Layer-by-Layer Modification of Cation Exchange Membranes Controls Ion Selectivity and Water Splitting

The present study investigates the possibility of inducing monovalent ion permselectivity on standard cation exchange membranes, by the layer-by-layer (LbL) assembly of poly­(ethyleneimine) (PEI)/poly­(styrenesulfonate) (PSS) polyelectrolyte multilayers. Coating of the (PEI/PSS)_<i>N</i> LbL multilayers on the CMX membrane caused only moderate variation of the ohmic resistance of the membrane systems. Nonetheless, the polyelectrolyte multilayers had a substantial influence on the monovalent ion permselectivity of the membranes. Permselectivity comparable to that of a commercial monovalent-ion-permselective membrane was obtained with only six bilayers of polyelectrolytes, yet with significantly lower energy consumption per mole of Na⁺ ions transported through the membranes. The monovalent ion permselectivity stems from an increased Donnan exclusion for divalent ions and hydrophobization of the surface of the membranes concomitant to their modification. Double-layer capacitance obtained from impedance measurements shows a qualitative indication of the divalent ion repulsion of the membranes. At overlimiting current densities, water dissociation occurred at membranes with PEI-terminated layers and increased with the number of layers, while it was nearly absent for the PSS-terminated layers. Hence, LbL layers allow switching on and turning off water splitting at the surface of ion exchange membranes