On the permselectivity of cation-exchange membranes bearing an ion selective coating

Monovalent ion selective membranes are permeable for monovalent ions while rejecting divalent ions to some extent. Different processes and applications like electrodialysis, microbial fuel cells, and flow batteries utilize such selective properties. Industrial membranes have a specific coating of equal charge to reject the bivalent counter ions. It has been suggested in the past that the current density regime at which such composite ion exchange membranes are operated can strongly influence their ion selectivity towards monovalent ions. Here, we now confirm this observation for a commercial CMS membrane as well as for a novel microgel modified membrane. The higher the applied current density is, the more acute the drop in selectivity becomes as opposed to the unmodified reference membrane where bivalent ions are transported preferentially. This work focuses on the relationship between selectivity and current density regime using (1) a conventional commercial membrane, (2) a commercial monovalent ion selective membranes as well as (3) a tailor-made selective membrane, the latter being surface-modified with quaternized Poly (2-vinylpyridine) (qP2VP) microgels. Experimental electrodialysis results in combination with (a) direct numerical simulations of mixed ion transport through the composite architecture and (b) a swelling analysis of the microgels by ellipsometry reveal that layer responsiveness and structural change of the modification layer is responsible for the decrease in permselectictivity upon an increased current density. The swelling reduces the charge density of the modification and therewith the rejection of divalent ions. It would be highly desirable to synthesize modification layers that have a low ion transport resistance while maintaining its volumetric charge density independent of the current density applied.