Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

# Tracking airborne CO2 mitigation and low cost transformation into valuable carbon nanotubes

## Abstract

Primary evidence of the direct uptake of atmospheric CO2 and direct transformation into carbon nanotubes, CNTs, is demonstrated through isotopic labeling and provides a new high yield route to mitigate this greenhouse gas. CO2 is converted directly to CNTs and does not require pre-concentration of the airbone CO2. This C2CNT (CO2 to carbon nanotube) synthesis transforms CO2-gas dissolved in a 750 °C molten Li2CO3, by electrolysis, into O2-gas at a nickel electrode and at a steel cathode into CNTs or carbon or nanofibers, CNFs. CNTs are synthesized at a 100-fold price reduction compared to conventional chemical vapour deposition, CVD, synthesis. The low cost conversion to a stable, value-added commodity incentivizes CO2 removal to mitigate climate change. The synthesis allows morphology control at the liquid/solid interface that is not available through conventional CVD synthesis at the gas/solid interface. Natural abundance 12CO2 forms hollow CNTs, while equivalent synthetic conditions with heavier 13CO2 favours closed core CNFs, as characterized by Raman, SEM and TEM. Production ease is demonstrated by the first synthesis of a pure 13C multiwalled carbon nanofiber.

## 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):

This C2CNT cost analysis assumes a generic renewable electric supply and does not include the additional efficiency advantages of a solar thermal and electric supply22,23,24,25,26,27,28,29,30. Electrolysis costs to produce CNFs, or CNTs, will be similar to infrastructure costs associated with the chlor-alkali and aluminum industries. The electrical energy costs are low, requiring 0.9 to 1.4 V3. CNFs or CNTs are consistently prepared here at 80 to 100% coulombic efficiency of the four electrons required to reduce CO2, which at $0.10 per kWh is equivalent to$800 to $1,600 per metric tonne of CNT. After washing off the electrolyte, the product consists of >80% pure carbon nanotubes1. The synthesis continues to be refined for higher purities and specific morphologies. The electrolyte tends to tenaciously bind onto the CNT product, but is water soluble and purification does not require toxins or costly washing materials. Analogous waste management and cleaning costs of recyclables (plastics and metals) is < <$10 per tonne and is assumed to be similar here. Lithium carbonate is not consumed during the CO2 electrolysis and at today’s cost of $6,000 per tonne, as amortized over ten year’s usage, cost an additional$140 per metric tonne CNT, additionally the amortized cost of the nickel and steel electrodes and ancillary equipment is low for a combined cost upper limit of ~$2,000 per metric tonne of CNT. These costs compare to today’s conventional chemical vapor deposition or electrospun production cost of ~$25,000 per tonne of CNF2 and $200,000 to 400,000 per metric tonne of industrial grade (90% purity) CNTs31. The low costs of C2CNT production opens high revenue windows and provides a significant incentive for CO2 removal and as costs decrease also provides impetus for CNF and CNT market growth. ### Raman isotope spectra and tracking the C2CNT transformation Figure 2 presents Raman spectra of carbon nanofibers that yield structural and isotope replacement information. Each was synthesized by electrolysis under identical conditions except one (top, Fig. 2A) was produced with natural abundance CO2 and Li2CO3, while others (middle, Fig. 2B) were grown in molten 13C Li2CO3. The natural abundance carbon nanotubes (Fig. 2A) Raman spectrum exhibits two sharp peaks at 1350 cm−1 and 1575 cm−1, which correspond to the disorder-induced mode (D band) and the high frequency E2g first order mode (G band), respectively. The intensity ratio between D band and G band (ID/IG) is an important parameter to evaluate the graphitization and here the ratio of 0.70, is consistent with that of commercial hollow carbon natural abundance carbon isotope nanofibers32. The Raman spectrum observed for 13C-CNFs in Fig. 2B have both bands downshifted compared to the 12C spectrum in Fig. 2A. The frequency of Raman modes in samples containing a concentration, C, of 13C is determined by the following equation which incorporates the increased mass of this isotope:11 In Equation 2, ω0 is the frequency of a particular mode in a natural abundance CNT sample, ω is the frequency of a particular mode in 13C enriched carbon sample and C0 = 0.0107 is the natural abundance of 13C. In accord with Equation 2, the D and G band for pure 13C-CNFs should peak at 1297 and 1514 cm−1, respectively. The close agreement between the theoretical calculated (1297 and 1514 cm−1) Raman peak position and the observed experimental peak (red curve, Fig. 2B) indicates the CNFs obtained approached 100% 13C isotope enrichment. The intensity ratio of ID/IG is 0.63, indicating a better degree of graphitization than for the natural-enriched CNTs (Fig. 2A). Generally in this study during the electrolyses, either 13CO2 was exposed to Li213CO3, or (natural abundance) 12CO2 was exposed to Li212CO3. Our calculations indicated that atmospheric CO2 can be absorbed and then converted into carbon with Li2O present in carbonate melts, by low energy electrolysis with the potential constrained by the reaction: Li2O + 2CO2 → Li2CO3 + C + O23. The hypothesis that the CNFs are formed directly from absorbed atmospheric CO2 via Equation 3 is significant as it provides a direct process to mitigate this greenhouse gas. The hypothesis is tested here by a study in which a 13C electrolyte (1 m Li2O in Li213CO3) is exposed to regular (natural abundance, 99% 12C) air containing 0.04% CO2 during the electrolysis. This experiment is conducted to determine if this carbon 12 from CO2 gas in the air was incorporated into the electrolyzed product. All other steps in the procedure were identical to the 13CO2 exposed synthesis. The product’s Raman spectrum is shown in the black curve in the Fig. 2B and is similar, but slightly up-shifted compared to the red curve spectrum, that of pure 13C-CNFs. The G-band shift is ~4 cm−1 towards higher frequency, indicating ~4% 12C was present in this sample using Equation 1, to provide evidence that (12C) CO2 is directly absorbed from the air in the formation of the carbon product. A broadening of the G-band in this sample, which can be seen by the larger FWHM (full width at half maximum) is further evidence of a 12C/13C mixture, because in the other, pure 12C, or pure 13C, cases the G-band is single peak and the FWHM is the most narrow. This result acts to confirm our proposed mechanism that the presence of Li2O in the electrolyte absorbs the greenhouse gas CO2 from air and transforms it by electrolysis into CNFs. The Raman G’ band of the products is presented in Fig. 2C. The peak frequency of pure 13C-CNFs is 2585 cm−1. The reported frequencies of the G’ band vary in the literature. For example, in ref. 4 the G’ band of SWCNTs is observed at 2526 cm−1, while in ref. 6 this band of SWCNTs at ~2580cm−1; both using 532 nm incident laser. The discrepancy may be due to a variation in morphology (such as zig-zag or arm-chair) and/or the relatively broad character of the peak. Our results are close to those in ref. 6. ### Isotope variation of nanostructure morphology: 13CNFs and 12CNTs We had previously established that multiwalled carbon nanofibers are deposited at high yield when carbon dioxide, dissolved in lithiated molten carbonates is reduced at a galvanized steel electrode in the presence of nickel (or other transition metal) oxides1. Specifically, we had observed that a transition metal oxide that is highly soluble in the lithium carbonate electrolyte, such as iron oxide, results in an electrolysis product consisting of a wide array of nanofiber morphologies, while a limited solubility transition metal oxide, such as nickel oxide, results in a highly uniform product consisting of either straight (without added Li2O) or tangled (in an electrolyte with excess, dissolved Li2O) nanofibers. The extent to which the fibers had a hollow core (nanotubes) or a solid core (nanofibers) was not discussed in that study. Other physical chemical parameters are expected to influence the synthesized fiber nanomorphology. Under identical electrosynthetic conditions, the heavier mass of the molten 13C carbonate anion compared to the 12C (natural abundance) will affect the relative rate of mass diffusion of carbonate towards the cathode compared to the rate of oxide away from the cathode, during the carbon (IV) reduction and hence might be expected to influence the morphology of the product nanofibers. This expectation is tested in this study. Figures 3 and 4 present microscopy comparing the nanomorphology when 12C is the dominant isotope (Fig. 3) and when 13C is the dominant isotope (Fig. 4) used in the electrolytic synthesis. Here we observe that carbon fibers formed with the natural isotope CO2 consistently exhibit hollow cores (carbon nanotubes), whereas those formed from 13CO2 have solid or nearly solid cores (carbon nanofibers). Figure 3 shows the typical SEM/TEM images of the multiwalled carbon nanotubes, MWCNTs, electrochemically synthesized from natural abundance CO2 in molten, natural abundance Li2CO3. Typically the MWCNTs have wall thickness of ~100–150 nm and inner diameter of ~160–210 nm. The distance between graphene layers from high resolution TEM is 0.342nm and therefore the wall contains about 300~450 layers. The layer spacing is consistent with pure 12C graphitic structures. We observe larger diameter nanotubes (with the same graphene layer spacing) when, rather than a galvanostatic DC current of 1 A (through the 10 cm2 galvanized steel cathode), the CNT growth current is pulsed at a low frequency (cycled at 9 minutes on (1 A), 1 minute off (0 A). As seen in Fig. 4, rather than the large core nanotubes produced by electrolysis from natural abundance (99% 12C) carbonate, the fibers grow with a nearly solid core structure from 13C (from 13CO2 in Li213CO3), except where the nickel catalyst remains present, as analyzed by EDS. The separation between graphene layers in graphite is approximately 0.335 nm33 and can vary near this value in multiwalled carbon nanotubes. The spacing between carbon layers is seen to decrease from 0.342 nm for the natural abundance carbon nanostructures in Fig. 3, to 0.338 nm as shown in Fig. 4. Higher resolution TEM of the intergraphene layer spacing calculations of natural abundance carbon (left) compared to the 13C isotope (right) spacings shown in Fig. 5 and interspatial graphene spatial layer separations are determined by repeat measurements on TEM analyses. The 13C products exhibit a carbon nanofiber morphology with an inner (void) volume that is less than 6% of the total. Specifically, the 13C-CNFs have an outer diameter of ranging from 150 to 350 nm, a wall thickness of 75 to 175 nm. The observed yield of fibers from the 13C synthesis is 60–80% during replicate measurements, while from the natural abundance C synthesis, the observed carbon nanotube yield is typically 80–90%. The slightly lower yield for the 13C synthesis may be related to the higher cost of the reactants, which precluded further (than three) replicate measurements or optimizations and as previously observed for natural carbon abundance electrolyses (as described in the supplementary information of ref. 1) the remaining product includes amorphous graphites and graphenes. The thicker wall and smaller diameter of the 13C products compared to the 12C carbon nanotubes may be due to the different diffusion behaviours in Li213CO3 electrolyte. CO2 is solvated in solution as carbonate and diffusion of carbonate to the cathode supplies carbon for the nanotube/nanofiber growth. The heavier 13CO32− species in the 13C electrolyte has a lower mobility with respect to 12CO32− in the natural abundance carbonate electrolyte and this can influence (i) the distribution of the transition metal nucleation sites, (ii) the curvature of the carbon cap growing on the nucleation site and (iii) the availability of carbon during the growth process. With the 13C nanostructures we observe by EDS, nickel both at the exterior and within the interior of the nanofiber (dark areas in the middle of Figs 4 and 5) and these inner core nucleation sites may promote additional inner core wall growth. It is likely that the closer-spaced walls observed in the 13C compared to 12C synthesized structures are indicative of a more stable structure, which would be consistent with the observed tendency of the 13C to form more layers within the carbon nanotube matrix. Two physical chemical characteristics have been suggested that could be linked to our observation that multi-walled carbon nanotubes synthesized by the electrolysis of CO2 in molten carbonate are nearly solid (CNFs) when formed from carbon-13, but are hollow (CNTs) when formed from natural abundance carbon. These two characteristics are (i) the increased stability evident in the observed closer packed walls in the 13C CNFs and (ii) the lower mobility of the heavier 13CO32− compared to 12CO32−. Whether either, or both of these characteristics dominate the growth mechanism which leads to these morphological difference requires further extended study. The 1.7% mass difference between 13CO32− compared to 12CO32− increases the relative rate at the growing carbon nanostructure on the cathode that electrogenerated oxide exits compared to the rate at which carbonate arrives and is consumed. Density functional calculations will be of interest to probe the hypothesis that heavier carbon isotopes would tend to form a smaller diameter carbon cap for carbon nanofiber growth, future molecular dynamic simulation studies will be of interest to probe the hypothesis that slower carbonate mobility will promote a more densely packed multiwalled carbon growth. ### Tangled compared to straight 13CNF morphology Raman spectroscopic analysis was conducted to determine the degree of graphitization of the synthesized carbon nanostructures. In Fig. 2B, the D band peak correspond to the disorder-induced vibration and the higher frequency E2g first order vibration mode G band peak corresponds to the graphitization vibration and generally to sp3 and sp2 hybridized carbon species, respectively. The intensity ratio between D band and G band (ID/IG), is an important parameter to evaluate the graphitization and hence the total relative ratio of defective carbons in the material. The addition of Li2O to the synthesis electrolyte leads to the observed curvature of the conventional isotope 12C carbon nanotubes and it is of interest there to investigate whether a similar occurrence here with the 13C carbon nanofibers, hence 13C carbon nanofibers were additionally grown with the addition of 1 m Li2O to the 750 °C Li2CO3 electrolyte. As shown in Fig. 2B, the ID/IG ratio of 0.9 for tangled 13CNFs is significantly higher than the 0.6 ratio for the straight 13CNFs At 750 °C, “pure” lithium carbonate spontaneously decomposes to form an equilibrium concentration of 0.3 m Li2O in Li2CO33. This occurs through the reverse of Equation 2 that is the dissociation of the molten carbonate to form an equilibrium concentration: Li2CO3 Li2O + CO23. Experimentally, we estimate KLi2CO3 by mass loss or mass gain of the molten carbonate. Li2CO3 heated to 750 °C in air (containing pCO2 = 4.0 × 10−4 atm CO2) evolves 0.017 mole fraction of CO2. The mass loss is equivalent to the equilibrium formation of 0.25 m Li2O (m ≡ molal = mol kg−1 Li2CO3) determined as 0.018/((1–0.018)*0.07391) kg mol−1 Li2CO3) in the molten lithium carbonate. However, this is regarded as a lower bound of this value to the equilibrium concentration at 750 °C value, as it does not include any initial Li2O impurities in the Li2CO3. pH titration by HCl of the original Li2CO3 dissolved as an aqueous solution indicates any Li2O impurity is <1% of the total carbonate. In accord with the reverse reaction of eq 5, when Li2O is added to the carbonate, the 750 °C Li2CO3 is observed to gain, rather than lose mass, that is absorb CO2 from air. We measure that a mix of 0.51 m Li2O in Li2CO3 heated to 750 °C gains mass equivalent to the absorption of 0.18 m CO2, for a final concentration of 0.33 m Li2O in solution. From the upper and lower bound measurements, the equilibrium concentration of Li2O in molten lithium carbonate at 750 °C is 0.29 (±0.04) m. This yields an experimental value of the 750 °C equilibrium constant of KLi2CO3 = 4.0 × 10−4 * 0.29 = 1.2 × 10−4. Higher concentrations of Li2O can be added to the electrolyte prior to the electrolysis and have two effects: (i) increased CO2 absorption from the gas and (ii) (as we had observed previously for natural abundance carbon) the high oxide environment near the cathode led to tangled, rather than straight, nanofiber growth. Here, this high oxide effect also occurs for the 13C carbon nanostructures. SEM/TEM were also conducted to study the structure properties of carbon nanofibers grown such a high oxide (1 m Li2O) environment and are shown in Fig. 6. Instead of straight CNFs or MWCNTs, curved or tangled CNFs were obtained. Again, the inner void is very small and sometimes some nickel was incorporated into these inner voids. The ID/IG is 0.9 in the Raman spectrum for the 13 CNFs grown in the 1 m Li2O 750 °C Li2CO3 electrolyte, indicate a lesser degree of graphitization and many more defects. This is confirmed by HRTEM in the lower portion of Fig. 6, that the tangled 13C nanomorphology, as synthesized from the high oxide electrolyte, exhibits a less ordered, curved, pitted structure, in other words, containing many more defects than the (straight) 13C-MWCNFs and 12C-MWCNTs. As we will show in an upcoming study, synthetic control of the D:G ratio in these nanostructured carbon material is useful to engineer and improve the intercalation properties for Li-anode and Na-anode of carbon-based electrodes. ## Conclusions We present a new high yield pathway to produce fully carbon 13 isotope enriched multiwalled carbon nanofibers in a straightforward synthesis directly from 13CO2 dissolved in Li213CO3 melt at inexpensive galvanized steel and nickel electrodes. The heavier mass of 13C-carbonate ions results in different diffusion conditions in the bulk electrolyte and during the electrolytic growth at the liquid/solid interfaces, resulting in a high yield 13C multiwalled carbon nanofiber morphology, rather than a high yield multiwalled carbon nanotube morphology observed when instead synthesized from natural abundance (99% 12C) carbon. Raman spectra, SEM and TEM indicate the successful synthesis of 13C-CNFs and also provide insight into the role of CO2 as a reactant during the nanocarbon growth from reduction of carbonates by electrolysis. The direct transformation of gas phase CO2 to carbon nanofibers is demonstrated through the isotopic tracking of (natural abundance) carbon 12CO2 introduced into a carbon-13 (L213CO3 molten electrolyte) media. Electrolyte dissolved Li2O serves as the absorbent for gas phase CO2 to add to the carbonate in the electrolyte. Li2O is released to the electrolyte during CNF formation to absorb further CO2 for the electrolytic formation of the MW13CNF. Despite their superior mechanical, thermal and electronic properties, carbon nanotubes applications had been limited to date by the high cost of their conventional chemical vapour deposition synthesis. This CNT cost is higher than for carbon fibers and 2 to 3 orders of magnitude higher than that of conventional graphitic or amorphous carbons31,34,35. Currently, bulk purified MWCNTs are sold for less than$200,000 per metric tonne, which is 10-fold greater than the cost of commercially available carbon fiber36. Hence, the finding here that the expensive MWCNT (hollow core) morphology product tends to be formed from the inexpensive 12CO2 reactant, while the less expensive product MWCNF (solid core) morphology tends to be formed from the expensive 13CO2 reactant provides an additional economic incentive to remove CO2 from the air or from stack emissions. Here, the facile synthesis of CNTs from natural abundance CO2, at high yield by electrolysis in molten carbonate at cost effective nickel and steel electrodes, provides an incentive to transform the greenhouse gas carbon dioxide into a useful, stable, valuable commodity.

How to cite this article: Ren, J. and Licht, S. Tracking airborne CO2 mitigation and low cost transformation into valuable carbon nanotubes. Sci. Rep. 6, 27760; doi: 10.1038/srep27760 (2016).

## References

1. Ren, J., Li, F.-F., Lau, J., Ganzalez-Urbina, L. & Licht, S. One-pot synthesis of carbon nanofibers from CO2 . Nano Lett. 15, 6142–6148 (2015).

2. Service, R. Tailpipe to Tank. Science, 349, 1158–1160 (2015).

3. Ren, J., Lau, J., Lefler, M. & Licht, S. The minimum electrolytic energy needed to convert carbon dioxide by electrolysis in carbonate melts. J. Phys. Chem. C., 119, 23342–23349 (2015).

4. Blackburn, J. L., Holt, J. M., Irurzun, V. M., Reasasco, D. E. & Rumbles, G. Confirmation of K-Momentum Dark Exciton Vibronic Sidebands Using C-13-labeled, Highly Enriched (6,5) Single-walled Carbon Nanotubes. Nano Lett. 12, 1398–1403 (2012).

5. Engtrakul, C. et al. Unraveling the C-13 NMR Chemical Shifts in Single-Walled Carbon Nanotubes: Dependence on Diameter and Electronic Structure. J. Am. Chem. Soc., 134, 4850–4856 (2012).

6. Costa, S. D. et al. Resonant Raman spectroscopy on enriched C-13 carbon nanotubes. Carbon, 49, 4719–4723 (2011).

7. Simon, F. Studying Single-Wall Carbon Nanotubes Through Encapsulation: From Optical Methods Till Magnetic Resonance. J. Nanosci. Nanotech., 7, 1197–1220 (2007).

8. Miyauchi, Y. & Maruyama, S. Identification of an excitonic phonon sideband by photoluminescence spectroscopy of single-walled carbon-13 nanotubes. Phys. Rev. B 74, 034515 (2006).

9. Fan, S., Liu, L. & Liu, M. Monitoring the growth of carbon nanotubes by carbon isotope labelling. Nanotech. 14, 1118–1123 (2003).

10. Abou-Hamad, E. et al. Structural properties of carbon nanotubes derived from 13C NMR. Phys. Rev. B, 84, 165417 (2011).

11. Kalbac, M., Kavan, L., Zukalova, M. & Dunsch, L. In Situ Raman Spectroelectrochemical Study of 13C-Labeled Fullerene Peapods and Carbon Nanotubes. Small 3, 1746–1752 (2011).

12. Singer, P. M., Wzietek, P., Alloul, H., Simon, F. & Kuzmany, H. NMR Evidence for Gapped Spin Excitations in Metallic Carbon Nanotubes. Phys. Rev. Lett. 95, 236403 (2005).

13. Xu, Y. et al. 13C-engineered carbon quantum dots for in vivo magnetic resonance and fluorescence dual-response. Analyst 139, 5134–5139 (2014).

14. Anno, Y., Takei, K., Akita, S. & Arie, T. Artificially controlled synthesis of graphene intramolecular heterojunctions for phonon engineering. Phys. Status Solidi RRL 8, 692–297 (2014).

15. Wang, S., Zuzuki, S. & Hibino, H. Raman spectroscopic investigation of polycrystalline structures of CVD-grown graphene by isotope Labeling. Nanoscale 6, 13838–13844 (2014).

16. Zhang, C. et al. Isotope effect of the phonons mean free path in graphene by micro-Raman measurement. Sci. China 57, 1817–1821 (2014).

17. Chen, S. et al. Thermal conductivity of isotopically modified graphene. Nature. Mat. 11, 203–207 (2012).

18. Li, X., Chen, J., Yu, C. & Zhang, G. Comparison of isotope effects on thermal conductivity of graphene nanoribbons and carbon nanotubes. Appl. Phys. Lett. 103, 013111 (2013).

19. Zurek, E., Pickard, C. J. & Autschbach, J. A Density Functional Study of the 13C NMR Chemical Shifts in Functionalized Single-Walled Carbon Nanotubes. J. Am. Chem. Soc., 129, 4430–4439 (2007).

20. Ogrinc, N., Kobal, I. & Senegacknik, M. Carbon-13 Kinetic Isotope Effects in the Catalytic Oxidation of Carbon Monoxide Over Pd/Al2O3 . J. Phys. Chem. A, 101, 7236–7242 (1997).

21. Bernatowicz, P. et al. Carbon-13 NMR Relaxation Study of the Internal Dynamics in Cyclodextrins in Isotropic Solution. J. Phys. Chem. B. 114, 59–65 (2010).

22. Licht, S. Efficient Solar-Driven Synthesis, Carbon Capture and Desalinization, STEP Production of Fuels, Metals, Bleach. Advanced Materials 47, 5592–5612 (2011).

23. Licht, S. & Wu, H. STEP Iron, a Chemistry of Iron Formation without CO2 Emission: Molten Carbonate Solubility and Electrochemistry of Iron Ore Impurities. J. Phys. Chem. C., 115, 25138–25157 (2011).

24. Licht, S. et al. STEP Cement: Solar Thermal Electrochemical Production of CaO without CO2 emission. Chem. Comm. 48, 6018–6922 (2012); Licht, S., Cui, B., Wang, B. STEP Carbon Capture: the barium advantage. J. CO2Utilization, 2, 58–63 (2013).

25. Licht, S. et al. Wang, B., Ghosh, S., Ayub, H., Jiang, D. & Ganley, J. A. New Solar Carbon Capture Process: STEP Carbon Capture. J. Phys. Chem. Lett. 1, 2363–2368 (2010).

26. Li, F.-F. et al. A One-pot Synthesis of Hydrogen and Carbon Fuels from Water and Carbon Dioxide. Adv. Energy Materials 5, 1401791 1–7 (2015).

27. Licht, S. STEP (solar thermal electrochemical photo) generation of energetic molecules: A solar chemical process to end anthropogenic global warming. J. Phys. Chem. C 113, 16283–16292 (2009).

28. Cui, B. & Licht, S. STEP advances for sustainable iron production. Green Chem. 15, 881–884 (2013).

29. Licht, S. & Wang, B. STEP High Solubility Pathway for the Carbon Dioxide Free Production of Iron. Chem. Comm. 47, 7004–7006 (2010), ibid, 46, 3081–3083 (2011).

30. Licht, S. et al. Ammonia synthesis by N2 and steam electrolysis in molten hydroxide suspensions of nanoscale Fe2O3 . Science 345, 637–640 (2014).

32. Saravanan, M., Sennu, P., Ganesan, M. & Ambalavana, S. Multi-Walled Carbon Nanotubes Percolation Network Enhanced the Performance of Negative Electrode for Lead-Acid Battery. J. Electrochem. Soc. 160, A70–A76 (2013).

33. Kalbac, M. et al. Raman Spectroscopy and in Situ Raman Spectroelectrochemistry of Bilayer C-12/C-13 Graphene. Nano Lett. 11, 1957–1963 (2011).

34. Liu, W. Synthesis and Characterization of Graphene and Carbon nanotubes: A review on the past and recent developments. J. Ind. Eng. Chem. 20, 1171–1185 (2014).

35. Song, H. H. Carbon nanofibers: synthesis and applications. J. Nanosci. nanotech. 14, 1799–1810 (2014).

36. De Folder, M. F. L., Tawfick, S. H., Baughman, R. H. & Hart, A. J. Carbon nanotubes: present and future applications. Science, 339, 535–539 (2013).

## Acknowledgements

This project was supported in part by a grant from the United States National Science Foundation 1230732. We are grateful to the Director of the George Washington University Institute for Nanotechnology Prof. Michael Keidar and his research group members Dr. Alexey Shashurin and Xiuqi Fang for their help in attaining the Raman spectroscopy.

## Author information

Authors

### Contributions

Experiments were largely designed by S.L. with input from J.R. and conducted and analyzed by J.R. and S.L. The manuscript was drafted by S.L. and reviewed by all authors.

## Ethics declarations

### Competing interests

The authors declare no competing financial interests.

## Rights and permissions

Reprints and Permissions

Ren, J., Licht, S. Tracking airborne CO2 mitigation and low cost transformation into valuable carbon nanotubes. Sci Rep 6, 27760 (2016). https://doi.org/10.1038/srep27760

• Accepted:

• Published: