CA2247471A1 - Low cost stable air electrode material for high temperature solid oxide electrolyte electrochemical cells - Google Patents

Abstract

A low cost, lanthanide-substituted, dimensionally and thermally stable, gas permeable, electrically conductive, porous ceramic air electrode composition of lanthanide-substituted doped lanthanum manganite is provided which is used as the cathode in high temperature, solid oxide electrolyte fuel cells and generators. The air electrode composition of this invention has a much lower fabrication cost as a result of using a lower cost lanthanide mixture, either a natural mixture or an unfinished lanthanide concentrate obtained from a natural mixture subjected to incomplete purification, as the raw material in place of part or all of the higher cost individual lanthanum. The mixed lanthanide primarily contains a mixture of at least La, Ce, Pr and Nd, or at least La, Ce, Pr, Nd and Sm in its lanthanide content, but can also include minor amounts of other lanthanides and trace impurities. The use of lanthanides in place of some or all of the lanthanum also increases the dimensional stability of the air electrode. This low cost air electrode can be fabricated as a cathode for use in high temperature, solid oxide fuel cells and generators.

Description

LOW COST STABLE AIR ELECTRODE MATERLAL FOR HIGH TEMPERATURE
SOLID OXIDE ELEC~OLYTE FT ~CTROt~ CAL CELLS

1. Go~ "e.~ Contract:
The Gov~ of the United States of America has rights in this invention ~ul~ua~ll to Contract No. DE-FC21-9lMC28055, awarded by the United States Department of Energy.
10 2. Field of the Invention:
The present invention relates to the field of high l~,--pr-,i.l.-.e, solid oxide electrolyte ele~;l,~h~ l ceUs and electr~hpm~ gPnPr~t rs for electrical power genPr~ti-)n plants.
This invention more particularly relates to air electrodes for such solid oxide electrolyte electr~chemic~l cells that are extremely cost effective to m~nllf~ctllre commerciaUy, since the 15 air electrodes are made from l~n~h~ni~le mixtures mainly comprising La, Ce, Pr, and Nd and other l~nth~ni~lçc, commercially available as ImfinichP~ nth~nide co~ P-c that are mined from rare earth oxide natural l~u~ces found in the earth and incolnl)letely purified.
And despite being made from such l."l~ hP~l raw m~tPri~lc, the resultant air electrodes have PYcPllPnt thermal eYr~ncion match properties with the solid oxide electrolyte and other 20 components of the cells and al)plu~lia~ low resistivities, porosities, and dimPncion~l stability at the high o~ldling ~ hlres of the cells. This invention further relates to the method for making such low cost stable air electrode m~t~ri~lc from these llnfinich~d l~nth~ni~lP
con~ s 3. ~ach~lvund of the Invention:
High ~ P~ e, solid oxide electrolyte fuel ceU config~ tionc and fuel ceU
gPnPI,l~ol~ are well hnown in the art, and are taught in U.S. Pats. 4,395,468 (Isenberg) and 4,490,444 (Isenberg). These fuel cell configurations include a plurality of individual, series and paraUel electricaUy col-n~lP~I, axially elon~tP~l, usuaUy tubular, solid oxide fuel ceUs ("SOFC"s) which g~ lr. Pl~trie~l energy through electrochPmi/~ ctirnc between air 30 and hydl~bon fuel gac to produce a flow of electrons in an PYtPrn~l circuit. Generators based on SOFCs offer a clean, poUution-free, approach for el~L,~h~P~ l genPr~tion of electricity with high effieien~ioc.
Each SOFC typicaUy in~ des a porous, annular, open- or close-ended, axially P1Ong~t~d, electric~lly conductive, ceramic air electrode (or cathode) usuaUy made of doped W 097/32349 PCTrUS97/03019 n~... m~n~nitP. The air electrode is a self-su~ g structure. The outer surface of the air electrode is mostly covered by a dense, gas-tight, oxygen ion con~luctive, thin ceramic film solid electrolyte usually made of yttria-st~hili7~1 7irconia. The outer surface of the solid electrolyte is mostly covered with a thin, porous, electrically conductive, cermet fuel S electrode (or anode) usually made of nickel-irconia cermet. Both the solid electro~yte and the fuel electrode are fliccQ~ lous in a selected radial se~ ler~dbly along the entire active length of the fuel cell, for infl~ )n of a dense, gas-tight, Pl~tri-~lly conductive, ceramic ~ ne~;~ usually made of doped 1~ cl~vll,ile, which is in turn mostly covered by an Pl~ctri~lly c~nductive metal, usually nickel, or a cerrnet usually made of 10 nickel-;~ ;onia cermet, to provide an electrir~l in~l~ ti~ n area for ~ rpnt fuel cells. A
compliant nickel felt is used to make series or parallel cell connections.
F~h SOFC gen~r~tPs electrical energy at approxim~tPly 1,000~C when air or oxygengas is supplied in the annulus of the air electrode (cathode) where it is reacted (reduced) with "~collling ~ ons from an eYtern~l circuit, to form oxygen ions. The oxygen ions 15 migrate through the solid electrolyte to the fuel electrode (anode). At the fuel electrode, hydrocarbon fuel gas is supplied over the fuel electrode rli~nsed on the outside of the tubular fuel cell, and the oxygen ions combine with hydrogen gas and/or carbon monQxille gas (collt~i,led in the hydrocarbon fuel gas) and oxidize the fuel, to form water (steam) and/or carbon ~lioYide7 and also lib~ldling electrons. Electrons flow from the fuel electrode (anode) 20 through the external circuit to the air electrode (cathode) and are collected for power ge.nPrz~tion .
The air electrode of the fuel cells are porous ceramic structures which g~Pner~lly have from about 20% to 40% porosity ~60% to 80% of theoretical density) and also have good electrical conductivities ~ow resistivities) in a heated air en~ unlllt;n~ for effective operation 25 as the air electrode in the fuel celI. Spe~ifit~ y~ the air electrode can be comrri~d of doped or undoped oxides in the perovskite (ABO3) family, such as LaMnO3 (with the A-site being the La ion and the B-site being the Mn ion), as discussed above, but may also include CaMnO3, LaCoO~, LaCrO3, and the like. The usual air electrode used in high ~Ill~ldl~lre, sol;d oxide fuel cells is LaMnO3 doped w;th Ca or Sr in the A-site as a substitute for part of 30 the La to improve the el~,trir~l cr)nfl~lctivity of the m~tPri~l, for example, LaO8CaO2MnO3 or Lao 8sro 2Mno3 Many improvements have been made to ~e air electrode used in the solid oxide fuel cells over the years. Self-supporting high bulk air electrodes are taught in U.S. Pats.

CA 0224747l l998-08-2l W 097/32349 PCTrUS97/03019 4,751,152 (Zymboly) and 4,888,254 (Reichner). The pl~re.l~d air electrode m7tPrirll in these patents comprises L~MnO3 doped with Sr. In both Zymboly and ~rirhner~ the air e~ectrode is formed by extruding a mixture of individual high purity oxides or c~ulJon~l~s of La, Mn, and Sr into a tubular form, and then ~intPring the extruded "green" tube at from S about 1,300~C to about 1,600~C to form a unitary self-s-~o,~ing tubular air electrode body, upon which the s~ llposed solid electrolyte, and then the fuel electrode, can be deposited.
Thin self-su~o~ g air electrodes are taught in U.S. Pat. 5,108,850 (Carlson et al.).
The ~lt;Ç~l'~d air electrode m?tPrirrll in this patent coln~lises sintered, doped LaMnO3 with Ca of the general formula La, ,~Ca,MnO3, where x = 0.1 to 0.25. In Carlson et al., the air 10 electrode tube is formed by mixing a formable co-..po~i~ion c~ n~rti~ particles of calcium-doped 17 .Ill7 ~ r~ l;le~ extruding or i~st~7tic pl~illg the composition into a tube of circular cross-section, plugging one end of the tube with ~7rl~litit~l1;7t formable co---po~ilion to close one end, and then heating to sinter the tube.
An Py~mplrrtry air electrode of Carlson et al. is more particularly formed by first 15 weighing out and then intim7tPly dry blending individual high purity powdered oxides of La and Mn, such as La203 and MnO2, l~ ely~ In~~ with individual ~wd~led call~n~es of Ca, such as CaCO3, in a~ lia~ plO ~lliolls to give the desired calcium-doped ~ 7~ m,7ng;7nitP composition after c7lcin7ti~)n The mixed powder is then pressed into a cylin(lrir7l pellet shape, r7lrinPd at from about 1,300~C to 1,700~C for from 20 about 3 to 5 hours, and crushed to form particles having a particle size belw~n about 0.5 and IOS miclu~ , which steps may be l~ ~~ated a ~-u-~-ber of times to provide the desired homogeneity and small palticle size. The ultim7tP- crushed r~7lrinPd powder of doped lrlllllrrll~lllll Illr7n~ is then i.lli-l ~I~]y m-ixed with from about 1% to 5% by weight of a ~l~crt~ ~k1~ cchP~ion agent, such as an organic starch, e.g., corn starch, rice starch, potato 25 starch, or the like, to provide c~-hP-~ion and ~ Licily for extrusion, from about 1% to 4% by weight of a deco---posal~le pore-forming agent, such as an organic Ct~ S1~7 e.g., maple wood flour, fiber cPlhllose, or the like, to provide gas permeability, from about 1% to 4%
by weight of an organic, water-soluble binder, e.g., polyvinyl rrtl~hOl, polyvinyl acetate, pald~ wax em~ on, or the like, to provide dry strength for hrlnrlling, and up to about 1%
c 30 by weight of an optional wetting agent, such as naphth~lPne-sulfonic acid cort-lPn~tPc, to help in t;~ u~;on, with the balance of the mixture cQI.~ g the crushed e~l~inP~ powder, pl~rel~ly from about 90 to 95% by weight. All dry powder~d ingredients are il~ llrrtl~ly dry mixed together and ~en wet mixed with the water-soluble binder in a water solution to W 097/32349 PCTrUS97/03019 provide a wet formable mixture, which is a~lu~liately aged for about 6 to 12 hours.
This aged forrnable rnixture is then extruded or i~ncf~ti~-pressed into a tubular shape.
An optional solid cylindrical plug of the formable rr~ixture is then pushed into one end of the tube a s~l~JPcT rli~t~n~e to close one end of the tube. The closed tube is then dried, and next 5 heated in air from about 1,300~C to 1,700~C for about 1 hour to sinter the tube walls and plug togethPr~ and to vaporize the c~ h~cion agent, binder and pore ro~ lg agent. The resultant structure is a cr~n~li-~tP~i sintered air electrode tube having a density between about 60% and 85% of theoretical density. The tube is then cut along the closed end and the closed end is smoothed or rounded or otherwise finished prior to de~o~iLion of the solid 10 electrolyte, fuel electrode, and i,l~l~lmf~-~
Both Sr-doped and Ca-doped 1~ --.. m~n~nitP. fnnn~ ti~n~, however, were found to be ~ c;on~lly lmct~hle, i.e., the air el~;ll~de sh~ank in length during thermal cycling, when such air electr~des were used during fuel cell op~r~tinn~ and, ~onse l~le"lLy, the life expectancy of the cell suffered. BP O 593 281 A2 Clakao et al.) taught that ~site 15 doping with Ni, Al or Mg in both Sr- and Ca-doped l~nth~n~lm Ill~ P. air electrodes improved the coeffiripnt of thermal expansion and cignifi-~ntly reduced the cyclic ~hrink~e and ~ c;onal stability problems of the fuel cell. However, a common d~wback with all of these compositions as well was that their coeffl~ içnt of therrnal PY~n~i~nl was not mzlt~h~d close enough with that of the yttria-ct~hili7P~I zirconia solid electrolyte to result in the most 20 effective air electrode m~tPri~l U.S. Pat. 4,562,124 ~Ruka) i~entified therrnal e7~ncion problems with the air electrnde m~tpri~l of the fuel cells. Ruk~ taught that the difficulty in constructing fuel cells using doped hl~ .. , m~ngzmitP ~MnO3) air electrodes was that when the l~-~ll.;1.. -m~n~nite was doped with calcium (Ca) or ~ u~lliunl (Sr) to have the highest Plectrir 25 c~?n~uct vity, the resulting air electr~de would have a higher coefflrilont of thermal ~A~ ,;on than some of the other m~trn~lc typically used in making the fuel cells, such as those used for the solid oxide electrolyte, for example, ythia- or calcia-stabilized ~ ol~a.
Accordingly, if the thermal exr~n~ n of the various fuel cell components are mi~m~tch~, the fuel cells tend to crack as a result of excess ~hrink~ge of the air electrode during therrnal 30 cycling between high ~"l~ldLures of fabrication or between operation and roomLt;~u~eldtures. This would render the fuel cells ~ ctir~lly less effective in power genel~alion operations.
Ruka taught that the addition of small amounts of cerium (Ce) into the air electrode CA 0224747l l998-08-2l W O 97132349 PCTrUS97/03019 _ 5 _ m~tPri~l of doped ~MnO3 or LaCrO3 doped with calcium (Ca) or strontium (Sr) reduced the cc~ffici~o-nt of thermal PYp~n~ion and helped the thermal eYr~ncion match with stabilized o~ electrolytes. Ruka taught for the air electr~de a sintered, single phase, solid col~ltion with a perovskite-like crystal structure of the general rull-lula Lal-xw(ML)x(ce)w(Ms~ x S (Ms~)yO3~ where ML = Ca, Sr or Ba; MSl = Mn or Cr; and MS2 = Ni, Fe, Co, Ti, Al, In, Sn, Mg, Y, Nb or Ta; w= O.OS to 0.25, x + w = 0.1 to 0.7; and y = 0 to 0.5. Preferred conl~ùuilds were LaO3CaOs to 06CeO2 ,0 OIMnO3. In Ruka, again these solid sol~ltinn, perovskite-like crystal structure, air electrode co",pos;Lions of l~ g~ P or .l. chru~ were forrned by homoge~ cly rnixing tc~ether individual high purity 10 ~wd~ ed oxides, carbonates, or other co",po~nds that form oxides upon heating, such as oY~l~tes~ of the air electrode elPmPnt~ in the aLJylulJliaL~ ~u~lLions~ p es~.ng the powderPd mixture into a tubular shape, and ~ ;,-g at about 1,400~C to 1,800~C for about 1 to 4 hours, to form the axially Pk n~t~l tubular, air ~l~;L ùde with a density of the sintered oxide that does not exceed about 80% of theoretical density, to perrnit surrounding oxidant 15 gases (air or oxygen) to perrneate to the air electrode-electrûlyte i"lt;lr~ce.
U.S. Pat. 5,342,704 (Vasilow et al.) taught a porous air electrode m~tPri~l with use of a rare earth metal additive, such as cerium, having improved s-,ltt; dl)ility, to control the percent porûsity of the sintered air electrode m~t~-ri~l to a final porosity from about 20% to 40% porosity ~60% to 80% of theoretical density). In Vasilow et al., the air electrode 2û m~tPri~l had the general formula Lal x(M)xCeO0,0,v0.04sMnO3~ where M = Ca, Sr or Cr and x = 0.2 to 0.4. The air electrodes of Vasilow et al. were also formed by forming a powder of doped 1~ -- .n~ ;le, such as calcium doped l~ n---.- m~n~ni~, made form individual high purity oxides or c~l,onales of the metals, and mixing this powder with an additive powder which co~ inc an individual high purity rare earth metal, such as individual 25 high purity oxides of cerium. The powdered mixture is then molded by i~ost~tic pressing, or usually by extrusion with ~ lidLt; organic binders, such as polyvinyl alcohol, methyl CP~ SP~ starch or the like, and then sintered in air at from about 1,000~C to 1,750~C for about 1 to 6 hours. The sintered structure is then cooled, to form a unitary sintered tubular mass with controlled porosity, and the other fuel cell components are depo~iL~d on this 30 structure.
As seen from the above patents, several form~ tic-nc of doped ~ .. m~n~nitP
air electrode m~tt~.n~l~ have been ~ oposed and also sllcceccfully used for fabricating solid oxide fuel cells. However, during thermal cycling, tubular, axially elongated, solid oxide W097/32349 PCT~US97103019 electrolyte fuel cells co~ ir-g the above air electrode formulations can still be improved in terms of thermal match properties with the solid electrolyte, to prevent the fuel cells from ç~rl~ng on ocr~ci~)n due to excessive $hrink~ge during thermal cycling and which in turn results in less effirient electrical power g~.nP.~tinn capabilities.
Moreover, all of the above de-cign~ air electrode formulations are very t;~ ive to formulate, since they are made from high purity individually ~~dl~d ingredients, namely individual high purity ~vd~l~d oxides and wbonales of the co~ liL~.e~l metals. In particular, the individual high purity 1~ ... oxide powders have proved to be t;,~L,c;l--ely costly COIl~pOl~ of the air electrode m~t~ri~h For ~ ."~, in order to obt~in individual 10 high purity ~ oxide, ~ mined from the rare earth oxide natural resources found in the earth has to go through a number of ~lective .s~r~tinn and pllrifi~tinn ~l~cesses to yield the desired individual 1~ oxide, which ~es greatly increase the cost of these individual m~tP.ri~l.c EJccessive cost of the air ele~;L,odes will 111tim~t~.1y hinder the attractiveness of any commercial production of SOFCs and SOFC gt~.nerAtnrc.
15 The cost of the air electrode is extremely i~ ulL~l because this com~.lent cnnt~inc the buLk of the fuel cell m~tt-ri~1 Ihus, in order for solid oxide electrolyte fuel cells and gene,al~
to become co-~ ;ally viable, the air electrode cost must be signifir~ntly reduced but without a co-l~lJonding ~l~ogr~hti~n in the thermal match pl~.Lies, porosities, e1ectrir~1 resistivities, and therrnal and .1;ll~ C;~nA1 st~i1iti~s during isoll.c..nal and therrnal cyclic 20 exposure con-litinnc of the air electrode.
What is needed is an air electrode for a solid oxide fuel cell and gt;ll~ldllJr which has a good thermal e~rAncicn match with the solid oxide electrolyte, good low resistivity in a heated air envi.~ "-~l~l, good porosity, and good thermal and riim~on.cion~l stability, and, rll~ll-ellllore, is cignifi~ntly less l_~nci~e to fabricate than conventional air electrodes.

W O 97/32349 PCTrUS97103019 4. Su~ / of the Invention:
It is an object of the invention to provide a high (e.. ~ , solid oxide fuel cell which co~ h~c an air electrode with good gas ~llll~ility, good r~l~trj~l con~luctivity in a heated ~I..,o~ , and good therrnal and .l;",~n~;on~l stability during iso~lGllllal and 5 therrnal cycling conrliti-)n~
It is another object of the invention to provide an air electrode for a solid oxide fuel cell that has a closer thermal e ~ on match with the solid oxide el~tl~ly~.
It is still another object of the invention to provide an air ele~llude for a solid oxide fuel cell from less pure ingl~li~ll~ than those used in conventional air electrodes that is cost 10 effective to fabricate in the construction of solid oxide fuel cells and ge,~It is yet another object of the invention to provide an air d~;llude for a solid oxide fuel cell from l~nth~ni-le Il~ixlul~, such as collllll~ ;ally available ul~ nth~nitle cc nc~ s that are mined from rare earth oxide natural l~oul~;es in the earth andinco,l,pletely pl-rifi~, as a s~-bstit~te for the more e~n~i~re individual l~nth~nnm oxides of 15 the air e}ectrode.
It is yet another object of the invention to provide a method of making an air electrode of a solid oxide fuel cell from relatively i,~ re l~nth~nide mixtures, such as co~.. t~,. ially available l~nth~ni~l~ col)c~ .s mined from rare earth oxide naturat r~uul~ces in the earth and in~rnp1et~'y purified.
20In one aspect, the invention resides in a method of making a low cost, 1~3nth~nir1~-s~ .~ imPn.cion~lly and thermaUy stable, el~tri~lty conduct ve, porous ceramic air electrode structure which is ch~r~'~d~ by the steps of: (a) providing powdered oxides or c~l,onates of a natural 1~nth~nirte mixture of at least two l~nth~nirl~ from the group of La, Ce, Pr, Nd, Sm and other 1~nth~ni-1e.s (i.e., Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu); ~b) 2~providing powdered oxides or carbonates of individual species of La and Mn, at least one A-site dopant from the group of individual species of Ca, Sr, Ba, and Ce, and, at least one B-site dopant from the group of individual species of Mg, Ni, Cr, Al and Fe; (c) blending lel said powdered oxides or call,onatt;s of the 1~nth~nide mixture with said powdered oxides or carbonates of said individual species of La and Mn, said at least one A-site dopant 30from the group of individual species of Ca, Sr, Ba, and Ce, and, said at least one B-site dopant from the group of individual species of Mg, Ni, Cr, Al and Fe, in a~ ia~
~ ,~lLions to provide a desired 1~nth~nir1e-substituted doped LaMnO3 con~posilion after c~1çining; (d) pressing the blended powder into a shape; (e) c~1cining the pressed shape at a CA 02247471 l99X-08-21 ;
o ~ 3~9 P~-;.~S~-'0~019 temperature of from about 1,300~C to 1,750~C for ~bout I to ~ hours; (f) pulverizing the cqlcined shape to powder torm; (g) blending the calcined powder with at leqst one from the group of a cohesion agent, a pore-forming agent, a water-soluble binder, a wetting agent, and water to provide a formable miYture, where the calcined powder constitutes from about 5 90~ to 95~ by weight of the formable mixture; (h) molding, preferably extruding, the formable composition into a shaped air electrode structure; and, (i) sintering the shaped air electrode structure in air at a temperature of from about 1,300~C to 1,750~ for about I to 6 hours, to form a porous, shaped, air electrode structure of the l~qnrh~qnid~-substituted doped LaMnO3 composition. lt is preferred that the air electrode is molded into a tube with a 10 closed end. The l,qnth,qni~ mixture preferably comprises primarily at least La, Ce, Pr and Nd or primarily at least La, Ce, Nd, Pr, and Sm, although minor amounts of otherl,qnrh,qni-lP5 and trace impurities can be present. The air electrode formed by the method preferably has a porosity from about 20 to 40% by volume (60~ to 80% of theoretical density), a coefficient of thermal expansion from about 10.4 x lOb to 10.6 x 10~/~C irl the 15 range of about 25~C to 1,000~C, and an electrical resistivity of from about 10 to 25 mQ-cm at about l,000~C.
In another aspect, the invention resides in a low cost, I,qnth,qni~ substituted,dimensionally and thermally stable, electrically conductive, porous air electrode composition, characterized by the ~h~-mie,ql formula (1):
(Lal w.~ yLnwCe~(M~)y)(Mnl z(MB)~)~3 ( 1 ), where Ln is a l,qnth~ni~l~ mixrure, natural or preferably llnfini.ched concentrate, selected from a mixture of at least two, at least three, at least four, or at least five or more of La, Ce, Pr, Nd, Sm, and other l,qnth~ni-lec, with the proviso that if Ln comprises a mixture of only two l,qnth,qni~lPs, the mixture is not the combination of La and Ce; La and Ce are selected from 25 individual species of La and Ce, respectively; M~ is an A-site dopant for electrical conductivity selected from individual species of at least one of Ca. Sr or Ba, or mixtures thereof; MB is a B-site dopant for dimensional stability selected from individual species of at least one of Mg, Ni, Cr, Al or Fe, or mixtures thereof; w is from about 0.05 to 0.9~ or from about 0.1 to 0.9, or from about 0.4 to 0.8; x is from about 0 to 0.1; y is from about 0.1 to 0.~; and, z is about 0.05 to 0.1 mole per mole of formula (1). The Ln of the air electrode composition preferably comprises a mixrure of primarily at least La, Ce, Pr, Nd or primarily at least La, Ce, Pr, and Sm. The air electrode composition pr~ferably has a porosity from about 20 to 40% porous by volume (60% to 80% of theoretical density), an electrical A~ENDED SltEEl CA 02247471 l99X-08-21 , W0 97~23~9 : F~,~S9~,/03019 ~ _ 9 _ resistivity at 1,000~C is from about lO to 25 mS2-cm. and, a coefficient of thermal expansion in the range of from about 25~C to l,000~C is from about 10.4 x 10-6 to 10.6 x 10-6/~C. The air electrode composition is preferably tubular and has a dense, gas-tight, oxygen ion conductive, yttria- or calcia-stabilized zirconia cerarnic solid electrolyte on the outer periphery S of the air electrode to contact and s~lh.st~nri~lly surround the air electrode, a porous nickel- or cobalt-zirconia cerrnet fuel electrode on the outer periphery of the solid electrolyte to contact and substantially surround the solid electrolyte, in order to forrn a solid oxide fuel cell.

AMENDED SH~ET

WO 97/32349 PCTrUS97/03019 5. Brief Description of the Drawin~s:
There are shown in the drawings certain Pxemrl~ry embo~limp-ntc of the invention as ~ s~llly plcrcrl~d. It should be ~ d~ oo~ that the invention is not limited to the embo limPnt~ os-p~ as e ~ r le ~ and is capable of variation within the scope of the 5 appended claims. In the drawings, FIGURE 1 is a .~ ', !~ctto~ view of a ~rtrcllcd embodiment of a tubular, solid oxide fuel cell which inr.ll~fles a low cost and stable air electrode made in a~cor~ ce with this invention, FIGURE 2 is a ~ f~ l shuwing total ~hrinl~ e for heat cycled low cost 10 air electrode m~tP.ri~l (LaO2LnO.6CaO2)(MnO95NiO05)03, where Ln is a l~nth~nifiP. m-ixture of at least La, Ce, Pr and Nd, a l)rcrclled air electrode m~tP.ri~l in acconla~lce with the invention;
FIGURE 3 is a graphical rli~gr~m showing total chrink~ge for heat cycled low cost air electrode m~tPri~l (LaO2LnO6Ca~2)(MnOgONiO l0)~3~ where Ln is a l~nth~nifle mixture of at least La, Ce, Pr and Nd, another ~lcr~lcd air electrode m~tP.ri~l in accc,lda~ce with the lS invention; and, FIGURE 4 is a gl~ c~ ... showing total .chrink~ge for heat cycled low cost air electrode material (LnO8CaO2)(MnO.9oMgolo)o3~ where Ln is a l~nth~ni(1e mixture of at least La, Ce, Pr and Nd, still another ~Jlcf~ d air electrode m~tPri~l in accol.lance with the invention.

W 097/32349 PCT~US97/03019 6. Detailed Description of the F~r~lr~l Embo~limPntc of the Invention:
A high ~"-~ ;, solid oxide fuel cell ge"e.d~f in~hlA~s a gas-tight, tht~rm~lly in~ t~A. housing which houses individual ch~lllh~ ;".~ g, without li---;l~lio-~, a ,~,e~ ..r cl~",her and co.,lbu~ c~ . The g~ , c~ .her, in which power S generation occurs, c~nt~in.~ a solid oxide fuel cell stack which is made up of an array of a plurality of axially ek)ng~t~, tubular, series-pa~allel co~n~rA solid oxide fuel cells, and ~ fuel and air di~,llil)ulion e~lui~;~lnenl. The solid oxide fuel cells contained in the ge~ ol c~ -l)eJ can take on a variety of well known confi~ -s, in~ rling tubular, flat plate, and corrugated designs which are taught in U.S. Pats. 4,395,468 (Isenberg) and 10 4,490,444 (Isenberg) for tubular SOFCs, U.S. Pat. 4,476,196 (Poppel et al.) for flat plate SOFCs, and U.S. Pat. 4,476,198 ~Ackrllll~l et al.) for colluga~d SOFCs, which li~losllres are h~col~l~led by fer~fence herein in their entireties. However, for ~ul~oses of ~implic~,ity, tubular solid oxide fuel cells will be ~ ls~s~d as an exemplary type useful in ~s invention, and the description h~lr~r~ will generally relate to that type, which shall in no 15 way be c~n~ ered limiting as to the scope of the invention.
Rert;l,ing now to FIGURE 1, a l)r~rrll~d, tubular, axially elong~t~l, high (r.---~ , solid oxide fuel cell 10 is shown. The plc~relled configuration is based upon a fuel cell system in which a flowing gaseous fuel, such as natural gas, hydrogen or carbon monoxide, is directed axially over the outside of the fuel cell, as in~ tPd by the arrow F, 20 and a flowing oxi-l~nt such as air or oxygen, is fed through an optional riser tube 12, positionPcl within the annulus of the fuel cell and eYtPnrling near the closed end of the fuel cell, and then out of the riser tuber and back down the fuel cell axially over the inside wall of the fuel cell, as in~ t~ by the arrow O. Where the fuel cell 10 is as shown, and operated at a high le~ of a~ rly 1,000~C, oxygen mohPcl~lPs pass from the oxidant 25 through a porous, electrically conductive, tubular air electrode 14 (or cathode), and are changed to oxygen ions at the air electrode-solid electrolyte interface. The oxygen ions then diffuse through a dense, gas-tight, oxygen ion c~nductive, solid oxide electrolyte 16, to combine with fuel gas at a porous, el~.trir~lly con(luctive, fuel electrode 18 (or anode), and release electrons at the fuel electrode-solid electrolyte interface, which are collected at the air 30 electrode, thus g~ g a flow of electrons in an eYtP-rn~l load circuit (not shown). For a more complete desc~ ion of the m~tPri~lc, confignr~tic)nc and operation of an e.~el~ ,y tubular solid oxide fuel cell and solid oxide fuel cell gr~ ol~ of tubular configurations which contain a plurality of series-parallel conn~tP~l fuel cells, reference can be made to W O 97/32349 PCTrUS97/03019 U.S. Pats. 4,395,468 (I~nberg) and 4,490,444 (I~nberg), which ~ osllres are incv.~ol~Lted by l~fer~,lce herein in their entireties.
The tubular solid oxide fuel cell design features a tubular, axially elongated (a~n~ ld~ly Sû to 230 cm long) air electrode 14. The air electrode 14 (or cathode) that S is, the eleetrode whieh will be in contact with the oxidant such as air or oxygen, is a porous, ic~lly con-~uctive, p.c;r~bly self-~u~l~lg structure, typically made of doped "-~ ~MnO3) ~ fc;~ ly doped with c~lrillm~ vllLiu~ barium or cerium in me A-site and chromium, niekel, ~ m, ~lllmimlm or iron in the B-site of its ABO3 perovskite erystal structure (a~lu*i~ Jy 1 to 3 mm thick) and, whieh is ~enf~lly extruded 10 or icost~tir~lly pressed into tubular shape and then sintered. An optional porous, calcia-~t~hili~i zireonia support tube ~not shown) gentor~lly surrounding the inside of the annulus of the air electrode ean be used, if n~çc~.y, to provide ~ litinn tl structural support to the air electrode. As shown in FIGURE 1, the air elec~.)de 14 is thin and of low volume design, so that only one oxidant feed tube 12 need be used. Reference ean be made to U.S.
15 Pat. 5,108,85û (Car}son et al.) for a more detailed description of this thin, low volume, self-ing air electrode configuration, which ~icrlnstlre is i~ uldts d by l~rt;le -ce herein in its entirety. Sueh a self-su~o~Ling air electrode ~L-U~U1~ iS relatively less ~ ensi~e, cimplifies the rn~nltf~-~t~lring process, and allows improved cell ~lrw-..anoe.
Surrol-n-ling most of the outer ~iph~y of the air electrode 14 is a layer of a dense, 20 gas-tight, oxygen ion ~lme~le, solid electrolyte 16, typically made of calcia- or yttna-stabilized zirconia (a~,u~i.na~ly 0.001 to 0.1 mm thick). The solid electrolyte 16 can be d~,osiL~cd onto the air electrode by well known, high l~ .me, elect~ochPmic~l vapor de~osilioll ~EVD) techniques as taught in U.S. Pats. 4,597,170 (Isenberg) and 4,609,562 (Isenberg et al.), which ~ n~,cs are il~cu.~oldLed by reference herein in their entireties. A
25 ~!lere;llt d solid electrolyte composition is (Y203)0 l(ZrO2)0 9.
A selected radial segm~ont 20 (a~ x~ t~ly 9 mm wide) of the air ~ ode 14, preferably exten-lin~ along the entire active cell length, is masked during fabrication of the solid electrolyte, and covers a thin, dense, gas-tight"~lL~lcolmection 22, which provides an electrical cont~ting area to an ~ Pnt cell (not shown) or to a power contact ~not shown), 30 as is well known in the art. The dense, gas-tight, ir~ ;on 22, covering the surface of the air electrode 14 along the radial sPg.-~..l 20, as shown, must be Plectnc~lly conductive in both an oxidant and fuel envil~n..lel-t at elevated ~Ill~l~Lules. The gas-tight i"t~ onl-ection 22, typically made of l~ ;l.ro-niL~ (LaCrO3) doped with c~lei-lm, , W 097/32349 PCTrUS97/03019 barium, strontium"~g.~ m or cobalt (a~ uAi"~ately 0.03 to 0.1 mm thick), is roughly similar in thit~l~nPc~ to the solid electrolyte. The in~l~;on~ ;on should be non-porous (over about 95% dense) and electri~ ly conductive at 1,000~C, the usual O~;~ g ~e~ re of the fuel cell. The h,~,~nl-ectinn can be de~)osi~d onto the air electrode by high ~"~ re, ele~ l vapor deposition (EVD) techniques as taught in U.S. Pats.
4,597,170 (Isenberg) and 4,609,562 (lsenberg et al.), both previously il~co~ ted by reference, or by plasma spraying as taught in U.S. Pat. 5,389,456 (Singh et al.), which Ai~ln~.~.c is incorporated by reference herein in its entirety. An el~octr~ lly cnn(l~ctive top layer 24 can be d~osi~d over the in~l~l~ n 22, typically made of nickel, nickel-zirconia or cobalt-zirconia cermet, typically of the same co,~ iol- as the fuel electrode (a~ u~ima~ly 0.05 to 0.1 mm thick).
Surrounding the rçm~inriçr of the outer ~li~h~ly of the fuel cell, on top of the solid electrolyte 16, except at the il~t~l~l-n~;l;on area, is the fuel electrode 18 (or anode), that is the electrode which will be in contact with the fuel. The fuel electrode 18 is a thin, electrically conductive, porous structure, typically made of nickel-zirconia or cobalt-zirconia cermet (i.e., a metal cerarnic) (a~lu,~ ly 0.03 to 0.1 mm thick). As shown, the solid electrolyte 16 and fuel electrode -18 are ~ ,l;n,lo~l$~ the fuel electrode being spaced-apart from the inL~l-;f)nl~cliol- 22 to avoid direct flr~t-;n~l contact. A major portion of the fuel electrode 18 is a skeletal ~Yt~n~ion of the yttria-stabilized zirconia solid electrolyte m~ttori~k The fuel dc~ }de 18 and top layer 24 can be dc;~osiLed on the solid electrolyte and ill~r~~ ecti~ n, respectively, by well known techniques such as dipping or spraying, and may be anchored more securely by electro~hemi~l vapor de~o~ilion (EVD) as taught in U.S. Pats. 4,582,766 (Isenberg et al.) and 4,597,170 (Isenberg), which ~ es are il~col~old~d by reference herein in their entireties. Both electrodes are Pl~tri~lly con~ ctive at 1,000~C, the usual fuel cell operating ~ . The self-~u~ g fuel cell configuration and the m~ttori~l~ and mPthf~ used for the solid electrolyte,in~l~,.l~-ection, and fuel electrode are well known, and described in U.S. Pats. 4,562,124 (Ruka), 4,751,152 (Zymboly), and 5,108,850 (Carlson, et al.), which ~li~losl-res are incw~.dled by reference herein in their entireties.
- 30 In operation at ;l~Jrv~illld~ly 1,000~C, a gaseous fuel, such as hydrogen (H~) or carbon monc-~ide (CO), or sc,.l.rl;lllt~s natural gas ~rim~rily ool~ g mPth~nt-), is directed over the outside of the fuel cell, and a source of oxygen, such as air or oxygen (O~), is passed through the inside of the fuel cell. The oxygen molecules pass through the porous W 097/32349 PC~US97/03019 electrically con~ ctive air electrode and forrn oxygen ions at the air electrode-solid electrolyte interface. The oxygen ions then migrate through the solid electrolyte m~t~ri~l to combine with the fuel at the fuel electrodc cle~ ulyte interface and release cl~;ll~,ns at the fuel electrode, which are ~en c~ llect~d at the air electrode through an external load circuit, S thus gen~ g a flow of ~l~trjr~l current in the external circuit from the fuel ~l~;llude (anode) to the air electrode (cathode). The electrorh~rnic~l reaction of oxygen with fuel thus ~ luces a ~olGnlial dirr~lG~ce across the ~xtern~l load which ~ a continuous cle~L-un and oxygen ion flow in a closed circuit during the genPr~tion of useful electricity. A
plurality of similar cells can be electrically col-n~L~I in series by contact between the 10 i lle~ .-ectiQn of one cell and the fuel electrode of another cell. The plurality of similar cells can also be tqloctrir~lly cnnnPct~d in palallel by contact between the fuel electrode of one cell and the fuel electrode of another cell. A more complete d~li~ion of the operation of tl~is type of fuel cell can be found in U.S. Patent E~e. 28,792 (Ruka), which disclosure is inco~ ~d by l~f~l~l)ce herein in its entirety.
The porous air electrode remains exposed to the hot oxidant gas ~trnosrh~re~ usually air, heated to a~pluki..-;.t~ ly l,000~C during ge~ c-r o~.~lion, and oxygen reduction takes place at the air electrodc ~l~;llolyte interface. In the tubular fuel cell configuration, the porous, Plectrir~lly con~1uctive, air electrode Ill~ contact with the dense, gas-tight, oxygen ion conductive, solid electrolyte, and dense, gas-tight, ~l~trir~lly c~ n-luctive, 20 int~ ~tion film, and also with the optional porous support tube when used. The s~ol~tic-n of a suitable air electrode must be done carefully to ensure that the air electrode has certain ~lu~lLies including, without limitation, high conductivity (low resistivity) at the high op~ g tem~r~hmes~ low re-~ict~nre contact to the solid electrolyte, good rhPm~
~ inter~tion or int~ldir~sion) stability and structural and rlimPn~ion~l stability at the high 25 ~ ;llg ~ ~J~ , 5l~ffiri~nt gas pPrmP~tinn porosity, and good match of the c~fficiPnt of thermal exp~n~iQn with the soLid electrolyte and il.lel~l-n~cl;nn.
Structural and ~limPn~ion~l stability of the air electrode, in particular, is an i~ t criteria for ~ inl;.i~ing long terrn m~ch~ni~l integrity n~ry for succP~ful fuel ce)l o~ iuns~ espe~ lly under isothermal or thermal cycling during cell r~,i~lion and30 orf~r~til~n For example, a ~pical air e}ectrode of a fuel cell ranges in leng~ from about 50 cm to 230 cm. If the air electrode length was 100 cm long, the tot~l heat ~hrink~ge of even 0.05% in length of the air electrode in contact with the solid electrolyte and inl~r~o,m~;lion would result in a 0.5 mm difference in length between the air electrode and ~e solid electrolyte or the in~ ;oll. This would result in severe stresses between the m~t~
A ~ illal total heat shrinkage of the air electrode length would be from about 0.03% to 0.04%, and long l;fe collllllel.;dlly ~r~ept~hle heat shrinkage values are thought to be below about 0.02%.
S All co... l~.. Pnl~ of the fuel cell except the air electrode gPnP.r~lly remain, under o~ldlillg c-)n-lition~, stable to shrink~ge when subjected to certain isothermal (i.e., oxygen partial ~ Ul~ cycling) con-lition-c or as a result of thermal cycling during cell fabrication and op-P~tirm This tendency to shrink is resisted by the adjoining solid electrolyte and ill~l~n~--Pction and l"~ s into stresses between the air electrode and adjoining10 components and, in some cases, can result in cr~king failure of individual fuel ceUs, l.~...~.;..g PlPrtrir~l power genPr~ti~n of a multi~ell ~ r. It is thus desired to produce an air electrode m~t~.ri~l with a better match of its co~ffirient of thermal eY~n~ion with that of the solid electrolyte and in~-;ol-l-Pction in order to reduce such tlimPn~ic)n~ hrink~ge of the air electrode material, while also producing a low cost structure in order to improve the 15 collll"wcial attractiveness of these fuel cells, but without i...l.~i.h.g the other desirable air electrode l~lu~llies such as good low resistivity and controlled porosity.
The porous, ~re~ bly self-~u~ g, air electrode of this invention provides a m~t~.ri~l which has better thermal ~-p~ ;o.- match with the adjoining co."pollell~ of the fuel cell, such as the solid electrolyte and i~ on~ -, in order to improve the ~ P~ion~l 20 stability and reliability of the ~fuel cells during cell r~licaLion and op~r~tirn, and further is attractive because of its relatively lower cost to fabricate as colll~d to fabrication costs for conventional fuel cells. The air electrode of this invention also provides a m~tP.ri~l which has good ~ clul~l and ~limPn~ion~l stability to self-support the air electrode tube, good porosity for oxidant ~lllle~iol~ and good low resistivity at high ~ re for effective Ploctricity 25 genP.r~ti~n The lower cost of fabrication of the air electrode of this invention results from the use of lower purity raw m~tPri~l~ rather than the typically used individually s~;;y~dl~d higher purity raw m~fP.ri~l~ in conventional air electrodes. In this invention, l~nth~nifie Il~ibclul~s mainly ~Ill~ ing at least La, Ce, Pr, and Nd, or at least La, Ce, Pr, Nd, and Sm, such as 30 collllllel,idlly available lmfini~hed l~nth~nirle ccnr~..-l,,.l~s mined from rare earth oxide natur~l resources in the ground and incompletely selectively se~ 1 into their individual col--pollellL~, are substituted in the air cl~;llude m~tPri~l for some higher cost individual species of l~nth~nllm, commercially available as individual l~nth~nllm oxides and carbonates, W O 97/32349 PCTrUS97/03019 that have been subjected to extensive selective sep~r~tinn and purifir~tion proce-c~-c The unfiniehe~ l~nth~nnm conf~ t~ in the past served as the raw m~tPri~l for the m~nnf~ lre of individual l~-.lh~ compounds, such as individual 1~ oxide. This low costl~nth~nif1e mixture sub.ctitutif ,l for individual l~ h~ .", in the air electrode also provides, S llneypect~P~ly, better results than prior art co",loc;l;nnc not only in terms of its lower f~hric~tion cost, but also in terms of better thermal e~r~ncion match with the solid electrolyte and i~ f~tion. Past air electrode formnl~tinnc suffered from both ~ ol~l sf~bility problems as a result of the thermal ~ nc;..~ micm~tr~h and high fabrication costs i~om the use of relatively expensive pure individual l~ h~ " ~l~lpounds, such as pure individual 10 l~f.ll,~nl-", oxides and l~"lh~ "~ f~bollales, as the air electrode raw m~fPri~lc Both of these problems are avoided in the present invention.
The l~nth~nifl~P5 (i.e., rare earth metals), include 15 pl~mPntc in the periodic table whiCh are k~ n~ cerium, pIaseodymium, neodymium, p",",~ I ;."", Guru~iu"" ~lolini~lm, terbium, dysprosium, hnlmillm, erbium, thulium, yl~lbiulll, and 15 lnt~tium (L~, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, ely). Except for Pm, the l~nth~ni~l~c (solllrl;lllP-s ~ ~d by rare earth col~ c ac Ln203) are not uncommon and occur naturally as ~ui~lulc~s in rock form~tionc and sands throughout t-he world although sizable ~iepQsitC are few in number. T ~nth~ni-lP5 are typically found in three minerals which are ~ and b~ctn~cite for light l~nth~nirles and xenotime 20 for heavy l~nth~niflP~s and yttrium (another rare earth metal). l~on~7it~ and bA'il..~ - are the two mint-,r~l~ that are mined co"~",~ -,ially to supply the world with most of the rare earth ~hemi~ lc, ~I.A.~llP.7 a fluoro~llol-a~ (Ln2F3(CO3)3), cont~inc about 90% La, Ce, Pr and Nd metals in its l~nth~niflP content in the natural mixture and is the chief source of co"~ r~;al supply of l~nth~ni~Ps Mt~n~7it~,, an thorium orthophncph~t~q (LnPO4) also 25 cont~inc about 90% La, Ce, Pr, and Nd metals in its l~nth~ni-le content in the natural mixture and some thorium ~hO~) and is the second source of conllll~ ial illl~l~lce. The main deposit of l~l"~;le is in the Mountain Pass area of California (90% of U.S. and 66.6% of world output).
An indllctri~l demand currently exists for individual l~nth:3ni-1es, and rare earth 30 co",~ c have devised extensive rare earth ore extraction, st;~a,~Lion, and pllrifi~tion systems to isolate the individual l~nth~ni~les from the natural l~nth~ni~ ll~lul~s present in the raw min~ l.c, Cerium is generally the most abundant l~nth~ni~e in the natural l~nth~nitle rmixture comprising up to about 50% of the natural mixture. Cerium is p~ y in the W O 97t32349 PCTtUS97/03019 highest demand and co.. ~n~ls the highest market price. Thclcrfu~c, the rare earth co~ ies routinely first selectively scll~udlc cerium from the rnineral deposits, leaving an mfinichPcl by-product l~nth~ni~lP con~Pntr~tP co~ ioi~-~ the r~ ncP~ ~t~ natural 1~nth~ni~lP ~ ulcs less most of its cerium content. For a detailed description of b~ctn~citP
S and mcm~7itP rare earth ore PYtr~-~tion, sep~r~ti-n, and pllrifi~tifm techniques, which are well known in the art, l~rclc,~ee can be made to Kirk-Othmer, Concise En4yclopedi~ of Chemical Technology, John Wiley & Sons, Inc., New York, NY (1985), pp. 204-242 and 997-998, which f1i~los~re is incol~l~t~d by ~cÇt;lc,~ce herein in its entiret.y.Further in-lnstri~l demand for other individual l~nth~ni(lps~ such as individual10 l~nth~num, Pcpe~ lly for use in air electrode col,.posilions of solid oxide fuel cells, also exists. However, sep~r~tion of individual l~nth~nirlPs, such as lanlll~~ , commercially from the Imfiniched l~nth~niflP mixture, such as the l~ h~ P cm-~ P- by-product, or even from a natural l~nth~ni~le mixture involves extensive s~pAr~tif~n and purifif~ti~n pl'~7 iec which in turn inc~~s the overall cost of these metals as raw m~tPri~lc flr~cti~lly.
15 Th~lcrolc, to keep raw m~tPri~l costs down for the doped t~nth~n~m ll~n~ , (I,aMnO3) air cl~ll~de m~tPri~lc, it is desirable to use l~ h;~ co---~ -ds in a less pure form other than its individual form, such as individual La203, LaCO3, or the like, which is ~;ulltl~tly the m~tPni~l of choice for fabrication of the air electrodes. The inventors have located a lower cost alternative previously cnnci~lPred as an unacceptable inlc .n-~ialc m~tPri~l which, 20 conse luen~y, he~ orf lc; was ignored in the solid oxide fuel cell industry as a raw m~tPri~l for the air ele;tlodes. This alternative m~tPri~l is this ImfinichPcl ~con~xPntrAtPrl) natural l~nth~ni~le (Ln) mixture which c~ t~ and other l~nth~nirlf$~ usually ~n La, Nd, Pr, and so...P~ s Sm in varying natural amounts, minor a",uunls of Ce ~
from the previous selective sPp~r~ti~rl, and trace hl~puliLies. This .".r~ nthzlni~P
25 concPntr~tP m~t~ri~l typically s rved as the raw m~tPri~l for the production of individual ~nth~nllm compounds.
The low cost l~nth~ni-le ,,~lu.~ are ~us s~lbstitut~l at the A-site of the air electrode for some of the la,Ll,anu"~ in this invention. In 1995"...ri~ hPd l~nth~nide mixtures ("l~nth~ni~le conccntna~s'') were priced at $3.00(USD)lkg, whereas individual pure 30 lanthanum ~ul,onales were priced at $13.75(USD)/kg and individual pure lanlll~u." oxides were priced at $ 17.60 (US)/kg. SubstitutiQn of l~nth~nicles for some of the pure l~nth~num is also believed to lower the thermal ex~n~ion of the air electrode material as a result of the smaller ionic radii of other l~nth~ni~les as co---pa~cd to that of l~l~fl.;"~.,... Thus, it was ::

WO 97/32349 PCT~US97/03019 discovered that the use of mixed l~nth~nides ~Ln), mainly comrri.cing La, Ce, Pr and Nd and minor ~molmtS of other l~ es and trace i~l~pulilies, as a substitute for at least part of La in the air electrodes, lowers the coeffir;ent of thermal eY~n~ion to ~e desired range and ~ignifi~ntly reduces the raw m~t~ri~l costs of such air electrodes.
The ylcrelled unfini.~h~-l mixed l~nth~ni-le. co"l~u"ds used in the air electrode of the invention are provided in their natural l~u~l.,..~;-le mixture state except for Ce, and mainly coml)ri~P at least La, Ce, Pr, and Nd and minor ~mount~ of other l~nth~ni~ s and i"~ s A co,.. - .c;aUy available lmfini~hP~ nth~nitle cQncPntr~t~- typicaUy co,.,~ prim~rily at least La, Ce, Pr, and Nd, or at least La, Ce, Pr, Nd and Sm, together with minor alllOUnl 10 of other l~nth~nides and impurities.
The low cost, flim~.n~ n~lly stable, porous, el~trir~lly c~ndllctive, air electrode of this invention is a solid sc)ll~tinn~ having a perovskite-like (AB03) crystal structure, of a l~nth~ni~le-substituted doped l~ g~ having the general rhemi~l forrnula (1).
~ X,T nwcex~y)~nl-z~o3 (1) 15 where Ln is a low cost l~nth~ni~le mLsture, being either in a naturally occllrring state or a partially s~ ~ and nnfini~h~d c~ nrPntrat~i state, sPl~t~l from a mixture of at least two, at least three, at least four, or at least five of without limit~ticn, La, Ce, Pr, Nd, and Sm and other l~nth~ni(les, with the proviso that if Ln co~ ises a mixture of only two l~nth~nirles, the mixture is not the co,-,bindLioll of La and Ce; La and Ce are ~lect~l from individual 20 species of La and Ce, ~ ely; MA is an A-site dopant for el~fric~l conductivity selected from individual species of at least one of Ca, Sr or Ba, or IIIi~lU1~ thereof; MB is a B-site dopant for dimPn~i- n~l stability selected from individu~l species of at least one of Mg, Ni, Cr, Al or Fe, or ,,~,,~Lu~s thereo~; w is from about 0.05 to 0.9, pl~rt;lda)ly about 0.1 to 0.9, most preferably about 0.4 to 0.8; x is from about 0 to 0.1; y is from about 0.1 to 0.2; and, 25 z is about 0.05 to 0.1 mole per mole of formula ~1). However, ~is equation is merely eY~mpl~ry and any range of La, Mn and A-site and B-site dopants are embodied by this invention so long as the co-,~ ion cont~ins a finite amount of Ln as a substitute for some or all of the La. These air electrodes are novel in that their formulations contain other l~nth~nides, such as Nd, Pr, etc., which were not found in the prior air electrode 30 formulations because such m~t~ri~ls were avoided in air electrodes and considered i~ ulilies in the art. But these lanthanide mixtures in the air electrodes now render the air electrodes less expensive to fabricate and unexpectedly provide a better thermal exp~n~i~ n match to the solid electrolyte. The l~nth~nifle mixture may be a substitute for part or all of the l~ n~

CA 0224747l l998-08-2l WO 97132349 PCTrUS97/03019 in the air electrode forml~lztti--n~.
Some p-~r~l~d air electrode compositions of this invention are:
(LaO,2LnO,6CaO,2)(MnO,gsNi 05)o3;
~tO.2Lno ~Cao.~)(Mno 9~Nio lo)o3; and, S (LnO 8CaO,~)(M ,nO.9~MgO.10)03~
In these ~I~;r~ d c~--po~ nc, the lztnthztni-l~ mixture (abbreviated "Ln") comprises a lztnthztni~ P of at least four l~ ni-lP~ of La, Ce, Pr, and Nd.
The actual cc,~po~ t- of the co,...~ ;;dlly available lztnthzmi~le c~ t~ in these p,~r~ d Pyztmples is a mixture of c~l~lales ((C03),~) or oxides (O,~) of the following 10 lzlnthztnid~os:
Ln = (LaO 598Ndo l84pro o8~ceo~l3lcao o~2sro oo4)~
where Sr and Ca are trace impurities. Other C~ of lzmthzlnide conc~
co~ ;ially available are a mixture of ca~ n~LLt;s or oxides of the following lzmth~ni~e5-Ln = (L~0 ~8CeO sPr0 7Nd0.2MnO3); and, Ln = (La067Ce00o7proo7Ndo~
for high or low Ce con~ os"~,~;Li~ely. Cle~rly, the molar ranges and types of lznthznides in the mi~cture will vary in these unfini~hed lzmthzlnirle IllL~Lùl~s, since such ll~i~lulc~S are based on natural Il~i~Lules which have been incolll~leL~ly purified to dirrt;,c~, -extents. However, for purposes of this invention, the lzlnthzlnide n~lules col-llll~r ;ally 20 available can be ch~r~teri7pd as c~~ at least a mixture of La, Ce, Pr and Nd colll~ unds.
The 1~nth~ni~le- iul, jLilu~d doped 1~ o~. .;l~ m~t-ri~lc of this invention are solid so1~ltic-nc whieh pl~r~dl~ly eonsist of a single phase. In these ceramie porous (i.e., about 20% to 40~ porous by volume) air eleetrode m~t~ri~lc, 1~ . is s~1bstit~ted with lower eost 1~nth~nide eol~ nds, sueh as natural 1~nth~ni~1e mixtures or Imfinich~cl 1~nth~nide eonc~l.,.t~i, in the perovskite erystal lafflee, to provide a lower eost air eleetrode m~t~-ri~1 whieh has excellent opt~r~tion~l properties, such as Px~11Pnt eoP-ffieient of thermal e~ ion mateh, porosity, resistivity, ~im~nci~n~1 stability, and meets all other air eleetrode requirements.
As a first step in making the porous, self-supporting, air eleetrode tube of this invention, powdered oxides, earbonates, or other eompounds that form oxides upon heating, such as oxalates, of low cost l~nth~niclt- mixtures, such as an Imfinichefl l~nth~nid~
concentr~t~ mainly eomprising a mixture of at least La, Ce, Pr and Nd, for example, a W 097/32349 PCTrUS97/03019 ~uxture of at least La203, CeO2, Pr60,l, and Nd203 or co,llp~udl)le m~tPri~l~, or som~otim~s a mixture mainly compri~in~ a mixture of at least La, Ce, Pr, Nd and Sm, are intim~tPly blended ~gG~.el with pure individual oxides, carbonates or other coll,pounds that form oxides upon h~ting, of the La and Mn base metals, for eY~mpl~, individual Id2O3 or S LaCO3, individual MnO2, or co".p~u~le m~tt-ri~lc, and also with pure individual oxides, carbonates, or other com~ m~ that form oxides upon he~ting, of the Ce, Sr, Ca, Ba, Mg, Cr, Al, Fe, or Ni dopants, for GA._ 1 le, CeO2, SrO, SrCO3, CaCO3, BaCO3, MgO, CrzO3, Al2O3, Fe2O3, and NiO, or c~ .n~ m~t~,ri~l~, Each m~n~l is acco,~lingly weighed out in the proper p~ ions to give the desired l~nth~ni~l~sul~sLiluled doped 10 m~n~ni~ air electrode colllpo~iiion after calcining.
The powdered mixture is next pressed ~ler~dbly by i~ost~ti~ ~lGS~il2g, into a shape, preferahly a cylindrical pellet form. The pellet is then ~k~,inPA, plcr~l~ly in air, at ~ tures from about 1,300~C to about 1,750~C, plGrGl~bly about 1,500~C, for about 1 to S hours. The calcined pellet of doped l~ n~.~;le powder with l~nth~ni~15 s~iluled for some of the pure l~nth~nllm is then pulverized, i.e., crushed or ground, and further s-;lce,~ed, to provide smaller particles and a more uniform particle size di~LIil)u~ion.
~lcin~tion and pulveri7ation can be lc~led a number of times, typically about 3 times, to provide the desired f~Qh~n~xd chemi~ homogeneity of the powder and small particle size distribution. The finich~l c~k~in~l and pulverized powder preferably has medium particle 20 size between about 0.5 to 100 microns, preferably about 10 microns. The particle size distribution, emrh~ci7ing small ~ icles~ is illl~ll~lt in providing strong, yet thin, porous air electrode tubes of this l~nth~ni-le-substituted doped 1;~ .. m~n~nit.q co.l,~osilion.
The crushed ~k~,inl~ci powder is then molded by jCoct~ti~ pressing, or more usually extruded, into tubular shape. Prior to Çulll~ g into a tube, the crushed c~ .in~d powder can 25 be intim~t~,ly blended with other ingredients, such as cohPci~n agents, pore-fornling agents, binders, and wetting agents for improving the forming opf~tion~ and structural l,l~pelLies of the "green" unsil~lel~d tube. The powder can then be mixed with from about 1% to 5% by weight of a decomposable cohesion agent, such as an organic starch, e.g., corn starch, Ace starch, potato starch, or the like, to provide c~ h~.si~ n and plasticity for extrusion, from about 30 1% to 4% by weight of a decol.-~sable pore-forming agent, such as an organic c~ ose~
e.g., maple wood flour, fibrous cellulose, methyl cPll~ c~, or the like, to provide pores for gas pel...~ n, from about 1% to 4% by weight of an organic, water-soluble binder, e.g., polyvinyl alcohol, polyvinyl acetate, paraffin wax emlllcion~ or the like, to provide dry W 097/32349 PCT~US97/03019 strength for h~tlfllinE, and up to 1% by weight of an optional wetting agent, to help in extrusion, such as n~rhth~lPnP-sulfonic acid conr~Pn~tPc, with the balance of the formable mixture con~tit~lting the c~lrinpr~ powder, pl~re;l~bly from about 90 to 95% by weight.
Plerel~bly, the particle size of the cohP-~;on agent and pore-forming agent should be below 75 microns, and shou}d deco"l~ose b~Lwee-- about 100~C and 550~C. The binder should also decou,l,ose in this te~ ; range. Pl~;f~ldbly, all dry ingredients are mixed tc~gethPr dry and then wet mixed with the water-soluble binder dissolved in a water s~ tinn, to provide a wet formable mixture. It is ~lt;f~ d that the wet forrnable mixture is aged for about ti to 12 hours, to ~JlUlllOk~ water di~L~ ulion and homogenPity. The forrnable mixture 10 is then either i~st~tic~lly pressed or p~rel~ly extruded into tubular shape in a "green"
unsi,~L~led CQn~ition In order to close one end of the air electrode tube, a solid cylin-lrir~l plug of the formable mixture can then be pushed into the annulus of the tube a sPl~tPd ~ t~nce~' usuaUy from about 2.5 to 7 cm, from one end of the tube. The plugged or clos~ended tube is then 15 dried in air, and next sintered in air at l~ -Al-- ~s from about 1,300~C to 1,750~C, ly about 1,550~C, for about 1 to 6 hours, to sinter togPthPr the air electrode tube walls and end plug, to drive off the binder, cohP-cion agent, pore-rul,liing agent and wetting agent, and to provide a concolitl~t~od, sintered tube of a l~nth~ni(lP-~ulJ~liLuL~d doped n. The sintered structure is then cooled to form a unitary 20 sintered mass. The tube can then be cut along a portion of the plugged closed end and then smoothed or rounded, or otherwise fini~hP~i for end use. The form of the air electrode is usually a thin walled, tubular forrn as shown in FIGURE 1, but it can also be in buLI~ form and in the form of flat or collu~Lt;d plates dPpen~ling on the multi-cell g~
config~ Lioll.
In a high Le~ dlllre, solid oxide fuel cell and g~ lol, this porous, self-~ù~l~olL~d, electrically corlductive, air electrode tube is sub~l~lLially covered on its exterior by a dense, gas-tight, oxygen ion conductive, solid oxide electrolyte, such as yttria-stabilized zirconia, e.g., (Y203)0,(ZrO2)09, except ~or an axially ekmg~tPfl radial qPglll~ u~r~dbly e~tPn-iing along the entire active cell length, which is covered on its exterior in this ~ ~-1 by a 30 dense, gas-tight, electric~lly conduchve, i"l~r~ l~n~;l such as doped l~nth~nnm chlulllile.
The solid electrolyte is s~sl~lially covered by a porous, elechicaUy conductive, fuel electrode, such as nickel-zirconia cermet, and a distinct layer of nickel-zirconia cermet also covers the inlel~ -Pct The solid elechrolyte and fuel electrode are disc~ntinuous along the W 097/32349 PCTrUS97/03019 radial ~Pgm~nt and are spaced apart from the inle.~;o,lllect to avoid direct p1~trj~l contact with the ir,t~.~;om~e.;~ and, consequently, short~i~ iling of the fuel ceU. A plurality of solid oxide fuel ceUs can be series-parallel c~....-Pcl~l to form a multi cell g~n~r~t~lr for eloc~r~
power ge~ ;nn as is well known in the art. The use of l~nth~ni-lPs and l~nth~ni~l~ Jn~lul~.
5 in the air electrode as a s~ for l~ -- results in a lower cost air eleet~de without degrading the air electrode opef~ti()n~1 requ,l~n~
The foUowing FYZ~ ' further ill~-5tf~tP the air electrodes of the invention and are intPnd.od to be purely PYe nrl~ry without lirniting the invention in any way whatsoever.

A low cost, stable, ei~tfi~lly con~ ctive, gas perrneable, self-suL~ thin-waUed air electrode tube of this invention was made from a low cost l~nth~nide mixture, i.e., a l~nth~nid~P conc~ntf~t~P, as a substitute for some pure l~i.ll.~.. ~, and co.n~d in terms of thermal expansion coPffl~jent, porosity, ele~i~l resistivity and cyclic ~hrink~e, to another air electrode tube made from pure individual 1~ .l.ll. The low cost air electrode tube was~5 ~ a,ed by first mixing together the dry powdered ingredients in Table 1.
TABL;E 1 Material Gra ns Grade T~ r~ Concentrate CarboDate ~c.,~ ,r;.. lg mainly La, Ce, Pr, and Nd~ 89.9 Molycor" 5211 CeO2 6.2Aldrich/99.9 %
CaCO~ 10.0Fisher/Certified MnO2 40.9,~h~mirDI/HP~
NiO 0.7Cerac/Pure Cr203 1.5Fisher/Certified Molycorp 5211 l~ ide conr~ntrate carbonate is sold by Molycorp of New York.
This l~nth~nifle co~ lP is a low cost l~nth~nide mixture that is derived from a natural 20 l~nth~nirlp mixture mined frs~m rare ear~ oxide mines in Mountain Pass, California and has been subjected to inco~ ~ separation of its cerium content. This m~tPri~l is sold as a carbonate and cont~inc a mixture of carbonates of La, Ce, Pr, and Nd and somPtimf!s trace Sm and other l~nth~nif1ec and other i,~ ilies. Of course, since this is a naturally derived product, the concPntr~tionc of the individual l~nth~ni(lP colll~ollenl~ varies from batch to 25 batch. Molycorp 5211 ls~nth~niflp concf~ntr~t~ has been expressed ~c 69.3% La203, 4.7%
CeO2, 7.6% Pr6O", and 18.06% Nd2O3, in weight percent on an oxide basis. Molycorp CA 0224747l l998-08-2l W O 97/32349 PCTrUS97/03019 5211 l~nth~ni~le con~ntr~tP has also been ~ ,~sed as a molar based La0.s98Ndo~l84pro.o8~ceol3lcaooo2srooo4. However, it should be l-n~lerst~l that the use of any l~nth~nifle mixture that inrludes a mixture of at least two to five of La, Ce, Pr, Nd, Sm and other l~nth~nide~c and trace impllriti~s for an air electrode comro~ition is en~r~ ~l by this S ~li.~lc)s~~re.
The above raw m~t~.ri~l~ listed in Table 1 were ;~ tPly mixed to~ -th~or and this ll~ixlur~ was c~le~ tpd to provide the desired sarnple eo,l.posilion upon ~ P~ . The n~ixed powder was co~n~ ed into pellets and then e~lrinecl three times at about 1,500~C in air for about 4 hours. After each r~lrin~tion, the r~lrined powder was pulveri~ed in order to 10 enh~nee rhPmi~l hornogen~ity and rep~llefi7~1 for the next c~lrin~tion The fini~hed ç~lrined powder had about a 10 micron ,.,~l;~.... particle size. The dry e~lein~l powder was then mixed with methylcellulose binder to forrn an aqueous formable paste for extrusion.
The forrnable paste was then extruded in a tubular form (65 cm long, 1.58 cm OD) and next fired at 1,550~C for about 4 hours to form a sintered, porous ~about 30% porosity), self-15 supported air electrode tubes of the composi~icn listed in Table 2. The p,ù~llies of this airelectrocle tube are also listed in Table 2. And, for co...~ on, another air electrode ~o~ osi~oll was made by using an individual species of pure l~ -... oxide (a~lu~ ~ly 99.9% pure) rather than a low cost l~nth~ni-le mixture, and its ~u~Lies are also inelu(l~l in Table 2.

Material Porosity Electrical Thermal Cyclic ( %) Resistivity F . ' Shrinkage at l,000~C Co~ at (% per (mQ-cm) 25~C to cycle) 1 ,000~C
(10~6m/m/~C) [(La0~516NdQ~30PrQQusmQoolsrQool) 31 20.8 10.5 0.002 CaQ200CeQ,Qs][MnQ94Nio~QcrQo~lo3 [Lao7cao~2coceQlQs]~no~NiQQ~crQo~]o3 30 12.5 10.8-10.9 0.001 The above results show that the low cost 1~nth~ni~l~-su~stit~lt~l doped l~nth~n~lm ~ ng~ . air electrode composition of this invention is gas permeable, electrically conducting, and 11im-~n~ion~lly stable. The advantage of this composition is its lower 25 ~bli~lion costs and that its coeffi~i~nt of thermal e~p~n~ic)n more closely n~tl~h~s that of a (Y2O3)0 I(Zr~2)0 9 solid electrolyte which has a coefficient of thermal expansion of about 10.5 W O 97t3Z349 PCTnUS97/03019 . 24 x 10~6m/m/~C.

Low cost, stable, e~ ly con~ ctive, gas ~l,.,~le, air electrode test bars were prepared based on the geneIal formula (2), a subset of general formula (1) listed 5 hereinabove:
a,al w~.2LnwCaO.2)(Mn, ,,(Ni or Mg)z)03 (2) where w = 0.4 to 0.8, preferably 0.4, 0.6, OF 0.8, and y= 0.05 to 0.1, ~rPrPl~ly 0.05 or 0.1. The tubes were made with l~nth~ni~1e mixtures of Molycorp 5211 l~nth~ni~lecon~ P ca,l)ol.~le as in Example 1 (abbreviated for cimrli~ity as "Ln") which again 10 comprises a mixture of at least La, Ce, Pr, and Nd and other trace l~n~h~nirlP~ and imr~riti~s The l~nth~ni-~e con~-pntr~tp used in this Example was lc~ll~d as a molar baced co--l~osilion of LaO.s98Ndo l84PrQO8lCeO ,3,CaO on2SrO~0o4~ with Ca and Sr being trace illl~lilies.
The Ln was il.~ Ply blended together with individual species of CaC03, MnO2, and NiO
or MgCO3 for calcination. The air electrode powder was prepared by three solid state 15 ~ ein~ti~nc similar to that described in Fy~mrl~. 1. The fini.ch~ k~in~1 powder was i~o5t~tic~lly pressed into ~ ular sarnple test bars 2.54 cm long by 0.635 cm thick by 0.635 cm wide, and sintered at about 1,550~C into bar ~ tes with about 30% porosity by the method flescrihe~ in ~;Y~mple. 1. Thermal eYr~n.ciorl coeffi~ientc were ~--~ul~d for each test bar and the results are listed in Table 3, and are con.~d to that of an air electrode 20 co~ n made from pure individual lan~anum cc~---~unds and that of t'ne yttria-stabilized zirconia solid oYide electrolyte.

Material Function C~fC-- of Thermal F- r - (l0-6m/m/~C) ~LnO~CaQ~)(MnQ95NiQO5)03 Air Electrode 9.6 ~a0~LnQ6caQ2)(Mno95NiQo5)o5 Air Electrode 10.5 (LaQ4LnO4CaO.~)(MnO95NiQQ~)0~ Air Electrode 10.7 (LrJO~CaO.~(MnOg~NiQ~)03 Air Electrode 9.5 a,aO 2LnO 6CaQ~)(MnO goMo ~0)03 Air Electrode 10.4 (LaO~LnQ4CaO.2)(MnO9oNio.l~)o3 Air Electrode 10.7 (LnQ8CaO ~(MnQgOMgQ,0)03 Air Electrode 10.6 a~aQ2Lno6caQ2)(~qrbsDMgQl~)o3 Air Electrode 10.9 (~b.4LnO ~cao~2)(MnQ9oMgQl~)o3 Air Electrode 11.2 CA 0224747l l998-08-2l Co~.~,~d~ F
(Y2~3)QI(zr~~)Q9 Solid Electrolyte 10.5 (LaQ,CaQ2)(MnO.ysNiQQ~O3 Air Electrode 10.8-10.9 (LaO,CaO2)MnO3 Air Electrode 10.9 Table 3 inf~ tps that the coPffi~iP-nt of therrnal eY~n~ion is rim~rily do.,~ 1 by the Ln/La ratios. For the air electrode of the invention the pl~r~ d Ln/I~ ratio is in the range of 3 to 4. Also, to a lesser degree, the coeffi~ient of thermal e~ was ~le~Pn~lpnt 5 on the Ni or Mg doping in the ~site.
Furtherrnore, three low cost air electrode co~ ilions listed in Table 3 which are ~0.2Lno.6Cao,2,~ o 95Nio 05)03; (LaO ~LnO 6CaO ~)(MnO gONiO l0)03; and, (LnO 8CaO~)(MnOgOMgO 10)~3~ were found to possess coeffi~i~nt.~ of thermal expansion (10.5 +
0.1 x 10~/~C) which closely match that of the yt~ia-stabilized ~ .,a solid electrolyte (10.5 10 x 106/~C). These three co,llpo~iLions were further tested for PlP-ctrif~l concll~ctivity and imPn~icm~l stability, which results are list~d in Table 4.

Material Porosity Electrical Thermal Cyclic (%) Resistivity F~ Shrinkage at 1,000~C Coef~rient at (% per (mQ-cm) 25~C to cycle) 1 ,000~C
(10'6m/m/~C) (LaQ2LnO 6CaQ2)(MnQg5NiQ05)03 30 14.4 10.5 0.000 (LaO2LnQ6CaQ2)(MnOgONiQ9~O3 31 18.9 10.4 0.004 (LnO ~CaQ2)(MnQg0MgQl~o3 31 20.3 10.6 0.000 Example 1 (LaQ5l6NdQl3oprQQusmQoo~sro~ool) 31 20.8 10.5 0.002 CaQ2ooceQlQ5(MnQ94NiQo2crQo~)o3 Cc,~ Example (LaO~,CaO~200CeO~105)(Mno9~Nio~o2crQo~)o3 30 12.~ 10.8-10.9 0.001 Results of total shrinkage for the heat cycled matenals of E~cample 2 listed in Table 4 are ~ gr~mm~ti~lly shown in FIGURES 1-3, ~ e;lively. In the Figures, the capital letter A shows the e~p~n~ion curve and the small letter a shows the l~ tulc; curve. Thus, re curve a proceeds up to 1 ,000~C and holds, drops to 600~C and holds, and raises W O 97/32349 PCTrUS97/03019 to l,000~C and again holds. The final chrink~ge is ~Lt;~ ed as the difference between peaks on the e~cr~n~ion curve A, at a le~ e of 1,000~C, shown as X-X'.
All of the U.S. patents mentiont?d in this ~erifir~tion are incorporated by .~r~ ce herein in their entireties.
The invention having been ~ o~ in conn~ction with the for~goillg embo(limrntc and examples, ~ itiorl~l exarnples and embor~imrntc wilI now be a~ L to persons sl~lled in the art. The invention is not int~.nde-i to be limited to the ernbo~1imrntc and ~ox~mple .cr~erifir~lly mPntion~l, and accordingly reference should be made to the appendecl claims rather than the ror~o.l,g ~licrll~Q;~ n of ~l~rell~d embodimentc and ~ " to assess the 10 spirit and scope of the invention in which exclusive rights are cl~im~.

Claims (23)

We claim:

1. A method of making a low cost, lanthanide-substituted, dimensionally and thermally stable, electrically conductive, porous ceramic air electrode structure, which comprises:
(a) providing a powdered natural lanthanide mixture either in a natural state or an unfinished concentrated state of at least two lanthanides from the group of La, Ce, Pr, Nd and, Sm;
(b) providing powdered oxides or carbonates of individual species of La and Mn, at least one A-site dopant from the group of individual species of Ca, Sr, Ba, and Ce, and, at least one B-site dopant from the group of individual species of Mg, Ni, Cr, Al and Fe, where the molar relationship between powdered individual species of La (La) and powdered natural lanthanide mixture (Ln) is La ~-W Ln W, where W is from about 0.1 to 0.9;
(c) blending together said powdered oxides or carbonates of the lanthanide mixture with said powdered oxides or carbonates of said individual species of La and Mn, said at least one A-site dopant from the group of individual species of Ca, Sr, Ba, and Ce, and, said at least one B-site dopant from the group of individual species of Mg, Ni, Cr, Al and Fe, in appropriate proportions to provide a desired lanthanide-substituted doped LaMnO3 composition after calcining;
(d) pressing the blended powder into a shape;
(e) calcining the pressed shape at a temperature of from about 1,300°C to1,750°C for about 1 to 5 hours;
(f) pulverizing the calcined shape to powder form;
(g) blending the calcined powder with at least one from the group of a cohesion agent, a pore-forming agent, a water-soluble binder, a wetting agent, and water to provide a formable mixture, where the calcined powder constitutes from about 90% to 95 % by weight of the formable mixture;
(h) molding the formable composition into a shaped air electrode structure; and,(i) sintering the shaped air electrode structure in air at a temperature of fromabout 1,300°C to 1,750° for about 1 to 6 hours, to form a porous, shaped, air electrode structure of the lanthanide-substituted doped LaMnO3 composition which is thermally stable and electrically conductive.

2. The method of claim 1, in which steps (d)-(f) are repeated one or more times.

3. The method of claim 1, in which step (h) further includes molding the formable mixture into a tubular shape.

4. The method of claim 3, in which sometime between steps (h) and (i) one end of the tube is plugged with additional formable mixture.

5. The method of claim 1, in which the lanthanide mixture comprises a mixture of at least La, Ce, Pr and Nd.

6. The method of claim 1, in which the lanthanide mixture comprises a mixture of at least La, Ce, Nd, Pr, and Sm.

7. The method of claim 1, in which the lanthanide-substituted doped lanthanum manganite air electrode material has the chemical formula (1):
(La ~-w-x-y Ln w Ce x(MA)y)(Mn ~-z(MB)z)O3 (1), where Ln is a lanthanide mixture either in a natural state or an unfinished concentrated state, selected from a mixture of at least two of without limitation, La, Ce, Pr, Nd, and Sm, with the proviso that if Ln comprises a mixture of only two lanthanides, the mixture is not the combination of La and Ce; La and Ce are selected from individual species of La and Ce, respectively; MA is an A-site dopant for electrical conductivity selected from individual species of at least one of Ca, Sr or Ba, or mixtures thereof; M B is a B-site dopant for dimensional stability selected from individual species of at least one of Mg, Ni, Cr, Al or Fe, or mixtures thereof; w is from about 0.10 to 0.9; x is from about 0 to 0.1; y is from about 0.1 to 0.2; and, z is about 0.05 to 0.1 mole per mole of formula (1).

8. The method of claim 1, in which after step (i), a dense, gas-tight, oxygen ion conductive, yttria- or calcia-stabilized zirconia ceramic solid electrolyte is applied to the outer periphery of the air electrode to contact and substantially surround the air electrode, and then a porous nickel- or cobalt-zirconia cermet fuel electrode is applied to the outer periphery of the solid electrolyte to contact and substantially surround the solid electrolyte, to form a solid oxide fuel cell.

9. The method of claim 1, in which in step (h) the electrode structure is molded by extrusion or isostatic pressing.

10. The method of claim 1, in which the air electrode has a porosity from about 20 to 40% by volume (60% to 80% of theoretical density), a coefficient of thermal expansion from about 10.4 x 10-6 to 10.6 x 10-6/°C in the range of about 25°C to 1,000°C, and an electrical resistivity of from about 10 to 25 m.OMEGA.-cm at about 1,000°C.

11. An air electrode made by the method of claim 1.

12. A low cost, lanthanide-substituted, dimensionally and thermally stable, electrically conductive, porous air electrode composition, which comprises the chemical formula (1):
(La l-w-x-y Ln wCe(MA)y)(Mn l-z(MB)z)O3 (1), where Ln is a natural lanthanide mixture either in a natural state or in an unfinished concentrated state selected from a mixture of at least two of La, Ce, Pr, Nd, Sm, and other lanthanides, with the proviso that if Ln comprises a mixture of only two lanthanides, the mixture is not the combination of La and Ce; La and Ce are selected from individual species of La and Ce, respectively; MA is an A-site dopant for electrical conductivity selected from individual species of at least one of Ca, Sr or Ba, or mixtures thereof; MB is a B-site dopant for dimensional stability selected from individual species of at least one of Mg, Ni, Cr, Al or Fe, or mixtures thereof; w is from about 0.10 to 0.9; x is from about 0 to 0.1; y is from about 0.1 to 0.2; and, z is about 0.05 to 0.1 mole per mole of formula (1).

13. The air electrode composition of claim 12, in which w is from about 0.4 to 0.8.

14. The air electrode composition of claim 12, in which the lanthanide mixture comprises a natural mixture of lanthanides.

15. The air electrode composition of claim 12, in which the lanthanide mixture comprises an unfinished lanthanide concentrate.

16. The air electrode composition of claim 12, in which Ln comprises at least three of La, Ce, Pr. Nd, Sm, and other lanthanides.

17. The air electrode composition of claim 12, in which the Ln comprises primarily a mixture of La, Ce, Pr, and Nd.

18. The air electrode composition of claim 12, in which the Ln comprises primarily a mixture of La, Ce, Pr, Nd, and Sm.

19. The air electrode composition of claim 12, in which the air electrode is from about 20 to 40% porous by volume (60% to 80% of theoretical density), the electrical resistivity at 1,000°C is from about 10 to 25 m.OMEGA.-cm, and, the coefficient of thermal expansion in the range of from about 25°C to 1,000°C is from about 10.4 x 10-6 to 10.6 x 10-6/°C.

20. The air electrode composition of claim 12, in which the composition has the chemical formula (2):
(La l-w-0.2LnwCa0.2)(Mn l-z(Ni or Mg)z)O3 (2), where w is from about 0.4 to 0 8; and, y is from about 0.05 to 0.1.

21. A tubular solid oxide fuel cell, which comprises:
a porous self-supporting, inner air electrode tube;
a gas-tight solid electrolyte substantially surrounding the outer periphery of the air electrode tube; and, a porous outer fuel electrode substantially surrounding the solid electrolyte, where the solid electrolyte and fuel electrode are discontinuous and have an interconnect disposed on the air electrode in the discontinuity, in which the air electrode is a low cost, lanthanide-substituted, dimensionally and thermally stable material, electrically conductive material selected comprising formula (1):
(La ~-w-x-y Ln w Ce x(MA)y)(Mn ~-z(MB)z)O3 (1), where Ln is a natural lanthanide mixture either in a natural state or in an unfinished concentrated state, selected from a mixture of at least two of without limitation, La, Ce, Pr, Nd, and Sm and other lanthanides (Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu), with the proviso that if Ln comprises a mixture of only two lanthanides, the mixture is not the combination of La and Ce; La and Ce are selected from individual species of La and Ce, respectively; M A is an A-site dopant for electrical conductivity selected from individual species of at least one of Ca, Sr or Ba, or mixtures thereof;
M B is a B-site dopant for dimensional stability selected from individual species of at least one of Mg, Ni, Cr, Al or Fe, or mixtures thereof; w is from about 0.10 to 0.9; x is from about 0 to 0.1; y is from about 0.1 to 0.2; and, z is about 0.05 to 0.1 mole per mole of formula (1).

22. The fuel cell of claim 21, in which the solid electrolyte is made of yttria- or calcia-stabilized zirconia, the interconnect is made of doped lanthanum chromite, and the fuel electrode is made of nickel- or cobalt-zirconia cermet.

23. A fuel cell generator made by series and parallel connecting a plurality of similar fuel cells of claim 22.