Thermodynamic studies of anion adsorption at the Pt(111) electrode surface in sulfuric acid solutions

The thermodynamics of the so-called perfectly polarized electrode were employed to analyze the total charge densities for a Pt(111) electrode in solutions of H₂SO₄ with an excess of an inert electrolyte (0.1 M HClO₄). A high quality Pt(111) single crystal electrode with a low defect density was employed in this study. The total charge densities were calculated by integration of cyclic voltammetry curves. A complete thermodynamic analysis using the electrode potential and the charge as independent variables has been performed. The best results were obtained when the charge was the independent variable. These results show that the maximum Gibbs excess of (bi)sulfate attains a value of ～3×10¹⁴ ions cm^?2, which corresponds to the packing density of ～0.2 monolayer. This number agrees well with the results of a radioactive labeling method (Kolics and Wieckowski, J. Phys. Chem. Sect. B 105 (2001) 2588) and with recent STM studies (Itaya, Prog. Surf. Sci. 58 (1998) 121; Funtikov, Linke, Stimming and Vogel, Surf. Sci. 324 (1995) L343). The calculated Esin–Markov coefficient and electrosorption valencies exhibit an important dependence on the electrode potential (or charge).