Oxygen reduction to water operating through the Direct (or Dissociative) Route: Descriptive and fitting capabilities of polarization curves

Highlights: • The Direct Route of oxygen reduction to water is analyzed without approximations. • Complete dependencies current density (j) vs. overpotential (η) are obtained. • The effects of the kinetic and adsorption parameters on the j(η) curves are analyzed. • The inapplicability of Tafel plots for analyzing this mechanism is demonstrated. • The model properly correlates experimental j(η) data measured on Pt electrodes. • The model fails in describing the electro-oxidation of intermediates at high potentials. - ABSTRACT: The 4-e⁻ oxygen reduction reaction (orr) to water operating through the Direct Route (dissociative chemical adsorption of molecular oxygen followed by a two-step electro-reduction of adsorbed atomic oxygen) in acid media was fully analyzed. An analytical equation for the dependence of the current density (j) on the overpotential (η) was derived in terms of the elementary kinetic constants and the adsorption parameters without aprioristic assumptions. The descriptive capability of this equation was thoroughly explored by simulations over wide ranges of these kinetic and adsorption parameters. It was demonstrated that the Direct Route predicts the existence of a maximum current density (j_max) that convolutes kinetic and mass-transport contributions. The value of j_max may approach the mass-transport limiting current density (j_L), although it will be smaller than this value when the rate of oxygen adsorption is small, and/or when j_L is very large. The application of Tafel plots for the analysis of the orr kinetics is contrasted to the rigorous equation derived in this work for this mechanistic route. The model was used to successfully correlate experimental j(η) dependencies measured on Pt electrodes, and the obtained kinetic parameters were scrutinized.