Non-noble metal electrocatalysts for oxygen depolarized cathodes and their uses
Highly anion resistant electrocatalysts suitable for catalyzing an oxygen reduction reaction (ORR) and methods of synthesizing the same are provided. The catalysts contain a transition metal, a heteroatom, and carbon. Preferred catalysts include N as the heteroatom and Fe as the transition metal, with active sites having Fe—N 4 stoichiometry (Fe x N y C z ) as part of a metal organic framework (MOF) or sequestered within a MOF. Electrocatalysts further including Fe nanoparticles (Fe NPs ) are also provided. The catalysts described herein are applicable in the preparation of oxygen decoupled cathodes (ODC) for chlorine evolution processes such as in chlor-alkali cells or HCl electrolyzers. The catalysts are also useful in preparing ODC for use in fuel cells, including phosphoric acid fuel cells.
1. A method of synthesizing an electrocatalyst for an oxygen reduction reaction, the method comprising the steps of:
(a) reacting an organic ligand, a first transition metal or salt thereof, and a first catalytic precursor to form a product, wherein the first catalytic precursor is a heteroatom-containing organic molecule, and wherein the product comprises a metal organic framework (MOF) comprising the first transition metal;
(b) reacting a second catalytic precursor with the product resulting from (a) until a precipitate is formed, wherein the second catalytic precursor is a second transition metal or salt thereof, whereby the first and the second catalytic precursors are encapsulated inside the MOF; and
(c) isolating the precipitate and subjecting it to pyrolysis, whereby the first transition metal evaporates yielding the electrocatalyst.
2. The method according to claim 1 , wherein the heteroatom-containing organic molecule provides a heteroatom capable of catalyzing an oxygen reduction reaction.
3. The method according to claim 1 wherein the pyrolysis is carried out at about 700° C. to about 1100° C.
4. The method according to claim 1 , wherein the heteroatom-containing organic molecule comprises one or more heteroatoms selected from the group consisting of nitrogen, oxygen, and sulfur.
5. The method according to claim 1 , wherein the second transition metal is a non-noble transition metal.
6. The method according to claim 5 , wherein the non-noble transition metal is selected from the group consisting of iron, cobalt, copper, nickel, and chromium.
7. The method according to claim 1 , wherein the first and the second transition metals have oxidation states selected from the group consisting of all known oxidation states for the respective transition metal.
8. The method according to claim 1 , wherein the first transition metal is zinc.
9. The method according to claim 1 , wherein the salt of each of the first and the second transition metals is selected from the group consisting of acetate, nitrate, sulfate, phosphate, and chloride.
10. The method according to claim 1 , wherein the organic ligand is selected from the group consisting of imidazole, methylimidazole, pyridine, pyridine derivatives, pyrimidine, triazole, tetrazole, napthylene, and napthyridine.
11. The method according to claim 1 , wherein the second transition metal is in the form of nanoparticles or a colloid accommodated within pores of the MOF.
12. The method according to claim 1 , wherein the first catalytic precursor is selected from the group consisting of phenanthroline, porphyrin, imidazole, pyridine, pyrimidine, and triazole.
13. The method according to claim 1 , wherein the electrocatalyst is cross-linked as a result of the pyrolysis in step (c).
14. The method according to claim 1 , wherein the electrocatalyst is resistant to anion poisoning when used in an oxygen reduction reaction.
15. An electrocatalyst comprising graphene sheets and a transition metal, wherein the graphene sheets consist of carbon atoms and heteroatoms, and wherein the transition metal is coordinated with the heteroatoms.
16. The electrocatalyst according to claim 15 , further comprising nanoparticles, the nanoparticles comprising or consisting of a non-oxidated metal (M) surrounded with a layer of metal oxide (M x O y ).
17. A cathode for an electrolytic process for chlorine evolution in a chlor-alkali electrolysis cell, the cathode comprising the electrocatalyst according to claim 15 .
18. A cathode for an electrolytic process for chlorine evolution in an HCl electrolyzer, the cathode comprising the electrocatalyst according to claim 15 .
19. A cathode for a phosphoric acid fuel cell comprising the electrocatalyst according to claim 15 .
20. A cathode for carrying out an oxygen reduction reaction in an electrolytic process, the cathode comprising the electrocatalyst of claim 15 , wherein the cathode is resistant to anion poisoning.
21. The cathode according to claim 20 that is resistant to poisoning by chloride ion.
22. The cathode according to claim 20 that is resistant to poisoning by dihydrogen phosphate ion.
23. A method of chlorine evolution, the method comprising the step of electrolyzing brine in a chlor-alkali electrolysis cell, wherein the cathode of the cell comprises the electrocatalyst according to claim 15 .
24. A method of chlorine evolution comprising electrolyzing HCl in an HCl electrolyzer, wherein the cathode of the electrolyzer comprises the electrocatalyst according to claim 15 .
25. The method according to claim 1 , wherein steps (a) and (b) are carried out in a single reaction vessel.