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Review

Insights into the Influence of Key Preparation Parameters on the Performance of MoS2/Graphene Oxide Composites as Active Materials in Supercapacitors

by
Catalin Alexandru Salagean
1,2,
Codrut Costinas
2,
Liviu Cosmin Cotet
1,3 and
Lucian Baia
1,2,4,*
1
Laboratory for Advanced Materials and Applied Technologies, Institute for Research, Development and Innovation in Applied Natural Sciences, Babeș-Bolyai University, Fantanele 30, RO-400294 Cluj-Napoca, Romania
2
Faculty of Physics, Babeș-Bolyai University, M. Kogalniceanu 1, RO-400084 Cluj-Napoca, Romania
3
Faculty of Chemistry and Chemical Engineering, Babeș-Bolyai University, Arany Janos 11, RO-400024 Cluj-Napoca, Romania
4
Centre of Nanostructured Materials and Bio-Nano Interfaces, Institute for Interdisciplinary Research on Bio-Nano-Sciences, Babeș-Bolyai University, Treboniu Laurian 42, RO-400271 Cluj-Napoca, Romania
*
Author to whom correspondence should be addressed.
Catalysts 2021, 11(12), 1553; https://doi.org/10.3390/catal11121553
Submission received: 12 November 2021 / Revised: 1 December 2021 / Accepted: 3 December 2021 / Published: 20 December 2021
(This article belongs to the Special Issue Advanced Nanomaterials for a Green World)
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Versions Notes

Abstract

:
Advances in energy storage and energy conversion play an essential role nowadays because the energy demands are becoming greater than ever. To overcome the actual performances of the materials used to build supercapacitors, a combination of transition metal dichalcogenides (TMDCs) and graphene oxide (GO) or reduced graphene oxide (rGO) as graphene-based structures are often studied for their excellent properties, such as high specific area and good electrical conductivity. Nevertheless, synthesis pathways and parameters play key roles in obtaining better materials as components for supercapacitors with higher technical performances. Driven by the desire to understand the influence of the structural and morphological particularities on the performances of supercapacitors based on MoS2/graphene oxide (GO) composites, a survey of the literature was performed by pointing out the alterations induced by different synthesis pathways and key parameters to the above-mentioned particularities.

Graphical Abstract

1. Introduction

In the last few years, due to the advances in the current technologies such as electric/hybrid powered vehicles and day to day electronics, there has grown a high demand for energy storage systems with high power and energy density combined with a long lifespan. Currently, batteries are capable of high energy density, but the charging process is very slow. On the other hand, supercapacitors can deliver high power density with a much higher lifespan but fall short in terms of energy density [1]. Despite the progress made in the last period, the scientific community has a strong interest in developing various techniques and materials to advance energy storage system technologies [2].
Due to their high-power density, supercapacitors and their high charge/discharge rate, long cycle life (>1,000,000 cycles) and simple operation mode, are to be considered one of the most promising energy storage units [3]. The advantage of supercapacitors related to batteries is that charging is not limited by the diffusion of ions in the material, so they could supplement or even replace batteries when high power is needed to be delivered [4].
In general, depending on the energy storage types, the involved technology consists of three types of supercapacitors [5,6,7], as presented in Figure 1.
The first type is the electrochemical double-layer capacitor (EDLC), which uses mostly porous carbon with a high specific surface area as the main active material. The electrostatic interactions between the electrolyte ions and electrode surface charges, are the basis for EDLCs, which operate as an “electrical sponge” that adsorbs and desorbs the ions in a charge–discharge mechanism. The second type of supercapacitors are represented by the pseudo-capacitor, which is mainly based on electro-chemical pseudo-capacitance provided by the redox reactions occurring at the electrolyte–active material interface, which is made up of various oxides and conductive polymers. Another type is the hybrid supercapacitor, which combines the two types of energy storage. The hybrid supercapacitors have come to life due to the desire for increased performances. Their functionality is based on the coupling mechanism between EDLCs and pseudo-capacitors, which are meant to increase the power output and cyclability [4,8].
Part of the reported literature [9,10] presents activated carbon with good capacitance (varying from 100 to 200 F·g−1) as the most commonly utilized material for supercapacitor electrodes, mainly due to their great specific surface area, high electrical conductivity, low production cost, etc. Significant research effort has been placed in the last few decades into the improvement of their performance by using composites such as conductive polymers and metallic oxides [11,12,13]. Moreover, nitrogen-enriched activated carbon was studied and its impact on the porosity of carbons and the dependence of capacity on the chemistry of surface groups were discussed [14]. Furthermore, in order to get higher energy density, various designs have been considered such as asymmetric capacitor systems consisting of different electrodes like (activated carbon)/LiMn2O4 [15,16], activated carbon/graphene/MnO2, [17] NaxMnO2/N-(rGO) [18], and also symmetric supercapacitors, which have similar electrodes, such as MoS2-rGO with poly(3,4-ehylenedioxythiophene) [5], and were tested in energy storage applications. Other intriguing approaches were reported about the composites based on graphene materials and MoS2 [19]. These combinations represent excellent choices for the use of novel electrode materials in terms of energy storage technologies, due to their good characteristics, such as a high specific surface area, and thermal stability.
Graphene, GO and rGO, are usually obtained by chemical vapor deposition (CVD), mechanical and/or oxidative exfoliation of the graphite followed by reduction techniques (in the case of rGO, the unreduced form being GO), and are generally termed as graphene materials. These materials have excellent features such as large specific area and are considered suitable options for obtaining electrode materials in supercapacitors [20,21,22]. However, graphene structures, taken as an individual component, limit the supercapacitor performances. A good strategy to overcome this drawback is to combine graphene materials with various components. In this respect, it is worth mentioning the use of transition-metal dichalcogenides (TMDCs) that gained a lot of interest in the past few years due to their physical and chemical properties [23,24,25]. For example, due to their large specific area and their rich redox chemistry, these structures are thought to have significant potential as supercapacitor electrode materials.
Besides this, many research groups [26,27,28,29,30] found that another promising material for supercapacitor construction is molybdenum disulfide (MoS2). Because MoS2 is easily intercalated by other compounds and has a great theoretical capacitance (±1000 F·g−1), it can be used to build layered electrode materials for the construction of supercapacitors [31]. These are built up from a graphite-like layered structure stacked by covalently bonded S-Mo-S layers, and bonded by weak van der Waals forces [32,33].
Taking into account the above presented information, it becomes obvious that there is a real interest to understand the synthesis pathways and parameters that lead to the alteration of the morphological, structural and surface particularities of the MoS2-GO composites from the perspective of the improvement of supercapacitor performances. In this respect, this review proposes, based on a comprehensive survey of the literature, to bring more light to these above-mentioned aspects by presenting the existent correlations between the obtaining procedures, properties and the achieved performances in the case of MoS2 nanostructures and MoS2-GO composites.
At the beginning of the review will be presented an overview of the synthesis methods and parameters involved in the obtaining of MoS2 nanostructures. Further on, the effects of each preparation parameter on the structure, morphology, and surface particularities of MoS2/MoS2-rGO composites will be discussed, with an emphasis on the essential properties required for designing efficient supercapacitors. Several promising electrode material results in terms of specific capacitance and cycle stability will be further pointed out. A list of these achievements based on certain essential criteria for supercapacitor applications, such as specific capacitance and cycle stability of electrode materials, and their ranking in order of performance, is also presented.

2. Synthesis Methods for MoS2/MoS2 Composites

The synthesis methods of molybdenum disulfide can be divided into two major categories, such as bottom-up and top-down [34,35], as depicted in Figure 2.
Each of them uses different techniques and, as expected, has advantages and disadvantages. In the first category, the chemical exfoliation, mechanical cleavage and ball milling is highlighted. In the second one chemical vapor deposition (CVD), and wet chemical synthesis, which is based on hydrothermal/solvothermal processes, are described.
The synthesis pathways are chosen depending on the targeted properties of the final MoS2 product; however, the most commonly used preparation method for MoS2 composites is the hydrothermal technique. One of the parameters that influences the morphology and properties of the composites is the reaction time of the hydrothermal process that affects directly the morphology of MoS2, which can vary from nanosheets, or nanospheres to flower-like hierarchical structures [36,37]. Another element that influences the synthesis is the reducing agent type, which is also the sulfur source. Taking this into account, the molybdenum species, as well as thiourea, sodium thiosulfate, L-cysteine and thio-acetamide, which act as sulfur sources by contributing to the sulfuration of the molybdenum species, were investigated [36]. Surfactants also play a decisive role in the synthesis of MoS2 composites because they act as a mediator between the two precursors and control the pore-size, ultimately affecting the specific surface area and the conductivity [6,38,39,40]. Moreover, the binding agent can also influence the conductivity of the active material, thereby influencing the final specific capacitance of the cell. Another parameter worth taking into account is the molar ratios between the Mo and S precursors, which affects directly the crystallinity of the composite [28]. One of the most common routes used to obtain MoS2 composites is the hydrothermal approach [28,29,36]. The hydrothermal method facilitates the formation of hierarchical porous structures that can improve the performances of supercapacitors by providing active unsaturated sulfur sites. Moreover, the addition of graphene materials can increase the composite’s electrical conductivity as a result of the rGO formation after hydrothermal treatment is applied, which turns this type of carbonic structure into a strong competitor for the novel electrode materials for energy storage cells [6,37,41].

2.1. Top-Down Methods for MoS2 Synthesis

From the top-down pathways, two common methods are discussed in this study, namely the chemical exfoliation and ball milling.
Chemical exfoliation is based on breaking the weak van der Waals forces between the MoS2 (S-Mo-S) layers (process assisted by sonication), thus facilitating the liquid-based exfoliation processes [42]. It is worth mentioning that in order to obtain MoS2 nanosheets by chemical exfoliation one can choose the organolithium method, in which the medium is a mixture with n-butyl lithium solutions or by choosing some organic solvents [7,42].
The exfoliation of semiconducting 2H molybdenum disulfide crystalline phase is based on intercalating Li ions after using organolithium compounds, which results in the formation of single layer MoS2 nanosheets in the form of LixMoS2. Afterwards, the molybdenum disulfide intercalated with lithium ions will interact with water during sonication, producing a MoS2 dispersion.
Another aspect that needs to be discussed here is the transition of MoS2 from the semiconducting crystalline 2H phase to the metallic 1T crystalline phase with defects in the lattice. In some cases, defects created by the transition to 1T metallic phase improves the electrochemical performances of the active material by generating additional sulfur active sites and affecting the band structure of the composite [43], giving it an advantage over the semiconducting 2H phase. Furthermore, the electrical conductivity of the 1T metallic phase is seven-orders of magnitude greater than that of the 2H-MoS2 phase [37,44]. MoS2 crystalline phases are depicted in Figure 3.
Another procedure for liquid phase exfoliation of molybdenum disulfide is sonication assisted by organic solvents such as N-methyl-2-pyrrolidone (NMP). MoS2 can be obtained easily using NMP without the use of any surfactants. Moreover, the surface tension of the TMDCs should closely match the solvent’s surface tension [45]. Although surfactant-assisted chemical exfoliation using organic solvents prevents the aggregation of MoS2 nanosheets by stabilizing the dispersion [46], it does not impact the structure of the material by creating phase changes like the one done using organolithium compounds.
Ball milling is also a top-down synthesis mechanism that is mostly used to obtain few-layered MoS2 nanosheets. This approach is based on the process of milling, which reduces the size and layers of the compound, increasing its specific surface area. It was also used to prepare graphene layers from bulk graphite, which is a similar structure to that of MoS2 [47]. The ball milling approach is also considered as a “green method”, without requiring harmful chemicals, and can also be used as a simple and efficient method for obtaining qualitative MoS2 nanosheets [48].

2.2. Bottom-Up Methods

Chemical vapor deposition (CVD) has always been a versatile bottom-up approach involved in obtaining large surface area MoS2 on a substrate as a few-layered structure with great crystallinity. In CVD processes, a specific substrate is exposed to reactive precursors in a high pressure and temperature medium, in which they react or decompose to form high purity ultrathin films. Furthermore, via this method, the size and thickness of films can be controlled to obtain great crystal quality and desired electronic properties [49]. Some particular CVD techniques are sulfurization of Mo-based films and decomposition of molybdenum and sulfur-containing precursors [50,51]. The direct sulfurization of MoO3 is one of the best ways to obtain great quality MoS2. Using MoO3 as a deposited surface on the substrate, it can react with sulfur precursors on its surface, generating the MoS2 layers. The entire mechanism of CVD is explained by Hong et al. [52] Yongjie et al. [50] reported a direct preparation of single- and few-layered MoS2 on SiO2 substrates by pre-depositing Mo films followed by a CVD approach. They concluded that the size and thickness of MoS2 films are dependent on the size of the substrate and the thickness of pre-deposited Mo, which are easily scalable and tuneable, making CVD an important approach for a large variety of applications.
Hydrothermal/solvothermal chemical synthesis is one of the most versatile approaches for obtaining MoS2 structures, usually adopted for the preparation of molybdenum disulfide containing composites [33,50,51]. It is used to synthesize MoS2 using Mo and S precursors via chemical reactions performed in high pressure of a tightly sealed autoclave placed in a medium temperature (i.e., <220 °C) [53,54]. During this synthesis, three steps take place: nucleation, aggregation and self-assembly of nanocrystals (Figure 4).
In this kind of process, precursors like MoO3, sodium molybdate, ammonium heptamolybdate and sulfur precursors such as thiourea, thioacetamide, L-cysteine and sodium thiosulfate are dissolved in a solvent. The precursors are mixed together and are assisted by sonication, and then the synthesis is placed in a controlled medium at high pressure and temperature [55,56,57]. The obtained nanosheets will then aggregate and start to self-assemble to form microspheres and flower-like morphologies [58,59,60]. As a particular case, MoS2/rGO composites were taken into account [33]. A scheme of MoS2/rGO composite formation by this approach is presented in Figure 5.
It is an approach based on a time-dependent evolution, starting with the nucleation phase, followed by nanosheet growth and aggregation, and the self-assembly of the nanosheets as a final step. Zheng et al. [36] carried out a series of time-dependent experiments in order to explore the formation mechanism of MoS2 porous flower-like microspheres.
In more detail, the nucleation step starts firstly with the development of a nucleus and is subsequently continued by a growing stage, known as Ostwald ripening process [61]. This final process is a two-phase transformation of a thermodynamically unstable mixture. An increase in the size scale of a second phase and a decrease in the total interfacial area can reduce the system’s energy. In other words, decreasing the system’s energy by lowering the total surface area greatly influences the morphology of the product [61]. In the first step of a hydrothermal method, under controlled conditions, the molecules and ions gather around the nucleation area due to a concentration gradient and thermal convection. Usually, sulfur sources such as thioacetamide, thiourea and L-cysteine can easily coordinate with molybdate species because of the reductive -NH2 functional group and combine with the oxygen atoms of the molybdate species. Afterwards, the residual Mo and S, can bond into MoS2 nuclei, which tend to aggregate into larger structures, minimizing the surface energy, in accordance with the Ostwald principle [36,62]. The second part of the formation mechanism consists of the aggregation of nuclei in nanosheets and the formation of petal-like layers on the surface of the larger particles [36]. Thus, the stacking process is uneven, due to the fact that in a layer structure the interlayer interactions and stress will be produced by local stacking. To reduce the internal stress and to minimize the surface energy over the structure, nanosheets will tend to bend [63]. Bending of the structure can also be seen in the grain boundaries of polycrystalline graphene [64]. Defects in the layered structure also affect the morphology of the composites. The two-phase hybridization of MoS2 can also play an important role in this process because MoS2 exhibits different electronic structures, such as a semiconducting ones (2H) and a metallic ones (1T). In other words, many processes, such as stacking, the existence of defects in the lattice and the presence of different phases of MoS2 play key roles in the aggregation and bending growth of the composite.
The self-assembly process is dictated by the unstable structures of MoS2, which tend to combine. The obtained hierarchical aggregated MoS2 nanosheets have a high surface area. Baig et al. [65] had obtained flower-like structures with a specific surface area of 391 m2g−1 for a MoS2/rGO composite with ammonium hepta-molybdate and thiourea as precursors. Moreover, Chao et al. [31] reported specific surface areas of different MoS2 composites such as 11.2 m2g−1 for MoS2/polyaniline (PANI), 31.4 m2g−1 for MoS2/rGO and 85.3 m2g−1 for MoS2/PANI/rGO. It was also reported that the size of nanosheet aggregates grows from 400 to 680 nm as the reaction period increases from 6 to 20 h [37], so they possess high surface energy. This is the primary reason for the MoS2 nanosheets to make contact. As a result, they begin to expand and bend in order to decrease the surface energy and internal stress. The charge difference between the inner surface and the outer surface of the edges tends to lower or even disappear due to the effect of electron shielding [61]. At this stage, many connections will be formed between the surface and the edges of MoS2 in order to minimize the specific surface area and energy, and the self-assembly process will continue until flower-like or sphere-like morphologies emerge, as was already reported [35,66,67,68].

3. The Influence of Preparation Parameters on Morphology, Structure and Surface Particularities of MoS2/MoS2-rGO Composites for Designing Efficient Supercapacitors

As pointed out above, there are some key preparation parameters and important processes that can influence the morphology, structure and surface particularities of MoS2/MoS2-rGO composites, such as reaction time, reducing agent, precursors, surfactants and molar ratios of molybdenum and sulphur sources. Further on, we will discuss the influence and interactions of these parameters from the perspective of the way they can affect the final properties. In order to understand the process as a whole mechanism, parameters and their contribution to this approach were taken into account.

3.1. Influence of the Reaction Time in MoS2 and Its Composites Synthesis

Reaction time is definitely an important parameter in the synthesis of MoS2 and its composites, especially when it comes down to hydrothermal technique, which affects the morphological, structural and surface area of the active material. Tang et al. [29] visualized by TEM technique the structural particularities of the formed MoS2 after 6, 12 and 48 h of reaction time (Figure 6).
It was observed that with an increase in the reaction time to 48 h, a perfect crystalline flower-like MoS2 structure was achieved. Based on the formation mechanism, which is time-dependent, it can be inferred that the sulfur sources are bound onto the surface of reduced graphene oxide via nucleation, forming MoS2 nanoflowers that overlay both sides of the rGO. In terms of morphology, Naz et al. [37] reported an increase in nanosheet size from 400 to 680 nm, with a reaction time increase from 6 to 20 h at a temperature of 220 °C, respectively, and the formation of small nanoflowers with numerous edge sites. They also reported that by increasing the reaction time, the thickness of MoS2 nanosheets rises from 4.8 to 15 nm, and the number of layers from 6–7 to more than 15.
Zheng et al. [36] also performed a series of time-dependent experiments to explore the formation mechanism of MoS2 flower-like microspheres, varying the synthesis reaction time (4, 8, 12, 16 and 22 h) and divided the whole mechanism into three phases, as discussed in the previous sections: nucleation, aggregation and self-assembly. It can be concluded that, in a hydrodynamic process, the formation of flower-like structures is time-dependent, meaning that a higher reaction time can deliver a more porous structure. However, other parameters, such as reducing agent and surfactants, which will be discussed later, must be taken into account to fully control the formed flower-like structures.
The specific surface area is also impacted by the reaction time as Manuraj et al. showed in their work [27]. They synthesised MoS2 nanostructures using different reaction times (i.e., 12, 24, 36, 48 h) and proceeded, with BET analyses, the measurements of the surface area. They obtained the following results for the specified times: 10.74, 13.68, 18.45 and 13.26 m2g−1 and pore diameters of 0.047421, 0.042381, 0.064782 and 0.035250 cm3g−1, respectively. These results showed that the maximum surface area and pore volume was exhibited at 36 h reaction time [27].
One should emphasize that which was reported by Naz et al. [37] relating to the presence of different defects in the composite lattice after 6 h of synthesis, after performing HAADF-STEM measurements. These defects consisted of the appearance of 3–5 nm holes, mainly due to varying quantities and shapes of atoms. They also concluded that the number of defects is higher in the sample synthesised after 6 h than those present into the 20 h synthesized one. They reported by Inductively Coupled Plasma Optical Spectroscopy (ICP-OS) analysis that the weight percentage of MoS2 rises by 5.2% (from 56.6 to 61.8%) with the increase of the reaction time. Their XRD analysis showed a shift of the (002) peak from about 2θ = 14° to lower angles compared with that observed at 2θ = 14.4° for bulk 2H-MoS2 phase, indicating a transition to the 1T phase. Moreover, another peak corresponding to (103) plane at 2θ = 39.7° confirmed the existence of a rich 2H phase as the reaction time was increased from 10 to 20 h [37].
In their work, Naz et al. [37] measured the XPS spectra of their composites synthesised with 6 h and 20 h of reaction time. The samples obtained after 6 h synthesis time show an Mo spectrum consisting of two major peaks at 228.7 and 231.8 eV, which correspond to 3d5/2 and 3d3/2 components, providing substantial evidence for the presence of 1T-phase of MoS2. It was also calculated that the 1T-phase percentage was around 70–76%, while for the 2H-phase it was around 24–30%. The sample obtained with a 20 h reaction time had both peaks shifted towards higher bonding energy, at 229.4 eV for 3d5/2 and 232.5 eV for 3d3/2. Additionally, a broadening of the peaks was noticed, indicating the presence of a dominant 2H-phase at a percentage of about 60–63%. It was found in the fine spectrum of S that the signals at 2p3/2 and 2p1/2 shift to higher energy values with the increase of reaction time as follows: from 161.6 to 162.6 eV, and from 162.8 to 163.8 eV, respectively. The C 1 s components were also analysed by keeping in mind the evidence of GO deoxygenation (i.e., the reduction of GO) during hydrothermal process as the reaction time increases [37]. As it is known, GO contains many oxygen-containing functional groups like C=O, C-OH, O-C-O, HO-C=O etc., with broad peaks ranging from 280 to 290 eV [69]. Naz et al. [37] performed XPS measurements under the same conditions, but without GO substrate, and the sharp peaks recorded at 229.5 eV, for 3d5/2, and 232.5 eV, for 3d3/2, indicating the presence of pure 2H-phase of MoS2. The obtained results demonstrated that the GO substrate has an important role in the growth and transition of 1T-phase MoS2, which can be associated with the functional groups of GO.

3.2. The Influence of Surfactants for Synthesising MoS2 and Its Composites

Usually, in the hydrothermal synthesis processes, the surfactant plays an essential role in controlling and altering the morphology of MoS2 composites and preventing the restacking of MoS2 and GO sheets [70]. Among the surfactants used, the followings stand out: cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), sodium dodecyl-benzene-sulfonate (SDBS), polyvinylpyrrolidone (PVP) or octyl phenol ethoxylate (Triton TX-100) [28,29,40].
Wu et al. [21] prepared MoS2 hollow nanospheres using CTAB as a surfactant in a hydrothermal process. They claim that the amount of CTAB surfactant used increases the stacking of MoS2 nanolayers, and that processing factors such as pH and CTAB surfactant can change the morphology. Nanosheets with a thickness of a few nanometers coil up and stretch towards the edge of hollow of flower-like microspheres and nanosheets according to their report. Since CTAB acts as a “soft” template for synthesising hollow nanomaterials, when the amount of CTAB surfactant is increased to 200 mg, well-defined hollow MoS2 is formed [21].
Tang et al. [27] synthesised MoS2 by the hydrothermal method, obtaining irregular nanosheets aggregated together to form flower-like structures of approximately 1–2 µm in size using about 180 mg of CTAB. They conducted a EDX analysis that revealed that the nanoflowers consisted of Mo and S elements. Moreover, the quantification of the peaks has shown an atom ratio of 1.98:1, which is very close to the stoichiometry of MoS2 [29].
Hongtao et al. [40] synthesised MoS2/rGO using the same procedure [27], but with three different surfactants such as CTAB, SDS and Triton X-100 to investigate their influence over composite’s morphology. In the case of SDS, the GO flakes were compacted and the MoS2 particles were scattered on them. Using Triton X-100 as surfactant they observed that the MoS2 particles were wrapped by the paper-like rGO flakes. They reported that by using CTAB the MoS2 particles were seen clearly and evenly distributed onto the rGO flakes and had a diameter of 1µm. They also reported that the three peaks (2θ = 14, 33.4, 58.9), which are indicative of the hexagonal MoS2 phase, are sharper for the MoS2/rGO-CTAB composite, indicating a higher crystallinity. In terms of specific surface area, they also reported that CTAB was the best candidate, the sample exhibiting a surface area of 133 m2g−1, while in the case of involving SDS and Triton X-100, the specific surface areas of 96 m2g−1 and 27 m2g−1 were achieved, respectively [40]. They performed XPS analysis on the CTAB-assisted sample, the recorded wide spectrum revealing the presence of C 1 s spectra at 285.1 eV, O 1s at around 533.2 eV, Mo 3d around 232.3 eV and S 2p at 163.1 eV. They divided the C 1s spectrum into three peaks corresponding to C=C bonds at 284.5 eV, C-O groups at 285.6 eV, and C=O bonds at 288.9 eV, which indicated the high deoxygenation of rGO. They also acquired the spectrum for Mo 3d and S 2p and highlighted the formation of crystalline MoS2 [40].
Cui et al. [28] also reported a MoS2 synthesis method using CTAB, SDBS and PVP, in which they pointed out the effects of surfactants on the morphology of MoS2 using an SEM technique (Figure 7). It was shown that the bigger and most regular MoS2 nanospheres were obtained by using CTAB as a surfactant. The positively charged CTAB attracts the negative charges of MoS2, forming highly dispersed microspheres. However, the anionic surfactant SDBS presented negative charges in a water-based solution, resulting in poor MoS2 dispersion. The macromolecular hydrophilic film of non-ionic surfactant PVP was formed on the surface of MoS2 particles, early nucleation of MoS2 was inhibited and agglomeration was avoided [28].

3.3. The Influence of Reducing Agents for the Synthesis of MoS2 and Its Composites

Another important parameter for the synthesis of MoS2 and MoS2 composites is the reducing agent, which can control the morphology. Zheng et al. [36] used different Mo and S precursors where the S precursor acts as a sulfur source and reducing agent and divided their combination into three groups. The first one, when (NH4)6Mo7O24/H2NCSNH2 and Na2MoO4·2H2O/CH3CSNH2 were used, a flower-like hollow microsphere was formed, while in the case of the second group, where a single compound in the obtaining for which (NH4)6Mo7O24/Na2S2O3·5H2O was used, morphology of a micron-sheet wrinkled surface was achieved, via a hydrothermal reaction. The third group, obtained using Na2MoO4·2H2O and (NH4)6Mo7O24 and thioacetamide was made up of a blocky amorphous structure with particle sizes between 10 and 50 nm. The XRD patterns of the first group exhibited sharp peaks at 2θ = 14°, 33°, 38° and 58° that correspond to (002), (001), (103) and (100) lattice planes of the hexagonal structure of MoS2, indicating a high crystallinity and a well-developed stacking. The second group’s crystallinity was slightly decreased and the third one’s were inconclusive, probably due to the amorphous structure [36].
Cui et al. [28] prepared MoS2 microspheres through a surfactant-assisted hydrothermal synthesis using thioacetamide as the reducing agent. They deducted that compared to a thiourea containing system, the thioacetamide has a slower breakdown rate to form H2S, which does contribute to the sulfuration of MoO3 species. Ammonium heptamolybdate decomposition is also aided by the presence of H+. However, the excess of H+ increases the rate at which the thioacetamide decomposes to generate H2S, potentially resulting in an inadequate reduction of MoO3, lowering the purity and crystallinity of the synthesised MoS2. It was also suggested that the electrostatic interactions might explain the modifying impact of surfactants, and that the effects of MoS2 particles with a net negative charge on the surface was attracted by the positive charges of CTAB [28].

3.4. The Influence of the Molar Ratios of Molybdenum and Sulfur Precursors for Synthesising MoS2 and Its Composites

Cui et al. [28] synthesised MoS2 microspheres through a surfactant-assisted hydrothermal method using thioacetamide as a reducing agent. Their synthesis was done at 220 °C for different Mo to S ratios of 1:1, 1:2, 1:3, 1:4, and 1:5. They analysed the structure by X-ray diffraction. The sample with a Mo/S ratio of 1:2 exhibited a major diffraction peak that matched the 2H-phase two-layered hexagonal MoS2. They stated that the recorded signals became broader with the decrease of Mo/S to 1:3, a result which indicated the presence of an amorphous structure. The same diffraction patterns as the one of 1:3 sample were observed in the other two samples with a Mo/S ratio of 1:4 and 1:5. XPS analyses of the MoS2 samples prepared at 220 °C, at a pH of 6, without any surfactant, were also performed. The binding energy of 230.9 eV and 233.7 eV were found to correspond to the Mo 3d5/2, and Mo 3d3/2 orbitals of Mo(IV). The orbitals of S(II) were associated with S 2p3/2 and S 2p1/2 with the binding energy around 163.1 eV and 164.5 eV, respectively. By calculating the peak areas, the atom ratio of Mo to S was of about 1:2, which corresponds to MoS2 and confirmed the previously reported results [28].
Sun et al. [71] synthesised 3D graphene/MoS2 composites as electrode materials for supercapacitors and used GO, Na2MoO4 and thiourea as precursors. They prepared samples using different amounts of sodium molybdate and thiourea corresponding to mass ratio of MoS2 to GO in composites of 1.1:1, 1.6:1, 2.1:1, 3.2:1, 4.2:1, and 5.3:1. For the first three samples, their SEM analysis showed MoS2 nanosheets growing on the GO substrate. With the increase of Na2MoO4, for the next two samples they observed the formation of MoS2 nanoflowers on the GO substrate and, ultimately the last sample with the highest Na2MoO4 concentration showed that the nanoflowers started to grow on top of the other MoS2 nanoflowers instead growing on the GO substrate. Their results demonstrated that the morphology can change with the increase of Na2MoO4 content [71]. The 4:2:1 mass ratio sample’s crystal phase and structure was compared to that of 3D GO, and MoS2 through XRD analysis and concluded that their XRD patterns were quite similar, thus pointing out that GO has little effect on nucleation phase and growth of MoS2. This observation highlights that 3D GO can be an appropriate platform for synthesising the composite, because graphene layers are dispersed and surrounded by MoS2 layers [71]. BET surface analysis was used to determine the specific surface area, and the results for the first five samples were consistent with their morphologies, with an increase from 64.4 m2g−1 to 165.7 m2g−1 for samples with a mass ratios of 1.1:1 and 4.2:1, respectively [71].
Liu et al. [1] prepared MoS2/rGO by hydrothermal method at 180 °C for 20 h using (NH4)Mo7O24·4H2O and L-cysteine as precursors. They used different percentages of MoS2 to obtain composites, as follows: 35.8 wt%, 50.5 wt% and 60.8 wt%. The morphology and structure of MoS2 show a highly porous structure enclosed by ultrathin sheets. The MoS2 composite has nanosheets with a thickness of 1–3 layers uniformly dispersed on the rGO. By increasing the weight percentage of MoS2, they obtained an increase in sheet thickness of 4–6, 7–9 and >10 for the simple MoS2. The XRD patterns of rGO and MoS2 can be assigned to the plane hexagonal structure of MoS2 and the d002 plane of graphite. They stated that with the increase in the molybdenum disulfide concentration, the diffraction peaks of MoS2 become higher in intensity, as expected, and the lattice spacing of the (002) plane of MoS2 observed was 0.619. The group characterised the electronic structure of the composites by XPS analysis. The Mo 3d spectra showed dominant Mo4+ and traces of Mo6+ species corresponding to 2H-phase of MoS2. The S 2p XPS spectra were also assigned to the 2H-phase. They observed that with the decrease in MoS2 content, the intensities of Mo6+-O and SOx species rose gradually due to the increased proportion of the strong interaction between rGO and MoS2 [1].
Peng et al. [41] reported a synthesis of MoS2/rGO by hydrothermal method using (NH4)6Mo7O24·4H2O and NH2CSNH2 at 200 °C for 24 h. The composites were prepared using 2:1 and 3:2 volume ratios of water and ethanol as solvents. The morphology of these composites revealed that the nanosheets of molybdenum disulfide were stacked and displayed in a 3D flower-like architecture. For the first sample with the volume ratio of 2:1, a porous flower-like morphology with corrugated and scrolled rGO sheets was obtained. With the decrease from 2:1 to 3:2 volume of water, the flower-like architecture vanished and changed to nanoflakes of MoS2 on the surface of the rGO sheets. The XRD patterns showed peaks at 2θ = 13.9°, 33.4° and 59.1° attributed to (002), (100) and (110) planes of MoS2 that indicated a successful load onto the rGO substrate. They calculated the corresponding d002 plane by the Bragg equation and found that it increased from 0.630 nm, for the blank, to 0.949 nm and 0.933 nm, for the 2:1 and 3:2 sample, respectively. The BET surface area was determined and the best results obtained were 28.7 m2g−1, 23.4 m2g−1, and 16.1 m2g−1, for the samples with the water to ethanol ratio of 2:1, 3:2, and the samples prepared in water as a solvent respectively. It was concluded that the solvent plays a substantial role and had an obvious effect on the surface structure of the resulting composites [41].

4. Electrical and Capacitive Properties for MoS2-Based Electrode Materials in Energy Storage Applications

In this section, several promising results of electrode materials regarding the specific capacity and cycling stability will be presented. The results are also briefly presented in Table 1. We compiled a list of accomplishments based on key parameters, such as specific capacitance and cycling stability of electrode materials for supercapacitor applications, and ranked them in order of performance.
A different strategy to synthesize MoS2 nanospheres was approached by Wang et al., [72] who used MnCO3 as a template to grow the nanospheres and removed them afterwards by acid washing. Using MnCO3, sodium molybdate and L-cysteine in a hydrothermal process, followed by acid washing to remove the template, they managed to obtain MoS2 nanospheres with a specific capacity of 142 F·g−1 at a current density of 0.59 A·g−1 and a cycling stability of 92.9% after 1000 cycles [72].
Another electrode material synthesized by Liu et al. is MoS2/GO [73]. After hydrothermal synthesis of molybdenum disulfide and its intercalation with N-buthyllithium, the GO was mixed with a poly(ethyleneimine) solution. Their material reported a specific capacitance of 153.9 F·g−1 at 5 A·g−1 with a cycling retention of 96% over 6800 cycles [73].
Wu et al. [21] prepared MoS2 as an electrode material for supercapacitors via a hydrothermal method, assisted with CTAB. The material’s morphology varied between flower-like, nanosheets and hollow structures, and is influenced by the pH and CTAB concentration. Their best results were reported to be the ones of hollow structures of MoS2, with a specific capacitance of 160 F·g−1 at a current density of 1 A·g−1 and a cycling stability of 94.8% after 3000 cycles [21].
Huang et al. [54] reported a layered MoS2-graphene composite with a sheet-like structure obtained by hydrothermal synthesis at 200 °C for 24 h using sodium molybdate and thiourea as the main precursors and GO obtained by a modified Hummers method. The maximum specific capacitance of the composite was recorded to be 243 F·g−1 at a discharge current density of 1 A·g−1 and a cycling stability with only 7.7% loss after 1000 cycles [54].
Vikraman et al. [74] obtained a mixed-phase MoS2 decorated rGO hybrid composite through a facile one-pot synthesis, using ammonium molybdate and thiourea as the main precursors and synthesized them on a hot plate at 90 °C for 8 h. Their hybrid material showed a symmetrical capacitance of 275 F·g−1 at a current density of 1 A·g−1 and a cycling stability of 97% over 5000 cycles.
Murugan et al. [33] obtained a two-dimensional composite with MoS2 nanosheets anchored on graphene by an in-situ hydrothermal method. Their method consisted of using ammonium heptamolybdate and thioacetamide as precursors and citric acid as a surfactant, proceeding in a stainless-steel autoclave for 24 h at 435 k (161.85 °C). Their nanosheet structure was reported to exhibit a specific capacitance of 334 F·g−1 at a scan rate of 10 mV·s−1 and also a retention of 95% after 500 cycles.
In terms of symmetrical supercapacitors, Liu et al. [1] synthesized an electrode material able to obtain high specific capacitance of 361 F·g−1 at 5 mV·s−1 and a good cycle stability in 1 M H2SO4. The materials were obtained by a hydrothermal method using ammonium molybdate tetrahydrate and L-cysteine accompanied by GO obtained by a modified Hummers method. The morphology obtained was presented as a highly porous structure enclosed by ultrathin MoS2 sheets uniformly distributed on the rGO substrate.
Gao et al. [53] synthesized, by a hydrothermal method, carbon-anchored MoS2 nanosheets using sodium molybdate, L-cysteine and β-cyclodextrine as precursors in the process at 200 °C for 12 h. Among their samples, C/MoS2—5.0% had the best electrical and capacitive performances. They reported that their material specific capacitance at a CV scan rate of 5 mV·s−1 was 394.2 F·g−1, and after increasing the scan rate at 200 mV·s−1, it remained at 249.6 F·g−1. It also presented a good cycling stability with a retention in capacity of 60.68% for 2000 cycles at a scan rate of 40 mV·s−1 [53].
Sun et al. [71] managed to synthesize a composite based on 3D GO and MoS2 with a flower-like structure, which was able to increase the performances of the electrode material in a supercapacitive cell. As main precursors, sodium molybdate and thiourea were chosen with different concentrations. Their best results showed a specific capacitance of 410 F·g−1 at 1 A·g−1, current density and a cycle stability of 80.3% over a continuous 10,000 cycles at 2 A·g−1.
Naz et al. [37] reported a highly defective 1T-phased MoS2 on 3D rGO composite with flower-like morphology using the hydrothermal route. They tested different synthesis times using, as precursors, ammonium molybdate and thiourea for MoS2 at 220 °C. The rGO was prepared by the Hummers method. Their best results were at a synthesis time of 6 h, capable of giving an exceptional specific capacitance of 442.0 F·g−1 at a discharge current density of 1 A·g−1 and also being able to retain 90.3% of its capacitance after 1000 cycles at 5 A·g−1.
Huang et al. [81] synthesized a composite based on MoS2/PANI by in situ chemical oxidative polymerization directed by molybdenum disulfide. Their material maximum specific capacitance was of about 575 F·g−1 at 1 A·g−1 and a retention of less than 2% after 500 cycles at the same current density.
Dutta et al. [6] prepared a hybrid electrode material of MoS2/rGO nanosheets by a hydrothermal route using SDS as a surfactant and MoO3 and potassium thiocyanate as the main precursors, keeping them in a stainless steel autoclave at 220 °C for 24 h. Their results showed a specific capacitance of approximately F·g−1 and a retention of 95% after 1000 cycles using 1 M H2SO4 as electrolyte, with measurements being made in a three-electrode system.
Baig et al. [65] reported a composite material made of MoS2/rGO with a specific capacitance of 850 F·g−1 at a current density of 1 A·g−1 and a cycle stability at 2 A·g−1 of 95.3% for over 10,000 cycles. To obtain the electrode material they used a hydrothermal method with ammonium heptamolybdate and thiourea as precursors and PVP as a main surfactant.
Hongtao et al. [40] tested different surfactants and their effect on the morphologies and electrical properties of a composite based on MoS2/rGO. In their hydrothermal synthesis, they used sodium molybdate and thioacetamide as precursors for MoS2 and a modified Hummers method for the rGO. They obtained different morphologies and properties based on the surfactant. However, the best results were given by using CTAB with the highest specific capacitance of 1022 F·g−1 at a current density of 0.3 A·g−1.
Self-assembled nanoflowers of MoS2 were obtained by Wei et al. [26] Their method was based on a one-pot hydrothermal reaction from which they obtained flower-like MoS2 capable of exhibiting 1120 F·g−1 at a current density of 0.5 A·g−1 in KOH electrolyte. Their cycling stability was measured at 10 A·g−1 current density and it showed a 96% retention in 2000 cycles.

5. Conclusions

A general conclusion might be that in developing more efficient electrode materials for supercapacitors, combining a conductive polymer like polyaniline (PANI) with a promising material like MoS2 to obtain loose and porous structures with pseudocapacitive behaviour is a must. Using conductive materials such as rGO to create a structure to intercalate porous MoS2 morphologies is also a good way to improve the electrode material’s electrochemical properties and to develop better active materials. The key in developing better electrode materials for supercapacitor applications appears to be engineering the phase structure of MoS2, as well as modelling its morphology and using the appropriate electrolyte.
To put it another way, better results can be obtained by selecting and engineering the appropriate materials for the development of efficient and long-lasting supercapacitors. Graphene, GO, and rGO, as well as conductive polymers like polyaniline, in combination with the porous, well-engineered phase MoS2, could be a promising path forward in the energy storage field. Although, in order to achieve better materials and results, the synthesis route and parameters such as precursors, reaction time, surfactants used and molar ratios must be carefully considered.

Author Contributions

Conceptualization, C.A.S., L.C.C. and L.B.; writing—original draft preparation, C.A.S.; writing—review and editing, C.C. and L.C.C.; visualization, L.B. and L.C.C.; supervision, L.B. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by UBB-TeMATIC-Art P_40_374 project.

Data Availability Statement

Not applicable.

Acknowledgments

C.C. acknowledges financial support from a special grant for scientific activity awarded by STAR-UBB (Babeş-Bolyai University).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Classification of supercapacitors based on their energy storage mechanism, specifying the active materials and the processes involved.
Figure 1. Classification of supercapacitors based on their energy storage mechanism, specifying the active materials and the processes involved.
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Figure 2. MoS2 synthesis pathways.
Figure 2. MoS2 synthesis pathways.
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Figure 3. Top and side-view of MoS2 monolayer crystalline phases [30]. Reprinted with permission from [30]. Copyright 2018. Elsevier.
Figure 3. Top and side-view of MoS2 monolayer crystalline phases [30]. Reprinted with permission from [30]. Copyright 2018. Elsevier.
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Figure 4. Formation of MoS2/rGO composites following the three-step process of hydrothermal synthesis.
Figure 4. Formation of MoS2/rGO composites following the three-step process of hydrothermal synthesis.
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Figure 5. Scheme representing the formation of defective 1T MoS2 nanoflowers on rGO substrate [37]. Reprinted with permission from [37]. Copyright 2019. Elsevier.
Figure 5. Scheme representing the formation of defective 1T MoS2 nanoflowers on rGO substrate [37]. Reprinted with permission from [37]. Copyright 2019. Elsevier.
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Figure 6. TEM images of MoS2 samples obtained under different reaction times ((a): 6 h, (b): 12 h, (c): 48 h) from [29]. Reprinted with permission from [29]. Copyright 2012. Elsevier.
Figure 6. TEM images of MoS2 samples obtained under different reaction times ((a): 6 h, (b): 12 h, (c): 48 h) from [29]. Reprinted with permission from [29]. Copyright 2012. Elsevier.
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Figure 7. SEM images of MoS2 microspheres prepared at 220 °C in pH 6 solution adding different surfactants (Mo/S ratio of 1:2): (a) no adding; (b) adding CTAB; (c) adding SDBS; (d) adding PVP [28]. Reprinted with permission from [28]. Copyright 2016. Elsevier.
Figure 7. SEM images of MoS2 microspheres prepared at 220 °C in pH 6 solution adding different surfactants (Mo/S ratio of 1:2): (a) no adding; (b) adding CTAB; (c) adding SDBS; (d) adding PVP [28]. Reprinted with permission from [28]. Copyright 2016. Elsevier.
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Table
Table 1. Various results from literature on electrode materials based on MoS2 composites for supercapacitor applications.
Table 1. Various results from literature on electrode materials based on MoS2 composites for supercapacitor applications.
Composite Precursors Surfactant Synthesis Method Morphology Synthesis Conditions Specific Capacitance (F·g−1)Specific Surface Area (m2·g−1) Cycling Stability Collectors Electrode Test System Electrolyte References
C/MoS2Na2MoO4
and C3H7NO2S		β-cyclodextrinHydrothermalFlower-like200 °C for 12 h394 at
5 mV/s		6.3660.68% after 2000 cycles at 40 mV/sStainless SteelThree electrode system1 M Na2SO4[53]
MoS2/rGO (NH4)6Mo7O24
and CH4N2S		-One pot reactionNanospheres90 °C for 8 h275 at 1
A·g−1		9.0297% after 5000 cycles at 1 A·g−1Ni foamTwo
electrode system		6 M KOH[75]
MoS2/G Na2MoO4
and CH4N2S		-Hydrothermal3D spheres180 °C for 36 h243 at 1
A·g−1		102.892.3% after 1000 cycles at 1 A·g−1Stainless SteelThree electrode system1 M Na2SO4[54]
MoS2/rGO(NH4)6Mo7O24
and C2H5NS		Citric AcidHydrothermalWrinkly paper like180 °C for 24 h334 at 0.5 mV/s-90% after 500 cycles at 10 mV/sPlatinum stripThree electrode system1 M LiClO4[33]
MoS2/rGO (NH4)6Mo7O24
and C3H7NO2S		-HydrothermalWrinkly paper like180 °C for 20 h361 at 5 mV/s-92% after 10,000 cyclesCarbon paperTwo & Three electrode system1 M H2SO4[1]
MoS2/Ni foam Na2MoO4
and CH4N2S		CTABHydrothermalHollow nanostructure160–200 °C for 24 h160.1 at 1
A·g−1		-94.8% after 3000 cycles at 1 A·g−1Ni foamThree electrode system1 M Na2SO4[21]
MoS2Na2MoO4
and C2H5NS		-HydrothermalBrush like180 °C for 36 h244 at 1
A·g−1		18.4592% after 9000 cycles at 5 A·g−1Ni foamThree electrode system1 M KOH[27]
MoS2/G (NH4)6Mo7O24
and CH4N2S		-One pot reactionNanosheet grains50 °C for 6 h756 at 0.5
A·g−1		-88% after 10,000 cycles at 0.5 A·g−1Stainless steelThree electrode system1 M KOH[74]
MoS2Na2MoO4
and C3H7NO2S		-HydrothermalNanospheres220 °C for 24 h142 at 0.59
A·g−1		-92.9% after 1000 cycles at 0.59 A·g−1Ni foamThree electrode system1 M KCl[72]
MoS2/PANI/
rGO		(NH4)6Mo7O24
and CH4N2S		-HydrothermalFlower-like200 °C for 48 h330.7 at 10
A·g−1		85.381.9 % after 40,000 cycles
at 10 A·g−1		Graphite paperThree electrode system1 M H2SO4[31]
MoS2/rGO MoO3·H2O
and C2H5NS		UreaHydrothermalFlake-like210 °C for 18 h441 at
1 A·g−1		163.684.2% after 1000 cyclesITOThree electrode system1 M Na2SO4[76]
MoS2/N-doped 3D Graphene (NH4)6Mo7O24
and CH4N2S		-HydrothermalFlower-like200 °C for 24 h301.2 at
0.2 A·g−1		-82% after 1000 cycles at 1 A·g−1Glassy carbonThree electrode system1 M Na2SO4[77]
MoS2/G aerogel Li intercalation-Hydrothermal3D porous network180 °C for 12 h268 at 0.5 A·g−1 149.393% after 1000 cyclesTi foilThree electrode system1 M Na2SO4[78]
MoS2/CNT MoO3 and KSCNSDBSHydrothermalFlower-like220 °C for 24 h74.05 at 2 A·g−192.2580.8% after 1000 cycles at 1 A·g−1Ni foamThree electrode system1 M Na2SO4[79]
MoS2(NH4)6Mo7O24
and CH4N2S		-HydrothermalFlower-like220 °C for 24 h255.65 at 0.25 A·g−1-70%Graphite foilTwo electrode system3 M KOH[80]
MoS2/rGO (NH4)6Mo7O24
and CH4N2S		PVPHydrothermalFlower-like180 °C for 16 h850 at 1 A·g−139195.33% after
10,000 cycles at 2 A·g−1		Ni foamThree electrode system1 M KCl[65]
MoS2/rGO (NH4)6Mo7O24
and CH4N2S		-HydrothermalFlower-like220 °C for 6 h442 at 1 A·g−129.2790.3% after 1000 cycles at 5 A·g−1Glassy carbonThree electrode system1 M H2SO4[37]
MoS2/3D-G Na2MoO4
and CH4N2S		-HydrothermalNanoflowers200 °C for 24 h410 at 1 A·g−1165.790.3% after 10,000 cycles at 2 A·g−1Ni foamThree electrode system1 M Na2SO4[71]
MoS2/PANI MoS2 powder and aniline-In situ chemical processFlower-like12 h ice bath
polymerization		575 at
1 A·g−1		->98% after 500 cycles at 1 A·g−1Stainless steelThree electrode system1 M H2SO4[81]
MoS2/rGO Na2MoO4
and C2H5NS		CTABHydrothermalNanoparticles180 °C for 12 h1022 at
0.3 A·g−1		133----[40]
MoS2MoO3 and C2H5NS-HydrothermalNanoflowers200 °C for 12 h112054.796% after 2000 cycles at 10 A·g−1Ni foamThree electrode system3 M KOH[26]
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Salagean, C.A.; Costinas, C.; Cotet, L.C.; Baia, L. Insights into the Influence of Key Preparation Parameters on the Performance of MoS2/Graphene Oxide Composites as Active Materials in Supercapacitors. Catalysts 2021, 11, 1553. https://doi.org/10.3390/catal11121553

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Salagean CA, Costinas C, Cotet LC, Baia L. Insights into the Influence of Key Preparation Parameters on the Performance of MoS2/Graphene Oxide Composites as Active Materials in Supercapacitors. Catalysts. 2021; 11(12):1553. https://doi.org/10.3390/catal11121553

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Salagean, Catalin Alexandru, Codrut Costinas, Liviu Cosmin Cotet, and Lucian Baia. 2021. "Insights into the Influence of Key Preparation Parameters on the Performance of MoS2/Graphene Oxide Composites as Active Materials in Supercapacitors" Catalysts 11, no. 12: 1553. https://doi.org/10.3390/catal11121553

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