**1. Introduction**

Global CO2 has risen rapidly, accelerating extinction risk [1–4]. CO2 is a highly stable molecule and is difficult to remove from the environment [5]. One means to mitigate CO2 under consideration is its low-energy chemical transformation to a (i) stable, (ii) useful, and (iii) valuable product, with a low cost and low carbon footprint of production. The transformed CO2's product stability prevents the captured CO2 from re-emission, the product's usefulness provides a buffer to store the captured carbon, and its high-value (ideally higher than the cost of CO2 transformation) provides an economic incentive to remove the greenhouse gas. Graphitic nanocarbons, such as carbon nanotubes made from CO2, may meet several of these transformed CO2 product requirements. For example, its basic structure of layered graphene retains the durability of graphite, whose hundredsof-millions-year-old mineral deposits attest to its long term stability, while CNTs' market value of \$100,000 to \$400,000/tonne can provide a revenue, rather than a cost, while removing CO2.

Multiwalled CNTs (Carbon NanoTubes) are comprised of concentric cylindrical graphene sheets. CNTs have a measured tensile strength of 93,900 MPa, which is the highest tensile strength of any material [6,7]. Other useful properties of CNTs include high electrical capacity, high thermal conductivity, high flexibility, high capacity for charge storage, and catalysis. CNTs applications range from stronger, lighter structural materials including cement,

**Citation:** Liu, X.; Licht, G.; Wang, X.; Licht, S. Controlled Transition Metal Nucleated Growth of Carbon Nanotubes by Molten Electrolysis of CO2. *Catalysts* **2022**, *12*, 137. https:// doi.org/10.3390/catal12020137

Academic Editor: Javier Ereña

Received: 17 December 2021 Accepted: 20 January 2022 Published: 22 January 2022

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aluminum, and steel admixtures [8], to medical applications [9,10], electronics, batteries and supercapacitors [11,12], sensors and analysis [13–15], plastics and polymers [16–20], textiles [21], hydrogen storage [22], and water treatment [23,24].

The deliberate thought of this study is that the superior physical chemical properties of CNTs, and in particular CNTs made by consuming CO2, will cause a demand for its application, thereby incentivizing CO2 consumption and driving climate change mitigation by decreasing emissions of the greenhouse gas CO2.

Increased pathways for the use of CO2 as a molten carbonate electrolysis reactant to synthesize value-added CNTs will provide the effect of opening a path to consume this greenhouse gas to mitigate climate change, and its transformation to CNTs will provide a stable material to store carbon removed from the environment.

To date, the CNT market has been limited due to a high cost of production. Commercially, CNTs are mainly produced by chemical vapor deposition (CVD) and not from CO2 [25,26]. CVD production of CNTs is chemical and energy intensive and expensive, leading to current costs of \$100 K to \$400 K per tonne of CNT, and CVD production has a high carbon footprint [27].

Prior attempts to transform CO2 to carbon nanotubes or graphene have been low yield and energy intensive, such as the production of graphitic flakes using high-pressure CO2 or by electrolysis in molten CaCl2 electrolytes. Undesired byproducts included Al2O3, hydrogen, and hydrocarbons, and from the electrolysis, carbon monoxide byproducts from an 850 ◦C electrolysis splitting in molten CaCl2 electrolytes [28,29].

However, CO2 has a strong affinity for certain molten carbonates. In 2009, a process was introduced to mitigate the greenhouse gas CO2 through molten electrolytic splitting and transformation at elevated temperatures. Pathways were opened to the high-purity, renewable energy electrolytic splitting of CO2 to solid carbon by demonstrating that in molten lithium carbonate (melting point 723 ◦C) electrolytes, the four-electron molten electrolysis reduction of tetravalent carbon to solid products dominates below 800 ◦C. With rising electrolysis temperature between 800 and 900 ◦C, the two-electron reduction to a CO product increasingly dominates, and by 950 ◦C, the transition to the alternative CO byproduct is complete [30].

The solid carbon product of CO2 electrolysis was further refined to graphitic nanocarbons through the discovery of the catalyzed molten electrolysis transition metal nucleated growth of carbon nanotubes and carbon nanofibers in lithium carbonate electrolytes [31–33]. The process was given the acronym C2CNT (Carbon dioxide to Carbon NanoTubes). The 13C isotope of CO2 was used to track carbon through the C2CNT process from its origin (CO2 as a gas phase reactant) through its transformation to a CNT or carbon nanofiber product, and the CO2 originating from the gas phase served as the renewable C building blocks in the observed CNT product [32]. The net reaction is:

> Dissolution: CO2(gas) + Li2O(soluble) -Li2CO3(molten) (1)

Electrolysis: Li2CO3(molten) → C(CNT) + Li2O (soluble) + O2(gas) (2)

$$\text{Net: CO}\_2\text{(gas)} \rightarrow \text{C(CNT)} + \text{O}\_2\text{(gas)}\tag{3}$$

The electrolytic CO2 splitting in molten carbonates could occur at electrolysis potentials of less than 1 volt [33]. Due to the high affinity of CO2 towards reaction 1, that is, for the Li2O present in the electrolyte, in the C2CNT process, the electrolytic splitting could occur as a direct capture of carbon in the air without CO2 pre-concentration, or with exhaust gas, or with concentrated CO2. As it directly captured CO2 from the air, no further introduction of CO2 was needed throughout the group's experiments. By means of variation of the electrolysis setup, the process could produce, in addition to conventional morphology CNT: doped CNTs, helical CNTs, and magnetic CNTs, as illustrated in Figure 1 [31–44]. Studied applications of electrolytic CNTs from CO2 include batteries [34], CO2 transformation from power station flue gas [45], and the substantial decrease in the carbon footprint of structural materials as CNT composites, including CNT-cement, CNT-steel, and CNT-

aluminum [8,46], as well as modification of the CO2 splitting process to yield other CNMs including carbon nano-onions, carbon platelets, and graphene [47–51].

**Figure 1.** High-yield electrolytic synthesis of carbon nanotubes from CO2, either directly from the air or from smokestack CO2, in molten carbonate.

The rise of this greenhouse gas is causing extensive climate change and damage to the planet's ecosphere, and its mitigation is one of the most pressing challenges of our time [1–5]. Technical, catalyst-driven solutions to mitigate climate change are of the highest significance to the catalyst, not only due to their probes of a new chemistry to catalyze nanocarbon formation, but also by galvanizing the community with action towards mitigation of the existential climate change threat facing the planet. This study provides four contributions to a catalyst-driven solution to climate change: (i) Ten distinct, new electrochemical procedures are presented to transform CO2 to CNTs at high purity. The procedures produce a variety of distinct CNT morphologies ranging from curled to straight, short to long, and thin to thick. (ii) This study explores the transition metal nucleation that catalyzes the process to produce high-purity carbon nanotubes. (iii) The study demonstrates new syntheses of macroscopic assemblies of CNTs using the C2CNT process, with structural implications towards their potential applications for nano-filtration and neural nets. (iv) This study provides an extensive carbon nanotube baseline to a companion study in which the same electrochemical components are utilized in new configurations to generate entire new classes of non-carbon nanotube graphitic nanocarbons.

#### **2. Results and Discussion**
