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

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 (h) 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 (jmax) that convolutes kinetic and mass-transport contributions. The value of jmax may approach the mass-transport limiting current density (jL), although it will be smaller than this value when the rate of oxygen adsorption is small, and/or when jL is very large. The application of Tafel plots for the analysis of the orrkinetics is contrasted to the rigorous equation derived in this work for this mechanistic route. The model was used to successfully correlate experimental j(h) dependencies measured on Pt electrodes, and the obtained kinetic parameters were scrutinized.