Preparation And Modification Of Graft-crosslinked Proton Exchange Membranes

As one of the vital components for the polymer electrolyte membrane fuel cell,the proton exchange membrane has an important influence on the performance of fuel cells.In this dissertation,a series of graft-crosslinked sulfonated poly(aryl ether sulfone)proton exchange membranes were prepared through themal crosslinking reaction from fluorinated copolymer and sulfonated oligomers.Properties of prepared membranes were investigated in detail such as ion exchange capacity(IEC),morphology,water uptaker,dimentional change,proton conductivity,thermo-oxidative stability and H2/O2 fuel cell performance.Fluorinated poly(aryl ether sulfone)polymer and-OK terminated sulfonated oligomers were firstly synthesized via nucleophilic substitution reaction using 9,9’-bis(4-hydroxyphenyl)fluorene,4’4-dihydroxybiphenyl,decafluorobiphenyl and 4,4-biphenol,3,3’-disulfonated-4,4’-difluorodiphenylsulfone,4,4’-difluorodiphenylsulfone as materials,respectively.A series of sulfonated oligomer-crosslinked fluorinated poly(aryl ether sulfone)membranes(c-SPFAES)have been prepared under semienclosed condition through themal crosslinking reaction from fluorinated polymers and sulfonated oligomers,where the later acted as hydrophilic grafts and crosslinkers at the same time.The obtained c-SPFAES membranes(1.04-1.78 mmol g-1)exhibited excellent mechanical toughness with Young’s moduli above 1.3 GPa and tensile strength beyond 35.8 MPa,good dimensional and chemical stability.They also exhibited clear hydrophilic-hydrophobic phase-separation morphology.The proton conductivity was higher than 100 mS cm-1 for the c-SPFAES membranes with IEC over 1.38 mmol g-1 at 60℃.In a H2/O2 single cell test,the maximum power density of the c-SPFAES-1.61 membrane achieved 587 mW cm-2 at 80℃ under fully hydrated state.A series of graft-crosslinked membranes with the same theoretical IEC value(1.50 mmol g-1)were prepared at different membrane-forming conditions to cast membrane.The effects of solid-liquid ratio and themal crosslinking reaction temperature on the properties of prepared membranes were investigated.The results showed that the graft-crosslinked membrane prepared with solid-liquid ratio of 10%and produced in the range of 80-200℃program-controlled temperature elevating exhibited the highest crosslink degree of 85.1%with high proton conductivity and excellent thermo-oxidative stability.The obtained c-SPFAES-1.78’ membrane exhibited an improved dimensional stability and hydrolytic stability with the mass loss decreasing from 35.1%to 15.5%,while the proton conductivity remained at a similar level,as compared with c-SPFAES-1.78 membrane.The sulfonated oligomers with different sulfonation degrees were synthesized by altering the structure of oligomers.A series of sulfonated poly(aryl ether sulfone)proton exchange membranes(c-SPFAES-x/y)with different graft-sulfonated oligomers as side chains were prepared via themal crosslinking reaction with fluorinated copolymer.The results indicated that the obtained crosslinking membrane with higer degree of sulfonation and more flexible sulfonated oligomers showed more excellent comprehensive properties.The c-SPFAES-15/5 membrane dispalyed a high proton conductivity of 188.6 mS cm-1 at 90℃,and the swelling ratios in the in-plane direction and through-plane direction of 5.3%and 16.9%,respectively.It also showed excellent oxidation and hydrolytic stability with weight loss rates of only 0.2%and 1.5%after the oxidation and hydrolytic tests.