Validated leverett approach for multiphase flow in PEFC diffusion media

This work is the second part of a series of papers to describe the multiphase transport mechanism in thin-film polymer electrolyte fuel cell (PEFC) diffusion media (DM). The present work is devoted to delineating the effects of compression. Direct measurements of drainage capillary pressure- saturation curves for SGL series carbon paper DM tailored with a range of mixed wettability were performed at room temperature under various compressions (0, 0.6, and 1.4 MPa) typically encountered in a fuel cell assembly. Based on these benchmark data, an appropriate form of the Leverett approach, including a Leverett- type empirical function that incorporates the effect of compression and the mixed wettability characteristics of the tested DM samples, was developed. The presented approach can determine the capillary pressure as a function of hydrophobic additive content, liquid saturation, compression, and uncompressed porosity of the DM. Compression leads to an increase in capillary pressure, effectively caused by the corresponding reduction in effective porosity. Any increase in hydrophobicity amplifies the compression effect, yielding a higher capillary pressure for the same saturation level. Furthermore, the fraction of connected hydrophilic pores is observed to be reduced with an increase in compression, leading to a favorable reduction in water storage capacity of the fuel cell DM. (c) 2007 The Electrochemical Society.