DC Conductivity Study
Figure 1 shows the typical impedance plot (imaginary part,Zi against real part, Zr of complex impedance, Z*) of CMC/CS doped with different NH4Br content at 303K. This impedance analysis is important in conductivity study and becomes a powerful tool in the investigation of ionically conduction behaviour in solid biopolymer electrolyte films. From the plot, it contains two-well defined regions, namely a high frequency region semicircle arc and low frequency region inclination spike. The semicircle represents the bulk effect of electrolyte which due to parallel combination of bulk resistance, R (proton migration) and bulk capacitance, C (immobile polymer chain). The inclination spike denotes the effect between two blocking electrodes which represent the formation of double layer capacitance of polymer electrolyte interface (proton migration at low frequency, f). The value of bulk resistance, Rb can be retrieved from the interception between low-frequency and high-frequency region on Zr-axis, which the value was then used to calculate ionic conductivity of the system.
Figure 1: The Cole-cole plots of the real impedance, Zr against imaginary impedance, -Zi biopolymer electrolyte system for (a) CMC/CS film, (b) CMC/CS-10 wt.% NH4Br, (c) CMC/CS-20 wt.% NH4Br and (d) CMC/CS-30 wt.% NH4Br at 303 K
Based on the plot, the value of Rb decrease with addition of NH4Br content up to 20 wt.%, and start to decrease beyond that concentration. Together with the decreasing in Rb value, the high semicircle arc seems to gradually fade away and completely disappears above 20 wt.% of NH4Br. Based on the previous study, the depressed semicircle and inclines spike (Figure 1a, 1b and 1 d) shows that the ion have different relaxation times and reveals non-Debye behaviour of the sample. From the calculation, 20 wt.% of NH4Br gives the highest conductivity value of 2.12 x 10-5 Scm-1. According to previous study, the increasing in conductivity value is due to the presence of ammonium salts (ionic dopant) in the system which believed to be a good proton donor in a polymer matrix and expected to enhance in conductivity value. Additionally, the low-frequency inclined spike beyond 20 wt.% of NH4Br prevails in the plot indicates that the system is monopolized by the only resistive component as illustrated in Figure 1c.
Further analysis has been done in this work with the aim to observe the effect of NH4Br addition with temperature on the electrical properties of biopolymer electrolyte film. Figure 2 shows the salts content dependence of conductivity value of CMC/CS-NH4Br at selected temperature. The conductivity together with amorphous nature of polymer blend (reduction in crystallinity with increasing amorphousity) increased gradually with increasing in salt content and reach the optimum value at 20 wt.% of NH4Br as proven in Cole-cole plot study (Figure 1).
Figure 2: Salts content dependence of CMC/CS-NH4Br...