PEI coated with hexagonal boron nitride nanosheets significantly outperforms competitive polymers at the operating temperatures needed for electric vehicles and aerospace power applications. Image: Feihua Liu/Penn State.Scientists at Penn State have shown experimentally that a new, lightweight composite material for energy storage in flexible electronics, electric vehicles and aerospace applications can store energy at operating temperatures well above current commercial polymers. This composite of a polymer and a two-dimensional (2D) material can be produced with techniques already used by industry.
"This is part of a series of work we have done in our lab on high-temperature dielectrics for use in capacitors," explained Qing Wang, professor of materials science and engineering at Penn State. "Prior to this work, we had developed a composite of boron nitride nanosheets and dielectric polymers, but realized there were significant problems with scaling that material up economically."
Scalability – or making advanced materials in commercially relevant amounts for devices – has been the defining challenge for many of the new, 2D materials being developed in academic labs. "From a soft materials perspective, 2D materials are fascinating, but how to mass produce them is a question," Wang said. "Plus, being able to combine them with polymeric materials is a key feature for future flexible electronics applications and electronic devices."
To solve this problem, Wang's lab collaborated with a group at Penn State working with 2D crystals. "This work was conceived in conversations between my graduate student, Amin Azizi, and Dr. Wang's graduate student, Matthew Gadinski," said Nasim Alem, assistant professor of materials science and engineering and a faculty member in Penn State's Center for 2-Dimensional and Layered Materials. "This is the first robust experiment in which a soft polymeric material and a hard 2D crystalline material have come together to create a functional dielectric device."
Azizi, now a post-doctoral fellow at the University of California, Berkeley, and Gadinski, now a senior engineer at Dow Chemical, developed a technique using chemical vapor deposition to make multilayer, hexagonal boron-nitride nanocrystal films and transfer the films to both sides of a polyetherimide (PEI) film. They then used pressure to bond the films together into a three-layer sandwich structure. In a result that surprised the researchers, pressure alone, without any chemical bonding, was enough to produce a free-standing film strong enough to potentially be manufactured in a high-throughput roll-to-roll process. The researchers report their results in a paper in Advanced Materials.
Hexagonal boron nitride is a wide band-gap material with high mechanical strength. Its wide band gap makes it a good insulator and protects the PEI film from dielectric breakdown at high temperatures, the reason for failure in other polymer capacitors. At operating temperatures above 176°F, the current best commercial polymers start to lose efficiency, but hexagonal-boron-nitride-coated PEI can operate at high efficiency at over 392°F. Even at these high temperatures, the coated PEI remained stable for over 55,000 charge-discharge cycles in testing.
"Theoretically, all these high-performance polymers that are so commercially valuable can be coated with boron nanosheets to block charge injection," Wang said. "I think this will make this technology feasible for future commercialization."
"There are many devices made with 2D crystals at the laboratory scale, but defects make them a problem for manufacturing," added Alem. "With a large band-gap material like boron nitride, it does a good job despite small microstructural features that might not be ideal."
First-principles calculations determined that the electron barrier, which is established at the interface between the PEI/hexagonal boron-nitride structure and the metal electrodes applied to the structure to deliver current, is significantly higher than typical metal electrode-dielectric polymer contacts. This makes it more difficult for charges from the electrode to be injected into the film.
This story is adapted from material from Penn State, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.