NATIONAL WORKSHOP ON FUNCTIONAL MATERIALS 2009

DC And AC Conductivity Of Chitosan-Silver Triflate PolymerElectrolyte

S.B. Aziz, M.S.Rusli, Z.H.Z. Abidin and A.K. Arof *

Center for Ionics University Malaya, Department a/Physics, Faculty a/Science,University 0/Malaya, 50603 Kuala Lumpur, Malaysia

*Corresponding Author: akaroj@um.edu.my

Abstract

T~e frequency and temperature dependence of to DC and AC conductivity of chitosan-silvertnflate electrolyte has been studied from 303 K to 423 K and over a frequency range of 50 Hzto 1000 ~Hz. The conductivity was found to increase with the increase in AgCF3S03c~nqentratlOn. The DC conductivity increases in the low temperature region but decreases athigh r temperatures. From the total conductivity versus frequency plots dispersion from DCto AC conductivity can be observed below 358 K. '

Keyw ords: chitosan, silver triflate, DC conductivity, AC conductivity

1. INTRODUCTION

Studies on solid polymer electrolytes (SPE)have been progressing actively due to theirpotential application in solid stateelectrochemical cells, high energy densitybatteries, fuel cells, sensors andelectrochromic devices [1-2]. Polymerelectrolytes have good mechanicalproperties, can be easily fabricated as thinfilms, can have a wide range of compositionallowing control of properties and are ableto form effective electrode-electrolytecontacts. Many types of polymerelectrolytes have been studied in the pursuitto develop solid electrolyte systems [3].These include PVA [4], PVC [5-7],PPG4000 [8], PEO [9-11] and PVdF [12].

Chitosan is a natural and low costbiopolymer. It is biocompatible, non-toxicand chemically and thermally stable.Chitosan has been widely studied as apromising source of membrane materials.According to Cui et al. [13] chitosan can beused in the preparation of polyelectrolytecomplexes via electrostatic interactions withpolyanions. Previous studies have proventhat chitosan can be used as a polymermatrix for ionic conduction. Each of the

nitrogen and oxygen atoms in chitosan haslone pair electrons where complexation canoccur. Thus, chitosan satisfies one of thecriteria as pointed out by Armand and Grayfor the chitosan to act as a polymer host forthe solvation of salts [14]. Usually bothcrystalline and amorphous phases arepresent in polymer electrolytes butconductivity mainly occurs In theamorphous phase [15]. Some ion-conducting polymer electrolytes based onchitosan have been reported. Examplesinclude chitosan doped with ammoniumnitrate (N~N03) [16], ammonium triflate(NH4CF3S03) [17] and silver nitrate(AgN03) [18] and ammonium iodide (NH4I)[19].To the best of our knowledge, there are noreports in the literature which discuss theeffect of silver triflate on the chitosanpolymer. Silver polymer electrolytescomprising silver salts dissolved in a polarpolymer matrix other than chitosan haveattracted much attention for their applicationin solid state membranes. These silver solidpolymer electrolytes (SPE) have manyadvantages, including high separationperformance, simple operation and lowenergy consumption [20]. In view f theabove reasons, the present paper aims to (i)

31

NATIONAL WORKSHOP ON FUNCTIONAL MATERIALS 2009

study the influence of silver triflate(AgCF3S03) salt at low concentration on theDC conductivity of chitosan, and (ii) tostudy the effect of temperature andfrequency on the AC conductivity ofchitosan-silver triflate.

2. EXPERIMENTAL

2.1 Preparation OJ SPE Films

Chitosan from crab shells (2: 75 %deacetylated, Sigma) and silver triflate withrelative molecular weight 256.94 (suppliedby Fluka, 2: 98 purity) have been used as theraw materials in this study. The SPE filmswere prepared by the solution casttechnique. For this purpose 1 g of chitosanwas dissolved in 100 ml of acetic acid (1 %)solution. The mixture was stirredcontinuously with magnetic stirrer forseveral hours at room temperature (333 K)until the chitosan powder has completelydissolved in acetic acid solution. To this setof solutions, 2 wt. % to 10 wt. % silvertriflate were added separately and themixtures were stirred continuously untilhomogeneous solutions were obtained. Thesolutions were then cast into different cleanand dry plastic petri dish and left to dry atroom temperature in order to allow completeevaporation of solvent for obtaining solvent-free films of the polymer host and polymersalt complexes, respectively. The films werekept in desiccators with silica gel desiccantfor further drying. Table 1 shows theconcentration of the prepared samples.

Table 1 The composition of chitosan doped withsilver triflateDesignation Chitosan AgCFJSOJ Silver triflate

(g) (g} (wt. %)CA 1.0 0.0000 0.0

CAl 1.0 0.0204 2.0

CA2 1.0 0.0416 4.0

CA3 1.0 0.0638 6.0

CA4 1.0 0.0869 8.0

CAS 1.0 0.1111 10.0

2.2 Complex Impedance Spectroscopy

The complex impedance spectroscopy isused to characterize the electrical properties

of the materials. The SPE films were cutinto small discs of 2 em diameter andsandwiched between two stainless steelelectrodes under spring pressure. Thecomplex impedance plot that is (Z" versus Z'where Z' and Z" represent the real andimaginary parts of impedance respectively)were used for the evaluation of the electricalconductivities of various samples as afunction of temperature. The electricalconductivity was calculated using therelation,

(1)

where, t is the thickness, A is the area of thefilm and Rb is the bulk resistance of the filmderived from the intercept of the impedanceplot on the real axis.

3. RESULTS AND DISCUSSION

3.1 DC conductivity

The room temperature conductivity ofchitosan-AgCF3S03 is shown in Fig. 1.

_,-..

'su 1.E-08rJJ'-'\:).....-:E-u-6 1.E-09coU

1.E-10 +----,-------.----.......1

o 5 10 15AgCF 3S03 content (wt. %)

Fig. 1 The ionic conductivity of chitosan withvarious concentrations of AgCF3S03

Note that the ionic conductivity or chitosanelectrolyte increases with increasingAgCF3S03 concentrations. The conductivityvalue in room temperature has been found toincrease by an order of magnitude from 1.73

32

NATIONAL WORKSHOP ON FUNCTIONAL MATERIALS 2009

x 10-9 for pure chitosan polymer to 4.25 x

10-8 S cm-I at 0.1 % AgCF3S03 dopinglevel. This indicates the fact thatdissociation of silver triflate has taken placewithin the polymer electrolyte.

xe general expression for conductivity is

a=nqJ-l (2)

wi ere n is the concentration of chargecarriers, q is 1.6 x 10-19 C, and p is themobility of the ions. Thus, a steep increasein conductivity can be explained by anincrease either in mobility or in theconcentration of charge carriers [21].Relatively few studies have been reportedon polymers complexed with silver salts.The reported conductivities are all withinthe lange from 10-8 to 10-5 S em", except forPEO-AgN03 where conductivity as high as9 x 10-3 S ern" has been found for extremelyhigh concentrations of the silver salt [22].The conductivity values reported forAgN03+PEO, Agl-lOj+polytpcntamcthyle-ne sulfide) and AgI+NaI+PEO [21] are veryclose to the present paper forchitosan+AgCF3S03.

Fig. 2 shows the variation of DCconductivity for chitosan electrolyte as afunction of temperature (log (J versus 103IT)for different concentrations of silver triflate.

-4

-5,-..

E -6CJrJ).._,

-7\:)tlJ)0~ -8

-9

-102.3

X~· ••6* • "~...... x~ •••

•• x XA ••• • X;.:6 •

• o ~¢ • xx:tf6 •• -x~ 6 •o .. ~ 6

c c •• "x¢¢ •• ~~~~

o.o¢ ••<><> •o

¢¢¢¢ ¢¢

+------r--

¢CA

• CA1

6CA2xCA3;.:CA4

• CA5

2.8 3.3 3.8

Fig. 2 Temperature dependence f ionic conductivityof chito an- ilver triflate

From Fig. 2, conductivity increases withincreasing temperature up to a certaintemperature, which changes with saltconcentration, after which the conductivitybegins to decrease. In the region whereconductivity increases with temperature,there is no sudden change in the value ofconductivity with temperature. According toYahya et al. [21], the non-existence of anyabrupt jump indicates the fact that thechitosan-based electrolyte is completelyamorphous.The linear increase in conductivity of thechitosan electrolyte with increase intemperature can be described by theArrhenius equation,

Here (J is the conductivity at temperature, 1(in Kelvin), kB is the Boltzmann constant, Eois the activation energy and (Jo is the pre-exponential factor. The activation energy, Eomay be deduced easily from the slope of log(J versus 1000lT plot. The activation energyindicates the energy barrier an ion has toovercome for a successful jump betweensites [24]. The calculated activation energy,Ea decreases from 0.99 eV to 0.81 eV as theconductivity of the sample increases. Theincrease in conductivity may be attributed tothe increase in dielectric constant of thesample. Dielectric constant is known toincrease with temperature. The decrease inconductivity with increasing temperaturemay be ascribed to the scattering of carriersdue to the increased vibrations of functionalgroups at these elevated temperatures [25] .

3.2 AC Conductivity Versus Frequency

The total conductivity, (J(O)) of chitosan basedelectrolyte for the highest conductingsample has been studied as a function offrequency from 50 Hz to 1000 kHz and303 K to 423 K. The total conductivity oncan be e pr ed by th following quatiofl25]

rr - rr + to \ 0<, < I (4)V(w) - V,k

NATIONAL WORKSHOP ON FUNCTIONAL MATERIALS 2009

Here 0" de IS the conductivity that IS

independent of frequency. The frequencyvariation of total conductivity (O"(W))' atdifferent temperatures is shown in Fig. 3.

~~----------------------~,-.'7S -5.5CJ -600'-~ -6.5

eJ

~ -7.5

-7

•~+-------~-------,------~1 3 5

Logf(Hz)-5~-----------------------.

:' -5.5·sCJ -600'-

~ -6.5

eJ -7o- •-7.5 +----------,----------,----------1

1 3 5Logf(llz)

~~-----------------------.

eJ -6.5o-

~ ~.8~ -6eJ..s: -6.2

-7

-5.4~.6

-6.4

•1

•1 3 5

Logf(Hz)

4.5~-----------------------,-. 4.7

4.9'7S

CJ00'- -5.3~ -5.5eJ -5.7..s: -5.9

-5.1

•-6.1 +------- __------~------_____1

1 73 5Logf(Hz)

,-.

4~-----------------------,

4.5

-57

eJ -5.5o--6+-~.------------~-----------1

1 73 5Logf(Hz)

-5,------------------------,-5.2-'7S -5.4

CJ -5600 •

~ -5.8~~ -6eJ..s: -6.2

-6.4+-------~------~------____l1 3 5

Logf(Hz)-5.3 ~-----------------------,

- -5.5 [.378K[ ••~·S -5.7

~ -5.9 ./,§ -6.1 ./~ .~ -6.3-

7

7

1·368KI .r•

•7

•

3 5Logf(Hz)

-5 ,---------------------,--

!.338KI .//..41

./•

-6.5+-------,-------~----~1 3 5

Logf(llz)7

Fig. 3 Frequency dependence of 0"( at various1lJ)

temperatures for CAS7

0"(1lJ) increases with increasing in equency

up to 338 K. Above this temperature 0"(w)

seems to saturate at high frequencie . Thisfrequency dependence of conductivity

34

NATIONAL WORKSHOP ON FUNCTIONAL MATERIALS 2009

suggests the hopping conductivity andfollows the Jonschers power law behavior[26]. At 308 K dispersion between electrode

polarization, CYde and 0'ae is not that

obvious. At 318 K to 348 K, the dispersion.'i clearer. However at 358 K, dispersion

etween CYde and CYae is not observed due

to limitation of the frequency range.Dispersion between electrode polarization

nd CYae is however clearly manifested. This

continues at even of 378 K. Electrodepolarization indicates the accumulation ofions at the electrode-electrolyte interfaceand that the ions have a distribution ofrelaxation times.

4. CONCLUSIONS

In conclusion, in the range of saltconcentration between 0 to 10 wt. %, theconductivity for the highest conductingsample in chitosan - AgCF3S03 is 4.25 x10 S cm-I The results for DC conductivityindicate that the ions conduct by hopping.The DC activation energy was found to beabout 0.81 eV for the highest conductingsample. The conductivity demonstrated afrequency and temperature dependence andobeys the power law. Conductivity in thelow frequency region indicates that the ionshave a distribution of relaxation times.

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