Balanced water management and mass transport with hierarchically structured gas diffusion layers in fuel cells

The key to improving water management and fuel cell performance is to enhance the efficiency of mass transport and establish a fast and direct way to remove the product water from the gas diffusion layer (GDL). In this study, three baseline GDLs with uniform, gradient, and segmented porosity are investigated with computational methods. We utilised a stochastic reconstruction process to create three different GDLs (segmented, gradient, uniform), enabling analysis of the impact of their micro-structures on various surface properties, gas diffusion, electrical conductivity, and two-phase flow phenomena. The simulation results demonstrate that compared to the uniform GDL, the gradient and segmented GDL exhibit optimal surface properties which positively impacts the contact resistance between the GDL and the catalyst layer and enhances the in-plane transport characteristics. However, their water management and transport properties in the through-plane direction require further optimisation. Additionally, the impact of bipolar plates on the behaviour of liquid water near the channels and lands is also taken into account. To further improve the water removal and reactants supply efficiency, patterned through-holes along the through-plane direction inside the GDL are proposed. Our analysis suggests that the GDL, featuring a porosity gradient with through-holes positioned exclusively beneath the flow channel, exhibits the most balanced interfacial contact, mass transport characteristics and effective water management. The findings of this study provide valuable insights for the design of GDLs to optimise the transport of gas, and electrons, as well as the liquid water distribution, thereby enhancing the performance and efficiency of fuel cell systems. It is important to note that the insights gained from this study are not limited to GDLs alone but can also be applied to various porous media, including porous transport layers.