Nanoporous Ag-Sn derived from codeposited AgCl-SnO2 for the electrocatalytic reduction of CO2 with high formate selectivity

Article abstract of DOI:10.1016/j.elecom.2019.03.017

Nanoporous Ag-Sn was prepared by direct electroreduction of a codeposited AgCl-SnO₂ mixture in 0.1 M HCl, and evaluated as an electrode catalyst for the reduction of CO₂ in 0.5 M KHCO₃. A volcano-type correlation between selectivity for the formate product and the atomic ratio of Ag to Sn in the nanoporous catalysts was revealed. It was found that the bimetallic catalyst with a Ag:Sn ratio of 3:2, mainly composed of the Ag₄Sn alloy, showed excellent catalytic performance for the conversion of CO₂ to formate. This catalyst delivered a current of about 10 mA cm^?2 with a high formate faradaic efficiency of about 85% at ?0.8 V vs. the reversible hydrogen electrode. Moreover, the catalytic activity remained reasonably stable during a 13.5-hour electrolysis.

Full text of DOI:10.1016/j.elecom.2019.03.017

Accepted Manuscript
Nanoporous Ag-Sn derived from codeposited AgCl-SnO2 for the
electrocatalytic reduction of CO2 with high formate selectivity
Xiaoyan Wang, Wei Xiao, Jichen Zhang, Zhiyong Wang, Xianbo
Jin
PII:
S1388-2481(19)30081-5
DOI:
Reference:
https://doi.org/10.1016/j.elecom.2019.03.017
ELECOM 6438
To appear in:
Electrochemistry Communications
Received date:
Revised date:
Accepted date:
4 March 2019
28 March 2019
28 March 2019
Please cite this article as: X. Wang, W. Xiao, J. Zhang, et al., Nanoporous Ag-Sn derived
from codeposited AgCl-SnO2 for the electrocatalytic reduction of CO2 with high formate
selectivity,
Electrochemistry
Communications,
https://doi.org/10.1016/
j.elecom.2019.03.017
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ACCEPTED MANUSCRIPT
Nanoporous Ag-Sn derived from codeposited AgCl-SnO2 for the electrocatalytic
reduction of CO2 with high formate selectivity
Xiaoyan Wang+, Wei Xiao+, Jichen Zhang, Zhiyong Wang and Xianbo Jin*
*Hubei Key Laboratory of Electrochemical Power Sources, College of Chemistry and
Molecular Sciences, Hubei International Scientific and Technological Cooperation Base of
Sustainable Resource and Energy, Wuhan University, Wuhan, 430072, P.R. China. Email:
xbjin@whu.edu.cn
+These authors contributed equally
Keywords: Ag-Sn bimetals, solid electroreduction, electrocatalysis, carbon dioxide, formate
Abstract: Nanoporous Ag-Sn was prepared by direct electroreduction of a codeposited
AgCl-SnO2 mixture in 0.1 M HCl, and evaluated as an electrode catalyst for the reduction of
CO2 in 0.5 M KHCO3. A volcano-type correlation between selectivity for the formate product
and the atomic ratio of Ag to Sn in the nanoporous catalysts was revealed. It was found that
the bimetallic catalyst with a Ag:Sn ratio of 3:2, mainly composed of the Ag4Sn alloy,
showed excellent catalytic performance for the conversion of CO2 to formate. This catalyst
delivered a current of about 10 mA cm-2 with a high formate faradaic efficiency of about 85%
at -0.8 V vs. the reversible hydrogen electrode. Moreover, the catalytic activity remained
reasonably stable during a 13.5-hour electrolysis.
1. Introduction
The growing carbon dioxide (CO2) content of the atmosphere has resulted in serious
greenhouse effects and global warming. Conversion of CO2 into various organic
compounds or high value-added chemical fuels is an attractive idea for reducing and
reclaiming CO2, and various techniques, including catalytic hydrogenation [1,2],
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photocatalytic [3,4] and electrocatalytic reduction [5,6] of CO2, have been extensively
investigated. In this regard, electrochemical reduction of CO2 has been considered as one
of the most promising approaches, because the process may efficiently balance the present
intermittent electrical energy supply of renewable solar, wind and tidal energies.
Unfortunately, it is difficult to activate the stubborn CO2 molecule [7], and great efforts
should be made to develop efficient electrocatalysts.
Among the previous catalysts screened, Sn is attractive as it allows generation of the
formic acid product, but its activity and selectivity need to be improved [8-10]. Sn-based
bimetallic catalysts have been investigated, expecting that the catalytic activity and
selectivity could be tuned by varying the composition of the material [11-13]. For example, it
was reported that a Sn-In alloy has a higher catalytic activity than pure Sn and In [14], and
Sn-Pb is better than single Sn and Pb [15]. Ag-Sn is of particular interest due to the
combined high activity and high selectivity of these metals [16-17]. However in practice, it is
generally required that a bimetallic electrocatalyst should have a nanostructure in which the
components are well dispersed. To achieve this, Zhang et al. prepared Sn-based bimetals
from metal stannates, in which the elements are dispersed at the atomic scale. However,
this method can only produce Ag:Sn with a fixed molar ratio of 2:1 [16]. Luc et al.
synthesized Ag-Sn catalysts with different Ag:Sn ratios through a displacing reaction route:
nanoparticles of Sn were first synthesized and dispersed in solution with the help of
polyvinylpyrrolidone (PVP), then Ag ions were added to replace some Sn with Ag, thus
forming the Ag-Sn bimetallic catalysts. Before use, the PVP in the catalysts should be
thoroughly removed by repeated washing. They found that Ag-Sn with a Ag:Sn ratio of
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about 3:1 performed best, with a formate faradaic efficiency (FE) of 80% and a current
density of 16 mA cm-1 at -0.8 V vs. RHE [17].
In this study, we report a simple codeposition strategy to prepare AgCl/SnO2 precursors,
which is followed by a solid state electroreduction to prepare nanoporous Ag-Sn electrodes
with different Ag:Sn ratios. We found that the codeposition method guaranteed highly
dispersed AgCl and SnO2 in the precursors, and subsequently Ag-Sn bimetallic catalysts
with a good distribution of metals were produced. When assessed as electrode catalysts for
the reduction of CO2 in 0.5 M KHCO3, the nanoporous Ag-Sn showed formate selectivity
with a volcano-type variation with Ag content. The catalyst with a Ag:Sn ratio of 3:2 (denoted
here as 3Ag2Sn) exhibited both high formate selectivity (FE is 85%) and high partial current
density (8.5–12 mA cm-2) at -0.8 to -0.93 V vs. RHE.
2. Experimental
2.1. Preparation of the AgCl-SnO2 precursors
AgNO3 (AR), SnCl4·5H2O (AR) and ammonia water were used as received. Silver
ammonia solution was prepared by adding ammonia water to 0.1 mol L-1 AgNO3 solution.
Under constant stirring, scheduled proportions of the silver ammonia solution and the SnCl4
solution (0.1 mol L-1) were mixed together, with the pH value being adjusted to 6 to allow
complete codeposition of AgCl and SnO2. The precipitate was centrifuged, washed, and
dried in a vacuum oven at 80C for 12 h before use.
2.2. Preparation of nanoporous Ag-Sn electrodes
Membranes (~10 mm thick; mass loading, ~5 mg cm-2; porosity, ~10%) of the AgCl-SnO2
precursors were prepared by the rolling-press method using polytetrafluoroethylene (PTFE,
10 wt%) as the binder. To prepare the working electrode, a piece of the precursor
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membrane with a geometric area of 0.4 cm2 was rolled onto a titanium mesh current
collector. The precursor electrodes were then reduced in 0.1 M HCl at -1.4 V vs. Ag/AgCl to
form the nanoporous Ag-Sn electrodes.
2.3. Electrocatalytic reduction of CO2
The reduction of CO2 was performed in a gas-tight two-compartment electrochemical
cell divided by an anion exchange membrane (Nafion® 117). The electrolyte was 0.5 M
KHCO3 solution containing saturated CO2, with a pH value of 7.2. The counter electrode
was a Pt sheet and the reference electrode was a saturated calomel electrode (SCE),
whose potential can be converted to that of the reversible hydrogen electrode (RHE) using
the formula ERHE = ESCE + 0.242 + 0.059pH (V). The current density is calculated based on
the geometric area of the electrode. The test was controlled using an Autolab potentiostat at
room temperature and ambient pressure.
During the constant potential electrolysis, CO2 was fed (25 mL min-1) continuously to
bring out the gaseous product, which was monitored by an online gas chromatograph (GC)
equipped with a thermal conductivity detector and with argon serving as the carrier gas. The
dissolved product in the catholyte was sampled (0.3 mL catholyte + 0.1 mL D2O and 0.1 mL
2.8 mM dimethyl sulfoxide) and quantified by a nuclear magnetic resonance spectrometer
(NMR, Bruker AVANCE III HD). The detected cathodic products included CO, H2 and
formate; their content was calculated using calibration curves produced using standard H2
and CO gases by GC and solutions of sodium formate in DMSO by NMR, respectively.
2.4. Materials Characterization
The crystalline structures of the samples were characterized by X-ray diffraction (XRD,
Bruker D8 Advance) using Cu Kα radiation at 40 mA and 40 kV. X-ray photoelectron
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spectroscopy (XPS, Thermo Fisher ESCALAB 250xi) was carried out with an Al Kα
excitation source. The sample morphology and composition were investigated using a
scanning electron microscope (SEM, Zeiss Merlin) equipped with an energy dispersive
X-ray (EDS) spectrometer.
3. Results and discussion
It has previously been reported that nanoporous Ag may be prepared by solid reduction of
AgCl or Sn by SnO2 [18-21]. Solid reduction of mixed precursors of Ag and Sn may in
principle be used to prepare nanoporous Ag-Sn catalysts. However, we found that common
silver compounds such as AgCl and Ag2O are too soft to mix with SnO2 (the most likely Sn
precursor) by a simple mechanical method. We therefore propose the codeposition strategy
shown schematically in Fig. 1 to prepare the uniformly dispersed AgCl-SnO2 precursor. The
precursor is then made into a membrane electrode for solid reduction to remove the O and
Cl, which will intrinsically produce pores in the membrane, resulting in a porous, uniformly
dispersed Ag-Sn bimetallic electrode.
The codeposition of AgCl and SnO2 was achieved through linkage reactions between the
silver ammonia solution and SnCl4. It is well known that the hydrolysis of SnCl4 is slow
unless an alkaline substance such as ammonia is added. When the silver ammonia and
-
SnCl4 solutions are mixed together, the Ag(NH3)2 ions can dissociate to Ag+ ions and
ammonia, inducing the hydrolysis of SnCl4 to SnO2 with Cl- ions released; the Cl- ions in turn
-
accelerate the dissociation of Ag(NH3)2 ions by precipitating the Ag+ ions to form AgCl.
These linkage reactions may generate a highly dispersed AgCl-SnO2 precursor. To confirm
this assumption, solutions of silver ammonia and SnCl4 with a Ag:Sn molar ratio of 3:2 were
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mixed and allowed to react and precipitate AgCl and SnO2, which was denoted as
3AgCl2SnO2 for convenience (3 before AgCl and 2 before SnO2 indicate the starting Ag:Sn
molar ratio). The SEM image in Fig. 2a reveals that 3AgCl2SnO2 comprised uniform
particles of nanometer size (70–100 nm). As expected, EDS element mapping analyses
shown in Fig. 2d and 2e indicate that the AgCl and SnO2 particles are highly dispersed and
well mixed.
The electrode membrane made from the 3AgCl2SnO2 powder was originally very dense
(Fig. 2b). However, after electroreduction in 0.1 M HCl, the resulting metallic electrode had
a nanoporous structure with a porosity of about 55%. As shown in Fig. 2c, the nanopores
and nanoparticles were uniformly dispersed in the electrode, with sizes ranging from 40 to
120 nm. This electrode was denoted as 3Ag2Sn for consistency with its precursor. Again,
EDS element mapping analyses (Fig. 2f and 2g) indicate that the distribution of Ag and Sn
in the 3Ag2Sn electrode was very uniform, as expected.
A number of AgCl-SnO2 precursors with different Ag:Sn molar ratios were then
synthesized and the corresponding nanoporous Ag-Sn electrodes produced. EDS analysis
indicates that the Ag:Sn molar ratio in the metallic product was in good agreement with the
starting (fed) ratio of silver ammonia to SnCl4 (Table S1). XRD patterns revealed that the
AgCl-SnO2 precursors consisted of amorphous SnO2 and crystalline AgCl (Fig. 3a). It was
found that the reduction of SnO2 alone was rather slow: a 4000-second reduction only led to
partial metallization of the SnO2 (Fig. 3b). This is likely due to difficulties in both
thermodynamics and kinetics. However, on adding some AgCl the precursor was easy to
reduce, as evidenced by the reduction of the 1AgCl4SnO2 precursor, noticing that the XRD
pattern of the resulting 1Ag4Sn shows much stronger peaks of Sn and Ag4Sn. This is
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reasonable considering that the reduction of AgCl is easy and the generated Ag dispersing
in the electrode may form many Ag/SnO2/electrolyte three-phase electrochemical
interlines [19] that facilitate the reduction of the remaining SnO2. Moreover, the reduction of
SnO2 on Ag is also more thermodynamically favourable due to the formation of the Ag4Sn
alloy, as predicted by the Ag-Sn binary diagram. Ag4Sn becomes predominant in the XRD
pattern of the 3Ag2Sn. In the XRD pattern of the 4Ag1Sn product, although Ag4Sn is still
predominant, some small peaks of Ag can be observed, indicating the presence of a small
amount of SnO2 in the amorphous structure. Ag becomes dominant when the Ag:Sn molar
ratio is larger than 9:1.
The electrocatalytic reduction of CO2 was conducted in a gas-tight H-type cell filled
with CO2-saturated 0.5 M KHCO3 solution [17, 21-25]. The cyclic voltammogams (Fig. S1)
indicate that Ag-Sn electrodes with different Ag/Sn ratios have a very different catalytic
performance for CO2 reduction, compared under constant potential electrolysis at -0.8 V vs.
RHE. The GC and NMR analyses indicate that carbon monoxide (CO), hydrogen (H2) and
formate were the main products (Fig. S2). The partial current of each product can be
derived from its productive rate determined by GC or NMR together with the calibration
curves measured using standard H2 and CO gases, and a standard solution of HCOONa
(Fig. S2). The Faradaic efficiency (FE) is the ratio between the partial current (or charge)
and the total current (or charge). Fig. 4a displays the change in FE of each product with the
Ag content in the bimetallic catalyst; the corresponding partial currents are shown in Fig. 4b.
It can be seen that the total current sharply increased on increasing the Ag content to about
80% and then slightly decreased. The Sn exhibited a formate FE of ~40% and a H2 FE of
~53%, in line with previous observations on a pure Sn electrode [26,27]. On increasing the
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Ag content, the formate FE first increased but then decreased. The highest formate FE was
observed for the 3Ag2Sn electrode. Further increasing the content of Ag decreased both the
FE and partial current of formate. The pure Ag catalyst generated no formate; the product
was mainly CO with a FE as high as ~85%, in agreement with previous results obtained
using nanoporous Ag electrodes [28,29].
The 3Ag2Sn catalyst, mainly composed of Ag4Sn alloy, not only showed a high selectivity
for formate (~85%), but also delivered a high partial current (8.5 mA cm-2), suggesting that
the Ag4Sn alloy is a high activity phase towards the formate product. As the Ag to Sn atomic
ratio is less than 4 in 3Ag2Sn, there should be excess Sn, which if in the form of metal would
lead to the generation of large amounts of H2. However, the H2 FE at the 3Ag2Sn was low.
The explanation might be that the Sn in the electrode has been partially oxidized, bearing in
mind that Sn is thermodynamically unstable in air or water, and the resulting SnOx has been
shown to be an assistant promoter for the reduction of CO2 to formate [30-32]. This is
confirmed in Fig.S3, where the XPS signal of Sn 3d shows two strong peaks of SnOx (486.5
and 495 eV) [17,30] and two weak peaks of Sn0 (484.3 and 492.7 eV), indicating that SnOx
is dominant on the surface of the 3Ag2Sn catalyst. On the other hand, according to Fig. 3b
and 4b, once Ag becomes dominant in the bimetallic catalyst, the CO partial current
increases significantly. This may suggest that the presence of elemental Ag should be
avoided if the Ag-Sn catalyst is to achieve a high formate efficiency.
The catalytic performance of 3Ag2Sn was further investigated. Fig. 4c shows the
potential-dependent FEs of different reduction products. As the polarization potential
increased from -0.6 V to -0.8 V vs. RHE, both the formate FE and partial current increased
strikingly. When the electrolysis overpotential was further increased, the formate partial
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current slowly increased to about 14.7 mA cm-2 at -1.13 V (Fig. 4d), but the FE deceased.
This is likely due to reaching the mass transport limitation of CO2 and the enhancement of
competitive hydrogen evolution [24,32]. As a compromise, a high formate FE (80–85%) and
a relatively high current density (11–12 mA cm-2) can be reached at potentials around -0.9 V.
To obtain kinetic insights into the mechanism of CO2 electroreduction to formate at the
3Ag2Sn electrode, it is necessary to study the Tafel plot depicted in Fig. 4e. The Tafel slope
is about 92 mV dec-1, lower than those (110–136 mV dec-1) reported previously with Ag–Sn
bimetallic catalysts [16,17]. As a slope of 118 mV dec-1 suggests that the rate-determining
step is the one-electron transfer reaction to form the CO-2. intermediate [33], a lower Tafel
slope of 92 mV dec-1 may suggest that at the 3Ag2Sn electrode, the electron transfer rate
has been speeded up and the reduction changed to mixed electron transfer and chemical
reaction control. This may be attributed to the high catalytic activity of the Ag4Sn alloy phase
and the nanoporous structure of the 3Ag2Sn catalyst providing more active sites for the
reduction of CO2.
The stability of the 3Ag2Sn catalyst during use was investigated through a programme of
short- and long-term operation at -0.8 V vs. RHE: five 1-hour electrolyses, followed by a
continuous 8.5-hour electrolysis. During the intervals, the electrolyte was replaced with
clean electrolyte. As shown in Fig. 4f, the 3Ag2Sn catalyst exhibited good catalytic stability,
with a current density of about 10 mA cm-2 and a mean FE of 85% for formate formation
throughout the 13.5 h electrolysis. The overall performance of the 3Ag2Sn catalyst is
comparable with that of the best catalyst reported for formate generation in the literature
[13-17,34-38]. During the electrolysis, the cell voltage remained basically stable at about 2.8
V after deducting the ohmic drop, indicating an energy consumption of about 4
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kWh/Kg-formate. It should be pointed out that the planar Pt counter electrode is not suitable
for practical use as here it caused a high overpotential of about 0.8 V for the oxygen
evolution reaction. Obviously, there should be further investigation of the anodic process in
order to replace the Pt with cheap porous electrodes and if possible to reduce the
overpotential at the same time.
Conclusions
In summary, we synthesized nanoporous Ag-Sn catalysts by direct electroreduction of
AgCl-SnO2 catalyst precursors in 0.1 M HCl. The uniform mixing of the AgCl-SnO2
precursors was achieved by a codeposition strategy, making use of linkage reactions
-
between Ag(NH3)2 ions and SnCl4 in water. Among the resulting Ag-Sn catalysts with
different Ag to Sn molar ratios, the 3Ag2Sn material exhibited both high catalytic activity and
selectivity for the electroreduction of CO2 to formate in KHCO3 solution, delivering a formate
partial current of 8.5 and 12 mA cm-2 at -0.8 V and -0.93 V vs. RHE, respectively, with a FE
of about 85%. It also exhibited high catalytic stability during a 13.5 h electrolysis. A
high-efficiency bimetallic Ag-Sn catalyst towards the formate product should consist mainly
of the Ag4Sn alloy phase. Our findings may prove helpful in the design and preparation of
multimetallic electrocatalysts with enhanced activity for the reduction of CO2 to formate.
Acknowledgements
We appreciate the funding support from NSFC (21673164, 21872107, 51722404), and the
Large scale Instrument and Equipment Sharing Foundation of Wuhan University.
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Figures and captions
Fig. 1. Schematic illustration of the synthesis of a nanoporous bimetallic Ag-Sn catalyst.
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Fig. 2. SEM images and EDS elemental mapping of a typical AgCl-SnO2 precursor and the
corresponding nanoporous Ag-Sn. (a) SEM image of 3AgCl2SnO2 powders; (b) the
3AgCl2SnO2 membrane made from (a); (c) the nanoporous 3Ag2Sn derived from (b).
EDS mappings of Ag (d, f) and Sn (e, g) corresponding to the views shown in (a) and (c).
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Fig. 3. XRD patterns of (a) the AgCl-SnO2 precursors and (b) the corresponding Ag-Sn
catalysts derived from the precursors through solid state electroreduction.
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Fig. 4. Electrocatalytic performance of the nanoporous Ag-Sn electrodes. Changes in (a)
Faradaic efficiencies and (b) partial current densities of the formate, H2 and CO products
with the Ag-Sn composition when the CO2 reduction was performed at -0.8 V vs. RHE in 0.5
M KHCO3. Changes in (c) Faradaic efficiencies and (d) partial current densities of the
formate, H2 and CO products with the reduction potential at the 3Ag2Sn electrode. (e) Tafel
plot of the 3Ag2Sn electrode for formate formation. (f) Stability of the 3Ag2Sn electrode
operated at −0.8 V vs. RHE.
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Highlights
 Codeposition was used to prepare uniform AgCl-SnO2 mixtures
 Preparation of nanoporous bimetallic Ag-Sn catalysts through solid state
electroreduction of the AgCl-SnO2 precursor
 Tuning electrocatalytic performance of Ag-Sn for CO2 reduction by changing the Ag:Sn
molar ratio
 The Ag-Sn catalyst with Ag:Sn = 3:2 shows high activity and selectivity for formate
formation
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