IIT Mandi researchers turn hydrophobic carbon materials to hydrophilic high performance supercapacitors

Mandi: Dr. Viswanath Balakrishnan, Associate Professor, School of Engineering, Indian Institute of Technology Mandi, and his research scholar, Mr. Piyush Avasthi, have recently developed aligned carbon nanotube-based electrodes that could enable high energy supercapacitors. Their research papers have been published in Advanced Materials Interfaces and ACS Applied Nanomaterials.

The quest for the ‘perfect’ battery that can store large amounts of energy, and be charged rapidly and repeatedly, has spurred considerable research all over the world. The lithium ion battery that now powers almost all our consumer electronics devices such as mobile phones, laptops and even electric cars, has its own limitations. One of the disadvantages with batteries is that charging takes a lot of time, which results in extended down-times of operation. In addition durability is another major concern. What if there is an energy storage device that not only delivers energy quickly, but can also be charged instantaneously?

“A promising route to improving the performance of energy storage devices, especially in terms of cycling life and charging times, is to move away from batteries towards supercapacitors”, says Dr. Balakrishnan. Supercapacitors can charge and discharge instantly and can ideally last across millions of charge-discharge cycles without performance degradation. They also have a higher power density than batteries. Where they have fallen short so far, is in the area of energy density; supercapacitors have forty times less ability to store energy than the state-of-art lithium ion battery.

A supercapacitor essentially consists of two conducting electrodes immersed in an electrolyte, which are separated by electrically insulating layer to separate the charges. While applying the current, potential difference develops between two electrodes and opposite charged ions physically adsorb on the respective surfaces of electrodes. This charge storage mechanism is highly reversible which makes supercapacitor to charge discharge very quickly. Carbon is usually used as the electrode material, but traditional carbon forms result in low energy density.

Carbon nanotubes are tiny tubes of carbon, a hundred thousand times thinner than the human hair. These materials, when used as electrodes, have the potential to considerably increase the energy density of supercapacitors. But typically high surface area carbon nanotube and fibers are hydrophobic, i.e., they cannot be ‘wetted’ by the electrolyte. “Tuning wetting behavior of supercapacitor electrode surface plays a crucial role in various interfacial processes as it directly affects mass transfer, formation of electric double layer, and electron delivery at the interface”, the researchers write in their paper.

IIT Mandi researcher Mr. Piyush Avasthi used a process called Chemical Vapor Deposition to produce ‘forests’ of vertically aligned carbon nanotubes that are wettable (hydrophilic) by the electrolyte. The perfectly aligned nanotubes, that were a few micrometers in height, were grown on a stainless-steel mesh and treated with two different ways to enhance their hydrophilic properties – in one, the forests were treated with potassium hydroxide (KOH), and in the other, they were coated with an ultrathin layer of titanium dioxide (titania), which made the nanotubes superhydrophilic. While KOH treatment resulted in better energy density than randomly oriented carbon nanotubes, treating with titania resulted in a 102-fold increase in energy density, 20-fold increase in specific capacitance, and 13-fold increase in power density. With that kind of improvement, supercapacitors can certainly give lithium ion batteries a run for their voltage.

The stainless-steel mesh on which the carbon nanotubes were grown, is physically flexible and would allow incorporation of the energy storage devices on wearable, miniaturized and portable electronic products and smart devices. Dr. Balakrishnan’s research on advanced materials would hasten the realization of commercially viable, standalone supercapacitor-based energy storage solutions that are safer, more powerful and longer lasting than current state-of-art batteries.