Reaction Pathways and the Underlying Mechanism of Ni<sub>4</sub>Cu Alloy Clusters Anchored on Graphene for CO<sub>2</sub> Electroreduction to Formic Acid.

Abstract

The electrochemical CO₂ reduction reaction (CO₂RR) offers a sustainable route for converting greenhouse gases into high-value fuels; however, its efficiency has long been constrained by the thermodynamic stability of CO₂ molecules and the competing hydrogen evolution reaction. Using density functional theory (DFT) calculations, this work systematically investigates the catalytic performance of Ni₅ and alloy Ni₄Cu clusters anchored on divacancy graphene (DVG) for CO₂RR. The results demonstrate that the introduction of Cu atoms significantly enhances the interfacial binding energy between the cluster and the support (shifting from -6.2 eV to -7.5 eV). Charge density difference analysis combined with Bader charge analysis further reveals that interfacial charge transfer and the formation of Ni-C bonds serve as the electronic origin of this improved stability. Free energy calculations show that, compared to Ni₅/DVG, Ni₄Cu/DVG substantially reduces the energy barrier of the rate-determining step for formic acid (HCOOH) formation from 1.18 eV to 0.26 eV, thereby significantly optimizing the reaction kinetics. Crystal orbital Hamilton population (COHP) analysis demonstrates that Cu doping modulates metal-oxygen bond strength in the key *OCHO intermediate (ICOHP: Ni-O bonds at -0.697 eV/-0.976 eV vs. Cu-O bonds at -0.408 eV/-0.492 eV), optimizing the adsorption-desorption balance and steering selectivity toward HCOOH. This work elucidates the atomic-scale electronic and bonding mechanisms underlying Ni-Cu synergistic effects, providing theoretical guidance for designing efficient non-noble metal CO₂RR electrocatalysts.