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

The CO 2 electroreduction reaction has been investigated on Cu(100) and Cu(111) surfaces in 0.1  m aqueous solutions of KClO 4 , KCl, KBr, and KI electrolyte. The formation of ethylene and ethanol on these surfaces generally increased as the electrolyte anion was changed from ClO 4 − →Cl − →Br − →I − . For example, on Cu(100) at −1.23 V versus RHE, as the electrolyte anion changed from ClO 4 − 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 C 2 and C 3 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 C 2 compounds better than Cu(111). The favorable reduction of CO 2 to C 2 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 C 2 products by tuning the coordination environment of adsorbed *CO, which gives rise to more efficient C−C coupling.