Ionic conductivity and thermal expansion of anion-deficient Sr11Mo4O23 perovskite

Transport properties of perovskite-type Sr₁₁Mo₄O₂₃ and composite Sr₁₁Mo₄O₂₃ - 1 wt% Al₂O₃ were studied at 400–1300 K in the oxygen partial pressure range from 0.21 down to 10⁻¹⁹ atm. The electromotive force and faradaic efficiency measurements, in combination with the energy-dispersive spectroscopy of the fractured electrochemical cells, unambiguously showed prevailing role of the oxygen ionic conductivity under oxidizing conditions. At temperatures above 600 K, protonic and cationic transport can be neglected. The oxygen ion transference numbers vary in the range of 0.95–1.00 at 973–1223 K. At temperatures lower than 550 K, the total conductivity of Sr₁₁Mo₄O₂₃ - 1 wt% Al₂O₃ composite measured by impedance spectroscopy tends to increase in wet atmospheres, thus indicating that hydration and protonic transport become significant. Reducing oxygen partial pressure below 10⁻¹⁰–10⁻⁹ atm leads to a significant increase in the n-type electronic conduction. The average thermal expansion coefficients in oxidizing atmospheres are (14.3–15.0) × 10⁻⁶ K⁻¹ at 340–740 K and (18.3–19.2) × 10⁻⁶ K⁻¹ at 870–1370 K.

     

The Mixed Electronic and Ionic Conductivity of Perovskite-Like Ba1 –xSrxFe1 –yTiyO3 – δ and BaTi0.5Fe0.5 –zCezO3 – δ Solid Solutions

Russian Journal of Electrochemistry - The work is focused on the studying of structural peculiarities, electronic and ionic conductivity, and thermomechanical properties of perovskite-like...     

Electrophysical and thermomechanical properties of perovskites La<Subscript>0.5</Subscript>A<Subscript>0.5</Subscript>Mn<Subscript>0.5</Subscript>Ti<Subscript>0.5</Subscript>O<Subscript>3–δ</Subscript> (A = Ca,Sr, Ba) used as fuel cell anodes: the effect of radius of alkali-earth cation

The effect of the radius of the alkali-earth cation substituted into the A sublattice of La_0.5A_0.5Mn_0.5Ti_0.5O_3–δ (А = Са, Sr, Ba) perovskites on their stability and transport and thermomechanical properties is considered. The increase in the cation radius is shown to improve the phase stability and decrease the conductivity under both oxidative and reductive conditions. The thermal and chemical expansion of La_0.5A_0.5Mn_0.5Ti_0.5O_3–δ ceramics is studied by dilatometry in controlled atmospheres and a wide temperature range at p(O₂)=10^–21–0.21 atm. The coefficients of thermal expansion of La_0.5A_0.5Mn_0.5Ti_0.5O_3–δ are in the interval of (10.7–14.3)× 10^–6 K^–1, i.e., compatible with those of standard solid electrolytes of solid-oxide fuel cells. The maximum chemical expansion does not exceed 0.2% at isothermal reduction in the CO?CO₂ mixture.     

Electrical Conductivity,Thermal Expansion and Electrochemical Properties of Perovskites PrBaFe<Subscript>2–<Emphasis Type="Italic">x</Emphasis></Subscript>Ni<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>5 + δ</Subscript>

In order to evaluate applicability of mixed-conducting PrBaFe_2–xNi _x О_{5 + δ} perovskites for cathodes of solid oxide fuel cells (SOFCs), their crystal structure, thermal and chemical expansion, electrical conductivity and electrochemical behavior were studied. The solubility limit of nickel in PrBaFe₂O_{5 + δ} corresponds to x = 0.8. At x > 0.2, the disordered cubic phase transformed into the tetragonal phase. The maximum level of conductivity (50–120 S/cm) at the operating temperatures of SOFC was found for the composition with the maximum nickel content, PrBaFe_1.2Ni_0.8О_{5 + δ}. This material is also characterized by moderate thermal and chemical expansion relative to other ferrite-nickelates. The polarization resistance of a porous PrBaFe_1.2Ni_0.8О_{5 + δ} cathode in a cell with a protective Ce_0.6La_0.4O_2–δ layer and a solid electrolyte (La_0.9Sr_0.1)_0.98Ga_0.8Mg_0.2O_3–δ was ~0.9 Ohm cm² at a temperature of 1073 K, atmospheric oxygen pressure, and current density of–120 mA cm^–2.     

Electrochemical behavior of membranes of ion-selective electrodes based on complex oxides

Lead-selective film electrodes with solid contact with membranes on the basis of niobates and vanadates of compositions Sr_2.75Pb_0.25La(VO₄)₃, Ba_3.8Pb_0.2Nb₂O₉, Ba_3.5Pb_0.5Nb₂O₉, Pb₃NiNb₂O₉, and Pb₂Nb₂O₇ are constructed and studied. Basic electrochemical characteristics of ion-selective electrodes, such as linearity region and steepness of the electrode function, working pH interval, type of electrode function, and reproducibility are determined. The electrode with a membrane on the basis of Pb₃NiNb₂O₉ is tested and recommended as an indicator in the method of potentiostatic titration. The optimum titrant for assaying ions of lead(II) in solution is potassium chromate.     

Electrical, electrochemical, and thermomechanical properties of perovskite-type (La1?x Sr x )1?y Mn0.5Ti0.5O3?δ (x?=?0.15–0.75, y?=?0–0.05)

Strontium additions in (La_1?x Sr _x )_1?y Mn_0.5Ti_0.5O_3?δ (x?=?0.15–0.75, y?=?0–0.05) having a rhombohedrally distorted perovskite structure under oxidizing conditions lead to the unit cell volume contraction, whilst the total conductivity, thermal and chemical expansion, and steady-state oxygen permeation limited by surface exchange increase with increasing x. The oxygen partial pressure dependencies of the conductivity and Seebeck coefficient studied at 973–1223?K in the p(O₂) range from 10^?19 to 0.5?atm suggest a dominant role of electron hole hopping and relatively stable Mn³⁺ and Ti⁴⁺ states. Due to low oxygen nonstoichiometry essentially constant in oxidizing and moderately reducing environments and to strong coulombic interaction between Ti⁴⁺ cations and oxygen anions, the tracer diffusion coefficients measured by the ¹⁸O/¹⁶O isotopic exchange depth profile method with time-of-flight secondary-ion mass spectrometric analysis are lower compared to lanthanum–strontium manganites. The average thermal expansion coefficients determined by controlled-atmosphere dilatometry vary in the range 9.8–15.0?×?10^?6?K^?1 at 300–1370?K and oxygen pressures from 10^?21 to 0.21?atm. The anodic overpotentials of porous La_0.5Sr_0.5Mn_0.5Ti_0.5O_3?δ electrodes with Ce_0.8Gd_0.2O_2-δ interlayers, applied onto LaGaO₃-based solid electrolyte, are lower compared to (La_0.75Sr_0.25)_0.95Cr_0.5Mn_0.5O_3?δ when no metallic current-collecting layers are introduced. However, the polarization resistance is still high, ~2 Ω?×?cm² in humidified 10?% H₂–90?% N₂ atmosphere at 1073?K, in correlation with relatively low electronic conduction and isotopic exchange rates. The presence of H₂S traces in H₂-containing gas mixtures did not result in detectable decomposition of the perovskite phases.     

Mixed conductivity, oxygen permeability and redox behavior of K2NiF4-type La2Ni0.9Fe0.1O4+δ

The total conductivity and Seebeck coefficient of La₂Ni_0.9Fe_0.1O_4+δ with K₂NiF₄-type structure, studied in the oxygen partial pressure range from 10⁻⁵ to 0.5 atm at 973-1223 K, were analyzed in combination with the steady-state oxygen permeability, oxygen non-stoichiometry and Mössbauer spectroscopy data in order to examine the electronic and ionic transport mechanisms. Doping of La₂NiO_4+δ with iron was found to promote hole localization on nickel cations due to the formation of stable Fe³⁺ states, although the electrical properties dominated by p-type electronic conduction under oxidizing conditions exhibit trends typical for both itinerant and localized behavior of the electronic sublattice. The segregation of metallic Ni on reduction, which occurs at oxygen chemical potentials close to the low-p(O₂) stability boundary of undoped lanthanum nickelate, is responsible for the high catalytic activity towards partial oxidation of methane by the lattice oxygen of La₂Ni_0.9Fe_0.1O_4+δ as revealed by thermogravimetry and temperature-programmed reduction in dry CH₄-He flow at 573-1173 K. A model for the oxygen permeation fluxes through dense La₂Ni_0.9Fe_0.1O_4+δ ceramics, limited by both bulk ionic conduction and surface exchange kinetics, was proposed and validated.     

Electrochemical Behavior of (Fe,Ni)Ox-Based Anodes for Solid-Oxide Fuel Cells in Methane-Containing Atmospheres

Russian Journal of Electrochemistry - The work is devoted to investigation of the electrochemical behavior of (Fe,Ni)Ox-based composite anodes in the hydrogen- and methane-containing fuel. Among...     

Transport, thermomechanical, and electrode properties of perovskite-type {\left( {{\hbox{L}}{{\hbox{a}}_{0.{75} - x}}{\hbox{S}}{{\hbox{r}}_{0.{25} + x}}} \right)_{0.{95}}}{\hbox{M}}{{\hbox{n}}_{0.{5}}}{\hbox{C}}{{\hbox{r}}_{0.{5} - x}}{\hbox{T}}{{\hbox{i}}_x}{{\hbox{O}}_{{3} - }}_\delta \left( {x = 0 - 0.{5}} \right)

Increasing Sr²⁺ and Ti⁴⁺ concentrations in perovskite-type $ {\left( {{\hbox{L}}{{\hbox{a}}_{0.{75} - x}}{\hbox{S}}{{\hbox{r}}_{0.{25} + x}}} \right)_{0.{95}}}{\hbox{M}}{{\hbox{n}}_{0.{5}}}{\hbox{C}}{{\hbox{r}}_{0.{5} - x}}{\hbox{T}}{{\hbox{i}}_x}{{\hbox{O}}_{{3} - }}_\delta \left( {x = 0 - 0.{5}} \right) $ results in slightly higher thermal and chemical expansion, whereas the total conductivity activation energy tends to decrease. The average thermal expansion coefficients determined by controlled-atmosphere dilatometry vary in the range (10.8?C14.5)?×?10^?6?K^?1 at 373?C1,373?K, being almost independent of the oxygen partial pressure. Variations of the conductivity and Seebeck coefficient, studied in the oxygen pressure range 10^?18?C0.5?atm, suggest that the electronic transport under oxidizing and moderately reducing conditions is dominated by p-type charge carriers and occurs via a small-polaron mechanism. Contrary to the hole concentration changes, the hole mobility decreases with increasing x. The oxygen permeation fluxes through dense ceramic membranes are quite similar for all compositions due to very low level of oxygen nonstoichiometry and are strongly affected by the grain-boundary diffusion and surface exchange kinetics. The porous electrodes applied onto lanthanum gallate-based solid electrolyte exhibit a considerably better electrochemical performance compared to the apatite-type La₁₀Si₅AlO_26.5 electrolyte at atmospheric oxygen pressure, while Sr²⁺ and Ti⁴⁺ additions have no essential influence on the polarization resistance. In H₂-containing gases where the electronic transport in $ {\left( {{\hbox{L}}{{\hbox{a}}_{0.{75} - x}}{\hbox{S}}{{\hbox{r}}_{0.{25} + x}}} \right)_{0.{95}}}{\hbox{M}}{{\hbox{n}}_{0.{5}}}{\hbox{C}}{{\hbox{r}}_{0.{5} - x}}{\hbox{T}}{{\hbox{i}}_x}{{\hbox{O}}_{{3} - }}_\delta $ perovskites becomes low, co-doping deteriorates the anode performance, which can be however improved by infiltrating Ni and $ {\hbox{Ce}}{{\hbox{O}}_{{\rm{2}} - }}_\delta $ v into the porous oxide electrode matrix.     

Transport and Electrochemical Properties of SrFe(Al,Mo)O<Subscript>3–δ</Subscript>

In this work, effects of molybdenum doping on the crystal structure, stability, electrical conductivity, oxygen permeability and thermomechanical properties of Sr(Fe,Al)O_3–δ-based perovskites, were studied. The electrochemical performance of model anodes of solid oxide fuel cells (SOFCs), made of SrFe_0.7Mo_0.3O_3–δ, was assessed. Whilst the introduction of Mo cations improves structural stability with respect to the oxygen vacancy ordering processes, excessive molybdenum content leads to a worse phase and mechanical stability under oxidizing conditions. Mo-doping was shown to decrease the thermal and chemical expansivity, to reduce p-type electronic conductivity and to increase n-type electronic conduction. The oxygen permeation fluxes through gas-tight Sr_0.97Fe_0.75Al_0.2Mo_0.05O_3–δ membranes are determined by both the bulk oxygen diffusion and surface exchange kinetics. The role of the latter factor increases on decreasing temperature and reducing oxygen partial pressure. Due to a relatively high electrical conductivity and moderate thermal expansion coefficients in reducing conditions, SrFe_0.7Mo_0.3O_3–δ-based anodes show a substantially high electrochemical activity.