To address the rising levels of CO₂ in the atmosphere, the conversion of CO₂ into secondary chemicals and materials offers the most economically viable approach to solve this global challenge. However, state-of-the-art efforts to convert CO₂ into low-value commodities and fuels, such as methanol, suffer from low efficiencies, low value of the commodity, and expensive catalysts and processes. Here we discuss an alternative approach where CO₂ is captured in a molten carbonate medium from air, and split into its elemental constituents in the presence of a catalytic electrode surface to form carbon nanotubes (CNTs). This general approach was developed in the 1960's for the deposition of carbon, [1-2] and recently shown to be adapted without composition control of bulk catalytic electrodes to the growth of large-diameter (> 100 nm) CNTs [3]. Here, we demonstrate the ability to rationally engineer the anode and cathode electrodes to controllably tune the catalytic material composition and size during the production of CNTs using this technique. This relies on existing knowledge of advances in the gas-phase synthesis of CNTs applied to this electrochemical system, where it is known that iron is the most catalytically active catalyst material, and that catalyst nanoparticle size controls the diameter of the grown CNTs. We show the ability to engineer the material composition of the electrodes to produce high yield (> 99%) growth of multi-walled CNTs with tight diameter ranges averaged < 30 nm, and we show that control on catalytic nanoparticle size can decrease this average diameter to even smaller CNTs. Most notably, early results indicate a minority presence of few-walled and single-walled CNTs, and we present a vision where the ability to transfer the mechanisms studied and understood for single-walled CNT growth over the past few decades can drive rapid forward progress in producing high quality single-walled CNTs in both tangled and aligned architectures using this technique. This paves a pathway to produce a material that prominent researchers for the past few decades have envisioned to revolutionize our modern world, with a precursor material derived from air that poses a significant long-term global threat to mankind.

References:

[1] B.B.M.D. Ingram and G.J. Janz, Electrochim Acta, 11, 1629 (1966)

[2] H.Y. Yin, X.H. Mao, D.Y. Tang, W. Xiao, L.R. Xing, H. Zhu, D.H. Wang, and D.R. Sadoway, Energ Environ Sci, 6, 1538 (2013).

[3] S. Licht, A. Douglas, J. Ren, R. Carter, M. Lefler, and C.L. Pint, ACS Cent Sci, 2, 162-168 (2016).