| The polymer electrolyte membrane is the core component of the proton exchange membrane fuel cell(PEMFC),which conducts proton,blocks fuel and isolates electron.Due to the harsh chemical environment inside the fuel cells,which contains high requirements on the performance of the proton exchange membrane(PEM),the only PEM that can be commercialized at present is the perfluorosulfonic acid(PFSA)polymer membrane represented by Nafion.With the maturity of PEMFC technology,the bottleneck of high performance and diversified fuel cell application requirements all point to PEM.The traditional PFSA polymer membranes can no longer meet the current development needs of various types of fuel cells,and it is urgent to develop membrane materials with new functions,specificity and high stability.Direct methanol fuel cells(DMFCs)fueled by methanol have been developed for many years,and one of the key problems that has not yet been commercialized is the lack of suitable PEMs.Due to the high methanol permeability of Nafion membranes,2 M methanol solution is usually used as the fuel,which greatly reduces the energy density.With the increasing demand for miniaturization and portability of DMFCs,it is an inevitable trend to use high-concentration methanol solution as fuel.Therefore,the development of PEM materials with excellent methanol resistance is an urgent problem to be solved.The sulfonated poly(arylene ether)(SPAE)materials has good mechanical property and thermal stability,and its methanol permeability is at least one order of magnitude lower than that of the Nafion membrane,making it an ideal material for DMFCs.However,traditional SPAE materials have low acidity and poor hydrophilic/hydrophobic microscopic phase separation morphology,and only high ion exchange capacity(IEC)can achieve acceptable proton conductivity,resulting in uncontrollable dimensional change and increased methanol permeability.Therefore,achieving a balance between proton conductivity,dimensional stability and methanol permeability is the focus and difficulty of research on SPAE PEMs for DMFCs.In this work,the structure and arrangement of the hydrophobic segment and the hydrophilic segment of the polymers are regulated from the perspective of molecular structure design.First,crystalline poly(arylene ether ketone)(PAEK)was innovatively introduced to replace the traditional amorphous poly(arylene ether)s as the hydrophobic backbone to suppress the dimensional change and methanol permeation of the membranes by enhancing the hydrophobic segment.In addition,the introduction of a hydrophilic segment with densely sulfonated structures promotes the formation of good hydrophilic/hydrophobic microscopic phase separation morphology,thereby enhancing proton conductivity.During the research,the microscopic morphology and macroscopic performance of the membranes were discussed and analyzed,and the molecular structures of the sulfonated polymers were continuously adjusted to achieve good comprehensive performance.In the first part of the work,semi-crystalline PAEK PEMs(Semi-SPEK-x)with densely sulfonated side chains were prepared by conventional high-temperature nucleophilic polycondensation and post-sulfonation reactions.By adjusting the molecular structure design,the balance of crystallinity and sulfonation was achieved.Compared with the amorphous SPAE membrane(PAES-SP18)with the same hydrophilic side chain,the introduction of the crystalline PAEK backbone effectively improved the dimensional stability of the Semi-SPEK-x membrane.Under fully hydrated conditions,the In-plane swelling ratio of Semi-SPEK-x membranes at 80 oC is approximately one-half that of PAES-SP18 membranes with similar IEC.In terms of methanol resistance,compared to amorphous PAES-SP18 membranes,Semi-SPEK-x membranes exhibit excellent dimensional stability in methanol while maintaining low methanol permeability.However,the water uptake and proton conductivity of Semi-SPEK-x membranes are slightly low,and further improvement of the molecular structure of the polymer is still required.The results of the first part of the study clearly demonstrate the potential of semi-crystalline sulfonated poly(arylene ether ketone)(SPAEK)PEMs.Due to the harsh conditions of high temperature polymerization and the complex synthesis process,in the second part of the work,the polymer synthesis and post-functionalization steps were optimized.The monomer N-phenyl(4,4'-difluorodiphenyl)ketamine instead of 4,4'-difluorobenzophenone was introduction to participate in the nucleophilic polycondensation reaction,and polymer precursors(PEKt-x)with high molecular weights were synthesized at lower temperature.Subsequently,the sulfonation of the active benzene ring and the hydrolysis of the Schiff base were realized through the sulfonation synergistic hydrolysis reaction process,and the semi-crystalline SPAEK PEMs(Cr-SPEK-x)was finally obtained.The improvement of the synthetic method lays the foundation for the follow-up work.The test results show that the Cr-SPEK-x membranes still perform well in terms of dimensional stability and methanol resistance.By increasing the IEC value,the water uptake of the semi-crystalline membrane increases without uncontrollable swelling behavior,thereby improving the proton conductivity of the membranes.Among them,the proton conductivity of Cr-SPEK-20membrane at 80 oC reached 0.137 S cm-1.It is the key to develop PEMs with good comprehensive properties to clarify the intrinsic relationship between the molecular structure of polymers,the microscopic morphology and macroscopic properties of membranes.Based on the polymer structure in Chapter two,in the third part of the work,three semi-crystalline SPAEK PEMs(m Cr-SPEK,d Cr-SPEK,t Cr-SPEK)with similar IEC value but different sizes of hydrophilic"microblocks"were prepared.The effects of hydrophilic segment size and hydrophobic segment regularity on the microscopic morphology and macroscopic properties of the membranes were investigated.As the length of the hydrophilic segment increases,more obvious hydrophilic/hydrophobic microphase separation morphology is formed,and the size of the hydrophilic ion cluster increases,forming good proton transport channels and improving the proton conductivities of the membranes.The increase in the regularity of the hydrophobic segment promotes the formation of PAEK crystals,improves the crystallinity of the membranes,and further improves the dimensional stability and methanol resistance.Ultimately,the t Cr-SPEK membrane with the best microscopic phase separation morphology and highest crystallinity exhibited the best comprehensive performance.Combining the above studies,the fourth part of the work designed a hydrophilic pendant with higher sulfonation density and introduced it into the crystalline PAEK backbone through flexible segments.The high concentration of densely sulfonated structures enhances the polarity difference between the hydrophilic and hydrophobic domains,which contributes to the formation of obvious microscopic phase separation morphology.The introduction of the flexible segment between the hydrophobic backbone and hydrophilic pendant reduces the influence of the pendant on the regularity of the main chain and improves the crystallinity of the PAEK backbone.Finally,the prepared SL-SPEK-x membranes shows excellent comprehensive performance,in which the SL-SPEK-12.5 membrane contains proton conductivity as high as 0.182 S cm-1 at 80 oC,In-plane swelling ratio only 13.6%and methanol permeability is 1.6×10-7 cm2 s-1,which is the membrane with the best comprehensive performance of the whole work.In the DMFC measured with high concentration methanol(12 M)as fuel,the maximum power density of SL-SPEK-12.5 membrane reached 161.7 m W cm-2,surpassing the performance of Nafion 117 membrane.In summary,a series of semi-crystalline SPAEK PEMs were prepared by molecular structure design.The relationship between the molecular structure of the polymers,the microscopic morphology and macroscopic performance of the membranes was explored and discussed.Finally,the semi-crystalline SPAEK PEMs achieve a balance between proton conductivity,dimensional stability and methanol permeability,and exhibits good performance in practical fuel cell tests.It is hoped that our research can arouse the interest of researchers in semi-crystalline PEMs and provide new research ideas for the development of PEMs for DMFCs. |
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