Flexible Asymmetric Supercapacitors Constructed by Reduced Graphene Oxide/MoO3 and MnO2 Electrochemically Deposited on Carbon Cloth

Abstract

A flexible asymmetric supercapacitor (ASC) is successfully developed by using the composite of MoO₃ and graphene oxide (GO) electrochemically deposited on carbon cloth (CC) (MoO₃/rGO/CC) as the cathode, the MnO₂ deposited on CC (MnO₂/CC) as the anode, and Na₂SO₄/polyvinyl alcohol (PVA) as the gel electrolyte. The results show that the introduction of the GO layer can remarkably increase the specific capacitance of MoO₃ from 282.7 F g⁻¹ to 341.0 F g⁻¹. Furthermore, the combination of such good electrode materials and a neutral gel electrolyte renders the fabrication of high-performance ASC with a large operating potential difference of 1.6 V in a 0.5 mol L⁻¹ Na₂SO₄ solution of water. Furthermore, the ASCs exhibit excellent cycle ability and the capacitance can maintain 87% of its initial value after 6000 cycles. The fact that a light-emitting diode can be lit up by the ASCs indicates the device’s potential applications as an energy storage device. The encouraging results demonstrate a promising application of the composite of MoO₃ and GO in energy storage devices.

1. Introduction

Supercapacitors (SCs) are a potential energy storage device that have been widely studied due to their high energy density, fast charging and discharging speed, and long cycle life [1,2]. Supercapacitors are widely used in hybrid vehicles, backup energy systems, mobile electronic devices, and more [3]. With the increasing requirement for personal electronics in modern society, to make electronic devices flexible and portable has become the research hotspot at present. Correspondingly, flexible and portable electronic devices need design of a new type of energy storage device different from previous ones [4,5,6,7,8]. Therefore, flexible asymmetric SCs (ASCs) have attracted much attention, because they can provide more energy density compared with symmetric SCs [9,10,11,12,13,14,15].

To fabricate flexible ASCs (FASCs), it is essential and crucial to select the current collector and active materials because they directly affect the performance of FASCs [16,17,18]. As a new flexible conductor, carbon cloth (CC) has received much attention, due to its three-dimensional (3D) network structure and moderate electrochemical stability [19]. By using CC as a substrate, conducting polymers deposited on it has shown high energy density and power density. For example, Horng et al. have prepared polyaniline nanowire/CC nanocomposite electrodes by employing electrochemical deposition, and the PANI had a C_s of 1079 F g⁻¹ [20]. An electrode of CC/MoS₂/PANI has been prepared via combining the facile hydrothermal method, and the PANI of this electrode showed a C_s value of 972 F g⁻¹ at 1 A g⁻¹ [21].

Transition metal oxides are the other commonly used electrode materials for SCs; at first, the noble metal oxides such as RuO₂ [22], RhO_x [23], etc., were extensively studied because of their high faradic capacitance and good conductivity. Despite the excellent capacitive performances, the rarity and high price of precious metal oxides limit their practicality. As a result, many studies have focused on the substitution of precious metal oxides due to their relative theoretical carbon content, availability of inexpensive transition metal oxides, and relative reversibility [24]. At present, layered porous manganese dioxide nanosheets with a maximum C_s of 268 F g⁻¹ have been prepared by the rapid hydrothermal method [25]. Different from the usually used electrode material of MnO₂, which possesses a capacitive property at a positive potential range, MoO₃ exhibits capacitance at a negative potential range and has attracted much attention [26,27]. Furthermore, the other attractive nature of MoO₃ is its particular 2D layered structure, which is conducive to provide high power density for capacitor applications because of the adequate intercalation between the layers and electrolyte ions [28]. Recently, Pujari et al. found that hexagonal MoO₃ microrods display a C_s of 194 F g⁻¹ [29,30]. Despite the efforts on MoO₃, the C_s values are still not very high due to its low conductivity. Therefore, it is necessary to develop a strategy to prepare the composites with relatively high conductivity by adding some highly conductive materials [22,31,32,33,34]. The co-electrochemical deposition of active materials and carbon materials has been proven to be an effective method in various methods of manufacturing composite materials; as a precursor of graphene, graphene oxide (GO) has attracted great interest in the manufacturing of composite materials due to its ease of operation in aqueous media and high specific surface area [35,36].

In this work, we choose CC as a flexible substrate to deposit MoO₃ as a cathode, during which the graphene oxide (GO) is added to form the composite of MoO₃ and reduced GO (rGO) through electrochemical co-deposition. The electrode prepared by electrochemically depositing MnO₂ on CC is used as an anode, MoO₃/rGO/CC as a cathode, and NaSO₄/polyvylene oxide as a gel electrolyte and separator to assemble ASCs. By using this strategy, we hope that the FASCs can possess high operating potential differences, energy density, and power density.

2. Results

2.1. Structural Properties’ Characterization of All Samples

Figure 1 depicts a schematic illustration of the procedure for the preparation of electrodes and the device. Based on the flexibility and conductivity of CC, and the potential ranges of electrochemical activity for MoO₃ and MnO₂ [37,38], the FASCs can be fabricated by using MoO₃/rGO/CC as a cathode and MnO₂/CC as an anode in a Na₂SO₄/PVA gel electrolyte.

From the SEM image shown in Figure 2, it is found that fibers contained in CC have a clean surface with some groves (Figure 2A); the diameter of the fibers is relatively uniform and many accumulated pores are observed among the fibers (Figure 2B). When the molybdenum oxide has been electrochemically deposited, the surface of the fiber will be covered by a layer of molybdenum oxide; it can be seen from the image of MoO₃/CC (Figure 2C) that the fiber is uniformly wrapped by a layer of molybdenum oxide, but the film has many cracks, which are caused during the drying process. Under the large magnification, a corrugated morphology is observed (Figure 2D); this rough surface structure is favorable to increase the surface area of MoO₃. It is worth noting that by changing the film-forming time, the thickness and mass load of the film can be well controlled. When the evaporation time is 1000 s, a “skeleton/skin film” structure is obtained with carbon fiber as the skeleton and MoO₃ as the skin film [39,40]. Figure S3 is the SEM image of the MnO₂/CC electrode, from which it can be found that MnO₂ particles are formed on the CC. In Figure 2E,F, the surface of CC is decorated with rGO flakes, embedded in the enlarged image (Figure 2G), which clearly shows that rGO smooths the surface of the carbon. In addition, uniform rGO nanosheets form a 2D layered structure on the surface of MoO₃/rGO/CC, which increases the surface area and improves the electrical conductivity compared with the blank CC. SEM results showed that the surface of the MoO₃/CC composite was successfully coated with a layer of rGO.

To confirm the properties of the product, X-ray diffraction was used to evaluate the prepared material. From Figure 3, it can be found that all the samples show obvious peaks related to the substrate of CC or rGO, but no distinguishable peaks can be seen for the components of MoO₃ and MnO₂, which may be attributed to the fact that the formed MoO₃ and MnO₂ are amorphous or the crystalline is too small, or the weak diffraction peaks related to MoO₃ and MnO₂ are covered up by the diffraction peak of C due to the small amount of the active materials [19,41,42,43]. The FTIR spectrum provides details on the band characteristics of composite materials. As shown in Figure S4, no strong characteristic peaks are seen in the wave number ranging from 400 to 2000 cm⁻¹. The spectrum of MoO₃/CC shows three main peaks at 955, 826, and 753 cm⁻¹. This is the Mo=O stretching vibration of terminal oxygen and the symmetric and asymmetric stretching vibration of Mo-O-Mo cross-linked oxygen, respectively [44]. However, the IR spectrum for GO/MoO₃/CC exhibits the characteristic of rGO [19] besides the MoO₃’s characteristics (C=C: 1738 cm⁻¹), confirming that rGO has successfully loaded on the surface of MoO₃/CC.

The analysis of the XPS can reveal the chemical states of the samples of MnO₂/CC (a), MoO₃/rGO/CC (b), MoO₃/CC (c), and CC (d) (Figure 4A). All the spectra show the XPS peaks related to the element of Mn or Mo together with that of C and O. Specially, the characteristic doublets of Mn2p (Figure 4B) 2p1/2 (654.1 eV) and Mn2p 2p3/2 (642.4 eV) from MnO₂/CC identify the formation of MnO₂ due to the difference between the two peaks of 11.7 eV [45]. Similarly, the double peaks observed at 233.0 and 236.2 eV in MoO₃/rGO/CC can be contributed to Mo 3d5/2 and Mo 3d3/2, respectively, due to spin-orbit splitting (Figure 4D) [44]. According to the fitted results, it can be found that the components of MoO₂ and MoO_x are exhibited in the sample besides the component of MoO₃ [46]. On the other hand, the above peaks in the Mo 3d spectrum of MoO₃/rGO/CC composite materials are shifted by 0.2 eV due to the interaction between rGO and MoO₃, to 232.8 eV and 236.0 eV, respectively. The peak intensity of the MoO₃/CC spectrum is higher than that of MoO₃/rGO/CC.

2.2. The electrochemical Properties of MoO₃/rGO/CC

The galvanostatic method is used to deposit MoO₃ on the pretreated CC, and the depositing current and depositing time are optimized by changing the current and depositing time. Firstly, the deposition current is optimized in the same deposition solution by changing the current, keeping the depositing charge (Q = 6 C) amount constant. The obtained electrodes’ CV curves are tested by employing a three-electrode system, and according to the formulae shown in the Supplementary Materials, the plot of the C_m value versus the currents is drawn in Figure S1. The C_m value reaches its maximum value of 282.8 F g⁻¹ at 6 mA. Subsequently, the current is fixed at 6 mA to change the deposition time to optimize the time. It can be found in Figure S1B that the maximal C_m value of 283.5 F g⁻¹ is achieved when the depositing time is 1000 s. Therefore, we use the condition of a 6 mA constant current and depositing time of 1000 s to prepare the electrode in the following experiments. For improving the performance, electrodes of MoO₃/CC are used as working electrodes to electrochemically deposit the rGO in a different GO solution; from Figure S1C, we can find that when the GO solution is 7.0 mg mL⁻¹, the modified electrode shows the largest C_m value. Therefore, the electrode prepared at the above-mentioned condition will be evaluated in detail.

Figure 5A shows the electrode of MoO₃/rGO/CC exhibiting far larger surrounded CV area than MoO₃/CC at the same scan rate. The CV curve of the MoO₃/rGO/CC electrode shows a similar shape within the range of a potential scanning rate of 5 to 100 mV s⁻¹; based on the curves, the C_m values of MoO₃/rGO/CC are calculated and shown in Figure 5C, and it is clear that the C_m values of MoO₃/rGO/CC decrease with the increase in the scan rates because of the limited diffusion time for ions when the scan rate is large, and it has the largest C_m value of 341.0 F g⁻¹ that is higher than that of MoO₃/CC (282.8 F g⁻¹) at 1 mV s⁻¹, which is believed to be because the conductive rGO flakes on the surface of MoO₃ shorten the transport pathways of electrons and electrolyte ions. As the scanning rate increases, the C_m value of MoO₃/rGO/CC shows a decreasing trend. The GCD curves of the MoO₃/rGO/CC electrode in a potential range of −1.0–0 V (Figure 5D) display almost symmetric shapes at different current densities; the iR drop is small at low current density and increases with the increment of the current densities, and the C_m obtained from the GCD curves are similar to those from CV profiles. The good capacitive performance of MoO₃/rGO/CC can be due to the close contact between the carbon fiber and MoO₃ layer, in addition to fast and effective charge transfer through a three-dimensional CC framework; graphene oxide sheets coated on MoO₃ also provide an electron transfer pathway. At an open circuit potential, the EIS spectrum has been recorded within the frequency range from 100 kHz to 0.01 Hz to reveal the kinetic property of MoO₃/rGO/CC.

Regarding the Nyquist curve (Figure 5E), the semicircles in the high-frequency region and the straight peaks in the low-frequency region are clearly seen [45,47,48]. The profile can be fitted by using Zview (2.3.1) software; the R_ct value of the GO/MoO₃/CC electrode is 1.82 Ω, indicating the enhanced charge transportation provided by the decorated rGO on the surface. The electrochemical stability of the electrodes investigated by successive CV scanning at 100 mV s⁻¹ (Figure 5F), and it is found that MoO₃/rGO/CC can retain 88.6%, indicating better cycling stability.

2.3. Electrochemical Performance of the FASC

The anode has been prepared by using CC as a substrate via the electrochemical depositing method. The conditions for depositing MnO₂ have been optimized in Supplementary Materials (Figure S2). The electrode prepared at a current of 4 mA for 1000 s exhibits the optimal C_m value (343.3 F g⁻¹), and the electrodes prepared under this condition are further investigated and used to assemble the devices. In a 0.5 mol L⁻¹ Na₂SO₄ solution, the CV curve of the MnO₂/CC electrode exhibits a quasi-rectangular shape at low scanning speeds, indicating excellent performance of MnO₂/CC (Figure 6A). It can be easily seen from the curves of C_m and scanning speed that as the scanning speed increases, the C_m value decreases. In 100 mV s⁻¹, the C_m value maintains 45.1% of the maximum C_m value (Figure 6B), demonstrating good rate capability. Based on the results obtained from CV tests in the three-electrode system, the area ratio of MoO₃/rGO/CC to MnO₂/CC is 1:1. Therefore, the devices are constructed by using the Na₂SO₄/PVA gel electrolyte to separate the cathode of MoO₃/rGO/CC and anode of MnO₂/CC. By employing the two-electrode system, we recorded the CV curves in different voltage windows at 20 mV s⁻¹ (Figure 6C,D) to determine the operating potential difference. It is clear that the operating potential difference of the device can be expanded to 1.6 V since MoO₃/rGO/CC is −1.0–0 V, while MnO₂/CC is 0–1.0 V, which are determined by the three-electrode system [49,50,51]. Notably, the CV curve of the ASC device is similar to the curve observed in the MnO₂/CC, maintaining stable redox pairs within 2–100 mV s⁻¹ (Figure 6E), and the scan rate increases and the C_m of ASC decreases at 2 mV s⁻¹ (Figure 6F); the device shows a C_m of 98.3 F g⁻¹ (C_a of 226.4 mF cm⁻²).

The GCD curve (Figure 7A) shows a quasi-linear symmetric shape, with fast voltage current response and good electrochemical reversibility. Due to the presence of pseudo-capacitance in the device, the result deviates slightly from the straight line. At low current density, the GCD curve shows a tiny decrease in iR. At 1.0 mA cm⁻², the device can achieve a C_m of approximately 88.3 F g⁻¹ and C_a of 203.4 mF cm⁻². The C_m value of ASC decreases with the increase in current density (Figure 7B), which is analogous to the result of CV.

In order to further investigate the detailed electrochemical characteristics of ASC, we performed EIS experiments (Figure 7C). The result shows that at low frequencies, lines almost perpendicular to the Z′ axis exhibit quite good capacitance characteristics [52,53,54]. After 6000 cycles, the capacitance of the ASC device remained at 88.1% of the initial value and showed good cycling stability, indicating that this type of ASC device has good electrochemical stability (Figure 7D).

In order to evaluate the practical performance of the device, E and P were calculated based on the discharge branch of the GCD curve using the formula provided in the Supplementary Information (Figure 8A). At a P of 533.3 W kg⁻¹, E is 32.1 Wh kg⁻¹. More noteworthy is that when P reaches 5333.3 W kg⁻¹, E remains at 20.2 Wh kg⁻¹. Due to the high electrochemical capacitance and good multiplication ability of the electrode, it is superior to other devices previously reported (Table 1), indicating the potential application of this material. In addition, LEDs can be driven by fully charged series batteries (Figure 8B), indicating the possibility of practicality. By combining the pseudo-capacitive material fully loaded on the CC substrate and mild gel electrolyte, an effective way for preparing high-performance ASC with excellent cycling performance can be obtained.

3. Materials and Methods

3.1. Materials

Manganese (II) acetate (Mn(Ac)₂·4H₂O), ammonium molybdate [(NH₄)₂MoO₄], ammonium chloride, polyvinyl alcohol (PVA, Mw: 85,000), and sodium sulfate (Na₂SO₄) were purchased from Aladdin Chemical Co. (Ontario, CA, USA) and used without further purification. The conductive carbon cloth (CC) substrate was obtained from Shanghai Chuxi Industrial Co., Ltd. (Shanghai, China)

3.2. Fabrication of the Electrodes of MnO₂/CC and MoO₃/rGO/CC

Pretreatment of the CC: CC is made from pre-oxidized polyacrylonitrile fabric that is carbonized or spun from carbon fiber. As a kind of flexible carbon-based template with special porous structure, high electrical conductivity, high mechanical stability, and corrosion resistance, carbon cloth not only has the inherent characteristics of carbon materials, but also has the machinability of fiber materials. It is often used as a substrate to carry active substances and improve the electrochemical performance of capacitors. The commercial CC was first immersed in concentrated nitric acid (HNO₃, 68 wt.%) and then heated at 60 °C for about 6 h in a water bath to make the surface hydrophilic. Subsequently, the CC was washed with distilled water thoroughly and then soaked in distilled water for use.

Electrochemically depositing MnO₂ on CC: By using a three-electrode system, MnO₂/CC was also prepared via an electrodeposition method. In the mixing solution of 15 mL Mn(Ac)₂ (0.05 mol L⁻¹) and 15 mL Na₂SO₄ (0.05 mol L⁻¹), the MnO₂ was deposited on the pretreated CC at a constant current of 4.0 mA cm⁻² when the CC was used as a working electrode, a Pt plate as a counter electrode, and a saturated calomel electrode (SCE) as a reference electrode, and the amount of MnO₂ was controlled by depositing time (600–1400 s). After the deposition process was completed, the electrode of MnO₂/CC was washed by deionized water and dried in a vacuum oven at 70 °C overnight. The electrode of MnO₂/CC prepared from a different depositing time was defined as MnO₂/CC-t (t is the depositing time).

Electrochemically depositing MoO₃ on CC: The used electrolyte here (30.0 mL) was an aqueous solution of 15 mL (NH₄)₂MoO₄ (0.05 mol L⁻¹) and 15 mL NH₄Cl (0.05 mol L⁻¹) and its pH value was adjusted to 2.0 by acetic acid. The pretreated CC slices (1 × 1 cm²) were used as a working electrode, a Pt sheet as a counter electrode, and a saturated calomel electrode (SCE) as a reference electrode; by using a constant current density of 4 mA cm⁻², different amounts of MoO₃ were deposited on CC by controlling the depositing time (700–1300 s). According to the capacitance measured by using a three-electrode system in a 0.5 M Na₂SO₄ electrolyte, the MoO₃/CC prepared from 1000 s was used to assemble the ASCs.

Electrochemically depositing MoO₃/rGO on CC: First, GO was prepared by oxidizing 300-mesh graphite powder according to the modified hummer method [63]. The sample of MoO₃/rGO/CC prepared by using the electrochemical deposition method according to the above was used as a work electrode here to deposit GO on the surface of MoO₃/CC by the cyclic voltammetry (CV) method in different concentrations of the GO solution, and the obtained electrode was defined as MoO₃/rGO/CC. The appropriate GO in the electrolyte is selected by optimizing the deposition time and the CV number. Place MoO₃/rGO/CC in a vacuum oven overnight. The electrochemical performance test was carried out in a three-electrode system after drying.

3.3. Fabrication of the FASC Devices

Gel electrolyte: 1.0 g PVA powders were slowly added into a 10.0 mL solution of Na₂SO₄ (0.5 mol L⁻¹) under stirring; then, the mixture was heated to 85 °C until the mixture became transparent, and then the solution was naturally cooled to room temperature for utilization.

Assembly of FASC device: Firstly, the CC was first pretreated with HNO₃ at 60 °C to remove the sizing agent on the surface of the fibers. Then, MnO₂/CC and MoO₃/rGO/CC were fabricated using a facile electrodeposition method. MnO₂ or MoO₃ or rGO was grown on the CC substrate using three-electrode configuration with CC as the working electrode, a Pt piece as the counter electrode, and Hg/HgCl₂ as the reference electrode. Finally, MoO₃/rGO/CC was used as the negative electrode and MnO₂/CC as the positive electrode. According to the capacitance measured by using the three-electrode system, it was clear that the capacitances of the MoO₃/rGO/CC negative electrode and MnO₂/CC positive electrode could be balanced when geometric area of the two electrodes was the same. When the FASC device was assembled, the Na₂SO_4/PVA gel electrolyte was uniformly covered on one electrode such as MnO₂/CC, and then the other electrode (MoO₃/rGO/CC) was placed on the face of MnO₂/CC, which was covered by the Na₂SO_4/PVA gel electrolyte and followed by pressure to bring the two electrodes together; subsequently, the assembled devices were frozen in a refrigerator for 30 min. Finally, the fabrication of FASC was completed after the frozen device was placed at room temperature to unfreeze.

4. Conclusions

A convenient and efficient method for depositing high-active pseudo-capacitive materials on CC (MoO₃/rGO/CC and MnO₂/CC) has been developed. The introduction of the rGO layer can remarkably improve the specific capacitance of the MoO₃ layer on the carbon fibers from 282.7 to 341.0 F g⁻¹. Furthermore, the assembled ASC by using the electrochemically deposited electrodes and neutral gel electrolyte Na₂SO₄/PVA possesses a large operation potential of 1.6 V, and exhibits a high energy density of 32.08 Wh kg⁻¹ at the power density of 0.53 kW kg⁻¹, and 5.33 Wh kg⁻¹ at 20.2 kW kg⁻¹. Furthermore, the ASC exhibits good cycle ability and the capacitance can maintain 87.1% of its initial value after 6000 cycles. The ability of these two ASC devices connected in series is that they are able to light up an LED, which indicates their potential applications as energy storage devices.

Author Contributions

Data curation, Z.L.; writing—original draft preparation, S.L.; writing—review and editing, S.L. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available either in this article or in the Supplementary Materials.

Acknowledgments

This work was supported by the Institute of Optoelectronic Functional Materials.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

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Figures and Table

Figure 1. Schematic diagram for preparing MoO3/rGO/CC and MnO2/CC electrodes and assembled ASC.

Figure 2. The SEM images in different magnification for CC (A,B), MoO3/CC (C,D), MoO3/rGO/CC (E,F), and inset of the enlarged image (G).

Figure 3. The XRD patterns of CC, MnO2/CC, MoO3/CC, and MoO3/rGO/CC (A); the magnified XRD patterns for MoO3/CC and MoO3/rGO/CC in the range of 10–50° (B).

Figure 4. (A) The whole XPS spectra for MnO2/CC (a), MoO3/CC (b), MoO3/rGO/CC (c), and CC (d); (B) the high-resolution XPS for Mn2p in MnO2/CC; (C) the C1s XPS for MnO2/CC; (D) the high-resolution XPS for Mo3d in MoO3/rGO/CC; (E) the C1s XPS; and (F) N1s XPS for MoO3/rGO/CC.

Figure 5. Electrochemical behaviors of the MoO3/rGO/CC electrode: (A) CV curves of CC, MoO3/CC, and MoO3/rGO/CC; (B) CV curves for the MoO3/rGO/CC electrode at different scan rates; (C) the plot of the Cm values versus scan rate for MoO3/rGO/CC; (D) GCD curves of the MoO3/rGO/CC electrode at different current densities (a–g represent GCD curves with current density of 1–7 mA cm−2, respectively); (E) EIS spectra for the MoO3/rGO/CC electrode (inset of E: equivalent circuit diagram of Nyquist plots); (F) the plot of capacitance versus the cyclic number for MoO3/rGO/CC. The electrolyte is a 0.5 mol L−1 Na2SO4 aqueous solution.

Figure 6. (A) CV curves for MnO2/CC at different scan rates (a-g represent scan rates of 5, 10, 20, 40, 60, 80, and 100 mV s−1, respectively), (B) the plot of Cm of MnO2/CC versus the scan rates, (C) the CV curves of the positive and negative electrodes in the three-electrode system at a scan rate of 20 mV s−1, (D) the FASC’s CV curves recorded at various potential difference windows at 20 mV s−1 (a-d represent 0–1.4, 1.6, 1.8, and 2.0 V, respectively), (E) the FASC’s CV curves recorded at different scan rates (a–g represent scan rates of 2, 5, 10, 20, 50, 80, and 100 mV s−1, respectively), (F) the plot of the FASC’s Cm based on active material versus scan rates.

Figure 7. (A) GCD curves of ASC recorded at different current densities (a-h represent current densities of 1.0, 2.0, 3.0, 4.0, 6.0, 8.0, 10.0, and 12.0 mA cm−2), (B) the plot of specific capacitances versus current densities, (C) Nyquist plots for the ASC of MoO3/rGO/CC//MnO2/CC, (D) the plot of the capacitance of ASC versus the cyclic number.

Figure 8. (A) The Ragone plots for ASC of MoO3/rGO/CC//MnO2/CC and some other devices from the previous literature for comparison [55,56,57,58,59,60,61,62]; (B) the photograph of an LED lit by two cells connected in series.

Table 1

Comparison of the energy density and cyclic stability of the ASCs.

Electrode Materials	Electrolyte	Energy Density (Wh kg−1)	Stability (CapacitanceRetention %, Cycles)	Ref.
MoO3-PPy/CNTs-MnO2	Na2SO4/PVA gel	21.0	76.0, 10,000 cycles	[55]
MnO2@PEDOT/PEDOT	LiClO4/PMMA gel	9.8	86.0, 1250 cycles	[56]
Fe3O4 embedded incarbon nanosheet/porous carbon	KOH/PVA gel	18.3	70.8, 5000 cycles	[57]
Graphene (IL-CMG)/RuO2-IL-CMG	H2SO4/PVA gel	19.7	95.0, 2000 cycles	[58]
CNTs/MnO2/CNTs/PANI	Na2SO4/PVP gel	24.8	-	[59]
Co3O4 nanowires/Ni foam/carbon aerogel	KOH/PVA gel	17.9	-	[60]
Mn3O4 nanoparticle/graphene/CNT/graphene	KCl/PVA gel	32.7	86.0, 10,000 cycles	[61]
Graphene/Ni(OH)2/graphene/CNT	KOH/PVA gel	18	-	[62]
MoO3/rGO/CC/MnO2/CC	Na2SO4/PVA gel	32.1	88.1, 6000 cycles	This work

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules29133116/s1, Figure S1. Specific capacitance variations of MoO₃/CC versus deposition currents, time, and concentrations of GO. Figure S2. The plots of specific capacitance of MnO₂/CC versus depositing currents and times. Figure S3. The SEM images in different magnification for MnO₂/CC (A,B). Figure S4. The FTIR spectra of MoO₃/CC and MoO₃/rGO/CC.

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