SOLID CARBON PRODUCTS FROM CARBON FEEDSTOCKS

Abstract

Disclosed herein are methods and systems for the electrochemical deposition of graphitic carbon from molten carbonate salts onto either mild steel or graphite electrodes. The carbon deposition methods disclosed herein allows for rapid formation of a deposit layer, with growth rate tunable by applied potential.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119 to U.S. provisional patent application Nos. 63/765,867 and 63/766,072 both of which were filed on 3 Mar. 2025, the contents of which are hereby incorporated in their entirety.

CONTRACTUAL ORIGIN

This invention was made with government support under Contract No. DE-AC36-08GO28308 awarded by the Department of Energy. The government has certain rights in this invention.

BACKGROUND

Solid carbon is vital to nearly every aspect of modern life. Graphite, a critical mineral, is especially important. Largely due to projected growth of the battery sector, the graphite market is expected to grow in the next decade. Despite being a major consumer of graphite, the US has no naturally occurring source of graphite. China dominates the graphite market but has restricted exports.

Other allotropes of carbon, including amorphous carbon, are also valuable. For example, the steel industry requires solid carbon for operation of electric arc furnaces, and the demand for carbon black for tires is another lucrative industry.

Despite the high value of carbon across industries, synthetic routes for the production of specific solid carbon structures are inefficient and expensive. For example, synthetic graphite production is extremely energy intensive, and current costs are significantly higher than for naturally occurring graphite. Synthetic graphite is typically produced in a furnace using carbon-based materials such as petroleum coke or coal. These raw materials are first crushed into a powder and blended with a binder, usually a tar pitch, and molded into a chosen shape. The first round of baking is performed in an anaerobic furnace heated to 900-1200° C. where carbonization of the binder occurs while most volatiles are released. Pitch is then readded to decrease porosity and then the part is baked again. In the next stage, the carbon block is converted to graphite in a process called “graphitization”, where it is baked in an aerobic environment at approximately 3000° C. This extreme heat transforms the amorphous carbon into crystalline graphite and purifies the product as most impurities will vaporize at these temperatures. This step is typically done in an Acheson furnace, which relies on resistive heating. The entire process results in graphite with a high purity of approximately 99.95%. The graphite can undergo further processing if, for example, the porosity is desired to be changed. Due to producing synthetic graphite being resource intensive and expensive, there remains a need for an improved way of producing synthetic graphite.

BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments of the present disclosure are illustrated in the referenced figures of the drawings. It is intended that the embodiments and figures disclosed herein are to be considered illustrative rather than limiting.

FIG. 1 illustrates a simplified electrochemical schematic of the reactor setup and illustration of key reactions, according to some aspects of the present disclosure.

FIG. 2 illustrates electrochemical response for different potentials and cathode materials; (a) cyclic voltammetry on steel and graphite cathodes; (b) current response to approximately 28 min of chronoamperometry on a steel cathode at approximately −1.8V (green circles) and approximately −2.4V (brown triangles), according to some aspects of the present disclosure.

FIG. 3 illustrates optical micrographs of a graphite electrode cleaved cross section after reaction at approximately −2.4 V for approximately 28 min; the graphite electrode is approximately 3.2 mm diameter, surrounded by an approximately 1 mm deposit layer consisting of deposited carbon (dark phase, yellow arrows) and trapped salt (white phase, red arrows); the high magnification inset (b) shows the deposit layer of approximately 1 mm on the electrode surface, according to some aspects of the present disclosure.

FIG. 4 illustrates EDS imaging for graphite electrode cleaved cross section, (a) low-magnification mapping of the outer salt layer, deposit layer, and electrode, (b) high-magnification mapping of the center of the deposit layer, according to some aspects of the present disclosure.

FIG. 5 illustrates electron micrographs of the graphite electrode deposit layer at increasing magnification (a-d), according to some aspects of the present disclosure.

FIG. 6 illustrates optical micrographs of electrode cross sections (a) approximately −1.8 V steel electrode, (b) and (c) approximately −2.4 V steel electrode, according to some aspects of the present disclosure.

FIG. 7 illustrates electron micrographs of the deposit layer for the steel electrode after reaction at approximately −2.4 V, (a) low magnification overview showing electrode (left) and deposit layer, (b) microstructure close to the electrode, and (c) microstructure far from the electrode, according to some aspects of the present disclosure.

FIG. 8 illustrates electron micrographs of radial cross sections showing extent of carburization at the electrode interface for (a) untreated mild steel control, (b) mild steel heated to approximately 715° C. for approximately 1 h, but not exposed to salt or electrochemical conditions, (c) electrode exposed to salt at approximately −1.8 V for approximately 1 h, and (d) electrode exposed to salt at approximately −2.4 V for approximately 1 h, inset (e) shows high-magnification view of a pearlite grain with fine (less than approximately 1 μm) lamellar spacing in (d), according to some aspects of the present disclosure.

FIG. 9 illustrates quantification of cementite content as a function of distance from the electrode edge for steel electrodes at approximately −2.4 V, approximately −1.8 V, heat treated, and untreated, according to some aspects of the present disclosure.

FIG. 10 illustrates cyclic voltammetry at approximately 10 mV/son mild steel cathodes, with the first and fourth scans shown for clarity, according to some aspects of the present disclosure.

FIG. 11 illustrates (a) powder X-ray diffraction results from the approximately −2.4 V graphite electrode deposit layer; (b) powder X-ray diffraction results from the approximately −2.4 V mild steel electrode deposit layer, according to some aspects of the present disclosure.

FIG. 12 illustrates optical imaging of the steel electrodes after reaction at (a) approximately −1.8 V and (b) approximately −2.4 V, according to some aspects of the present disclosure.

FIG. 13 illustrates EDS mapping of the approximately −2.4 V mild steel deposit layer near the electrode surface, showing the electron image (left) as well as corresponding maps for Fe, C, K, and O, according to some aspects of the present disclosure.

FIG. 14 illustrates Fe—C paraequilibrium binary phase diagram, calculated with FactSage, according to some aspects of the present disclosure.

FIG. 15 illustrates electron micrographs showing representative areas for the electrode exposed to molten salt at approximately −1.8 V for about 1 h (a) at the interface, (b) approximately 100 μm from the interface, (c) approximately 300 μm from the interface, according to some aspects of the present disclosure.

FIG. 16 illustrates electron images showing representative areas for the electrode exposed to molten salt at approximately −2.4 V for approximately 1 h (a) at the interface, (b) approximately 100 μm from the interface, (c) approximately 300 μm from the interface, according to some aspects of the present disclosure.

FIG. 17 illustrates optical micrographs of radial cross sections after etching for (a) approximately −1.8 V mild steel electrode and (b) approximately −2.4 V mild steel electrode, according to some aspects of the present disclosure.

FIG. 18 depicts Raman spectroscopy at varying points on the carbon deposited on a graphite electrode using methods disclosed herein.

FIGS. 19a, 19b, 19c, and 19d depict electron micrographs of carbon deposit layers on graphite electrodes at −2.4V. FIG. 19a depicts flakes; FIG. 19b depicts rod-like structures; and FIG. 19c depicts amorphous phases interspersed with salt. FIG. 19d depicts carbon phases on steel and shows a mixture of rods/whiskers and flakes distributed throughout the deposit. Yellow arrows mark carbon phases and red arrows mark recrystallized salts.

FIG. 20 depicts total electron yield (TEY) C 1s spectra of (green), (red) and (blue). Dashed lines belong to graphite (black) and MSC-30 (orange) standards. Three characteristic regions of the spectra are highlighted, and respective peaks are labelled.

DETAILED DESCRIPTION

The embodiments described herein should not necessarily be construed as limited to addressing any of the particular problems or deficiencies discussed herein. References in the specification to “one embodiment”, “an embodiment”, “an example embodiment”, “some embodiments”, etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

As used herein the term “substantially” is used to indicate that exact values are not necessarily attainable. By way of example, one of ordinary skill in the art will understand that in some chemical reactions 100% conversion of a reactant is possible, yet unlikely. Most of a reactant may be converted to a product and conversion of the reactant may asymptotically approach 100% conversion. So, although from a practical perspective 100% of the reactant is converted, from a technical perspective, a small and sometimes difficult to define amount remains. For this example of a chemical reactant, that amount may be relatively easily defined by the detection limits of the instrument used to test for it. However, in many cases, this amount may not be easily defined, hence the use of the term “substantially”. In some embodiments of the present invention, the term “substantially” is defined as approaching a specific numeric value or target to within 20%, 15%, 10%, 5%, or within 1% of the value or target. In further embodiments of the present invention, the term “substantially” is defined as approaching a specific numeric value or target to within 1%, 0.9%, 0.8%, 0.7%, 0.6%, 0.5%, 0.4%, 0.3%, 0.2%, or 0.1% of the value or target.

As used herein, the term “about” is used to indicate that exact values are not necessarily attainable. Therefore, the term “about” is used to indicate this uncertainty limit. In some embodiments of the present invention, the term “about” is used to indicate an uncertainty limit of less than or equal to ±20%, ±15%, ±10%, ±5%, or ±1% of a specific numeric value or target. In some embodiments of the present invention, the term “about” is used to indicate an uncertainty limit of less than or equal to ±1%, ±0.9%, ±0.8%, ±0.7%, ±0.6%, ±0.5%, ±0.4%, ±0.3%, ±0.2%, or ±0.1% of a specific numeric value or target.

Among other things, the present disclosure relates to a method of electrochemical synthesis of solid carbon, including graphite and other graphitic carbon, in molten salts. In an embodiment, graphitic carbon means various forms of carbon including, but not limited to, graphite, graphene, turbostratic graphene and any carbon characterized by ordered, hybridized hexagonal sheets of carbon. Incumbent graphite synthesis technologies rely on extremely high temperatures to be maintained for extended periods of time. In contrast, the process described herein occurs at approximately 715° C. and carbon deposition occurs in a relatively short period of time. Thus, this process has potential to revolutionize the production of graphite, a critical mineral that is required for applications including lithium-ion batteries and industrial processes. The method of the present disclosure utilizes lithium potassium carbonate (LiKCO₃) as an electrolyte and graphite or mild steel electrodes. The deposition rate is shown to be tunable by applied voltage. Furthermore, deposition of carbon on mild steel causes relatively rapid carburization in the steel microstructure, with a pearlite layer formed at the electrode surface and further carburization observed along the grain boundaries. The process of the present disclosure has potential to substantially increase the efficiency of scrap-based electric arc furnace steelmaking, which accounts for approximately ⅔ of US steelmaking. The observed carburization in the methods of the present disclosure could both increase the carbon content of scrap for optimal processing and exclude copper, a major and problematic contaminant in scrap steel.

FIG. 1 illustrates a simplified electrochemical schematic of the reactor setup and illustration of key reactions, according to some aspects of the present disclosure. In experiments, lithium carbonate (≥99%, Sigma Aldrich) and potassium carbonate (≥99%, Sigma Aldrich) were used as received for the electrolyte. The electrodes used were mild steel rods (AISI 1018, diameter: 2 mm; length: 300 mm, McMaster-Carr), SnO₂ rods (purity: 98.5%; diameter: 10 mm; length: 300 mm, Dyson Industries Ltd.), and conductive graphite rods (diameter: 3.175 mm; length: 300 mm, McMaster-Carr). The reactor setup for experiments included several alumina pieces. Alumina sheaths and crucibles were purchased from AdValue Technology. Single bore alumina tube (diameter: ¼″, length: 12″, AL-T-N1/4-N3/16-12, AdValue Technology), 2 bore hole alumina tube (diameter: ¼″, length: 18″, AL-T2-N25-N062-18, AdValue Technology), 250 ml alumina crucible (AL-1250, AdValue Technology), and 60 ml and 35 ml alumina dishes (AL-4060, AL-4025, AdValue Technology) were among the main pieces used.

For experiments, salts were stored and mixed in a glovebox. Salt was prepared by bulk mixing the lithium and potassium carbonate in a 1:1 molar ratio. Crucibles were pre-weighed outside the glovebox, filled within to approximately ⅞ full, and sealed in airtight bags to prevent air exposure. This is done due to the high fluctuations in measuring mass inside the glovebox. Shortly before use, the filled crucibles were removed from the glovebox and weighed with the mass calculated from the mass of the empty crucible subtracted from the full crucible.

A custom stainless-steel bucket and lid was designed and used in a Thermo-Fisher Scientific CF56622C Lindberg/Blue M™ Crucible Furnace. The experimental setup used in these experiments is shown in FIG. 1. An alumina crucible was filled with the selected salt, approximately 200 g. The electrodes were substantially immersed in the salt with the exposed end wrapped in copper wire, which is threaded through alumina tube sheaths to insulate the wires and prevent shorting through the lid. The alumina sheaths were passed through the lid of the bucket and held with welded Swagelok fittings and PTFE ferrules.

In addition to the electrodes, two closed-end alumina sheaths with type-K thermocouples were inserted into the bucket. One thermocouple is placed in the center and immersed in the salt (TC1) Finally, a ¼″ gas inlet line was attached to a multiple-bore hole alumina tube to pump gas into the salt mixture. A ⅜″ diameter outlet line served as the gas exhaust. All alumina sheaths and tubes were held in place on the bucket lid with ¼″ Swagelok fittings and Teflon ferrules, with a 1/16″ fitting to seal around the Cu wire. Inert gas was pumped into the apparatus for approximately 12 h at a flow rate of approximately 90 sccm to purge the reactor. Leaks were checked using a GL Sciences Inc® Leak Detector LD239.

The Cu wire connecting the electrodes were attached in a 2-electrode configuration with alligator clips to an Autolab Vionic Potentiostat/Galvanostat 3500001080) for electrochemical measurements and data was collected with Intello software. The measured voltages were internally referenced against the oxygen evolution reaction occurring at the anode, which is assumed to be relatively stable in the short time spans of the electrochemical experiments.

In some embodiments, the process of the present disclosure includes heating under inert gas flow at approximately 150 sccm and heats to approximately 200° C. at a rate of approximately 5.83° C./min. This temperature is held for approximately 1 h to release any absorbed water. The furnace then ramps to approximately 800° C. at a rate of approximately 4° C./min and is held there for approximately 1 h to ensure thermal equilibration in the bucket.

In some embodiments, the process next includes measuring the open circuit potential (OCP) for 1 h to monitor the cell. Cyclic voltammetry (CV) is cathodically scanned at 10 mV/s from −1 V to −2.4 V.

In some embodiments, the process next includes performing chronopotentiometry at a substantially set voltage (approximately −1.8 V or approximately −2.4 V) performed for approximately 28 min.

In some embodiments, the process next includes lifting electrodes out of the molten salt by pulling the copper wire through the lid, but are not removed from the bucket completely, instead remaining in the headspace above the molten salt.

In some embodiments, the process next includes cooling the furnace to room temperature at approximately 4° C./min.

The conditions used in experiments described herein are shown in Table 1. All tests were carried out in the same electrolyte (50/50 mol % Li₂CO₃:K₂CO₃), temperature (approximately 715° C.), anode (SnO₂), and duration (approximately 28 min). Two different applied potentials above and below the cathodic limit were utilized for their influence on C deposition: approximately −2.4V and approximately −1.8V. For comparison, a mild steel cathode which was heated to the reaction temperature (referred to herein as “heat-treated”) and an untreated mild steel cathode used as received (referred to herein as “untreated”) were utilized in experiments.

TABLE 1
Electrochemical conditions for carbon deposition
Name	Cathode	Applied Potential (V)
−2.4	Mild Steel	−2.4
−1.8	Mild Steel	−1.8
Heat-treated	Mild Steel	none
Untreated	Mild Steel	none
Graphite	Graphite	−2.4

After the reaction, the mild steel electrodes and attached deposit layer were imaged with optical microscopy, then vacuum infiltrated with liquid epoxy (Buehler Epothin 22) to stabilize porosity. The mounted samples were sectioned radially and mechanically polished to P4000 grit with SiC grit sheets. The polished cross-sections were then ion milled at approximately 5 kV for approximately 15 min to produce a mirror finish. After milling, the polished cross-sections were immersed in an aqueous solution of approximately 10 wt % sodium metabisulfite for approximately 15 s while agitating to etch the surface, revealing carbides and grain boundaries. After etching, the surfaces were washed with isopropanol and air dried. Electron microscopy characterization was performed on a ThermoFisher Nova 650, utilizing both SE and BSE detectors. EDS mapping was performed on the same instrument with an Oxford EDS detector. Optical microscopy was performed with a Keyence 7000.

To quantify carbide content in the polished cross sections, the electron micrographs captured with BSE imaging were converted to binary images in ImageJ to distinguish the bright carbide phase from the darker ferrite phase. The count of white and black pixels was used to determine the carbide area fraction in each micrograph in intervals of approximately 10 μm proceeding inward from the electrode surface.

For graphite electrodes, after the reaction the electrode remained embedded in the salt electrolyte. After cooling to room temperature, the electrode was removed from the salt by dissolving the surrounding salt with approximately 1 M acetic acid. Once removed, the bottom approximately 4 cm of the graphite electrode was cleaved off. This portion of the electrode was soaked in approximately 0.5 M acetic acid for approximately 15 min to remove excess salt around the exterior, then rinsed with DI water and dried for approximately 4 h in an approximately 60° C. oven, then dried for approximately 12 h in a vacuum oven to ensure all moisture is removed. The dried approximately 4 cm electrode segment was further cleaved into approximately 1 cm segments and imaged without further processing.

The electro-carburization process involves electrochemically generating carbon formed by the decomposition of CO₂. This process starts with the dissolution of gaseous CO₂ into the molten salt and reaction with oxide ions to form carbonate, Reaction 1.

These carbonates are then reduced to deposit carbon at the cathode, Reaction 2.

At an inert anode, the generation of O₂ and/or CO₂ can occur, as shown in Reactions 3 and 4.

Overall, the reaction is the decomposition of CO₂:

The electrochemically deposited carbon atoms at the surface of a steel cathode can diffuse into the steel crystal lattice to form Fe₃C. A mixture of Li—K carbonates was adopted for this study due to the advantage of the lower melting point of these mixtures (approximately 503° C.). Therefore, a bath temperature of approximately 715° C. was chosen to increase crystallinity of C deposits while avoiding production of CO. Additionally, Li₂CO₃ may be required in the electrochemical reduction of molten alkali carbonates for carbon formation. Thermodynamically, it prevents the formation of alkali metals such as K and possibly acts as an intermediate for the transformation of CO₂.

Cyclic voltammetry was performed in a 50/50 mol % Li₂CO₃:K₂CO₃ electrolyte at approximately 715° C. in a two-electrode configuration with either a mild steel or graphite cathode and SnO₂ anode. Cathodic scans at approximately 10 mV/s were performed to assess the reduction reactions and are shown in FIG. 2(a). A reduction peak ca approximately −1.6V is present on mild steel cathode, and similar cathodic limits ca approximately −2.0V for both mild steel and graphite cathodes corresponding to electrolyte decomposition.

Chronopotentiometry tests were conducted on steel cathodes by applying either approximately −1.8V or approximately −2.4 V, and on graphite cathode at approximately −2.4V. All tests were conducted for approximately 28 min. The results are shown in FIG. 2(b). In FIG. 2, grey markers indicate the current response to approximately −2.4V on a graphite cathode.

On the graphite electrode, no cathodic peaks are observed prior to the cathodic limit, consistent with the literature on glassy carbon and Pt cathodes. In contrast, the mild steel cathode displays a cathodic peak at approximately −1.6 V. The observation of a cathodic peak prior to the cathodic limit has been attributed to the direct reduction of carbonate ions, followed by alkali metallization as the potential is scanned to more negative values. In this present disclosure, a smaller reduction peak (R2) initially appeared at approximately −1.4 V in the CV scans with a mild steel cathode as shown in FIG. 10, right. However, this peak disappeared with subsequent scans while the peak at approximately −1.6V (R1) grows. This phenomenon is attributed to reactions of transition metals in the mild steel, as is consistent in the literature of Ni and steel cathodes. As C is deposited, the transition metals in the mild steel no longer have access to the electrolyte, leading to the disappearance of this peak and the concurrent growth of the peak at approximately −1.6 V.

The current response is relatively steady in all tested conditions. Applying higher voltage results in higher current, as expected for chemically controlled reactions due to the greater driving force at approximately −2.4 V to drive the kinetics. At the same voltage of approximately −2.4 V, the current is higher with a steel cathode compared to graphite, most likely due to steel's higher reactivity. For the electrodes tested at approximately −1.8 V, just below the cathodic limit, the current rapidly drops off in the first approximately 500 s of impressed voltage before stabilizing near approximately 65 mA. In contrast, the steel electrode subjected to approximately −2.4V shows a stable and slow increase in current from ca. approximately 73 mA to approximately 78 mA after approximately 28 min of constant voltage. With a sufficient supply of

${\mathrm{CO}}_3^{2-}$

to drive the generation of conductive C at the cathode, the surface area effectively increases as the reaction proceeds, resulting in the higher observed current. For the steel electrode at approximately −1.8 V, C deposition was not significant enough to observe the same effect.

Optical microscopy of the cleaved graphite electrode cross section after reaction at approximately −2.4 V is shown in FIG. 3. After the reaction, the approximately 3 mm diameter graphite electrode shows a large (approximately 1 mm) deposit layer. The deposit layer macrostructure consists of a mixture of carbon (dark color) and entrapped salt (white color, red arrows). Near the electrode surface, the carbon deposit shows little to no salt entrapment, but as the deposit grows beyond approximately 200 μm, greater salt incorporation is observed in the dark deposit. This initially salt-deficient layer near the electrode could be due to relatively homogeneous C deposition on the smooth graphite electrode in the initial stages of deposition. As the C surface grows, local variations in the electrolyte composition and different chemistries propagating on surface defects may lead to nonhomogeneous C growth, increased surface roughness, and thus greater salt entrapment. A layer of salt encases the exterior of the deposit layer, likely due to molten salt that remains wetted on the deposit surface after the electrode is removed from the bath at high temperature. Powder XRD analysis of the deposit layer, shown in FIG. 11a, indicates the presence of graphite and amorphous carbon, as well as a significant amount of LiKCO₃, LiOH, and Li₂O. These results correspond to the large volume of salt observed in the deposit layer, with the exact nature of which Li and K compounds are present will depend on the post-processing of the deposit layer.

FIG. 4(a) shows EDS mapping of the graphite electrode cross section macrostructure. As expected from the optical micrographs, the portion of the deposit layer closest to the electrode is carbon rich, while areas further from the electrode are richer in salt, as indicated by the high K content. FIG. 4(b) shows EDS mapping of the deposit layer microstructure, showing the coexistence of three distinct phases (i) carbon-rich (red) assumed to be deposited hard carbon, (ii) potassium-rich (blue) assumed to be LiKCO₃ or K₂CO₃, and (iii) oxygen-rich (green) assumed to be Li₂O or LiOH. These EDS maps indicate that the salt trapping phenomenon occurs at a microscale throughout the deposit, expanding to macroscale after the initial, carbon rich deposit layer is formed. The maps also confirm that the deposit layer contains fully carbonaceous products, not just Li and K compounds.

FIG. 5(a) shows electron micrographs of the carbon-rich area of the deposit layer for the graphite electrode to demonstrate different morphologies of carbon observed. Two distinct phases with different brightness are observed. The dark, flake-like phase (yellow arrows) corresponds to the carbon rich phase observed in FIG. 5(b). The lighter phase (red arrows) corresponds to the K- and O-rich phases, assumed to be recrystallized salt or related compounds (oxides and hydroxides).

The EDS results shown in FIG. 4 and corresponding micrographs shown in FIG. 5 indicate that much of the deposit layer consists of carbon flakes, approximately 5 μm in size, surrounded by microscale deposits of salt that is likely trapped in the growing carbon layer as a liquid, then solidifies during cooling. The powder XRD results indicate that these flakes are likely graphitic.

To assess the applicability of the carbon deposition reaction for steel and iron processing, the same electrochemical reaction was performed using a mild steel electrode rather than graphite. The goal of this experiment is to characterize the carbon deposition, carburization, and electrochemical control for the ubiquitous mild steel that makes up steel scrap.

FIG. 6 shows optical micrographs of the steel electrode after reaction at (a) approximately −1.8 V and (b) approximately −2.4 V. The thickness of the deposit layer is approximately 130 μm for the approximately −1.8 V electrode, but approximately 1.1 mm for the approximately −2.4 V electrode. In the approximately −2.4 V electrode deposit layer, significant salt content (red arrows) can be seen entrapped in the carbon (yellow arrows), and an outer layer of salt encases the entire deposit; these results closely resemble that of the graphite electrode for the same conditions. In the approximately −1.8 V electrode deposit layer, salt (red arrow) is seen with minimal carbon remaining in the deposit layer after cooling to room temperature. FIG. 12 shows side views of the electrodes before sectioning, showing the cylindrical morphology, rough texture, and large difference in deposit layer thickness between the two conditions. These results prove that the chosen applied potential has a strong effect on the amount of carbon deposited, and, by proxy, the amount of salt entrapped in the deposit layer.

FIG. 7 shows microstructural characterization of the deposit layer for the approximately-2.4 V electrode reveals a highly porous structure consisting of hard carbon sheets and high salt content. Much of the salt is concentrated into large deposits (greater than approximately 100 μm), as marked by red arrows in FIG. 7a; these large deposits greatly increase the overall thickness of the deposit. Close to the electrode (FIG. 7b), sheets of carbon are observed, with salt still present throughout. The carbon phase morphology is similar to that observed in the graphite electrode under the same conditions. Far from the electrode (FIG. 7c), the porous microstructure contains larger approximately 10 μm size salt deposits, with a higher portion of spheroidal, amorphous carbon.

EDS mapping shown in FIG. 13 reveals the prevalence of K throughout the deposit layer, indicative of entrapped salt.

Powder X-ray diffraction of the deposit layer supports this as well, with LiKCO₃, Li₂CO₃, and Li₂O identified, along with graphite; these results are shown in FIG. 11b. The broad peak between about 15° and about 30° is indicative of amorphous carbon phases. As compared to the phase composition of the graphite electrode experiment, both contain a large amount of LiKCO₃, Li₂CO₃, amorphous carbon, and Li₂O, and a minor amount of graphite and K₂CO₃·(H₂O). LiOH was observed in the graphite electrode deposit but not the steel electrode deposit, likely as a result of post-reaction processing rather than as an indicator of a difference in the reaction.

Despite a similar overall thickness and salt content of the deposit on steel and graphite (~1 mm, 43%), the macroscopic structure of the deposits vary substantially. Near the graphite electrode surface, the carbon deposit shows little to no salt entrapment, with greater salt incorporation as the deposit grows beyond 200 μm. In contrast, the steel electrode shows radially consistent salt entrapment throughout the deposit, with band-like formations of denser carbon that do not span the entire circumference. This phenomenon could be due to the different surface properties of the electrodes and the relative rates of reaction.

Transition metals in steel initially catalyze the carbon deposition reaction, causing rapid carbon growth (high current of ~0.72 A) which inadvertently entraps salt and increases the surface area. Additionally, the mixture of metals in steel and presence of surface oxides and passivation layers inherently leads to greater surface inhomogeneity, which could result in locally variable carbon growth and greater salt entrapment in the initial stages of deposition on steel compared to the smooth and homogeneous surface of the graphite electrode. Without these catalysts and inhomogeneities, carbon grown directly on graphite is slower, which results in less entrapped salt and initially concentric carbon growth. As the carbon encompasses the surface, the electrochemical reaction transitions from carbon-on-steel or carbon-on-graphite to carbon-on-carbon. At this stage, the deposition reaction becomes substrate-blind and the change in growth rate of carbon is similar on both cathodes, with a ~200 mA shift in current due to the higher surface area of the nonhomogeneous carbon growth on steel. These results highlight the effect that the electrode surface properties have on the quantity and macrostructure of carbon deposited.

The purity of the deposited carbons was evaluated by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES). No Sn was detected in any of the carbon samples, suggesting that the SnO2 anode is a robust choice as an inert counter electrode. Li, K, and Fe were detected on the steel sample. Li and K are likely remnants of entrained salt, and Fe is likely from the steel surface. No metals were detected on the sample grown on graphite.

Raman spectroscopy was conducted to investigate the structural evolution of the deposited carbon as a function of distance from the electrode surface, see FIG. 18. Spectra were collected at multiple points along the cross section of the carbon layer to assess gradients in graphitic order. Entrapped salt residues were evident in both the steel and graphite electrode deposits.

Large broadband fluorescence and background emissions obscured the Raman features of the steel sample at both 633 nm and 785 nm wavelengths. Presence of Fe in the salt deposits as evidenced by the pink color of the salt after testing and ICP results is likely responsible for these interfering effects. In contrast, the lower degree of salt entrapment and absence of Fe in the carbon deposits formed on graphite enabled acquisition of high signal-to-noise spectra, showing the characteristic D (~1350 cm⁻¹) and G (~1582 cm⁻¹) bands. Notably, the relative intensity ratio of D/G is 1.476 on the graphite rod, consistent with graphite electrodes which have been exposed to high temperatures33,34. The relative intensity ratio of D/G remain largely consistent at ~1.0-1.1 across the sampled regions of the deposit. This trend suggests that the structural ordering and defect density of the carbon phase remain relatively uniform throughout the deposition layer, with no significant gradient in carbon quality as the film grows on graphite. These observations indicate that under the applied electrochemical conditions, carbon deposition on graphite proceeds via a steady growth mechanism that preserves the degree of local ordering in the sp² carbon network.

The uniformity in Raman response further supports that once an initial carbon layer is established, subsequent deposition occurs on carbon itself, leading to consistent growth kinetics. Scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDS) was used to investigate the microstructure of carbon deposits. EDS was used to identify grains belonging to carbon vs entrained salt. FIG. 19a-c shows electron micrographs of the denser carbon region near the buried graphite electrode surface, and FIG. 19d shows carbon phases deposited on steel. Two distinct phases with different brightnesses were observed which are attributed to carbon-rich phases of different morphologies (yellow arrow, FIG. 19 a,b) and K- and O-rich phases (red arrow, FIG. 19a). These K and O-rich phases are assumed to be recrystallized salt or related compounds (oxides and hydroxides) which become trapped in the growing carbon layer. The ~5 μm carbon flakes shown in FIG. 3a are indicative of graphitic carbon. Additionally, rod-like (FIG. 19b) and amorphous (FIG. 19c) were observed on the same deposit layer, indicating the variety of carbon structures that are produced. The diversity of structures implies that there are different mechanistic parameters which are affecting the resulting architecture, which is not solely dependent on the distance away from the graphite rod during deposition. The carbon phases on steel (FIG. 19d) similarly show a mixture of rods/whiskers and flakes distributed throughout the deposit.

To further investigate the ordered nature of the carbon deposit, X-ray Absorption Near Edge Structure (XANES) was employed to characterize the carbon deposited on both steel and graphite substrates, as well as in a washed variant of the steel-supported sample. These results are shown in FIG. 20. C K-edge XANES probes the electronic structure and bonding environments in the carbon deposit, providing insight into both the local bonding geometry and long-range ordering of the carbon network as influenced by the deposition substrate and subsequent washing,

All samples exhibit a π* resonance centered at ~285.5 eV, indicative of sp²-hybridized carbon35-38. The sharpness and consistent energy position of this peak across steel electrode, graphite electrode, and washed samples suggests that the local bonding environment is dominated by planar C═C motifs, largely independent of the substrate or processing. The alignment of these features with the π* resonance of graphitic standards confirms reliable energy calibration and supports the conclusion that π*-states are robust against structural disorder at the scale of nearest-neighbor interactions 39,40. Compared to the MSC-30 standard, samples have relatively sharper π* resonance, affirming the higher degree of structural order.

The presence and clarity of the π* peak—together with the absence of spectral signatures associated with sp³-hybridized carbon—suggests that the electrochemical deposition process does not result in amorphous carbon formation. However, the absence of a well-defined long-range order sp2 excitonic peak (D*) at ~291.7 eV in the steel and graphite electrode samples, alongside the emergence of a broad, redshifted σ*-like feature (C) centered at ~290.7 eV, indicates that these materials lack the interlayer coherence characteristic of crystalline graphite. Instead, the XANES features are consistent with the formation of turbostratic graphitic domains—sp²-rich carbon regions with in-plane ordering but rotational or translational disorder between layers. This structural motif reflects a partially ordered form of graphitic carbon that retains π-conjugation locally while lacking long-range stacking order.

In the functionalized carbon region (C—H/COOH) between the π* and σ* resonances, all samples exhibit two characteristic features: a broad shoulder near ~288 eV and a sharper peak at ~289 eV. These features are frequently reported in carbon K-edge spectra of disordered graphitic materials and are often associated with structural disorder or oxygen-containing functionalities. The exact identification of these features requires complementary spectroscopic techniques, but these features were frequently assigned to C—H and COOH groups in the literature.

In the graphite standard, a sharp long-range order sp2 excitonic peak (D*) at ~291.7 eV confirms the expected crystallinity.5,6,15,16 In contrast, the steel and graphite electrode samples display a broadened and redshifted feature (C), consistent with σ*-like transitions into localized states that likely arise from turbostratic stacking, constrained domain sizes, and bond angle disorder. Instead, these samples only form broad σ* sp2 peaks (D). These effects suppress the formation of an excitonic resonance, reflecting a disordered carbon architecture with limited coherence lengths.

These observations from the XANES data delineate a clear structural progression: between crystalline (graphite standard) and short-range ordered MSC-30, samples exhibit partially ordered yet intact sp² bonding (steel and graphite electrode samples), to a partially restored or exposed long-range ordering after washing. The final spectral observation are the residual potassium L-edge features at ~297.5 and ~300.2 eV that corresponds to 2p3/2 and 2p1/2 transitions 50-52. These residual K+ ions from the LiKCO3 appear to be higher in the graphite electrode compared to steel and decreases with the washing process.

In an embodiment, disclosed herein are methods for the rapid electrochemical deposition of turbostratic graphene on mild steel or graphite cathodes and SnO₂ anodes with no additional catalyst. While not fully graphitic, turbostratic carbons possess local ordering and can serve as an indicator of initial graphitization, indicating the utility of this structure in the production of other useful carbon phases such as needle coke. Our process shows tunability from amorphous to ordered carbons.

Steel Electrode Sample

The steel electrode sample shows a notable spectral transformation after going through acid washing to remove surface-bound impurities and disordered phases. The broad, redshifted σ*-like feature (C) at ~290.7 eV diminishes significantly in intensity, while a weak but distinct shoulder (D*) appears at ~291.7 eV—precisely the energy of the long-range order sp2 peak in graphitic carbon 5,6,16. This emergence implies that the washing process either exposes previously buried ordered regions or facilitates a local reorganization of carbon layers. Rather than amorphizing the material, the post-treatment appears to selectively attenuate the contribution of highly disordered components, enhancing the spectral visibility of residual ordered domains.

The electrodes were characterized with metallographic techniques to assess microstructural changes indicative of carbide formation. The nominal temperature of the electrode (approximately 715° C.) falls below the eutectoid temperature (approximately 727° C.) for the transition from ferrite to austenite, so carbides observed are expected to have formed during the reaction, not just during the cooling stage.

Understanding the phase changes expected in the Fe—C system is useful in providing context to microstructural results. The Fe—C binary phase diagram is shown in FIG. 14. Mild steel is composed of α-ferrite, the body-centered cubic phase of Fe; this phase has relatively low solubility for carbon, approximately 0.022 wt %. When pure Fe is heated above approximately 912° C., it transforms from α-ferrite to Υ-austenite, the face-centered cubic phase of Fe. Austenite has much higher solubility for C (approximately 2.14 wt %). If an austenite matrix with significant carbon in solution is slowly cooled back to ferrite, Fe₃C precipitates out of the ferritic matrix as pearlite, a eutectoid phase composed of alternating lamellae of α-ferrite and Fe₃C. The lamellar pearlite structure can coarsen into spheroidal cementite particles, particularly if the lamellae are finely spaced (less than about 1 μm). A similar structure, bainite, is differentiated from pearlite by an acicular rather than lamellar shape for the cementite, and both are observed here. For these experiments, the AISI 1018 mild steel used has a nominal carbon content of about 0.15 wt % to about 0.2 wt %, indicating that some carbide formation is expected as the solubility limit for carbon in ferrite is, at most, approximately 0.02 wt %.

FIG. 8 shows electron micrographs of radial electrode cross sections for untreated, heat treated, approximately −1.8 V, and approximately −2.4 V conditions. Each set of micrograph shows a representative section of the electrode cross section from the surface to approximately 200 μm in, to demonstrate how the microstructure changes with distance from the surface. The high magnification inset in (e) demonstrates the fine pearlitic structure of the pearlite grains in (d).

The untreated mild steel electrode in FIG. 8a does not show any variation in microstructure with respect to distance from the surface, as expected. The grain boundaries are visible after etching, showing approximately 10 μm equiaxed grains. Minor amounts of bainite (white particulate) are observed as well, as expected for the initial approximately 0.15-0.2 wt % C content.

The electrode microstructure after heat treatment at approximately 715° C. for approximately 28 min in a CO₂ environment, but substantially no molten salt exposure or applied potential is shown in FIG. 8b. Like the untreated electrode, no variation in microstructure is observed as a function of radial distance from the electrode edge. The grain boundaries are visible, and significant grain growth is observed as compared to the initial mild steel grain structure. The grains after heat treatment have coarsened to approximately 50 μm grain size as a result of the annealing treatment. The carbide structure is unaltered from the untreated state.

The electrode microstructure after molten salt exposure for approximately 28 min at approximately 715° C. under an applied voltage of about −1.8 V is shown in FIG. 8c. Pearlite colonies, identified by their distinct lamella are observed throughout the microstructure, concentrated most heavily near the surface of the electrode but existing within a dominant ferritic matrix. Moving inward, pearlite grains grow smaller, from approximately 10 μm near the surface to in the range of about 1 μm to about 2 μm when about 200 μm from the surface. The pearlite-rich layer extends approximately 20 μm inwards, after which pearlite grains are observed exclusively along the grain boundaries. From approximately 20 μm to approximately 150 μm inward the grain boundaries are composed entirely of pearlite grains, with no simple grain boundaries observed. Beyond approximately 150 μm inward a mixture of pearlite grains and simple grain boundaries are observed. Additional images of the approximately −1.8 V steel electrode microstructure are shown in FIG. 15. Near the surface there exists a cluster of adjacent 10 μm pearlite colonies seated within a ferritic matrix. 100 μm in, the microstructure consists of large ferritic grains outline with large (10 μm) pearlite colonies. 300 μm in, the microstructure consists of large ferritic grains with sparse pearlite precipitation along the grain boundaries.

The electrode microstructure after molten salt exposure for about 1 h at approximately 715° C. under an applied voltage of approximately −2.4 Vis shown in FIG. 8d. Like the approximately −1.8 V electrode, the approximately −2.4 V electrode shows a higher concentration of pearlite near the surface. Unlike the approximately −1.8 V electrode, however, the coarse pearlite grains observed in the heavily carburized region within approximately 30 μm of the electrode surface exist within a fine pearlite matrix (FIG. 8e) rather than a ferritic matrix, indicating a much higher degree of carburization. The heavily carburized region extends to approximately 40 μm inward, after which the pearlite matrix transitions back to a ferritic matrix. Carburization at the grain boundaries is observed as well, with no simple grain boundaries observed, but pearlite colonies at the grain boundaries were observed. Additional images of the −2.4 V steel electrode microstructure are shown in FIG. 16. Near the surface, the microstructure is heavily carburized, consisting of a mixture of fine and coarse pearlite, with some fully cementite grains (blue arrows) observed as well. Approximately 20 μm in, pearlite is still present but the fine pearlitic matrix changes back to a ferritic matrix. Approximately 200 μm in, large ferrite grains are outlined by coarse pearlite.

The cementite fraction in each electrode condition was quantified using digital image processing based on the high contrast between cementite (light) and ferrite (dark) in electron micrographs. The results of the quantification are shown in FIG. 9. Individual data points represent quantification for individual electrodes at each condition, and the highlighted region indicates the variation observed between individual electrodes for each condition.

As expected from the micrographs in FIG. 8, the approximately −2.4 V condition can produce significantly more carburization than the approximately −1.8 V condition near the surface of the electrode, and both produce more carburization than the heat-treated and untreated electrodes. After the initial high carburization layer near the surface, the approximately −2.4 V electrodes show a drop in cementite fraction at approximately 150 μm, as the carbide structure shifts from a consistent layer to present at grain boundaries. The variation in carbide fraction stems from how many grain boundaries and grain boundary intersections are captured at any given distance from the surface, and from radial variation in the carbide layer thickness around the perimeter of the electrode. These radial variations are shown in optical micrographs of etched radial cross sections in FIG. 17. A thin pearlite layer is observed in FIG. 17(a), with cementite along the grain boundaries throughout the electrode indicated by the black, thick grain boundaries. A thicker pearlite layer is clearly observed in FIG. 17(b), though it varies in width along the perimeter of the electrode; cementite at the grain boundaries is observed throughout the electrode as well.

The difference in carburization behavior between the two voltage conditions is attributed to the rate of carbon deposition on the electrode surface. Once deposited, the diffusion of carbon inward and the precipitation of carbides as either pearlite or cementite is expected to depend on the duration of time held at high temperature, without influence from the electrochemical reaction. Therefore, under the approximately −1.8 V condition it is assumed that all the carbon deposited is consumed by the electrode, and that the rate of consumption of the carbon deposit layer is higher than the rate of deposition. This is supported by post-mortem examination of the −approximately 1.8 V electrode surface, which shows a deposit layer consisting of the salt that remains wetted to the electrode surface when it is removed from the bath, and minimal carbonaceous material observed at the surface. Additionally, radial analysis of both the approximately −2.4 V and approximately −1.8 V electrodes reveal variations in pearlite layer thickness with respect to the radial coordinate. This result indicates that the carbon deposition is not consistent around the circumference of the electrode.

For the approximately −2.4 V electrode, however, the large amount of black, carbonaceous material sets an upper bound for carburization in this system, as there is clearly excess carbon deposition that results in heavier carburization of the electrode interior. Further increasing the deposition rate is not expected to alter the carbide formation within the electrode. In both the approximately −1.8 V and approximately −2.4 V electrodes, pearlite formation occurs not just at the electrode surface, but at all grain boundaries throughout the area of the electrode. This is due to the short-circuit diffusion of carbon along grain boundaries, which allows the grain boundary network to become rapidly saturated with carbon and precipitate carbides much faster than in the bulk ferrite.

Further indication of carbon buildup along grain boundaries can be seen by examining the morphology of pearlite formation near the surface in the approximately −1.8 V electrode in FIG. 8c. The pearlite seen in the cross section is present not directly at the surface boundary, but along the first inner grain boundary near the surface, forming a ring (cylinder in 3D) concentric with the electrode surface but inset by one grain. The grain boundary short-circuit diffusion is relevant, as it indicates the homogeneity with which carbon is distributed throughout the electrode. For an electrode with coarse grains, the carbon distribution is expected to be less homogeneous due to the large distance between grain boundaries, while a fine-grained electrode is expected to show more homogeneous carbon and carbide distribution. This is controlled both by the initial grain structure of the piece, and the amount of time held at temperature before the reaction.

Electrochemical deposition of carbon from molten carbonate salts onto either mild steel or graphite electrodes is demonstrated. The carbon deposition reaction allows for rapid formation of a deposit layer, with growth rate tunable by applied potential.

Overall, in some embodiments, for graphite electrodes, the growth of the deposit layer was slower than that of the steel electrode, though the graphite-electrode deposit layer contained significantly less entrapped salt. For mild steel electrodes, the deposition of carbon on the surface causes rapid carburization of the steel, with pearlite structures observed throughout the cross-section at grain boundaries, and a pearlite layer at the surface of the electrode. In both cases, the rapid deposition of hard carbon is observed. These carbon deposition reactions show promise for efficient production of solid carbon products including graphite. Carburization reaction may increase the efficiency of scrap-based steelmaking. Future work will demonstrate the application of this technique for copper separation from steel scrap.

In conclusion, disclosed herein are methods for the rapid electrochemical deposition of carbon from molten carbonate salts onto either mild steel or graphite electrodes with inert SnO2 anodes and no additional catalyst was demonstrated. Acid washing the carbon deposit revealed turbostratic graphene domains in the carbon deposited on steel, perhaps indicative of the changing nature of the deposit during deposition. With no detectable Sn observed to contaminate the carbon deposit on either electrode, our system is robust and further tuning of the applied potential, time, and temperature will influence the carbon deposition route and resulting morphology.

On both graphite and steel electrodes, the rapid <30 min) deposition of hard carbon is observed, with both amorphous and ordered carbon phases observed. For mild steel electrodes, the deposition of carbon on the surface immediately entrapped salt, which was encased with the growing carbon layer. For graphite electrodes, the growth of the deposit layer was slower than that of the steel electrode and initially formed a salt-free carbon rich layer, with greater salt entrapment as the layer grows. Tuning the carbon deposition rate by applied voltage and surface properties appear to be viable pathways to reduce salt entrapment. The carbon deposition reactions disclosed herein in our system show promise for the production of high-purity ordered solid carbon products including graphite. Understanding their growth during electrodeposition is key towards development of a tightly controlled process optimized for greater graphitic structures, with less entrapped salt.

The foregoing discussion and examples have been presented for purposes of illustration and description. The foregoing is not intended to limit the aspects, embodiments, or configurations to the form or forms disclosed herein. In the foregoing Detailed Description for example, various features of the aspects, embodiments, or configurations are grouped together in one or more embodiments, configurations, or aspects for the purpose of streamlining the disclosure. The features of the aspects, embodiments, or configurations may be combined in alternate aspects, embodiments, or configurations other than those discussed above. This method of disclosure is not to be interpreted as reflecting an intention that the aspects, embodiments, or configurations require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment, configuration, or aspect. While certain aspects of conventional technology have been discussed to facilitate disclosure of some embodiments of the present invention, the Applicants in no way disclaim these technical aspects, and it is contemplated that the claimed invention may encompass one or more of the conventional technical aspects discussed herein. Thus, the following claims are hereby incorporated into this Detailed Description, with each claim standing on its own as a separate aspect, embodiment, or configuration.

Claims

1. A method for making graphitic carbon comprising:

applying a voltage to an electrode at least partially submerged in a molten salt; and

providing a carbon source to the molten salt; and depositing graphitic carbon on the electrode and isolating the graphitic carbon from the electrode.

2. The method of claim 1 wherein the graphitic carbon is selected from the group consisting of graphite, graphene, and turbostratic graphene.

3. The method of claim 1, wherein:

the carbon source comprises at least one of carbon dioxide, biocarbon, petroleum coke, coal, or fossil fuel-derived feedstocks.

4. The method of claim 1, wherein:

the molten salt comprises an electrolyte.

5. The method of claim 4 wherein the electrolyte comprises molten carbonate salts of alkali metals.

6. The method of claim 4, wherein:

the electrolyte comprises at least one of lithium potassium carbonate (LiKCO3) and dipotassium carbonate (K2CO3).

7. The method of claim 1, wherein:

the electrode comprises a graphite electrode or a steel electrode.

8. The method of claim 1, wherein:

the applied voltage ranges from approximately −1.6 V to approximately −2.4 V.

9. The method of claim 1, wherein:

the applying of a voltage is performed for less than an hour.

10. The method of claim 1, wherein:

the applying of a voltage is performed for less than 30 minutes.

11. The method of claim 1, further comprising:

heating the molten salt to a temperature less than approximately 1000° C.

12. The method of claim 1, further comprising:

heating the molten salt to a temperature less than approximately 900° C.

13. The method of claim 1, further comprising:

heating the molten salt to a temperature less than approximately 800° C.

14. The method of claim 1, further comprising:

heating the molten salt to a temperature less than approximately 720° C.

15. A system comprising:

a cathode;

an anode

a molten salt;

a carbon source; and

a voltage source.

16. The system of claim 15, wherein:

the cathode comprises a graphite electrode.

17. The system of claim 15, wherein:

the cathode comprises a SnO2 anode.

18. The system of claim 15, wherein:

the molten salt comprises an electrolyte consisting of molten carbonate salts of alkali metals.

19. The system of claim 15, wherein:

the voltage source is configured to apply a voltage in the range of approximately −1.6 V to approximately −2.4 V.

20. The system of claim 15, wherein:

the carbon source comprises at least one of carbon dioxide, biocarbon, petroleum coke, coal, or fossil fuel-derived feedstocks.