Trang 9 2 LIST OF TABLES Table 1.1 Carbon-based the electrode materials for supercapacitors 25 Table 1.2 Metal oxide-based the electrode materials for supercapacitors 25 Table1.3 Conduct
INTRODUCT ION
DOUBLE LAYERS MODEL
In 1853, Helmholtz introduced the concept of the electric double layer, which describes the electrostatic charge separation occurring at the interface between a solid electrode and electrolyte solutions when a voltage is applied This phenomenon prevents the reversible transfer of charges, such as electrons or ions, from the solid electrode to the electrolyte solutions The model resembles that of a conventional capacitor, allowing for the calculation of the Helmholtz capacitance of the double layer using a specific equation.
The Helmholtz model is particularly effective for high concentrations of electrolyte solutions, which is why it remains relevant in the practical application of supercapacitors that typically utilize concentrated ion solutions This effectiveness makes the model a preferred choice for simple calculations in the field.
The Helmholtz model of the electric double layer does not account for the diffusion effects of other ions in the electrolyte solution on the first absorbed layer Consequently, the capacitance results derived from this model do not fully align with practical observations.
Therefore, scientists continued to develop other models for enhancing the accuracy compared to the real electrochemical process.
Georges Gouy found that the capacitance of the electric double layer is influenced by the applied voltage and random thermal motion As a result, the concentration of reversible charge ions decreases as one moves further away from the electrode.
In 1913, Chapman utilized both Poisson's equation and the Boltzmann distribution to mathematically formulate the diffusion layer, leading to the development of a significant equation.
Here, o , z, n o , k and T is surface potential, valence of electrolytic ions, cation concentration at thermodynamics equilibrium, Boltzmann constant and the absolute temperature, respectively
This model outperforms the Helmholtz model; however, treating ions as point charges presents a challenge, as they can theoretically approach the solid electrode interface infinitely This scenario is impractical for experimental applications, highlighting the need for a more accurate model to address these limitations.
In 1923, Stern advanced the Helmholtz and Gouy-Chapman models by introducing a new framework that recognized the finite size of ions, allowing them to approach the electrode interface closely This innovative model divided the electric double layer into two distinct regions: the Stern layer, consisting of a uniform layer of ions adjacent to the electrode, and a secondary layer formed by the diffusion of ions from the electrolytic solution towards the electrode.
Stern’s theory is mathematically described in the following equation, which shows that capacitance of the solid electrode depends on two factors [14]:
(4) With Cc, Cd is capacitance of Helmholtz and diffuse layer, respectively
In 1947, Grahame enhanced Stern's model by incorporating specific ion adsorption to better align with experimental results This led to the development of the Stern-Chapmann model, which comprises three distinct regions: the Inner Helmholtz Plane (IHP), the Outer Helmholtz Plane (OHP), and the diffuse layer.
The electrical double-layer at a positively charged surface can be represented by three models: the Helmholtz model, the Gouy–Chapman model, and the Stern model These models illustrate the arrangement of charges and potentials, highlighting key features such as the inner Helmholtz plane (IHP) and outer Helmholtz plane (OHP) In this context, ψ0 refers to the potential at the electrode surface, while ψ denotes the potential at the electrode/electrolyte interface.
Bockris, Devanathan and Muller suggested the BDM model, which is the most accurate model applied for electric double layer (EDL) nowadays, which is an
The upgraded version of the Stern-Chapmann model incorporates solvent molecules, specifically water, suggesting that some of these molecules are absorbed within the inner Helmholtz plane of the electrode surface The dipole formation of water molecules is influenced by their proximity to the electrode, with their alignment being stabilized by interactions with the electrode's electronic charges Subsequent layers of water molecules align without fixed dipole orientations This model is illustrated in Figure 1.2.
Figure 1 2 A double layer model including layers of solvent [43]
BACKGROUND OF SUPERCAPACITOR
Supercapacitors, which also called ultracapacitors, is fundamentally electrochemical
Supercapacitors have garnered significant research interest over the past few decades due to their remarkable cycle life stability, higher power density for rapid charge and discharge compared to batteries, and superior energy density relative to conventional capacitors A basic electrochemical capacitor consists of two solid electrodes connected to current collectors, along with electrolytic solutions and a separator to prevent short circuits Detailed information about each component is provided in the following sections, while Figure 1.3 illustrates the fundamental structure of supercapacitors, highlighting both theoretical modeling, including ELD phenomena, and their practical appearance.
The principal setup of an Electric Double Layer Capacitor (EDLC) features porous carbon electrodes positioned on current collectors, separated by an ion-conducting electrolyte Additionally, the construction of a spirally wound EDLC is highlighted, showcasing an assembled device with a capacitance of 2600F housed alongside a flat 5F coin device.
It is very necessary to have a separator in an electrochemical capacitor in order to prevent a short circuit between the positive and negative electrode Therefore,
Separators in battery technology must possess high electrical resistance while allowing easy permeability for ions in electrolytic solutions during charge and discharge cycles This necessitates that separators have tailored ionic conductivity and sufficient porosity to facilitate ion movement.
Numerous studies have focused on identifying suitable materials for supercapacitor separators The choice of separator material is significantly influenced by the type of electrolytic solution used For organic electrolytes, polymer and paper separators demonstrate exceptional performance, whereas ceramic or glass fiber materials are more effective for aqueous electrolytic solutions.
The electrolyte between the electrodes is a critical component of supercapacitors, serving as a source of ions that facilitates reversible ion transfer in electric double-layer capacitors (EDLCs) and fast reversible redox reactions in pseudocapacitors Significant research efforts have been dedicated to developing electrolytes that are compatible with the active materials used in electrodes The classification of electrolytes utilized in supercapacitors is illustrated in Figure 1.4.
Electrolytes play a crucial role in determining the performance of supercapacitors, particularly in terms of energy density and power density, which are key metrics for evaluation The energy and power density values are significantly influenced by the operating potential; a larger voltage window results in higher energy and power densities However, the maximum operating potential is closely tied to the stability of the electrolyte used Additionally, power density is inversely related to the equivalent series resistance, which varies depending on the type of electrolyte employed.
The operational temperature range of supercapacitors is influenced by the type of electrolyte used This impact is illustrated in Figure 1.5.
Figure 1 5 Diagram of effects of range of working temperature on electrolyte in some ways [58]
Electrolytes play a vital role in the performance of supercapacitors, making their selection essential for optimal device operation.
1.2.3 Energy density and power density
Energy density and power density are two main factors not only supercapacitor but also all kind of energy storage systems/devices to evaluate their qualifications The
17 energy density is represented by a capability of energy storage, whereas power density shows performance in term of how fast they charge or discharge
When evaluating a supercapacitor, it is essential to consider capacitance, as it plays a vital role in determining energy and power density values Consequently, many studies on electrochemical capacitors emphasize presenting their findings in terms of capacitance values.
Based on the model presented, the total capacitance of a simple supercapacitor can be determined in the expression: n p
(5) Where, C p and C n are capacitance of positive and negative electrode, respectively
For simple calculation of capacitance of supercapacitors, the capacitance of each electrode can be treat as one electrostatic capacitor, rely on the Helmholtz model And therefore, d
To achieve optimal results, the capacitance of each electrode can be calculated using the advanced BDM method, which considers Helmholtz as both the Inner Helmholtz Plane (IHP) and the Outer Helmholtz Plane (OHP) This approach enhances the accuracy of the capacitance measurements.
Getting total capacitance of supercapacitors, the maximum values of energy density can be obtained by formula when being applied an external voltage U [33 36]: –
Q is total charges stored at the electrodes of ECs
This happens in case symmetric supercapacitors, C p =C n But when C p is different from C n (asymmetric supercapacitor) and supposes that C p < C n , the value of E is determined by:
The power density of the supercapacitors is given in a common way by the express:
And the maximum value of P is [34 36]: –
Here, RESR is equivalent in series resistance of the supercapacitors
Figure 1 6 Ragone plot shows a comparison of some main types of energy storage devices in term of power density and energy density [19, 37]
The Ragone plot (Figure 1.6) effectively illustrates the energy storage capabilities of different devices, highlighting the time required for charging and discharging processes While conventional capacitors offer high power density, they significantly lag behind batteries and fuel cells in terms of energy density.
Supercapacitors are classified into three main types based on their energy storage principles: electric double layer capacitors, pseudo-capacitors, and hybrid capacitors Understanding these charge storage mechanisms is crucial, and the following sections will provide a concise overview of each type: Electric Double Layer Capacitors, Pseudo-capacitors, and Hybrid Supercapacitors.
Figure 1 7 Classification of supercapacitor based on electrode materials [22]
Figure 1 8 A schematic diagram of EDLCs and description of potential change through interface of electrode/electrolytic solutions when applied an external voltage [52]
Electric Double Layer Capacitors (EDLCs) are electrochemical devices that store energy directly in the double-layer at the electrode/electrolyte interface, where charges accumulate without any faradaic reactions This design allows EDLCs to achieve higher energy uptake than conventional capacitors and superior power performance compared to batteries Additionally, EDLCs boast an impressive cycle life, capable of enduring millions of charge/discharge cycles, in stark contrast to the thousands of cycles typical of high-quality batteries A crucial aspect to note is the effective isolation between the electrolyte and the underlying mechanism.
The charge storage capacity in Li-ion batteries is influenced by the solid-electrolyte interphase, allowing for the use of various solvents in electrochemical capacitors to adapt to different operational environments.
It is a truth in term of the experiment that there is a vacuum with a length of several Angstroms separating the electrolytic ions layer and surface of electrode [73]
Unlike Electric Double Layer Capacitors (EDLCs), this type of supercapacitor utilizes faradaic reactions, or electrochemical reversible redox reactions, for energy storage When an external voltage is applied to the current collectors, rapid and highly reversible redox reactions occur on the surfaces of the electrodes This mechanism, akin to that in various battery types, involves the transfer of charges between the electrodes and the electrolytes The phenomena occurring at both the positive and negative electrodes can be illustrated as follows: [59].
//: surface of electrodes and electrolyte
e - : electrosorption valence (involve in the redox reactions at surface of the electrodes)
While E 1 , E 2 are notations for positive the positive and negative electrode, respectively
ELECTRODE MATER IALS
1.4.1 Principle, classification and recent development
First of all, it is necessary to make sense the general principle of developments of the electrode materials with the purpose of enhancing energy density capacity
The energy density of an electrode is directly proportional to its capacitance, as indicated in equation 8 Additionally, from equation 6, it can be inferred that capacitance is proportional to both surface area and absolute permittivity (which relates to conductivity), while being inversely proportional to the distance between ions and the electrode surface, according to the Helmholtz model.
The capacitance value of supercapacitors, whether based on electric double layer capacitors (EDLCs) or pseudo-capacitors, is significantly influenced by the surface area and conductivity of the electrode materials Extensive research has focused on discovering new electrode materials to enhance the energy capacity of supercapacitors However, studies have shown that the relationship between capacitance and the total surface area of these materials is not linear, as it is affected by the dimensions of ions in the electrolyte solutions To address this, the concept of "electrochemically accessible surface area" was introduced to better define the surface area that directly contributes to capacitance.
25 value Moreover, reducing preparation cost and enhancing friendliness with the environment are two factors more and more important nowadays
With respect to electrode materials developed so far, they could be divided into three main groups, as summarized in Table 1.1-1.3 [62 65] –
Table 1.1 Carbon-based the electrode materials for supercapacitors
Table 1.2 Metal oxide-based the electrode materials for supercapacitors
Table 1.3 Conductive polymers-based the electrode materials for supercapacitors
1.4.2 MoS 2 /rGO-based electrode materials of supercapacitor
The electrochemical properties of MoS 2 based materials have attracted numerous research interest for close to a decade up to now due to its exceptional characteristics
In 2013, Ke-Jing Huang et al [28] showed some very first results relating to the electrochemical properties of the layer MoS 2 /rGO composites for sensing purpose
The synthesis of graphene oxide (GO) as an intermediate substance for composite production was achieved through the modified Hummer method, incorporating potassium chlorate (KClO3) as a reactant, with a prolonged stirring time of up to 96 hours Subsequently, the composites were fabricated via the hydrothermal method.
The study utilized Na2MoO4·2H2O and l-cysteine as precursors, adjusting the solution's pH to 6.5 with NaOH before subjecting it to a hydrothermal process at 180°C for 36 hours The Electrochemical Impedance Spectroscopy (EIS) results demonstrated that the composites exhibited very low charge transfer resistance (Rct), indicating excellent electron conductivity Additionally, BET analysis revealed that the specific surface area of the composites significantly exceeded that of pure reduced graphene oxide (rGO), which contributed to a substantial enhancement in their electrochemical properties.
In 2013, researchers published a paper on MoS2/rGO composites synthesized via a modified l-cysteine-assisted solution-phase method using the hydrothermal technique The synthesis process involved using CH4N2S as the sulfur precursor, with the pH value adjusted to less than 1 for MoS2 using HCl 12M, and 6.5 for MoS2/rGO using NaOH 0.1M solution The resulting composites were then heated to 200°C for 24 hours for MoS2 and 180°C for 36 hours for MoS2/rGO The specific capacitance of the MoS2/rGO composite reached a maximum value of 243 F/g at a current density of 1 A/g in 1M Na2SO4 solution, significantly outperforming the individual MoS2 and rGO materials, which exhibited values of 120 F/g and 35 F/g, respectively.
The MoS2/rGO hybrid material demonstrated exceptional cycling stability, retaining 92.3% of its specific capacitance after 1000 cycles Notably, the 3D structure formed by integrating MoS2 sheets with reduced graphene oxide (rGO) provided a large surface area, facilitating efficient ion and charge transfer through the electrode.
In 2014, Firmiano et al pioneered a novel method for preparing supercapacitor electrodes by directly bonding 2D MoS2 on reduced graphene oxide (rGO) using microwave heating This approach led to the creation of a composite MoS2/rGO material with varying weight percentages of MoS2 (5.6%, 17.6%, and 44.5%) The resulting electrodes exhibited specific capacitances of 128, 265, and 148 F/g at 10mA/g, significantly outperforming rGO (40F/g) Notably, the electrode with low MoS2 concentration maintained 92% of its capacitance after 1000 cycles, while the medium concentration electrode retained 70% The authors attributed the storage charge to the nanostructure of the hybrid materials, highlighting the importance of double-layer capacitance and fast faradaic reactions in the electrochemical process.
A notable publication in 2014 highlighted the fabrication of a graphene-based electrode using a MoS2/rGO composite, which was synthesized in under 1 minute via a hybrid microwave annealing method This innovative approach significantly reduced synthesis time, although it still required the preparation of graphene oxide (GO) using the modified Hummer method as a precursor.
The synthesis process involved mixing 28 substances with graphene oxide (GO), thiourea, ethanol, and MoCl5, followed by strong stirring for 1 hour The mixture was then subjected to a drying oven to remove residual ethanol, before being irradiated in a 1000W household microwave oven for 45 seconds Notably, this method offers two key advantages: the simultaneous reduction of GO to graphene and crystallization of MoS2, and the absence of an additional loading step of MoS2 on graphene, streamlining the overall process.
The thermal treatment process of MoS2/rGO is remarkably simple, ultrafast, and energy-efficient due to its extremely short treatment time Furthermore, scaling up this process is relatively easy Notably, MoS2/rGO exhibits superior electrochemical properties, with a significantly higher capacitance value of 1246 F compared to pure MoS2 (56.64 μF) and commercial MoS2 (16.81 μF) Additionally, Electrochemical Impedance Spectroscopy (EIS) results reveal that MoS2/rGO has a lower charge transfer resistance (Rct) of 35 Ohm, outperforming pure MoS2 (1130 Ohm) and commercial MoS2 (3600 Ohm), thereby enhancing charge transfer capability.
Researchers led by Peng Chen published a notable paper in 2015, exploring the electrochemical properties of hybrid fibers composed of MoS2, reduced graphene oxide (rGO), and multi-wall carbon nanotubes (MWCNT) The team employed the chemical vapor deposition method to prepare MWCNT fibers, while single-layer MoS2 was fabricated via electrochemical intercalation Notably, the rGO was synthesized using the modified Hummer method, followed by a distinct reduction process By drop-casting a mixture of MoS2 and GO nanosheets onto MWCNT nanosheets, the researchers created samples that demonstrated a voltage window of -0.6V to 0.8V and a volumetric capacitance of 6.3 F/cm3, with remarkable stability after 7000 cycles in 1M H2SO4.
In 2015, a research study titled "Characterization of MoS2/Graphene Composites for High-Performance Coin Cell Supercapacitor" by Mark A Bisset et al demonstrated the potential of MoS2/graphene composites in enhancing supercapacitor performance Few-layer MoS2 was produced through solution phase exfoliation and combined with exfoliated graphene to fabricate composite-based electrodes The study found that all MoS2/graphene composites exhibited improved specific capacitance compared to pure samples, with the 1:3 composite ratio achieving the highest specific capacitance of 11 mF/cm2 at 5 mV/s in 1M electrolyte.
Na 2 SO 4 A very remarkable thing of the 1:3 composite made by this method was that specific capacitance strongly increased by 250% after 10000 cycles and figure for pure MoS2 membrane was super brilliant with an increase in 800% after 3000 cycles (still maintained after 7000 cycles) EIS results indicated that equivalent solution resistance (ESR) was the highest of the MoS2 membrane (1.6 Ohm) and figures for the 1:3 composites, graphene were 0.7 and 0.96 Ohm, respectively
A 2016 study by Nguyen Van Hoa and colleagues demonstrated the successful decoration of layer-structure 3D nanohybrid MoS2/rGO on nickel foam, resulting in a significant enhancement in supercapacitor electrode storage capacity The researchers employed a self-assembly mechanism to grow MoS2 on reduced graphene oxide (rGO), which was obtained by reducing graphene oxide (GO) with 0.1M N2H4 solution at 70°C for 24 hours Notably, the composite exhibited a specific capacitance of 575 F/g, outperforming bare MoS2 with a specific capacitance of 309 F/g at a current density of 20 A/g in 1M KOH solution Furthermore, the composite retained 94% of its initial capacitance after 2000 cycles, compared to 87% for bare MoS2, which was attributed to the increased equivalent series resistance (ERS) observed in the electrochemical impedance spectroscopy (EIS) results.
In 2016, Wei Xiao and colleagues introduced an innovative approach to create a MoS2/rGO composite in the form of hollow spheres through hydrothermal synthesis, aimed at enhancing supercapacitor electrodes Prior to this, commercial graphene oxide (GO) sheets were utilized in their research.
OBJECTIVE RESEARCH AND OUTLINE OF T HESIS
The primary goal of this study is to create a simple, cost-effective, and scalable synthesis method for producing MoS2/rGO nanocomposite materials intended for supercapacitor applications The research systematically investigates how various synthesis conditions influence the materials' characteristics and their electrochemical properties in Na2SO4 and KCl electrolyte solutions, aiming to achieve high-performance supercapacitors Additionally, the study employs the Electrochemical Impedance Spectroscopy (EIS) method to gain a deeper understanding of the electrochemical phenomena occurring on the surface of the electrode materials.
This master thesis book was organized in three main chapters
This article commences by introducing readers to the fundamental models that describe the phenomena occurring at the interface between electrolytic solutions and solid electrodes, providing essential background information on supercapacitors Additionally, it presents an overview of MoS2/rGO-based composite materials used in supercapacitor electrodes Recent studies on the application of MoS2/rGO nanocomposites in supercapacitors are also highlighted, alongside the objectives of this research, laying the groundwork for a comprehensive exploration of the subject.
The second chapter gave some information about experiments: chemical substrates as precursors, experimental procedure to prepare samples and methodologies using to characterize structural and electrochemical properties
The pivotal third component of this master thesis presented a comprehensive analysis of the obtained results throughout the research process, with a detailed discussion of the findings Grounded in objective research, two primary objectives were fulfilled: demonstrating the successful synthesis of samples through structural analysis via XRD and spectral analysis via Raman, and determining the electrochemical properties of the samples using CV and EIS methods.
Finally, this master thesis is ended by a conclusion and outlook in concise words
EXPERIMENTAL SECTION
MATERIALS, EQUIPMENT AND STEPS OF PREPARING MATERIALS
The experiments utilized high-purity chemical substances, including 99.99% graphite, 98% concentrated sulfuric acid, 99% potassium manganate, 36% hydrochloric acid, and hydroperoxide, as well as reagent-grade ammonium heptamolybdate tetrahydrate, 99.8% thiourea, and 98.9% hydroxylamine hydrochloride, all of which were used without further purification due to their high reagent-grade quality.
2.1.2 Preparation of materials powder for electrode fabrication
Detailed steps were used to synthesize GO (with 0.5 gram objective mass of product) as below:
1 Take 0.5 gram of graphite and 0.5 gram NaNO3 into a clean beaker
3 Stir the mixture in 10 minutes at 300 rpm
4 Added slowly 2.5 gram KMnO4 (2.5 gram/ 20 min), keep stirring in 45 min
(4 above steps were done with the beaker put into a ice-bath in order to keep temperature was less than 5 o C)
5 Change the beaker to another water-bath with temperature around 35 o C (error
5 o C), keep stirring at 200 rpm in 1 hour
6 Added gradually 40ml of DI H2O (stir at 200 rpm) and rise temperature up to
90 o C (error 5 o C) After that, added 50 ml DI H2O
7 Added slowly 3 ml H2O2 30% (color of the solution change from brown to yellow)
8 Until finish escaping of bubble, filter and clean the products:
8.2 Second, using DI water and centrifugal machine
9 dried the obtained product at 60 o C (after filtering and cleaning process) in a dried oven
(b) Preparation of rGO powder rGO was prepared from GO as steps shown below:
1 Dissolve mixture of 0.1 gram GO powder and 2gram ascorbic acid by 80ml of DI water into a clean beaker
2 Maintain the system at 60 o C for 24 hours
3 Dry obtained products at 60 o C in the dried oven
(c) Preparation of composite MoS 2 /rGO
MoS 2 /rGO nanocomposites with different compositions were synthesized by a hydrothermal method, following by the procedure shown in Figure 2.1 Figure 2.1 presents the steps of the hydrothermal method to prepare the MoS2/rGO composite First, 0.1 gram GO would be added to (NH 4 ) 6 Mo 7 O 24 4H 2 O, CH 4 N 2 S (resulting in a suitable content of MoS2 in each the MoS2/rGO composite 1:3, 1:1 and 3:1) The molar ratio between NH2OH.HCl and (NH4)6Mo7O24.4H2O was 14:1 The obtained mixture was dissolved into 50ml of DI water and then, stirred in 15 minutes To get a more homogeneous solution, the solution was put into an ultrasonic path (100 W) in 15 minutes The final solution was added 50 ml DI water and transferred into a steel autoclave and maintained at 200 o C in 24 hours
Figure 2 1 The synthetic procedure of the MoS2/rGO composite
To synthesize MoS2, the same procedure used for creating the MoS2/rGO composite was employed, but without the incorporation of graphene oxide (GO) Prior to utilizing the composite for electrochemical applications, it is crucial to establish the optimal synthetic conditions, which were determined through a series of experiments and analyses.
Effect of synthesis temperatures on specific capacitance of the electrode of supercapacitor was got by changing reaction temperature at 160 o C,
To understand reaction time dependence of the specific capacitance of the electrode of supercapacitor, 24h, 36h and 48h were utilized to synthesize MoS2 samples
PREPARATION OF ELECTRODE AND ELECTROLYTIC SOLUTIONS
The obtained powders from the hydrothermal process were combined with Polytetrafluoroethylene (PTFE) at a 9.5:1 mass ratio and thoroughly milled using a clean agate mortar for 20-30 minutes to achieve a uniform mixture The resulting mixture was then formed into circular electrode materials with a diameter of 3 millimeters Subsequently, each circular material was carefully placed onto a titanium net-electrode, weighed, and pressed to ensure secure adhesion.
In this research, electrolytic solutions of Na2SO4 and KCl were prepared at three distinct concentrations (0.5 M, 1 M, and 2 M) by dissolving measured weights of the solid electrolytes in deionized water The solutions were then homogenized using a magnetic bead and stir-magnetic machine over a period of 10-15 minutes, followed by sonication in a bath to achieve optimal uniformity.
METHODOLOGY OF STRUCTURAL CHARACTERIZATION AND
analysis a, Materials characterization by SEM, XRD and RAMAN
Firstly, the obtained products were investigated the morphological by using Scanning Electron Microscopy (JEOL7600, AIST institute, HUST; FESEM images got at Physics department, VNU of Science)
Hence, the powder was taken to investigate the crystal structure by using XRD (Chemicals department, VNU of Science) and Raman spectroscopy (Physical engineering department, HUST) methods b, Cyclic Voltammetry
The electrochemical characteristics of the fabricated materials were analyzed using a three-electrode setup, depicted in Figure 2.2 Measurements of the samples' electrochemical properties were conducted using the Keithley 2460-EC, a device from the iSensors group, ITIMS, HUST.
Figure 2 2 (A) Schemetic of the CV (three electrodes system) configuration, (B) real image of (A)-system c, Electrochemical Impedance Spectroscopy
To deeply understand about electrochemical processes happened at the surface of the electrode, EIS results were exhibited by using the Vertex Ivium instrument at Physics department, HUST
RESULTS AND DISCUSSION
MOS 2 RESULTS
The morphology and quality of MoS2 significantly impact the electrochemical properties of MoS2/rGO nanocomposites, making it crucial to optimize its synthesis The synthesis conditions of MoS2 play a pivotal role in determining its morphology and quality By focusing on optimizing the synthesis of MoS2, the optimal conditions can be established and subsequently applied to prepare high-performance MoS2/rGO nanocomposites, ultimately enhancing their electrochemical properties.
Figure 3 1 X-ray diffraction patterns of MoS2 synthesized by hydrothermal method at different temperature 160, 180 and 200 o C
Figure 3.1 displays the XRD patterns of MoS2 synthesized via the hydrothermal method at temperatures of 160, 180, and 200°C, with the primary peak at 2θ ≈ 2.7° and the secondary peak at 2θ ≈ 3.7° The primary peak at 2θ ≈ 2.7° corresponds to a d-spacing of approximately 0.64 nm, indicating that MoS2 samples grow well along the c-axis during the hydrothermal process Notably, this peak is slightly shifted to the low-angle region compared to the reference pattern in JCPDS no 37-1492, suggesting possible intercalation.
The hydrothermal process successfully synthesized MoS2 materials at various temperatures, with 39% of ions like hydroxyl present Notably, each pair of peaks in the MoS2 samples can be indexed to the (002) and (110) crystal planes of the hexagonal phase MoS2 (JCPDS no 37-1492), confirming the successful synthesis of MoS2 materials through a facile hydrothermal method.
Figure 3 2 Specific capacitance value of MoS2 prepared by hydrothermal processes in 24h depends on temperature treatments
The hydrothermally synthesized samples, prepared at temperatures of 160, 180, and 200°C, were confirmed to be MoS2 through characterization These samples were then utilized to fabricate electrodes, which were subsequently investigated for their electrochemical properties using cyclic voltammetry (CV) measurements in a 1M electrolyte solution.
The specific capacitance values of the electrodes were calculated based on the CV results in a Na2SO4 solution at a scan rate of 5mV/s The results, as shown in Figure 3.2, reveal the specific capacitance values of MoS2 prepared at 24h, which vary depending on the temperature treatment in the hydrothermal processes Notably, the specific capacitance value of the MoS2-based electrode prepared at different temperatures exhibits distinct differences.
The highest capacitance of 35 F/g was achieved with the MoS2-based electrode prepared at 200°C in 24 hours In contrast, the electrode prepared at 160°C in 24 hours showed a significantly lower capacitance of approximately 20 F/g The lowest capacitance was recorded for the MoS2-based electrode prepared at 180°C in 24 hours, with a value just over 15 F/g.
Figure 3.3 exhibits Raman spectra of MoS 2 samples synthesized by hydrothermal at different reaction-time of 24 h, 36 h and 48 h Active Raman modes located at 378,
The synthesized MoS2 samples exhibited characteristic peaks at 377 and 379 cm-1, corresponding to the E1 2g mode, which involves in-plane vibrations of two S atoms and a Mo atom The A1g vibrational mode, attributed to out-of-plane S atom vibrations, was observed at 404-406 cm-1 for the 24-48 hour samples An additional peak at 454 cm-1 was assigned to double longitudinal acoustic phonons 2LA(M) These peaks are consistent with the hexagonal crystal structure of MoS2, confirming the successful synthesis of MoS2 samples via hydrothermal methods at reaction times of 24, 36, and 48 hours.
The Raman shift spectrum of MoS2 synthesized through the hydrothermal method at varying reaction times of 24 hours, 36 hours, and 48 hours confirms the successful preparation of three distinct samples.
The fabrication of electrodes was followed by an investigation into their electrochemical properties, which was conducted using cyclic voltammetry (CV) measurements in a 1M Na2SO4 solution at a scan rate of 5mV/s Subsequent analysis of the CV results enabled the determination of specific capacitance values for all MoS2-based electrodes The findings obtained from this analysis are presented in Figure 3.4, providing a visual representation of the results.
Figure 3.4 presented the results of specific capacitance values of MoS2-based electrodes prepared by hydrothermal method at 200 o C depending on reaction times
The specific capacitance of MoS2-based electrodes fabricated at 200°C exhibited varying results based on fabrication time Notably, the electrode fabricated at 200°C for 24 hours achieved the highest specific capacitance value, exceeding 35 F/g In contrast, extending the fabrication time to 36 hours resulted in a lower specific capacitance of approximately 33 F/g, while the lowest value was recorded for the electrode fabricated at 200°C for 48 hours, with a specific capacitance of over 28 F/g.
Figure 3 4 Reaction time (24 h, 36 h and 48 h) dependence of specific capacitance (C m ) of MoS 2 -based electrodes prepared by hydrothermal method at 200 o C
After investigating the effect of temperature and reaction time on the specific capacitance of MoS2-based electrodes, I decided to select a procedure of
42 hydrothermal method, which led to the highest value of specific capacitance (at
200 o C and 24 hours reaction time) to synthesize the composite MoS 2 /rGO
This section presents the findings on the impact of varying mass ratios of MoS2 on reduced graphene oxide (rGO), focusing on its morphological, structural, and electrochemical properties A series of samples, including rGO, MoS2/rGO 1:1, MoS2/rGO 1:3, MoS2/rGO 3:1, and MoS2, were synthesized to investigate the effects of different mass ratios on the properties of rGO-based composites.
COMPOSITE MOS 2 /RGO RESULTS
3.2.1 Crystal structure, morphological properties and CV results
Figure 3 5 man spectra of: Ra a) Graphite; b) Graphite oxide (GO)
Figure 3.5 illustrates the Raman spectra of graphite and graphene oxide (GO) synthesized via a modified Hummer method The D band at 1330 cm⁻¹ in the spectrum of graphite indicates defects in carbon materials, while the G band at 1580 cm⁻¹ corresponds to the sp² hybridized C=C bond's vibrational mode In graphite, the D band peak is lower than the G band peak, whereas in GO, the D band peak is elevated and broadened The intensity ratio of D to G (I_D/I_G) for graphite is 0.33, compared to approximately 1.28 for GO, suggesting an increase in disorder and defects in the GO structure.
43 the appearance of functional groups in GO after the oxidation process of graphite, indicating that GO is prepared successfully by the modified Hummer method
C ur re nt d en si ty (A /g )
Figure 3 6 a) X-ray diagram of rGO; b) SEM image of rGO; CV curve of rGO-c) based electrode
X-ray diffraction (XRD) is a valuable technique for characterizing reduced graphene oxide (rGO) as it provides crucial information about interlayer spacing and corresponding diffraction angles The XRD pattern of the rGO sample, as shown in Figure 3.6a, exhibits two intense peaks at 24.5 and 43 degrees, which are characteristic of the (002) and (100) planes, respectively, and are indicative of rGO.
The successful preparation of reduced graphene oxide (rGO) is further confirmed Scanning electron microscopy (SEM) analysis, as shown in Figure 3.6b, reveals the hierarchical surface morphology of rGO, comprising large sheets or flakes, which is beneficial for conductivity but may limit specific capacitance The electrochemical properties of the rGO-based electrode are evaluated through cyclic voltammetry (CV) measurements, providing valuable insights into its performance.
figure 3.6c In the range of voltage window tested, CV curves were quasi-rectangles shapes indicating a good capacitive storage relied on the EDLC mechanism
C ur re nt d en si ty (A /g )
Figure 3 7 a) X-ray diagram of the nanocomposites MoS2/rGO 1:3; b) SEM image of the nanocomposites MoS 2 /rGO 1:3; c) CV curve of the nanocomposites
The primary peak of MoS2 at 14.2° undergoes a shift to 12° due to the intercalation of ions such as OH-, NH4+, or Cl- between sulfur atom layers during the synthetic process Notably, the 0.11nm shift in distance corresponds to the presence of OH- ions Furthermore, the peaks at 24.2° and 43° are well-indexed to (002) and (102) planes, characteristic of reduced graphene oxide (rGO), confirming the successful synthesis of the MoS2/rGO composite The scanning electron microscopy (SEM) image of the MoS2/rGO (1:3) sample reveals a morphology comprising crumpled and rippled sheets or flakes, primarily consisting of rGO, which may contribute to its high performance.
The MoS2/rGO 1:3 based electrode exhibits excellent electrochemical properties, as demonstrated by the CV curve in Figure 3.7c, which showcases a quasi-rectangle shape indicative of good capacitive storage This unique characteristic is attributed to the electrode's ability to maintain continuity of charges conductivity within a specific area, resulting in enhanced storage capacity Furthermore, the observed CV curve is consistent with the Electric Double-Layer Capacitor (EDLC) mechanism, underscoring the electrode's potential for efficient energy storage.
Cu rr en t d en si ty (A /g )
Figure 3 8 a) Raman spectrum of the nanocomposites MoS2/rGO 1:1, b) SEM image of the nanocomposites MoS2/rGO 1:1, c) CV curve of the nanocomposites
The Raman spectrum of the MoS2/rGO (1:1) composite, as shown in Figure 3.8a, exhibits characteristic peaks at 376 and 404 cm-1, corresponding to the in-plane vibration of S and Mo atoms and the out-of-plane vibration of the S atom, respectively, which are indicative of phase MoS2 Notably, the ID/IG ratio of the composite, which is used to evaluate the disorder of rGO and GO, is 1.47, exceeding that of GO (1.28), suggesting the formation of defects during the reduction process.
The MoS2/rGO composite exhibits a unique morphology, as seen in Figure 3.8b, with a pre-flower-like formation consisting of several nanopetals on a ripple-flaked rGO background, potentially leading to a high surface area and enhanced charge storage capacity Notably, the CV curves of the MoS2/rGO-based electrode, comprising 50 wt.% MoS2, display a rectangular shape, indicative of good capacitive storage and EDLC mechanism, when tested in a 1M Na2SO4 solution at a 5mV/s scanning rate and a voltage window of -1V to -0.5V.
The Raman spectrum of the MoS2/rGO (3:1) composite reveals characteristic peaks at 230 and 336 cm-1, indicating the presence of the 1T phase of MoS2, while a peak at 280 cm-1 is assigned to E1g, characteristic of 2H-MoS2 Additionally, peaks at 814 and 993 cm-1 are attributed to Mo=O vibration in the MoO3 phase, which may have formed due to oxidation of the MoS2 sample Scanning electron microscopy (SEM) analysis of the MoS2/rGO 3:1 sample reveals a flower-like morphology with an average diameter of 250nm, comprising leaf-structure sheets with a thickness of several nanometers, resulting in a porous structure with promising storage capacity.
The Na2SO4 electrolyte solution, tested at a scan rate of 5 mV/s and a voltage window of -1V to -0.5V, exhibited a rectangular shape in its cyclic voltammetry profile, indicating a strong energy storage capacity of the electrode based on the electric double-layer capacitance (EDLC) mechanism.
Figure 3 9 a) Raman spectrum of the MoS2/rGO 3:1 composite, b) SEM image of the MoS2/rGO 3:1 composite c) CV curve of the MoS2/rGO 3:1 composite - based electrode
The Raman spectrum of the MoS2 sample, prepared via the hydrothermal method, reveals two characteristic peaks at 378 cm⁻¹ and 405 cm⁻¹, corresponding to the in-plane E₁₂g and out-of-plane A₁g vibrational modes of hexagonal MoS2, confirming the successful synthesis of the material Additionally, the presence of the 2LA(M) peak indicates the asymmetric translation of Mo and S atoms along the c-axis Scanning Electron Microscopy (SEM) analysis shows that the MoS2 sample has a sheet-like morphology with a thickness of approximately 20 nm, arranged in a petal-like structure that enhances its porosity This unique morphology is promising for achieving high specific capacitance, which is significant for storage capacity applications.
The bare MoS2-based electrode was tested in a 1M Na2SO4 electrolyte solution at a scan rate of 5 mV/s, within a voltage window of -1V to -0.5V The resulting 48 curves exhibited a rectangular shape, indicating a strong storage capacity of the electrode, which is attributed to the electric double-layer capacitance (EDLC) mechanism.
Figure 3 10 a) Raman spectrum of MoS2, b) SEM image of MoS 2 , c) CV curves of
In this study, various composites of MoS2 and rGO were synthesized in different mass ratios (0:1, 1:3, 1:1, 3:1, 1:0) to fabricate electrodes The electrochemical properties were evaluated using cyclic voltammetry (CV) in a 1M Na2SO4 solution at a scan rate of 100 mV/s, which helped identify the optimal voltage window for electric double layer capacitors (EDLCs) The findings revealed that the ideal voltage window ranged from -1V to -0.5V.
After that, the electrodes were tested for specific capacitance in 1M Na2SO4 at different MoS 2 concentrations and with the voltage window as determined above
The results of the experiment are presented in Figure 3.11, which displays the CV curves with rectangular shapes, indicating excellent characteristics for Electric Double-Layer Capacitors (EDLCs) Notably, the areas under these curves demonstrate ideal features for EDLCs Furthermore, the specific capacitance values of individual electrodes were calculated from the CV curves and are illustrated in the subsequent figure, providing a comprehensive analysis of the electrode performance.
Figure 3 11 CV curves comparison of the composites MoS 2 /rGO with different contents of MoS 2
Figure 3 12 Specific capacitance values of the composites MoS2/rGO-based electrodes with different contents of MoS2
The specific capacitance of MoS2/rGO-based electrodes with varying MoS2 content (0, 25, 50, 75, and 100 wt.%) is presented in Figure 3.12 Notably, the energy capacity of the electrodes is significantly influenced by the change in MoS2 concentrations in the composites The optimal mass ratio of 1:3 yielded the highest specific capacitance value of approximately 96 F/g, while the bare MoS2-based electrode exhibited the lowest value of about 15 F/g In comparison, the specific capacitance values for the 1:1, 3:1, and rGO-based electrodes were 55 F/g, 29 F/g, and over 25 F/g, respectively.
Figure 3 13 CV curves of the composites MoS2/rGO 1:3 at different scan rates
The electrochemical properties of MoS2/rGO composites were investigated, with a focus on the 1:3 wt.% composition, to gain a deeper understanding of their behavior Notably, the cyclic voltammetry (CV) curves of MoS2/rGO composites, comprising 25 wt.% of MoS2, were analyzed at various scan rates, including 5, 10, 20, 50, and 100 mV/s, as depicted in Figure 3.13.