Defect-Tolerant Axial Ligand Modification Enables Robust NH2OH Electrosynthesis over Co Single-Atom Catalysts

Abstract Hydroxylamine (NH2OH) electrosynthesis from nitrogen oxides (NOx) stands at the forefront of electrocatalysis. Although Co–N4 single-atom catalysts (SACs) hold great promise for selective NH2OH production, their performance is severely constrained by the sluggish *NO-to-*NHO kinetics. Herein, we demonstrate that planar ligand change (N → C/O) impedes the *NO-to-*NHO step, whereas axial ligand modification (Co–N4–X, X = Cl, F, OH, O) accelerates it. Among these, Co–N4–O SAC exhibits the lowest kinetic barrier for this step, delivering high activity and selectivity toward NH2OH electrosynthesis. Furthermore, we extend axial ligand modification to Co SACs featuring altered planar ligands and show that it preserves fast *NO-to-*NHO kinetics even under nonideal planar coordination environments (e.g., N2C2, N3O). These findings uncover the defect-tolerant nature of axial coordination engineering: the limited NH2OH activity of pristine Co–N4 SAC rapidly declines without ideal planar coordination, whereas the high performance of axially modified SACs remains robust against coordination variations, regardless of the specific axial ligands. Our study not only proposes an axial-ligand-regulation strategy to boost NH2OH electrosynthesis on Co SACs but, more importantly, demonstrates that this enhanced activity is defect-tolerant across diverse planar coordination environments.