Ion Exchange Membranes: Constructing and Tuning Ion Transport Channels

Abstract

Ion exchange membranes (IEMs) that can selectively transport ions are crucial to a variety of applications, such as ion extraction/separation, fuel cells, redox flow batteries, and water electrolysis. IEM performance, in terms of membrane permeability/conductivity and selectivity, relies heavily on the formation of effective ion transport channels within the membranes, and there exists a tradeoff between permeability/conductivity and selectivity, which obstructs the development and widespread adoption of IEM-based processes. To overcome this tradeoff and to advance IEM-related applications, extensive research efforts are devoted to the construction and tuning of ion transport channels, and various strategies are proposed. These strategies mainly include 1) inducing microphase separation by properly regulating the polymer architecture, 2) introducing a third phase/region for ion transfer, 3) realizing a high free volume with polyrotaxanes or polymers with sterically bulky groups, and 4) constructing ion transport channels with nanoporous materials. These strategies are outlined and summarized in this review. Perspectives and future research directions are also discussed.

Branched Poly(arylene ether ketone sulfone)s with Ultradensely Sulfonated Branched Centers for Proton Exchange Membranes: Effect of the Positions of the Sulfonic Acid Groups

“…Such a significant gap in the water affinity between two types of membranes is mainly due to two reasons, one of which is the different regularity in the polymer architecture. Many published papers demonstrate that sulfonated polymers with regular architecture (such as block and densely sulfonated) present less water absorption than the polymer bearing the disordered distribution of sulfonic acid groups. ,, The sulfonic acid groups are ultradensely tethered on the branched centers of B- x -SPAEKS; in contrast, those are relatively randomly modified on the branched arms of B-10-SPAEKS-30. In addition, the discrepancy in free volume is likely the second reason. , As listed in Table , the densities of B- x -SPAEKS membranes in a dry state were higher than that of the B-10-SPAEKS-30 membrane.…”

“…The swelling variations of the PEMs are usually directly correlated to water absorption. , Accordingly, the SRs of B- x -SPAEKS followed a rising trend in terms of IP, TP, and volume as the WUs increased. B-10-SPAEKS-30 exhibited stronger SRs in any dimension than B-10-SPAEKS due to larger WU.…”

“…Many published papers demonstrate that sulfonated polymers with regular architecture (such as block and densely sulfonated) present less water absorption than the polymer bearing the disordered distribution of sulfonic acid groups. 30,31,41 The sulfonic acid groups are ultradensely tethered on the branched centers of B-x-SPAEKS; in contrast, those are relatively randomly modified on the branched arms of B-10-SPAEKS-30. In addition, the discrepancy in free volume is likely the second reason.…”

Highly Ion-Conductive and Alkaline Stable Anion Exchange Membranes Containing Bulky Aryl Units and Symmetric bis-Piperidinium Branches

“…It is generally believed that an effective strategy involves balancing the trade-off relationship between OH − conductivity and dimensional stability and building efficient ion transport channels, by reasonably designing the chemical structures of AEMs. 18,19 Up to now, various methods have been developed to facilitate the formation of ion transport channels, including the introduction of structural components such as blocks, 20 ionized side chains, 21 branched structures, 22 and hydrophobic side chains. 23 For example, AEMs based on branching side chains have good hydroxide conductivity and dimensional stability.…”

Ionic-Nanophase Hybridization of Nafion by Supramolecular Patching for Enhanced Proton Selectivity in Redox Flow Batteries

“…The development of a sustainable society needs advanced energy technology. Vanadium redox flow batteries (VRFBs) are promising candidates to provide a reliable power supply because of their high safety, flexible design, and long-term operation durability. − Proton exchange membranes, which serve as a separator for positive/negative electrolyte solutions and a conductor to complete the internal circuit, play a key role in VRFBs. Currently, Nafion series membranes are predominantly applied in VRFBs for their high conductivity and superior chemical stability under highly acidic and oxidative conditions.…”