High performance electrocatalysis for hydrogen evolution reaction using nickel-doped CoS2 nanostructures: experimental and DFT insights

Article abstract of DOI:10.1016/j.electacta.2017.01.103

Development of non-noble-metal electrocatalysts with high activity and durability is a competitive strategy for the practical applications of water splitting which including hydrogen evolution reaction (HER) as well as oxygen evolution reaction (OER). In this work, high-quality CoS₂ ultrathin nanosheets were synthesized through a simple and efficient hydrothermal method. After merging 10% wt Ni, the bimetal catalyst (Co_0.9Ni_0.1S₂) displays excellent catalytic activity (10 mA cm^?2 at an overpotential of 156 mV and a small Tafel slope of 52 mV dec^?1) as well as stability (over 3000 cycles without obvious decay) in acid condition. Based on the results of Density Functional Theory (DFT) calculations, H⁺ adsorbed competition, hydrogen formation energetic and kinetics are proposed to help us understand the high efficient HER process over the Co_0.9Ni_0.1S₂ (CNS) ultrathin nanosheets.

Full text of DOI:10.1016/j.electacta.2017.01.103

Electrochimica Acta 228 (2017) 428–435
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
High performance electrocatalysis for hydrogen evolution reaction
using nickel-doped CoS nanostructures: experimental and DFT
2
insights
a
a
a
a
b,
Yan Cui , Chunwu Zhou , Xinze Li , Yuan Gao , Jian Zhang *
a
Key Laboratory of Broadband Wireless Communication and Sensor Network Technology, Nanjing University of Posts and Telecommunications, Ministry of
Education, Nanjing 210003, PR China
b
Key Laboratory for Organic Electronics and Information Displays & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for
Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications, Nanjing 210023, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 1 November 2016
Received in revised form 5 January 2017
Accepted 17 January 2017
Available online 18 January 2017
Development of non-noble-metal electrocatalysts with high activity and durability is a competitive
strategy for the practical applications of water splitting which including hydrogen evolution reaction
(HER) as well as oxygen evolution reaction (OER). In this work, high-quality CoS ultrathin nanosheets
2
were synthesized through a simple and efficient hydrothermal method. After merging 10% wt Ni, the
ꢀ
2
bimetal catalyst (Co0.9Ni0.1
1
2
S ) displays excellent catalytic activity (10 mA cm at an overpotential of
ꢀ
1
Keywords:
Electrocatalysis
water splitting
56 mV and a small Tafel slope of 52 mV dec ) as well as stability (over 3000 cycles without obvious
+
decay) in acid condition. Based on the results of Density Functional Theory (DFT) calculations, H
adsorbed competition, hydrogen formation energetic and kinetics are proposed to help us understand
the high efficient HER process over the Co0.9Ni0.1 (CNS) ultrathin nanosheets.
2017 Published by Elsevier Ltd.
CoS
2
S
2
Ni doped
HER
©
1. Introduction
distribution of atom on the interface of solid- liguid [11]. Recently,
d transition metal (Ni, Co, Fe, and Mn) dichalcogenides have
3
As an idea candidate, hydrogen evolution through electro-
catalysis water splitting will open up a promising avenue to relieve
the tension of the growing demand for energy and environmental
concerns [1]. It is well known that platinum and other precious
metals are the best hydrogen evolution reaction (HER) and oxygen
reduction reaction (ORR) electrocatalysts [2,3]. However, their high
cost and low reserves limit their applicability. Actually, earth-
abundant, low cost electrocatalysts with competitive HER and ORR
catalytic activity to platinum have been designed and investigated
including transition metal nitrides [4], borides [5], and carbides [6]
and also the enormous reported transition-metal dichalcogenides
attracted a growing amount of attention because of the earth-
abundant and environmentally benign [12,13]. As a significant
class of semiconductors in TMDs family, cobalt sulfides with
2 3 4 9 8
different chemical formulas (CoS , CoS, Co S , Co S , and Co1ꢀxS)
have been investigated in supercapacitors [14,15], lithium ion
batteries [16–18], catalysis [19], and dye-sensitized solar cell
(DSSC) [20,21]. Compared with the other 3d transition metal
chalcogenides, the remarkable advantages in thermal stability and
electronic conductivity make cobalt sulfides more attractive in
energy storage and transformation. For the pyrite phase transition
metal disulfides (Ni, Co, Fe, and their allays), Jin et al. [12] have
(
TMDs) such as MoS
2
[7], MoSe
2
[8], and WS
2
[9].
demonstrated that CoS
2
exhibited higher overall performance than
For natural hydrogenase systems with metal sulfur clusters, the
FeS and NiS in HER. Besides, nanostructure based CoSe
2
2
2
active sites for HER can attribute to the permanent ligands in a
distorted octahedral ligation shell [10]. Actually, the catalytic
capability focus on the kinetic energy barrier of hydrogen
evolution pathway and hydrogen adsorption energy as well, which
exhibited a strong dependence on the crystal phase and
electrocatalyst has been proved excellent catalytic activity for
not only HER [22] but also ORR [23]. Different from Nb and Ta
doping [24], Fe doping process could improve HER performance in
catalytic activity as well as stability and provide a novel method
through modulate original materials in electronic state and crystal
structure [25,26]. Merging Co into ultrathin earth abundant iron
2
pyrite FeS nanosheets could also greatly improve the HER
catalytic activity which was reported by Yang’ group [27] last
year. Besides, Wang et al. [11]. have found that the metallic iron-
*
Corresponding author.
E-mail address: iamjzhang@njupt.edu.cn (J. Zhang).
http://dx.doi.org/10.1016/j.electacta.2017.01.103
013-4686/© 2017 Published by Elsevier Ltd.
0

Y. Cui et al. / Electrochimica Acta 228 (2017) 428–435
429
nickel sulfide nanostructures show a low overpotential (105 mV at
(99.7%, Aladdin) and 6 mmol of sodium diethyldithiocarbamate
trihydrate (99.0%, Aladdin) were dissolved in 38 mL deionized
water, followed by addition of 0.5, 1.0, and 2.0 mL of 0.6 M nickel
ꢀ2
10 mA cm ) and good durability (over 40 h). Motivated by these
advances, it is reasonable to expect a similar influence of Ni on the
HER performance of cobalt sulfides. Hence, we have designed and
2 2
chloride hexahydrate NiCl 6H O (99.0%, Aladdin) aqueous solu-
ꢁ
implemented the preparation of Ni doped CoS
2
(Co1-xNi
x
S
2
)
tion, respectively. Then, 0.02 g poly(vinylpyrrolidone) (PVP, AR,
Shanghai Chemical Factory of China) was added to the above
solution. After magnetic stirring over 30 min, the mixture solution
was transferred to a stainless steel autoclave (50 mL) and
multilevel nanostructure and used as electrocatalysts for HER
application.
Herein, uniform CoS nanoballs assembled from ultrathin nano-
2
sheets were successful synthesized through an efficient hydrother-
ꢂ
maintained at 180 C for 24 h. After cooling to room temperature
mal process. Incorporated into a small quantity of Ni element (10%
naturally, the precipitate was collected by centrifugation and
washed with distilled water and absolute ethanol two times each
and dried in a vacuum oven at 50 C (12 h). As a benchmarked
wt), catalytic activity and durability of Co0.9Ni0.1
for HER were fundamentally enhanced compared with single CoS
catalyst, matching the activity of many reported TMDs based HER
catalysts in acidic solutions, such as MoS [28] and WS [29]
especially for 1T polymorph). In conjunction with theoretical
2
S (CNS) nanoballs
ꢂ
2
sample, pure CoS
Co1-xNi in the absence of the Ni source in the hydrothermal
process.
2
sample was prepared with same method as the
2
2
x 2
S
(
calculations, we have demonstrated the origins and mechanism of
the advanced catalytic properties. Based on the experimental and
Density functional theory (DFT) results, our work undoubtedly
provides prospective insights into the exploration of highly efficient
and durable electrocatalysts for HER from water.
2.2. Sample Characterization
The morphologies of the samples were characterized by
Transmission electron microscopy (TEM) and scanning transmis-
sion electron microscopy (STEM) were carried out on an FEI Tecnai
F20 transmission electron microscope at an acceleration voltage of
2. Experimental
2
00 kV. Scanning transmission electron microscopy (SEM) images
2
.1. Synthesis of Co1-xNi
x
S
2
Nanoballs
were taken on a Hitachi S-4800 scanning electron microscope. XRD
measurements were carried out on a BRUKER D8 Advance X-
In a typical synthesis of Co0.95Ni0.05
S
2
, Co0.9Ni0.1
S
2
, and
diffractometer with Cu K
a radiation. The BET surface area was
Co0.8Ni0.2 catalysts, 2 mmol of anhydrous cobalt chloride CoCl
S
2
2
measured using the nitrogen gas adsorption-desorption method
2 2 2 2
Fig. 1. Morphology and structure characterizations. FESEM images of CoS (a) and Co0.9Ni0.1S (b). TEM images of CoS (c) and Co0.9Ni0.1S (d) and the corresponding HRTEM
images (insets).

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Y. Cui et al. / Electrochimica Acta 228 (2017) 428–435
(
TriStar II 3020) at 77 K. X-ray photoelectron spectra (XPS) were
2.4. Computational Methods and Details
acquired on an ESCALAB MK II with Mg K as the excitation source.
a
All calculations were performed using DFT with the program
package of DMol3 in Materials Studio. The exchange-correlation
energy was calculated within GGA-PW91 approximation. The
double-numeric quality basis set with polarization functions
(DNP) was used in the system. The density functional semicore
pseudopotential (DSPP) method was employed for the metal
atoms. The bottom two layers were kept fixed to the bulk
coordinates and full atomic relaxations were allowed for the top
four layers. For these calculations, a 3 ꢄ 3 ꢄ1 k-Point mesh was
2
.3. Electrochemical Measurements
Electrochemical measurements were collected through a three-
electrode system on an electrochemical workstation (Chenhua,
CHI660E). All the measurements were carried out in 30 mL of 0.5 M
H
2
SO
powder and 30
dispersed in 2 mL water isopropanol solution with a volume ratio
of 3: 1 by sonicating for 1 h. Then, 20 L (60 g catalyst) of the
4
electrolyte (pH = 0.157, purged with H
2 x 2
). 4 mg of Co1-xNi S
mL Nafion solution (Sigma Aldrich, 5% wt) were
m
m
used in the 2 ꢄ 2 supercell. In CoS
2
(110) (Co: 8, S: 16) and CNS (110)
dispersion was dripped onto a glassy carbon electrode as the
working electrode, an Ag/AgCl (in 3 M KCl solution) electrode as
the reference electrode and a graphite rod (Alfa Aesar, 99.9995%) as
a counter electrode. The electrochemical stability of the catalyst
(Co: 7, Ni: 1, S: 16) slab models. A fermi smearing of 0.005 Hartree
and a real-space cutoff of 4.0 Å were used to improve the
computational performance. All calculations were performed with
spin-polarization. The energy per adatom was calculated from the
ꢀ
1
was evaluated (0.0 ꢃ ꢀ0.4 V vs. RHE) with a scan rate of 50 mV s
.
differential adsorption energy,
D
EH, and was derived as
Þ , where n is the number
À
Á
The Nyquist plots were measured in the same electrochemical
workstation with an overpotential of 100 mV.
1
n
D
E
H
¼
ðsurf þ nHÞ ꢀ EðsurfÞ ꢀ EðH
2
n
2
of hydrogen atoms adsorbed on the surface and E (surf + nH) is the
total energy of the surface with n atoms adsorbed. The negative
x 2
To determine Cdl for each Co1-xNi S electrode, two CV cycles
were recorded (50 ꢃ 200 mV vs. RHE) in the standard measure-
sign of E
H
corresponds to the energy gain of the system because of
ment configuration described above at scan rates of 4, 10, 20, 40,
H atom adsorption.
ꢀ1
6
0, 80, 100, and 120 mV s . The two CV cycles at each scan rate
were compared to ensure consistency, and the second cycle was
always used for Cdl determination. Then, for each scan rate, J was
3
. Results and discussion
D
measured at ꢀ 0.15 V vs. RHE and plotted against the scan rate.
These data were then fit to a line, the slope of which is equal to
twice the geometric Cdl.
A series of Co1-xNi
hydrothermal method. The morphology and structure of the
representative CNS and CoS electrocatalysts were investigated by
FESEM and TEM as shown in Fig. 1. From Fig. 1a, the original CoS
sample has almost homogenous nanoballs profile which has highly
x 2
S catalysts were synthesized by a simple
2
2
2 2
Fig. 2. (a) XRD patterns and (b) N adsorption-desorption isotherm of CoS and CNS catalysts. HAADF-STEM images (c) and element mapping images (d,e,f) of CNS catalyst.

Y. Cui et al. / Electrochimica Acta 228 (2017) 428–435
431
rough surface and composed of sheet-like subunits in random
orientation. These nanoballs are three-dimensional flower-like
structures with relatively uniform size of ꢃ 100 nm.
(220), (221), (311), (222), (230), and (321) diffraction planes, which
are consistent well with a cubic CoS standard pattern (JCPDS no.
41-1471) [34]. Compared with the pure CoS , the main character-
istic diffraction peaks of the CNS sample did not obviously change
after doping with 10% wt Ni. No peaks typical of crystalline NiS
2
appeared in the XRD pattern of the bimetal sample indicated that
the crystal structure maintained well after slight Ni atom
substitution, which in accordance with the results of HRTEM
(Fig. 1d, inset).
2
2
Doped with 10% wt Ni, it is clear to find that the morphology
and structure of Co0.9Ni0.1
size (ꢃ 50 nm) as compared with the CoS
S
2
sample maintained well except the
(Fig. 1b), which can
2
attribute to the doping effect for the size control [30]. Abundant
edge-terminated sites of layered TMDs have been proved as active
site for the majority of catalytic reaction including HER such as
MoS
2
[31] and WS
2
[32]. Except for the uniformity and nanoscale,
Specific surface area of the catalysts is one of crucial factors of
the electrocatalysis efficiency in HER which is mainly dependent
on surface contacts, especially for 2D TMDs electrocatalysts [35]
ultrathin nanosheet edge provides a large number of active sites,
which are benefit to enhance the HER catalytic activity. Compared
to FESEM images, the corresponding TEM images could further
confirm the formation of spherical particles and the flower-like
structure was assembled from abundant uniform nanosheets. The
representative HRTEM image from inset of Fig. 1c shows the
parallel lattice fringes (ꢃ0.28 nm), which can be indexed to (200)
and, hence, N
pressure of 0.1-0.9) is performed to reveal the specific surface
area of the as-synthesized CNS and pure CoS . The BET surface area
of CNS is 62.63 m g , which is much higher than the values of
2
adsorption-desorption measurement (relative
2
2
ꢀ1
2
ꢀ1
CoS
2
(45.97 m g ) as shown in Fig. 2b. The pore volumes for CNS
ꢀ
1
facet diffractions of cubic CoS
2
[33]. The same lattice fringe was
and pure CoS are 0.49 and 0.26 cc g , respectively. It is clear to
2
detected from the inset of Fig. 1d (CNS) for the similar atomic
radius of Co and Ni.
XRD was used to investigate the crystal structure of the
samples. Fig. 2a shows the powder XRD patterns of the as-prepared
find that the surface area of the CNS increases, which could provide
more active sites for HER. To further confirm the composition and
distribution of elements in CNS catalyst, high-angle annular dark-
field scanning TEM (HAADF-STEM) and selected area EDX mapping
were conducted as illustrated in Fig. 2c–f. These images (Figs. 2c–f)
further confirm the uniform distributions of Co, Ni, and S elements
2 2
CoS and CNS catalysts. For pure CoS , ten primary diffraction
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
peaks at 27.4 , 32.2 , 36.8 , 39.8 , 46.6 , 49.2 , 54,7 , 57.3 , 60.3 ,
ꢂ
and 62.5 are respectively ascribed to the (111), (200), (210), (211),
Fig. 3. XPS spectrum of CNS. Survey spectrum (a), Co 2p (b), Ni 2p (c) and S 2p (d).

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32
Y. Cui et al. / Electrochimica Acta 228 (2017) 428–435
in the selected single nanoball. Besides, the elemental composition
of Co: Ni: S close to 0.9: 0.1: 2.
780.5 and 796.2 eV, which can be assigned to Co 2p3/2 and Co 2p1/
2, respectively [36], confirming the existence of a Co-S bond for the
XPS measurements were then conducted to analyze the atomic
species and bonding characteristics information of the represen-
tative CNS sample. From the binding energy ranges of
2
formation of CoS [37]. For the Ni 2p electrons (Fig. 3c), the high-
resolution spectrum presents two predominant peaks at 855.3 and
873.0 eV, which shows a slight shift compared with that of pure Ni
(853.8 and 871.3 eV) [38], assign to the binding energies of Ni 2p3/2
and Ni 2p1/2, respectively [39]. The other two weaker peaks are
satellite peaks [40]. The S 2p spectra can be divided into three
peaks. The signals at 163.6 and 162.7 eV are typical of metal-
100 ꢃ1100 eV, the typical Co, Ni, S, O, and C elements are detected
in the full spectrum (Fig. 3a). The Co 2p, Ni 2p, and S 2p spectra ere
fitted by the Gaussian fitting method as presented in Fig. 3b, c, and
d, respectively. From Fig. 3b, two distinct peaks can be identified at
Fig. 4. (a) Polarization curves and (b) corresponding Tafel plots of a series Co1-xNi
control experiment). (c) Nyquist plots of CoS and CNS samples. (d) Durability test for CNS after 3000 CV cycles. iR-corrected for all HER test curves. (e) Cyclic voltammograms
of CNS within the range of no faradaic reactions. (f) Plot showing the extraction of the double-layer capacitance (Cdl) for CoS and CNS electrodes.
x 2 2 4
S as indicated in the acidic electrolytes of 0.5 M H SO (including glassy carbon and Pt/C for
2
2

Y. Cui et al. / Electrochimica Acta 228 (2017) 428–435
433
sulphur bonds and the low coordinate sulphur ion at surface [41].
The third peak at 168.2 eV is S-O interaction resulting from air
exposure [42]. Besides, XPS analysis of CNS catalyst exhibited an
elemental composition of Co: Ni: S close to 0.9: 0.1: 2, consistent
with the EDX result.
obtain the exchange current density normalized to the real surface
area (J0,real) of each electrode as shown in Table 1. J0,real value of CNS
ꢀ2
was 12.56 mA cm , almost 4 times more than a value of
ꢀ
2
3.36
m
A cm
2 2 4
presented by CoS in 0.5 M H SO . A higher
electrochemically active surface area in CNS indicated that a rapid
charge transfer between active sites and electrolyte was achieved.
To understand the effect of Ni doping on the catalytic activity of
cobalt for HER, DFT calculations was employed to investigate the
The electrocatalytic HER activities (Fig. 4) of the Co1-xNi
electrocatalysts with different amount of Ni doping deposited on
the glassy carbon electrode were performed in 0.5 M H SO
electrolyte continuously purged with H , as described in the
x 2
S
2
4
2
2
detailed HER pathways and process on CoS and CNS catalysts. The
experimental section. As a control experiment, a commercial Pt
catalyst (10% wt Pt) was also conducted and nearly zero onset
overpotential with high current density was achieved as shown in
Fig. 4a. Compared with the negligible current density from a bare
scheduling of proton in HER process mainly involved three steps
(in acid solution): proton adsorption (Fig. 5a), hydrogen atom
formation (Fig. 5b), H formation and desorption (Fig. 5c), which all
2
process occur on the edge of catalyst [43]. According to previous
reports, hydrogen evolution pathway was depended on the kinetic
energy barrier of the catalysts, including platinum [44]. Mean-
while, the sulfur atom on the edge of TMDs was recognized as HER
active site. Based on the well-known mechanism above, the kinetic
glassy carbon control, all of the Co1-xNi
lower onset overpotentials. Co0.9Ni0.1
value of 55 mV, suggesting the superior HER activity. Notably, the
overpotential required for the Co0.9Ni0.1 sample to produce
cathodic current densities of 10 mA cm is just ꢀ152 mV, which is
over 2 times larger than the current detected in pure CoS nanoball
catalyst and the other two synthesized doped samples (-165 and
198 mV, respectively). Moreover, the corresponding Tafel plots
Fig. 4b) were fitted to the Tafel equation, yielding Tafel slopes of
x
S
2
catalysts possess much
S
2
sample exhibits the lowest
S
2
ꢀ
2
2
energy barrier profiles of hydrogen evolution on CoS and CNS
2
were investigated by DFT (Fig. 5d). From Fig. 5d, the rate-
determining step is thought to be the combination of Hads with H+
ꢀ
2
(Step two). For pure CoS sample, the energy barrier for adsorption
(
3
was calculated to be 0.21 eV. Compared with pure catalyst, the
energy barrier of the doped sample (CNS) was found to be only
0.03 eV. Besides, the hydrogen generated on the surface of CNS
ꢀ1
0, 52, 59, 67, and 92 mV decade
for Pt, Co0.9Ni0.1
2
S (CNS),
Co0.95Ni0.05
S
2
, Co0.8Ni0.2
S
2
and pure CoS , respectively. The lowest
2
ꢀ
1
value (ꢃ 50 mV decade ) observed from CNS indicated a possible
Volmer-Heyrovsky reaction pathway with electrochemical de-
sorption of hydrogen as the rate-limiting step [43].
2
catalyst is much easier to release compared to CoS , which can be
attributed to the larger energy for desorption (Step three, ꢀ2.21 eV
vs. ꢀ2.15 eV). Therefore, based on the simulation results, the higher
catalytic activity achieved from the CNS catalyst than the undoped
To further investigate the electrode kinetics under HER process
of the as-prepared catalysts, electrochemical impedance spectros-
copy (EIS) measurements were performed as illustrated in Fig. 4c.
The inset of Fig. 4c shows equivalent circuit diagram for modeling
the measured impedance curves. Re is associated with the
electrolyte resistance, the first semicircle in the high frequency
region (Rs/CPE1) is associated with the migration and surface film
resistance, and the second semicircle in the moderate frequency
region (Rct/CPE2) is ascribed to the charge transfer resistance. Z
represents the Warburg impedance. The Nyquist plots reveal an
obvious decrease of the Rct from 189.7 (CoS ) to 92.2 (CNS),
W
V
2
V
which can be attributed to the homogeneous distribution of Ni
element. Stability in acid media is another important criterion to
evaluate an advanced electrocatalyst. For CNS sample, after 3000
cyclic voltammetric (CV) sweeps in 0.5 M H SO aqueous solution,
2 4
a slightly larger overpotential is displayed in Fig. 4d. The
remarkable durability indicates that the CNS (10% wt Ni doped
2
CoS ) catalyst is highly stable over the course of the cycling
investigation.
2
The exchange current density (J0,geo) of CoS based catalysts, an
inherent measure of HER activity, was deduced from the Tafel plots
ꢀ
2
(
Fig. 4b) as shown in Table 1. Obviously, the J0,geo of 4.98
m
A cm in
ꢀ2
CNS was higher than the values of 2.02
m
A cm for CoS sample.
2
Electrochemical double-layer capacitances capacitance (Cdl) be-
tween the catalysts and electrolyte can be used to evaluate the
effective surface area of various catalysts by cyclic voltammetry, as
ꢀ
2
shown in Fig. 4c. From Fig. 4d, the Cdl value of 23.2 mF cm for the
CNS was detected, remarkably higher than the undoped CoS (15.3
mF cm ), meanwhile comparable recently reported literature on
Co-based electrocatalysts for hydrogen production [22,42]. More-
over, Since Cdl is proportional to the real surface area, it was used to
Fig. 5. (a,b,c) Schematic reaction pathway of HER on sulfur atom of CNS catalyst
edge in acid environment. The yellow, red, green, and blue spheres in (a–c)
represent S, Ni, Co, and H atoms, respectively. (d) The kinetic energy barrier profiles
2
ꢀ2
2
of HER on CNS and pure CoS catalysts. (For interpretation of the references to
colour in this figure legend, the reader is referred to the web version of this article.)
Table 1
2
Summary of the electrochemical properties of CoS and CNS electrodes.
ꢀ2
ꢀ1
ꢀ2
ꢀ2
ꢀ2
Sample
CoS
CNS.
h
(mV vs RHE) for J = –10 mA cm
Rct (
V
)
Tafel (mV decade
)
J
0,geo
(m
A cm
)
C
dl (mF cm
)
J
0,real
(
mA cm
)
2
152
197
189.7
92.2
92
52
2.02
4.98
15.3
23.2
3.36
12.56

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Y. Cui et al. / Electrochimica Acta 228 (2017) 428–435
2
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In summary, uniform Ni doped CoS nanoballs assembled from
[
ultrathin nanosheets were prepared as a highly active electro-
catalyst for HER through a simple and low-cost hydrothermal
process. Ni doping can be employed as an effective way to enhance
the HER performance of CoS and the optimal doping ratio is 10%
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