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Thermal, vibrational and Ac impedance studies on proton conducting polymer electrolytes based on poly(vinyl acetate)
Journal of Non-Crystalline Solids 358 (2012) 531–536
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Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Thermal, vibrational and Ac impedance studies on proton conducting polymer electrolytes based on poly(vinyl acetate) D. Arun Kumar a, b, 1, S. Selvasekarapandian a, c,⁎, R. Baskaran d, T. Savitha e, H. Nithya b, d a
Department of Physics, Bharathiar University, Coimbatore-641 046, Tamil Nadu, India DRDO-BU Center for Life Sciences, Bharathiar University, Coimbatore-641 046, Tamil Nadu, India Kalasalingam University, Krishnankoil, Srivilliputhur-626190, Tamil Nadu, India d IMRAM, Tohoku University, Sendai, Japan e Solid State Physics and Magnetism Section, Department of Physics and Astronomy Celestijnenlaan 200D, B-3001 Leuven, Belgium b c
a r t i c l e
i n f o
Article history: Received 28 July 2011 Received in revised form 17 November 2011 Available online 16 December 2011 Keywords: Proton conductors; PVAc; FTIR; DSC; Impedance analysis
a b s t r a c t Proton conducting polymer electrolytes based on poly(vinyl acetate) (PVAc) and perchloric acid (HClO4) have been prepared by solution casting technique with various compositions. The X-ray diffraction analysis confirms the polymer–HClO4 complex formation. FTIR spectra analysis reveals the interaction between proton and ester oxygen of poly(vinyl acetate) (PVAc). The shift in Tg towards the lower temperature indicates that the polymer salt interaction takes places in the amorphous phase of the polymer matrix. Ac impedance spectroscopy reveals that 75 mol% PVAc:25 mol% HClO4 exhibits maximum conductivity, 3.75 × 10− 3 S cm − 1 at room temperature (303 K). The increase in conductivity with increase in dopant concentration and temperature may be attributed to the enhanced mobility of the polymer chains, number of charge carriers and rotations of side chains. The temperature dependence of conductivity shows non-Arrhenius behavior at higher temperatures. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Since the pioneering works of Wright et al. [1,2] a lot of facts have been found out about the ionic conductivity in polymeric complexes. In recent years, proton conducting polymer electrolytes have attracted considerable attention due to the possibility of their application in a variety of electrochemical devices, such as fuel cells, humidity, gas sensors, electro chromic displays and electro chromic windows [3]. In general proton-conducting polymers are usually based on polymer electrolytes, which have negatively charged groups attached to the polymer backbone. Complexes of basic polymers such as poly(ethylene oxide) (PEO), poly(ethylene imine) (PEI), poly(acryl amide) (PAAM) and poly(vinyl alcohol) (PVA) with strong acids have been shown to possess high proton conduction in the range of 10− 4–10− 3 S cm− 1 [4]. Many investigators have studied the behavior of acid-based polymer electrolyte complexes as proton conductors and their applications in solid state devices at room temperature have been demonstrated [5]. High proton conductivity at high temperature and low relative humidity can be achieved using acid–base polymer complexes between basic polymers
⁎ Corresponding author at: Kalasalingam University, Krishnankoil, Srivilliputhur626190, Tamil Nadu, India. Tel.: + 91 9443703089. E-mail address: [email protected] (S. Selvasekarapandian). 1 Present address: Energy Materials Center, Korea Institute of Energy Research, Daejeon, Korea. 0022-3093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2011.11.031
and strong acids or polymeric acids [6]. The proton-accepting polymers include poly(ethylene oxide) (PEO), poly(vinyl acetate) (PVAc), polyacrylamide (PAAM), poly vinyl alcohol (PVA) and poly(ethylene imine) (PEI ) [7,8]. Complexes of strong inorganic acids (phosphoric acid [9,10], sulfuric acid [10] or ammonium salts [11] with commercially available polymers such as poly(ethylene oxide)(PEO), poly(ethylene imine), poly vinyl alcohol [12,13], or poly vinyl acetate [14,15] have mostly been investigated. PVA complexed with inorganic acid has been already reported by Vargas et al., and has reported the plasticization effect of acids and water in PVA [16]. Dielectric and conduction properties of PVA complexed with H3PO4 have been studied by Kufian et al. [17]. Dielectric properties, vibrational and NMR studies on PVA doped with H3PO4 and dielectric properties on PVA doped with H2SO4 have been studied by Singh et al. [18,19]. PVAc happens to be one of the polymers which possess large dipole moments and high relaxation time which are due to its side chains connected to ester oxygen [20]. PVAc is a suitable candidate to achieve high ionic conductivity at ambient temperatures when doped with inorganic acids and salts. Electrical properties of PVAc doped with inorganic salts have been studied and reported, but the reports on the electrical properties of acid doped PVAc are scarce. Hence in the present work, proton conducting polymer electrolytes based on poly(vinyl acetate) (PVAc) and perchloric acid (HClO4) have been prepared by solution casting technique with various compositions and its thermal vibrational and electrical properties are studied. Plasticization effect of acid on poly(vinyl acetate) (PVAc) has been discussed using FTIR. Impedance spectroscopy
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has been performed to analyze effect of acid on the conductivity of pure polymer.
(f) 75PVAc : 25HClO4
2. Experimental analysis
Poly(vinyl acetate) (PVAc) Himedia, perchloric acid (HClO4) Qualigens and acetone (Merck) were used as received without no other treatment.
(e) 80PVAc : 20HClO4
Intensity (arb unit)
2.1. Materials
(d) 85PVAc : 15HClO4 (c) 90PVAc : 10HClO4
2.2. Preparation of proton conducting polymer membrane
(b) 95PVAc : 5HClO4
The polymer complexes have been prepared by solution casting technique. The host polymer PVAc has been dissolved using acetone as solvent. Then the dopant HClO4 is added to this solution and stirred well to get a homogenous solution. The solution is then poured in to small glass Petri dishes to form thin film, and kept in room temperature for 24 hours to remove acetone and then in vacuum oven at a temperature of 60 °C to further remove the traces of acetone.
(a) Pure PVAc
2.3. Structural analysis 2.3.1. XRD analysis XRD measurements were performed on the polymer membrane by using PANalytical Xpert pro X-ray diffractometer at an average of scans at the resolution of 2° s − 1. 2.3.2. FTIR analysis FTIR spectrum was recorded for the thin polymer membrane using Shimadzu 8000-Spectrophotometer instrument in the range 4000–400 cm − 1 at a resolution of 2 cm − 1.
20
40
80
60
2 (Degree) Fig. 1. XRD pattern of (a) pure PVAc, (b) 5 mol% HClO4, (c) 10 mol% HClO4, (d) 15 mol% HClO4, (e) 20 mol% HClO4, and (f) 25 mol% HClO4 doped PVAc.
the polymer electrolytes are shown in Fig. 2, which shows vibration bands at different positions corresponding to the vibrational modes of the polymer salt complex. 3.3. Thermal analysis The DSC studies have been employed to study the thermal behavior of the PVAc:HClO4 polymer electrolyte system. The salt concentration dependence of the glass transition temperature of the polymer has been studied and the DSC thermogram is shown in Fig. 3.
2.4. Thermal analysis 3.4. Impedance spectra analysis The thermal property of the polymer membranes has been investigated using NETZSCH DSC 204F1 differential scanning calorimeter. Dry nitrogen gas at the rate of 20 ml min − 1 is purged through the cell during all the measurements. Heating and cooling rate was fixed at 5 °C min − 1 for all the samples. The samples are cooled from 30 °C to − 100 °C and kept at this temperature for 2 min, then heated from −100 °C to 50 °C.
Impedance spectroscopy is relatively powerful technique of characterizing many solid electrolytes and it may be used to investigate the dynamics of bound or mobile charge in bulk and interfacial regions of polymer electrolytes. Fig. 4(a) shows the Nyquist plot or cole–cole plot of 95 mol% PVAc–5 mol% HClO4, 90 mol%
2.5. Electrical analysis
3. Results 3.1. XRD analysis The complexation of PVAc with HClO4 has been studied using X-ray diffraction studies. The XRD patterns of complexed PVAc–HClO4 of different compositions are presented in Fig. 1(a–f).
(c) % Transmittance
To study the ionic conductivity of the samples Ac impedance spectroscopy is performed using HIOKI 3532 LCR Hitester in the frequency range 42 Hz–5 MHz for the temperature range 303 K–373 K using aluminum as blocking electrodes. The impedance data were taken at an average of 32 at each frequency within the measured frequency range.
610
(b) 3723 625
(a)
(a) Pure PVAc (b) 85PVAc:15HClO4
1245 1730
(c) 80PVAc:20HClO4
3.2. Vibrational analysis 3.2.1. FT-IR spectroscopic analysis The vibrational analysis for the prepared polymer electrolytes has been studied using FTIR spectroscopy. The FTIR spectra observed for
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1) Fig. 2. FT-IR spectrum of (a) pure PVAc, (b) 15 mol% HClO4, and (c) 20 mol% HClO4 doped PVAc.
D.A. Kumar et al. / Journal of Non-Crystalline Solids 358 (2012) 531–536
PVAc–10 mol% HClO4, 85 mol% PVAc–15 mol% HClO4, 80 mol% PVAc–20 mol% HClO4, and 75 mol% PVAc–25 mol% HClO4 proton exchange membranes at 303 K.
(a) 95PVAc : 5HClO4
Exo
(b) 85PVAc : 15HClO4 (c) 75PVAc : 25HClO4
3.5. Conductance spectra analysis
(c)
Fig. 5(a) shows the conductance spectra of 95 mol% PVAc–5 mol% HClO4, 90 mol% PVAc–10 mol% HClO4, 85 mol% PVAc–15 mol% HClO4, 80 mol% PVAc–20 mol% HClO4, and 75 mol% PVAc–25 mol% HClO4 proton exchange membranes at 303 K. Fig. 5(b) shows the conductance spectra of 5 mol% HClO4 doped polymer electrolytes at different temperatures.
Endo
(b) Tg
200
220
240
260
(a)
280
300
3.6. Temperature dependent conductivity
Temperature (K) Fig. 3. DSC thermogram of (a) 5 mol% HClO4, (b) 15 mol% HClO4, (c) and 15 mol % HClO4 doped PVAc.
a
533
30000
Temperature dependence of conductivity has been employed to study the conduction of ions in the polymer electrolytes. Fig. 6 shows the temperature dependent conductivity plot.
a
-2.0
2500
25000
-2.5
-Z''
2000
-3.0
1500
4 4
Log T (Scm-1K)
1000
20000
-Z''
500 0
15000
0
500
1000 1500 2000 2500
Z'
303 K 95PVAc:5HClO4 (PH1)
10000
90PVAc:10HClO4 (PH2)
-3.5 -4.0 -4.5 -5.0 -5.5
95PVAc:5HClO4 (PH1) 90PVAc:10HClO4 (PH2) 85PVAc:15HClO4 (PH3) 80PVAc:20HClO4 (PH4) 75PVAc:25HClO4 (PH5)
-6.0
85PVAc:15HClO4 (PH3)
5000
80PVAc:20HClO4 (PH4)
-6.5
75PVAc:25HClO4(PH5)
-7.0
0
3 0
5000
10000
15000
20000
25000
5
Log
6
7
(Hz)
b -3.0
100000 95PVAc:5HClO4 (PH1)
-3.5
Log T (Scm-1K)
303 K 313 K 323 K
80000
4
30000
Z'
b
303 K
-Z''
60000
40000
-4.0
303 K 313 K 323 K 333 K 343 K 353 K 363 K
-4.5 -5.0 -5.5
20000 -6.0 95PVAc:5HClO4 (PH1) 0
-6.5 0
25000
50000
75000
100000
Z' Fig. 4. Nyquist plot for (a) PVAc–HClO4 complexes at 303 K and (b) 5 mol% HClO4 doped PVAc at various temperatures.
3
4
5
Log
6
7
(Hz)
Fig. 5. Conductance spectra for (a) PVAc–HClO4 complexes at 303 K and (b) 95 mol% PVAc–5 mol% HClO4 doped polymer electrolyte.
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D.A. Kumar et al. / Journal of Non-Crystalline Solids 358 (2012) 531–536
4. Discussion
0.0 95 PVAc:5 HClO4 Linear Fit of 95 PVAc:5 HClO4
-0.5
Log T (Scm-1K)
R2 = 0.98753 -1.0
-1.5
-2.0
-2.5
-3.0 0.0026 0.0027 0.0028 0.0029 0.0030 0.0031 0.0032 0.0033
1000/T(K-1) Fig. 6. Temperature dependent conductivity for 5 mol% and 15 mol% PVAc:HClO4 complexes.
3.7. Dielectric spectra analysis The complex permittivity (ε) or dielectric constant of a system is defined by
ε ¼ ε′ þ iε″ ¼ ε′−jðσ=ωε 0 Þ
ð1Þ
where ε′ ε″ σ ω ε0
real part of the dielectric constant, imaginary part of the dielectric constant of the material, conductivity of the material, angular frequency, and permittivity of free space.
Fig. 7 shows the log ω vs ε′ plot for 95 mol% PVAc–5 mol% HClO4, 90 mol% PVAc–10 mol% HClO4, 85 mol% PVAc–15 mol% HClO4, 80 mol% PVAc–20 mol% HClO4, and 75 mol% PVAc–25 mol% HClO4 proton exchange membranes at 303 K.
400000
95PVAc:5HClO4 (PH1) 90PVAc:10HClO4 (PH2) 85PVAc:15HClO4 (PH3)
300000
80PVAc:20HClO4 (PH4) 75PVAc:25HClO4 (PH5)
200000 303 K
100000
0
3
4
5
Log
6
(Hz)
Fig. 7. Variation of dielectric constant with frequency for PVAc:HClO4 complexes.
7
From Fig. 1(a–f) the complexation of the salt with the polymer is confirmed. Fig. 1(a), the pure polymer PVAc shows two broad peaks at 2θ = 15.07° and 22.7° which reveal the amorphous nature of the polymer [15]. The two broad peaks observed in PVAc were found suppressed to one broad peak at 2θ value 20°, which suggests that the addition of perchloric acid into the polymer matrix decreases the degree of crystallinity with the increase in amorphous nature of the sample. This indicates that the amorphous nature of the polymer matrix increases with the addition of perchloric acid concentration. This result can be interpreted by Hodge et al. [21] criterion, which establishes a correlation between the intensity of the peak and the degree of crystallinity. Hence the change in intensity and the broad nature of the peak suggest that the polymer and acid complexation takes place in the amorphous region of the polymer matrix [22]. In Fig. 1(b–d) for the concentration of acid ranging from 5 to 15 mol%, no sharp peaks corresponding to perchloric acid have been observed in the XRD pattern. This contributes a lot to the increment of proton and transport in the amorphous phase and thus results in the improvement of conductivity [23]. However when the acid concentration is 20 mol% and 25 mol%, less intense peak at 2θ = 28° corresponding to perchloric acid [JCPDS card no. 78-2454] appears which may be due to the presence of perchloric acid precipitation. From the FTIR spectra of PVAc–HClO4 complexes (Fig. 2), the vibrational bands observed at 2923, 2865 and 1375 cm − 1 are ascribed to CH3 asymmetric stretching, symmetric stretching and symmetric bending vibrations of pure PVAc respectively. The peaks at 1245, 1100 and 1090 cm − 1 are ascribed to C–O–C symmetrical stretching, C–O and C–C stretching vibrations of pure PVAc respectively. The peak observed at 610 cm− 1 (Fig. 2(b)) is ascribed to the ClO4− ion which is slightly shifted to lower wave numbers 595 cm− 1 at higher concentrations of HClO4. This result indicates the coordination of ClO4− ion with the polar group present in the polymer of the polymer complexes. In Fig. 2(b–c), the peak observed at 3728 cm− 1 is ascribed to the weak inter ionic bonding between H+ and ClO4− as (H+ ClO4− ——— H+ ClO4−). Fig. 2(b) and (c) well reflect the different degree of coordination of cation to both the C–O–C and C=O moieties. The appearance of strong band in the spectrum at 1730 cm− 1 which corresponds to C=O stretching frequency of pure PVAc is slightly shifted to lower wave number (1719–1705 cm− 1) in the polymer–acid complexes. This effect is due to the coordination of the cation with the oxygen, which results in the weakening of the C O bond and hence the wave number of absorption decreases. Similar results have been reported by Weihua Zhu et al. and Selvasekarapandian et al. for the PEG-PU/ NaClO4 and PVAc–NH4SCN complexes, respectively [24,25]. Moreover the addition of HClO4 causes a small decrease (1238–1232 cm− 1) of the C–O–C stretch down to lower wave numbers due to the coordination of the ester oxygen with the cation. Similar effect was reported by Wieczorek et al. for polyether–poly(methyl methacrylate) blend based system [26]. The band assignments for the polymer salt complex have been tabulated and shown in Table 1. The polymer–acid complex formation and the proton interaction have been confirmed from the above analysis. DSC results are shown in Fig. 3. The glass transition temperature of pure PVAC is about 303 K as reported by Baskaran et al. [15]. A well resolved step which is associated to a glass transition (Tg) is observed to be 275 K, 257 K and 244 K for 5 mol%, 15 mol% and 25 mol% of HClO4 concentration respectively. Fig. 3(a–c) represents decrease in Tg with the increase in HClO4 concentration (5 mol%, 15 mol%, and 25 mol%) which could be accounted by the availability of ester oxygen and the plasticizing effect of increased ions associated [15,25]. The same effect has also been observed by Pu et al. for the PVA/Imi/ NH4H2PO4 system [23]. The decrease in Tg indicates the increase in mobility of the polymer chains [26]. The effect of the plasticizing additives on the polymers can be explained by the free volume theory.
D.A. Kumar et al. / Journal of Non-Crystalline Solids 358 (2012) 531–536 Table 1 Vibrational band assignments of the prepared polymer electrolytes. Polymer electrolyte complex wavenumber (cm− 1) Pure PVAc 1090 1100 1245 1375 1730 2865 2923
Table 2 Conductivity values of prepared polymer electrolyte.
Band assignments
85PVAc:15HClO4 80PVAc:20HClO4 610 1070 – 1238 1354 1719 2853 2904 3723
595 1070 – 1236 1348 1705 2844 2895 3712
ClO4 C–C stretching C–O stretching C–O–C symmetric stretching CH3 symmetric bending C=O stretching CH3 symmetric stretching CH3 asymmetric stretching Weak inter ionic bonding between H+ and ClO4−
In accordance with this theory the presence of plasticizers in the polymer matrix increases the free volume available in the polymer chain segments and therefore allows greater internal chain rotation and an increase in the segmental mobility. As the ion transport is dependent on the local segmental motion, the increase in free volume with the increase in HClO4 concentration influences the conductivity by the easy flow of protons through the polymer chain network when there is an applied electric field. Hence the result confirms that HClO4 exerts a plasticizing effect on the polymer matrix [27]. From Fig. 4(a) the impedance spectra/cole cole plot shows two well defined regions, a chord in the high frequency region which is related to the conduction process in the bulk of the electrolytes and a spike in the low frequency region. The observed inclined spike represents a finite or infinite diffusion of mobile charge carrier which occurs into the electrode materials. The curvature in the spike is observed which is attributed to the blocking effect of the electrodes [28]. The appearance of semi-circle in the high frequency region can be represented by a parallel combination of a resistor and a capacitor. The resistor occurs due to the migration of ions through the free volume of the polymer matrix, and the capacitor to the polarized polymer chains due to the alternating field applied. The disappearance of the high frequency chord portion in the complex impedance plot for the high salt concentration polymer electrolyte illustrates that the total conductivity is mainly due to the result of ionic conduction [29]. Fig. 4 (b) shows the temperature dependent cole cole plot for the proton exchange membrane 95 mol% PVAc–5 mol% HClO4. From Fig. 4(b) it is observed that the impedance value decreases with increase in temperature. As the temperature increases, the segmental motion of the polymer chain and dissociation of the acid increases resulting in the decrease in impedance, hence high conductivity. The bulk resistance has been calculated by extrapolating the spike to the x-axis (Z′) and the conductivity has been calculated using the relation σ¼
l −1 S cm RB A
ð2Þ
where l RB A
535
thickness of the sample and bulk resistance area of the electrolyte
The RB values are calculated using ZPlot software and the error in the RB value is 0.8%. The maximum ionic conductivity of 3.75 × 10 − 3 S cm− 1 at 303 K has been observed for the sample 75 mol% PVAc:25mol% HClO4. The conductivity values calculated for all the samples prepared are tabulated in Table 2. The conductance spectra (Fig. 5) show three regions: the low frequency dispersion which is followed by a mid frequency plateau and
Sample no.
Sample
1 2 3 4 5
95 mol% 90 mol% 85 mol% 80 mol% 75 mol%
a
Conductivity (S cm− 1) 303 Ka PVAc–5 mol% HClO4 PVAc–10 mol% HClO4 PVAc–15 mol% HClO4 PVAc–20 mol% HClO4 PVAc–25 mol% HClO4
7.09 × 10− 6 8.73 × 10− 5 1.71 × 10− 4 1.81 × 10− 3 3.75 × 10− 3
Error—0.8%.
a high frequency dispersion region. The low frequency dispersion may be attributed to the space-charge polarization that occurs at the electrode–electrolyte interface. The mid frequency plateau represents the DC conductivity region caused due to the hopping of ions in the polymer chains and the high frequency dispersion obeys the power law feature Aω n. According to Jonscher's Power law [30], the frequency dependent of Ac conductivity of polymer electrolytes is described by the following equation: σ ¼ σ 0 þ Aω
σ0 A, n
n
ð3Þ
DC conductivity and material parameters.
The value of σ0 has been found by fitting the equation to non linear least square fit. The DC conductivity observed form the conductance spectra are in good agreement with the values obtained from impedance plot. Fig. 5(b) shows the conductance plot for the sample (95 mol% PVAc:5 mol% HClO4) at different temperatures. From the plot it is clear that as the temperature increased the high frequency dispersion disappears and the low frequency dispersion increases due to the increase in mobile charge carrier and the DC plateau region shifts towards high frequency region. From Fig. 6 it is observed that for low concentration of HClO4 (5 mol%), conductivity value increases linearly with increase in temperature up to 345 K and for higher temperature it shows nonArrhenius behavior. As the acid concentration is increased the proton exchange membrane shows non-Arrhenius behavior in the temperature range studied. This non-Arrhenius behavior may be attributed to the reduction of Tg due to the plasticization effect of acid [16]. The same phenomenon has been observed by Vargas et al. in the system PVA complexed with KHSO4. Fig. 7 shows the dielectric constant variation with respect to salt concentration at 303 K. The dielectric permittivity raises sharply towards low frequencies due to electrode polarization effects. At high frequencies the periodic reversal of the electric field occurs so fast that there is no excess ion diffusion in the direction of the field. The polarization due to the charge accumulation decreases leading to the decrease in the dielectric constant (ε′) values [18]. Fig. 8 shows the plot between logω and ε″ for 95 mol% PVAc–5 mol% HClO4, 90 mol% PVAc–10 mol% HClO4, 85 mol% PVAc–15 mol% HClO4, 80 mol% PVAc–20 mol% HClO4, and 75 mol% PVAc–25 mol% HClO4 at 303 K. The dielectric loss is very large at low frequencies due to the free charge motion within the material. These values do not correspond to the bulk dielectric processes but are due to the free charges build up at the interface between the material and the electrodes. From the plot, high dielectric loss has been observed for the sample 75 mol% PVAc:25 mol% HClO4 which has high conductivity than other samples. 5. Conclusion Proton conducting polymer electrolytes based on poly(vinyl acetate) (PVAc) and perchloric acid (HClO4) have been prepared. The XRD analysis
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References
400000 350000
95PVAc:5HClO4 (PH1) 90PVAc:10HClO4 (PH2)
300000
85PVAc:15HClO4 (PH3) 250000
80PVAc:20HClO4 (PH4) 75PVAc:25HClO4 (PH5)
200000
303 K 150000 100000 50000 0 3
4
5
Log
6
7
(Hz)
Fig. 8. Dielectric loss spectra for PVAc:HClO4 complexes at 303 K.
confirms the complete complexation of the polymer–acid. The thermal analysis reveals that the ionic transport takes place in the amorphous phase of the polymer matrix. FTIR spectra analysis reveals the interaction between proton and ester oxygen of poly(vinyl acetate) (PVAc). 75 mol% PVAc–25 mol% HClO4 has been found to exhibit maximum conductivity of 3.75×10− 3 S cm− 1 at 303 K. The temperature dependence of conductivity shows non-Arrhenius behavior at higher temperatures. Acknowledgements One of the authors, Dr. D. Arun Kumar, acknowledges DRDO-BU Center for Life Sciences for providing Senior Research Fellowship.
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Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Thermal, vibrational and Ac impedance studies on proton conducting polymer electrolytes based on poly(vinyl acetate) D. Arun Kumar a, b, 1, S. Selvasekarapandian a, c,⁎, R. Baskaran d, T. Savitha e, H. Nithya b, d a
Department of Physics, Bharathiar University, Coimbatore-641 046, Tamil Nadu, India DRDO-BU Center for Life Sciences, Bharathiar University, Coimbatore-641 046, Tamil Nadu, India Kalasalingam University, Krishnankoil, Srivilliputhur-626190, Tamil Nadu, India d IMRAM, Tohoku University, Sendai, Japan e Solid State Physics and Magnetism Section, Department of Physics and Astronomy Celestijnenlaan 200D, B-3001 Leuven, Belgium b c
a r t i c l e
i n f o
Article history: Received 28 July 2011 Received in revised form 17 November 2011 Available online 16 December 2011 Keywords: Proton conductors; PVAc; FTIR; DSC; Impedance analysis
a b s t r a c t Proton conducting polymer electrolytes based on poly(vinyl acetate) (PVAc) and perchloric acid (HClO4) have been prepared by solution casting technique with various compositions. The X-ray diffraction analysis confirms the polymer–HClO4 complex formation. FTIR spectra analysis reveals the interaction between proton and ester oxygen of poly(vinyl acetate) (PVAc). The shift in Tg towards the lower temperature indicates that the polymer salt interaction takes places in the amorphous phase of the polymer matrix. Ac impedance spectroscopy reveals that 75 mol% PVAc:25 mol% HClO4 exhibits maximum conductivity, 3.75 × 10− 3 S cm − 1 at room temperature (303 K). The increase in conductivity with increase in dopant concentration and temperature may be attributed to the enhanced mobility of the polymer chains, number of charge carriers and rotations of side chains. The temperature dependence of conductivity shows non-Arrhenius behavior at higher temperatures. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Since the pioneering works of Wright et al. [1,2] a lot of facts have been found out about the ionic conductivity in polymeric complexes. In recent years, proton conducting polymer electrolytes have attracted considerable attention due to the possibility of their application in a variety of electrochemical devices, such as fuel cells, humidity, gas sensors, electro chromic displays and electro chromic windows [3]. In general proton-conducting polymers are usually based on polymer electrolytes, which have negatively charged groups attached to the polymer backbone. Complexes of basic polymers such as poly(ethylene oxide) (PEO), poly(ethylene imine) (PEI), poly(acryl amide) (PAAM) and poly(vinyl alcohol) (PVA) with strong acids have been shown to possess high proton conduction in the range of 10− 4–10− 3 S cm− 1 [4]. Many investigators have studied the behavior of acid-based polymer electrolyte complexes as proton conductors and their applications in solid state devices at room temperature have been demonstrated [5]. High proton conductivity at high temperature and low relative humidity can be achieved using acid–base polymer complexes between basic polymers
⁎ Corresponding author at: Kalasalingam University, Krishnankoil, Srivilliputhur626190, Tamil Nadu, India. Tel.: + 91 9443703089. E-mail address: [email protected] (S. Selvasekarapandian). 1 Present address: Energy Materials Center, Korea Institute of Energy Research, Daejeon, Korea. 0022-3093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2011.11.031
and strong acids or polymeric acids [6]. The proton-accepting polymers include poly(ethylene oxide) (PEO), poly(vinyl acetate) (PVAc), polyacrylamide (PAAM), poly vinyl alcohol (PVA) and poly(ethylene imine) (PEI ) [7,8]. Complexes of strong inorganic acids (phosphoric acid [9,10], sulfuric acid [10] or ammonium salts [11] with commercially available polymers such as poly(ethylene oxide)(PEO), poly(ethylene imine), poly vinyl alcohol [12,13], or poly vinyl acetate [14,15] have mostly been investigated. PVA complexed with inorganic acid has been already reported by Vargas et al., and has reported the plasticization effect of acids and water in PVA [16]. Dielectric and conduction properties of PVA complexed with H3PO4 have been studied by Kufian et al. [17]. Dielectric properties, vibrational and NMR studies on PVA doped with H3PO4 and dielectric properties on PVA doped with H2SO4 have been studied by Singh et al. [18,19]. PVAc happens to be one of the polymers which possess large dipole moments and high relaxation time which are due to its side chains connected to ester oxygen [20]. PVAc is a suitable candidate to achieve high ionic conductivity at ambient temperatures when doped with inorganic acids and salts. Electrical properties of PVAc doped with inorganic salts have been studied and reported, but the reports on the electrical properties of acid doped PVAc are scarce. Hence in the present work, proton conducting polymer electrolytes based on poly(vinyl acetate) (PVAc) and perchloric acid (HClO4) have been prepared by solution casting technique with various compositions and its thermal vibrational and electrical properties are studied. Plasticization effect of acid on poly(vinyl acetate) (PVAc) has been discussed using FTIR. Impedance spectroscopy
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has been performed to analyze effect of acid on the conductivity of pure polymer.
(f) 75PVAc : 25HClO4
2. Experimental analysis
Poly(vinyl acetate) (PVAc) Himedia, perchloric acid (HClO4) Qualigens and acetone (Merck) were used as received without no other treatment.
(e) 80PVAc : 20HClO4
Intensity (arb unit)
2.1. Materials
(d) 85PVAc : 15HClO4 (c) 90PVAc : 10HClO4
2.2. Preparation of proton conducting polymer membrane
(b) 95PVAc : 5HClO4
The polymer complexes have been prepared by solution casting technique. The host polymer PVAc has been dissolved using acetone as solvent. Then the dopant HClO4 is added to this solution and stirred well to get a homogenous solution. The solution is then poured in to small glass Petri dishes to form thin film, and kept in room temperature for 24 hours to remove acetone and then in vacuum oven at a temperature of 60 °C to further remove the traces of acetone.
(a) Pure PVAc
2.3. Structural analysis 2.3.1. XRD analysis XRD measurements were performed on the polymer membrane by using PANalytical Xpert pro X-ray diffractometer at an average of scans at the resolution of 2° s − 1. 2.3.2. FTIR analysis FTIR spectrum was recorded for the thin polymer membrane using Shimadzu 8000-Spectrophotometer instrument in the range 4000–400 cm − 1 at a resolution of 2 cm − 1.
20
40
80
60
2 (Degree) Fig. 1. XRD pattern of (a) pure PVAc, (b) 5 mol% HClO4, (c) 10 mol% HClO4, (d) 15 mol% HClO4, (e) 20 mol% HClO4, and (f) 25 mol% HClO4 doped PVAc.
the polymer electrolytes are shown in Fig. 2, which shows vibration bands at different positions corresponding to the vibrational modes of the polymer salt complex. 3.3. Thermal analysis The DSC studies have been employed to study the thermal behavior of the PVAc:HClO4 polymer electrolyte system. The salt concentration dependence of the glass transition temperature of the polymer has been studied and the DSC thermogram is shown in Fig. 3.
2.4. Thermal analysis 3.4. Impedance spectra analysis The thermal property of the polymer membranes has been investigated using NETZSCH DSC 204F1 differential scanning calorimeter. Dry nitrogen gas at the rate of 20 ml min − 1 is purged through the cell during all the measurements. Heating and cooling rate was fixed at 5 °C min − 1 for all the samples. The samples are cooled from 30 °C to − 100 °C and kept at this temperature for 2 min, then heated from −100 °C to 50 °C.
Impedance spectroscopy is relatively powerful technique of characterizing many solid electrolytes and it may be used to investigate the dynamics of bound or mobile charge in bulk and interfacial regions of polymer electrolytes. Fig. 4(a) shows the Nyquist plot or cole–cole plot of 95 mol% PVAc–5 mol% HClO4, 90 mol%
2.5. Electrical analysis
3. Results 3.1. XRD analysis The complexation of PVAc with HClO4 has been studied using X-ray diffraction studies. The XRD patterns of complexed PVAc–HClO4 of different compositions are presented in Fig. 1(a–f).
(c) % Transmittance
To study the ionic conductivity of the samples Ac impedance spectroscopy is performed using HIOKI 3532 LCR Hitester in the frequency range 42 Hz–5 MHz for the temperature range 303 K–373 K using aluminum as blocking electrodes. The impedance data were taken at an average of 32 at each frequency within the measured frequency range.
610
(b) 3723 625
(a)
(a) Pure PVAc (b) 85PVAc:15HClO4
1245 1730
(c) 80PVAc:20HClO4
3.2. Vibrational analysis 3.2.1. FT-IR spectroscopic analysis The vibrational analysis for the prepared polymer electrolytes has been studied using FTIR spectroscopy. The FTIR spectra observed for
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1) Fig. 2. FT-IR spectrum of (a) pure PVAc, (b) 15 mol% HClO4, and (c) 20 mol% HClO4 doped PVAc.
D.A. Kumar et al. / Journal of Non-Crystalline Solids 358 (2012) 531–536
PVAc–10 mol% HClO4, 85 mol% PVAc–15 mol% HClO4, 80 mol% PVAc–20 mol% HClO4, and 75 mol% PVAc–25 mol% HClO4 proton exchange membranes at 303 K.
(a) 95PVAc : 5HClO4
Exo
(b) 85PVAc : 15HClO4 (c) 75PVAc : 25HClO4
3.5. Conductance spectra analysis
(c)
Fig. 5(a) shows the conductance spectra of 95 mol% PVAc–5 mol% HClO4, 90 mol% PVAc–10 mol% HClO4, 85 mol% PVAc–15 mol% HClO4, 80 mol% PVAc–20 mol% HClO4, and 75 mol% PVAc–25 mol% HClO4 proton exchange membranes at 303 K. Fig. 5(b) shows the conductance spectra of 5 mol% HClO4 doped polymer electrolytes at different temperatures.
Endo
(b) Tg
200
220
240
260
(a)
280
300
3.6. Temperature dependent conductivity
Temperature (K) Fig. 3. DSC thermogram of (a) 5 mol% HClO4, (b) 15 mol% HClO4, (c) and 15 mol % HClO4 doped PVAc.
a
533
30000
Temperature dependence of conductivity has been employed to study the conduction of ions in the polymer electrolytes. Fig. 6 shows the temperature dependent conductivity plot.
a
-2.0
2500
25000
-2.5
-Z''
2000
-3.0
1500
4 4
Log T (Scm-1K)
1000
20000
-Z''
500 0
15000
0
500
1000 1500 2000 2500
Z'
303 K 95PVAc:5HClO4 (PH1)
10000
90PVAc:10HClO4 (PH2)
-3.5 -4.0 -4.5 -5.0 -5.5
95PVAc:5HClO4 (PH1) 90PVAc:10HClO4 (PH2) 85PVAc:15HClO4 (PH3) 80PVAc:20HClO4 (PH4) 75PVAc:25HClO4 (PH5)
-6.0
85PVAc:15HClO4 (PH3)
5000
80PVAc:20HClO4 (PH4)
-6.5
75PVAc:25HClO4(PH5)
-7.0
0
3 0
5000
10000
15000
20000
25000
5
Log
6
7
(Hz)
b -3.0
100000 95PVAc:5HClO4 (PH1)
-3.5
Log T (Scm-1K)
303 K 313 K 323 K
80000
4
30000
Z'
b
303 K
-Z''
60000
40000
-4.0
303 K 313 K 323 K 333 K 343 K 353 K 363 K
-4.5 -5.0 -5.5
20000 -6.0 95PVAc:5HClO4 (PH1) 0
-6.5 0
25000
50000
75000
100000
Z' Fig. 4. Nyquist plot for (a) PVAc–HClO4 complexes at 303 K and (b) 5 mol% HClO4 doped PVAc at various temperatures.
3
4
5
Log
6
7
(Hz)
Fig. 5. Conductance spectra for (a) PVAc–HClO4 complexes at 303 K and (b) 95 mol% PVAc–5 mol% HClO4 doped polymer electrolyte.
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4. Discussion
0.0 95 PVAc:5 HClO4 Linear Fit of 95 PVAc:5 HClO4
-0.5
Log T (Scm-1K)
R2 = 0.98753 -1.0
-1.5
-2.0
-2.5
-3.0 0.0026 0.0027 0.0028 0.0029 0.0030 0.0031 0.0032 0.0033
1000/T(K-1) Fig. 6. Temperature dependent conductivity for 5 mol% and 15 mol% PVAc:HClO4 complexes.
3.7. Dielectric spectra analysis The complex permittivity (ε) or dielectric constant of a system is defined by
ε ¼ ε′ þ iε″ ¼ ε′−jðσ=ωε 0 Þ
ð1Þ
where ε′ ε″ σ ω ε0
real part of the dielectric constant, imaginary part of the dielectric constant of the material, conductivity of the material, angular frequency, and permittivity of free space.
Fig. 7 shows the log ω vs ε′ plot for 95 mol% PVAc–5 mol% HClO4, 90 mol% PVAc–10 mol% HClO4, 85 mol% PVAc–15 mol% HClO4, 80 mol% PVAc–20 mol% HClO4, and 75 mol% PVAc–25 mol% HClO4 proton exchange membranes at 303 K.
400000
95PVAc:5HClO4 (PH1) 90PVAc:10HClO4 (PH2) 85PVAc:15HClO4 (PH3)
300000
80PVAc:20HClO4 (PH4) 75PVAc:25HClO4 (PH5)
200000 303 K
100000
0
3
4
5
Log
6
(Hz)
Fig. 7. Variation of dielectric constant with frequency for PVAc:HClO4 complexes.
7
From Fig. 1(a–f) the complexation of the salt with the polymer is confirmed. Fig. 1(a), the pure polymer PVAc shows two broad peaks at 2θ = 15.07° and 22.7° which reveal the amorphous nature of the polymer [15]. The two broad peaks observed in PVAc were found suppressed to one broad peak at 2θ value 20°, which suggests that the addition of perchloric acid into the polymer matrix decreases the degree of crystallinity with the increase in amorphous nature of the sample. This indicates that the amorphous nature of the polymer matrix increases with the addition of perchloric acid concentration. This result can be interpreted by Hodge et al. [21] criterion, which establishes a correlation between the intensity of the peak and the degree of crystallinity. Hence the change in intensity and the broad nature of the peak suggest that the polymer and acid complexation takes place in the amorphous region of the polymer matrix [22]. In Fig. 1(b–d) for the concentration of acid ranging from 5 to 15 mol%, no sharp peaks corresponding to perchloric acid have been observed in the XRD pattern. This contributes a lot to the increment of proton and transport in the amorphous phase and thus results in the improvement of conductivity [23]. However when the acid concentration is 20 mol% and 25 mol%, less intense peak at 2θ = 28° corresponding to perchloric acid [JCPDS card no. 78-2454] appears which may be due to the presence of perchloric acid precipitation. From the FTIR spectra of PVAc–HClO4 complexes (Fig. 2), the vibrational bands observed at 2923, 2865 and 1375 cm − 1 are ascribed to CH3 asymmetric stretching, symmetric stretching and symmetric bending vibrations of pure PVAc respectively. The peaks at 1245, 1100 and 1090 cm − 1 are ascribed to C–O–C symmetrical stretching, C–O and C–C stretching vibrations of pure PVAc respectively. The peak observed at 610 cm− 1 (Fig. 2(b)) is ascribed to the ClO4− ion which is slightly shifted to lower wave numbers 595 cm− 1 at higher concentrations of HClO4. This result indicates the coordination of ClO4− ion with the polar group present in the polymer of the polymer complexes. In Fig. 2(b–c), the peak observed at 3728 cm− 1 is ascribed to the weak inter ionic bonding between H+ and ClO4− as (H+ ClO4− ——— H+ ClO4−). Fig. 2(b) and (c) well reflect the different degree of coordination of cation to both the C–O–C and C=O moieties. The appearance of strong band in the spectrum at 1730 cm− 1 which corresponds to C=O stretching frequency of pure PVAc is slightly shifted to lower wave number (1719–1705 cm− 1) in the polymer–acid complexes. This effect is due to the coordination of the cation with the oxygen, which results in the weakening of the C O bond and hence the wave number of absorption decreases. Similar results have been reported by Weihua Zhu et al. and Selvasekarapandian et al. for the PEG-PU/ NaClO4 and PVAc–NH4SCN complexes, respectively [24,25]. Moreover the addition of HClO4 causes a small decrease (1238–1232 cm− 1) of the C–O–C stretch down to lower wave numbers due to the coordination of the ester oxygen with the cation. Similar effect was reported by Wieczorek et al. for polyether–poly(methyl methacrylate) blend based system [26]. The band assignments for the polymer salt complex have been tabulated and shown in Table 1. The polymer–acid complex formation and the proton interaction have been confirmed from the above analysis. DSC results are shown in Fig. 3. The glass transition temperature of pure PVAC is about 303 K as reported by Baskaran et al. [15]. A well resolved step which is associated to a glass transition (Tg) is observed to be 275 K, 257 K and 244 K for 5 mol%, 15 mol% and 25 mol% of HClO4 concentration respectively. Fig. 3(a–c) represents decrease in Tg with the increase in HClO4 concentration (5 mol%, 15 mol%, and 25 mol%) which could be accounted by the availability of ester oxygen and the plasticizing effect of increased ions associated [15,25]. The same effect has also been observed by Pu et al. for the PVA/Imi/ NH4H2PO4 system [23]. The decrease in Tg indicates the increase in mobility of the polymer chains [26]. The effect of the plasticizing additives on the polymers can be explained by the free volume theory.
D.A. Kumar et al. / Journal of Non-Crystalline Solids 358 (2012) 531–536 Table 1 Vibrational band assignments of the prepared polymer electrolytes. Polymer electrolyte complex wavenumber (cm− 1) Pure PVAc 1090 1100 1245 1375 1730 2865 2923
Table 2 Conductivity values of prepared polymer electrolyte.
Band assignments
85PVAc:15HClO4 80PVAc:20HClO4 610 1070 – 1238 1354 1719 2853 2904 3723
595 1070 – 1236 1348 1705 2844 2895 3712
ClO4 C–C stretching C–O stretching C–O–C symmetric stretching CH3 symmetric bending C=O stretching CH3 symmetric stretching CH3 asymmetric stretching Weak inter ionic bonding between H+ and ClO4−
In accordance with this theory the presence of plasticizers in the polymer matrix increases the free volume available in the polymer chain segments and therefore allows greater internal chain rotation and an increase in the segmental mobility. As the ion transport is dependent on the local segmental motion, the increase in free volume with the increase in HClO4 concentration influences the conductivity by the easy flow of protons through the polymer chain network when there is an applied electric field. Hence the result confirms that HClO4 exerts a plasticizing effect on the polymer matrix [27]. From Fig. 4(a) the impedance spectra/cole cole plot shows two well defined regions, a chord in the high frequency region which is related to the conduction process in the bulk of the electrolytes and a spike in the low frequency region. The observed inclined spike represents a finite or infinite diffusion of mobile charge carrier which occurs into the electrode materials. The curvature in the spike is observed which is attributed to the blocking effect of the electrodes [28]. The appearance of semi-circle in the high frequency region can be represented by a parallel combination of a resistor and a capacitor. The resistor occurs due to the migration of ions through the free volume of the polymer matrix, and the capacitor to the polarized polymer chains due to the alternating field applied. The disappearance of the high frequency chord portion in the complex impedance plot for the high salt concentration polymer electrolyte illustrates that the total conductivity is mainly due to the result of ionic conduction [29]. Fig. 4 (b) shows the temperature dependent cole cole plot for the proton exchange membrane 95 mol% PVAc–5 mol% HClO4. From Fig. 4(b) it is observed that the impedance value decreases with increase in temperature. As the temperature increases, the segmental motion of the polymer chain and dissociation of the acid increases resulting in the decrease in impedance, hence high conductivity. The bulk resistance has been calculated by extrapolating the spike to the x-axis (Z′) and the conductivity has been calculated using the relation σ¼
l −1 S cm RB A
ð2Þ
where l RB A
535
thickness of the sample and bulk resistance area of the electrolyte
The RB values are calculated using ZPlot software and the error in the RB value is 0.8%. The maximum ionic conductivity of 3.75 × 10 − 3 S cm− 1 at 303 K has been observed for the sample 75 mol% PVAc:25mol% HClO4. The conductivity values calculated for all the samples prepared are tabulated in Table 2. The conductance spectra (Fig. 5) show three regions: the low frequency dispersion which is followed by a mid frequency plateau and
Sample no.
Sample
1 2 3 4 5
95 mol% 90 mol% 85 mol% 80 mol% 75 mol%
a
Conductivity (S cm− 1) 303 Ka PVAc–5 mol% HClO4 PVAc–10 mol% HClO4 PVAc–15 mol% HClO4 PVAc–20 mol% HClO4 PVAc–25 mol% HClO4
7.09 × 10− 6 8.73 × 10− 5 1.71 × 10− 4 1.81 × 10− 3 3.75 × 10− 3
Error—0.8%.
a high frequency dispersion region. The low frequency dispersion may be attributed to the space-charge polarization that occurs at the electrode–electrolyte interface. The mid frequency plateau represents the DC conductivity region caused due to the hopping of ions in the polymer chains and the high frequency dispersion obeys the power law feature Aω n. According to Jonscher's Power law [30], the frequency dependent of Ac conductivity of polymer electrolytes is described by the following equation: σ ¼ σ 0 þ Aω
σ0 A, n
n
ð3Þ
DC conductivity and material parameters.
The value of σ0 has been found by fitting the equation to non linear least square fit. The DC conductivity observed form the conductance spectra are in good agreement with the values obtained from impedance plot. Fig. 5(b) shows the conductance plot for the sample (95 mol% PVAc:5 mol% HClO4) at different temperatures. From the plot it is clear that as the temperature increased the high frequency dispersion disappears and the low frequency dispersion increases due to the increase in mobile charge carrier and the DC plateau region shifts towards high frequency region. From Fig. 6 it is observed that for low concentration of HClO4 (5 mol%), conductivity value increases linearly with increase in temperature up to 345 K and for higher temperature it shows nonArrhenius behavior. As the acid concentration is increased the proton exchange membrane shows non-Arrhenius behavior in the temperature range studied. This non-Arrhenius behavior may be attributed to the reduction of Tg due to the plasticization effect of acid [16]. The same phenomenon has been observed by Vargas et al. in the system PVA complexed with KHSO4. Fig. 7 shows the dielectric constant variation with respect to salt concentration at 303 K. The dielectric permittivity raises sharply towards low frequencies due to electrode polarization effects. At high frequencies the periodic reversal of the electric field occurs so fast that there is no excess ion diffusion in the direction of the field. The polarization due to the charge accumulation decreases leading to the decrease in the dielectric constant (ε′) values [18]. Fig. 8 shows the plot between logω and ε″ for 95 mol% PVAc–5 mol% HClO4, 90 mol% PVAc–10 mol% HClO4, 85 mol% PVAc–15 mol% HClO4, 80 mol% PVAc–20 mol% HClO4, and 75 mol% PVAc–25 mol% HClO4 at 303 K. The dielectric loss is very large at low frequencies due to the free charge motion within the material. These values do not correspond to the bulk dielectric processes but are due to the free charges build up at the interface between the material and the electrodes. From the plot, high dielectric loss has been observed for the sample 75 mol% PVAc:25 mol% HClO4 which has high conductivity than other samples. 5. Conclusion Proton conducting polymer electrolytes based on poly(vinyl acetate) (PVAc) and perchloric acid (HClO4) have been prepared. The XRD analysis
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References
400000 350000
95PVAc:5HClO4 (PH1) 90PVAc:10HClO4 (PH2)
300000
85PVAc:15HClO4 (PH3) 250000
80PVAc:20HClO4 (PH4) 75PVAc:25HClO4 (PH5)
200000
303 K 150000 100000 50000 0 3
4
5
Log
6
7
(Hz)
Fig. 8. Dielectric loss spectra for PVAc:HClO4 complexes at 303 K.
confirms the complete complexation of the polymer–acid. The thermal analysis reveals that the ionic transport takes place in the amorphous phase of the polymer matrix. FTIR spectra analysis reveals the interaction between proton and ester oxygen of poly(vinyl acetate) (PVAc). 75 mol% PVAc–25 mol% HClO4 has been found to exhibit maximum conductivity of 3.75×10− 3 S cm− 1 at 303 K. The temperature dependence of conductivity shows non-Arrhenius behavior at higher temperatures. Acknowledgements One of the authors, Dr. D. Arun Kumar, acknowledges DRDO-BU Center for Life Sciences for providing Senior Research Fellowship.
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