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.     

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     

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 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.     

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.     

Characterization of nonstoichiometric nickel manganite spinels NixMn3−x□3δ/4O4+δ by temperature programmed reduction

Cation deficient spinels Ni_xMn_3−x□_3δ/4O_4+δ (0≤x≤1) have been prepared by thermal decomposition of mixed oxalates Ni_x/3Mn_(3−x)/3(C₂O₄)·nH₂O in air at 623 K. They have been characterised by temperature programmed reduction (TPR) under H₂, the reaction being followed by gravimetric and powder X-ray diffraction measurements. It has been shown that TPR proceeds in several steps. The first steps correspond to the loss of nonstoichiometric oxygen leading to the formation of a stoichiometric oxide. During the following stages the manganese cations are reduced, causing the spinel structure to be destroyed, and the formation of solid solution of NiO in a cubic MnO. Subsequently, Ni²⁺ cations undergo a reduction to metallic nickel, and, finally, a mixture of nonstoichiometric MnO_1−δ and metallic nickel is formed. These oxides contain a high level of vacancies which vary with the nickel content with a maximum of δ≈1 near x=0.6. This nonstoichiometry is ascribed both to the presence of Ni³⁺ and excess of Mn⁴⁺.     

Aging behavior and ionic conductivity of ceria-based ceramics: a comparative study

An understanding of high-temperature aging effects on the electrical properties of electrolytes is very important in selecting optimum compositions for practical applications. The aging behavior and mechanisms of doped zirconia ceramics have been extensively studied. However, little information is available regarding the aging behavior of ceria-based electrolytes. The present study has demonstrated that a high-temperature aging at 1000 °C has a significant effect on the ionic conductivity of the Y- or Gd-doped ceria (Ce_1−xY_xO_2−δ and Ce_1−xGd_xO_2−δ), especially in the case of the Gd doping. The aging behavior is characterized by a critical dopant concentration, above which the aging has a detrimental effect on the conductivity of the doped ceria ceramics. The aging behavior in the doped ceria cannot be explained using the aging mechanisms applied to the doped zirconia. Instead, the formation of the microdomains in the doped ceria has been acknowledged to be the main contribution to the aging behavior of the Y- or Gd-doped ceria ceramics. The formation ability of microdomains has been estimated to be in the order of La³⁺>Gd³⁺>Y³⁺, based on the degree of size mismatch between the dopant ion and Ce⁴⁺ ion. The critical dopant concentrations at which the microdomains start to form for La³⁺, Gd³⁺ and Y³⁺ in the doped ceria ceramics are x=0.15, 0.2 and 0.25, respectively. This critical dopant concentration is also an important indication: below which the conductivity is governed by only the association enthalpy, and above which the conductivity is dominated mainly by the microdomains rather than the association enthalpy.     

Superconducting Nd1.85Ce0.15CuO4 films grown by the pulsed electron deposition technique

Nd_1.85Ce_0.15CuO_4−δ superconducting thin films were prepared on (1 0 0) SrTiO₃ substrates by pulsed electron deposition technique without reducing atmosphere. Oxygen content is finely controlled by high temperature vacuum annealing, and optimal superconductivity has been obtained. The deposition conditions of the film are discussed in details. Higher deposition temperature and lower gas pressure result in the loss of copper and the appearance of the foreign phase Ce_0.5Nd_0.5O_1.75. High quality Nd_1.85Ce_0.15CuO_4−δ epitaxial films are deposited at 840–870 °C in the mixed gas with a ratio of O₂:Ar = 1:3.     

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.     

Magnetic properties and composition range of non-stoichiometric m-type hexagonal ferrites prepared by ion exchange

Single crystals of Ba, Sr M-type hexagonal ferrites were prepared by ion exchange in Ba, Sr containing molten salts from single crystals of β″ -ferrites. A fast diffusion of the divalent Ba²⁺, Sr²⁺ is observed leading to a non-stoichiometric M-type ferrite with chemical formula: Ba_1+xFe_10.5Co_0.25O_17+x (0 ≤ x ≤ 0.25). x depending on the exchange reaction time. Saturation magnetization ranges from 19 to 64 emu/g depending on exchange conditions. The Curie temperature is (470 ± 5)°C. An easy axis direction (M_s c) has been determined in all cases. The observed anisotropy is considerably lower than that of M ferrite. The calculated anisotropy constants are, in 10⁶ erg/cm³, K₁ = 0.8 and K₂ = 1.0 at room temperature.