The Preparation And Ion Transport Performance Research Of Ion Exchange Membranes With A Mobile Ion Shuttle

Ion exchange membranes (IEMs) are polymer electrolytes that conduct anions or cations, as they contain positively or negatively charged groups bound covalently to polymer backbones. Over the last decades, IEMs have been widely used on a large industrial scale in processes such as diffusion dialysis, electrodialysis, electrolysis, for seawater desalination, the treatment of industrial effluents, and chlorine-alkaline production. More recently, to reduce reliance on fossil fuels, there has been increasing interest in the use of IEMs in energy conversion and storage systems, such as reverse electrodialysis (RED), polymer electrolyte fuel cells (PEFCs), microbial fuel cells and redox flow batteries (RFB). In particular, the polymer electrolyte membrane fuel cell (PEMFC) is now considered as one of the most promising green energy-conversion technologies for stationary and mobile applications, due to the high fuel conversion efficiency. As one of the key componets of the PEMFC, IEMs are required to posses sufficient ionic conductivity for the commercialization of this technique. Prior investigations have noted that conventional IEMs in which the ionic groups are commonly connected to the polymer backbones or pended on side chains exhibit a unexpected dependence of ionic conductivity on the ionic content. Although increasing the ionic content in a covalent IEM can help to reach the ostensible goal of "higher conductivity," it, in turn, causes undesirable, excessive dimensional swelling from the associated water of hydration. Thus, to mitigate this "trade-off" effect, this study describe the preparation of a novel IEM containing "ion shuttles" by introducing "rotaxane" into polymer mainchains to increase the mobility of the side chains, rather than the ionic content of IEMs.One of the striking features of these polymers compared with covalently-bounded polymers is their structural resemblance to polymer blends, wherein the linear embedded axle components can reversibly vary the aggregation and relative spatial positions with respect to changes in the physical and chemical environment (e.g., temperature and pH)." Rotaxane" is derived from the Latin words "wheel" and "axle", and describes a compound that consists of a linear species and cyclic species bound together in a threaded structure by no ncovalent forces. The two components is incorporated together through hydrogen bonding. The thermal- and pH-triggered mechanical motion of polyrotaxane suggests a fundamentally new direction for developing novel IEMs with movable side chains. Polyrotaxanes represent a significant class of non-covalently-bonded polymers, in which linear molecular embedded axles (guests), have mobility through macrocyclic cavities (hosts) in the main chains. Inspired by this unique molecular architecture, we prepare IEMs by threading linear guests containing two dibenzylammonium centers into an aromatic main chain bearing dibenzo [24]crown-8 (DB24C8) cavities via host-guest molecular recognition, followed by confining the guest with bulky end groups. Their ionic conductivities (e.g. H+, OH- and HCO3-) have been investigated.(1) Firstly, we synthesized a polymer containing crown ether by Friedel-Crafts acylation. Proton conducting membrane was prepared by threading a linear guests containing two dibenzylammonium centers into an aromatic main chain bearing dibenzo[24]crown-8 (DB24C8) cavities via host-guest molecular recognition, followed by confining the guest with bulky 1-naphthol-3-sulfonic acid sodium salt. The result suggests that the membrane with a low IEC of 0.78 exhibits a maxium proton conductivity of 280 mS/cm at 85 ℃. The mobile ion shuttles are responsible for it, and also endow the membrane better stability than others with the similar proton conductivity.(2) In addition to proton conducting membrane, we prepared hydroxide ions conducting membrane by confining the shuttles with bulky ammonium end groups. The pH-and thermal-triggered mobile ion shuttles endow the membrane excellent hydroxide conductivity, which increases from 58 mS/cm at 30℃ and reaches a maximum of 189 mS/cm at 90℃.This result is among the best reported for covalent AAEMs to date-but with a much lower IEC (0.68 mmol/g).(3) Based on this new concept, we also prepared bicarbonate conducting membranes by confining the shuttles with bulky triphenylphosphine end groups. The conductivity can reach up to 110 mS/cm at 85℃ with a IEC of 1.05 mmol/g. Moreover, the high stability suggests its potential in fuel cell application.In conclusion, we report a new approach that provides the first example of IEMs with a mobile ion shuttle, made by threading ionic linear chains into poly (crown ether) main chains via host-guest molecular recognition. The pH- and thermally-triggered disruption of this interaction allows ion shuttle mobility, which accelerates ions transport. The mobile shuttle architecture provides a novel mode of ion transport for IEMs, which can be utilized in a number of ion exchange applications, and which have conductivities higher than conventional membranes. Furthermore, this study provides a new vehicle for improving the understanding of ions transport through hydrated polymer media, and will catalyze exploration of new applications of this novel class of polymers with a unique mode of ion transport.