Anion Adsorption on Gold Electrodes Studied by Electrochemical Surface Forces Measurement

The adsorption of ions on electrodes determines the surface potential and charge density of the electrode, thus, quantitative evaluation of the ion adsorption on an electrode is necessary and has been one of the central questions in electrochemistry. Electrochemical Surface Forces Apparatus (EC-SFA) can provide an efficient characterization method of these properties. The interactions between two gold electrodes in various electrolyte solutions, that is, 1 mM aqueous KClO₄, K₂SO₄, and KCl, were measured by controlling of the electrochemical potential (E). The long-range, double layer repulsion and the jump-in due to the van der Waals attraction at the surface separation of about 20 nm were observed between the electrodes in all the solutions. We evaluated the ψ₀ and σ values employing DLVO fitting of these interactions. The signs of ψ₀ and σ were determined from the interaction between the electrode and negatively charged mica surfaces. This study demonstrated that the σ values were negative and similar in all the solutions when the applied potential E was lower than the potential of zero charge (pzc). When the potential E was increased to near the pzc, the σ values were negative and low and in the order of KClO₄ ≈ K₂SO₄ > KCl. When the potential E was further increased to the pzc, the σ value was positive in aqueous KClO₄ because of less anion adsorption on the gold electrode, while those in the K₂SO₄ and KCl aqueous solutions were negative due to higher adsorption amount of the anions. The method demonstrated in this study enabled us to quantitatively evaluate the influences of the ion adsorption on the effective surface potential and charge density of the electrodes, which should determine the performances of electrochemical devices.