Indian Institute of Technology (IIT) Bombay offers a fabricated wearable super-capacitor by the researchers that has the capacity to store and deliver a huge amount of electrical energy, exceeding other similar devices. The wearable energy storage device is capable to stitch on to any fabric and deliver power ranging from microwatt to milliwatt. The energy stored in the device can power GPS location-based transmitters or a 1.8 volt LED. “The idea is when the super-capacitor is integrated with a piezoelectric energy generator then it will become completely self-sustaining. And when stitched to the fabric, the super-capacitor can be used for powering GPS location-based devices or LED lamp or even charge small electronic devices,” claims Prof. Chandramouli Subramaniam, from the Department of Chemistry at IIT Bombay and the lead author of a paper published in the journal ACS Applied Materials & Interfaces.
The laminated super-capacitor expressed unmodified performance even when subjected to extreme and harsh mechanical testing that involved striking constantly with a hammer, complete flexing, bending and rolling, and washing in a laundry machine in the presence of hot water, detergents and high spinning action. “This is possibly the first demonstration of a wearable device that can withstand rigorous washing conditions,” stated Jha. In addition, being a lightweight device, it looks after no interruption to the user movement in any way.
The electrode of the super-capacitor was fabricated by uniformly coating cotton yarn with carbon nanotubes (CNTs). The coating is completed by dipping the yarn into carbon nanotube ink, where the CNTs are dispersed in water using a surfactant (detergent). The coating converts the electrical insulating yarn into a metallic conductor hence, behaving like an electrode. “The yarns coated with carbon nanotubes exhibited a finite electrical conductivity,” marks Prof. Subramaniam.
As the super-capacitor focuses on wearable and portable electronics, liquid electrolytes are not countable. So the researchers formulated a solid electrolyte film just 150 micrometer thick by combining poly vinyl alcohol and potassium hydroxide in appropriate proportions. “We stitched the solid electrolyte with CNT-coated yarn both vertically and horizontally. Capacitors were formed wherever the CNT wires criss-crossed each other and sandwiched the electrolyte,” claimed Prof. Subramaniam. “By increasing the number of stitches, and therefore, the number of capacitors, the amount of energy stored can be increased.” A 1×1 sqcm. electrolyte would consist of at least a minimum of hundred capacitors. The researchers laminated the electrolyte film containing CNT wire electrodes to prevent it. The laminated capacitors retained flexibility and sturdiness without compromising on performance and power.
The ions in a solid matrix are conditionally trapped and come in the way of energy storage abilities. To overcome this, the polymer matrix was controllably hydrated with water vapour to improve the mobility of the ions. Likewise, to enhance the interaction between the CNT wire and electrolyte, the wires were treated with acid. Acid treatment improved the interface between the CNT wire and the electrolyte. “The combination of mobile ions in the electrolyte and better interface between the wire and the electrolyte increases the capacity to store electrical energy,” stated Prof. Subramaniam. “Energy stored in just nine capacitors can power LED of 1.8 volts,” marked Mihir Kumar Jha from the Department of Chemistry at IIT Bombay and the first author of the paper. “Depending on the application, we can increase the number of capacitors made in a small area and integrate to increase the total amount of energy stored in the system.”