It is prior art to manufacture electrodes for fuel cells with solid oxide electrolytes from mixed oxides of the Perowskite type containing lanthanum. Lanthanum manganite LaMnO3, lanthanum cobaltite LaCoO3 and lanthanum strontium man- ganite (La, Sr) Mn03 are examples for said oxides. These compounds have the ad- vantage of a good heat resistance being necessary for the high working tempera- ture of the fuel cell of more than 1000 °C. Furthermore is the heat extension coef- ficient of these mixed oxides similar to the one of the oxide electrolyte. Thus it is possible to avoid strain in the cell caused by different heat extension coefficients by using these materials.

It is preferred to use electrodes manufactured from mixed oxides comprising lan- thanum, strontium and manganese. This material is porous with a high specific surface. Therefore this material provides an excellent penetration of oxygen and oxygen ions what guarantees the demanded high electrical conductivity of the electrodes.

It has already been suggested to manufacture these electrodes made from lantha- num strontium manganite (La, Sr) Mn03 in a process wherein in a first reaction stage the solid oxides or carbonates of lanthanum, strontium and manganese are mixed mechanically. Afterwards this mixture is reacted in a second reaction stage to the mixed oxide. High temperatures between 1300 and 1600 °C are necessary

in order to get a satisfactory yield. Then the mixed oxide is ground, kneaded with water and organic binders, molded and sintered at the aforementioned tempera- tures to form the electrode (EP-A 0 633 619).

Electrodes being produced according to this procedure are porous and are pro- viding a good permeability for air. Therefore this process should provide material for electrodes with a good electrical conductivity. But it is known that because of the high temperature the said process for preparing said oxides is suffering from bad reproducibility. The process can result in material being too porous with the consequence of a decreased specific density. It is a disadvantage of this decreased specific density that therefore the electrical resistance is increased and conse- quently the electrical conductivity is decreased (EP-A 0 361 383).

Another method to prepare the mixed oxides of the Perowskite type comprising lanthanum, strontium and manganese from the carbonates relates to the use of microwaves as energy source (US 5 462 009).

Further it has been described to prepare said oxides by plasma pyrolysis of liquids containing the components of the mixed oxide in form of dissolved metal salts. (J.

Am. Ceram. Soc. (1997), 80 (1), 261-263); Mater. Sci. Eng. , B (1997), B49 (1), 36- 41)).

The disadvantage of both methods is the low flow of materials coupled with a high energy consumption. Both features are extremely disadvantageous for an industrial production.

Another method for the preparation of the mixed oxides comprises the pyrolysis of the metal citrates and cellulose. It is said that for this reaction relative low tem- peratures are necessary only (Gongneng Cailiao (1998), 29,1190-1191).

But it is well known that in particular the technical manufacturing process for the mixed oxides suffers from problems in homogenous mixing of the starting com- pounds providing homogenous mixed oxides formed in the sintering process.

Even the aforementioned high temperatures in the sintering process of more than 1000 °C cannot guarantee the formation of homogenous material. Material being not homogeneous results in a decline of the demanded good electrical conductiv- ity. In addition mixed phases of lanthanum oxides and lanthanum carbonate char- acterized by a formula like La2C05 can be formed. These compounds reduce the electrical conductivity in a disadvantageous way, too.

It is the object of the present invention to provide an improved process for the preparation of mixed oxides comprising lanthanum, strontium and manganese providing said oxides with excellent homogeneity, good reproducibility and ex- cellent electrical conductivity.

To obtain the aforementioned object, the present invention provides a process wherein in a first reaction stage the water is removed from an aqueous solution of salts of lanthanum, strontium and manganese with organic or inorganic acids by convection drying and in a second reaction stage the resulting powder or granulate is sintered to form said mixed oxides.

It was completely unexpected and therefore it is extremely surprising that with the new process mixed oxides of the Perowskite type comprising lanthanum, stron- tium and manganese can be formed at temperatures being considerably lower than the temperatures of the prior art. Therefore mixed oxides with the superior fea- tures are already formed in a temperature range of from 250 to 900 °C. Said ox- ides have an excellent homogeneity, are free from mixed phases of lanthanum oxide and lanthanum carbonate, have a high specific density but are nevertheless porous and permeable for oxygen and oxygen ions. Consequently it is possible to manufacture electrodes from said mixed oxides from said new process with ex- cellent electrical conductivity.

In particular the object of the invention is a process for the preparation of mixed oxides comprising lanthanum, strontium and manganese characterized by the gen- eral formula (Lal xSrx) yMnO3 with x and y being numbers independent from each other and x means a number of from 0.01 and 0.99 and y means a number of from 0.01 and 1, wherein in a first reaction stage water is removed from an aqueous solution of salts of lanthanum, strontium and manganese by convection drying and in a second reaction stage the resulting powder or granulate of said convection drying is sintered to form the mixed oxides, wherein the weight ratio of the salts in the aqueous solution is selected in a way to form in the mixed oxides a ratio of elements of La: Sr = (1-x) : x and (Lal-xSrx) : Mn = y: 1.

It is another object of the invention to provide said mixed oxides from said proc- ess of the invention as material for electrodes. In particular these oxides are espe- cially useful for manufacturing cathodes for fuel cells with solid oxide electrolytes (solid oxide fuel cell, SOFC).

The salts of lanthanum, strontium and manganese used in the new process are well known in the art or can be produced with known processes. It is possible to use the salts of lanthanum, strontium and manganese with inorganic and organic ac- ids.

It is preferred to use those salts which have a good solubility in water and which can be easily decomposed to the desired mixed oxides.

For example the inorganic acids are selected from the group comprising the dif- ferent types of sulfuric acids, as for example sulfurous acid or sulfuric acid, dif- ferent types of nitric acids, as for example nitrous acid and nitric acid, different types of phosphoric acids, as for example phosphoric (III) acid and phosphoric (V) acid, carbonic acid, hydrochloric, hydrobromic and hydriodic acid.

For example the organic acids are selected from the group comprising the linear and branched carboxylic acids, as for example formic acid, acetic and pivalic acid, aromatic carboxylic acids, as for example benzoic acid, cycloaliphatic acids, as for example cyclohexanecarboxylic acid, acids with two or more carboxylic groups, as for example oxalic acid.

If salts of lanthanum, strontium and manganese with inorganic acids are used, for example the sulfates, nitrates or chlorides, in the thermal stage of reaction oxides of sulfur and nitrogen or hydrogen halide can be emitted. These gases produced during sintering can cause corrosion in the devices being used in said sintering. Therefore it is necessary to clean the waste air what is costly. Therefore the use of said salts is less preferred.

It is preferred to use the salts of lanthanum, strontium and manganese derived from the organic acids. Normally the volatile fission products being formed dur- ing sintering are considerably less aggressive than the fission products aforemen- tioned.

It is especially preferred to use as salts the formates. But it is mostly preferred to use the acetates. These latter compounds show an excellent solubility in water and can easily decomposed to said mixed oxides. In addition it is easy to prepare these salts by reacting the oxides with formic or acetic acid. This good availability is an advantage, too.

The salts can be dissolved in water by stirring. It is possible to dissolve each com- pound separately and then to combine the solutions. But one can dissolve the compounds together, too. Then the sequence of the addition of the salts to the water is irrelevant.

It is possible to use the aqueous solutions of the salts in relatively diluted or con- centrated form. For example one can use solutions of from 1 to 60 % by weight in

the reaction stage of convection drying. But it is preferred to use solutions of from 5 to 50 % by weight, especially of from 10 to 40 % by weight. The salts normally are very soluble within these ranges of concentration. Therefore concentration precipitates are avoided. So the powders and granulates which are formed in said convection drying are especially useful in the second reaction stage of the sinter- ing process.

The temperature of the solutions should be in a range to provide sufficient solu- bility. To improve the solubility and to avoid concentration precipitates of high concentrated solutions it can be useful to warm up the solutions during dissolving the salts. As a result heated salt solutions are used. A range of temperature of from 20 and 80 °C is especially useful.

In the first reaction stage of convection drying water is removed to the greatest possible extent from the salt solutions. The devices for this process are well known in the art of production technology.

It is an object of the invention to run said convection drying by means of fluidized drying or conveying air drying. It is especially preferred to use spray drying or fluidized bed spray granulation drying in said reaction stage.

When the especially preferred embodiment of spray drying is used the salt solu- tion is sprayed in known manner from the top of a wide, cylindrical vessel by means of atomizing nozzles or by means of a rotating-disc atomizer to produce atomized droplets. These atomized droplets are mixed with hot air or an inert gas, which are introduced into the dryer round the atomizing zone. As atomizing de- vices spiral nozzles, spiral pressure nozzles, pneumatic nozzles or centrifugal at- omizers are used.

It is possible to introduce the salt solutions of the single components separated from each other into the spray dryer. Then the solutions are mixed before passing

the atomizing device. For example it is possible, too, to introduce a solution con- taining two components into the spray dryer and to mix it with the third dissolved component before leaving the atomizing device. But it is possible to use an aque- ous solution containing the salts of the three components. It is preferred to use this latter embodiment of the invention.

It is preferred to run the convection drying within a temperature range of the in- coming air of from 200 to 400 °C. It is especially preferred to use a temperature range of from 250 to 300 °C.

The resulting products of said convection drying are microscopic homogeneous powders or granulates. The range of particle size can be between 0,0005 mm and 1 mm, preferable 0, 001 and 0,1 mm. It is an advantage of the described process to produce particles within a well defined range of particle size being surprisingly good reproducible. This fact is extremely advantageous for the second reaction stage of the sintering process resulting in the mixed oxides.

It is preferred to obtain the products from said drying process in form of a powder.

Normally these powders have a residual moisture of less than 4 % by weight.

In the next processing step the products formed in said convection drying are sin- tered. For that purpose they are preferably heated up to a temperature of from 250 °C to 900 °C. A temperature range of from 600 to 900 °C is preferred. Approxi- mately after 20 to 30 hours the formation of the mixed oxide is finished. During the reaction residual water and organic volatile fission products are eliminated from the system. It is recommended to regulate the heating rate and the cooling rate within a range of approximately 3 to 5 °C/min to obtain products being ho- mogeneous as possible.

If necessary the said mixed oxides being processed by said sintering can be ground to powders according to methods well known in the art. The particles can have a diameter of from 0,0001 to 0,05 mm.

The mixed oxides comprising lanthanum, strontium and manganese which are prepared according to said process of the invention can have a stoichiometric structure as described in the Japanese laid open No. 2-293,384 (y = 1).

But compounds containing the ions of lanthanum and strontium in an amount be- low the stoichiometric structure can be prepared, too (y < 1). Examples for such compounds are disclosed in EP-A 0 633 619. It depends from the weight ratio of the used salts which type of mixed oxide is formed.

It is an preferred embodiment of the process of the invention to use an aqueous solution of the salts or solutions of the salts which provides mixed oxides of the formula (Lao. 5-0.95Sr0.5-0.05)0.7-1MnO3 (0.05#x#0.5 and 0.7#y#1).

It is especially preferred to use an aqueous solution of the salts or solutions of the salts which provides mixed oxides of the formula (Lao. 7-0.9Sr0.3-0.1)0.9-1MnO3 (0. 1x0. 3and0. 9<yl).

But it is mostly preferred to use an aqueous solution of the salts or solutions of the salts which provides a mixed oxide of the formula Lao. stro. 2MnO3 (x = 0.2 and y = 1). With a mixed oxide being characterized by such formula and prepared according to the process of the invention electrodes with excellent prop- erties can be produced.

The process of the invention guarantees an excellent reproducibility. Therefore said mixed oxides are highly homogeneous. They are porous with a large specific surface. But at the same time they have a high specific density. Therefore they have an excellent electrical conductivity. Because of these superior features said mixed oxides of said process of the invention are very valuable components for manufacturing electrodes for fuel cells.

The methods of manufacturing said electrodes made from mixed oxides compris- ing lanthanum, strontium and manganese are well known in the art. It is preferred to use said oxides for the manufacture of cathodes.

For example the mixed oxides of the process of the invention can be kneaded in the presence of a water-soluble organic binder. Examples for organic binders are polyacrylic acid or polyvinyl alcohol. After the paste is molded to form the elec- trode. If necessary further substances which form pores can be admixed to the paste. Examples are esters of polyacrylic acid or carbon powder. It is preferred to use substances in an amount between 2 and 7 %'by weight being able to form pores. After the form is dried and calcined to the electrode.

However it is possible, too, to coat an electrolyte directly with the said mixed ox- ides. Mostly this electrolyte is zirconium oxide, which is stabilized with the oxide of an alkaline earth metal or a rare earth metal. For this process the sinter method has been described, wherein the electrolyte is coated with the pasty electrode ma- terial. It is possible to use screen printing as a coating method.

Other methods comprise the coating of the electrolyte with the powdered material for the electrodes, wherein thermal methods as laser spraying, plasma spray coat- ing, laser-plasma hybride spraying or other methods can be used. Thus a variety of methods well known in the art can be used to apply the electrode material to the surface of a solid electrolyte to thereby produce a membrane as electrode of a solid electrolyte fuel cell (GB 2 284 599).

According to said methods it is possible to coat the counter electrode, for example the anode, with the electrolyte.

The present invention will be further described below by an example. But the in- vention is not limited thereto.

Example Preparation of a mixed oxide comprising lanthanum, strontium and manganese of the composition Lao. 8Sro. 2MnO3 as electrode material: a) Preparation of an aqueous solution of salts 5745 g (17.6 moles) lanthanum oxide (La203) were suspended in a mixture of 7630 g (122.1 moles) acetic acid and 74970 g water with stirring. After the mix- ture was heated up to a temperature of 70 °C. Approximately after one hour a clear solution was formed. The solution was cooled down to room temperature and treated with 10804 g (44.1 moles) manganese (Il) acetate (as tetrahydrate; Mn (CH3CO0) 2x4 H20) and 1814 g (8. 8 moles) strontium acetate (Sr (CH3CO0) 2). The composition was stirred till a clear solution was formed.

After the solution was adjusted with acetic acid to PH 4. The 100 kg solution had a solids content of 22, 8 % by weight. b) Spray drying of the aqueous solution Afterwards the solution was spray dried. The spray drying was carried out by means of a spray device Lab Plant Drier Typ SD 04. The volume flow rate of the aqueous solution was 1 to 2 1/h. The incoming air had a temperature of approxi- mately 280 °C. The resulting powder had a faint brownish color and was good flowable.

c) Sintering 10 kg of the powder from reaction stage b) were sintered in a quartz cup according to the following temperature program: 1. Heating up to 105 OC (0,5 h) 2. Hold at 105 °C (3 h) 3. Heating up to 300 °C (3 h) 4. Hold at 300 °C (3 h) 5. Heating up to 850 °C (3 h) 6. Hold at 850°C (10 h) 7. Cooling down to room temperature (10 h) 5020 g of a black product with a faint metallic gloss were obtained. Afterwards the product was ground.

The X-ray diffraction revealed that a compound of the formula Lao. sSro. 2MnO3 had been formed. Undesired phases of lanthanum oxides with carbonates could not be detected.

Maufacture of an electrode The properties of the mixed oxide of the formula Lao, 8SrO, 2MnO3 of the process of the invention were checked in a SOFC test cell. Herein the electrodes and the electrolyte were arranged in stacked layers. According to the aforementioned known methods of the prior art a printable paste was prepared from the mixed oxide of the formula Lao, 8Sr0, 2MnO3 as prepared as described before. This paste was printed with screen printing on an arrangement of an anode made from nickel oxide which was coated with zirconium oxide doped with yttrium oxide as the solid oxide electrolyte. After the arrangement was sintered. Thereby the electrode

was formed as the cathode with a layer thickness of approximately 150 microme- ter.

As electrical properties of the fuel cell the area specific resistance and the power per surface unit were determined. It is well known to use these two quantities for the characterization of fuel cells. With increasing electrical conductivity the value of the area specific resistance is decreased and the value of the power is increased and vice versa.

This test cell was compared with two cells having the same arrangement of elec- trodes and electrolytes as the test (cell 1 and 2). The anodes were manufactured from nickel oxide and the electrolytes consisted of zirconium oxide doped with yttrium oxide. The cathodes were manufactured from different customary mixed oxides consisting of lanthanum, strontium and manganese. The results are listed in the table, clearly showing the superior properties of the electrode material of the process of the invention. area specific resistance [Q * cm2] power [mW/cm2] cell 1 0,455 552 cell 2 0,431 579 test cell 0,409 625