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Nickel-doped MoSe2 nanosheets with Ni–Se bond for alkaline electrocatalytic hydrogen evolution
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 5 ( 2 0 2 0 ) 1 0 7 2 4 e1 0 7 2 8
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Short Communication
Nickel-doped MoSe2 nanosheets with NieSe bond for alkaline electrocatalytic hydrogen evolution Yaqian Yang, Xu Zhao, Han Mao, Rui Ning, Xiaohang Zheng*, Jiehe Sui, Wei Cai** School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, 150001, China
highlights In synthesized process, ethanol makes Ni source evenly disperse in MoSe2 nanosheets. The Ni was doped in the inert planes of MoSe2, which increase active sites. The Ni was firmly adhered in MoSe2 nanosheets for the formation of NieSe bond.
article info
abstract
Article history:
Molybdenum diselenide (MoSe2) is a potential catalytic material for the electrocatalytic
Received 15 October 2019
hydrogen evolution reaction (HER). However, due to the low density of its active sites,
Received in revised form
MoSe2 nanosheets feature high overpotential in HER, which limits its practical application.
27 December 2019
This describes the method of doping the Ni in MoSe2 nanosheets to increase active sites.
Accepted 28 December 2019
The NiO2 evenly dispersed on MoSe2 by ethanol solution reduces to ~4 nm Ni nanoclusters
Available online 28 February 2020
under annealing process, which is firmly adhered to MoSe2 nanosheets with NieSe bond. The electrochemical active surface area of Ni-doped MoSe2 expands, proving that Ni
Keywords:
dopants produce more activity sites in MoSe2 nanosheets. The overpotential of MoSe2 (at
Hydrogen evolution reaction
10 mA cm2) decreases from 335 mV to 181 mV with 4.5 at.% Ni doped in 1 M KOH. The Ni
Molybdenum diselenide
eMoSe2 also characterizes excellent stability for 12 h with the formation of NieSe bond.
Doping Ni
The study of doping Ni in MoSe2 nanosheets is of great guiding significance to the design
Electrocatalysts
and production of non-noble electrocatalysts for HER in alkaline media.
Alkaline media
© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction With the depletion and pollution of traditional fossil energy, hydrogen has become one of the most promising substitutions for fossil energy. Among all the hydrogen production methods,
electrocatalytic water splitting is one of the most efficient and renewable strategies [1e3]. Considering the conversion efficiency of electricity to hydrogen, it is vital to choose appropriate catalytic material for HER. The noble metals such as Pt exhibits high catalytic activity for HER due to their zeroapproaching the Gibbs free energy for hydrogen absorption
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (X. Zheng), [email protected] (W. Cai). https://doi.org/10.1016/j.ijhydene.2019.12.212 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 5 ( 2 0 2 0 ) 1 0 7 2 4 e1 0 7 2 8
(DGH*z0), but their scarcity and high costs hinder the mass commercialization of such catalysts [4]. Therefore, it is urgent to design non-noble catalysts for HER. In the past few years, 2D transition metal dichalcogenides (TMDs) have emerged as potential electrocatalysts to replace Pt for that theoretical DGH* on its active sites is similar to Pt, particularly MX2 (M ¼ Mo or W, X ¼ S or Se) [5e7]. Among these popular MX2 materials, MoSe2 shows more metallic in nature and higher electrical conductivity compared to the general MoS2 (2HeMoS2) [8]. Gholanyand et al. [9] concluded that the best candidate is MoSe2 for its both minimum overpotential and maximum Tafel slope in a comparative study of these TMDs [10]. However, the MoSe2 nanosheets have large proportion of inert planes and its catalytic activity mainly comes from the unsaturated Se atoms on edge sites[11]. The low density of active sites hinders the electrocatalytic activity of MoSe2. Doping with heteroatoms can increase the density of activities leading to a higher electrocatalytic activity. Kuraganti et al. prepared Mn-doped MoSe2 [12] and Qian et al. synthesized Zn-doped MoSe2 catalysts [13], which confirmed the lower overpotential compared with MoSe2 in acidic media for more active sites produced by the dopants. For the more, DFT calculation showed that the new catalytic sites were produced at the dopant atoms or the adjacent Mo- or Seatoms [14e17]. Although the acidic condition can promote the reaction to some extent, acidic electrolyzers and electrodes are susceptible to corrosion. The alkaline electrocatalytic reactions have a potential for long-lifetime and practical applications [18]. However, the doping some heteroatoms such as Zn is not suitable to alkaline media. Since the catalytic efficiency is related to the contents of Hþ in media, the lack of Hþ in alkaline condition causes low catalytic efficiency. Thus, alkaline catalysts need the ability to improve water
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dissociation. Sargent et al. [19] prepared catalysts with Ni doped onto the Cu surface and verified that Ni sites have strong bonding energies for hydrogen and hydroxyl groups. Theoretical calculations further confirmed that Ni sites have an excellent ability to promote HOeH bond cleavage in alkaline media [20]. Herein, we prepare Ni-doped MoSe2 nanosheets with NieSe bond through hydrothermal method and subsequent annealing treatment under a reducing atmosphere. During the synthesized process, we used ethanol solution to make NiCl4 evenly distribute in MoSe2 nanosheets, which converted to NiO2 during drying. After that, the annealing atmosphere made NiO2 reduced and formed 4e5 nm Ni nanoclusters with high activity, which were firmly adhered to in MoSe2 nanosheets under the formation of NieSe bonds. The Ni-doped MoSe2 (4.5 at.%) shows good catalytic performance with large electrochemically active surface area and lower charge transfer resistance. In alkaline media, the 4.5 at.% NieMoSe2 has a low overpotential of 184 mV to produce 10 mA cm2 and excellent long-term stability for 12 h.
Results and discussion The XRD, Raman spectroscopy and XPS of Fig. 1 are conducted to evaluate NieMoSe2 and undoped MoSe2 nanosheets the detailed crystal structure. (All of the following marks NieMoSe2 represents 4.5 at.% Ni-doped MoSe2.) As shown in Fig. 1a and Fig. S2, the XRD patterns of Ni-doped MoSe2 and pure MoSe2 reveal that all of the samples have the 2H-hexagonal crystal structure, showing the typical Bragg reflections of MoSe2 (JCPDS No. 29e0914). The peak located at 44.5 of NieMoSe2 is attributed to the (111) lattice plane for Ni (JCPDS No. 87-0712). When the contents of NiCl4 increases to 10 wt%, the peak of Ni
Fig. 1 e (a)XRD patterns of pristine MoSe2 and NieMoSe2. (b) Raman spectroscopy of pristine MoSe2 and NieMoSe2. XPS spectra of pristine MoSe2 and NieMoSe2: (c) Full spectrum of NieMoSe2. (d) Mo 3d and (e) Se 2s XPS spectra for MoSe2 and NieMoSe2. (f) XPS analyses of Ni 2p regions for NieMoSe2.
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appears (Fig. S2). In Fig. 1b, the characteristic signatures of Raman peaks at 238.7 cm1 and 284.8 cm1 are respectively attributed to the A1g and E12g modes of MoSe2 without other identifiable peaks in NieMoSe2 [21]. As shown in Fig. 1cef, XPS is further carried out to confirm the chemical composition and valence states of the elements in pristine MoSe2 and NieMoSe2. All peak positions are calibrated using the C-1s standard position (284.8 eV). The discovery of the Ni, Mo and Se elements in the full spectrum (Fig. 1c) is in good agreement with elemental mapping results (Fig. 2c). From Fig. 1d and e, the highresolution XPS results show the presence of Mo 3d and Se 3d. Fig. 3d shows a high resolution Se 3d XPS spectrum, which can be fitted to the Se 3d5/2 and Se 3d3/2 centered at 54.18 eV and 55.13 eV in MoSe2 [22]. In Fig. 1e, the high-resolution Mo 3d spectrum of MoSe2 could be divided into two bands (228.7 eV and 231.8 eV), ascribed to Mo 3d5/2 and Mo 3d3/2 respectively. Compared with the pristine MoSe2, the peaks of Se 3d and Mo 3d in NieMoSe2 are slightly shifted to higher binding energies. The shift of bonding energy of Mo 3d and Se 3d may be caused by the doping of Ni. In the high-resolution Mo 3d spectrum, another peak located 230.1 eV is the Se 3s which is probably caused by NieSe bonds [23]. High-resolution Ni 2p spectra of NieMoSe2 are presented in Fig. 1f. The XPS spectrum of Ni 2p shows two obvious scopes corresponding to Ni 2p3/2 and Ni 2p1/ 2. The 2p3/2 and 2p1/2 peaks are resolved into three bands which are labeled with S1 (854.5 eV and 871.6 eV), S2 (856.6 eV and 874.5 eV) and satellite peaks (862.3 eV and 880.5 eV). The S1 peaks located at 854.4 eV and 871.6 eV are most likely originated from Nidþ ions which contact different oxidation states. The gap in the bonding energy between the dominant peaks of Ni 2p3/2 and Ni 2p1/2 is 17.9 eV, confirming the oxidation state of Ni. The existence of Nidþ ions suggests the formation of NieSe
bonding which may be caused by the remove of Se and the substitute of Ni and the reduced valence state of Ni which could facilitate the proton reduction for H2 generation [24]. Another characteristic peak located at 854.5 eV could be indexed to oxidized Ni due to surface oxidation [25]. The SEM and TEM images of MoSe2 and NieMoSe2 are shown in Fig. 2. Both pristine MoSe2 and NieMoSe2 have the same nanoflakes morphology, which is staked by thin nanosheets to form fluffy agglomerates with a flower-like morphology. After dispersing in ethanol solution and postannealing at 300 C in H2/Ar2 treatment, Ni is symmetrically doped in MoSe2 corresponding to elemental mapping images, as shown in Fig. 2c. Further insights into the morphology of NieMoSe2 are obtained by TEM and HRTEM images (Fig. 2def and Fig. S3). Compared with the HRTEM images of pristine MoSe2, the lattice fringes of NieMoSe2 are more disorderly and abundant defects appear on MoSe2 nanoflakes, labeled in Fig. 2d and e. Interestingly, HRTEM image (Fig. 2d) exhibits the layered crystalline structure of MoSe2 nanoflakes with the large interlayers spacing about 0.71 nm, suggesting an expansion in d-spacing of (002) peak of NieMoSe2 compared with pristine MoSe2(0.67 nm). To further validated by XRD, the (002) peak for NieMoSe2 could be found to be slightly downshifted as compared to MoSe2 (Fig. 1a). The expansion in (002) inter-planar spacing could be devoted to the jailing of Ni in MoSe2 nanosheets under annealing process. Moreover, the HRTEM image of NieMoSe2 in Fig. 2d also obviously displays the crystal planes where the lattice fringes with an interplanar distance of 0.2 nm have corresponded to the (111) plane of Ni. The earth of Ni also is shown in Fig. 2f, with an inter-layer distance of 0.17 nm corresponding to the (200) of Ni and ~4 nm Ni nanoclusters.
Fig. 2 e (a) SEM image of pristine MoSe2. (b) SEM image of NieMoSe2. (c) SEM images of elemental mapping images of NieMoSe2. (d),(e) and (f) HRTEM images of NieMoSe2.
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Fig. 3 e HER performance of MoSe2 and NieMoSe2 catalysts in 1 M KOH. (a) Polarization curves. (b) the corresponding Tafel plots of obtained sample for MoSe2, NieMoSe2 and Pt/C. (c) EIS Nyquist plots of MoSe2 and NieMoSe2. (d) Double-layer capacity currents vs. scan rates of for MoSe2 and NieMoSe2. (e) Time-dependent current density curve of NieMoSe2. (f) After 12 h cycle, SEM images of elemental mapping images of NieMoSe2.
The electrochemical catalytic properties of the samples modified on the glassy carbon electrode (3 mm in diameter) are measured by using a three-electrode system in N2-saturated 1 M KOH. The HER performances of different Ni contents samples are shown in Fig. S3. Fig. 3a shows the polarization curves of undoped MoSe2, NieMoSe2 and Pt/C, and the data are all iRcorrected. The NieMoSe2 (4.5 at.%) shows the best catalytic properties in different Ni contents samples (Fig. S4a) with the best overpotential of 184 mV to achieve a current density of 10 mA cm2, which is significantly lower than that of the original MoSe2 nanosheets (335 mV). The decreasing of overpotential of Ni-doped MoSe2 may ascribe to the water dissociation ability of Ni dopants [26,27] and new active sites produced by Ni dopants or at the adjacent Mo- or Se-atoms [28,29]. The HER kinetics of the electrocatalysts are determined by the Tafel slopes obtained from their homologous polarization curves. Fig. 3b and Fig. S4b display the iR-corrected Tafel plots, overpotential versus log (-j). From the Tafel slope values, the NieMoSe2 reduces the Tafel slope from 118 mV dec1 to 83 mV dec1 and Tafel results clearly show that doping Ni enhances the HER activity of MoSe2. According to the three principal steps involved in the hydrogen evolution (Volmer reaction, Tafel slope 120 mV dec1; Heyrovsky reaction, Tafel slope 40 mV dec1; Tafel reaction, Tafel slope 30 mV dec1) [30], the hydrogen evolution reaction on the NieMoSe2 electrode follows the Heyrovsky-Volmer mechanism which was depended on the electrochemical desorption step [31]. Shown in Fig. 3c and Fig. S4c, electrochemical impedance spectroscopy (EIS) is further used to reflect the fast charge transfer and the HER kinetics at the interface between the electrode and electrolyte of MoSe2 and Ni-doped MoSe2. The NieMoSe2 shows the lowest Rct values and suggests a lower charge transfer resistance, which improves HER performance
and leads to a fast electron transfer rate at the interfaces of electrode and electrolyte. In addition, the electrochemical double-layer capacitance (Cdl) has been proven to be an effective parameter to evaluate the electrochemically active surface area (ECSA) of the catalysts. From Fig. 3d, the Cdl of NieMoSe2 (4.5 at.%) is ~21 mF/cm2, which is three times of that of MoSe2, indicating that 4.5 at.% NieMoSe2 has high exposure of effective active sites compared with MoSe2. As shown in Fig. S4d, the ECSA of the lower doping contents and undoped MoSe2 have slight changes, which may ascribe that some Ni dopants occupy edge sites of MoSe2 [32] and another Ni dopants produce some new active sites. To assess the stability of NieMoSe2, the continuous i-t test is conducted. As shown in Fig.3e and Fig. S6, the current density of NieMoSe2 has slightly changed and retains good stability after 12 h, which shows good stability compared with MoSe2. After the stability test of the NieMoSe2, Ni dopants are still firmly adhered to in nanosheets as shown in Fig. 3f. Fig. S7 shows the XRD patterns of NieMoSe2 before and after the reactions, which is to further verify the states of Ni doping and MoSe2 in the NieMoSe2 after HER. The phase of MoSe2 has no changes and the (111) plane of Ni disappears after HER. The disappearing of Ni may be contributed to the formation of NiOOH at the process of CV activation which provides the actives sites to decrease the overpotential [33]. The 4.5 at.% NieMoSe2 has low overpotential and excellent stability. The low overpotential and large ESCA may devote to the increasing of active sites, which are produced by Ni dopants and defects. Ni dopants and defects can be observed in HRTEM images. The Ni dopants are deposited in MoSe2 nanosheets in the form of nanoclusters and the defects are produced in H2 annealing atmospheres. The excellent stability may ascribe to the NieSe bonds. The strong chemical bonds
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make the Ni dopants firmly adhered to MoSe2 nanosheets after a long-time cycle.
Conclusion In summary, NieMoSe2 was successfully prepared through the hydrothermal and following the annealing process. The Ni dopants were doped in MoSe2 nanosheets as the formation of Ni nanoclusters. In alkaline media, the Ni-doped MoSe2 with 4.5 at.% contents shows the best electrocatalytic ability with low overpotential and excellent stability. The 4.5 at.% NieMoSe2 has an overpotential of 184 mV to produce 10 mA cm2 with a small Tafel slope of 83 mV dec1, which is significantly lower than that of the original MoSe2 (335 mV). The catalyst also shows excellent stability and a low transfer resistance. The catalytic ability of NieMoSe2 has improved, which is attributed to the increase in active sites. This study of Ni-doped MoSe2 may broaden our horizons in the design of no-noble electrocatalysts in alkaline media.
Acknowledgements This work is financially supported by the National Natural Science Foundation of China (No.51971083 and No.51731005).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.12.212.
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[1] Xie J, Zhang H, Li S, Wang R, Sun X, Zhou M, Zhou J, Lou XW, Xie Y. Adv Mater 2013;25:5807e13. [2] An L, Huang L, Zhou P, Yin J, Liu H, Xi P. Adv Funct Mater 2015;25:6814e22. [3] Zhou W, Lu J, Zhou K, Yang L, Ke Y, Tang Z, Chen S. Nanomater Energy 2016;28:143e50. [4] Choi Y-H, Lee J, Parija A, Cho J, Verkhoturov SV, AlHashimi M, Fang L, Banerjee S. ACS Catal 2016;6:6246e54. [5] Hinnemann B, Moses PG, Bonde J, Jørgensen KP, Nielsen JH, Horch S, Chorkendorff I, Nørskov JK. J Am Chem Soc 2005;127:5308e9. [6] Yu Y, Nam GH, He Q, Wu XJ, Zhang K, Yang Z, Chen J, Ma Q, Zhao M, Liu Z, Ran FR, Wang X, Li H, Huang X, Li B, Xiong Q, Zhang Q, Liu Z, Gu L, Du Y, Huang W, Zhang H. Nat Chem 2018;10:638e43.
[7] Lin J, Yan Y, Li C, Si X, Wang H, Qi J, Cao J, Zhong Z, Fei W, Feng J. Nano-Micro Lett 2019;11:55. [8] Tang H, Dou KP, Kaun CC, Kuang Q, Yang SH. J Mater Chem A 2014;2:360e4. [9] Gholamvand Z, McAteer D, Backes C, McEvoy N, Harvey A, Berner NC, et al. Nanoscale 2016;8:5737e49. [10] Ali Eftekhari. Int J Hydrogen Energy 2017;42:11053e77. [11] Huang Y, Miao YE, Fu J, Mo S, Wei C, Liu T. J Mater Chem A 2015;3:16263e71. [12] Vasu K, Akash J, Ronen B, Ashwin R, Maya B. ACS Appl Mater Interfaces 2019;11:25155e62. [13] Qian J, Wang T, Xia B, Xi P, Gao D. Electrochim Acta 2019;296:701e8. [14] Gao D, Xia B, Zhu C, Du Y, Xi P, Xue D, Ding J, Wang. J Mater Chem A 2018;6:510e5. [15] Wang H, Tsai C, Kong D, Chan K, Abild-Pedersen F, Nørskov JK, Cui Y. Nano Res 2015;8:566e75. [16] Deng J, Li H, Xiao J, Tu Y, Deng D, Yang H, Tian H, Li J, Ren P, Bao X. Energy Environ Sci 2015;8:1594e601. [17] Zhu Y, Zhang J. Superlattice Microst 2018;117:155e62. [18] Li S, Cheng C, Sagaltchik A, Pachfule P, Zhao C, Thomas A. Adv Funct Mater 2019;29:1807419. [19] Dinh CT, Jain A, de Arquer FPG, De Luna P, Li J, Wang N, Zheng X, Cai J, Gregory BZ, Voznyy O, Zhang B, Liu M, Sinton D, Crumlin EJ, Sargent EH. Nat Energy 2019;4:107e14. [20] Lu S, Zhang L, Dong Y, Zhang J, Yan X, Sun D, Shang X, Chi J, Chai Y, Dong B. J Mater Chem A 2019;7:16859e66. [21] Yin Y, Zhang Y, Gao T, Yao T, Zhang X, Han J, Wang X, Zhang Z, Xu P, Zhang P, Cao X, Song B, Ji S. Adv Mater 2017;29:1700311. [22] Zhang L, Wang T, Sun L, Sun Y, Hu T, Xu K, Ma F. J Mater Chem A 2017;5:19752e9. [23] Matoba M, Anzai S, Fujimori A. J Phys Soc Jpn 1991;60(12):4230. [24] Wang Q, Zhao Z, Dong S, He D, Lawrence MJ, Han S, Cai C, Xiang S, Rodriguez P, Xiang B, Wang Z, Liang Y, Gu M. Nanomater Energy 2018;53:458e67. [25] Kong C, Min S, Lu G. ACS Catal 2014;4(8):2763e9. [26] Martinez LM, Delgado JA, Saiz CL, Cosio A, Wu Y, Villagran D, Gandha K, Karthik C, Nlebedim IC, Singamaneni SR. J Appl Phys 2018;124:153903. [27] Song X, Chen H, Huang C, Qin Y, Li H. Electrochim Acta 2018;283:121e31. [28] Zhang J, Wang T, Liu P, Liu S, Dong R, Zhuang X, Chen M, Feng X. Energy Environ Sci 2016;9:2789e93. [29] Zhong Y, Chang B, Shao Y, Xu C, Wu Y, Hao X. ChemSusChem 2019;12:2008e14. [30] Zhu Y, Chen G, Xu X, Yang G, Liu M, Shao Z. ACS Catal 2017;7:3540e7. [31] Zhu Y, Chen H, Hsu C, Lin T, Chang C, Chang S, Tsai L, Ming Chen H. ACS Energy Lett 2019;4:987e94. [32] Shi Y, Zhou Y, Yang D-R, Xu W-X, Wang C, Wang F-B, Xu J-J, Xia X-H, Chen H-Y. J Am Chem Soc 2017;139:15479e85. [33] Zhang Q, Zhang C, Liang J, Yin P, Tian Y. ACS Sustainable Chem Eng 2017;5:3808e18.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Short Communication
Nickel-doped MoSe2 nanosheets with NieSe bond for alkaline electrocatalytic hydrogen evolution Yaqian Yang, Xu Zhao, Han Mao, Rui Ning, Xiaohang Zheng*, Jiehe Sui, Wei Cai** School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, 150001, China
highlights In synthesized process, ethanol makes Ni source evenly disperse in MoSe2 nanosheets. The Ni was doped in the inert planes of MoSe2, which increase active sites. The Ni was firmly adhered in MoSe2 nanosheets for the formation of NieSe bond.
article info
abstract
Article history:
Molybdenum diselenide (MoSe2) is a potential catalytic material for the electrocatalytic
Received 15 October 2019
hydrogen evolution reaction (HER). However, due to the low density of its active sites,
Received in revised form
MoSe2 nanosheets feature high overpotential in HER, which limits its practical application.
27 December 2019
This describes the method of doping the Ni in MoSe2 nanosheets to increase active sites.
Accepted 28 December 2019
The NiO2 evenly dispersed on MoSe2 by ethanol solution reduces to ~4 nm Ni nanoclusters
Available online 28 February 2020
under annealing process, which is firmly adhered to MoSe2 nanosheets with NieSe bond. The electrochemical active surface area of Ni-doped MoSe2 expands, proving that Ni
Keywords:
dopants produce more activity sites in MoSe2 nanosheets. The overpotential of MoSe2 (at
Hydrogen evolution reaction
10 mA cm2) decreases from 335 mV to 181 mV with 4.5 at.% Ni doped in 1 M KOH. The Ni
Molybdenum diselenide
eMoSe2 also characterizes excellent stability for 12 h with the formation of NieSe bond.
Doping Ni
The study of doping Ni in MoSe2 nanosheets is of great guiding significance to the design
Electrocatalysts
and production of non-noble electrocatalysts for HER in alkaline media.
Alkaline media
© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction With the depletion and pollution of traditional fossil energy, hydrogen has become one of the most promising substitutions for fossil energy. Among all the hydrogen production methods,
electrocatalytic water splitting is one of the most efficient and renewable strategies [1e3]. Considering the conversion efficiency of electricity to hydrogen, it is vital to choose appropriate catalytic material for HER. The noble metals such as Pt exhibits high catalytic activity for HER due to their zeroapproaching the Gibbs free energy for hydrogen absorption
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (X. Zheng), [email protected] (W. Cai). https://doi.org/10.1016/j.ijhydene.2019.12.212 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 5 ( 2 0 2 0 ) 1 0 7 2 4 e1 0 7 2 8
(DGH*z0), but their scarcity and high costs hinder the mass commercialization of such catalysts [4]. Therefore, it is urgent to design non-noble catalysts for HER. In the past few years, 2D transition metal dichalcogenides (TMDs) have emerged as potential electrocatalysts to replace Pt for that theoretical DGH* on its active sites is similar to Pt, particularly MX2 (M ¼ Mo or W, X ¼ S or Se) [5e7]. Among these popular MX2 materials, MoSe2 shows more metallic in nature and higher electrical conductivity compared to the general MoS2 (2HeMoS2) [8]. Gholanyand et al. [9] concluded that the best candidate is MoSe2 for its both minimum overpotential and maximum Tafel slope in a comparative study of these TMDs [10]. However, the MoSe2 nanosheets have large proportion of inert planes and its catalytic activity mainly comes from the unsaturated Se atoms on edge sites[11]. The low density of active sites hinders the electrocatalytic activity of MoSe2. Doping with heteroatoms can increase the density of activities leading to a higher electrocatalytic activity. Kuraganti et al. prepared Mn-doped MoSe2 [12] and Qian et al. synthesized Zn-doped MoSe2 catalysts [13], which confirmed the lower overpotential compared with MoSe2 in acidic media for more active sites produced by the dopants. For the more, DFT calculation showed that the new catalytic sites were produced at the dopant atoms or the adjacent Mo- or Seatoms [14e17]. Although the acidic condition can promote the reaction to some extent, acidic electrolyzers and electrodes are susceptible to corrosion. The alkaline electrocatalytic reactions have a potential for long-lifetime and practical applications [18]. However, the doping some heteroatoms such as Zn is not suitable to alkaline media. Since the catalytic efficiency is related to the contents of Hþ in media, the lack of Hþ in alkaline condition causes low catalytic efficiency. Thus, alkaline catalysts need the ability to improve water
10725
dissociation. Sargent et al. [19] prepared catalysts with Ni doped onto the Cu surface and verified that Ni sites have strong bonding energies for hydrogen and hydroxyl groups. Theoretical calculations further confirmed that Ni sites have an excellent ability to promote HOeH bond cleavage in alkaline media [20]. Herein, we prepare Ni-doped MoSe2 nanosheets with NieSe bond through hydrothermal method and subsequent annealing treatment under a reducing atmosphere. During the synthesized process, we used ethanol solution to make NiCl4 evenly distribute in MoSe2 nanosheets, which converted to NiO2 during drying. After that, the annealing atmosphere made NiO2 reduced and formed 4e5 nm Ni nanoclusters with high activity, which were firmly adhered to in MoSe2 nanosheets under the formation of NieSe bonds. The Ni-doped MoSe2 (4.5 at.%) shows good catalytic performance with large electrochemically active surface area and lower charge transfer resistance. In alkaline media, the 4.5 at.% NieMoSe2 has a low overpotential of 184 mV to produce 10 mA cm2 and excellent long-term stability for 12 h.
Results and discussion The XRD, Raman spectroscopy and XPS of Fig. 1 are conducted to evaluate NieMoSe2 and undoped MoSe2 nanosheets the detailed crystal structure. (All of the following marks NieMoSe2 represents 4.5 at.% Ni-doped MoSe2.) As shown in Fig. 1a and Fig. S2, the XRD patterns of Ni-doped MoSe2 and pure MoSe2 reveal that all of the samples have the 2H-hexagonal crystal structure, showing the typical Bragg reflections of MoSe2 (JCPDS No. 29e0914). The peak located at 44.5 of NieMoSe2 is attributed to the (111) lattice plane for Ni (JCPDS No. 87-0712). When the contents of NiCl4 increases to 10 wt%, the peak of Ni
Fig. 1 e (a)XRD patterns of pristine MoSe2 and NieMoSe2. (b) Raman spectroscopy of pristine MoSe2 and NieMoSe2. XPS spectra of pristine MoSe2 and NieMoSe2: (c) Full spectrum of NieMoSe2. (d) Mo 3d and (e) Se 2s XPS spectra for MoSe2 and NieMoSe2. (f) XPS analyses of Ni 2p regions for NieMoSe2.
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appears (Fig. S2). In Fig. 1b, the characteristic signatures of Raman peaks at 238.7 cm1 and 284.8 cm1 are respectively attributed to the A1g and E12g modes of MoSe2 without other identifiable peaks in NieMoSe2 [21]. As shown in Fig. 1cef, XPS is further carried out to confirm the chemical composition and valence states of the elements in pristine MoSe2 and NieMoSe2. All peak positions are calibrated using the C-1s standard position (284.8 eV). The discovery of the Ni, Mo and Se elements in the full spectrum (Fig. 1c) is in good agreement with elemental mapping results (Fig. 2c). From Fig. 1d and e, the highresolution XPS results show the presence of Mo 3d and Se 3d. Fig. 3d shows a high resolution Se 3d XPS spectrum, which can be fitted to the Se 3d5/2 and Se 3d3/2 centered at 54.18 eV and 55.13 eV in MoSe2 [22]. In Fig. 1e, the high-resolution Mo 3d spectrum of MoSe2 could be divided into two bands (228.7 eV and 231.8 eV), ascribed to Mo 3d5/2 and Mo 3d3/2 respectively. Compared with the pristine MoSe2, the peaks of Se 3d and Mo 3d in NieMoSe2 are slightly shifted to higher binding energies. The shift of bonding energy of Mo 3d and Se 3d may be caused by the doping of Ni. In the high-resolution Mo 3d spectrum, another peak located 230.1 eV is the Se 3s which is probably caused by NieSe bonds [23]. High-resolution Ni 2p spectra of NieMoSe2 are presented in Fig. 1f. The XPS spectrum of Ni 2p shows two obvious scopes corresponding to Ni 2p3/2 and Ni 2p1/ 2. The 2p3/2 and 2p1/2 peaks are resolved into three bands which are labeled with S1 (854.5 eV and 871.6 eV), S2 (856.6 eV and 874.5 eV) and satellite peaks (862.3 eV and 880.5 eV). The S1 peaks located at 854.4 eV and 871.6 eV are most likely originated from Nidþ ions which contact different oxidation states. The gap in the bonding energy between the dominant peaks of Ni 2p3/2 and Ni 2p1/2 is 17.9 eV, confirming the oxidation state of Ni. The existence of Nidþ ions suggests the formation of NieSe
bonding which may be caused by the remove of Se and the substitute of Ni and the reduced valence state of Ni which could facilitate the proton reduction for H2 generation [24]. Another characteristic peak located at 854.5 eV could be indexed to oxidized Ni due to surface oxidation [25]. The SEM and TEM images of MoSe2 and NieMoSe2 are shown in Fig. 2. Both pristine MoSe2 and NieMoSe2 have the same nanoflakes morphology, which is staked by thin nanosheets to form fluffy agglomerates with a flower-like morphology. After dispersing in ethanol solution and postannealing at 300 C in H2/Ar2 treatment, Ni is symmetrically doped in MoSe2 corresponding to elemental mapping images, as shown in Fig. 2c. Further insights into the morphology of NieMoSe2 are obtained by TEM and HRTEM images (Fig. 2def and Fig. S3). Compared with the HRTEM images of pristine MoSe2, the lattice fringes of NieMoSe2 are more disorderly and abundant defects appear on MoSe2 nanoflakes, labeled in Fig. 2d and e. Interestingly, HRTEM image (Fig. 2d) exhibits the layered crystalline structure of MoSe2 nanoflakes with the large interlayers spacing about 0.71 nm, suggesting an expansion in d-spacing of (002) peak of NieMoSe2 compared with pristine MoSe2(0.67 nm). To further validated by XRD, the (002) peak for NieMoSe2 could be found to be slightly downshifted as compared to MoSe2 (Fig. 1a). The expansion in (002) inter-planar spacing could be devoted to the jailing of Ni in MoSe2 nanosheets under annealing process. Moreover, the HRTEM image of NieMoSe2 in Fig. 2d also obviously displays the crystal planes where the lattice fringes with an interplanar distance of 0.2 nm have corresponded to the (111) plane of Ni. The earth of Ni also is shown in Fig. 2f, with an inter-layer distance of 0.17 nm corresponding to the (200) of Ni and ~4 nm Ni nanoclusters.
Fig. 2 e (a) SEM image of pristine MoSe2. (b) SEM image of NieMoSe2. (c) SEM images of elemental mapping images of NieMoSe2. (d),(e) and (f) HRTEM images of NieMoSe2.
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Fig. 3 e HER performance of MoSe2 and NieMoSe2 catalysts in 1 M KOH. (a) Polarization curves. (b) the corresponding Tafel plots of obtained sample for MoSe2, NieMoSe2 and Pt/C. (c) EIS Nyquist plots of MoSe2 and NieMoSe2. (d) Double-layer capacity currents vs. scan rates of for MoSe2 and NieMoSe2. (e) Time-dependent current density curve of NieMoSe2. (f) After 12 h cycle, SEM images of elemental mapping images of NieMoSe2.
The electrochemical catalytic properties of the samples modified on the glassy carbon electrode (3 mm in diameter) are measured by using a three-electrode system in N2-saturated 1 M KOH. The HER performances of different Ni contents samples are shown in Fig. S3. Fig. 3a shows the polarization curves of undoped MoSe2, NieMoSe2 and Pt/C, and the data are all iRcorrected. The NieMoSe2 (4.5 at.%) shows the best catalytic properties in different Ni contents samples (Fig. S4a) with the best overpotential of 184 mV to achieve a current density of 10 mA cm2, which is significantly lower than that of the original MoSe2 nanosheets (335 mV). The decreasing of overpotential of Ni-doped MoSe2 may ascribe to the water dissociation ability of Ni dopants [26,27] and new active sites produced by Ni dopants or at the adjacent Mo- or Se-atoms [28,29]. The HER kinetics of the electrocatalysts are determined by the Tafel slopes obtained from their homologous polarization curves. Fig. 3b and Fig. S4b display the iR-corrected Tafel plots, overpotential versus log (-j). From the Tafel slope values, the NieMoSe2 reduces the Tafel slope from 118 mV dec1 to 83 mV dec1 and Tafel results clearly show that doping Ni enhances the HER activity of MoSe2. According to the three principal steps involved in the hydrogen evolution (Volmer reaction, Tafel slope 120 mV dec1; Heyrovsky reaction, Tafel slope 40 mV dec1; Tafel reaction, Tafel slope 30 mV dec1) [30], the hydrogen evolution reaction on the NieMoSe2 electrode follows the Heyrovsky-Volmer mechanism which was depended on the electrochemical desorption step [31]. Shown in Fig. 3c and Fig. S4c, electrochemical impedance spectroscopy (EIS) is further used to reflect the fast charge transfer and the HER kinetics at the interface between the electrode and electrolyte of MoSe2 and Ni-doped MoSe2. The NieMoSe2 shows the lowest Rct values and suggests a lower charge transfer resistance, which improves HER performance
and leads to a fast electron transfer rate at the interfaces of electrode and electrolyte. In addition, the electrochemical double-layer capacitance (Cdl) has been proven to be an effective parameter to evaluate the electrochemically active surface area (ECSA) of the catalysts. From Fig. 3d, the Cdl of NieMoSe2 (4.5 at.%) is ~21 mF/cm2, which is three times of that of MoSe2, indicating that 4.5 at.% NieMoSe2 has high exposure of effective active sites compared with MoSe2. As shown in Fig. S4d, the ECSA of the lower doping contents and undoped MoSe2 have slight changes, which may ascribe that some Ni dopants occupy edge sites of MoSe2 [32] and another Ni dopants produce some new active sites. To assess the stability of NieMoSe2, the continuous i-t test is conducted. As shown in Fig.3e and Fig. S6, the current density of NieMoSe2 has slightly changed and retains good stability after 12 h, which shows good stability compared with MoSe2. After the stability test of the NieMoSe2, Ni dopants are still firmly adhered to in nanosheets as shown in Fig. 3f. Fig. S7 shows the XRD patterns of NieMoSe2 before and after the reactions, which is to further verify the states of Ni doping and MoSe2 in the NieMoSe2 after HER. The phase of MoSe2 has no changes and the (111) plane of Ni disappears after HER. The disappearing of Ni may be contributed to the formation of NiOOH at the process of CV activation which provides the actives sites to decrease the overpotential [33]. The 4.5 at.% NieMoSe2 has low overpotential and excellent stability. The low overpotential and large ESCA may devote to the increasing of active sites, which are produced by Ni dopants and defects. Ni dopants and defects can be observed in HRTEM images. The Ni dopants are deposited in MoSe2 nanosheets in the form of nanoclusters and the defects are produced in H2 annealing atmospheres. The excellent stability may ascribe to the NieSe bonds. The strong chemical bonds
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make the Ni dopants firmly adhered to MoSe2 nanosheets after a long-time cycle.
Conclusion In summary, NieMoSe2 was successfully prepared through the hydrothermal and following the annealing process. The Ni dopants were doped in MoSe2 nanosheets as the formation of Ni nanoclusters. In alkaline media, the Ni-doped MoSe2 with 4.5 at.% contents shows the best electrocatalytic ability with low overpotential and excellent stability. The 4.5 at.% NieMoSe2 has an overpotential of 184 mV to produce 10 mA cm2 with a small Tafel slope of 83 mV dec1, which is significantly lower than that of the original MoSe2 (335 mV). The catalyst also shows excellent stability and a low transfer resistance. The catalytic ability of NieMoSe2 has improved, which is attributed to the increase in active sites. This study of Ni-doped MoSe2 may broaden our horizons in the design of no-noble electrocatalysts in alkaline media.
Acknowledgements This work is financially supported by the National Natural Science Foundation of China (No.51971083 and No.51731005).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.12.212.
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