Synthesis,Stability Improvement And Electrochemical Performance Of Poly(Arylene Alkylene)s-based Anion Exchange Membrane

Anion exchange membrane fuel cell（AEMFC）have attracted much attention due to their high oxygen reduction kinetics in alkaline working environment and low fuel permeability.However,as one of the vital components of AEMFC,anion exchange membrane（AEM）still suffers from some intrinsic disadvantages,such as poor stability（chemical stability,dimensional stability）and low ion transport efficiency,which will seriously affect the electrochemical performance of AEMFC.In this dissertation,AEMs based on poly（arylene alkylene）backbone were synthesized,and not only the chemical stability of the membranes was evaluated,the balance between ion conductivity and other AEM-pertinent properties also studied.By adjusting the chemical composition of the AEMs,we also investigated the influence of the ionomer chemical structure optimization on the basic properties of the corresponding AEMs,electrochemical performance and cell durability.（1）The bication crosslinked AEM with the poly（m-terphenylene alkylene）backbone were designed and prepared.The results showed that the formation of hydrophilic cross-linking structure can effectively inhibit the water absorption and swelling,and improve the dimensional stability of the membrane.Moreover,the flexible hydrophilic segments crosslinks did not change the morphology of the membrane.The m-XPTPA40-2N membrane with the highest crosslinking degree showed best oxidative stability,and its mass and conductivity remained above 98%and 99%of the initial value after immersed in Fenton’s reagent at 50℃ for 24 h.In addition,the membrane electrode assembly（MEA）using m-XPTPA40-2N membrane was prepared and evaluated,and peak maximum power density can reach 302 m W cm^-2 at 60℃,exhibiting the good single cell performance.（2）Although the stability of AEM can be improved by introducing of the flexible crosslinking network,the crosslinking will make membrane insoluble in organic solvents,Therefore,in this section,we introduce the para-quaterphenylene unit with high rigidity into polymer backbone to enhance the AEM’s chemical and dimensional stability without compromise the solubility of the membrane.Specifically,biphenyl and para-quaterphenyl are copolymerized by superacid catalyzed electrophilic polycondensation,and the copolymerization also can improve the irregularity of polymer backbone,which may endow the ionomer with better solubility and film-forming property.The results showed that the prepared P（4PA-co-2PA）containing rigid para-quaterphenyl segments have excellent dimensional stability,high ionic conductivity and ion transport efficiency.In addition,no substantial degradation,neither in conductivity change nor head cation degradation,was observed for P（4PA-co-2PA）-47,after soaking 1 M NaOH aqueous solution at 80℃ for 30 days.Finally,we assembled the membrane P（4PA-co-2PA）-47 in MEA,and the maximum power density reached 397 m W cm^-2 at 80℃.The voltage decay rate was only 0.8 m V h^-1 when the cell was operated at constant current density of 200m A cm^-2 for 100 h.（3）In order to find more alkaline resistant functional cations and apply them in anion exchange membranes,we further designed and prepared pyrazolium and imidazolium functionalized AEM,i.e.,P（4PA-co-2PA）-Py and P（4PA-co-2PA）-Im.They have the same functional cation substitution degree and ion exchange capacity,and their water uptake,swelling ratio and conductivity are similar,which indicates that the uniformly physicochemical properties of P（4PA-co-2PA）-Py and P（4PA-co-2PA）-Im AEM.However,P（4PA-co-2PA）-Py has better alkaline resistance.The conductivity of P（4PA-co-2PA）-Py was well retained after the alkaline stability test in 1 M NaOH aqueous solution at 80℃ for 14 days,however,the counterpart P（4PA-co-2PA）-Im decreased by53%under the same conditions.In addition,the maximum power density of the membrane electrode assembled by P（4PA-co-2PA）-Py functionalized with pyrazolium group reaches 502 m W cm^-2 at 80℃,and the voltage decay rate is only 3.4 m V h^-1 when the electrode is operated at constant current density of 200 m A cm^-2 for 50 hours.On the contrary,the polarization curve of P（4PA-co-2PA）-Im membrane electrode can be measured at 20℃,but the maximum power density was only 45 m W cm^-2,and the cell performance decays rapidly as the temperature increasing.We speculated that that the rapid degradation of P（4PA-co-2PA）-Im occurred during the fuel cell operating may responsible for the poor electrochemical performance,which is in accordance with its poor ex-situ alkaline stability.