Synthesis Of High Performance Polytetrafluoroethylene (PTFE)-Reinforced Composite Proton Conductive Membranes And Their Performance Studies For Polymer Electrolyte Fuel Cells

Currently, the development of key materials, the proton exchange membranes (PEMs) and the electrocatalysts, is now considered as the impediment to the success of the PEM fuel cell technology and thus should be given high priority. The PEM, located in the central of the fuel cell, has gained a more and more central importance in fuel cell R&D. The main task of the PEM is to separate the anodic and cathodic chambers in terms of electric isolation and reactant separation, and provide the function of a proton conductive electrolyte. Composite PEMs are of much importance in fuel cell membrane fields.In this thesis, with the two targets of cost and performance in mind, also required for PEM fuel cells for transportation applications, ultrathin composite membrane based on micro-porous expanded PTFE and ionomer (such as Nafion or disulfonated poly(arylene ether sulfone) etc.) were developed and investigated. In detail, the composite membranes based on PTFE and ionomer (Nafion or SPSU), can be made ultrathin for PEMFC due to PTFE reinforcement effect. The strategy of implement of thin membrane could great contribute to the reduced areal specific resistance and voltage drop, thus leading to high cell performance. However, a key issue exists in the synthesis or fabrication process of ionomer/PTFE membranes, namely, interface compatibility between hydrophobic PTFE and hydrophilic ionomer components. The surface modification of PTFE could improve interface compatibility. These twoconsiderations promoted successful development of Ionomer/PTFE membranes with excellent fuel cell performances.To well meet current low or none humidification requirement of PEMFC, sandwich-structured self-humidifying membrane (nanometer-sized SiO2 supported Pt catalyst, Nafion and PTFE composites, donated as Pt-SiO2/NP) was fabricated, characterized and experimentally analyzed. Surprisingly, this membrane could be applied in not only low temperature PEMFC, but high temperature PEM fuel cell (operated at 120 oC, R.H. = 25%). In detail, the SiO2 supported Pt catalyst (Pt-SiO2) in both side layers of the membrane suppresses gas-crossover by chemically catalyzing mutually permeable H2 and O2 to produce H2O, which can in situ hydrate the membrane and facilitate water balance. The recombination reaction inside the membrane occurs chemically not electrochemically, since the insulation of SiO2 results in lack of electron channel, which is essential for three phase interfaces required for electrochemical reaction. The reduction in membrane thickness can facilitate water back-diffusion. In the design, the anode side layer is used for membrane self-humidification and the cathode side layer aims to decrease oxygen electrode polarization, especially at low current density, and accordingly improve the cell OCV. Hydrophilic nanometer-sized SiO2 inside the membrane is expected to adsorb water at low current density, and to release water at high current density to satisfy the electro-osmotic drag requirement. The fuel cells performance data employing above three membranes (such as Nafion/PTFE, Pt-SiO2/NP and SPSU/PTFE) were far superior than virtually all existing literature data.Moreover, interestingly, the phenomenon of ionic cluster aggregation aligned along PTFE nano-rods and nodes in ionomer/PTFE membrane was observed and investigated. Furthermore, the behavior of ionic cluster aggregation was correlatedwith proton conduction mechanism and fuel cell performance.