FOCUS ON MATERIALS
Figure 4 : Stainless steel foil can be produced in thicknesses down to 0.05mm .
heat generated by the reaction . The bipolar plates play a structural role in separating the individual cells in a stack from each other .
A single PEM fuel cell will produce only around 1V . To provide the voltage and power required for transportation applications , multiple cells are connected in series in a stack . Typically , a stack will be built up from 200 to 400 individual bipolar plates .
Bipolar plate material
A complex pattern of channels is pressed or machined into the surface of a bipolar plate , as shown in Figure 3 overleaf . This pattern is essential to create a ‘ flow field ’ that will enable the hydrogen and oxygen to flow over the membrane electrode assembly ( MEA ). In some designs , the plate may also feature internal channels for liquid cooling .
Bipolar plates place considerable demands on the materials used in their construction . They require a high level of physical strength , good electrical and thermal conductivity , corrosion resistance against the acidic nature of the MEA and gas impermeability and tightness . For some time , it has been an open race between graphite , composites and metals such as aluminium , titanium and stainless steel .
The early generations of PEM fuel cells , with relatively low production runs , made effective use of CNC-machined graphite for their bipolar plates . The downside is the high cost of graphite and the need for complex
Figure 2 : 200 to 400 bipolar plates are assembled to create a fuel cell stack .
The bipolar plates play a structural role in separating the individual cells in a stack from each other
machining processes . Since the bipolar plates could account for some 40 per cent of the total cost of a fuel cell , manufacturers are keen to investigate alternative materials that are more cost-effective for mass production on a very large scale .
Switching from graphite to a thin stainlesssteel foil offers considerable potential to reduce the size and weight of the fuel cell stacks . This is a vital consideration as the bipolar plates are estimated to make up around 80 per cent of the total weight of the stack . Furthermore , stainless steel makes it possible to form the complex flow patterns in the surface by press forming , a process that increases productivity and reduces manufacturing costs . An added benefit is that stainless steel enables cells and stacks to be constructed by laser welding .
Outokumpu ’ s investigations into the manufacture of stainless steel have indicated that the grade EN 1.4404 ( Supra 316L ) is
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