A New Path to Sustainable Nanotube Materials

A hollow tube of foam streams out of a metal reactor in the lab of Adam Boies, a mechanical engineer at Stanford University. It looks a bit like charcoal gray cotton candy, but it is something much more special. With some stretching and further treatment, scientists could make this carbon nanotube aerogel into fibers whose properties rival those of industrial materials like steel. And these fibers could be a much more sustainable alternative.

Boies is part of an international, multidisciplinary collaboration led by Rice University chemical engineer Matteo Pasquali and working toward a radical idea first proposed by Pasquali in 2021: Carbon nanotube materials could be produced at scale as a viable alternative to steel.

Carbon nanotubes are molecular straws made of rolled-up, atomically thin sheets of pure carbon. Individually, they are more conductive than copper, have greater tensile strength than steel, and offer myriad other outstanding properties. Processed and bundled into carbon nanotube fibers, they can still often outperform hard-to-decarbonize materials like steel. The challenge is making them at a reasonable cost and in massive quantities. To compete with materials like steel, carbon nanotubes must be produced at gigaton scales, says Boies. Meeting that challenge will require a better understanding of the fundamental chemistry and physics that govern how nanotubes form.

In 2023, The Kavli Foundation brought together researchers to explore what would be required to make Pasquali’s idea possible. The group concluded that achieving this goal would require improvements of several orders of magnitude in the synthesis process. Incremental gains would not be enough. Crucially, these advances would require a deeper understanding of how the process works. That convening helped launch the international collaboration behind this work.

Now, guided in part by early insights from this collaboration, Boies’ research group demonstrated a new reactor that is 446 times more efficient than previous designs — a step change that brings large-scale production significantly closer to reality. The process, described in Nature Energy, converts methane into carbon nanotubes while producing hydrogen gas, a clean-burning fuel.

A new approach to nanotubes
Pure carbon nanotubes were first made in the 1990s, and since that time, scientists have been excited about their superlative properties: Electrical and thermal conductivity, strength and toughness are chief among them. Over the decades, researchers have found

ways to use them in a variety of small-scale applications. But turning those properties into bulk materials proved far more difficult than expected, and progress toward large-scale uses remained slow.

Pasquali and others began rethinking the problem from the ground up. In 2013,

researchers working with Pasquali made carbon nanotube fibers — materials made from assembled nanotubes — with properties that were “a reasonable fraction” of what had been predicted, he says, and continued improving them. As their research advanced, carbon nanotube fibers began matching, and even exceeding, the properties of steel, aluminum and Kevlar. “We showed you could actually displace them. [Carbon nanotube fibers] could do the same function. It was a matter of cost,” Pasquali says.

Then, a question about making carbon nanotubes from methane — the main component in natural gas — inspired Pasquali to take his work in a new direction. If scientists could use methane as a raw material to produce carbon nanotubes, they could tackle multiple sustainability challenges at once: sequestering carbon while producing materials that could serve as an alternative to steel at meaningful scale.

There is a critical need to find sustainable solutions for structural materials. Concrete accounts for about 3% of global greenhouse emissions, and steel accounts for 7%. Carbon nanotube-based materials could do the same work and be either carbon neutral or carbon negative if made from methane, particularly when that methane is sourced from waste sources, such as landfills or agriculture. If done efficiently, Pasquali says, synthesizing carbon nanotubes from methane could also produce hydrogen, a clean-burning fuel.

For a while, Pasquali says, his vision of a large-scale carbon nanotube industry was not taken seriously. “Most people thought I was a lunatic because this is a material that was produced hundreds of grams at a time, and I was saying we will make millions of tons, the size of the steel market,” he recalls. “But we started making progress, and we started building a community.” In 2023, that work led to a $1.9 million Kavli Exploration Award in Nanoscience for Sustainability, an early investment to address the fundamental science behind scaling up carbon nanotube synthesis, that helped spur an additional $2.2 million in funding from other sources.

Jeff Miller, science program officer leading the nanoscience portfolio at The Kavli Foundation, says nanotubes could offer an alternative to whole classes of materials that are considered hard to decarbonize, such as steel and concrete. “We’re looking for innovative materials where understanding more about basic nanoscience might move the field forward.”

Inside the reactor
The Kavli-supported collaboration is investigating the fundamental chemistry behind nanotube synthesis, much of which remains poorly understood. Its goal is to dramatically improve efficiency, ensuring every carbon atom that goes into the reactor becomes part of the carbon nanotube aerogel that comes out, without requiring excessive energy to keep the reaction going.

“We have to break down scientific principles to understand how complex phenomena interact inside a reactor and [we have] to model them,” Pasquali says.

Boies’ research group is not the first to produce carbon nanotubes from methane. But previous reactors consumed hydrogen rather than producing it, relying on hydrogen-rich mixtures that limited how much methane could be used and forced the process to run in batches. In Boies’ upgraded design, the reactor runs continuously. Hydrogen produced during methane breakdown helps keep the reaction running cleanly, while the reactor simultaneously produces excess hydrogen alongside the carbon nanotubes. Researchers can then refine the resulting aerogel into fibers and other materials.

This reactor design led to a 446-fold improvement in molar process efficiency, a count of the number of carbon and hydrogen atoms in the starting materials that end up in useful products. It also demonstrates that carbon nanotubes and hydrogen can be co-produced efficiently from methane, including methane derived from biogas.

Iron and sulfur catalysts aid these transformations. Controlling the concentration of these catalysts proved key to improving efficiency, says Boies. To push the process even further, his research group needs a better understanding of how these catalysts behave and how the reactions unfold.

“It’s surprising that sulfur is needed because it typically poisons catalysts. It’s necessary, but we don’t yet understand why,” says Boies. Scientists think iron atoms coalesce into tiny spheres whose surfaces are broken up by spots of sulfur. Carbon nanotubes then grow like blades of grass from these iron islands. Without sulfur, the iron spheres might get completely coated with carbon, effectively shutting down the reaction.

Boies and the rest of the team want to understand what is actually going on inside the reactor in greater detail. His collaborators are modeling these reactions, and Boies is building a new reactor with imaging components that will give scientists a literal window into the process.

Pasquali says Boies’ recent work shows that the dream of a large-scale carbon nanotube industry is “not that far-fetched.”

“It’s not hype,” he adds. “It’s just taking time.”

Miller agrees: “We need the time and patience to do the fundamental work nobody has done before. Thermodynamics is on our side, but there are still many fundamental — and extremely interesting — questions to answer before this vision can be realized.”