| Anion exchange membrane fuel cells(AEMFCs)as clean energy devices possess the facile kinetics of electrode reactions,less methanol crossover and the choice of using non-noble or low cost metal electrocatalysts.The AEMFC is considered as portable power sources to be used in electric vehicles,power stations and aerospace shuttles.Great efforts have been made to improve the performance of the AEMFCs for promoting their practical applications widely.However,the ionic conductivity and the chemical stability of the anion exchange membranes(AEMs),using as electrolyte in AEMFCs,are desired to be improved.The properties of AEMs can directly affect the efficiency and lifetime of AEMFCs.Therefore,AEMs with superior properties for a reliable performance of the devices are needed.The durability of the AEMs in alkaline conditions is a critical issue from a point of view of applications.The aromatic polymer is expected to be used as AEM backbone to ensure a reasonable mechanical property and chemical stability in the alkaline medium.Besides the stability of polymer backbones,the stability of functional groups is also concerned and investigated.The quaternary ammonium cation is the most widely used one in AEMs.However,the quaternary ammonium cations are very susceptible to the nucleophilic substitution,Hofmann elimination or El elimination by the attack of hydroxide ions in alkaline conditions.For the improved stabilities of AEMs,other functional cation groups such as phosphoniums and imidazoliums have been developed.Generally,grafting more functional groups onto polymer backbones is the direct way to improve the conductivity of the AEMs.However,the much introducted hydrophilic functional groups normally bring about significant increases in water uptake and dimensional swelling,which could result in the deterioration of mechanical properties.To solve this problem,covalent crosslinking has been demonstrated an effective way to balance the ionic conductivity and mechanical stability of the AEMs.Various crosslinked AEMs were prepared based on tetramethylbiphenyl containing polymer of poly(aryl ether ketone).The backbones of the AEMs were crosslinked by linear or star crosslinking the polymer chains using micromolecular or macromolecular crosslinkers,respectively.In order to know the influence of the different crosslinking structures on the physicochemical properties,characterizations of the prepared AEMs including water uptake,swelling,ion exchange capacity,conductivity,mechanical property and alkaline stability were performed.The prepared crosslinked AEMs exhibit high conductivity and excellent alkaline stability.The main contents are summarized as follows:(1)Phosphonium-based crosslinked AEMs with 1,4-bis(diphenylphosphino)butane as the crosslinker were prepared by grafting triphenylphosphine or tributylphosphine onto poly(aryl ether ketone)polymers.The crosslinking and quaternized reactions accomplished simultaneously.When the crosslinking degree is around 20%,the crosslinked triphenylphosphonium and tributylphosphonium functionalized AEMs exhibit the tensile stresses at break of 45 MPa and 30 MPa under ambient atmosphere,and maintain the conductivities of 3.0 mS cm-1(81%)and 3.4 mS cm-1(69%)at 60?,respectively,after exposed to 3 mol L-1 KOH at 60? for 120 h.The results indicate that the crosslinked triphenylphosphonium functionalized AEMs exhibit higher tensile stress and better alkaline stability than the crosslinked tributylphosphonium functionalized ones at the same crosslinking degree.However,the conductivity of the phosphonium-based crosslinked AEMs(lower than 15 mS cm-1 at 80?)should be improved furthermore.Imidzolium-based crosslinked AEMs with three dibromoalkane(1,4-dibromobutane,1,5-dibromopentane or 1,8-dibromooctane)crosslinkers were prepared by grafting 1-methylimidazole and 2-undecylimidazole onto poly(aryl ether ketone)polymers in different mole ratios.The crosslinking reaction was performed by linking the imine of 2-undecylimidazole with crosslinkers in order to improve the conductivity and mechanical stability.All the crosslinked imidzolium-based AEMs with the crosslinking degree of 20%exhibit the tensile stress at break of about 20 MPa under ambient atmosphere,the conductivity in purified water of 10.6-11.5 mS cm-1 at 25?,and 40.3-44.9 mS cm-1 at 80 ?,respectively.Moreover,the AEMs crosslinked with a long alkyl spacer(1,8-dibromooctane)have high tolerance to the nucleophilic attack.After exposed to 1 mol L-1 KOH at 60? for 120 h,the 1,8-dibromooctane crosslinked membranes maintain the conductivity of 8.6 mS cm-1(31%)at 60? and the ion exchange capacity of 1.12 mmol g-1(59%),respectively.(2)Ionically crosslinked 1-butyl-2-methylimidazolium or 1,2-dimethylimidazolium functionalized poly(aryl ether ketone)membranes were prepared by using sulfonated poly(ether ether ketone)as a macromolecular crosslinker to improve the ion conductivity and alkaline stability.The generated imidazolium cations could electrostatically react with sulfonate ions of the sulfonated poly(ether ether ketone)forming the ionic crosslinking structure of the membranes,which ensured that the crosslinked membranes with a high content of cationic groups also exhibited excellent dimensional stability.The changes in crosslinking degree and the alkyl chain-length on N3 site of the imidazoliums could highly affect the properties of the membranes.The crosslinked membranes with crosslinking degrees of 10%and 15%exhibit the conductivity in the range of 36.7 to 56.9 mS cm-1 at 80? and the tensile stress at break in the range of 4.86 to 15.1 MPa under ambient atmosphere,respectively.The AEMs functionalized by 1-butyl-2-methylimidazole exhibit superior properties comparing to those functionalized by 1,2-dimethylimidazole according to the tolerance tests of the AEMs towards hot alkaline solutions.After exposed to 1 mol L-1 KOH at 60? for 200 h,the 1-butyl-2-methylimidazolium functionalized AEMs maintain the conductivity of 26.1 mS cm-1(73%)at 60? and the ion exchange capacity of 2.24 mmol g-1(93%),respectively.(3)In order to improve the conductivity and alkaline stability,the AEMs crosslinked with star-main chains or star-side chains were prepared by grafting the 1-butyl-2-methylimidazole and/or 1-aminopropyl-2-methyl-3-butylimidazolium hydroxide onto branched or linear poly(aryl ether ketone)polymers,respectively.The introduced star structures of the membranes exhibit conductivities of about 40 mS cm-1 at 60? and about 60 mS cm-1 at 80?,respectively.The tensile stress at break of the membrane with star-main chains or side chains varied within 23-45 MPa under ambient atmosphere.Moreover,the durability of the membranes in alkaline medium was detected by monitoring the changes in conductivity and ion exchange capacity.The results indicate that the introduced star structures of both main chains and side chains are effective to enhance the alkaline stability of the membranes.The membranes with star-main chains exhibit a better alkaline stability than that with star side chains,which retained the conductivity of 36.1 mS cm-1(83%)at 60? and the ion exchange capacity of 1.93 mmol g-1(95%),respectively,after exposed to 1 mol L-1 KOH at 80? for 300 h. |
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