Preparation And Properties Of The Anion Exchange Membranes Having Functional Macrocycle In Molecular Structure

Presently,anion exchange membranes fuel cells?AEMFCs?draw much attention,due to their convenient water management,low cost and high-activity electrode reaction.But the development and application of anion exchange membranes?AEMs?are restricted by low conductivity and stability.To overcome dilemma,novel structure need to be introduced into the molecular design of AEMs.As we all known,rational molecular structure contributes to the performance improvement,such as conductivity,mechanical performance,anti-swelling capability and durability.Based on conventional strategy including cross-linking,block,graft and local high-density functional groups,macrocyclic compounds are applied in AEMs as functional component or assisting moiety.The macrocyclic compounds can be divided into five categories as follow:?1?crown ether;?2?cucurbituril;?3?cyclodextrin;?4?pillararenes;?5?calixarene.Considering the stability in alkaline condition,difficulty of modification and demand of molecular design,pillararenes and cyclodextrin were selected to introduce AEM.Specifically,the reasons for choosing pillararenes are as follows:?1?The frame of pillararenes consisting of benzene ring and alternate methylene avoided alkali sensitive group such as nitrogen and oxygen;?2?pillararenes having ten functional sites satisfied the demand of transport ion;?3?common pillararenes were directly synthesized in high yield.But the study about pillararenes in AEMs has not reported.The?-cyclodextrins as an oligosaccharide,were applied in AEMs,basing on the advantages including?1?cost of cyclodextrins is low and reaction routes for polymer grafted cyclodextrins and modified cyclodextrin are mature;?2?cyclodextrins have abundant hydroxyls in surface of truncated cone with hollow.However,the reports about cyclodextrins in AEMs were insufficient.This study about macrocyclic compounds in AEM involves four parts including:?1?cross-linked net-works fabricated by polyethersulfone containing pillararene units in backbone was applied as AEMs.Utilizing pillararenes-type bisphenol,the high-molecular-weight polyethersulfone containing pillararenes in backbone was synthesized for the first time.After a series of modification,pillaraenes moiety is grafted high-density long flexible alkyl chain containing quaternary ammonium heads.And the backbone of polyethersulfone was linked via hydrophilic glycol chain to fabricate the net-work.Due to local high-density quaternary ammonium,large-volume pillararenes and hydrophilic cross-linker,the resulting AEM showed a high conductivity under low ion exchange capacity?IEC?and improved alkaline stability.The highest conductivity can reach 93.2 mS cm^-1 at 80 ?,but corresponding swelling ratio is 6.8%.Benefited from the long flexible alkyl chain,the alkaline stability of novel AEM was enhanced.?2?cross-linked net-works were fabricated by aminated polyethersulfone and pillararenes containing multiple alkyl chain with bromine heads as AEMs.Considering the difficulty of synthesizing pillararene-type polyelectrolyte,the route of functional pillararenes directly mixing with aminated polyethersulfone was more valuable.And the obtaining AEMs had excellent conductivity and alkaline stability.Compared with chapter 1,the optimized route was favor of higher density of functional groups,higher IEC.The highest conductivity reached 155 mS cm^-1 and simultaneously swelling ratio was 23.5%.High cross-linking density enhanced the mechanical performance and the highest tensile strength was 33.5 MPa.In addition,benefited from improved conductivity,the power density was 135.3 mW cm^-1?H₂/O₂,60 ?,100%relative humidity?.?3?cyclodextrin-grafted polyphenylene oxide was synthesized as AEMs.Monosubstituted cyclodextrin was attached on polyphenylene oxide containing quaternary ammonium via flexible alkyl chain.The AEMs with same cyclodextrin content but different bromination degree or same cyclodextrin content but different bromonation degree,were synthesized respectively.Hydrophilic hydroxyls in cyclodextrin boosted water uptake and ion transport.The result indicated AEMs containing cyclodextrin had improved performance compared with conventional AEMs.The highest conductivity can reach 110.8 mS cm^-1 at 80 ? and corresponding swelling ratio just is 22.8%.In addition,slight improvement in alkaline stability resulted from hindrance of cyclodextrins to OH^-attack.What's more,the maximum power density154 mW cm^-2?H₂/O₂,60 ?,100%relative humidity?,suggesting suitable application as AEMs.?4?The cyclodextrin-grafted polyphenylene oxide with different-length alkyl chain is synthesized.The performance of grafted polymer is affected by length of side chain.Specifically,the short side chain positive impact on conductivity but adverse effect on swelling ratio,mechanical performance and fuel cell performance.AEMs with one methylene group in side chain had the highest conductivity of 107.3 mS cm^-1,water uptake of 108.9%and swelling ratio of 18.3%at 80 ?,but the lowest tensile strength of 19.6 MPa.