Reactive Molecular Dynamics Simulations of Proton Transport in PFSA Membranes: Hydration–Temperature Mechanism Crossover and Cold-Start Implications

Proton transport through perfluorosulfonic acid (PFSA) membranes sets the ohmic resistance of proton exchange membrane fuel cells and is most constrained during cold start, when reduced hydration coincides with sub-ambient temperatures, yet the conditions under which the Grotthuss and vehicle mechanisms dominate remain unresolved. Here, ReaxFF reactive molecular dynamics with explicit proton-transfer event statistics characterizes this crossover across 48 trajectories spanning λ = 6 and λ = 14 at 230–300 K. At λ = 6, every trajectory shows zero Grotthuss hops at all temperatures and no hydronium-like motif; the acidic proton stays bound to its parent sulfonate, so the nonzero tracer diffusivity reflects localized motion rather than vehicle transport, and the apparent activation energy of 0.09 eV serves as an order-of-magnitude anchor for continuum cold-start models. Because λ = 6 remains hop-free even at 300 K, rehydration rather than heating is required to restore structural diffusion. At λ = 14, nine of twenty-four trajectories sustain structural diffusion (2.1–3.1 hops/ps) while the rest stay kinetically locked; velocity-reseed controls show this identity is configuration-determined. Individual hops displace the excess charge by only ≈0.4 Å, so net transport relies on the minority of cage-escaping hops, implying Haven-ratio-type corrections of up to ≈10× on the tracer-based conductivities. Two distinct mechanistic regimes are thus resolved, separated by a crossover in 6 < λ ≤ 14; density-related uncertainty chiefly bounds the quantitative rates at 230 K.