The usage of polymeric electrolytes has been primer work due to the wide range
of possible applications. Proton conducting polymeric electrolytes is used in
application such as batteries, capacitors, sensors, electrochromic displays,
photo-electrochemical solar cells and fuel cells (Wieczorek
and Stevens, 1997). In order to enhance the transport properties of Polymer
Electrolytes (PE) to make it more viable for various applications, various researches
has been done and reported in the past years. Through extensive and laborious
investigations, it can be concluded that the polymer can be group into two major
classes, i.e. Solid Polymer Electrolytes (SPEs) and Gel Polymer Electrolytes
(GPEs). Advantages in space and mass, structural stability and low volatility
are some of the reminiscent of polymer electrolytes comparable to the other
types of solid state electrolytes. The GPEs are reported as suitable electrolytes
for their optimized properties (Kreuer, 1997; Gary,
1997). For instance, cheap and easily produced electrolytes are needed for
large-scale fuel cell applications.
In this study, the conductivity, dielectric behavior and ionic transport parameter was investigated at ambient and elevated temperature of 293-323 K. The activation energy and the relaxation energy for the polymer gel electrolyte also calculated in order to prove the hopping mechanism of the contributing ions.
MATERIALS AND METHODS
Sample preparation: The solvents (EC-PC) were mix at room temperature. Then the salicylic acid was added and dissolved. Later the oleic acid was added and stirred until homogenous solutions obtained. Finally, poly methyl methacrylate (PMMA) (MW = 60,000 g mol-1) was added and the whole mixture was heated to 70°C to promote gel formation. Finally the mixture was caste into petri dishes and let dry at room temperature for 24 h before further drying in the desiccators.
Electrical impedance spectroscopy (EIS) study: The sample was sandwich between two stainless steel electrodes for impedance spectroscopy analysis. The EIS was measured using a computer-interfaced HIOKI 3531-01 LCR bridge with frequency ranging from 42 Hz to 1 MHz. The EIS was measured at room and elevated temperatures (298-353 K). The electrical conductivity was obtained by inserting the bulk resistance (Rb), thickness (t) and area of contact between the gel and the electrode (A) into the Eq. 1:
The bulk resistance Rb was obtained from the intercept at the x-axis of the complex impedance plot, -Zi vs. Zr as shown in Fig. 1.
RESULTS AND DISCUSSION
From the complex impedance plot of Fig. 1, it can be observed that the bulk resistance (RL) of the samples decreases as temperature increases. From the inset of Fig. 1, it can be observed that the depressed circle corresponds to the parallel combination of bulk resistance and Constant Phase Elements (CPE), in series with another CPE from the inclined straight line assigned to the double layer capacitance for the gel electrolyte. From Eq. 1, the value of conductivity was calculated for temperature between 298 and 353 K and depicted in Fig. 2.
||The complex impedance plot, -Zi vs. Zr
for sample PMMA-SA-OA at room and elevated temperature (inset: depressed
circle of sample at 323 K)
|| Log conductivity (σ) vs. 1000/T plot
|| The values of Ea at various temperatures
From the temperature dependence of conductivity plot, the linear relationship
is observed which confirms that the variation in ionic conductivity with temperature
obeys the Arrhenius type thermally activated process. This indicates that there
is no phase transition occurring in the polymer matrix or that domains are formed
by the addition of acid (Idris et al., 2009).
Therefore, no dynamic conformational change in the polymer matrix and H+
ions might migrate through the conduction path formed by the lattice structure
of the polymer chains (Hema et al., 2009).
From the plot of log σ versus 1000/T, the activation energy was calculated using the Arrhenius equation:
where, σo is the pre-exponential factor; Ea is the activation energy; T is the absolute temperature and k is the Boltzmanns constant. The Ea was calculated for every temperature and depicted in Fig. 3.
Figure 3 shows that the activation energy decreases with
the temperature. The sample with highest conductivity has the lowest activation
energy. The decrease in activation energy is due to the density of ions in the
polymer electrolyte increase with increasing temperature; hence, the energy
barrier to the proton transport decreases which would lead to a decrease in
the Ea (Filho et al., 2007). Rice
and Roth (1972) hypothesized that energy gap, ε, exists in the ionic
conductor which ions of mass (M) belonging to the conducting species can be
thermally excited from localized ionic states to free ion-like states in which
an ion propagates throughout the solid with a velocity (υ). The velocity
is given by υ = (2E/M)1/2. Such an excited free-ion-like state
is supposed to have a finite lifetime (t). The mean free path or distance from
one complexed site to another (ℓ) is given by ℓ = υt (Zhao
et al., 2007).
The Rice and Roth model (Rice and Roth, 1972) was employed
in this study in order to determine the transport parameters for the samples.
The Rice and Roth model expresses the conductivity as:
The exponential term tends to unity. Hence τ calculated from the Rice and Roth conductivity is of the order 10-14 s. Using the value of τ and the Ea from the log σ versus 1000/T plot, the number density of mobile ions, n, can be calculated at every temperature. Using the Nernst-Einstein equation and the values of n obtained from earlier, mobility of mobile ions (μ) and diffusion coefficient of the mobile ions (D) were calculated. The ionic mobility is defined as:
and diffusion coefficient is given by Nernst-Einstein equation:
Table 1 lists the estimated values of n, μ and D for the OA-plasticized gel electrolyte of the present work at varied temperature studied.
From Table 1, the increase in conductivity with temperature is observed to be influenced by the n and μ, since the D of the ions remains constant throughout the temperature range in the present investigation. n increases with temperature while μ decreases with temperature. The decrease in μ with temperature is attributed to the blocking effect due to ion overcrowding, thereby making the ions less mobile. Hence, temperature helps to dissociate the acid into ions thereby leading to an increase in conductivity.
The value of n and D calculated from the Rice and Roth equation is in reasonable
agreement with that obtained by Wieczorek and Stevens (1997),
Kreuer (1997), Gary (1997) if
τ takes the value of ~10-14 sec. The mobility is of the order
10-6 cm2 V-1 sec-1 and is the same
for the samples studied by other workers.
|| The estimated values of n, μ and D for the OA-plasticized
gel electrolytes at ambient and elevated temperature
Permittivity is a measure of the degree to which a medium can resist the flow of charge. Since ac conductivity is known to increase with frequency, permittivity must decrease with frequency so that the flow of charge will not encounter much resistance in the high frequency regime. Information on relaxing dipole in a medium may be obtained from an analysis of complex permittivity. The relaxation frequency and strength are characteristics of the relaxing dipoles. In order to detect any relaxation peak in the high frequency region, the modulus formalism is adopted. If the modulus formalism did not show any relaxation peaks, loss tangent which can be calculated from the ratio of imaginary modulus and real modulus will be adopted. The permittivity was calculated from the equations below:
εr versus frequency, f, is as shown in Fig. 4.
εr shows an increasing trend with decrease in frequency. The
plots exhibit frequency dispersion at all temperatures investigated. The decrease
in εr arises from the fact that polarization does not occur
instantaneously with the application of the electric field due to inertia (Damay
and Klein, 2003). The delay in response toward the applied electric field
leads to the lost and decline in εr. This implies plurality
of relaxation times (Murugaraj and Govindaraj, 2005)
as indicated by the tilt in the circle of the Cole-Cole plot.
At low frequencies, all types of polarization contribute. As frequency is increased,
the mobile charge (or ions) with large relaxation times cease to respond leading
to a decrease in εr. At low frequencies, contribution from space
charge polarization is high. This reduces with increase in frequency.
|| Dielectric constant vs. frequency at different temperature
for OA-plasticized sample
|| Dielectric loss vs. frequency at different temperature for
Space charge arises from ion accumulation at the electrode-electrolyte interface.
Since these dipoles do not respond at higher frequencies, εr
drops. The εr represents the fractional increase in the stored
energy per unit voltage. This means that εr represents the fractional
increase in charge. The charge must come from the protons of the gel electrolytes.
Hence a fractional increase in charge implies an increase in the number of ions.
Since εr for the gel increases with temperature as can be easily
observed at low frequencies the number of ions also increases. As a rule, the
conductivity, σ is given by σ = n q μ where n is the number of
charged species, q the electron charge and μ the mobility of the charged
species. The frequency dependent conductivity of the gel increases with increasing
temperature. This is consistent with the decrease in εr with
frequency of mobile ions although μ decreases with increasing temperature.
The εi versus frequency, f, plot is shown in Fig.
5. The dielectric loss, εi, probes a wide variety of phenomena
along with any relaxation time which may be present in the material. The relaxation
time is a characteristic time that determines the sluggishness of the dipole
segment to an applied ac field (Aziz et al., 2010a).
It is the mean time for the dipole to lose its alignment with the field due
to its random interactions with other molecules.
Dipole relaxation occurs when the frequency of the applied ac field is such that there is a maximum energy transfer from the ac voltage source to heat in the dielectric through the molecular collisions and lattice vibrations. The peak occurs when the angular frequency of the ac field is the reciprocal relaxation time. By the above argument it is sufficient to say that at high temperatures the gel contains more ions, as the dielectric loss is higher at high gel temperature. This is because the higher number of ions will result in more random interactions which will finally lead to a longer relaxation time.
The real part of complex electrical modulus Mr versus frequency, f, plot is shown in Fig. 6. It can be observed that there is a long tail exhibited beginning from 104 Hz to lower frequencies. This long tail feature is characteristic of a highly capacitive material. At high frequencies (>104 Hz), Mr continues to increase in the frequency range studied.
Imaginary electrical modulus, Mi versus frequency, f, plot is shown
in Fig. 7. Mi increases with increasing frequency.
The value of Mi is lower as the temperature increases in the Mi
spectrum. This implies that the relaxation time for the protons at high temperature
is shorter than that at low gel temperature.
||Real part of dielectric modulus vs. frequency at different
temperatures for OA-plasticized sample
||Imaginary part of dielectric modulus vs. frequency at different
temperatures for OA-plasticized sample
This also explains why the conductivity of the gel is higher at higher temperatures.
Since no relaxation peaks are observed, the loss tangent formalism will be adopted
(Aziz et al., 2010b).
For the sake of clarity in presentation, some loss tangent versus frequency
plots are presented. The plot of loss tangent versus frequency is shown in Fig.
8. It can be observed that the loss tangent curve increases with the frequency,
passes through a loss peak and decreases with further increase in frequency.
The peak shifts to higher frequencies as temperature increases, again supporting
the fact that conductivity increases with temperature. It can be observed in
the figure, the peak of the plot shifts towards higher temperatures as the frequency
increased. This is because temperature facilitates dipole orientation (Khaled
and Vafai, 2003). This in turn reduces the relaxation time and increases
conductivity as temperature increases.
|| Loss tangent vs. frequency at selected temperatures for OA-plasticized
||The highest ionic conductivity at room temperature is 8.65x10-4
S cm-1 and the conductivity increases to 1.93x10-2
S cm-1 at 368 K
||The ionic mobility and diffusion coefficient values calculated using the
σ = nqμ and the Nernst-Einstein equations are in reasonable agreement
with the results evaluated from experiments to further strengthen that protons
are the dominant charge carriers
||The number density of mobile ions was obtained from the Rice and Roth
||The conductivity shows significant influenced of the ionic transport properties
i.e. Number of ions, n, mobility of ions, μ and diffusion coefficient
of ions (D)
||The dielectric study supports the transport study, where the conductivity
is significantly influence by the temperature
The author would like to thank A.K. Arof and A.S. Samsudin for the technical support.