Effect of Halide Anions on the Electroreduction of CO2 to C2H4: A Density Functional Theory Study**

Abstract The halide anions present in the electrolyte improve the Faradaic efficiencies (FEs) of the multi‐hydrocarbon (C 2+ ) products for the electrochemical reduction of CO 2 over copper (Cu) catalysts. However, the mechanism behind the increased yield of C 2+ products with the addition of halide anions remains indistinct. In this study, we analysed the mechanism by investigating the electronic structures and computing the relative free energies of intermediates formed from CO 2 to C 2 H 4 on the Cu (100) facet based on density functional theory (DFT) calculations. The results show that formyl *CHO from the hydrogenation reaction of the adsorbed *CO acts as the key intermediate, and the C−C coupling reaction occurs preferentially between *CHO and *CO with the formation of a *CHO‐CO intermediate. We then propose a free‐energy pathway of C 2 H 4 formation. We find that the presence of halide anions significantly decreases the free energy of the *CHOCH intermediate, and enhances desorption of C 2 H 4 in the order of I − >Cl − >Br − >F − . Lastly, the obtained results are rationalized through Bader charge analysis.