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Conduction mechanism of solid biopolymer electrolytes system based on carboxymethyl cellulose--ammonium chloride.

INTRODUCTION

Recently, the problem of solid waste has become more than an issue, especially electronic waste such as batteries because the chemicals used in commercial batteries are hazardous and nonbiodegradable. In addition, the chemicals used in commercial products are toxic and expensive, such as titanium and cadmium sulphate. Thus, it would put the environment and also humans nation in jeopardize [1, 2]. To overcome this problem, many researchers are looking for a better material for solid polymer electrolytes refer to their desirable characteristics that show good compatibility with electrodes, no leakage, low self-discharge, elasticity, and easy processability [3, 4].

Natural polymers are particularly interesting due to their abundance in nature, very low cost and principally, biodegradation properties [1]. Therefore, among the polymers which become the focus of the present study were polyethylene oxide (PEO) [5, 6], polyvinyl alcohol (PVA) [7], methyl cellulose [3], polyvinyl chloride (PVC) [8-10] and chitosan [11-13].

Since Wright discovered ionic conductivity in a PEO/Na+ complex in 1975, researchers started to be more concerned with solid polymer electrolytes (SPEs), in particular for improvement of the ionic conductivity [14]. SPEs are suitable for high-temperature operation and possess good thermal stability, electrolyte proof, less flammability and low toxicity [12, 15].

In this research, carboxyl methylcellulose (CMC) was chosen due to its superior properties as a polymer host and doped with ammonium chloride (AC). Ammonium chloride (AC) was chosen since ammonium salts are claimed to be good proton donors to the polymer matrix ion [16]. Besides that, N[H.sub.4.sup.+] is believed to be responsible for the ionic conduction [3].

Experimental method: Sample preparation

Carboxymethyl cellulose (CMC) was obtained from Acros Organic Co. without further petrification. In the preparation, 2.0g CMC was then diluted into distilled water. The solution was stirred for a few hours at room temperature. Different weight percentages of AC (R & M Marketing) were added and further stirred until it become homogenous. To prepare the CMC-AC solid bio-polymer electrolyte, the amount of CMC was fixed while the amount of AC was varied from 2-20 wt. %. All the mixtures were then cast into petri dishes and allowed to evaporate to form films. The films were kept in the desiccator (with silica gel desiccant) to ensure that the films are dry.

Impedance spectroscopy:

The electrical conductivity of the electrolytes was determined by using EIS via HIOKI 3532-02 LCR Hi-Tester that was interfaced to a computer with frequency in the range of 50Hz to 1MHz. The electrolyte samples were sandwiched between two stainless steel blocking electrodes of a conductivity cell which are connected to a computer. The value of the bulk resistance ([R.sub.b]) was determined from the Cole-Cole plots. Conductivity ([sigma]) was calculated using equation:

[sigma] = t/[R.sub.b]A

Here t is the thickness of the electrolyte samples and A is the electrode-electrolyte contact area. The impedance measurement on each electrolyte sample at room temperature was done for six times and the average value was considered for the calculation of the conductivity value. At various temperatures for temperature dependence analysis, the impedance measurement was carried out for one time. A digital thickness gauge (Mapra Technik Co.) was employed to measure the thickness of the electrolytes. The thickness of the electrolytes was found to be 0.008-0.080 mm.

Result and discussion

Impedance analysis:

Figure 1a, b, c, d, e, f shows the Cole-Cole plots of sample CMC-AC system tested at room temperature doped with different concentration of AC at ambient temperature. The Cole-Cole plot of the electrolyte containing 0, 2, 8, 12, 16, 20 wt. % AC shows incomplete semicircle curve. These figures show two well defined regions; a high frequency incomplete semicircle related to the parallel combination of a resistor and a capacitor and a low frequency spike representing formation of double layer capacitance at the blocking electrode-electrolyte interface which causes the inclination and is better known as the polarization effect due to migration of ions [17-19].

The semicircle part of the impedance spectrum in the high frequency region is related to ionic conduction in the bulk of the electrolyte and can be modelled as a parallel resistor due to the mobile ions inside the polymer matrix and capacitor due to the immobile polymer chain circuit network where the capacitance is usually of the order of nano-Farads [20, 21]. With increasing salt composition, a local effective pathway is constructed in the liquid phase for ionic conduction, as a result, ions can transport quickly in the liquid phase as the electric potential alternates between the positive and negative electrodes in an alternating field.

Ionic conductivity:

The conductivity, [sigma], plot was show in figure 2. According to Figure 2, the addition of AC beyond 16wt% lessens the conductivity. The highest conductivity obtained for AC-16wt.% is 1.43 x [10.sup.-3] S/cm. The effect of increases of conductivity CMC-AC could be due to the enhancement of ionic mobility and the number of charge carrier or free ions [18, 22, 23]. Meanwhile, the decreases in conductivity could be due to the formation of ion pairs and aggregates. The formation of ion pairs produces neutral species and thus reduces the number of free ions [22, 23].

Temperature-dependent:

In order to analyze the mechanism of ionic conduction in the SBEs system, the temperature-dependent study was carried out. Log [sigma] against 1000/T plot for various compositions of SBEs system based on CMC-AC in the temperature range of 303-353 K is displayed in Figure 3. The plot shows that the ionic conductivity of the SBEs system increases with increasing temperature for all salt composition which obeys the Arrhenius behaviour which R2 ~ 1. This is because the ionic mobility and the degree of salt dissociation are also temperature dependent [12]. The plot can be considered as Arrhenius behaviour by the relation,

[sigma] = [[sigma].sub.o] exp ([E.sub.a]/kT) (2)

Where [[sigma].sub.o] is the pre-exponential factor, [E.sub.a] is activation energy, k is the Boltzmann constant, and T is absolute temperature. The calculated [E.sub.a] value for each AC electrolyte is tabulated in Table 1. The value of [E.sub.a] found to ranging from 0.14-0.37 eV for AC electrolytes. The higher conducting electrolyte has the lower [E.sub.a] value. This is in agreement with the fact that the density of ions in the electrolyte increases with the increase in AC content causes the activation energy drop and vice versa.

Conduction Mechanism:

Ionic conduction mechanism of an electrolyte can be determined by equation 3 and 4 by employing Jonscher's universal power law [11, 12, 24, 25]:

[sigma]([omega]) = [[sigma].sub.ac] + [[sigma].sub.dc] (3)

[sigma]([omega]) = A[[omega].sup.s] + [[sigma].sub.dc] (4)

Where [sigma] ([omega]) is the total conductivity, [[sigma].sub.ac] is the ac conductivity, and [[sigma].sub.dc] is the dc conductivity. The ac conductivity is represented by A[[omega].sup.s], where A is constant at a particular temperature, [omega] is the angular frequency ([omega] = 2[pi]f) and s is the power law exponent. The ac conductivity can also be obtained using equation [12]:

[[sigma].sub.ac] = [[epsilon].sub.o][[epsilon].sub.i][omega] (5)

By substituting [[sigma].sub.ac] = A[[omega].sup.s] into equation (3), the value of exponent s was determined by plotting the following relation:

ln [[epsilon].sub.i] = ln A/[[epsilon].sub.o] + (s - 1)ln [omega] (6)

and;

[[epsilon].sub.i] = [Z.sub.r]/[omega][C.sub.o]([Z.sup.2.sub.r] + [Z.sup.2.sub.i])

Figure 4 depicts the frequency, ln[omega] dependence of dielectric loss, [[epsilon].sub.i] at selected temperatures. The dielectric loss increases with increasing temperature for a particular frequency but it decreases with the increase in frequency at a specified temperature. From figure, it can be seen clearly that there are two different slopes. In the low frequency region, stronger frequency dependence of dielectric loss is observed. This dielectric loss-frequency behaviour in the low frequency region is contributed to space charge polarization, which occurs when there is more than one phase having different conductivities existing within the same material and electrode polarization [26]. Electrode polarization arises due to the accumulation of ions at the electrode-electrolyte interface. In the high frequency region, the direction of the electric field changes at a faster rate. Hence, most of the ions will be in the bulk of the sample and slighter ions can pile up at the interface [27].

Therefore, the values of [[epsilon].sub.i] decrease with the increase in frequency. The exponent s was determined from the slope of plots in Figure 5. In this work, the slope is only taken in the frequency region of 12 < In[omega] < 16 since at high frequency region, no or minimal impact of electrode polarization occurs [12, 28].

CMC-AC SBEs can be fitted to the equation s = 0. 0023T+1.0281. From this fitting equation, s[right arrow]1 when T [right arrow] 0. To determine the electrical conduction mechanisms of 4 types theoretical models have been proposed based on the analysis of ac conductivity [11, 29] such as:

i. Overlapping large polaron tunnelling (OLPT) model: In the OLPT model, the exponent s is temperature dependent. The exponent s is predicted to decrease with increasing temperature, reaching a minimum value at a given temperature before continuing to increase with increasing [11, 12, 28, 30].

ii. Correlated barrier hopping (CBH) model: In the CBH model, exponent s decreases with increasing working temperatures, but no minimum in exponent s is observed [11, 18, 31]. The exponent s is predicted to be temperature dependent with exponent s [right arrow] 1 as T [right arrow] 0K [32].

iii. Small polaron hopping (SPH) model: In the SPH model, the exponent s increases with increasing temperature [24, 33]. In this model, small polaron is formed by the addition of the charge carrier to a site, which results in a large degree of local lattice distortion. These small polarons are assumed to be localized so that their distortion clouds do not overlap, so the hopping energy is not dependent on intersite separation [16].

iv. Quantum mechanical tunnelling (QMT) model: In the QMT model, the exponent s is almost equal to 0.8 and increases slightly with temperature or independent of temperature [24, 30]. According to Psarras [34], ionic hopping from one site to neighboring site occurs not just by jumping over a potential barrier but can also be accompanied by quantum mechanical tunneling. Hence, from the observations on the behavior of the exponent s with the temperature in the CMC-AC system (Figure 5), it can be fitted to the equation s= 0.0005x + 0.2767. Thus, it can be implied that QMT model is more applicable in explaining the conduction mechanism of carboxymethyl cellulose-ammonium chloride biopolymer electrolyte. Buraidah and Arof [13] reported that the conduction mechanism for chitosan-PVA-N[H.sub.4]I occurred by way of QMT model in which the plot of exponent s against T can be fitted to the equation s=0.00004T + 0.1256 [13]. Their result is comparable with the present work which the polarons is made up by the protons and their stress fields are able to tunnel through the potential barrier that exists between two possible complexation sites [11].

Conclusion:

SBEs system based on CMC-AC has been prepared via solution casting technique. In CMC-AC system, the SBE containing 16wt% AC obtains the highest conductivity value of (1.43 [+ or -] 0.02) x [10.sup.-3] S/cm at room temperature and it has [E.sub.a] value of 0.14 eV. The effect of increase with increasing temperature for the highest conducting SBE in CMC-AC system is due to the enhancement of ionic mobility and the number of charge carrier or free ions. The ionic conduction mechanism for the highest conducting in CMC-AC SBEs system is best explained by QMT model.

Acknowledgement

The authors would like to thank Ministry of Education Malaysia (MOE) for financial support through Fundamental Research Grant Scheme (FRGS) Vot. 59271 and Exploratory Research Grant Scheme (ERGS) Vot. 55101, and University Malaysia Terengganu (UMT) for technical support and facilities.

Published Online 11 February 2015.

Received: 31 December 2014;

Revised: 26 January 2015;

Accepted: 28 January 2015

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Composites: Part A, 37: 1545-1553.

(1) Nur Hidayah Ahmad, (2) Mohd. Ikmar Nizam Mohamad Isa

(1) School of Fundamental Science, Universiti Malaysia Terengganu, 21030 Terengganu, Malaysia.

Corresponding Author: Mohd Ikmar Nizam Mohamad Isa, School of Fundamental Science, Universiti Malaysia Terengganu, 21030 Terengganu, Malaysia. Tel: +609 6684930/4931, E-mail: ikmar_isa@umt.edu.my

Table 1: Ionic Conductivity and Activation Energy of SBEs Based on
CMC-AC System Measured at Room Temperature.

Sample AC,   Conductivity, [sigma] (S/cm)    Activation energy,
                                               [E.sub.a] (eV)

0                      1.11E-07                     0.37
2                      3.45E-06                     0.34
8                      1.62E-05                     0.32
12                     2.85E-04                     0.20
16                     1.43E-03                     0.14
20                     4.50E-05                     0.28

Abbreviation: wt. %. Weight percentage
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Title Annotation:Research Article
Author:Ahmad, Nur Hidayah; Isa, Mohd. Ikmar Nizam Mohamad
Publication:American-Eurasian Journal of Sustainable Agriculture
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Date:Jan 1, 2015
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