Conventional (A, B) and new (C) methods for synthesizing carbon fibers from various polymer precursors. (A) Pure PAN is electrospun into a fiber mat, oxidized at 280°C in air to crosslink PAN (blue), and then pyrolyzed at 800°C in N2 to generate carbon fibers (grey). An individual polymer fiber (purple) is magnified for illustration. (B) PAN is mixed with sacrificial PMMA (red) to form a polymer blend. After oxidation, the polymer blend macrophase-separates and forms non-uniform domains. After pyrolysis, PMMA is removed, resulting in non-uniform pores. (C) PAN-b-PMMA block copolymer microphase-separates into uniform PMMA nanodomains (red) in a matrix of PAN (blue) after oxidation and self-assembly. After pyrolysis, the porous carbon fibers contain well-controlled and uniformly distributed pores.

Carbon fiber reinforced plastics (CFRP) are mechanically strong, chemically resistant, electrically conductive, fire retardant, and lightweight, making them suitable materials in supercars and other specialty automotive applications.

Guoliang “Greg” Liu, an assistant professor in the Virginia Tech University’s Department of Chemistry, however, thinks the materials could take on an additional role. Phone and computer companies use polymers that have regularly sized micro-porous holes throughout their volume, creating space for energy storage.

Liu says ideally the carbon fibers could be designed to have micro-holes uniformly scattered throughout, similar to a sponge, that would store ions of energy. However, attempts at adding micro-porosity to carbon fiber had led to materials with holes that were too large or uneven to do any good – lowering material strength in some cases.

Liu tweaked a longtime conventional method of chemically producing carbon fibers to create a porous material uniform size and spacing. He details this work in a recently published article in Science Advances.

“We designed, synthesized, and then processed these polymers in the lab, and then we made them into porous carbon fibers,” Liu says.

Liu’s multistep chemical process used polyacrylonitrile (PAN) and polyacrylonitrile-block-methyl methacrylate) (PMMA). PAN is a precursor compound to carbon fibers, and PMMA acts as a place-holding material that is later removed to create the pores.

Images from a scanning electron microscope (SEM) of carbon fibers made from (left) PAN, (middle) PAN/PMMA, and (right) PAN-b-PMMA. Liu's lab used PAN-b-PMMA to create carbon fibers with more uniformly sized and spaced pores.

Other chemists had tried mixing the polymers separately, creating carbon fiber with irregularly sized and shaped pores. Liu’s team bonded PAN and PMMA, creating a block copolymer. One half of the compound polymer is PAN, and the other half is PMMA, and they're covalently bonded in the middle.

“This is the first time we used block copolymers to make carbon fibers and the first time we used block copolymer-based porous carbon fibers in energy storage,” Liu says. “Often, we’re only thinking from the process point-of-view, but here we’re thinking from the materials design point-of-view.”

After synthesizing the block copolymer in the lab, Liu’s team put it through an electrospinning process using electric force to create fibrous strands that harden into a paper-like material. Oxidation heating then separated the PAN and PMMA into self-assembled strands of PAN and uniformly scattered domains of PMMA. Using pyrolysis, researchers heated the polymer to a higher temperature, solidifying PAN into carbon and removing PMMA, leaving behind interconnected pores throughout the fiber.

“It opens the way we think about designing materials for energy storage,” Liu says. “Now we can also start to think about functionality. We not only use (carbon fibers) as a structural material but also a functional material.”

Virginia Tech
https://www.cals.vt.edu