Supercapacitors for Clean Energy Technologies

Neutron beams provide insights into the nanoscale workings of supercapacitors, an enabling technology for clean energy innovations such as wind turbines, solar cells, light-rail trains, and electric vehicles.

Source: Canadian Neutron Beam Centre (CNBC)
Image: Siemens

Wind turbines use supercapacitors to change the angle of the blades against the wind to maximize energy production, as well as to stabilize energy production when wind gusts cause rapid fluctuations. (Image: Aka, CC BY-SA 2.5)

In recent years, there has been an exponential increase in interest related to clean and renewable energy technologies. One hot area of research relates to the development of better supercapacitors, which are proving to be a key component for many clean energy innovations.

For instance, wind turbines and solar panels use supercapacitors to stabilize their electricity output when wind gusts or moving clouds can cause fluctuations in energy production. Light-rail trains that travel without overhead wires use supercapacitors to recharge quickly during stops.

Supercapacitors, which can store and deliver energy much faster than batteries, are also playing an important role in the development of cleaner vehicles, particularly when it comes to regenerative braking (i.e., storing the energy a car usually loses during braking for later reuse). Regenerative braking is especially important for hybrid and electric vehicles, where the energy savings can be massive—up to 50 percent. Small but significant energy savings have also been realized in gas-powered cars such as the Mazda6, whose 2014 model was the first mass-produced car to use regenerative braking.

During the day, enough energy can be gathered by solar panels and stored in supercapacitors to run LED street lights all night. (Image: Cgwalther, CC BY-SA 2.5)

During the day, enough energy can be gathered by solar panels and stored in supercapacitors to run LED street lights all night. (Image: Cgwalther, CC BY-SA 2.5)

Supercapacitors also have the potential to one day reduce the charging time for electric vehicles from hours to minutes. More research is needed, however, because so far supercapacitors have lacked the ability to store as much energy as comparable batteries.

Professor David Mitlin of Clarkson University in New York State has published extensive research on materials that underpin clean and renewable energy technologies, including the materials needed for the development of better supercapacitors. One of Mitlin’s research avenues aims to help design materials that match the optimum specifications for supercapacitors by getting a better understanding of ‘pseudo-capacitors,’ a kind of supercapacitor that uses a chemical reaction at the surface of the electrode material to store energy.

Due to defects or other complicating factors, the energy density in a real device is normally expected to be lower than what would be anticipated for a theoretically ideal device. In pseudo-capacitors, however, the energy density of the chemical reaction at the surface is often found to be higher than the theoretical limits predicted by the prevailing model—as much as three times higher in some cases. This systematic discrepancy indicates a significant gap in the understanding of how pseudo-capacitors work at the molecular level.

To help bridge this knowledge gap, Mitlin set out to study the exact nature of the chemical reactions that take place at the surface of pseudo-capacitors. In research published in 2014, Mitlin’s research group used thin films of cobalt oxide (Co3O4), a material attractive for use in pseudo-capacitors because its very high capacitance is complemented by low cost and low environmental footprint. The Co3O4 films were incredibly thin—about 45 nanometres (i.e., only a few hundred molecules) thick. Thus, the researchers accessed the Canadian Neutron Beam Centre (CNBC) to conduct their experiments, as neutron reflectometry was the only technique available to probe the surface of such thin material with nanoscopic precision while the chemical reaction was in progress.

The findings were surprising. Conventional wisdom had suggested that the chemical reaction would only involve the outermost layer the film, several molecules deep. However, what they found was that the chemical reaction took place throughout the entire thickness of the film.

“This finding could not only explain the discrepancy between theory and experiment, it could be transformative in how researchers think about these materials,” says Mitlin. “Their supercapacitance is not a purely surface effect, but a quasi-bulk effect.”

This experiment provided invaluable insights for Mitlin’s parallel research efforts on pseudo-capacitors. In fact, he has already patented several of his ideas for better supercapacitors and is now actively pursuing commercialization.

“I’m optimistic there’s a huge potential for better supercapacitors,” he says, adding that realizing such potential “will enable us to extend our use of clean and renewable energy technologies and reduce the use of fossil fuels.”

DOI: 10.1149/2.081405jes