Study On The Annealed And EPTFE Reinforced Nafion Membranes

Proton exchange membrane fuel cell (PEMFC) is a device converting the chemical energy of the fuel to electrical energy through electrochemical reaction. The characteristics of fast start-up, high efficiency and low emission make it the promising candidate for the mobile and stationary power.The major hurdle for the commercialization of fuel cell is the durability issue of the fuel cell. Proton exchange membrane is the key component of PEMFC. It is believed that the fast degradation of the proton exchange membrane would lead to the early failure of the PEMFC. Extensive researches have demonstrated the durability of the membrane could be enhanced by reinforcing the membrane with a base material or annealing. Although there are now many papers concerning to the fabrication and evaluation of the ePTFE reinforced composite PEM, no study has revealed the effect of ePTFE content on the mechanical behavior of the composite membrane. The durability of the membranes could be improved through annealing but the origin of the improvement in durability is still unknown. Besides, the annealed membranes may show a lower conductivity compared to the as-received membranes. In this thesis, we carefully investigated the effect of ePTFE content on the mechanical properties of the ePTFE/Nafion membrane and the annealing effect on Nafion 211 membrane. The conclusions have been drawn as followed:(1) Annealing Nafion 211 membrane between 120℃-160℃would help to increase the membrane crystallinity and the mechanical properties of the membrane could be enhanced as a result. But the annealing procedure may disturb the cluster structure in the membrane. Therefore, the conductivity of the membrane is lower after annealing. Annealing the Nafion 211 membrane in Na-form also improves the mechanical properties of the membrane. At the same time, the conductivity of the membrane is also improved because annealing with the introduction of Na +ion could reform the membrane to a structure with lower constrain to the side chain of the membrane. Annealing the Na-form membrane (i.e. at 240℃) could balance the mechanical properties and conductivity of the membrane. Besides, The durability of the membrane could be expected to be enhanced because the decrease of the humidity induced stress after annealing.(2) In the ePTFE/Nafion composite membrane, there is a PTFE phase content threshold value (ca. 22.7vol%, volume fraction) beyond which the composite membrane could be reinforced. The tensile strengthen of the composite membrane goes up with the growing phase content of the ePTFE when the ePTFE content is greater than the threshold value and the tensile strengthen of the composite membrane decreases with the growing ePTFE content when the ePTFE content does not reach the threshold value. The yield strengthens change of the composite membranes with the ePTFE content shows similar trend. The yield strengthens increase with the ePTFE content only when the relative ePTFE content is greater than the threshold value.