How Electrolytes Control the Activity and Stability of Fe‐N‐C for Oxygen Reduction Reaction in Acidic Media

ABSTRACT Fe‐N‐C single‐atom catalysts (SACs) are widely recognized as the most promising platinum‐free candidates for the oxygen reduction reaction (ORR) in acidic media. However, their activity and stability remain insufficient, primarily because the active site configuration in acidic electrolytes remains unclear, which limits mechanistic understanding and rational design. Here, with the aid of the grand canonical method, complemented by experimental validation, the behavior of FeN ₄ sites in acidic electrolytes was systematically investigated. It is shown that electrolyte anions, rather than oxygenated intermediates as commonly presumed, dictate the axial ligation of FeN ₄ sites under acidic conditions and thereby control both catalytic activity and durability. Using H ₂ SO ₄ and HClO ₄ as prototypical acidic electrolytes, we reveal distinct coordination environments: in H ₂ SO ₄ , SO ₄ ²⁻ binds strongly to Fe, forming SO ₄ ‐Fe ^* sites with a higher onset potential (0.94 V) and improved kinetic stability, whereas in HClO ₄ , hydration dominates, yielding H ₂ O‐Fe ^* sites with lower activity (0.67 V) and greater demetallation susceptibility. These computational insights, corroborated by electrochemical measurements, establish electrolyte identity as a decisive factor in shaping the activity and stability of Fe‐N‐C catalysts and highlight electrolyte engineering as a promising strategy for durable platinum‐free ORR catalysis.