Copper
catalyzes the electrochemical reduction of CO to valuable
C<sub>2+</sub> products including ethanol, acetate, propanol, and
ethylene. These reactions could be very useful for converting renewable
energy into fuels and chemicals, but conventional Cu electrodes are
energetically inefficient and have poor selectivity for CO vs H<sub>2</sub>O reduction. Efforts to design improved catalysts have been
impeded by the lack of experimentally validated, quantitative structure–activity
relationships. Here we show that CO reduction activity is directly
correlated to the density of grain boundaries (GBs) in Cu nanoparticles
(NPs). We prepared electrodes of Cu NPs on carbon nanotubes (Cu/CNT)
with different average GB densities quantified by transmission electron
microscopy. At potentials ranging from −0.3 V to −0.5
V vs the reversible hydrogen electrode, the specific activity for
CO reduction to ethanol and acetate was linearly proportional to the
fraction of NP surfaces comprised of GB surface terminations. Our
results provide a design principle for CO reduction to ethanol and
acetate on Cu. GB-rich Cu/CNT electrodes are the first NP catalysts
with significant CO reduction activity at moderate overpotential,
reaching a mass activity of up to ∼1.5 A per gram of Cu and
a Faradaic efficiency >70% at −0.3 V
