Influence of Water Uptake, Charge, Manning Parameter and Contact Angle on Water and Salt Transport in Commercial Ion Exchange Membranes

Abstract

In electrochemical processes such as electrodialysis or redox flow batteries, where ion exchange membranes (IEMs) play a critical role in process performance, energy losses can be reduced by minimizing the permeability of IEMs to water and salt. In pure, homogeneous polymer membranes, water permeability is known to be controlled by the size of the free volume elements. However, there is very limited evidence concerning the extent to which this theory applies to practical, commercial IEMs, which frequently have more complex structures. We recently reported water and salt transport characteristics (i.e., permeability, partition, and diffusion coefficients) of 20 commercial IEMs, and demonstrated that water and salt transport were governed primarily by the microstructure of the membrane rather than the polymer chemistry. To further investigate the factors that determine water and salt transport in commercial IEMs, in this study we adopted a statistical approach informed by free volume theory and other literature to examine relationships between transport characteristics and water uptake (i.e., swelling) in addition to fixed charge concentration, ion exchange capacity (IEC), Manning parameter, and contact angle. Our analysis shows that water uptake had the strongest correlation with water and salt transport in commercial IEMs, which is consistent with the predictions of free volume theory for homogeneous polymers; however, the relationship observed between water uptake and permeability in commercial membranes was not as straightforward as that reported in the literature for homogeneous polymers. Membrane charge (IEC) was also correlated with permeability and diffusion coefficients, but to a more limited extent than water uptake, while the Manning parameter and contact angle did not appear to be correlated to any transport properties. Furthermore, there are indications that microstructural differences among membranes may significantly affect permeability. Therefore, further study of IEM microstructure, e.g., phase separation, is an important strategy for advancing the development of commercial IEMs.