Adsorption of Iodide and Bromide on Au(111) Electrodes from Aprotic Electrolytes: Role of the Solvent

Abstract

AbstractThe adsorption of iodide and bromide ions on Au(111) single‐crystal electrodes in three organic solvents [propylene carbonate (PC), diglyme (DG), and dimethyl sulfoxide (DMSO)] was studied by differential capacity measurements, cyclic voltammetry (CV), X‐ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). The anion adsorption charge in DMSO is approximately half as large as in propylene carbonate and that in water. Quantification of the adsorbed iodide amount from XPS spectra confirms the coverage estimated from the adsorption charge in CVs. As expected, the rate of anion adsorption on Au(111) is higher for I− than for Br− due to better solvation of the latter. The extent of anion adsorption on Au(111) electrode decreases and the corresponding adsorption rate decreases in the solvent order H2O>PC>DG>DMSO. Since the donor number (DN) of the solvent increases in the same order (PC<DG<DMSO), we assume that it plays a decisive role since it determines the chemical interaction of the solvent with the metal surface. Moreover, the adsorption rate of iodide and the extent of its adsorption is increased with increasing water content in PC and DMSO. This effect, and the much higher adsorption rate in water is explained by the closer approach of the solvated halogen ion to the surface and the resulting stronger interaction of the transition state with the electrode.