Axial coordination modification of M–N4 single-atom catalysts to regulate the electrocatalytic CO2 reduction reaction

Abstract

Earth-abundant Fe, Co, Ni based single-atom catalysts (SACs) show great promise in catalyzing the electrochemical CO₂ reduction reaction (CO₂RR) to CO, yet the reaction activity and selectivity are still unsatisfactory. Recent experiments have witnessed great activity enhancement of M–N–C SACs by axial coordination, but the reported ligands are only limited to O, Cl and N. Various organic ligands have been widely applied in stabilizing metal nanoclusters/nanoparticles, which can, in principle, be covalently grafted onto the metal SACs. Thus rationalizing this axial modification tactic is important for the design of promising CO₂RR SACs. Herein, we systematically studied MN₄ (M = Fe, Co, Ni) SACs modified by a series of organic ligands via density functional theory (DFT) computations. The introduction of axial ligands can significantly affect the adsorption and reaction characteristics. The *COOH and *CO adsorption are both weakened at Fe and Co, which changes the thermodynamically most sluggish step from *CO desorption at the Fe site and CO₂ adsorption at the Co site to *COOH formation. By contrast, the adsorption of *COOH is greatly enhanced while the physisorbed CO has barely changed at the Ni site, leading to greatly improved *COOH formation. The CO₂RR selectivity of ligand coordinated Fe and Co is enhanced, and that of Ni is well maintained by –CH₃ and –NH₂. The ligand-induced regulation of the reaction thermodynamics is related to the changes in the d-band center gap of the spin state and the electron density around the Fermi level. Our findings open new avenues for developing highly effective CO₂RR catalysts.