A Solution-based Method for Synthesizing Pyrite-type Ferrous Metal Sulfide Microspheres with Efficient OER Activity

Article abstract of DOI:10.1002/asia.202000504

Simple and stable synthesis of transition metal sulfides and clarification of their growth mechanisms are of great importance for developing catalysts, metal-air batteries and other technologies. In this work, we developed a one-step facile hydrothermal approach to successfully synthesize NiS₂ microspheres. By changing the experimental parameters, the reason that affects the formation of nanostructured spheres is investigated and discussed in detail, and the formation mechanism of microspheres is proposed innovatively. Furthermore, electrochemical testing results show that the 7 h-NiS₂ catalyst exhibits a remarkable oxygen evolution reaction (OER) activity with an overpotential of 311 mV at 10 mA cm^?2 in 1.0 M KOH, superior to precious metal RuO₂. The NiS₂ catalyst also exhibits a robust durability. This work will contributes to the rational design and the understanding of growth mechanism of transition metal chalcogenide electrocatalysts for diverse energy conversion technologies.

Full text of DOI:10.1002/asia.202000504

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: A Novel Solution-based Method for Synthesizing Pyrite-type
Ferrous Metal Sulfide Microspheres with Efficient OER Activity
Authors: Wendi Xiang, Qianqiu Tian, Cheng Zhong, Yida Deng,
Xiaopeng Han, and Wenbin Hu
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A Novel Solution-based Method for Synthesizing Pyrite-type
Ferrous Metal Sulfide Microspheres with Efficient OER Activity
Wendi Xiang[a], Qianqiu Tian[a], Cheng Zhong[a,b], Yida Deng[a], Xiaopeng Han* [a,c], Wenbin Hu* [a,b]
[a]
W. D. Xiang, Q. Q. Tian, C. Zhong, Y. D. Deng, X. P. Han, W. B. Hu
School of Materials Science and Engineering, Tianjin Key Laboratory of Composite and Functional Materials, Key Laboratory of Advanced Ceramics and
Machining Technology (Ministry of Education)
Tianjin University
Tianjin 300072, China
E-mail: xphan@tju.edu.cn; wbhu@tju.edu.cn
[b]
[c]
Joint School of National University of Singapore and Tianjin University
International Campus of Tianjin University
Binhai New City, Fuzhou 350207, China
Jiangsu Key Laboratory of Electrochemical Energy-Storage Technologies, College of Materials Science and Technology, Nanjing University of Aeronautics
and Astronautics, Nanjing 210016, China
Abstract: Simple and stable synthesis of transition metal sulfides
and clarification of their growth mechanisms are of great importance
for developing catalysts, metal-air batteries and other technologies.
In this work, we develop a one-step facile hydrothermal approach to
successfully synthesize NiS2 microspheres. By changing the
experimental parameters, the reason that affects the formation of
nanostructured spheres is investigated and discussed in detail and
the formation mechanism of microspheres is proposed innovatively.
Furthermore, electrochemical testing results show that the 7h-NiS2
catalyst exhibits remarkable oxygen evolution reaction (OER) activity
with an overpotential of 311 mV at 10 mA cm-2 in 1.0 M KOH,
superior to precious metal RuO2. The NiS2 catalyst also exhibits
robust durability. This work will contributes to the rational design and
the understanding of growth mechanism of transition metal
chalcogenide electrocatalysts for diverse energy conversion
technologies.
utilization, the use of surfactant or un-environmentally friendly
solvent. Moreover, to the best of our knowledge, most reports
mainly focused on the electrochemical performance, while few
systematic studies have been conducted on the discussion
about the growth mechanism of metal sulfide materials. One of
the major constraints is the reaction process cannot be directly
observed in solvothermal reactions and the complexity of the
multiple crystal forms of nickel sulfide systems, making it rather
difficult to excavate the growth mechanism of this type of
material. Therefore, it is still a challenge to synthesize phase-
pure pyrite-type disulfides with a simple, universal and green
method and explain its growth mechanism under mild
environments.
In this work, we present a one-step facile solvothermal
strategy to successfully prepare NiS2 and CoS2 microspheres. In
contrast to the conventional approach, our approach does not
require complex precursors or polluting solvent and the
morphologies of the products are well controlled. Moreover, a
growth mechanism of the pyrite-type disulfides microspheres
has been proposed by adjusting the added ratio of sulfur to
nickel in the raw materials, reaction time and solvent ratio.
Electrochemical testing results indicated that the 7h-NiS2
Introduction
The transition metal chalcogenides have the advantages of low
cost, high abundance, good intrinsic conductivity and good
structural stability [1-3]. Particularly, it has attracted extensive
attentions in magnetic, optical, electrical and catalytic fields,
such as catalysts, supercapacitors, lithium-ion batteries, dye-
sensitized solar cells, [4, 5] etc. In recent years, some synthetic
strategies, such as microwave assisted method, high
temperature solid phase method and electrochemical methods,
have been widely developed [6-8]. Among them, hydrothermal
and solvothermal methods based synthetic strategies, under
relatively low temperature and mild atmosphere, have shown
unique charm in the liquid-phase synthesis of nanoparticles and
the synthesis and control of low dimensional materials [9, 10]. For
example, Zhang et al. synthesized a series of the Co-doped
FeS2 nanospheres, delivering discharge capacity of 0.220 Ah g-1
microspheres
exhibited
the
best
catalytic
activities:
overpotentials of 311 mV to achieve a current density of 10 mA
cm-2 in 1.0 M KOH for OER. The 7h-NiS2 microspheres also
exhibited long-term catalytic durability. This work not only
contributes to the controllable synthesis strategy of pyrite-type
disulfides electrode materials, but also provides deeper
understanding of the nucleation reaction and growth mechanism.
Results and Discussion
after 5000 cycles at 2 A g-1 with an ether-based electrolyte and
[11]
in a voltage range of 0.8–2.9 V for sodium-ion batteries
.
Moreover, a series of NiSx (i.e., NiS, Ni3S2, NiS2) nanocrystals
with controllable phase and composition have been synthesized
via a facile polyol solution process and the corresponding
electrocatalytic properties towards oxygen evolution reaction
(OER) and hydrogen evolution reaction (HER) have been
systematically investigated [12]. However, up to date, most
synthetic strategies mainly depend on sulfur sources with low
Figure 1. Schematic illustrating the preparation process of NiS2 microsphere.
1
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Figure 2. (a) XRD patterns of the NiS2 with different reaction time. (b-e) SEM images of the NiS2 with different reaction time: (b) 1h, (c) 3h, (d) 7h, and (e) 12h. (f-
h) EDS mapping images of NiS2 sample with reaction time of 7h.
A
schematic illustration for the preparation process is
The scanning electron microscopy (SEM) images of the as-
prepared NiS2 for different reaction time (1, 3, 7, 12 h) were
displayed in Fig. 2b–e and S2. SEM images reveal a high
similarity in morphology and structure of different samples,
which are composed of numerous interconnected spheres with
typical average diameter of 150 nm, 600 nm, 1 μm and 1.6 μm
for 1h-NiS2, 3h-NiS2, 7h-NiS2 and 12h-NiS2, respectively. The
detailed morphology and structural features of NiS2 were further
demonstrated by TEM measurement (Fig. S3a and S3b). TEM
results of NiS2 showed the shape of the microspheres connected
to each other and both solid spheres and a small amount of
core-shell hollow spheres were observed. Moreover, HRTEM
image show that observed neighbouring distances of
approximately 0.326 nm correspond to the distances of the (230)
planes of NiS2; 0.143 nm and 0.151nm correspond to the
distances of the (121) and (311) planes of sulfur, respectively
(Fig. S3c). It can be seen some residual sulfur still existed.
According to the elemental mapping of NiS2 (Fig. 1f-h), the Ni
(green) and S (blue) distribute homogeneously. In order to
explain the internal structure of the microsphere, the cross
section of a NiS2 microsphere was observed. TEM images were
taken on the grinded NiS2 microsphere, showing that the inner
structure of NiS2 microspheres is composed of tiny NiS2
nanocrystallites with an average size of 30–50 nm (Fig. S4).
X-ray photoelectron spectroscopy (XPS) was employed to
characterize the chemical states and surface valence states of
the prepared samples. The collected chemical compositions are
consistent with the results from EDS (Fig. S5a). The Ni 2p
spectra of NiS2 are consisted of two shakeup satellites and four
primary core level peaks (Fig. S5b). The peaks located at 869.2
eV and 872.85 eV are assigned to Ni1/2 of Ni2+ and Ni3+,
demonstrated in Fig. and the detailed programmed
1
temperature and time are also shown (Note: room temperature
was set to be 25 °C and the heating rate was constant. When
they reached the set temperature, it was held for 12h). In our
synthesis strategy, sulfur powder and Ni (CH3COO)2·4H2O were
used as the sulfur resource and the Ni resource to synthesize
NiS2. N, N-Dimethylformamide (DMF) and ethanol were selected
as solvents. Most synthesis methods took complex molecular
[13]
[14]
precursor
solvent
,
surfactant
or un-environmentally friendly
[11]
as the raw material. Compared with them, the
solvents used in our synthesis strategy have a relatively low
toxicity, viscosity and cost. This also improves the security and
convenience in the experimental operation process. In
comparison to CS2 [15], thiourea [16], and other sulfur sources, the
use of sulfur powder can improve the utilization rate of sulfur
element. In addition, the mechanism of Oswald ripening and the
coordination between DMF and Ni ions are involved in the
synthesis process. Because Fe2+, Co2+ and Ni2+ have similar
properties and can coordinate with DMF, the method and growth
mechanism are expected to be suitable for synthesizing other
pyrite-type disulfides.
As shown in Fig. 2, X-ray diffraction (XRD) was employed to
analyze the phase and purity of the product. All diffraction peaks
belong to cubic NiS2 (JCPDS 11-0099), similar to the previous
report about Ni-based system [12]. Moreover, energy dispersive
spectroscopy (EDS) was performed to determine the chemical
composition. The results reveal that no other element was
observed except Ni and
quantification of the peaks shows that the molar ratio of S and Ni
is 2.02, which is quite close to the stoichiometric value of NiS2.
S (Fig. S1). Furthermore, the
2
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respectively [8, 17]. The peaks located at 852.05 eV and 855.04
eV are assigned to the Ni 2p3/2 peaks of Ni2+ and Ni3+. Other
peaks at 859.17 and 878.22 eV are shakeup satellites, indicating
the existence of the Ni2+ oxidation state. Fig. S5c shows the S
2p spectra. The peak appearing at binding energy of 160.04 eV
part of S2- cannot be reduced to S22-. Instead, NiS was
generated via combining S2- with Ni2+. Therefore, diffraction
peaks of NiS ((101), (102)) were observed in sample 1/4 and 1/6.
2-
is assigned to S2- and the peak at 162.01 eV corresponds to S2
of S 2p3/2. The other two fitting peaks at 160.89 and 163.05 eV
are ascribed to the S 2p1/2 of S2- and S22-. The peaks at 167.2 eV
and 168.3 eV are assigned to the S 2p3/2 and S 2p1/2 of S-O
bond [18-21]. Accordingly, it can be concluded that the NiS2
sample contain Ni2+, Ni3+, S2- and S22-, in accordance with the
previous observations in the literature [22]
.
Optical absorption spectrum of NiS2 was shown in Fig. S6.
The spectrum was analyzed to acquire the band-gap energy
(Ebg) of the NiS2 by Tauc’s formula. Tauc’s formula shows the
relationship between the absorption coefficient and the incident
photon energy [23, 24], which is illustrated by αhν = A(hν−Eg )n,
Where ‘α’ is the absorption coefficient, ‘A’ is a constant, and n =
1/2 (for a direct transition semiconductor) or n = 2 (for indirect
transition semiconductors). From the chart inserted into Figure
S6, the value of the Ebg was found to be ∼ 2.74 eV, which is in
good accordance with the band structure of NiS2
Figure 4. SEM images of NiS2 with different Ni/S ratios: (a) 1/4, (b) 1/6, (c) 1/8,
(d) 1/12.
[23,
24]. To
investigate the porous characteristics of the NiS2, the nitrogen
adsorption and desorption isotherms was carried out at 77 K.
The hysteresis loop presented in Fig. S7 can be classified as
type IV and the Brunauer-Emmett-Teller (BET) surface area is
determined to be about 35 m2 g-1, which can be possibly further
enhanced by introducing nano/mesopore structures.
As shown in Fig. 4 and S8, large quantities of spheres with
diameters of approximately 500 nm–1.7 μm were observed. The
smaller nanospheres coalesced and stacked together irregularly
to form bulky nanostructures with 1/4 Ni/S molar ratio. With Ni/S
molar ratio decreased, the bonding between the spheres is
closer and tends to reunite into larger spheres (Fig. 4b). Thus,
the decreased Ni/S molar ratio leads to the increased diameter
of the final microspheres until to the maximal size, which is
determined by the aggregation barrier of the system (Fig. 4c and
d) [29]. It can be clearly seen that as the sulfur content increases,
small spheres gradually integrated into large spheres. On the
basis of the above analysis, it is believed that Ostwald ripening
[30,
play an important role in NiS2 growth process
31]. In this
process, small-sized NiS2 microspheres gradually disappeared
and accompanied by the growth of large-sized microspheres.
Moreover, sulfur is liquid at 180 °C with fluidity and viscosity and
it tends to form a spherical surface to reduce the surface energy
of the system. Therefore, with the sulfur content increases, the
aggregation of sulfur droplets becomes more intense, resulting
in the generation of larger NiS2 microspheres.
In this binary solution system, the phase and the morphology
of the products are strongly influenced by used solvents of DMF
or ethanol. The XRD patterns of prepared samples in a mixed
solvent of DMF and ethanol (volume ratio is 1:1, Fig. 2a) indicate
that the product is pure NiS2 while other impurities will form
when only use the DMF or ethanol as the solvent (Fig. 5a). The
morphologies of the surfaces were characterized using scanning
electron microscopy (Fig. 5b, c and Fig. S9). The DMF-only
sample is composed of the accumulated microspheres but the
ethanol-only sample contains a huge number of chunks and
irregular particles. Moreover, DMF and ethanol act as reducing
agents to provide S2- for the nucleation reaction. At the same
time, sulfur atoms can replace oxygen atoms in DMF to form N,
N-dimethylthioamide at high temperature [32]. When using only
DMF, a large amount of sulfur is consumed via the above
Figure 3. XRD pattern of the NiS2 with different Ni/S molar ratios. The bottoms
show the standard peaks of NiS and NiS2.
The Ni/S molar ratio has an important effect on the
morphology and phase of the product. The powder XRD
patterns of the NiS2 with different Ni/S molar ratios were shown
in Fig. 3. For the samples prepared with high sulfur content (1/12,
1/8), all the diffraction peaks can be indexed to the cubic NiS2
phase (JCPDS 11-0099). As the Ni/S molar ratio decreased to
1/6 and 1/4, the diffraction peaks are assigned to the hexagonal
NiS (JCPDS no.02-1280) and the cubic NiS2 phase (JCPDS 11-
0099). Since no strong reducing agents were added in reaction,
it is thought that DMF and ethanol can act as reducing agent at
high temperature [25-28]. Therefore, sulfur can be reduced to S2- in
2-
reaction, so that S2- can’t be converted into S2 completely.
2-
Therefore, the phase of NiS appeared in XRD pattern of DMF-
only samples. In addition to being used as a reducing agent and
solvents and then form S2 with the surplus of sulfur and finally
react with Ni2+ to form NiS2. As the Ni/S molar ratio increased, a
3
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a reaction raw mate-rial, DMF is also used as a ligand for Ni2+. It
has been reported that DMF is O-bonded to nickel as
coordination via nitrogen and it is believed that DMF plays an
important role in controlling precipitation processes [33]. The
loss of sulfur between 220 °C and 340 °C later on [40]. A final
20.72 % weight loss between 380 °C to 550 °C is observed and
the phase of the residues collected after calcination at 380 °C for
2 h has also been examined. The XRD patterns in Figure S11
indicated that the product collected after 380 °C treatment was a
mixture of Ni0.96S and NiS2 phases. In the differential scanning
calorimetry (DSC) curve, the endothermic peaks around 90 °C
and 120 °C are attributed to the residual H2O and DMF. The
peaks between 260 °C and 300 °C present the violent
volatilization of sulfur and the peak between 350 °C to 550 °C is
endothermic caused by the decomposition of NiS2. The above
analysis indicates that the prepared NiS2 sample contains a
certain amount of sulfur. This is ascribed to the use of excess
sulfur powder during the solvothermal reaction, which will be
further optimized in the next research stage.
sample without DMF presents irregular bulk aggregations, which
2−
may be attributed to the fast reaction between Ni2+ and S2
.
Herein, another function of DMF is to form a complex with Ni2+
ions through a coordination interaction that controls the release
rate of Ni2+ from the complex, thus reducing the precipitation rate
of NiS2 and enhancing the product regularity [34, 35]. As shown in
Figure S10, DMF shows characteristic absorption bands at 1660
and 655 cm-1 [36]. When DMF binds to Ni2+, the band at 1660 cm-
1 shifts to 1659.16 cm-1 due to the weakening of the double-bond
nature of the carbonyl bond. Meanwhile, the peak at 655.8 cm-1
shifts to 658.28 cm-1, possibly due to the increase in the order of
the amide C-N bond [37, 38]. It should be noted that a new peak
appear at 410.47 cm-1, which can be attributed to Ni-O bond
formation [39]
.
Figure 6. TG and DSC curves of the 12h-NiS2 under N2 flow with a heating
rate of 10 °C min-1.
During the synthesis, the NiS2 nanospheres constructed by
nanocrystalline grains can be obtained through a hydrothermal
process. In the reaction, the Ni2+, DMF, ethanol and sulfur can
form a similar oil/water system. DMF and ethanol react with
sulfur to release S2- at the DMF/sulfur interface and S2- react with
2-
sulfur to generate S2 to provide sulfur source for the formation
of NiS2. The whole reaction can be simplified as the following：
S (DMF, ethanol) →S2-
(1)
(2)
(3)
2-
S2- + S →S2
Ni2+ + S22- →NiS2
Based on the above experimental observations, a plausible
formation mechanism of the pyrite-type disulfides microspheres
is proposed and illustrated in Fig. 7. The formation process
mainly can be divided into three stages, including the formation
of DMF-Sulfur system, crystallization processes and growing
processes. Firstly, DMF is a strong Lewis base known to form
bond with metallic cation via its oxygen atom and it coordinate
with Ni2+ to form a stable [Ni(dmf)6]2+ complex [41-43]. After the
temperature rises to 180 °C, the sulfur is converted into a liquid
state and forms a liquid-liquid heterogeneous solution with DMF.
During the middle stage of the hydrothermal process, the DMF
can carry Ni2+ from the solvent phase to the Liquid-liquid
interface. The DMF reacts with sulfur to release S22-, and then
reacts with Ni2+ to form NiS2 nuclei. Afterwards, NiS2
nanocrystalline is aggregated or coalesced to assemble the NiS2
microspheres due to van der Waals force (intermolecular force)
and violent collisions caused by Brownian motion [44, 45]. Lastly,
Ostwald ripening process proceeded, eliminating crystallites of
Figure 5. XRD pattern (a) and SEM images of prepared samples with solvent
of (b) DMF-only, (c) ethanol-only.
Thermogravimetric analysis (TGA) was performed to
characterize the thermal stability. The TGA curve in Fig. 6
reveals a 53 % weight loss of the NiS2 sample in total, which is
composed of 10 % weight loss of residual H2O and DMF
between 70 °C and 110 °C at first and then a 21.53 % weight
4
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smaller sizes. Whilst the growth of the larger ones moves along,
the morphology of interconnected microspheres is finally formed.
catalyst exhibits a considerable overpotential of 311 mV (Table
1), which is lower than those of 12h-NiS2 (341 mV) and 1h-NiS2
(390 mV) and the precious metal IrO2 catalyst (350 mV).
Moreover, the Tafel slope of the 7h-NiS2 electrode is fitted to be
60 mV dec-1 (Fig. 8b), which is smaller than those of 12h-NiS2
(72 mV dec-1), 1h-NiS2 (102 mV dec-1) and IrO2 (70 mV dec-1). It
has been widely accepted that, the elementary reaction steps of
the complete OER mechanism in alkaline solution are as follows
[46]: (1) M + OH- → MOH + e-, (2) OH- + MOH → MO + H2O + e-,
(3) MO + OH- → MOOH + e-, (4) MOOH + OH- → O2 + H2O + e-,
where M is the active site on the surface. Generally, the smaller
Tafel slope indicates faster kinetics for OER catalysis and more
rapid increase in the rate of O2 generation with the applied
[47,
overpotential
48]. Electrochemical impedance spectroscopy
(EIS) reveals that the charge-transfer resistance (Rct) values are
around 4.13, 1.35 and 1.4 Ω for 1h-NiS2, 7h-NiS2 and 12h-NiS2,
respectively (Fig. 8c), suggesting that 7h-NiS2 has the fastest
charge transfer ability. This trend is in accordance with the
polarization results and Tafel slopes. Apart from the high activity,
the 7h-NiS2 catalyst also exhibits catalytic durability which is
further corroborated by the overlaid OER curves after the 1st
and 1000th cyclic voltammetry (CV) cycles. XPS and HRTEM
reveal that an amorphous oxide layer appears after 1000 OER
Figure 7. Schematic illustration of formation mechanism of NiS2 microspheres.
To further demonstrate the versatility of this method, we
extended the synthesis to preparation of other ferrous metal
sulfide materials. The example of CoS2 microspheres shown in
Fig. S12 and S13 is a testament to the feasibility and generality
of this approach in controlling the morphology of pyrite-type
disulfides. XRD was used to analyze the phase and purity of the
product. All diffraction peaks of the samples are assigned to
cubic CoS2 (JCPDS 41-1471). EDS was performed to determine
the chemical composition. The results reveal that no other
element was observed except Co and S (Fig. S12). Furthermore,
the quantification of the peaks shows that the molar ratio of S to
Co is 1.9, agreeing with the standard value of CoS2. SEM
images indicate the microsphere structure morphology of
prepared CoS2 that is very similar to that of NiS2 (Fig. S13).
cycles (Fig. S14 and S15), consistent with previous literatures
[49-52]
.
Among different samples, 7h-NiS2 shows the best
performance for OER in alkaline media and is comparable to
many previously reported Ni sulfides (Table 2). The poor activity
of 1h-NiS2 should be due to that NiS2 crystallized incompletely
and the unreacted sulfur powder covered the OER active site
after one-hour synthesis procedure. With longer reaction time,
the NiS2 microspheres gradually grew larger with the increased
crystallinity and decreased content of sulfur powder. Therefore,
7h-NiS2 shows the best performance. As the reaction continued,
the specific surface area of the microspheres decreased due to
the Oswald process, presenting inferior catalytic activity.
Table 1. Comparison of performance parameters of 1h-NiS2, 7h-NiS2 and 12h-
NiS2 in this work.
Overpotential
@10mA cm-2
(mV)
Tafel slope
(mV dec-1)
Catalysts
Reaction
Rct (Ω)
1h-NiS2
7h-NiS2
12h-NiS2
IrO2
OER
OER
OER
OER
390
311
341
350
101
60
4.13
1.35
1.41
72
70
Figure 8. (a) OER polarization curves of Bare CP, 12h-NiS2, 7h-NiS2, 1h-NiS2,
and IrO2 in 1.0 M KOH at a scanning rate of 5mV s-1. (b) Tafel plots of 12h-
NiS2, 7h-NiS2, 1h-NiS2, and IrO2. (c) EIS spectra of 12h-NiS2, 7h-NiS2 and 1h-
NiS2. (d) Catalytic durability of 7h-NiS2.
Table 2. Comparison of OER performance parameters in this work with the
results in references.
Catalysts
Support
Electrolyte
Current
density
Overpotential
(mV)
Refs
(mA cm-2
)
The electrochemical properties of the NiS2 sample were
further evaluated. The NiS2 synthesized at three different
reaction times were evenly spread onto 1 × 1 cm carbon paper
(CP), and then the catalyst-modified CP was used as the
working electrode. The performance of the bare CP and IrO2
was also tested for comparison. Fig. 8a displays the OER
polarization curves in 1.0 M KOH at 5 mV s-1. Specifically, to
reach an anodic current density of 10 mA cm-2, the 7h-NiS2
NiS2
microsphere
Carbon
paper
This
work
1.0M KOH
1.0M KOH
1.0 M KOH
10
10
10
314
345
320
Nanocrystallin
e NiS particles
Carbon
paper
53
54
NiS2 hollow
microspheres
Carbon
paper
5
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Copper
foil
integrated sphere. TG and DSC curves were obtained by using
a TG-DSC 6300 Mettler Toledo. The purge gas was nitrogen
with a flow rate of 100ꢀmL min−1 and a heating rate of 10ꢀ°C
min−1 was adopted.
Ni@NiO@C
1.0 M KOH
10
100
10
380
330
375
55
56
12
Nickel
foam
N-doped Ni3S2
1.0 M KOH
1.0 M KOH
NiS2
nanocatalysts
Carbon
paper
Electrocatalytic Measurements
Glassy
carbon
1.0 M
NaOH
Ni0.85Se films
NiS spheres
10
50
320
335
57
58
The electrochemical measurement is tested using the Iviumstat
electrochemical workstation. The OER performances are
measured in a three electrodes system in a 1.0 M KOH
electrolyte, platinum electrode and saturated calomel electrode
(SCE) are used as counter electrode and reference electrode,
respectively and prepared samples are designed as working
electrode. For making the working electrode for OER, first to
dispose the CP and mix 10 mg NiS2 samples into 960μl
isopropanol, then add 35 μl Nafion to form a homogeneous
solution. The working electrode is a CP (1 cm*1 cm) covered
with 150 μl prepared solution. Cyclic voltammograms (CV) are
conducted firstly to reach a steady state. The linear sweep
voltammetry (LSV) curves of OER LSV curves are at 0.9 V ~ 0 V
and the scanning speed is 5 mV s-1. The collected data is
calculated according to the iR-compensation, the equation is
Ecorr = Emea – iRs. And all data is transformed to reversible
hydrogen electrode (RHE) following the equation Evs. RHE = Evs.
Nickel
foam
1.0M KOH
Conclusion
In conclusion, we have successfully synthesized NiS2
microspheres using new one-step facile solution-based
a
strategy. This method is simple, cheap, green, non-toxic and
easy to operate due to the low viscosity of the solution. In
addition, the route for the synthesis of NiS2 microstructures will
be a promising way in the industrial application of pyrite-type
disulfides. Among prepared products, the 7h-NiS2 exhibits the
best catalytic activity with low overpotential of 311 mV to achieve
a current density of 10 mA cm-2 for OER and remarkable long-
term catalytic durability. Besides, it was found that the formation
of NiS2 microspheres was attributed to the oriented aggregations
and the Ostwald ripening. The work provides insight into the
controlled synthetic mechanism of earth-abundant transition
metal sulfide based compounds.
+ 0.059 * pH + 0.241.[59-61] Electrochemical impedance
SCE
spectroscopy (EIS) is measured with the frequencies from 10
kHz to 0.01 Hz. To testify the stability of samples,
ChronoPotentiometry data at 10 mA cm-2 current density is
collected.
Experimental Section
Acknowledgements
Materials synthesis
In this work, NiS2 microspheres are synthesized by
a
This work was supported by the Young Elite Scientists
Sponsorship Program by CAST (2018QNRC001), the National
Natural Science Foundation of China (51972224, U1601216),
and the Tianjin Natural Science Foundation (17JCQNJC02100).
solvothermal method. The specific steps are as follows: 0.246 g
Ni(CH3COO)2·4H2O is weighed and dissolved in the mixed
solution of 15 ml ethanol and 15 ml DMF, followed by magnetic
stirring for 30 minutes. Then, 0.39 g sulfur powder is added into
the solution and stirred for 30 minutes to form a uniform mixture.
The autoclave is placed in a furnace and then the temperature is
evaluated to 180 °C at a rate of 10 °C/min and maintained for 12
h. After naturally cooling to room temperature, the samples are
separated by centrifugation, washed using deionized water and
ethanol for three times. Finally, the NiS2 product is collected
after lyophilizing. The synthetic procedure for CoS2 is similar to
that of NiS2 except replacing the Ni (CH3COO)2·4H2O by the
same molar amount of Co(CH3COO)2·4H2O.
Keywords: SOLVOTHERMAL SYNTHESIS•GROWTH
MECHANISM•OXYGEN EVOLUTION•METAL
SULFIDE•MICROSPHERES
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7
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Entry for the Table of Contents
NiS2 and CoS2 microspheres were prepared by a one-step facile solvothermal strategy. In contrast to the conventional approach, this
method does not require complex precursors or polluting solvent and the morphologies of the products are well controlled. Moreover,
a growth mechanism of the pyrite-type disulfides microspheres has been proposed. Electrochemical testing results indicated that the
7h-NiS2 microspheres exhibited the best OER catalytic activities with overpotential of 311 mV to achieve a current density of 10 mA
cm-2 in 1.0 M KOH as well as robust long-term durability.
8
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