Applicability of gas-phase isotope exchange method for investigation of porous materials

Journal of Solid State Electrochemistry - Oxygen transport in ceramic oxide materials has been actively explored over the past decades. This is due to the desire to design high-temperature...     

Oxygen isotope exchange in the LSM-YSZ composite under the conditions of long-term tests

The stability of the properties of the composite 40 vol % La_0.6Sr_0.4MnO_{3 ? δ}-60 vol % Zr_0.83Y_0.17O_1.92 (LSM-YSZ) held for 1000 h at a temperature of 800°C and an oxygen pressure of 1.01 kPa and changes in its microstructure were studied by isotope exchange and gas-phase analysis. The oxygen exchange was found to occur by the dissociative adsorption mechanism involving the adsorption forms of oxygen on the triple-phase LSM-YSZ-O₂ boundary. The rate-determining step of the oxygen exchange of the composite did not change with time. The rate of oxygen exchange decreased by 22%, which is comparable to the decrease in the length of the LSM-YSZ-O₂ triple-phase boundary (～20%), on which oxygen exchange is dominant. The conductivity of the LSM-YSZ composite increased due to the connectivity of the material.     

Effect of oxygen nonstoichiometry on kinetics of oxygen exchange and diffusion in lanthanum-strontium cobaltites

The method of isotopic exchange was used to study the kinetics of oxygen exchange and diffusion in complex oxides of La_{1 − x} Sr _x Co_{1 − y} Fe _y O_{3 − δ} (x = 0.0, y = 0.0; x = 0.6, y = 0.2, 0.4). The rates of oxygen interfacial exchange and its diffusion coefficient were determined for LaCoO_{3 − δ} at the pressure of 5 torr in the temperature range of 600–850°C and at the temperature of 700°C in the pressure range of 1–70 torr. The contributions of the three exchange types were calculated. The order of the dependence of the interfacial exchange rate on the oxygen pressure was 0.51 ± 0.01. In the case of La_0.4Sr_0.6Co_{1 − y} Fe _y O_{3 − δ} (y = 0.2, 0.4) in the temperature range of 600–900°C at the oxygen pressure of 10 torr, the oxygen exchange rates and diffusion coefficients were determined in the material bulk and in the subsurface region; contributions of the three types of exchange were calculated. The paper considers the mechanism of oxygen exchange and diffusion as compared to nonstoichiometry in the oxygen sublattice of the La_{1 − x} Sr _x Co_{1 − y} Fe _y O_{3 − δ} oxides.     

Oxygen isotope exchange and electrical conductivity of CaZr1–x Sc x O3–x/2

Journal of Solid State Electrochemistry - The effect of scandium content in CaZr1–x Sc x O3–x/2 on electrical conductivity and oxygen exchange kinetics was investigated. The electrical...     

Phase equilibria,water dissolution,and peculiarities of charge transfer in Ca-doped La<Subscript>2</Subscript>Zr<Subscript>2</Subscript>O<Subscript>7–α</Subscript>

A series of oxides La_{2 - x} Ca _x Zr₂O_7–α (x = 0.00, 0.05, 0.10, 0.15, 0.20) is synthesized. It is found that in samples with the calcium content x = 0.15, 0.20, the second phase Ca_0.9La_0.2Zr_0.9O₃ is present in the fraction increasing with the increase in x. The solubility limit of calcium to form solid solutions based on La₂Zr₂O₇ corresponds to x = 0.1. By high-temperature gravimetry, the proton concentration in La_1.95Са_0.05Zr₂O_7–α is obtained as a function of temperature in the interval of 300–950°С in Н₂О–О₂ atmosphere. According to temperature programmed desorption studies, in the temperature range of 400–900°С at least two types of OH defects with different binding energies are present in the oxide lattice. The temperature dependences of conductivity are obtained for La_1.95Са_0.05Zr₂O_7–α in dry and humid air atmosphere in the temperature range of 350–800°С by the method of impedance spectroscopy. The electrolyte conductivity in humid air is shown to substantially exceed the corresponding values in dry air, which can be associated with manifestation of protonic conductivity in humid atmosphere. The dependences of oxide conductivity on the oxygen content in the gas phase are determined. The conductivity is divided into its ionic and hole components.     

Oxygen isotope exchange in praseodymium nickelate

Oxygen surface exchange kinetics and diffusion were studied in Pr₂NiO_4?+?δ (PNO) by the isotope exchange method with gas phase equilibration in the temperature range of 600–800 °C and oxygen pressure range of 0.33–1.62 kPa. The oxygen heterogeneous exchange rate (r_H), oxygen diffusion coefficient (D), rates of oxygen dissociative adsorption (r_a), and oxygen incorporation (r_i) were calculated along with the apparent activation energies of oxygen surface exchange and diffusion processes. The temperature dependence of r_H was found to benon-linear in Arrhenius coordinates. The apparent activation energy changed from 1.4?±?0.2 eV at T?>?700 °C to 2.0?±?0.1 eV. This might be attributed to the change in the rate-determining stage of oxygen exchange for Pr₂NiO_4?+?δ at T ~?700 °C, because of a shift in the ratio between r_a and r_i caused by the difference in their activation energies. Possible reasons for the observed changes in the rate-determining stage are discussed.     

Options for adjustment of microstructure and conductivity of cathodic substrates of La(Sr)MnO<Subscript>3</Subscript>

The electrodes of solid-oxide fuel cells (SOFCs) must be characterized by high conductivity to decrease ohmic losses and sufficient porosity to provide high gas diffusion rate. In the cases, when the SOFC electrodes are substrates, they must be synthesized at the temperature above the temperature of formation of their solid-electrolyte coating. Herewith, manufacturing of supporting electrodes with the required micro-structure is rather complicated. The present paper studies the effect of the method of manufacturing of the initial La_0.6Sr_0.4MnO₃ (LSM) powders, their degree of dispersion, introduction of sintering additives and pore agents on their microstructure, conductivity, and possibility of adjusting the temperature of SOFC cathodic substrate formation at which the required characteristics are reached. It is shown that sintering of cathodic substrates to the relative density of 65–70% can be carried out at the temperatures from 1050 to 1350–1400°C, which would allow obtaining electrolyte films of powders with different sintering ability on such substrates. The average pore size in cathodic substrates can be varied in the range of 0.4 to 2.5 μm by using the initial LSM powder with different dispersion degree and by employing graphite as a pore agent. At 900°C, conductivity of cathodic substrates of LSM grows at an increase in their relative density from 50% to 70% approximately from 50 to 100 S/cm and weakly depends on the dispersion degree of the initial powders.     

Catalase activity of coarse-grained and nanosized oxide tungsten bronzes obtained by electrolysis of molten salts

Nanocrystalline samples of K _x Li _y WO₃ with hexagonal structure obtained by electrodeposition of molten salts were several times more active in catalytic decomposition than the coarse-grained materials.     

Oxygen isotope exchange between the gas-phase and the electrochemical cell O<Subscript>2</Subscript>, Pt | YSZ | Pt,O<Subscript>2</Subscript> under conditions of applied potential difference

The kinetics of oxygen isotope exchange between gas-phase oxygen and the electrochemical cell O₂, Pt | ZrO₂ + 10 mol % Y₂O₃ (YSZ) | Pt, O₂ with applied potential difference (ΔU = ±1.2 V) is studied in the temperature range of 600–800°С and the oxygen pressure interval of 3–13 kPa. An original design of a vacuum electrochemical cell with the separated gas space is put forward for studying how the potential difference on the electrochemical cell influences the kinetics of interaction of gas-phase oxygen with the gas electrode O₂, Pt | YSZ in the electrochemical cell. It is shown that the oxygen interphase exchange rate is the higher the more negative the charge on the electrode studied; moreover, the mechanism of gas-phase oxygen exchange with the gas electrode O₂, Pt | YSZ in the electrochemical cell depends fundamentally on the electrode charge sign. The possible reasons for the revealed differences are discussed; the corresponding models are proposed.     

Effect of defect structure of lanthanum manganite on oxygen exchange kinetics and diffusion

The method of oxygen isotopic exchange was used to study the oxygen exchange kinetics and diffusion in the LaMnO_{3 + δ} oxide at the temperatures of 600–850°C and in the range of oxygen pressures of 133.3–9332.4 Pa. The rate of interface exchange and diffusion coefficient of oxygen are much lower in the case of LaMnO_{3 + δ} as compared to LaCoO_{3 − δ}, which may be due to the different defect structure of these oxides. It is shown that the first exchange type prevails in LaMnO_{3 + δ} occurring without participation of oxygen from the oxide surface. At the same time, in the case of LaCoO_{3 - δ}, an increase in the temperature results in a significant contribution of both the second and third exchange types with participation of one and two oxygen atoms of the oxide surface, accordingly. The rate determining exchange stage is the process of dissociative oxygen adsorption/desorption on the oxide surface.