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ISSN: 1023-666X (Print) 1563-5341 (Online) Journal homepage: https://www.tandfonline.com/loi/gpac20
Investigation of the Ionic Conduction Mechanism in Carboxymethyl Cellulose/Chitosan Biopolymer Blend Electrolyte Impregnated with Ammonium Nitrate
M. S. A. Rani, N. S. Mohamed & M. I. N. Isa
To cite this article: M. S. A. Rani, N. S. Mohamed & M. I. N. Isa (2015) Investigation of the Ionic Conduction Mechanism in Carboxymethyl Cellulose/Chitosan Biopolymer Blend Electrolyte Impregnated with Ammonium Nitrate, International Journal of Polymer Analysis and Characterization, 20:6, 491-503, DOI: 10.1080/1023666X.2015.1050803
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Published online: 11 Aug 2015.
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ISSN: 1023-666X print/1563-5341 online DOI:10.1080/1023666X.2015.1050803
Investigation of the Ionic Conduction Mechanism in Carboxymethyl Cellulose/Chitosan Biopolymer Blend
Electrolyte Impregnated with Ammonium Nitrate
M. S. A. Rani,1N. S. Mohamed,2 and M. I. N. Isa3
1Institute of Graduate Studies, University of Malaya, Kuala Lumpur, Malaysia
2Centre for Foundation Studies in Science, University of Malaya, Kuala Lumpur, Malaysia
3Centre for Corporate Communication and Image Development, Universiti Malaysia Terengganu, Terengganu, Malaysia
In the present work, an attempt has been made to prepare a new natural biopolymer blend electrolyte of carboxymethyl cellulose/chitosan impregnated with NH4NO3by the solution casting technique. The conductivity for the system was measured by impedance spectroscopy. The incorporation of 40 wt.% NH4NO3 optimized the ambient temperature conductivity of the electrolyte up to 1.03105 S cm1. All electrolytes were found to follow the Arrhenius relationship. Dielectric studies confirmed that the electrolytes obey non-Debye behavior. The temperature dependence of the power law exponent s for the highest conducting film can be represented by the correlated barrier hopping model.
Keywords: Arrhenius; Carboxymethyl cellulose; Chitosan; Conductivity; Correlated barrier hopping
INTRODUCTION
Solid proton conducting polymer electrolytes are of immense interest and have attracted much attention in solid state ionics due to their potential applications in electrochemical devices like rechargeable batteries, photo-electrochemical solar cells, sensors, fuel cells, and supercapacitors.[1,2]In contrast to liquid electrolytes, solid polymer electrolytes (SPEs) offer dynamic properties such as satisfactory mechanical properties, good shelf life, no leak- age, good compatibility with electrode materials, and ease of fabrication.[3]Various types of SPEs have been explored in the pursuit of developing solid electrolyte systems using synthetic materials such as poly(ethylene oxide), poly(methyl methacrylate), poly(vinyl alcohol), and poly(vinylidene fluoride).[4–7]However, these types of polymers are synthetic based, which means that their origins are from petroleum, so they possess a high cost of materials and are nonbiodegradable in the natural environment. Therefore, researchers have been working on the development of natural biopolymers as an alternative to the use of
Submitted 25 March 2015; accepted 31 March 2015.
Correspondence: M. I. N. Isa, Centre for Corporate Communication and Image Development, Universiti Malaysia Terengganu, 21300 Kuala Terengganu, Terengganu, Malaysia. E-mail:[email protected]
Color versions of one or more of the figures in the article can be found online athttp://www.tandfonline.com/gpac.
petroleum-based polymers. Recently, extensive work has been done to investigate the potential of natural polymers because of their abundance, biodegradability, environmental friendliness, and good mechanical and electrical properties.[8,9]
A few years ago, the attention of researchers turned towards the blending of polymers, one of the effective methods to enhance the electrical and mechanical properties of electrolyte systems.
Chitosan is one of the polysaccharides commonly used in a blending system with various poly- mers. Buraidah and Arof[10]reported that the highest conductivity value obtained at ambient temperature was 1.77106S cm1for the chitosan-PVA-NH4I system. As a comparison, the ionic conductivity achieved for an unblended system, chitosan- NH4I was 3.73107S cm1. A similar result was reported by Khiar and Arof[11]for the blend of two natural polymers, chitosan and starch. The highest ionic conductivity was found to be 3.89105 S cm1 at ambient temperature at the doping concentration of 35 wt.%NH4NO3. These results proved that the blending of polysaccharides is a promising technique that can be used to optimize the ionic conductivity of polymer systems. According to Buraidah et al.,[10]blending of two polymers provides more complexation sites, which raise the ion migration and exchange, resulting in an increment of ionic conductivity.
In the present work, a natural biopolymer blend electrolyte (NBBE) system based on car- boxymethyl cellulose (CMC)/chitosan (CS) impregnated with different compositions of NH4NO3were prepared by the solution casting technique in the presence of 1%acetic acid solvent. CS and CMC are miscible and thus homogeneous films are expected to be formed.[12]
To the best of the authors’ knowledge, there are few reports on NBBE comprised of CMC blended with CS, especially on the ionic conduction mechanism and electrical property.
For this work, electrochemical impedance spectroscopy (EIS) was applied to investigate the dependence of complex conductivity on frequency and of the CMC/CS-based electrolyte system.
EXPERIMENTAL SECTION
CMC was obtained from Acros Organic Co., and CS was supplied by W.A. Hammond Drierite Company Ltd. The solution casting technique was employed to obtain films with varied amounts of ammonium nitrate concentration (10–50 wt.%). Pure NBBE film without dopant was also prepared as a control. In a clean beaker, weighed amounts of CMC and CS powder with 2:1 weight ratio with NH4NO3crystals (purity 99%) were dissolved in 100 mL of 1%acetic acid at room temperature.[12]Complete dissolution was achieved after 24 h of stirring at room temperature using a magnetic stirrer. The final light yellowish solution was then poured into separate Petri dishes and left to dry in an oven to form yellowish transparent thin films. NBBE films were transferred to a desiccator for further drying prior to characterization.
The impedance study was performed by using a Hioki 3532-50 LCR Hi-Tester. Impedance data were collected for frequencies ranging from 50 Hz to 1 MHz. Impedance spectroscopic measurement was performed to determine the ionic conductivity of biopolymer electrolyte films over a temperature range from 30° to 100°C. The biopolymer electrolyte samples were sand- wiched between stainless steel electrodes that have a surface contact area of 2.0 cm2, and the samples were mounted onto the sample holder under spring pressure.
RESULTS AND DISCUSSION Conductance Plot Analysis
Figure1presents the conductance spectra of highest conducting sample of CMC/CS-40 wt.% NH4NO3sample with temperature. The ionic conductivity enhances with increase in tempera- ture from 303 to 373 K. The enhancement of conductivity with temperature can be explained by a decrease in viscosity and thus increase in polymer chain flexibility.[13]Application of higher temperature to the polymer results in faster ion movement, which is attributed to the increment of bond rotations, hence resulting in higher ionic conductivity.[14,15]The conductivity disper- sion can be observed to be less predominant at low temperatures, however the frequency at which the dispersion becomes prominent shifts to a higher frequency region when the tempera- ture increases.[16]Based on Figure1, more charges are accumulated at the electrode-electrolyte interface with decreasing frequency. This phenomenon leads to a drop in the number of mobile ions and hence a decrease in conductivity at low frequency. As the frequency increases, the mobility of charge carriers also increases, resulting in enhancement of conductivity of the NBBE system.[17]
Complex Impedance Plot
Figure2 illustrates complex impedance plot of NBBE film of CMC/CS doped with 40 wt.% NH4NO3recorded at different temperatures. From the plot, there are two-well defined regions that can be observed: a high-frequency region semicircular arc and a low-frequency region inclined spike. The appearance of the semicircle in this present study can be explained by the bulk effect of NBBE. This is due to the parallel combination of a resistor (the migration
FIGURE 1 Conductance spectra of CMC/CS-40 wt.%NH4NO3sample in the temperature range of 303–373 K.
of the ions occurs through the free volume matrix) and a capacitor (the immobile polymer chains).[18]The bulk conductivity of the sample was calculated using:
r¼ t
RbA ð1Þ
where A is the area of electrode–electrolyte contact in cm2,tis the thickness of the sample in cm, andRbis the bulk resistance, which can be retrieved by plotting the negative imaginary impedance,Zi, versus the real part,Zr, of impedance.
The spectra in Figure2also show that the value ofRbdecreases with increasing temperature.
This can be explained by the decrease in resistance of the NBBE sample resulting in enhance- ment in the charge carriers’mobility with temperature.[19]The high-frequency semicircular arc seems to gradually fade away and completely disappeared at 60°C. The absence of the semi- circle in the complex impedance plot and formation of inclined spikes indicates the prevailing of the resistive components of the NBBE.[18]However, the spikes inclined at an angle less than 90° instead of the vertical axis in this present work may be attributed to the roughness of the electrode/electrolyte interface.[20]
Salt Concentration Dependence of Ionic Conductivity
Figure3 displays the graph of conductivity against concentration of NH4NO3at ambient tem- perature (303 K). As we can see from Figure3, pure polymer blend (NH4NO3-0 sample) shows the ionic conductivity of 1.79108S cm1. The conductivity increases upon addition of 10 up to 40 wt.%of NH4NO3. The maximum ionic conductivity achieved is 1.03105S cm1. The increase of ionic conductivity with addition of NH4NO3concentration is due to the increase of the number of charge carriers and mobility of ions/conducting species.[21]However, the con- ductivity dramatically declines after addition of 40 wt.% NH4NO3. Ng and Mohamad[22]
reported that as NH4NO3 concentration increases, the host matrix (polymer blend) becomes crowded with the dopant ions. Hence, the transportation of charge carriers is reduced due to the limitation of ionic mobility.[23] For further understanding of the ionic conductivity
FIGURE 2 Complex impedance spectra for CMC/CS-40 wt.%NH4NO3NBBE at different temperatures.
mechanism, the ionic conductivity of biopolymer systems was tested at elevated temperatures, 303 to 338 K.
Temperature Dependence of Conductivity
Figure4shows the plot of log conductivity,ragainst 1000/T, for the samples with 0 to 50 wt.% of NH4NO3. The regression values of the plots are approximated to 1, indicating that the tem- perature dependence of the ionic conductivity for all samples is linear and obeys the Arrhenius rule.[24]This relation implies that the conductivity is thermally assisted, which means that the variation of temperature affects the conductivity of the samples.
FIGURE 3 Ambient temperature ionic conductivity of CMC/CS-NH4NO3.
FIGURE 4Temperature dependence of ionic conductivity.
The activation energy, Ea, was also calculated from the slope of the plots in Figure5 by using the equation:
r¼ ro expEa
kT ð2Þ
whererois the pre-exponential factor,kis the Boltzmann constant, andTis the temperature. It can be observed that the trend ofEaillustrated in Figure 6 is the inverse of the conductivity trend presented in Figure3. According to Selvasekarapandian et al.,[25]when NH4NO3concen- tration increases, the barrier known as the band gap for proton transport to pass through is reduced, thusEadecreases. Samples with lowerEaprovide a smaller band gap, which allows the conducting ions to move more easily to a free ion-like state.
Dielectric Study
In this work, a dielectric study was done to understand and explain the conductivity behavior of the CMC/CS NBBEs, which brings important insight into the ionic transport phenomenon.[26,27]
FIGURE 5Activation energy vs. NH4NO3concentration.
FIGURE 6 (a) Dielectric constant and (b) dielectric loss for the sample with 40 wt.%of NH4NO3.
According to Hema et al.,[27] important insights into the ionic transport phenomenon can be gleaned from the dielectric behavior of the system. The impedance data measured were used to calculate the real and imaginary part of the complex permittivity. Liedermann and Lapčik[28]
stated that information about the motion of entities having an electric charge or an electric dipole moment can be obtained from dielectric analysis. For dielectric studies, two important parameters have been studied, the dielectric constant,er, known as the storage component, and dielectric loss, ei; a component used to measure the energy loss for each cycle of the applied electric field.[27]
Both components can be calculated using the equations below:
erð Þ ¼x Zi
xC0 Zr2þZi2 ð3Þ eið Þ ¼x Zr
xC0 Zr2þZi2 ð4Þ where Co¼eοA/t, x¼2πf, Zr is the real part of impedance, and Zi is the imaginary part of impedance.
Dielectric constant and dielectric loss for the sample with 40 wt.%of NH4NO3at various temperatures are shown in Figures6(a) and 6(b), respectively. In the frequency range between 1 and 6 Hz, there are no definitive relaxation peaks observed. This shows that the increment of conductivity is affected by the increasing concentration of mobile ions in the system.[29]Besides that,erandeirise sharply towards low frequencies, which indicates that electrode polarization effects have occurred, which confirms the non-Debye dependence.[30,31] As the frequency increases, the rate of reversal of the electric field also increases, indicating there is no charge buildup at the interface that brings about a decrease in the values oferandeidue to the decrease of the polarization effect by the charge accumulated.[32]In addition, all samples demonstrate an increased trend oferas well aseiwith the increase in temperature. This is because as the tem- perature increases, the number of free ions increases resulting from the increase in degree of salt dissociation and re-dissociation of ion aggregates.[26]
Modulus Study
Further analysis of the electrical behavior was done by studying the electrical modulus, which suppresses the effect of the electrode polarization. According to Majid and Arof,[33]an indicator that the polymer electrolyte films are ionic conductors is the presence of peaks in the modulus formalism at higher frequencies. The real part of modulus,Mr, and the imaginary part of modu- lus,Mi, can be calculated using the equations below:
Mrð Þ ¼x er
er2þei2
ð Þ ð5Þ
Mið Þ ¼x ei
er2þei2
ð Þ ð6Þ
whereeris the dielectric constant andeiis dielectric loss.
Figures7(a) and 7(b) depict the frequency dependence of the real,Mr, and imaginary,Mi, parts of the modulus formalism, respectively. BothMrandMiapproach zero at low frequencies, indi- cating that the electrode polarization is negligible. The appearance of a long tail at low frequencies shows that there might be a large capacitance associated with the electrodes used in EIS measure- ment, which further confirms non-Debye behavior in the samples.[34] However, no definitive peaks can be observed for theMiplot, therefore the loss tangent, tand formalism was adopted.
Tangent Loss
The loss tangent was calculated using the equation:
tand¼er
ei
¼Mr
Mi ð7Þ
For the sake of clarity in presentation, the plot of tandversus logfat selected temperatures is shown in Figure8. From the figure, it is noticed that tandcurves increase with frequency and pass through a loss peak and then decrease, supporting previous results that conductivity increases with temperature. Besides that, the maximum peak of tan d shifted to higher fre- quency, indicating that the increase of charge carriers resulted in decreasing resistivity of the dual-blend biopolymer electrolytes.[35]
Conduction Mechanism
The ac conductivityracis described as:
rac¼ eoerxtand ð8Þ
By substituting er tand¼ei
rac¼ eoeix ð9Þ
FIGURE 7 (a) Real part and (b) imaginary part of modulus for 40 wt.%of NH4NO3.
The total conductivity,r(x), is the sum ofracand dc conductivity,rdc:
r xð Þ ¼ racþrdc ð10Þ Generally,rac can be analyzed at a high-frequency region in order to determine the ionic conduction mechanism using Jonscher’s universal power law:[36–38]
r xð Þ ¼ Axsþrdc ð11Þ whereA is the temperature-dependent parameter,sis the power law exponent, and rdcis the frequency-independent dc conductivity:
rdc¼ Axs ð12Þ
By substituting Equation (12) into Equation (9), the value ofs can be evaluated from the following relation:
lnei¼lnA
eoþðs1Þlnx ð13Þ Figure9illustrates the frequency dependence ofeiat selected temperatures for CMC/CS- 40 wt.% NH4NO3 while Figure 10 shows the plot of ln ei at high-frequency range. The values of exponent s for the optimized conducting NBBE sample were determined from the slope of the plot of lnei versus lnxin the frequency range where there is minimal or no electrode polarization: 12.1<ln x<15.6.[39] The variation of s with temperature is shown in Figure11.
Four theoretical models of conduction mechanism have been suggested based on the frequency exponentsbehavior. The models are as follows:
1. Quantum mechanical tunneling (QMT) model: the exponent s is temperature independent; the exponent s is almost equal to 0.8 and increases slightly with temperature.[40,41]
FIGURE 8 Frequency dependence for CMC/CS-40 wt.%NH4NO3of tangent loss, tandat selected temperatures.
2. Overlapping large-polaron tunneling (OLPT) model: the exponentsdepends on both temperature and frequency; s decreases with increasing temperature and exhibits a minimum value and then increases with a further increment of temperature.[38,42]
3. Small-polaron hopping (SPH) model: the exponent sincreases with increasing tem- perature.[39]
4. Correlated barrier hopping (CBH) model: the value ofsincreases towards unity asT→ 0 K; the frequency exponentsranges from 0.7 to 1 at room temperature and decreases with increasing temperature.[38,43]
The exponents, which is observed in Figure11, tends to decrease with increasing tempera- ture and can be best represented by the equations¼ 0.0016Tþ0.7105. This equation suggests that s→1 when T→0 K. This means that the conduction mechanism in CMC/CS-40 wt.%
FIGURE 9 lneivs. lnxat selected temperatures for CMC/CS-40 wt.%NH4NO3.
FIGURE 10lneivs. lnxat different temperatures for NBBE doped with 40 wt.%NH4NO3at high-frequency region.
NH4NO3NBBE can be best explained using the CBH model. According to Shukur et al.,[44]the conduction mechanism for corn starch-LiI occurs by way of a CBH model in which the plot of exponents against T can be fitted to the equation of s¼ 0.0023Tþ0.9297. Their result is similar with that of the present study.
CONCLUSIONS
CMC/CS natural biopolymer blend electrolytes incorporated with NH4NO3were successfully prepared using the solution casting technique. Ionic conductivity of the prepared sample increased with addition of NH4NO3concentration as well as temperature. The maximum con- ductivity value of 5.77105 S cm1 was obtained for the system containing 40 wt.% of NH4NO3 at ambient temperature. Variation in conductivity with temperature showed that all electrolytes followed the Arrhenius rule. Dielectric analysis reveals that the er andei values decreased with increase in frequency, indicating that all samples in this work possessed non- Debye behavior. The variation of ionic conductivity in this system was due to the variation of concentration and mobility of ions. The conduction mechanism for the system with 40 wt.%NH4NO3was best represented by the CBH model.
FUNDING
The authors would like to extend their gratitude towards the Ministry of Education (MOHE), Malaysia for the Explanatory Research Grant Scheme ERGS Vot.55101 and the scholarship under MyBrain15 program awarded to M. S. A. Rani.
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