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 %, FEethanol 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.