The effects of halide anions on the electroreduction of CO2 to C2H4: a density functional theory study

TL;DR

This study uses density functional theory to analyze how halide anions influence the electroreduction of CO2 to ethylene on copper catalysts, revealing that halides lower energy barriers and enhance product desorption.

Contribution

It provides a detailed mechanistic understanding of how different halide anions affect C2H4 formation during CO2 electroreduction on Cu(100) surfaces, highlighting the role of halides in energy pathway modulation.

Findings

Halide anions improve C2+ product yields in CO2 electroreduction.

Presence of halides decreases free energy of key intermediates.

Order of halide effectiveness: F- < Cl- < Br- < I-.

Abstract

The halide anions present in the electrolyte gradually improves the Faradaic efficiencies (FEs) of the multi-hydrocarbon (C2+) products for the electrochemical reduction of CO2 over copper (Cu) catalysts in the order of F- < Cl- < Br- < I-. However, the mechanism behind the increased yield of C2+ products with the addition of halide anions still 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 species from the hydrogenation reaction of the adsorbed *CO acts as the key intermediate, and the C-C coupling reaction occurs preferentially between the *CHO and *CO with the formation of a *CHO-CO intermediate. Subsequently, the free-energy pathway of C2H4 formation has been proposed, and we found that the presence of halide anions significantly decreases the free energy of the *CHOCH intermediate, and enhances the desorption capacity of C2H4 in the order of F- < Cl- < Br- < I-. Lastly, the obtained results are rationalized by the Bader charge analysis.