The Langmuir adsorption isotherms of electroadsorbed hydrogens for the cathodic hydrogen evolution reactions at the Pt(100)/H 2SO 4 and LiOH aqueous electrolyte interfaces

Abstract

The Langmuir adsorption isotherms of the under-potentially deposited hydrogen (UPD H) and the over-potentially deposited hydrogen (OPD H) at the Pt(100)/0.5 M H 2 SO 4 and 0.5 M LiOH aqueous electrolyte interfaces have been studied using cyclic voltammetric and ac impedance techniques. The phase-shift profile (− φ vs. E) for the optimum intermediate frequency can be used as an experimental method to estimate the Langmuir adsorption isotherm ( θ vs. E) at the interfaces. At the Pt(100)/0.5 M H 2 SO 4 electrolyte interface, the equilibrium constant ( K) and the standard free energy (Δ G ads) of the OPD H are 1.5×10 −4 and 21.8 kJ/ mol , respectively. At the Pt(100)/0.5 M LiOH electrolyte interface, K transits from 1.9 (UPD H) to 6.1×10 −6 (OPD H) depending on the cathode potential ( E) and vice versa. Similarly, Δ G ads transits from −1.6 kJ/ mol (UPD H) to 29.7 kJ/ mol (OPD H) depending on E and vice versa. The transition of K and Δ G ads is attributed to the two distinct adsorption sites of the UPD H and the OPD H on the Pt(100) surface. The UPD H and the OPD H on the Pt(100) surface act as two distinguishable electroadsorbed H species. The UPD H and the OPD H at the Pt(100) interfaces are the independent processes depending on the H adsorption sites rather than the sequential processes for the cathodic H 2 evolution reactions. The criterion of the UPD H and the OPD H is the H adsorption sites and processes rather than the H 2 evolution reactions and potentials.