Impact of microporous layer pore properties on liquid water transport in PEM fuel cells: carbon black type and perforation

Abstract

The oxygen and water transport through various microporous layers (MPLs) is investigated by fuel cell tests in a 5 cm2 active area cell under differential-flow conditions, analyzing polarization curves, the associated high-frequency resistance, and the oxygen transport resistance extracted from limiting current density measurements. In this study, MPLs with two different carbon blacks are prepared and compared to a commercial material, all coated on the same GDL-substrate (Freudenberg); furthermore, perforated MPLs with large pores produced by a thermally decomposable polymeric pore former with a particle diameter of ≈30 μm are examined. The materials are characterized by mercury porosimetry, nitrogen adsorption and scanning electron microscopy. While at dry conditions (Tcell = 80°C, RH = 70%, pabs = 170 kPa) the performance of all materials is similar, at conditions of high water saturation (Tcell = 50°C, RH = 120%, pabs = 300 kPa), MPLs with larger pores or perforations exhibit a performance improvement due to a ≈30% reduction in oxygen transport resistance. The results indicate that liquid water is transported exclusively through these large pores, while the oxygen transport occurs in the small pores defined by the carbon black structure.