Tuning nanocavities of Au?Cu2O yolk-shell nanoparticles for highly selective electroreduction of CO2to ethanol at low potential

Article abstract of DOI:10.1039/d0ra02482a

The electrosynthesis of high-value ethanol from carbon dioxide and carbon monoxide addresses the need for the large-scale storage of renewable electricity and reduction of carbon emissions. However, the electrosynthesis of ethanol by the CO₂reduction reaction (CO₂RR) has suffered from low selectivity and energy efficiency. Here, we report a catalyst composed of Au nanoparticles in Cu₂O nanocavities (Au?Cu₂O) that is very active for CO₂reduction to ethanol through the confinement of the CO intermediate. The architecture shows tandem catalysis mechanisms in which CO₂reduction on Au yolks produces CO filling Cu nanocavities, where a sufficiently high CO concentration due to the confinement effect promotes ethanol formation and then results in an ethanol faradaic efficiency of 52.3% at -0.30 Vversusthe reversible hydrogen electrode (vs.RHE)viaregulating the hollow size of the Cu₂O nanocavities. Such a strategy provides a new way of fabricating various tandem catalysts with high selectivity and efficiency for the CO₂RR.

Full text of DOI:10.1039/d0ra02482a

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Tuning nanocavities of Au@Cu O yolk–shell
2
nanoparticles for highly selective electroreduction
Cite this: RSC Adv., 2020, 10, 19192
of CO to ethanol at low potential†
2
Bin-Bin Zhang, Ya-Hui Wang, Shan-Min Xu, Kai Chen, Yu-Guo Yang*
and Qing-Hua Kong
*
The electrosynthesis of high-value ethanol from carbon dioxide and carbon monoxide addresses the need
for the large-scale storage of renewable electricity and reduction of carbon emissions. However, the
electrosynthesis of ethanol by the CO
energy efficiency. Here, we report a catalyst composed of Au nanoparticles in Cu
Au@Cu O) that is very active for CO reduction to ethanol through the confinement of the CO
intermediate. The architecture shows tandem catalysis mechanisms in which CO reduction on Au yolks
2
2
reduction reaction (CO RR) has suffered from low selectivity and
2
O nanocavities
(
2
2
2
produces CO filling Cu nanocavities, where a sufficiently high CO concentration due to the confinement
effect promotes ethanol formation and then results in an ethanol faradaic efficiency of 52.3% at ꢀ0.30 V
Received 17th March 2020
Accepted 12th May 2020
2
versus the reversible hydrogen electrode (vs. RHE) via regulating the hollow size of the Cu O
DOI: 10.1039/d0ra02482a
nanocavities. Such a strategy provides a new way of fabricating various tandem catalysts with high
selectivity and efficiency for the CO RR.
rsc.li/rsc-advances
2
22
production. Sargent's group constructed a core–shell vacancy
Introduction
engineering catalyst (Cu S–Cu–V) to steer products beyond
2
23
The electrocatalytic reduction of CO
fuels and chemicals offers a route to reduce CO
and facilitate the long-term storage of renewable electricity.
2
to valuable carbon-based alkenes and toward ethanol at certain potentials. More
emissions recently, theoretical and experimental investigations demon-
2
1
–6
2
strate that the activity and selectivity for C and C2+ of Cu-based
In particular, C2 and C2+ products from the CO2 reduction catalysts can be greatly advanced by bimetallic strategy or con-
reaction (CO RR) have attracted considerable attention due to straining the local CO concentration at the catalyst–electrolyte
2
24,25
their relatively high energy density, added-value and wide- interface.
Au/Cu bimetallic electrocatalyst was obtained with
spread use as feedstocks in polymer synthesis, etc. Excellent improved activity and selectivity for electrochemical trans-
electrocatalysts have been developed to boost the activity and formation of CO to alcohols over hydrocarbons at low over-
potentials, and a tandem catalysis mechanism has been
Among currently available materials, Cu-based catalysts have proposed where Au nanoparticles (Au NPs) reduce CO to CO
7–9
2
10–14
2 2
selectivity of the CO RR towards C and C2+ productions.
2
25
been reported as the most promising electrodes for producing near the copper surface, driving a high CO coverage. When
C and C compounds under aqueous conditions in CO RR, supplied directly with CO instead of CO as a feedstock, oxide-
However, there still derived nanocrystalline copper electrodes produce ethanol
present some scientic challenges, such as poor selectivity, low with up to 30% faradaic efficiency at modest overpotentials
2
2+
2
2
8,12,15–21
and have been extensively studied.
26
faradaic efficiency and durability, which need to be managed under alkaline conditions. In addition, the connement effect
primarily in further work.
is known to alter thermodynamic and transport properties of
27–30
To date, several avenues have been employed to improve the uids.
Prior studies of porous catalysts exploited conne-
selectivity of Cu-based catalysts for CO
including altering size, structure, composition, surface state, retention of C
and so on.
2
RR to specic products, ment effect to implement a selectivity shi by extending the
or C species, and then boost C or C produc-
Hori et al. declared that the product selectivity of tion, which lack in-depth exploration. As such, developing
1
2
2
3
17–20
16,24
CO RR shied greatly with the crystal orientation, where a new strategy of combining bimetallic strategy and conne-
2
Cu(100) yielded mainly C H and Cu(111) beneted CH4 ment effect is highly imperative for boosting the selectivity to
2
4
2
ethanol in CO electroreduction and its practical application.
Herein, we further apply the connement effect with
Department of Chemistry, School of Science, Beijing Jiaotong University, Beijing
a bimetallic yolk–shell structure of Au nanoparticle in Cu O
2
1
†
1
00044, China. E-mail: ygyang@bjtu.edu.cn; qhkong@bjtu.edu.cn
Electronic supplementary information (ESI) available. See DOI:
0.1039/d0ra02482a
nanocavity (Au@Cu O), whereas Au shows high catalytic activity
2
31,32
for converting CO
2
to CO at low potentials,
and Cu is able to
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catalyze the second step of the tandem reaction, the reduction Preparation of working electrode
33–35
of CO to C2 products at low potentials.
The Au@Cu O
2
To prepare the catalyst inks, 3 mg of catalyst powder, 10 mL of
Naon solution (5%) and 1200 mL of isopropyl alcohol were mixed
and treated under sonication for 30 min. Then, 600 mL of the as-
prepared ink was pipetted onto the two sides of a carbon cloth
catalyst has higher selectivity for ethanol compared with
hydrocarbons at lower overpotentials. We propose that the
selectivity shiing from C
catalysis mechanism, where Au NPs reduce CO
1
to C
2
originates from a tandem
to CO in the Cu
2
2
ꢀ2
with area of 1 ꢃ 1.5 cm giving a catalyst loading of 1 mg cm
.
nanocavities, producing a high CO concentration, thus boosts
the ethanol production due to the connement of CO inter-
mediate. Meanwhile, we adjust the CO concentration in the Characterizations
nanocavities by regulating the hollow size of the Cu O nano-
2
X-ray diffraction (XRD) pattern was carried out on a Rigaku D/
Max-2500 diffractometer equipped with a Cu Ka1 radiation (l
cavities, and then achieve an ethanol faradaic efficiency of
2.3% at ꢀ0.30 V vs. RHE.
5
˚
¼
1.54 A). Scanning electron microscopic images (SEM) were
collected on a JEOL scanning electron microscope (S-4800,
Japan). Transmission electron microscopic images (TEM) were
obtained by a JEM-2100F microscope (JEOL, Japan) equipped
with an EDS detector (Oxford Instrument, UK). X-ray photo-
electron spectroscopy (XPS) was performed on an ESCALab220i-
XL electron spectrometer (VG Scientic, UK) with a mono-
Experimental
Chemicals
Hydrogen tetrachloroaurate(III) trihydrate (HAuCl $3H O, 99.99%),
4
2
citric acid, trisodium salt (98%), copper(II) nitrate trihydrate (99%),
hydrazine hydrate (hydrazine, 64%), polyvinylpyrrolidone (PVP,
average M.W 58000), 2-propanol (99.7+%), Naon (5%). All
chemicals were obtained directly without further purication.
2
chromatic Al Ka source. The gas products for CO reduction
were measured on a gas chromatography (GC, Agilent Tech-
nologies 7890B). The liquid products were analyzed with
a Bruker AVANCE 600 using dimethyl sulphoxide (DMSO) as an
internal standard.
Preparation of Au NPs
The Au NPs with a diameter of about 22 ꢁ 2 nm were synthe-
sized and used as the core materials for the Au@Cu O yolk–
2
36,37
Electrochemical performance test
shell particle fabrication.
Au NPs were prepared by a stan-
dard citrate reduction procedure. Typically, 150 mL of 0.025 M Electrochemical measurements were performed on a CHI 660E
HAuCl solution was added into a three-necked round bottom electrochemical workstation in a typical H-type electrolysis cell
ask and heated to boiling under continuously magnetic stir under ambient pressure and room temperature using platinum
4

2
with condensing and reuxing conditions. Then 7.5 mL of mesh (1 ꢃ 1 cm ) and Ag/AgCl electrode (saturated KCl) as
0
3
.02 M sodium citrate solution was added. Aer reaction for counter and reference electrodes, respectively. The cathode and
0 min, heating resource was removed and the solution was anode compartments were separated by a proton-exchange
allowed to cooled down naturally to room temperature. Finally, membrane (Naon 117). The 0.1 M KHCO aqueous solution
3
the Au NPs were collected from the solution by centrifugation was used as electrolyte directly without any purication. The
(10 000 rpm), washed with deionized water several times and electrode potentials were converted to the reversible hydrogen
redispersed in 10 mL of deionized water.
electrode reference (RHE) scale using the following Nernst
equation:
Preparation of Au@Cu O yolk–shell NPs with different hollow
2
E(RHE) ¼ E(Ag/AgCl) + 0.197 V + 0.0591 ꢃ pH.
sizes
Au@Cu O yolk–shell NPs was prepared by hydrazine hydration
2
3
6,38
reduction method.
Typically, 1 g of PVP powders was added
solution under constant stirring
Before test, the catalyst was measured at ꢀ0.2 V vs. RHE for
into 10 mL of 0.01 M Cu(NO )
3 2
3
3
0 min in Ar-saturated 0.1 M KHCO solution to ensure all
until the PVP powders dissolved completely. Then a certain
amount of as-obtained Au NPs solution was added, followed by
immediate introduction of trace hydrazine hydrate. Aer reac-
Cu
2
O-shells had been reduced to Cu-shells. Then, the catalyst
-saturated 0.1 M KHCO
was immediately transfer into CO
solution to perform linear scanning voltammetry (LSV) test
2
3
tion, the Au@Cu O yolk–shell NPs were synthesized and
2
from 0.2 to ꢀ1.1 V vs. RHE until the performance of the elec-
centrifuged (10 000 rpm), washed three times with water and
2
trode became stable. Finally, CO RR experiment at a constant
ꢂ
isopropyl alcohol, and then dried in an oven at 60 C for later
applied voltage spanned over 3600 seconds. The electrochemi-
cally surface area (ECSA) was determined by measuring the
double layer capacitance (Cdl), which was derived from the CV
curves at various scan rates.
2
use. Au@Cu O NPs with different hollow sizes can be controlled
through adjusting the amount of reducing agent and the reac-
tion time (Fig. 1a).
Preparation of Cu
2
O NPs
Analysis of CO
2
reduction products
Cu O NPs was prepared in parallel by the same method as
2
Au@Cu
2
O yolk–shell NPs except for no addition of Au NPs and We measure the reduction performance of the catalyst by
PVP powders.
2
calculating the faradaic efficiency (FE) of the CO reduction
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2 2 2
Fig. 1 (a) Schematic illustration of the synthetic process of different samples. (b–e) SEM, (f–i) TEM images of Cu O, Au@Cu O-SC, Au@Cu O-
MC, Au@Cu O-LC, respectively.
2
product. The FE of products can be calculated using the and the radius of the hollow size is donated as R1. In the
39
following formula:
structure, C chemical selectivity can be tuned by systemati-
2
cally altering the R1 of 14, 35, 48 nm, which are named small
Q
i
N
i
ꢃ n ꢃ F
FE
i
¼ Q
¼
cavity Au@Cu
2
O (Au@Cu
2
O-SC), middle cavity Au@Cu
2
O
total
Q
total
(
Au@Cu O-MC) and large cavity Au@Cu
2
2
O (Au@Cu O-LC),
2
ꢀ
i: the specic reduction product, CO, HCOO , C
total: the total charge of CO RR, C; Q : charge used for the
reduction of certain product, C; N : number of moles for certain
product, mol; n: number of electrons transferred for the CO
CO, HCOO and C H OH conversion or water-to-H reduction,
2 5 2
H OH or H ;
respectively. Scanning electron microscopy (SEM) images in
Fig. 1b–e reveal that the surfaces of all obtained samples are
uneven, and as reaction time went on, a clear porous structure
Q
2
i
i
2
-to-
was seen on the surface of Au@Cu O-LC. Corresponding to
2
ꢀ
2
5
2
SEM images, transmission electron microscopy (TEM) images
ꢀ
which is 2, 2, 12, 2 for CO, HCOO , C H OH and H , respec-
tively; F: faradaic constant, which is 96 485 C mol
2
5
2
in Fig. 1f–i show all the samples have a porous Cu O shell. And
2
ꢀ
1
.
the inner hollow size became larger along with the increased
reaction time and the shell structure gradually grew looser. In
2
addition, the samples except porous Cu O NPs belong to yolk–
Results and discussion
shell structure and exhibit an average diameter of 140 ꢁ 10 nm
with an average Au-cores diameter of 22 ꢁ 2 nm. Moreover, the
statistical analysis based on over 100 nanoparticles indicates
that they are in narrow size distribution (Fig. S3†), agreeing
The Au NPs with uniform size and morphology were obtained
according to previous literature (Fig. S1†).
the Au nanoparticle was encapsulated within a porous Cu
shell with an average size of 140 nm and the hollow size was
precisely controlled by regulating the reaction time and the
amount of reducing agents, as depicted in Fig. 1a. Meanwhile,
Fig. S2† shows the geometry of the multilayer particle we
3
6,37
Subsequently,
2
O
well with SEM and TEM results. Take Au@Cu O-MC catalyst
2
for example, more detailed structural information was char-
acterized in Fig. 2. High-resolution TEM image (HRTEM)
clearly shows the lattice fringes in spacings of 0.30 and
0
2
.25 nm corresponding to (110) and (111) planes of cubic Cu O
2
simulated, in which the radius of the Cu O NPs is donated as R
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ꢂ
with a characteristic interplanar angel of 90 on the shell Consequently, it is suggested that the active Cu sites of the
4
0
(
Fig. 2a). Dark-eld scanning transmission electron micros- catalyst during CO RR are derived from Cu O.
2 2
copy (DF-STEM) image (Fig. 2b) exhibits the loose and porous
The CO RR performances were evaluated over the as-
2
structure of the shell material, which will promise efficient obtained samples supported on the carbon cloth in 0.1 M
mass transport for potential electrochemical applications. KHCO solution saturated with CO using H-cell set-up. The
Meanwhile, the corresponding energy dispersive X-ray spec- products of CO RR were analyzed and quantied by online gas
3
2
2
1
troscopic (EDS) elemental mapping images (Fig. 2c–f) clearly chromatography (GC) for the gas products and H nuclear
display that Au distributes homogeneous on the core part magnetic resonance spectroscopy ( H NMR) for the liquid-
1
while Cu and O are distributed mainly across the nanoparticle phase products. We can determine the electrochemical
shell section, conrming the yolk–shell structure of activity by linear sweep voltammetry (LSV) towards CO RR,
2
Au@Cu O.
initially. Compared with the current density under Ar-saturated
2
The composition and crystalline structure of these samples 0.1 M KHCO , it has a signicant increase under CO -saturated
3
2
were investigated by X-ray diffraction (XRD) technique. All of the electrolyte, indicating that Au@Cu
2
2
O-MC has superior CO RR
recorded diffraction peaks in the typical XRD patterns (Fig. 3a) performance (Fig. S4a†). From Fig. S4b,† it can be seen the
can be well indexed to cubic Au (JCPDS no. 89-3697) and cubic difference of current density between these samples, and
Cu
2
O (JCPDS no. 78-2076), corresponding to the HRTEM Au@Cu
2
O-MC has the maximum current density at the same
RR performance. For
results. Further clues can be seen in X-ray photoelectron spec- potential, showing the excellent CO
2
troscopic spectra (XPS) (Fig. 3b and c). By comparing the Cu 2p porous Cu O NPs, at low applied cathodic potential of ꢀ0.3 and
2
and Cu-LMM XPS peaks of Cu O and Au@Cu O-MC, it could be ꢀ0.35 V vs. RHE, the only detectable product is ethanol and the
2
2
+
clearly seen that the Cu atom in Au@Cu O-MC is Cu , which is faradaic efficiency (FE
2 5
C H OH
) reaches 16% at ꢀ0.3 V vs. RHE
2
1
5,16,26,41
benecial for CO
2
RR according to previous reports.
2
In (Fig. 4a), demonstrating the Cu O NPs has intrinsic C–C
addition, since XPS is a surface analysis technique with inves- coupling potential. At more cathodic potentials, a substantial
tigation depth of 2–5 nm and the Au-core is coated by Cu
signal of Au element is not detected. Based on these results, it ucts of C
2
O, the difference in the product distribution is found with the prod-
42
ꢀ
2
and signicant increases in the CO, HCOO
can be concluded that Au-core encapsulates in porous Cu
shell particle, which constitute the tandem catalyst. (Fig. 4b), the selectivity shis clearly and CO, HCOO , C H OH
2
O- production rates. Aer introducing Au NPs in the cavities
ꢀ
2
5
products are generated at ꢀ0.3 and ꢀ0.35 V vs. RHE. The
2
Fig. 2 (a) HRTEM, (b) DF-STEM and (c–f) EDS elemental mapping images of Au@Cu O-MC.
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2 2
Fig. 3 (a) XRD patterns of different samples, (b) high-resolution Cu 2p and (c) Cu-LMM XPS spectra for Cu O and Au@Cu O-MC.
production of CO and HCOOH on the Au@Cu
2
O-SC as well as the FEC2H5OH at higher cathodic potentials from ꢀ0.45 to ꢀ0.6 V
its absence on the pure porous Cu O NPs, together with the vs. RHE indicates that the previously described tandem mech-
2
promoted FEC2H5OH (36%) at low potentials, prove the effect of anism is almost inactive in this potential window for Au@Cu O-
2
the suggested tandem reaction mechanism within the nano- MC due to the overwhelmingly competitive HER from the
conned space with both catalytic sites located in cavities. exposed metal species (Fig. S5†). Further increasing the hollow
The Au-core is active for electroreduction of CO
Cu-shell is able to reduce the retention of CO in the cavities to (Fig. 4d), which is slightly less than that of Au@Cu O-MC.
2
2
to CO, yet the size to 48 nm, the largest FEC2H OH is 38% at ꢀ0.35 V vs. RHE
5
improve ethanol production (Fig. 4e). When the hollow size of However, the active potential window of tandem reaction
the catalyst was increased to 35 nm, the largest FEC2H OH was mechanism for Au@Cu O-LC is broader than that of Au@Cu O-
improved to 52.3% at ꢀ0.3 V vs. RHE (Fig. 4c). The reduction of MC, and all of the FEC2H5OH at whole potentials from ꢀ0.27 to
5
2
2
2
Fig. 4 (a–d) The faradaic efficiencies of carbon monoxide, formic acid, and ethanol from the CO RR products under a range of potentials of
different samples. (e) Schematic illustration of tandem catalysis mechanism in the Au@Cu
FEC2H OH of Au@Cu O-MC in CO -saturated 0.1 M KHCO
solution at ꢀ0.3 V vs. RHE.
2
O cavity. (f) Time-dependent current density curve and
5
2
2
3
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0.45 V vs. RHE is higher than that of Au@Cu O-SC, as well as
RSC Advances
ꢀ
Notes and references
a reduction of the detected CO at ꢀ0.27 and ꢀ0.3 V vs. RHE for
Au@Cu O-MC and Au@Cu O-LC, suggesting the reaction
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43
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ꢀ1
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ꢀ2
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,
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5
2
5
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catalyst, comparable with or outperforming most of other state-
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2
1
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potential. It was demonstrated that the spatial connement of
different active sites with a tandem catalysis mechanism leads
to the selectivity shi from C to C . The Au-core can reduce CO
1
2
2
1
to CO in the copper nanocavity, producing a high CO concen-
tration, thus the Cu-shell transforms CO to ethanol production.
In addition, we conne the CO concentration in the nanocavity
2
by optimizing the hollow size of the Cu O nanocavity, and then
perform an ethanol faradaic efficiency of 52.3% at ꢀ0.30 V vs.
RHE. These results suggest that the present strategy may shed
light on the design and preparation of highly active tandem
catalysts for other electrochemical reactions.
1
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1
Conflicts of interest
19 Y. Zhou, F. Che, M. Liu, C. Zou, Z. Liang, P. De Luna,
H. Yuan, J. Li, Z. Wang, H. Xie, H. Li, P. Chen, E. Bladt,
R. Quintero-Bermudez, T. K. Sham, S. Bals, J. Hoens,
D. Sinton, G. Chen and E. H. Sargent, Nat. Chem., 2018, 10,
There are no conicts to declare.
9
74–980.
Acknowledgements
2
0 S. Zhong, X. Yang, Z. Cao, X. Dong, S. M. Kozlov, L. Falivene,
This work was supported by the Fundamental Research Funds
for the Central Universities (No. 2018JBM067) and the National
Natural Science Foundation of China (No. 21603011).
J. K. Huang, X. Zhou, M. N. Hedhili, Z. Lai, K. W. Huang,
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