The bipolar plate is one of the most used components in a fuel cell stack, alongside the gaskets and the membrane electrode assembly. This is why it’s important in the overall scheme of things to bring down their cost. Manufacturers, regardless of whether they produce metal or graphite solutions, are increasingly looking to automate and link up individual processes on the one hand and to optimize the products themselves on the other, for instance by further reducing sheet thicknesses. Already plans are afoot to scale up production and could result in several million bipolar plates being manufactured for use in over 100,000 stacks every year.
In a proton exchange membrane fuel cell, the bipolar plate is a key component. It accounts for up to 80 percent of stack weight and up to 65 percent of stack volume, hence its enormous importance in terms of the power density. It’s equally significant for the functioning of the fuel cell: The bipolar plate or BPP separates and distributes process gases and removes product water. Not only that, this component is responsible for performing the essential tasks of conveying the generated current and evenly distributing all media.
BPPs are principally made from graphite carbon or metal. The various materials are associated with different properties and have different advantages for plate functionality. Because of low efficiency benefits and a lack of manufacturing processes for competitive metal BPPs, it is the graphite variant that has dominated in the past. However, graphite-based BPPs exhibit volumetric and gravimetric shortfalls compared with their metal counterparts, particularly when it comes to demanding applications. Plus, graphite is extremely brittle and can therefore break easily. Nevertheless, graphite plates are frequently deployed in stationary applications in which the volume of the structure is not a limiting factor.
On matters of cost, it’s metal plates that take the lead. “With the right production process the sheet thicknesses can be reduced down to 0.05 mm. Here, metal is at a completely different price level to graphite,” emphasizes German manufacturer CellForm. Given that several hundred plates are used in a single stack, the financial ramifications for the final application are huge. A further advantage of metal BPPs cited by CellForm is their positive impact on the cold-start capability of the fuel cell.
The company, based in Baienfurt in Baden-Württemberg, covers the entire manufacturing process for metal BPPs with a multistage forming process and subsequent laser welding. Commenting on working with metal, company representatives point out that the “extremely thin” sheet thicknesses are a particular challenge: Shaping such fine initial sheets and creating the highly precise and complex geometry of the channels can, due to physical constraints, quickly lead to fractures that would render the BPP unusable. On top of this come the stringent quality requirements with low margins for error which need to be met when producing in large volumes. “Only those who satisfy this requirement will be able to maintain their position in this growing and fiercely competitive market,” says the company.
These challenges, according to CellForm, are putting a certain degree of selective pressure on the manufacturing processes which are still under development. “Physical restrictions – such as heat generation – will limit how much these processes can make in future mass production,” states the manufacturer. This problem, it says, isn’t noticeable when dealing with small volumes, but will become increasingly evident in the years ahead as demand grows.
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