Self-supported CoMoS nanosheet array as an efficient ...Self-supported CoMoS 4 nanosheet array as an efficient catalyst for hydrogen evolution reaction at neutral pH Xiang Ren1,§,
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Self-supported CoMoS4 nanosheet array as an efficientcatalyst for hydrogen evolution reaction at neutral pH
Xiang Ren1,§, Dan Wu1,§, Ruixiang Ge2, Xu Sun1, Hongmin Ma1, Tao Yan3, Yong Zhang1, Bin Du3, Qin Wei1 (),
and Liang Chen2 ()
1 Key Laboratory of Interfacial Reaction & Sensing Analysis in Universities of Shandong, School of Chemistry and Chemical Engineering,
University of Jinan, Jinan 250022, China 2 Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China 3 School of Resources and Environment, University of Jinan, Jinan 250022, China § Xiang Ren and Dan Wu contributed equally to this work.
only shows two peaks at 26° and 43° (CC: JCPDS no.
75-2078), indicating the formation of an amorphous
species. XPS is used to characterize the as-synthesized
CoMoS4. Figure S1 in the Electronic Supplementary
Material (ESM) is the XPS survey spectrum for CoMoS4
NS/CC, which illustrates the amorphous product
consisting of Co, Mo and S elements. The CoMoS4
catalyst may be superficially oxidized before the XPS
measurement [9], leading to a slight shift compared
with the pure product. In Fig. 1(b), the peaks at 779.6
and 794.7 eV correspond to Co 2p3/2 and Co 2p1/2,
respectively. Meanwhile, the binding energies (BEs)
at 783.1 and 801.0 eV with two shakeup satellites
Figure 1 (a) XRD patterns of Co(OH)F NS/CC and CoMoS4 NS/CC. XPS spectra of CoMoS4 nanosheet in the (b) Co 2p, (c) Mo 3d, and (d) S 2p regions.
(identified as “Sat.”) also correspond to Co2+/Co3+
[45, 46]. In Fig. 1(c), the BEs at 229.5, 232.9 and 236.1 eV
are well matched to Mo 3d5/2, Mo 3d3/2, and Mo6+,
respectively, indicating that Mo exists in its VI
oxidation state form, which is in accordance with that
of MoS42– [47–49]. S 2p3/2 and S 2p1/2 appear at BEs of
162.1 and 163.4 eV (Fig. 1(d)), respectively, suggesting
the existence of S2–. The ICP-MS analysis suggests an
atomic ratio of nearly 1:1:4 for Co:Mo:S, indicating
the formation of CoMoS4, which is consistent with
the XPS and EDX results (Table S1 in the ESM). Based
on the above characterizations, it has been proven that
Co(OH)F has been successfully converted to CoMoS4.
The SEM images (Fig. 2(a)) of Co(OH)F NS/CC show
the entire surface of CC is completely covered with
Co(OH)F nanosheet array. As observed in Fig. 2(b), the
anion-exchanged CoMoS4 maintains the nanosheet
morphology. The cross-section SEM images for
Co(OH)F NS/CC and CoMoS4 NS/CC (Fig. S2 in the
ESM) indicate that all the nano arrays are about 2.5 μm
in thickness, before and after anion exchange. Figure S3
in the ESM shows the TEM image of the CoMoS4
nanosheet, which is in accordance with Fig. 2(b).
According to the EDX mappings in Fig. 2(c), the Co,
Mo, and S elements in the amorphous product are
uniformly distributed on the CC. The HRTEM image
(Fig. 2(d)) presents a well-resolved lattice fringe
with an interplanar distance of 1.56 Å indexed to the
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2028 Nano Res. 2018, 11(4): 2024–2033
Figure 2 SEM images of (a) Co(OH)F NS/CC and (b) CoMoS4 NS/CC (inset: high-magnification image). (c) SEM image and EDX elemental mapping images of Co, Mo, and S in CoMoS4 NS/CC. (d) HRTEM image and (e) SAED spectrum of the Co(OH)F nanosheet. (f) HRTEM image and (g) SAED spectrum of the amorphous CoMoS4 nanosheet.
(002) plane of Co(OH)F. The corresponding selected
area electron diffraction (SAED) spectrum shows a
discernible ring indexed to (002) plane of Co(OH)F
(JCPDS no. 50-0827), as shown in Fig. 2(e). Figures 2(f)
and 2(g) indicate that the CoMoS4 nanosheet array is
amorphous after anion exchange.
We further investigated the HER activity of CoMoS4
NS/CC (loading: ~ 1.48 mg·cm–2) using a typical
three-electrode system with a scan rate of 2 mV·s–1 in
1.0 M PBS. Owing to the direct reflection of the
intrinsic behavior of catalysts, the iR correction is
used to eliminate the effect of ohmic resistance unless
otherwise specified [50] and all potentials were reported
on a RHE scale. Commercial Pt/C on CC (loading:
~ 1.48 mg·cm–2) was examined for comparison.
Figure 3(a) shows the linear sweep voltammetry (LSV)
curves. As expected, Pt/C displays excellent HER acti-
vity, requiring an overpotential of only 59 mV to drive
a current density of 10 mA·cm–2. Co(OH)F NS/CC is
also capable of HER catalysis, requiring 323 mV to
drive 10 mA·cm–2. In sharp contrast, CoMoS4 NS/CC
exhibits superior HER activity compared to Co(OH)F
NS/CC, only requiring a much smaller overpotential
of 183 mV to drive 10 mA·cm–2; 140 mV lower than
that for Co(OH)F NS/CC. The overpotential of CoMoS4
and CoMoS3/fluorine-doped tin oxide (FTO) (η5 mA·cm–2 =
206 mV) [53]. A more detailed comparison is provided
in Table S2 in the ESM. The performance of the
CoMoS4 NS/CC catalyst under acidic and alkaline
conditions was also assessed (Fig. S4 in the ESM).
The Tafel plots are well fitted by the following
equation: η = b·logj + a, where j is the current density
and b is the Tafel slope. In Fig. 3(b), the Tafel slopes
are 75, 116 and 193 mV·dec–1 for Pt/C, CoMoS4 NS/CC
and Co(OH)F NS/CC, respectively. This suggests
that the HER occurs on Co-based electrodes through
a Volmer rate-determining step mechanism [54, 55].
Figure 3(c) shows the multi-step chronopotentiometric
curve for CoMoS4 NS/CC in 1.0 M PBS, with the current
increasing from 16 to 52 mA·cm–2 (4 mA·cm–2 per 500 s).
At the initial current value, the potential immediately
levels off at –1.03 V and is unchanged for the remaining
500 s. The other steps exhibit similar results, indicating
the excellent conductivity and mass transportation
properties, as well as the mechanical robustness of
the 3D CoMoS4 NS/CC electrode [1, 56]. Meanwhile,
the stability of CoMoS4 NS/CC was also evaluated in
Figure 3 (a) LSV curves of Pt/C, Co(OH)F NS/CC and CoMoS4 NS/CC for HER. (b) Corresponding Tafel plots for Pt/C, Co(OH)F NS/CC and CoMoS4 NS/CC. (c) Multi-current process of CoMoS4 NS/CC. (d) LSV curves of CoMoS4 NS/CC before and after 1,000 cyclic voltammetry cycles and time-dependent current density curve of CoMoS4 NS/CC. All experiments were performed in 1.0 M PBS.
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2029 Nano Res. 2018, 11(4): 2024–2033
this study (Fig. 3(d)). The LSV curves of CoMoS4 NS/CC
show no obvious changes after 1,000 cyclic voltammetry
(CV) cycles in 1.0 M PBS, suggesting that the catalytic
activity is fully retained, in accordance with the 25 h
long-term durability results.
The electrochemical double layer capacitance (Cdl)
of CoMoS4 NS/CC was tested to calculate electro-
chemical surface area by CV. In Figs. S5(a) and S5(b)
in the ESM, scan rates of 20, 40, 60, 80 and 100 mV·s–1
were used to evaluate the performance of the catalysts.
As shown in Figs. S5(c) and S5(d) in the ESM, Cdl of
18.6 and 0.975 mF·cm–2 is obtained for CoMoS4 NS/CC
and Co(OH)F NS/CC, respectively, suggesting that the
topotactic conversion of the anion exchange reaction
increases the surface roughness, which is beneficial
for enhancement of HER activity. The significantly
rougher catalyst surface [57, 58] and much higher
performance of the amorphous product [59, 60] indicates
higher electrochemical double-layer capacitance.
Meanwhile, electrochemical impedance spectroscopy
(EIS) was also performed to evaluate the performance
of the catalysts. EIS is considered to be a powerful
tool for studying electrode kinetics in a catalytic
reaction. CoMoS4 NS/CC possesses a smaller semicircle
radius than Co(OH)F NS/CC (Fig. S6 in the ESM), which
suggests lower charge-transfer resistance (Rct) and
more rapid catalytic kinetics when amorphous CoMoS4
is generated.
The TOF was calculated in this study to demonstrate
the HER activity of CoMoS4 NS/CC at a constant
overpotential. Figure S7 in the ESM shows the cyclic
voltammograms in the region of −1.0 to + 0.6 V vs.
SCE for CoMoS4 NS/CC at pH 7. As the product
evolution rate per mole of active sites, the TOF is
calculated to be 0.2 and 1.13 s–1 at overpotentials of 200
and 600 mV for CoMoS4 NS/CC, smaller than those for
analysis, which was quantified with a calibrated pressure
sensor of a H-type electrolytic cell. The FE of the HER
process is calculated to be in approximate agreement
with the theoretical value (assuming 100% FE), which
suggests that nearly 100% FE is achieved by this
catalyst electrode (Fig. S8 in the ESM).
Generally, the hydrogen evolution activity of a
catalyst is related to ΔGH. In detail, a thermoneutral
ΔGH value of 0 eV can result in optimal HER activity,
with a balance between proton reduction and removal
of adsorbed hydrogen from the surface [44]. For further
understanding, the side-view structure of Co(OH)F
and CoMoS4 is shown in Figs. 4(a) and 4(b), respectively.
First-principles DFT was used to calculate ΔGH for
the (110) surface of the catalyst, in order to illustrate
the superior HER activity of CoMoS4. Figure 4(c) shows
the free energy diagram for HER on Pt, Co(OH)F and
CoMoS4 surfaces. Pt, the most active HER catalyst,
presents a ΔGH of approximately –0.09 eV. With regard
to the Co(OH)F catalyst, it has been found that both
the Co top- and O-sites can accommodate H atoms,
whereas H adsorption on top of the F site is not
stable. The strong interactions between the H and O
ions yield an undesirable ΔGH of –0.773 eV. In sharp
contrast, the Co sites are well matched with the OH
and F coordinates, and bind H quite loosely, so that
ΔGH shifts from 0 to 0.576 eV. Furthermore, the same
chemical environments exist between the exposed Mo
and Co sites of CoMoS4 and the Co ions of Co(OH)F.
As a result, these sites also bind H weakly, yielding
large ΔGH values of 0.755 and 0.514 eV for the Mo
and Co sites, respectively. Meanwhile, the exposed S
atoms, which act as the stable binding sites, can also
accommodate hydrogen. Note that the S sites, which
have lower electronegativity than the O sites, can
interact moderately with H, leading to a favorable
ΔGH. In addition, the ΔGH of the S site is calculated to
be 0.191 eV, which is significantly closer to zero than
Figure 4 Side view of (a) Co(OH)F and (b) CoMoS4 model structures. The purple, blue, yellow, red, light blue and white balls represent Mo, Co, S, O, F and H atoms, respectively. (c) Free energy diagram for HER on Pt, Co(OH)F and CoMoS4 surfaces.
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any of the other sites. Considering that the catalyst is
amorphous, we also calculated the ΔGH values for the
(001) and (111) faces of CoMoS4. The calculated ΔGH
values for the (001) and (111) faces are –0.11 and 0.63 eV,
respectively. However, the amorphous transition metal
compound possesses more defects or vacancies that
can serve as active sites for enhanced water splitting
[59, 60, 62]. Thus, the amorphous product exhibits
higher activity than the crystal structure. In the
calculation steps, the crystalline CoMoS4 based on
different faces exhibits excellent activity, indicating
that the amorphous CoMoS4 has superior performance.
4 Conclusions
In summary, the self-supported CoMoS4 nanosheet
array on carbon cloth has been successfully derived
in situ from a Co(OH)F nanosheet array on carbon cloth
through hydrothermal anion exchange in (NH4)2MoS4
solution. As a 3D non-noble metal electrode, this
catalyst shows superior activity towards hydrogen
evolution, with a geometrical current density of
10 mA·cm–2 at an overpotential of only 183 mV in
1.0 M PBS (pH 7). This electrode also demonstrates
excellent long-term electrochemical durability and a
high TOF (1.13 s–1, η = 600 mV) for HER. This study
not only provides an attractive earth-abundant catalyst
for efficient HER, but also paves a new way to the
rational design and scalable self-templating fabrication
of metallic thiomolybdate nanoarrays for a wide range
of applications [63–66].
Acknowledgements
This work was supported by the National Key Scientific
Instrument and Equipment Development Project of
China (No. 21627809), the National Natural Science
Foundation of China (Nos. 21375047, 21377046, 21405059,
21575137, 21575050, and 21601064), Natural Science
Foundation of Shandong Province (Nos. ZR2016JL013
and ZR2016BQ10), Graduate Innovation Foundation
of University of Jinan (No. YCXB15004), and the
Special Foundation for Taishan Scholar Professorship
of Shandong Province (No. ts20130937).
Electronic Supplementary Material: Supplementary
Material (experimental section; SEM and TEM images;
XPS spectrum; CVs; LSV curves; capacitive current vs.
scan rate plots; EIS spectra; oxidation peak current
vs. scan rate plots; Faradaic efficiency; Tables S1 and
S2) is available in the online version of this article at
https://doi.org/10.1007/s12274-017-1818-6.
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