The Effects of Impurities on the Electrochemical Characterization of


Mar 7, 2019 - Habib M.N. Ahmad , Sujoy Ghosh , Gaurab Dutta , Alec G. Maddaus , John G Tsavalas , Shawna Hollen , and Edward Song. J. Phys. Chem...
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C: Energy Conversion and Storage; Energy and Charge Transport

The Effects of Impurities on the Electrochemical Characterization of Liquid Phase Exfoliated Niobium Diselenide Nanosheets Habib M.N. Ahmad, Sujoy Ghosh, Gaurab Dutta, Alec G. Maddaus, John G Tsavalas, Shawna Hollen, and Edward Song J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00485 • Publication Date (Web): 07 Mar 2019 Downloaded from http://pubs.acs.org on March 8, 2019

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The Effects of Impurities on the Electrochemical Characterization of Liquid Phase Exfoliated Niobium Diselenide Nanosheets Habib M. N. Ahmad1, Sujoy Ghosh1,4, Gaurab Dutta1, Alec G. Maddaus2, John G. Tsavalas3,5, Shawna Hollen4,5, and Edward Song1,5, *

1Department

of Electrical and Computer Engineering, University of New Hampshire, Durham,

NH 03824 USA 2Department

of Chemical Engineering, University of New Hampshire, Durham, NH 03824 USA

3Department

of Chemistry, University of New Hampshire, Durham, NH 03824 USA

4Department

of Physics, University of New Hampshire, Durham, NH 03824 USA

5Materials

Science Program, University of New Hampshire, Durham, NH 03824 USA

*Corresponding author. Tel: +1-603-862-5498. E-mail: [email protected]

ABSTRACT Commercially available bulk niobium diselenide (NbSe2) reduced into nanomaterials upon exfoliation typically contains oxide and carbide impurities. Liquid phase exfoliated twodimensional (2D) nanosheets of NbSe2 obtained from bulk powders provide high charge mobility and large surface area but become self-passivated and chemically inert as the presence of oxide impurities make them behave more semi-metallic. In this article, we report the effects of inherent impurities of liquid phase exfoliated 2D NbSe2 (intended to be integrated as supercapacitor electrodes) on electrochemical performance. The highest specific capacitances achieved using 1butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6) and sulfuric acid (1M H2SO4) electrolytes were 4955.5±21.5% mF/cm2 and 13361.6±31.8% mF/cm2, respectively, which were affected by the impurities, the oxophilicity of niobium defects and the moisture adsorption in the

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cell. Galvanostatic charge-discharge profiles show moisture adsorption affecting the high energy charging procedure in the cell for BMIMPF6 resulting in leakage and decomposition of the electrolyte. Electrochemical impedance spectroscopy (EIS) provided insight into the solidelectrolyte interphase and charge transfer mechanisms at exfoliated 2D NbSe2 nanosheets, which affect the ion intercalation through heterogenous phases of the nanosheets. Overall, the NbSe2 nanosheets offer heterogenous phases due to the coexistence of Nb2O5 that influences the charge transfer mechanism at the exfoliated surfaces.

1.

Introduction Recently, energy storage research has been pursuing materials that have the potential to

combine the benefits of a high energy density of batteries with the long cycle life and short charging times of supercapacitors1–4. Among the favorable materials, transition metal dichalcogenides (TMDs) exhibit favorable properties for potential applications in catalysis5–10, energy storage and conversion devices1–3, and therefore have spurred renewed interest. Furthermore, some insulating transition metal oxides (TMOs) on a TMD surface provide better properties for electrochemical double layer capacitor (EDLC) and battery applications11–16. High electrical conductivity, chemical stability, and large specific surface area are among the utmost desirable properties of electrode materials for electrochemical energy storage

17–19.

Among several TMDs, MoS2, WS2, and others have been widely studied for potential use as electrode materials5,20–22. However, limited electrical conductivity in these TMDs poses significant challenges for rapid charge transfer and thus limits the energy storage capabilities. Recently, Acerce et al. demonstrated high specific capacitance with 1T exfoliated MoS2 due to enhanced hydrophilicity and high electrical conductivity when compared to exfoliated 2H-MoS21. However,

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1T MoS2 is unstable over time and temperature and further requires a more complex exfoliation processes23. In this regard, low-dimensional exfoliated NbSe2 can possibly provide enhanced charge storage capabilities due to its superior metallic properties with the high electrical conductivity of ~104 S/cm24. NbSe2 is metallic at ambient temperature and exhibits superconductivity at low temperatures (~7.0 K)25. Niobium atoms at the center of the trigonal selenium prisms of NbSe2 nanocrystals have the same layered hexagonal structure as 2H-MoS2 with van der Waals interactions between the selenium layers26,27. Among different available synthesis methods, a simple, economical and universal large-scale synthesis route that results in 2H-phase pure TMD bulk powders has been reported by Sofer et al. via a thermal reaction25. Excess chalcogens are typically introduced into the system to improve the crystallinity of TMD bulk powder. However, such methods can cause impurities in the crystals such as niobium oxides and carbides14,25,28. Few or single-layer NbSe2 flakes are obtained during exfoliation due to cleaving between selenide layers from the bulk powder12,29. The 2D NbSe2 contains point defects which include monoselenium and diselenium vacancies coupled with anti-site defects (where two atoms of Se replace the Nb) along the grain boundaries or edge planes27. The non-stoichiometric oxide impurity phases are formed via the oxidation in the exfoliated NbSe2 samples, which occurs at the defects due to oxophilicity of niobium11,12,30. Niobium and selenium oxides have different electrical properties: NbO is conductive, NbO2 is semiconducting, Nb2O5 is insulating, and selenium oxides are conductive11–13,30,31. Forming a higher fraction of the Nb2O5 phase during annealing in vacuum or exposure in the air by the NbO-NbO2-Nb2O5 reaction sequence is possible in bulk and exfoliated NbSe211,16. Nanoscale thin Nb2O5 has transparent features, which are indistinguishable in their surface morphology11,13,15. Native NbC impurities in both bulk and exfoliated samples originate

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from the undesirable reaction with carbon contamination during thermal synthesis of the bulk NbSe2 powders14,28. These impurities introduce surface inhomogeneity in exfoliated NbSe2 affecting the charge storage. So far, the effects on the electrochemical and electrocatalytic properties of group V TMDs coupled with native impurities (such as oxides and carbides) have not been well-studied. Hence, the focus of this research is to study the effects of such impurities on the charge storage capability of the liquid phase exfoliated NbSe2 nanosheets. Here, we report the surface chemistry, as well as the electrochemical charge storage properties of the liquid phase, exfoliated 2D NbSe2 sheets derived from commercially available bulk NbSe2 powder as a precursor with native carbide and oxide impurities. Based on our elemental analysis using XPS, we have identified Nb2O5 as a significant impurity with trace amounts of NbC, critically affecting the electrochemical charge transfer at exfoliated NbSe2. We have also performed cyclic voltammetry (CV) measurements and galvanostatic charge-discharge (CD) characterizations in two different types of electrolytes, namely, acidic medium (1M H2SO4) and ionic liquid (IL) medium (BMIMPF6) to investigate the charge transfer mechanisms at the exfoliated surfaces.

2. MATERIALS AND METHODS 2.1. Exfoliation of NbSe2 Nanosheets from Bulk powder NbSe2 nano-sheets were exfoliated from the bulk powder (Alfa Aesar, 99.8% purity) via the liquid phase exfoliation technique32–34. Bulk NbSe2 powder (5.0 mg/mL) was added to a solution mixture of 35% ethanol and 65% deionized water. The final solution was sonicated with an average power of ~ 60W for 30 minutes using a horn-tip sonicator (Branson 550W). A stable temperature was

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maintained during sonication by using an ice bath. After exfoliation, the dispersed NbSe2 was centrifuged at 3000 rpm for 30 minutes to isolate the few layered nanosheets. If the centrifuge speed was higher than 3000 rpm, the resulting NbSe2 dispersion became colorless, indicating less availability of NbSe2 nanoflakes in the supernatant34,35. The exfoliation process is schematically shown in Figure 1a. The collection of the supernatant solution was then followed by electrode fabrication. Figure S6 shows a TEM image featuring aggregation of nanoparticles and sedimentation on the NbSe2 flakes due to further sonication during exfoliation, which is in agreement with the literature34. 2.2. Exfoliated NbSe2 Electrode Fabrication NbSe2 electrodes were fabricated using vacuum filtration to deposit exfoliated NbSe2 on hydrophilic PTFE (Polytetrafluoroethylene) membranes with a 0.1 µm pore size (Sterlitech). After deposition of the supernatant solution, the membranes were carefully dried with a light stream of air. The mass of the membrane was measured before and after deposition to achieve an effective mass loading of ~6.8 mg (3.8 mg-cm-1) of the exfoliated NbSe2. The membranes were then cut into electrodes with the same dimensions such that only one side contained the active material with an effective area of ~1.77 cm2. 2.3. Integration of NbSe2 Electrodes in a Test Cell for Electrochemical Characterization A stainless-steel split test cell (MTI Corporation; Model: XA16008) was used for measurements. The electrode integration was done by first placing a single electrode inside with the insulating membrane side facing up to maintain contact between the exfoliated NbSe2 layer with the bottom current collector. A small amount of liquid electrolyte was added on top of the membrane and then another blank PTFE membrane was placed over the electrode to serve as the separator. A second

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electrode with the deposited NbSe2 side facing up was placed to connect with the top current collector. The electrodes were then pressed together and the excess electrolyte was removed before closing the test cell to prevent short-circuiting between the electrode connections26. Figure 1b depicts the exploded view of the split test cell configuration. 2.5. Morphological, Structural and Optical Characterization of Liquid Phase Exfoliated NbSe2 The exfoliated NbSe2 nanosheet morphology was examined by Transmission Electron Microscopy (Zeiss / LEO 922 Omega TEM). Also, the nanosheets were characterized by UV-Vis spectroscopy (SPELEC 1050; wavelength range 350−1050 nm, Dropsens). XPS studies were performed using a Thermo Scientific K-Alpha system with a base pressure of 5 × 10 ―9 mbar using the AlK (1486.8 eV) X-ray monochromatized radiation with a pass energy of 50 eV (resolution: 0.5 eV). Peak fitting was carried out using the Thermo Avantage Data System. 2.6. Sample Preparation for TEM, XPS, and UV-vis Absorbance Spectra Measurements Samples for the TEM characterization were prepared by placing a small droplet of the supernatant solution containing the exfoliated NbSe2 nanosheets (0.5 mg/mL) on a lacey carbon grid and drying at room temperature in air for several hours. For XPS measurements, the bulk and the exfoliated NbSe2 sample solutions (5 mg/mL) were drop-casted onto a Si/SiO2 substrate and dried at 100ºC for 5 mins. UV-Vis was performed on the supernatant solution after exfoliation (5 and 0.5 mg/mL). 2.7. Electrochemical Characterization of Liquid Phase Exfoliated NbSe2 Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge-discharge (CD) characterization were performed using a Gamry Instruments Reference

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600+ potentiostat/galvanostat/ZRA. Two different electrolytes, sulfuric acid (1M H2SO4) and ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6) were used to test the prepared electrodes. CVs were performed at scan rates of 10, 50, 100, 200, 500, 700 and 1000 mVs-1 from 0.0 to +0.8 V for sulfuric acid and 0.0 to +2.5 V for BMIMPF6 electrolyte. Galvanostatic charge-discharge (CD) was performed at constant current loads of 2.0, 2.8, 4.0, 5.2 and 6.8 mAcm-2. Figure 5 indicates the IR drops which were observed from the CD curves for the two different electrolytes measured at the abovementioned constant current loads. Each of the CV and CD curves were repeated for five cycles in both electrolytes. The EIS measurements were performed within the frequency range of 0.1 Hz to 100 KHz for H2SO4 and 0.1 Hz to 1 MHz for BMIMPF6 with zero DC bias. For sulfuric acid, the maximum frequency of 100 KHz was selected to minimize the inductance effect. The equivalent circuit fitting was also performed by a nonlinear least squares method as shown in Figure 6a and 6b with circuit parameters and error range (n = 3) listed in Table 1.

3. RESULTS AND DISCUSSION 3.1. The Absorbance Spectrum and TEM Characterization of Exfoliated and Bulk NbSe2 The UV-vis spectra in Figure 2a indicate a weak absorbance spectrum with no prominent peaks for the exfoliated NbSe2 samples independent of concentration. Metallic NbSe2 requires little energy in d−d transition indicated by no visible absorption in the spectrum after exfoliation, which is in agreement with the previous reports36. Zhou et al. reported that liquid phase exfoliated NbSe2 flakes show a reduction in size upon increment in centrifuge speed, while also demonstrating a size-independent nonlinear optical property indicating a fast charge transfer process at 2D NbSe2

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which is also consistent with our spectra34. The absorbance spectra for Nb2O5 (from 200 – 600 nm) and NbC (from 350 – 850 nm) were not observed in our UV-vis measurements37,38. The optical absorption due to the presence of atomic- and nano-scale Nb2O5 and NbC impurities present in our exfoliated NbSe2 samples (confirmed by XPS) were not visible in the UV-Vis spectra indicating the overall metallic optical property of NbSe213,34,37,39. In agreement with the previous report34, Figure 2c and 2d shows the TEM images featuring a transparent morphology of exfoliated NbSe2 nanosheets with low rigidity, suggesting that the exfoliation was successful in producing few-layer flakes of NbSe234. 3.2. XPS Characterization of the Exfoliated and Bulk NbSe2 Surface functionalities by X-ray photoelectron spectroscopy (XPS) in Table S5 and Figure 3 show that both bulk and exfoliated NbSe2 nanosheets have significant Nb2O5 impurities with a negligible fraction of NbC. The XPS studies also reveal that defect induced phases of the NbSe2 nanosheets oxidize to Nb2O5 during the synthesis which induces heterogeneous electrochemical activity. The Nb 3d spectra curve in Figure 3a and 3b fitted in both exfoliated and bulk samples reveal that the core level spectrum has three peaks at 203.4 eV, 206.2 eV, and 207.2 eV. The peaks corresponding to the Nb4+ 3d5/2 spectra (i.e., 203.4 eV and 206.2 eV) can be assigned to NbSe2 which coexists with the Nb5+ 3d3/2 spectra peak (i.e., 210.2 eV) that corresponds to Nb2O5 10–12,27. Table S5 affirms that both the bulk powder and exfoliated nanosheets of NbSe2 have traces of Nb2O5. C1 and O1 core spectra suggest the presence of carbon and oxide contamination on both types of NbSe2 samples, which may have occurred during sample synthesis and exposure to atmosphere11,40,41. The Nb2O5 phases have a peak at 530.8 eV in O1 spectra as reported in the literature15. Figure 3 also displays increased intensity of the Nb2O5 peak in both Nb3d core and O1 spectra in exfoliated samples compared to their bulk counterparts suggesting more oxide impurities at the surface.

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Figure S4a and S4b show the charge reference in C1 spectra for both sample types at 284.8 eV which indicates the graphitic peak in both samples14. The presence of non-stoichiometric NbC impurities was confirmed in Table S5 for C1 spectra having a peak at 282.8 eV for both bulk and exfoliated samples14. The exfoliated samples shown in Table S5 have attenuated atomic percentage (At%) of NbC, suggesting the dissolution of impurities during liquid phase exfoliation14. The Se3d core level spectrum fitted with Se3d5/2 (53.2 eV and 54.8 eV) and Se3d3/2 (55.6 eV) agrees with the spectra of NbSe210,12,27. The Se3d spectrum at 60.5 eV for both sample types shows significant oxidation of the Se phases which is indicative of the presence of nonstoichiometric selenium oxides or acidic SeO42- groups on NbSe2 phases42. Javey and co-workers evaluated the SnSe2 films and found oxygen impurities bonded to Se forming diselenium oxides, a potential impurity in exfoliated TMDs which contributes to n-type degeneracy resulting in a more metallic behavior31. XPS data confirms the presence of impurities, i.e., NbC, Nb2O5 and selenium oxides with pure NbSe2 phase having a variation in At% after exfoliation. Also, the O1 spectra in Figure 3e and 3f reveal peaks at 532.5 eV for silicon dioxide (SiO2) occurring from the wafer substrate at which samples were deposited for XPS measurement39. No peaks attributed to metallic niobium (Nb0) and chalcogenide selenium (Se0) were found in either samples11,42. 3.3. Electrochemical Studies of Exfoliated NbSe2: Specific Capacitance and Charge Build-Up Characterization. Energy storage is typically divided into three mechanisms: purely capacitive (the electrochemical double layer capacitors (EDLCs))26, pseudo-capacitive, and finally the battery-like charge storage regime4,43–45. The EDLC stores energy in the electrostatic mode, relying on high specific surface area and thus potential dependent charge storage at the surface4,43,44. Therefore, an EDLC results in high power density and low energy density4,43,45. The pseudo-capacitive charge storage involves

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partial faradaic charge transfer mechanism along with the EDLC present at the surface45. The battery-like charge storage relies predominantly on faradaic transfer involving only redox reaction, resulting in higher energy density and lower power density45. The electrochemical activities at exfoliated NbSe2−electrolyte interface are mainly governed by the diffusion-limited charge transfer and electrical double layer currents as reported earlier6,10,46. Figure S1 demonstrates that the exfoliated nanosheets offer more specific surface area, increased double layer capacitance and overall specific capacitance compared to the bulk NbSe2 powder. The CVs of exfoliated NbSe2 in 1M H2SO4 scanned from 0 to +0.8V have anodic peaks at different scan rates, indicating hydrogen adsorption or impurity/defect induced phase oxidation19,47–49. The initial shoulder peak in Figure 4a is assigned to hydrogen (H+) adsorption, which may arise from the impurity (mainly Nb2O5) induced NbSe2 phases46,50. Table S1 and Figure 4b provide evidence that such an adsorption peak is moderately distinct for the scan rates of 10 – 700 mV.s-1. Table S1 shows that the shoulder peak shifts towards the positive potential and appears suppressed with increasing scan rate. This peak shift results from the drift of charge carriers due to surface stress and limited diffusion at faster scan rate45,46. A small oxidation peak was observed at around +585 ± 0.5% mV at 200 mV.s-1 scan rate. Table S1 shows such oxidation peak shifts to +692 ± 0.7% mV for the scan rate 1000 mV.s1.

The oxidation peaks around +585 mV to +692 mV arise possibly due to the faradaic electron

transfer for irreversible reactions at exfoliated surfaces6,10,16,46. The occurrence of such oxidation peaks may signify the redox state of impurity and defects (i.e., Nb2O5) induced phases at exfoliated surface14,15,38,46,50. The corresponding oxidation peaks shift anodically as indicated in Table S1, ranging from +64 ± 3.2% mV (for scan rate 500 mV.s-1) to +107 ± 3% mV (for scan rate 1000 mV. s-1) with respect to the scan rate of 200 mV.s-1, which occurs due to the charge carrier drift13,16,45,50. Such drift indicates greater resistance towards faradaic charge transfer in exfoliated

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NbSe2 nanosheets, and this may also arise due to the surface roughness46. This peak is not present at scan rates less than 50 mV.s-1 suggesting the existence of electrochemically less active phases at the exfoliated NbSe2. For the scan rate of 1000 mV.s-1 in Figure 4a, the shoulder peak is flattened out resulting in broader rectangular shaped voltammograms indicating more EDLC dominated charge storage at the electrode-electrolyte interface6,26,51. The CV in 1M H2SO4 at scan rate 1000 mV.s-1 shows enhanced charging current which suggests the simultaneous activities of both Nb2O5 and NbSe2 phases46,50. Zhang et al. have shown that nanometer thin Nb2O5 can result in a suppressed oxygen reduction reaction (ORR) affecting the electrocatalytic activity at metal and glassy carbon surfaces13. The irregular phases of the native Nb2O5 at exfoliated NbSe2 nanosheets can act as an electron extraction inhibitor30. Similar suppression of ORR during a cathodic sweep was also observed in Figure 4a for CVs of 1M H2SO4 with irreversible reaction and partial redox state at exfoliated NbSe2 nanosheets13,16. Lim et al. demonstrated a complementary non-reversible specific capacity and a sluggish rate capability affecting the electrochemical activity for TT−Nb2O5 (pseudohexagonal phases) at carbon core-shell nanocrystals compared to T−Nb2O5 (orthorhombic phases)16,18. Possible formation of TT−Nb2O5 phases can occur from the hydrolysis of quasi-stable niobium (V) ethoxide which may form during liquid phase exfoliation12,15,16,34. The NbSe2 shows improved diffusion-controlled charge transfer at the surfaces in acidic medium (1M H2SO4) and promotes the EDLC behavior exploiting the pseudo-capacitance for enhanced capacitive charge storage52. Compared to the acidic medium, the room temperature IL (i.e., BMIMPF6) shows less faradaic charge transfer due to the absence of the redox state at the surface53. Usually, ILs offer less conductivity at ambient temperature and lower Debye length, promoting higher double layer capacitance at the interface45,51,53,54. BMIMPF6 provides a greater potential window, allowing positive potentials near +2.5 V, suitable for

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assessing charge storage capabilities in supercapacitor and rechargeable batteries45,51,53. Lower scan rate CV in BMIMPF6 with forwarding scan shows subdued oxidation states indicating partial redox behavior, suppressed diffusion of PF6− ions at heterogenous NbSe2 phases45,51,54. The reverse scans in Figure 4d and 4e indicate no visible BMIM+ ion intercalation at the exfoliated surface, signifying no reversible faradaic charge transfer. The suppressed anodic peaks at +1000 to +1300 mV (Peak 1) and +2000 to +2200 mV (Peak 2), which are visible at lower scan rates (10 – 200 mVs-1) with no prominent reduction peaks, represent irreversible reactions at the interphase16,53. Figure 4d and 4e and Table S2 show Peak 1 visible for scan rates of 10 – 200 mVs-1 and Peak 2 for 50 – 200 mVs-1 but not evident in scan rates greater than 200 mVs-1. Such suppression may have possibly occurred for restricted PF6− ion diffusion at the surface10,46,53,54. According to Table S2 during the forward CV scan in BMIMPF6, Peak 2 appears to be more anodic indicating interphases restricting ionic diffusion concerning the interphases related to Peak 1. Shifts of Peak 2 (from +166 to +253 mV) and Peak 1 (from +579 to +722 mV ) indicate electron drift50,51,53,54. A sharp and asymmetric peak in Figure 4d that appears around +2450 mV for scan rates greater than 200 mVs-1 shows a stretched anodic scan with inclined nature of CVs in IL26,51. More inclined CVs in BMIMPF6 possibly signifies a large IR drop at exfoliated NbSe2 with increased overall resistance towards PF6− ion diffussion26,55–57. Such sharp current in an anodic scan of CV is possibly due to charge puddling, which can be attributed to surface roughness, inhomogeneity, or adsorption of moisture resulting in possible hydrolysis at the exfoliated surface26,51,55. Xiao and coworkers observed similar effects in MnO2 by diluting BMIMPF6 resulting in an attenuation of the adsorption effect of [BMIM]+ ion at the surface indicating reduced viscosity and resistance of IL51. Such effects in BMIMPF6 can occur from interaction with moisture, thus weakening the hydrogen bonds and Van der Waals forces5,17,19,26,48,51,55. Ionic liquid (IL) medium (BMIMPF6)

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shows less involvement of a pseudocapacitive nature compared to the sulfuric acid medium due to the electrostatic manner of charge storage at the exfoliated surfaces4. The charge storage with IL at exfoliated NbSe2 is affected by parallel leakage and solution decomposition due to the presence of impurities and formation of the solid-electrolyte interphase (SEI)4,52. The SEI is composed of precipitates from the decomposition of electrolytes and oxide impurities at exfoliated NbSe2 nanosheets4,43. The SEI is electrochemically more distinguishable in IL compared to acidic medium which can be assessed by the electrochemical impedance spectroscopy (EIS) studies4,43. Specific capacitance (𝐶𝑠𝑝) of NbSe2 is calculated from the CV curve by using the relationship 𝐶𝑠𝑝 =

∫𝐼(𝑉)𝑑𝑣 𝑣𝐴∆𝑉

where, ∫𝐼(𝑉) is the integrated area under the CV curve, 𝑣 is the scan

rate, 𝐴 is the electrode area of electrode, and ∆𝑉 is the potential window26. Both CVs in Figure 4a and 4d suggest that exfoliated NbSe2 nanosheets exhibit enhanced surface area for non-insertion charge storage with reduced ion diffusion path, offered potential-independent specific capacitance45. The 𝐶𝑠𝑝 values with respect to 𝑣 for both electrolytes obtained from Figure S2 were tabulated in Table S3. At lower scan rate (i.e. 10 mVs-1) the 𝐶𝑠𝑝 is 13361.6 ± 31.8% mF.cm-2 in 1M H2SO4 and 4955.5 ± 21.5% mF.cm-2 in BMIMPF6. The 2-fold increase indicates improved EDLC behavior in 1M H2SO456,57. At higher scan rates 1M H2SO4 (𝐶𝑠𝑝:1.41 ± 20% mF.cm-2) has resulted in 5-fold higher values compared to BMIMPF6 (𝐶𝑠𝑝:0.28 ± 6.1% mF.cm-2). A decline in 𝐶𝑠𝑝 values in IL can be attributed to less capacitance due to limited diffusion, resulting in a overall lowered synergistic effect during charge transfer at NbSe2 nanosheets26. Figure S2 and the inset in Table 1 illustrates the relationship between 𝐶𝑠𝑝 and 𝑣 after fitting for different data, represented

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𝐴

as 𝐶𝑠𝑝 = 𝑣𝑚 where, 𝑚 and 𝐴 are arbitrary constants, representing the overall EDLC behavior. Depending on the electrolyte, the lower “𝑚” value results in lower“𝐴” value, signifying enhanced diffusion-controlled pseudocapacitance behavior and elevated synergistic effect of charge storage in 1M H2SO46,46,56. Higher “𝑚” value leads to higher “𝐴” value indicating less pseudocapacitance resulting in low coulombic efficiency and slower kinetics at the surface in BMIMPF616,26,46,53,54,56. Further investigation is needed in understanding the effects of 𝑚 and 𝐴 values on the charge storage mechanism at nanoscale. The CD in Figure 5 measures the IR drop under different media, which depends the ion diffusion resistance at electrode-electrolyte interfaces. The IR drop is also influenced by pseudocapacitance at interface51,54,57. The CD behavior indicates the parallel leakage process and solvent decomposition and thus has significant impact on the workable potential range, a self discharge, efficiency and the supercapacitor-like performance for the NbSe251,52,58. The CD recorded in Figure 5 for both 1M H2SO4 and BMIMPF6 have two distinct plateaus during charging resulting from non-uniform acceptance of charges across the inhomogeneous surface14,52. The first plateau (labeled P1 in Figure 5) represents oxidation of niobium phases (mainly Nb4+) at surfaces, forming further oxidized phases4,14. Such oxidation initiates SEI layer formation at exfoliated surface4,43,59. The second plateau (P2) represents solvent decomposition at the NbSe2 nanosheets4,14,43,52,59. The SEI layer formation initiates parallel leakage and electrolyte decomposition4,43,52. During charging in 1 M H2SO4 in Figure 5a and 5b, the exfoliated NbSe2 resulted in steeper P1 followed by concise P2 which indicates enhanced SEI activity in 1M H2SO4 with faster charging4,14,59. This results in a reduction of the maximum steady state cell potential from the expected value of +0.8 V down to +0.7 V in 1M H2SO4. Figure 5a and 5b manifest near linear behavior in 1M H2SO4 for the potential (E) vs. time (t) response at lower current loads (~2.0 mA.cm-2), indicating voltage-independent parallel leakage at the exfoliated surfaces4,14,16,46,52. The

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CD in 1M H2SO4 indicates electron transfer reactions at the chemical equilibrium providing enhanced mass transport4,6,10,16,52. The CD in Figure 5c and 5d for BMIMPF6 show a shorter P1 followed by a broader and much flatter P2 indicating voltage-independent parallel leakage coupled with electrolyte decomposition during charging14,52. The electrolyte decomposition occurs from two phases co-existing at the electrode surfaces58,59. The span of plateaus represents the miscibility gap at the nanoscale surface in strong polarized medium58,60. Li et. al. have shown that, upon reducing the materials to nanoscale, the specific surface area increases with reduced miscibility gap58,59. However, in BMIMPF6, the difference in surface free energies of the two phases (pure NbSe2 and impurity phases) can lead to the decomposition of PF6- ions due to the decreasing cation-anion interaction55,58,60. The electrolyte decomposition depicted in Figure 5c and 5d at low current loads is possibly due to moisture interactions and the decline in cation-anion interactions in the IL55. At higher current loads, P2 becomes less evident resulting in low decomposition51,52. Exfoliated NbSe2 shows parallel leakage and solution decomposition in ILs which have affected the electrochemical performance in our cell significantly52. The mass transfer limitation results in an enhanced IR drop, as shown in Figure 5c and 5d, and Table S3 and S451,53,54. Figure S3 (and the data in Table S4) shows the plot for IR drop vs. current density load which presents a greater slope for 𝐾𝐵𝑀𝐼𝑀𝑃𝐹6= 0.13883 ± 26.86% Ω.𝑐𝑚 ―2, compared to 𝐾𝐻2𝑆𝑂4= 0.02371 ± 27.19% Ω.𝑐𝑚 ―2. This underlines that the exfoliated NbSe2 has a higher equivalent series resistance in the ionic liquid (IL) compared to that in the acidic medium57. From Figure 5, the discharge time recorded for the IL at high current load (6.8 mA.cm-2) is about 2.4 seconds compared to the low current load (2.0 mA.cm-2) which is about 33 seconds. In H2SO4 the discharge time is about 1.5-fold less (at 6.8 mA.cm-2) and about 2-fold less (at 2.0 mA.cm-2) with respect to the IL. Figure 5a and 5b show

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that the acidic medium experiences a sharp drop in discharge voltage which resulted in reduced discharge time intends low polarization losses coupled with elevated self-discharge 43,52.

3.4. Electrochemical Impedance Spectroscopy (EIS) Characterization of Exfoliated NbSe2 in a Split Test Cell Arrangement: The Effects of NbSe2 on the Interfacial Properties upon Exfoliation.

In this section, the heterogeneous reaction kinetics at exfoliated NbSe2 is investigated using EIS in frequency domain44,51,61. EIS reveals the presence of SEI and the distinct phases (i.e., Nb2O5, NbSe2) at the exfoliated NbSe2 surface. Figure 6a and 6b show the Nyquist plots for both liquid media (i.e., H2SO4 and BMIMPF6). The depressed semicircle at high frequency can be attributed to the combined interfacial impedances of SEI (impurity phase) and phase-pure NbSe2 surface for charge transfer. The straight line at low frequency indicates ion diffusion at exfoliated surfaces4,43,61. The equivalent circuit fitting shown in Table 1 indicates that the sulfuric acid-based coin cell results in a lower series ohmic resistance Rs (3.83 ± 1.4% Ω) compared to BMIMPF6based cell (9.16 ± 0.7% Ω) due to the higher resistivity of ionic diffusion in the IL26,53. The ohmic resistance Rs measured at the electrode-electrolyte interface includes contributions from the filter papers as well as the charge barrier between the stainless-steel current collector and NbSe2 layer26. According to the circuit model, the exfoliated NbSe2 surface comprises two regions, namely Region 1 and Region 2 showing non-uniform electrochemical acitivity43. The three elements fitted for these two regions are resistance (R), capacitance (C), and a constant phase element CPE (Q). Region 1, which is in series with Rs, represents SEI and charge-transfer mechanism at the surface, modeled by a parallel combination of R1 and Q1. The CPE Q1 can be attributed to frequency-

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dispersed non-ideal capacitive behavior for double layer capacitance that originates from spatial inhomogeneity of the surface. R1 is attributed to the charge transfer resistance in Region 14,43,59,61. Apparently, Region 1 is reflected by the suppressed semicircle in both Nyquist plots in Figure 6a and 6b. The impedance 𝑍𝑄𝑚contributed by Q is described below:

𝑍 𝑄𝑚 = 𝑄

1 𝑚(𝑗𝜔)

Eq.1

𝑛

Where, m is either 1 or 2, representing Region 1 or Region 2, respectively, n is a parameter (0 < n < 1) that governs the CPE behavior, and ω is frequency. For ideal Q behavior, n = 0.5. The impedance Z1 contributed by the Region 1 is described below: 𝑅1

Eq.2

𝑍1 = 1 + 𝑅 𝑄 (𝑗𝜔)𝑛 1 1

At higher frequencies (1 ≪ 𝑅1𝑄1(𝑗𝜔)𝑛) the imaginary part is the dominant factor in Region 1. For lower frequencies (1 ≫ 𝑅1𝑄1(𝑗𝜔)𝑛) it behaves in a resistive manner. Region 1 fitting for 1M H2SO4 shown in Table 1 suggests that Q1 values are ~60-fold higher compared to that of BMIMPF6 show much lower impedance for capacitive build up at the electrode−electrolyte interface. R1 shows ~60-fold lower resistance towards charge transfer with respect to BMIMPF6 suggests simultaneous reactivity of Nb2O5 and NbSe2 phases in 1M H2SO458,59,62. The corresponding n1 values are very close to the ideal Q behavior, having a range of 0.39 − 0.42 for both electrolytes. Region 2 in Figure 6b is modeled by Q2 in parallel with R2 which is similar to Region 1 in 1M H2SO4. The Q and R values of Region 2 are higher compared to those of Region 1 which indicates ion diffusion coupled with leakage resistance in 1M H2SO4. The Q arises from chemically, microstructurally and morphologically heterogenous phases at surfaces and surface roughness, contributing to pseudocapacitance process4,43,45. In Region 2, the Q2 represents a finite

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diffusion through the defect-induced grain boundaries at exfoliated NbSe2 nanosheets44. Table 1 shows that the Q value is 4 fold higher in Region 2 compared to that in Region 1 in 1M H2SO4, and this signifies less impedance due to ion diffusion. The R2 represents a leakage pathway in 1M H2SO4 due to the difference in ion diffusion between interphases influenced by adsorption effect26,44,62. The corresponding n2 value of ~0.73 signifies that Q2 is more capacitive than Q1. The fitted model for Region 2 shown in Figure 6a suggests that C2 is attributed to pure NbSe2 phases in parallel with a combination of R2 and Q226,53,54. A single RC time constant in Region 2 indicates the presence of chemically homogeneous pure metallic NbSe2 phases. Lower R2 indicates less ion diffusion resistance at pure NbSe2 phases. Lower frequencies show a low value of Q2 reflected by the stretched straight line, indicating restricted diffusion of PF6- anion compared to SO42- at the exfoliated surface26,43. The SEI layers formed at exfoliated NbSe2 surfaces have a synergistic effect on charge transport in acidic medium4. The Bode plot in Figure S5 shows that the IL offers much higher impedance at lower frequencies than the acidic medium, suggesting impeded ionic diffusion at interphases of exfoliated NbSe2 nanosheets. The Bode phase plot for IL in Figure 6c shows a broader peak (𝜃 of 18.8°) at 2.3kHz which signifies the presence of oxidized phases at the exfoliated surface.

4. CONCLUSION In conclusion, a systematic electrochemical characterization of the liquid phase exfoliated NbSe2 nanosheets has revealed that the impurities present in the nanosheets alters the charge transfer mechanism at the surface-electrolyte interface. XPS reveals an altered At% fraction of impurities before and after exfoliation. CV shows that 1M H2SO4 has simultaneous reactivity with impurities (i.e., Nb2O5 and NbSe2). CD shows that NbSe2 samples exfoliated in 1M H2SO4 operate at a

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thermodynamically stable lower potential range (0.1 V – 0.6 V instead of 0 V – 0.8 V). At higher current loads, NbSe2 exfoliated in 1M H2SO4 shows a steeper charge-discharge profile, suitable for supercapacitors requiring fast charge-discharge behavior52. Parallel leakage and electrolyte decomposition prominent in CD profiles in BMIMPF6 may result in gaseous evolution, electrolyte crystallization at the separator, and dissolution of current collectors. This can affect the assembly quality and electrochemical performance of the cell. Leakage and decomposition significantly affect rate capability and duty cycle of the system in BMIMPF6 at lower current loads52. In BMIMPF6, the exfoliated NbSe2 nanosheets show less ionic diffusion and slow kinetics indicating low coulombic efficiency. Change in impedance in EIS at lower frequencies depend on the cumulative resistivity and permittivity at the interface4,43,45,61. EIS shows rapid diffusion of SO42ions compared to PF6- ions at the exfoliated surfaces. SEI formed at the exfoliated NbSe2 surfaces facilitates the charge transfer in 1M H2SO4, suggesting simultaneous reactivity of heterogeneous phases (i.e., Nb2O5, NbSe2). However, charge transfer was affected by self-passivated behavior of oxide impurities with respect to the liquid medium/electrolyte. The impact of pseudocapacitance from heterogeneous phases (mainly Nb2O5) at the exfoliated NbSe2 surfaces tends to vary depending on the type of the liquid medium4,43. The NbSe2 nanosheets demonstrate enhanced charge transfer in 1M H2SO4 than IL and therefore can be chosen for intercalating H+ ions63. NbSe2 resulting in no significant redox state signifies better cycle stability in electrochemical charge storage mechanism59. Upon tailoring the Nb2O5 structure (from hexagonal to orthorhombic) and distribution during the exfoliation process, this can possibly provide a tunable 2D NbSe2 surface with enhanced rate capability, thus significantly impacting applications related to energy storage and supercapacitance.

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AUTHOR INFORMATION Corresponding Author: Edward Song Email: [email protected]

AUTHOR CONTRIBUTIONS S.G and E.S. conceived the work. All of the authors collectively performed the experiments, collected the data, analyzed the results and prepared the manuscript.

SUPPLEMENTARY INFORMATION Cyclic voltammogram (CV) of bulk and exfoliated NbSe2 in ionic liquid; Plot of specific capacitance vs. scan rates; Plot of IR drop vs. current densities for the exfoliated NbSe2; XPS spectra for bulk and exfoliated NbSe2 samples; Bode plot obtained from electrochemical impedance analysis; TEM image of the exfoliated NbSe2 sample; Distinctive anodic peak potentials and the peak shift of exfoliated NbSe2 in H2SO4 and BMIMPF6; Calculated specific capacitance of exfoliated samples; Calculated IR drop of the exfoliated samples; Nb3d and Se3d core spectra along with C1 and O1 spectra for bulk and exfoliated NbSe2 samples.

ACKNOWLEDGEMENT Authors would like to thank Prof. Jeffrey Halpern in the Department of Chemical Engineering at the University of New Hampshire for his assistance with the electrochemical characterization. This

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work was supported in part by the Center for Advanced Materials and Manufacturing Innovation (CAMMI) at the University of New Hampshire.

NOTES The authors declare no competing financial interest.

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FIGURE CAPTIONS Figure 1. (a) Illustration is demonstrating the NbSe2 exfoliation and deposition procedures with pictures of solution before vacuum filtration and material after deposition. (b) Exploded view of the test cell geometry embedded with exfoliated NbSe2 nanosheets used for electrochemical double layer device.

Figure 2. UV vis spectra for exfoliated NbSe2 samples (a) at different concentrations shows metallic behavior with no absorbance peak. Inset (b) visual representation of dispersed exfoliated NbSe2 samples in solvent mixture composed of 35% ethanol and 65% deionized water having different concentration of 0.5 mg/mL (light brown) and 5mg/mL (wine) respectively. (c) & inset (d) are TEM images taken at different magnification (8000x & 10000x respectively) for exfoliated NbSe2 samples. Legends: scale bar for (c) and (d) are respectively 0.5 and 0.2 µm respectively. The red arrow marking shows exfoliated NbSe2 nanosheets on lacey carbon grid.

Figure 3. XPS spectra for fitted for exfoliated NbSe2 (a, c & e) and bulk NbSe2 (inset b, d & f) samples. For NbSe2 (a &b) Nb3d spectra have Nb3d5/2 spectra having peaks at 203.4 (wine),206.2 (grey) & 207.2 eV(green); Nb3d3/2 spectra having peak at 210.2 eV(violet) respectively. The Se3d spectra (c &d) comprises of Se3d5/2 spectra peaks at 53.2 (orange) and 54.8 (green) eV; Se3d3/2 spectra have peak at 55.6 (magenta) and Se3d spectra at 60.5 (cyan) eV respectively. Also, O1 spectra have peaks at 530.8eV (green) and 532.5 eV (wine). Figure (a-f) legends real data (blue dots), fitted (red curves) and background (black curves). Figure 4. Cyclic voltammograms (CVs) with different scan rates 10−200mVs-1 in forward and reverse scan region of exfoliated NbSe2 electrode using (a) 1M H2SO4 electrolyte having a potential window of 0 to +0.8V shows early shoulders marked in a solid box and oxidation peaks at anodic region marked in a dotted box. The inset (b) represents distinct early shoulders appeared in CV from 0 to +0.25V region in forwarding scan and (c) inset of oxidation peaks in anodic regions (+0.4 to +0.8V). (d) BMIMPF6 electrolyte having a potential window of 0 to +2.5V shows distinctive oxidation peaks in the anodic scan and (e) inset represents distinct anodic oxidation peaks Peak1 and peak 2 respectively in the anodic region (+1.0 to + 2.45V). Arrows are for CV directions for forward and reverse scan respectively. Legends: 1000mVs-1 (wine curve), 700mVs-1 (orange curve), 500mVs-1 (purple curve), 200mVs-1 (green curve), 100mVs-1 (red curve), 50mVs-1 (blue curve), 10mVs-1 (black curve). Arrows are for CV directions for forward and reverse scan respectively.

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Figure 5. Galvanostatic charge−discharge curves for range of 2 to 6.8 mA.cm-2 current densities change in cell voltage recorded Vs time for exfoliated NbSe2 in coin cell arrangement in (a) 1M H2SO4 electrolyte having potential window of 0 to +0.8V shows two plateau voltage behavior (P1 and P2) during charging with respect to time for respected current densities. The inset (b) represents such behavior for higher current densities from 5.2 to 6.8 mA.cm-2 (c) BMIMPF6 electrolyte having potential window of 0 to +2.5V shows more distinctive appearance of plateau behavior at lower current densities ranging from 2.0 to 4.0 mA.cm-2 (d) inset represents charge−discharge behavior having less evident plateau behavior at higher current densities from 5.2 to 6.8 mA.cm-2 . Legends: 2 mA.cm-2 (red dotted curve), 2.8 mA.cm-2 (blue dotted curve), 4.0 mA.cm-2 (orange dotted curve), 5.2 mA.cm-2 (wine dotted curve), 6.8 mA.cm-2 (green dotted curve). Figure 6. Electrochemical Impedance Spectroscopy for exfoliated NbSe2 split test cell arrangement displays the change in charge transfer predicting the capacitive nature build up in 1M H2SO4 (red color dots) and BMIMPF6 (blue color dots) electrolytes for a range of frequency from 100kHz to 100mHz (1M H2SO4) and 1MHz to 100mHz (BMIMPF6). (a) The Nyquist behavior for exfoliated NbSe2 samples in the different electrolyte with circuit fitting model in an ionic medium. (b) inset is showing Nyquist behavior in acidic medium coupled with the fitted circuit. (c) Bode phase plot is indicating a distinctive phase in BMIMPF6 at 2.3 KHz with respect to 1M H2SO4.

TABLE: Table 1 Values of interfacial parameters of exfoliated NbSe2 nanosheets in a split test cell arrangement obtained by fitting the circuit to experimental data. The % errors are mentioned with circuit elements.

Circuit Parameters Exfoliated NbSe2 Region 1 Electrolyte

Region 2

Rs (Ω)

Q1 (mF. sn-1)

n1

R1 (Ω)

C2 (mF)

R2 (Ω)

Q2 (mF. sn-1)

n2

1M H2SO4

3.83±1.4%

49.66±7.4%

0.42±2.2%

1.39±2.8%

NA

57.83±11.4%

216.7±4.4%

0.73±1%

BMIMPF6

9.16±0.7%

0.798±0.7%

0.39±12.8%

80.8±0.98%

0.82±3.3%

3.34±20%

6.12±1.1%

0.39±14%

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FIGURES: Figure 1.

Figure 2.

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Figure 3.

Figure 4.

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Figure 5.

Figure 6.

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