Langmuir Adsorption Isotherms of Overpotentially Deposited Hydrogen at Poly-Au and Rh/H[sub 2]SO[sub 4] Aqueous Electrolyte Interfaces

Abstract

The Langmuir adsorption isotherms of over-potentially deposited hydrogen ͑OPD H͒ for the cathodic hydrogen evolution reaction ͑HER͒ 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°р Ϫ р 90°) for the optimum intermediate frequency can be linearly related to that of the fractional surface coverage (1 у у 0) of OPD H for the cathodic HER at the interfaces. The phase-shift profile ͑Ϫ vs. E͒ 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 ͑ vs. E͒ 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 ͑K͒ and the standard free energy (⌬G ads ) of the OPD H are 2.3 ϫ 10 Ϫ6 and 32.2 kJ/mol, respectively. At the poly-Rh/0.5 M H 2 SO 4 aqueous electrolyte interface, K and ⌬G ads of the OPD H are 4.1 ϫ 10 Ϫ4 or 1.2 ϫ 10 Ϫ2 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.

Mechanism of the Hydrogen Evolution Reaction in Mildly Acidic Environments on Gold

“…13,21 Considering the rate determining Volmer reaction throughout the full range of cathodic currents, either as an ordinary charge transfer reaction or a barrierless reaction, the surface coverage of H ads is expected to be low (θ→0), as discussed in more detail in H + adsorption rate determining step section below. However, this was found to be in contrast with the findings from a study by Chun et al 6 where a significant coverage of H ads (θ→1) was reported in the 120 mV Tafel slope region.…”

“…6 Their measurements showed a low surface coverage at the low cathodic overpotentials, which was rapidly increased to full coverage at higher cathodic overpotentials. Based on these results, authors report the value of K = 2.3 × 10 −6 M −1 for the Langmuir equilibrium constant.…”

“…3,6,16,18,21,22 A number of different explanations for the underlying mechanism based on these observed Tafel slopes have been proposed in the literature. In a study by Ives 20 in 0.1 N HCl solutions, the author reported polarization curves with an uncharacterized region at low current densities preceding to the 120 mV Tafel slope range.…”

Developments in Electrochemistry: The Phase-Shift Method and Correlation Constants for Determining the Electrochemical Adsorption Isotherms at Noble and Highly Corrosion-Resistant Metal/Solution Interfaces

“…8 show the four numerically calculated Langmuir adsorption isotherms of (H + D) corresponding to K = 5.3 × 10 , respectively. For 0.2 < θ < 0.8, all of the Langmuir adsorption isotherms are always parallel to each other [13,16,42]. Correspondingly, all of the slopes of the Langmuir adsorption isotherms, i.e.…”

“…Thus, the tedious experimental procedures presented there [e.g. 13,16,[19][20][21]27] have been used to verify or confirm the validity and correctness of the phase-shift method. This is discussed in more detail in the section on theoretical and experimental backgrounds of the phase-shift method.…”