Angewandte
Communications
Chemie
Heterogeneous Catalysis
Controllable Hydrocarbon Formation from the Electrochemical
Reduction of CO2 over Cu Nanowire Arrays
Abstract: In this work, the effect of Cu nanowire morphology
on the selective electrocatalytic reduction of CO2 is presented.
Cu nanowire arrays were prepared through a two-step syn-
thesis of Cu(OH)2 and CuO nanowire arrays on Cu foil
substrates and a subsequent electrochemical reduction of the
CuO nanowire arrays to Cu nanowire arrays. By this simple
synthesis method, Cu nanowire array electrodes with different
length and density were able to be controllably synthesized. We
show that the selectivity for hydrocarbons (ethylene, n-
propanol, ethane, and ethanol) on Cu nanowire array electro-
des at a fixed potential can be tuned by systematically altering
the Cu nanowire length and density. The nanowire morphology
effect is linked to the increased local pH in the Cu nanowire
arrays and a reaction scheme detailing the local pH-induced
formation of C2 products is also presented by a preferred CO
dimerization pathway.
intermediates (such as CO and COH) that could influence
the formation of final products.[11,15] One of the important
parameters in the electroreduction of CO2 is the pH that is
related to the formation of intermediates in certain reaction
pathways, which could have a significant effect on products
formation. The effect of local pH at the electrode/electrolyte
interface on the selectivity of hydrocarbon products in the
electroreduction of CO2 was proposed by Hori in 1989,[16]
showing that a locally high pH formed near Cu electrodes
could facilitate the reduction of the intermediate CO to C2H4
and alcohols. Recently, Koper et al.[17–19] demonstrated that
the electrolyte pH could play a key role in the product
selectivity towards different hydrocarbons and proposed
a CO coupling mechanism, indicating that C2H4 could be
formed from a CO dimer adsorbed on Cu catalysts. Further-
more, Mul et al. reported that the formation of C2H4 from
a CO coupling mechanism is favorable at a high local pH.[20]
Recently, we showed that CuO-derived Cu nanowire
(NW) array electrodes are capable of reducing CO2 to CO at
a moderate overpotentials.[21] At more negative potentials,
hydrocarbon gas phase products on Cu NW arrays were
observed.[21] In this work, we tailor the selectivity of hydro-
carbon products on Cu NW arrays by systematically varying
the length and density of the Cu NWs which can offer a high
local pH within the NW arrays. In addition, this study
provides further insight into the mechanism of the hydro-
carbon formation due to the enhanced CO dimerization
afforded by the NW morphology.
Cu(OH)2 NWs were first synthesized on Cu foils by
immersing Cu foils into a solution mixture containing 0.133m
(NH3)2S2O8 and 2.667m NaOH.[22,23] The increased length of
Cu(OH)2 NWs was obtained by immersing Cu foils in the
solution mixture for longer time. After a discrete synthesis
time, the Cu foils were taken out from the solution, rinsed
with de-ionized water and absolute ethanol, and dried with
nitrogen. CuO NWs were then fabricated by annealing the
Cu(OH)2 NW arrays at 1508C for 2 hours in air.[21] The
resulting CuO NW arrays were directly used in the electro-
reduction of CO2, and were electrochemically reduced to Cu
NW arrays during electrolysis.[21] Thus, the annealed Cu(OH)2
NWs with gradually increased length and density were
electrochemically reduced to Cu NWs with corresponding
increased length and density.
T
he electrochemical reduction of CO2 to fuel by using
renewable energy has attracted considerable attention for
closing the anthropogenic carbon cycle.[1–6] From electro-
reduction of CO2, the captured CO2 at the large emission
sources could be used as a sustainable feedstock to be
electrochemically reduced to high energy density hydro-
carbons (such as CH4 and C2H4).[6–9] Hydrocarbon products
can be conveniently used as fuels within the existing energy
infrastructure.[7] For achieving this goal, it is critical to
develop a stable and cost-effective catalyst with high selec-
tivity and efficiency. Researchers have identified various
electrocatalyst materials that are capable of reducing CO2
electrochemically in CO2-saturated aqueous solutions.[6–14]
Among currently identified catalyst materials, Cu is the only
known material that is capable of catalyzing the formation of
significant amounts of hydrocarbons at high reaction rates in
CO2-saturated aqueous solutions at ambient temperature and
pressure.[8,10] However, controlling the selectivity of the
catalytic reduction of CO2 to a desired hydrocarbon product
on a Cu catalyst remains a significant scientific challenge.
The electrochemical reduction of CO2 to hydrocarbons on
Cu catalysts is a complex process with many adsorbed
[*] M. Ma, Dr. W. A. Smith
Materials for Energy Conversion and Storage (MECS), Department of
Chemical Engineering, Delft University of Technology
P.O. Box 5045, 2600 GA Delft (The Netherlands)
Figure 1 shows typical SEM images of Cu(OH)2 NW
arrays synthesized under different synthesis time. The corre-
sponding length of Cu(OH)2 NWs prepared at different
synthesis time were characterized by SEM (see Figure S4 in
the Supporting Information), and the NW length as a function
of synthesis time is shown in Table S1. The increase of NW
length follows an enhanced NW density with increasing
Dr. K. Djanashvili
Department of Biotechnology, Delft University of Technology
P.O. Box 5045, 2600 GA Delft (The Netherlands)
Supporting information for this article can be found under:
6680
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 6680 –6684