Determination of Adsorption Isotherms of Overpotentially Deposited Hydrogen on Platinum and Iridium in KOH Aqueous Solution Using the Phase-Shift Method and Correlation Constants

Abstract

The phase-shift method and correlation constants, that is, unique electrochemical impedance spectroscopy (EIS) techniques for studying the linear relationship between the behavior (-phi vs E) of the phase shift (90 degrees >= -phi >= 0 degrees) for the optimum intermediate frequency and that (theta vs E) of the fractional surface coverage (0 <= theta <= 1), are proposed and verified to determine the Frumkin, Langmuir, and Temkin adsorption isotherms of overpotentially deposited hydrogen (OPD H) and related electrode kinetic and thermodynamic parameters of noble metals (alloys) in aqueous solutions. On Pt and Ir in 0.1 M KOH aqueous solution, the Frumkin and Temkin adsorption isotherms (theta vs E), equilibrium constants (K), interaction parameters (g), rates (r) of change of the standard free energy of OPD H with theta, and standard free energies (AG(0)(0)) of OPD H are determined using the phase-shift method and correlation constants. The Frumkin adsorption isotherm is more accurate, useful, and effective than the Temkin adsorption isotherm. At 0.2 < theta < 0.8, the negative (positive) values of the interaction parameter for the Frumkin (Temkin) adsorption isotherms of OPD H are determined. A lateral attraction or repulsion interaction between the adsorbed OPD H species appears. The duality of the lateral attraction and repulsion interactions is probably a unique feature of OPD H on Pt, Ir, and Pt-Ir alloys in aqueous solutions.