Numerical simulation of multi-scale transport processes and reactions in pem fuel cells using two-phase models

Sammanfattning: A numerical study for the cathode of a PEM fuel cell has been performed in this study. The results have been limited to cathode only because, in PEM fuel cells, the oxygen reduction reactions, ORRs, are considered the rate limiting reactions and govern the fuel cell performance. The modeling approach utilized the two-phase models involving water phase change for PEM fuel cells i.e. two-phase current (solid and membrane), two-phase flow (gas and liquid water) and two-phase temperature (fluid and solid). The catalyst layer has been modeled using the microscale agglomerate approach where diffusion of oxygen into the agglomerate structure was used to model the reaction rates. For comparison of the PEM fuel cell performance, detailed study was performed at load conditions of current densities of 0.22, 0.57 and 0.89 A/cm2 explicitly. A varying fuel cell performance was observed under different loads. At low current densities, the temperature, electro-osmotic drag, irreversible and losses are quite low but the membrane phase conductivity showed a decreasing pattern along the length of the cathode. At higher current density (0.89 A/cm2), a sharp decrease in the current was observed due to the mass limitation effects, and due to higher water content, the water flooding effect was observed as more prominent than at lower current densities. The maximum power density for the present case was observed at 0.55 V. By comparing the results of this study and previous study with single phase flow model, it can be seen that this model is more conservative and captures the mass limitation effects to a great extent and the maximum power density as predicted by the single phase models falls in the mass limitation zone.