Understanding the liquid water transport behaviour in a polymer electrolyte membrane (PEM) fuel cell is key to achieving high performance. The gas diffusion layer (GDL) provides transport pathways for electrons, reactant gases, and product water. However, when liquid water accumulates within the pores of the GDL, the effective porosity of the material is decreased, resulting in a reduced effective diffusivity of reactant gases. This can lead to reactant starvation at the catalyst during high current density operation, causing significant efficiency losses. The aim of this thesis is to investigate the impact of the GDL microstructural properties on water transport in an operating fuel cell. Synchrotron X-ray radiography was used as a probing tool for identifying the water distribution in the GDL in situ.

The role of the microporous layer (MPL) on liquid water management was identified through investigating in-plane water distribution in the GDL. Nanopores of the MPL reduced the amount and the size of water clusters in the GDL. Next, the influence of MPL thickness on the water accumulation in the GDL was investigated. The application of a thick MPL on the carbon fibre substrate resulted in less water content at the MPL-substrate interface than that in the GDL with a thin MPL. The effect of substrate thickness was also studied in terms of the fuel cell performance and liquid water distribution. The fuel cell incorporating a thin substrate exhibited superior electrochemical performance;​ however, unusual material movements were observed in the synchrotron images. It was concluded that the thin substrate was not able to provide necessary mechanical support to prevent material movements (e.g. membrane swell). Lastly, performance benefit of an MPL based on multiwall carbon nanotubes (MWCNTs) was studied. The MWCNTs additive to the MPL resulted in a high-porosity GDL compared to the conventional MPL. The high porosity region was prone to liquid water accumulation, but it also facilitated improved oxygen transport. The main findings of this thesis provide insights to fuel cell manufactures for design next generation GDLs.