Tuning the Catalytic Activity and Selectivity of Cu for CO₂ Electroreduction in the Presence of Halides

In the present study we demonstrate that the activity and selectivity of copper during CO₂ electrochemical reduction can be tuned by simply adding halides to the electrolyte. Comparing the production rate and Faradaic selectivity of the major products as a function the working potential in the presence of Cl^–, Br^–, and I^–, we show that the activity and selectivity of Cu depends on the concentration and nature of the added halide. We find that the addition Cl^– and Br^– results in an increased CO selectivity. On the contrary, in the presence of I^– the selectivity toward CO drops down and instead methane formation is enhanced up to 6 times compared with the halide-free electrolyte. Even though Br^– and I^– can induce morphology changes of the surface, the modification in the catalytic performance of Cu is mainly attributed to halides adsorption on the Cu surface. We hypothesizes that the adsorption of halides alters the catalytic performance of Cu by increasing the negative charge on the surface according to the following order: Cl^– < Br^– < I^–. In the case of adsorbed I^–, the induced negative charge has a remarkably positive effect favoring the protonation of CO. These results present an easy way to enhance CH₄ production during the CO2RR on Cu. Furthermore, understanding this effect can contribute to the design of new and more efficient catalysts.