The effect of oxygen vacancy on the oxide ion mobility in LaAlO3-based oxides

The electrical property of (La_1−xSr_x)_1−z(Al_1−yMg_y)O_3−δ (LSAM; x≤0.3, y≤0.15 and z≤0.1) was measured using the DC four-probe method as a function of temperature (500–1000°C) and oxygen partial pressure (1–10⁻²² atm). Among LSAMs, (La_0.9Sr_0.1)AlO_3−δ showed the highest ionic conductivity, σ_i=1.3×10⁻² S cm⁻¹ at 900°C. A simultaneous substitution at A and B sites or A site deficiency is expected to create larger oxygen vacancy and higher ionic conductivity. However, it showed a negative effect. The effect of the vacancy increase did not effect monotonously the ionic conductivity. It was found that the concentration of oxygen vacancy, [V_O], influences not only the oxide ion conductivity, σ_i, but also the mobility, μ_v, of [V_O]. These properties exhibit a maximum at around [V_O]=0.05. With the increase in [V_O], the activation energy, E_a, of the ionic conduction dropped from 1.8 to ca. 1.0 eV at [V_O]=0.05 and became almost constant at [V_O]>0.05. The dependency of the pre-exponential term, μ⁰_v, and E_a on [V_O] was analyzed and their effect on μ_v and σ_i was discussed with respect to crystal structure and defect association. It was estimated that the crystal structure mainly governs these properties. The effect of defect association could not be ignored but is considered to be a complicated correlation.     

Deuterium conductivity, diffusion and implantation in Sr-doped LaYO3

A study of deuterium conductivity and diffusion in the oxide perovskite La_0.9Sr_0.1YO_3−δ is presented in this work. Deuterium ions were implanted into La_0.9Sr_0.1YO_3−δ (50 keV, 1×10¹⁶ atoms/cm²) and the corresponding deuterium depth profile was determined by SIMS and compared with a Monte Carlo simulation (TRIM96). This implant was used as a standard for the determination of deuterium concentration in a La_0.9Sr_0.1YO_3−δ sample pre-treated in D₂O atmosphere. In this way, it was fully confirmed that La_0.9Sr_0.1YO_3−δ incorporates water at high temperatures. The conductivity of La_0.9Sr_0.1YO_3−δ was measured in D₂O atmosphere and compared with other proton (deuteron) conductors. Concentration and conductivity data were used in conjunction to estimate the deuterium diffusivity and the constant of reaction of (heavy) water incorporation into LaYO₃. Some comments on the catalytic activity of this oxide are made.     

Oxygen tracer diffusion in La1 − xSrxCoO3

Tracer diffusion of ¹⁸O in dense, polycrystalline La_1−xSr_xCoO₃ for x = 0.1 has been measured in the temperature range 400 to 600 °C and at 500 °C for x = 0.2 at an oxygen partial pressure of 1 × 10⁵ Pa. Depth profiles were obtained by secondary ion mass spectrometry. The diffusion coefficient for La_0.9Sr_0.1CoO₃ is given by D = (17–247) exp[(−232 ± 8 kJ/mole)/RT] cm²/s. This value is several orders of magnitude lower than D extrapolated from the results for x = 0.2 measured in the 700–900 °C temperature range. One possible explanation for the discrepancy is that the two measurements reflect different diffusion paths. As expected, La_0.8Sr_0.2CoO₃ exhibits a higher diffusivity at 500 °C than does La_0.9Sr_0.1CoO₃.     

Origin of cathodic degradation and new phase formation at the La0.9Sr0.1MnO3/YSZ interface

New phase formation at the La_0.9Sr_0.1MnO₃/YSZ interface and its effects on the cathodic performances were studied at 900 °C in air. The resistance caused by the interfacial product layer kept increasing with time to reach up to 40% of the total resistance after 500 h. The interfacial product was identified as La₂Zr₂O₇ by XRD measurement. The electrical conductivity of La₂Zr₂O₇ (2.4 × 10⁻⁵ S cm⁻¹ at 1000 °C), measured by AC impedance and current interruption methods, was 4 to 7 orders of magnitude smaller than those of La_0.9Sr_0.1MnO₃ electrode or YSZ electrolyte. Either the electronic conductivity or the electrochemical O₂ reduction activity of La₂Zr₂O₇ was negligible. Combining these results, a conclusion was made that the cathodic degradation comes mainly from the growth of interfacial product layer and its contribution to the cell resistance increment is ohmic in nature.     

Oxygen permeability of La2Cu(Co)O4+δ solid solutions

Formation of the La₂Cu_1−xCo_xO_4+δ solid solutions with orthorhombic K₂NiF₄-type structure was found to be in the range of 0≤x≤0.30 at temperatures above 1270 K. Incorporating cobalt into the copper sublattice of lanthanum cuprate leads to increasing oxygen hyperstoichiometry and decreasing electrical conductivity. Thermal expansion coefficients of the La₂Cu_1−xCo_xO_4+δ (x=0.02–0.30) ceramics at 470–1100 K were calculated from the dilatometric data to vary in the range (12.2–13.2)×10⁻⁶ K⁻¹. Studying the dependence of oxygen permeation fluxes through La₂Cu(Co)O_4+δ on the membrane thickness demonstrated that the oxygen transport at the thickness values below 1 mm is limited by both surface exchange rate and bulk ionic conductivity. Oxygen permeability of the La₂Cu_1−xCo_xO_4+δ solid solutions was ascertained to increase with cobalt concentration at x=0.02–0.10 and to decrease with further dopant additions, indicating a participation of interstitial oxygen in the ionic transport.     

Niobium based tetragonal tungsten bronzes as potential anodes for solid oxide fuel cells: synthesis and electrical characterisation

In this paper we report studies on a range of niobate based tungsten bronzes, with a view to analysing their potential as anode materials in SOFCs. Six systems were studied, (Sr_1−xBa_x)_0.6Ti_0.2Nb_0.8O₃, Sr_0.6−xLa_xTi_0.2+xNb_0.8−xO₃, (Sr_0.4−xBa_x)Na_0.2NbO₃, (Ba_1−xCa_x)_0.6Ti_0.2Nb_0.8O₃, Ba_0.5−xA_xNbO₃ (A=Ca, Sr), and Ba_0.3NbO_2.8, and the electrical conductivities were examined over a range of oxygen partial pressures (10⁻²⁰–1 bar). All the systems showed good conductivity in low oxygen partial pressures, with values as high as 8 S cm⁻¹ at 930°C (P(O₂)=10⁻²⁰ bar). As the oxygen partial pressure was raised the conductivity dropped showing in most cases an approximate [P(O₂)]^−1/4 dependence and good re-oxidation kinetics. Of all the samples studied the (Sr_1−xBa_x)_0.6Ti_0.2Nb_0.8O₃ and (Ba_1−xCa_x)_0.6Ti_0.2Nb_0.8O₃ systems appear most promising for potential use as anode materials in SOFCs.     

Perovskite-like system (Sr,La)(Fe,Ga)O3−δ: structure and ionic transport under oxidizing conditions

The maximum solid solubility of gallium in the perovskite-type La_1−xSr_xFe_1−yGa_yO_3−δ (x=0.40–0.80; y=0–0.60) was found to vary in the approximate range y=0.25–0.45, decreasing when x increases. Crystal lattice of the perovskite phases, formed in atmospheric air, was studied by X-ray diffraction (XRD) and neutron diffraction and identified as cubic. Doping with Ga results in increasing unit cell volume, while the thermal expansion and total conductivity of (La,Sr)(Fe,Ga)O_3−δ in air decrease with gallium additions. The average thermal expansion coefficients (TECs) are in the range (11.7–16.0)×10⁻⁶ K⁻¹ at 300–800 K and (19.3–26.7)×10⁻⁶ K⁻¹ at 800–1100 K. At oxygen partial pressures close to atmospheric air, the oxygen permeation fluxes through La_1−xSr_xFe_1−yGa_yO_3−δ (x=0.7–0.8; y=0.2–0.4) membranes are determined by the bulk ambipolar conductivity; the limiting effect of the oxygen surface exchange was found negligible. Decreasing strontium and gallium concentrations leads to a greater role of the exchange processes. As for many other perovskite systems, the oxygen ionic conductivity of La_1−xSr_xFe_1−yGa_yO_3−δ increases with strontium content up to x=0.70 and decreases on further doping, probably due to association of oxygen vacancies. Incorporation of moderate amounts of gallium into the B sublattice results in increasing structural disorder, higher ionic conductivity at temperatures below 1170 K, and lower activation energy for the ionic transport.     

Oxygen nonstoichiometry and electrical conductivity of the solid solutions La2−xSrxNiOy (0≤x≤0.5)

Oxygen nonstoichiometry and electrical resistance of a series La_2−xSr_xNiO_y solid solutions, where x=0.0, 0.2 and 0.5 in argon flows at oxygen partial pressures 1.5, 10.2, 49.2, 100 and 286 Pa within the temperature range of 20–1050°C were studied. Nickelate oxygen desorption/sorption spectra when heating–cooling at constant rate demonstrated strong dependence of cation composition of the samples. Unlike La_1.5Sr_0.5NiO_y compounds those of La₂NiO_y and La_1.8Sr_0.2NiO_y have weakly bonded oxygen, capable to exchange reversibly with the gas phase at the temperatures higher than 250°C. The equilibrium values of oxygen nonstoichiometry and specific resistance for the these nickelates were determined at 300–1050°C and p_O2=1.5–286 Pa as a functions of temperature versus oxygen partial pressure. All nickelate studied appear to be p-type conductors with metal electric conductivity at equilibrium states.     

Germanium and iron co-substituted SrCoO2.5+δ as oxygen permeable membrane

Germanium and iron co-doped SrCoO_2.5+δ was investigated in terms of phase stability, oxygen permeability and electrical conductivity. The favorable high-temperature cubic structure of SrCoO_2.5+δ was stabilized to lower temperatures by co-doping Ge (10 mol%) and Fe (10 mol%) that substituted for Co, which however could not be achieved by doping Ge (20 mol%) alone. In contrast to SrCo_0.8Ge_0.2O_2.5+δ sample which showed a sharp decrease in oxygen permeability at temperature of 875 °C upon cooling, SrGe_0.1Co_0.8Fe_0.1O_3−δ sample remained well-permeable to oxygen at lower temperatures down to at least 820 °C; an abrupt change in electrical conductivity in SrCo_0.8Ge_0.2O_2.5+δ also occurred accompanying the phase transition. The oxygen permeation flux for SrGe_0.1Co_0.8Fe_0.1O_3−δ increased significantly with the decrease of the membrane thickness, indicating the transport of oxygen ions in the bulk of the membrane as the rate-limiting step.     

Transport properties of SrCe0.95Y0.05O3−δ and its application for hydrogen separation

Transport properties of SrCe_0.95Y_0.05O_3−δ were studied by impedance spectroscopy and by measuring open-cell voltage (OCV) and gas permeation. Ionic transference numbers were determined by measuring the OCV of concentration cells and water vapor evolution of an O₂/H₂ fuel cell. We observed interfacial polarization on the basis of the I–V curves obtained by discharging a hydrogen concentration cell or an O₂/H₂ fuel cell. The observed high protonic conductivity (high proton and low oxide ion transference numbers) makes SrCe_0.95Y_0.05O_3−δ a potential material for hydrogen separation. From proton conductivity measurements, under a given hydrogen partial pressure difference of 4%/0.488%, the hydrogen permeation rate (of a dense membrane with 0.11 cm in thickness) was calculated to be ≈0.072 cm³ (STP) cm⁻² min⁻¹ at 800°C, whereas the permeation rate calculated from short-circuit current measurements was ≈0.023 cm³ (STP) cm⁻² min⁻¹ at 800°C. The difference between calculated and observed permeation rates is probably due to interfacial polarization.     

Oxygen diffusion and surface exchange in La1−xSrxFe0.8Cr0.2O3−δ (x=0.2, 0.4 and 0.6)

Oxygen tracer diffusion (D*) and surface exchange rate constant (k*) have been measured, using isotopic exchange and depth profiling by secondary ion mass spectrometry (SIMS), in La_1−xSr_xFe_0.8Cr_0.2O_3−δ (x=0.2, 0.4 and 0.6). Measurements were made as a function of temperature (700–1000 °C) and oxygen partial pressure (0.21–10⁻²¹ atm) in dry oxygen, water vapour and water vapour/hydrogen/nitrogen mixtures. At high oxygen activity, D* was found to increase with increasing temperature and Sr content. The activation energies for D* in air are 2.13 eV (x=0.2), 1.53 eV (x=0.4) and 1.21 eV (x=0.6). As the oxygen activity decreases, D* increases as expected qualitatively from the increase in oxygen vacancy concentration. Under strongly reducing conditions, the measured values of D* at 1000 °C range from 10⁻⁸ cm² s⁻¹ for x=0.2 to 10⁻⁷ cm² s⁻¹ for x=0.4 and 0.6. The activation energies determined at constant H₂O/H₂ ratio are 1.21 eV (x=0.2), 1.59 eV (x=0.4) and 0.82 eV (x=0.6).

The surface exchange rate constant of oxygen for the H₂O molecule is similar in magnitude to that for the O₂ molecule and both increase with increasing Sr concentration.     

Electrochemical performance of mixed ionic–electronic conducting oxides as anodes for solid oxide fuel cell

Mixed ionic–electronic conducting (MIEC) oxides, SrFeCo_0.5O_x, SrCo_0.8Fe_0.2O_3−δ and La_0.6Sr_0.4Fe_0.8Co_0.2O_3−δ have been synthesized and prepared on yttria-stabilized zirconia as anodes for solid oxide fuel cells. Power output measurements show that the anodes composed of such kinds of oxides exhibit modest electrochemical activities to both H₂ and CH₄ fuels, giving maximum power densities of around 0.1 W/cm² at 950°C. Polarization and AC impedance measurements found that large activation overpotentials and ohmic resistance drops were the main causes for the relative inferior performance to the Ni-YSZ anode. While interlayered with an Ni-YSZ anode, a significant improvement in the electrochemical performance was observed. In particular, for the SrFeCo_0.5O_x oxide interlayered Ni-YSZ anode, the maximum power output reaches 0.25 W/cm² on CH₄, exceeding those of both SrFeCo_0.5O_x and the Ni-YSZ, as anodes alone. A synergetic effect of SrFeCo_0.5O_x and the Ni-YSZ has been observed. Future work is needed to examine the long-term stability of MIEC oxide electrodes under a very reducing environment.     

Electrons and holes in undoped Nd2CuO4

Nd₂CuO_4±δ is the non-superconducting prototype of the Re_2−xM_xCuO₄t−y family (Re=Pr, Nd, Sm and M=Ceor Th) of n-type oxide superconductors. Four-probe DC conductivity, EMF in P(O₂) gradient, and thermopower measurements have been used to characterise its electric transport and defect structure between 300 and 900°C and between 5×10⁻⁴ and 1 atm oxygen partial pressure.

The results show that Nd₂CuO_4±δ can be oxygen under-stoichiometric (with n-type conductivity), near-stoichiometric, and over-stoichiometric (with p-type conductivity) in different T, P(O₂) ranges.     

Intercalation thermodynamics and chemical diffusion of oxygen in the solid solution YBa2Cu3−xCoxO6+δ

The equilibrium oxygen content as a function of the temperature and oxygen pressure was measured for the solid solution YBa₂Cu_3−xCo_xO_6+δ, where x=0, 0.2, 0.4, 0.6, 0.8, by using coulometric titration in the temperature range 600–850°C and oxygen pressures between 10⁻⁵ and 1.0 atm. The change in the partial molar enthalpy and entropy of the intercalated oxygen was determined at different oxygen and cobalt contents. The oxygen chemical diffusion was studied by thermogravimetric relaxation in the oxygen-controlled atmosphere. The thermodynamic data were employed to determine how the chemical diffusion coefficient, the thermodynamic factor and the random-diffusion coefficient depend on oxygen content in specimens with different cobalt concentration. The oxygen intercalation thermodynamics and diffusivity results provide evidence of ordering phenomena on a microscopic scale in the basal plane of the tetragonal solid solution YBa₂Cu_3−xCo_xO_6+δ. A model for the oxygen diffusion is suggested to explain the large difference between the random and tracer diffusion coefficients in YBa₂Cu₃O_6+δ     

Conductivity anisotropy in directionally solidified CaZrO3---CaSZ and MgO-MgSZ eutectics

The results of the impedance spectroscopy measurements on eutectic samples based on zirconium oxide are presented here. Samples of CaZrO₃---ZrO₂(cubic) and MgO---ZrO₂(cubic) have been grown by a directional solidification procedure such that the different phases appear nearly oriented along the growth direction (lamellae in the system of CaZrO₃-ZrO₂(cubic) and fibers of MgO in a ZrO₂ matrix in the other system). The DC electrical conductivity has been measured by impedance spectroscopy along and across the growth axis. For CaZrO₃---ZrO₂ the coductivity is clearly anisotropic. The following values for σT have been obtained: the conductivity at 600 °C equals 2.0 × 10⁻⁶ Ω⁻¹ cm⁻¹ perpendicular to the fiber axis and 1.4 × 10⁻⁵ Ω⁻¹ cm⁻¹ parallel to it and with an activation energy of 1.3 eV for σT. For MgO---ZrO₂(cubic) the isotropic value of the conductivity at 600 °C is 10⁻⁴ Ω⁻¹ cm⁻¹ with activation energy for σT of 1.5 eV. The anisotropic conductivity in the CaZrO₃---ZrO₂ (cubic) system has been explained by a model of an ordered stacking of oxygen conducting (cubic ZrO₂) and non-conducting (CaZrO₃ or MgO) phases.     

Ionic conductivity of crystalline and glassy Mg4.5Na7(P2O7)4

In our work single crystals of Mg_4.5Na₇(P₂O₇)₄ were prepared, pulverized, pressed into pellets and sintered in order to measure the electrical conductivity of polycrystalline specimens. The conductivity was also measured on glassy specimens obtained by the melting of previously prepared crystals. The electrical conductivities at 25°C with values of the order of 10⁻¹⁶ Ω⁻¹ cm⁻¹ for polycrystalline samples and a value of the order of 10⁻¹⁴ Ω⁻¹ cm⁻¹ for glass, show that the glassy phase of Mg_4.5Na₇(P₂ because of its greater molar volume and loosely packed structure, is a better matrix for ionic motion.     

Oxygen transfer processes in (La,Sr)MnO3/Y2O3-stabilized ZrO2 cathodes: an impedance spectroscopy study

Impedance spectroscopy was used to study the oxygen reaction kinetics of La_0.8Sr_0.2MnO₃ (LSM)-based electrodes on Y₂O₃-stabilized ZrO₂ (YSZ) electrolytes. Three types of electrodes were studied: pure LSM, LSM–YSZ composites, and LSM/LSM–YSZ bilayers. The electrodes were formed by spin coating and sintering on single-crystal YSZ substrates. Measurements were taken at temperatures ranging from 550 to 850°C and oxygen partial pressures from 1×10⁻³ to 1 atm. An arc whose resistance R_el had a high activation energy, E_a=1.61±0.05 eV, and a weak oxygen partial pressure dependence, (P_O2)^−1/6, was observed for the LSM electrodes. A similar arc was observed for LSM–YSZ electrodes, where R_el(P_O2)^−0.29 and the activation energy was 1.49±0.02 eV. The combination of a high activation energy and a weak P_O2 dependence was attributed to oxygen dissociation and adsorption rate-limiting steps for both types of electrodes. LSM–YSZ composite cathodes showed substantially lower overall interfacial resistance values than LSM, but exhibited an additional arc attributed to the resistance of YSZ grain boundaries within the LSM–YSZ. At 850°C and low P_O2, an additional arc was observed with size varying as (P_O2)^−0.80 for LSM and (P_O2)^−0.57 for LSM–YSZ, suggesting that diffusion had become an additional rate limiting step. Bilayer LSM/LSM–YSZ electrodes yielded results intermediate between LSM and LSM–YSZ. The results showed that most of the improvement in electrode performance was achieved for a LSM–YSZ layer only ≈2 μm thick. However, a decrease in the grain-boundary resistance would produce much better performance in thicker LSM–YSZ electrodes.     

A semi-empirical equation for oxygen nonstoichiometry of perovskite-type ceramics

Explicit equations correlating oxygen nonstoichiometry to oxygen partial pressure and temperature are important for applications of perovskite-type ceramics as membranes, adsorbents and catalysts in various chemical reaction and separation processes. A semi-empirical equation for oxygen nonstoichiometry on perovskite-type ceramics is reported in this paper. Though derived from the results of a point defect model on a perovskite-type ceramic material, La_0.1Sr_0.9Co_0.5Fe_0.5O_3−δ, this equation describes very well the experimentally measured oxygen nonstoichiometry data for two perovskite-type ceramics measured in this work and three perovskite-type ceramics reported in the literature. The major advantage of this semi-empirical equation lies in its simplicity, explicitness and accuracy. This equation is coupled with oxygen permeation equation to predict oxygen permeation current density through two perovskite-type ceramic membranes. The predicted data agree very well with the results reported in the literature using a complex defect reaction model.     

Studies on strontium substituted rare earth manganites

Sintering, electrical conductivity and thermal expansion behaviour of combustion synthesised strontium substituted rare earth manganites with the general formula Ln_1−xSr_xMnO₃ (Ln=Pr, Nd and Sm; x=0, 0.16 and 0.25) have been investigated as solid oxide fuel cell cathode materials. The combustion derived rare earth manganites have surface area in the range of 13–40 m²/g. Strontium substitution increases the electrical conductivity values in all the rare earth manganites. With the decreasing ionic radii of rare earth ions, the conductivity value decreases. Among the rare earth manganites studied, (Pr/Nd)_0.75Sr_0.25MnO₃ show high electrical conductivity (>100 S/cm). The thermal expansion coefficients of Pr_0.75Sr_0.25MnO₃ and Nd_0.75Sr_0.25MnO₃ were found to be 10.2×10⁻⁶ and 10.7×10⁻⁶ K⁻¹ respectively, which is very close to that of the electrolyte (YSZ) used in solid oxide fuel cells.