Draft: Modeling and Experiments of Voltage Transients of Pem Fuel Cells with the Dead-ended Anode

The operation of PEM fuel cells (PEMFC) with dead-ended anode (DEA) leads to severe voltage transients due to accumulation of nitrogen, water vapor and liquid water in the anode channels and the gas diffusion layer (GDL). Accumulation of nitrogen causes a large voltage transient with a characteristic profile whereas the amount of water vapor in the anode is limited by the saturation pressure, and the liquid water takes up very small volume at the bottom of the anode channels in the case of downward orientation of the gravity. Here, we present a transient 1D along-the-channel model of PEMFCs operating with periodically-purged DEA channels. In the model, transport of species is modeled by the Maxwell-Stefan equations coupled with constraint equations for the cell voltage. A simple resistance model is used for the membrane to express the permeance of nitrogen and transport of water through the membrane. The model results agree very well with experimental results for the voltage transients of the PEMFC operating with DEA. In order to emphasize the effect of nitrogen accumulation in the anode, we present experimentally obtained cell voltage measurements during DEA transients, when the cathode is supplied with pure oxygen. In the absence of nitrogen in the cathode, voltage remained almost constant throughout the transient. Then, the model is used to determine the effect of oxygen-to-nitrogen feed ratio in the cathode on the voltage transient behavior for different load currents. Lastly, the model is used to show the effect of the small amount of leak from the anode exit on the voltage transient; even for leak rates as low as less than 10 ml/h, nitrogen accumulation in the anode channels is alleviated and the cell voltage remained almost constant throughout the transient. INTRODUCTION The complexity of the PEMFC balance-of-plant design is a major drawback that impedes its advance especially in transportation applications. For example, the effective utilization of the on-board-stored fuel requires expensive components and advanced mechatronic solutions. A simpler approach to improve the fuel utilization, once its shortcomings are addressed, is to operate the PEMFC with the DEA by using a simple pressure regulator instead of expensive mass flow controllers [1,2,3]. Major shortcomings of the DEA operation are: first, membrane humidification depends on the humidification and product water from the cathode side; second, accumulation of nitrogen dilutes the concentration of hydrogen in the anode channels insofar as hydrogen cannot reach to reaction sites especially near the exit. Along with nitrogen, water vapor accumulates in the anode as well, partially alleviating the first problem, but contributing to the second one. Accumulation of nitrogen and water vapor in the anode blocks hydrogen transport to portions of the active area near the anode exit. Moreover, especially at high current density and relative humidification in the cathode, accumulation of liquid water in the anode-GDL leads to further adverse conditions that render transport of hydrogen difficult throughout the anode [2,3,4,5,6]. Lastly, hydrogen 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, the operation of PEMFCs with DEA is not well understood and a closer look at the transport and degradation mechanisms is necessary. Here, we present a time-dependent one-dimensional along the channel model of DEA transients of PEMFCs between purge cycles. The proposed model is compared with recent ex-