Probing CO2 Reduction Pathways for Copper Catalysis Using an Ionic Liquid as a Chemical Trapping Agent

Article abstract of DOI:10.1002/anie.202009498

The key to fully leveraging the potential of the electrochemical CO₂ reduction reaction (CO2RR) to achieve a sustainable solar-power-based economy is the development of high-performance electrocatalysts. The development process relies heavily on trial and error methods due to poor mechanistic understanding of the reaction. Demonstrated here is that ionic liquids (ILs) can be employed as a chemical trapping agent to probe CO2RR mechanistic pathways. This method is implemented by introducing a small amount of an IL ([BMIm][NTf₂]) to a copper foam catalyst, on which a wide range of CO2RR products, including formate, CO, alcohols, and hydrocarbons, can be produced. The IL can selectively suppress the formation of ethylene, ethanol and n-propanol while having little impact on others. Thus, reaction networks leading to various products can be disentangled. The results shed new light on the mechanistic understanding of the CO2RR, and provide guidelines for modulating the CO2RR properties. Chemical trapping using an IL adds to the toolbox to deduce the mechanistic understanding of electrocatalysis and could be applied to other reactions as well.

Full text of DOI:10.1002/anie.202009498

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
International Edition Chemie
www.angewandte.org
Accepted Article
Title: Probing CO2 Reduction Pathways in Copper Catalysts using
Ionic Liquid as a Chemical Trapping Agent
Authors: Gui-Rong Zhang, Sascha-Dominic Straub, Liu-Liu Shen,
Yannick Hermans, Patrick Schmatz, Andreas M. Reichert, Jan
P. Hofmann, Ioannis Katsounaros, and Bastian J.M. Etzold
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202009498
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Angewandte Chemie International Edition
10.1002/anie.202009498
RESEARCH ARTICLE
Probing CO
2
Reduction Pathways in Copper Catalysts using Ionic
Liquid as a Chemical Trapping Agent
Gui-Rong Zhang+[a], Sascha-Dominic Straub+[a], Liu-Liu Shen+[a], Yannick Hermans , Patrick Schmatz ,
[b]
[a]
[c]
[b]
[c]
[a]
Andreas M. Reichert , Jan P. Hofmann , Ioannis Katsounaros , and Bastian J. M. Etzold*
[a]
Dr. G.-R. Zhang, S. Straub, Dr. L.-L. Shen, P. Schmatz, Prof. B. J. M. Etzold
Ernst-Berl-Institut für Technische und Makromolekulare Chemie
Technical University of Darmstadt
Alarich-Weiss-Str. 8, 64287 Darmstadt, Germany
E-mail: bastian.etzold@tu-darmstadt.de
[
b]
c]
Dr. Y. Hermans, Prof. J. P. Hofmann
Surface Science Laboratory, Department of Materials and Earth Sciences
Technical University of Darmstadt
Otto-Berndt-Str. 3, 64287 Darmstadt, Germany
A. M. Reichert, Dr. I. Katsounaros
[
Helmholtz-Institute Erlangen-Nürnberg for Renewable Energy (IEK-11)
Forschungszentrum Jülich GmbH
Egerlandstraße 3, 91058 Erlangen, Germany
[+]
These authors contributed equally to this work
Supporting information for this article is given via a link at the end of the document
Abstract: The key to fully leveraging the potential of electrochemical
CO reduction reaction (CO2RR) to achieve a sustainable solar
provides the basis of steering the CO2RR toward desired
products, remains controversial. Although the adsorbed *CO
species is well-accepted as a key intermediate leading to various
2
power-based economy is the development of high-performance
electrocatalysts. The development process relies heavily on trial and
error methods due to poor mechanistic understanding of the reaction.
Here we demonstrate that ionic liquids (ILs) can be employed as a
chemical trapping agent to probe CO2RR mechanistic pathways. This
method is implemented by introducing a small amount of IL
C2+ products, it remains an open challenge to elucidate the
mechanistic pathways from *CO to C2+ products on Cu. Especially,
the formation mechanisms of ethylene and ethanol have long
been the subject under debate in both experimental and
theoretical studies.[5]
(
[BMIm][NTf
2
]) to a copper foam catalyst, on which a wide range of
Mechanistic understandings of the CO2RR are derived almost
exclusively through in situ/operando spectroscopic techniques
(e.g., IR, Raman).[6] Early in situ spectroscopic studies of Cu
CO2RR products, including formate, CO, alcohols, and hydrocarbons,
can be produced. IL can selectively suppress the formation of
ethylene, ethanol and n-propanol while having little impact on others.
Thus reaction networks leading to various products can be
disentangled. The results shed new light on the mechanistic
understanding of the CO2RR, and provide guidelines for modulating
CO2RR properties. Chemical trapping using IL adds to the toolbox to
deduce mechanistic understanding of electrocatalysis and could be
applied to other reactions as well.
electrodes suggest that hydrogenation of *CO to *CH
the precursor to ethylene and ethanol,[6c,7] while others suggest
that formation of these C species would mainly proceed through
forming a *CO dimer (*C
-) which is subsequently protonated to
2
would be
2
2
O
2
*CO-COH.[8] These discrepancies may stem from the inherent
limitations of spectroscopic techniques. The limitations include
[
8b,9]
the interference from the solvent or spectator species,
limited
temporal/spatial resolution due to the low coverage and short
residual time of key intermediates,[6c,6d] and ill-defined background
signals that are sensitive to electrode pretreatment history and
Introduction
[6d,10]
cell configurations,
and all these may add to the uncertainty
of the measurement and make interpretation of resultant spectra
a non-trivial task.[6d] Complementary ways of analyzing the
CO2RR mechanism are highly desirable.
The electrochemical CO
promising solution to offset the increased atmospheric CO
2
reduction reaction (CO2RR) provides a
2
concentration, and also represents an excellent option for storing
intermittent renewable electricity (e.g. solar, wind energy) by
producing value-added chemicals.[1] However, poor energy
conversion efficiency and broad product spectrum are major
barriers to achieving economic viability of the CO2RR. Intensive
effort has been spent searching for high performance
electrocatalysts. Copper (Cu) is identified as the only metal that
produces hydrocarbons and alcohols with appreciable Faradaic
efficiency (FE),[3] due to its moderate binding strength with key
intermediate species.[4] Despite its unique catalytic properties,
mechanistic understanding of the reaction pathways which
Chemical trapping is regarded as an effective way to study
reaction mechanisms. It originated in organic chemistry and was
widely applied in catalysis.[11] The reaction mechanism is deduced
using a compound (trapping agent) that reacts specifically with
one or more reaction intermediate(s) to form a stable product(s).
The trapping agent stops/decelerates specific reactions, and
reaction mechanisms can then be deduced by examining the
products. Bell et al demonstrated in their exemplary works that
the production of hydrocarbons from CO hydrogenation involved
adsorbed methylene species as a key intermediate, as shown by
[2]
1
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Angewandte Chemie International Edition
10.1002/anie.202009498
RESEARCH ARTICLE
the suppressed formation of hydrocarbons in presence of
methylene scavengers.[12] This chemical trapping method has not
yet been applied to electrocatalysis, largely due to the lack of
suitable chemical trapping agents that can selectively interact with
specific intermediates while without being oxidized/reduced under
electrochemical conditions. Inspired by previous works where
ionic liquids (ILs) were employed as surface modifiers to modulate
the catalytic properties of a variety of electrocatalysts, IL is used
here as a chemical trapping agent to analyze the CO2RR
pathways in Cu catalysts. This idea is realized by analyzing the
IL-induced perturbation in the product spectrum. The rationales
for choosing ILs also include their coordination ability with CO2RR
intermediates and good stability over a wide potential window.[13]
ILs have been used as either pure electrolyte or electrolyte
additive to change the CO2RR properties in various metal
catalysts (e.g., Ag, Pb).[14] ILs are reported to lower the
overpotential and explicitly favor the formation of CO, presumably
(F, N, and S) can be identified on Cu-Foam-IL using both energy
dispersive X-ray spectroscopic (EDS) and X-ray photoelectron
spectroscopy (XPS) (Figures S4 & S5), confirming the successful
incorporation of the IL in Cu-Foam. To explore the spatial
distribution of the IL, EDS elemental mapping analyses were
performed (Figure S6). The EDS signals of F and S from the
2
[BMIm][NTf ], are distributed over the porous Cu foams and
surround the macropores without any localized aggregation,
suggesting a homogeneous distribution of the IL. To probe any
possible change in the surface electronic structure of Cu after IL
modification, XPS (Cu 2p3/2) and Auger spectra (Cu LMM) of Cu
were recorded (Figure S7). Both samples exhibit a major XPS
peak at a binding energy (BE) of 932.5 eV, which associates with
0
+
Cu /Cu . The Cu LMM Auger spectra confirm that the surface Cu
+
O),[20] which is not
O occurs immediately
upon air exposure.[21] A minor shoulder peak at a BE of 934.7 eV,
which associates with Cu(OH)
,[20] can also be observed on
pristine Cu-Foam, indicating that a small portion of Cu O in Cu-
Foam are prone to further oxidation to form Cu(OH) . This
on both samples mainly exists as Cu (i.e., Cu
2
surprising since the oxidation of Cu to Cu
2
through coordinating with reduction intermediates (e.g., CO
2
−•) by
2
either stabilizing the intermediates or preventing their spatial
approach.[
2
13c,15]
2
In the current study, the IL is introduced by immobilizing a small
consequent oxidation process was also reported by Tannenbaum
et al when studying the initial oxidation behavior of Cu in air.[21]
Intriguingly, this shoulder peak is absent on Cu-Foam-IL, implying
that the IL can help suppress surface oxidation, which is in line
with our previous study on Pt-based catalysts.[17e-g]
Notwithstanding this difference, considering the well documented
amount
of
1-butyl-3-methylimidazolium
]) on a Cu-Foam
bis(trifluoromethylsulfonyl)imide ([BMIm][NTf
2
catalyst (Figure S2). This method follows the concept of “solid
catalyst with ionic liquid layer (SCILL)”, which was first invented
in heterogeneous catalysis,[16] and was soon successfully
transferred to electrocatalysis particularly in improving
readiness of copper oxide reduction under CO2RR
[
17]
[10,22]
electrocatalysts for the oxygen reduction reaction (ORR). The
hydrophobic nature of the IL and capillary force ensure the
confinement of the IL within the catalysts even in aqueous
electrolytes.[17d,17e] We demonstrate that IL can act as a chemical
trapping agent in the CO2RR. Its presence significantly alters the
product spectrum by selectively suppressing the formation of
ethylene, ethanol, and n-propanol, while without disturbing either
FE or partial current density of the others. These findings
demonstrate selective interactions between the IL and one or
more reaction intermediate(s), while the altered product
distribution provides a unique perspective to track the CO2RR
pathways. This work may represent a simple approach to gaining
mechanistic insights into the CO2RR, and also paves a new way
in modulating the CO2RR activity and selectivity.
conditions,
the presence of a small portion of Cu(OH)
2
species on initial Cu-Foam is not expected to play a significant
role in altering the product distribution. The CO2RR performance
on Cu is sensitive to surface facetings of Cu.[23] To find out
whether the IL can change the surface facetings by selectively
blocking certain facets, we performed PbUPD stripping
experiments on both samples (Figure S8). The comparable
integrated areas of PbUPD stripping peaks verify that (selective)
blocking of Cu facets by the IL can be ruled out.
Results and Discussion
Cu foams were chosen because of the unique catalytic property
of Cu and the abundance of porous structure which is beneficial
to IL immobilization. Cu-Foams were prepared using a hydrogen
evolution reaction (HER) assisted electrodeposition method,[18]
with a Cu plate as the substrate and copper sulfate as the
Figure 1. Representative SEM images of a) Cu-Foam and b) Cu-Foam-IL; the
insets show magnified images, and the red arrow in the inset of panel (b) marks
a curved meniscus of the IL.
precursor (Figure S1). [BMIm][NTf
hydrophobic nature and ability to coordinate with CO
anion radical.[15,19] Figure 1a displays the representative scanning
electron microscopy (SEM) image of pristine Cu-Foam, featuring
an open porous structure. A magnified image (the inset in Figure
2
] was chosen because of its
and/or its
2
The CO electrolysis experiments were performed in a gas-tight
2
electrochemical cell with anode and cathode separated by an
anion exchange membrane (Figure S9). Figure 2a compares the
overall current densities of both samples obtained from
chronoamperometry experiments at various potentials (Figure
S10). Despite the fluctuation, the electrolysis current densities are
more or less comparable at the beginning and end of the
electrolysis on both samples. This result indicates that Cu foams
are stable during the electrolysis regardless of IL modification,
which is also evidenced by the intact dendritic structures of both
Cu foams after the electrolysis (Figure S11). The stability of IL on
1a) discloses a dendritic structure composing of irregularly
shaped particles. Meanwhile, IL modification has not induced any
pronounced difference in either the morphology or their average
macropore sizes (31.8 ± 8.1 µm vs. 31.7 ± 8.4 µm) (Figures 1b &
S3). The IL can be seen (the inset in Figure 1b), existing as blur
on the dendritic nanostructures. Characteristic elements of the IL
2
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RESEARCH ARTICLE
Cu foams during the CO2RR was also probed by performing post-
mortem analyses of Cu-Foam-IL using both XPS and diffuse
reflectance infrared Fourier transform spectroscopy techniques.
The characteristic signals of IL can be clearly resolved using both
techniques after the electrolysis (Figure S12), implying that the IL
can be well-maintained within the Cu foams during electrolysis.
The overall current densities are comparable between these two
samples, despite a slight current increase in Cu-Foam-IL at
potentials of −0.7 and −0.8 V versus reversible hydrogen
electrode (vs. RHE). These results verify that the presence of IL
has not induced any change in mass transport properties of
at around −0.7 V, due to the liberation of strongly adsorbed *CO
intermediate from Cu surfaces. Different from other studies of the
CO2RR on Cu catalysts, on which methane is a major product, in
the current work, methane is produced with a rather low FE(< 1%)
reactant molecules (CO
surfaces, and also imply that the majority of the CO
2
) from bulk electrolyte to Cu-Foam
molecules
2
may approach the catalyst surface in a free form instead of an IL-
coordinated form. Figure 2b compares the FEs of various
products on both samples at −0.7 V. A variety of products,
including CO, formate, ethylene glycol (EG), ethylene, ethane,
ethanol, n-propanol, methane and acetate can be detected, with
CO, formate, and EG identified as the major products (in addition
to hydrogen). Various CO2RR products can usually be observed
in Cu foams, while the major product depends on their
morphology, active surface area, and foam thickness.[2b,18,24]
Intriguingly, herein we observe that EG, which is usually identified
as a minor product in Cu catalysts, is produced with impressively
high FEs (~20%) on both samples. These results showcase that
Cu foams are a versatile platform in producing value-added
CO2RR products.
Figure 2. (a) Current densities recorded at various electrolysis potentials, and
(b) Faradaic efficiency of CO2RR over Cu-Foam and Cu-Foam-IL at -0.7 V.
Electrolysis was performed for 1 hour in 0.1 M CO saturated KHCO solution.
2
3
The potential dependent FEs of various products on pristine and
IL-modified Cu-Foams are compared in Figure 3. The HER, a
major competing reaction of the CO2RR, still dominates the
2
product spectra on both catalysts. A surge in H production is
observed at electrode potentials lower than -0.7 V, relating to the
liberation of surface sites from adsorbed *CO.[25] Meanwhile, the
HER is promoted by the IL modification. This may stem from the
inherent acidity and superior proton transfer capability of the IL
being used, which offers greater proton availability for the
HER.[14a,26] The H
production rates on both samples converge at
2
lower electrode potentials (< -0.85 V), indicating that at higher
reaction rate, the HER is mainly limited by the diffusion of proton
(
or proton source) from bulk solution to the catalyst surface, and
the influence of IL modification is not pronounced. Similar
potential-dependent FEs for major CO2RR products, including
formate, EG, CO, ethylene, and ethane, can be observed on both
samples despite some minor difference in FEs for EG and formate
Figure 3. Summary of the FEs for the CO2RR products on both Cu-Foam
catalysts at different potentials. The suppressed products are marked in red.
The arrows emphasize the changes in FEs of the suppressed products.
3
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RESEARCH ARTICLE
on both catalysts. Similarly, Broekmann et al also observed that
as a chemical trapping agent, which provides the basis for
deducing the CO2RR pathways by analyzing the altered product
spectrum in presence of the IL.
Despite understandings of reaction pathways on Cu catalysts are
rife with controversy, some consensus has been reached, which
C
1
pathway to methane was almost completely suppressed on Cu
foams.[ The morphology or surface faceting of Cu catalysts
plays a crucial role in determining the product selectivity of
CO2RR.[27] For instance, Cu(100) facets favor the formation of
ethylene while Cu(111) facets facilitate the formation of
methane.[27b] This structure sensitive behavior of the CO2RR on
Cu catalysts originates from the differences in binding energy of
18]
enables discussion of the observed chemical trapping results.
−•
Transferring the first electron to CO
2
to form CO
2
anion is
considered as the rate determining step for CO
2
activation
*
CO and/or energetic barrier for the C-C coupling or
because of the high reorganization energy needed to activate a
hydrogenation step between different Cu facets.[8b,28] Herein, the
low FEs of methane on both catalysts imply that the Cu foams
linear CO
geometry.
molecule to form CO
8d,15,29]
−•
anion with bent coordination
2
2
[
Moreover, CO is identified as a key intermediate
2
during the reduction of CO to various C2+ products, since CO is
after the initial reduction of surface Cu
2
O species during the
CO2RR might be enclosed by abundant (100) facets as
suggested by Broekmann et al.[18,24] The comparable FEs and
onset potentials for major products such as CO and formate on
Cu-Foam and Cu-Foam-IL also verify that the presence of IL has
not induced any fundamental structural change on the Cu foam
itself, and at the same time, the possible blockage or surface
rearrangement of specific faceting by IL molecules during the
CO2RR can be excluded. The most striking effect induced by IL
modification is that ethanol and n-propanol, giving a maximum FE
of 7% and 5% on pristine Cu-Foam, respectively, are completely
absent on Cu-Foam-IL (Figure 3h & 3i). Meanwhile, the FE of
ethylene is solely suppressed in the high overpotential region (<
the only C
on a Cu catalyst.
1
molecule that gives similar product spectrum as CO
2
[3a,3d]
However, it remains elusive how the
adsorbed CO intermediate is further converted to various
products. Intrigued by the altered product spectrum after IL
modification, we clarify several elusive reduction pathways by
referring to the widely reported yet controversial mechanism in
literature, as summarized in Figure 4.
Among various products, ethylene shows the most interesting
response to IL modification. Its formation is only suppressed at
high overpotentials, while at low overpotentials both FE (Figure
3d) and the partial current density of ethylene (Figure S13d) are
almost the same regardless of IL modification. This result strongly
suggests that ethylene could form via two separate pathways at
high and low overpotentials. A dual pathway mechanism for
ethylene production was proposed by Koper et al when studying
−0.7 V), with the highest FE decreasing from 10.2% to 5.2% after
IL modification (Figure 3d), while little difference can be observed
in the low overpotential region (i.e. from −0.6 to −0.7 V). The same
conclusion can also be drawn by comparing the partial current
densities of CO2RR products (Figure S13). The IL has selectively
slowed down the production rate of ethylene in the high
overpotential region and ceased the production of ethanol and n-
propanol. These results demonstrate that the feasibility of the IL
[
28b]
CO reduction on Cu.
One pathway (Pathway II) involves the
dimerization of two adjacent CO at low overpotentials, which is
later reduced and protonated to form ethylene. The dimerization
would proceed by forming a hydrogenated CO dimer (*CO-COH)
Figure 4. Proposed reaction roadmaps of CO2RR on Cu catalysts. Selected intermediates are presented for clarity. Unfeasible pathways are marked by red crosses.
4
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RESEARCH ARTICLE
as confirmed by spectroscopic and theoretical studies.[8b] On the
other pathway (Pathway I), CO is converted to either *CHO[4a,28b]
or *COH[30] at high overpotentials, which is then reduced to
However, herein both Cu foam catalysts exhibit fairly high FE of
EG: up to 25% and 19% on Cu-Foam and Cu-Foam-IL,
respectively. The formation of EG is double-checked by analyzing
the liquid products using GC-MS (Figure S14). Consensus on the
reaction pathway to EG has not yet been reached, although it is
carbene-like *CH
between two *CH
2
species, followed by either C-C coupling
, or CO insertion as in the Fischer-Tropsch
2
process, to produce ethylene.[30] The dual pathway mechanism
may also hold its validity for ethylene production in Cu-Foams.
The IL could selectively quench one or more intermediates in
Pathway I, which eventually suppresses the formation of ethylene
at high overpotentials, while it appears that Pathway II, which
starts at relatively low overpotentials and involves the C-C
coupling through CO dimerization, is undisturbed by the IL.
Ethane is not a typical CO2RR product on Cu catalysts.[31] The
production of ethane with a significant FE is explicitly observed on
nanostructured porous Cu catalysts.[31-32] Ethane can be seen a
reduction product of ethylene after two more protonation steps.
The porous structure of Cu catalysts seems to increase the
retention time of pre-formed products in a confined space.
Therefore, for a long time, the formation of ethane has been
attributed to the re-adsorption and reduction of pre-formed
ethylene on Cu catalysts (Pathways I & IIA).[31-32] However, both
FE and partial current density of ethane are actually insensitive to
the IL modification (Figures 3e and S13e). The entirely different
responses of FEs for ethylene and ethane to IL modification imply
that ethane is formed via an independent pathway. Recent works
report that ethane is produced via the CO dimerization pathway
involving ethoxy intermediate,[33] which reconciles with our
observation that the pathway involving CO dimerization is
undisturbed by the IL. These findings suggest that production of
ethane would mainly proceed through Pathway IIB (Figure 4).
inferred that EG formation might proceed through a CO
dimerization mechanism.[31,35] Herein, EG formation is always
accompanied by formate, and their FEs exhibit similar potential-
dependent behavior, i.e. higher FEs obtained at lower
overpotentials and maximum FEs obtained at around −0.7 V.
These results imply that these two products probably share the
-
same intermediate, e.g. *CO
confirmed as
2
, which has been experimentally
a
key intermediate to produce formate.[36]
Brennecke et al suggested that C-C coupling could also take
-
to form oxalate species.[19a]
place between two adsorbed CO
2
The hypothesis here is that EG is produced via dimerization of
-
two adsorbed *CO
2
species, instead of *CO, followed by multi-
step reduction and protonation to give EG (Figure 4). The
predominant product at −0.7 V switches from EG on Cu-Foam to
formate on Cu-Foam-IL. The IL may inhibit the dimerization
−∙
process of the co-adsorbed CO
2
species by preventing their
close approach.[19a] It is also intriguing to observe that IL
modification exhibits little impact on the methane formation. Two
reaction pathways are usually proposed for the methane
formation. One pathway involves carbene (*CH
2
) as an
intermediate, which is further reduced to *CH and finally to CH .
3
4
The other pathway is through hydrogenation of *CO to form *CHO,
followed by a multiple electron-proton transfer process to produce
CH (Figure S15). Considering that Pathway I (carbene pathway)
4
has been significantly suppressed by the IL, herein, comparable
FEs of methane on both Cu foams leads us to hypothesize that
methane is mainly produced through the latter pathway (Figure 4).
Ethanol is considered to share the similar formation mechanism
as ethylene.[
3d,8d]
Two reaction pathways, which involve either
(Pathway I) or
2
formation of carbene intermediate (*CH )
dimerization of two adjacent CO (Pathway II), are usually
proposed (Figure 4). We found that formation of ethanol is
completely suppressed on Cu-Foam-IL, which suggests that IL
traps the key intermediate(s) leading to the formation of ethanol.
Similarly, n-propanol is not produced on Cu-Foam-IL. It is
generally accepted that the formation of n-propanol undergoes
intramolecular C-C coupling between CO and hydrogenated
carbon (e.g., carbene *CH ), followed by proton/electron transfer
2
to form propionaldehyde, an intermediate that is further reduced
to n-propanol (Figure 4).[3d,8d,34] It can be seen that the formation
2
of both ethanol and n-propanol involves carbene species (*CH ),
which is also the intermediate to produce ethylene at high
overpotentials. The IL-induced suppression of ethanol, n-
propanol and ethylene (at high overpotentials, pathway I) infers
that these products likely share one or more common
intermediate(s) selectively trapped by the IL.
Figure 5. A simplified diagram summarizing the proposed CO2RR pathways on
Cu. The IL suppressed pathways and products are highlighted in yellow.
Regarding the other CO2RR products including CO and formate,
their differences in FE and partial current density are quite minor
or within measurement error between two catalysts, determining
their pathways conclusively becomes challenging. Nevertheless,
some inspiring information can be deduced. For instance, the
formation of CO and formate is insensitive to IL modification,
indicating that starting from the adsorption of CO on Cu surfaces
2
to the formation of adsorbed CO, the IL seems to play a negligible
role, or in other words, the IL does not take effect through
Analyzing the IL-induced change in CO2RR product distribution
provides a unique perspective to gain some unprecedented
mechanistic insights into the Cu catalyzed CO2RR which actually
bypasses the necessity of explicit understandings about the
chemical identity of surface intermediate(s). Based on the above
results, a simplified overview of the reaction pathways that lead to
varied CO2RR products is summarized in Figure 5, where the IL
suppressed products and pathways are highlighted in yellow. The
bifurcation of intermediates leading to the suppressed products
coordinating with CO
2
molecules which are more likely
transported to the catalysts surface in a free form. Moreover, EG
is usually detected as a minor product of the CO2RR.[31,35]
starts right after the formation of adsorbed carbene (i.e. *CH
2
) is
especially intriguing (Figure 4). This hints that the key
5
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
intermediate(s) are either *CH
2
or other species (e.g., *COH,
(Figure
complicated CO
2
reduction pathways. This approach is
*
CHO, *C, *CH) that can further be converted to *CH
2
transferable to other electrocatalytic reactions and materials. This
work demonstrates the possibility of moderating the CO2RR
product spectrum by rationally leveraging the IL modification
effect, which can be key to finely tuning the catalytic properties of
S16). Another key question is how the IL molecules can trap the
surface intermediate(s). IL molecules are reported to adopt a
charge-separated layered structure with alternating cation-/anion-
rich layer at electrified surfaces.[17f,37] Accordingly, [BMIm]+
cations should be enriched at the innermost (Stern) layer of the
electrode-electrolyte interface when the electrode is negatively
2
a CO reduction catalyst at a molecular level.
polarized (i.e., the CO
2
electrolysis conditions). Therefore,
Acknowledgements
+
understanding of how [BMIm] cations can possibly interact with
other species would be crucial to extrapolate the role of ILs during
the CO2RR. It is well documented that an imidazolium cation can
The authors acknowledge the funding from the European
Research Council (ERC) under the European Union’s Horizon
2020 research and innovation program (grant agreement No.
681719). We thank Dr. Marc Ledendecker for his comments and
suggestions on the manuscript. We also thank Prof. Hongbin
Zhang, Mr. Yi Xiao and Prof. Bo Li for fruitful discussions.
easily be deprotonated at its C2-site, thus converting the C2-site
into a reactive center due to its nucleophilicity.[38] Accordingly, to
+
clarify whether [BMIm] interacts with surface intermediate(s) via
its C2-site, an imidazolium-based IL on which the C2-site at the
imidazolium cation ring is “neutralized” by a methyl group
+
(
denoted as [BMMIm] , Figure S17a), was used for modifying Cu
foams. It turns out that the chemical trapping effect of the IL is not
pronounced. Both ethanol and propanol can be detected, and the
formation of ethylene at high overpotentials is not suppressed
2
Keywords: CO reduction • chemical trapping • ionic liquid • Cu
foam • mechanism
(
Figure S18). Furthermore, another IL, [HMIm][NTf
2
] which shares
[1] a) C. Costentin, M. Robert, J. M. Saveant, Chem. Soc. Rev. 2013, 42, 2423-
2436; b) J. L. Qiao, Y. Y. Liu, F. Hong, J. J. Zhang, Chem. Soc. Rev. 2014,
structural similarity with [BMIm][NTf ] but features a longer
2
43, 631-675; c) P. Strasser, M. Gliech, S. Kuehl, T. Moeller, Chem. Soc.
cationic chain, was also tested. Although both ethanol and
propanol can still be detected, their FEs are much lower than
those on unmodified counterpart, and ethylene formation is also
suppressed (Figure S18). Two more common ILs (i.e.,
Rev. 2018, 47, 715-735.
[2] a) C. Reller, R. Krause, E. Volkova, B. Schmid, S. Neubauer, A. Rucki, M.
Schuster, G. Schmid, Adv. Energy Mater. 2017, 7, 1602114; b) K. Klingan,
T. Kottakkat, Z. P. Jovanov, S. Jiang, C. Pasquini, F. Scholten, P. Kubella,
A. Bergmann, B. Roldan Cuenya, C. Roth, H. Dau, ChemSusChem 2018,
1
1, 3449-3459; c) S. Gao, Y. Lin, X. Jiao, Y. Sun, Q. Luo, W. Zhang, D. Li,
[
2 2
MTBD][NTf ], [P1444][NTf ]) were also tested for comparison. Not
J. Yang, Y. Xie, Nature 2016, 529, 68-71; d) K. Jiang, R. B. Sandberg, A.
J. Akey, X. Liu, D. C. Bell, J. K. Nørskov, K. Chan, H. Wang, Nat. Catal.
2018, 1, 111-119; e) H. Xiao, W. A. Goddard, T. Cheng, Y. Liu, Proc. Natl.
Acad. Sci. U.S.A. 2017, 10.1073/pnas.1702405114, 201702405.
surprisingly, no pronounced chemical trapping effect can be
identified using either IL (Figure S18). Their product spectra are
comparable to that of the unmodified Cu-Foam, except for a
[
3] a) Y. Hori, in Modern Aspects of Electrochemistry, 10.1007/978-0-387-
2 2
slightly higher FEs of H on Cu-Foam modified with [MTBD][NTf ],
49489-0_3 (Eds.: C. G. Vayenas, R. E. White, M. E. Gamboa-Aldeco),
Springer New York, New York, NY, 2008, pp. 89-189; b) H. Yoshio, K.
Katsuhei, S. Shin, Chem. Lett. 1985, 14, 1695-1698; c) Y. Hori, H. Wakebe,
T. Tsukamoto, O. Koga, Electrochim. Acta 1994, 39, 1833-1839; d) Y. Hori,
R. Takahashi, Y. Yoshinami, A. Murata, J. Phys. Chem. B 1997, 101, 7075-
probably due to the protonic nature of this IL. These results lead
us to hypothesize that the IL traps the surface key intermediates
through bonding with carbene (or other hydrogenated carbon
species) on Cu surfaces. This process may involve deprotonation
and following alkylation reactions at the C2-site of the imidazolium
ring.[39]
7081; e) P. De Luna, R. Quintero-Bermudez, C.-T. Dinh, M. B. Ross, O. S.
Bushuyev, P. Todorović, T. Regier, S. O. Kelley, P. Yang, E. H. Sargent,
Nat. Catal. 2018, 1, 103-110.
a) A. A. Peterson, F. Abild-Pedersen, F. Studt, J. Rossmeisl, J. K. Nørskov,
Energy Environ. Sci. 2010, 3, 1311-1315; b) A. A. Peterson, J. K. Nørskov,
J. Phys. Chem. Lett. 2012, 3, 251-258.
[4]
[5] Y. Y. Birdja, E. Pérez-Gallent, M. C. Figueiredo, A. J. Göttle, F. Calle-
Vallejo, M. T. M. Koper, Nat. Energy 2019, 4, 732-745.
Conclusion
[
6] a) A. D. Handoko, F. Wei, Jenndy, B. S. Yeo, Z. W. Seh, Nat. Catal. 2018,
1, 922-934; b) T.-C. Chou, C.-C. Chang, H.-L. Yu, W.-Y. Yu, C.-L. Dong,
J.-J. Velasco-Vélez, C.-H. Chuang, L.-C. Chen, J.-F. Lee, J.-M. Chen, H.-
L. Wu, J. Am. Chem. Soc. 2020, 10.1021/jacs.9b11126; c) Y. Zheng, A.
Vasileff, X. Zhou, Y. Jiao, M. Jaroniec, S.-Z. Qiao, J. Am. Chem. Soc. 2019,
This work outlines a new strategy to probe CO2RR pathways. IL
alters the product spectrum during the CO2RR on Cu foams.
Analyzing the responses of CO2RR products to IL modification is
a unique way to gain new insights into CO2RR pathways: 1)
Ethanol and n-propanol form explicitly through a “carbene”
mechanism, while formation of ethylene could proceed through
two independent pathways which involve carbene and dimerized
CO as key intermediates at high and low overpotentials,
respectively; 2) The presence of IL can selectively suppress the
formation of those products involving carbene intermediates,
likely by forming stable imidazolium-carbene compound(s); 3)
Ethane, which has long been considered a reduction product of
re-adsorbed ethylene during CO2RR, is identified as proceeding
with an independent pathway that involves CO dimerization
process. Considering the great structural flexibility in ILs,
141, 7646-7659; d) S. Zhu, T. Li, W.-B. Cai, M. Shao, ACS Energy Letters
2019, 4, 682-689.
[
7] B. D. Smith, D. E. Irish, P. Kedzierzawski, J. Augustynski, J. Electrochem.
Soc. 1997, 144, 4288-4296.
8] a) R. Kortlever, J. Shen, K. J. P. Schouten, F. Calle-Vallejo, M. T. M. Koper,
J. Phys. Chem. Lett. 2015, 6, 4073-4082; b) E. Pérez-Gallent, M. C.
Figueiredo, F. Calle-Vallejo, M. T. M. Koper, Angew. Chem. Int. Ed. 2017,
[
56, 3621-3624; c) K. D. Yang, C. W. Lee, K. Jin, S. W. Im, K. T. Nam, J.
Phys. Chem. Lett. 2017, 8, 538-545; d) Z. Sun, T. Ma, H. Tao, Q. Fan, B.
Han, Chem 2017, 3, 560-587.
[
[
[
[
[
9] R. Kas, O. Ayemoba, N. J. Firet, J. Middelkoop, W. A. Smith, A. Cuesta,
ChemPhysChem 2019, 20, 2904-2925.
10] S. Jiang, K. Klingan, C. Pasquini, H. Dau, J. Chem. Phys. 2019, 150,
041718.
11] H. Y. Wang, J. P. Liu, J. K. Fu, H. B. Zhang, K. R. Tsai, Res. Chem.
Intermed. 1992, 17, 233-242.
12] a) J. G. Ekerdt, A. T. Bell, J. Catal. 1980, 62, 19-25; b) J. A. Baker, A. T.
Bell, J. Catal. 1982, 78, 165-181.
13] a) J. Feng, S. Zeng, J. Feng, H. Dong, X. Zhang, Chin. J. Chem . 2018, 36,
9
61-970; b) G.-R. Zhang, B. J. M. Etzold, J. Energy Chem. 2016, 25, 199-
2
identification of reaction pathways for CO products by carefully
207; c) M. Alvarez-Guerra, J. Albo, E. Alvarez-Guerra, A. Irabien, Energy
designing task-specific ILs to selectively interact with intermediate
species may be feasible. The success of this will bring IL
modification closer to being a generic strategy for analyzing
Environ. Sci. 2015, 8, 2574-2599; d) S. J. Zhang, J. Sun, X. C. Zhang, J. Y.
Xin, Q. Q. Miao, J. J. Wang, Chem. Soc. Rev. 2014, 43, 7838-7869.
6
This article is protected by copyright. All rights reserved.

Angewandte Chemie International Edition
10.1002/anie.202009498
RESEARCH ARTICLE
[
14] a) J. H. Koh, H. S. Jeon, M. S. Jee, E. B. Nursanto, H. Lee, Y. J. Hwang,
B. K. Min, J. Phys. Chem. C 2015, 119, 883-889; b) B. A. Rosen, A. Salehi-
Khojin, M. R. Thorson, W. Zhu, D. T. Whipple, P. J. A. Kenis, R. I. Masel,
Science 2011, 334, 643-644.
[15] B. A. Rosen, I. Hod, Adv. Mater. 2018, 30, 1706238.
[16] U. Kernchen, B. Etzold, W. Korth, A. Jess, Chem. Eng. Technol. 2007, 30,
9
85-994.
17] a) J. Snyder, T. Fujita, M. W. Chen, J. Erlebacher, Nat. Mater. 2010, 9, 904-
07; b) Y. M. Tan, C. F. Xu, G. X. Chen, N. F. Zheng, Q. J. Xie, Energy
[
9
Environ. Sci. 2012, 5, 6923-6927; c) J. Snyder, K. Livi, J. Erlebacher, Adv.
Funct. Mater. 2013, 23, 5494-5501; d) G.-R. Zhang, M. Munoz, B. J. Etzold,
ACS Appl. Mater. Interfaces 2015, 7, 3562-3570; e) G.-R. Zhang, M. Munoz,
B. J. Etzold, Angew. Chem., Int. Ed. 2016, 55, 2257-2261; f) G.-R. Zhang,
T. Wolker, D. J. S. Sandbeck, M. Munoz, K. J. J. Mayrhofer, S. Cherevko,
B. J. M. Etzold, ACS Catal. 2018, 8, 8244-8254; g) M. George, G.-R. Zhang,
N. Schmitt, K. Brunnengräber, D. J. S. Sandbeck, K. J. J. Mayrhofer, S.
Cherevko, B. J. M. Etzold, ACS Catal. 2019, 9, 8682-8692.
[18] A. Dutta, M. Rahaman, N. C. Luedi, M. Mohos, P. Broekmann, ACS Catal.
2016, 6, 3804-3814.
[19] a) L. Y. Sun, G. K. Ramesha, P. V. Kamat, J. F. Brennecke, Langmuir 2014,
30, 6302-6308; b) Z. G. Lei, C. N. Dai, B. H. Chen, Chem. Rev. 2014, 114,
1289-1326.
[20] M. C. Biesinger, Surf. Interface Anal. 2017, 49, 1325-1334.
[21] I. Platzman, R. Brener, H. Haick, R. Tannenbaum, J. Phys. Chem. C 2008,
112, 1101-1108.
[
22] a) L. Mandal, K. R. Yang, M. R. Motapothula, D. Ren, P. Lobaccaro, A.
Patra, M. Sherburne, V. S. Batista, B. S. Yeo, J. W. Ager, J. Martin, T.
Venkatesan, ACS Appl. Mater. Interfaces 2018, 10, 8574-8584; b) D. Ren,
Y. Deng, A. D. Handoko, C. S. Chen, S. Malkhandi, B. S. Yeo, ACS Catal.
2
015, 5, 2814-2821.
23] Y. Huang, A. D. Handoko, P. Hirunsit, B. S. Yeo, ACS Catal. 2017, 7, 1749-
756.
[
1
[24] S. Sen, D. Liu, G. T. R. Palmore, ACS Catal. 2014, 4, 3091-3095.
[25] D. Raciti, C. Wang, ACS Energy Letters 2018, 3, 1545-1556.
[26] Q. Wang, Y. Gao, Z. Ma, Y. Zhang, W. Ni, H. A. Younus, C. Zhang, Z. Chen,
S. Zhang, J. Energy Chem. 2020, 150, 041718.
[27] a) W. Tang, A. A. Peterson, A. S. Varela, Z. P. Jovanov, L. Bech, W. J.
Durand, S. Dahl, J. K. Nørskov, I. Chorkendorff, Phys. Chem. Chem. Phys.
2
012, 14, 76-81; b) A. Loiudice, P. Lobaccaro, E. A. Kamali, T. Thao, B. H.
Huang, J. W. Ager, R. Buonsanti, Angew. Chem. Int. Ed. 2016, 55, 5789-
792; c) D. Gao, I. Sinev, F. Scholten, R. M. Arán-Ais, N. J. Divins, K.
Kvashnina, J. Timoshenko, B. Roldan Cuenya, Angew. Chem. Int. Ed. 2019,
8, 17047-17053.
28] a) J. H. Montoya, C. Shi, K. Chan, J. K. Nørskov, J. Phys. Chem. Lett. 2015,
, 2032-2037; b) K. J. P. Schouten, Z. Qin, E. Pérez Gallent, M. T. M. Koper,
5
5
[
[
6
J. Am. Chem. Soc. 2012, 134, 9864-9867.
29] a) A. Hailu, S. K. Shaw, Energy & Fuels 2018, 32, 12695-12702; b) W. Lu,
B. Jia, B. Cui, Y. Zhang, K. Yao, Y. Zhao, J. Wang, Angew. Chem., Int. Ed.
2
017, 56, 11851-11854.
30] X. Nie, M. R. Esopi, M. J. Janik, A. Asthagiri, Angew. Chem. Int. Ed. 2013,
2, 2459-2462.
31] S. Nitopi, E. Bertheussen, S. B. Scott, X. Liu, A. K. Engstfeld, S. Horch, B.
Seger, I. E. L. Stephens, K. Chan, C. Hahn, J. K. Nørskov, T. F. Jaramillo,
I. Chorkendorff, Chem. Rev. 2019, 119, 7610-7672.
[
[
5
[
32] a) K. D. Yang, W. R. Ko, J. H. Lee, S. J. Kim, H. Lee, M. H. Lee, K. T. Nam,
Angew. Chem. Int. Ed. 2017, 56, 796-800; b) H. Song, M. Im, J. T. Song,
J.-A. Lim, B.-S. Kim, Y. Kwon, S. Ryu, J. Oh, Appl. Catal., B 2018, 232,
391-396; c) C. S. Chen, J. H. Wan, B. S. Yeo, J. Phys. Chem. C 2015, 119,
26875-26882.
[33] a) M. Ma, K. Djanashvili, W. A. Smith, Angew. Chem. Int. Ed. 2016, 55,
6680-6684; b) A. Vasileff, Y. Zhu, X. Zhi, Y. Zhao, L. Ge, H. M. Chen, Y.
Zheng, S. Z. Qiao, Angew. Chem. Int. Ed. 2020, 10.1002/anie.202004846.
34] a) D. Kim, C. S. Kley, Y. Li, P. Yang, Proc. Natl. Acad. Sci. U.S.A. 2017,
[
114, 10560-10565; b) D. Ren, N. T. Wong, A. D. Handoko, Y. Huang, B. S.
Yeo, J. Phys. Chem. Lett. 2016, 7, 20-24; c) R. A. Geioushy, M. M. Khaled,
K. Alhooshani, A. S. Hakeem, A. Rinaldi, Electrochim. Acta 2017, 245, 456-
4
62.
35] K. P. Kuhl, E. R. Cave, D. N. Abram, T. F. Jaramillo, Energy Environ. Sci.
012, 5, 7050-7059.
36] I. V. Chernyshova, P. Somasundaran, S. Ponnurangam, Proc. Natl. Acad.
Sci. U.S.A. 2018, 115, E9261-E9270.
[
[
[
[
[
2
37] R. Wen, B. Rahn, O. M. Magnussen, Angew. Chem. Int. Ed. 2015, 54,
6062-6066.
38] K. Noack, P. S. Schulz, N. Paape, J. Kiefer, P. Wasserscheid, A. Leipertz,
Phys. Chem. Chem. Phys. 2010, 12, 14153-14161.
39] E. Ennis, S. Handy, Molecules 2009, 14, 2235-2245.
7
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RESEARCH ARTICLE
Table of Contents
The presence of a small amount of ionic liquid significantly alters the product spectrum of CO
2
reduction on a Cu catalyst. The ionic
liquid acts as a chemical trapping agent, selectively suppressing the formation of C2+ products that involve carbene as a key intermediate.
The response in product distribution to ionic liquid modification offers a new way to disentangle the complex reaction networks of CO
reduction in Cu catalysts.
2
8
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