Determination of the Langmuir adsorption isotherms of under- and over-potentially deposited hydrogen for the cathodic H 2 evolution reaction at poly-Ir/aqueous electrolyte interfaces using the phase-shift method

Abstract

The Langmuir adsorption isotherms of under-potentially deposited hydrogen (UPD H) and over-potentially deposited hydrogen (OPD H) for the cathodic H 2 evolution reaction (HER) at poly-Ir/0.5 M H 2SO 4 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 (Δ G ads 0) of H (UPD H, OPD H) for the cathodic HER at the interfaces. At the poly-Ir/0.5 M H 2SO 4 aqueous electrolyte interface, K and Δ G ads 0 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 Δ G ads 0 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 Δ G ads 0 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, Δ G ads 0) of H (UPD H, OPD H) for the cathodic HER at the interfaces.