Overlimiting mass transfer through cation-exchange membranes modified by Nafion film and carbon nanotubes

Eight cation-exchange membranes different in the surface morphology and the degree of hydrophobicity were studied by contact angle, voltammetry and mass transfer rate measurements. One series of membranes was prepared starting from heterogeneous MK-40 membranes, and another, from homogeneous Nafion (TM) 117 membranes. Coating a membrane with a thin film of Nation resulted in increasing surface hydrophobicity, while the doping of the Nation surface film with carbon nanotubes (CNT) led to an unexpected decrease in hydrophobicity. It was found however that after 100 h operation of a Nafion (TM) 117 membrane coated with a Nation film doped with CNT, the contact angle increased from 51 to 81 degrees. This increase in the surface hydrophobicity was accompanied by a significant rise in overlimiting transfer rate, more than 1.5 times, under the same voltage. High correlation between the overlimiting mass transfer rate and the degree of hydrophobicity was observed also in all studied cases: more hydrophobic surface leads to a higher mass transfer rate. The effect is explained by increasing electroconvection occurring as electroosmosis of the second kind: the slip of water over a hydrophobic surface enhances the tangential velocity of electroconvective vortex having its maximum at a distance of several hundreds of nm from the surface. (C) 2011 Elsevier Ltd. All rights reserved.