Sustainable Future for Carbon Nanotubes

Rice University researchers published a study in the journal Carbon, demonstrating that carbon nanotube (CNT) fibers can be recycled without losing their structure or properties, supporting efforts toward a more sustainable and circular economy.

This finding positions CNT fibers as a sustainable alternative to traditional materials such as metals, polymers, and larger carbon fibers, which are challenging to recycle.

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.

Matteo Pasquali, A.J. Hartsook Professor of Chemical and Biomolecular Engineering, Materials Science and NanoEngineering and Chemistry, Rice University

He added, “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.”

Using chlorosulfonic acid, a common industrial solvent, the team dissolved fiber-grade commercial CNTs to produce solution-spun CNT fibers. Evaluating the impact of different material sources on the fiber manufacturing process and fiber properties was important, as end-of-life recycling often involves combining materials from various manufacturers using different production methods.

Fibers made from different types of CNTs from various manufacturers were first processed into distinct single-source virgin fibers and then recycled by mixing them with chlorosulfonic acid. Notably, the two fibers fully redissolved without separating into distinct liquid phases. The redissolved material was spun into a mixed-source recycled fiber with the same structure and alignment as the virgin fiber.

By using two different sources of carbon nanotubes, we ensured that our recycling process was representative of real-life conditions. Remarkably, the recycled fibers demonstrated equivalent mechanical strength, electrical conductivity, thermal conductivity and alignment, which is unprecedented in the field of engineered materials.

Michelle Durán-Chaves, Study Co-First Author and Graduate Student, Rice University

The study identified several key findings that highlight CNT fibers as a viable material for sustainable practices. Foremost among these is their complete recyclability. Unlike traditional materials such as polymers and carbon fibers, which degrade in quality during recycling, CNT fibers retain 100 % of their original properties after recycling.

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.

Ivan R. Siqueira, Study Co-First Author and Associate Professor, Pontifícia Universidade Católica

The efficiency of the recycling process is another significant advantage. The researchers observed that CNT fiber recycling is far more efficient than conventional metal and polymer recycling methods, which often involve high energy consumption, hazardous chemicals, or labor-intensive sorting.

In contrast, CNT fibers can be recycled without sorting, as fibers from various sources can be combined to produce high-quality recycled materials. At scale, this straightforward recycling process could substantially reduce waste, energy use, and carbon dioxide emissions associated with production.

Durán-Chaves added, “The ability to fully recycle CNT fibers has broad implications for industries like aerospace, automotive, and electronics. 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.”

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

This research is part of the Carbon Hub, a Rice-led initiative aiming for a zero-emission future by efficiently and sustainably co-producing advanced carbon materials and clean hydrogen from hydrocarbons.

The study received funding 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, the Ken Kennedy Institute Graduate Fellowship from Schlumberger and Rice, and the Riki Kobayashi Fellowship from Rice’s chemical and biomolecular engineering department.

Video Credit: Rice University

Journal Reference:

Siqueira, I. R., et. al. (2024) Fully recyclable carbon nanotube fibers. Carbon. doi.org/10.1016/j.carbon.2024.119899