Redox behaviour,chemical compatibility and electrochemical performance of Sr2MgMoO6 − δ as SOFC anode

The double perovskite Sr₂MgMoO_6 − δ (SMM) has been proposed as a potential anode material for direct hydrocarbon oxidation in solid oxide fuel cells (SOFCs). The oxygen nonstoichiometry and electrical conductivity dependence of Sr₂MgMoO_6 − δ have been determined as a function of the oxygen partial pressure by coulometric titration and impedance spectroscopy techniques. The chemical compatibility of Sr₂MgMoO_6 − δ with most of the typical electrolytes commonly used in SOFCs i.e. La_0.8Sr_0.2Ga_0.8Mg_0.2O_3 − δ (LSGM), Ce_0.8Gd_0.2O_2 − δ (CGO) and Zr_0.84Y_0.16O_2 − δ (YSZ), was investigated. Reactivity between SMM and all these electrolytes has been found above 1000 °C, although the reaction is most severe with ZrO₂-based electrolytes. Area-specific polarisation resistance of the SMM/LSGM/SMM symmetrical cells indicates that the polarisation resistance increases with the firing temperature of the electrodes due to chemical interaction between LSGM and SMM layers. A CGO buffer layer between the anode and electrolyte was also used to prevent an excessive interdiffusion of ionic species between these components, resulting in better performance. Power densities of 330 and 270 mW cm^− 2 were reached at 800 °C for SMM/CGO/LSGM/LSCF and SMM/LSGM/LSCF electrolyte-supported cells, respectively; with 600-μm-thick LSGM electrolyte, using humidified H₂ as fuel and air as oxidant. XPS and XRPD studies on SMM powders annealed in air and diluted CH₄ atmospheres showed that the surface of SMM powders is mainly formed by SrMoO₄ and metal carbonates.     

Synthesis and investigations on the stability of La0.8Sr0.2CuO2.4+δ at high temperatures

For application in solid oxide fuel cells La_0.8Sr_0.2CuO_2.4+δ was synthesized and the phase evolution was characterized after quenching from different temperatures and after slow cooling. A single phase perovskite was found after quenching from 950 °C. The electrical conductivity of the La_0.8Sr_0.2CuO_2.4+δ perovskite exhibited metallic behavior reaching values of about 270 S/cm at 800 °C in air. The thermal expansion between 30 and 800 °C gave a thermal expansion coefficient of 11.1 × 10^− 6 K^− 1.At higher temperatures, the perovskite was transformed to the K₂NiF₄-type structure via an intermediate stage that can be best described as a LaSrCuO₄ phase with preferential growing of {020} lattice planes. After sintering at 1100 °C and slow cooling in the furnace a phase mixture of (La,Sr)CuO_4+δ and (La,Sr)CuO_2.4+δ perovskite was obtained. This phase mixture showed higher electrical conductivity (400 S/cm at 800 °C) and smaller thermal expansion coefficient (9.6 × 10^− 6 K^− 1) than the single phase La_0.8Sr_0.2CuO_2.4+δ perovskite.     

Determination of the ionic conductivity of Sr-doped lanthanum manganite by modified Hebb–Wagner technique

The Hebb–Wagner polarization method with the electron blocking electrode has been discussed in this paper in aim to determine a partial ionic conductivity of Sr-doped lanthanum manganite. The “limiting current” in the proposed system was measured using the two-point DC technique with additional Pt electrode between LSM and blocking electrode. The electrochemical model based on bulk diffusion processes and Boltzmann statistics has been also described. The ionic conductivity calculated with the use of proposed model for La_0.7Sr_0.3MnO_3+δ was 5.3×10⁻⁴ S cm⁻¹ at 800 °C and the activation energy of ionic conductivity was found to be (0.60±0.02) eV. This result is in agreement with previous literature reports and indicates the workability of the modified Hebb–Wagner system.     

Electrical behaviour of LSGM–LSM composite cathode materials

Composite cathode materials were prepared by mixing La_0.83Sr_0.17Ga_0.83Mg_0.17O_2.83 (LSGM) and La_0.8Sr_0.2MnO₃ (LSM) powders fired at 1300 °C. Several compositions were set up containing 1, 5, 25, 50, 75 wt.% of LSM. Their microstructure and electrical behaviour were investigated by XRPD, SEM/EDS and EIS. In composites containing 50 and 75 wt.% of LSM, the electronic contribution to conductivity is predominant, then there is only a single point at the low frequency end of the Nyquist plot. On the contrary, in the composites with up to 25 wt.% of LSM, there is a significant amount of ionic transport, then the IS spectra show complex features: at least three different arcs can be devised and their interpretation depends upon temperature. LSGM bulk and grain boundary conductivity, as well as interface polarization between the ionic (LSGM) and electronic (LSM) phases can be separated at temperatures below 600 °C; total LSGM contribution, i.e. bulk plus grain boundary, LSGM–LSM interface and electrode polarizations are attributed above 600 °C.     

Electrochemical characterization of mixed conducting and composite SOFC cathodes

The electrochemical performance of La_0.58Sr_0.4Co_0.2Fe_0.8O_3−δ (L58SCF), La_0.9Sr_1.1FeO_4−δ (LS2F) and LSM (La_0.65Sr_0.3MnO_3−δ)/LSM–YSZ (50 wt.% LSM–50 wt.% ZrO₂ (8 mol% Y₂O₃)) cathode electrodes interfaced to a double layer Ce_0.8Gd_0.2O_2−δ (CGO)/YSZ electrolyte was studied in the temperature range of 600 to 850 °C and under flow of 21% O₂/He mixture, using impedance spectroscopy and current density–overpotential measurements. The L58SCF cathode exhibited the highest electrocatalytic activity for oxygen reduction, according to the order: LS2F/CGO/YSZ ≤ LSM/LSM–YSZ/CGO/YSZ < L58SCF/CGO/YSZ.     

Performance of La-doped strontium titanate (LST) anode on LaGaO3-based SOFC

Ni-containing anode is currently used with many electrolytes of solid oxide fuel cells (SOFCs). However, Ni is easily oxidized and deteriorates the LaGaO₃-based electrolyte. A La-doped SrTiO₃ (LST, La_0.2Sr_0.8TiO₃) is a candidate as an anode material to solve the Ni poisoning problem in LaGaO₃-based SOFC. In this study, a single-phase LST and an LST-Gd_0.2Ce_0.8O_2 ? δ (GDC) composite were tested as the possible anodes on La_0.9Sr_0.1Ga_0.8Mg_0.2O_3 ? δ (LSGM) electrolyte. In order to further improve the anodic performance, Ni was impregnated into the LST-GDC composite anode. The performance was examined from 600 °C to 800 °C by measuring impedance of the electrolyte-supported, symmetric (anode/electrolyte/anode) cells. A polarization resistance (R_p) of LST-GDC anode was much reduced from that of LST anode. When Ni was impregnated into LST-GDC composite, the R_p value was further reduced to ~ 10% of the single-phase LST anode, and it was 1 Ωcm² at 800 °C in 97% H₂ + 3% H₂O atmosphere. A single cell with Ni-impregnated LST-GDC as an anode, Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3 ? δ (BSCF) as a cathode and LSGM as an electrolyte exhibited the maximum power density of 275 mW/cm² at 800 °C, increased from ~ 60 mW/cm² for the cell using the LST-GDC as an anode. Thus, LST-GDC composite is promising as a component of anode.     

Proton conduction in LaSrScO3 single crystals

Transparent single crystals of La_1−xSr_xScO₃ (x = 0.01, 0.03) were grown by the floating zone method. The optical and the electrical properties were studied by infrared absorption spectra and AC-impedance measurements, respectively. Our results showed that proton conduction becomes dominant in La_0.97Sr_0.03ScO₃ single crystals below 700 °C. The electrical conductivity increases as the amount of Sr increases. At 600 °C, La_0.97Sr_0.03ScO₃ had a proton conductivity of 4.89 × 10^− 3 S/cm. This value is almost the same as that in the literature for La_0.9Sr_0.1ScO₃ ceramic samples. It was revealed from infrared absorption spectra that several different sites are occupied by protons in this material. Furthermore, it was found that the intensity of high-frequency O–H bands can be related to the dopant-concentration dependence of the electrical conductivity.     

Structure,stability and electrical properties of the La(2−x)SrxMnO4±δ solid solution series

Single phase materials of the La_(2−x)Sr_xMnO_4±δ (0.6 ≤ x ≤ 2.0) solid solution series were prepared via solid state reaction. The structure of each material was examined at room temperature and determined to be tetragonal for all phases examined. An expansion in lattice volume was observed on increasing lanthanum content. The stability and thermal expansion of each member of the solid solution series was determined via the use of in situ high temperature X-ray diffraction. It was found that all materials remained stable up to a temperature of 800 °C. Thermal expansion coefficients were found to be in the region of 15 × 10^− 6 K^− 1 for La_(2−x)Sr_xMnO_4±δ compounds where x > 1.4. The electrical conductivity of each phase was also determined over a similar temperature range with a maximum value of ∼6 Scm^− 1 at 900 °C for the x = 1.8 phase.     

Preparation of cadmium stannate films by spray pyrolysis technique

Cadmium stannate thin films were prepared by spray pyrolysis technique using cadmium acetate and tin(II) chloride precursors at substrate temperatures 450 °C and 500 °C. XRD pattern confirms the formation of orthorhombic (1 1 1) cadmium stannate phase for the film prepared at substrate temperature of 500 °C, whereas, films prepared at 450 °C are amorphous. Film formation does not occur at substrate temperature from 300 to 375 °C. SEM images reveal that the surface of the prepared Cd₂SnO₄ film is smooth. The average optical transmittance of ∼86% is obtained for the film prepared at substrate temperature of 500 °C with the film thickness of 400 nm. The optical band gap value of the films varies from 2.7 to 2.94 eV. The film prepared at 500 °C shows a minimum resistivity of 35.6 × 10⁻⁴ Ω cm.     

Oxygen nonstoichiometry and transport properties of strontium substituted lanthanum cobaltite

Oxygen nonstoichiometry, structure and transport properties of the two compositions (La_0.6Sr_0.4)_0.99CoO_3−δ (LSC40) and La_0.85Sr_0.15CoO_3−δ (LSC15) were measured. It was found that the oxygen nonstoichiometry as a function of the temperature and oxygen partial pressure could be described using the itinerant electron model. The electrical conductivity, σ, of the materials is high (σ > 500 S cm^− 1) in the measured temperature range (650–1000 °C) and oxygen partial pressure range (0.209–10^− 4 atm). At 900 °C the electrical conductivity is 1365 and 1491 S cm^− 1 in air for LSC40 and LSC15, respectively. A linear correlation between the electrical conductivity and the oxygen vacancy concentration was found for both samples. The mobility of the electron-holes was inversely proportional with the absolute temperature indicating a metallic type conductivity for LSC40. Using electrical conductivity relaxation the chemical diffusion coefficient of oxygen was determined. It was found that accurate values of the chemical diffusion coefficient could only be obtained using a sample with a porous surface coating. The porous surface coating increased the surface exchange reaction thereby unmasking the chemical diffusion coefficient. The ionic conductivity deduced from electrical conductivity relaxation was determined to be 0.45 S cm^− 1 and 0.01 S cm^− 1 at 1000 and 650 °C, respectively. The activation energy for the ionic conductivity at a constant vacancy concentration (δ = 0.125) was found to be 0.90 eV.     

Oxygen diffusion and surface exchange studies on (La0.75Sr0.25)0.95Cr0.5Mn0.5O3−δ

Oxygen tracer diffusion coefficient (D^⁎) and surface exchange coefficient (k^⁎) have been measured for (La_0.75Sr_0.25)_0.95Cr_0.5Mn_0.5O_3−δ using isotopic exchange and depth profiling by secondary ion mass spectrometry technique as a function of temperature (700–1000 °C) in dry oxygen and in a water vapour-forming gas mixture. The typical values of D^⁎ under oxidising and reducing conditions at ∼ 1000 °C are 4 × 10^− 10 cm² s^− 1 and 3 × 10^− 8 cm² s^− 1 respectively, whereas the values of k^⁎ under oxidising and reducing conditions at ∼ 1000 °C are 5 × 10^− 8 cm s^− 1 and 4 × 10^− 8 cm s^− 1 respectively. The apparent activation energies for D^⁎ in oxidising and reducing conditions are 0.8 eV and 1.9 eV respectively.     

Extraordinary fast oxide ion conductivity in La1.61GeO5−δ thin film consisting of nano-size grain

Thin films of La_1.61GeO_5−δ, a new oxide ionic conductor, were fabricated on dense polycrystalline Al₂O₃ substrates by a pulsed laser deposition (PLD) method and the effect of the film thickness on the oxide ionic conductivity was investigated on the nanoscale. The deposition parameters were optimized to obtain La_1.61GeO_5−δ thin films with stoichiometric composition. Annealing was found necessary to get crystalline La_1.61GeO_5−δ thin films. It was also found that the annealed La_1.61GeO_5−δ film exhibited extraordinarily high oxide ionic conductivity. Due to the nano-size effects, the oxide ion conductivity of La_1.61GeO_5−δ thin films increased with the decreasing thickness as compared to that in bulk La_1.61GeO_5−δ. In particular, the improvement in conductivity of the film at low temperature was significant .The electrical conductivity of the La_1.61GeO_5−δ film with a thickness of 373 nm is as high as 0.05 S cm^− 1 (log(σ/S cm^− 1) = − 1.3) at 573 K.     

Synthesis and characterization of (Pr0.75Sr0.25)1 − xCr0.5Mn0.5O3 − δ as anode for SOFCs

The complex perovskite (Pr_0.75Sr_0.25)_1 − xCr_0.5Mn_0.5O_3 − δ (PSCM) has been prepared and studied as possible anode material for high-temperature solid oxide fuel cells (SOFCs). PSCM exhibits GdFeO₃-type structure and is both physically and chemically compatible with the conventional YSZ electrolyte. The reduction of PSCM resulted in structural change from orthorhombic Pbnm to cubic Pm-3m. Selected area electron diffraction (SAED) analysis on the reduced phases indicated the presence of a √2 × √2 × 2 superlattice. The total conductivity values of ∼ 75% dense Pr_0.75Sr_0.25Cr_0.5Mn_0.5O_3 − δ at 900 °C in air and 5% H₂/Ar are 9.6 and 0.14 S cm^− 1 respectively. The conductivity of PSCM drops with decreasing Po₂ and is a p-type conductor at all studied Po₂. The average TEC of Pr_0.75Sr_0.25Cr_0.5Mn_0.5O_3 − δ is 9.3 × 10^− 6 K^− 1, in the temperature range of 100–900 °C and is close to that of YSZ electrolyte. The anode polarization resistance of PSCM in wet 5%H₂ is 1.31 Ω cm² at 910 °C and in wet CH₄ at 930 °C; the polarization resistance is 1.29 Ω cm². PSCM was unstable at 900 °C in unhumidified hydrogen. Cell performance measurements carried out using graded PSCM and La_0.8Sr_0.2MnO₃ as anode and cathode respectively yielded a maximum power density of 0.18 W cm^− 2 in wet 5%H₂/Ar at 910 °C and the corresponding current density was 0.44 A cm^− 2 at 0.4 V. The activation energy for the electrochemical cell operating in wet (3% H₂O) 5%H₂/Ar fuel is 85 kJ mol^− 1.     

Fast and wide-band response infrared detector using porous PZT pyroelectric thick film

Porous lead zirconate titanate (PbZr_0.3Ti_0.7O₃, PZT30/70) thick films and detectors for pyroelectric applications have been fabricated on alumina substrates by screen-printing technology. Low temperature sintering of PZT thick films have been achieved at 850 °C by using Li₂CO₃ and Bi₂O₃ sintering aids. The microstructure of PZT thick film has been investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The dielectric properties were measured using HP 4284 at 1 kHz under 25 °C. The permittivity and loss tangent of the thick films were 94 and 0.017, respectively. Curie temperature of PZT thick film was 425 °C as revealed by dielectric constant temperature measurement. The pyroelectric coefficient was determined to be 0.9 × 10⁻⁸ Ccm⁻² K⁻¹ by dynamic current measurement. Infrared detector sensitive element of dual capacitance was fabricated by laser directly write technology. Detectivity of the detectors were measured using mechanically chopped blackbody radiation. Detectivity ranging from 1.23 × 10⁸ to 1.75 × 10⁸ (cm Hz^1/2 W⁻¹) was derived at frequency range from 175.5 Hz to 1367 Hz, and D^*’s −3 dB cut-off frequency bandwidth was 1.2 kHz. The results indicate that the infrared detectors based on porous thick films have great potential applications in fast and wide-band frequency response conditions.     

Fe alloying effect on the performance of the Ni anode in hydrogen fuel

Composite materials of YSZ (yttria-stabilized zirconia) with various Ni–Fe alloys were synthesized and evaluated as the solid oxide fuel cell (SOFC) anode using a 200-µm thick YSZ electrolyte as support and YSZ +La_0.8Sr_0.2MnO₃ (LSM) as cathode. The single cell with the YSZ + Ni_0.75Fe_0.25 anode exhibited the highest performance among all the investigated cells, e.g. a peak power density of 403, 337, 218 and 112 mW/cm² was achieved with H₂ fuel at 900, 850, 800 and 750 °C, respectively. The composite anode with the Ni_0.75Fe_0.25 alloy also had the lowest polarization resistance of 0.55 Ω·cm² at 800 °C among all the alloy compositions, indicating that this specific alloy offered a better anode composition than pure Ni. The possible mechanism for the improved performance of Ni with the Fe alloying addition towards H₂ oxidation was discussed.     

La0.75Sr0.25Cr0.5Mn0.5O3−δ + Cu composite anode running on H2 and CH4 fuels

La_1−xSr_xCr_1−xM_xO_3−δ (M = Cr, Fe, V) system has been studied as anode materials for solid oxide fuel cells (SOFCs). The perovskite La_0.75Sr_0.25Cr_0.5Mn_0.5O_3−δ (LSCM) is stable in both H₂ and CH₄ atmospheres at temperatures up to 1000°C. However, in the reducing atmospheres of H₂ and CH₄, its electronic conductivity is greatly reduced from its value in air. We have characterized LSCM as the anode of a SOFC having 250 μm-thick La_0.8Sr_0.2Ga_0.83Mg_0.17O_2.815 (LSGM) as the electrolyte and SrCo_0.8Fe_0.2O_3−δ (SCF) as the cathode. We report a comparison of the overpotentials at the following anodes: (1) La_0.4Ce_0.6O_1.8 (LDC) + NiO composite in H₂, (2) porous LSCM in H₂ and CH₄, (3) porous LSCM impregnated with CuO in H₂ and CH₄ and (4) porous LSCM impregnated with CuO and sputtered with Pt in H₂ and CH₄. An LSCM + CuO + Pt anode gave a maximum power output at 850 °C of 850 mW/cm² and 520 mW/cm², respectively, with H₂ and CH₄ as fuel whereas anode (1) gave 1.4 W/cm² at 800 °C in H₂. There was no noticeable coke formation in CH₄ with anodes (2), (3) and (4), which demonstrates that the perovskite oxide is a plausible option for the anode of a SOFC operating with hydrocarbon fuels. We also report the moisture effect in the H₂ and CH₄ fuel-oxidation process.     

Sensor application-related defect chemistry and electromechanical properties of langasite

The operation of langasite (La₃Ga₅SiO₁₄) resonators as sensors at elevated temperature and controlled atmospheres is examined. This paper focuses on mapping the regimes of gas-insensitive operation of uncoated langasite resonators and the correlation to langasite's defect chemistry for temperatures up to 1000 °C. As a measure of sensitivity, the fundamental resonant mode at 5 MHz is estimated to be determined to within ± 4 Hz by network analysis for resonators operated in air at temperatures below 1000 °C. The calculated frequency shift induced by redox-related reactions in langasite only exceeds the limit of ± 4 Hz below p_O2 ≈ 10^− 17 bar at 1000 °C, below 10^− 24 bar at 800 °C and below 10^− 36 bar at 600 °C. Water vapor is found to shift the resonance frequency at higher oxygen partial pressures. In the hydrogen-containing atmospheres applied here, langasite can be regarded as a stable resonator material above oxygen partial pressures of about 10^− 13 and 10^− 20 bar at 800 and 600 °C, respectively.     

Thermal and chemical expansion of mixed conducting La0.5Sr0.5Fe1−xCoxO3−δ materials

Oxygen deficiency, thermal and chemical expansion of La_0.5Sr_0.5Fe_1−xCo_xO_3−δ (x = 0, 0.5, 1) have been measured by thermogravimetry, dilatometry and high temperature X-ray diffraction. The rhombohedral perovskite materials transformed to a cubic structure at 350 ± 50 °C. The thermal expansion of the materials up to the onset of thermal reduction was 14–18 × 10^− 6 K^− 1. Above 500 °C in air (400 °C in N₂), chemical expansion contributed to the thermal expansion and the linear thermal expansion coefficients were significantly higher, 16–35 × 10^− 6 K^− 1. The chemical expansion, ε_c, showed a maximum of 0.0045 for x = 0.5 and 0.0041 for x = 1 at 800–900 °C. The normalized chemical expansion, ε_c/Δδ, was 0.036 for x = 0.5 and 0.035 for x = 1 at 800 °C. The chemical expansion can be correlated with an increasing ionic radius of the transition metals with decreasing valence state.     

Growth and characterization of indium oxide films

In₂O₃ films have been deposited using chemical spray pyrolysis technique at different substrate temperatures that varied in the range, 250–450 °C. The structural and morphological properties of the as-deposited films were studied using X-ray diffractometer and scanning electron microscope as well as atomic force microscope, respectively. The films formed at a temperature of 400 °C showed body-centered cubic structure with a strong (2 2 2) orientation. The structural parameters such as the crystallite size, lattice strain and texture coefficient of the films were also calculated. The films deposited at a temperature of 400 °C showed an optical transmittance of >85% in the visible region. The change of resistivity, mobility, carrier concentration and activation energies with the deposition temperature was studied. The highest figure of merit for the layers grown at 400 °C was 1.09 × 10⁻³ Ω⁻¹.     

Thermal expansion behavior and chemical compatibility of BaxSr1−xCo1−yFeyO3−δ with 8YSZ and 20GDC

Perovskite oxides of the composition Ba_xSr_1−xCo_1−yFe_yO_3−δ(BSCF) were synthesized via a modified Pechini method and characterized by X-ray diffraction, dilatometry and thermogravimetry. Investigations revealed that single-phase perovskites with cubic structure can be obtained for x ≤ 0.6 and 0.2 ≤ y ≤ 1.0. The as-synthesized BSCF powders can be sintered in several hours to nearly full density at temperatures of over 1180 °C. Thermal expansion curves of dense BSCF samples show nonlinear behavior with sudden increase in thermal expansion rate between about 500 °C and 650 °C, due mainly to the loss of lattice oxygen caused by the reduction of Co⁴⁺ and Fe⁴⁺ to lower valence states. Thermal expansion coefficients (TECs) of BSCF were measured to be 19.2–22.9 × 10^− 6 K^− 1 between 25 °C and 850 °C. Investigations showed further that Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ is chemically compatible with 8YSZ and 20GDC for temperatures up to 800 °C, above which severe reactions were detected. After being heat-treated with 8YSZ or 20GDC for 5 h above 1000 °C, Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ was completely converted to phases like SrCoO_3−δ, BaCeO₃, BaZrO₃, etc.