Single Fe atom coordinated with four planar nitrogen atoms codoped graphene layer ($\text{Fe-}{\text{N}}_{\text{4}}\text{-C}$) has emerged as one of the promising single atom electrocatalyst (eSAC) for the remediation of nitrate (${{\text{NO}}_{\text{3}}}^{\text{-}}$). Herein, based on density functional theory (DFT) calculations, it is found that the ligands X (X = O, OH, F, Cl, Br, and I) axially ligated to the Fe center of $\text{Fe-}{\text{N}}_{\text{4}}\text{-C}$ can significantly improve its catalytic activity toward electrocatalytic nitrate reduction (${\text{eNO}}_{\text{3}}\text{RR}$) to ammonia (${\text{NH}}_{\text{3}}$) with limiting potentials (${\text{U}}_{\text{L}}$) of 0, -0.36, -0.28, -0.41, -0.25 and -0.19 eV, respectively. The remarkable catalytic activity should be associated with the well-matched orbitals of the ${\mathit{Fe}}^{{3d}_{\mathit{xz}}}$/${\mathit{Fe}}^{{3d}_{\mathit{yz}}}$ and ${\mathit{NO}}^{\pi \ast }$, together with moderate $\ast NO$ adsorption free energy (${\mathrm{\Delta }G}_{\ast NO}$). Although ${\mathit{eNO}}_{3}RR$ on $O-Fe-N_4$ is highly energy favorable, extremely low ${\mathrm{\Delta }G}_{\ast NO}$ (-0.02 eV) may lead to inferior selectivity toward ${\text{NH}}_{\text{3}}$ product. These findings highlight the role of axial ligand for the ${\mathit{eNO}}_{3}RR$ properties of $\mathit{Fe}-N_4-C$.

单个铁原子与四个平面氮原子共掺杂石墨烯层配位（$\text{铁-}{\text{否}}_{\text{4个}}\text{-C}$) 已成为一种很有前途的单原子电催化剂 (eSAC) 用于修复硝酸盐 (${{\text{不}}_{\text{3个}}}^{\text{-}}$). 在此，基于密度泛函理论 (DFT) 计算，发现配体 X (X = O、OH、F、Cl、Br 和 I) 轴向连接到 Fe 中心$\text{铁-}{\text{否}}_{\text{4个}}\text{-C}$可以显着提高其对电催化硝酸盐还原的催化活性（${\text{氧化氮}}_{\text{3个}}\text{RR}$) 到氨 (${\text{氨氮}}_{\text{3个}}$) 具有极限电位 (${\text{ü}}_{\text{大号}}$) 分别为 0、-0.36、-0.28、-0.41、-0.25 和 -0.19 eV。显着的催化活性应该与匹配良好的轨道有关${\mathit{铁}}^{{\mathrm{3个}d}_{\mathit{xz}}}$/${\mathit{铁}}^{{\mathrm{3个}d}_{\mathit{yz}}}$和${\mathit{不}}^{\pi ＊}$, 加上适度的$＊否欧$吸附自由能（${\mathrm{△}G}_{＊否欧}$). 虽然${\mathit{氧化氮}}_{\mathrm{3个}}RR$在$欧-F\mathrm{电子}-{否}_{\mathrm{4个}}$非常节能，极低${\mathrm{△}G}_{＊否欧}$(-0.02 eV) 可能导致较差的选择性${\text{氨氮}}_{\text{3个}}$产品。这些发现突出了轴向配体对${\mathit{氧化氮}}_{\mathrm{3个}}RR$的属性$\mathit{铁}-{否}_{\mathrm{4个}}-C$.