Synthesis and Investigation of Ceramic Materials for Medium-Temperature Solid Oxide Fuel Cells

Abstract

Finely dispersed (СeO2)1-x(Sm2O3)x (x = 0.02; 0.05; 0.10); La1-xSrxNiO3, La1-xSrxCoO3 and La1-xSrxFe0.7Ni0.3O3 (x = 0.30; 0.40) mesoporous xerogel powders are synthesized by co-crystallization of the corresponding nitrates with ultrasonic processing and used to obtain nanoscale ceramic materials with cubic fluorite-like, orthorhombic, and perovskite-like tetragonal crystal structure, respectively, with CSR ∼ 64–81 nm (1300°C). Physicochemical characterization of the obtained ceramics revealed that (СeO2)1-x(Sm2O3)x features with open porosity 2–6%, while for La1-xSrxNiO3, La1-xSrxCoO3, and La1-xSrxFe0.7Ni0.3O3, this value is about 21–29%. Ceria-based materials possess a predominantly ionic conductivity (ion transport numbers ti = 0.82–0.71 in the temperature range 300–700°C, σ700°С = 1.3·10−2 S/cm) determined by the formation of mobile oxygen vacancies upon heterovalent substitution of Sm3+ for Се4+. For solid solutions based on lanthanum nickelate and cobaltite, a mixed electronic-ionic conductivity (σ700°С = 0.80·10−1 S/cm) with ion transport numbers (te = 0.98–0.90, ti = 0.02–0.10) was obtained. The obtained ceramic materials are shown to be promising as solid oxide electrolytes and electrodes for medium-temperature fuel cells.

1. Introduction

In view of the currently increasing demand for energy resources, gradual depletion of fossil fuels, and growing environmental problems, the development of alternative hydrogen energy is an essential and highly promising R&D direction. For further progress in this area, the development of advanced materials for electrochemical power systems is required. Particularly, promising solid oxide fuel cells (SOFCs) provide the efficiency of up to 85% in couple with almost double cost savings and a 100 times reduction of harmful emissions compared with conventional power sources due to the absence of direct chemical contact between the fuel and the oxidizer [1]. The application of SOFC-based power sources a significant power and fuel savings. The operation temperatures as high as 700–950°C afford increased rates of electrode reactions without using expensive catalysts. Another advantage of SOFCs is the absence of strict requirements for fuel purity. Besides hydrogen, any hydrocarbons converted into synthesis gas (H₂-CO) can be used. High thermodynamic efficiency, continuous operation, and environment-friendly performances make solid oxide fuel cells more promising compared with such conventional systems as internal combustion engines, solar panels, and wind turbines. As a promising power storage system, reversible SOFCs can provide an economically effective approach to the power management using discontinuous energy sources [2, 3].

Thus, the development and implementation of SOFC-based fuel cells for commercial production are becoming a priority to address the problems of distributed power supply, energy saving, cogeneration, and saving fuel resources.

The decrease of working temperature and development of medium-temperature SOFCs is an important goal in materials science since high-operating temperatures cause problems with the compatibility of electrode materials and electrolytes. The implementation of medium-temperature SOFCs can provide an extension of the range of applied materials, reduce degradation of the devices, and increase their operational lifetime. The main components of SOFC include cathode, anode, and electrolyte. The applied electrolytes differ in their anionic, protonic, or ion-mixed ion-transport mechanism. The basic principle of fuel cell operation is that the transport of oxygen ions (O²⁻) from the cathode to the anode can only proceed in the presence of oxygen vacancies. In this regard, the optimal electrolyte materials must contain anion vacancies in the crystal lattice. Currently, cerium dioxide (CeO₂)-based nanomaterials with oxygen-ionic conductivity are considered promising as medium-temperature electrolytes affording the reduction of the fuel cell operating temperature by 300–400°C. In respect of electric performances, these electrolytes are not inferior to conventional zirconia-based YSZ materials (particularly, (ZrO₂)_0.92(Y₂O₃)_0.08 ceramics) [4, 5]. Furthermore, they are thermodynamically stable at relatively low-operating temperatures of 600–800°C that provide a long lifetime. Moreover, relatively low-operating temperatures prevent from the interlayer diffusion and interfacial layers between SOFC components exclude solid-phase interaction. All the existing SOFC cathodes have certain disadvantages that determine the growing interest in R&D in the development of new cathode materials, particularly for medium-temperature (500–700°C) SOFCs.

During SOFC operation in the middle-temperature range, the power characteristics of the cell are limited by the cathode operation [6]. Since the SOFC cathode reduction of molecular oxygen and transport of oxygen ions to the electrolyte surface take place, the developed medium-temperature cathode materials should meet several requirements. Particularly, they should be ultrafine and have high electronic or mixed electron-ion conductivity. To reduce the diffusion drag at the cathode, a well-developed porous structure is required, since the rate of molecular oxygen reduction depends on the specific surface area. Furthermore, the mechanical compatibility of the cathode and electrolyte is important. These requirements are fulfilled by complex metal oxides and their nanocomposites, particularly complex oxides based on rare earth elements (REE) and 3D transition metals combining high electric and catalytic properties. Currently, single-phase complex perovskite materials such as LnMO_3-δ (M-Cr, Mn, Fe, Ni, and Co) are proposed as new materials for medium-temperature SOFC cathodes, featuring stability in an oxidizing atmosphere in a wide temperature range and sufficiently high p-type electrical conductivity [6, 7]. Among them, the highest conductivity is observed for manganese-, cobalt-, and nickel-containing oxides. The conductivity of such perovskites can be further enhanced by increasing the concentration of charge carriers (holes) due to the heterovalent substitution of La³⁺ for cations of alkaline earth elements M²⁺ = Ca, Sr, or Ba [8]. Oxides with a perovskite structure constitute a large class of complex oxides with ABO₃-type unit cells (Figure 1). A distinctive feature of perovskites is the possibility of cationic substitution in both A and B positions in a wide range of concentrations [9]. In practice, most crystals with a cubic perovskite structure crystallize in a lower symmetry with the distortion of the cubic structure to orthorhombic, hexagonal, or tetragonal one.

Figure 1.

Perovskite structure.

The variation of the substituting cations percentage within a fairly wide range and changing their oxidation degrees allow the simulation of functional properties of perovskite-like oxide systems.

Strontium lanthanum manganites such as La_1-xSr_xMnO₃ (LSM) with perovskite structure are extensively used as cathodes of solid oxide fuel cells, providing the best performances in the high-temperature range (800–1000°C) [10]. For medium-temperature fuel cells, lanthanum cobaltites and nickelates seem to be most promising, since their electrical conductivity exceeds that of lanthanum manganites due to a higher specific surface reactivity determined by a lower strength of metal-oxygen (Me–O) bond [11]. The development of processes for obtaining efficient electrolyte and cathode materials for SOFCs is an important R&D goal.

The most economically efficient approach to this problem is the use of liquid-phase synthetic methods, including co-crystallization of salts, co-precipitation of hydroxides, sol-gel, and hydrothermal. These approaches afford fine powders and nanoceramic materials-based thereon, also providing a reduced energy consumption due to the reduced temperature of powder synthesis and ceramics sintering [12].

The preparation of solid oxide electrolyte and cathode materials and their characterization to find the relationships such as “composition – synthesis technology – structure – properties” afford the determination of their optimal characteristics and synthesis conditions.

2. Synthesis and investigation of ceramic materials

2.1 Synthesis of ceramic materials

The synthesis of xerogels and nanodispersed powders with different oxide concentration ratios in the CeO₂–Sm₂O₃, La₂O₃–SrO–Ni₂O₃(Со₂O₃), and La₂O₃–SrO–Ni₂O₃–Fe₂O₃ systems was carried out by co-crystallization of the corresponding nitrates followed by the ultrasonic treatment [13]. Nitrates of cerium Се(NO₃)₃·6H₂O (analytical grade), samarium Sm(NO₃)₃·6H₂O (chemically pure), lanthanum La(NO₃)₃·6H₂O (chemically pure), strontium Sr.(NO₃)₂ (analytical grade), nickel Ni(NO₃)₂·6H₂O (pure), cobalt Co(NO₃)₂·6H₂O (pure), and iron Fe(NO₃)₃·9H₂O (pure) as ∼0.5 M solutions were used for the synthesis. The resulting solutions were mixed in the proportions corresponding to the stoichiometric ratio of oxides, followed by evaporation on a water bath for 3 h to obtain a supersaturated solution. Then, the supersaturated solution was cooled at a temperature of 3–5°C to facilitate the adsorption of the crystallizing substance on the surface of the crystals formed during the evaporation of mixtures of the nitrate solutions. To improve the dispersity of crystalline particles and to make their size distribution more narrow, the crystalline hydrate was subjected to ultrasonic treatment for 30 min in distilled water, resulting in an almost monodispersed powder.

The subsequent drying (110°C, 0.5 h) yielded X-ray amorphous xerogels that were subjected to heat treatment (600°C, 1 h) to form nanopowders with a stable crystal structure. The synthesized powders of a given composition were ground in a mortar, followed by uniaxial cold pressing into tablets of 1.0 and 1.5 cm in diameter using a PGR400 at the pressure of 100–150 MPa and then sintering installation for 2 hours and temperature 1300°C.

2.2 Characterization techniques

XRD characterization was performed using a Bruker D8-Advanced diffractometer. The ICDD-2006 international database was used to interpret the diffraction patterns, and the analysis results were processed using the WINFIT 1.2.1 software involving the Fourier transform of the reflection profile. The sizes of coherent scattering regions (CSR) were estimated according to the Selyakov-Scherer equation:

$$D_{\mathit{CSR}}=\frac{0.9\times \mathrm{\lambda }}{\mathrm{\beta }\times cos\mathrm{\theta }}$$E1

where λ = 15,418 Ǻ is the СuKα wavelength, and β is the XRD peak full width at half maximum (FWHM) [14].

The thermolysis processes occurring in coprecipitated xerogels and powders upon heating in the temperature range of 20–1000°C were studied using the Q-1000 derivatograph (MOM). The specific surface area of ​​the synthesized nanopowders was measured by low-temperature nitrogen adsorption using a QuantaChrome Nova 4200B analyzer. Based on the obtained data, the specific surface area S_BET of the samples was calculated using the Brunauer-Emmett-Teller (BET) model. The calculation of the pore size distribution was carried out based on nitrogen desorption isotherms according to the Barret-Joyner-Halenda (BJH) method; the thermal treatment of the powders was carried out in a Naberterm furnace with program control in the temperature range of 25–1300°C for 3 hours, followed by slow cooling of the furnace. The open porosity of the samples was determined by hydrostatic weighing in distilled water in accordance with the Russian standard GOST 473.4–81 [15]. The electrical resistance of the obtained ceramic materials was measured by the two-contact method at a direct current in the temperature range of 250–1000°C using the “Hardware-software complex for studying the electrical properties of nanoceramics in various gaseous media” [16].

The transfer numbers of ions and electrons in bulk solid electrolytes [17] were determined using the West-Tallan method. A mixture of CO₂ + CO was used as an inert gas (corresponding to a partial pressure of oxygen 10³ Pa). The measurements were carried out at a direct current in weak (U = 0.5 V) fields after a long (up to 30 min) current drop. The contributions of ionic and electronic conductivities were estimated according to the formulas:

$$t_e=\frac{R_{\mathit{air}}}{R_e}$$E2

$$t_i=1-t_e$$E3

where t_e and t_i are electron and ion transport numbers, respectively; R_air and R_e are electric resistance values measured in air and inert gas, respectively.

2.3 Results and discussion

For all the obtained compositions, thermolysis processes were studied. As an example, DTA thermograms of La_0.6Sr_0.4CoO₃ xerogel synthesized with (a) and without ultrasonic treatment (b) are shown in Figure 2.

Figure 2.

Differential thermal analysis results for La_0.6Sr_0.4CoO₃ xerogel prepared with (a) and without (b) xerogel freezing at −25°C (24 h).

As shown in Figure 2, ultrasonic treatment during the synthesis results in the reduction of the crystalline hydrate dehydration temperature from 115–110°C, as well as the temperatures of all thermal transformations. This effect is determined by weakening the bonds between the molecules of nitrate salts and crystallization water molecules upon the impact of ultrasonic waves, facilitating the dehydration and decomposition of salts. Ultrasonic treatment also affects the powder crystallization reducing the temperature of its transition into the crystalline phase (320 → 290°C).

Compared with the considered data, the differences in the temperatures of exo- and endo-effects for other compositions were no more than 10–15°C.

The microstructural parameters of the synthesized powders were determined using low-temperature nitrogen adsorption.

Microstructural performances of the synthesized powders are summarized in Table 1.

Composition	Specific surface area Ss, m^2/g	Average pore size Dpor, nm	Specific pore volume Vpor, cm^3/g
Electrolyte materials
(CeO2)0.98(Sm2O3)0.02	40	11	0.075
(CeO2)0.95(Sm2O3)0.05	63	8	0.082
(CeO2)0.90(Sm2O3)0.10	74	2	0.087
Cathode materials
La0.7Sr0.3NiO3	30	2.3	0.029
La0.6Sr0.4NiO3	28	2.4	0.026
La0.7Sr0.3CoO3	15	2.1	0.018
La0.6Sr0.4CoO3	15	2.2	0.020
La0.7Sr0.3Fe0.7Ni0.3O3	31	2.2	0.035
La0.6Sr0.4Fe0.7Ni0.3O3	30	2.0	0.033

Table 1.

Microstructural performances of the synthesized powders [18].

XRD characterization revealed that at 600°C a cubic solid solution of the fluorite type is formed in the studied CeO₂–Sm₂O₃ powders with an average CSR size of ∼10 nm. Subsequent annealing at higher temperatures (1300°C) does not disrupt the single-phase structure of the nanopowders and ceramics-based thereon. As an example, consecutive steps of fluorite-type cubic solid solution formation for the (CeO₂)_0.95(Sm₂O₃)_0.05 sample are shown in Figure 3. Crystal structures and specific electrical conductivity of synthesized powders and ceramics in the system CeO₂–Sm₂O₃ are shown in Table 2.

Figure 3.

XRD patterns of (CeO₂)_0.90(Sm₂O₃)_0.10 (a = 5.41396 Å) based xerogel (a—150°C), nanopowder (b—600°C), and ceramics (c—1300°C) samples [18].

Composition	Fluorite-like structure	Lattice parameter, a (Ǻ)	CSR, nm		σ·10^−2, S·cm^−1; (700°C)	Ea, eV
			600°C	1300°C
(CeO2)0.98(Sm2O3)0.02	cubic	a = 5.43204	11	81	0.4	1.15
(CeO2)0.95(Sm2O3)0.05	cubic	a = 5.41848	9	70	0.7	1.13
(CeO2)0.90(Sm2O3)0.10	cubic	a = 5.41396	8	68	1.3	1.00

Table 2.

Crystal structures and specific electrical conductivity of powders and ceramics in the system CeO₂–Sm₂O₃ [18].

The phase compositions of powders and corresponding ceramics of all the compositions in the systems La₂O₃–SrO–Ni₂O₃(Со₂O₃) and La₂O₃–SrO–Ni₂O₃–Fe₂O₃ are summarized in Table 3. As an example, Figure 4 shows the phase composition of La_0.6Sr_0.4NiO₃ xerogel powder and ceramics, indicating the formation of a solid solution with a tetragonal perovskite structure at 1300°C.

Composition (preparation conditions)	Perovskite-like structure	Parameters	CSR, nm	σ·10^−1, S·cm^−1 (700°C)	Ea, eV
La1-xSrxCoO3
La0.7Sr0.3CoO3; (600°C, holding 1 h)	Orthorhombic; (crystallization start)	a = 4.1132; b = 5.429; c = 9.804; V = 218.93	34	Non-sintered powder	—
La0.7Sr0.3CoO3; (1300°C, heating 3 h + holding 5 h)	Orthorhombic	a = 4.1027; b = 5.420; c = 9.792; V = 217.75	70	Ceramics; 0.20	1.82
La0.6Sr0.4CoO3; (1300°C, heating 3 h + holding 5 h)	Tetragonal	a = 3.9052; c = 12.8412; V = 195.84	67	Ceramics; 0.35	1.67
La1-xSrxNiO3
La0.7Sr0.3NiO3; (900°C, holding 3 h)	Tetragonal	a = 3.8168; c = 12.7498; V = 185.74	40	Non-sintered powder	—
La0.7Sr0.3NiO3; (1300°C, heating 3 h + holding 2 h)	Tetragonal	a = 3.8140; c = 12.7421; V = 185.35	68	Ceramics; 0.25	1.78
La0.6Sr0.4NiO3; (600°C, holding 1 h)	Orthorhombic	a = 14.6033; b = 9.7345; c = 6.9057; V = 981.69	32	Non-sintered powder	—
La0.6Sr0.4NiO3; (900°C, holding 3 h)	Tetragonal	a = 3.8187; c = 12.7496; V = 185.92	44	Non-sintered powder	—
La0.6Sr0.4NiO3; (1300°C, heating 3 h + holding 2 h)	Tetragonal	a = 3.8152; c = 12.7436; V = 185.49	65	Ceramics; 0.80	1.63
La1-xSrxFe1-yNiyO3
La0.6Sr0.4Fe0.7Ni0.3O3 (600°C, holding 1 h)	Orthorhombic	a = 15.10; b = 9.687; c = 6.9297; V = 1013.63	25	Non-sintered powder	—
La0.6Sr0.4Fe0.7Ni0.3O3 (1300°C, heating 3 h + holding 2 h)	Tetragonal	a = 3.9102; c = 12.7903; V = 195.33	65	Ceramics; 0.75	1.59
La0.7Sr0.3Fe0.7Ni0.3O3 (1300°C, heating 3 h + holding 2 h)	Tetragonal	a = 3.8965; c = 12.7752; V = 193.96	63	Ceramics; 0.30	1.84

Table 3.

Crystal structures and specific electrical conductivity of powders and ceramics in the systems La₂O₃–SrO–Ni₂O₃(Со₂O₃) and La₂O₃–SrO–Ni₂O₃–Fe₂O₃.

Figure 4.

XRD patterns of La_0.6Sr_0.4NiO₃ xerogel powder and ceramics; heat treatment at a) 600°C, b) 900°C, c) 1000°C, and d) 1300°C.

The data in Table 3 show that the synthesized powders and ceramic materials in the temperature range of 600–1300°C have an orthorhombic and tetragonal perovskite-type structure.

Physicochemical properties of all the ceramic samples obtained on the basis of nanopowders in the СeO₂-Sm₂O₃ La₂O₃–SrO–Ni₂O₃(Со₂O₃) and La₂O₃–SrO–Ni₂O₃–Fe₂O₃ systems are summarized in Table 4. These data show that an increase in the content of samarium oxide in the obtained samples in the СeO₂-Sm₂O₃ system leads to a decrease in their density, which is probably due to the distortion of the cerium dioxide lattice upon Sm₂O₃ dissolving.

Composition	∆L/L, %	ρteor, g/cm^3	Apparent density ρexp, g/cm^3	P, %	TEC α, ×10^−6 K^−1
Electrolyte materials
(CeO2)0.98(Sm2O3)0.02	15.8	7.15	6.35	4.7	12.5 [20]
(CeO2)0.95(Sm2O3)0.05	15.0	6.92	6.21	2.3	12.5 [20]
(CeO2)0.90(Sm2O3)0.10	14.5	6.83	6.01	6.0	12.5 [20]
Cathode materials
La0.6Sr0.4NiO3	15.1	5.67	5.41	25.2	14.2 [21]
La0.6Sr0.4CoO3	14.2	5.87	5.61	26.0	18.9 [22]
La0.7Sr0.3NiO3	15.1	5.56	5.24	21.0	14.2 [21]
La0.7Sr0.3CoO3	14.3	5.89	5.61	23.5	17.7 [22]
La0.6Sr0.4Fe0.7Ni0.3O3	14.7	5.69	5.49	28.3	12.8–13.1 [20]
La0.7Sr0.3Fe0.7Ni0.3O3	14.6	5.73	5.52	27.9	12.8–13.1 [20]

Table 4.

Physicochemical properties of ceramics samples synthesized by co-precipitation of salts [18].

To create a porous structure in ceramics based on the La₂O₃–SrO–Ni₂O₃(Со₂O₃) and La₂O₃–SrO–Ni₂O₃–Fe₂O₃ systems, a pore-forming additive (10% aqueous solution of polyvinyl alcohol) in an amount of 10% over the bulk of the charge was added [19]. The values of open porosity are in the range of 21–29%, which is one of the main conditions for the optimal operation of cathode solid oxide materials as shown in Table 4.

One of the main conditions for SOFC operation is the compatibility of its components in terms of the thermal expansion coefficient (TEC). TEC of electrolytes based on cerium oxide doped with samarium oxide is α = 12.5 · 10⁻⁶ K⁻¹ [20]. Cathode materials based on systems La₂O₃–SrO–Ni₂O₃ (TEC—14.2 · 10⁻⁶ K⁻¹) and La₂O₃–SrO–Ni₂O₃–Fe₂O₃ (TEC—12.8–13.1 · 10⁻⁶ K⁻¹) have comparable TEC values with ones of electrolytes based on cerium oxide. As can be seen in Table 4, strontium lanthanum cobaltites have a TEC much higher than the TEC of samples of the composition (CeO₂)_1-x(Sm₂O₃)_x.

The electrical conductivity of (СeO₂)_1-x(Sm₂O₃)_x samples (x = 0.02; 0.05; 0.10) was measured using the two-contact method at direct current (Figure 5). The appearance of high oxygen ionic conductivity in CeO₂-Sm₂O₃-based solid electrolytes is determined by the formation of oxygen vacancies in the СеО₂ matrix when Се⁴⁺ is replaced by Sm³⁺ during the dissolution of Sm₂O₃ in CeO₂, which can be described by the following quasi-chemical equation in the Kroeger-Winke notation [23]:

Figure 5.

Temperature dependence for specific electrical conductivity of 1—(CeO₂)_0.95(Sm₂O₃)_0.02, 2—(CeO₂)_0.90(Sm₂O₃)_0.05, 3—(CeO₂)_0.80(Sm₂O₃)_0.10 [18].

$${\mathrm{Sm}}_2{\mathrm{O}}_3\stackrel{{\mathrm{CeO}}_2}{\to }2\mathrm{Sm}{\prime }_{\mathrm{Ce}}+3{\mathrm{O}}_{\mathrm{O}}^{\mathrm{x}}+{\mathrm{V}}_{\mathrm{O}}^{••}$$E4

where Sm′_Ce is a samarium ion replacing Ce⁴⁺ and yielding a negative charge, $V_O^{••}$ is a positively charged oxygen vacancy compensating the dopant charge, and $O_O^{\times }$ is oxygen atom in a regular site with a neutral charge.

As can be seen in Figure 5, the temperature growth in the range from 500 to 1000°C leads to the increase in electrical conductivity of all the samples. In addition, with an increase in the concentration of samarium oxide, the specific electrical conductivity of the ceramics increases in the entire temperature range in the study. The highest specific electrical conductivity in the temperature range of 500–1000°C (σ_700°C = 1.3 · 10⁻² S/cm) is observed for the sample containing 10 mol. % Sm₂O₃.

The temperature dependence of the specific electrical conductivity of solid solutions in the systems La₂О₃–SrO–Co₂O₃ and La₂О₃–SrO–Ni₂O₃ (La_0.6Sr_0.4CoO₃, La_0.7Sr_0.3CoO₃, La_0.6Sr_0.4NiO₃, La_0.7Sr_0.3NiO₃) is shown in Figure 6. The conductivity grows with temperature in the range from 300 to 700°C up to the saturation plateau at 600–800°C. Particularly, for La_0.6Sr_0.4NiO₃ and La_0.6Sr_0.4CoO₃, the plateau is reached at 600 and 700°C, respectively, while at higher temperatures, no conductivity increase is observed due to a change in the conductivity mechanism from semiconductor to metallic. The electrical conductivity in the considered solid solutions can proceed via several possible mechanisms, mainly by itinerant electrons along the Ni³⁺–O–Ni³⁺ chain and electron or hole jump directly between Ni³⁺ and Ni²⁺ ions. The authors of ref. [6] believe that the appearance of metallic conductivity in the synthesized solid solutions is due to the delocalization of Ni d-electrons during the interaction of nickel and oxygen atoms in the Ni-O-Ni chains.

Figure 6.

Temperature dependence for specific electrical conductivity of 1—La_0.6Sr_0.4NiO₃, 2—La_0.6Sr_0.4CoO₃, 3—La_0.7Sr_0.3NiO₃, and 4—La_0.7Sr_0.3CoO₃.

Figure 6 also indicates that the conductivity of the studied samples grows with an increase in strontium oxide content in the solid solutions.

The highest conductivity in the temperature range 500–1000°С (σ_700°С = 0.80·10⁻¹ S/cm) is observed for the composition La_0.6Sr_0.4NiO₃. The conductivity values σ_700°C for La_0.7Sr_0.3NiO₃, La_0.6Sr_0.4CoO₃, and La_0.7Sr_0.3CoO₃ samples are 0.25·10⁻¹, 0.35·10⁻¹, and 0.20·10⁻¹ S/cm, respectively. Figure 7 shows the temperature dependence plots for samples of the compositions La_0.6Sr_0.4Fe_0.7Ni_0.3O₃ and La_0.7Sr_0.3Fe_0.7Ni_0.3O₃, indicating that the former one features with higher conductivity compared with the latter one. The observed plot shapes and level of conductivity are similar to those for lanthanum nickelate and cobaltite with only small differences. However, the addition of iron results in a prominent increase in the transition temperature from semiconductor to metallic conductivity.

Figure 7.

Temperature dependence for specific electrical conductivity of a) La_0.6Sr_0.4Fe_0.7Ni_0.3O₃ and b) La_0.7Sr_0.3Fe_0.7Ni_0.3O₃.

Using the West-Tallan method, the ratio of the electronic and ionic conductivity in the studied ceramic samples was determined. As an example, Table 5 presents the data on the ratio of the transfer numbers of ions and electrons for the studied samples of the composition (CeO₂)_0.90(Sm₂O₃)_0.10. These data indicate that these solid electrolytes have mixed conductivity with the ion transport number—t_i = 0.82 at 300°C and 0.71 at 700°C. The temperature growth leads to a sharp increase in the contribution of the electronic component to the total value of electrical conductivity that relates to a partial transition Ce⁴⁺ → Ce³⁺.

Т, °С	ti	tе
300	0.82	0.18
400	0.78	0.22
500	0.76	0.24
600	0.72	0.28
700	0.71	0.29

Table 5.

Performances of electronic and ionic conductivity of (CeO₂)_0.90(Sm₂O₃)_0.10 [18].

The ratio between the electronic and ionic conductivity determined according to the West-Tallan method is exemplarily illustrated in Table 6, indicating the ratios of ion and electron transfer numbers for La_0.6Sr_0.4CoO₃, La_0.7Sr_0.3CoO₃, La_0.6Sr_0.4NiO₃, and La_0.7Sr_0.3NiO₃. The presented data show that these materials have mixed conductivity with a predominance of electronic components with the transfer numbers t_e = 0.92–0.98 and t_i = 0.08–0.02 at 800°C. The electronic component contribution to the total electrical conductivity sharply grows with temperature due to the appearance of metallic conductivity.

Composition	La0.6Sr0.4CoO3		La0.7Sr0.3CoO3		La0.6Sr0.4NiO3		La0.7Sr0.3NiO3
T, °C	ti	te	ti	te	ti	te	ti	te
500	0.14	0.86	0.16	0.84	0.13	0.87	0.10	0.90
600	0.11	0.89	0.13	0.87	0.10	0.90	0.07	0.93
700	0.08	0.92	0.09	0.91	0.07	0.93	0.03	0.97
800	0.04	0.96	0.05	0.95	0.02	0.98	0.02	0.98
Composition	La0.6Sr0.4Fe0.7Ni0.3O3				La0.7Sr0.3Fe0.7Ni0.3O3
Т, °С	ti		te		ti		te
500	0.22		0.78		0.25		0.75
600	0.18		0.82		0.20		0.80
700	0.13		0.87		0.15		0.85
800	0.06		0.94		0.08		0.92

Table 6.

Performances of electronic and ionic conductivity of La_0.6Sr_0.4CoO₃, La_0.7Sr_0.3CoO₃, La_0.6Sr_0.4NiO₃, La_0.7Sr_0.3NiO₃, La_0.6Sr_0.4Fe_0.7Ni_0.3O₃, and La_0.7Sr_0.3Fe_0.7Ni_0.3O₃ ceramics.

3. Conclusions

Finely dispersed xerogel nanopowders of the compositions (СeO₂)_1-x(Sm₂O₃)_x (x = 0.02, 0.05, 0.10), La_1-xSr_xCo(Ni)O₃, and La_1-xSr_xFe_0.7Ni_0.3O₃ (x = 0.3, 0.4) with an average crystallite size of ∼8–10 nm are obtained by co-crystallization of salts with ultrasonic treatment. Their consolidation by cold uniaxial pressing at the pressure of 150 or 100 MPa followed by sintering at 900–1300°C provided ceramic electrolyte and cathode materials. At 600–1300°C, the obtained ceramic materials are single-phase solid solutions with a fluorite type cubic structure for the СeO₂–Sm₂O₃ system, perovskite-type tetragonal, and orthorhombic structure for La₂О₃–SrO–Co(Ni)₂О₃ and La₂О₃–SrO–Fe₂O₃-Ni₂О₃ systems. The obtained ceramic materials are characterized by CSR 64–81 nm (1300°C), open porosity in the range of 2–30%, and relative density of 97–94%. Materials based on cerium oxide have the conductivity σ_700°С = 1.3·10⁻² S/cm of predominantly ionic type ion transfer numbers t_i = 0.82–0.71 in the temperature range of 300–700°С, due to the formation of mobile oxygen vacancies during the heterovalent substitution of Ce⁴⁺ for Sm³⁺. Solid solutions based on nickelate and lanthanum cobaltite have mixed electron-ionic conductivity σ_700°С = 0.80·10⁻¹ S/cm with transfer numbers t_e = 0.98–0.90, t_i = 0.02–0.10. Lanthanum nickelate features a higher electrical conductivity compared to lanthanum cobaltite; both solid solutions are characterized by an increase in electrical conductivity with the content of strontium oxide. In addition, ceramics with a perovskite-type tetragonal crystal structure show higher electrical conductivity compared to materials with an orthorhombic perovskite-type crystal structure.

The commensurability of TEC values of the resulting electrolyte material based on cerium dioxide (12.2–12.5·10⁻⁶ K⁻¹) and cathode materials based on lanthanum nickelate (12.8–14.2·10⁻⁶ K⁻¹) has been established, which makes it possible to consider this pair of ceramic materials as SOFC component.

The obtained ceramics materials according to their mechanical (open porosity, density and thermal expansion coefficient) and electrophysical properties are promising as solid oxide electrolytes and cathodes of medium-temperature fuel cells.