L. S. Leonova

Protonic conductivity of neutral and acidic silicotungstates

Abstract The salts of composition Me 4− x H x SiW 12 O 40 · n H 2 O (Me=Na, K, Rb, Cs, NH 4 ) are synthesized; their thermal stability, temperature dependence of proton conductivity and structure of protonhydrate shell are studied. The proton conductivity is shown to depend on both the number of acidic protons and polarizability of salt-forming cation of alkaline metal. The presence of differently hydrated protons is discovered in both acidic and neutral salts of 12-silicotungstic acid.

Crystal structure and ionic conductivity of solid electrolytes M5RESi4O12, where M=Na, Ag, RE=Sm→Lu☆

Abstract Conductivity measurements of the crystals of the N5RESi4O12 family, where RE=Sm→Lu, have been made. A reversible phase transition R32 R 3 c has been established. The conductive channels have been determined using a high temperature X-ray structural analysis of the Na5RESi4O12 crystals, as well as the ion exchanged Ag phase.

Thermodynamic equilibria and kinetic reversibility of the solid electrolyte/electron conductor/gas boundary at low temperature

The relation of the concentration dependence of the emf of low-temperature potentiometric gas sensors to the adsorption isotherms of the detected species is considered on the basis of the literature data and the experimental results of the authors. The causes for the deviation of measured emf of the sensors from their theoretical values are discussed. The conditions for the formation of electrochemical products and their elimination from the triphase boundaries of solid electrolyte/electron conductor/gas are also discussed.

Low-temperature proton conductivity in hydrated and nonhydrated tin dioxide.

Abstract The protonic and electronic conductivity of the specimens SnO 2 · n H 2 O, SnO 2 ·Sb 2 O 4 · n H 2 O and SnO 2 at the temperature range from −20 to +60°C and the processes of ion transport through the interface SnO 2 · n H 2 O|Pt in hydrogen containing environments are investigated. A similarity of mechanisms of protonic transport in hydrated and non-hydrated specimens is shown.

Tungsten oxide bronzes with alkali metals

Single-phase samples of tungsten bronzes MxWO3 (M = K+, Rb+, Cs+) are prepared by solid-state synthesis. The reversibility of the M0.33WO3/M+-solid electrolyte interface is studied subject to the alkali metal nature and humidity over a wide temperature interval. The exchange current density at 24°C and 58%-relative humidity is 3.6 × 10−4 A/cm2 for the Rb0.33WO3/Rb+-solid electrolyte interface; 2.2 × 10−4 A/cm2 for the Cs0.33WO3/Cs+-solid electrolyte interface; and 1.3 × 10−4 A/cm2 for the K0.33WO3/K+-solid electrolyte interface. A correlation between the reversibility of the bronze|solid electrolyte interface, which is characterized by the exchange current density, and the rate of potential equilibration in sensor systems, where the bronze is a reference electrode, is revealed. Ionic component of the conductivity of the synthesized tungsten oxide bronzes is measured at a background of the predominant electronic conductivity. The ionic conductivity is three orders of magnitude lower than the electronic conductivity; it decreases in the series Rb0.33WO3 > Cs0.33WO3 > K0.33WO3, amounting to 2.3 × 10−2, 2.1 × 10−3, and 2 × 10−4 S cm−1, respectively. The working capacity of the M0.3WO3 bronzes as reference electrodes in sensor systems for carbon dioxide detection is evaluated. The plots of the cell potential vs. the CO2 concentration in the electrochemical cells are linear, their slopes (59 ± 1 mV/decade) are characteristic for one-electron process. The fastest response to changes in the CO2 concentration was obtained with the sensor system that used Rb0.33WO3 as reference electrode.

Vibrational spectra, structure, and proton conduction in hydrous tin dioxide

SnO2 · nH2O (hydrous tin dioxide, HTD, n = 1.5) and SnO2 · nD2O (deuterated hydrous tin dioxide, DHTD) samples were studied by IR and Raman spectroscopy. The using of these spectroscopic methods elucidated some structural features of the hydrogen-bond network in HTD, where two types of water molecules and two types of hydroxide groups are present. Type 1 water molecules and hydroxide groups are found to be linked to one another by weak hydrogen bonds, as in liquid water. Type 2 water molecules and hydroxide groups are linked to type 1 water molecules and hydroxide groups by rather strong hydrogen bonds. The existence of these strong hydrogen bonds is interpreted as arising from the effect of tin ions on some water molecules and hydroxide groups. Proton conductivity in HTD was found by the impedance method to be a nearly linear function of n. The break of the line at n = 1.3 corresponds to the percolation threshold. The roles of type 1 and type 2 water molecules and hydroxide groups in the generation of proton conduction in HTD are discussed.

Lanthanum-strontium cuprate: A promising cathodic material for solid oxide fuel cells

Samples of lanthanum-strontium cuprate LaSrCuO3.61 are synthesized by a solid-phase method at 1473 K, in air. Electron diffraction patterns reveal low-intensity satellites whose position is described in the reciprocal space by the (3 + 2)-dimensional space group I4/mmm(αα0, α-α0)00mg. Using a five-layer cell LaSrCuO4-δ|YSZ|LaSrCuO4-δ|YSZ|LaCuO4-δ prepared by isostatic hot pressing, the ionic component of conductivity of LaSrCuO4-δ is determined (σ1179 K = 3.8 × 10−3 S cm−1), which is commensurate with other mixed conductors based on complex oxides of cobalt and iron.

Chemical and electrochemical processes in low-temperature superionic hydrogen sulfide sensors

Effect the morphology of the surface of the working electrode (PbS) exerts on the sensitivity of a low-temperature potentiometric hydrogen sulfide sensor is studied. The sensor, which is based on electrochemical cell NaxWO3/NASICON/PbS, may be used for fast selective detection of hydrogen sulfide in air in natural conditions. It is demonstrated that the sensors with PbS that are deposited out of solution have a faster response than the pressed-to ones. The dependence of EMF on the hydrogen sulfide concentration for the former is linear in semilogarithmic coordinates. Thus difference is explained by the microstructure of the lead sulfide layer. It is shown that the lead sulfide interaction with hydrogen sulfide involves a reversible partial reduction of sulfur and lead at the surface. The species that form in so doing contain sulfur atoms in lower oxidation degrees (poly-and oligo sulfides, sulfite). A mechanism of the sensor operation is proposed on the basis of data yielded by experiment and quantum-chemical simulation. The mechanism includes reversible transport of hydrogen from sulfur atoms to oxygen atoms.

The synthesis and transport properties of the cuprates La2 − xSr1 + xCu2O6 + δ

The cuprates La2 − xSr1 + xCu2O6 + δ(x = 0, 0.1, 0.2) are synthesized by solid-state method. Their dc conductivity was studied over the 373 to 1173 K temperature range and 10 to 2.1 × 104 Pa oxygen partial pressure range by using four-probe technique. It is shown that the cuprate conductivity in air is maximal at ∼673 K; it is 60 S/cm for La2SrCu2O6.09; 68 S/cm, for La1.9Sr1.1Cu2O6.18; and 81 S/cm, for La1.8Sr1.2Cu2O6.10. The thermal expansion coefficient of La2SrCu2O6.09 is determined by thermomechanical method and high-temperature X-ray diffraction method; its value (16 ppm K−1) shows that the material is compatible with the ceria-based solid electrolytes during the thermal cycling.

Electrochemical sensors based on platinized Ti1 − xRuxO2

Electrocatalysts based on platinized titania modified with ruthenia (0–9 mol %) were studied. The synthesized materials were investigated as working electrodes in potentiometric sensors sensitive to hydrogen and carbon monoxide. All electrocatalysts showed reproducible behavior at pure gas concentrations from 400 to 4000 ppm. In CO-H2 mixtures with comparable concentrations of both gases, the sensors were selective toward hydrogen at ≥0.05 mol % Ru, but not selective to hydrogen or CO at less than 0.05 mol % Ru in the substrate.