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High-performance supercapacitors based on polymeric binary composites of polythiophene (PTP)–titanium dioxide (TiO2)
Synthetic Metals 220 (2016) 25–33
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High-performance supercapacitors based on polymeric binary composites of polythiophene (PTP)–titanium dioxide (TiO2) Anukul K. Thakur, Ram Bilash Choudhary* Nanostructured Composite Materials Lab, Department of Applied Physics, Indian School of Mines, Dhanbad 826004, India
A R T I C L E I N F O
Article history: Received 8 January 2016 Received in revised form 17 May 2016 Accepted 20 May 2016 Available online xxx Keywords: Polythiophene Titanium dioxide Specific capacitance
A B S T R A C T
Polythiophene (PTP)-titanium dioxide (TiO2) composites were prepared via oxidative polymerization process using varying ratios of TiO2 content. Fourier transform-infrared (FTIR) spectroscopy was used to confirm the incorporation of TiO2 into PTP. X-ray diffraction spectroscopy was used to examine the nature of pure PTP and PTP/TiO2 composites. Distribution of TiO2 in PTP was examined by field emission scanning electron microscopy (FESEM). The electrochemical properties were studied by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) with 1 M aq. H2SO4 as an electrolyte with two electrode system. From charging-discharging measurement, it was found that the capacitance of PTP/TiO2 composites initially increased and then it decreased gradually. The maximum specific capacitance (250 F/g) and energy density (5.54Wh/Kg) were recorded for PTP/TiO2 (10:2) composite at a current density of 1A/g. Furthermore, the power density was achieved up to 263.8W/kg at a current density of 1A/g. ã 2016 Elsevier B.V. All rights reserved.
1. Introduction Since last few decades, enormous research activities have been devoted towards the design and development of different categories of energy storage devices. The development of energy storage devices requires highly efficient and synergistic combination of materials of hybrid origin. Specifically, the electrochemical device such as supercapacitor has been globally recognized as one of the important class of energy storage devices [1–3]. There has been tremendous interest among the researchers for fabricating supercapacitors as efficient energy storage devices because of their simple working principle, fast charge-discharge rate, long cyclic life, and high power capability [4–6]. Supercapacitors have been classified on the basis of energy storage action mechanisms: (i) electric double layer capacitor (EDLC), and (ii) pseudocapacitor. Among these, pseudocapacitors have become the most promising candidates for next-generation power devices because of their higher energy density. Further, they show fast reversible redox reaction, wider thermal operating range, and low maintenance cost [7,8]. Supercapacitors have applications in digital
* Corresponding author. E-mail addresses: [email protected], [email protected] (R.B. Choudhary). http://dx.doi.org/10.1016/j.synthmet.2016.05.023 0379-6779/ã 2016 Elsevier B.V. All rights reserved.
communication systems, power tools, office machines, electronic devices, stationary power generation, load cranes, handheld gaming devices, mass rapid transit, and elevators [9,10]. For the successful fabrication of energy storage devices, conducting polymers have been employed in suitable combination with inorganic fillers. These conducting polymer mainly include pdoped polymers such as polythiophene (PTP), polyaniline (PANI), and polypyrrole (PPY). These are exclusively used because of their higher stability towards degradation than n-doped polymers and have high power density [11–15]. PTP has been selectively used over other polymers due to its high mobility, environmental stability, stability in oxidized form, light weight, ease of synthesis, controllable doping/dedoping chemistry, and reversible electrical properties by controlled charge processes [16–18]. However, rapid decrease in capacitance due to the structural change of PTP during charge-discharge process remains a serious matter of concern for the researcher [15]. The incorporation of inorganic fillers into PTP results into hybrid composite material that proves to be an effective electrode material for electrochemical energy storage device fabrication. The pseudo capacitance of the hybrid material is mostly contributed by PTP while the electric double-layer capacitance is generated by carbon and inorganic fillers [16]. The measurement of capacitance has been carried out by several researchers for different
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combinations of PTP and organic-inorganic fillers. Fu et al. [17] reported that by adding MWCNT to PTP, there was an increase in the capacitance up to 116 F/g. In another, communication, Senthilkumar et al. [18] reported electrochemical properties of PTP in the presence of various surfactants and they recorded the highest capacitance up to 117 F/g. Similarly, Hur et al. [19] reported the capacitance of PTP in the range of mF/g. Therefore, the addition of inorganic fillers to PTP enhances the energy and power capability to a great extent. Hence, the composite of PTP with TiO2 is expected to impart high electrochemical stability required for supercapacitor application. TiO2 is a material which possesses essential electrochemical properties with lower cost, better chemical stability, high surface area and fairly good eco-friendliness [20–23]. PTP/TiO2 composites carry the donor-acceptor interactions (p–n junction) between PTP and TiO2 surface on account of the reason that TiO2 is a typical n-type material whereas PTP is a p-type material [15,24]. In this paper, we report for the laboratory synthesis of PTP/TiO2 composite by the in-situ oxidative polymerization process. The effect of TiO2 content on its electrochemical properties was investigated. PTP/TiO2 composites of four different compositions were synthesized. These were examined by XRD, FESEM, EDX, FTIR, TGA, and UV–vis techniques. Their electrochemical measurements were carried out using CV, galvanostatic charging-discharging (GCD), and EIS methods. In the GCD curves, PTP/TiO2 composite (10:2) showed a high specific capacitance of 250 F/g at 1A/g and it also delivered a high energy density of 5.54 Wh/kg at a 263.8 W/kg power density. However, the PTP/TiO2 composite (10:2.5) showed a lower value of specific capacitance of 122 F/g at 1 A/g. Hence, this composition appeared to be an ineffective and antagonistic one and was ignored for the further studies.
The resulting precipitate was dried at 70 C in vacuum to get the composites. Following the above procedure, four different composites of PTP/TiO2 were synthesized by varying the TiO2 content in the composites viz. 10:1, 10:1.5,10:2 and 10:2.5 (weight%). 2.4. Structural characterization FTIR (Perkin Elmer-1600) spectroscopic technique was used to investigate the bonding properties in the wavelength range 400– 4000 cm1 of the resultant composites. For this purpose, diskshaped pellets were prepared by mixing the composite material (PTP/TiO2) with spectroscopic grade potassium bromide (KBr) in 1:20 (w/w) ratio. Crystallographic nature of the as-prepared composites was examined using X-ray diffractometer (BrookerD8) with nickel-filtered CuKa target (l = 1.5406 Å) at the scan rate of 0.5 /min. Raman spectroscopy was employed to investigate the interaction of different species present in the composites and also to characterize the TiO2 with Micro-Raman (Renishaw) spectrophotometer under 532 nm laser excitation. The microstructure and morphological studies of the assynthesized composites were performed using FESEM (SUPRA 55, CARL- ZEISS Germany) with accelerating voltage of 5 kV and vacuum (108–1010 mm Hg). In order to validate the existence of different elements in the composites, energy dispersive X-ray (EDX) analysis was employed. Ultraviolet– visible (UV–vis) spectroscopic technique was used and absorption spectra were observed on (Ocean optics, HR 4000) spectrometer in the wavelength 200–800 nm at a scanning rate of 480 nm/min. The thermal stability was examined using TGA (NETZSCH 0798-M) technique in the temperature range 30– 800 C and at the heating rate of 10 C min1 in the nitrogen environment.
2. Materials and method 2.5. Electrochemical characterization 2.1. Materials Thiophene monomer (Merck), oxidant anhydrous iron(III) chloride (Merck), chloroform (Merck), ethanol (Merck), and titanium dioxide (Merck) were procured for synthesis purpose. All other chemicals were used without any further purification. Distilled water was used throughout the whole preparation process. 2.2. Synthesis of polythiophene PTP was prepared by chemical oxidative polymerization. 3 mL thiophene monomer was dispersed in 50 mL chloroform and it was stirred for 30 min at room temperature. Then 10 g anhydrous FeCl3 solution was added drop wise to the above mixture and reaction was allowed to continue for 24 h in the nitrogen atmosphere at room temperature. The resulting suspension was filtered and washed 3–4 times with deionized water and ethanol. Finally, the product was dried under vacuum at 70 C for 24 h to obtain a brown powder of PTP.
In order to analyze the electrochemical properties of the assynthesized composites, cyclic voltammetry (CV), galvanostatic charging-discharging (GCD), and electrochemical impedance spectroscopic (EIS) measurements were employed using two electrochemical workstations (CHI-760D) at room temperature. A slurry of the electrode material was prepared by mixing 80 wt% active material, 15 wt% carbon black, and 5 wt% polyvinylidene difluoride (PVDF) binder in N-methyl pyrrolidine and was grinded adequately. In order to make the electrodes, the as-prepared slurry was coated onto 1 1 cm2 area of current collector carbon paper. Finally, it was dried at 100 C in the oven overnight. The total weight of the electrode material on both the electrodes was 1 mg for all electrochemical measurements. Electrochemical studies based on CV, GCD, and EIS were done by using two conventional electrode systems in 1 M H2SO4 aqueous electrolyte. CV curves were recorded from 0.2 to 0.8 V at varying scan rates. The galvanostatic charge-discharge measurement was performed in the range 0–0.8 V at various current densities. EIS was performed in the frequency range 0.1 Hz to 100 KHz at open circuit potential with ac perturbation of 5 mV.
2.3. Synthesis of PTP/TiO2 composites 3. Results and discussion PTP/TiO2 composites were synthesized by chemical oxidative polymerization. 3 mL thiophene monomer and a given amount of TiO2 have added to 50 mL CHCl3 in 500 mL three-necked flask. Thereupon, 10 g anhydrous FeCl3 solution was added drop wise to the above mixture over a period of 4–5 h. The reaction was continued for another 24 h with constant stirring in the nitrogen atmosphere at room temperature. The resultant suspension was filtered and washed with distilled water and ethanol respectively.
3.1. FTIR study Fig. 1 shows FTIR spectra for pure PTP and PTP/TiO2 composites in the region 400 cm1–4000 cm1. Several low-intensity peaks were observed in the range 400 cm1–4000 cm1 for pure PTP. The peak at 826 cm1occurred due to C H out-of-plane vibration of the 2,5 substituted thiophene monomer. The peak due to C H
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3.2. XRD study Fig. 2 shows the XRD pattern for pure PTP and PTP/TiO2 composites with varying content of TiO2. Broad diffraction peak at 2u = 10 –20 was attributed to the amorphous nature of pure PTP [18,25]. Diffraction peaks for PTP/TiO2 composites were found at 25.43 , 36.86 , 37.73 , 38.53 , 48.91, 53.82 , 55.02 , 62.63 , and 68.75 [31]. The XRD peaks for TiO2 were in good agreement with the bulk TiO2 data which had a tetragonal structure with space group 141/amd, and unit cell parameters a = 3.78, b = 3.78, and c = 9.51. It was observed that with an increase in TiO2 content in the composites, the position of the crystalline peak (101) was shifted slightly towards the lower Bragg diffraction angle. Also, there was a gradual decrease in the intensity of the amorphous peak of pure PTP with an increase in TiO2 content in the said composite. These results elucidated successful mix-up of the two constituents in PTP/TiO2 composites. Fig. 1. FTIR spectra for (a) Pure PTP, (b) PTP/TiO2 (10:1), (c) PTP/TiO2 (10:1.5), and (d) PTP/TiO2 (10:2) composites.
bending band was observed at 1055 cm1. The peak at 1410 cm1 was attributed to C¼C stretching band [25–27]. The most important characteristic peak was observed at 670 cm1 which denoted C-S stretching in thiophene rings [18,27]. The absorption band at 1640 cm1corresponded to the asymmetric stretching vibration of thiophene [18,28,29]. The above characteristic peaks confirmed the formation of PTP. From the FTIR spectra of PTP/TiO2 composites, it was observed that almost all peaks shifted towards lower wave number. Interestingly, the intensities of the peaks were observed to increase with the increase in TiO2 content in the composites which revealed the better interaction between PTP and TiO2 during the process of in-situ polymerization. Further, the broad parabolic curve peak at 3395 cm1–3400 cm1 indicated the presence of OH stretching of water (due to moisture in KBr) in pure PTP and PTP/TiO2 composites [18]. The peak at 471 cm1 represented Ti-O-Ti stretching, which could be attributed to the addition of TiO2 into polythiophene [23,30]. All together, these results revealed that polythiophene and TiO2 particles were not only blended or mixed up rather these also developed strong interactions between them.
Fig. 2. XRD spectra for (a) Pure PTP, (b) PTP/TiO2 (10:1), (c) PTP/TiO2 (10:1.5.), and (d) PTP/TiO2 (10:2) composites.
3.3. Raman spectra Fig. 3 represents Raman spectra for pure PTP and its composites with varying amount of TiO2 content. The bands at 1500 cm1 and 1450 cm1 corresponded to C¼C stretching region of pure PTP [25]. The band in the weaker range 1050–1100 cm1 corresponded to C C stretching, CH wagging component and at 636 cm1 for C– S–C ring deformation [22,25]. PTP/TiO2 composites showed a peak at 1425 cm1 and it was attributed to the quinoid unit. The peak at 630 cm1 showed the presence of thiophene in PTP/TiO2 composites. While specific peak at 150 cm1was assigned to the signature of the anatase TiO2 [32]. 3.4. FESEM analysis Fig. 4 shows the surface morphology of pure PTP and PTP/TiO2 composites. This helps to understand the nature of the level of dispersion and hence, the crystal formation can be understood accordingly. FESEM image of pure PTP showed that polythiophene had a uniform surface. The particles of pure PTP had a size ranging in nanometer scale possessing some fibrils or thread-like structure. The FESEM images of the PTP/TiO2 composites revealed a uniform thin coating of pure PTP by TiO2. Fig. 4d revealed that due to the presence of TiO2, these acquired globular or spherical shape emerging together randomly of nanometer size. It could also be
Fig. 3. Raman spectra for (a) Pure PTP, (b) PTP/TiO2 (10:1), (c) PTP/TiO2 (10:1.5), and (d) PTP/TiO2 (10:2) composites.
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Fig. 4. FESEM images for (a) Pure PTP, (b) PTP/TiO2 (10:1), (c) PTP/TiO2 (10:1.5), and (d) PTP/TiO2 (10:2) composites.
observed that the length of pure PTP and PTP/TiO2 was around 100 nm and TiO2 were deposited on the entire surface of PTP. However, the FESEM image of PTP/TiO2 (10:2.5) composite has been shown as supporting information in Fig. S1 which was of least interest.
composites. The pure PTP specimen comprised the elemental content of C, O, and S whereas PTP/TiO2 composites consisted the elemental content of C, O, S and Ti. For all the EDX spectra, the additional peak for Platinum (Pt) was also observed. This occurred due to the presence of platinum (pt) coating onto the specimen surface.
3.5. EDX analysis 3.6. Optical studies Fig. 5 shows EDX spectra for pure PTP and PTP/TiO2 (10:2) composite. It describes quantitative and qualitative measurements of the chemical constituents of pure PTP and PTP/TiO2
Fig. 6 shows UV–vis absorption spectra for pure PTP and PTP/ TiO2 composites with varying content of TiO2 and their
Fig. 5. EDX spectra for (a) Pure PTP and (b) PTP/TiO2 (10:2) composite.
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Fig. 6. Tauc plots for (a) Pure PTP, (b) PTP/TiO2 (10:1), (c) PTP/TiO2 (10:1.5), and (d) PTP/TiO2 (10:2) composites and their corresponding absorption spectra.
corresponding Tauc plots. The absorption peak for PTP was around 370 nm which was attributed to p-p transition in thiophene ring corresponding to the band gap of 2.74 eV. This was in good agreement with the reported band gap of polythiophene [18,33]. In the case of PTP/TiO2 (10:1) composite, the absorption peak occurred around 410 nm corresponding to a band gap of 2.4 eV. Whereas, in the case of PTP/TiO2 composite (10:1.5) and PTP/TiO2 (10:2) composite, the absorption peaks extended to 420 nm and 430 nm corresponding to the band gap of 2.3 eV and 2.2 eV respectively. Further, it was observed that with an increase in TiO2 content in the PTP/TiO2 composites, the positions of absorption peaks were shifted slightly towards the higher wavelength (red shift) with a decrease in the energy gap. The absorption coefficient a was correlated to the photon energyh# according to the following relation [34]. ah# ¼ A h# Eg n where A—constant, h is the Planck constant, # is the photon frequency, Eg is the optical band gap, and n is the index related to the optical absorption process (n = 2 for an indirect allowed transition). 3.7. Cyclic voltammetry Fig. 7 represents the electrochemical behavior of PTP and PTP/ TiO2 composites. It provided cyclic voltammetric curves in the potential range 0.2V-0.8 V at the scan rate of 5 mV/sec, 10 mV/ sec, 20 mV/sec, 50 mV/sec, and 100 mV/sec. The positive current in these curves represented oxidation and negative current represented reduction process. It was also observed that the cathodic peak shifted positively and anodic peaks shifted negatively with an increase in potential sweep rates from
5 mV/s to 100 mV/s. The cyclic voltammograms of pure PTP showed nonrectangular shape. This asymmetric cyclic voltammograms can be ascribed to the pseudocapacitive contribution. The CV curves were plotted with different scan rates and it displayed oxidation and reduction properties of conducting polymer electrode. Where the scan rate was doubled, the area under CV loop and the peak current response increased for PTP and its composites. For a fixed potential window, at higher scan rates the increasing trend of CV loop area and the current response would be attributed to the fact that the ion only interacted with the outer surface of the electrode. On the other hand at slower scan rates, the current response was low and ion from the electrolyte could be reach almost all the pores of the electrode and take a longer time to record a voltammogram as compared to fast scan rate [35]. Hence, due to very little interaction of the ion with the bulk material of electrode inside the pores, a very little pseudocapacitive was contributed resulting in the overall decrease in the pseudocapacitance [36]. This fact was supported by the observed results as shown in Fig. 7d. The effect was most pronounced in the case of PTP/TiO2 (10:2) composite where the current response was maximum and the nature of the CV curve was nearly rectangular indicating the aptness of the composite for pseudocapacitor application as compared to the other composites. The CV plot of PTP/TiO2 (10:2.5) composite has been shown as supporting information in Fig. S2. 3.8. Galvanostatic charging-discharging Fig. 8 shows galvanostatic charging-discharging (GCD) curves for PTP and PTP/TiO2 composite electrodes with varying amount of TiO2 content at the current density of 1A/g, 2A/g, 3A/g, and 5A/ g. It was observed that the GCD curve of PTP/TiO2 composites
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Fig. 7. Cyclic voltammograms for (a) Pure PTP, (b) PTP/TiO2 (10:1), (c) PTP/TiO2 (10:1), and, (d) PTP/TiO2 (10:2) composites.
presented triangular behavior, good linearity, and specific capacitance of PTP increased with the incorporation of TiO2. The linearity of charge-discharge curves indicated fast reversible Faradic reaction and good Columbic efficiency. At higher current density, due to the lack of sufficient time for ions to complete doping/dedoping process, only outer layers of the composite could contribute to the charge-discharge process leading to a triangular behavior of GCD plots [35,37,38]. PTP/TiO2 composites showed longer charge-discharge time curves as compared to pure PTP. This might be caused due to the unique crystal-amorphous structure of the composites. Such behaviour of the electrode materials could be attributed to the fact that PTP is a p-doped polymer with good electrical conductivity and uniform distribution of TiO2 particles on the surface of PTP facilitating the charge transport from TiO2 leading to a rapid redox reaction. This revealed that the supercapacitor carried good capacitive behavior with good charging-discharging reversibility. The specific capacitance was calculated by using the relation as given below [7,12].
density of 1A/g. The value of specific capacitance followed and increasing trend with an increase in the amount of TiO2 content in the composites. Further, it decreased to 122 F/g for PTP/TiO2 (10:2.5) (see supporting information Fig. S3). In the case of PTP/ TiO2 (10:2.5), the specific capacitance value was lowered due to excess TiO2 content which remained uncoated (see FESEM supporting information Fig. S1). The specific capacitance of the as-prepared PTP/TiO2 composites in comparison with other reported PTP composites has been shown in Table 1. The galvanostatic charge-discharge curves for PTP and PTP/TiO2 composite electrodes were recorded at a different current density as shows in Fig. 8. The energy density (E) and power density (P) were estimated using the following equation [7].
Csp = 2 [I/(m dV/dt)]
where, V is the potential window (V) and t is the discharge time (h). The calculated energy density of PTP and PTP/TiO2 composites were found around 1.77 Wh/kg, 2.21 Wh/kg, 3.98 Wh/kg and 5.54 Wh/kg at power density of 177 W/kg, 194.2 W/kg, 199 W/kg and 263.8 W/kg respectively.
(1)
where, Csp is the specific capacitance (F/g), I is the applied chargedischarge current (A), dV/dt is the slope of the discharge curve (V/ s), and m is the mass of the active martial coated on the carbon paper electrode (mg). Multiplication of capacitance by a factor of 2 is included because the series capacitance formed in two electrode system. The specific capacitance calculated for PTP, PTP/TiO2 (10:1), PTP/TiO2 (10:1.5), and PTP/TiO2 (10:2) were found around 80 F/g, 100 F/g, 180 F/g, and 250 F/g respectively at the current
E = 1/8CspV2 1000/3600
(2)
P = E/t
(3)
3.9. Impedance study Fig. 9 shows electrochemical impedance spectra for PTP and PTP/TiO2 composites. Nyquist plots were obtained to study the
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Fig. 8. Galvanostatic charging-discharging curves for (a) Pure PTP, (b) PTP/TiO2 (10:1), (c) PTP/TiO2 (10:1.5), and (d) PTP/TiO2 (10:2) composites.
Table 1 Comparative values of capacitance for PTP/TiO2 and many other composites. Capacitor
Specific Capacitance (F/g)
Current Density (A/g)
Energy Density (Wh/kg)
Power Density (W/kg)
Reference No.
PTP/CNT PEDOTa PTP/CNT GO/PTPb PTP/TiO2
110 130 216 296 250
1 1 1 0.3 1
– – – – 5.54
– – – – 263.8
[17] [39] [40] [41] Present Work
– Not available. a PEDOT-Poly (3,4 ethylenedioxythiophene). b GO/Graphene oxide.
capacitive properties of PTP and PTP/TiO2 composites by evaluating their interfacial properties at the open-circuit potential in the frequency range 0.1Hz-100 KHz. The observed semi-circular portion for all the plots at higher frequencies corresponded to the electron-transfer-limited process. For pure PTP, Nyquist plot had a semicircle of larger diameter as compared to PTP/TiO2 composites. This could be attributed to the higher interfacial charge transfer resistance which in turn indicated a lower conductivity for PTP than the PTP/TiO2 composites. A smaller diameter in the case of PTP/TiO2 composites indicated that the electrolyte ions could easily access the inner layers of the polymer. The Nyquist plots for PTP/TiO2 composites became more vertical
with a gradual increase in the amount of TiO2. Since a higher value of the imaginary part of the impedance (at low frequency) can lead to an enhanced capacitive behaviour. Hence, the addition of TiO2 causes to improve the capacitive nature of PTP in the composites [13]. As the frequency increased, enhanced inclined features of the spectra were observed due to stronger surface redox reactions at the electrode-electrolyte interface. The EIS measurements of the composites indicated that these possessed a reduced ion diffusion resistance and charge transfer resistance, which collectively resulted in an improvement of the electrochemical capacitance. The best result was witnessed in the case of 10:2 composition, thereby indicating that PTP/TiO2 composite (10:2) had most
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Fig. 9. Nyquist plots for (a) Pure PTP, (b) PTP/TiO2 (10:1), (c) PTP/TiO2 (10:1.5), and (d) PTP/TiO2 (10:2) composites.
enhanced electron collection and transportation mechanism within the electrode amongst all the composites. The Nyquist plot for PTP/TiO2 (10:2.5) composite have been shown as supporting information in Fig. S4. 3.9.1. TGA analysis Fig. 10 shows thermograms for pure PTP and PTP/TiO2 composites. The TGA measurements showed a marked difference in temperature stability for pure PTP and PTP/TiO2 composites. For pure PTP, the first weight loss of 8.5% was observed in the range of 90–100 C which was probably due to the de-intercalation of water molecules or volatile impurities as shown in Fig. 10a. The gradual weight loss between 150 C and 550 C was ascribed to higher amorphous nature (very low crystallinity) of PTP [42]. The third step of weight loss at 650 C was related to the degradation and decomposition of PTP. As compared to the TGA of the PTP, PTP/TiO2 composites showed higher thermal stability. Fig. 10b,c,d revealed that the higher stability of PTP/TiO2 composites might occur due to the stabilizing effect of TiO2 on PTP. The TGA results of PTP and PTP/ TiO2 composites have been summarized in Table 2. Pure PTP showed a weight loss around 41.92% at 200 C this loss got reduced to 29.75%, 15.01%, and 9.94% for PTP/TiO2 composites corresponding to the addition of 10%, 15%, and 20% of TiO2 content respectively. In the case of PTP/TiO2 (10:2) composites, highest thermal stability was observed in the temperature 300–400 C. The mass reduction was just 1.33% in this temperature range for the above composite. The composite (10:2) was seen to have 81.86% weight retention when the temperature reached 800 C which could be attributed to the incorporation of TiO2. A similar result was reported related to the improved thermal stability of PTP and various polymers due to increasing weight% of TiO2 and many other metal oxides [23,43–47].
Fig. 10. TGA thermograms for (a) PTP, (b) PTP/TiO2 (10:1), (c) PTP/TiO2 (10:1.5), and (d) PTP/TiO2 (10:2) composites.
4. Conclusions In summary, we conclude that the PTP/TiO2 composites were successfully prepared via in-situ oxidative polymerization method. The morphological study revealed that PTP and TiO2 had a uniform distribution in the polymeric composites at the nanometer scale. UV–vis study revealed that there was a successive decrease in the band gap (2.7–2.2 eV) of the PTP/TiO2 corresponding to the increase in TiO2 content. The EIS studies revealed that the electrochemical performance of pure PTP was markedly improved due to the successful incorporation of TiO2. The highest value of specific capacitance (250F/g) was recorded for PTP/TiO2 (10:2) composite at the current density of 1 A/g. This value was quite higher than that of pure PTP (80 F/g). Furthermore, the energy density reached to 5.54Wh/kg at the power density of 263.8W/kg. Acknowledgements The authors acknowledge their sincere thanks to Professor N. R. Mandre Department of Fuel and Mineral Engineering, Indian School of Mines, Dhanbad for providing TGA facility. The authors are very grateful to Dr. M.V.Shelke (Scientist NCL, India) and Dr. G.C. Nayak (Assist. Professor Dept. of App. Chemistry, ISM, India) for their constant support, fruitful discussion, and valuable suggestions. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. synthmet.2016.05.023.
Table 2 Thermgravimetric analysis. Sample
Weight loss (%) at 200 C
Weight loss (%) at 300 C
Weight loss (%) at 400 C
Weight loss (%) at 500 C
Retention of weight (%) at 800 C
PTP PTP/TiO2(10:1) PTP/TiO2 (10:1.5) PTP/TiO2 (10:2)
41.92 29.75 15.01 9.94
45.02 35.03 17.42 11.12
48.98 36.10 19.63 12.45
52.29 47.43 24.09 16.59
24.98 49.20 64.92 81.86
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
High-performance supercapacitors based on polymeric binary composites of polythiophene (PTP)–titanium dioxide (TiO2) Anukul K. Thakur, Ram Bilash Choudhary* Nanostructured Composite Materials Lab, Department of Applied Physics, Indian School of Mines, Dhanbad 826004, India
A R T I C L E I N F O
Article history: Received 8 January 2016 Received in revised form 17 May 2016 Accepted 20 May 2016 Available online xxx Keywords: Polythiophene Titanium dioxide Specific capacitance
A B S T R A C T
Polythiophene (PTP)-titanium dioxide (TiO2) composites were prepared via oxidative polymerization process using varying ratios of TiO2 content. Fourier transform-infrared (FTIR) spectroscopy was used to confirm the incorporation of TiO2 into PTP. X-ray diffraction spectroscopy was used to examine the nature of pure PTP and PTP/TiO2 composites. Distribution of TiO2 in PTP was examined by field emission scanning electron microscopy (FESEM). The electrochemical properties were studied by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) with 1 M aq. H2SO4 as an electrolyte with two electrode system. From charging-discharging measurement, it was found that the capacitance of PTP/TiO2 composites initially increased and then it decreased gradually. The maximum specific capacitance (250 F/g) and energy density (5.54Wh/Kg) were recorded for PTP/TiO2 (10:2) composite at a current density of 1A/g. Furthermore, the power density was achieved up to 263.8W/kg at a current density of 1A/g. ã 2016 Elsevier B.V. All rights reserved.
1. Introduction Since last few decades, enormous research activities have been devoted towards the design and development of different categories of energy storage devices. The development of energy storage devices requires highly efficient and synergistic combination of materials of hybrid origin. Specifically, the electrochemical device such as supercapacitor has been globally recognized as one of the important class of energy storage devices [1–3]. There has been tremendous interest among the researchers for fabricating supercapacitors as efficient energy storage devices because of their simple working principle, fast charge-discharge rate, long cyclic life, and high power capability [4–6]. Supercapacitors have been classified on the basis of energy storage action mechanisms: (i) electric double layer capacitor (EDLC), and (ii) pseudocapacitor. Among these, pseudocapacitors have become the most promising candidates for next-generation power devices because of their higher energy density. Further, they show fast reversible redox reaction, wider thermal operating range, and low maintenance cost [7,8]. Supercapacitors have applications in digital
* Corresponding author. E-mail addresses: [email protected], [email protected] (R.B. Choudhary). http://dx.doi.org/10.1016/j.synthmet.2016.05.023 0379-6779/ã 2016 Elsevier B.V. All rights reserved.
communication systems, power tools, office machines, electronic devices, stationary power generation, load cranes, handheld gaming devices, mass rapid transit, and elevators [9,10]. For the successful fabrication of energy storage devices, conducting polymers have been employed in suitable combination with inorganic fillers. These conducting polymer mainly include pdoped polymers such as polythiophene (PTP), polyaniline (PANI), and polypyrrole (PPY). These are exclusively used because of their higher stability towards degradation than n-doped polymers and have high power density [11–15]. PTP has been selectively used over other polymers due to its high mobility, environmental stability, stability in oxidized form, light weight, ease of synthesis, controllable doping/dedoping chemistry, and reversible electrical properties by controlled charge processes [16–18]. However, rapid decrease in capacitance due to the structural change of PTP during charge-discharge process remains a serious matter of concern for the researcher [15]. The incorporation of inorganic fillers into PTP results into hybrid composite material that proves to be an effective electrode material for electrochemical energy storage device fabrication. The pseudo capacitance of the hybrid material is mostly contributed by PTP while the electric double-layer capacitance is generated by carbon and inorganic fillers [16]. The measurement of capacitance has been carried out by several researchers for different
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combinations of PTP and organic-inorganic fillers. Fu et al. [17] reported that by adding MWCNT to PTP, there was an increase in the capacitance up to 116 F/g. In another, communication, Senthilkumar et al. [18] reported electrochemical properties of PTP in the presence of various surfactants and they recorded the highest capacitance up to 117 F/g. Similarly, Hur et al. [19] reported the capacitance of PTP in the range of mF/g. Therefore, the addition of inorganic fillers to PTP enhances the energy and power capability to a great extent. Hence, the composite of PTP with TiO2 is expected to impart high electrochemical stability required for supercapacitor application. TiO2 is a material which possesses essential electrochemical properties with lower cost, better chemical stability, high surface area and fairly good eco-friendliness [20–23]. PTP/TiO2 composites carry the donor-acceptor interactions (p–n junction) between PTP and TiO2 surface on account of the reason that TiO2 is a typical n-type material whereas PTP is a p-type material [15,24]. In this paper, we report for the laboratory synthesis of PTP/TiO2 composite by the in-situ oxidative polymerization process. The effect of TiO2 content on its electrochemical properties was investigated. PTP/TiO2 composites of four different compositions were synthesized. These were examined by XRD, FESEM, EDX, FTIR, TGA, and UV–vis techniques. Their electrochemical measurements were carried out using CV, galvanostatic charging-discharging (GCD), and EIS methods. In the GCD curves, PTP/TiO2 composite (10:2) showed a high specific capacitance of 250 F/g at 1A/g and it also delivered a high energy density of 5.54 Wh/kg at a 263.8 W/kg power density. However, the PTP/TiO2 composite (10:2.5) showed a lower value of specific capacitance of 122 F/g at 1 A/g. Hence, this composition appeared to be an ineffective and antagonistic one and was ignored for the further studies.
The resulting precipitate was dried at 70 C in vacuum to get the composites. Following the above procedure, four different composites of PTP/TiO2 were synthesized by varying the TiO2 content in the composites viz. 10:1, 10:1.5,10:2 and 10:2.5 (weight%). 2.4. Structural characterization FTIR (Perkin Elmer-1600) spectroscopic technique was used to investigate the bonding properties in the wavelength range 400– 4000 cm1 of the resultant composites. For this purpose, diskshaped pellets were prepared by mixing the composite material (PTP/TiO2) with spectroscopic grade potassium bromide (KBr) in 1:20 (w/w) ratio. Crystallographic nature of the as-prepared composites was examined using X-ray diffractometer (BrookerD8) with nickel-filtered CuKa target (l = 1.5406 Å) at the scan rate of 0.5 /min. Raman spectroscopy was employed to investigate the interaction of different species present in the composites and also to characterize the TiO2 with Micro-Raman (Renishaw) spectrophotometer under 532 nm laser excitation. The microstructure and morphological studies of the assynthesized composites were performed using FESEM (SUPRA 55, CARL- ZEISS Germany) with accelerating voltage of 5 kV and vacuum (108–1010 mm Hg). In order to validate the existence of different elements in the composites, energy dispersive X-ray (EDX) analysis was employed. Ultraviolet– visible (UV–vis) spectroscopic technique was used and absorption spectra were observed on (Ocean optics, HR 4000) spectrometer in the wavelength 200–800 nm at a scanning rate of 480 nm/min. The thermal stability was examined using TGA (NETZSCH 0798-M) technique in the temperature range 30– 800 C and at the heating rate of 10 C min1 in the nitrogen environment.
2. Materials and method 2.5. Electrochemical characterization 2.1. Materials Thiophene monomer (Merck), oxidant anhydrous iron(III) chloride (Merck), chloroform (Merck), ethanol (Merck), and titanium dioxide (Merck) were procured for synthesis purpose. All other chemicals were used without any further purification. Distilled water was used throughout the whole preparation process. 2.2. Synthesis of polythiophene PTP was prepared by chemical oxidative polymerization. 3 mL thiophene monomer was dispersed in 50 mL chloroform and it was stirred for 30 min at room temperature. Then 10 g anhydrous FeCl3 solution was added drop wise to the above mixture and reaction was allowed to continue for 24 h in the nitrogen atmosphere at room temperature. The resulting suspension was filtered and washed 3–4 times with deionized water and ethanol. Finally, the product was dried under vacuum at 70 C for 24 h to obtain a brown powder of PTP.
In order to analyze the electrochemical properties of the assynthesized composites, cyclic voltammetry (CV), galvanostatic charging-discharging (GCD), and electrochemical impedance spectroscopic (EIS) measurements were employed using two electrochemical workstations (CHI-760D) at room temperature. A slurry of the electrode material was prepared by mixing 80 wt% active material, 15 wt% carbon black, and 5 wt% polyvinylidene difluoride (PVDF) binder in N-methyl pyrrolidine and was grinded adequately. In order to make the electrodes, the as-prepared slurry was coated onto 1 1 cm2 area of current collector carbon paper. Finally, it was dried at 100 C in the oven overnight. The total weight of the electrode material on both the electrodes was 1 mg for all electrochemical measurements. Electrochemical studies based on CV, GCD, and EIS were done by using two conventional electrode systems in 1 M H2SO4 aqueous electrolyte. CV curves were recorded from 0.2 to 0.8 V at varying scan rates. The galvanostatic charge-discharge measurement was performed in the range 0–0.8 V at various current densities. EIS was performed in the frequency range 0.1 Hz to 100 KHz at open circuit potential with ac perturbation of 5 mV.
2.3. Synthesis of PTP/TiO2 composites 3. Results and discussion PTP/TiO2 composites were synthesized by chemical oxidative polymerization. 3 mL thiophene monomer and a given amount of TiO2 have added to 50 mL CHCl3 in 500 mL three-necked flask. Thereupon, 10 g anhydrous FeCl3 solution was added drop wise to the above mixture over a period of 4–5 h. The reaction was continued for another 24 h with constant stirring in the nitrogen atmosphere at room temperature. The resultant suspension was filtered and washed with distilled water and ethanol respectively.
3.1. FTIR study Fig. 1 shows FTIR spectra for pure PTP and PTP/TiO2 composites in the region 400 cm1–4000 cm1. Several low-intensity peaks were observed in the range 400 cm1–4000 cm1 for pure PTP. The peak at 826 cm1occurred due to C H out-of-plane vibration of the 2,5 substituted thiophene monomer. The peak due to C H
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3.2. XRD study Fig. 2 shows the XRD pattern for pure PTP and PTP/TiO2 composites with varying content of TiO2. Broad diffraction peak at 2u = 10 –20 was attributed to the amorphous nature of pure PTP [18,25]. Diffraction peaks for PTP/TiO2 composites were found at 25.43 , 36.86 , 37.73 , 38.53 , 48.91, 53.82 , 55.02 , 62.63 , and 68.75 [31]. The XRD peaks for TiO2 were in good agreement with the bulk TiO2 data which had a tetragonal structure with space group 141/amd, and unit cell parameters a = 3.78, b = 3.78, and c = 9.51. It was observed that with an increase in TiO2 content in the composites, the position of the crystalline peak (101) was shifted slightly towards the lower Bragg diffraction angle. Also, there was a gradual decrease in the intensity of the amorphous peak of pure PTP with an increase in TiO2 content in the said composite. These results elucidated successful mix-up of the two constituents in PTP/TiO2 composites. Fig. 1. FTIR spectra for (a) Pure PTP, (b) PTP/TiO2 (10:1), (c) PTP/TiO2 (10:1.5), and (d) PTP/TiO2 (10:2) composites.
bending band was observed at 1055 cm1. The peak at 1410 cm1 was attributed to C¼C stretching band [25–27]. The most important characteristic peak was observed at 670 cm1 which denoted C-S stretching in thiophene rings [18,27]. The absorption band at 1640 cm1corresponded to the asymmetric stretching vibration of thiophene [18,28,29]. The above characteristic peaks confirmed the formation of PTP. From the FTIR spectra of PTP/TiO2 composites, it was observed that almost all peaks shifted towards lower wave number. Interestingly, the intensities of the peaks were observed to increase with the increase in TiO2 content in the composites which revealed the better interaction between PTP and TiO2 during the process of in-situ polymerization. Further, the broad parabolic curve peak at 3395 cm1–3400 cm1 indicated the presence of OH stretching of water (due to moisture in KBr) in pure PTP and PTP/TiO2 composites [18]. The peak at 471 cm1 represented Ti-O-Ti stretching, which could be attributed to the addition of TiO2 into polythiophene [23,30]. All together, these results revealed that polythiophene and TiO2 particles were not only blended or mixed up rather these also developed strong interactions between them.
Fig. 2. XRD spectra for (a) Pure PTP, (b) PTP/TiO2 (10:1), (c) PTP/TiO2 (10:1.5.), and (d) PTP/TiO2 (10:2) composites.
3.3. Raman spectra Fig. 3 represents Raman spectra for pure PTP and its composites with varying amount of TiO2 content. The bands at 1500 cm1 and 1450 cm1 corresponded to C¼C stretching region of pure PTP [25]. The band in the weaker range 1050–1100 cm1 corresponded to C C stretching, CH wagging component and at 636 cm1 for C– S–C ring deformation [22,25]. PTP/TiO2 composites showed a peak at 1425 cm1 and it was attributed to the quinoid unit. The peak at 630 cm1 showed the presence of thiophene in PTP/TiO2 composites. While specific peak at 150 cm1was assigned to the signature of the anatase TiO2 [32]. 3.4. FESEM analysis Fig. 4 shows the surface morphology of pure PTP and PTP/TiO2 composites. This helps to understand the nature of the level of dispersion and hence, the crystal formation can be understood accordingly. FESEM image of pure PTP showed that polythiophene had a uniform surface. The particles of pure PTP had a size ranging in nanometer scale possessing some fibrils or thread-like structure. The FESEM images of the PTP/TiO2 composites revealed a uniform thin coating of pure PTP by TiO2. Fig. 4d revealed that due to the presence of TiO2, these acquired globular or spherical shape emerging together randomly of nanometer size. It could also be
Fig. 3. Raman spectra for (a) Pure PTP, (b) PTP/TiO2 (10:1), (c) PTP/TiO2 (10:1.5), and (d) PTP/TiO2 (10:2) composites.
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Fig. 4. FESEM images for (a) Pure PTP, (b) PTP/TiO2 (10:1), (c) PTP/TiO2 (10:1.5), and (d) PTP/TiO2 (10:2) composites.
observed that the length of pure PTP and PTP/TiO2 was around 100 nm and TiO2 were deposited on the entire surface of PTP. However, the FESEM image of PTP/TiO2 (10:2.5) composite has been shown as supporting information in Fig. S1 which was of least interest.
composites. The pure PTP specimen comprised the elemental content of C, O, and S whereas PTP/TiO2 composites consisted the elemental content of C, O, S and Ti. For all the EDX spectra, the additional peak for Platinum (Pt) was also observed. This occurred due to the presence of platinum (pt) coating onto the specimen surface.
3.5. EDX analysis 3.6. Optical studies Fig. 5 shows EDX spectra for pure PTP and PTP/TiO2 (10:2) composite. It describes quantitative and qualitative measurements of the chemical constituents of pure PTP and PTP/TiO2
Fig. 6 shows UV–vis absorption spectra for pure PTP and PTP/ TiO2 composites with varying content of TiO2 and their
Fig. 5. EDX spectra for (a) Pure PTP and (b) PTP/TiO2 (10:2) composite.
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Fig. 6. Tauc plots for (a) Pure PTP, (b) PTP/TiO2 (10:1), (c) PTP/TiO2 (10:1.5), and (d) PTP/TiO2 (10:2) composites and their corresponding absorption spectra.
corresponding Tauc plots. The absorption peak for PTP was around 370 nm which was attributed to p-p transition in thiophene ring corresponding to the band gap of 2.74 eV. This was in good agreement with the reported band gap of polythiophene [18,33]. In the case of PTP/TiO2 (10:1) composite, the absorption peak occurred around 410 nm corresponding to a band gap of 2.4 eV. Whereas, in the case of PTP/TiO2 composite (10:1.5) and PTP/TiO2 (10:2) composite, the absorption peaks extended to 420 nm and 430 nm corresponding to the band gap of 2.3 eV and 2.2 eV respectively. Further, it was observed that with an increase in TiO2 content in the PTP/TiO2 composites, the positions of absorption peaks were shifted slightly towards the higher wavelength (red shift) with a decrease in the energy gap. The absorption coefficient a was correlated to the photon energyh# according to the following relation [34]. ah# ¼ A h# Eg n where A—constant, h is the Planck constant, # is the photon frequency, Eg is the optical band gap, and n is the index related to the optical absorption process (n = 2 for an indirect allowed transition). 3.7. Cyclic voltammetry Fig. 7 represents the electrochemical behavior of PTP and PTP/ TiO2 composites. It provided cyclic voltammetric curves in the potential range 0.2V-0.8 V at the scan rate of 5 mV/sec, 10 mV/ sec, 20 mV/sec, 50 mV/sec, and 100 mV/sec. The positive current in these curves represented oxidation and negative current represented reduction process. It was also observed that the cathodic peak shifted positively and anodic peaks shifted negatively with an increase in potential sweep rates from
5 mV/s to 100 mV/s. The cyclic voltammograms of pure PTP showed nonrectangular shape. This asymmetric cyclic voltammograms can be ascribed to the pseudocapacitive contribution. The CV curves were plotted with different scan rates and it displayed oxidation and reduction properties of conducting polymer electrode. Where the scan rate was doubled, the area under CV loop and the peak current response increased for PTP and its composites. For a fixed potential window, at higher scan rates the increasing trend of CV loop area and the current response would be attributed to the fact that the ion only interacted with the outer surface of the electrode. On the other hand at slower scan rates, the current response was low and ion from the electrolyte could be reach almost all the pores of the electrode and take a longer time to record a voltammogram as compared to fast scan rate [35]. Hence, due to very little interaction of the ion with the bulk material of electrode inside the pores, a very little pseudocapacitive was contributed resulting in the overall decrease in the pseudocapacitance [36]. This fact was supported by the observed results as shown in Fig. 7d. The effect was most pronounced in the case of PTP/TiO2 (10:2) composite where the current response was maximum and the nature of the CV curve was nearly rectangular indicating the aptness of the composite for pseudocapacitor application as compared to the other composites. The CV plot of PTP/TiO2 (10:2.5) composite has been shown as supporting information in Fig. S2. 3.8. Galvanostatic charging-discharging Fig. 8 shows galvanostatic charging-discharging (GCD) curves for PTP and PTP/TiO2 composite electrodes with varying amount of TiO2 content at the current density of 1A/g, 2A/g, 3A/g, and 5A/ g. It was observed that the GCD curve of PTP/TiO2 composites
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Fig. 7. Cyclic voltammograms for (a) Pure PTP, (b) PTP/TiO2 (10:1), (c) PTP/TiO2 (10:1), and, (d) PTP/TiO2 (10:2) composites.
presented triangular behavior, good linearity, and specific capacitance of PTP increased with the incorporation of TiO2. The linearity of charge-discharge curves indicated fast reversible Faradic reaction and good Columbic efficiency. At higher current density, due to the lack of sufficient time for ions to complete doping/dedoping process, only outer layers of the composite could contribute to the charge-discharge process leading to a triangular behavior of GCD plots [35,37,38]. PTP/TiO2 composites showed longer charge-discharge time curves as compared to pure PTP. This might be caused due to the unique crystal-amorphous structure of the composites. Such behaviour of the electrode materials could be attributed to the fact that PTP is a p-doped polymer with good electrical conductivity and uniform distribution of TiO2 particles on the surface of PTP facilitating the charge transport from TiO2 leading to a rapid redox reaction. This revealed that the supercapacitor carried good capacitive behavior with good charging-discharging reversibility. The specific capacitance was calculated by using the relation as given below [7,12].
density of 1A/g. The value of specific capacitance followed and increasing trend with an increase in the amount of TiO2 content in the composites. Further, it decreased to 122 F/g for PTP/TiO2 (10:2.5) (see supporting information Fig. S3). In the case of PTP/ TiO2 (10:2.5), the specific capacitance value was lowered due to excess TiO2 content which remained uncoated (see FESEM supporting information Fig. S1). The specific capacitance of the as-prepared PTP/TiO2 composites in comparison with other reported PTP composites has been shown in Table 1. The galvanostatic charge-discharge curves for PTP and PTP/TiO2 composite electrodes were recorded at a different current density as shows in Fig. 8. The energy density (E) and power density (P) were estimated using the following equation [7].
Csp = 2 [I/(m dV/dt)]
where, V is the potential window (V) and t is the discharge time (h). The calculated energy density of PTP and PTP/TiO2 composites were found around 1.77 Wh/kg, 2.21 Wh/kg, 3.98 Wh/kg and 5.54 Wh/kg at power density of 177 W/kg, 194.2 W/kg, 199 W/kg and 263.8 W/kg respectively.
(1)
where, Csp is the specific capacitance (F/g), I is the applied chargedischarge current (A), dV/dt is the slope of the discharge curve (V/ s), and m is the mass of the active martial coated on the carbon paper electrode (mg). Multiplication of capacitance by a factor of 2 is included because the series capacitance formed in two electrode system. The specific capacitance calculated for PTP, PTP/TiO2 (10:1), PTP/TiO2 (10:1.5), and PTP/TiO2 (10:2) were found around 80 F/g, 100 F/g, 180 F/g, and 250 F/g respectively at the current
E = 1/8CspV2 1000/3600
(2)
P = E/t
(3)
3.9. Impedance study Fig. 9 shows electrochemical impedance spectra for PTP and PTP/TiO2 composites. Nyquist plots were obtained to study the
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Fig. 8. Galvanostatic charging-discharging curves for (a) Pure PTP, (b) PTP/TiO2 (10:1), (c) PTP/TiO2 (10:1.5), and (d) PTP/TiO2 (10:2) composites.
Table 1 Comparative values of capacitance for PTP/TiO2 and many other composites. Capacitor
Specific Capacitance (F/g)
Current Density (A/g)
Energy Density (Wh/kg)
Power Density (W/kg)
Reference No.
PTP/CNT PEDOTa PTP/CNT GO/PTPb PTP/TiO2
110 130 216 296 250
1 1 1 0.3 1
– – – – 5.54
– – – – 263.8
[17] [39] [40] [41] Present Work
– Not available. a PEDOT-Poly (3,4 ethylenedioxythiophene). b GO/Graphene oxide.
capacitive properties of PTP and PTP/TiO2 composites by evaluating their interfacial properties at the open-circuit potential in the frequency range 0.1Hz-100 KHz. The observed semi-circular portion for all the plots at higher frequencies corresponded to the electron-transfer-limited process. For pure PTP, Nyquist plot had a semicircle of larger diameter as compared to PTP/TiO2 composites. This could be attributed to the higher interfacial charge transfer resistance which in turn indicated a lower conductivity for PTP than the PTP/TiO2 composites. A smaller diameter in the case of PTP/TiO2 composites indicated that the electrolyte ions could easily access the inner layers of the polymer. The Nyquist plots for PTP/TiO2 composites became more vertical
with a gradual increase in the amount of TiO2. Since a higher value of the imaginary part of the impedance (at low frequency) can lead to an enhanced capacitive behaviour. Hence, the addition of TiO2 causes to improve the capacitive nature of PTP in the composites [13]. As the frequency increased, enhanced inclined features of the spectra were observed due to stronger surface redox reactions at the electrode-electrolyte interface. The EIS measurements of the composites indicated that these possessed a reduced ion diffusion resistance and charge transfer resistance, which collectively resulted in an improvement of the electrochemical capacitance. The best result was witnessed in the case of 10:2 composition, thereby indicating that PTP/TiO2 composite (10:2) had most
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A.K. Thakur, R.B. Choudhary / Synthetic Metals 220 (2016) 25–33
Fig. 9. Nyquist plots for (a) Pure PTP, (b) PTP/TiO2 (10:1), (c) PTP/TiO2 (10:1.5), and (d) PTP/TiO2 (10:2) composites.
enhanced electron collection and transportation mechanism within the electrode amongst all the composites. The Nyquist plot for PTP/TiO2 (10:2.5) composite have been shown as supporting information in Fig. S4. 3.9.1. TGA analysis Fig. 10 shows thermograms for pure PTP and PTP/TiO2 composites. The TGA measurements showed a marked difference in temperature stability for pure PTP and PTP/TiO2 composites. For pure PTP, the first weight loss of 8.5% was observed in the range of 90–100 C which was probably due to the de-intercalation of water molecules or volatile impurities as shown in Fig. 10a. The gradual weight loss between 150 C and 550 C was ascribed to higher amorphous nature (very low crystallinity) of PTP [42]. The third step of weight loss at 650 C was related to the degradation and decomposition of PTP. As compared to the TGA of the PTP, PTP/TiO2 composites showed higher thermal stability. Fig. 10b,c,d revealed that the higher stability of PTP/TiO2 composites might occur due to the stabilizing effect of TiO2 on PTP. The TGA results of PTP and PTP/ TiO2 composites have been summarized in Table 2. Pure PTP showed a weight loss around 41.92% at 200 C this loss got reduced to 29.75%, 15.01%, and 9.94% for PTP/TiO2 composites corresponding to the addition of 10%, 15%, and 20% of TiO2 content respectively. In the case of PTP/TiO2 (10:2) composites, highest thermal stability was observed in the temperature 300–400 C. The mass reduction was just 1.33% in this temperature range for the above composite. The composite (10:2) was seen to have 81.86% weight retention when the temperature reached 800 C which could be attributed to the incorporation of TiO2. A similar result was reported related to the improved thermal stability of PTP and various polymers due to increasing weight% of TiO2 and many other metal oxides [23,43–47].
Fig. 10. TGA thermograms for (a) PTP, (b) PTP/TiO2 (10:1), (c) PTP/TiO2 (10:1.5), and (d) PTP/TiO2 (10:2) composites.
4. Conclusions In summary, we conclude that the PTP/TiO2 composites were successfully prepared via in-situ oxidative polymerization method. The morphological study revealed that PTP and TiO2 had a uniform distribution in the polymeric composites at the nanometer scale. UV–vis study revealed that there was a successive decrease in the band gap (2.7–2.2 eV) of the PTP/TiO2 corresponding to the increase in TiO2 content. The EIS studies revealed that the electrochemical performance of pure PTP was markedly improved due to the successful incorporation of TiO2. The highest value of specific capacitance (250F/g) was recorded for PTP/TiO2 (10:2) composite at the current density of 1 A/g. This value was quite higher than that of pure PTP (80 F/g). Furthermore, the energy density reached to 5.54Wh/kg at the power density of 263.8W/kg. Acknowledgements The authors acknowledge their sincere thanks to Professor N. R. Mandre Department of Fuel and Mineral Engineering, Indian School of Mines, Dhanbad for providing TGA facility. The authors are very grateful to Dr. M.V.Shelke (Scientist NCL, India) and Dr. G.C. Nayak (Assist. Professor Dept. of App. Chemistry, ISM, India) for their constant support, fruitful discussion, and valuable suggestions. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. synthmet.2016.05.023.
Table 2 Thermgravimetric analysis. Sample
Weight loss (%) at 200 C
Weight loss (%) at 300 C
Weight loss (%) at 400 C
Weight loss (%) at 500 C
Retention of weight (%) at 800 C
PTP PTP/TiO2(10:1) PTP/TiO2 (10:1.5) PTP/TiO2 (10:2)
41.92 29.75 15.01 9.94
45.02 35.03 17.42 11.12
48.98 36.10 19.63 12.45
52.29 47.43 24.09 16.59
24.98 49.20 64.92 81.86
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