Nanostructured carbon materials and methods of making and use thereof

1. A method of making a plurality of carbon nanotubes, comprising:

applying a current across a catalytically passive anode and a catalytic cathode;

wherein the catalytic cathode comprises a catalyst and the catalyst comprises Fe, Co, Mo, Cr, Cu, or a combination thereof;

wherein the catalytically passive anode comprises a first conductive substrate coated with a first passivating layer, the first passivating layer comprising Al 2 O 3 , TiO 2 , MgO, TiN, VN, or combinations thereof;

wherein the catalytically passive anode and the catalytic cathode are in electrochemical contact with a molten carbonate electrolyte and a source of CO 2 ; and

wherein the current is applied at a current density of from 25 mA/cm 2 to 500 mA/cm 2 ;

thereby forming a plurality of carbon nanotubes on the catalytic cathode.

2. The method of claim 1 , wherein the current is applied for an amount of time of from 1 minute to 10 hours.

3. The method of claim 1 , wherein the first passivating layer has a thickness of 2 nm to 100 nm.

4. The method of claim 1 , wherein the catalytically passive anode comprises a metal, a metal oxide, a carbon material, or a combination thereof, with the proviso that the catalytically passive anode does not comprise nickel.

5. The method of claim 1 , wherein the catalytic cathode comprises steel.

6. The method of claim 1 , wherein the catalytic cathode comprises a second conductive substrate and a catalytic layer deposited on said second conductive substrate, the catalytic layer comprising the catalyst and the catalytic layer having a thickness of from 0.1 nm to 25 nm.

7. The method of claim 6 , wherein the catalytic cathode further comprises a second passivating layer; wherein the second passivating layer is disposed between the second conductive substrate and the catalytic layer; and wherein the second passivating layer comprises Al 2 O 3 , TiO 2 , MgO, TiN, VN, or combinations thereof.

8. The method of claim 1 , wherein the catalytic cathode comprises a second conductive substrate and a plurality of catalytic particles deposited on said second conductive substrate, wherein the plurality of catalytic particles comprise the catalyst and wherein the plurality of catalytic particles have an average particle size of from 0.5 nm to 50 nm.

9. The method of claim 8 , wherein the catalytic cathode further comprises a second passivating layer; wherein the second passivating layer is disposed between the second conductive substrate and the plurality of catalytic particles; and wherein the second passivating layer comprises Al 2 O 3 , TiO 2 , MgO, TiN, VN, or combinations thereof.

10. The method of claim 1 , wherein the catalyst comprises Fe.

11. The method of claim 1 , wherein the catalyst comprises Fe and the catalytic cathode comprises Fe in an amount of 75% or more.

12. The method of claim 1 , wherein the source of CO 2 is provided by injecting the CO 2 source into the molten carbonate electrolyte.

13. The method of claim 1 , wherein the source of CO 2 comprises CO 2 , air, exhaust from an industrial process, exhaust from an internal combustion engine, or a combination thereof.

14. The method of claim 1 , wherein the plurality of carbon nanotubes are crystalline.

15. The method of claim 1 , wherein the average length of the plurality of carbon nanotubes is from 100 nm to 1 cm; the average outer diameter of the plurality of carbon nanotubes is 100 nm or less; or a combination thereof.

16. The method of claim 1 , wherein:

the catalyst comprises Fe;

the catalytic cathode comprises steel;

the current density is from 25 mA/cm 2 to 100 mA/cm 2 ;

the current is applied for an amount of time of from 1 minute to 1 hour;

the first conductive substrate comprises nickel;

the first passivating layer comprises Al 2 O 3 ; and

the first passivating layer has a thickness of from 45 nm to 55 nm.

17. The method of claim 1 , further comprising collecting the plurality of carbon nanotubes from the catalytic cathode; wherein collecting the plurality of carbon nanotubes from the catalytic cathode comprises:

sonicating the catalytic cathode to separate the plurality of carbon nanotubes from the catalytic cathode by dispersing the plurality of carbon nanotubes into a solvent and centrifuging or filtering the solvent with the plurality of carbon nanotubes dispersed therein to thereby collect the plurality of carbon nanotubes; or

mechanically scraping the catalytic cathode to separate the plurality of carbon nanotubes from the catalytic cathode.