Electrically aligned ion channels in cation exchange membranes and their polarized conductivity

The formation of ion channels is crucial in preparation of high-performance ion exchange membranes. We studied the alignment of ion channels in a desired direction to improve the ion conductivity of a membrane. In this study, sulfonic groups, attached to poly(2,6-dimethyl-1,4-phenylene oxide) (SPPO), were aligned according to the direction of the applied electric field. For the proof-of-concept experiment, the ion channels were aligned on a glass plate under an electric field while drying the polymer solution. The formation of in-plane ion channels was confirmed by polarized ion conductivity, atomic force microscopy (AFM), and field emission transmission electron microscopy (FE-TEM). The electrical alignment method was further tested to fabricate an ion exchange membrane with aligned ion channels in the through-plane (thickness) direction, which was then characterized in terms of ion conductivity, transport number, and water uptake. The results show that ion conductivity of the membrane was enhanced up to 12 times without loss of ion selectivity. This study successfully demonstrated an electrical alignment of nanoscale ion channels to increase the ion conductivity of an ion exchange membrane at a given ion exchange capacity.