0903_SUPERCAP-1 feature

0903_SUPERCAP-1 featureComposite clay/RTIL electrolyte may enable fabrication of supercapacitors that operate at temperatures to 200°C or more, according to Rice University scientists. (Credit: Ajayan Group/Rice University.)

With so much research focused on graphene-based supercapacitors, it’s refreshing to be able to report on a bit of a different approach.

Even more interesting is the low-tech material at the heart of a new supercapacitor design developed by researchers at Rice University, Houston, Tex.: Bentonite clay. Oh, and full disclosure, there’s a bit of graphene in there, too. But the breakthrough here lies in how the researchers combined the clay with a room temperature ionic liquid (RTIL) to develop a new type of membrane electrolyte.

Led by professor of chemistry and materials science and nanoengineering Pulickel Ajayan, the group recently reported in the online, open access journal Scientific Reports that a supercapacitor with a clay-based membrane electrolyte is reliable at temperatures to at least 200°C. According to a news release, the device could be useful for providing power in extreme environments such as aerospace and defense applications.

“Our intention is to completely move away from conventional liquid or gel-type electrolytes, which have been limited to low-temperature operation of electrochemical devices,” Arava Leela Mohana Reddy, lead author and former Rice research scientist, says in the release. “We found that a clay-based membrane electrolyte…overcomes one of the key limitations of high-temperature operation of electrochemical energy devices. By allowing safe operation over a wide range of temperatures without compromising on high energy, power, and cycle life, we believe we can dramatically enhance or even eliminate the need for expensive thermal management systems.”

The Rice researchers combined room temperature ionic liquids, which have relatively low conductivity at room temperature but become less viscous and more conductive when heated. They combined equal amounts of RTIL and bentonite into a composite paste, then layered the material with reduced graphene oxide to form supercapacitor membranes. According to the release, electrical testing and subsequent microscopy showed no change in the composite material after heating to 200°C, and minimal change after heating to 300°C.

“The ionic conductivity increases almost linearly until the material reaches 180°C, and then saturates at 200°C,” Reddy says in the release, which also reports that the devices remained stable through 10,000 test cycles.

To enable easier fabrication of devices using the composite electrolyte, the scientists added 10 wt.% thermoplastic polyurethane to the clay/RTIL mixture. Cast into a film, the resulting material maintained capacitance at the test temperature of 200°C and could be cut and shaped to fit a variety of applications, the scientists say.