Scalable Production of Few-layer Niobium Disulfide Nanosheets via

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Scalable Production of Few-layer Niobium Disulfide Nanosheets via Electrochemical Exfoliation for Energy#Efficient Hydrogen Evolution Reaction Jincheng Si, Qiang Zheng, Hanlin Chen, Chaojun Lei, Yange Suo, Bin Yang, Zhiguo Zhang, Zhongjian Li, Lecheng Lei, Yang Hou, and Kostya (Ken) Ostrikov ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22052 • Publication Date (Web): 18 Mar 2019 Downloaded from http://pubs.acs.org on March 19, 2019

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ACS Applied Materials & Interfaces

Scalable Production of Few-layer Niobium Disulfide Nanosheets via Electrochemical Exfoliation for Energy Efficient Hydrogen Evolution Reaction Jincheng Si,a,b Qiang Zheng,c Hanlin Chen,b Chaojun Lei,b Yange Suo,a,* Bin Yang,b Zhiguo Zhang,a Zhongjian Li,b Lecheng Lei,b Yang Hou,b,* Kostya (Ken) Ostrikovd

a

Department of Energy and Environmental Systems Engineering, Zhejiang University of

Science and Technology, Liuhe Road 318#, Hangzhou, Zhejiang Province 310023, China b

Key Laboratory of Biomass Chemical Engineering of Ministry of Education, College of

Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China c

Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA d

School of Chemistry, Physics, and Mechanical Engineering, Queensland University of Technology, Brisbane, QLD 4000, Australia

Keywords: Two-dimensional nanosheets, few-layer NbS2, electrochemical exfoliation, scalable production, energy efficient HER

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Abstract Two-dimensional niobium disulfide (NbS2) materials feature unique physical and chemical properties leading to highly-promising energy conversion applications. Herein, we developed a robust synthesis technique consisting of electrochemical exfoliation under alternating currents and subsequent liquid-phase exfoliation to prepare highly-uniform few-layer NbS2 nanosheets. The obtained few-layer NbS2 material has a 2D nanosheets structure with an ultrathin thickness of ~3 nm and a lateral size of ~ 2 µm. Benefiting from the unique 2D structure and highly exposed active sites, the few-layer NbS2 nanosheets drop-casted on carbon paper exhibited an excellent catalytic activity for hydrogen evolution reaction (HER) in acid with an overpotential of 90 mV at current density of 10 mA cm-2 and a low Tafel slope of 83 mV dec-1, which are superior to those for other reported NbS2-based HER electrocatalysts. Furthermore, the few-layer NbS2 nanosheets are effective as a bi-functional electrocatalyst for hydrogen production by overall water splitting where urea and hydrazine oxidation reactions replace the oxygen evolution reaction.

Introduction As a green renewable energy, hydrogen energy is widely regarded as the most promising energy source. Electrochemical water splitting has become a green, energy-saving and efficient way to produce hydrogen. So far, Pt-based materials remain the most effective hydrogen-producing electrocatalyst. However, the high cost, low durability, and low richness hinder their practical applications. Therefore, development of low cost, highly efficient, and durable hydrogen evolution reaction (HER) catalysts to replace noble metal materials is particularly attractive in the field.1-3 In recent years, since the discovery of graphene,4 more and more attention has been


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paid to the two-dimensional (2D) transition metal dichalcogenides (TMDs).5-8 Compared to the bulk crystals, 2D TMDs materials possess the promising chemical and physical properties owing to their unique surface chemistry, high aspect ratio, high exposed active site, and quantum-size effects.9 Among the 2D TMDs family, 2D metallic niobium disulfide (NbS2) has been received wide attention due to its attractive properties, anisotropic structure, and application prospects in energy and catalysis fields.10-11 Currently, 2D TMDs materials can be produced in a variety of ways such as mechanical12-13 or surfactant/polymer-assisted exfoliation,14-15 and liquid sonication exfoliation in organic solvents,16-18 however these synthesis methods not only require multiple synthesis steps, but also are difficult to control the numbers of layers and thickness of nanosheets.5 Recently, although there have been significant precedents in using electrochemical lithium intercalation strategy to exfoliate the 2D TMDs materials,19-20 and these electrochemically exfoliated 2D TMDs materials have been studied for HER electrocatalysis,18, 21-25

the lithium intercalation treatments usually result in the formation of byproducts such as

Li2S and H2S, together with a low exfoliation efficiency, which largely limits the development of electrochemical lithium intercalation methods for synthesis of 2D TMDs materials. Therefore, radically new methods of rapid and effective mass production are highly warranted. Moreover, replacing oxygen evolution reaction with oxidation of more advantageous species such as urea and hydrazine are necessary to reduce the total voltage required for water electrolysis. Herein, we developed a robust and scalable electrochemical exfoliation treatment of bulk NbS2 under alternating currents with liquid-phase exfoliation to synthesize highly-uniform few-layer NbS2 nanosheets, which was composed of ultrathin nanosheets with the thickness of ~3 nm and length of ~2 µm. The resulting few-layer NbS2 nanosheets drop-casted on the carbon paper (fewlayer NbS2 nanosheets/CP) exhibited an excellent HER activity in acid, featured by a low


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overpotential of 90 mV at current density of 10 mA cm-2 and a low Tafel slope of 83 mV dec-1, which are superior to those for other reported NbS2-based HER electrocatalysts. The superior HER performances are attributed to the unique 2D nanosheets structure and highly exposed active sites. Furthermore, the few-layer NbS2 nanosheets bifunctional electrocatalyst acts as both anode and cathode, enabling the urea oxidation reaction (UOR) and hydrazine oxidation reaction (HzOR) that replace the oxygen evolution reaction (OER), leading to energy-efficient hydrogen production.

Experimental section Synthesis of few-layer NbS2 nanosheets: The bulk NbS2 was fixed on the copper sheets using conductive silver paint. Next, the bulk NbS2 was directly installed on the clamp of the cathode with Pt plate as negative pole in 1.5 M H2SO4 electrolyte. Before switching the anode and cathode, a voltage of 3.0 V was applied for 5 min. Then, the process was repeated several times, and the resultant NbS2 powder was initially dispersed in the H2SO4 solution. Further, the NbS2 powder was filtered and mixed with N-Methyl-2-pyrrolidone (NMP) solution, and the sonication was performed for 10 h. Finally, the few-layer NbS2 nanosheets were obtained through 10,000 rpm of centrifugation, followed by freeze-dried treatment using a freeze dryer. Notably, this approach is easy for scale up. The few-layer NbS2 nanosheets can be continuously produced by adding bulk NbS2 supported on copper sheet into 1.5 M H2SO4 solution at a specific rate. For instance, using four beakers (250 mL) as reactors, and bulk NbS2 supported on copper sheet (thickness of 0.3 mm and width of 20 mm) as raw material, few-layer NbS2 nanosheets were continuously produced with a production rate of about 120 mg h-1, showing the great


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potential of this way for mass production of few-layer NbS2 nanosheets. In China, the cost of few-layer NbS2 nanosheets gained through electrochemical exfoliation with subsequent liquidphase exfoliation is estimated to be $ 8.123/120 mg, which is below the present-day market price. The detailed cost estimates are presented in the Supporting Information. Characterization: X-ray diffraction (XRD) was carried out on a RIGAKU D/MAX 2550/PC Xray diffractometer for

J radiation. Electron micrographs were obtained using a field

emission scanning electron microscope (FESEM, Supra 55). The microstructure and morphology were investigated by transmission electron microscopy (TEM, HT7700) and a high-resolution TEM (HRTEM, JEM-2100, 200 kV). Raman data were collected using Laser Confocal Raman Microspectroscopy (LabRAM HR Evolution, wavelength = 532 nm). X-ray photoelectron spectroscopy (XPS) was performed using Thermo Fisher Scientific, Escalab 250Xi with Al Ka radiation. The thickness of products was investigated by Atomic Force Microscope (AFM MultiMode VEECO America with tapping Mode). The samples were deposited on lacey carbon copper grids and then dried in air for scanning transmission electron microscopy (STEM) observations. High-angle annular dark-field-STEM (HAADF-STEM) imaging was acquired on an aberration-corrected Nion UltraSTEM 100TM operated at 100 kV, with a beam convergence semi-angle of 30 mrad.26 Electrochemical

tests:

All

electrochemical

measurements

were

performed

on

an

electrochemical analyzer (CHI 760E), in which a glassy carbon electrode (3 mm in diameter, 0.07 cm2) with catalyst acted as the working electrode, an Ag/AgCl electrode was used as reference electrode, and a graphite rod was used as counter electrode. The electrolyte was 0.5 M H2SO4. Typically, 3.5 mg of catalyst and 3.5 mg of carbon black with 35 µL of Nafion solution (5 wt.%) were dispersed in 315 µL of ethanol to form a homogeneous ink by sonication for 30


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min. Next, the homogeneous ink was loaded on the surface of glassy carbon electrode. In order to facilitate easier integration of the electrode into devices for practical applications, the fewlayer NbS2 nanosheets was further drop-casted onto carbon paper (loading amount of 0.285 mg cm-2) as cathode for HER. Before the electrochemical tests, the 0.5 M H2SO4 electrolyte was saturated with N2 gas for 30 min. Linear sweep voltammetry (LSV) was carried out from 0 to 0.8 V (vs. Ag/AgCl) with a scan rate of 5 mV s-1. All polarization curves were presented with iR correction. The electrochemical impedance spectroscopy (EIS) test of samples was performed at a potential of -0.24 V (vs. RHE). The electrochemically active surface area (ECSA) of samples was evaluated by cyclic voltammogram (CV) cycling from 0.09 to 0.21 V (vs. RHE) in 0.5 M H2SO4 at sweep rates of 20~100 mV s-1. The few-layer NbS2 nanosheets/CP electrodes were acted as both cathode and anode in a single electrolytic cell without membrane for whole water splitting measurements. The potentials were converted into the reversible hydrogen electrode (RHE) reference scale via the following Nernst equation: E(vs. RHE) = E(vs. Ag/AgCl) + 0.0591 V × pH + 0.197 V.

Results and discussion The synthesis process of few-layer NbS2 nanosheets is shown in Figure 1. First, bulk NbS2 material was synthesized by a solid-phase synthesis by fusing Nb and S powders in a vacuum tube. Next, electrochemical exfoliation of bulk NbS2 was completed by using the voltage of 3.0 V under alternating currents for 20 min. Afterwards, the exfoliated NbS2 was dispersed in NMP solution after 10 h of the sonication treatment. Figure 1a details the electrochemical exfoliation process of bulk NbS2 under alternating currents. Figure 1b shows the process of bulk NbS2 ACS Paragon Plus Environment

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shedding during the electrochemical exfoliation process. Specifically, the bulk NbS2 was directly installed on the clamp of positive pole with a Pt plate as a negative pole and immersed in 1.5 M H2SO4 solution (Figure 1b). When a voltage of 3.0 V was used, the applied voltage caused the anion (SO42-) migration and insertion as well as O2 gas generation from positive pole, which could result in the enlarged spacing of NbS2. 2H2O Q O2 + 4H+ + 4eIn the meantime, the free radicals produced by the anode can attack the edges or inherent defects of the NbS2, thus resulting in opening at the edges of the NbS2. When the negative voltage was applied, the inserted SO4- anions could be converted into gas bubbles (SO2). 2SO42- + 4H+ + 2e- Q 2H2O + 2SO2 (hazardous) At the same time, the H+ ions migrated and accumulated on the surface of bulk NbS2. The resulting H2 bubbles generated from negative pole could provide a powerful force between the NbS2 layers, pushing the layers further apart. 2H+ + 2e- Q H2 As observed, the exfoliated NbS2 with the yellow/brown color was obtained from bulk NbS2 (Figure 1c). The process was then repeated several times, splitting bulk NbS2 into exfoliated NbS2, which was dispersed in the H2SO4 solution (Figure 1d). Next, the exfoliated NbS2 were further purified by washing with water and filtering through the membrane, as shown in Figure 1e. The color of the solution was changed from brown to bright yellow (Figure 1f), depending on the centrifugation speed increase, and an obvious Tyndall phenomenon was observed. Figure 1g


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shows that few-layer NbS2 nanosheets were evenly distributed throughout the NMP solution after 10 h sonication in mass production.12 In addition, we further explored the dispersibility of the few-layer NbS2 nanosheets in other three organic solvents (dimethyl sulfoxide, N,NDimethylformamide, and dimethylcarbinol) for HER. The corresponding polarization curves show that the HER performance of few-layer NbS2 nanosheets dispersed in NMP solution was clearly higher than that of few-layer NbS2 nanosheets dispersed in other solvents (Figure S1). The morphology of few-layer NbS2 nanosheets was first investigated by field-emission scanning electron microscope (FESEM). The bulk NbS2 exhibited a typical hexagonal shape in a denselypacked stack (Figure S2a). After only the electrochemical exfoliation treatment, the anionic and cationic intercalations caused bulk NbS2 to expand significantly (Figure 2a and Figure S2b). In contrast, after only the sonication treatment, the bulk NbS2 went from a large piece to a uniform sheet (Figure S2c). After the combined exfoliation and sonication treatments, the few-layer NbS2 nanosheets with a lateral size of ~2 µm was obtained (Figure S2d). The thickness of few-layer NbS2 nanosheets was ~3 nm, corresponding to ~5 layers (single layer thickness is 0.59 nm) based on the images from atomic force microscopy (AFM, Figure 2b). Figure 2c-2d and Figure S2e-2f show typical transmission electron microscopy (TEM) images of few-layer NbS2 nanosheets, and the formation of few-layer NbS2 nanosheets at different magnification levels was confirmed. Figure 2e showed a Tyndall effect of few-layer NbS2 nanosheets. The selected area electron diffraction (SAED) of few-layer NbS2 nanosheets displayed the (006), (100), and (103) facets (Figure 2f), corresponding to the lattice spacings of 0.198, 0. 287, and 0.233 nm, respectively. Continuous lattice fringes are clearly visible from the high-resolution TEM (HRTEM) images (Figure 2f) of few-layer NbS2 nanosheets, further confirming its high crystallinity. The lattice distance of few-layer NbS2 nanosheets was measured to be 0.279 nm,


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corresponding to the (101) plane of NbS2 crystal. Figure 2g shows a typical high-angle annular dark field scanning TEM (HAADF-STEM) image for few-layer NbS2 nanosheets in the projection of [001]. The region marked by a white rectangle was further magnified and displayed in Figure 2h, clearly revealing the Nb and S atomic columns. Since the HAADF-STEM image intensity was roughly proportional to Z2 (Z is the atomic number), atomic columns involving Nb atoms reveal much stronger contrast compared to the columns involving S atoms. The uniform distribution of Nb and S in the few-layer NbS2 nanosheets was evidenced by the corresponding elemental mapping using energy dispersive X-ray spectroscopy (EDX) (Figure 2i). Structures of the NbS2 monolayer and stacking bilayer are shown in Figure 3a. The NbS2 layers are superimposed along the crystallographic vertical axis, which is maintained by the weak van der Waals force.27 X-ray diffraction (XRD) pattern of few-layer NbS2 nanosheets is presented in Figure 3b. All the XRD peaks are attributed to the PDF 41-0980, and the strong intensity of diffraction peaks located at 14.7o, 31.0o, and 55.3o corresponded to the (002), (100), and (110) planes, respectively.16 Compared with bulk NbS2, the peak intensity of few-layer NbS2 nanosheets was weaker or even disappeared, indicating that the bulk NbS2 was successfully exfoliated. The successful exfoliation of few-layer NbS2 nanosheets from bulk NbS2 can be further supported by Raman spectroscopy. As shown in Figure 3c, the Raman peaks at 263 cm-1 (E1g), 304 cm-1 (E2g), 379 cm-1 (A1g), and 458 cm-1 (A2u) are significantly reduced from bulk to few-layer nanosheets,28-29 further confirming the successful exfoliation. X-ray photoelectron spectroscopy (XPS) survey spectrum of few-layer NbS2 nanosheets provides a proof of the presence of Nb, S, and O elements (Figure 3d). The strong oxidation peaks of fewlayer NbS2 nanosheets are mainly attributed to the ultrathin structure of few-layer NbS2 nanosheets that is easier to be oxidized at the time of testing than the bulk NbS2. The high-


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resolution Nb 3d XPS spectra of few-layer NbS2 nanosheets showed the main peaks at 202.8 eV and 203.8 eV (Figure 3e), and the two peaks can be assigned to Nb4+ 3d5/2. The two peaks at 205.4 eV and 206.2 eV can be attributed to Nb4+ 3d3/2. The oxidation state of Nb5+ was assigned to Nb 3d centered at 209.9 eV and 206.9 eV, which can be attributed to the surface of nanosheets being oxidized under the test atmosphere.30-31 In the high-resolution S 2p XPS spectra (Figure 3f), the main peaks between 160 eV and 163 eV can be attributed to the S 2p1/2 and S 2p3/2 binding energies of Nb-S bonding (S 2p1/2 peaks at 160.3 eV and 161.7 eV, and S 2p3/2 peaks at 161.2 eV and 162.4 eV), and oxidation state of S at 168.4 eV

32.

These results provide further

evidence of the formation of few-layer NbS2 nanosheets. The electrocatalytic performance of few-layer NbS2 nanosheets for HER was examined in 0.5 M H2SO4 by using a typical three-electrode system.33-34 All the reported potentials were converted to the RHE.35-36 The five control samples are bulk NbS2, NbS2 prepared only by sonication (sonNbS2), NbS2 prepared only by electrochemical exfoliation (ex-NbS2), few-layer NbS2 nanosheets, and Pt/C. All the polarization curves were iR-corrected. As shown in Figure 4a, the commercial Pt/C exhibited the highest HER activity.37 In contrast, the bulk NbS2 showed poor HER activity. The few-layer NbS2 nanosheets delivered an overpotential of 236 mV to reach the current density of 10 mA cm-2, which was much lower than that of ex-NbS2 (300 mV at 10 mA cm-2) and son-NbS2 (379 mV at 10 mA cm-2), confirming that ultrathin NbS2 nanosheets boost the HER performance due to its unique 2D structure.38 Further, the few-layer NbS2 nanosheets/CP exhibited a small overpotential of 90 mV at the current density of 10 mA cm-2, much lower than other reported NbS2-based electrocatalysts, such as, NbS2/rGO (420 mV at 10 mA cm-2)16 and MoSx@NbS2/GC (164 mV at 10 mA cm-2) (Table S1).39 To be specific, the current density of few-layer NbS2 nanosheets was about 12.3 mA cm-2 at 250 mV, which was


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much higher than that of ex-NbS2 (3.3 mA cm-2), son-NbS2 (2.1 mA cm-2), and bulk NbS2 (1.5 mA cm-2) at the same potential, further revealing the excellent HER activity of few-layer NbS2 nanosheets. Figure 4b quantifies the HER kinetics with Tafel slope. A Tafel slope of 125 mV dec-1 was obtained for few-layer NbS2 nanosheets, while it was 110 mV dec-1 for ex-NbS2, 146 mV dec-1 for son-NbS2, and 249 mV dec-1 for bulk NbS2. Figure 4c shows a multi-current steps curve for few-layer NbS2 nanosheets with the current densities cumulative from 10 to 60 mA cm2.

For the start current density, the potential remains stable at -0.27 V over the reaction time of

100 s. Likewise, the corresponding multi-potential steps of few-layer NbS2 nanosheets are displayed in Figure S3a. All other steps showed the same trends, suggesting the outstanding mass transport property and superior mechanical robustness.40 Figure 4d illustrates the changes in the polarization curves of few-layer NbS2 nanosheets before and after 1,000 cycles. No obvious changes was observed for few-layer NbS2 nanosheets, suggesting its robust catalytic activity. This conclusion was further supported by the inset of Figure 4d, in which the few-layer NbS2 nanosheets showed a high stability with a negligible recession of potential over 10 h of continuous HER electrolysis. To further study the catalytic kinetics during the HER process, the Nyquist plots of few-layer NbS2 nanosheets are shown in Figure 4e. The few-layer NbS2 nanosheets produced the smallest radius among all control samples, corresponding to the lowest charge-transfer resistance.41 The charge transfer resistance (Rct) is related to the actual electron transfer resistance of catalysts, and the fitting results are summarized in Table S2. The Rct of 99 was much smaller than those of ex-NbS2 (1,284

for few-layer NbS2 nanosheets

), son-NbS2 (4,048

), and bulk NbS2 (5,519

). The double-layer capacitance (Cdl) was used to estimate the ECSA of few-layer NbS2 nanosheets because of their positive proportion relationship.38, 42-43 As shown in Figure 4f and


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Figure S4a-e, the capacitance of few-layer NbS2 nanosheets was calculated to be 10.9 mF cm-2, three times larger than that of bulk NbS2 (3.6 mF cm-2), indicating that more active sites were exposed after the exfoliation and sonication treatments. Based on the above results, one can conclude that the larger ECSA and higher electron transfer rates of few-layer NbS2 nanosheets contributed to the high HER activity. We further explore the catalytic properties collected at different centrifugation speeds (Figure S3b). By comparison, the few-layer NbS2 nanosheets obtained by 10,000 rpm of centrifugation exhibited the highest HER catalytic performance, and the corresponding EIS results showed that it has the minimum radius (Figure S3c). Although the few-layer NbS2 nanosheets were active for HER in 0.5 M H2SO4, our further research showed that the few-layer NbS2 nanosheets can also be effectively to produce the hydrogen under 1.0 M KOH (Figure S5). The efficiency of H2 production by electrolysis largely depends on the anodic OER process with high activation energy barrier for O-O bond formation.3,

44-45

Consequently, OER process

replaced with oxidation of more favorable species like hydrazine (N2H4), urea (CH4N2O), methanol, and ethanol46 can lead to a decrease in the anode potential, thereby reducing the voltage required for hydrogen production. The electrocatalytic HzOR and UOR activity of fewlayer NbS2 nanosheets/CP was investigated in a typical three-electrode system. Figure 5a-b and Figure S6 show the electrochemical performance of few-layer NbS2 nanosheets/CP in 1.0 M KOH with and without 0.5 M N2H4 (or 0.5 M CH4N2O). As observed, the presence of 0.5 M N2H4 and 0.5 M CH4N2O had little effect on the HER performance of few-layer NbS2 nanosheets/CP, but significantly promoted the anodic reaction, suggesting that the few-layer NbS2 nanosheets/CP can be acted as efficient electrocatalyst for both HzOR and UOR. These


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results implied the potential applications of few-layer NbS2 nanosheets/CP as a bifunctional catalyst for energy-saving electrolytic hydrogen production. Based on the above results, we made a two-electrode electrolyzer with few-layer NbS2 nanosheets/CP bifunctional electrocatalyst as cathode for the HER and anode for the UOR or HzOR. As shown in Figure 5c, in the absence of N2H4, the few-layer NbS2 nanosheets/CP needs a potential of 1.7 V to achieve the current density of 10 mA cm-2. However, the potential was reduced to 0.4 V to obtain the current density of 10 mA cm-2 when 0.5 M N2H4 was added. Figure 5d shows that the chronopotentiometric curve of few-layer NbS2 nanosheets/CP with a current density of 10 mA cm-2 for 10 h. Likewise, when 0.5 M CH4N2O was added, the potential was reduced to 1.38 V to achieve the current density of 10 mA cm-2 with strong long-term stability (Figure 5e-5f). These results suggest a method of using the few-layer NbS2 nanosheets as a bifunctional electrocatalyst, enabling UOR and HzOR instead of OER on the anode for energy efficient electrolysis for HER.

Conclusion In conclusion, a 2D ultrathin few-layer NbS2 nanosheets with thickness of ~3 nm and ~ 2 µm in lateral size was synthesized by using electrochemical exfoliation of bulk NbS2 under alternating currents and subsequent sonication treatment. The few-layer NbS2 nanosheets/CP exhibited excellent electrocatalytic activity towards HER, featured by an overpotential of 90 mV at a current density of 10 mA cm-2 and a low Tafel slope of 83 mV dec-1. Such a low overpotential of 90 mV for few-layer NbS2 nanosheets/CP represented the excellent performance among the previously reported results for NbS2. The unique 2D structure as well as highly exposed active sites contributed to the excellent HER activity. Furthermore, replacing OER with UOR and


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HzOR at the anode offered an effective method for energy-efficient HER by using few-layer NbS2 nanosheets as a bifunctional catalyst. This electrochemically exfoliated few-layer NbS2 nanosheets presented in this work provide new avenues for exfoliation of other 2D TMDs materials and is promising for many important applications in energy and catalysis fields, including but not limited to CO2 reduction, nitrogen reduction, and oxygen reduction reactions.

Supplementary information

FESEM, TEM, structure of bulk NbS2, electrochemical HER measurements (Figures S1-S8); comparison of HER activity of few-layer NbS2 nanosheets with recently reported catalyst (Table S1-S3); and additional references.

Corresponding author

Yange Suo, E-mail: [email protected]

Yang Hou, E-mail: [email protected]

Notes

The authors declare no competing V

interest.

Acknowledgements

Y. Hou thanks the support of National Natural Science Foundation of China (51702284, 21878270), Zhejiang Provincial Natural Science Foundation of China (LR19B060002), and the Startup

Foundation

for

Hundred-Talent

Program

of

Zhejiang

University

(112100-


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193820101/001/022). Yange Suo thanks the support of National Natural Science Foundation of China 21805244. Q. Zheng thanks the support by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), Materials Sciences and Engineering Division. STEM in this work was conducted at the ORNL’s Center for Nanophase Materials Sciences (CNMS), which is a DOE Office of Science User Facility.

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Page 23 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60





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b

c

ex-NbS2

-25

son-NbS2 few-layer NbS2 nanosheets

-50

-75

0 -25

Pt/C few-layer NbS2 nanosheets/CP

-50 -75

-100 -0.8

-100 -0.6

-0.4

-0.6

-0.4

-0.2

0

14

6

d mV

-1

0 11

de

mV

c

-1

mV

c de

-1

0.2 bulk NbS2 ex-NbS2

83

mV

de

c

few-layer NbS2 nanosheets/CP

son-NbS2

few-layer NbS2 nanosheets

0.0

1.0

e

ec

5 12

0.0

Potential (V vs. RHE)

d

9m

c

0.4

0.0


-0.2

24

0.0

-1

-1

e Vd

1.5

2000 (Rct)

1500

-120

-Z'' (8)


-40 -80

0.6 0.0

10 mA cm-2

-0.6

-0.6

-0.4

bulk NbS2 son-NbS2 ex-NbS2

500

few-layer NbS2

-1.2 0.0

2.5

5.0

7.5

Time (h)

-160

1000

-0.2


nanosheets

10.0

0 0.0

0

500

1000

1500

50 mA cm-2 20 mA cm-2 40 mA cm-2

-0.6

150

2000

Z' (8)

60 mA cm-2

300

450

600

Time (s)

f

(Rs)

1000th cycle 1st cycle

30 mA cm-2

-0.4

-0.8

2.0

Log J (mA cm-2)

-0.2 10 mA cm-2

Current density (mA cm-2)

bulk NbS2


0.6

0



a


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Potential (V vs.RHE)

Page 25 of 28

few-layer NbS2 nanosheets

1.2

ex-NbS2

-2

son-NbS2 bulk NbS2

0.8

. 10

9

m

F

cm

-2

9.

F 6m

cm -2

4 .3

0.4

mF

cm

-2

3.6 m

F cm

0.0 20

40

60

80

100

Scan rate (mV s-1)

Figure 4. (a) Polarization curves of bulk NbS2, ex-NbS2, son-NbS2, few-layer NbS2 nanosheets. Inset: polarization curves of few-layer NbS2 nanosheets/CP and Pt/C. (b) Corresponding Tafel plots. (c) Multi-current steps of few-layer NbS2 nanosheets. (d) Polarization curves of few-layer NbS2 nanosheets for the first and 1000th CV cycles. Inset: time-dependent potential of few-layer NbS2 nanosheets under overpotential of 290 mV for 10 h. (e) Nyquist plots of bulk NbS2, exNbS2, son-NbS2, and few-layer NbS2 nanosheets. Inset: an equivalent circuit for fitting impedance date. (f) Cdl values of bulk NbS2, ex-NbS2, son-NbS2, and few-layer NbS2 nanosheets. All measurements were carried out in 0.5 M H2SO4.


25


b Current density (mA cm-2)

80

1.0 M KOH with 0.5 M N2H4 1.0 M KOH

60 HzOR

40

20 OER

0 0.0

0.4

0.8

1.2

1.6

Potential (V vs. RHE) 80

1.0 M KOH with 0.5 M CH4N2O 1.0 M KOH

75

50 UOR

25

0 1.2

OER

1.4

1.6

1.8


d 0.9

1.0 M KOH with 0.5 M N2H4 1.0 M KOH

60

Potential (V)


c

100

HER&HzOR

40

HER&OER

20

0 0.0

10 mA cm-2

0.3

0.0 0.4

0.8

1.2

e

0

1.6

Potential (V) 100

0.6

1.39 V

2

4

6

8

10

8

10

Time (h)

f 3

1M KOH with 0.5 M CH4N2O 1M KOH

75

50 HER&UOR

25

HER&OER

Potential (V)


a


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 28

2

10 mA cm-2 1

V

0 1.0

0

1.2

1.4

1.6

1.8

0

Potential (V)

2

4

6

Time (h)

Figure 5. (a) Polarization curves for few-layer NbS2 nanosheets/CP in 1.0 M KOH with and without 0.5 M N2H4. (b) Polarization curves of few-layer NbS2 nanosheets/CP in 1.0 M KOH with and without 0.5 M CH4N2O. (c) Polarization curves of few-layer NbS2

E 2^( 9:

layer NbS2 nanosheets/CP couple in the presence and absence of 0.5 M N2H4 in 1.0 M KOH. (d) Chronopotentiometric curve of few-layer NbS2 nanosheets/CP with a constant density of 10 mA


26

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cm-2 in 1.0 M KOH with 0.5 M N2H4. (e) Polarization curves of few-layer NbS2 E 2^( 9:

! NbS2 nanosheets/CP couple in the presence and absence of 0.5 M

CH4N2O in 1.0 M KOH. (f) Chronopotentiometric curve of few-layer NbS2 nanosheets/CP with a constant density of 10 mA cm-2 in 1.0 M KOH with 0.5 M CH4N2O.


27



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Scalable Production of Few-layer Niobium Disulfide Nanosheets via

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