Tandem electrocatalytic-thermocatalytic conversion can help reduce greenhouse gas emissions by converting carbon into a useful material.
Researchers at the U.S. Department of Energy’s Brookhaven National Laboratory and Columbia University have developed a method to convert carbon dioxide (CO2) into carbon nanofibers, which have a wide range of unique properties and numerous long-term uses. Their technique utilizes electrochemical and thermochemical reactions run at low temperatures and ambient pressure. As described in the journal Nature Catalysis, this approach could effectively transform carbon into a useful solid form to offset or even achieve negative carbon emissions.
Jingguang Chen, a professor of chemical engineering at Columbia with a joint appointment at Brookhaven Lab who led the research, said, “You can use carbon nanofibers to reinforce cement. This will trap carbon in concrete for at least 50 years, potentially longer. By that time, the world should have shifted to renewable energy sources that do not emit carbon.”
Additionally, the process generates hydrogen gas (H2), which is a promising alternative fuel that produces zero emissions when used.
Capturing or converting carbon
Although the idea of capturing CO2 or converting it into different materials to combat climate change is not new, storing CO2 gas can lead to leaks. Furthermore, many CO2 conversions produce carbon-based chemicals or fuels that are used immediately, leading to the re-release of CO2 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,” Chen said.
Solid carbon materials, such as carbon nanotubes and nanofibers, have various desirable properties like strength, thermal and electrical conductivity. However, it is a complex process to extract carbon from carbon dioxide and assemble it into these microscopic structures. One method involves extreme heat at temperatures exceeding 1,000 degrees Celsius.
“It’s very unrealistic for large-scale CO2 mitigation,” Chen said. “In contrast, we identified a process that can occur at 400 degrees Celsius, a more practical and achievable temperature for industry.”
The tandem two-step
The key was to break the reaction into stages and use two different types of catalysts, materials that make it easier for molecules to 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,” said Brookhaven Lab and Columbia research scientist Zhenhua Xie, lead author on the paper.
The scientists discovered that carbon monoxide (CO) is a superior starting material than carbon dioxide (CO2) for creating carbon nanofibers (CNF). They then worked backwards to find the most efficient method for generating CO from CO2.
Previous research from their team led them to use a commercially available electrocatalyst composed of palladium supported on carbon. Electrocatalysts drive chemical reactions using an electric current. In the presence of flowing electrons and protons, the catalyst breaks down both CO2 and water (H2O) into CO and H2.
For the second step, the scientists utilized a heat-activated thermocatalyst made of an iron-cobalt alloy. It operates at temperatures of roughly 400 degrees Celsius, which is significantly less extreme than what would be required for a direct CO2-to-CNF conversion. They also discovered that adding a little extra metallic cobalt significantly 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.
Catalyst characterization
The scientists performed various experiments to gain insights into how the catalysts function. These experiments included physical and chemical characterization studies at Brookhaven Lab’s National Synchrotron Light Source II (NSLS-II) with the Quick X-ray Absorption and Scattering (QAS) and Inner-Shell Spectroscopy (ISS) beamlines, computational modeling studies, and microscopic imaging at the Electron Microscopy facility at the Lab’s Center for Functional Nanomaterials (CFN).
The researchers used “density functional theory” (DFT) calculations to analyze the atomic arrangements and other characteristics of the catalysts when interacting with the active chemical environment. They examined the structures to determine the stable phases of the catalyst under reaction conditions. They also looked at active sites and how they bonded with reaction intermediates. By determining the barriers or transition states from one step to another, they learned exactly how the catalyst functioned during the reaction.
X-ray diffraction and X-ray absorption experiments at NSLS-II tracked how the catalysts changed physically and chemically during the reactions. Synchrotron X-rays showed how the presence of electric current transformed metallic palladium in the catalyst into palladium hydride, a metal that is crucial to producing both H2 and CO in the first reaction stage.
For the second stage, the researchers wanted to know the structure of the iron-cobalt system under reaction conditions and how to optimize the iron-cobalt catalyst. The X-ray experiments confirmed that both an alloy of iron and cobalt plus some extra metallic cobalt were present and needed to convert CO to carbon nanofibers.
“The two work together sequentially,” said Liu, whose DFT calculations helped explain the process.
“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,” she explained.
Recycle-ready, carbon-negative
“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,” he said.
This process benefits from the ease of catalyst recycling, commercial availability of the catalysts, and relatively mild reaction conditions for the second reaction, resulting in lower energy and other costs, according to the researchers.
“For practical applications, both are really important—the CO2 footprint analysis and the recyclability of the catalyst,” said 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.”
If these processes are powered by renewable energy, they could result in carbon-negative outcomes, thus creating new opportunities for CO2 reduction.
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