Synthesis And Properties Of Concentrated Sulfonated Poly(arylene Ether) Based Proton Exchange Membrane

As a low-temperature fuel cell,PEMFC has the advantages of fast start-up,wide range of applications and ease of assembly and processing.As a key component of PEMFC,proton exchange membrane(PEM)material has the function of proton conduction,fuel barrier and catalyst loading.In recent decades,researchers have made some progress in the development of PEM materials.Sulfonated poly(arylene ether)is considered as one of the most promising alternative proton exchange membrane materials(PEM)attributed to its excellent thermal stability and chemical stability,superior mechanical properties and low cost.However,the conductivity of the conventional sulfonated random copolymer is not sufficient to fulfill the requirements of PEMFC.In addition,the higher water swelling results in a significant reduction in mechanical properties.From the molecular structure point of view,the rigid aromatic polymer chain hinder the the movement of ion-conducting groups.And the distribution of sulfonic acid groups on the backbone are disorganized,resulting in formation of ambiguous hydrophilic / hydrophobic phase separation.Herein,based on molecular design,our work will be carried out on both of the hydrophilic segments and hydrophobic segments to overcome the shortcomings in conventional sulfonated poly(arylene ether)s.By adjusting the position and distribution of the sulfonic acid groups in the hydrophilic segments,the sulfonic acid groups are introduced on the polymer side chains with enhanced mobility and the concentrated sulfonic acid groups are able to form ion cluster structure which facilitate proton conduction.Further,we also alter the polymer backbone structure to improve the hydrophobicity and stability of the hydrophobic segments resulting in better mechanical properties.The optimized performance of PEM with higher proton conductivity and dimensional stability is able to fulfill the requirement of PEMFC applicaton.First,a multi-phenyl containing diphenol monomer was successfully obtained from 2,6-difluorobenzoyl chloride through a series of reactions.And poly(arylene ether sulfone)polymers(PES-x)were synthesized by nucleophilic polycondensation with this diphenol,4,4'-difluorodiphenylsulfone and 4,4'-dihydroxydiphenylsulfone.The multi-phenyl structure in PES-x was sulfonated by concentrated sulfuric acid in the post-sulfonation reaction to give concentrated sulfonated poly(arylene ether sulfone).Compared with the conventional random sulfonated copolymer,this series of polymers have moderate water uptake and swelling ratio,as well as higher proton conductivity.SPES-16 with IEC of 1.61 meq.g-1 owns comparable proton conductivity with Nafion 117.The result of small angle x-ray scattering(SAXS)measurement demonstrate that SPES-x polymers have hydrophilic ion clusters structure,forming a well-defined hydrophilic / hydrophobic phase separation morphology.In addition,proton conductivity and dimensional stability of SPES-x are enhanced attributed to this morphology.Secondly,based on multi-phenyl containing diphenol,we construct highly poly(arylene ether)backbone with decafluorobiphenyl and 6FBPA.And fluorine-containing concentrated sulfonated polymer was prepared by the post-sulfonation of chlorosulfonic acid.The higher fluorine content enhances the hydrophobicity of the polymer,thus FPAE-x own lower water absorption and better dimensional stability.Although the water absorption has a certain degree of reduction,but the F-SPAE-x series of polymers still maintain a high proton conductivity.The conductivity of F-SPAE-20 was superior to Nafion throughout the test range.Compared to the non-fluorinated SPES-x,F-SPAE-x owned enhanced dimensional stability and comparable conductivity with the same hydrophilic segment content.F-SPAE-16 showed the best DMFC performance in this series,while the performance of F-SPAE-20 declined at a higher methaol concentration.In the third part,polyether ketone imine(PEKt)was initially synthesized via a low temperature polymerization of multi-phenyl diphenol,4,4'-difluorobenzophenone imine and 4,4'-dihydroxybenzophenone.The sulfonic acid group was introduced into the benzene ring by the post-sulfonation reaction,which also trigger the hydrolysis reaction of the ketimine structure.As a result,the semi-crystalline sulfonated poly(ether ketone)(semi-SPEK-x)was successfully obtained.Due to the presence of the crystalline structure,semi-SPEK-x showed excellent stability in conventional organic solvents.The XRD results demonstrate that the crystallinity of the dried sulfonated polymer membrane is higher than 10%,and the crystallinity decreases with the increase of IEC,and the crystallinity of the wet membrane decreases slightly.Through the study of the water absorption behavior of the polymer membrane,we consider that the microcrystalline region in the polymer restrain the diffusion of water molecules,thus reducing the water absorption and swelling ratio of semi-SPEK-x.At the same time,the crystal structure also hinders the transmission of protons and the diffusion of methanol,leading to the membrane proton conductivity and methanol permeability decreased.The fuel cell performance test indicates that the crystallinity should be optimized to obtain a high performance membrane.In the last part,we ultilized 1,5-bis(4-fluorobenzoyl)-2,6-dimethoxynaphthalene to prepare the dimethoxy-containing poly(arylene ether)(MPAE).Using boron tribromide as demethylating reagent,the corresponding dihydroxyl-containing poly(arylene ether)(HPAE)was obtained.The structure of the polymer was confirmed by NMR spectroscopy.The graft reaction occurred between HPAE and pentafluorophenyl-containing monomer at lower temperature.1H NMR and 19 F NMR were performed to characterize the structure of graft polymer and demonstrate the accomplishment of graft reaction.The graft octa-sulfonated poly(arylene ether)polymer(SPAE-x)was prepared by post-sulfonation with chlorosulfonic acid.SPAE-x has longer hydrophobic segment which enhance the dimensional stability and mechanical property of the membrane.In the case of equivalent IEC level,with a higher proton conductivity.The SAXS test showed that SPAE-x had a distinct hydrophilic / hydrophobic phase separation morphology.With the increase of IEC,the average distance between the clusters decreases and the hydrophilic phase is gradually expanded to form proton transport channels.Considering its lower methanol permeability,SPAE-x can be utilized in direct methanol fuel cells applications.In summary,we adjust the molecular structure of both hydrophilic and hydrophobic segments in the sulfonated polymer to prepare several series of concentrated sulfonated poly(arylene ether)polymers,and investigate the proton conductivity,dimensional stability,morphology,fuel cell performance,etc.We discuss the relationship between its structure and properties in detail.We hope that our work could provide new thoughts for the development of this kind of PEM.