at the Shima Seiki Haute Tech Lab.

The NFW process includes several steps which were perfected by Drexel University doctoral student Kristy Jost and Commander David P. Durkin at the U.S. Naval Academy. In the lab, ionic liquid was applied to the cellulose fibers, causing the biopolymer to swell, and the individual polymer chains to start separating. At this point instead of allowing the material to completely dissolve, the ionic liquid was mixed with carbon particles, and when the yarn was partially swelled, pressure was applied to it in order to embed the carbon into the fiber surface. Finally, the yarn was spooled and rinsed with water, which helped remove the ionic liquid and re-solidify the cellulose.

Despite the seemingly complex nature of the process, Durkin was able to develop a small NFW machine to continuously create tens of meters of yarn at a time.

"The only thing driving the cost of NFW would be the ionic liquids,” says Durkin. “They can be expensive, but are attractive alternatives to organic solvents because they are not volatile. They can be recycled during the water reconstitution process." Durkin believes that if the NFW process were scaled up, ionic liquid cost would be a critical parameter.

Although we will probably not be using energy storage textiles to power our smartphones any time soon due to the low energy density of the current technology, it can still have many other applications. Jost explains that she had started working on textile energy storage because she noticed the growing number of e-textiles on the market that incorporated knitted components, but they all had solid battery packs attached to the shirt, rather than incorporating an energy storing fabric. Her ultimate goal is to produce an energy storage device that can seamlessly power these kinds of technologies, including diverse types of knitted sensors and low energy communication devices.

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