Cloride-derived copper electrode for efficient electrochemical reduction of CO2 to ethylene

Article abstract of DOI:10.1016/j.cclet.2018.07.003

The electrochemical reduction of carbon dioxide can convert the greenhouse gas into value-added chemical products or fuels, which provides a promising strategy to address current energy and environmental issues. Increasing the selectivity for C2&C2+ products, particularly ethylene, remains an important goal in this field. We chose cuprous chloride as the catalyst precursor for electrochemical reduction of CO₂, which efficiently converted carbon dioxide to ethylene. CuCl powder exhibited a maximum ethylene faradaic efficiency (FE) of 37%, ethylene partial current density of 14.8 mA/cm², and selectivity of 57.5% for C2&C2+ products at ?1.06 V (vs. reversible hydrogen electrode, RHE). Electron microcopy (TEM, SEM) and time-resolved ex situ X-ray diffraction (XRD) demonstrated that the catalyst was transformed gradually into a mixed phase of copper and cuprous oxide, with the morphological change into a cubic structure during reduction process. The presence of Cu¹⁺ and the unique electrode morphology may simultaneously lead to the enhanced electrochemical activity.

Full text of DOI:10.1016/j.cclet.2018.07.003

Accepted Manuscript
Title: Cloride-derived copper electrode for efficient
electrochemical reduction of CO2 to ethylene
Authors: Tian Qin, Yao Qian, Fan Zhang, Bolin Lin
PII:
DOI:
Reference:
S1001-8417(18)30279-1
https://doi.org/10.1016/j.cclet.2018.07.003
CCLET 4605
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Chinese Chemical Letters
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6-4-2018
29-6-2018
3-7-2018
Please cite this article as: Qin T, Qian Y, Zhang F, Lin B, Cloride-derived copper
electrode for efficient electrochemical reduction of CO2 to ethylene, Chinese Chemical
Letters (2018), https://doi.org/10.1016/j.cclet.2018.07.003
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Communication
Cloride-derived copper electrode for efficient electrochemical reduction of CO2 to
ethylene
Tian Qina,b,c, Yao Qianb, Fan Zhangb, Bolin Linb, 
aShanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China.
bSchool of Physical Science and Technology (SPST), ShanghaiTech University, Shanghai 201210, China.
cUniversity of Chinese Academy of Sciences, Beijing 100049, China.
ARTICLE INFO
ABSTRACT
Article history:
The electrochemical reduction of carbon dioxide can convert the greenhouse gas into
value-added chemical products or fuels, which provides a promising strategy to address
current energy and environmental issues. Increasing the selectivity for C2&C2+
products, particularly ethylene, remains an important goal in this field. We chose
cuprous chloride as the catalyst precursor for electrochemical reduction of CO2, which
efficiently converted carbon dioxide to ethylene. CuCl powder exhibited a maximum
ethylene faradaic efficiency (FE) of 37%, ethylene partial current density of 14.8
mA/cm2, and selectivity of 57.5% for C2&C2+ products at −1.06 V (vs. reversible
hydrogen electrode, RHE). Electron microcopy (TEM, SEM) and time-resolved ex situ
X-ray diffraction (XRD) demonstrated that the catalyst was transformed gradually into
a mixed phase of copper and cuprous oxide, with the morphological change into a cubic
structure during reduction process. The presence of Cu1+ and the unique electrode
morphology may simultaneously lead to the enhanced electrochemical activity.
Received 4 Apirl 2018
Received in revised form 29 June 2018
Accepted 2 July 2018
Available online
Keywords: Cuprous chloride
Ethylene
Electrochemistry
Carbon dioxide reduction
Morphology
Large-scale combustion of non-renewable fossil fuels leads to the rapid growth of the concentration of
atmospheric CO2, which gives rise to a series of problems [1-4]. Electrochemical reduction of CO2 using renewable
energy to generate high value-added chemicals or fuel becomes an attractive strategy for utilization of CO2
resources [5]. A lot of efforts have been made to screen electrocatalysts that can reduce carbon dioxide efficiently,
and inhibit the competitive hydrogen evolution reaction in the aqueous solution, such as oxidation treatment of
metal electrodes including Au [6], Ag [7, 8], Sn [9], Pb [10], Co [11] and nanostructured optimization including
nanoparticle [12], nanowire [13], nanoneedle [14] and nanoporous structure [15]. However, these efforts are
limited to the conversion of carbon dioxide to generate carbon monoxide or formic acid through a two-electron
pathway. The synthesis of electrocatalysts for production of higher value-added C2&C2+ products, especially
ethylene as an important monomer in polymer synthesis, is still a great scientific challenge [16].
Copper-based electrodes have been attracting extensive research efforts as copper has great potential to
catalyze electrochemical CO2 reduction reaction (CRR) towards ethylene. CRRs on polycrystalline copper electrode
mainly lead to mixtures of methane, ethylene, and carbon monoxide [17]. The partial current density for ethylene
is only ca. 1.5 mA/cm2 with a low selectivity. Although copper single crystal electrode [18], copper electrode with
rough surface [19] and copper nanocubic crystal [20] show improved faradaic efficiency for ethylene, the partial
current density for ethylene is relatively low.
Recently, copper electrode covered with oxides exhibited excellent performance for ethylene formation. It has been proposed that
copper oxide precursor was reduced to metallic copper with a unique microstructure under a negative potential environment during CO2
reduction reaction [21]. However, in-situ characterizations pointed to the existence of oxidized copper species even after the electrolysis
process [22, 23]. A large number of Cu1+ sites might facilitate the stable adsorption of intermediate products and thus promote the
selective generation of ethylene and other multi-carbon compounds [24].
Electrochemical treatment of copper in a KCl solution can also significantly improve the faradaic efficiency of C2H4 [25, 26]. The
chloride ion presumably induced the formation of cubic particles on the copper surface with a large amount of exposed (1 0 0) facets,
often considered as the active facet for ethylene production [27]. In addition, the chloride ion might stabilize the oxidized state and delay
 Corresponding author.
E-mail address: linbl@shanghaitech.edu.cn

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the reduction of cuprous oxide to metallic copper, which could promote the formation of reduction products with longer carbon chain
[24].
Herein, we report the direct use of cuprous chloride as the catalyst precursor for electrochemical reduction of
carbon dioxide. During the CRR, the precursor gradually turned into biphasic Cu2O-Cu cubes (CB-Cu), showing a
relatively high faradaic efficiency and partial current density for the formation of ethylene, ethanol and other
C2&C2+ products.
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Fig.1. Characterization of CuCl electrode. SEM images of (a) CuCl electrode before the electrolysis;
(b,c) CuCl electrode at −1.06 V after 3680 s’ electrolysis; TEM images of selected cuboid particle for
CuCl after 3680 s’ electrolysis (d) low magnification; (e) high magnification; (f) SAED pattern; STEM-
EDS images of selected cuboid particle for CuCl after 3680 s’ electrolysis showing elemental
distribution of (h) copper; (i) oxygen; and line-scan mode (j) (along line in (g)); X-ray diffraction(k) and
XPS data(l) for CuCl electrode before and after 3680 s’ electrolysis. The hashes in (k) corresponds to
carbon substrate. F 1s peak in (l) results from carbon substrate with PTFE treated.
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The electrode was prepared by printing a catalyst precursor ink uniformly onto a PTFE treated carbon paper
(Supporting information). Large particles with the size of 1.5-2.5 μm were formed initially (Fig. 1a). XRD indicates
that CuCl2•2H2O (eriochalcite) was formed with other unknown species during the electrode-preparation processes
(Fig. 1k). After CRR for ca. 1 h, the large particles were transformed into smaller ones with the size of 150-350 nm
(Fig. 1b). Flower-like aggregates of these particles were observed. High-magnification SEM and TEM images (Figs.
1c and d) show the formation of cuboid particles. XRD indicates the presence of both copper and cuprous oxide
(Fig. 1k). The high-resolution TEM image in Fig.1e shows lattice fringe spacing of 0.21 nm and 0.31 nm, indexed to
Cu2O (2 0 0) and (1 1 0) facet direction, respectively, as well as lattice fringe spacing of 0.184 nm and 0.203 nm
corresponding to Cu (2 0 0) and Cu (1 1 1), respectively. SAED further confirms the existence of both copper and
cuprous oxide (Fig. 1f). Scanning transmission electron microscopy equipped with energy dispersive X-ray
spectroscopy (STEM-EDS) shows that copper element distributes densely on the whole particle, while oxygen
element is less dense (Figs. 1g-j). The combined results of STEM-EDS and HRTEM suggest a random mixture of
copper and cuprous oxide at the shell of the cuboid particles.
No Cl element was detected by XPS after the CRR (Fig. 1l). Peaks at 935.4 and 955.4 eV detected for the
electrode before CRR were attributed to Cu 2p3/2 and Cu 2p1/2 (Fig.S1 in Supporting information). Satellite peaks
at 942.9 eV indicated the presence of Cu2+ [28]. The broad peak of Cu 2p3/2 was fitted into two peaks with
binding energy of 935.8 (major) and 933.2 eV (minor), corresponding to Cu2+ and Cu1+/0, respectively. Because
the binding energy of Cu1+ and Cu0 is very close, it is very difficult to distinguish the two states based on Cu

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2p3/2 data [29]. After the CRR, the characteristic peak of Cu2+ disappeared, and the Cu 2p peak moved to lower
binding energy. These observations are also consistent with a depletion of Cl element and a formation of a
biphasic Cu2O-Cu for the electrode during the CRR.
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Fig.2. Time resolved ex situ XRD patterns of CuCl electrode during the electrolysis at −1.06 V. (a) CO2 was purged
for 30 min, (b) after LSV, (c) 60 s, (d) 600 s, (e) 3680 s, and (f) 6 hours after the start of the CRR.
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Fig.3. Time resolved ex situ SEM images of CuCl electrode at −1.06 V. (a) CO2 was purged for 30 min, (b) after
LSV, (c) 60 s, (d) 600 s, (e) 3680 s, and (f) 6 h after the start of CRR.
To further explore the changes of the morphology and the phase of the catalyst in the reaction process, the
CuCl electrodes at six different reaction stages were compared by time resolved ex situ XRD (Fig. 2) and SEM
(Fig. 3). Intensities of the peaks corresponding to CuCl2•2H2O decreased gradually and virtually disappeared
after 600 seconds’ electrolysis. While those corresponding to copper and cuprous oxide increased as the CRR
proceeded. When the reaction time was extended to six hours, the intensity of (1 1 1) peak corresponding to
Cu2O decreased relative to the Cu (1 1 1) peak, suggesting that the cuprous oxide was partially reduced to
copper during a long period of CRR [24, 30]. The significantly different intensities of the peaks corresponding to
Cu2O for the electrodes with different electrolysis time but same air exposure time strongly suggest that aerobic
oxidation only cannot explain the observation of Cu2O and there should be Cu2O on the electrode even after a
certain period of CRR. It can be seen from Fig. 3, the cubic crystals began to form after 60s’ CRR, and the
electrode surface was covered with a large number of cubes with a size of 150-350 nm after electrolysis for 600
s. After a long period of electrolysis, the shape of the cubic crystal was still reserved, showing good
morphological stability.
The potentiostatic electrolysis measurement was carried out in a CO2 saturated 0.1 mol/L KHCO3 solution. Gas
and liquid products were detected by gas chromatography and nuclear magnetic resonance, respectively. LSV was
used to compare the overall activity of the CuCl electrode, the carbon paper substrate and the copper foil. As
shown in Fig. 4a, the catalyst led to significant increase of the current density. For carbon paper substrate,
hydrogen evolution dominated at −0.9 V. The onset potential of methane was −1.1 V, while ethylene emerged at
−1.2 V with faradaic efficiency of only 1.2%. We investigated the electrochemical activity of CB-Cu to reduce carbon
dioxide under a series of potentials. As shown in Fig.4c, under the low overpotential (−0.7 V~−0.9 V), the gaseous
products were dominated by H2 and CO, with faradaic efficiency of 40% and 21%, respectively. Ethanol was the
main C2&C2+ product. A considerable amount formate formation (FE: 9%-20%) was also formed in the liquid
phase, as well as a small amount of methanol and acetate (FE ≤ 5%). With increasing overpotential (−0.9 V~−1.16
V), the FE of H2, CO, and formate decreased. At −1.16 V, the minimum FE of H2 was reached (17%), and the overall
selectivity for CRR was 76.5%.
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Fig.4. Catalytic behavior for electroreduction of CO2 on CB-Cu. (a) LSV comparison for CB-Cu electrocatalyst,
carbon paper and the copper foil. (b) gaseous products distribution on carbon paper, (c) faradaic efficiency of
products on CB-Cu, (d) current densities as a function of time at different applied potentials, (e) partial current
density of ethylene as a function of applied potentials, (f) Six hours’ stability performance of CB-Cu at −1.06 V vs.
RHE.
When the potential shifted from −0.9 V to −1 V, the FE of ethylene increased 3.4 times, and n-propanol and
methane started to appear. At −1.06 V, a maximum ethylene FE of 37% was obtained and the overall C2&C2+
products selectivity reached 57.5%, including ethanol FE of 15%, n-propanol FE of 5%, and acetate FE of 0.5%. The
selectivity of the C2&C2+ products (Fig. S6 in Supporting information) is at the same level with that obtained using
the Cu2O electrode as the starting catalytic material [25, 31, 32]. When the potential was negatively shifted to
−1.16 V, the faradaic efficiency of ethylene decreased slightly, which may be related to limited mass transfer of
carbon dioxide [33].
Cl in the electrolyte is known to affect activity and selectivity of CO2 electroreduction on Cu catalysts [34, 35].
Control experiment was carried out by replacing the electrolyte for the activation of the chloride catalyst precursor
with freshly prepared electrolyte during a continuous electrolysis (SI), and only little change was observed on
current density and product selectivity, suggesting that the chloride dissolved in the original electrolyte is probably
not important in the electrolysis. In comparison, the presence of Cl was proposed to delay the reduction of Cu2O

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[24], which might facilitate the formation of C2+ products. The high selectivity of ethylene may benefit from the in-
situ formation of biphasic Cu2O-Cu induced by Cl [36] during the reaction. Recent studies have shown that the
interface between Cu1+ and Cu0 contributes to the dimerization of CO adsorbed on the electrode surface to
generate ethylene [37]. In addition, Yang et al. [38] emphasized the role of the structural transformation of Cu
nanoparticles into cube particles during the reaction to increase the selectivity of the C2&C2+ product. The cubic
mesocrystals were formed during the present in-situ reduction process, which may also contribute to the improved
ethylene selectivity [25, 26].
Fig.4d showed that the current density increased from 3.35 mA/cm2 to 40.51 mA/cm2 with a negative shift in the
applied potential. As shown in Fig. 4e, CB-Cu produced a maximum ethylene current density of 14.8 mA/cm2 at
−1.06 V and showed fairly good stability in the 6-h test (Fig. 4f).
Table S1 (Supporting information) compares ethylene production performance of CB-Cu catalyst with other
copper-based catalysts in the literature. As for the partial current density of ethylene, CB-Cu was comparable or
better than the reported catalysts. We tentatively attribute this enhancement to the increased surface roughness
of our chloride-derived copper compared to the oxide-derived copper reported previously.
In summary, we studied the catalytic activity of a CuCl-based electrode for electrochemical reduction of carbon
dioxide and found that CuCl was gradually transformed into a cubic structure of biphasic Cu2O-Cu presumably due
to the depletion and induction of chlorine ions in the reduction process. Enhanced faradaic efficiency and partial
current density for the formation of ethylene were observed for this new catalyst with a good stability. The synergy
of Cu1+ and cubic structure may account for this improvement. The utilization of cuprous chloride as a catalyst
precursor in replace of cuprous oxide provides a new strategy to improve the selectivity of C2&C2+ products.
Acknowledgments
This work was financially supported by Shell-CAS Frontier Sciences Program (No. PT48809) from Shell and start-up funding from
ShanghaiTech University.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/
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