CO 2 Reduction
Electrocatalytic CO 2 Reduction on CuO x Nanocubes : T racking the
Evolution of Chemical State , Geometric Structure , and Catalytic
Selectivity using Operando Spectroscop y
T im Mçller , F abian Sc holten, T rung Ngo Thanh, Ilya Sinev , J anis T imoshenko , Xingli W ang,
Zarko J ovanov , Manuel Gliec h, Beatriz Roldan Cuenya,* Ana Sofia V arela,* and P eter Strasser*
Abstract : The direct electrochemical conversion of carbon
dioxide (CO 2 ) into multi-carbon (C 2 + ) products still faces
fundamental and technological challenges . While facet-con-
trolled and oxide-derived Cu materials have been touted as
promising catalysts , their stability has remained problematic
and poorly understood. Herein we unco ver changes in the
chemical and morphological state of supported and unsup-
ported Cu 2 O nanocubes during operation in lo w-current H-
Cells and in high-current gas diffusion electrodes (GDEs)
using neutral pH buffer conditions . While unsupported nano-
cubes achieved a sustained C 2 + F aradaic efficiency of around
60 % for 40 h, the dispersion on a carbon support sharply
shifted the selectivity pattern towards C 1 products . Operando
XAS and time-resolved electron microscop y revealed the
degradation of the cubic shape and, in the presence of a carbon
support, the formation of small Cu-seeds during the surpris-
ingly slow reduction of bulk Cu 2 O . The initially (100)-rich
facet structure has presumably no controlling role on the
catalytic selectivity , whereas the oxide-derived generation of
under -coor dinated lattice defects , can support the high C 2 +
product yields .
Introduction
A society fueled by intermittent electricity from wind and
solar power plants invariably requires electrochemical tech-
nologies to store recurring electricity supply surpluses . [1] Th e
direct CO 2 electrochemical reduction reaction (CO 2 RR) has
emerged as one promising technology to use electricity to
convert CO 2 into carbon-based chemicals or fuels , thereby
closing the anthropogenic energy carbon cycle . [2]
T he product distribution of the CO 2 RR depends sensi-
tively on the chemical nature of the catalyst, in particular Cu
has been intensively studied and is arguably one of the most
interesting materials for the CO 2 RR due its capability of
facilitating the direct reduction of CO 2 into C 2 + products . [3, 4]
However, the required kinetic overpotentials , issues of
selectivity and stability still largely limit commercial interest.
T o date , most Cu-based catalysts employed in CO 2 RR
revealed a complex temporal variation of the faradaic
efficiency . T he origin of these efficiency variations remains ,
however, poorly explored. Some of these changes can be
assigned to the deposition of electrolyte contaminations ,
while others arise from the complex structural and chemical
transformation of the catalysts . [5]
In recent years , much scientific work has been focused on
understanding the fundamental factors that control the
catalytic activity of Cu. Different studies have repeatedly
shown that the selectivity between C 2 H 4 /CH 4 is strongly
dependent on the surface structure and composition of the
catalyst, as well as on the reaction electrolyte . [5, 6] It has been
established, that while ethylene is the predominant hydro-
carbon in alkaline pH, the production of CH 4 is favored in
acidic conditions , due to a difference in the mechanistic
protonation pathway . [7] In the case of nanoparticles , both the
size and the interparticle distance showed an effect on the
reaction activity and selectivity . [6d, 8] W ork on polycrystalline
Cu implied that the C 2 H 4 to CH 4 ratio is strongly dependent
on the surface pretreatment. In particular, oxide derived Cu
(OD-Cu) exhibited a clearly altered catalytic performance
from that of metallic Cu, suppressing CH 4 selectivity and
lowering the onset potential for CO and C 2 H 4 formation. [2-
d, 6a,j, 9] Despite the promising results obtained on OD-Cu, the
molecular origin of its unique selectivity has remained
controversial. A number of studies worked on the hypothesis
that the outstanding catalytic performance of OD-Cu may
[*] T . Mçller , T . N. Thanh, X. W ang, Z. Jovanov , M. Gliech, P. Strasser
The Electrochemical Energy , C atalysis, and Materials Science Labo-
ratory , Department of Chemist ry , Chemical Engineering Division,
T echnical University Berlin
Berlin (Germany)
E-mail : pstrasser@ tu-berlin.de
F. Scholten, J. Timoshenko, B. Roldan Cuenya
Department of Interface Science, F ritz-Haber Institute of the Max
Planck Society
14195 Berlin (Germany)
E-mail : roldan@fh i-berlin.mpg.de
I. Sinev
Department of Physics, Ruhr-University Bochum
44780 Bochum (Germany)
A. S. V arela
Institute of Chemistry , National Autonomous University of Mexico
Mexico City (Mexico)
E-mail : asvarela@iq uimica.unam.mx
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under :
https ://doi.org/ 10.1002/anie.2020071 36.
2020 The Authors. Published by Wiley-VCH GmbH. This is an
open access article under the terms of the C reative Commons
Attribution License, which permits use, distributi on and reproduc-
tion in any medium, provided the original work is properly cited.
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International Edition: doi.org/10.1002/anie.202007136
German Edition: doi.org/10.1002/ange.202007136
17974 2020 The Autho rs. Published by Wiley-VCH GmbH Angew . Chem. Int. Ed. 2020 , 59 , 17974 – 17983
origin in an altered binding strength of reactive intermediates .
Here , different causes such as grain boundaries , [10] the
presence of remaining Cu I[2d, 11] and of subsurface oxygen
were suggested. [6g, 9b , 12]
On a device site , Gas Diffusion Electrodes (GDEs) have
proved indispensable for integration of catalytic reactions in
larger -scale electrolyzer devices , as shown for the case of CO 2
to CO with GDEs based on Ag catalysts , pioneered by the
Kenis group . [13] T o date, Cu catalyst-based GDE studies have
largely included electrodeposited Cu films for the production
of hydrocarbons and oxygenates , yet their yields , reactivity
and performance stability remained low . [14] Later, more
efficient Cu catalyst-based GDEs operated in electrolyzer
cells were reported, at the price of using highly alkaline , and
therefore quite impractical electrolytes or hazardous pure CO
input feeds . [15]
In this contribution, we explore structure and composi-
tion-selectivity relations of cubic , Cu 2 O nanoparticles of
about 35 nm edge-length. The nanocubes were initially tested
in a two-chamber H-Cell as a carbon-supported and unsup-
ported powder catalyst. W e observed a clear effect of the
carbon support, steering the selectivity away from C 2 + of the
unsupported nanocubes towards C 1 products . W e correlated
this difference to an altered morphological evolution of the
nanocubes by spatial isolation of the particles on the
conductive support. Furthermore , we monitored the surpris-
ingly slow and incomplete electrochemical reduction of the
unsupported Cu 2 O particles on the molecular scale. Our
operando XAS allowed us to trace the electrochemical
reduction of the initially cubic Cu 2 O into predominantly
metallic particles of ill-defined morphology , showing an
abundance of Cu lattice defects , which are often associated
with catalytic sites of extraordinary activity . In this , we
succeed at presenting the formation of a defective Cu
structure by electrochemical reduction of an oxidized pre-
cursor in real time .
Furthermore , deposition of the catalysts on a Gas Dif-
fusion Electrode allowed for tests at industrially relevant
current densities of 50 to 700 mA cm 2 in an electrolyzer flow
cell. W e were able demonstrate a high and stable faradaic
efficiency of unsupported Cu 2 O nanocubes toward C 2 +
products in an electrolyzer set up at neutral pH. Carbon-
supported Cu 2 O nanocubes , on the other hand, undergo
distinctly different structural dynamics in electrolyzers . Over
the course of 40 h, they first form very small Cu seeds that
grow , sinter, and eventually resemble the unsupported Cu
particles in structure and efficiency .
Results and Discussion
Unsupported and carbon-supported cubic Cu 2 O nanocatalysts
T he unsupported Cu 2 O nanocubes will be referred to as
“U-NC”, while the supported nanocubes at a loading of 23
weight% will be denoted as “S-NC”. F igure 1 a–e and F ig-
ure S1b show the local TEM-based microstructural morphol-
ogies of the as-prepared unsupported and supported nano-
catalysts , while the insets in F igure 1 d,e display the selected
area of the electron diffraction patterns and F igure 1 c the X-
ray diffraction patterns of the crystalline phases , respectively .
F igure 1 d,e and Figure S1b confirm the targeted cubic
morphology of the Cu-based NPs with an edge length of
35 6 nm (see histogram in F igure 1 f), which showed no
Figure 1. T ransmission electron microscop y (TEM) images of a),d) the unsupported Cu 2 O nanocubes, U-NC, and b),e) the carbon-supported
Cu 2 O nanocubes (23 wt %), S-NC. Insets in (d,e) show selected area electron diffraction (SAED) patterns of the respective material. c) X-ray
diffraction (XRD) patterns of both catalysts and f ) particle size histogram derived from TEM images of the U-NC.
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apparent change once supported on the carbon. T he crystal
phase analysis in F igure 1 d,e,c revealed a single Cu 2 O phase
for both S-NCs and the U-NCs , confirming the stability of the
crystalline cubic Cu 2 O phase during the supporting proce-
dure .
Electrochemical CO 2 RR activity and stability in a liquid-
electrolyte H-C ell configuration
Subsequently , both CO 2 RR electrocatalysts , S-NC and U-
NC , were tested in a two-compartment H-Cell at constant
applied electrode potentials in aqueous 0.1 m KHCO 3 buffer
electrolyte . While the favorable faradaic ethylene efficiencies
of cubic-shaped Cu-based nanoparticles have been docu-
mented in previous works , [16] catalytic support effects have
been proven important but remain less studied. [5] T herefore ,
we place emphasis on the comparison between unsupported
nanocubes , U-NC (red color code and symbols), and the
supported ones , S-NC (black code and symbols).
Catalytic CO 2 RR Activity : T he S-NC showed a clearly
higher geometric current density compared to the U-NC at
more cathodic potentials than 0.9 V RHE (F igure 2 d). Com-
parison to a supported sample of higher particle loading
(44 wt %) showed a similar enhancement in activity , agreeing
with the notion of a higher availability of active Cu sites by
dispersion on the support, see Figure S5. T o exclude effects of
the substrate (glassy carbon plate) and support (V ulcan
Figure 2. a),b) F aradaic product efficiencies (FEs) as a function of the applied electrode potential after one hour of reaction time for a) the
unsupported Cu nanocu bes, U-NC and b) the carbon-supported Cu nanocu bes (23 wt %), S-NC. C olor coded bars denote products as given in the
legend. c) Chronoamperometric efficienc y stability at a constant applied electrode potential of 0.86 V RHE for the U-NC and the S-NC catalysts .
C olors as in (a). d) The electrochemi cal CO 2 reduction polarization curves (geometric current-density vs. IR-corrected applied electrode potential)
and e) (C 2 + /C 1 ) FE ratios versus IR corrected applied electrode potential. Lines to guide the eye.
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carbon) reference measurements were performed, which
showed only minor catalytic activity of the two , mainly
attributable to HER (F igure S4). W e further performed
measurements of lead under potential deposition (Pb-UPD)
in order to quantify this increase in accessible surface, which
we estimated to be in the order of 3 times for the S-NC in
comparison to the U-NC based on the respective Pb-mono-
layer stripping charge (F igure S3).
Catalytic CO 2 RR Selectivity : F igure 2 a,b shows the
F aradaic efficiency (FE) values of all CO 2 RR products for
the U-NC and S-NC as function of the applied IR-corrected
potential. At potentials of 0.65 V RHE , both catalysts exhib-
ited high CO FEs , early onset potentials of ethylene combined
with a suppression of methane formation. At 0.97 V RHE , the
U-NCs displayed high ethylene selectivity , given the neutral
pH conditions , exceeding 30 % FE. T he high C 2 + product
yield is consistent with previous reports on CO 2 electro-
reduction on cubic-shaped Cu NPs in H-Cell, and is consid-
ered to constitute the key advantage of Cu(100) facet-rich
nanocatalysts over electropolished Cu foils . [16b, 17]
In addition to the distinct catalytic activity , the dispersion
of the Cu nanocubes on the carbon support induced
important differences in terms of selectivity : T he S-NC
displayed a clearly lower ethylene FE, while the FEs of CO
and HCOO were higher. The HCOO FE peaked at 19 %,
CO at 24 %, while the C 1 FEs values generally remained at
elevated levels . T his observation is in good agreement with
previous studies . [5] Again, our supported 44 wt % Cu nano-
cubes sample is in agreement with the observed FE shift,
caused by the support, see F igure S1.
Next, we turn to a comparison of the time-stability of the
FE values of the S-NC and the U-NC catalysts at 0.86 V RHE ,
displayed in F igure 2 c. It is evident that both catalysts
displayed distinct, yet similarly time-stable trajectories of
the FE values of their major gas products , resulting from
CO 2 RR, over at least 5 hours . T ests at other kinetic over -
potentials as 0.95 V RHE and 0.66 V RHE showed again
constant FEs over 5 hours of testing time , confirming the
stability of the systems (see F igure S6).
Support effects : T he dispersion of Cu 2 O nanocubes on
a carbon support altered their catalytic selectivity in a char -
acteristic way : C 2-3 products , such as ethylene , ethanol and
propanol became suppressed, whereas the production of C 1
products , such as CO , HCOO and CH 4 , was favored. T his
resulted in a clearly smaller ratio of (C 2 + /C 1 ) products over
the tested potential range (F igure 2 e). Previously , simple
physical mixing of Cu NPs with Ketjenblack carbon prior to
electrode casting showed similar shifts in faradaic selectivities ,
and was attributed to a disrupted morphological particle
evolution during CO 2 RR from spherical to cubic morpholo-
gies . [18] While that view placed emphasis on dynamic struc-
tural changes of Cu particles on carbon surfaces , it neglects
the contributing effect of an effectively larger mean inter -
particle distance on a support, associated with less likely re-
adsorption of reactive intermediates such as CO . [6d, 8a] Such an
effect can be of significance for the S-NCs , as the dispersion of
the Cu nanocubes on the high surface area carbon support, as
evidenced by our TEM and Pb-UPD measurements , results in
an effective physical separation of active surface sites of
adjacent Cu cubes . In this , we believe that our observation is
fundamentally comparable to the work of W ang et al.
mentioned above . T he reduction of geometric mass loading
of spherical CuO x particles on planar carbon substrate is
similar to the support of Cu 2 O on a porous carbon. In both
cases , the spatial density of the particles was reduced in the
process , which resulted in a decrease of FE for multi-carbon
products . Furthermore, Grosse et al. recently showed a strong
dependence of the experimental CO 2 RR reactivity of electro-
deposited, several hundred nanometer -sized Cu cubes on
their underlying substrates , which was chosen as either a Cu
foil, or a carbon paper. Their analysis suggested next to
a difference in the morphological stability , a difference in
chemical stability as well, caused on the substrate . [5]
The stability and role of {100} facets : T o further deconvo-
lute the possible origins of the observed catalytic difference ,
we tracked the morphological and crystal phase evolution of
the S-NC and U-NC catalysts . Figure 3 a,b and F igure d,e
show electron micrographs of both catalysts before and after
60 min of continuous CO 2 electrolysis at constant potential of
0.95 V RHE . T he particles appear to have agglomerated and
merged, without a defined resulting morphology . Further
microscopy of higher resolution clearly shows this strong
morphological transition, as seen in F igure S2. F rom this , we
conclude that it is unlikely that the observed stable FE over
5 hours (F igure 2 c, F igure S6) originate from the well-defined
initial cubic morphology . Previous studies on metallic Cu
cubes , have attributed their high C 2 selectivity to the sustained
stable presence of {100} facets , [16b] which are known to
catalyze the CO dimerization at low overpotentials . [19]
Oxide-derived Cu : Wh ile the mechanistic origins of
enhanced dimerization products on {100} facets are well
described, the molecular reasons for the improved perfor -
mance of oxide-derived Cu (OD-Cu) surfaces are still
controversial. Different factors , such as an increase of the
local pH due to large surface roughness compared to
a polished metallic Cu foil, [9c] the presence of grain bounda-
ries after reduction, [20] remaining oxygen species within the
catalyst surface [2d, 16c] and the formation of undercoordinated
Cu sites [6b] have all been correlated and associated with the
high selectivity towards C 2 + products .
Origins of sustained high C 2 + yields : . Our results suggest
that stable C 2 + FE similar to OD-Cu bulk catalysts can be
obtained on unsupported Cu I oxide nanoparticles as well. [20b]
W e hypothesize that factors , such as the detailed geometric
nature of the resulting stepped OD-Cu surfaces , or the
chemical state of the Cu atoms at the catalyst surface control
the experimental product selectivity during CO 2 RR. Such
possible origins have been discussed in recent literature :
Mistry et al. showed high ethylene selectivity during the
CO 2 RR for an oxygen-plasma treated copper foil, which
partially reduced under reaction conditions , yet STEM-EDS
analyses after the catalytic tests suggested the sustained
presence of a small fraction of oxygen atoms and Cu I . [2d] A
possible theoretical explanation for this observation was
provided by Xiao et al. , who suggested that a mixed matrix of
metallic and oxidized copper can facilitate the dimerization of
adsorbed CO . [21] Furthermore , the effect of residual sub-
surface oxygen, generated during the reduction of oxidized
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copper, is discussed in multiple publications and an amor -
phous metallic Cu layer was suggested as a possible stabilizing
site for them. [9b , 12a–c, 22] Moreover, the stabilization of Cu I
species has also been demonstrated in the presence of Br
and I. [6j, 12d,e]
Post-reaction, quasi in situ and operando catalyst
characterization
T o gain insight into structural and chemical changes of the
surface and bulk of the S-NCs and U-NC we investigated first
the phase structure of the nanocubes before and after CO 2 RR
using grazing incidence X-ray powder Diffraction (GI-XRD)
(F igure 3 c,f). As expected from the strongly reducing reac-
tion environments , the characteristic metallic Cu(200) Bragg
diffraction reflection at 50
8
indicated the formation of
metallic Cu after 60 min of reaction at a potential of
0.65 V RHE . Interestingly , the presence of a Cu 2 O(111) facet
reflection at 36
8
remained visible for both catalysts , while the
Cu 2 O pattern intensity decreased under more negative
potentials (see patterns at 0.95 V RHE ). This implies a surpris-
ing chemical stability of Cu 2 O , contrary to thermodynamic
expectations . Nevertheless , we acknowledge the limitations of
ex situ XRD to get a proper assessment of the phase-
evolution of the catalyst, especially due to the highly reactive
nature of near -surface metallic copper under ambient con-
ditions . Additionally , changes in nanoparticle orientation can
also influence the intensity ratio of observed reflexes in GI-
XRD . T his is why we resorted to quasi in situ X-ray
Photoelectron Spectroscopy (XPS) to trace the evolution of
the chemical state of the surface with operando X-ray
Absorption Spectroscopy (XAS) to assess the changes in
the bulk phase under reaction conditions .
Th e quasi in situ XPS measurements (F igure 4 a–c)
suggested a complete chemical reduction to metallic Cu of
the near surface region after about one hour of reaction at low
( 0.65 V RHE ), as well as more negative ( 0.95 V RHE ) poten-
tials . T his change in redox state was comparable for the U-NC
and the S-NC sample . Whi le our XPS results show a fast
reduction of the near surface region, it does not allow for
a statement about possible defects introduced during the
reduction or the oxidation state of deeper catalyst layers .
Here , our analysis by operando XAS helps to add further
insight into this system.
T he U-NCs were deployed in an operando X-ray analysis
cell and their chemical state and local structure was tracked
during the electrochemical reduction of CO 2 . F igure 5 shows
operando XANES (F igure 5 a) and EXAFS (F igure 5 c)
spectra recorded approximately every 10 minutes under
chronoamperometric conditions at 0.66 V RHE and subse-
quently at 0.95 V RHE . T here were drastic changes taking
place in the sample already during the first 10 minutes under
the moderate applied potential. Whi le the potential was kept
constant, the pre-edge features became broader , less intense ,
and the absorption edge shifted to lower energy towards its
position in the spectrum of a metallic foil. At the same time ,
the feature above the edge gradually changed its shape
towards a two-peak feature characteristic of metallic Cu.
Figure 3. a) SEM images of the unsupported catalyst, U-NC, as prepared and b) after 1 hour of CO 2 RR at 0.95 V RHE . d) TEM images of supported
Cu nanocu be, S-NC, (23 wt %) catalyst, as prepared and e) after 1 hour of CO 2 RR testing at 0.95 V RHE . c) XRD patterns of the U-NC catalyst and
f ) the S-NC catalysis after initial deposition on a glassy carbon electrode (black), after 1 hour of CO 2 RR at 0.66 V RH E (blue) and after 1 hour at
0.95 V RHE (red). XRD patterns of the blank glassy carbon plate and of the as prepared catalyst powders are displayed in Figure S7 for reference.
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A linear combination fitting analysis of the operando
XANES data, using Cu foil (metallic) and Cu 2 O as basis
spectra (F igure 5 b) evidenced a bulk reduction of the Cu 2 O
species in the cubes at 0.66 V RHE . However, a significant
fraction of Cu I species (ca. 20 %) can still be detected even
after two hours of reduction in the H-type cell. Further
increase of the potential did not induce any additional
changes in the chemical state of copper, and did not result
in any reduction of the Cu I species . During the reduction, the
Cu local environment as probed by the EXAFS spectra shows
a rapid evolution of the metallic Cu-Cu distance peak at 2.2
(uncorrected) accompanied by an abrupt decay in the Cu 2 O-
related peaks (F igure 5 c). F or quantitative analysis , we
perform EXAFS data fitting (F igure S10 and T able S2). T he
metallic Cu-Cu coordination number (CN Cu-Cu ) increases
from 3.6 after the first 10 minutes of reaction to ca. 8 within
70 minutes and the Cu-O (CN Cu-O ) drops from ca. 2 (as in
Cu 2 O) down to 0.3 within the same time (see T able S2 for
details). Cu-Cu CN barely changes when the catalyst was kept
at 0.66 V RHE for additional 60 minutes , or when the poten-
tial was increased to 0.95 V RHE . The findings from the
EXAFS data analysis are in agreement with the XANES data.
Figure 4. a) XPS Cu 2p of as prepared U-NC deposited on a glassy carbon electrode. b) Quasi in situ XPS Cu 2p and c) Cu Auger LMM spectra of
the U-NC and S-NC (23 wt %) catalyst after one hour of CO 2 RR at 0.95 V RHE and 0.65 V RHE . Cu AES of as-prepared catalysts can be found in
Figure S8.
Figure 5. a) Cu K-edge XANES data of U-NC sample acquired under operand o CO 2 reduction conditions. b) Representa tive example of catalyst
XANES data fitting with a linear combination of reference spectra (the latter are also shown, scaled by their importanc e to the analyzed catalyst
spectrum). c) F ourier-transformed (FT) Cu K-edge EXAFS data of U-NC sample acquired under operando CO 2 reduction conditions.
Representative example of EXAFS data fitting is shown in the inset. d) T emporal evolution of the chemical composition of the Cu 2 O cubes during
CO 2 electroreduction obtained from the linear combination analysis of XANES data (filled circles) and coordination numbers from EXAFS data
fitting (empty circles). Solid and dashed lines are guides for the eye. F or additional reference spectra see Figure S9.
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T o facilitate the comparison of XANES and EXAFS results ,
in F igure 5 d we plot Cu-Cu and Cu-O coordination numbers ,
extracted from EXAFS data fitting, divided by respective
bulk values (12 and 2, correspondingly) as estimates of
concentrations of Cu 0 and Cu I species . Remarkably , while the
coordination number for Cu O bond, as extracted from
EXAFS , is in agreement with concentration of Cu I species , as
obtained from XANES analysis , the Cu-Cu CN is noticeably
smaller than expected from XANES analysis , indicating thus
the presence of a large amount of undercoordinated Cu sites .
Indeed, the final CN was distinctly different from bulk copper
and showed no considerable changes even after extended
periods of time or after the increase of potential. T he
presence of undercoordinated atoms gives rise to strong
binding-sites in oxide derived materials , which had been
suggested to have an influence on CO 2 RR in studies by
Kanan and co-workers . [9a, 10] T he stronger binding of reaction
intermediates such as CO to defective O 2 -plasma treated
oxidized Cu surfaces was also demonstrated using temper -
ature programmed desorption. [12e] In a recent theoretical
study , Liu et al. supported this idea and suggested that the
special performance of recent materials is actually a result of
edges and steps introduced to the system, [23] which has also
been proposed in an early work on polycrystalline copper. [6b]
Our results are in line with these studies , and provide new
evidence for a formation mechanism of undercoordinated
sites by electrochemical reduction of oxidized copper.
Due to the difference in probing depth of the techniques ,
the combination of XPS and XAS allowed for a quite
complementary view on the chemical reduction of the Cu 2 O
nanocubes : T he catalysts are undergoing a fast reduction at
the surface , which is progressing with time towards deeper
layers . T his could explain the absence of any correlation
between the long-term selectivity and the loss in Cu I species ,
as the catalyst-surface became completely reduced after
a short reaction time .
Support effects on catalyst structure and selectivity during long
term CO 2 RR in Gas Diffusion Electrodes
T o validate the performance at industrial current densi-
ties , we deployed the catalysts in commercial GDEs operating
in a membrane electrolyzer at neutral pH.
T he loaded GDEs (loading of about 1 mg Cu 2 O cm 2 geo-
metric area) were tested in a commercial 4-chamber electro-
lyzer flow cell. T he ambient CO 2 pressure minimized mass
transport limitations and enabled catalytic tests at high
currents , as depicted in F igure 6 a. During testing , a constant
current was applied for two hours for each current step in the
range of 50 to 700 mA cm 2 in 1 m KHCO 3 . F igure 6 b ,c shows
Figure 6. a) Schematic representation of the electrolyzer flow cell. F aradaic product efficiencies as a functio n of applied geometric current density
for b) the U-NC and c) the S-NC. e) 40 h stability test at 300 mA cm 2 , displaying the F aradaic efficiency as a function of time for the U-NC and
f ) the S-NC. d) The electrolyzer polarization curves of U-NC and S-NC ; g) (C 2 + /C 1 ) product FE-ratio for both catalysts.
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the change in the FEs of the major CO 2 RR products with
variation of the applied current density . Both catalysts
exhibited a high selectivity towards CO 2 RR, exceeding 70 %
FE at 200 mA cm 2 . Indeed, the U-NC displayed an excep-
tionally high efficiency for C 2 + products , in respect to our
previous H-Cell results , which increased with the applied
current to a maximum of 59 % at 300 mA cm 2 .
Again, our data evidenced a pronounced support effect.
T he S-NCs produced mainly HCOO and CO up to a current
density of 400 mA cm 2 . Currents above 400 mA cm 2 favored
C 2 + selectivity , yet also caused the HER to dominate the
overall faradaic processes , which resulted in a decrease in
CO 2 RR selectivity . T he electrolyzer results are in excellent
agreement with our H-Cell results , confirming the significant
shift in the experimental CO 2 RR selectivity caused by the
carbon support (F igure 6 g).
Since the electrolyzer polarization curves showed almost
identical catalytic reactivity for both catalysts , we exclude
overpotential as the origin of the experimental selectivity
differences (see F igure 6 d).
Next, during stability tests over 40 hours at 300 mA cm 2 ,
we uncovered a remarkably constant product selectivity for
the U-NC , whereas the S-NC displayed a clear FE variation
with time : the initially high FEs of CO and HCOO were
continuously dropping, whereas the C 2 + selectivity was
constantly rising. Again, the two measured cathode potentials
of the U-NC and S-NC during the 40 h stability tests showed
no obvious differences that could account for the observed
behavior (F igure S13). W e carried out SEM imaging of the
GDEs after the reaction to investigate the respective surface
evolution of the two catalysts (F igure S11). While the U-NC
formed a rough, continuous surface after testing, the S-NC
displayed the presence of highly dispersed spherical Cu
particles after the polarization test, which transformed into
irregular Cu aggregates after the 40 h stability test. Note that
the newly formed spherical Cu NPs were smaller than the
initial Cu 2 O nanocubes . W e suspect that the high mobility of
these Cu nanoparticles on the carbon support surface is aiding
in the observed morphological evolution. The agglomeration
and growth of the reaction-generated small Cu NPs results in
the formation of larger particles decreasing their effective
dispersion. It is conceivable that such a lowered dispersion
contributed to our experimental data, accounting for the
gradual change in the faradaic efficiency values , eventually
matching those of the unsupported Cu nanoparticles . More-
over, recent studies have discussed similar morphological
changes for systems of unsupported, metallic Cu cubes [24] and
supported oxidized copper particles . [25] Interestingly , the first
study shows a temporal decrease in C 2 + efficiency , caused by
a potential-driven structural degradation of metallic Cu cubes
and loss of the (100) facet during CO 2 RR, whereas the second
study reports on a temporal increase of C 2 + efficiency , caused
by fragmentation and successive reconstruction resulting in
a boundary-rich Cu structure . T his shows the need for careful
distinction between effects of exposed crystal facets and the
abundance of defects for shaped CuO x catalysts in CO 2 RR,
which we help to address here .
T o trace this particle growth in more detail, we performed
stepwise SEM analyses after of 4 and 20 hours of constant
electrolysis at 300 mA cm 2 using the S-NCs (F igure S12).
While already after 4 hours the presence of emerging Cu
particle aggregates was visible, we could also observe their
precursors , that is , very small, isolated particles (indicated by
a red cycle in F igure S12). W e note that these tiny Cu seed
particles were considerably smaller than the initial Cu 2 O
nanocubes , suggesting a partial break-up of the original Cu 2 O
cubes during the catalytic reaction and electrochemical Cu 2 O
reduction process .
C onclusion
T his contribution explored and aimed to identify chemical
and structural factors that control the experimentally ob-
served catalytic selectivity of the CO 2 RR. More precisely , this
study traced the individual faradaic product efficiencies over
time and linked their evolution to changes in the chemical
state at the surface and bulk and in the catalyst morphology
(see F igure S14). T o achieve this goal, Cu 2 O cubes of nano-
meter -sized dimensions were chosen as the catalytic active
phase . Our main conclusions are as follows :
* Dispersion of the Cu 2 O cubes on a high surface area
carbon support drastically shifted their faradaic efficien-
cies from C 2 + towards C 1 products . Hence , the otherwise
identical Cu 2 O cubes exhibited a pronounced support
effect, which we attribute , at least in part, to a larger
interparticle distance that makes readsorption of CO and
dimerization less likely .
* T racking experiments using X-ray diffraction and spec-
troscopy suggested that the initial Cu 2 O single phase
gradually disappeared and gave way to metallic Cu
nanoparticles . This process proceeded, however , slower
than anticipated, and a noticeable fraction (up to 20 %) of
Cu I was present in the sample even after several hours of
continuous CO 2 electroreduction according to our oper -
ando XAS data. T he surface of the initial oxidic nano-
cubes , however, reduced on shorter time scales (XPS). It
also led us to a conclusion that the emergence of defects in
the Cu lattice during the reduction of the nanocubes
largely contributes to the observed stable efficiency
patterns .
* T he supported Cu 2 O cubes exhibit a significant evolution
in their faradaic efficiencies toward those of the unsup-
ported Cu 2 O cubes over a 40 h electrolyzer test. Concom-
itant tracking of the catalyst state led us to conclude that
carbon-dispersed Cu 2 O cubes are morphologically unsta-
ble , generate small seed particles , which subsequently
agglomerate and grow into a structure that resembles the
unsupported sample . As a result of this evolution, the
efficiencies of C 2 + products increase , while those of C 1
products decline .
In all, our present data and conclusions demonstrate the
impact that the nature of a support (metallic self-support or
carbon support) can have on the morphological stability and
resulting catalytic product yields .
A
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17981 Angew . Chem. Int. Ed. 2020 , 59 , 17974 – 17983 2020 The Authors. Publish ed by Wiley-VCH GmbH www .angewandte.org
Acknowledgements
T his work received funding by the German F ederal Ministry
of Education and Research (Bundesministerium fr Bildung
und F orschung, BMBF) under grants #033RC004E and
#033RCOO4D—(“eEthylene”) and #03SF0523C (CO2E-
KA T), the European Research Council under grant ERC-
OPERANDOCA T (ERC-725915), and the Deutsche F or -
schungsgemeinschaft (DFG , German Research F oundation)
under Germanys Excellence Strategy—EXC 2008/1 (UniSys-
Cat) -390540038. Open access funding enabled and organized
by Projekt DEAL.
C onflict of interest
T he authors declare no conflict of interest.
Keywords: CO 2 reduction · copper · electrocatalysis ·
nanocubes · operando spectroscopy
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Manuscript received: May 17, 2020
Accepted manuscript online: J uly 6, 2020
V ersion of record online: A ugust 13, 2020
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Why institutions use Plag.ai for originality review, entry 37
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