Gas phase modification of solid oxide fuel cells

Abstract

A solid oxide fuel cell comprising an electrolyte, an anode and a cathode. In this fuel cell at least one electrode has been modified with a promoter using gas phase infiltration.

Claims

1. A solid oxide fuel cell, comprising: an electrolyte; an anode; and a cathode, wherein at least one electrode has been modified with a promoter layer, using gas phase infiltration, having a thickness from 1 nm to 1000 nm, wherein the electrode is infiltrated with the promoter selected from the group consisting Pr, Ce, Sn, Sm, Gd or combinations thereof, and wherein the anode is pre-reduced at a temperature from 400 C. to 800 C. in a reducing atmosphere containing 1-100% hydrogen or other reducing gas atmospheres.

2. The solid oxide fuel cell of claim 1, wherein the gas phase infiltration technique involves chemical vapor deposition.

3. The solid oxide fuel cell of claim 1, wherein the gas phase infiltration technique involves physical vapor deposition.

4. The solid oxide fuel cell of claim 1, wherein the electrode infiltrated is the anode.

5. The solid oxide fuel cell of claim 1, wherein the electrode infiltrated is the cathode.

6. The solid oxide fuel cell of claim 1, where the promoter is an oxide or a hydroxide.

7. The solid oxide fuel cell of claim 1, where the promoter is an oxide or an hydroxide.

8. The solid oxide fuel cell of claim 1, wherein the gas phase infiltration occurs at temperatures equal to or less than the temperature used for electrolyte sintering.

9. The solid oxide fuel cell of claim 1, wherein the gas phase infiltration occurs at temperatures less than 1000 C.

10. The solid oxide fuel cell of claim 1, wherein the gas phase infiltration occurs at temperatures less than 500 C.

11. The solid oxide fuel cell of claim 1, wherein the infiltration occurs after the formation of the fuel cell.

12. The solid oxide fuel cell of claim 1, wherein the electrolyte is selected from the group comprising of yittria stabilized zirconia, scandia stabilized zirconia, gadolinium doped ceria, samarium doped ceria, doped barium zirconate cerate, or combinations thereof.

13. The solid oxide fuel cell of claim 1, wherein the cathode is selected from the group comprising of: lanthanum strontium iron cobalt oxide, doped ceria, strontium samarium cobalt oxide, lanthanum strontium iron oxide, lanthanum strontium cobalt oxide, barium strontium cobalt iron oxide, or combinations thereof.

14. The solid oxide fuel cell of claim 1, wherein the anode is selected from the group comprising of: nickel oxide, nickel, yittria stabilized zirconia, scandia stabilized zirconia, gadolinium doped ceria, samarium doped ceria, doped barium zirconate cerate, or combinations thereof.

15. The solid oxide fuel cell of claim 1, wherein the electrolyte comprises a Sc doped BZCY.

16. The solid oxide fuel cell of claim 1, wherein the electrolyte comprises a porous BZCYYb electrolyte.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) A more complete understanding of the present invention and benefits thereof may be acquired by referring to the follow description taken in conjunction with the accompanying drawings in which:

(2) None

DETAILED DESCRIPTION

(3) Turning now to the detailed description of the preferred arrangement or arrangements of the present invention, it should be understood that the inventive features and concepts may be manifested in other arrangements and that the scope of the invention is not limited to the embodiments described or illustrated. The scope of the invention is intended only to be limited by the scope of the claims that follow.

(4) The current embodiments describe a solid oxide fuel cell (SOFC) comprising an electrolyte, an anode and a cathode. In this SOFC at least one electrode has been modified with a promoter using gas phase infiltration.

(5) In one embodiment of the SOFC the anode is typically porous to allow the fuel to flow towards the electrolyte. Anodes are typically chosen for their (1) high electrical conductivity; (2) a thermal expansion that matches those of the adjoining components; (3) the capacity to avoid coke deposition; (4) fine particle size; (5) chemical compatibility with other cell components (electrolyte and interconnector) under a reducing atmosphere at the operating temperature; (6) large triple phase boundary; (7) high electrochemical or catalytic activity for the oxidation of fuel; (8) high porosity (20-40%) adequate for the efficient transport of fuel and reaction products; (9) good electronic and ionic conductive phases and (10) good catalytic activity for hydrocarbon reforming. In the current SOFC any known anode electrodes can be utilized. Types of anodes that can be used include Ni/YSZ, Cu/Ni, perovskite structures with a general formula of ABO.sub.3. In the perovskite structure the A cations can be group 2, 3, or 10 elements or more specifically cations such as Ba, La, Sr, Ca or Sm. Also in the perovskite structure the B cations can be group 4, 6, 7, 8, 9, or 10 elements or more specifically cations such as Ti, Cr, Ni, Fe, Co, Mn or Zr. Other materials that the anode could be include nickel oxide, nickel, yittria stabilized zirconia, scandia stabilized zirconia, gadolinium doped ceria, samarium doped ceria, doped barium zirconate cerate, or combinations thereof.

(6) In one embodiment the anode can be pre-reduced at a temperature from about 400 C. to about 800 C. in a reducing atmosphere containing 1-100% hydrogen or other reducing gas atmospheres.

(7) In one embodiment of the SOFC the cathode is typically porous to allow the oxygen reduction to occur. Any cathode material known to those skilled in the art can be used. One example of cathode materials that are typically used include perovskite-type oxides with a general formula of ABO.sub.3. In this embodiment the A cations are typically rare earths doped with alkaline earth metals including La, Sr, Ca, Pr or Ba. The B cations can be metals such as Ti, Cr, Ni, Fe, Co, Cu or Mn. Examples of these perovskite-type oxides include LaMnO.sub.3. In one differing embodiment the perovskite can be doped with a group 2 element such as Sr.sup.2+ or Ca.sup.2+. In another embodiment cathodes such as Pr.sub.0.5Sr.sub.0.5FeO.sub.3; Sr.sub.0.9Ce.sub.0.1Fe.sub.0.8Ni.sub.0.2O.sub.3; Sr.sub.0.8Ce.sub.0.1Fe.sub.0.7Co.sub.0.3O.sub.3; LaNi.sub.0.6Fe.sub.0.4O.sub.3; Pr.sub.0.8Sr.sub.0.2Co.sub.0.2Fe.sub.0.8O.sub.3; Pr.sub.0.7Sr.sub.0.3Co.sub.0.2Mn.sub.0.8O.sub.3; Pr.sub.0.8Sr.sub.0.2FeO.sub.3; Pr.sub.0.6Sr.sub.0.4Co.sub.0.8Fe.sub.0.2O.sub.3; Pr.sub.0.4Sr.sub.0.6Co.sub.0.8Fe.sub.0.2O.sub.3; Pr.sub.0.7Sr.sub.0.3Co.sub.0.9Cu.sub.0.1O.sub.3; Ba.sub.0.5Sr.sub.0.5Co.sub.0.8Fe.sub.0.2O.sub.3; Sm.sub.0.5Sr.sub.0.5CoO.sub.3; or LaNi.sub.0.6Fe.sub.0.4O.sub.3 can be utilized. Other materials that the cathode could be include lanthanum strontium iron cobalt oxide, doped ceria, strontium samarium cobalt oxide, lanthanum strontium iron oxide, lanthanum strontium cobalt oxide, barium strontium cobalt iron oxide, or combinations thereof.

(8) The electrolyte used in the SOFC is responsible for conducting ions between the electrodes, for the separation of the reacting gases, for blocking internal electronic conduction, and for forcing the electrons to flow through the external circuit. Some of the typical characteristics that electrolytes typically invoke include (1) an oxide-ion conductivity greater than 10.sup.3 S.Math.cm.sup.1 at the operating temperature; (2) negligible electronic conduction, which means an electronic transport number close to one; (3) high density for gas impermeability; (4) thermodynamic stability over a wide range of temperatures and oxygen partial pressures; (5) a thermal expansion coefficient that is compatible with that of the electrodes and other cell materials from ambient temperature to cell operating temperature; (6) suitable mechanical properties, with fracture resistance greater than 100 MPa at room temperature; (7) negligible chemical interaction with electrode materials under operation and fabrication conditions to avoid formation of blocking interface phases; (8) ability to be fabricated as thin layers (less than 30 m) and (9) low cost of starting materials and fabrication.

(9) In the current SOFC the electrolyte can be any electrolyte known to those skilled in the art. In one embodiment the electrolyte comprises a porous BZCYYb as the backbone and carbonate as the secondary phase within the pores of.

(10) The weight ratio of BZCYYb in the composite electrolyte may vary, as long as the composite electrolyte can reach higher conductivity as well as current density as compared to non-composite electrolyte. In one embodiment, the weight ratio of BZCYYb in the composite electrolyte ranges from 9:1 to 1:1, but more preferably ranges from 50-90% or 70-80%. In another embodiment, the weight ratio of BZCYYb is about 75%.

(11) The weight percentage of carbonate in the composite electrolyte also may vary, as long as the composite electrolyte can maintain physical integrity during operation. In one embodiment, the weight percentage of carbonate in the composite electrolyte ranges from 10 to 50 wt %. In another embodiment, the weight percentage of carbonate in the composite electrolyte ranges from 20 to 30 wt %, in yet another embodiment, the carbonate is about 25%.

(12) In one example of preparing BZCYYb lithium-potassium carbonate is typically made first. Stoichiometrical amount of Li.sub.2CO.sub.3 and K.sub.2CO.sub.3 were mixed in the weight proportion of 45.8:52.5 and milled in a vibratory mill for 1 hour. The mixture was then heated to 600 C. for 2 hours. The heated mixture was then quenched in air to the room temperature and ground. The resulting lithium-potassium carbonate was used later in the preparation of composite electrolyte with BZCYYb.

(13) In one embodiment the BZCYYb powder was prepared by solid-state reaction, but other methods could also be used. Stoichiometric amounts of high-purity barium carbonate, zirconium oxide, cerium oxide, ytterbium oxide and yttrium oxide powders (all from Sigma-Aldrich Chemicals) were mixed by ball milling in ethanol (or other easily evaporated solvent) for 24 h, followed by drying at 80 C. for overnight and calcinations at 1100 C. in air for 10 h. The calcinated powder was ball milled again, followed by another calcination at 1100 C. in air for 10 h to produce single phase BZCYYb.

(14) The resulted BZCYYb powder and the carbonate obtained above were mixed at weight ratio of 75:25 and thoroughly ground again for one hour. The mixture was then heated to 680 C. for 60 minutes until only the carbonate melted and wet the BZCYYb grain boundaries in the mixture. Next, it was quenched (i.e. fast cooling) in air to room temperature. The quenched mixture was ground again to get the composite electrolyte powder.

(15) In another example an alternate way of preparing BZCYYb powder can be described. In this embodiment stoichiometric amounts of high-purity barium carbonate, zirconium oxide, cerium oxide, ytterbium oxide, and yttrium oxide powders (all from Sigma-Aldrich Chemicals) were mixed by ball milling in ethanol for 48 h, followed by drying in an oven and calcination at 1100 C. in air for 10 h. The calcined powder was ball milled again, followed by another calcination at 1100 C. in air for 10 h.

(16) The CeO.sub.2 and ZrO.sub.2 powders with different particle sizes were used to optimize the fabrication procedures. To prepare electrolyte samples for the conductivity measurement, we pressed the calcined powders isostatically into a disk at 274.6 MPa. The green disks had a diameter of 10 mm, with a typical thickness of 1 mm. The disks were then sintered at 1500 C. for 5 h in air (relative density >96%).

(17) In some embodiments a Sc-doped BZCY powder can be prepared. In one example of this embodiment BZCY-Sc with a nominal composition of BaCe.sub.0.7Zr.sub.0.1Y.sub.0.1Sc.sub.0.1O.sub.3- (BZCY-Sc) was synthesized by a conventional solid state reaction (SSR) method. Stoichiometric amount of high-purity barium carbonate, zirconium oxide, cerium oxide, yttrium oxide and scandium oxide powders (BaCO.sub.3: ZrO2: CeO.sub.2: Y.sub.2O.sub.3: Sc.sub.2O.sub.3=167.33: 12.32: 120.48: 22.58: 13.79, all from Sigma-Aldrich Chemicals) were mixed by ball milling in ethanol for 24 hours, followed by drying at 80 C. for overnight and calcinations at 1100 C. in air for 10 hours. The calcined powder was ball milled again, followed by another calcination at 1100 C. in air for 10 hours to produce single phase BZCY-Sc.

(18) In the present method, the calcining step is carried out at preferably higher than 1000 C. in air for 10 hours. However, the temperature and the length of calcination can vary, depending on different factors to be considered, such as the particle size chosen. The particle size of the zirconium oxide powder is preferably between 50 nm and 200 nm, and more preferably between 50 nm and 100 nm. The particle size of the cerium oxide powder is preferably between 1 m and 20 m, and more preferably between 5 and 10 m.

(19) As described above the current embodiments describe a SOFC wherein at least one electrode has been infiltrated with a promoter using gas phase infiltration. This infiltration can occur either before the formation of the fuel cell or after the formation of the fuel cell.

(20) In another embodiment the electrolyte can be yittria stabilized zirconia, scandia stabilized zirconia, gadolinium doped ceria, samarium doped ceria, doped barium zirconate cerate, or combinations thereof.

(21) Different types of gas phase infiltration can be utilized. In one embodiment the gas phase infiltration involves chemical vapor deposition (U.S. Pat. No. 5,261,963) or even physical vapor deposition. Examples of physical vapor deposition can include evaporative deposition or sputter deposition. The infiltration can either occur at the anode or the cathode.

(22) In one embodiment the promoter is selected from the group consisting of a group 1, group 2, group 4, group 6 or lanthanide elements when infiltrating the anode electrode. More specifically, the infiltration can be with the group selected from Mo, Pr, Mg, Ca, Sr, Ba, K, Ce, La, Zr, Mn, Fe, Pd, Cu, Sn, Pt, Ag, Ru, Ir, Sm, Gd elements, or combinations thereof. In some embodiments the promoter can be an oxide or a hydroxide.

(23) In another embodiment the promoter is selected from the group consisting of a group 2, group 8, group 9, group 10, group 15 or lanthanide elements when infiltrating the cathode electrode. More specifically, the infiltration can be with the group selected from Pr, Sr, Ce, Fe, Co, La, Sm, Ni, Gd, Ca, Ba, Bi, Ga, Mg, Pt, Ag, Ru elements, or combinations thereof. In some embodiments the promoter can be an oxide or a hydroxide.

(24) In one embodiment during chemical vapor deposition the promoter would be volatile element or compounds such as Ba, Mo, TiCl, or metal-organic compounds.

(25) During this gas phase infiltration the promoter layer added to either the anode or the cathode can vary in thickness from about 1 nm to 100 nm or even 1000 nm both on the surface and the interior of the electrode.

(26) The temperature of the gas phase infiltration can occur at temperatures less than 1000 C., 500 C. or even 250 C.

(27) In one embodiment the SOFC infiltrated is comprises a porous BZCYYb electrolyte or a Sc-doped BZCY.

(28) In closing, it should be noted that the discussion of any reference is not an admission that it is prior art to the present invention, especially any reference that may have a publication date after the priority date of this application. At the same time, each and every claim below is hereby incorporated into this detailed description or specification as an additional embodiment of the present invention.

(29) Although the systems and processes described herein have been described in detail, it should be understood that various changes, substitutions, and alterations can be made without departing from the spirit and scope of the invention as defined by the following claims. Those skilled in the art may be able to study the preferred embodiments and identify other ways to practice the invention that are not exactly as described herein. It is the intent of the inventors that variations and equivalents of the invention are within the scope of the claims while the description, abstract and drawings are not to be used to limit the scope of the invention. The invention is specifically intended to be as broad as the claims below and their equivalents.