Stable and selective electrochemical reduction of carbon dioxide to ethylene on copper mesocrystals

Article abstract of DOI:10.1039/c4cy00906a

Stable and selective electrochemical reduction of carbon dioxide to ethylene was achieved using copper mesocrystal catalysts in 0.1 M KHCO₃. The Cu mesocrystal catalysts were facilely derived by the in situ reduction of a thin CuCl film during the first 200 seconds of the CO₂ electroreduction process. At -0.99 V vs. RHE, the Faradaic efficiency of ethylene formation using these Cu mesocrystals was ～18× larger than that of methane and forms up to 81% of the total carbonaceous products. Control CO₂ reduction experiments show that this selectivity towards C₂H₄ formation could not be replicated by using regular copper nanoparticles formed by pulse electrodeposition. High resolution transmission electron microscopy reveals the presence of both (100)_Cu facets and atomic steps in the Cu mesocrystals which we assign as active sites in catalyzing the reduction of CO₂ to C₂H₄. CO adsorption measurements suggest that the remarkable C₂H₄ selectivity could be attributed to the greater propensity of CO adsorption on Cu mesocrystals than on other types of Cu surfaces. The Cu mesocrystals remained active and selective towards C₂H₄ formation for longer than six hours. This is an important and industrially relevant feature missing from many reported Cu-based CO₂ reduction catalysts.

Full text of DOI:10.1039/c4cy00906a

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Stable and selective electrochemical reduction
of carbon dioxide to ethylene on copper
mesocrystals†
Cite this: DOI: 10.1039/c4cy00906a
Chung Shou Chen, Albertus D. Handoko, Jane Hui Wan, Liang Ma, Dan Ren
*
and Boon Siang Yeo
Stable and selective electrochemical reduction of carbon dioxide to ethylene was achieved using
copper mesocrystal catalysts in 0.1 M KHCO3. The Cu mesocrystal catalysts were facilely derived by the
in situ reduction of a thin CuCl film during the first 200 seconds of the CO2 electroreduction process.
At −0.99 V vs. RHE, the Faradaic efficiency of ethylene formation using these Cu mesocrystals was
~18× larger than that of methane and forms up to 81% of the total carbonaceous products. Control CO2
reduction experiments show that this selectivity towards C2H4 formation could not be replicated by using
regular copper nanoparticles formed by pulse electrodeposition. High resolution transmission electron
microscopy reveals the presence of both (100)Cu facets and atomic steps in the Cu mesocrystals which we
assign as active sites in catalyzing the reduction of CO2 to C2H4. CO adsorption measurements suggest
that the remarkable C2H4 selectivity could be attributed to the greater propensity of CO adsorption on
Cu mesocrystals than on other types of Cu surfaces. The Cu mesocrystals remained active and selective
towards C2H4 formation for longer than six hours. This is an important and industrially relevant feature
missing from many reported Cu-based CO2 reduction catalysts.
Received 11th July 2014,
Accepted 9th August 2014
DOI: 10.1039/c4cy00906a
www.rsc.org/catalysis
of the CO2 reduction process.8,9 For the above reasons,
considerable effort has been dedicated to understand the
Introduction
Carbon dioxide reduction to hydrocarbons and alcohols has
the potential of generating a sustainable supply of valuable
feedstock for our chemical industries and fuels for our energy
needs.1 This process also mitigates excessive CO2 buildup in
the atmosphere which contributes to global climate warming.
CO2 can be electrochemically reduced to hydrocarbons such
as ethylene and methane via 2CO2 + 12e− + 8H2O → C2H4 +
12OH− and CO2 + 8e− + 6H2O → CH4 + 8OH−, respectively.
C2H4 is a particularly valuable product as it has widespread
applications in many industries including agriculture and
polymer manufacturing. To date, the most promising cata-
lyst that can electroreduce CO2 to C2H4 is copper metal.2,3
However, alongside C2H4, many carbonaceous side-products
including methane (CH4), carbon monoxide (CO) and formate
(HCOO−) are also simultaneously formed.4–7 Furthermore,
the Cu catalysts are highly susceptible to poisoning and
deactivation, commonly within 30 minutes from the start
structure and composition of materials with the aim of devel-
oping catalysts that can selectively reduce CO2 to C2H4 over a
long period of time.7,10,11
Polycrystalline Cu surfaces do not show significant prefer-
ence towards ethylene formation, with a C2H4/CH4 product
ratio of around 1 to 2.3–5,12,13 It is thought that their lack of
selectivity originates from the great heterogeneity of sites
present on the polycrystalline surface, each with different cata-
lytic activities. This is underscored by the work of Hori et al.
which investigated the effect of different Cu facets on the
selectivity of CO2 electroreduction.7,14 Single crystal (100)Cu
surfaces were found to favor the formation of C2H4 more than
(111)Cu, as indicated by their C2H4/CH4 ratios of 1.3 and 0.2,
respectively.7 Interestingly, when the high index (711)Cu
,
(911)Cu, and (810)Cu planes, formed by cleaving (100)Cu, were
examined, they displayed even higher selectivity towards C2H4
with the C2H4/CH4 ratio value increasing to 10 for (711)Cu
.
Whether the selectivity is due to increased population of
atomic steps exposed in such high index facets or a certain
periodic spacing in the exposed copper terraces, it is clear that
certain Cu facets do exhibit preference towards different
hydrocarbons and it is possible to tune them.
Department of Chemistry, National University of Singapore, 3 Science Drive 3,
Singapore 117543. E-mail: chmyeos@nus.edu.sg; Fax: +65 67791691;
Tel: +65 65162836
† Electronic supplementary information (ESI) available: Calculation of Faradaic
efficiencies, representative raw data, electron micrographs and electrochemical
data. See DOI: 10.1039/c4cy00906a
Besides single crystal Cu surfaces, enhancements in C2H4
selectivity during CO2 reduction were also observed on CuO
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and CuI-halide coated electrodes.15,16 Sustaining stable cata-
lytic activities on these catalysts is however more challenging
because CuII or CuI will inevitably be reduced to Cu metal
during the CO2 reduction process. Thus, in the case of
Cu oxide, intermittent anodic pulses of +1 to +2 V had to be
significantly. Our discovery of a facilely prepared, robust
and selective catalyst based on an earth-abundant metal such as
Cu represents a major step forward towards the realization of
industrial-scale reduction of CO2 to C2H4.
applied during the CO2 reduction process to maintain the Experimental
Cu oxide in its catalytically active oxidation state.15 Large
cathodic potentials of up to −3 V (vs. Ag/AgCl) were also
Catalyst preparation
Only deionized Type I water (18.2 MΩ cm, Barnstead, Thermo
required for the CO2 reduction process, which significantly
increased the energy input of the system. High surface area
Cu nanoparticles have been shown to offer good selectivity
towards hydrocarbon formation, especially C2H4.6 It was pro-
posed that the numerous steps and edges formed on the sur-
faces of the Cu nanoparticles could be crucial for selective
C2H4 formation. In support of this, quantum chemical simu-
lations indicated that reaction intermediates like CHO*
are more stable on the (211)Cu surface steps than on (100)Cu
terraces. This could lead to their concentration build up and
eventual dimerization to C2H4.17 More recently, thick Cu2O
films have also shown promising selectivity towards C2H4
formation.18 Local pH changes associated with the thickness
of the films were proposed to induce this selectivity, as pH
has been shown to alter the production rates of various CO2
reduction products.10
These preceding studies have inspired us to develop a
stable C2H4-selective electrocatalyst based on copper and
understand how this selectivity came to fruition. Herein, we
report the activity and characteristics of novel copper meso-
crystals for the selective electroreduction of CO2 to C2H4.
These mesocrystals were facilely prepared by electrochemically
roughening a Cu electrode in KCl electrolyte to produce a thin
overlayer of CuCl. CuCl was then reduced in situ during
the CO2 electroreduction process to yield Cu mesocrystals.
These catalysts are highly active for CO2 reduction to C2H4,
which was demonstrated by the high C2H4/CH4 Faradaic effi-
ciency (FE) ratio of ~18. The FE of C2H4 results in up to ~81%
of the total carbonaceous products. Thorough materials
analysis reveals that the morphology of these mesostructurally
arranged 30–50 nm copper particles displayed both (100)Cu
facets and numerous steps/edges. Cyclic voltammetry studies
indicate that the C2H4 selectivity of these mesocrystals can
be correlated with the propensity and stronger adsorption
of CO intermediates on their surfaces. Control experiments
performed on copper nanoparticles prepared by pulse electro-
deposition or electropolished Cu surfaces show that this
selectivity could not be simply replicated by ordinary nano-
particulate or bulk copper surfaces.
Scientific) was used for washing and for preparing solutions.
10 mm diameter Cu metal discs (99.99%, Goodfellow Inc.)
were used as the base to prepare all catalysts. These discs were
mechanically polished with SiC paper and alumina slurries,
resulting in a mirror-like finish.19 Between each step, the copper
discs were ultrasonicated in deionized water and 0.1 M KOH
to remove any alumina particles left on their surface.
The following catalysts were prepared.
1. Catalyst A: Cu mesocrystals. Polished copper discs
were electrochemically roughened in aqueous 0.1 M KCl
using five triangular potential scans ranging from 0.24 V to
1.74 V (vs. RHE) at a rate of 500 mV s−1. During each cycle,
the potential was held at the positive and negative limits for
10 and 5 seconds, respectively. They were then rinsed and
washed with copious amounts of deionized water several
times. Cu mesocrystals were then formed in situ during the
CO2 electroreduction process in CO2-saturated 0.1 M KHCO3
(99.99%, Sigma Aldrich). These parameters were the optimum
for obtaining a mechanically stable layer of Cu mesocrystals.
This sample shall be addressed in this article as catalyst A or
Cu mesocrystals.
2. Catalyst B: Cu nanoparticles. Cu nanoparticles were
electrodeposited on polished Cu discs using 3000 cycles of
galvanostatic pulse deposition. A square wave pulsed current
setting was used, alternated between −4.94 and +2.49 mA cm−2,
with a 100 ms duration for both anodic and cathodic pulses.
The electrolyte consisted of 0.01 M CuSO4 (extra pure, GCE
Laboratory Chemicals), 0.1 M Na2SO4 (≥99.0%, Sigma Aldrich)
and 0.1 M H2SO4 (98%, RCI Labscan). This sample shall be
addressed as catalyst B or Cu nanoparticles.
3. Catalyst C: electropolished Cu. Mechanically-polished
Cu disks were electropolished in phosphoric acid (85%,
RCI Labscan) at 259.7 mA cm−2 anodic current for 60 seconds
and then rinsed with deionized water.6 This sample shall be
addressed as catalyst C or electropolished Cu.
Online electrochemical Gas Chromatography (GC)
Electrochemical measurements were performed using a
Gamry 600 galvanostat/potentiostat in a three-electrode cell con-
figuration with an Ag/AgCl reference electrode (ET072, eDAQ)
and a Pt mesh counter electrode. The potential of the Ag/AgCl
reference electrode was checked daily against a reversible
hydrogen electrode (HydroFlex®, Gaskatel).
Our results demonstrate that both (100)Cu facets and
steps formed on copper mesocrystals are essential for the
selective reduction of CO2 to C2H4. These copper meso-
crystals remain very active and selective for C2H4 production
for over six hours. They are also robust enough to be taken
out mid-reaction, exposed to the environment and reintroduced
to fresh electrolyte for a full round of CO2 electroreduction
without significant loss in activity. Addition of Cl− to the
electrolyte did not affect the activity of the copper mesocrystals
A
custom-made, gas-tight two compartment poly-
tetrafluoroethylene cell was used for CO2 reduction experiments.4
The anodic and cathodic compartments were separated by an
anion-exchange membrane (Selemion AMV, AGC Asahi Glass).
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A holder gripped the working electrode firmly in place and
exposed only a circular geometric surface area of 0.385 cm2.
The cathodic compartment was filled with 32 ml of electrolyte,
leaving approximately 3 ml of headspace. Before each mea-
surement, the aqueous 0.1 M KHCO3 (99.99%, Sigma Aldrich)
electrolyte was saturated with CO2 gas (99.999%, Linde Gas)
for at least 30 minutes. All experiments were performed at 298 K.
CO2 was continuously pumped into both the catholyte and
anolyte during the CO2 reduction process at a rate of 20 sccm
using calibrated mass flow controllers (MC-100SCCM-D,
Alicat Scientific). The gas outlet of the cathodic compartment
was connected to a gas chromatograph (GC-7890A, Agilent
Technologies) for periodical sampling. The GC is equipped
with a thermal conductivity detector (TCD) for detecting H2
and two flame ionization detectors (FID, one fitted with a
methanizer) for detecting hydrocarbons and CO. The carrier gases
for the TCD and FIDs are nitrogen and helium, respectively.
The GC was calibrated regularly using standard gas mixtures
(Singapore Oxygen Air Liquide and National Oxygen Pte Ltd)
under standard conditions (1 atm, 298 K).
drop-casted on a 300 mesh nickel grid coated with lacey
carbon (LC325-Ni, Electron Microscopy Sciences). Only smaller
particles, with sizes ≤50 nm, could be analyzed by TEM as the
electron beam cannot penetrate through larger particles.
X-ray photoelectron spectroscopy (XPS) was performed
using a Kratos AXIS Ultra (Al Kα emission source). An ultrathin
Pt film was sputter-coated onto the catalyst surface just before
XPS measurement as the internal standard for binding energy
calibration. X-ray diffraction (XRD) of the films was performed
using a Bruker D8 (Cu Kα, 40 kV, 40 mA). The incoming X-ray
angle was kept at 0.1° to minimize the diffraction signal from
the underlying copper disc.
iR drop compensation and reporting of working electrode
potentials and currents
iR drop compensation was performed during the electro-
chemical measurements using the current interrupt technique.
All of the potentials measured in this work are referenced to
the RHE using the following conversion:
A typical CO2 electroreduction experiment at a constant
applied voltage spanned over 4200 seconds. A total of six gas
aliquots (2.5 cm3 each) were measured. They were injected
into the GC every 10 minutes by an automated sampler. The
first injection is performed 230 seconds after the start of
the CO2 reduction reaction; this ensures adequate flushing of
the transfer line of atmospheric contaminants. The GC data
collected in this work are translated to Faradaic efficiencies
(FE) (ESI† section S1). The Faradaic efficiencies of the
products in all measurements amount to ~90% or better,
demonstrating that all of the major electroreduction products
have been detected.
ERHE(V) = EAg/AgCl(V) + 0.205 V + (0.059 V × pH)
The pH values of the electrolytes are listed in the ESI† S2.
The current density values reported in this work were nor-
malized to the geometric surface area.
Results and discussion
GC results: stable ethylene selectivity of copper mesocrystals
Representative chronoamperograms of catalysts A, B and C
(A = Cu mesocrystals, B = Cu nanoparticles and C = electro-
polished Cu) during CO2 electroreduction at −0.99 V are
presented in Fig. 1A. Catalyst A exhibited significantly higher
reduction current during the first ~200 seconds that peaked
at −65 mA cm−2 before stabilizing to ca. −25 mA cm−2.
On the basis of ex situ characterization of catalyst A at
different times during the electrochemical CO2 reduction
(see the following section), this temporal reduction current
was attributed to the reduction of CuCl to Cu mesocrystals.
When steady state currents were compared, catalyst A always
displayed the highest current density, which is approximately
twice and three times those of catalysts B and C, respectively
(ESI† section S3). We have determined, from their cyclic
voltammograms, that the ratio of the electroactive surface
areas of catalysts A, B and C is approximately 5 : 2 : 1. Hence,
the difference in the measured currents can be attributed to
differences in the surface areas of the catalysts.
The detected products in catalysts A, B and C are
methane, ethylene, carbon monoxide, formate and hydrogen,
and their Faradaic efficiencies vary with the applied working
potentials (Fig. 1B, ESI† section S4). Strikingly, only catalyst A
exhibited significant preference towards C2H4 formation. At
−0.99 V, the FE values of C2H4 and CH4 are 27.2% and
1.47%, respectively, which gives a C2H4/CH4 ratio of ~18
(Fig. 1B–C). This ratio is greater than the C2H4/CH4 selectivity of
~10 found for high index (711)Cu single crystal copper surfaces.7
NMR detection of formate
After each CO2 electroreduction experiment, 2 mL of the
catholyte was mixed with 0.1 mL of an internal standard
consisting of 25 mM phenol (99.5%, Scharlau) and 5 mM
dimethyl sulfoxide (99.9%, Quality Reagent Chemical). 0.5 mL
of this aqueous mixture was then added to 0.7 mL of D2O
(99.96% deuterium, Cambridge Isotope Laboratories). Solvent
suppression was applied to decrease the intensity of the water
peak. The results presented here are the average of 52 1D
1H NMR scans (Avance 300, Bruker) and processed using the
WIN-NMR software (Bruker). Formate (chemical shift 8.3 ppm)
was the only CO2 reduction product detected in the electrolyte.4
Characterization of copper catalysts
The surface morphologies of the Cu catalysts were analyzed by
scanning electron microscopy (SEM, JEOL JSM-6710F) operated
in secondary electron mode (5 keV, 10 mA probe current).
High resolution transmission electron micrographs of selected
catalysts were obtained using a JEOL TEM-3010. For the TEM
measurements, the top layer of the Cu samples was scraped
off the substrate and suspended in isopropanol using ultra-
sonication. A drop of the homogenized solution was then
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Fig. 1 (A) CO2 reduction current as a function of time for catalysts A (Cu mesocrystals), B (Cu nanoparticles) and C (electropolished Cu). Potential
applied: −0.99 V. The inset is a zoomed-in picture of the reduction currents at the start of the CO2 reduction process. (B) Faradaic efficiencies of
the CO2 electroreduction products of catalyst A as a function of potential. A comparison of the (C) Faradaic efficiencies and (D) production rates
of CO2 electroreduction products on catalysts A, B and C at −0.99 V.
As a comparison, catalysts B and C exhibited C2H4/CH4 ratios
of only 2.3 and 3.1, respectively, consistent with the ratios
reported previously for polycrystalline and sputtered Cu electrodes
(Fig. 1C).4–6,20 C2H6 was also detected on Cu mesocrystals,
with a Faradaic efficiency of ~0.3%, (ESI† section S1) but not
on the other two catalysts. The Faradaic efficiency of C2H4 for-
mation using catalyst A is ~81% of the total carbonaceous
product yield (Fig. 1C). This is significantly higher than the
C2H4 yield of 45% and 40% found for catalysts B and C. This
figure is also higher than the 61% C2H4 yield found for
(711)Cu.7 In terms of the production rate, C2H4 is produced at
a rate of ~17 μmol cm−2 h−1 using catalyst A (Fig. 1D). This
is approximately an order of magnitude higher than that
observed for catalyst C. These figures of merit demonstrate
that there is a significant improvement in the selectivity
of CO2 reduction to C2H4 when the Cu mesocrystal catalyst
is used.
The rapid deactivation of the Cu catalysts is a major obstacle
that will hinder any efforts to industrially scale up the CO2
electroreduction process to produce C2H4.8,9 It is therefore
important to assess the stability of our copper electrodes over
longer CO2 electroreduction periods. A bias of −0.99 V was
chosen as it showed the optimum selectivity towards C2H4,
especially for copper mesocrystals (catalyst A, Fig. 1B). Results
presented in Fig. 2 demonstrate that the copper mesocrystals
displayed superior and stable catalytic activity towards C2H4
formation for >6 hours (Fig. 2A). In contrast, catalysts B and C
showed rapid catalytic deactivation. The Faradaic efficiencies
Fig. 2 Composite plot of current density vs. time (black trace) at −0.99 V and Faradaic efficiencies of CO (blue trace), C2H4 (green), CH4 (red) and
2 (inset, magenta) over six hours of CO2 reduction on (A) catalyst A, (B) catalyst B, and (C) catalyst C.
H
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of their CH4 and C2H4 products declined over the course
of the electrolysis, with the FE of C2H4 using catalyst C
decreasing by 75%. H2 production has also increased for
these two catalysts. Deactivation of Cu catalysts has been
previously attributed to contamination of trace amounts
of heavy metals present in the electrolyte.8 The subsequent
sections of this work will be devoted to in-depth materials
and electrochemical investigation of the origin of the higher
ethylene selectivity and stability of our Cu mesocrystal catalyst.
converted to cuboids approx. 400–500 nm across (Fig. 3E). A
longer exposure time (~100 seconds) resulted in the growth
of smaller nanoparticles (30–50 nm) over the well-defined
cuboids' surface (Fig. 3F–H; see also the ESI† section S5).
These small and rough nanoparticles were determined by
XRD and XPS to be crystalline copper metal (Fig. 3C–D). No
CuCl, Cl, or metal contaminants such as Fe or Zn could be
detected on the catalyst's XPS spectra after 4200 seconds of
the CO2 electrochemical reduction process. As there should
be no Cu ions present in the KHCO3 electrolyte initially, the
formation of Cu mesocrystals from CuCl must have proceeded
via dissolution–redeposition21 followed by oriented attachment22
or a self-limiting electrochemical aggregative growth mechanism,23
with the large Cu cuboids' surface acting as the orientation
template.24
The representative high resolution transmission electron
micrograph of the 30–50 nm Cu nanoparticle clusters, obtained
by scraping catalyst A after 4200 seconds of CO2 electro-
reduction, is shown in Fig. 3I. The marked d-spacings in these
nanoparticles correspond to those of the Cu metal only (Fig. 3J).
The most striking observation in the HRTEM images is the
clear presence of numerous atomic steps and (100)Cu terraces
on the surfaces of these copper nanoparticles (Fig. 3K–L, also
Chemical and structural analyses of the Cu mesocrystals
Cu mesocrystals (catalyst A) were analyzed before, during and
after CO2 reduction. The initial steps of Cu mesocrystal for-
mation involved five cycles of electrochemical roughening of
the polished Cu discs in 0.1 M KCl solution. After this rough-
ening procedure, aggregated particles of ~400–1000 nm in
size were found on top of the polished Cu (Fig. 3A–B). X-ray
diffraction and XPS revealed that these particles are CuCl
(Fig. 3C–D).‡ The CuCl-covered Cu discs were then rinsed with
deionized water and used directly for electrochemical CO2
reduction.
Within 10–20 seconds from the application of −0.99 V to
the electrode in CO2-saturated KHCO3, the CuCl layer was
Fig. 3 SEM micrographs of (A) polished Cu and (B) catalyst A before CO2 reduction. (C) X-ray diffraction and (D) XPS data for catalyst A before
(black trace) and after 4200 seconds of CO2 reduction (red trace). The hashes in (C) indicate trace amounts of CuCl2 while the asterisks in
(D) indicate the internal Pt standard. The inset in panel (D) shows a higher resolution scan region where the Cl 2p XPS peak occurs. Time
resolved ex situ SEM micrographs of catalyst A after (E) 10, (F) 100, and (G–H) 4200 seconds of CO2 electroreduction. (I–L) HRTEM micrographs
of the same catalyst in (G–H) under increasing magnifications. Only lattice parameters belonging to the Cu metal were detected. The dark
arrows indicate some of the steps and edges present on the Cu mesocrystals.
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see the ESI† S6). This arrangement is reminiscent of the high
index facet exposed by off-cutting (100)Cu single crystals, exposing
numerous high index atomic steps with (100)Cu terraces in
between.7 However, as our Cu mesocrystals have larger
surface areas than single crystal copper surfaces, there will
be significantly more high index atomic steps. The (100)Cu
surface terminations of the Cu mesocrystals may have been
promoted by the underlying Cu cuboid template when it was
formed from CuCl (Fig. 3E). This process usually has a lower
activation energy.25 Similar mesocrystals were also observed
after CO2 electroreduction at different potentials between
−0.69 and −0.89 V (ESI† section S7). We thus conclude that
the selective reductions of CO2 to ethylene at other potentials,
while showing variable product ratios, were catalyzed by the
same Cu mesocrystals.
Catalyst B (Cu nanoparticles) was introduced to assess the
importance of the well-defined cuboid surface (in Fig. 3E) as
the template for the mesocrystals' nucleation and meso-
structural arrangement. These electrodeposited copper nano-
particles are relatively small and possess high surface areas
(Fig. 4A–B). However, as they were grown directly on polycrys-
talline copper discs, which do not have a significant number
of exposed (100)Cu growth templates like catalyst A, rounded
particles are formed without any clear (100)Cu termination.
This morphology was retained throughout the CO2 reduction
at −0.99 V (Fig. 4C). High resolution TEM revealed that the
surfaces of these rounded nanoparticles were smoothly gradated,
possibly with many steps, but no terraces could be found
(Fig. 4D inset, see also the ESI† S8). This surface can thus be
classified as an extremely high index plane with many steps
but with no terraces. Although Hori et al. have shown that
high index stepped surfaces are important for C2H4 selectivity,
exposing an even higher index plane than (711)Cu appears to
be detrimental to C2H4 selectivity.7 This suggests that (100)Cu
terraces are important and high C2H4 selectivity can only be
achieved by balancing the step and terrace population. Thus
unsurprisingly, catalyst B only displayed a C2H4/CH4 ratio of
2.3, not very different from regular polycrystalline copper.4
Catalyst C was introduced to represent smooth polycrystal-
line Cu surfaces. As seen in Fig. 4E, electropolishing strips
the Cu surface of protrusions by dissolving them in
phosphoric acid, resulting in a very smooth surface. As a
result, catalyst C was not found to be selective towards C2H4
formation, which is in agreement with previous reports on
polycrystalline copper.4,5 After CO2 reduction, the surface
becomes slightly rougher with newly formed protrusions
(Fig. 4F). The roughening could be attributed to the prolonged
application of cathodic potentials. However, this roughened
surface does not exhibit any selectivity towards C2H4 (Fig. 2C).
CO adsorption studies and robustness of Cu mesocrystals
Thus far, we have demonstrated that copper mesocrystals are
highly selective towards the electrocatalytic reduction of CO2
to C2H4. We have also established, by comparing different
copper surfaces, that the presence of both (100)Cu facets and
atomic steps/edges is essential for their selectivity.
The dimerization of CO (or CHO*) on copper has been
postulated to be the key step for C2H4 formation during CO2
reduction.26–28 This suggests that the population of active
sites responsible for C2H4 formation can be probed by CO
adsorption.
Baseline measurements of these three catalysts in
Ar-saturated 0.1 M KHCO3 were first taken. The oxidation
and reduction potentials of the Cu catalysts in all three cata-
lysts agree well with previous reports (Fig. 5A).6 Cu meso-
crystals (catalyst A) show the highest current density and a
more complex set of oxidation/reduction peaks. These obser-
vations can be respectively attributed to their larger surface
area and to the greater number of exposed steps/facets
on their surface.6,29 The CV measurements in CO- and
CO2-saturated electrolytes were very similar (Fig. 5B–C),
which suggests that these two compounds can react similarly
on Cu surfaces.20 The shoulders at −0.7 to −0.9 V in the CVs
shown in Fig. 5B–C are usually attributed to the formation of
adsorbed CO.30 However, more recent work by Kortlever et al.
indicates that these peaks are instead related to the direct
reduction of bicarbonate to formate.31
Fig. 4 SEM micrographs of electrodeposited copper nanoparticles
(A) before and (B) after CO2 reduction. (C) A representative HRTEM
micrograph of electrodeposited Cu nanoparticles after CO2 reduction.
(D) Increased magnification of the marked area in (C). SEM micrographs
of the electropolished Cu surface (E) before and (F) after CO2 reduction.
When the CVs of the electrodes in the CO-saturated electro-
lyte were examined between 0 to −0.5 V, an anodic peak
feature around −0.14 V was observed exclusively on the
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Conclusion
In this work, we report the highly selective and stable
electroreduction of CO2 to C2H4 on Cu mesocrystals. A
C2H4/CH4 FE ratio of ~18 was achieved. The in situ forma-
tion of this novel electrocatalyst from CuCl resulted in the
mesostructurally arranged 30–50 nm (100)Cu terminated
particles bearing many steps and edges. These specific sur-
face arrangements lead to higher availability of active sites
on Cu mesocrystals, indicated by higher and stronger CO
adsorption, a crucial factor that leads to high C2H4 selectivity.
Control CO2 electroreduction experiments on electrodeposited
Cu nanoparticles and electropolished Cu could not replicate
the high C2H4 selectivity exhibited by Cu mesocrystals. Despite
having large surface areas, electrodeposited Cu nanoparticles
appeared more rounded and did not exhibit clear (100)Cu
facets or terraces. They displayed a similar product distribu-
tion as the electropolished Cu surface. These control experi-
ments affirmed that the presence of (100)Cu with steps as
imaged in the Cu mesocrystals is indeed essential for the
selective reduction of CO2 to C2H4.
Fig. 5 Cyclic voltammograms of the three Cu catalysts in (A) Ar-,
(B) CO2- and (C) CO-saturated 0.1 M KHCO3. (D) Close comparison of
the CVs of (i) Cu mesocrystals, (ii) Cu nanoparticles and (iii) electro-
polished Cu in Ar- (broken line) and CO- (solid line) saturated 0.1 M
KHCO3. The arrows indicate CO adsorption/desorption features
around −0.14 V (only on Cu mesocrystals) and −0.35 V (on all samples).
These CV measurements were performed after 4200 seconds of CO2
electroreduction.
Most remarkably, our Cu mesocrystals are stable and selec-
tive towards C2H4 for over six hours. Exposure to chloride
anions during reaction or atmospheric contaminants also
does not significantly change the catalytic activity of the
Cu mesocrystals. We believe that the development of facilely
prepared, stable and selective Cu mesocrystals is a significant
step towards the realization of industrial-scale CO2 reduction
to ethylene.
Cu mesocrystals (Fig. 5D(i)). In contrast, both the Cu nanoparticles
and the electropolished Cu showed only a faint peak at −0.35 V.
These peaks are reminiscent of the CO adsorption/desorption
features observed in single crystal copper surfaces.7 The more
positive position and greater peak intensity in the CV of the
Cu mesocrystals indicate that they adsorb CO more readily as
compared to the other two catalysts.32
Since dimerization of CO (or CHO*) is a key step in ethylene
production, the fact that CO can be better adsorbed on the
Cu mesocrystals can explain their preferred selectivity for
ethylene production. The likely consequence is that CO and
CH4 production rates would be correspondingly lower on the
Cu mesocrystals. This was indeed what we observed (Fig. 1D).
The Faradaic efficiency of CO during CO2 electroreduction on
different Cu single crystal surfaces has also been shown to be
inversely proportional to that of C2H4 in the literature.7
Acknowledgements
This work is supported by a start-up grant (R143-000-515-133)
from the National University of Singapore. A. D. H. is grateful
for a research fellowship from Singapore-Peking-Oxford Research
Enterprise (R-706-000-100-414).
The authors thank Dr. Yuan Ze Liang (NUS Chemical
and Biomolecular Engineering) for XPS measurements and
Mr. Lee Ka Yau (NUS Chemistry) for TEM measurements.
Unlike catalysts B and C, catalyst A's C2H4 production rate
is constant for six hours (Fig. 2). A reason for this enhanced
stability could be the minimum exposure of the Cu meso-
crystals to the atmosphere as they were formed in situ from
CuCl during the CO2 reduction process. However, we found
that catalyst A remained very active even when it was taken
out mid-reaction, exposed to ambient atmosphere for several
minutes, and then reintroduced back to the cell for further
CO2 reduction (ESI† section S9). The presence of Cl− has also
been reported to alter product selectivity in CO2 electro-
reduction.3,16 However, we found that the activity and product
distribution of CO2 reduction on the Cu mesocrystals were
not affected by Cl− added to the KHCO3 electrolyte (ESI† S9).
These experiments demonstrate that the Cu mesocrystals are
inherently more stable and resistant to atmospheric and trace
contaminants than ordinary Cu surfaces or nanoparticles.
Notes and references
‡ Trace amounts of CuCl2 were also detected on the CuCl film. Its reflexions in
the XRD data were marked with hashes in Fig. 3C.
1 D. T. Whipple and P. J. A. Kenis, J. Phys. Chem. Lett., 2010, 1,
3451–3458.
2 Y. Hori, H. Wakebe, T. Tsukamoto and O. Koga, Electrochim.
Acta, 1994, 39, 1833–1839.
3 Y. Hori, in Modern Aspects of Electrochemistry, ed. C. Vayenas,
R. White and M. Gamboa-Aldeco, Springer New York, 2008,
ch. 3, vol. 42, pp. 89–189.
4 K. P. Kuhl, E. R. Cave, D. N. Abram and T. F. Jaramillo,
Energy Environ. Sci., 2012, 5, 7050–7059.
5 Y. Hori, A. Murata and R. Takahashi, J. Chem. Soc., Faraday
Trans. 1, 1989, 85, 2309–2326.
This journal is © The Royal Society of Chemistry 2014
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
6 W. Tang, A. A. Peterson, A. S. Varela, Z. P. Jovanov, L. Bech,
W. J. Durand, S. Dahl, J. K. Norskov and I. Chorkendorff,
Phys. Chem. Chem. Phys., 2012, 14, 76–81.
7 Y. Hori, I. Takahashi, O. Koga and N. Hoshi, J. Mol. Catal. A:
Chem., 2003, 199, 39–47.
8 Y. Hori, H. Konishi, T. Futamura, A. Murata, O. Koga,
H. Sakurai and K. Oguma, Electrochim. Acta, 2005, 50,
5354–5369.
19 B. S. Yeo and A. T. Bell, J. Am. Chem. Soc., 2011, 133,
5587–5593.
20 Y. Hori, H. Wakebe, T. Tsukamoto and O. Koga, Surf. Sci.,
1995, 335, 258–263.
21 L. Yu, H. Sun, J. He, D. Wang, X. Jin, X. Hu and G. Z. Chen,
Electrochem. Commun., 2007, 9, 1374–1381.
22 E. J. H. Lee, C. Ribeiro, E. Longo and E. R. Leite, J. Phys.
Chem. B, 2005, 109, 20842–20846.
9 D. W. DeWulf, T. Jin and A. J. Bard, J. Electrochem. Soc.,
1989, 136, 1686–1691.
10 K. J. P. Schouten, E. Pérez Gallent and M. T. M. Koper,
J. Electroanal. Chem., 2014, 716, 53–57.
23 J. Ustarroz, J. A. Hammons, T. Altantzis, A. Hubin, S. Bals
and H. Terryn, J. Am. Chem. Soc., 2013, 135, 11550–11561.
24 M. Tian, J. Wang, J. Kurtz, T. E. Mallouk and M. H. W. Chan,
Nano Lett., 2003, 3, 919–923.
11 K. J. P. Schouten, E. Pérez Gallent and M. T. M. Koper,
ACS Catal., 2013, 3, 1292–1295.
25 K. R. Sarma, P. J. Shlichta, W. R. Wilcox and R. A. Lefever,
J. Cryst. Growth, 1997, 174, 487–494.
12 C. W. Li and M. W. Kanan, J. Am. Chem. Soc., 2012, 134,
7231–7234.
13 M. Gattrell, N. Gupta and A. Co, J. Electroanal. Chem., 2006,
594, 1–19.
26 K. J. P. Schouten, Y. Kwon, C. J. M. van der Ham, Z. Qin and
M. T. M. Koper, Chem. Sci., 2011, 2, 1902–1909.
27 F. Calle-Vallejo and M. T. M. Koper, Angew. Chem., Int. Ed.,
2013, 52, 7282–7285.
14 Y. Hori, I. Takahashi, O. Koga and N. Hoshi, J. Phys. Chem. B,
2001, 106, 15–17.
15 J. Yano, T. Morita, K. Shimano, Y. Nagami and S. Yamasaki,
J. Solid State Electrochem., 2007, 11, 554–557.
16 K. Ogura, H. Yano and F. Shirai, J. Electrochem. Soc., 2003,
150, D163–D168.
28 A. A. Peterson, F. Abild-Pedersen, F. Studt, J. Rossmeisl and
J. K. Norskov, Energy Environ. Sci., 2010, 3, 1311–1315.
29 A. Hamelin and A. M. Martins, J. Electroanal. Chem.,
1996, 407, 13–21.
30 Y. Hori, A. Murata, R. Takahashi and S. Suzuki, J. Chem.
Soc., Chem. Commun., 1988, 17–19.
17 W. J. Durand, A. A. Peterson, F. Studt, F. Abild-Pedersen and
J. K. Nørskov, Surf. Sci., 2011, 605, 1354–1359.
18 R. Kas, R. Kortlever, A. Milbrat, M. T. M. Koper, G. Mul and
J. Baltrusaitis, Phys. Chem. Chem. Phys., 2014, 16, 12194–12201.
31 R. Kortlever, K. H. Tan, Y. Kwon and M. T. M. Koper, J. Solid
State Electrochem., 2013, 17, 1843–1849.
32 S. K. Shaw, A. Berna, J. M. Feliu, R. J. Nichols, T. Jacob and
D. J. Schiffrin, Phys. Chem. Chem. Phys., 2011, 13, 5242–5251.
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2014