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# Determination of the Adsorption Isotherms of Overpotentially Deposited Hydrogen on a Pt−Ir Alloy in H_{2}SO_{4} Aqueous Solution Using the Phase-Shift Method and Correlation Constants

journal contribution

posted on 2011-02-10, 00:00 authored by Jinyoung Chun, Nam Y. Kim, Jang H. ChunThe phase-shift method and correlation constants, which are unique electrochemical impedance spectroscopy techniques for studying the linear relationship between the phase shift (90° ≥ −φ ≥ 0°) versus electric potential (

*E*) behavior for the optimum intermediate frequency and the fractional surface coverage (0 ≤ θ ≤ 1) vs*E*behavior, are proposed and verified to determine the Frumkin, Langmuir, and Temkin adsorption isotherms and related electrode kinetic and thermodynamic parameters. On a Pt−Ir [90:10 % (by weight)] alloy in 0.5 M H_{2}SO_{4}aqueous solution, the Frumkin and Temkin adsorption isotherms (θ vs*E*), equilibrium constants [*K*= 3.3 · 10^{−5}exp(2.5θ) mol^{−1}for the Frumkin and*K*= 3.3 · 10^{−4}exp(−2.1θ) mol^{−1}for the Temkin adsorption isotherm], interaction parameters (*g*= −2.5 for the Frumkin and*g*= 2.1 for the Temkin adsorption isotherm), standard Gibbs energies of adsorption of overpotentially deposited (OPD) H [(25.6 ≥ Δ*G*_{θ}^{0}≥ 19.4) kJ·mol^{−1}for*K*= 3.3 · 10^{−5}exp(2.5θ) mol^{−1}and 0 ≤ θ ≤ 1 and (20.9 < Δ*G*_{θ}^{0}< 24.0) kJ·mol^{−1}for*K*= 3.3 · 10^{−4}exp(−2.1θ) mol^{−1}and 0.2 < θ < 0.8], and rates of change of Δ*G*_{θ}^{0}of OPD H with θ (*r*= −6.2 kJ·mol^{−1}for*g*= −2.5 and*r*= 5.2 kJ·mol^{−1}for*g*= 2.1) have been determined and are compared using the phase-shift method and correlation constants. For 0.2 < θ < 0.8, a lateral attractive (*g*< 0) or repulsive (*g*> 0) interaction between the adsorbed OPD H species appears. On Pt, Ir, and Pt−Ir alloys in 0.5 M H_{2}SO_{4}aqueous solution, the values of*K*for the Frumkin adsorption isotherms of OPD H decrease with increasing mass ratio of Ir. Negative values of*g*for the Frumkin adsorption isotherms of OPD H on the Pt, Ir, and Pt−Ir alloys in acidic and alkaline H_{2}O and D_{2}O solutions are experimentally and consistently determined. The duality of the lateral attractive and repulsive interactions is a unique feature of the adsorbed OPD H species on the Pt, Ir, and Pt−Ir alloys in acidic and alkaline H_{2}O and D_{2}O solutions.