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Polymer nanocomposites for efficient energy storage applications
Journal of Energy Storage 28 (2020) 101275
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Biocompatible supercapacitor electrodes using green synthesised ZnO/Polymer nanocomposites for efficient energy storage applications
T
Sohini Chakrabortya, Amal Raj Mb, N.L. Marya,
⁎
a b
Department of Chemistry, Stella Maris College (Autonomous), University of Madras, Chennai 600 086 Department of Chemistry, Arul Anandar College (Autonomous), Madurai Kamaraj University, Madurai 625 514
ARTICLE INFO
ABSTRACT
Keywords: Green synthesis Structural modification Specific capacitance Copolymer Nanoparticles Cell viability
In the era of rapid decline of renewable sources of energy, novel supercapacitor materials are essential to provide impetus to the growing energy demands arising from the commercialization of technology towards an electricpowered future which aims at both efficient and sustainable use of energy. The inspiration is to develop materials that effect specific applications without causing any damage to the environment. The use of inherently non-conducting but biocompatible polymers that can be functionally modified are thus imperative in this respect. Here, styrene maleic anhydride (SMA) copolymer has been structurally modified with a thiadiazole moiety along with the incorporation of green synthesized ZnO nanoparticles to prepare a polymer nanocomposite. The green synthesized ZnO nanoparticles display a flake-like structure with a size of about 20 nm. UV, FT-IR and XRD analyses validates the incorporation of ZnO in to the polymer matrix to form the polymer nanocomposite. Satisfactory supercapacitor behaviour with a specific capacitance of 268.5 F g−1 at 0.1 A g−1 is estimated for the polymer nanocomposite which is larger than ZnO nanoparticles and the bare polymer. It is demonstrated that the innate conductivity of the SMA copolymer is enhanced upon modification. The materials also exhibit good cycling stability with maximum capacitance retention. The biocompatibility of the nanocomposites has been established from the preliminary cell viability studies performed on 3T6 mouse fibroblast cells. The developed material shows great promise to provide a green alternative to existing supercapacitors.
1. Introduction Supercapacitors are energy storage devices which provide a major solution to the increased energy consumption of the present generation. The fabrication of new materials and the modification of existing materials have revolutionized the energy sector and have opened new pathways to balance the demand-supply ratio in this sector [1,2]. These devices possess large specific capacitance as well as cycle life. The research in this field involves improving their efficacy by increasing the number and stability of the charge-discharge cycles [3]. Supercapacitors are classified on the basis of the charge storage mechanism of the material. Electrochemical double layer capacitors (EDLC) uses a non-faradaic process of charge transfer with the accumulation of charge at the interface between the electrode and electrolyte; on the other hand, psuedocapacitors stores charge through faradaic redox reactions [4]. Metal oxides and conducting polymers are psuedocapacitors with the advantages of enhanced specific capacitance and energy densities [5]. The metal oxides that show good electrochemical performance include RuO2 and NiO [6,7]. RuO2 is however expensive and also
⁎
highly toxic. NiO cannot be used beyond the potential window of 0.6 V [8]. Therefore, it is particularly necessary to identify metal oxides that offer good electrochemical performance and are also cost-effective and less toxic. In this context, green synthesis techniques can be exploited, which addresses the issue of toxic environmental effects [9]. A wide variety of conducting polymers have been used to fabricate supercapacitor devices [10]. The domain of these polymers can be extended further by modifying certain biodegradable and biologically significant polymers to induce electrochemical properties into them or enhance their existing properties [11]. The advantage would be to obtain more environment-friendly sources of energy which are easily biodegradable and are adequately biocompatible to be used inside the human systems as medical implants that survive longer on a single charge [12]. The flexibility in structural modification offered by these polymers helps in arriving at a plethora of new materials that can be obtained from a single parent material [13,14]. This approach not only results in novel materials that are tailored to accomplish specified applications but also reduce the cost of fabrication by utilizing the same precursor. The
Corresponding author. E-mail address: [email protected] (N.L. Mary).
https://doi.org/10.1016/j.est.2020.101275 Received 25 October 2019; Received in revised form 27 January 2020; Accepted 7 February 2020 2352-152X/ © 2020 Elsevier Ltd. All rights reserved.
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incorporation of heterogeneous atoms such as nitrogen, sulphur, oxygen and surface oxygenous groups such as the hydroxyl or the carboxylic group provides electron enrichment which in turn results in higher electrochemical activity [15]. Styrene maleic anhydride (SMA) is a biocompatible polymer, customarily employed as drug carriers by conjugating it with various proteins such as neocarzinostatin (NCS) which is an antitumor protein. This conjugation effectively reduces its toxicity and increases its plasma half-life [16]. SMA incorporated with Ag nanoparticles has been reported to possess higher antibacterial activities [17]. Thiadiazoles are also biologically important molecules with excellent pharmacological properties [18]. Apart from this, thiadiazole and its derivatives has also been observed to possess satisfactory electrochemical properties and can also function as anticorrosion agents due to the heteroatoms present in its structure –providing electron mobility in the matrix [19]. Zinc oxide is an inexpensive transition metal oxide and has effective psuedocapacitance due to its facile redox properties and semiconducting nature [20]. This study focuses on combining the benefits of all these materials in a single compound. A paradigm shift in the arena of fabrication of supercapacitor electrodes has been aimed at, by making use of a biologically significant polymer and modifying it with a thiadiazole moiety along with the incorporation of zinc oxide nanoparticles to achieve enhanced specific capacitance. These biologically significant polymers apart from providing the advantage of being environmentally benign also proves to be beneficial in being compatible to specific cell lines in the human body [21–23]. Therefore, combining the properties would lead to energy storage devices that can be used within the human system without raising compatibility limitations. In the present work, we have made use of styrene maleic anhydride as the polymer matrix, while 2-amino-5-mercapto-1,3,4-thiadiazole is employed to modify the polymer matrix and bring about enhanced mobility of electrons in the matrix. The anhydride group in SMA promotes effective modification by the thiol group present in the thiadiazole moiety. Green synthesized ZnO nanoparticles are incorporated into the matrix to enhance the supercapacitor behaviour. Thus, this study analyses the infusion of polymers with established biological applicability into the field of supercapacitors envisioning a pathway that juxtaposes them in a new realm of chemistry and biotechnology.
drops of the piper nigrum extract was added with constant stirring at 70–80 °C and the pH was maintained by using 1 M NaOH. The resulting white precipitate was calcined in an alumina crucible and heated at 400 °C for 3 h. 2.4. Synthesis of modified copolymer (SMA-1) The modified copolymer was prepared by dissolving styrene maleic anhydride (SMA) and 2-amino-5-mercapto-1,3,4-thiadiazole (AMT) in a 1:1 ratio in 10 ml of dimethyl acetamide (DMAc). The solutions were homogenously dispersed using an ultrasonicator for 3 h. The mixture was heated at 160 °C in an oil bath and evaporation of the solvent leads to the desired copolymer. Scheme 1 shows the schematic representation of the preparation of the modified copolymer. 2.5. Synthesis of polymer nanocomposites (SMZ) SMA and AMT were taken in a 1:1 ratio in DMAc. 0.1 g of ZnO nanoparticles were dispersed into the above solution and it was stirred for 3 h. The mixture was kept in an oil bath and heated at 160 °C for about 2 h. After complete evaporation of the solvent, the nanocomposite was dried and finely powdered. 2.6. Preparation of working electrodes The glassy carbon electrode was polished with 0.3 mm alumina slurry and sonicated in water and acetone for 15 min each to remove any surface adherent impurities. 1 mg mL−1 dispersion of the samples (SMA-1, SMZ, ZnO) were prepared by ultrasonication using ethanol as the solvent and polytetrafluoroethylene (PTFE) as the binder. The working electrode was modified by drop casting 3 µL of the sample on to its surface. The modified electrodes were obtained after evaporation of the solvent. 2.7. Instrumentation The UV-Visible absorption spectra were obtained in the wavelength range of 200–800 nm using a Jasco V 750 UV-Visible spectrophotometer. The FTIR spectra of the samples were recorded using a Bruker ATR-FTIR Model Alpha-T in the range of 400–4500 cm−1 at a scan rate of 24 per second. The SEM images were acquired using the FEI Quanta FEG 200-High Resolution Scanning Electron Microscope (HRSEM). X-ray diffraction analysis was performed using a D8 advance Bruker Powdered X-ray diffractometer. Thermal analysis of the sample was carried out using an universal TA instrument (Model SDT Q600) at a heating rate of 20 °C min−1 under inert atmosphere up to 800 °C. The data was analysed using the TA universal software. The electrochemical measurements were carried out using a CH 608 E Electrochemical Workstation (CH Instruments, USA).
2. Materials and methods 2.1. Materials used The chemicals zinc nitrate, dimethyl acetamide, polytetrafluoroethylene powder, Poly(styrene co-maleic anhydride) (SMA, ¯ W = 1600) and 2-amino-5-mercapto-1,3,4-thiadiazole (AMT) were M procured from Sigma Aldrich and were used without further purification. Commercially available Black Pepper (Piper nigrum) was used for the green synthesis. All the solutions were prepared with double distilled water.
2.8. Electrochemical measurements Electrochemical measurements were carried out using a three electrode system with modified glassy carbon electrode as the working electrode, Pt wire being the counter electrode and Ag/AgCl (1 M KCl) as the reference electrode. All the electrochemical experiments were carried out at room temperature (25 °C ± 1). Cyclic voltammograms were recorded in the potential window of −0.2 V to 0.4 V at scan rates ranging from 10–100 mV s−1. Chronopotentiometry was performed at different current densities and the impedance measurements were carried out at the frequency range of 10−2–105 Hz at the open circuit potential (0.02 V). Mott Schottky data was collected to obtain the charge carrier densities and the flat band potential.
2.2. Preparation of piper nigrum extract Piper nigrum was washed thoroughly with distilled water to remove any surface-adherent impurities and subsequently powdered. It was boiled in distilled water for 30 min to prepare an extract. The extract was cooled and then centrifuged at 160 rpm. The clear solution was obtained on filtration with a Whatman filter paper and the extract was stored at 4–5 °C for further use. 2.3. Synthesis of zinc oxide nanoparticles (ZnO NPs) The synthesis of zinc oxide nanoparticles was carried out using zinc nitrate as the precursor. 1 mM zinc nitrate solution was prepared by dissolving the crystals in distilled water. To 10 ml of zinc nitrate, 5
2.9. Cell viability studies The cytotoxic effect of the nanocomposite was assessed by 3-[4,52
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Fig. 1. UV-Visible spectrum of (a) ZnO nanoparticle prepared from Piper nigrum (b) SMA-1 and SMZ (c) Tauc plot of SMA-1 (d) Tauc plot of SMZ.
noticed in the absorption maxima (340–330 nm). This can be attributed to the change in polarizability of the environment due to the secondary interaction between ZnO and the modified copolymer matrix [26]. The peak between 320–350 nm evident in SMA-1 corresponds to the π absorption of the functional groups in the copolymer. Since the excitation of π electrons requires more energy than the excitation of the electrons at the nonbonding level, the latter occurs at a longer wavelength as depicted in the spectra. The Tauc plot for obtaining the optical band gap between SMA-1 and SMZ is shown in Fig. 1c and d. The optical energy band gap values are obtained from the intercept of the plot between (αhν)2 and photon energy (hν) [27]. The band gap values of SMA-1 and SMZ are 2.90 eV and 2.43 eV respectively [28,29]. The band gap reduction of 0.47 eV confirms the effective incorporation of ZnO into the polymer matrix [30]. The lowering of the band gap can be attributed to the structural changes associated with the incorporation of nanoparticles. This leads to the formation of defects in the matrix which can also be inferred from the SEM images. This subsequently alters the optical properties of the nanocomposite. ZnO is a n-type semiconductor and its band gap value is nearly 3.30 eV [31]. The n- type nature of ZnO helps in imparting greater electronic density to the localized states of the band gap and the magnitude of the band gap energy decreases as observed in SMZ. This implies a significant overall increase in the electrical conductivity of the nanocomposite when compared to the copolymer and the
dimethlythiazol-2-yl]-2,5-diphenly tetrazolium bromide (MTT) assay. 8 × 103 cells per well (3T6 fibroblast cells) were seeded in 24 well plates and allowed to adhere and proliferate overnight in a CO2 incubator at 37 °C. After 24 h of incubation, the cells were treated with samples at various concentrations and incubated overnight at 37 °C. After 24 h, the cells were treated with MTT (0.5 mg/mL) for 3 h. The resulting formazan complex was solubilised with dimethyl sulphoxide (DMSO) and the absorption was measured at 570 nm with a reference wavelength of 630 nm, in a Biorad micro plate reader. The percentage viability was calculated using the optical density of the control and treated cells [24]. 3. Results and Discussion 3.1. UV-visible spectroscopy The UV-Visible spectra of the synthesized ZnO nanoparticles (Fig. 1a) exhibit a peak at 350 nm, which is characteristic of ZnO NPs [25]. This confirms the efficient synthesis of ZnO nanoparticle from black pepper. Fig. 1b depicts the spectrum of the modified copolymer and the polymer incorporated with ZnO NPs. SMA-1 exhibits a peak in the region 340–350 nm which is attributed to the transition arising from the nonbonding electrons at the carbonyl oxygen of the anhydride group. On comparing the spectra of SMA-1 and SMZ, a blue shift is 3
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polymer matrix. Fig. 3b provides the layered surface of the modified copolymer matrix on which the ZnO NPs are distributed. The nanoparticles are well adhered to the matrix while both small and large sized flakes are perceptible in a random arrangement throughout the matrix of the copolymer. The EDAX analysis of SMZ is shown in Fig. 4 indicates the elemental compositions of Carbon, Nitrogen, Oxygen and Zinc (Table 2). This confirms the presence of ZnO NPs in the polymer matrix. 3.4. Transmission electron microscopy
nanoparticles.
The TEM images in Fig. 5 shows the morphology of the ZnO nanoparticles and the polymer nanocomposite, SMZ. The TEM image of ZnO exhibits a nanoflake arrangement with evidences of agglomeration. The distribution of ZnO in the polymer matrix is exhibited in Fig. 5b. The presence of ZnO nanoflakes is confirmed in the polymer matrix of SMZ and they are distributed in a random manner [30].
3.2. Fourier transform infrared spectral analysis
3.5. X-Ray Diffraction Analysis
The FTIR spectra for SMA-1 and SMZ are given in Fig. 2 and the peaks assigned to various functional groups are represented in Table 1. The prominent absorption band in the range of 1717 cm−1 is assigned to the symmetrical stretching vibration of C]]O groups of the maleic anhydride moieties in the copolymer. The absorptions at 3500–3000 cm−1 and 1630–1640 cm−1 indicate the presence of aromatic ring. The band at 653 cm−1 corresponds to CeSeC symmetric ring structure. The band at 1212 cm−1 is due to CeN symmetric stretching. The peak intensity at 3440 cm−1 and 763 cm−1 is due to NeH bending and NeH wag of thiadiazole ring respectively. The presence of ZnO nanoparticles in SMZ was identified by the absorption band at 531 cm−1 which is typical of the ZneO stretching vibration [32]. The band at 1387 cm−1 is due to thiadiazole ring stretching present in both SMA-1 and SMZ. The absorption band at 3201 cm−1 corresponds to the free amine group [33]. This confirms that the thiadiazole is attached to SMA-1 through the sulphur atom of the mercapto linkage at the fifth position of AMT, while the amine group at the second position is left free. Sulphur being softer and more polarizable due to its greater size, has a greater ability to coordinate to the anhydride moiety of the SMA copolymer when compared to nitrogen validating the suggested structure of the compound (Scheme 1). On juxtaposing the spectra of SMA-1 and SMZ, the peaks appear to be more prominent in SMA-1 and a broadening of the peaks is observed in the case of SMZ indicating the slight modification of properties of the bare copolymer by the inclusion of ZnO nanoparticles. This is indicative of the successful formation of SMZ polymer nanocomposite [34].
Fig. 6 represents the X-ray diffraction analysis of SMZ and ZnO nanoparticles. The diffraction peaks at 31.64°, 34.44°, 36.44°, 47.47°, 56.28°, 62.68° and 68.10° correspond to the planes at (010), (002), (011), (012), (110), (013) and (112). These peaks imply the hexagonal wurtzite structure of ZnO and are consistent with the JCPDS data number 36-1451 [36]. The crystallite size was estimated from the Scherrer formula [37]:
Fig. 2. FTIR spectra of SMA-1 and SMZ.
D = 0.9 / cos where, D = average crystallite size, λ = wavelength of the incident Xray beam (1.5406 Ӑ), β = full width at half maxima in radians and θ = Bragg's diffraction angle. Using the above equation, the crystallite size was estimated as 18.4 nm. The XRD pattern of SMZ shown in Fig. 6b implies a relatively amorphous structure. The crystalline structure of ZnO is very minimally retained in the polymer nanocomposite. A peak at nearly 30° is observed indicating the presence of ZnO in the polymer matrix [38]. A comparison of of the sizes of ZnO nanoparticles obtained from various plant-mediated synthetic procedures is represented in Table 3. 3.6. Thermogravimmetric analysis In order to analyse the thermal stability of SMA-1 and SMZ, TGA was carried out (Fig. 7). The TGA data of SMA-1 and SMZ exhibits two main weight loss regions [47]. The first weight loss region at 150–200 °C corresponds to the loss of water from the samples. The second weight loss region is due to the degradation of the polymer backbone. SMA-1 decomposes at 316 °C with a final residue of 8.7% and SMZ decomposes at 324 °C with final residue being 13.2%. On the addition of ZnO NPs, the thermal stability of the polymer increases by 8 °C. The nanoparticles protect the polymer against thermal degradation [48]. The char residue also increases as the ZnO NPs leaves behind a solid residue upon heating [49].
3.3. Scanning electron microscopy The SEM micrographs in Fig. 3a indicate that the nanoparticles have a flake-like structure with an average size of 20.4 nm [35]. These nanoflakes are vertically aligned and randomly distributed throughout the Table 1 Assignment of peaks of the FTIR spectra of SMA-1 and SMZ Wavenumber (cm−1)
Functional Group
1717 653 1212 3440 763 1387 3201 450–500
symmetrical stretching vibration of C]O CeSeC symmetric ring structure CeN symmetric stretching NeH bending NeH wag thiadiazole ring stretching eNH2 MeO (M]Zn)
3.7. BET analysis The surface area and pore size of the green synthesised ZnO nanoparticles have been investigated from the BET isotherm as shown in Fig. 8. The pore diameter and the surface area are found to be 2.579 nm and 16.352 m2/g respectively from the BET analysis data. The isotherm is a type IV isotherm with a hysteresis loop observed in the P/P0 range of 0.8–0.95. This indicates a mesoporous structure of the sample [50]. 4
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Fig. 3. SEM image of (a) ZnO nanoparticles (b) SMZ nanocomposite.
Fig. 4. SEM-EDAX of SMZ nanocomposite.
techniques were performed.
Table 2 Elemental compositions in the SMZ Element
Wt%
At %
3.9. Cyclic voltammetry (CV)
C N O S Zn
74.27 06.24 08.89 04.95 05.65
83.27 06.00 07.48 02.08 01.16
Fig. 9 depicts the cyclic voltammograms of the polymer nanocomposite SMZ at various scan rates. The voltammograms exhibits a rectangular shaped loop symmetrical around 0 V indicating a satisfactory capacitive behaviour of SMZ [51]. With the increase in the scan rate, the current increases. The redox nature of the zinc oxide nanoparticles in the polymer nanocomposite is overshadowed by the capacitance arising from the combined effects of the polymer and the nanoparticle [52]. Hence, the voltammograms have a rectangular shape vis a vis the absence of redox peaks. The linear dependence of the current on the square root of the scan
3.8. Electrochemical studies In order to study the feasibility of the modified electrode for supercapacitor applications, various electrochemical characterisation 5
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Fig. 5. TEM image of (a) ZnO nanoparticles (b) SMZ.
Fig. 6. XRD patterns of (a) ZnO nanoparticles (b) SMZ. Table 3 Typical comparison of the sizes of ZnO nanoparticles obtained from various plant-mediated synthetic procedures. Plant Source
Size (nm)
Shape
Reference
Aloe vera
8–20 (XRD)
[39]
Neem Red Clover Water Hyacinth Coconut
18(XRD) 60–70 (XRD) 32–36 (SEM and TEM) 20–80 (TEM), 21.2 (XRD)
Mexican mint Sandalwood
50–180 (TEM) 100 (DLS and SEM), 70–140 (TEM) 24(XRD), 16–20 (SEM)
Spherical, Hexagonal Spherical Spherical Spherical Spherical, hexagonal Rod shaped Nanorods
[44] [45]
Spherical
[46]
Drumstick tree
[40] [41] [42] [43]
rate (Fig. 10) implies a diffusion controlled behaviour of SMZ [53]. Further cyclic voltammetric studies shown in Fig. 11 provides a comparison of the bare polymer (SMA-1), the polymer nanocomposite (SMZ) and ZnO at a scan rate of 10 mV s−1. Interestingly, the SMZ nanocomposite has a significantly higher current than ZnO and SMA-1 modified electrodes. The supporting information in Fig. S1 further represents the cyclic voltammograms of
Fig. 7. Thermogram of SMA-1 and SMZ.
6
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of the semicircle provides the charge transfer resistance (Rct) and is estimated as 8100 Ω, 6300 Ω and 4900 Ω for the ZnO nanoparticles, SMA-1 and SMZ respectively. Hence SMZ has the maximum conductivity followed by SMA-1 and ZnO nanoparticles. Due to the bare polymer possessing a larger conductivity than ZnO NPs, their synergistic effects validate the enhanced supercapacitive characteristics of SMZ [56]. The obtained impedance data for SMZ was fitted by the Randle's equivalent circuit using the ZSimpWin software in order to further comprehend the electrochemical properties at the interface and in between the electrodes and the electrolyte solution. The impedance fitted data is shown in Fig. 13 where Cdl is the double-layer capacitance, Rs is the solution resistance, Rct is the charge-transfer resistance and Zw is the Warburg impedance. The values obtained for these parameters are depicted below in table 4. The impedance measurements at different potentials are carried out to construct the Mott Schottky plots (Fig. 14) and the results are given in Table 5. All the three samples exhibit a positive slope suggesting ntype semiconductor behaviour. The flat band potential (Vfb) is deduced to be −0.69 V for SMZ and −0.37 V for the bare polymer. The higher negative value of Vfb for SMZ is ascribed to the presence of ZnO nanoparticles in the nanocomposite [57]. Further, the charge carrier concentration is maximum for SMZ and is consistent with larger conductivity of the sample than the bare polymer and ZnO.
Fig. 8. BJH plot and pore size distribution plot (inset) of ZnO nanoparticles.
3.11. Galvanostatic charge-discharge curves Fig. 15 provides the chronopotentiometric response of the nanocomposite wherefrom the triangular charge-discharge curves are inferred and are in accordance with the anticipated pattern for supercapacitive materials [58]. However, the bare polymer SMA-1 also exhibits a triangular shape with a lower discharge time than that for the nanocomposite as observed in Fig. 15a. At a current density of 0.1 A g−1, maximum specific capacitance is observed. At higher current densities also, the shape of the curve is retained although the specific capacitance decreases. A discharge time of 290 s and 537 s was observed for SMA-1 copolymer and SMZ nanocomposite respectively as inferred from Fig. 15a and 15b. Fig. 15c shows a comparative study of the relative discharge times for SMA-1, SMZ and ZnO. The cycling stability of SMZ is exhibited in Fig. 16 and it shows maximum capacitance retention with good cycling stability. The volumetric capacitance of 268.5 F g−1 dropped only by 27% (initial capacitance to final capacitance value) even after 1000 cycles. The morphology of SMZ also remains largely unaffected after the cycling stability test (Fig. S8 and S9). Fig. 17 provides the variation of the specific capacitance with current density for SMZ and SMA-1. The specific capacitance is calculated from the formula 1. [20]:
Fig. 9. Cyclic voltammogram of SMZ modified GC electrode at a scan rate of 10 mV s−1 in 0.5 M KCl.
unmodified SMA, modified SMA (SMA-1), ZnO nanoparticles and the polymer nanocomposite (SMZ). The current response of SMA-1 is much greater than that of SMA. This justifies the modification of the polymer matrix with the thiadiazole moiety which increases the conductivity of otherwise feebly conducting SMA. From Cottrell's equation for a diffusion controlled process, the current is directly proportional to the number of ions when all other factors are constant [54]. Since the voltammograms are recorded under identical conditions, the increase in current for SMZ is attributed to the increased mobility of the electrons in the nanocomposite matrix. The structural modifications and the incorporation of nanoparticles into the polymer matrix of SMZ are responsible for the greater access of electrons by all the three electrodes [55].
Csp =
i t m V
(1)
The specific capacitance for the bare polymer, SMA-1 is 145 F g−1 and it increases to 268.5 F g−1 for SMZ. This implies that the synergistic effect of the faradic redox nature of the ZnO nanoparticles and the psuedocapacitance of the polymer has led to enhanced specific capacitance of the nanocomposite [59]. Table 6 represents the specific capacitances obtained for ZnO coated GC electrodes. Fig. 18 shows the energy density versus current density curves of SMA-1 and SMZ. The energy density is calculated using the formula 2:
3.10. Electrochemical impedance spectroscopy
E=
In order to obtain further mechanistic insights, electrochemical impedance measurements were carried out at various frequency ranges. The Nyquist plots exhibit a semicircle at the high frequency region and a spike at the low frequency region as depicted in Fig. 12. The diameter
CV 2 8
(2)
The energy density of SMZ is significantly higher than the bare polymer. The highest energy density of 13.425 Wh kg−1 and 7.25 Wh kg−1 is achieved at 0.1 A g−1 for the SMZ nanocomposite and 7
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Fig. 13. Impedance spectra and corresponding fitting results for SMZ.
Fig. 10. Current versus square root of scan rate for SMZ modified GC electrode in 0.5 M KCl.
Table 4 Simulated results of SMZ using Randles equivalent circuit Sample Code
Rs (Ω)
Rct (Ω)
Cdl (µF/ cm−2)
Zw *1000(Ω s(1/2))
SMZ
997.3
5005
13.02
5.67
Fig. 11. CV curves of SMA-1, SMZ and ZnO nanoparticles at the scan rate of 10 mV s−1 in 0.5 M KCl.
Fig. 14. Mott Schottky plots of SMA-1, SMZ and ZnO nanoparticles. Table 5 The charge carrier density, carrier type and flat band potential of SMA-1, SMZ and ZnO Material
Charge carrier concentration (ND) (cm−3)
Carrier type
Flat-band potential (Vfb) (V)
SMA-1 SMZ ZnO
2.4041 × 1019 2.5680 × 1019 5.52 × 1018
N N N
−0.37 −0.69 −0.77
the bare polymer respectively [67,68]. The energy density gradually decreases with the increase in current density as anticipated. 3.12. Cell viability studies Fig. 12. Impedance spectra of SMA-1, SMZ and ZnO nanoparticles.
The cytotoxicity studies of SMZ were performed on the fibroblast 8
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Fig. 15. Charge discharge curves of (a) SMA-1(b) SMZ(c) SMA-1, SMZ and ZnO at 0.1 A g−1 in 0.5 M KCl.
cells so as to analyse its biocompatibility. MTT assay of Swiss 3T6 cell proliferation was treated with different concentration of SMZ [24]. The results indicate that the proliferation of the cell lines is concentrationdependent. After 24 h of incubation, the cell viability remained > 80% from 0.1–10 µg sample concentration, while concentrations > 1 μg, was found to be statistically highly toxic at the time interval of 24 h (Fig. 19). The cell viability studies establish that the polymer matrix of the modified copolymer stimulates cell viability (Scheme 2). The previous reports on SMA have demonstrated that a theraupetic dosage of 60 mg SMA in 120 ml DMSO on mouse adherent fibroblast cell lines at a dose level of 10−6–10−9 M is safe at all-time intervals and promotes mitochondrial integrity as well as activity of the cells [69]. However, metal oxide nanoparticles have been known to promote apoptosis by generating reactive oxygen species and hence the amount of these nanoparticles becomes crucial for the viability of the cells [70]. The XRD data obtained here indicate that the individual domain of ZnO in the polymer nanocomposite is retained as is evident from its characteristic peaks. The incorporation of the nanoparticles is also effectively inferred from the SEM images. Thus, the cytotoxic effect of the nanoparticle on the cell lines is an essential feature, dictating the viability of the cells.
Here, the preparation of a polymer nanocomposite using SMA-1 lowers the amount of ZnO nanoparticles used and also synergistically balances the adverse effects of these nanoparticles thereby exhibiting enhanced biocompatibility. The substantial improvement in the biocompatibility of ZnO/Chitosan nanocomposite coated textile, on human foreskin fibroblast cells have also been reported earlier [71]. 4. Conclusion The polymer matrix of styrene maleic anhydride was successfully modified with the help of a thiadiazole moiety. The green-synthesized ZnO nanoparticles were incorporated into the modified polymer matrix. The specific capacitance (268.5 F g−1) of the nanocomposite is higher than that of the bare polymer (145 F g−1) and ZnO nanoparticles (55 F g−1) at a current density of 0.1 A g−1. The nanocomposite also shows excellent cycling stability with maximum capacitance retention. The preliminary cytotoxicity tests of these materials on fibroblast cells display high biocompatibility and hence can be used as energy-storage devices inside the human body or as medical implants which are charged potentially by drawing power within the body cells. Hence this study is of immense significance to the development of 9
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Fig. 18. Energy density versus Current density for SMA-1 and SMZ nanocomposite.
Fig. 16. Specific Capacitance versus cycle number of SMZ.
Fig. 19. Cell viability studies on SMZ nanocomposite.
the ability to provide new breakthroughs pertaining to the commercial arena. Fig. 17. Specific capacitance versus current density of SMZ at 0.1 A g−1.
Data availability statement
Table 6 Typical specific capacitances of ZnO coated GC electrodes System Carbon aerogel/ZnO Functionalized CNT/ZnO Activated Graphene/ZnO Graphene/ZnO Graphene/ZnO (Solvothermal) Ppy-ZnO nanorod Carbon sphere/ZnO core-shell nanocomposite ZnO nanopetals ZnO/MnO2 ZnO/PAH
Specific Capacitance −1
25 F g 59 F g−1 84 F g−1 61.7 F g−1 122.4 F g−1 240 mF cm−2 630 F g−1 365 F g−1 2648 μF cm−2 204 μF cm−2
The data for this work cannot be shared as it is also a part of another similar ongoing project,
Reference
CRediT authorship contribution statement
[60] [61] [62] [63] [59] [55] [64] [65] [66] [66]
Sohini Chakraborty: Formal analysis, Investigation, Resources, Data curation, Writing - original draft, Visualization. Amal Raj M: Conceptualization, Methodology, Writing - review & editing. N.L. Mary: Conceptualization, Methodology, Writing - review & editing, Supervision, Project administration. Declaration of Competing Interest There are no conflicts of interest to declare.
biosupercapacitors and provides integrating the concepts of energy storage and conservation by exploiting the readily available energy from the human body. This also aims at utilizing the available energy and thereby aiming at sustainable use of energy. By fine-tuning the electrochemical properties and incorporating specific functionalities that contribute to the immunological efficiency, these materials have
Acknowledgement We would like to thank the DST-FIST programme-2015 LEVEL 0 for providing us with the instrumentation facilities to carry out this research work. The data has been presented at the International 10
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Scheme 2. Schematic representation of cell viability.
Conference on Polymer Science and Technology (SPSI-Macro 2018) held in IISER Pune and subsequent inputs from the meet has been included in the present paper. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.est.2020.101275. References [1] A.W. Lang, Flexible, aqueous-electrolyte supercapacitors based on water-processable dioxythiophene polymer/carbon nanotube textile electrodes, J. Mater. Chem. A 5 (2017) 23887–23897. [2] Z.S. Iro, C. Subramani, S.S. Dash, A brief review on electrode materials for
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Journal of Energy Storage journal homepage: www.elsevier.com/locate/est
Biocompatible supercapacitor electrodes using green synthesised ZnO/Polymer nanocomposites for efficient energy storage applications
T
Sohini Chakrabortya, Amal Raj Mb, N.L. Marya,
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a b
Department of Chemistry, Stella Maris College (Autonomous), University of Madras, Chennai 600 086 Department of Chemistry, Arul Anandar College (Autonomous), Madurai Kamaraj University, Madurai 625 514
ARTICLE INFO
ABSTRACT
Keywords: Green synthesis Structural modification Specific capacitance Copolymer Nanoparticles Cell viability
In the era of rapid decline of renewable sources of energy, novel supercapacitor materials are essential to provide impetus to the growing energy demands arising from the commercialization of technology towards an electricpowered future which aims at both efficient and sustainable use of energy. The inspiration is to develop materials that effect specific applications without causing any damage to the environment. The use of inherently non-conducting but biocompatible polymers that can be functionally modified are thus imperative in this respect. Here, styrene maleic anhydride (SMA) copolymer has been structurally modified with a thiadiazole moiety along with the incorporation of green synthesized ZnO nanoparticles to prepare a polymer nanocomposite. The green synthesized ZnO nanoparticles display a flake-like structure with a size of about 20 nm. UV, FT-IR and XRD analyses validates the incorporation of ZnO in to the polymer matrix to form the polymer nanocomposite. Satisfactory supercapacitor behaviour with a specific capacitance of 268.5 F g−1 at 0.1 A g−1 is estimated for the polymer nanocomposite which is larger than ZnO nanoparticles and the bare polymer. It is demonstrated that the innate conductivity of the SMA copolymer is enhanced upon modification. The materials also exhibit good cycling stability with maximum capacitance retention. The biocompatibility of the nanocomposites has been established from the preliminary cell viability studies performed on 3T6 mouse fibroblast cells. The developed material shows great promise to provide a green alternative to existing supercapacitors.
1. Introduction Supercapacitors are energy storage devices which provide a major solution to the increased energy consumption of the present generation. The fabrication of new materials and the modification of existing materials have revolutionized the energy sector and have opened new pathways to balance the demand-supply ratio in this sector [1,2]. These devices possess large specific capacitance as well as cycle life. The research in this field involves improving their efficacy by increasing the number and stability of the charge-discharge cycles [3]. Supercapacitors are classified on the basis of the charge storage mechanism of the material. Electrochemical double layer capacitors (EDLC) uses a non-faradaic process of charge transfer with the accumulation of charge at the interface between the electrode and electrolyte; on the other hand, psuedocapacitors stores charge through faradaic redox reactions [4]. Metal oxides and conducting polymers are psuedocapacitors with the advantages of enhanced specific capacitance and energy densities [5]. The metal oxides that show good electrochemical performance include RuO2 and NiO [6,7]. RuO2 is however expensive and also
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highly toxic. NiO cannot be used beyond the potential window of 0.6 V [8]. Therefore, it is particularly necessary to identify metal oxides that offer good electrochemical performance and are also cost-effective and less toxic. In this context, green synthesis techniques can be exploited, which addresses the issue of toxic environmental effects [9]. A wide variety of conducting polymers have been used to fabricate supercapacitor devices [10]. The domain of these polymers can be extended further by modifying certain biodegradable and biologically significant polymers to induce electrochemical properties into them or enhance their existing properties [11]. The advantage would be to obtain more environment-friendly sources of energy which are easily biodegradable and are adequately biocompatible to be used inside the human systems as medical implants that survive longer on a single charge [12]. The flexibility in structural modification offered by these polymers helps in arriving at a plethora of new materials that can be obtained from a single parent material [13,14]. This approach not only results in novel materials that are tailored to accomplish specified applications but also reduce the cost of fabrication by utilizing the same precursor. The
Corresponding author. E-mail address: [email protected] (N.L. Mary).
https://doi.org/10.1016/j.est.2020.101275 Received 25 October 2019; Received in revised form 27 January 2020; Accepted 7 February 2020 2352-152X/ © 2020 Elsevier Ltd. All rights reserved.
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incorporation of heterogeneous atoms such as nitrogen, sulphur, oxygen and surface oxygenous groups such as the hydroxyl or the carboxylic group provides electron enrichment which in turn results in higher electrochemical activity [15]. Styrene maleic anhydride (SMA) is a biocompatible polymer, customarily employed as drug carriers by conjugating it with various proteins such as neocarzinostatin (NCS) which is an antitumor protein. This conjugation effectively reduces its toxicity and increases its plasma half-life [16]. SMA incorporated with Ag nanoparticles has been reported to possess higher antibacterial activities [17]. Thiadiazoles are also biologically important molecules with excellent pharmacological properties [18]. Apart from this, thiadiazole and its derivatives has also been observed to possess satisfactory electrochemical properties and can also function as anticorrosion agents due to the heteroatoms present in its structure –providing electron mobility in the matrix [19]. Zinc oxide is an inexpensive transition metal oxide and has effective psuedocapacitance due to its facile redox properties and semiconducting nature [20]. This study focuses on combining the benefits of all these materials in a single compound. A paradigm shift in the arena of fabrication of supercapacitor electrodes has been aimed at, by making use of a biologically significant polymer and modifying it with a thiadiazole moiety along with the incorporation of zinc oxide nanoparticles to achieve enhanced specific capacitance. These biologically significant polymers apart from providing the advantage of being environmentally benign also proves to be beneficial in being compatible to specific cell lines in the human body [21–23]. Therefore, combining the properties would lead to energy storage devices that can be used within the human system without raising compatibility limitations. In the present work, we have made use of styrene maleic anhydride as the polymer matrix, while 2-amino-5-mercapto-1,3,4-thiadiazole is employed to modify the polymer matrix and bring about enhanced mobility of electrons in the matrix. The anhydride group in SMA promotes effective modification by the thiol group present in the thiadiazole moiety. Green synthesized ZnO nanoparticles are incorporated into the matrix to enhance the supercapacitor behaviour. Thus, this study analyses the infusion of polymers with established biological applicability into the field of supercapacitors envisioning a pathway that juxtaposes them in a new realm of chemistry and biotechnology.
drops of the piper nigrum extract was added with constant stirring at 70–80 °C and the pH was maintained by using 1 M NaOH. The resulting white precipitate was calcined in an alumina crucible and heated at 400 °C for 3 h. 2.4. Synthesis of modified copolymer (SMA-1) The modified copolymer was prepared by dissolving styrene maleic anhydride (SMA) and 2-amino-5-mercapto-1,3,4-thiadiazole (AMT) in a 1:1 ratio in 10 ml of dimethyl acetamide (DMAc). The solutions were homogenously dispersed using an ultrasonicator for 3 h. The mixture was heated at 160 °C in an oil bath and evaporation of the solvent leads to the desired copolymer. Scheme 1 shows the schematic representation of the preparation of the modified copolymer. 2.5. Synthesis of polymer nanocomposites (SMZ) SMA and AMT were taken in a 1:1 ratio in DMAc. 0.1 g of ZnO nanoparticles were dispersed into the above solution and it was stirred for 3 h. The mixture was kept in an oil bath and heated at 160 °C for about 2 h. After complete evaporation of the solvent, the nanocomposite was dried and finely powdered. 2.6. Preparation of working electrodes The glassy carbon electrode was polished with 0.3 mm alumina slurry and sonicated in water and acetone for 15 min each to remove any surface adherent impurities. 1 mg mL−1 dispersion of the samples (SMA-1, SMZ, ZnO) were prepared by ultrasonication using ethanol as the solvent and polytetrafluoroethylene (PTFE) as the binder. The working electrode was modified by drop casting 3 µL of the sample on to its surface. The modified electrodes were obtained after evaporation of the solvent. 2.7. Instrumentation The UV-Visible absorption spectra were obtained in the wavelength range of 200–800 nm using a Jasco V 750 UV-Visible spectrophotometer. The FTIR spectra of the samples were recorded using a Bruker ATR-FTIR Model Alpha-T in the range of 400–4500 cm−1 at a scan rate of 24 per second. The SEM images were acquired using the FEI Quanta FEG 200-High Resolution Scanning Electron Microscope (HRSEM). X-ray diffraction analysis was performed using a D8 advance Bruker Powdered X-ray diffractometer. Thermal analysis of the sample was carried out using an universal TA instrument (Model SDT Q600) at a heating rate of 20 °C min−1 under inert atmosphere up to 800 °C. The data was analysed using the TA universal software. The electrochemical measurements were carried out using a CH 608 E Electrochemical Workstation (CH Instruments, USA).
2. Materials and methods 2.1. Materials used The chemicals zinc nitrate, dimethyl acetamide, polytetrafluoroethylene powder, Poly(styrene co-maleic anhydride) (SMA, ¯ W = 1600) and 2-amino-5-mercapto-1,3,4-thiadiazole (AMT) were M procured from Sigma Aldrich and were used without further purification. Commercially available Black Pepper (Piper nigrum) was used for the green synthesis. All the solutions were prepared with double distilled water.
2.8. Electrochemical measurements Electrochemical measurements were carried out using a three electrode system with modified glassy carbon electrode as the working electrode, Pt wire being the counter electrode and Ag/AgCl (1 M KCl) as the reference electrode. All the electrochemical experiments were carried out at room temperature (25 °C ± 1). Cyclic voltammograms were recorded in the potential window of −0.2 V to 0.4 V at scan rates ranging from 10–100 mV s−1. Chronopotentiometry was performed at different current densities and the impedance measurements were carried out at the frequency range of 10−2–105 Hz at the open circuit potential (0.02 V). Mott Schottky data was collected to obtain the charge carrier densities and the flat band potential.
2.2. Preparation of piper nigrum extract Piper nigrum was washed thoroughly with distilled water to remove any surface-adherent impurities and subsequently powdered. It was boiled in distilled water for 30 min to prepare an extract. The extract was cooled and then centrifuged at 160 rpm. The clear solution was obtained on filtration with a Whatman filter paper and the extract was stored at 4–5 °C for further use. 2.3. Synthesis of zinc oxide nanoparticles (ZnO NPs) The synthesis of zinc oxide nanoparticles was carried out using zinc nitrate as the precursor. 1 mM zinc nitrate solution was prepared by dissolving the crystals in distilled water. To 10 ml of zinc nitrate, 5
2.9. Cell viability studies The cytotoxic effect of the nanocomposite was assessed by 3-[4,52
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Fig. 1. UV-Visible spectrum of (a) ZnO nanoparticle prepared from Piper nigrum (b) SMA-1 and SMZ (c) Tauc plot of SMA-1 (d) Tauc plot of SMZ.
noticed in the absorption maxima (340–330 nm). This can be attributed to the change in polarizability of the environment due to the secondary interaction between ZnO and the modified copolymer matrix [26]. The peak between 320–350 nm evident in SMA-1 corresponds to the π absorption of the functional groups in the copolymer. Since the excitation of π electrons requires more energy than the excitation of the electrons at the nonbonding level, the latter occurs at a longer wavelength as depicted in the spectra. The Tauc plot for obtaining the optical band gap between SMA-1 and SMZ is shown in Fig. 1c and d. The optical energy band gap values are obtained from the intercept of the plot between (αhν)2 and photon energy (hν) [27]. The band gap values of SMA-1 and SMZ are 2.90 eV and 2.43 eV respectively [28,29]. The band gap reduction of 0.47 eV confirms the effective incorporation of ZnO into the polymer matrix [30]. The lowering of the band gap can be attributed to the structural changes associated with the incorporation of nanoparticles. This leads to the formation of defects in the matrix which can also be inferred from the SEM images. This subsequently alters the optical properties of the nanocomposite. ZnO is a n-type semiconductor and its band gap value is nearly 3.30 eV [31]. The n- type nature of ZnO helps in imparting greater electronic density to the localized states of the band gap and the magnitude of the band gap energy decreases as observed in SMZ. This implies a significant overall increase in the electrical conductivity of the nanocomposite when compared to the copolymer and the
dimethlythiazol-2-yl]-2,5-diphenly tetrazolium bromide (MTT) assay. 8 × 103 cells per well (3T6 fibroblast cells) were seeded in 24 well plates and allowed to adhere and proliferate overnight in a CO2 incubator at 37 °C. After 24 h of incubation, the cells were treated with samples at various concentrations and incubated overnight at 37 °C. After 24 h, the cells were treated with MTT (0.5 mg/mL) for 3 h. The resulting formazan complex was solubilised with dimethyl sulphoxide (DMSO) and the absorption was measured at 570 nm with a reference wavelength of 630 nm, in a Biorad micro plate reader. The percentage viability was calculated using the optical density of the control and treated cells [24]. 3. Results and Discussion 3.1. UV-visible spectroscopy The UV-Visible spectra of the synthesized ZnO nanoparticles (Fig. 1a) exhibit a peak at 350 nm, which is characteristic of ZnO NPs [25]. This confirms the efficient synthesis of ZnO nanoparticle from black pepper. Fig. 1b depicts the spectrum of the modified copolymer and the polymer incorporated with ZnO NPs. SMA-1 exhibits a peak in the region 340–350 nm which is attributed to the transition arising from the nonbonding electrons at the carbonyl oxygen of the anhydride group. On comparing the spectra of SMA-1 and SMZ, a blue shift is 3
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polymer matrix. Fig. 3b provides the layered surface of the modified copolymer matrix on which the ZnO NPs are distributed. The nanoparticles are well adhered to the matrix while both small and large sized flakes are perceptible in a random arrangement throughout the matrix of the copolymer. The EDAX analysis of SMZ is shown in Fig. 4 indicates the elemental compositions of Carbon, Nitrogen, Oxygen and Zinc (Table 2). This confirms the presence of ZnO NPs in the polymer matrix. 3.4. Transmission electron microscopy
nanoparticles.
The TEM images in Fig. 5 shows the morphology of the ZnO nanoparticles and the polymer nanocomposite, SMZ. The TEM image of ZnO exhibits a nanoflake arrangement with evidences of agglomeration. The distribution of ZnO in the polymer matrix is exhibited in Fig. 5b. The presence of ZnO nanoflakes is confirmed in the polymer matrix of SMZ and they are distributed in a random manner [30].
3.2. Fourier transform infrared spectral analysis
3.5. X-Ray Diffraction Analysis
The FTIR spectra for SMA-1 and SMZ are given in Fig. 2 and the peaks assigned to various functional groups are represented in Table 1. The prominent absorption band in the range of 1717 cm−1 is assigned to the symmetrical stretching vibration of C]]O groups of the maleic anhydride moieties in the copolymer. The absorptions at 3500–3000 cm−1 and 1630–1640 cm−1 indicate the presence of aromatic ring. The band at 653 cm−1 corresponds to CeSeC symmetric ring structure. The band at 1212 cm−1 is due to CeN symmetric stretching. The peak intensity at 3440 cm−1 and 763 cm−1 is due to NeH bending and NeH wag of thiadiazole ring respectively. The presence of ZnO nanoparticles in SMZ was identified by the absorption band at 531 cm−1 which is typical of the ZneO stretching vibration [32]. The band at 1387 cm−1 is due to thiadiazole ring stretching present in both SMA-1 and SMZ. The absorption band at 3201 cm−1 corresponds to the free amine group [33]. This confirms that the thiadiazole is attached to SMA-1 through the sulphur atom of the mercapto linkage at the fifth position of AMT, while the amine group at the second position is left free. Sulphur being softer and more polarizable due to its greater size, has a greater ability to coordinate to the anhydride moiety of the SMA copolymer when compared to nitrogen validating the suggested structure of the compound (Scheme 1). On juxtaposing the spectra of SMA-1 and SMZ, the peaks appear to be more prominent in SMA-1 and a broadening of the peaks is observed in the case of SMZ indicating the slight modification of properties of the bare copolymer by the inclusion of ZnO nanoparticles. This is indicative of the successful formation of SMZ polymer nanocomposite [34].
Fig. 6 represents the X-ray diffraction analysis of SMZ and ZnO nanoparticles. The diffraction peaks at 31.64°, 34.44°, 36.44°, 47.47°, 56.28°, 62.68° and 68.10° correspond to the planes at (010), (002), (011), (012), (110), (013) and (112). These peaks imply the hexagonal wurtzite structure of ZnO and are consistent with the JCPDS data number 36-1451 [36]. The crystallite size was estimated from the Scherrer formula [37]:
Fig. 2. FTIR spectra of SMA-1 and SMZ.
D = 0.9 / cos where, D = average crystallite size, λ = wavelength of the incident Xray beam (1.5406 Ӑ), β = full width at half maxima in radians and θ = Bragg's diffraction angle. Using the above equation, the crystallite size was estimated as 18.4 nm. The XRD pattern of SMZ shown in Fig. 6b implies a relatively amorphous structure. The crystalline structure of ZnO is very minimally retained in the polymer nanocomposite. A peak at nearly 30° is observed indicating the presence of ZnO in the polymer matrix [38]. A comparison of of the sizes of ZnO nanoparticles obtained from various plant-mediated synthetic procedures is represented in Table 3. 3.6. Thermogravimmetric analysis In order to analyse the thermal stability of SMA-1 and SMZ, TGA was carried out (Fig. 7). The TGA data of SMA-1 and SMZ exhibits two main weight loss regions [47]. The first weight loss region at 150–200 °C corresponds to the loss of water from the samples. The second weight loss region is due to the degradation of the polymer backbone. SMA-1 decomposes at 316 °C with a final residue of 8.7% and SMZ decomposes at 324 °C with final residue being 13.2%. On the addition of ZnO NPs, the thermal stability of the polymer increases by 8 °C. The nanoparticles protect the polymer against thermal degradation [48]. The char residue also increases as the ZnO NPs leaves behind a solid residue upon heating [49].
3.3. Scanning electron microscopy The SEM micrographs in Fig. 3a indicate that the nanoparticles have a flake-like structure with an average size of 20.4 nm [35]. These nanoflakes are vertically aligned and randomly distributed throughout the Table 1 Assignment of peaks of the FTIR spectra of SMA-1 and SMZ Wavenumber (cm−1)
Functional Group
1717 653 1212 3440 763 1387 3201 450–500
symmetrical stretching vibration of C]O CeSeC symmetric ring structure CeN symmetric stretching NeH bending NeH wag thiadiazole ring stretching eNH2 MeO (M]Zn)
3.7. BET analysis The surface area and pore size of the green synthesised ZnO nanoparticles have been investigated from the BET isotherm as shown in Fig. 8. The pore diameter and the surface area are found to be 2.579 nm and 16.352 m2/g respectively from the BET analysis data. The isotherm is a type IV isotherm with a hysteresis loop observed in the P/P0 range of 0.8–0.95. This indicates a mesoporous structure of the sample [50]. 4
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Fig. 3. SEM image of (a) ZnO nanoparticles (b) SMZ nanocomposite.
Fig. 4. SEM-EDAX of SMZ nanocomposite.
techniques were performed.
Table 2 Elemental compositions in the SMZ Element
Wt%
At %
3.9. Cyclic voltammetry (CV)
C N O S Zn
74.27 06.24 08.89 04.95 05.65
83.27 06.00 07.48 02.08 01.16
Fig. 9 depicts the cyclic voltammograms of the polymer nanocomposite SMZ at various scan rates. The voltammograms exhibits a rectangular shaped loop symmetrical around 0 V indicating a satisfactory capacitive behaviour of SMZ [51]. With the increase in the scan rate, the current increases. The redox nature of the zinc oxide nanoparticles in the polymer nanocomposite is overshadowed by the capacitance arising from the combined effects of the polymer and the nanoparticle [52]. Hence, the voltammograms have a rectangular shape vis a vis the absence of redox peaks. The linear dependence of the current on the square root of the scan
3.8. Electrochemical studies In order to study the feasibility of the modified electrode for supercapacitor applications, various electrochemical characterisation 5
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Fig. 5. TEM image of (a) ZnO nanoparticles (b) SMZ.
Fig. 6. XRD patterns of (a) ZnO nanoparticles (b) SMZ. Table 3 Typical comparison of the sizes of ZnO nanoparticles obtained from various plant-mediated synthetic procedures. Plant Source
Size (nm)
Shape
Reference
Aloe vera
8–20 (XRD)
[39]
Neem Red Clover Water Hyacinth Coconut
18(XRD) 60–70 (XRD) 32–36 (SEM and TEM) 20–80 (TEM), 21.2 (XRD)
Mexican mint Sandalwood
50–180 (TEM) 100 (DLS and SEM), 70–140 (TEM) 24(XRD), 16–20 (SEM)
Spherical, Hexagonal Spherical Spherical Spherical Spherical, hexagonal Rod shaped Nanorods
[44] [45]
Spherical
[46]
Drumstick tree
[40] [41] [42] [43]
rate (Fig. 10) implies a diffusion controlled behaviour of SMZ [53]. Further cyclic voltammetric studies shown in Fig. 11 provides a comparison of the bare polymer (SMA-1), the polymer nanocomposite (SMZ) and ZnO at a scan rate of 10 mV s−1. Interestingly, the SMZ nanocomposite has a significantly higher current than ZnO and SMA-1 modified electrodes. The supporting information in Fig. S1 further represents the cyclic voltammograms of
Fig. 7. Thermogram of SMA-1 and SMZ.
6
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of the semicircle provides the charge transfer resistance (Rct) and is estimated as 8100 Ω, 6300 Ω and 4900 Ω for the ZnO nanoparticles, SMA-1 and SMZ respectively. Hence SMZ has the maximum conductivity followed by SMA-1 and ZnO nanoparticles. Due to the bare polymer possessing a larger conductivity than ZnO NPs, their synergistic effects validate the enhanced supercapacitive characteristics of SMZ [56]. The obtained impedance data for SMZ was fitted by the Randle's equivalent circuit using the ZSimpWin software in order to further comprehend the electrochemical properties at the interface and in between the electrodes and the electrolyte solution. The impedance fitted data is shown in Fig. 13 where Cdl is the double-layer capacitance, Rs is the solution resistance, Rct is the charge-transfer resistance and Zw is the Warburg impedance. The values obtained for these parameters are depicted below in table 4. The impedance measurements at different potentials are carried out to construct the Mott Schottky plots (Fig. 14) and the results are given in Table 5. All the three samples exhibit a positive slope suggesting ntype semiconductor behaviour. The flat band potential (Vfb) is deduced to be −0.69 V for SMZ and −0.37 V for the bare polymer. The higher negative value of Vfb for SMZ is ascribed to the presence of ZnO nanoparticles in the nanocomposite [57]. Further, the charge carrier concentration is maximum for SMZ and is consistent with larger conductivity of the sample than the bare polymer and ZnO.
Fig. 8. BJH plot and pore size distribution plot (inset) of ZnO nanoparticles.
3.11. Galvanostatic charge-discharge curves Fig. 15 provides the chronopotentiometric response of the nanocomposite wherefrom the triangular charge-discharge curves are inferred and are in accordance with the anticipated pattern for supercapacitive materials [58]. However, the bare polymer SMA-1 also exhibits a triangular shape with a lower discharge time than that for the nanocomposite as observed in Fig. 15a. At a current density of 0.1 A g−1, maximum specific capacitance is observed. At higher current densities also, the shape of the curve is retained although the specific capacitance decreases. A discharge time of 290 s and 537 s was observed for SMA-1 copolymer and SMZ nanocomposite respectively as inferred from Fig. 15a and 15b. Fig. 15c shows a comparative study of the relative discharge times for SMA-1, SMZ and ZnO. The cycling stability of SMZ is exhibited in Fig. 16 and it shows maximum capacitance retention with good cycling stability. The volumetric capacitance of 268.5 F g−1 dropped only by 27% (initial capacitance to final capacitance value) even after 1000 cycles. The morphology of SMZ also remains largely unaffected after the cycling stability test (Fig. S8 and S9). Fig. 17 provides the variation of the specific capacitance with current density for SMZ and SMA-1. The specific capacitance is calculated from the formula 1. [20]:
Fig. 9. Cyclic voltammogram of SMZ modified GC electrode at a scan rate of 10 mV s−1 in 0.5 M KCl.
unmodified SMA, modified SMA (SMA-1), ZnO nanoparticles and the polymer nanocomposite (SMZ). The current response of SMA-1 is much greater than that of SMA. This justifies the modification of the polymer matrix with the thiadiazole moiety which increases the conductivity of otherwise feebly conducting SMA. From Cottrell's equation for a diffusion controlled process, the current is directly proportional to the number of ions when all other factors are constant [54]. Since the voltammograms are recorded under identical conditions, the increase in current for SMZ is attributed to the increased mobility of the electrons in the nanocomposite matrix. The structural modifications and the incorporation of nanoparticles into the polymer matrix of SMZ are responsible for the greater access of electrons by all the three electrodes [55].
Csp =
i t m V
(1)
The specific capacitance for the bare polymer, SMA-1 is 145 F g−1 and it increases to 268.5 F g−1 for SMZ. This implies that the synergistic effect of the faradic redox nature of the ZnO nanoparticles and the psuedocapacitance of the polymer has led to enhanced specific capacitance of the nanocomposite [59]. Table 6 represents the specific capacitances obtained for ZnO coated GC electrodes. Fig. 18 shows the energy density versus current density curves of SMA-1 and SMZ. The energy density is calculated using the formula 2:
3.10. Electrochemical impedance spectroscopy
E=
In order to obtain further mechanistic insights, electrochemical impedance measurements were carried out at various frequency ranges. The Nyquist plots exhibit a semicircle at the high frequency region and a spike at the low frequency region as depicted in Fig. 12. The diameter
CV 2 8
(2)
The energy density of SMZ is significantly higher than the bare polymer. The highest energy density of 13.425 Wh kg−1 and 7.25 Wh kg−1 is achieved at 0.1 A g−1 for the SMZ nanocomposite and 7
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Fig. 13. Impedance spectra and corresponding fitting results for SMZ.
Fig. 10. Current versus square root of scan rate for SMZ modified GC electrode in 0.5 M KCl.
Table 4 Simulated results of SMZ using Randles equivalent circuit Sample Code
Rs (Ω)
Rct (Ω)
Cdl (µF/ cm−2)
Zw *1000(Ω s(1/2))
SMZ
997.3
5005
13.02
5.67
Fig. 11. CV curves of SMA-1, SMZ and ZnO nanoparticles at the scan rate of 10 mV s−1 in 0.5 M KCl.
Fig. 14. Mott Schottky plots of SMA-1, SMZ and ZnO nanoparticles. Table 5 The charge carrier density, carrier type and flat band potential of SMA-1, SMZ and ZnO Material
Charge carrier concentration (ND) (cm−3)
Carrier type
Flat-band potential (Vfb) (V)
SMA-1 SMZ ZnO
2.4041 × 1019 2.5680 × 1019 5.52 × 1018
N N N
−0.37 −0.69 −0.77
the bare polymer respectively [67,68]. The energy density gradually decreases with the increase in current density as anticipated. 3.12. Cell viability studies Fig. 12. Impedance spectra of SMA-1, SMZ and ZnO nanoparticles.
The cytotoxicity studies of SMZ were performed on the fibroblast 8
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Fig. 15. Charge discharge curves of (a) SMA-1(b) SMZ(c) SMA-1, SMZ and ZnO at 0.1 A g−1 in 0.5 M KCl.
cells so as to analyse its biocompatibility. MTT assay of Swiss 3T6 cell proliferation was treated with different concentration of SMZ [24]. The results indicate that the proliferation of the cell lines is concentrationdependent. After 24 h of incubation, the cell viability remained > 80% from 0.1–10 µg sample concentration, while concentrations > 1 μg, was found to be statistically highly toxic at the time interval of 24 h (Fig. 19). The cell viability studies establish that the polymer matrix of the modified copolymer stimulates cell viability (Scheme 2). The previous reports on SMA have demonstrated that a theraupetic dosage of 60 mg SMA in 120 ml DMSO on mouse adherent fibroblast cell lines at a dose level of 10−6–10−9 M is safe at all-time intervals and promotes mitochondrial integrity as well as activity of the cells [69]. However, metal oxide nanoparticles have been known to promote apoptosis by generating reactive oxygen species and hence the amount of these nanoparticles becomes crucial for the viability of the cells [70]. The XRD data obtained here indicate that the individual domain of ZnO in the polymer nanocomposite is retained as is evident from its characteristic peaks. The incorporation of the nanoparticles is also effectively inferred from the SEM images. Thus, the cytotoxic effect of the nanoparticle on the cell lines is an essential feature, dictating the viability of the cells.
Here, the preparation of a polymer nanocomposite using SMA-1 lowers the amount of ZnO nanoparticles used and also synergistically balances the adverse effects of these nanoparticles thereby exhibiting enhanced biocompatibility. The substantial improvement in the biocompatibility of ZnO/Chitosan nanocomposite coated textile, on human foreskin fibroblast cells have also been reported earlier [71]. 4. Conclusion The polymer matrix of styrene maleic anhydride was successfully modified with the help of a thiadiazole moiety. The green-synthesized ZnO nanoparticles were incorporated into the modified polymer matrix. The specific capacitance (268.5 F g−1) of the nanocomposite is higher than that of the bare polymer (145 F g−1) and ZnO nanoparticles (55 F g−1) at a current density of 0.1 A g−1. The nanocomposite also shows excellent cycling stability with maximum capacitance retention. The preliminary cytotoxicity tests of these materials on fibroblast cells display high biocompatibility and hence can be used as energy-storage devices inside the human body or as medical implants which are charged potentially by drawing power within the body cells. Hence this study is of immense significance to the development of 9
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Fig. 18. Energy density versus Current density for SMA-1 and SMZ nanocomposite.
Fig. 16. Specific Capacitance versus cycle number of SMZ.
Fig. 19. Cell viability studies on SMZ nanocomposite.
the ability to provide new breakthroughs pertaining to the commercial arena. Fig. 17. Specific capacitance versus current density of SMZ at 0.1 A g−1.
Data availability statement
Table 6 Typical specific capacitances of ZnO coated GC electrodes System Carbon aerogel/ZnO Functionalized CNT/ZnO Activated Graphene/ZnO Graphene/ZnO Graphene/ZnO (Solvothermal) Ppy-ZnO nanorod Carbon sphere/ZnO core-shell nanocomposite ZnO nanopetals ZnO/MnO2 ZnO/PAH
Specific Capacitance −1
25 F g 59 F g−1 84 F g−1 61.7 F g−1 122.4 F g−1 240 mF cm−2 630 F g−1 365 F g−1 2648 μF cm−2 204 μF cm−2
The data for this work cannot be shared as it is also a part of another similar ongoing project,
Reference
CRediT authorship contribution statement
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Sohini Chakraborty: Formal analysis, Investigation, Resources, Data curation, Writing - original draft, Visualization. Amal Raj M: Conceptualization, Methodology, Writing - review & editing. N.L. Mary: Conceptualization, Methodology, Writing - review & editing, Supervision, Project administration. Declaration of Competing Interest There are no conflicts of interest to declare.
biosupercapacitors and provides integrating the concepts of energy storage and conservation by exploiting the readily available energy from the human body. This also aims at utilizing the available energy and thereby aiming at sustainable use of energy. By fine-tuning the electrochemical properties and incorporating specific functionalities that contribute to the immunological efficiency, these materials have
Acknowledgement We would like to thank the DST-FIST programme-2015 LEVEL 0 for providing us with the instrumentation facilities to carry out this research work. The data has been presented at the International 10
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