Synthesis and Characterization of MnWO₄-CNT for Supercapacitor Applications

Abstract

This study reveals, for the first time, the excellent capability of MnWO₄-CNT as a supercapacitor electrode compared to MnWO₄. In previous research conducted on this compound, RGO was used to enhance its electrochemical properties. The objective of this study is to investigate the effects of CNT on the electrochemical properties of the compound, which also yielded promising results. The physical and morphological analysis of MnWO₄ and MnWO₄-CNT was conducted using Raman, XRD, BET, and SEM-EDX techniques. The electrochemical performance of the samples was assessed through cyclic voltammetry (CV), impedance electrochemical spectroscopy (EIS), and galvanostatic charge–discharge (GCD). Notably, MnWO₄-CNT exhibited a significant specific capacitance of 1849.14 F·g⁻¹ at a scan rate of 10 mV·s⁻¹. The stability evaluation of the samples demonstrated a high capacitance retention of 81.2% and 89.4% for MnWO₄ and MnWO₄-CNT, respectively. The substantial specific capacity, along with the favorable stability of MnWO₄-CNT, positions it as a highly promising material for utilization in supercapacitor electrodes.

1. Introduction

With the depletion of energy resources and growing concerns over environmental pollution caused by fossil fuel usage, high-power energy production and storage systems have garnered significant attention in recent years as potential alternatives [1,2,3].

Although the advancement of technology and transition to modern life has brought relative prosperity to mankind, on the other hand, it has also caused an increasing number of pollution. Pollution such as environmental pollution, water pollution, soil pollution, air pollution, noise pollution, etc. seriously threatens the health of communities. This has caused all countries to think of a way to reduce this pollution in the form of planned projects that often investigate, predict, and estimate the level of pollution in the short and long term in the countries, as well as evaluate the level of pollution. On the other hand, in their macro policies, they provide engineered and effective solutions and preventive laws to reduce the pollution process. For example, in many developing countries in recent years, large industries, which are the main consumers of energy and the producers of pollution caused by fossil fuels, have been obliged to reduce a percentage of their energy consumption by building power plants and providing clean electricity from renewable fuel sources.

Investigating the sources of contamination and correcting these sources is also very important. With the development of mining industries and urban life, we are witnessing a significant increase in environmental pollution resulting from the entry of gases and toxic substances into the air and soil. Most of this pollution comes from mines, factories, and urban traffic. As mentioned, one of the most important sources of pollutants in the world is the extraction and excessive use of fossil fuel resources to provide energy to society, which is highly dependent on electricity and energy, and it is impossible to imagine a world without electricity. Policy, management, and sustainability in the field of energy are among the most important and necessary issues that should be taken into consideration by all countries. The increase in greenhouse gases from fossil fuels threatens the health of many people and imposes a huge cost on governments. For pollution prevention and control resulting from fossil fuels, a solution has been proposed at the world level: the use of renewable fuel sources for energy supply. This solution is welcomed worldwide because renewable fuel resources are available almost all over the world, and various factors, such as political sanctions, war, etc., do not affect the price and sanctions of these resources. However, the high cost of exploration, extraction, transportation, and transfer of fossil fuels should not be ignored.

Although compared to fossil fuel sources, renewable sources have advantages, applying and producing energy from these fuel sources requires modern and in some cases expensive equipment that is not easily available to countries. Many studies, research projects, and grants in modern and developing countries have been devoted to the technology of manufacturing energy production equipment from renewable sources, and almost every day, we see the introduction of new and efficient equipment in this field. The emergence of new technology and energy consumption management, as well as culture building in countries for correct energy consumption, will undoubtedly have an impact on energy sustainability.

According to the above, finding suitable and alternative sources for fossil fuels is always one of the concerns of researchers and governments to overcome the energy crisis. Alternatives are always available to everyone regardless of political sanctions and the economic situation of the countries. Undoubtedly, the principal and attractive option in the way of energy supply is the use of renewable energy sources. Sunlight, wind, geothermal energy, and solar energy are among the most common sources of energy in the world. Although these fuel sources are available almost all over the world depending on the geographical location of the countries, the most critical challenge in the way of using renewable fuel sources is the energy conversion equipment.

Among the most important and famous of this equipment, we can mention solar cells, wind turbines, etc. The use of this equipment, in addition to the high cost, also requires a lot of space for installation, which sometimes causes problems. Today’s human inclination to use portable and small equipment has caused attention to modern energy production and storage equipment such as fuel cells, electrochemical batteries such as aqueous ammonium-ion batteries, Li-ion batteries, and Zn-air batteries, and types of supercapacitors [4,5,6]. Of these, supercapacitors hold particular promise as the next generation of energy devices due to their high specific power, rapid charging and discharging capabilities, long cycle life, and low maintenance costs [7,8,9,10].

Valuable and in-depth scientific studies in the field of applying energy storage equipment and energy production have brought together different sciences. Every day, we see the introduction of new and efficient materials and catalysts in this field.

Supercapacitors utilize an electrical charge stored on the surface of an electrode, making them a form of physical energy storage. This charge storage can be achieved through either the formation of an electrical double layer or via Faradaic reactions. The first category, known as electric double-layer capacitors (EDLCs), employs carbon-based materials such as activated carbon (AC), carbon nanotubes (CNT), and graphene as active electrode materials. In EDLCs, the capacitance of the supercapacitor relies solely on the surface area of the electrode used for energy storage. As a result, EDLCs exhibit high levels of output power since the release of energy from the surface is facilitated in this configuration [11]. Furthermore, EDLCs boast a long cycle life.

On the other hand, pseudocapacitors utilize conductive polymers [12] and metal oxides like NiO, RuO₄, and MnO₂ as electrode materials. While materials like NiO, RuO₄, and MnO₂ have higher specific capacitance compared to carbon-based materials, they exhibit lower conductivity and have a shorter cycle life. To enhance the properties of these materials, binary metal oxides can be utilized in combination with single metal oxides. Reports suggest that the performance of these materials is improved when binary metal oxides are used, thanks to their high electrical conductivity and large electrochemically active surface [13].

On the other hand, carbon-based materials have been extensively studied due to their chemical stability, high electrical conductivity, adequate thermal resistance, large specific surface area, and the ability to create highly porous electrode materials [14]. Among these materials, carbon nanotubes (CNTs) offer significant benefits as electrode materials in supercapacitors. They possess the aforementioned electrical properties and also exhibit high rigidity. In some studies, pure CNT electrodes have been examined, demonstrating a specific capacitance of approximately 100 F·g⁻¹. These electrodes combine the desirable characteristics of both EDLCs and pseudocapacitors. Additionally, when combined with metal oxides, the porous structure of CNTs facilitates the transition of ions from the electrolyte to the surface of the composite, resulting in reduced resistance [15,16,17]. Binary tungsten oxides represent a suitable and efficient choice for electrode materials due to their superior conductivity compared to single metal oxides. Manganese oxides and tungstate are commonly employed in numerous research efforts due to their low toxicity, wide availability, environmental friendliness, and high theoretical capacitance [18,19]. Previous reports have indicated that combining manganese oxide with a carbon-based material can optimize the electrochemical properties of both components. Recently, there has been significant attention directed toward two-dimensional electrode materials due to their expansive active surface area [20,21,22]. However, upon further investigation, it became apparent that the electrochemical behavior of MnWO₄-CNT has received relatively less study and its supercapacitor properties remain unexplored. In a previous effort by J. Tang et al., MnWO₄/RGO was studied and the results were pleasing [23]. Also, in another research, F. Li et al. investigated MnWO₄ as a binary compound [24]. In this study, we aim to investigate the electrochemical properties and application of a binary combination of MnWO₄ with CNT in supercapacitors. The previous studies conducted on this specific combination are limited, making it a relatively new area of research. However, considering the individual properties of carbon nanotubes and MnWO₄, we anticipate promising results from their combined utilization. The hydrothermal method is a highly efficient and practical approach for synthesizing nanoparticles. Its simplicity and cost-effectiveness further contribute to its significant advantages. This method can be utilized to produce a wide range of compounds including hydroxides, phosphates, sulfides, and nitrides. The hydrothermal synthesis process involves the formation and growth of crystals through chemical reactions and alterations in compound solubility under suitable temperature and pressure conditions.

In this study, we employed the hydrothermal method to synthesize MnWO₄ and MnWO₄-CNT on a nickel foam substrate. Subsequently, we conducted various electrochemical investigations such as galvanostatic charge–discharge (GCD), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). Furthermore, we examined the stability of the synthesized electrodes.

2. Materials and Methods

All the materials used were Merck Company products with 99% purity. The X-ray diffraction (XRD) analysis was conducted by the XRD Xpert Pro Panalytical device (Malvern PANalytical Ltd., Almelo, The Netherlands). The scanning electron microscope MIRA3 (Tescan, as., Brno, Czech Republic) equipment was used for SEM imaging. The Raman spectroscopy was conducted by confocal microscopy and a Raman spectroscopy device (Takram P50C0R10, Teksan Co., Tehran, Iran). All the electrochemical analyses were carried out by Zive SP1 galvanostat/potentiostat (WonATech Co., Ltd., Seoul, Republic of Korea) in a three-electrode system. MnCl₂.tetrahydrate (0.396 g) and NaWO₄ (0.588 g) were added to distilled water (50 mL and 20 mL, respectively) and the mixtures were stirred. Ascorbic acid was then added to the mix and the solution was stirred for 10 min. Nickel foam was rinsed with distilled water and ethanol, then it was sonicated and finally placed inside an autoclave together with the mentioned solution. The synthesis was then carried out for 10 h at 110 °C. After this process, the nickel foam was rinsed and dried at 60 °C. To synthesize the MnWO_4–CNT composite, the same process was repeated but the nanotubes (1:10 weight ratio with ascorbic acid) were added to the solution and the mix was placed inside an autoclave together with the nickel foam. The synthesis took place under the same conditions as the previous process. Scheme 1 shows the experimental process of MnWO₄ nanocomposite synthesis.

3. Results

3.1. Physical Characterization

X-ray diffraction patterns for MnWO₄ and MnWO₄-CNT are visible in Figure 1. The main peaks at different diffraction angles are signified in the figure. These are representations of Miller indices (010), (100), (110), (−111), (020), (120), (−102), (121), (−211), (022), (220), (031), (−131), (212), (−113), (−132) and (−232). The depicted peaks hint at a monoclinic structure for MnWO₄; this is in agreement with data from JCPDS, No.13-0434 [25]. In the figure dedicated to MnWO₄-CNT, a peak marked with * is visible at 2θ = 29.2° which is an indication of CNT’s presence in the composite’s structure MnWO₄-CNT. By comparing the diffraction patterns drawn in Figure 1, it is visible that the addition of CNT decreases the intensity of the peaks. It can also be concluded from the figure that no meaningful changes have taken place in the phase of the samples. The grain size of the samples can also be calculated using the X-ray graph; Debye–Scherrer is used to calculate the size of the crystallites [26]:

$$\mathrm{D}=\left(\mathrm{K}\mathsf{\lambda }\right)/\left(\mathsf{\beta }\mathrm{C}\mathrm{o}\mathrm{s}\mathsf{\theta }\right)$$

where λ is the wavelength, θ is the angle of the crystal plane, and β is the full width at half maximum (FWHM). Using this equation, the mean crystallite size for MnWO₄ and MnWO₄-CNT was calculated; the values were equal to 47.83 and 37.5 nm, respectively. This points to the decreased grain size in MnWO₄-CNT.

Figure 2a depicts a Raman spectrum of MnWO₄-CNT. Raman analysis is a great method for determining the structure of carbon-based material. This analysis reveals whether carbon is present in the structure; XRD is not capable of such evaluations. As it can be observed in Figure 2a that is related to MnWO₄-CNT, three peaks are formed which are associated with carbon nanotubes. The peak formed at approximately 1584 cm⁻¹ is the G band and it is a result of first-order scattering in the E_2g phonons of graphite structure [27]. The second peak, which is visible at about 2697 cm⁻¹, is the G’ band related to the second-order diffraction of phonons (the second-order two phonon process), and a third weaker peak around 1364 cm⁻¹ indicates the D band, which is associated with defects in the structure, or, in other words, it is related to phonons with an A_1g symmetry near the K point. The existence of these peaks is strong evidence of CNT’s presence in the substance. The Raman spectrum of MnWO₄ displayed in Figure 2b depicts rises around 182, 330, 912 and 960, which are characteristic peaks of MnWO₄; the same pattern is also visible in the Raman spectrum of MnWO₄-CNT, but as it is visible, the peaks have shifted and an alteration in intensity can be noticed, which is due to CNT’s presence in the structure. According to Figure 2a, the value of I_D/I_G is equal to 0.16.

The EDX spectrum of MnWO₄-CNT is presented in Figure 3. We used EDX elemental analysis and mapping to determine the existing elements in the MnWO₄-CNT. The presented chart confirms the presence of Mn, W, O, and C elements in the sample. Also, in this figure, which presents the mapping analysis of the sample, the homogeneous distribution of the elements in the hybrid is visible.

Figure 4 shows the morphology of MnWO₄ and MnWO₄-CNT at three different magnifications of 200 nm, 500 nm, and 1 μm. As demonstrated in Figure 4a–c, nanotubes of MnWO₄-CNT are dispersed within the sample structure, and MnWO₄ particles are located between nanotubes individually. Figure 4d–f depicts larger grains of MnWO₄ and reveals that adding nanotubes to the sample structure causes the grains to reduce in size and a structure of greater porosity. Therefore, the MnWO₄-CNT composite may present greater supercapacitive potential and performance than the MnWO₄ compound.

The specific surface area of the electrode materials (MnWO₄ and MnWO₄-CNT) is one of the most important parameters that must be considered for energy storage for use in supercapacitor devices. The higher the specific surface area and porosity in the structure of the electrode materials, the more the electrode and the electrolyte will be in contact with each other, and naturally, the penetration of the electrolyte into the depth of the electrode will be facilitated. The BET method is applied to examine samples’ specific surface area and porosity based on nitrogen adsorption–desorption isotherms at the nitrogen boiling temperature of 77° K. Figure 5 shows the adsorption–desorption isotherms of samples MnWO₄ and MnWO₄-CNT, a linear curve that determines the specific surface area of samples [28]. According to AUPAC isotherms, MnWO₄ and MnWO₄-CNT showed the IV type isotherms, indicating that samples have a mesoporous structure. Examining isotherms of MnWO₄-CNT and MnWO₄, adding CNT to MnWO₄ increases the specific surface area, causing the electrode to be in contact with more electrolyte ions and improving the supercapacitive properties. The specific surface area for MnWO₄ and MnWO₄-CNT is 12.031 and 21.067 m²·g⁻¹, respectively, indicating this parameter augmentation by adding CNT to pure MnWO₄.

3.2. Electrochemical Characteristics

The cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) analyses are conducted for both materials MnWO₄ and MnWO₄-CNT in an electrolyte containing 3 M KOH. Figure 6 presents the CV curves of both materials MnWO₄ and MnWO₄-CNT. Evaluations are performed at different scan rates of 10, 30, 50, 70, and 100 mV·s⁻¹. In the curve of MnWO₄ at a scan rate of 10 mv.s⁻¹, redox peaks appear in the potential range of 0.1–0.4 V; however, the enclosed area of this curve is lower than in higher scan rates, indicating the low capacity of this material at this scan rate. Redox peaks are formed in a wider potential range by increasing the scan rate, and their magnitude reaches 50 mV·s⁻¹. In higher scan rates, like 70 and 100 mV·s⁻¹, for MnWO₄, the oxidation peak is seen at high potentials, and as the scan rate increases, the distance between the two oxidation and reduction peaks increases. Figure 6a indicates the CV curve of MnWO₄-CNT, where redox peaks formed at all scan rates are in the range of 0.2–0.4 V. At higher scan rates, an oxidation peak is observed, showing the adsorption of electrolyte ions on the catalyst surface even at higher rates [29]. As shown in Figure 6, reduction peaks of MnWO₄-CNT become smother than those of MnWO₄ because of the CNT involved in the material structure. It can be said that the material of EDLC exhibits improved properties. Based on Figure 6, the redox peaks of MnWO₄-CNT are more symmetrical compared to those of MnWO₄, indicating the better supercapacitive properties of MnWO₄-CNT. The electrode capacitance of the supercapacitor is calculated from CV curves using the following equation [30]:

$$C=\raisebox{1ex}{$\int i dV$}\!\left/ \!\raisebox{-1ex}{$\nu m \Delta V$}\right.$$

where i is the current, V is the potential, ν is the scan rate, m is the mass of the electrode’s active material, and $\Delta V$ is the potential window.

In the CV curves of Figure 6, the potential window is between 0 and 0.5 V. The capacitances of MnWO₄ electrodes calculated from CV curves are 640, 493, 380, 300, and 218 F·g⁻¹ at scan rates of 10, 30, 50, 70, and 100 mV·s⁻¹, respectively. By adding CNT to MnWO₄ and tube location across the material structure, the storage space increases, expanding the active surface area. As evident from the greater surface area under the curve of Figure 6a compared to that of Figure 6b, the electrode capacitance of MnWO₄ significantly increases by adding CNT. MnWO₄-CNT exhibits capacitances of 1300, 1146.6, 1008.8, and 758 F·g⁻¹ at scan rates of 10, 30, 50, 70, and 100 mV·s⁻¹, respectively.

Figure 7 illustrates the specific capacitance curve at different scan rates, where the specific capacitance decreases by increasing the scan rate.

Figure 8 demonstrates the stability of electrodes MnWO₄ and MnWO₄-CNT after 5000 cycles at the scan rate of 100 mV·s⁻¹. As can be seen, MnWO₄ and MnWO₄-CNT have 81.2 and 89.4% stability, respectively. It can be said that MnWO₄-CNT electrodes show better stability and capacitance after 5000 cycles.

The GCD analysis is performed to measure the capacitance of both materials MnWO₄ and MnWO₄-CNT. Figure 9 shows these measurements figuratively. These measurements are conducted in the potential range of 0–0.5 V and at current densities of 1, 1.5, 2, 4, and 6 A·g⁻¹.

Based on curves MnWO₄ and MnWO₄-CNT composed of vertical and horizontal lines, it is concluded that Faradaic reactions have occurred [31]. For MnWO₄-CNT, Figure 9a indicates that the discharge range is broader than that of the MnWO₄ in Figure 9b. This increase in capacitance can be due to adding nanotubes to the compound. The GCD curve, obtained in the following equation, is used to calculate the supercapacitor’s specific capacitance [32].

$$\mathrm{C}=\raisebox{1ex}{$i \Delta t$}\!\left/ \!\raisebox{-1ex}{$m \Delta V$}\right.$$

where i is current in A, $\Delta t$ is the discharge time in S, $\Delta V$ is the potential range in V, and m is the mass of active material on the electrode in g.

The specific capacitances for MnWO₄ are 293.998, 181.815, 151.776, 112.192, and 93.696 F·g⁻¹ at current densities of 1, 1.5, 2, 4, and 6 A·g⁻¹, respectively, while they are 1849.148, 1308.657, 1147.044, 810.472, and 663.504 F·g⁻¹ for MnWO₄-CNT at the same current densities. It is observed that adding nanotubes to the MnWO₄ compound raises porosity and expands the active surface area, causing the supercapacitor capacitance to increase. As shown by capacitances calculated for both materials MnWO₄ and MnWO₄-CNT, increasing the current density leads to a decrease in the specific capacitance of the supercapacitor due to impaired redox reactions in the electrode originating from the decreased adsorption of electrolyte ions on the active sites of the electrode. It is concluded that reactions are only conducted on the electrode surface for high current density values [23,33].

Electrochemical impedance spectroscopy (EIS) is used for electrochemical evaluation of the electrode in response to different frequencies. MnWO₄ and MnWO₄-CNT electrodes are analyzed in a dual-electrode system for the frequency range of 0.1 to 100 kHz. The Nyquist diagram has two semi-circular parts in high frequencies showing the resistance of charge transfer in the electrode–electrolyte interface. Another part of this diagram is a linear line in low frequencies (called the Warburg region) associated with the velocity of ion movement in the electrode. Figure 10 indicates that the value of charge transfer resistance of MnWO₄-CNT is about 23.8 Ω and about 17.8 Ω for MnWO₄, but the line in the Warburg region is clearer for MnWO₄-CNT and shows good impedance and capacitance behavior. It also shows the speed of ion transfer through the electrode or the low resistance of the electrode, but the slope of this line is lower in MnWO₄ and has an almost ohmic behavior. The intense activity of the electrode is because the presence of CNTs in the composite materials increases the conductivity and improves the supercapacitor behavior of the materials [34,35].

In

Table 1. Comparison of supercapacitor performance of nanocomposite with other recent research.

Nanocomposite	Specific Capacitance (F g−1)	Potential Range (V)	Current Density (Ag−1)	Scan Rate (mVs−1)	Reference
MnWO4/CNT	1849	0–0.5	1	_	This work
MnWO4/RGO	288	−0.35–0.55	_	5	[23]
ZnWO4/CNT	4552	−0.1–0.8	1	_	[36]
MnCoP/(Co,Mn)(Co Mn)2O4	123.43	_	0.5	_	[37]
ACNFs@Ni-Mn	1077	_	1	_	[38]
MnWO4 nanorods	247	0–1.1	_	5	[39]
CNPFs/MoS2/rGO	144.3	0–1.0		2	[40]
Ni-Co-S@N-pCNFs	520.2	_	0.2	_	[40]
N-doping grape-based honeycomb-like porous carbon (NGHPC)	275	−1.0–0	0.5	_	[41]

, we have compared the supercapacitor performance of nanocomposite MnWO₄ with other recent research.

4. Conclusions

In this study, the MnWO₄-CNT was synthesized and then deposited on nickel foam by a cost-effective hydrothermal method to be used as a supercapacitor electrode. The physical characterization of the active material was conducted through the Raman, XRD, BET and SEM-EDX analyses. BET showed that the MnWO₄-CNT has a higher specific area than bare MnWO₄, which can be a crucial component for enhancing the electrochemical activity in MnWO₄-CNT. In this binary combination, CNT increases the conductivity by providing electrons to the complex, thus reducing the resistance. On the other hand, with the help of CNT, the effective surface area increases, which plays an important role in charge transfer and energy storage. The electrochemical performance of electrodes was investigated through the CV, EIS, and GCD. The high specific capacitance and suitable stability of MnWO₄-CNT were higher than MnWO₄. According to our results, the supercapacitor application of MnWO₄-CNT can introduce it as a promising candidate for use in energy storage devices.