*2.4. Electrochemical Conditions to Synthesize Nanocarbon Dragon, Tree, Belt and Rod Allotropes from CO2*

Variation of the electrochemical conditions of CNT product formation to those of Electrolysis XVI leads to a change in allotrope from carbon nanotubes to another fascinating morphology referred to here as nano-dragons and presented in Table 3 and Figure 8. The changes from the earlier syntheses that produced CNTs under similar circumstances, include an Inconel 718 anode, rather than Nichrome C, a higher current density of 0.4, rather than 0.1, A/cm2, (exhibiting a 100% coulombic efficiency), and that the electrolyte is not aged. Unlike the other unique electrolytically synthesized nanocarbon morphologies, carbon nano-dragons do not consist of a a simple, repeated geometric shape, but rather a complex combination of cylinders, platelets and spheres. The small "legs" observed in the Figure 8 SEM images could be smaller branched CNTs or small metal nodules of metal growth.

The addition of low levels of lithium oxide has led to high quality of CNTs [21]. With use of a specific anode (Inconel 718 with two layers of Inconel 600), the quality of the product is retained, but the morphology of the CNT changes substantially. We have previously observed larger transition metal nodule growth from the CNTs [18]. With the addition of Li2O, branched carbon nano-trees are included as Electrolysis XVII in Table 3 and Figure 8. The electrolysis is conducted at 0.13 A/cm2 and exhibits a coulombic efficiency of 98.7%. The nano-trees exhibit distinct growth of smaller CNT branches emanating from larger CNT trunks. The red circled area on the right panel of Electrolysis XVII in Figure 8 shows an example of y section branching. Addition of low levels of iron oxide leads to high quality CNTs. However, with 24 h aging of the electrolyte followed by subsequent addition, as in Electrolysis XVIII in Table 3 and Figure 8, an alternative flattened nanocarbon morphology is observed, which is referred to here as nano-belts. The electrolysis is conducted at 0.08 A/cm2 and exhibits a coulombic efficiency of 79%. The nano-belt structure appears to consist of a flattened (or "deflated") carbon nanotube.

TEM and HAADF elemental analysis of the nano-dragon, nanobelt and nanotree structures are presented in Figures 9–13. In Figure 9, the nano-dragon structure is seen as a graphitic structure, albeit complex. A similar looking Pt, rather than C, structure has been previously observed and described as a bumpy surface on one-dimensional Pt nanowires [66]. The nano-tree allotrope is seen in Figure 10 to consist of CNTs, but differs from the conventional CNT structures, which generally do not contain merged CNTs. However, the nano-tree morphology includes intersecting CNTs as seen in Figure 10, whose structures merge and appear to branch off one another. A nanocarbon CVD growth branching mechanism has been suggested and is shown in Figure 11, catalyzed by fractionation of the nucleation sites leading to carbon branches [67]. In Figures 10 and 12, it can be seen that the interior of nano-trees and nano-belts can respectively contain nickel and iron, or nickel in the structure interior. As seen in Figure 12, the nano-belt product is flat and consists of graphene layers, but other than the measured presence of nickel, the mechanism of this unusual flat morphology is evident from the TEM. CVD nano-belt CNT structures have been previously synthesized with a schematic structure illustrated on the right side of Figure 11 [68].

**Figure 8.** SEM of the synthesis product of nano-dragons, nano-trees, nano-belts and nano-rod allotropes of carbon by electrolytic splitting of CO2 in 770 ◦C Li2CO3. Moving left to right in the panels, the product is analyzed by SEM with increasing magnification. Scale bars in panels (starting from left) are for panels k: 50, 10, 5 and 5 μm; for panels Q: 100, 100, 5, 1 and 100 μm; for panels Z: 50, 5 and 5 μm; for panels 9: 30, 10, 5 and 1 μm.

**Figure 9.** TEM and HAADF elemental analysis of the nano-dragons carbon allotrope synthesized by molten carbonate electrolysis. (**A**,**C**,**D**): Nano-dragon; (**B**) and B-1: Nano-dragon wall and measured graphene layer thickness (**E**); Nano-drageon and (right side) elemental analysis.

**Figure 10.** TEM and TEM HAADF elemental analysis of the nano-trees carbon allotrope synthesized by molten carbonate electrolysis. (**A**,**B**), B-2, B-3, (**C**), C-1, C-1-1, (**D**), D-1, (**E**), E-1, E-2-1, (**F**), F-1, (**G**), G-1. G-2, TEM of Nano-trees; B-1, B-4, D-1-1, E1, G1-1, G1-2, G2-1; TEM of Nano-trees with measured graphene layer thickness; (**H**,**I**) HAADF of Nano-tree with elemental profile (right side).

**Figure 11. Left**: A scheme illustrating the growth of an observed CVD synthesized amorphous branched carbon nano-tree catalyzed by iron carbide (include as the yellow domains). a–f and g–k show fractionation of the yellow iron carbide nucleation site leading to one or more purple-colored carbon branches. Modified from Reference [64]. **Right**: A scheme illustrating the structure of a CVD synthesized carbon nano-belt. Modified from Reference [65].

**Figure 12.** TEM and TEM HAADF elemental analysis of the Nano- belt carbon allotrope synthesized by molten carbonate electrolysis. (**A**–**C**) TEM of Nano-belts; B-1, B-2, C-1, C-2 TEM of Nano-trees with measured graphene layer thickness; (**D**) HAADF of Nano-belt with elemental analysis (**E**) and profile (middle and right side).

**Figure 13.** TEM and HAADF elemental analysis of nano-rod carbon allotrope synthesized by molten carbonate electrolysis. TEM and TEM HAADF elemental analysis of the Nano-rod carbon allotrope synthesized by molten carbonate electrolysis. (**A**,**B**) TEM with (right side) HAADF elemental analysis of Nano-rods; A-1, B-1, B-2, B-3 TEM of Nano-rods.

Without aging the electrolyte, the low current density (0.08 A/cm2, exhibiting a coulombic efficiency of 80%), long-term growth (18 h) growth of carbon nanotubes with a Monel cathode, iridium anode, 0.81% Ni, and no ramped current activation step, leads to squat, ring-like nano-rod allotropes seen in Figure 13, and included in Table 4, as Electrolysis XIX. Of the electrolyses presented here, the product is singularly unusual from two physical chemical perspectives: (1) The TEM in Figure 13 reveals no evidence of a layered graphene structure. However, as shown in a later section, this morphology does exhibit an XRD peak and Raman spectrum typical of graphitic layered graphene structures. (2) As seen in the elemental analysis in Figure 13, the nano-rods are the only one of the new molten synthesized nanocarbon allotropes in which a significant concentration of oxygen (7.0 to 9.4%) is observed. With the bulbous rod-like morphology, rather than a growth which increases a CNT's length along with its diameter in time, this appears consistent with a long-term growth dominated by diameter, rather than length, increases.


**Table 4.** Raman spectra of a diverse range of carbon CNMs formed by molten electrolysis.
