Structural and Ionic Transport Study on CMC Doped NH4Br: A New Types of Biopolymer Electrolytes
The development of new solid Biopolymer Electrolyte (BEs) system based on Carboxy Methylcellulose (CMC) is creating opportunity for new types of electrochemical devices which may themselves, in turn, revolutionize many industrial areas. Biodegradable CMC doped with ammonium bromide (NH4Br) as BEs were prepared via solution-casting method. The polymer-salt complexes have been analyzed through FTIR spectroscopy, XRD measurement and impedance measurement. The sample containy 25 wt.% NH4Br exhibited the highest room temperature conductivity of 1.12x10-4 Scm-1. Small Polaron Hopping (SPH) model has been found to be most appropriate for fitting the experimental conductivity data.
Received: August 04, 2011;
Accepted: November 24, 2011;
Published: January 18, 2012
Polymer Electrolytes (PEs) have attracted much attention due to potential applications
in electrochemical devices (Mika et al., 2007;
Bozkurt and Meyer, 2001). Many aspects of PEs can be
investigated such as ionic conductivity, nature of films and vibrational properties
of functional groups. There are several advantages have been discovered on PEs.
The main advantages of the PEs are their good mechanical properties, ease of
fabrication into thin films of desirable sizes and the ability to form good
electrode/electrolyte contact (Armand, 1994; Hashmi
et al., 1990). The polymers have been complexed with various salts
which provide the ions for conduction. Ammonium salts have already been reported
as a good proton donor to the polymer matrix (Stainer et
al., 1984; Daniela et al., 1988).
One promising aspirant to act as polymer host for bio-PEs is Carboxy Methylcellulose
(CMC). Recently, due to the good biocompatibility and biodegradable, CMC attracted
more attentions as representative water-soluble polysaccharide in many research
fields (Barbucci et al., 2000; Kulkarni
and Sa, 2008; Marci et al., 2006). To the
best of our knowledge, there has been no previous study of biopolymer electrolyte
based on CMC. The present work aims on developing new type of biopolymer electrolyte
with CMC as the host polymer. Ammonium bromide (NH4Br) has been chosen
as the dopant since ammonium salts are considered as a good proton donor to
the polymer matrix (Kumar and Sekhon, 2002). The prepared
polymer electrolytes have been characterized by Fourier Transform Infrared (FTIR),
X-ray diffraction (XRD) and by impedance spectroscopic techniques.
MATERIALS AND METHODS
Sample preparation: Two grams of CMC obtained from Acros Organic Co. was dissolved in distilled water. Then, varied amount of NH4Br in weight percent (5-35 wt.%) was added. The mixture was stirred continuously until complete dissolution of the NH4Br. The mixture was then poured into different Petri dishes and left dried at room temperature for the film to form. The films were kept in desiccators for about 1 month before being characterized to ensure no water present in the Bes system. The CMC-BEs were cut into suitable sizes for further analysis.
FTIR spectroscopy: FTIR spectroscopy measurement was carried out using Thermo Nicolet 380 FTIR spectrometer. The spectrometer was equipped with an Attenuated Total Reflection (ATR) accessory with a germanium crystal. The sample was put on germanium crystal and infrared light was passed through the sample with the frequency ranging from 4000 to 675 cm-1 with spectra resolution of 4 cm-1.
X-ray diffraction: To study the nature of the BEMs system, the X-ray
diffraction (XRD) measurements were performed using Rigaku MiniFlex 2. Prior,
samples were cut into suitable sizes (2x2 cm) and then adhered onto a glass
slide. The glass slide was then placed at the sample holder of the diffractometer
and the samples were directly scanned at 2θ angles between 5 and 80°
with X-rays of 1.5406 Å wavelength generated by a Cu Ka source.
Impedance spectroscopy: The polymer electrolyte samples were cut into small discs of 2 cm diameter and sandwiched between two stainless steel electrolytes under spring pressure. The samples were characterized via Electrical Impedance Spectroscopy (EIS) using HIOKI 3532-50 LCR Hi-Tester interfaced to a computer in a frequency range between 50 Hz and 1 MHZ. The measurements were carried out at room temperature of 303 until 383K. The conductivity of electrolyte can be calculated from the equation:
Here, A (cm2) is the electrode-electrolyte contact area of the film and t its thickness. Rb is bulk resistance obtained from the complex impedance plot at the intersection of the plot and the real impedance axis.
RESULTS AND DISCUSSION
Ftir spectroscopic analysis: Figure 1 shows the spectra
of CMC-NH4Br complexes in the region from 1200 to 1700-1.
In the present work, the band at 1592 cm-1 is assigned to asymmetrical
COO¯ stretching of the carboxylate anion in the CMC (Pushpamalar
et al., 2006). With the addition of AB in the system, this peak is
expected to be effected due to the lone pair electrons will attract the salt
molecules to be attach to it. Upon addition of AB salt, the gap for 1592 cm-1
due to COO¯ is observed to decrease up to 35 cm-1. In ammonium
bromide, the hydrogen bonding occurs with the N-H bond within the tetrahedral
ion, NH4+, pointing directly toward the bromide ion, Br¯
and forming an N-HþBr hydrogen bond (Reed
and Williams, 2006). Two of the four hydrogens of NH4+
ions are bound identically, one hydrogen is bound more rigidly and the fourth
more weakly. The weakly bound H of NH4+ can easily be
dissociated under the influence of a dc electric field (Hema
et al., 2008). These H+ ions can hop via each coordinating
site (oxygen) at the band 1592 cm-1 of the host polymer (CMC) and
thus conduction takes place. The conduction occurs through structure diffusion
(Grotthus mechanism), i.e., the conduction occurs through the exchange of ions
between complexed sites (Hashmi et al., 1990).
||FTIR spectra of (a) pure CMC film (b) 5 wt.% NH4Br
(c) 10 wt.% NH4Br (d) 15 wt.% NH4Br (e) 20 wt% NH4Br
and (f) 25 wt.% NH4Br
Hence from FTIR spectroscopy the interaction between CMC and NH4Br
has been confirmed and the conduction mechanism in the polymer electrolytes
has been well established.
Figure 2 shows the XRD patterns for the various electrolyte
films. It can be inferred from the X-ray diffractograms, the amorphousness of
the polymer electrolyte decreased after addition of more than 25 wt.% NH4Br.
Conductivity increases with the increase in amorphous domain of the sample (Shuhaimi
et al., 2010). It can be predicted that the sample containing 25
wt.% salt exhibits the highest conductivity at room temperature. However, with
increment of salt composition above 25 wt.%, new peaks at 2θ = 22.25 and
31.20° corresponding to the undissociated salt have been observed and the
sample has become more crystalline. Changes in amorphousness of the CMC-NH4Br
BEs system contribute to the change of conductivity of the samples. This amorphous
nature is responsible for greater ionic diffusivity resulting in high ionic
conductivity (Shuhaimi et al., 2010; Balasubramanyam
et al., 2007). This observation confirms that complexation has taken
place in the amorphous phase.
Impedance studies: The conductivity of the CMC-NH4Br based
BEs system at room temperature is tabulated in Table 1. It
can be observed that the ionic conductivity of the sample increases with addition
of salt content.
|| Transport parameters of the CMC-NH4Br biopolymer
electrolytes at room temperature
||XRD patterns XRD patterns for (i) pure NH4Br, (ii)
CMC-35 wt.% NH4Br, (iii) CMC-30 wt.% NH4Br, (iv) CMC-25
wt.% NH4Br, (v) CMC-20 wt.% NH4Br, (vi) CMC-15 wt.%
NH4Br, (vii) CMC-10 wt.% NH4Br, (viii) CMC-5 wt.%
NH4Br, (xi) pure CMC film and (x) CMC powder
The highest conductivity is 1.12x10-4 Scm-1 for the sample
containing 25 wt.% NH4Br. Above 25 wt.% NH4Br the conductivity
decreases and can be attributed to the reassociation of the ions into neutral
aggregates (Schantz and Torell, 1993; Teeters
et al., 1996) and also the excess salt recrystallized out of the
polymer as previously proven by XRD result in Fig. 2.
The log σ versus 1000/T plot for the CMC-NH4Br systems shown
in Fig. 3 confirms that the ionic conductivity of the biopolymer
electrolyte increase with increasing temperature for all compositions. Since
all polymer complexes do not show any abrupt jump with temperature, it indicates
that these electrolytes exhibit a completely amorphous structure (Rajendran
and Uma, 2000). The regression values are close to unity, (R2 =
1) suggesting that the temperature-dependent ionic conductivity for all complexes
obeys Arrhenius behavior (Micheal et al., 1997)
by the relation,
where, σo is the pre-exponential factor, Ea the activation energy and k is the boltzman constant.
|| Temperature dependence of ionic conductivity
The activation energy (combination of the energy of defect formation and the
energy for migration of ion) listed in Table 1 was calculated
by linear fit of the Arrhenius plot. It shows that Ea for the conduction
decreased gradually with the increase in the salt content implying that the
ions in highly conducting samples require lower energy for migration. In the
situation of biopolymer electrolyte, when the ion has acquired enough energy,
it is able to split away from the donor site and travel to another donor site.
The movement from one site to another result in the conduction of charge and
the energy for this conduction is the Ea (Buraidah
et al., 2009). Thus, in order to analyze quantitatively the conductivity
trend observed, the number of free ions, η has been calculated based on
the Rice and Roth model (Rice and Roth, 1972). Knowing
η and combining the result with conductivity, the mobility of the ionic
charge carrier, μ can then be determined. Table 1 lists
the transport parameters at room temperature for complexes of CMC-NH4Br
Table 1 depict the values of number density of mobile ions,
η, ionic mobility, μ and the diffusion coefficient, D. The transport
parameters were found to be related to the conductivity of the samples. It can
be explained that as the conductivity increase, the Ea as mentioned
before require lower energy to move the ion due to increasing of η, μ
and D in this BEs system.
||Salt content dependence of dielectric constant εr
at selected frequencies
||Temperature dependence of dielectric constant εr
for sample containing 25 wt.% NH4Br at selected frequencies
The mechanism of transport parameter can be more describe in dielectric studies.
Dielectric studies: The dielectric constant is representative for the stored charge in a material. In polymer electrolytes, the charge carriers are ions. The dielectric constant (the real part of complex permittivity, εr) is given by:
Here, C0 = εoA/t and ω = 2πf. εo
is permittivity of free space, Zi and Zr is the imaginary
and real parts of the complex permittivity and f is frequency. The salt content
dependence of the dielectric constant (εr) at selected frequencies
is shown in Fig. 4. It can be observed that the εr
increase with increment of salt content for every frequency until after 25 wt.%
NH4Br when the value of εr drops. This implies that,
as the salt content increase, the stored charge in the sample rise which means
that the number density of mobile ions has increased. Decrement in dielectric
constant at 30 wt.% NH4Br is due to decrement in density of charge
carriers which is attributed to the reassociation of ions. Results shown in
Fig. 5 imply that the phenomenon of polarization rise with
temperature in this system. As temperature increases, the degree of salt dissociation
an redissociation of ion aggreagates start to grow resulting in the increase
in number of charge carrier density.
Frequency dependence AC conductivity: The ac conductivity can be obtained
from dielectric loss, εi at every frequency according to:
The phenomenon of ac conductivity can be analyzed using Jonschers Universal
Power Law (UPL) (Winie and Arof, 2004).
Here, σ (ω) is the total dc and ac conductivity. The dc conductivity,
σdc is the frequency independent component, A is a parameter
dependent on temperature and s is the power law exponent with value in the range
between 0 and 1. From Eq. 4 and 6:
Figure 6 depicts the frequency dependence of dielectric loss,
εi at selected temperatures for higher conductivity. Form the
plot, the value of exponent s can be obtained from the slope at the higher frequency
region where there is no or minimal space charge polarization (Buraidah
et al., 2009).
|| ln εi versus ln μ at elevated temperatures
|| Variation of exponent s versus temperature
The variation of s with temperature for biopolymer electrolyte system
is plotted in Fig. 7.
From Fig. 7, it can be observed that s increase with increasing
temperature. Thus, the frequency dependence of sample can be explained in terms
of Small Polaron Hopping (SPH) model due to the variation of the index s with
temperature (Mott and Davis, 1979). In this model, a small
polaron is formed by the addition of a charge carrier to a site which results
in large degree of local lattice distortion.
The development of new solid biopolymer electrolytes based on CMC doped with
NH4Br has been prepared by solution cast technique. FTIR spectroscopy
provides an insight into the possible interaction between CMC and NH4Br
which confirm via Grotthus mechanism and the ionic species in the polymer electrolytes
has been well established as a proton conductor (H+). XRD measurements
confirmed that the BEs system predominantly amorphous in nature. The highest
conductivity obtained was 1.12x10-4 cm-1 at room temperature.
The temperature dependence of ionic conductivity of the BEs system exhibits
Arrhenius behavior where the samples conductivity exclusively affected by the
temperature and composition of NH4Br. From the calculations carried
out, it can be inferred that the conductivity is governed by the number density
of ions, mobility of ions and diffusion coefficient. The conduction mechanism
studies shown the biopolymer electrolyte can be best represented by the SPH
The authors would like to thank MOHE for the FRGS Vot59185, Faculty Sciences and Technology and University Malaysia Terengganu for the technical and research support.
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