Weaving carbon nanotube wires into high-performance, wearable supercapacitors

(Nanowerk Spotlight) Textiles and apparel constitute the primary interface between skin and surroundings, offering physical comfort and protection. Wearable electronics constitute the broad class of electronics that can be integrated directly on to fabrics and textiles.
Such devices offer enormous portability of operation and would be used for a range of applications such as health-care monitoring, point-of-care diagnostics and homeland security.
An energy storage device that can store electrical energy and power the devices forms the heart of such wearable electronics and technologies. Conventional energy storage devices such as Li-ion batteries are not flexible and therefore not amenable for such applications. Besides posing health hazards and being incompatible with clothing or skin, their rigid structure also poses problems for the movement and comfort of the user. Further, charging and discharging of Li-ion batteries is a major problem when it comes to wearable and portable applications.
Therefore, alternative strategies for energy storage devices for wearable applications are in demand. The potential devices demand high energy density (measure of how much electrical energy a device can store) and high power density (how fast it can charge or discharge during usage). Importantly, it should be able to withstand all mechanical movements and duress that a clothing undergoes during daily usage.
The most important technical challenge is to blend the chemical nature of raw materials with fabrication techniques and processability, all of which are diametrically conflicting for textiles and conventional energy storage devices. A team from Indian Institute of Technology Bombay has come out with a comprehensive approach involving simple and facile steps to fabricate a wearable energy storage device. Several scientific and technological challenges were overcome during this process.
First, to achieve user-comfort and computability with clothing, the scaffold employed was the the same as what a regular fabric is made up of – cellulose fibers. However, cotton yarns are electrical insulators and therefore practically useless for any electronics. Therefore, the yarns are coated with single-wall carbon nanotubes (SWNTs).
SWNTs are hollow, cylindrical allotropes of carbon and combine excellent mechanical strength with electrical conductivity and surface area. Such a coating converts the electrical insulating cotton yarn to a metallic conductor with high specific surface area. At the same time, using carbon-based materials ensures that the final material remains light-weight and does not cause user discomfort that can arise from metallic wires such as copper and gold. This CNT-coated cotton yarn (CNT-wires) forms the electrode for the energy storage device.
Next, the electrolyte is composed of solid-state electrolyte sheets since no liquid-state electrolytes can be used for this purpose. However, solid state electrolytes suffer from poor ionic conductivity a major disadvantage for energy storage applications. Therefore, a steam-based infiltration approach that enhances the ionic conductivity of the electrolyte is adopted. Such enhancement of humidity significantly increases the energy storage capacity of the device.
Weaving carbon nanotube wires into high-performance, wearable supercapacitors
Figure 1. (a) Schematic representation of various steps such as sewing and packaging to fabricate the sewcap with CNT-wires and solid electrolyte, culminating in integration onto clothing. Scanning electron microscope (SEM) image of the CNT-wire and photograph of the sewcap worn on a T-shirt are also provided. (b) SEM image of the interconnected CNT network on the polyester wire. (c) SEM image of the cross junction created by sewing indicating the device architecture. (d) Cyclic voltammograms of sewcap at different scan rates. (e) Gravimetric Ragone plot comparing the performance of sewcap with other reported literature. (f) Areal Ragone plot. (Reprinted with permission from American Chemical Society) (click on image to enlarge)
The integration of the CNT-wire electrode with the electrolyte sheet was carried out by a simple and elegant approach of interweaving the CNT-wire through the electrolyte (see Figure 1). This resulted in cross-intersections which are actually junctions where the electrical energy can be stored. Each such junction is now an energy storage unit, referred to as sewcap.
The advantage of this process is that several 100s and 1000s of sewcaps can be made in a small area and integrated to increase the total amount of energy stored in the system. This scalability is unique and critical aspect of this work and stems from the approach of interweaving.
Further, this process is completely adaptable with current processes used in textile industries. Hence, a proportionately large energy-storage is achieved by creating sewcap-junctions in various combinations.
All components of the final sewcap device are flexible. However, they need to be protected from environmental effects such as temperature, humidity and sweat while retaining the mechanical flexibility. This is achieved by laminating the entire device between polymer sheets. The process is exactly similar to the one used for protecting documents and ID cards.
The laminated sewcap can be integrated easily on clothing and fabrics while retaining the flexibility and sturdiness. This is demonstrated by the unchanged performance of the device during extreme and harsh mechanical testing such as striking repeatedly with a hammer, complete flexing, bending and rolling and washing in a laundry machine.
In fact, this is the first device that has been proven to be stable under rigorous washing conditions in the presence of hot water, detergents and high torque (spinning action of washing machine). This provides the device with comprehensive mechanical stability.
The final performance of the device in terms of energy density and power density is higher than several other reported devices (Figure 1e and 1f above). This shows that the device delivers outstanding performance in terms of both weight-normalised performance and area-normalised performance. This signifies that the device is both lightweight and small and is yet, able to perform better that several other known devices.
These findings have been published in ACS Applied Materials & Interfaces ("Interwoven Carbon Nanotube Wires for High-Performing, Mechanically Robust, Washable, and Wearable Supercapacitors").
CNTs have high surface area and electrical conductivity. The CNT-wire combines these properties of CNTs with stability and porosity of cellulose yarns. The junction created by interweaving is essentially comprised of two such CNT-wires that are sandwiching an electrolyte. Application of potential difference leads to polarization of the electrolyte thus enabling energy storage similar to the way in which a conventional capacitor acts.
"We use the advantage of the interweaving process and create several such junctions. So, with each junction being able to store a certain amount of electrical energy, all the junctions synchronized are able to store a large amount of energy. This provides high energy density to the device," Prof. C. Subramaniam, Department of Chemistry, IIT Bombay and corresponding author of the paper points out.
The device has also been employed for lighting up an LED. This can be potentially scaled to provide electrical energy demanded by the application.
Wearables is an emerging area of commercial and industrial interest and importance. The wearables market is expected to touch USD 20 billion by 2020. An Indian patent based on the technique and materials have been applied for. The authors are currently looking for avenues for collaboration with industries to take this technology forward.
Provided by Indian Institute of Technology Bombay as a Nanowerk exclusive

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