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Mo2C@NC@MoSx porous nanospheres with sandwich shell based on MoO42--polymer precursor for efficient hydrogen evolution in both acidic and alkaline media
Accepted Manuscript 2Mo2C@NC@MoSx porous nanospheres with sandwich shell based on MoO4 polymer precursor for efficient hydrogen evolution in both acidic and alkaline media Jing-Qi Chi, Xiao Shang, Shan-Shan Lu, Bin Dong, Zi-Zhang Liu, Kai-Li Yan, WenKun Gao, Yong-Ming Chai, Chen-Guang Liu PII:
S0008-6223(17)30902-8
DOI:
10.1016/j.carbon.2017.09.027
Reference:
CARBON 12357
To appear in:
Carbon
Received Date: 30 May 2017 Revised Date:
19 August 2017
Accepted Date: 8 September 2017
Please cite this article as: J.-Q. Chi, X. Shang, S.-S. Lu, B. Dong, Z.-Z. Liu, K.-L. Yan, W.-K. Gao, Y.2M. Chai, C.-G. Liu, Mo2C@NC@MoSx porous nanospheres with sandwich shell based on MoO4 polymer precursor for efficient hydrogen evolution in both acidic and alkaline media, Carbon (2017), doi: 10.1016/j.carbon.2017.09.027. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract
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Mo2C@NC@MoSx porous nanospheres with sandwich shell based on MoO42--Polymer precursor
for efficient hydrogen evolution in both acidic and
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alkaline media Jing-Qi Chi a, Xiao Shang a, Shan-Shan Lu a, b, Bin Dong *a, b, Zi-Zhang Liu a, b, Kai-Li Yan a, Wen-Kun Gao a, b, Yong-Ming Chai *a, Chen-Guang Liu a
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a State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China),
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Qingdao 266580, PR China
b College of Science, China University of Petroleum (East China), Qingdao 266580, PR China
Abstract
Mo2C@NC@MoSx porous nanospheres with sandwich shell have been
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synthesized for efficient hydrogen evolution in both acidic and alkaline media. Firstly, porous MoO2-Mo2C@NC nanospheres have been obtained with ultrafine Mo2C nanocrystallines as core and ultrathin N-doped carbon (NC) as shell through the
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carbonization of MoO42--/aniline-pyrrole (MoO42--Polymer) as precursor. Secondly,
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Mo2C@NC@MoSx porous nanospheres with sandwich shell have been synthesized by hydrothermal sulfurization, which can be confirmed by XPS and HRTEM. The sandwich shell is composed of ultrathin MoSx, NC layer and Mo2C nanocrystallines, which may reduce the strong Mo-H bonding energy of pure Mo2C and lead to the suitable ∆GH* for HER. In addition, the ultrathin NC can prevent the aggregation of
* Corresponding author. Email: [email protected] (B. Dong), [email protected] (Y. M. Chai) Tel: +86-532-86981376, Fax: +86-532-86981787 1
ACCEPTED MANUSCRIPT active
sites
and
improve
charge
transfer
rate
due
to
rich
N-doping.
Mo2C@NC@MoSx exhibits enhanced performance and long-time durability in both acidic and alkaline solution. It requires a low onsetpotential of 119 mV, Tafel slope of
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56 mV dec-1 in acidic solution and onsetpotential of 86 mV, Tafel slope of 90 mV dec-1 in alkaline solution. Therefore, designing sandwich nanostructure with better conductivity and optimized Mo-H bonding energy may be a promising strategy for
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excellent Mo-based HER electrocatalysts.
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Keywords: MoSx; Mo2C; sandwich shell; N-doping; electrocatalyst; hydrogen evolution reaction
1. Introduction
Hydrogen (H2) has been considered as an attractive and the most potential energy
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carrier because of its high energy density, renewability and zero carbon emission [1-5]. One effective strategy to generate H2 relies on electrochemical water splitting for efficient hydrogen evolution reaction (HER), in which an efficient electrocatalyst is
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demanded to accelerate the gas production efficiency [6-11]. As we known, Platinum
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(Pt) and its alloys are the most efficient catalysts now due to its suitable hydrogen adsorption Gibbs free energy and potential H desorption activation energy, however, the high cost and scarcity of Pt noble metal restrict its large-scale applications on earth [12-15]. Thus, developing alternative electrocatalysts based on earth-abundant and cost-effective elements has attracted a lot of attentions recently [16, 17]. Mo-based compounds, including Mo2C [18, 19], MoN [20], MoP [21, 22] and MoS2 [23-26] have been widely explored as effective HER electrocatalyst due to their 2
ACCEPTED MANUSCRIPT Pt-like electrochemical behaviors. As one of the most representative Mo-based compounds, Mo2C has been highlighted due to its wide pH flexibility, tunable phase and composition and excellent HER activity. To expose more active sites and improve
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the intrinsic conductivity of Mo2C, two strategies usually has been made including designing unique nanostructures [27] with enrich porosity and doping other element [28]. For example, Lou’s group synthesized porous molybdenum carbide
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nano-octahedrons deriving from metal-organic frameworks for efficient HER [29].
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Tian et al. found that Mo2C nanowires showed higher activity with a smaller Tafel slope of 55.8 mV dec-1 compared the HER activity of nanosheets [30]. Although limited progress has been made for synthesizing unique nanostructures, most reported Mo2C materials are irregular in shape due to the inevitable aggregation of
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nanoparticles at high carbonization temperature [31]. So it is still a great challenge to synthesize Mo2C with ultrafine nanocrystallite and high activity. More importantly, the hydrogen-binding free energy (∆GH*) of an electrocatalyst
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has been considered as a good symbol of the intrinsic activity for HER [32]. The
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intrinsic activity of Mo2C is limited by the strong interaction between crystal phase of Mo2C and Hads (∆GH* ≈ -0.8 eV) [19, 33], which would subsequently result in poor HER performance due to the foreseeable difficulty of H2 release [34]. In order to obtain the best reaction efficiency, the Gibbs free energy of H* adsorption of electrocatalyst should be close to zero [35]. Some strategies have been adopted to adjust the value of ∆GH* of electrocatalysts. For example, nonmetallic elements with low electronegativity such as N and S emerge as a suitable option to weak Mo-H 3
ACCEPTED MANUSCRIPT binding energy thus accomplish the intrinsically improved HER activity [36]. Lan’s group synthesized N-doped Mo2C exhibiting enhanced intrinsic activity due to the synergistic effect between Mo2C and N leading to a favorable ∆GH* [33]. Moreover,
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molybdenum sulfides nanostructures with positive ∆GH*, which indicates a relative weakened strength of Mo-H of Mo2C, has been proposed to optimize Mo-H bonding energy accomplishing the intrinsically improved HER activity [37, 38]. For example,
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Zhu et al. have synthesized ultrathin MoS2 nanosheets anchored on N-doped
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Mo2C-embedded carbon nanotubes to optimize Mo-H bonding energy, showing a small Tafel slope of 69 mV dec-1 [39]. Wang’s group has prepared MoSx@Mo2C exhibiting the enhanced HER activity ascribed to the synergistic effect between MoSx and Mo2C [40]. So it is highly desirable to construct hybrid of MoSx/Mo2C
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nanostructures to optimize Mo-H bonding energy for efficient HER. Herein, we have synthesized Mo2C@NC@MoSx porous nanospheres with sandwich shell through covering ultrathin MoSx on MoO2-Mo2C@NC nanospheres for efficient
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HER. Scheme 1 shows the synthesis process of Mo2C@NC@MoSx nanospheres by
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adopting MoO42--/aniline-pyrrole (MoO42--Polymer) as precursors. Firstly, the carbonization of MoO42--Polymer can provide the porous nanospheres with ultrafine Mo2C nanocrystallines as core and ultrathin N-doped carbon (NC) as shell. Secondly, a facile hydrothermal sulfurization was adopted to cover ultrathin MoS2 on MoO2-Mo2C@NC nanospheres, which may construct the novel sandwich shell composed of ultrathin MoS2, NC layer and Mo2C nanocrystallines. The obtained Mo2C@NC@MoSx demonstrates superior HER performance with a small onset 4
ACCEPTED MANUSCRIPT potential of 119 mV and Tafel slope of 56 mV dec-1 in acidic media and onset potential of 86 mV and Tafel slope of 90 mV dec-1 in alkaline media. The excellent HER performances may be ascribed to the formation of sandwich shell, which can
2. Experimental 2.1 Synthesis of MoO42--Polymer nanospheres
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N-doping.
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optimize ∆GH* of Mo-H on Mo2C and provide fast charge transfer rate due to rich
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All chemicals were of analytical reagent grade and were used without further purification. To prepare MoO42--Polymer nanospheres, 26 mM (NH4)6Mo7O24·4H2O were dissolved in 60 ml deionized water containing 0.08 g Triton X-100. Then 0.4 mL aniline and 0.3 mL pyrrole were added into the above mixed solution under a strong
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ultrasonication bath for 1 h to yield a homogeneous solution. Then the aqueous solution of containing (NH4)2S2O8 cooled at 0 °C for 0.5 h was dropped into the precooled aqueous solution for polymerization for 12 h at 0 °C. Finally, the
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synthesized product was washed with ethanol and deionized water for three times and
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dried under vacuum. The obtained spherical-like product was denoted as MoO42--Polymer. The coexistence of both aniline and pyrrole is crucial to the formation of porous MoO42--Polymer spheres precursor. The formation mechanism of porous MoO42--Polymer nanospheres can be illustrated as follows: As we know, pyrrole monomers are hydrophobic substances compared with aniline, so aniline and pyrrole comonomers enter into Triton X-100 template and remained at different positions of the micelles owning to their different hydrophobic properties. After 5
ACCEPTED MANUSCRIPT adding oxidation, polymerization of aniline and pyrrole occurs at the micelle-water interface thus forming porous nanostructures in the inner part of spheres, and in the meantime (NH4)6Mo7O24·4H2O diffuse into various position of solution to form
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homogeneous distributed porous MoO42--Polymer nanospheres. 2.2 Synthesis of MoO2-Mo2C@NC nanospheres (MoO2-Mo2C@NC)
The as obtained MoO42--Polymer nanospheres were loaded on a quartz boat and
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calcined at 650 °C for 3 h in a tube furnace under argon atmosphere with a heating
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rate of 5 °C/min to produce hybrids of MoO2-Mo2C@NC.
2.3 Synthesis of ultrathin MoSx/Mo2C-embedded N-doped carbon nanospheres (Mo2C@NC@MoSx)
As-prepared MoO2-Mo2C@NC (0.1 g) and 0.2 g of thioacetamide were mixed in
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30 mL of distilled water. After a strong ultrasonication for 1 h, the solution was placed in a 100 mL Teflon-lined stainless autoclave and maintained at 200 °C for 24 h. The final products were obtained by centrifugation, washing with distilled water and
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ethanol for three times and drying at 60 °C for 8 h.
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2.4 Preparation of contrast samples For comparison, MoO42--Polymer@MoSx was also prepared following the same
procedure using MoO42--Polymer as the raw material to replace the as-prepared MoO2-Mo2C@NC with the same amount of thioacetamide molar weight for the sulfuration reaction. 2.5 Characterizations X-ray powder diffraction (XRD) patterns were recorded using a X’Pert PRO MPD 6
ACCEPTED MANUSCRIPT system with a Cu Kα irradiation source (λ = 0.154 Å) with a scanning rate of 8° min-1 and the 2θ ranges from 10 to 80˚. X-ray photoelectron spectroscopy (XPS, VG ESCALABMK II) was adopted to analyze the surface compositions and chemical
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states of surface constituents. Surface morphologies and elemental compositions of all samples were characterized by scanning electron microscopy (SEM, Hitachi, S-4800) using an instrument equipped with an energy-dispersive X-ray analyzer (EDX, Octane
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Ultra). Transmission electron microscopy (TEM, FEI Tecnai G2) and high-resolution
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transmission electron microscopy (HRTEM) were utilized to confirm structure information.
2.6 Electrochemical measurements
The electrochemical measurements were conducted on a standard three-electrode
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configuration in solution of 0.5 M H2SO4 and 1 M KOH (purging N2 for 30 min in advance to saturate the electrolyte) on an electrochemical workstation (Gamry Reference 600 Instruments), respectively. A carbon rod and saturated calomel
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electrode (SCE) were used as the counter electrode and reference electrode,
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respectively. To obtain the working electrode, typically, 5 mg of sample and 20 µL Nafion solution (5 wt %) were dispersed in 1 mL water-ethanol solution with volume ratio of 1:1 by sonicating to form a homogeneous ink. Then 5 µL of the dispersion was loaded onto a glassy carbon electrode of 4 mm diameter. For comparison, measurements were also conducted using a commercial Pt/C catalyst exhibiting high HER catalytic performance. All of the potentials were calibrated with respect to a RHE via the Nernst equation: ERHE = ESCE + 0.059pH + 0.243. All the potentials 7
ACCEPTED MANUSCRIPT reported in the manuscript were against RHE. The electrocatalytic activity of all the samples towards HER was tested by obtaining polarization curves using linear sweep voltammetry (LSV) with a scan rate of 10 mV·s-1 in 0.5 M H2SO4 solutions and 1 M
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KOH, respectively. The ohmic resistance was obtained from EIS measurements with frequencies ranging from 105 Hz to 0.1 Hz with an AC voltage of 5 mV. Experimental EIS data were analyzed and fitted with the software of Zsimpwin to extract the series
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resistances (Rs) and charge-transfer resistances (Rct). The double-layer capacitances
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(Cdl) were estimated by CV at various scan rates (40-140 mV s-1) to evaluate the effective surface area of various catalysts. The electrochemical stability of catalysts was evaluated by CV from 0 to -0.2 V (vs RHE) with a scan rate of 100 mV s-1 for 1000 times. In addition, amperometric i-t curve was also tested at a constant potential
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of 0.2 V (vs RHE) for 10 h to evaluate the stability.
3. Results and Discussion
The crystalline phase structure of all the samples including MoO42--Polymer,
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MoO2-Mo2C@NC, MoO42--Polymer@MoS2 and Mo2C@NC@MoS2 is characterized
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by XRD. Fig. S1 shows XRD patterns of MoO42--Polymer. A broad peak at 26.5° can be indexed to the amorphous carbon matrix. After the carbonization process at 650 °C, Fig. 1 show that main peaks at 37.5°, 43.1°, 61.9° and 74.8° corresponds to the (111), (200), (220) and (311) facets of cubic structure of Mo2C (PDF no. 00-015-0457) with a relative low intensity. Besides, peaks at 26.2° are indexed to (011) facets of MoO2 (PDF no. 03-065-5787), which are inactive species toward HER. After hydrothermal sulfurization at 200 °C, there are no dramatically changes observed in XRD pattern of 8
ACCEPTED MANUSCRIPT Mo2C@NC@MoSx except that the density of cubic structure of Mo2C reduces compared with MoO2-Mo2C@NC, which implies that the existence of MoSx anchored on the surface of MoO2-Mo2C@NC results in the reduction of intensity of main peaks.
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Due to the poor crystallinity and low content, diffraction peaks of MoSx can’t be observed in XRD. The actual compositions of Mo2C@NC@MoSx will be further analyzed by XPS and HRTEM. As comparison, MoO42--Polymer as the raw material
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for hydrothermal sulfurization, pure MoS2 (PDF no. 00-073-1508) can be obtained
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individually with three typical diffraction peaks of (002), (101) and (110), located at 14.8°, 32.4° and 57.5°, respectively. According to previous reports adopting the similar hydrothermal method [41], the low intensity of a (002) peak deriving from MoO42--Polymer@MoS2 indicates that the as-prepared MoS2 shows poor crystallinity.
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The Brunauer-Emmett-Teller (BET) surface area of Mo2C@NC@MoSx measured by the N2 sorption isotherms is 175 m2 g-1 (Fig. S2). Moreover, the corresponding pore size distribution of Mo2C@NC@MoSx is mainly distributed in the range from 3 to 10
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nm, which is characteristic of a mesoporous structure. Overall, the large surface area
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and mesporous nanostructures of Mo2C@NC@MoSx can effectively promote electrolyte penetration and electron transfer. Detailed composition and valence of Mo2C@NC@MoSx porous nanospheres are
investigated by XPS. As illustrated in Fig. 2a, the characteristic peaks of Mo, C, S and N can be clearly observed. In addition, the peaks O 1s may derive from surface oxidation forming MoO2 and MoO3 species owing to the calcination-hydrothermal reaction. Fig. 2b shows a high-resolution spectrum of Mo 3d in the binding energy 9
ACCEPTED MANUSCRIPT range from 224-238 eV. The main peaks between 226 and 236 eV clearly split as three separated doublets based on the spin-orbital coupling feature. The Mo 3d peaks located at 228.6 and 231.7 eV demonstrate the characteristic doublets of cubic Mo2C
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phase [42]. Besides, the Mo 3d peaks located at 228.2 and 231.3 eV are attributable to Mo4+ state of MoSx [43]. As a consequence of surface oxidation, the peaks at 232.1 and 234.7 eV are attributable to MoO3, which are inert species toward the
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HER [44]. In the C 1s region (Fig. 2c), the peaks at 284.6 and 285.1 eV can be
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indexed to carbide species and sp2 hybridized graphite-like carbon, respectively [45]. In the high-resolution of S 2p region (Fig. 2d), the two separated doublets between 160 and 166 eV suggests the existence of both S2- and S22- ligands. The peaks at 161.7 and 162.9 eV are detected, corresponding to S2- 2p3/2 and S2- 2p1/2, respectively.
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Their energy separation of about 1.2 eV is ascribed to characteristic of S2- of MoS2 species [46]. Meanwhile, the peaks at 163.7 and 164.9 eV are clearly observed, confirming the existence of S22- 2p3/2 and S22- 2p1/2, respectively [47]. Besides, their
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energy separation of 1.2 eV is a characteristic of S22- deriving from MoS3 species
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[48]. Therefore, molybdenum sulfides with poor crystallinity in Mo2C@NC@MoSx is demonstrated as the mixture of MoS2 and MoS3 species and thus donated as “MoSx”. In the N 1s region (Fig. 2e), the peaks at 395.8, 398 and 400.6 eV can be indexed to N-Mo binding, pyridinic-N and pyrrolic-N, respectively [49]. The doping of N elements into Mo2C and amorphous carbon matrix can largely change electronic structures and improve the charge transfer rate, which is favorable for HER. Spherical-like core-shell-structured Mo2C@NC@MoSx with rich N-doped 10
ACCEPTED MANUSCRIPT nanostructures is successfully synthesized by facile calcination-hydrothermal method. In contrast, the fitting peaks of Mo, C, N and O can be clearly observed in XPS spectra of MoO2-Mo2C@NC, as presented in Fig. S3a-d. The existence of Mo2+,
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Mo4+ and Mo6+ reveals the composition of MoO2-Mo2C@NC comprising of Mo2C, MoO2 and MoO3, respectively (Fig. S3a) [42, 44]. By comparing the two samples of MoO2-Mo2C@NC and Mo2C@NC@MoSx, it can be clearly seen that when the
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sulfurization reaction occurs, the ratio of Mo4+ to Mo6+ gradually becomes larger,
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implying the enhancement of sulfurization reaction of the surface of MoO3 species. SEM images in Fig. 3a shows that the obtained MoO42--Polymer exhibit uniform nanospherical morphology with homogeneous distribution. Fig. 3b displays a more clear morphology of MoO42--Polymer nanosphere with the diameter about 100 nm.
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SEM images of MoO2-Mo2C@NC after carbonization process at a temperature of 650 °C are shown in Fig. 3c and 3d, which confirm that the morphology of nanospheres has been well maintained after carbonization treatment. However, the
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surface of MoO2-Mo2C@NC becomes obviously rough and exhibits a shrunken size
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about 80 nm compared with that of MoO42--Polymer, which may be favorable for the formation of sulfides phase on the surface. For the as-prepared Mo2C@NC@MoSx sample, as shown in Fig. 3e and 3f, it is obvious that the morphology has nearly no change compared with MoO2-Mo2C@NC. Under high magnification (Fig. 3f), each nanosphere of Mo2C@NC@MoSx becomes more rough, indicating the formation of homogeneous MoSx ultrathin film anchored on the surface of MoO2-Mo2C@NC nanospheres. SEM equipped with mapping is utilized to further confirm the 11
ACCEPTED MANUSCRIPT existence of ultrathin MoSx film, as shown in Fig. 3g, elements of S, Mo, C, N, O are homogeneously distributed on the surface of sample. For comparison, Fig. S4 exhibits SEM morphology of as-obtained MoO42--Polymer@MoS2. It can be seen
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from Fig. S4a and S4b that MoO42--Polymer nanospheres are covered by thick sheet-like structures with severe aggregation. The nanospheres are greatly enlarged, which may result in the reduction of exposed active sites. Simultaneously,
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SEM-EDX in Fig. S5 are conducted to probe the element composition and atomic
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percentage of MoO2-Mo2C@NC, MoO42--Polymer@MoS2 and Mo2C@NC@MoSx. It can be seen that N atom are successfully doped into Mo composition and amorphous carbon matrix although going through carbonization process resulting in the reduction of the N atomic percentage.
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To confirm porous nanostructures of MoO2-Mo2C@NC and Mo2C@NC@MoSx, TEM and HRTEM images are presented in Fig. S6 and Fig. 4, respectively. As shown in Fig. S6a and S6b, TEM image of MoO2-Mo2C@NC displays homogeneously
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distributed nanospheres morphology with porous and loose nanostructures embedded
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into the carbon layers and a large of nanoparticles encapsulated in each nanosphere. In HRTEM image of Fig. S6c, an ultrathin outer shell can be clearly observed with the thickness below 2 nm. The ultrafine Mo2C nanocrystallites with the lattice fringes spacing of 0.23 nm inside of shell are indexed to (111) planes of cubic structure of Mo2C. Fig. 4 shows TEM and HRTEM images of Mo2C@NC@MoSx. As expected, Mo2C@NC@MoSx maintains the intact spherical-like morphology compared with those of their corresponding MoO42--Polymer precursors and MoO2-Mo2C@NC, 12
ACCEPTED MANUSCRIPT which implies the stable structure of porous core-shell nanomaterials (Fig. 4a and 4b). As shown in Fig. 4c, a large amount of nanoparticles inside of shells with porous core-shell structure are unchangeable, which implies that successive hydrothermal
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sulfurization does not have influence on the inside of core nanostructures. In HRTEM image in Fig. 4d, clear interplanar distances can be detected and measured to be 0.23 nm, corresponding to the (111) planes of cubic structure of ultrafine Mo2C crystalline
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with the size below 5 nm. Meanwhile, compared with pure MoO2-Mo2C@NC, the
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thickness of shell exceeding 2 nm for Mo2C@NC@MoSx is observed over the shell thickness of MoO2-Mo2C@NC, which implies that ultrathin MoSx homogeneously covered on the surface of MoO2-Mo2C@NC without obvious aggregation. In order to confirm the existence of MoSx, the interplanar distance anchored on the surface of
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amorphous carbon is calculated to be about 0.62 nm, consistent with the (002) crystal facets of MoS2. It is worth mentioning that the ultrafine Mo2C nanoparticles embedded in NC shell and ultrathin MoSx film supported on the surface of NC shell
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can prevent the aggregation and excessive growth of nanoparticles thus exposing
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more active sites, which are favorable for HER. The obtained results powerfully demonstrate the sandwich nanostructures of Mo2C@NC@MoSx. So far, the exact composition of Mo2C@NC@MoSx can be convincingly confirmed as a hierarchical porous core-shell structure with Mo2C as core confirmed by XRD, XPS and HRTEM and the mixture of MoS2 and MoS3 species as shell detected by XPS and HRTEM. In the acidic solution of 0.5 M H2SO4, HER performances of all the as-prepared samples are investigated in Fig. 5. Fig. 5a shows LSV curves of MoO42--Polymer, 13
ACCEPTED MANUSCRIPT MoO2-Mo2C@NC, MoO42--Polymer@MoSx, Mo2C@NC@MoSx and commercial 20 wt% Pt/C. As a benchmark electrocatalyst, 20 wt% Pt/C exhibits the highest electrochemical activity for HER with a nearly zero onset overpotential [50],
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whereas MoO42--Polymer exhibits a poor HER activity with a severely retarded catalytic onset, eliminating the effect on HER performances of other samples. Mo2C@NC@MoSx exhibits the smallest overpotential of 119 mV (vs. RHE)
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compared with MoO2-Mo2C@NC (195 mV), MoO42--Polymer@MoSx (175 mV),
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implying that Mo2C@NC@MoSx displays the best activity for HER in acidic solution. To achieve a current density (j) of 10 mA cm-2, Mo2C@NC@MoSx requires an overpotential of 189 mV, which is obviously lower than those of MoO2-Mo2C@NC (320 mV) and MoO42--Polymer@MoSx (252 mV). This suggests
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a synergistic effect derived from sandwich shell of Mo2C@NC@MoSx porous nanospheres. For MoO2-Mo2C@NC, the relatively inferior electrochemical activity may be due to the existence of MoO2 species resulting in the degradation of of
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samples compared with pure Mo2C. Moreover, the obtained Mo2C embedded into
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carbon shells belongs to cubic phase of Mo2C as shown in Fig. 1, which exhibits inferior electrochemical activity compared with highly active hexagonal phase of Mo2C.
To elucidate HER mechanism, Tafel slopes are obtained by simulating Tafel
equation (ŋ=b log (j)+a, where b as the Tafel slope and j as the current density), as shown in Fig. 5b. The Tafel slope of 20 wt% Pt/C is 30 mV dec-1, which is highly consistent with the previously reported value [29], thus confirming the accuracy of 14
ACCEPTED MANUSCRIPT our measurements. Tafel slope of 462 mV dec-1 for MoO42--Polymer, 99 mV dec-1 for MoO2-Mo2C@NC, 70 mV dec-1 for MoO42--Polymer@MoSx and 56 mV dec-1 for Mo2C@NC@MoSx reveals the fast increase of hydrogen generation rate of
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Mo2C@NC@MoSx. According to the previous report [51], the mechanism of HER is a two principle steps taking place on the catalyst-modified electrode surface. The first step is Volmer reaction for H+ reduction with a Tafel slope of 116 mV dec-1 (eq. 1)
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and the second step is Heyrovsky reaction for Hads desorption with a Tafel slope of 38
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mV dec-1 (eq. 2) or the Tafel reaction for atom combination with a Tafel slope of 29 mV dec-1 (eq. 3). Although the Tafel slope is not enough to present the specific HER mechanism, the evident lower slope for Mo2C@NC@MoSx still confirms the promoted Volmer reaction in HER kinetics. Even when compared with that of various
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reported MoSx-based electrocatalysts, HER performance of Mo2C@NC@MoSx in acidic solution is outstanding, as shown in Table S1. H3O+ + e- → Hads + H2O
(1) (2)
Hads + Hads → H2
(3)
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Hads + H3O + e- → H2 + H2O
To gain further insight into the electrochemical activity of Mo2C@NC@MoSx for
HER, EIS data of all samples are compared and Rct values of all samples are obtained by fitting an equivalent circuit inserted in Fig. 5c utilizing Zview software (Fig. 5c and Table S2). Compared with the Nyquist plot of and Rct value of all samples, Mo2C@NC@MoSx shows a much smaller semicircle and Rct values increases in the order of Mo2C@NC@MoSx (53.3 Ω) < MoO42--Polymer@MoS2 15
ACCEPTED MANUSCRIPT (141.6 Ω) < MoO2-Mo2C@NC (518Ω) < MoO42--polymer (1753 Ω), suggesting that Mo2C@NC@MoSx
has
the
lower
impedance.
This
result
proves
that
Mo2C@NC@MoSx with sandwich shell affords markedly faster HER kinetics.
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MoO2-Mo2C@NC exhibits inferior electrochemical activity due to the strong hydrogen binding on Mo2C and the consequently restricted Hads desorption, whereas the sandwich shell composed of ultrathin MoS2, NC and Mo2C would reduce the
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strength of Mo-H bonding energy for the promoted Hads desorption and thus
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remarkably improve HER activity. Besides, rich NC layers in the sandwich shell of Mo2C@NC@MoSx can greatly improve electronic charge transfer rate. Electrochemical active surface (ECSA) can be visualized through evaluating the double-layer capacitances (Cdl) deriving from CV curves due to their proportional
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relationship. As shown in Fig. S7a and S7b, the capacitance values of Mo2C@NC@MoSx in 0.5 M H2SO4 is estimated to be 1.16 mF cm-2, which is higher than other samples. The large active surface may be ascribed to its unique sandwich
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shell and porous structures with high porosity.
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Long-term durability is another critical factor for HER electrocatalysts. To probe the durability of Mo2C@NC@MoSx, continuous CV is performed between -0.2 and 0.1 V in 0.5 M H2SO4 solution at a 100 mV s-1 scan rate (Fig. 5d). As observed, the polarization curve for Mo2C@NC@MoSx remains almost unchanged after 1000 cycles. In addition, the durability of Mo2C@NC@MoSx is examined by chronoamperometry test at a static overpotential. Fig. S8a shows that the catalytic current is sustainable with a negligible current loss, suggesting its robust stability. 16
ACCEPTED MANUSCRIPT Moreover, the structure and components information of Mo2C@NC@MoSx after long-time continuous CV test is investigated by SEM (Fig. S9a) and SEM mapping (Fig. S9c), respectively. As expected, Mo2C@NC@MoSx keeps intact spherical-like
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morphorlogy with homogeneous elemental distribution after stability test, further proving its robust stability.
HER electrocatalysts which can work effciently in a wide range of pH is highly
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desirable for hydrogen evolution. To further evaluate the potential of series of
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samples for widespread HER applications, HER performance of all the samples in 1 M KOH is investigated at a scan rate of 10 mV s-1. Obviously, the synergistic effect between Mo2C, NC and MoS2 in the sandwich shell also promotes HER performance in a alkaline electrolyte due to the optimized electronic properties and promoted Hads
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desorption of Mo species. As shown in Fig. 6a, the measured onset overpotential is mere 86 mV and only 249 mV is required to drive apparent hydrogen evolution with the current density of 10 mA cm-2. Compared with MoO42--Polymer@MoS2,
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Mo2C@NC@MoSx exhibits much superior electrochemical activity, which may be
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ascribed to the introduction of highly active Mo2C species leading to the performance enhancement of Mo2C@NC@MoSx. Besides, as displayed in Fig. 6b, the estimated Tafel slope of 90 mV dec-1 highlights its superior HER kinetic in alkaline solution. Moreover, by fitting an equivalent circuit inserted in Fig. 6c and Table S2, it can be seen that Rct value for Mo2C@NC@MoSx samples is of 32.52 Ω much smaller than that of other samples, indicating more favorable charge transport kinetics for Mo2C@NC@MoSx. Ascribed to the unique sandwich shell and porous 17
ACCEPTED MANUSCRIPT core-shell structure of Mo2C@NC@MoSx with more active sites and optimizing ∆GH*, this electrode presents high Cdl of 8.02 mF cm-2 in KOH solution, which is much higher than other samples (Fig. S7c and S7d). In addition, its excellent
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durability is further identified by CV test of 1000 cycles (Fig. 6d) and continuous 10 h electrolysis reaction (Fig. S8b). As shown in Fig. S9b and S9d, the Mo2C@NC@MoSx nanosphere structure remains unchanged after CV tests and
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SEM mapping exhibiting homogeneous element distribution suggests the stability of
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ultrathin MoSx, NC layer and Mo2C nanocrystallines. All the above noticeable performances prove the potential of Mo2C@NC@MoSx electrocatalyst for HER in both acidic and basic media.
4. Conclusions
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In summary, we have successfully designed and synthesized Mo2C@NC@MoSx nanospheres with sandwich shell based on MoO42--polymer as precursor for the first time. The obtained Mo2C@NC@MoSx shows better electrocatalytic performances
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both in acid and alkaline solution compared with MoO2-Mo2C@NC and
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MoO42--Polymer@MoSx. The enhancement of Mo2C@NC@MoSx can be attributed to the introduction of ultrathin MoS2 reducing the strength of Mo-H bonding energy of Mo2C thus promot Hads desorption. Besides, the NC shell can provide the protection to avoid the aggregation and fast growth of Mo2C nanocrystallines and MoSx film. This work proposes a promising strategy to bond highly active HER electrocatalyst by designing sandwich shell of Mo-based hybrids.
Acknowledgements 18
ACCEPTED MANUSCRIPT This work is financially supported by the National Natural Science Foundation of China (U1662119) and the Fundamental Research Funds for the Central Universities (15CX05031A) and Postgraduate Innovation Project of China University of
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Petroleum (YCX2017032).
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1
Schematic
illustration
of
the
synthesis
process
for
porous
Mo2C@NC@MoSx nanospheres.
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Fig. 1 XRD of MoO2-Mo2C@NC, MoO42--Polymer@MoS2 and Mo2C@NC@MoSx. Fig. 2 (a) XPS high-resolution scans for Mo2C@NC@MoSx in (b) Mo 3d; (c) C 1s; (d) S 2p and (e) N 1s regions.
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Fig. 3 SEM images of (a,b) MoO42--Polymer; (c,d) MoO2-Mo2C@NC and (e,f)
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Mo2C@NC@MoSx. (g) SEM mapping of Mo2C@NC@MoSx. Fig. 4 TEM images of Mo2C@NC@MoSx.
Fig. 5 (a) Polarization curves; (b) corresponding Tafel plots; (c) Nyquist plots of Pt/C, MoO42--Polymer,
MoO2-Mo2C@NC,
MoO42--Polymer@MoS2
and
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Mo2C@NC@MoSx in 0.5 M H2SO4. (d) LSV curves of Mo2C@NC@MoSx before and after 1000 CV cycles in 0.5 M H2SO4.
Fig. 6 (a) Polarization curves; (b) corresponding Tafel plots; (c) Nyquist plots of Pt/C, MoO2-Mo2C@NC,
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MoO42--Polymer,
MoO42--Polymer@MoS2
and
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Mo2C@NC@MoSx in 1 M KOH. (d) LSV curves of Mo2C@NC@MoSx before and after 1000 CV cycles in 1 M KOH.
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S0008-6223(17)30902-8
DOI:
10.1016/j.carbon.2017.09.027
Reference:
CARBON 12357
To appear in:
Carbon
Received Date: 30 May 2017 Revised Date:
19 August 2017
Accepted Date: 8 September 2017
Please cite this article as: J.-Q. Chi, X. Shang, S.-S. Lu, B. Dong, Z.-Z. Liu, K.-L. Yan, W.-K. Gao, Y.2M. Chai, C.-G. Liu, Mo2C@NC@MoSx porous nanospheres with sandwich shell based on MoO4 polymer precursor for efficient hydrogen evolution in both acidic and alkaline media, Carbon (2017), doi: 10.1016/j.carbon.2017.09.027. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract
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Mo2C@NC@MoSx porous nanospheres with sandwich shell based on MoO42--Polymer precursor
for efficient hydrogen evolution in both acidic and
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alkaline media Jing-Qi Chi a, Xiao Shang a, Shan-Shan Lu a, b, Bin Dong *a, b, Zi-Zhang Liu a, b, Kai-Li Yan a, Wen-Kun Gao a, b, Yong-Ming Chai *a, Chen-Guang Liu a
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a State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China),
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Qingdao 266580, PR China
b College of Science, China University of Petroleum (East China), Qingdao 266580, PR China
Abstract
Mo2C@NC@MoSx porous nanospheres with sandwich shell have been
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synthesized for efficient hydrogen evolution in both acidic and alkaline media. Firstly, porous MoO2-Mo2C@NC nanospheres have been obtained with ultrafine Mo2C nanocrystallines as core and ultrathin N-doped carbon (NC) as shell through the
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carbonization of MoO42--/aniline-pyrrole (MoO42--Polymer) as precursor. Secondly,
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Mo2C@NC@MoSx porous nanospheres with sandwich shell have been synthesized by hydrothermal sulfurization, which can be confirmed by XPS and HRTEM. The sandwich shell is composed of ultrathin MoSx, NC layer and Mo2C nanocrystallines, which may reduce the strong Mo-H bonding energy of pure Mo2C and lead to the suitable ∆GH* for HER. In addition, the ultrathin NC can prevent the aggregation of
* Corresponding author. Email: [email protected] (B. Dong), [email protected] (Y. M. Chai) Tel: +86-532-86981376, Fax: +86-532-86981787 1
ACCEPTED MANUSCRIPT active
sites
and
improve
charge
transfer
rate
due
to
rich
N-doping.
Mo2C@NC@MoSx exhibits enhanced performance and long-time durability in both acidic and alkaline solution. It requires a low onsetpotential of 119 mV, Tafel slope of
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56 mV dec-1 in acidic solution and onsetpotential of 86 mV, Tafel slope of 90 mV dec-1 in alkaline solution. Therefore, designing sandwich nanostructure with better conductivity and optimized Mo-H bonding energy may be a promising strategy for
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excellent Mo-based HER electrocatalysts.
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Keywords: MoSx; Mo2C; sandwich shell; N-doping; electrocatalyst; hydrogen evolution reaction
1. Introduction
Hydrogen (H2) has been considered as an attractive and the most potential energy
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carrier because of its high energy density, renewability and zero carbon emission [1-5]. One effective strategy to generate H2 relies on electrochemical water splitting for efficient hydrogen evolution reaction (HER), in which an efficient electrocatalyst is
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demanded to accelerate the gas production efficiency [6-11]. As we known, Platinum
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(Pt) and its alloys are the most efficient catalysts now due to its suitable hydrogen adsorption Gibbs free energy and potential H desorption activation energy, however, the high cost and scarcity of Pt noble metal restrict its large-scale applications on earth [12-15]. Thus, developing alternative electrocatalysts based on earth-abundant and cost-effective elements has attracted a lot of attentions recently [16, 17]. Mo-based compounds, including Mo2C [18, 19], MoN [20], MoP [21, 22] and MoS2 [23-26] have been widely explored as effective HER electrocatalyst due to their 2
ACCEPTED MANUSCRIPT Pt-like electrochemical behaviors. As one of the most representative Mo-based compounds, Mo2C has been highlighted due to its wide pH flexibility, tunable phase and composition and excellent HER activity. To expose more active sites and improve
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the intrinsic conductivity of Mo2C, two strategies usually has been made including designing unique nanostructures [27] with enrich porosity and doping other element [28]. For example, Lou’s group synthesized porous molybdenum carbide
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nano-octahedrons deriving from metal-organic frameworks for efficient HER [29].
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Tian et al. found that Mo2C nanowires showed higher activity with a smaller Tafel slope of 55.8 mV dec-1 compared the HER activity of nanosheets [30]. Although limited progress has been made for synthesizing unique nanostructures, most reported Mo2C materials are irregular in shape due to the inevitable aggregation of
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nanoparticles at high carbonization temperature [31]. So it is still a great challenge to synthesize Mo2C with ultrafine nanocrystallite and high activity. More importantly, the hydrogen-binding free energy (∆GH*) of an electrocatalyst
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has been considered as a good symbol of the intrinsic activity for HER [32]. The
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intrinsic activity of Mo2C is limited by the strong interaction between crystal phase of Mo2C and Hads (∆GH* ≈ -0.8 eV) [19, 33], which would subsequently result in poor HER performance due to the foreseeable difficulty of H2 release [34]. In order to obtain the best reaction efficiency, the Gibbs free energy of H* adsorption of electrocatalyst should be close to zero [35]. Some strategies have been adopted to adjust the value of ∆GH* of electrocatalysts. For example, nonmetallic elements with low electronegativity such as N and S emerge as a suitable option to weak Mo-H 3
ACCEPTED MANUSCRIPT binding energy thus accomplish the intrinsically improved HER activity [36]. Lan’s group synthesized N-doped Mo2C exhibiting enhanced intrinsic activity due to the synergistic effect between Mo2C and N leading to a favorable ∆GH* [33]. Moreover,
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molybdenum sulfides nanostructures with positive ∆GH*, which indicates a relative weakened strength of Mo-H of Mo2C, has been proposed to optimize Mo-H bonding energy accomplishing the intrinsically improved HER activity [37, 38]. For example,
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Zhu et al. have synthesized ultrathin MoS2 nanosheets anchored on N-doped
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Mo2C-embedded carbon nanotubes to optimize Mo-H bonding energy, showing a small Tafel slope of 69 mV dec-1 [39]. Wang’s group has prepared MoSx@Mo2C exhibiting the enhanced HER activity ascribed to the synergistic effect between MoSx and Mo2C [40]. So it is highly desirable to construct hybrid of MoSx/Mo2C
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nanostructures to optimize Mo-H bonding energy for efficient HER. Herein, we have synthesized Mo2C@NC@MoSx porous nanospheres with sandwich shell through covering ultrathin MoSx on MoO2-Mo2C@NC nanospheres for efficient
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HER. Scheme 1 shows the synthesis process of Mo2C@NC@MoSx nanospheres by
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adopting MoO42--/aniline-pyrrole (MoO42--Polymer) as precursors. Firstly, the carbonization of MoO42--Polymer can provide the porous nanospheres with ultrafine Mo2C nanocrystallines as core and ultrathin N-doped carbon (NC) as shell. Secondly, a facile hydrothermal sulfurization was adopted to cover ultrathin MoS2 on MoO2-Mo2C@NC nanospheres, which may construct the novel sandwich shell composed of ultrathin MoS2, NC layer and Mo2C nanocrystallines. The obtained Mo2C@NC@MoSx demonstrates superior HER performance with a small onset 4
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2. Experimental 2.1 Synthesis of MoO42--Polymer nanospheres
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N-doping.
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optimize ∆GH* of Mo-H on Mo2C and provide fast charge transfer rate due to rich
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All chemicals were of analytical reagent grade and were used without further purification. To prepare MoO42--Polymer nanospheres, 26 mM (NH4)6Mo7O24·4H2O were dissolved in 60 ml deionized water containing 0.08 g Triton X-100. Then 0.4 mL aniline and 0.3 mL pyrrole were added into the above mixed solution under a strong
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ultrasonication bath for 1 h to yield a homogeneous solution. Then the aqueous solution of containing (NH4)2S2O8 cooled at 0 °C for 0.5 h was dropped into the precooled aqueous solution for polymerization for 12 h at 0 °C. Finally, the
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synthesized product was washed with ethanol and deionized water for three times and
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dried under vacuum. The obtained spherical-like product was denoted as MoO42--Polymer. The coexistence of both aniline and pyrrole is crucial to the formation of porous MoO42--Polymer spheres precursor. The formation mechanism of porous MoO42--Polymer nanospheres can be illustrated as follows: As we know, pyrrole monomers are hydrophobic substances compared with aniline, so aniline and pyrrole comonomers enter into Triton X-100 template and remained at different positions of the micelles owning to their different hydrophobic properties. After 5
ACCEPTED MANUSCRIPT adding oxidation, polymerization of aniline and pyrrole occurs at the micelle-water interface thus forming porous nanostructures in the inner part of spheres, and in the meantime (NH4)6Mo7O24·4H2O diffuse into various position of solution to form
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homogeneous distributed porous MoO42--Polymer nanospheres. 2.2 Synthesis of MoO2-Mo2C@NC nanospheres (MoO2-Mo2C@NC)
The as obtained MoO42--Polymer nanospheres were loaded on a quartz boat and
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calcined at 650 °C for 3 h in a tube furnace under argon atmosphere with a heating
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rate of 5 °C/min to produce hybrids of MoO2-Mo2C@NC.
2.3 Synthesis of ultrathin MoSx/Mo2C-embedded N-doped carbon nanospheres (Mo2C@NC@MoSx)
As-prepared MoO2-Mo2C@NC (0.1 g) and 0.2 g of thioacetamide were mixed in
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30 mL of distilled water. After a strong ultrasonication for 1 h, the solution was placed in a 100 mL Teflon-lined stainless autoclave and maintained at 200 °C for 24 h. The final products were obtained by centrifugation, washing with distilled water and
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ethanol for three times and drying at 60 °C for 8 h.
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2.4 Preparation of contrast samples For comparison, MoO42--Polymer@MoSx was also prepared following the same
procedure using MoO42--Polymer as the raw material to replace the as-prepared MoO2-Mo2C@NC with the same amount of thioacetamide molar weight for the sulfuration reaction. 2.5 Characterizations X-ray powder diffraction (XRD) patterns were recorded using a X’Pert PRO MPD 6
ACCEPTED MANUSCRIPT system with a Cu Kα irradiation source (λ = 0.154 Å) with a scanning rate of 8° min-1 and the 2θ ranges from 10 to 80˚. X-ray photoelectron spectroscopy (XPS, VG ESCALABMK II) was adopted to analyze the surface compositions and chemical
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states of surface constituents. Surface morphologies and elemental compositions of all samples were characterized by scanning electron microscopy (SEM, Hitachi, S-4800) using an instrument equipped with an energy-dispersive X-ray analyzer (EDX, Octane
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Ultra). Transmission electron microscopy (TEM, FEI Tecnai G2) and high-resolution
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transmission electron microscopy (HRTEM) were utilized to confirm structure information.
2.6 Electrochemical measurements
The electrochemical measurements were conducted on a standard three-electrode
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configuration in solution of 0.5 M H2SO4 and 1 M KOH (purging N2 for 30 min in advance to saturate the electrolyte) on an electrochemical workstation (Gamry Reference 600 Instruments), respectively. A carbon rod and saturated calomel
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electrode (SCE) were used as the counter electrode and reference electrode,
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respectively. To obtain the working electrode, typically, 5 mg of sample and 20 µL Nafion solution (5 wt %) were dispersed in 1 mL water-ethanol solution with volume ratio of 1:1 by sonicating to form a homogeneous ink. Then 5 µL of the dispersion was loaded onto a glassy carbon electrode of 4 mm diameter. For comparison, measurements were also conducted using a commercial Pt/C catalyst exhibiting high HER catalytic performance. All of the potentials were calibrated with respect to a RHE via the Nernst equation: ERHE = ESCE + 0.059pH + 0.243. All the potentials 7
ACCEPTED MANUSCRIPT reported in the manuscript were against RHE. The electrocatalytic activity of all the samples towards HER was tested by obtaining polarization curves using linear sweep voltammetry (LSV) with a scan rate of 10 mV·s-1 in 0.5 M H2SO4 solutions and 1 M
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KOH, respectively. The ohmic resistance was obtained from EIS measurements with frequencies ranging from 105 Hz to 0.1 Hz with an AC voltage of 5 mV. Experimental EIS data were analyzed and fitted with the software of Zsimpwin to extract the series
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resistances (Rs) and charge-transfer resistances (Rct). The double-layer capacitances
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(Cdl) were estimated by CV at various scan rates (40-140 mV s-1) to evaluate the effective surface area of various catalysts. The electrochemical stability of catalysts was evaluated by CV from 0 to -0.2 V (vs RHE) with a scan rate of 100 mV s-1 for 1000 times. In addition, amperometric i-t curve was also tested at a constant potential
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of 0.2 V (vs RHE) for 10 h to evaluate the stability.
3. Results and Discussion
The crystalline phase structure of all the samples including MoO42--Polymer,
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MoO2-Mo2C@NC, MoO42--Polymer@MoS2 and Mo2C@NC@MoS2 is characterized
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by XRD. Fig. S1 shows XRD patterns of MoO42--Polymer. A broad peak at 26.5° can be indexed to the amorphous carbon matrix. After the carbonization process at 650 °C, Fig. 1 show that main peaks at 37.5°, 43.1°, 61.9° and 74.8° corresponds to the (111), (200), (220) and (311) facets of cubic structure of Mo2C (PDF no. 00-015-0457) with a relative low intensity. Besides, peaks at 26.2° are indexed to (011) facets of MoO2 (PDF no. 03-065-5787), which are inactive species toward HER. After hydrothermal sulfurization at 200 °C, there are no dramatically changes observed in XRD pattern of 8
ACCEPTED MANUSCRIPT Mo2C@NC@MoSx except that the density of cubic structure of Mo2C reduces compared with MoO2-Mo2C@NC, which implies that the existence of MoSx anchored on the surface of MoO2-Mo2C@NC results in the reduction of intensity of main peaks.
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Due to the poor crystallinity and low content, diffraction peaks of MoSx can’t be observed in XRD. The actual compositions of Mo2C@NC@MoSx will be further analyzed by XPS and HRTEM. As comparison, MoO42--Polymer as the raw material
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for hydrothermal sulfurization, pure MoS2 (PDF no. 00-073-1508) can be obtained
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individually with three typical diffraction peaks of (002), (101) and (110), located at 14.8°, 32.4° and 57.5°, respectively. According to previous reports adopting the similar hydrothermal method [41], the low intensity of a (002) peak deriving from MoO42--Polymer@MoS2 indicates that the as-prepared MoS2 shows poor crystallinity.
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The Brunauer-Emmett-Teller (BET) surface area of Mo2C@NC@MoSx measured by the N2 sorption isotherms is 175 m2 g-1 (Fig. S2). Moreover, the corresponding pore size distribution of Mo2C@NC@MoSx is mainly distributed in the range from 3 to 10
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nm, which is characteristic of a mesoporous structure. Overall, the large surface area
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and mesporous nanostructures of Mo2C@NC@MoSx can effectively promote electrolyte penetration and electron transfer. Detailed composition and valence of Mo2C@NC@MoSx porous nanospheres are
investigated by XPS. As illustrated in Fig. 2a, the characteristic peaks of Mo, C, S and N can be clearly observed. In addition, the peaks O 1s may derive from surface oxidation forming MoO2 and MoO3 species owing to the calcination-hydrothermal reaction. Fig. 2b shows a high-resolution spectrum of Mo 3d in the binding energy 9
ACCEPTED MANUSCRIPT range from 224-238 eV. The main peaks between 226 and 236 eV clearly split as three separated doublets based on the spin-orbital coupling feature. The Mo 3d peaks located at 228.6 and 231.7 eV demonstrate the characteristic doublets of cubic Mo2C
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phase [42]. Besides, the Mo 3d peaks located at 228.2 and 231.3 eV are attributable to Mo4+ state of MoSx [43]. As a consequence of surface oxidation, the peaks at 232.1 and 234.7 eV are attributable to MoO3, which are inert species toward the
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HER [44]. In the C 1s region (Fig. 2c), the peaks at 284.6 and 285.1 eV can be
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indexed to carbide species and sp2 hybridized graphite-like carbon, respectively [45]. In the high-resolution of S 2p region (Fig. 2d), the two separated doublets between 160 and 166 eV suggests the existence of both S2- and S22- ligands. The peaks at 161.7 and 162.9 eV are detected, corresponding to S2- 2p3/2 and S2- 2p1/2, respectively.
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Their energy separation of about 1.2 eV is ascribed to characteristic of S2- of MoS2 species [46]. Meanwhile, the peaks at 163.7 and 164.9 eV are clearly observed, confirming the existence of S22- 2p3/2 and S22- 2p1/2, respectively [47]. Besides, their
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energy separation of 1.2 eV is a characteristic of S22- deriving from MoS3 species
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[48]. Therefore, molybdenum sulfides with poor crystallinity in Mo2C@NC@MoSx is demonstrated as the mixture of MoS2 and MoS3 species and thus donated as “MoSx”. In the N 1s region (Fig. 2e), the peaks at 395.8, 398 and 400.6 eV can be indexed to N-Mo binding, pyridinic-N and pyrrolic-N, respectively [49]. The doping of N elements into Mo2C and amorphous carbon matrix can largely change electronic structures and improve the charge transfer rate, which is favorable for HER. Spherical-like core-shell-structured Mo2C@NC@MoSx with rich N-doped 10
ACCEPTED MANUSCRIPT nanostructures is successfully synthesized by facile calcination-hydrothermal method. In contrast, the fitting peaks of Mo, C, N and O can be clearly observed in XPS spectra of MoO2-Mo2C@NC, as presented in Fig. S3a-d. The existence of Mo2+,
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Mo4+ and Mo6+ reveals the composition of MoO2-Mo2C@NC comprising of Mo2C, MoO2 and MoO3, respectively (Fig. S3a) [42, 44]. By comparing the two samples of MoO2-Mo2C@NC and Mo2C@NC@MoSx, it can be clearly seen that when the
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sulfurization reaction occurs, the ratio of Mo4+ to Mo6+ gradually becomes larger,
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implying the enhancement of sulfurization reaction of the surface of MoO3 species. SEM images in Fig. 3a shows that the obtained MoO42--Polymer exhibit uniform nanospherical morphology with homogeneous distribution. Fig. 3b displays a more clear morphology of MoO42--Polymer nanosphere with the diameter about 100 nm.
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SEM images of MoO2-Mo2C@NC after carbonization process at a temperature of 650 °C are shown in Fig. 3c and 3d, which confirm that the morphology of nanospheres has been well maintained after carbonization treatment. However, the
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surface of MoO2-Mo2C@NC becomes obviously rough and exhibits a shrunken size
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about 80 nm compared with that of MoO42--Polymer, which may be favorable for the formation of sulfides phase on the surface. For the as-prepared Mo2C@NC@MoSx sample, as shown in Fig. 3e and 3f, it is obvious that the morphology has nearly no change compared with MoO2-Mo2C@NC. Under high magnification (Fig. 3f), each nanosphere of Mo2C@NC@MoSx becomes more rough, indicating the formation of homogeneous MoSx ultrathin film anchored on the surface of MoO2-Mo2C@NC nanospheres. SEM equipped with mapping is utilized to further confirm the 11
ACCEPTED MANUSCRIPT existence of ultrathin MoSx film, as shown in Fig. 3g, elements of S, Mo, C, N, O are homogeneously distributed on the surface of sample. For comparison, Fig. S4 exhibits SEM morphology of as-obtained MoO42--Polymer@MoS2. It can be seen
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from Fig. S4a and S4b that MoO42--Polymer nanospheres are covered by thick sheet-like structures with severe aggregation. The nanospheres are greatly enlarged, which may result in the reduction of exposed active sites. Simultaneously,
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SEM-EDX in Fig. S5 are conducted to probe the element composition and atomic
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percentage of MoO2-Mo2C@NC, MoO42--Polymer@MoS2 and Mo2C@NC@MoSx. It can be seen that N atom are successfully doped into Mo composition and amorphous carbon matrix although going through carbonization process resulting in the reduction of the N atomic percentage.
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To confirm porous nanostructures of MoO2-Mo2C@NC and Mo2C@NC@MoSx, TEM and HRTEM images are presented in Fig. S6 and Fig. 4, respectively. As shown in Fig. S6a and S6b, TEM image of MoO2-Mo2C@NC displays homogeneously
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distributed nanospheres morphology with porous and loose nanostructures embedded
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into the carbon layers and a large of nanoparticles encapsulated in each nanosphere. In HRTEM image of Fig. S6c, an ultrathin outer shell can be clearly observed with the thickness below 2 nm. The ultrafine Mo2C nanocrystallites with the lattice fringes spacing of 0.23 nm inside of shell are indexed to (111) planes of cubic structure of Mo2C. Fig. 4 shows TEM and HRTEM images of Mo2C@NC@MoSx. As expected, Mo2C@NC@MoSx maintains the intact spherical-like morphology compared with those of their corresponding MoO42--Polymer precursors and MoO2-Mo2C@NC, 12
ACCEPTED MANUSCRIPT which implies the stable structure of porous core-shell nanomaterials (Fig. 4a and 4b). As shown in Fig. 4c, a large amount of nanoparticles inside of shells with porous core-shell structure are unchangeable, which implies that successive hydrothermal
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sulfurization does not have influence on the inside of core nanostructures. In HRTEM image in Fig. 4d, clear interplanar distances can be detected and measured to be 0.23 nm, corresponding to the (111) planes of cubic structure of ultrafine Mo2C crystalline
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with the size below 5 nm. Meanwhile, compared with pure MoO2-Mo2C@NC, the
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thickness of shell exceeding 2 nm for Mo2C@NC@MoSx is observed over the shell thickness of MoO2-Mo2C@NC, which implies that ultrathin MoSx homogeneously covered on the surface of MoO2-Mo2C@NC without obvious aggregation. In order to confirm the existence of MoSx, the interplanar distance anchored on the surface of
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amorphous carbon is calculated to be about 0.62 nm, consistent with the (002) crystal facets of MoS2. It is worth mentioning that the ultrafine Mo2C nanoparticles embedded in NC shell and ultrathin MoSx film supported on the surface of NC shell
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can prevent the aggregation and excessive growth of nanoparticles thus exposing
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more active sites, which are favorable for HER. The obtained results powerfully demonstrate the sandwich nanostructures of Mo2C@NC@MoSx. So far, the exact composition of Mo2C@NC@MoSx can be convincingly confirmed as a hierarchical porous core-shell structure with Mo2C as core confirmed by XRD, XPS and HRTEM and the mixture of MoS2 and MoS3 species as shell detected by XPS and HRTEM. In the acidic solution of 0.5 M H2SO4, HER performances of all the as-prepared samples are investigated in Fig. 5. Fig. 5a shows LSV curves of MoO42--Polymer, 13
ACCEPTED MANUSCRIPT MoO2-Mo2C@NC, MoO42--Polymer@MoSx, Mo2C@NC@MoSx and commercial 20 wt% Pt/C. As a benchmark electrocatalyst, 20 wt% Pt/C exhibits the highest electrochemical activity for HER with a nearly zero onset overpotential [50],
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whereas MoO42--Polymer exhibits a poor HER activity with a severely retarded catalytic onset, eliminating the effect on HER performances of other samples. Mo2C@NC@MoSx exhibits the smallest overpotential of 119 mV (vs. RHE)
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compared with MoO2-Mo2C@NC (195 mV), MoO42--Polymer@MoSx (175 mV),
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implying that Mo2C@NC@MoSx displays the best activity for HER in acidic solution. To achieve a current density (j) of 10 mA cm-2, Mo2C@NC@MoSx requires an overpotential of 189 mV, which is obviously lower than those of MoO2-Mo2C@NC (320 mV) and MoO42--Polymer@MoSx (252 mV). This suggests
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a synergistic effect derived from sandwich shell of Mo2C@NC@MoSx porous nanospheres. For MoO2-Mo2C@NC, the relatively inferior electrochemical activity may be due to the existence of MoO2 species resulting in the degradation of of
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samples compared with pure Mo2C. Moreover, the obtained Mo2C embedded into
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carbon shells belongs to cubic phase of Mo2C as shown in Fig. 1, which exhibits inferior electrochemical activity compared with highly active hexagonal phase of Mo2C.
To elucidate HER mechanism, Tafel slopes are obtained by simulating Tafel
equation (ŋ=b log (j)+a, where b as the Tafel slope and j as the current density), as shown in Fig. 5b. The Tafel slope of 20 wt% Pt/C is 30 mV dec-1, which is highly consistent with the previously reported value [29], thus confirming the accuracy of 14
ACCEPTED MANUSCRIPT our measurements. Tafel slope of 462 mV dec-1 for MoO42--Polymer, 99 mV dec-1 for MoO2-Mo2C@NC, 70 mV dec-1 for MoO42--Polymer@MoSx and 56 mV dec-1 for Mo2C@NC@MoSx reveals the fast increase of hydrogen generation rate of
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Mo2C@NC@MoSx. According to the previous report [51], the mechanism of HER is a two principle steps taking place on the catalyst-modified electrode surface. The first step is Volmer reaction for H+ reduction with a Tafel slope of 116 mV dec-1 (eq. 1)
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and the second step is Heyrovsky reaction for Hads desorption with a Tafel slope of 38
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mV dec-1 (eq. 2) or the Tafel reaction for atom combination with a Tafel slope of 29 mV dec-1 (eq. 3). Although the Tafel slope is not enough to present the specific HER mechanism, the evident lower slope for Mo2C@NC@MoSx still confirms the promoted Volmer reaction in HER kinetics. Even when compared with that of various
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reported MoSx-based electrocatalysts, HER performance of Mo2C@NC@MoSx in acidic solution is outstanding, as shown in Table S1. H3O+ + e- → Hads + H2O
(1) (2)
Hads + Hads → H2
(3)
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Hads + H3O + e- → H2 + H2O
To gain further insight into the electrochemical activity of Mo2C@NC@MoSx for
HER, EIS data of all samples are compared and Rct values of all samples are obtained by fitting an equivalent circuit inserted in Fig. 5c utilizing Zview software (Fig. 5c and Table S2). Compared with the Nyquist plot of and Rct value of all samples, Mo2C@NC@MoSx shows a much smaller semicircle and Rct values increases in the order of Mo2C@NC@MoSx (53.3 Ω) < MoO42--Polymer@MoS2 15
ACCEPTED MANUSCRIPT (141.6 Ω) < MoO2-Mo2C@NC (518Ω) < MoO42--polymer (1753 Ω), suggesting that Mo2C@NC@MoSx
has
the
lower
impedance.
This
result
proves
that
Mo2C@NC@MoSx with sandwich shell affords markedly faster HER kinetics.
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MoO2-Mo2C@NC exhibits inferior electrochemical activity due to the strong hydrogen binding on Mo2C and the consequently restricted Hads desorption, whereas the sandwich shell composed of ultrathin MoS2, NC and Mo2C would reduce the
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strength of Mo-H bonding energy for the promoted Hads desorption and thus
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remarkably improve HER activity. Besides, rich NC layers in the sandwich shell of Mo2C@NC@MoSx can greatly improve electronic charge transfer rate. Electrochemical active surface (ECSA) can be visualized through evaluating the double-layer capacitances (Cdl) deriving from CV curves due to their proportional
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relationship. As shown in Fig. S7a and S7b, the capacitance values of Mo2C@NC@MoSx in 0.5 M H2SO4 is estimated to be 1.16 mF cm-2, which is higher than other samples. The large active surface may be ascribed to its unique sandwich
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shell and porous structures with high porosity.
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Long-term durability is another critical factor for HER electrocatalysts. To probe the durability of Mo2C@NC@MoSx, continuous CV is performed between -0.2 and 0.1 V in 0.5 M H2SO4 solution at a 100 mV s-1 scan rate (Fig. 5d). As observed, the polarization curve for Mo2C@NC@MoSx remains almost unchanged after 1000 cycles. In addition, the durability of Mo2C@NC@MoSx is examined by chronoamperometry test at a static overpotential. Fig. S8a shows that the catalytic current is sustainable with a negligible current loss, suggesting its robust stability. 16
ACCEPTED MANUSCRIPT Moreover, the structure and components information of Mo2C@NC@MoSx after long-time continuous CV test is investigated by SEM (Fig. S9a) and SEM mapping (Fig. S9c), respectively. As expected, Mo2C@NC@MoSx keeps intact spherical-like
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morphorlogy with homogeneous elemental distribution after stability test, further proving its robust stability.
HER electrocatalysts which can work effciently in a wide range of pH is highly
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desirable for hydrogen evolution. To further evaluate the potential of series of
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samples for widespread HER applications, HER performance of all the samples in 1 M KOH is investigated at a scan rate of 10 mV s-1. Obviously, the synergistic effect between Mo2C, NC and MoS2 in the sandwich shell also promotes HER performance in a alkaline electrolyte due to the optimized electronic properties and promoted Hads
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desorption of Mo species. As shown in Fig. 6a, the measured onset overpotential is mere 86 mV and only 249 mV is required to drive apparent hydrogen evolution with the current density of 10 mA cm-2. Compared with MoO42--Polymer@MoS2,
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Mo2C@NC@MoSx exhibits much superior electrochemical activity, which may be
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ascribed to the introduction of highly active Mo2C species leading to the performance enhancement of Mo2C@NC@MoSx. Besides, as displayed in Fig. 6b, the estimated Tafel slope of 90 mV dec-1 highlights its superior HER kinetic in alkaline solution. Moreover, by fitting an equivalent circuit inserted in Fig. 6c and Table S2, it can be seen that Rct value for Mo2C@NC@MoSx samples is of 32.52 Ω much smaller than that of other samples, indicating more favorable charge transport kinetics for Mo2C@NC@MoSx. Ascribed to the unique sandwich shell and porous 17
ACCEPTED MANUSCRIPT core-shell structure of Mo2C@NC@MoSx with more active sites and optimizing ∆GH*, this electrode presents high Cdl of 8.02 mF cm-2 in KOH solution, which is much higher than other samples (Fig. S7c and S7d). In addition, its excellent
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durability is further identified by CV test of 1000 cycles (Fig. 6d) and continuous 10 h electrolysis reaction (Fig. S8b). As shown in Fig. S9b and S9d, the Mo2C@NC@MoSx nanosphere structure remains unchanged after CV tests and
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SEM mapping exhibiting homogeneous element distribution suggests the stability of
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ultrathin MoSx, NC layer and Mo2C nanocrystallines. All the above noticeable performances prove the potential of Mo2C@NC@MoSx electrocatalyst for HER in both acidic and basic media.
4. Conclusions
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In summary, we have successfully designed and synthesized Mo2C@NC@MoSx nanospheres with sandwich shell based on MoO42--polymer as precursor for the first time. The obtained Mo2C@NC@MoSx shows better electrocatalytic performances
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both in acid and alkaline solution compared with MoO2-Mo2C@NC and
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MoO42--Polymer@MoSx. The enhancement of Mo2C@NC@MoSx can be attributed to the introduction of ultrathin MoS2 reducing the strength of Mo-H bonding energy of Mo2C thus promot Hads desorption. Besides, the NC shell can provide the protection to avoid the aggregation and fast growth of Mo2C nanocrystallines and MoSx film. This work proposes a promising strategy to bond highly active HER electrocatalyst by designing sandwich shell of Mo-based hybrids.
Acknowledgements 18
ACCEPTED MANUSCRIPT This work is financially supported by the National Natural Science Foundation of China (U1662119) and the Fundamental Research Funds for the Central Universities (15CX05031A) and Postgraduate Innovation Project of China University of
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Petroleum (YCX2017032).
19
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ACCEPTED MANUSCRIPT Figure Captions Scheme
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Fig. 1 XRD of MoO2-Mo2C@NC, MoO42--Polymer@MoS2 and Mo2C@NC@MoSx. Fig. 2 (a) XPS high-resolution scans for Mo2C@NC@MoSx in (b) Mo 3d; (c) C 1s; (d) S 2p and (e) N 1s regions.
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Fig. 5 (a) Polarization curves; (b) corresponding Tafel plots; (c) Nyquist plots of Pt/C, MoO42--Polymer,
MoO2-Mo2C@NC,
MoO42--Polymer@MoS2
and
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Fig. 6 (a) Polarization curves; (b) corresponding Tafel plots; (c) Nyquist plots of Pt/C, MoO2-Mo2C@NC,
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MoO42--Polymer,
MoO42--Polymer@MoS2
and
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Mo2C@NC@MoSx in 1 M KOH. (d) LSV curves of Mo2C@NC@MoSx before and after 1000 CV cycles in 1 M KOH.
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