Preparation Principle And Structure Design Of Composite Polymer Electrolyte Membrane For Fuel Cell

Polymer electrolyte membrane is currently meeting requirements of decreasing proton resistance and improving operation lifetime. Replaced the pure PFSA membrane with PFSA/ePTFE composite membranes is very favourable to obtain a highly stable and super thin proton exchange membrane. However, the PFSA impregnation in the porous PTFE matrix is presently far from 100% density because the incompatibility between the hydrophilic PFSA ionomer and the hydrophobic PTFE polymer. The practical improvement of the PEM operation lifetime is also dissatisfied due to the lack of understanding of degradation mechanism. Furthermore, various demands such as elevated-temperature performance and low methanol crossover have also emerged to the recent PEMs research. This thesis aim to these issues and have made the following achievements:(1) The impregnation of PFSA ionomers in the porous ePTFE matrix can be improved by decrease of inner gas pressure in the ePTFE micropores and chemical modification of the ePTFE matrix. In a capillary experiment, the Nafion solution can occupy 98.2% (4.91 cm vs. 5 cm) of the end-sealed PTFE capillary when the P_inner is lowered to 5×10² Pa. The hydrogen crossover of the PFSA/ePTFE compisite memrbane prepared at this condition was 60% lower than that prepared at standard atmosphere pressure. Chemical modification based on sodium-naphthalene treatment and NMA-grafling had also been shown to be effective in introducing the hydrophilicity to the original hydrophobic ePTFE matrix. It has been found that the composite membranes based on the hydrophilic ePTFE have a higher PFSI impregnation loading, much lower porosity and lower gas crossover or permeability, as compared to that prepared with as-received hydrophobic ePTFE matrix.(2) The decomposition of Nafion polymer starts from the defect ends of the main chain and result in the loss of the polymer repeat units. With the increase of repeat unit loss, little voids and pinholes would appear in the proton exchange membrane. These little voids and pinhole can be enlarged by the RH-induced stress, which was higher than membrane safety stress in long time condition, then make the membrane high permeated to reactant gas and failure. Highly durable PEMs can be fabricated by fixing the polymer ionomers in the PTFE matrix micropores to increase the membrane physical stability, and followed by heat-treating the polymer at 270℃after converted the polymer to Na⁺ form to decrease the defect groups in the polymer. The experiment in Fenton's reagent revealed that the fluorine emission rate of the home-made PEM was much lower than that of the commercial Nafion membrane. By fixing the polymer ionomers into the PTFE matrix micropores, the home-made PEM showed excellent stability. The RH-generated stress was 0.6 MPa from soaking to drying in 25 RH% gas at 90℃, compared to 3.1 MPa of the Nation membrane at the same condition. On the other hand, the limited PTFE micropore size prevented to enlarge the pinholes during the chemical attack.(3) Water-retention polymer electrolyte membrane with uniformly silica distribution and homogeneous polymer/silica interface can be developed with the assistance of Nation ionomer stabilized SiO₂ nanoparticles. By completely dispersing of TEOS in Nation ionomers without water and then employing fourfold excessive water, the diameter of Nation stabilized 5wt% SiO₂ can be lower to 3～4 nm. The as-prepared 5wt% SiO₂ membrane possessed perfect water retention after heated at 60℃for 10h. Theλ_H2O/SO3H in the composite membrane was 7.02. This is about 5 times higher than that of the Nation 211 membrane (1.33 at the same condition). The good water-retention property gave the composite membrane a good performance at very low humidity and elevated temperature. Even under absolute dry gas feeding, the decreasing rate of the cell voltage performed at 600mA/cm² was 0.29 mV/min at a period of 340 min. For the elevated temperature of 100℃, the degradation rate of the cell voltage assembled with Nafion/SiO₂ nanocomposite membrane was 0.12 mV/min for the period of 750 min against 2.33 mV/min for the period of 180 min for the Nafion 212 membrane.(4) Nafion/ePTFE membrane has been improved to meet the requirement of DMFC use by self-assembling of charged Pd nanoparticles. The loading of the Pd nanoparficles assembled on the membrane was 1.6～1.8μgcm^-2 after a self-assembly precedure of 48 h and had little effect on the high conductivity of the Nafion membrane. With the Pd nanoparticles self-assembly, the methanol permeation have a noticeable decrease from 340 mA/cm² to 28 mA/cm². As a result, the OCVof the Nafion/ePTFE membrane have a more significant increase from0.55 V to 0.73V. The decrease of methanol crossover also increase the DMFC U-I performance, which gives the self-assembled PEMs a promised prospect in DMFC.