Diagnostic Method for Structural Degradation of Porous Catalyst Layers in PEMFCs Using Low-Frequency Impedance and Undershoot Variations in Current Steps

Abstract

In this study, we developed an in situ electrochemical diagnostic method for polymer electrolyte membrane fuel cells (PEMFCs), utilizing voltage drop and specific frequency impedance responses observed during current steps. At lower current densities, efficient gas supply and water removal ensure stable cell operation. However, at higher current densities, increased load leads to excessive water generation, which can obstruct reactant gas inflow and hinder water removal, resulting in increased internal resistance. By leveraging these degradation mechanisms, we propose a diagnostic method to predict pore structures and accurately assess degradation in PEMFCs. Current steps ranging from 200 mA cm⁻² to 600 mA cm⁻² were employed to analyze voltage undershoot and impedance at 1 Hz. The voltage undershoot is linked to increased contact resistance caused by ionomer instability, indicating structural degradation in the carbon support. Meanwhile, the impedance at 1 Hz correlates with mass transport resistance, providing an indirect method for predicting pore structure. This method enables efficient diagnostics without requiring complex equivalent circuit fitting, and allows measurements to be conducted under stable operational conditions, avoiding changes in temperature, humidity, or gas composition, which prevents further degradation during the diagnostic process. This approach offers a straightforward and effective means for diagnosing degradation in PEMFCs, enhancing fuel cell performance and durability by enabling rapid identification of problematic components and timely maintenance.