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Journal ArticleDOI

Constructing Mono-/Di-/Tri-Types of Active Sites in MoS2 Film toward Understanding Their Electrocatalytic Activity for the Hydrogen Evolution

26 Nov 2019-Vol. 2, Iss: 12, pp 8974-8984

AbstractThe availability and catalytic activity of the cost-efficient electrocatalysts are the dominant factors for the hydrogen evolution reaction (HER) performance in the renewable hydrogen economy. Exte...

Topics: Hydrogen economy (61%), Catalysis (52%)

Summary (2 min read)

1. INTRODUCTION

  • In view of the forthcoming fossil fuel exhaustion, rapid global population growth and environmental issues, the immediate deployment and development of renewable energy resources become paramount.
  • Hydrogen fuel is considered to be one of the most promising sustainable and clean energy sources since the raw material for the hydrogen production is water.
  • It was real no way to define the contribution of the electrocatalysis active sites in the randomly restacked MoS2 electrode on HER performance by weight, because their relative proportion was unknown and their electrocatalytic activities were also unclear.
  • The current collector is cleaned carbon fibre (CF).

2. EXPERIMENTAL DETAILS

  • The MoS2 films were directly fabricated on the CF substrate without any additives by using a magnetron sputtering strategy in the Ar plasma.
  • The maximum fabrication area of film depends on the target size.
  • The cross-sectional structure of representative 3050 MoS2 film was observed by a high-resolution transmission electron microscope (HRTEM, FEI Titan Themis Cubed 300) and the chemical component was analysed by the attached energy dispersive X-ray spectroscopy (EDS).
  • All electrochemical measurements were performed with a standard three-electrode electrochemical cell in 0.5 M H2SO4 solution.
  • Hz and an amplitude of 10 mV under the overpotential of 200 mV vs. RHE, and spectra were appraised through the Levenberg-Marquardt minimization procedure.

3. RESULTS AND DISCUSSION

  • The morphologies of MoS2 films with thicknesses of ~15 nm, ~35 nm, 76 nm, 880 nm and 3050 nm were characterized by FESEM, and the cross-sectional and surface morphologies are shown in Figure 2(a).
  • 7,15 The height of the formed stepped-termination surfaces of each columnar MoS2 platelet was 15~20 nm, as shown in Figure 1(f).
  • Here, the authors aim to evaluate the electrocatalytic activity of various active sites in MoS2 electrode.
  • The ~15 nm film possesses the immature basal planes decorated by terrace active sites (tB), and the ~35 nm film constitutes of Zone I with thickness of ~20 nm and the upper Zone II (~15 nm) with sS active sites, in which the Zone I was covered by the upper Zone II.

4. CONCLUSIONS

  • The porous and ordered MoS2 film with various active sites (exposed edge of columnar sheets (eE), stepped-termination surface (sS) and terraces on basal plane (tB)) was successfully fabricated on carbon fibre by one-step physically-sputtering strategy in energetic plasma environment.
  • The amount of active sites at the edge of columnar platelets can be customized via simply controlling the length of the 2H-MoS2 columns.
  • This new approach proposes and evaluates a facile strategy to investigate the electrocatalytic activities of various active sites (eE, sS and tB) and the dependence of eEs’ electrocatalytic activities on the distance to the current collector in columnar MoS2 platelets electrocatalyst for HER.
  • The results reveal that the tB active site has the same electrocatalytic activity to the eE active site but it is higher than that of sS active site.
  • It should be attributed to the limited charge transfer from the current collector to the far-end active sites of the MoS2 columnar crystal platelets.

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Version: Accepted Version
Article:
Xu, S, Xu, J, Liu, Y-Z et al. (5 more authors) (2019) Constructing Mono-/Di-/Tri-Types of
Active Sites in MoS2 Film toward Understanding Their Electrocatalytic Activity for the
Hydrogen Evolution. ACS Applied Energy Materials, 2 (12). pp. 8974-8984. ISSN 2574-
0962
https://doi.org/10.1021/acsaem.9b02084
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Constructing Mono-/Di-/Tri-Types of Active Sites in MoS
2
Film toward Understand Their Electrocatalytic Activity for
the Hydrogen Evolution
Journal:
ACS Applied Energy Materials
Manuscript ID
ae-2019-020843.R2
Manuscript Type:
Article
Date Submitted by the
Author:
n/a
Complete List of Authors:
Xu, Shusheng; University of Leeds, Institute of Functional Surfaces,
School of Mechanical Engineering
Xu, Jiao; Chinese Academy of Sciences, State Key Laboratory of Solid
Lubrication, Lanzhou Institute of Chemical Physics; Southern University
of Science and Technology, School of Material Science and Engineering
Liu, Yu-Zhen; Yonsei University, Centre for Nano-Wear, School of
Mechanical Engineering
Hua, Yong; University of Leeds, Institute of Functional Surfaces, School
of Mechanical Engineering
Duan, Zewen; Chinese Academy of Sciences, State Key Laboratory of
Solid Lubrication, Lanzhou Institute of Chemical Physics
Wang, Yanan; Chinese Academy of Sciences, State Key Laboratory of
Solid Lubrication, Lanzhou Institute of Chemical Physics
Neville, Anne; University of Leeds, School of Mechanical Engineering
Gao, Xiaoming; Chinese Academy of Sciences, State Key Laboratory of
Solid Lubrication, Lanzhou Institute of Chemical Physics
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Constructing Mono-/Di-/Tri-Types of Active Sites in MoS
2
Film toward
Understand Their Electrocatalytic Activity for the Hydrogen Evolution
Shusheng Xu
†,#
, Jiao Xu
‡,§,#
, Yu-Zhen Liu
, Yong Hua
, Zewen Duan
, Yanan Wang
‡,
*, Anne Neville
†,
*, Xiaoming Gao
Institute of Functional Surfaces, School of Mechanical Engineering, University of Leeds, Leeds, LS2 9JT, UK
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou
730000, People’s Republic of China
§
School of Material Science and Engineering, Southern University of Science and Technology, Shenzhen 518055, People’s
Republic of China
Centre for Nano-Wear, School of Mechanical Engineering, Yonsei University, Seoul 03722, Republic of Korea
ABSTRACT
The availability and catalytic activity of the cost-efficient electrocatalysts are the dominant factors for
the hydrogen evolution reaction (HER) performance in the renewable hydrogen economy. Extensive
efforts have been devoted to maximize the amount of various active sites in non-noble metal
electrocatalysts for HER. This work reported a physically-sputtering strategy to construct porous and
ordered 2H-MoS
2
films with mono-/di-/tri-types of active sites via controlling the film thickness (from
~15 nm to 3050 nm) in the energetic plasma. As the pure (2H-) MoS
2
for HER electrocatalyst, the as-
fabricated 3050 nm additive-free columnar film electrode shows a stable electrochemical activity for
HER (an overpotential of 204 mV at a current density of -10 mA/cm
2
). Interestingly, the MoS
2
film
with controllable thickness can serve as an innovative platform to study the electrocatalytic activity of
the customized different active sites (the exposed active edge of sheets (eE), stepped-termination
surfaces (sS) and terrace on the basal planes (tB)) and the dependence of electrocatalytic efficiency of
the vertically-aligned MoS
2
eE active sites on their distance to the current collector. The results firstly
revealed that the tB active sites possessed almost the same electrocatalytic activity as that of the eE
active sites but higher than sS active sites. The electrocatalytic efficiency of the eE active sites
decreased as their distances to the current collector were gradually increasing, due to the limited
conductivity of the semi-conductive 2H-MoS
2
sheets. This work proposes and evaluates a facile
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strategy for replying the question on how to investigate the electrocatalytic activities of various active
sites in the electrocatalysts.
KEYWORDS: MoS
2
, physically-sputtering strategy, various active sites, electrocatalytic activity,
hydrogen evolution reaction
1. INTRODUCTION
In view of the forthcoming fossil fuel exhaustion, rapid global population growth and environmental
issues, the immediate deployment and development of renewable energy resources become paramount.
Hydrogen fuel is considered to be one of the most promising sustainable and clean energy sources since
the raw material for the hydrogen production is water.
1-4
Solar energy is a rival source but with some
issues due to the intermittent nature. By comparison, hydrogen fuel can be produced by simply splitting
water driven by electrocatalyst and the production process is paralleled. The low abundance and high
cost feature of Pt has limited its wide adoption for the hydrogen evolution reaction (HER) even though
Pt based electrocatalyst are demonstrated to have the most effective catalysis performance.
4-7
Currently,
one challenge is to develop a low cost but high efficiency electrocatalyst, as an alternative to the earth-
rare Pt for HER.
The race was started to improve the HER performance of non-noble-metal candidate materials
(carbide: W
2
C, Mo
2
C, etc.;
8,9
phosphide: MoP, Ni
x
Co
y
P, etc.;
10,11
nitride: Ni
3
N, WN, etc.;
12,13
oxide:
Co
3
O
4
,
14
transition metal dichalcogenides (TMDs)
6,7,15,16
) since the natures of their active sites for
electrocatalytic activity had been identified. One of the main strategies was to create more active sites
per unit area, and the other was to improve the electric conductivity to further enhance the
electrocatalytic activities of the existing active sites. Among the aforementioned candidates, the TMDs
have been widely studied due to their promising high activity and high stability in many strong acids.
Up to now, extensive efforts have been devoted to developing the TMDs (MoS
2
, WS
2
, MoSe
2
, WSe
2
,
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MoTe
2
, NbSe
2
, etc.)
7,15,17,18-26
and tailoring their nanostructure (the ratio of effective atoms at the
surface and subsurface) to maximize the amount of active sites to ultimately enhance HER performance.
The efficient strategy to increase the density of active sites included (i) reduction of the TMDs’ size to
enlarge the ratio of the exposed active edge of sheets (eE),
21,27-29
and (ii) induction of the heterogeneous
growth of TMDs crystals to fabricate the stepped-termination surface (sS),
7,15,30
and (iii) activating the
inert basal plane by creating the active terraces (tB),
21,24,31-35
and (iv) switching the semi-conductive 2H
phase to the metallic and active 1T’ phase TMDs.
7,36-39
Grain boundary was also known as another
type of active site but it had lower electrocatalytic activity than eE, sS and tB.
40
Thus, additive
manufacturing of more active sites in MoS
2
electrode held broad interests and significances to fully
accelerate the HER kinetics. Hu et al have simply pointed out that the more loading mass of porous
active material in electrode film, the higher hydrogen yield.
17,18
Nevertheless, it was still unclear of the
contribution of the high-loading mass active material on the enhanced hydrogen production. The key
challenge was in lack of the understanding of the contributions of different active sites on the HER
kinetics. David et al have used porous MoS
2
electrodes with various thicknesses as model to identify
the dominant factors of active sites for HER activity.
38,41-43
However, it was real no way to define the
contribution of the electrocatalysis active sites in the randomly restacked MoS
2
electrode on HER
performance by weight, because their relative proportion was unknown and their electrocatalytic
activities were also unclear. In fact, the explicit definition of the electrocatalytic activities of various
active sites for HER is essential to design the well-defined structure for further enhancing their
electrocatalytic performance.
Recently, the novel physical approach is triggering interests in manufacturing the additive-free
vertically-aligned active materials on current collector to explore the enhanced electrochemical
performances.
44-47
This stimulates us to explore a straightforward physically-sputtering strategy to
directly synthesize the porous and ordered TMDs film on the current collector to define electrocatalytic
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Frequently Asked Questions (2)
Q1. What have the authors contributed in "Constructing mono-/di-/tri-types of active sites in mos2 film toward understand their electrocatalytic activity for the hydrogen evolution" ?

This work reported a physically-sputtering strategy to construct porous and ordered 2H-MoS2 films with mono-/di-/tri-types of active sites via controlling the film thickness ( from ~15 nm to 3050 nm ) in the energetic plasma. This work proposes and evaluates a facile Page 1 of 34 ACS Paragon Plus Environment ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Page 2/33 strategy for replying the question on how to investigate the electrocatalytic activities of various active sites in the electrocatalysts. As the pure ( 2H- ) MoS2 for HER electrocatalyst, the asfabricated 3050 nm additive-free columnar film electrode shows a stable electrochemical activity for HER ( an overpotential of 204 mV at a current density of -10 mA/cm2 ). 

Their further research plan is to present how to arrange the conductive agent along the edge-orientated columnar platelets but without any cover or passivate the active sites to fast transfer electron to the far-end active sites, to approach its intrinsic activity rate for HER.