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Electrocatalysis Very Important Paper
Angewandte
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International Edition: DOI: 10.1002/anie.201612617
German Edition:
DOI: 10.1002/ange.201612617
Understanding of Strain Effects in the Electrochemical Reduction of
CO2 : Using Pd Nanostructures as an Ideal Platform
Hongwen Huang+, Huanhuan Jia+, Zhao Liu+, Pengfei Gao, Jiangtao Zhao, Zhenlin Luo,
Jinlong Yang,* and Jie Zeng*
Abstract: Tuning the surface strain of heterogeneous catalysts
represents a powerful strategy to engineer their catalytic
properties by altering the electronic structures. However,
a clear and systematic understanding of strain effect in
electrochemical reduction of carbon dioxide is still lacking,
which restricts the use of surface strain as a tool to optimize the
performance of electrocatalysts. Herein, we demonstrate the
strain effect in electrochemical reduction of CO2 by using Pd
octahedra and icosahedra with similar sizes as a well-defined
platform. The Pd icosahedra/C catalyst shows a maximum
Faradaic efficiency for CO production of 91.1 % at @0.8 V
versus reversible hydrogen electrode (vs. RHE), 1.7-fold higher
than the maximum Faradaic efficiency of Pd octahedra/C
catalyst at @0.7 V (vs. RHE). The combination of molecular
dynamic simulations and density functional theory calculations
reveals that the tensile strain on the surface of icosahedra
boosts the catalytic activity by shifting up the d-band center and
thus strengthening the adsorption of key intermediate COOH*.
This strain effect was further verified directly by the surface
valence-band photoemission spectra and electrochemical analysis.
The ever-increasing consumption of fossil fuels has led to
dramatically rising atmospheric CO2 levels, which is primarily
responsible for global warming.[1] Electrochemical reduction
of CO2 in aqueous solutions is a promising solution to this
climate issue by converting CO2 into value-added fuels and
chemical feedstocks in a sustainable fashion.[2] Moreover,
powered by solar and other sources of renewable electricity,
the electrochemical reduction of CO2 provides a strategy to
store these intermittent sources of energy into high-energy
chemicals.[3] To enhance the efficiency of energy conversion,
rational design of highly active and robust electrocatalysts to
strengthen the adsorption and activation of inert CO2 is
important, which can be boosted by the fundamental understanding of the correlations between structure and proper[*] Dr. H. Huang,[+] H. Jia,[+] Z. Liu,[+] P. Gao, J. Zhao, Prof. Z. Luo,
Prof. J. Yang, Prof. J. Zeng
Hefei National Laboratory for Physical Sciences at the Microscale,
Key Laboratory of Strongly-Coupled Quantum Matter Physics of
Chinese Academy of Sciences, National Synchrotron Radiation
Laboratory, Department of Chemical Physics
University of Science and Technology of China
Hefei, Anhui 230026 (P.R. China)
E-mail: [email protected]
[email protected]
[+] These authors contributed equally to this work.
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201612617.
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ties.[4] To this end, numerous studies have focused on the
understanding of the size, facet, and alloying effects in
electrochemical reduction of CO2.[5]
Surface strain, which is generally generated by the lattice
mismatch between different kinds of compositions and some
twin structures like icosahedra, is present extensively in
heterogeneous catalysts.[6] Engineering the surface strain
represents a powerful method to regulate the catalytic
properties of heterogeneous catalysts by modifying their
electronic structures in oxygen reduction, formic acid oxidation, aerobic oxidation, and other reactions.[7] As demonstrated by theoretical studies, the d-band center of Pt can be
shifted by circa 0.1 eV with only 1 % surface strain, which can
further appreciably alter the adsorption energies of reactive
intermediates.[8] It is thus believed that the surface strain can
also be used as an important knob to tune the catalytic
properties in electrochemical reduction of CO2. Cu overlayers
with different atomic-scale thickness on a Pt substrate as
electrocatalysts were recently studied for electrochemical
reduction of CO2, revealing a combination of strain effect and
electronic effect to control the activity and selectivity on Cu
surfaces.[9] The difficulty in distinguishing the strain effect
from the electronic effect impeded an unambiguous understanding of solely strain effect in electrochemical reduction of
CO2 in this case. To our best knowledge, a clear and
systematic understanding of strain effect in electrochemical
reduction of CO2 is still lacking at present, which restricts the
use of surface strain as a tool to optimize the performance of
electrocatalysts.
Herein, we design an ideal platform based on Pd
octahedra and icosahedra to explore the strain effect on the
activity in CO2 electrochemical reduction. The electrochemical measurements indicate that the Pd icosahedra/C catalyst
shows much a higher catalytic activity towards electrochemical reduction of CO2 with respect to the Pd octahedra/C
catalyst. The combination of molecular dynamics (MD)
simulations and density functional theory (DFT) calculations
reveals that the improvement in catalytic activity stems from
the tensile strain on the surface of Pd icosahedra, which shifts
up the d-band center and thus strengthens the adsorption of
key intermediate COOH*. Such strain effect was further
directly verified by the surface valence-band photoemission
spectra and electrochemical analysis.
To construct an ideal model, the Pd octahedra and
icosahedra with similar sizes were prepared based on
previously reported synthetic methods (see Supporting information for details).[10, 11] Figure 1 shows the structural characterizations of the produced Pd octahedra and icosahedra. The
representative transmission electron microscopy (TEM)
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larger lattice constant of Pd icosahedra. Taken together,
both the atomic-resolution HAADF-STEM and synchrotron radiation XRD analyses demonstrate the
presence of tensile strain on the icosahedron and the
negligible strain on the octahedron. These two types of
nanocrystals, which show an identical exposed facet and
similar sizes, except for the different surface strains,
could serve as an ideal platform to study the strain
effect in electrochemical reduction of CO2.
To evaluate the catalytic properties of Pd octahedra
and icosahedra towards electrochemical reduction of
CO2, both were uniformly dispersed onto separate
Vulcan XC-72 carbon supports with a Pd loading
content of 20 % (Supporting Information, Figure S3).
Controlled potential electrolysis of CO2 was then
Figure 1. A) TEM and B) atomic-resolution HAADF-STEM images of the Pd
performed at different applied potentials between
octahedra. C) Intensity profile recorded from the area indicated by the
@0.6 and @1.0 V versus reversible hydrogen electrode
rectangular box in panel (B). D) TEM and E) atomic-resolution HAADF-STEM
(vs. RHE) in a CO2-saturated 0.1m KHCO3 solution
images of the Pd icosahedra. (F) Intensity profile recorded from the area
(pH 6.8) at room temperature under atmospheric
indicated by the rectangular box in panel (E). The insets in panel (B) and (E)
show the corresponding models of Pd nanostructures.
pressure. Under these reaction conditions, only CO
and H2 were detected by online micro gas chromatography (GC). Figure 2 A shows Faradaic efficiencies
images in Figure 1 A and D indicate the successful prepara(FEs) for the formation of CO on Pd octahedra/C and Pd
tion of Pd octahedra and icosahedra in high purity and
icosahedra/C catalysts at different reduction potentials.
uniformity. The average sizes of Pd octahedra and icosahedra
Clearly, the Pd icosahedra/C presented much higher FEs at
were determined to be of 19.8 : 3.7 and 19.4 : 2.8 nm,
various reduction potentials compared with the Pd octahedra/
respectively, by counting more than 100 particles of each
C, suggesting that the intrinsic activity of Pd icosahedra/C is
shape (Supporting Information, Figure S1). Because the key
much higher than that of Pd octahedra/C.[14] Specifically, the
difference in the atomic-scale structure between octahedron
Pd icosahedra/C showed a maximum FE of 91.1 % at @0.8 V
and icosahedron is the interatomic distance, the atomic(vs. RHE), 1.7-fold higher than the maximum FE at @0.7 V
resolution high-angle annular dark-field scanning TEM
(vs. RHE) of Pd octahedra/C. Consistently, the Pd icosahedra/
(HAADF-STEM) images of a single octahedron and icosaC exhibited larger CO partial current densities at various
hedron were thus analyzed to obtain the related information,
reduction potentials (Figure 2 B). The mass activities of the
as shown in Figure 1 B and E. As both the octahedron and
catalysts at various reduction potentials followed a similar
icosahedron are enclosed by {111} facets, we compared their
interplanar spacing of {111} planes. To reduce the measurement error, the total distance of 10 groups of successive {111}
planes was measured and then divided by 10 to obtain the
interplanar spacing of the {111} planes. The interplanar
spacing of {111} planes for the octahedron was 2.24 c
(Figure 1 C), close to the value of 2.25 c for single-crystalline
bulk. For comparison, the interplanar spacing of {111} planes
at the center of a face for the icosahedron was determined to
be 2.32 c (Figure 1 F), slightly larger than that of octahedron.
The result agrees well with the previously reported values,
proving the existence of tensile strain on the icosahedron.[12]
Synchrotron radiation X-ray diffraction (XRD) was also
employed to distinguish structural differences between Pd
octahedra and icosahedra. As shown in Figure S2, the
diffraction pattern of the octahedra presented three symmetric diffraction peaks, corresponding well to {111}, {200}, and
{220} planes of face-centered-cubic (fcc) Pd. However, the
diffraction pattern of icosahedra showed obviously different
features in terms of peak number and peak position.
Specifically, the diffraction peak of {111} planes around
Figure 2. Reduction potential dependence on A) FEs, B) current densi17.488 split into at least three peaks, which can be ascribed to
ties for CO production, and C) mass activities over Pd octahedra/C
[13]
the inhomogeneous strain distributed on the icosahedron.
and Pd icosahedra/C catalysts. D) Long-term durability tests of Pd
Moreover, all the main peaks shifted toward lower angles
octahedra/C and Pd icosahedra/C catalysts at the reduction potential
with respect to those from the octahedra, suggesting the
of @0.9 V (vs. RHE).
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trend with their CO partial current densities, further confirming the higher CO2-reduction activity of Pd icosahedra/C
catalyst (Figure 2 C). The long-term durability, another crucial criterion to evaluate a catalyst, of the catalysts was also
measured at a constant potential of @0.9 V (vs. RHE) in
a CO2-saturated 0.1m KHCO3 solution, as shown in Figure 2 D. Both the Pd icosahedra/C and Pd octahedra/C
showed negligible variations in the current densities, as well
as their FEs for the formation of CO over 10 h, suggesting
their remarkable stability. The above experimental results
clearly indicate that the CO2-reduction activity of Pd icosahedra/C is much higher than that of Pd octahedra/C.
To figure out the reason for the improved activity, we
examined all possible factors that may influence the activity in
electrochemical reduction of CO2. The size effect can be
safely excluded for this enhancement in activity because the
fabricated Pd octahedra and icosahedra have the similar sizes.
Considering that both structures are bounded by {111} facets
and capped by PVP, the influences of facet and capping agent
can also be ruled out. We thus turned our attention to the
different coordination numbers (CN) of edge atoms and
different surface strains in Pd octahedra and icosahedra.
Based on early research, the electrochemical conversion of
CO2 to CO typically includes the following elementary steps
[Equations (1)–(3)]:[15]
CO2 ðgÞ þ Hþ ðaqÞ þ e@ þ * ! COOH*
ð1Þ
COOH* þ Hþ ðaqÞ þ e@ ! CO* þ H2 OðlÞ
ð2Þ
CO* ! COðgÞ þ *
ð3Þ
where * refers to a catalytic site at which a species can adsorb.
The theoretical studies have indicated that step (1), generally
inhibited by weak COOH binding, serves as the rate-limiting
step.[16] Thus, it is rational to improve the catalytic activity of
the catalyst by strengthening the adsorption of CO2 to some
extent. Generally, the CN of atoms on the edge of octahedron
and icosahedron is 7 and 8, respectively. If the reaction is
mainly controlled by edge atoms, a higher activity of Pd
octahedra/C would be expected due to the stronger adsorption strength on the edge atoms of Pd octahedra associated
with their lower CN number.[3] However, this conjecture is
completely inconsistent with our experimental results. Therefore, the difference in surface strain should be the dominant
factor responsible for the enhancement in CO2-reduction
activity of Pd icosahedra/C.
To gain an intrinsic understanding of the surface strain on
CO2-reduction activity, we performed a series of theoretical
calculations. We first used MD simulations to obtain the strain
fields on the surfaces of Pd octahedra and icosahedra. To
more accurately reveal the strain fields on the surfaces of real
Pd octahedra and icosahedra, we constructed the atomic
models of Pd octahedron and icosahedron with similar sizes to
our real nanoparticles. As shown in Figure 3 A, the surface of
an icosahedron exhibits a distinct tensile strain (averaging
+ 1.8 %), which is consistent with the trend we observed
based on the HAADF-STEM images and XRD patterns. By
contrast, a slightly compressive strain on the surface of an
octahedron (averaging @0.5 %) was observed. Subsequently,
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Figure 3. A) Surface strain fields of a Pd octahedron and icosahedron.
Color indicates strain labeled in the color map. The surface strain is
calculated based on the equilibrium bond length in bulk Pd. B) Projected d-density of states (PDOS) of surface atoms on Pd (111)
surfaces with different surface strains. The calculated d-band centers
are marked with white lines.
the electronic band structures of Pd (111) planes with
different strains were calculated by DFT calculations. As
shown in Figure 3 B, the d-band centers (with regard to Fermi
level) of Pd (111) planes under different surface strains of
@0.5 %, 0 %, and 1.8 % were presented, exhibiting the upward
shift of d-band center from @1.48 to @1.40 eV with the surface
strain shifting from compression to tension. Such a trend was
further confirmed by the surface valence band photoemission
spectra collected from Pd octahedra and icosahedra (Supporting Information, Figure S4). It is well-established that the
upward shift of d-band center pushes more of the antibonding states above the Fermi level, resulting in the
decreasing occupation and stronger adsorbate bonding.[17]
Accordingly, we can conclude that the tensile strain on the
surface of an icosahedron shifts up the d-band center of
surface atoms, thereby strengthening the adsorption of CO2
relative to that on an octahedron.
To investigate the variation in adsorption of intermediates
induced by surface strain, the Gibbs free energy diagrams for
CO2 reduction into CO at 0 V (vs. RHE) on Pd (111) under
different surface strains were further achieved by using the
methodology proposed by Nørskov and co-workers.[18] As
shown in Figure 4 and Figure S5 in the Supporting Information, the formation of COOH* on Pd (111) is associated with
an increase in free energy of 0.15 eV under a tensile stain of
+ 1.8 % and 0.19 eV under a compressive strain of @0.5 %,
suggesting the formation of key intermediate COOH* is
easier on a tensile surface. This result is in accordance with the
improved adsorption of CO2 on the tensile surface of an
icosahedron due to the upward shift of d-band center. The
changes of Gibbs free energy for the formation of CO*,
another reactive intermediate, are analogous to that of
COOH* because the conversion of COOH* to CO* is
typically facile.[3] As the formation of COOH* is a dominant
electrochemical step for CO2 reduction to CO, the higher
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believe that our work not only provides an in-depth understanding of the strain effect but also offers an effective knob
to tune the catalytic properties for electrochemical reduction
of CO2.
Acknowledgements
Figure 4. Free energy diagrams for CO2 reduction to CO on Pd (111)
under different surface strains.
CO2-reduction activity of Pd icosahedra can be extracted
from the calculated Gibbs free energy diagrams, which is well
consistent with our experimental results. Taken together, the
enhancement in CO2-reduction activity for Pd icosahedra can
be attributed to their tensile strain, which can shift up the dband center and further strengthen the adsorption of key
intermediate COOH*.
Electrochemical analysis was also performed to directly
verify the increased adsorption energies of reactive intermediates. As the adsorption of CO2, a rate-determining step,
can be estimated by using the adsorption of OH@ as
a surrogate, we thus studied cyclic voltammograms of Pd
octahedra and icosahedra at 20 mV s@1 in N2-saturated 0.1m
HClO4 (Supporting Information, Figure S6).[19, 20] For the
hydroxide peaks, the Pd icosahedra show a negative shift in
the position of the reduction peak of Pd(OH)2 in the cathodic
scan relative to Pd octahedra, indicating a stronger adsorption
of CO2 on the icosahedra.[21] Another experimental evidence
for the increased adsorption energy on icosahedra derives
from the electrochemical CO stripping voltammetry measurements (Supporting Information, Figure S7).[3] A broad CO
stripping profile with a dominant peak around 0.87 V (vs.
RHE) was observed on Pd octahedra, compared with a sharp
peak around 0.89 V (vs. RHE) with a shoulder peak around
0.91 V (vs. RHE) on Pd icosahedra. The positive shift of the
peak potential and the increased current density from Pd
octahedra to icosahedra suggest the strengthened adsorption
of CO on Pd icosahedra. Both of these electrochemical
analyses confirmed the stronger adsorption on Pd icosahedra,
experimentally supporting the strain effect proposed by the
DFT calculations.
In summary, we have demonstrated the surface-strain
dependence of catalytic activity in electrochemical reduction
of CO2 by using Pd octahedra and icosahedra with similar
sizes as a well-defined platform. The Pd icosahedra/C catalyst
show a maximum FE for CO production of 91.1 % at @0.8 V
(vs. RHE), 1.7-fold higher than the maximum FE at @0.7 V
(vs. RHE) of Pd octahedra/C catalyst. The combination of
MD simulations and DFT calculations indicates that the
tensile strain on the surface of icosahedra boosts the catalytic
activity by shifting up the d-band center and thus strengthening the adsorption of key intermediates COOH*. This strain
effect was further verified by the surface valence band
photoemission spectra and electrochemical analysis. We
Angew. Chem. Int. Ed. 2017, 56, 3594 –3598
This work was supported by the Collaborative Innovation
Center of Suzhou Nano Science and Technology, MOST of
China (2014CB932700), the China Postdoctoral Science
Foundation (2015M580536 and 2016T90569), NSFC
(21603208, 21573206, 21233007, 51371164, 11374010, and
11434009), Key Research Program of Frontier Sciences of
the CAS (QYZDB-SSW-SLH017), Strategic Priority
Research Program B of the CAS (XDB01020000), and
Fundamental Research Funds for the Central Universities.
The authors thank the Super-computing Center of USTC for
the use of computational resources. The authors also thank
the staff at beamlines BL14B and BL19U of SSRF for their
support.
Conflict of interest
The authors declare no conflict of interest.
Keywords: adsorption energies · CO2 reduction ·
electrocatalysis · palladium · strain effects
How to cite: Angew. Chem. Int. Ed. 2017, 56, 3594 – 3598
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Manuscript received: December 29, 2016
Final Article published: February 20, 2017
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Angew. Chem. Int. Ed. 2017, 56, 3594 –3598