New process spins carbon nanofibers from CO₂

A new approach could help lock away carbon dioxide emissions for the long term. Researchers report a catalytic strategy to convert CO₂ into carbon nanofibers (Nat. Catal. 2024, DOI: 10.1038/s41929-023-01085-1). Carbon nanofibers (CNFs), which are similar to carbon nanotubes, could find use as cement additives, in battery electrodes, and in tough composites for sports equipment and vehicles.

Researchers have been seeking ways to convert captured CO₂ into valuable materials, such as fuels and chemicals. But those “are used within a few years, promptly releasing CO₂ back into the atmosphere,” says Jingguang Chen, a chemist at Columbia University who led the new work.

A few groups have reported the conversion of CO₂ into solid carbon materials. But the methods use expensive metal catalysts, use energy-intensive temperatures above 600 °C, or have low yields.

Chen and colleagues devised a two-step process to make CNFs. They first used a commercially available palladium catalyst to electrolyze CO₂ and water and produce syngas, a mix of carbon monoxide and hydrogen. They fed the gas into a thermochemical reactor containing an iron-cobalt alloy that triggered the precipitation and growth of carbon into CNFs on its surface. Adding extra metallic cobalt boosted nanofiber growth, and the process produced on average 2.5 g of CNFs per gram of catalyst per hour.

Together with researchers at Brookhaven National Laboratory, the team used computational modeling, X-ray absorption and scattering, spectroscopy, and electron microscopy to unravel the process behind nanofiber growth. They found that the alloy helps break C–O bonds in carbon monoxide, and the metallic cobalt facilitates the formation of C–C bonds. The process works at “relatively mild, industry-applicable temperatures of 370–450 °C,” Chen says.

Stuart Licht, a chemist at the George Washington University and founder of C2CNT,says the new method uses additional energy-consuming processes such as catalyst regeneration, and uses higher voltages compared with the one-step process his team reported in 2015 and is now commercializing (Nano Lett., DOI: 10.1021/acs.nanolett.5b02427). In that method, CO₂ dissolved in molten lithium carbonate is electrolytically converted to CNFs.

But Chen says the catalyst regeneration can be powered by renewable electricity, in which case his team’s analysis shows that their overall tandem process should have net-negative CO₂ emissions.

By integrating two well-known catalytic reactions into a single process, the team has “achieved high-quality CNFs with an appreciable production rate,” says Edman Tsang, a chemist at the University of Oxford. “I found this work elegant.”