_TECHNOLOGY Material Engineering

_Recharged

A research team at Drexel recently engineered a supercapacitor system that can operate at very low temperatures—a development that could eventually eliminate the use of traditional batteries.

_Yury Gogotsi

Gogotsi is trustee chair professor of materials science and engineering and director of the A.J. Drexel Nanotechnology Institute.

In China, a new type of electric-powered bus travels quietly down a busy Shanghai street. As passengers step on and off, a pair of parallel metal poles atop the bus rises to touch an overhead charging line suspended over the stop. In just a few minutes, rows of energy-storing supercapacitors hidden under the seats are fully charged and the bus rolls on.

Researchers racing to develop the next generation of energy storage devices are closely watching the Chinese experiment to see if supercapacitors might one day replace batteries in our cars, computers, phones and other electronic gadgets. Batteries are the industry standard today, but supercapacitors charge faster, are safer, more efficient and could cost less to produce. Their only drawback? They don’t store as much energy as batteries.

And whether you’re operating a bus or an electric screwdriver, neither option works particularly well in extremely cold climates. The reason it’s more difficult to start a car in in the winter is that the ions stored in the electrolyte solution of the battery move much slower at freezing temperatures.

But a Drexel research team, in collaboration with the University of Texas at Austin and Paul Sabatier University in Toulouse, France, has recently engineered a supercapacitor system that can operate efficiently at very low temperatures – as low as 50 below zero Celsius (58 below zero Fahrenheit).

“What we’ve done is to make supercapacitor electrodes which have both high energy density and can operate in wider temperature ranges than lithium ion batteries or conventional supercapacitors,” says Drexel Department of Materials Science and Engineering Professor Yury Gogotsi.

For seven years, his lab has been developing new applications using a widely studied nanomaterial called graphene, an atomically thin sheet or layer of carbon riddled with tiny holes. The carbon nanomaterial—in this case activated microwave exfoliated graphite oxide (a-MEGO)—is rolled into a tube-like structure. A liquid salt electrolyte solution conveys electrical voltage from the charging stop to the a-MEGO material where it’s stored.

Current devices use an electrolyte solution that will fail at temperatures below 25 below zero Celsius (13 below zero Fahrenheit). However, the devices Gogotsi’s team designed use a mixture of ionic liquids that allow for better performance at even colder temperature ranges.

“In this experiment, we showed this material works in the same temperature range and can store roughly five times more energy compared to carbon nanotubes we used in our initial experiments just two years ago,” Gogotsi says.

Another advantage to using the a-MEGO material is its high surface area; about two grams has the surface area of a football field. As a result, a-MEGO is able to store larger amount of charge on its surface than batteries or other supercapacitors. Additionally, supercapacitors can last for more than 10 years and produce up to 1 million charges/discharge cycles, compared to batteries that will last a couple years and are limited to about 1,000 cycles.

If used more widely in vehicles, supercapacitors energy cells could help manufacturers realize an additional 20- to 30-percent energy gain simply by recovering energy used during braking, Gogotsi says.

Until recently, an undervalued but increasingly important advantage supercapacitors have over batteries is their safety performance record. Supercapacitors store energy electrostatically, so there’s a reduced risk of fire or explosion. An electrochemical battery fire aboard a Boeing 787 jet recently forced the grounding of the entire fleet.

“These are all the issues we’re trying to overcome with this research, by finding better materials and extending the temperature range of the operation of supercapacitors,” Gogotsi says.

In only five years, supercapacitors have replaced lithium ion batteries not just in hybrid buses, but also in some power tools, computers, camera flashes, and for controlling giant wind turbine blade operations. In the next 15 to 20 years, as more engineers learn to incorporate supercapacitors into their product designs, capacitor energy storage may become as important as battery storage in our lives, Gogotsi says.

“I believe this area has a bright future and hopefully the materials like the ones we’ve developed will widen and broaden the use of supercapacitors,” he says.