Non-uniform Hydrophobicity in Gas Diffusion Media of Polymer Electrolyte Fuel Cell Components

Abstract

Polymer electrolyte fuel cells with their high gravimetric energy density face a water balance problem especially under variable loads, which represents normal automotive operation: 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, is responsible for a passive water management of the cell. The adjustment of the hydrophobicity of the GDL is crucial for stable operation, and non-uniform hydrophobicity has already been shown to be advantageous. In polymer electrolyte fuel cells (PEFC) they typically 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. Modifications of the hydrophobicity were applied, in particular with the goal to form a non-uniform wettability to the gas diffusion media, thus introducing in parallel more hydrophilic pathways for water while maintaining more hydrophobic areas. Modifications applied by irradiation with laser, x-ray, and ion-beam were assessed and correlated to the alterations of hydrophobicity and tested for improvements. 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. References [1] P. Kuttanikkad, M. Prat, J. Pauchet, Pore-network simulations of two-phase flow in a thin porous layer of mixed wettability: Application to water transport in gas diffusion layers of proton exchange membrane fuel cells, J. Power Sources 196 (2011) 1145. [2] M. Schulze, C. Christenn, XPS investigation of the PTFE induced hydrophobic properties of electrodes for low temperature fuel cells, App. Surf. Sci. 252.