Kinetic Approach to Investigate the Mechanistic Pathways of Oxygen Reduction Reaction on Fe-Containing N‑Doped Carbon Catalysts

The mechanism of the oxygen reduction reaction (ORR) on polyimide-based N-doped carbon catalysts with and without Fe (i.e., Fe–N–C and N–C) in acidic media was studied, using a rotating ring-disk electrode voltammetry, based on the estimation of the rate constants of the ORR, i.e., k₁, k₂, and k₃ corresponding to the 4-electron direct reduction of O₂ to H₂O, the 2-electron reduction of O₂ to H₂O₂, and the 2-electron reduction of H₂O₂ to H₂O, respectively as functions of the loading density of Fe–N–C or N–C and the electrode potential, in which two different kinetic models for the ORR, i.e., Damjanovic and Wroblowa models were used to estimate the values of the k₁, k₂, and k₃. The H₂O₂ disproportionation reaction rate constants (k₄) of the Fe–N–C and N–C catalysts were also estimated. For both catalysts, the ORR follows, as a whole, a 4-electron pathway (k₁ ≫ k₂) when the loading density is high (>ca. 200 μg/cm²), whereas at the lower loading density the 2-electron reduction of O₂ becomes more dominant (k₁ < k₂). The value of k₄ was found to be negligible compared with that of k₁, k₂, and k₃. In addition, the individual rate constants estimated by extrapolating rate constant vs loading density plots to the zero loading density demonstrate that in the case of the Fe–N–C catalyst, 2-electron and 4-electron reduction sites coexist with a higher population of the former, whereas in the case of the N–C catalyst, only 2-electron active sites exist. The kinetic analysis strongly suggests that the Fe in the Fe–N–C catalyst plays a crucial role in making the 4-electron active sites as well as it may catalyze the H₂O₂ reduction to H₂O, resulting in the enhanced ORR performance.