DE10223746A1 - Process for the production of mixed oxides containing lanthanum, strontium and manganese for electrodes in fuel cells - Google Patents

Abstract

Process for the preparation of mixed oxides containing lanthanum, strontium and manganese of the general formula (La¶1-x¶Sr¶x¶) ¶y¶MnO¶3¶, in which x is independently a number from 0.01 to 0.99 and y is a number from 0.01 to 1, characterized in that the water is largely removed by convection drying from an aqueous solution of lanthanum, strontium and manganese salts and then the powder or granules obtained are converted into the mixed oxides by heating, the Weight ratios of the salts are selected so that an element ratio of La: Sr = (1 - x): x and (La¶1-x¶Sr¶x¶): Mn = y: 1 is established in the mixed oxides.

Description

The invention relates to a process for producing mixed oxides containing Lanthanum, strontium and manganese and the use of these mixed oxides as Electrode material in fuel cells with solid oxidic electrolytes (Solid oxide fuel cell, SOFC).

It is known to produce electrodes for fuel cells with electrolytes from solid oxides from mixed oxides with a perovskite structure which contain lanthanum. Examples of this are lanthanum manganite LaMnO ₃ , lanthanum cobaltite LaCoO ₃ or lanthanum strontium manganite (La, Sr) MnO ₃ . These compounds have the advantage of good heat resistance, which is necessary because of the high operating temperature of the cells of over 1000 ° C. In addition, their coefficients of thermal expansion mostly correspond very well to those of the solid oxidic electrolyte, so that stresses in the cell caused by different thermal expansion are avoided when using these materials.

Electrodes can preferably be used using the Mixed oxides containing lanthanum, strontium and manganese are produced. This Material is porous with a high specific surface. It therefore has one excellent permeability to oxygen and oxygen ions. This will ensures the required high conductivity of the electrode.

To produce these electrodes from lanthanum strontium manganite (La, Sr) MnO ₃ , it has already been proposed to first mechanically mix solid oxides or carbonates of lanthanum, strontium or manganese and then to convert them to the mixed oxide. In order to obtain a satisfactory reaction, high temperatures of 1300 to 1600 ° C are necessary. The mixed oxide is then ground, kneaded with water and organic binder, shaped and fired to the shape of an electrode at the temperatures mentioned above (EP-A 0 633 619).

The electrodes thus produced are porous and therefore have a good one Permeability to air. You should therefore have good conductivity. But is known to be able to manufacture the mixed oxides because of the very high Temperatures and because of the poor reproducibility of the reaction material accumulate, which is too porous and therefore has a low specific density. This disadvantageously increases the electrical resistance and thus the electrical conductivity undesirable (EP-A 0 361 383).

The production of mixed lanthanum-strontium-manganese oxides is also known of the perovskite type from the metal carbonates using Microwave radiation as an energy source (US 5 462 009) and the production by Plasma pyrolysis of liquids in the form of the components of the mixed oxide dissolved metal salts contain (J. Am. Ceram. Soc. (1997), 80 (1), 261-263; Mater. Sci. Eng., B (1997), B49 (1), 36-41.).

The disadvantages of both methods are the low material throughputs at the same time high energy consumption, what for technical production is extremely disadvantageous.

It is also known that the mixed oxides by pyrolysis of a solid mixture the corresponding metal citrates and cellulose to perform, already relatively low temperatures are said to lead to implementation (Gongneng Cailiao (1998), 29, 1190-1191).

However, it is also known that it is difficult, particularly in the technical production of mixed oxides, to produce homogeneous mixtures with solid starting compounds, which then also lead to homogeneous mixed oxides. This is the case even when the high temperatures of above 1000 ° C. described at the beginning of the reaction are used. Inconsistent material also leads to impairment of the required good conductivity. In addition, mixed phases of lanthanum oxides and lanthanum carbonates of the approximate formula La ₂ CO _{5 can also} form, which likewise undesirably reduce the conductivity.

It was therefore the task of an improved method of manufacture to provide mixed oxides containing lanthanum, strontium and manganese, the does not have the disadvantages described and is too homogeneous, good leads to reproducible products with excellent conductivity.

This object could be achieved according to the invention in that first of all from an aqueous solution, the lanthanum, strontium and manganese salts contains, the water largely removed by convection drying and then the powder or granules obtained sinters, the desired mixed oxides arise.

It was not foreseeable and can therefore be described as surprising that in the new method compared to that in the prior art required temperatures at significantly lower sintering temperatures in one A range of approx. 250-900 ° C form mixed oxides that are superior to those required Possess properties. They have excellent homogeneity and are free of mixed phases of lanthanum oxides and lanthanum carbonates and are at high specific density sufficiently porous and permeable to oxygen. Therefore can be made from the mixed oxides advantageously produced by the new process Represent electrodes that have excellent electrical conductivity.

The invention thus relates to a process for the preparation of mixed oxides containing lanthanum, strontium and manganese of the general formula (La _1-x Sr _x ) _y MnO ₃ , in which, independently of one another, x is a number from 0.01 to 0.99 and y is a number from 0.01 to 1, characterized in that from an aqueous solution which contains lanthanum, strontium and manganese salts, the water is largely removed by convection drying and then the powder or granules obtained are converted into the mixed oxides by heating, the Weight ratios of the salts in the aqueous solution are selected such that an element ratio of La: Sr = (1 - x): x and (La _1-x Sr _x ): Mn = y: 1 is established in the mixed oxides.

Another object of the invention is the use of the after Mixed oxides produced according to the invention as electrode material especially for cathodes in fuel cells with solid oxidic electrolytes (Solid oxide fuel cell, SOFC).

The lanthanum, strontium and manganese salts used in the new process the metals are known or can be produced by known processes become. Salts of lanthanum, strontium and manganese can be used with both inorganic and organic acids can be used.

The salts which have good water solubility are particularly preferred possess, and which decompose well to the desired mixed oxides in the sintering step to let.

Inorganic acids are, for example, the various sulfuric acids, such as Sulphurous acid or sulfuric acid, nitric acids such as nitrous acid and Nitric acid, phosphoric acids such as phosphorous acid and orthophosphoric acid Carbonic acid, chloric, bromic and hydroiodic acid.

Organic acids are, for example, linear and branched carboxylic acids, such as z. B. formic, acetic and pivalic acid, aromatic carboxylic acids, such as. B. Benzoic acid, cycloaliphatic carboxylic acids, such as. B. cyclohexane carboxylic acid, Dicarboxylic acids, such as. B. oxalic acid.

When using the salts of lanthanum, strontium and manganese with however, inorganic acids, for example sulfates, nitrates or chlorides in the thermal reaction step of sintering with emissions of sulfur and Nitrogen oxides or hydrogen halide. These gases can Corrosion of equipment lead and make a complex preparation of the Process exhaust air necessary. Their use is therefore less preferred.

Salts of lanthanum, strontium and manganese with organic are preferred Acids are used because the volatile fission products formed during sintering are much less aggressive than in the case described above.

The formates are particularly preferred as salts, but in particular those Acetate used. The latter have excellent solubility in water. They are also easy to manufacture, for example by reacting the oxides with the carboxylic acids. That is why they are also readily available.

The salts can be dissolved in water by stirring, where possible is to solve each component separately and then the solutions if necessary to unite. But you can also put the components together in solution bring, the order of addition then does not matter.

The aqueous solutions of the salts can be in relatively dilute as well as relatively concentrated form can be used. For example, it is possible to use 1 to 60% by weight To use solutions. However, 5 to 50 are preferred. in particular 10 to 40 wt .-% solutions used in convection drying, because in In this weight range, the salts are still very soluble in water, Concentration precipitation can be avoided and thus particularly suitable powder have it made.

The temperature of the aqueous solutions should be chosen so that a there is sufficient solubility. To improve solubility and Avoidance of precipitation, especially more concentrated ones Solutions can therefore make sense to warm up during the dissolving process and use heated salt solutions. A temperature range of 20-80 ° C has proven particularly successful.

The salt solutions are then in a convection dryer, for example by spray drying or fluidized bed drying, largely from water exempt, using the usual devices used in technology.

According to the invention, convection drying by means of fluid drying, Conveying air or flight drying take place.

Spray or fluidized bed spray granulation drying is particularly preferred applied.

In the particularly preferred embodiment of spray drying, this becomes drying saline solution in a known manner at the top of a wide, cylindrical container through nozzles or by means of a rapidly rotating Atomizer disk sprayed into a drop mist. The resulting droplet fog is included hot air or an inert gas mixed around the atomization zone the dryer be started. Swirl Pressure nozzles, pneumatic nozzles or centrifugal atomizers in question.

One can proceed in such a way that the salt solutions of the individual components separately from the spray dryer and the solutions only before Atomizer nozzle mixes. However, it is also possible, for example one Saline solution containing two of the components to the spray dryer, and them mix with the third, dissolved component in front of the atomizing nozzle. you can also use an aqueous solution in which the salts of the three Components exist together.

Drying is preferably at a supply air temperature in the range of 200 up to 400 ° C. A range between 250 and is particularly preferred 300 ° C.

The products obtained are microscopically homogeneous powders or granules a grain size range of about 0.0005 mm to 1 mm, preferably 0.001 to 0.1 mm. The process allows particles in an adjustable and Obtain surprisingly reproducible grain size range, what for Mixed oxide formation described below is of great advantage.

The products, preferably obtained in powder form, normally have one Residual moisture below 4%.

In the next process step, those obtained by convection drying are obtained Powder sintered. For this purpose, it is preferably brought to a temperature of 250 ° C to 900 ° C, preferably 600 to 900 ° C. After about 20 to 30 h Mixed oxide formation is usually completed, with residual water and organic volatile components are eliminated from the system. To be as homogeneous as possible To receive products, it is recommended that the heating and cooling phase temperature controlled at about 3 to 5 ° C / min.

If necessary, the mixed oxides produced by sintering can after usual methods are ground, for example to powders with particle diameters from 0.001 to 0.05 mm.

The lanthanum, strontium produced by the process according to the invention and mixed oxides containing manganese can, depending on the The composition of the component template used has a stoichiometric structure (y = 1) as described in Japanese Patent Laid-Open No. 2-293,384 is. According to EP-A 0 633 619, however, they can also contain lanthanum and strontium ions stoichiometric deficit included (y <1).

A salt solution or salt solutions are preferably used which, according to the process of the invention, mixed oxides of the formula

(La _0.5-0.95 Sr _0.5-0.05 ) _0.7-1 MnO ₃

result (0.05 ≤ x ≤ 0.5 and 0.7 ≤ y ≤ 1).

However, a salt solution or salt solutions in which mixed oxides of the formula are used are particularly preferred

(La _0.7-0.9 Sr _0.3-0.1 ) _0.9-1 MnO ₃

form (0.1 ≤ x ≤ 0.3 and 0.9 ≤ y ≤ 1).

In a very particularly preferred embodiment, however, a salt solution or salt solutions are used which result in a mixed oxide of the formula

La _0.8 Sr _0.2 MnO ₃

lead (x = 0.2 and y = 1). With this mixed oxide, which is produced by the method according to the invention, electrodes can be produced which have very particularly favorable properties.

The method according to the invention and very reproducible Mixed oxides obtained are porous with high specificity with high homogeneity Surface with high specific density. That's why they assign one excellent conductivity. These properties in particular make them closed a valuable starting product for the production of electrodes for Fuel cells.

This production of electrodes, preferably cathodes, from the lanthanum, Mixed oxides containing strontium and manganese are known or can be known methods.

For example, the mixed oxides according to the invention with water and in Presence of a water-soluble organic binder, such as. B. Polyacrylic acid or polyvinyl alcohol, kneaded and the electrode formed from the dough. If necessary, other pore-forming substances, such as. B. other polyacrylates or carbon powder, the dough are added. The addition of pore-forming substances is preferably between 2 and 7% by weight on the mixed oxide used. Then the shape dried and then burned to the electrode.

However, it is also possible to apply the mixed oxides directly to the electrolyte mostly made of zirconium oxide, which is combined with the oxide of an alkaline earth metal or Rare earth metal is stabilized to apply. For this procedure For example, the sintering method described, the electrolyte with pasted Electrode material is coated. You can use the electrode material as Make printable paste and use it as a coating method Use screen printing.

However, it is also possible to powder the electrolyte with Coating electrode material using thermal methods, the laser spray method, the Plasma spraying method, the laser-plasma hybrid spraying method and the like come into use. The electrode material is placed on the surface of the solid electrolyte to form a membrane there, which the Electrode of a fuel cell (GB 2 284 599).

In the method described above, the electrolyte can already on the Counter electrode, for example the anode, may be applied.

The invention is described in more detail by the following examples, without however, restrict them.

example

The example describes the preparation of a mixed oxide containing lanthanum, strontium and manganese with the composition La _0.8 Sr _0.2 MnO ₃ :

a) Preparation of an aqueous salt solution

5745 g (17.6 mol) of lanthanum oxide (La ₂ O ₃ ) were suspended in a mixture consisting of 7630 g (122.1 mol) of acetic acid and 74,970 g of water with stirring. The mixture is then brought to a temperature of 70.degree. After about an hour, a clear solution had been found. 10804 g (44.1 mol) of manganese diacetate (as tetrahydrate; Mn (CH ₃ COO) ₂ × 4H ₂ O) and 1814 g (8.8 mol) of strontium diacetate (Sr (CH ₃ COO) were then added to the solution, which had cooled to room temperature. ₂ ) entered and stirred until a clear solution was obtained. The solution was then adjusted to a pH of approximately 4 by adding acetic acid. 100 kg of solution with a solids content of 22.8% were obtained.

b) spray drying the aqueous solution

The aqueous solution was then spray dried. This was done with the help of a spray tower Lab Plant Spray Drier type SD 04. The The volume flow of the starting solution was 1-2 l / h. The supply air temperature was about 280 ° C. The product was a free-flowing, slightly brownish colored Get powder.

c) sintering

• 1. Aufheizen auf 105°C (0,5 h)
• 2. Halten bei 105°C (3 h)
• 3. Aufheizen auf 300°C (3 h)
• 4. Halten bei 300°C (3 h)
• 5. Aufheizen auf 850°C (3 h)
• 6. Halten bei 850°C (10 h)
• 7. Abkühlen auf Raumtemp. (10 h)

10 kg of the powder obtained by spray drying were sintered in the quartz crucible according to the temperature program described below:

• 1. Heating up to 105 ° C (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. Cool to room temp. (10 h)

5020 g of black, slightly metallic glossy product were obtained. Then it was crushed in a grinding process.

The X-ray diffractogram showed that the desired connection was present and crystallized in the perovskite lattice. Undesired phases of using lanthanum oxides No carbonates were found.

Manufacture of an electrode

The properties of the mixed oxide of the formula La _0.8 Sr _0.2 MnO ₃ produced by the process according to the invention were checked in a SOFC test cell in which the electrodes and the electrolyte were in a stacked construction. For this purpose, a printable paste was produced from the mixed oxide of the formula La _0.8 Sr _0.2 MnO ₃ from the example described above using the conventional methods described in the prior art from the mixed oxide produced by the process according to the invention. With this paste, an already prepared arrangement of a nickel oxide anode, which was coated with an electrolyte layer made of zirconium oxide doped with yttrium oxide, was printed by means of the screen printing method. Then it was burned. The electrode formed as a cathode with a layer thickness of approximately 150 micrometers.

The electrical properties of the fuel cell were determined by the area-specific resistance and the performance of the cell per surface unit are determined. It is common to use these sizes to characterize fuel cells use. The numerical value increases as the conductivity of the cathode increases of the area-specific resistance lower and the numerical value of the power bigger and vice versa.

The SOFC test cell was compared with two cells that were constructed analogously to the test cell with the same arrangement of electrodes and electrolytes (cells 1 and 2). Here, too, the anodes were made of nickel oxide and the electrolytes of zirconium oxide doped with yttrium oxide. The cathodes were each made from LaSrMn oxides available on the market. The results obtained are listed in the table below, which illustrates the superior properties of the electrode material produced by the method according to the invention.

Claims (12)

1. A process for the preparation of mixed oxides containing lanthanum, strontium and manganese of the general formula (La _1-x Sr _x ) _y MnO ₃ , in which, independently of one another, x is a number from 0.01 to 0.99 and y is a number from 0.01 to 1, characterized in that the water is largely removed by convection drying from an aqueous solution of lanthanum, strontium and manganese salts and then the powder or granules obtained are converted into the mixed oxides by heating, the weight ratios of the salts being chosen so that an element ratio of La: Sr = (1 - x): x and (La _1-x Sr _x ): Mn = y: 1 is established in the mixed oxides. 2. A process for the preparation of mixed oxides according to claim 1, characterized characterized that using lanthanum, strontium and manganese salts organic acids can be used. 3. A process for the preparation of mixed oxides according to claim 1 or 2, characterized in that the organic acid is acetic acid. 4. Process for the preparation of mixed oxides according to one of the preceding Claims, characterized in that the convection drying in Form of spray drying or fluidized bed spray granulation drying he follows. 5. Process for the preparation of mixed oxides according to one of the preceding Claims, characterized in that by convection drying obtained powder or granules for mixed oxide formation at temperatures heated from 250 to 900 ° C. 6. A process for the preparation of mixed oxides according to one of the preceding claims, characterized in that the weight ratios of the salts in the aqueous solution are chosen so that mixed oxides of the formula (La _0.7-0.9 Sr _0.3-0.1 ) _0.9-1 MnO _{3 are} formed. 7. A process for the preparation of mixed oxides according to one of the preceding claims, characterized in that the weight ratios of the salts in the aqueous solution are selected so that a mixed oxide of the formula La _0.8 Sr _0.2 MnO _{3 is} formed. 8. Mixed oxides containing lanthanum, strontium and manganese of the general formula (La _1-x Sr _x ) _y MnO ₃ , in which, independently of one another, x is a number from 0.01 to 0.99 and y is a number from 0.01 to 1, can be produced by a process comprising the following reaction stages: 1. Extensive removal of water from an aqueous solution of lanthanum, strontium and manganese salts by convection drying 2. converting the powder or granules obtained from reaction stage 1 into the mixed oxides by heating, the weight ratios of the salts being chosen so that an element ratio of La: Sr = (1 - x): x and (La _{1- x} Sr _x ): Mn = y: 1 is established in the mixed oxides. 9. Use of the according to one of claims 1 to 7 or according to claim 8 produced mixed oxides as electrode material in fuel cells. 10. Use of one of claims 1 to 7 or of claim 8 produced mixed oxides as electrode material in fuel cells oxidic electrolytes (solid oxide fuel cell, SOFC). 11. Electrode in fuel cells containing a mixed oxide manufactured according to Claims 1 to 7 or a mixed oxide according to claim 8. 12. A fuel cell containing an electrode according to claim 11.