A slab-like pore model consisting of a thin water slab that is confined by a metal wall on one side and a dense ionomer skin layer on the other side was simulated with classical molecular dynamics. The model mimics thin-film structures of Pt/support (referred to as the metal), water layer, and ionomer phase encountered in cathode catalyst layers of polymer electrolyte fuel cells. The equilibrium proton density near the metal surface is the key variable required to predict the ORR activity and the Pt dissolution rate under fuel cell operating conditions. Here, we explored the equilibrium proton density distribution in the confined water layer as a function of oxide coverage at the metal surface, excess surface charge density, water layer thickness, and ionomer film structure. The effect of ionomer hydrophobicity on the water dynamics in the nanopore was also investigated. The electric dipole field created by the oxide layer on the Pt surface interacts strongly with protons, and concentrates protons at the metal surface. A direct relation was found between the surface proton concentration and the oxide layer dipole moment. Performing simulations with different ionomer structures and pore widths clarified the sensitivity of the proton density distribution to properties that are specific for the structure and distribution of the ionomer phase in catalyst layers. (C) 2015 Elsevier B.V. All rights reserved.