In the race to lighter, safer and more efficient electronics - from electric vehicles to transcontinental energy grids - one component literally holds the power: the polymer capacitor. Seen in such applications as medical defibrillators, polymer capacitors are responsible for quick bursts of energy and stabilizing power rather than holding large amounts of energy, as opposed to the slower, steadier energy of a battery. However, current state-of-the-art polymer capacitors cannot survive beyond 212 degrees Fahrenheit (F), which the air around a typical car engine can hit during summer months and an overworked data center can surpass on any given day.
Now, today (Feb. 18) in Nature, a team led by Penn State researchers reported a novel material made of cheap, commercially available plastics that can handle four times the energy of a typical capacitor at temperatures up to 482 F.
"Advances in the full systems for electric vehicles, data centers, space exploration and more can all hindered by the polymer capacitor," said co-first author Li Li, postdoctoral scholar in Penn State's Department of Electrical Engineering. "Conventional polymer capacitors need to be kept cool to operate. Our approach solves that issue while enabling four times the power - or the same amount of power in a device four times smaller,"
Capacitors store less energy than batteries, but they charge and discharge their power much quicker. A mobile phone, for example, has a battery that charges from a power source. The energy it stores comes from many internal chemical-electrical reactions over a period of time that keep the phone working. Extra functions, like the flash on the phone's camera, require a burst of energy. A capacitor is responsible for discharging that extra bang of power.
Most polymer capacitors fail at high temperatures because they are made of polymers with long chains of molecules that have low glass-transition temperatures, meaning the molecules turn from rubbery and malleable to brittle and fragile like glass at relatively low temperatures. Polymers can be found in natural materials, but are also synthetically produced to make thin, flexible films, thick, rigid plastics and everything in between. When the polymers and other material mix, their nanostructures - at the atomic level - form interfaces to varying degrees. They can leak electric charges, the researchers said, and the problem worsens at high temperatures.
"Normally, you can't have both high energy density and high temperature tolerance in one dielectric polymer - we achieved both by mixing two commercially available high-temperature polymers," said co-first author Guanchun Rui, postdoctoral scholar in the Department of Electrical Engineering and in Penn State's Materials Research Institute (MRI).
The researchers combined PEI, originally produced by General Electric and often used in pharmaceutical production, and PBPDA, a polymer with high heat resistance and electric insulation. When mixed together at suitable temperatures, the molecular components of the polymers self-assembled into 3D structures, which the researchers used to make thin films. The key, according to Rui, was finding the correct level of polymers' immiscibility, or inability to mix. Like oil and water, immiscible materials separate and organize in 3D structures based on their individual properties.
"You can mix different ratios to see how the performance shifts, very much like how metal alloy works," Rui said. "By properly controlling the immiscibility, we ended up with - to our knowledge - the first polymer alloy with these highly desirable qualities."
It's unusual for the properties of a product to be so much better than those of the individual components, according to corresponding author Qiming Zhang, Harvey F. Brush Chair and Professor of Electrical Engineering.
"If you put two similar materials together, you'd expect a similar material with a similar performance level as the two ingredients," Li said, pointing out that some small changes can sometimes lead to incremental performance improvements, like others in the field had achieved at much smaller scales. He explained that the measurement for how much energy a polymer can store and use - called the dielectric constant or K - for each individual polymer the researchers used was less than four. "Together, the polymer alloy had a K of 13.5, and it kept constant from -148 F to 482 F. That's remarkable."
The leap comes from the nanostructure of the polymers, the researchers found when they microscopically assessed the material and confirmed with computational modeling. Without the stiff, brittle restrictions implemented by ceramic or metal materials, the polymer molecules can adapt to accommodate energy without breaking down. Their self-assembled interfaces act as barriers that block mobile charge leaks and tighten the capacitor's ability to carry and discharge energy.
"The dielectrics are cheap and commercially available, the process to make large quantities is simple," Li said. "This is a cost-effective solution to energy crisis and could significantly help across multiple applications. We can put four times the power into a device, or shrink a device to one-fourth its size while it keeps the original amount of power. We can put a lot of function into something very compact in easily achievable way."
Next, the researchers are working to bring the polymer capacitors, for which they have filed a patent, to market.
Other contributors from Penn State include co-first author Wenyi Zhu, doctoral student in electrical engineering; Zitan Huang, doctoral student in materials science and engineering; Yiwen Guo, doctoral student in chemical engineering; Zi-Kui Liu, Corning Faculty Fellowship in materials science and engineering; and Ralph H. Colby, professor of materials science and engineering and of chemical engineering; and Seong H. Kim, department head and Robb Family Endowed Chair of chemical engineering and professor of materials science and engineering and of chemistry; and Qing Wang, professor of materials science and engineering. Siyu Wu, Brookhaven National Laboratory; and Wenchang Lu and J. Bernholc, North Carolina State University, also co-authored the paper.
The Office of Naval Research, the U.S. National Science Foundation, the Axalta Coating Systems and the Penn State College of Engineering's Harvey F. Brush Chair endowment supported this research.
