Storage Key to Energy on Demand

Dr. Wesley Henderson was in the Iraqi desert in the early 1990s when he experienced an epiphany, realizing his life’s work would be in energy research. “I really thought a lot about what energy independence and the use of alternative fuels would mean to the U.S.,” says Henderson, who served as a Private First Class in the Army during the Gulf War and is now an assistant professor of chemical engineering. He is among several NC State researchers trying to build better storage options, such as batteries and capacitors, to prevent generated energy from being lost when not immediately used.

“There are many ways of storing electricity, but our research will allow devices to store more than ever before.”

Henderson’s research focuses on ionic liquids, organic salts with complex structures and relatively low melting points. Using such liquids as electrolytes in heavy-duty applications like vehicle batteries would provide longer life and improved safety, he says, because they are less volatile at higher operating temperatures than the solvents now used. But ionic liquids are more viscous than other solvents, which limits their conductivity—and battery power. So, the Army Research Office is funding Henderson to find ways to boost the liquids’ transport properties without sacrificing their safety benefits.

Henderson is testing various additives that exhibit properties of both ionic liquids and conventional solvents. “We’re trying to design molecules that are somewhere in between,” he says. “We want to produce an electrolyte that’s both highly conductive and stable because, as we move toward plug-in hybrid and fully electric vehicles, we need to revolutionize battery chemistries.”

Physics professors Marco Buongiorno-Nardelli and Jerry Bernholc, post-doctoral researcher Vivek Ranjan, and graduate student Liping Yu believe they have found another revolutionary way to boost energy storage. Working with scientists at Penn State, they have shown in supercomputer simulations that a chemical additive boosts the storage capacity of polyvinylidene fluoride (PVDF) polymer capacitors by up to seven times, which could improve the performance of hybrid cars, lasers, and other devices.

Capacitors rely on a polarized electric field to store power and release it quickly. PVDF normally provides little energy storage, but adding chlorotrifluoroethylene (CTFE) to it as it’s formulated allows the material to polarize more easily, increasing its storage capacity, Ranjan says. The large CTFE molecules create spaces in the PVDF polymer chains, Buongiorno-Nardelli says, allowing the molecules to rotate more freely and become polarized when the capacitor is charged up. The researchers are running simulations to determine the optimal concentration of CTFE for peak performance and are studying whether other polymers exhibit the same characteristics. “There are many ways of storing electricity,” Buongiorno-Nardelli says, “but our research will allow devices to store more than ever before.”

 

Dr. Wesley Henderson peers through the antechamber of a glove box at a bottle of ionic liquid, or liquid salt—used in his battery electrolyte research. The electrolyte is the portion of the battery that transports ions between the electrodes where electrochemical reactions occur.

This diagram shows the difference between polarized and non-polarized PVDF molecules. The non-polarized molecules (top box) produce the polymer chain on the left, while the chain on the right is produced by the polarized molecules (bottom box).