Effect of Polypyrrole on the Capacitance Enhancement of the Spherical ZnS–ZnO/g-C3N4 Nanocomposite for Supercapacitor Applications

The ZnS–ZnO/graphitic carbon nitride (g-C3N4) nanocomposite was synthesized using the hydrothermal method, incorporating 0.05 g of g-C3N4 during the composite formation. The crystalline nature of the materials was confirmed through X-ray diffraction analysis. Scanning electron microscope and transmission electron microscope analyses revealed the morphological properties of the materials. The nanocomposite exhibited 2D g-C3N4 nanosheets with a thickness of 7.8 nm. Additionally, the nanocomposite consisted of spherical particles with an average particle size of 15 nm, which were distributed over the 2D g-C3N4 sheets. ZnS–ZnO/g-C3N4 nanocomposite consists of spherical particles with an average particle size of 15 nm that covers the 2D g-C3N4 sheets. ZnS–ZnO/g-C3N4 nanocomposites without/with polypyrrole are used as a paste for the two-symmetric electrodes hybrid supercapacitor. The charge (0.5 to 1.3 A/g), cyclic voltammetry (0.0 to 1.0 potential window), impedance, and stability for the prepared supercapacitor are studied using electrochemical measurements. ZnS–ZnO/g-C3N4 supercapacitors have a specific capacitance of 15.1 and 152.2 F/g without/with the physical addition of polypyrrole, respectively. Also, they have energy density values of 1.17 and 11.87 W h/kg, respectively. Through the Randles cell, The Rs in the supercapacitor without/with polypyrrole are 6.9 and 6.65 Ω, respectively. While the values of the semicircle diameter (Rct) are 0.37 and 0.25 Ω, respectively. Also, the capacitance retention values are 88.6 and 90.2% without/with polypyrrole addition until 500 runs of charge/discharge. These values confirm the behavior of the polypyrrole material in the enhancement of the supercapacitor efficiency, storage capacity, and stability.


Introduction
Supercapacitors are considered among the best energy stores, and this is due to their very high energy density, the speed of their movement in a very short time without return, in addition to their low cost and long life [1][2][3].Now, the electrodes used are carbon which has doublelayered electrical properties [4][5][6].The carbon electrodes did not serve the required purpose and did not meet the market requirements due to their rather low energy density, this is considered a major drawback, so we find that researchers are looking for non-carbon electrode materials such as electrodes with Faraday capacitance.They also expect in the future to produce capacitors with a very large capacity (pseudo capacitors) [7][8][9].
Material (graphitic carbon nitride) (g-C 3 N 4 ) is one of the very important chemicals in physics and electrical engineering due to its ease of preparation and low manufacturing cost in addition to its non-pollution to the environment and it has a very promising future for large use in many applications [10][11][12].
To develop the characteristics of g-C 3 N 4 , and to improve its action, scientists have proposed several hypotheses, such as the use of anabolic steroids with other substances.Such as metal oxides and sulfides, noble metals, and nanoscale carbon materials [13][14][15].Metal oxides or sulfides can be used to improve the function of g-C 3 N 4 by increasing the electrical conductance and stability properties.This is due to their appropriate structures [16][17][18][19][20][21][22].
The production of electrode materials with high-speed capacitors at a low cost is a great effort for energy storage devices, so it was necessary to conduct intensive research and advanced studies to know the materials that can be used in the production of high-speed capacitors.Zinc oxide (ZnO) has high electrical conductivity and optical properties.It is a compound that is used in many photovoltaic applications, gas sensors, energy storage, and conversion [23][24][25][26].Lu et al. they reached a maximum C S of 61.7 F/g for the thin film of carbon and zinc oxide, which was prepared by pyrolysis technique [27], and another report between the maximum C S of about 140 F/g for the G-ZnO nanocomposite [28].The preparation of metal oxide with high control morphology is performed through highly complex techniques with high cost [29][30][31][32]; moreover, the small efficiency or stability produced is still a great limitation for industrial applications.The incorboration of g-C 3 N 4 in the composite is proven to have a great effect on the charge storage; through this idea, some studies have fabricated supercapcitors by adding Ni(OH) 2 with g-C 3 N 4 in the paste of the device, the produced C S is affected by this behavior, in which the CS before and after g-C 3 N 4 is 14.2 and 20.5 F/g, respectively [33].Avinash et al. studied some composites for supercapacitor fabrications, but the discharge time still has a problem.Using the NiO, TiO2, and CeO2 composites, the discharge time is about 10, 5, and 20 s, respectively [34][35][36].
Recent studies have investigated the use of additive sulfide-graphene composites, including CuS/graphene, ZnS/ graphene, SnS/graphene, and CdS/graphene, for supercapacitors and charge storage applications.The incorporation of graphene into these materials has led to improved charge storage capabilities and enhanced electrochemical properties [37][38][39][40].Moreover, polymers produced by chemical or electrochemical oxidation, exhibit good conductivity as a result of polymerization reaction [41,42].Compared with other polymers, polypyrrole possesses high conductivity, high C S value, and perfect mechanical properties, and is considered one of the polymers that can be applied to electrodes [43,44].
In this study, the effect of polypyrrole materials on the performance of inorganic sulfur and carbon materials is demonstrated for charge storage applications for enhancement of the supercapacitor's efficiency and stability.A novel ZnS-ZnO/g-C 3 N 4 nanocomposite and polypyrrole material are prepared using the hydrothermal and oxidative polymerization methods, respectively.XRD, XPS, TEM, and SEM analyses have been carried out for confirming all the chemical and morphological properties.This ZnS-ZnO/g-C 3 N 4 nanocomposite without/with the physical addition of polypyrrole is applied for the supercapacitor (two-symmetric electrodes) through the study of the electrochemical parameters; Charge and cyclic voltammetry are studied, from which the supercapacitor Cs, E, and P values are determined.Moreover, the impedance and stability are studied well.The physical addition of polypyrrole represents great advantages for the enhancement of all the chemical and mechanical properties of this supercpacitor.

Preparation of ZnS-ZnO/g-C 3 N 4
ZnS-ZnO/g-C 3 N 4 nanocomposite is prepared depending on g-C 3 N 4 .This material is the product of the combustion of (10 g) urea (organic material rich with nitrogen) in the air environment.The combustion process is carried out for 2 h at 550 °C.Through this reaction, the urea material decomposes, and the g-C 3 N 4 material forms with its faint yellow color.
For the preparation of ZnS-ZnO/g-C 3 N 4 nanocomposite, 0.05 g g-C 3 N 4 is added to the zinc solution at pH 10.This solution is composed of 0.1 M (CH 3 COO) 2 Zn and (0.2 M) thiourea with the addition of oxidant material ((0.025M) K 2 S 2 O 8 ).After 12 h at 160 °C of hydrothermal reaction, the product powder is collected and dried at 60 °C for 3 h.Finally, the prepared composite is annealed at 300 °C (for 5 min), this process led to the insertion of ZnO in the composite (Fig. 1).

Polypyrrole Preparation
The preparation of polypyrrole is carried out through the oxidation process of (0.1 M) pyrrole using the oxidant (0.15 M) K 2 S 2 O 8 .This preparation process occurred from an acid medium (0.5 M HCl) and under room temperature conditions.Then the dark green precipitate is collected and dried at 60 °C for 6 h.

Hybrid Supercapacitor's Fabrication and the Electrochemical Study
The fabrication of this hybrid supercapacitor is carried out by loading a composite paste on each electrode with 1.0 cm 2 surface area.This paste is prepared using 0.04 g of ZnS-ZnO/g-C 3 N 4 mixed with 0.005 g graphite powder in a solution of 750 µl isopropanol and 100 µl nafion.For making this paste with high uniformity, it is stirred using a magnetic stirrer for 2 days.Then this paste is loaded on each electrode (0.003 g) for 1.0 cm 2 Au-electrode area.
The effect of polypyrrole is carried out through physical mixing 0.01 g of it with 0.03 g of ZnS-ZnO/g-C 3 N 4 composite in the previous solution.
The electrochemical study is carried out for the prepared hybrid supercapacitor using an electrochemical workstation (CHI608E), in which the separator, Whatman filter paper, is inserted between these electrodes.This paper is wetted with 1.0 M Na 2 SO 4 .The supercapacitor is tested by measuring the charge/discharge and cyclic voltammetry, through these parameters, the efficiency and capacitance values are determined (CS, E, and P).Moreover, impedance and stability are studied without/with the physical addition of polypyrrole.

Characterization
The chemical structure is characterized using analytical tools XRD(device model: X'Pert Pro, Holland) and photoelectron analyses (XPS).While the morphology of the materials is confirmed using the SEM (device model: ZEISS SUPRA, Germany) and TEM (JEOL JEM-2100, USA).

Analyses
The XRD of the graphitic carbon nitride is investigated in Figs.2a, from this figure, the main characteristic peak for g-C 3 N 4 is located at 27.2° for the growth direction (002), moreover, there is a small peak related to g-C 3 N 4 is located at 13.02° for the growth direction (100).These peaks confirm the construction of the g-C 3 N 4 materials under the combustion of urea, and they match with the previous literature (JCPDS) 87-1526 card [45,46].
Under the composite formation, ZnS-ZnO/g-C 3 N 4 , there are additional peaks are formed that are related to these materials.The peak that appears at 28.3° is related to the growth of ZnS for the growth direction (111), and, at the same time, it is related to g-C 3 N 4 .There are additional two characteristic peaks located at 47.3° and 76.6° for the growth direction (220) and (331) confirming the construction of ZnS materials [47].The ZnO material is inserted inside the composite during the combustion step at 300 °C for 5 min, this material is confirmed through the peak located at 56.3° for the growth direction (110) [48].
The XRD of the polypyrrole is shown in Fig. 2b, the curve shows the formation of semicrystalline material related to the semi-sharp peak located at 28.8°.The broad curve is normal for polymer materials related to the nature of their chemical structure [49].
XPS analyses of the ZnS-ZnO/g-C 3 N 4 nanocomposite are shown in Fig. 3 and the summarized data is mentioned in Table 1.The survey spectra for the nanocomposite are  spectra are mentioned in Fig. 3d, in which their peaks are located at 533.1 eV, this element is related to the ZnO material inside the composite.
On the other hand, the g-C 3 N 4 material is confirmed through the N and C elements spectra for 1S, in which their peaks are located at 400.9 and 286.2 eV, respectively.These peaks are related to many formed bonds such as N-C=N and C-C [11].
The morphological properties of the prepared g-C 3 N 4 , ZnS-ZnO/g-C 3 N 4 , and polypyrrole are studied well using  the SEM and TEM analyses as shown in Fig. 4. Figure 4a illustrates the morphology of the prepared g-C 3 N 4 , in which 2D sheets are formed with a high crystal structure.These 2D sheets have an average length, width, and thickness of 170, 80, and 8 nm, respectively.On the other hand, the composite ZnS-ZnO/g-C 3 N 4 has spherical shape particles (about 15 nm) that are agglomerate on each other, in which these particles are mixed well with the 2D g-C 3 N 4 sheets.The great sheets of the g-C 3 N 4 motivate the composite formation, in which these sheet morphologies increase the surface area and the active sites of the prepared composite.
Moreover, the morphology of the prepared polypyrrole material appears in Fig. 4c, this prepared material has great brain-like shapes with high wrinkles properties.Small particles around 10 nm agglomerate to form a larger shape with a diameter of about 220 nm.These properties for the polymer are promising, in which these small particles can physically composite well with the prepared ZnS-ZnO/g-C 3 N 4 .
The great morphology of the ZnS-ZnO/g-C 3 N 4 composite or the polymer material motivates energy storage, then these materials can be applied well in the supercapacitor.
The TEM image (Fig. 4d) confirms the preparation of 2D g-C 3 N 4 sheets with a dark color, in which the ZnS-ZnO particles are collected and agglomerate well around these sheets.The thickness of these 2D g-C 3 N 4 sheets is about 7.8 nm.
Theoretical simulated figures for g-C 3 N 4 and ZnS-ZnO/g-C 3 N 4 , respectively, using the Gwydion program are illustrated in Fig. 4e, f), respectively.Through these figures, the composite has a great roughness related to the incorporation of ZnS-ZnO over the smooth g-C 3 N 4 sheets.

The Electrochemical Study for the Hybrid Supercapacitor
The electrochemical study of the hybrid supercapacitor based on ZnS-ZnO/g-C 3 N 4 without/with polypyrrole added using the physical mixing is shown in Fig. 5a and b, respectively.while the calculated specific capacitance (CS) values for these supercapacitors are mentioned in Fig. 5c and d, respectively.The electrochemical study is carried out using the electrochemical workstation CHI608E.Each Au-metal electrode is loaded with 0.003 g paste of the prepared ZnS-ZnO/g-C 3 N 4 that is mixed with/without polypyrrole.Whatman filter paper wetted with 1.0 M Na2SO4 is used as a separator between these two plates.The supercapacitor testings are carried out through the study of the charge/discharge curves in potential windows from 0.0 to 1.0 V, by applying current density (J) from 0.5 to 1.3 A/g. Figure 5a and b have the same behavior, in which the charge curves decrease with increasing the current density from 0.5 to 1.3 A/g. this is normal in the previous literature, in which with increasing the current density, the supercapacitor can not carry charges well, this causes a decrease in the produced charge time [51,52].
For the composite, ZnS-ZnO/g-C 3 N 4 , the values of the charge time, at 0.5 and 1.3 A/g, are 52 and 3.2 s, respectively.These values enhances many times through the incorporation of polypyrrole in the past using the physical reaction, in which the charge time values are 274 and 25.5 s, respectively.This enhancement confirms the role of the prepared polypyrrole material for the enhancement of the composite adhesion on the electrode surface.Incorporating polypyrrole materials into the supercapacitor composites led to a remarkable improvement in the discharge time.Specifically, the discharge time increased from 5 to 60 s after the introduction of polypyrrole.This enhancement highlights the significant role played by this polymer in energy conservation, allowing for a longer duration of sustained energy release related to the redox reaction.The incorporation of polypyrrole materials appears to contribute positively to the overall performance and efficiency of the supercapacitor system.Moreover, this polymer material has good conductivity that plays a great role in the charge transfer to the ZnS-ZnO/g-C 3 N 4 composite material.In addition to the great surface area of the polypyrrole, it motivates the construction of more active sites for the materials.
Using the 1.0 M Na 2 SO 4 as an electrolyte is a good choice, as this electrolyte does not react with the composite materials, moreover, the Na + ions have great mobility that increases the charge transfer and then the charge storage through the hybrid capacitor properties of the composite supercpacitor [53,54].
The efficiency of the prepared ZnS-ZnO/g-C 3 N 4 hybrid supercapacitor without/with the physical addition of polypyrrole is determined through the calculation of the electrochemical parameters: specific capacitance (C S ) (Eq. 1) and the energy density (E) (Eq.2) [55,56].Moreover, the powder density (p) (Eq. 3) is calculated through the E value [55,56].These Equations depend on the current (I) value and potential window ( ΔV).Also, the Equation depends on charge time ( Δt) , the loaded mass (m), and the square values of potential ( ).The C S values are calculated through Fig. 5c and d for the ZnS-ZnO/g-C 3 N 4 supercapacitor without/with the physical addition of polypyrrole material, in which these values are 15.1 and 152.2 F/g, respectively (at 0.7 A/g).these values decrease with increasing the current density till reaching the minimum value at 1.3 A/g.At current density, 0.7 A/g, the E values for the ZnS-ZnO/g-C 3 N 4 supercapacitor without/with polypyrrole addition are 1.17 and 11.87 W h/kg, respectively.While the P values are 1053 and 737 W h/kg, respectively.These values confirm the role of the polypyrrole in the enhancement of the supercapacitor efficiency.
On the other hand, the effect of the incorporation of polypyrrole materials, inside the prepared ZnS-ZnO/g-C 3 N 4 composite, appears well through the enhancement of the cyclic voltammetry behavior.Through the potential window 0 to 1.0 V, the produced current density and the cyclic voltammetry area under the curve increase well in Fig. 6b in comparison to Fig. 6a.This behavior confirms the increase of the charge storage capacity inside the paste containing pyrrole, and the role of the surface area and conductivity of this material for the enhancement of the charge transfer and storage inside the supercapacitor.In both supercpacitor, the capacitance increases directly with the scan rate from 50 to 300 mV s −1 which confirms the effect of this scan rate on the charge stored on the plates [57,58].The construction material of the supercapacitor motivates the pseudo reaction for oxidation and reduction behavior.The incorporation of polypyrrole accepts this composite with the same properties by increasing these redox reactions, so the area of the cyclic voltammetry curve increases well after the incorporation of this polymer material with the ZnS-ZnO/g-C 3 N 4 composite.
The electrochemical impedance of the prepared ZnS-ZnO/g-C 3 N 4 composite supercapacitor without/with polypyrrole material is measured through the Nyquist plot using the real (Z) and imaginary (Z − ) resistance as shown in Fig. 7a and b, respectively.This relation represents the facility of charge transfer inside the electrolyte and through the electrodes and the electrolyte using 1.0 M Na 2 SO 4 solution.The small diameter semicircle after the incorporation of polypyrrole with the composite ZnS-ZnO/g-C 3 N 4 confirms (3) P = 3600E∕Δt Fig. 6 The electrochemical cyclic voltammetry of the prepared ZnS-ZnO/g-C 3 N 4 composite supercapacitor a without and b with the physical incorporation of polypyrrole in the paste the enhancement in the charge transfer through the enhanced paste [59].
Randles cell in Fig. 7b represents the various resistance used Rct and Rs represent the resistance of charge transfer and solution, respectively.While Cd and W represent the capacitor and Warburg impedance, respectively [60,61].The R s in the supercapacitor without/with polypyrrole are 6.9 and 6.65 Ω, respectively.While the values of the semicircle diameter (R ct ) are 0.37 and 0.25 Ω, respectively, these values confirm the great enhancement in the electric behavior of the composite after the incorporation of polypyrrole material.
The stability of the prepared hybrid supercapacitor, ZnS-ZnO/g-C 3 N 4 , without/with polypyrrole material is mentioned in Fig. 8a and b, respectively.The stability study is carried out by measuring the charge curve till 1000 runs using 0.1 M Na 2 SO 4 as an electrolyte.The capacitance retention values are enhanced by the incorporation of polypyrrole in the composite paste, this appears well through the charge/ discharge till 1000 runs, in which the capacitance retention values are 88.6 and 90.2% without/with polypyrrole addition till 500 runs of charge/discharge, respectively.These values decrease to 80.5 and 80.6% at 1000 cycles, respectively.So the polypyrrole material has an additional enhancement role in increasing the lifetime and stability of the prepared supercapacitor.

Conclusions
Spherical ZnS-ZnO/g-C 3 N 4 nanocomposite has been prepared by the hydrothermal technique, in which g-C 3 N 4 2D sheets of material are prepared using the combustion of urea.On the other hand, polypyrrole material is prepared using the oxidation polymerization method.All the composite properties are confirmed using the XRD, XPS, SEM, and TEM analyses.The g-C 3 N 4 material has an average length, width, and thickness of 170, 80, and 8.0 nm, respectively.The spherical particles of the ZnS-ZnO agglomerate around the 2D g-C 3 N 4 sheets.ZnS-ZnO/g-C 3 N 4 nanocomposite has pseudocapacitance properties that were confirmed through the study of the electrochemical parameters; cyclic voltammetry, charge, stability, and impedance.The C S values are 15.1 and 152.2 F/g, and the E values are 1.17 and 11.87 W h/kg without/ with the physical addition of polypyrrole, respectively.Also, the capacitance retention values are 88.6 and 90.2% without/with polypyrrole addition till 500 cycles.Soon, our team works on designing a prototype of the supercapacitor for industrial applications that is related to its high capacitance and easy preparation technique.

Fig. 2 Fig. 3
Fig. 2 The XRD for a g-C 3 N 4 and ZnS-ZnO/g-C 3 N 4 and b polypyrrole nanomaterials

4
The SEM analyses of a g-C 3 N 4 , b ZnS-ZnO/g-C 3 N 4 , and c polypyrrole.d TEM image of ZnS-ZnO/g-C 3 N 4 .e and f Theoretical simulated figures for g-C 3 N 4 and ZnS-ZnO/g-C 3 N 4 , respectively, using the Gwydion program

Fig. 5
Fig. 5 The electrochemical charge for the prepared ZnS-ZnO/g-C 3 N 4 supercapacitor a without and b with the incorporation of polypyrrole.The specific capacitance for this capacitor a without and b with the incorporation of polypyrrole under Na 2 SO 4 electrolyte

Fig. 7 Fig. 8
Fig. 7 The Nyquist plot of the prepared ZnS-ZnO/g-C 3 N 4 composite supercapacitor a without and b with polypyrrole material

Table 1
The XPS obtained peak location and the elements percent of the prepared ZnS-ZnO/g-C 3 N 4 nanocomposite