Catalytic Combo Effectively Produces Carbon Nanofibers from CO2

Researchers at Columbia University and the US Department of Energy’s (DOE) Brookhaven National Laboratory have created a method for turning carbon dioxide (CO₂) into carbon nanofibers—materials with a variety of special qualities and several long-term applications.

Their approach uses simultaneous thermochemical and electrochemical processes carried out at room temperature and atmospheric pressure. This method might successfully lock carbon away in a useful solid form to balance or even reach negative carbon emissions, as the authors detail in the journal Nature Catalysis.

You can put the carbon nanofibers into cement to strengthen the cement. 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.

Jingguang Chen, Professor, Chemical Engineering, Columbia University

In addition, the process yields hydrogen gas (H₂), a viable alternative fuel with zero emissions when consumed.

Capturing or Converting Carbon

It is not a novel concept to absorb CO₂ or transform it into other elements to slow global warming. However, even holding CO₂ gas may result in leakage. Furthermore, a lot of CO₂ transformations result in immediately usable carbon-based compounds or fuels, which immediately return CO₂ to the environment.

Chen added, “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.”

These solid carbon materials have many desirable qualities, such as strength and electrical and thermal conductivity. Examples of these materials are carbon nanotubes and nanofibers with diameters measured in billionths of meters.

However, getting carbon to separate from carbon dioxide and assemble into these intricate structures is no easy task. More than 1,000 degrees Celsius are needed for one direct, heat-driven process.

Chen further added, “It is very unrealistic for large-scale CO₂ mitigation. In contrast, we found a process that can occur at about 400 degrees Celsius, which is a much more practical, industrially achievable temperature.”

The Tandem Two-Step

The secret was to employ two distinct kinds of catalysts—materials that facilitate molecular interactions and reactions—and to divide the process into phases.

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.

Zhenhua Xie, Study Lead Author and Research Scientist, Columbia University

Initially, scientists discovered that carbon monoxide (CO) is a far superior precursor to carbon dioxide (CO₂) in the process of creating carbon nanofibers (CNF). Then, they went back to figure out how to produce CO from CO₂ as efficiently as possible.

Their group’s earlier research advised them to employ palladium supported on carbon electrocatalysts sold commercially. Electrocatalysts use an electric current to accelerate chemical processes. Water (H₂O) and CO₂ are divided into CO and H₂ by the catalyst when protons and electrons are moving through it.

The scientists used an iron-cobalt alloy thermocatalyst that was heat-activated for the second step. It runs at temperatures close to 400 degrees Celsius, which is far colder than what would be needed for a direct conversion of CO₂ to CFR. They also observed that adding a bit of additional metallic cobalt dramatically increases the creation of the carbon nanofibers.

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

Catalyst Characterization

The researchers conducted several investigations to learn more about the specifics of these catalysts’ workings. These included microscopic imaging at the Lab’s Center for Functional Nanomaterials (CFN) Electron Microscopy facility, computational modeling studies, and physical and chemical characterization studies at Brookhaven Lab’s National Synchrotron Light Source II (NSLS-II) using the Quick X-Ray Absorption and Scattering (QAS) and Inner-Shell Spectroscopy (ISS) beamlines.

In terms of modeling, the researchers examined the atomic configurations and other properties of the catalysts in relation to the active chemical environment via “density functional theory” (DFT) computations.

We are looking at the structures to determine what are the stable phases of the catalyst under reaction conditions. We are looking at active sites and how these sites are bonding with the reaction intermediates. By determining the barriers, or transition states, from one step to another, we learn exactly how the catalyst is functioning during the reaction.

Ping Liu, Study Co-Author, Brookhaven National Laboratory

At NSLS-II, X-Ray diffraction and X-Ray absorption experiments monitored the physical and chemical changes in the catalysts during the reactions. For instance, synchrotron X-Rays demonstrated how the catalyst’s metallic palladium changes into palladium hydride when the electric current is present. This metal is essential for the first reaction stage's production of both H₂ and CO.

Xie further stated, “We wanted to know what the structure of the iron-cobalt system under reaction conditions and how to optimize the iron-cobalt catalyst.”

The X-Ray experiments verified the presence of an iron-cobalt alloy in addition to some additional metallic cobalt, which is required to transform CO into carbon nanofibers.

“The two work together sequentially,” Liu noted.

She explained, “According to our study, the cobalt-iron sites in the alloy help to break the C-O bonds of carbon monoxide. That makes atomic carbon available to serve as the source for building carbon nanofibers. Then the extra cobalt is there to facilitate the formation of the C-C bonds that link up the carbon atoms.”

Recycle-Ready, Carbon-Negative

CFN scientist and study co-author Sooyeon Hwang added, “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.”

The images demonstrate how the catalyst is pushed up and away from the surface as the carbon nanofibers expand. According to Chen, this facilitates the recycling of catalytic metals.

He added, “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.”

The catalysts’ commercial availability, simplicity of recycling, and the second reaction’s comparatively moderate reaction conditions all help to appraise the process’s energy and other expenses favorably, according to the researchers.

Chen noted, “For practical applications, both are really important—the CO₂ footprint analysis and the recyclability of the catalyst. 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.”

These operations would have genuinely carbon-negative outcomes if they were powered by renewable energy, creating new avenues for CO₂ reduction.

The DOE Office of Science (BES) provided funding for this study. Computational resources at CFN and the DOE's Lawrence Berkeley National Laboratory’s National Energy Research Scientific Computing Center (NERSC) were used to carry out the DFT calculations. NERSC, CFN, and NSLS-II are user facilities of the DOE Office of Science.

Journal Reference:

Xie, Z., et. al. (2023) CO₂ fixation into carbon nanofibres using electrochemical–thermochemical tandem catalysis. Nature Catalysis. doi:10.1038/s41929-023-01085-1

Source: https://www.bnl.gov/