Study On Fabrication And Performances Of Crystalline SEBS Based Anion Exchange Membranes

Anion exchange membrane fuel cells(AEMFCs)have attracted much attention due to their non-noble metal electrocatalysts,rapid electrode reaction kinetics and good corrosion resistance of metal catalysts under operating conditions.As a key component of AEMFCs,the performance of anion exchange membrane(AEMs)directly affects the practical application of AEMFCs.The structure of polymer skeleton plays a decisive role in the performance of AEMs.It is found that the AEMs with microphase separation structure can be obtained by using poly(styrene-b-(ethylene co butene)-b-styrene)(SEBS)as the framework material,which can realize efficient ion transport.However,SEBS based AEMs often have high water absorption and expansion.How to balance ionic conductivity and dimensional stability is a problem that must be considered.Different from other methods of forming cross-linking structure to limit the over expansion of AEMs,the crystal structure was introduced to improve the dimensional stability of AEMs.Then,the effect of crystal structure on the properties of SEBS based AEMs was systematically studied by adjusting the molecular structure of SEBS.The main contents are as follows:(1)Two kinds of poly(styrene-b-butadiene-b-styrene)copolymers(SBS-30 and SBS-40)with 86～87%1,4-butadiene structural units were hydrogenated to obtain SEBS-30 and SEBS-40 with crystallinity of 21.7%and 18.4%respectively.A series of SEBS based AEMs with crystalline structure were prepared by chloromethylation,quaternization and alkalization.The results show that compared with commercial SEBS based AEMs, crystalline AEMs not only have good microphase separation structure,but also show better mechanical properties and dimensional stability.SEBS-40-OH(IEC=1.48 mmol/g) showed low expansion,high water absorption and moderate OH~-conductivity(～12%, 80%and 81 m S/cm at 80 ℃).In the single cell test,when the current density is 860 m A/cm~2,the peak power density is as high as 320 m W/cm~2.(2)A series of crystalline AEMs were prepared from star shaped and linear SBS with high content of 1,4-butadiene.The effects of polymer topological structure on the crystal structure and properties of AEMs were studied.The results show that compared with linear SEBS based AEMs,star SEBS based AEMs have higher crystallinity(18.4%vs 22.4%),which not only shows better mechanical properties and dimensional stability,but also forms a more obvious microphase separation structure conducive to the efficient transport of OH~-,and has better alkali resistance stability.(3)SBS copolymers with 69.0%,82.7%and 92.5%1,4-butadiene units were synthesized by reactive anionic polymerization with n-Bu Li as initiator,butadiene and styrene as monomers,cyclohexane as solvent and tetrahydrofuran as structural modifier.Then SEBS with crystallinity of～0%,16.3%and 25.9%were prepared by hydrogenation.A series of crystalline SEBS based AEMs(SEBS-70-OH,SEBS-80-OH and SEBS-90-OH)were prepared by chloromethylation,quaternization and alkalization.The effects of SEBS crystal structure on the properties of AEMs were studied.The results show that SEBS-90-OH has the highest tensile strength,the lowest expansion rate and the best alkali resistance stability,but its high crystallinity hinders the ion transport and reduces the ion transport performance of the membrane:its ionic conductivity at 80℃ is only 66.35 m S/cm(IEC=1.52 mmol/g).In contrast,SEBS-80-OH has moderate crystallinity but excellent ion transport performance(83.70 m S/cm,IEC=1.06 mmol/g),while SEBS-70-OH with low crystallinity has similar ion transport performance as commercial SEBS based AEMs(73.26 m S/cm,IEC=1.49 mmol/g).Therefore,the introduction of appropriate crystal structure is conducive to the preparation of anion exchange membrane with good mechanical/chemical stability and ionic conductivity.