ARTICLE
Received 12 Apr 2016
| Accepted 29 Jul 2016 | Published 16 Sep 2016
Efficient hydrogen evolution by ternary
molybdenum sulfoselenide particles on
self-standing porous nickel diselenide foam
Haiqing Zhou
1
, Fang Yu
1
, Yufeng Huang
2
, Jingying Sun
1
, Zhuan Zhu
3
, Robert J. Nielsen
2
, Ran He
1
, Jiming Bao
3
,
William A. Goddard III
2
, Shuo Chen
1
& Zhifeng Ren
1
With the massive consumption of fossil fuels and its detrimental impact on the environment,
methods of generating clean power are urgent. Hydrogen is an ideal carrier for renewable
energy; however, hydrogen generation is inefficient because of the lack of robust catalysts
that are substantially cheaper than platinum. Therefore, robust and durable earth-abundant
and cost-effective catalysts are desirable for hydrogen generation from water splitting via
hydrogen evolution reaction. Here we report an active and durable earth-abundant transition
metal dichalcogenide-based hybrid catalyst that exhibits high hydrogen evolution activity
approaching the state-of-the-art platinum catalysts, and superior to those of most transition
metal dichalcogenides (molybdenum sulfide, cobalt diselenide and so on). Our material is
fabricated by growing ternary molybdenum sulfoselenide particles on self-standing porous
nickel diselenide foam. This advance provides a different pathway to design cheap, efficient
and sizable hydrogen-evolving electrode by simultaneously tuning the number of catalytic
edge sites, porosity, heteroatom doping and electrical conductivity.
DOI: 10.1038/ncomms12765
OPEN
1
Department of Physics and TcSUH, University of Houston, Houston, Texas 77204, USA.
2
Materials and Process Simulation Center (139-74), California
Institute of Technology, Pasadena, California 91125, USA.
3
Department of Electrical and Computer Engineering, University of Houston, Houston, Texas 77204,
USA. Correspondence and requests for materials should be addressed to S.C. (email: schen34@uh.edu) or to Z.R. (email: zren@uh.edu).
NATURE COMMUNICATIONS | 7:12765 | DOI: 10.1038/ncomms12765 | www.nature.com/naturecommunications 1
T
he large consumption of fossil fuels and its impact on the
environment make it urgent to develop environmentally
friendly and renewable energy sources. Hydrogen (H
2
)isan
attractive and promising energy carrier because of its high energy
density and no pollution gas emission
1,2
. One direct and effective
route to generate H
2
is based on electrocatalytic hydrogen
evolution reaction (HER) from water splitting, in which an
efficient catalyst is required to ensure the energy efficiency
3–5
.
Platinum (Pt)-based noble metals are by far the most active
catalysts; however, they are not suitable for large-scale applications
because of the high cost and scarcity of Pt on earth
6
. Thus, we aim
to identify alternative electrocatalysts based on earth-abundant
and cost-effective elements
7,8
. Until now, various classes of
earth-abundant transition metal compounds are confirmed to
be promising candidates
9,10
, such as metal sulfides, selenides,
phosphides, carbides and the composites. However, thus far most
of the catalysts exhibit inferior efficiency to Pt, while many involve
complicated preparation methods and multiple steps that
increase costs. Great progress has been obtained for HER based
on layered transition metal dichalcogenides (LTMDs) such as
molybdenum disulfide (MoS
2
) either in the form of crystalline or
amorphous states
9–12
, and even in molecular mimics
13
; however, it
remains a challenge to get catalytic performance comparable to
that of Pt, which is probably due to the low density and reactivity
of active sites, poor electrical transport and inefficient electrical
contact to the catalyst
14–16
.
On the basis of the above results, fabricating MoS
2
or its
derivatives into hybrids or composites might be an interesting
strategy to promote the catalytic performance
17
.Currently,
carbon-based materials are generally used as the catalyst support
because of their high surface area and good conductivity
17,18
.
However, complex catalyst synthesis procedures are typically
required. As an alternative, arranging the catalysts into double-
gyroid structures with numerous nanopores might lead to
improved HER activity because of preferential exposure of
catalytic active edges rather than the inactive basal planes
19
.This
approach reminds us of the necessity to make three-dimensional
(3D) catalysts with high surface area loaded on porous supports,
fast proton transfer and greater contact areas with reactants during
the catalytic process. Meanwhile, the bottleneck of the
double-gyroid structures is the intrinsically poor conductivity of
the catalysts. Thus, even though MoS
2
is established as an effective
HER catalyst since 2005 (ref. 11), it is still difficult to obtain
satisfactory catalysts in low costs on par with the current Pt
catalysts. In the light of these challenges, we conclude that the best
strategy is to improve the dispersion and electrical conductivity of
these catalysts on the supports and to expose a large number of
active edge sites. Furthermore, we consider that arranging two
different materials into hybrids might lead to synergistic effects
20,21
that utilize the best properties of each component.
In this work, we propose a strategy of growing ternary
molybdenum sulfoselenide MoS
2(1 x)
Se
2x
particles with vertically
aligned layers on a 3D porous HER-active conductive nickel
diselenide (NiSe
2
) scaffold, which takes advantage of the merits of
highly conductive support, double-gyroid structures (3D, porous
and lots of exposed edge sites) and synergistic effects between two
different catalysts. Indeed, we measure excellent HER performance
for this hybrid catalyst that is superior to most reported transition
metal dichalcogenides (MoS
2
,cobaltdiselenideCoSe
2
and so on).
Results
Preparation of 3D porous hybrid electrocatalyst. To the best of
our knowledge, the majority of HER catalysts reported thus far
are based on nanostructures (nanoparticles, nanosheets and so
on); thus, binder polymers (for example, nafion solution) are
necessary to fasten the catalysts on the conducting substrates such
as glassy carbon electrodes, which somewhat increases the cost.
This problem can be avoided by growing the active catalysts
directly on self-standing conducting skeletons as the current
collectors
22–24
. The key challenge is to find a suitable 3D supports
with high surface area, high porosity and good conductivity.
Graphene or carbon nanotube is not feasible because of their high
costs. Instead, nickel (Ni) foam is suitable because of its low price,
commercial availability and 3D skeleton structure
25
(Fig. 1a).
However, Ni foam is not stable in acidic electrolytes because of
corrosion. Interestingly, our previous work shows that direct
selenization in Ar atmosphere can convert Ni foam to porous
NiSe
2
foam (Fig. 1b,c, Supplementary Fig. 1 and Supplementary
Note 1) that is HER-active and very stable in acid
26
. We find that
numerous additional pores are generated in the NiSe
2
, which
provides preferential sites for growing LTMD catalysts with
high-density active edges
27
. Thus, we propose using 3D
porous NiSe
2
foam as a conductive skeleton to load ternary
MoS
2(1 x)
Se
2x
catalysts (Supplementary Note 2), thereby
utilizing the excellent electrical conductivity, porous structures
and high surface area of the NiSe
2
foam (Fig. 1d,e). Indeed,
scanning electron microscopy (SEM; Fig. 1d,e and Supplementary
Note 3) images clearly show the uniform distribution of small
ternary particles on porous NiSe
2
foam, which is important for
the electrocatalytic performance of LMDT catalysts.
Structural characterizations of the electrocatalyst. The chemical
composition of the as-grown particles was examined using
high-resolution transmission electron microscopy (TEM), X-ray
photoelectron spectroscopy (XPS), Raman spectroscopy and
energy-dispersive X-ray spectroscopy. TEM images (Fig. 2a,b and
Supplementary Fig. 2) clearly resolve a large amount of vertically
aligned MoS
2(1 x)
Se
2x
layers, suggesting that many active edge
sites are exposed at the surface of MoS
2(1 x)
Se
2x
particles. It is
reasonable since the porous structure of NiSe
2
foam with high
surface area is favourable for the growth of layered materials with
vertically aligned layers
27
. Meanwhile, XPS spectra in the hybrid
reveal the presence of Ni, Mo, S and Se elements (Fig. 2c–e).
However, since the Se in NiSe
2
foam has a similar state to that in
MoS
2(1 x)
Se
2x
, it is difficult to demonstrate the selenization of
MoS
2
on porous NiSe
2
foam. Instead, to confirm the chemical
composition of the molybdenum compound, we put a precursor-
decorated Si substrate underlying the NiSe
2
foam during the
second selenization. It is clear that the (NH
4
)
2
MoS
4
precursor has
been converted to a distinctive ternary alloy phase at 500 °C
from the prominent Mo, S and Se signals in the XPS spectra
28
(Fig. 2c–e). Especially in the Raman spectra (Fig. 2f), in
comparison with pure MoS
2
that exhibits two prominent peaks
at 380 cm
1
(E
1g
) and 406 cm
1
(A
2g
), there is another obvious
peak located at 264 cm
1
for the samples with a ternary phase,
which can be ascribed to the A
1g
mode of the Mo–Se bond
29
.
Compared with the Raman mode of the bulk MoSe
2
crystals
(B242 cm
1
), the blueshifts of this peak to 264 cm
1
suggest a
ternary MoS
2(1 x)
Se
2x
compound rather than a mixture of two
solid phases. This Raman feature is also observed from the
ternary phase grown on porous NiSe
2
foam, which is consistent
with previously reported results on ternary MoS
2(1 x)
Se
2x
single
crystals
29
. By comparing the relative peak intensity between 264
and 380 cm
1
, we estimate that the atomic ratio between S and
Se is B1, which is further supported by the energy-dispersive
X-ray spectroscopy analysis (Supplementary Fig. 3).
Hydrogen evolution catalysis. Considering the metallic and
porous feature in the NiSe
2
foam, and the good dispersion
and preferential layer orientation of ternary MoS
2(1 x)
Se
2x
ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms12765
2 NATURE COMMUNICATIONS | 7:12765 | DOI: 10.1038/ncomms12765 | www.nature.com/naturecommunications
particles, it is expected that this 3D architecture should have
outstanding HER activity, such as low overpotentials, low Tafel
slopes and large exchange current densities. To evaluate the
catalytic performance of these ternary MoS
2(1 x)
Se
2x
particles on
3D porous NiSe
2
foam, we performed detailed electrocatalytic
measurements via a standard three-electrode set-up in a 0.5 M
H
2
SO
4
electrolyte de-aerated with high-purity N
2
. The loading of
MoS
2(1 x)
Se
2x
catalysts is B4.5 mg cm
2
. Figure 3a shows that
the self-standing porous hybrid catalyst can afford a geometric
current density of 10 mA cm
2
at a very low overpotential of
69 mV for the ternary MoS
2(1 x)
Se
2x
/NiSe
2
hybrid electrode
(Supplementary Table 1). In contrast, for binary MoS
2
on
NiSe
2
foam and pure NiSe
2
foam, overpotentials of 118 and
153 mV are needed to achieve 10 mA cm
2
, respectively.
The catalytic overpotential ( 69 mV) of the MoS
2(1 x)
Se
2x
/
NiSe
2
hybrid is also much lower than those of the best
catalysts thus far based on LTMDs MoS
2
( 110 mV)
18
,
WS
2
( 142 mV)
15
and WS
2(1 x)
Se
2x
( 170 mV)
30
, and first-
row transition metal dichalcogenides CoSe
2
( 139 mV)
24
, NiSe
2
( 136 mV)
26
and CoS
2
( 142 mV)
31
, suggesting that our
ternary MoS
2(1 x)
Se
2x
particles/NiSe
2
foam hybrid is an
outstanding HER catalyst. Meanwhile, a Tafel slope, which is
an inherent property of the catalyst, can be obtained by extracting
the slopes from the linear regions in Tafel plots (Fig. 3b). We find
that the ternary electrode possesses a smaller Tafel slope of
42.1 mV per decade than that of binary MoS
2
on NiSe
2
foam
(58.5 mV per decade) and pure NiSe
2
foam (46.4 mV per decade).
In addition, our hybrid catalyst leads to a Tafel slope much lower
than many previously reported cheap and efficient HER
catalysts in the same electrolyte (Supplementary Table 2). More
interestingly, based on the intercept of the linear region of
the Tafel plots, the exchange current densities (j
0
,
geometrical
) at the
thermodynamic redox potential (Z ¼ 0) can be calculated to
be 299.4 mAcm
2
for the ternary-phase hybrid catalysts. This
exchange current density is one to two orders of magnitude larger
than those of well-known LTMDs MoS
2
and WS
2
, or first-row
transition metal dichalcogenides CoSe
2
and CoS
2
catalysts
(Supplementary Table 2). Thus, considering the small
overpotential ( 69 mV to reach 10 mA cm
2
), low Tafel slope
(B42.1 mV per decade) and large exchange current density
(B299.4 mAcm
2
), it is worth pointing out that the catalytic
performance of our as-prepared catalyst is superior to most of the
MoS
2
-based catalysts.
Aside from a stringent requirement for high HER activity,
stability is another important criterion in evaluating the
performance of an electrocatalyst. In our experiment, a
long-term cyclic voltammetry (CV) test between 0.20 and
0.07 V versus RHE shows no significant degradation of cathodic
current densities for the hybrid catalyst after 1,000 cycles
(Fig. 3c). Particularly, the cathodic current density for the hybrid
catalyst remains stable and exhibits no obvious degradation for
electrolysis at a given potential ( 69 or 121 mV) for over a
long period (416 h; Fig. 3d), suggesting the potential use of this
catalyst over a long time in an electrochemical process. Even after
long-term stability and cyclability tests, the catalytic performance
of this hybrid catalyst still shows no degradation compared with
3D Ni foam
600 °C
a
bc
ed
Selenization
in Ar gas
Selenization
at 500 °C
(NH
4
)
2
MoS
4
modification
3D porous
NiSe
2
foam
3D porous
MoSSe/NiSe
2
foam
Figure 1 | The schematic diagram and morphology characterizations. (a) The procedures for growing ternary MoS
2(1 x)
Se
2x
particles on porous
NiSe
2
foam. (b,c) Typical SEM images showing the surface roughness of the NiSe
2
foam grown at 600 °C from commercial Ni foam. (d,e) Typical SEM
images showing the morphologies of ternary MoS
2(1 x)
Se
2x
particles distributed on porous NiSe
2
foam grown at 500 °C. (b,d) Scale bar, 50 mm.
(c,e) Scale bar, 1 mm.
NATURE COMMUNICATIONS | DOI: 10.1038/ncomms12765 ARTICLE
NATURE COMMUNICATIONS | 7:12765 | DOI: 10.1038/ncomms12765 | www.nature.com/naturecommunications 3
its initial state (Fig. 3c). In addition, the Faradaic efficiency for
hydrogen evolution of this hybrid catalyst was evaluated
(Supplementary Note 4). The efficiency is determined to be
nearly 100% during 60 min of electrolysis (Supplementary Fig. 4).
To elucidate the origin of the differences in the overall
catalytic performance among different catalysts, a simple CV
method
15,20,24
was utilized to measure the corresponding
electrochemical double-layer capacitances (C
dl
) for evaluation of
the electrochemically effective surface areas (Supplementary
Fig. 5). Taking consideration of the direct proportion between
the effective surface area and double-layer capacitance, we just
need to compare the capacitance values C
dl
. By plotting the
positive and negative current density differences (Dj ¼ j
a
j
c
)ata
given potential (0.15 V versus RHE) against the CV scan rates, we
can directly get the C
dl
, which is equal to half the value of the
linear slopes of the fitted lines in the plots. As shown in Fig. 3e,
the MoS
2(1 x)
Se
2x
/NiSe
2
hybrid electrode exhibits a C
dl
value of
319.15 mF cm
2
, which is one order of magnitude larger than
that of the pure MoS
2
/NiSe
2
foam (30.88 mF cm
2
), and
B43 times larger than that of pure NiSe
2
foam
(7.48 mF cm
2
), demonstrating the proliferation of active sites
in the porous hybrid catalyst, which accordingly results in the
improved catalytic performance. From these capacitance values,
we can roughly calculate the electrochemically effective surface
area, and thus the turnover frequency per site (0.030 s
1
at
100 mV and 0.219 s
1
at 150 mV, see Supplementary Table 3) by
using a similar calculation method developed by Jaramillo et al.
32
(Supplementary Note 5). The turnover frequency values are larger
than many values reported on MoS
2
-like catalysts, and close to
that of transition metal phosphide-based electrocatalysts
(Supplementary Table 3). On the other hand, electrochemical
impedance spectroscopy was carried out to examine the electrode
kinetics under the catalytic HER-operating conditions (Fig. 3f).
According to the Nyquist plots and data fitting to a simplified
Randles circuit, our results clearly reveal that the charge-transfer
resistance (R
ct
B0.5 O) for the MoS
2(1 x)
Se
2x
/NiSe
2
hybrid is
much smaller than that for pure MoS
2
/NiSe
2
(R
ct
B8 O) or for
porous NiSe
2
foam alone (R
ct
B22 O). In addition, all the
catalysts have very small series resistances (R
s
B0.6 1.2 O),
suggesting high-quality electrical integration of the catalyst with
the electrode.
Quantum mechanics calculations. To understand the
improvement on the catalytic hydrogen evolution of the
MoS
2(1 x)
Se
2x
/NiSe
2
hybrid catalyst, quantum mechanics
calculations at the density functional theory (DFT) level (PBE-D3
flavor, see Supplementary Note 6) were performed to calculate
the binding free energies of hydrogen on the Mo atom
11,23
.
Although it was originally suggested that the edge S atom is the
catalytic atom in hydrogen evolution on MoS
2
(ref. 11), we find
that H
2
formation going through the Mo atom via the Heyrovsky
reaction
33
has a lower barrier than the Heyrovsky and Volmer
34
reaction on the S atom. Therefore, we use a lower hydrogen-
binding energy on the Mo atom as the indicator of a lower
barrier in the Heyrovsky step. Since there are various exposed
facets in our as-prepared NiSe
2
foam (Supplementary Fig. 1), we
modelled the reaction on the simple low-index (100), (110) and
(111) surfaces of NiSe
2
. Molybdenum dichalcogenide with Se:S
ratios of 0:1, 1:1, 1:0 are modelled, and, in the 1:1 case, the S and
Se alternate above and below the plane to avoid strain. As shown
in Fig. 4a, DG
H
* is 8.4 kcal mol
1
for hydrogen adsorbed on
Binding energy (eV)
a
b
c
d
e
f
Binding energy (eV)
158 160 162 164 166 168
50 52 54 56 58 60
100
Raman intensity(a.u.)
200 300 400
Raman shfit (cm
–1
)
500 600 700 800
Binding energy (eV)
225
Intensity (a.u.)
Intensity (a.u.)
Intensity (a.u.)
228 231 234 237
S 2s
Se 3d
S 2p
3/2
Se 3p
3/2
Se 3p
1/2
S 2p
1/2
Mo 3d
MoSSe/NiSe
2
foam
MoSSe/NiSe
2
Pure NiSe
2
MoS
2
/NiSe
2
MoSSe/NiSe
2
foam
MoSSe/NiSe
2
foam
MoS
2
/Si
MoS
2
/Si
MoS
2
/Si
MoSSe/Si
MoSSe/Si
MoSSe/Si
MoSSe/Si
Mo 3d
5/2
Mo 3d
3/2
240
0.62 nm
0.62 nm
Figure 2 | Characterization of the ternary MoS
2(1 x)
Se
2x
/NiSe
2
foam hybrid catalysts. (a,b) TEM images showing the vertical layer orientation of
MoS
2(1 x)
Se
2x
particles grown on different regions of porous NiSe
2
foam. Scale bar, 5 nm. (c–e) Detailed XPS analysis of the Mo 3d,S2p and Se 3d spectra
in different samples, such as binary MoS
2
particles on Si, MoS
2(1 x)
Se
2x
particles on Si and MoS
2(1 x)
Se
2x
particles on porous NiSe
2
foam. (f) Raman
spectra measured on different samples.
ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms12765
4 NATURE COMMUNICATIONS | 7:12765 | DOI: 10.1038/ncomms12765 | www.nature.com/naturecommunications
MoS
2(1 x)
Se
2x
/MoS
2(1 x)
Se
2x
, which is more reactive than
MoS
2
/MoS
2
with a DG
H
* of 10.6 kcal mol
1
, agreeing with the
reported experimental results (Supplementary Figs 6 and 7)
35,36
.
In contrast, once the MoS
2(1 x)
Se
2x
particles are hybridized with
porous NiSe
2
foam, the relevant DG
H
* on MoS
2(1 x)
Se
2x
/NiSe
2
(100) and MoS
2(1 x)
Se
2x
/NiSe
2
(110) are further decreased to 2.7
and 2.1 kcal mol
1
, making these hybrid catalysts much
more active than MoS
2
/MoS
2
and MoS
2(1 x)
Se
2x
/MoS
2(1 x)
Se
2x
in the HER process. To understand the reason for the improved
reactivity of MoS
2(1 x)
Se
2x
/NiSe
2
hybrid catalysts, we examined
the intermediate structures (Fig. 4b). When the MoS
2(1 x)
Se
2x
particles are placed on top of the NiSe
2
substrate, they
relax to form a chemically bonded hybrid on the (100) and
(110) surfaces of NiSe
2
, while remaining unbonded from
the (111) surface of NiSe
2
. Thus, DFT calculations corroborate
that the MoS
2(1 x)
Se
2x
/NiSe
2
hybrid is a promising
electrocatalyst (Fig. 4).
Discussion
In general, the as-prepared hybrid catalysts possess the merits of
all the MoS
2
catalysts that have ever been reported on improving
the relevant catalytic performance
18,19
. Namely, the outstanding
HER activity as well as good stability for ternary MoS
2(1 x)
Se
2x
particles/porous NiSe
2
foam can be attributed to the synergistic
effects from the dense catalytic edge sites at the MoS
2(1 x)
Se
2x
surface, good dispersion of the MoS
2(1 x)
Se
2x
particles on
NiSe
2
foam, good electrical contact and chemical bonding
between MoS
2(1 x)
Se
2x
and NiSe
2
catalysts, and 3D porous
structures of HER-active NiSe
2
foam: first, similar to MoS
2
,
the catalytic property of MoS
2(1 x)
Se
2x
is greatly related to the
number of exposed edge sites
11,37
. Indeed, in our experiments,
because of the porous structure and curved surface of as-grown
NiSe
2
foam, ternary MoS
2(1 x)
Se
2x
layers tend to exhibit
vertical orientation on the NiSe
2
surface as demonstrated using
high-resolution TEM, indicating that abundant active edge sites
Potential (V versus RHE)
Potential (V versus RHE)
–0.16
–10
–8
–6
–4
–2
0
–0.12
–0.08
–0.04
0.00
Pure NiSe
2
foam
Pure Nise
2
foam
MoS
2
/NiSe
2
MoS
2
/Nise
2
foam
C
dl
= 30.88 mF cm
–2
C
dl
= 7.48 mF cm
–2
C
dl
= 319.15 mF cm
–2
MoS
2(1–×)
Se
2×
/NiSe
2
MoS
2(1–×)
Se
2×
/Nise
2
Pt wire
–0.6
–0.20
–600
–500
–400
–300
–200
–100
0
Initial
After 1,000 cycles
After 1,000 cycles
and stability test
–0.15
–0.10
Potential (V versus RHE)
–0.05
0.00
Time (h)
0
0
0
1
2
3
–Im(Z) (Ω)
4
5
6
7
8
–300
–250
–200
–150
–100
–50
0
Potential: –69 mV
Potential: –121 mV
4
4
Re(Z) (Ω)
Scan rate (mV s
–1
)
0
∆j
0.15 V versus RHE
(mA cm
–2
)
0
2
4
6
8
10
12
14
16
20
40 60 80 100 120 140 160 180 200
R
s
CPE
R
ct
2
8
810
6
12
16
Current density (mA cm
–2
)
Current density (mA cm
–2
)
Current density (mA cm
–2
)
Current density (mA cm
–2
)
–100
–80
–60
–40
–20
0
–0.5
–0.5
0.00
Overpotential (V)
0.07
0.14
0.21
Slope = 46.4 mV dec
–1
, j
0
= 11.4 µA cm
–2
Slope = 58.5 mV dec
–1
, j
0
= 104.9 µA cm
–2
Slope = 42.1 mV dec
–1
, j
0
= 299.4 µA cm
–2
Pt: 30 mV dec
–1
,1.078 mΑ cm
–2
0.28
–0.4
–0.3
–0.2
–0.1
0.0
0.0
0.5 1.0
1.5
Log
j (mA cm
–2
)
2.0
2.5
3.0
a
b
c
d
e
f
Figure 3 | Electrocatalytic performance of different catalysts. (a) The polarization curves recorded on MoS
2(1-x)
Se
2x
/NiSe
2
foam hybrid, MoS
2
/NiSe
2
foam hybrid and pure NiSe
2
foam electrodes compared with a Pt wire. (b) Tafel plots recorded on the catalysts in a.(c) Polarization curves showing
negligible current density loss of ternary MoS
2(1 x)
Se
2x
/NiSe
2
hybrid electrodes initially, after 1,000 CV cycles and after the stability test. (d) Time
dependence of current densities 10 and 140 mA cm
2
recorded on the MoS
2(1 x)
Se
2x
/NiSe
2
hybrid electrode under given potentials of 69 and
121 mV, respectively. (e) Plot showing the extraction of the C
dl
from different electrodes. (f) Electrochemical impedance spectroscopy (EIS) Nyquist
plots of different electrocatalysts. The data were fit to the simplified Randles equivalent circuit shown in the inset. The loading of MoS
2(1 x)
Se
2x
catalyst is
4.5 mg cm
2
.
NATURE COMMUNICATIONS | DOI: 10.1038/ncomms12765 ARTICLE
NATURE COMMUNICATIONS | 7:12765 | DOI: 10.1038/ncomms12765 | www.nature.com/naturecommunications 5