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Proton exchange membrane fuel cells (PEMFCs) have garnered considerable attention for transportation applications owing to their high energy efficiency. Understanding their long-term durability is essential because the performance deteriorates over time. The water transport characteristics of the gas diffusion layer (GDL), aged by inserting hydrogen peroxide solutions, are investigated. The dynamics of the water meniscus inside the GDL is visualized via synchrotron phase-contrast radiography, and the temporal variations in the pressure are measured simultaneously. The pressure and time at breakthrough (BT) when the water firstly emerged from GDL were compared. The degraded GDL exhibits a larger BT pressure and requires a longer time to achieve the first water BT than the pristine GDL. Unlike the pristine GDL showing snapoff patterns, water continuously penetrates the degraded GDL representing the piston-like movement, and repetitive increases and decreases in the pressure are not observed. This difference represents the dominant transport mechanisms. GDL degradation induces the loss of polytetrafluoroethylene (PTFE), which is generally used for the effective transport of fuel and water. The PTFE loss reduces the hydrophobicity and pore size, which can increase the actual path length of the water flow. The increase in the BT time and BT pressure, as well as continuous transport, can disrupt fuel supply to chemical reaction sites, thereby deteriorating the PEMFC performance.

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