LOW-TEMPERATURE METHOD FOR ELECTROCHEMICAL CONVERSION OF CARBON DIOXIDE TO CARBON

Abstract

A method for converting carbon dioxide to carbon (e.g., graphite, graphene, or amorphous porous carbon), the method comprising passing the carbon dioxide through a molten anhydrous salt maintained at a temperature within a range of 400 C.-800 C. while the molten anhydrous salt is in contact with a cathode and an anode that are electrically interconnected to impart a voltage to the molten anhydrous salt, wherein the cathode has a metal composition comprising at least one of nickel, iron, and cobalt, wherein the voltage is within a range of 2 V to 3.5 V, and wherein the temperature and voltage are applied for a period of time of 5 minutes to 10 hours (or, e.g., 0.1-10 or 1-10 hours) to result in conversion of the carbon dioxide to carbon.

Claims

1. A method for converting carbon dioxide to carbon, the method comprising passing the carbon dioxide through a molten anhydrous salt maintained at a temperature within a range of 400 C.-800 C. while the molten anhydrous salt is in contact with a cathode and an anode that are electrically interconnected to impart a voltage to the molten anhydrous salt, wherein the cathode has a metal composition comprising at least one of nickel, iron, and cobalt, wherein the voltage is within a range of 2 V to 3.5 V, and wherein the temperature and voltage are applied for a period of time of 5 minutes to 10 hours to result in conversion of the carbon dioxide to carbon.

2. The method of claim 1, wherein the molten anhydrous salt comprises a lithium salt or lithium oxide and at least one salt selected from non-lithium alkali metal salts and alkaline earth metal salts, provided that the molten anhydrous salt has a melting point within a range of 400 C.-800 C.

3. The method of claim 1, wherein the molten anhydrous salt comprises a lithium salt or lithium oxide and at least one salt selected from non-lithium alkali metal salts and alkaline earth metal salts and at least one metal hydroxide selected from sodium hydroxide and potassium hydroxide, provided that the molten anhydrous salt has a melting point within a range of 400 C.-800 C.

4. The method of claim 1, wherein the molten anhydrous salt comprises lithium carbonate and at least one salt selected from non-lithium alkali metal salts and alkaline earth metal salts, provided that the molten anhydrous salt has a melting point within a range of 400 C.-800 C.

5. The method of claim 1, wherein the molten anhydrous salt comprises lithium carbonate and at least one salt selected from sodium carbonate and potassium carbonate.

6. The method of claim 1, wherein the molten anhydrous salt comprises lithium carbonate, sodium carbonate, and potassium carbonate.

7. The method of claim 1, wherein the molten anhydrous salt is maintained at a temperature within a range of 400 C.-700 C.

8. The method of claim 1, wherein the molten anhydrous salt is maintained at a temperature within a range of 400 C.-600 C.

9. The method of claim 1, wherein the molten anhydrous salt is maintained at a temperature within a range of 400 C.-550 C.

10. The method of claim 1, wherein the cathode is a porous cathode.

11. The method of claim 10, wherein the cathode is a nickel foam cathode.

12. The method of claim 10, wherein the carbon produced by the method is substantially graphitic, with a degree of graphitization of at least 0.95.

13. The method of claim 10, wherein the molten anhydrous salt is maintained at a temperature within a range of 500 C.-800 C. at a voltage within a range of 2.5 V to 3.5 V and the carbon produced by the method is substantially graphitic, with a degree of graphitization of at least 0.95.

14. The method of claim 13, wherein the voltage is within a range of 2.5 V to 3.0 V.

15. The method of claim 13, wherein the temperature and voltage are maintained for a period of time of 0.1-3.5 hours to result in conversion of the carbon dioxide to the substantially graphitic carbon.

16. The method of claim 10, wherein the molten anhydrous salt is maintained at a temperature within a range of 400 C.-500 C. and the carbon produced by the method comprises graphene.

17. The method of claim 1, wherein the cathode is a nickel or steel plate, and the carbon produced by the method is substantially amorphous carbon.

18. The method of claim 17, wherein the substantially amorphous carbon is porous carbon.

19. The method of claim 18, wherein the porous carbon comprises microporous or mesoporous carbon.

20. The method of claim 1, wherein the molten anhydrous salt is maintained at a temperature within a range of 500 C.-700 C. and the carbon produced by the method is substantially amorphous carbon.

21. The method of claim 20, wherein the temperature and voltage are maintained for a period of time of 3-10 hours to result in conversion of the carbon dioxide to the substantially amorphous carbon.

22. The method of claim 20, wherein the temperature and voltage are maintained for a period of time of 4-10 hours to result in conversion of the carbon dioxide to the substantially amorphous carbon.

23. The method of claim 20, wherein the cathode is a nickel or steel plate.

24. The method of claim 20, wherein the substantially amorphous carbon is porous carbon.

25. The method of claim 24, wherein the porous carbon comprises microporous or mesoporous carbon.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] FIGS. 1a-1f. FIGS. 1a-1c show powder X-ray diffraction (PXRD) patterns of as synthesized graphitic carbon samples NF-550-2.8-1H (FIG. 1a), NF-550-2.9-1H (FIG. 1b), and NF-550-3.5-1H (FIG. 1c), respectively. FIGS. 1d-1f show Raman spectra of carbon samples NF-550-2.8-1H (FIG. 1d), NF-550-2.9-1H (FIG. 1e), and NF-550-3.5-1H (FIG. 1f), respectively. These samples were prepared at 550 C. for 1 hour experiment time at different voltage.

[0008] FIGS. 2a-2e. Scanning electron microscope (SEM) images of carbon sample NF-550-2.8-1H (FIG. 2a); NF-550-2.9-1H (FIG. 2b); and NF-550-3.5-1H (FIG. 2c). HR-TEM (transmission electron microscope) images of carbon samples NF-550-2.8-1H (FIG. 2d) and NF-550-2.9-1H (FIG. 2e).

[0009] FIGS. 3a-3d. FIGS. 3a-3b show PXRD patterns of as-synthesized graphitic carbon samples NF-550-2.9-2H (FIG. 3a) and NF-550-3.5-2H (FIG. 3b). FIGS. 3c-3d show Raman spectra of carbon samples NF-550-2.9-2H (FIG. 3c) and NF-550-3.5-2H (FIG. 3d). These samples were prepared at 550 C. for 2 hours experiment time at different voltage.

[0010] FIGS. 4a-4d. FIGS. 4a-4b show PXRD patterns of as-synthesized graphene samples NF-450-2.9-20M (FIG. 4a) and NF-450-2.9-20M (FIG. 4b). FIGS. 4c-4d show Raman spectra of as-synthesized graphene samples NF-450-2.9-20M (FIG. 4c) and NF-450-2.9-20M (FIG. 4d). These samples were prepared at 450 C. for 20 minutes experiment time at different voltage.

[0011] FIGS. 5a-5b. FIG. 5a shows PXRD pattern of as-synthesized graphene sample NF-450-2.9-1H (FIG. 5a), and FIG. 5b shows a Raman spectrum of NF-450-2.9-1H (FIG. 5a). These samples were prepared at 450 C. for 1 hour experiment time at different voltage.

[0012] FIGS. 6a-6d. FIGS. 6a-6b show PXRD pattern of as-synthesized graphene sample NP-550-2.9-5M (FIG. 6a) and amorphous carbon sample NP-550-2.9-1H (FIG. 6b). FIGS. 6c-6d show Raman spectra of as-synthesized graphene sample NP-550-2.9-5M (FIG. 6c) and amorphous carbon sample NP-550-2.9-1H (FIG. 6d).

[0013] FIGS. 7a-7f. FIGS. 7a-7c shows the PXRD patterns of as-synthesized amorphous carbon samples SS-550-2.9-2H (FIG. 7a), SS-550-3.2-2H (FIG. 7b), and SS-550-3.5-2H (FIG. 7c). FIGS. 7d-7f show Raman spectra of as synthesized amorphous carbon samples SS-550-2.9-2H (FIG. 7d), SS-550-3.2-2H (FIG. 7e), and SS-550-3.5-2H (FIG. 7f).

[0014] FIGS. 8a-8f. FIGS. 8a-8c show the PXRD pattern (FIG. 8a), Raman spectrum (FIG. 8b), and pore size distribution (FIG. 8c, with average pore size of 1.1 nm) of NF-500-2.9 for 8 hrs electrolysis time. FIGS. 8c-8f show the PXRD pattern (FIG. 8d), Raman spectrum (FIG. 8e), and pore size distribution (FIG. 8f, with average pore size of 1-13 nm) of NF-450-3.2 for 4 hrs electrolysis time.

[0015] FIGS. 9a-9f. FIGS. 9a-9c show the PXRD pattern (FIG. 9a), Raman spectrum (FIG. 9b), and pore size distribution (FIG. 9c, with average pore size of 1.1 nm) of SS-500-2.9 for 10 hrs electrolysis time. FIGS. 9d-9f show the PXRD pattern (FIG. 9d), Raman spectrum (FIG. 9e), and pore size distribution (FIG. 9f, with average pore size of 1-13 nm) of SS-450-3.2 for 4 hrs electrolysis time.

[0016] FIGS. 10a-10c show the electrochemical performance of as-synthesized graphitic carbon (NF-550-2.9-1H) as an anode for lithium-ion batteries. FIG. 10a shows the cyclic voltammetry plot, which exhibit the lithium intercalation and de-intercalation profile below 0.3 V vs. Li/Li.sup.+, like the state-of-the-art graphite. FIG. 10b shows the battery cycling data at 0.3 C (1 C=372 mAh g.sup.1) and FIG. 10c shows the excellent rate capability even under fast charging conditions (5 C, 10 C).

[0017] FIGS. 11a-11c show the electrochemical performance of as-synthesized porous carbon (NF-450-3.2) as an anode for lithium-ion batteries. FIG. 11a shows the cyclic voltammetry plot, which exhibit the pseudocapacitive type profile like hard carbon. FIG. 10b shows the battery cycling data at 100 mA g.sup.1 and FIG. 10c shows the excellent rate capability even under fast charging conditions (0.05 A g.sup.1 to 5 A g.sup.1).

[0018] FIGS. 12a-12d. FIGS. 12a-12d show powder X-ray diffraction (PXRD) patterns of as synthesized graphitic carbon samples NF-450-2.7-6H (FIG. 12a), NF-450-2.7-12H (FIG. 12c), and Raman spectra of carbon samples NF-450-2.7-6H (FIG. 12b), NF-450-2.7-12H (FIG. 12d), respectively. These samples were prepared at 450 C. for a 6-hour experiment time (FIG. 12a-12b) and 12-hour experiment time (FIG. 12c-12d) at 2.7 voltage.

[0019] FIGS. 13a-13b. FIG. 13a shows a powder X-ray diffraction (PXRD) pattern of as-synthesized graphitic carbon sample NF-600-2.8-3H (FIG. 13a), and Raman spectrum of carbon sample NF-600-2.8-3H (FIG. 13b). This sample was prepared at 600 C. for a 3-hour experiment time at 2.8 voltage.

DETAILED DESCRIPTION

[0020] The present disclosure is foremost directed to a thermo-electrochemical method for converting carbon dioxide to carbon. Depending on the conditions employed, the carbon may be, for example, graphitic, graphenic, or amorphous carbon. The method generally involves passing carbon dioxide through a molten anhydrous salt at a temperature within a range of 400 C.-800 C. while the molten anhydrous salt is in contact with a cathode and an anode that are electrically interconnected to impart a voltage to the molten anhydrous salt, wherein the voltage is within a range of 2 V to 3.5 V. The temperature and voltage are typically applied for a period of time of 5 minutes to 10 hours to result in conversion of the carbon dioxide to carbon.

[0021] The molten anhydrous salt is composed of one or more salts that together (in admixture) provide a melting point of at least 400 C. and up to 800 C. (i.e., within a range of 400 C.-800 C.). The term salt, as used herein, refers to any ionic compound other than an oxide or sulfide compound. The ionic (salt) compound may be, for example, a carbonate, halide (e.g., F, Cl, Br, or I), sulfate, or nitrate ionic compound of an alkali or alkaline earth metal. In some embodiments, the molten anhydrous salt includes at least one alkali metal salt or an alkali metal oxide, or more particularly, at least one lithium salt or lithium oxide. In different embodiments, the melting point may be precisely or about, for example, 400 C., 450 C., 500 C., 550 C., 600 C., 650 C., 700 C., 750 C., or 800 C., or the melting point is within a range bounded by any two of the foregoing values, e.g., 400 C.-750 C., 400 C.-700 C., 400 C.-650 C., 400 C.-600 C., 400 C.-550 C., 400 C.-500 C., 400 C.-450 C., 450 C.-800 C., 450 C.-750 C., 450 C.-700 C., 450 C.-650 C., 450 C.-600 C., 450 C.-550 C., 450 C.-500 C., 500 C.-800 C., 500 C.-750 C., 500 C.-700 C., 500 C.-650 C., 500 C.-600 C., or 500 C.-550 C.

[0022] In some embodiments, the molten anhydrous salt includes or exclusively contains a lithium salt or lithium oxide (Li.sub.2O) and at least one salt (i.e., one or more salts) selected from non-lithium alkali metal salts and alkaline earth metal salts, provided that the molten anhydrous salt has a melting point within a range of 400 C.-800 C. or any of the sub-ranges provided above. The lithium salt may be, for example, lithium carbonate, a lithium halide (e.g., LiF, LiCl, LiBr, or LiI), lithium sulfate, or lithium nitrate, or a combination thereof. The one or more non-lithium alkali metal salts may be selected from sodium, potassium, and rubidium salts. Some examples of sodium salts include sodium carbonate, sodium halides (e.g., NaF, NaCl, NaBr, or NaI), sodium sulfate, and sodium nitrate. Some examples of potassium salts include potassium carbonate, potassium halides (e.g., KF, KCl, KBr, or KI), potassium sulfate, and potassium nitrate. The alkaline earth metal salt may be, for example, a magnesium (Mg), calcium (Ca), strontium (Sr), or barium (Ba) salt, such as a carbonate, halide, sulfate, or nitrate salt of any of these alkaline earth metals.

[0023] In some embodiments, the molten anhydrous salt contains a lithium salt (e.g., lithium carbonate) or lithium oxide in combination with one or more non-lithium alkali metal salts (e.g., sodium carbonate and/or potassium carbonate). In some embodiments, the molten anhydrous salt contains a lithium salt (e.g., lithium carbonate) or lithium oxide in combination with one or more alkaline earth metal salts (e.g., Mg or Ca carbonate or halide). In particular embodiments, the molten anhydrous salt includes or exclusively contains lithium carbonate and at least one salt selected from sodium carbonate and potassium carbonate. In further particular embodiments, the molten anhydrous salt includes or exclusively contains lithium carbonate, sodium carbonate, and potassium carbonate. In further embodiments of any of the embodiments disclosed in this application, the molten anhydrous salt further includes at least one metal hydroxide selected from sodium hydroxide (NaOH) and potassium hydroxide (KOH). In some embodiments, a lithium oxide (or metal oxides altogether) is/are excluded from the molten anhydrous salt. In some embodiments, any one or more non-lithium alkali metal salts is/are excluded from the molten anhydrous salt. In some embodiments, any one or more alkaline earth metal salts is/are excluded from the molten anhydrous salt.

[0024] The molten anhydrous salt is maintained at a temperature within a range of 400 C.-800 C. In different embodiments, the temperature being maintained is precisely or about, for example, 400 C., 450 C., 500 C., 550 C., 600 C., 650 C., 700 C., 750 C., or 800 C., or the temperature being maintained is within a range bounded by any two of the foregoing values, e.g., 400 C.-750 C., 400 C.-700 C., 400 C.-650 C., 400 C.-600 C., 400 C.-550 C., 400 C.-500 C., 400 C.-450 C., 450 C.-800 C., 450 C.-750 C., 450 C.-700 C., 450 C.-650 C., 450 C.-600 C., 450 C.-550 C., 450 C.-500 C., 500 C.-800 C., 500 C.-750 C., 500 C.-700 C., 500 C.-650 C., 500 C.-600 C., or 500 C.-550 C.

[0025] The phrase maintained at a temperature within a range may, in a first embodiment, mean maintaining the molten anhydrous salt at a precise or approximate (i.e., single) temperature within a specified temperature range, as provided above, during the period of time the carbon dioxide is passed through and in contact with the molten anhydrous salt. In a second embodiment, the phrase maintained at a temperature within a range permits a change or fluctuation in temperature to occur in the molten anhydrous salt, provided that the temperature of the molten anhydrous salt remains within the specified temperature range. The change or fluctuation in temperature may be, for example, 1, 2, 5, or 10 C. from a given selected temperature in the range, provided the varying temperatures remain within the range. The molten anhydrous salt can be heated by any suitable means known in the art, e.g., by being placed in an electric furnace or by being wrapped in heating tape, while contained in a suitable crucible or other heat-resistant vessel.

[0026] The molten anhydrous salt is subjected to a voltage within a range of 2 V to 3.5 V while being maintained at any of the above temperature or ranges thereof. In different embodiments, the voltage is precisely or about, for example, 2 V, 2.2 V, 2.5 V, 2.7 V, 3 V, 3.2 V, or 3.5 V, or the voltage is within a range bounded by any two of the foregoing values, e.g., 2-3.5 V, 2-3.2 V, 2-3 V, 2-2.7 V, 2-2.5 V, 2.2-3.5 V, 2.2-3.2 V, 2.2-3 V, 2.2-2.7 V, 2.2-2.5 V, 2.5-3.5 V, 2.5-3.2 V, 2.5-3 V, 2.5-2.7 V, 2.7-3.5 V, 2.7-3.2 V, or 2.7-3 V. In some embodiments, the voltage is kept precisely or substantially constant while the current is varied. In other embodiments, the current is kept constant while the voltage is varied within any of the ranges provided above. Any of the above temperatures or temperature ranges provided above can be combined with any of the voltages or ranges provided above to convert carbon dioxide to a carbon product.

[0027] To provide the voltage, the molten anhydrous salt is in contact with a cathode and an anode that are electrically interconnected. The cathode has a metal composition containing or exclusively composed of at least one of nickel, iron, and cobalt. A cathode or anode that includes iron may be steel. The cathode or anode may be porous or non-porous. If the electrode is porous, it may be a foam electrode, such as a nickel foam cathode, as well known in the art. In some embodiments, the cathode contains or is exclusively composed of nickel. If the electrode is non-porous, it may be in the form of a metal plate. In particular embodiments, the cathode is a nickel foam cathode. In other embodiments, the cathode is a nickel or steel plate cathode. The anode may be constructed of any of the electrode compositions known in the art provided it does not react with the anhydrous molten salt or carbon dioxide or otherwise hinder the conversion process. The anode may be, for example, a glassy carbon anode, alumina, titania, or zirconia coated glassy carbon, or tin oxide.

[0028] The molten anhydrous salt is maintained at any of the above temperatures or ranges thereof while under the influence of any of the above voltages or ranges thereof for a suitable period of time that converts the carbon dioxide to carbon. The period of time is typically within a range of 5 minutes to 10 hours. In different embodiments, the period of time is precisely or about, for example, 5 minutes (approximately 0.1 hour), 10 minutes, 20 minutes, 30 minutes, 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours, 4.5 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, or 10 hours, or a period of time within a range bounded by any two of the foregoing values, e.g., 0.1-10, 0.1-5, 0.1-3.5, 0.5-10, 0.5-3.5, 1-10, 1-8, 1-6, 1-5, 1-4, 1-3.5, 1-3, 1-2.5, 1-2, 2-10, 2-8, 2-6, 2-5, 2-4, 2-3, 3-10, 3-8, 3-6, 3-5, 3-4, 4-10, 4-8, 4-6, or 4-5 hours. Any of the temperatures or ranges thereof provided above can be combined with any of the voltages or ranges provided above, and this further combined with any of the periods of time or ranges thereof provided above, to convert carbon dioxide to a carbon product.

[0029] In a first set of embodiments, the carbon produced by the method is substantially graphitic, with a degree of graphitization of at least 0.95, 0.96, 0.97, 0.98, or 0.99. In some embodiments, the carbon produced by the method is completely graphitic (i.e., a degree of graphitization of 1.0). Certain conditions have herein been found to promote the conversion of carbon dioxide to a substantially graphitic carbon material. A first condition is the use of a porous cathode, such as a nickel foam cathode. A second condition is selection of a temperature (or variation of temperature) within a range of 500 C.-800 C. or any sub-range thereof as provided earlier above, or more particularly, a temperature (or variation of temperature) in a range of 525 C.-800 C., 550 C.-800 C., 575 C.-800 C., 600 C.-800 C., 500 C.-750 C., 525 C.-750 C., 550 C.-750 C., 575 C.-750 C., 600 C.-750 C., 500 C.-700 C., 525 C.-700 C., 550 C.-700 C., 575 C.-700 C., 600 C.-700 C., 500 C.-650 C., 525 C.-650 C., 550 C.-650 C., 575 C.-650 C., 600 C.-650 C., 500 C.-600 C., 525 C.-600 C., or 550 C.-600 C. A third condition is selection of a voltage within a range of 2.5-3.5 V, 2.5-3.2 V, or 2.5-3 V. A fourth condition is selection of a period of time of 5 minutes to 3.5 hours or 1-3.5 hours or any range therein (e.g., 0.1-3.5, 0.1-1.5, 0.5-3.5, 0.5-1.5, 1-1.5, 1-2, 1-2.5, 1-3, 1.5-2, 1.5-2.5, 1.5-3, 1.5-3.5, 2-2.5, 2-3, 2-3.5, 2.5-3.5, or 3-3.5 hours) at which any of the above temperatures and/or voltages are applied (each of which may be substantially constant or varied). Any one, two, three, or all conditions provided above may be employed to promote production of a substantially graphitic carbon material.

[0030] In a second set of embodiments, the carbon produced by the method is substantially or completely graphenic (i.e., partially or completely graphene). Certain conditions have herein been found to promote the conversion of carbon dioxide to a substantially graphenic carbon material. A first condition is the use of a porous cathode, such as a nickel foam cathode. A second condition is selection of a temperature (or range in temperature) of at least 400 C. or 425 C. and up to or less than 500 C. or 475 C., or more particularly, within a range of 400 C.-500 C. The temperature may alternatively be within a range of 400 C.-475 C., 400 C.-450 C., 400 C.-425 C., 425 C.-500 C., 425 C.-475 C., 425 C.-450 C., 450 C.-500 C., 450 C.-475 C., or 475 C.-500 C. A third condition is selection of a voltage within a range of 2.5-3.5 V, 2.5-3.2 V, or 2.5-3 V. A fourth condition is selection of a period of time of 5 minutes to 3.5 hours or 1-3.5 hours or any range therein (e.g., 0.1-3.5, 0.1-1.5, 0.5-3.5, 0.5-1.5, 1-1.5, 1-2, 1-2.5, 1-3, 1.5-2, 1.5-2.5, 1.5-3, 1.5-3.5, 2-2.5, 2-3, 2-3.5, 2.5-3.5, or 3-3.5 hours) at which any of the above temperatures and/or voltages are applied (each of which may be substantially constant or varied). Any one, two, three, or all conditions provided above may be employed to promote production of a substantially graphenic carbon material.

[0031] In a third set of embodiments, the carbon produced by the method is substantially or completely amorphous (i.e., partially or completely amorphous). The amorphous carbon may more particularly be a porous carbon material. The porous carbon may more particularly be a microporous or mesoporous carbon material. Certain conditions have herein been found to promote the conversion of carbon dioxide to a substantially amorphous carbon material. A first condition is the use of a non-porous cathode, such as a nickel or steel plate cathode. A second condition is selection of a temperature (or variation of temperature) within a range of 500 C.-700 C. or any sub-range thereof as provided earlier above, or more particularly, a temperature (or variation of temperature) in a range of 525 C.-700 C., 550 C.-700 C., 575 C.-700 C., 600 C.-700 C., 500 C.-650 C., 525 C.-650 C., 550 C.-650 C., 575 C.-650 C., 600 C.-650 C., 500 C.-600 C., 525 C.-600 C., or 550 C.-600 C. A third condition is selection of a voltage within a range of 2.5-3.5 V, 2.5-3.2 V, 2.5-3 V, or 3-3.5 V. A fourth condition is selection of a period of time of 3-10 hours or any range therein (e.g., 3-8, 3-6, 3-5, 4-10, 4-8, 4-6, 5-10, 5-8, 6-10, 6-8, 7-10, or 8-10 hours) at which any of the above temperatures and/or voltages are applied (each of which may be substantially constant or varied). Any one, two, three, or all conditions provided above may be employed to promote production of a substantially amorphous carbon material.

[0032] In some embodiments, the method for converting carbon dioxide to carbon, as described above, is integrated with a CO.sub.2 production process. The CO.sub.2 is typically captured and isolated in the CO.sub.2 production process before being used in the presently described process. Typically, the CO.sub.2 is produced as an undesirable byproduct. The gaseous source can be, for example, air, waste gas from an industrial or commercial process, flue gas from a power plant, exhaust from an engine, or sewage or landfill gas.

[0033] Examples have been set forth below for the purpose of illustration and to describe certain specific embodiments of the invention. However, the scope of this invention is not to be in any way limited by the examples set forth herein.

EXAMPLES

[0034] The eutectic mixtures of anhydrous Li.sub.2CO.sub.3Na.sub.2CO.sub.3K.sub.2CO.sub.3 (300 g in a molar ratio of 43.5:31.5:25 with melting point 397 C.) was placed in an alumina/graphite crucible (inner diameter=64 mm and height=151 mm) inside a sealed quartz reactor under N.sub.2 atmosphere. The salt was pre-dried at 325 C. for 24 hrs under N.sub.2 flow to remove the residual moisture. Then, the reactor was heated slowly to 450 C. or 500 C., or 550 C. in an electric furnace. For CO.sub.2 electrolysis, either pure or 10% CO.sub.2 in nitrogen was bubbled through the molten carbonate salts as a source for carbon for graphite/graphene/porous synthesis. A potentiostat was used for constant potential (2.8 V, 2.9 V, 3.2 V, or 3.5 V) polarization of the electrochemical cell made of a glassy carbon rod anode and nickel foam/nickel plate/stainless steel plate cathode, where electro-reduced carbon was deposited. The electrolysis was run for different times (20 minutes, 1 hr, 2 hrs, 4 hrs, and 10 hrs). After the CO.sub.2 electrolysis, the cathode was removed from the reactor, cooled, and dissolved the nickel foam in 20% HNO.sub.3, filtered, and washed with distilled water to obtain clean powder samples. The carbon powder samples were characterized by XRD, Raman spectroscopy, SEM/HR-TEM to identify the structure of the products (graphite, graphene, or amorphous carbon). The resulting graphitic (graphite/graphene) products are herein referred to as NF-450-3.5-20M, NF-550-3.5-2H, NF-550-3.5-1H, NF-450-2.9-20M, NF-550-2.9-2H, NF-550-2.9-1H, NF-550-2.8-1H, NP-550-3.5-5M and the amorphous carbon products are herein referred to as NP-550-3.5-1H, SS-550-2.9-2H, SS-550-3.2-2H, and SS-550-3.5-2H, where NF, NP, and SS refers to nickel foam, nickel plate, and stainless steel plate cathode, first number represents the electrolysis temperature, second number represents voltage, and last number with letter represent indicates the experiment time (e.g., 1H and 2H indicate 1 hour and 2 hours, respectively). Similarly, the porous carbon products are herein referred to as NF-500-2.9/SS-500-2.9 and NF-450-3.2/SS-450-3.2, wherein NF and SS are nickel foam and stainless-steel cathodes, respectively, and the numbers in the middle and last represent the electrolysis temperature and voltage, respectively. Furthermore, the electrochemical performance of as-synthesized graphite and amorphous carbon as an anode for lithium-ion batteries were investigated.

[0035] Example 1. When the CO.sub.2 electrolysis was run at 550 C. in the eutectic mixtures of anhydrous Li.sub.2CO.sub.3Na.sub.2CO.sub.3K.sub.2CO.sub.3 using nickel foam cathode at different voltages (2.8 V, 2.9 V, 3.5 V) and different times (1 hour and 2 hours), the as electro-reduced carbon was significantly graphitic, which is confirmed by the presence of a sharp crystalline peak at 26 in XRD patterns, the presence of a sharp G-band, reduced D-band, and presence of 2D-band in Raman spectra as well as SEM and HR-TEM images (see FIGS. 1a-1f, 2a-2e, and 3a-3d). The degree of graphitization was higher at lower voltages and shorter electrolysis time.

[0036] Example 2. When the CO.sub.2 electrolysis was run at 450 C. in the eutectic mixtures of anhydrous Li.sub.2CO.sub.3Na.sub.2CO.sub.3K.sub.2CO.sub.3 using nickel foam cathode at different voltages (2.9 V and 3.5 V) and different times (20 minutes and 1 hour), the as electro-reduced carbon had significantly graphene-like structure, which is verified by the XRD patterns and Raman spectral features (see FIGS. 4a-4d and 5a-5b). The sample shows higher crystallinity at lower voltages and shorter electrolysis time.

[0037] Example 3. When the CO.sub.2 electrolysis was run at 550 C. in the eutectic mixtures of anhydrous Li.sub.2CO.sub.3Na.sub.2CO.sub.3K.sub.2CO.sub.3 using nickel plate cathode at 3.5 V and different times (5 minutes and 2 hour), the as electro-reduced carbon had a graphene-like structure at shorter electrolysis time (see FIGS. 6a and 6c) and completely amorphous carbon at longer electrolysis time (see FIGS. 6b and 6d).

[0038] Example 4. Using a stainless steel plate cathode generally yielded amorphous carbon. For example, when the CO.sub.2 electrolysis was run at 550 C. in the eutectic mixture of anhydrous Li.sub.2CO.sub.3Na.sub.2CO.sub.3K.sub.2CO.sub.3 using a stainless steel plate cathode at different voltages (2.9V, 3.2 V, and 3.5 V) for 2 hours, the as electro-reduced carbon was predominantly amorphous (see FIGS. 7a-7f).

[0039] Example 5. When the CO.sub.2 electrolysis was run at 450 C. (for 4 hrs)/500 C. (for 8 hrs) in the eutectic mixture of anhydrous Li.sub.2CO.sub.3Na.sub.2CO.sub.3K.sub.2CO.sub.3 with 3 weight % NaOH as additive using a nickel foam cathode at different voltages (2.9V, 3.2 V), the as electro-reduced carbon was predominantly microporous carbon with an average pore size of 1 nm at 500 C. and mesoporous/microporous carbon with average pore size of 1-13 nm at 450 C. Increasing the temperature and voltage reduces the average pore size and surface area, whereas decreasing the temperature increases surface area and pore size (see FIGS. 8a-8f)).

[0040] Example 6. When the CO.sub.2 electrolysis was run at 450 C. (for 4 hrs)/500 C. (for 10 hrs) in the eutectic mixtures of anhydrous Li.sub.2CO.sub.3Na.sub.2CO.sub.3K.sub.2CO.sub.3 with 3 weight % NaOH as additive using stainless steel plate cathode at different voltages (2.9V, 3.2 V), the as electro-reduced carbon was predominantly microporous carbon with average pore size 1nm at 500 C. and mesoporous/microporous carbon with average pore size 1-13 nm at 450 C. Increasing temperature and voltage reduces the average pore size and surface area, whereas decreasing the temperature increases surface area and pore size (see FIGS. 9a-9f)).

[0041] With 100% CO.sub.2 gas, the conversion rate of CO.sub.2 to graphite/graphene was 1% with a faradaic efficiency of 65-70%, whereas >9% conversion rate was achieved with >70% faradaic efficiency with 10% CO.sub.2 in nitrogen. Similarly, with 100% CO.sub.2 gas, the conversion rate of CO.sub.2 to amorphous porous carbon was 9% with a faradaic efficiency of 70%, whereas >21% conversion rate was achieved with >85% faradaic efficiency with diluted CO.sub.2 (10% CO.sub.2 in N.sub.2). CO.sub.2 conversion rate is equal to the ratio of recovered graphite product/theoretical carbon yield from supplied CO.sub.2*100%, where recovered graphite product=total recovered graphite productrecovered graphite product from blank electrolysis. The blank electrolysis refers to the electrolysis conducted in the absence of CO.sub.2 gas.

[0042] The electrochemical performance of as-synthesized graphite and porous carbon as an anode for lithium-ion battery were tested in a half cell configuration in a coin cell with lithium as counter and reference electrode. The electrodes of graphite and porous carbon were made by a slurry casting method using 80% (either graphite or porous carbon), 10% conductive carbon, and 10% polymer binder in N-methyl-2-pyrrolidone in copper foil with active mass loading 1-1.5 mg cm.sup.2 (for graphite) and 2.5-3 mg cm.sup.2 (for porous carbon) with electrode area of 1.27 cm.sup.2. 1.2 molar LiPF.sub.6 in ethylene carbonate/ethyl methyl carbonate (3:7 ratio by volume) and Celgard 2325 were used as an electrolyte and a separator, respectively. All coin cells were made inside a high purity argon gas-filled glove box with moisture and oxygen at <0.5 ppm. The cyclic voltammetry and galvanostatic charge/discharge cycling test were conducted to measure the electrochemical performance.

[0043] The electrochemical performance of one graphite sample (NF-550-2.9-1H) was investigated as an anode for the lithium-ion battery. FIGS. 10a-10c show the excellent electrochemical performance. FIG. 10a exhibits the typical de/intercalation cyclic voltammogram like state-of-the-art graphite, which indicates that as-synthesized graphite is highly crystalline. FIGS. 10b-10c exhibit enhanced capacity, excellent cycling performance, and rate capability even under fast charging conditions, much higher than commercial graphite. This indicates that these graphite products could be better alternatives for fast charging anodes for electric vehicles.

[0044] Similarly, the electrochemical performance of one of the porous carbon samples (NF-450-3.2) was investigated as an anode for the lithium-ion battery. FIGS. 11a-11c show the excellent electrochemical performance. FIG. 11a shows the typical cyclic voltammogram like other amorphous carbon materials. However, these porous carbon anodes stored much higher capacity (FIG. 11b) and excellent rate capability even under extremely high current rate (FIG. 11c), which makes these porous carbon electrodes suitable for fast charging high energy density battery for electric vehicles.

[0045] Example 7. When the CO.sub.2 electrolysis was run at 450 C. in the eutectic mixtures of anhydrous Li.sub.2CO.sub.3Na.sub.2CO.sub.3K.sub.2CO.sub.3 using nickel foam cathode at 2.7 V and longer times (6 hours and 12 hours), the as electro-reduced carbon had significantly crystalline (graphene-like structure), which is confirmed by the XRD patterns and Raman spectral features shown in FIGS. 12a-12d. The lowered voltage electrolysis enables the production of the highly crystalline product even at longer electrolysis.

[0046] Example 8. When the CO.sub.2 electrolysis was run at 600 C. in the eutectic mixtures of anhydrous Li.sub.2CO.sub.3Na.sub.2CO.sub.3K.sub.2CO.sub.3 using nickel foam cathode at 2.8 V and 3 hours, the as electro-reduced carbon had a significantly crystalline structure (comparable to commercial graphite-like structure), which is confirmed by the XRD patterns and Raman spectral features shown in FIGS. 13a-13b. The lowered voltage and higher temperature electrolysis enables the production of the highly crystalline graphite product even at longer electrolysis.

[0047] While there have been shown and described what are at present considered the preferred embodiments of the invention, those skilled in the art may make various changes and modifications which remain within the scope of the invention defined by the appended claims.