Evaluating The Alkaline Stability Of Adamantyl-substituted Quaternized Ammonium And Imidazolium Salts And Their Applications In Anion Exchange Membrane

As the core component of Alkaline Polymer Fuel Cell-Anion Exchange Membrane(AEM),its properties directly decided the performance of fuel cell,acted as a fuel barrier and anion conduction.However,due to the inherent low ion conductivity of AEM and its functional cationic ions were sensitive to OH-under the alkaline condition,the large-scale commercialization of AEM were limited.Recently,researchers could obtain AEM with high OH-conductivity by adjusting the polymer aggregation structure and constructing nano-porous structure and so on.Nonetheless,the alkaline stability of AEM were still an intricate issue and needed to be addressed urgently.According to the problems mentioned above,firstly,we took action to introduce the bulky sterically hindered substituted group next to the functional cationic ions(quatrernized ammonium(QA)and imidazolium)in order to impede the attack of OH-and to study the discipline between stability of cationic ions and effect of hindrance.Secondly,we evaluated the alkaline stability of these cationic ions and chose the stable one to tether to the PPO backbone in a reasonable method,we further discuss the difference between the alkaline stability of small cationic compound and polymer,meanwhile,we probed the other performance of AEM(water uptakedimensional stability?ion conductivity?fuel cell performance and so on).(1)We introduced the Admantane(Ad)group to the benzyl type quaternized ammonium(DMBAdA)and evaluated the alkaline stability of DMBAdA via in-situ 1H NMR spectra systematically.There were no new peaks showed after immersing the DMBAdA in 2M NaOH,at 80 ? for 7 days which meant DMBAdA had superior alkaline stability.Then,we tethered DMBAdA to the PPO side chain via Menshutkin reaction which was a facile way to obtain the AEM(PPO-AdA-x).After soaking the membrane in the 1M NaOH at 60 ? for 7 days,the conductivity only degraded 11.7%.Comparing to traditional benzyl type PPO-QA,the PPO-AdA-x had higher conductivity and much more stable in the alkaline condition.(2)Meanwhile,we studied the relationship between imidazolium stability and bulky sterically hindered substituted group.We designed six kinds of imidazolium which have different substituted group on the C-2 and N-3 position(EMIm,BEIm,AdEIm,EDMIm,BMEIm,AdMEIm).From the results of 1H NMR spectra,after aging the cations in 2M NaOH at 80 ? for 168h,Ad substituted at N-3 position of imidazolium(AdEIm and AdMEIm)have better alkaline stability.After that,we tethered these cationic ions to the PPO side chain via "click" chemistry to get the AEM(PPO-AdIm,PPO-AdMeIm).The obtained AEM possess comparable conductivity but insufficient dimensional stability,a possible explanation is that the introduction of rigid adamantane moiety at N3-position of imidazolium created additional free volume in the membrane matrix and impeded the chain packing and thereby reduced the interchain interactions largely.Meanwhile,the alkaline stability of AEM is not excellent like the small imidazolium compound,only 60%of its origin conductivity remained after soaking the membrane in 1M NaOH at 60 ? for 7 days.