Modeling Impact of Crack Density on the Performance of the Microporous Layer in PEMFCs

The crack density (CD) of the microporous layer (MPL) has a positive impact on the water management and electrochemical performance for PEMFCs. As there is still a lack of strategies for designing optimal CD under varying relative humidity (RH) for enhancing MPL performance, a numerical model is proposed with integration of the capillary pressure weakening coefficient, and the effects of cracks on water-gas transport, the capillary pressure drop, and water flooding are quantitatively considered. Furthermore, experiments are set up to adopt a kind of thickener (methylcellulose) to adjust the slurry viscosity for preparing MPLs with different crack densities, and the main parameters related to cracks, e.g., crack density, porosity, permeability, and water contact angle, are systematically characterized as input parameters for the model. Results show that at 100% RH and 50% RH, the maximum power density increases first and then decreases with increased CD, reaching its maximum value when the CD is 5.24%, which is due to the impairment of gas mass transfer caused by flooding in the CL at low CD and in the GDL at high CD. However, the lower RH and faster evaporation rate of water at 50% RH result in a small difference in performance. At 25% RH, membrane hydration becomes the primary factor governing cell performance. An increase in MC partially reduces the hydrophobicity of the MPL, thereby modestly enhancing membrane hydration and improving the fuel cell performance. Although CD is the main parameter discussed in this paper, MC also causes changes in pore structure, permeability, and wettability. Therefore, the improvement in the MPL performance under different RHs is attributed to the coupling effect of CD and other physical properties.