Molecular dynamics simulation study on the effect of perfluorosulfonic acid side chains on oxygen permeation in hydrated ionomers of PEMFCs.

We prepared two types of perfluorosulfonic acid (PFSA) ionomers with Aquivion (short side chain) and Nafion (long side chain) on a Pt surface and varied their water contents (2.92 ≤ λ ≤ 13.83) to calculate the solubility and permeability of O in hydrated PFSA ionomers on a Pt surface using full atomistic molecular dynamics (MD) simulations. The solubility and permeability of O molecules in hydrated Nafion ionomers were greater than those of O molecules in hydrated Aquivion ionomers at the same water content, indicating that the permeation of O molecules in the ionomers is affected not only by the diffusion coefficient of O but also by the solubility of O. Notably, O molecules are more densely distributed in regions where water and hydronium ions have a lower density in hydrated Pt/PFSA ionomers. Radial distribution function (RDF) analysis was performed to investigate where O molecules preferentially dissolve in PFSA ionomers on a Pt surface. The results showed that O molecules preferentially dissolved between hydrophilic and hydrophobic regions in a hydrated ionomer. The RDF analysis was performed to provide details of the O location in hydrated PFSA ionomers on a Pt surface to evaluate the influence of O solubility in ionomers with side chains of different lengths. The coordination number of C(center)-O(O) and O(side chain)-O(O) pairs in hydrated Nafion ionomers was higher than that of the same pairs in hydrated Aquivion ionomers with the same water content. Our investigation provides detailed information about the properties of O molecules in different PFSA ionomers on a Pt surface and with various water contents, potentially enabling the design of better-performing PFSA ionomers for use in polymer electrolyte membrane fuel cells.