Langmuir Adsorption Isotherms of Overpotentially Deposited Hydrogen at Poly-Au and Rh / H 2 SO 4 Aqueous Electrolyte Interfaces Qualitative Analysis Using the Phase-Shift Method

The Langmuir adsorption isotherms of over-potentially deposited hydrogen (OPD H) for the cathodic hydrogen evolution reaction (HER) at poly-Au and Rh/0.5 M H 2 SO 4 aqueous electrolyte interfaces have been studied using cyclic voltammetric and ac impedance techniques. The behavior of the 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 OPD H for the cathodic HER at the interfaces. The phase-shift profile (-Φ vs. E) for the optimum intermediate frequency, i.e., the phase-shift method, can be used as a new electrochemical method to determine the Langmuir adsorption isotherm (θ as E) of the OPD H for the cathodic HER at the interfaces. At the poly-Au/(0.5 M H 2 SO 4 aqueous electrolyte interface, the equilibrium constant (K) and the standard free energy (ΔG ads )of the OPD H are 2.3 X 10 -6 and 32.2 kJ/mol, respectively. At the poly-Rh/0.5 M H 2 SO 4 aqueous electrolyte interface, K and ΔG ads, of the OPD H are 4.1 x 10 -4 or 1.2 X 10 -2 and 19.3 or 11.0 kJ/mol depending on E, respectively. In contrast to the poly-Au electrode interface, two different Langmuir adsorption isotherms of OPD H are observed at the poly-Rh electrode interface. The two different Langmuir adsorption isotherms of OPD H correspond to two different adsorption sites of OPD H on the poly-Rh electrode surface.

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