Optimizing CO Coverage on Rough Copper Electrodes: Effect of the Partial Pressure of CO and Electrolyte Anions (pH) on Selectivity toward Ethylene

Abstract

The conversion of the initial intermediate CO in the electrochemical reduction reaction of CO₂ on the surface of oxide-derived Cu electrodes has been investigated as a function of partial pressure and pH, manipulated by the composition of the electrolyte. We show that in inert gas, an increase in partial pressure of CO results in a continuous increase in Faradaic efficiency (FE) for ethylene, at various potentials ranging from -0.7 to -1.1 V versus RHE, with the highest FE of ∼28% obtained using 1 bar CO at -0.8 V. When the partial pressure of CO is increased in a mixture of CO and CO₂, an optimum in the ethylene FE was found for the partial pressure of CO in the range from 0.5 bar (at -1.1 V, FE is ∼45%) to 0.8 bar (at -0.9 V, FE is ∼35%). At lower negative potentials (-0.8 to -0.7 V), the presence of CO₂ has negligible influence, and similar data to reduction of CO in inert gas were obtained. Variation of the anion in solution (0.1 M concentration) shows that the optimized FE toward ethylene increases from 5.2% in KH₂PO₄ to 43.2% in KOH. The observed differences in selectivity are attributed to anion buffering capacity and the associated local pH near the surface of the electrode. Using in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS), it was determined that the CO coverage increases as a function of increasing pH, confirming that CO coverage and pH correlate. Collectively, the data herein outline the critical role of reactant partial pressures and the significant effect of anion composition (pH) on the surface coverage of CO and concomitant selectivity in electrochemical reduction of CO₂ to ethylene.