Styrene-based Anion Exchange Membrane:syntheses And Properties Studies

Fossil fuels,such as coal and oil,are the world’s largest consumption energy.However,a large amount of polluting smoke is generated during the use of fossil fuels,which threatens the entire world’s ecological environment.Therefore,it is necessary to develop a green and clean renewable energy source to replace chemical energy.This is also the main direction of new energy development in the future.As a kind of green and clean new energy,fuel cell only produces very few harmful gases,which is environmentally friendly.It also has other advantages,such as high conversion efficiency,low noise during work,and wide fuel sources,which have attracted more and more attention.Among fuel cells,polymer electrolyte membrane fuel cells are widely used in the field of communication,transportation,military and aerospace.Because non-precious metals are used as the electrode catalysts in anion exchange membrane fuel cells（AEMFC）,the cost of AEMFC can be reduced to a large extent.Therefore,more and more researchers continue to explore AEMFC recently.It is also necessary to study the anion exchange membrane（AEM）,which is the core component of AEMFC,which transports OH^-while blocking the fuel and oxidant on both sides.The current AEM mainly has the problems of low ion conductivity and poor alkali stabilityand cannot be commercialized.Therefore,the research of an AEM with better performance in all aspects is the goal of researchers.AEM is mainly composed of the skeleton structure of the membrane material and cationic groups.Therefore,preparing the AEM with excellent performance in all aspects should consider the synergistic effect of skeleton and cationic groups.Therefore,this thesis focused on the molecular structure design to improve the ionic conductivity and alkali stability of AEM,and further optimize the overall performance of AEM.Studies have shown that,due to the presence of ether bonds in the main chain of poly（arylene ether ketone）,OH^-easily attacks the ether bonds in the main chain under alkaline conditions,which reduces the alkali resistance stability of AEM.At the same time,due to the positive charge of the cation conducting group,OH^-is easy to attack the cationic group in the AEM,and then AEM is degraded by the cleavage of cationic group.When the cationic group is located far away from the backbone of AEM,the positive charge of the cationic group is weakened and the attack on the main chain is also avoided.Therefore,OH^-is not easy to attack the cationic group and thus improves the alkali resistance stability of AEM.Therefore,the molecular design of AEM requires a comprehensive consideration of the polymer backbone structure as well as the position and structure of the cationic group.For solving the above problems,the purpose of this paper is to investigate"the influence of the different content of fluorine-containing aromatic structure in the main chain on the performance of AEM,and the influence of different grafting rates of flexible alkyl side chains on the performance of AEM"as well as"the influence of the degree of cross-linking on the performance of cross-linked AEM".The specific work is described as follows:（1）For improving the ionic conductivity and alkali resistance stability of AEM,this chapter first synthesized 2,3,4,5,6-pentafluorostyrene-r-styrene copolymer（P）with the all-carbon backbone.Then,the long alkyl side chain BHC was grafted onto the copolymer by a Friedel-acylation reaction to obtain the side-chain-type polymer.After quaternization reaction and ion exchange,four kinds of AEMs（P-CH₂-TMA-X）were obtained from solvent casting method.The results showed that all the four kinds of AEMs exhibited good thermal stability for application in AEMFC.Especially,P₁-CH₂-TMA-1.5 with high content of fluorine-containing aromatic ring in the main chain exhibits a more obvious hydrophilic-hydrophobic phase separation structure,and a lower WU and SR,because the increase of the fluorine-containing aromatic ring structure in the main chain inhibits water absorption and then improves the dimensional stability of AEM.At the same time,the self-aggregation of the hydrophobic structure facilitates the formation of a more obvious hydrophilic/hydrophobic phase separation.Furthermore,after immersing the four membranes in 1M Na OH solution at 60℃for720 h,the loss of ion conductivity is below 30%,indicating a good alkali resistance stability.In addition,although the P₁-CH₂-TMA-1.5 membrane has the highest IEC value and the highest ionic conductivity,the membrane has a higher WU and SR at80℃,which will damage the dimensional stability of the membrane as well as a series of other properties.Therefore,the comprehensive performance of AEMs can be further improved by reducing the WU and SR while maintaining high ionic conductivity.（2）From the work in the previous chapter,it is found that P₁-CH₂-TMA-1.5exhibited high WU and SR,thus resulting in poor dimensional and alkaline stability.In order to resolve this problem,cross-linking was proposed in this chapter to improve the dimensional and alkaline stability of AEMs.In this chapter,three kinds of crosslinked AEMs with different crosslinking degrees were prepared by using N,N,N’,N’-tetramethyl-1,6-hexanediamine as the crosslinker.The results indicate that the mechanical properties of the cross-linked membrane have been improved,and WU and SR have been significantly reduced.The WU and SR of the crosslinked membrane P₁-CH₂-Br-1.5-c-0.25 at 80°C are 30.96%and 25.57%,respectively,which are much lower than the WU and SR of the P₁-CH₂-TMA-1.5 membrane at 80°C,which are 130.63%and 99.97%,respectively.The alkali stability test shows that the conductivity loss of cross-linked membrane P₂-CH₂-Br-1.5-c-0.75,P₂-CH₂-Br-1.5-c-0.5,P₂-CH₂-Br-1.5-c-0.25 are 13.58%,16.47%,and 19.93%,respectively,which are significantly lower than that of the uncrosslinked membrane P₂-CH₂-TMA-1.5（31.93%）.Although the IEC value of the cross-linked membrane P₂-CH₂-Br-1.5-c-Br-0.25 is slightly lower than that of the uncross-linked membrane P₂-CH₂-TMA-1.5,the WU and SR show a significant decrease,indicating that the dimensional stability of AEMs has been enhanced.Meanwhile,the mechanical properties and the alkali resistance stability of crosslinked membrane P₂-CH₂-Br-1.5-c-Br-0.25 have been improved.Moreover,P₂-CH₂-Br-1.5-c-Br-0.25 exhibits better fuel cell performance than P₂-CH₂-TMA-1.5.Therefore,it could be concluded that moderate cross-linking can improve the overall performance of AEMs.