Rice researchers unveil 'surprising' breakthrough in carbon nanotube recycling

"Recycling has long been a challenge in the materials industry - metals recycling is often inefficient and energy intensive, polymers tend to lose their properties after reprocessing and carbon fibers cannot be recycled at all, only downcycled by chopping them up into short pieces," said corresponding author Matteo Pasquali, director of Rice's Carbon Hub and the A.J. Hartsook Professor of Chemical and Biomolecular Engineering, Materials Science and NanoEngineering and Chemistry. "As CNT fibers are being scaled up, we asked whether and how these new materials could be recycled in the future so as to proactively avoid waste management problems that emerged as other engineered materials reached large-scale use. We expected that recycling would be difficult and would lead to significant loss of properties. Surprisingly, we found that carbon nanotube fibers far exceed the recyclability potential of existing engineered materials, offering a solution to a major environmental issue."

The research team used solution-spun CNT fibers created by dissolving fiber-grade commercial CNTs in chlorosulfonic acid, a widely used industrial solvent. Because end-of-life recycling invariably brings together materials that were manufactured by different companies in different processes, it was important to assess the effect of multiple material sources on the fiber manufacturing process and fiber properties. Fibers made from different types of CNTs produced by different manufacturers were initially processed into separate single-source virgin fibers, then recycled by combining them and mixing in chlorosulfonic acid. Surprisingly, mixing the two fibers led to complete redissolution and no sign of separation of the two source materials into different liquid phases. This redissolved material was spun into a mixed-source recycled fiber that retained the same structure and alignment of the virgin fiber.

"By using two different sources of carbon nanotubes, we ensured that our recycling process was representative of real-life conditions," said co-first author Michelle Duran-Chaves, a graduate student in chemistry. "Remarkably, the recycled fibers demonstrated equivalent mechanical strength, electrical conductivity, thermal conductivity and alignment, which is unprecedented in the field of engineered materials."

The research revealed several significant findings that position CNT fibers as a promising material in the journey toward sustainable practices. Foremost among these is the full recyclability of CNT fibers. Unlike traditional materials, particularly polymers and carbon fibers that degrade in quality during recycling, CNT fibers retained 100% of their original properties after being recycled.

"This preservation of quality means CNT fibers can be used and reused in demanding applications without compromising performance, thus extending their lifecycle and reducing the need for new raw materials," said co-first author Ivan R. Siqueira, a recent doctoral graduate in Rice's Department of Chemical and Biomolecular Engineering who is now associate professor of mechanical engineering at the Pontificia Universidade Catolica in Rio de Janeiro.

Equally significant is the efficiency of the recycling process. The researchers demonstrated that CNT fiber recycling is notably more efficient than traditional recycling methods for metals and polymers, which often involve high energy use, hazardous chemicals or labor-intensive sorting. CNT fibers, however, can be recycled without sorting as fibers from various sources can be combined to produce high-quality recycled materials. Once these materials reach scale, this simple recycling process will significantly reduce waste, energy consumption and carbon dioxide emissions associated with materials manufacturing.

"The ability to fully recycle CNT fibers has broad implications for industries like aerospace, automotive and electronics," Duran-Chaves said. "We hope this could pave the way for fully recyclable composites in aircraft, vehicles, civil infrastructures and more, ultimately reducing environmental impacts across a wide range of sectors."

Other co-authors of the paper include Rice graduate alumni Oliver Dewey, now of DexMat; Steven Williams; Cedric Ginestra, now of LyondellBasell; Yingru Song, now a postdoctoral fellow at Purdue University; Rice undergraduate alumnus Juan De La Garza, now of Axiom Space; and Geoff Wehmeyer, assistant professor of mechanical engineering.

This research is part of the broader program of the Carbon Hub, a Rice-led initiative developing a zero emissions future, where advanced carbon materials and clean hydrogen are co-produced efficiently and sustainably from hydrocarbons.

The work was supported by the Department of Energy's Advanced Research Project Agency, the Air Force Office of Scientific Research, the Robert A. Welch Foundation, the National Science Foundation, the Novo Nordisk Foundation CO2 Research Center, the Ken Kennedy Institute Graduate Fellowship from Schlumberger and Rice and a Riki Kobayashi Fellowship from Rice's chemical and biomolecular engineering department.

Research Report:Fully recyclable carbon nanotube fibers