Electrodialysis (ED) is a cost-effective and environmentally friendly process; it is considered a key step in Zero Liquid Discharge systems. However, the ED process producing a fairly concentrated solution is not well understood. An ED process where 10 L of 0.5 M NaCl was circulated in diluate stream (DS) and 0.1 L of initially 2.0 M NaCl circulated in concentrate stream (CS) was studied. The concentration of concentrate increased up to 3.5 M. The electrodialyzer ' s design eliminated current leakage. Ion -exchange membranes, MK -40, MA -41 (Shchekinoazot) and CJMA-3 (Chemjoy Polymer Materials), were characterized prior to ED. Based on the concentration dependences of membrane conductivity and diffusion permeability, the " true " ( t i *) ion transport numbers in the membranes were found; the water transport number ( t w ) was found from volumetric measurements. It was shown that the current efficiency, eta , found using t i * is quantitatively consistent with eta found in the ED experiments. To calculate the time dependences of solution concentration and volume in CS, a mathematical model was built. For the first time, t i * and t w determined in independent experiments, were used as parameters. The only fitting parameter was the osmotic permeability coefficient. A good agreement with experimental results is obtained.

The influence of the type and amount of an inert component in perfluorinated MF-4SC sulfonic cation-exchange membrane on its equilibrium physicochemical and transport properties has been studied. The exchange capacity, water content, specific conductivity, and diffusion and electroosmotic permeability of two series of MF-4SC membranes obtained by casting from polymer solutions in dimethylformamide with variable contents of inert fluoropolymers have been investigated. A relationship between the equilibrium and dynamic hydration characteristics of the samples has been found as a result of studying water distribution over water binding energies and effective pore radii and the numbers of water transport in an external electric field. The type and content of an inert component in the perfluorinated membrane have been found to affect more significantly the state of water under equilibrium conditions than the electroosmotic water transport.

The electrotransport properties and the structure of polyaniline-modified sulfocationic membranes MK-40 and MF-4SK are studied in solutions of sulfuric acid and sulfates of nickel and chromium. The modification by polyaniline is performed in elelectrodialyzer. The decrease in conductivity and diffusion permeability of membranes after their modification with polyaniline is assessed in electrolyte solutions of different nature. The key effect of the counter-ion charge on the conductivity of original and modified membranes is confirmed. The unusual effect of the decrease in the MF-4SK/PANI membrane conductivity with an increase in the concentration of solutions containing multiply-charged cations is observed. The information acquired by porosimetry on the effect of multiply-charged ions on the structure of homogeneous and heterogeneous membranes is supplemented by calculations of the transport and structure parameters using the microheterogeneous model of ion-exchange membrane. Based on the analysis of parameters of current–voltage curves in solutions of nickel sulfate and sulfuric acid, the prospects of using the modified membranes in the electrodialysis of solutions containing sulfuric acid and multiply-charged ions are assessed.

The results of application of platinum bulk modified perfluorinated membranes in proton exchange membrane fuel cell (PEMFC) are presented. The change in physicochemical and transport characteristics of the membranes after modification with platinum and at different stages of their operation in the PEMFC are also discussed. The thickness, radius pore distribution obtained by the standard contact porosimetry method, the concentration dependences of the conductivity, and the current-voltage characteristic are studied. The influence of copper in bimetallic electrocatalyst on the characteristics of the perfluorinated membrane are considered. An increase in the efficiency of PEMFC with both commercial and bimetallic catalysts and platinum bulk modified membranes due to the formation of a self-humidifying structure is found.

Comprehensive characterization of basic MK-40 and Ralex CMH heterogeneous cation-exchange membranes and composite membranes with polyaniline based on them is performed including the determination of specific electrical conductivity and diffusion permeability; measurement of current–voltage curves in solutions of sodium, calcium, and magnesium chlorides and hydrochloric acid and curves of distribution of water with respect to the bond energies and effective pore radii as well as assessment of the transport structural parameters of a microheterogeneous model. The time of synthesis of polyaniline on the surface of cation-exchange membranes for obtaining samples with an anisotropic structure and asymmetric electric transport properties is determined by successive diffusion of a solution of an oxidizing agent and a monomer through the membrane into water. It is shown based on the analysis of the electric transport properties, structural characteristics, and model transport structural parameters of the membranes in solutions of singly and doubly charged ions that the obtained materials are promising for use in the processes of electrodialysis desalination of multicomponent solutions.

The physicochemical properties of experimental heterogeneous MK-40 and MA-41 membranes with different ratios of ion-exchange resin and polyethylene binder in their composition are studied. It is shown that the specific electrical conductivity in 0.01–1 M NaCl solution decreases by more than 3 times for cation-exchange membranes and by 2 times for anion-exchange ones with a decrease in the content of ion-exchange resin in the membranes from 69 to 55%. It has been established that the diffusion permeability of anion-exchange membranes is more sensitive to their composition and naturally increases with an increase in the ion-exchange resin proportion in the composition. Information on the influence of the heterogeneous membrane composition on its structure, obtained by method of standard contact porosimetry, is supplemented by the calculation of transport-structural parameters of the microheterogeneous and extended three-wire models of the ion-exchange membrane.

The free solvent transport number in an MF-4SK perfluorinated membrane in solutions of alkaline metal chlorides and hydrochloric acid is for the first time calculated within the framework of a capillary model based on the data of standard contact porosimetry and membrane conductometry. The reasons for the change in the structural characteristics and specific conductivity upon varying the nature of the counterion are discussed. The portion of through mesopores in MF-4SK homogeneous and MK-40 heterogeneous sulfonated cation-exchange membranes is estimated using the experimental data on the water transport numbers in solutions of electrolytes of different natures.

A procedure has been developed for determining the water transport numbers in an ion-exchange membrane by the gravimetric method. Based on a comparative study of this characteristic by the volumetric and gravimetric methods, the experimental conditions (current density, duration of the experiment, and the concentration range of the electrolyte solution) have been found under which the water transport numbers differ by no more than 5%. The electroosmotic permeability, water content, and electrical conductivity of a heterogeneous cation-exchange membrane MK-40 have been studied in a wide range of concentrations of sodium chloride and sulfate solutions. The influence of the nature of the coion on the equilibrium and dynamic hydration characteristics of a heterogeneous membrane has been evaluated. Using the representation of the membrane as a two-phase system, the structure of the hydrated fixed ion–counterion complex has been quantitatively characterized and the hydration numbers of the sulfo group, the sodium counterion, and the sulfate ion in solution have been calculated.

Results of a study of the transport and structural characteristics of an MF-4SK membrane at different stages of membrane operation as part of the membrane electrode assembly of a proton-exchange membrane fuel cell have been described. Membrane degradation has been assessed by the membrane conductometry and voltammetry, standard contact porosimetry, and optical microscopy methods. A quantitative assessment of the effect of a thermomechanical impact exerted during the pressing of the membrane electrode assembly and the different conditions of the assembly operation on the thickness, exchange capacity, water uptake, pore structure characteristics, electrical conductivity, and current–voltage curve parameters of the membrane has been conducted. The main factors that have the most significant effect on the degradation of the perfluorinated membrane have been identified.

Polyaniline has been synthesized on the surface of anion exchange membranes by the successive diffusion of an oxidizer and monomer solutions through a membrane in water. Current–voltage curves of initial and modified membranes in solutions of sodium chloride, calcium chloride, hydrochloric acid, and their equimolar mixtures have been studied. The asymmetry of current–voltage curves of composite membranes due to the polyaniline layer present on their surface have been observed. This effect depends on the nature of ionogenic groups of anion exchange membranes. An increase in the limiting current density as a result of the modification of ion-exchange membranes with polyaniline was found for the first time.

The possibility of calculating the ionic composition of a membrane and the ion-exchange equilibrium constant on the basis of the concentration dependences of the specific conductivity of the membrane in individual and mixed solutions has been studied for an ion-exchange membrane–solution system, in which the solution contains two types of counterions and a common coion. The calculation has been carried out for MF-4SK and MK-40 membranes in a mixed solution of calcium and sodium chlorides, and a satisfactory agreement of the obtained values with those reported in the literature has been shown. This approach has been used to estimate the ion-exchange equilibrium constant for polyaniline-modified MF-4SK and MK-40 membranes, and a correlation has been shown between the results of the conductivity studies and the assessments of the counterion fluxes during the electrodialysis of the mixed solutions.

Several single-and bilayer sulfocationite membranes MF-4SC modified by PANI-halloysite nanotubes are manufactured utilizing dimethylformamide as a solvent and applying two different methods: airbrushing and casting. Electrical conductivity, diffusion permeability, current-voltage curves are examined for the produced nanocomposite single-and bilayer membranes. It is shown that hybrid bilayer perfluorinated sulfocationic membranes demonstrate the asymmetry of transport properties in dependence on their ori-entation towards the counterions' flux. The introduction of PANI-halloysite into the perfluorinated matrix of the MF-4SC membrane leads, in some samples, to decrease in diffusion permeability and in an increase in conductivity, without substantial change in the limiting current density. Comparing the theoretically calculated quantities, like the integral coefficient of diffusion permeability, specific electrical conductivity and the limiting current density in dependence on electrolyte concentration to their experimental values for studied samples, we have found the physicochemical parameters of one-and bilayer cation-exchange perfluorinated membranes modified with PANI-halloysite nanotubes in order to predict their transport properties upon changing the external conditions. This is important for evaluation of capability of the separating membranes in low-temperature fuel cells and electrical devices, like sensors and diodes. (c) 2021 Elsevier Ltd. All rights reserved.

The limiting diffusion current density in electromembrane systems is theoretically estimated by using refined Pears equation and different model approaches for the calculating of the counter-ion transport number in the membrane and its diffusion permeability differential coefficient. To this purpose, experimental data on specific conductivity, diffusion and electroosmotic permeability, as well as the apparent transport numbers of counter-ions in perfluorinated sulfocationite MF-4SK membranes with different specific water content over wide range of sodium chloride solution concentrations are used. Special features of different approaches and models used in the evaluating of the membrane parameters necessary for calculating the electrodiffusion characteristics and the limiting diffusion current are analyzed. The possibility of adequate theoretical estimation of the limiting diffusion current for ion-exchange membranes modified by organic and inorganic dopants is shown. This allows predicting the effectiveness of membranes in electromembrane processes basing on relatively simple measurements of the transport characteristics of the modified ion-exchange membranes.

The effect of the nature, concentration of nanoparticle stabilizers, and pH of the reducing agent solution on morphological features of platinum deposited on the perfluorinated membrane surface is studied. It is found that ethylene glycol and polyethylene glycol stabilize platinum dispersion not only in the solution, but also in the membrane. Efficiency of application of modified membranes as polymer electrolyte in a low-temperature hydrogen–air fuel cell is estimated. It is shown that power characteristics of a membrane-electrode assembly with hybrid membranes obtained using ethylene glycol as platinum dispersion stabilizers are higher than those obtained using polyethylene glycol.

Diffusion permeability and electrical conductivity have been studied for bilayer perfluorinated MF-4SC membranes modified with halloysite nanotubes (HNTs) containing noble metal nanoparticles (NPs) deposited onto their surface. It has been found that the diffusion permeability of the bilayer membranes depends on their orientation with respect to an electrolyte flow: the diffusion flux is higher when the modified side of the membrane faces a flow. The study of the electrodiffusion characteristics of HNT-modified bilayer perfluorinated membranes has resulted in the factors being revealed that have the strongest influence on the development of the asymmetry of their current–voltage curves. It has been found that, to obtain a membrane with a fortiori asymmetric properties, it is necessary to synthesize a material the layers of which differ from each other in only one component—either HNTs or metal NPs. The efficiency of the modified membranes as polymer electrolytes for hydrogen-air fuel cells has been determined, and it has been shown that the modification with HNTs reduces the specific power of a fuel cell, while the presence of platinum NPs on a nanotube surface increases this parameter. The reason for the increase in the specific power is the catalytic activity of platinum NPs in the interaction between oxygen and hydrogen in the membrane bulk, which leads to the self-humidifying of the membrane and a decrease in its ohmic resistance.