Study On Alkaline Stability And OH~- Transport Characteristics Of Imidazolium Anion Exchange Membranes

As a critical component of the alkaline anion exchange membrane fuel cell(AEMFC),anion exchange membrane(AEM)still faces the problems of poor alkali stability and low OH~-conductivity.Functional cations conduct hydroxide ion(OH~-)from cathode to anode,and thus dominate performance of the AAEMFC.Imidazoliums have the advantages of low price,easy synthesis and modification,and its alkali stability can be significantly improved by introducing substituents.In addition,the introduction of alkyl side chain in AEMs can effectively construct phase separation and improve OH~-conductivity.However,the structure-performance relationship between the steric hindrance of substituents,the types and lengths of alkyl side chains and the performance of AEMs is not clear.To date,the design of imidazolium-based AEMs with high OH~-transport and excellent alkali stability is still facing challenge.In this study,poly(2,6-dimethyl-1,4-phenylene oxide)(PPO)with excellent mechanical and thermal properties was selected as the main chain of polymer,and imidazolium-based AEMs with side chain was synthesized.By comparing the performance of imidazolium-based AEMs without alkyl side chains,the influence of alkyl side chain on the performance of imidazolium-based AEMs was obtained.The influences of steric hindrance of substituents and the types and lengths of alkyl side chains on the alkali stability and OH~-transport of imidazolium-based AEMs were studied by molecular simulation,aiming to provide theoretical basis for the structural design of imidazolium-based AEMs.The main contents and conclusions are as follows:(1)The alkyl side chain was grafted to PPO by Friedel-Crafts acylation,and then 1,2,4,5-tetramethylimidazole was introduced into the side chain by carbonyl reduction and Menschutkin reaction to prepare side chain imidazolium-based AEMs.Compared with the imidazolium-based AEMs without alkyl spacer chain,the introduction of alkyl spacer chain slightly reduces the alkaline stability,while the activation energy of OH~-transport in the AEMs is reduced and thus increases the ionic conductivity.(2)Density functional theory(DFT)simulation and molecular dynamics(MD)simulation were used to explore the influence of steric hindrance of substituents and the type and length of alkyl side chains on the stability of imidazolium-based AEMs.The introduction of phenyl to imidazolium transferes LUMO orbital of imidazolium to phenyl,and reduces Fukui nucleophilic function of each site of the imidazolium,which reduces the degradation possibility of imidazolium.MD simulation showed that steric hindrance of substituents and alkyl pendent chain could make it difficult for OH~-to approach imidazolium,which increased the difficulty for OH~-to attack imidazolium.However,the introduction of alkyl spacer chain weakened the steric effect of OH~-by PPO backbone,which allows more OH~-around imidazolium and increased the degradation possibility of imidazolium.With the increase of alkyl spacer chain length,the possibility of imidazolium degradation also increased.In addition,when the water content increased,a complete hydration structure formed around OH~-,which decreased the possibility of OH~-approaching imidazoliums,and thus increasing the alkaline stability of the imidazolium-based AEMs.(3)The mechanism of steric hindrance of substituents and the introduction type and length of alkyl side chain on OH~-transport was explored by MD simulation.The steric hindrance of substituents and the introduction of alkyl pendent chain weakened the hydration structure of imidazolium,decreased the continuity of OH~-transport channel and reduced the number of hydrogen bonds,which inhibited OH~-transport.However,the introduction of alkyl spacer chain enhenced the hydration structure around imidazolium,which enhenced the OH~-transport channel more continuous and accelerates the transport of OH~-.With the increase of alkyl spacer chain length,the OH~-transport increased initially and then decreases.The imidazolium-based AEMs with alkyl spacer chains of 6-8 carbon lengths has the highest OH~-transport.