Determination of the Adsorption Isotherms of Hydrogen and Deuterium Isotopes on a Pt—Ir Alloy in LiOH Solutions Using the Phase-Shift Method and Correlation Constants

摘要

The 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 alloy (90:10 mass ratio) in 0.1 M LiOH (H2O) and 0.1 M LiOH (D2O) solutions, the Frumkin and Temkin adsorption isotherms (θ vs E), equilibrium constants (K), interaction parameters (g), standard Gibbs energies (ΔG°_∞) of hydrogen (H) and deuterium (D) adsorption, and rates of change (r) of ΔG°_∞ of H and D with θ have been determined and are compared using the phase-shift method and correlation constants. The value of K decreases in going from H2O to D2O. The values of K for both H and D increase with increasing E and θ. Over the θ range (i.e., 1 ≥ θ ≥ 0), the value of K for H is 3.7 to 4.1 times greater than that for D. For 0.2 < θ < 0.8, a lateral attractive (g < 0) or repulsive (g > 0) interaction between the adsorbed H or D species appears. The duality of the lateral attractive and repulsive interactions is a unique feature of the adsorbed H and D species on Pt, Ir, and Pt—Ir alloys in acidic and alkaline H2O and D2O solutions.