Bipolar Plates

• good electrical conductivity (>100 S cm 1 bulk conductivity), • high thermal conductivity (>20 W cm ), • high chemical and corrosion resistance, • mechanical stability toward compression forces, • low permeability for hydrogen, • low-cost material being processable with mass production techniques, • low weight and volume, and • recyclable materials. Two different kinds of materials have been used in the past: metallic and graphitic. For mobile applications of fuel cells, the requirement of high power densities at very low cost is difficult to fulfill, though the lifetime in terms of operational hours is limited to several thousands. Here, stainless steel seems to be the material of choice – the material already being a mass product, its forming processes are well established in the automotive industry. Thin metal sheets show sufficient mechanical strength. Two sheets of thin and structured metal plates can be combined into a bipolar plate with flow fields on both sides and cooling channels in between. For improving lifetime, a corrosion-protective coating is typically required. For stationary applications, lifetime expectations are usually much higher; 40 000–80 000 h would be advantageous for residential power supply systems. Therefore, graphitic materials are preferred as bipolar plate materials for this purpose. Graphite itself is not sufficiently gas-tight to separate hydrogen and air from the cooling channels. Various methods to improve the properties of graphite have been investigated in the past: impregnation of graphite plates with a resin or composite materials with polymer binders. Here, forming processes have to be developed or at least adapted to the new material combinations. Both compression molding and injection molding have been investigated. Flow field design is, to some extent, dependent on the materials used. But it is mainly influenced by the requirements of gas distribution and water management in the plane of larger electrode areas, low pressure drop mainly at the air electrode, and good electrical contact over the cell. Particularly, multiple serpentine flow fields have been realized, which withstand a certain pressure drop and favor removal of condensing water droplets.