The conversion of
the initial intermediate CO in the electrochemical
reduction reaction of CO
2
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 CO
2
, 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 CO
2
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 KH
2
PO
4
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 CO
2
to ethylene.