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The thickness of the microporous layer (MPL) in polymer electrolyte membrane (PEM) fuel cells with gas diffusion layers (GDLs) has a dominating impact on oxygen transport resistance, particularly compared to the thickness of the GDL substrate. Specifically, GDLs with thinner MPLs showed superior oxygen transport resistance despite possessing relatively restrictive GDL substrates. Concurrent operando imaging revealed inconsequential liquid water accumulation at the catalyst layer interface and within the GDL substrate despite the high relative humidity conditions. The highly beneficial absence of liquid water that typically lowers oxygen transport resistance, enabled a closer examination of how GDL morphology impacts the oxygen transport resistance during operation. • Operando imaging reveals low liquid water content in GDL despite high RH. • MPL dominates oxygen transport resistance compared to GDL substrate. • MPL thickness has strong influence on oxygen transport resistance.
Parekh et al. (2025) studied this question.