Structure Design And Properties Study Of Cross-linked Polymer Electrolyte Membranes

As the energy crisis and environmental problem become more and more serious in recentdecades, clean and renewable resources are attracting more and more attention. Fuel cells, as atype of clean and high efficiency energy conversion device, are with great promising ofwidely applying in our daily life in the future. Proton exchange membrane fuel cells(PEMFCs) are thought as reliable power in future vehicles, portable power, area station powersource for their advantages, such as quick start, high energy density, safe and portable.Proton exchange membranes (PEMs) as one of the crucial part in PEMFCs, affect theperformances of fuel cells directly. According to the working mechanism of PEMFCs, PEMsneed to meet the following requirements: isolate fuel from oxygen, conduct proton andinsulation. Meanwhile, they need to have good dimension stability, mechanical stability andoxidation resistant. Unfortunately, some of these properties contradict to others. For example,when using high proton carrier concentration to increase proton conductivity, it oftenincreases the water content and brings adverse effect to dimensional stability sometimes alsoto mechanical stability. To balance the relationship between these properties, people havedeveloped many methods to modification membranes, in order to get membranes withsuperior properties.In these methods, crosslinking is a facial and efficient method to get improveddimensional stability and high methanol resistance. However, there are also drawbacks of thismethod, such as high crosslink density often causes membranes brittle, lower water or acidabsorption ability, as well as lower proton conductivity. To solve these problems and find aefficient way to modify membranes, we investigate crosslink modification method through two aspects: crosslinker structure design and crosslink networks design.We design and synthesis crosslinkers bearing proton conductivegroups--polybenzimidazole oligomer ended with caroxyl groups and bearing sulfonate acids.We use this novel crosslinker to modify sulfonated poly(etheretherketone)(SPEEK). From theexperimental results, we found that this novel crosslinker can lower the effect of crosslinkeron the conducting group concentration and improve the dimensional stability and protonconductivity at the same time. Meanwhile, the mechanical properties and the chemicalresistance are enhanced. All these results testified the successful modification of SPEEK withthis novel crosslinker.Fuel cells operating at higher temperature will accelerate electrochemcial reaction,increase the tolerant of catalyst to CO, as well as simplify the management system. Based onthese advantages, proton exchange membranes using at elevated temperature attracted moreand more attentions in these years.In Chapter Three, we design novel PEM for high temperature fuel cells. Phosphorate acid(PA) doped polybenzimidazole (PBI) is a typical type of high temperature fuel cellmembranes. PBI itself do not have proton conductivity, acid doped PBI can conduct proton bythe hoping of proton from imdazole ring to PA moleculars. So the higher PA concentration thehigher proton conductivity would be. But this also leads to poor mechanical properties of themembranes. To solve this problem, people often crosslink PBI by covalent bond betweenimdazole ring and certain kind of crosslinker. There are two drawbacks of this method, one isthat imdazole rings, which can interact with PA, would be consumed during crosslinkingreaction; the other is that covalent crosslinking on the main chain would affect membranemechanical properties, make membranes brittle and affect further manufacture.To cope with these problems, we try to optimize the crosslinking structure in themembranes. We synthesized end-group crosslinable polybenzimidazole (E-PBI), and make anend-group crosslinking network with E-PBI and TMBP. And then make high molecularweight polybenzimidazole (m-PBI) penetrated in the crosslinking networks, got a series ofblend membranes with different crosslinking network content (b-PBI-X). The introducing ofthis network improve the thermal stability, oxidization stability, PA doping levels as well asthe proton conductivities of the membranes, especially for b-PBI-90, which proton conductivity reached0.035S cm-1at170oC. As we expected, membranes showed goodmechanical properties. After96h Fenton test under70oC, the mechanical properties still canmeet the requirement of application. Acid doped b-PBI-20showed similar strain to acid dopedPBI, but the tensile strength is two times higher of the latter, which showed we solve thebrittle problem successfully.Although the membranes’ properties increased with the above method, the conductivitystill has room to increase. We were trying to further increase the PA content and theconductivity of the membranes. We use carboxyl terminated and sulfonatedpolybenzimidazole trimer (s-BI) as crosslinker crosslink polybenzimidazole. Both thebenzimidazole group and the sulfonate acid are benefit to acid uptake. The acid uptakes of allthe crosslinked membranes are higher than pure PBI membranes. So was the protonconductivity of the crosslinked membranes. For s-BI concentration at3%, its conductivity at180oC reached to0.060S cm-1, which is nearly twice of the acid doped polybenzimidazolemembrane.We want to improve the mechanical properties of high conductivity membranes further.,and balance the relationship between conductivity and mechanical properties.We applied thetoughen mechanism of soft matter to the PA doping membranes--building up double networks.By preparing the second network in PAETA hydrogel, we got a series of double networkhydrogels with varied second network crosslinking density. Doping this double network in PAsolution obtain the PA doping double network crosslinked membranes. Membrane with thelowest crosslinking density have the highest doping level14.56(PA/unit). Tensile strengthincrease and strain decrease with the crosslinking density increasing. PAETA4%/PAAm0%with the lowest crosslinking density showed the highest strain, up to500%. The toughelectrolyte also with high conductivity not only at high temperatures (0.047S cm-1at180oC)but also at low temperature (0.027S cm-1at40oC)Through the above modification method, membranes with high conductivity but also highconductivity were obtained.