Scientists Unveil New Method to Convert Carbon Dioxide into Carbon Nanofibers

TAGS:  Polymer Reinforcement     Sustainability / Natural Solutions    

Scientists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory and Columbia University have developed a way to convert carbon dioxide (CO₂) into carbon nanofibers.

Their strategy uses tandem electrochemical and thermochemical reactions run at relatively low temperatures and ambient pressure.

Producing Hydrogen Gas as By Product

As the scientists describe in the journal Nature Catalysis, this approach could successfully lock carbon away in a useful solid form to offset or even achieve negative carbon emissions.

“You can put the carbon nanofibers into cement to strengthen the cement,” said Jingguang Chen, a professor of chemical engineering at Columbia with a joint appointment at Brookhaven Lab who led the research. “That would lock the carbon away in concrete for at least 50 years, potentially longer. By then, the world should be shifted to primarily renewable energy sources that don’t emit carbon.”

As a bonus, the process also produces hydrogen gas (H₂), a promising alternative fuel that, when used, creates zero emissions.

Using Electrocatalytic-thermocatalytic Tandem Strategy

The idea of capturing CO₂ or converting it to other materials to combat climate change is not new. But simply storing CO₂ gas can lead to leaks. And many CO₂ conversions produce carbon-based chemicals or fuels that are used right away, which releases CO₂ right back into the atmosphere.

“The novelty of this work is that we are trying to convert CO₂ into something that is value-added but in a solid, useful form. It’s very unrealistic for large-scale CO₂ mitigation,” Chen said. “In contrast, we found a process that can occur at about 400 degrees Celsius, which is a much more practical, industrially achievable temperature.”

Such solid carbon materials—including carbon nanotubes and nanofibers with dimensions measuring billionths of a meter—have many appealing properties, including strength and thermal and electrical conductivity. But it’s no simple matter to extract carbon from carbon dioxide and get it to assemble into these fine-scale structures. One direct, heat-driven process requires temperatures in excess of 1,000 degrees Celsius.

The electrocatalytic-thermocatalytic tandem strategy for CNF production circumvents thermodynamic constraints by combining the co-electrolysis of CO₂ and water into syngas (CO and H₂) with a subsequent thermochemical process under mild conditions (370-450 °C, ambient pressure). This yields a high CNF production rate. The optimal synergy of iron-cobalt (FeCo) alloy and extra metallic Co enhanced the dissociative activation of syngas, promoting carbon-carbon bond formation for CNF production. (Zhenhua Xie/Brookhaven National Laboratory and Columbia University)

Converting CO₂ into CO to Make CNF

The trick was to break the reaction into stages and to use two different types of catalysts—materials that make it easier for molecules to come together and react.
“If you decouple the reaction into several sub-reaction steps you can consider using different kinds of energy input and catalysts to make each part of the reaction work,” commented Brookhaven Lab and Columbia research scientist Zhenhua Xie, lead author on the paper.

The scientists started by realizing that carbon monoxide (CO) is a much better starting material than CO₂ for making carbon nanofibers (CNF). Then they backtracked to find the most efficient way to generate CO from CO₂. Earlier work from their group steered them to use a commercially available electrocatalyst made of palladium supported on carbon.

Electrocatalysts drive chemical reactions using an electric current. In the presence of flowing electrons and protons, the catalyst splits both CO₂ and water (H₂O) into CO and H₂. For the second step, the scientists turned to a heat-activated thermocatalyst made of an iron-cobalt alloy. It operates at temperatures around 400 degrees Celsius, significantly milder than a direct CO₂-to-CNF conversion would require. They also discovered that adding a bit of extra metallic cobalt greatly enhances the formation of the carbon nanofibers.

“By coupling electrocatalysis and thermocatalysis, we are using this tandem process to achieve things that cannot be achieved by either process alone,” Chen said.

New Opportunities for CO₂ Mitigation

“Transmission electron microscopy (TEM) analysis conducted at CFN revealed the morphologies, crystal structures, and elemental distributions within the carbon nanofibers both with and without catalysts,” said CFN scientist and study co-author Sooyeon Hwang.

“The images show that, as the carbon nanofibers grow, the catalyst gets pushed up and away from the surface. That makes it easy to recycle the catalytic metal,” Chen said. “We use acid to leach the metal out without destroying the carbon nanofiber so we can concentrate the metals and recycle them to be used as a catalyst again. For practical applications, both are really important—the CO₂ footprint analysis and the recyclability of the catalyst,” stated Chen. “Our technical results and these other analyses show that this tandem strategy opens a door for decarbonizing CO₂ into valuable solid carbon products while producing renewable H₂.”

This ease of catalyst recycling, commercial availability of the catalysts, and relatively mild reaction conditions for the second reaction all contribute to a favorable assessment of the energy and other costs associated with the process, the researchers said. If these processes are driven by renewable energy, the results would be truly carbon-negative, opening new opportunities for CO₂ mitigation.

Source: Brookhaven National Laboratory