Relationship between the electron-transfer coefficients of the oxygen reduction reaction estimated from the Gibbs free energy of activation and the Butler-Volmer equation.

The rate of electron-transfer reactions, irrespective of whether electrochemical or electrocatalytic, is universally explained on the basis of Butler-Volmer (B-V) theory. The charge-transfer coefficient () obtained is typically in the range of 0.0-1.0, and is 0.6 ± 0.1 for the oxygen reduction reaction (ORR) on Pt, which is the subject of the present investigation. Alternatively, can be estimated from the derivative of the change in Gibbs free energy of activation (Δ) with respect to the overpotential () and has the unreasonably high value of 1.1 ± 0.2. The origin of the difference in the values obtained from these two methods is investigated. The value of greater than 1.0 stems from the alternative potential-dependent lower energy barrier path for the formation of the activated complex, offered by the electrified catalyst surface. For the electrocatalytic reaction, the value derived from the Δ is the true kinetic parameter. The theoretical background of such processes is presented to justify our claims.