Preparation And Performance Of Proton Exchange Membranes Based On Sulfonated Vinyl-addition Type Polynorbornene And Their Nano-Silica Hybrid Membranes

The direct-methanol fuel cell (DMFC), having high energy efficiency and simple design and operation, is currently considered one of the most promising alternative power-delivery systems. The proton exchange membrane (PEM), which has the essential functions of a proton-conductive medium as well as a barrier to prevent direct contact between fuel and oxidant, is the crucial part determining the performance of the fuel cells. So far, the widely used proton exchange membranes are perfluorosulfonic acid membranes, such as DuPont’s Nafion. Although their chemical and electrochemical stability and high proton conductivity, it hinders their further applications by the shortcomings of sharp drop of proton conductivity at low humidity or high temperature, high methanol permeation, and exorbitant cost. Therefor, many promising polymers and their composites with sulfonic acid functionality as substitutes for Nafion were studied for PEM applications.Vinyl-addition type polynorbornene (PNB) are one of these interesting highly stable polymers as a consequence of its fully saturated and contains bicyclic units in the backbone. This material is of particular interest for uses at low methanol permeation. Taking advantage of the basic character, sulfonated vinyl-additon type PNB is suitable for development as a direct methanol fuel cell proton exchange membrane material.Novel copolynorbornenes bearing pendant sultone groups (designated as P(BN/SulNBOH) and P(BN/SulNBOMe)) have been successfully synthesized via vinyl-addition copolymerization of functionalized norbornenes bearing sultones (designated as SulNBOH and SulNBOMe) with2-butoxymethylene norbornene (BN) by using bis-(P-ketonaphthylamino)nickel(Ⅱ)/B(C6F5)3catalytic system. The catalyst system showed high catalyst activity (104gpoiymer/molNi·h) and the obtained copolymers have high molecular weight and a narrow molecular weight distribution. Furthermore, the achieved copolymers P(BN/SulNBOH) and P(BN/SulNBOMe) were converted into sulfonated copolymers sP(BN/NBOH) and sP(BN/NBOMe) with high thermal properties and good mechanical properties by ring-openning. Both sP(BN/NBOH) and sP(BN/NBOMe) membranes displayed low water uptake and better proton exchange membranes properties. The proton conductivities measured in the hydrated state at80℃ranged from10"5to7.19×10-3S·cm-1.Hybrid cross-linked proton exchange membranes (SPBN/SiO2) are synthesized from sulfonated copoly(norbornene)s (SPBN) and a zwitterionic silica containing sulfonic acid and ammonium groups,3-[[3-(triethoxysilyl)-propyl]amino]butane-1-sulfonic acid (TPABS), using a sol-gel process. As these membranes were well processed as self-supporting film, they showed high stabilities and proton conductivity and low methanol permeability. Obtained SPBN/SiO2-50(50wt.%of TPABS to SPBN in the membrane matrix) showed the best performance with proton conductivity of6.34×10-2S cm-1, methanol permeability of3.64×10-7cm2s-1, ion-exchange capacity value of1.21meq g-1and comparable selectivity parameter of1.74×104S s cm-3. Finally the membrane electrode assembly (MEA) was fabricated using the SPBN/SiO2-50as PEM, the open circuit voltage of SPBN/SiO2-50at1M methanol (80℃) is0.563V, and its power density is50.2mW cm-2, which is lower than that of Nafion117(124.2mW cm-2). The new designed cross-linked membranes, though its power density is low, can be a promising candidate to satisfy the requirements of PEMs for direct methanol fuel cells.