Three ion-exchange membranes (an AMX homogeneous anion-exchange membrane, a MK-40 heterogeneous cation-exchange membrane, and a Nafion-117 homogeneous cation-exchange membrane) have been studied by electrochemical impedance spectroscopy. Processing of the experimental impedance spectra according to the model developed previously has made it possible to find the Nernst diffusion boundary layer (DBL) thickness δ as a function of current density. The behavior of the AMX membrane has been shown to be close to the “ideal” one described by the model: the impedance spectrum of the membrane is close to the theoretical spectrum and the value of δ is only slightly smaller than the quantity δ Lev calculated by the Leveque equation derived in terms of classical convective diffusion theory. The behavior of the MK-40 and Nafion membranes markedly differs from the “ideal” behavior: the reactive component of the impedance in the region of medium frequencies corresponding to the maximum point in the low-frequency range of a Warburg type finite-length impedance spectrum is significantly lower than its theoretically predicted value. The value of δ is less than δ Lev even for underlimiting currents, and the deviation increases with the increasing current density. This specific behavior of the membranes correlate well with the voltammetry data. The behavior of the studied membranes is associated with the surface properties: the heterogeneity (case of MK-40) and, especially, high hydrophobicity of the (Nafion-117) surface facilitate the development of electroconvection. Homogeneity and high hydrophilicity of the surface of the AMX membrane determine its behavior, which is close to the ideal.

Surface properties were measured together with electrochemical characteristics of a CMX (Neosepta, Tokuyama Corp.) cation-exchange membrane. Relative hydrophobicity was controlled by the contact angle; XPS and SEM were used for characterizing chemical composition and microrelief of the surface, respectively. Voltammetry, chronopotentiometry, and mass transfer rate measurements were made as well. A “fresh” membrane and samples after 10, 25, 100, and 150 h of operation in an electrodialysis cell at an overlimiting current equal to 3 theoretical limiting currents, in a 0.02 M NaCl solution, were characterized. Some electrochemical properties were also measured for a Neosepta cation-exchange membrane, aged 2 years, in an industrial food process. It was found that the hydrophobicity of the CMX membrane has increased after the first 10 h of operation; more and more cavities of the dimension of the order of 1 μm have appeared with time testifying electrochemical erosion of the surface. The limiting current density (ilim) and the overlimiting transfer rate through the CMX membrane increased with time of its operation under overlimiting current. In the case of new CMX, ilim was very close to the theoretical value ilimtheor calculated by the Lévêque equation. After 10 h of operation, ilim increased by 5%, and after 25, 100, and 150 h, the increase was by 30%, 70%, and 100%, respectively. Similarly, the mass transfer rate was found to increase up to 5 times (when desalting 0.005 M NaCl under 3 V) in comparison with the theoretical value. The ensemble of data was explained by the hypothesis that the passage of intensive current produces erosion of the ion-exchange polymer forming a continuous phase in CMX. This erosion results in exposure at the surface of the other constituent of CMX: small (about 100 nm) particles of relatively hydrophobic polyvinylchloride. Increasing surface hydrophobicity facilitates the slip of electroconvective vortexes along the surface. Besides, the geometry of the cavities gives rise to appearing tangential electric force applied to the extended space charge density at cavity’s walls. As the local limiting current density within a cavity is lower than at the flat surface, electroconvective vortices arise at current densities lower than ilimtheor. With time, the number and the size of cavities increase (apparently, due to paired electroconvective vortices occurring inside them) that seems the main reason for overlimiting transfer increase.

Usually in electrochemical systems, the direct current densities not exceeding the limiting current density are applied. However, the recent practice of electrodialysis evidences the interest of other current modes where either the imposed direct current is over the limiting one or a non-constant asymmetrical (such as pulsed) current is used. The paper is devoted to make the mechanisms of mass transfer under these current regimes more clear. The theoretical background for mathematical modelling of mass transfer at overlimiting currents is described. Four effects providing overlimiting current conductance are examined. Two of them are related to water splitting: the appearance of additional charge carriers (H+ and OH− ions) and exaltation effect. Two others are due to coupled convection partially destroying the diffusion boundary layer: gravitational convection and electroconvection. These effects result from formation of concentration gradients (known as concentration polarization) caused by the current flowing under conditions where ionic transport numbers are different in the membrane and solution. Similar effects take place not only in electrodialysis membrane systems, but in electrode ones, in electrophoresis and electrokinetic micro- and nanofluidic devices such as micropumps. The relation of these effects to the properties of the membrane surface (the chemical nature of the fixed groups, the degree of heterogeneity and hydrophobicity, and the geometrical shape of the surface) is analyzed. The interaction between the coupled effects is studied, and the conditions under which one or another effect becomes dominant are discussed. The application of intensive current modes in electrodialysis, the state-of-the-art and perspectives, are considered. It is shown that the intensive current modes are compatible with new trends in water treatment oriented towards Zero Liquid Discharge (ZLD) technologies. The main idea of these hybrid schemes including pressure- and electro-driven processes as well as conventional methods is to provide the precipitation of hardness salts before the membrane modules and that of well dissolved salts after.

A way of improvement of electrornembrane technology for water production required in heat-and-power engineering by applying modified anion exchange membranes is proposed. The novel membranes are produced by treatment of commercial Russian heterogeneous MA-40 membranes with a polyelectrolyte bearing quaternary ammonium groups. It is found that this modification results in lowering the rate of H+ and OH- ions generation (water splitting) at overlimiting currents and in a considerable increase in current efficiency due to the decrease in the OH- ions transfer across the anion-exchange membrane. Moreover, we observed an increment in the salt counterion transfer through the membrane. The decrease in water splitting rate at the interface 'anion-exchange membrane/depleted solution' leads to increasing pH in the desalting compartment and decreasing pH in the concentrating one. As a consequence, the risk of the salt precipitation on the membrane surface in the concentrating compartment is reduced, and the removal of weak acids from the desalting water is found enhanced.

Several laws governing ampholyte transport through ion-exchange membranes are established by a comparative analysis of the concentration dependence of electrical conductivity for homogeneous (CMX, AMX) and heterogeneous (MK-40, MA-41) membranes in NaCl, LysHCl, and NaH2PO4 solutions. The increase in the electrical conductivity of membranes in ampholyte solutions as the solutions become more dilute is explained by the increased fraction of divalent ions of the amino acid (cation-exchange membrane) or from phosphoric acid (anion-exchange membrane) in the membrane as a result of Donnan exclusion of hydrolysis products (hydroxide ions or protons, respectively).

Mechanisms responsible for the overlimiting ion transfer in membranes systems are discussed. The overlimiting transfer is shown to be due largely to the action of four effects coupled with the concentration polarization of the system. Two of these are connected with the water dissociation near the membrane/solution interface: the emergence of additional charge carriers (ions H + or OH − ) in the depleted solution layer and the exaltation of transfer of salt counterions. The latter effect is connected with the perturbation of electric field caused by the water dissociation products. The other two effects are two versions of coupled convection, which leads to partial destruction of the depleted diffusion layer. These include gravitational convection and electroconvection. The former is caused by the emergence of the solution’s density gradient. The latter develops via a mechanism of electroosmotic slip. In this work, methods of voltammetry and chronopotentiometry and pH measurements are used to study the transfer of ions through homogeneous membranes Nafion-117 and AMX as a function of the concentration of sodium chloride solutions in the underlimiting and overlimiting current regimes. In a 0.1 M NaCl solution, gravitational convection makes a considerable contribution to the transfer of salt ions near the membrane surface in intensive current regimes. The influence of this effect on the electrochemical behavior of membrane systems weakens with the solution dilution and with increasing relative transfer of the H + and OH − ions that are generated at the membrane/solution interface. In conditions where gravitational convection is suppressed and the water dissociation near the membrane/solution interface is not great, the major contribution to the overlimiting growth of current is made by electroconvection. Topics for discussion in the paper include the mutual influence of effects on one another, in particular, the effect the rate of generation of the H + and OH − ions exerts on the gravitational convection and electroconvection and the reasons for the different behavior of cation-and anion-exchange membranes in intensive current regimes.

Electrodialysis is mostly used for drinking water production but it has gained applicability in different new fields in recent decades. Membrane characteristics and ion transport properties strongly influence the efficiency of electrodialysis and must be evaluated to avoid an intense energy consumption and ensure long membrane times of usage. To this aim, conducting studies on ion transport across membranes is essential. Several dynamic characterization methods can be employed, among which, chronopotentiometry has shown special relevance because it allows a direct access to the contribution of the potential in different states of the membrane/solution system. The present paper provides a critical review on the use of chronopotentiometry to determine the main membrane transport properties and to evaluate mass transfer phenomena. Properties, such as limiting current density, electrical resistances, plateau length, transport number of counter-ions in the membrane, transition times, and apparent fraction of membrane conductive area have been intensively discussed in the literature and are presented in this review. Some of the phenomena evaluated using this technique are concentration polarization, gravitational convection, electroconvection, water dissociation, and fouling/scaling, all of them also shown herein. Mathematical and experimental studies were considered. New trends in chronopotentiometric studies should include ion-exchange membranes that have been recently developed (presenting anti-fouling, anti-microbial, and monovalent-selective properties) and a deeper discussion on the behaviour of complex solutions that have been often treated by electrodialysis, such as municipal wastewaters. New mathematical models, especially 3D ones, are also expected to be developed in the coming years.

The operational range of electro-driven processes can be extended by (i) reducing diffusion resistances at the membrane/electrolyte interface using spacers and (ii) by electroconvective mixing of the boundary layer at overlimiting current densities. Although many studies have addressed each of these phenomena separately, the details of the interplay of spacer-engineered hydrodynamics and electroconvection are unexplored. In this study, we establish a methodology of a new membrane cell design and the use of a high-speed microparticle tracking velocimetry setup for direct observation and quantification of the 3D velocity field in a spacer-filled electrodialysis channel under process conditions relevant for real electrodialysis applications, i.e., overlimiting currents and elevated Reynolds. We unravel details of the superposition of electroconvective vortices and forced flow velocity with and without an inserted spacer. Overall, the forced flow flattens the vortex structure of electroconvection at increasing Reynolds numbers. In terms of vortex heights, a spacer with intentional dead zones outperforms a commercial spacer that decreases the boundary layer thickness. This study suggests a counter-intuitive spacer design: to exploit electroconvection, an engineered spacer design for maximum use of electroconvection in the far overlimiting region should aim to create intentional regions of low flow velocity.

Four effects providing overlimiting current transfer in ion-exchange membrane systems are examined. Two of them are related to water splitting: the appearance of additional current carriers (H+ and OH- ions) and exaltation effect. Two others are due to coupled convection partially destroying the diffusion boundary layer: gravitational convection and electroconvection. Three anion-exchange membranes, which differ in surface morphology and the nature of ion-exchange sites within a surface layer, are examined. The ion transfer across these membranes in NaCl solutions is studied by voltammetry, chronopotentiometry, and pH-metry. By excluding the effects of water splitting and gravitational convection, it is shown that the main mechanism of overlimiting mass transfer in narrow membrane cells at low salt concentrations is electroconvection. The reasons explaining why water splitting suppresses electroconvection are discussed. The scenario of development of potential oscillations with growing current and time is compared with that described theoretically by Rubinstein and Zaltzman.