As rising levels of atmospheric greenhouse gases continue to threaten drastic climate change, some scientists are scouring for ways to scrub the key offender, carbon dioxide (CO2), right out of the air. Now, a team of chemists led by Stuart Licht at George Washington University in Washington, D.C., thinks it has discovered a method that could possibly pay for itself by making something potentially useful out of the CO2 pulled from the air. Today in Nano Letters, the group presents a process that turns atmospheric CO2 into carbon nanofibers similar to valuable materials used in industries such as aerospace, construction, and electronics.
“It’s clearly interesting and exciting research, but it’s a first step,” cautions James Tour, a nanoengineering and materials scientist at Rice University in Houston, Texas, who is not part of the study. “There’s still a lot of work to be done.”
The conversion process, called STEP (for Solar Thermal Electrochemical Process), is powered by sunlight, heat, and a small electrical kick. The researchers simply run a current between two electrodes in a crucible filled with molten lithium carbonate, which melts at 723°C, and a few types of metal. To start the process, the molten lithium carbonate absorbs CO2 from the air. The electrical current then busts apart the CO2 molecules through a process called electrolysis, and the carbon atoms swarm to the negatively charged electrode. Ordinarily, the carbon just forms useless gunge—which makes this form of carbon capture a sure loser. But the researchers found that if they seasoned the lithium carbonate with just the right metals, they could form more valuable carbon nanotubes instead. First, zinc sparks the process of nanofiber formation. Then, one of four other metals (nickel, cobalt, copper, or iron) acts as a scaffold, known as a nucleation site, on which the nanofiber continues to grow. The team found that with the right combination of metals, voltage, and temperature, STEP’s coulombic efficiency (a classic measure of chemical yield used to determine how efficiently the product is made) hovered around 95%, meaning that in STEP’s case, 95% of the current worked toward producing the carbon nanofibers, and only 5% powered other side reactions.
“It’s very exciting, and I think it’s a viable option in providing a scientific and technological solution to decreasing the CO2 in the atmosphere,” Licht says. In particular, he and his team hope STEP could provide a less expensive way to make carbon nanofibers. The materials—noncylindrical cousins of carbon nanotubes, composed of chains of carbon atoms less than a micrometer wide—are used in making composite materials, chemical catalysts, and even scanning probe microscopes. But they are frustratingly costly to manufacture. Cutting the price through STEP would be a real “game-changer,” Licht says: Cheaper nanofibers would lead to greater commercial use and, in turn, to less carbon in the atmosphere. Licht says that using STEP in an area less than 10% the size of the Sahara desert could bring CO2 in the atmosphere down to preindustrial levels in just 10 years. (Side note: 10% of the Sahara desert is more than twice the area of California—that’s a lot of molten lithium.)
First, though, STEP would require some serious upscaling. One major hurdle, Tour says, is that STEP can’t yet produce usable nanofibers. “It’s like when you shear a sheep and you get wool,” he says. “Those little wool fibers that first come off its back—you can’t make a blanket out of that. You have to somehow get it spun into long fibers that you can then put into a machine to get a wool sweater or blanket.” Until scientists learn to spin STEP’s carbon nanofibers into long commercially usable fibers, he says, it’s too early to hail the process as an environmental breakthrough. But Andrew Bocarsly, a physical inorganic chemist at Princeton University who is not part of the study, says the team’s results are encouraging despite such open questions. “Even if Licht’s work only uses up 0.1%, or even 0.01% of the CO2 that we’re generating, it would be a great start,” Bocarsly says. “It’s not the solution, but it might be part of the solution.”