Determination of the Langmuir adsorption isotherms of under- and over-potentially deposited hydrogen for the cathodic H2 evolution reaction at poly-Ir/aqueous electrolyte interfaces using the phase-shift method

The Langmuir adsorption isotherms of under-potentially deposited hydrogen (UPD H) and over-potentially deposited hydrogen (OPD H) for the cathodic H2 evolution reaction (HER) at poly-Ir/0.5 M H2SO4 and 0.05 M KOH aqueous electrolyte interfaces have been studied using cyclic voltammetric, differential pulse voltammetric, and ac impedance techniques. The behavior of the lagged phase shift (0°⩽−φ⩽90°) for the optimum intermediate frequency can be linearly related to that of the fractional surface coverage (1⩾θ⩾0) of H (UPD H, OPD H) for the cathodic HER at the interfaces. A linear relationship between the phase-shift profile (−φ vs. E) for the optimum intermediate frequency and the Langmuir adsorption isotherm (θ vs. E) of H (UPD H, OPD H), i.e., the phase-shift method, can be used as a new electrochemical method to determine or estimate the fractional surface coverage (θ), equilibrium constant (K), and standard free energy (ΔGads0) of H (UPD H, OPD H) for the cathodic HER at the interfaces. At the poly-Ir/0.5 M H2SO4 aqueous electrolyte interface, K and ΔGads0 of the OPD H for the cathodic HER are 9.9×10−5 and 22.8 kJ/mol, respectively. At the poly-Ir/0.05 M KOH aqueous electrolyte interface, K and ΔGads0 of the UPD H for the cathodic HER are 3.9 and −3.4 kJ/mol, respectively. At the poly-Ir/0.05 M KOH aqueous electrolyte interface, K and ΔGads0 of the OPD H for the cathodic HER are 5.8×10−4 and 18.5 kJ/mol, respectively. The two different Langmuir adsorption isotherms for H (UPD H, OPD H) correspond to two different adsorption sites of H (UPD H, OPD H) on the poly-Ir electrode surface. The two different adsorption sites of UPD H and OPD H act as two distinguishable electroadsorbed H species. The phase-shift method is a simple and efficient tool for determining the adsorption, electrode kinetic, and thermodynamic parameters (θ, K, ΔGads0) of H (UPD H, OPD H) for the cathodic HER at the interfaces.