Unconventional Oxygen\nReduction Reaction Mechanism\nand Scaling Relation on Single-Atom Catalysts

Abstract

The\nelectrochemical oxygen reduction reaction (ORR) mechanism was\ngenerally considered to be O₂ → OOH* →\nO* → OH* → H₂O (O* mechanism).\nThis O* mechanism predicted reasonable ORR half-wave potential (<i>E</i>_1/2) of Co/N/C but abnormally underestimated\nthe one of Fe/N/C. Herein, we highlight an unconventional 2OH* ORR\nmechanism (O₂ → OOH* → 2OH* →\nOH* → H₂O), which was often ignored because\nthe free energies (Δ<i>G</i>) of 2OH* and O* are equal,\naccording to the famous scaling relation: 2Δ<i>G</i>(OH*) = Δ<i>G</i>(O*). This scaling relation is true\nfor traditional catalysts with near-continuous active sites. We find\na different scaling relation: Δ<i>G</i>(2OH*) = Δ<i>G</i>(O*) + 1.5 eV on single-atom catalysts (Me/N/C, Me = Fe,\nCo, etc.) and suggest that the 2OH* mechanism should not be overlooked.\nIn consideration of both O* and 2OH* mechanisms, the ORR <i>E</i>_1/2 values of Co/N/C and Fe/N/C are in good agreement\nwith experimental results. This work reveals the structure dependence\nof ORR reaction mechanisms and scaling relations in single-atom catalysis,\nand it is also heuristic for other reactions, such as O₂ evolution and N₂ reduction on single-atom catalysts.