Determination of adsorption isotherms of hydrogen and hydroxide at Pt–Ir alloy electrode interfaces using the phase-shift method and correlation constants

At the Pt–Ir (Pt:Ir; 70:30 wt%) alloy/0.5 M H2SO4H2SO4 aqueous solution interface, the Frumkin and Temkin adsorption isotherms (θHθH vs. EE), equilibrium constants (KH=3.1×10-5exp(2.5θH)mol-1 for the Frumkin and KH=3.1×10-4exp(-2.1θH)mol-1 for the Temkin adsorption isotherm), interaction parameters (g=-2.5g=-2.5 for the Frumkin and g=2.1g=2.1 for the Temkin adsorption isotherm), rates of change of the standard free energy of H with θHθH (r=-6.2kJmol-1 for g=-2.5g=-2.5 and r=5.2kJmol-1 for g=2.1g=2.1), and standard free energies (25.7⩾ΔGθ0⩾19.5kJmol-1 for KH=3.1×10-5exp(2.5θH)mol-1 and 0⩽θH⩽10⩽θH⩽1 and 21.1<ΔGθ0<24.2kJmol-1 for KH=3.1×10-4exp(-2.1θH)mol-1 and 0.2<θH<0.80.2<θH<0.8) of H are determined using the phase-shift method and correlation constants. The negative value of g implies a lateral attraction interaction between the adsorbed H. The capability of two-dimensional phase formation is experimentally presented. At the Pt–Ir (Pt:Ir; 70:30 wt%) alloy/0.1 M KOH aqueous solution interface, the Frumkin and Temkin adsorption isotherms (θOHθOH vs. EE), equilibrium constants (KOH=4.7×10-10exp(-1.8θOH)mol-1 for the Frumkin and KOH=4.7×10-9exp(-6.4θOH)mol-1 for the Temkin adsorption isotherm), interaction parameters (g=1.8g=1.8 for the Frumkin and g=6.4g=6.4 for the Temkin adsorption isotherm), rates of change of the standard free energy of OH with θOHθOH (r=4.5kJmol-1 for g=1.8g=1.8 and r=15.9kJmol-1 for g=6.4g=6.4), and standard free energies (53.2⩽ΔGθ0⩽57.7kJmol-1 for KOH=4.7×10-10exp(-1.8θOH)mol-1 and 0⩽θOH⩽10⩽θOH⩽1 and 50.7<ΔGθ0<60.2kJmol-1 for KOH=4.7×10-9exp(-6.4θOH)mol-1 and 0.2<θOH0.2<θOH<0.8<0.8) of OH are also determined using the phase-shift method and correlation constants. At the intermediate values of θH,OHθH,OH, i.e., 0.2<θH,OH<0.80.2<θH,OH<0.8, the Temkin adsorption isotherms correlating with the Frumkin adsorption isotherms, and vice versa, are readily determined using the correlation constants. The phase-shift method and correlation constants are accurate and reliable techniques to determine the Langmuir, Frumkin, and Temkin adsorption isotherms and related electrode kinetic and thermodynamic parameters.