Fraunhofer Institute for Solar Energy Systems ISE| Duration: | August 2015 - December 2017 |
| Contracting Authority/ Sponsors: | Ministerium für Finanzen und Wirtschaft Baden-Württemberg |
| Project Partners: | Deutsche Institute für Textil Forschung DITF Denkendorf, CHT Germany, Johns Manville Europe, NuCellSys |
| Project Focus: |       |
Resource Efficiency – Development of Gas Diffusion Layers for Polymer-Electrolyte Membrane Fuel Cells based on Environmentally Friendly and Energy-Saving Raw Materials
Polarization curve (solid lines) and high-frequency resistance curve (dashed) for over-humidified gases (120% relative humidity) of the commercial reference GDL (SGL 25 BC) and a wet proofed commercial carbon fiber GDL (SGL 35BA) with fluorine-free ISE-MPL.
Current/voltage characteristics and high-frequency resistance (HFR, dashed) at a gas inlet RH of 15%. Cell temperature: 80°C, Air/hydrogen at 2 barabs, 6/4nl/min, 12cm² active area. CCM with 0,4/0,1 mgPtcm-2, membrane thickness 18µm.
Current/voltage characteristics and high-frequency resistance (HFR, dashed) at a gas inlet RH of 100%. Cell temperature: 80°C, Air/hydrogen at 2 barabs, 6/4nl/min, 12cm² active cell surface. CCM with 0,4/0,1mgPtcm-2, membrane thickness 18µm
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).