Altering Ligand Microenvironment of Atomically Dispersed CrN 4 by Axial Ligand Sulfur for Enhanced Oxygen Reduction Reaction in Alkaline and Acidic Medium.

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)-N  electrocatalyst containing extraterrestrial S formed axial S -Cr N  bonds (S Cr N 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, S Cr N C exhibits enhanced the intrinsic activity (half-wave potential (E ) is 0.90 V versus reversable hydrogen electrode, RHE) compared with that of CrN 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 S Cr N C moiety has the better ORR activity than Cr N C moieties. The axial S ligand in S Cr N C moiety can break the electron localization around the planar Cr N  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.