Presolvation Dynamics Preceding the Hydrated Proton Transfer in the Electrical Double Layer

Hydrated proton transfer (PT) mechanisms in a solid-liquid electric double layer (EDL) remain challenging, as the solvation structure is influenced by the operating potential. Under constant potential framework, we combined the fixed-potential method with ab initio molecular dynamics (AIMD) to simulate the PT in the EDL. Near the interface of EDL, negative potential (vs zero-charge potential) promotes the effective diffusion of the excess proton via reducing the proportion of trapping and revisiting processes of PT, but stiffens the hydrogen bond network manifesting as the restriction of water orientation and the elongation of the O···O pairs. At high negative potentials, hydrated hydrogen ions tend to form the Eigen cation (H₂O)₃H₃O⁺ during presolvation processes, but this cation exhibits a higher energy barrier for PT than the pentamer (H₂O)₄H₃O⁺ with a 4-fold coordinated shell. Further simulations reveal that the stiffening effect and the Eigen cation formation suppress proton conductivity near the electrode surface.