Modeling and Simulations of PEMFCs Operating With Periodically Purged Dead-Ended Anode Channels

PEMFC operation with dead-ended anode has inherent transient behavior: the cell operates between purge cycles that replenish fuel and discharge accumulated gases, such as nitrogen and water vapor, and liquid water. During the operation when the anode exit is shut, gases that cross-over from the cathode accumulate and stratify in the anode channels above the liquid water when the gravity is acting in the flow direction. In this work, we present a transient two-dimensional along the channel model and simulations of the PEMFC operating with a dead-ended anode. The transport of gas species in flow channels and gas diffusion layers is modeled by Maxwell-Stefan equations. Flow in the channels is modeled by laminarized Navier-Stokes equations, where the inertial terms are dropped from the force balance, but the buoyancy effect due to the variation of the composition of gas mixture is included at the anode side. Flow in the gas diffusion layers is modeled by Darcy’s Law. Permeation of nitrogen in the membrane is considered since it can accumulate in the anode as opposed to instant reaction of oxygen (hydrogen) at the anode (cathode) catalyst layer(s). The membrane is considered as a resistance (interface) to transport of water vapor and nitrogen. Ohm’s Law is used to model the transport of charged particles, i.e. electrons in the electrodes and flow plates and protons in the membrane. A finite-element representation of the governing equations in the 2D PEMFC geometry is solved using a commercial multiphysics software, COMSOL. The imposed boundary conditions are chosen to match the experimental conditions. According to model results the reversible voltage degradation between purge cycles is mostly caused by nitrogen accumulation in the anode that leads to partial fuel starvation in the cell.