The mechanisms that control a powerful technique that captures and converts atmospheric CO₂ into carbon nanotubes (CNTs) have been revealed by researchers at Vanderbilt University. The electrochemical approach yielded CNTs with median diameters between ∼33 and ∼23 nm, which are the smallest diameters known for this category of synthetic route. The results are published in a recent issue of ACS Applied Materials & Interfaces.

“What we’re working on not only provides a blueprint for sustainable conversion of CO₂ into materials that are more valuable than the energy required for conversion, but is also an efficient process to produce interesting carbon-based nanostructures with numerous exciting technological applications [e.g., flexible electronics, nanosensors, and nanocomposites],” says Anna Douglas, first author of the article and a doctoral candidate in the group of Cary L. Pint, who leads the research.

Douglas and Pint used their successful results to co-found SkyNano LLC, a clean energy start-up company that focuses on the production and scale-up of high-value functional materials using greenhouse gases as chemical inputs.

The main focus of the team’s work is to control the diameter of the CNTs produced from ambient CO₂ via the electrochemical reduction of CO₂ to CNTs through molten salt electrolysis. The method is based on splitting Li₂CO₃ electrolyte to its components; O₂, which is collected at the anode; Li₂O, which accumulates in the electrolyte; and carbon, which precipitates on the cathode and grows into CNTs at the interface between the cathode and the molten salt. Li₂O regenerates the Li₂CO₃ electrolyte through a chemical reaction with ambient CO₂.   

“The main challenge that we overcame was to understand—through a series of control experiments where we changed out the cathode material—why we ended up with the same, low quality, large diameter, hollow microscale fibers or tubes in every single case,” Pint says. “After a bit of frustration, we eventually realized that our anode, which was nickel, was corroding into the electrolyte and depositing metal onto our cathode,” he says. A corrosion-resistant anode was finally fabricated by passivating the anode surface by means of atomic layer deposition of aluminium oxide (Al₂O₃) on the Ni surface.

“From there, we have been able to isolate the catalytic mechanisms at the cathode and really make major progress in controlling the diameter of the carbon nanotubes that we’re producing,” Pint says.

The diameter of the CNTs was directly correlated to the thickness of Fe catalyst layers, which were deposited by e-beam evaporation onto the stainless steel surface of the cathode.  The researchers were able to achieve smaller and tighter diameter distributions (from ∼10 nm to 38 nm and median diameter of ∼23 nm), with a catalyst layer thickness of 0.5 nm. When using Fe layers as thick as 5 nm, the result was a much wider diameter distribution (from ∼19 nm to 62 nm, with a median diameter of ∼33 nm).

Although these dimensions are the best known in literature so far achieved by an electrochemical process, even the smaller median diameters measured are still much larger than observed for CNTs prepared with the more common chemical vapor deposition (CVD) process. The study showed that the reason for this is that the CNT growth is also affected by catalyst coarsening throughout the duration of growth. Experiments with the longest duration (around 60 min) almost doubled the average diameters of the CNTs produced, while after only 3 min, the nanotubes had a much tighter diameter distribution, centered near ∼10 nm.

“The study is a major advance in this new field of electrochemical synthesis of CNTs,” says Placidus B. Amama, professor at Kansas State University, who did not participate in the study. “I believe it will fill the current knowledge gap in electrochemical synthesis of CNTs and prevent scientists from simply extrapolating mechanisms observed in catalytic CVD growth to electrochemical synthesis,” he says.

“Although this electrochemical route has been known for a long time, the group of Cary Pint refined it to a point close to real application,” says Vincent Jourdain, expert in carbon nanotube growth and professor at the University of Montpellier, France, who was not involved in this research. Jourdain stresses, though, that this work was not intended to and will not provide a solution for large-scale removal of CO₂ from the atmosphere. “This would require producing hundreds of millions of tons of nanotubes each year … and there is clearly no market for that,” he says. However, he sees a high potential in the new method for producing CNTs with reduced cost and environmental footprint.

However, Pint and his team demonstrate in their work the lower energy consumption for this process compared to conventional CVD, meaning lower cost CNTs. With cheaper CNTs come more products that can cost-effectively adopt them, and more market opportunity, which Pint and Douglas are working to achieve with SkyNano LLC.

The research group is now aggressively working toward solving the technical challenges of maintaining catalysts only a few nanometers in diameter at a liquid-solid interface to yield single-walled carbon nanotube growth. The researchers are also adapting the process for other types of materials, such as diamond coatings.

“There are a lot of reasons to be excited about repurposing carbon dioxide from air into the most valuable all-carbon building blocks in the world around us,” Pint says. “I hope that our work emphasizes that there is still exciting science in synthesizing these materials and that there are still challenges we as researchers need to overcome to put these materials into markets,” he says.

Read the abstract in ACS Applied Materials & Interfaces.