Controlling the Selectivity of the CO2 Electrochemical Reduction through Anion Effects

Despite over two decades of research on the CO₂ electrochemical reduction the selectivity and the energy efficiency of this reaction still need to be improved. A better understanding of the factors that influence this reaction is crucial in the designing new and more efficient catalyst.

Recently, Salazar-Villalpando and co-workers have suggested that the presence of halides in the solution can facilitate the reduction of CO₂ on copper. This conclusion, however, is based only the reduction currents and the reaction products were not analyzed.

In the present work we have performed CO₂ electrolysis, in KHCO₃ in the presence of halides (Cl^-, Br^-, I^-).  Interestingly, we observed that besides the positive effect that the presence of halides has in the reduction activity, there is also a clear effect on the selectivity of the reaction.

On copper, the efficiency towards hydrocarbons is enhancedby the presence of bromide and iodine. Remarkably, the ratio between methane and ethylene depends on which halide is in solution. In the presence of Cl^- and I^- both methane and ethylene are formed. By contrast, when bromide is added to the electrolyte ethylene formation is the completely suppressed and methane is the predominant product from the CO₂ electrochemical reduction.

These results indicate that halides affect differently the reaction paths for methane and ethylene. Understanding this difference could be key in the overall understanding on the reaction mechanism to C2 products which is still on debate.