Current ramping: A strategy for rapid start-up of PEMFCs from subfreezing environment

A rapid start-up of proton exchange membrane fuel cells (PEMFCs) from subzero temperatures is essential for fuel cell vehicle commercialization. We explore the limits of the start-up time and the required cell material properties and operating conditions using an experimentally validated model. A linear class of current-ramping protocols is proposed and optimized for a rapid cold start. A PEMFC with a standard cell thermal mass of 0.4 J/ cm² K starting from -30°C is of primary interest and is extensively studied in this work. Either a small initial current density (e.g., 100 mA/ cm²) combined with an intermediate ramping rate or a relatively large initial current density (e.g., 200 mA/ cm²) in combination with a small ramping rate can lead to a successful self-start if the membrane electrode assembly (MEA) is sufficiently dry before the start-up. However, a current-ramping cold start using an insufficient initial current density (e.g., 50 mA/ cm²) shuts down with any ramping rate. A more rapid self-start can be achieved by increasing the initial current density, which is mainly limited by the initial water content in the MEA. Hence, keeping the MEA mildly hydrated before the cold start can be favorable to a rapid start-up of a PEMFC using current ramping. This strategy is particularly effective for the rapid start-up of next-generation PEMFCs with reduced thermal mass. A PEMFC with a thermal mass of 0.2 J/ cm² K and a relatively wet MEA can be successfully started from -30°C in about 5 s by applying an initial current density of as high as 1 A/ cm².