An ambitious new geoengineering approach promises to scrub carbon dioxide from the atmosphere and transform it into carbon nanofibers. That would turn a greenhouse gas into a highly valuable product that is stronger and lighter than steel, is used in race cars, Boeing's new lightweight, high efficiency "Dreamliner" airplane and wind turbine blades.
Carbon nanofibers are typically made through vapor deposition, a complex and energy-intense process where carbon is heated until it turns into a gas and is then cooled to form into fibers. Now researchers have developed an electrochemical process that captures carbon dioxide from the atmosphere, heats it until it turns into a liquid then uses an electric current to draw out the carbon in the form of nanofibers.
The new approach shows promise as a way to remove some of the planet's most pervasive greenhouse gas from the atmosphere. But even if the new technique proves commercially viable, climate scientists say, the relatively small amount of carbon dioxide it could transform into carbon nanofiber would be insignificant in the larger fight against climate change.
Calling it "diamonds from the sky," this novel process was described by Jessica Stuart of George Washington University at the American Chemical Society's annual meeting in Boston today. The process is named for its ability to turn atmospheric carbon dioxide into a high-value product that is, like diamonds, made from carbon.
It is the latest and perhaps most promising among a number of recent attempts to find new uses for the greenhouse gas. It was first introduced in a paper published by Stuart's professor, George Washington University chemistry professor Stuart Licht, on August 3.
"We developed a process that we believe is a viable technology to significantly decrease the concentration of carbon dioxide in the air that can help reduce the impact of climate change," Licht said.
To capture C02 and transform it into carbon nanofibers, Licht, Stuart and their colleagues heated a mixture of carbonate salts to 750 degrees C, causing the molten salts to draw CO2 from the atmosphere. Using a pair of nickel-and-steel electrodes, the group then applied a low-voltage current to the molten material, causing carbon nanofibers to form on the negatively charged steel electrode. The process was powered by a high-efficiency concentrated solar-energy system that generates heat and electricity.
The process requires one-tenth the energy of today's leading methods of carbon nanofiber production and, at $1,000 a ton, can be done for roughly 1/25th the cost, Licht said.
Carbon nanofibers are similar to carbon nanotubes in that both are made of carbon and are at least 1000 times smaller in diameter than a human hair. Carbon nanotubes are uniform in their composition and more than 100 times stronger than steel per unit weight. The strength of the new, less uniform carbon nanofibers made by Licht and colleagues have not yet been tested, but they expect to find they are somewhat less strong than nanotubes but stronger than the larger-diameter carbon fiber that is roughly twice as strong as steel.
In a peer-reviewed paper published in 2011 Licht posited that similar solar arrays and carbon dioxide capture and conversion equipment deployed across an area of less than 10 percent the size of the Sahara Desert, could remove enough CO2 to reduce atmospheric levels of the greenhouse gas to pre-industrial levels within 10 years. The Sahara desert is nearly the size of the United States and 10 percent of it would be bigger than Texas and Utah combined.
The figures, however, amount to something of a catch-22. Transforming such massive amounts of C02 into carbon nanofiber would far and away exceed the market for the material; sales of the carbon nanofiber couldn't keep up with production, leaving no mechanism to pay for additional carbon dioxide removal. Conversely, producing only enough carbon nanofiber to meet market demand wouldn't significantly reduce levels of the greenhouse gas in the atmosphere.
"It's not valuable at a scale that matters," says Daniel Schrag, director of Harvard University's Center for the Environment.
Global carbon dioxide emissions are approaching 40 billion [metric] tons per year. Finding a solution that would remove 1 billion tons of CO2 from the atmosphere each year would be significant, but something that removes only 1 million tons a year would be far from significant, Schrag says. Global carbon fiber production, including larger non-"nano" fibers, is expected to grow to just 73,000 metric tons per year in 2016, according to MarketsandMarkets, a global market research and consulting company.
"We are a factor of a million off," Schrag said. "If it isn't in the billions of tons of carbon, I'm not interested from a climate perspective."
Licht concedes the market is currently small but argues that by dramatically reducing costs, his method of production could turn carbon nanofibers into a replacement for steel, aluminum and concrete in building materials.
"If we start really using it as a significant building material then there is a massive market out there that is combined with a massive need of taking the carbon dioxide out," Licht says. "We are at the state we're in because we consume a great deal of materials and many of them can be advantageously replaced by carbon composites."
Turning Licht's laboratory-scale production into a conventional building material would be a long process, and would likely take too long to make a difference in climate change.
"Even if the global supply of building materials became carbon fiber, it would be barely a dent in terms of the amount of fossil fuels we are burning," Schrag said.