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membranes exhibits exceptionally high chemical and thermal stability, and is stable against chemical attack in strong bases, strong oxidizing and reducing acids, Cl2, H2, and O2 at ® temperatures up to 125°C (6). Nafion consists of a fluoropolymer backbone, similar to Teflon , upon which sulfonic acid groups are chemically bonded (7,29). Nafion membranes have exhibited long life in selected applications, operating conditions, and electrochemical applications. In selected fuel cell tests and water electrolysis systems, lifetimes of over 50,000 hours have been demonstrated. The Dow Chemical Company produced an electrolyte membrane, the XUS 13204.10, that contained a polymeric structure similar to that of Nafion, except that the side chain length was shortened (8). As a result, the membrane properties were significantly impacted, including a higher degree of water interactions within the membrane. This translated to lower electrical resistance and permited higher current densities than the Nafion membrane, particularly when used in thinner form (9). These short side-chain membranes exhibited good performance and stability, but are no longer supplied by Dow. Furthermore, due to Nafion’s expense and other engineering issues, new alternative membranes are being developed by a number of different companies. Progress in manufacturing techniques has been made. Although melt-extruded films were the norm, the industry is moving to a solution-cast film process to reduce costs and improve manufacturing throughput efficiency. In this process, the ionic form of the polymer is solubilized in alcoholic solution, such as propanol, and then fabricated into a film of desired thickness. The conversion of the non-ionic polymer to an ionic phase, ready for use in a fuel cell, is carried out prior to the solubilization step. Another advancement in membrane technology is that of using an internal support layer to enhance the mechanical strength of the membrane film, especially as the membrane thickness is decreased. The Primea 55 and 56 series membranes manufactured by W.L. Gore are examples of such internally-supported membranes. Porous Backing Layer The polymer membrane is sandwiched between two sheets of porous backing media (also referred to as gas diffusion layers or current collectors). The functions of the backing layer8 are to: (1) act as a gas diffuser; (2) provide mechanical support, (3) provide an electrical pathway for electrons, and (4) channel product water away from the electrodes. The backing layer is typically carbon-based, and may be in cloth form, a non-woven pressed carbon fiber configuration, or simply a felt-like material. The layer incorporates a hydrophobic material, such as polytetrafluoroethylene. The function of polytetrafluoroethylene is to prevent water from “pooling” within the pore volume of the backing layer so that gases freely contact the catalyst sites. Furthermore, it facilitates product water removal on the cathode as it creates a non-wetting surface within the passages of the backing material. One PEFC developer (10) devised an alternative plate structure that provides passive water control. Product water is removed by two mechanisms: (1) transport of liquid water through the porous bipolar plate into the coolant, and (2) evaporation into the reactant gas streams. The cell is similar in basic design to other PEFCs with membrane, catalysts, substrates, and bipolar plate components. However, there is a difference in construction and composition of the bipolar plate: 8. Commonly referred to as the gas diffusion layer (GDL) even though it has additional functions. 3-4PDF Image | Fuel Cell Handbook (Seventh Edition)
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