MF-4SK Membranes Research Articles

Ion transport in MF-4SK membranes modified with hydrous zirconia

Composites based on perfluorinated cation-exchange membranes MF-4SK with embedded hydrous zirconia were synthesized. The composites have higher proton conductivity, lower diffusion permeability, and higher ion-transport selectivity than unmodified MF-4SK membranes. Diffusion permeability anisotropy was found in samples with heterogeneous dopant distribution across the membrane.

Investigation of poly(vinylidene chloride) distribution in perfluorinated cation-exchange membranes MF-4SK upon UV- and γ-initiated graft polymerization

A new process for grafting poly(vinylidene chloride) (PVDC) to the membrane material MF-4SK by UV-initiated graft polymerization of the monomer from the gas phase has been developed. Modified membranes containing up to 20 wt % of UV-grafted PVDC have been obtained. Microphotographs of thin sections of the modified membranes have been investigated. It has been shown that the pretreatment of the membranes and variation of UV- or γ-grafting conditions make it possible to achieve an uniform distribution of grafted PVDC both along the thickness of the membrane and in a thin surface layer. The values of the parameters determining the character of the distribution have been estimated. Numerical simulation of the UV- and γ-initiated graft polymerization of VDC gave solutions for the grafted-PVDC distribution fitting with the experimental data.

Transport properties of MF-4SK membranes modified with inorganic dopants

Proton transport in MF-4SK perfluorinated sulfo cationic membranes modified by silica and zirconium hydrogen phosphate were studied using pulsed-field gradient NMR and impedance spectroscopy. At high water contents, water molecules are involved in proton transfer. At low water contents, the proton conductivity of modified membranes considerably exceeds the conductivity of an unmodified membrane. The most likely reason for this effect is the generation of an extra H-bond network involving the dopant and sulfo groups of the membrane.

Structural and electrokinetic characteristics of track-etched initial membranes and membranes modified with perfluorinated sulfocationic ionomer

Structural and electrokinetic characteristics of track-etched microfilters with pore radii of 1 μm and microfilters modified with perfluorinated sulfocationic ionomer are studied as functions of pH and concentration of KCl solutions. Colloidochemical properties of modified microfilters and perfluorinated sulfocationic MF-4SK membrane are compared.

Electrochemical behavior of MF-4SK/polyaniline composites as investigated by membrane voltammetry

Membrane voltammetry was used to investigate composites consisting of perfluorinated MF-4SK membranes and polyaniline (PANI) and synthesized under various conditions. A theoretical analysis of the influence of transport-structure parameters of the ion-exchange membranes on their selectivity and limiting-current value is carried out. For MF-4SK/PANI composites, the increase in the exchange potential at the overlimiting state is found to be more than 2V compared to the original membrane. An analysis of how various factors influence the parameters of a current-voltage curve shows that the presence of anilinium and Fe3+ ions in the electromembrane system has no effect on the value of the potential at the onset of the overlimiting state. A phenomenological model is proposed to account for the increased plateau length at the limiting current through a change in the energetic state of water in the perfluorinated-membrane matrix resulting from the template synthesis of polyaniline.

Electroluminescence of ion-exchange polymeric membranes in the swollen state

The first results on the electroluminescence of the MK-40L, MK-40K, MF-4SK, MA-40L, MA-41L, MA-40K, and MA-41K ion-exchange polymeric membranes in the swollen state are reported. It was found that electroluminescence had the character of flashes. Intensity and time characteristics of electroluminescence were determined over the range of NaCl solution concentrations from 0 to 0.1 M. The special features of electroluminescence from the ion-exchange membranes were determined from their photographs. It was found that the membranes in the air-dry state did not exhibit electroluminescence.

Transport properties of separating membranes MF-4SK during alkaline electrolysis of water

The transport properties of separating membranes MF-4SK are studied during electrolysis of H2O in solutions of KOH. The effective diffusion coefficients of molecules of KOH and H2O and the transfer coefficients of ions K+ and OH− and molecules of H2O are measured at KOH concentrations reaching 11 M, currents reaching 0.31 A cm−2, at ambient temperature and at 80°C. In contact with a KOH solution in the concentration interval 0.1 to 11 M, the membranes that initially swelled in H2O lose a considerable fraction of water that was present in them and the overall volume of clusters and solution-filled channels in them noticeably decreases. The coefficients of transfer by current of ions K+ out of anodic compartment into cathodic and the OH− ions in the reverse direction, respectively, happen to be equal to about 0.6 and 0.4 at ambient temperature and 0.8 and 0.2 at 80°C. The coefficients of transfer of water molecules out of the anodic volume into the cathodic volume in the process of electrolysis happen to be in the limits 1.6–1.9 at ambient temperature and in the limits 2.2–2.8 at 80°C. The effective diffusion coefficients of molecules of KOH and H2O at moderate concentrations of KOH (5.6 M) amount to ∼2.6 × 10−7 and 30 × 10−7 cm2s−1 at ambient temperature and ∼4 × 10−7 and 61 × 10−7 cm2s−1 at 80°C, respectively. At a high concentration of KOH (∼10 M) these quantities substantially diminish.

Effect of modification of ion-exchange membrane MF-4SK on its polarization characteristics

Modifications of perfluorinated membrane MF-4SK are studied by a membrane voltammetry method. Various techniques used for physicochemical modification of the membrane (variation of conditions of its chemical conditioning, insertion of tetrabutylammonium cations) exert predicted action on its structural and electrotransport properties. Correlation is established between variations in the slope of the ohmic segment of a voltammetric curve, the magnitude of the limiting electrodiffusion current, and the potential of the system’s conversion into an overlimiting state and the electrotransport properties of membranes and their structural characteristics that are obtained by independent methods of impedance conductimetry and standard porosimetry. The increase in the limiting current for membranes subjected to a thermal oxidizing treatment is explained by a change in the content and state of water in the membrane bulk. Effects of acceleration of the occurrence of an overlimiting state of the membrane saturated with tetrabutylammonium cations is discovered and possible reasons for this phenomenon are discussed. For membranes saturated with tetrabutylammonium cations, a correlation is established between the limiting current and electroosmotic phenomena.

Influence of modification-regeneration cycles and the charge of the modifying ion on the water permeability of a MF-4SK sulfocationite membrane

The water permeability of the MF-4SK membrane (analogue of the Nafion membrane) modified with singly, doubly, and triply charged metal ions and then regenerated is examined. It is shown that the modification of the membrane with metal ions followed by regeneration substantially increases its water permeability because of a decrease in its thickness. It is found that the introduction of multiply charged ions more significantly affects the characteristics of the membrane.

Adsorption and luminescence properties of Tb3+-modified perfluorosulfonic acid membrane

After treatment of perfluorosulfonic acid membrane MF-4SK with an aqueous solution of TbCl3, the protons in sulfo groups are completely replaced by the hydrated terbium cations. Specific features of the water adsorption and its influence on the luminescence of the modified membrane were examined.

Ion transport in MF-4SK cation-exchange membranes modified with acid zirconium phosphate

An MF-4SK cation-exchange membrane has been modified to obtain composite materials containing acid zirconium phosphate particles. It is demonstrated by electron microscopy and X-ray diffraction that acid zirconium phosphate in the resulting membrane is in the crystalline state. As compared to the initial MF-4SK membrane, the modified membrane shows a somewhat lower diffusion permeability and a higher ion selectivity.

Peculiarities of electrotransport characteristics of composite membranes PANi/MF-4SK in sulfuric acid solutions

A methodological approach towards assessing the conducting, diffusion, and selective properties of perfluorinated membranes MF-4SK and also their composites with polyaniline (PANi/MF-4SK) is presented. Studying these properties in NaCl and H2SO4 solutions makes it possible to elucidate the role played by the nature of counter-ions and co-ions in the charge and mass transfer. It is shown that both individual and composite membranes in their protonic forms are highly selective and, on average, have a conductivity that is higher by a factor of 3.5 as compared with their sodium forms. In contrast, the diffusion permeability is 4-times lower, in line with variation in the charge of the ions that define the mass transfer (co-ions Cl− or HSO4−). Within the framework of model assumptions on the structure of ion-exchange membranes, the transport and structure parameters of MF-4SK and PANi/MF-4SK are determined in H2SO4 solutions for a PANi-saturation of 25%. It is found that aromatic PANi chains localized in aqueous clusters of the template inhibit the diffusion transfer through the PANi/MF-4SK films, whereas the conductivity and transport numbers of PANi/MF-4SK in both NaCl and H2SO4 remain virtually unchanged.

Electrokinetic Phenomena in Sulfonated Cation-Exchange Membranes with Tetraalkylammonium Ions

The electric transport of solvent ions and molecules through membrane systems containing cations of tetraalkylammonium bases is studied. The interrelation between electrokinetic characteristics of sulfonated cation-exchange membranes at different degrees of their saturation with tetrabutylammonium ions and the content of water in membranes at equilibrium is established. A new type of conductor-insulator percolation transitions is revealed for homogeneous MF-4SK perfluorinated membranes saturated with organic counterions. This transition is interpreted with allowance for the concept of the governing role of narrow channels in the membrane microstructure. The electroosmotic permeability of MF-4SK membranes in solutions of sodium and tetraethylammonium chloride, as well as in their mixtures, is studied. An experimentally observed abnormally high amount of water transferred by tetraethylammonium ions is discussed with allowance for the dynamic hydration characteristics of ions.

Composite Polyaniline/MF-4SK Membranes: A Chemical Template Synthesis and the Sorption and Conduction Properties

The sorption and hydrophilic properties of composite membranes PAN/MF-4SK, obtained by a chemical template synthesis, are studied. The synthesis is performed in static and dynamic conditions for perfluorinated sulfo-cationite membranes MF-4SK in 0.01 M aniline solutions in 0.5 M H2SO4 and 0.01 M FeCl3 in 0.5 M H2SO4. The aniline sorption and polymerization kinetics is studied by conductimetry, radioisotope, and optical methods. The polymerization rate is found to depend on the oxidant concentration and the sequence of the membrane saturation with aniline. An increase in the intensity of the color of a composite film, which depends on the balance between the quinoimine, amine, and radical-cation fragments in the perfluorinated matrix, does not lead to any substantial changes in the moisture content or the ac electroconductivity. Implanting polyaniline chains into the perfluorinated matrix presumably leads to a molecular reorganization of water at the expense of a change in its association at the point where redox fragments of polyaniline converge with side segments of the template matrix.

New Optical Sensors for Oxygen Based on Phosphorescent Cationic Water-Soluble Pd(II), Pt(II), and Rh(III) Porphyrins

A new material is proposed for optical sensors for molecular oxygen; the material is obtained by the introduction of Pd(II), Pt(II), and Rh(II) complexes of cationic water-soluble porphyrins into an MF-4SK ion-exchange polymer membrane. The phosphorescence of immobilized metal porphyrins is efficiently quenched by molecular oxygen both in the gas phase and in aqueous solutions. The Stern–Volmer relationships for phosphorescence quenching are linear over the entire range of oxygen concentrations. The new material exhibits good response times and high photochemical stability.

Structure alterations of perfluorinated sulfocationic membranes under the action of ethylene glycol (SAXS and WAXS studies)

Small-angle X-ray scattering and wide-angle X-ray scattering studies were carried out for perfluorinated sulfocationic membranes MF-4SK (membranes of a Nafion type) in the normal ‘as-manufactured’ state and soaked in ethylene glycol at 110 °C and subsequently washed with water. It was shown that such a treatment of MF-4SK membranes resulted in alterations of their nanostructure and these alterations were stable for a long time. On the basis of the paracrystalline layered model of the perfluorinated membrane nanostructure these structure alterations were interpreted as an increase in the thickness of the liquid layers (ionic channels) separating clusters of ionomer groups in MF-4SK membranes. The importance of these structure alterations was confirmed by the twofold growth of the quantum yield of anthracene photo-oxidation catalyzed by tetraphenylporphyrin (TPP) immobilized on MF-4SK membranes treated with ethylene glycol.

Macrokinetics of electrolysis of dilute solutions in systems with a solid polymer electrolyte

A model of contact between an electrode and a MF-4SK membrane in sulfuric acid solutions is studied by the impedance method. It is shown that a layer of higher electrolyte concentration forms near the free electrode surface in heavily diluted solutions. The layer ensures participation of this surface in the electrode process. Electrical fields in the region of contact between the membrane and a current collector are calculated