Atomic-scale insights into how mechanical stress affects oxygen transport performances in fuel cells

Local mechanical stress in the catalyst layer of proton exchange membrane fuel cells (PEMFCs) can alter the ionomer and water thin film structures and thus affect their oxygen transport performances. In this work, all-atom molecular dynamics simulations are employed to investigate how the applied compressive stress affects the local oxygen transport resistance to the catalyst surface. The results show that the evolution of ionomer film structures exhibits a non-monotonic behavior with a critical transition around 2 MPa. Below this threshold, a more connected water network facilitates continuous oxygen transport pathways and lowers the diffusion resistance. When the applied compressive stress exceeds this value, the ionomer chains gradually coil and disrupt local water clusters, breaking them into smaller fragments. These structural changes collectively restrict oxygen transport and increase the diffusion resistance. Nevertheless, the oxygen adsorption resistance increases monotonically due to progressively reduced Interfacial permeability. For the water film on catalysts, the increase in mechanical stress leads to a larger oxygen transport resistance, and a second characteristic transition appears around 7.5 MPa, beyond which a pronounced confinement effect emerges due to strong compression of the water layers.