Tafel Slope Analysis from Inherent Rate Constants for Oxygen Reduction Reaction Over N-doped Carbon and Fe–N-doped Carbon Electrocatalysts

Nitrogen-doped carbon with and without Fe additives is a promising alternative for commercial Pt/C catalysts for the oxygen reduction reaction (ORR) in proton and anion exchange membrane fuel cells. To understand the nature of the rate-determining steps (RDSs) of the ORR over newly developed catalysts, the analysis of the Tafel slopes of ORR voltammograms is beneficial for elucidating the number of electrons involved in the RDS. Conventionally, the Tafel slope is evaluated from the measured total current, which involves several different reaction pathways: the four-electron pathway from O₂ to H₂O described with a kinetic constant k₁, the two-electron pathway from O₂ to H₂O₂ with k₂, and the two-electron pathway from H₂O₂ to H₂O with k₃. This method provides reasonable Tafel slopes as long as the measured ORR is selective to a particular reaction pathway, such as the four-electron pathway over a Pt/C catalyst; however, typical Fe/N/C and N/C catalysts have mixed reaction pathways and analyzing the Tafel slopes from the total current does not provide meaningful information. To address this, we propose a new methodology for analyzing Tafel slopes. In this study, the measured ORR currents were converted into inherent kinetic constants (k₁⁰, k₂⁰, and k₃⁰) using the Nabae model, which was previously developed by our group, and the Tafel plots for k₁⁰, k₂⁰, and k₃⁰ were analyzed to determine the Tafel slopes of each reaction pathway. Four ORR systems (Fe/N/C and N/C catalysts in acid and base) were analyzed using the proposed method, and the differences in the reaction mechanisms were successfully reflected in the determined parameters.