Orientation-Dependent Charge Transfer at the Degraded Nafion/Pt(111) Interface: Insights from First-Principles Calculations

Abstract

Durability of proton exchange membrane fuel cells is limited by degradation at the Nafion–platinum interface, yet the atomistic role of degraded Nafion species remains insufficiently understood. Although most first-principles studies have focused on pristine Nafion, the electronic structure and interfacial behavior of degraded Nafion at Pt contacts remain largely unexplored. Here, we use density functional theory with Hubbard corrections (DFT+U) to investigate three representative configurations of a degraded Nafion monomer adsorbed on hydrated Pt(111), differing in orientation and initial separation. Interfacial properties were analyzed using charge-density differences, local density of states, Bader charge analysis, interaction energies, and climbing-image nudged elastic band calculations. When the sulfonic group was oriented toward Pt at an initial separation of 2.8 Å, relaxation led to interfacial OH formation, pronounced charge redistribution, Pt–O/Pt–OH bonding, and local Pt surface relaxation, indicating enhanced reactivity. This chemically reactive configuration showed an interaction energy of approximately –2.8 eV. Increasing the initial separation to 4 Å produced the most stable interface, with an interaction energy of approximately –4.4 eV, while suppressing surface distortion and favoring proton redistribution through hydronium formation. Representative CI-NEB calculations showed that proton transfer between the Nafion sulfonic/sulfonate group and interfacial water molecules depends strongly on local water–Nafion distance and molecular orientation, with pathways ranging from nearly spontaneous proton relocation to activated transfer barriers up to approximately 0.90 eV. In contrast, orienting the hydrophobic CF3 end toward Pt gave the weakest interaction, approximately –1.5 eV, and negligible charge redistribution. These results show that degraded Nafion/Pt(111) stability and functionality are controlled by interfacial orientation, hydration-mediated interactions, charge redistribution, structural rearrangement, and proton-transfer energetics, providing molecular insight for designing more durable proton exchange membrane fuel cells.