Trivalent ion overcharging on electrified graphene

Amanda J Carr; Sang Soo Lee; Ahmet Uysal

doi:10.1088/1361-648X/ac4a58

Trivalent ion overcharging on electrified graphene

J Phys Condens Matter. 2022 Jan 28;34(14). doi: 10.1088/1361-648X/ac4a58.

Authors

Amanda J Carr¹, Sang Soo Lee¹, Ahmet Uysal¹

Affiliation

¹ Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, IL 60439, United States of America.

PMID: 35016162
DOI: 10.1088/1361-648X/ac4a58

Abstract

The structure of the electrical double layer (EDL) formed near graphene in aqueous environments strongly impacts its performance for a plethora of applications, including capacitive deionization. In particular, adsorption and organization of multivalent counterions near the graphene interface can promote nonclassical behaviors of EDL including overcharging followed by co-ion adsorption. In this paper, we characterize the EDL formed near an electrified graphene interface in dilute aqueous YCl₃solution usingin situhigh resolution x-ray reflectivity (also known as crystal truncation rod) and resonant anomalous x-ray reflectivity (RAXR). These interface-specific techniques reveal the electron density profiles with molecular-scale resolution. We find that yttrium ions (Y³⁺) readily adsorb to the negatively charged graphene surface to form an extended ion profile. This ion distribution resembles a classical diffuse layer but with a significantly high ion coverage, i.e., 1 Y³⁺per 11.4 ± 1.6 Å², compared to the value calculated from the capacitance measured by cyclic voltammetry (1 Y³⁺per ∼240 Å²). Such overcharging can be explained by co-adsorption of chloride that effectively screens the excess positive charge. The adsorbed Y³⁺profile also shows a molecular-scale gap (⩾5 Å) from the top graphene surfaces, which is attributed to the presence of intervening water molecules between the adsorbents and adsorbates as well as the lack of inner-sphere surface complexation on chemically inert graphene. We also demonstrate controlled adsorption by varying the applied potential and reveal consistent Y³⁺ion position with respect to the surface and increasing cation coverage with increasing the magnitude of the negative potential. This is the first experimental description of a model graphene-aqueous system with controlled potential and provides important insights into the application of graphene-based systems for enhanced and selective ion separations.

Keywords: CTR; RAXR; adsorption; graphene; interface; overcharging; x-ray reflectivity.