Researchers have made a significant advancement towards establishing a sustainable and circular economy by demonstrating that carbon nanotube (CNT) fibers can be completely recycled without any degradation in their structure or properties. This finding positions CNT fibers as an eco-friendly alternative to conventional materials such as metals, polymers, and larger carbon fibers, which are notoriously hard to recycle.
Researchers from Rice University have published an important study in the journal Carbon, revealing that carbon nanotube (CNT) fibers can be fully recycled without any loss in their structural integrity or properties. This breakthrough highlights CNT fibers as a sustainable option compared to traditional materials like metals, polymers, and the much larger carbon fibers, which face challenges in the recycling process.
“Recycling has always posed challenges for the materials industry. While metal recycling can be inefficient and energy-heavy, polymers often lose their essential properties after reprocessing, and carbon fibers cannot be recycled at all; they are simply downcycled into shorter pieces,” said Matteo Pasquali, the corresponding author and a director at Rice’s Carbon Hub. Pasquali holds the A.J. Hartsook Professorship in Chemical and Biomolecular Engineering, Materials Science, NanoEngineering, and Chemistry. “Given the increasing production of CNT fibers, we aimed to explore their recycling potential proactively to prevent waste issues related to large-scale use of other engineered materials. We anticipated difficulties and significant loss in properties during recycling. Interestingly, we discovered that CNT fibers have far superior recyclability compared to existing engineered materials, thus presenting a solution to an essential environmental challenge.”
The research team fabricated solution-spun CNT fibers by dissolving fiber-grade commercial CNTs in chlorosulfonic acid, a common industrial solvent. Since end-of-life recycling generally involves mixing materials produced by different manufacturers through varying processes, it was critical to evaluate how these different sources impact fiber production and properties. The fibers made from various types of CNTs produced by different companies were initially processed into distinct single-source virgin fibers before being recycled by mixing and re-dissolving them in chlorosulfonic acid. Surprisingly, the mixture achieved complete redissolution without any indication of separation between the source materials into distinct liquid layers. The re-dissolved material was then spun into a mixed-source recycled fiber, maintaining the same structure and orientation as the original virgin fiber.
“By utilizing two different sources of carbon nanotubes, we ensured that our recycling process mirrored real-life conditions,” stated Michelle Durán-Chaves, a co-first author and graduate student in chemistry. “Remarkably, the recycled fibers exhibited mechanical strength, electrical conductivity, thermal conductivity, and alignment equivalent to those of the virgin fibers, which is unprecedented in engineered materials.”
The research uncovered several crucial findings that highlight CNT fibers as a promising option for sustainable practices. A key takeaway is the complete recyclability of CNT fibers. Unlike traditional materials, notably polymers and carbon fibers that deteriorate in quality during recycling, CNT fibers preserved 100% of their original properties post-recycling.
“This ability to maintain quality implies that CNT fibers can be repeatedly used in demanding applications without sacrificing performance, thereby extending their lifespan and diminishing the need for new raw materials,” explained Ivan R. Siqueira, a co-first author and recent doctoral graduate in Rice’s Department of Chemical and Biomolecular Engineering, who is now an associate professor of mechanical engineering at Pontifícia Universidade Católica in Rio de Janeiro.
Equally important is the recycling process’s efficiency. The researchers showed that recycling CNT fibers is significantly more efficient than traditional recycling methods, which for metals and polymers can demand high energy inputs, hazardous chemicals, or detailed sorting. CNT fibers can be recycled without the need for sorting, as fibers from various sources can be blended to create high-quality recycled materials. Once scaled up, this straightforward recycling process could greatly lessen waste, energy consumption, and carbon dioxide emissions connected with material production.
“The capacity to completely recycle CNT fibers has wide-reaching implications for industries such as aerospace, automotive, and electronics,” Durán-Chaves remarked. “We hope that this research can facilitate the development of fully recyclable composites for aircraft, vehicles, civil infrastructure, and more, ultimately lessening environmental impacts across multiple sectors.”
Other co-authors of this study include Rice graduate alumni Oliver Dewey, currently with DexMat; Steven Williams; Cedric Ginestra, now with LyondellBasell; Yingru Song, currently a postdoctoral fellow at Purdue University; and Rice undergraduate alumnus Juan De La Garza, now with Axiom Space, along with Geoff Wehmeyer, an assistant professor of mechanical engineering.
This research is a part of the broader initiative at the Carbon Hub, which is spearheaded by Rice and aims to develop a future with zero emissions, wherein advanced carbon materials and clean hydrogen are produced efficiently and sustainably from hydrocarbons.
The project received support from 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, and the Ken Kennedy Institute Graduate Fellowship from Schlumberger and Rice, along with a Riki Kobayashi Fellowship from Rice’s chemical and biomolecular engineering department.
