Surface Modifications and Analysis of a non-uniform Hydrophobicity in Polymer Electrolyte Fuel Cell Components

1. Introduction Polymer electrolyte fuel cells with their high gravimetric energy density face a water balance problem especially under variable loads, in particular under automotive conditions: The excess product water needs to be removed from the fuel cell while maintaining a humidified membrane. The gas diffusion layer (GDL), which also provides contact to the electrochemically active components, has to achieve the passive management of the water balance. The adjustment of the hydrophobicity of the GDL is crucial for stable operation, and non-uniform hydrophobicity has already been proven shown to be advantageous. 2. Experiment Typically, gas diffusion electrodes for polymer electrolyte fuel cells (PEFC) consist of conductive medium, either a carbon based powder in the microporous layer (MPL) or carbon felt/fibres/cloth in the macroporous backing, and a hydrophobicity impregnation agent like polytetrafluoroethylene (PTFE). The ratio determines the hydrophobicity and thus the performance. The surface hydrophobicity of GDL components were characterized by x-ray photoemission spectroscopy and infrared absorption spectroscopy. Modifications applied by irradiation with laser, x-ray, and ion-beam were assessed and correlated to the alterations of hydrophobicity. In particular, non-uniform hydrophobicity was applied and tested for improvements. 3. Summary It was observed, that while a direct improvement in output power could not be achieved with pure hydrophobicity modifications, the current density distribution in a running fuel cell was more homogeneous. Both the areas of extremely high and low current densities were reduced. Due to the close correlation of these extreme conditions and the degradation of PEFCs, it can be expected, that the durability is increased with the applied non-uniform surface modifications. 4. References [1] M. Schulze, C. Christenn, XPS investigation of the PTFE induced hydrophobic properties of electrodes for low temperature fuel cells, App. Surf. Sci. 252 5. Acknowledgements The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) for Fuel Cell and Hydrogen Joint Technology Initiative under Grant No. 303446 (IMPALA)