Syntheses And Alkali-resistant Evaluation Of Adamantyl-Substituted Imidazoliums And Their Application In Anion Exhange Membranes

Anion exchange membranes(AEMs)have lots of applications in material and chemical industry such as fuel cell membranes,water treatment,soda manufacturing,electrodialysis and so on.The development of high performanced AEMs was lasted many decades in scientific and industrial communities.As a core component for alkaline polymer fuel cell,AEM is responsible for ion conduction and the separation of the fuels,and it determines the performance of the fuel cell.However,the low conductivity and poor chemical stability of the AEM remain the two major limitations in the fuel cell field.Nowadays,two major methods,designing of novel alkaline-stable cations and separating cation from polymer backbone by flexible spacers,were applied for the improvement the alkaline durability of the AEM.In this dissertation,bulky adamantyl moiety was introduced into the N3-and C2-position of imidazolium,and then tethered this cation into polyolefin backbone by ring-opening metathesis polymerization.The properties of the corresponding AEMs,such as water uptake,ion conductivity,swelling ratio,and alkaline stability,were also investigated.Specifically,the following works have been performed:(1)Adamane has steric and hydrophobic effects because of its special cage structure.We introduced adamantyl and norbornyl groups into N3-and N1-positions of imidazolium,respectively,and appended these cations into polyolefin backbone by ring-opening metathesis copolymerization with dicyclopentadine and di-norbornyl compound to the fabrication of crosslinked AEMs.The membrane derived from di-norbornyl compound exhibited good comprehensive properties with the chloride conductivity of 37 m S/cm at 80°C,however,the water uptake and swelling were well suppressed,only 101 wt.%and 20%were observed,respectively.Unfortunately,although bulky adamantyl group was introduced into imidazolium,the alkaline stability of the resultant PEM was not mitigated accordingly,for instance,after 7 days of alkaline hydrolysis at 1 M Na OH 60 ~oC,the conductivity of all cross-linked films decreased by more than 40%,which is similar to the conventional imidazolium-based AEMs.(2)In order to systemically study the effect of steric hinderance to the alkaline stability of the imidazoliums,we further introduced bulky adamantyl group to the C2-position of benzimidazolium and synthesized two molecular coumpounds,i.e.,1-methyl-2-adamantyl-3-butyl-4,5-benzimidazolium and 1,3-dibutyl-2-admantyl-4,5-benzimidazolium,and ~1H NMR was used to evaluate their alkaline stability.Moreover,the ionomers containing these admantyl group-protected benzimidazolium group were synthesized by ring-opening metathesis polymerization.The alkaline stability measurement suggested that this type of imidazolium molecular comounds were totally decomposed after 7 days aging in 1 M Na OH/Me OH solution at 60°C,indicating the inferior alkaline stability.Moreover,the membrane based on 2-admantyl-4,5-benzimidazolium was considerably degraded in natural water at 80°C,which was totally contradicted to our design strategy.By the analysis from the degradation products of the molecular coumponds by ~1H NMR,we speculated that the dominant degradation pathway was the nucleophilic attack of OH~-to the C2-position of imidazolium ring,resulting the degradation of cationic imidazolium ring to the natural amide compound.Possibly,the high strain energy of the benzimidazolium ring resulted from the introduction of bulky admantayl group was responsible to the chemical instability of these novel cations as well as the corresponging AEMs.