Characterization of mass transport processes to enable PEM fuel cell start-up from low temperatures

A PEM fuel cell assembly requires the simultaneous delivery of reactants to the electrodes of multiple cells in order to produce electrical power. The mass transport of reactants to the electrode surfaces is complicated by the presence of liquid water during operation in low temperature environments. The water transport characteristics of an operational fuel cell were experimentally determined to build a low ambient temperature control strategy for a full scale automotive application. Two key topics for fuel cell control are covered: (a) purge of the fuel cell at shutdown, and (b) the fuel cell stack power management during freeze-start. After a PEMFC system is shut down, excess liquid water is often present in the MEA and flow field. Upon exposure to sub-freezing temperatures, this water can block the flow of reactants to the cell and prevent the fuel cell system from operating. To mitigate the effects of residual water in the fuel cell, this study tests the effects of various parameters during shutdown purge on freeze-start reliability. Both cell orientation and water content in the membrane-electrode assembly (as inferred by measurement of high frequency resistance) were shown to influence freeze-start reliability.;The transport of product water away from the cell electrodes during start-up is significantly limited at lower temperatures (below about 45°C), where the water carrying capacity of the reactant streams is reduced as a result of low water vapor saturation pressure. As a result of the reduced water transport, proper control of the fuel cell current is necessary to prevent the electrode from filling with water or ice before the fuel cell system warms to its normal operating temperature of 60°C or greater. This study presents a point model of the low temperature water transport and proposes strategies for low temperature fuel cell power control, as a function of the heat capacity of the cell assembly and the fraction of produced electrical power used for supplemental heating.