Distinction and relationship between local and pore-scale saturation in fuel cell gas diffusion layers clarified via high-speed operando tomography

In this work, we achieved fast 18s tomographic scans of an operating polymer electrolyte membrane (PEM) fuel cell at high resolution (voxel edge length of 2.44 μm) and achieved commercially relevant current and power densities (up to 2.0 A cm −2 and 0.45 W cm −2 , respectively). We define and quantify pore-scale saturation (for the first time) in fuel cell gas diffusion layers (GDLs) at statistically representative scales. Partial filling of pores was observed, and pores were more saturated closer to the catalyst layer (CL)-GDL interface (43.7% saturated on average) than at the flow field (FF)-GDL interface (27.9% saturated on average). Additionally, we found that pore-scale saturation increased with increasing local saturation in the GDL. The oftentimes default assumption of fully saturated pores in numerical models could lead to significant underestimation of GDL permeability. A GDL with partially filled pores has a 26.4% higher through-plane permeability and 50.3% higher in-plane permeability of air compared to a GDL with fully saturated pores. • 18s tomographic scans of an operando fuel cell at high resolution 2.44 μm/pixel. • Liquid water formation and accumulation in the gas diffusion layers. • Partial filling of pores, with higher saturation closer to the catalyst layer interface. • Pore saturation increased with increasing bulk saturation in the GDL. • Assuming fully saturated pores could lead to underestimations of GDL permeability.