Synthesis And Properties Of Sulfonated Poly (Arylene Ether Sulfone) And Sulfonated Poly (Arylene Ether Ketone) Containing Fluoro-phenyl Pendent Group As Proton Exchange Membranes

Proton exchange membrane is a crucial component in proton exchange membrane fuel cells?PEMFCs?,Which plays an important role in improving the performance of PEMFCs and reducing their cost.At present,commercial Nafion membranes have long been deemed as the most suitable PEMs due to their excellent proton conductivity and chemical stability.Unfortunately,there are some disadvantages associated with Nafion membranes,such as high cost production and drastically proton conductivity drop at elevated temperature.In addition,due to high methanol permeability of Nafion membranes,methanol crossover easily causes catalyst to deactivate.These drawbacks have forced scientists to develop alternative membrane materials.There have been considerable efforts in searching sulfonated aromatic PEM including sulfonated poly?aryl ether sulfone??SPAES?and poly?aryl ether ketone??SPAEK?.These two kinds of sulfonated aromatic polymers are promising candidates for PEM due to their excellent thermal stability,good proton conductivity,high mechanical strength,low methanol permeability,low cost and easy preparation.In general,proton conductivity of PEM relies heavily on the sulfonation degree.On one hand,PEM with high sulfonation degree suffers from excessive swelling and decreases its oxidative stability,which reduces the performance of PEMFCs.On the other hand,low sulfonation degree of PEM affords good dimensional stability,but leads to low proton conductivity.Therefore,trade-off between proton conductivity and dimensional stability of PEM is a key issue for SPAES or SPAEK membrane.To balance the proton conductivity and dimensional stability of PEM,researchers have found some promising approaches toward achieving this goal,which are including regulating the hydrophobicity of and sulfonic acid group location of sulfonated polymer,and modifying sulfonated polymer with inorganics.According to the above concept,four series of SPAES or SPAEK were synthesized,and SPAES were modified with graphene oxide?GO?.The relationship was explored between molecular structure,microstructure and properties of sulfonated aromatic polymers as proton exchange membrane.The main contents and results are as follows:First,phenyl hydroquinone?0F?,4-fluoro-phenyl hydroquinone?1F?and 3,4-fluoro-phenyl hydroquinone?2F?monomers have been synthesized by coupling diazonium salt with benzoquinone.The hydroquinone monomers were characterized by FT-IR,¹HNMR and ¹⁹FNMR,which confirmed the chemical structures of the monomers as designed.DSC curves showed that the melting range of0F,1F and 2F monomers were within 20 ^oC,which was a solid evidence that the three monomers are with high purity.Secondly,to investigate the effect of phenyl pendant on the microstructure and properties of SPAES,a novel series of SPAES containing phenyl side groups with different sulfonation degree?0F-SPAES-xx?have been synthesized through phenyl hydroquinone,3,3'-disulfonate-4,4'-dichlorodiphenylsulfone?SDCDPS?and 4,4-dichlorodiphenylsulfone?DCDPS?.Hydroquinone,SDCDPS and DCDPS have been used to synthesize SPAES-HQ-40.The chemical structures of 0F-SPAES-xx and SPAES-HQ-40 were confirmed by FT-IR and ¹HNMR.Inherent viscosity values of0F-SPAES-xx and SPAES-HQ-40 were in the range of 0.77-0.80 dL/g,which indicated the molecular weight of 0F-SPAES-xx and SPAES-HQ-40 were enough to cast membranes.DSC and TG analysis showed that phenyl group incorporated into SPAES enhanced the thermal stability of SPAES.Compared with those of SPAES-HQ-40 membrane,introduction of phenyl side groups increased the dimensional stability,tensile strengths and oxidative stability of 0F-SPAES-40membrane;but it declined the proton conductivity of 0F-SPAES-40 membrane.AFM and SAXS studies revealed that ionic clusters size of 0F-SPAES-40 membrane was small and isolated,which led to reducing the proton proton conductivity but inhibited 0F-SPAES-40 membrane swelling.In the third part,strong polar fluoro-phenyl pendant group was introduced into SPAES to further regulate the ionic clusters size and enhance the effect between hydrophilic phase and hydrophobic phase of SPAES membrane.Two series of SPAES?1F-SPAES-xx and 2F-SPAES-xx?have been obtained from 1F and 2F monomers along with SDCDPS and DCDPS,respectively.Chemical structures of 1F-SPAES-xx and 2F-SPAES-xx were confirmed by FT-IR,¹HNMR and¹⁹FNMR.The TG and DSC results of xF-SPAES-40 showed that fluoro-phenyl pendant group resulted in reducing the thermal stability of SPAES.The inherent viscosity values of 1F-SPAES-xx and 2F-SPAES-xx polymers showed that those polymers also had enough molecular weight to cast membranes.Compared with 0F-SPAES-40 membrane,it was found that fluoro-phenyl increased the proton conductivity of SPAES membrane and inhibited the swelling ratio of SPAES membrane.The proton conductivities of 2F-SPAES-40 and 1F-SPAES-40 membranes reached 0.142 S/cm and0.121 S/cm at 80 ^oC and 100%relative humidity,respectively.AFM and SAXS were performed to investigate microstructure of xF-SPAES-40 membrane.The results showed fluoro-phenyl promoted the phase separation and increased the ion cluster size of membrane.0F-SPAES-40 membrane exhibited a continuous hydrophobic phase.4-fluoro-phenyl group helped to form bi-continuous microstructure in the 1F-SPAES-40 membrane;while 3,4-difluoro-phenyl group resulted in the hydrophobic phase of 2F-SPAES-40 membrane reversing and forming continuous hydrophilic phase.The microstructures of xF-SPAES-40 membranes were consistent with its proton conductivity and dimensional stability.The fluoro-phenyl group was helpful to improve the oxidative stability.4-difluoro-phenyl group was in favor of improving the mechanical properties of SPAES membrane,while 3,4-difluoro-phenyl group had a negative impact on the mechanical properties of SPAES membrane.In the fourth part,to further improve the proton conductivity and dimensional stability of 0F-SPAES-40 and 1F-SPAES-40 membranes,0F-SPAES-40 and 1F-SPAES-40 were modified with graphene oxide?GO?with hydrophilic groups using proton conducting sites.The effect of GO on properties of 0F-SPAES-40-GO-x and 1F-SPAES-40-GO-x composite membrane as PEM was investigated.The proton conductivities of 0F-SPAES-40-GO-x and 1F-SPAES-40-GO-x composite membranes were enhanced with increasing GO content.Compared with pure membrane,the proton conductivity of 0F-SPAES-40-GO-3%composite membrane were up to 0.141 S/cm at 80 ^oC and100%relative humidity,which was increased by 46.87%;the proton conductivity of 1F-SPAES-40-GO-3%composite membrane reached 0.133 S/cm,which was improved by 9.92%.The swelling ratio testing results showed GO inhibited the swelling ratio of composite membranes.Compared to pure membrane,the swelling ratio of 0F-SPAES-40-GO-2%?27.73%?composite membrane was reduced by 54.78%;that of 1F-SPAES-40-GO-2%?26.27%?composite membrane decreased by39.08%at 90 ^oC.FE-SEM results illustrated that GO had better compatibility with 0F-SPAES-40than with 1F-SPAES-40.The SAXS results had shown ion-cluster size of composite membranes were larger than that of pure membrane.The TG and DSC results showed GO had a side effect on thermal stability of composite membranes.The oxidative stability of composite membranes decreased with increasing GO content.The tensile strength of 0F-SPAES-40-GO-x composite membranes have been improved by incorporating GO.However,the tensile strength of the 1F-SPAES-40-GO-x composite membranes decreases with increasing GO content.At last,to explore the effect location of sulfonic acid groups on the microstructure and properties of sulfonated aromatic polymer,a novel series of PAEK containing amino group?PAEK-x-NH₂?were synthesized with 4-fluoride phenyl phenol and 4,4'-difluobenzophenone as well as2,2-double?3-amino-4-hydroxy phenyl?six fluoride propane.Poly?arylene ether ketone?containing alkylsulfonate side chain?SPAEK-G-xx?was prepared by PAEK-x-NH₂ grafted 1,4-butanesultone.The chemical structures of PAEK-x-NH₂ and SPAEK-G-xx were confirmed by FT-IR,¹HNMR and¹⁹FNMR.The inherent viscosity values of SPAEK-G-xx indicated them with high molecular weight.SPAEK-G-xx membranes were prepared by solution casting.Compared to main-chain-acid membrane with a similar IEC value,side-chain-acid membrane had better dimensional stability,but lower proton conductivity.The proton conductivity of SPAEK-G-50 was up to 0.18 S/cm,which was higher than that of Nafion 117 membranes?0.15 S/cm?at 80 ^oC and 100%relative humidity.The swelling ratio of SPAEK-G-50 membranes(2.97 meq.g^-1)was up to 289.3%,but this membrane did not dissolve at 90 ^oC.To explore the microstructure of SPAEK-G-50 and 1F-SPAES-40membrane using AFM and SAXS,the results showed that compared with main-chain-acid membrane with a similar IEC value,side-chain-acid membrane had a smaller ion-cluster size and worse continuous hydrophilic phase leading to decreasing its proton conductivity.The tensile strength of SPAEK-G-xx membrane were better than that of 1F-SPAES-40 membrane.The oxidative stability of SPAEK-G-xx membranes decreases with sulfonation degree increasing.To sum up,it is clear that the macro-phase structure of the membrane is closely related to proton conductivity and swelling ratio.Adjusting the hydrophobicity of sulfonated aromatic polymer with hydrophobic component and the location of sulfonic acid group are effective way to achieve the balance between proton conductivity and dimensional stability of PEM.In addition,SPAES membrane modified with GO improved its proton conductivity and dimensional stability to achieve the same goal.