Permeability Measurement and Water Transport Visualization in PEMFC Gas Diffusion Layers: A Review

ABSTRACT The performance of proton exchange membrane fuel cells (PEMFCs) is significantly influenced by the mass transport behavior of the gas diffusion layer (GDL), which plays a key role in reactant delivery and water removal. This review systematically explores current methods for characterizing GDL gas and liquid permeability and for visualizing water transport using in situ and ex situ techniques. A comparative analysis of different water transport visualization methods, including direct optical imaging, fluorescence microscopy, environmental scanning electron microscopy, neutron radiography, and X‐ray computed tomography, is presented, focusing on their spatial resolution, real‐time imaging capabilities, and potential for quantitative two‐phase flow analysis. Rather than treating these methods as interchangeable diagnostics, this review distinguishes the transport quantities that each method can constrain, including permeability anisotropy, saturation‐dependent transport, breakthrough pathways, water‐cluster connectivity, and the scale gap between pore‐level observation and cell‐level operation. Recent progress in operando neutron imaging, time‐resolved X‐ray tomography, micro‐CT‐informed simulations, machine‐learning‐assisted image analysis, and device‐compatible electrothermal or embedded‐sensor diagnostics indicate that GDL characterization is moving from qualitative visualization toward quantitative, model‐constrained water management design. This review is intended to support the rational design of GDL materials and improved water management strategies in PEMFC systems.