Performance And Morphology Studies Based On Composite Proton Exchange Membranes

Proton exchange membrane fuel cells (PEMFCs) are the preferred power sources to spaceflight, military affairs, vehicles and portable devices, because of the virtues of the lowest working temperature highest specific energy, the fastest startup, the longest service life, and the widest applications. Proton exchange membrane is the"heart"of PEMFC for transferring protons from the anode to cathode as well as providing a barrier to the fuel gas cross-leaks between the electrodes. In the present, the membranes traditionally used in PEMFC are perfluorosulfonic polymers such as Dupont Nafion?, however, high cost, low proton conductivity under low relative humidity and high temperature, high methanol permeability have limited their usages.Hence, new types of proton exchange membranes with low cost and high performance attract lots of researchers. Polybenzimidazole (PBI) possesses good mechanical strength, high chemical and thermal stability at high temperatures, at the same time, the PBI membrane is highly impermeable to methanol, and the water drag coefficient is nearly zero. So,Polybenzimidazole he excellent material to be used as polymer electrolyte membrane. However, the shortcoming is the disability of proton conduct. Sulfonated poly(aryl ether)s (SPAEs) show very good potential usages in PEM, however, SPAEs with low IEC exhibited good mechanical strength and methanol resistance and low proton conductivity, SPAEs with high IEC exhibited superior proton conductivity, bad mechanical performance and relatively high methanol crossover.In order to solve these problems we have prepared a series of the composite membranes.At first, Polybenzimidazole was synthesized and mixed with highly sulfonated polyether-ether ketone (SPEEK) and 12-phosphotungstic acid (PWA), respectively. The aim was to keep the virtue and overcome its defect used as proton exchange membrane. In chapter 3, the SPEEK/PBI composite membranes were prepared. The morphology characteristics of the composite membranes were studied in detail by SEM, AFM, SAXS, etc. The relationships between the performance of the composite membrane and morphology were analyzed. Because of the interaction between sulfonic acid groups and amine groups the composite membranes became more compact and the intercluster distance decreased with the increasing content of PBI (0–20%),which lead to the cut down of water uptakes and following the decrease of proton conductivity (25 C: 0.048 0.01 S/cm). However, the lowest the proton conductivity of the composite membranes was still higher than 10 2 S/cm. The thermal stabilitiesand mechanical properties have shown a raise tendency. Therefore, considering all the performance of the composite membranes, we could conclude that the SPEEK/PBI composite membranes are promising for usage in DMFC.In chapter 4, PWA was successfully introduced into PBI membranes. In this paper, we also proposed a fast proton-conducting structure and mechanism at 100% relative humidity. The proton conductivity was dependent on the ratio between PWA and PBI. The interaction between PWA and PBI was studied by IR and the morphology of the membrane was studied by scanning electron microscope. The performance such as thermal stabilities and mechanical properties were tested and showed the elevated results. The PWA-encapsulated material may have a potential for polymer electrolyte membrane fuel cells operating at low temperatures under 100% relative humidity.Secondly, the research above found that the inorganic can improve the mechanical property of the polymer. So in chapter 5, Ga2O3 was blended into SPEEK. Aim to improve the mechanical property, methanol resistance and keep high proton conductivity. The results proved that the mechanical and thermal stabilities of the hybrid SPEEK membranes were dramatically improved by introduction of Ga2O3. The water uptakes of the hybrid membranes were decreased with the increasing content of inorganic section besides 10% wt. content of Ga2O3. which was caused by the dispersion of Ga2O3 as been proved by SEM. The water retentions at 80 C and methanol resistances of the hybrid membranes had been obviously enhanced. The proton conductivities of SPEEK/ Ga2O3 hybrid membranes decreased slightly at lower temperature compared with the pristine SPEEK membrane, however, it was higher than pristine SPEEK membrane at relativiely higher temperature ( above 600C). The selectivity of SPEEK/ Ga2O3 hybrid membranes showed higher results associate with SPEEK. These results indicated that the SPEEK/ Ga2O3hybrid membrane possessed good physical and chemical property and was promising to be used as PEMs in DMFCs.