Mechanistic Insights into the Enhanced OER Activity of Single-Atom Iron Catalysts Compared to Metal Clusters

Atomically dispersed transition metals on nitrogen-doped carbon materials represent a highly active class of electrocatalysts that may replace platinum-based catalysts in electrochemical water-splitting applications. Single-atom catalysts (SACs) in particular have shown great attraction, yet the influence of metal loading and coordination environment on their stability, reactivity, and tunability remains unclear. Here, we use spin-polarized DFT and AIMD simulations to investigate the oxygen evolution reaction on Fe n @C 2 N ( n = 1–5) model catalysts. Starting from the Fe 1 @C 2 N system, we systematically increase the number of anchored iron atoms and compute reaction free-energy profiles for each cluster. We find that the single-atom Fe 1 @C 2 N exhibits the lowest overpotential, driven by the *OOH formation step as the potential-determining step (PDS) which is consistent with its d-band center and density of states. In contrast, clusters with n = 2–5 atoms are limited by the *O formation as a PDS step. Crystal orbital Hamilton population analysis quantifies the bonding interactions that underlie these trends. Finally, a linear scaling relationship between *OH and *O adsorption energies, together with volcano-plot analysis, suggests pathways for optimizing multinuclear catalysts by tuning *OH binding.