Altering Ligand Microenvironment of Atomically Dispersed CrN₄ by Axial Ligand Sulfur for Enhanced Oxygen Reduction Reaction in Alkaline and Acidic Medium

Abstract Because of the instability and Fenton reactivity of non‐precious metal nitrogen‐carbon based catalyst when processing the oxygen reduction reaction (ORR), seeking for electrocatalysts with highly efficient performance becomes very highly desired to speed up the commercialization of fuel cell. Herein, chromium (Cr)‐N₄ electrocatalyst containing extraterrestrial S formed axial S₁‐Cr₁N₄ bonds (S₁Cr₁N₄C) is achieved via an assembly polymerization and confined pyrolysis strategy. Benefiting from the adjusting coordination configuration and electronic structure of the metal center through axial coordination, S₁Cr₁N₄C exhibits enhanced the intrinsic activity (half‐wave potential (E_1/2) is 0.90 V versus reversable hydrogen electrode, RHE) compared with that of CrN₄C and Pt/C catalysts. More notably, the catalyst is almost inert in catalyzing the Fenton reaction, and thus shows the high stability. Density functional theory (DFT) results further reveal that the existence of axial S atoms in S₁Cr₁N₄C moiety has the better ORR activity than Cr₁N₄C moieties. The axial S ligand in S₁Cr₁N₄C moiety can break the electron localization around the planar Cr₁N₄ active center, which facilitated the rate‐limiting reductive release of OH* and accelerated overall ORR process. The present work opens up a new avenue to modulate the axial ligand type of the single‐atoms (SAs) active center to enhance intrinsic SAs performances.