Quantitative decoupling of electro-thermal degradation in PEMFCs: H₂ crossover through a single Sub-10 μm pinhole under asymmetric pressure

Proton exchange membrane fuel cells suffer from membrane degradation induced by pinhole defects under asymmetric pressure. However, the quantitative influence of sub-10 μm pinholes, critical to understanding early-stage failure, is poorly understood. We present a novel mechanistic model of two-phase flow through a 3 μm pinhole, validated experimentally with a custom in-situ test bench and infrared thermography. The results quantitatively decouple the degradation mechanisms: (1) H₂ crossover shifts from diffusion- to convection-dominated when the anode-cathode pressure gradient exceeds 0.8 kPa, with a tenfold flux increase at 10 kPa. Sensitivity analysis identifies temperature as the dominant factor governing convective crossover. (2) Voltage loss comprises 50–100 mV from the pinhole itself and an additional 13.9–41.5 mV from asymmetric pressure, mainly due to increased oxygen reduction reaction activation loss. (3) Under symmetric pressure, the pinhole generates a negative current at the carbon paper surface at open circuit and a catalyst-layer hotspot 9.5 °C above operating temperature (105 °C). Although the negative current vanishes at 1500 mA cm⁻², local current drops by 70% and the hotspot temperature rises by 5 °C. (4) Asymmetric pressure does not significantly change local current but further raises the catalyst-layer hotspot by 28 °C. Yet these intense localized hotspots remain macroscopically undetectable, producing only a 1–2 °C rise on the carbon paper surface and evading conventional infrared detection. Thus, while a pinhole primarily degrades electrical performance, asymmetric pressure dominates thermal degradation by exacerbating H₂ convection crossover. By establishing quantitative performance relationships and a defect-sensitivity framework, this work provides predictive insights and practical guidelines for enhancing PEMFC durability.