Performance of advanced automotive fuel cell systems with heat rejection constraint

Although maintaining polymer electrolyte fuel cells (PEFC) at temperatures below 80 °C is desirable for extended durability and enhanced performance, the automotive application also requires the PEFC stacks to operate at elevated temperatures and meet the heat rejection constraint, stated as Q/ΔT < 1.45 kW/°C, where Q is the stack heat load for an 80-kWe net power PEFC system and ΔT is the difference between the stack coolant temperature and 40 °C ambient temperature. We have developed a method to determine the optimum design and operating conditions for an automotive stack subject to this Q/ΔT constraint, and illustrate it by applying it to a state-of-the-art stack with nano-structured thin film ternary catalysts in the membrane electrode assemblies. In the illustrative example, stack coolant temperatures >90 °C, stack inlet pressures >2 atm, and cathode stoichiometries <2 are needed to satisfy the Q/ΔT constraint in a cost effective manner. The reference PEFC stack with 0.1 mg/cm2 Pt loading in the cathode achieves 753 mW cm−2 power density at the optimum conditions for heat rejection, compared to 964 mW cm−2 in the laboratory cell at the same cell voltage (663 mV) and pressure (2.5 atm) but lower temperature (85 °C), higher cathode stoichiometry (2), and 100% relative humidity.