N. A. Kononenko

Electrotransport properties and morphology of MF-4SK membranes after surface modification with polyaniline

The method of template synthesis is used for the surface modification of MF-4SK membranes with polyaniline. The influence of the time of polyaniline synthesis in the surface layer of a perfluorinated MF-4SK membrane on its morphology and electrotransport properties is investigated. It is established that under the synthesis conditions, a gradient distribution of polyaniline develops across the thickness of the membrane, and as a result of this, an anisotropic composite structure is formed. It is shown that the specific electrical conductivity and the electroosmotic and diffusion permeability exhibit an extremal character as functions of the time of polyaniline synthesis. When the orientation of these composite membranes is changed with respect to electrolyte flow, an asymmetry effect in their diffusion characteristics is found. With the application of the bilayer fine porous membrane model, the modified-layer thickness is estimated, and the determining influence of the difference in absolute values of effective fixed-charge volume densities on the development of the asymmetry effect is found.

Theoretical and experimental study of asymmetry of diffusion permeability of composite membranes

Theoretical approaches to the description of asymmetry effects of transport characteristics of two-layer membranes are discussed. Based on a uniform finely porous model membrane, a new method is proposed for calculating mass transfer through asymmetric membranes with allowance for the physicochemical properties of their layers. The difference between the effective densities of charges fixed in membrane layers is shown to be the key factor that governs the degree of asymmetry of diffusion permeability. The adequacy of the proposed approach is supported by the comparison between theoretical calculations and experimental data on the diffusion permeability of modified ion-exchange membranes formed on polystyrene and perfluorinated matrices. It is shown that this approach is applicable to the description of the asymmetry effects of diffusion characteristics of MF-4SK perfluorinated membranes, the surface layer of which is modified with polyaniline.

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.

Porous structure of ion exchange membranes investigated by various techniques

A comparative review of various techniques is provided: mercury intrusion porosimetry, nitrogen sorption porosimetry, differential scanning calorimetry (DSC)-based thermoporosimetry, and standard contact porosimetry (SCP), which allows determining pore volume distribution versus pore radius/water binding energy in ion-exchange membranes (IEMs). IEMs in the swollen state have a labile structure involving micro-, meso- and macropores, whose size is a function of the external water vapor pressure. For such materials, the most appropriate methods for quantifying their porosity are DSC and SCP. Especially significant information is given by the SCP method allowing measuring porosimetric curves in a very large pore size range from 1 to 105nm. Experimental results of water distribution in homogeneous and heterogeneous commercial and modified IEMs are presented. The effect of various factors on water distribution is reviewed, i.e. nature of polymeric matrix and functional groups, method for membrane preparation, membrane ageing. A special attention is given to the effect of membrane modification by embedding nanoparticles in their structure. The porosimetric curves are considered along with the results of electrochemical characterization involving the measurements of membrane conductivity, as well as diffusion and electroosmotic permeability. It is shown that addition of nanoparticles may lead to either increase or decrease of water content in IEMs, different ranges of pore size being affected. Hybrid membranes modified with hydrated zirconium dioxide exhibit much higher permselectivity in comparison with the pristine membranes. The diversity of the responses of membrane properties to their modification allows for formation of membranes suitable for fuel cells, electrodialysis or other applications.

Influence of conditions of polyaniline synthesis in perfluorinated membrane on electrotransport properties and surface morphology of composites

The conductivity, diffusion, and electroosmotic permeability of composites, obtained in different conditions of polyaniline synthesis on the surface or in volume of perfluorinated MF-4SK membrane, were studied. The oxidative polymerization of aniline was carried out in static conditions in mixture of monomer and oxidant solutions, in concentration field by method of successive diffusion of polymerization solutions through the membrane in water, and in an external electrical field. The concentration of polymerization solutions, the type of electron acceptors, duration of synthesis, and current density were varied to obtain a set of composites. The certain advantages of every method of polyaniline chemical template synthesis were revealed to prepare the materials with required set of the electrotransport characteristics.

New approach to the characterization of ion-exchange membranes using a set of model parameters

An extended three-wire model of the conductivity of ion-exchange membranes is proposed. The model includes a set of three-wire model equations supplemented with coupling equations between three-wire model parameters and the parameters of a microheterogeneous model. To calculate the complete set of parameters for evaluating the structural inhomogeneity of a membrane and calculating its effective electrotransport properties, it is necessary and sufficient to get only the concentration dependence of the specific conductivity of the membrane, which simplifies the characterization procedure. The model was verified based on independent experimental data and the calculations of transport-structural parameters with the use of the microheterogeneous model. It was found that the proposed model approach makes it possible to obtain a reliable combination of structural parameters to characterize the volume fractions of conductive phases and their mutual arrangement in the ion exchange material peculiar to the membrane test sample.

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.

The three-wire model and Lichtenecker’s equation for calculations of the conductivity of ion-exchange columns

The approaches to the evaluation of the conductivity of ion-exchange columns are analyzed. Specifically, the three-wire model and Lichtenecker’s power equation are examined. The problem of how the coefficient of ion-exchange column filling and the parameter characterizing the state of conducting phases with respect to the current flow are related to the parameters involved in the equations of the three-wire model is treated theoretically. The common points and the coincident portions of the functions obtained in each of the approaches and some discrepancies between the functions are found. The discrepancies are determined by the special features of the models under consideration.

Ion Transport in sulfuric acid solution through anisotropic composites based on heterogeneous membranes and polyaniline

Anisotropic composites based on heterogeneous ion-exchange membranes MK-40 and polyaniline have been prepared in a constant electric field. The diffusion and electric conduction properties of these materials in sulfuric acid solutions have been studied, and the diffusion permeability of the composites has been shown to decrease compared to the base membrane, with the conductivity remaining relatively unchanged. Investigation of mass transport of ions through the composite membranes during electrodialytic demineralization of sulfuric acid solutions has revealed that the performance of such materials in high-intensity current modes is higher than that of the base MK-40 membranes. In particular, the energy consumption for demineralization of sulfuric acid solutions is reduced by 40–60% and the current efficiency increases approximately twofold under certain conditions. The mass transfer and energy characteristics of the electrodialysis process have been found to depend on the orientation of the modified layer of the composite membranes with respect to the counterion flow.

The polyaniline/MF-4SK composite systems with modified surface layer

Processes of aniline polymerization in perfluorinated polysulfonic acid MF-4SK solution are studied by UV-spectroscopy. Membranes with polyaniline-modified surface layer are synthesized. It is shown by voltammetry and potentiometry that the composite materials demonstrate asymmetric ion transfer with respect to protons and sodium cations.