Anisotropic Etching Induced Construction of Co‐N ₄ S ₁ Single‐Atom Sites on 2D Hierarchical Porous Honeycomb Carbon With Enhanced Mass Transfer for Efficient Electrocatalysis

Abstract The meticulous modulation of asymmetric active sites and the enhancement of mass transfer efficiency are both of paramount importance to the comprehensive performance of single‐atom catalysts. Herein, a facile in situ anisotropic etching induced construction of a 2D monolayered hierarchical porous honeycomb carbon anchored with Co‐N ₄ S ₁ single‐atom sites is developed. This “two‐in‐one” strategy not only achieves precise modulation of the axial coordination environment of Co single atoms but also enhances multiphase mass transfer capabilities and accessibility of catalytic active centers offered by the unique 2D monolayered honeycomb architecture. The construction of the Co‐N ₄ S ₁ coordination environment can cause the d ‐orbital energy level of Co sites to shift toward the Fermi level, reduce the reaction barrier of the rate determining step ( ^* NOH→ ^* N), and promote the adsorption of ^* N during the nitrate reduction reaction (NO ₃ RR) process. By using operando electrochemical impedance spectroscopy and distributed relaxation times analysis, the intricate relationship between mass transfer and catalytic performance is deeply revealed. The synergistic integration of metal active center modulation and mass transfer optimization is demonstrated to significantly enhance catalytic performance, as evidenced by the superior NO ₃ RR performance (NH ₃ yield activity = 9.46 mg h ⁻¹ mg _cat ⁻¹ ) and oxygen reduction reaction activity ( E _1/2 = 0.895 V).