Determination of the Adsorption Isotherms of Overpotentially Deposited Hydrogen on a Pt−Ir Alloy in H2SO4 Aqueous Solution Using the Phase-Shift Method and Correlation Constants

The phase-shift method and correlation constants, which are unique electrochemical impedance spectroscopy techniques for studying the linear relationship between the phase shift (90° ≥ −φ ≥ 0°) versus electric potential (E) behavior for the optimum intermediate frequency and the fractional surface coverage (0 ≤ θ ≤ 1) vs E behavior, are proposed and verified to determine the Frumkin, Langmuir, and Temkin adsorption isotherms and related electrode kinetic and thermodynamic parameters. On a Pt−Ir [90:10 % (by weight)] alloy in 0.5 M H2SO4 aqueous solution, the Frumkin and Temkin adsorption isotherms (θ vs E), equilibrium constants [K = 3.3 · 10−5 exp(2.5θ) mol−1 for the Frumkin and K = 3.3 · 10−4 exp(−2.1θ) mol−1 for the Temkin adsorption isotherm], interaction parameters (g = −2.5 for the Frumkin and g = 2.1 for the Temkin adsorption isotherm), standard Gibbs energies of adsorption of overpotentially deposited (OPD) H [(25.6 ≥ ΔGθ0 ≥ 19.4) kJ·mol−1 for K = 3.3 · 10−5 exp(2.5θ) mol−1 and 0 ≤ θ ≤ 1 and (20.9...