Abstract:
A perovskite lanthanum gallate electrolyte doped with strontium and magnesium and a solid oxide fuel cell incorporating a doped lanthanum gallate electrolyte with a cathode on one side, an anode on the other side and a buffer layer comprising a mixed electronic and oxide-ion conductor between the anodes and/or the cathode and the electrolyte to block unwanted chemical reactions while permitting electronic and oxide-ion transport.

Description:
BRIEF DESCRIPTION OF THE INVENTION 
     This invention relates generally to solid oxide fuel cells and more particularly to a solid electrolyte for solid oxide fuel cells and to the use of a buffer layer between the electrolyte and one or both of the electrodes to improve solid oxide fuel cells incorporating the electrolyte. 
     BACKGROUND OF THE INVENTION 
     The solid oxide fuel cell (SOFC) promises a high conversion efficiency (40 to 60%) of chemical energy to electric power with negligible pollution and is attractive for use in the co-generation of electric power. The prototype SOFCs now being marketed use yttria-stabilized zirconia (YSZ) as the oxide-ion electrolyte. This requires an operating temperature T op  ≈1000° C. if conventional ceramic membranes are used. The interconnect between individual cells must be stable in both the oxidizing atmosphere at the cathode and the reducing atmosphere at the anode, and at T op  ≈1000° C. it is necessary to use a conducting ceramic for the interconnects. However, even the ceramic of choice, Ca-doped LaCrO 3 , loses oxygen from the side exposed to the anodic atmosphere and gains oxygen on the side exposed to the cathodic atmosphere. This causes the interconnect membranes to warp. An operating temperature in the range 600° C.&lt;T&lt;800° C. could allow the use of an oxidation-resistant stainless steel or another alloy as the interconnect material. The lower operating temperature would also reduce operating costs, increase durability, extend service life, and permit more frequent cycling. 
     Two approaches to a T op  &lt;800° C. are under active consideration: (1) reduction of the thickness of the YSZ electrolyte membrane to l≦10 μm and (2) use of a solid electrolyte having an oxide-ion conductivity at or below 800° C. that is comparable to that of YSZ at 1000° C. The most promising traditional material for the second approach is CeO 2  doped with an alkaline-earth oxide, AO, or a rare earth oxide, Ln 2  O 3 , but reduction of Ce 4+   to Ce 3+   in the anodic gas introduces into the electrolyte an unwanted polaronic conduction. 
     There is a need for a solid electrolyte having a high oxide-ion conductivity at a reduced operating temperature, negligible electronic conductivity over a wide range of oxygen partial pressure, viz 10 -22  &lt;Po 2  &lt;1 atm, and stable performance over extended periods of time. 
     The pseudo-cubic perovskite system La 1-x  Sr x  Ga 1-y  Mg y  O 3-0 .5(x+y) is attracting increasing attention as an oxide-ion solid electrolyte competitive with yttria-stabilized zirconia; it demonstrates an oxide-ion conductivity σ o  ≧0.10 S/cm at 800° C., a negligible electronic conduction at temperatures T&lt;1000° C. over a broad range of oxygen partial pressure from pure oxygen Po 2  =1 atm) to moistened hydrogen (Po 2 .sup.˜ 10 -22  atm), and a stable performance over long operating periods. These superior electrical and chemical properties make it a strong candidate for use as the solid electrolyte in reduced-temperature solid oxide fuel cells (RTSOFCs) operating at or below 800° C. 
     A typical prior art doped gallate, La 0 .9 Sr 0 .1 Ga 0 .8 Mg 0 .2 O 2 .85, has an ionic conductivity of 0.07-0.1 S/cm 2  at 800° C. However, at room temperature it always contains an undesirable nonconducting second phase LaSrGaO 4 . The presence of this phase, which may gradually disappear at higher temperatures, may explain the unusually rapid decrease of ionic conductivity with decreased temperature. For instance, the conductivity, measured by a 4 probe DC measurement technique, was 0.075 S/cm at 800° C., but only 0.028 S/cm at 700° C. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is a general object of the present invention to provide a solid electrolyte for solid oxide fuel cells (SOFCs) having superior electrical and chemical properties. 
     It is another object of the present invention to provide a strontium and magnesium doped lanthanum gallate perovskite solid electrolyte for reduced temperature solid oxide fuel cells (RTSOFCs). 
     It is a further object of the present invention to provide a phase pure strontium, magnesium doped lanthanum gallate electrolyte exhibiting increased ionic conductivity. 
     It is a further object of the present invention to provide an improved reduced temperature solid oxide fuel cell. 
     It is a further object of the present invention to provide a fuel cell having a phase pure strontium, magnesium doped lanthanum gallate electrolyte exhibiting increased ionic conductivity with anode and cathode on opposite sides of the electrolyte and a buffer layer between the anode and electrolyte. 
     There is provided a lanthanum gallate perovskite electrolyte for solid oxide fuel cells in which the A/B ratio of atoms of the perovskite [A (La, Sr)] and [B (Ga, Mg)] is smaller than or equal to one. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     The foregoing and other objects of the invention will be more clearly understood from the following description and the accompanying drawings in which: 
     FIG. 1 shows a solid oxide fuel cell incorporating a solid electrolyte and an anode buffer layer in accordance with the present inventions. 
     FIG. 2A is a phase diagram of the LaO 1 .5 -SrO-GaO 1 .5 -MgO system showing the compositions of a pseudo-cubic perovskite in accordance with the inventions. 
     FIG. 2B is an enlarged view of a portion of the phase diagram of FIG. 2A. 
     FIGS. 3A, 3B are iso-conductivity contours for the La 1-x  Sr x  Ga 1-y  Mg y  O 3-0 .5(x+y) system showing the highest conductivity region at 800° C. and at 700° C., respectively, in accordance with the inventions. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows a fuel cell including a pseudo-cubic perovskite system as the solid electrolyte 11. This solid electrolyte comprises a phase pure lanthanum (La) gallium (Ga) perovskite doped with strontium (Sr) and magnesium (Mg). The doping is such that an A/B ratio less than or equal to one is achieved either by reducing the Sr content on the A site of the lattice or increasing the Mg content on the B site below or above that required for strict stoichiometry. That is, a non-stoichiometric compound may be formed. We have found that a phase pure perovskite compound having excellent oxide-ion conductivity without detrimental electronic conductivity in either highly oxidizing or highly reducing conditions is obtained with the following non-stoichiometric range. 
     
         La.sub.1-x-w Sr.sub.x-w Ga.sub.1-y Mg.sub.y+z O.sub.3-0.5(x+y+5w-2z) 
    
     where 
     0.1≦x≦0.3 
     0.1≦y≦0.3 
     0.01≦w≦0.04 
     0.03≦z≦0.15 
     FIG. 2 is a phase diagram of compositions of a pseudo-cubic perovskite in accordance with the above compositions. Compositions falling in this range generally exhibit a conductivity ≧0.12 S/cm at 800° C. and ≧0.06 S/cm at 700° C. Maximum conductivities of 0.166 S/cm and 0.079 S/cm at 800° C. and 700° C. respectively, are found in the following, preferred composition range: La0.8-0.85 Sr0.15-0.2 Ga 0 .8-0.85 Mg. 0 .18-0.3 O 2 .753-2.810. The preferred compositions fall within the 0.12 S/cm conductivity field, while the optimum compositions fall within the 0.14-0.16 S/cm line. Iso-conductivity contours for the La 1-x  Sr x  Ga 1-y  Mg y  O 3-0 .5(x+y) system are shown in FIGS. 3A and 3B for 800° C. and 700° C., respectively. 
     The non-stoichiometric A-site Sr deficient and B-site Mg excessive La 1-x-w  Sr x-w  Ga 1-y  Mg y+z  O 3-0 .5 (x+y+5w-2z) compounds were prepared by mixing non-stoichiometric amounts of La 2  O 3 , SrCO 3 , Ga 2  O 3  and MgO where w, x, y and z range from 0.01 to 0.04, 0.1 to 0.3, 0.1 to 0.3, and 0.03 to 0.15, respectively. The compounds were mixed with the aid of acetone and fired at 1250° C. overnight. After regrinding and pelletizing, the mixture was sintered for 36 hours at 1470° C. 
     The fuel cell of FIG. 1 includes an La 0 .6 Sr 0 .4 CoO 3- δ (LSCo) cathode 12. SrCO 3 , La 2  O 3  and Co 3  O 4  were used to make the La 0 .6 Sr 0 .4 CoO 3- δ  cathode. Two types of anodes 13 were formed by the reduction of LSGM/NiO or CeO 2  /NiO composites to give porous LSGM or CeO 2  anodes with metallic Ni particles in the walls of the porous channels. These composites represent standard anode materials in SOFCs. 
     In one example the electrodes were fabricated on the top and bottom of the 500-μm-thick electrolyte membrane 11 by screen-printing a slurry of an intimate mixture of electrode powder and organic binder. After baking at 1150° C. for two hours, Pt meshes with Pt leads and an electrode paste to achieve good contact were fixed on top of each electrode to act as current collectors. The effective electrode area was 2.5 cm 2 . The cells were glass-sealed into ZrO 2  tubes at 1100° C. for 30 minutes since ZrO 2  has a thermal expansion coefficient similar to that of LSGM. The glass sealant used was developed by Ceramatec, Inc. 
     The cells were tested by placing them in the hot zone of a vertical furnace. Air was supplied directly to the cathode surface; water-moistened hydrogen (at .sup.˜ 30° C.) was fed to the anode surface at a rate of 100 ml/min. All of the tests and the heating/cooling of the furnace was controlled by computer; the tests were carried out in the temperature range 600° C.&lt;T op  &lt;800° C. with an interval of 50° C. 
     Single-cell performance tests of a solid oxide fuel cell (SOFC) constructed as described above and without the interlayer (14) shown in FIG. 1 using Sr- and Mg-doped LaGaO 3  (LSGM) with a Sr-doped LaCoO 3  (LSCO) cathode and an LSGM+Ni or CeO 2  +Ni composite anode have shown improved power output compared with a SOFC using a stabilized ZrO 2  or CeO 2  as the solid electrolyte. For example, a single-cell test with an LSGM membrane 500 μm thick gave a stable peak power density of 270 mW/cm 2  at 800° C. with an LSCo cathode and a Ca:CeO 2  +Ni anode; the power output with an LSGM+Ni anode was 300 mW/cm 2  at 800° C., but it was not stable. A three-electrode configuration was used to monitor the overpotential at the two electrodes as well as across the electrolyte. A high overpotential was found on the anode side: 480 mV for the Ca:CeO 2  +Ni anode and 300 mV for LSGM+Ni anode at 0.65 A/cm 2  compared with 110 mV for the LSCo cathode at the same current density. We found that the cause of the high anode overpotential was a reaction between LSGM and Ni. An independent investigation of the reactivity between the LSGM electrolyte and the NiO revealed the formation of the metallic perovskite LaNiO 3 , which is a poor oxide-ion conductor. In the formation of an anode, a NiO/LSGM or NiO/Ca:CeO 2  composite is reduced by the fuel to a porous LSGM or Ca:CeO 2  with elemental Ni on the sides of the pores. In order to avoid the formation of LaNiO 3  at the interface of the electrolyte and anode, we introduced a thin buffer layer between the anode and the electrolyte layer (14) of FIG. 1, that would block the unwanted chemical reaction without suppressing the oxide-ion permeability. For this purpose, we chose a material that conducts O 2-   ions and electrons under reducing atmosphere. Our initial candidate was CeO 2  doped with a lower-valent cation substituting for Ce so as to introduce oxide-ion vacancies as in Ce 1-x  Sm x  O 2-0 .5x or Ce 1-x  Ca x  O 2-x . Such a buffer layer not only blocked migration of La 3+   and Ni 2+  across it, thus preventing formation of LaNiO 3  ; it also conducted O 2-   ions and was itself a catalyst for the oxidation of the fuel, e.g. H 2  to OH -   and eventually to H 2  O. 
     Single-cell tests were made with our LSGM electrolyte 500 μm thick and an LSCo cathode area of 2.5 cm 2 . With this standardized three-electrode configuration, two anodes of similar area were compared: Sm:CeO 2  +Ni with and without Sm:CeO 2  buffer layer. 
     The cells with buffer layer were: Air, Pt, LsCo|optimized LSGM|Sm:CeO 2  |Sm:CeO 2  +Ni, Pt, H 2  +H 2  O, and 
     without buffer layer were: Air, Pt, LSCo|optimized LSGM|Sm:CeO 2  +Ni, Pt, H 2  +H 2  O 
     The performances of these single cells with 500-μm thick electrolyte in the temperatures range 600≦T op  ≦800° C. are summarized in the following table. 
     
                       TABLE I______________________________________OCV,         current density at peak                      peak power density,V                  power density, A/cm.sup.2                              mW/cm.sup.2______________________________________cell withbuffer layer800° C.                    1.10                     540700° C.                    0.45                     226600° C.                    0.12                     64cell withoutbuffer layer800° C.                    0.52                     270700° C.                    0.24                     180600° C.                    0.07                     37.9______________________________________ 
    
     Thus there has been provided a superior solid electrolyte for solid oxide fuel cells, improved solid oxide fuel cells employing the novel electrolyte and superior solid oxide fuel cells employing the novel electrolyte and a buffer layer between the fuel cell anode and the electrolyte.