Figure 4 shows the cole-cole plot of pure iota-carrageenan and iota-carrageenan with different concentrations of NH4HCO2 at room temperature. Generally, the Cole-Cole plot consists of a high-frequency semicircle represented a capacitor parallel to a bulk resistor and a low frequency spike represented by a constant phase element. Migration of ions may occur through the free volume of the polymer matrix which can be represented by a resistor. The immobile polymer chains become polarized in the alternating field which can be represented by a capacitor. The ionic migration and bulk polarization are physically in parallel and therefore a semicircle at high frequency can be observed. In the present study, the cole–cole plot shows the disappearance of high frequency semicircular portion which leads to a conclusion that the charge carriers are ions and hence the total conductivity is mainly the result of ion conduction. All plots depict a tilted spike indicating resistive component only for the polymer electrolytes. The data can be well fitted with the equivalent circuit consists of a Rb and CPE for which Rb is bulk electrolyte resistance and CPE is a constant phase element as presented in insert of Figure 4. The existence of CPE could be due to a capacitive behavior that changes with the frequency and arises if there is air presence in between the electrode-electrolyte surfaces 33. The resistance of the bulk electrolyte (Rb) has been retrieved from the intercept of the straight line on the Z? axis. Electrochemical impedance spectroscopy (EIS) parameters have been extracted by fitting the complex impedance plot using EQ software developed by Boukamp 34. The impedance of the constant phase element is given as
ZCPE = 1/Q0(j?)n
where Q0 and n are frequency independent parameters. n varies from 0 to 1. If n = 1, it represents a pure capacitor. If n =0, It denotes a pure resistor. The calculated CPE, n values are listed in Table 3 for all the samples at 303 K. The ionic conductivity of the polymer electrolytes are calculated using the equation,
where, t and A are the thickness and area of the electrolyte film respectively, Rb is the bulk resistance of the polymer electrolyte. The calculated conductivity values have been tabulated in Table 3. As shown in table.3, the ambient temperature ionic conductivity of ammonium formate complexed polymer electrolyte system increases with increasing salt concentration and the maximum ionic conductivity has been found for the 0.4wt% of NH4HCO2 system, as 1.11×10-3 S/cm. The increase in conductivity with increasing salt concentration may be due to the increase in the number of mobile charge carriers. The decrease in conductivity at higher salt concentrations has been attributed to either incomplete dissociation of salt or the formation of ion aggregates 35, 36.
3.4.2 Conductance Spectra
The conductance spectra of the polymer electrolytes for pure iota-carrageenan and iota-carrageenan with different concentration of ammonium formate are shown in Figure 5. In general, the conductance spectra displays three well-defined regions, a low frequency dispersion region which describes the electrode–electrolyte interfacial phenomena which are attributed to the space charge polarization at the blocking electrodes Frequency-independent plateau region in the mid frequency range corresponding to DC conductivity of the bulk material and a high frequency dispersive region corresponding to AC conductivity. In the present case, a conductance spectrum Figure 5 consists of two regions, low frequency dispersion region and plateau region. The DC conductivity (?dc) of the polymer electrolytes has been obtained by extrapolating the plateau region on the log ? axis. It has been observed that the ?dc increases with increasing of NH4HCO2 up to 0.4 wt%. This may be due to increase in amorphous nature of the polymer complex 37. The highest conductivity of 1.11×10-3 S/cm has been observed for the electrolyte iota-carrageenan with 0.4 wt% of NH4HCO2. The conductivity values obtained from the conductance spectra are found to agree very well with those obtained from the cole-cole plot (AC conductivity).
3.4.3 Arrhenius plot
The temperature dependence of ionic conductivity of the polymer electrolytes iota-carrageenan/ammonium formate system has been depicted in Figure 6. It is clear that bulk resistance reduces with rising temperature and there is a linear relationship between natural logarithm of conductivity (log sigma) and reciprocal temp (1/T). This means that the ionic conductivity obeys the Arrhenius equation
?= ?0 exp (-Ea/kT)
where ?0 is the pre-exponential factor, k is the Boltzmann constant and Ea is the activation energy. The calculated activation energy values and Regression values (R) have been tabulated in Table 4. The increase of conductivity with temperature could be elucidated from enhanced polymer chain flexibility with increasing temperature, the polymer complex could be expanded, and therefore the mobile charge carriers would have more hopping or pathways between adjacent interstitial sites for migration from one site to another. Higher temperature, larger the amplitude of oscillation of macromolecules. An increase in the vibration of molecules could favor the polymer chain segment mobility 38, 39.
Table 4 revealed that the activation energy decreases with increasing in salt concentration. This is due to the increase in amorphous nature of the polymer electrolyte with addition of salt that facilitates the ionic motion in the polymer network. At higher salt concentrations, the activation energy increases due to aggregation of ions, leading to the formation of ion clusters, thus decreasing the number of mobile charge carriers. The activation energy has been found to be low (0.32 eV) for the highest conducting polymer electrolyte iota- carrageenan (1 g) with 0.4 wt% of NH4HCO2. The relation between salt concentration versus conductivity and activation energy plot as shown in Figure 7. Figure 7 revealed that the highest conducting sample has got low activation energy than that of the other polymer electrolytes iota-carrageenan/ NH4HCO2 complex.
3.5 Transference number measurement
Wagner’s polarization technique 40, 41 is used to measure transference number which identifies whether the conductivity in the polymer electrolyte is due to presence of ions or electrons. Transference number is calculated using the formula:
t+ = (Ii – If) / Ii
t- = (If / Ii)
where Ii is the initial current and If is the final residual current, t+ is the ionic and t- is the electronic transference number. The polarization current passing through the cell is monitored as a function of time. The dc polarization graph of the iota-carrageenan (1 g) with 0.4 wt% NH4HCO2 polymer electrolyte at room temperature is shown in Figure 8. The initial current (Ii) decreases with time due to the depletion of the ionic species in the electrolyte and becomes constant in depleted situation. The cationic (t+) transference number for all composition of NH4HCO2 has been found in the range of 0.94 to 0.99. The highest ionic conducting membrane having the ionic transference number (t+) which is 0.99 and which leads to charge transport in polymer electrolyte is predominantly accompanied by H+ ions. The measured values of conductivity and ionic transference number have been used to calculate the Mobility and diffusion coefficient of H+ ions of all polymer membranes using the following equations 42.
n1 = N ? × molar ratio of salt/molecular weight of the salt
where n1 = number of molecules cm-3, N = Avogadro number, and ? = density of the salt
Mobility (µ) ? = ?/n1e
Diffusion coefficient (D) D = kT?/n1e2
Where k = Boltzmann constant, T = Temperature, ? = Conductivity, and e = charge of an electron, D+ = Diffusion coefficient of cation, D- = Diffusion coefficient of anion, ?+ mobility of cation and ?- mobility of anion. Table 5 provides the value of ?+, ??, D+, and D? for the biopolymer electrolytes investigated at ambient temperature.
From table 4, it can be observed that the values of ?+ is higher than ?? and also the values of D+ is higher than D? for all samples. Therefore, it can be concluded that the sample is more cationic (+) than anionic (?) conductor. The value of (iota-carrageenan (1 g): 0.4 wt% of NH4HCO2) ?+ is (1.37× 10?6 cm2/Vs) greater than ?? (5.04×10?8 cm2/Vs) by two orders of magnitude. Similarly, the diffusion coefficient is divided in two terms D+ which is due to cation and D? which is due to anion. The value of D+ is (3.58×10?8 cm2/s) greater than D? (1.32×10?9 cm2/s) by one order of magnitude. The study of transference number leads to the conclusion that conductivity is influenced by ?+ and D+.
3.6 Electrochemical stability
The electrochemical stability window i.e. the working voltage range of a polymer electrolyte is another important parameter for its potential application in electrochemical devices. Electrochemical stability of the maximum conducting polymer electrolyte (iota-carrageenan: 0.4 wt% of NH4HCO2) is calculated by linear sweep voltammetry. The measurement has been done at room temperature. A linearly varying potential, from 0 to 5 V at the scanning rate of 1 mV/s has been applied and the change in current is recorded and shown in Figure 9. As shown in Figure 9, the electrolyte is stable up to 2.98 eV for iota-carrageenan with 0.4 wt% of NH4HCO2 from Figure 9. This amount of voltage is good enough to allow use of this biopolymer for the fabrication of protonic batteries, since the electrochemical window standard of a protonic battery is about ~1 V 43. The electrochemical stability window of this polymer complex is higher than the reported value for NH4NO3 (2.47 V) 17 and NH4Br (2.1 V) 16 with I- carrageenan polymer electrolytes.