A Fractal, Flower Petal-like CuS-CuO/G-C3N4 Nanocomposite for High Efficiency Supercapacitors

Abstract

A fractal, flower petal-like CuS-CuO/G-C3N4 nanocomposite is prepared and applied in a symmetric two-electrode supercapacitor. The preparation of CuS-CuO/G-C3N4 is carried out through the hydrothermal method, in which salts of copper are dissolved and mixed with a suspension of G-C3N4 nanoparticles. A symmetric two-electrode supercapacitor, formed from CuS-CuO/G-C3N4 paste on Au-plates is investigated. The measurements are carried out in diluted 0.5 M HCl, and Whatman filter paper is used as a separator. The supercapacitor electric properties are determined by measuring the charge/discharge, cyclic voltammetry, impedance, and lifetime parameters. An enhancement in the charge/discharge time from 65 to 420 s was recorded while decreasing the current density (J) from 1.0 to 0.3 A/g. The cyclic voltammetry behavior is studied from 50 to 300 mV·s⁻¹, causing a direct increase in the produced J values. The specific capacitance (C_S) and energy density (E) values are 370 F/g and 37 W·h·kg⁻¹, respectively. The magnificent properties of the prepared supercapacitor qualify it for industrial applications as an alternative to batteries.

1. Introduction

Recent supercapacitor development challenges battery use. Such devices have seen an increased interest in their use as energy storage devices owing to their high ability to provide sustainable energy [1,2,3]. The three categories of supercapacitors, i.e., double-layer, hybrid, and pseudo-capacitor [4,5], ensure an easy and fast charging time compared to their high discharging time. Carbon supercapacitors are also known as a double-layer supercapacitors due to the charge storage in a double layer between the carbon and electrolyte. Whereas pseudo-capacitors prepared from oxides, sulfides, or nitrides owe their high ability to redox reactions inside the capacitor. Nevertheless, both these supercapacitors are limited by their small energy power [6]. In contrast, hybrid supercapacitors combine all the advantages of the above-mentioned types. Indeed, this is due to its particular construction, combing both carbon and normal materials that possess excellent redox capabilities and double-layer storage behavior at the same time [7].

In this regard, the proper choice of base material may enhance the supercapacitor. Indeed, utilizing a composite material will provide additional advantages related to the combination of the electrical conductivity properties of various materials present in the compound [8,9,10,11]. Moreover, oxide and sulfide materials with large surface areas provide an extra active site for charge storage [6].

Graphitic carbon nitride (G-C3N4), recognized for its magnificent properties of charge storage associated with its high surface area and charge and chemical durability, has become one of the most promising materials used for supercapacitors. This material can be prepared in high masses by the combustion of some organic materials rich in nitrogen, such as urea and thiourea. This explains its chemical structure of tris-triazine (C₆N₇) connected through a ternary amino group [12]. For the electrical conductivity which is 0.9 × 10⁻⁹ Sm⁻¹ [8], this material still needs additional enhancement.

G-C3N4 has been widely used for supercapacitor applications [13,14]. Santos et al. studied the synthesis of CoO and CuO on G-C3N4, where a specific capacitance (CS) was found equal to 84.28 at 0.5 A g [13]. Kuila et al. studied Gd on G-C3N4 for supercapacitor applications, where a capacitance of 2.59 mF cm⁻² at 10 mV s⁻¹ was found [14]. Rani et al. similarly investigated CoFe₂O₄/G-C3N4 for supercapacitor applications, demonstrating a C_S > 300 F/g at 2 mV/s [15]. On the other hand, G-C3N4/graphene was prepared as an electrode where a C_S equal to 265 F/g was measured for the supercapacitor [16]. Furthermore, G-C3N4/carbon nanotubes were also applied in supercapacitors with polyvinyl alcohol/H₂SO₄ electrolytes, the produced C_S value was about 148 F/g [17]. Similarly, G-C3N4/activated carbon in 1 M Na₂SO₄ was examined for supercapacitor use, where a C_S = 266 F/g was obtained [18]. Moreover, there are additional sulfide materials that are used for the supercapacitor enhancement, such as Ni/Co sulfide, Zn/Co sulfides, and Ni/Cu sulfides, which have shown great performance [19,20,21].

Here, a hybrid supercapacitor based on a CuS-CuO/G-C3N4 fractal, flower petal-like nanocomposite was synthesized using the hydrothermal method. The chemical structure was confirmed using XRD analysis, while the morphological analyses were confirmed using SEM, TEM, and theoretical modeling images. The composite was used as an electrode for a symmetric two-electrode supercapacitor for energy storage. The electrochemical testing was carried out with 0.5 M HCl as the electrolyte, and the parameters of charge/discharge, cyclic voltammetry, impedance, and supercapacitor stability were determined. The specific capacitance, energy density, and lifetime were calculated for the novel supercapacitor.

2. Experimental Section

2.1. Materials

Copper sulfate pentahydrate (CuSO₄·5H₂O), ammonia (NH₄OH), hydrochloric acid (HCl), ammonium persulfate ((NH₄)₂S₂O₈), graphite powder, ethanol, urea, and thiourea were purchased from Piochem company, Egypt. Nafion was dissolved in methanol (5 wt.%) (Sigma–Aldrich, St. Louis, MO, USA).

2.2. Preparation of CuS-CuO/G-C3N4 Nanocomposite

First, graphitic carbon nitride (G-C3N4) was prepared from the combustion of 10 g urea in a nitrogen atmosphere at 550 °C for 2 h. Then, the CuS-CuO/G-C3N4 nanocomposite was prepared through the hydrothermal method, in which 0.1 M CuSO₄·5H₂O was dissolved in water, then the pH was increased to 9 using NH₄OH solution. In another flask, 0.05 M (NH₄)₂S₂O₈ was mixed with 0.2 M thiourea under ultrasonication. Both solutions were mixed in an autoclave in the presence of 0.05 g G-C3N4. The hydrothermal process was carried out at 160 °C for 12 h, then the powder was dried at 60 °C for 5 h. Finally, for the synthesis of CuO inside the CuS/G-C3N4 composite, the powder was annealed at 300 °C for 10 min, leading to the synthesis of the CuS-CuO/G-C3N4 nanocomposite.

2.3. Two Symmetric Electrodes Supercapacitor Fabrication

The supercapacitor consisted of two symmetric electrodes separated by a Whatman filter paper wet with 0.5 M HCl. Each electrode consisted of Au-metal loaded with CuS-CuO/G-C3N4 paste. The paste was made by mixing 0.04 g of the composite with 0.005 g graphite powder. Then, this mixture was suspended in 100 µL nafion and 700 µL ethanol. The prepared paste was stirred for 2 days, then 0.003 g of the paste was loaded on each electrode. Then, the supercapacitor was closed carefully using a plastic tip cover. The electrochemical measurements were carried out using an electrochemical workstation (CHI608E). Different electrochemical parameters were measured: charge/discharge, cyclic voltammetry, lifetime, and impedance. Then, the specific capacitance (C_S) and energy density (E) of the fabricated supercapacitor were calculated.

2.4. Characterization

The chemical structures and morphological properties of the synthesized nanocomposite were confirmed using several characterization techniques. X-ray diffraction (X’Pert Pro, Almelo, Holland, The Netherlands) was used to confirm the chemical structure of the prepared materials. The morphologies were determined via scanning electron microscopy, SEM, (ZEISS SUPRA 55 VP, Oberkochen, Germany), and transmitted electron microscopy (TEM), (JEOL JEM-2100).

3. Results and Discussion

The chemical structures of the prepared nanomaterials: G-C3N4 and CuS-CuO/G-C3N4 are confirmed through the XRD pattern as shown in Figure 1. The XRD pattern of G-C3N4 XRD (black curve) has two characteristic peaks at 13.03° for the growth direction (100) and 27.20° for the growth direction (002) [22,23]. After composite formation, the XRD pattern (red curve) shows three characteristic peaks for G-C3N4, the characteristic peak at 27.2° has almost the same position, while the peak at 13.03° has shifted, related to the composite formation.

The measurements for monoclinic CuO had six characteristic peaks at 32.66°, 36.99°, 39.58°, 49.85°, 52.73°, and 59.34° for the growth directions (110), (002), (111), (−202), (020), and (202), respectively. This result matches well with a previous analysis [24] and CPDS data (45-0937) for the monoclinic phase of CuO [25]. On the other hand, the CuS XRD pattern has four characteristic peaks at 29.19° and 31.70°, 32.96°, and 47.90° for the growth directions (102), (103), (006), and (110), respectively, this matches with CPDS 06-0464 [26].

SEM, TEM, and theoretical modeling images have been used for the morphological and structural analysis of the samples. SEM images of both G-C3N4 and CuS-CuO/G-C3N4 composites are illustrated in Figure 2a,b, respectively. The G-C3N4 displays a fractal, flower-like shape. The flower size is about 5.7 nm in width and its length varies from 100 nm to 200 nm. The flower ‘petal’ walls are formed from a thin sheet structure, visibly distinguished in the TEM image (see Figure 2c), of G-C3N4, which illustrates flat sheets of maximum 200 nm width. On the other hand, several dark flakes of different sizes in the sheets were also located, confirming CuS-CuO incorporation into the G-C3N4 sheets.

Theoretical modeling is shown in Figure 2d, the petals forming the fractal, flower petal-like shape could be clearly distinguished, consisting of small walls of different sizes. The formed CuS-CuO/G C3N4 composite shows a high homogeneous roughness where small particles have coated the sheets of G-C3N4. Indeed, fractality enhancement observed here plays a key role in the supercapacitor enhancement. Thus, the prepared fractal, flower petal-like CuS-CuO/G-C3N4 nanocomposite provides high efficiency for supercapacitors.

Testing of the Prepared Supercapacitor

The electrochemical testing of the prepared CuS-CuO/G-C3N4 nanocomposite supercapacitor was carried out using the electrochemical workstation (CHI608E) in 0.5 M HCl. Two supercapacitor plates were prepared, in which a 0.003 g paste of the nanocomposite was deposited onto each plate with a dimension of 1 cm². Whatman filter paper was used as a separator. The measurements were carried out in 0.5 M HCl electrolyte under different electrochemical parameters: cyclic voltammetry, charge/discharge, impedance, and stability. The cyclic voltammetry was carried out in a potential window of 0.0 to 1.0 V at room temperature. The charge/discharge was carried out under a current density of 0.3 to 1.0 A/g.

The charge/discharge and cyclic voltammetry studies of the CuS-CuO/G-C3N4 nanocomposite supercapacitor have been investigated (see Figure 3a,b, respectively). The produce time increases with decreasing the current density (J) from 1.0 to 0.3 A/g during the charge/discharge study. These results reflect the strong effect that the J values has on the charge storage, and the general capacitance of the prepared supercapacitor. The enhancement located at the charge/discharge time from 65 to 420 s, while decreasing the J value from 1.0 to 0.3 A/g, illustrates the required time for the supercapacitor to arrange the charge inside its plates [27,28]. The large value of discharge time (140 s) at 0.3 A/g confirms the role of both redox and the double layer for CuS-CuO and G-C3N4, respectively, in the prepared hybrid supercapacitor. The HCl, as a strong acid, provides a high density of H⁺ ions ensuring a high activity for better charge storage in the supercapacitor [29].

The cyclic voltammetry of the prepared supercapacitor confirms the same behavior as seen in the charge/discharge study (see Figure 3b). This is illustrated through the large J value produced that ranges from −1.1 to 1.3 mA·cm⁻², associated with the appearance of oxidation and reduction peaks, indicating the redox behavior of the used paste material. Furthermore, the produced J value increases with an increase in the scan rate from 50 to 300 mV·s⁻¹. Such behavior confirms the diffusion of more H⁺ ions inside the paste with a rising scan rate [30,31]. This process reflects the increase in the charge storage inside the plates of the prepared supercapacitor.

The resistance of the prepared CuS-CuO/G-C3N4 nanocomposite supercapacitor is determined through the Nyquist plot from the relation of the real (Z) and imaginary (Z⁻) values (see Figure 4a). This relationship represents the charge transfer between the electrolyte and the plates of the supercapacitor. The insertion cell in Figure 4a represents Rundle’s cell, in which W, Rs, Cd, and Rct correspond to Warburg impedance, solution resistance, capacitor, and charge transfer [32,33], respectively. R_CT characterizes the rate of redox reactions at the electrode/electrolyte interface, while C_dl occurs at the interfaces between solids and ionic solutions due to separations of ionic and electronic charges. The values of R_S and R_CT are 162 and 15 Ω, respectively, while the C_dl value is 40 µF.

The curve shows a nice semi-circuit illustrating the charge transfer between the solution and the plates. Thus, the prepared supercapacitor demonstrates a high ability of charge transfer, i.e., charge storage inside its plates.

The stability of the prepared supercapacitor was determined by measuring the electrochemical charge/discharge for 300 cycles (see Figure 4b). The measurements are carried out with 0.5 M HCl at J values of 0.3 A/g. The supercapacitor displays a good stability of 51% for 300 cycles.

On the other hand, the prepared supercapacitor has a high specific capacitance (C_S) and energy density (E). Both C_S and E parameters depend on m, I, ΔV, and Δt corresponding to the loaded mass, current, potential window, and discharge time, respectively. Equations (1) and (2) describe C_S and E values [34,35]. From these equations, the C_S and E values are 370 F/g and 37 W·h·kg⁻¹, respectively.

$${\mathrm{C}}_{\mathrm{s}}=4\mathrm{I}·\Delta \mathrm{t}/ \Delta V·m$$

(1)

$$E=0.5{\mathrm{C}}_{\mathrm{s}}· \left(V_{max }^2-V_{min }^2\right)$$

(2)

4. Conclusions

A CuS-CuO/G-C3N4 nanocomposite has been prepared using the hydrothermal method, following preparation of the G-C3N4 composite via combustion in ambient air. From the morphological analyses, G-C3N4 has a nanosheet structure. After composite formation, flower petal-like nanomaterials are formed. The symmetric two-electrode supercapacitor haa great properties for supercapacitor applications; the charge/discharge time is 65 and 420 s at a current density of 0.0 and 0.3 A/g, respectively. The C_S and E values are 370 F/g and 37 W.h.kg⁻¹, respectively. In future work, our team will work on a supercapacitor prototype for industrial application as a promising solution for energy storage instead of batteries.