Boosting Electrochemical Reduction of CO2 at a Low Overpotential by Amorphous Ag-Bi-S-O Decorated Bi0 Nanocrystals

Article abstract of DOI:10.1002/anie.201908735

Bimetal-S-O composites have been rarely researched in electrochemical reduction of CO₂. Now, an amorphous Ag-Bi-S-O decorated Bi⁰ catalyst derived from Ag_0.95BiS_0.75O_3.1 nanorods by electrochemical pre-treatment was used for catalyzing eCO₂RR, which exhibited a formate FE of 94.3 % with a formate partial current density of 12.52 mA cm^?2 at an overpotential of only 450 mV. This superior performance was attributed to the attached amorphous Ag-Bi-S-O substance. S could be retained in the amorphous region after electrochemical pre-treatment only in samples derived from metal-S-O composites, and it would greatly enhance the formate selectivity by accelerating the dissociation of H₂O. The existence of Ag would increase the current density, resulting in a higher local pH, which made the role of S in activating H₂O more significantly and suppressed H₂ evolution more effectively, thus endowing the catalyst with a higher formate FE at low overpotentials.

Full text of DOI:10.1002/anie.201908735

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Accepted Article
Title: Boosting electrochemical reduction of CO2 at a low overpotential
by amorphous Ag-Bi-S-O decorated Bi0 nanocrystals
Authors: Yawen Zhang, Jun-Hao Zhou, Kun Yuan, Liang Zhou, Yu
Guo, Ming-Yu Luo, Xiao-Yan Guo, and Qing-Yuan Meng
This manuscript has been accepted after peer review and appears as an
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201908735
Angew. Chem. 10.1002/ange.201908735
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http://dx.doi.org/10.1002/ange.201908735

Angewandte Chemie International Edition
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COMMUNICATION
Boosting electrochemical reduction of CO
2
at a low overpotential
0
by amorphous Ag-Bi-S-O decorated Bi nanocrystals
Jun-Hao Zhou, Kun Yuan, Liang Zhou, Yu Guo, Ming-Yu Luo, Xiao-Yan Guo, Qing-Yuan Meng and
Ya-Wen Zhang*
Abstract: Bimetal-S-O composites have been rarely researched in
investigated the catalytic performance of metal-S-O composites
in eCO
RR.[7] There is every reason to believe that metal-S-O
composites would exhibit some unique quality for catalyzing
eCO RR.
Except single metal sulfides, there exists a large class of
ternary metal sulfides, and they have exhibited some unique
electrochemical reduction of CO
2
field. In this work, an amorphous
2
0
Ag-Bi-S-O decorated Bi catalyst derived from Ag0.95BiS0.75
O
3.1
nanorods by electrochemical pre-treatment was used for catalyzing
eCO RR, which exhibited a formate FE of 94.3% with a formate
partial current density of 12.52 mA cm at an overpotential of only
50 mV. This superior performance was attributed to the attached
2
2
-
2
4
characteristics in H
evolution reactions (such as NiCo
bioimaging and photochemistry (such as AgInS
2
evolution, O
2
evolution and O
, FeNiS
, etc.),[8] or in
, CuInS
, etc.).[9]
2
reduction
amorphous Ag-Bi-S-O substance. S could be remained in the
amorphous region after electrochemical pre-treatment only in
samples derived from metal-S-O composites, and it would greatly
enhance the formate selectivity by accelerating the dissociation of
2
S
4
2
2
2
By annealing ternary metal sulfides in air at high temperature,
we could get a series of bimetal-S-O composites. Considering
that bimetallic catalysts often exhibit more superior catalytic
performance compared to the corresponding single metal
H
2
O. The existence of Ag would increase the current density,
resulting in a higher local pH, which made the role of S in activating
O more significantly and suppressed H evolution more effectively,
H
2
2
catalysts due to the synergistic geometric and electronic effects
between hetero metallic components,[
10]
we could infer
thus endowing the catalyst with a higher formate FE at low
overpotentials.
reasonably that bimetal-S-O composites may show attractive
activity and selectivity in eCO RR field. However, almost none of
research has examined the eCO RR catalytic performance of
bimetal-S-O composites. Therefore, it is of great significance for
introducing bimetal-S-O composites into the eCO RR field,
which may help us finding a large class of novel electrocatalysts
with satisfactory performance for eCO RR.
Herein, we synthesized the bimetal-S-O composite
3.1 nanorods by annealing the AgBiS nanorods in
2
2
Since the last decades, for making life of human beings more
convenient and comfortable, fossil fuels have been consumed
2
without restraint and excess carbon dioxide CO
2
has been
emitted into the atmosphere, which causes a series of problems
such as energy crisis, global climate change and ocean
2
[
1]
acidification. As a result, exploring renewable energy sources
for replacement of fossil fuels and developing effective methods
Ag0.95BiS0.75
O
2
air at 500 °C. After electrochemical pre-treatment, this
for fixing the atmospheric CO
reduction of CO (eCO RR), which could utilize the electricity
generated from renewable energy sources and convert waste
CO into valuable products such as CO, formate, C etc., has
drawn more and more attention in recent years.[3]
Owing to the inert CO molecule, although various materials
have been developed for eCO RR, it is still a great challenge to
2
become urgent.[2] Electrochemical
0
Ag0.95BiS0.75
O
3.1 nanorod was converted into Bi nanocrystals
2
2
attached by amorphous substance composed of Ag, Bi, S and O
(
the whole synthesis procedure is shown in Fig.1a). The Ag-Bi-
2
2
H
4
0
S-O decorated Bi catalyst exhibited a formate FE of 94.3% with
a formate partial current density of 12.52 mA cm-2 at an
2
overpotential of only 450 mV, while a long-time measurement
illustrates that this catalyst also showed a quite satisfactory
stability.
2
catalyze this reaction with satisfactory activity and selectivity at a
low overpotential.[4] Among the reported catalysts, metal oxides,
metal sulfides and their derivants exhibit quite attractive
2
AgBiS nanorods (Fig. S1) were firstly synthesized and then
annealed in air at 200 °C or 500 °C, and the obtained samples
were named as AgBi-200b and AgBi-500b (‘b’ means before
electrochemical pre-treatment), respectively (seeing Supporting
Information for more details). The results of transmission
electron microscopy (TEM), X-ray diffraction (XRD), High-
resolution TEM (HRTEM), high angle annular dark field scanning
TEM (HAADF-STEM) and energy dispersive X-ray spectroscopy
[
5]
eCO
eCO
2
RR performance. It’s commonly accepted that during the
2
RR process, metal oxides and sulfides would be reduced
partially or entirely, endowing the catalyst with unique surface
structures and local environments, such as defects and
oxygen/sulfur modifiers, which would facilitate the reduction of
[
6]
2
CO .
However, most of the research only focused on the
simplex metal oxides or metal sulfides, and only a little work has
(
EDS) (Fig.S2 to Fig.S4) demonstrate that AgBi-200b could still
maintain the composition of AgBiS while AgBi-500b was
composed of AgBiS , Ag O and Bi28 10. For comparison,
2
2
2
4
O32(SO )
[
*]
J. Zhou, K. Yuan, L. Zhou, Y. Guo, M. Luo, X. Guo, Q. Meng, Prof.
Y. Zhang
Bi-200b and Bi-500b were also prepared through a similar
procedure, and the relative characterization (Fig.S5 to Fig.S8)
Beijing National Laboratory for Molecular Sciences
College of Chemistry and Molecular Engineering
Peking University
No.5 Yiheyuan Road Haidian District, Beijing 100871, China
Email: ywzhang@pku.edu.cn
illustrates that Bi-200b could maintain the composition of Bi
2 3
S
while Bi-500b was composed of Bi , Bi and Bi28O32(SO ) .
2
S
3
O
2 3
4 10
The shifting of Ag 3d, Bi 4f, S 2p and S 2s peaks to higher
binding energies in X-ray photoelectron spectroscopy (XPS)
spectra (Fig.S9 and Fig.S10) suggests the formation of Ag-O,
Bi-O and S-O bonds in AgBi-500b and Bi-500b. Finally, XPS
analysis illustrates the composition of AgBi-500b and Bi-500b to
Supporting information for this article is given via a link at the end of
the document.
be Ag0.95BiS0.75O3.1 and BiS0.68O2.7 (Table S1)
.
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Figure 1. a) Schematic of the synthesis process of AgBi-500; b) to d) HRTEM
images of AgBi-500, while c) and d) is a magnification of the orange squares
in b) and c) respectively; e) to h) EDS elemental mappings of AgBi-500.
Figure 2. a) formate FE and b) formate partial current density for four samples
in CO
2
-saturated 0.1M KHCO
3
solution; c) stability test for AgBi-500 at -0.7 V
and N saturated electrolyte.
To simplify the problem that Bi species would be reduced in
vs. RHE; d) LSV curves of AgBi-500 in CO
2
2
2
eCO RR process (the reduction degree depends on the testing
Linear sweep voltammetry (LSV) were first conducted for
investigation of the electrochemical activity of the four samples.
In Fig.S20, we could find that at potentials more negative than -
potential) and the valence of Bi would greatly influence the
performance,[
11]
catalytic
we conducted electrochemical
reduction pre-treatment on the four samples and the obtained
catalysts were denoted as AgBi-200, AgBi-500, Bi-200 and Bi-
0.45 V vs. reverse hydrogen electrode (RHE) (all potentials
hereafter are provided with respect to RHE unless otherwise
stated), current densities of these four catalysts followed the
order of AgBi-500 > AgBi-200 > Bi-500 > Bi-200, meaning the
introduction of Ag and the derivation from metal-S-O composites
would ensure a large current density. The four samples were
then examined as electrocatalysts for eCO RR by the controlled
2
potential electrolysis method. Fig.2a and Fig.S21 provide the
Faradaic efficiency of all products for these four samples. At all
5
00, respectively. Fig.S11 to Fig. S14 show that all four
nanorods decomposed into irregular nanoparticles after
electrochemical pre-treatment. Only lattice spacing and peaks
corresponding to Bi could be observed in HRTEM images and
XRD patterns of these samples, suggesting the formation of Bi0
nanocrystals and that no Ag-Bi alloy was formed (Fig.1b to 1d,
Fig.S11 to Fig.S15). In some part of the nanoparticles, no
obvious lattice fringe could be observed (Fig.1c, Fig.S12b,
Fig.S13b), suggesting the existence of amorphous substance.
EDS elemental mappings (Fig.1e to 1h, Fig.S12, Fig. S13)
demonstrate the uniform distribution of Ag, Bi, S, O in AgBi-500,
0
2
applied potentials, formate and H were the main products, while
trace CO could be detected at some potentials for AgBi-500 and
Bi-500. AgBi-500 exhibited the most extraordinary formate
selectivity among the four catalysts. It could reach the maximum
formate FE of 94.3% at -0.7 V (only a 450 mV overpotential) with
a formate partial current density of 12.52 mA cm-2 (Fig.2b), and
the formate FE could maintain over 90% in a wide potential
range from -0.7 V to -1.0 V. Compared to other reported Bi-
based electrocatalysts, this AgBi-500 catalyst showed the
highest formate FE at the lowest overpotential (Fig.S22). As for
Bi-500, although it could reach a formate FE over 90% with a
large overpotential (-0.9 V), it performed unimpressively at more
positive potentials. At -0.7 V, its formate FE is ~70% and its
formate partial current density was much lower than that of
AgBi-500. The counterparts AgBi-200 and Bi-200 showed very
poor performance at all applied potential, while AgBi-200
performed a little better than Bi-200. Thus, we could conclude
that the existence of Ag and the derivation from metal-S-O
Ag, Bi,
O in AgBi-200 and Bi, S, O in Bi-500. Peaks
corresponding to S 2s and S 2p (sulfate, +6) could be observed
in XPS spectra of AgBi-500 and Bi-500, which did not appear in
XPS spectra of AgBi-200 and Bi-200 (Fig.S16 to Fig.S19).
Therefore, we could point out that after electrochemical pre-
treatment: 1) A large proportion of Bi would be reduced to Bi0
with fine crystallization; 2) Ag, S, O and the rest of Bi would
attach to Bi in an amorphous form of metal-O-sulfate in AgBi-
0
500, while in Bi-200 and AgBi-200, the amorphous species were
composed of Bi, S, O metal-O-sulfate, and Ag, Bi, O metal oxide,
respectively (seeing Table S2 for more information); 3) S would
be totally removed out from the surface of AgBi-200 and Bi-200
while remained in the form of sulfates in the amorphous part of
AgBi-500 and Bi-500, which could be explained by the different
valence of S. In AgBi-200b and Bi-200b, all S presented a
valence of -2, and S2- would react with H+ in the electrolyte to
2
composites would facilitate the eCO RR process, which brought
AgBi-500 with the most superior catalytic performance. A 12 h
long-term electrolysis test (Fig.2c) demonstrates that whether
the total current density or the formate FE of AgBi-500 were
generally stable, suggesting a good stability of AgBi-500. Finally,
2
form H S when the metal ions were reduced and leave out from
the electrodes; in AgBi-500b and Bi-500b, part of S was oxidized
to a positive valence of +6, and this part of S would not become
2
H S and could remain in the samples.
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to rule out the possibility that the formate could be produced by
AgBi-500 exceeded Bi-500 on the electrical conductivity,
confirming that the existence of Ag in the catalyst would
decrease the charge-transfer resistance and thus facilitate the
the decomposition of electrolyte, electrolysis on AgBi-500 in N
saturated KHCO was performed, and the much lower current
density and no formate being detected in -saturated
electrolyte confirm the formate to be produced from the
reduction of CO (Fig. 2d and Fig. S23).
2
-
3
N
2
2
charge-transfer process from electrode to the adsorbed CO .
Besides, the Tafel slope of AgBi-500 was a little smaller than
that of Bi-500, also evidencing a faster charge-transfer process
on AgBi-500. Both results could account for the enhancement in
geometric current density of AgBi-500 at -0.7 V. It should be
noted that this enhancement differed essentially from the
increase of formate partial current density brought by the surface
2
To figure out the origin of the superior performance of AgBi-
5
00, Tafel plots (overpotentials versus logjHCOOH) were firstly
2
used for investigating the reaction kinetics for CO reduction to
formate (Fig.3a). Compared to AgBi-200 and Bi-200, AgBi-500
and Bi-500 exhibited much lower Tafel slopes of 104 mV dec-1
and 137 mV dec-1 respectively, which illustrates the great
S spices because S activated the H
2
O and made the adsorbed
RR
CO
2
react with H+ more easily, resulting in a faster eCO
2
enhancement of kinetics activity for eCO
from metal-S-O composites. Besides, Tafel slopes of AgBi-500
2
RR on samples derived
kinetics and thus larger formate partial current density. Moreover,
we loaded 2 mol% Ag on Bi-500, and the new Ag/Bi-500 catalyst
showed higher current density compared to Bi-500 at the applied
potentials, confirming the role of Ag in enhancing the current
density (Fig. S26 and S27a).
-
1
and Bi-500 were close to the theoretical value of 118 mV dec ,
which means that the first electron transfer step (the adsorbed
-
•-
[12]
CO
2
+ e → CO
2
) was the rate-determining step (RDS). For
AgBi-200 and Bi-200, their Tafel slopes were much larger than
-
1
1
18 mV dec , suggesting that the adsorbed CO
2
receiving an
+
electron and combining with H from the electrolyte to form
2
+
-
OCHO* (the adsorbed CO + [H + e ] → OCHO*) became the
RDS.[ This difference may reflect that H in the electrolyte was
13]
+
more difficult to be accessed when using AgBi-200 and Bi-200
for catalyzing eCO RR. Considering that no S species existed
2
on the surface of AgBi-200 and Bi-200, while in the amorphous
part of AgBi-500 and Bi-500, S would be remained, we could
infer that it may be S species on the surface of AgBi-500 and Bi-
5
00 that made H+ in the electrolyte more accessible and thus
2
greatly enhanced the kinetics activity. Electrolysis in N -
saturated 0.1 M KOH solution shows larger current density of
AgBi-500 compared to that of AgBi-200 at the applied potentials,
which means higher H formation rate of AgBi-500, confirming
2
that S on the surface could accelerate the dissociation of H
2
O
-
(
Fig. S24). HCO
3
concentration dependence experiment
(
Fig.3b) points out that there was no correlativity between
-
formate partial current density and HCO
3
concentration when
RR catalysts,
adopting AgBi-500 or Bi-500 as the eCO
2
evidencing that H+ anticipating the formation of OCHO* was
provided from the dissociation of H
2
O.[ Thus, it was S on the
14]
surface of AgBi-500 and Bi-500 that accelerated the dissociation
+
of H
2
O to form H , which could react with CO
2
to form OCHO*,
RR kinetics and formate FE. Wang’s
group also discovered that S species in S-doped In would
O and thus enhance the formate
resulting in a faster eCO
2
2 3
O
accelerate the activation of H
selectivity.[ That is why samples derived from metal-S-O
2
7]
composites (AgBi-500 and Bi-500) exhibited a much better
eCO RR performance than their counterparts.
2
Then, we need to figure out why AgBi-500 exhibited higher
current densities and formate FE at lower overpotentials
-
Figure 3. a) Tafel plots for four samples; b) HCO
3
concentration dependence
-
compared to Bi-500. Firstly, the adsorption of OH as a surrogate
experiment on AgBi-500 and Bi-500; c) Nyquist plots for AgBi-500 and Bi-500;
d) an illustration of local pH effect; e) pH dependence experiment on AgBi-500
•
-
of CO2 was examined for investigating the binding affinity of
•-
-2
CO2 on AgBi-500 and Bi-500, and the reduction peak of AgBi-
00 and Bi-500 appeared at nearly the same place (Fig.S25),
and Bi-500 in 0.1 M K
2
HPO
4
, KHCO
3
, K
2
SO
4
solution at 7.11 mA cm .
5
+
2
As illustrated in Fig.3d, the consumption of H in H evolution
2
RR would increase the local pH near the cathode, and
a higher current density would result in a larger local pH.[
phenolphthalein color transition experiment was conducted on
AgBi-500 and Bi-500,[17] and the result showed that after 1 min
electrolysis at -0.7 V, the electrolyte’s color would change to pink
•
-
demonstrating that their ability to stabilize CO2 intermediate
and eCO
differed very little.[ Thus, the stabilization of CO2 intermediate
could not account for the different formate FE of AgBi-500 and
Bi-500.
15]
•-
16]
A
Secondly, electrochemical impedance spectroscopy (EIS) was
conducted and the obtained Nyquist plots (Fig.3c) indicates that
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Angewandte Chemie International Edition
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COMMUNICATION
only in AgBi-500 system, confirming that the local pH around the
cathode was truly higher in AgBi-500 system (Fig. S28). At a
higher local pH value, as reported by Wang’s work, the role of S
This work was financially supported by the National Key
Research and Development Program of China (no.
2016YFB0701100), and the National Natural Science
in accelerating the dissociation of H O would be more significant, Foundation of China (Nos. 21832001, 21771009, 21573005 and
2
which would enhance the formate selectivity; besides, H
evolution would also be suppressed more effectively.[7, 18]
2
21621061).
To verify the local pH effect, we examined the formate FE of
AgBi-500 and Bi-500 in three electrolytes with different buffer
capacity at a constant current density. Fig.3e illustrates that with
the local pH increasing, the formate FE of AgBi-500 and Bi-500
both increased, which confirms that a high local pH was
Keywords: Bimetal-S-O composites • Amorphous Ag-Bi-S-O •
CO
2
electroreduction • Formic acid • Low overpotential
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preferred for eCO RR than H evolution. Besides, in the same
electrolyte, AgBi-500 and Bi-500 showed comparable formate
FE, which may be attributed to their similar ability for stabilizing
•-
[2]
P. H. Stauffer, G. N. Keating, R. S. Middleton, H. S. Viswanathan, K. A.
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CO2 intermediate. Furthermore, Ag/Bi-500 exhibited a higher
formate FE (81.5%) at -0.7 V compared to Bi-500 (70.3%), also
confirming the higher local pH resulted from the larger current
density facilitated the formation of formate (Fig. S27b). Now, we
could conclude that at more positive potentials, the existence of
Ag in AgBi-500 would greatly enhance the geometric current
density, resulting in a higher local pH and thus S species on the
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more
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2
significantly. That’s why AgBi-500 exhibited a superior formate
FE at a low overpotential. At more negative potentials, the
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larger overpotential and higher geometric current density. As for
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and H
important role of the local pH effect. Finally, Ag is commonly
believed to convert CO to CO, and in AgBi-500 there existed a
large proportion of Ag, but CO was still reduced to formate with
high selectivity. This could be ascribed to the stronger
adsorption of acidic CO molecule on alkaline main group metal
Bi (Ag is a kind of more inert noble metal) and Bi would reduce
2
FE could maintain beneath 8%, also suggesting the
2
2
[
[
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CO
2
to formate due to the intrinsic property of Bi (seeing
supplementary remark in supporting information).[19]
In summary, we have developed an amorphous Ag-Bi-S-O
0
decorated Bi nanocatalyst derived from Ag0.95BiS0.75
O
3.1
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partial current density of 12.52 mA cm-2 at a low overpotential.
Because it was derived from the metal-S-O composites, S could
be remained in the amorphous region after electrochemical pre-
treatment, which could facilitate the eCO
2
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2
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Entry for the Table of Contents
COMMUNICATION
Jun-Hao Zhou, Kun Yuan, Liang Zhou, Yu
Guo, Ming-Yu Luo, Xiao-Yan Guo, Qing-
Yuan Meng and Ya-Wen Zhang*
Boosting electrochemical reduction of
CO
2
at a low overpotential by amorphous
0
Ag-Bi-S-O decorated Bi nanocrystals
Low overpotential selective eCO
amorphous Ag-Bi-S-O decorated Bi nanocatalyst derived from bimetal-S-O composite Ag0.95BiS0.75
2
RR: A formate efficiency of 94.3% at a low overpotential of 450 mV was achieved by an
0
O3.1 due to the role of S in
accelerating the dissociation of H
2
O and the higher local pH resulted from the existence of Ag.
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