Optimising next-generation bipolar plates for fuel cell drives with innovative coating systems.
Thanks to recent technological advancements, hydrogen is emerging as a low-carbon alternative fuel option for a range of industries, including road transport, maritime, aviation and rail. Hydrogen offers great potential for the decarbonisation of the mobility sector, however widespread commercialisation of the technology is still in its early stages.
Hydrogen technologies largely rely on polymer electrolyte membrane (PEM) fuel cells as they offer high current density, greater energy efficiency and small mass-volume compared to other alternatives. All fuel cell systems rely on bipolar plates. These perform several vital functions, including providing channels both for the separation and distribution of the process gases and coolant, and for the removal of the water resulting from the chemical reaction.
Motion technology leader Schaeffler Group is demonstrating its development capabilities in the field of hydrogen-powered mobility with a new generation of bipolar plates for PEM fuel cells. The next-generation plates feature a new design optimised for large-series production and leverage an innovative coating process for long fuel cell life. Significantly, fuel cell stacks made using the new Schaeffler plates achieve a power density of around 20% greater than that of stacks made using previous-generation plates.
“When it comes to drive systems for commercial vehicles, Schaeffler is also investing in hydrogen, especially for long-haul applications,” says Matthias Zink, CEO Automotive Technologies at Schaeffler. “We are developing both individual components and complete subsystems for fuel cells and are gearing up for their full-scale industrialisation.”
INCREASING POWER DENSITY
Despite being only the size of an A4 envelope and weighing just 60g, bipolar plates are the core component of fuel cells.
“Schaeffler has developed an innovative design that makes use of the plate’s surface area,” explains Dr. Jochen Schröder, Schaeffler’s Head of E-Mobility. “The finer and more precise the structures on the bipolar plate, the more efficient the plate is.”
Thanks to a new, optimised design, the Schaeffler plates enable a fuel cell stack power density of 4.6kW per litre of fuel cell volume, including end plates and compression hardware. For vehicle applications, several hundred of these plates are layered on top of one another, each separated by a membrane electrode assembly (MEA), to form a stack. The plates account for up to 80% of the stack’s weight, and up to 65% of its volume. Fuell cell stacks comprising up to 400 of these cell units have a total power output of up to 140kW – enough to power light commercial vehicles. Heavy commercial vehicles up to 40 tonnes generally require two stacks.
Not only are the plates designed with power density in mind, but also for large-scale industrialisation using a Design for Manufacturing (DfM) approach. By taking this approach, the company hopes to achieve a level of cost-effectiveness and scalability of manufacturing that is sufficient for hydrogen-powered mobility to reach market breakthrough. In terms of production, the company is leveraging its years of experience in metal stamping and forming to achieve the high level of precision needed to stamp the necessary ultra-fine structures on the surface of the plates, which have a thickness of only 50-100 micrometres.
INNOVATIVE COATING SYSTEM
One particularly unique feature of the metallic bipolar plates is the coating system Schaeffler used. Coatings are typically used to maintain a high level of electrical conductivity over the fuel cell’s entire service life. Schaeffler’s Enertect family of high-performance coating systems is developed specifically for bipolar plates, and can be engineered for maximum plate service life, minimum carbon footprint or optimised price-performance ratio in line with customer requirements.
“Thanks to our capabilities in surface technology, we are able to offer each customer an application-specific coating solution,” says Schröder. “This means we can meet each customer’s requirements in terms of balancing cost, performance, and manufacturing-related CO2 emissions.
The Enertect coatings are applied using a specially adapted and fine-tuned version of the physical vapour deposition (PVD) process that Scheaffler has previously used for the production of millions of highly-stressed valvetrain components. From a vital safety perspective, fuel cells must be both gas- and water-tight. To achieve this, the company uses either injection-moulded, silk screen gaskets, or a special laser welding process developed in-house, depending on requirements.
Schaeffler is now manufacturing the new bipolar plates in runs of up to several tens of thousands of units at its purpose-built pilot production facility in Herzogenaurach, Germany. The plates will be used by international vehicle manufacturers in prototype and small-series production. The fully automated facility is part of Schaeffler’s centre of excellence for hydrogen, which has been designed so that it can also manufacture large plates of the same type used for electrolysers. Schaeffler will start production of the new bipolar plates under the name Innoplate at the beginning of 2024. The plates will be customised for fuel cell systems in a joint venture with hydrogen technology and systems developer, Symbio.