In this study, hydrated Nafion film in the catalyst layer of the cathode for a polymer electrolyte membrane fuel cell is investigated using the molecular dynamics simulation method, exhibiting different structural characteristics on Pt and carbon surfaces. First, it is found that water molecules, hydronium ions, and sulfonate groups are highly concentrated at the interfacial region between the Nafion phase and the Pt surface, whereas Nafion backbone chains are present in a high concentration at the interface between the Nafion phase and the carbon surface. Second, it is also found from pair correlation function analysis that the water molecules and sulfonate groups in the hydrated Nafion phase are more associated with the Pt surface compared to the carbon surface, which is due to their strong attractive interactions with the Pt surface that makes the dimension of the hydrated Nafion phase 4–7% thinner on the Pt surface. Third, it is observed from water-occupied volume analysis that water molecules on the carbon surface can form large-size water phase between the Nafion phase and the carbon surface because the Nafion–carbon interface is not tightly integrated due to their weak interaction. In these structural characteristics, it is demonstrated that the water diffusion and proton vehicular diffusion are suppressed in the interfacial region of the Pt surface due to the highly packed structures in the water phase as well as the polymer phase in addition to the strong molecular interaction with the Pt surface, whereas the proton hopping diffusion is enhanced due to the well-developed organized water phase via the hydrogen bonding network.

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Proton exchange membranes with high through-plane proton conductivity are a critical component of high-performance fuel cells, electrolyzers, and batteries. However, isotropically distributed proton-conducting channel structures of current membranes present a limitation. Herein, a proton exchange membrane with straight proton-conducting channels aligned in the thickness direction is fabricated, achieved by magnetic field-induced alignment of proton-conductive, paramagnetic, and one-dimensional (1D) tungsten disulfide nanotubes (pms-WS2) distributed in a perfluorinated sulfonic acid (Nafion) membrane. The pms-WS2 nanotubes feature straight WS2 nanotubes as a core, a polystyrenesulfonate (PSS) skin layer, and surface-decorated Fe3O4 nanoparticles. A molecular dynamics simulation suggests that straight proton-conducting channels are constructed at the interface of Nafion/pms-WS2 due to densely populated sulfonic acids. Spectroscopic investigation and magnetization measurements verify the through-plane alignment of pms-WS2 under a weak through-plane magnetic field (0.035 T) during the removal of solvent from the membrane cast. Compared with a recast Nafion membrane with the same thickness, the through-plane aligned composite membrane exhibits 69% higher proton conductivity and 51% higher power performance in a proton exchange membrane fuel cell, demonstrating its efficacy. The through-plane alignment of a proton-conductive inorganic 1D material promises improved power performance of advanced electrochemical devices.

In this study, molecular dynamics simulations were used to understand the effects of the surface properties of the catalyst on ionomer film morphology by switching the nature of the catalyst surface from hydrophilic to hydrophobic. Equilibrium structures and radial distribution functions reveal that the surface properties affect the ionomer film morphology and the dispersion solvent on the catalyst surface. For a Pt surface, water molecules and the sulfonate groups of the ionomer are mainly situated in close proximity to the Pt surface, which is due to the hydrophilicity of water and the sulfonate groups that interact strongly with the hydrophilic Pt surface. In the case of the alkylthiol-modified Pt surface, dipropylene−glycol molecules and the backbones of the Nafion ionomers are distributed in the immediate proximity of the alkylthiol-modified Pt surface owing to favorable hydrophobic−hydrophobic interactions. In addition, the accompanying transport properties of water molecules also depend on the nature of the catalyst surface. The calculated diffusion coefficients show that water molecules diffuse faster on the alkylthiol-modified Pt surface than on the pure Pt surface and that the diffusion coefficient gradually decreases as the amount of the dipropylene−glycol/water solvent used to disperse the Nafion solution is reduced.

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 O2 in hydrated PFSA ionomers on a Pt surface using full atomistic molecular dynamics (MD) simulations. The solubility and permeability of O2 molecules in hydrated Nafion ionomers were greater than those of O2 molecules in hydrated Aquivion ionomers at the same water content, indicating that the permeation of O2 molecules in the ionomers is affected not only by the diffusion coefficient of O2 but also by the solubility of O2. Notably, O2 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 O2 molecules preferentially dissolve in PFSA ionomers on a Pt surface. The results showed that O2 molecules preferentially dissolved between hydrophilic and hydrophobic regions in a hydrated ionomer. The RDF analysis was performed to provide details of the O2 location in hydrated PFSA ionomers on a Pt surface to evaluate the influence of O2 solubility in ionomers with side chains of different lengths. The coordination number of C(center)–O(O2) and O(side chain)–O(O2) 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 O2 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.