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MoS2 for hydrogen evolution reaction
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Synthesis and electrocatalytic activity of Ni0.85Se/ MoS2 for hydrogen evolution reaction Zhaoxia Yang a, Qiong Cai c, Chuanqi Feng a, Huimin Wu a,*, He Mei b,** a
Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials & Ministry-of-Education Key Laboratory for the Synthesis and Application of Organic Functional Molecules, Hubei Key Laboratory of Polymer Materials, Hubei Key Laboratory of Ferro & Piezoelectric Materials and Devices, Faculty of Physics and Electronic Science, College of Chemistry & Chemical Engineering, Hubei University, Wuhan 430062, PR China b Department of Environmental Sciences, Zhejiang Provincial Key Laboratory of Watershed Science and Health, Wenzhou Medical University, Wenzhou, 325035, PR China c Department of Chemical and Process Engineering, Faculty of Engineering and Physical Sciences, University of Surrey, Guildford GU2 7XH, UK
highlights Ni0.85Se/MoS2 with different mole ratios were synthesized. Ni0.85Se/MoS2 exhibited better catalytic activity than Ni0.85Se and MoS2. Ni0.85Se/MoS2 is expected to be a promissing substitute for Pt.
article info
abstract
Article history:
Recently, the replacement of expensive platinum-based catalytic materials with non-
Received 5 June 2019
precious metal materials to electrolyze water for hydrogen separation has attracted
Received in revised form
much attention. In this work, Ni0.85Se, MoS2 and their composite Ni0.85Se/MoS2 with
11 August 2019
different mole ratios are prepared successfully, as electrocatalysts to catalyze the hydrogen
Accepted 15 August 2019
evolution reaction (HER) in water splitting. The result shows that MoS2/Ni0.85Se with a
Available online 10 September 2019
molar ratio of Mo/Ni ¼ 30 (denoted as M30) has the best catalytic performance towards HER, with the lowest overpotential of 118 mV at 10 mA cm2, smallest Tafel slope of
Keywords:
49 mV$dec1 among all the synthesized materials. Long-term electrochemical testing
Hydrogen evolution reaction
shows that M30 has good stability for HER over at least 30 h. These results maybe due to the
Water splitting electrocatalysts
large electrochemical active surface area and high conductivity. This work shows that
Transition metal selenides
transition metal selenides and sulfides can form effective electrocatalyst for HER.
Disulfides
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Composite
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H. Wu), [email protected] (H. Mei). https://doi.org/10.1016/j.ijhydene.2019.08.118 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Introduction It is well known that the world faces energy crisis as the conventional fossil fuels not only have limited reserves, but also pollute the environment [1]. The development of new energy sources is extremely urgent to provide clean and sustainable energy, and to meet the ever increasing energy demand [2]. As an efficient and clean energy carrier, hydrogen has attracted much attention [3,4]. The most established method of producing hydrogen is via reforming of hydrocarbons which is associated with CO2 emission [5]. Electrolysis of water provides a clean route, and has been commonly used in industry to produce hydrogen, at the same time releasing oxygen by-product [6]. A typical electrolysis cell involves hydrogen evolution reaction (HER) at cathode and oxygen evolution reaction (OER) at anode, and theoretically encounters an electric potential of 1.23 V [7,8]. In practice, the actual electric voltage required to operate an electrolysis cell is much greater than 1.23 V. This is because extra electric energy is needed to overcome the reaction barriers associated with the processes of multi-step electron transfer in HER and OER [9,10]. Therefore, it is of great significance to find good catalysts to reduce the reaction barriers in the process of HER and OER [11,12]. In particular, the design and preparation of highly active catalysts for HER to have low overpotential is the key for efficient water splitting. Platinum has the best catalytic performance among the various catalyst materials [13,14], but is difficult to be widely used due to its scarce resource and high price [15e17]. Non-precious metal materials as cheaper HER catalysts have been widely studied [18e20]. It mainly includes transition metals and their alloys, transition metal selenide and sulfide compounds (e.g. MoSe2, MoS2) [21], carbides (e.g. TiC, aMo2C) [22], phosphate compounds (e.g. FeP, NiP) [23], and nitrides (e.g. W2N, NiMoNx) [24,25]. In particular, transition metal selenide has recently drawn significant attention due to its promising performance [26e28]. For example, Nguyen et al. studied MoSe2 with the overpotential of 270 mV at 10 mA cm2, Tafel slope of 60 mV$dec1, and showed their activities for HER [29]. NiSe2 were also studied as potential high efficiency electrocatalysts [30]. The large electrochemical active surface area and high conductivity of such Ni0.85Se/ MoS2 catalysts, may be confirmed to be responsible for the high HER performance. However, the catalytic performance of these materials has not been optimized and it has been reported that pure materials normally have limited amount of active sites [31e33]. In order to enhance the catalytic performance of transition metal based catalysts and improve the efficiency of HER, many researchers have studied the modification of the catalysts via various strategies such as doping, morphology control, and composition with other materials [34e38]. These strategies have helped to improve the performance of catalytic materials [39e44]. In particular, composition is a relatively easier way to achieve enhanced HER performance. So far, researchers have reported the composition of transition metal compounds of the same type [33e35,41], for example, the composition of two sulphides (CoS and MoS2, MoS2 and Ni3S2) [45], and the composition of two selenides (ZnSe and MoSe).
Here, we report a two-step synthesis of a series of composite materials by combining molybdenum disulfide MoS2 with nickel selenide (NiSe), whilst the proportion of the two materials is controlled to achieve different composites. The as-synthesized materials are denoted as Mx (x(atomic ratio) ¼ Mo/Ni ¼ 10, 20, 30, 40, 50), with the ratios confirmed by materials characterization. Different morphologies are revealed with different Mo/Ni ratios. For example, M30 has a uniformly dispersed nanosheet structure, while M20 and M40 are more dense and disordered. Among the synthesized composites, M30 shows the best catalytic performances towards HER. This study can provide a reference for improving the performance of nonnoble metals and has potential application in water splitting.
Experimental Materials All of the reagents including NiCl2$6H2O, Na2SeO3, MoS2, (NH4)6Mo24$4H2O and SC(NH2)2 were purchased from Sinopharm Chemical Reagent Co. Ltd. The chemical agents were analytical grade and used as obtained without further treatment.
Synthesis of Ni0.85Se/MoS2 Preparation of MoS2 1.24 g (NH4)6Mo24$4H2O and 2.28 g SC(NH2)2 were dissolved in 35 ml ultrapure water under stirring. Then, the solution was transferred to a 100 ml Teflon-lined stainless steel autoclave and heated at 200 C for 24 h. After this, the MoS2 was obtained by centrifuging and washed several times with water and ethanol.
Preparation of Ni0.85Se/MoS2 Na2SeO3, NiCl2$6H2O and the synthesized MoS2 (Table 1) were added to the solution containing the 6 mL ultrapure water, 8.5 ml N2H4$H2O and 24 mL ethanolamine (EA) solvent. After stirring, the solution was put into the Teflon-lined stainless steel high-pressure reactor, and heated at 140 C for 24 h. Finally, the product was cooled down to room temperature gradually, centrifuged and washed several times with water and ethanol, then dried under vacuum to obtain Ni0.85Se/ MoS2.
Table 1 e The name and amounts of the materials in the synthesis reaction. Material number Ni0.85Se M10 M20 M30 M40 M50
NiCl2$6H2O (g)
Na2SeO3 (g)
MoS2 (g)
At%(Mo/ Ni)
0.238 0.238 0.238 0.238 0.238 0.238
0.845 0.845 0.845 0.845 0.845 0.845
0 0.017 0.04 0.069 0.107 0.16
0 10 20 30 40 50
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Characterization X-ray diffraction (XRD) measurement was carried out using a GBC MMA X-ray diffractometer with Cu Ka radiation. Jobin Yvon Horbia Confocal Micro Raman Spectrometer (Model HR800) was used to record Raman spectra, with a 532 nm diode laser excitation on a 300 lines/mm grating at room temperature. X-ray photoelectron spectroscopy (XPS) data was collected by an ESCALAB 250Xi device. Transmission electron microscopy (TEM) and energy-dispersive spectroscopy (EDS) were respectively performed on a Tecnai G2 F30 instrument and a JSM6510LV instrument. For electrochemical measurements, CHI 750E electrochemical workstation (CH Instrument Company, Shanghai, China) was employed. All tests were carried out using a conventional three-electrode system. The counter electrode is carbon rod and the reference electrode is saturated calomel electrode (SCE). The catalysts prepared above were used to make the working electrode. 5 mg of the catalyst was added to
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0.5 mL of absolute ethanol, and 35 L of Nafion (5%) was added and dispersed. For individual electrode preparation, the ink was drop-casted onto a glass carbon electrode (GCE, diameter 3 mm). The electrode has a geometric area of 0.07 cm2, which was used in the calculation of current densities. 0.5 M H2SO4 was used as the electrolyte solution. All of the potentials we measured were converted to reversible hydrogen electrode (RHE), according to the Nernst equation (ERHE(V) ¼ ESCE þ 0.059 pH þ 0.241). The electrochemical impedance spectroscopy (EIS) was conducted with frequency (Hz) from 1 to 1000000.
Results and discussion Material selection Fig. 1 (A) represents the XRD spectra of (a) M10, (b) M20, (c) M30, (d) M40, (e) M50, (f) Ni0.85Se and (g) MoS2. The peaks at 36.0 , 47.4 , 53.2 , 62.7 , 64.3 , 72.4 (Fig. 1(A)-f), could be
Fig. 1 e (A) XRD patterns of (a) M10, (b) M20, (c) M30, (d) M40 (e) M50, (f) Ni0.85Se and (g) MoS2; (B) Raman spectras of M30; XPS high-resolution spectras of (C) Ni 2p, (D) Mo 3d, (E) Se 3d, and (F) S 2p of M30.
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Fig. 2 e (A) LSV curves of Ni0.85Se, MoS2, M10, M20, M30, M40, M50, and Pt/C; (B) Tafel plots of Ni0.85Se, MoS2, M10, M20, M30, M40, M50, and Pt/C; (C) Nyquist plots and (D) the dotted line diagram of Resistence of Ni0.85Se, MoS2, M10, M20, M30, M40, M50.
indexed to the (101), (102), (110), (103), (201), (202) of Ni0.85Se (JCPDS No.18-0888). While the peaks at 12.6 , 31.2 , 34.6 (Fig. 1(A)-g) were attributed to the (002), (100), (102) lattice plane of MoS2 (JCPDS No.37-1492). Meanwhile, in Fig. 1(A), the positions of the peaks are the same for different composites (M10, M20, M30, M40 and M50), with both Ni0.85Se and MoS2 peaks clearly identifiable, confirming that composites of Ni0.85Se/MoS2 have been successfully synthesized. Therefore, with the increase of MoS2, the peak to the (002) became more and more obvious. But there is a slight shift in the peak positions of the materials, which may be caused by the content of MoS2 [46e48]. Fig. 1(B) presents the Raman spectra of M30. The peaks at 180 cm1 and 240 cm1 could be indexed to NieSe. The other two peaks at 375 cm1 and 405 cm1 could be ascribed to MoeS [49e51]. It demonstrates that the Ni0.85Se/MoS2 has been successfully synthesized [52,53]. To investigate the chemical compositions and valence states of catalysts, the XPS measurements were used in experiment [54,55]. Fig. 1(C)e(F) shows the surface survey XPS data of Ni0.85Se, MoS2 and M30. As shown in Fig. 1(C), the peaks at 870.6 eV and 853.1 eV corresponded to Ni2þ. The Ni 2p 1/2 at 873.7 eV and Ni 2p 3/2 at 855.7 eV may belong to Ni 3 þ from the surface oxide phase [56,57]. The peaks of M30 are slightly deviated from these of Ni0.85Se. The two satellite peaks at 879.7 eV and 860.8 eV are oxidation state of Ni2þ. Mo 3d3/2 at 231.6 eV and Mo 3d5/2 at 228.4 eV are attributed to Mo4þ in Fig. 1(D) [58]. Se 3d3/2 at 54.5 eV and Se 3d5/2 at 54.0 eV could be observed in Fig. 1(E), corresponding to Se2 [41,49]. The other peak at 58.6 eV is characteristic with surface oxidation SeOX.
In Fig. 1(F), S 2P1/2 at 162.5 eV and S 2P3/2 at 161.4 eV are attributed to S2 [59e61]. It indicates the existence of Ni, Mo, Se, S elements in M30. All the data indicate that the composite was successfully prepared. Overpotential is one of the parameters to evaluate the performance of catalytic materials. Under the same current density, smaller overpotential means better catalytic performance of materials [30]. The overpotential can be compared by linear sweep voltammetry (LSV). M30 has the lowest overpotential of 118 mV to drive 10 mA cm2 in Fig. 2 (A). The Tafel slope (i.e. the slope of the applied overpotential h vs. log jA of) derived from LSV is often used to indicate the HER mechanism [62]. In Fig. 2 (B), M30 has the smallest Tafel slope of 49 mV$dec1. Electrochemical impedance spectroscopy (EIS) can prove catalytic kinetics of hydrogen evolution. The inset in Fig. 2(C) demonstrates the electrical equivalent circuit of the impedance that includes the solution resistance (Rs), double layer capacitance (Cdl), charge transfer resistance (Rct) and Warburg impedance (Zw). The semicircle diameter corresponds to a charge transfer resistance (Rct), which can evaluate the charge transfer restriction process. The smaller diameter semicircle represents the lower Rct of HER, meaning higher current density at the same overpotential. Fig. 2 (C)e(D) show the resistance of M30 is minimum with 27.72 U. In brief, Fig. 2 shows that the catalytic performance of M30 is the best among all of the as-prepared materials. These indicate the combination of 30% mole ratio of MoS2 make the performance of Ni0.85Se greatly improve.
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Fig. 3 e (A)TEM images of M30; (B) HRTEM image of M30; Mapping images (C) Ni (D) Mo (E) Se (F) S of M30.
As can be seen from Fig. 3 (A), the material has a nanosheet structure, which may mean more exposure of active sites and active edges [23,41]. Fig. 3(B) is the high-resolution TEM (HRTEM) image of M30. The distance between the lattice fringe is 0.27 nm, which corresponds to the MoS2 (100); while the distance of 0.19 nm corresponds to the Ni0.85Se (102) crystal face. This confirms that the as-synthesized material is the composite of Ni0.85Se and MoS2, which is consistent with the XRD results. Hu and coworkers found that amorphous MoS2 films are particularly HER active and the increased coordinately and structurally unsaturated sulfur atoms led to the enhancement [15]. Fig. 3(C)~(F) respectively represent the distribution of Ni, Mo, Se, and S element in M30. It is clear that all elements are evenly distributed throughout the catalyst M30. Energy Dispersive Spectrometer (EDS) was used to
provide qualitative and quantitative analysis of the element distribution in the microscopic region (as shown in Table S1), showing the synthesis were successfully prepared.
Characterization Fig. 4(A) is the LSV curves for Ni0.85Se, MoS2, M30 and Pt/C. The M30, Ni0.85Se, MoS2 overpotential were 118 mV, 190 mV, 257 mV respectively at 10 mA cm2. Obviously, M30 performs better than Ni0.85Se and MoS2. The enhanced performance of M30 could be attributed to the enlarged specific surface area and the increased active sites compared to Ni0.85Se and MoS2 [29,41]. The HER activities of M30 are compared with that of other as-prepared materials (Table 2) and reported materials in Table S2, in terms of overpotential at the same current
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Fig. 4 e (A) LSV curves; (B) Tafel plots; (C) Nyquist plots with an equivalent circuit (inset); (D) Estimated Cdl and relative electrochemically active surface areas of the materials.(E) i-t testing of M30 under static potential of ¡0.21 V vs. RHE. Inset is the enlargement of the area denoted by the rectangle. (F) LSV curves of M30, after 30 h i-t testing.
density of 10 mA cm2. As can be seen, M30 outperformed all the selenides-based electrocatalysts reported in the literature. Under acidic conditions, HER mechanism is mainly divided into two parts, the first part is electrochemical reaction, also known as Volmer reaction (1), the second part is divided into electrochemical desorption (2-a) and complex desorption (2-b) due to different catalyst properties, the specific reaction formula is as follows [63]. H3 Oþ þ e % m H þ H2 O ðVolmer reactionÞ
(1)
H3 Oþ þ e þ mH%H2 þmþ H2 O ðHeyrovsky reactionÞ
(2-a)
2m H % H2 ðTafel reactionÞ
(2-b)
where e represents an electron, and m-H represents a
hydrogen atom which adsorbed onto the catalyst m. The Tafel equation (h ¼ b log jA þ a, where b is the Tafel slope). As shown in Fig. 4(B), the Tafel slope of M30 is 49 mV$dec1. This indicates that M30 followed the Volmer Heyrovsky reaction in the process of HER reaction [64]. The height of the
Table 2 e HER catalytic performance data of all asprepared catalysts at 10 mA/cm2. Material number M10 M20 M30 M40 M50 Ni0.85Se MoS2
h Tafel slope （mV/ (mV) dec） 180 152 118 136 186 190 257
76 64 49 51 82 57 113
Cdl (mF/ cm2)
Rct (U)
0.72 1.31 1.49 1.44 0.58 0.53 0.29
43.79 36.35 27.72 31.01 47.19 48.24 112.05
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Tafel slope can also be used to evaluate the HER performance of the catalytic material [30]. Fig. 3(B) compares the Tafel slopes of different catalysts, showing 49 mV$dec1, 57 mV$dec1, 113 mV$dec1, and 55 mV$dec1 for M30, Ni0.85Se, MoS2 and Pt/C respectively. It is clear that M30 has the best HER activity among the investigated catalysts. M30 also outperforms other reported electrocatalysts in literatures when comparing the Tafel slope, as shown in Table S2. From Fig. 4(C), the Rct value of M30 is 27.72 U, lower than that of Ni0.85Se (48.24 U) and MoS2 (112.05 U), indicating that M30 has the fastest electron transfer rate [65]. Double layer capacitance Cdl, which can be obtained from the CV analysis, is directly related to the electrochemical active surface area (EASA) and the scanning rate v in a linear relationship (Cdl ¼ v EASA) [66]. Therefore, the values of Cdl can be used to indicate the EASA. As shown in Fig. 4(D), the Cdl of M30 is 1.49 mF/cm2, higher than that of Ni0.85Se (0.53 mF/ cm2) and MoS2 (0.29 mF/cm2) (Fig. S6), suggesting that M30 has the highest EASA thus the highest amount of active sites. In practical applications of hydrogen production, the stability of the electrocatalyst is very important. Chronoamperometry testing (i-t) at 0.21 V was conducted to evaluate the stability of M30, and the result is shown in Fig. 4(E). The current shows a slight drop at the beginning, which may be caused by the consumption of Hþ or the remaining of H2 bubbles on the electrode surface [67]. Meanwhile, from Fig. 4(F), we can find that the Rct of M30 only has a slight increase after 30 h of i-t testing. Moreover, the LSV data of M30 were plotted and shown in Fig. 4(F). Only 6 mV shift at the current density of 10 mA cm2 can be observed. Fig. S7 shows the Nyquist plots of M30 of the initial and after 30 h of i-t testing. Obviously, the impedance barely changed. The above results confirm that M30 has good stability, and can be used as potential electrocatalyst for hydrogen production via water splitting.
Conclusions In this paper, Ni0.85Se, MoS2 and Mx (x ¼ 10, 20, 30, 40, 50) were prepared by hydrothermal method. The experimental results show that the physical performance of Ni0.85Se/MoS2 is different from that of Ni0.85Se, leading to different catalytic performance towards HER in the process of water splitting. Among the different catalysts, M30 has lower overpotential of 118 mV to drive 10 mA$cm2 than these of Ni0.85Se (190 mV), MoS2 (257 mV), and smaller Tafel slope of 49 mV$dec1 than these of Ni0.85Se (57 mV$dec1), MoS2 (113 mV$dec1). This may be because the introduction of MoS2 leads to more exposure to active sites, increasing EASA and conductivity. These together can give accelerated electron transfer rate in HER process, resulting in both high catalytic activity and stability.
Acknowledgements We acknowledge financial support from the National Natural Science Foundation of China (Grant No. 21205030), the Key
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Project of Hubei Provincial Education Department (D20171001), Hubei Key Laboratory of Ferro & Piezoelectric Materials and Devices (201710), Hubei Key Laboratory for Processing and Application of Catalytic Materials (201829303), and (111 project, B12015).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.08.118.
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Available online at www.sciencedirect.com
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Synthesis and electrocatalytic activity of Ni0.85Se/ MoS2 for hydrogen evolution reaction Zhaoxia Yang a, Qiong Cai c, Chuanqi Feng a, Huimin Wu a,*, He Mei b,** a
Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials & Ministry-of-Education Key Laboratory for the Synthesis and Application of Organic Functional Molecules, Hubei Key Laboratory of Polymer Materials, Hubei Key Laboratory of Ferro & Piezoelectric Materials and Devices, Faculty of Physics and Electronic Science, College of Chemistry & Chemical Engineering, Hubei University, Wuhan 430062, PR China b Department of Environmental Sciences, Zhejiang Provincial Key Laboratory of Watershed Science and Health, Wenzhou Medical University, Wenzhou, 325035, PR China c Department of Chemical and Process Engineering, Faculty of Engineering and Physical Sciences, University of Surrey, Guildford GU2 7XH, UK
highlights Ni0.85Se/MoS2 with different mole ratios were synthesized. Ni0.85Se/MoS2 exhibited better catalytic activity than Ni0.85Se and MoS2. Ni0.85Se/MoS2 is expected to be a promissing substitute for Pt.
article info
abstract
Article history:
Recently, the replacement of expensive platinum-based catalytic materials with non-
Received 5 June 2019
precious metal materials to electrolyze water for hydrogen separation has attracted
Received in revised form
much attention. In this work, Ni0.85Se, MoS2 and their composite Ni0.85Se/MoS2 with
11 August 2019
different mole ratios are prepared successfully, as electrocatalysts to catalyze the hydrogen
Accepted 15 August 2019
evolution reaction (HER) in water splitting. The result shows that MoS2/Ni0.85Se with a
Available online 10 September 2019
molar ratio of Mo/Ni ¼ 30 (denoted as M30) has the best catalytic performance towards HER, with the lowest overpotential of 118 mV at 10 mA cm2, smallest Tafel slope of
Keywords:
49 mV$dec1 among all the synthesized materials. Long-term electrochemical testing
Hydrogen evolution reaction
shows that M30 has good stability for HER over at least 30 h. These results maybe due to the
Water splitting electrocatalysts
large electrochemical active surface area and high conductivity. This work shows that
Transition metal selenides
transition metal selenides and sulfides can form effective electrocatalyst for HER.
Disulfides
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Composite
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H. Wu), [email protected] (H. Mei). https://doi.org/10.1016/j.ijhydene.2019.08.118 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Introduction It is well known that the world faces energy crisis as the conventional fossil fuels not only have limited reserves, but also pollute the environment [1]. The development of new energy sources is extremely urgent to provide clean and sustainable energy, and to meet the ever increasing energy demand [2]. As an efficient and clean energy carrier, hydrogen has attracted much attention [3,4]. The most established method of producing hydrogen is via reforming of hydrocarbons which is associated with CO2 emission [5]. Electrolysis of water provides a clean route, and has been commonly used in industry to produce hydrogen, at the same time releasing oxygen by-product [6]. A typical electrolysis cell involves hydrogen evolution reaction (HER) at cathode and oxygen evolution reaction (OER) at anode, and theoretically encounters an electric potential of 1.23 V [7,8]. In practice, the actual electric voltage required to operate an electrolysis cell is much greater than 1.23 V. This is because extra electric energy is needed to overcome the reaction barriers associated with the processes of multi-step electron transfer in HER and OER [9,10]. Therefore, it is of great significance to find good catalysts to reduce the reaction barriers in the process of HER and OER [11,12]. In particular, the design and preparation of highly active catalysts for HER to have low overpotential is the key for efficient water splitting. Platinum has the best catalytic performance among the various catalyst materials [13,14], but is difficult to be widely used due to its scarce resource and high price [15e17]. Non-precious metal materials as cheaper HER catalysts have been widely studied [18e20]. It mainly includes transition metals and their alloys, transition metal selenide and sulfide compounds (e.g. MoSe2, MoS2) [21], carbides (e.g. TiC, aMo2C) [22], phosphate compounds (e.g. FeP, NiP) [23], and nitrides (e.g. W2N, NiMoNx) [24,25]. In particular, transition metal selenide has recently drawn significant attention due to its promising performance [26e28]. For example, Nguyen et al. studied MoSe2 with the overpotential of 270 mV at 10 mA cm2, Tafel slope of 60 mV$dec1, and showed their activities for HER [29]. NiSe2 were also studied as potential high efficiency electrocatalysts [30]. The large electrochemical active surface area and high conductivity of such Ni0.85Se/ MoS2 catalysts, may be confirmed to be responsible for the high HER performance. However, the catalytic performance of these materials has not been optimized and it has been reported that pure materials normally have limited amount of active sites [31e33]. In order to enhance the catalytic performance of transition metal based catalysts and improve the efficiency of HER, many researchers have studied the modification of the catalysts via various strategies such as doping, morphology control, and composition with other materials [34e38]. These strategies have helped to improve the performance of catalytic materials [39e44]. In particular, composition is a relatively easier way to achieve enhanced HER performance. So far, researchers have reported the composition of transition metal compounds of the same type [33e35,41], for example, the composition of two sulphides (CoS and MoS2, MoS2 and Ni3S2) [45], and the composition of two selenides (ZnSe and MoSe).
Here, we report a two-step synthesis of a series of composite materials by combining molybdenum disulfide MoS2 with nickel selenide (NiSe), whilst the proportion of the two materials is controlled to achieve different composites. The as-synthesized materials are denoted as Mx (x(atomic ratio) ¼ Mo/Ni ¼ 10, 20, 30, 40, 50), with the ratios confirmed by materials characterization. Different morphologies are revealed with different Mo/Ni ratios. For example, M30 has a uniformly dispersed nanosheet structure, while M20 and M40 are more dense and disordered. Among the synthesized composites, M30 shows the best catalytic performances towards HER. This study can provide a reference for improving the performance of nonnoble metals and has potential application in water splitting.
Experimental Materials All of the reagents including NiCl2$6H2O, Na2SeO3, MoS2, (NH4)6Mo24$4H2O and SC(NH2)2 were purchased from Sinopharm Chemical Reagent Co. Ltd. The chemical agents were analytical grade and used as obtained without further treatment.
Synthesis of Ni0.85Se/MoS2 Preparation of MoS2 1.24 g (NH4)6Mo24$4H2O and 2.28 g SC(NH2)2 were dissolved in 35 ml ultrapure water under stirring. Then, the solution was transferred to a 100 ml Teflon-lined stainless steel autoclave and heated at 200 C for 24 h. After this, the MoS2 was obtained by centrifuging and washed several times with water and ethanol.
Preparation of Ni0.85Se/MoS2 Na2SeO3, NiCl2$6H2O and the synthesized MoS2 (Table 1) were added to the solution containing the 6 mL ultrapure water, 8.5 ml N2H4$H2O and 24 mL ethanolamine (EA) solvent. After stirring, the solution was put into the Teflon-lined stainless steel high-pressure reactor, and heated at 140 C for 24 h. Finally, the product was cooled down to room temperature gradually, centrifuged and washed several times with water and ethanol, then dried under vacuum to obtain Ni0.85Se/ MoS2.
Table 1 e The name and amounts of the materials in the synthesis reaction. Material number Ni0.85Se M10 M20 M30 M40 M50
NiCl2$6H2O (g)
Na2SeO3 (g)
MoS2 (g)
At%(Mo/ Ni)
0.238 0.238 0.238 0.238 0.238 0.238
0.845 0.845 0.845 0.845 0.845 0.845
0 0.017 0.04 0.069 0.107 0.16
0 10 20 30 40 50
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Characterization X-ray diffraction (XRD) measurement was carried out using a GBC MMA X-ray diffractometer with Cu Ka radiation. Jobin Yvon Horbia Confocal Micro Raman Spectrometer (Model HR800) was used to record Raman spectra, with a 532 nm diode laser excitation on a 300 lines/mm grating at room temperature. X-ray photoelectron spectroscopy (XPS) data was collected by an ESCALAB 250Xi device. Transmission electron microscopy (TEM) and energy-dispersive spectroscopy (EDS) were respectively performed on a Tecnai G2 F30 instrument and a JSM6510LV instrument. For electrochemical measurements, CHI 750E electrochemical workstation (CH Instrument Company, Shanghai, China) was employed. All tests were carried out using a conventional three-electrode system. The counter electrode is carbon rod and the reference electrode is saturated calomel electrode (SCE). The catalysts prepared above were used to make the working electrode. 5 mg of the catalyst was added to
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0.5 mL of absolute ethanol, and 35 L of Nafion (5%) was added and dispersed. For individual electrode preparation, the ink was drop-casted onto a glass carbon electrode (GCE, diameter 3 mm). The electrode has a geometric area of 0.07 cm2, which was used in the calculation of current densities. 0.5 M H2SO4 was used as the electrolyte solution. All of the potentials we measured were converted to reversible hydrogen electrode (RHE), according to the Nernst equation (ERHE(V) ¼ ESCE þ 0.059 pH þ 0.241). The electrochemical impedance spectroscopy (EIS) was conducted with frequency (Hz) from 1 to 1000000.
Results and discussion Material selection Fig. 1 (A) represents the XRD spectra of (a) M10, (b) M20, (c) M30, (d) M40, (e) M50, (f) Ni0.85Se and (g) MoS2. The peaks at 36.0 , 47.4 , 53.2 , 62.7 , 64.3 , 72.4 (Fig. 1(A)-f), could be
Fig. 1 e (A) XRD patterns of (a) M10, (b) M20, (c) M30, (d) M40 (e) M50, (f) Ni0.85Se and (g) MoS2; (B) Raman spectras of M30; XPS high-resolution spectras of (C) Ni 2p, (D) Mo 3d, (E) Se 3d, and (F) S 2p of M30.
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Fig. 2 e (A) LSV curves of Ni0.85Se, MoS2, M10, M20, M30, M40, M50, and Pt/C; (B) Tafel plots of Ni0.85Se, MoS2, M10, M20, M30, M40, M50, and Pt/C; (C) Nyquist plots and (D) the dotted line diagram of Resistence of Ni0.85Se, MoS2, M10, M20, M30, M40, M50.
indexed to the (101), (102), (110), (103), (201), (202) of Ni0.85Se (JCPDS No.18-0888). While the peaks at 12.6 , 31.2 , 34.6 (Fig. 1(A)-g) were attributed to the (002), (100), (102) lattice plane of MoS2 (JCPDS No.37-1492). Meanwhile, in Fig. 1(A), the positions of the peaks are the same for different composites (M10, M20, M30, M40 and M50), with both Ni0.85Se and MoS2 peaks clearly identifiable, confirming that composites of Ni0.85Se/MoS2 have been successfully synthesized. Therefore, with the increase of MoS2, the peak to the (002) became more and more obvious. But there is a slight shift in the peak positions of the materials, which may be caused by the content of MoS2 [46e48]. Fig. 1(B) presents the Raman spectra of M30. The peaks at 180 cm1 and 240 cm1 could be indexed to NieSe. The other two peaks at 375 cm1 and 405 cm1 could be ascribed to MoeS [49e51]. It demonstrates that the Ni0.85Se/MoS2 has been successfully synthesized [52,53]. To investigate the chemical compositions and valence states of catalysts, the XPS measurements were used in experiment [54,55]. Fig. 1(C)e(F) shows the surface survey XPS data of Ni0.85Se, MoS2 and M30. As shown in Fig. 1(C), the peaks at 870.6 eV and 853.1 eV corresponded to Ni2þ. The Ni 2p 1/2 at 873.7 eV and Ni 2p 3/2 at 855.7 eV may belong to Ni 3 þ from the surface oxide phase [56,57]. The peaks of M30 are slightly deviated from these of Ni0.85Se. The two satellite peaks at 879.7 eV and 860.8 eV are oxidation state of Ni2þ. Mo 3d3/2 at 231.6 eV and Mo 3d5/2 at 228.4 eV are attributed to Mo4þ in Fig. 1(D) [58]. Se 3d3/2 at 54.5 eV and Se 3d5/2 at 54.0 eV could be observed in Fig. 1(E), corresponding to Se2 [41,49]. The other peak at 58.6 eV is characteristic with surface oxidation SeOX.
In Fig. 1(F), S 2P1/2 at 162.5 eV and S 2P3/2 at 161.4 eV are attributed to S2 [59e61]. It indicates the existence of Ni, Mo, Se, S elements in M30. All the data indicate that the composite was successfully prepared. Overpotential is one of the parameters to evaluate the performance of catalytic materials. Under the same current density, smaller overpotential means better catalytic performance of materials [30]. The overpotential can be compared by linear sweep voltammetry (LSV). M30 has the lowest overpotential of 118 mV to drive 10 mA cm2 in Fig. 2 (A). The Tafel slope (i.e. the slope of the applied overpotential h vs. log jA of) derived from LSV is often used to indicate the HER mechanism [62]. In Fig. 2 (B), M30 has the smallest Tafel slope of 49 mV$dec1. Electrochemical impedance spectroscopy (EIS) can prove catalytic kinetics of hydrogen evolution. The inset in Fig. 2(C) demonstrates the electrical equivalent circuit of the impedance that includes the solution resistance (Rs), double layer capacitance (Cdl), charge transfer resistance (Rct) and Warburg impedance (Zw). The semicircle diameter corresponds to a charge transfer resistance (Rct), which can evaluate the charge transfer restriction process. The smaller diameter semicircle represents the lower Rct of HER, meaning higher current density at the same overpotential. Fig. 2 (C)e(D) show the resistance of M30 is minimum with 27.72 U. In brief, Fig. 2 shows that the catalytic performance of M30 is the best among all of the as-prepared materials. These indicate the combination of 30% mole ratio of MoS2 make the performance of Ni0.85Se greatly improve.
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Fig. 3 e (A)TEM images of M30; (B) HRTEM image of M30; Mapping images (C) Ni (D) Mo (E) Se (F) S of M30.
As can be seen from Fig. 3 (A), the material has a nanosheet structure, which may mean more exposure of active sites and active edges [23,41]. Fig. 3(B) is the high-resolution TEM (HRTEM) image of M30. The distance between the lattice fringe is 0.27 nm, which corresponds to the MoS2 (100); while the distance of 0.19 nm corresponds to the Ni0.85Se (102) crystal face. This confirms that the as-synthesized material is the composite of Ni0.85Se and MoS2, which is consistent with the XRD results. Hu and coworkers found that amorphous MoS2 films are particularly HER active and the increased coordinately and structurally unsaturated sulfur atoms led to the enhancement [15]. Fig. 3(C)~(F) respectively represent the distribution of Ni, Mo, Se, and S element in M30. It is clear that all elements are evenly distributed throughout the catalyst M30. Energy Dispersive Spectrometer (EDS) was used to
provide qualitative and quantitative analysis of the element distribution in the microscopic region (as shown in Table S1), showing the synthesis were successfully prepared.
Characterization Fig. 4(A) is the LSV curves for Ni0.85Se, MoS2, M30 and Pt/C. The M30, Ni0.85Se, MoS2 overpotential were 118 mV, 190 mV, 257 mV respectively at 10 mA cm2. Obviously, M30 performs better than Ni0.85Se and MoS2. The enhanced performance of M30 could be attributed to the enlarged specific surface area and the increased active sites compared to Ni0.85Se and MoS2 [29,41]. The HER activities of M30 are compared with that of other as-prepared materials (Table 2) and reported materials in Table S2, in terms of overpotential at the same current
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Fig. 4 e (A) LSV curves; (B) Tafel plots; (C) Nyquist plots with an equivalent circuit (inset); (D) Estimated Cdl and relative electrochemically active surface areas of the materials.(E) i-t testing of M30 under static potential of ¡0.21 V vs. RHE. Inset is the enlargement of the area denoted by the rectangle. (F) LSV curves of M30, after 30 h i-t testing.
density of 10 mA cm2. As can be seen, M30 outperformed all the selenides-based electrocatalysts reported in the literature. Under acidic conditions, HER mechanism is mainly divided into two parts, the first part is electrochemical reaction, also known as Volmer reaction (1), the second part is divided into electrochemical desorption (2-a) and complex desorption (2-b) due to different catalyst properties, the specific reaction formula is as follows [63]. H3 Oþ þ e % m H þ H2 O ðVolmer reactionÞ
(1)
H3 Oþ þ e þ mH%H2 þmþ H2 O ðHeyrovsky reactionÞ
(2-a)
2m H % H2 ðTafel reactionÞ
(2-b)
where e represents an electron, and m-H represents a
hydrogen atom which adsorbed onto the catalyst m. The Tafel equation (h ¼ b log jA þ a, where b is the Tafel slope). As shown in Fig. 4(B), the Tafel slope of M30 is 49 mV$dec1. This indicates that M30 followed the Volmer Heyrovsky reaction in the process of HER reaction [64]. The height of the
Table 2 e HER catalytic performance data of all asprepared catalysts at 10 mA/cm2. Material number M10 M20 M30 M40 M50 Ni0.85Se MoS2
h Tafel slope （mV/ (mV) dec） 180 152 118 136 186 190 257
76 64 49 51 82 57 113
Cdl (mF/ cm2)
Rct (U)
0.72 1.31 1.49 1.44 0.58 0.53 0.29
43.79 36.35 27.72 31.01 47.19 48.24 112.05
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Tafel slope can also be used to evaluate the HER performance of the catalytic material [30]. Fig. 3(B) compares the Tafel slopes of different catalysts, showing 49 mV$dec1, 57 mV$dec1, 113 mV$dec1, and 55 mV$dec1 for M30, Ni0.85Se, MoS2 and Pt/C respectively. It is clear that M30 has the best HER activity among the investigated catalysts. M30 also outperforms other reported electrocatalysts in literatures when comparing the Tafel slope, as shown in Table S2. From Fig. 4(C), the Rct value of M30 is 27.72 U, lower than that of Ni0.85Se (48.24 U) and MoS2 (112.05 U), indicating that M30 has the fastest electron transfer rate [65]. Double layer capacitance Cdl, which can be obtained from the CV analysis, is directly related to the electrochemical active surface area (EASA) and the scanning rate v in a linear relationship (Cdl ¼ v EASA) [66]. Therefore, the values of Cdl can be used to indicate the EASA. As shown in Fig. 4(D), the Cdl of M30 is 1.49 mF/cm2, higher than that of Ni0.85Se (0.53 mF/ cm2) and MoS2 (0.29 mF/cm2) (Fig. S6), suggesting that M30 has the highest EASA thus the highest amount of active sites. In practical applications of hydrogen production, the stability of the electrocatalyst is very important. Chronoamperometry testing (i-t) at 0.21 V was conducted to evaluate the stability of M30, and the result is shown in Fig. 4(E). The current shows a slight drop at the beginning, which may be caused by the consumption of Hþ or the remaining of H2 bubbles on the electrode surface [67]. Meanwhile, from Fig. 4(F), we can find that the Rct of M30 only has a slight increase after 30 h of i-t testing. Moreover, the LSV data of M30 were plotted and shown in Fig. 4(F). Only 6 mV shift at the current density of 10 mA cm2 can be observed. Fig. S7 shows the Nyquist plots of M30 of the initial and after 30 h of i-t testing. Obviously, the impedance barely changed. The above results confirm that M30 has good stability, and can be used as potential electrocatalyst for hydrogen production via water splitting.
Conclusions In this paper, Ni0.85Se, MoS2 and Mx (x ¼ 10, 20, 30, 40, 50) were prepared by hydrothermal method. The experimental results show that the physical performance of Ni0.85Se/MoS2 is different from that of Ni0.85Se, leading to different catalytic performance towards HER in the process of water splitting. Among the different catalysts, M30 has lower overpotential of 118 mV to drive 10 mA$cm2 than these of Ni0.85Se (190 mV), MoS2 (257 mV), and smaller Tafel slope of 49 mV$dec1 than these of Ni0.85Se (57 mV$dec1), MoS2 (113 mV$dec1). This may be because the introduction of MoS2 leads to more exposure to active sites, increasing EASA and conductivity. These together can give accelerated electron transfer rate in HER process, resulting in both high catalytic activity and stability.
Acknowledgements We acknowledge financial support from the National Natural Science Foundation of China (Grant No. 21205030), the Key
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Project of Hubei Provincial Education Department (D20171001), Hubei Key Laboratory of Ferro & Piezoelectric Materials and Devices (201710), Hubei Key Laboratory for Processing and Application of Catalytic Materials (201829303), and (111 project, B12015).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.08.118.
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