*2.3. Electrochemical Conditions to Synthesize Nickel-Coated CNTs, and Onion and Flower Nanocarbon Allotropes from CO2*

A nickel anode or an excess of added nickel leads to nickel-coated CNT. Rather than forming alternative allotropes, such as nano-bamboo or nano-pearl, the use of excess nickel, particularly (i) when employed with a stainless steel cathode, (ii) when utilized at higher electrolysis current densities, and (iii) with the activation by an initial current ramp, tends to coat the carbon nanotube with nickel. This is summarized in the top row of Table 3 as Electrolysis X, in which 0.81 wt% Ni powder is added to the Li2CO3 electrolyte, and Nichrome C is used as the anode. The electrolysis is conducted at 0.20 A/cm<sup>2</sup> and exhibits a coulombic efficiency of 98.9%. The Ni coating is further improved (appearing more uniform in the SEM) in Electrolysis XI in Table 3 and as the top row in Figure 7, when a pure nickel, rather than Nichrome C, anode is used, but no Ni powder is added to the electrolytes, and there is no current ramp employed. The electrolysis is conducted at 0.15 A/cm<sup>2</sup> and exhibits a coulombic efficiency of 93.4%.


**Table 3.** Systematic variation of CO2 splitting conditions in 770 ◦C Li2CO3 to optimize formation of nickel-coated CNTs and onion, flower, dragon, belt and rod nanocarbon allotropes.

The exclusion of transition metals from the molten electrolysis environment prevents their activity as nucleation points for carbon growth and suppresses the growth of carbon nanotubes. Suppression of the metal nucleated growth of CNTs, such as through use of a noble metal anode, was found to be an effective means to promote the growth of another nanocarbon: carbon nano-onions [25]. Here, another molten electrolysis pathway is found to ensure a high nano-onion product yield, that is through addition of lithium phosphate to the electrolyte. As summarized in Electrolyses XII and XIII in Table 3, with the addition of 8 wt% Li3PO4 to the Li2CO3 electrolyte, the product is nearly pure (97–98%) carbon nano-onions as summarized in Table 3. This nano-onion product is the observed to be the case for a wide range of electrolysynthesis current densities (0.08 to 0.20 A/cm2), with either Muntz Brass or Monel as the cathode, and with (Electrolysis XII) or without (Electrolysis XIII) inclusion of an initial current ramp step during the electrolysis. In a future study, it will be interesting to probe whether phosphates bind or suppress specific free metal availability in molten carbonates in a manner comparable to their tendency to chelate certain metals under ambient aqueous conditions.

A variation of the low current density, Muntz brass cathode, Nichrome C anode, utilizing an aged electrolyte leads to a fascinating new high-purity molten electrolysis nanocarbon allotrope: nano-flowers. Specifically, after the 24 h aging of the electrolyte, an excess (0.081 wt%) of chromium metal powder is added to the electrolyte. The electrolysis is conducted at 0.08 A/cm2 and exhibits a coulombic efficiency of 78%. The electrolyses are repeated (as Electrolyses XIV and XV) and yield the same results as summarized in Table 3 and shown by SEM in Figure 7. As seen in the lower right panel of Figure 7, the product does appear as hollow tubes within the flower morphology. However, the product morphology is highly unusual in several aspects. Collections of tubes seem to burst from a single point, giving the flower-like arrangement. This will require further study and could represent base, rather than tip, growth and multiple growth patterns activated from singular activation points. An alternative mechanism to be explored is tip based, in which the metal nucleation tip is sintered (decreasing in size) as growth progresses, which with continued growth would decrease the diameter of the nanocarbon product. The tubes appear as short, very straight spikes. The spikes have a diameter which diminishes towards the end of the spike. A small percentage of platelets and garnet-like

material is interspersed throughout the floral arrangement. Although new as a majority molten electrolytic synthesis product, nano-flowers have been observed not only from carbon, but also from gold, platinum, and silver as well as from zinc and titanium oxides, and have been described as "a newly developed class of nanoparticles showing structure similar to flower" [62–65]. Chromium may drive both of the proposed mechanisms for nano-flower growth by making nano-metal nucleation points less fluid or bound to the electrode, promoting base growth, or, when it grows from the tip, the chromium does not keep pace with the growing CNM, causing the particle to decrease in size.

**Figure 7.** SEM of the synthesis product of nano-flowers, nano-broccoli, nano-onions and Ni-coated CNT 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 XI: 150, 20, 15 and 2 μm; for panel XII: 50 μm; for panels XIII: 50 and 15 μm; for panels XIV: 300, 80 and 20 μm; for panels XV upper row: 100, 30, 15 and 10 μm; for panels XV lower row: 100 μm 30 and 5 μm.
