Calcium metaborate induced thin walled carbon nanotube syntheses from CO2 by molten carbonate electrolysis

An electrosynthesis is presented to transform the greenhouse gas CO2 into an unusually thin walled, smaller diameter morphology of Carbon Nanotubes (CNTs). The transformation occurs at high yield and coulombic efficiency of the 4-electron CO2 reduction in a molten carbonate electrolyte. The electrosynthesis is driven by an unexpected synergy between calcium and metaborate. In a pure molten lithium carbonate electrolyte, thicker walled CNTs (100–160 nm diameter) are synthesized during a 4 h CO2 electrolysis at 0.1 A cm−2. At this low current density, CO2 without pre-concentration is directly absorbed by the air (direct air capture) to renew and sustain the carbonate electrolyte. The addition of 2 wt% Li2O to the electrolyte produces thinner, highly uniform (50–80 nm diameter) walled CNTs, consisting of ~ 75 concentric, cylindrical graphene walls. The product is produced at high yield (the cathode product consists of > 98% CNTs). It had previously been demonstrated that the addition of 5–10 wt% lithium metaborate to the lithium carbonate electrolyte boron dopes the CNTs increasing their electrical conductivity tenfold, and that the addition of calcium carbonate to a molten lithium carbonate supports the electrosynthesis of thinner walled CNTs, but at low yield (only ~ 15% of the product are CNTs). Here it is shown that the same electrolysis conditions, but with the addition of 7.7 wt% calcium metaborate to lithium carbonate, produces unusually thin walled CNTs uniform (22–42 nm diameter) CNTs consisting of ~ 25 concentric, cylindrical graphene walls at a high yield of > 90% CNTs.

Scientific RepoRtS | (2020) 10:15146 | https://doi.org/10.1038/s41598-020-71644-0 www.nature.com/scientificreports/ An important component of the C2CNT growth process is transition metal nucleated growth, such as the addition of nickel powder which leads to clearly observable CNT walls as shown in Fig. 1. However, when these nucleation additives are purposely excluded during the synthesis, then the high yield synthesis of carbon nanoonions (spheres) (as shown in Fig. 2) or graphene are accomplished 19,22 .
Many different carbon allotropes can be produced by molten CO 2 splitting also referred to as the Genesis Device™. The wide range of carbon nanomaterial morphologies observed shows the potential for tuning the product for uses in many different useful products. Here, we present synthesis of unusually thin walled CNTs, (1) Dissolution : CO 2 gas + Li 2 O(soluble) → Li 2 CO 3 (molten)    High yield electrolytic synthesis of carbon nanomaterials from CO 2 , either directly from the air or from smokestack CO 2 , in molten carbonate 11,19,22,23  Electrolysis and purification. The electrolyte is pre-mixed in the noted ratios. Unlike early studies, such as shown in Fig. 1, that used 1 cm separated, horizontally aligned anodes and cathodes disks comprised of coiled wires, this study uses electrodes that are sheet metal and vertically immersed into the molten salt electrolyte. 0.25-inch-thick Muntz brass sheet is used as the cathode, and 0.04-inch-thick Nichrome (chromel A) sheet is used as the anode. The cathode is aligned (sandwiched) between two series connected anodes, and the cathode is spaced 1 cm from each of the anodes. The electrolyte and electrodes are contained in a rectangular stainless steel 304 case. Unlike, the experiments described in Fig. 1 which was at a constant current of 0.2 A/cm 2 for different short intervals of time (15,30 or 90 min), here, for the vertically immersed planar electrodes a constant current of 0.1 A/cm 2 is applied for a constant time of 4 h. The electrolysis temperature is 770 °C. The raw product is collected from the brass cathode after the experiment and cooled down, followed by an aqueous wash procedure. The washed carbon product is separated by vacuum filtration. The washed carbon product is dried overnight at 60 °C in an oven yielding a black power product. The coulombic efficiency of electrolysis is the percent of applied, constant current charge that was converted to carbon determined as: This is measured by the mass of washed carbon product removed from the cathode, C experimental , and calculated from the theoretical mass, C theoretical = (Q/nF) × (12.01 g C mol −1 ) which is determined from Q, the time integrated charged passed during the electrolysis, F, the Faraday (96,485 As mol −1 e -), and the n = 4 e-mol −1 reduction of tetravalent carbon.
Characterization. Samples are analyzed by PHENOM Pro Pro-X SEM, FEI Teneo LV SEM, and by FEI Teneo Talos F200X TEM.

Results and discussion
The electrolytic splitting of CO 2 in molten carbonate electrodes can be conducted with a wide range of cathode materials including iron, steels, nickel, nickel alloys, Monel, copper and brass. The diameter of the CNTs grown on copper or on brass cathodes tends to be similar. In Fig. 1, concentric CNT walls separated by 0.335 nm, which is typical of the distinctive one atom thick separation of multiple graphene layers are observed. Figure 1 demonstrates when the electrolyte is conducted in pure Li 2 CO 3 an increase in CNT diameter from 22 to 116 nm occurs when the constant current electrolysis time is increased from 15 to 90 min. The CNT is composed of concentric, cylindrical graphene walls spaced 0.335 nm apart. Alongside the increased diameter is an increase in the number of concentric CNT walls on each of the inner sides of the nanotube increase from 18 to 142 graphene layers. In pure Li 2 CO 3 , for 4 h, rather than 1.5 h electrosynthesis, the CNT continues to grow, and on the average the CNT diameter ranges from 100 to 160 nm, for example with repeat a 4 h constant current electrolyses.
The electrolyte composition can affect the CNT diameter. Figure 3 presents SEM of the thinnest 4 h grown CNTs that had been observed. This is accomplished by addition of low concentrations of lithium oxide to the electrolyte. The CNTs are electrosynthesized in 770 °C Li 2 CO 3 with 2 wt% Li 2 O electrolyte (0.67 mol of Li 2 O per kg Li 2 CO 3 ) using a nickel alloy anode and brass cathode. At the relatively low current density of 0.1 A/cm 2 applied (aluminum smelting by electrolysis of aluminum oxide typically occurs at 0.5-0.6 A/cm 2 ), CO 2 from the air (direct air capture) is sufficient to renew the electrolyte in accord with Eq. (3) and maintain the electrolyte level in accord with Eq. (1), and concentrated addition of CO 2 is not required and not added. Nickel chromium alloy anodes and brass cathodes have been shown to be particularly stable for repeated use in CO 2 splitting by molten carbonate electrolysis 21 . After the synthesis, the extracted cathode is cooled and the solid product readily is peeled off the cathode and washed to remove excess electrolyte prior to microscopy. Panel B of Fig. 3 is of interest as it constitutes SEM of a product removed from the rear side (not facing the anode) of the cathode. In particular, a piece of the multilayer graphene sheet which first forms on the cathode, and from which the CNTs growth is evident in a manner consistent with the tip growth mechanism presented in reference 3. The product is ~ 98% uniform CNTs as determined by visual inspection of multiple SEMs and TEM. Repeat experiments using the 2 wt% Li 2 O in Li 2 CO 3 electrolyte, the coulombic efficiency was consistently 97% to 100%. Lower concentrations of lithium oxide resulted in thicker diameter and CNTs, and greater than 2 wt% added lithium oxide did not further decrease the observed CNT product thickness. The diameter of representative samples of the CNTs was It had previously been demonstrated that the addition of 5-10 wt% LiBO 2 to a Li 2 CO 3 electrolyte, used in CO 2 electrolysis, boron dopes the CNTs increasing their electrical conductivity tenfold 18 . It had also been shown the addition of alkali earth metal carbonates to a lithium electrolyte have a substantial effect on the carbon nanomaterial electrolysis product. For example, the addition of magnesium carbonate prevented the formation of CNTs, and the addition of CaCO 3 inhibited, and diminished, but allowed the formation of CNTs resulting in a yield of only ~ 15 CNT product 15 . Interestingly, it was observed that those CNTs which did form in the Ca/Li mixed carbonate electrolyte had much thinner walls than those synthesized in pure lithium carbonate 15 .
Of the metaborate salts, and their molten phase counterparts, sodium metaborate is that which is most studied, which is likely due to its use in certain formulations of glass [26][27][28][29][30] . To a lesser extent calcium borate, CaB 2 O 4 or CaO·B 2 O 3 , has also been studied [31][32][33] . Note that the boron in calcium metaborate has a ratio Ca to B to O ratio of 1:2:4, whereas the ratio in calcium borate, common name Gersely borate, Ca 3 (BO 3 ) 2 is 1:2/3:2.
Calcium metaborate in this study, was synthesized by the addition of calcium oxide and boric acid: www.nature.com/scientificreports/ Specifically, 0.2 mol of Ca and 0.4 mol of boric acid were added to 300 g Li 2 CO 3 and heated at 770 °C overnight to release all water as steam. The electrolysis of CO 2 uses a molten electrolyte mix of 0.67 molal (7.7 wt%) CaO·B 2 O 3 in Li 2 CO 3 , with a 6 cm by 7 cm brass cathode sandwiched between nichrome anodes. The electrolysis approaches 100% coulombic efficiency as measured according to Eq. (4), and the product consists of 2-6 µm length CNTs, and is marginally less pure (90% yield of CNTs) than the 0.67 molal Li 2 O synthesis. The cathode is extracted and cooled, after a 4 h electrolysis is shown on the left side of Fig. 4. White cylinders in the photo are alumina placed on the cathode to prevent shorting with the anode. The graphitization of the thin walls is demonstrated in the inset of panel F by the individual carbon nanotubes which are flat and separated by 0.34 nm, which is the same as graphene layers in graphite 11 .
The diameter of representative CNTs of the calcium borate in Li 2 CO 3 electrosynthesized CNTs varied from ~ 22 to 42 nm which is considerably smaller than in similar pure lithium carbonate or lithium carbonate with lithium oxide electrolytes. The distribution of CNT diameter size by count is compared in Fig. 5

conclusions
We present a new high yield pathway to produce thin walled carbon nanotubes. The process uses a calcium metaborate dissolved in a molten Li 2 CO 3 electrolyte, and splits and consumes CO 2 as the carbon source building blocks of the carbon nanotubes. Using equivalent 4 h CO 2 770 °C electrolyses at 0.1 A cm −2 , the carbon CNT products of electrolyses in a pure molten lithium carbonate electrolyte, have a diameter of 100 to 160 nm. Those  www.nature.com/scientificreports/ In accord with Fig. 1, it is likely that the diameter may be decreased approximately eightfold by a 15 min, rather than 4 h electrolysis. In a similar manner, electrolyses conducted for the same period, but with a lower current density will likely also exhibit fewer CNT walls and smaller CNT diameters. The high yield, high coulombic efficiency molten carbonate electrosynthesis of single and double walled CNTs may be within reach and achieved by combining appropriate electrolyte additives, such as calcium metaborate, with low current density and short electrolysis times.
Received: 10 May 2020; Accepted: 17 August 2020 Figure 5. The distribution of CNT diameter size by count is compared between the CNT product formed om lithium carbonate electrolyte, either containing 0.67 m Li 2 O (top), or 0.67 m CaO·B 2 O 3 (bottom). After the same electrolysis, but in a pure Li 2 CO 3 electrolyte (without additives), the average CNT diameter after a 4 h electrolysis is ~ 130 to 160 nm.