Selectivity of ion exchange membranes: A review

Ion exchange membranes (IEMs) have been established as a key component in industrial water desalination and electrolysis processes. Thus, nowadays, they are being studied and developed for application in new energy conversion and storage systems as well as efficient desalination and wastewater treatment processes. These processes include redox flow batteries, reverse electrodialysis, membrane capacitive deionization, microbial fuel cells, and ion exchange membrane bioreactors. Ion permselectivity between counter- and co-ions, the most essential property in IEMs, makes these processes possible and/or efficient. Additionally, ion selectivity between different counter-ions is required for these novel processes to be efficient. This review aims to provide a comprehensive overview of ion exchange membrane permselectivity by summarizing the developments in this field over the past decade. Membrane microstructure and possible mechanisms for ion transport in the membrane phase are discussed with respect to permselectivity, along with the influence of current density and related membrane-solution boundary conditions. A selectivity order for the transport of common anions through conventional anion exchange membranes (AEMs) is generalized, the same is done for cation transport through conventional cation exchange membranes (CEMs). Two types of experimental methods for the determination of ion permselectivity: electrodialysis and the membrane potential method, are summarized. Finally, relevant membrane preparation methods and surface modification of IEMs reported over the past decade are classified and discussed. In summary, we conclude that synthetic methods have evolved rapidly; however, development of fundamental knowledge on the many complex phenomena occurring in the adaptive membrane bulk and solution-interface environments is not sufficiently established. Thus, much effort is required to combine experimental, theoretical, and simulation data for a more comprehensive and coherent understanding of these phenomena.