Computational studies for boosting nitrate electroreduction activity of Fe-N4-C Single-Atom catalyst via axial fifth ligand

Single Fe atom coordinated with four planar nitrogen atoms codoped graphene layer (Fe-N4-C) has emerged as one of the promising single atom electrocatalyst (eSAC) for the remediation of nitrate (NO3-). 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 Fe-N4-C can significantly improve its catalytic activity toward electrocatalytic nitrate reduction (eNO3RR) to ammonia (NH3) with limiting potentials (UL) 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 Fe3dxz/Fe3dyz and NOπ∗, together with moderate NO∗ adsorption free energy (ΔGNO∗). Although eNO3RR on O-Fe-N4 is highly energy favorable, extremely low ΔGNO∗ (-0.02 eV) may lead to inferior selectivity toward NH3 product. These findings highlight the role of axial ligand for the eNO3RR properties of Fe-N4-C.