The fuel cell is gaining importance as an alternative to battery technology in electrified vehicles. This is due to a number of reasons, including Germany’s federal government adopting a National Hydrogen Strategy in June 2020. When it comes to fuel cells for electric vehicles, a high degree of efficiency and a compact design are the two key factors. In both respects, the steel material used to manufacture the bipolar plates plays a critical role. Waelzholz supplies the pioneers of mobile fuel cell applications with high-quality precision steel strip specifically for this purpose.
“When it comes to our materials for the bipolar plates used in fuel cells for mobile applications, we operate at the intersection of the conflicting demands of efficiency, thickness, stability, and formability,” says Jan Ullosat, materials engineer at Waelzholz, outlining the requirements the steel grades used in this area of application need to meet. In fuel cell vehicles, known as FCVs, the “stacks,” i.e. the series of cells which make up a complete fuel cell, need to consist of approximately 400 cells in order to deliver the necessary power. The cells are separated from each other using bipolar plates, with each cell having one bipolar plate to its left and one to its right. Each of these bipolar plates consists of two pieces of sheet metal. “As such, there are approximately 800 sheet metal blanks in a fuel cell for an FCV. Since space for the power system is limited in the vehicle, the plates have to be as thin as possible so that the entire system takes up as little installation space as possible,” explains Ralf Sauer from Sales.
Maximum stability and formability thanks to optimized manufacturing processes
Ullosat explains precisely what “as thin as possible” means: “We are capable of manufacturing extremely thin stainless precision steel strip in the range of 75 to 100 micrometers (0.003 to 0.004 in) for this application. For purposes of comparison, this is roughly equivalent to the thickness of a human hair. The challenging part of this process is reconciling two requirements. On the one hand, the material must be easy to form, since the bipolar plates feature a complex structure of gas channels – known as flow fields. On the other hand, the plates must be extremely dimensionally stable, which is largely determined by our material and our customers’ plate design.” Dimensional stability is particularly relevant to the steel’s use in mobile applications, since the entire fuel cell system is stressed by shocks and vibrations when the vehicle is in motion. According to Ullosat, “bipolar plates have an extremely complex shape that gives them stability in later use. To create these shapes, our customers require materials with outstanding durability for the complex forming processes. The way to achieve this is through a production routing with specially coordinated rolling and heat treatment processes to give the extremely thin precision steel strip outstanding forming properties. Our customers receive a completely isotropic material from us that is literally made for complex forming processes, even with tight bending radii.” Another requirement in connection with forming and subsequent use is flatness – bipolar plates have a multidimensional structure with ultra-fine channels for fluid and gas flow. Exceptional flatness ensures that these channels can be formed precisely and accurately over the entire plate surface.
Stainless steel with extremely high purity requirements
Waelzholz uses a special grade of stainless steel for fuel cell applications, which is then coated by customers to achieve even greater electrical performance. Ullosat explains the advantages of the Waelzholz material: “The specific details of the coating are always highly unique to the customer and have a significant impact on the performance of the fuel cell. With regard to the topology of the surface, we can offer every conceivable option thanks to our material and our expertise. We have the ability to precisely customize the roughness of the material surface to meet the customer’s coating needs.” And should the coating ever be damaged in the course of the forming processes or in later use, the rust-free Waelzholz material prevents the corrosion of the bipolar plates. “This is a significant safety aspect, because if the plates were to rust through, it could lead to failure of the individual cell and, as a result, the total failure of the entire fuel cell stack,” Ullosat explains. Sales expert Sauer adds another advantage of the material: “Fuel cell manufacturers expect us to supply materials with an extremely high degree of purity. This is due to the fact that inclusions in the material could adversely affect the forming properties in this low material thickness range. To effectively ensure that our products possess the required degree of purity, we closely and meticulously inspect all of the raw materials supplied to us.”