Mechanistic Insights into Transport Resistance Redistribution Induced by Through-Plane Porosity Gradients in Cathode GDLs of PEM Fuel Cells

The gas diffusion layer (GDL) is a representative porous medium governing coupled heat and mass transfer in proton exchange membrane fuel cells (PEMFCs), where the through-plane porosity distribution critically affects reactant transport, water migration, and thermal behavior. To clarify the regulation mechanisms of porosity gradients on multi-physical transport, a three-dimensional PEMFC model is established. Uniform, exponential-gradient, and linear-gradient cathode GDL structures are systematically compared in terms of mass transport, heat transfer and electrochemical coupling characteristics. The results reveal that through-plane porosity gradients reconstruct the spatial distribution of effective diffusion resistance, eventually redistributing oxygen transport, water vapor removal, liquid water accumulation, and heat dissipation within the porous medium. Compared with the uniform GDL, the linear-gradient structure mitigates oxygen concentration decay near the CL, suppresses local water accumulation, and reduces the through-plane temperature gradient, resulting in more uniform heat–mass coupling. In contrast, exponential gradients induce locally intensified resistance due to abrupt porosity variations, leading to constrained transport regulation. Unlike previous studies focusing mainly on porosity size or averaged performance, this work demonstrates that the continuity and smoothness of through-plane porosity gradients govern the redistribution of transport resistance and reaction intensity. These findings provide mechanistic guidance for designing gradient structured porous media in coupled heat and mass transfer systems.