Low-cost ternary Fe/N/C catalyst has been considered as the most promising candidate to replace Pt for oxygen reduction reaction (ORR) in fuel cells. However, poor stability of Fe/N/C catalyst severely hampers its applications. A widely accepted mechanism for the instability of Fe/N/C catalysts is the oxidative attack by reactive oxygen species, especially by aggressive •OH radical. The •OH is speculated mainly via the decomposition of H₂O₂ intermediate through Fenton’s reaction. However, there are few reports about direct detection, especially quantitatively analysis of •OH radical during ORR on pyrolyzed Fe/N/C catalysts. The exact mechanism for generating •OH is unclear yet.

In this study, we use coumarin as a fluorescent probe to detect •OH generated from ORR on Fe/N/C catalyst. During the Fe/N/C ORR non-fluorescent coumarin will react with the •OH to generate the intense fluorescent 7-hydroxyl coumarin. According to the fluorescent intensity of 7-hydroxyl coumarin, the •OH can be quantitatively analyzed. Meanwhile, this probe molecule is highly specific to •OH radical, and is insensitive to O₂, H₂O₂ and electrochemical oxidation.

We tested the •OH produced by the ORR at different potentials, and investigated its correlation with H₂O₂ intermediate. As shown in Figure 1, with potential decreased from 0.70 to 0.60 V, the amount of •OH generated from the ORR process on Fe/N/C catalyst increased and then declined, reaching a maximum at 0.65 V. This variation of •OH amount is greatly different from that of H₂O₂, and the latter increased linearly with decreasing potentials. Our result indicates that •OH is not mainly generated by the H₂O₂ decomposition.