Stabilization of Cu+by tuning a CuO-CeO2interface for selective electrochemical CO2reduction to ethylene

Article abstract of DOI:10.1039/d0gc02279a

Electrochemical conversion of carbon dioxide (CO2) into multi-carbon fuels and chemical feedstocks is important but remains challenging. Here, we report the stabilization of Cu+ within a CuO-CeO2 interface for efficient and selective electrocatalytic CO2 reduction to ethylene under ambient conditions. Tuning the CuO/CeO2 interfacial interaction permits dramatic suppression of proton reduction and enhancement of CO2 reduction, with an ethylene faradaic efficiency (FE) as high as 50.0% at -1.1 V (vs. the reversible hydrogen electrode) in 0.1 M KHCO3, in stark contrast to 22.6% over pure CuO immobilized on carbon black (CB). The composite catalyst presents a 2.6-fold improvement in ethylene current compared to that of CuO/CB at similar overpotentials, which also exceeds many recently reported Cu-based materials. The FE of C2H4 remained at over 48.0% even after 9 h of continuous polarization. The Cu+ species are believed to be the adsorption as well as active sites for the activation of CO2 molecules, which remain almost unchanged after 1 h of electrolysis. Further density functional theory calculations demonstrate the preferred formation of Cu+ at the CuO-CeO2 interface. This work provides a simple avenue to convert CO2 into high-value hydrocarbons by rational stabilization of Cu+ species. This journal is

Full text of DOI:10.1039/d0gc02279a

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ARTICLE
Stabilization of Cu+ by tuning CuO-CeO2 interface for selective
electrochemical CO2 reduction to ethylene
Senlin Chu,a,# Xupeng Yan,b,# Changhyeok Choi,c,# Song Hong,a Alex W. Robertson,d Justus Masa,e
Received 00th January 20xx,
Accepted 00th January 20xx
Buxing Han,b Yousung Jungc,* and Zhenyu Suna,*
DOI: 10.1039/x0xx00000x
Abstract: Electrochemical conversion of carbon dioxide (CO2) to multi-carbon fuels and chemical feedstocks is important but
remains challenging. Here we report the stabilization of Cu+ within a CuO-CeO2 interface for efficient and selective
electrocatalytic CO2 reduction toward ethylene under ambient conditions. Tuning the CuO/CeO2 interfacial interaction
permits dramatic suppression of proton reduction and enhancement of CO2 reduction, with an ethylene faradaic efficiency
(FE) as high as 50.0% at −1.1 V (versus the reversible hydrogen electrode) in 0.1 M KHCO3, in stark contrast to 22.6% over
pure CuO immobilized on carbon black (CB). The composite catalyst presents a 2.6-fold improvement in ethylene current
compared to CuO/CB at similar overpotentials, which also exceeds many recently reported Cu-based materials. The FE for
C2H4 maintained over 48.0% even after 9 h of continuous polarization. The Cu+ species are believed to be the adsorption
as well as active sites for activation of CO2 molecules, which remains almost unchanged after 1 h of electrolysis. Further
density functional theory calculations demonstrate preferred formation of Cu+ at the CuO-CeO2 interface. This work provides
a simple avenue to converting CO2 into high-value hydrocarbons by rational stabilization of Cu+ species.
www.rsc.org/
and enjoy global demand in comparison to C1 products.19, 20 For
instance, ethylene is widely used as an industrial feedstock for
manufacturing plastics and diesel, and its selective production
Introduction
Electrochemical carbon dioxide (CO2) reduction (ECR) shows
promise in reducing greenhouse gas emissions and storing
intermittent renewable electricity, as well as attaining energy
security and sustainability.1, 2 Although this energy conversion
process can be conducted under mild temperature and
atmospheric pressure, there are still many challenges such as
low conversion efficiency and poor product selectivity that have
to be overcome.3, 4 To enable progress towards this goal, the
development of catalysts with high efficiency, sufficient
in lieu of methane is important.
Copper, with its unique electronic properties, has been
shown to stabilize a CO intermediate (*CO), and enable it to be
further reduced to multi-carbon products via CO dimerization
to yield an *OCCO adsorbate and subsequent hydrogenations.19
However, Cu is intrinsically limited by the scaling relations
between the binding energies of various reaction intermediates
on the metallic surfaces, which leads to wide product
distributions and undesirable hydrogen evolution, thus
hampering large-scale practical implementation.21 Selective
reduction of CO2 into industrially important C2+ species remains
an ongoing challenge. Recent investigations indicate that
preferential conversion of CO2 to C2+ products can be achieved
using Cu-based materials doped with foreign atoms,22 Cu
alloys,23, 24 or through control of exposed crystal lattice,25, 26
selectivity, and low cost is necessary.5, The synthesis of
6
valuable hydrocarbons and other chemicals through ECR has
drawn significant attention as a potential scheme for recycling
of CO2.7-18 In particular, C2+ (containing two or more carbon
atoms) compounds such as ethylene have high energy densities
28
29-31
oxidation state,27,
and surface morphology.25,
For
a
State Key Laboratory of Organic-Inorganic Composites, Beijing University of
Chemical Technology, Beijing 100029, China. E-mail: sunzy@mail.buct.edu.cn
example, single-crystal Cu(100) was demonstrated to display
good selectivity for ethylene evolution with a faradaic efficiency
(FE) of about 40.0%, which can be further improved to 50.0%
over Cu(711) at 5.0 mA cm−2 in 0.1 M KHCO3.25 A recent study
showed that CuAg bimetallic catalysts have enhanced
selectivity to C2+ products, which was attributed to suppression
of the hydrogen evolution reaction (HER) due to the formation
of compressively strained CuAg surface alloys.32 In addition,
Cu/oxide interfaces are regarded to be critical to inhibit the
parasitic HER during electrocatalytic CO2 reduction.33, 34 Oxides
of copper exhibit enhanced ECR activity and increased
selectivity towards multi-carbon products. The selectivity of
b
Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Colloid
and Interface and Thermodynamics, CAS Research/Education Center for Excellence in
Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing
100190, China
c
Department of Chemical and Biomolecular Engineering, Korea Advanced Institute
of Science and Technology (KAIST), Daejeon 34141, Republic of Korea. E-mail:
ysjn@kaist.ac.kr
d Department of Materials, University of Oxford, Oxford, OX1 3PH, United Kingdom
e
Analytische Chemie-Elektroanalytik & Sensorik, Ruhr University Bochum, D-44780
Bochum, Germany
f
Key Laboratory of Low-Carbon Conversion Science
& Engineering, Shanghai
Advanced Research Institute, Chinese Academy of Sciences, China
# These authors contributed equally to this work.
Electronic Supplementary Information (ESI) available: Experimental details, XPS
spectra, STEM image and EDS elemental maps, and FE versus electrolysis
temperature. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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these catalysts is dependent on copper oxidation state.31 Some with individual CuO/CB and CeO2/CB are shown inVFieigw.A1rtaic.leAOpnalinret
DOI: 10.1039/D0GC02279A
computational studies have suggested that the coexistence of a from a broad peak at 22.2° originating from carbon black with
Cu+/Cu0 mixture synergistically promotes CO2 reduction to C2+ low crystallinity, diffraction peaks at about 28.1, 47.6, and 56.0°
products due to improved CO2 activation and CO were observed in both CeO2/CB and CuO-CeO2/CB,
36
dimerization.35,
Experimentally, however, evidence of the corresponding to the (111), (220), and (311) planes of CeO2
stability of the active Cu+ species during CO2 reduction remains (PDF# 34-0394). These indicate the formation of fluorite (Fm—
unclarified thus far.
3
m) CeO2 with a face-centered cubic (fcc) structure in the
Herein, we report on the stabilization of Cu+ by controlling composites. Unlike the bare CuO/CB that displayed
the interplay between lattice-mismatched CuO and CeO2. This representative monoclinic CuO peaks (PDF# 44-0706), no
scheme allows one to design an efficient and selective catalyst diffraction peaks of any Cu compounds were discernible in the
for electrocatalytic CO2 reduction to produce ethylene, among XRD pattern of CuO-CeO2/CB, likely due to the low loading
other products (methane, carbon monoxide, formic acid, and/or small size of CuO in the composite.
ethanol). Catalytic selectivity can be greatly improved by taking
X-ray photoelectron spectroscopy (XPS) was employed to
advantage of CuO-CeO2 interactions in different composition acquire information about the surface composition and
regimes and interfacial structures. A remarkable FE for ethylene chemical state of the Cu species, as well as possible interactions
production of up to 50.0% was obtained at mild overpotentials, between copper and cerium oxides. Fig. 1b shows the Cu 2p
outperforming many previously reported Cu-based signal of CuO/CB and CuO-CeO2/CB. The Cu 2p core-level
electrocatalysts. Furthermore, density functional theory (DFT) spectrum of CuO/CB reveals pronounced CuO features, that is,
calculations revealed that CeO2 changes the oxidation state of Cu 2p1/2 and 2p3/2 peaks with binding energies (BEs) at 954.0
Cu atoms toward Cu+ at the CuO-CeO2 interface.
and 934.0 eV, respectively. Strong Cu2+ satellites at 962.6, 944.0,
and 941.6 eV were also clearly observed.37 Nevertheless, no
apparent peaks assigned to Cu+ can be identified. In contrast,
two peaks located at 954.3 and 951.9 eV were observed in CuO-
Results and discussion
CeO2/CB, which can be attributed to Cu2+ 2p1/2 and Cu+ 2p3/2
,
(a)
(c)
(e)
(b)
Cu 2p
. C
 CeO2
CuO
CuO/CB
Cu(‖) satellite
respectively.10 This unambiguously verifies the formation and
stabilization of Cu+, likely owing to electron transfer from Ce3+
to Cu2+. The relative Cu+ percentage was determined to be
23.4% in CuO-CeO2/CB, based on the peak area ratio of all
copper oxidation states in the Cu 2p regions. Fig. 1c depicts the
Ce 3d signals of CuO-CeO2/CB having a satellite structure due to
hybridization of Ce 3d orbitals with O 2p orbitals and partial
occupation of the 4f levels.38 The 3d5/2 and 3d3/2 spin-orbit
components (spin-orbit splitting, ~18.5 eV) are denoted as ν and
ν′, respectively, which are in line with the previous literature on
Ce(IV).39 The peaks of ν0 and ν1 were attributed to a mixing
configuration of the 3d9 4f2 (O 2p4 ) and 3d9 4f1 (O 2p5 ) Ce4+
states and ν2 to the 3d9 4f0 (O 2p6 ) Ce4+ state.38 The same
assignment could be applied to the ν′ structures, which
correspond to the Ce 3d3/2 level. This illustrates the major
valence of Ce(IV) in the sample, consistent with the XRD result.
Four peaks u0 (BE ≈ 885.8 eV), u1 (BE ≈ 880.6 eV), u0 (BE ≈ 904.1
eV), and u1 (BE ≈ 899.5 eV) associated with Ce3+ were identified,
indicating the presence of Ce2O3 in the sample.11 The well-
defined peak ν2′ typical of Ce4+ can be used to estimate the
fraction of Ce4+.40 Given that the area of the ν2′ component
comprises 14.0% of the overall area of the Ce 3d region, the Ce3+
percentage was estimated to be 27.0%. The deconvoluted O 1s
XPS spectrum of CuO-CeO2/CB (Fig. S1a) displays a predominant
peak at 529.7 eV arising from lattice oxygens in the metal
oxides, and two less intense peaks at 531.2 and 532.6 eV that
can be assigned to defective sites (surface oxygen vacancies)
and physisorbed water, respectively.
(111)
.
.
.

(220)
(311)


CeO2/CB



CuO-CeO2/CB
CuO/CB
2p3/2
CuO-CeO2/CB
(002)
(111)
Cu(I) satellite
Cu(‖)
2p1/2
Cu(ǀ)
Cu(ǀ)
Cu(‖)
CuO (PDF#44-0706)
CeO2 (PDF#34-0394)
960
950
940
930
15
30
45
60
75
Binding energy/eV
2/degree
(d)
 CeO2
CuO  Cu2O
Ce 3d
2'
0'
0
2
CuO/CB
1'
u0'
u1'
409.5 cm-1
u0
661.1 cm-1
1

u1




CuO-CeO2/CB
200
400
600
800
920
910
900
890
880
Raman shift/cm-1
Binding energy/eV
(f)
5
4
3
2
1
0
CuO-CeO2/CB
B
C
/
2
O
e
C
-
O
u
C
CeO2/CB
CuO/CB
B
C
/
O
u
C
0.0
0.2
0.4
0.6
0.8
1.0
100 200 300 400 500 600 700
Relative pressure/(P/P0)
Temperature/C
Fig. 1 (a) XRD patterns of CeO2/CB, CuO/CB, and CuO-CeO2/CB.
(b) Cu 2p XPS spectra of CuO/CB and CuO-CeO2/CB. (c) Ce 3d
XPS spectrum of CuO-CeO2/CB. (d) Raman spectra of CuO/CB
and CuO-CeO2/CB. (e) H2-TPR profiles of CeO2/CB, CuO/CB, and
CuO-CeO2/CB. (f) CO2 adsorption isotherms of CuO/CB and CuO-
CeO2/CB.
The presence of Cu+ in CuO/CeO2 heterostructure was also
evidenced by Raman scattering experiments. The bands
centered at about 258.0, 462.9, and 595.0 cm−1 were identified
in Fig. 1d, which can be well assigned to the F2g mode, second-
order transverse acoustic (2TA) mode, and defect-induced (D)
The X-ray diffraction (XRD) patterns of CuO-CeO2/CB together
2 | J. Name., 2012, 00, 1-3
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mode of fluorite CeO2, respectively.41 It is noteworthy that the
three peaks at 290.4, 337.0, and 622.8 cm−1 that appeared for aberration corrected high-angle annular dark-field scanning
CuO/CB, are attributed to the respective single Ag mode and transmission electron microscopy (HAADF-STEM) was
two Bg optical modes of cupric oxide.42 However, for CuO- performed on CuO-CeO2/CB. Fig. 2a and b showed the
CeO2/CB, the feature around 290.4 cm−1 disappeared and a new formation of many reticular NPs homogeneously distributed on
band at 230.4 cm−1 was observed being tentatively assigned to carbon black. The (111) and (100) planes of CeO2 were indexed
ARTICLE
To decipher the morphological features of our catalyst,
View Article Online
DOI: 10.1039/D0GC02279A
one-magnon
scattering,
which
arose
from
the with the aid of fast Fourier transformation (FFT) (inset of Fig.
antiferromagnetic ordering of the Cu2+ ions.42 Additional two 2b). Furthermore, the energy-dispersive X-ray spectroscopy
distinct peaks at 409.5 and 661.1 cm−1 are observable, typical of (EDS) maps (Fig. 2c-g and Fig. S2b-f) along with the EDS
Cu+ Raman fingerprints.42 This further confirms that CuO was spectrum (Fig. 2h) confirmed that the NPs were composed of
partially converted to Cu2O, possibly induced by adjacent CeO2 CuO and CeO2 crystallites. EDS elemental mapping revealed
nanoparticles (NPs). These results are consistent with the XPS almost full overlap of Cu-rich and Ce-rich domains, indicating
data in Fig. 1b.
large interfaces between the two metal oxides. The crystallite
Temperature-programmed reduction by hydrogen (H2-TPR, sizes of CuO and of CeO2 were found to be similar with sizes of
Fig. 1e) manifested two marked H2 consumption peaks at 110.0 less than 5 nm by high-resolution TEM (Fig. 2i and j). By FFT, fcc
and 178.0 °C for CuO-CeO2/CB, being ascribed to the reduction CuO and CeO2 NPs were discerned (Fig. 2k and l). Most of the
of the subsurface Cuδ+ to Cu+ and further to Cu0 respectively, by CuO crystals exhibited faceted cuboidal morphology and were
consuming reducible oxygen from the CuOx species. Notably, surrounded by CeO2 NPs (Fig. 2i and j).
the TPR reduction peaks shifted to lower temperatures relative
(a)0
(b)
60
H2
CO
CH4
C2H4
to CuO/CB, which is likely a result of hydrogen spillover to CuO
at the CuO/CeO2 interface. Furthermore, the CuO-CeO2/CB
exhibited a CO2 uptake capacity of 4.7 cm3 g−1 (Fig. 1f), 2.3 fold
as large as that of CuO/CB. This could lead to enriched CO2 on
the local surface of the working electrode, thus boosting *CO
coverage and dimerization. The significant enhancement in CO2
capture ability is due to the introduction of CeO2, which can
effectively adsorb CO2, forming carbonates and hydrogen
carbonates.43 It can be envisioned that the as-made hybrid
catalyst may facilitate multiple interesting functionalities such
as adsorption, electronic, activation, and catalysis, among
others, based on the synergistic interaction between CuO and
CeO2.
HCOOH
C2H5OH
Ar
-2
-4
-6
-8
45
30
15
0
-0.1
-0.2
-0.3
Ar
CO2
CO2
-0.8
-0.6
-0.4
-0.2
0.0
Potential/V (vs. RHE)
-1.3 Potential/V (vs.-R1.0HE) -0.9
-1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0
-1.2
-1.1
Potential/V (vs. RHE)
12
(c)
(d)
H2
CO
CH4
C2H4
CuO-CeO2/CB
CeO2/CB
CuO/CB
60
45
30
15
0
HCOOH
C2H5OH
CuO-CeO2/CBmix
9
6
3
0
-1.3
-1.2
-1.1
-1.0
-0.9
-1.2
-1.1
-1.3 Potential/V (vs.-R1.0HE) -0.9
Potential/V (vs. RHE)
(e)
(f)
CuO-CeO2/CB
50
45
40
35
30
25
CuO/CB
CuO-CeO2/CBmix
This work
CuOzCly nanocube
15
10
5
Ag/Cu
Cu2(OH)3Cl sol-gel
Cu2O film(1.7m)
CuO/NxC
Ag1-Cu1.1 NDs
Cu NDs (Br)
Nanoporous Cu
Oxide-derived Cu
Cu2O/ILGS
CuPc
-1.0
0
-1.3
-1.2
-1.1
-1.0
-0.9
-1.3
-1.2
-1.1
-0.9
-0.8
Potential/V (vs. RHE)
Potential/V (vs. RHE)
Fig. 3 (a) LSV results of CuO-CeO2/CB on a glassy carbon
electrode in Ar- (dashed black line) or CO2 (solid red line)-
saturated 0.1 M KHCO3 solutions at a scan rate of 5 mV s-1. (b)
Faradaic efficiencies and (c) partial current densities for ECR
products over CuO-CeO2/CB at various applied potentials. (d)
C2H4 FEs of CuO-CeO2/CB, CuO/CB, CeO2/CB, and CuO-
CeO2/CBmix in the potential range from −0.9 to −1.3 V. (e)
Production rates of C2H4 at different potentials over CuO-
CeO2/CB, CuO/CB, and CuO-CeO2/CBmix. (f) C2H4 FEs of CuO-
CeO2/CB and other reported Cu-based electrocatalysts.
Fig. 2 (a-b) HAADF-STEM images of CuO-CeO2/CB. The inset in
image (b) shows the corresponding FFT of CuO-CeO2/CB. EDS
elemental maps of (c) C, (d) O, (e) Ce, (f) Cu, and (g) overlay of C
(blue), Ce (red) and Cu (green) over the region shown in image
(b), along with corresponding EDS spectrum (h). (i) HRTEM
image of CuO-CeO2/CB. (j-l) Magnified image of the blue
rectangle in image (i) and FFTs of the regions encased by the
yellow and green rectangles in image (j).
This journal is © The Royal Society of Chemistry 20xx
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ECR is very sensitive to operating conditions, such as the −1.1 V over fresh and treated CuO-CeO2/CB under Ar, H2O, 8.0%
View Article Online
DOI: 10.1039/D0GC02279A
nature and properties of the electrocatalyst, electrolyte H2/Ar, and air. (d) Tafel plots of the partial geometric current
composition, and electrochemical cell type. To evaluate the density for C2H4 production over CuO-CeO2/CB, CuO/CB, and
intrinsic catalytic properties of the as-prepared hybrids, we CuO-CeO2/CBmix. (e) Geometric current-, C2H4 FE-, and H2 FE-
conducted the ECR in CO2-saturated 0.1 M KHCO3 aqueous time responses of CuO-CeO2/CB at −1.1 V. (f) Cu 2p XPS spectra
electrolyte (pH 6.8) using a reported design of liquid H-type cell of CuO-CeO2/CB before and after 1 h of electrolysis.
with continuous CO2 bubbling.44 The potential-dependent
geometric current densities of CuO-CeO2/CB within the
The C1 products were obtained at similar yields for CuO/CB
potential range of 0.0 to −1.4 V (vs. RHE) were recorded by
and CuO-CeO2/CB, but C2+ product selectivity, C2H4 FE and
linear sweep voltammetry (LSV), as shown in Fig. 3a.
production rate (Fig. 3e) were remarkably boosted on the latter
Significantly higher cathodic currents were observed in a CO2
sample. C2H4 emerged at an onset potential of −0.7 V (vs. RHE)
environment than in an Ar environment within the entire
over CuO-CeO2/CB and rose to a maximum with FE up to 50.0%
potential region. CO, H2, CH4, HCOOH, C2H4, and C2H5OH were
at −1.1 V (vs. RHE), in contrast to that of 22.6% and 1.2% for
detected at applied potentials ranging from −0.9 to −1.3 V (vs.
CuO/CB and CeO2/CB, respectively (Fig. 3b and d). The C2H4
RHE) in a CO2-saturated 0.1 M KHCO3 solution. The ECR
selectivity even outperforms many recently reported Cu-based
preferably occurred over HER in the potential range from −0.9
electrocatalysts under similar overpotentials (Fig. 3f), such as
to −1.2 V (vs. RHE), while HER dominated at more negative
state-of-the-art Cu nanocubes with exposed (100) facets
potentials (Fig. 3b and c). As demonstrated in Fig. 3d, CeO2/CB
(maximum C2H4 FE 32.0%),45, 46 Ag-Cu nanodimers (maximum
generated exclusively H2 with a very small amount of ECR
C2H4 FE 40.0%).24
products (FE < 3.0%). Commercial Cu2O decorated on CB
The ECR activity was tunable by adjusting the amounts of
exhibited an FE of 31.7% for ECR (vs. RHE) (Fig. S3), whereas the
CeO2 and CuO (Tables S1 and S2). As seen in Fig. 4a,
FE for C2H4 formation was as low as 8.5%. Although CuO/CB can
incorporation of CeO2 at various contents was found to thwart
reduce CO2 at overpotentials larger than 0.97 V, the highest FE
hydrogen evolution and facilitate C2H4 generation. The optimal
for ECR was below 32.0%, with the selectivity for C2+ products
loading of CeO2 was 30.0 wt%. Continuous increase in CeO2
being less than 23.0%. Noteworthy is that the CuO-CeO2/CB
loading led to a slight decrease of ECR activity, probably owing
nanocomposites substantially promoted the activity toward
to reduction in electrical conductivity. Likewise, the ECR activity
CO2 reduction with an FE > 63.0%.
to yield C2H4 increased with the mass percentage of CuO in the
(a)
(b)
C2H5OH
CO
HCOOH
CH4
C2H5OH
CO
HCOOH
CH4
16
12
8
16
12
8
range from 0.75 to 1.5 wt%, above which the C2H4 FE tended to
diminish upon further increase of the CuO loading (Fig. 4b). This
may be due to a combination of less extended interface and
formation of larger CuO particles, resulting in weakened binding
of the reactants and intermediates. Furthermore, it was found
that there is an optimum in the particle size of CuO which
maximized C2H4 generation (Table S3), in line with the results
observed for Cu in the literature.24 The effect of electrolytic
temperature on ECR was also explored. The FE for ECR was
found to be maximized at 3  3 °C (Fig. S4), indicating that HER
tends to be inhibited at low reaction temperatures.
To check if Ce3+ impacted the ECR, the synthesis of catalysts
was performed in an air-free glove-box under otherwise similar
conditions. The resulting CuO-Ce2O3/CB provided a much lower
C2H4 FE (22.4% at −1.1 V vs. RHE) compared to CuO-CeO2/CB.
This indicates that Ce3+ is unlikely to contribute to the enhanced
ECR.
75
C2H4
C2H4
60
45
30
15
0
60
45
30
15
0
4
4
0
0
B
B
B
B
C
/
)
B
C
C
B
C
/
0.75
1.5
3
6
12
B
C
/
C
/
)
/
/
)
)
)
C
/
%
0
%
%
%
0
5
%
O
2
O
e
C
0
0
0
u
1
2
4
3
(
CuO loading/wt%
(
(
(
(
O2
C
2
O2
O2
O2
e
C
u
O
e
e
e
e
C
C
C
C
-
-
-
-
-
O
O
O
O
O
(c)Cu
u
u
u
C
C
C
C
(d)
C2H5OH
CO
HCOOH
CH4
75
CuO-CeO2/CB
-1.2
-1.1
-1.0
-0.9
-0.8
C2H4
CuO/CB
CuO-CeO2/CBmix
60
45
30
15
0
1
-
c
e
d
V
1
-
m
7
.
c
e
0
d
6
1
V
m
9
.
8
4
1
1
-
c
e
d
V
m
5
.
9
6
1
0
0
0
0
2
_
-1.0
-0.5
0.0
0.5
1.0
h
0
0
0
s
0
2
_
e
2
_
r
2
r
A
Log (|JC H |/mA cm-2)
_
)
r
r
i
A
F
g
(
A
/
O
2 4
2
H
2
H
%
8
The role of Cu+ during ECR was investigated by treating CuO-
CeO2/CB at 200 °C under different atmospheres. The relative
fractions of Cu0, Cu+, and Cu2+ in the treated CuO-CeO2/CB
samples were probed by XPS (Table S4 and Fig. S5). As observed
in Fig. 4c, the CO2 reduction activity and selectivity dropped
slightly in an Ar environment that may favor the transformation
of a small fraction of Cu2+ to Cu+ and Cu+ to Cu0, (Table S4 and
Fig. S5a). The C2H4 FE mildly decreased with a simultaneous
increase of CH4 FE, probably resulting from aggregation of metal
0
54
48
42
36
30
24
(e)
(f)
FE(C2H4)
Fresh
Cu 2p
2p3/2
Cu(‖) satellite
2p1/2
-5
-10
-15
-20
After 1 h
Cu(I) satellite
Cu(ǀ)
Cu(ǀ)
FE(H2)
Cu(‖)
Cu(‖)
0
1
2
3
4
5
6
7
8
9
960 Binding energy/eV
950
940
930
Time/h
Fig. 4 FE and geometric current density at −1.1 V as a function oxide NPs and presence of Cu0. This phenomenon became a
of (a) CeO2 loading at a fixed CuO content of 6.0 wt% and (b) little more pronounced after being subjected to water vapor,
CuO loading at a constant CeO2 loading of 30.0 wt%. (c) FEs at which may be due to promotion of CH4 formation by the
4 | J. Name., 2012, 00, 1-3
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adsorbed surface water molecules, in addition to the
ARTICLE
The long-term performances of the catalysts were examined
View Article Online
transformation of Cu+ (Table S4 and Fig. S5b). However, by chronoamperometric measurementsD. OTIh: 1e0.r1e03s9u/lDts0G(FCi0g2.2479eA)
annealing of the catalyst in 8% H2/Ar led to increased HER with showed that the FE for C2H4 remained steady exceeding 48.0%
a distinct drop in ECR performance. Despite the CO FE being even after 9 h of continuous polarization at −1.1 V (vs. RHE). XPS
improved to 33.7%, the C2H4 FE fell down to 33.8%. This analysis (Figs. 4f and S1b) indicated that the surface
suggests that the reduction of Cu+/Cu0 ratio (Table S4 and Fig. concentration of Cu+ was preserved after 1 h of polarization
S5c) is detrimental to CO-CO coupling. Also, the decrease of at−1.1 V (vs. RHE), reflecting its good stability owing to the
Cu+/Cu2+ (Table S4 and Fig. S5d) by exposure of the sample to strong interplay between ceria and copper oxide.
air at elevated temperature degraded C2H4 production,
accompanied with substantially more H2 evolution.
To further investigate the role of CeO2 in stabilizing Cu+, we
performed density functional theory (DFT) calculations (Fig. 5).
To probe the role of the CuO-CeO2 interface, we made efforts We modeled the interface between CuO and CeO2 (denoted as
to tailor the interfacial structure by fine-tuning of synthetic CuO-CeO2) by constructing a small CeO2 cluster (Ce3O6) on the
parameters such as the feeding sequence of metal precursors. CuO(100) surface.47, 48 The (100) facet, which has been known
When the precursor Cu(Ac)2 was first added followed by as the active site for C2 production in electrochemical CO2
addition of Ce(NO3)3 to prepare the catalyst, only 22.0% of C2H4 reduction, is considered.49 We considered the Cu-terminated
FE was attained (Table S5). Alternatively, a cascade addition of surface of CuO since the surface O species would be reduced at
cerium precursor and copper precursor in sequence also gave the experimental electrode potential range (–0.9 ~ –1.3 V vs.
rise to a lower C2H4 FE of about 37.8%. In both cases, the RHE). Previous studies have shown that subsurface oxygen in
accessible CuO-CeO2 interfaces with exposed copper domains copper oxides plays an important role in facilitating the CO2
were markedly reduced, which accounted for the declined ECR reduction,35, 36 and hence, we focus on subsurface O rather than
performance. A physical mixture of CuO/CB and CeO2/CB (CuO- surface O.
CeO2/CBmix) with equivalent metal oxide loadings was also
We focus on the change/trend of the Bader charges50 of the
evaluated for ECR. It showed even worse CO2 reduction activity surface Cu atoms with and without CeO2 cluster since the Bader
than CuO/CB (Fig. 3d and e), most likely due to poor mass charge agrees with the oxidation state qualitatively (albeit not
transport. Taken together, we conclude that intelligent design quantitatively). Assuming that the Bader charges of surface Cu
of CuO-CeO2 interfaces to yield and stabilize Cu+ is essential to atoms in Cu(100), Cu2O(100), and CuO(100) correspond to the
facilitate CO2-to-C2H4 conversion.
oxidation states of 0, +1, and +2, respectively, we obtain a linear
The interfacial reaction kinetics was explored by Tafel relationship between the Bader charge vs. oxidation state (Fig.
analysis. A Tafel slope of 148.9 mV dec−1 was observed for CuO- 5d), and from the latter correlation we obtain the oxidation
CeO2/CB, much lower than 160.7 mV dec−1 for CuO/CB and state of Cu atoms in CuO-CeO2.
169.5 mV dec−1 for CuO-CeO2/CBmix. (Fig. 4d). This indicates that
This Bader charge analysis shows that the oxidation states of
the CuO-CeO2/CB has a comparatively faster kinetics for CO2 several Cu atoms at the CuO-CeO2 interface lie between that of
reduction. The formation of the *CO intermediate for tandem Cu2O and CuO (Fig. 5d), indicating that the interfacial CeO2
catalysis on the surface of the catalysts determines the reaction cluster changes the oxidation state of neighboring Cu atoms in
rate.
CuO toward that of Cu2O. More specifically, the oxidation states
of two Cu atoms at the CuO-CeO2 interface are highly similar to
those of surface Cu atoms in Cu2O (i.e. Cu+). This result agrees
with the presence of Cu+ in the XPS characterization of CuO-
CeO2/CB (Fig. 1b), and indicates that the CeO2 plays an
important role in stabilizing Cu+.
Conclusions
In summary, we present CuO-CeO2/CB as a highly promising
electrocatalyst for enhancing selective reduction of CO2 to
ethylene. By utilizing the strong synergistic interaction between
CuO and CeO2, stabilization of the Cu+ species at the metal oxide
interface is realized, while H2 production is simultaneously
considerably suppressed, resulting in boosted ethylene
production with a high FE of up to 50.0%. The existence of Cu+
species was confirmed by XPS and Raman spectroscopy, as well
as TPR, which are believed to be the adsorption as well as active
sites for activation of CO2 molecules. This work provides a
simple way to enhance the conversion of CO2 into ethylene, and
it is hoped that the findings will inspire the rational design of
active copper domains for efficient electroreduction of CO2.
Fig. 5 Top-view of the optimized geometries of (a) Cu2O, (b)
CuO, and (c) CuO-CeO2. (d) Oxidation states of surface Cu atoms
obtained by Bader charge analysis. Only the surface Cu atoms
adjacent to CeO2 (denoted as grey balls in (c)) are considered
for the Bader charge analysis of CuO-CeO2. In (d), black, blue,
cyan, and red color represent Cu, Cu2O, CuO, and CuO-CeO2,
respectively.
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ARTICLE
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Conflicts of interest
The authors declare no conflicts of interest.
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Acknowledgements
This work was supported by National Natural Science
Foundation of China (No. 21972010); the State Key Laboratory
of Organic-Inorganic Composites (No. oic-201901001); Beijing
Natural Science Foundation (No. 2192039); Beijing University of
Chemical Technology (XK180301, XK1804-2); the Foundation of
Key Laboratory of Low-Carbon Conversion Science
&
Engineering, Shanghai Advanced Research Institute, Chinese
Academy of Sciences (No. KLLCCSE-201901, SARI, CAS); the
DCCEM at Department of Materials, Oxford, and the Henry
Royce Institute (EP/R010145/1). Y.J. acknowledges the National
Research Foundation of Korea (NRF-2019M3D1A1079303).
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