Researchers say they have hit on a way to make ‘“diamonds” from the sky’ – by taking carbon dioxide from the air and using it to make valuable carbon nanofibres, they reported at the ACS meeting in Boston in August 2015. The one-pot process requires just a few volts of electricity, at an estimated energy cost of $1000/t of carbon nanofibre product, according to lead researcher Stuart Licht at George Washington University in Ashburn, VA, US.
This should be far cheaper than the whopping $25,000/t price for carbon nanofibres - used in composites such as in the Boeing Dreamliner as well as in high-end sports equipment and wind turbine blades - made by current chemical vapour deposition (CVD) and polymer ‘pulling and spinning’ technologies, Licht said.
‘Substituting [our solar electrochemical process process] could bring down the cost of nanofibres, which would drive up demand,’ he added – while at the same time mitigating climate change by removing greenhouse gas CO2 from the air.
The new process is based on the electrolysis of molten lithium carbonate, he explained. At the steel cathode, the molten carbonate breaks down at temperatures of 750°C to carbon and lithium oxide, which reacts with added CO2 from air to replenish the lithium carbonate and generate oxygen at the nickel or iridium anode. Carbon nanofibres effectively ‘grow’ on the cathode, and their structures can be controlled or tailored for various end-applications by nucleation on different transition metals, such as iron, nickel or cobalt, on the electrode surface.
Oxygen from air contains 0.04% CO2 or just 1.7x10-5 mol of reducible tetravalent carbon per litre, compared with ca20 mol of tetravalent carbon per litre, Licht and cowokers reported (NanoLetters, doi: 101.1021/acsnanolett.5b02427). ‘By absorbing CO2 from the air, molten carbonates provide a million-fold concentration increase of reducible tetravalent carbon for splitting (to carbon) in the electrolysis chamber.’
Heat and electricity to power the synthesis are provided by a process comparable to that used in solar power towers which work by concentrating light on hot water to generate steam, converted to electricity via a turbine. ‘Our system simply replaces the turbine with an electrochemical generator,’ Licht explained.
The group is currently testing out the technology in Washington DC, where CO2 from the air is being extracted to make the fibres. Initial 1A experiments produced 0.1g/hour of nanofibre, while scaling up to 100A produced 10g/hour with no reported losses per unit mass.
‘It is a surprise and a real bonus that this “super-green” process [developed by Licht and coworkers] yields carbon in a very useful form of nanofibres,’ commented Andrei Khlobystov, director of the University of Nottingham’s Nanotechnology & Nanoscience Centre. ‘It will be interesting to see how the functional properties of these nanofibres, such as mechanical strength, heat and electric conductivities, and their chemical stability, compare with carbon nanofibres synthesised by traditional means. The method is clearly highly efficient at the laboratory scale, so scaling-up to the industrial scale will be next technological challenge.’
Other comments posted on the GreenTechMedia website, however, refer to the high energy requirement for the CO2 to carbon conversion and point out that a thorough investigation of the thermodynamics is needed to see if the approach makes sense.