Side-chain-type Anion Exchange Membranes: Morphology Regulation And Fixation

Anion exchange membrane?AEM?is widely used in the field of separator of anion exchange membrane fuel cells and vanadium redox flow battery as well as in electrodialysis.The most important parameter for AEM is the ion conductivity,which is highly related to the ion exchange capacity?IEC?,water uptake and the morphology of the membrane.In general,increasing the IEC of an AEM to improve the conductivity is a direct and simple way.However,high ion contents often lead to excessive water absorption and impair the intermolecular interactions,result in the unfavorable swelling and even mechanical loss of the AEM.Inspired by the chemical strcture and morphology of Nafion,constructing the AEM with separated microphase separation morphology is key to balance the trade-off property between conductivity and dimensional stability and mechanical properties.Therefore,in this dissertation,two thermodynamic immisible grafts were introduced into the side chain of the PPO backbone,aiming to the induction of distinct and stable microphase separation morphology in the AEM.In addition,we further fixed the morphology of the AEM by UV-triggered in-situ crosslinking,and other AEM pertinent properties,such as water uptake,dimensional and chemical stabilities as well as conductivity,were investigated.?1?Poly?2,6-dimethyl phenylene oxide??PPO?was used as the main chain,and the imidazolium with ethane oxide as the connecting group was introduced as the hydrophilic side chain by the"Click"reaction,and octyl or perfluoroctyl as the hydrophobic side chain AEMs.Among them,the AEMs?PPO-x-Im_y8F_z?perfluorooctyl-containing side chain showed obvious microphase separation structure,and its ion cluster size was 2-5 nm,and the ion cluster domain spacing was about 6 nm.AEMs?PPO-x-Im_y8C_z?with octyl hydrophobic side chain or AEMs?PPO-x-Im?without hydrophobic side chain only have weak or no obvious microphase separation structure.The results show that the introduction of hydrophobic side chains and the hydrophobic strength of the hydrophobic side chains play an important role in the formation of AEM ion clusters.In addition,PPO-x-Im_y8F_z not only has better ionic conductivity,but also can inhibit the water absorption of AEM,so as to better trade-off the relationship between water uptake and ionic conductivity.?2?Since the perfluoroctyl side chain can effectively induce the formation of obvious microphase separation structure of the AEM,we further prepared the multiquaternary ammonium salt side chain with good alkali stability as the functional side chain of AEM.Meanwhile,AEMs tethering tri-quaternary ammonium?PPO-8-3QA?and benzyltrimethylammonium?PPO-22-QA?were synthesized for comparison.The results show that PPO-22-3QA8F has better microphase separation and ion channel has better coherence.In addition,PPO-22-3QA8F hydroxide conductivity at 90?to 83 mS/cm,far above the PPO-8-3QA?39 mS/cm?and PPO-22-QA?27 mS/cm?;PPO-22-3QA8F also has very excellent swelling resistance and alkali resistance,in 1 M NaOH 80?alkaline hydrolysis after 168 h,PPO-22-3QA8Fconductivity almost no decline,and PPO-8-3QA and PPO-22-QA keep only 60%and 55%respectively;In the PPO-22-3QA8F H₂/O₂ single fuel cell performance test,the power density reached 102mW/cm²,while in the same test condition,the PPO-8-3QA only had 38 mW/cm².?3?To improve the dimensional stability and chemical stability of anion exchange membrane by in situ chemical crosslinking.An anion exchange membrane?PPO-24-3QA?with side chains of nultiquaternary ammonium salt was synthesized and prepared.Then the membrane was immersed in 5 mg/mL 4,4'-azodistyrene-2,2'-disodium disulfonate tetrahydrate aqueous solution?DAS?.The DAS small molecule crosslinking agent was adsorbed into the AEM by electrostatic.AEM was subjected to UV illumination,and azobenzene was generated by azide under UV irradiation,and then addition or insertion reaction was conducted between AEM and adjacent groups?phenyl,methylene,and quaternary ammonium?.Finally,in-situ crosslinked AEM?XPPO-24-3QA-3030?was obtained.The results show that the size stability of AEM is greatly improved after crosslinking.For example,the water uptake of PPO-24-3QA at room temperature is 88 wt.%,while that of XPPO-24-3QA-3030 is 42 wt.%.XPPO-24-3QA-3030 also has more excellent alkali resistance performance,under the condition of the 1 M NaOH 80?alkaline hydrolysis after 168 h and 336 h,the conductivity remains at 97%and 93%.Compared with the traditional AEM,the alkali stability is improved greatly.In addition,the crosslinking degree of AEM can be conveniently adjusted by controlling the soaking time and UV irradiation time of DAS solution.Moreover,the action mechanism of this method is electrostatic interaction,which is universal to AEM.