E. Yu. Safronova

Synthesis and characterization of MF-4SK + SiO2 hybrid membranes modified with tungstophosphoric heteropolyacid

Composites based on MF-4K perfluorinated cation-exchange membranes doped with hydrous silica nanoparticles, which were precipitated under various conditions, and with tungstophosphoric heteropolyacid nanoparticles were synthesized. The proton conductivity of the composites was studied as a function of temperature and relative ambient humidity. As a result of modification, the water content and ion conductivity of the membranes in low humidities increase by 2.5 orders of magnitude compared to unmodified MF-4SK membranes and the material is rendered less water-dependent.

Prospects of practical application of hybrid membranes

Membrane technologies using novel types of ion-exchange membranes comprising nanoparticles of inorganic substances—hybrid materials—have been vigorously developed in recent years. This modification leads to significant changes in the system of pores and channels and in the transport properties of the membranes. Owing to the changes, these materials exhibit considerable advantages over conventional membranes. This brief review analyzes the prospects for the use of hybrid ion-exchange membranes in water treatment systems, electrolysis cells, electrochemical current-generation devices, and sensors and in catalysis and gas separation processes.

Ionic conductivity of hybrid membranes

The recent data obtained by the authors on the conducting properties of certain hybrid membranes have been summarized. To explain the phenomenon of enhancement of ionic conductivity in these systems, the theory of the semielasticity of membrane pore walls has been proposed. It has been shown that in a low humidity region, transport along the surface of incorporated nanoparticles makes an additional contribution to the increment in conductivity.

Water self-diffusion and ionic conductivity in perfluorinated sulfocationic membranes MF-4SK

Water self-diffusion and ion mobilities in various ionic forms (H+, Li+, Na+, Rb+, Cs+, and Ba2+) of perfluorinated sulfocationic membranes MF-4SK were studied by NMR and impedance spectroscopy. When degrees of hydration are low, the self-diffusion coefficients of water and ionic conductivities are considerably affected by the water content of the membrane. The self-diffusion coefficients decrease in the order H+ > Ba2+ > Cs+ > Rb+ > Na+ > Li+, whereas the ion mobility decreases in the order H+ > Li+ > Na+ > Cs+ > Ba2+.

Diffusion mobility of alkali metals in perfluorinated sulfocationic and carboxylic membranes as probed by 1H, 7Li, 23Na, and 133Cs NMR spectroscopy

The hydration, state, and ion mobility of protons and alkali metal cations (Li+, Na+, and Cs+) in perfluorinated sulfocationic membranes (MF-4SK) and carboxylic membranes (F-4KF) were studied using NMR and impedance spectroscopy. The hydration numbers and translational mobilities of ions and the formation conditions for cation/ionogenic group tight ion pairs were ascertained proceeding from analysis of chemical shifts and 1H, 7Li, 23Na, and 133Cs NMR line widths. NMR data correlate with the results of ion conductivity measurements. The translational mobility of cations and ionic conductivity in the sulfocationic membranes are higher than in the carboxylic membranes and increase in the order Cs+-Na+-Li+.

Transport properties of materials based on MF-4SC membranes and silica manufactured by a casting method

Composites based on MF-4SC perfluorinated cation-exchange membranes were manufactured by casting from a polymer solution with a precursor for subsequent use in the synthesis of hydrous silica. Their proton conductivity was studied as a function of composition, temperature, and relative humidity. Modification of MF-4SC with small dopant amounts increases the water content and ion conductivity, in a reduced humidity, too. Materials containing 3% SiO2 by weight have a maximal conductivity.

Hybrid perfluorosulfonated membranes with zirconia(IV) nanoparticles as an electrode-active material for potentiometric sensors

The effect that modifying MF-4SC and Nafion membranes with ZrO2 nanoparticles has on the sensitivity of PD-sensors (devices with an analytical signal in the form of a Donnan potential) to bulky organic ions in acid solutions of novocaine and lidocaine hydrochlorides and in alkaline solutions of glycine and cysteine is studied. The hybrid membranes are in situ synthesized by introducing dopant particles into the matrix of a finished membrane. An examination of the diffusion permeability of the membranes reveals that modification with ZrO2 nanoparticles leads to a deceleration of the anion transport rate and an improvement in the cation transport selectivity when compared to the original membranes. Potentiometric measurements show that the sensitivity of PD-sensors based on hybrid membranes to cations in acid solutions of NovHCl and LidHCl significantly increases compared to sensors based on unmodified membranes. An increase in the ZrO2 concentration in the membrane matrix results in a considerable increase in the sensitivity of PD-sensors to anions in alkaline solutions of amino acids, while the sensitivity to cations exhibits a tendency to decrease. The results show that hybrid membranes can be used in potentiometric sensors as electrode-active materials that are highly sensitive to various ions.

Determination of glycine, alanine, and leucine at different solution ph with the aid of donnan potential sensors based on hybrid membranes

The cross sensitive sensors whose analytical signal is the Donnan potential (PD-sensors) were developed for the determination of the amino acids glycine, alanine, and leucine in acidic and alkaline solutions. Hybrid materials based on perfluorinated sulfo cation-exchange membranes Nafion and MF-4SC with incorporated zirconium dioxide and silicon dioxide nanoparticles, including those with modified surfaces containing proton-acceptor groups, were used in the PD-sensors. The sensitivity of the PD-sensors to hydronium ions, which interfere with the determination of amino acids at pH < 7, was considerably decreased due to the use of the membranes obtained by an in situ method that contained silicon dioxide nanoparticles with amine-containing groups. The greatest sensitivity of the PD-sensors to the anions of amino acids at pH > 7 and the smallest sensitivity to the cations K+ were observed in hybrid membranes, which combined an increased rate of anion transfer and a low moisture capacity. The use of the PD-sensors based on hybrid membranes makes it possible to determine the cations, anions, and zwitterions of amino acids over a wide range of pH with a sufficiently high accuracy.

Proton conductivity of MxH3–xPX12O40 and MxH4–xSiX12O40 (M = Rb, Cs; X = W, Mo) acid salts of heteropolyacids

We have studied the electrical conductivity of rubidium and cesium acid salts of tungstophosphoric, tungstosilicic, molybdophosphoric, and molybdosilicic heteropolyacids. The materials have the form of hollow spheres up to 2 μm in size, and their surface layer is formed by nanoparticles. The ionic conductivity of the salts has been measured at different relative humidities (from 30 to 95%) in a wide temperature range (from–40 to 90°C). The highest conductivity is offered by the RbxH4–xSiW12O40 samples (σ = 5.3 × 10–4 S/cm at 30°C and 30% RH). Near 0°C, the materials undergo a phase transition which involves water crystallization and a change in activation energy, but their high proton conductivity persists even at negative temperatures.

Ion transport mechanism in hybrid MF-4SC membranes modified by silica and posphotungstic heteropoly acid

Transport properties of MF-4SC perfluorinated cation-exchange membrane composites containing hydrous silica and phosphotungstic heteropoly acid (PTA) nanoparticles were studied at various temperatures. Modification of an MF-4SC membrane increases its ion conductivity under reduced humidity by 2.5 orders of magnitude compared to the unmodified membrane and makes the material less humidity-dependent. Comparison of NMR and impedance spectroscopy data implies that water molecules are involved in ion transport in the test samples. Most likely that modification changes the membrane structure at the nanoscale (broadens channels and increases pores) and generates an additional net of hydrogen bonds involving a dopant and sulfo groups of the membrane.