Solvothermally-Prepared Cu2O Electrocatalysts for CO2 Reduction with Tunable Selectivity by the Introduction of p-Block Elements

Article abstract of DOI:10.1002/cssc.201601578

The electroreduction of CO₂ to fuels and chemicals is an attractive strategy for the valorization of CO₂ emissions. In this study, a Cu₂O electrocatalyst prepared by a simple and potentially scalable solvothermal route effectively targeted CO evolution at low-to-moderate overpotentials [with a current efficiency for CO (CE_CO) of ca. 60 % after 12 h at ?0.6 V vs. reversible hydrogen electrode, RHE], and its selectivity was tuned by the introduction of p-block elements (In, Sn, Ga, Al) into the catalyst. SEM, HRTEM, and voltammetric analyses revealed that the Cu₂O catalyst undergoes extensive surface restructuring (favorable for CO evolution) under the reaction conditions. The modification of Cu₂O with Sn and In further enhanced the current efficiency (CE) for CO (ca. 75 % after 12 h at ?0.6 V). In contrast, the introduction of Al altered the selectivity profile of the catalyst significantly, decreasing the selectivity toward CO but promoting the reduction of CO₂ to ethylene (CE≈7 %), n-propanol, and ethanol (CE≈2 % each) at ?0.8 V vs. RHE. This result is related to a decreased reducibility of Al-doped Cu₂O that might preserve Cu⁺ species (favorable for C₂H₄ production) under the reaction conditions, which is supported by XRD, X-ray photoelectron spectroscopy, and H₂ temperature-programmed reduction observations.

Full text of DOI:10.1002/cssc.201601578

Accepted Article
Title: Solvothermally-Prepared Cu2O Electrocatalysts for CO2
Reduction with Tunable Selectivity by the Introduction of p-Block
Elements
Authors: Gastón O Larrazábal, Antonio J Martín, Frank Krumeich,
Roland Hauert, and Javier Pérez-Ramírez
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To be cited as: ChemSusChem 10.1002/cssc.201601578
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ChemSusChem
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ARTICLE
Solvothermally-Prepared Cu O Electrocatalysts for CO
2
2
Reduction with Tunable Selectivity by the Introduction of p-Block
Elements
[
a]
[a]
[b]
[c]
Gastón O. Larrazábal, Antonio J. Martín, Frank Krumeich, Roland Hauert, and Javier Pérez-
[
a]
Ramírez *
Abstract: The electroreduction of CO
2
to fuels and chemicals is an
production of liquid fuels and plastics by well-established
processes in industry. On the other hand, the direct conversion
attractive strategy for valorizing CO
show that Cu
2
emissions. In this study, we
a
2
O
electrocatalyst prepared by
a
simple and
2
of CO into more reduced products with greater economic value
potentially scalable solvothermal route effectively targeted CO
evolution at low to moderate overpotentials (CECO ca. 60%
after 12 h at −0.6 V vs. RHE), and that the introduction of p-block
elements (In, Sn, Ga, Al) into the catalyst can be employed to tune
its selectivity. SEM, HRTEM, and voltammetric analyses revealed
(such as hydrocarbons and alcohols) is very promising, although
this route has been hindered so far by the high overpotentials
required and the difficulty of targeting the production of a single
[
4]
compound.
[
5–7]
that the Cu
2
O catalyst undergoes extensive surface restructuring
Noble metal electrocatalysts, such as Au-
and Ag-based
electrodes, are among the best
2
performing materials for CO reduction to CO known so far.
[
8–11]
[12]
(
favorable for CO evolution) under reaction conditions. Modification
and Pd nanoparticles,
of Cu
2
O with Sn and In further enhanced the current efficiency for
CO (ca. 75% after 12 h at −0.6 V). On the other hand, the
introduction of Al significantly altered the selectivity profile of the
catalyst, decreasing the selectivity toward CO but promoting the
Nevertheless, their large-scale application would be hindered by
their scarcity and high price. Copper, which is abundant and
inexpensive, has been widely studied due to its unique ability to
reduction of CO
2
to ethylene (CE ca. 7%), n-propanol, and ethanol
reduce CO
2
to hydrocarbons and oxygenates at high
[
4,13,14]
(
CE ca. 2% each) at −0.8 V vs. RHE. We relate this result to a
overpotentials,
but regular polycrystalline Cu electrodes
+
decreased reducibility of Al-doped Cu
species (favorable for C production) under reaction conditions, as
supported by XRD, XPS and H -TPR observations.
2
O which might preserve Cu
2
perform poorly at milder conditions because the eCO RR is
2
H
4
outcompeted by the hydrogen evolution reaction (HER). In
contrast, Cu electrodes prepared from the reduction of thick
2
2
oxide films (oxide-derived Cu) showed much better eCO RR
performance at low overpotentials, producing a mixture of CO
and COOH (i.e. ca. 35% current efficiency each at
1
. Introduction
[15]
−
0.5 V vs. RHE).
Subsequent efforts have demonstrated the
ability of oxidized Cu films to yield more reduced products, such
The development of technologies able to transform waste
materials into valuable products is a key pillar for sustainable
growth. Owing to its scalability and facile coupling with
[16–19]
as ethylene and ethanol,
from CO was recently reported by Ren et al. over
Cu O/Cu(OH) Although the irregular structure of these
and the production of n-propanol
2
a
[20]
2
2
foil.
renewable energies, the electroreduction of CO
attractive approach to utilize anthropogenic CO
resource for the synthesis of fuels and chemicals under ambient
2
(eCO
2
RR) is an
OD Cu materials is commonly acknowledged to play an
important role, a precise rationalization of their catalytic behavior,
particularly in regard to the production of highly reduced
compounds, has remained elusive. It has been postulated that
2
emissions as a
[
1]
conditions.
electrocatalysts that are also sufficiently active and selective at
low overpotentials is major barrier toward its practical
The efficient reduction of CO to CO, which
along with HCOOH is the most commonly observed eCO RR
product at the lower overpotential range (i.e. below
0.8 V vs. RHE), would provide a versatile compound for the
However, the lack of inexpensive and stable
thicker Cu
2
O layers favor the formation of C
2
H
4
due to local pH
whereas grain
a
[16,21]
effects on the (rougher) electrode surface,
[
2,3]
implementation.
2
boundary sites that bind CO strongly have been associated with
2
[22,23]
the reduction of CO to C2 compounds.
spectroscopic studies, Mistry et al. recently postulated that Cu
species that are resistant to reduction under eCO RR conditions
over OD Cu
Based on operando
+
−
2
are key for driving the formation of
C
2
H
4
[
24]
electrodes.
[
a]
G. O. Larrazábal, Dr. A. J. Martín, Prof. J. Pérez-Ramírez
Institute for Chemical and Bioengineering
Another route for the design of better eCO
2
RR catalysts is to
Department of Chemistry and Applied Biosciences, ETH Zurich
Vladimir-Prelog-Weg 1, 8093 Zurich (Switzerland)
E-mail: jpr@chem.ethz.ch
develop multicomponent systems able to break the linear
relationship between the binding energies of CO and COOH
[
25,26]
intermediates on transition metal surfaces.
For example,
[
[
b]
c]
Dr. F. Krumeich
early studies by Watanabe et al. demonstrated changes in the
selectivity patterns of electrodeposited Cu alloys compared to
Laboratory of Inorgamic Chemistry
Department of Chemistry and Applied Biosciences, ETH Zurich
Vladimir-Prelog-Weg 1, 8093 Zurich (Switzerland)
[
27]
bare copper.
More recently, Takanabe and co-workers
[
28]
[29]
showed that the electrodeposition of indium
and tin
on
Dr. Roland Hauert
Empa, Swiss Federal Laboratories for Materials Science and
Technology, Überlandstrasse 129, 8600 Dübendorf (Switzerland)
thermally oxidized Cu foils yielded electrodes with outstanding
selectivity for CO in the lower overpotential range (e.g. CECO ca.
9
0% at –0.6 V vs. RHE), and similar observations were reported
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ChemSusChem
10.1002/cssc.201601578
ARTICLE
by Zhao et al. over Sn-decorated Cu O nanowires (NWs) 2. Results and Discussion
x
[
30]
anchored to a Cu substrate.
These findings have been
ascribed to the formation of alloys on the surface and the
2 2
2.1. CO electroreduction over pristine Cu O
[
28,29]
inhibiting effect of In and Sn on the HER,
although high
selectivities for CO have only been observed when oxidized
substrates are used as starting materials. In contrast, bulk Cu-
The unmodified Cu
solvothermal reduction in ethylene glycol of a Cu(II) precursor.
Figure 1 shows the performance of Cu O-ud over a long 12-
hour electrolysis at –0.6 V in CO -saturated 0.1 M KHCO . A
commercial sample of Cu O (Cu O-cs) was evaluated as a
2 2
O catalyst (Cu O-ud) was prepared by the
[
29]
[31]
Sn
and Ag-In
alloy electrodes have shown comparatively
2
poor performance at similar potentials, indicating that
nanostructure plays a key role in the emergence of synergistic
effects in multicomponent electrocatalysts.
2
3
2
2
benchmark at the same conditions. Both catalysts showed an
increase of the total current density over the first hour of the
reaction followed by a plateau. Nevertheless, the commercial
sample showed stable (and low, ca. 10%) selectivity for CO
throughout the whole run, whereas the selectivity profile of the
Despite the promising results achieved with modified oxide-
derived Cu electrodes, these materials display modest current
densities and require elaborate synthetic procedures combining
thermal and electrochemical steps on bulk substrates.
Comparatively few efforts have been geared toward transferring
the performance of such systems to powdered catalysts that can
be synthesized in large scale and easily integrated into a gas
diffusion electrode (GDE) of a practical electrolyzer. A step in
this direction was recently demonstrated through the in situ
Cu
2
O-ud catalyst evolved significantly during the experiment.
O-ud catalyst showed a preference for the HER
Initially, the Cu
2
which gradually shifted toward CO evolution even as the total
current remained stable. Electrolyses performed at other
potentials revealed a similar evolving behavior of the current
efficiency for CO, although more reducing potentials resulted in
a faster equilibration of the catalyst (Figure 1b). Compared to
formation of Cu-In alloys from the reduction of the CuInO
2
[
32]
delafossite,
resulting in high selectivity for CO (i.e. CECO ca.
any other reported Cu
2
O-derived electrodes, either prepared by
[
15]
6
0% at –0.6 V vs. RHE) following the generation of
a
the thermal oxidation of Cu foils or by the electrodeposition of
[
16]
heterogeneous nanostructure after successive CO
2
reduction
Cu
2
O films on Cu substrates,
the evolved Cu
2
O-ud catalyst
[
33]
cycles.
shows higher selectivity for the reduction of CO
2
to CO over a
range of potentials (i.e. ca. 55%, compared to ca. 30% at
−0.6 V) and much lower production of formate. This
In this contribution, we adopted a simple and potentially scalable
solvothermal route for preparing Cu O catalysts in powdered
form, and evaluated their performance at low to moderate
overpotentials (i.e. up to –0.80 V vs. RHE) in 0.1 M KHCO . This
a
2
observation suggests that the solvothermal synthesis (followed
by equilibration) might be more effective than other treatments in
generating OD Cu catalysts with highly active sites for CO
evolution while avoiding the “dead-end” formate pathway.
However, we remark that this comparison is limited by the fact
that tests reported in the literature are usually much shorter and
might not be fully representative of the final catalytic
performance of such oxide-derived Cu electrodes. Nonetheless,
the high initial selectivity for CO and low production of formate
3
approach enabled us to assess in a comparable basis the effect
of introducing different p-block elements (Sn, In, Ga, and Al) into
Cu
2
O by adding the corresponding precursor to the synthesis
O catalyst
medium. While the performance of the unmodified Cu
2
compared favorably to a commercial sample and to OD Cu
electrodes reported in the literature, marked improvements in
selectivity and stability for CO production were attained by the
2
over Cu O-ud contrast markedly with available works. The
addition of Sn and In to Cu
2
O. On the other hand, the
results in Figure 1 highlight the suitability of a simple and
potentially scalable solvothermal synthesis for preparing a Cu-
based powder catalyst with solid performance for this reaction.
introduction of Al into the Cu O catalyst promoted the production
2
of C2 and C3 compounds, such as ethylene, ethanol and n-
propanol, at moderate overpotential.
Figure 1. (a) Current efficiency (CE) and total current density (j) in CO
2
reduction electrolyses at −0.6 V vs. RHE in CO
2 3
-saturated 0.1 M KHCO (pH
6
.75) over a commercial Cu O sample (Cu O-cs, left) and the unmodified Cu
2
2
2
O catalyst (Cu O-ud, right) prepared by solvothermal reduction. (b) Average
2
current efficiency for CO (bars) and partial current density for CO (diamonds) during the final 2 hours of 12-hour electrolyses at different potentials. The
dashed horizontal lines indicate the average CECO during the first 2 hours of each electrolysis, evidencing the catalyst evolution toward higher CO
selectivity. The difference between the starting and final points becomes less pronounced at more cathodic potentials.
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ChemSusChem
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ARTICLE
Figure 3. Scanning electron (SEM) and high-resolution transmission electron (HRTEM) micrographs of solvothermally-prepared Cu
row) and a commercial Cu O sample (Cu O-cs, bottom row), before and after CO reduction electrolyses at –0.6 V vs. RHE. The insets show the
selected area electron diffraction (SAED) patterns of the used catalysts, evidencing their polycrystalline nature and the presence of Cu and Cu
2 2
O (Cu O-ud, top
2
2
2
2
O
(
diffraction rings associated to the corresponding (111) crystallographic planes are highlighted). The black arrows point to some of the steps and edges
present on the surface of the used Cu
pattern.
2
O-ud catalyst, while the blue dashed box highlights a region of overlapping lattice fringes leading to a Moiré
Figure 2 shows the XRD patterns of the pure Cu
before and after electrolyses at –0.6 for 12 h (the
corresponding patterns of the powders are shown in Figure S1).
The diffractogram of the fresh Cu O-ud electrode confirms that
Cu O was obtained from the solvothermal synthesis, albeit a
fraction of the precursor was reduced to metallic Cu. The
Cu O-cs electrode shows only Cu O, as expected. The average
crystallite size calculated with the Scherrer equation (based on
the (111) peak of the Cu O powders) was found to be 14 and
7 nm for the prepared and commercial Cu samples,
2
O electrodes
V
2
2
2
2
2
3
2
O
respectively. The diffractrograms of both electrodes after the
electrolysis are practically identical and evidence the almost
complete bulk reduction of Cu
conditions. In addition, the change of the peak shape in the Cu
LMM Auger spectra of the Cu O-ud electrode (taken after 4 min
2 2
O to metallic Cu under eCO RR
Figure 2. XRD patterns of fresh and used (i.e. after 12-hour CO
reduction electrolyses) Cu O-ud and Cu O-cs electrodes. The fresh
electrodes are comprised mainly of (▼) Cu O and are mostly reduced to
s) metallic Cu under eCO RR conditions. Reflections from the carbon
2
2
2
2
2
+
0
of sputtering with Ar ) confirm the formation of Cu (Figure S2).
(
2
gas diffusion layer (GDL) used as substrate are marked with (Ø).
Even though the solvothermally-prepared Cu O catalyst and the
2
the early stages of the electrolysis (Figure 1a). HRTEM analysis
commercial sample are chemically similar, their microstructures
are markedly different, as evidenced by the SEM micrographs of
2
of the used Cu O-ud catalyst (scraped off a glassy carbon
substrate after 4 h of electrolysis at –0.6 V) showed a very
irregular surface populated by nanoparticles, causing numerous
the electrodes (Figure 3). The Cu
large irregular aggregates (with a size of ca. 1 µm) with a jagged
surface. In contrast, the particles of the fresh Cu O-cs catalyst
2
O-ud catalyst is comprised by
2
steps and edges (Figure 3). Moreover, the used Cu O-ud
2
catalyst is characterized by an abundance of Moiré fringes, with
the overlapping lattices possibly being a consequence of the
faulted layering of platelet-like structures upon restructuring
are much finer and have a smoother surface, and do not form
large aggregates on the electrode surface. The electrolysis
leads to a roughening of both catalysts, as reflected by the
appearance of more defined nanometric polyhedral features on
the surface of the particles, which are particularly abundant in
2
under eCO RR conditions (HRTEM of the fresh catalyst is
shown in Figure S3). It is reasonable to postulate that this
restructuring leads to a highly stepped surface. Similar features
are present to a more limited extent in the used benchmark
2
the used Cu O-ud catalyst. This roughening conceivably results
in a growth of the electrochemically active surface area (ECSA),
as reflected by the increase of the double-layer (DL) capacitance
of the electrodes (Table S1) and of the total current density in
sample, suggesting that Cu
2
O-cs does not undergo such
RR
extensive surface transformation when exposed to eCO
2
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ChemSusChem
10.1002/cssc.201601578
ARTICLE
−
1
Figure 4. Cyclic voltammograms (scan rate 20 mV s ) in CO
2
-saturated
O-cs electrodes, and following
reduction electrolyses at −0.6 V vs. RHE.
0
1
.1 M KHCO
2-hour CO
3 2 2
of fresh Cu O-ud and Cu
2
conditions, likely as a result of its higher crystallinity and larger
crystallite size (as evidenced by XRD patterns of the fresh
powders, Figure S1), and of its significantly lower total surface
2
–1
2
–1
area (0.9 m g vs. 5.4 m g in the Cu
determined by N physisorption). Moreover, this observation is
consistent with the lower ECSA (after the reaction) of the Cu O-
cs electrode compared to Cu O-ud, as reflected by DL
capacitance measurements (Table S1) and by its overall lower
activity. These results suggest that only Cu O powders which
are initially quite irregular would undergo in situ deep
restructuring process favorable for CO reduction, mirroring the
2
O-ud catalyst, as
2
2
Figure 5. Average current efficiency (bars), total current density (circles)
2
2
and partial current density for CO in CO reduction electrolyses (2 h
duration each) at different potentials over doped Cu O catalysts.
2
2
a
The elemental analysis of the thus prepared catalysts showed
2
that the foreign elements were mostly added to Cu
intended amounts, but that only little aluminum could be
incorporated, resulting in an Al-doped Cu O catalyst (Table 1).
We remark that the optimization of the synthetic procedure might
improve the control of the modifier content in the resulting Cu
catalysts. In particular, additional experiments showed that by
switching to Al(NO as the aluminum source (without any other
changes to the synthetic procedure), it was possible both to
increase the amount of modifier introduced into the Cu
2
O in the
differences commonly observed between smooth polycrystalline
Cu electrodes and oxide-derived electrodes prepared by treating
[
15,16]
2
Cu bulk substrates.
2
O
The Cu O-ud
2
catalyst
after
the
reaction
showed
electrochemically active sites that are not originally present in
the fresh material, as extracted from the cyclic voltammograms
3 3
)
(
CVs) shown in Figure 4. The oxidation features at −0.13 and
.25 V seem to be correlated to overlapping reduction features
forming the broad peak at −0.21 V. None of these peaks are
detectable in the CVs of the fresh Cu O-ud or the used Cu O-cs
2
O
0
catalyst (up to similar ratios present in the In- and Sn-modified
materials) and to more precisely target lower loadings. However,
these variations did not alter significantly the catalytic behavior
2
2
electrodes, suggesting a possible fingerprint of sites that are
particularly active for CO evolution. Though the exact nature of
these sites cannot be determined at this stage, similar features
in the investigated potential range (Figure S4), so that Cu
2
O:Al-
1
40 can be reasonably assumed to be representative of the
[
34]
behavior of Al-modified Cu
presented method.
2
O catalysts prepared by the herein
have been previously associated to the presence of defects,
which is consistent with the HRTEM analysis of the used
Cu O-ud catalyst (Figure 3).
2
Table 1. Target and actual loadings of the modified Cu
2
O catalysts
expressed as the atomic ratio of Cu to the second element (Cu/M).
2
.2. Evaluation of p-block elements as modifiers
Target
Cu/M
Actual
M content
(wt.%)
Modifier
Catalyst
[a]
After identifying Cu
2
O-ud as a promising and potentially scalable
Cu/M
catalyst for CO reduction, we assessed the catalytic effect of
2
Sn
In
Cu
Cu
Cu
Cu
2
O:Sn-20
20
20
20
20
28.1
20.8
5.6
7.2
3.1
0.3
incorporating p-block elements as modifiers. Previous reports
have shown that the electrodeposition of small amounts of
2
O:In-20
O:Ga-20
O:Al-140
[
28]
[29]
indium
and tin
on oxidized Cu foils leads to enhanced
selectivity for CO at low overpotentials. In this context, we
hypothesized that a similar effect could be achieved via a
Ga
Al
2
30.5
2
139.4
2
solvothermal synthesis. Consequently, the modified Cu O
catalysts were prepared by the reduction in ethylene glycol of
the Cu(II) precursor along with an appropriate amount of the
chloride of the second element. In addition to indium and tin, this
synthetic approach allowed us to evaluate the effect of gallium
[
a] X-ray fluorescence (XRF) spectroscopy
As an initial assessment, we compared the electrocatalytic
performance of the modified catalysts in short (2 h) eCO RR
electrolyses in 0.1 M KHCO at −0.6 V and −0.8 V (Figure 5).
2
and aluminum on Cu
2
O, which would be more difficult to
3
incorporate into the catalyst via electrochemical methods.
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ChemSusChem
10.1002/cssc.201601578
ARTICLE
Cu
2
O-ud was tested under the same conditions and is shown as
early stages of the electrolysis (ca. 80% in the first 2 h) but then
decreases steadily. In fact, at the end of each run the selectivity
for CO over the modified catalysts was slightly lower than over
a benchmark. The addition of Sn and In had a marked
enhancing effect on the selectivity for CO, which is most
pronounced in the In-modified catalyst at −0.6 V, while at −0.8 V
the addition of Sn and In resulted in very similar behavior. The
addition of Ga had a small positive effect at –0.6 V but
significantly decreased the CECO at –0.8 V. It is interesting to
pristine Cu
suggests a gradual deactivation of the In- and Sn-containing
catalysts. Previous reports have linked the eCO RR activity of In
and Sn electrodes to oxidic species that are stable at the
2
O, but with opposite temporal trends. This initially
2
[
35,36]
note that the addition of Al to Cu
2
O (even at the small loading
cathodic potentials of the electrolysis.
Based on these
that resulted from the synthesis) had a large effect on the
selectivity profile, significantly decreasing the selectivity for CO
but increasing the current efficiency for ethylene at –0.8 V (no
ethylene was detected over any catalyst at –0.6 V). Apart from
the gaseous products shown in Figure 5, only trace amounts of
methane and ethane were observed at –0.8 V, comprising a
negligible share of the current efficiency (i.e. less than 0.1% in
all cases).
observations, we have postulated that the synergistic effect in
[
31]
[33]
Ag-In
and Cu-In
electrocatalysts might be based on the
, with the
interaction of non-reducible species, such as In(OH)
3
metallic components. In this context, a possible cause for
deactivation at −0.8 V could be the gradual reduction of such
oxidic species. Nevertheless, the partial current density for CO
2 2
over the Cu O:Sn-20 and Cu O:In-20 catalysts remains fairly
stable after 4 h, implying that the decrease of the selectivity is
not caused by a loss of preferential active sites for CO evolution
(sites that are possibly located at the interfaces between Cu and
oxidic Sn/In species) but rather by the generation of new
“unmodified” sites more favorable for hydrogen evolution.
2
2
.3. Stability of Sn- and In-modified Cu O catalysts
Since the 2-hour electrolyses showed that the addition of tin and
indium had the largest effect in enhancing the selectivity of Cu
for CO at −0.6 and −0.8 V, the Cu O:Sn-20 and Cu O:In-20
2
O
Considering that Cu
resistance to reduction even at −1.0 V vs. RHE, it is possible
that new HER sites appear from the reduction of Cu O that was
2
O has been reported to show unexpected
[
24]
2
2
catalysts were selected for additional experiments. Their stability
was assessed by performing 12-hour electrolyses at −0.6 V and
2
0
not reducible at −0.6 V but is transformed to Cu as the potential
is increased. It is interesting to note that the Cu-Sn NWs catalyst
−
0.8 V (Figure 6). At −0.6 V, the Sn- and In-modified catalysts
equilibrated more rapidly and achieved a considerably higher
selectivity for CO (> 70%) compared to pristine Cu O. The In-
[
30]
developed by Zhao et al.
for CO at −0.8 V (initial CECO ca. 90%) compared to Cu
However, the CECO also decreased steadily throughout a 12-
hour run along an increase of the current efficiency for H and of
the total current density (although the increase of the current
density was less pronounced than in Cu O:Sn-20). This
observation suggests that this catalyst might also suffer from a
similar effect as Cu O:Sn-20. On the other hand, the CECO and
showed an even higher selectivity
2
2
O:Sn-20.
modified catalyst performed better than its Sn counterpart,
although this might be a reflection of the slightly higher amount
2
2
of the modifier in Cu O:In-20.
2
In terms of stability, a different picture emerges at −0.8 V. The
current efficiency for CO over both catalysts is very high in the
2
Figure 6. Current efficiency (CE) and partial current density for CO (jCO) in 12-hour CO
vs. RHE over Sn- (left) and In-modified (right) Cu O catalysts with Cu/M = 20. The arrows between the plots indicate the evolved CECO (i.e. average over
the last 2 h in 12 h electrolyses) over the unmodified Cu O catalyst at each corresponding potential.
2
reduction electrolyses at (a) −0.6 V vs. RHE and (b) −0.8 V
2
2
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ChemSusChem
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ARTICLE
Figure 7. (a) Current efficiency (CEC2H4) and partial current density for ethylene (jC2H4) in 12-hour CO
2
reduction electrolyses at −0.8 V vs. RHE over
unmodified (left) and Al-doped Cu
2
O (right) catalysts. (b) Average current efficiencies for different products in 12-hour electrolyses. The inset shows the
O:Al-140 catalyst.
distribution of minor liquid phase products obtained over the Cu
2
the current density at −0.6 V over the electrodeposited Cu-Sn
and Cu-In catalysts demonstrated by Takanabe and
attempted a similar approach using Cu-Sn mixed (hydr)oxides,
namely CuSnO and CuSn(OH) . These materials were very
poor CO reduction catalysts at −0.6 V and at more cathodic
potentials formate (and not CO) was the favored eCO RR
3
6
[
28,29]
coworkers
appears to be rather stable at −0.6 V. Along with
2
the data in Figure 6, these results suggest that the generation of
HER sites on these catalysts can be minimized at −0.6 V (and at
even less cathodic potentials), thus preserving the high
selectivity for CO.
2
product by a large margin. Exposing of these catalysts to
successive electrolytic runs did not alter much their selectivity,
suggesting that neither CuSnO
restructuring process favorable for CO evolution as CuInO
(Figure S6). In fact, the performance of these materials was
practically identical to that of pure SnO , particularly at higher
3
nor CuSn(OH)
6
undergo a
2
does
Overall, the enhancement of the current efficiency for CO
following the addition of In and Sn to Cu
2
O is in line with recent
2
results from Cu-In and Cu-Sn electrodes prepared by electro-
overpotentials (Figure S7). This effect might occur if a surface
enrichment of Sn in the mixed (hydr)oxides ‘masks’ the activity
of Cu, in an analogous manner to Cu-Sn electrodes when the
[
28,29]
[30]
and electroless deposition. Consequently, we show that
a simpler and more scalable synthesis is capable of effectively
transferring the outstanding current efficiencies for CO observed
amount of Sn electrodeposited exceeded
a
monolayer
[
29]
over these model materials to Cu
2
O catalysts in a form more
coverage.
amenable to practical implementation. However, it is important
to note that the aforementioned electrodes generally achieved
even higher selectivities for CO. Compared to the solvothermal
synthesis presented in this contribution, it is likely that electro-
and electroless deposition achieve a finer dispersion of Sn and
In on the Cu surface, resulting in a larger amount of sites that
benefit from the interaction between Cu and Sn/In species.
Nevertheless, the exploratory nature of this work leaves an open
space for optimization efforts that might close the performance
gap in future developments.
2.4. CO reduction to C H over Al-doped Cu O
2
2
4
2
The introduction of Al had a significant influence on the
selectivity pattern of Cu O. Even though it negatively affected
2
the production of CO at −0.8 V, a beneficial effect was the
increase in the current efficiency for ethylene at this potential
(Figure 5). This observation was particularly intriguing because
this behavior was not observed in any of the other modified
Cu
showed a stable current efficiency for C
an initial equilibration phase. In contrast, the selectivity for C
over pristine Cu O (max. ca. 3.5%) was lower than over
Cu O:Al-140 and then decreased steadily as the run progressed
(Figure 7a). Both the partial current density for C and the
total current density over Al-modified Cu O were significantly
2
O catalysts. In a 12-hour run the Al-doped Cu
2
O catalyst
2
H
4
of ca. 7% following
As was the case for the aforementioned Cu-In and Cu-Sn
electrodes, the performance of the catalysts changed with the
amount of Sn and In added. At very low contents (Cu/M = 250),
2 4
H
2
2
the Sn-modified catalyst was very similar to pristine Cu
the advantage of faster stabilization), whereas the In-modified
catalyst had a significantly worse selectivity for CO than Cu O-
2
O (with
2 4
H
2
2
higher than over the unmodified catalyst, which is probably
linked to a higher ECSA as shown by the DL capacitance
measurements (Table S1). The Al-doped catalyst also produced
more formate, as well as several highly reduced products in the
ud (Figure S5). Increasing the loading in the synthesis medium
beyond Cu/M = 20 resulted in very heterogeneous materials (i.e.
2
Cu O alongside the oxides of In and Sn, as well as CuCl), which
prevented a straightforward comparison with the catalysts with
lower loadings. Nevertheless, an interesting ‘high loading
scenario’ occurs in mixed equimolar oxides in which the
distribution of Cu and the second element are expected to be
completely homogeneous. We have previously shown that the in
2
liquid phase that were not detected over undoped Cu O,
although the presence of these products at amounts below the
detection limit of the NMR analysis cannot be discounted
(Figure 7b). Out of this fraction, the presence of n-propanol and
ethanol (CE ca. 2% each) are noteworthy, alongside smaller
amounts of methanol and acetate. In comparison, a recent study
situ reduction of CuInO
CO reduction cycles results in the gradual segregation of Cu
and In and the formation of In(OH)
2
followed by its exposure to successive
[
20]
2
by Ren et al. demonstrated a current efficiency for n-propanol
of ca. 10% over Cu O/Cu(OH) foils biased at −0.95 V vs. RHE.
3
, alongside an increase of the
In light of these results, we
2
2
[
33]
activity and selectivity for CO.
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ChemSusChem
10.1002/cssc.201601578
ARTICLE
Figure 8. XRD patterns of fresh and used (i.e. after 12-hour CO
2
reduction electrolyses) modified Cu
are comprised mainly of (▼) Cu O and are mostly reduced to (s)
metallic Cu under eCO RR conditions except for the case of Cu O:Al-
40, where reflections from the oxide phase are still apparent.
Reflections from the carbon GDL are marked with (Ø).
2
O electrodes. The fresh electrodes
2
2
2
1
The production of small amounts of ethylene at comparable
potentials over unoptimized Cu electrodes has been previously
[
4]
reported (e.g. CE around 2% over polycrystalline Cu and the
[
15]
original oxide-derived Cu electrode at −0.8 V).
selectivity for C over Cu O-ud (and over the Sn- and In-
modified catalysts) is in line with these results. On the other
hand, the increased selectivity for C over the Al-doped
catalyst mimics to some extent the performance of materials that
have been tailored to target the production of C , such as
The low
2
H
4
2
2
H
4
Figure 9. SEM micrographs of the fresh and used electrodes following
1
2-hour electrolyses over Sn-, In- and Al-modified Cu
and In at −0.6 V, Al at −0.8 V). The scale bar in the insets represents
00 nm.
2
O catalysts (Sn-
2
H
4
[
16,18]
[24]
thick Cu
2
O films on Cu sheets,
plasma-activated Cu foils,
2
and nanocubes formed from the reduction in the presence of
[
37]
halides of Cu(I) oxide formed on Cu electrodes.
the appearance of n-propanol alongside ethylene in the Cu
40 catalyst is consistent with previous reports proposing a C-C
In addition,
in Figure S1). The formation of other oxides (e.g. In
2 3 2
O , SnO ,
O:Al-
2 3
Al O ) was not observed, although the XRD patterns of the fresh
2
1
powders indicate the existence of CuCl in trace amounts (Figure
S1). Although the presence of halides in large amounts has
coupling mechanism of carbon monoxide and ethylene
precursors. Moreover, ex situ measurements, discussed in more
detail in the following section, point toward a stabilizing role from
[
39]
been reported to influence the eCO
2
RR performance and the
we remark that intensity of
[
17,37]
nanostructure of Cu electrodes,
+
Al on Cu species, which has been previously linked to
the peak from CuCl (which is proportional to the content) is small
and comparable in all the modified catalysts. Consequently, the
presence of trace amounts of CuCl is unlikely to greatly
[
24,38]
increased selectivity for C
2
H
4
over OD Cu electrodes.
2
.5. Structural characterization of modified Cu
2
O catalysts
2
influence the catalytic behavior of the modified Cu O catalysts,
or be the reason behind the divergent influence of Sn and In
compared to Al.
Considering that the effect of adding Sn and In to Cu
enhanced selectivity for CO) was different from the effect of
doping with Al (i.e. lower CECO but increased selectivity for C
2
O (i.e.
2
H
4
The diffractograms of the used In- and Sn-modified catalysts
evidenced the bulk reduction of the materials to metallic Cu
and alcohols), we evaluated whether the structural changes
underwent by these catalysts might be associated with the
diverging effect of the modifiers.
2
under eCO RR conditions, as expected (Figure 8). No
intermetallic Cu-In and Cu-Sn compounds were detected after
the reaction, although their formation cannot be completely
disregarded due to their expectedly low concentration and the
large background signal from the carbon GDL.
The XRD patterns of the modified catalysts in their fresh state
confirm the presence of Cu
small fraction of metallic Cu, as was the case in the pristine
Cu O catalyst (Figure 8, and the corresponding powders shown
2
O as the main phase alongside a
2
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ChemSusChem
10.1002/cssc.201601578
ARTICLE
−
1
Figure 10., Cyclic voltammograms (scan rate 20 mV s
2
) in CO -
saturated 0.1 M KHCO
0.6 V vs. RHE. of (a) In-modified Cu
two different loadings and (c) at -0.8 V vs. RHE of Al-doped Cu
-TPR profiles of fresh Cu O-ud and Cu O:Al-140.
3
following 12 h CO
O and (b) Sn-modified Cu
O. (d)
2
reduction electrolyses at
−
2
2
O, at
2
H
2
2
2
2
The addition of p-block elements to Cu O seemed to have only
minor effects on the microstructure of the resulting catalysts, as
examined by SEM (Figure 9, lower magnification micrographs
are shown in Figure S8). The introduction of indium and
aluminum appeared to favor the formation of micrometric
particles with sparsely dispersed cubes, while the addition of tin
favored the formation of smaller, spherical aggregates.
Nevertheless, the surface of the particles in the modified
Figure 11. Elemental maps over fresh and used Sn-, In- and Al-doped
2
Cu O catalysts following 12-hour electrolyses. The scale bar in the inset
represents 200 nm.
3
+
after the reaction, but rather indicates that it is present as In
(
Figure S2). These observations strongly suggest that indium
exists in the Cu
2
O:In-20 catalyst as an electrochemically silent
, as observed previously on
oxidic species, such as In(OH)
3
2
catalysts appears jagged and irregular (as in Cu O-ud material)
[
35]
[33]
anodized In electrodes
and Cu-In catalysts.
On the other
and the small differences in morphology evidenced in the
micrographs do not seem to reflect a change of the preferred
growth orientation of the crystallites, as can be deduced from the
hand, the presence of Sn has a clear effect on the voltammetric
response (Figure 11b). Since redox features labeled “1” and “3”
correspond respectively to the oxidation and reduction of the
predominance of the (111) peak of Cu
2
O in the XRD patterns.
0
[40]
2
Sn /SnO pair, it is inferred that isolated Sn must exist in the
As in the Cu O-ud catalyst, post-reaction micrographs of the
2
used catalyst, as confirmed by the elemental mapping. However,
the oxidation feature “2” likely corresponds to stabilized (i.e. less
modified materials showed a roughening of the surface due to
the formation of fine polyhedral features, particularly on the
spherical particles. These features appear to be more dense and
0
oxidizable) Sn species associated with Cu-Sn intermetallic
compounds. These compounds would be formed by the
abundant in Cu
2
O:Al-140, although this is most likely a result of
more aggressive potential (i.e. −0.8 V
[
41,42]
interdiffusion of metallic Cu and Sn
formed in situ under
its exposure to
a
reaction conditions, which is consistent with the redistribution of
Sn after the reaction as observed by EDX mapping. In the case
of the Al-doped catalyst, the CV lacks the redox features that
were associated to CO evolution in the undoped catalyst. This
fact, together with the low selectivity for CO observed over the
Al-doped catalyst, reinforces the premise of fingerprint peaks
associated to defect sites particularly active for CO evolution.
compared to −0.6 V for the In- and Sn-doped samples), as
reflected by the more pronounced relative increase of the
current density (Figure 7).
The cyclic voltammograms unveiled stark differences in the
electrochemically active sites of the catalysts. Remarkably, the
presence of indium led to similar (though better defined) redox
features as in the parent material (Figure 10a). The lack of any
detectable signal from In contrasts with its clear presence on the
catalyst surface, as shown by the elemental mapping (Figure
In contrast to Sn and In, Al appears to be very finely dispersed in
the fresh catalyst (Figure 11). An indication that Al affects Cu
2
O
in a different way to In and Sn is provided by the XRD diffraction
1
1). XPS measurements did not provide evidence of metallic In
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ChemSusChem
10.1002/cssc.201601578
ARTICLE
degree of resistance to reduction under eCO
compared to the unmodified catalyst.
2
RR conditions
Based on operando studies, Mistry et al. have recently proposed
+
that Cu species that are resistant to reduction are behind the
very high selectivity for ethylene (i.e. CE ca. 40% at
−
0.8 V vs. RHE)
observed
In accordance with this result, it is possible that
O in the Al-doped catalyst drives its increased
4. It is interesting to note that we also observed
a stabilizing effect of Al on Cu O in temperature-programmed
reduction (TPR) experiments with H over the fresh catalyst
Figure 10d). In this case, the addition of Al clearly decreased
over
plasma-activated
Cu
[
24]
electrodes.
the preserved Cu
selectivity for C
2
2
H
2
2
(
the reducibility of Cu
2
O, as evidenced by the shift of the
O:Al-140 compared to the
reduction peak by ca. 100 °C in Cu
2
undoped sample. This result suggests at least some degree of
parallelism between the gas-phase and the electrochemical
reducibility. Nevertheless, we remark that elucidating the
mechanism through which Al might hinder the reduction of Cu
requires further studies.
2
O
2 2
Figure 12. (a) Cu LMM Auger spectra of Cu O-ud and Cu O:Al-140
electrodes after eCO RR electrolyses (12 h, −0.8 V) following
2
+
successive cycles of Ar -sputtering (indicated by the black arrow). The
unsputtered surfaces (bottom, red curve) show only (▼) Cu O, whereas
2
3
. Conclusions
characteristic features from (s) metallic Cu become apparent after
0
sputtering. (b) Variation of the atomic concentration of Cu (on a Cu
basis) with increasing depth, as obtained by peak fitting of the Cu LMM
Auger spectra after each sputter cycle. As a reference, the sputter yield
In this work, we employed a simple and potentially scalable one-
pot solvothermal route for preparing Cu O catalysts for the
electrochemical reduction of CO , and studied the catalytic effect
of introducing Sn, In, Ga, and Al into the Cu O material by
adding the corresponding precursors to the synthesis medium.
The pristine Cu O catalyst showed enhanced selectivity for CO
and high activity at reduced overpotentials relative to
commercial sample and to oxide-derived Cu electrodes
described in the literature, while inhibiting the reduction of CO
to formate. The addition of Sn and In to Cu O was found to have
a promotional effect on the reduction of CO to CO, in line with
2
−
1
2 5
on Ta O was found to be 4 nm min .
2
2
2
patterns (Figure 8), since reflections from Cu O are still visible
in the Al-doped material after 12 hours of electrolysis at −0.8 V.
This result initially suggests that the presence of Al hinders the
2
a
bulk reduction of Cu
The Cu LMM Auger spectra of unmodified and Al-doped Cu
electrodes following the electrolysis (Figure 12a) shows only
Cu O at the surface, although this is possibly a consequence of
surface oxidation upon exposure to air following the reaction.
2
O to metallic Cu at the evaluated potentials.
2
O
2
2
2
[
43]
2
previous results obtained over Cu-In and Cu-Sn electrodes but
with the advantage of a more practical catalyst. In contrast to Sn
and In, Al was incorporated only at low amounts and had a
detrimental effect on the current efficiency for CO. Instead, the
catalyst was driven toward ethylene production, as it showed a
Nevertheless, depth profiling reveals the presence of metallic Cu
in both electrodes, as shown by the appearance of characteristic
features in the Cu LMM Auger peak. From a qualitative point of
view, these features are more prominent in the spectra of the
2
used Cu O-ud electrode, suggesting a more metallic character in
0
sevenfold increase of the current efficiency for C
2 4
H at –0.8 V vs.
comparison to the Al-doped material. Figure 12b shows the Cu
RHE compared to pristine Cu O and to the Sn- and In-modified
2
content (based on the total Cu content) as determined by peak
0
catalysts. Ex situ characterization of the catalysts showed that
the Sn- and In modified catalysts were mostly reduced to
metallic Cu after the electrolysis conditions, as was the case of
the parent material. However, the Al-doped catalyst showed a
fitting of Cu and Cu
2
O contributions to the Auger spectra after
each sputtering cycle employing experimentally-derived line
shapes (Figure S9). The fraction of metallic copper in both
electrodes shows an increasing trend versus the sputter time
clearer presence of Cu
that the reduction of the oxide matrix was hindered. In this
context, we postulate that the increased selectivity for C
observed over the Al-doped Cu O catalyst might be linked to a
2
O even after 12 h of reaction, suggesting
(
and hence versus depth) after the removal of the surface oxide
layer with 30 s of sputtering (depth ca. 2 nm). Nevertheless, it is
0
2 4
H
interesting to note that the Cu content is significantly higher in
2
2
the Cu O-ud electrode than in its Al-doped counterpart at all
+
stabilizing effect of Al on Cu species under reaction conditions.
We foresee that the optimization of these catalysts might close
the performance gap compared to electrodes prepared by less
practical synthetic methods.
points, matching the qualitative observations from the Auger
spectra in Figure 12a. Although it is difficult to directly
extrapolate these ex situ measurements to the actual state of
0
the surface under reaction conditions, the lower Cu content in
the Cu
2
O:Al-140 electrode after the electrolysis is consistent with
O with a higher
the proposition that doping with Al endows Cu
2
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ChemSusChem
10.1002/cssc.201601578
ARTICLE
vacuum desiccator at ca. 3 mbar for at least 60 min prior to the
measurement. X-ray photoelectron spectroscopy (XPS) analyses were
carried out on a Physical Electronics (PHI) Quantum 2000 photoelectron
spectrometer using monochromated Al Kα radiation (1486.6 eV)
generated from an electron beam operated at 15 kV and 32.3 W and a
Experimental Section
Catalyst synthesis. Copper(I) oxide catalysts were prepared from the
reduction of a Cu(II) precursor by a one-pot solvothermal route based on
[
44]
the procedure described by Deng et al.
pristine Cu O catalyst (Cu O-ud), the Cu(NO
For the synthesis of the
·3H O precursor (4 mmol,
hemispherical capacitor electron energy analyzer equipped with
a
2
2
3
)
2
2
channel plate and a position-sensitive detector. The binding energy (BE)
scale was calibrated with the Au 4f signal being at 84.0 ± 0.1 eV.
Compensation of surface charging during spectra acquisition was
obtained by simultaneous operation of an electron and an argon ion
neutralizer. Depth profiles were recorded by employing alternating cycles
Sigma-Aldrich, purum p.a.) was dissolved in ethylene glycol (40 ml,
Sigma-Aldrich, 99%) under vigorous magnetic stirring (750 rpm for at
least 15 min) and the solution was then transferred to a 50 ml Teflon-
lined autoclave. The sealed autoclave was heated at 140 °C for 10 h and
then allowed to cool down to room temperature. The precipitate was
collected by centrifugation, washed and centrifuged three times with
ultrapure water, and dried overnight at 80 °C in a vacuum oven. The
modified catalysts were prepared in a similar manner by dissolving an
appropriate amount of chloride salt of the second element in ethylene
glycol along with the Cu(II) precursor in the first step of the synthesis (e.g.
+
of XPS analysis and sputtering with a focused 2 kV Ar beam rastered
2
over an area of 4 mm . The sputter yield was determined to be
−
1
4
nm min in a 100 nm Ta
Auger spectra was carried out in the CasaXPS software using
experimentally-derived line shapes from reference Cu and Cu
2 5
O reference film. Peak fitting of the Cu LMM
2
O
[
46,47]
samples.
provided in the Supporting Information. Scanning transmission electron
STEM) micrographs in high-angle annular dark field (HAADF) mode, as
A detailed description of the peak fitting procedure is
0
.2 mmol for a target Cu:modifier atomic ratio of 20). The employed
precursors were SnCl
4
·5H
(Acros Organics, 99.99%) and AlCl
·9H
2
O
(Sigma-Aldrich, 98%), InCl (ABCR,
3
(
9
9
9.99%), GaCl
9.99%). Al(NO
3
3
·6H O (Fluka,
2
well as energy-dispersive x-ray spectroscopy (EDX) element maps of the
electrocatalysts, were obtained in a FEI Talos instrument operated at
3
)
3
2
O (Fluka, 99%) was also evaluated as an Al
O (Sigma-Aldrich, 99.99%) was
employed as a benchmark catalyst (Cu O-cs). The catalysts are coded
as Cu O:M-N, where M denotes the introduced p-block element (M = Sn,
In, Ga, Al) and N is the Cu/M atomic ratio. CuSn(OH) and CuSnO
powders were prepared according to a previously reported procedure.
Briefly, concentrated ammonia solution (1.5 ml, Sigma-Aldrich, 25%) was
added dropwise under stirring to an aqueous solution of Cu(SO ·5H
25 ml, 50 mM, Sigma-Aldrich, puriss.), followed by the addition of an
aqueous solution of Na SnO ·3H O (25 ml, 75 mM, Sigma-Aldrich, 95%)
source. A commercial sample of Cu
2
2
00 kV, while high-resolution transmission electron microscopy (HRTEM)
analyses were carried out in a FEI Tecnai F30 microscope operated at
00 kV. For these analyses, the samples were ultrasonically dispersed in
2
2
3
6
3
[
45]
a few drops of ethanol and then dropcasted onto lacey-carbon-coated Ni
grids. Samples of the used electrocatalysts were obtained by scratching
the catalyst off the electrode with a utility knife. Standard scanning
electron microscopy (SEM) of the fresh and used electrodes was carried
out in a FEI Quanta 200F instrument operated at 15 kV. Temperature-
4
)
2
2
O
(
2
3
2
programmed reduction with hydrogen (H
out in a Thermo Scientific TPDRO 1100 unit equipped with a thermal
conductivity detector. The H -TPR experiments over the fresh catalysts
comprised an isothermal drying step in He (150 °C, 1 h) followed by a
2
-TPR) experiments were carried
over ca. 5 min. The precipitate was aged for 15 min and then collected by
centrifugation, washed with water and ethanol, and dried overnight at
2
8
0 °C in a vacuum oven to obtain CuSn(OH)
6 3
. Amorphous CuSnO was
obtained by calcination of this material in static air at 400 °C for 3 h.
−
1
temperature ramp from 50 to 900 °C (10 °C min ) in a flow of 5 vol.% H
2
3
−1
Electrode preparation. Electrodes were prepared by airbrushing a
catalyst ink on a gas diffusion layer (GDL). The ink was prepared by
dispersing the catalyst (50 mg) in a mixture of ultrapure water (5 ml), 2-
propanol (5 ml, Sigma-Aldrich, 99.8%) and Nafion solution (50 µl, 5 wt.%,
Sigma-Aldrich) for 15 minutes with an ultrasonic processor (VibraCell
VCX130). This dispersion was then painted with an airbrush (Iwata
Eclipse HP-SBS) on the microporous layer of a GDL (Sigracet 35BC,
in N
2
(20 cm STP min ).
Electrochemical tests.
A
custom gastight glass cell with two
®
compartments separated by a Nafion 212 membrane (Alfa Aesar,
0
0
.05 mm thickness) was employed for all electrochemical experiments. A
.1 M KHCO solution (Sigma-Aldrich, 99.95% trace metals basis)
3
prepared with 18.2 MΩ cm ultrapure water was used as the electrolyte.
Each compartment contained 45 ml of the electrolyte that was saturated
2
SGL Group, 12 cm cross-sectional area) which had been mounted on a
–
2
hot plate at a temperature of 90 °C. A catalyst loading of 1 to 2 mg cm
with CO
2 2
(PanGas, purity 4.5), with a resulting pH of 6.75, and CO was
was typically achieved in this manner. The electrodes were produced by
cutting the GDL into L-shaped pieces and attaching them through the
protrusion to a flat silver contact soldered to a copper wire. The resulting
joint and the wire were covered with abundant PTFE tape to avoid
contact with the electrolyte, yielding square electrodes with an area of
bubbled continuously into the catholyte during the electrolysis at a flow
3
−1
rate of 20 cm min . All electrochemical measurements were carried out
at room temperature with an Autolab PGSTAT302N potentiostat, using a
Pt wire as the counter electrode and a Ag/AgCl reference electrode (3 M
NaCl, model RE-1B, ALS). The GDL-based working electrodes were
completely dipped into the electrolyte, and a new electrode was used for
each test unless indicated otherwise. All potentials reported in this work
are referenced to the reversible hydrogen electrode (RHE) scale. The
potentiostatic electrolyses were carried out with the iR compensation
2
2
1
cm or 2.25 cm (larger electrodes were used for tests at lower
potentials). Current densities reported in this work are referred to the
geometric area of the corresponding electrode.
Characterization of catalysts and electrodes. The chemical
composition of the modified catalysts (i.e. the Cu/M atomic ratio N) was
determined by X-ray fluorescence (XRF) spectroscopy in an Orbis Micro-
XRF analyzer equipped with a 35 kV Rh anode and a silicon drift detector.
X-ray diffraction (XRD) patterns of the powder catalysts and the
u
function set at 85% of the uncompensated resistance R , which was
determined before the start of the electrolysis and updated every 7.5 min
by electrochemical impedance spectroscopy measurements at the
electrolysis potential. Following each electrolysis, the recorded potentials
were converted to the RHE scale after manually correcting for the
[
4]
electrodes were obtained with
a
PANalytical X’Pert PRO-MPD
remaining uncorrected R
(DL) capacitances of the electrodes (normalized to the geometric area) in
the reaction medium (CO -saturated 0.1 M KHCO ) before and after 12-
hour electrolyses at −0.8 V were estimated by performing CVs at different
u
, as described by Kuhl et al. Double-layer
diffractometer with Bragg-Brentano geometry using Ni-filtered Cu Kα
radiation (λ = 0.1541 nm). The instrument was operated at 40 mA and
2
3
4
0 kV, and the patterns were recorded in the 10−70° 2θ range with an
−
1
angular step size of 0.05° and a counting time of 180 s per step. The
used electrodes were analyzed after rinsing them copiously with
ultrapure water following removal from the cell, and drying them in a
scan rates (range: 2 to 40 mV s ) narrowly centered (30 mV) on the
open circuit potential (OCP), following stabilization of the full CV
response.
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ChemSusChem
10.1002/cssc.201601578
ARTICLE
Product analysis. The outlet gas from the cathodic compartment flowed
continuously through a sample loop and was periodically injected into an
SRI 8610C gas chromatograph (Multi-Gas #3 configuration) for analysis,
using Ar (PanGas, purity 5.0) as the carrier gas at a head pressure of
[16]
[17]
[18]
[19]
[20]
[21]
[22]
R. Kas, R. Kortlever, A. Milbrat, M. T. M. Koper, G. Mul, J.
Baltrusaitis, Phys. Chem. Chem. Phys. 2014, 16, 12194–12201.
C. S. Chen, A. D. Handoko, J. H. Wan, L. Ma, D. Ren, B. S. Yeo,
Catal. Sci. Technol. 2015, 5, 161–168.
2
.3 bar. The GC was equipped with packed HayeSep D and Molecular
D. Ren, Y. Deng, A. D. Handoko, C. S. Chen, S. Malkhandi, B. S.
Yeo, ACS Catal. 2015, 5, 2814–2821.
Sieve 13X columns, a methanizer, and thermal conductivity and flame
ionization detectors. GC runs were initiated 10 min after the start of the
electrolysis and thereafter every 15 min. Following the electrolysis, a
sample of the catholyte was taken for liquid-phase analysis. The
concentration of formate was determined by high-performance liquid
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This article is protected by copyright. All rights reserved.

ChemSusChem
10.1002/cssc.201601578
ARTICLE
Entry for the Table of Contents
ARTICLE
One way or another. The selectivity
Gastón O. Larrazábal, A. J. Martín, F.
Krumeich, J. Pérez-Ramírez
2 2
in CO reduction over Cu O
electrocatalysts prepared by a
simple and potentially scalable
solvothermal route can be tuned by
the introduction of p-block elements.
The addition of tin and indium further
enhances the selectivity for CO,
while the introduction of aluminum
promotes the formation of valuable
C2 and C3 products at moderate
overpotential.
Page No. – Page No.
Solvothermally-Prepared Cu
2
O
Electrocatalysts for CO Reduction
2
with Tunable Selectivity by the
Introduction of p-Block Elements
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