Water dynamics and sum-frequency generation spectra at electrode/aqueous electrolyte interfaces

The dynamics of water at interfaces between an electrode and an electrolyte is essential for the transport of redox species and for the kinetics of charge transfer reactions next to the electrode. However, while the effects of electrode potential and ion concentration on the electric double layer structure have been extensively studied, a comparable understanding of dynamical aspects is missing. Interfacial water dynamics presents challenges since it is expected to result from the complex combination of water-water, water-electrode and water-ion interactions. Here we perform molecular dynamics simulations of aqueous NaCl solutions at the interface with graphene electrodes, and examine the impact of both ion concentration and electrode potential on interfacial water reorientational dynamics. We show that for all salt concentrations water dynamics exhibits strongly asymmetric behavior: it slows down at increasingly positively charged electrodes but it accelerates at increasingly negatively charged electrodes. At negative potentials water dynamics is determined mostly by the electrode potential value, but in contrast at positive potentials it is governed both by ion-water and electrode-water interactions. We show how these strikingly different behaviors are determined by the interfacial hydrogen-bond network structure and by the ions' surface affinity. Finally, we indicate how the structural rearrangements impacting water dynamics can be probed via vibrational sum-frequency generation spectroscopy. We perform molecular dynamics simulations of aqueous NaCl solutions at the interface with graphene electrodes, and examine the impact of both ion concentration and electrode potential on interfacial water reorientational dynamics.

Automated summary

The dynamics of water at interfaces between an electrode and an electrolyte play a crucial role in the transportation of redox species and the kinetics of charge transfer reactions. In this study, molecular dynamics simulations were used to investigate the water dynamics at the interface between aqueous NaCl solutions and graphene electrodes, and the effects of ion concentration and electrode potential were examined. The results showed that water dynamics exhibited asymmetric behavior, slowing down at positively charged electrodes and accelerating at negatively charged electrodes. The behavior was mainly determined by the electrode potential at negative potentials, while it was influenced by both ion-water and electrode-water interactions at positive potentials.