Ternary Sn-Ti-O Electrocatalyst Boosts the Stability and Energy Efficiency of CO2 Reduction

Article abstract of DOI:10.1002/anie.202004149

Simultaneously improving energy efficiency (EE) and material stability in electrochemical CO₂ conversion remains an unsolved challenge. Among a series of ternary Sn-Ti-O electrocatalysts, 3D ordered mesoporous (3DOM) Sn_0.3Ti_0.7O₂ achieves a trade-off between active-site exposure and structural stability, demonstrating up to 71.5 percent half-cell EE over 200 hours, and a 94.5 percent Faradaic efficiency for CO at an overpotential as low as 430 mV. DFT and X-ray absorption fine structure analyses reveal an electron density reconfiguration in the Sn-Ti-O system. A downshift of the orbital band center of Sn and a charge depletion of Ti collectively facilitate the dissociative adsorption of the desired intermediate COOH* for CO formation. It is also beneficial in maintaining a local alkaline environment to suppress H₂ and formate formation, and in stabilizing oxygen atoms to prolong durability. These findings provide a new strategy in materials design for efficient CO₂ conversion and beyond.

Full text of DOI:10.1002/anie.202004149

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
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Accepted Article
Title: Ternary Sn-Ti-O Electrocatalyst Boosts the Stability and Energy
Efficiency of CO2 Reduction
Authors: Zhongwei Chen, Guobin Wen, Bohua Ren, Moon G. Park, Jie
Yang, Haozhen Dou, Zhen Zhang, Yaping Deng, Zhengyu
Bai, Lin Yang, Jeff Gostick, Gianluigi A. Botton, and Yongfeng
Hu
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10.1002/anie.202004149
Angewandte Chemie International Edition
RESEARCH ARTICLE
Ternary Sn-Ti-O Electrocatalyst Boosts the Stability and Energy
Efficiency of CO2 Reduction
Guobin Wen+, Bohua Ren+, Moon G. Park, Jie Yang, Haozhen Dou, Zhen Zhang, Ya-Ping Deng,
Zhengyu Bai*, Lin Yang, Jeff Gostick, Gianluigi A. Botton, Yongfeng Hu, and Zhongwei Chen*
[*]
Dr. G. Wen,[+] Prof. Z. Bai, Prof. L. Yang
School of Chemistry and Chemical Engineering
Key Laboratory of Green Chemical Media and Reactions, Ministry of Education, Henan Normal University
Xinxiang 453007 (China)
E-mail: baizhengyu@htu.cn
Dr. G. Wen,[+] Dr. B. Ren,[+] Dr. M. Park, Dr. H. Dou, Dr. Z. Zhang, Dr. Y. Deng, Prof. J. Gostick, Prof. Z. Chen
Department of Chemical Engineering
Waterloo Institute for Nanotechnology, Waterloo Institute for Sustainable Energy, University of Waterloo
Waterloo, Ontario N2L 3G1 (Canada)
E-mail: zhwchen@uwaterloo.ca
Dr. J. Yang, Prof. G. Botton
Department of Materials Science and Engineering and Canadian Centre for Electron Microscopy
McMaster University
1280 Main Street West, Hamilton, Ontario L8S 4M1 (Canada)
Prof. G. Botton, Prof. Y. Hu
Canadian Light Source
University of Saskatchewan
Saskatoon, Saskatchewan S7N 0X4 (Canada)
[+]
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of the document.
Abstract: Simultaneously improving energy efficiency (EE) and
binary-metal hybrid effect,[5] tuning oxidation states,[6] and
synthesizing single-atom catalysts.[7] However, there are still
technical challenges that hinder the commercial scale up of
CO2RR processes, such as limited EE (generally lower than 60%),
shorter operating lifetime (generally lower than 100 hours), and
difficult separation of end products.
Tin (Sn) and partially oxidized SnOx based materials are one
of the most promising non-precious materials for practical CO2RR
systems,[8] being able to convert CO2 to both CO and formic acid
(Table S1).[5a, 6b] Noticeably, oxygen (O) atoms in SnOx play an
important role in the adsorption of intermediates.[9] Nevertheless,
the energy efficiency (EE) of CO2RR is typically limited by a low
selectivity of desired product, a large overpotential (> 0.8 V), and
material stability in electrochemical CO2 conversion remains an
unsolved challenge. Among
a
series of ternary Sn-Ti-O
electrocatalysts, 3D ordered mesoporous (3DOM) Sn0.3Ti0.7O2
achieves a trade-off between active sites exposure and structural
stability, demonstrating up to 71.5% half-cell EE over 200 hours and
94.5% Faradaic efficiency for CO at an overpotential as low as 430
mV. Density functional theory (DFT) and X-ray absorption fine
structure analyses reveal an electron density reconfiguration in the
Sn-Ti-O system. A downshift of the orbital band centre of Sn and a
charge depletion of Ti collectively facilitate the dissociative adsorption
of desired intermediate COOH* for CO formation. It is also beneficial
in maintaining a local alkaline environment to suppress H2 and
formate formation and in stabilizing oxygen atoms to prolong durability. a narrow potential window.[10] Additionally, the introduction of O
These findings provide a new strategy of material design for efficient
CO2 conversion and beyond.
atoms also causes catalysts to become unstable due to the
competition between metallic oxide reduction and CO2RR under
highly cathodic conditions.[11]
Here, we propose to decorate Sn with titanium (Ti) and
construct a 3D ordered mesoporous (3DOM) structure to
overcome existing limitations of CO2RR. First, added Ti atoms
are revealed to preferably adsorb the oxygen atom in the COOH*
intermediate, tuning the selectivity toward CO instead of formate.
Second, Ti atoms are further unearthed to stabilize the lattice
oxygen atoms in SnOx and maintain the 3DOM structure,
prolonging catalyst lifetime. Third, the robust 3DOM structure
results in a local environment with higher alkalinity, not only
supressing the hydrogen evolution reaction (HER), but also
further hindering formate formation. Finally, the decorative Ti
atoms and the high local alkaline environment are experimentally
proven to reduce CO2 activation potential and expand the
potential window. Therefore, our work offers a new 3DOM ternary
Introduction
The conversion of CO2 to valuable chemicals via
electrochemical CO2 reduction reactions (CO2RR) offers a clean
and sustainable approach towards closing the carbon loop.[1] The
CO2RR process also allows for harvesting intermittent renewable
energy sources such as wind and solar.[2] Among a variety of
derived chemicals, carbon monoxide (CO) and formic acid are of
great interest due to their relatively lower activation potentials and
their large potential market (CO for synthetic gas and formic acid
for hydrogen carrier).[3] To seek efficient electrocatalytic materials,
many strategies have been explored extensively, including, but
not limited to, engineering morphology,[4] exploration of the
1
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RESEARCH ARTICLE
Sn-Ti-O electrocatalyst that can efficiently convert CO2 to CO at
high cathodic energetic efficiency (EEca) for extended time
periods, advancing Sn-based catalytic materials for scale up and
practical applications of CO2RR.
the breaking of C-O bond and weakening of Sn-C bond during
adsorption process - both benefit the dissociative adsorption of
COOH* intermediate, as illustrated in Figure 1d. These results
additionally reveal the change in the catalytic mechanism
associated with Ti substitution. As depicted in Figure 1a, the
intermediate COOH*, which is derived from CO3H* and deemed
as the main intermediate of CO formation, bonds on the SnO2-δ
surface (Model I) through a carbon atom (Figure 1a). Upon further
transfer of H+ from the electrolyte, COOH* dissociates into CO*
and OH* with a reaction energy of -2.96 eV (Table 2). However,
for the Sn-Ti-O system, COOH* dissociative adsorption takes
place on the Sn(Ti)O2-δ surface (Model II), leading to direct CO*
formation (Figure 1b). This transforms CO* formation from a two-
step reaction (CO3H* →COOH* → CO*) to a one-step reaction
(CO3H* → CO*), which facilitates CO* production. (Figure S1d
and e).
Results and Discussion
Density functional theory (DFT) analyses were performed to
examine the atomic interactions in the Sn-Ti-O metal−oxide
interface. SnOx is reported to have good activity for CO2RR.[3b, 12]
While TiO2 plays an important role to maintain the structure. The
samples with lower ratios of Ti (<0.7) cannot construct stable
3DOM structure and pure TiO2 sample is inactivity for CO2RR
(Electrochemical Performance session). Consequently, two
Models were considered in DFT simulation to consider the effect
of Ti on active SnOx site. Model I was generated (i.e. SnO2 with 6
oxygen vacancies, SnO2-δ)
[13] owing to the fact that the surface tin
oxide layer can be partially reduced under reaction conditions.[3b]
Model II was built as Model I with three Ti atoms substituting Sn
on the surface (i.e. Sn(Ti)O2-δ) in order to investigate the influence
of Ti on active SnO2-δ site. Model II in DFT analyses was used as
a representation of the local atomic structure of active sites.[5e, 13]
Two main pathways for CO and formic acid (HCOOH) production
were considered on two models (Figure S1a-c in the Supporting
Information).[7a, 11-12]
Table 1. Calculated average Bader charge (e) of surface Ti atoms and
neighboring Sn and O atoms in Model I and II.
Surface Models
Variation
(∆II – I
Atom[a]
)
Model II: Sn(Ti)O2-δ
Model I: SnO2-δ
O (av)
Sn (av)
Ti (av)
-1.063
+1.390
-
-1.061
+0.843
+1.745
+0.002
-0.547
-
Figure 1. DFT simulations. (a) COOH and OH species on the surface of Model
I before and after adsorption (Ads.). (b) COOH* dissociative adsorption on
Model II. (c) PDOS of s, p, and d orbitals of Sn and Ti atoms on Model I and
Model II. Dashed lines indicate the orbital band centres and Fermi energy level.
(d) Schematic of COOH* dissociative adsorption on the Sn-Ti surface. (e)
Approximate schematic of antibonding and bonding states between the reaction
surface and the adsorbate before (gray) and after (blue) Ti substitution. (f)
Schematic of HCOO* adsorption on the Sn-Ti surface.
[a] O (av) and Sn (av) entries indicate the average (av) charges of three oxygen
atoms and three Tin atoms, respectively, located at the nearest neighbouring
sites of Ti.
From Bader charge analysis (Table 1), we found that the
surface Sn atoms near Ti atoms possess a formal charge
variation of -0.547 e, while O atoms near Ti atoms retain its
valence state (+0.002 e variation). This infers that Ti tends to
delocalize charge by releasing it to the Sn atom (Figure 1d). The
projected density of states (PDOS) of the surface Sn and Ti atoms
were further analyzed in Figure 1c-e. The substitution of Ti atoms
causes the s, p, and d band centres of surface Sn atoms to
downshift by over 2.3 eV (Table S2).[14] The electron density
transfer from Ti to Sn has two important impacts:
(I) In favor of CO production (shortened reaction pathway).
The marked downshifts of the band centres indicate that the
antibonding states are filled with more electrons. The back-
donation antibonding states therefore decrease (Figure 1e).[15]
Specifically, the back-donation from the valence band of Sn to the
unoccupied orbital of COOH* is reduced,[15c] weakening the Sn-C
bond. On the other hand, Ti atoms are depleted of electron
density and further possess enhanced O affinity, seeking to bond
with the oxygen atom of COOH*.[16] The corresponding effects -
(II) Suppression of HCOOH production (higher reaction
energy): For Model I (pure SnO2-δ), HCOO* is adsorbed on the
surface by forming Sn-O bond. However, for Model II (Sn(Ti)O2-δ),
HCOO* tends to adsorb by forming Ti-O bond instead (Figure 1f).
The Ti atoms introduced in the pure SnO2-δ system interacts with
neighboring Sn atoms. As shown in Figure 1f, Sn atoms deplete
Ti atoms of electron density. Consequently, Ti atoms further
possess enhanced O affinity, which indicates that Ti-O bond is
stronger than Sn-O bond. This is also supported by the stronger
adsorption of HCOO* on Model II. As indicated by the definition
of adsorption energy (Eads) in the Supporting Information, a more
negative value of Eads means stronger adsorption. As shown is
Table S3, Eads of HCOO* on Model II is -3.11 eV, while Eads is -
2.20 eV on Model I. The much higher adsorption strength (more
negative) makes HCOO* intermediate overly stabilized, making it
2
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10.1002/anie.202004149
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more difficult to break the Ti-O bond and produce HCOOH*, as
shown in Figure S1b. In addition, the higher adsorption energy of
HCOOH* on Model II, as shown in Table S3, leads to unfavorable
desorption. The above reasons justify the higher reaction energy
for HCOOH* production in Model II than Model I, causing a
suppression of HCOOH* in Model II. Accordingly (Table S4), the
variation in reaction energies indicate that CO formation is more
spontaneous and formation of HCOOH is suppressed for Model
II.
line scan, chemical compositional maps and HRTEM of
Sn0.2Ti0.8O2 material which are provided in Figure S4.
Table 2. Calculated reaction energies (∆E) of Model I and II.
Surface Models
Reactions
Model I: SnO2-δ
Model II: Sn(Ti)O2-δ
COOH* + OH* + H+ + e− + *
 CO* + H2O* + OH*
-2.96 eV
-3.33 eV[a]
HCOO* + OH* + H+ + e−
 HCOOH* + OH*
-3.59 eV
-2.49 eV
[a] the reaction energy for CO* formation step is calculated by using
E(CO*+2OH*)-E(CO3H*+H+) due to dissociative adsorption of COOH* for Model
II (Figure S1).
As disclosed by DFT calculations, with appropriate
construction of ternary Sn-Ti-O catalysts, Ti atoms tend to
delocalize charge by releasing it to the Sn atoms. This may
explain why selectivity is shifted towards CO rather than formate.
In light of Sn-Ti interactions and previously reported superiority of
mesoporous structure,[17] we sought to produce 3DOM ternary
Sn-Ti-O compounds, which were prepared according to our
previous work.[18] A series of ternary Sn-Ti-O catalysts with
varying surface ratios of Sn-Ti were synthesized and
characterized: Sn0.1Ti0.9O2, Sn0.2Ti0.8O2, Sn0.3Ti0.7O2, Sn0.4Ti0.6O2,
Sn0.6Ti0.4O2, Sn0.8Ti0.2O2 (Table S5). Among these, Sn0.3Ti0.7O2
catalyst had the best selectivity for CO formation as discussed in
the later Electrochemical Performance section.
Figure 2. Material characterizations. (a) SEM images, (b) TEM images, and (c)
Line scan from EELS spectra on the edge of 3DOM-Sn0.3Ti0.7O2 material. (d)
Elemental mapping from EELS spectra of O (red), Ti (green) and Sn (blue)
atoms. (e) HRTEM image of Sn0.3Ti0.7O2 material. (1) 0.261nm, SnO2(101); (2)
0.330nm, SnO2 (110); (3) 0.167nm, anatase TiO2 (211); (4) 0.170 nm, anatase
TiO2 (105); (5-7) amorphous layer; Red dash line highlights the interface
between SnOx and TiO2. (f) XRD patterns of 3DOM-TiO2, Sn0.1Ti0.9O2,
Sn0.2Ti0.8O2, Sn0.3Ti0.7O2 and 3DOM-SnO2 electrodes. SnO2: JCPDS 41-1445;
TiO2 in anatase phase: JCPDS 21-1272; TiO2 in rutile phase: JCPDS 78-1510.
Hierarchically ordered and interconnected mesoporous
structure of synthesized Sn-Ti-O materials were found to feature
a type IV N2 adsorption-desorption isotherm,[18] as shown in
Figure S5. Particularly, Sn0.3Ti0.7O2 material showed Brunauer-
Emmet-Teller (BET) specific surface area of 39.9 m2 g−1 and a
less broad pore size distribution. This special structure appears to
achieve an ideal balance between exposure of active sites (more
Sn-Ti-O interface) and appropriate electron density shift to enable
Sn-Ti-O ternary synergistic effect and display optimum CO2RR
performance as depicted by electrochemical tests, to be
discussed later. The formation of such special structures, with
nanoparticles attached to the surface of 3DOM framework can be
ascribed to the selective surface migration, phase separation and
oxidation of the metals during calcination.[19]
Due to the nature of sample fabrication (co-precipitation and
calcination at 600 ᵒC), it is possible to thermodynamically observe
formation of Sn-Ti intermetallic as well as mixed-oxide phases as
indicated in the phase diagrams (Figure S6 a, b). The X-ray
diffraction (XRD) technique has deep penetration and provides a
glimpse of the material compositions. As depicted from XRD
patterns (Figure 2f, S7a), the special Sn0.3Ti0.7O2 composition
displayed a major SnO2 phase. However, as shown in Figure 2c-
e (Line scan from EELS spectra; Elemental mapping; HRTEM
images), there were still coexisting amorphous and anatase TiO2
The produced ternary Sn-Ti-O materials were characterized
using scanning electron microscopy (SEM) and observed an
evident honeycomb porous structure, as shown in Figure 2a and
Figure S2. The structure exhibited consistent pores with a
diameter of approximately 100 nm, (Figure 2b) providing a strong
structural support and high surface area with many available sites
for catalytic activity. Interestingly, only for certain Sn-Ti-O ratios
(Sn0.2Ti0.8O2 and Sn0.3Ti0.7O2, Figure S3 and S4), there were 5 nm
nanoparticles attached on the 3DOM framework (Scheme 1 and
Figure 2b inset).
Figure 2c depicts a compositional line scan on the edge of the
porous structure of 3DOM Sn0.3Ti0.7O2 material, showing the
composition of Sn, Ti and O elements. As further shown in Figure
2d and S3, the chemical compositional maps with Electron Energy
Loss Spectroscopy (EELS) were obtained over the region shown
within the green box. The results showed a 3DOM framework with
adherent nanoparticles composed of SnOx with dispersed
amorphous/crystallographic (anatase) TiO2 in Sn0.3Ti0.7O2
material (Scheme 1). High resolution TEM (HRTEM) was
performed on the 3DOM structure, which showed a d-spacing
corresponding to the (110) and (101) planes of SnO2 and the (211)
and (105) planes of anatase TiO2 and amorphous layer (Figure
2e). The same conclusions were derived from the compositional
3
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RESEARCH ARTICLE
phases presumably near the surface of the 3DOM framework.
Due to TiO2 being mainly amorphous and the limited amount of
anatase TiO2, the peaks of SnO2 dominated and TiO2 peaks could
not be observed in the XRD patterns.
the 3DOM SnO2 electrode after activation. Further, for lower Sn-
Ti ratios (Sn0.1Ti0.9O2 and Sn0.2Ti0.8O2), the oxide-to-metal phase
exchange (i.e. reduction of SnO2 to Sn) was not noticeable from
XRD analysis. This resistance to phase change is understood to
be due to the oxygen atoms being much more stable in Sn-Ti-O
ternary structure during CO2RR.
To explore the complex interactions and the electronic
structure between Sn, Ti, and O atoms, the calculated charge
density reconfiguration is disclosed in Figure 3a and shows that
For the higher Sn-Ti ratios (Sn0.8-0.4Ti0.2-0.6O2), XRD patterns
still clearly revealed only peaks of SnO2, and the Ti atoms likely
existed as poorly crystalline oxides. Although the pure SnO2
sample formed extensive 3DOM structure, with low additions of
TiO2, the SnO2 phase tended to congregate and formed
nanoparticles with the influence of Ti additions, thereby
interrupting the framework, as shown in Figure S8. From TEM
images (Figure S9), the 3DOM formation became more stable for
Sn0.4Ti0.6O2, suggesting higher additions of TiO2 eventually
stabilize the structure.
the charge density is accumulated around O and Sn atoms, while
23]
depleted around Ti atoms.[6a,
To further verify the electron
density configuration of Ti in the catalysts, X-ray absorption near-
edge spectroscopy (XANES) at the Ti K-edge was performed to
illustrate the coordination environments of Ti in different cases.
Three characteristic features are denoted A1, A2’ and A3 in the
pre-edge structure of the Ti K-edge spectra of compounds with
high structural symmetry, such as six-coordinated anatase
(Figure 3b).[24] For all Sn-Ti-O materials, A1and A3 signals were
weak; only a pronounced A2 peak was perceived instead,
indicating the existence of less symmetric Ti atoms predominately
on the surface of the oxides.[25] The low-coordination nature of Ti
is also supported by the amorphous structure of TiO2 as depicted
by XRD patterns (i.e. no clear discrete diffraction peaks
associated with anatase TiO2, Figure S7), HRTEM (Figure 2e),
and extended X-ray absorption fine structure (EXAFS) spectra of
Ti K-edge (where lower EXAFS amplitude is observed for Sn-Ti-
O samples as shown in Figure S10a, b). For the Ti atom in
Sn0.1Ti0.9O2, it showed the highest intensity for A2 peak with
negligible A1 peak, suggesting its Ti coordination was more
similar to four-coordinate TiO2.[25b] On the other hand, the Ti
atoms in Sn0.3Ti0.7O2 had the lowest pre-edge peak area with
more defined A1, A2, and A3 peaks (inset of Figure 3b). This
indicates that Ti in Sn0.3Ti0.7O2 specifically had a more distorted
symmetry,[25b] due to the strongest interaction between Ti 3d and
Sn/O (2p) and electron density reconfiguration.
Scheme 1. Schematic of the 3DOM ternary Sn0.3Ti0.7O2 electrocatalyst and local
putative reactive Sn-Ti-O interface. (Am. indicates amorphous and Ana.
indicates anatase.)
On the other hand, for a lower Sn-Ti ratio (Sn0.2Ti0.8O2,
Sn0.1Ti0.9O2), there was evidence of peak broadening and peak
shoulder/separation at around 27°, 35°, and 51° in the XRD
patterns (Figure S7a), presumed to correspond to the spinodal
decomposition region (Figure S6a), displaying a more defined
coexistence of SnO2 and anatase TiO2 phases. In this case,
formation of TiO2 rich regions caused shifting and transition of
SnO2 peaks towards TiO2 anatase. At 100 mol% TiO2 composition,
the phase diagram predicts a rutile phase.[20] Interestingly, our
3DOM structure appeared as anatase, which has higher catalytic
activity than the rutile phase.[21] Considering the differences
between nano-material and bulk material solidification, the
deviation from the phase diagram is understandable. The 3DOM
nanostructure presents itself with larger solubility range and
formation of anatase rather than rutile due to: (1) the extra energy
applied during the sonication procedure, (2) possibility of liquid
and solid phase coexistence in small dimensions,[22] and (3) more
Ti accommodation due to atomic segregation towards the surface
which avoids nucleation of another phase.[20] The special Sn-Ti
ratio (Sn0.3Ti0.7O2) is most likely the crucial turning point for
spinodal decomposition for the nano-binary materials (Figure
S6c). Sn0.3Ti0.7O2 not only avoided complete spinodal
decomposition, presenting mainly SnO2 phase with minor
amorphous/ anatase TiO2 phase, but also maintained a stable
3DOM structure.
As we previously disclosed,[12a] the Sn-based catalysts need
to undergo accelerated degradation and activation under -1.13 V
vs RHE before performing CO2RR.[3b] The emergence of Sn
peaks was found for the electrodes with Sn-Ti ratios higher than
3:7 after the in-situ activation step (Figure S7). After activation
step, notably, the SnO2 peaks were still conserved for Sn0.3Ti0.7O2
electrodes, while there were no remaining strong SnO2 peaks for
Figure 3. Electron density transfer among Sn, Ti, and O atoms. (a) Scheme of
calculated charge densities. The yellow and cyan blue isosurfaces correspond
to the increase in the number of electrons and the depletion zone, respectively.
(b) XANES spectra of Ti K-edge; the red area highlights the near-edge
absorption energy. (c) O 1s, (d) Ti 2p and (e) Sn 3d core level XPS spectra of
3DOM-TiO2, Sn0.1Ti0.9O2, Sn0.2Ti0.8O2, Sn0.3Ti0.7O2 and 3DOM-SnO2 materials.
4
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RESEARCH ARTICLE
Sn-L3 edge XANES spectra (Figure S10c) further confirmed
Sn atoms in Sn0.3Ti0.7O2 exhibited an oxidation state between the
Sn2+ and Sn4+ state.[26] In summary, the interaction between Sn
and Ti atoms exists as amorphous structure with partial electron
density reconfiguration among Sn, Ti, and O according to the XRD
(Figure S7), XPS (Figure S11) and Ti K-edge and Sn-L3 edge
XANES spectra (Figure 3 and Figure S10c). Additionally, our
simulated XANES spectra[27] using DFT Model II (Figure S10d) is
consistent with the experimental measurements, which indicates
the DFT surface model is valid to depict the interaction of local
Sn-Ti-O interface.
XPS analysis was performed to further prove the oxide states
in ternary Sn-Ti-O catalysts (Figure 3c-e and Figure S11a-c). Both
Ti and Sn in the matrix were present in their oxide states (Sn4+
and Ti4+, respectively). Figures 3c shows the obvious shifts of O
1s with different ratios of Sn-Ti. These shifts resulted from
interactions between O, Sn and Ti atoms. With increasing Sn-Ti
ratio, the O 1s and Ti 2p binding energies shifted higher, while all
Sn 3d peaks of Sn-Ti-O materials shifted to lower binding energy
compared with the 3DOM-SnO2, with the shift of Sn0.3Ti0.7O2 being
the largest. This indicates the strongest interaction between Ti
and Sn/O was experienced within Sn0.3Ti0.7O2, as the maximum
electron density was accumulated around Sn atoms, and the most
depletion of electron density occurred for Ti 2p.[28] These shifts
are attributed to a partial electron density reconfiguration among
Sn, Ti, and O (Figure S11g), which also corresponds with the Ti
K-edge XANES and DFT analyses.
(GC) and 1H- Nuclear Magnetic Resonance (NMR) spectrometer,
respectively, and an overall Faradaic efficiency (FE) of
approximately 100% was achieved (Table S6, Figure S14). As
depicted in Figure 4a, Sn0.3Ti0.7O2 electrode showed a noticeable
FE(CO), reaching 92% ± 2.5% at a low potential of around -0.54
V vs. RHE (430 mV overpotential). The cathodic energy
conversion efficiency is projected to reach 71.5 % (methods in
Supporting Information), which surpasses most Sn-based
catalysts (Table S1) and satisfies the basic requirement for
electrosynthesis to compete with fossil fuel–derived feedstocks.[2]
Meanwhile, the remarkable CO selectivity (above 90%) was
maintained over a wide potential window, from -0.54 V to -0.94 V,
as highlighted in light blue in Figure 4a. It is deemed that a wide
potential window is essential for scaling CO2RR process into
practical and grid-scale applications,[15c] and will enhance the
robustness and stability during inherent potential fluctuations
inherent in electrolysis.
From the comparation of XPS spectra before and after the
activation step (Figure S11d-f), the activation step resulted in a
high number of oxygen defect sites (531.0 eV) with a low oxygen
coordination[29] and the presence of Sn2+/4+ (485.9 and 494.4 eV)
and Sn0 (484.0 and 492.5 eV) on the Sn0.3Ti0.7O2 catalyst surface.
To further consider the effect caused by metallic Sn atoms, we
added a new DFT Sn5-Sn(Ti)O2 model (Model III), as shown in
Figure S12, which contains a Sn5 cluster – the most stable
configuration containing 5 Sn atoms[30] – mounted on the Sn(Ti)O2
surface. Sn5-Sn(Ti)O2 model indicated the preferential formation
of formate over CO. This outcome contradicts experimental
results, and it is concluded that this model configuration is unlikely
the active site for CO2-to-CO reduction. Only by appropriate
construction of ternary Sn-Ti-O catalysts (Sn0.3Ti0.7O2), one
appears to achieve a balance between exposure of active sites
and appropriate electron density shift. It was demonstrated that Ti
atoms tended to delocalize maximum charge by releasing it to the
Sn atoms in the compositions of Sn0.3Ti0.7O2, which altered the
selectivity between CO and formate. Though the main cause of
charge delocalization is electronegativity difference between Sn,
Ti, and O, other factors can also play a role, such as synthesis
method, composition, and contact interface.
The CO2RR activities of ternary Sn-Ti-O electrodes were
investigated in an H-type electrochemical cell separated by a
Nafion membrane with CO2-saturated 0.5 M KHCO3 as the
electrolyte (methods in Supporting Information).[12a] In this work,
the potentials reported are all converted into RHE. Linear sweep
voltammetry (LSV) was performed firstly in the CO2-purged and
N2-purged electrolyte (Figure S13a, b), which showed the
catalysts induce higher currents in the CO2- saturated solution.
Then the constant potential electrolysis (CPE) tests for 2 hours
and repeated three times; the results presented are the averaged
values under a sequence of potentials (Figure S13c). H2, CO and
formate were the only products detected by gas chromatography
Figure 4. CO2RR performances. (a) FE toward CO, formate and H2 on
Sn0.3Ti0.7O2 electrode at a series of potentials from −0.34 to −1.21 V. (b) FE and
(c) Partial current density (PCD) of CO on 3DOM-TiO2, Sn0.1Ti0.9O2, Sn0.2Ti0.8O2,
Sn0.3Ti0.7O2, Sn0.4Ti0.6O2, and 3DOM-SnO2 electrodes under different potentials.
(d) FE toward CO, formate and H2 on 3DOM-SnO2 electrode. (e) Calculated pH
values in the mesopores via FEM simulations. (f) Stability test for 3DOM-SnO2
and Sn0.3Ti0.7O2 measuring current density (line plot) and FE (scatter plot). Test
α (orange curve) of Sn0.3Ti0.7O2 is continues electrolysis at -0.94 V, while Test β
(green curve) is altering the potentials during electrolysis. The error bars
represent the standard deviations of three independent measurements of the
same sample.
Comparing the entire series of produced 3DOM catalysts
toward CO formation (Figure 4b), 3DOM TiO2 had nearly zero
FE(CO), while FE(CO) on the 3DOM-SnO2 reached a maximum
of about 50% at -0.34 V and decreased with more negative
5
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10.1002/anie.202004149
Angewandte Chemie International Edition
RESEARCH ARTICLE
potential. From the studied 3DOM ternary Sn-Ti-O catalysts,
Sn0.3Ti0.7O2 obtained an optimum selectivity for CO, which is a
more desirable product than formate in terms of separation and
broader use.[6b] As depicted in Figure 4c, the optimal current
density was achieved by Sn0.3Ti0.7O2 and also demonstrated
superiority over reported results (Table S1). Especially compared
with the SnO2 nanosheets[12a] we previously reported, 3DOM-
was well maintained under a wide potential window (-0.54 V to -
0.94 V). From stability tests, it was evident that the existence of Ti
was essential to not only donate electron density and stabilize the
lattice oxygen in the ternary Sn-Ti-O materials but also to
strengthen 3DOM structure. The demonstrated electrochemical
performance and resistance to variations in potentials can
broaden the utilization of catalysts and is essential for a practical
CO2RR system.
SnO2 exhibited
a slightly lower formate selectivity (65%
FE(HCOO−) for 3DOM-SnO2 vs. 77% for SnO2 nanosheets) and
competitively higher CO selectivity (32% FE(CO) for 3DOM-SnO2
vs. 18% for SnO2 nanosheets) at its optimum potential of -1.03 V.
The observed differences in selectivity is attributed to the high
local alkalinity provided by the mesopores, as shown in the Figure
4e. From finite element modelling (FEM), the local pH value was
determined to theoretically reach up to 9.2 in the pores of 3DOM.
Our FEM calculation results further validate the ability of our
mesoporous structure to increase CO selectivity, by not only
suppressing the HER,[31] but also hindering formate formation.
The bicarbonate electrolyte has low buffer capacity with limited
migration rates of ionic species.[32] It results in two consequences:
1) limited hydroxide ions migration rates and increased local
alkalinity restrict the availability of protons in the mesopores,
which inhibits the HER activity;[1c, 31b, 32] 2) limited formate ions
migration rates and increased formate ions concentration in the
mesopores (Figure S15) further hinder formate formation.
Additionally, a highly alkaline region is also beneficial for the
catalytic activation of CO2 in the electrolyte.[10, 33] The authors
Conclusion
In summary, 3DOM ternary Sn-Ti-O catalysts were
successfully produced, representing a simple but highly effective
artificial material for CO2RR. DFT calculations combined with
different physical and chemical characterizations revealed the
nature of the electron density reconfiguration among Sn, Ti and O
atoms. A balance between exposure of active sites and degree of
electron density shift was imperative for achieving a highly
efficient 3DOM catalyst. In particular, it was inferred that the
crucial electron density transfer from Ti to Sn enabled the
dissociative adsorption of COOH*, directly producing CO with
cathodic energy efficiency over 71.5%. The existence of Ti not
only effectively creates an electron bank which contributes to
activation of catalytic reactions, but it also plays an important role
in strengthening the 3DOM structure in order to maintain a high
EEca for over 200 hours within a wide potential window. Insights
gained through these quantitative analyses of electron density
configuration can be further exploited as promising design
principles for high-performance catalysts, making the
electrochemical process more sustainable for practical
applications.
acknowledge that intentionally creating
a
high alkaline
environment with the use of KOH electrolyte in the H-Type cell
experiment would directly demonstrate these favorable effects.
However, KOH participates in a side reaction that consumes
CO2 and decreases the available CO2 concentration around the
electrodes needed for the reduction reaction.[34] For example,
researchers have previously proposed to use alkaline flow cells
with a gas diffusion electrode (GDE) operated in a flow-by
mode.[35] It can provide higher current densities (CD) and energy
efficiencies. However, it was also shown to suffer from carbonate
salt precipitation in the stagnant pores of the GDE. Moreover, CO2
is parasitically converted to bicarbonate in KOH electrolyte.
Noticeably, the high local alkalinity in our engineered mesopores
remained confined and appeared to be unaffected by the strong
stirring turbulence in the experimental setup. It was therefore able
to maintain CO2 availability, which is an impressive feature for
CO2RR.
Although the 3DOM SnO2 showed improved CO selectivity
relative to the nanosheets, 3DOM SnO2 was not stable and only
had stability for around 20 hours. Afterwards, the FE(CO) was
suddenly reduced and the porous structure collapsed (Figure
S16). Remarkably, the produced optimum Sn0.3Ti0.7O2 electrode
had good stability of over 200 hours of continuous electrolysis
(orange Curve α in Figure 4f) without any obvious degradation
confirmed by LSV curves and XRD, XANES and EXAFS analyses
(Figure S16 e, f, and S17). SEM image (Figure S16) displayed the
collapse of the porous morphology, which is likely attributed to
further reduction of SnOx and the removal of oxygen atoms from
the crystal lattice, as proven by XPS spectra (Figure S18). To
maintain a constant ionic concentration in the electrolyte during
prolonged testing, the electrolyte was replaced with fresh solution
every 20 hours and the recorded time step was set to 30 mins. To
demonstrate structural robustness, we altered the potential during
the stability test. As the Sn0.3Ti0.7O2-β curve depicts, the selectivity
Acknowledgements
The authors gratefully acknowledge the support provided by the
Natural Sciences and Engineering Research Council of Canada
(NSERC), University of Waterloo, and Waterloo Institute for
Nanotechnology. This work was financially supported by the
National Natural Science Foundation of China (Grant No.
U1909213, 51922008 and 51872075), the 111 Project (Grant No.
D17007), Henan Centre for Outstanding Overseas Scientists
(Grant No. GZS2018003) The authors thank Dr. Qunfeng Xiao
and Dr. Mohsen Shakouri from Soft X-ray Microcharacterization
Beamline (SXRMB) of Canadian Light Source (CLS) for X-ray
absorption measurements.The CLS is supported by the NSERC,
the National Research Council Canada, the Canadian Institutes
of Health Research, the Province of Saskatchewan, Western
Economic Diversification Canada, and the University of
Saskatchewan. The authors acknowledge the facilities, scientific
and technical assistance from the Canadian Centre for Electron
Microscopy (CCEM). G.W. acknowledges the support of the
China Scholarship Council (CSC). The authors also thank
Catherine Silva for helping with revision of the manuscript. The
authors also kindly acknowledge the computational resources
used for FEM calculations supported by CMC Microsystems
(www.cmc.ca) and DFT calculations provided by Shared
6
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10.1002/anie.202004149
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RESEARCH ARTICLE
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7
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RESEARCH ARTICLE
Entry for the Table of Contents
With a trade-off between active sites exposure and structural stability, 3D ordered mesoporous (3DOM) structured Sn-Ti-O material
achieves cathodic energy conversion efficiency (EEca) as high as 71.5% over 200 hours for electrochemical conversion of CO2 to CO.
It benefits from an electron density reconfiguration in the Sn-Ti-O interface and a local alkaline environment to improve CO selectivity
and stabilizing lattice oxygen atoms for prolonged stability.
8
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