| The energy crisis and environmental pollution caused by fossil fuel have been taken seriously,and it is urgent to find an efficient,clean and sustainable green energy source to alleviate the current dilemma.Fuel cell is a power device that converts hydrogen energy into electrical energy,which has the advantages of high efficiency and pollution-free products.Anion-exchange membrane fuel cells（AEMFC）are becoming a focus of research because of their high oxygen reduction reaction kinetics under alkaline environment and the possibility of using non-precious metal catalysts.Anion exchange membrane（AEM）,as one of the core components,will directly affect the power generation efficiency of AEMFC.Therefore,it is very important to design and develop anion exchange membranes with high ion transport capacity,good dimensional stability and alkaline stability.However,in practical applications,anion exchange membranes exhibit low ion conductivity and poor chemical stability.It has been shown that constructing a good microphase separation structure by designing different molecular skeletons and ion transport groups and regulating the ion group positions is an effective strategy to improve the hydroxide conductivity due to the formation of wide and continuous ionic transport channels.Meanwhile,the formation of microphase separation structure improves the chemical stability and dimensional stability of AEMs to a certain extent.Based on the above strategy,we designed and prepared a series of poly（pentafluorophenyl-carbazoly）based side-chain-type AEMs and investigated the relationship between their structures and properties.The details of the work and the results of the study are as follows:（1）The presence of C-F bonds is beneficial to increase the hydrophobicity of the backbone and the dimensional stability of AEMs,so in this work,pentafluorobenzaldehyde monomer was used,which was subjected to superacid-catalyzed polymerization with 9-（6-bromohexyl）-9H-carbazole monomer with a bromohexyl side chain at the 9 position to obtain an all-carbon backbone that does not contain ether bonds.Subsequently,three anion-exchange membranes（PC-PF-X）modified with different quaternized cation groups were prepared by Menshutkin reaction with tertiary amine（trimethylamine QA,1-methylpyrrolidine Py,1-methylpiperidine Pip）solutions.SAXS and AFM results showed that all AEMs exhibited well-defined microphase separation structures.The scattering peaks for all three AEMs appear around 1.2 nm-1 and the corresponding ionic cluster size is around5.2 nm.In the dimensional stability tests,three kinds of AEMs exhibited moderate water uptake and low swelling ratio values.Among them,PC-PF-QA exhibited the highest water uptake and the largest area swelling ratio at high temperature.In addition,the PC-PF-QA membrane also exhibited the highest ionic conductivity of 123 m S cm-1at 80°C.This is attributed to the higher water uptake allowing more water molecules to participate in ion transport within the membrane.The ionic conductivity of AEMs was found to decrease slightly when the membranes were immersed in 1 M Na OH solution for 40 days at 60°C for alkaline stability testing.PC-PF-QA and PC-PF-Pip showed comparable stability.Despite the small size of the QA-type AEM ionic groups and small spatial site resistance,they still showed good chemical stability,which may be related to the stable polymer backbone.It can be seen that the choice of polymer molecular backbone and cationic groups has a significant influence on the properties of AEMs.（2）Based on the study of PC-PF-QA AEM,we aimed to further increase the difference between hydrophilic and hydrophobic phase regions to regulate the microphase separation structure by extending the length and local density of flexible side chains containing quaternary ammonium cations.Therefore,AEMs containing double cation strings（PC-PF-BQA-X）were prepared by adjusting the grafting ratio of the flexible chains.Meanwhile,the AEMs with high grafting rate were reinforced by microporous PTFE layer to obtain a composite membrane（PTFE-BQA-2）.It was observed from the surface and cross-sectional morphology of the filled composite membranes that the PTFE-BQA-2 membrane had a dense and homogeneous structure.Meanwhile,a three-layer structure could be seen in the cross-sectional view,demonstrating that the polymer solution could penetrate well into the porous supporting membrane.SAXS curves showed that the three kinds of AEMs had a well-defined phase separation structure.The ionic cluster size became larger with increasing IEC values and the presence of PTFE matrix did not affect the microscopic morphology of the membranes.Due to the lower IEC value and the well-defined microphase separation structure exhibited,BQA-1 had excellent dimensional stability,BQA-2 with higher IEC value exhibited higher water absorption and swelling.In contrast,the water absorption and swelling rate of PTFE-BQA-2 composite membrane decreased due to the presence of reinforced layer.Similarly,due to the introduction of PTFE supporting layer,the reinforced membrane has good mechanical properties compared to the two homogeneous membranes,with a tensile strength five times higher than that of the original homogeneous membrane（BQA-2）.It can be seen that the balances the IEC and mechanical properties to some extent.For the alkaline stability,the retention rate of ionic conductivity for BQA-2 and PTFE-BQA-2 also reached 90%,which showed considerable chemical stability,proving the feasibility of the composite membrane preparation method.However,the conductivity of the reinforced membrane PTFE-BQA-2 is reduced compared to the original homogeneous membrane because the PTFE matrix is non-conductive.The above results indicate that changing the ionic group arrangement within the membrane and increasing the local ionic density can help improve the microphase separation structure.In addition,the reinforced membranes have better dimensional stability and mechanical properties without affecting the microstructure,which is beneficial to improve the performance of AEMs. |
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