| Polymer electrolyte membrane fuel cell is one of the fuel cells,which is promising in industrialized applications due to its high specific power,low operating temperature and convenient operation.According to the conducting ion species,polymer electrolyte membrane fuel cells can be devided into proton exchange membrane fuel cells and alkaline anion exchange membrane fuel cells.Compared to the proton exchange membrane fuel cells,anion exchange membrane fuel cells(AEMFCs)have some advantages,such as available to use non-precious metal electro catalysts.Due to the alkaline working environment,anion exchange membranes are easy to degrade because of the nucleophilic attack of hydroxide ions,which could bring on dramatic decline in the conductivity and mechanical properties of the membranes,and thus limit the development of AEMFCs.It is well known that there are three recognized routes to elucidate the degradation of quaternary ammonium functionalized polymer membranes,including Hofmann elimination,SN2 reaction and the reaction of forming ylide intermediates.It is reported that the degradation of the AEMs might be related to O2·-and ·OH radicals,since the AEMs even lose initial I EC in deionised water for electrolyser/fuel cell operations.However,if the degradation of the AEMs in alkaline medium is dominated by free radicals or not,and the influence degree of free radicals on the alkaline stability of AEMs is still unknown.Aming to improve the alkaline stability of the AEMs,some strategies are used in this work including addition of radical inhibitors in alkaline solutions,preparation of AEMs with different radical inhibitor structures and doping with inorganic components,respectively.They are summarized as follows:(1)Four radical inhibitors(phenol(Phol),p-ethyl phenol(PEP),p-tert-butylcatechol(PTB)and p-benzoquinone(PBQ))were added in alkaline solutions,repectively,to investigate their influences on the degradation of poly(4-vinylbenzyl chloride-styrene)based anion exchange membranes(AEMs).In the alkaline solution,the conductivity of AEM is easy to decrease because of the attack of hydroxixe ions(OH-).According to the conductivity measurements results,it is found that the presence of the radical inhibitors could improve the alkaline stability of the AEMs at various degrees.Among the four kinds of radical inhibitors,PEP exhibits the most significant improvement on the alkaline stability of the AEMs.When immersed in 8 M KOH in the presence of PEP at 80℃ for 24 h,the AEMs remained 87%of the initial conductivity.While in the presence of Phol,PTB or PBQ in the alkaline solutions,the AEM remained its original conductivity of 83%,32%and 17%,respectively.As a comparison,the AEMs only remained 16%of the initial conductivity without any radical inhibitors in the same conditions.According to the 1H NMR spectra,the quaternary ammonium groups of the AEMs are mainly degraded into tertiary amines after immersed in 8 M KOH at 80℃ for 24 h,while in the presence of 0.025 M PEP,this degradation is restrained and the quaternary ammonium groups are maintained by reaction with PEP.Fenton test results further indicate that high oxidative stability of the AEMs is benefited from the presence of a small amount of PEP,which could reduce the generation of the ·OH radicals in the Fenton solution.An inhibition reaction mechanism of the radical inhibitors preventing the AEMs from alkaline degradation is proposed.(2)Polymers were synthesized by polymerization of 4-vinylbenzyl chloride with three kinds of aromatic monomers,respectively,and quaternization.of these polymers were performed to fabricate associated AEMs possessing different pendants with a conductivity ranging from 32.6 to 39.5 mS cm-1 in water at 80℃.The obtained AEMs showed excellent stability in alkaline solutions at elevated temperatures.In particular,the AEM having a pendant of p-ethyl phenol could maintain the initial conductivity of 89%after immersed in 1 M KOH at 80℃ for 1000 h.Even soaked in 8 M KOH solution at 80℃ for 24 h,this AEM still exhibited a conductivity of 26.4 mS cm-1 at 80℃(about 80%of its original conductivity).However,the rest of prepared AEMs showed significant decline in conductivity and they only remained 8%and 24%of the initial conductivities,respectively.(3)Six nucleophilic reagents of 3-dimethylamino-1-propanol,(dimethylamino)-acetaldehyde diethyl acetal,4-(dimethylamino)phenol,(dimethylamine methyl)phenol,trimethylamine and triethylamine were used to prepare a series of AEMs with different cationic groups,and the influence of the cationic groups(having hydroxyl or radical inhibitor structure)on the alkaline stability of the AEMs was investigated.The conductivities of the AEMs having different cationic groups are in a range of 15.4-47.2 mS cm-1 at 80℃ in water.The AEMs modified with both 3-dimethylamino-1-propanol and trimethylamine have better alkaline stabilities than rest of others when immersed in 8 M KOH at 80℃ for 24 h.These two AEMs remained 66%and 85%of the initial conductivities,respectively.The best alkaline stability of the AEM modified with trimethylamine is resulted from the absence of Hβ in the quaternary ammoniums,which avoids the Hoffman degradation of the AEM.However,the AEMs modified with 4-(dimethylamino)phenol and(dimethylamine methyl)phenol,respectively,shown a similar alkaline stability with the AEMs modified with trimethylamine or(dimethylamino)acetaldehyde diethyl acetal,and these AEMs remained 15-24%of the initial conductivities when immersed in 8 M KOH at 80℃ for 24 h.(4)Five kinds of radical inhibitors(p-tert-butylphenol,4-amino-tempo,benzophenone,phenothiazine and N-phenyl-l-naphthylamine)were used,repectively,to prepare the radical inhibitor doped anion exchange membranes based on poly(4-vinylbenzyl chloride-styrene).The physical and chemical properties of the prepared AEMs with different concentration of radical inhibitors were investigated.As a result,the radical inhibitors show different influence on the alkaline stability of the AEM.When immersed in 8 M KOH at 80 ℃ for 3 h,the pristine QP(VBC-St)AEM remained 64%of the initial conductivity,while the AEMs having radical inhibitors(70%of the chloromethyl group in the polymer)remained 93%,58%,70%,41%and 47%of the initial conductivity,respectively.According to the fluorescence spectra,it is found that hydroxyl radicals,which would lead to the degradation of the AEMs,formed in the alkaline solutions during the alkaline stability test.When the AEM doped with p-tert-butylphenol,which reacts with the hydroxyl radicals,the content of hydroxyl radicals in the alkaline solution decreased,and the alkaline stability of the AEM was improved.The alkaline stability of the AEMs doped with different kinds of phenolic radical inhibitors was investigated as well.However,when the phenol-containing radical inhibitors have a large substituent group in its benzene ring,the membranes tend to be brittle during the stability test.(5)Organic-inorganic hybrid anion exchange membranes were synthesized by introducing 3-glycidoxypropylthrimethoxysilane(GPTMS)into the substrate of quaternary triethylamonium functionalized poly(2,6-dimethyl-1.4-phenyleneoxide)(QPPO).The resultant hybrid membranes have good thermal stability and mechanical properties.The introduction of GPTMS increased the number of hydroxyl groups in the hybrid membranes,which benefits the water uptake and conductivity of the hybrid membranes.A highest conductivity of 46.0 mS cm-1 is reached at 80℃ in water by the QPPO0.4-G0.55 membrane,and its pristine membrane of QPPO0.4 is 27.8 mS cm-1.In addition,the introduction of the inorganic component of GPTMS brings on the hybrid membrane with a high tolerance toward 1 M KOH at 60℃.The hybrid membranes remained about 73-79%of the initial conductivity after immersed in 1 M KOH at 60℃ for 480 h.As a contrast,QPPO0.4 membrane remained its initial conductivity of 57%. |
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