Molecular-scale experimental characterizations of multivalent ion distributions near electrified graphene

Ion and water organization near charged surfaces is fundamentally interesting and relevant to separation processes such as capacitive deionization. The Poisson-Boltzmann equation can describe the distribution of an ideal, monovalent ion near charged surfaces including the structure of the electrical double layer. However, modeling multivalent ion behavior near electrodes is more complicated and directly relevant to many aqueous systems, including those demonstrating overcharging. In these instances, information about both the cation and anion distributions specifically near the interface are critical. We utilize high resolution x-ray reflectivity and elementally-sensitive resonant anomalous x-ray reflectivity to measure the in situ water and metal ion organization near graphene electrodes. Graphene is promising electrode material because it is atomically smooth and has an ideal surface lacking specific functional groups. We reveal ion distributions with molecular-scale resolution and demonstrate significant trivalent cation overcharging in dilute aqueous systems. Additionally, we consider the effects of anion choice and characterize halide organization likely in response to cation overcharging.