Next-gen car solution? Scientists expand uses for electrostatic capacitor

Research coming out of the University of Maryland’s Maryland NanoCenter is up-ending the conventional wisdom on electrostatic capacitors as a method for storing energy.

Professors Gary Rubloff and Sang Bok Lee used principles in nanotechnology to increase the energy density of electrostatic capacitors, which store energy as an electric charge.

Today, electrostatic capacitors aren’t even part of the conversation on energy storage because of the extremely low energy density, said Rubloff, director of the NanoCenter and an engineering professor. In fact, a U.S. Department of Energy report on electrical energy storage doesn’t even mention the category, he said.

“It’s striking, but there’s no reason to because the energy density is so low you wouldn’t expect it to affect the global energy crisis,” Rubloff told the Cleantech Group. “Now that’s changed.”

To date, capacitors have been restricted by low energy density that prevents them from releasing energy fast enough to provide power to devices. Capacitors show promise because of their high power density, allowing fast recharge and bursts of power (see New nanocomposite process improves capacitors).

Alternately, batteries, which store energy chemically, have high energy density and lower power density. One of the major barriers to adoption of electric vehicles is the lengthy charge time and low power of batteries (see Scientists discover 9-second lithium-ion recharge).

Rubloff and Lee say their method improves the electrostatic capacitor’s energy density to 10 times what’s available today. The process, detailed in Nature Nanotechnology this week, could—for the first time—make the device a useful add-on to rechargeable lithium ion batteries to increase power and drastically reduce the time it takes to recharge.

“This work being reported is exciting, and it may change the picture of how you design electric energy storage systems,” Rubloff said. “You could now feasibly use a hybrid system of electrostatic and electrochemical storage. I think this demonstrates that innovations from nanotechnology have the capability to change the entire picture of all these devices.”

In the future, electrostatic capacitors could be used alone to power vehicles or electronic devices. The pair is working to further improve energy density of the devices to meet that goal.

“It’s a big step to think of the electrostatic capacitor as a single device that could provide electrical energy to electronic devices,” said Lee, an associate professor in the Department of Chemistry and Biochemistry. “This research is the first big step to show proof of that concept.”

Lee—also a professor at the Korea Advanced Institute of Science and Technology—led the research in nanostructures, using the principle that anodization of aluminum forms anodic aluminum oxide, which has very uniform and parallel nanopores. Those nanopores vastly increase the surface area of the material. Rubloff brought in knowledge of the semiconductor industry, which uses chemical deposition of thin-film materials.

The combination of the two principles allowed the professors to deposit alternating layers of metal and insulator, about six nanometers thick each, inside the nanopores to create a huge surface area, allowing the system to greatly increase its ability for storing an electric charge, Rubloff said. The resulting system is about 25 nanometers thick.

“Electrostatic capacitors were already very good at storing high power, but by making very deep nanopores and filling them up with metal and insulator, we get a much higher energy density than ever before,” Rubloff said. “This moves them so that they can compete with more conventional ways for storing energy, like today’s conventional electrochemical capacitors.”

Conventional electrostatic capacitors provide less than 0.1 watt-hours per kilogram of energy density, while the NanoCenter device provides more than 0.7 watt-hours per kilogram, falling into the range of electrochemical capacitors, Lee said. The system still has lower energy density than batteries, but the findings provide hope that electrostatic capacitors could be the one device that addresses the opposite problems of low energy density of capacitors and low power density of batteries.

“The ultimate goal from both industries would be the same, single device that would provide extremely high power and extremely high energy density,” Lee said. “That’s the motivation for why we used nanostructures to improve the energy density in the electrostatic capacitor.”

The professors are continuing their work in order to increase the energy density by another factor of 10, which would make the electrostatic capacitor competitive with alkaline batteries, Lee said. The current technological findings took about a year of research.

Rubloff said the duo still has to address technical challenges in order to make the device a viable manufacturable technology, including improvements to the amount of voltage the device is able to accept and the lifetime the device can hold the charge.

Additionally, the researchers would have to decide on the types of equipment needed for mass production. But one advantage is that the nanotechnology process to create the nanopores is much cheaper than what’s used today. Such nanopores are also used in memory chips but etched by precise, expensive machines. And the atomic layer deposition process is widely used in the semiconductor industry, meaning there could be cheap, mass produced equipment on the horizon, Rubloff said.