Microstructure changes induced by capillary condensation in catalyst layers of PEM fuel cells

摘要

This paper proposes a hypothesis for explaining Pt/C particles' coarsening inside the catalyst layers of a PEM fuel cell. The hypothesis includes the two parts: (1) due to capillary condensation a water-bridge could be formed between two neighboring nano-scale Pt/C particles at relative humidity under 100% when the surfaces of the Pt/C particles are hydrophilic; (2) the capillary force of the water-bridge tends to pull together the Pt/C particles. The relation is derived in this paper between the capillary force and the factors including the diameter of Pt/C particles, relative humidity, temperature, distance between the two neighboring Pt/C particles and water contact angle. A parametric study is performed showing some details about water-bridge formation. Finally, the stress level induced by the capillary force inside the Nafion thin-film connecting with the Pt/C particles is calculated. The result shows that the capillary force could be large enough to break apart the Nafion thin-film, facilitating the movement of Pt/C particles towards each other.