*2.3. Tailored Electrochemical Growth Conditions Producing High-Aspect-Ratio CNTs from CO2*

Figure 4 presents the product's SEM of the electrochemical configuration that yielded the longest (100 to 500 μm long) and highest purity (98%) CNTs at high coulombic efficiency (99.5%) of those studied here (described as Electrolysis F in Table 1). As with the previous configuration that yielded nearly as high-purity, but shorter, CNTs. The synthesis used an 0.1 wt% Fe2O3 additive to the Li2CO3 electrolyte, a Muntz Brass cathode, and an Inconel 718 anode with a layered Inconel 600 screen. However, this synthesis found an optimization in CNT purity and length using two, rather than three layers of Inconel 600, and using a higher current density (0.4, rather than 0.08, A/cm2) and shorter electrolysis time (4, rather than 15, hours). With a diameter of <0.2 μm, these CNTs could have an aspect ratio of >1000. As correlated with the alloy composition in Table 1, the smaller number of Inconel 600 layers reflected the need for the inclusion of anodic molybdenum available in that alloy, but at a controlled, lower concentration, to achieve the resultant high-purity, high-aspect-ratio CNTs. As seen in the figure, the CNTs were densely packed and largely parallel, and as discussed in sect. 2.5, would be a useful candidate for use in nano-filtration.

Figure 5 presents TEM and HAADF probes of the high-aspect-ratio CNT product of Electrolysis F (as described in Table 1, and by SEM in Figure 4). As seen on the right side of the middle row of the figure, the CNT walls consisted of parallel carbon (layers separated by the characteristic 0.33–0.24 nm graphene layer spacing). As seen in the elemental analysis of Area 1 in the lowest row, the areas consisted of hollow tubes composed of 100% carbon. However, as seen in the TEM of the top two rows and in the bottom row as the HAADF elemental profiles, there were also extensive portions of the tubes that were intermittently filled with metal. In the bottom row of the figure, a lateral cross sectional elemental CNT profile scanned through Area 2 from the outside, through the CNT and then out the opposite wall, shows that the wall was composed of carbon, while the inner region also contained iron as the dominant metal coexisting with some nickel.
