Optimization and scalability assessment of supercapacitor electrodes based on hydrothermally grown MoS2 on carbon cloth

MoS2 is a well-known 2D transition metal dichalcogenide (TMD) with feasibility for energy storage applications due to its eco-friendliness and high electroactive surface area. Electrodes based on MoS2 are typically made by either immobilizing its multiphase nanocomposites, having binders and conductive fillers, or by directly growing the materials on current collectors. In this work, we follow and optimize this latter approach by applying a hydrothermal route to directly synthesize MoS2 nanostructures on carbon cloth (MoS2@CC) hence enabling binder-free current collector electrodes. Raman spectroscopy and electron microscopy analyses confirmed the formation of 2H MoS2 nanosheets with hexagonal structure. The as-prepared electrodes were used to assemble symmetric supercapacitor cells, whose performance were tested in various types of electrolytes. Electrochemical measurements indicate that both precursor concentration and growth time significantly affect the device performance. Under optimized conditions, specific capacitance up to 226 F g−1 (at 1 A g−1 in 6 M KOH) was achieved, with corresponding energy and power densities of 5.1 W h kg−1 and 2.1 W kg−1. The device showed good stability, retaining 85% capacitance after 1000 cycles. Furthermore, the electrodes assessed in PYR14-TFSI showed energy and power densities of up to 26.3 W h kg−1 and 2.0 kW kg−1, respectively, indicating their feasibility not only in aqueous but also in ionic liquid electrolytes. In addition, galvanostatic charge/discharge measurements conducted on devices having footprint sizes from 1 cm2 to 25 cm2 show very similar specific capacitances, which proves scalability and thus the practical relevance of the binder-free electrodes demonstrated in this study.


Introduction
Molybdenum disulde (MoS 2 ) is one of the most explored 2D materials aer graphene.MoS 2 exists in both metallic (1T trigonal) and semiconducting (2H hexagonal and 3R rhombohedral) forms, among which the 2H phase is the most stable.[11][12][13] MoS 2 can be synthesized by top-down methods such as chemical/mechanical exfoliation, [14][15][16] and bottom-up techniques including chemical vapor deposition, 4,17,18 atomic layer deposition, 19,20 pulsed laser deposition (PLD), 21,22 and RFmagnetron sputtering, [23][24][25] as well as wet chemical routes under normal 26 and hydrothermal conditions. 27,28Among these methods, hydrothermal growth is probably the most practical for scale-up synthesis with high yield. 8upercapacitor electrodes are usually prepared using the conventional slurry method, which involves applying a paste (having a typical composition of 80% active material, 10% polymer binder, and 10% conductive ller) on the collectors by means of spray coating, 29 doctor blading, 30 and printing or painting. 313][34] The use of inactive polymeric binders reduces the performance of supercapacitors.][41][42] In this work, we report scalable synthesis of binder-free MoS 2 -based supercapacitor electrodes and their practicality in scaling up the device by directly synthesizing vertically aligned MoS 2 nanosheets on highly conducting and exible carbon cloth (CC) current collectors using a hydrothermal route.Symmetric supercapacitors based on the obtained MoS 2 @CC structures were assembled and their electrochemical performance was optimized by varying the precursor concentration and growth time of the MoS 2 synthesis.The developed SCs showed a specic capacitance of up to 226 F g −1 with a retention of 85% aer 1000 cycles, suggesting feasibility for high-power supercapacitor applications.To increase the voltage window, the developed electrodes were also tested in a solvent-free ionic liquid (IL), 1-butyl-1-methylpyrrolidinium bis(tri-uoromethanesulfonyl)imide (PYR14-TFSI) as well as in PYR14-TFSI mixed in acetonitrile (ACN), and it was found that the electrode in the IL/ACN mixture showed good electrochemical performance with the highest measured energy and power density of 26.3 W h kg −1 and 2.0 kW kg −1 , respectively.The practical feasibility of scaling-up the developed supercapacitor (SC) electrode was evaluated using sandwich-type SCs with varying electrode sizes ranging from 1 cm 2 to 25 cm 2 .The specic capacitance of the scaled-up devices is consistent with that of the 1 cm 2 device, affirming the developed electrode's practical utility in high-performance supercapacitors.

Materials
Sodium molybdate dihydrate (Na 2 MoO 4 $2H 2 O) and thiourea (CH 4 N 2 S) were obtained from Sigma Aldrich.Plain carbon cloth (#1071, ber diameter of 5-10 mm) was purchased from the Fuel Cell Store, USA.Other solvents of analytical grade were used without further purication.

Surface treatment of carbon cloth
Bare carbon cloth is hard to wet in an aqueous solution due to its hydrophobic nature.To obtain a hydrophilic surface and uniform growth of MoS 2 , carbon cloth was subjected to a surface treatment using a protocol similar to that of Zhang et al. 43 Pieces of CC were cut to a size of 5 × 5 cm 2 , then cleaned with a mixture of ethanol and acetone (1 : 1 vol.) and rinsed in DI water.The cleaned CCs were immersed in a mixture of cc.H 2 SO 4 : HNO 3 (1 : 1 vol.) and subjected to an ultrasonic treatment for 60 minutes.Finally, the surface-treated carbon cloth was washed with DI water and then dried at 60 °C overnight.

Synthesis of MoS 2 on carbon cloth
MoS 2 nanosheets were grown on CC using a hydrothermal technique from sodium molybdate dihydrate (Na 2 MoO 4 $2H 2 O) and thiourea (CH 4 N 2 S) as molybdenum and sulfur sources, respectively. 39,41,44,45The precursors were dissolved in 600 mL DI water.The solution was magnetically stirred for 1 h and then transferred into a 750 mL Teon-lined vessel in an autoclave.
Aer the CC was immersed in the solution, the vessel was closed and heated to 200 °C.Aer synthesis, MoS 2 coated carbon cloth (MoS 2 @CC) was washed with DI water followed by rinsing in ethanol and drying at 60 °C overnight.To investigate the effect of synthesis parameters, reaction time and precursor concentrations were varied, and samples were named based on molybdenum precursor concentrations as MC005 (0.005 M sodium molybdate dihydrate and 0.025 M thiourea), MC01 (0.01 M sodium molybdate dihydrate and 0.05 M thiourea), and MC02 (0.02 M sodium molybdate dihydrate and 0.1 M thiourea).

Materials characterization
The crystal structure of the grown MoS 2 on CC was characterized by X-ray diffraction (XRD, Rigaku SmartLab 9 kW, Co source) and Raman spectroscopy (Thermo Fisher Scientic A DXR TM 2xi, l = 532 nm), whereas the morphology and microstructure were assessed using eld-emission scanning electron microscopy (FESEM, Zeiss Ultra Plus) and transmission electron microscopy (TEM, JEOL JEM-2200FS).

Electrochemical measurements
As-prepared MoS 2 @CC was punched into circular discs of 1 cm in diameter and then symmetric supercapacitor cells were assembled in a Swagelok cell together with a lter paper as the separator.Three different electrolytes were used in the experiments: (i) 6 M KOH (aq.), (ii) 1-butyl-1-methylpyrrolidinium bis(triuoromethanesulfonyl)imide (PYR14-TFSI), and (iii) a mixture of PYR14-TFSI and acetonitrile (ACN) with 1 : 1 mass ratio.In scaling-up experiments, the supercapacitors were assembled between stainless steel sheets of varying sizes and clamped together.Electrochemical measurements including cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS) were carried out using a VersaSTAT 3 instrument.The specic capacitances of the SCs were calculated from the GCD curves as: where C dev is the device capacitance, I is the current, Dt is the discharge time, m is the total mass of active materials, and DV is the voltage window.Note: the specic capacitance of a single electrode is calculated as C elec = 4C dev .The energy density (E in W h kg −1 ) and power density (P in W kg −1 ) of the SC are calculated using eqn ( 2) and (3), respectively. 40-42 where Dt is the discharge time in seconds.

Results and discussion
The three-dimensional ber-like structured CC acts as a template for the growth of MoS 2 nanosheets.Surface-treated

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CC was characterized with FESEM to analyze the surface morphology of carbon bers.The surface of bare CC was very smooth, as shown in Fig. 1a, while some cracks were observed in acid-treated CC (Fig. 1c).In addition, as suggested by multiple studies for carbon bers 43,46 and nanotubes, 47    X-ray diffraction patterns of MoS 2 @CC electrodes (Fig. 2a-c) conrm the formation of 2H MoS 2 nanosheets on carbon cloth.The reections could be indexed to the corresponding lattice planes (002), ( 004), (100), (102), and (110) of 2H MoS 2 (JCPDS: 37-1492).
XRD peaks of carbon ber are also observed in samples while they disappear in sample MC02 deposited for 24 h (Fig. 2c) due to the presence of an increased amount of MoS 2 nanoowers.No other impurity peaks are observed in the samples, con-rming the growth of pure MoS 2 nanosheets.From the Raman spectra (Fig. 2d-f), the presence of two prominent vibration modes E 2g 1 at around 379 cm −1 and A 1g at around 405 cm −1 in all samples indicate in-plane and out-of-plane vibrations of two S atoms in respect of the Mo atom, respectively, further conrming the growth of 2H MoS 2 . 51No other impurity peaks are observed in the Raman spectra either.
FESEM imaging was used to analyze the coverage and uniformity of MoS 2 on carbon cloth.The concentrations of the precursors and growth time affect the uniformity and mass loading of MoS 2 on the surface of the carbon cloth template (Fig. S1 †).Therefore, by modifying the precursor concentrations and growth time, it is possible to avoid the agglomeration of MoS 2 on the carbon cloth while achieving sufficient mass loading.Fig. 3a-d show the FESEM images of the MC02 electrodes.Uniform growth of vertically aligned MoS 2 nanosheets was observed on the surface in the 6 h experiment MC02 electrodes (Fig. 3a and b).Some spherical nanostructured microscopic features (nanoowers) are also present on the surface.As the deposition time increases, more MoS 2 nanoowers form on the vertically grown MoS 2 (Fig. 3c and d), leading to their agglomeration and entire population throughout the surface.Therefore, the mass loading of the electrode varied signicantly depending on the synthesis parameters, from 5.1 g m −2 Fig. 3 FESEM images of MC02 electrodes at different growth times of (a and b) 6 h, (c) 12 h, and (d) 24 h, and (e and f) TEM images of the MC02@6 h electrodes.

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(MC005@12 h) to 184.6 g m −2 (MC02@24 h) on carbon cloth of 132 g m −2 specic mass (Table S1 †).TEM imaging (Fig. 3e and f) of the MC02 electrode at a growth time of 6 h (MC02@6 h) also conrms uniform growth of the vertically aligned MoS 2 with an interlayer spacing of 0.63 nm.
The electrochemical performance of the electrodes was analyzed using CV and GCD techniques.Fig. 4a illustrates the electrochemical system of the SCs.The CV and GCD curves of MC005 and MC01 electrodes are shown in Fig. S2.† The obtained discharge time is low in MC005 and MC01 devices.Fig. 4b shows the CV curves (acquired at a scan rate of 50 mV s −1 ) of SCs made with the MC02 electrodes.The quasirectangular shapes of the CV curves indicate that the devices have both pseudocapacitive (faradaic) and EDLC (non-faradaic) features.[54] MoS 2 + K + + e − 4 MoS − SK + (7) The area of the CV loops of MC02-based SCs decreases with growth time (6 h, 12 h, and 24 h), which implies that the surplus MoS 2 in the form of nanoowers does not contribute signicantly to the charge storage, which can be explained by their poor electrical contacts with the current collector.These results align well with the GCD curves measured at a current density of 1 A g −1 (Fig. 4c), showing a reduction of charge-discharge times Fig. 4 (a) Schematic of the electrochemical system, (b and c) CV at 50 mV s −1 and GCD curves at 1 A g −1 of the MC02 electrode based SC at different reaction times (note: some error data points are visible in the CV data), (d) GCD curves of the MC02@6 h-based SC at different current densities, (e) variation of specific capacitance versus current densities, (f) Nyquist plots of MC02 based SCs with different synthesis times, (g) cycling stability of the MC02@6 h based SCs for 1000 cycles, at a current density of 5 A g −1 (inset shows the GCD profiles of the 1 st , 500 th and 1000 th cycles for MC02@6 h based SCs, at a current density of 5 A g −1 ).

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Nanoscale Advances for the devices made of MC02 electrodes synthesized at a growth time of 12 and 24 h.The corresponding specic capacitances of the devices were calculated to be 56.5 F g −1 , 48 F g −1 , and 6.8 F g −1 , respectively.(The specic capacitance of respective electrodes is 4 times that of the device, i.e., 226 F g −1 , 192 F g −1 , and 27.2 F g −1 for MC02 electrodes at growth times of 6 h, 12 h and 24 h electrodes, respectively).Table S1 † shows detailed information about mass loading and electrochemical performance of the assembled MoS 2 -based SCs.From these results we can draw a conclusion that generally increasing the precursor concentrations increases the mass loading and the electrode performance whereas increasing the synthesis time increases the mass loading but aer a certain point will only add more mass to the electrode without having contribution to charge and energy storage.The electrochemical performance of the MC02@6 h-based SC was further analyzed in detail by systematically varying charge/discharge currents in GCD (Fig. 4d) and voltage scan rates in CV measurements (Fig. S3 †).The device exhibits good stability, and no noticeable deviation of the shapes of the CV curves was observed.As displayed in Fig. 4e, the specic capacitances calculated from the GCD curves decreased with increased current densities (230 F g −1 , 226 F g −1 , 206 F g −1 , and 178 F g −1 at current densities of 0.5 A g −1 , 1 A g −1 , 2 A g −1 , and 5 A g −1 , respectively) because of slow ion adsorption and charge transfer at the electrode/electrolyte interface.It is important to note that no obvious voltage drop was observed at the start of discharge even at high specic current densities, indicating the low equivalent series resistance of the device.as well as with the addition of further metal oxides/suldes 42,58 and also by applying conductive polymers with pseudocapacitive properties 59 reaching capacitances over 3000 F g −1 . 60EIS measurements were performed in the frequency range from 100 mHz to 100 kHz, from which the equivalent series resistances (ESR) and charge transfer resistances (R ct ) were assessed according to the Nyquist plot (Fig. 4f).The ESR (total resistance of the current collector, electrolyte, and electrode material) of the MC02@6 h based SC was found to be 0.45 U.The diameter of the semicircle on the real axis in the highfrequency region gives a charge transfer resistance of 0.2 U, denoting extremely good ion conducting pathways provided by MoS 2 nanosheets.The Warburg impedance (Z W ), visible as the slope in the low frequency region aer the semicircle, generally represents the diffusion of ions within the electrolyte.In SCs based on longer synthesis times the overall resistances are very similar (<1 U), with only a small increase of resistances compared to SCs with electrodes having lower MoS 2 loadings (Fig. S4 †).The cycling stability of the MC02@6 h-based SC was assessed using GCD measurements at a current density of 5 A g −1 (Fig. 4g).No noticeable voltage drops were found even aer 500 or 1000 cycles at the start of the discharge cycles (inset of Fig. 4f), and the capacitance retention was 85% aer 1000 charge/discharge cycles.
The electrochemical performance of the optimized electrode was further assessed by using an ionic liquid (IL), specically 1butyl-1-methylpyrrolidinium bis(triuoromethanesulfonyl) imide (PYR14-TFSI) as an electrolyte.ILs are considered promising electrolytes for energy storage applications because of their non-volatility, thermal stability, and wide electrochemical window, approximately 3.5 V. 66 However, ILs exhibit higher viscosity compared to other organic electrolytes, which can reduce their ionic conductivity.Recent research has shown that adding solvents like acetonitrile (ACN) to ILs can decrease their viscosity, thus improving ionic conductivity. 66,67The electrochemical performance of Swagelok-type cells assembled with the optimized electrodes (i.e.MC02@6 h) was evaluated in both solvent-free PYR14-TFSI IL and a mixture of acetonitrile and PYR14-TFSI in a 1 : 1 mass ratio at a voltage window of 3 V.Fig. 5a shows the CV curves of the MC02@6 h electrode-based SC at a scan rate of 50 mV s −1 .A CV with an enlarged area was observed for the SC when using a mixture of ACN and PYR14-TFSI electrolyte, in comparison to using PYR14-TFSI IL alone.The CV and GCD curves for the MC02@6 h based SC in PYR14-TFSI are presented in Fig. S5.† The specic capacitance of the SCs was measured from the GCD curves (Fig. 5b) using eqn (1).The specic capacitances of the SC with PYR14-TFSI and the mixture of ACN and PYR14-TFSI were obtained as 10.6 F g −1 and 23.8 F g −1 at a current density of 0.1 A g −1 , respectively (the specic capacitances of the electrodes are 42.4F g −1 and 95.2 F g −1 , respectively).The enhancement in capacitance is primarily attributed to the reduced viscosity of the ILs upon solvent addition, compared to the viscosity of solvent-free ILs.The detailed electrochemical performance of the MC02@6 h-based SC, utilizing a mixture of IL and ACN, was further analyzed.Fig. 5c shows the CV curves of the device at various scan rates ranging from 10 mV s −1 to 500 mV s −1 , while Fig. 5d shows the GCD proles at different current densities.
Even with the addition of a solvent to ILs, there is an observed increase in voltage drop as the current density rises, with a signicant voltage drop of approximately 1.2 V noted at a high current density of 2 A g −1 , attributed to the slow kinetic properties of the ILs.Fig. 5e shows the comparison of the Ragone plots for the SCs using KOH, PYR14-TFSI, and PYR14-TFSI + ACN as electrolytes.The highest measured energy density and power density of KOH are obtained as 5.1 W h kg −1 and 2.1 kW kg −1 , respectively.The PYR14-TFSI + ACN -based SC shows the highest energy density of 26.3 W h kg −1 and power density of 2 kW kg −1 , respectively, which is higher than that of the solvent-free PYR14-TFSI-based SC.These results indicate that the optimized electrode is also effective with an ionic liquid electrolyte, although it performs better at lower current densities.The lower capacitances measured with ILs compared to KOH can be explained by their larger ions that are unable to intercalate into the MoS 2 structure.Therefore, without modication of the electrode material, for example by making a MoS 2 -graphene composite, 64

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The feasibility of scaling up the optimized MC02@6 h electrode was assessed by assembling sandwich-type supercapacitors of varying footprint areas from 1 cm 2 to 25 cm 2 .GCD curves of these devices (Fig. 5f), recorded at a current density of 1 A g −1 , exhibit very similar proles, and calculated specic device capacitances of 69 F g −1 , 63 F g −1 , 64 F g −1 , 66 F g −1 , and 67 F g −1 for 1 cm 2 , 2 cm 2 , 5 cm 2 , 9 cm 2 and 25 cm 2 , respectively.(The corresponding specic capacitances calculated for the electrodes are 276 F g −1 , 252 F g −1 , 256 F g −1 , 264 F g −1 and 268 F g −1 ).The negligible deviations of data may be attributed to variations in mass loadings.Accordingly, our experiments suggest that the devices can be effectively scaled up to manufacture robust supercapacitors for use in large-scale applications.[70]

Conclusions
The present work demonstrated the in situ growth of vertically aligned MoS 2 nanosheets on carbon cloth for high-power supercapacitors using a simple hydrothermal technique.Electrochemical studies infer that the growth time has a signicant effect on device performance most likely due to the lack of intimate electrical contacts of MoS 2 nanoowers (which populate the collector surface aer 12 and 24 h synthesis) with the carbon cloth.Therefore, it is important to nd the correct synthesis parameters especially the growth time, as too long growth adds only an inactive mass of material to the electrode.The binder-free MoS 2 based electrodes grown at a reaction time of 6 h exhibited the highest specic capacitance of 226 F g −1 at a current density of 1 A g −1 .The device showed good stability and capacitance retention of 85% aer 1000 charge/discharge cycles.Moreover, the optimized electrode (MC02@6 h) was also effective in an ionic liquid electrolyte providing energy and power densities of 26.3 W h kg −1 and 2.0 kW kg −1 , respectively.The consistent specic capacitance achieved for devices in all scaled-up device sizes indicates the electrode viability for largescale supercapacitor (SC) applications.
treatment with oxidizing acids results in the partial etching/cutting of the carbonaceous structure48 as well as in the formation of polar functional groups (such as -COOH, -OH, -C]O) turning the originally hydrophobic character of the materials to hydrophilic.Accordingly, the acid treatment helps to improve the wettability of the surface by the precursor solution and allows for uniform nucleation and growth of subsequently anchored nanoparticles49 -MoS 2 nanosheets in our study.Hydrothermal reaction of a precursor solution containing MoO 4 2− and SO 4 2− leads to the formation of MoS 2 nanosheets.During the process, CH 4 N 2 S decomposes partly into H 2 S, whereas Na 2 MoO 4 $2H 2 O into MoO 3 and reacts to eventually form MoS 2 (eqn (4)-(6)): 50

Fig. 1
Fig. 1 FESEM images of (a and b) bare CC and (c and d) acid-treated CC.
energy and power densities cannot reach the highest values reported for MoS 2 based SC electrodes based on aqueous electrolytes.60

Fig. 5
Fig. 5 (a) CV curves and (b) GCD curves of the MC02@6 h electrode based SC in IL and the mixture of IL and ACN at 0.1 A g −1 , (c) CVs at various scan rates (d) GCD at different current densities for the MC02@6 h electrode based SC in the mixture of IL and ACN electrolyte, (e) Ragone plot of MC02@6 h based SCs in aqueous electrolyte and PYR14-TFSI and PYR14-TFSI + ACN ILs and (f) GCD curves of MC02@6 h electrode-based SCs with different electrode sizes ranging from 1 cm 2 to 25 cm 2 measured at 1 A g −1 .

Table 1
Hydrothermally grown binder-free MoS 2 SC electrodes on carbon cloth