Turning carbon dioxide (CO2) into solid carbon, like carbon nanofibers (CNF), is a good way to store carbon for a long time. This helps in reducing the overall amount of carbon in the air. However, the usual methods have challenges. Using heat and catalysts to change CO2 into CNF directly has some limits. Also, using electricity for this often results in a type of carbon that’s not very structured and doesn’t give much product, or it needs a lot of energy to work.
Researchers at the U.S. Department of Energy’s Brookhaven National Laboratory and Columbia University have found a method to transform carbon dioxide (CO2), a strong greenhouse gas, into carbon nanofibers. These nanofibers have various unique properties and could serve multiple purposes over a long time.
The process combines electrochemical and thermochemical reactions at relatively low temperatures and normal pressure. This approach can potentially effectively store carbon in a useful solid form, helping offset or even achieve negative carbon emissions.
The bonus part is that the process also generates hydrogen.
Jingguang Chen, a professor of chemical engineering at Columbia with a joint appointment at Brookhaven Lab who led the research, said, “The novelty of this work is that we are trying to convert CO2 into something value-added but in a solid, useful form.”
Solid carbon materials like carbon nanotubes and nanofibers, which are extremely tiny, have excellent properties like strength and efficient heat and electricity conduction. However, extracting carbon from carbon dioxide and making it form these small structures is a complex process. One common method involves high temperatures exceeding 1,000 degrees Celsius, making it challenging.
Chen said, “It’s 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 key was to divide the reaction into stages and employ two different types of catalysts. Catalysts are substances that facilitate the coming together and reaction of molecules. Separating the reaction into multiple smaller steps made it possible to use various energy inputs and catalysts for each part of the process, making it more manageable.
The scientists found that carbon monoxide (CO) is a better starting material than carbon dioxide (CO2) for producing carbon nanofibers (CNF). They then worked backward to identify the most efficient way to generate CO from CO2.
Previous research from their team led them to use a commercially available electrocatalyst of palladium supported on carbon. Electrocatalysts drive chemical reactions using an electric current. In the presence of flowing electrons and protons, this catalyst splits CO2 and water (H2O) into CO and H2.
For the second step, the researchers employed a heat-activated thermocatalyst made of an iron-cobalt alloy. It operates at temperatures around 400 degrees Celsius, much milder than the high temperatures required to convert CO2 to CNF directly. They also found that adding a small amount of extra metallic cobalt significantly improves the formation of carbon nanofibers.
By combining electrocatalysis and thermocatalysis in a tandem process, researchers are accomplishing tasks that would be challenging with either process.
To understand how these catalysts work, the scientists conducted various experiments. This included computational modeling studies, 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, and microscopic imaging at the Electron Microscopy facility at the Lab’s Center for Functional Nanomaterials (CFN).
Using “density functional theory” (DFT) calculations for modeling, the researchers analyzed the catalysts’ atomic arrangements and other characteristics when interacting with the active chemical environment.
Study co-author Ping Liu of Brookhaven’s Chemistry Division, who led these calculations, said, “We are looking at the structures to determine what the stable phases of the catalyst are 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.”
X-ray experiments at NSLS-II tracked how catalysts changed physically and chemically during reactions. For instance, they revealed how electric current transforms metallic palladium into palladium hydride, crucial for producing both H2 and CO in the first reaction stage.
In the second stage, the researchers optimized the iron-cobalt catalyst by confirming the presence of an alloy of iron and cobalt, along with some extra metallic cobalt. Both were necessary to convert CO into carbon nanofibers.
Prof Liu said, “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.”
CFN scientist and study co-author Sooyeon Hwang said, “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.”
Chen said, “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.”
“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.”
“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.”
“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.”