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A mixed ionic and electronic conducting membrane includes a two-phase solid state ceramic composite, wherein the first phase comprises an oxygen ion conductor and the second phase comprises an n-type electronically conductive oxide, wherein the electronically conductive oxide is stable at an oxygen

A mixed ionic and electronic conducting membrane includes a two-phase solid state ceramic composite, wherein the first phase comprises an oxygen ion conductor and the second phase comprises an n-type electronically conductive oxide, wherein the electronically conductive oxide is stable at an oxygen partial pressure as low as 10-20 atm and has an electronic conductivity of at least 1 S/cm. A hydrogen separation system and related methods using the mixed ionic and electronic conducting membrane are described.

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What is claimed is: 1. A mixed ionic and electronic conducting membrane, comprising: a two-phase solid state ceramic composite, wherein the first phase comprises an oxygen ion conductor and the second phase comprises a donor-doped, n-type electronically conductive oxide, wherein the electronically

What is claimed is: 1. A mixed ionic and electronic conducting membrane, comprising: a two-phase solid state ceramic composite, wherein the first phase comprises an oxygen ion conductor and the second phase comprises a donor-doped, n-type electronically conductive oxide, wherein the electronically conductive oxide is stable at an oxygen partial pressure less than about 10-7 atm and has an electronic conductivity of at least 1 S/cm. 2. The membrane of claim 1, wherein the electronically conductive oxide comprises an n-type semiconductor. 3. The membrane of claim 1, wherein the membrane is stable at an oxygen partial pressure in the range of 10-7-10-20 atm. 4. The membrane of claim 1, wherein the membrane is stable at an oxygen partial pressure in the range of 10-16-10-20 atm. 5. The membrane of claim 1, wherein the oxygen ion conductor is selected from the group consisting of rare earth doped ceria, rare earth doped zirconia, rare earth doped thoria, rare earth doped hafnia and alkaline earth doped lanthanum gallium oxide. 6. The membrane of claim 1, wherein the oxygen ion conductor comprises a mixed metal oxide having a fluorite structure. 7. The membrane of claim 1, wherein the electronically conductive oxide comprises a donor-doped perovskite. 8. The membrane of claim 5, wherein the electronically conductive oxide comprises a donor-doped perovskite. 9. The membrane of claim 1, wherein the electronically conductive oxide is selected from the group consisting of donor-doped indium oxides and donor-doped tin oxides. 10. The membrane of claim 1, wherein the electronically conductive oxide comprises donor-doped strontium titanate. 11. The membrane of claim 10, wherein the donor-doped strontium titanate comprises RxSr1-xTi1-yR'yO3-δ, wherein R is a rare earth or alkaline earth element, R' is Al, x is in the range of about 0.01 to 0.5 and Y is in the range of about 0 to 0.2. 12. The membrane of claim 1, wherein the membrane has a thickness of about 1 μm to 3 mm. 13. The membrane of claim 1, wherein the volume ratio of first phase to second phase is in the range of from about 20:80 to about 80:20. 14. The membrane of claim 1, wherein the volume ratio of the first phase to the second phase is in the range of from about 40:60 to about 60:40. 15. The membrane of claim 1, wherein the first phase forms a percolative path in the composite. 16. The membrane of claim 1, wherein the second phase forms a percolative path in the composite. 17. The membrane of claim 1, wherein the membrane is stable at a temperature greater than about 500° C. 18. The membrane of claim 1, wherein the membrane is stable at a temperature in the range of 700-1000° C. 19. The membrane of claim 1, wherein the membrane is stable in the presence of steam. 20. A hydrogen purification system, comprising: a source of reforming gas; a source of steam; a flow cell comprising a first oxidizing compartment and a second reducing compartment separated by a two-phase solid state ceramic membrane, wherein the first phase comprises an oxygen ion conductor and the second phase comprises a donor-doped n-type electronically conductive oxide, wherein the electronically conductive oxide is stable at an oxygen partial pressure less than about 10-7 atm and has an electronic conductivity of at least 1 S/cm; a conduit for directing the reforming gas across the membrane in the first compartment; a conduit for directing the steam across the membrane in the second compartment; and a condenser downstream from the second compartment for separating steam from hydrogen. 21. The system of claim 20, wherein the electronically conductive oxide comprises an n-type semiconductor. 22. The system of claim 20, wherein the membrane is stable at an oxygen partial pressure in the range of 10-7-10-20 atm. 23. The system of claim 20, wherein the membrane is stable at an oxygen partial pressure in the range of 10-16-10-20 atm. 24. The system of claim 20, wherein the oxygen ion conductor is selected from the group consisting of rare earth doped ceria, rare earth doped zirconia, rare earth doped thoria, rare earth doped hafnia and alkaline earth doped lanthanum gallium oxide. 25. The system of claim 20, wherein the oxygen ion conductor comprises a mixed metal oxide having a fluorite structure. 26. The system of claim 20, wherein the electronically conductive oxide comprises a donor-doped perovskite. 27. The system of claim 25, wherein the electronically conductive oxide comprises a donor-doped perovskite. 28. The system of claim 20, wherein the electronically conductive oxide is selected from the group consisting of donor-doped indium oxides and donor-doped tin oxides. 29. The system of claim 20, wherein the electronically conductive oxide comprises donor-doped strontium titanate. 30. The system of claim 29, wherein the donor-doped strontium titanate comprises RxSr1-xTi1-yR'yO3-δ, wherein R is a rare earth or alkaline earth element, R' is Al, x is in the range of about 0.01 to 0.5 and Y is in the range of about 0 to 0.2. 31. The system of claim 20, wherein the membrane has a thickness of about 1 μm to 3 mm. 32. The system of claim 20, wherein the volume ratio of first phase to second phase is in the range of from about 20:80 to about 80:20. 33. The system of claim 20, wherein the volume ratio of the first phase to the second phase is in the range of from about 40:60 to about 60:40. 34. The system of claim 20, wherein the first phase forms a percolative path in the membrane. 35. The system of claim 20, wherein the second phase forms a percolative path in the membrane. 36. The system of claim 20, wherein the membrane is stable at a temperature in the range of 700-1000° C. 37. A method of purifying hydrogen purification, comprising: providing a flow cell comprising a first oxidizing compartment and a second reducing compartment separated by a two-phase solid state ceramic membrane, wherein the first phase comprises an oxygen ion conductor and the second phase comprises a donor-doped n-type electronically conducting oxide; directing the reforming gas across the membrane in the first compartment; directing the steam across the membrane in the second compartment; and collecting purified hydrogen downstream from the second compartment by separating steam from hydrogen.