Nanoporous metal-carbon composite

1. A metal-carbon composite, comprising (i) a porous scaffold comprising one or more of carbon nanotubes, graphene and graphene oxide, and (b) metal nanoparticles of pure metal disposed on said porous scaffold, wherein the metal nanoparticles are obtained from reduction of a metal salt in a gel that comprises the metal salt and the one or more of carbon nanotubes, graphene and graphene oxide, wherein the metal-carbon composite has a density of 1 g/cm 3 or less, and wherein the metal nanoparticles account for 1 wt. % or more of the metal-carbon composite.

2. The composite of claim 1 , wherein the pure metal is selected from the group consisting of Cu, Ag, Au, Pt, Pd, Co, Ni, W, Mo, Fe, and Ru.

3. The composite of claim 1 , wherein the pure metal is selected from the group consisting of Cu, Ag, Au, and Pt.

4. The composite of claim 1 , wherein the porous scaffold comprising a three-dimensional network of graphene sheets crosslinked by covalent bonds.

5. The composite of claim 1 , wherein the porous scaffold comprising a three-dimensional network of graphene oxide sheets crosslinked by covalent bonds.

6. The composite of claim 1 , wherein the porous scaffold comprising a three-dimensional network of carbon nanotubes crosslinked by carbon nanoparticles.

7. The composite of claim 1 , wherein the metal-carbon composite has a density of 100 mg/cm 3 or less, and wherein the metal nanoparticles account for 5 wt. % or more of the metal-carbon composite.

8. The composite of claim 1 , wherein carbon and metal account for 90 at. % or more of the metal-carbon composite.

9. A method for making the metal-carbon composite of claim 1 , comprising (a) providing an aqueous mixture comprising (i) at least one of carbon nanotube and graphene oxide and (ii) at least one metal salt; (b) freezing the mixture to obtain a cryogel; (c) drying the cryogel to obtain a dry gel; and (d) reducing the dry gel to obtain the metal-carbon composite.

10. The method of claim 9 , wherein the metal salt is a copper or silver salt.

11. The method of claim 9 , wherein step (d) comprises pyrolyzing the dry gel in a reducing environment.

12. A method for making the metal-carbon composite of claim 1 , comprising (a) providing an aqueous mixture comprising (i) graphene oxide, (ii) at least one metal salt, and (iii) at least one catalyst; (b) curing the reaction mixture to produce a wet gel; (c) washing and drying the wet gel to obtain a metal-graphene oxide aerogel comprising a graphene oxide scaffold.

13. The method of claim 12 , wherein the metal salt is a gold salt.

14. The method of claim 12 , further comprising (d) reducing the metal-graphene oxide aerogel to produce a metal-graphene aerogel comprising a graphene scaffold, wherein carbon and metal account for 90 at. % or more of the metal-graphene aerogel.

15. The method of claim 12 , further comprising (e) removing the graphene oxide scaffold to obtain a metal aerogel.

16. A method for making the metal-carbon composite of claim 1 , comprising (a) providing a carbon aerogel; (b) immersing said carbon aerogel in an aqueous metal salt solution; (c) freeze drying the carbon aerogel in a vacuum to obtain a metal salt-impregnated carbon aerogel; and (d) reducing the metal salt-impregnated carbon aerogel to obtain the metal-carbon composite.

17. The method of claim 16 , wherein the metal salt is selected from platinum salt and copper salt.