technical

Volume 2, Issue 4, July-August 2012 Available Online at www.gpublication.com/jcer ISSN No.: 0976-8324 ©Genxcellence Publication 2011, All Rights Reserved

RESEARCH PAPER

Ionic conductivity and relaxation process in CMC-G.A solid biopolymer electrolytes. Mohd Fatihah MohdOthman 1 , Ahmad Salihin Samsudin and Mohd Ikmar Nizam Mohamad Isa*2 1 Department of Physical Sciences, University Malaysia Terengganu, 21300 Kuala Terengganu, Terengganu, Malaysia [email protected] 2* Advanced Material Research Group; Renewable Energy Research Interest Group, Department of Physical Sciences, University Malaysia Terengganu, 21300 Kuala Terengganu, Terengganu, Malaysia [email protected]* Abstract The electrical conductivity and thermal conductivity of Carboxyl methylcellulose (CMC) and glycolic acid (G.A) have been measured by the electrical impedance spectroscopy method in the temperature range of 303 – 383 K. The composition of glycolic acid (G.A) was varied between 10-50wt. % and the samples were prepared via solution casting technique. The highest ionic conductivity at ambient temperature, (303K) is 1.04x10-4 Scm-1 for sample containing 40 wt.% of G.A. The system was found to obey Arrhenius rule where R2~1 and prove that in this CMC-G.A system conductivity is thermally activated.

Keywords Solid polymer electrolyte, carboxyl methylcellulose, glycolic acid, ionic conductivity, dielectric study, activation energy

INTRODUCTION Lots of study had been done for natural polymer such as chitosan, starch and cellulose to be applied as biopolymer electrolytes. The aim of all these research is to find a new type of biopolymer electrolytes that can be use as alternative material to current industrial materials. With a wide variety of biomaterial choices in market the increment in material cost can be avoided. Carboxymethyl cellulose (CMC) is one of the promising materials that had been recently studied. CMC shows potential aspirant to act as polymer host for proton conducting biopolymer electrolyte. CMC was choose because of its unique properties such as provide a good electrode electrolyte contact, a water – soluble materials [1], abundant in nature, low cost material, and most significant characteristic of CMC, it is biodegradable. CMC has been utilized in wide applications such as textile industry and food industry [2].It is well known that CMC is a poor conductors of electricity because of its unavailability of large number of free electrons to participate in the conduction process, thus in this biopolymer electrolytes system, glycolic acid (G.A) was added to increase the number of free electrons. In this paper the potential of Carboxymethyl cellulose (CMC) as biopolymer electrolytes were studied by it electrical properties.

MATERIAL AND METHODS In this study, biopolymer electrolyte films were prepared by using solution cast technique [3]. CMC from sigma Aldrich (0.5g, dry basis) was stirred in 15 ml absolute ethanol, this is to prevent the agglomeration of CMC in the solution. To this solution 25 ml of distilled water was added and stirred for 4 hour until a homogenous, transparent and clear solution was

obtained, later 1 g of glycerol was added as the plasticizer in these polymer electrolytes system [4],[7]. Finally glycolic acid (G.A) were added and varied from 10-50wt. % and continuously stirred until clear and transparent solutions were obtained. These solutions were cast into glass Petri dishes and put in the oven for about 48 hours at 45 degree Celsius (oC) in temperature. Further drying was made where samples were left in desiccators for 30 days. Conductivity measurement Biopolymer electrolyte films obtained were cut into small round pieces with 2cm in diameter and sandwiches between two blocking stainless steel electrode under spring pressure [4]. The impendence of samples was determined by using complex impendence technique. Hioki-3530 Z Hi Tester Impedance Spectrometer was used to determine the impendence with frequency ranging from 50Hz – 1MHz at temperature ranging from 303K-383K., The conductivity value calculated from the equation:

(1) Where, A is the area of the sample electrode and t is the thickness. Rb is bulk resistance of the sample obtained from the complex impendence graph. RESULTS & DISCUSSION Electrical and Conductivity study. The conductivity-G.A content (wt. %) graph at ambient temperature (303K) is shown in Figure 1. This graph 6

Please Cite this Article at: M.F.Mohd. Othman et al, Journal of Current Engineering Research, 2 (4),July-August 2012, 6-10

reveals the information on the specific interaction between CMC and G.A.

depicts the temperature dependence of these biopolymer electrolytes films.

( wt. % )

(R )

E a ( eV) (t urnc a t ed to t w o de ci ma l p lac es)

10 20 30 40 50

0.996 0.912 0.998 0.982 0.995

0.3 5 0.3 4 0.3 3 0.3 1 0.3 7

Sa mp le

R egre ssion 2

Figure 1: The conductivity-G.A variation graph at ambient temperature (303K) The reliance on conductivity toward salts concentration provides certain information on the complexation between salts and polymer matrix. The behavior of conductivity-salts variations can be explained in terms of weak electrolytes theory Mohamed, (1995). The weak electrolytes theory states that = nqµ, where n is the number of mobile charge carrier which in motion under the action of an electric field, q is the electronic charge and µ is the mobility of ionic species. From Figure 1, it is shown that the conductivity is increased gradually with the increment in salts concentration until the addition of 40wt. % G.A, the highest conductivity achieved at room temperature is 1.04x10-4 Scm-1. The increment in conductivity with G.A addition is due to the increment in number of mobile ions or charge carrier (n) [4] this explained the increasing of conductivity with the increment in G.A addition. As the concentration of salts increase, more protons (H+) ions are supplied due to the dissociation of glycolic acid. With the addition of G.A content higher than 40wt. % the conductivity is found to decrease, the decrement of conductivity is due to the formation of ions clustering or ions overcrowding due to too many ions were doped in this systems [5]. Based from Figure 1, it can be concluded that the concentration of ionic conducting species or number of ions ( ) influencing the conductivity value in this present biopolymer electrolyte study.

Temperature dependence of conductivity. In order to investigate the thermal behavior of these biopolymer electrolytes, the CMC-G.A system was introduced to higher temperature at 303-383K. Figure 2

Figure 2: log

versus 1000/T plot for every sample

A linear relationship can be observed from the graph of log versus 1000/T, which confirms that all samples are Arrhenius in behavior. From the graph, it is clearly shown that the conductivity increase with the increment on temperatures for all investigated samples. The Arrhenius plot are almost unity, (R2~1) shows that the conductivity is thermally activated [6]. Furthermore, it also indicates that there is no phase transition occurs in the polymer matrix or domains formed by the addition of salts [8]. Therefore, dynamic conformational changes in the polymer matrix and H+ ions might migrate through the conduction path formed by the lattice structure of the carboxymethyl cellulose chains. The activation energy, Ea (combination of the energy of defect formation and the energy for migration of ion) is calculated from the slope of the log conductivity, versus 1000/T graph by using equation:

(2)

Where is the pre-exponential factor, Ea is the activation energy, k is the Boltzmann constant and T is the absolute temperature. Activation energy, Ea is the energy required for an ion to begin movement. In the context of polymer electrolytes, the ion is usually ‘‘loosely bound’’ to a site with donor electrons. When the ion has acquired sufficient energy, it is able to break away from the donor site and move to another donor site [5]. The movement from one site to another result 7


in the conduction of charge and the energy for this conduction is the activation energy. The value of the regression value, R2, and activation energy, Ea for the samples is shown in Figure 2.

Ionic transport properties. By obtaining the activation energy, Ea, the number of ions ( ) can be determined for every sample by using Rice and Roth model. Rice and Roth model hypothesized that for an ionic carrier of mass, m after receiving energy, E, gets excited from a localized ionic state to another state, in which the ion propagates throughout the solid with velocity, v [9],[10],[11]. The velocity of charge carrying species is given by:

(3)

Based from the transport parameter in Table 1 it is clearly shows the value of mobility and diffusion coefficients are related to the salt concentration of the samples. The value of ionic mobility calculated from the Rice and Roth model lies between 1.55 x10-18 to 13.0 x10-18 cm2V-1s-1 and diffusion coefficient was 2.53 x10-1 to 21.3 x10-1 cm2s-1. The conductivity of the biopolymer electrolyte is found to be controlled by the mobility and diffusion coefficient.

Dielectric study. Based on plots in Figure 3(a) it is clearly show that dielectric constant is increase at lower frequency and reaches constant value at high frequencies. The increase is more prominent towards low frequencies and the plots exhibit, a frequency dispersion at all temperatures investigated. This indicates that electrode polarization and space charge effects have occurred confirming the non-Debye dependence [13].

The Rice and Roth model express the conductivity as: 3(a) (4) here e is the charge of conducting species and k is the Boltzman constant. is the time to travel between sites, and can be determined from = . Where the monomer length, for CMC is 5.15Å [12]. The ionic mobility (µ) and diffusion coefficient (D) could be identified by using equations (5) and (6); (5) and;

3(b) (6)

The conductivity ( ), number of ions ( ), ionic mobility (µ) and diffusion coefficient (D) for CMC-G.A biopolymer electrolytes system at 303K is tabulated in Table 1 below.

Table 1: Transport parameter for every investigated sample at 303K Sampl e (wt. %) 10 20 30 40 50

ó (Scm1 )x10-5 1.08 1.44 4.02 10.4 4.53

ç (cm3)x1031 1.99 1.64 3.58 4.98 18.32

µ (cm2V-1s1 )x10-18 3.39 5.48 7.01 13 1.55

D (cm2s1 )x10-1 5.53 8.96 11.5 21.3 2.53

Figure 3: (a) Shows dielectric constant, r and (b) dielectric loss, i for sample containing 40wt.% G.A at every investigated temperature.

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The dielectric loss, i versus is shown in Figure 3(b) shows that the dielectric loss, i, has the same trends as the dielectric constant, r, where the dielectric loss value also decreasing with the increasing of frequency value. A clear loss peak is also observed (10000 Hz), which shifts to the higher frequency with increasing in temperature. These peaks is attributed to the energy absorbs by permanent dipoles in the materials. The peak occurrence is observed for every sample shows that there is energy absorbs by permanent dipole in this CMC-G.A system with increasing temperatures [15]. These peaks also indicate the existence of relaxation time in CMC system where it is the characteristic which reflect the degree of interaction between polymer and its doping salt. This possibly because of at high frequency relaxation process, presumably involves a dipolar activity and in this system it could be associated with the co-operative motion between the side group of CMC (-OCH2COONa) and G.A (-COOH) [14]. Dielectric loss, i, also becomes very large at low frequencies, due to the free charge motion within the materials. The values correspond due to free charge build up at the interface between the materials and the electrodes [15]. Modulus study

Figure 4: (a) Shows imaginary modulus, Mi and (b)real modulus, Mr for sample containing 40wt.% G.A at every investigated temperature. Figure 4(a) shows plots of imaginary part of the electrical modulus, Mi. It can be observed from the figure, the Mi values are increasing with the increase in frequency values. The lower value of Mi at lower frequency can be attributing to the large value of capacitance associated with the electrode. The capacitance, C can be deriving from C= r oA/t, so with increase in r brings increase in the capacitance value. The increase in Mi at the high frequency end is attributed to the bulk effect. With the raise of temperature, the height of the peak decreases suggesting a plurality of relaxation mechanism [13] which confirms the non-Debye behavior in the samples [16].No definitive peaks can be observed for the imaginary part plot. This frequency range on the lower side of the peak frequency determines the range in which charge carriers are mobile over long distance. Figure 4(b) shows the real part of electrical modulus, Mr increased as the frequency increases. From this figure it also can be observed that the value of Mr decreased in the lower frequency regime tend to zero. The appearance of this long tail at low frequency is due to the highly capacitive material [17]. The value of Mr also decreased as the temperatures increased indicate the relaxation time at high temperature is shorter than at low temperature.

CONCLUSION

4(b)

The CMC-G.A biopolymer electrolytes system was successfully obtained. The results of impedance spectroscopy, shows that amount the amount of G.A in the system affecting the ionic conduction. From the calculation using Rice and Roth model (1972) shows that in this biopolymer electrolyte ionic mobility ( ) and diffusion coefficient (D) was dependence on the quantity of G.A. The temperature dependence on ionic conductivity of these polymer electrolytes obeys Arrhenius relationship. The electrical study shows the plurality of relaxation time, . become shorter at higher temperature and high f relaxation phenomena is attributed from dipolar activity associated with the cooperative motion between the side group of CMC and G.A. From this study it can be concluded that this CMC-G.A biopolymer electrolyte are very attractive to be used for electrochemical device applications such as electrochromic smart windows. ACKNOWLEDGEMENT The authors would like to thank the Department of Physical Sciences under the Faculty of Science and Technology, University Malaysia Terengganu, for the help and support given for this work.

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Characterization of PVA–NH4Br Polymer Electrolyte System. Journal of Physica B. 403: 2740-2047.

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