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Research Papers

Electrocatalytically Active Niobium Sulfide Modified Carbon Cloth for Lithium–Sulfur Batteries PUBLIC ACCESS

[+] Author and Article Information
Leela Mohana Reddy Arava

Department of Mechanical Engineering,
Wayne State University,
5050 Anthony Wayne Drive,
Room# 2140,
Detroit, MI 48202
e-mail: leela.arava@wayne.edu

Deepesh Gopalakrishnan

Department of Mechanical Engineering,
Wayne State University,
5050 Anthony Wayne Drive,
Room# 2140,
Detroit, MI 48202
e-mail: deepesh@wayne.edu

Andrew Lee

Department of Mechanical Engineering,
Wayne State University,
5050 Anthony Wayne Drive,
Room# 2140,
Detroit, MI 48202
e-mail: andrewlee1030@gmail.com

1Corresponding author.

Manuscript received June 13, 2017; final manuscript received September 19, 2017; published online October 17, 2017. Assoc. Editor: Partha P. Mukherjee.

J. Electrochem. En. Conv. Stor. 15(1), 011005 (Oct 17, 2017) (5 pages) Paper No: JEECS-17-1069; doi: 10.1115/1.4038020 History: Received June 13, 2017; Revised September 19, 2017

We report a simple novel annealing technique for the synthesis of NbS2 nanoflakes. The synthesized NbS2 flakes were characterized well with different spectroscopic and microscopic techniques and confirmed they are in 3R-NbS2 polymorph structure, which is semiconducting in nature. Later, they were successfully deposited onto carbon cloth (CC) and tested for Li–S cell. Lithium–sulfur batteries suffer from polysulfide (PS) shuttling effects which hinder the performance of the cell. High capacity fade, slow redox kinetics, and the low cyclability of cells are just some of the many problems caused by the shuttling effect that hinder the viability of the battery. Herein, we utilized the catalytic nature of NbS2 along with the high conductivity of CC for better PS adsorption, their liquid to solid conversion, fast PS redox kinetics which substantially enhanced the overall Li–S performance.

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In the past decade, developments in renewable energy technologies have increased interest in replacing traditional fossil fuels with clean electricity. While lithium-ion batteries have mostly satisfied our energy storage needs, the next generation of green innovation will call for batteries with higher energy densities, lower costs, and fewer environmental impacts [1,2]. Toward this, researchers have developed many novel mechanisms and chemistries which are superior over the existing type of battery systems [24]. Thus, the demand for superior batteries can be achieved with lithium–sulfur (Li–S) batteries which have a ten times higher theoretical capacity (1673 mA h g−1) than the conventional Li ion batteries and can be produced with cheaper and greener materials [1,3,5]. Despite its superiority to current battery technologies, Li–S batteries suffer from redox shuttle reactions causing lithium polysulfides (LiPS) to deposit onto the electrodes, hindering the performance of the cell. As sulfur is reduced during battery discharge, short-chain lithium polysulfides (Li2Sx; 1 ≤ x ≤ 4) fall out of solution due to its high polarity in the nonpolar electrolyte and form a passivation layer on the Li-anode which lowers cycle life and leads to higher charge/discharge times [68]. A strong adhesion to the cathode is critical for charge transference back into the LiPS to convert it to long chain polysulfides, yet highly polar LiPS cannot adhere to carbon very well [9]. Poor cyclability, anode corrosion, and lower capacity are few problems that plague Li–S batteries because of the shuttling effect, so a solution must be researched to make Li–S batteries practical for everyday use [10,11]. Various techniques have been studied to prevent LiPS from deteriorating battery performance [12]. As the electron flow is crucial for polysulfides to oxidize back into sulfur upon battery recharge, conductive substrates such as carbon nanotubes [13], porous carbon compounds [14], and nanocomposites [15] have been widely studied in hopes of improving battery performance. For example, Fu et al. utilized multiwalled carbon nanotubes and its unique shape to distribute electron charges to reduce the effects of shuttling [13]. Kamphaus and Balbuena have used ultrasonication to synthesize compounds with nanopores that trap LiPS [14]. They found smaller pore sizes to be more effective in adsorbing the polysulfides. Fu et al. also produced another work which involved agglomerated spherical nanoparticles with high surface areas and sulfur loading capabilities that improved capacity and lowered resistance in the cell [15]. Zhang et al. studied transition metal disulfides, metal chlorides, and metal oxides which work as catalysts to adsorb LiPS, which improves battery cyclability [9]. Oxides were found to have the highest binding energies to LiPS, followed by sulfides and then chlorides, though sulfides were focused on for this experiment.

Similarly, sulfiphilic CoS2 showed a greater enhancement in liquid polysulfide (PS) adsorption and their conversion redox activity with a final deposition of insoluble solid Li2S [16]. The adsorption between the sulfide and LiPS proved to be depending on the interfacial interaction between the two species; a high exposed surface area would be needed to provide maximum effect. Also, MnO2 nanosheets were capable for fast liquid to solid Li2S conversion through catenation reaction which clarifies the different reaction pathway toward addressing the capacity fade and short cycle life of Li–S batteries [10]. These reports inform the significance of conductive surface in the deposition of solid PS end products during charge discharge process and their role in Li–S redox reaction kinetics. However, the electron transfer rate and mass transport during the redox reactions occurring in presence of these conductive surfaces are not well studied. Recently, our group has opened up a new avenue demonstrating the electrocatalytic surface effects on PS reaction in Li–S by using conventional catalytic surfaces like Pt or Ni in the presence of conductive matrices like graphene [12,1719].

The binding energy of metal sulfide toward Li2S4 was found to be substantially higher and favorable than that of graphene. This shows the sulfide’s greater ability compared to traditional material in latching on to LiPS and preventing them from diffusing throughout the cell. Metal sulfides like SnS2, MoS2, WS2, CoS2, etc., were proven for their high potential for anchoring the LiPS and further reducing the PS shuttling effect [9,18]. NbS2 has already been studied for catalyzing processes such as hydrodesulphurization, hydrodenitrogenation, hydrodemetalation, and hydrocracking [20,21]. But the preparation of stoichiometrically stable NbS2 nanomaterials is very hard which hinders it from many practical applications. Nb2O5 or NbO2 requires a high sulfur pressure around 6 atm unlike WO3 and MoO3 (which requires very low sulfur pressure to form corresponding sulfides) and a high temperature to form highly stable NbS2 crystals [2224]. Thus, developing a facile synthesis route for few layered NbS2 flakes still remains as a challenge [24]. Structure of NbS2 is similar to MoS2, which has active catalysis edges upon cleavage of the bulk material [25]. In addition, NbS2 monolayers were actually more stable than MoS2, making them a good candidate as a catalyst.

Herein, we aim to further examine the ability of transition metal disulfides in the presence of high surface area matrix like carbon cloth (CC) to adsorb LiPS by investigating bare CC, NbS2/CC, and their interaction with PS, being an electrocatalyst for enhanced performance in Li–S batteries. Further, the adsorption of LiPS onto the catalytic surface and the electrocatalysis driven electrochemical performance of NbS2 nanosheets loaded on CC were studied in detail.

The coalesced NbS2 nanoflakes were prepared using a simple annealing technique with a tube furnace which uses commercial powdered niobium (1 g, Alfa Aesar, >99% purity) and elemental sulfur (3 g, Sigma Aldrich, >99% purity) as precursors kept in first and middle zone of the tube, respectively. The annealing procedure was carried out in the presence of argon with a constant flow rate in order to prevent niobium metal from oxidizing into niobium oxide. The furnace was ramped up to 1050 °C in 100 min and stayed at that temperature for another 80 min for the sulfurization to happen.

For loading the semiconducting NbS2 samples on carbon cloth for improving the conductivity of whole system, the prepared NbS2 nanoflakes were sonicated in water:ethanol (3:4 ratio) for 1 h. The disintegrated NbS2 individual flakes were filtered through the carbon cloth which was kept on the whatman filter membranes. The NbS2 flakes got deposited onto the CC and was used for further characterization and electrochemical measurements.

Different spectroscopic and microscopic techniques were carried out for morphological and compositional analyses of NbS2. The as-synthesized NbS2 samples were examined using a scanning electron microscope (SEM, JSM 7600 FE SEM), transmission electron microscopy (TEM, JEOL, Peabody, MA), and a confocal Raman microscope with a laser excitation wavelength of 532 nm (Instruments). The crystalline nature of the as grown samples were analyzed using X-ray diffractometer (XRD, Bruker, Billerica, MA) using Cu–Ka radiation (1.5418Å).

To evaluate the electrochemical properties of as synthesized electrode materials, 200 mM and 0.06 M of electro-active species containing catholyte solution has been used. Such electrolyte solutions were prepared using calculated amounts of Li2S and S to get long-chain LiPS (Li2S8) in tetra ethylene glycol dimethyl ether at 90 °C for 24 h under continuous stirring. The NbS2/CC and CC were used as electrodes which are circular disks of 12.7 mm diameter. Standard CR2032 type coin cells were used to perform the electrochemical measurements, where CC and NbS2 loaded CC as working electrodes and lithium metal as counter and reference electrodes. For the charge discharge measurements, 10 μL of 200 mM molar concentration of catholyte solution was used as the active material along with a blank electrolyte (10 μL, without polysulfides) consisting of 1 M of lithium bis(trifluoromethanesulfonyl) imide and lithium nitrate in tetra ethylene glycol dimethyl ether. Celgard was used as separator for CR2032 cells. Cyclic voltammetry studies to understand the LiPS redox kinetic were conducted using a CH760E bipotentiostat with a potential window of 3.0–1.5 V at a scan rate of 0.1 mV/s.

To confirm the crystal structure and composition of as grown NbS2, XRD and raman spectroscopy have been used. The crystal structure of coalesced NbS2 flakes synthesized at 1050  °C was examined using XRD technique and found to have 3R-type rhombohedral structure of NbS2 with lattice constants, a = b = 3.330 A, c = 7.918 A (PDF: 03-065-3655), shown in Fig. 1(a). The sharp peak (003) at 2θ = 15 deg indicates the layered nature of prepared NbS2 with layers piled up with respect to the C axis. The diffraction peaks from (105), (015) planes with corresponding 2θ values at 33 deg and 40 deg match to the disorder and randomly distributed NbS2 nanosheets. During the growth, at a high temperature, the sulfur pressure got increased up to 1 atm and vapors of sulfur reacted with Nb particles to form NbS2 nanoflakes which are distinct with precise edges.

To reconfirm the crystalline nature of NbS2 nanoflakes synthesized at 1050 °C, Raman scattering measurements were carried out and are shown in Fig. 1(b). The typical four major nondegenerate Raman active modes, at around 290, 330, 386, and 450 cm−1 were detected representing E modes (E1 and E2) and A modes (A1 and A2), respectively, predicting the 3R crystal structure of NbS2 [25]. Also, E mode peaks are found to be less sharp which indicates the lesser thickness of the NbS2 nanoflakes. Thus, confirming the formation of NbS2 nanoflakes with crystalline nature elucidates its capability for catalyzing the redox activity of adsorbed LiPS on the exposed active catalyst surface.

To expose the more preferable sites on the catalyst surface, the coalesced NbS2 nanoflakes had undergone sonication to disintegrate into single flakes. These morphological changes are analyzed using SEM and are shown in Fig. 2. Figure 2(a) shows the SEM image of as synthesized NbS2 nanoflakes which are found to be thin layered with definite edges which can be associated to the absence of restacking. However, the bunches of NbS2 flakes were got fragmented into single flakes upon sonication in water:ethanol (4:3) mixture as shown in Fig. 2(b). Single individual flakes can make more active surface available for LiPS adsorption thereby improving the overall redox kinetics. These fragmented flakes were deposited uniformly through the filtration method which is shown in Fig. 2(d) and the smooth surface of CC fibers can be seen in SEM image of bare CC (Fig. 2(c)). The NbS2 deposited CC was taken further for electrochemical Li–S battery measurements.

To reveal the electrochemical performance of the NbS2/CC composite, bare NbS2, and bare CC, galvanostatic charge discharge was carried out by fabricating 2032 coin cells using them as cathode versus metallic lithium as anode. Figures 3(a) and 3(b) shows the charge discharge behavior of these NbS2/CC and bare CC electrodes at a current rate of 0.1 C (based on sulfur mass in the cell), respectively. The discharge curves showed the typical discharge plateaus at 2.35 V and 1.97 V corresponding to the formation of soluble high order polysulfides and their dissociation to low order insoluble Li2SX, respectively. During charging process, reversible conversion reaction was occurred at 2.3 V which can be assigned to the transition of lower LiPS to high order PS. There were no significant changes observed in the charge discharge profiles of the NbS2/CC and bare CC electrodes. NbS2/CC composite with its improved conductivity along with the ability of NbS2 to catalyze the LiPS during charge discharge helped in excellent reversibility and improved capacity, which took them a step ahead of bare NbS2 and carbon electrodes. The NbS2/CC electrode exhibits stable charge–discharge capacities of 810 mA h g−1 at a c-rate of 0.1 C whereas the bare CC showed a capacity around 600 mA h g−1 over 25 cycles, which exposes strength of catalytic NbS2 on CC electrode to adsorb and increase the LiPS redox reaction kinetics along with the improved conductivity from CC. Figure 4 shows the charge discharge behavior of bare semiconducting NbS2 flakes which undergo huge capacity fade upon cycling explaining the inability to hold the PS, its reversible conversion (from liquid LiPS to solid and vice versa). Notably, we can understand the effect of conducting surface toward PS adsorption, mass transport and electron transfer capability, etc. Also, the excellent coulombic efficiency can be seen for the NbS2/CC electrodes when compared with that of CC alone (Fig. 5). Figure 3(c) shows the cyclic life of both the electrodes and confirms that the capacity retention is good for both which supports the need of a conducting matrix for enhanced electrochemical performance.

The potential difference between the charge discharge plateaus reflects the polarization of the electrode which in turn related to the reaction kinetics of the whole system. Here, the potential difference between the plateaus are measured and found that the potential difference becomes larger for bare carbon, i.e., 0.57 V (as shown in Fig. 3(d)) when compared to that of NbS2/CC (0.39 V). This low polarization suggests the better reaction kinetics which is due to the improved electrical conductivity and efficient contact between the NbS2 on CC.

This synergistic effect of catalytic and conducting surface toward better PS adsorption and kinetics are reconfirmed with AC impedance measurements. Electrochemical impedance of both bare CC and NbS2/CC were measured to confirm the enhanced electrical conductivity, structural stability and Li ion diffusion as we designed our approach (Fig. 6(a)). Clearly, the presence of CC with NbS2 nanoflakes contribute toward the improved electrical conductivity, Re from 136 Ω (bare CC) to 93 Ω. Also, from the impedance data, it can be inferred that the porosity and structure of CC is well maintained.

Again, electrocatalysis of LiPS redox activity at different electrode surfaces was measured using potentiodynamic cyclic voltammetry technique. From Fig. 6(b), LiPS reduction peak potential of NbS2/CC was found to be 2.3 V which gets anodically shifted from 2.29 V when compared to that of carbon. In addition, the anodic and cathodic peak potential difference of each catalytic surface gives major information regarding the advantage of having electrocatalytic surface toward polysulfide redox activity. The reduction in the peak potential difference values minimizes the cell polarization, increases the rate capability and cycle life. Here, the difference between anodic and cathodic peak potentials for NbS2/CC was 380 mV, which is lower than that of carbon (410 mV) attributing toward a catalytic nature of NbS2/CC. Similarly, LiPS oxidation peak potential was cathodically favored for NbS2/CC (2.68 V) while compared with carbon (2.7 V) which again confirms the influence of catalytic surface toward better polysulfide catalysis. The cyclic voltammetry derived parameter like peak potential difference clearly attribute towards the surface dependency on LiPS redox kinetics [26,27]. Polysulfide interaction with metal sulfides is purely depending on the charge transfer mechanism rather than the direct bond formation. The unsaturated dangling bonds present in the edge sites of NbS2 will interact more with the polysulfides which will allow the polysulfide redox reactions on their surface. In our previous studies, we have proved that the metal sulfide can act as a catalyst which enhances the polysulfide adsorption, accelerates the liquid to solid PS conversion and improves oxidation kinetics of Li2S [12,17,18]. Thus, aqueous PS redox can be activated by the catalytic surface which experimentally proves our concept of the surface involved electrocatalysis driven sulfur redox process in the Li–S battery system.

In summary, we demonstrate an easy route for the synthesis of NbS2 under a controlled atmosphere. The as synthesized NbS2 nanoflakes were well-characterized and found with 3R polymorph with rhombohedral crystal structures. Also, we have explained successfully the role of conducting and catalytic surfaces for improving the lithium–sulfur redox kinetics while effectively adsorbing the LiPS onto them. Thus, catalytic surface holds the LiPS from dissolution, reduces the redox overpotential and improves the diffusion properties of PS anions, which ends up in high capacity gaining and enhanced cell reversibility. The improvement in conductivity through the incorporation of carbon cloth along with the catalytic behavior of NbS2 helped to give a stable capacity of 810 mA h g−1 for NbS2/CC electrodes. Impedance analysis supported the results by exhibiting a lower charge transfer resistance attributed toward the better conductivity of the electrode.

  • NSF REU program (Award No. 1461031).

  • Faculty start-up funds from Wayne State University.

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References

Ji, X. , and Nazar, L. F. , 2010, “ Advances in Li-S Batteries,” J. Mater. Chem., 20(44), pp. 9821–9826. [CrossRef]
Bruce, P. G. , Freunberger, S. A. , Hardwick, L. J. , and Tarascon, J.-M. , 2012, “ Li-O2 and Li-S Batteries With High Energy Storage,” Nat. Mater., 11(1), pp. 19–29. [CrossRef]
Demir-Cakan, R. , Morcrette, M. , Gangulibabu, Gueguen, A. , Dedryvere, R. , and Tarascon, J.-M. , 2013, “ Li-S Batteries: Simple Approaches for Superior Performance,” Energy Environ. Sci., 6(1), pp. 176–182. [CrossRef]
Zhang, H. , Yu, X. , and Braun, P. V. , 2011, “ Three-Dimensional Bicontinuous Ultrafast-Charge and -Discharge Bulk Battery Electrodes,” Nat. Nanotechnol., 6(5), pp. 277–281. [CrossRef] [PubMed]
Manthiram, A. , Fu, Y. , Chung, S.-H. , Zu, C. , and Su, Y.-S. , 2014, “ Rechargeable Lithium–Sulfur Batteries,” Chem. Rev., 114(23), pp. 11751–11787. [CrossRef] [PubMed]
Zhang, S. S. , 2013, “ Liquid Electrolyte Lithium/Sulfur Battery: Fundamental Chemistry, Problems, and Solutions,” J. Power Sources, 231, pp. 153–162. [CrossRef]
Song, M.-K. , Zhang, Y. , and Cairns, E. J. , 2013, “ A Long-Life, High-Rate Lithium/Sulfur Cell: A Multifaceted Approach to Enhancing Cell Performance,” Nano Lett., 13(12), pp. 5891–5899. [CrossRef] [PubMed]
Mikhaylik, Y. V. , and Akridge, J. R. , 2004, “ Polysulfide Shuttle Study in the Li/S Battery System,” J. Electrochem. Soc., 151(11), pp. A1969–A1976. [CrossRef]
Zhang, Q. , Wang, Y. , Seh, Z. W. , Fu, Z. , Zhang, R. , and Cui, Y. , 2015, “ Understanding the Anchoring Effect of Two-Dimensional Layered Materials for Lithium–Sulfur Batteries,” Nano Lett., 15(6), pp. 3780–3786. [CrossRef] [PubMed]
Liang, X. , Hart, C. , Pang, Q. , Garsuch, A. , Weiss, T. , and Nazar, L. F. , 2015, “ A Highly Efficient Polysulfide Mediator for Lithium–Sulfur Batteries,” Nat. Commun., 6, p. 5682. [CrossRef] [PubMed]
Liang, X. , Garsuch, A. , and Nazar, L. F. , 2015, “ Sulfur Cathodes Based on Conductive MXene Nanosheets for High-Performance Lithium–Sulfur Batteries,” Angew. Chem., Int. Ed., 54(13), pp. 3907–3911. [CrossRef]
Babu, G. , Masurkar, N. , Al Salem, H. , and Arava, L. M. R. , 2017, “ Transition Metal Dichalcogenide Atomic Layers for Lithium Polysulfides Electrocatalysis,” J. Am. Chem. Soc., 139(1), pp. 171–178. [CrossRef] [PubMed]
Fu, Y. , Su, Y.-S. , and Manthiram, A. , 2014, “ Li2S-Carbon Sandwiched Electrodes With Superior Performance for Lithium-Sulfur Batteries,” Adv. Energy Mater., 4(1), p. 1300655. [CrossRef]
Kamphaus, E. P. , and Balbuena, P. B. , 2016, “ Long-Chain Polysulfide Retention at the Cathode of Li–S Batteries,” J. Phys. Chem. C, 120(8), pp. 4296–4305. [CrossRef]
Fu, Y. , Su, Y.-S. , and Manthiram, A. , 2012, “ Sulfur–Carbon Nanocomposite Cathodes Improved by an Amphiphilic Block Copolymer for High-Rate Lithium–Sulfur Batteries,” ACS Appl. Mater. Interfaces, 4(11), pp. 6046–6052. [CrossRef] [PubMed]
Yuan, Z. , Peng, H.-J. , Hou, T.-Z. , Huang, J.-Q. , Chen, C.-M. , Wang, D.-W. , Cheng, X.-B. , Wei, F. , and Zhang, Q. , 2016, “ Powering Lithium–Sulfur Battery Performance by Propelling Polysulfide Redox at Sulfiphilic Hosts,” Nano Lett., 16(1), pp. 519–527. [CrossRef] [PubMed]
Babu, G. , Ababtain, K. , Ng, K. Y. S. , and Arava, L. M. R. , 2015, “ Electrocatalysis of Lithium Polysulfides: Current Collectors as Electrodes in Li/S Battery Configuration,” Sci. Rep., 5, p. 8763. [CrossRef] [PubMed]
Thangavel, N. K. , Gopalakrishnan, D. , and Arava, L. M. R. , 2017, “ Understanding Heterogeneous Electrocatalysis of Lithium Polysulfide Redox on Pt and WS2 Surfaces,” J. Phys. Chem. C, 121(23), pp. 12718–12725.
Al Salem, H. , 2016, “ Stabilizing Polysulfide-Shuttle in a Li–S Battery Using Transition Metal Carbide Nanostructures,” RSC Adv., 6(111), pp. 110301–110306. [CrossRef]
Aray, Y. , Zambrano, D. , Cornejo, M. H. , Ludeña, E. V. , Iza, P. , Vidal, A. B. , Coll, D. S. , Jímenez, D. M. , Henriquez, F. , and Paredes, C. , 2014, “ First-Principles Study of the Nature of Niobium Sulfide Catalyst for Hydrodesulfurization in Hydrotreating Conditions,” J. Phys. Chem. C, 118(48), pp. 27823–27832. [CrossRef]
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Figures

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Fig. 6

Electrocatalytic properties: (a) AC impedance measurements of NbS2/CC and bare CC electrode and (b) comparative cyclic voltammograms of NbS2/CC and bare CC as a working electrode versus Li/Li+ in 0.06 M catholyte solution at scan rate of 0.1 mV/s

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Fig. 5

Coulombic efficiency of NbS2/CC and bare CC electrodes

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Fig. 4

Charge–discharge profiles of bare NbS2 at 0.1 C rate

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Fig. 3

Electrochemical behavior: (a) and (b) charge–discharge profiles of bare CC and NbS2/CC at 0.1 C rate, respectively, (c) cycling study of electrocatalytically active NbS2/CC and bare CC as working electrode versus Li/Li+ with catholyte consisting of 0.2 M Li2S8 at 0.1 C rate, and (d) comparison of charge discharge curves of NbS2/CC and bare CC for analyzing the polarization behavior

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Fig. 2

Morphological characterization of NbS2 nanoflakes, NbS2/CC: (a) SEM image of NbS2 flakes, (b) the fragmented single NbS2 nanoflakes after sonication process, (c) SEM image of bare CC, and (d) fragmented NbS2 flakes on CC after deposition

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Fig. 1

Characterization of NbS2 nanoflakes: (a) XRD pattern of NbS2 flakes and (b) raman spectrum showing characteristic vibration modes of NbS2 nanoflakes

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