Preparation of PPy-WO3 Nano-composite for Supercapacitor Applications

In this work, polypyrrole (PPy) composites were chemically prepared by a chemical oxidation method. Also, Tungsten Trioxide (WO3) nanoparticles were prepared and added in certain proportions to PPy. The structure properties were studied for the polypyrole and tungesten trioxide separately before mixing them together. The X-ray diffraction (XRD) analysis revealed a hexagonal WO3 and a triclinic PPy. It was observed that the nano-composite prepared by the addition of WO3 with 10 and 20% volume ratios to PPy shows a triclinic phase with the presence of hexagons. The molecular structures of PPy, WO3, and PPy–WO3 nano composites were depicted by Fourier-transform infrared spectroscopy (FTIR) in the scope of 400–4400 cm -1 . The supercapcitors of PPy and PPy-WO3 were examined by calculating the figure of merit of the electrode (PPy and PPy-WO3) using cyclic voltammetry (CV), galvanostatic charge and discharge (GCD), and electrochemical impedance spectroscopy (EIS) techniques. It was found that the highest value of capacity (266.62 F/g) is obtained at 20% WO3 ratio.


Introduction
The conducting polymers (CPs) are suitable for favorable conditions to allow conducting the charge current of a simple electron by the polymer network in electrochemical procedures [1][2][3]. PPy is one of the most commonly used CPs considering its simplicity of blending, good conductivity, and

ISSN: 0067-2904
Hameed and Ibrahim Iraqi Journal of Science, 2021, Vol. 62, No. 5, pp: 1503-1512 1504 environmental stability [4]. Bonding is a primary necessity for PPy's electrical effects. The electrical conductivity of PPy is likely to be upgraded through catalysis processes that lead to the joining of counters such as cations, inorganic anions, natural molecules, etc. [5]. The unique properties of CPs open up new potential outcomes of mechanical processes; for example, in recharging batteries or biochemical allergens. CPs can store the capacitance by the mechanism of accumulation/release of countercharges due to redox reactions arising under the electric field [6][7][8][9][10]. The introduction of electrostatic polymers remains the subject of extensive examination by researchers around the world. These conductive polymers have unique electrochemical properties in terms of the ability to rapidly discharging steroids and doping. The aim of this process could be to trap the charge in all volumes. Only few types of conductive polymers have the property of super-condensation, such a poly (PPy), poly (1, 5-diamino anthraquinone), and polyaniline (PANI). PPy is a specially oriented polymer due to its high conductivity, high storage capacity, environmental stability, and strong redox current [11][12]. Generally, capacitors are built using a set of parallel connection plates that are separated by an insulator. The capacitive charge of the opposite sign builds up when voltage is applied to the plates in series. These conventional capacitors have low (mF) power values, but the supercapacitors have the high capacities (F) [20]. This study aimed to prepare an electronic electrode, investigating its structural properties, and then measuring the figure of merit for super capacitor, such as GCD, EIS, CD, and impedance 2. Experimental Work

Synthesis of PPy Nanostructure
The combination of polypyrrole nanostructure was built up based on the methodology described by Yang [14]. The initial step was built up with complete scattering of 0.818 g of methyl orange in 60 mL distilled water. Under slow stirring of 80 rpm and submerging in an ice bath, 2.237g of ferric chloride was scattered at fluid arrangement of methyl orange with incorporation of 1 mL of pyrrole. The blend procured a blackish appearance and was kept under stirring for 24 hours. The precipitant was filtered and washed with ethanol and distilled water many times, then vacuum-dried for 60 minutes.

Synthesis of WO 3 Nanoparticles
For the preparation of WO 3 nanoparticles, 0.1 g of WO 3 was dissolved with 100 ml of distilled water and stirred for 10 min at a temperature of 60 o C. Drops of ammonium hydroxide (NH 4 OH) were added until the pH was adjusted to 8. The mixture was filtered and washed with ethanol and distilled water many times, then vacuum-dried at 60 o C for 60 minutes. After that, the resulted material was placed in an oven at 400 °C for 1 hour to obtain a stable phase of tungsten oxide.

Preparation of PPy/WO 3 nanocomposite
An appropriate amount of WO 3 was dispersed in distilled water and added to PPy with volume ratios of 10%:90% and 20%:80% and subjected to ultrasonic waves for 1 hour to obtain a ready-to-use homogenous dispersion.

Preparation of the electrode
Nickel foam (1*1 cm 2 ) substrate was used for the preparation of the PPy:WO 3 as an electrode. PPy:WO 3 nanocomposite with ratios of 10%:90% and 20%:80%, respectively, was deposited on nickel foam substrate, by using the drop casting method. The prepared sample (PPy:WO 3 / nickel foam) was placed in 1 ml of concentrated sulphuric acid (100% H 2 SO 4 ) for testing the cyclic voltammetry (CV), galvanostatic charge-discharge examination (GCD), and electrochemical impedance spectroscopy (EIS). The mass of the electrode (PPy:WO 3 ) was 0.7mg. Figure 1 shows a schematic diagram of the prepared sample which was used as an electrode for the supercapacitor.

Analyzing X-ray diffraction
The structural analysis of PPy and the two concentrations of PPy-WO 3 hybrid nanocomposite were achieved by XRD. Figure 2(A) represents the XRD of PPy which indicates the wide diffraction peak at 2Ɵ = 17 -82, which characterized PPy for its one peak form arranged by substance oxidative polymerization strategy [15].  (111). From these results, it is concluded that PPy has a triclinic shape whereas WO 3 is hexagonal. Nonetheless, PPy-WO 3 nanocomposite appeared as a mix of triclinic and hexagonal shapes. In addition, it was observed that the WO 3 nanoparticles have significant effects on PPy, making a hybrid nanocomposite material with polycrystalline structure [16]. This observation proposes homogeneous scattering of the WO 3 nanoparticulates in the PPy network without any agglomeration by a specified percentage of WO 3 . This result corresponds to that shown by an earlier work [17].

Fourier Transform Infrared analyzes
The chemical composition of pure PPy and PPy-WO 3 nanocompounds was examined by FTIR spectrometry. within the scope of 400-4000 cm -1 , as illustrated in Fig. 3. A characteristic peak at 786 cm -1 is assigned to the C-H vibration, which shows deformation of planes at 1039 cm -1 . The peak at 1116 cm -1 is ascribed to C-H inside and outside the planes deformation. The N-C stretch band and the C-H located inside the planes vibration are observed at 1209 cm -1 and1290 cm -1 , respectively. The C-C asymmetric wave extension and the stretching modes for the pyrrole ring are seen at 1467cm -1 and 1533 cm -1 , respectively [18][19]. Figure 3 displays the large frequency band at 700 cm -1 as compared to the W-O-W stretch vibrations of the crystallized WO 3 . The conspicuous spikes at 1633 cm -1 might be because of the O-H vibrations. The spike at 3014 cm -1 indicates an -OH assembly of tungsten oxide [19]. The results indicate that the WO 3 nanoparticles have a well crystalline structure when calcinated at 400 0 C. The FTIR spectra of PPy-WO 3

Figure of merit of supercapacitor 3.3.1 Cyclic voltammetry
The results of cyclic voltammetry analysis of PPy and PPy-WO 3 films are portrayed in Fig. 4 (A-C). By repeating the scan rate (50, 70,100) mV several times, we notice the clear peaks of the anode and cathode at (3.47, 3.54, 5.27), (3.41, 5.89, 8.82), and (6.96, 8.92, 12.5) mA, respectively. CV outcomes showed that the materials are electrochemically dynamic in the potential window. The PPy-WO 3 mixture film incorporates the electrochemical activities of both PPy and WO 3 . As observed by figure 4, the nano WO 3 -PPy membrane shows an observable electrochemical response below a positive potential of 0.3 V. The cathode electrode will be the charge storage for positive charge carriers inside the WO3, while the anode electrode will be the positive charge emitter. This mechanism takes place during the discharge process. During the forms of CVs, the cathode side becomes black while the anode becomes white. The Gaussian shape in figure 4 reveals that the electrochemical interaction has a great significance in PPy-WO 3 composites, in contrast to pure PPy or WO 3 . This could be well clarified using the synergistic impact of PPy and WO 3 . In each curve in figure 4(A-C), the two strong redox peaks indicate a supercapacitor pseudo-capacitative property due to the reactions to the Faradaic redox [20]. Similar behavior was reported for the polyaniline-WO 3 composite membrane [20].

3.3-2 The stability of Cyclic voltammetry
The long-term cycle stabilization of PPy and PPy-WO 3 was evaluated by setting the cyclic voltammetry to 300 cycles in 1M H 2 SO 4 , as an electro-chemical solution, at a velocity of 50 (mV.S -1 ). The results of the specific capacity, which is determined by the cyclic voltammetry test, are demonstrated in figures 5A and 5B. PPy-WO 3 was shown to maintain 80% specific capacity after 300 cycles. However, the specific capacity was increased with the increase in cycle number through the charge/discharge operation. The contact molecular collaboration between the WO 3 and PPy nanostructures confines the differences in the normal system frame through the charge/discharge process and improves the cycle stabilization of PPy-WO 3 . The all-encompassing system frame is likewise liable for improved cycle stabilization of PPy-WO 3 [21]. The mass of the electrode of PPy or PPy-WO 3 is 0.7mg. The electrode stability can be calculated from the following equation [27]: Stability= (C n /C 1 ) x100 where C n is the capacitance of the electrode in each cycle and C 1 is the capacitance of the first cycle. We found that the capacity of the pure PPy is (160, 215, 175, 181) F.g −1 . However, by adding 10% vol. of WO 3 to the PPy, we obtained a capacity of (166, 240, 209,213) F.g −1 , while when the ratio was increased to 20%, we recorded a capacity of (236, 266, 262,262) F.g −1 .

Galvanostatic charge and discharge examination
The electrochemical performance of the hybrid PPy-WO 3 electrode was conducted using galvanostatic charge / discharge test. Potential varieties for single electrodes were reported separately when a Nifoam reference anode is added to ensure that the hybrid system will operate at the most notable work potential window. The potential varieties are practically slanting with one and two stages for the negative and positive electrodes (presence of protrusions in the case of charging and discharging), respectively, which concur with the CV results shown in Fig.4. This is credited to the potentialsubordinate nature of redox responses. The PPy is a p-doped (anion-inserting) polymer. This endless supply of the electrons from an unbiased fragment of their quinoid chains leads to some local positive charge distortion, making a polaron. In the event that the oxidation procedure proceeds further, more electrons must be expelled from the electrodes, bringing about the development of cation radicals, or bipolarons, which are vigorously liked to the arrangement of two separate polarons. In the event that the weight proportion PPy-WO 3 is not actually tuned, during the charging procedure, the oxidation and reduction of the WO 3 and the PPy electrodes, respectively, would not be complete, and the hybrid framework would work at a lower working potential window than anticipated [22,24]. Nonetheless, during the charge/discharge forms (Fig. 6), both the PPy and the PPy-WO 3 electrodes work sensibly over the normal potential extents. Also, the hybrid composite PPy-WO 3 electrode can work at the working potential window 0.3V. The specific capacitance can be calculated from the CD curve using the following equation [23] C= I. ∆t/∆V. m where C (F.g −1 ) is specific capacitance, ∆V (V) is potential window (here 0.3V), I (A) is discharge current (here 8 mA), ∆t (s) is discharge time (4.22 s for pure PPy, 6.2 s for 10% vol. of WO 3 , 7 s for 20% vol. of WO 3 ), and m (g) is the mass of the electrode (PPY or PPY-WO 3 , here 0.7mg) [24]. We obtained a capacity of pure PPy of 160 (F.g −1 ) and, by adding 10% vol. of WO 3 to the PPy, we obtained a capacity of 240(F.g −1 ), whereas when adding a quantity of up to 20% vol. of WO 3 , we reached a capacity of 266.62 (F.g −1 ). This might be due to the attraction between the tungsten oxide and polypyrrol nanoparticles, which leads to an increase in the surface area and, consequently, to an increase in the capacitance. The maximum value of the capacity was 266.6 (F/g), which it was obtained in 1 M H 2 SO 4 electrolysis [13] and PPy plate -WO 3 nanocomposites by the oxidative polymerization technique.  Figure 7 displays the subsequent Nyquist EIS spectrum plots for the PPy, WO 3 , and PPy-WO 3 complex anodes. A total spectrum made out of a high frequency arc was running and rising in the weak frequency extension. The arc on the real axis at high frequency indicates the equal series impedance. It is identified with the impedance of the electrical chemical solution, the total impedance of the internal and interfacial touch resistance of the electrolyte, the surface substance, and the current collector. The arc width is connected to the anode's load-transfer resistance (Rct). This proposes that the simultaneous arrangement of a PPy-WO 3 covering has a contrast between the synergistic effect of both PPy and WO 3 and that of the individual segments alone. The decreased resistance of the PPy-WO 3 terminal additionally shows a quicker Faradaic response, which is answerable for increased performance quality. Furthermore, at low frequencies, the PPy-WO 3 anode has a more perpendicular form than that of the pure PPy and pure WO 3 , with good capacitive conduct and decreased diffusion impedance [25,26].

Figure 7
Nyquist plots of the electrochemical impedance spectroscopy for the PPy, WO 3 and PPy-WO 3 hybrid composite electrodes.

Conclusions
The PPy-WO 3 nanocomposites were prepared successfully by a simple oxidative method. The PPy-WO 3 nanocomposite has a higher electrochemical movement than that of the pure PPy or WO 3 . The highest cyclic voltammetry stability of PPy-WO 3 was at 20% of WO 3 , where a large specific capacity of 266.62 (F.g −1 ) was estimated. This capacitance percentage is greater than that of the pure PPy 215 (F.g -1 ). At low frequencies, the PPy-WO 3 anode has a more perpendicular form as compared to that of the pure PPy and pure WO 3 .