Effects of Electrolyte Anions on the Reduction of Carbon Dioxide to Ethylene and Ethanol on Copper (100) and (111) Surfaces

The CO₂ electroreduction reaction has been investigated on Cu(100) and Cu(111) surfaces in 0.1 m aqueous solutions of KClO₄ , KCl, KBr, and KI electrolyte. The formation of ethylene and ethanol on these surfaces generally increased as the electrolyte anion was changed from ClO₄^- →Cl^- →Br^- →I^- . For example, on Cu(100) at -1.23 V versus RHE, as the electrolyte anion changed from ClO₄^- to I^- , the faradaic efficiency (FE) of ethylene formation increased from 31 to 50 %, FE_ethanol increased from 7 to 16 %, and the associated current densities increased five- and sevenfold, respectively. A remarkable total FE of up to 74 % for C₂ and C₃ products was obtained in the presence of KI. Despite surface roughening in the presence of the electrolytes, the Cu(100) electrode still enhanced the formation of C₂ compounds better than Cu(111). The favorable reduction of CO₂ to C₂ products in KI electrolyte was correlated with a higher *CO population on the surface, as shown using linear sweep voltammetry. In situ Raman spectroscopy indicated that the coordination environment of *CO was altered by the used electrolyte anion. Thus, apart from affecting the morphology of the electrode and local pH value, we propose that the anion plays a critical role in enhancing the formation of C₂ products by tuning the coordination environment of adsorbed *CO, which gives rise to more efficient C-C coupling.