Investigation of Hydrogen Oxidation/Evolution Reactions Based on Charge-Transfer Coefficients Derived from Butler–Volmer and Eyring Analyses
Geetanksha Gupta, Debittree Choudhury, Rajan Maurya, Shreya Sharma, Manoj Neergat
Abstract
The rate of hydrogen oxidation/evolution reactions is examined on Pt black in 0.5 M H 2 SO 4 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 ( iR s ), and mass-transport effects to obtain the kinetic current. The kinetic parameters [exchange current density ( j 0 ) 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 (Δ H 0 ) and that at various overpotentials (Δ H # ) are estimated from the Eyring analysis. In an acidic medium, Δ H # 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 Δ H # with η in alkaline medium results in unexpected α values, indicating electro-inhibition of the reaction. The higher Gibbs free energy of adsorption (Δ G H upd ) 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 H upd 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.