RU2823277C1 - Method for synthesis of electrode powder based on lanthanum-barium ferrite - Google Patents

Abstract

FIELD: various technological processes.

SUBSTANCE: invention relates to synthesis of electrode materials based on lanthanum-barium ferrite. Electrode powder is obtained by controlled two-jet coprecipitation from a solution of water-soluble salts of barium, lanthanum and iron with a molar ratio of 2:3:5, respectively, using a precipitant solution containing 2 mol/dm³ ammonia and 3 mol/dm³ ammonium carbonate, wherein controlled double-jet coprecipitation of metals of the obtained powder is carried out by simultaneous supply of a solution of water-soluble salts and a solution of a precipitator into a reaction medium – distilled water with continuous stirring and maintaining a constant pH value at 8.0, after which, by filtration, the precipitate is separated from the mother solution, which is dried at temperature of 150 °C until complete removal of residues of mother solution and calcined in air atmosphere at temperature from 1,100 to 1,300 °C for 6 hours.

EFFECT: obtaining electrode powder based on lanthanum barium ferrite, suitable for use as material of oxygen, fuel, as well as symmetrical electrodes of solid oxide or proton-ceramic fuel cells.

1 cl, 1 dwg, 1 tbl, 2 ex

Description

FIELD OF TECHNOLOGY TO WHICH THE INVENTION RELATES

The invention relates to the production of materials for electrochemical devices, namely, to electrode materials based on lanthanum-barium ferrite, which can be used as a material for oxygen, fuel, and also symmetrical electrodes of solid oxide fuel cells (SOFC) or proton-ceramic fuel cells ( PKTE).

Solid oxide fuel cells are devices that allow you to effectively convert the energy of the chemical reaction of fuel oxidation (hydrogen, methane, methanol, etc.) into electrical energy. Classic SOFCs consist of an anode, which is typically a nickel-based cermet, a dense electrolyte (such as yttrium-stabilized zirconia), and a cathode, which is traditionally lanthanum strontium manganite.

Barium ferrite-based materials are widely studied as electrodes for proton-conducting electrochemical cells. This is due, among other things, to the high degree of hydration associated with the pronounced basic properties of these materials and the large volume of the lattice. Ferrites are materials with mixed electronic and ionic conductivity, the additional advantage of which is their high redox stability, which allows them to be used as fuel and oxygen electrodes in so-called symmetric SOFCs or SCFCs. This advantage can be used to optimize the technological process for producing electrode powders and reduce the cost of manufacturing electrochemical devices.

BACKGROUND OF THE ART

Traditional methods for the synthesis of powder materials for medium-temperature SOFCs include the sol-gel method, co-precipitation method, polymer-complex synthesis, etc. [doi.org/10.1016/j.pmatsci.2011.08.002].

Most electrode materials are usually obtained by solid-phase synthesis due to the simplicity and low cost of this method. For example, there is a known method for producing electrode material based on lanthanum-strontium ferrite using solid-phase synthesis [doi.org/10.1016/S1872-2067(15)61116-0]. In this method, strontium carbonate, iron oxide and lanthanum oxide, pre-calcined at 1000°C, are used as precursors. Weighed powders in a stoichiometric ratio are mixed, ground in ethanol and subjected to heat treatment, first at 800°C for 2 hours, and then at 1300°C for 3 hours. The disadvantages of this method traditionally include the heterogeneity of the composition of the resulting material, the uncontrolled distribution of particles over sizes, low specific surface area and high sintering temperatures.

To obtain strontium lanthanum manganite, the method of polymer complex synthesis is used [10.1016/j.electacta.2016.12.170]. In this method, citric or ethylenediaminetetraacetic acid is used as a chelating agent. The reagents are introduced into a solution of metal salts in a certain ratio to metal ions, followed by alkalization of the solution to a pH value of 7-8 using an aqueous ammonia solution. The resulting solution is heated until the water completely evaporates and the residue ignites spontaneously. The resulting ash is subjected to heat treatment at 1000°C for 3 hours.

The method is characterized by high purity of the resulting materials and the ability to control the composition of the resulting powders, however, its significant disadvantages include low product yield, the impossibility of obtaining a powder of a narrow fraction and the difficulty of scaling.

It is known to produce electrode materials based on lanthanum ferrite by combustion method [US 10833333 B2, publ. 11/10/2020]. According to this method, nitrates of the corresponding metals were dissolved in distilled water, glycine was introduced into the resulting solution in a ratio of 2:1 to the total content of cations, and kept with constant stirring and heating until the residue spontaneously ignited. The resulting ash was crushed and fired at a temperature of 1200°C for 12 hours. This method is characterized by simple hardware design and the possibility of a high level of control over the production of powder with the required characteristics, but the low productivity of the process does not allow its use on a production scale.

In terms of technical essence, the known method for producing electrode material powder based on lanthanum ferrite by co-precipitation can be considered the closest to the claimed one [10.1016/j.ijhydene.2012.09.063]. The method involves preparing a solution by dissolving a stoichiometric amount of lanthanum, iron and strontium nitrates in distilled water at a temperature of 60°C to obtain a solution with a metal ion concentration of about 1 mol/dm ³ . To do this, a solution of nitrates is placed in a solution of ammonium carbonate with vigorous stirring, the resulting suspension is kept for 3 hours at 60°C to balance the coprecipitation process, and then filtered under pressure using membranes with a pore diameter of 0.22 mm. The resulting cake is washed to remove any remaining nitrates and dried at a temperature of 110°C. The dried powders are crushed in a mortar and fired at temperatures from 300 to 1350°C.

As a result, a homogeneous particle morphology and low sintering temperatures compared to the solid-phase method are obtained, but it was not possible to obtain a completely single-phase structure even after firing at 1350°C for 10 hours.

No methods for synthesizing electrode powder based on barium lanthanum ferrite using the co-precipitation method have been found in the prior art.

DISCLOSURE OF THE INVENTION

The technical problem solved by the claimed invention is the possibility of using the co-precipitation method for the synthesis of electrode powder based on barium lanthanum ferrite as a material for oxygen, fuel, as well as symmetrical electrodes of solid oxide or proton-ceramic fuel cells.

For this purpose, a method has been proposed for the synthesis of electrode powder based on lanthanum-barium ferrite, including coprecipitation of metal ions of the resulting powder from a solution of water-soluble salts of the corresponding metals using a precipitant containing ammonia and ammonium carbonate, filtration of the suspension to obtain a precipitate, which is subjected to drying and subsequent heat treatment.

The new method is distinguished by the fact that electrode powder based on lanthanum-barium ferrite is obtained by controlled double-jet coprecipitation from a solution of water-soluble salts of barium, lanthanum and iron with a molar ratio of 2:3:5, respectively, using a precipitant solution containing 2 mol/ ^dm3 of ammonia and 3 mol/dm ³ ammonium carbonate, while the controlled two-jet coprecipitation of the metals of the resulting powder is carried out by simultaneously feeding a solution of water-soluble salts and a precipitant solution into the reaction medium - distilled water with continuous stirring and maintaining a constant pH value at 8, after which it is separated by filtration sediment from the mother liquor, which is dried at a temperature of 150 °C until the remaining mother liquor is completely removed and fired in an air atmosphere at a temperature of 1100 to 1300 °C for 6 hours.

The essence of the claimed method is as follows. During the controlled double-jet coprecipitation of barium, lanthanum and iron, while maintaining a constant pH value of 8, close to the dissolution of barium carbonate, small crystals are formed that are captured by amorphous lanthanum and iron hydroxides. Thus, the formed deposit ensures maximum contact between all deposited forms, which promotes the formation of the perovskite phase during firing. In addition, maintaining a constant pH value of a solution of water-soluble salts, a precipitant and distilled water as a reaction medium at 8, prevents a decrease in this pH with subsequent partial dissolution of barium carbonate during dosing of a solution of metal salts, and also prevents an increase in this pH with subsequent partial dissolution iron hydroxide throughout the controlled two-jet coprecipitation. The particles obtained after firing are loosely bound aggregates with an average particle size of 25 to 40 μm. If necessary, these particles, after gentle disaggregation for 10 minutes, are broken down into particles with an average size of less than 5 microns.

The technical result achieved by using the invention is to obtain an electrode powder based on barium lanthanum ferrite, suitable for use as a material for oxygen, fuel, and symmetrical SOFC or PCFC electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated by a table showing the phase composition, average particle size, as well as the cationic composition of the mother liquor after filtration, characterizing the completeness of coprecipitation, as well as a figure showing a diffraction pattern of a powder sample obtained according to Example 1.

IMPLEMENTATION OF THE INVENTION

To synthesize electrode powder based on lanthanum barium ferrite, an aqueous solution of lanthanum, barium and iron salts was prepared in laboratory conditions, preferably by dissolving in distilled water. As metal salts, water-soluble salts of inorganic acids, preferably nitrates or chlorides, can be used. Weighed salts are selected in such a way that the mole fractions of barium, lanthanum and iron in the solution are 2:3:5, respectively. It is recommended to select the acidic residue of the salt based on the solubility of the salt in water and its resistance to hydrolysis.

To prepare solutions of metal salts, lanthanum nitrate, iron nitrate and barium carbonate were introduced sequentially into distilled water until each component was completely dissolved.

To obtain a precipitant solution, an aqueous solution of a mixture of ammonia and ammonium salt was prepared, with an acid residue that can form insoluble salts of barium, iron and lanthanum. The molar ratio of ammonia and ammonium salt in the precipitant solution should be 2:3. The use of alkali solutions as a source of hydroxide ions will lead to contamination of the electrode powder with alkali metal cations, which can negatively affect both the phase composition and the electrochemical characteristics of the material.

For controlled two-jet coprecipitation, a mixture of an aqueous solution of ammonia and ammonium carbonate with a concentration of 2 and 3 mol/dm ³ , respectively, was used. Controlled two-jet coprecipitation of barium carbonate, as well as lanthanum and iron hydroxides, was carried out by simultaneously feeding a solution of water-soluble salts and a precipitant solution into the reaction medium - distilled water with continuous stirring and maintaining a constant pH value at 8. The amount of distilled water has little effect on the quality of the synthesized electrode material , but to reduce the volume of the filtered suspension and the consumption of the precipitant solution, it is recommended to add distilled water in minimal quantities. The formation of a precipitate in the reaction medium is carried out by achieving the solubility product of the precipitated forms with the help of a constant and uniform supply of solutions of metal salts and a precipitant, which are sources of metal cations, as well as hydroxide and carbonate cations.

Maintaining a constant pH value of 8.0 by automatically controlling the supply of precipitant solution ensures identical conditions for the formation of each particle. The pH value is recorded using a glass combined electrode and ion meter.

The resulting suspension of lanthanum-barium ferrite was filtered to separate the precipitate from the mother liquor. To remove the remaining mother liquor, the resulting precipitate was dried at a temperature of 150 °C to constant weight.

The dried sediment was fired in an air atmosphere at a temperature from 1100 to 1300 °C, during which the formation of a perovskite phase occurs, as well as decomposition and subsequent removal of impurity reaction products.

To obtain a fine fraction, the calcined powder was subjected to gentle disaggregation for 10 minutes, preferably in a grinding mill.

The final control of the average particle size of the resulting powder was carried out using a Mastersizer 2000 laser granulometric analyzer. The phase composition was controlled using a D/MAX-2200 X-ray diffractometer. To analyze the mother liquor obtained after filtration for the presence of non-coprecipitated cations, an Optima 4300 DV inductively coupled plasma atomic emission spectrometer was used.

Example 1

To prepare a solution of metal salts, 750 cm ³ of distilled water, 81.60 g of lanthanum nitrate, 101.23 g of iron nitrate and 33.04 g of barium carbonate were sequentially added to a beaker, excluding water of crystallization. The resulting mixture was stirred until a clear solution was formed, followed by dilution with distilled water to 1000 cm ³ . Dissolution was carried out with constant stirring and at a temperature of 80 °C to accelerate the homogenization process.

At the same time, a precipitant solution was prepared. 600 cm ³ of distilled water, 145 cm ³ of a commercial aqueous solution of ammonia and 288 g of ammonium carbonate were placed in a beaker, excluding adsorbed water, followed by bringing the resulting solution to 1000 cm ³ with distilled water. Dissolution was carried out with constant stirring and at a temperature of 80 °C to accelerate the homogenization process.

After preparing the solutions, controlled two-jet coprecipitation was carried out. With constant stirring, a solution of metal salts and a solution of a precipitant were added to 200 cm ³ of distilled water at the same speed. The pH of the reaction medium was maintained at 8.0 by automatically controlling the supply of precipitant solution. Co-precipitation was considered complete when the entire solution of metal salts was transferred into the reaction medium.

After coprecipitation, the resulting suspension was filtered to separate the precipitate from the mother liquor, and the wet precipitate was successively dried in a drying cabinet at a temperature of 150 °C for 24 hours, fired in a muffle furnace at a temperature of 1300 °C for 6 hours, and disaggregated in a ball mill with rotation speed 250 rpm for 10 minutes. We obtained a powder based on lanthanum-barium ferrite with an average particle size of 2.74 μm and a content of barium, lanthanum, and iron cations in the matrix solution in an amount of <1 wt.%.

Example 2

The preparation of the salt solution was carried out similarly to Example 1, but 815.96 g of lanthanum chloride, 680.15 g of ferric chloride and 348.65 g of barium chloride were used as the starting metal salts, excluding water of crystallization. The preparation of the precipitant solution was carried out similarly to example 1, but 2000 cm ³ of distilled water, 725 cm ³ of a commercial aqueous solution of ammonia and 1440 g of ammonium carbonate were added without taking into account the absorbed water, followed by dilution with distilled water to 5000 cm ³ . Controlled double-jet coprecipitation, filtration and drying were carried out similarly to example 1.

The dried sediment was fired in a muffle furnace at a temperature of 1100°C for 6 hours, but the fired powder was not disaggregated. We obtained a powder based on lanthanum-barium ferrite with an average particle size of 34.88 μm and a content of barium, lanthanum, and iron cations in the mother solution in an amount of <1 wt.%.

Thus, the new method makes it possible to obtain an electrode material based on lanthanum-barium ferrite, which can be used as a material for oxygen, fuel, and symmetrical electrodes of solid oxide fuel cells (SOFC) or proton-ceramic fuel cells (PCFC).

Claims (1)

A method for synthesizing electrode powder based on lanthanum-barium ferrite, including coprecipitation of metal ions of the resulting powder from a solution of water-soluble salts of the corresponding metals using a precipitant containing ammonia and ammonium carbonate, filtration of the resulting mother solution to obtain a precipitate, which is subjected to drying and subsequent heat treatment, characterized in that electrode powder based on lanthanum-barium ferrite is obtained by controlled double-jet coprecipitation from a solution of water-soluble barium, lanthanum and iron salts with a molar ratio of 2:3:5, respectively, using a precipitant solution containing 2 mol/dm ³ ammonia and 3 mol/dm dm ³ of ammonium carbonate, while the controlled two-jet coprecipitation of the metals of the resulting powder is carried out by simultaneously feeding a solution of water-soluble salts and a precipitant solution into the reaction medium - distilled water with continuous stirring and maintaining a constant pH value at 8.0, after which the precipitate is separated by filtration from the mother liquor, which is dried at a temperature of 150°C until the remaining mother liquor is completely removed and fired in an air atmosphere at a temperature of 1100 to 1300°C for 6 hours.