Untapped Potential of Polymorph MoS2: Tuned Cationic Intercalation

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Untapped potential of polymorph MoS2: Tuned cationic intercalation for high-performance symmetric supercapacitors Basant A. Ali, Asmaa M Omar, Ahmed S. G. Khalil, and Nageh K. Allam ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b11444 • Publication Date (Web): 26 Aug 2019 Downloaded from pubs.acs.org on August 27, 2019

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ACS Applied Materials & Interfaces

Untapped potential of polymorph MoS2: Tuned cationic intercalation for high-performance symmetric supercapacitors Basant A. Ali a, Asmaa M. A. Omar b, Ahmed S. G. Khalil b, Nageh K. Allam a,* aEnergy

bPhysics

Materials Laboratory, School of Sciences and Engineering, The American University in Cairo, New Cairo 11835, Egypt.

Department, Center for Environmental and Smart Technology, Faculty of Science, Fayoum University, Fayoum 63514, Egypt.

Abstract Supercapacitors have been the key target as energy storage devices of modern technology that needs fast charging. Although supercapacitors have large power density, modifications should be done to manufacture electrodes with high energy density, longer stability, and simple device structure. The polymorph MoS2 has been one of the targeted materials for supercapacitor electrodes. However, it was hard to tune its phase and stability to achieve the maximum possible efficiency. Herein, we demonstrate the effect of the three main phases of MoS2 (the stable semiconductor 2H, the metastable semiconductor 3R, and the metastable metallic 1T) on the capacitance performance. The effect of the cation intercalation on the capacitance performance was also studied in Li2SO4, Na2SO4, and K2SO4 electrolytes. The performance of the electrode containing the metallic 1T outperforms those of the 2H and 3R phases in all electrolytes, with the order 1T > 3R > 2H. The 1T/2H phase showed a maximum performance in the K2SO4 electrolyte with a specific capacitance of 590 Fg-1 at a scan rate of 5 mVs-1. MoS2 showed a well-standing performance in both positive and negative potential windows allowing the fabrication of symmetric supercapacitor devices. The 1T MoS2 symmetric device showed a power density of 225 W/Kg with an energy density of 4.19 Wh/Kg. The capacitance retention was 82% after 1000 cycles, which is an outstanding performance for the metastable 1Tcontaining electrode. Keywords: polymorph MoS2; symmetric supercapacitor; intercalaction; cycling stability; energy density. * Corresponding Author: [email protected]

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1. Introduction Speed is the given name to our modern civilization that is totally dependent on electrical/electronic tools. This necessitates the search for the fastest and most efficient way to charge our devices. Batteries have been widely used for centuries but due to their low power density, supercapacitors are becoming the new generation of energy storage devices, which enjoy higher power density. The power density of supercapacitors is mainly controlled by the nature of the used electrode materials and the potential window of the fabricated devices.1 However, the energy density of supercapacitor electrodes is very low compared to batteries.2–5 To this end, increasing the energy density of the supercapacitor electrodes, while enjoying their high power density, requires the discovery and design of new materials that depend on faradic processes besides adsorption.4,6–10 The widely used supercapacitors are made of carbonaceous materials that store charges in the form of an electrical double layer (EDL), where their performance is limited by the surface area of the materials.4,11–14 On the other hand, the faradic supercapacitors can add the advantages of diffusion, intercalation, or redox reactions to the EDL counterparts in order to achieve higher energy densities.4,11,12,15,16 However, the faradic processes affect the stability and potential window of the devices and reduce their life time.7 Although potential window is related to the nature of the reaction or intercalation process on the surface of the electrode material, it is also limited by the electrolyte used 17 as it might decompose under the applied voltage. For aqueous electrolytes, it is very hard to reach a potential window higher than 1.23 V due to the splitting of water.17 Although the use of ionic liquids may overcome the potential window limitation, they reduce the faradic behavior of the electrode material and limit the capacitance to the EDL mechanism due to the inert large ions of the ionic liquids.17 In this regard, it is better to identify an


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aqueous electrolyte that withstands the decomposition and a material that is compatible with that electrolyte in the operating potential window. Transition metal dichalgonides such as MoS2,18–20 and WS221–23 have been the target as supercapacitor electrode materials due to their 2D layered structure that enjoys high surface area for electrical double layer (EDL) capacitance. Besides, the empty orbitals and large interlayer structure would provide the possibility of intercalation of ions.20,24 However, dichalgonides are not conductive in all their possible structures. The polymorph MoS2 has three main phases; the stable hexagonal 2H phase, the metastable rhombohedral 3R phase, and the metastable octahedral 1T phase, of which only the 1T phase has a metallic character.20,25,26 However, the 1T phase is unstable over large retention cycles in different electrolytes.26,27 Therefore, many attempts were done to enhance the performance of MoS2 by mixing with carbon materials,28–30 synthesis of different morphologies31–34 or tuning the phase of MoS2.20,26,27 Xuyen et al.26 studied the effect of the percentage of the 1T phase on the performance of the MoS2, reaching a specific capacitance of 259 F/g in KCl electrolyte. Despite this high performance, higher capacitance is needed to achieve high energy density.26 Moreover, the 1T/2H polymorph MoS2 was also studied in KOH electrolyte, resulting in a specific capacitance of 338.8 Fg-1 at a sweep rate of 20 mVs-1 on Ni-sheet substrate with very good cycling stability.27 However, the used Ni substrate may interfere with the KOH, increasing the faradic behavior of the electrode.35,36 Acerce et al.20 studied the 1T MoS2 capacitance performance in different electrolytes reaching a capacitance of 300 F/cm3 without shedding the light on the charge storage mechanism or device performance. The charge storage mechanism in a monolayer of 1T and 2H MoS2 was studied computationally using first principles.37 However, charge storage mechanism of the 3R phase was not studied. Besides, the



monolayer MoS2 is not a real practical example. On the other hand, the charge storage mechanism transition was studied in 1T MoS2 upon changing morphology and mixing with carbon materials.19 Herein, we demonstrate a facile hydrothermal fabrication method of flower-shaped 2D structures of the three phases of MoS2. The work sheds the light on the electrochemical performance of polymorph MoS2 as supercapacitor electrode materials in the presence of different cations in the electrolyte solution. Moreover, the cheap carbon sheet substrate is used instead of the expensive Ni-foam substrate to exclude the possibility of any contribution from the substrate and to provide accurate results regarding the charge storage behavior of MoS2. Tuning all the studied aspects enabled us to fabricate a cheap symmetric supercapacitor device that covers a large potential window and provides a power density of 225 W/Kg with an energy density of 4.19 Wh/Kg.

2. Experimental 2.1 Synthesis of polymorph MoS2 Polymorph MoS2 with different phase compositions were fabricated via a hydrothermal method. Thiourea and ammonium molybdate were dissolved in 60 ml of DI water under constant stirring. The whole mixture was transferred into a 134 ml Teflon-lined stainless-steel autoclave. The autoclave was kept in an oven at 200 oC for 24 h, then left to cool down to room temperature. The resulting black precipitate was collected by centrifugation, washed three times with distilled water then with ethanol, and dried in an oven for 12 h at 80 oC. Polymorph 1 (1T/2H) was prepared using a Mo:S ratio of 1:5 at 45% filling of the Teflon reactor. Increasing the sulfur content with a Mo:S ratio of 1:25 at 75% filling of the reactor resulted in the formation of hybrid 2H/3R phase


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(polymorph 2). Pure 2H phase was the result upon the thermal treatment of either polymorph 1 or polymorph 2 under argon stream at 800 °C for 2 h. 2.2 Morphological characterization The morphology of the nanostructured samples was explored using field-emission scanning electron microscopy (FESEM, Zeiss SEM Ultra 60, 5 kV). The high resolution transmission electron microscopy (HRTEM, JOEL JEM-2100) was used selected area diffraction analysis. The crystal structure of the polymorph MoS2 was studied using an X-ray powder diffractometer (Panalytical X’pert PRO MPD X-ray diffractometer) with Cu Kα radiation (λ = 0.15418 nm, 30 mA, 40 kV), and a dispersive Raman microscope (Pro Raman-L Analyzer) with a laser power of 1 mW and an excitation wavelength of 512 nm was used for Raman spectroscopy analysis. The phase ratio and composition of the polymorph MoS2 were studied using XPS (ESCALAB 250XI, Thermo Scientific). The active surface area was investigated via BET analysis of the nitrogen adsorption/desorption isotherms at −196 °C using accelerated surface area and porosimetry.

2.3 Electrochemical characterization The electrochemical characterizations were carried out in both 3-electrode and two-electrode systems. The working electrodes were fabricated as a slurry composed of 80% active material, 10% carbon black, and 10% polyvinylidene fluoride in dimethyl formamide (PVDF) on a graphite sheet (1 × 1 cm2) as the substrate using a BioLogic SP-300 work station. For the 3-electrode cell, a Pt coil was used as the counter electrode, a calomel electrode was used as the reference electrode, and different electrolytes were used in the system (0.5 M Li2SO4, 0.5 M Na2SO4, and 0.5 M K2SO4). Cyclic voltammetry (CV) measurements were carried out over a wide potential window 5 ACS Paragon Plus Environment


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(−0.4 to 0.5 V) at different scan rates (from 5 to 300 mV s−1). The galvanostatic charge/discharge (GCD) experiments were performed at different current densities (0.5, 0.7, 1, 2, 3, 4, 5, 7 and 10 A/g). The electrochemical impedance spectroscopy (EIS) measurements were performed in the frequency range of 10 kHz to 100 mHz. The materials were tested for 2000 cycles and the EIS was re-measured after the 2000 cycles. The data from the CV and GCD were used to calculate the specific capacitance using Eqs. 1 and 2, respectively. Testing the capacitance of the supercapacitor material and/or device can be performed using the voltammetry technique according to Eq. 1.6,18 ∫𝐼 𝑑𝑉

(1)

𝐶𝑠 = 𝑣𝑚 ∆𝑉

where 𝑣 is the scan rate (V/s), m is the mass of the electrode (g), ΔV is the potential window (V) and ∫𝐼 𝑑𝑉 is the integration of the CV curve. Since the supercapacitor is mainly a charge storage and delivery device so, measuring the charge and discharge time will reflect more significantly the capacitance of the material and/or device. The specific capacitance can be calculated from the charge/discharge test using Eq. 2. 6,18 𝑖∆𝑡

Cs= ∆𝑉

(2)

where i is the current density (A/g) and ∆𝑡 is the discharge time (s). For further investigation of the charge storage mechanism in the material, some electrochemical calculations can be used. The current resulting from the voltammetric response at a specific voltage can be related to the scan rate according to Tafel’s relation (Eq. 3).

18,19,38

For battery-like or

pseudocapacitors the chosen current is the one of the peaks and the relation between the current and the scan rate is not linear (b=0.5 for typical battery-like behavior). However, in a typical EDL behavior, there no peaks appear and the chosen current is barely changed and the variation of


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voltage with current is linearly related to the scan rate value (b=1). This method can give an insight on the majority of the current origin either EDL or diffusion. i= a𝑣b

(3)

For the two-electrode system, the 1T phase of MoS2 was used as both the positive and negative electrodes, the masses of the positive and negative active materials in the electrodes in the symmetric device were balanced according to eq. 4.6,7 𝑚+ 𝑚―

𝐶𝑠+ ∆𝑉 +

(4)

= 𝐶 ― ∆𝑉 ― 𝑠

The energy density (E) of the device was calculated according to eq. 5.6,7 𝐶𝑠∆𝑉2

(5)

𝐸 = 2 ∗ 3.6 The power density (P) of the device was calculated according to eq. 6.6,7 𝑃=

3600 ∗ 𝐸 ∆𝑡

=

1000 𝑖 ∆𝑉

(6)

2

3. Results and Discussion 3.1 Synthesis and characterization of the MoS2 phases For the synthesis of MoS2 nanomaterials, ammonium molybdate and thiourea were used as precursors for molybdenum and sulfur, respectively. The formation of MoS2 goes through a fourstage growth process:39 CS (NH2)2 + 2H2O

→

(NH4)6Mo7O24 →

7MoO3 + 6NH3 + 3H2O

MoO3 + 3H2S + H2O →

H2S (g) + 2NH3 (g) + CO2 (g) MoO2 +SO42- + 2H+

MoO2 + 2H2S → MoS2 + 2H2O 7 ACS Paragon Plus Environment


Note that thiourea was used as a precursor for sulfur as well as a reducing agent, which can precisely and effectively reduce MoO3 to MoS2.39 Ammonia in the reaction could effectively diffuse beneath the parallel planes of the MoS2 nanosheets, leading to expansion of the lattice and generation of more active unsaturated sulfur atoms in more disordered structure (i.e. 1T/2H phase).40 Water serves as a solvent and is adsorbed onto the surface of MoS2 to form a molecular layer that controls layer stacking.41 As more thiourea is used (in case of polymorph 2), MoO3 becomes insufficient due to the following two reasons. One is simply due to the fact that higher thiourea concentration consumes more MoO3. Secondly, with a higher thiourea concentration, the pH value will be higher, leading to the conversion of MoO3 to MoO4. This counter effect reduces the amount of MoO3 and therefore the formation of 1T MoS2 phase.41,42 In addition, as the sulfur concentration increases, the probability of having lattice mismatch during layer stacking increases, leading to the partial formation of 3R phase. Phase transformation upon thermal treatment of either 1T/2H and/or 3R/2H materials into stable and pure 2H phase was previously reported for MoS2 prepared either via hydrothermal or CVD routes.43,44 The morphology of the as-synthesized MoS2 phases was identified using FESEM as presented in Figure 1a-c. The FESEM images showed that all synthesized MoS2 samples have flower-shaped morphology with different diameters. The1T/2H mixed MoS2 sample (Figure 1a) has the lowest diameter of the flowers (400-700 nm), while the 3R/2H mixed MoS2 sample (Figure 1b) has the largest diameter of the flowers (1-3 µm). However, their wettability (inset in Figure 1) shows very close contact angles for the three samples, indicating hydrophobic nature of the fabricated MoS2 samples. The surface area of the fabricated MoS2 samples was measured using BET isotherm (Table 1), indicating very close surface area. The 3R/2H MoS2 sample showed the largest surface area of ~5 m2/g and the 2H MoS2 sample showed the smallest surface area of ~3.5 8 ACS Paragon Plus Environment

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m2/g. As the MoS2 was shown to have a very low wettability, the surface charge was measured using the zeta potential to identify the possibility of intercalation of ions and electrosorption upon the use of those materials as electrodes in electrochemical capacitors. The surface charge of the three materials presented in Table 1 showed the three samples to have high negative charge on their surfaces, indicating the potential to attract positive ions for intercalation.

Table 1 BET surface area and Zeta potential of the as-synthesized MoS2 phases. Polymorph MoS2

1T/2H-MoS2

3R/2H-MoS2

2H-MoS2

Surface area (m2/g)

4.712

5.024

3.553

Zeta potential (mV)

-36.900

-28.000

-36.000

Figure 1. FESEM images of (a)1T/2H MoS2, (b) 3R/2H MoS2, and (c) 2H MoS2 samples. Contact angle measurements of (d) 1T/2H MoS2, (e) 3R/2H MoS2, and (f) 2H MoS2 samples.

Figure 2a shows the diffraction pattern of the synthesized MoS2. The diffraction peaks are all broad and noisy due to the formation of nanosize domains and few layers of MoS2,31,45 making it difficult to identify the exact phase of the MoS2 crystals.46 However, the 2H MoS2 showed sharper 9 ACS Paragon Plus Environment


peaks than the 1T/2H MoS2 and the 3R/2H MoS2 counterparts. The (002) plane showed a clear peak in the three samples with 2θ at 14.04° for both 2H MoS2 and 3R/2H MoS2 and at 13.26° for the 1T/2H MoS2. The d-spacing of the (002) plane indicates an interlayer spacing between the adjacent layers of ~6.1 Å for all samples. The 2H MoS2 showed diffraction peaks at 25.9°, 33.4°, 39.6°, 44.9°, and 58.3° that can be ascribed to the (004), (100), (103), (105), and (110) planes, respectively, matching the hexagonal lattice of MoS2.26,45 All peaks of the 1T/2H MoS2 were shifted due to the distorted lattice structure that can be attributed to the mixed 1T/2H phases.31,47 It is usually hard to differentiate experimentally between the phases of the MoS2 using the XRD since the peaks usually appear in the same position or overlap, besides being noisy and broad.45,47 Therefore, the vibrational modes of the MoS2 samples were characterized using the Raman spectroscopy, Figure 2b. The 1T/2H MoS2 sample showed vibrations at 239 cm-1, 281 cm-1, and 335 cm-1 attributed to the Raman active J2, E1g, and J3 vibrations of the 1T MoS2 lattice, respectively. The peaks at 407 cm-1 and 378 cm-1 can be attributed to the out of plane A1g vibration and the in-plane E12g vibration of the 2H MoS2, respectively.45,46,48 For the 3R/2H MoS2 and 2H MoS2 samples, the Raman spectra were not distinguishable due to the similar vibration modes of the 2H MoS2 phase and the 3R MoS2 phase. The 3R/2H MoS2 sample and the 2H MoS2 sample both showed active peaks at 382 cm-1 and 407 cm-1 corresponding to the E12g and A1g vibration modes.45,48,49 The difference in Raman shift (Δ) between the E12g and A1g was 29 cm-1 for the 1T/2H MoS2 and 25 cm-1 for both 3R/2H MoS2 and 2H MoS2, indicating bulk thickness of the MoS2 sheets for all samples.24,31,50 Therefore, Raman spectroscopy is a good tool to confirm the presence of the 1T phase, however, it was not very successful to identify the 3R phase.25


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Figure 2. (a) X-ray diffraction peaks and (b) Raman spectra of the studied phases of the MoS2. (I: 1T/2H MoS2, II: 3R/2H MoS2, III: 2H MoS2).

The morphology and the phases of the fabricated samples were further identified using HRTEM as presented in Figure 3a-c. The HRTEM images showed the formation of inhomogeneous structure with some parts being more crystalline than the others and the edges are not curled indicating non-defected MoS2 sheets. The HR-TEM images were further analyzed using imageJ51 to determine the d-spacing and to identify the phases. The 1T/2H-MoS2 sample (Figure 3a) showed multi d-spacing areas indicating multi-phases of MoS2. The 0.63 nm d-spacing represents the (002) plane of the 2H MoS231 while the 0.31 nm and 0.42 nm represent the distances between the adjacent atoms in the 1T phase. It is observed that the distances between atoms in the 1T phase is elongated than normal (0.29 and 0.34 nm, respectively),52 which can be attributed to the insertion of solvent molecules between layers or intercalation of some residual atoms from the precursor used during synthesis.52 The selected area electron diffraction (SAED) patterns of the 1T/2H-MoS2 sample was identified using the CrystBox software53, based on the reported crystal structures for the three MoS2 phases from the Crystallography Open Database54 and the Cambridge Crystallographic Data Centre.55 The SAED image in Figure 3a shows that the diffraction patterns



represent both the 2H and 1T planes with high accuracy.56,57 For the 3R/2H-MoS2, the HR-TEM presented in Figure 3b showed that the interlayer spacing has two main values, one is the original 0.63 nm and the other is the elongated 0.83 nm. It was observed that the interlayer spacing near the edges tend to be more elongated, which can be attributed to the adsorption and intercalation of impurity atoms either during the synthesis or from the environment. The SAED patterns of the 3R/2H-MoS2 sample, Figure 3b, showed perfect match of the 3R planes58 of the MoS2 in some areas and the 2H planes of MoS2 in other areas.56 For the 2H-MoS2 sample, the interlayer spacing was homogeneous with some elongation (0.67 nm) and a mixed crystalline-amorphous part. The SAED of the 2H-MoS2 sample presented in Figure 3c showed the existence of only 2H patterns.

Figure 3. HR-TEM and SAED (a) 1T-MoS2, (b) 3R/2H-MoS2, and (c) 2H-MoS2.

In addition to the HRTEM, the X-ray photoelectron spectroscopy (XPS) is a very useful tool to distinguish between the 1T and 2H phases and to give insights on the ratio of the different phases or other forms of MoS2.26,59 Figure 4 shows the XPS spectra of the different MoS2 samples. The Mo 3d peak of the 1T/2H MoS2 (Figure 4a) can be deconvoluted into four peaks. The peaks at 232.9 eV and 229.9 eV can be attributed to the 3d5/2 and 3d3/2 orbitals of Mo in the 2H MoS2, respectively.26,28,60 However, other two peaks with larger intensity and lower energy are observed


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at 228.28 eV and 231.8 eV corresponding to 3d5/2 and 3d3/2 orbitals of the Mo in the 1T MoS2, respectively.24,26,61 Another peak at 235.6 eV was observed corresponding to the Mo-O bond due to the presence of impurities of MoO3.19,24,26 The analysis of the XPS peaks showed that MoO3 is as low as 10% while the 1T phase is 70% of the 1T/2H MoS2 sample. The S 2p peak of the 1T/2H MoS2 sample can be deconvoluted into three peaks as shown in Figure 4b. The peaks at 161.25 eV and 161.42 eV can be attributed to the 2p3/2 of the 1T MoS2 and the 2p3/2 of the 2H MoS2, respectively, while the peak at 162.55 can be attributed to the 2P1/2 of the 1T and/or the 2H MoS2.26,61 For the 3R/2H MoS2 sample, Mo 3d peaks can be seen at 228.9 and 232.0 eV (Figure 4c), while S 2p peaks were shown at 161.8 eV and 162.9 eV (Figure 4d), which are the peaks characteristic of both 2H and 3R MoS2 phases.25 However, the 2H MoS2 sample showed another peak for the impurity MoO3 (Figure 4e) with a ratio less than 5%.



Figure 4. XPS peaks of the MoS2 phases (a-b) 1T/2H MoS2, (c-d) 3R/2H MoS2, and (e-f) 2H MoS2.


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3.2 Electrochemical performance of the polymorph MoS2 The zeta potential measurements presented in Table 1 indicate negatively charged potential, suggesting high affinity of the material to intercalation with cations such as Li+, Na+, and K+. Consequently, the MoS2 electrodes were tested in different electrolytes, namely Li2SO4, Na2SO4, and K2SO4. For the 2H MoS2, the calculated specific capacitance in different electrolytes from both cyclic voltammograms (CV) and GCD showed the same trend of Li2SO4 > Na2SO4 > K2SO4. This trend may be attributed to the fact that Li+ ions are smaller in size, allowing them to diffuse easily into the interlayers of MoS2 forming new electronic states in the band structure of MoS2 due to the formation of LixMoS2.62 The three electrolytes showed the same shape of both CVs (Figure 5a) and GCDs (Figure 5b), which gives insights on the charge storage mechanism.63 Both the shape of CVs and GCDs show that the 2H MoS2 has a pseudocapacitive along with EDL capacitive behavior. The 2H MoS2 showed a very high stability with retention of ≥100% in the three electrolytes after 2000 cycles (Figure 5c), which may be attributed to the fact that the 2H MoS2 phase is the most stable polymorph. Despite the high stability of the 2H MoS2, it showed significantly low specific capacitance. However, to show the effect of adding the metastable 3R phase to the 2H phase, the 3R/2H MoS2 was studied in the three electrolytes. The 3R/2H MoS2 showed a different behavior than that of pristine 2H MoS2. The 3R/2H MoS2 electrode showed almost similar capacitance in K2SO4 and Li2SO4 electrolytes with partial peaks started to appear in the CV (Figure 5d) indicating extra redox and/or diffusion process. The new behavior of the 3R/2H phase in K2SO4 may be attributed to the packing structure of the 3R MoS2 (ABCA) that allows the intercalation of the large K+ cation between its layers. Although the packing structure of the 2H MoS2 (ABA) has almost the same interlayer spacing as the 3R MoS2,64,65 the sulfur atoms result in a steric effect, making it difficult for the large K+ cations to diffuse. The redox



peaks in the CV of the 3R/2H MoS2 in K2SO4 indicate not only the surface pseudocapacitive behavior but also electron transfer between the localized valence electrons of MoS2 and that of the K+ ion.66 On the other hand, testing the material in Na2SO4 electrolyte did not show any redox peaks, indicating no electron transfer process from the Na+, hence lower capacitance than that obtained in K2SO4. The GCD of the 3R/2H MOS2 electrode also showed similar capacitance upon testing in Li2SO4 and the K2SO4 electrolytes as presented in Figure 5e. However, the capacitance retention of the 3R/2H MoS2 electrode increases dramatically during the first 1000 cycles then stabilizes at a retention of ≥120% (Figure 5f), this may be attributed to the increase in the diffusion of ions to the 3R structure of MoS2. This increase in capacitance may be attributed to the activation of the electrodes until all active sites are subject to the diffused cations.8,18 As the 1T MoS2 has the highest conductivity due to its metallic bandgap, the 1T/2H MoS2 has been studied in the three electrolytes as well. The behavior of the 1T/2H MoS2 was found to be different than those of the 2H MoS2 and the 3R/2H MoS2 counterparts. The 1T/2H MoS2 showed a capacitance trend that depends on the electrolyte used as K2SO4 > Na2SO4 > Li2SO4 with only the electrodes tested in K2SO4 showing redox peaks in the CV scans as presented in Figure 5g. The steric effect of sulfur atoms in the packed structure of the 1T MoS2 is very low, which explains the possibility of diffusion of both K+ and Na+ ions. However, the very low capacitance of Li+ intercalation can be explained by its larger hydrated radius, low transference number, and ionic conductivity in comparison to the K+ and the Na+ ions.67 The 1T/2H MoS2 showed a drop in capacitance retention after the first 50 cycles then stabilized at ~72%, 50%, and 80% in the Li2SO4, Na2SO4, and K2SO4 electrolytes, respectively since the 1T phase is a metastable structure of the 2H MoS2 and usually turn back to the stable 2H MoS2 structure. The Coulombic efficiency of all MoS2 phases in all electrolytes is very low but it increases to 100% after ~100 cycles due to the activation and


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stabilization of the diffusion and intercalation process. In general, the 2H MoS2 electrode prefers the cations with small size to easily intercalate due to the steric effect of the sulfur atoms in its packing structure. However, the 3R MoS2 can allow larger ions to intercalate but only reactive cations with high transference number will have an observable effect. On the other hand, the electrodes with the lowest steric structures such as 1T MoS2 will allow the intercalation of larger cations with higher ionic conductivity.



Figure 5. Electrochemical behavior of the polymorph MoS2 in different electrolytes (a) cyclic voltammograms of the 2H MoS2 at 10 mV/s, (b) GCD of the 2H MoS2 at 0.5 A/g, (c) cycling stability of the 2H MoS2 at 2 A/g, (d) cyclic voltammograms of the 3R/2H MoS2 at 10 mV/s, (e) GCD of the 3R/2H MoS2 at 0.5 A/g, (f) cycling stability of the 3R/2H MoS2 at 2 A/g, (g) cyclic voltammograms of the 1T/2H MoS2 at 10 mV/s, (h) GCD of the 1T/2H MoS2 at 0.5 A/g, and (i) cycling stability of the 1T/2H MoS2 at 2 A/g. 18 ACS Paragon Plus Environment

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The rate capability is an important metric for the material to be used in advanced electronic devices. The specific capacitance was calculated from both CV and GCD data at different scan rates and current densities, respectively. For the 2H MoS2 electrode (Figure 6a-b), the highest specific capacitance is 63 F/g in the Li2SO4 electrolyte at 5 mV/s that is decreased to 26.6 F/g at 0.5 A/g, 9 F/g at 300 mV/s, and 1.5 F/g at 10 A/g. Although the stability of the 2H MoS2 electrode increases with increasing cycling, its rate capability is very low. For the 3R/2H MoS2 electrode (Figure 6d-e), the specific capacitance in the Li2SO4 electrolyte is 107.5 F/g and 62.2 F/g at 5 mV/s and 0.5 A/g, respectively that were reduced to 19.8 F/g and 6.9 F/g at 300 mV/s and 10 A/g, respectively. The presence of the 3R phase enhanced the rate capability of the 2H MoS2, however, the enhancement is still limited. For the 1T/2H MoS2 electrode (Figure 6g-h), the specific capacitance in K2SO4 electrolyte is 590.6 F/g and 193 F/g at 5 mV/s and 0.5 A/g, respectively that were reduced to 80.13 F/g and 17.22 F/g at 300 mV/s and 10 A/g, respectively. Although the rate capability of the 1T/2H MoS2 is still not very high, it is acceptable due to the high starting capacitance. To identify the charge storage mechanism, Tafel’s plot was constructed for MoS2 in different electrolytes as presented in Figure 6c,f,i using Eq. 3. It was found that Tafel’s slope is between 0.54 and 0.65, indicating a pseudocapacitive behavior with equirectangular shape of the CVs.18,68 Table 2 lists the recent development in the capacitance behavior of MoS2 for supercapacitor application showing that different electrolytes and different substrates can result in different capacitance values. Our current work shows that K2SO4 can be a good electrolyte that can be used to assemble 1T MoS2-based supercapacitor devices.



Figure 6. Electrochemical behavior of the polymorph MoS2 in different electrolytes (a) rate capability of the 2H MoS2 at different scan rates, (b) rate capability of the 2H MoS2 at different current densities, (c) Tafel’s representation of the 2H MoS, (d) rate capability of the 3R/2H MoS2 at different scan rates, (e) rate capability of the 3R/2H MoS2 at different current densities, (f) Tafel’s representation of the 3R/2H MoS2, (g) rate capability of the 1T/2H MoS2 at different scan rates, (h) rate capability of the 1T/2H MoS2 at different current densities, and (i) Tafel’s representation of the 1T/2H MoS2. 20 ACS Paragon Plus Environment

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Table 2 Chosen comparative capacitance data of MoS2 electrodes studied between 2017-2019. Material

Substrate

Ammoniated 1T/2H MoS2 Water coupled 1T MoS2 1T/2H MoS2 2H MoS2/GNF/CNT High defect density 1T MoS2 MoS2 flakes 1T MoS2 Carbon nanoparticle/MoS2 1T MoS2 hydrogel 1T MoS2 flowers

Ni Foil Glassy Carbon Filter Paper Graphite Sheet Ni Foam Carbon Papers TiO2/Ti Stainless Steel Filter Paper Graphite Sheet

Potential window 0.6 to -0.1 0.2 to -1 0.8 to -0.8 0.6 to -0.6 0.0 to 0.5 0.0 to 1.0 0.2 to -0.7 0.2 to -0.8 0.0 to 0.7 0.5 to -0.4

Electrolyte 2.0 M KOH 0.5 M Li2SO4 1 M KCl 1 M Na2SO4 1 M KOH 1 M H2SO4 1M Na2SO4 1 M Na2SO4 1 M H2SO4 0.5 M K2SO4 0.5 M Na2SO4

Specific Capacitance 346 F/g at 1 A/g 380 F/g at 5 mV/s 259 F/g at 5 mV/s 104 F/g at 0.5 A/g 379 F/g at 1 A/g 283 F/g at 1 A/g 428 F/g at 0.2 A/g 394.2 F/g at 5 mV/s 147 F/g at 0.1 A/g 590 F/g at 5 mv/s 208 F/g at 0.5 A/g

3.3 Electrochemical performance of the symmetric device To test the real behavior of the 1T/2H MoS2 electrode in K2SO4 electrolyte, a symmetric device was assembled and tested. The CVs of the device presented in Figure 7a show a stable quasi-rectangular behavior that maintained its shape up to 300 mV/s. For the charge/discharge curves at different current densities (Figure 7b), it was found that the Coulombic efficiency is enhanced from that measured in the 3-electrode system and the GCD behavior is stable up to 10 A/g. The electrochemical impedance spectroscopy measurements were performed for the device before and after 2000 cycles, with the Nyquist plot fitted to the circuit shown in Figure 7c. It was found that the electrolyte resistance (R1) increased from 2.5 Ω to 2.56 Ω after 2000 cycles, while the diffusion resistance (R2) increased from 1.188 Ω to 1.659 Ω after 2000 cycles. The specific capacitance of the device was 79.4 F/g and 37.2 F/g at 5 mV/s and 0.5 A/g, respectively that was reduced to 31 F/g and 13.44 F/g at 300 mV/s and 10 A/g, respectively indicating a dramatically increase in rate capability than that obtained in the 3-electrode system as presented in Figure 7d-


Reference 27 69

26 18 70 71 72 73 74

This work


e. The power density and the corresponding energy density are presented in the Ragon’s plot shown in Figure 7f. The obtained energy densities are 4.18 and 1.5 Wh/Kg at power densities of 225 and 4500 W/Kg, respectively. The stability of the device is very close to that of the 3-electrode system maintaining 82% capacitance after 1000 cycles and 72% after 2000 cycles, Figure 7g shows the time of each 500 cycles in different color. The enhanced behavior of the device over that of the 3electrode system may be attributed to the adjustment of the weight of the active material in both the positive and negative electrodes and due to the absence of resistance of other parts in the 3electrode system.


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Figure 7. Electrochemical behavior of the 1T/2H MoS2 symmetric device in K2SO4 electrolyte (a) CVs at different scan rates, (b) GCDs at different current densities, (c) Nyquist plot of the electrode before and after 2000 cycles, (d) rate capability of the at different scan rates, (e) rate capability at different current densities, (f) Ragon plot of the device at different power densities, (g) Stability of the device over the 2000 cycles. 23 ACS Paragon Plus Environment


4. Conclusions High-performance supercapacitor devices are urgently needed for our modern technology. Although transition metal dichalcogenides are promising for supercapacitor applications, there have been some problems that need optimization. Herein, we demonstrated a hydrothermal method to fabricate different phases of MoS2. XRD was used to prove the presence of the 2H phase in the three MoS2 samples, while Raman spectroscopy and XPS were used to identify the 1T phase of MoS2. The 3R MoS2 phase was proven using the SAED images of the samples. The electrochemical behavior of the three phases was studied using 3-electrode system. It was found that the 2H MoS2 prefers the small sized cation intercalation due to the steric effect of the sulfur atoms in its packing structure. The 3R MoS2 allows the intercalation of larger ions but only the ions with high transference number showed an observable effect. On the other hand, the lowest steric structures such as the 1T MoS2 preferred the larger cations with a higher transference number and higher ionic conductivity. A symmetric device of 1T/2H MoS2 was assembled and tested in K2SO4 electrolyte. The overall electrochemical behavior of the symmetric device was better than the 3-electrode system behavior. The device can deliver a specific capacitance of ~80 F/g at 5 mV/s, an energy density of 4.19 Wh/Kg, and a power density of 225 W/Kg, with cyclic stability of 82% after 1000 cycles.

Acknowledgment Support of this work by the American University in Cairo is gratefully acknowledged. Basant Ali is supported by an Allehedan Ph.D. Endowed Scholarship.


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Untapped Potential of Polymorph MoS2: Tuned Cationic Intercalation

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