Atomic Scale Characterization of the Voltage-Dependent Electrical Double Layer Structure*

The adsorption of ions at charged surfaces into an electrical double layer (EDL) is a driving factor in many phenomena including energy storage, nanoparticle stabilization, and biomolecule transport. The ability to control these processes relies on a detailed understanding of the atomic-scale interactions at play. Several EDL models with different assumptions have been proposed, including the diffuse ion cloud of Gouy-Chapman (GC) theory and complex ion-ion correlation phenomena. However, significant debate persists over the accuracy and limitations of these theories. Direct experimental probes of the structures at different surface charges can help to elucidate many open questions. Here, we use in situ resonant anomalous X-ray reflectivity (RAXR) to study the EDL structure of Rb⁺ at a graphene/SiC electrode in aqueous 0.1 M RbCl as a function of applied potential. The results are compared to molecular dynamics (MD) predictions. Differences in RAXR and MD results are evaluated in the context of GC and ion-ion correlation theories with an eye on the possibility of overcharging at the most negative potentials.