In Situ Reconstruction of a Hierarchical Sn-Cu/SnOx Core/Shell Catalyst for High-Performance CO2 Electroreduction

Article abstract of DOI:10.1002/anie.201916538

The electrochemical CO₂ reduction reaction (CO₂RR) to give C₁ (formate and CO) products is one of the most techno-economically achievable strategies for alleviating CO₂ emissions. Now, it is demonstrated that the SnO_x shell in Sn_2.7Cu catalyst with a hierarchical Sn-Cu core can be reconstructed in situ under cathodic potentials of CO₂RR. The resulting Sn_2.7Cu catalyst achieves a high current density of 406.7±14.4 mA cm^?2 with C₁ Faradaic efficiency of 98.0±0.9 % at ?0.70 V vs. RHE, and remains stable at 243.1±19.2 mA cm^?2 with a C₁ Faradaic efficiency of 99.0±0.5 % for 40 h at ?0.55 V vs. RHE. DFT calculations indicate that the reconstructed Sn/SnO_x interface facilitates formic acid production by optimizing binding of the reaction intermediate HCOO* while promotes Faradaic efficiency of C₁ products by suppressing the competitive hydrogen evolution reaction, resulting in high Faradaic efficiency, current density, and stability of CO₂RR at low overpotentials.

Full text of DOI:10.1002/anie.201916538

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: In Situ Reconstruction of Hierarchical Sn-Cu/SnOx Core/Shell
Catalyst for High-Performance CO2 Electroreduction
Authors: Ke Ye, Zhiwen Zhou, Jiaqi Shao, Long Lin, Dunfeng Gao, Na
Ta, Rui Si, Guoxiong Wang, and Xinhe Bao
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201916538
Angew. Chem. 10.1002/ange.201916538
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10.1002/anie.201916538
Angewandte Chemie International Edition
RESEARCH ARTICLE
In Situ Reconstruction of Hierarchical Sn-Cu/SnOx Core/Shell
Catalyst for High-Performance CO2 Electroreduction
Ke Ye,+ Zhiwen Zhou,+ Jiaqi Shao,+ Long Lin, Dunfeng Gao, Na Ta, Rui Si, Guoxiong Wang,* and
Xinhe Bao
Abstract: Electrochemical CO2 reduction reaction (CO2RR) into
valuable C1 (formate and CO) products has been identified as one of
the most techno-economically achievable strategies for alleviating
CO2 emissions. However, high C1 Faradaic efficiency and stability
are generally attained at high overpotential and low current density.
Herein, we demonstrate that the SnOx shell in Sn2.7Cu catalyst with
hierarchical Sn-Cu core can be in situ reconstructed under cathodic
potentials of CO2RR. The resulting Sn2.7Cu catalyst achieves a high
current density of 406.7±14.4 mA cm–2 with C1 Faradaic efficiency of
98.0±0.9% at –0.70 V versus reversible hydrogen electrode (RHE),
and remains stable at 243.1±19.2 mA cm–2 with C1 Faradaic
efficiency of 99.0±0.5% for 40 h at –0.55 V vs. RHE. Density
functional theory calculations indicate that the in situ reconstructed
Sn/SnOx interface facilitates formic acid production by optimizing the
binding of the reaction intermediate HCOO* while promotes Faradaic
efficiency of C1 products by suppressing the competitive hydrogen
evolution reaction, resulting in high Faradaic efficiency, current
density and stability of CO2RR at low overpotentials.
only the kinetically sluggish CO2RR but also more favorable
proton reduction course, which is usually along with the
competitive hydrogen evolution reaction (HER).[5−7] Thus,
efficient catalysts with excellent activity and Faradaic efficiency
(FE) are indispensable for accelerating the CO2RR process in
order to reduce energy consumption.[8]
Current techno-economic evaluation manifests that the
yields of high-value C1 (formate and CO) products are more
economically practical.[9−15] Various non-noble metals (e.g. Sn, In,
Bi, Cd, Zn and Co) have been investigated as catalysts for
selectively reducing CO2 to C1 products.[5−7,16−32] Among these
materials, Sn is the most attractive catalyst owing to its high
activity and stability towards C1 production.[6,7,9,13,16−21] Besides,
Cu-based catalysts are also able to catalyze CO2RR to C1
products.[32−37] Nonetheless, controlling the surface structure of
these catalysts is a great challenge on account of their oxyphilic
properties in air. In addition, the electrical conductivity of most
metal oxides is poor, resulting in pure metal oxides with
undesirable activity in CO2RR.[7] Hence, pure metal oxides
usually can’t be used as efficient catalysts for CO2RR.
There is a feasible remedy to solve the above problems by
preparing core-shell structured catalysts with a highly conductive
metal core and thin metal oxide shell. Jiao et al. prepared core-
shell Ag-Sn catalysts with surface-oxidized Sn shell, which can
efficiently catalyze CO2RR to formate with high Faradaic
efficiency of 80% and desirable current density (j=16 mA cm–2).[7]
Li et al. synthesized Cu-SnO2 catalysts with a core-shell
structure, which showed a favorable catalytic performance for
CO production in an H-type cell.[17] In order to overcome the
limitation of CO2 solubility in aqueous solutions using H-type
cells, Ajayan et al. fabricated SnO2 nanoparticles gas diffusion
electrode and used 1 M KOH as electrolyte in a flow cell to
further enhance the CO2RR current density to 147 mAꢀcm−2 with
C1 Faradaic efficiency of 90% staying only for 0.5 h at −0.95 V
versus reversible hydrogen electrode (RHE).[38] Obviously, the
CO2RR current density, C1 Faradaic efficiency and stability are
also insufficient even by using flow cell reactors. The low CO2RR
activity of these materials may be ascribed to the homogeneous
metal core with desired stoichiometric ratio, which makes the
outer shell steady. The catalytic shell structure can’t be in situ
reconstructed during CO2RR to provide a highly efficient
interface for boosting CO2RR performance.
Introduction
Electrochemical conversion of carbon dioxide (CO2) into value-
added fuels and chemicals utilizing renewable electricity is a
promising strategy for alleviating atmospherical CO2
emissions.[1−4] However, electrochemical CO2 reduction reaction
(CO2RR) usually requires high energy penalty due to the
thermodynamic stability of CO2 molecule. The barriers root in not
[*]
Dr. K. Ye,[+] Z. Zhou,[+] J. Shao,[+] L. Lin, Dr. D. Gao, Dr. N. Ta, Prof.
Dr. G. Wang, Prof. Dr. X. Bao
State Key Laboratory of Catalysis
Dalian National Laboratory for Clean Energy
Dalian Institute of Chemical Physics
Chinese Academy of Sciences
Zhongshan Road 457,Dalian 116023 (China)
E-mail: wanggx@dicp.ac.cn
Prof. Dr. K. Ye,[+] J. Shao[+]
Key Laboratory of Superlight Materials and Surface Technology of
Ministry of Education
College of Materials Science and Chemical Engineering
Harbin Engineering University
Nantong Street 145, Harbin 150001 (China)
Z. Zhou,[+] L. Lin
For practical application of CO2RR, it is imperative to further
design Sn-based catalysts so that the CO2RR course is able to
proceed with high current density, C1 Faradaic efficiency and
stability at low overpotentials. Herein, we present a novel Sn-Cu
catalyst with hierarchically heterogeneous Sn-Cu alloy/Sn
metallic core responsible for high electrical conductivity and
amorphous thin SnOx shell for catalyzing CO2RR. Under the
cathodic potentials of CO2RR, the SnOx shell can be in situ
reconstructed due to that the hierarchical Sn-Cu alloy/Sn core
University of Chinese Academy of Sciences
Beijing 100039 (China)
Prof. Dr. R. Si
Shanghai Synchrotron Radiation Facility
Zhangjiang Laboratory
Shanghai 201204 (China)
[+]
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.201916538
Angewandte Chemie International Edition
RESEARCH ARTICLE
can provide sufficient Sn sources for Sn diffusion into the shell
and trigger the shell reconstruction, which is proved by in situ
and ex situ experiments. The surface structure and composition
of catalytic shell can be accurately tuned by the devisable bulk
composition. The in situ reconstructed Sn2.7Cu catalyst exhibits
the highest performance of CO2RR among the Sn-Cu catalysts.
It achieves a high current density of 406.7±14.4 mA cm–2 with C1
Faradaic efficiency of 98.0±0.9% at –0.70 V vs. RHE, and
remains stable at 243.1±19.2 mA cm–2 with C1 Faradaic
Figure 1c discloses that the outer amorphous layer is composed
of Sn and O species to form the SnOx shell, while the inner layer
is constituted by Sn-Cu alloy, Sn and Sn-Cu alloy species to
form the hierarchically heterogeneous Sn-Cu alloy/Sn core,
which can also be observed in its HRTEM image (Figure 1b).
Although the synthesis of amorphous SnOx shell has been
occasionally reported,[7,15,39] the as-prepared Sn2.7Cu catalyst
obviously differs from previous literatures owing to its unique
heterogeneous Sn-Cu alloy/Sn core with hierarchical
characteristics, which might exhibit a suitable precursor for the in
situ structural reconstruction in the shell under CO2RR
conditions. Additional TEM, HRTEM, ADF-STEM and EDX
elemental maps explorations for other compositions (Sn, SnCu1.1
and SnCu4.0 catalysts) are shown in Figures S5-S7. In brief, the
pure Sn catalyst surface contains some SnO2 crystals that
caused by spontaneous oxidation of Sn in air (Figure S5), while
the SnCu1.1 catalyst owns a core-shell structure, in which the
core is Sn-Cu alloy and the 2.0 nm-thick amorphous shell is
SnOx (Figure S6). Furthermore, an excess CuSO4 concentration
ratio in the electrodeposition solution makes the redundant Cu
species grow on the surface of SnCu4.0 catalyst (Figure S7). X-
ray photoelectron spectroscopy (XPS), X-ray absorption near
edge structure (XANES) and extended X-ray absorption fine
structure (EXAFS) measurements reveal that Sn species are
easily oxidized to SnOx in air on the catalysts surface and Sn
atoms are locally enriched in the core of Sn2.7Cu catalyst (Figure
2, Tables S1 and S2).
CO2RR performance of the Sn and Sn-Cu catalysts on Cu
foam was explored in an H-type cell. A volcano-type relationship
of formate Faradaic efficiency on the catalyst composition is
observed at various potentials. Sn2.7Cu catalyst shows the
highest formate Faradaic efficiency of 83.0±1.7% at potentials
higher than −0.93 V vs. RHE (Figure 3a). On the contrary,
SnCu4.0 catalyst produces the largest amount of CO among the
four catalysts in the measured potential window, due to Cu
species exposed on the surface (Figure S7).[32−34] To explore the
intrinsic property of the as-prepared catalysts, electrochemically
active surface area (ECSA) was measured.[9,31] The geometrical
partial current density of formate (Figure S8) is corrected by their
ECSA (Figure S9) and the ECSA-corrected current density of
formate is shown in Figure 3b. Distinctly, there is a volcano-type
correlation between ECSA-corrected current density of formate
and catalyst composition among the four catalysts at various
potentials (Figure 3b). Besides, Tafel plots also reveal that the
Tafel slope for formate production on Sn2.7Cu catalyst presents
the minimum value (Figure S10). Take the Faradaic efficiency
and intrinsic activity into consideration, the Sn2.7Cu catalyst
shows the optimal CO2RR performance. During the CO2RR for
32 h, the Sn2.7Cu catalyst reaches a steady geometrical total
current density with C1 Faradaic efficiency of ~ 96.0% at –0.93 V
vs. RHE (Figure S11).
efficiency of 99.0±0.5% for 40
h at –0.55 V vs. RHE,
outperforming the previous Sn-based catalysts. Density
functional theory (DFT) calculations indicate that the in situ
reconstructed Sn/SnOx interface facilitates formic acid
production by optimizing the binding of the reaction intermediate
HCOO* while promotes Faradaic efficiency of C1 products by
suppressing the competitive HER, resulting in high Faradaic
efficiency, current density and stability of CO2RR at low
overpotentials.
Results and Discussion
Three-dimensional (3D) hierarchical Sn-Cu catalysts
supported on Cu foam were fabricated by a facile one-step
electrodeposition technique using hydrogen bubble as
a
dynamic template (Scheme 1 and Figure S1). In order to design
the structure and composition of electrodeposited Sn-Cu
catalysts, we rationally tuned the molar ratios of SnSO4/CuSO4
in the electrodeposition solutions. The composition of as-
prepared Sn-Cu catalysts was confirmed by scanning electron
microscopy (SEM) equipped with energy-dispersive X-ray (EDX)
analysis. Briefly, we labeled the catalysts as Sn, Sn2.7Cu,
SnCu1.1 and SnCu4.0 according to their compositions in atomic
ratio.
The Sn2.7Cu catalyst is verified to be a hybrid structure that
consists of Sn-Cu alloy accompanied by enriched Sn from its X-
ray diffraction (XRD) analysis (Figure S2). SEM images show
that the ear of wheat-like Sn2.7Cu catalyst uniformly distributes
on Cu foam surface (Figures 1a and S3b). Meanwhile, surface
morphology and composition of Sn-Cu catalysts can be simply
controlled from pine needle-like to dendritic structure through
decreasing the molar ratios of SnSO4/CuSO4 in the
electrodeposition solutions (Figure S3). Transmission electron
microscopy (TEM) images further identify that the Sn2.7Cu
catalyst has an ear of wheat-like structure, which additionally
owns a defined core-shell construction (Figure S4a and b). High-
resolution TEM (HRTEM) image shows that the 3.2 nm-thick
shell is amorphous, while the core emerges to be crystalline with
hierarchical crystals (Figure 1b). The core of Sn2.7Cu catalyst
displays two types of evident lattice fringes with d spacing of
0.30 nm and 0.29 nm measured by their neighbouring lattice
planes, in accordance with the interplanar spacing of Cu6Sn5
(22Ī) and Sn (200) planes, respectively, indicating that the Sn-
Cu alloy and metallic Sn species coexist in the inner core. In
order to further investigate the detailed core-shell structure of
Sn2.7Cu catalyst, annular dark field-scanning TEM (ADF-STEM)
and corresponding energy-dispersive X-ray (EDX) elemental
maps measurements were carried out (Figures 1c and S4c).
To survey the detailed structure of SnOx on the Sn2.7Cu
surface during CO2RR, we carried out in situ EXAFS tests. The
in situ electrochemical cell was tailored and illustrated in Figure
3c. All the electrochemical parameters were conducted the
same as those in the H-type cell. The in situ EXAFS spectra
(Figure 3d) suggest a peak in the range of 1.5~2.0 Å, which
indexes to the Sn-O bonding[9] that appears in both Sn and
Sn2.7Cu catalysts at open circuit potential (OCP). After applying
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10.1002/anie.201916538
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RESEARCH ARTICLE
a potential of –0.93 V vs. RHE, the Sn-O peak almost
disappears in the pure Sn catalyst, suggesting that the
crystalline SnO2 on Sn catalyst surface is highly reduced under
the actual CO2RR condition. It is worth noting that the scattering
intensity of Sn-O peak weakens but preserves strength to some
extent at –0.93 V vs. RHE, indicative of partial reduction of the
SnOx shell in the Sn2.7Cu catalyst during CO2RR. The ex situ
XPS analysis further confirms that the surface SnOx composition
of Sn2.7Cu catalyst distinctly lessens after the CO2RR stability
test (Figure 3e and Table S3).
Faradaic efficiency and reducing the activation energy of
CO2RR.[5,38,44] In the meantime, previous literatures and standard
Pourbaix diagram (E–pH) propose that the Sn and SnOx phase
can be retained in alkaline media at the applied reduction
potentials.[38,44,46,47] As a result, 1 M KOH is used as both
catholyte and anolyte to evaluate the CO2RR performance in
flow cell. It is evident that the dependence of C1 (formate and
CO) Faradaic efficiency on the catalyst composition
demonstrates a volcano-type feature. The Sn2.7Cu catalyst
presents the highest C1 Faradaic efficiency and maintains
~99.0% over a wide potential window from –0.32 V to –0.70 V vs.
RHE (Figure 4b). Conversely, SnCu4.0 catalyst yields the highest
CO Faradaic efficiency among the four catalysts at the applied
potentials between –0.32 V and –0.60 V vs. RHE, due to the
excessive Cu species exposed on the surface of SnCu4.0
catalyst (Figures S2 and S7). Similarly, a distinct volcano-type
relationship can also be observed between the geometrical
current density and catalyst composition in the four catalysts at
each potential (Figure 4a and c). In view of the Faradaic
efficiency and current density, the Sn2.7Cu catalyst exhibits the
highest CO2RR performance. Notably, the high geometrical total
current density in flow cell achieves 406.7±14.4 mA cm–2 at –
0.70 V vs. RHE (Figure 4a) with a Faradaic efficiency of
98.0±0.9% for C1 products (Figure 4b).
The electrochemical stability of catalysts is also a key
parameter of CO2RR performance. Remarkably, the Sn2.7Cu
catalyst unveils a desirable stability in flow cell during the
CO2RR for 40 h (Figure 5c). The geometrical total current
density remains steady at 243.1±19.2 mA cm–2 and C1 Faradaic
efficiency keeps 99.0±0.5% at –0.55 V vs. RHE, representing
that the Sn2.7Cu catalyst is stable during CO2RR. After the
CO2RR stability test, ex situ HRTEM images of Sn2.7Cu catalyst
also show that the SnOx shell is in situ reconstructed (Figures 5d
and S19) and efficiently catalyzes CO2RR at low overpotentials.
However, the geometrical total current densities of both Sn
(Figure 5a) and SnCu1.1 (Figure 5e) catalysts obviously decay
lower than 50.0 mA cm–2 and their C1 Faradaic efficiency drop
below 89.0% even within 10 h, indicating that both Sn and
SnCu1.1 catalysts can’t provide a stable CO2RR performance. Ex
situ HRTEM images display that the Sn catalyst still has many
SnO2 crystals on its surface because of spontaneous oxidation in
air (Figure 5b and S18),[42,43] while the SnCu1.1 catalyst maintains
its core-shell structure with the homogeneous Sn-Cu alloy core
and a tiny shrinkage of amorphous SnOx shell from 2.0 nm
(Figure S6) to 1.8 nm (Figure 5f and S21) after CO2RR stability
test. Notably, the SnOx shell in Sn2.7Cu catalyst can be in situ
reconstructed owing to its hierarchically heterogeneous Sn-Cu
alloy/Sn core structure under the cathodic potentials of CO2RR
(Figure S20), but the in situ reconstructed Sn/SnOx interface
tends to be immediately oxidized to SnO2/SnOx after exposure to
air for ex situ tests (Figures 5d and S19).[42,43] Hence, the
presence of in situ reconstructed Sn/SnOx interface is
indispensable for boosting the CO2RR performance. More
importantly, by comparing the CO2RR performance of current
Sn-based catalysts, Sn2.7Cu catalyst obviously surpasses all of
them in terms of their current density, C1 Faradaic efficiency and
stability (Table S4).
The used Sn2.7Cu catalyst was characterized by HRTEM
(Figures 3f and S12). The ex situ HRTEM images show that the
thickness of the amorphous SnOx shell visibly shrinks from 3.2
nm (Figure 1b) to 2.7 nm (Figure 3f) but some sites change to
be thicker and present crystalline SnO2 species with the d
spacing of 0.33 nm[17,39] embedded in the amorphous SnOx shell.
The reason may be that different surface free energies are
between Sn (0.509 eV atom–1) and Cu (1.323 eV atom–1)
species.[40,41] Under the cathodic potential of CO2RR, the outer
amorphous SnOx shell is partially reduced (Figure 3d), and the
surface free energy balance of original core-shell structure
(Figure 1b) is destroyed. Therefore, with the purpose of
minimizing the surface free energies of Sn2.7Cu catalyst, the
lower surface free energies of Sn make the redundant Sn atoms
in the core region automatically migrate outwards into the SnOx
shell, constructing Sn/SnOx interface. However, once the used
catalysts are examined by ex situ HRTEM and contact with air,
the surface Sn species of catalysts will be instantly oxidized in
air and combine with the SnOx matrix to form SnO2/SnOx (Figure
3f).[7,20] The crystalline Sn are prone to form the SnO2 crystals
with a spontaneous oxidation in air, which is similar to the
surface oxidation of crystalline Sn in the pure Sn catalyst (Figure
S5) and also in accordance with previous literatures.[42,43]
Furthermore, ex situ XRD and SEM results suggest no evident
variation for the integral structure and morphology of Sn2.7Cu
catalyst after CO2RR stability measurement (Figures S13 and
S14), indicating that the Sn2.7Cu catalyst is stable during the
CO2RR process. Interestingly, the in situ reconstruction
phenomenon only exists in the Sn2.7Cu catalyst because of the
hierarchically heterogeneous Sn-Cu alloy/Sn core structure,
which triggers the dramatic improvement for CO2RR
performance (Figure 3a and b). TEM image of the used SnCu1.1
catalyst further verifies that the thickness of amorphous SnOx
shell shrinks from 2.0 nm (Figure S6) to 1.8 nm (Figure S15) and
the SnOx shell can’t be in situ reconstructed after CO2RR, which
may be caused by the steady Sn-Cu alloy core without
redundant Sn metal in the SnCu1.1 catalyst (Figures S2 and S6).
To further improve the CO2RR current density of the as-
prepared catalysts, we fabricated gas diffusion electrode (GDE)
with the electrodeposited Sn-Cu catalysts powder exfoliated
from their Cu foam substrates (Figure S16) and evaluated their
CO2RR performance utilizing a flow cell (Figure S17), in which
the CO2 gas and catholyte were fed to the two sides of GDE,
respectively and CO2 was directly electroreduced at their gas-
liquid-solid three-phase interfaces to significantly promote the
current density of CO2RR without the limitation of CO2
solubility.[38,39,44,45] Compared to the H-type cell, flow cell allows
the use of alkaline electrolyte, which has the merits of
suppressing the competitive HER, enhancing the product
DFT calculations were carried out to shed light on the role
of in situ reconstructed Sn/SnOx interface on CO2RR
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10.1002/anie.201916538
Angewandte Chemie International Edition
RESEARCH ARTICLE
performance. The facet of (211) was used for modeling metallic
Sn since the stepped facets has been reported to enable
generally lower limiting potential during the CO2RR process than
close-packed facets.[48] To represent the SnOx surface, we
adopted SnO2(110) surface which was reported to be the
thermodynamically most stable facet.[49,50] According to the
above experimental results, under CO2RR conditions, the SnOx
demonstrates reduced states. For describing the surface of
reduced SnOx, we consider removing bridge O atoms from
stoichiometric SnO2(110) surface obtaining the reduced
SnO2(110) surface, which has been observed by STM
measurements.[49,50] To confirm whether the reduced SnO2(110)
surface is preferred under CO2RR conditions, we calculated the
Gibbs free energy change for removing bridge O atoms with
proton-electron pair, which turns out to be thermodynamically
spontaneous process (Figure S22). The in situ reconstructed
Sn/SnOx interface under CO2RR conditions is modeled as the
ribbon of two Sn layers deposited on the reduced SnO2(110)
surface (Figure S23). To avoid the structural fluxionality due to
relatively small size of the ribbon model, only the Sn atoms at
one of the interface that directly interact with reaction
intermediates were fully relaxed during structural optimizations.
We calculated the Gibbs free energy diagram of CO2RR
process (Figure 6a) and the corresponding adsorption
configurations of reaction intermediates on Sn(211) (Figure
S24a-c), in situ reconstructed Sn/SnOx interface (Figure 6d-f)
and the reduced SnO2(110) surface (Figure S24e-g). The
formation of intermediate HCOO* is an exothermic process while
such process for *COOH is endothermic in reverse. Therefore,
for the path of generating formic acid, the limiting potential is
determined by the step of HCOO* hydrogenation while for
producing CO, the potential-limiting step is the formation of
*COOH. The participation of the in situ reconstructed Sn/SnOx
interface weakens the binding of intermediates HCOO* and
*COOH compared with those of corresponding intermediates on
Sn(211) and the reduced SnO2(110) surface (Figure 6a), which
contributes to the decreasing of limiting potential for producing
formic acid and the increasing of that for generating CO at the
interface. In addition, we calculated the Gibbs free energy
diagram for HER process (Figure 6b) and the corresponding
structures for the reaction intermediates (Figures 6g, S24d and
h), revealing that the in situ reconstructed Sn/SnOx interface
shows the superior inhibition of hydrogen evolution over either
the metal or oxide surface through weakening the binding of the
intermediate *H. Figure 6c shows the calculated limiting potential
of each product for three systems. For all three systems, the
limiting potential for formic acid production is the lowest among
those of the three products, indicating that formic acid is the
main product of CO2RR for systems of Sn-based catalysts. More
importantly, for the in situ reconstructed Sn/SnOx interface (blue
bars in Figure 6c), the limiting potential of producing formic acid
is further lowered while those of generating CO and H2 are
raised compared with Sn(211) and the reduced SnO2(110)
surface (pink and orange bars in Figure 6c). Therefore, DFT
calculations suggest that the selectivity of both formic acid and
C1 products is improved via the participation of the in situ
reconstructed Sn/SnOx interface.
Conclusion
In summary, we demonstrate a strategy to fabricate the
Sn2.7Cu catalyst with a hierarchical Sn-Cu alloy/Sn core and
amorphous SnOx shell by a facile one-step electrodeposition. In
situ EXAFS and ex situ structural examinations indicate that the
amorphous SnOx shell in Sn2.7Cu catalyst can be in situ
reconstructed to Sn/SnOx interface owing to its hierarchically
heterogeneous Sn-Cu alloy/Sn core structure under the CO2RR
condition. The Sn2.7Cu catalyst maintains C1 Faradaic efficiency
of ~99.0% over a wide potential range. It achieves a high
geometrical current density of 406.7±14.4 mA cm–2 with a
Faradaic efficiency of 98.0±0.9% for C1 products at –0.70 V vs.
RHE in flow cell, as well as a high stability of CO2RR. DFT
calculations suggest that in situ reconstructed Sn/SnOx interface
facilitates formic acid production by optimizing the binding of the
reaction intermediate HCOO* while promotes Faradaic efficiency
of C1 products by suppressing the competitive HER. These
results highlight the significant role of in situ reconstructed
Sn/SnOx interface on improving the CO2RR performance for
practical applications.
Acknowledgements
We gratefully acknowledge financial support from the National
Key R&D Program of China (Grant 2017YFA0700102), the
National Natural Science Foundation of China (Grants 21573222,
91545202, 21802124 and 91845103), Dalian National
Laboratory for Clean Energy (DNL180404), Dalian Institute of
Chemical Physics (Grant DICP DMTO201702), Dalian
Outstanding Young Scientist Foundation (Grant 2017RJ03) and
the Strategic Priority Research Program of the Chinese
Academy of Sciences (Grant XDB17020200). G.X. Wang thanks
the financial support from CAS Youth Innovation Promotion
(Grant 2015145). K. Ye thanks the financial support from the
China Postdoctoral Science Foundation (2018M630307 and
2019T120220).
Conflict of interest
The authors declare no conflict of interests.
Keywords: in situ reconstruction • tin oxide • hierarchical tin-
copper • interface • electrochemical carbon dioxide reduction
reaction
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10.1002/anie.201916538
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RESEARCH ARTICLE
Scheme 1. Schematic diagram of synthetic route of the 3D hierarchical Sn-Cu catalysts on Cu foam.
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Figure 1. SEM image (a), HRTEM image (b), Annular dark field-STEM image (c) and corresponding EDX elemental maps image
(Red represents Sn L, Green represents Cu K, Blue represents O K and combination of three elements) for the Sn2.7Cu catalyst.
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Figure 2. XPS spectra of Sn 3d (a) and Cu 2p (b); Sn K-edge XANES spectra (c) and fitting of Sn K-edge EXAFS spectra (d) for the
Sn, Sn2.7Cu, SnCu1.1 and SnCu4.0 catalysts. Sn foil and SnO2 were measured as standards.
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Figure 3. CO2RR performance in H-type cell. Potential-dependent Faradaic efficiency of formate, CO and H2 (a) and potential
dependence of ECSA-corrected current density for formate (b) on the Sn, Sn2.7Cu, SnCu1.1 and SnCu4.0 catalysts in H-type cell,
respectively. Schematic diagram of in situ EXAFS liquid cell for CO2RR (c). In situ Sn K-edge EXAFS spectra of the Sn and Sn2.7Cu
catalysts operated at open circuit potential (OCP) and −0.93 V vs. RHE (d). Sn 3d XPS spectra of Sn2.7Cu catalyst before and after
CO2RR stability test (e). Ex situ HRTEM image of Sn2.7Cu catalyst after CO2RR stability test (f).
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Figure 4. CO2RR performance in flow cell. Geometrical total current density (a), potential-dependent Faradaic efficiency of formate,
CO and H2 (b) and geometrical partial current density of formate (c) on the Sn, Sn2.7Cu, SnCu1.1 and SnCu4.0 gas diffusion electrode
(GDE) in flow cell. The catalysts were exfoliated from their Cu foam substrates by ultrasonication and subsequently painted on the
microporous layer of carbon paper to prepare GDEs.
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Figure 5. CO2RR stability test in flow cell. Electrochemical stability measurement at –0.55 V vs. RHE (Black represents current
density, Red symbols represent Faradaic efficiency (FE)) on the Sn (a), Sn2.7Cu (c) and SnCu1.1 (e) GDE in flow cell. Ex situ HRTEM
images of Sn (b), Sn2.7Cu (d) and SnCu1.1 (f) GDE after CO2RR stability test.
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10.1002/anie.201916538
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Figure 6. Computational results of CO2RR and HER. Calculated Gibbs free energy diagrams for (a) CO2RR and (b) HER paths
occurred on Sn(211), in situ reconstructed Sn/SnOx interface and the reduced SnO2(110) surface. (c) Calculated limiting potentials for
HCOOH, CO and H2 production on Sn(211), in situ reconstructed Sn/SnOx interface and the reduced SnO2(110). (d-g) Optimized
adsorption configurations for reaction intermediates at the interface. * indicates the adsorption sites.
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Entry for the Table of Contents
RESEARCH ARTICLE
Ke Ye,+ Zhiwen Zhou,+ Jiaqi Shao,+
Long Lin, Dunfeng Gao, Na Ta, Rui
Si, Guoxiong Wang,* and Xinhe Bao
In situ reconstructed Sn/SnOx
interface facilitates formic acid
production by optimizing the
binding of the reaction intermediate
HCOO* while promotes Faradaic
efficiency of C1 products by
Page No. – Page No.
In
Situ
Reconstruction
of
suppressing
hydrogen
resulting
the
evolution
competitive
reaction,
Hierarchical
Sn-Cu/SnOx
Core/Shell Catalyst for High-
Performance CO2 Electroreduction
in
high
Faradaic
efficiency, current density and
stability of electrochemical CO2
reduction
reaction
at
low
overpotentials.
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