WO2000064814A1

WO2000064814A1 - Mixed electronic and ionic conducting ceramics

Info

Publication number: WO2000064814A1
Application number: PCT/CA2000/000426
Authority: WO
Inventors: Anthony Petric; Shiquiang Hui
Original assignee: Mcmaster University
Priority date: 1999-04-27
Filing date: 2000-04-25
Publication date: 2000-11-02
Also published as: AU4095600A

Abstract

There is disclosed new materials exhibiting high levels of mixed ionic and electronic conductivity at high temperatures (500-1000 °C) and low oxygen pressures (10?-13 - 10-20¿ atm) suitable as a fuel cell anode. The material is based on the perovskite, strontium titanate, having a formula Sr¿1-x?TiO3:n1%R1, where x is the fraction of vacancies and dopant atoms R1 on the strontium sublattice wherein 0.10 ≤ x ≤ 0.20, wherein 6-15 mol% of the strontium is replaced by R1 which is preferably yttrium or a late rare earth element from the group samarium, europium, gadolinium, terbium, disprosium, holmium, erbium, thulium, ytterbium and lutetium. Similar novel materials having the formula Sr1-xTi1-yO3:n1%R1,n2%R2 in which a fraction of the titanium is replaced by R2 which may be other transition metal ions or lower valent ions such as aluminium, gallium, also form part of the present invention. Electronic conductivities greater than 60 S/cm at 800 °C and at oxygen pressures of 10?-20¿ atm have been observed with the composition Sr¿.88?Y.08TiO3-δ and more than 80 S/cm at the composition Sr.86Y.08TiO3-δ.

Description

MIXED ELECTRONIC AND IONIC CONDUCTING CERAMICS

FIELD OF THE INVENTION

The present invention relates to electronically and ionically conducting mixed metal oxide ceramics, and more particularly the present invention relates to strontium titanate doped with yttrium as a fuel cell anode material.

BACKGROUND OF THE INVENTION

High temperature fuel cells with ceramic electrolytes require anodes and cathodes to increase reaction kinetics with gases at their surfaces. In effect, the electrodes are both catalysts and current collectors/distributers. Typical operating temperatures are 700 - 1000°C although long term trends are towards temperatures of 500 - 700°C. In the operation of a fuel cell, oxygen ions are transported from air through the cathode/electrolyte/anode laminate to oxidize the fuel on the other side. Since the electrodes must have sufficient thickness to transport a current of electrons along the surface, the electrode itself is a barrier to oxygen flow normal to the surface, unless it is also an oxygen ion conductor. Although cathodes of mixed ionic and electronic conductivity have been known for some time, to date, no mixed conducting oxide anodes are known. Instead, mixtures of nickel metal (Ni) and yttria-stabilized zirconium oxide (YSZ) are commonly used, see N.Q. Minh, "Ceramic Fuel Cells", J. Am. Ceram. Soc, 76, 563-88 (1993).

To date, there is no known oxide ceramic with high electronic conductivity in reducing atmospheres exhibiting efficacy as an anode in for example fuel cells. Typical conductivities of n-type oxides like Ce0₂ are around 1 S/cm under reducing conditions. To be stable as an anode material, the oxide must be stable to reduction. Therefore, from the point of view of stability, compounds of nickel or cobalt are not suitable, although these metals are the preferred catalysts for anodic reaction. Earlier studies have focussed on improving the properties of LaCr0₃ by substitution of Ni on Cr sites since undoped LaCr0₃ has poor catalytic activity, see H.E. Hofer, "Electronic Conductivity in the La(Cr,Ni)O₃ Perovskite System", J. Electrochem. Soc, 141 , 782-86 (1994); M. Stojanovic, R.B. Haverkamp, CA. Mims, H. Moudallal and A.J. Jacobson, "Methane Oxidation over LaCr₁._xNi_xO₃ Perovskite Oxides I: Synthesis and Characterization of Catalysts", J. Catalysis, 165, 315 (1997); and M.

Stojanovic, CA. Mims, H. Moudallal, Y.L. Yang and A.J. Jacobson, "Methane Oxidation over LaCr₁ Ni_xO₃ Perovskite Oxides II: Reaction Kinetics", J. Catalysis, 165, 324 (1997). Although LaNi0₃ is more conductive than LaCr0₃ in air, the conductivity of the mixed Cr-Ni perovskite, even when doped with 20% strontium, decreases with decreasing oxygen pressure to levels below that of

LaCr0₃. This is primarily because both Cr and Ni form p-type oxides which require high oxygen pressure for high conductivity.

Perovskites are preferred as anodes because they have the potential for mixed ionic and electronic conduction in contrast to simpler compounds like spinels which are not ionic conductors and in addition exhibit good environmental stability. N-type perovskites are based on transition metals such as Ti, V, Nb, Mo and W. Of these, perovskites such as SrTi0₃ and SrV0₃ (with Ca/Ba or rare earth elements substituted for Sr) are the most likely candidates for anode materials. Nb is too difficult to reduce to the 3+ or 4+ valence state; Mo and W have limited 4+ stability ranges. With respect to vanadates, LaV0₃ or other rare earth vanadates are not good conductors, nor are CaV0₃ or BaV0₃. SrV0₃ is an excellent electronic conductor with a conductivity at 800°C of 1000 S/cm at low oxygen pressure (Po₂). However, at higher oxygen pressures, SrV0₃ oxidizes to Sr₃V₂0₈, an apatite-type phase which is extremely stable and which cannot be reversed to perovskite at oxygen pressures common to fuel cell anode environments.

All of the known oxides that are good electronic conductors at ambient conditions (room temperature and near atmospheric oxygen pressure) are unstable at either high temperature and/or low oxygen pressure. Examples are Re0_3l Cr0₂, Fe₃0₄ and other spinel compound oxides, see W.D. Kingery, Introduction to Ceramics, John Wiley & Sons, New York, 1960. Some oxides such as SrV0₃, LaTi0₃, SrNb0₃ and SrMo0₃ must be reduced at extremely low Po₂ and high temperature and cooled to ambient conditions where they exhibit high conductivity. Every known oxide compound is unstable at high temperatures and either oxidizing or reducing conditions and suffers from

(possibly irreversible) phase transformations with respect to the required normal working conditions.

A number of studies have been devoted to finding a suitable high temperature, electronically conducting oxide at low oxygen pressures. B.C.H. Steele, "Materials for High Temperature Fuel Cells", Phil. Trans. R. Soc. Lond.

A, 354, 1695-1710 (1996) investigated compounds such as CrTi₂0₅, Ca-doped LaCr0₃ and Nb-doped Ti0₂ , Ti₃O_s, and Sm₂Ti₂0₇ as fuel cell anodes. Based on his criteria for a suitable oxide anode, he concluded that further pursuit of this goal is futile. Porat et al. have reported a conductivity of 70 S/cm for Gd₂(Ti ₃Mo ₇)₂0₇ but this phase is not stable to high Po₂, nor is the transformation reversible, see O. Porat, C. Heremans and H.L. Tuller, "Stability and Mixed Ionic Eectronic Conduction in Gd₂(Ti₁._xMo_x)₂O₇ under Anodic Conditions", Solid State Ionics, 94, 75-83 (1997).

CaTi0₃ with 10 - 50% Fe substitution for Ti has shown conductivities close to 1 S/cm, see L.A. Dunyushkina, A.K. Demin and B.V. Zhuravlev,

"Electrical Conductivity of Iron-doped Calcium Titanate", Solid State Ionics, 116, 85-88 (1999). J.T.S. Irvine, P.R. Slater and P.A. Wright, Ionics, 2, 213 (1996) have reported conductivities for SrTi0₃ doped with Nb as high as 5.6 S/cm at 930 °C With La doping, they found a maximum conductivity of 7 S/cm, see P.R. Slater, D.P. Fagg and J.T.S. Irvine, "Synthesis and Electrical

Characterisation of Doped Perovskite Titanates as Potential Anode Materials for Solid Oxide Fuel Cells", J. Mater. Chem., 7, 2495-98 (1997).

In contrast, a recent report showed conductivities of 60 S/cm at 800°C for a similar composition of strontium titanate with 40 mol% La substitution, see G. Pudmich, W. Jungen and F. Tietz, "Characterization of New Ceramic Anode Materials for Direct Methane Oxidation in SOFC", proc. SOFC VI, Electrochemical Society, Vol. 99-19, p.577, 1999. The difference in the latter report may be due to lower moisture content in the hydrogen gas, resulting in stronger reducing ability. Compounds of SrTi0₃ doped with Ba and Nb which are not perovskites, but Ti bronzes, had conductivities near 7 S/cm, see P.R. Slater and J.T.S. Irvine, "Synthesis and Electrical Characterization of New Anode Materials for SOFCs", 3rd Euro. SOFC Forum, Nantes, June, 1998. As disclosed in I. Burn and S. Neirman, "Dielectric properties of donor-doped Polycrystalline SrTiO3", J. Mater. Sci., 17, 3510-24 (1982); J. Mater. Sci. Lett., 4, 1152-56 (1985) low temperature conductivities of 1%Y-doped SrTi0₃ have been reported.

There is therefore a need for economical anode materials exhibiting high enough ionic and electronic conductivity which are also thermodynamically stable in the demanding environment of high temperature fuel cells.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a mixed metal oxide with high electronic and ionic conductivities thermodynamically stable in the temperature range in which high temperature solid oxide fuel cells operate. In one aspect of the invention there is provided a compound having a formula I;

S^ \0₃ u %R, [I] wherein R- are first dopant atoms replacing some strontium atoms on a strontium sublattice, x is a fraction of vacancies and said dopant atoms R on the strontium sublattice and 0.10 < x < 0.20, said dopant atoms R., being selected from the group consisting of yttrium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium, and wherein n.,% is a mole percent of R and n. > 5.

In this aspect of the invention R^ may be yttrium (Y) and n-% is in a range from about 6% to about 15%. In another aspect of the invention there is provided a compound having a formula II;

wherein R₁ is first dopant atom replacing some strontium atoms on a strontium sublattice, x is a fraction of vacancies and said dopant atoms R^ on the strontium sublattice and 0J0 < x ≤ 0.20, said dopant atoms R₁ being selected from the group consisting of yttrium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium, and wherein n.,% is a mole percent of R₁ and n., > 5; and R₂ is second dopant atoms replacing some titanium atoms on a titanium sublattice, y is a fraction of vacancies and said dopant atoms R₂ on the titanium sublattice and 0.01 < y ≤ 0.20, said dopant atoms R₂ being selected from the group consisting of aluminum, gallium, transition metal ions and any combination thereof, wherein n₂% is the mole percent of R₂ and is in the range from about 1% to about 15%.

In this aspect of the invention R may be yttrium (Y) and n.,% is in a range from about 6% to about 15%.

In another aspect of the invention there is provided a solid oxide fuel cell, comprising; a laminate of a cathode, an anode and an electrolyte sandwiched between said cathode and anode, said anode comprising an oxide compound having a formula Sr^- iG^n^/oR.,, wherein R is first effective dopant atoms replacing some strontium atoms on a strontium sublattice, x is a fraction of vacancies and said dopant atoms R^ on the strontium sublattice and 0.10 < x < 0.20, said dopant atoms R being selected from the group consisting of yttrium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium, and wherein n.,% is a mole percent of R and n., > 5.

In this aspect of the invention R^ may be yttrium and n.,% may be in a range from about 6% to about 15%. The invention also provides a compound having a formula III; Sr₁. iO₃:n₁%Y [III] wherein x is in the range 0J4 < x < 0J5 and _ is between 8% to 10%. The present invention also provides a compound having a formula Sr₈₆ Y₀₈TiO₃._δ wherein δ is in a range from 0 to about 0.03 when said compound is in the presence of a reducing atmosphere.

BRIEF DESCRIPTION OF THE DRAWINGS

The new mixed oxides and fuel cells incorporating these oxides forming the subject invention will now be described, by example only, reference being had to the accompanying drawings, in which:

Figure 1 is a prior art plot of ionic conductivities of various perovskites as a function of temperature in air: 1-LaAI0₃, 2-CaTi0₃, 3-SrTi0₃, 4-La₀₇Ca₀₃AIO₃._α 5-La₀₉Ba_{0 1} AI0₃._α 6-SrTi₀₉AI₀ ,0^_, 7-CaTi₀₉₅Mg₀₀₅O₃._α 8-CaTi₀₅AI₀₅0₃._α, 9-CaTi₀₇AI₀₃O₃._α, taken from T. Takahashi and H. Iwahara, "Energy

Conversion", 11, 105 (1971);

Figure 2 shows the change in electrical conductivity of strontium titanate doped with 10 mol% Y and 5 mol% Co during cycling between oxidizing and reducing environments which are indicated by the corresponding partial oxygen pressures;

Figure 3 is a plot of electrical conductivity of strontium titanate containing 8 mol% Yb, Sm and Y; strontium titanate containing 10 and 50 mol% La; and calcium titanate containing 8 mol% Y;

Figure 4 is a plot of electrical conductivity of Y-doped SrTi0₃ at 800°C in low Po₂;

Figure 5 is a plot of conductivity versus Log Po₂ (atm) for 10% at yttrium- doped SrTi0₃ with 5% of different dopants on the B-site at 800°C;

Figure 6 is a perspective view of a solid oxide fuel cell constructed using the present anode material; and Figure 7 is a plot of voltage and power as a function of current for a fuel cell comprising a YSZ electrolyte, a strontium titanate anode containing 10% Y and 5% Ga, and a Pt cathode running on air and pure H₂ fuel.

DETAILED DESCRIPTION OF THE INVENTION The inventors have investigated numerous mixed perovskites prepared in air and measured under reducing conditions and all with conductivities less than 1.5 S/cm (Table 1 ). Although the inventors have found that vanadium-based perovskites prepared and measured in a reducing atmosphere have good electronic conduction as high as 1000 S/cm (Table 2), none of these oxides are stable at, or reversible from, high Po₂.

The inventors have produced new materials that exhibit relatively high levels of mixed ionic and electronic conductivity at temperatures in the range from 500 to 1000°C and low oxygen pressures (10 ¹³ - 10^~20 atm) suitable as fuel cell anodes. The new materials are oxide compounds based on the perovskite, strontium titanate (SrTi0₃), having the formula Sr₁._xTi0₃:n_l%R₁ where x is the fraction of vacancies and dopant atoms R on the strontium sublattice and 0J0 ≤ x ≤ 0.20, wherein 6-15 mol% of the strontium is replaced by R which is preferably yttrium or a late rare earth element from the group samarium, europium, gadolinium, terbium, disprosium, holmium, erbium, thulium, ytterbium and lutetium. The strontium vacancies compensate for some of the charge.

Additional vacancies in excess of that required for charge compensation may be added to improve conductivity. Some of the charge may also be compensated by oxygen vacancies or by B-site dopants. A small but significant amount of the charge may be compensated by Ti³⁺ ions. Similar novel materials having the formula Sr₁._xTi₁._y0₃:n₁%R₁,n₂%R₂, in which a fraction of the titanium is replaced by R₂ which may be other transition metal ions or lower valent ions such as aluminum, gallium and the like, also form part of the present invention. Several compounds of the new mixed oxides disclosed herein exhibit electronic conductivities greater than 60 S/cm at 800°C and at oxygen pressures of 10^"20 atm and ionic conductivity values between 0.0001 and 0.01 S/cm, see Figure 1. The oxygen ion conductivity occurs due to vacancies on the oxygen sublattice.

One method of producing this material comprises mixing powders of SrC0₃, Y₂0₃ and Ti0₂, pressing the mixture into pellets and sintering the pellets. More particularly, this material was processed in a reducing atmosphere at temperatures of 1400°C Hydrogen-5% argon was used as the reducing gas. The conductivity of the pellets in a reducing atmosphere of carbon monoxide and carbon dioxide at 800°C was measured to be over 60 S/cm. When the pellets were retested at 800°C in air, the conductivity dropped to a level below 1.0 S/cm. When the atmosphere was replaced by reducing gas, the conductivity returned to its initial value in a period of several days, see Figure 2. Furthermore, there is no observable phase change in the material between oxidizing and reducing atmospheres. However, processing the material under reducing conditions at 1400°C yields a better conductor than processing at 1400°C in air and then reducing the oxidized material at 800°C

Yttrium is the preferred dopant for this material, and although other late rare earths are contemplated by the inventors to produce similarly high conductivities, the highest conductivities have been observed with yttrium, as seen from the data of Figure 3, and yttrium is by far the least expensive. These other rare earths include samarium, europium, gadolinium, terbium, disprosium, holmium, erbium, thulium, ytterbium and lutetium. Doping by La, one of the early rare earths, is not as effective as Y, even at levels of 50%, as shown in Figure 3. An alternative perovskite,

is far less conductive when doped with Y in a similar manner, as can also be seen from the data in Figure 3. Referring to Figure 4, the maximum conductivity occurs at a dopant level of about 8 mol% yttrium. When n, is between 8% to 10% in

x is in the range 0J4 < x < 0J5 which give the highest conductivities.This is the concentration of Y at the limit of solubility. The conductivity can be further increased by adding excess vacancies to the strontium site. The present new mixed oxide exhibits a maximum in electronic conductivity of more than 60 S/cm at the composition Sr ₈₈Y ₀₈TiO₃._δ and more than 80 S/cm at the composition Sr₈₆Y₀₈TiO₃._δ. (In air δ=0 and in the presence of a typical fuel anode gas δ is approximately equal to 0.03 with a corresponding Po₂ approximately equal to 10-²⁰ atm. ) Additional doping of the A-site by La appears to increase the Y solubility but decreases the conductivity. In addition to replacing some of the strontium, some of the titanium atoms may be replaced by dopants to produce materials with compositions given by Sr₁__xTi₁._y0₃:n₁%R₁,n₂%R_2; where y is the fraction of vacancies and dopant atoms R₂ on the titanium sublattice and 0.01 < y < 0.20. The dopant atoms R₂ may be from the transition metals or lower valency metal ions, wherein n₂% is the mole percent of R₂ and is in the range from about 1 % to about 15%. Examples of dopant atoms R₂ include aluminum, gallium and the transition metal ions. When R₂ is 5%, the value of R^ corresponding to the maximum conductivity is between 10 and 11% which is also at the limit of solubility. With 10 mol% Y on the A-site, doping the B-site with 5 mol% Co, Ga,

Cu, V, Al, Mn, Ni, Mo or Fe decreases the total conductivity by approximately half, see Figure 5. However, the B-site dopants may add beneficial catalytic properties or increase oxygen ion conductivity when the material is used as an anode or in other applications. The surprisingly high level of conductivity for these new compositions is unprecedented for an oxide under the normal reducing conditions of common hydrocarbon fuels, and the high conductivity is recoverable from oxidizing to reducing conditions which makes this material useful for numerous practical applications. A fuel cell was constructed to demonstrate the feasibility of the new mixed metal oxide anode materials. Referring to Figure 6, a solid oxide fuel cell shown generally at 10 comprises a laminate structure including a first interconnect layer 12 in electrical contact with a cathode 14 and a second interconnect layer 20 in electrical contact with an anode 18. Sandwiched between anode 18 and cathode 14 is a solid electrolyte 16. The electrolyte 16 was a yttria-stabilized zirconia (YSZ) ceramic and approximately 0.5 mm thick which provided support for the other layers. The anode layer 18 and cathode layer 14 are formed on the opposed surfaces of the electrolyte layer 16. The anode 18 comprises the novel doped strontium titanate disclosed herein and more particularly the anode was strontium titanate doped with 10 mol% Y and 5 mol% Ga. The cathode 14 was porous Pt with air as the oxidant. The fuel was a mixture of H₂ gas.

In operation fuel cell 10 showed normal output, although performance was low because of the thick electrolyte 16, see Figure 7. It will be understood that the same anode may be used in tubular, segmented or other fuel cell designs and is not restricted to the planar geometry shown in Figure 6.

It will be appreciated by those skilled in the art that the new materials disclosed herein will have utility in other technological applications such as, but not limited to, oxygen pumps, partial oxidation reactors, sensors and the like. There are several surprising advantages obtained with the new mixed oxide anode materials based on SrTi0₃ disclosed herein. SrTi0₃ is thermodynamically very stable and does not form a reaction product with typical electrolytes such as YSZ and lanthanum gallate (LSGM), nor causes a phase change, which means the mixed oxide disclosed herein is very compatible with solid oxide fuel cell components. The SrTi0₃ with some of the Sr substituted by Y has a relatively high electronic conductivity of more than 60 S/cm, compared to Ni-YSZ which, depending on the mixture, is around 500 S/cm. It will be understood that the new metal oxides disclosed herein may be used in a mixture including a metallic second phase to increase total conductivity and/or catalytic activity. Non-limiting exemplary examples of such metals includes such as Fe, Ni, Co and Cu.

The oxide exhibits ionic conductivity of up to 0.01 S/cm (Figure 1 ) compared to 0.035-0J5 S/cm for typical electrolytes. The new material will support reaction between oxygen ions and fuel over the entire surface of the anode, rather than only at the junction between ionically and electronically conducting phases as in 2-phase anodes like Ni-YSZ. The oxide anode will allow a fuel cell to operate in the range of high current densities not possible with a Ni- YSZ anode where the nickel metal is subject to oxidation. Although it is a single phase material, grain growth is restricted to a size near 5 microns by a phenomenon known as the donor dopant anomaly. A fine grain size has the advantage of higher strength and fracture toughness.

The new perovskite materials disclosed herein exhibiting high ionic and electronic conductivity are not as prone to carbon deposition from hydrocarbon fuels on the surface as other materials. The new mixed oxide anode materials will not react with sulfur in the fuel, or sulfur compounds such as H₂S. (Currently, H₂S needs to be scrubbed from natural gas to prevent fouling and corrosion.) Further, when used with the LSGM electrolyte, it will be lattice matched, eliminating any interphase boundary, and reducing resistance losses.

The inventors are not aware of any phase diagrams of SrO-Ti0₂ containing rare earths. The foregoing description of the preferred embodiments of the invention has been presented to illustrate the principles of the invention and not to limit the invention to the particular embodiment illustrated. It is intended that the scope of the invention be defined by all of the embodiments encompassed within the following claims and their equivalents.

Table 1. Conductivity of Selected Oxides at 800 °C

Theoretical Composition Conductivities (S/cm) Conductivities (S/cm)

(prepared in air) (Po, = 10^"20 atm) (in air)

LaNi_{0 75}Mo_{0 75}O,. 1.2xl0^"2 5.98

LaNi₀₆Nb₀₄O₃- 2JxlO^"3 7.9x10^"2

LaFeO₃ 7.8xl0^"3 0J2

LaNi₀₇₅W₀₂₅O₃. 9Jxl 0^"3 0.58

LaFe₀₅V_{0 5}O₃. 0.13

LaNi₀ N_{0 5}O₃. 0.79 8.82

LaΝi₀₅Ti₀₂₅V₀₂₅O₃. 3.31xl0^"2 0.58

La₀₈Sr₀₂Ni₀₅V₀₅O₃. 0.25 2.65

La₀₈Sr₀₂Ni₀₄V₀₆O₃_ 1.04 1.46

La_0.4Sr_{0 fi}Ni₀₂V_{0 8}O . 1.42 4.56x10^"2 Table 2. Conductivity of Selected Vanadium Oxides

Theoretical Composition Conductivities (S/cm) Conductivities (S/cm) (prepared in 7% H₂ +Ar) (Po₂= 10-²⁰atmat800°C) (air at 25 °C)

SrVO_3. 10³ 3xl0³

SrV₀₉Tι₀,O₃. 94.4 lJxlO³ SrV₀₅Ti₀₅O₃- 0.97 7Jxl0³

SrV₀₅Zr₀₅O₃- 1J8 470

SrV₀₇Ga₀₃O₃. 139J 1.63x10 ,3 La_n7Sr_0.NO 47.2 300

Claims

THEREFORE WHAT IS CLAIMED IS:

1. A compound having a formula I;

Sr_1.xTi0₃:n₁%R₁ [I] wherein R is first dopant atoms replacing some strontium atoms on a strontium sublattice, x is a fraction of vacancies and said dopant atoms R₁ on the strontium sublattice and 0J0 < x < 0.20, said dopant atoms R^ being selected from the group consisting of yttrium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium, and wherein n.,% is a mole percent of R₁ and n., > 5.

2. The compound according to claim 1 wherein R₁ is yttrium (Y) and wherein n^/o is in a range from about 6% to about 15%.

3. The compound according to claim 2 wherein 0J4 < x ≤ 0J5 and n^/o is in a range from about 8% to about 10%.

4. The compound according to claims 1 , 2 or 3 wherein said compound has a mixed electronic and ionic conductivity greater than about 10 S/cm at a temperature ranging from 500°C to 1000°C and an oxygen pressure ranging from about 10^"13 to about 10^"20 atm.

5. The compound according to claims 1 , 2, 3 or 4 including second dopant atoms R₂ present in an amount of n₂% replacing some titanium atoms on a titanium sublattice, said compound having a formula

Sr^Ti^Os: n^/oR-, n₂%R₂ wherein y is a fraction of vacancies and said dopant atoms R₂ on the titanium sublattice and 0.01 < y < 0.20, said dopant atoms R₂ being selected from the group consisting of aluminum, gallium, transition metal ions and any combination thereof, wherein n₂% is a mole percent of R₂ and is in the range from about 1% to about 15%.

6. A compound having a formula I;

Sr₁._xTi0₃:n_l%R₁ [I] wherein R^ is first dopant atoms replacing some strontium atoms on a strontium sublattice, x is a fraction of vacancies and said dopant atoms R., on the strontium sublattice and 0J0 < x ≤ 0.20, said dopant atoms R being selected from the group consisting of yttrium and the late rare earth elements in the lanthanide series excluding lanthanum, and wherein n.,% is in a range from about 6% to about 15%.

7. A compound having a formula Sr₈₆ Y₀₈TiO₃._δ wherein δ is in a range from 0 to about 0.03 when said compound is in the presence of a reducing atmosphere.

8. The compound according to claim 1 wherein R^ is yttrium and wherein _ι > 5 and spans a range in which said yttrium is soluble in strontium titanate.

9. A compound having a formula II;

Sr₁._xTi₁._y0₃: n^/oR^ n₂%R₂ [II] wherein R., is first dopant atom replacing some strontium atoms on a strontium sublattice, x is a fraction of vacancies and said dopant atoms R₁ on the strontium sublattice and 0J0 < x < 0.20, said dopant atoms R., being selected from the group consisting of yttrium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium, and wherein n.,% is a mole percent of R^ and n., > 5; and

R₂ is second dopant atoms replacing some titanium atoms on a titanium sublattice, y is a fraction of vacancies and said dopant atoms R₂ on the titanium sublattice and 0.01 < y < 0.20, said dopant atoms R₂ being selected from the group consisting of aluminum, gallium, transition metal ions and any combination thereof, wherein n₂% is the mole percent of R₂ and is in the range from about 1% to about 15%.

10. The compound according to claim 9 wherein R^ is yttrium and wherein n.,% is in a range from about 6% to about 15%.

11. The compound according to claim 10 wherein 0J4 < x ≤ 0J5 and n^/o is in a range from about 8% to about 10%.

12. The compound according to claim 9 wherein R., is yttrium and wherein n., > 5 and spans a range in which said yttrium is soluble in strontium titanate.

13. A compound having a formula III;

Sr₁._xTi0₃:n₁%Y [III] wherein x is in the range 0J4 ≤ x ≤ 0J5 and n₁ is between 8% to 10%.

14. A solid oxide fuel cell, comprising; a laminate of a cathode, an anode and an electrolyte sandwiched between said cathode and anode, said anode comprising an oxide compound having a formula Sr^TiO^n^/oR.,, wherein R^ is first effective dopant atom replacing some strontium atoms on a strontium sublattice, x is a fraction of vacancies and said dopant atoms R., on the strontium sublattice and 0J0 < x < 0.20, said dopant atoms R^ being selected from the group consisting of yttrium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium, and wherein n.,% is a mole percent of R., and n^ δ.

15. The solid oxide fuel cell according to claim 14 wherein R^ is yttrium and wherein n.,% is in a range from about 6% to about 15%.

16. The solid oxide fuel cell according to claim 15 wherein 0J4 ≤ x < 0J5 and n.,% is in a range from about 8% to about 10%.

17. The solid oxide fuel cell according to claims 14, 15 or 16 including second dopant atoms R₂ present in an amount of n₂% replacing some titanium atoms on a titanium sublattice of said anode, said compound having a formula Sr_l._xTi₁__y0₃: n^/oR.,, n₂%R₂ wherein y is a fraction of vacancies and said dopant atoms R₂ on the titanium sublattice and 0.01 < y < 0.20, said dopant atoms R₂ being selected from the group consisting of aluminum, gallium, transition metal ions and any combination thereof, wherein n₂% is a mole percent of R₂ and is in the range from about 1% to about 15%.

18. The solid oxide fuel cell according to claims 14, 15, 16 or 17 wherein said anode includes an effective metal mixed with said oxide compound for increasing conductivity or catalytic activity of said anode.

19. The solid oxide fuel cell according to claim 18 wherein said metal is selected from the group consisting of iron, cobalt, nickel and copper.

20. The solid oxide fuel cell according to claims 14, 15, 16, 17, 18 or 19 operating at a temperature ranging from 500 °C to 1000°C and wherein an anode compartment is operated at an oxygen pressure ranging from about 10^"13 to about 10^"20 atm.

21. The solid oxide fuel cell according to claims 15, 16, 17, 18, 19 or 20 wherein said compound is Sr₈₆ Y₀₈TiO₃._δ wherein δ is in a range from 0 to about 0.03 when said compound is in a presence of a reducing atmosphere.