| With an increase on the awareness of environmental protection,fuel cells have been drawn much attention as one of the most promising clean and green energy generators.Among the various types of fuel cells,anion exchange membrane fuel cells（AEMFCs）based on alkaline polymer electrolytes stand out due to their advantages of low cost and high efficiency,and have attracted much attention.Anion exchange membranes（AEMs）are one of core components in AEMFCs,and their performance directly affects the efficiency of fuel cells.In the study of AEMs,there are still some major problems（e.g.poor stability and low ionic conductivity）to be resolved.To obtain the improved properties related to membrane’s applications,such as dimensional stability,chemical stability and tensile strength,some cross-linkable moieties have been incorporated to construct the stable cross-linked networks.However,a compact cross-linked structure may lead to a reduced“free volume”and water absorption capacity,making it difficult to build“ion transport channels”and achieve a high level of conductivity.In addition,a more compact cross-linked structure will affect the flexibility of the membranes.In this thesis,in order to enhance an overall performance of AEMs,we design and prepare two types of novel polyphenylene oxide（PPO）based cross-linked blend anion exchange membranes having a“flexible hydrophilic”component and a“linear-hyperbranched”structure,respectively.The specific research work is as follows:Based on the synthesis of quaternized PPO containing aldehyde groups,a series of PPO-PVA cross-linked AEMs were prepared by blending with polyhydroxy polyvinyl alcohol（PVA）through solution casting.The cross-linked structure could effectively improve the dimensional stability of the membranes.The tensile strengths of PPO-PVA membranes were 25.1～28.9 MPa,which were about 4 times higher than that of the PPO-PVA-0 membrane.PPO-PVA membranes exhibited a better water uptake ability and a better competence for constructing“ion transport channels”,due to the introduction of hydrophilic and flexible PVA component.The hydroxide ion conductivity of PPO-PVA-5 membrane reached a relatively high level of 125.1 m S·cm-1,which was twice that of 48%Br-PPO membrane.In addition,the elongations at break of the membranes increased after the introduction of PVA.Therefore,it is believed that the cross-linked blend membranes with hydrophilic PVA structure are a new type of high-performance AEMs,which may be promising for AEMFC applications.In order to further explore the structure of cross-linked membranes,we proposed a method for the preparation of cross-linked blend anion exchange membranes with“linear-hyperbranched”structure.First,a hyperbranched poly（4-vinylbenzyl chloride）（HVBC）was synthesized,and then a mixed solution containing HVBC and brominated polyphenylene oxide was prepared.After tetramethylenediamine was added to the mixed solution,a series of PPO-HVBC cross-linked membranes were successfully prepared via a normal solution casting process.PPO-HVBC membranes had a higher porosity and a lower density than PPO-Cr membrane.At the same IEC value(IEC～1.62mmol·g-1),the ionic conductivity of PPO-HVBC-60 membrane was 1.9 times higher than that of PPO-Cr.The maximum conductivity of PPO-HVBC membranes reached135.9 m S·cm-1 at 80℃.The tensile strengths of PPO-HVBC membranes were17.3～32.3 MPa,and their elongations at break were above 20%.The results reveal that the hyperbranched HVBC with a unique multi-cation structure is an efficient“performance-enhancing component”for the PPO-based membranes,and this study also provides a new idea for the preparation of high-performance AEMs. |
