Start-Up Transient Comparison of Constant-Power and Constant-Resistance Loading in an Air-Cooled PEMFC Stack with Ultra-Thin Vapor Chamber Thermal Management

The start-up of air-cooled proton exchange membrane fuel cell (PEMFC) stacks involves strong electrical and thermal transients that can affect operational stability and durability. This study experimentally compares two start-up loading strategies - constant power and constant resistance - for an air-cooled PEMFC stack integrated with ultra-thin vapor chambers (UTVCs) as an auxiliary thermal management component. A dedicated test platform was built to measure stack voltage/current dynamics, cell-level temperature evolution, and temperature uniformity using multi-point thermocouple instrumentation. Experiments were performed under constant power loads of 70-130 W and constant resistances of 0.1-0.5 Ω under controlled ambient conditions. Both strategies produce voltage undershoot during start-up; constant power loading induces a coupled response with voltage undershoot and current overshoot whose magnitude and settling time increase with power, whereas constant resistance loading produces voltage undershoot without current overshoot but can lead to excessive steady-state current under low resistance. Thermal measurements show that UTVC-assisted cooling improves heat dissipation and limits intra-cell temperature differences to approximately 2.5-2.9 °C in the tested cases, while constant resistance loading at high current levels yields higher steady-state temperatures and periodic uniformity fluctuations. The results provide practical guidance for selecting start-up loading strategies and demonstrate the effectiveness of UTVC-assisted thermal management for improving the start-up behavior of air-cooled PEMFC stacks.