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Graphene confined MoS2 particles for accelerated electrocatalytic hydrogen evolution
international journal of hydrogen energy xxx (xxxx) xxx
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Graphene confined MoS2 particles for accelerated electrocatalytic hydrogen evolution Yinchang Li 1, Bing He 1, Xueqin Liu*, Xiaoqin Hu, Jing Huang, Siqin Ye, Zhu Shu, Yang Wang***, Zhen Li** Engineering Research Center of Nano-Geomaterials of Ministry of Education, Faculty of Materials Science and Chemistry, China University of Geosciences, 388 Lumo Road, Wuhan, Hubei 430074, PR China
article info
abstract
Article history:
MoS2 is a promising noble-metal-free electrocatalyst for the hydrogen evolution reaction.
Received 7 November 2018
Extensive trials have been carried out to increase its low electrical conductivity and
Received in revised form
insufficient active sites. Here, a remarkable electrocatalyst for hydrogen evolution is
9 February 2019
developed based on the in-situ preparation of MoS2 confined in graphene nanosheets.
Accepted 13 February 2019
Graphene effectively controls the growth of MoS2 and immensely increases the conduc-
Available online xxx
tivity and structural stability of the composite materials. Remarkably, because of the plentiful active sites, sufficient electrical contact and transport, MoS2 particles confined in
Keywords:
graphene nanosheets exhibit an onset overpotential as small as 32 mV, an overpotential
MoS2
approaching 132 mV at 10 mA cm2, and a low Tafel slope of 45 mV dec1. This work
Graphene
presents a reasonable architecture for practical applications in efficient electrocatalytic H2
Confined
generation.
Hydrogen evolution
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction With its high energy density, environmental friendly and natural abundance, hydrogen has been proved to be an efficient and safe energy carrier [1e4]. However, the inherently slow kinetics of hydrogen evolution reaction (HER) gives low efficiency of the hydrogen production [5e7]. Therefore, advanced electrocatalysts should be innovated to provide adequate active sites for hydrogen production and reduce the overpotential of HER [8]. Platinum (Pt) is proved to be the most effective and stable electrocatalyst [9,10], but its further application is heavily hindered by its high-cost and scarcity. In
the last years, significant studies have been executed in exploring new effective HER electrocatalysts, especially those low-cost and abundant materials. As shown in a density functional theory (DFT) calculations based database [11,12], molybdenum disulfide (MoS2) with an adequate hydrogen binding energy (△GH) is a promising system for HER among transition and noble metals. In addition, edges of MoS2 nanoparticles are active for HER [13,14], S-Mo-S trilayers structure of MoS2 could provide enough active sites for hydrogen production. Furthermore, elemental abundance and environmental friendliness make MoS2 being a competitive member of HER electrocatalyst [15e18]. However, with MoS2 being a semiconductor, its electronic conductivity is
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (X. Liu), [email protected] (Y. Wang), [email protected] (Z. Li). 1 These authors contributed equally. https://doi.org/10.1016/j.ijhydene.2019.02.089 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Li Y et al., Graphene confined MoS2 particles for accelerated electrocatalytic hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.02.089
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poor which limits its overall electrocatalytic activity. Furthermore, the insufficient exposed active sites and inert basal-plane of MoS2 are impediments to its use as an electrocatalyst [19,20]. So, more attentions have been paid to the modification of MoS2 electrocatalysts to increase the electronic conductivity and maximize the quantity of active sites. For example, MoS2 vertical sheets discontinued with a stepped surface structure have been fabricated to tackle the problem of scarce active sites [21]. A strongly coupled MoS2carbon nanotubes nanocomposite was constructed to accelerate the HER via accelerating the electron transport [22]. It is desirable to tackle the impeded electron conductivity and scarce active sites of MoS2 at the same time. Recently, the restriction of electrocatalysts within carbon materials, such as porous carbon and graphene, is a practical way to enhance the HER activity of catalysts, which can be anticipated to not only enhance electrical conductivity of the hybrid catalysts, but also effectively increase the number of reaction sites because of the restriction on the size of active particles and few aggregation of electrocatalysts due to the confinement effect [23e25]. To date, a few confined electrocatalysts have been explored to replace Pt for several reactions. For example, Xie et al. constructed a model of graphene confined ultrathin tin layers, which showed remarkably enhanced electrocatalytic activity and stability because of the abundant active sites, the protection and highly-conductive of graphene [26]. Feng et al. reported Mo2C nanoparticles with diameters down to 5 nm confined in Ndoped porous carbon [27]. The results showed that such confinement provided sufficient exposed active sites for electrocatalysis and ensured strong interaction between the two components of the composites. In recent years, the combination of MoS2 particles with graphene have been extensively investigated as high-performance electrocatalysts for HER [28e30]. However, the investigation into the confinement of MoS2 particles in graphene as HER electrocatalysts has been rarely reported. Herein, we report MoS2 particles confined in graphene (MoS2@G) via an in-situ hydrothermal strategy, and superior HER electrocatalytic performance has been achieved. Asprepared MoS2@G featured abundant exposed active sites for electrocatalysis attributed to the evenly distributed MoS2 particles with downsized diameters, high charge transport pathway and diffusion space for electrolyte and products provided by graphene. The superior MoS2@G catalyst represented low onset overpotential of ~32 mV, which was comparable to that of Pt/C catalyst (~24 mV). Moreover, the MoS2@G has excellent electrochemical stability, which was proved to be a promising electrocatalyst for HER.
Experimental Materials Thiourea, ammonium molybdate, n-methyl pyrrolidone, sulfuric acid, and platinum-carbon (Pt/C, 10 wt%) catalyst were purchased from Sinopharm. Acetylene black and polyvinylidene fluoride were obtained from Aladdin Bio-Chem Technology Co., LTD. Nafion solution (5 wt%) was obtained
from Sigma. The graphene oxide (GO) was prepared by improved Hummers method and reduced to reduced graphene oxide (rGO) by hydrothermal method [31].
Preparation of catalysts Typically, a certain concentration of GO solution was prepared and formed a homogeneous solution by ultrasonic dispersion method. Subsequently, 0.72 mM of ammonium molybdate was dissolved into the GO solution thoroughly and keep stirring for 2 h. After that, 21.6 mM of thiourea was dissolved into the above-solution. Then, the solution was sealed in a 50 mL autoclave and heated for 24 h at 220 C. In the end, the sediments were cleaned by deionized water and ethyl alcohol thoroughly by centrifugation. In our study, the mass fraction of graphene in the MoS2@G composites can be readily tuned by changing the amount of GO. Accordingly, the resulting MoS2@G composites were referred to as MoS2@G-1 (wt % ¼ 10%), MoS2@G-2 (wt% ¼ 30%), MoS2@G-3 (wt% ¼ 50%), and MoS2@G-4 (wt% ¼ 70%). As reference, conventional MoS2 anchored graphene (MoS2/G) was prepared by a solvothermal method reported elsewhere [32]. The preparation process was operated without hours of stirring and preliminary assembly between [Mo7O24]6- and GO [33]. Bare MoS2 was also obtained by a similar process in the absence of GO.
Electrochemical tests 4 mg of catalysts and 0.1 mL of 5 wt% Nafion solution were mixed in 4 mL ethanol and sonicated for 0.5 h to form a homogeneous seriflux. After that, 4 mL of the seriflux was deposited on a glassy carbon electrode with the working area of ~0.07 cm2 (loading ~57 mg cm2). The electrochemical HER tests were performed in 0.5 M H2SO4 aqueous solution (pH ¼ 1) with a standard Ag/AgCl electrode and a Pt slice as the reference and counter electrodes, respectively. Potentials were converted to the standard reversible hydrogen electrode (RHE) potential by adding 0.256 V (0.197 þ 0.059 pH).
Results and discussion The fabrication steps of MoS2@G are illustrated in Scheme 1. Firstly, with hours of continuous moderate stirring, a homogeneous dispersion solution with [Mo7O24]6- and ultrathin GO (Fig. S1 in the Supplementary Information) was obtained. Interestingly, due to the chelating connection between [Mo7O24]6- and GO [34], [Mo7O24]6- embedded layered-GO structure was assembled in this process (MoeO stretching vibrations at 950 cm1 advocated the hypothesis that MoS2 and graphene are connected via CeOeMo, Fig. S2) [35,36]. Finally, graphene confined MoS2 composites were formed via hydrothermal treatment with the help of thiourea. To probe the morphology of the as-prepared samples, the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were carried out. Fig. 1a and b shows the SEM and TEM images of MoS2@G composites. It is observed that the downsized MoS2 particles with the diameter of ca. 100 nm are uniformly distributed and covered by graphene without obvious agglomeration. The high resolution TEM
Please cite this article as: Li Y et al., Graphene confined MoS2 particles for accelerated electrocatalytic hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.02.089
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Scheme 1 e The synthetic process of MoS2 confined in graphene.
(HRTEM) image of MoS2@G in Fig. 1c shows a layered structure with interlayer spacing of ca. 0.61 nm, which corresponds to the (002) plane of hexagonal MoS2, and a looming graphene morphology is observed around the parallel lines. Then the energy-dispersive X-ray (EDX) mapping images from the selected areas of MoS2@G hybrid exhibit a uniform distribution of MoS2 throughout the graphene sheets (Fig. 1d). Furthermore, the atomic ratio of Mo/S was calculated to be 1:2.03 (as shown in Fig. S3), which is according with the stoichiometry of MoS2. The downsized MoS2 particles in MoS2@G were attributed to the introduction and pre-assembling of graphene nanosheets in the synthesis process. In sharp contrast, in the absence of GO, the MoS2 particles were deeply aggregated with large radial sizes of about 1.5 mm (Fig. 1e), which lead to a lower specific surface area (as shown in Fig. S4). It is undoubtable that the larger specific surface area could provide more active sites for HER as well as adsorption of reactant, causing to a higher HER efficiency [29]. Furthermore, the morphology of MoS2/G is shown in Fig. 1f. Obviously, most of MoS2 edges were exposed on the surface of graphene, which is different from the structure of graphene confined MoS2 framework. Compared to MoS2 and MoS2/G, the smaller size of MoS2 in MoS2@G indicated the confinement effect of graphene on MoS2 growth. To obtain the phase structure of the catalysts, X-ray diffraction (XRD) and Raman spectroscopy were employed. For MoS2 microspheres (Fig. 2a), four obviously diffraction peaks centered at around 2q ¼ 14.06 , 33.37 , 39.94 , and 59.14 were observed, which can be indexed respectively to (002), (100), (103), and (110) planes of hexagonal phase of MoS2 (JCPDS card. NO. 75-1539). In comparison to the bare MoS2, the intensity of four characteristic diffraction peaks of MoS2 in MoS2@G diminishes progressively, indicating that the combination of graphene limits the growth of MoS2 particles of high crystallinity. Therefore, the downsized MoS2 particles fabricated in the confined environment between graphene layers can provide abundant catalytic active sites. Moreover, the (002) diffraction peak of the graphite cannot be detected in
MoS2@G, indicating that there was no stack of the graphene nanosheets in the time of reaction for the existence of MoS2. Raman spectroscopy was employed to identify the components of the as-prepared samples. In the case of MoS2 and MoS2@G, two Roman peaks at 381 and 402 cm1 are observed (Fig. 2b), corresponding to the in-plane E12g and out-of-plane A1g modes of 2H MoS2, respectively [33,37]. Significantly, both GO and MoS2@G composites show two distinct peaks at about 1349 and 1577 cm1, which has well matching degree with the D and G bands of graphene, respectively [38]. In our case, MoS2@G composites show a higher ID/IG value than that of the pristine GO (1.03 and 0.83, respectively). This distinction might be caused by the structural defects within the sp2 carbon network that arose upon reduction of the as-used GO. X-ray photoelectron spectroscopy (XPS) measurements were conducted to gain insight into the atomic valence states and the composition of the MoS2@G composites. The XPS survey spectrum (Fig. S5) confirms that the MoS2@G is mainly composed of Mo, S, C and O, which is in according with the EDX result. As shown in Fig. 2c and d, a successful reduction of GO to rGO was verified by the C 1s signal, according to the relative decreases of characteristic peaks from C-O and C]O/ COOH [19]. The reduction of GO ensures the high conductivity of graphene in the MoS2@G composite. The high-resolution XPS spectra of Mo 3d and S 2p regions were explored to verify the preparation of MoS2. Fig. 2e shows the high resolution Mo 3d spectrum of MoS2@G, where four peaks could be deconvoluted, including 226.4 eV for S 2s of MoS2, 229.2 eV and 232.4 eV for Mo 3d5/2 and Mo 3d3/2 of MoS2, and 235.6 eV for Mo 6þ may 3d of MoO3 or MoO2 4 , respectively. The presence of Mo attribution to the mild oxidation of MoS2 [1]. The strong peaks (Fig. 2f) located at binding energies of 162.1 and 163.2 eV correspond to the S 2p3/2 and S 2p1/2 lines of MoS2, respectively. Meanwhile the binding energy at 168.7 eV can be ascribed to the terminal S4þ species on the surface or edges of MoS2 [39]. The results further suggest the successful preparation of MoS2@G composites. To test our expectation, the electrocatalytic HER activities of MoS2@G and MoS2/G were investigated using a typical
Please cite this article as: Li Y et al., Graphene confined MoS2 particles for accelerated electrocatalytic hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.02.089
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Fig. 1 e (a) SEM, (b) TEM and (c) HRTEM images of MoS2@G. (d) TEM image and corresponding EDX mappings of MoS2@G. SEM images of (e) MoS2 and (f) MoS2/G. three-electrode configuration. Bare MoS2, graphene and Pt/C were also taken as references to estimate the contribution from graphene and confinement effect. Linear sweep voltammetry (LSV) polarization curves of various catalysts with a sweep rate of 5 mV s1 are shown in Fig. 3a. Obviously, the graphene shows almost no HER catalytic activity, and bare MoS2 exhibits inferior HER activity. In sharp contrast, MoS2@G electrocatalyst delivers a low onset overpotential approaching 32 mV and achieves a cathodic current density of 10 mA cm2 at 132 mV. Such a low overpotential is slightly lower than that of Pt/C but highly superior to that of MoS2/G catalyst, which is ascribed to the numerous active sites and ultrafast electrons transport derived from the unique framework of graphene confined MoS2. The HER performance of MoS2@G is markedly better than or at least comparable to those of the most active HER electrocatalysts (Table S1).
In addition, the Tafel plots were derived to analyze the quantitative kinetics of the HER process. Fig. 3b shows the Tafel plots of as-prepared catalysts and commercial Pt/C catalyst which were fitted to the Tafel equation (h ¼ b log j þ a, where j is the current density and b is the Tafel slope). Graphene reveals a largest Tafel slope value of 137 mV dec1. The Tafel slope is 45 mV dec1 for sample MoS2@G, which is closed to that of Pt/C (33 mV dec1) and lower than that of MoS2/G (76 mV dec1) and pure MoS2 (103 mV dec1). It is noticeably that smaller Tafel slope means a faster increase of the HER rate with the increase of potential. Hence, the MoS2@G electrocatalyst possesses the fastest HER process among all prepared samples. Furthermore, exchange current density (j0) calculated by extrapolating the Tafel plot is also a crucial parameter for HER, which usually used to evaluate the inherent HER activity [39e41]. For the value of j0, the
Please cite this article as: Li Y et al., Graphene confined MoS2 particles for accelerated electrocatalytic hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.02.089
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Fig. 2 e (a) XRD patterns of MoS2 and MoS2@G. (b) Roman spectra of MoS2, GO and MoS2@G. XPS of GO and MoS2@G. (c) High resolution C 1s spectrum of GO. High resolution (d) C 1s, (e) Mo 3d, and (f) S 2p spectra of MoS2@G.
descending order is Pt/C＞MoS2@G＞MoS2/G＞MoS2＞rGO, with the value of 0.464, 0.419, 0.284, 0.094 and 0.087 mA cm2, implying a much higher catalytic activity of MoS2@G. The LSV polarization curves and Tafel plots of MoS2@G with different proportions of graphene were also conducted to verify the effect of graphene amount on the electrochemical properties of MoS2@G electrocatalysts, as shown in Fig. S6 and S7, which reveal that MoS2@G composite with the GO addition proportion of 50 wt% obtains the optimal properties. Moreover, the details of HER performance parameters for various electrocatalysts are displayed in Table S2. Long-term stability is another important criterion for the HER catalysts. After continuous operation of 5000 potential cycles from 0.1e0.2 V (vs. RHE), the polarization curve of MoS2@G exhibits negligible decay on current while compared with the initial one, as shown in Fig. 3c, manifesting excellent cycling stability under the operating conditions. To further verify the durability of MoS2@G composite catalyst for HER, the current-time plot was operated for 24 h at the potential of
0.132 V (vs. RHE). As shown in Fig. 3d, there has no obvious decline of the current density, revealing that the MoS2@G catalysts retained a steady HER activity. In order to motivate the comprehending of the HER performance of the catalysts, the electrochemically active surface areas (ECSA) of MoS2@G and MoS2/G catalysts were estimated from the electric double layer capacitance (Cdl) by employing cyclic voltammetry (CV) test (Fig. S8). The Cdl of MoS2@G reaches 32.73 mF cm2, which is much higher than that of MoS2/G (26.33 mF cm2). This suggests a large electrochemical surface area of graphene confined MoS2 electrocatalyst [42], which is consistent with the result of Brunauer Emmett Teller (BET). Therefore, a mechanism for enhancing electrocatalytic property of MoS2@G composite was proposed and schematically illustrated in Fig. 4a. Firstly, for the MoS2@G, the size of MoS2 particles was severely refrained due to the introduction of graphene, which could afford ample catalytic active sites for HER [43e45]. Secondly, the architecture of graphene
Please cite this article as: Li Y et al., Graphene confined MoS2 particles for accelerated electrocatalytic hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.02.089
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Fig. 3 e (a) Polarization curves and (b) Tafel plots of the rGO, MoS2, MoS2/G, MoS2@G, and Pt/C (10%) catalysts. (c) Polarization curves for MoS2@G before and after 5000 CV cycles. (d) Current-time plot of the MoS2@G at a static overpotential of ¡0.132 V vs. RHE. All the HER measurements were conducted in 0.5 M H2SO4 electrolyte at 25 C.
Fig. 4 e (a) Schematic illustration of electrons transport pathway on MoS2@G for HER. (b) Nyquist plots of the MoS2, MoS2@G, MoS2/G, rGO and Pt/C (10 wt%) catalysts.
Please cite this article as: Li Y et al., Graphene confined MoS2 particles for accelerated electrocatalytic hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.02.089
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confined MoS2 particles facilitated electron transfer during the hydrogen evolution process [46,47]. Due to the limitation of the space constructed by graphene, the downsized MoS2 particles confined into graphene lead to a more continuous electron transport network, resulting in a fast and efficient electron transfer of the MoS2@G based electrode [48,49]. On the other hand, the structure of MoS2@G increased the contact between the edges of MoS2 particles and graphene, which is more conductive than lay-down configuration of MoS2-graphene stacked structure [50,51]. To glean this effect, the impedance measurements had been performed, as shown in Fig. 4b. Obviously, the MoS2@G exhibited much lower impedance than MoS2/G and MoS2 (3.8, 10.1, and 15.8 U, respectively, as shown in Tab. S2). The significantly reduced Rct afforded markedly faster electron transfer and stronger HER kinetics with the MoS2@G electrocatalyst. Furthermore, the graphene protected MoS2 structure ascertained the stability of the catalyst, which lead to a durable electrochemical application [52]. Therefore, an excellent electrocatalytic performance is obtained in the MoS2@G based electrocatalyst.
Conclusions In summary, we have logically engineered and prepared a highly active HER catalyst of MoS2@G where the MoS2 particles were space-confined in graphene nanosheets via an insitu hydrothermal strategy. Graphene increased the specific surface area of MoS2@G composites and was used as conductive skeleton that provided a peculiar microenvironment and conductively multiplexed path-ways to boost the rapid diffusion of ions and electrons. The MoS2@G composite electrocatalysts exhibited superior HER property with low onset overpotential of 32 mV, and a small Tafel slope of 45 mV dec1. These excellent HER properties were owing to both the abundant HER active sites and the enhanced conductivity. Thus, the graphene confined MoS2 architecture with facile preparation process and high electrocatalytic activity makes it a potential cost-effective catalyst for hydrogen evolution.
Acknowledgements This work was supported by the National Natural Science Foundation of China (41502030), the Natural Science Foundation of Hubei Province of China (2017CFB190), the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (CUG170638), the Open Foundation of Engineering Research Center of NanoGeomaterials of Ministry of Education (NGM2017KF002, NGM2018KF017), and the Fund for Outstanding Doctoral Dissertation of China University of Geosciences.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.02.089
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Available online at www.sciencedirect.com
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Graphene confined MoS2 particles for accelerated electrocatalytic hydrogen evolution Yinchang Li 1, Bing He 1, Xueqin Liu*, Xiaoqin Hu, Jing Huang, Siqin Ye, Zhu Shu, Yang Wang***, Zhen Li** Engineering Research Center of Nano-Geomaterials of Ministry of Education, Faculty of Materials Science and Chemistry, China University of Geosciences, 388 Lumo Road, Wuhan, Hubei 430074, PR China
article info
abstract
Article history:
MoS2 is a promising noble-metal-free electrocatalyst for the hydrogen evolution reaction.
Received 7 November 2018
Extensive trials have been carried out to increase its low electrical conductivity and
Received in revised form
insufficient active sites. Here, a remarkable electrocatalyst for hydrogen evolution is
9 February 2019
developed based on the in-situ preparation of MoS2 confined in graphene nanosheets.
Accepted 13 February 2019
Graphene effectively controls the growth of MoS2 and immensely increases the conduc-
Available online xxx
tivity and structural stability of the composite materials. Remarkably, because of the plentiful active sites, sufficient electrical contact and transport, MoS2 particles confined in
Keywords:
graphene nanosheets exhibit an onset overpotential as small as 32 mV, an overpotential
MoS2
approaching 132 mV at 10 mA cm2, and a low Tafel slope of 45 mV dec1. This work
Graphene
presents a reasonable architecture for practical applications in efficient electrocatalytic H2
Confined
generation.
Hydrogen evolution
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction With its high energy density, environmental friendly and natural abundance, hydrogen has been proved to be an efficient and safe energy carrier [1e4]. However, the inherently slow kinetics of hydrogen evolution reaction (HER) gives low efficiency of the hydrogen production [5e7]. Therefore, advanced electrocatalysts should be innovated to provide adequate active sites for hydrogen production and reduce the overpotential of HER [8]. Platinum (Pt) is proved to be the most effective and stable electrocatalyst [9,10], but its further application is heavily hindered by its high-cost and scarcity. In
the last years, significant studies have been executed in exploring new effective HER electrocatalysts, especially those low-cost and abundant materials. As shown in a density functional theory (DFT) calculations based database [11,12], molybdenum disulfide (MoS2) with an adequate hydrogen binding energy (△GH) is a promising system for HER among transition and noble metals. In addition, edges of MoS2 nanoparticles are active for HER [13,14], S-Mo-S trilayers structure of MoS2 could provide enough active sites for hydrogen production. Furthermore, elemental abundance and environmental friendliness make MoS2 being a competitive member of HER electrocatalyst [15e18]. However, with MoS2 being a semiconductor, its electronic conductivity is
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (X. Liu), [email protected] (Y. Wang), [email protected] (Z. Li). 1 These authors contributed equally. https://doi.org/10.1016/j.ijhydene.2019.02.089 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Li Y et al., Graphene confined MoS2 particles for accelerated electrocatalytic hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.02.089
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poor which limits its overall electrocatalytic activity. Furthermore, the insufficient exposed active sites and inert basal-plane of MoS2 are impediments to its use as an electrocatalyst [19,20]. So, more attentions have been paid to the modification of MoS2 electrocatalysts to increase the electronic conductivity and maximize the quantity of active sites. For example, MoS2 vertical sheets discontinued with a stepped surface structure have been fabricated to tackle the problem of scarce active sites [21]. A strongly coupled MoS2carbon nanotubes nanocomposite was constructed to accelerate the HER via accelerating the electron transport [22]. It is desirable to tackle the impeded electron conductivity and scarce active sites of MoS2 at the same time. Recently, the restriction of electrocatalysts within carbon materials, such as porous carbon and graphene, is a practical way to enhance the HER activity of catalysts, which can be anticipated to not only enhance electrical conductivity of the hybrid catalysts, but also effectively increase the number of reaction sites because of the restriction on the size of active particles and few aggregation of electrocatalysts due to the confinement effect [23e25]. To date, a few confined electrocatalysts have been explored to replace Pt for several reactions. For example, Xie et al. constructed a model of graphene confined ultrathin tin layers, which showed remarkably enhanced electrocatalytic activity and stability because of the abundant active sites, the protection and highly-conductive of graphene [26]. Feng et al. reported Mo2C nanoparticles with diameters down to 5 nm confined in Ndoped porous carbon [27]. The results showed that such confinement provided sufficient exposed active sites for electrocatalysis and ensured strong interaction between the two components of the composites. In recent years, the combination of MoS2 particles with graphene have been extensively investigated as high-performance electrocatalysts for HER [28e30]. However, the investigation into the confinement of MoS2 particles in graphene as HER electrocatalysts has been rarely reported. Herein, we report MoS2 particles confined in graphene (MoS2@G) via an in-situ hydrothermal strategy, and superior HER electrocatalytic performance has been achieved. Asprepared MoS2@G featured abundant exposed active sites for electrocatalysis attributed to the evenly distributed MoS2 particles with downsized diameters, high charge transport pathway and diffusion space for electrolyte and products provided by graphene. The superior MoS2@G catalyst represented low onset overpotential of ~32 mV, which was comparable to that of Pt/C catalyst (~24 mV). Moreover, the MoS2@G has excellent electrochemical stability, which was proved to be a promising electrocatalyst for HER.
Experimental Materials Thiourea, ammonium molybdate, n-methyl pyrrolidone, sulfuric acid, and platinum-carbon (Pt/C, 10 wt%) catalyst were purchased from Sinopharm. Acetylene black and polyvinylidene fluoride were obtained from Aladdin Bio-Chem Technology Co., LTD. Nafion solution (5 wt%) was obtained
from Sigma. The graphene oxide (GO) was prepared by improved Hummers method and reduced to reduced graphene oxide (rGO) by hydrothermal method [31].
Preparation of catalysts Typically, a certain concentration of GO solution was prepared and formed a homogeneous solution by ultrasonic dispersion method. Subsequently, 0.72 mM of ammonium molybdate was dissolved into the GO solution thoroughly and keep stirring for 2 h. After that, 21.6 mM of thiourea was dissolved into the above-solution. Then, the solution was sealed in a 50 mL autoclave and heated for 24 h at 220 C. In the end, the sediments were cleaned by deionized water and ethyl alcohol thoroughly by centrifugation. In our study, the mass fraction of graphene in the MoS2@G composites can be readily tuned by changing the amount of GO. Accordingly, the resulting MoS2@G composites were referred to as MoS2@G-1 (wt % ¼ 10%), MoS2@G-2 (wt% ¼ 30%), MoS2@G-3 (wt% ¼ 50%), and MoS2@G-4 (wt% ¼ 70%). As reference, conventional MoS2 anchored graphene (MoS2/G) was prepared by a solvothermal method reported elsewhere [32]. The preparation process was operated without hours of stirring and preliminary assembly between [Mo7O24]6- and GO [33]. Bare MoS2 was also obtained by a similar process in the absence of GO.
Electrochemical tests 4 mg of catalysts and 0.1 mL of 5 wt% Nafion solution were mixed in 4 mL ethanol and sonicated for 0.5 h to form a homogeneous seriflux. After that, 4 mL of the seriflux was deposited on a glassy carbon electrode with the working area of ~0.07 cm2 (loading ~57 mg cm2). The electrochemical HER tests were performed in 0.5 M H2SO4 aqueous solution (pH ¼ 1) with a standard Ag/AgCl electrode and a Pt slice as the reference and counter electrodes, respectively. Potentials were converted to the standard reversible hydrogen electrode (RHE) potential by adding 0.256 V (0.197 þ 0.059 pH).
Results and discussion The fabrication steps of MoS2@G are illustrated in Scheme 1. Firstly, with hours of continuous moderate stirring, a homogeneous dispersion solution with [Mo7O24]6- and ultrathin GO (Fig. S1 in the Supplementary Information) was obtained. Interestingly, due to the chelating connection between [Mo7O24]6- and GO [34], [Mo7O24]6- embedded layered-GO structure was assembled in this process (MoeO stretching vibrations at 950 cm1 advocated the hypothesis that MoS2 and graphene are connected via CeOeMo, Fig. S2) [35,36]. Finally, graphene confined MoS2 composites were formed via hydrothermal treatment with the help of thiourea. To probe the morphology of the as-prepared samples, the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were carried out. Fig. 1a and b shows the SEM and TEM images of MoS2@G composites. It is observed that the downsized MoS2 particles with the diameter of ca. 100 nm are uniformly distributed and covered by graphene without obvious agglomeration. The high resolution TEM
Please cite this article as: Li Y et al., Graphene confined MoS2 particles for accelerated electrocatalytic hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.02.089
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Scheme 1 e The synthetic process of MoS2 confined in graphene.
(HRTEM) image of MoS2@G in Fig. 1c shows a layered structure with interlayer spacing of ca. 0.61 nm, which corresponds to the (002) plane of hexagonal MoS2, and a looming graphene morphology is observed around the parallel lines. Then the energy-dispersive X-ray (EDX) mapping images from the selected areas of MoS2@G hybrid exhibit a uniform distribution of MoS2 throughout the graphene sheets (Fig. 1d). Furthermore, the atomic ratio of Mo/S was calculated to be 1:2.03 (as shown in Fig. S3), which is according with the stoichiometry of MoS2. The downsized MoS2 particles in MoS2@G were attributed to the introduction and pre-assembling of graphene nanosheets in the synthesis process. In sharp contrast, in the absence of GO, the MoS2 particles were deeply aggregated with large radial sizes of about 1.5 mm (Fig. 1e), which lead to a lower specific surface area (as shown in Fig. S4). It is undoubtable that the larger specific surface area could provide more active sites for HER as well as adsorption of reactant, causing to a higher HER efficiency [29]. Furthermore, the morphology of MoS2/G is shown in Fig. 1f. Obviously, most of MoS2 edges were exposed on the surface of graphene, which is different from the structure of graphene confined MoS2 framework. Compared to MoS2 and MoS2/G, the smaller size of MoS2 in MoS2@G indicated the confinement effect of graphene on MoS2 growth. To obtain the phase structure of the catalysts, X-ray diffraction (XRD) and Raman spectroscopy were employed. For MoS2 microspheres (Fig. 2a), four obviously diffraction peaks centered at around 2q ¼ 14.06 , 33.37 , 39.94 , and 59.14 were observed, which can be indexed respectively to (002), (100), (103), and (110) planes of hexagonal phase of MoS2 (JCPDS card. NO. 75-1539). In comparison to the bare MoS2, the intensity of four characteristic diffraction peaks of MoS2 in MoS2@G diminishes progressively, indicating that the combination of graphene limits the growth of MoS2 particles of high crystallinity. Therefore, the downsized MoS2 particles fabricated in the confined environment between graphene layers can provide abundant catalytic active sites. Moreover, the (002) diffraction peak of the graphite cannot be detected in
MoS2@G, indicating that there was no stack of the graphene nanosheets in the time of reaction for the existence of MoS2. Raman spectroscopy was employed to identify the components of the as-prepared samples. In the case of MoS2 and MoS2@G, two Roman peaks at 381 and 402 cm1 are observed (Fig. 2b), corresponding to the in-plane E12g and out-of-plane A1g modes of 2H MoS2, respectively [33,37]. Significantly, both GO and MoS2@G composites show two distinct peaks at about 1349 and 1577 cm1, which has well matching degree with the D and G bands of graphene, respectively [38]. In our case, MoS2@G composites show a higher ID/IG value than that of the pristine GO (1.03 and 0.83, respectively). This distinction might be caused by the structural defects within the sp2 carbon network that arose upon reduction of the as-used GO. X-ray photoelectron spectroscopy (XPS) measurements were conducted to gain insight into the atomic valence states and the composition of the MoS2@G composites. The XPS survey spectrum (Fig. S5) confirms that the MoS2@G is mainly composed of Mo, S, C and O, which is in according with the EDX result. As shown in Fig. 2c and d, a successful reduction of GO to rGO was verified by the C 1s signal, according to the relative decreases of characteristic peaks from C-O and C]O/ COOH [19]. The reduction of GO ensures the high conductivity of graphene in the MoS2@G composite. The high-resolution XPS spectra of Mo 3d and S 2p regions were explored to verify the preparation of MoS2. Fig. 2e shows the high resolution Mo 3d spectrum of MoS2@G, where four peaks could be deconvoluted, including 226.4 eV for S 2s of MoS2, 229.2 eV and 232.4 eV for Mo 3d5/2 and Mo 3d3/2 of MoS2, and 235.6 eV for Mo 6þ may 3d of MoO3 or MoO2 4 , respectively. The presence of Mo attribution to the mild oxidation of MoS2 [1]. The strong peaks (Fig. 2f) located at binding energies of 162.1 and 163.2 eV correspond to the S 2p3/2 and S 2p1/2 lines of MoS2, respectively. Meanwhile the binding energy at 168.7 eV can be ascribed to the terminal S4þ species on the surface or edges of MoS2 [39]. The results further suggest the successful preparation of MoS2@G composites. To test our expectation, the electrocatalytic HER activities of MoS2@G and MoS2/G were investigated using a typical
Please cite this article as: Li Y et al., Graphene confined MoS2 particles for accelerated electrocatalytic hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.02.089
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Fig. 1 e (a) SEM, (b) TEM and (c) HRTEM images of MoS2@G. (d) TEM image and corresponding EDX mappings of MoS2@G. SEM images of (e) MoS2 and (f) MoS2/G. three-electrode configuration. Bare MoS2, graphene and Pt/C were also taken as references to estimate the contribution from graphene and confinement effect. Linear sweep voltammetry (LSV) polarization curves of various catalysts with a sweep rate of 5 mV s1 are shown in Fig. 3a. Obviously, the graphene shows almost no HER catalytic activity, and bare MoS2 exhibits inferior HER activity. In sharp contrast, MoS2@G electrocatalyst delivers a low onset overpotential approaching 32 mV and achieves a cathodic current density of 10 mA cm2 at 132 mV. Such a low overpotential is slightly lower than that of Pt/C but highly superior to that of MoS2/G catalyst, which is ascribed to the numerous active sites and ultrafast electrons transport derived from the unique framework of graphene confined MoS2. The HER performance of MoS2@G is markedly better than or at least comparable to those of the most active HER electrocatalysts (Table S1).
In addition, the Tafel plots were derived to analyze the quantitative kinetics of the HER process. Fig. 3b shows the Tafel plots of as-prepared catalysts and commercial Pt/C catalyst which were fitted to the Tafel equation (h ¼ b log j þ a, where j is the current density and b is the Tafel slope). Graphene reveals a largest Tafel slope value of 137 mV dec1. The Tafel slope is 45 mV dec1 for sample MoS2@G, which is closed to that of Pt/C (33 mV dec1) and lower than that of MoS2/G (76 mV dec1) and pure MoS2 (103 mV dec1). It is noticeably that smaller Tafel slope means a faster increase of the HER rate with the increase of potential. Hence, the MoS2@G electrocatalyst possesses the fastest HER process among all prepared samples. Furthermore, exchange current density (j0) calculated by extrapolating the Tafel plot is also a crucial parameter for HER, which usually used to evaluate the inherent HER activity [39e41]. For the value of j0, the
Please cite this article as: Li Y et al., Graphene confined MoS2 particles for accelerated electrocatalytic hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.02.089
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Fig. 2 e (a) XRD patterns of MoS2 and MoS2@G. (b) Roman spectra of MoS2, GO and MoS2@G. XPS of GO and MoS2@G. (c) High resolution C 1s spectrum of GO. High resolution (d) C 1s, (e) Mo 3d, and (f) S 2p spectra of MoS2@G.
descending order is Pt/C＞MoS2@G＞MoS2/G＞MoS2＞rGO, with the value of 0.464, 0.419, 0.284, 0.094 and 0.087 mA cm2, implying a much higher catalytic activity of MoS2@G. The LSV polarization curves and Tafel plots of MoS2@G with different proportions of graphene were also conducted to verify the effect of graphene amount on the electrochemical properties of MoS2@G electrocatalysts, as shown in Fig. S6 and S7, which reveal that MoS2@G composite with the GO addition proportion of 50 wt% obtains the optimal properties. Moreover, the details of HER performance parameters for various electrocatalysts are displayed in Table S2. Long-term stability is another important criterion for the HER catalysts. After continuous operation of 5000 potential cycles from 0.1e0.2 V (vs. RHE), the polarization curve of MoS2@G exhibits negligible decay on current while compared with the initial one, as shown in Fig. 3c, manifesting excellent cycling stability under the operating conditions. To further verify the durability of MoS2@G composite catalyst for HER, the current-time plot was operated for 24 h at the potential of
0.132 V (vs. RHE). As shown in Fig. 3d, there has no obvious decline of the current density, revealing that the MoS2@G catalysts retained a steady HER activity. In order to motivate the comprehending of the HER performance of the catalysts, the electrochemically active surface areas (ECSA) of MoS2@G and MoS2/G catalysts were estimated from the electric double layer capacitance (Cdl) by employing cyclic voltammetry (CV) test (Fig. S8). The Cdl of MoS2@G reaches 32.73 mF cm2, which is much higher than that of MoS2/G (26.33 mF cm2). This suggests a large electrochemical surface area of graphene confined MoS2 electrocatalyst [42], which is consistent with the result of Brunauer Emmett Teller (BET). Therefore, a mechanism for enhancing electrocatalytic property of MoS2@G composite was proposed and schematically illustrated in Fig. 4a. Firstly, for the MoS2@G, the size of MoS2 particles was severely refrained due to the introduction of graphene, which could afford ample catalytic active sites for HER [43e45]. Secondly, the architecture of graphene
Please cite this article as: Li Y et al., Graphene confined MoS2 particles for accelerated electrocatalytic hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.02.089
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Fig. 3 e (a) Polarization curves and (b) Tafel plots of the rGO, MoS2, MoS2/G, MoS2@G, and Pt/C (10%) catalysts. (c) Polarization curves for MoS2@G before and after 5000 CV cycles. (d) Current-time plot of the MoS2@G at a static overpotential of ¡0.132 V vs. RHE. All the HER measurements were conducted in 0.5 M H2SO4 electrolyte at 25 C.
Fig. 4 e (a) Schematic illustration of electrons transport pathway on MoS2@G for HER. (b) Nyquist plots of the MoS2, MoS2@G, MoS2/G, rGO and Pt/C (10 wt%) catalysts.
Please cite this article as: Li Y et al., Graphene confined MoS2 particles for accelerated electrocatalytic hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.02.089
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confined MoS2 particles facilitated electron transfer during the hydrogen evolution process [46,47]. Due to the limitation of the space constructed by graphene, the downsized MoS2 particles confined into graphene lead to a more continuous electron transport network, resulting in a fast and efficient electron transfer of the MoS2@G based electrode [48,49]. On the other hand, the structure of MoS2@G increased the contact between the edges of MoS2 particles and graphene, which is more conductive than lay-down configuration of MoS2-graphene stacked structure [50,51]. To glean this effect, the impedance measurements had been performed, as shown in Fig. 4b. Obviously, the MoS2@G exhibited much lower impedance than MoS2/G and MoS2 (3.8, 10.1, and 15.8 U, respectively, as shown in Tab. S2). The significantly reduced Rct afforded markedly faster electron transfer and stronger HER kinetics with the MoS2@G electrocatalyst. Furthermore, the graphene protected MoS2 structure ascertained the stability of the catalyst, which lead to a durable electrochemical application [52]. Therefore, an excellent electrocatalytic performance is obtained in the MoS2@G based electrocatalyst.
Conclusions In summary, we have logically engineered and prepared a highly active HER catalyst of MoS2@G where the MoS2 particles were space-confined in graphene nanosheets via an insitu hydrothermal strategy. Graphene increased the specific surface area of MoS2@G composites and was used as conductive skeleton that provided a peculiar microenvironment and conductively multiplexed path-ways to boost the rapid diffusion of ions and electrons. The MoS2@G composite electrocatalysts exhibited superior HER property with low onset overpotential of 32 mV, and a small Tafel slope of 45 mV dec1. These excellent HER properties were owing to both the abundant HER active sites and the enhanced conductivity. Thus, the graphene confined MoS2 architecture with facile preparation process and high electrocatalytic activity makes it a potential cost-effective catalyst for hydrogen evolution.
Acknowledgements This work was supported by the National Natural Science Foundation of China (41502030), the Natural Science Foundation of Hubei Province of China (2017CFB190), the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (CUG170638), the Open Foundation of Engineering Research Center of NanoGeomaterials of Ministry of Education (NGM2017KF002, NGM2018KF017), and the Fund for Outstanding Doctoral Dissertation of China University of Geosciences.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.02.089
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