Design And Properties Of Poly(arylene Ether)s With Sulfonated Tetraarylmethane Structure For Proton Exchange Membranes

Presently,fossil fuels are the primary energy supply option worldwide,and their burning leads to large amounts of carbon dioxide emissions that add to the global greenhouse effect.In view of this fact,many countries are developing sustainable and renewable energy sources as alternatives to fossil fuels.Emerging green energy sources include fuel cells,solar cells,geothermal energy,and nuclear energy.Fuel cells,as eco-friendly energy conversion devices,are regarded as promising candidates.Among the various types of fuel cell,polymer electrolyte membrane fuel cells(PEMFCs)are attracting increasing attention due to their low operating temperature and simple operation.Furthermore,the proton-exchange membrane(PEM),which is a crucial component of a PEMFC,serves not only as a conductor for proton transportation,but also as barrier to prevent reagent mixing.Perfluorosulfonic acid(PFSA)polymers,represented by Nafion,are extensively used PEM materials in current fuel cell applications because of their high proton conductivities and excellent chemical stabilities that meet the requirements of PEM.However,PFSA membranes are still limited by restricted operating temperatures and high fuel permeation;in addition,high cost and synthetic complexities hamper their manufacturing development.Consequently,sulfonated aromatic polymers have been studied as alternatives to perfluorosulfonic acid polymers because of advantages including low cost and high stability.However,many of these sulfonated aromatic polymers membranes generally depend on high ion-exchange capacities(IECs)to achieve proton conductivities that are comparable with those of perfluorosulfonic acid polymers,which is mostly due their less-developed hydrophobic/hydrophilic nano-separations than that of Nafion.Although sufficiently high conductivities,when compared to Nafion,profit from higher IEC values,excessive water uptake may lead to significant declines in the dimensional stabilities and mechanical strengths of these membranes.In this paper,from the perspective of molecular design,poly(arylene ether ketones)s copolymer with highly and densely sulfonated structure was designed and synthesized by taking advantage of the diversity of sulfonated poly(arylene ether)s.The unique structure promotes the formation of hydrophilic-hydrophobic phase seperation.The microscopic phase separation structure achieves the effect of promoting high proton conductivity while maintaining better dimensional stability.In addition,three series of sulfonated poly(arylene ether)s copolymers with different with different chain structures were designed and synthesized to investigate the effect of chain structure on the microphase structure and macroscopic properties of sulfonated polymers.The first part of this paper: A new bisphenol monomer containing a pair of electron-rich tetraarylmethane units was designed and synthesized.Based on this monomer,along with commercial 4,4’-(hexafluoroisopropylidene)diphenol A and 4,4’-difluorobenzophenone,a series of novel poly(arylene ether ketone)s containing octasulfonated segments of varying molar percentage(x)(6F-SPAEK-x)were successfully synthesized by polycondensation reactions,followed by sulfonation.The well-defined nanophase-separated structures were promoted by strong polarity differences between highly ionized segments and fluorinated hydrophobic backbone,and more well connected ion nanochannels are formed with larger ionic content.6F-SPAEK-x samples exhibited appropriate water uptake and swelling ratios at moderate IEC value(1.68 meq.g-1),and excellent proton conductivities.The highest proton conductivity(215 m S cm-1)is observed for hydrated 6F-SPAEK-15 at 100 °C,which is more than 1.5 times that of Nafion 117.The second part of this paper,three kinds of bisphenol monomers with single,double,and triple tetraphenylmethane structures were designed and synthesized.These three bisphenol monomers were then synthesized with 4,4’-sulfonyldiphenol and 4,4’-difluorobenzophenone by nucleophilic polycondensation reaction.The obtained copolymers were subjected to post-sulfonation and cast into the membranes to successfully prepare three series of poly(arylene ether)s with concentrated sulfonation structures containing single,double,and triple sulfonated tetraarylmethane,respectively(mt-SPAE-y,dt-SPAE-y,tt-SPAE-y).Each of membranes contains three sulfonated copolymers with different IEC values.Similar to the first part,as the IEC value increases,the distance between the hydrophilic ion clusters decreases.With the same IEC value,the size of the hydrophilic segment increases with increasing the number of sulfonated tetraarylmethane,and the size of the hydrophilic ion clusters of the three series of sulfonated copolymers also increases.By increasing the IEC value or increasing the size of the hydrophilic segments,the microscopic phase separation structure of the sulfonated copolymers can be enhanced to form a more continuous hydrophilic phase,thereby increasing the water absorption,swelling rate,and proton conductivity of the sulfonated polymer.At 80 oC,the highest values of water uptake and swelling ratio of the three series of sulfonated copolymers were 61.41% and 20.91%,respectively,and the highest value of proton conductivity was 76.97 m S cm-1,and the lowest value of proton conduction activation energy was 10.2 k J mol-1.In summary,from the perspective of molecular design,several bisphenol monomers with different amounts of tetraarylmethane were designed and synthesized,and these monomers were successfully introduced into different polymer systems.Well difined hydrophilic-hydrophobic microscopic phase separation structure was formed between the concentrated sulfonated structure and the polymer backbone,thereby effectively improving the electrochemical performance of the membranes.Furthermore,the effects of chain structure on the microcosmic morphology and macroscopic properties of sulfonated polymers were discussed.It was found that by increasing the IEC value and increasing the size of the hydrophilic segments,the sulfonated polymer membrane could form continuous hydrophilic phase.The unique microcosmic morphology increases the water absorption,the swelling ratio,and the proton conductivity.