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4th February 2019, Blacksburg, VA

Researchers create carbon fibres with uniform porous structure

From left to right: Tianyu Liu, a postdoctoral associate in the Liu Lab; Assad Khan, a fifth-year chemistry doctoral student; and Guoliang "Greg" Liu, an assistant professor of chemistry and a member of the Macromolecules Innovation Institute. © Virginia Polytechnic Institute and State University

From left to right: Tianyu Liu, a postdoctoral associate in the Liu Lab; Assad Khan, a fifth-year chemistry doctoral student; and Guoliang "Greg" Liu, an assistant professor of chemistry and a member of the Macromolecules Innovation Institute. © Virginia Polytechnic Institute and State University

Guoliang “Greg” Liu, an assistant professor in the Department of Chemistry, in Virginia Tech’s College of Science, wants to power planes and cars using energy stored in their exterior shells. He has used block copolymers to create first carbon fibres with uniform porous structure that be both structurally and functionally useful.

“What if we can design them to have functionality, such as energy storage?” said Liu, also a member of the Macromolecules Innovation Institute. “If you want them to store energy, you need to have sites to put ions in.” Liu said ideally the carbon fibres could be designed to have micro-holes uniformly scattered throughout, similar to a sponge, that would store ions of energy.

After tweaking a conventional method of chemically producing carbon fibres, Liu now has developed a process to synthesise porous carbon fibres with uniform size and spacing. He details this work in a recently published article in Science Advances.

Conventional (A, B) and new (C) methods for synthesizing carbon fibres from various polymer precursors. © Virginia Polytechnic Institute and State University

Conventional (A, B) and new (C) methods for synthesizing carbon fibres from various polymer precursors. © Virginia Polytechnic Institute and State University

“Making porous carbon fibres is not easy,” said Liu. “People have tried this for decades. But the quality and the uniformity of the pores in the carbon fibres were not satisfactory. “We designed, synthesised, and then processed these polymers in the lab, and then we made them into porous carbon fibres.”

Using block copolymers

Liu used a multistep chemical process using two polymers – polyacrylonitrile (PAN) and poly(acrylonitrile-block-methyl methacrylate) (PMMA). PAN is well-known as a precursor compound to carbon fibres, and PMMA acts as a place-holding material that is later removed to create the pores.

But in the past, other chemists had typically mixed PAN and PMMA separately into a solution. This created porous carbon fibres but with differently sized and spaced pores. Energy storage can be maximised with greater surface area, which occurs with smaller, uniform pores, the researcher explains. He came up with the new idea of bonding PAN and PMMA, creating what is known as 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 utilise block copolymers to make carbon fibres and the first time to use block copolymer-based porous carbon fibres in energy storage,” said Liu.

Images from a scanning electron microscope (SEM) of carbon fibres made from (left) PAN, (middle) PAN/PMMA, and (right) PAN-b-PMMA. © Virginia Polytechnic Institute and State University

Images from a scanning electron microscope (SEM) of carbon fibres made from (left) PAN, (middle) PAN/PMMA, and (right) PAN-b-PMMA. © Virginia Polytechnic Institute and State University

After synthesising the block copolymer in the lab, the viscous solution then underwent three chemical processes to achieve porous carbon fibres. The first step is electrospinning, a method that uses electric force to create fibrous strands and harden the solution into a paper-like material. Next, Liu put the polymer through an oxidation heating process. In this step, the PAN and PMMA naturally separated and self-assembled into the strands of PAN and uniformly scattered domains of PMMA. In the final step, known as pyrolysis, Liu heated the polymer to an even higher temperature. This process solidified PAN into carbon and removed PMMA, leaving behind interconnected mesopores and micropores throughout the fibre.

New possibilities in energy storage

Although this improves an already high-performing engineering material, perhaps the greater breakthrough is the ability to use block copolymers to create uniform porous structures for energy storage possibilities, Liu believes. “It opens the way we think about designing materials for energy storage,” he said. “Now we can also start to think about functionality. We not only use (carbon fibres) as a structural material but also a functional material.”

Liu had been working on this idea since he joined Virginia Tech in 2014, but he started formal research after submitting a winning proposal through the Air Force Young Investigator Program (YIP) in 2016. The first author of the paper is Zhengping Zhou, Liu’s former postdoctoral associate who now works as an assistant professor at North Dakota State University. Also involved in the research were Tianyu Liu, another postdoctoral associate in the Liu Lab, and Assad Khan, a fifth-year doctoral student in the Department of Chemistry.

www.vt.edu

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