Researchers from QUT's Battery Interest Group developed supercapacitor technology intended for EV body panels. The discovery was made by Postdoctoral Research Fellow Dr. Jinzhang Liu, Professor Nunzio Motta and PhD researcher Marco Notarianni, from QUT's Science and Engineering Faculty - Institute for Future Environments. Also involved were PhD researcher Francesca Mirri and Professor Matteo Pasquali, from Rice University in Houston, TX.
Supercapacitors used for the EV body panels were made using carbon nanotube (CNT) films as current collectors and graphene films as electrodes.They were fabricated using all carbon electrodes, with volume energy in the order of 10−3 Whcm−3, comparable to Li-ion batteries, and power densities in the range of 10 Wcm−3, better than laser-scribed-graphene supercapacitors.
Figure. Charging a Nissan Leaf EV built with solar panels.
All-carbon supercapacitor electrodes are made by solution processing and filtering electrochemically-exfoliated graphene sheets mixed with clusters of spontaneously entangled multiwall carbon nanotubes. Capacitance was maximized by using a 1:1 weight ratio of graphene to multi-wall carbon nanotubes and by controlling their packing in the electrode film so as to maximize accessible surface and further enhance the charge collection. This electrode is transferred onto a plastic-paper-supported double-wall carbon nanotube film used as a current collector. These all-carbon thin films are combined with plastic paper and gelled electrolyte to produce solid-state bendable thin film supercapacitors. Supercapacitors were assembled with cells in series in a planar configuration to increase their operating voltage. It was found that the shape of the supercapacitor film strongly affects its capacitance. An in-line superposition of rectangular sheets is superior to a cross superposition in maintaining high capacitance when subject to fast charge/discharge cycles.
It was found that the thin graphene film with thickness <1 μm can greatly increase the capacitance. Using only CNT films as electrodes, the device exhibited a capacitance as low as ~0.4 mF cm−2, whereas by adding a 360 nm thick graphene film to the CNT electrodes led to a ~4.3 mF cm−2 capacitance. The conductive CNT film is equivalent to gold as a current collector, while it provides a stronger binding force to the graphene film.
The supercapacitors “sandwich” was made into a thin and extremely strong film that could be embedded in a car's body panels, roof, doors, hood and floor - storing enough energy to turbocharge an electric car's battery (see figure) in just a few minutes. Mr. Notarianni noted that a car partly powered by its own body panels could be a reality within five years.
Vehicles need an extra energy spurt for acceleration, which is made possible by the supercapacitors. They hold a limited amount of charge, but they can deliver it very quickly, making them the perfect complement to mass-storage batteries.
"Supercapacitors offer a high power output in a short time, meaning a faster acceleration rate of the car and a charging time of just a few minutes, compared to several hours for a standard electric car battery." Dr Liu said currently the "energy density" of a supercapacitor is lower than a standard lithium ion (Li-Ion) battery, but its "high power density", or ability to release power in a short time, is "far beyond" a conventional battery.
Supercapacitors are presently combined with standard Li-Ion batteries to power electric cars, offering a substantial weight reduction and increase in performance. In the future, it is hoped that supercapacitor will be able to store more energy than a Li-Ion battery while retaining the ability to release its energy up to 10 times faster - meaning the car could be entirely powered by the supercapacitors in its body panels. After one full charge this car should be able to run up to 500 km - similar to a gasoline-powered car and more than double the current limit of an EV.
Dr. Liu said the technology would also potentially be used for rapid charges of other battery-powered devices. "For example, by putting the film on the back of a smart phone to charge it extremely quickly," he said.
"We are using cheap carbon materials to make supercapacitors and the price of industry scale production will be low," Professor Motta said. "The price of Li-Ion batteries cannot decrease a lot because the price of Lithium remains high. This technique does not rely on metals and other toxic materials either, so it is environmentally friendly, if it needs to be disposed of."