BiFoilStack: Stack designs for fuel cells

Fuel cell drives are a promising technology for replacing combustion engines in commercial vehicles. The main obstacles to widespread use are the currently still high system and stack costs and the short service life of fuel cells. The core objective of the BiFoilStack project is the development of customized stack concepts for NT-PEM fuel cells for the target application "commercial vehicles" based on compound bipolar foils from the Fraunhofer UMSICHT calendering process. The thermoplastic bipolar foils with up to 90 percent carbon have high electrical conductivity and low contact resistance, can be adapted to specific requirement profiles and can also be welded with similar material counterparts. This opens up completely new design possibilities and thus approaches for innovative stack concepts.

In the project, a sealed as well as a (partially) welded construction method are being considered. Stack designs with frame elements are compared to frameless concepts. Furthermore, concepts with bipolar plates made of structured and joined half plates, as well as bipolar plates constructed in layers, are evaluated. Additionally, stack configurations with bipolar plates that do not have their own flow field and with an innovative gas distribution system, are examined.

The focus of the developments is on modularity, high reliability coupled with a long service life and the reduction of manufacturing and assembly costs among others by using welded components.

The bipolar plates are adapted to the requirements of the novel stack concepts in terms of materials, with the process iteratively transitioning between material and stack development. Furthermore, development steps are taken regarding the formability and laminability of the bipolar plates, aiming to achieve a bipolar full plate with a flow field.

The increasing occurrence of weather extremes such as heatwaves, drought periods, floods, and tornadoes in atypical locations underscores the urgency of swift action for climate-neutral and sustainable solutions. Accounting for more than a third of global CO₂ emissions, traffic is one of the biggest drivers of climate change. While the transition from fossil fuels to sustainable solutions for passenger cars is in full swing, heavy commercial vehicles and road freight transport are still lagging behind. This is mainly due to the much higher energy requirements of these vehicles compared to passenger cars, which makes battery solutions and the corresponding charging infrastructure a challenge. Here, fuel cells are widely recognized as a cornerstone for the electrification of the heavy-duty sector. They convert hydrogen into electricity, which powers the vehicle's electric drive system. Unfortunately, the industrialization of fuel cells is currently at a level where battery electric vehicles were five to ten years ago, and there are still some hurdles to overcome. One major challenge for the application of fuel cells in heavy commercial vehicles, for example, is durability. The fuel cells must have a service life of over 30,000 hours. This applies in particular to long-distance transportation.

Polymer electrolyte membrane fuel cells (PEM) are currently the preferred type of fuel cell in transportation. Their advantages are their fast start-up time, high power density and efficiency. They consist of several hundred cells, which are characterized by a membrane electrode assembly (MEA) embedded between two bipolar plates (BP). The BPs provide separate flow channels for the educt gases, the product water and the cooling medium, connect the individual cells electrically and dissipate heat to the cooling circuit.

The project "BiFoilStack – Development of stack designs for NT-PEM fuel cells with novel compound bipolar foils" is based on thin-walled compound bipolar foils from the Fraunhofer UMSICHT calendering process. The aim is to further develop the novel foils with regard to the specific requirements of fuel cell stacks for heavy-duty applications and to develop cost-effective, scalable industrial processes for converting the compound foils into bipolar plates. The project also involves the simulation-based development of innovative cell and stack designs using the additional design freedom of the material. The compound material has the potential to be a disruptive technology by combining the properties of metallic and graphitic plates – substrate thickness close to metal, superior corrosion resistance and cost-effective production processes.

In particular, the following development paths are being evaluated in the project:

• Assembly, pressure losses and performance: BP made of structured and joined half plates versus multi-layer BP with different flow fields on the front and back sides
• Modularity, durability and assembly: Sealed versus (partially) welded construction
• Structure and design: Stack with frame elements versus frameless design
• BPP manufacturing costs and assembly: Stack design with BP without its own flowfield with innovative gas distribution system

• Material-side optimization of compound bipolar foils from the calendering process for the application field of "commercial vehicles" and further development of unstructured bipolar foils into bipolar plates with flow field
• Development and implementation of tailored stack concepts
• Increasing the service life of mobile fuel cells (> 20,000 hours)
• Achievement of a volumetric power density at stack level of > 5 kW_net/l
• Electrochemical and degradation characterization of the developed components (bipolar foils and stack)
• Evaluation of the developments with regard to technical, ecological and economic aspects and comparison with state-of-the-art components

• FEV Europe GmbH
• Dätwyler Sealing Solution Deutschland GmbH & Co KG
• Fraunhofer-Institut für Lasertechnik ILT
• Rheinisch-Westfälische Technische Hochschule Aachen
• Hydrogenics GmbH (assoziierter Partner)
• Imerys Graphite & Carbon (assoziierter Partner)