The CNT wool process builds on earlier work by Professor Licht and his team to develop a one-pot process that transforms CO₂ into a controlled selection of nanotubes (CNTs) via molten electrolysis: C2CNT (CO₂ into carbon nanotubes). This synthesis consumes only CO₂ and electricity, and is constrained only by the cost of electricity. The initial synthesis pathways, however, led only to short CNTs.

Whereas our previous molten carbonate synthesized CNTs have nanometer-sized diameter and lengths, this CNT wool reaches diameters over 1 mm and length over 1 mm. Hence, the question arises whether this new CNT wool should be classified as a nanomaterial. The physical properties, such as the unusually high electrical conductivity and Raman spectra of these materials demonstrated in the linked Data in Brief paper are that of multi-walled carbon nanotubes, and are due to the morphology as demonstrated by TEM.

The length to diameter ratio and the 0.342 nm inter-wall spacing of these confined cylindrical graphene layers suggests these new CNT wools should be classified as a (albeit unusual, but particularly useful as a cloth precursor, class of) CNT material. Monel cathode substrates, electrolyte equilibration, and a mixed metal (NiChrome) nucleation facilitate the synthesis of this CNT wool. The process is constrained by the (low) cost of electricity. Carbon dioxide is the sole reactant in this CNT transformation, providing a financial impetus for the removal of this greenhouse gas.

—Johnson et al.

Schematic representation of new synergistic pathways to form a high yield of macroscopic length CNT “wool” by electrolysis in molten carbonate. Left side: experimental cell configuration used in these C2CNT experiments. Middle: Prior C2CNT syntheses were dependent on a zinc coated steel cathode, a pure Ni anode, and a low current pre-electrolysis activation step. An intermediate, new C2CNT electrolysis removes the requirement of a zinc coating leading to the exploration of a variety of new cathode substrates. Right side: The optimized C2CNT pathway utilizes Monel cathodes and Nichrome anodes, molten electrolyte equilibration for 24 h, and the electrolysis is conducted directly without pre-electrolysis activation steps. This pathway produces a high yield of macroscopic length CNT wool. Johnson et al. Click to enlarge.

The new process grows the CNTs on Monel (nickel alloy) instead of steel. Monel, electrolyte equilibration, and a mixed metal nucleation facilitate the synthesis.

Efficacious climate mitigation by CO₂ transformation requires a massive market, and product stability and compactness. The most compact form of captured carbon is through its transformation to solid carbon. CNTs are among the highest strength and most stable materials. CNT cost reduction by C2CNT, provides a preferred (lower mass per unit strength) to the mass metal market, and the CNT wool introduced here accelerates CNT demand as a building industry and textile material. Together these principal societal staples, when produced from CO₂, comprise an ample demand to markedly decrease atmospheric carbon.

… Initial scaling is efficiently applied to available concentrated, hot sources of CO₂, such as eliminating the CO₂ emission from industrial smoke stacks and simultaneously forming valuable CNT wool. Larger scale C2CNT can be achieved through direct elimination of atmospheric CO₂ using solar heat and solar to electric PVs.

—Johnson et al.

The team calculated that an area equal to only 4% of the Sahara Desert would be sufficient to bring atmospheric CO₂ concentrations back to pre-industrial levels in ten years, and that a wind speed of 1 km per hour would be sufficient to deliver that CO2 to those STEP (solar thermal electrochemical process (STEP) CNT plants (earlier post).

Schematic representation of an ocean-based solar thermal and photovoltaic field to drive both water purification and C2CNT splitting of CO2 to useful products. Johnson et al. Click to enlarge.

Resources

• Marcus Johnson, Jiawen Ren, Matthew Lefler, Gad Licht, Juan Vicini, Xinye Liu, Stuart Licht (2017) “Carbon nanotube wools made directly from CO2 by molten electrolysis: Value driven pathways to carbon dioxide greenhouse gas mitigation,” Materials Today Energy, Volume 5, Pages 230-236 doi: 10.1016/j.mtener.2017.07.003