Iodide adsorption at Au(111) electrode in non-aqueous electrolyte: AC-voltammetry and EIS studies

The adsorption/desorption of iodide ions on the Au (111) single crystal plane in propylene carbonate has been investigated by cyclic voltammetry, AC-voltammetry, and electrochemical impedance spectroscopy for various electrolyte compositions. In non-aqueous electrolyte (propylene carbonate), two broad reversible peaks are present in the cyclic voltammetry due to the adsorption/desorption of iodide at the electrode surface; they are broader than in the corresponding aqueous electrolyte. AC voltammetry shows sharp peaks of the iodide adsorption/desorption process at the same potential. Similar to the aqueous solution, the adsorption charge is in the range of 80 μC cm−2 and thus corresponds to a complete monolayer. The rate of iodide adsorption increases with iodide concentration in the solution as expected for a simple charge transfer. When hexafluorophosphate (PF6−) is the anion in the supporting electrolyte, the rate is also larger than when it is perchlorate (ClO4−). When the cation is changed, the rate also decreases (the adsorption resistance increases) by more than an order of magnitude in the sequence K+ > Na+ > Li+ > TBA+ and, thus, in the same sequence as the radii of the solvated ions.