Stripping away ion hydration shells in electrical double-layer formation: Water networks matter

Abstract

The double layer at the solid/electrolyte interface is a key concept in electrochemistry. Here, we present an experimental study combined with simulations, which provides a molecular picture of the double-layer formation under applied voltage. By THz spectroscopy we are able to follow the stripping away of the cation/anion hydration shells for an NaCl electrolyte at the Au surface when decreasing/increasing the bias potential. While Na+ is attracted toward the electrode at the smallest applied negative potentials, stripping of the Cl− hydration shell is observed only at higher potential values. These phenomena are directly measured by THz spectroscopy with ultrabright synchrotron light as a source and rationalized by accompanying molecular dynamics simulations and electronic-structure calculations.

Nature of the Electrical Double Layer on Suspended Graphene Electrodes

“…Such an asymmetric behavior was also observed on supported graphene electrode/pure water interfaces. 36 The nonlinear response of the water orientation to different gating potentials and ion species indicates again that the Stern−Gouy Chapman model does not describe properly the EDL at the microscopic scale 37 and that more advanced EDL models that include effects of solvent dipoles, 38,39 ion solvation structure, 40 ion finite size, 41 and nonelectrostatic forces between molecular species and electrode surfaces 37,42 should be used. Since the alignment of the water dipoles by the electric field is the result of competition between the torque on the water molecules by the field−dipole interaction and the hydrogen bonding network near the interface, the electric field E DC in eq 1, its dependence on distance to the interface, solute type, and ion species should be considered more carefully and needs correction when deducing the surface change density at the interface.…”

Highlight on H-Bond Interaction-Associated Multiple Ion Layer Formation of an Imidazolium-Based Ionic Liquid on a Potential-Bias Surface: Molecular Dynamics Simulations

“…Such analysis was used to characterize the EDL formation’s experimental spectra in the dilution of NaCl in the aqueous solution. In the solution, the H-bond network at the positively charged surface influences the dynamics of EDL formation …”

“…In the solution, the H-bond network at the positively charged surface influences the dynamics of EDL formation. 60 In this study, the PCA was performed using the R package. 61 PCA is a multivariate analysis for reducing the dimensionality of datasets into a few principal components (PCs).…”

Electrochemistry under confinement

“…122,123 By analogy to the solvated ions, the molecular dipoles can orient along the electric field, 124 leading to local changes in polarity and thus in free energy of solvation or adsorption. Especially the effect of the latter impede the predictability of the EDL structures under biased potentials as shown by Alfrano et al 125 Unusual alignment of single molecules and formations of ice-like networks from solvent molecules 126 under confinement might facilitate these deviations even further and will be introduced in the next section.…”