Modeling water phenomena in the cathode side of polymer electrolyte fuel cells

Water exerts a crucial influence on the performance of a polymer electrolyte fuel cell as both "catalyst activating agent" and "oxygen blocker". Therefore, fine-tuning the water distribution is imperative for high performance. In this work, we present a water balance model to calculate the distribution of liquid water in cathode catalyst layer and diffusion media. The model incorporates the influence of the local liquid water saturation on the effective transport properties. Liquid water saturation is both a composition variable determining the effective properties and a variable that depends on the solution of the transport equations that use the effective properties. The model reveals the formation of a thin water layer in the diffusion medium adjacent to the catalyst layer at high current density. This interfacial water layer strongly impedes oxygen transport and reduces the oxygen concentration in the catalyst layer, which causes a drastic increase in the voltage loss at high current density that drastically reduces the cell performance. We elucidate the origin of the water layer, present parametric studies of this effect, and propose mitigation strategies. The fundamental understanding gained will aid the development of membrane electrode assemblies with tailored pore network properties to achieve vital improvements in performance.