6

Chemical CO₂ Conversion to Elemental Carbon Materials

6.1 INTRODUCTION TO ELEMENTAL CARBON PRODUCTS

Carbon accounts for ~27.3 wt% of the total mass of CO₂. Captured CO₂ could supply the elemental carbon needed for products in a net-zero future. These elemental carbon products can be divided into two categories according to the need for additional reactants and subsequent processes: directly and indirectly derived elemental carbon materials. The products in each category are described in Section 6.2. Elemental carbon materials can be directly derived from CO₂ noncatalytically or catalytically by CO₂ decomposition or by reacting with a reducing agent. Other elemental carbon materials are produced indirectly with CO₂ as their carbon source via multiple steps (e.g., reduction or decomposition of CO₂-derived chemicals). There are several CO₂ reduction reaction pathways by which captured CO₂ can result in useful elemental carbon materials, as discussed in Section 6.3.

As introduced in Chapter 2, the market for elemental carbon materials could increase by 400 percent from 2020 to 2050 (see Table 2-1), owing to increased demand for materials with novel structural and electronic properties. Increased understanding of structures and characteristics of such materials has expanded potential applications to include energy conversion (e.g., supercapacitors [Luo et al. 2023]), novel chemical and material syntheses (e.g., nonprecious-metal electrocatalysts [Collins et al. 2023]), construction materials, health care (e.g., for bioimaging), and environmental protection (e.g., photocatalytic degradation of pollutants [Yao et al. 2023]), among others (Dabees et al. 2023; Malode et al. 2023; Sasikumar et al. 2023; Son et al. 2023; Zhang et al. 2023a). As discussed in Section 2.2.5.5 of this report, some elemental carbon products of CO₂ utilization can provide durable storage of carbon, can replace highly carbon-emitting processes via material substitution, or can produce high value products, although most products in this class are likely to be small-volume and thus will not utilize large amounts of CO₂. Status and research needs for CO₂ conversion to carbon nanotubes (CNTs) were discussed in Chapter 4, Chemical Utilization of CO₂ into Chemicals and Fuels, in the 2019 National Academies report Gaseous Carbon Waste Streams Utilization: Status and Research Needs (NASEM 2019), although other classes of elemental carbon materials were not addressed. They were briefly discussed as long-lived CO₂-derived products, or Track 1 products, in Section 3.3.4 of the first report of this committee, Carbon Dioxide Utilization Markets and Infrastructure: Status and Opportunities: A First Report (NASEM 2023). Chapter 2 of the present report provides market information on elemental carbon materials, and this chapter provides an up-to-date review of research and development (R&D) on CO₂ conversion to elemental carbon materials.

CO₂-derived carbon materials can adopt multidimensional (zero- to three-dimensional [0D–3D]) structures from among more than 1500 hypothetical 3D-periodic allotropes of carbon found so far (Hoffman et al. 2016). Zero-dimensional (0D) nanocarbon materials are those having three dimensions only at the nanoscale, with no

dimension larger than ~100 nm—for example, carbon quantum dots (CQDs) in sphere form and with crystal lattice or cross-linked network structures consisting of sp² and sp³ hybridized¹ carbon and heteroatoms, such as O and N; graphene quantum dots (GQDs) in the form of a single truncated atomic layer of graphite (Bacon et al. 2014); and fullerene, representing a class of carbon allotropes in the form of spherical, cage molecules with carbon atoms located at the surface and vertices of a polyhedral structure consisting of pentagons and hexagons (Dhall 2023). One-dimensional (1D) nanocarbon materials are those with only one dimension beyond the nanoscale—that is, larger than ~100 nm; for example, carbon nanorods (CNRs), CNTs, carbon nanowires (CNWs), and carbon tubular clusters (CTCs). Despite their common 1D structures, CNRs, CNTs, and CNWs each possess distinguishing features. For example, CNRs have aspect ratios (length/width) of 3–5 with lengths typically of 10–120 nm (Abraham et al. 2021), whereas CNW aspect ratios can be higher than 10³, while CNT aspect ratios can be >10⁸. Thus, their aspect ratios are significantly different. Also, CNWs have very high specific surface areas. Moreover, CNTs are chiral materials, which governs their metallic or semiconducting character. Among CNTs are novel CTCs, which are very stable. Interestingly, the electronic properties of metallic CTCs are not affected by their diameters, number of walls, or chirality. Two-dimensional (2D) nanocarbon materials have only two dimensions beyond the nanoscale—that is, larger than ~100 nm. These include carbon nanofilms with thin layers of material spanning from a fraction of a nanometer to several micrometers in thickness (Ranzoni and Cooper 2017); carbon nanolayers with monolayer, bilayers, and multilayers (Schaefer 2010); graphene; and nanocoatings. Three-dimensional (3D) carbon nanomaterials (CNMs) are those with three dimensions beyond nanoscale—that is, all larger than ~100 nm; for example, bulk carbon powders, graphite, carbon fibers (CFs), carbon foams, carbon-carbon composites (CCCs), bundles of CNWs and CNTs, as well as multinanolayers, and dispersions of nanoparticles (Chung 2002; Park 2015; Spradling and Guth 2003; Terrones et al. 1998; Windhorst and Blount 1997; Wissler 2006; Zhao et al. 2023).

Each type of 0D–3D carbon material consists of atoms with unique electronic orbital hybridization state(s), and thus specific characteristics and applications. As shown in Figure 6-2, the carbon materials with sp, sp², and sp³ hybridizations, created intrinsically and extrinsically via different defect engineering approaches, typically have different applications in energy conversion and storage areas—for example, Li-ion, Na-ion, and K-ion batteries (Rajagopalan et al. 2020), supercapacitors, the hydrogen evolution reaction, and the oxygen reduction reaction (Luo et al. 2023). CQDs typically have sp² carbon cores and sp³ carbon shells terminated with –OH, –COOH, –NH₂, and other functional groups resulting from their preparation processes, which determine the properties and applications of CQDs. Also, the amount of sp² carbon and the ratio of sp² to sp³ carbon can significantly affect the photoluminescence properties, and thus the bioimaging ability, of CQDs (Jana and Dev 2022; Jhonsi 2018). Furthermore, the amount of sp² + sp³ bonds in the carbon shell or on the surface of CQDs has a substantial effect on the solubilities of CQDs in water and other solvents (e.g., ethanol). The water solubilities of CQDs determine their various applications in many fields, including solar cells (Kim et al. 2021), drug delivery (Jana and Dev 2022; Zoghi et al. 2023), and gene delivery (Rezaei and Hashemi 2021). The carbon atom hybridization structures of 0D–3D carbon materials can change significantly upon chemical modification—for example, ozone and high-temperature oxidation and irradiation. For example, all carbon atoms in pristine or pure graphite have sp² hybridization. Pristine graphene with its sp² hybridization, although extremely interesting owing to its large specific surface area, unusual physicochemical properties and extraordinary anisotropic mechanical strength, and exceptional thermal and electronic conductivity, is a zero-band-gap semiconductor (a semi-metal), which makes it uninteresting for a number of device applications (Gui et al. 2008; Mbayachi et al. 2021; Rani and Jindal 2013). To make graphene widely useful in the electronics space, its band gap needs to be opened and tunable according to application requirements. Pristine graphene therefore needs to be modified in various ways (e.g., UV-light-assisted oxidation [Güneş et al. 2011]) to enable its carbon atom hybridization structures to change from 100 percent sp² to mixtures of sp² and sp³ with different sp²/sp³ ratios to achieve improved properties and more applications, including in electronic, electromagnetic, and optical devices and for catalysis. Representative 0D–3D carbon materials, their typical carbon hybridization characteristics, and their major properties are summarized in Table 6-1.

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¹ Orbital hybridization of carbon atoms in different types of elemental carbon materials are described as sp, sp², and sp³, describing the amount of s- and p-type orbital character of the carbon atoms. The amount of s or p character impacts the bonding between the carbon atoms, with sp having linear, sp² having trigonal planar, and sp³ tetrahedral character. See Figure 6-2 for images showing the orbital hybridization and illustration of the impact on bonding in elemental carbon materials.

TABLE 6-1 0D–3D Carbon Materials and Their Structural Characteristics and Major Properties

NOTES: Orbital hybridization patterns of carbon atoms in different types of materials are described as sp, sp², and sp³, where the s and p describes the amount of s- or p-type orbital character around the carbon atoms. The amount of s or p character impacts the bonding around the carbon atoms, with sp having linear, sp² having trigonal planar, and sp³ having tetrahedral character which in turn affects the material properties. See Figure 6-2 for images showing the orbital hybridization and illustration of the impact on bonding.

SOURCES: Based on data from Budyka et al. (2017); Cartwright et al. (2014); He et al. (2021); Khan and Alamry (2022); Lee et al. (2020); Lesiak et al. (2018); Liu et al. (2020a, 2020b); Tsang et al. (2006); Van Tran et al. (2022); Zhang et al. (2023b). Zhao et al. (2003); Zhou and Zhang (2021).

Carbon materials with different hybridization structures sometimes have been (at laboratory scale) utilized similarly, as evidenced in Figure 6-2. For example, both sp- and sp²-hybridized carbon materials could be used for synthesizing supercapacitors owing to their similar structures and properties, such as high conductivity and stability, and thus can provide the same function (Luo et al. 2023). Both CQDs and CNRs can be used as sensing materials because both contain sp² hybridized structures; their sensing capabilities increase with the amount of sp² hybridized carbon present. CQDs are being used as components of light-emitting diodes, luminescent solar concentrators, and photovoltaic cells; they also can be functionalized to act as catalysts (Zhou et al. 2024).

As indicated in Figure 6-1, a variety of pathways exist for producing 0D–3D carbon materials from CO₂. The major challenge of converting CO₂ into elemental carbon materials is to reduce the formal oxidation state of C from +4 to 0 via various reduction reactions, which may be realized with thermochemical, electrochemical, photochemical, plasmachemical, or hybrid/tandem processes. All of these technologies are still in the R&D phase at the present time, and each has its advantages and disadvantages from energy consumption and environmental impact perspectives, as discussed in the following sections of this chapter.

6.2 CO₂-DERIVED ELEMENTAL CARBON MATERIALS

6.2.1 Directly Derived Elemental Carbon Materials

6.2.1.1 Fullerenes

Fullerenes consist of sp² hybridized carbon atoms linked by single and double bonds, forming a spherical- or cylindrical-shaped closed structure (Ramazani et al. 2021). The most famous of the closed spherical fullerenes is C₆₀, for the 60 carbon atoms in the molecule, also known as buckminsterfullerene or a buckyball, for the molecule’s resemblance to both the geodesic domes of Buckminster Fuller and a standard soccer ball. Fullerene C₆₀ has a van der Waals diameter of about 1.1 nm and a nucleus-to-nucleus diameter of about 0.71 nm. Open-ended cylindrical fullerenes are known as CNTs; single-walled CNTs typically have 0.5–2 nm diameters. Owing to their unique structure and electronic characteristics, the applications of fullerenes are extensive, including photodynamic therapy, drug and gene delivery, nano-sensors, battery electrodes, and organic solar cells, covering many industries (Anctil et al. 2011; Ramazani et al. 2021). Hence, there is significant interest among chemical and material scientists in developing fullerene synthesis methods. General strategies for fullerene synthesis involve generation of fullerene-rich soot through arc discharges, combustion, laser ablation, or microwaves, and then purification of the soot by toluene or benzene washing, Soxhlet extraction, or active carbon filtering (Keypour et al. 2013; Komatsu et al. 2004; Parker et al. 1992; Ramazani et al. 2021; Taylor et al. 1993).

The conventional carbon sources for fullerenes are graphite, aliphatic and olefinic hydrocarbons, chloroform, and aromatics (e.g., naphthalene) (Ramazani et al. 2021); however, there is increasing interest in using captured CO₂ as a feedstock for fullerene synthesis (Chen and Lou 2009; Motiei et al. 2001). Chen and Lou (2009) reported that CO₂ can be successfully reduced to C₆₀ by metallic lithium at 700°C and 100 MPa, as confirmed by ultraviolet (UV)-visible absorption spectroscopy, matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass (TOF) mass spectrometry (MS), and high-performance liquid chromatography. Higher fullerenes (C₇₀, C₇₈, etc.) were not detected using this synthetic approach. The authors postulated that CO₂ radical anion (CO₂^•−) or other single carbon radicals, resulting from the transfer of an electron from elemental Li to supercritical CO₂ (whose polarity increases with its density, thus facilitating electron transfer [Tucker 1999]), are possible key intermediates during the reduction of CO₂ to C₆₀. The generation of the CO₂/CO₂^•− couple in the reaction system is the key step for the formation of C₆₀ owing to the fact that the standard potential of the redox CO₂/CO₂^•− couple in an aprotic solvent such as N,N-dimethylformamide (DMF) containing a counter-cation (tetraethylammonium, NEt₄⁺) can be as negative as −2.2 V versus saturated calomel electrode (Bhugun et al. 1996) and the standard potential of the redox couple Li⁺/Li, 3.04 V, although the potentials of the redox CO₂/CO₂^•− and Li⁺/Li couples in this reaction system were not measured (Chen and Lou 2009; Motiei et al. 2001). The finding by Motiei et al. (2001) was used by Chen and Lou to explain the feasibility of C₆₀ formation.

6.2.1.2 Hollow Carbon Spheres

Unlike 0D fullerene, hollow carbon spheres (HCSs) can have either 2D or 3D structure (Deshmukh et al. 2010; Liu et al. 2019) and both sp² and sp³ hybridized carbon atoms on their surface (Deshmukh et al. 2010). HCSs have diameters between 2 nm and several microns. HCSs have unique properties, including encapsulation ability, controllable permeability, surface functionality, high surface-to-volume ratios, and excellent chemical and thermal stabilities (Li et al. 2016). HCSs can be synthesized with hard-templating, soft-templating, and template-free processes. The properties of HCSs vary, depending on the raw carbon materials and synthesis conditions used. For example, the surface areas of HCS can change from a few m²/g to more than 1000 m²/g, which determines their potential applications, especially those of functionalized HCSs.

Synthesis of HCSs from CO₂ has been demonstrated using the microbubble-effect-assisted electrolytic method in a CaCO₃-containing LiCl–KCl melt electrolyte at 450°C (Deng et al. 2017). The authors employed precise control of the electrode potential to tune the electrochemical reduction rate of carbonate ions and the CO microbubble effect to shape the hollow spheres within the resultant carbon sheets. The produced HCSs exhibited good plasticity and capacitance, which are desirable properties of HCSs used for battery, capacitor, and fuel cell

applications, and composite materials. Li et al. (2018) also synthesized HCSs from CO₂ using molten carbonate electrolyzers and proposed the following mechanism for the conversion:

6.2.1.3 Carbon Nanofiber

1D carbon nanofibers (CNFs) with sp² hybridized carbon atoms (Deng et al. 2013; Zhou et al. 2020) can be used for energy storage (Zhang et al. 2016), electrochemical catalysis (Shakoorioskooie et al. 2018), sensor manufacturing (Sengupta et al. 2020), and high-strength building material development (Ren et al. 2015). Traditional CNF synthesis methods include electrospinning/carbonization, arc/plasma, and chemical vapor deposition (CVD), with electrospinning/carbonization being the principal approach. Conventional raw materials for CNF preparation include polyacrylonitriles, pitch, acetone, and hydrocarbon gases (Ren et al. 2015) but recently, renewable feedstocks (e.g., lignin and cellulose) for CNF manufacturing have received considerable attention (Lallave et al. 2007; Wu et al. 2013).

Several research groups have demonstrated that CO₂ could be a good candidate for CNF synthesis (Lau et al. 2016; Novoselova et al. 2007; Ren et al. 2015; Xie et al. 2024). Ren et al. (2015) reported a one-pot synthesis method for synthesizing CNFs via electrolytic conversion of CO₂. The technology, based on Li₂O looping for CO₂ reduction, employs low-cost, scalable nickel and steel electrodes to decompose CO₂ into CNFs and O₂, producing CNFs with diameters of 200–300 nm and lengths of 20–200 μm. The Coulombic efficiency is higher than 80 percent and can be close to 100 percent (i.e., complete decomposition) if all the products can be collected. Lau et al. (2016) developed a system integrating Li₂CO₃ electrolysis with a combined cycle natural gas power plant to produce CNFs and pure O₂. The system consumes 219 kJ to convert 1 mol of CO₂ to 1 mol of carbon, while the pure O₂ generated from CO₂ decomposition is sent back to the gas turbine, improving electricity generation efficacy. Xie et al. (2024) reported an electrochemical–thermochemical tandem catalysis system to convert CO₂ to CNF using renewable hydrogen, which achieved an average yield of 2.5 g_carbon g_metals⁻¹h⁻¹ (“metals” here refers to the catalyst used for conversion; in Xie et al. [2024] this was an FeCo alloy and extra metallic Co). In summary, CO₂ conversion to CNFs shows promise as a method to reduce CO₂ emissions while generating high-value CNFs with market potential in nanoelectronics, energy storage, and construction materials.

6.2.1.4 Carbon Nanotubes

Carbon nanotubes (CNTs) are rolled graphene sheets. CNTs are 1D carbon materials with sp² or sp² + sp³ hybridized carbon atoms and are classified as single-walled nanotubes (SWNTs) or multiwalled nanotubes (MWNTs). SWNTs are open-ended cylinders with only one wrapped graphene sheet, while MWNTs are an assembly of homocentric SWNTs. The dimensions of SWNTs and MWNTs differ in both length and diameter, leading to considerably different properties (Kozinsky and Mazari 2006; Rathinavel et al. 2021). For example, unlike SWNTs, the mechanical properties (e.g., Young’s modulus) of MWNTs depend not only on diameter and chirality but also on the number of sidewalls (Rathinavel et al. 2021). Various methods have been developed for preparing CNTs, including arc discharge, laser ablation, CVD, and injection of carbon atoms into metal particles (Rodríguez-Manzo et al. 2007). Factors affecting CNT syntheses include the carbon source (e.g., hydrocarbons, alcohols), catalyst (e.g., Al₂O₃), temperature, pressure, and flow of gases. CNTs have been explored for use in many applications, including sensing (Wang et al. 2023), cancer therapy (Mishra et al. 2023), preparation of biological fuel cells (ul Haque et al. 2023), light-weight reinforced high-strength materials (Hong et al. 2023), Li/Na-ion batteries (Qu et al. 2024), and others (Kordek-Khalil et al. 2024).

CO₂ can be used as a carbon source for CNT synthesis. Research to-date primarily has explored electrochemical conversion processes (Douglas et al. 2018; Li et al. 2018; Licht et al. 2016; Moyer et al. 2020). For example,

Licht et al. (2016) reported that ambient CO₂ could be successfully captured and converted to CNTs and CNFs in molten lithium carbonates at high yield via electrolysis using inexpensive steel electrodes, with the resultant carbon materials exhibiting good, stable capacities for energy storage. Douglas et al. (2018) synthesized CNTs with small diameters (~10 nm) using ambient CO₂ and Fe catalysts. They concluded that (1) the energy input costs for the conversion of CO₂ into CNTs are $50/kg_CNT and $5/kg_CNT using Al₂O₃ and ZrO₂ as thermal insulation materials, respectively, and (2) the CO₂ to small-diameter CNTs technology is superior to other CO₂ conversion technologies with lower-value materials as their products. Li et al. (2018) found that the electrolyte composition in a molten carbonate electrolyzer that captures CO₂ from air and converts it to CNTs and HCSs plays a key role in the selectivity toward CNTs, as well as in determining the diameter of the synthesized CNTs.

6.2.1.5 Graphene

2D graphene, with its sp² hybridized carbon atoms and very stable structure, is the thinnest (sheet thickness of 0.34 nm) and strongest nanomaterial known (Yu et al. 2020). Graphene itself has limited applications owing to its easy agglomeration, and difficult processing (Yu et al. 2020), and it requires modification and functionalization to increase its potential applications. Graphene synthesis techniques include exfoliation of graphite, reduction of graphene oxide, thermal and plasma CVD of hydrocarbons, thermal decomposition of silicon carbide, and unzipping of CNTs. Graphene functionalization processes are based on (1) the formation of covalent bonds between graphene and introduced functional groups (e.g., −OH and −COOH); (2) the formation of non-covalent bonds (e.g., π–π interactions, hydrogen, ionic, and dative bonding); and (3) element doping (Yu et al. 2020). The primary challenges facing production and use of graphene and functionalized graphene are high costs and carbon/environmental footprint, determined by the characteristics of typical synthesis methods. For example, graphene oxide (GO) reduction requires the use of highly corrosive agents and thus a long washing process after reduction, energy-intensive CVD, and low quality of large-scale production (Liu et al. 2020c; Urade et al. 2023). Nonetheless, graphene and functionalized graphene could have a wide range of applications—including in supercapacitors, solar cells, electrodes, and e-textiles—owing to their many desirable properties (Su et al. 2020; Urade et al. 2023; Yu et al. 2024), including rich functional group variability and density, environmental stability and compactness (Su et al. 2020). For example, the deleterious ion migration that reduces operational stability of iodide perovskite solar cells synthesized with organic–inorganic halide perovskite materials, resulting from the weak Coulomb interactions in the perovskite lattice, can be suppressed by using graphene, which has a lattice parameter (0.246 nm) smaller than the radius of I⁻ (0.412 nm) (Su et al. 2020).

CO₂ also has been explored as a feedstock for graphene synthesis (Chakrabarti et al. 2011; Hu et al. 2016; Liu et al. 2020c; Strudwick et al. 2015; Wei et al. 2016). For example, Liu et al. (2020c) used molten carbonate electrolysis to synthesize graphene, where the conversion occurs via carbonate formation in Li₂O, electrolysis of Li₂CO₃, and then exfoliation of the resultant carbon nanoplatelets:

Addition of zinc and increased electrolysis current led to the selective (more than 95 percent yield) formation of high-purity carbon nanoplatelets rather than CNTs, and exfoliation of the carbon nanoplatelets produced graphene in 83 percent yield by mass of the original carbon nanoplatelets (Liu et al. 2020c).

6.2.1.6 Graphite

Graphite is a 3D material with a stacked planar sp²-hybridized C6 fused ring structure—that is, stacked layers of graphene with AB stacking (Jara et al. 2019). It is known for its high specific surface area, thermal conductivity,

fracture strength, and special charge transport phenomena. It can be obtained as naturally occurring graphite or produced by synthesis (Surovtseva et al. 2022). Natural occurring graphite is mined but requires energy- and chemical-intensive beneficiation and purification thereafter, especially for its use in batteries (Surovtseva et al. 2022). Synthetic graphite is tunable in microstructure and morphology. The synthesis typically includes two sequential steps: formation of amorphous carbon via carbonization of a carbon precursor and subsequent graphitization of the amorphous carbon. Both steps occur at high temperature and are therefore energy-intensive and generally CO₂-emitting. Synthetic graphite can be prepared by various methods, including graphitizing nongraphitic carbons (e.g., cokes [Gharpure and Vander Wal 2023]), processing hydrocarbons (e.g., agricultural wastes or biomass materials [Yap et al. 2023]), CVD at >2500°C, and decomposing unstable carbides. Graphite is used in batteries (Kim et al. 2024), refractories (Chandra and Sarkar 2023), metallurgical processing (Li et al. 2023a), and other fields.

Some researchers have synthesized graphite from CO₂ (Chen et al. 2017a; Hu et al. 2015, 2019; Hut et al. 1986; Liang et al. 2021; Ognibene et al. 2003; Yu et al. 2021). For example, Liang et al. (2021) prepared synthetic graphite submicroflakes by heating CO₂ in the presence of lithium aluminum hydride (LiAlH₄) at 126°C. This synthetic graphite was compared to commercial graphite to test its potential application as an anode for lithium storage materials, and both showed stable reversible capacities around 320 mAh g^–1 from the 1st to 100th cycles at a current density of 0.1 A g^–1. After 100 cycles, the synthetic graphite and commercial graphite achieved 99 percent and 95.4 percent retention efficiencies, respectively, suggesting that the synthetic graphite prepared with CO₂ is superior to its commercial counterpart, especially considering that it was generated without a separate graphitization step. Electrochemical methods also can be used to produce graphite from CO₂, with Chen et al. (2017a) demonstrating that CO₂ captured from synthetic flue gas by a molten salt (Li₂CO₃ – Na₂CO₃ – K₂CO₃ – Li₂SO₄) at as low as 775°C (the electrolysis temperature) without the use of any catalyst can produce nano-structured graphite.

6.2.2 Indirectly Derived Carbon-Rich Carbon Materials

6.2.2.1 Carbon Fiber

Carbon fibers (CFs), with a $7.1 billion market in 2023 and annual growth rate of 12.6 percent (marketsandmarkets 2024), have extremely useful properties, including high elastic moduli, compressive and tensile strengths, and thermal and electrical conductivities, as well as low coefficients of thermal expansion (Hiremath et al. 2017). CFs are reinforcing materials widely used in airplanes, cars, and wind turbine blade manufacturing (Liu and Kumar 2012). The most common carbon source for CF synthesis is polyacrylonitrile (Le and Yoon 2019). Polyacrylonitrile-based CF manufacturing includes fiber spinning, stabilization, carbonization, and graphitization steps (Kaur et al. 2016). Synthesis of polyacrylonitrile is based on the free-radical polymerization of acrylonitrile (Pillai et al. 1992), which is produced via catalytic ammoxidation of propylene. Propylene, in turn, is generally produced from the reaction of ethylene with 2-butene, where the latter is synthesized via ethylene dimerization (Pillai et al. 1992). Thus, ethylene is a critical intermediate in CF production.

As discussed in Chapter 7 of this report, ethylene can be produced via thermochemical, electrochemical, or potentially photochemical conversion of CO₂, and thus, CO₂ can play an indirect role in CF syntheses (Li et al. 2020; Pappijn et al. 2020). The development of highly active, selective, and stable CO₂ to ethylene catalysts would facilitate the CO₂-to-CF pathway.

6.2.2.2 Carbon-Carbon Composites

As noted in Section 6.2.2.1, CO₂ can be a raw material for producing the critical precursor to CFs, which then could be used subsequently to produce CCCs. CCCs are lightweight, high-strength materials with good electrical properties, making them attractive for a wide spectrum of applications. The CCC manufacturing process involves saturation (impregnation) of other materials into carbon matrices, followed by graphitization or carbonization (a pyrolysis process) to form a graphitic structure (Windhorst and Blount 1997). During pyrolysis, voids form because of volatilization, which is deleterious for the mechanical properties of the CCCs. Repeated impregnation

and carbonization can address this problem, but repetition increases manufacturing time and thus cost. Additionally, as-made CCCs may not possess adequate microstructure, porosity, interlaminar shear strength, flexural, ultrasonic and vibration damping behavior for certain applications (Bansal et al. 2013). Further modifications may be necessary to achieve the desired qualities, including physical treatments (e.g., plasma-based surface changes) and chemical treatments or use of additives.

Among the possible additives are carbon nanomodifiers (CNMOs), or the carbon nanostructure materials introduced in Table 6-1. Some CNMOs, such as nanographene, have been synthesized from CO_2, as discussed in Section 6.2.1. CNMOs might be able to overcome the abovementioned challenges faced during CCC manufacturing, combining with other matrices, such as pitch, to enhance the properties of CCCs. CNMOs can mitigate shrinkage and tailor the properties of CCCs. For example, Bansal et al. (2013) introduced nanographene platelets (NGPs) to fill defects such as pores and cracks during manufacturing of CCCs. When the NGP-to-CCC ratio was 1.5 wt%, the interlaminar shear strength, flexural strength, and Young’s modulus of the CCCs increased by 22 percent, 27 percent, and 15 percent, respectively, compared to CCCs that did not contain NGPs. Meanwhile, the porosity of the modified CCCs was reduced by 17.5 percent. Eslami et al. (2015) filled carbon-fiber/phenolic composites with MWNTs. When the sample was modified by 1 wt% MWNTs, the thermal stability of the CCCs increased, according to thermogravimetric analysis, and the linear and mass ablation rates decreased by about 80 percent and 52 percent, respectively. Scanning electron microscopy showed the formation of a strong carbon network in CCCs resulting from the addition of MWNTs. These examples suggest that CO₂-derived CNMs can play an important role in the development of high-quality CCCs.

6.3 EMERGING TECHNOLOGIES TO REDUCE CO₂ TO ELEMENTAL CARBON

6.3.1 Introduction

As introduced above, the reduction of CO₂ to elemental carbon (CTEC) can be performed by four major chemical reaction processes: thermochemical, photochemical, electrochemical, and plasmachemical reduction (see Figure 6-1). Table 6-2 summarizes the strengths and shortcoming of these four chemical conversion pathways for CTEC. The four pathways could be coordinated to develop potentially more efficient and less expensive CTEC processes by combining their strengths and overcoming their shortcomings, as listed in Table 6-2. For example, the temperature required for thermochemical CO₂ conversion to CNTs can be as high as 700°C (Lou et al. 2006), however, a combined photo-thermochemical CO₂-to-CNT process can proceed at as low as 80°C (Duan et al. 2013).

The majority of CTEC conversion technologies are still in the lab-scale study phase, and thus their development status can be described as “emerging,” although the specific technology readiness levels (TRLs) of different conversion technologies vary. Initial research on CTEC processes examined thermochemical pathways, and as early as 1978, cation-excess magnetite was used to convert CO₂ to carbon via CTEC with an efficiency of nearly 100 percent at 290°C, although the structure of the generated carbon wasn’t reported by the researchers (Tamaura and Tahata 1990). Thermochemical CTEC processes are at higher TRLs than other pathways, with bench-scale² and pilot-scale³ conversions being successfully demonstrated. As of April 2024, no CTEC technology has been commercialized.

Some CTEC processes are effective at forming specific high-value carbon materials but are very energy-intensive. For example, high-quality CNMs, such as CNFs and CNTs, can be produced by CVD, but this method requires very high temperatures and low pressures over long periods of time and is not easily scalable. As a result, this method is estimated to have an unusually large carbon footprint of up to 600 tonnes of CO₂ emitted per tonne of CNM produced (Wang et al. 2020). Alkali and alkaline earth metals, such as lithium, sodium, magnesium, and calcium, can be used as reductants to reduce CO₂ to various carbon products, including carbon spheres, graphene, and CNTs. However,

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² For example, 100 percent direct conversion of CO₂ to C has been demonstrated at the bench scale with Ni_0.39Fe_2.6O₄ (reaction conditions: flow rate of simulated flue gas: 9 dm³/h; composition of the simulated flue gas flue gas: 20% CO₂ and 80% N₂) (Taylor et al. 1993).

³ The bench-scale result described in Taylor et al. (1993) was tested at pilot scale by designing a system with the capacity to treat 1000 Nm³ h⁻¹ flue gas (composition: 9.4% CO₂, 74.8% N₂, 0.8% O₂, 15% H₂O, 50 ppm NO_x) from a liquefied natural gas combustion boiler (Yoshida et al. 1997).

TABLE 6-2 Comparison of the Strengths and Weaknesses of the Four Major State-of-the-Art (SOA) Chemical Reduction Processes Being Explored for CTEC Material Conversion

SOURCES: Based on data from the following: Thermochemical: Álvarez et al. (2017); Kondratenko et al. (2013); Kosari et al. (2022); Ye et al. (2019); Zuraiqi et al. (2022). Photochemical: Duan et al. (2013); Han et al. (2023a); Li et al. (2014); Yaashikaa et al. (2019). Electrochemical: Lu and Jiao (2016); Overa et al. (2022); Pérez-Gallent et al. (2020); Sajna et al. (2023); Spinner et al. (2012); Tackett et al. (2019). Plasmachemical: George et al. (2021); Lerouge et al. (2001); Martirez et al. (2021); Snoeckx and Bogaerts (2017).

regeneration of these reductants is also very energy intensive. For CTEC processes to be competitive against other alternative carbon sources or processes, energy efficiency must be maximized and low-carbon energy sources used.

The distribution percentages of the 161 journal papers found with CO₂ being the sole carbon source in the four CTEC research areas is shown in Figure 6-3. Clearly, research work in the thermochemical area dominates all those four areas, accounting for 70 percent of the total work reported in published papers. The thermochemical research largely involves CTEC studies of decomposition-based reactions between CO₂ and cation-excess materials and reactions between CO₂ and strong reducing agents. Note that only published journal papers as of January 2024 were collected in the analysis given in Figure 6-3.

The history of the annual publications resulting from global CTEC R&D efforts is presented in Figure 6-4, where it is evident that this is an understudied phenomenon, yet to take off. The first CTEC paper was published in 1978, followed by a drought of CTEC research for 12 years. CTEC research productivity was stable from 1990 to 2001, during which about three papers were published annually. The average quantity of annual CTEC publications has tripled since 2015, a significant increase. However, the pace of CTEC R&D activities has been much slower than other CO₂ utilization technologies. Figure 6-4 reveals that thermochemical methods have dominated CTEC technology history, while photochemical and plasmachemical approaches have only been studied occasionally. Note that researchers are increasingly interested in developing electrochemical CTEC technologies owing to their advantages compared to thermochemical ones, as given in Appendix K.

6.3.2 Reaction Processes

6.3.2.1 Thermochemical Conversion Pathways

Thermochemical CTEC can involve CO₂ decomposition or CO₂ reaction with strong reducing agents—for example, H₂ or alkali and alkaline earth metals, with and without the use of other reactants or catalysts. The strengths and weaknesses of these two types of CTEC technologies are described below.

6.3.2.1.1 Decomposition-Based CTEC

The CO₂ decomposition into C can be via the R6.7 pathway:

Synthesizing elemental carbon from CO₂ via R6.7 is difficult because of its large, positive ΔG° value. Research efforts have aimed to identify materials that can enable more favorable reduction pathways and thus allow lower operating temperatures to be used (Kim et al. 2001, 2019; Kormarneni et al. 1997; Lin et al. 2011; Sim et al. 2020; Tamaura and Tahata 1990; Tsuji et al. 1996a; Yoshida et al. 1997). The strategy of exposing CO₂ to a cation-excess metal oxide works because the material is oxygen-deficient and therefore at much lower temperatures than in the gas phase it is possible to strip oxygen from CO₂ by absorbing that oxygen in the metal oxide lattice, filling oxygen vacancy sites. For example, Tamaura and Tahata (1990) found that reacting CO₂ with cation-excess magnetite (Fe_3+δO₄, δ = 0.127) can reduce CO₂ to carbon with an efficiency of nearly 100 percent at 290°C. During CO₂ reduction, all of the oxygen in CO₂ transfers as described above, in the form of O²⁻ to the cation-excess magnetite, because only carbon and no CO was detected (Tamaura and Tahata 1990).

Tsuji et al. (1996a) investigated the reactivity toward CO₂ decomposition of metallic iron formed on oxygen-deficient Ni(II)-bearing ferrite. 86 percent CO₂ conversion was observed, yielding 97 percent elemental carbon and 3 percent CO. The same group also achieved excellent elemental carbon selectivity with an ultrafine Ni(II) ferrite prepared with coprecipitation of Ni²⁺, Fe²⁺, and Fe³⁺ at 60°C with 36 percent Ni²⁺ substitution for Fe²⁺ in magnetite at 300°C (Tsuji et al. 1996b). The associated CO₂ decomposition mechanism can be written as:

where M represents divalent metals, and δ and δ' are the initial oxygen deficiency and the oxygen deficiency at any reaction time (t), respectively (Tsuji et al. 1996b). The change in oxygen deficiency (δ − δ') directly reflects the degree of CO₂ conversion to elemental carbon.

Kim et al. (2019) examined SrFeCo_0.5O_x for its ability to reduce CO₂ to elemental carbon. The highest CO₂ decomposition efficiency achieved with SrFeCo_0.5O_x reached ~90 percent, while decomposition efficiencies of ≥80 percent at 550°C to 750°C lasted for more than 60 minutes. The reaction mechanism of the interaction between SrFeCo_0.5O_x and CO₂, proposed by Kim et al. (2019), is shown in Figure 6-5.

The same research group examined another reactant, SrFeO_3–δ, for CO₂ reduction under the same test conditions, achieving a CO₂ decomposition efficiency of ≥90 percent, with decomposition ≥80 percent lasting for ~170 min, 70 percent longer than that realized with SrFeCo_0.5O_x (Sim et al. 2020). However, the elemental carbon production selectivity achieved with SrFeCo_0.5O_x and SrFeO_3–δ, despite their activity, stability, and reproducibility, are not as good as that obtained with other solid oxide reactants—for example, Ni(II) ferrites (Tsuji et al. 1996b). In other words, unlike Ni(II) ferrites, both SrFeCo_0.5O_x and SrFeO_3–δ based CTEC processes have a common shortcoming—that is, generation of CO as a by-product, which needs to be overcome when elemental carbons are the targeted products. One possible method is to add another reactor to consequently split the CO generated in Step I in Figure 6-5 subsequently into elemental carbon. Another difference between Kim et al. (2019) and previously reported work in this area is that a continuous CO₂ decomposition system was used, which is beneficial to eventual commercialization of the cation-excess materials based catalytic CTEC technology.

Note that use of these solid-oxide reactants to decompose CO₂ is closely related to research and development being pursued for solar thermochemical syngas production, in which oxygen-deficient metal oxides at high

temperature (heated by concentrated sunlight) strip oxygen from water and/or carbon dioxide to produce hydrogen and carbon monoxide (Wexler et al. 2023a; Zhai et al. 2022). Some of the same materials as mentioned above have been explored for this purpose (Gautam et al. 2020; Wexler et al. 2021, 2023b). As reduction to elemental carbon by these materials therefore likely involves stepwise stripping of oxygen from carbon dioxide to form carbon monoxide (the end goal in solar thermochemical syn gas production) first, it is unsurprising that it is even more difficult to form elemental carbon, given that the second C-O chemical bond in carbon monoxide is a stronger triple bond whereas the first C-O bond to be stripped is a double bond (see Figure 6-5).

6.3.2.1.2 CTEC with Strong Reducing Agents

In this type of CO₂ to elemental carbon conversion, H₂, alkali metals, and alkaline earth metals are used as reductants—for example,

All CTEC reactions of this type are thermodynamically favorable under standard conditions. The temperatures required for R6.9 and R6.10 are lower or much lower than those for the above-mentioned decomposition-based reduction reactions, depending on the reducing agent used. Magnesium (Mg) is the primary metal used as a strong reducing agent to react with CO₂ and produce CNMs in this type of CTEC process (R6.10). The major elemental carbon product of such processes is graphene, which can combine with Mg to form Mg matrix composites (Li et al. 2022c, 2023b; Samiee and Goharshadi 2014; Wei et al. 2022; Zhang et al. 2014). R6.10 was first reported in 1978 (Driscoll 1978). In recent years, multiple research groups have demonstrated its use for simultaneous reduction of CO₂ and generation of elemental carbon materials (Li et al. 2022c, 2023b; Samiee and Goharshadi 2014; Wei et al. 2022; Zhang et al. 2014).

Ideally, the products of R6.9 and R6.10 are elemental carbon and H₂O or metal oxides. However, completely avoiding the generation of by-products is difficult. For example, R6.11 could occur simultaneously with R6.9 (Pease and Chesebro 1928)

and form an undesirable product, CH₄. Also, MgO generated in R6.10 can further react with CO₂ to form MgCO₃ via

The formation of undesired products (e.g., CH₄, CO, carbonates) in R6.11 and R6.12 decreases elemental carbon selectivity and yield and complicates the separation of elemental carbon during decomposition-based strong reducing agents based CTEC processes. Carbon materials—for example, graphene, produced via the metal reduction CTEC process can exhibit excellent practical properties. For example, Wei et al. (2022) reported successful preparation of few-layer graphene through the reaction between molten Mg and CO₂ gas at 750°C, 100°C above the Mg melting temperature. The produced graphene, exhibiting a high degree of graphitization and nanoscale thickness, served as a lithium storage material and achieved excellent rate capability and cycling performance with a reversible capacity of 130 m Ah g⁻¹ after 1,000 cycles at a current density of 1.0 A g⁻¹.

Compared to decomposition-based CTEC technologies, CTEC using strong reducing agents has several notable advantages. First, the reactions of CO₂ with strong reducing agents typically have negative Gibbs free energy (ΔG⁰) and enthalpy (ΔH⁰) changes, indicating their thermodynamic feasibility under standard conditions. Second, the very negative ΔG⁰ and ΔH⁰ values for reactions of CO₂ with strong reducing agents to form elemental carbon materials imply that 100 percent CO₂ conversion efficiency potentially can be achieved, a precondition for reaching a high yield of elemental carbon. Also, this class of CTEC processes does not require catalysts because moderate temperature elevation alone can significantly increase the rates of the reactions between CO₂ and strong reducing agents (e.g., H₂, alkali or alkaline earth metals). Moreover, the reaction set-ups and operation are relatively simple, which will reduce costs of the CTEC processes.

Despite having advantages of thermodynamic feasibility, high conversion efficiency, and less dependence on catalysts, CTEC processes using strong reducing agents are not without shortcomings. Selecting an appropriate reactive metal or reducing agent for the CTEC process is crucial, as the choice will affect CO₂ conversion efficiency and elemental carbon selectivity. When a metal is used instead of H₂ as the reducing agent, the reaction has to proceed at temperatures above the melting point of that metal. These temperatures are typically higher than those needed for the thermal-decomposition-based CTEC (~300°C). Moreover, achieving precise control over the morphology and properties of the elemental carbon produced is not easy, especially when specific structures of elemental carbon materials are desired—for example, 2D graphene and graphite with sp² carbon hybridization, and 0D CQDs with both sp² core and sp³ shell carbon hybridizations. However, as noted above, the metal reduction technology generates metal oxides and sometimes by-products like carbonates that are not only long-lived materials to store carbon but that also could be sold for industrial applications owing to their high purities after they are separated from the elemental carbon materials and the metal oxides. Separation of carbon and noncarbon materials is relatively easy. The resulting metal oxides can be reduced to elemental metals (e.g., Mg) for cyclic CTEC conversion but this requires the use of energy-intensive processes. Also, as with any technology, transitioning from the current laboratory-scale experiments to large-scale production for practical applications could present additional challenges.

Last, although using CO₂ as a feedstock to produce long-lived elemental carbon materials could provide environmental benefits (Cossutta and McKechnie 2021; Goerzen et al. 2024; NASEM 2023), the overall environmental impact of thermochemical CTEC conversion needs to be assessed holistically, considering requirements for temperature, pressure, product separation, and reactant material recycling, if applicable.

6.3.2.2 Electrochemical and Electrically Driven Thermal Conversion Pathways

6.3.2.2.1 Electrochemical

The use of high-temperature molten carbonate electrolysis to produce elemental carbon products from CO₂ has advanced considerably over the past decade. Derived from earlier solar thermal electrochemical processes (Ren et al. 2019), these systems use metal carbonate electrolytes, which have high melting points, so the reactions

are typically run at 450°C–700°C. High-temperature molten carbonate electrolysis can form a large spectrum of carbon products, including carbon powder, nanoplatelets, graphene, nano-onions, high-quality nanotubes, and other nanocarbon allotropes. Additives such as calcium or lithium chloride or fluorides can improve the performance of these systems by increasing the solubility of CO₂ and the calcium oxide (Tomkute et al. 2013). The mechanism of this reaction is currently unclear (Han et al. 2023b).

High-temperature molten carbonate electrolysis is scalable because all the elemental components are abundant and economical, although high temperatures are required to prevent the molten carbonate electrolyte from solidifying. The molten carbonate precursor, often an alkali metal oxide, reacts with CO₂ selectively at low concentrations, including from air, to generate the electrolyte. Thus, this method can be used to integrate CO₂ capture from atmospheric or flue gas streams for conversion. Such integration of molten salt CO₂ capture and electrochemical transformation eliminates the need for distinct separations processes and the accompanying energy and capital requirements. These systems might be able to operate on waste heat from high-temperature flue gas exhaust in a fossil fuel plant, and the oxygen generated at the anode could be used to facilitate combustion. It has been suggested that this method might be able to achieve a 100× price reduction for the production of CNTs compared to conventional CVD methods (Ren and Licht 2016).

A more recent development is room-temperature CO₂ electrolysis into carbon materials, which uses metals that are liquid at room temperature, such as gallium, alloyed with redox-active metals, such as cerium. In one such system, analysis of the liquid revealed a thin film of cerium oxide on the surface, and it is believed redox reactions occur by cycling cerium between the zero and tetravalent oxidation state (Esrafilzadeh et al. 2019). The proposed reactions are shown below, where the R6.13–R6.16 reduction reactions occur at the liquid metal working electrode (a cathode made of a gallium-indium-tin alloy, galinstan), and R6.17 is the oxidation reaction at the anode. The carbonaceous nanoflakes have been applied as carbon-based supercapacitor materials (Esrafilzadeh et al. 2019).

This approach has also been reported with other liquid metal electrode formulations (Irfan et al. 2023; Ye et al. 2023). Other examples of CO₂ conversion to carbon materials that use liquid metals apply mechanical energy (Tang et al. 2022) or heat (Zuraiqi et al. 2022) as the energy source. The use of liquid metals provides several advantages over CVD and high-temperature molten carbonate electrolysis. The components are abundant and economical and have low toxicity and vapor pressure. Unlike the other electrochemical methods discussed, the process operates at low or room temperature. The high surface tension of the liquid metal also acts as an intrinsic coking-resistant surface. The carbonaceous products spontaneously flake off the surface of the electrode, which facilitates separations and prevents catalyst poisoning or inhibition.

6.3.2.2.2 Electrically Driven Thermal Reduction

Solid oxide electrolyzer cells (SOECs) can be used to combine thermal and electrochemical energy to reduce CO₂ to elemental carbon materials. CO₂ electrolysis in the SOEC forms carbon monoxide (CO) and solid carbon at the cathode, while oxygen is evolved at the anode. The electrodes again are typically made of oxide perovskites similar to the materials discussed earlier. This method has been applied to produce CNTs, albeit at high temperatures (>800°C) (Tao et al. 2014).

A related technology is the use of tandem electro-thermochemical looping to access carbon materials from CO₂. In these systems, CO₂ is initially reduced to CO in a low-temperature electrolytic cell or a high-temperature SOEC (Luc et al. 2018; Mori et al. 2016). The CO is then heated to high temperatures (500°C–700°C), where it undergoes disproportionation in a Boudouard reaction, forming solid carbon and CO₂. This method has been used to produce densely packed and aligned CNTs. Challenges to this approach include integration of the CO stream to the reactor, as well as the purity of the former. Minimizing concomitant evolution of H₂ with CO is critical to prevent hydrogen gas accumulation in the system. Additionally, recovered CO₂ must be free of carbon materials before being looped back into the electrolysis cell to avoid poisoning the electrodes in the low-temperature electrolytic cell (Luc et al. 2018).

When CO₂ decomposes to CO and ½ O₂, CO can disproportionate via the Boudouard reaction to produce ½ CO₂ and ½ C(solid), which, with recycle of CO₂, can result in an overall conversion of CO₂ → C(solid) + O₂ via this two-step pathway. Solid carbon is more readily generated from CO than CO₂ below 971K (Chery et al. 2015; Han et al. 2023b). Hence, pathways through CO can provide indirect conversion of CO₂ to solid carbon products.

6.3.2.3 Photochemical Conversion Pathways

Photocatalytic CO₂ reduction has been widely researched to produce C1 and C2 gas and liquid products (Fu et al. 2019). The direct production of solid carbon materials from CO₂ by photocatalysis is challenging due the limited reaction pathways in photochemistry (Li et al. 2019a). However, many photocatalytic products like methane and CO are valuable substrates for downstream carbon material production through chemical synthesis (Duan et al. 2013). A viable approach for carbon material production leveraging photocatalysis is a tandem process, where photocatalytic CO₂ reduction first converts CO₂ into gaseous products like methane and CO, followed by conversion of methane and CO into carbon materials via (e.g., pyrolytic) decomposition (Anisimov et al. 2010; Shen and Lua 2015).

6.3.2.4 Plasmachemical Conversion Pathways

Plasmachemical processes can facilitate thermodynamically unfavorable reactions, such as CO₂ activation, at relatively low temperatures. Several nonthermal plasma sources have been applied to transform CO₂, including dielectric barrier discharge, microwave discharge, and gliding arc (Mei et al. 2014). Plasma processing parameters such as discharge power, gas composition, feed flow rate, and dielectric material affect the conversion of reactants. Catalysts can be used to assist plasma reactions, especially when packed-bed reactors are employed, which can integrate catalysts to enhance the synergies between catalysts and plasmas.

Plasma catalysis can be performed via two main types of reactor configurations, in-plasma catalysis and post-plasma catalysis (George et al. 2021; Wang et al. 2018). For the in-plasma catalysis configuration, catalysts are placed in the plasma discharge region, which facilitates a direct reaction between the plasma species and the catalyst surface, potentially enhancing the conversion of reactants to desired products (e.g., nanocarbon materials). In post-plasma catalysis, the reaction occurs in two steps. First, CO₂ could participate in in-plasma reactions if plasma is used to split CO₂ into CO and O₂, while in the second stage, CO and/or CO₂ could undergo catalytic surface reactions. The chemical and physical interactions between the nonthermal plasma and catalyst strongly affect the percentage of CO₂ transformed and the product selectivity (George et al. 2021). Plasma can impact the catalyst by generating excited species and radicals, lowering the activation barrier and enhancing pathways for surface reactions. At the same time, the catalyst can affect the electric field of the plasma, discharge type, generation of micro discharge in pores, and impurity concentration in the plasma. Consequently, plasma cataly-

sis may be able to increase energy efficiency, reaction rate, product yield, and catalyst durability; enhance the concentration of active species; and improve selectivity (Neyts et al. 2015). However, Bogaerts et al. (2020) note that good thermal CO₂ activation catalysts might not be good plasma catalysts for CTEC conversion because the plasma could change the properties of the catalyst surface and introduce new species (e.g., excited species and reactive species) for surface reactions, thereby changing surface interactions with the catalyst and thus the reaction pathways.

As of April 2024, research on splitting CO₂ into carbon materials and O₂ with nonthermal plasma remains at the concept stage. Only one paper is known to have been published in this area, which discusses the feasibility of the concept in detail (Centi et al. 2021). However, no experiments have been performed using plasma with or without the help of catalysts to confirm the reaction.

6.3.3 Potential Alternative or Competitor Routes to Elemental Carbon Materials

In a net-zero future, alternative routes to produce elemental carbon materials include methane pyrolysis and lignocellulosic biomass processing. While a detailed discussion of these approaches is out of scope for this study, brief descriptions are provided below, along with their advantages and disadvantages relative to the production of elemental carbon materials from CO₂. Life cycle and techno-economic assessments (see Chapter 3) will be needed to help evaluate and compare the emissions impacts and economic viability of these different approaches for producing elemental carbon materials.

6.3.3.1 Methane Pyrolysis

Methane pyrolysis entails the decomposition of methane to form solid carbon and hydrogen (Amin et al. 2011) and is a potential sustainable pathway to produce both elemental carbon products and clean hydrogen, provided the solid carbon products remain sequestered. The process is endothermic (Lewis et al. 2001) with its enthalpy change being 74.5 kJ/mol-CH₄ or 37.3 kJ/mol-H₂ produced (Catalan et al. 2023):

Methane pyrolysis is attractive as a source of hydrogen because, in principle, solid carbon can be separated from the gas-phase hydrogen product without CO₂ capture and sequestration, as would be required for sustainable conventional steam methane reforming (Korányi et al. 2022; Sánchez-Bastardo et al. 2021; Timmerberg et al. 2020). Formation of carbon fibers and nanotubes from methane (fully reduced carbon) may be less difficult than a similar pathway from CO₂ (fully oxidized carbon), given that a number of the CNT pathways from CO₂ are postulated to proceed through methane or other reduced hydrocarbon intermediates (Kim et al. 2020a, 2020b).

6.3.3.2 Lignocellulosic Biomass Processing

Elemental carbon materials derived from plant lignocellulosic biomass represents a competitive platform to CO₂-derived carbon materials. Of course, biomass-based carbon materials also are derived from CO₂ ultimately, as plants fix CO₂ through photosynthesis and convert it into biopolymers such as cellulose and lignin in the plant cell wall. These biopolymers subsequently can be converted into carbon materials like carbon fiber, graphene, CNTs, carbon foam, and others (Li et al. 2022b). Among different biopolymers, lignin has the greatest potential for carbon material manufacturing owing to its high carbon content and aromatic ring structure (Zhang et al. 2022). Recent studies have advanced fundamental understanding of structure–function relationships of how lignin composition (molecular weight, uniformity, linkage profile, and functional group) can impact its structure and performance. This understanding guides the design of new lignin structures to improve the quality and performance of the resulting carbon materials (Li et al. 2022a, 2022b).

6.3.4 Challenges for CO₂ Conversion to Elemental Carbon Materials

As noted in Section 6.3.2, except for plasmachemical processes, research into CTEC processes has been ongoing for quite some time, especially thermochemical CTEC research that started as early as 1990 (Tamaura and Tahata 1990). However, CTEC research activities and processes are still limited, despite the large market potential for elemental carbon materials discussed in Chapter 2. All four major types of CTEC technologies (thermochemical, electrochemical, photochemical, and plasmachemical) are still at low TRLs, in concept development stages. The sections below identify challenges common to all four CTEC approaches and those unique to each CTEC approach.

6.3.4.1 Common Challenges for the Four CTEC Approaches

1. Research efforts have been limited: Although research on CTEC conversions, especially in thermochemical approaches, has been ongoing for several decades, research intensity in the four areas have not been high. The first thermochemical Mg-CO₂–to–MgO-C conversion reaction was reported in 1978, and the first thermochemical CO₂ splitting with cation-excess magnetite was published in 1990. Nonetheless, only 161 papers have been published on thermochemical, electrochemical, photochemical, and plasmachemical CTEC conversion as of April 2024. Therefore, theoretical and experimental work on CTEC conversions are still limited, especially in the areas of photochemical and plasmachemical CTEC, as illustrated in Figure 6-3.
2. Difficult to compare CTEC approaches: Research activities on the four CTEC approaches are not balanced; thus, it is difficult to assess the advantages and disadvantages of the different reaction pathways.
3. Substantial energy requirements: Breaking carbon-oxygen bonds in CO₂ is energy intensive; thus, all four CTEC approaches require substantial external energy. For these products and processes to have net-zero emissions, this energy will have to be provided by zero-carbon-emission sources of electricity or heat.
4. Limited understanding of catalyst/reactant material stability and carbon product selectivity: The stabilities of the catalysts/reactant materials used and carbon materials generated during CTEC processes have not been well characterized.
5. A grand challenge for all of these CTEC technologies is how to convert CO₂selectively at high yield to a particular morphological form of solid carbon.

6.3.4.2 Specific Challenges for Each CTEC Approach

6.3.4.2.1 Thermochemical CTEC

Both the decomposition-based and strong reducing agents–based CTEC have low overall energy utilization efficiencies owing to the low mass and heat transfer efficiencies of fixed bed reactors in SOA thermochemical processes, the need to heat the reaction systems to moderately high temperatures to initiate the reaction, and loss of the heat released during the reaction. The two thermochemical CTEC technologies also have their own specific challenges.

• Decomposition-based CTEC:
  • Fast reactant material deactivation and the resultant slow reaction kinetics owing to coking via direct active site poisoning and pore plugging from the nanoscale carbon particles generated from CO₂ reduction, which lowers the reaction rate.
  • Stabilities of cation-excess reactants during their reactions with CO₂ at high temperatures are not well studied.
  • The purity and structure of the carbon materials generated have not been systematically evaluated.
• CTEC using strong reducing agents:
  • Initiation of the elemental-metals-based CTEC processes are slow owing to the need to reach high temperatures (higher than metal melting points) prior to the initiation of the reactions.

6.3.4.2.2 Electrochemical CTEC

Most electrochemical CTEC processes require high temperatures, and therefore greater energy input. Room-temperature electrolysis to produce elemental carbon materials is comparatively nascent and does not generate as high-value materials as the high-temperature systems. Deposition of carbon on electrode surfaces can lead to the significant reduction of active sites and a decrease in electrode activity. It is difficult to successfully convert CO₂ from gas to solid carbon owing to the complexity in overcoming kinetic barriers and achieving efficient nucleation and solid carbon structure growth.

6.3.4.2.3 Photochemical CTEC

Only one paper has been published using this CTEC technology (Duan et al. 2013). The research was performed with the help of temperature management. Thus, knowledge of reaction mechanisms for photocatalytic CTEC area is entirely lacking.

6.3.4.2.4 Plasmachemical CTEC

This concept has been proposed but not yet confirmed experimentally.

6.3.5 R&D Opportunities for CO₂ Conversion to Elemental Carbon Materials

There are many R&D activities that can advance CTEC processes. Research is needed into how the reaction conditions (e.g., temperature, pressure, composition of CO₂-containing feedstock), use of catalyst, and other factors affect the types and qualities of carbon materials produced. Work is also needed to fully characterize the carbon materials generated via CTEC technologies and identify their corresponding markets, including the preparation of organic solar cells and light-emitting diodes, supercapacitors, batteries, sensors, and catalysts. The following lays out specific R&D opportunities across thermochemical, electrochemical, photochemical, and plasmachemical CTEC.

Thermochemical CTEC:

• Decomposition-based CTEC:
  • There is an opportunity to build on knowledge of solid-oxide reactants, including ferrites and SrFeO₃-σ, to develop new types of cation-excess materials with high CO₂ conversion activity, ~100 percent carbon formation selectivity, and good stability and regeneration ability for decomposition-based CTEC. Utilizing knowledge already gleaned from the solar thermochemical hydrogen solid-oxide materials research could prove fruitful (Wexler et al. 2021, 2023b). Similarly, exploiting knowledge already gathered from solid oxide fuel cell materials characterization and optimization also could be helpful (Muñoz-García et al. 2014; Ritzmann et al. 2016).
  • The carbon materials obtained with decomposition-based CTEC, mainly CNT and graphene, can differ from those generated by reacting CO₂ with strong reducing agents because their reaction temperatures are different. Thus, more R&D is needed to discover how to generate different high-value carbon materials with decomposition-based CTEC processes.
• Strong-reducing-agents-based CTEC:
  • R&D is needed to diversify the elemental carbon products that can be made from the reaction of Mg and CO₂. Current systems primarily yield CNTs and graphene. By changing the reaction conditions, other products such as CNFs and CQDs might be accessible.
  • The primary metal used as a strong reducing agent for CTEC is Mg. R&D is needed to explore the pros and cons of using non-Mg metals for metal reduction CTEC processes.

Electrochemical CTEC:

• Lowering the operating temperature for molten electrolysis or solid oxide electrolyzer cells or coupling these processes with exothermic reactions could lower the overall energy requirements.

• Low-temperature electrolysis with liquid metals is a promising approach for generating elemental carbon materials, but more research is required to access higher-value carbon products. In the reported electrochemical processes, the mechanism of reduction is not well understood.

Photochemical CTEC:

• According to Duan et al. (2013), the photochemical method does not work on its own; thermocatalysis needs to be coupled to photocatalysis for the reduction of CO₂ to C to occur. Thus, dual-function catalysts that can simultaneously accelerate both thermal- and photo-splitting reactions may play a key role and could be a promising future research direction in this area.

Plasmachemical CTEC:

• A plasmachemical CTEC scheme (Mei et al. 2014) was proposed about 10 years ago. However, no realizable experimental data have been published to confirm the concept. Accordingly, multiple R&D opportunities in this area exist in this area, including:
  • Theory, modeling, and simulation—While such research has begun for some proposed methods (e.g., methane pyrolysis), none is available yet for understanding plasmachemical CO₂ reduction mechanisms.
  • Experiments—Experiments need to be conducted to confirm the feasibility of plasmachemical CTEC, especially regarding the use of catalysts both in situ and post-plasma.

6.4 CONCLUSIONS

6.4.1 Findings and Recommendations

Finding 6-1: CO₂ to elemental carbon technologies are far from commercialization—Thermochemical, electrochemical, photochemical, and plasmachemical conversion of CO₂ to elemental carbon technologies are generally at technology readiness levels of 3, 2 (for room-temperature electrolysis), 1, and 1, respectively, indicating that all are far from commercialization.

Recommendation 6-1: Support basic research to advance CO₂ to elemental carbon technologies—Basic Energy Sciences within the Department of Energy’s Office of Science and the National Science Foundation should invest in building the knowledge foundation and accelerating the maturities of the four CO₂ to elemental carbon technology areas: thermochemical, electrochemical, photochemical, and plasmachemical.

Finding 6-2: Demanding materials and energy requirements for CO₂ to elemental carbon technologies—All CO₂ to elemental carbon (CTEC) technologies need strong reducing agents (e.g., Mg or H₂) or very negative electrochemical potentials, oxygen-deficient reactant materials (e.g., cation-excess magnetite), and other materials (e.g., molten salts) as part of their conversion process. To develop CTEC technologies with net-zero or net-negative CO₂ footprints, the materials used in the CTEC technologies need to be generated from low-carbon-emission sources. In addition, all CTEC technologies need external energy to initiate and/or maintain the reactions.

Finding 6-3: Challenges with activity, selectivity, and stability of redox-active materials key to CO₂ to elemental carbon conversion—Redox-active materials are key to the success of CO₂ to elemental carbon technologies (CTEC). However, current catalysts for these technologies lack sufficient activity, selectivity, and stability to achieve high performance. Carbon nanotubes and graphene are the primary carbon materials generated from CO₂ via CTEC, with CO₂-derived graphene performing better than its commercial counterpart. Several other carbon materials, such as carbon nanofibers, are less frequently produced from CO₂.

Recommendation 6-2: Fund research into catalysts and materials for CO₂ to elemental carbon conversion—Basic Energy Sciences within the Department of Energy’s Office of Science (DOE-BES) and the National Science Foundation should fund basic research into the discovery of high-performance catalysts that are active, morphologically selective, and robust for low-cost CO₂ to elemental carbon (CTEC) conversion. DOE-BES, DOE’s Office of Energy Efficiency and Renewable Energy, and DOE’s Office of Fossil Energy and Carbon Management should, jointly or independently, fund research on materials (e.g., catalysts, reducing agents) used in CO₂ to elemental carbon processes. These investigations should aim to discover new, optimal materials for catalysis and separation; understand how to control CTEC reactions to increase the diversity of products and selectively generate desired morphologies; and increase energy efficiency of CTEC reaction processes that can be powered by clean energy.

Finding 6-4: Tandem systems have potential to optimize CO₂ to elemental carbon conversion—Combining multiple CO₂ to elemental carbon technologies—for example, photo/thermal or electro/thermal combinations, either in one-pot or sequential systems, could be more efficient than any single process alone. These superior efficiencies could include increased carbon yield, optimized systems, minimized energy input, and control of desired carbon material morphology.

Recommendation 6-3: Fund the development of tandem CO₂ to elemental carbon technologies to maximize economic and environmental benefits—Basic Energy Sciences within the Department of Energy’s (DOE’s) Office of Science, DOE’s Office of Energy Efficiency and Renewable Energy, and the Advanced Research Projects Agency–Energy should fund independently and/or collectively the development of tandem CO₂ to elemental carbon (CTEC) technologies that can combine the advantages of different types of CTEC processes to maximize the economic and environmental benefits of the converted carbon materials.

Finding 6-5: Combined capture and conversion of CO₂ to elemental carbon can lead to savings—Combining CO₂ capture with CO₂ to elemental carbon (CTEC) conversion can lead to reductions in capital and operation costs, and carbon footprints, of CTEC technologies, in contrast to discrete operations of CO₂ capture and subsequent CO₂ reduction to carbon materials.

Recommendation 6-4: Fund research on integrated CO₂ capture and conversion to elemental carbon materials to maximize economic and environmental benefits—The Department of Energy’s Office of Fossil Energy and Carbon Management, Office of Energy Efficiency and Renewable Energy, and the Advanced Research Projects Agency–Energy should fund research on integrated CO₂ capture and conversion to elemental carbon materials, with particular consideration of technology integration and economic and environmental benefit enhancement.

6.4.2 Research Agenda for Chemical CO₂ Conversion to Elemental Carbon Materials

Table 6-3 presents the committee’s research agenda for chemical CO₂ conversion to elemental carbon materials, including research needs (numbered by chapter), and related research agenda recommendations (a subset of research-related recommendations from the chapter). The table includes the relevant funding agencies or other actors; whether the need is for basic research, applied research, technology demonstration, or enabling technologies and processes for CO₂ utilization; the research theme(s) that the research need falls into; the relevant research area and product class covered by the research need; whether the relevant product(s) are long- or short-lived; and the source of the research need (chapter section, finding, or recommendation). The committee’s full research agenda can be found in Chapter 11.

TABLE 6-3 Research Agenda for CO₂ Conversion to Elemental Carbon Materials

NOTE: ARPA-E = Advanced Research Projects Agency–Energy; BES = Basic Energy Sciences; DOE = Department of Energy; EERE = Office of Energy Efficiency and Renewable Energy; FECM = Office of Fossil Energy Carbon Management; NSF = National Science Foundation.

6.5 REFERENCES