Already from the start, fuel cells should be designed taking into account an environmentally friendly production and at the same time to facilitate market entry by using cost-effective materials. The gas diffusion layers used in fuel cells are made of electrically conductive carbon fibers. The production of polyacrylonitrile fibers is energy-intensive due to multiple compression and carbonization steps at very high temperatures. The carbon fibers are equipped with fluorocarbons to reinforce the water-repellent properties.
In the project, the following alternatives are evaluated:
- Electrically conductive systems based on glass fibers have been tested and developed to replace the energy-intensive carbon fibers.
- Toxicologically questionable fluoropolymers are replaced by alternative, environmentally friendly hydrophobic agents.
Development of a Fluorine-Free Microporous Layer (MPL)
In the project, an MPL with a fluorine-free hydrophobic agent could be developed. When comparing the characteristic curve of a carbon fiber GDL (SGL 35BA) coated with the fluorine-free ISE-MPL, it becomes clear that the performance of the fluorine-free MPL corresponds to that of a commercial MPL even under heavily over-humidified operating conditions. The lower high-frequency resistance indicates improved electrical conductivity of the ISE-MPL, possibly due to the lower proportion of non-conductive hydrophobic agent compared to the SGL 26 BC (Fig. 1).
Optimization of Electrical Conductivity
Glass fibers are electrical insulators. High electrical conductivities can be achieved by optimized carbon coatings.Non-conductive components are incorporated into the coating for stabilization and water repellency. Our investigations revealed that with increasing loading of the conducting phase, the conductivity converges to a value independent of the carbon used and that the maximum achievable conductivity strongly depends on the stabilizer content. The minimum requirement of a conductivity of 200Sm-1 has been achieved in the project. The diffusion properties for the reaction gases remained within an acceptable range, despite the high coating and filling degrees, using a suitable combination of different carbons. The finally achieved performance characteristics correspond to commercial materials (Fig. 2 and 3).