Insights into the Inﬂuence of Key Preparation Parameters on the Performance of MoS 2 /Graphene Oxide Composites as Active Materials in Supercapacitors

: 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 MoS 2 /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. applications, such as speciﬁc capacitance and cycle stability of electrode materials, and their ranking in order of performance, is also presented.


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 Fꞏg −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]. 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)/LiMn 2 O 4 [15,16], activated carbon/graphene/MnO 2 , [17] Na x MnO 2 /N-(rGO) [18], and also symmetric supercapacitors, which have similar electrodes, such as MoS 2 -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 MoS 2 [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 (MoS 2 ). Because MoS 2 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 MoS 2 -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 MoS 2 nanostructures and MoS 2 -GO composites.
At the beginning of the review will be presented an overview of the synthesis methods and parameters involved in the obtaining of MoS 2 nanostructures. Further on, the effects of each preparation parameter on the structure, morphology, and surface particularities of MoS 2 /MoS 2 -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.

Synthesis Methods for MoS 2 /MoS 2 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.
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 Fꞏg −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.

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 MoS 2 product; however, the most commonly used preparation method for MoS 2 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 MoS 2 , 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 MoS 2 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 MoS 2 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].

Top-Down Methods for MoS 2 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 MoS 2 (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 MoS 2 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 MoS 2 nanosheets in the form of Li x MoS 2 . Afterwards, the molybdenum disulfide intercalated with lithium ions will interact with water during sonication, producing a MoS 2 dispersion.
Another aspect that needs to be discussed here is the transition of MoS 2 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-MoS 2 phase [37,44]. MoS 2 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). MoS 2 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 surfactantassisted chemical exfoliation using organic solvents prevents the aggregation of MoS 2 Catalysts 2021, 11, 1553 5 of 18 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.
Another procedure for liquid phase exfoliation of molybdenum disulfide is s cation assisted by organic solvents such as N-methyl-2-pyrrolidone (NMP). MoS2 ca obtained easily using NMP without the use of any surfactants. Moreover, the surface sion of the TMDCs should closely match the solvent's surface tension [45]. Although factant-assisted chemical exfoliation using organic solvents prevents the aggregatio MoS2 nanosheets by stabilizing the dispersion [46], it does not impact the structure o material by creating phase changes like the one done using organolithium compound Ball milling is also a top-down synthesis mechanism that is mostly used to ob few-layered MoS2 nanosheets. This approach is based on the process of milling, w reduces the size and layers of the compound, increasing its specific surface area. It also used to prepare graphene layers from bulk graphite, which is a similar structu that of MoS2 [47]. The ball milling approach is also considered as a "green method", w out requiring harmful chemicals, and can also be used as a simple and efficient me for obtaining qualitative MoS2 nanosheets [48].

Bottom-Up Methods
Chemical vapor deposition (CVD) has always been a versatile bottom-up appr involved in obtaining large surface area MoS2 on a substrate as a few-layered struc with great crystallinity. In CVD processes, a specific substrate is exposed to reactive cursors in a high pressure and temperature medium, in which they react or decompo form high purity ultrathin films. Furthermore, via this method, the size and thickne films can be controlled to obtain great crystal quality and desired electronic prope [49]. Some particular CVD techniques are sulfurization of Mo-based films and decom sition of molybdenum and sulfur-containing precursors [50,51]. The direct sulfuriza of MoO3 is one of the best ways to obtain great quality MoS2. Using MoO3 as a depos surface on the substrate, it can react with sulfur precursors on its surface, generating MoS2 layers. The entire mechanism of CVD is explained by Hong et al. [52] Yongjie e [50] reported a direct preparation of single-and few-layered MoS2 on SiO2 substrate pre-depositing Mo films followed by a CVD approach. They concluded that the size thickness of MoS2 films are dependent on the size of the substrate and the thickne pre-deposited Mo, which are easily scalable and tuneable, making CVD an importan proach for a large variety of applications.
Hydrothermal/solvothermal chemical synthesis is one of the most versatile proaches for obtaining MoS2 structures, usually adopted for the preparation of mo denum disulfide containing composites [33,50,51]. It is used to synthesize MoS2 using and S precursors via chemical reactions performed in high pressure of a tightly se autoclave placed in a medium temperature (i.e., <220 °C) [53,54]. During this synth three steps take place: nucleation, aggregation and self-assembly of nanocrystals (Fi 4).  [30]. Reprinted with permission from [30]. Copyright 2018. Elsevier.
Ball milling is also a top-down synthesis mechanism that is mostly used to obtain few-layered MoS 2 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 MoS 2 [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 MoS 2 nanosheets [48].

Bottom-Up Methods
Chemical vapor deposition (CVD) has always been a versatile bottom-up approach involved in obtaining large surface area MoS 2 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 MoO 3 is one of the best ways to obtain great quality MoS 2 . Using MoO 3 as a deposited surface on the substrate, it can react with sulfur precursors on its surface, generating the MoS 2 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 MoS 2 on SiO 2 substrates by pre-depositing Mo films followed by a CVD approach. They concluded that the size and thickness of MoS 2 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 MoS 2 structures, usually adopted for the preparation of molybdenum disulfide containing composites [33,50,51]. It is used to synthesize MoS 2 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 MoO 3 , 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 selfassemble to form microspheres and flower-like morphologies [58][59][60]. As a particular case, MoS 2 /rGO composites were taken into account [33]. A scheme of MoS 2 /rGO composite formation by this approach is presented in Figure 5.  In this kind of process, precursors like MoO3, sodium molybdate, ammonium hep tamolybdate and sulfur precursors such as thiourea, thioacetamide, L-cysteine and so dium 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 star to self-assemble to form microspheres and flower-like morphologies [58][59][60]. As a partic ular case, MoS2/rGO composites were taken into account [33]. A scheme of MoS2/rGO composite formation by this approach is presented in Figure 5.  [37]. Reprinted with permission from [37]. Copyright 2019. Elsevier.
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 experi ments in order to explore the formation mechanism of MoS2 porous flower-like micro spheres.
In more detail, the nucleation step starts firstly with the development of a nucleus  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.  [37]. Reprinted with permission from [37]. Copyright 2019. Elsevier.
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 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 MoS 2 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 -NH 2 functional group and combine with the oxygen atoms of the molybdate species. Afterwards, the residual Mo and S, can bond into MoS 2 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 MoS 2 can also play an important role in this process because MoS 2 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 MoS 2 play key roles in the aggregation and bending growth of the composite.
The self-assembly process is dictated by the unstable structures of MoS 2 , which tend to combine. The obtained hierarchical aggregated MoS 2 nanosheets have a high surface area. Baig et al. [65] had obtained flower-like structures with a specific surface area of 391 m 2 g −1 for a MoS 2 /rGO composite with ammonium hepta-molybdate and thiourea as precursors. Moreover, Chao et al. [31] reported specific surface areas of different MoS 2 composites such as 11.2 m 2 g −1 for MoS 2 /polyaniline (PANI), 31.4 m 2 g −1 for MoS 2 /rGO and 85.3 m 2 g −1 for MoS 2 /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 MoS 2 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 MoS 2 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].

The Influence of Preparation Parameters on Morphology, Structure and Surface Particularities of MoS 2 /MoS 2 -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 MoS 2 /MoS 2 -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.

Influence of the Reaction Time in MoS 2 and Its Composites Synthesis
Reaction time is definitely an important parameter in the synthesis of MoS 2 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 MoS 2 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 MoS 2 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 MoS 2 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 MoS 2 nanosheets rises from 4.8 to 15 nm, and the number of layers from 6-7 to more than 15.

Influence of the Reaction Time in MoS2 and Its Composites Synthesis
Reaction time is definitely an important parameter in the synthesis of MoS composites, especially when it comes down to hydrothermal technique, which af morphological, structural and surface area of the active material. Tang et al. [29 ized by TEM technique the structural particularities of the formed MoS2 after 6, 1 h of reaction time ( Figure 6).  [29]. Reprinted with permission from [29]. Copyright 2012. Elsevier.
It was observed that with an increase in the reaction time to 48 h, a perfect cr flower-like MoS2 structure was achieved. Based on the formation mechanism, time-dependent, it can be inferred that the sulfur sources are bound onto the su reduced graphene oxide via nucleation, forming MoS2 nanoflowers that overlay bo of the rGO. In terms of morphology, Naz et al. [37] reported an increase in nanos from 400 to 680 nm, with a reaction time increase from 6 to 20 h at a temperatu °C, respectively, and the formation of small nanoflowers with numerous edge sit also reported that by increasing the reaction time, the thickness of MoS2 nanoshe 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 the formation mechanism of MoS2 flower-like microspheres, varying the synthe tion time (4, 8, 12, 16 and 22 h) and divided the whole mechanism into three ph discussed in the previous sections: nucleation, aggregation and self-assembly. I concluded that, in a hydrodynamic process, the formation of flower-like structure dependent, meaning that a higher reaction time can deliver a more porous s However, other parameters, such as reducing agent and surfactants, which wil cussed later, must be taken into account to fully control the formed flower-like str The specific surface area is also impacted by the reaction time as Manu showed in their work [27]. They synthesised MoS2 nanostructures using different times (i.e., 12, 24, 36, 48 h) and proceeded, with BET analyses, the measuremen surface area. They obtained the following results for the specified times: 10.74, 13. and 13.26 m 2 g −1 and pore diameters of 0.047421, 0.042381, 0.064782 and 0.03525 respectively. These results showed that the maximum surface area and pore volu exhibited at 36 h reaction time [27].
One should emphasize that which was reported by Naz et al. [37] relating to  [29]. Reprinted with permission from [29]. Copyright 2012. Elsevier.
Zheng et al. [36] also performed a series of time-dependent experiments to explore the formation mechanism of MoS 2 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 MoS 2 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 m 2 g −1 and pore diameters of 0.047421, 0.042381, 0.064782 and 0.035250 cm 3 g −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 MoS 2 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-MoS 2 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 3d 5/2 and 3d 3/2 components, providing substantial evidence for the presence of 1Tphase of MoS 2 . 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 3d 5/2 and 232.5 eV for 3d 3/2 . 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 2p 3/2 and 2p 1/2 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 3d 5/2 , and 232.5 eV, for 3d 3/2 , indicating the presence of pure 2H-phase of MoS 2 . The obtained results demonstrated that the GO substrate has an important role in the growth and transition of 1T-phase MoS 2 , which can be associated with the functional groups of GO.
Wu et al. [21] prepared MoS 2 hollow nanospheres using CTAB as a surfactant in a hydrothermal process. They claim that the amount of CTAB surfactant used increases the stacking of MoS 2 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 MoS 2 is formed [21].
Tang et al. [27] synthesised MoS 2 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 MoS 2 [29].
Hongtao et al. [40] synthesised MoS 2 /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 MoS 2 particles were scattered on them. Using Triton X-100 as surfactant they observed that the MoS 2 particles were wrapped by the paper-like rGO flakes. They reported that by using CTAB the MoS 2 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 MoS 2 phase, are sharper for the MoS 2 /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 m 2 g −1 , while in the case of involving SDS and Triton X-100, the specific surface areas of 96 m 2 g −1 and 27 m 2 g −1 were achieved, respectively [40]. They performed XPS analysis on the CTABassisted 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 MoS 2 [40].
Cui et al. [28] also reported a MoS 2 synthesis method using CTAB, SDBS and PVP, in which they pointed out the effects of surfactants on the morphology of MoS 2 using an SEM technique (Figure 7). It was shown that the bigger and most regular MoS 2 nanospheres were obtained by using CTAB as a surfactant. The positively charged CTAB attracts the negative charges of MoS 2 , forming highly dispersed microspheres. However, the anionic surfactant SDBS presented negative charges in a water-based solution, resulting in poor MoS 2 dispersion. The macromolecular hydrophilic film of non-ionic surfactant PVP was formed on the surface of MoS 2 particles, early nucleation of MoS 2 was inhibited and agglomeration was avoided [28]. 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 m 2 g −1 , while in the case of involving SDS and Triton X-100, the specific surface areas of 96 m 2 g −1 and 27 m 2 g −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].   [28]. Reprinted with permission from [28]. Copyright 2016. Elsevier.

The Influence of Reducing Agents for the Synthesis of MoS 2 and Its Composites
Another important parameter for the synthesis of MoS 2 and MoS 2 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 (NH 4 ) 6 Mo 7 O 24 /H 2 NCSNH 2 and Na 2 MoO 4 ·2H 2 O/CH 3 CSNH 2 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 (NH 4 ) 6 Mo 7 O 24 /Na 2 S 2 O 3 ·5H 2 O was used, morphology of a micron-sheet wrin-kled surface was achieved, via a hydrothermal reaction. The third group, obtained using Na 2 MoO 4 ·2H 2 O and (NH 4 ) 6 Mo 7 O 24 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 MoS 2 , 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 MoS 2 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 H 2 S, which does contribute to the sulfuration of MoO 3 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 H 2 S, potentially resulting in an inadequate reduction of MoO 3 , lowering the purity and crystallinity of the synthesised MoS 2 . It was also suggested that the electrostatic interactions might explain the modifying impact of surfactants, and that the effects of MoS 2 particles with a net negative charge on the surface was attracted by the positive charges of CTAB [28].

The Influence of the Molar Ratios of Molybdenum and Sulfur Precursors for Synthesising MoS 2 and Its Composites
Cui et al. [28] synthesised MoS 2 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 MoS 2 . 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 MoS 2 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 3d 5/2 , and Mo 3d 3/2 orbitals of Mo(IV). The orbitals of S(II) were associated with S 2p 3/2 and S 2p 1/2 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 MoS 2 and confirmed the previously reported results [28].
Sun et al. [71] synthesised 3D graphene/MoS 2 composites as electrode materials for supercapacitors and used GO, Na 2 MoO 4 and thiourea as precursors. They prepared samples using different amounts of sodium molybdate and thiourea corresponding to mass ratio of MoS 2 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 MoS 2 nanosheets growing on the GO substrate. With the increase of Na 2 MoO 4 , for the next two samples they observed the formation of MoS 2 nanoflowers on the GO substrate and, ultimately the last sample with the highest Na 2 MoO 4 concentration showed that the nanoflowers started to grow on top of the other MoS 2 nanoflowers instead growing on the GO substrate. Their results demonstrated that the morphology can change with the increase of Na 2 MoO 4 content [71]. The 4:2:1 mass ratio sample's crystal phase and structure was compared to that of 3D GO, and MoS 2 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 MoS 2 . This observation highlights that 3D GO can be an appropriate platform for synthesising the composite, because graphene layers are dispersed and surrounded by MoS 2 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 m 2 g −1 to 165.7 m 2 g −1 for samples with a mass ratios of 1.1:1 and 4.2:1, respectively [71].
Liu et al. [1] prepared MoS 2 /rGO by hydrothermal method at 180°C for 20 h using (NH 4 )Mo 7 O 24 ·4H 2 O and L-cysteine as precursors. They used different percentages of MoS 2 to obtain composites, as follows: 35.8 wt%, 50.5 wt% and 60.8 wt%. The morphology and structure of MoS 2 show a highly porous structure enclosed by ultrathin sheets. The MoS 2 composite has nanosheets with a thickness of 1-3 layers uniformly dispersed on the rGO. By increasing the weight percentage of MoS 2 , they obtained an increase in sheet thickness of 4-6, 7-9 and >10 for the simple MoS 2 . The XRD patterns of rGO and MoS 2 can be assigned to the plane hexagonal structure of MoS 2 and the d 002 plane of graphite. They stated that with the increase in the molybdenum disulfide concentration, the diffraction peaks of MoS 2 become higher in intensity, as expected, and the lattice spacing of the (002) plane of MoS 2 observed was 0.619. The group characterised the electronic structure of the composites by XPS analysis. The Mo 3d spectra showed dominant Mo 4+ and traces of Mo 6+ species corresponding to 2H-phase of MoS 2 . The S 2p XPS spectra were also assigned to the 2H-phase. They observed that with the decrease in MoS 2 content, the intensities of Mo 6+ -O and SO x species rose gradually due to the increased proportion of the strong interaction between rGO and MoS 2 [1].
Peng et al. [41] reported a synthesis of MoS 2 /rGO by hydrothermal method using (NH 4 ) 6 Mo 7 O 24 ·4H 2 O and NH 2 CSNH 2 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 MoS 2 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 MoS 2 that indicated a successful load onto the rGO substrate. They calculated the corresponding d 002 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 m 2 g −1 , 23.4 m 2 g −1 , and 16.1 m 2 g −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].

Electrical and Capacitive Properties for MoS 2 -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 MoS 2 nanospheres was approached by Wang et al., [72] who used MnCO 3 as a template to grow the nanospheres and removed them afterwards by acid washing. Using MnCO 3 , sodium molybdate and L-cysteine in a hydrothermal process, followed by acid washing to remove the template, they managed to obtain MoS 2 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 MoS 2 /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 MoS 2 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 MoS 2 , 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 MoS 2 -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 MoS 2 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 MoS 2 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 H 2 SO 4 . 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 MoS 2 sheets uniformly distributed on the rGO substrate.
Gao et al. [53] synthesized, by a hydrothermal method, carbon-anchored MoS 2 nanosheets using sodium molybdate, L-cysteine and β-cyclodextrine as precursors in the process at 200°C for 12 h. Among their samples, C/MoS 2 -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 MoS 2 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 MoS 2 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 MoS 2 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 MoS 2 /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 MoS 2 /rGO nanosheets by a hydrothermal route using SDS as a surfactant and MoO 3 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 H 2 SO 4 as electrolyte, with measurements being made in a three-electrode system.
Baig et al. [65] reported a composite material made of MoS 2 /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 MoS 2 /rGO. In their hydrothermal synthesis, they used sodium molybdate and thioacetamide as precursors for MoS 2 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 MoS 2 were obtained by Wei et al. [26] Their method was based on a one-pot hydrothermal reaction from which they obtained flower-like MoS 2 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.

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 MoS 2 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 MoS 2 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 MoS 2 , 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 MoS 2 , 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.