Investigation of Hydrogen Oxidation/Evolution Reactions Based on Charge-Transfer Coefficients Derived from Butler–Volmer and Eyring Analyses

The rate of hydrogen oxidation/evolution reactions is examined on Pt black in 0.5 M H2SO4 and in 0.1 M KOH. The investigations are conducted in the temperature range 283–323 K using a three-electrode rotating disk electrode assembly at a rotation rate of 1600 rpm. The measured current is corrected for background, Ohmic drop (iRs), and mass-transport effects to obtain the kinetic current. The kinetic parameters [exchange current density (j0) and charge-transfer coefficient (α)] are obtained from the Butler–Volmer analysis, and in both the media, the α values obtained are in the sensible range of 0.6–0.9. The activation enthalpy at equilibrium potential (ΔH0) and that at various overpotentials (ΔH#) are estimated from the Eyring analysis. In an acidic medium, ΔH# decreases with overpotential (η), suggesting electrocatalysis of the reactions, leading to α values in the sensible range of 0.6–0.9. On the other hand, the increasing trend of ΔH# with η in alkaline medium results in unexpected α values, indicating electro-inhibition of the reaction. The higher Gibbs free energy of adsorption (ΔGHupd) in alkaline medium, obtained from the Temkin–Frumkin isotherm, suggests stronger binding of the adsorbed species to the catalyst surface. Consequently, the inhibiting effect of OH– ions on Pt is manifested by a decrease in the Hupd area and the voltammetric features in the alkaline medium. Thus, the electrocatalysis and electro-inhibition of the reactions are established from the Eyring analysis. A method for recovering the active surface sites is also presented.