Abstract:
A solid-oxide electrolyte operable at between 600° C. and 800° C. and a method of producing the solid-oxide electrolyte are provided. The solid-oxide electrolyte comprises a combination of a compound having weak metal-oxygen interactions with a compound having stronger metal-oxygen interactions whereby the resulting combination has both strong and weak metal-oxygen interaction properties.

Description:
CONTRACTUAL ORIGIN OF THE INVENTION 
     The United States has rights in this invention pursuant to contract No. W-31-109-ENG-38 between the United States Government and Argonne National Laboratory. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to high temperature ionic conductors for solid-oxide fuel cells and more particularly to a Bi-Al-O system that displays superior conductivity levels at temperatures between 500° C. and 800° C. 
     2. Background of the Invention 
     Solid-state oxide fuel cells (SOFC&#39;s) can become one of the most durable and economic fuel systems for utility and transportation applications. Using solid electrolytes virtually eliminates corrosion reactions and electrolyte losses that are common in liquid electrolyte fuel cells. Furthermore, fuel processing for SOFC&#39;s is simpler and less expensive than other types of fuel cells. 
     Presently, SOFC&#39;s operate at temperatures of approximately 1000° C. The requirement of high operating temperatures to obtain adequate conductivity levels limits the number of materials available for SOFC fabrication as most of the materials become compromised thermally, chemically and mechanically under these high temperature conditions. For example, the conductivity of the commonly used yttrium-stabilized zirconium oxide is 10 -1  ohm -1  cm -1  at 1000° C. This conductivity decreases to 4×10 -2  ohm -1  cm -1  at 800° C. Examples of yttria-stabilized zirconia electrolytes used at high temperatures can be found in U.S. Pat. Nos. 4,476,196; 4,476,197 and 4,476,198, wherein the electrolytes facilitate ion transfer in electrochemical fuel cells operating in temperatures exceeding 1000° C. As with the above-mentioned teachings, most fuel cells incorporating the yttria-stabilized zirconia also rely on standard materials such as zirconia-based cermet and LaMnO 3  as constituents for the accompanying electrodes. 
     Operating an SOFC at more moderate temperatures, such as 600°-800° C., would allow much greater flexibility in engineering the fuel stack because metals could be used as interconnect and gasket materials. This would ultimately reduce the cost and open up new applications. With the present technology, it is not possible to lower the operating temperature of the fuel cell because the electrical resistance of the electrolyte increases exponentially as temperature decreases. To decrease the operating temperature, a new electrolyte must be found. 
     New electrolytes have been produced which provide superior conductance at temperatures ranging from 500°-800° C. The materials are selected and produced based on an observed relationship between the conductivity of a material and the metal-oxygen interaction strength in the material. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a class of electrolytes that overcomes many of the disadvantages of prior art arrangements. 
     It is another object of the present invention to provide a class of electrolytes for use in utility and transportation applications. A feature of the present class of electrolytes is their use at temperatures of between approximately 500° C. and 800° C. An advantage of the present invention is the ability to now incorporate a wider range of materials in the fabrication of solid-oxide fuel cells. 
     Still another object of the present invention is to provide a highly conductive electrolyte to operate at temperatures below 1000° C. A feature of the invention is the combination of compounds having weak intramolecular metal-oxygen interactions with compounds having relatively stronger intramolecular metal-oxygen interactions. An advantage of the present invention is that the resulting material has a relatively larger Gibbs free energy of formation than the more standard electrolytes now in use, leading to a more conductive electrolyte material. 
     Yet another object of the present invention is to provide materials that are more conductive than standard yttria-stabilized zirconium oxides. A feature of the invention is that the constituents of the invented electrolytes have properties concomitant with both strong and weak metal-oxygen interactions. The advantage of the invention is that the electrolytes represent a compromise between the high thermodynamic stability of refractory metal oxides, such as alumina, and the high ionic conductivity of reducible oxides, such as Bi 2  O 3 . 
     In brief, the objects and advantages of the present invention are achieved by a solid-oxide electrolyte. The electrolyte is comprised of a combination of a compound having weak metal-oxygen interactions with a compound having strong metal-oxygen interactions whereby the resulting combination has properties indicative of both strong and weak metal-oxygen interactions. The invention also provides a method of fabricating solid-oxide electrolytes comprised of a combination of a compound having weak metal-oxygen interactions with a compound having strong metal-oxygen interactions whereby the resulting combination has properties indicative of both strong and weak metal-oxygen interactions. Such materials can be used in fuel cells, sensors, batteries and other electrochemical devices. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The present invention together with the above and other objects and advantages may best be understood from the following detailed description of the embodiment of the invention illustrated in the drawings, wherein: 
     FIG. 1 is an Arrhenius plot depicting the variation of conductivity with temperature for an exemplary electrolyte having the general formula Bi 2  Al 4  O 9  of the present invention; and; 
     FIG. 2 is a plot depicting the comparison between the log of conductivity and bismuth content for an exemplary electrolyte having the general formula (La,Bi)AlO 3  of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The new electrolytes of the present invention described herein operate at temperatures ranging from 500°-800° C. to provide conductivities higher than presently used solid-oxide electrolytes operating at similar temperatures. 
     The invention centers around the selection of electrolyte materials having phases containing both strong and weak metal-oxygen interactions. The addition of compounds with a stronger metal-oxygen bond serves to add chemical stability towards hydrogen and oxygen at elevated temperatures which are environs in which the fuel cell operates. 
     Conductivity of the invented electrolytes can be measured by ac impedance spectroscopy in either air or in a humidified hydrogen/oxygen cell, the latter serving to mimic fuel stack conditions. Also, by measuring the electromotive force between the two electrodes and humidified hydrogen/oxygen and comparing the experimental to the theoretical values, the ionic transference number can be determined as additional assurance that the conductivity occurring is ionic and not electronic. Unity is the theoretical value depicting a one to one ion transfer through the electrolyte. 
     Material Types 
     Materials used as the electrolyte in the invention represent many compound systems, including Bi-M-O (wherein M is one or more metals), Re-M-O (wherein RE is a rare earth and M is one or more metals), and La-M-O (wherein M is one or more metals). These systems were chosen based on an observed inversely proportional relationship between the conductivity of material fabricated from compounds representing the system and the metal-oxygen interaction strength in the material. Interaction strength was gauged with model reactions using the end-member oxides, such as those illustrated in Equations 1 and 2, below: 
     
         M.sub.x O.sub.y +H.sub.2 →M.sub.x O.sub.y-1 +H.sub.2 OEq. 1 
    
     
         M.sub.x O.sub.y +yH.sub.2 →xM+yH.sub.2 O            Eq. 2 
    
     This invention exploits the classical thermodynamic phenomenon wherein the standard free energy of formation of a compound, ΔG f  °, is a measure of the stability of the substance. If a compound has a large negative value of ΔG f  °, the compound cannot be readily decomposed into its elements. Rather, that compound is likely to be formed as a product in any reaction in which its constituent elements are involved. Conversely, a compound with a relatively more positive ΔG f  20  is more unstable and has a tendency to decompose into its elements. 
     Thus, the inventors have observed that oxides with a more positive ΔG f  ° tend to be more conductive. For example, yttria-doped ZrO 2 , with the lowest ΔG f  ° value is the least conductive and yttria-doped Bi 2  O 3 , with the highest ΔG f  ° value is the most conductive. These trends are depicted in Table 1, below: 
     
                       TABLE 1______________________________________ΔG values of formation and conductivities forselected metal oxides.         ΔG.sub.f °/mol                    ConductivityMaterial      O.sub.2 at 727° C.,                    Ω.sup.-1 cm.sup.-1(Y-doped)     kcal/mol   at 700° C.______________________________________ZrO.sub.2     -220       1.8 × 10.sup.-2CeO.sub.2     -208       2.5 × 10.sup.-2Bi.sub.2 O.sub.3          -58       1.6 × 10.sup.-1______________________________________ 
    
     Yttrium-, erbium-, and niobium-doped Bi 2  O 3  are known to be excellent oxide ion conductors in the temperature range of 500°-800° C. However, these doped bismuth oxides are easily reduced in hydrogen. Ideally, an electrolyte should display high conductivity characteristics and good stability in fuel cell environments which are laden with H 2  and O 2 . 
     Surprisingly and unexpectedly, it has been found that when compounds having weak metal-oxygen interactions, such as Bi 2  O 3 , transition metal oxides, and rare earth metal oxides are combined with compounds having stronger metal-oxygen interactions, such as Al 2  O 3 , Y 2  O 3 , La 2  O 3 , and alkaline earth oxides, or when Bi 2  O 3  is incorporated in a refractory host lattice, a more stable and conductive electrolyte is obtained. 
     When the reducible oxide Bi 2  O 3  is combined with more stable refractory metal oxides, for example, the newly formed compound is more stable in hydrogen than doped bismuth oxide and also exhibits higher conductivity than any of the more stable metal oxides enumerated above. 
     The invented electrolytes cannot be categorized into any one structural class, although many structural classes, one being the perovskites, can be represented by the invention. Electrolytes were produced representing many systems, including Bi-M-O (wherein M is one or more metals), RE-M-O (wherein RE is a rare earth and M is one or more metals), and La-M-O (wherein M is one or more metals). Examples of the invented electrolytes include, but are not limited to, BiAlO 3 , Bi 2  Al 4  O 9 , BiYO 3 , La 0 .33 NbO 3 , La-M-Bi-Al-O wherein M is a metal, La 0 .8 Bi 0 .1 Ba 0 .1 AlO 3-x  wherein x is the oxygen content of the various oxides used in combination to make the compound, La 0 .7 Bi 0 .315 Al 0 .95 Zn 0 .05 O 3-x  wherein x is the oxygen content of the various oxides used in combination to make the compound, La 0 .6 Bi 0 .4 Al 0 .95 Zn 0 .05 O 2 .975, and M 2  LnBiO 6  wherein the M is an alkaline earth and Ln is a rare earth. Obviously, the x-value will change for any of the compounds enumerated above if oxidation states for some of the metals, such as Bi and Zn, change. For example, in the compound La 0 .8 Bi 0 .1 Ba 0 .1 AlO 3-x , the 3-x value is 2.95, assuming an oxidation state for Bi of +3. If this +3 oxidation state changes, the x-value will correspondingly change. 
     Any of the alkaline earth metals can be used in the electrolyte structure, such as Mg, Ca, Sr, and Ba. A myriad of rare earths are acceptable, including, but not limited to, La, Gd, Dy, Ho, Er and Lu. 
     To illustrate that the oxide-ion conductivity of a material is affected by the strength of the metal-oxygen bonds, an electrolyte was produced by starting with Bi 2  O 3 , which has weak, easily reducible metal-oxygen bonds, and then strengthening the bonds by adding another material with very strong metal-oxygen bonds, such as Al 2  O 3 . The combination, in approximately a 1:2 molar ratio, yielded Bi 2  Al 4  O 9 . In an oxygen gradient, the conductivity of this material at 800° C. was 5-8×10 -3  ohm -1  cm -1 , with transference numbers in the 0.6 to 0.7 range. At 800° C. the combination had conductivity values of 7-9×10 -2  ohm -1  cm -1 . FIG. 1 depicts the increase in conductivity values as a function of temperature for bismuth-containing compounds. 
     Bi 2  Al 4  O 9  is different from other Bi 2  O 3  materials in that the amount of Al 2  O 3  used is far above the so-called &#34;doping&#34; level. Furthermore, Bi 2  Al 4  O 9  represents a compromise between the high conductivity, relatively unstable, low Gibbs free energy Bi 2  O 3  species and the low conductivity, highly stable Al 2  O 3 . 
     Fabrication Methods 
     Two methods are employed to make the electrolyte combinations discussed above: combustion of a solution of metal nitrates, known as the glycine-nitrate process; and a solid state reaction of oxides which is a process of substituting, for example, the Bi-based ceramic into a refractory host lattice. The glycine-nitrate process provides the synthetic step of producing pre-calcined powders wherein an aqueous solution of the metal nitrate and glycine first undergoes evaporation and is then combusted. A more complete disclosure of the process can be found in Materials Letters 10:6, 1990, and is incorporated herein by reference. 
     Bi-Al-O Perovskite System Fabrication: Practically, pellets of the perovskite BiAlO 3  are difficult to make due to peritectic decomposition, and also because of the presence of the orthorhombic phase, Bi 2  Al 4  O 9 . Conversely, phase-pure samples of Bi 2  Al 4  O 9  could not be sintered into dense pellets. Instead, an approximately six mole percent of Bi 2  O 3  was combined with Bi 2  Al 4  O 9  via the glycine-nitrate process, and then formed into pellets. After calcining and sintering this material, at 800° C. and 975° C., respectively, the pellets were characterized by AC impedance spectroscopy in an oxygen gradient. These pellets had a density of 93-96%. 
     Calcining temperatures can range from 400° C. to 800° C. Calcining times are a matter of convenience but typically require at least an hour. Calcining overnight is not atypical. Sintering temperatures vary widely, depending on the material being treated. Such temperatures can be as high as 1650° C. for La-Al-O compounds. For Bi-Al-O compounds, sintering temperatures range from 975° C. to just below the decomposition temperature of 1080° C. 
     Alternative methods were tried in an effort to make better quality powder and purer 1:1 material. Pellets made by solution methods were white to pastel yellow in color; when made by solid-state methods, they were orange. 
     La-Al-O Perovskite System Fabrication: Another method to gain thermodynamic stability in electrolytes is to substitute materials having weak metal-oxygen interactions, such as Bi-based ceramics, into a refractory host lattice exhibiting stability at high temperatures and in environments ladened with H 2  and O 2 . A requirement here is a host metal site that is large enough to incorporate the reducible metal center. For example, LaAlO 3  is an ionic conductor when doped with divalent cations. As such, this refractory oxide is a good choice as an electrolyte lattice when combined with Bi. 
     The following compound was produced: 
     
         La.sub.1.0-x Bi.sub.x AlO.sub.3 
    
     where x ranges from 0 to 0.4. These powders were produced via the glycine-nitrate process with bismuth added in the synthetic step. Sintered pellets produced from these powders were tested for conductivity in an oxygen gradient (0.1% v. 100%). At 800° C., ionic transference numbers for these materials were approximately 0.50. At temperatures greater than 750° C., the compositions with higher Bi concentrations were more highly conductive; at temperatures lower than 750° C., the conductivity decreased as x increased from 0.2 to 0.4. These trends are illustrated in FIG. 2. A least-squares analysis of the data show an increase in the activation energy and an increase in the preexponential factor of the Arrhenius equation with increasing Bi content. 
     Another La-Al-O perovskite produced has the formula La 0 .8 Bi 0 .1 Ba 0 .1 AlO 3-x  with a density 58.8% of the theoretical value, and wherein x represents the oxygen content of the various other metal oxides combined to produce the material. This material was prepared by dissolving La 2  O 3 , Bi(NO 3 ) 3 .5H 2  O, Ba (NO 3 ) 2  and Al(NO 3 ) 3 .9H 2  O in concentrated HNO 3 . The water was evaporated from the solution, and the resultant solid was calcined for 10 hours at 500° C. The calcined material was milled to reduce particle size. A pellet was pressed and sintered according to the following thermal profile: 1200° C. for 10 hours, 1600° C. for 10 hours, and 1650° C. for 16 hours. The conductivity results are given in Table 2. 
     
                       TABLE 2______________________________________Conductivity values from La.sub.0.8 Bi.sub.0.1 Ba.sub.0.1 AlO.sub.3-x.degree.Temperature    Conductivity°C.     Ω.sup.-1 cm.sup.-1______________________________________560            1.40 × 10.sup.-6660            1.67 × 10.sup.-5760            5.50 × 10.sup.-5860            1.22 × 10.sup.-4______________________________________ 
    
     La-Al-O perovskites containing bismuth and zinc yielded exceptionally good conductivity rates, as depicted in Tables 3 and 4 below. 
     
                       TABLE 3______________________________________Conductivity values fromLa.sub.0.7 Bi.sub.0.315 Al.sub.0.45 Zn.sub.0.05 O.sub.3-xTemperature    Conductivity°C.     Ω.sup.-1 cm.sup.-1______________________________________505            2.6 × 10.sup.-5603            2.8 × 10.sup.-4693            2.6 × 10.sup.-3790            8.0 × 10.sup.-2795            9.6 × 10.sup.-2799            1.4 × 10.sup.-1802            1.1 × 10.sup.-1805            1.6 × 10.sup.-1______________________________________ 
    
     
                       TABLE 4______________________________________Conductivity values fromLa.sub.0.6 Bi.sub.0.4 Al.sub.0.95 Zn.sub.0.05 O.sub.2.975Temperature    Conductivity°C.     Ω.sup.-1 cm.sup.-1______________________________________495            1.80 × 10.sup.-5603            2.09 × 10.sup.-4700            9.25 × 10.sup.-4750            4.00 × 10.sup.-3800            1.17 × 10.sup.-1850            3.04 × 10.sup.-1898            3.44 × 10.sup.-1______________________________________ 
    
     La-M-O Perovskite System Fabrication: La 0 .33 NbO 3  was obtained by milling La(OH) 3  and Nb 2  O 5  together and calcining for 10 hours at 1175° C. for 10 hours. The geometric density of the pellet was 84% of theoretical density. Table 5 contains the conductivity data for this material. 
     Measurements of the ionic transference numbers were obtained using 100% O 2  in one compartment and 0.01% O 2  (balance inert) in the other. 
     
                       TABLE 5______________________________________Conductivity values from La.sub.0.33 NbO3 in 0.01%O.sub.2 /100% O.sub.2.Temperature    Conductivity°C.     Ω.sup.-1 cm.sup.-1______________________________________503            9.18 × 10.sup.-6600            2.52 × 10.sup.-5699            1.05 × 10.sup.-4801            5.26 × 10.sup.-4896            1.58 × 10.sup.-3996            5.69 × 10.sup.-3______________________________________ 
    
     While the invention has been described with reference to details of the illustrated embodiments, these details are not intended to limit the scope of the invention as defined in the appended claims.