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 transient two-dimensional along the channel model and simulations of the PEMFC operating with a deadended anode. 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). 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. Finite-element representation of the governing equations in the 2D PEMFC geometry and subject to boundary conditions mimicking experimental conditions is solved using a commercial multiphysics software, COMSOL. According to model results reversible voltage degradation between purge cycles is mostly due to nitrogen accumulation in the anode that leads to partial fuel starvation in the cell. INTRODUCTION To improve the fuel utilization of PEMFCs one has to implement mechatronic solutions with expensive hydrogen grade components and hardware on the anode side, which add cost and complexity to overall balance of the plant. Alternatively, anode side of the PEMFC can be operated at the dead-ended mode with periodic brief purges of dry H2, using a pressure regulator instead of mass flow controllers [1,2,3]. There are major drawbacks of the dead-ended anode (DEA) operation. First, since the fuel that is supplied to the anode is not humidified, membrane humidification depends solely on the water produced in the cathode side. Second, large partial pressure of N2 in the air supplied to the cathode causes a slow crossover to anode through the membrane. Along with N2, water vapor accumulates in the anode as well, partially alleviating the first problem, but contributing to the second one. Accumulation of N2 and H2O vapor in the anode hinders the delivery of H2 to portions of the active area of the fuel cell near the anode exit. Moreover, at higher current densities and cathode humidification, accumulation of liquid water in the anode-GDL leads to further adverse conditions which render the transport of H2 more difficult in the anode [2,3,4,5,6]. Lastly, H2 starvation in the PEMFC operating in the DEA mode leads to degradation of the catalyst support through the carbon corrosion mechanism [7]. Despite all its adverse conditions, DEA mode of the PEMFC is not well understood and a closer look at the transport and degradation mechanisms is necessary.