Electric double layer structure in concentrated aqueous solution.

Toward tailored electrocatalysis, significant attention has been directed to the electrode-electrolyte interface. The electric double layer provides a crucial microenvironment for electrochemical reactions. However, its atomic-scale structure remains unresolved, particularly for non-dilute electrolyte concentrations relevant to practical systems. A notable example is the camel-to-bell shape transition in the capacitance curve, where two peaks merge as the concentration increases, which is still poorly understood at the molecular level. Herein, using all-atom simulations, we elucidate the electric double layer structures and their phase transitions which give rise to capacitance peaks. The predicted transition potentials match the experimental peak positions. We observe collective water reorientation in the cathodic region and anion surface condensation in the anodic region, which are further validated by in situ spectroscopy. Finally, we construct an electric double layer structural phase diagram to provide detailed insight into the electric double layer microenvironment. This work presents a valuable framework for design of improved interfaces.