*2.2. Electrolytic Conditions to Synthesize High-Purity, High-Yield CNTs from CO2*

The syntheses listed in Table 1 delineated the electrochemical growth conditions for the high-purity growth of carbon nanotubes, each exhibiting the characteristic concentric multiple-graphene cylindrical walls. This can be observed in Figure 3, which presents Transmission Electron Microscopy (TEM) and High Angle Annular Dark-Field TEM (HAADF) results of a typical example (the product of Electrolysis E, as further described in Table 1 and Figure 2), and which provides general structural and mechanistic information of carbon nanotubes synthesized by molten electrolysis. As seen in the top row of the figure, the carbon nanotubes were formed by successive, concentric layers of cylindrical graphene. The graphene can be identified by its characteristic inter-graphene layer separation of

0.33 to 0.34 nm as measured in the figure by the spacing between the dark layers of uniform blocked electron transmission on the magnified top right side of the figure. This CNT had an outer diameter of 74 nm, and inner diameter of 46 nm, and by counting dark rows, it can be determined that the number of graphene layers in this CNT was 41. The right side of the third row of the figure measures the carbon elemental profile of the CNT. This profile was swept laterally from the tube's exterior (no carbon) through the left wall (carbon), then through the void of interior of the tube (low carbon from the exterior backside wall), then through the right wall (carbon), and finally to the exterior of the tube on the outer left side (no carbon). Additionally, the integrated elemental profile of area 1 of this panel is shown, which exhibits 100.0% carbon (fit error 1.3%).

**Figure 3.** TEM and HAADF of the synthesis product of high-purity, high-yield carbon nanotubes under the Electrolysis E (Table 1) electrochemical conditions by electrolytic splitting of CO2 in 770 ◦C Li2CO3. In the top row, the product is analyzed by TEM with scale bars of 20 nm (left panel) or 1 nm (right). Moving left to right in the second row, there are scale bars of 100, 5, 5, and 1 nm. The third row's scale bars are 100 or 50 nm. The bottom row scale bars are 20, 1, and 1 nm. Panels: (**A**,**B**), B-1, B-2, B-3, B-3-1 TEM; A-1, B-1-1 TEM, with measured graphene layer thickness. (**C**): Elemental HAADF elemental analysis; (**D**): HAADF element analysis with (right side) elemental profile.

In Figure 3 on the right side of row 2, the parallel 0.34 nm spacing for the graphene layers in the CNT walls is again observable. This panel also includes dark areas of metal trapped within the CNT, and serves as a snapshot in time of the growth of the CNT. In the third row of the figure, it can be seen that HAADF analysis of Area 1 revealed an elemental composition for this area, including the walls with the trapped interior metal, of 94.4% carbon, 2.5% Fe, and 3.2% Ni, as distributed according to the individual C, Fe, and Ni HAADF maps included in the figure. The second row of the figure also shows the tip of the CNT, which included trapped metal. The transition metal served as a nucleating agent, which supported the formation of the curved graphene layers shown at the tip of the CNT, which is a major component of the CNT growth mechanism. While occurring in an entirely different physical chemical environment than chemical vapor deposition (CVD), this molten carbonate electrolysis process of transition metal nucleated growth of CNTs appeared to be similar to those noted to occur for CVD CNT growth. This was the case despite the fact that CVD is a chemical/rather than electrochemical process, and occurs at the gas/solid, rather than liquid/solid interface.
