SrCo1−xSbxO3−δ perovskite oxides as cathode materials in solid oxide fuel cells

The SrCo_1−xSb_xO_3−δ (x = 0.05, 0.1, 0.15 and 0.2) system was tested as possible cathode for solid oxide fuel cells (SOFCs). X-ray diffraction results show the stabilization of a tetragonal P4/mmm structure with Sb contents between x = 0.05 and x = 0.15. At x = 0.2 a phase transition takes place and the material is defined in the cubic Pm-3m space group. In comparison with the undoped hexagonal SrCoO₃ phase, the obtained compounds present high thermal stability without abrupt changes in the expansion coefficient. In addition, a great enhancement of the electrical conductivity was observed at low and intermediate temperatures (T ≤ 800 °C). The sample with x = 0.05 displays the highest conductivity value that reaches 500 S cm⁻¹ at 400 °C and is over 160 S cm⁻¹ in the usual working conditions of a cathode in SOFC (650-900 °C). Moreover, the impedance spectra of the SrCo_1−xSb_xO_3−δ/Ce_0.8Nd_0.2O_2−δ/SrCo_1−xSb_xO_3−δ (x ≥ 0.05) symmetrical cells reveal polarization resistances below 0.09 Ω cm² at 750 °C which are much smaller than that displayed by the pristine SrCoO_3−δ sample. The composition with x = 0.05 shows the lowest ASR values ranging from 0.009 to 0.23 Ω cm² in the 900-600 °C temperature interval with an activation energy of 0.82 eV.     

SrCo1−yTiyO3−δ as potential cathode materials for intermediate-temperature solid oxide fuel cells

The perovskites SrCo_1−yTi_yO_3−δ (SCTy, y = 0.00-0.20) are synthesized and assessed as potential cathode materials for intermediate-temperature solid oxide fuel cells (IT-SOFCs) based on the La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ (LSGM) electrolyte. SCTy composites with y ≥ 0.05 adopt a cubic perovskite structure with thermal stability between 30 °C and 1000 °C in air. Substitution of Ti significantly enhances the electrical conductivity of the SCTy composites relative to the undoped SrCoO_3−δ. The highest electrical conductivity of the sample with y = 0.05 varied from 430 S cm⁻¹ to 160 S cm⁻¹ between 300 °C to 800 °C in air. The area-specific resistances of the SCTy cathodes on the LSGM electrolyte gradually increase from 0.084 Ω cm² at y = 0.05 to 0.091 Ω cm² at y = 0.20 with increasing Ti content at 750 °C. Single-cells that used SCTy cathodes with y = 0.05, 0.10, 0.15, and 0.20 on a 300 μm-thick LSGM electrolyte achieve peak power densities of 793, 608, 525, and 425 mW cm⁻² at 800 °C, respectively. These novel SCTy cubic perovskites demonstrate considerable potential for application in IT-SOFC cathodes.     

New SrMo1−xCrxO3−δ perovskites as anodes in solid-oxide fuel cells

Oxides of composition SrMo_1−xCr_xO_3−δ (x = 0.1, 0.2) have been prepared, characterized and tested as anode materials in single solid-oxide fuel cells, yielding output powers higher than 700 mW cm⁻² at 850 °C with pure H₂ as a fuel. All the materials are suggested to present mixed ionic–electronic conductivity (MIEC) from neutron powder diffraction (NPD) experiments, complemented with transport measurements; the presence of a Mo⁴⁺/Mo⁵⁺ mixed valence at room temperature, combined with a huge metal-like electronic conductivity, as high as 340 S cm⁻¹ at T = 50 °C for x = 0.1, could make these oxides good materials for solid-oxide fuel cells. The magnitude of the electronic conductivity decreases with increasing Cr-doping content. The reversibility of the reduction–oxidation between the oxidized Sr(Mo,Cr)O_4−δ scheelite and the reduced Sr(Mo,Cr)O₃ perovskite phases was studied by thermogravimetric analysis, which exhibit the required cyclability for fuel cells. An adequate thermal expansion coefficient, without abrupt changes, and a chemical compatibility with electrolytes make these oxides good candidates for anodes in intermediate-temperature SOFC (IT-SOFCs).     

Scandium-doped PrBaCo2−xScxO6−δ oxides as cathode material for intermediate-temperature solid oxide fuel cells

Scandium-doped PrBaCo_2−xSc_xO_6−δ(PBCS-x, x = 0.00–1.00) oxides have been evaluated as cathode materials of intermediate-temperature solid oxide fuel cells (IT-SOFCs) with respect to phase structure, oxygen content, thermal expansion behavior and electrical and electrochemical properties. The XRD results have demonstrated a phase transition in PBCS-x due to Sc³⁺ doping from tetragonal double-layered perovskite structure at x = 0.00–0.20, bi-phase mixtures at x = 0.30–0.40, to cubic perovskite structure at x = 0.50–0.90. The oxygen contents (6-δ) and average valences of cobalt ions in PBCS-x decrease with the higher Sc³⁺ content and increasing temperatures in air. Sc³⁺ doping has also led to decreased thermal expansion coefficients, lowered electrical conductivities and enhanced electrochemical reaction activities for PBCS-x characterized by decreased area-specific resistances (ASRs) and smaller reaction activation energies. Among the studied samples, the PBCS-0.50 oxide with Sc³⁺-doping content of x = 0.50 exhibits the best electrochemical performance on Ce_0.9Gd_0.1O_1.95 electrolyte. Its ASR values range from 0.123 Ω cm² at 600 °C to 0.022 Ω cm² at 750 °C, which are much lower than the related cathode materials. These results have demonstrated that the PBCS-0.50 oxide is a promising cathode material for IT-SOFCs.     

La0.9−xCaxCe0.1CrO3−δ as potential anode materials for solid oxide fuel cells

LaCrO₃ doped with calcium and cerium on the A-site in the series of La_0.9−xCa_xCe_0.1CrO_3−δ (LCCC3060, LCCC4050, LCCC5040, LCCC6030 corresponding to x = 0.6, 0.5, 0.4, and 0.3 respectively), is synthesized by a sol–gel combustion method and evaluated as anode material for solid oxide fuel cells (SOFCs). Relatively higher Ca-doping on La in LaCrO₃ is found to improve both electronic and ionic conductivity. LCCC compositions have demonstrated good chemical stability in reducing atmospheres. Evaluation of the LCCC material as anode in symmetrical cell configuration shows that the highest Ca-doping composition results in the lowest activation energy and the lowest polarization resistance. La_0.8Sr_0.2Ga_0.83Mg_0.17O_3−δ (LSGM) electrolyte-supported single cells with LCCC3060 as the anode and La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) as the cathode show that LCCC3060 can be a potential anode material for H₂, but not for CH₄.     

New Nb-doped SrCo1−xNbxO3−δ perovskites performing as cathodes in solid-oxide fuel cells

The high-temperature cubic phase of SrCoO_3−δ is a promising cathode material for solid oxide fuel cells (SOFC) due to its high electrical conductivity and oxygen permeation flux. However, this phase is not stable below 900 °C where a 3C-cubic to 2H-hexagonal phase transition takes place when the sample is slowly cooled down. In this work we have stabilized a 3C-tetragonal P4/mmm structure for SrCo_1−xNb_xO_3−δ with x = 0.05. We have followed the strategy consisting of introducing a highly-charged cation at the Co sublattice, in order to avoid the stabilization of the unwanted 2H structure containing face-sharing octahedra. The characterization of this oxide included X-ray (XRD) and neutron powder diffraction (NPD) experiments. SrCo_0.95Nb_0.05O_3−δ adopts a tetragonal superstructure of perovskite with a = a₀, c = 2a₀ (a₀ ≈ 3.9 Å) defined in the P4/mmm space group containing two inequivalent Co positions. Flattened and elongated (Co,Nb)O₆ octahedra alternate along the c axis sharing corners in a three-dimensional array (3C-like structure). In the test cell, the electrodes were supported on a 300-μm-thick pellet of the electrolyte La_0.8Sr_0.2Ga_0.83Mg_0.17O_3−δ (LSGM). The test cells gave a maximum power density of 0.4 and 0.6 W/cm² for temperatures of 800 and 850 °C, respectively, with pure H₂ as fuel and air as oxidant. The good performance of this material as a cathode is related to its mixed electronic-ionic conduction (MIEC) properties, which can be correlated to the investigated structural features: the Co³⁺/Co⁴⁺ redox energy at the top of the O-2p bands accounts for the excellent electronic conductivity, which is favored by the corner-linked perovskite network. The considerable number of oxygen vacancies, with the oxygen atoms showing high displacement factors suggests a significant ionic mobility.     

La0.8Sr0.2Mn1−xRuxO3−δ as a cathode material for solid oxide fuel cells

Oxides of La_0.8Sr_0.2Mn_1−xRu_xO_3−δ (LSMR) (x = 0, 0.25, 0.50, 0.75, or 1.0) were prepared to fabricate cathodes in solid oxide fuel cells. The crystal structure changed from trigonal (x = 0 or 0.25) to a mixture of trigonal and orthorhombic (x = 0.5) and to orthorhombic (x = 0.75 or 1.0). X-ray photoelectron spectroscopy analysis after electrochemical testing indicated that the relative concentrations of Ru⁴⁺ to Ru⁶⁺ and Mn⁴⁺ to Mn²⁺ influence the performance of a single cell. The transformation from Ru⁴⁺ to Ru⁶⁺ releases two electrons but that from Mn⁴⁺ to Mn²⁺ creates two electron holes (an oxygen vacancy). The relative concentrations in LSMR were determined using the stoichiometric ratio (x) of Ru, and then, the concentrations of electrons and electron holes for influencing the cathode electrochemical catalytic reactivity were estimated. x = 0.25 represented the better cell performance, and Ru may stabilize the LSMR grain size during electrochemical testing.     

Investigation on scandium-doped manganate La0.8Sr0.2Mn1−xScxO3−δ cathode for intermediate temperaturesolid oxide fuel cells

Scandium-doped lanthanum strontium manganate La_0.8Sr_0.2Mn_1−xSc_xO_3−δ (LSMS) combined with YSZ as composite cathode for anode-supported solid oxide fuel cell is investigated. The LSMS powders are prepared using the modified Pechini method. The XRD and H₂-TPR results reveal that non-stoichiometric defects are introduced into the perovskite lattice of LSMS samples as a result of Sc substitution, which leads to increased oxygen ion mobility in the Sc containing samples. But high level doping of Sc may results in the segregation of the Sc₂O₃ secondary phase at elevated temperature. The cells with the LSMS-containing cathodes exhibit higher performances, especially at lower temperatures, which can be ascribed to the increased oxygen anionic vacancies in the LSMS.     

Electrode redox properties of Ba1−xLaxFeO3−δ as cobalt free cathode materials for intermediate-temperature SOFCs

Cobalt-free perovskite oxides Ba_1−xLa_xFeO_3−δ (x = 0.1–0.4) were synthesized by glycine-nitrate combustion method and investigated as a candidate cathode material for intermediate temperature solid oxide fuel cells (IT-SOFCs). Cubic perovskite structure was obtained when 10–20 mol% La was substituted at Ba-site in Ba_1−xLa_xFeO_3−δ, and the crystal structure was transformed from cubic structure into orthorhombic one at x ≥ 0.2 with an addition of lanthanum doping. The thermal expansion coefficients of Ba_1−xLa_xFeO_3−δ oxides decreased gradually with La content due to increasing electrostatic attraction forces. A gradual increase existed in electrical conductivity tendency with La content due to disproportionation of Fe³⁺ and the larger extent of electron clouds. The electrode redox performance was investigated by electrochemical impedance spectroscopy. Among Ba_1−xLa_xFeO_3−δ series oxides, Ba_0.9La_0.1FeO_3−δ exhibited the best electrochemical performance. The area specific resistance (ASR) of Ba_0.9La_0.1FeO_3−δ was 0.079 Ω cm², 0.37 Ω cm², and 2.15 Ω cm² at 800, 700 and 600 °C under open circuit potential. To investigate electrochemical performances after cathodic polarization, bias potentials were employed on Ba_1−xLa_xFeO_3−δ cathode at 650–800 °C. The results demonstrated the potential applications for Ba_0.9La_0.1FeO_3−δ as cathode materials for IT-SOFCs as a tradeoff between electrochemical and thermal expansion performance.     

Preparation and properties of BaxSr1−xCoyFe1−yO3−δ cathode material for intermediate temperature solid oxide fuel cells

Ba_xSr_1−xCo_yFe_1−yO_3−δ (BSCF) materials with perovskite structure were synthesized via solid-state reaction. Their structural characteristics, electrical-conduction behavior and cathode performance were investigated. Compared to A-site elements, B-site elements show a wide solid-solution range in BSCF. The electrical-conduction behavior of BSCF obeys the small polaron-hopping mechanism. An increase of Ba or Co content in the BSCF samples results in a decrease of electrical conductivity, which is mainly attributable to the preferential existence of B³⁺ rather than B⁴⁺ in Ba- or Co-rich samples. At the same time, this leads to increases in the lattice parameter a and the number of oxygen vacancies. BSCF samples with high Ba content show a high structural stability (high oxygen-loss temperature). Ba_0.6Sr_0.4Co_0.8Fe_0.2O_3−δ and Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ materials present good thermal-cycling stability of the electrical conductivity. Compared with Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ, Ba_0.6Sr_0.4Co_0.8Fe_0.2O_3−δ exhibits a better cathode performance in a Ce_0.8Gd_0.2O_2−δ (GDC)-supported half cell. The cell performance can be improved by introducing a certain amount of GDC electrolyte into the BSCF cathode material.     

Sr1−xPrxCo0.95Sn0.05O3−δ ceramic as a cathode material for intermediate-temperature solid oxide fuel cells

In this study, the physical properties of the Sr_1−xPr_xCo_0.95Sn_0.05O_3−δ ceramics were measured and their potential for use as a cathode material of intermediate-temperature solid oxide fuel cells (IT-SOFCs) was evaluated. A cubic phase was retained in all of the Sr_1−xPr_xCo_0.95Sn_0.05O_3−δ ceramics. Analysis of the temperature-dependent conductivity found the SrCo_0.95Sn_0.05O_3−δ and Sr_0.9Pr_0.1Co_0.95Sn_0.05O_3−δ ceramics exhibiting semiconductor-like behavior below 550 °C and metal-like behavior above the same temperature. The Sr_0.8Pr_0.2Co_0.95Sn_0.05O_3−δ and Sr_0.7Pr_0.3Co_0.95Sn_0.05O_3−δ ceramics, however, reported a metal-like conductivity in the whole temperature range. The electrical conductivities of the Sr_0.8Pr_0.2Co_0.95Sn_0.05O_3−δ ceramic at 500 °C and 700 °C read respectively 1250 S/cm and 680 S/cm, both of which were superior than those in most of the common perovskites. Single cells with a structure of NiO–Sm_0.2Ce_0.8O_2−δ (SDC)/SDC/Sr_0.8Pr_0.2Co_0.95Sn_0.05O_3−δ-SDC were built and characterized. Addition of SDC in Sr_0.8Pr_0.2Co_0.95Sn_0.05O_3−δ emerged to be a crucial factor reducing the ohmic resistance (R₀) and polarization resistance (R_P) of the cell by facilitating a better adhesion to and electrical contact with the electrolyte layer. The R₀ and R_P of the cell read respectively 0.068 Ω cm² and 0.0571 Ω cm² at 700 °C and 0.298 Ω cm² and 1.310 Ω cm² at 550 °C. With no microstructure optimization and hermetic sealing of the cells, maximum power density (MPD) and open circuit voltage (OCV) reached respectively 0.872 W/cm² and 0.77 V at 700 °C and 0.482 W/cm² and 0.86 V at 550 °C. It is evident that Sr_1−xPr_xCo_0.95Sn_0.05O_3−δ is a promising cathode material for IT-SOFCs.     

A novel Ba0.6Sr0.4Co0.9Nb0.1O3−δ cathode for protonic solid-oxide fuel cells

Ba_0.6Sr_0.4Co_0.9Nb_0.1O_3−δ (BSCN), originated from SrCo_0.9Nb_0.1O_3−δ (SCN), is investigated as a cathode material in a protonic solid-oxide fuel cell (SOFC-H⁺) with a BaZr_0.1Ce_0.7Y_0.2O₃ (BZCY) electrolyte. The surface-exchange and bulk-diffusion properties, phase reaction with the electrolyte, electrochemical activity for oxygen reduction, and performance in the real fuel cell condition of SCN and BSCN electrodes are comparatively studied by conductivity relaxation, XRD, EIS and I-V polarization characterizations. Much better performance is found for BSCN than SCN. Furthermore, water has a positive effect on oxygen reduction over BSCN while it has the opposite effect with SCN. A peak power density of 630 mW cm⁻² at 700 °C is achieved for a thin-film BZCY electrolyte cell with a BSCN cathode compared to only 287 mW cm⁻² for a similar cell with an SCN cathode. The results highly recommend BSCN as a potential cathode material for protonic SOFCs.     

Electrochemical performance of nanostructured La0.6Sr0.4CoO3−δ and Sm0.5Sr0.5CoO3−δ cathodes for IT-SOFCs

The electrochemical performance of nanostructured cathodes for IT-SOFCs based on perovskite-type mixed ionic/electronic conductors (MIECs) is investigated. Different compounds (La_0.6Sr_0.4CoO_3−δ and Sm_0.5Sr_0.5CoO_3−δ) and synthesis methods (freeze-drying and citrate complexation) were evaluated. These materials exhibited excellent performance (area-specific resistance values in the range of 0.05-0.20 Ω cm² for an operating temperature of 700 °C), which improved with decreasing grain size. This performance can be attributed to the high specific surface area of these nanostructured cathodes, thus dramatically increasing the number of active sites for the oxygen reduction reaction. Under these conditions, the electrochemical properties are mainly controlled by oxide ion diffusion through the MIEC cathode, which becomes faster with decreasing grain size.     

Synthesis and properties of Sm-deficient Sm1−xBaCo2O5+δ perovskite oxides as cathode materials

Double-layered perovskite oxides of Sm_1−xBaCo₂O_5+δ (S_1−xBCO) with various A-site Sm³⁺-deficiencies (x = 0.00–0.08) were synthesized and evaluated as cathode materials of intermediate-temperature solid oxide fuel cells (IT-SOFCs). The Sm³⁺-deficiency content in S_1−xBCO was limited up to x = 0.05, and higher content x = 0.08 caused impurity phase. S_1−xBCO oxides were chemically stable with GDC electrolyte at 1050 °C and below. Introduction of Sm³⁺-deficiency caused decreased oxygen content and increased concentration of oxygen vacancy in S_1−xBCO. Electrical conductivities of S_1−xBCO decreased with increasing temperature in air, and also changed with the Sm³⁺-deficiency content. Electrochemical performance of S_1−xBCO cathodes were characterized by impedance spectra measurement based on symmetric cells. Higher Sm³⁺ deficiency content has resulted in decreased area specific resistances (ASRs) and activation energy (Ea), i.e. enhanced electrochemical reaction reactivity for the S_1−xBCO cathodes. Among the studied samples, the S_0.95BCO (x = 0.05) oxide showed the best electrochemical performance with ASR values of 0.316 Ω cm² at 600 °C, 0.137 Ω cm² at 650 °C, 0.068 Ω cm² at 700 °C and 0.038 Ω cm² at 750 °C respectively, thus it's a promising cathode material of IT-SOFCs.     

Synthesis and electrochemical properties of (Pr–Nd)1−ySryMnO3−δ and (Pr1−xNdx)0.7Sr0.3MnO3−δ as cathode materials for IT-SOFC

(Pr–Nd)_1−ySr_yMnO_3−δ (P-NSM, y = 0.2, 0.25, 0.3, 0.35) powders made from commercial Pr–Nd mixed oxide, as well as (Pr_1−xNd_x)_0.7Sr_0.3MnO_3−δ (PN3SM, x = 0, 0.5, 0.7, 1) were synthesized by a glycine-nitrate process and characterized as cathode materials for intermediate temperature solid oxide fuel cell (IT-SOFC). XRD patterns showed the powders had formed pure perovskite phase after being calcined at 800 °C for 2 h. (Pr–Nd)_0.7Sr_0.3MnO_3−δ (P-N3SM) achieved a high conductivity of 194 S cm⁻¹ at 500 °C and showed a good chemical stability against YSZ at 1150 °C. And the thermal expansion coefficient of P-N3SM/YSZ cathode was 11.1 × 10⁻⁶ K⁻¹, which well matched YSZ electrolyte film. The tubular SOFC with P-N3SM/YSZ cathode exhibited the maximum power densities of 415, 367, 327 and 282 mW cm⁻² at 850, 800, 750 and 700 °C, respectively, which indicated P-N3SM was potentially applied in SOFC for low cost.     

Electrochemical performance of Pr1−xYxBaCo2O5+δ layered perovskites as cathode materials for intermediate-temperature solid oxide fuel cells

Pr_1−xY_xBaCo₂O_5+δ (x = 0.3, 0.5 and 0.7) oxides were prepared and evaluated as cathode materials for intermediate-temperature solid oxide fuel cells. The effect of Y-doping on the crystal structure, oxygen vacancy concentration, thermal expansion coefficient (TEC), electrical conductivity and cathode performance of Pr_1−xY_xBaCo₂O_5+δ was investigated. These properties were compared with that of GdBaCo₂O_5+δ having a middle element of lanthanides. Pr_1−xY_xBaCo₂O_5+δ shows TEC (∼17.6 × 10⁻⁶ K⁻¹) lower than that of undoped PrBaCo₂O_5+δ, but similar to the one for GdBaCo₂O_5+δ. Y-doping causes a decrease in electrical conductivity, but at the same time induces an increase in oxygen vacancy concentration. With increasing Y-doping level, the area specific resistance (ASR) of Pr_1−xY_xBaCo₂O_5+δ-based electrode in a symmetrical cell increases, and correspondingly, the peak power density of single-cell decreases slightly. Nevertheless, comparing to GdBaCo₂O_5+δ-based electrode, Pr_1−xY_xBaCo₂O_5+δ (x = 0.3–0.7) exhibits significantly lower ASR, and allows to obtain cells with higher maximum power density.     

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.     

La2−xSrxCoO4−δ (x = 0.9, 1.0, 1.1) Ruddlesden-Popper-type layered cobaltites as cathode materials for IT-SOFC application

La_2−xSr_xCoO_4−δ (x = 0.9, 1.0, 1.1) compounds with Ruddlesden-Popper K₂NiF₄-type structure have been investigated as potential cathode materials for IT-SOFC application. Materials have been prepared by citrate-nitrate combustion method. Structural evolution analysed by XRD shows a shortened Co–O–Co bond length within the perovskite layer as Sr substitution increases, while the interlayer distance at the same time increases. An oxygen stoichiometry close to 4 has been found for all compositions at room temperature. Thermal expansion coefficients have been obtained from temperature-dependent XRD analysis and show large values (>20 × 10⁻⁶ K⁻¹) compared to the currently utilized electrolyte materials. Electrochemical characterisation has been performed by means of impedance spectroscopy on symmetric cells with CGO electrolyte. Pure electrodes have a high Area Specific Resistance, probably due to limited oxygen ion diffusion. By using composite electrode (50 wt.% CGO), an Area Specific Resistance of 0.25 Ω cm² is obtained at approximately 700 °C for all the three compounds suggesting promising electrochemical properties for IT-SOFCs.     

Investigation of Sm0.5Sr0.5CoO3−δ/Co3O4 composite cathode for intermediate-temperature solid oxide fuel cells

The electrochemical properties of an Sm_0.5Sr_0.5CoO_3−δ/Co₃O₄ (SSC/Co₃O₄) composite cathode were investigated as a function of the cathode-firing temperature, SSC/Co₃O₄ composition, oxygen partial pressure and CO₂ treatment. The results showed that the composite cathodes had an optimal microstructure at a firing temperature of about 1100 °C, while the optimum Co₃O₄ content in the composite cathode was about 40 wt.%. A single cell with this optimized C₄₀-1100 cathode exhibited a very low polarization resistance of 0.058 Ω cm², and yielded a maximum power density of 1092 mW cm⁻² with humidified hydrogen fuel and air oxidant at 600 °C. The maximum power density reached 1452 mW cm⁻² when pure oxygen was used as the oxidant for a cell with a C₃₀-1100 cathode operating at 600 °C due to the enhanced open-circuit voltage and accelerated oxygen surface-exchange rate. X-ray diffraction and thermogravimetric analyses, as well as the electrochemical properties of a CO₂-treated cathode, also implied promising applications of such highly efficient SSC/Co₃O₄ composite cathodes in single-chamber fuel cells with direct hydrocarbon fuels operating at temperatures below 500 °C.     

Performance of double-perovskite Sr2−xSmxMgMoO6−δ as solid-oxide fuel-cell anodes

Double-perovskite Sr_2−xSm_xMgMoO_6−δ (SSMM, 0 ≤ x ≤ 0.8) is investigated as a possible anode material for solid-oxide fuel cells on La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ (LSGM) electrolytes. Single-phase SSMM samples with 0 ≤ x ≤ 0.4 are prepared. At x ≥ 0.6, a small amount of SrMoO₄ and Sm₂O₃ impurities are observed. The Mg/Mo ordering in SSMM decreases with increasing Sm content. Substitution of Sm for Sr significantly improves the electrical conductivity of SSMM. At x = 0.6, the sample yields the highest conductivity, with values reaching 16 S cm⁻¹ in H₂ at 800 °C. The maximum power densities of single cells achieved with x = 0.0, 0.2, 0.4, 0.6, and 0.8 anodes on a 300 μm-thick LSGM electrolyte are 693, 770, 860, 907, and 672 mW cm⁻², respectively, in H₂ at 850 °C. The SSMM sample with x = 0.4 is considered as the best anode candidate because of the impurity formation seen in x ≥ 0.6 samples. The x = 0.4 sample not only has a thermal-expansion coefficient closer to that of the LSGM electrolyte but also exhibits good electrochemical performance and stability in commercial city gas containing H₂S, where the maximum power density achieved is 726 mW cm⁻² at 850 °C.