Two-Phase Unit Cell Model for Slow Transients in Polymer Electrolyte Membrane Fuel Cells

A model is presented of a polymer electrolyte fuel cell including slow transient effects of liquid water accumulation and evaporation in gas diffusion electrodes GDEs and gas channels. The model is reduced dimensionally, coupling a one-dimensional 1D model of gas and coolant channel flow to 1D models of transport through the membrane electrode assembly MEA and bipolar plates. An asymptotic reduction of the two-phase flow to a sharp interface model is used, in which phase change occurs at a front that evolves in time. The asymptotic reduction is based on an immobile water fraction in the GDE and a large capillary pressure. The water content in the membrane and channels is also tracked in time. Gas and thermal transport are taken to be at quasi-steady state on the time scale of liquid accumulation. The model is fit to Ballard Mk9 cells and validated against experimental measurements of both steady-state and transient MEA water content distributions along the length of the channel. Predictions of slow cyclovoltammograms are presented based on the model. Polymer electrolyte membrane PEM fuel cells are complex electrochemical devices that display transient behavior and hysteresis on a wide range of time scales. We present a model that captures the slow transient behavior associated to the accumulation of liquid water within the membrane electrode assembly MEA and gas flow fields of PEM fuel cells. Liquid water is widely known to play a critical role in overall cell performance, influencing the protonic conductivity of the Nafion membrane, the transportation of oxygen to reaction sites within the catalyst layers, 1 and the transportation of gases within the flow fields. Moreover, the time scale for liquid water buildup, on the order of tens of minutes, has an important impact on the dynamic loading of fuel cells typical for driving cycles. 2,3 Indeed, any predictive model designed for the optimization

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