Preparation Of Highly Alkaline Stable Anion Exchange Membranes Based On Electron-Withdrawing C=O Link-Free Poly(arylene Ether)s

Fuel cells have emerged as one of the most promising clean energy conversion devices,in which anion exchange membrane fuel cells(AEMFCs)become a research hot-spot in the field of fuel cells due to the fast kinetics for oxygen reduction and feasibility for non-precious metal catalysts.As one of the key components,the hydroxide conductivity and alkaline stability of anion exchange membranes(AEMs)will greatly determine the performance and lifetime of AEMFCs.However,the electron-withdrawing links(such as ketone and sulfone groups)and benzylic ortho-type cations in the structure of commonly used main-chain poly(arylene ether)s AEMs will accelerate the aryl-ether cleavage,which limits the widespread application of poly(arylene ether)s in AEMs.The works of literature generally synthesize aryl ether-free polyaromatics or graft side chains in poly(arylene ether)s to improve the alkali stability of AEMs.However,the application of most methods is limited by the requirement of noble metal catalysts,harsh reaction conditions and complex synthesis processes.In this work,highly alkali stable poly(arylene ether)s AEMs based on electron-withdrawing link-free backbones are prepared through a different approach of eliminating the electron-withdrawing C=O links in the main chains by a simple and precious metal catalyst-free way.In addition,long side-chain ionic liquids with strong mobility are introduced into the electron-withdrawing C=O link-free backbones to successfully construct side-chain type AEMs and improve the ionic conductivity.Aiming at resolving the key problem that ether-cleavage trigger factors in the main chain structure of commonly used poly(arylene ether)s AEMs(electron-withdrawing links and ortho-benzyl cations)can easily cause poor alkaline stability,a new strategy is proposed to eliminate electron-withdrawing C=O links in the main chains of poly(arylene ether)s.First,the electron-withdrawing C=O links in the conventional poly(arylene ether)s backbones are converted into the electron-donating C-NH2 links through the Leuckart reaction without a precious metal catalyst.Then the C-NH2 links are quaternizated to prepared novel AEMs based on electron-withdrawing link-free poly(arylene ether)s backbones with para-type quaternary ammonium(QA)groups(QA-PEAM).Density functional theory(DFT)calculations indicate that the electron-withdrawing C=O link-free poly(arylene ether)s backbones and para-type cations increase the electron cloud density on the aryl-ether carbon and the steric hindrance of the benzyl carbon,resulting in higher barrier heights of aryl-ether cleavage and benzyl QA group degradation.Meanwhile,by tuning the quaternization degree,the remaining C-NH2 groups in the polymer backbones could form hydrogen bonding networks to enhance the mechanical properties.The QA-PEAM-80 AEM exhibits a high hydroxide conductivity(80?,92.2mS cm-1)and excellent mechanical properties(48.2MPa,51.3%).The novel AEM also exhibits excellent alkaline stability with no polymer backbone degradation even in harsh conditions(4M KOH,80?,400 h).In order to solve the problem of competition restriction in the above-mentioned QA-PEAMs that the C-NH2 group acts as both the quaternized active site and hydrogen bond strengthening donor.The chloromethyl quaternization sites are introduced into poly(arylene ether)s backbones to prepare novel high alkali resistance and high strength poly(arylene ether)s AEMs with C-NH2 group repeating units(QA-PNH2).The competitive restriction between the hydrogen bond network and the quaternized functional group is lifted,so that the QA-PNH2 AEMs not only maintain high alkali stability but also exhibit excellent mechanical properties and dimensional stability under high IEC.DFT calculations indicate that high-density electron-donating C-NH2 linkages can not only enhance the electronic cloud density on both the ether-connected carbon and the benzylic carbon,increasing barrier heights for aryl-ether cleavage and benzylic QA group degradation,but also enhance hydrogen-bond networks and intermolecular interaction and provide additional hydroxide transport sites.The experimental results show that QA-PNH2 with high IEC(2.06mmol g-1)also exhibits a low swelling ratio(80?,12.5%),excellent mechanical properties(48.4MPa,50.8%)and hydroxide conductivity(80 ?,108.2mS cm-1).The AEM also shows excellent alkaline stability with no polymer backbone degradation even in harsh conditions(4M KOH,80?,400 h).Although the above-mentioned main-chain type electron-withdrawing link-free poly(arylene ether)s AEMs exhibit high main chain alkali stability,the benzyl functional groups directly connected to the main chains have limited mobility.In order to further improve the microphase separation and membrane performance,the advantage of electron-withdrawing C=O link-free poly(arylene ether)s backbones in excellent alkaline stability and the advantage of strong mobility of long alkyl spacer side chains are combined to effectively promote the microphase separation,improve ionic conductivity while maintaining alkali stability.Bromide ionic liquids with different alkyl spacer lengths are grafted through the Menshutkin reaction to obtain the spacer-tunable single ion side-chain AEMs based on electron-withdrawing C=O link-free poly(arylene ether)s backbones(PEAM-Cn).The introduction of the long flexible side chains can effectively enhance the aggregation of cations,make the microphase separation more clear,and form a more connected ion transport channel.Among them,the AEM with hexyl spacers(PEAM-C6)shows the best microphase separation structure(9.2nm)and then exhibits the highest hydroxide conductivity(80?,112.5mS cm-1)and fuel cell performance(80?,300mW cm-2).Alkali stability test shows that the long flexible spacer cationic side chain can not only inhibit the degradation of the poly(arylene ether)s backbone by increasing electron cloud density on the ether-connected carbon atom.but also enhance the higher barrier height of the quaternary ammonium(QA)group degradation by providing stronger steric hindrance effect.After immersion in 4M KOH at 80°C for 400h,the conductivity loss of PEAM-Cn AEMs is less than 20%.Finally,in order to further improve the conductivity and fuel cell performance,double ion side-chain membranes with the branched cationic side chain(PEAM-BC6)and the ionic string side chain(PEAM-SC6)are prepared.The stronger hydrophilicity of the double ion side chains can more effectively promote the OH-conduction,and form larger ion clusters and more obvious microphase separation structure.Therefore,compared with the previous sections,the PEAM-BC6 membrane shows the highest conductivity(80?,128.2mS cm-1)and fuel cell performance(80?,499mW cm-2).In addition,since main chains do not contain electron-withdrawing C=O links and side chains have hexyl spacers,double ion side-chain AEMs(PEAM-SC6 and PEAM-BC6)maintain excellent alkali stability.After immersion in 4M KOH at 80? for 400h,the poly(ary lene ether)s backbones show no degradation,and the loss of ion conductivity is less than 23%.