| Proton exchange membrane fuel cells(PEMFCs)show great potentials in future applications such as energy generation because of the reduction of environmental pollution compared with the burning of fossil fuels.The challenges for PEMFCs are the restricted operation conditions and the high cost of perfluorosulfonic acid(PFSA)types as the commercial proton exchange membrane(PEM).Besides,PEM as one of the key components in a PEMFC needs to have good mechanical properties,excellent oxidative and hydrolytic stability as well as good proton conductivity under fully and partially humidified conditions.This dissertation focuses on sulfonated poly(aryl ether)materials with medium and high ion exchange capacity(IEC),including sulfonated poly(arylene ether sulfone)(SPAES)and poly(ether ether ketone)(SPEEK).Multicomponent polymer blending was used as the strategy to balance the excessive water swelling behavior of sulfonated polymers with high IECs and low proton conductivity of sulfonated polymers with low IECs to ensure good stability and proton conductivity of the developed blend the membranes.In order to solve the compatibility problem of composite membrane,a four-stage sulfonation degree gradient transition composite strategy was selected.A series of novel multi-component blend membranes were prepared from four SPAES copolymers,SPAES-20,SPAES-30,SPAES-40 and SPAES-50 with sulfonation degree(DS)of 20%,30%,40%and50%,respectively.The blend membranes showed considerably improved dimensional,thermal,hydrolytic and oxidative stabilities,together with good methanol resistance at a similar IEC level while comparing with the control membrane.Among the blend membranes,the B(0.5:1:2:2)membrane showed the highest proton conductivity(203.1 m S cm-1 at 90 oC and 169.2 m S cm-1 at 94.1%RH)with excellent oxidative(5.1%mass loss in Fenton's reagent at 80 oC for 1 h)and dimensional stability(<15%at 80 oC),and maximum power output(MPD)of 468 m W cm-2in the H2/O2 fuel cell.In order to improve the conductivity and battery performance of SPAES composite membrane,a three-component SPEEK/SPAES composite membrane was prepared by using sulfonated polyether ether ketone(SPEEK)polymer instead of SPAES polymer with low IEC level.SPEEK polymer was used to replace the SPAES with low IEC value in the blend system.The polymeric blends were simply fabricated by a three-component system,which contained SPEEK(10-50 wt%,1.83 mmol/g),and SPAES-40(1.72 mmol/g)/SPAES-50(2.0mmol/g)at 1:1 in weight.The IEC and proton conductivity increased while the dimensional and chemical stability of PEMs decreased with the increment of SPEEK polymer amount.The SPEEK/SPAES blend membranes showed lower water uptake,better dimensional and oxidative stability,together with higher proton conductivity beyond 70 oC with SPEEK content of 10-40%than the pristine SPEEK membrane.In an H2/O2 fuel cell test,the B30membrane containing 30 wt%SPEEK polymer showed a MPD of 700 m W cm-2.In order to further improve the dimensional stability and conductivity of SPAES,non sulfonated polyethersulfone(PES)and sulfonated titanium dioxide(stio2)were introduced to form a five component composite membrane with SPAES with different sulfonation degrees.Multi-component nanocomposite blend membranes containing SPAES,polyethersulfone(PES)and inorganic nanofiller of titanium oxide(Ti O2)or sulfonate titanium oxide(s Ti O2)were prepared.The blend system contained SPAES-30,SPAES-40,SPAES-50 at a weight ratio of 1:2:2(1.79 mmol g-1),PES(5 wt%)and s Ti O2or Ti O2(0.75 wt%)nanoparticles.The PES and s Ti O2significantly increased both the compatibility and the effective proton mobility of the blend system,resulting in the higher proton conductivity.Dimensional and oxidative stability were also enhanced.The B-SPAES/PES/s Ti O2 and B-SPAES/PES/Ti O2membranes displayed high?/SR of 15.3 m S cm-1and 14.6 m S cm-1,respectively,at 80°C,both higher than the control B-SPAES membrane(10.7 m S cm-1).The B-SPAES/PES/s Ti O2membrane exhibited a MPD of 619 m W cm-2,which was higher than that of control B-SPAES(589 m W cm-2)at 80 oC and 100%RH.As a facile but effective strategy,thermal crosslinking was applied for the SPAES/SPEEK blend system to enhance the dimensional,hydrolytic and oxidation stability and proton conductivity at high temperatures.Two SPEEK polymers(1.6 and 2.0 mmol g-1)and one SPAES polymer(2.0 mmol g-1)were selected.The homogeneous and flexible thermally-crosslinked SPEEK/SPAES membranes displayed excellent mechanical toughness(27-46 MPa),suitable water uptake(<60%),high dimensional stability(swelling ratio<15%)and high proton conductivity(>120 m S cm-1)at 80 oC.They also showed significantly enhanced hydrolytic and oxidative stability.The t-SPEEK/SPAES(1:2:2)membrane achieved a MPD of 665 m W cm-2 at 80 oC in a H2/O2 fuel cell. |
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