The synthesis of nano carbon fibers and modified CNFs has been of increasing interest, with applications ranging from capacitors, Li-ion batteries, and electrocatalysts to the principal component of lightweight, high strength building materials; today, CNF demand is mainly limited by the complexity and cost of the synthetic process, which requires 30−100-fold higher production energy compared to aluminum. Carbon nanofibers have been synthesized from a variety of materials including pitch, rayon, polyacrylonitrile, solid carbon materials, acetone, or hydrocarbon gases, by employing electrospinning/carbonization, chemical vapor deposition (CVD), arc/plasma techniques, etc. Recent interests are focusing on renewable feedstocks, i.e., lignin and cellulose, rather than conventional chemicals from the natural gas or petroleum industry.

Here, we synthesize a valuable commodity, CNFs, directly from atmospheric CO₂ in a one-pot synthesis. The “production of CNFs by electrolysis in molten lithium carbonate is impossible” according to a prior report in the literature. Yet here, we present exactly that—a high yield process for the electrolytic conversion of CO₂, dissolved in molten carbonates, directly to CNFs at high rates using scalable, inexpensive nickel and steel electrodes.

—Ren et al.

Previously, the researchers had made fertilizer and cement without emitting CO₂, which they reported. Now, the team, which includes postdoctoral fellow Jiawen Ren, Ph.D., and graduate student Jessica Stuart, says their research make CO₂ a feedstock for the manufacture of in-demand carbon nanofibers.

In the process, CO₂ is broken down in a high-temperature electrolytic bath of molten carbonates at 750 ˚C (1,380 ˚F). Atmospheric air is added to an electrolytic cell. Once there, the CO₂ dissolves when subjected to the heat and direct current through electrodes of nickel and steel. The carbon nanofibers build up on the steel electrode, where they can be removed, Licht says.

To power the syntheses, heat and electricity are produced through a hybrid and extremely efficient concentrating solar-energy system. The system focuses the sun’s rays on a photovoltaic solar cell to generate electricity and on a second system to generate heat and thermal energy, which raises the temperature of the electrolytic cell.

Licht estimates electrical energy costs of this “solar thermal electrochemical process” to be around $1,000 per ton of carbon nanofiber product, which means the cost of running the system is hundreds of times less than the value of product output.

We calculate that with a physical area less than 10% the size of the Sahara Desert, our process could remove enough CO₂ to decrease atmospheric levels to those of the pre-industrial revolution within 10 years.

—Stuart Licht

At this time, the system is experimental, and Licht’s biggest challenge will be to ramp up the process and gain experience to make consistently sized nanofibers. Licht said the team is scaling the process up quickly, and soon should be in range of making tens of grams of nanofibers an hour.

One advance the group has recently achieved is the ability to synthesize carbon fibers using even less energy than when the process was initially developed. Carbon nanofiber growth can occur at less than 1 volt at 750 ˚C,—less than the 3-5 volts used in the 1,000 ˚C industrial formation of aluminum, he said.

We hope this will provide motivation to remove carbon dioxide from the air and to mitigate the effects of climate change, and will decrease the cost of carbon composites for lighter weight electric vehicle components, and provide new higher capacity Li-ion anode opportunities for Li batteries.

—Stuart Licht

The team’s research has been funded primarily by the National Science Foundation.

Resources

• Jiawen Ren, Fang-Fang Li, Jason Lau, Luis Gonzaĺez-Urbina, and Stuart Licht (2015) “One-Pot Synthesis of Carbon Nanofibers from CO₂” Nano Letters doi: 10.1021/acs.nanolett.5b02427