Studies On Composite Membranes For Proton Exchange Membrane Fuel Cells

At present the problems of lowering the prices of perfluorosulfonated ionomer (PFSI) membranes used in proton exchange membrane fuel cells (PEMFCs) and simplifying the operating systems of PEMFC are deeply concerned. The work in this thesis focuses on the two problems mentioned above, /'. e. the functions of reinforcing, lowering cost and self-humidifying are obtained by synthesizing the PFSI/polytetrafluoroethylene (PTFE) composite membranes.The PFSI/PTFE composite membranes were synthesized by dissolving PFSI polymer and impregnating the obtained PFSI solution into the pores of porous PTFE films. In the process, the gravity dragged PFSI solution into the pores of the PTFE films. PFSI/PTFE composite membranes have less amount of expensive PFSI polymer than commercialized PFSI membranes, so they have much lower prices. Due to enforcement of PTFE films, in both dry and wet conditions, the strength of the composite membrane is larger than that of Nafion?membrane. Therefore very thin membrane can be obtained. The fuel cell performances of the composite membranes are equal or superior to those of commercialized Nafion?membranes. After 180 hours' life test, the composite membrane showed good stability, delamination of the composite membrane did not occur. So we conclude that it is very promising to be used in fuel cells.New PEMs (Pt-PEM and Pt/C-PEM) were prepared by pouring Nafion?solution containing Pt black or Pt/C catalyst particles on one piece of PTFE porous film. PTFE film used not only serves to reinforce the composite membrane but also functions as a good method to retain Pt particles adjacent to the anode side in the compositemembrane. Thin composite membranes could accelerate back-diffusion of water from cathode to anode; and the catalyst layer adjacent to the anode side could chemically catalyze the reactant gases (H2 and Oi) to produce water, thus humidifying the PEM in the fuel cell. If the water amount of back-diffusion together with that generated through self-humidifying is larger than that through electro-osmotic drag, then the fuel cell could operate stably. Furthermore, the self-humidifying layer can suppress the crossover of reactant gases, so the open circuit voltage of the cell can be enhanced.