Probing far-from-equilibrium dynamics of electrical double layers.

Electrified solid-liquid interfaces are central to energy and matter conversion in biological1 and electrochemical systems2-4, in which intense local electric fields govern reaction kinetics5-9. Yet, under realistic electrocatalytic conditions involving rapid charge transfer and far-from-equilibrium dynamics, the molecular structure and evolution of the electrical double layer (EDL) remain poorly understood. Classical EDL models, derived under equilibrium and non-reactive conditions, cannot capture the interfacial processes emerging at reactive interfaces10-16. Here we develop an integrated experimental-computational framework to directly resolve EDL dynamics under the hydrogen evolution reaction (HER). Chemically stable nanostructured Pt film electrodes enable high-sensitivity, time-resolved surface-enhanced infrared absorption spectroscopy (SEIRAS) at increased overpotentials, whereas machine-learning molecular dynamics (MLMD) captures interfacial charge fluctuations and solvent dynamics over nanosecond timescales. This combined approach reveals a nonlinear, two-phase evolution of the inner layer that intensifies the local electric field. Time-resolved spectra further uncover irreversible restructuring of interfacial water during cyclic potential modulation. These findings show that ions and interfacial water respond asynchronously under the condition far from equilibrium, establishing a quantitative molecular framework for understanding electrostatic potential variations, interfacial electrostriction of ions17-19, electrolyte effects20-24 and rational electrolyte design for energy conversion technologies.