Selectivity of transport processes in ion-exchange membranes

Andrey Yaroslavtsev
Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Moscow,  Russia

 Ion exchange membranes are widely used in different applications. Each of them has its own requirements. The presence of high ionic conductivity and selectivity of transport processes are basic/main requirements usually. Selectivity can be estimated by the ratio of transfer rates of coions (or gas molecules) and counterions. The purpose of this report is to describe the approaches used to design membranes which combine high selectivity of transport processes with high ionic conductivity.

The structure of cation-exchange membranes can be described by the Gierke model. Hydrophilic functional groups with sorbed water form a system of pores and channels in the hydrophobic polymer matrix. Most of the cations are located in the Debye layer, near the negative charged pore walls. The transport in this layer determines the ionic conductivity of the membranes. The remaining pore volume is filled with an electrically neutral solution with low concentration of coions or gas molecules which determines their slow transfer and a decrease in membrane selectivity.

The large pores in heterogeneous membranes determine a decrease in their selectivity. Grafting polymerization of styrene inside an activated polymer film with subsequent sulfonation allows to obtain grafted membranes with the composition close to heterogeneous one, but the absence of secondary porosity makes them much more selective1. It is possible to optimize the ionic conductivity and selectivity of membranes by varying the degree of grafting and crosslinking.

An increase in the ionic conductivity can be achieved by membrane doping by acidic nanoparticles. On the contrary, zirconia with basic properties can bind the part of the current curriers, decreasing membrane conductivity and water uptake and increasing its selectivity. In the case of acidic silica, negatively charged particles should be localized in the pore center. It results in the decrease in alcohol transfer and methanol crossover in fuel cells.