E. A. Sanginov

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.

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.

Effect of the structure of calix[4]arene-para-sulfonic acid on its transport properties

The effect of a change in the ambient temperature and humidity on the behavior of calix(4)arene-para-sulfonic acid was studied. For the calixarene under study, at least two stable hydrates were observed: tetra- and octahydrate. The number of water molecules in the crystalline hydrate of the acid was confirmed by simultaneous thermal analysis data and the shape of the IR spectra and diffraction patterns. The change in the composition of the hydrate from n = 4 to n = 20 affected the conductivity but slightly; in the region of phase transitions between the hydrates, the activation energy of conductivity changed abruptly.

Mechanism of proton conductivity in polyvinyl alcohol-phenolsulfonic acid membranes from 1H and 13C NMR data

With line narrowing during magic angle spinning in solid-state NMR, molecular mobility and hydration in composite membranes based on polyvinyl alcohol (PVA) and phenol-2,4-disulfonic acid (PSA) were studied as functions of the ratio of the acidic and polymeric components, the degree of cross-linking in the polymeric matrix, and the moisture content. It is shown that at high relative humidity proton transport takes place by means of the network of hydrogen bonds, which are formed by the H+ counterions, sulfonate groups, and water molecules. At low moisture content, the hydroxyl groups in PVA play an active role in proton transport.

Proton conductivity of perfluorinated and nanocomposite ion exchange membranes in aqueous and water-methanol solutions

The proton conductivity of commercial perfluorinated membranes Nafion-115, MF-4SC and synthesized nanocomposite membranes (polyvinylidenefluoride-sulfonated polystyrene, ultra high molecular weight polyethylene-sulfonated polystyrene, polypropylene-sulfonated polystyrene) is studied as a function of concentration of the water-methanol solution sorbed by membranes, by means of contact impedancemetry with the regulated pressure on the electrode-membrane contacts. The optimal experimental conditions are shown to correspond to a pack of several membranes pressed between gold electrodes with the strength of no less than 40 kg/cm2. It is found that the specific conductivity of synthesized membranes in water at 24°C is 50–120 mS/cm, which virtually coincides with the corresponding values for Nafion-115 and MF-4SC. As the concentration of the water-methanol solution increases from 0 to 60%, the specific conductivity of Nafion-115 and MF-4SC membranes decreases by 30%. The specific conductivity of synthesized nanocomposite membranes decreases by 40–55% in this concentration range.

Synthesis of polymer nanocomposite ion-exchange membranes from sulfonated polystyrene and study of their properties

Methods for the preparation of composite ion-exchange membranes from polymer (polyvinylidene fluoride (PVDF), ultrahigh molecular weight polyethylene (UHMWPE), and polypropylene (PP)) matrices were considered. Polystyrene (PS) was introduced in the matrices by thermal polymerization of the monomer followed by sulfonation of the implant. The fundamentals of membrane synthesis from industrial polytetrafluoroethylene (PTFE, Teflon F-4) films by thermal polymerization of styrene in a film stretched in a monomer solution followed by sulfonation of incorporated PS were described. The literature on radiation- chemical synthesis of composite ion-exchange membranes based on polymer matrices with embedded polystyrene and its subsequent sulfonation was analyzed. Some problems of the kinetics and mechanism of thermal implantation of PS into various polymer matrices under different conditions were discussed. The physicochemical characteristics, structure, and transport properties of the membranes synthesized by thermal implantation of PS were reported. The obtained membranes were tested in low-temperature fuel cells.

FTIR spectroscopic study of the complex formation between H+ and DMSO in Nafion

Nafion membranes plasticized with dimethyl sulfoxide (DMSO) have been examined at room temperature using the vacuum ATR - FTIR spectroscopic technique in the range 50-4000cm-1. The amount of the plasticizer corresponds to the molecular ratio n=DMSO/H+=1.2, 2.3, 4.8, 7.0, 9.7 and 13.3. The medium intensity band with two maxima at 780 and 853cm-1 have been assigned to the ν(SO) stretching vibrations of the H+(DMSO)2 complex. The possible reason of ν(SO) splitting is symmetry decrease of hydrogen bond under the influence of the anion group SO3- electric field. Whereas the mutual association of free DMSO molecules in Nafion leads to appearance of weak band at 86cm-1 assigned to the dipole-dipole interactions.

Aspects of proton transport in calix[6]arene sulfonic acid

The features of proton transport and proton hydration shell structure of calix[6]arene sulfonic acid were studied. XRD and NMR data indicate the existence of crystalline and amorphous phases and various conformations in the acid structure. The hydration process occurs in three stages by thermal analysis and IR spectroscopy. The change in the composition of the hydrate affected the conductivity only at low humidity; an inverse relationship between the water uptake and the activation energy of conductivity is observed in the entire investigated range of humidity.

New Composite Proton-Conducting Membranes Based on Nafion and Cross-Linked Sulfonated Polystyrene

New composite membranes based on commercial perfluorinated Nafion-115 membrane and cross-linked sulfonated polystyrene were synthesized and investigated. The membranes were prepared by radical polymerization of styrene in the presence of a cross-linking agent divinylbenzene in Nafion polymer matrix and subsequent sulfonation of formed polystyrene. The membranes containing approximately 5 and 10 wt % of cross-linked polystyrene with ion-exchange capacity of 1.1 to 1.3 mg-eq/g were obtained. Modification with sulfonated polystyrene leads to an increase in the moisture content and proton conductivity of membranes in the humidity range of 15 to 100 RH.

Transport Rate of Liquid Water and Saturated Water Vapors across Polymer Proton-Exchange Membranes

Dependences of the transport rate of liquid water and saturated water vapor across commercial membranes (Nafion, MF-4SK) and proton-exchange membranes synthesized by the authors (PVDF, PP, UHMWPE, PTFE films modified with sulfonated polystyrene) on the membrane thickness have been studied. It has been found that at room temperature (17–25°C), the transport rate of liquid water and saturated water vapor across the membranes into an air stream hardly depends on the membrane type and thickness (60–240 μm), with the transport rate of saturated vapor being almost an order of magnitude below that of liquid water contacting one of the membrane surfaces. The fact that the flux of water and water vapor across the membrane does not depend on membrane thickness under conditions of maximum moistening suggests that the flow resistance is determined by the resistance at the feed and permeate interfaces. If one of the membrane surfaces is in contact with liquid water, the transport rate is equal to the rate of water removal from the permeate surface of the membrane; in the case of contact with saturated vapor, the transport rate is determined by the rate of water sorption from the vapor phase by the membrane. The results can be used to optimize the operation of fuel cells based on polymer proton-exchange membranes.