Mapping the Atomistic Structure of the Electrical Double Layer with X-ray Reflectivity and Molecular Dynamics

A driving factor in electrochemical energy storage is the adsorption of ions in an electrical double layer (EDL) at the electrode-electrolyte interface. A complete understanding of the structure-property relationships among charged surfaces, ions, and solvents that give rise to the EDL is needed to optimize capacitive energy storage devices. Here, we present in situ X-ray reflectivity (XR) and resonant anomalous XR (RAXR) results of the atomistic, element-specific structure of the EDL at the interface of a graphene/SiC electrode and aqueous RbCl. RAXR data are analyzed using increasingly complex levels of theory of the EDL structure, starting from the simple Gouy-Chapman model of an exponentially decaying charge distribution, to assess the level of theory needed to describe the data. Classical MD simulations of the system qualitatively agree with the experimental structures. A quantitative analysis of the MD predictions is performed via direct comparison of the experimental data with RAXR signals expected from the predicted structures.