Composites Based on Poly(ortho-toluidine) and WS₂ Sheets for Applications in the Supercapacitor Field

Abstract

In this work, three methods for the synthesis of composites based on poly(ortho-toluidine) (POT) and WS₂ are reported: (a) the solid-state interaction (SSI) of POT with WS₂ nanoparticles (NPs); (b) the in situ chemical polymerization (ICP) of ortho-toluidine (OT); and (c) the electrochemical polymerization (ECP) of OT. The preparation of WS₂ sheets was performed by the ball milling of the WS₂ NPs followed by ultrasonication in the solvent N,N’-dimethyl formamide. During the synthesis of the POT/WS₂ composites by SSI and ICP, an additional exfoliation of the WS₂ NPs was reported. In this work, we demonstrated the following: (a) the ICP method leads to POT/WS₂ composites, which contain repeating units of POT in the leucoemeraldine salt (LS) state, while (b) the ECP method leads to POT/WS₂ composites, which contain repeating units of POT in the emeraldine salt (ES) state. Capacitances equal to 123.5, 465.76, and 751.6 mF cm⁻² in the cases of POT-ES/WS₂ composites, synthesized by SSI, ICP, and ECP, respectively, were reported.

1. Introduction

Composites based on conducting polymers and transition metal dichalcogenides (TMDs) have been of particular interest as a result of applications in the fields of photocatalysis [1], gas sensors [2], and, last but not least, supercapacitors [3,4,5]. The methods often used for the synthesis of polyaniline (PANI)/WS₂ composites are as follows: (i) the method of the chemical interaction of nanolayers of WS₂ with PANI in a solid state [3]; (ii) the ECP of a monomer in an anhydrous solution of H₂SO₄, acetonitrile, and WS₂ microparticles [4]; and (iii) the exfoliation of WS₂ microparticles in butyl lithium, followed by the ICP of aniline [6]. The use of TMDs in the bulk state in supercapacitors (SCs) has often been avoided due to their small surface area (SA). The large SA of individual TMD sheets and higher oxidation numbers (e.g., in MoS₂ from +2 to +4) allow TMDs to store electrostatic charge as well as ionic interlacing in the interlayer space, leading to a specific capacitance and an energy density with higher values [7,8]. The main methods for TMD exfoliation are as follows: (a) mechanically; (b) in the liquid phase, using organic solvents or surfactants; (c) ion intercalation; and (d) grinding [9]. Concerning the particular case of WS₂ nano- and microparticles, among the main strategies adopted for the exfoliation of these TMDs are (i) liquid-phase exfoliation assisted by the ultrasonication of the WS₂ crystals in surfactant aqueous solutions [10] or organic solvents [11] as well as superacid (chlorosulfonic acid) [12]; (ii) chemical exfoliation performed in three stages involving magnetic stirring, horn-tip sonication, and shear mixing [13]; (iii) electrochemical exfoliation at a DC potential of 10 V [14]; (iv) solvothermal reaction [15], etc. The methods often used to illustrate TMD exfoliation are X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman scattering, and X-ray photoelectron spectroscopy (XPS), e.g., in [10,11,12,13,14,15]. A disadvantage of liquid-phase exfoliation assisted by the ultrasonication of WS₂ crystals is the long time taken to obtain the nanolayers as a consequence of the successive ultrasonication and centrifugation stages. In this work, special attention will be paid to obtaining WS₂ sheets by grinding WS₂ NPs followed by ultrasonication for a short period in N,N’-dimethyl formamide (DMF).

The low conductivity in the most stable phases of TMDs prevents their use as potential electrode materials in supercapacitor cells. Conductive polymers such as PANI [16] are often used in the field of supercapacitors as a result of their high specific capacitance, electrical conductivity >0.001 S/cm, manufacturing methods that do not require expensive equipment and high flexibility, ability to be obtained in different oxidation states such as leucoemeraldine, emeraldine, and pernigraniline, which correspond to reduced, semi-oxidized and oxidized states, and obtaining of polymers in undoped and doped states, respectively [17]. The following methods have been used to synthesize PANI/WS₂ composites: (i) the ICP of aniline using WS₂ sheets resulting from the exfoliation of WS₂ powder in water/isopropanol mixtures [18]; (ii) the interaction of the two constituents, e.g., in [3,19]; and (iii) the ECP of WS₂ dispersed in a mixture of aniline and acid solution, e.g., in [4]. According to Ref. [20], a disadvantage of PANI is its rigid structure, which can influence the capacitance of supercapacitors. An improvement in the processability of PANI was reported by replacing aniline with ortho-substituted aniline, with functional groups of the types -CH₃, -OCH₃, etc. [21]. One of the important properties of POT is its conductivity, which varies between 10⁻⁴ and 2 × 10⁻² S/cm [22,23]. In this study, our effort will be given to the synthesis of composites based on WS₂ and POT and the evaluation of their optical, structural, and electrochemical properties. The following strategies will be used for the synthesis of POT/WS₂ composites: (i) the SSI of POT with WS₂ NPs; (iii) the ICP of ortho-toluidine (OT) with the addition of WS₂ NPs; and (iii) the ECP of OT in a semi-aqueous acid solution containing WS₂ sheets in DMF/H₂O (the volumetric ratio was 1:1). Using correlated studies of XRD and SEM, new evidence concerning the exfoliation of WS₂ NPs in the absence and presence of POT will be shown. The chemical interactions at the interface of the two constituents of the POT/WS₂ composites will be highlighted by Raman scattering, FTIR spectroscopy, and X-ray photoelectron spectroscopy (XPS). Using cyclic voltammetry (CV), new information concerning the capacitive and diffusion-controlled intercalation processes and their contributions to the current densities of cyclic voltammograms of the symmetrical supercapacitor cells that have electrodes containing the POT/WS₂ composites as active materials will be shown. An assessment of the dependence of the capacitance of the symmetrical supercapacitors as a function of the potential scan rate and cyclic voltammogram number will also be reported.

2. Materials and Methods

The supplier for the compounds OT, H₂SO₄, DMF, K₂Cr₂O₇, NH₄OH, CH₃CN, poly(vinylidene fluoride) (PVDF), dibutyl phthalate (DBP), acetone, diethyl ether, Nafion 117 membrane, and WS₂ NPs was Sigma-Aldrich (St. Louis, MO, USA).

To prepare the WS₂ sheets, a powder containing WS₂ NPs was used, which were subjected to mechanical exfoliation using a ball mill model PM 100—RETSCH and SamplePrep 8000 M shaker mill from SPEX, USA. Using the first ball mill, 1 g of WS₂ NPs was subjected to a grinding time of one hour at 150, 300, and 600 rpm. With the second mill, the grinding of 1 g of WS₂ NPs was carried out for an hour of effective milling time at 900 cycles/min. Milling periods of 5 min were interspersed with 5 min pauses to avoid excessive heating of samples. The changes induced in the crystalline structure of the WS₂ NPs by the exfoliation process were analyzed by XRD. An additional exfoliation after grinding WS₂ NPs for one hour at 900 rpm was performed by ultrasonicating a suspension of WS₂ in DMF for 15 min, leading to a yellowish solution with a concentration of 0.5 mg/mL.

The chemical synthesis of poly(o-toluidine)–emeraldine salt (POT-ES) and poly(o-toluidine)–emeraldine base (POT-EB) involved the use of two solutions of OT 0.18 M and K₂Cr₂O₇ 0.05 M, each in 50 mL H₂SO₄ 2M. Adding the solution of K₂Cr₂O₇ in H₂SO₄ to the solution of OT in H₂SO₄ induces a change in the reaction mixture color from orange to dark green. After two hours, the chemical polymerization reaction led to a green precipitate, which corresponds to POT-ES. The filtration of POT-ES and its interaction with 1 M NH₄OH solution led to obtaining POT-EB, when a change in the color of the precipitate to dark blue occurred. Further, the interaction with 500 mL of distilled water and 100 mL of CH₃CN allowed the removal of oligomers from the reaction product. The polymerization reaction products, i.e., POT-ES and POT-EB, were dried at room temperature for 24 h to constant mass.

The ICP of the POT/WS₂ composites, having TMD concentrations of 20 wt.% and 10 wt.%, was performed as described above; the only difference was the addition of 0.1 g and 0.01 g of WS₂ NPs to the OT solution in H₂SO₄.

Another method for the preparation of the POT-EB/WS₂ composites was the SSI of POT-EB with WS₂ NPs when the TMD concentration in the composite mass was equal to 33 wt.% and 50 wt.%.

Electrochemical synthesis of POT-ES involves the use of a semi-aqueous solution of OT 0.1 M in H₂SO₄ 0.5 M with a volumetric ratio of DMF/H₂O equal to 1:1.

The electrosynthesis of the POT-ES/WS₂ composite involved the preparation of (i) a WS₂ solution in DMF (50 mL, 0.5 mg/mL) by ultrasonication of the TMD in DMF for 15 min and (ii) an aqueous solution of 0.1 M OT in 0.5 M H₂SO₄ (50 mL). Subsequently, the solutions were interacted and homogenized by ultrasonication for 5 min. The reaction mixture was placed in a single-compartment cell, which was accessorized with three electrodes as follows: a Pt working electrode with an area of 1 cm², an auxiliary electrode made of Pt, and a Ag/AgCl reference electrode. Cyclic voltammograms were recorded in the potential range (−200; +900 mV) vs. Ag/AgCl at a potential scan rate of 50 mV/s, with a Voltalab 80 potentiostat/galvanostat, purchased from Radiometer Analytical.

XRD diagrams of the POT/WS₂ and POT-ES/WS₂ composites were recorded with Bruker’s D8 Advance X-ray diffractometer (Bruker, Hamburg, Germany).

The SEM images of WS₂ NPs before and after exfoliation as well as their composites with POT-EB were recorded with the Zeiss Gemini 500 scanning electron microscope (Zeiss, Oberkochen, Germany).

The Raman spectra of WS₂, POT-EB, POT-ES, and the POT-ES/WS₂ and POT-EB/WS₂ composites were recorded using an FTRaman spectrophotometer, MultiRam model, from Bruker (λ_exc = 1064 nm, (Ettlingen, Germany)).

To record IR spectra of POT-EB, POT-ES, and the POT-ES/WS₂ and POT-EB/WS₂ composites, we used an FTIR spectrophotometer, model Vertex 80, from Bruker (Billerica, MA, USA), in the transmission geometry mode for compounds prepared by SSI and ICP, while for composites deposited as films on the Pt electrode, the totally attenuated reflection geometry was used.

To record the UV-VIS spectrum of WS₂ sheets, we used a Lambda 950 UV-VIS-NIR spectrophotometer from Perkin-Elmer (PerkinElmer, Inc., Waltham, MA, USA).

The XPS spectra of WS₂, POT-EB, POT-ES, and the POT-ES/WS₂ and POT-EB/WS₂ composites were recorded on a SPECS spectrometer (SPECS Gmbh, Berlin, Germany) with a Phoibos 150 electron energy analyzer and a monochromatic X-ray source of the type Al Kα 1486.74 eV.

The testing of composites as electrode active materials (EAMs) in SC cells was performed by CV. To prepare electrodes, we used a mixture of 80 wt.% EAM, 5 wt.% PVDF, 15 wt.% super-P conductive black carbon, 0.1 mL DBP, and 1 mL acetone, which were mixed for 12 h under magnetic stirring to obtain a homogeneous paste. A film was obtained after the evaporation of the acetone from the paste poured onto the glass slides. To remove DBP from the film to be used as an electrode in SCs, an interaction with diethyl ether was performed. The electrodes for SC cells were in a circular form with a diameter of 8 mm obtained by cutting above the film. The electrolyte used in the SC study was a Nafion 117 membrane treated with a 1 M H₂SO₄ solution [21].

3. Results and Discussion

3.1. Exfoliation of the WS₂ Nanoparticles as Well as Their Optical and Structural Properties

Figure 1(a₁,a₂) show SEM images of WS₂ flakes. According to Figure 1(a₂), WS₂ flakes have a diameter between 260 and 460 nm and a width between 30 and 78 nm. Figure 1 shows that the thinnest WS₂ sheets were obtained when the grinding of WS₂ flakes took place at 900 rpm for one hour.

Figure 2 shows an SEM image of the WS₂ sheets deposited onto the Si plate after the evaporation of DMF from the solution prepared using WS₂ sheets, resulting from grinding flakes for one hour at 900 rpm, and then its dispersion by ultrasonication in DMF.

Figure 2 shows WS₂ sheets of various sizes, some of which are rolled.

Information confirming the formation of WS₂ sheets is presented by XRD in Figure 3. Figure 3a shows the XRD diagram of WS₂ nanoparticles, where diffraction maxima are observed at 2θ angles of cca. 14.3°, 28.8°, 32.8°, 33.5°, 39.6°, 43.9°, 49.7°, 58.4°, 59.8°, and 60.3°, belonging to the crystalline planes (002), (004), (100), (101), (103), (006), (105), (110), (008), and (112) [PDF 00-008-0237]. An important detail highlighted in Figure 3a is that the peak intensity of the crystalline plane (002) is 3.58 × 10⁵ counts. According to Figure 3c, it is observed in the XRD diagram that mechanical exfoliation induces a significant decrease in the intensity of the maximum corresponding to the crystal plane (002) to cca. 6.53 × 10³ counts, accompanied by a diminution in the intensity of the maxima corresponding to the crystalline planes (004), (006), and (008).

These variations are typical for the exfoliation of WS₂ particles, and to support this claim, an example may be the case of exfoliation of WS₂ particles mixed with lithium halides [24] or organic solvents such as ethanol, acetone, methanol, isopropyl alcohol, dimethyl formamide, and N-methyl pyrrolidone [24].

The Raman spectra of WS₂ NPs before and after their exfoliation are shown in Figure 4. According to Figure 4a, two intense Raman lines of WS₂ NPs with maxima of 357 and 422 cm⁻¹ are observed. These are attributed to the E_2g¹ and A_1g vibrational modes, respectively, that must be understood as related to in- and out-of-plane vibrations of W and S atoms [25].

Raman lines at 126, 168–172, and 314 cm⁻¹ were assigned to the vibrational modes A_g, B_u, and A_g of 1T’-WS₂ [26]. The Raman line at 200 cm⁻¹ belongs to the vibrational mode E’’(M) ‒ ZA(M) [27]. The Raman line that peaks at 150 cm⁻¹ is situated no longer in terms of wavelength than that reported at 148 cm⁻¹, which is characteristic of monolayer WS₂ [28]. In Figure 4b, we can observe two Raman lines of WS₂ sheets associated with vibrational modes E_2g¹ and A_1g, which show a shift at cca. 354 and 420 cm⁻¹, which has been attributed to the exfoliation of WS₂ microparticles [24]. In the context of the exfoliation process of WS₂ NPs, an important parameter is the distance between the two Raman lines associated with vibrational modes E_2g¹ and A_1g and the ratio between their intensities (I_Eg/I_Ag). Careful analysis of Figure 4a,b reveals that (a) the distance between the Raman lines associated with vibrational modes E_2g¹ and A_1g is equal to 65 and 66 cm⁻¹, respectively, and (b) the I_Eg/I_Ag ratio is equal to 1.8 and 1.1, respectively.

The UV-VIS spectrum of WS₂ sheets mechanically exfoliated at 900 rpm, which were further dispersed into DMF by ultrasonication for 15 min (Figure 5), shows a band at 632 nm, accompanied by another band presenting one shoulder at 530 nm and another one at 454 nm, which were attributed to the exciton band, the transition of the indirect exciton, and the optical transition between the valence and conduction bands specific to the semiconductor character of the direct forbidden band [29].

3.2. POT/TMD Composites Obtained by SSI and ICP

According to Figure 6a, the SSI of the constituents of the POT-EB/WS₂ composite involves the exfoliation of WS₂ nanoparticles, which was evidenced in the XRD by decreasing the peak intensity of the crystal planes (002) from 6.35 × 10⁵ counts/sec (black curve in Figure 6a) to 3.58 × 10⁵ counts/sec (blue curve in Figure 6a) to 1.11 × 10⁵ counts/sec (red curve in Figure 6a).

This exfoliating process of WS₂ nanoparticles is also observed in the ICP of OT assisted by WS₂ NPs. In this context, the diffraction peak intensity of the crystalline plane (002) of WS₂ varies from 6.35 × 10⁵ counts/sec (black curve in Figure 6a) to 1.74 × 10⁴ counts/sec (Figure 6b) for the sample with a WS₂ concentration equal to 20 wt.% in POT-EB/WS₂ composite mass.

The analysis of Figure 7a,b indicates that Raman spectra of the POT-EB and POT/WS₂ composite show Raman lines that belong to (a) WS₂, these being situated at 359 and 421 cm⁻¹, and (b) POT, these being localized at 1119, 1176, 1259, 1329–1363, 1499, and 1620 cm⁻¹ and attributed to the vibrational modes of deformation of the benzene ring (B) + C-H bond in the CH₃ group, the C-H bond in B, the stretching C-N bond + stretching C-C + C-H bonds in B, the stretching C-C bond in B, the stretching C-C bond in the quinoid ring (Q) + C-H bond in B, and the stretching C-N bond and stretching C-C + C-H bond in B [30,31]. The Raman lines at 1119, 1259, 1363, and 1620 cm⁻¹ are closely situated to those reported in the POT-EB (Figure 7a) that peaked at 1117, 1254, 1367, and 1618 cm⁻¹. In contrast with the Raman line of POT-EB at 1163 cm⁻¹ (Figure 7a), the Raman line of the POT/WS₂ composite is situated at 1176 cm⁻¹ (Figure 7b) as a consequence of the generation of POT–emeraldine salt (ES).

The difference between vibrational modes E_2g¹ and A_1g of the POT/WS₂ composite is less than that obtained by the mechanical exfoliation of WS₂ NPs followed by ultrasonication in DMF. This fact indicates that the protocol for the preparation of WS₂ sheets, shown in the previous section, is most suitable for preparing composites based on POT and WS₂ sheets.

Information on potential interactions between constituents of the POT-EB/WS₂ composite, synthesized by ICP of OT in the presence of WS₂ NPs, is shown in Figure 8. Figure 8a shows the IR spectrum of POT-EB, which is characterized by IR bands located at 444, 816, 879, 1004, 1111, 1151, 1242, 1306, 1497, and 1601 cm⁻¹, attributed to vibrational modes of B deformation, out-of-plane deformation of the substituted B in the even position, deformation of triple-substituted B, deformation of B, deformation of the B + C-H bond in B, the C-H bond in-plane deformation of Q, stretching of the C-N bond in the B-N=Q structure, stretching of the C-N bond in Q, stretching of C-C + C-H + N-H bonds, and the structure of N=Q=N [32]. According to Figure 8b, the following vibrational changes are induced in POT by WS₂: (a) the IR bands are down-shifted from 1111, 1242, 1497, and 1601 cm⁻¹ (Figure 8a) to 1105, 1213, 1490, and 1587 cm⁻¹ (Figure 8b), accompanied by an up-shift in the IR band from 1306 cm⁻¹ (Figure 8a) to 1313 cm⁻¹ (Figure 8b); (b) a change in the absorbances ratios of the IR bands situated at (b₁) 1242–1213 cm⁻¹ and 1497 cm⁻¹ (I_1242-1213/I₁₄₉₇) as well as (b₁) 1497 cm⁻¹ and 1587–1601 cm⁻¹ (I₁₄₉₇/I_1587-1601) from 1.16 and 1.76 to 3.73 and 1, respectively. The IR bands that peaked at 3381 and 3730 cm⁻¹ (Figure 8a) were assigned to the N-H symmetric stretching [33] and free OH group [34] vibrational modes. In contrast to Figure 8a, the FTIR spectrum of the POT/WS₂ composite highlights the two IR bands situated at 3371 and 3729 cm⁻¹, the last one being accompanied by a new IR band at 3628 cm⁻¹ assigned to the stretching OH vibrational mode [35]. An explanation for the OH vibrational mode must consider the generation of new hydrogen bonds between POT/WS₂ and H₂O molecules, according to Scheme 1.

In the case of the chemically synthesized POT/WS₂ composite, the following is observed by deconvolution: (a) In the XPS C1s spectrum, there are two bands of high intensity, with the maximum at 284.5 and 286.3 eV, which are accompanied by another five bands of lower intensity located at 287.3, 285.3, 284.8, 288.9, and 291 eV belonging to POT-EB (Figure 9a). The band from 291 eV corresponds to the π-π* transition of the aromatic ring of POT [36]. The bands that peak at 284.5, 284.8, 285.3 and 286.3 eV are assigned to bonds C=C (C hybridized sp²), C-C/C-H, C-N/C=N, and C-O-H [37]. The bands that peak at 287.6 and 288.9 eV are assigned to the C=O and -COOR bonds existing as contaminants in the XPS installation. (b) In the XPS N1s spectrum, there are three bands located at 399.2, 399.9, and 401.4 eV (Figure 9b) that are assigned to bonds C-N=C [37], C-NH-C [37] and C-N⁺ [38] of POT. (c) In the XPS W4f spectrum, three bands at 32.8, 34.9, and 35.9 eV (Figure 9c) are highlighted. The intense bands from 32.8 and 34.9 eV correspond to the doublet W 4f^7/2 and W 4f^5/2, which are separated by cca. 2.1 eV, the ratio of the areas of the two bands being 4:3 [31]. The low-intensity band from 36.7 eV is associated with the 4f^7/2 band of the tungsten sulfate adsorbed from the manufacturing process. (d) In the XPS S2p spectrum, four bands with maxima at 162.4, 163.6, 168.4, and 169.6 eV (Figure 9d) are remarked. The first two bands correspond to the 2p^3/2 and 2p^1/2 states, which are separated by cca. 1.18 eV, the ratio of the areas of the two bands being 2:1. The doublet from 162 to 163 eV represents 78% S from WS₂, while the doublet from 169 to 170 eV corresponds to 22% S from W(SO₄)₃. This fact indicates the presence of W(SO₄)₃ as a reaction byproduct in the WS₂ sample [31].

Considering the variations presented above, the total reaction of ICP of OT is shown in Scheme 1.

In our opinion, Scheme 1 explains the existence of the C-N=C, C-NH-C, and C-N⁺ bonds in the POT-EB/WS₂ composites resulting from the ICP of OT in the presence of WS₂ NPs.

3.3. Electrochemical Synthesis of POT/WS₂ Composite and Its Optical Properties

Figure 10 shows the first 50 cyclic voltammograms recorded in the potential range (−200; +900) mV vs. Ag/AgCl, using a semi-aqueous solution of 0.1 M OT in 0.5 M H₂SO₄ (the volumetric ratio of DMF/H₂O was 1:1) and 0.5 mg/mL WS₂ sheets exfoliated according to the protocol reported in this work.

The main changes in the cyclic voltammogram profile whilst increasing the cycle number recorded on the working electrode surface (Figure 10) can be quantified in the case of the following: (a) the first cyclic voltammogram, when the potential scanning takes place from (a₁) −200 to +900 mV vs. Ag/AgCl at the oxidation maximum with a potential of +418 mV, which is accompanied by another one with a potential of +246 mV, with the current densities of the two oxidation maxima at +418 and +246 mV equal to 2.12 mA/cm² and 1.06 mA/cm², respectively, and (a₂) +900 mV to −200 mV vs. Ag/AgCl, by a reduction maximum characterized by a potential equal to +334 mV, which is accompanied by another one with a potential of +200 mV; the two reduction maxima from +334 and +200 mV are characterized by current densities equal to 2.25 and 1.83 mA/cm², respectively; (b) the 50th cyclic voltammogram, when the potential scanning occurs from (b₁) −200 to +900 mV vs. Ag/AgCl, by an oxidation maximum with potential of +414 mV that is accompanied by another maximum with potential of +248 mV (the current densities of the two oxidation maxima at +414 and +248 mV are equal to 2.67 mA/cm² and 1.48 mA/cm²), and (b₂) +900 mV to −200 mV vs. Ag/AgCl, where a reduction maximum characterized by a potential equal to +328 mV is observed, which is accompanied by another one with a potential of +198 mV; the two reduction maxima from +328 and +198 mV are characterized by current densities equal to 2.73 and 2.33 mA/cm², respectively. Regardless of the number of cyclic voltammograms recorded at the electrolyte/electrode interface, it is observed that two oxidation–reduction processes take place, which are evidenced by oxidation and reduction maxima with anodic and cathodic potentials located at (i) +418 mV and +334 mV as well as +246 mV and +200 mV in the first cyclic voltammogram, and (ii) +414 mV and +328 mV as well as +248 mV and +198 mV, respectively, in the 50th cyclic voltammogram. The above variations are accompanied by a progressive increase in the density of anodic and cathodic currents as the cyclic voltammogram number increases. The two oxidation–reduction processes are characterized by a potential difference of ∆E = E_a − E_c, (where E_a and E_c are anodic and cathodic peak potential), equal to (i) 84 mV and 46 mV for the first cyclic voltammogram and (ii) 88 mV and 50 mV in the case of the 50th cyclic voltammogram. The ratio between the peak current densities of the anode to cathode (i_a/i_c) is equal to (i) 0.94 and 0.58 for the first cyclic voltammogram and (ii) 0.97 and 0.63 in the case of the 50th cyclic voltammogram. Analyzing the above results, it is noticed that there are oxidation–reduction processes for which ∆E differs by 56.5 mV/n, where n is the number of electrons involved in the oxidation–reduction process, and/or the ratio of anodic to cathodic current densities (i_a/i_c) differs from one, facts which allows us to conclude that the ECP of OT onto a Pt electrode in the presence of WS₂ sheets is an irreversible process. After 50 cyclic voltammograms, a dark green film is generated on the Pt electrode surface as a consequence of the generation of the POT-ES/WS₂ composite. To confirm the formation of this composite, Figure 11 shows the Raman spectrum of the POT-ES/WS₂ composite synthesized electrochemically on the Pt electrode surface, after recording the 50th cyclic voltammogram.

According to Figure 11a, the Raman spectrum of the POT-ES/WS₂ composite is characterized by Raman lines located at (a) 359 and 420 cm⁻¹, coming from WS₂ sheets, being related to vibrational modes E_2g¹ and A_1g [24], and (b) 1118, 1178, 1256, 1317, 1363, 1500, and 1618 cm⁻¹, belonging to POT-ES [31,32]. The FTIR spectrum of the electrochemically synthesized POT-ES/WS₂ composite after recording 50 cyclic voltammograms in the range (−200; +900) mV (Figure 11b) is characterized by IR bands situated at 758, 881, 1045, 1161, 1465 −1495, 1529, and 1632 cm⁻¹. According to Ref. [39], the FTIR spectrum of POT-ES is characterized by IR bands localized at 754, 889, 1045, 1145–1185, 1481–1489, and 1610 cm⁻¹ that are attributed to the following vibrational modes: B deformation, Q deformation, out-of-plane deformation of triple-substituted B, (B)-NH⁺=(Q), N-B-N, and N=Q. The differences concerning the wavenumbers of IR bands associated with the vibrational modes of B–NH⁺=Q and N=Q can be explained by steric hindrance effects due to the covalent coordination bonds established between W atoms in WS₂ and amine groups of POT-ES. According to Figure 11b, the IR bands assigned to the stretching OH and free OH group vibrational modes peaked at 3610 and 3740 cm⁻¹. A careful analysis of Figure 8b and Figure 11b highlights that the ratio between the absorbance of the OH stretching vibrational mode and free OH groups is equal to 1 and 0.5, respectively. The low absorbance of the IR band assigned to OH bond vibration indicates a small proportion of hydrogen bonds and a low degree of connectivity of molecules through hydrogen bonds for the samples resulting from ECP. More information about the bonds existing in the electrochemically synthesized POT-ES/WS₂ composite is presented in the following by XPS. Figure 12 illustrates the XPS spectra of the electrochemically synthesized POT/WS₂ composite, for which the deconvolution of (a) the XPS C1s spectrum highlights a band of high intensity at 284.3 eV and another three bands of lower intensity situated at 285.1, 285.7, and 286.8 eV belonging to POT-ES (Figure 12a). The bands from 284.3, 285.1, 285.7, and 286.8 eV have been assigned to the bonds C=C (C hybridized sp²), C-C/C-H, C-N/C=N, and C-O-H [34]. (b) The XPS N1s spectrum highlights three bands located at 399.2, 399.9, and 401.4 eV (Figure 12b), which have been assigned to bonds C-N=C [37], C-NH-C [37], and C-N⁺ [38]. (c) The XPS W4f spectrum highlights two bands at 32.6 and 34.7 eV (Figure 12c), corresponding to the doublet W 4f^7/2 and W 4f^5/2, the two bands being separated with cca. 2.1 eV, the ratio of areas being 4:3. (d) The XPS S2p spectrum leads to four bands with maxima at 162.1, 163.3, 168, and 169.2 eV (Figure 12d).

The first two bands correspond to the 2p^3/2 and 2p^1/2 states, which are separated by cca. 1.18 eV, the ratio of the areas of the two bands being 2:1. The doublet from 162 to 163 eV represents S from WS₂, while the doublet from 168 to 169 eV corresponds to S derived from W(SO₄)₃. This fact indicates that the polymerization reaction leads to a byproduct of the type W(SO₄)₃. All these changes clearly prove the generation of POT-ES/WS₂ composites by the ECP method. The chemical mechanism that can explain the electrochemical generation of the POT/WS₂ composite on the Pt electrode surface takes into account (i) the oxidation reaction of OT when a cation radical of OT is produced, which is unstable and consequently reacts with itself, forming the dimer of OT; (ii) further, macromolecular chain growth involves a propagation reaction when the dimer is successively transformed into a trimer, oligomer, and polymer, with POT thus obtained being in a leucoemeraldine base (LB) state, which reacts with H₂SO₄ and leads to POT in leucoemeraldine salt (LS). According to Scheme 2, the oxidation reaction of POT-LS leads to the generation of POT- ES, which chemically interacts with WS₂, resulting in the POT-ES/WS₂ composite.

3.4. Electrochemical Properties of the POT/WS₂ Composites as Promising EAM in Supercapacitor Cells

Figure 13 shows cyclic voltammograms of the POT-EB/WS₂ and POT-ES/WS₂ composites, prepared by SSI and ICP, respectively, when a Nafion membrane that interacted with H₂SO₄ 1M was used as an electrolyte. A careful analysis of Figure 13a,b highlights higher values of the voltametric output currents of the POT-EB/WS₂ and POT-ES/WS₂ composites than of POT-EB and POT-ES. These results allow us to anticipate that the high capacitance value of the POT-EB/WS₂ and POT-ES/WS₂ composites would be obtained.

Using the equation i(V) = a x v^b, where I, v, a, and b correspond to current density (A cm⁻²), potential scan rate (mV s⁻¹), and the two constants, we calculate the contribution of the capacitive process due to the double-layer charge at the electrolyte/electrode interface and diffusion processes [40]. According to Ref. [40], depending on the value of the b constant, (a) a redox reaction at the electrode surface and the charge/discharge processes of the electric double-layer capacitors (EDLCs) occur when b = 1; (b) a pseudocapacitive process takes place when 0.8 < b < 1; and (c) a behavior of the type battery is invoked when 0.5 < b < 0.8. With all these in mind, from plot log(i) to log(v), we have determined that all samples show battery behavior. The contribution of the surface capacitive and diffusion-controlled intercalation (DCI) processes in cyclic voltammograms can be assessed using the protocol published in Ref. [31]. Thus, using the equation i(V) = k₁v + k₂v^1/2, where i(V), k₁v, and k₂v^1/2 are current density for a fixed potential, the weight of the capacitive effect, and the intercalation process, respectively, and applying a linear fit in the case of the plot i(V)/v^1/2 vs. v^1/2, the values of k₁ and k₂ were determined. The contribution of the two processes is shown in Figure 14 and Figure 15.

According to Figure 14 and Figure 15, all samples show a significant contribution given the diffusion-controlled intercalation process, which is accompanied by a capacitive process. Scheme 3 shows the reactions at the electrolyte/electrode interface when the electrodes contain the composites prepared by ICP and ECP.

Figure 16 shows the variation in capacitance vs. scan rate of the electrodes prepared in this study.

At a scan rate of 10 mV s⁻¹, the capacitance of the SCs with composites prepared by ICP and ECP was equal to 2.82 and 12.5 F cm⁻². A decrease in capacitance when increasing the potential scan rate from 10 to 200 mV/s takes place for (i) the POT-EB/WS₂ composites resulting from the SSI method, which have WS₂ concentrations equal to 33 wt.% and 50 wt.%, from 462.18 and 299.68 mF cm⁻² to 123.52 and 108.16 mF cm⁻²; (ii) the POT/WS₂ composites, synthesized by the ICP method, when the TMD concentration in the composite mass is 10 wt.% and 20 wt.%, from 2386.2 and 2820.71 mF cm⁻² to 382.69 and 465.76 mF cm⁻²; and (iii) the POT-ES/WS₂ composite synthesized by the ECP method from 6136.75 to 751.6 mF cm⁻². Figure 17 shows the variation in the capacitance of the SCs with the cyclic voltammogram numbers, in the case of electrodes based on (i) the POT-EB/WS₂ composite resulting from the interaction of the two constituents when the WS₂ concentration is 50 wt.%; (ii) the POT/WS₂ composite, synthesized by the chemical polymerization of OT, when the WS₂ concentration is 20 wt.%; and (iii) the electrochemical synthesized POT-ES/WS₂ composite.

According to Figure 17, an important decrease in the capacitance takes place in the first 750 cyclic voltammograms for the POT/WS₂ composites prepared by the three synthesis methods. This behavior should be explained by considering various expansions and contractions of the volume of the electrode as a consequence of the expulsion and insertion of ions during discharge and charge processes, as shown in Scheme 3 and Scheme 4.

Scheme 3 highlights, based on Equations (1) and (2), that during the discharge half-cycle of the first cyclic voltammogram, the HSO₄⁻ and S²⁻ ions leave the POT-ES/WS₂ composites, with the macromolecular compounds having an LB structure. In the charge half-cycle of the first cyclic voltammogram, both the HSO₄⁻ and S²⁻ ions will inserted into the composite matrix. Scheme 4 shows the chemical reaction that can occur after the discharge half-cycle of the first cyclic voltammogram considering the H⁺ and HSO₄⁻ ions of the Nafion membrane treated with H₂SO₄, which interact with the amine groups according to Equation (1) when POT-LB/WS₂ is transformed into POT-LS/WS₂. This fact indicates that, in the following cyclic voltammograms, the charge/discharge processes take place according to Equation (2). Increasing the number of cyclic voltammograms will induce more and more amine groups to react with H₂SO₄ according to Equation (1), which, in our opinion, may explain the decrease in SC capacitance in the first 750 cyclic voltammograms and the degradation of EAM. After recording 5000 cyclic voltammograms, the values of the capacitance of the supercapacitor cells with electrodes based on the POT/WS₂ composites obtained by SSI, ICP, and ECP are equal to 95.03 (Figure 17a), 327.71 (Figure 17b), and 375.33 mF/cm² (Figure 17c).

The differences between the capacitance values of supercapacitors containing the PANI/WS₂ and POT/WS₂ composites, prepared in different ways and using various synthesis methods or suppliers for WS₂ particles, are summarized in

Table 1. The performance of SCs based on PANI/WS₂, POT/WS₂, PANI/MnO₂, and polybenzidine/MnO₂ composites.

EAMs; Inorganic Particle Synthesis Method or Supplier	Synthesis Method of Composite	Electrolyte	Current Density or Potential Scan Rate	Capacitance	Ref.
PANI/WS2/C; Merck	ICP	1M Na2SO4	10 mVs−1	464 F g−1	[41]
PANI nanofibers/WS2; Sigma Aldrich	Electrodeposition by CV	1M Na2SO4	1Ag−1; 5000 V s−1	72.27 F g−1; 331 mF cm−2	[41]
PANI/WS2; hydrothermal	ICP	1M H2SO4	1Ag−1	560 F g−1	[42]
PANI/WS2; mechano-chemically assisted NaCl	Mechano-chemical reaction	1M H2SO4	1Ag−1	580 F g−1	[43]
POT/WS2; ball milling	SSI	1M H2SO4	50 mV s−1	177 mF cm−2	This work
POT/WS2; ball milling	ICP	1M H2SO4	50 mV s−1	1.15 F cm−2	This work
POT/WS2; ball milling	ECP by CV	1M H2SO4	50 mV s−1	2.64 F cm−2	This work
MnO2 Nanorods@PANI; hydrothermal	ICP	1M KOH	50 mVs−1	259 F g−1	[44]
γ-MnO2/PANI; from precursor of KMnO4	ICP	0.5M H2SO4	0.5 A g−1	493 F g−1	[45]
Polybenzidine/MnO2	Chemical interaction	1M H2SO4	0.5 mAcm−2	950 mF cm−2	[46]
POT/WS2; ball milling	ICP	1M H2SO4	10 mVs−1	2.82 F cm−2	This work
POT/WS2; ball milling	ECP by CV	1M H2SO4	10 mVs−1	12.5 F cm−2	This work

[41,42,43,44,45,46]. According to Table 1, the capacitance value depends on the shape of the WS₂ particles, the synthesis method of the composites used as active materials in the SC cells, the type of electrolyte, and the applied current densities. Table 1 further presents the performance of SCs with PANI/MnO₂ and polybenzidine/MnO₂ composites as EAMs. The comparison between SCs with composites of the type PANI/WS₂ [42,44] and γ-MnO₂/PANI [45] as the EAM indicates that the former shows higher values of capacitance. In the case of SCs based on POT/WS₂ composites, which were prepared by ICP and ECP, higher capacitance values of 2.82 and 12.5 F cm⁻² at a potential scan rate of 10 mV s⁻¹ are reported in comparison with the SC containing polybenzidine/MnO₂ as the EAM [46]. In this stage of our investigations, to optimize the SCs developed in this work, we plan to test SC cells by preparing new POT/WS₂ composites, in which WS₂ sheets will be replaced by WS₂ rods. Considering the work reported by Zhang et al [16], which demonstrated that by doping PANI with HClO₄ and p-toluenesulfonic acid (p-TSA), respectively, higher energy density values can be obtained only in the former case, another parameter to be considered for improving the performance of SCs containing POT/WS₂ is co-doping with a suitable dopant agent such as HClO₄. An electrolyte often used in the SC field containing sulfides [47,48] or MnO₂ [44] is KOH, which will be considered by us in future studies.

4. Conclusions

The synthesis methods of the POT/WS₂ composites, their optical and structural properties, and their applications in the symmetrical SC field were reported. Using SEM, XRD, FTIR spectroscopy, Raman scattering, XPS, UV-VIS spectroscopy, and CV, the following conclusions are highlighted:

(i)

Mechanical exfoliation by grinding WS₂ NPs using a ball mill followed by exfoliation in DMF allows the transformation of nanoparticles of WS₂ in sheets and induces a diminution in the intensity of the crystalline plane (002), a change in the distance between the Raman lines related to vibrational modes E_2g¹ and A_1g, and the presence of three bands situated at 632, 530, and 454 nm in the UV-VIS spectrum.

(ii)

SSI of POT-EB with WS₂ induces an exfoliation of the WS₂ NPs.

(iii)

ICP of OT in the presence of WS₂ sheets induces the formation of covalent coordination bonds between the imine groups of EB and the transition metal W, when a POT/WS₂ composite is created, simultaneously with the generation of a byproduct such as tungsten sulfate.

(iv)

ECP of OT in the presence of WS₂ sheets involves oxidation and reduction reactions at the electrolyte/electrode interface that have an irreversible character as a result of the generation of the POT-ES/WS₂ composite when the formation of covalent coordination bonds between imine groups of POT and W atoms of TMD takes place.

(v)

The electrochemical processes at the electrode/electrolyte interface in symmetrical SCs show a significant contribution given the performance of a diffusion-controlled intercalation process and an additional capacitive-type process; the best values of capacitance of the SCs with EAMs of the composites prepared by ICP and ECP were reported at 10 mV s⁻¹, these being equal to 2.82 and 12.5 F cm⁻²; the increase in the scan rate at 200 mV s⁻¹ led to values of capacitance equal to 123.5, 465.76, and 751.6 mF cm⁻², respectively, for the POT/WS₂ composites prepared by SSI, ICP, and ECP. After recording 5000 cyclic voltammograms at 200 mV s⁻¹, a decrease in the capacitance of the three SCs to 95.03, 327.71, and 375.33 mF cm⁻² was reported.