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

The halide anions present in the electrolyte improve the Faradaic efficiencies (FEs) of the multi-hydrocarbon (C2+) products for the electrochemical reduction of CO2 over copper (Cu) catalysts. However, the mechanism behind the increased yield of C2+ 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 CO2 to C2H4 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 C2H4 formation. We find that the presence of halide anions significantly decreases the free energy of the *CHOCH intermediate, and enhances desorption of C2H4 in the order of I->Cl->Br->F-. Lastly, the obtained results are rationalized through Bader charge analysis