N. G. Bukun

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.

Impedance of solid electrolyte systems

The main regularities of studying electrode processes in the systems with solid electrolytes are described using the method of electrochemical impedance with the relationships of Ershler-Randles and Frumkin-Melik-Gaykazyan and analysis of results based on equivalent electric circuits.

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.

Percolation model of conductivity of calix[n]arene-p-sulfonic acids

Proton conductivity of special class of aromatic sulfonic acids is described, in particular, calixarene sulfonic acids that consist of flat anionic layers interlinked by labile two-dimensional hydrogen-bond network. High proton conductivity of their hydrates was observed earlier. The dependence of their transport characteristics (the proton conductivity, the activation energy of conductivity) was shown to have threshold character. The studied systems’ behavior is described on basis of percolation model that assumes changing of the proton transport mechanism at low water content in the structure.

Oxide supported platinum electrocatalysts for hydrogen and alcohol fuel cells

Efficient oxide supported electrocatalysts for hydrogen and alcohol fuel cells are developed. They are characterized by a low content of platinum, exhibit high activity in the oxidation of low-molecular alcohols and tolerance to the CO poisoning. It is shown that the application of catalysts developed (Pt/SnO2-SbOx) enables one to raise the power of fuel cells operating on ethanol approximately by two times as compared with similar fuel cells with commercial PtRu/C catalysts.

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.

Superionic Transitions in NASICON-Type Solid Electrolytes

Four different methods, temperature dependent X-ray diffraction, impedance measurements, calorimetry and dilatometry, were employed to investigate the superionic transitons of NASICON type materials. Two singularities are found at 160 – 170 °C and about 230 °C. The first one is related to a second order phase transition close to the temperature of structural phase transition; the second one coincides with the inflection in the temperature dependence of conductivity.

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.

Electrochemical properties of systems with rubidium-tungsten-oxide bronze

Single-phase rubidium-tungsten-oxide bronze of the composition Rb0.3WO3 with hexagonal syngony (a = 0.7387 ± 0.0004 nm, c = 0.7547 ± 0.0004 nm) is synthesized. The ionic constituent of the bronze conductivity, determined for the first time ever, is nearly three orders of magnitude as small as the electronic constituent and varies from 1.1 × 10−2 to 2.7 × 10−2 S cm−1 at 30–84°C. The reversibility of the Rb0.3WO3/Rb6La2Si6O18 interface increases with the environment humidity. The exchange current is equal to 6.4 × 10−4 A cm−2 at a temperature of 30°C and a relative humidity of 58%. It is shown that the synthesized bronze may, in principle, be utilized in the role of a reference electrode in solid-state potentiometric sensors for carbon dioxide and hydrogen sulfide that would be capable of operating in natural conditions.

Effect of hydration of solid electrolytes on the behavior of boundaries Na0.5WO3/Na+-solid electrolyte

Abstract The effect of hydration of sodium-conducting solid electrolytes (SEs) on the reversibility of Na 0.5 WO 3 /Na + -solid electrolyte interfaces was studied. It was shown that solid electrolytes with higher tendency to hydration form the interfaces with higher room-temperature reversibility with respect to Na + ions. According to IR spectra, the ability to hydration decreases in series Na 5 GdSi 4 O 12 >Na 3 Zr 2 Si 2 PO 12 >Na 3 Sc 2 (PO 4 ) 3 . It is supposed that hydrated layers at the surfaces of solid electrolyte grains exhibit a more disordered structure than the bulk of the grains and, thus, promote the electrochemical reactions.