Cage Effect-Induced Intermittent Dynamics Govern Proton Conductivity in Nafion Proton Exchange Membrane

Understanding the microscopic dynamics of proton transport within proton exchange membranes (PEMs) is crucial for the design of high-performance proton exchange membrane fuel cells (PEMFCs). The transport dynamics of hydrated protons in Nafion membranes is governed by the Grotthuss and vehicle mechanisms, yet the variability in kinetic rates among protons following the same mechanism remains poorly understood. To address this, we employ molecular dynamics simulations to study electric-field-driven proton diffusion. We find that hydrated protons exhibit significant dynamic heterogeneity characterized by intermittent behavior, manifesting as either two-phase dynamics (alternating cage rattling and cage jumping) or an exclusive cage rattling motion. This intermittency correlates directly with diffusion rates, with cage jumps leading to a substantially higher mobility. The local chemical environment of the proton-coordinating cage dictates its mobility: water-rich, fluorine-poor cages facilitate jumps, while rigid fluorine-rich cages restrict motion. Cage jumps occur via cooperative motion with water molecules, primarily as the H₉O₄⁺ structural motif. Finally, we conclude that jump probability is synergistically regulated by the hydration level (λ) and electric field strength, with the dominant factor shifting according to conditions. At low hydration (λ = 3), water availability and its localized aggregation become the decisive limiting factors for cage jumps, while at high hydration levels (λ = 9, 15, and 20), the external electric field strength emerges as the primary determinant. The observed intermittent dynamics, while identified within the vehicle mechanism, likely reflect a universal feature of proton diffusion in Nafion as analogous back-and-forth motions have also been reported in Grotthuss-type transport (Choe. Phys. Chem. Chem. Phys. 2009, 11, 3892-3899). Our results directly link intermittent dynamics to macroscopic conductivity, advancing the mechanistic understanding of proton transport. Moreover, since this intermittency stems from the cage effect─a consequence of the ubiquitous local coordination environment in membranes, the insights offered here provide broadly applicable guidance for designing high-performance PEMs from microstructural and dynamic perspectives.