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

Abstract

At the Pt–Ir (Pt:Ir; 70:30 wt%) alloy/0.5 M H 2 SO 4 aqueous solution interface, the Frumkin and Temkin adsorption isotherms ( θ H vs. E ), equilibrium constants ( K H = 3.1 × 10 - 5 exp ( 2.5 θ H ) mol - 1 for the Frumkin and K H = 3.1 × 10 - 4 exp ( - 2.1 θ H ) mol - 1 for the Temkin adsorption isotherm), interaction parameters ( g = - 2.5 for the Frumkin and g = 2.1 for the Temkin adsorption isotherm), rates of change of the standard free energy of H with θ H ( r = - 6.2 kJ mol - 1 for g = - 2.5 and r = 5.2 kJ mol - 1 for g = 2.1 ), and standard free energies ( 25.7 ⩾ Δ G θ 0 ⩾ 19.5 kJ mol - 1 for K H = 3.1 × 10 - 5 exp ( 2.5 θ H ) mol - 1 and 0 ⩽ θ H ⩽ 1 and 21.1 < Δ G θ 0 < 24.2 kJ mol - 1 for K H = 3.1 × 10 - 4 exp ( - 2.1 θ H ) mol - 1 and 0.2 < θ H < 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 ( θ OH vs. E ), equilibrium constants ( K OH = 4.7 × 10 - 10 exp ( - 1.8 θ OH ) mol - 1 for the Frumkin and K OH = 4.7 × 10 - 9 exp ( - 6.4 θ OH ) mol - 1 for the Temkin adsorption isotherm), interaction parameters ( g = 1.8 for the Frumkin and g = 6.4 for the Temkin adsorption isotherm), rates of change of the standard free energy of OH with θ OH ( r = 4.5 kJ mol - 1 for g = 1.8 and r = 15.9 kJ mol - 1 for g = 6.4 ), and standard free energies ( 53.2 ⩽ Δ G θ 0 ⩽ 57.7 kJ mol - 1 for K OH = 4.7 × 10 - 10 exp ( - 1.8 θ OH ) mol - 1 and 0 ⩽ θ OH ⩽ 1 and 50.7 < Δ G θ 0 < 60.2 kJ mol - 1 for K OH = 4.7 × 10 - 9 exp ( - 6.4 θ OH ) mol - 1 and 0.2 < θ OH < 0.8 ) of OH are also determined using the phase-shift method and correlation constants. At the intermediate values of θ H , OH , i.e., 0.2 < θ H , 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.