**1. Introduction**

High atmospheric CO2 levels are the largest cause of global warming. Atmospheric CO2 concentration, which had cycled at 235 ± ∼50 ppm for 400 millennia until 1850 and the advent of rising anthropogenic CO2 emissions, is currently at 416 ppm and rising at a rapid annual rate, creating global planetary climate disruptions, habitat loss, and species extinction [1–4]. To date, international efforts to decrease CO2 emissions have failed. The Earth is in the grips of an existential threat and a mass extinction event generally defined as loss of 75% of planetary species. This emphasizes the critical imperative of alternative pathways of CO2 conversion into another, stable non-greenhouse material (CO2 utilization). CO2 is regarded as such a stable molecule that its transformation into a non-greenhouse material poses a major challenge, as summarized in our NSF workshop on Chemical Recycling and Utilization of CO2 [5].

Graphitic carbon nanomaterials (CNMs) are high value, highly stable (with a graphitelike geologic stability) state-of-the-art materials, which have the potential to be attractive CO2 utilization products. However, conventional methodologies of CNM production have a high CO2 footprint. For example, chemical vapor deposition (CVD) is an energy-intensive, expensive process to produce CNM associated with an unusually massive release, as much as 600 tons of CO2 emitted per ton of CNM product [6]. Despite the attractive properties

**Citation:** Liu, X.; Licht, G.; Wang, X.; Licht, S. Controlled Growth of Unusual Nanocarbon Allotropes by Molten Electrolysis of CO2. *Catalysts* **2022**, *12*, 125. https://doi.org/ 10.3390/catal12020125

Academic Editor: Javier Ereña

Received: 17 December 2021 Accepted: 12 January 2022 Published: 21 January 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

and application of CNMs, which range from extraordinary strength as materials with the highest ultimate tensile strength to advanced electronic, thermal, electrical storage, shielding and tribological properties, its demand is limited by a high market cost. Due to high CVD production costs, the market price of CNMs is unusually high (USD 100,000 to over USD 1 million per ton). A "green", rather than high carbon footprint CNT, will increase demand. The use of CO2 as a reactant to generate value-added CNM products will provide motivation to consume this greenhouse gas to mitigate climate change, and its transformation to CNMs can provide a stable material to store carbon removed from the environment.

In 2009 (fundamental) and 2010 (experimental), a novel solar driven methodology to split CO2 into C and O2 by molten carbonate electrolysis was demonstrated as a tool to mitigate climate change [7,8]. This molten carbonate process is not limited to sunlight as the electrolysis energy source. It was demonstrated that by using molten carbonate and a variety of electrolytic configurations, the product can be pure CNM, such as CNT [8–29]. For example, this novel chemistry transforms carbon dioxide to carbon nanotubes (C2CNT process), and as illustrated in Figure 1 directly captures atmospheric CO2, or concentrated anthropogenic CO2, such as from industrial processes. Power plant and cement exhausts have been investigated [23,24], and after a five-year-long international competition, this C2CNT process was awarded the 2021 XPrize XFactor Award for transforming CO2 from flue gas into a valuable product using flue gas from the 860 MW Shepard natural gas power plant (Calgary, AB, Canada) [30,31]. Composites of these high-strength CNTs can be mixed with structural materials, such as CNT-cement, CNT-steel and CNT-aluminum, greatly reducing the carbon footprint of structural materials, and acting to amplify the CNT's CO2 emission reduction [11]. As presented in Figure 1, several different CNMs can be produced by molten carbonate splitting including: magnetic CNTs [18], thin CNTs [19,29], long CNTs [15,16,29], doped, high electrical conductivity CNTs [13,15,16], high Li-ion anode storage CNTs [22], macroscopic CNT assemblies [29], a novel nanocarbon scaffold [28], graphene and carbon nanoplatelets (CNP) [27], and carbon nano-onions (CNO) [25] and helical CNTs (HCNT) [20]. This study introduces entirely distinct nanocarbon morphologies discovered by systematic variation of the electrochemical parameters of the molten carbonate splitting of CO2. With the exception of a new methodology to form CNOs, there are no overlaps or redundancies with the previous electrolytically formed CNM morphologies that have been discovered.

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

The wide range of CNM morphologies observed shows the potential for product tuning. Different CNMs have different applications. For example, CNTs are the strongest known and most thermally conductive material along their major axis [32,33]. CNOs can make excellent lubricants and their high surface area and other characteristics make them appealing for batteries or supercapacitors, and they may have uses in refrigerants and in EMF shielding [20,25,34–39]. Platelets can contribute to the formation of lightweight and porous, but strong, nano-gels [40–42]. HCNTs have strong chiral, magnetic, piezo-electric properties, and may act as nano-springs [43–52]. Even within the same group, such as CNTs, different lengths, thicknesses, layers of graphene walls, etc. morphologies can have different properties such as differing rigidity or surface areas [15,19,47]. This would incentivize even greater CO2 transformed to removable carbon by allowing products to have access to a wide range of markets, which collectively could increase technological progress and provide a growing demand as a buffer to remove anthropogenic CO2.

The C2CNT process has quantified the high affinity of molten carbonates to absorb both atmospheric and flue gas CO2 levels. It has been shown, utilizing the 13C isotope of CO2 to track the carbon from its origin (CO2 a gas phase reactant) through its transformation to nanocarbon product, that the CO2 originating from the gas phase serves as the renewable C building blocks in the observed CNT product [8,9]. The molten carbonate is not consumed, but renewed, catalyzing the ongoing electrolysis of CO2. The net reaction is:

$$\text{Dissolution:}\ \text{CO}\_2\ \text{(gas)} + \text{Li}\_2\text{O}\ \text{(soluble)} \rightleftharpoons \text{Li}\_2\text{CO}\_3\ \text{(molten)}\tag{1}$$

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

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

An important component of the C2CNT growth process is transition metal nucleated growth. These catalysts lead to clearly observable CNT walls with thick graphene interlayer separations. However, when these nucleation additives are excluded, rather than CNTs, instead high-yield synthesis of CNOs or graphene is accomplished [25,27].

This study systematically explores the possibility to synthesize a variety of new molten carbonate synthesized carbon "fullerene" allotropes, and opens a new world of inexpensive nanocarbons, made from CO2, to be explored as incentivized (valuable) products for the transformation and stable removal of CO2 and climate change mitigation. Here the conventional definition of allotrope of "different physical forms in which an element can exist" is employed, rather than the alternative "structural modifications of an element bonded together in a different manner". Specifically, this study explores which reactive pathway condition leads to the selection of new nanocarbon allotropes over another and can lead to higher purity products and better formation of a single product

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