Hydration-dependent transport mechanisms of Ce3+ ions in Nafion membranes revealed by molecular dynamics simulations

Proton exchange membrane fuel cells are promising clean energy devices, but their chemical durability remains a major barrier to their practical adoption. Ce ions are incorporated into membranes as radical scavengers to mitigate degradation. However, their nonuniform distribution and leakage during operation still lead to membrane deterioration. To address this issue we investigated the transport mechanisms of Ce ions using molecular dynamics simulations. We found Ce ions diffuse by hopping between the cages formed by sulfonate groups, and diffusivity depends on both local structural changes and the connectivity of water clusters in Nafion membrane. When the ratio of water molecules to sulfonate groups, hereafter referred to as the water content (λ), is 3, Ce ions are coordinated to sulfonate groups. However, as hydration increases, the ions gradually dissociate from sulfonate groups and become fully dissociated at λ = 7. A percolation threshold emerges between λ = 7 and 9, enabling long-range ion migration through water channels. Pore size analysis revealed Ce ions migrate only through regions with pore diameters ≥ 6 Å, suggesting controlling pores smaller than 6 Å could effectively suppress ion leakage. These insights provide key design principles for developing durable membranes capable of retaining Ce ions.