Effects of interfacial confinement induced heterogeneous reconstruction of ionomer films on oxygen transport.

In the cathode catalyst layer of proton exchange membrane fuel cells (PEMFCs), oxygen transport through ionomer films strongly affects performance at high current densities. In this work, all atom molecular dynamics simulations were employed to investigate the effects of interfacial confinement on nanoscale ionomer film structure and oxygen transport. The results show that oxygen diffusion coefficient of ionomer films is reduced due to the interfacial confinement effects, and gradually increases with film thickness. The interaction of ionomer film with platinum/carbon interface induces pronounced heterogeneous ionomer structures along the film thickness direction, characterized by a dense interfacial region and a gradually relaxed region with increased water connectivity and extended Nafion chain conformations away from interfaces. Besides, the anisotropic oxygen transport behaviors of ionomer films would be more significant under the interfacial confinement, with its diffusion coefficient much higher in the direction parallel to the platinum/carbon interface. Furthermore, introducing hydrophilic functional groups on carbon surfaces would enhance interfacial water enrichment and further strengthen structure heterogeneity within the ionomer film. These findings demonstrate that oxygen transport in ionomer films is jointly regulated by film thickness, water content, and interfacial chemical environment.