Synthesis And Stability Of Alkaline Anion Exchange Membranes

The increasing demand for high-quality and clean energy is directed towards the rapid development of fuel cells as an environmentally friendly power source and alternative for traditional fossil fuel.Among many types,alkaline anion exchange membrane fuel cells(AAEMFCs)have elicited wide attention because of its significant oxygen reduction kinetics under high pH conditions,lower dependence on precious metal catalysts,and low cost of membrane production.Unfortunately,as the main component of alkaline fuel cells,a trade-off between conductivity,dimensional and chemical stabilities under alkaline conditions especially at elevated temperature has to be faced by the most common alkaline anion exchange membranes(AAEMs).AAEMs with high ion exchange capacity usually possess high water uptake and poor dimensional stability,which will affect the continuous and stable operation of AEMFCs.Hydroxide ions also need sufficient ion exchange capacity and water uptake,due to their own limitations,to reach a satisfying level of hydroxide conductivity.Therefore,improving the longterm alkaline stability and hydroxide ion conduction of AAEMs is critical to improving the speed of research and development of alkaline anion exchange membranes.The details are as follow:(1)Based on density functional theory,the structures of the alkyl chain length(2→4→6),substituents at C2,4,5 positions(-CH3,-Ph,-PhCH3…),the values of Mulliken charges,LUMO energy,electrostatic potential,and reaction energy barrier,and alkaline stability of imidazolium cations were investigated in detail.The simulation results are verified by experiments,and the relationships between the structures,the values of Mulliken charges,LUMO energy,electrostatic potential,and reaction energy barrier,and alkaline stability are summarized for the screening of imidazolium cations with high alkali stability.Cationic imidazolium derivatives and corresponding main-chain cationic polymers were prepared by Radziszewski reaction.The water uptake,swelling ratio,hydroxide ion conductivity,alkaline stability,thermal stability,and morphology of the prepared membranes were characterized.The polyimidazolium-based anion exchange membrane(CM-DHPDMIm)with phenyl-substitution at C2 and methylsubstitution at C4,5 positions processed the best chemical stability in alkaline solution.After the stability test,the conductivity and IEC remained about 87.4%and 81.3%of the initial values,respectively.The main-chain polyimidazoliumtype AAEMs is expected to be equipped with fuel cell devices.(2)Polyolefin-based AAEMs were designed and synthesized to explore the relationships between quaternary ammonium salt,imidazolium,and pyrrolidinium cationic functional groups with mono-or double-cationic structures,physicochemical properties,and ionic conductivity in AAEMs.The results showed that the AAEMs with double-cationic structures processed better properties than that with mono-cationic in water uptake,swelling ratio,and ionic conductivity.Based on the experimental data,the six kinds of AAEMs were mathematically modeled,and the molecular dynamics simulation results were consistent with the experimental results.The chemical stability of cationic functional groups in alkaline solution was also predicted by molecular dynamics simulation.(3)On account of strong acid catalysis and alkylation reactions,polyaromatic ionomers without aromatic ether bonds were synthesized with diphenyl,trifluoroacetophenone,7-bromo-2-methyl heptanol,and 6-(dimethylamino)-Nhexyl-N-dimethylhexane ammonium bromide.The prepared ionomer-based AAEMs showed excellent electrochemical performance.The double-quaternary ammonium salts with methyl,hydroxyl,and methoxy groups at the end were selected as cationic functional groups to study the molecular dynamics of three kinds of double-quaternary ammonium salt functionalized AAEMs,in terms of solvation,transport properties,chemical stability,and so on.The results showed that the hydroxyl and methoxy groups at the end of cationic functional groups are helpful to improve the hydroxyl ion conductivity of AAEMs.(4)A series of multi-hydroxyl substituted ethanolamine cations were prepared by the alkylation reaction,and the relationship between the number of hydroxyl substituents and the alkaline stability of ethanolamine cations was investigated systematically.With the increase in the number of hydroxyl substituents,the chemical stability of ethanolamine cations decreased.PPO was selected as the main chain structure of ionomer and ethanolamine cation as cationic functional groups for molecular dynamics simulation.The simulation results indicated that the chemical stability of AAEMs in alkaline solution was PPO-3OH-30<PPO2OH-30<PPO-OH-30.Besides,the increase of hydroxyl-substitution on ethanolamine cationic groups is beneficial to the hydroxyl ion conductivity of AAEMs.