Preparation And Properties Of Bisimidazole Cross-linked Anion Exchange Membranes

Compared with Proton exchange membrane fuel cells,Anion exchange membrane fuel cells can use non-precious metal catalysts,they have great potential as energy conversion devices for automobiles and receive widespread attention in recent years.As a key component in Anion exchange membrane fuel cells,Anion exchange membranes'(AEM)performance to some extent determine the performance of the fuel cell.However,AEMs with excellent comprehensive properties like perfluorosulfonic acid membranes have not been prepared up to now.Most of the researches aim to increasing the ionic conductivity of the membrane by increasing the ion exchange capacity of membrane,but it is prone to swelling and sacrificing the dimensional stability of the membrane.Crosslinking is a common method for balancing ionic conductivity and AEM swelling at high ion exchange capacity,but the alkaline stability of cross-linked AEMs still need to improve.Therefore,it is urgent to develop a highly stable anion exchange membrane material having high conductivity,alkaliane stability and swelling resistance.In this paper,commercial poly(2,6-dimethyl-1,4-phenylene oxide)was selected as the main chain,and the bisimidazolium ionic liquid was grafted and cross-linked.By adjust the number of methyl substituents on the bisimidazolium and the alkyl chain space between the main chain and imidazolium,we obtained 2 series cross-linked AEMs PPO-IM-n and PPO-6C-IM-n,(n=0,1,2,represent the number of methyl groups on the imidazole ring;6C represents the main chain and the imidazolium cation is connected by hexane,and the absence of 6C is through methylene-linked between backbone and imidazolium cation).The results show that the PPO-IM-1 AEM with the methyl group at the C2 position has the highest hydroxide conductivity,which can reach 115 mS/cm at80°C,is comparable to the commercial Nafion membrane.By increasing the number of methyl groups on the imidazole ion,PPO-IM-2 and PPO-6C-IM-2 with methyl groups on both C2 and C4 have the highest alkaline stability in each series,indicating that imidazole ring's C2 and C4 position substitute by a methyl group can effectively prevent the attack of the OH~-and improve the stability of the AEM.The PPO-6C-IM-n series polymer has a better microphase separation than the PPO-IM series polymer due to the introduction of 6 carbon hydrophobic alkyl chain between the polymer backbone and the imidazolium cation.With better water management,it is less prone to swelling and has a significant increase in alkaline stability.Among of them,PPO-6C-IM-1 has a swelling ratio of only 5.8%at 60°C;PPO-6C-IM-2 immersed in a 1 mol/L NaOH solution at80°C for 7 days,the conductivity is only decreased by 17%.In addition,this paper also used Nafion membrane and home-made AEM to assemble ion exchange membrane fuel cells and conduct preliminary exploration of battery performance.In summary,this paper demonstrates that the use of methylation is a viable method that does not significantly reduce the conductivity of AEM while improving its chemical stability.The alkyl chain between the imidazolium and the main chain can effectively reduce AEM swelling and enhance AEM alkaline stability.These studies provide new ideas for synthesis and practical application of high performance anion exchange membranes.