Amorphous Sn/Crystalline SnS2 Nanosheets via In Situ Electrochemical Reduction Methodology for Highly Efficient Ambient N2 Fixation

Article abstract of DOI:10.1002/smll.201902535

Electrochemical nitrogen reduction reaction (NRR) as a new strategy for synthesizing ammonia has attracted ever-growing attention, due to its renewability, flexibility, and sustainability. However, the lack of efficient electrocatalysts has hampered the development of such reactions. Herein, a series of amorphous Sn/crystalline SnS₂ (Sn/SnS₂) nanosheets by an L-cysteine-based hydrothermal process, followed by in situ electrochemical reduction, are synthesized. The amount of reduced amorphous Sn can be adjusted by selecting electrolytes with different pH values. The optimized Sn/SnS₂ catalyst can achieve a high ammonia yield of 23.8 μg h^?1 mg^?1, outperforming most reported noble-metal NRR electrocatalysts. According to the electrochemical tests, the conversion of SnS₂ to an amorphous Sn phase leads to the substantial increase of its catalytic activity, while the amorphous Sn is identified as the active phase. These results provide a guideline for a rational design of low-cost and highly active Sn-based catalysts thus paving a wider path for NRR.

Full text of DOI:10.1002/smll.201902535

FULL PAPER
Electrocatalysis
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Amorphous Sn/Crystalline SnS2 Nanosheets via In Situ
Electrochemical Reduction Methodology for Highly Efficient
Ambient N2 Fixation
Pengxiang Li, Wenzhi Fu, Peiyuan Zhuang, Yudong Cao, Can Tang,
Angelica Blake Watson, Pei Dong, Jianfeng Shen,* and Mingxin Ye*
per year, which operates at high tem-
Electrochemical nitrogen reduction reaction (NRR) as a new strategy for
synthesizing ammonia has attracted ever-growing attention, due to its
renewability, flexibility, and sustainability. However, the lack of efficient
electrocatalysts has hampered the development of such reactions. Herein,
a series of amorphous Sn/crystalline SnS2 (Sn/SnS2) nanosheets by an
L-cysteine-based hydrothermal process, followed by in situ electrochemical
reduction, are synthesized. The amount of reduced amorphous Sn can be
adjusted by selecting electrolytes with different pH values. The optimized
Sn/SnS2 catalyst can achieve a high ammonia yield of 23.8 µg h−1 mg−1,
outperforming most reported noble-metal NRR electrocatalysts. According
to the electrochemical tests, the conversion of SnS2 to an amorphous Sn phase
leads to the substantial increase of its catalytic activity, while the amorphous
Sn is identified as the active phase. These results provide a guideline for a
rational design of low-cost and highly active Sn-based catalysts thus paving a
wider path for NRR.
peratures (300–500 °C) and pres-
sures (200–300 atm) on iron-based
catalysts.[2,3] It is an energy-intensive
and environmentally hazardous industry,
which accounts for ≈2% of the global
annual energy consumption and ≈1%
of the world CO2 emission.[4–6] There-
fore, much efforts have been focused
on developing an alternative, effi-
cient process for NH3 synthesis using
electrocatalysts,[7–9] photocatalysts,[10,11]
biological nitrogenase[12,13] at ambient
conditions, which can provide the possi-
bility to overcome the limitations of the
Haber–Bosch process. Recently, electro-
catalytic nitrogen fixation attracts more
and more attention, due to the utilization
of electricity generated from intermittent
renewable energy resources such as solar
and wind, etc.[14–17]
Despite these potential advantages, there are two main prob-
lems which greatly hampered the development of nitrogen
reduction reaction (NRR). First, most studies of the NRR in
aqueous electrolyte under ambient conditions showed a low
NH3 yield rate and Faradaic efficiency (FE) due to the competing
hydrogen evolution reaction (HER).[6,18] Second, this process
has been dramatically impeded by the lack of low cost electro-
catalysts. To solve the above problems, we focused our atten-
tion on materials with low HER activity and earth-abundant
elements like Sn. As a semiconductor, it is reported that Sn can
limit the surface electron accessibility for a suppressed HER
process.[19] Moreover, previous work by Nørskov and co-workers
demonstrated that unreactive metals are able to bind *N2H
more strongly without affecting the binding energy of *NH2
or *NH.[20] However, pristine Sn is not competent enough to
preform high NRR activity. Inspired by the amorphous Au and
amorphous Bi4V2O11 that demonstrated enhanced NRR activity
over the crystalline counterpart,[21,22] it is anticipated that struc-
tural optimization via the amorphization strategy may be of high
promise in achieving high NRR activities under mild condi-
tions. Different from crystals, amorphous catalysts can induce
defective sites and reduce energy barrier to promote the catalytic
activity.[23–26] Therefore, finding a strategy to prepare an amor-
phous Sn catalyst might enable new opportunities to achieve
highly active catalysts in NRR.
1. Introduction
The Haber–Bosch process is an epoch-making industrial
nitrogen supply method, which chemically converts free
nitrogen into ammonia, resulting in the huge increase of
agricultural productivity and world population witnessed
in the previous century.[1] Currently, roughly 500 million
tons of ammonia is produced by the Haber–Bosch process
P. Li, Dr. W. Fu, Dr. P. Zhuang, Dr. Y. Cao, Prof. J. Shen, Prof. M. Ye
Institute of Special Materials and Technology
Fudan University
Shanghai 200433, P. R. China
E-mail: jfshen@fudan.edu.cn; mxye@fudan.edu.cn
Dr. C. Tang
Department of Chemistry and Shanghai Key Laboratory of Molecular
Catalysis and Innovative Materials
Fudan University
Shanghai 200433, P. R. China
Dr. A. B. Watson, Prof. P. Dong
Department of Mechanical Engineering
George Mason University
Fairfax, VA 22030, USA
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/smll.201902535.
DOI: 10.1002/smll.201902535
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In this work, we report that the amorphous Sn/crystalline
SnS2 nanosheet, synthesized by an L-cysteine-based hydro-
thermal process followed by in situ electrochemical reduction,
can be a superior catalyst for electrocatalytic reduction of N2
into NH3 with high activity and selectivity under mild condi-
tions. The Sn/SnS2 samples with different amorphous Sn con-
tents were synthesized by controlling the pH of the electrolyte.
In N2-saturated 0.1 m phosphate buffer solution (PBS), such
amorphous Sn/crystalline SnS2 electrocatalyst achieve a high
ammonia yield of 23.8 µg h−1 mg−1 at −0.8 V versus revers-
ible hydrogen electrode (RHE) and a high Faradaic efficiency
of 6.5% at −0.7 V versus RHE, superior to most reported
NRR noble-metal electrocatalysts under ambient conditions.
Further electrochemical test demonstrates that electrochem-
ical reduction activity toward N2 reduction arises from the
presence of reduced amorphous Sn formed from SnS2 under
cathodic electrolysis conditions and the mass of amorphous
Sn in Sn/SnS2 samples is proportional to their NRR yield,
which indicates that amorphous Sn is the active phase in
NRR. This pioneer work will be a starting point for further
research to synthesize and application of high performance
Sn-based NRR catalysts.
and subjected to characterizations immediately after several
washing with deionized water and ethanol.
Figure 1a–d shows the typical scanning electron microscopy
(SEM) images of SnS2 and three Sn/SnS2 samples. The SEM
image of the SnS2 sample (Figure 1a) illustrates that the flower
morphology was composed of a large number of nanosheets
with uniform size ≈500–1000 nm. However, when SnS2
begins to transform into Sn, the nanosheets began cracking
and coalescing until cross-linked with each other. To further
examine the conversion from the SnS2 to metallic Sn, XPS was
collected. The high-resolution XPS spectra of Sn 3d peaks are
exhibited in Figure 1e–h. The fitting results indicate that only
Sn4+ ions in the SnS2 sample. However, the Sn 3d XPS spectra
of electrochemical reduction products (Sn/SnS2) can be decon-
voluted into speaks at 486.6, 495.0 (Sn4+) and 485.2, 493.7 eV
(Sn). This result indicates that the electrochemical reduction
products under cathodic condition consists of both Sn0 and
SnS2 components, which indicates a partial reduction of the
SnS2 nanosheets to metallic Sn0 consistent with the prereduc-
tion process. In addition, Figure S3 (Supporting Information)
shows high-resolution XPS spectrum of the N 1s region, indi-
cating that no N species were observed in SnS2 and 2-Sn/SnS2
sample within the XPS detection limit. Energy dispersive X-ray
spectrum (EDX) and inductively coupled plasma (ICP) analyses
are adopted to identify the elemental compositions of SnS2 and
three Sn/SnS2 samples (Figure S4, Supporting Information),
which shows that the content of element S decreases due to the
transformation from SnS2 to Sn. The amount of Sn0 was quan-
tified through measuring the elemental S and Sn molar ratio
using EDX and ICP of the samples. For example, in 2-Sn/SnS2
sample, the ratio between Sn and S are 0.38 from EDX data
and 0.35 calculated from the ICP, with the mean value of 0.365,
showing excellent agreement. A Sn0 to SnS2 molar ratio of
4.12 can be obtained from a simple calculation. In this way,
we calculated the molar ratios of Sn0 to SnS2 in the samples,
0.47 for 1-Sn/SnS2, 4.12 for 2-Sn/SnS2, and 0.99 for 3-Sn/SnS2
(Figure 1i). In the electrochemical reduction process of SnS2,
lower pH value means a greater degree of conversion to Sn. So,
2-Sn/SnS2 has more Sn than 3-Sn/SnS2 probably because of
the corrosion of Sn in the higher concentrations of acid. The
samples were further investigated by Raman spectral analysis
as shown in Figure 1j. The prominent peaks at 311 cm−1 are
attributed to the A1g mode of SnS2,[28] No peaks of other Sn
compounds are shown in Raman spectral of three Sn/SnS2
samples. The sharp decrease of peak signal intensity means
the transformation of partial SnS2. The X-ray diffraction (XRD)
of four samples was shown in Figure 1k. The XRD pattern
for SnS2 with diffraction peaks shows clear and sharp diffrac-
tion peaks at 15.1°, 28.4°, 32.2°, 50.2°, and 52.6°. These peaks
correspond to crystal indexes of (001), (100), (110), and (101),
which can be indexed to the hexagonal phase SnS2 (JCPDS card
No.23-0677). However, no signal peaks other than SnS2 were
shown in the three Sn/SnS2 samples. We hypothesize that Sn
reduced by the in situ electrochemical reduction is amorphous.
This hypothesis was further verified by transmission electron
microscopy (TEM) results.
2. Results and Discussion
The SnS2 nanosheets were synthesized by a simple hydro-
thermal method, using SnCl4·5H2O and L-cysteine at 180 °C
for 12 h.[27] SnS2 is stable under ambient conditions, but under-
goes a reductive reaction at cathodic potentials. This reductive
transformation under cathodic condition can be written as
Equation (1)
SnS2 +2H2O+ 4e− →Sn +2HS− +2OH−
(1)
Because of the hydroxide ions formation in the process of
cathodic electrolysis, the reduction degree of SnS2 may depend
on the pH of the electrolyte. As prepared SnS2 was loaded onto
glassy carbon electrode (1 × 1 cm2) as the working electrode.
Cyclic voltammetry (CV) cycles were carried out in Ar saturated
four electrolytes with different pH (0.1 m NaOH, 0.1 m PBS,
0.1 m HCl, and 0.5 m HCl solution) at a scan rate of 50 mV s−1
to better understand the reduction reaction of SnS2 at cathode
potential (Figure S1, Supporting Information). In 0.1 m PBS,
0.1 m HCl, and 0.5 m HCl solution, a reduction peak appears at
0.7 V versus RHE in the second CV cycle, which is attributed
to the conversion of SnS2 to metallic Sn based on the following
X-ray photoelectron spectroscopy (XPS) data. The conversion of
SnS2 to Sn decreases upon cycling of potential and eventually
stays stable after 20 CV cycles. Hydrogen sulfide gas is further
formed and detected by lead acetate test paper (Figure S2, Sup-
porting Information) indicating that S2− is released into the
solution during the CV cycles. The same results did not occur
in the CV cycles under alkaline conditions, which mean that
SnS2 is still stable (the following data are further proof). After
20 CV cycles under 0.1 m PBS, 0.1 m HCl, and 0.5 m HCl solu-
tion conditions. The reduction products (accordingly denoted as
1-Sn/SnS2, 2-Sn/SnS2, and 3-Sn/SnS2 hereafter) attached
to the glassy carbon electrode were collected by ultrasound
Figure 2a,c shows the TEM images of the SnS2 and
2-Sn/SnS2, respectively. The insets are the corresponding selec-
tive area electron diffraction (SAED) patterns. The SAED of
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Figure 1. SEM images of a) SnS2, b) 1-Sn/SnS2, c) 2-Sn/SnS2, and d) 3-Sn/SnS2. XPS survey spectra of e) SnS2, f) 1-Sn/SnS2, g) 2-Sn/SnS2, and
h) 3-Sn/SnS2. i) Chemical compositions of SnS2 and three Sn/SnS2 samples. j) Raman spectra of SnS2 and three Sn/SnS2 samples. k) XRD pattern of
SnS2 and three Sn/SnS2 samples.
Figure 2. TEM images of a) SnS2. Inset in (a) is corresponding SAED patterns of SnS2. b) HRTEM lattice fringes of SnS2. c) TEM images of 2-Sn/SnS2.
Inset in (c) is corresponding SAED patterns of 2-Sn/SnS2. d) HRTEM lattice fringes of 2-Sn/SnS2.
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Figure 3. a) Yield rate of NH3 production and b) Faradaic efficiency at different applied potentials on 2-Sn/SnS2. c) Chronoamperometry curves of NRR
over 2-Sn/SnS2 at different applied potentials. d) UV–vis absorption spectra over 2-Sn/SnS2 at different potentials.
SnS2 reveals the polycrystalline nature and dominant (001),
(100), (101), and (102) diffraction planes. High-resolution
TEM (HRTEM) study illustrates obvious lattice fringes cor-
responding to the (102) plane of SnS2, shown in Figure 2b.
The amorphous nature of reduced Sn in 2-Sn/SnS2 samples is
directly evidenced by the SAED patterns, which do not reveal
any measurable diffraction rings or spots. HRTEM in Figure 2d
also indicates the amorphous structure of reduced Sn due to no
clear crystal lattice fringes.
In order to understand the role of Sn reduced under cathode
condition in NRR, we further investigated the NRR perfor-
mance of SnS2 and three Sn/SnS2 samples within a gas-tight
two-compartment electrochemical cell separated by a Nafion
117 membrane (Figure S5, Supporting Information). Electro-
catalytic N2 fixation of SnS2 was performed in a 0.1 m NaOH
aqueous solution for guarantee the stable existence of catalysts
during electrolysis. A glassy carbon electrode (1 × 1 cm2) coated
with SnS2 catalysts was used as a working electrode. All poten-
tials were converted to the RHE scale via calibration (Figure S6,
Supporting Information). After the electrolytic reaction for
4 h at −1.0 V versus RHE. The SnS2 attached to the working
electrode were collected. The elemental analysis (Figure S7,
Supporting Information) and SEM image (Figure S8, Sup-
porting Information) indicate that the morphology and element
composition of SnS2 remain unchanged after electrolysis in
alkaline electrolyte condition, which also means that SnS2 is
stable in an alkaline electrolytic environment. Since Sn is an
amphoteric metal, in order to ensure that Sn reduced is not
corroded by the electrolyte. The catalytic performance of three
Sn/SnS2 samples was further tested in N2-saturated 0.1 m PBS
electrolyte (pH = 7.4) toward N2 electrochemical reduction. To
prevent the Sn reduced from being oxidized by the air during
the transfer process. After 20 CV cycles under 0.1 m PBS; 0.1 m
HCl solution conditions. A glassy carbon electrode (1 × 1 cm2)
coated with cathodic reduction product (1-Sn/SnS2; 2-Sn/SnS2)
was immediately used as a working electrode for next nitrogen
fixation process. For electrocatalytic N2 fixation of 3-SnS2. Due
to the severe corrosion of 3-Sn/SnS2 in 0.5 m HCl solution, it
is difficult to estimate the quality of Sn reduced in 3-Sn/SnS2
sample. 0.1 mg 3-Sn/SnS2 was reattached to the glassy carbon
electrode for NRR test. A platinum foil (2.5 × 2.5 cm2) serves
as a counter electrode, the Ag/AgCl/KCl electrode (saturated
KCl electrolyte) as the reference electrode. Figure S9 (Sup-
porting Information) shows linear sweep voltammetry curves
of 2-Sn/SnS2 in Ar and N2-saturated 0.1 m PBS solution. It is
clearly seen that 2-Sn/SnS2 attains a high current density in
N2-saturated solution, indicating efficient NRR occurs. The
reduction products, NH3, was quantified by a calibrated indo-
phenols blue method.[29] The calibration curves for quantita-
tively determining NH3 are shown in Figure S10 (Supporting
Information). Another possible solution-phase product, N2H4,
was also quantified by a spectrophotometric method developed
by Watt and Chrisp.[29,30] However, the absorption spectrum of
the cathode electrolyte indicates no difference after 2 h of elec-
trolysis, indicating that NH3 is the only product in this study
(Figure S11, Supporting Information).
The highest NH3 yields rate for three Sn/SnS2 electro-
catalysts were then compared. The NH3 yields rate and
corresponding Faradaic efficiency, ultraviolet–visible light
(UV–vis) absorption spectra and current densities of 2-Sn/SnS2
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Figure 4. a) The composition analysis and calculated mass of Sn loading on glassy carbon electrode in different samples. b) The highest NH3 yield
rate of different samples. c) The relationship between the mass of amorphous Sn loading on glassy carbon electrode and the highest ammonia yield.
d) Yield rate of three Sn/SnS2 samples based on mass of unit amorphous Sn.
at potentials between −0.7 and −1.1 V are plotted in Figure 3. As
shown in Figure 3a,b, the NH3 yields rate reaches the highest
value of 23.8 µg h−1 mg−1 and FE of 3.4% at −0.8 V versus RHE.
Beyond this negative potential, the reduction rate and Faradaic
efficiency decrease significantly, which is attributed to the com-
petitive adsorption of N2 and hydrogen species on the electrode
surface.[31–33]
in Figure 4a. The highest NH3 yield rate of different sam-
ples are shown in Figure 4b. The mass of amorphous Sn in
Sn/SnS2 samples is proportional to their NRR yield (Figure 4c).
The drop of the 3-Sn/SnS2 point may be due to the oxidation
of Sn in the 3-Sn/SnS2 sample by air during the transfer pro-
cess. The yield rate of three Sn/SnS2 samples based on mass of
unit amorphous Sn did not differ greatly (Figure 4d). Based on
this result, it can be concluded that the amorphous Sn is active
phase in the nitrogen reduction process.
In addition, our experimental results also show that amor-
phous Sn is superior over their crystalline counterparts in NRR.
As shown in Figure S15a (Supporting Information), crystal-
line Sn was successfully synthesized by reducing SnCl2 with
NaBH4 in aqueous solution,[34] the SEM image (Figure S15b,
Supporting Information) of crystalline Sn suggests the forma-
tion of roughed nanoparticles with an average particle size of
≈100 nm. UV–vis absorption spectra and current densities
of crystalline Sn at are plotted in Figure S15c,d (Supporting
Information). The obtained crystalline Sn nanoparticles
endow the electrocatalytic NRR with the high ammonia yield
of 3.5 µg h−1 mg−1 at −0.7 V versus RHE, much lower than
that of 2-Sn/SnS2 (Figure S16, Supporting Information). The
improved NRR performances of 2-Sn/SnS2 may be attributed to
more reactive sites for nitrogen molecules and the presence of
defect sites.[21,22,35]
Moreover, the stability of 2-Sn/SnS2 was further investi-
gated. A durability test consisting of 25 h of chronoampero-
metric runs showed a slight change in current density
(Figure 5a). After five cycles of chronoamperometric runs
at −0.8 V versus RHE (Figure 5b), no obvious change was
found in the NH3 yield rate (80% performance retention)
and Faradaic efficiency (Figure 5c,d), suggesting that 2-Sn/
SnS2 is still highly active for the NRR. In addition, in order to
further explore whether 2-Sn/SnS2 could be further reduced
in 0.1 m PBS solution under cathode condition, SEM, XRD,
and elemental analysis of 2-Sn/SnS2 before and after NRR
Respectively, the other two Sn/SnS2 samples (1-Sn/SnS2 and
3-Sn/SnS2) also showed significant decrease of both NH3 pro-
duction rates and Faradaic efficiency. UV–vis absorption spectra
and current densities of 1-Sn/SnS2 and 3-Sn/SnS2 are plotted
in Figures S12 and S13 (Supporting Information). The highest
ammonia yield rate (9.2 µg h−1 mg−1) of 1-Sn/SnS2 are achieved
at −0.9 V versus RHE, while the highest ammonia yield rate
(12.5 µg h−1 mg−1) of 3-Sn/SnS2 are obtained at −0.9 V versus
RHE based on the corresponding UV–vis absorption spectra.
SnS2 sample was carried out at four different voltages for 2 h
under 0.1 m NaOH electrolytes condition. Chronoamperom-
etry curves of NRR at different applied potentials are shown in
Figure S14a (Supporting Information). In the cathode electro-
lyte after the electrolysis reaction, UV–vis absorption spectra at
different potentials show no difference indicated that ammonia
is hardly detected (Figure S14b, Supporting Information).
SnS2 alone shown no activity for NRR, which indicates
that the conversion of SnS2 to the amorphous Sn phase
leads to a substantial increase in NRR catalytic activity. To
further verify the relationship between amorphous Sn con-
tent and the nitrogen reduction performance, the amorphous
Sn mass of four samples during NRR was calculated based
on the molar ratio of Sn to SnS2 and the mass of the corre-
sponding Sn/SnS2 sample. Since S2− is released into the solu-
tion during the CV cycles. The mass of the catalyst attached to
the glassy carbon electrode was reduced from 0.1 mg to about
0.089 and 0.073 mg in 1-SnS2 and 2-SnS2 based on chemical
compositions. The mass of amorphous Sn loading on glassy
carbon electrode during NRR in different samples are shown
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Figure 5. a) Time-dependent current density curve for 2-Sn/SnS2 at −0.8 V. RHE. b) Chronoamperometry results of 2-Sn/SnS2 for a stability test
recorded at the potential of −0.8 V versus RHE. c) Yield rate of NH3 production and d) FE during the recycling test on 2-Sn/SnS2. e) UV–vis absorp-
1
tion spectra over Sn/SnS2 under differences conditions. f) H-NMR spectra of PBS solutions after electrochemical reduction at −0.8 V versus RHE
using 15N2 and Ar as the feeding gas.
reaction were conducted (Figure S17, Supporting Informa-
tion). These results did not change significantly, which means
that 2-Sn/SnS2 has certain stability in NRR reaction in 0.1 m
PBS electrolyte under cathodic potential. In order to verify
the NH3 generated from N2 reduction catalyzed by 2-Sn/SnS2,
control experiments, including i) N2 gas is fed into the cell
at open-circuit; ii) An Ar gas flow is fed into the cell are per-
formed. The photographs of their colorimetric assay showed
no color difference (Figure 5e), indicated that no ammonia
was detected. Furthermore, nuclear magnetic resonance (The
isotopic labeling NMR) was further used to verify the compo-
work further expands the possibilities of low-cost and readily
available Sn-based catalysts in NRR.
4. Experimental Section
Preparation of SnS2 Sample: SnS2 electrocatalyst materials were
synthesized by a simple hydrothermal method. A typical synthesis of
SnS2 was given as follows: 1 × 10−3 m of tin (IV) chloride pentahydrate
(SnCl4 ·5H2O) and 5 × 10−3 m L-cysteine were added to a 60 mL of distilled
water under vigorous magnetic stirring. The mixture was sonicated for
30 min and then transferred to a 100 mL teflon-lined stainless autoclave,
followed by being heated to 180 °C and kept for 12 h. The sample was
collected after several washing with deionized water and ethanol, and
vacuum dried at 60 °C overnight.
Preparation of Sn/SnS2 Sample: Typically, about 2 mg SnS2 and
0.1 mL of Nafion solution (5 wt%) were dispersed in 4 mL of absolute
ethanol by ultrasonication for 1 h to form a homogeneous ink. The ink
(0.2 mL) was then loaded onto a glassy carbon electrode (1 × 1 cm2)
and dried under ambient conditions. The electrode was assembled
into in a gas-tight H-type cell separated by a Nafion 117 membrane
in different electrolytes (0.1 m PBS solution, 0.1 m HCl solution, and
0.5 m HCl solution) purged with Ar gas. After 20 CV cycles, SnS2 was
partially reduced to metallic Sn. The electrode coated with reduction
products (Sn/SnS2) was ultrasonic and the sample was collected after
several washing with deionized water and ethanol, and vacuum dried
at 60 °C overnight. The Sn/SnS2 samples reduced in 0.1 m PBS solution
were named 1-Sn/SnS2. The Sn/SnS2 samples reduced in 0.1 m HCl
solution were named 2-Sn/SnS2.The Sn/SnS2 samples reduced in 0.5 m HCl
solution were named 3-Sn/SnS2.
sition of the synthetic product (Figure 5f).[36,37] Doublet peaks
1
(J1
= 73 Hz) were found in the H-NMR spectrum of the
NH
+
PBS solutions. When comparing with a standard 15NH4
sample (Figure S18, Supporting Information), 1H-NMR
+
spectra reveal that 15NH4 could be detected when 15N2 was
fed. This result further proves that our cathode reduction
products are derived from the reduced N2.
3. Conclusion
To summarize, we successfully synthesize the amorphous
Sn/crystalline SnS2 nanosheets via the in situ transformation of
SnS2 nanosheet under cathodic electrochemical environments.
Comparative studies of the Sn/SnS2 and SnS2 samples reveal
that the reduced Sn forming from the SnS2 under cathodic con-
dition operates as the catalytic phase for N2 reduction. We also
demonstrate that Sn/SnS2 are efficient NRR catalysts with an
ammonia production rate of 23.8 µg h−1 mg−1 at −0.8 V versus
RHE and FE of 6.5% at −0.7 V versus RHE. Therefore, this
Preparation of Crystalline Sn Sample: To prepare crystalline Sn,
190 mg of anhydrous SnCl2 was added into 50 mL of aqueous solution
containing 380 mg of NaBH4, and the mixture was stirred at 25 °C for
30 min. The sample was collected after several washing with deionized
water and ethanol, and vacuum dried at 60 °C overnight.
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Cathode Preparation: For electrochemical NRR measurement of
Sn/SnS2. A glassy carbon electrode (1 × 1 cm2) coated with 0.1 mg SnS2
was assembled into in a gas-tight H-type cell separated by a Nafion
117 membrane in different electrolytes (0.1 m PBS solution, 0.1 m HCl
solution, and 0.5 m HCl solution) purged with Ar gas. After 20 CV cycles
in 0.1 m PBS and HCl solution, the electrode coated with reduction
products (1-Sn/SnS2; 2-Sn/SnS2) was used as a working electrode for
next nitrogen fixation process. For electrochemical NRR measurement
of 3-SnS2 and SnS2. A glassy carbon electrode (1 × 1 cm2) coated with
0.1 mg 3-Sn/SnS2 and SnS2 was used as a working electrode for nitrogen
fixation process.
for 4 h, 20 mL of the electrolyte was taken out and acidized to pH ≈3 by
adding 0.5 m H2SO4, and then concentrated to 1 mL by heating at 70 °C.
Afterward, 0.95 mL of the resulting solution was taken out and mixed
with 0.05 mL d6-DMSO for 1H-NMR measurement (nuclear magnetic
resonance, 500 MHz).
Elemental Analysis: EDX and ICP-optical emission spectrometry were
performed to determine elemental compositions of SnS2 and three Sn/
SnS2 samples. The amount of Sn0 was quantified through measuring the
elemental S and Sn molar ratio using EDX and ICP of the samples.
Electrochemical
measurements were performed using in
NRR
Measurements:
All
electrochemical
Supporting Information
a
gas-tight H-type cell
separated by a Nafion 117 membrane and an electrochemical working
station (CHI 660E, Shanghai CH Instruments Co., China). The three-
electrode system included the as-prepared electrode (1 × 1 cm2)
coated with SnS2 or three Sn/SnS2 samples as the working electrode,
platinum foil (2.5 × 2.5 cm2) as the counter electrode, and a silver/
silver chloride electrode (Ag/AgCl; saturated KCl electrolyte) as the
reference electrode. Prior to the NRR measurements, the Nafion
117 membrane was pretreated by heating it in 5% H2O2, 0.5 m H2SO4, and
ultrapure water at 80 °C for 1 h. For electrochemical NRR measurement
of three Sn/SnS2 samples, potentiostatic tests were conducted in N2-
saturated 0.1 m PBS solution (pH = 7.4, 20 mL) at different potentials.
For electrochemical NRR measurement of SnS2, the potentiostatic
tests were conducted in N2-saturated 0.1 m NaOH solution (pH = 12.9,
20 mL). The potentiostatic tests were performed at each potential for
2 h. Pure N2 was continuously fed into the cathodic compartment
during the measurements.
Characterizations: XRD patterns of the samples were acquired on
Bruker D8 Advance X-ray powder diffractometer with a scan range of
5°–80°. Raman spectra were obtained on Nicolet Magna-R760 Raman
tester, with a scan range of 50–2000 cm−1. XPS was taken from the PHI
5000C ESCA system. The morphology and microstructural properties of
the as-prepared materials were characterized by SEM (Tescan MAIA3
XMH) and TEM (JEOL 2010). Its elemental composition was given by
the Bruker Xflash Model 6160 EDX and inductively coupled plasma
optical emission spectrometry (ICAP 7400).
Supporting Information is available from the Wiley Online Library or
from the author.
Conflict of Interest
The authors declare no conflict of interest.
Keywords
ambient conditions, electrocatalysis, N2 reduction, Sn-based catalysts,
SnS2
Received: May 17, 2019
Revised: July 21, 2019
Published online:
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