Langmuir adsorption isotherms of overpotentially deposited hydrogen at poly-Au and Rh/H2SO4 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 H2SO4 aqueous electrolyte interfaces have been studied using cyclic voltammetric and ac impedance techniques. The behavior of the phase shift (0degreesless than or equal to-philess than or equal to90degrees) for the optimum intermediate frequency can be linearly related to that of the fractional surface coverage (1greater than or equal tothetagreater than or equal to0) of OPD H for the cathodic HER at the interfaces. The phase-shift profile (-phi 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 (theta vs. E) of the OPD H for the cathodic HER at the interfaces. At the poly-Au/0.5 M H2SO4 aqueous electrolyte interface, the equilibrium constant (K) and the standard free energy (DeltaG(ads)) of the OPD H are 2.3x10(-6) and 32.2 kJ/mol, respectively. At the poly-Rh/0.5 M H2SO4 aqueous electrolyte interface, K and DeltaG ads of the OPD H are 4.1x10(-4) or 1.2x10(-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. (C) 2003 The Electrochemical Society.