Molecular Structure Design Of Imidazole-Based Anion Exchange Membranes

Based on high energy conversion efficiency and minimal harmful emission,fuel cell technique has attracted tremendous research interests in the past decades.Compared with acid proton-exchange membrane fuel cell(PEMFC),alkaline anion-exchange membrane fuel cell(AEMFC)exhibits potential utilization of non-precious metal catalysts and faster kinetic reaction efficiency,which has been considered as a promising candidate for next generation of novel energy conversion device.As a significant component of AEMFC,anion-exchange membrane(AEM)serves to separate fuels and conduct water molecules and hydroxide ions.However,there are two main obstacles that hinder the development of AEM.First,hydroxide ion conductivity cannot be comparable to proton conductivity of PEM.Since the volume of OH-ion is much bigger than that of proton,its mobility is relatively smaller,resulting in poor ion transport efficiency of AEM.Second,the functional groups of AEM exhibit poor alkaline stability.The electrophilic cationic functional group is vulnerable to nucleophilic OH-ions under strong alkaline condition,which causes the severe degradation of AEM.In this essay,we have summarized our works in improving ion conductivity and alkaline stability of AEM.The main contents are described as following:(1)Two ethylene oxide spacers are incorporated into ionic side-chain of imidazolium-functionalized AEM.By using atom force microscopy and molecular dynamic simulation,we confirm that the molecular flexibility is beneficial for the self-assembly process of polymer,which further construct highly ordered ion channels.Moreover,the unique hydrophilicity of ether group is able to improve the affinity between ionic side-chain and H2O and OH-ions,and thus accelerate the ion transport in AEM.(2)The modification of imidazolium can be varied a lot due to its five-membered ring molecular structure.We incorporate imidazoliums with different methyl groups on molecular ring into brominated poly(2,6-dimethyl-1,4-phenylene oxide)to obtain different imidazolium-functionalized AEMs.In-situ 1H NMR technique is employed to monitor the degradation process of polymer under alkaline condition.The results demonstrate methyl groups at C2-,C4-and C5-positions are able to alleviate the deprotonation of imidazolium ring,thus improving the alkaline resistance of AEM.In single cell evaluation,the power density increases steadily with elevated temperature and reaches 340 mW cm-2 at 80 ?.