Continuous research and experiments are underway to find the best possible way to capture carbon dioxide from the atmosphere. Researchers at the U.S. Department of Energy’s Brookhaven National Laboratory and Columbia University have developed an innovative method for this. The process of solid carbon nanofiber production from CO2 catalysts will not only capture carbon but also produce hydrogen as byproduct.
Scientists at the U.S. Department of Energy’s Brookhaven National Laboratory and Columbia University have found a method to turn carbon dioxide (CO2), into carbon nanofibers. These materials have various special properties and could be used in many ways in the future. The strategy they employ involves the combination of electrochemical and thermochemical reactions carried out at low temperatures and regular pressure. As detailed by the scientists in their publication, this innovative method effectively sequesters carbon in a valuable solid state, leading to the mitigation or even reversal of carbon emissions. Additionally, the process also generates hydrogen gas (H2), a highly promising alternative fuel that, when utilized, produces zero emissions.
Leader of the research and Chemical Engineering Professor Jingguang Chen at Columbia, with a joint appointment at Brookhaven Lab, said, “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.”
Carbon Capture and Conversion
Capturing and converting CO2 to combat climate change is not a new concept. However, simply storing CO2 gas can result in leaks. Many CO2 conversions generate carbon-based chemicals or fuels that are immediately used, releasing CO2 back into the atmosphere.
Prof. Chen said, “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.”
Carbon nanofibers and nanotubes are solid carbon materials that have dimensions of billionths of a meter. Strength, thermal & electrical conductivity are included in the list of their properties. Extracting carbon from CO2 and assembling it into fine-scale structures is not easy. One process that relies on heat demands temperatures above 1,000° Celsius.
“It’s very unrealistic for large-scale CO2 mitigation. In contrast, we found a process that can occur at about 400° Celsius, which is a much more practical, industrially achievable temperature,” Prof Chen added.
The tandem 2-step
Reaction was divided into 2 stages for using 2 different types of catalyst. This makes it easier for molecules to organize and react.
Lead author of the paper and research scientist of Brookhaven Lab and Columbia, Zhenhua Xie, said, “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.”
The scientists discovered that using carbon monoxide (CO) as a starting material is more effective than using CO2 for producing carbon nanofibers (CNF). They subsequently worked back to determine the most efficient method for generating CO from CO2. Their previous work led them to use a commercially available electrocatalyst made of palladium supported on carbon. This electrocatalyst can split both CO2 and water into CO and H2 with the help of an electric current.
Then for solid carbon nanofiber production from CO2 catalysts the scientists turned to a heat-activated thermocatalyst made of an iron-cobalt alloy. It operates at temperatures around 400° 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.
Prof Chen said, “By coupling electrocatalysis and thermocatalysis, we are using this tandem process to achieve things that cannot be achieved by either process alone.”
The scientists conducted various experiments to understand how these catalysts work. In terms of modeling, the scientists employed density functional theory (DFT) calculations to examine the atomic structures and other properties of the catalysts under various chemical interactions. X-ray experiments at NSLS-II monitored catalysts’ physical and chemical transformations throughout the reactions.
Co-author and leader of calculations of the study, Ping Liu of Brookhaven’s Chemistry Division, said, “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.”
“For the second stage, we wanted to know what’s the structure of the iron-cobalt system under reaction conditions and how to optimize the iron-cobalt catalyst,” Xie added.
The x-ray experiments confirmed the presence of an alloy of iron and cobalt. It also shows some additional metallic cobalt, necessary for converting CO to carbon nanofibers. According to Liu, “The two work together sequentially. 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,”
Environmentally Friendly & Carbon-Neutral
CFN scientist and co-author of the study, Soo Yeon 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.”
According to Prof. Chen, “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 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.”
The process is cost-effective due to the following reasons:
- Ease of catalyst recycling
- Commercial availability of catalysts
- Relatively mild reaction conditions for the second reaction
“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,” Prof. Chen added.
If solid carbon nanofiber production from CO2 catalysts is powered by renewable resources, it will be truly carbon neutral. This will further open new opportunities for CO2 mitigation.