Kinetic Analysis of Electrochemical Oxygen Reduction over a Fe/N/C Catalyst Considering the Chemical Decomposition of H2O2

Iron and nitrogen codoped carbon catalysts (Fe/N/C catalysts) are promising nonprecious-metal catalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells. Understanding the ORR kinetics over Fe/N/C catalysts is crucial for active catalyst development. Here, we propose an analytical method for rotating ring–disk electrode (RRDE) voltammetry to separate the ORR for O 2 reduction to H 2 O via the 4-e pathway ( k 1 ), O 2 reduction to H 2 O 2 via the 2-e pathway ( k 2 ), H 2 O 2 reduction to H 2 O via the 2-e pathway ( k 3 ), H 2 O 2 chemical decomposition to O 2 and H 2 O ( k 4 ), and H 2 O 2 oxidation to O 2 by the 2-e pathway ( k –2 ). First, RRDE voltammetry in an H 2 O 2 solution was performed, yielding k 3, k 4, and k –2 . The obtained parameters were used to analyze the ORR voltammogram to calculate I 1 ′, I 2 ′, I 3 ′, I 4 ′, k 1 ′, and k 2 ′. These currents and kinetic constants were corrected by studying the effect of the catalyst loading density to obtain I 1 0, I 2 0, I 3 0, I 4 0, k 1 0, and k 2 0, thus avoiding the overestimation of I 1 0 and k 1 0 caused by the quasi-four-electron reduction of O 2 . The contribution of k 4 during the ORR is negligible at considerable anodic and cathodic overpotentials but was detected at 0.8 V, for which k 3 and k –2 are relatively small.