Numerical investigation of transport phenomena and electrochemical reactions in PEM fuel cell cathode

Abstract

Cathode over potential represents the single largest cell voltage loss mechanism in PEM fuel cells. The loss is mainly attributed to the slow nature of oxygen transport and sluggish electrochemical kinetics. These form the focus of the present study. The cathode catalyst layer is assumed to be composed of a uniform distribution of catalyst, liquid water, electrolyte, and void space. A serpentine flow field is used to distribute the oxidant over the active cathode electrode surface, with pressure loss in the flow direction along the channel. Both the convection and diffusion process occur in the electrode backing layer and the catalyst layer. The Stefan-Maxwell equation is used to model the multi-species diffusion. The two-dimensional numerical simulation highlights the transport process of oxygen, electron and proton in the catalyst layer, and their impact on the electrochemical process and the current density distribution. It is found that electron transfer to the reaction site leads to more cell losses than proton transfer. Most of the losses from electron transfer occur in the bipolar plate and backing layer. Thus, efforts should be focused on the improvement of those two domains. In addition, the assumption of water being in the vapour form everywhere cannot hold when the inlet relative humidity is high. Therefore, modeling liquid water is essential for a better understanding of the electrochemical process.