Design And Preparation Of Side-chain-type Anion Exchange Membranes For Fuel Cells

Fuel cells are regarded as one of the most promising energy conversion devices for mobile application in the 21st century due to their high efficiency,high energy density and low pollution.Of those,anion exchange membrane fuel cells(AEMFC)exhibited higher faster reduction kinetic under alkaline condition than proton exchange membranes fuel cell(PEMFC).Non-previous-metal electrocatalysts can be used to reduce the cost of fuel cell devices.Anion exchange membranes(AEMs),as one of the critical component of AEMFC,determine the performance and service time of the fuel cells.However,two major obstacles that severely hamper practical application of AEMFC are low conductivity and poor alkaline stability of the membrane materials.Recently,a few researchers focus on developing AEMs with side chain structure since the side-chain-type polymer will self-assemble and form microphase separation morphology,which is beneficial for fabricating high-efficient ion conducting channels and thus enhancing ion conductivity.Nevertheless,side-chain-type AEMs are still on the initial stage of development.The relationship between the structure and performance of the AEMs is unclear,which is need to be further investigated.In this thesis,various kinds of side-chain-type AEMs are designed and prepared,the relationship between the membrane structure and performance is studied in detail.The major progress is summarized as follows,(1)A series of novel phenolphthalein-based poly(ether sulfone)AEMs(PES-Bx-C16)with side chain are prepared.The introduction of long alkyl side chain results in obvious microphase separation morphology in the membrane.Compared with the main-chain type AEMs,the side-chain-type AEMs not only have an improved dimensional stability but also exhibit high ionic conductivity.The highest ionic conductivity of PES-B100-C16 reaches up to 44.0 mS·cm-1 at 80℃,nd is higher than that of the main-chain type PES-B60-C1(22 mS·cm-1).Furthermore,the PES-B100-C16 membrane still remained about 70%of its initial conductivity after immersing the membrane into a 2 M aqueous KOH solution at 60 ℃ for 360 h.Based on the AEMs with high conductivity,a single H2-O2 cell achieves a maximum power density of 43 mW·cm-2.This certify that the side-chain-type AEMs hold promise for fuel cells.(2)In order to improve the local mobility of cationic groups and enhance the conductivity,the ionic groups are located on the end of the side chain.A series of piperidinium-functionalized side-chain-type AEMs(PEK-CQA-x)are prepared via nucleophilic polycondensation,demethylation and Williamson reaction.The as-prepared AEMs show a clear hydrophilic/hydrophobic separated morphology resulting in a high conductivity of 72.7 mS·cm-1 at 80 ℃.Furthermore,the electron-withdrawing effect of the cationic groups is weaken since the distance between the piperidinium groups and backbone is far.Meanwhile,the piperidinium groups can provide good chemical stability in alkaline condition.As a result,the conductivity of PEK-CQA-0.8 only decreased by 8.8%after immersing the membrane into a 1 M aqueous KOH solution at 60 ℃ for 360 h.(3)In order to study the structure-property relationship,especially the effect of the length of flexible spacer on the performance of the membrane,a series of novel side-chain-type AEMs(PES-n-QA,where n is the number of carbon atoms in spacer unit,n=3,4,6,8 and 12)with various lengths of flexible spacer linking the cationic groups and the polymer backbone are prepared.Increasing the length of flexible spacer,an increase in water uptake(swelling ratio)is observed for n<6 and a decrease in water uptake(swelling ratio)is observed for 6≤n≤12.The tendency of hydroxide conductivity is found to be similar to water uptake.PES-6-QA exhibits the maximum conductivity of 62.7 mS·cm-1 at 80 ℃.Furthermore,Hofinann degradation of quaternary ammonium groups in the membrane can be inhibited by increasing the length of flexible spacer(n≥4)between the backbone and cationic groups.(4)In order to enhance the dimensional stability of the side-chain-type AEMs,we prepared a kind of novel cross-linked AEMs via heating without using catalyst.The cross-linked structure is responsible for improving the intermolecular interaction between the polymer chains and is beneficial for fabricating microphase separated morphology.The as-prepared AEM(CPES-IM0.95-TFH0.05)exhibits good dimensional stability(the swelling ratio is lower than 16.0%)and a highest conductivity of 77.1 mS·cm-1 at 80 ℃.Furthermore,the cross-linked AEMs also have robust mechanical properties,good thermal stability and reasonable alkaline stability.(5)The local density of ionic groups in the side chain is improved to promote the fabrication of microphase separated morphology and enhance the conductivity.A series of novel side-chain-type AEMs composed of poly(ether sulfone)s backbone and pendent imidazolium-functionalized polyphenylene oxide(PPO)are prepared.The densely functionalized side chains are found to be beneficial to the formation of ion clusters,as demonstrated by transmission electron microscopy(TEM).As a result,the highest hydroxide conductivity of the PAES-15-IMPPO membranes is 78.8 mS·cm-1 at 80 ℃ and is higher than that of PES-Bx-C16.However,the imidazolium groups are found to be unstable under alkaline condition since PAES-15-IMPPO retained about only 50%of its initial conductivity after immersed into a 1 M aqueous KOH solution at 80 ℃ for 600 h.There is still room for improvement on alkaline stability.(6)In previous work,we found that the backbone of poly(ether sulfone)s-based AEMs degraded slightly under alkaline condition since the existence of electron-withdrawing groups such as sulfone group has a negative effect on the stability of backbone resulting in a decline of mechanical property.Herein,we present the preparation of quaternized triblock copolymer polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene(SEBS)via Friedel-Crafts reaction,ketone reduction reaction and quaternization for AEMs.The quaternary ammonium groups are attached to the SEBS backbone free of C-O linkages via long flexible alkyl spacers,which are responsible for the robust alkaline stability of the AEMs.Thus,the hydroxide conductivity of SEBS-CH2-QA-1.5 only decreased by 7.7%and 13.7%after immersing the membranes into a 1 M aqueous KOH solution at 60 and 90 ℃ for 360 h,respectively.Furthermore,the mechanical property has hardly changed after the alkaline stability test because of the good chemical stability of the SEBS backbone.(7)To explore highly conductive and alkaline stable AEMs materials,triblock copolymers bearing alkyl-tethered cycloaliphatic quaternary ammonium-head-groups(ABA-QA-x)are prepared.The morphology of the membrane was controlled by adjusting the length of hydrophilic segment.TEM results show that a well-connected ion conducting region is formed in the membrane.The highest conductivity,up to 105.1 mS·cm-1 at 80℃ is achieved for ABA-QA-3.Moreover,the ABA-QA-3 membrane shows robust alkaline stability.High retention of hydroxide conductivity(88.9%)is observed for the AEMs via degradation test in a 1 M aqueous KOH solution at 80 ℃for 480 h.Based on the AEMs with high conductivity,a single H2-O2 cell achieves a peak power density of 176.5 mW-cm-2 at 80℃.(8)Inspired by the advantages of block copolymer structure and multication side chain,we designed a novel multi-quaternary ammonium functionalized triblock copolymer AEMs(ABA-TQA-x)to improve the conductivity and constrain the swelling behavior of the membrane.Three cationic groups are located in one side chain,which will enhance the local ion concentration in favor of fabricating microphase-separated morphology.As a result,the as-prepared AEM(ABA-TQA-44)demonstrated considerably higher conductivities,up to 130.5 mS· cm-1 at 80 ℃,than the AEM with monocation side chain(ABA-MQA).Additionally,the AEMs exhibited good dimensional stability and robust alkaline stability.The ABA-TQA-44 membrane retained 84.7%and 83.1%of its original conductivity and ionic exchange capacity(IEC),respectively,after immersed in a 1 M aqueous KOH solution at 80℃ for 480 h.The peak power density of a single H2-O2 cell using ABA-TQA-44 is 204.6 mW·cm-2 at 80 ℃.