Yu. A. Dobrovol’skii

The effect of continuous CO2 laser radiation on the thermal and molecular—Topological properties of polytetrafluoroethylene

The impact of high-intensity laser radiation on a polymer in vacuum is accompanied by the release of gaseous products of degradation and, in some cases, of clusters of the partially destroyed polymer. Polytetrafluoroethylene (PTFE) exhibits an abnormal behavior in this process: being exposed to continuous CO2 laser radiation, it degrades at a high rate and its clusters have a fibrous form. Depending on the irradiation conditions, the fibrous fraction forms two types of product, “cotton wool” and “felt”. Polytetrafluoroethylene and its laser-modified “cotton wool“ product have a semicrystalline topological structure. The preliminary γ-irradiation of PTFE enhances the laser ablation process.

Proton-exchange membranes for hydrogen-air fuel cells

The review is devoted to recent advances in the development of polymer electrolytes for low-temperature (operating temperatures ∼80°C) and medium-temperature (operating temperatures 160–180°C) fuel cells. At present the most used are perfluorinated polymer membranes, such as Nafion®, Aciplex®, Flemon®, and Dow®, owing to their high chemical stability and proton conductance, as well as good machinability. However, successful commercialization of fuel cells with such membranes is prevented by their high cost, as well as low proton conductance at low humidities and temperatures above 100°C. Much effort is underway to develop membranes alternative to perfluorinated ones, with emphasis on aromatic hydrocarbon polymers.

Proton conductivity of calix[4]arene-para-sulfonic acids

The proton conductivity of hydrates of calix[4]arene-para-sulfonic acids is studied. The high proton conductivity is observed at ambient temperatures in the wide ranges of environmental humidity and, correspondingly, water content in the hydrate. For the air humidity of 10 rel. %, the conductivity of compounds is 10−4-10−3 S cm−1 and approaches 10−2-10−1 S cm−1 as the humidity increases up to 70 rel. %.

Electrochemical noise diagnostics: Analysis of algorithm of orthogonal expansions

The algorithm of orthogonal expansions is applied to the problem of extraction of diagnostic information from random noise. It is shown that the spectral analysis of electrochemical noise can be performed by a unified algorithm regardless of the type of orthogonal expansion used. A multi-channel indicator of color of electrochemical noise can be constructed on the basis of the orthogonal expansions. It is concluded that the algorithm of orthogonal expansions is an advantageous tool for noise monitoring and noise diagnostics of practically important electrochemical devices, including the devices of electrochemical energetics and systems of protection against corrosion.

Silicon- and carbon-based anode materials: Quantum-chemical modeling

With the aim of searching for promising anode materials for lithium-ion batteries, we performed quantum-chemical modeling of the structure, stability, and electronic properties of silicon-coated carbon nanotubes, silicon rods, and silicon carbide fibers by the density functional theory method including gradient correction and periodic boundary conditions. It has been demonstrated that nanotubes poorly hold silicon, whereas silicon firmly adheres to the SiC surface. Silicon rods are more favorable than clusters and have the stability close to that of the crystal. The band gap in the rods is close to zero. Silicon carbide can be transformed into a conductor by doping with nitrogen.

Modeling of processes in fuel cells based on sulfonic acid membranes and platinum clusters

The density functional theory with account for gradient correction (DFT/PBE) and periodical boundary conditions was used to model the main stages of processes occurring in hydrogen low-temperature fuel cells. Modeling was carried out at the example of calculation of catalytic anodic and cathodic processes occurring on the surface of the Pt19 catalyst supported on a SnO2 and water adsorption processes on the surface of a membrane represented by a crystal of metisylene sulfonic acid dihydrate [(CH3)3C6H2SO3− · H5O2+]. It was shown that the most energy-efficient process in the membrane is formation of crystals, in which two stoichiometric water molecules correspond to a single SO3H group. Superstoichiometric water is adsorbed on the crystal surface with the adsorption energy of 0.3–0.6 eV; its transition inside the crystal is energy-consuming (2 eV). Barriers of surface proton conductivity are 0.2–0.3 eV.

Lithium-ion conductivity of the Nafion membrane swollen in organic solvents

The lithium-ion conductivity of the lithium form of the commercial perfluorinated membrane Nafion-115 was studied in a series of aprotic solvents in a wide temperature range. The highest ion conductivity was obtained for the sample kept in N,N-dimethyl formamide (DMF); its use in lithium-ion batteries, however, is restricted by its low electrochemical stability. Mixed solvents based on dimethyl sulfoxide (DMSO) with propylene carbonate (PC) and 1,2-dimethoxyethane (DME) additions were optimum solvents for ion transport. The Nafion membrane in the lithium form kept in a mixed solvent DMSO: PC: DME = 1: 1: 2 was characterized by high ion conductivity at room temperature (2.0 mS/cm) and the absence of phase transitions in the temperature range from–40 to +60°C. The activation energy of conductivity of the samples kept in DMF, DMSO, and mixed solvents based on DMSO above the transition temperature was 15–20 kJ/mol.

Interaction of platinum nanoparticles with different types of tin dioxide surface: Quantum-chemical modeling

The interaction of Pt29 nanoparticles with pristine and reduced (110), (100), (011), and (001) SnO2 surfaces has been modeled using the density functional theory method within the generalized gradient approximation (GGA). It has been demonstrated that, in some cases, the reduction of the surface leads to a considerable increase in the energy of interaction with platinum. The second oxidation of such structures should lead to the platinum fixation on the surface.

Synthesis and transport properties of proton-conducting membranes based on polyvinylidene fluoride films with introduced and sulfonated polystyrene

Thermal polymerization of styrene sorbed into a polyvinylidene fluoride (PVdF) film from a toluene solution followed by sulfonation of the resulting material was performed. The kinetics of polystyrene (PS) accumulation in the PVdF film during thermal polymerization was studied. Samples with 6–30 wt % PS and ∼100% PS sulfonation were obtained. Proton-exchange membranes wsith an ion-exchange capacity of up to 2 mg-eq/g and proton conductivity of up to 0.008 S/cm at 75% relative humidity were prepared. The permeability coefficients of water, methanol, and hydrogen and their dependences on the amount of introduced PS, ion-exchange capacity, and water uptake of membranes were measured. The synthesized materials proved similar to MF-4SK membranes in their basic transport characteristics and can be used as proton-exchange membranes in hydrogen-air and alcohol fuel cells.

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.