Treated Carbon Nanotube Electrodes Boost Supercapacitor Energy Density 100X
Engineers from the Indian Institute of Technology Mandi have developed aligned carbon nanotube-based electrodes that could enable high energy supercapacitors.
Dr. Viswanath Balakrishnan, Associate Professor, School of Engineering, Indian Institute of Technology Mandi and his research assistant Mr. Piyush Avasthi, published their research in Advanced Materials Interfaces and ACS Applied Nanomaterials.
The quest for the ideal battery that can store large amounts of energy, and can be charged rapidly and repeatedly, has driven substantial research all over the world. Supercapacitors can charge quickly, and unlike the lithium-ion batteries that now powers almost all our consumer electronics devices, they take a considerable amount of time to charge up.
On the other hand, over time, the charging, discharging, and recharging of lithium-ion batteries reduces their capacity to hold a charge.
"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," said Dr. Balakrishnan, Associate Professor, School of
Engineering, Indian Institute of Technology Mandi.
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. However, they have a significantly lower energy density. Supercapacitors have about forty times less ability to store energy than the state-of-art lithium-ion battery.
A supercapcitor is comprised of two conducting electrodes submerged in an electrolyte. The electrodes are separated by an electrically-insulating layer that separates the charges.
When current is applied, an electric field is created. A capacitor stores energy electrostatically in an electric field. This charge storage mechanism is known to be highly reversible which makes supercapacitor to charge and discharge very quickly.
Carbon is often used as the electrode material, but conventional carbon results in low energy density. However, carbon nanotubes, which are about a hundred thousand times thinner than the human hair, when used as electrodes, have the potential to significantly increase the energy density of supercapacitors. One difficulty is that typically high surface area carbon nanotube and fibers are hydrophobic, meaning 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 explained in their paper.
IIT Mandi researcher Mr. Piyush Avasthi used CVD to fabricate ‘forests' of vertically aligned carbon nanotubes that are wettable (hydrophilic) by the electrolyte. The perfectly aligned nanotubes, which were a few micrometers in high, were grown on a stainless-steel mesh. They were treated two different ways to enhance their hydrophilic properties.
The researchers treated one sample of nanotubes with potassium hydroxide (KOH), and they coated the other with an ultrathin layer of titanium dioxide (titania), which made the nanotubes super-hydrophilic.
The KOH treatment resulted in improved energy density than randomly oriented carbon nanotubes.
However, treating with titania lead to a 102-fold increase in energy density, a 20-fold increase in specific capacitance, and a 13-fold increase in power density.
This improved performance could make such supercapacitors a significant competitor to lithium-ion batteries, the researchers claimed.
The researchers also noted that the stainless-steel mesh upon which the carbon nanotubes were grown, is physically flexible. This flexible mesh could allow incorporation of the energy storage devices on wearables, the researchers speculated.