A negative value of the interaction parameter for over-potentially deposited hydrogen at Pt, Ir, and Pt–Ir alloy electrode interfaces

The phase-shift method, i.e., the electrochemical impedance spectroscopy (EIS) technique for studying the linear relationship between the behavior (−φ vs. E) of the phase-shift (90° ⩾ −φ ⩾ 0°) for the optimum intermediate frequency and that (θ vs. E) of the fractional surface coverage (0 ⩽ θ ⩽ 1), has been proposed and verified to determine the adsorption isotherms at the noble metal (alloy)|aqueous solution interfaces. At the Pt–Ir (90:10 wt%) alloy|0.1 M KOH aqueous solution interface, the Frumkin adsorption isotherm for over-potentially deposited hydrogen (OPD H) and related electrode kinetic and thermodynamic parameters are determined using the phase-shift method. The standard free energies (ΔGθ0), equilibrium constants (K), and interaction parameters (g) for OPD H and hydroxide (OH) at the noble metal (alloy)|aqueous solution interfaces are summarized. The negative values of g for OPD H at the Pt, Ir, and Pt–Ir alloy|aqueous solution interfaces imply a lateral attraction interaction between the adsorbed OPD H species, which is probably a unique feature of the Frumkin adsorption isotherms at these interfaces.