Excess Proton Diffusion in Bulk and Interfacial Water: Profound Anomalies Beneath Surface Normality

Abstract Guiding the diffusion of excess protons in liquid water is a significant challenge in fields ranging from electrocatalysis to energy conversion in biological systems. Interfaces have been proposed to provide kinetically favored pathways for channeling protons between their source and sink. However, the mechanism and kinetics of proton diffusion at aqueous interfaces are far less understood compared to the bulk. Here, we use density functional theory-based deep potential molecular dynamics simulations to reveal that the interface exerts opposing effects on the diffusion coefficients of water and excess protons. We show that unlike their anomalously fast diffusion in the liquid, protons at the interface diffuse at the same rate as water. At the air/water interface, the excess proton remains localized on one water molecule at a time, but with a different surrounding hydrogen-bond arrangement than in the bulk. This facilitates fast back-and-forth proton rattling within a pair of water molecules but causes the slowdown of the proton hops which govern proton diffusion. We explain why the reduced hydrogen-bond coordination of interfacial water enhances water diffusion while hindering the proton transport rate-limiting formation of a hydrogen bond on the proton donor. Finally, we show how bulk-enhanced surface diffusion of excess protons exhibits different limiting regimes with important implications for aerosol chemistry, electrocatalysis and biochemistry.