Enhanced sinterability of BaZr0.1Ce0.7Y0.1Yb0.1O3−δ by addition of nickel oxide

The effect of nickel oxide addition on the sintering behavior and electrical properties of BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ (BZCYYb) as an electrolyte for solid oxide fuel cells was systematically studied. Results suggest that the addition of a small amount (∼1 wt%) of NiO to BZCYYb greatly promoted densification, achieving ∼96% of the theoretical density after sintering at 1350 °C in air for 3 h (reducing the sintering temperature by ∼200 °C). Further, a sample sintered at 1450 °C for 3 h showed high open circuit voltages (OCVs) when used as the electrolyte membrane to separate the two electrodes under typical SOFC operating conditions, indicating that the electrical conductivity of the electrical conductivity of the BZCYYb was not adversely affected by the addition of ∼1 wt% NiO.     

Electrochemical behaviors of Y-doped La0.7Sr0.3CrO3−δ anode in sulfur-containing fuel SOFC

Fuel gas containing sulfur to feed solid oxide fuel cell is a challenge for extending the application of SOFC. Yttrium doped into La_xSr_1−xCrO₃ as potential anode tolerant to H₂S was investigated by XRD, XPS and electrochemical impedance spectra (EIS). Good sinter characteristic for (La,Y)_0.7Sr_0.3CrO_3−δ (LYSC) observed by SEM contributes to the low ohmic loss (high conductivity) in SOFC fueled by H₂(3%)–H₂S(1%). Maximum power density of 20 mW/cm² and open circuit voltage of 0.95 V for SOFC with LYSC can be obtained at 700 °C. The results by EIS indicate charge transfer loss in polarization resistance dominates in the total resistance, especially lower than 650 °C. Compared to ohmic loss, polarization resistance in LYSC is still the main cause to hinder the improvement of SOFC performance. Thus, LYSC with doped non-variant valence Y maintains good sulfur tolerance determined by XPS without improved electro-catalytic activity as EIS suggest.     

Properties and performance of Ba0.5Sr0.5Co0.8Fe0.2O3−δ + Sm0.2Ce0.8O1.9 composite cathode

The properties and performance of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) + Sm_0.2Ce_0.8O_1.9 (SDC) (70:30 in weight ratio) composite cathode for intermediate-temperature solid-oxide fuel cells were investigated. Mechanical mixing of BSCF with SDC resulted in the adhesion of fine SDC particles to the surface of coarse BSCF grains. XRD, SEM-EDX and O₂-TPD results demonstrated that the phase reaction between BSCF and SDC was negligible, constricted only at the BSCF and SDC interface, and throughout the entire cathode with the formation of new (Ba,Sr,Sm,Ce)(Co,Fe)O_3−δ perovskite phase at a firing temperature of 900, 1000, and ≥ 1050 °C, respectively. The BSCF + SDC electrode sintered at 1000 °C showed an area specific resistance of ∼0.064 Ω cm² at 600 °C, which is a slight improvement over the BSCF (0.099 Ω cm²) owing to the enlarged cathode surface area contributed from the fine SDC particles. A peak power density of 1050 and ∼382 mW cm⁻² was reached at 600 and 500 °C, respectively, for a thin-film electrolyte cell with the BSCF + SDC cathode fired from 1000 °C.     

Preparation and characterization of La0.9Sr0.1Ga0.8Mg0.2O3 − δ thin film on the porous cathode for SOFC

A dense and crack-free La_0.9Sr_0.1Ga_0.8Mg_0.2O_3 − δ thin film has been prepared by RF magnetron sputtering. The XRD, FESEM, XPS and four-probe technique are employed to characterize the La_0.9Sr_0.1Ga_0.8Mg_0.2O_3 − δ film. Results show that after annealing at 1000 °C, the La_0.9Sr_0.1Ga_0.8Mg_0.2O_3 − δ film presents a polycrystalline perovskite structure with grain size of 100–300 nm. XPS data show that both La and Ga are in their +3 state. Sr element has two chemical states which are related to Sr²⁺ in the perovskite lattice and SrO_1 − δ suboxide. The O 1s spectrum also shows two chemical states which can be assigned to molecularly adsorbed O₂ species and O²⁻ in the lattice. The electrical conductivity reaches to 0.093 S cm⁻¹ at 800 °C. The microstructure and conductivity analysis indicates that the La_0.9Sr_0.1Ga_0.8Mg_0.2O_3 − δ thin film prepared by RF magnetron sputtering is suitable for intermediate temperature Solid oxide fuel cell.     

Fabrication and evaluation of the electrochemical performance of the anode-supported solid oxide fuel cell with the composite cathode of La0.8Sr0.2MnO3−δ-Gadolinia-doped ceria oxide/La0.8Sr0.2MnO3−δ

The anode-supported single cell was constructed with porous Ni-Yittria-stabilized zirconia (YSZ) as the anode substrate, an airtight YSZ as the electrolyte, and a screen-printed La_0.8Sr_0.2MnO_3−δ (LSM)-Gadolinia-doped ceria (GDC)/LSM double-layer cathode. The SEM results show that the YSZ thin film is highly integrated, fully dense with a thickness of 13 μm, and exhibits excellent compatibility between cathode and electrolyte layers. The effects of feed rates of the reactants, temperature, and contact pressure between the current collector and the unit cell were systematically investigated. The results are based on the assumption that the anode contribution to the polarization resistance is negligible. Our analysis showed that the electrochemical reaction is limited by mass transfer control when the airflow rate is decreased to 500 ml min⁻¹. The maximum power density is 204.6 mW cm⁻² at 800 °C with H₂ and air at flow rates of 800 and 2000 ml min⁻¹, respectively. According to the AC-impedance data, the resistances of charge transfer at the electrode/electrolyte interface are 3.79 and 1.90 Ω cm². The resistances of oxygen-reduction processes are 3.63 and 1.01 Ω cm² at 700 and 800 °C, respectively. The results from the sensitivity analysis of the variation of contact pressure between current collectors and membrane electrode assembly (MEA) show that the influence is enhanced at the regions of the high current density.     

Synthesis and properties of La0.6Sr0.4CoO3−δ nanopowder

Uniform nanopowders of La_0.6Sr_0.4CoO_3−δ (LSC) were synthesized by the combined citrate–EDTA method. The precursor solution was prepared from nitrates of the constituent metal ion, citric acid and EDTA with a pH value controlled by ammonia. The obtained product was characterized by TG/DTA, XRD, SEM, and BET measurements. The single perovskite phase could form completely after sintering at the temperature of 900 °C. There was no significant effect of the precursor solution pH value on the perovskite phase formation temperature; however, LSC powders prepared from the precursors with different pH values showed specific shapes. The morphology of La_0.6Sr_0.4CoO_3−δ powder was also optimized with proper surfactant addition. The sintered La_0.6Sr_0.4CoO_3−δ bulk samples exhibited an electrical conductivity of 1867 S cm⁻¹ in air at 800 °C. The impedance spectra of a symmetric LSC cathode on a GDC electrolyte substrate were measured and polarization resistance (R_p) values of 0.17 Ω cm² at 700 °C and 0.07 Ω cm² at 750 °C in air were obtained.     

Oxidation resistance of Ni/La0.7Sr0.3AlO3−δ catalyst for steam reforming of model aromatic hydrocarbon

Oxidative resistance of Ni catalysts supported on various oxides La_0.7Sr_0.3AlO_3−δ, LaAlO₃, and α-Al₂O₃ were investigated for hydrogen production by steam reforming of model aromatic hydrocarbons. Ni/α-Al₂O₃ lost its steam reforming activity by oxidation treatment. In contrast, Ni/La_0.7Sr_0.3AlO_3−δ and Ni/LaAlO₃ catalysts showed steam reforming activity even after the oxidation treatment. The XANES (X-ray absorption near-edge structure) spectra at Ni K-edge for Ni/La_0.7Sr_0.3AlO_3−δ and Ni/α-Al₂O₃ after oxidation treatment revealed that the supported Ni on La_0.7Sr_0.3AlO_3−δ and α-Al₂O₃ were oxidized completely. Although the mean particle size of Ni on Ni/α-Al₂O₃ increased by oxidation treatment or reduction treatment, Ni particles on Ni/La_0.7Sr_0.3AlO_3−δ retained the fine structure after oxidation treatment or reduction treatment. Moreover, TPR (temperature programmed reduction) and XPS (X-ray photoelectron spectroscopy) measurements for elucidating the reducibility of Ni/La_0.7Sr_0.3AlO_3−δ showed that the supported Ni on La_0.7Sr_0.3AlO_3−δ was easily reduced even after the oxidation treatment.     

Electrical conductivity of cobalt doped La0.8Sr0.2Ga0.8Mg0.2O3−δ

La_0.8Sr_0.2Ga_0.8Mg_0.2O_3−δ (LSGM8282), La_0.8Sr_0.2Ga_0.8Mg_0.15Co_0.05O_3−δ (LSGMC5) and La_0.8Sr_0.2Ga_0.8Mg_0.115Co_0.085O_3−δ (LSGMC8.5) were prepared using a conventional solid-state reaction. Electrical conductivities and electronic conductivities of the samples were measured using four-probe impedance spectrometry, four-probe dc polarization and Hebb–Wagner polarization within the temperature range of 973–1173 K. The electrical conductivities in LSGMC5 and LSGMC8.5 increased with decreasing oxygen partial pressures especially in the high (>10⁻⁵ atm) and low oxygen partial pressure regions (<10⁻¹⁵ atm). However, the electrical conductivity in LSGM8282 had no dependency on the oxygen partial pressure. At temperatures higher than 1073 K, PO₂$P_{{\text{O}}_2}$ dependencies of the free electron conductivities in LSGM8282, LSGMC5 and LSGMC8.5 were about −1/4, and PO₂$P_{{\text{O}}_2}$ dependencies of the electron hole conductivities were about 0.25, 0.12 and 0.07, respectively. Oxygen ion conductivities in LSGMC5 and LSGMC8.5 increased with decreasing oxygen partial pressures especially in the high and low oxygen partial pressure regions, which was due to the increase in the concentration of oxygen vacancies. The change in the concentration of oxygen vacancies and the valence of cobalt with oxygen partial pressure were determined using a thermo-gravimetric technique. Both the electronic conductivity and oxygen ion conductivity in cobalt doped lanthanum gallate samples increased with increasing concentration of cobalt, suggesting that the concentration of cobalt should be optimized carefully to maintain a high electrical conductivity and close to 1 oxygen ion transference number.     

Characteristics of nano La0.6Sr0.4Co0.2Fe0.8O3−δ-infiltrated La0.8Sr0.2Ga0.8Mg0.2O3−δ scaffold cathode for enhanced oxygen reduction

The chemical compatibility and electrochemical properties of nanoLa_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF)-infiltrated La_0.8Sr_0.2Ga_0.8Mg_0.2O_3−δ (LSGM) scaffold were manufactured and assessed for the application as a solid oxide fuel cell cathode with an LSGM electrolyte. When the LSCF and LSGM powder mixture was fired above 950 °C, the characteristic peaks of the two materials merged and an insulation peak (derived from LaSrGaO₄) was observed. To prevent reactions between LSCF and LSGM, an infiltration technique was utilized with the LSGM as a scaffold. Using this infiltration technique, nano LSCF particles (approximately 100 nm) can be uniformly coated on the LSGM scaffold surface. Good nano particle adhesion was observed at the LSGM/LSCF interface, even at relatively low firing temperatures (850 °C). The cathode polarization resistance (R_p) of the nano LSCF infiltrated LSGM scaffold cathode was lower than that of a conventional LSCF cathode. The improvement in performance of the nano LSCF-infiltrated cathode was attributed to an increase in the number of triple phase boundaries (TPB) as a result of the nano LSCF coating. In addition, the oxygen reduction reaction (ORR) paths were extended from the TPBs to the LSCF surface because LSCF particles are considerably smaller than the LSCF oxygen ion penetration depth (3–4 μm) over the temperature range of 700 °C–800 °C.     

Evaluation of A-site cation-deficient (Ba0.5Sr0.5)1−xCo0.8Fe0.2O3−δ (x > 0) perovskite as a solid-oxide fuel cell cathode

A-site cation-deficient (Ba_0.5Sr_0.5)_1−xCo_0.8Fe_0.2O_3−δ ((BS)_1−xCF) oxides were synthesized and evaluated as cathode materials for intermediate-temperature solid-oxide fuel cells (ITSOFCs). The material's thermal expansion coefficient, electrical conductivity, oxygen desorption property, and electrocatalytic activity were measured. A decrease in both the electronic conductivity and the thermal expansion coefficient was observed for increasing values of the stoichiometric coefficient, x. This effect was attributed to the creation of additional oxygen vacancies, the suppression of variation in the oxidation states of cobalt and iron, and the suppression of the spin-state transitions of cobalt ions. The increase in A-site cation deficiency resulted in a steady increase in cathode polarization resistance, because impurities formed at the cathode/electrolyte interface, reducing the electronic conductivity. A single SOFC equipped with a BS_0.97CF cathode exhibited peak power densities of 694 and 893 mW cm⁻² at 600 and 650 °C, respectively, and these results were comparable with those obtained with a Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ cathode. Slightly A-site cation-deficient (BS)_1−xCF oxides were still highly promising cathodes for reduced temperature SOFCs.     

Desirable performance of intermediate-temperature solid oxide fuel cell with an anode-supported La0.9Sr0.1Ga0.8Mg0.2O3 − δ electrolyte membrane

An anode-supported La_0.9Sr_0.1Ga_0.8Mg_0.2O_3 − δ (LSGM) electrolyte membrane is successfully fabricated by simple, cost-effective spin coating process. Nano-sized NiO and Ce_0.8Gd_0.2O_3 − α (GDC) powders derived from precipitation and citric-nitrate process, respectively, are used for anode support. The dense and uniform LSGM membrane of ca. 50 μm in thickness is obtained by sintering at relatively low temperature 1300 °C for 5 h. A single cell based on the as-prepared LSGM electrolyte membrane exhibits desirable high cell performance and generates high output power densities of 760 mW cm⁻² at 700 °C and 257 mW cm⁻² at 600 °C, respectively, when operated with humidified hydrogen as the fuel and air as the oxidant. The single cell is characterized by field-emission scanning electron microscope (FESEM), X-ray diffraction (XRD) and electrochemical AC impedance. The results demonstrate that it is feasible to fabricate dense LSGM membrane for solid oxide fuel cell by this simple, cost-effective and efficient process. In addition, optimized anode microstructure significantly reduces polarization resistance (0.025 Ω cm² at 700 °C).     

Cathode reaction mechanism of porous-structured Sm0.5Sr0.5CoO3−δ and Sm0.5Sr0.5CoO3−δ/Sm0.2Ce0.8O1.9 for solid oxide fuel cells

The cathode reaction mechanism of porous Sm_0.5Sr_0.5CoO_3−δ, a mixed ionic and electronic conductor (MIEC), is studied through a comparison with the composite cathode Sm_0.5Sr_0.5CoO_3−δ/Sm_0.2Ce_0.8O_1.9. First, the cathodic behaviour of porous Sm_0.5Sr_0.5CoO_3−δ and Sm_0.5Sr_0.5CoO_3−δ/Sm_0.2Ce_0.8O_1.9 are observed for micro-structure and impedance spectra according to Sm_0.2Ce_0.8O_1.9 addition, thermal cycling and long-term properties. The cathode reaction mechanism is discussed in terms of frequency response, activation energy, reaction order and electrode resistance for different oxygen partial pressures p(O₂) at various temperatures. Three elementary steps are considered to be involved in the cathodic reaction: (i) oxygen ion transfer at the cathode-electrolyte interface; (ii) oxygen ion conduction in the bulk cathode; (iii) gas phase diffusion of oxygen. A reaction model based on the empirical equivalent circuit is introduced and analyzed using the impedance spectra. The electrode resistance at high frequency (R_c,HF) in the impedance spectra represents reaction steps (i), due to its fast reaction rate. The electrode resistance at high frequency is independent of p(O₂) at a constant temperature because the semicircle of R_c,HF in the complex plane of the impedance spectra is held constant for different values of p(O₂). Reaction steps (ii) and (iii) are the dominant processes for a MIEC cathode, according to the analysis results. The proposed cathode reaction model and results for a solid oxide fuel cell (SOFC) well describe a MIEC cathode with high ionic conductivity, and assist the understanding of the MIEC cathode reaction mechanism.     

Electrical conductivity and electrochemical performance of cobalt-doped BaZr0.1Ce0.7Y0.2O3−δ cathode

To develop efficient cathode materials for solid oxide fuel cells (SOFCs) based on Ba(Zr_0.1Ce_0.7Y_0.2)O_3−δ (BZCY) electrolyte, we have examined a series of cobalt-doped BZCY samples with the intended composition of BaZr_0.1Ce_0.7Y_0.2−xCo_xO_3−δ (where x = 0, 0.02, 0.05, 0.075, 0.1, 0.2). It is found that the solubility of cobalt is less than 10 mol% and the electrical conductivity of BaZr_0.1Ce_0.7Y_0.2−xCo_xO_3−δ decreased with the content of cobalt within this solubility. When the content of cobalt is greater than its solubility, a BaCoO₃-based phase forms, which markedly increases the conductivity of the sample (e.g., 2.48 S/cm for a composite material with an intended composition of BaZr_0.1Ce_0.7Co_0.2O_3−δ at 700 ^°C). Typical cells with the cobalt-doped BZCY cathode display much-improved performance than cells with other transition metal doped barium cerate ever reported, yielding a polarization resistance of 0.085 Ω cm² at 750 ^°C.     

Conductivity study of dense BaCexZr(0.9−x)Y0.1O(3−δ) prepared by solid state reactive sintering at 1500 °C

A cost and time effective process was used to prepare the solid solutions BaCe_xZr_(0.9−x)Y_0.1O_(3−δ) (0 ≤ x ≤ 0.4). 98% dense samples were obtained by solid state reactive sintering at 1500 °C for 4 h, with the addition of 1 wt% of NiO to the quantity of synthesized/sintered compound. Scanning electron micrographs reveal polygonal grains of 1–5 microns, whose size increases from the compound with no cerium (BCZY09) to the samples containing cerium (BCZY18–BCZY45). The conductivity, measured in wet reducing atmosphere (9% H₂ in N₂, p(H₂O) = 0.015 atm) by impedance spectroscopy, increases with the cerium content. Some samples have also been prepared using barium sulfate (BaSO₄) as barium precursor (instead of barium carbonate BaCO₃) due to its non toxicity. The corresponding samples (prepared at 1575 °C) showed similar properties as the ones prepared with barium carbonate. Furthermore, different geometries (rods, tubes, pellets) could be made.     

Effect of iron substitution content on structure, thermal expansion behavior and electrochemical properties of La0.5Ba0.5Co1−yFeyO3−δ

Cubic perovskite oxides La_0.5Ba_0.5Co_1−yFe_yO_3−δ (LBCF-y) with Fe content y = 0.1–0.7 were synthesized by sol-gel method and were studied with respect to their oxygen content, defect chemistry, thermal expansion behavior, and electrical and electrochemical properties. LBCF-y had increased oxygen content and cell volumes with higher Fe content. The chemical defects at B-sites of LBCF-y are Fe⁴⁺, Co⁴⁺ and Co³⁺ ions for y = 0.1–0.3, and Fe⁴⁺, Fe³⁺ and Co³⁺ ions for y ≥ 0.5 respectively. Thermal expansion coefficients of LBCF-y oxides firstly increased to a maximum at y = 0.3 then decreased gradually with bigger y. Conductivities of LBCF-y decreased with higher Fe content, and the maximum conductivity was 800 S cm⁻¹ at 500 K for LBCF-0.1 sample. Fe substitutions for Co in LBCF-y cathodes increase the high-frequency resistance associated with oxygen ionic diffusion process while hardly influence the low-frequency gas diffusion process. Very low area-specific resistances, <0.1 Ω cm² at 923 K, were obtained for LBCF-y (y = 0.1–0.7) oxides, demonstrating their potential applications as cathode materials for intermediate-temperature solid oxide fuel cells.     

Performance of SrSc0.2Co0.8O3−δ + Sm0.5Sr0.5CoO3−δ mixed-conducting composite electrodes for oxygen reduction at intermediate temperatures

In order to improve the electrical conductivity of the SrSc_0.2Co_0.8O_3−δ (SrScCo) electrode, a composite of 70 wt% SrSc_0.2Co_0.8O_3−δ and 30 wt% Sm_0.5Sr_0.5CoO_3−δ (SrScCo + SmSrCo) was prepared and investigated for electrochemical oxygen reduction at intermediate temperatures. The phase reaction between SrScCo and SmSrCo and its effect on the electrical conductivity, oxygen vacancy concentration and oxygen mobility were examined by XRD, 4-probe DC conductivity measurement, iodometry titration and O₂-TPD experiment, respectively. The results showed that the composite reached a maximum conductivity around 123 S cm⁻¹ at 600 °C, nearly five times that of SrScCo. AC impedance results showed that the electron charge-transfer process was greatly improved by forming the composite electrode, while the oxygen-ion charge-transfer process was somewhat deteriorated. By firing at 1000 °C for 2 h, a SOFC with the SrScCo + SmSrCo cathode and thin-film SDC electrolyte delivered peak power densities of 1100 and 366 mW cm⁻² at 600 and 500 °C, respectively, which were only modestly lower than those of a similar cell with a pure SrScCo cathode.     

Effects of bi-layer La0.6Sr0.4Co0.2Fe0.8O3−δ-based cathodes on characteristics of intermediate temperature solid oxide fuel cells

In this study, anode-supported planar IT-SOFCs, with a thin Sm_0.2Ce_0.8O_2−δ (SDC) electrolyte film and a bi-layer cathode, are fabricated using tape-casting and screen-printing processes. The bi-layer cathode consists of a current collector La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) layer and a functional LSCF-SDC composite layer in various thicknesses. Microstructure studies reveal that the interfaces among various layers show good adhesion, except for Cell A equipped with a cathode of pure LSCF. Cell A reports the lowest ohmic (R₀) and polarization (R_P) resistances. R_P, which increases with the thickness of the LSCF-SDC composite layer in the cathode, rises rapidly as the temperature drops, particularly at temperatures ≤550 °C. This indicates the high electrical conductivity of the cathode as a major contribution to the decrease of R_P at 500 °C. The best cell performances are observed at 650 °C for all cases, in which Cell A shows a maximum power density of 1.51 W cm⁻² and an open circuit voltage of 0.80 V. Considering both of the electrical and the mechanical integrity of the single cell, insertion of the composite layer is required to guarantee a good adhesion of cathode layer to electrolyte layer. However, the thickness of the composite layer should be retained as thin as possible to minimize the R₀ and R_P and maximize the cell performance.     

Microstructural characterization and electrochemical properties of Ba0.5Sr0.5Co0.8Fe0.2O3−δ and its application for anode of SOEC

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) was synthesized successfully by a novel citric acid–nitrate combustion method and employed as the anode of solid oxide electrolysis cells (SOEC) for hydrogen production for the first time in this paper. The crystal structure, chemical composition and electrochemical properties of BSCF were investigated in detail. The results showed that BSCF is in good stoichiometry of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−σ formation. ASR of BSCF/YSZ is only 0.077 Ω cm² at 850 °C, remarkably lower than the commonly used oxygen materials LSM as well as the current focus materials LSC and LSCF. Also, BSCF electrode exhibited much better performance than LSM under both SOEC and SOFC operating modes. The hydrogen production rate of BSCF/YSZ/Ni-YSZ can be up to 147.2 mL cm⁻² h⁻¹, about three times higher than that of LSM/YSZ/Ni-YSZ, which indicates that BSCF could be a very promising candidate for the practical application of SOEC technology.     

Characterization of perovskite-type cathode,La0.75Sr0.25 Mn0.95−xCox Ni0.05O3+δ (0.1 ≤ x ≤ 0.3), for intermediate-temperature solid oxide fuel cells

Phase evolution, structure, thermal property, morphology, electrical property and reactivity of a perovskite-type cathode system, La_0.75Sr_0.25 Mn_0.95−xCo_xNi_0.05O_3+δ (0.1 ≤ x ≤ 0.3), are reported. The samples are synthesized using metal acetates by the Pechini method. A perovskite-type phase is formed after calcination at ∼700 °C and a rhombohedral symmetry of R – 3c space group is stabilized at ∼1100 °C. An increase in x decreases the unit cell volume linearly, accompanying with a linear decrease in bond lengths and tilt angle. The differential thermal analysis suggests the phase stabilization for a temperature range, 50–1100 °C. The thermo-gravimetric, thermal expansion, and electrical and ionic conductivities studies suggest presence of a Jahn–Teller transition at ∼260–290 °C. The samples with x = 0.1 mol exhibit electrical conductivity of ∼55 S cm⁻¹ at ∼600 °C, activation energy of ∼0.13 eV, coefficient of thermal expansion of ∼12 × 10⁶ °C⁻¹, crystallite size of ∼45 nm, Brunauer–Emmett–Teller (BET) surface area of 1.26 m² g⁻¹ and average particle size of ∼0.9 μm. A fairly high ionic conductivity, 5–9 × 10⁻² S cm⁻¹ makes the sample with x = 0.1 mole suitable for intermediate-temperature solid oxide fuel cell cathode applications. The experimental results are discussed with the help of the defect models proposed for La_1−xSr_xMnO_3+δ.     

Characteristics of Ba0.5Sr0.5Co0.8Fe0.2O3−δ–La0.9Sr0.1Ga0.8Mg0.2O3−δ composite cathode for solid oxide fuel cell

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ–La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ composite cathodes are prepared successfully using combustion synthesis method. Microstructure, chemical compatibility and electrochemical performance have been investigated and analyzed in detail. SEM micrographs show that a structure with porosity and well-necked particles forms after sintering at 1000 °C in the composites. Grain growth is suppressed by addition of La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ phase and grain sizes decrease with increasing weight percent of La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ phase in the composites. Phase analysis demonstrates that chemical compatibility between Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ and La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ is excellent when the weight percent of La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ in the composite is not more than 40%. Through fitting ac impedance spectra, it is found that the ohmic resistance and polarization resistance decrease with increasing La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ content. The polarization resistance reaches a minimum at about 30 and 40 wt.% La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ in the composite.