Porosity of the GDL directly impacts the flow of reactant gases and ensures uniform distribution to the CL and the removal of water from the fuel cell. In addition to porosity, the tortuosity and pore size distribution within the gas diffusion layer are also critical factors that influence the overall performance of the proton exchange membrane fuel cell. Geometric tortuosity of the GDL in a PEMFC plays a crucial role in its performance. Geometric tortuosity refers to the ratio of the actual diffusive path length to the geometric thickness of the porous medium. Hence, geometric tortuosity affects the transport of gases, liquids, and heat within the fuel cell system. It influences the permeability, which in turn impact the power density and efficiency of the fuel cell. Higher tortuosity can lead to longer diffusive path lengths, affecting the transport of reactants and products within the GDL. Furthermore, water management within the microstructure of the GDL is fundamental for facilitating the passage of water in both vapor and liquid states through its fibers. Therefore, determining the relative diffusivity of water vapor within the GDL microstructure is critical to mitigate flooding by modulating the fiber and binder concentrations, thereby influencing the porosity of the system. Integration of hydrophobic polytetrafluoroethylene aids in water repellence, thereby averting flooding; however, excessive drying, which compromises GDL performance and subsequently affects the performance of the PEMFC, must be avoided. The diffusivity characteristics of the GDL are paramount in influencing the distribution of temperature and water saturation within the proton exchange membrane, consequently impacting fuel cell performance and longevity. A comprehensive understanding of diffusivity is pivotal for optimizing GDL design and function, as it directly governs the interplay among gas, liquid, and thermal dynamics within the fuel cell system. In addition, the electrical conductivity of the GDL directly impacts the power density and efficiency of the fuel cell system. Higher in-plane resistivity of the GDL can have a negative impact on the current density in certain areas, while lower resistivity can lead to improved current density. Additionally, the local current density for the entire catalyst layer is affected by the electrical conductivity of the GDL. The thermal conductivity of the GDL in PEMFCs is intricately linked with water management and thermal regulation. As the electrochemical reactions within the membrane generate heat, effective management of thermal gradients across the single cell and fuel cell stack becomes imperative. This necessitates the provision of a pathway for heat dissipation from the MEA to the bipolar plates, ensuring optimal operating temperatures to prevent membrane degradation. Anisotropic thermal conductivity of the GDL significantly influences temperature distribution and water saturation within the proton exchange membrane, thereby impacting cell performance and durability. Notably, proper thermal management is crucial for sustaining fuel cell performance, as the temperature of the MEA profoundly influences overall efficiency. Furthermore, it is observed that in GDLs with notable anisotropic thermal conductivity, the maximum and minimum temperatures in the cathode catalyst layer are primarily contingent upon average current density rather than local current density. And ultimately, increased contact resistance between the GDL and the MPL causes great ohmic losses by acting as an electron bottleneck, translating to lower voltage output and overall efficiency. In addition, it also impedes efficient transfer of electrons between the catalyst layer and the current collector (through the GDL). This ultimately reduces the PEMFC's ability to generate electricity efficiently. Hence, a low contact resistance between the GDL and the MPL is crucial for optimal PEMFC function. There exists substantial prior research, encompassing experimental, computational, and combined experimental-computational studies that has investigated transport properties of GDLs. Extensive experimental investigations have significantly advanced our comprehension of mass transport, heat transfer and structural characteristics of GDLs, contributing valuable insights to our understanding of GDL performance. For example, Yoshimune et al. developed an experimental method to evaluate the through-plane effective diffusion coefficient in GDLs using an infrared-absorption carbon dioxide sensor. They measured the Through-plane (TP) effective gas diffusivity of a commercial GDL and found to be 0.36 ± 0.02 at 25 o C which they claimed to be consistent with the data presented in literature. James et al. used x-ray computed tomography reconstruction to investigate a commercially available GDL under non-homogeneous compression, claiming that nonuniform compression significantly impacts transport properties within the material, leading to a roughly twofold difference between the land and channel regions. In addition, Xia et al. investigated how GDL thickness and porosity in high-temperature PEMFCs affect electron conduction and gas distribution. By optimizing these parameters (anode: 80 -120 μm, cathode: 140 -170 μm; porosity: 35 -45 %), the researchers achieved a remarkable 7.7 % improvement in cell performance, underscoring the critical role of precise GDL design in maximizing fuel cell efficiency. Moreover, Pfrang et al. used X-ray computed tomography to investigate the thermal properties of several commercial GDLs without the MPL, concluding that the TP thermal conductivities are lower than the in-plane (IP) thermal conductivities. Furthermore, Chen et al. subjected a GDL to a series of freeze-thaw cycles to assess the impact on its IP electrical resistivity under low compression, effectively assessing the impact of thermal aging on electrical performance. Results indicated weak anisotropy and greatly impacted by the transformation of carbon fiber connections within the GDL's porous structure. Notably, the initial 100 freeze-thaw cycles appeared to have the most pronounced effect on the IP electrical degradation performance, highlighting the sensitivity of the GDL to thermal stress.