Polymer electrolyte fuel cells (PEFCs) are expected as power sources because of their low emissions and high efficiencies. However, it is required to improve the power density at lower Pt loading to develop high power systems at a lower cost. It is known that the losses of the power density are caused by activation losses, ohmic losses, and mass transport losses, and that the mass transport losses are dominant at high current density. Moreover, it is suggested that one of the causes of the mass transport losses is the shortage of oxygen in the cathode side of PEFC. In the cathodic catalyst layer, there are Pt catalysts on supported carbon microparticles, and those particles are covered with ionomer thin films. The film has two properties for the water-generating reactions in the cathodic catalyst layer: the proton conductivity and the oxygen permeability. These two properties depend on the film thickness and water content in the film, however, as one of the problems, the dependence of the oxygen permeability on water content is required to be clarified.

In our previous research, we analyzed the oxygen transport properties—i.e., solubility, diffusivity, and permeability—in an ionomer thin film on a Pt surface based on the solution-diffusion theory using molecular dynamics (MD) simulations. As a result, the oxygen permeability decreased with increasing water content. The local oxygen permeability in each region was obtained, and it was found that the oxygen permeability in the ionomer/Pt interface is the lowest for all water contents. Moreover, compared with the oxygen diffusivity, the oxygen solubility has a larger contribution to the water content dependence of oxygen permeability. Then, the oxygen permeability of the film was obtained from the local oxygen permeability in each region of the film; however, the total permeability was not in good agreement with that estimated using the oxygen flux and the pressure differential. It is suggested as the causes that the local equilibrium condition at each boundary of the region is not satisfied in the non-equilibrium system for the oxygen permeation through the film.

In this study, therefore, without an assumption of the local equilibrium condition, a new theory was constructed to describe the oxygen transport phenomena in the film using the chemical potential distribution of oxygen in the system. In this theory, it is assumed that the oxygen flux is generated by the spatial gradient of the chemical potential in the thickness direction of the film. Based on the theory, the oxygen transport resistances in the film are discussed using the oxygen mobilities obtained by MD simulations. Moreover, since oxygen pathways in the film are found to be important to estimate the chemical potential distribution of oxygen, the structure of the film is also analyzed. Then, the relationship between the film structure and the oxygen transport properties is investigated.