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Ionic Conduction Mechanism of Solid Biodegradable Polymer Electrolytes Based Carboxymethyl Cellulose Doped Ammonium Thiocyanate

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Mohd Ikmar Nizam Mohamad Isa at Universiti Sains Islam Malaysia
Mohd Ikmar Nizam Mohamad Isa
Noor Azniza Mohd Noor at University of Malaysia, Terengganu
Noor Azniza Mohd Noor
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Abstract

A conducting solid biodegradable polymer electrolytes based carboxymethyl cellulose (CMC) doped ammonium thiocyanate (NH4SCN) system with concentration in the range 0 – 25 wt.% of NH4SCN have been prepared via solution casting method. The impedance study of CMC-NH4SCN system was measured via Electrical Impedance Spectroscopy (EIS) in the temperature range 303 K – 353 K. The highest ionic conductivity at room temperature (303 K) is 6.48 x 10⁻⁵ Scm⁻¹ for sample containing 25 wt.% NH4SCN. The temperature dependence of CMC-NH4SCN system was found to obey the Arrhenius behaviour where the ionic conductivity increases with increase of temperature. Dielectric data were analyzed using complex permittivity, Ɛi for sample with the highest ionic conductivity at various temperatures and found was non Debye behavior. The conduction mechanism of the charge carrier of CMC-NH4SCN system can be presented by quantum mechanical tunneling (QMT) model.
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Ionic Conduction Mechanism of Solid Biodegradable Polymer
Electrolytes Based Carboxymethyl Cellulose
Doped Ammonium Thiocyanate
M.I.N. Isa1,2,a and N.A.M. Noor 1,b
1Advanced Materials Research Group, School of Fundamental Science, Universiti Malaysia
Terengganu, 21030 Kuala Terengganu, Terengganu, Malaysia
2Corporate Communication and Image Development Center, Universiti Malaysia Terengganu,
21030 Kuala Terengganu, Terengganu, Malaysia
aikmar_isa@umt.edu.my, bahniza88@yahoo.com
Keywords: solid biodegradable polymer electrolytes; CMC-NH4SCN system; ionic conductivity
Abstract. A conducting solid biodegradable polymer electrolytes based carboxymethyl cellulose
(CMC) doped ammonium thiocyanate (NH4SCN) system with concentration in the range
0 – 25 wt.% of NH4SCN have been prepared via solution casting method. The impedance study of
CMC-NH4SCN system was measured via Electrical Impedance Spectroscopy (EIS) in the
temperature range 303 K – 353 K. The highest ionic conductivity at room temperature (303 K) is
6.48 x 10-5 Scm-1 for sample containing 25 wt.% NH4SCN. The temperature dependence of
CMC-NH4SCN system was found to obey the Arrhenius behaviour where the ionic conductivity
increases with increase of temperature. Dielectric data were analyzed using complex permittivity, Ɛi
for sample with the highest ionic conductivity at various temperatures and found was non Debye
behavior. The conduction mechanism of the charge carrier of CMC-NH4SCN system can be
presented by quantum mechanical tunneling (QMT) model.
Introduction
The pioneering work in the field of solid polymer electrolytes (SPE) was carried out by Wright
and co-workers in 1975, who reported ionic conductivity of the order of 10-5 Scm-1 at 330 K in
highly crystalline PEO-NaSCN complexes [1]. Since then, studies on SPE have been progressing
actively due to their possible application as solid electrolytes in a variety of electrochemical devices
such as energy conversion units (batteries/fuel cells), electrochromic display devices,
photochemical solar cells, supercapacitors and sensors [2]. Among the advantages of SPE as
compared to conventional liquid electrolytes include prevention of internal short-circuiting due to
formation of dendrites, leakage of electrolyte and volume expansion due to electrochemical
reactions on the electrolyte/electrode interface and reactive explosion. The development of SPE has
led to improved overall safety, portability and durability of various electrochemical devices [3].
Several polymers have been studied as polymer hosts in SPE, such as poly(ethylene oxide)
(PEO), poly(vinyl alcohol) (PVA), poly(vinyl chloride) (PVC) and poly(acrylo nitrile) (PAN). Over
the past few years, researchers have been working in the development of natural polymers due to
their renewable, sustainable and biodegradable properties. Many natural polymers have been
extensively studied as polymer hosts in electrolytes, such as chitosan, starch and cellulose with its
derivative [4]. This present study focused on developing CMC as polymer host doped with
NH4SCN as ionic dopant of solid biodegradable polymer electrolytes (SBPE). Polymer host from
cellulose or cellulose derivative which is carboxyl methylcellulose ( CMC) was choosen because of
its superior properties such as provide a good electrode electrolyte contact, a water soluble
materials, abundant in nature, low cost material and biodegradable [5]. The effect of temperature on
conductivity, dielectric and AC conductivity were investigated to study the conduction mechanism
of the CMC-NH4SCN system.
Applied Mechanics and Materials Vols. 719-720 (2015) pp 114-118 Submitted: 28.08.2014
© (2015) Trans Tech Publications, Switzerland Accepted: 03.11.2014
doi:10.4028/www.scientific.net/AMM.719-720.114
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,
www.ttp.net. (ID: 103.255.170.4-08/01/15,05:24:28)
Methodology
Sample preparation. The CMC-NH4SCN films were prepared via solution casting method. 2 g of
CMC (Acros Organic Co.) was dissolved in 100 ml distilled water. The CMC solution was stirrer
continuosly with magnetic stirrer for several hours at room temperature until the CMC have
completely dissolved. Then, the various concentration of NH4SCN (0 - 25 wt.%) was added to
CMC solution and stirred until homogenous solution were obtained. The mixture were poured into
petri dish and left to dry at room temperature for films to form. The films were kept in dessicators
for further drying to ensure there is no water content in films.
Characterization. Impedance analysis of the CMC-NH4SCN films was characterized via HIOKI-
LCR Hi-tester 3525-50 interfaced to a computer in frequency of 50 Hz - 1 MHz. The measurement
were carried out at the temperature range of 303 K to 353 K. The CMC-NH4SCN films was
sandwiched between two stainless steel electrodes with diameter 2 cm under spring pressure. The
ionic conductivity of CMC-NH4SCN films can be calculated using equation.
σ = t / Rb A (1)
Here A (cm2) is the electrode-electrolyte contact area of the film, t (cm) is thickness of film and Rb
is bulk resistance. Rb was obtained from the complex impedance plot (Cole-Cole plot) at the
intersection of the imaginary and the real impedance axis.
The dielectric loss (the imaginary part of complex permittivity), Ɛi is defined as,
Ɛi (ω) = Zr /ωCo (Zi2 + Zr2) (2)
where Co= Ɛo A/t, Ɛo is permittivity of free space, ω = 2πf (f is frequency), Zr is the real part of the
complex permittivity and Zi is the imaginary part of the complex permittivity.
Results and Discussion
Ionic conductivity analysis. Conductivity is related to the number of charge carriers (ƞ) and their
mobility (µ) according to the following equation:
σ = Ʃ ƞ . q . µ (3)
where q is the charge on each charge carrier. The ionic conductivity of CMC-NH4SCN system at
room temperature is depicted in Fig. 1. It can be observed from Fig. 1, by increasing the ionic
dopant contentration, the conductivity was found to increase. This is may be due to the increase in
the number of mobile charge carriers [6]. The highest ionic conductivity obtained is
6.48 x 10-5 Scm-1 for sample containing 25 wt% NH4SCN.
Fig. 1. Ionic conductivity of CMC-NH4SCN system at room temperature.
Applied Mechanics and Materials Vols. 719-720 115
The temperature dependence of ionic conductivity measurements have also been carried out to
examine the conductivity mechanism of the CMC-NH4SCN system. The plot of log conductivity vs
1000/T was constructed for various concentration of CMC-NH4SCN system in temperature ranges
from 303 - 353K depicted in Fig. 2. It can be observed in Fig. 2, conductivity increases when
temperature was increase. The increase in conductivity with temperature can be attributed to the
increase in number density of ions and or increase in mobility of ions [7]. The conductivity-
temperature relationship for CMC-NH4SCN system obeys Arrhenius behaviour where regression
value is almost unity (R2 ~ 1) suggesting that all points lie on a straight line indicating that the
conductivity mechanism is thermally assisted [8].
Fig. 2. Temperature dependence for ionic conductivity of CMC-NH4SCN system.
Conduction mechanism analysis. The study of dielectric in polymer electrolytes is a powerful
approach for obtaining information about the characteristics of ionic and molecular interactions.
From dielectric study, some of the physical and chemical properties of the polymer can evaluate and
help in understanding the conductive behavior of polymer electrolytes [9]. Fig. 3 presents frequency
dependence of dielectric loss for highest ionic conductivity of CMC-NH4SCN system at various
temperature. From Fig. 3, it can be observed that the dielectric loss rise sharply at low frequencies
indicating that electrode polarization and space charge effects have occurred confirming non-Debye
dependence. On the other hand, at high frequencies, periodic reversal of the electric field occurs so
fast that there is no excess ion diffusion in the direction of the field. Polarization due to charge
accumulation decreases, leading to the observed decrease in dielectric loss [10].
Fig. 3. Dielectric loss versus log frequency for the highest ionic conductivity of
CMC-NH4SCN system at various temperature.
116 Materials and Engineering Technology
The phenomenon of ac conductivity can be analyzed using Jonscher’s universal power law (UPL)
[4].
σ(ω) = Aωs + σdc (4)
Here, tan δ is the loss tangent. Substituting Ɛr tan δ = Ɛi,
σac = Ɛo Ɛi ω (5)
where A is a parameter dependent on temperature, s is the power law exponent with value in the
range between 0 and 1. The value of s can be evaluated from the following relation:
ln Ɛi = ln A/Ɛo + (s-1) ln ω (6)
From the Fig. 4, exponent s can be calculated from the slope at high frequency region. The
acceptable range was at high frequency where there is no minimal space charge polarization. In this
frequency, the electronic hops between pairs of sites can be explained when the relaxation process
occur with local character. It was contributed by the superposition of the potential which yield a
single ion potential that is actually felt by ion [11]. In this work, the acceptable frequency range is
from 11< ln ω < 16.
Fig. 4. Ln Ɛi versus ln ω for the highest ionic conductivity of CMC-NH4SCN system at various
temperature.
Up to now, a few of theoretical models have been proposed based on the analysis of ac conductivity
such as quantum mechanical tunneling (QMT), overlapping large polaron tunneling (OLPT), small
polaron hopping (SPH) and correlated barrier hopping (CBH) [12]. Fig. 5 illustrates the variation of
frequency exponent s versus temperature. Thus, it can be inferred that QMT model is most suitable
in explaining the conduction mechanism of CMC-NH4SCN system due to the variation of the index
s with temperature. In this QMT model, it can be assumed that the polarons (in this case is made up
of the protons and their stress fields) are able to tunnel through the potential barrier that exists
between two possible complexation sites [13].
Applied Mechanics and Materials Vols. 719-720 117
Fig. 5. Variation of exponent s versus temperature for the highest conductivity of CMC-NH4SCN
system.
Conclusion
Solid biodegradable polymer electrolytes based on CMC and NH4SCN were successfully
prepared via solution casting method and found to be transparent film. The CMC-NH4SCN system
obtained the highest conductivity of 6.48 x 10-5 Scm-1 at room temperature for sample with
NH4SCN concentration of 25 wt.%. The temperature dependence of ionic conductivity of the CMC-
NH4SCN system exhibits Arrhenius behavior. The conduction mechanism of the CMC-NH4SCN
system can be best presented by QMT model.
Acknowledgement
The author would like to thank Advanced Materials Team members, Ministry of Education
Malaysia (MOE) for the FRGS Grant (Vot. 59271) and MyPhD Scholarship and School of
Fundamental Science, Universiti Malaysia Terengganu for technical and financial supports given
for this work to be successfully completed.
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118 Materials and Engineering Technology
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  • Mar 2007
  • PHYSICA B
Dielectric and AC conductivity studies have been carried out for chitosan-based proton-conducting electrolytes. Chitosan-phosphoric acid (CP) and chitosan composite (CPS) electrolytes were prepared using the solution cast method. In both systems, the frequency dependence of epsilon(r), M-i and tan delta is non-Debye type. The AC conductivity in both electrolytes follows the Jonscher power law. Conduction mechanism studies show that the CP sample follows the quantum mechanical tunneling (QMT) model and the CPS electrolyte follows the overlapping large polaron-tunneling (OLPT) model. (c) 2006 Elsevier BN. All rights reserved.
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Ionic conductivity by correlated barrier hopping in NH4I doped chitosan solid electrolyte
Article
  • Apr 2009
  • PHYSICA B
Chitosan–NH4I and chitosan–NH4I–EC films have been prepared by solution cast technique. The sample containing 45 wt% ammonium iodide (NH4I) exhibited the highest room temperature conductivity of 3.7×10−7 S cm−1. The conductivity of the sample increased to 7.6×10−6 S cm−1 when 40 wt% ethylene carbonate (EC) was added to the 55 wt% chitosan-45 wt% NH4I sample. The conductivity–temperature relationship is Arrhenian. From dielectric loss variation with frequency, the power law exponent was obtained. The temperature dependence of the power law exponent for chitosan–NH4I system follows the correlated barrier hopping (CBH) model while conduction mechanism of the plasticized system can be represented by the small polaron hopping (SPH) model.
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Effect of oleic acid plasticizer on chitosan–lithium acetate solid polymer electrolytes
Article
  • May 2003
  • EUR POLYM J
Plasticized polymer electrolytes composed of chitosan as the host polymer, oleic acid (OA) as the plasticizer and lithium acetate (LiOAc) as the doping salt were prepared by the solution cast technique. These complexes with different amounts of salts and plasticizers were investigated as possible ionic conducting polymers. The highest ionic conductivity of the plasticized chitosan–LiOAc was ∼10−5 S cm−1 for the film containing 40.0 wt.% LiOAc and 10.0 wt.% of OA. Conductivity for the plasticized LiOAc-doped chitosan polymer was also studied as a function of temperature between 300 and 363 K. The plot of ln(σT) versus 103/T for each sample obeys Arrhenius rule indicating the conductivity to be thermally assisted. XRD and FTIR spectroscopy techniques have been used for the structural studies.
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