Electron transfer in electrocatalytic reactions: effective symmetry factors

Abstract The evaluation of electron transfer kinetics at electrochemical interfaces plays a fundamental role in optimizing the efficiency of electrocatalytic reactions of technological interest. Different approaches to this topic arose from the analysis of experimental data that did not fit conventional theories. Thus, the influence of adsorbed intermediates on the electrode kinetics of heterogeneous catalytic processes requires further analysis. Neither the Marcus–Hush theory (parabolic-type energetics in redox reactions) nor the Butler–Volmer (linear intersections of Morse curves) expressions fully explain all the experimental data. Therefore, it is necessary to include (electro)sorption properties as true components in Morse curves of reactants and products to develop an electrocatalytic kinetic theory. From the intersection point of both curves, it is possible to obtain the effective symmetry factor by first differentiating the electrochemical activation-free energy modified by electrosorption as a function of the over-potential. The influence of enthalpic and entropic components on the symmetry factor and electrosorption parameters is discussed for the hydrogen electro-oxidation reaction. Experimental results are compared with the emerging symmetry factor and some analysis done in other papers.