Freeze start strategies for fuel cell electric vehicles: balancing durability, reliability and practicability – an experimental approach on automotive-sized short stacks

Fuel cells are considered a key technology for the energy transition, as hydrogen can serve as a sustainable energy carrier when produced from renewable sources. However, water generated during operation can freeze under sub-zero conditions, leading to start-up failure and accelerated degradation. Reliable freeze-start capability is therefore a critical requirement for fuel cell electric vehicles (FCEVs). In this study, the different phases of a freeze-start strategy, including freeze preparation, stack cooling, and freeze start with internal heat-up, were experimentally investigated on an automotive-sized 10-cell short stack equipped with an adapted cooling circuit to reproduce vehicle-relevant thermal boundary conditions. The influence of freeze-preparation parameters and freeze-start operating conditions was analyzed. The results show that the extent of membrane drying is the dominant factor determining freeze-start capability. Increasing the high-frequency resistance (HFR) threshold after drying from 0.20 to 0.25 to 0.60 Ω·cm²/cell improved the minimum achievable start temperature from approximately − 10 °C to − 18 °C without negative cell voltages. Furthermore, lower stack voltages during potentiostatic heat-up increased the heating rate from 0.26 to 0.32 K s⁻¹ while simultaneously reducing the total water production from 10.3 g to 8.8 g. Variations in cathode airflow had only a minor influence on freeze-start performance. Overall, the study demonstrates that combining an aggressive but controlled freeze preparation with low-voltage potentiostatic heat-up provides the most robust and efficient freeze-start strategy for automotive PEM fuel cell stacks under severe winter conditions.