Electrochemical Reduction of CO2 Toward C2 Valuables on Cu@Ag Core-Shell Tandem Catalyst with Tunable Shell Thickness

Article abstract of DOI:10.1002/smll.202102293

Electrochemical CO₂ reduction reaction (CO₂RR) is critical to converting CO₂ to high-value multicarbon chemicals. However, the Cu-based catalysts as the only option to reduce CO₂ into C₂₊ products suffer from poor selectivity and low activity. Tandem catalysis for CO₂ reduction is an efficient strategy to overcome such problems. Here, Cu@Ag core-shell nanoparticles (NPs) with different silver layer thicknesses are fabricated to realize the tandem catalysis for CO₂ conversion by producing CO on Ag shell and further achieving C–C coupling on Cu core. It is found that Cu@Ag-2 NPs with?the proper?thickness of Ag shell exhibit the Faradaic efficiency (FE) of total C₂ products and ethylene as high as 67.6% and 32.2% at ?1.1?V (versus reversible hydrogen electrode, RHE), respectively. Moreover, it exhibits remarkably electrocatalytic stability after 14 h. Based on electrochemical tests and CO adsorption capacity analyses, the origin of the enhanced catalytic performance can be attributed to the synergistic effect between Ag shell and Cu core, which strengthens the bonding strength of CO on Cu/Ag interfaces, expedites the charge transfer, increases the electrochemical surface areas (ECSAs). This report provides a Cu-based catalyst to realize efficient C₂ generation via a rationally designed core-shell structured catalyst.

Full text of DOI:10.1002/smll.202102293

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Electrochemical Reduction of CO2 Toward C2 Valuables
on Cu@Ag Core-Shell Tandem Catalyst with Tunable
Shell Thickness
Shuaishuai Zhang, Shulin Zhao, Dongxue Qu, Xiaojing Liu, Yuping Wu,* Yuhui Chen,*
and Wei Huang
After a great research on CO2 electrore-
Electrochemical CO2 reduction reaction (CO2RR) is critical to converting CO2
to high-value multicarbon chemicals. However, the Cu-based catalysts as
the only option to reduce CO2 into C2+ products suffer from poor selectivity
and low activity. Tandem catalysis for CO2 reduction is an efficient strategy to
overcome such problems. Here, Cu@Ag core-shell nanoparticles (NPs) with
different silver layer thicknesses are fabricated to realize the tandem catalysis
for CO2 conversion by producing CO on Ag shell and further achieving C–C
coupling on Cu core. It is found that Cu@Ag-2 NPs with the proper thick-
ness of Ag shell exhibit the Faradaic efficiency (FE) of total C2 products and
ethylene as high as 67.6% and 32.2% at −1.1 V (versus reversible hydrogen
electrode, RHE), respectively. Moreover, it exhibits remarkably electrocata-
lytic stability after 14 h. Based on electrochemical tests and CO adsorption
capacity analyses, the origin of the enhanced catalytic performance can
be attributed to the synergistic effect between Ag shell and Cu core, which
strengthens the bonding strength of CO on Cu/Ag interfaces, expedites the
charge transfer, increases the electrochemical surface areas (ECSAs). This
report provides a Cu-based catalyst to realize efficient C2 generation via a
rationally designed core-shell structured catalyst.
duction catalysts, scientists found that
copper (Cu) is the only metal that can
reduce CO2 into C2+ products. However,
Cu itself usually has poor selectivity due to
the wide product distribution, which limits
their potential application in CO2RR.[3]
Extensive distinguished works have been
devoted to regulating the selectivity of
Cu catalysts, such as optimizing crystal
facets,[4] alloying,[5] modifying oxidation
state,[6] surface doping,[7] introducing
defects,[8] and modifying ligand.[9] Even
so, the reduction of CO2 into C2+ products
with high selectivity is still a challenging
work. Thus, the rational design and prep-
aration of efficient catalysts hold great
importance in their fundamental study
as well as the technical advancement of
CO2RR.
It is widely accepted that CO related
intermediates are the key species to C2+
products in CO2RR.[10] Therefore, the
designing of a two-step route (i.e., CO2
to CO and CO to C2+) provides an appropriate pathway for
the highly selective conversion of CO2 to C2+, which can be
achieved by the fabrication of tandem catalysts with multicom-
posite or hierarchical structure.[11] For multicomposite catalysts,
a synergistic effect between different components can be used
to achieve in situ CO generation, promoting the subsequently
reduced to C2+. Due to the excellent CO formation ability of Au
and Ag,[12] many studies revealed that the combining Au or/and
Ag with Cu could significantly improve the selectivity of C2+
products for CO2RR. Many composite catalysts, such as Au-bipy-
1. Introduction
Electrochemical CO2 reduction reaction (CO2RR) is a highly
promising strategy for valuable chemicals and fuels for real-
izing the carbon cycle.[1] Among different products of CO2RR,
multicarbon compounds (C2+) such as ethylene and ethanol are
very attractive for chemical raw materials and energy carriers.[2]
S. Zhang, Dr. S. Zhao, D. Qu, Dr. X. Liu, Prof. Y. Wu, Prof. Y. Chen
State Key Laboratory of Materials-Oriented Chemical Engineering
School of Energy Science and Engineering
Nanjing Tech University
Nanjing 211816, China
E-mail: wuyp@njtech.edu.cn; cheny@njtech.edu.cn
Cu,[13] Au/Cu,[14] Cu nanowire/Ag nanoparticles (NPs),[15] lay-
[18]
ered Cu/Ag,[16] Ag@Cu NPs,[17] Cu500Ag1000
,
Ag1-Cu1.1,[19]
and Cu-Au/Ag nanoframes,[20] have been developed. In addi-
tion to the composition control, the finely engineered structure
of the catalysts can also improve their performance. Among
them, the fabrication of core-shell structure has been verified to
be an effective strategy to boost the performance of nanomate-
rials through the short diffusion path, high active surface area,
low internal resistance, and excellent stability.[21] Therefore,
core-shell nanomaterials are appealing as electrocatalysts since
the core materials are the main active component with specific
functions, while the shell materials act as protective layers to
S. Zhang, Prof. Y. Wu, Prof. W. Huang
Key Laboratory of Flexible Electronics (KLOFE)
Institute of Advanced Materials (IAM)
Nanjing Tech University
Nanjing 210009, China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/smll.202102293.
DOI: 10.1002/smll.202102293
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Figure 1. Transmission electron microscopy (TEM) images of a) Cu@Ag-1, b) Cu@Ag-2, c) Cu@Ag-3, and d) Cu@Ag-4. e,f) High-resolution TEM
(HRTEM) images of Cu@Ag-2. g–j) Energy-dispersive spectroscopy (EDS) elemental mappings images for Cu@Ag-2.
enhance the core materials’ performance or to produce new
properties.
shell region, the lattice fringe spacing of 0.23 nm is indexed to
the (111) plane of pure cubic-phase Ag. Energy-dispersive spec-
troscopy (EDS) mapping of Cu and Ag clearly shows the Cu
(red) core was entirely covered by Ag (green) shell (Figure 1g–j),
which further confirmed the core-shell structure of Cu@Ag-2.
X-ray diffraction (XRD) was conducted to confirm the crystal-
line structure and composition of all these Cu@Ag catalysts.
The characteristic peaks of Cu and Ag were observed in both
the related XRD patterns of Cu@Ag NPs (Figure 2a). The dif-
fraction peaks at 43.30°, 50.43°, and 74.13°, which were indexed
to the (111), (200), and (220) planes of cubic Cu (JCPDS-04-
0836), respectively. Besides, the diffraction peaks at 38.12°,
44.28°, 64.43°, and 77.47° were assigned to the (111), (200), (220),
and (311) planes of cubic Ag (JCPDS-04-0783), respectively. It
is found that the intensities of silver diffraction peaks increase
from Cu@Ag-1 to Cu@Ag-4, indicating the increase of the
amount of Ag shell. In addition, the diffraction peaks of Ag (111)
in Cu@Ag NPs shifted to higher diffraction angles (Figure S3,
Supporting Information), which could be due to the compres-
sion of the lattice of Ag shell.[22] In order to further prove the
thicknesses of Ag shell are increased with the increasing amount
of silver precursor, Cu LMM Auger spectra for all Cu@Ag
NPs were measured (Figure 2b). Cu LMM peaks (564–570 eV)
and Ag 3p3/2 peaks (573 eV) can be observed in all the spectra,
while the ratios of intensity for Ag to Cu are increased with the
increasing amount of silver precursor, which is strong evidence
that the thicknesses of Ag layer are increased from Cu@Ag-1 to
Cu@Ag-4. The accurate molar percentages of Ag of these cata-
lysts identified by ICP-OES are 11.2%, 15%, 18.3%, and 24.7%,
respectively. The above characterizations suggest that the Cu@
Ag core-shell NPs with different thicknesses of Ag shell have
been synthesized successfully.
In this work, we report the synthesis of Cu@Ag core-shell
NPs with tunable shell thickness for C2 products during the
CO2RR process. A series of Cu@Ag core-shell NPs with average
sizes of 10.7, 11.2, 11.8, and 12.2 nm (denoted as Cu@Ag-1, Cu@
Ag-2, Cu@Ag-3, and Cu@Ag-4) were prepared. The Cu@Ag-2
catalyst with optimal thickness of Ag shell exhibited the highest
catalytic activity and selectivity toward C2, with a Faradaic effi-
ciency (FE) of 67.6% and a current density of −22.7 mA cm−2 at
−1.1 V (versus RHE). Based on experimental results, the excel-
lent performance of Cu@Ag-2 catalysts can be attributed to the
strong bonding strength to CO on Cu/Ag interfaces, large elec-
trochemical surface areas (ECSAs), and fast interfacial charge
transfer.
2. Results and Discussion
2.1. Component and Microstructure Characterization
The Cu@Ag core-shell NPs were prepared via a two-step reduc-
tion process. The Cu NPs were firstly prepared by the reduction
of copper (II) acetylacetonate (Cu(acac)2) in an inert atmos-
phere. These obtained Cu NPs exhibited spherical shapes with
an average size of about 9.7 nm (Figure S1, Supporting Infor-
mation). After that, AgNO3 was added into the above system
to form the shell of Ag on the surface of Cu NPs. By changing
the amount of silver precursor, four kinds of Cu@Ag NPs with
different thicknesses of Ag shell were prepared. Transmission
electron microscopy (TEM) images of all these Cu@Ag NPs are
shown in Figure 1a–d, which took spherical morphologies. The
sizes of Cu@Ag NPs increased consistently to 10.7, 11.2, 11.8,
and 12.2 nm (denoted as Cu@Ag-1, Cu@Ag-2, Cu@Ag-3, and
Cu@Ag-4) (Figure S2, Supporting Information). Represented
by Cu@Ag-2 NPs, the high-resolution TEM (HRTEM) images
are shown in Figure 1e,f, from which we can see the clear lat-
tice fringes, and the lattice spacing is measured to be 0.21 nm
for the core, assigned to the (111) plane of metallic Cu. In the
The surface electronic structures and chemical states of ele-
ments of Cu@Ag NPs were further characterized by X-ray photo-
electron spectroscopy (XPS). The survey XPS spectra (Figure S4,
Supporting Information) indicate the presence of Cu, Ag, O,
and C elements in the Cu@Ag NPs catalysts. The intensities of
Cu 2p XPS spectra decrease with the increase of the thickness
of the Ag shell. The high-resolution Cu 2p XPS spectra of the
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Figure 2. a) X-ray diffraction (XRD) patterns of Cu NPs, Cu@Ag NPs, and Ag nanoparticles (NPs). b) Cu LMM X-ray photoelectron spectroscopy (XPS)
spectra of Cu@Ag catalysts. High-resolution XPS spectra of c) Cu 2p region and d) Ag 3d region for Cu NPs, Cu@Ag NPs, and Ag NPs.
Cu@Ag NPs in Figure S5 (Supporting Information) indicates
that both Cu(II) and Cu(I)/Cu(0) species exist on the Cu sur-
face, suggesting that these Cu@Ag catalysts contained a very
small amount of oxidized copper. Meanwhile, the content of the
Cu(II) is decreased with the increase of the silver layer thick-
ness, indicating that the Ag shell could protect the Cu core from
oxidation. Furthermore, a small amount of oxidized copper
can effectively reduce the overpotential, increase the activity,
and facilitate the production of C2 products for CO2RR.[23] To
show the evolution trends of the electronic structures for Cu
and Ag, the higher resolution scans of the Cu 2p region and Ag
3d region for Cu NPs, Ag NPs, and all Cu@Ag NPs are placed
together for comparison, as shown in Figure 2c,d. The high-
resolution Cu 2p XPS spectra show that the band positions
of Cu 2p1/2 and Cu 2p3/2 shifted to the lower binding energy
with thicker Ag shell. On the contrary, the trends of Ag 3d5/2
and Ag 3d3/2 shifted to the opposite direction, which indicated
that the existence of electrons transfer between the Cu core
and Ag shell. As we know, in comparison with Cu, Ag is richer
in electrons and can transfer electrons to Cu. Meanwhile, Cu
acts as the electron acceptor, which leads to the shift of binding
energy.[17] Moreover, the core-shell structure is beneficial for the
charge transfer between Cu and Ag.
a carbon gas diffusion electrode (GDE) and subsequently tested
in Ar- and CO2-saturated 1.0 m KOH solutions. Linear sweep
voltammetry (LSV) curves of Cu@Ag core-shell catalysts are
depicted in Figure 3a. The current density of all catalysts in CO2-
saturated solution shows a dramatic increase and onset poten-
tial shifted to the positive direction compared with that in the
Ar-saturated solution, suggesting the CO2RR occurs readily on
the Cu@Ag catalysts. A series of chronoamperometry measure-
ments were performed to investigate the potential dependence
of the electrode on the FEs of the reduction products. The gas-
eous and liquid products were analyzed by gas chromatography
(GC) and 1H nuclear magnetic resonance (1H NMR) (Figure S6,
Supporting Information) measurements, respectively. In addi-
tion, Cu NPs without coating with Ag shell were studied as con-
trols. Figure 3b shows the FEs of all products for Cu@Ag-2 at
the different applied potentials. Specifically, the FEs of total C2
products for Cu@Ag-2 reached a maximum of 67.6% at −1.1 V
(versus RHE), in which acetate, ethanol, and C2H4 accounted
for 5%, 30.4%, and 32.2%, respectively. Moreover, the FEs of
total C2 products of Cu@Ag-2 are 16.1, 1.3, 1.6, and 2.0 times
higher than those of Cu NPs (4.2%), Cu@Ag-1 NPs (53.9%),
Cu@Ag-3 NPs (41.7%), and Cu@Ag-4 NPs (34.0%), respec-
tively, as shown in Figure S7 (Supporting Information) and
Figure 3c. In order to demonstrate the core-shell structure of
the catalyst is beneficial for the electrochemical reaction, the
Cu and Ag NPs were mixed together (mass fraction of Ag is
15%) for CO2RR. In comparison with the mixture of Cu-Ag
catalyst, the core-shell Cu@Ag catalyst can inhibit the produc-
tion of hydrogen, while the yields of CO and C2 products were
2.2. Electrochemical Properties of Cu@Ag Catalysts
The CO2RR performance over the Cu@Ag catalysts was tested
in a flow cell system. The electrocatalysts were spray-coated on
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Figure 3. a) Linear sweep voltammetry (LSV) curves for Cu@Ag NPs in Ar- and CO2-saturated 1.0 m KOH. b) Faradaic efficiency (FE) values of all
products at various applied potentials for Cu@Ag-2 NPs. c) FEs, d) Current density of C2 products for CO2RR over all Cu@Ag NPs. e) FEs of C2H4
over all Cu@Ag NPs. f) Stability test of Cu@Ag-2 NPs at −1.1 V (versus RHE).
increased (Figure 3b and Figure S8, Supporting Information).
Furthermore, the C2 partial current densities (jC2) of all Cu@
Ag samples increased gradually with the negative variation of
potentials (Figure 3d). Notably, Cu@Ag-2 presented the highest
Supporting Information) of Cu@Ag-2 catalyst indicate that
there was no change in both composition and structure of the
sample. All these results proved that Cu@Ag-2 catalyst can
operate stably for a long time under this condition.
j
C2 of −22.7 mA cm−2 at −1.1 V (versus RHE), which is superior
From the analysis of the results given above, it is worthy
to understand the relationship between the activity and selec-
tivity and structure for the series of Cu@Ag. Figure 4a sum-
marizes the FEs of CO and C2 products at −1.1 V (versus RHE)
for various Cu@Ag NPs. An obvious volcano-type curve for C2
was observed for four Cu@Ag electrocatalysts. While the FEs
for CO presented the opposite trend. From the FEs of products
for Cu@Ag-1 and Cu@Ag-2, it can be seen that the increase
of Ag layer thickness promotes the generation of C2. However,
when the Ag layer thickness keeps growing, such as Cu@Ag-3
and Cu@Ag-4, FEs of C2 decrease sharply, and CO increases
correspondingly. The results demonstrate that there is a deli-
cate balance between the Cu core and Ag shell for the CO2RR
performance. Therefore, we can infer that a two-step process
of CO2 electroreduction was achieved through the Cu@Ag
core-shell structure that acted as a tandem catalyst, that is,
CO2 was firstly adsorbed, activated and reduced to CO on the
Ag shell, and subsequently converted to C2+ products on Cu
to the other Cu@Ag samples in this work. The CO2RR perfor-
mance parameters of Cu@Ag-2 and some reported Cu-based
catalysts are summarized in Table S1 (Supporting Information).
The above findings demonstrate the great effect of the thick-
ness of Ag shell on selectivity and activity for CO2RR. Among
all products, C2H4 has been regarded as one of the important
chemical materials. Figure 3e and Figure S9 (Supporting Infor-
mation) show the FEs and current densities for C2H4 of Cu@
Ag catalysts at different applied potentials. The FEC2H4 and jC2H4
achieved on the Cu@Ag-2 catalyst are 32.2% and 9.0 mA cm−2
at −1.1 V (versus RHE), respectively. Besides the catalytic selec-
tivity and activity, the excellent durability of the CO2RR catalysts
is another crucial condition for practical applications. Further-
more, long-term operation was conducted at the potential of
−1.1V versus RHE on Cu@Ag-2 catalyst. There was no signifi-
cant decay in both current density and FE of C2H4 during 14 h
(Figure 3f). The morphology and XRD pattern (Figure S10,
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Figure 4. a) Faradaic efficiencies (FEs) of CO and C2 for the Cu@Ag NPs at −1.1 V (versus RHE). b) Schematic illustration of tandem catalysis for
CO2RR to C2 over Cu/Ag core-shell NPs. c) Current density as a function of the scan rate for Cu@Ag NPs. d) Electrochemical impedance spectroscopy
(EIS) spectrum for Cu@Ag NPs.
core, as shown in Figure 4b. Thus, the reason why C2 products
of Cu@Ag-1 are lower than that of Cu@Ag-2 may be due to
the silver shell of the former is thinner than the latter, which
cannot provide enough active sites to produce CO intermediate
that could be converted to C2 products by C–C coupling. How-
ever, as the thickness of silver shell continually grows to form
the Cu@Ag-3 and Cu@Ag-4 NPs, the silver shell becomes the
dominant phase to control the overall behavior of CO2RR. Such
a thick silver shell prevents CO intermediates from reaching
Cu core efficiently, resulting in the dramatically different
product distribution from Cu@Ag-2 on Cu@Ag-3 and Cu@
Ag-4 NPs for C2 products. In addition, the adsorption capacity
of CO on Cu@Ag-2 NPs was investigated by CO temperature
programmed desorption (CO-TPD), and Cu@Ag-4 was taken
as the contrast sample. As shown in Figure S11 (Supporting
Information), compared with Cu@Ag-4, in addition to the
CO desorption peak at 295 °C, the peak of Cu@Ag-2 at 341 °C
is attributed to the chemical adsorption of CO on Cu/Ag
interface, which was similar to the reported phenomena.[21a]
This result indicates that the Cu@Ag-2 has stronger bonding
strength to CO, which is beneficial to the further C-C coupling
to form C2 products during CO2RR.
charging current density against scan rates on series of Cu@Ag
catalysts. Obviously, the Cdl value for Cu@Ag-2 (9.9 mF cm−2)
is larger than the others Cu@Ag catalysts in this work, which
suggested that Cu@Ag-2 exposed more active sites to promote
the CO2RR process. In addition, the Tafel plots (Figure S13,
Supporting Information) of all catalysts were obtained to reveal
the reaction kinetics. From the picture, we can intuitively see
that the Tafel slope of Cu@Ag-2 was the smallest, indicating
the Cu@Ag-2 possessed the most favorable kinetic rate for
CO2RR. Moreover, EIS was further conducted to explore the
charge transfer kinetics of Cu@Ag catalysts. The Nyquist plots
(Figure 4d) and equivalent circuit (Figure S14, Supporting
Information) show that Cu@Ag-2 with an interfacial charge
transfer resistance (Rct) of 17.5 Ω, which was lower than those of
Cu@Ag-1 (43.2 Ω), Cu@Ag-3 (79.8 Ω), and Cu@Ag-4 (33.5 Ω),
confirming that Cu@Ag-2 processes the fastest interfacial
electron-transfer dynamics. Therefore, the excellent CO2RR
performance of the Cu@Ag-2 NPs might be attributed to the
synergistic effect of components, the strong bonding strength
to CO on the Cu/Ag interfaces, high availability of active sites,
and fast electron transfer rate.
In order to investigate the essential reason why the CO2
reduction performance of Cu@Ag-2 is optimal, ECSA and elec-
trochemical impedance spectroscopy (EIS) measurements were
conducted. ECSA was examined through the method of double
layer capacitance (Cdl), which was obtained from the cyclic
voltammograms (CVs) under different scan rates (Figure S12,
Supporting Information). Figure 4c shows the differences of
3. Conclusions
In summary, Cu@Ag core-shell NPs with tunable shell thick-
ness were fabricated as efficient CO2RR electrocatalysts. It was
found that the thickness of Ag shell have an important impact
on the selectivity of C2 products. The optimized Cu@Ag-2 NPs
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with a Valcoplot HayeSep Q column and a Molsieve 5 A column. The
liquid products were analyzed by 1H NMR recorded on a Bruker AVANCE
AV-300 spectrometer. The FE for a target product was calculated using
the following formula:
exhibited the total FE of C2 products and ethylene are as high as
67.6% and 32.2% at −1.1 V (versus RHE), respectively. The Cu@
Ag-2 catalysts with proper Ag shell thickness could produce
and boost the local CO concentration on the Cu core efficiently,
which help to improve the selectivity for C2 products. Electro-
chemical tests and CO adsorption capacity analyses confirmed
that the Cu@Ag-2 catalysts process stronger bonding strength
of CO on Cu/Ag interfaces, more active sites, and faster charge
transfer. This report provides an efficient core-shell tandem
catalyst and a rational design approach of electrocatalysts for
boosting CO2 conversion toward the C2 and C2+ products.
z·n·F
Q
(1)
FE % =
(
)
Where z is the number of electrons required to produce a target
molecule, n is the number of moles for a target product, F is the Faradaic
constant of 96 485 C mol−1, Q is the total charge of the electrochemical
reaction.
To estimate the ECSA, Cdl were conducted by measuring CVs with
a potential range of 0.054–0.154 V (versus RHE) that non-Faradaic
capacitive current. The scan rates were changed from 20 to140 mV s−1.
The Cdl was estimated by plotting the capacitance current density
(Δj = ja − jc, where ja and jc are the cathodic and anodic current densities,
respectively) at the potential of 0.104 V (versus RHE) with respect to the
CV scan rate of CV curves. The slope of the line was twice of the Cdl, and
the value of ECSA was positively correlated with Cdl. EIS measurement
was carried out by applying an AC voltage with 10 mV amplitude in a
frequency range from 0.01 to 106 Hz at open-circuit voltage.
4. Experimental Section
Synthesis of Cu NPs and Cu@Ag NPs: Cu NPs were prepared based
on the typical approach with modifications. In briefly, of Copper
(II) acetylacetonate (Cu(acac)2) (1.0 mmol, 261.6 mg) and 10 mL of
oleylamine (OAm) were added into a 50 mL three-necked flask with
a condenser pipe, which was connected to a Schlenk line. First, the
solution was heated to 80 °C at a rate of 5 °C min−1 and maintained at
80 °C for 1 h. Then, the solution was gradually heated to 170 °C and 210 °C
at a rate of 10 °C min−1, and each temperature section was maintained for
1 h to obtain Cu NPs. The obtained purple solution was cooled naturally
to room temperature. For the preparation of Cu@Ag-2 NPs, the mixture
of AgNO3 (0.2 mmol, 33.9 mg) and 10 mL of OAm was poured dropwise
into the above purple solution of Cu NPs at the rate of 30 mL h−1. After
vigorous stirring, the mixture solution was heated to 60 °C at 5 °C min−1
for 3 h under Ar atmosphere. The products were washed with hexane
and acetone, and centrifuged at 3000 rpm for 10 min. The Cu@Ag-2 NPs
were dried under vacuum at room temperature overnight for further
characterization. The entire experiment process was kept under strict
anaerobic conditions to prevent the possibility of NPs being oxidized.
The synthetic procedures for other Cu@Ag catalysts were the same
as that for Cu@Ag-2 NPs. The main difference lies on the amount of
used AgNO3 was adjusted. The amount of substance of AgNO3 for Cu@
Ag-1, Cu@Ag-3, and Cu@Ag-4 were 0.1, 0.5, and 1.0 mmol, respectively.
Characterization and Instruments: The morphology of these materials
was performed on TEM (JEOL JEM-2100F). The crystal structure and
phase analysis of the material was carried out by XRD on a Rigaku
SmartLab diffractometer equipped with a Cu Kα radiation. Elemental
analysis was tested by ICP-OES using a Perkin Elmer OPTIMA7000DV.
XPS tests were conducted on a PHI 5000 Versa Probe spectrometer with
Al Kα radiation (1486.6 eV). The carbon peak at 284.6 eV was used as a
reference to correct for charging effects.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
S.Z. and S.Z. contributed equally to this work. All authors have given
approval to the final version of the manuscript. This work was supported
by the financial support from the National Natural Science Foundation
of China (51773092, 51425301, 21603209, and 21975124), the Research
Foundation of State Key Lab (ZK201717), and the Natural Science
Foundation of Jiangsu Province (BK20190685).
Conflict of Interest
The authors declare no conflict of interest.
Electrochemical Measurements: Electrocatalysis experiments of all the
catalysts were measured on a Biologic VMP3 potentiostat controlled by
EC-Lab. The flow cell configuration that consisted of GDE (1.8 × 1.8 cm)
modified with catalysts as the working electrode, proton exchange
membrane (Nafion 115, Dupont), and nickel foam (1.8 × 1.8 cm) as the
anode. GDEs were prepared using an air-brush method. Five milligrams
of catalyst was dispersed in 500 µL water, 450 µL isopropyl alcohol,
and 50 µL Nafion solution (5 wt%) and sonicated for 30 min to form a
uniform catalyst ink. Then, the ink was sprayed on the gas diffusion layer
(GDL, 1.8 × 1.8 cm) with catalyst loading was to be 1 mg cm−2. Here, the
geometric area of the working electrode and the anode were designed to
be 1.0 cm2. The electrolyte (1.0 m KOH) with a flow rate of 2.5 mL min−1
was circulated using a peristaltic pump. Before testing, high-purity
CO2 was pumped into the cell for 20 min by a mass flow controller
with the flow rate was 5 mL min−1. All potentials were converted to a
RHE values by the following formula: ERHE (V) = EAg/AgCl(3.5 M) (V) +
0.209 V + (0.0591 V × pH). The LSV curves were measured in Ar- or
CO2-saturated 1.0 m KOH solutions at a scan rate of 5 mV s−1. The
chronoamperometry measurement was conducted under different
potentials. The quantification of the gas phase products was conducted
using a gas chromatograph (Nanjing Hope, GC-9860-5C) equipped
Data Availability Statement
Research data are not shared.
Keywords
CO2 reduction reaction, core-shell structure, Cu@Ag, synergy
Received: April 19, 2021
Revised: June 1, 2021
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