## Introduction

One climate change mitigation pathway is to transform CO2 into a stable, valuable commodity, which will provide an incentive to consume smokestack or atmospheric CO2. Recently, we reported on a high yield synthesis of CNF carbon, nanofibers, from carbon dioxide in molten lithium carbonate1. This electrolytic synthesis provides a facile route for the transformation of the greenhouse gas into a high-value commodity2 and occurs at low energy with low electrolysis potentials3. Here, we show that the synthesis produces an even more valuable product, CNTs, carbon nanotubes, rather than CNFs, carbon nanofibers and provide cost analysis and critical evidence that the product is not synthesized by consuming the electrolyte, but rather that the CNT is formed directly from atmospheric CO2. As illustrated in Fig. 1, the synthesis transforms CO2 gas, dissolved in a molten carbonate electrolyte by electrolysis at a nickel anode and at a galvanized steel cathode. At the anode the product is O2 and at the cathode the product is uniform carbon nanofibers, CNFs, or CNTs, which are CNFs with hollow interiors. Due to their superior strength than steel, conductivity, flexibility and durability CNTs and CNFs have applications ranging from capacitors, Li-ion batteries and nanoelectronics to the principal component of lightweight, high strength building materials, such as used in replacing steel and concrete in bridge construction, wind turbines and lighter-weight structural materials for jets, cars and athletic equipment.

In the C2CNT (CO2 to carbon nanotube) synthesis, metallic zinc at the galvanized steel initiates chemical carbon formation and carbon nanofiber electrochemical growth is promoted by transition metal nucleation sites (such as nickel, iron, cobalt or copper). The nano-morphology is tuned by controlling the electrolysis conditions, such as the concentration of added alkali oxide, the temperature, the current density and the transition metal concentration. Control of the latter variable is demonstrated both by the direct addition of transition metal oxides to the molten carbonate electrolyte, or (in the case of nickel) by the controlled low concentration release of nickel from the anode1. In the synthesis, the electrodes are conventional metals and the lithium electrolyte if not consumed is inexpensive.

Critical to this alternative CTCNT pathway to transform the principal greenhouse gas into a stable, useful and valuable commodity is the demonstration that the product is not synthesized by consuming the carbonate electrolyte, but rather the CNF is formed directly from atmospheric CO2. Here, we use carbon isotope tracking to substantiate that atmospheric CO2 is directly incorporated into the synthesized carbon nanostructure. We also report on electrolysis conditions to yield the first synthesis of pure carbon 13 isotope multi-walled carbon nanofiber, MWCNF and conditions which favor formation of either the carbon nanotube or carbon nanofiber morphology.

Whereas syntheses of single4,5,6,7,8,9 (SWCNT) and double10,11,12 (DWCNT) walled 13C isotope carbon nanotubes, quantum dots13 and two dimensional 13C nanomorphologies such as graphenes have been reported14,15,16,17, studies of 13C multiwalled carbon nanofibers have not been evident, presumably due to the complexity and expense of conventional electrospinning/carbonization or chemical vapor deposition syntheses which would require additional synthetic steps to form the requisite 13C polymers or 13C organometallic reactants. Similarly, density functional and molecular dynamic simulation studies are found on 13C isotope SWCNTs18,19, but not on nano-morphologies which would require modeling of a larger numbers of carbon centers. In general 13C isotopes have been of interest from the perspective of kinetic effects on catalysis, thermal conductivity, as well as on diffusion and NMR effects18,20,21.

Here, (i) through isotope tracing (12CO2 air into molten Li213CO3 electrolyte) we demonstrate the direct uptake of gas phase CO2 and its transformation into carbon nanotubes at high yield. Atmospheric concentrations (0.04%) of CO2 are directly consumed and transformed and this occurs without the pre-concentration that is required from CO2 sequestration processes. We present (ii) a short derivation of costs and (iii) the first synthesis of a pure carbon 13 isotope multiwalled carbon nanofiber, MWCNF. (iv) Finally, we show that under the electrolytic synthesis conditions studied, natural abundance 12CO2 favours formation of CNTs, while equivalent synthetic conditions with heavier 13CO2 favours formation of (solid core) carbon nanofibers.

## Experimental

Experimental details of the solar thermal electrochemical process, STEP, synthesis of a variety of societal staples and carbon capture have been delineated in previous publications22,23,24,25,26,27,28,29,30. This study focuses primarily on the STEP for carbon electrochemical reactor component to form a high-yield CNF component.13C lithium carbonate (Sigma-Aldrich, 99 atom% 13C) 13C carbon dioxide (Sigma-Aldrich, 99 atom% 13C), lithium carbonate (Alfa Aesar, 99%) and lithium oxide (Alfa Aesar, 99.5%) are combined to form various molten electrolytes.

Electrolyses are driven galvanostatically, at a set constant current as described in the text. The electrolysis is contained in a pure nickel 100 ml crucible (Alfa Aesar). Electrolyses in the Ni crucible used the inner walls of the crucible as the anode. A wide variety of steel wires for coiled cathodes are effective, an economic form (used in this study) is Fi-Shock 14 gauge, galvanized steel wire model #BWC-14200. Following an initial low current (0.05 A for 10minutes, 0.1A for 10 minutes, 0.2A for 5 minutes and 0.4A for 5 minutes) step to grow Ni nucleation sites on the cathode, CNTs or CNFs are grown on an immersed 10 cm2 galvanized steel cathode at 1.00 A for 1 hour. The electrolysis potentials are consistent with those we have recently reported for generic carbon deposition in a molten Li3CO3 electrolyte as a function of current density and as previously reported, the potential decreases with addition of Li2O to the electrolyte3. Two nanostructures are generated, straight CNTs or CNFs that are grown in electrolyte without added Li2O, or tangled CNTs or CNFs that are grown when Li2O has been added to the electrolyte. During electrolysis, the carbon product accumulates at the cathode, which is subsequently removed and cooled. Subsequent to electrolysis the product remains on the cathode, but falls off with electrolyte when the cathode is extracted, cooled and uncoiled. The product is washed with 11 m HCl and separated from the washing solution by centrifugation.

The carbon product is washed and analyzed by PHENOM Pro-X Energy Dispersive Spectroscopy (EDS) on the PHENOM Pro-X SEM or Carl Zeiss Sigma VP Field Emission SEM and by TEM with a JEM 2100 LaB6 TEM. Raman spectroscopy was measured with a LabRAM HR800 Raman microscope (HORIBA) with 532.14 wavelength incident laser light, with a high resolution of 0.6 cm−1.

### C2CNT cost analysis

CO2 removal here occurs by the electrolytic reduction of tetravalent carbon in proximity to the cathode to solid, zerovalent carbon. The concentration of tetravalent carbon available for reduction is much higher as carbonate than as airborne or soluble CO2. Molten carbonate, such as Li2CO3(liquid), contains ~10 molar reducible tetravalent carbon/liter. Air contains 0.04% CO2, equivalent to 1 × 10−5 molar of tetravalent carbon/liter. Hence, molten carbonates formed by the dissolution of atmospheric CO2 and its conversion to carbonate, provide a spontaneous, million-fold concentration increase of reducible tetravalent carbon sites per unit volume, compared to air, facilitating the direct conversion of atmospheric CO2. In this process, the production of carbon, such as CNFs, or their hollow morphology, CNTs, by electrolysis in lithium carbonate occurs simultaneously with the production of oxygen and dissolved lithium oxide:

Li2CO3 consumed by electrolysis is continuously replenished by reaction of this excess Li2O, formed as a product in the Equation 1 electrolysis, with CO2 from the air (or CO2 available in higher concentration from stack emissions):

for the net reaction (combining Equations 1 and 2):