Copper dopants improved the hydrogen evolution activity of earth-abundant cobalt pyrite catalysts by activating the electrocatalytically inert sulfur sites

Article abstract of DOI:10.1039/c7ta05433e

Cobalt pyrite (CoS₂) is considered to be a promising catalyst for the hydrogen evolution reaction (HER) due to its intrinsic metallicity and high catalytic activity. However, the catalytically inert S-sites and sluggish reaction kinetics severely impede its commercial application. Herein, combining systematic theoretical and experimental approaches, a highly active and stable Cu doped CoS₂ catalyst for the HER is demonstrated. Cu dopants are proven to not only reduce the hydrogen adsorption free energy (ΔG_H?) of the Co sites from 0.41 eV to -0.13 eV, but also arouse the inert S sites with the low ΔG_H? of -0.11 eV. A large cathode current density (10 mA cm^-2 at 52 mV), low Tafel slope (42 mV dec^-1), large exchange current density (0.68 mA cm^-2), and good stability were observed in the Co_0.93Cu_0.07S₂ catalyst, which are better than those found for the previously reported CoS₂-based catalysts. The success of improving the electrochemical performance via the introduction of Cu dopants offers new opportunities in the development of high performance CoS₂-based electrodes for other energy-related applications.

Full text of DOI:10.1039/c7ta05433e

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS nanocages as a lithium ion battery anode material
2
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DOI: 10.1039/C7TA05433E
Journal Name
ARTICLE
Copper dopants improved hydrogen evolution activity of earth-
abundant cobalt pyrite catalysts by activating the electrocatalysis
inert sulfur sites
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Jingyan Zhang ,‡Baorui Xiao,‡Xiaoli Liu, Peitao Liu, Pinxian Xi, Wen Xiao, Jun Ding, Daqiang Gao,*
and Desheng Xue *
www.rsc.org/
Cobalt pyrite (CoS ) is considered to be a promising catalyst for hydrogen evolution reaction (HER) owing to its intrinsic
2
metallicity and high catalytic activity. However, the catalytic inert S-sites and sluggish reaction kinetics severely impede its
commercial application. Herein, combining with systematic theoretical and experimental approaches, a highly active and
stable Cu doped CoS
free energy (ΔꢀH
2
catalyst for HER is demonstrated. Cu dopants are proven to not only reduce the hydrogen adsorption
∗
) of Co sites from 0.41 eV to -0.13 eV, but also arouse the inert S sites with the low ΔꢀH
∗
of -0.11 eV. The
2
large cathode current density (10 mA/cm at 52 mV), low Tafel slope (42 mV/dec), large exchange current density (0.68
2
mA/cm ), and good stability are observed in the catalyst Co0.93Cu0.07
S
2
, which are better than that of CoS -based catalysts
2
as reported. The success of improving the electrochemical performance via the introduction of Cu dopant offers new
opportunities in the development of high performance CoS -based electrodes for other energy-related applications.
2
performance (hydrogen production yield, overpotential for
hydrogen evolution and stability), attributed to either catalytic inert
Introduction
Developing hydrogen economy is considered to be one of the most from part of Non-element sites or high electrical resistance, hinders
important way to solve the problem of global energy crisis and their practical application. Pt-based noble metals are the excellent
environmental issues, because hydrogen has the advantage of zero candidates as the efficient electro-catalyst for HER, whereas their
carbon footprints and high gravimetric energy density compared high cost and limited abundance largely restricts the further electro
1
,2
5
with other fuels.
electrolysis, which could promote the hydrogen evolution reaction earth-abundant material as HER electrocatalysts, which possess low
HER) of electro-catalysts occurs, has been deemed as extremely electrical resistance as well as high catalytic activity of all
Of all the methods as reported, water catalysts applications. Therefore, developing a cost-effective and
(
3
,4
promising approach for abundant hydrogen production. During components is of utmost importance.
the past decades, various earth-abundant transition metal Recently, cobalt pyrite (CoS2) shows the intrinsic metallic
6
,7
selenides,8,9 nitrides,10,11 properties, making it exclusive as an HER electrocatalyst material
compounds, such as sulphide,
12,13
14,15
20, 21
carbides,
and phosphides
have been identified as the among these pyrite-phase transition metal dichalcogenides.
promising HER electrocatalysts. However, their low catalytic Faber et.al has employed nanostructuring strategies to increase the
activity of CoS electrodes toward HER to expose more active Co
sites. Moreover, it is proposed that substituting half of the S atoms
2
22
J. Zhang, B. Xia, P. Liu, Prof. D. Gao ,Prof. D. Xue
Key Laboratory for Magnetism and Magnetic Materials of MOE, Key Laboratory of
Special Function Materials and Structure Design of MOE, Lanzhou University, Lanzhou
by P atoms could tune the Δ ꢀH
∗
which result in further
23
improvements in catalytic activity. In addition, phosphorus
substitution is proved to be crucial for improving chemical stability
and catalytic durability. Our recent results also demonstrated that
the proper combinations of N dopants and S vacancies could yield
the minimized overpotential of CoS2 catalyst. Although the
calculated Gibbs free energy of the adsorbed H (ΔGH*) for hydrogen
2
adsorption at the Co sites of CoPS (P:S=1:1) and N-CoS systems is
indeed more favourable than that of CoS2, their S sites, which
constitutes the majority of the bulk material, are still inert.
730000, P. R. China
Email: gaodq@lzu.edu.cn; xueds@lzu.edu.cn
X. Liu
24
CAS Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety, National
Center for Nanoscience and Technology, Beijing, 100190, People’s Republic of China
Prof. P. Xi
Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu
Province and the Research Center of Biomedical Nanotechnology, Lanzhou University,
Lanzhou, 730000, P. R. China
W. Xiao, Prof. J. Ding, Prof. D. Gao,
Department of Materials Science and Engineering,National University of Singapore, 9
Engineering Drive 1, Singapore 117575
25
‡
The two authors have equal contribution to this work.
Electronic Supplementary Information (ESI) available: [Supporting Information
shows the optimized atomic structures of CoS and Cu doped CoS , XRD, Raman,
Therefore, to fully utilize common CoS
methods for simultaneous activating the catalytic activity of Co site
2
materials, alternative
2
2
SEM and XPS results, Cyclic voltammetry curves results, time-dependent current and inert S sites are needed.
density, overpotentials of sample Cu-0, Cu-1, Cu-2 and Cu3. And the optimized
atomic structure and their calculated bader charge for the surface atoms.]. See
DOI: 10.1039/x0xx00000x
Of many approaches to alter the electronic structure and
consequently enhance the HER performance of catalysis, doping is
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proved to play a significant role in optimizing the ΔG of the which is close to the value on the Pt [111] surface, indicating that
H∗
View Article Online
DOI: 10.1039/C7TA05433E
26
27
catalysts, for example Co doped Mo
2
C,
Co doped NiSe
2
,
Mo the optimal catalytic activity can be achieved in S sites. Compare to
2
8
29
30
doped W18
O
49
,
Ni doped CoS
2
,
Se doped NiP
2
,
2 2
Zn doped the pristine CoS with ΔGH* of 1.3 eV for S sites, the Cu doped CoS
are more active than that of pure binary shows the lower ΔGH* both at Co sites and S sites, where the
31
32
Co
3 4
S ,
Co doped WS
2
ones, which were demonstrated from experimental and theoretical optimized atomic structures with H adsorption at Co site and S site
studies. Recently, Peng et.al has demonstrated Cu dopants could are shown in inset of Figure 1c and Figure 1d, respectively.
provide a great number of active catalytic sites in Cu–CoS Therefore, it can be concluded that Cu dopants could improve the
nanosheets and eventually lead to high electro-catalytic activity for activity of Co sites and also further activate the HER activity of inert
quantum-dot sensitized solar cells. This indicates that Cu element S sites in CoS
2
2
, providing that the Cu doped CoS system is indeed a
33
could be as another potential dopants candidate for the catalysts.
Considering the similarity in atomic size, electro-negativity and
bonding format between Cu and Co, density functional theory
DFT) calculations are firstly employed. Our calculation results
very promising earth-abundant HER catalyst.
34
(
reveal that the remarkably enhanced HER performance can be
obtained, where the doped Cu atoms could not only reduce the
ΔꢀH
adjacent S site with the low ΔꢀH
dopants can generate more active sites in CoS
intrinsic metallicity and appearance of more band states near the
Fermi level of Cu doped CoS ensure the fast charge transfer
between the catalyst and electrode to further improve electro-
catalytic reactions. Experimentally, a series of Co1−xCu catalysts
with x ranging from 0 to 0.11 are synthesized through hydrothermal
method. By studying the impact of stoichiometry of Co1−xCu on
the HER performance, catalyst of Co0.93Cu0.07 exhibits the highest
∗
of Co sites from 0.41 eV to -0.13 eV, but also arouse the
of -0.11 eV, indicating that Cu
. Besides, the
∗
2
2
x 2
S
x 2
S
S
2
catalytic activity and outstanding stability, with overpotential of 133
2
mV at 100 mA/cm and Tafel slope of 42 mV/dec in 0.5 M H
2
SO
. All the
experimental and theoretical results clearly reveal that the Cu
4
,
which are more excellent than that of pristine CoS
2
Figure 1. (a) Electronic band structures and density of states (DOS) plots of the
bulk CoS and Cu doped CoS , respectively. (b) The projected density of states
PDOS) plots of Cu doped CoS for Cu-d (red line), Co-d (green line) and S-p
purple line) orbitals. Free-energy diagram for H adsorption on the CoS and Cu
with the [001] surfaces for (c) Co sites and (d) S sites. Insets show the
optimized atomic structures with H adsorption at Co site and S site.
dopants could largely improve the HER activity of pristine CoS
thus has potential applications in future hydrogen production.
2
and
2
2
(
(
2
2
Results and Discussion
doped CoS
2
Systematic density functional theory is firstly carried out to
determine the influences of Cu dopants on the electronic structure
Encouraged by this design concept, herein, we synthesized a series
of Cu doped CoS catalysts (Co1−xCu , x=0, 0.04, 0.07, and 0.11)
and the surface adsorption energy of the H atoms of pristine CoS
The calculated electronic band structures and density of states
DOS) plots of the pristine CoS and Cu doped CoS are shown in
Figure 1a. As can be seen that the pristine CoS shows the metallic
state, like other previous reports. In the case of Cu doped CoS
2
.
2
x 2
S
through simple hydrothermal method (the detail can be seen in the
Experiment part). For convenience, these samples are named as Cu-
(
2
2
2
0
, Cu-1, Cu-2 and Cu-3 as the increasing of the Cu dopants,
35
2
,
respectively. X-ray diffraction (XRD) is first adopted to investigate
the crystalline and phase structure of these samples, and the results
there is still no band gap exist, revealing its metallic nature, which
would facilitate the electron transfer in catalysis. Further, it can be
seen that there are two new bands appear near the Fermi level,
caused by the hybridization of the Cu-d, Co-d and S-p orbitals, as
demonstrated in the projected density of states (PDOS) plots
are shown in Figure 2a, where standard diffraction peaks of CoS
JCPDS- No. 41-1471) are marked below the XRD spectra. It can be
seen that all the spectra exhibit several diffraction peaks and these
peaks accord well with the standard peaks sites of CoS . So it is easy
2
(
2
(
Figure 1b). The appearance of new bands in Cu doped CoS
2
points
o
o
o
o
o
to identify the peaks located at 27.8 , 32.3 , 36.2 , 39.8 , 46.3 ,
to a lower intermedia adsorption energy and may allow favourable
o
o
o
5
4.9 , 60.3 and 62.7 are diffraction peaks of panels (111), (200),
210), (211), (220), (311), (230) and (321), respectively. This result
36
hydrogen adsorption in HER. Next, we evaluate the HER activity of
catalysts CoS and Cu doped CoS by calculating the hydrogen
adsorption free energy. Here, we used [001] surface of CoS and
Cu doped CoS to determine the values of ΔGH* at different sites. As
38
(
2
2
demonstrates good crystalline of sample Cu-0, Cu-1, Cu-2 and Cu-3.
37
2
o
o
It should be noticed that the two peaks emerge at 29.3 and 47.9
2
in the pattern of sample Cu-3, which can be ascribed to the impurity
shown in Figure 1c, the ΔGH* of Co site is about 0.41 eV, when
introducing Cu dopants, the ΔGH* of Co site reduces to -0.13 eV,
revealing that the Cu dopants could enhance the catalytic activity of
Co sites in HER. Importantly, as can be seen from Figure 1d, the
calculated ΔGH* of S site near the Cu dopants is about -0.11 eV,
39
phases of CuS owing to the excess Cu dopants. As further
increasing Cu dopant, two obvious diffraction peaks of phase CuS
o
o
emerge at 29.3 and 48.1 , further confirming the existence of CuS
in the case of higher Cu doping concentration (Figure S2a,
supporting information). Moreover, the diffraction peaks shift to
2
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the low angle as the increasing of Cu dopants (Figure S2b, reported of CoS
supporting information), indicating that Cu atoms substitution of Co 0 and Cu-2, which both exhibits two peaks inDOtwI: o10s.1a0m39p/lCe7sTaAt01546323.E5
atoms in CoS matrix because of the ionic radius of Cu is larger than eV (S 2p 3/2) and 163.5 eV (S 2p 1/2). Besides, Cu 2p region is also
that of Co. Figure 2b shows Raman scattering spectra of sample Cu- given in Figure 2f, wherein the peaks of Cu 2p 1/2 (951.8 eV) and 2p
.43 Figure 2 e shows S 2p XPS spectra of sample Cu-
2
View Article Online
2
0
, Cu-1, Cu-2 and Cu-3, which are excited by 532 line in air ambient. 3/2 (931.8 eV) exist in Cu-2, however, are absent in Cu-0, which is a
2+ 44
g g
There are two peaks of E and A modes can be observed in all the typical feature of Cu . Combining with the above XRD and Raman
samples at 287 cm and 390 cm , consisting with the pristine CoS
indicating that Cu dopants don't change the
-1
-1
2
,
results, we demonstrate that Cu atoms are doped into CoS
by replacing the Co atoms.
2
matrix
Figure 3. (a)-(b) Scanning electron microscope (SEM) results of Cu-0 (a) and Cu-2
(b), the insets are zoom-in SEM images of two samples. (c) Transmission electron
microscope (TEM) image of Cu-2. Inset shows the corresponding EDS result of
sample Cu-2. (d) High-resolution TEM image of Cu-2. (e)-(h) Energy Dispersive X-
ray (EDX) mappings of Cu-2 sample: the detected regions (e), Co-K (f), S-K (g) and
Cu-K (h).
Figure 2. (a) X-ray diffraction (XRD) patterns and (b) Raman scattering spectra of
sample Cu-0, Cu-1, Cu-2 and Cu-3. (c)-(d) X-ray photoelectron spectra (XPS)
results of Cu-0 and Cu-2 : (c) wide, (d) Co 2p, (e) S 2p and (f) Cu 2p regions.
Scanning electron microscope (SEM) is also utilized to investigate
the morphology of our synthesized samples, and the results of Cu-0
and Cu-2 are selected and are shown in Figures 3a and 3b. In SEM
results, Cu-0 nanoparticles aggregate as spherical shapes, whose
diameters are within 3-4 µm, however, the aggregation is much
weaker in Cu-2, resulting in more cavities among nanoparticles
compared with Cu-0, which might provide more active sites. To
illustrate more clearly, zoomed-in images of these two samples are
added as insets in the top right corners of Figures 3a and 3b,
respectively. It can be seen that the grain size increases after Cu
doping, consisting with the broaden of FWHMs in XRD patterns.
Subsequently, sample Cu-2 is selected and dispersed in ethanol
under ultrasonic for being observed by transmission electron
microscope (TEM). Result of sample Cu-2 in Figure 3c exhibits
several small stacked nanoparticles while the corresponding high-
resolution transmission electron microscope (HRTEM) image is
presented in Figure 3d. The fringes along different directions are
observed in HRTEM and the distances between two adjacent fringes
are measured as 0.25 nm and 0.20 nm, indicating the panels (210)
phase structure of CoS
2
and Cu dopants may doped into the CoS
matrix. Besides, the full width at half maximum (FWHMs)
becomes more broad in Cu doped CoS samples compared with that
of pristine CoS , which can be attributed to the lattice distortion
caused by Cu doping. Besides, the Raman peak appeared at 469
2
40
2
2
41
-1
cm indicates existing the impurity phase of CuS as the Cu dopants
increasing gradually (Figure S3, supporting information). Further, in
order to investigate the element components of these four
samples, X-ray photoelectron spectra (XPS) is employed. Figures 2c-
2
f present the XPS results of two representative samples of Cu-0
and Cu-2, corresponding to wide, Co 2p, S 2p and Cu 2p regions,
respectively. In Figure 2c, the two spectra are almost same in wide
range (0-1200 eV) except the lack of Cu peaks in Cu-0. Additional O
1
s and C 1s peaks are also can be seen in Cu-0 and Cu-2, which are
42
probably caused by the adhered O
2 2
and CO from air ambient.
High-resolution XPS results of Co 2p region are presented in Figure
2d, in which there are four decomposed components in sample Cu-
0 at 778.6 eV (Co 2p 3/2), 780.8 eV (satellite peak), 793.7 eV (Co 2p
1/2) and 794.6 eV (satellite peak), according well with the results
and (220) of CoS
2
, respectively. Figures 3e-3h provide Energy
Dispersive X-ray (EDX) mappings of Cu-2 and all the mappings
demonstrate the uniform distribution of Co, S and Cu in the sample
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Cu-2, further confirming that the Cu atoms are doped of 96 mV/dec and sample Cu-3 of 64 mV/dec, demonstrating that
View Article Online
DOI: 10.1039/C7TA05433E
the kinetics of the water splitting is effectively facilitated on the
homogeneously into the CoS
2
matrix.
The electrochemical HER activities of all Cu doped CoS
2
catalysts prepared Cu-2 electrode. As well as, Cu-2 electrode shows the
(
Co1−xCu
x
S
2
) are investigated by using a simple three-electrode excellent HER performance compare to other reported CoS
setup in 0.5 M H SO solution. Figure 4a presents the linear sweep catalysts (Table S1, supporting information). It can be seen that all
voltammograms (LSV) curves of these electrodes. As can be seen the observed Tafel slopes for the Cu doped CoS catalysts are
that their HER catalytic performance shows the variation of between 40 and 120 mV/dec, revealing that electrochemical
2
-based
2
4
2
enhancement first and then decreases as the increasing of the Cu
desorption is the rate-limiting step in these catalysts and that the
45
HER process occurs through a Volmer–Heyrovsky mechanism.
Besides, the exchange current density (j ) is also a kinetic parameter
0
Figure 4. (a) The HER polarization curves and (b) Tafel plots for Cu doped CoS
2
(
Co1-xCu
x
S
2
) electrocatalysts and 20% Pt/C in 0.5 M H
current density of Δj at 0.1 V versus RHE plotted against scan rate fits to a linear
regression (d) Nyquist plots of the Cu doped CoS (Co1-xCu ) electrocatalysts. (e)
Polarization curves of catalyst Cu-2 (Co0.93Cu0.07 ) for the initially and after 10000
CV scans. Inset shows the electrochemical stability of the catalyst Cu-2
) measured at different current densities and times. (f) Volcano
as a function of the ΔGH* for common catalysts, CoS and Cu doped
(red star).
2 4
SO . (c) The differences in
Figure 5. LSV curves of (a) Cu-2 and (b) Cu-0 before and after blocking their Co
sites and S sites, respectively. (c) The overpotentials of sample Cu-0 and Cu-2 at
2
x 2
S
2
current density of 100 mV/cm before and after blocking their Co sites and S sites.
S
2
(
d) The calculated bader charges for Co, S and H atoms, respectively. (e)
Projected density of states (PDOS) plots of S 3p orbitals for the pristine and Cu-
doped CoS . (f) PDOS plots of H 1s orbitals for the H adsorbed at pristine and Cu-
surface, showing the bonding and antibonding states relative to the
(Co0.93Cu0.07S
2
2
plots of i
0
2
doped CoS
2
CoS
2
Fermi level.
2 2
dopants in the CoS matrix, where the sample Cu-2 (Co0.93Cu0.07S )
to evaluate the intrinsic catalytic performance for the HER of the
catalysts, which could be obtained by extrapolating the Tafel plots
to an overpotential of 0 V. It is proposed that an excellent
electrocatalyst with outstanding catalytic activities for the HER
exhibits the best HER catalytic performance with the smallest onset
potential of 20 mV and the lowest overpotential of 133 mV at the
46
2
current density of 100 mA/cm . This observed onset potential and
overpotential values of the electrode Cu-2 is smaller than that of
should possess a high exchange current density. In our case, the
pristine CoS
that the Cu doping is playing a positive effect in the enhancement
of HER performance. The HER properties of other Cu doped CoS
2
, even close to the commercial Pt/C catalyst, suggesting
2
catalyst Cu-2 exhibits a remarkable j
much greater than the value of pristine CoS
also better than that of other Cu doped CoS
0
of 0.68 mA/cm , which is
2
2
(0.04 mA/cm ) and
catalysts, indicating
2
2
are summarized in Table 1. Above structure analysis results
indicates that there are impurity phase of CuS appearance as the
increasing of Cu dopants (Cu-3), this may cause the reduction of the
the fastest electron-transfer rate in sample Cu-2 during the HER
progress.
Further, to explore the functions of Cu dopants on the
HER performance of the Cu doped CoS
increasing the Cu dopants. What's more, we obtained the Tafel
slopes of the Cu doped CoS catalysts to investigate the reaction
2
catalysts for further
enhancement of HER activity for CoS
the relation between Cu dopants and amount of active HER
catalytic sites and conductivity of the Cu-doped CoS catalysts,
2
, we systematically investigate
2
2
mechanism and their inherent properties for the HER. Apparently,
sample Cu-2 possesses the smallest Tafel slope of 42 mV/dec
compared with those of sample Cu-0 of 105 mV/dec, sample Cu-1
where the two aspects are known as the key factors to affect the
HER catalytic performance of electro-catalysts. Firstly, the method
4
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of measuring the electrochemical double-layer capacitance (Cdl) is revealing that Cu doped CoS
employed to estimate the relative electrochemical active surface shows potential applications in future water splitting.
2
has excellent HER performance and
View Article Online
DOI: 10.1039/C7TA05433E
4
7
area (ECSA) of the catalysts.
Figure S6, Supporting Information) obtained at various scan rates dopants indeed enhance the HER performance by arousing more
in the potential range of 0 V to 0.2 V (vs RHE) were employed to active sites and increasing the electrical conductivity of the catalyst
calculate the Cdl of Cu doped CoS catalysts, where the value of Cdl CoS . What's more, to confirm that Co-sites and S-sites are both
was calculated by plotting Δj/2 at 0.1 eV against the scan rates. activated as theoretical expected for the HER in Cu doped CoS , the
Cyclic voltammetry (CV) curves
Above theoretical and experimental results indicate that Cu
(
2
2
2
2
-
Figure 4c shows that the Cdl are 19 mF/cm for sample Cu-0, 34 effect of thiocyanate ion (SCN ) and dodecanethiol on the HER
mF/cm for sample Cu-1, 38 mF/cm2 for sample Cu-2 and 25 activity of sample Cu-2 is further studied, where SCN and
mF/cm for Cu-3, indicating the catalyst Cu-2 possesses the largest dodecanethiol are known to poison the Co-sites and S-sites,
active surface area among all samples. The enlarged ECSA for Cu respectively.
doped CoS compare to that of pristine CoS indicate that the in 0.5 M H SO
2
-
2
5
0,51
-
As shown in Figure 5a, once 10 mM SCN is added
2
2
2
4
, the overpotential decreases significantly from -133
2
number of electrochemically active sites for HER significantly (-52) mV to -246 (-98) mV at current density of 100 (10) mV/cm ,
increased because of the Cu dopants. Furthermore, electrochemical inferring that most of active Co-sites of Cu doped CoS
2
catalyst are
impedance spectroscopy (EIS) measurement for Cu doped CoS blocked by the SCN- ions. Further, it also can be seen that the
2
electro-catalysts was also performed to investigate the electro- overpotential decreases significantly to -191 (-82) mV at current
2
catalytic kinetics during the HER process and the Nyquist plots as density of 100 (10) mV/cm for catalyst Cu-2 after the catalyst's S-
well as the fitted results are shown in Figure 4d. Generally, the sites are blocked. This results indicate that both the Co-sites and S-
semicircle in the high-frequency range represents the charge sites are the catalytic active sites in Cu doped CoS
2
. To compare, we
+
transfer resistance (Rct) of H reaction at the electrode–electrolyte also test the LSV curves of pristine CoS
2
(Cu-0) after blocking its Co-
interface, where the value of Rct varies inversely to the electro- sites and S-sites. As can be seen from Figure 5b, the HER
48
catalytic reaction rate. It can be seen that the catalyst of Cu-2 performance of Cu-0 decays by Co-sites blocking, however, LSV
exhibits the lowest Rct of 43 Ω , compare to that of 230 Ω for sample curves show no obviously change for the original and S-sites
Cu-0, 70 Ω for Cu-1 and 86 Ω for Cu-3, demonstrating that Cu blocked result. As can be seen from the summrized results in Figure
dopants could reduce charge transfer resistance and provide high 5c that only Co-sites are catalytic active sites and S-sites are catalyst
electrical conductivity of CoS
dramatically promotes electron charge transfer and offers faster active sites in Cu doped CoS
HER kinetics in Cu doped CoS catalyst, which thereby leads to the inert S-sites could be activated by introducing the Cu dopants,
better HER activity. In short, above Cdl measurements and EIS well agree with our calculation results. Further, to understand the
results reveal the dual-functions of Cu dopants in CoS2, that are the mechanism of enhancing HER performance of Cu doping on CoS by
activation of additional catalytic sites for enhancing HER efficiency arousing the inert S-sites, an analysis of the electronic properties
2 2
. The smallest Rct for sample Cu-2 inert in pristine CoS , while both Co-sites and S-sites are catalytic
2
. In short, above results indicate that
2
2
[
52]
2 2
and the increasing of the electronic conductivity, which could for CoS and Cu doped CoS has been made. As we know that the
promote fast charge transport over catalytic electrode and induce combination of H 1s orbital and S 3p orbital will form a bonding
rapid reaction kinetics in HER progress. These observations are orbital (σ) and anti-bonding orbital (σ*) when H atom is absorbed
53
consistent with our calculation results, indicating that Cu dopants in on surface S atom.
CoS indeed activates more electric catalytic active sites and between H atom and S atom determines the H–S bonding strength.
enhance the speed of charge transport between the catalysts and Calculations results reveal that the average bader charge of S
electrode, resulting in the excellent HER performance. elements in pristine CoS and Cu doped CoS is about 6.270 and
Durability is another important parameter to evaluate the 6.273, the average bader charge of Co elements in pristine CoS and
electro-catalyst in HER. Here, the electrochemical stability of Cu doped CoS is about 8.459 and 8.457, showing no obvious
And the degree of energy level matching
2
2
2
2
2
sample Cu-2 was performed via 10000 potential cycling difference (supporting Figures S9). However, as shown in Figure 5d,
measurements in the potential range of -0.2 and 0 V at a scan rate when absorbing one H atom on Co site, the calculated values of
of 100 mV/s (shown in Figure 4e). The results reveal that the bader charge are 8.558 for Co atom and 1.128 for H atom on
current decay on the catalyst Cu-2 is slight, demonstrating its pristine CoS
2
, 9.118 for Co atom and 0.942 for H atom on Cu doped
is
excellent stability. Further, the long-term electrochemical stability CoS
2
, indicating that H adsorption on Co atom in pristine CoS
2
tests of catalyst Cu-2 is continuously conducted at 10, 30, and 50 weaker than Cu doped system (supporting Figures S10). Importantly,
2
mA/cm for 10 h, respectively. As can be seen from the inset in when absorbing one H atom on S site, the calculated values of
Figure 4e, the catalyst of Cu-2 exhibits high activity and stable life bader charge are 5.504 for S atom and 1.877 for H atom on pristine
over long-time testing. Even at high current densities of 30 and 50 CoS
2
, 6.630 for S atom and 0.901 for H atom on Cu doped CoS
2
,
2
mA/cm , the voltage is still well kept. When the current density indicating that H adsorption on S atom in pristine CoS
2
is weaker
2
returns to 10 mA/cm after 30 h hydrogen evolution reaction, the than Cu doped system (supporting Figures S11). These results are
corresponding overpotential keeps consistent with the initial value, consitent with above experiment results. What's more, as proposed
again demonstrating the excellent durability of Cu doped CoS
catalysts. Meanwhile, volcano plots of i as a function of ΔGH* for orbital of S element is shifted downward with more filled states and
common metal catalysts, MoS -based catalysts, CoS and Cu doped simultaneously the H 1s orbital possesses more filled antibonding
CoS are shown in Figure 4f. It can be seen that the position of i
and ΔGH* for Cu doped CoS
2
by Nørskov et al., the H-S bond will be weakened when the 3p
0
2
2
5
4
2
0
2
states. The calculated ΔGH* of pristine CoS at S site is 1.3 eV,
49
2
reach up to the volcano peak,
indicating that the H atom is hardly adsorbed on the S site.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
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Jo uP rl ne aa sl eo df oM na ot et r ai ad l jsu sCt hme am r gi si nt rs y A
Page 6 of 7
ARTICLE
However, when introducing the Cu dopants in the CoS
p orbital of S element shifts downward with more filled states, The sample Cu-0 is pristine CoS2, the others are Cu doped CoS2.
which further results in more filled anti-bonding states of H 1s Materials Characterizations: X-ray diffractometer (XRD; X’pert Pro
orbital (shown in Figures 5c and 5d, respectively). Therefore, the Cu Philips with Cu Kα radiation), Raman spectrometer (Jobin-Yvon
Journal Name
2
matrix, the and Cu-4 according to the order from less to more of Cu content.
View Article Online
DOI: 10.1039/C7TA05433E
2
dopants in CoS weakens the H-S bond, pushes the ΔGH* to -0.11 eV LabRam HR80) with the 532 nm laser excitation were employed to
at S site and activates the HER activity homologous.
reveal the crystal structure of the catalysts. Further, scanning
electron microscopy (SEM; Hitachi S-4800) and transmission
electron microscopy (TEM; Tecnai G2 F30, FEI) were used to study
the micro morphology and atomic structure of the catalysts.
Energy-dispersive X-ray spectroscopy (EDX) attached in the TEM
and X-ray photoelectron spectroscopy (XPS; Kratos AXIS Ultra) were
used to obtain elemental information and bonding characteristics of
the catalysts.
Table 1. HER catalytic performance of Cu doped CoS
2
catalysts.
Onset
η
at
(mV) η (mV) at
Tafel
slope
Concentra
tion of Cu
Exchange
current density
overpote
ntial
J = 100
Catalyst
J = 10
mA/cm2
(mV/d
(at.%)
(mA/cm2)
(mV vs. mA/cm
ec)
2
RHE)
Electrochemical Measurements: We used CHI 660E electrochemical
workstation to perform all electrochemical tests with a standard
three-electrode cell, where the catalysts adhere on glassy carbon
electrode was used as working electrode (WE), Ag/AgCl and carbon
rod were used as reference and counter electrodes, respectively.
Linear sweep voltammetry (LSV) measurements were conducted in
Cu-0
Cu-1
Cu-2
Cu-3
Pt
0
167
97
20
35
0
239
158
52
364
252
133
241
79
105
96
0.04
0.09
0.68
0.47
0.78
4±0.2
7±ꢁ. ꢂ
11±0.2
-
42
111
10
64
30
2 2 4
N -saturated 0.5 M H SO electrolyte at a scan rate of 2 mV/s. The
stability of the catalysts was evaluated by carried out cyclic
voltammetry measurement at a scan rate of 100 mV/s for 10000
cycles followed by LSV measurement at a scan rate of 2 mV/s. The
Tafel slope was obtained from the LSV plots with a scan rate of 2
mV/s. All polarization curves were iR compensation unless noted.
All the potentials were calibrated with respect to a reversible
hydrogen electrode (RHE) by adding a value of 0.197 V. The EIS was
performed at the polarization voltage of 260 mV vs. RHE in a
frequency range from 100 kHz to 0.1 Hz with an AC amplitude of 10
mV. A simplified Randles circuit is used to fit the impedance data to
Conclusion
In summary, we propose and validate a simple and efficient
strategy to synthesize the efficient HER electro-catalysts of Cu
doped CoS
principle calculations, we demonstrate that Cu dopants could
increase the HER performance of CoS by activating the Co sites, as
2
particles. According to experimental results and first
2
well as, arousing the inert S sites. The optimized catalyst of
2
extract the solution and charge-transfer resistances (R
S
and RCT).
Co0.93Cu0.07
S
2
has the large cathode current density of 10 mA/cm at
First-principles Calculations: We used Vienna ab initio simulation
package (VASP) based on density functional theory (DFT) to carry
out our first-principles calculations. Electron-ion interactions was
modeled by projector augmented wave (PAW) potentials. The
generalized gradient approximation with Perdew-Burke-Ernzerhof
5
2 mV, low Tafel slope of 42 mV/dec, and large exchange current
2
density of 0.68 mA/cm , which is superior to that of the widely
reported TMD catalysts (MoS and so on) and first-row transition
metal pyrites (CoS and so on). This finding paves a rational
pathway to strengthen the electrocatalytic HER performance of
CoS via the Cu ions doping, and the involved concept and strategy
2
2
(
GGA-PBE) functional were performed to approximate exchange
2
and correlation effects to relax the atomic structures, with all
can be extended to other first-row transition metal pyrites.
-5
models fully relaxed in self-consistency accuracy of 10 eV. For the
plane-wave basis restriction, we set the cut-off energy as 450 eV
and the K-points were taken under Monkhorst-Pack for the
Methods
Synthesis: Both pristine CoS
one step hydrothermal method with the cobalt nitrate hexahydrate calculate the effective charge of Mo atoms in the supercells. A
Co(NO •6H O), sodium thiosulfate pentahydrate (Na O • 5H
O) 2×2×2 supercell was used to calculate the electronic property and
and copper dinitrate (Cu(NO • 3H
O) as the Co, S and Cu hydrogen evolution activity. For Cu doped CoS , we used one Cu
resources. In detail, 0.7276 g (~2.5 mmol) of Co(NO •6H
O and atom to replace one Co atom. To prevent interaction between two
.6201 g (~2.5 mmol) of Na O • 5H
O with 0 g, 0.0060 g (~0.025 neighboring surfaces, a vacuum slab of 15 Å was employed in (001)
mmol), 0.0181 g (~0.075 mmol), 0.0362 g (~0.15 mmol) and 0.048 g direction of CoS for the calculations of hydrogen evolution activity.
~0.2 mmol) Cu(NO • 3H
O were added to 60 ml water and The activity of hydrogen evolution reaction was reflected by the
stirred until dissolved, respectively. Then the solution was Gibb’s free energy change (∆GH*) and the values of ∆GH* is obtained
transferred to 100 mL Teflon-lined stainless autoclaves one by one, from the formula of ∆GH*=∆EH*+∆EZPE-T∆S, and ∆EH* = E(surface+H*)
reacted at 180 ℃ for 12h. They were taken out after cooling to
surface-1/2EH2. Here, E(surface+H*) and Esurface are the total energy of the
room temperature, washed two or three times with anhydrous surface model with and without H adsorption, respectively. EH2 is
ethanol and deionized water respectively, placed in the constant the energy of a single H molecule isolated in vacuum. ∆EZPE is the
2 2
and Cu doped CoS
are synthesized by Brillouin-zone integration. Bader charge analysis was performed to
(
3
)
2
2
S
2 2
2
3
)
2
2
2
3
)
2
2
0
S
2 2
2
2
(
3
)
2
2
-
E
*
2
temperature oven at 60 ℃ for 12h to dry. Finally we got five black zero-point energy change, which can be obtained from vibrational
powder samples that were labeled severally Cu-0, Cu-1, Cu-2, Cu-3 frequency calculation as implemented in VASP. T∆S is estimated to
be 0.2 eV to consider the entropy change at room temperature.
6
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
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Page 7 of 7
Jo uP rl ne aa sl eo df oM na ot et r ai ad l jsu sCt hme am r gi si nt rs y A
Journal Name
ARTICLE
2
6 H. Lin, N. Liu, Z. Shi, Y. Guo, Y. Tang, Q. GaoV,ieAwdAvrt.icleFuOnnlcinte.
Acknowledgements
Mater., 2016, 26, 5590-5598.
DOI: 10.1039/C7TA05433E
This work is supported by the National Natural Science Foundation 27 B. Liu, Y.F. Zhao, H.Q. Peng, Z.Y. Zhang, C.K. Sit, M.F. Yuen,
T.R. Zhang, C.S. Lee, W.J. Zhang, Adv. Mater., 2017.
8 X. Zhong, Y. Sun, X. Chen, G. Zhuang, X. Li, J.-G. Wang, Adv.
Funct. Mater., 2016, 26, 5778-5786.
of China (Grant No. 11474137), the Fundamental Research Funds
for the Central Universities (Grant No. lzujbky-2016-130).
2
2
3
3
9 Y. Cui, C. Zhou, X. Li, Y. Gao, J. Zhang, Electrochim. Acta, 2017,
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28, 428-435.
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