Unconventional Oxygen Reduction Reaction Mechanism and Scaling Relation on Single-Atom Catalysts
Lixiang Zhong, Shuzhou Li
Abstract
The electrochemical oxygen reduction reaction (ORR) mechanism was generally considered to be O2 → OOH* → O* → OH* → H2O (O* mechanism). This O* mechanism predicted reasonable ORR half-wave potential (E1/2) of Co/N/C but abnormally underestimated the one of Fe/N/C. Herein, we highlight an unconventional 2OH* ORR mechanism (O2 → OOH* → 2OH* → OH* → H2O), which was often ignored because the free energies (ΔG) of 2OH* and O* are equal, according to the famous scaling relation: 2ΔG(OH*) = ΔG(O*). This scaling relation is true for traditional catalysts with near-continuous active sites. We find a different scaling relation: ΔG(2OH*) = ΔG(O*) + 1.5 eV on single-atom catalysts (Me/N/C, Me = Fe, Co, etc.) and suggest that the 2OH* mechanism should not be overlooked. In consideration of both O* and 2OH* mechanisms, the ORR E1/2 values of Co/N/C and Fe/N/C are in good agreement with experimental results. This work reveals the structure dependence of ORR reaction mechanisms and scaling relations in single-atom catalysis, and it is also heuristic for other reactions, such as O2 evolution and N2 reduction on single-atom catalysts.