RU2741920C1 - Method for liquid-phase synthesis of a nanostructured ceramic material in a ceo2 - sm2o3 system for producing an electrolyte of a solid oxide fuel cell - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to modern materials for electrochemical energy generators. Solid oxide fuel cells (SOFC) are promising among them. Method is carried out by selecting as starting reagents nitrate salts of cerium Ce(NO₃)₃⋅6H₂O and samarium Sm(NO₃)₃⋅6H₂O, from which solutions with concentration of ~0.5 M are prepared. Obtained solutions are mixed taking into account the given stoichiometric ratio of oxides and evaporated in a water bath for 3 hours until a supersaturated solution forms, which is cooled at temperature 3-5°C, after which the crystalline hydrate is subjected to ultrasound treatment for 30 minutes. Synthesized powder of solid solution of composition (CeO₂)_0.98(Sm₂O₃)_0.02, (CeO₂)_0.95(Sm₂O₃)_0.05 and (CeO₂)_0.90(Sm₂O₃)_0.10 are subjected to thermal treatment at 600°C, compactions are then molded by method of uniaxial cold pressing at pressure of 150 MPa, which are fired at temperature of 1300°C in a tubular furnace with isothermal holding at 2 hours and heating rate of 350-400°/h.

EFFECT: promising technology of liquid phase synthesis of ceramics of composition (CeO₂)_0.98(Sm₂O₃)_0.02, (CeO₂)_0.95(Sm₂O₃)_0.05 and (CeO₂)_0.90(Sm₂O₃)_0.10 to obtain solid electrolyte based on cerium dioxide for solid oxide fuel cells.

1 cl, 4 tbl, 6 dwg

Description

At present, the demand for energy resources is increasing, while fossil fuels are gradually depleting, and the environmental situation in the world is deteriorating. In this regard, alternative, hydrogen energy is actively developing. For its further development, it is necessary to search for and create modern materials for electrochemical energy generators. Solid oxide fuel cells (SOFCs) are promising among them.

Fuel cells have a huge range of applications - from batteries in portable electronic devices to large-scale power generation and autonomous use (for example, in remote regions of the Far North). Power plants based on fuel cells are almost twice as economical as traditional ones. Their efficiency can reach 85%, and the amount of harmful emissions is almost 100 times lower due to the absence of direct chemical contact of the fuel with the oxidizer [1].

Solid oxide fuel cells (SOFCs) are attractive electrochemical generators that efficiently convert the chemical energy of hydrogen-oxygen interaction into electrical energy with minimal environmental impact. On the basis of such elements, pilot plants with a capacity of up to 100 kW were created. SOFCs are mainly applicable for power plants of high power, in which it is possible to minimize the relative share of heat losses into the surrounding space. One of the main advantages of this type of fuel cell is the absence of a liquid electrolyte and the possibility of creating a miniature unit cell consisting of thin layers of electrodes and electrolyte.

Reducing the operating temperature and developing medium-temperature solid oxide fuel cells is an important task of materials science at the world level, since at a high operating temperature of SOFCs, difficulties arise in the compatibility of electrode and electrolyte materials, which leads to a significant increase in their cost. As a result of lowering the operating temperature when using medium-temperature SOFCs, it is possible to increase the range of used construction materials, reduce the degradation of these devices and thereby increase their service life. In medium-temperature SOFCs, it is possible to use natural hydrocarbons (in particular, methane) as fuel, which are converted into carbon monoxide and hydrogen directly inside the fuel cell itself [2]. Such possibilities make medium-temperature SOFCs especially attractive in the absence of a developed hydrogen infrastructure in our country. One of the main parts of a fuel cell cell is the electrolyte. Electrolytes are distinguished by their ion transport mechanism: anionic, protonic and ionic mixed. The principle of operation of medium-temperature and high-temperature fuel cells is based on the transport of an oxygen ion (O ^2- ) from the cathode to the anode. This process is carried out only in the presence of oxygen vacancies; therefore, the electrolyte material with anionic vacancies in the crystal lattice is optimal. At present, cerium dioxide-based nanomaterials with oxygen-ionic conductivity are of increasing interest as medium-temperature electrolytes, which make it possible to reduce the operating temperature of the fuel cell by 300-400 ° C, but at the same time are not inferior in their electrophysical characteristics to traditionally used materials based on dioxide. zirconium YSZ (in particular, ceramics of composition (ZrO ₂ ) _0.92 (Y ₂ O ₃ ) _0.08 ) [3]. However, with high ionic conductivity, electrolyte materials based on zirconium dioxide have low specific electrical conductivity [4]. Thus, the development of new materials based electrolyte multicomponent systems of transition metal oxides, such as GeO ₂ -Sm ₂ O ₃ when creating the medium-temperature fuel cell with high performance is an important task of modern materials [3]. An important task of obtaining efficient electrolyte materials for solid oxide fuel cells is the development of an economical technology for their production. One of the ways to solve this problem is the use of liquid-phase, low-temperature synthesis methods, namely: methods of co-crystallization of salts, co-precipitation of hydroxides, sol-gel, hydrothermal, etc. The use of these synthesis methods makes it possible to obtain highly dispersed nanopowders and nanoceramic materials based on them at the same time, by reducing the temperature of powder synthesis and sintering of ceramics, to reduce energy consumption [5]. Obtaining and researching solid oxide electrolyte materials according to the scheme "composition - synthesis technology - structure - properties" allows achieving their necessary and optimal characteristics [6, 7]. Ceramic materials for electrolytes should have an ultradispersed, low-porosity structure and have ionic conductivity at the operating temperature of the SOFC.

The object of the invention is to provide a directional liquid-phase synthesis on the basis of co-crystallization method of nitrate salts for precursor powders and ceramic oxide nanocomposites given chemical composition in CeO ₂ -Sm ₂ O ₃ system.

According to the invention, the method of liquid-phase synthesis of nanostructured ceramic material in the CeO ₂ -Sm ₂ O _{3 system} to create an electrolyte of a solid oxide fuel cell is carried out by choosing cerium nitrate salts Ce (NO ₃ ) ₃ ⋅6H ₂ O and samarium Sm (NO ₃ ) as starting reagents ₃ ⋅6H ₂ o, from which solutions with a concentration of ~ 0.5 M are prepared, then the resulting solutions are mixed taking into account the given stoichiometric ratio of oxides and evaporated in a water bath for 3 hours until a supersaturated solution is formed, which is cooled at a temperature of 3-5 ° C, whereupon crystalline subjected to ultrasonic treatment for 30 minutes, then the synthesized powder of the solid solution composition (CeO _{2) 0.98} (Sm ₂ O _{3) 0.02,} (CeO _{2) 0.95} (Sm ₂ O _{3) 0.05} and (CeO _{2) 0.90} (Sm ₂ O ₃ ) _{0.10 is} subjected to heat treatment at 600 ° C, then compacts are formed by uniaxial cold pressing at a pressure of 150 MPa, which are fired at a temperature of 1300 ° C in a tubular Furnaces with isothermal holding for 2 hours and heating rate - 350-400 ° / h.

The technical result achieved by using the claimed set of essential features is as follows. The use of dilute solutions for co-crystallization of salts is more expedient than concentrated solutions, since this allows a more dispersed crystallization product to be obtained. As the evaporation process proceeds, the degree of saturation increases, which contributes to the onset of the crystallization process: the nucleation of a new phase in the form of crystallization centers, which gradually turn into small crystals, concentrating on the surface of the solution. A supersaturated solution cooled at a temperature of 3-5 ° C promotes the adsorption of the crystallizing substance on the surface of the crystals formed at the stage of evaporation of mixtures of salt solutions. Ultrasonic treatment for 30 minutes reduces the dispersion of crystalline particles, because weakens the forces of interaction of crystalline particles with each other, and also reduces the interval of their size distribution. The powder was dispersed in distilled water; after ultrasonic treatment, the resulting powder was almost monodisperse.

The essence of the invention is illustrated by drawings, where FIG. 1 shows a scheme for the synthesis of nanopowders with the composition (CeO ₂ ) _0.90 (Sm ₂ O ₃ ) _0.10 by the method of joint crystallization of salts; in fig. 2 - stages of the process of thermolysis of xerogel of composition (CeO ₂ ) _0.95 (Sm ₂ O ₃ ) _0.05 before ultrasonic treatment - curve 1 and after ultrasonic treatment - curve 2; in fig. 3 - diffraction patterns of xerogel (1-150 ° C), nanopowders (2-600 ° C) and ceramics (3-1300 ° C) of composition (CeO ₂ ) _0.95 (Sm ₂ O ₃ ) _0.05 . According to the results of X-ray phase analysis, it was found that at 600 ° C in the studied powders a cubic solid solution of the fluorite type is formed, with an average CSR size of ~ 10 nm [9], in Fig. 4 - dependent values OCD ceramic composition (CeO _{2) 0.95} (Sm ₂ O _{3) 0.05} on temperature; in fig. 5 - adsorption-desorption isotherm and differential pore size distribution curve for nanopowder (heat treatment at 600 ° C) of composition (СеО ₂ ) _0.98 (Sm ₂ O ₃ ) _0.02 ; in fig. 6 - curves of the temperature dependence of the electrical conductivity of ceramic samples of the composition: (CeO ₂ ) _0.98 (Sm ₂ O ₃ ) _0.02 - (1); (CeO ₂ ) _0.95 (Sm ₂ O ₃ ) _0.05 (2); (CeO ₂ ) _0.90 (Sm ₂ O ₃ ) _0.10 (3) [10-12].

The claimed method is carried out as follows.

Cerium nitrate salts Ce (NO ₃ ) ₃ ⋅6H ₂ O and samarium Sm (NO ₃ ) ₃ ⋅6H ₂ O were chosen as starting reagents, from which dilute solutions with a concentration of ~ 0.5 M were prepared. The method of joint crystallization of salts is carried out by converting a mixture of solutions of the initial reagents (cerium and samarium nitrates) into a solid state while maintaining the homogeneity of the resulting substance. The process of joint crystallization occurs in three stages: at the first stage, the initial reagents are dissolved and mixed, then the solvent is removed and a solid-phase product is obtained, followed by its heat treatment with the formation of the target solid solution or compound. The resulting solutions were mixed taking into account a given stoichiometric ratio of oxides and evaporated in a water bath for 3 h until a supersaturated solution was formed, in which the primary nucleation and formation of small crystals on the surface of the solution gradually took place. For crystallization of a substance, it is necessary to create a supersaturation of the initial mixture of reagents with a crystallized substance, for which it must be supercooled. The resulting supersaturated solution was cooled at a temperature of 3-5 ° C, which promoted the adsorption of the crystallizing substance on the surface of the crystals formed during the evaporation stage; in fig. 1 shows a scheme for the synthesis of nanopowders of the composition (CeO _{2) 0.90} (Sm ₂ O _{3) 0.10.} To reduce the dispersion of crystalline particles, as well as to reduce the range of their size distribution, the crystalline hydrate was subjected to ultrasonic treatment for 30 min [13, 14]. The powder was dispersed in distilled water; after ultrasonic treatment, the resulting powder was almost monodisperse. The thus obtained crystalline hydrate was subjected to heat treatment at 200 ° C (1 h) to remove adsorbed water, and then it was heat treated at 600 ° C (1 h) to form a stable crystalline structure of nanopowders.

This synthesis method promotes the interaction of chemical components at the ion-molecular level, allows minimizing the role of high-temperature diffusion due to the high degree of homogenization, and is also quite simple and does not require complex equipment.

The X-ray amorphous xerogels obtained as a result of drying (200 ° C, 1 h) were heat treated (600 ° C, 1 h) to form a stable crystal structure of nanopowders.

The study of the thermolysis of the synthesized crystal hydrates was carried out by the method of differential thermal analysis; by way of example, in FIG. 2 shows the process of thermolysis of a xerogel with the composition (CeO ₂ ) _0.95 (Sm ₂ O ₃ ) _0.05 before ultrasonic treatment - curve 1 and after ultrasonic treatment - curve 2. It was found that the main stage of the dehydration process of the initially synthesized crystalline hydrate without ultrasonic treatment occurs in two stages in the temperature range 60-240 ° C. In the temperature range 270-320 ° C, an exo effect is observed associated with the onset of crystallization of a cubic solid solution of the fluorite type based on cerium oxide. The processes of dehydration and decomposition of nitric acid salts correspond to a weight loss of ~ 40%. The temperature range of dehydration of the ultrasonic treated crystalline hydrate is narrower and is carried out in almost one stage. Such features are associated with the fact that ultrasonic waves, acting on the crystalline hydrate powder, weaken the bonds between the molecules of nitric acid salts and the molecules of crystallization water. At the same time, the dispersion of the powder decreases, facilitating the processes of dehydration and decomposition of salts. Ultrasonic treatment also affects the crystallization of the powder, namely, it reduces the temperature of its transition to the crystalline phase (290 ° C → 250 ° C).

According to the results of X-ray phase analysis, it was found that at 600 ° C, a cubic solid solution of the fluorite type is formed in the studied powders, with an average CSR size of ~ 10 nm. Subsequent firing at higher temperatures (1300 ° C) does not lead to a violation of the single-phase nature of nanopowders and ceramics based on them. As an example, the formation sequence of a cubic fluorite type solid solution of the sample composition (CeO _{2) 0.95} (Sm ₂ O _{3) 0.05} is shown in FIG. 3.

Table 1 shows X-ray diffraction characteristics of the sample of the test composition (CeO _{2) 0.95} (Sm ₂ O _{3) 0.05.}

From the synthesized powder of a given composition, thermo-treated at 600 ° C, compacts were formed by uniaxial cold pressing at a pressure of 150 MPa, which were fired in air at a temperature of 1300 ° C in a tubular furnace with program control from Nabertherm. The isothermal holding was 2 h, and the heating rate was 350-400 ° / h. A sufficiently high heating rate was chosen to reduce the rate of crystallite growth [8]. According to the XRF data in FIG. 4 shows changes in the average size of the OCD sample formulation (CeO _{2) 0.95} (Sm ₂ O _{3) 0.05} as a function of sonification in the temperature range 600-1300 ° C.

The determination of the parameters of the microstructure of the synthesized powders was carried out by the method of low-temperature nitrogen adsorption.

By way of example, in FIG. 5 shows the adsorption-desorption isotherms and differential curves of pore size distribution for the nanopowders (heat treatment 600 ° C) of composition (CeO _{2) 0.98} (Sm ₂ O _{3) 0.02.}

As seen in FIG. 5, the characteristic shape of the adsorption-desorption curve belongs to type IV according to the IUPAC classification, which indicates the mesoporous structure of the powder. The type of capillary-condensation hysteresis of the H1 type according to the IUPAC classification indicates the predominance of cylindrical mesopores (11 nm) [15].

The characteristics of the microstructure of the synthesized powders of all compositions are presented in Table 2.

Optimal values of density, porosity, and microhardness of the obtained materials are a necessary condition for the functionality of electrolytes of solid oxide fuel cells [16, 17]. The importance of these characteristics is due to the fact that electrolytes must be gastight [18]. Table 3 shows the physicochemical properties of the ceramics of the synthesized compositions.

Temperature dependence of the conductivity nanoceramics formulation (CeO _{2) 0.98} (Sm ₂ O _{3) 0.02,} (CeO _{2) 0.95} (Sm ₂ O ₃₎ and _0.05 (CeO _{2) 0.90} (Sm ₂ O _{3) 0.10} over the temperature range 25-1000 ° C is shown in FIG. 6.

As seen in FIG. 6, with an increase in temperature in the range from 500 to 1000 ° C, the electrical conductivity of all three samples increases. In addition, with an increase in the concentration of samarium oxide, the ionic conductivity of ceramics increases over the entire temperature range studied. The highest ionic conductivity is possessed by a sample containing 10 mol. % Sm ₂ O ₃ : σ _{700 ° C} = 1.3⋅10 ^-2 Scm ^-1 . The achieved effect is due to the formation of mobile oxygen vacancies, providing ionic conductivity of electrolytes in the CeO ₂ -Sm ₂ O _{3 system} upon heterovalent substitution of Ce ⁴⁺ cations by Sm ³⁺ . The formation of oxygen vacancies is described by the following quasi-chemical equation in the Kroeger-Wink notation: [19]

- ион самария с отрицательным эффективным зарядом на месте иона церия,

- кислородная вакансия с положительным эффективным зарядом, компенсирующим заряд допанта,

- атом кислорода в регулярном узле с нейтральным зарядом.Where

- samarium ion with a negative effective charge in place of the cerium ion,

- oxygen vacancy with a positive effective charge that compensates for the dopant charge,

- an oxygen atom in a regular site with a neutral charge.

Table 4 shows the ratio of the ionic and electronic conductivity for the studied samples of the composition (CeO ₂ ) _0.90 (Sm ₂ O ₃ ) _0.10 . As follows from the table. 4, these solid electrolytes have mixed conductivity with the ion transfer number t _i = 0.82 (300 ° C) and t _i = 0.70 (700 ° C). It is seen that with increasing temperature, the contribution of the electronic component to the total value of electrical conductivity increases sharply.

As seen in FIG. 6 and tab. 3 and 4, the temperature dependence of the specific electrical conductivity of the studied solid solutions based on CeO ₂ is of semiconducting nature, the resulting solid solutions have an oxygen-ionic conduction mechanism with ion transport numbers t _i = 0.82-0.71 in the temperature range 300-700 ° C ...

The obtained ceramic nanocomposites, in terms of their mechanical (open porosity, density, microhardness) and electrophysical (value, type and mechanism of electrical conductivity) properties, are promising as solid oxide electrolyte materials and can be recommended for use as an electrolyte for SOFCs.

); характеризующаяся ОКР 68-81 нм (1300°С), открытой пористостью в интервале 2-6%, высокими значениями относительной плотности 96% и ионной проводимости σ_700°C=1,3*10^-2 См/см, числами ионного переноса t_i=0,82-0,71 в интервале температур 300-700°С.When creating new materials for modern power engineering, the key issue is the development of technologies for obtaining high-quality oxide nanocomposites with specified physicochemical, electrical and mechanical properties. As a result of research carried out in this study, proposed technology phase synthesis of the ceramic material in the system CeO ₂ -Sm ₂ O ₃ to create a solid oxide electrolyte fuel cell. The ratio of oxides and the optimal conditions for obtaining nanocrystalline mesoporous powder of a solid solution based on cerium dioxide have been selected, and the conditions for its consolidation have been identified. It was found that the synthesized ceramics in the СеО ₂ -Sm ₂ O _{3 system} are solid solutions with a crystalline cubic structure of the fluorite type (a = 5.41848

); characterized by CSR 68-81 nm (1300 ° C), open porosity in the range 2-6%, high values of relative density 96% and ionic conductivity σ _{700 ° C} = 1.3 * 10 ^-2 S / cm, ion transfer numbers t _i = 0.82-0.71 in the temperature range 300-700 ° C.

Thus, the results obtained indicate that the proposed technology for the synthesis of ceramics with the composition (CeO ₂ ) _0.98 (Sm ₂ O ₃ ) _0.02 , (CeO ₂ ) _0.95 (Sm ₂ O ₃ ) _0.05 and (CeO ₂ ) _0.90 (Sm ₂ O ₃ ) is _promising. ) _0.10 to obtain a solid electrolyte based on cerium dioxide for solid oxide fuel cells.

Literature

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Claims (1)

The method of liquid-phase synthesis of nanostructured ceramic material in the CeO ₂ -Sm ₂ O _{3 system} to create an electrolyte of a solid oxide fuel cell is carried out by choosing cerium nitrate salts Ce (NO ₃ ) ₃ ⋅6H ₂ O and samarium Sm (NO ₃ ) ₃ ⋅ as initial reagents 6H ₂ O, from which solutions with a concentration of ~ 0.5 M are prepared, then the resulting solutions are mixed taking into account the given stoichiometric ratio of oxides and evaporated in a water bath for 3 hours until a supersaturated solution is formed, which is cooled at a temperature of 3-5 ° C, whereupon crystalline subjected to ultrasonic treatment for 30 minutes, then the synthesized powder of the solid solution composition (CeO _{2) 0.98} (Sm ₂ O _{3) 0.02,} (CeO _{2) 0.95} (Sm ₂ O _{3) 0.05} and (CeO _{2) 0.90} (Sm ₂ O ₃ ) _{0.10 is} subjected to heat treatment at 600 ° C, then compacts are formed by the method of uniaxial cold pressing at a pressure of 150 MPa, which are fired at a temperature of 1300 ° C in a tube furnace with an isothermal holding for 2 hours and heating rate 350-400 ° / h.

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