One-Pot Synthesis of Polyoxometalate Decorated Polyindole for Energy Storage Supercapacitors.

The demand for energy storage supercapacitor devices has increased interest in completing all innovative technologies and renewable energy requirements. Here, we report a simple method of two polyoxomolybdate (H4[PVMo11O40] and H5[PV2Mo10O40]) doped polyindole (PIn) composites for electrochemical supercapacitors. The interactions between polyoxomolybdates and PIn were measured by Fourier transform infrared spectroscopy (FTIR), and powder XRD, and stability was measured by thermogravimetry. The field emission scanning microscopy (FESEM) was employed to investigate the morphology of the materials. The electrochemical measurements show that the PIn/PV2Mo10 electrode exhibits a higher capacitance of 198.09 F/g with an energy density of 10.19 Wh/kg and a power density of 198.54 W/kg at 0.2 A/g current density than the PIn/PVMo11 electrode. Both electrodes show a pseudocapacitance behavior due to the doping of redox-active polyoxomolybdates on the PIn surface and enhance the electrochemical properties. The electrodes' capacitive nature was measured by electrochemical impedance spectroscopy (EIS), which shows that the PIn/PVMo11 electrode has a resistive nature within the electrode-electrode interface. Moreover, the PIn/PV2Mo10 electrode offers remarkable cycle stability, retaining ∼84% of its capacitance after 10,000 cycles (∼83% for the PIn/PVMo11 electrode). The higher specific capacitance, faster charge/discharge rates, and higher cycle stability make them promising electrodes in supercapacitors.

Introduction

The requirements of efficient energy storage devices have attracted tremendous interest in recent times due to the day-to-day utilization of portable electronic devices, mainly laptops, watch, mobile phones, etc. Moreover, the depletion of fossil fuels urge researchers to develop alternative green and sustainable energy storage systems.[1−3] On the other hand, sustainable and renewable energy is easily accessible and environment-friendly but cannot fulfill storage for the distant future because of its irregularities.[4,5] In this regard, electrochemical supercapacitors can store energy and deliver it even at higher rates than conventional batteries[6,7] because of their high power density, fast charge–discharge process, and long cycle life. However, less energy density placed the electrochemical capacitors below the Li-ion battery in the Ragone plot.[8] Nevertheless, the structural properties of the electrode materials define the performance of supercapacitors.[9−11]

To meet the demand for electronic storage devices, mainly carbonaceous-based supporting materials, such as graphene/metal oxides, graphene/conducting polymers, graphene/conducting polymers/metal oxides,[12,13] activated carbon (AC)/metal oxide,[14,15] carbon nanotube (CNT)/Co3O4,[16] graphdiyne,[17−19] SWCNT-polyoxometalate,[20] etc., were studied extensively. Recently, conducting organic polymers (COPs), mainly, polypyrrole (PPy), polyaniline (PAni), polythiophene (PTH), and poly(3,4-ethylene dioxythiophene) (PEDOT), attracted great interest in the field of energy storage device and were mostly explored due to the redox properties despite the swelling (oxidation)/shrinking (reduction) limitations during the charging–discharging process.[21] Furthermore, another COP, named polyindole (PIn), has immensely gained much attention in electrical energy storage. However, PIn is the least studied among the COPs owing to its low conductivity, mechanical strain over a long cycle, and material degradation, which leads to a loss in capacity and degradation (structural irregularities in the chain) of the storage device.[21] Despite suffering all the above properties, it bears some multiple advantages, including good thermal stability, stable redox activity,[22] photoluminescent properties,[23] electrochromic ability,[23] and slow degradation compared with polypyrrole and polyaniline.[24] Interestingly, PIn is a fused structure of poly(p-phenylene) and polypyrrole. Hence, it bears the combination of both cyclic ring properties, which makes this material even more suitable for energy storage applications. The available literature on PIn in energy storage is scary due to its low conductivity.

The COPs were combined with the appropriate metal oxide to overcome all the challenges mentioned above, which would enhance the electrochemical performance because of the synergistic effect between them.[25] Among the various metal oxides investigated over the years; polyoxometalates are found attractive due to the unmatched range of physical and chemical properties, namely, redox properties (electrochemical performance), tuneable properties, thermal stability, etc.[26] Polyoxometalates are a class of multinuclear metal oxide nanoclusters that could be combined with conducting polymers (CPs). These composite materials were studied the most for the supercapacitor due to their ″electron sponge″ behaviors and enhanced electrochemical stability.[27] For example, White et al. prepared heteropolyacid-doped conducting polypyrrole, which was grown in the vapor phase, and a specific capacitance of about 422 F/g was reported.[28] Vaillant et al. synthesized the hybrid material PAni/PMo12 with the aid of H2O2, and the material showed a capacitance value of 168 F/g. The same group has prepared a new PEDOT/PMo12 hybrid electrode materials by a chemical method but obtained a low capacitance value of 130 F/g without H2O2.[29] Recently, Vannathan et al. synthesized two PPy/H4[PVMo11O40] and PPy/H5[PV2Mo10O40] composites in situ, and a very high capacitance of 562 F/g was observed at 0.2 A/g in a H2SO4 electrolyte medium in two-electrode system.[30] Pedro Gomez-Romero et al. synthesized the PPy-Npipes, PW12/PPy, and PMo12/PPy and reported 204.5, 341, and 294.1 F/g of capacitance by using the three-electrode system in a 1 M H2SO4 electrolyte.[31] Cuentas-Gallegos et al. prepared H3PMo12O40/PAni and found a 120 F/g specific capacitance in a two-electrode system, and they used Nafion117 and sulfuric acid as an electrolyte.[32] Jingjing Lin et al. synthesized Fe-Anderson-type polyoxometalate/polyaniline/graphene (PPG) and reported a 1366 F/g capacitance in the three-electrode system using 1 M H2SO4 as electrolyte.[33]

Surprisingly, there has not been any literature report on supercapacitors using doped polyoxometalates on polyindole-based conducting polymers to date due to the polymer material’s low conductivity. We have decided to investigate the electrochemical studies of polyoxometalates doped on PIn and chose vanadium-based polyoxometalates for the oxidation–reduction nature of the vanadium.[20]

Herein, we report the first two new acidic Keggin polyoxomolybdates (H4[PVMo11O40], PVMo and H5[PV2Mo10O40], PV) doped into the PIn matrix for the electrochemical performance and study the cycle stability of PIn-POMs in an aqueous acid electrolyte medium. Finally, all materials’ supercapacitive properties were investigated by cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS) analysis. Hereafter, we refer to the two POMs using the abbreviations PVMo11 and PV2Mo10, respectively.

Results and Discussion

Physical Characterization of PIn, PIn/PVMo11, and PIn/PV2Mo10 Composites

Figure depicts the comparative FTIR of polyindole (PIn), PIn/PVMo11, and PIn/PV2Mo10. PIn revealed the characteristic absorption bands at 3400 (νN–H), 3000 (sp2 C–H), 1619 and1490 (νC–C, Ar), 1460 (νC–N), 1365 (νC=N), and 741 cm–1 out-of-plane deformations (νC–H, Ar), respectively.[34] The FTIR of the two POMs (PVMo11 and PV2Mo10) is reported elsewhere.[30]Figure B, C represents the FTIR of the PIn/PVMo11 and PIn/PV2Mo10 hybrid electrodes, which showed the retained chemical structure of both POMs (PVMo11 and PV2Mo10) and characteristic bands at 1058 (1055, νP–O), 960 (961, νMo=O), and 744 (νV=O) cm–1, respectively. Furthermore, the POM's characteristic bands were also observed in the hybrid materials and confirmed the impregnation of POMs on the PIn surface.

Figure 1

FTIR spectra of (A) PIn, (B) PIn/PVMo11, and (C) PIn/PV2Mo10.

Figure depicts the TGA of PIn, PVMo11, PV2Mo10, PIn/PVMo11, and PIn/PV2Mo10 electrode materials. As seen from the thermal decomposition, doping of redox-active polyoxometalates increases the thermal stability of the electrodes. The DTG and the TGA curve of PIn (Figure S1a,b) show the first weight loss of 2.95% at 61.71 °C because of the moisture removal.[35] Several weight losses were observed after the glass transition temperature, which indicate the stagewise decomposition of the polymer backbone. The 50% weight loss of PIn was observed until 500 °C, which was prepared from the aqueous solution using FeCl3 as an oxidant. The higher thermal stability of the PIn was varied, and it depends on the synthesis medium. For example, 50% weight loss was observed at 681.85 °C when synthesized from the CH3CN/Bu4NBF4 medium.[36] In general, the PIn has a higher thermal stability, maybe due to the absence of counterions and a change in the molecular structure.[37,38] The weight loss of pure PVMo11 and PV2Mo10 (22.17 and 13.26%, Figure S1d,f) was observed until 123 and 101 °C for both POMs, ascribed to the expelling of the moisture and crystal water molecules, and further heating of both POMs exhibited utmost phase transition stability at 343.8 and 291.5 °C, respectively (Figure S1c,e). The thermal decomposition of both POMs is well documented in a published article.[30] The weight losses of doped PVMo11 and PV2Mo10 on PIn are depicted in Figure . The first weight loss of 3.55% at 63.56 °C corresponds to the moisture removal from the hybrid material (Figure S1g,h). The final weight loss (14.3%) until 765.81 °C is ascribed to the degradation of inorganic moieties of the PIn/PVMo11. Likewise, the inorganic moieties in PIn/PV2Mo10 decompose above 750 °C (Figure S1i,j). Figure shows that both the PIn/PVMo11 and PIn/PV2Mo10 electrode materials are more stable than the pure PIn. In other words, the doping of POMs increased the stability of the electrode materials by many folds. This phenomenon can be explained by the synergistic interaction between the anionic PVMo11 and PV2Mo10 with PIn.

Figure 2

TGA of pure PIn, PVMo11, PV2Mo10, PIn/PVMo11, and PIn/PV2Mo10.

In agreement with the powder X-ray diffraction (P-XRD) patterns (Figure ), the deposition of pure PVMo11 and PV2Mo10 on PIn shows the amorphous nature of PIn/PVMo11 and PIn/PV2Mo10 electrodes. The PIn shows characteristic broad peaks at 2θ = 7.89, 18.75, and 23.834o with 11.2, 4.73, and 3.73 Å lattice spacing and broad peaks indicating the microsphere structure of the material.[30] The P-XRD pattern of PVMo11 showed peaks at 2θ of 9.7, 16.8, 21.8, 26.8, 27.6, 28.98, and 31.8° with a high percentage of crystalline material. Similarly, the P-XRD pattern of PV2Mo10 showed similar diffraction angles (2θ) (9.76, 16.96, 21.41, 26.45, 27.75, 28.99, and 31.72°) and exhibited high crystalline material.[30] After doping both POMs on the backbone of PIn in situ, the number of diffraction angles was reduced, indicating the amorphous nature of the materials, and the loss of crystallinity of POMs is distinctly visible in Figure .

Figure 3

P-XRD pattern of PIn, PVMo11, PV2Mo10, PIn/PVMo11, and PIn/PV2Mo10.

To have further insight, the surface morphology (FESEM) of all the hybrid materials (PIn, PIn/PVMo11, and PIn/PV2Mo10) was studied under identical conditions, which are presented in Figure A–C. FESEM reveals porous, rough surfaces with a nonuniform distribution of the PIn with 0.964 μm size (Figure A). Figure B, C shows the FESEM images of the two new PIn/PVMo11 and PIn/PV2Mo10 hybrid electrodes synthesized in situ by doping redox-active POMs on indole.

Figure 4

FESEM images of (A) pure PIn, (B) PIn/PVMo11, and (C) PIn/PV2Mo10. EDX spectra of (D) pure PIn, (E) PIn/PVMo11, and (F) PIn/PV2Mo10, respectively.

The morphologies of all materials reveal a characteristic nonuniform distribution rather than an accumulation of the composite’s electrodes. Such morphology may arise due to the intercalation of the POMs in the PIn domain. Nevertheless, the morphology of PIn varies with the synthetic method and hence the capacitance. For example, Koiry et al. prepared long fiber-shaped PIn using FeCl3 as an oxidant in an organic medium (dichloromethane) in a stationary interface. However, the PIn shape was changed to spherical when the interface was disturbed.[39] An et al. synthesized PIn by interfacial polymerization using (NH4)2S2O8 as an oxidizing agent in a chloroform medium. They observed a uniform nanoparticle formation with 1–3 mm diameter in size using cetyltrimethylammonium bromide as a surfactant.[40]

The energy dispersive X-ray (EDX) spectroscopy analysis was supported by the presence of all elements in PIn, PIn/PVMo11, and PIn/PV2Mo10, as shown in Figure D–F. The EDX spectra confirmed C and N in pure PIn, and C, N, O, P, V, and Mo in the PIn/PVMo11 and PIn/PV2Mo10 hybrid electrodes, respectively. The FESEM and EDX of pure PVMo11 and PV2Mo10 are shown in Figure S2. The elemental compositions from EDX are presented in Table .

Table 1

Elemental Composition of PIn, PIn/PVMo11, and PIn/PV2Mo10 from EDX

	atomic (%)
materials	N	O	C	P	V	Mo
PIn	23.73	50.84	25.42
PIn/PVMo11	1.79	53.83	28.71	1.02	1.11	13.54
PIn/PV2Mo10	6.06	52.62	26.31	1.20	1.05	12.76

Figure shows the N2 adsorption/desorption isotherm of PIn, PIn/PVMo11, and PIn/PV2Mo10 materials. The specific BET surface area of PIn, PIn/PVMo11, and PIn/PV2Mo10 composites is 3.3, 7.52, and 9.3 m2/g. The composite materials exhibited a type IV nitrogen adsorption isotherm without a defined hysteresis loop suggesting a nonporous nature, as shown in Figure . The lower surface area of newly synthesized electrode materials indicated a high amount of doping of PVMo11 and PV2Mo10 into the PIn surface and was fully integrated into the polymer matrix surfaces and may deliver substantial capacitance.

Figure 5

Nitrogen adsorption/desorption isotherm of PIn, PIn/PVMo11, and PIn/PV2Mo10 electrodes.

Electrochemical Characterization of the PIn/PVMo11 and PIn/PV2Mo10 Composites

The pseudocapacitive materials (e.g., transition metal oxide, conduction organic polymers) exhibit high specific capacitance and large energy density, making them ideal electrode materials for supercapacitors.[41−43] The electrochemical measurements of PIn, PIn/PVMo11, and PIn/PV2Mo10 electrodes were studied by cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) at 0.25 M H2SO4 electrolyte solution using two-electrode systems, as shown in Figure .

Figure 6

CVs of (A) PIn, (C) PIn/PVMo11, and (E) PIn/PV2Mo10 at various scan rates. GCD curves of (B) PIn, (D) PIn/PVMo11, and (F) PIn/PV2Mo10 with different current densities.

As depicted in Figure A, the pure PIn electrode’s CV curves were determined at various scan rates from 5 to 100 mV/s. It reveals a quasi-rectangular shape without noticeable redox peaks, indicating an excellent electrical double-layer charge storage performance. The PIn has been considered a superior electrode material for supercapacitor due to the high thermal stability, slow degradation, and fast redox reaction.[44] However, the lower specific capacitance of pure PIn has been documented, which varies from the 93 to 114 F/g in various electrolyte solutions in three-electrode systems.[45,46] It exhibits low specific capacitance because of the low electrical conductivity compared to other CPs. Figure B shows a linear plot in the potential range of 0–1 V of charge–discharge curves, which exhibits a lower specific capacitance of 33.50 F/g at a lower current density (0.2 A/g).

Moreover, the PIn/PVMo11 and PIn/PV2Mo10 electrodes show the distorted rectangular shapes with a few redox peaks in the CV curves (Figure C, and E) with similar scan rates of 50–100 mV/s. This could be because of the pseudocapacitive behavior of both PIn and redox-active polyoxometalates (PVMo11 and PV2Mo10). The specific capacitance was calculated to be 26.4 F/g for PIn/PVMo11 and 60.81 F/g for PIn/PV2Mo10 electrodes at the same scan rates of 5 mV/s (S1). The capacitance decreases with increasing scan rates for all the above cases due to the faster kinetics of ions at higher scan rates. Eventually, the electrolyte ion gets less time to penetrate the electrode.[38] The electrolyte ion’s kinetic energy is reduced and has enough time to diffuse to the electrode surface, causing higher specific capacitance upon lowering the scan rates. The capacitance of PIn/PVMo11 and PIn/PV2Mo10 electrodes was estimated by GCD (Figure D, and F) measurements at various current densities. The pseudocapacitive nature of the electrodes could be visible in the charge–discharge curves, suggesting reversibility characteristics. The specific capacitance was estimated to be 177.36 and 198.54 F/g for PIn/PVMo11 and PIn/PV2Mo10 electrodes, respectively, at 0.2 A/g current density, significantly higher than the pure PIn under the same conditions (Table S1). The equation used for the calculations of specific capacitance, energy, and power density is given the Supporting Information (S1). The much higher capacitance is probably due to the increment in the conductivity derived from the combination of redox-active polyoxometalates ions (PVMo11 and PV2Mo10) with PIn. Simultaneously, the PIn/PVMo11 and PIn/PV2Mo10 electrodes displayed an energy density of 9.77 and 10.19 Wh/kg, respectively, at the same 0.2 A/g current density (Figure B, Table S1). As seen in Figure D, F, the specific capacitance values decrease with an increase in current density (Figure A), which illustrates that the electrochemical diffusion mechanism leads to energy storage. We envisaged the electrolyte ions’ higher kinetic energy with less diffusion time with increasing current density.[47]

Figure 7

(A) Specific capacitance vs current density and (B) power density vs energy density of PIn/PVMo11 and PIn/PV2Mo10 electrodes.

The electrochemical impedance spectroscopy (EIS) analysis has been performed to examine the resistance of both composite's electrode–electrode interface. All the data are presented as the Nyquist plot over 1 to 10 5 Hz frequency range (Figure ) with 0.01 mV dc applied potential. In Figure , the insert pictured a small arc in the high-frequency region for PIn/PV2Mo10 electrodes, indicating the dominating supercapacitor’s resistive nature within the electrode–electrode interface. Pure PIn and PIn/PVMo11 have a minimal arc in the high frequency, suggesting the flawed resistive character.

Figure 8

Nyquist plots for the PIn, PIn/PVMo11, and PIn/ PV2Mo10 electrodes.

The cell capacitance calculated based on the following eq (vide infra) is shown in Table , and it was observed that the PIn/PV2Mo10 electrode material could be applicable for a small cell.

Table 2

Fitting Data of Equivalent Circuit Elements Obtained by Nyquist Plots

material	Rs (Ω)	Rct (Ω)	Rp (Ω)	cell capacitance, CTf = 1/2πf*Im[z]mF	normalized to capacitance by using an active mass of the electrode (F/g)
PIn	0.98	4.03	5.01	0.56	1.4
PIn/PVMo11	0.88	4.93	5.82	1.13	2.825
PIn/PV2Mo10	1.32	5.03	6.35	5.90	14.75

CTf is the cell capacitance, f is the minimum frequency applied, and Im(z) is the complex impedance at the minimum frequency. So, the pseudocapacitance arises either from the electrolyte or redox reaction of PIn and POMs.[48−50]

The cell capacitance and impedance were estimated using the EIS data for the device (laboratory scale) applications, and details are available in the Supplementary Information (S1). The cell capacitance values of PIn, PIn/PVMo11, and PIn/PV2Mo10 were calculated and observed to be 0.56, 1.13, and 5.9 mF, respectively (Table ). The normalized capacitance by using an active mass of the electrode is 1.4, 2.8, and 14.75 F/g, respectively, for PIn, PIn/PVMo11, and PIn/PV2Mo10.

The Rs, Rp, and Rct were calculated from Figure , representing the electrolyte resistance in contact with the current collector/electrode and the electrode’s internal resistance (Rp), and the charge transfer resistance of the redox reaction under course. As we know, the charge transfer resistance rises with the arch’s diameter, eventually lowering the charge stored or discharged at the higher value of Rct, indicating a higher pseudocapacitance. The PIn/PV2Mo10 electrode shows a higher Rct value of 5.03 Ω than other electrodes at room temperature (Table ). It can be due to the presence of more vanadium atoms in the PV2Mo10 Keggin polyanion, which helps to participate in the redox reaction.

An electrode’s capacitance is hugely related to the mass and the device’s size, considering the real-life application. For example, Pan et al. reported that for a small cell supercapacitor, the characteristic range of the cell capacitance is 1–10 mF (Table ).[51] In this case, the PIn/PV2Mo10 composite electrode shows the cell’s capacitance value of ∼6 mF within the range of 1–10 mF and can be applied for small SC cell purposes.

Stability Performance of the Hybrid Electrodes

The capacitance retention plots of the PIn, PIn/PVMo11, and PIn/PV2Mo10, respectively, shown in Figure a, which was measured based on the GCD curves at 4 A/g current density. After 10,000 cycles, the PIn, PIn/PVMo11, and PIn/PV2Mo10 electrodes retain their cycling stability of 78.4, 83.4, and 84.5%, respectively. The cycle stability of two pure POMs (PVMo11 and PV2Mo10) has been reported elsewhere using the scan rate of 500 mV/s, and the PVMo11 and PV2Mo10 electrodes exhibit 68.7 and 71.3% retention stability, respectively, after 4500 cycles based on CV.26 The PIn/PV2Mo10 electrode offers the highest cyclic stability, and the first (Figure b) and last three cycles (Figure S3) of the PIn/PV2Mo10 electrode show the electrode’s excellent stability as compared to other electrodes materials. The first and last three cycles of other electrodes are represented in Figures S4 and S5.

Figure 9

(a) Cycle stability of PIn, PIn/PVMo11, and PIn/PV2Mo10. (b) The first three cycles of PIn/PV2Mo10 are based on GCD.

Conclusions

In summary, we have synthesized two new PIn/PVMo11 and PIn/PV2Mo10 composite electrodes for the supercapacitor application. Furthermore, in the two-electrode cell configuration, the PIn/PVMo11 and PIn/PV2Mo10 nanostructure exhibits an upgraded specific capacitance value of 177.36 and 198.54 F/g, respectively in 0.25 M H2SO4 electrolyte. The PIn/PV2Mo10 shows superior electrochemical faradic charge storage performance than PIn/PVMo11, which might be attributed to the former’s higher conductivity than the latter, and shows good reversibility of the electrode material. Again, the PIn/PV2Mo10 electrode shows a high energy density of 10.19 Wh/kg and adequate cycle stability after 10,000 cycles at 4 A/g current density.

Materials and Method

Indole (99%) as a monomer was purchased from SRL Chem. Pvt. Ltd. NMP (N-methyl pyrrolidone) and carbon black were purchased from Sigma-Aldrich. Polyvinylidene (PDVF) was obtained from the Alfa Aesar. NaVO3, Na2MoO4, Na2HPO4, HCl, H2SO4, and diethyl ether were purchased from Loba Chem. Pvt. Ltd. Carbon cloth was purchased from Sinergy Fuel Cell India Pvt. Ltd. Unless otherwise specified, all the reagents used were of analytical grade, and the solutions were made using HPLC-grade water. The redox-active polyoxometalate clusters H4[PVMo11O40] and H5[PV2Mo10O40] were synthesized using the original synthesis method reported by Akba et al.[52]

Pure PIn was synthesized by a simple and inexpensive chemical method at room temperature. In this method, 1.0 g (0.04 mmol) of indole and 1.00 g (0.27 mmol) of FeCl3·6H2O were mixed in 50 mL of distilled water. The dark brown solid PIn started to form. The resulting solution was continuously stirred for 24 h at room temperature to complete the reaction. The dark brown solid was filtered off under vacuum using a membrane filter paper and washed several times with HPLC-grade water until the colorless filtrate’s appearance. For polymerization reaction, monomers have to be oxidized to initiate the reaction. Here, FeCl3 acts as an oxidizing agent and undergoes a free radical reaction for further polymerization. The synthesized composite's purity was checked by different analytical techniques, mainly FTIR, powder XRD, and finally EDX.[53]

Synthesis of PIn/PVMo11 and PIn/PV2Mo10 Composite Materials

Synthesis of the PIn/PVMo11 Composite

The PIn/PVMo11 was synthesized via an oxidative polymerization in situ method where PVMo11 acts as an oxidant and Bronsted acid. One gram (0.12 mmol) of indole (monomer) was mixed with 50 mL of distilled water in a beaker. The indole/water mixture was stirred for a few minutes. Then, 2 g (4.75 mmol) of acidic H4[PVMo11O40] was added slowly to the indole/water mixture. The orange solution slowly turned to brown, and brown salt began to form. The resulting reaction was continuously stirred for 24 h at room temperature (Scheme ). The deep brown solid was filtered off under vacuum by using a membrane filter paper and washed many times with HPLC-grade water until the filtrate appeared colorless to wash out excess POM present in the reaction medium. Finally, the substantial solid was air-dried, and we used it for our study. FTIR and powder XRD checked the purity of the synthesized composite. EDX measurements confirmed the elemental composition of all elements.

Scheme 1

Schematic Representation of the Synthesis of the PIn/PVMo11 and PIn/PV2Mo10

Synthesis of the PIn/PV2Mo10 Composite

The PIn/PV2Mo10 electrode was prepared similarly to PIn/PVMo11, except that H5[PV2Mo10O40] was used instead of H4[PVMo11O40] (Scheme ).

Electrode Preparation

The electrode preparation is essential to determine the electrochemical properties of prepared materials. The electrodes were fabricated on a piece of carbon cloth (1 × 1, Synergic Pvt. Ltd.) by coating a slurry of active materials (PIn, PIn/PVMo11, or PIn/PV2Mo10), carbon black, and polyvinylidene fluoride in the ratio of 80:10:10 with a few drops of N-methyl-2-pyrrolidone (NMP). The resulting slurry was sonicated for about 11/2 h, and the electrode materials (0.8 mg of the mass of the active material) were dispersed using a micropipette on the carbon cloth, followed by drying it over 24 h at 80 °C. The carbon cloth was rinsed with acetone and washed with HPLC grade water several times prior to use.

Instrumentation

The synthesized electrode materials were characterized by using different characterization techniques. FTIR spectra were acquired using a Bruker 4000 (USA) FTIR spectrometer in the wavenumber range of 4000–400 cm–1 to explore the material’s chemical structure. The electrode material’s thermal stability measurement was performed using a PerkinElmer TGA4000 (USA) using a heating rate of 5 °C/min inflowing of 20 mL/min N2. The X-ray diffraction (XRD) patterns were acquired using a Rigaku Smartlab (Japan) diffractometer in the 2θ range of 10 to 90°. The field emission scanning electron microscopy (FESEM) (Carl Zeiss Sigma, Germany) image was acquired for all electrode materials to explore morphological properties. The specific BET surface area was determined in a Micromeritics physisorption analyzer (Model ASAP 2020, USA) using N2 adsorption isotherm data collected at 77 K. The electrochemical performance of the prepared electrode material was performed using an electrochemical workstation (IVIUM Technologies BV Co., The Netherlands, Model: Vertex).