Pore-network simulations of two-phase flow in a thin porous layer of mixed wettability: Application to water transport in gas diffusion layers of proton exchange membrane fuel cells

Pore network simulations are performed to study water transport in a model gas diffusion layer (GDL) of polymer electrolyte membrane fuel cells (PEMFCs) in relation with the change in hydrophobicity that might be due to aging or temperature effect. The change in hydrophobicity is taken into account by changing randomly the fraction of hydrophilic elements, pores or throats, in the network. The transport and equilibrium properties of the model GDL are computed as a function of liquid saturation as well as at breakthrough varying the fraction of hydrophilic elements. The results indicate that the hydrophilic element percolation threshold marks the transition between two domains. The system is found to be weakly dependent on the fraction of hydrophilic elements as long as this fraction is below the percolation threshold whereas an increase in wettability above the percolation threshold favours a greater blockage of the pore space by the water and therefore a diminished access of gas to the catalyst layer. This model may help assess the effect of a change in wettability on the fuel cell performance and may also help suggest better GDL designs in relation with the water management problem in PEMFCs.