Influence of structural changes on current-voltage characteristics of the anion exchange membrane after their long contact with an ampholyte solution

Abstract

This work aims to study the evolution of the transport properties and electrochemical characteristics of the anion exchange membrane Ralex AMH-PES (Mega, Czech Republic) after prolonged contact with a solution of sodium dihydrogen phosphate. The investigated membrane was divided into two pieces, one of which was balanced with a solution of NaH2PO4 (0.02 mol/dm3) and exposed there for 336 hours, while the other sample was kept in the NaCl solution (0.02 mol/dm3). The transport and current-voltage characteristics of both samples were studied in NaCl solution.
The membrane electrical conductivity was measured by the differential method using a clip-type cell. The values of the inter-gel volume fractions were found by the tilt angles of the conductivity concentration dependences, constructed in the bilogarithmic coordinates. The membrane thickness was controlled by the precision digital micrometer. The membrane diffusion permeability was measured in a two-chamber flow cell. Current-voltage characteristics of the samples were obtained in the four-chamber flow cell.
The results showed that two weeks exposure of the anion exchange membrane in the sodium dihydrogen phosphate solution led to an increase of its thickness and diffusion permeability compared to the sample located in the NaCl solution. At the same time, it did not cause significant changes in the values of conductivity and effective transport numbers of co- and counter-ions. However, the length of the plateau on the current-voltage characteristic of the membrane decreased, and the value of limiting current increased by 9%. The detected changes in transport and electrochemical characteristics of the investigated anion exchange membrane apparently caused by a substantial swelling of the ion exchange material in the presence of strongly hydrated anions of phosphoric acid. The later led to the amplification of osmotic and electroosmotic transport of water through the central part of the membrane pores and to the development of equilibrium electroconvection