Metal air cathode: manganese oxide contained in octahedral molecular sieve
View Patent ↗An oxygen reduction electrode, e.g., an air cathode, comprising manganese oxides having octahedral molecular sieve structures as active catalyst materials and use of such an electrode as a component of a metal-air cell.
1. A metal-air electrochemical cell comprising:
an anode;
an aqueous alkaline electrolytic solution; and
an oxygen reduction electrode comprising a mixed manganese oxide catalyst comprising an octahedral molecular sieve structureand Mn 2 O 3 wherein the manganese oxide in the octahedral molecular sieve structure is cryptomelane the manganese having an average oxidation state between 3.19 and 3.59.
2. A metal-air electrochemical cell as set forth in claim 1 , wherein said octahedral molecular sieve structure has unit cell parameters: a=9.848 ű0.02 Å, b=2.862 ű0.02 Å, c=9.843 ű0.06 Å, and β=90.08°±0.4°, when calculated as a monoclinic unit cell.
3. A metal-air electrochemical cell as set forth in claim 1 , wherein the framework of said octahedral molecular sieve further comprises one or more secondary cations, said secondary cations being selected from the group consisting of Mg, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Zr, Nb, Ga, In, Ge, Sn, Pb, La, and combinations thereof.
4. A metal-air electrochemical cell as set forth in claim 3 wherein ions of said secondary cation(s) constitute up to 30 atom % of the total of metal cations in said framework.
5. A metal-air electrochemical cell as set forth in claim 3 wherein ions of said secondary cation(s) constitute between about 1 and about 30 atom %, or between about 2 and about 30 atom %, or between about 3 and about 30 atom %, or between about 5 and about 30 atom %, or between about 1 and about 20 atom %, or between about 2 and about 20 atom %, or between about 3 and about 20 atom %, or between about 5 and about 20 atom %, or between about 10 and about 20 atom % of the total of metal cations in said framework.
6. A metal-air electrochemical cell as set forth in claim 1 , wherein the oxygen reduction electrode comprises the manganese oxide catalyst in a calcined octahedral molecular sieve structure.
7. A metal-air electrochemical cell as set forth in claim 6 wherein the octahedral molecular sieve structure has been calcined at a temperature between about 95° C. and about 600° C.
8. A metal-air electrochemical cell as set forth in claim 6 wherein the manganese oxide catalyst in said calcined octahedral molecular sieve structure comprises particles having a mean particle size between about 0.01 μm and about 50 μm.
9. A metal-air electrochemical cell as set forth in claim 6 wherein the manganese oxide catalyst in said calcined octahedral molecular sieve structure comprises particles having a B.E.T. surface area between about 50 m 2 /g and about 500 m 2 /g.
10. A metal-air electrochemical cell as set forth in claim 1 wherein preparation of the manganese oxide present in said other form comprises reducing an alkali metal permanganate by reaction with carbon.
11. A metal-air electrochemical cell as set forth in claim 10 wherein preparation of the manganese oxide present in said other form comprises contacting said alkali metal permanganate with activated carbon in a weight ratio of alkali metal permanganate to carbon between about 0.01:1 and about 0.2:1.
12. A metal-air electrochemical cell as set forth in claim 1 wherein the manganese in said octahedral molecular sieve constitutes between about 5% and about 40% of the total of the manganese content of said octahedral molecular sieve and the manganese content of said other form of manganese oxide.
13. A metal-air electrochemical cell as set forth in claim 1 wherein the manganese in said octahedral molecular sieve constitutes between about 15% and about 30% of the total of the manganese content of said octahedral molecular sieve and the manganese content of said other form of manganese oxide.
14. A metal-air electrochemical cell as set forth in claim 1 , wherein the oxygen reduction electrode further comprises carbon; and wherein said octahedral molecular sieve comprises a framework of octahedra in a 2×2 arrangement forming atomic scale tunnels, the atomic scale tunnels containing metal ions selected from the group consisting of Ba 2+ , Pb 2+ , Sr 2+ , Cu 2+ , Mg 2+ , Fe 3+ , Co 2+ , Ni 2+ , combinations thereof, and combinations thereof with K + ions.
15. A metal-air electrochemical cell as set forth in claim 14 wherein the atomic scale tunnels comprise K + and Ni 2+ metal ions.
16. A metal-air electrochemical cell as set forth in claim 15 wherein about 10% of the metal ions in the atomic scale tunnels are Ni 2+ ions.