Polymer Electrolyte Membrane Based On Poly(arylene Ether)s For Fuel Cell Applications

Polymer electrolyte membrane fuel cells offer a promising power choice,which can effectively alleviate energy shortage and rationally solve environmental pollution problems due to their high efficiency and near-zero pollutant emissions.As a core component of fuel cell,polymer electrolyte membranes can be divided into proton exchange membrane(PEM)and anion exchange membrane(AEM)according to the conductive ions,and the performance directly affects power density and service life of fuel cells.To date,PEM and AEM have been suffered from low ionic conductivity and poor physicochemical stability,which severely restrict the commercial applications.To address these issues,strategies such as optimizing chemical structure,building cross-linking network and doping inorganic nanocomposite have been devoted to designing novel polymer electrolyte membranes which have hydrophilic/hydrophobic phase separation structure like Nafion membrane.In this dissertation,several kinds of poly(arylene ether)-based polymer electrolyte membranes with outstanding comprehensive performance have been developed,and the structure-property-performance relationships of the membranes have been explored in detail.Firstly,a series of side-chain fluorinated PEMs with similar sulfonation degrees were synthesized by decafluorobiphenyl and different diphenol monomers.Fundamental properties and structure-property relationships of the prepared PEMs were investigated.The results show that the SPFAE(S)membranes display high physicochemical stability,mechanical strength and proton conductivity.Especially,proton conductivity of the SPFAES-KDP membrane reaches 253 m S/cm@80°C,which is much higher than that of the Nafion 112membrane(180 m S/cm@80°C).Besides,the structure of diphenol monomers has a great impact on water absorption,mechanical properties and proton conductivity of the prepared PEMs due to the structural differences in rigidity and free volume.Among them,flexible groups and larger free volume in the membranes are beneficial to the water absorption,while rigid groups are favorable to improve the mechanical strength and stability.To further improve the water absorption and proton conductivity of this kind of membrane without deteriorating the mechanical properties and stability,sulfonated carbon nitride nanosheets(SCN)were introduced in the membrane matrix to prepare a series of nanocomposite membranes.The interfacial interactions between SCN and polymer motivates the formation of ionic transport channels and phase separation structures.Besides,the acid-base interaction of SCN and sulfonic acid groups facilitates the dissociation of sulfonic acid groups.As a result,the proton conductivity,mechanical strength and dimensional stability of the composite membranes significantly improved.The SPFAE/SCN-0.5 membrane displays an excellent H₂/O₂ fuel cell performance of 717 m W/cm² at 80°C.To construct microphase separation structure in polymer electrolyte membrane and restrict excessive water absorption,a series of multi-cationic oligomers(DABCO-x)with different chain lengths were synthesized and subsequently introduced into the chloromethylated poly(arylene ether sulfone)s to prepare cross-linked AEMs.As the cross-linking agent length and ionic groups amount increase,ion channels of the cross-linked membrane become better connected,water absorption and ionic conductivity also showed a significant increase.Among all the prepared membranes,the CQPAES-4 cross-linked membrane achieves the highest water uptake(80.7%)and ionic conductivity(78.7 m S/cm)at80°C.In addition,the DABCO molecules and aliphatic segments increase the steric hindrance of ionic groups and the electron cloud density of?hydrogen,respectively,resulting in the inhibition of the nucleophilic substitution reaction.Furthermore,quaternary amine-modified graphene oxides(QBGO)were synthesized by grafting the multi-cationic oligomer(DABCO-4)onto graphene oxide.Subsequently,the QBGO was employed as cross-linker to prepare poly(arylene ether sulfone)s AEMs with low cross-linking degree.Compared with the CQPAES cross-linked membranes,the QBGO cross-linked membranes possess higher water uptake and ionic conductivity,and clearer nano-phase separation structure.In this design,the polymer chains are more orderly distributed on both sides of the graphene oxide sheet due to the cross-linking effect,which leads to the more connected ion channels.The QPAES/QBGO-2.0 membrane displays an excellent H₂/O₂ fuel cell performance of 75.7m W/cm² at 80°C.In this dissertation,the designed poly(arylene ether)-based polymer electrolyte membranes exhibit good comprehensive performances,which can meet the essential requirements of fuel cell applications.In addition,this dissertation summarizes the structure-property-performance relationships of poly(arylene ether)-based polymer electrolyte membranes,which provides valuable methods and strategies for the subsequent design of high-performance polymer electrolyte membranes.