Selectivity of Transport Processes in Ion-Exchange Membranes: Relationship with the Structure and Methods for Its Improvement

Abstract

Nowadays, ion-exchange membranes have numerous applications in water desalination, electrolysis, chemistry, food, health, energy, environment and other fields. All of these applications require high selectivity of ion transfer, i.e., high membrane permselectivity. The transport properties of ion-exchange membranes are determined by their structure, composition and preparation method. For various applications, the selectivity of transfer processes can be characterized by different parameters, for example, by the transport number of counterions (permselectivity in electrodialysis) or by the ratio of ionic conductivity to the permeability of some gases (crossover in fuel cells). However, in most cases there is a correlation: the higher the flux density of the target component through the membrane, the lower the selectivity of the process. This correlation has two aspects: first, it follows from the membrane material properties, often expressed as the trade-off between membrane permeability and permselectivity; and, second, it is due to the concentration polarization phenomenon, which increases with an increase in the applied driving force. In this review, both aspects are considered. Recent research and progress in the membrane selectivity improvement, mainly including a number of approaches as crosslinking, nanoparticle doping, surface modification, and the use of special synthetic methods (e.g., synthesis of grafted membranes or membranes with a fairly rigid three-dimensional matrix) are summarized. These approaches are promising for the ion-exchange membranes synthesis for electrodialysis, alternative energy, and the valuable component extraction from natural or waste-water. Perspectives on future development in this research field are also discussed.

Ionic Mobility in Ion-Exchange Membranes

“…With an increase in the water uptake, the size of membrane pores increases [ 58 ]. This also leads to an increase in the size of the channels, which limits the conductivity [ 113 , 134 , 135 ].…”

“…Nevertheless, even in this case, it is necessary to limit gas diffusion through the membrane as much as possible, which determines the so-called fuel crossover—the undesired passage of fuel, not accompanied by energy production [ 55 , 56 , 57 ]. The ratio of fluxes of desired and undesired components determines the value of selectivity, which is one of the most important membrane operating parameters [ 37 , 58 ]. Selectivity is determined by the ratio of the mobility of different ions or molecules in the membrane.…”

The influence of poly(3,4‐ethylenedioxythiophene) modification on the transport properties and fuel cell performance of Nafion‐117 membranes

“…Based on the comparison of ionic conductivity values (Figure 6), we can assume that the diffusion rate is limited by the migration of Na + ions, the transfer rate of which increases by an average of 2.7 times. It can be assumed that the reason for this is a slight increase in the pore size and, as a consequence, of the channels connecting them 21 upon PEDOT introduction. It is the size of these channels, which limits the ionic transfer in membranes, leads to this effect 40 .…”

“…Compatibility is an important parameter for the formation of copolymers and composite membranes 18–20 . A large number of electronegative atoms (N, S, O) in polyaniline, polypyrrole, and other conducting polymers determines the possibility of their incorporation into the system of membrane pores and channels containing a large number of hydrophilic SO 3 H groups 21 . Polypyrrole and polyaniline are shown to be effective in improving the transport properties of perfluorosulfonic acid membranes and the performance of direct methanol fuel cells (DMFCs) based on these membranes 13,22–25 .…”

Ion and Molecule Transport in Membrane Systems 2.0

“…Structure–properties relationships in membrane systems were studied in [ 11 , 12 ]. M. Izquierdo-Gil [ 11 ] studied the relationship between NaCl electrolyte permeability and water content for three sulfonated cation-exchange membranes largely used in the practice of electro-membrane processes: MK-40, Nafion N324, and Nafion 117 membranes.…”