PROCESS FOR THE FACILE ELECTROSYNTHESIS OF GRAPHENE FROM CO2

Abstract

The present invention relates to the production of graphene from CO.sub.2 through electrolysis and exfoliation processes. One embodiment is a method for producing graphene comprising (i) performing electrolysis between an electrolysis anode and an electrolysis cathode in a molten carbonate electrolyte to generate carbon nanomaterial on the cathode, and (ii) electrochemically exfoliating the carbon nanomaterial from a second anode to produce graphene. The exfoliating step produces graphene in high yield than thicker, conventional graphite exfoliation reactions. CO.sub.2 can be the sole reactant used to produce the valuable product as graphene. This can incentivize utilization of CO.sub.2, and unlike alternative products made from CO.sub.2 such as carbon monoxide or other fuels such as methane, use of the graphene product does not release this greenhouse gas back into the atmosphere.

Claims

1-40. (canceled)

41. A method for producing graphene carbon nano-scaffolds comprising: (a) heating a carbonate salt to obtain a molten carbonate electrolyte enriched in non-lithium carbonates; (b) disposing the molten carbonate electrolyte between an electrolysis anode and an electrolysis cathode in a cell, wherein the electrolysis anode and/or the molten carbonate electrolyte optionally further comprises a transition metal nucleation agent; and (c) applying an electrical current to the electrolysis cathode and the electrolysis anode in the cell to electrolyze the carbonate and generate carbon nano-scaffolds, wherein if a transition metal nucleation agent is present, inhibiting activation of the transition metal nucleation agent during step (c).

42. The method of claim 41, wherein the non-lithium salt enrichment decreases the weight percent of electrolytic lithium carbonate to 70% weight percent or less, based upon 100% total weight of carbonate salts in the electrolyte.

43. The method of claim 41, wherein formation of transition metal nucleation sites is inhibited by conducting step (c) at a temperature less than about 700° C.

44. The method of claim 41, wherein the anode does not release a transition metal nucleation agent during the process.

45. The method of claim 41, wherein step (c) is performed at a current density of at least 0.4 A cm.sup.−2.

46. The method of claim 41, wherein the conditions for electrolysis reduce the solubility of one or more transition metal nucleating agents.

47. The method of claim 46, wherein the transition metal nucleating agents for which solubility has been reduced are selected from nickel, chromium, iron, and any combination of any of the foregoing.

48. The method of claim 46, wherein the conditions for reducing the solubility of one or more transition metal nucleating agents during electrolysis include (a) an electrolyte comprising (i) a lithium carbonate and (ii) one or both of sodium carbonate and potassium carbonate, (b) decreasing the electrolysis temperature, (c) decreasing the concentration of lithium in the electrolyte, (d) increasing the electrolysis current density, or (e) any combination of any of the foregoing.

49. The method of claim 41, wherein the electrolyzed carbonate in step (c) is replenished by addition of carbon dioxide.

50. The method of claim 49, wherein the source of the added carbon dioxide is one of air, pressurized CO.sub.2, concentrated CO.sub.2, a power generating industrial process, an iron generating industrial process, a steel generating industrial process, a cement formation process, an ammonia formation industrial process, an aluminum formation industrial process, a manufacturing process, an oven, a smokestack, or an internal combustion engines.

51. The method of claim 41, wherein the electrolysis cathode comprises stainless steel, cast iron, a nickel alloy, a material that resists corrosion in the presence of the molten carbonate electrolyte, or any combination of the foregoing.

52. The method of claim 41, wherein the electrolysis cathode is coated with zinc.

53. The method of claim 41, wherein in step (c), electrical current is applied with stepwise increases.

54. The method of claim 41, wherein the molten carbonate electrolyte comprises an alkali metal carbonate, an alkali earth metal carbonate, or any combination thereof.

55. The method of claim 54, wherein the alkali metal carbonate or alkali earth metal carbonate is lithium carbonate, sodium carbonate, potassium carbonate, rubidium carbonate, cesium carbonate, francium carbonate, beryllium carbonate, magnesium carbonate, calcium carbonate, strontium carbonate, barium carbonate, radium carbonate, or any mixture thereof.

56. The method of claim 41, wherein the molten carbonate electrolyte comprises lithium carbonate.

57. The method of claim 41, wherein the molten carbonate electrolyte further comprises one or more additional oxygen, sulfur, halide, nitrogen or phosphorous containing inorganic salts.

58. The method of claim 41, wherein the method results in a coulombic efficiency of greater than about 80%.

59. The method of claim 41, wherein the method results in a coulombic efficiency of about 100%.

60. The method of claim 41, wherein the electrolysis reaction is performed at a current density of between about 5 and about 1000 mA cm.sup.2.

61. The method of claim 41, wherein step (c) also produces molecular oxygen (O.sub.2).

62. The method of claim 41, wherein transition metal nucleating agents are suppressed by performing the electrolysis in step (c) under conditions which reduce the solubility of one or more transition metal nucleating agents.

63. The method of claim 62, wherein the transition metal nucleating agents for which solubility has been reduced are selected from nickel, chromium, iron, and any combination of any of the foregoing.

64. The method of claim 62, wherein the conditions for reducing the solubility of one or more transition metal nucleating agents during electrolysis include (a) an electrolyte comprising (i) a lithium carbonate and (ii) one or both of sodium carbonate and potassium carbonate, (b) decreasing the electrolysis temperature, (c) decreasing the concentration of lithium in the electrolyte, (d) increasing the electrolysis current density, or (e) any combination of any of the foregoing.

65. The method of claim 41, wherein the method further comprises electrochemically exfoliating the graphene carbon nano-scaffolds to produce graphene.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0095] In the figures, which illustrate, by way of example only, embodiments of the present invention:

[0096] FIG. 1 depicts an exemplary illustration of the method of electrosynthesis of graphene from CO.sub.2 In FIG. 1A, CO.sub.2 from the air or flue gas is electrolytically split to carbon nanoplatelets by molten carbon electrolysis. In FIG. 1B, the carbonate synthesis cathode is placed in a cellulose tube containing e.g., aqueous (NH.sub.4).sub.2SO.sub.4. In FIG. 1C, the cellulose tube is placed in an (NH.sub.4).sub.2SO.sub.4 bath and exfoliated.

[0097] FIG. 2 is a scanning electron microscopy (SEM) image of the electrolysis product formed by splitting CO.sub.2 in molten carbonate in the absence of nickel nucleation and in the presence of zinc. FIG. 2 shows the formation of carbon platelets.

[0098] FIG. 3 shows photographs (FIGS. 3A and 3C, electrodes before and after electrolysis, respectively), SEM (FIGS. 3D and 3E), Raman spectroscopy (FIG. 3F) and X-ray diffraction (XRD) (FIG. 3G) of the electrolysis product formed by splitting CO.sub.2 in molten carbonate, using a zinc coated stainless steel cathode, illustrating electrosynthesis of carbon platelets from CO.sub.2. FIG. 3B depicts the measured cell potential during electrolysis.

[0099] FIG. 4 shows the solubility of Li.sub.2CO.sub.3 in water (top graph) and various aqueous (NH.sub.4).sub.2SO.sub.4 solutions (bottom graph) as a function of temperature.

[0100] FIG. 5 shows tunneling electron microscopy (TEM) (FIGS. 5A and 5B, prior to exfoliation and subsequent to exfoliation, respectively), Raman spectroscopy (FIG. 5C) and atomic force microscopy (ASM) images (FIG. 5D) of a graphene product prepared according to the present invention.

[0101] FIG. 6 is a SEM image of the electrolysis product formed by splitting CO.sub.2 in molten carbonate in the absence of in the absence of a transition metal nucleating agents and in the presence of lithium oxide. FIG. 6 shows the formation of carbon nano-onions.

[0102] FIG. 7 shows TEM images showing a carbon nano-onion product. The bottom of FIG. 7 shows the interspatial graphene layer between the individual CNT walls in the adjacent SEM, and that the distance between 8 walls is 2.841 nm amounting to 0.255 nm between layers.

[0103] FIG. 8 shows SEM images showing a carbon nano-onion product subsequent to extended duration electrolysis.

[0104] FIG. 9 shows SEM images of an electrolysis product produced in various pure or mixed electrolytes.

[0105] FIG. 10 shows the formation of carbon nano-scaffolds. FIG. 10A shows a scheme of an electrolysis cell. FIG. 10B shows the electrolysis electrodes before and after the electrolysis. FIGS. 10C1-10C6 show SEM images of the electrolytic product produced under conditions of a decrease in electrolysis temperature and a decrease in concentration of lithium carbonate.

[0106] FIG. 11 shows SEM images of the electrolytic product produced under high or low current density conditions in various binary carbonate electrolytes at various temperatures.

DETAILED DESCRIPTION OF THE INVENTION

[0107] It will be understood that any range of values described herein is intended to specifically include any intermediate value or sub-range within the given range, and all such intermediate values and sub-ranges are individually and specifically disclosed.

[0108] It will also be understood that the word “a” or “an” is intended to mean “one or more” or “at least one”, and any singular form is intended to include plurals herein.

[0109] It will be further understood that the term “comprise,” including any variation thereof, is intended to be open-ended and means “include, but not limited to,” unless otherwise specifically indicated to the contrary.

[0110] When a list of items is given herein with an “or” before the last item, any one of the listed items or any suitable combination of two or more of the listed items may be selected and used.

[0111] The term “nanomaterial” generally refers to a material (i) having at least one limiting dimension of size less than 1000 nm, but other dimensions in the material can be larger (for example, carbon nanotubes with length much longer than 1000 nanometers are still carbon nanomaterials when their diameter (rather than their length) is less than 1000 nanometers), (ii) where the structure of the material may be nanometer dimension building blocks (e.g., many layers of graphene) repeated to a greater than 1000 nm size, or (iii) composed of walls which have a nanoscopic thickness (even if the diameter of the material is greater than 1000 nanometers).

[0112] The processes described herein include the synthesis of carbon nanomaterials and their subsequent conversion to graphene.

[0113] The present process splits carbon dioxide by electrolysis in molten carbonate. Isotopic .sup.13C tracking may be used to follow the consumption of CO.sub.2, as it is dissolved in molten carbonate and is split by electrolysis to form carbon nanomaterials, such as carbon nanoplatelets. CO.sub.2 dissolution in molten lithium carbonate is exothermic and rapid, which along with heat generated by the electrolysis provides thermal balance during carbon deposition on the cathode. The process (in the absence of a transition metal nucleating agent) where electrolysis is performed with lithium carbonate forms carbon nanomaterials (CNM), oxygen and dissolved lithium oxide:

Electrolysis: Li.sub.2CO.sub.3.fwdarw.C.sub.CNM+O.sub.2+Li.sub.2O  (1a)

[0114] The electrolyte used in the electrolysis step to produce the carbon nanomaterials may be pure lithium carbonate (Li.sub.2CO.sub.3) or may contain lithium carbon with one or more of added oxides, added sodium, calcium, or barium carbonates, or added boron, sulfur, phosphorus or nitrogen dopants, or any combination of any of the foregoing. CO.sub.2 added to the electrolyte dissolves and chemically reacts with lithium oxide to renew and reform Li.sub.2CO.sub.3:

Chemical Dissolution: CO.sub.2+Li.sub.2O.fwdarw.Li.sub.2CO.sub.3  (2)

[0115] In the processes described herein, carbon nanomaterials, such as carbon nanoplatelets, are formed by molten carbonate electrolysis when transition metal nucleating agents (e.g., transition metals other than zinc) are excluded. The processes described herein may be facilitated by increasing the electrolysis current in a step-wise manner prior to the constant current electrolysis:

Electrolysis: Li.sub.2CO.sub.3.fwdarw.C.sub.platelets+O.sub.2+Li.sub.2O  (1b)

[0116] In one embodiment, to avoid formation of carbon nanotubes (CNT), the electrolyte and cathode surface are substantially free or free of transition metal nucleating agents, such as nickel or chromium, which can nucleate CNT formation.

[0117] The carbon nanomaterials, such as carbon platelets, are then converted to graphene by exfoliation:

Exfoliation (DC voltage): C.sub.platelets.fwdarw.C.sub.graphene  (3)

[0118] In addition to carbon nanomaterial (such as carbon platelet) formation, the second product of molten carbonate CO.sub.2 electrolysis in Equation 3 is the evolution of pure oxygen, O.sub.2, during the electrolysis. As illustrated in FIG. 1, the net reaction of Equations 1b, 2 and 3 is CO.sub.2 split by electrolysis into graphene and oxygen:

CO.sub.2.fwdarw.C.sub.graphene+O.sub.2  (4)

[0119] CO.sub.2 electrolysis in molten carbonate production of carbon nanomaterials readily scales upward linearly with the area of the electrolysis electrodes, facilitating larger scale synthesis of graphene. The molten carbonate carbon nanomaterial electrolysis anode is not consumed and emits oxygen. The molten carbonate electrolysis does not consume carbon as a reactant and uses a no-cost oxide as the reactant to be reduced.

[0120] The carbon nanomaterial product resides on the cathode, which therefore may be stacked vertically in a low physical footprint configuration. The carbon nanomaterial molten carbonate electrolysis process can operate under relatively mild conditions (such as 770° C.) in a molten carbonate electrolyte at 0.8 to 2 V potential. The electricity costs per tonne are estimated as $360 compared to the known costs of $602 per tonne for aluminum. These inexpensive costs provide a significant incentive to use the greenhouse gas carbon dioxide as a reactant to produce graphene. The processes described herein provide a useful path forward to help break the anthropogenic carbon cycle to mitigate climate change.

EXAMPLES

Example I

[0121] Small transition metal clusters, including nickel, chromium and others, act as nucleation points to facilitate high yield C2CNT carbon nanotube growth. Zinc, although liquid at molten carbonate temperatures, lowers the energy of the initial carbon deposition. In the absence of a solid transition metal as nucleating agent (nucleating point), galvanized (zinc coated) steel was still shown to be an effective cathode for carbon growth, but CNTs were scarce, comprising <1% of the carbon product. Instead the product, as shown in FIG. 2, is an impure mix of ultra-thin carbon platelets, other carbon nanostructures and amorphous carbon. FIG. 2 shows an SEM image of the washed cathode product from a nickel free, 90 minute, 1 A constant current electrolysis in 730° C. molten Li.sub.2CO.sub.3 with 6 m (6 moles/kg Li.sub.2CO.sub.3) Li.sub.2O (Alfa Aesar 99.5%). The electrolysis used a 5 cm.sup.2 Pt foil anode and a 5 cm.sup.2 0.12 cm diameter coiled galvanized steel wire cathode.

[0122] The noble iridium/platinum anode utilized in this example was purposely selected to inhibit carbon nanotube (CNT) formation. This enhances the observed formation of the desired graphene product by preventing introduction from the anode, migration, reduction and formation of nickel or chromium nucleation sites on the cathode that favor formation of alternative CNT products. However, an iridium, platinum or iridium alloy anode is not a prerequisite for high yield platelet or graphene growth. The inhibition of low levels of nickel migration from a nickel or nickel containing alloy anode or use of a thin film (e.g., between about 10 and about 100,000 nm thick, such as between about 50 and about 10,000 nm thick, or between about 100 and about nm thick) iridium anode is viable. The following references describe thin film iridium deposition: Grushina et al., J. Appl. Chem. USSR, 2015, 1992, 65; Kamegaya et al., Electrochimica Acta, 1995, 40, 889; Ohsaka et al., Int. J. Surface Eng. Coatings 2007, 85, 260; Ohsaka et al., Electrochem, Solid-State Lett., 2010, 13, D65; Shuxin et al., Rare Metal Mat. Eng., 2015, 44, 1816; Lopez et al., Int. J. Electrochem. Sci., 2015, 10, 9933; Allahyarzadeh et al., Surface Rev. Lett., 2016, 23, 1630001; and Sheela et al., Int. J. Surface Eng. Coatings, 2017, 8:5, 191.

[0123] A mixture of nanostructures including a large fraction of platelets forms during the first few minutes (e.g., 5 minutes) of electrolysis, even in the presence of nickel. However, in the presence of nickel with extended electrolysis time (such as, e.g., 15 minutes), the product quickly resolves into carbon nanotubes. This is the case with a wide range of lithiated electrolytes, using a wide range of metal cathodes, including galvanized steel and copper, and over a range of electrolysis temperatures from 730 to 790° C. Higher temperatures, which were not used in this study, increasingly favor the two electron reduction of CO.sub.2 to CO, and by 950° C. the product is pure carbon monoxide.

Example II

[0124] In this example, it is shown that performing the electrolysis in the absence of other transition metal nucleating agents, but in the presence of zinc, carbon nano platelets, rather than carbon nano-onions (CNOs) or carbon nanotubes (CNTs), form. Zinc is present as the surface coating on the (galvanized) steel cathode. The yield of carbon platelets observed in FIG. 2 increases to 70% when the electrolyte is pure Li.sub.2CO.sub.3 rather than 6 m (6 molal) Li.sub.2O, and to over 95% when increasing constant current steps (FIG. 3B) are first applied prior to the constant current. Specifically, in this electrolysis, graphite platelets are grown on a 5 cm.sup.2 galvanized (zinc coated) steel cathode with a 5 cm.sup.2 Pt Ir foil anode in 770° C. Li.sub.2CO.sub.3 when the electrolysis current is increased stepwise for 10 min. at 0.05 and 0.10 A, then 5 min. at 0.2 and 0.4 A followed by a constant of 1 A for 2 hours. These experimental conditions (zinc on the cathode, pure Li.sub.2CO.sub.3 electrolyte, neither Ni nor Cr in the anode, and increasing constant current steps) were chosen to increase the yield of the carbon platelets. Replicate experiments produced similar results of over 95% carbon platelets yield. The 2-hour constant current electrolysis occurs at 0.2 A cm.sup.−2, consuming during the 2-hour electrolysis 0.82 g CO.sub.2 and producing 0.21 g carbon platelets. The potential of the stepped current electrolysis and the electrolysis product are presented in FIG. 4B. The product purity is over 95%. The remainder includes smaller particles, which also contain smaller platelets. X-ray diffraction (XRD) of the product (FIG. 3G) exhibits a sharp peak at 26.3° 2θ, indicative of a high degree of graphitic allotrope crystallinity. Raman spectroscopy (FIG. 3F) and TEM (FIG. 3A), indicates the platelets have a relatively low number (25 to 125) graphene layers. Without wishing to be bound by theory, the inventor theorizes that by starting with fewer graphene layers compared to graphite, these ultrathin platelets electrochemically exfoliate to a higher quality (thinner) graphene for an overall production of graphene from CO.sub.2 by electrolysis and electrochemical exfoliation, in accordance with Equation 5.

[0125] An important feature for the conversion of graphite to graphene is a red shift in the Raman spectrum 2D peak compared with graphite (2720 cm.sup.−1) (see, e.g., Zhou et al., Mat. Lett., 2019, 235, 153). The 2D-band is highly sensitive to the number of graphene layers, with single layer exhibiting a peak at 2679 cm.sup.−1, and 1-4 layers exhibiting a peak at 2698 cm.sup.−1. Even prior to electrochemical exfoliation, the ultrathin carbon platelets produced by molten carbonate synthesis (FIG. 3F) exhibit a significant red shift to 2708 cm.sup.−1. In FIG. 3F, the intensity ratio I.sub.D/I.sub.D′ is 1.3, demonstrating that for the whole range of I.sub.D/I.sub.D′, the defect level is always below the benchmark for graphene boundary defects (I.sub.D/I.sub.D′=3.5). (The ratio I.sub.D/I.sub.D′ represents the intensity ratio for the D peak (1350 cm.sup.−1) and D′ peak (1620 cm.sup.−1).) The ratio of Raman D or 2D to the G peaks are respectively associated with the number of defects and degree of graphitization. In FIG. 3F, the intensity ratio of the Raman I.sub.D/I.sub.G peak is a low (0.4), and that of Raman I.sub.2D/I.sub.G is 0.6, which both indicate a small quantity of defects. (The ratio I.sub.D/I.sub.G represents the intensity ratio for the D peak (1350 cm.sup.−1) and G peak (1583 cm.sup.−1).)

Example III

[0126] In this example, it is shown that lithium carbonate entrapped with the carbon platelets produced during the electrolysis described in Example II can be readily removed by dissolution in aqueous ammonium sulfate solutions.

[0127] Unlike Na.sub.2CO.sub.3 and K.sub.2CO.sub.3 which are highly soluble in water, Li.sub.2CO.sub.3 has a low solubility (30.6, 113 and 1.2 g per 100 g H.sub.2O, respectively, at 25° C.). Aqueous ammonium sulfate is one of the few media in which Li.sub.2CO.sub.3 solubility is enhanced.

[0128] An aqueous medium was investigated capable of both sustaining exfoliation and conducive to the dissolution of excess lithium carbonate electrolyte that congealed on the cathode during the molten lithium carbon electrolytic production and extraction and cooling of the cathode containing the carbon product. These solubility measurements are summarized below. Solubility is measured both by incremental addition of lithium carbonate (Alfa Aesar) to water, or ammonium sulfate (Alfa Aesar) in water until observation of excess lithium carbonate, and by dilution of excess lithium carbonate until observation of complete dissolution.

[0129] Interestingly, whereas the aqueous solubility of sodium and potassium carbonate are high (30.6, and 113 per 100 g H.sub.2O respectively at 25° C.) and increase with temperature (43.9/46, and 140/156 g H.sub.2O, respectively, at 80/100° C.), the measured aqueous solubility of lithium carbonate is low and decreases with increasing temperature, as shown in the top trace of FIG. 4. The aqueous solubility of lithium carbonate (1.2 g per 100 g H.sub.2O at 25° C.) is low compared to the aqueous solubilities of lithium chloride and lithium bromide (18.0 g and 17.5 per 100 g H.sub.2O respectively at 25° C.), and increases with temperature (to 112/128, and 245/266 g H.sub.2O, respectively, at 80/100° C.).

[0130] Next, the dissolution of ammonium sulfate in water (without lithium carbonate) was verified both at room temperature and approaching the solution boiling point. See Table 1. These measurements were conducted to verify dissolution, not to establish ammonium sulfate solubility limits, which are estimated at 15% to 20% higher than the observed maximum dissolution at each temperature. The solubility, as measured mass (grams), of lithium carbonate soluble in 100 ml of either 1.07, 2.33, 4.06 or 6.64 molal (NH.sub.4).sub.2SO.sub.4 is presented in the lower trace of FIG. 4. The 100° C. and 108.9° C. data in the lower trace of FIG. 5 are the measured solubility limits of lithium carbonate respectively in 6.45 or 6.64 molal ammonium sulfate. Increasing concentrations of aqueous ammonium sulfate considerably enhances lithium carbonate solubility.

TABLE-US-00001 TABLE 1 Dissolution of Aqueous Ammonium Sulfate Solutions as a Function of Temperature (NH.sub.4).sub.2SO.sub.4 in water C C C (mol/ (mol/ (mol/ H.sub.2O (NH.sub.4).sub.2SO.sub.4 per L per kg per kg Temperature (g) (g) solution) solution) H.sub.2O) 25° C. 93.3 13.2 1 0.94 1.07 25° C. 85.7 26.4 2 1.78 2.33 25° C. 77.1 49.5 3.75 2.7 4.56 100° C. 77.1 65.6 / 3.48 6.45 (64.3; 65.2; 65.6) 108.5° C. 77.1 67.5 / 3.54 6.64 (66.2; 67.3; 67.5)

Example IV

[0131] In this example, it is shown that the carbon platelets formed in Example II are converted to graphene by electrochemical exfoliation.

[0132] Securing the electrochemical exfoliation electrode within a cellulose dialysis membrane can isolate the graphene product from the bulk electrolyte. The electrode within a cellulose membrane assembly is used as the anode in a two-compartment electrochemical cell, but rather than using graphite, using the cooled cathode, unwashed (carbon nanoplatelet) cathode in 0.1 M (NH.sub.4).sub.2SO.sub.4 as shown in FIG. 1C. Specifically, the carbonate synthesis cathode containing product is cooled and placed in a cellulose tube containing aqueous 0.1 M (NH.sub.4).sub.2SO.sub.4. The cellulose tube is an inexpensive premium commercial cellulose dialysis membrane, (see, e.g., https://www.amazon.com/s?k=Premium-Dialysis-Tubing-Regenerated-Cellulose) listed as a cutoff of 12-14 kdals, equivalent to 1 to 2 nm pore size. As shown in FIG. 1C, the cellulose tube is placed in an 0.1 M (NH.sub.4).sub.2SO.sub.4 bath with a counter electrode. DC voltage is then applied that generates gas bursts between the graphene layers, exfoliating the thin platelets and producing graphene. As graphene layers are peeled, the cellulose traps them within the anode compartment.

[0133] Before exfoliation, the platelets range from 25 to 125 graphene layers as measured by TEM (see, e.g., FIG. 5A). This is consistent with the measured Raman spectrum 2D peak (graphite red shifted) at 2708 cm.sup.−1. After exfoliation, the lateral dimensions of the exfoliated layers are 3 to 8 μm, as measured by SEM (FIG. 5B). After exfoliation, the product is filtered, rinsed and freeze dried to remove water, then analyzed by TEM, atomic force microscopy (AFM), and Raman spectroscopy. The exfoliation product yield is 83% by mass of the original carbon platelets. The product yield would likely rise with longer exfoliation times (such as more than 10 hours).

[0134] Raman spectra of sample carbon nano-platelets produced by the C2CNT technique is shown in FIG. 5C top trace and of a sample graphene produced the C2CNT technique in FIG. 5C bottom trace. The presence of the D′-band is indicative of the layered single and multiple (platelet) graphene layers, and the left shift of the 2-D band indicates the thin graphene layer.

[0135] An important feature for the conversion of graphite to graphene is a red shift in the Raman spectrum 2D peak compared with graphite (2720 cm.sup.−1) (see, e.g., Zhou et al., Mat. Lett., 2019, 235, 153). The 2D-band is highly sensitive to the number of graphene layers, with single layer exhibiting a peak at 2679 cm.sup.−1, and 1-4 layers exhibiting a peak at 2698 cm.sup.−1. Even prior to electrochemical exfoliation, the ultrathin carbon platelets produced by molten carbonate synthesis (FIG. 3F) exhibit a significant red shift to 2708 cm.sup.−1. In FIG. 3F, the intensity ratio I.sub.D/I.sub.D′ is 1.3, demonstrating that for the whole range of I.sub.D/I.sub.D′, the defect level is always below the benchmark for graphene boundary defects (I.sub.D/I.sub.D′=3.5). (The ratio I.sub.D/I.sub.D′ represents the intensity ratio for the D peak (1350 cm.sup.−1) and D′ peak (1620 cm.sup.−1).) The ratio of Raman D or 2D to the G peaks are respectively associated with the number of defects and degree of graphitization. In FIG. 3F, the intensity ratio of the Raman I.sub.D/I.sub.G peak is a low (0.4), and that of Raman I.sub.2D/I.sub.G is 0.6, which both indicate a small quantity of defects. (The ratio I.sub.D/I.sub.G represents the intensity ratio for the D peak (1350 cm.sup.−1) and G peak (1583 cm.sup.−1).

[0136] Raman spectra of sample carbon nano-platelets produced by the process described herein is shown in the FIG. 5C bottom and compared to the Raman spectra of the sample graphene produced the process described herein in. The presence of the D′-band (1620 cm.sup.−1) is indicative of the layered single and multiple (platelet) graphene layers, and the left shift of the 2-D band indicates the thin graphene layer Raman spectra of sample carbon nano-platelets produced by the process described herein is shown in the FIG. 5C bottom and compared to the Raman spectra of the sample graphene produced the process described herein in. The presence of the D′-band (1620 cm.sup.−1) is indicative of the layered single and multiple (platelet) graphene layers, and the left shift of the 2-D band indicates the thin graphene layer.

[0137] In FIG. 5C, the Raman 2D peak exhibits a significant red shift from 2708 cm.sup.−1 to 2690 cm.sup.−1 from platelets (pre-exfoliation) to graphene (post-exfoliation) product. Both the platelets (pre-exfoliation) and graphene (post-exfoliation) are red shifted from graphite (2720 cm.sup.−1). This shift to 2690 cm.sup.−1 is indicative of graphene ranging from to 1 to 5 graphene layers thick. Edge TEM cross section of the exfoliation product also exhibits graphene ranging from 1 layer (shown in the inset to FIG. 5B) to 5 layers thick. This is verified by AFM (see FIG. 5D). Dispersion of the graphene product for AFM characterization remains a challenge. Sonication and freeze drying effectively disperses the product, but is overly aggressive and converts the graphene from a continuous flake to “swiss cheese” like, which has the benefit of providing extra locations for depth determination (see FIG. 5D). For comparison, using graphite foil as the exfoliating reactant, rather than the molten carbonate synthesized carbon nanoplatelets, in the same experimental configuration produces multi-layered graphene that is approximately 5 fold thicker, and ranges from 6 to 25 graphene layers thick, that exhibits a Raman 2D-band peak at ˜2703 cm.sup.−1, rather than 2690 cm.sup.−1 observed for the carbon nanoplatelet exfoliated product of Example II.

[0138] It is expected that the graphene products prepared by the processes described herein may provide improved structural materials. For example, it was observed that a key measurable characteristic correlated to strength is a low defect ratio as measured by the ratio of the ordered (G peak (1583 cm.sup.−1), reflecting the cylindrical planar sp.sup.2 bonding amongst carbons) as compared to disorder (D peak (1350 cm.sup.−1), reflecting the out of plane sp.sup.a tetrahedral bonding amongst carbons) in the Raman spectra.

[0139] Raman spectroscopy of the graphene products prepared according to the processes described herein indicates that the exfoliation product exhibits increased defects compared to thicker pre-exfoliation platelets formed during electrolysis in molten carbonate, but that the defect level remains low and within tolerated levels for graphene. From FIG. 6C, peak ratios for graphene are compared to ratios for the platelets: the I.sub.D/I.sub.D′ is 1.5 (for the graphene product, compared to 1.3 for the nano-platelets), again demonstrating that for the whole range of I.sub.D/I.sub.D′ the defect level is always below the benchmark for graphene boundary defect ratio of I.sub.D/I.sub.D′=3.5. The intensity ratio of the Raman I.sub.D/I.sub.G peak is 0.64 (for the graphene product, compared to 0.4 for the nano-platelets) and that of Raman I.sub.2D/I.sub.G is 0.70 (for the graphene product, compared to 0.6 for the nano-platelets), which both indicate a small amount of defects.

[0140] The majority of the applied exfoliation voltage is lost through resistance drop over the 0.1 M ammonium sulfate solution. This may be avoided by placing the electrodes closer together and/or higher ionic strength to lower energy requirements. The temperature can be increased and the cellulose membrane can also be modified to minimize the voltage drop and also increase the sustainable current density (and rate of exfoliation).

Example V

[0141] The processes and systems described herein can also be modified and used to produce other carbon nanomaterials (CNMs), including graphene, nano-onions, nano-platelets, nano-scaffolds and helical carbon nanotubes. It is observed that each of these CNMs exhibit unusual and valuable physical chemical properties, such as, for example, lubrication (nano-onions), batteries (graphene) and environmental sorbents (nano carbon aerogels) prior to addition to structure materials, and enhanced properties including improved electrical conductivity and sensing ability for CNM-structural material composites. In each case, the product may be synthesized to a high coulombic efficiency of over 95%, and in most cases the product had a purity over 95%.

Example VI

[0142] In this example, it is shown that performing the electrolysis in the absence of a nickel and the near exclusion of any other impurity level transition metal nucleating agents, and in the absence of a stepwise current increase, but in the presence of lithium oxide, which can serve to decrease solubility of any impurity presence of other transition metals, results in the formation of another graphene based morphology consisting of concentric spherical layers of graphene and resulting in a high yield of carbon nano-onions (CNOs), rather than the carbon nano platelets comprising two planar layered graphene as observed in Example II. Zinc is present as the surface coating on the (galvanized) steel cathode. The yield of carbon nano-onions shown in FIG. 6 is over 95%. Applications for inexpensive CNOs include supercapacitors, battery anodes, and solid-lubricants. The geologic (graphite-like durability) stability of graphene allotrope carbon materials may provide a long-term repository to store atmospheric CO.sub.2. SEM, EDS, and TEM characterization provides fundamental evidence of the high yield and purity of the CNO synthesis. Specifically, in this electrolysis, highly uniform carbon spheroids are grown on a 5 cm.sup.2 galvanized (zinc coated) steel cathode with a 5 cm.sup.2 Pt Ir foil anode in 770° C. Li.sub.2CO.sub.3 containing 5.9 molal Li.sub.2O when the electrolysis current is held constant at 1 A (0.2 A/cm.sup.2) for 1.5 hours. As measured by EDS, the carbon content of the product is over 99%, the purity of carbon spheroids in the product is over 95%, and the coulombic efficiency of the electrolysis is over 95%. FIG. 6A shows an SEM trace of the product following 5 minutes, 15 minutes or 90 minutes of electrolysis. As can be seen, the distinct carbon spheroid shape is evident even with an electrolysis duration of 15 minutes or less. FIG. 6C presents an overview (lower magnification SEM) of the various syntheses presented (at higher magnification) in FIG. 6B. In each case, the product is highly uniform diameter carbon spheroids. Each of the spheroids in FIG. 6B is in turn formed from clusters of nano-onions.

[0143] FIG. 7 shows a TEM of the carbon nano-onion (CNO) product after 30 minutes of electrolysis. The distinctive concentric, shell morphology of carbon nano-onions with a 0.35 nm interlayer separation typical of layered graphitic structures is evident. Not shown in the figure is that the separated, as well as individual bundled nano-onions in the spheroids, have an increasing average diameter with increasing electrolysis time, as measured by ImageJ SEM automated optical counting software. Respectively after 5, 30, and 90 minutes of electrolysis, the individual CNOs have an increasing diameter of 38±10 nm, 66±6 nm and 96±2 nm, while the spheroids (bundled nano-onions) have a combined ten-fold higher respective diameter of 400, 600, and 900 nm, as seen in FIG. 6B. While the short duration (5 minutes) electrolysis formed nano-onions have a distinctive size, unlike the longer duration syntheses, the product after 5 minutes of electrolysis does not yet exhibit the distinctive concentric spherical graphene shells evident in FIG. 7.

[0144] As seen in the SEM of FIG. 8A, extended electrolysis (15 hours, rather than 1.5 hours), at lower current density (0.1, rather than 0.2, A/cm.sup.2) produces more of the carbon nano-onion product, but not a significantly larger size of the carbon nano-onion product.

[0145] The SEM traces shown in FIG. 8B depict the product of a procedure in which carbon nano-onions are formed even in the presence of a transition metal which has been inhibited from promoting carbon nucleation. Generally, in an aged lithium carbonate electrolyte a high purity, uniform CNT product is obtained during an electrolysis at a controlled temperature in the 700° C. range, and the degree to which the carbon nanotube product is tangled or straight, long or short, or thick or thin can be controlled by additives to the lithium carbonate electrolyte, current density, electrolysis duration, and choice of anode or cathode material. However, when the electrolyte is not aged, the product can be a partial or pure carbon nano-onion product instead. Aging refers to allowing the electrolyte to sit in a molten state for a period of several hours to several days prior to use. Subsequent to initiation of an electrolysis in a freshly melted solution, it is observed there is a time, for example one hour, before CO.sub.2 is fully absorbed in the electrolyte. After that period, CO.sub.2 is fully absorbed up to a rate equivalent to the 4 Faraday per mole CO.sub.2 of the constant current applied in the electrolysis. It is observed that the activation period for CO.sub.2 to be absorbed during the electrolysis start-up can be shortened by 2 to 3-fold when Li.sub.2O has been added to the lithium carbonate electrolyte. This period of time appears to correlate with the necessary time for the molten carbonate to achieve a steady state concentration of Li.sub.2O, for example in accord with the equilibrium reaction:

Li.sub.2CO.sub.3 CO.sub.2+Li.sub.2O

[0146] Without wishing to be being bound by any theory, it is proposed that transition metal nucleation of carbon nanotube growth is inhibited during this initiation period of electrolyte activation. Specifically, an electrolysis is conducted in freshly melted 770° C. molten Li.sub.2CO.sub.3 using a Muntz brass cathode and Inconel 718 anode both with active area of 2450 cm.sup.2. The electrolysis is conducted at 0.2 A/cm.sup.2 for a duration of 16 hours. As shown in FIG. 8B, the washed cathode product is pure carbon nano-onions without any evidence of carbon nano-tubes.

Example VII

[0147] In this example, it is shown that performing the electrolysis in a high concentration sodium or potassium molten carbonate electrolyte forms an alternative graphene product, carbon nano-scaffolds. Rather than a flat, multilayered graphene platelet morphology, carbon nano-scaffolds consist of a morphology in which multilayered graphene is stacked at sharp angles in an open structure, This open structure is not only aesthetically distinct, but exposes a larger surface area of graphene, which has the potential to increase activity in graphene capacitor, battery, EMF shielding and catalytic applications. Furthermore, the conditions of carbon nano-scaffold growth are distinctive from the platelet growth conditions described above. Specifically, unlike the avoidance of transition metals to prevent competitive growth of an alternative carbon nanotube product, here (i) transition metal ions are permitted, for example as introduced by the anode, and the molten carbonate CO.sub.2 electrolysis is conducted in (ii) electrolytes and/or at (iii) temperature conditions that are specifically not conducive to carbon nanotube (CNT) growth.

[0148] It has been shown (see, e.g., Wu et al., Carbon., 2016, 106, 208) that temperatures greater than 700° C. are more conducive to CNT growth during molten carbonate electrolysis. Here, it is also demonstrated that electrolytes with an increasing fraction of Na.sub.2CO.sub.3 or K.sub.2CO.sub.3 in a mixed Li.sub.2CO.sub.3 electrolysis are less conducive to CNT growth even in the presence of nucleating transition metals. FIG. 9 shows SEM of the electrolysis product in various mixed electrolytes compared to that in FIG. 9A conducted in a pure, 24 hour aged, 770° C. Li.sub.2CO.sub.3 electrolyte subsequent to a 5 hour electrolysis. Each of the electrolysis reactions was conducted at a current density of 0.2 A/cm.sup.2 with a cathode of Muntz Brass (an alloy of 60% Cu and 40% Zn) and an anode of Inconel 718 (an alloy of 50-55% Ni, 17-21% Cr, 2, 4.75-5.5% Nb&Ta, 2.8-3.3% Mo, the remainder Fe and low concentrations of Ti, Co, Al, Mn, Cu, Si and C). The addition of 8% LiBO.sub.2 to the electrolyte further improves the morphology, uniformity and purity of the carbon nanotube product. For example, addition of 8% LiBO.sub.2 to the pure Li.sub.2CO.sub.3 increased the aspect ratio (length to diameter) of the CNT product (not shown), and this LiBO.sub.2 was added to each of the mixed electrolytes to improve the lower quality of the CNT product. (As discussed below, H.sub.3BO.sub.3 can be partially or completed substituted for LiBO.sub.2 after water is allowed to leave the system.) The scale bars are 50 μm for FIGS. 9A and 9F, 20 μm for FIG. 9D, and 10 μm for FIGS. 9B, 9C and 9E. As can be seen by comparing FIG. 9A to FIG. 9F at the same scale, there are no CNTs readily observed in the 60% Na.sub.2CO.sub.3/40% Li.sub.2CO.sub.3 electrolysis product, while the product is highly pure CNTs in the 100% Li.sub.2CO.sub.3 electrolysis product. The 10% or 20% Na.sub.2CO.sub.3 electrolysis products contain over 90% CNT, while 30% Na.sub.2CO.sub.3 (not shown), and 50% Na.sub.2CO.sub.3 exhibit a diminishing yield of CNTs and an increasing fraction of carbon nanospheres and carbon platelets. CNT aspect ratio decreases and the diameter increases with increasing Na.sub.2CO.sub.3 percentage in the electrolyte (10% Na.sub.2CO.sub.3: ˜80 nm, 20% % Na.sub.2CO.sub.3: ˜100 nm, 30% % Na.sub.2CO.sub.3: ˜200 nm, 50% % Na.sub.2CO.sub.3: ˜1 μm). For the 20 wt % K.sub.2CO.sub.3 in Li.sub.2CO.sub.3, SEM shown FIG. 9D and 20 wt % K.sub.2CO.sub.3 (not shown) electrolyses, the loss of aspect ratio drop in CNT purity occurs more rapidly with increasing K.sub.2CO.sub.3 weight fraction than the electrosynthesis with increasing Na.sub.2CO.sub.3 fraction. Energy-dispersive X-ray spectroscopy (EDS) tests were employed to probe the elemental analysis of products from the mixed electrolyte electrolyses. EDS of both the 20% Na.sub.2CO.sub.3 and 20% K.sub.2CO.sub.3 samples are 100% carbon, while the 50% Na.sub.2CO.sub.3 and 50% K.sub.2CO.sub.3 spectra are respectively 97.0% carbon (and 3.0% Na) and 97.8% carbon (and 2.2% K); boron in the CNTs is below the limits of EDS detection. The calculated thermodynamic potential for the reduction of the alkali carbonates increases in the order E.sub.Li2CO3<E.sub.Na2CO3<E.sub.K2CO3. The higher voltage of an increasing concentration of the latter salts would increase the possibility for reduction of the alkali cation to the alkali metal, rather than the desired reduction of carbonate to carbon. The coulombic efficiencies, comparing the mass of the product to the applied 4e.sup.− per mole of charge, approach 100% (98-100%) for the three cases of 100% Li.sub.2CO.sub.3, 10% Na.sub.2CO.sub.3, and 20% Na.sub.2CO.sub.3 electrolyte experiments. Coulombic efficiency is still high, but decreased in binary lithium carbon electrolytes containing over 20% of sodium or potassium carbonate. For example, the coulombic electrolysis efficiency drops from 95% for 30% Na.sub.2CO.sub.3 electrolyte to 93% 50% Na.sub.2CO.sub.3 electrolyte, and to 90% for the 60% Na.sub.2CO.sub.3 electrolyte. Carbonate electrolysis is decreasingly conducive to a CNT product in electrolytes containing. >20 wt % Na.sub.2CO.sub.3 or >20 wt % K.sub.2CO.sub.3.

[0149] In FIG. 10 is shown the distinctive carbon nano-scaffold product when the electrolysis is conducted at 670° C., rather than 770° C., in a similar 50% Na.sub.2CO.sub.3/50% Li.sub.2CO.sub.3 electrolyte. While transition metal elements can again be release from the Inconel 718 anode, and while the Muntz brass cathode is comprised of copper and zinc, there is no evidence that the carbon nano-scaffold growth is based transition metal nucleation. In the electrolyte 10 wt % H.sub.3BO.sub.3, rather than LiBO.sub.2, was added as a cost saving measure. H.sub.3BO.sub.3 can be partially or completed substituted for LiBO.sub.2 after water is allowed to leave the system. A scheme of the electrolysis cell is shown in FIG. 10A, and the electrolysis electrodes before and after the electrolysis in FIG. 10B. SEM images of the product is shown in FIG. 10 C1-C6 with various magnifications. In total the electrolyte consisted of 250 g of Na.sub.2CO.sub.3, 250 g of Li.sub.2CO.sub.3, and 50 g of H.sub.3BO.sub.3. The electrolysis was conducted at 670° C. for 4.0 hours at a constant current of 5 A with 5 by 5 cm electrodes. Voltage throughout the electrolysis was consistently 2.0 V, and over 85% of theoretically calculated CO.sub.2 was converted to carbon. Over 80% of the product was the unusual carbon nano-scaffold morphology. The morphology consists of a series of asymmetric 50 to 200 nm thick flat multilayer graphene platelets 2 to 20 μm long oriented in a 3D neoplasticism-like geometry.

[0150] FIG. 11 shows SEM of the electrolytic product produced under high (panels D through F) and low (panels) G or current density conditions in various binary carbonate electrolytes at various temperatures. FIGS. 11D-11F show carbon products with a higher current density (0.4 A/cm.sup.2), at a range of temperatures, with a different anode, Nichrome C (61% Ni, 15% Cr, 24% Fe, the same cathode, and without any borate additive. As seen in FIGS. 11D and 11E1, there is a significant carbon nano-scaffold product even at the higher temperature of 750° C. At this temperature and current density, the product of the 30% Na.sub.2CO.sub.3 electrolysis large proportions of both carbon nano-scaffolds and carbon nano-onions. Not shown is that carbon nano-scaffolds are also observed in a 70 wt % Na.sub.2CO.sub.3 electrolyte, but the structures are smaller and are surrounded by amorphous carbon. At this temperature and current density, as seen in FIGS. 11E1 and 11E2, the product of the 30% K.sub.2CO.sub.3 electrolysis consists mainly of carbon nano-scaffolds and ˜10% very thick carbon nanotubes. EDS verifies that the carbon nano-scaffold structures are largely carbon (98.3%) with a small amount of potassium (1.7%). The carbon nano-scaffold is observed at 50 wt % K.sub.2CO.sub.3 (not shown), but as seen in FIGS. 11F1-11F3, carbon nano-scaffolds are not observed in electrolytes with high wt % of K.sub.2CO.sub.3 (70% K.sub.2CO.sub.3/30% Li.sub.2CO.sub.3) In this electrolyte, at 570° C. the F1 panel product consists of small rounded, carbon assemblies, at 650° C. the F2 panel product consists of coral-like carbon assemblies, and at 750° C. the F3 panel product consists of larger, but less defined, coral-like carbon structures. Carbonate electrolysis is conducive to a carbon nano-scaffold product in electrolytes containing 30 to 70 wt % Na.sub.2CO.sub.3 or 30 to 50 wt % K.sub.2CO.sub.3 at 650° C. or higher (e.g., 750° C. or higher). While transition metal elements can be included in the electrolysis system that produces the nano-scaffold product, there is no evidence that the carbon nano-scaffold growth is based on transition metal nucleation. The inset of panel 11D shows that with the high current density of 0.4 A/cm.sup.2 in a 60/40 wt % Na.sub.2/Li.sub.2CO.sub.3 electrolyte, the nano-scaffold morphology is still observed when the temperature is decreased to 660° C. Carbon nano-scaffolds can also synthesized at a low current density of 0.1 A/cm.sup.2 when the temperature is decreased further to 570° C. as shown in FIG. 11 panel G, although the cross sectional width of each scaffold unit is approximately 3-fold smaller than in Figure C1-C6 when synthesized at high current density (0.4 A/cm.sup.2), higher temperature (670° C.) and with more lithium carbonate (50%) in the electrolyte.

[0151] Of course, the above described embodiments are intended to be illustrative only and in no way limiting. The described embodiments are susceptible to many modifications of form, arrangement of parts, details and order of operation. The invention, rather, is intended to encompass all such modification within its scope, as defined by the claims.