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Atomically dispersed transition metals anchored on N-doped carbon are identified as a class of promising candidates alternatively to Pt-based catalysts for oxygen reduction reaction but still restricted by the low activity and durability in acidic media. Herein, we report a new strategy for precisely tuning the number of Fe atoms in clusters anchored on N-doped carbon. Such a tunability offers the capabilities of altering O₂ adsorption from superoxo-like to peroxo-like configuration as well as tailoring N species in carbon matrix during pyrolytic catalyst synthesis. As a result, Fe₂ clusters on N-doped carbon display excellent acidic ORR activity with a half-wave potential of 0.78 V versus reversible hydrogen electrode (RHE) as well as remarkable durability with only a −20 mV shift after 20,000 cycles in 0.5 M H₂SO₄ solution. This work provides insights into non-noble-metal electrocatalyst design at the atomic level from the …