Cation-Exchange Membranes with Controlled Porosity in Electrodialysis Application

Abstract

Numerous attempts have been made to develop ion-exchange membranes with low resistance for various applications such as electrodialysis and fuel cells. In this study, the strategies of immersion precipitation and dry-casting were combined, to control the membrane porosity with the purpose of improving the physical and electrochemical properties of ion-exchange membranes. The porosity was tuned using the time of membrane exposure to an elevated-temperature environment. In addition to controlling the porosity to balance the membrane electrical resistance with the diffusion caused by the concentration gradient, it was experimentally shown that the porosity can influence the IEC and water uptake of the membrane and, thus, further affect the resistance. Furthermore, the surface hydrophilicity was characterized by water contact angle measurements; the results revealed that the porous membranes were more hydrophilic than the dense membranes. As demonstrated by experimental data for desalination by electrodialysis, it was found that a membrane dried at 60 °C for 1 h had the highest desalination efficiency. This is mainly because porous membranes facilitate the transport of ions. Compared to membranes with higher porosity, the membrane prepared with a 1-h aging time had more steric hindrance, which can decrease the diffusion of ions, so that a superior desalination efficiency can be obtained. To further investigate the impact of the density of −SO32– functional groups on the electrodialysis process, membranes with various weight ratios of poly(ether sulfone) (PES) to sulfonated poly(ether sulfone) (SPES) were prepared. With increasing content of SPES, the physical and electrochemical properties of the newly developed porous membranes were changed. A membrane with higher density of functional groups was found to have a higher desalination efficiency, because of the electrostatic effect of the membrane. These results were consistent with the current efficiency. Under optimal membrane preparation conditions, the obtained membrane had a high IEC (1.75 mmol/g) and water uptake (168%). The desalination efficiency reached 95%, and the current efficiency reached 100%. It was concluded that the performance of a porous membrane with controllable porosity can enhance the electrodialysis (ED) process with respect to energy efficiency and desalination efficiency. New methods of fabricating membranes with pores such as immersion precipitation and dry-casting are thought to be potential routes to decreasing the electrical resistance.