Qualitative Analysis of the Frumkin Adsorption Isotherm of the Over-Potentially Deposited Hydrogen at the Poly-Ni/KOH Aqueous Electrolyte Interface Using the Phase-Shift Method

Abstract

The phase-shift method of the Frumkin adsorption isotherm of the over-potentially deposited hydrogen (OPD H) for the cathodic H2evolution reaction (HER) at the poly-Ni/0.05 M KOH aqueous electrolyte interface has been studied using cyclic voltammetric and ac impedance techniques. The behavior of the phase shift (0° ≤ -φ ≤ 90°) for the optimum intermediate frequency corresponds well to that of the fractional surface coverage (1 ≥ θ ≥ 0) at the interface. The phase-shift method, i.e., the phase-shift profile (-φ vs. E) for the optimum intermediate frequency, can be used as a new method to estimate and analyze the Frumkin adsorption isotherm (θ vs. E) of the OPD H for the cathodic HER at the interface. At the poly-Ni/0.05 M KOH aqueous electrolyte interface, the rate (r) of change of the standard free energy of the OPD H with θ, the interaction parameter (g) for the Frumkin adsorption isotherm, the equilibrium constant (K) for the OPD H with θ, and the standard free energy (ΔGθ) of the OPD H with θ are 24.8 kJ/mol, 10, 5.9 × 10-6≤ K ≤ 0.13, and 5.1 ≤ ΔGθ≤ 29.8 kJ/mol, respectively. The electrode kinetic and thermodynamic parameters (r, g, K, ΔGθ) depend strongly on θ (0 ≤ θ ≤ 1).