Revealing the Potential-Dependent Rate-Determining Step of Oxygen Reduction Reaction on Single-Atom Catalysts.

Single-atom catalysts (SACs) have attracted widespread attention due to their potential to replace platinum-based catalysts in achieving efficient oxygen reduction reaction (ORR), yet the rational optimization of SACs remains challenging due to their elusive reaction mechanisms. Herein, by employing ab initio molecular dynamics simulations and a thermodynamic integration method, we have constructed the potential-dependent free energetics of ORR on a single iron atom catalyst dispersed on nitrogen-doped graphene (Fe-N/C) and further integrated these parameters into a microkinetic model. We demonstrate that the rate-determining step (RDS) of the ORR on SACs is potential-dependent rather than invariant within the operative potential range. Specifically, under the charge-neutral condition, the RDS is calculated to be water desorption with the highest barrier, while as the potential increases, it gradually transitions to the protonation of *OH species, O* species, and O* species, regardless of the protonation of *OH species as the potential-determining step. Moreover, we reveal the critical role of the dynamic adsorption of axially adsorbed water in facilitating the release of the single-atom site, thus enhancing the ORR rate. Our work has resolved the long-standing controversies over the RDS of ORR on SACs and implies that the step with the lowest exothermicity is not always synonymous with the RDS, highlighting the importance of examining the kinetic barriers under realistic potential conditions for understanding the electrocatalytic performance.