Structural Characterization
As discussed in the introduction section, phase separation/ crystallization of salts, and brittleness of lattice etc., with the increasing salt concentration, are the major concerns associated with PISEs. Hence, after synthesis, confirming the acceptability of salt host matrix is of prime importance. The photographs of all three systems are given in Fig. 1, which indicates that irrespective of the origin of the starch, membranes have a smooth and homogeneous look indicating that salt has been well accepted by cross-linked starches. No trace of phase separation is visibly observed. The prepared materials are flexible also. They can be easily twisted and stretched. No sign of brittleness or phase separation is observed in any of the three samples.
Figure 2 shows the SEM for pure and complexed system for all three starches. As expected, granule structures are clearly visible in host matrix. Missing granule structure in complexed system confirms that starch molecules have lost their original granule shape. No Phase separation is observed. This confirms that salt breaks the starch molecules in smaller one andstarchloses its granule structure. Uniform nature of the morphology indicates homogeneous mixing of salt and starch. It also confirms the absence of recrystallization of salt at the studied salt concentration.
The acceptance of salt has been further confirmed by XRD analysis. XRD spectra are given in Fig. 3, where it is clearly visible that all the signature peaks of added salt are missing inthe synthesized systems confirming the complete dissolution and dissociation of salt in the starch matrix. Here it should be remembered that in the three selected starches, corn has the highest amylose content whereas the arrowroot has the lowest one and the capacity of intra-molecular bonding is directly proportional to amylose content. In other words, it can be said that higher amylopectin content increases the probability of hydrogen bonding with additives. Hence in the arrowroot starch-based complex, the starch molecules make more hydrogen bonding with salt ions in comparison to corn starch-based complex, and hence in case of arrowroot basedsystem maximum change are observed in the XRD pattern of starch. It has also confirmed our hypothesis that crosslinked starch is able to accept a large amount of salt because hydrogen bonding plays an important role in cluster formation in these systems by making a bridge between different salt molecules and with starch molecules also. A detailed discussion will be given in the coming sections.
FTIR spectra of pure salt, starch + GA, and the complexed systems are given in Fig. 4. Table 1 summarizes the major observedchanges in salt and starch FTIR spectra after complexation. Peak observed at ~ 1000 cm− 1 is associated with C-O-C stretching[34]. In complexed materials, this is shifted towards higher wave number indicating that mass of the molecule is reduced. This confirms that addition of salt breaks the starch in smaller molecules. For Arrowroot starch-based system this shift is smallest (1024 cm− 1). The reason is associated with lower weight % amylose in arrowroot. Amyloses have alpha (1–4) bonding whereas the amylopectin have alpha (1–4) and alpha (1–6) bonding and the latter one results in branched structure. Amylose also have larger intramolecular bonding and hence can be easily broken into smaller molecules, hence shifting is more prominent (shifted to 1071cm− 1) in corn and potato starches in comparison to arrowroot starch. The peak observed at 3600 cm− 1 in complexed system is very broad and can be easily deconvoluted into three peaks as summarized in Table 1. The three deconvoluted peaks can be associated to cluster forming capabilities of water molecules. As discussed by Quiang, water molecules tend to form cluster where they defined different type of cluster formation possibilities such as DDAA, DDA, DAA and DA as indicated in Fig. 5.
Table 1
Summary of important FTIR peaks
Wavenumber (cm− 1)
|
AS_Complex
|
PS_Complex
|
CS_Complex
|
H-Bands
|
3563
3369
3232
|
3557
3367
3239
|
3557
3369
3230
|
O-H Bending Bands
|
1638
|
1638
|
1638
|
C-O-C stretching Bands
|
1081
|
1080
|
1081
|
Cl-O
|
691
|
690
|
688
|
Cl = O
|
618
|
616
|
618
|
In present case wavenumbers related to DDA, DAA and DA could be observed, whereas no signature peak for DDAA range (~ 3000 cm-1) could be identified. As discussed by Quiang [35], DDAA belongs to large waterclusters which are present in pure liquid water. Its absence in present case is intelligible as water molecules present in synthesized sample are present in the form of small water clusters and are assisting in stabilizing the conductive molecular structure of PISE electrolyte. In our ongoing work where effect of variation of salt concentration with starch is being studied, it has been observed that at low salt concentration, peak related to DDAA type bonding is also present.
The 3600 cm-1 peak in host matrix is very broad and its intensity is very small in comparison to peaks observed in complexed systems as indicated in Fig. 4. That is why they are not separately discussed here, the only point to be noted here is that the broadness of the peak is increased indicating the enhanced variegated hydrogen bonding after complexation of starch and salt which is obvious as the addition of salt will certainly have its signature on the starch hydrogen bonding as indicated in Fig. 4.
The XRD and FTIR results have indicated that hydrogen bonding and presence of water molecules are playing an important role in cluster formation. Figure 4 indicates the pictorial presentation of these different interactions and its possible effect on cluster formationwithout resulting in crystallization. Different hydrogen bonding seems to be an important factor in maintaining the good flexible mechanical properties.
Nyquist plot and Conductivity at normal temperature and humidity
Electrochemical impedance spectroscopy (EIS) has been used for the analysis of the electrochemical properties of the system. In this type of analysis, the frequency response of the synthesized system is determined by measuring impedance and phase angleover a range of frequencies, here, the frequency range from 5 MHz to 50 Hz is used. Nyquist plot for all the synthesized samples are given in Fig. 6(a). All the three curves are tilted straight lines intersecting the Z’axis at very small values.
No semicircle is observed in any of the Nyquist plots of the synthesized systems. Absence of the semicircle implies that R-C series combination circuit is system’s equivalent circuit and fast ion diffusion at electrode-electrolyte interface [36–38].
Conductivity of synthesized systems is calculated by using the formula, (σ = l/ (AR), where 𝜎 is the conductivity of the sample, R is the bulk resistance of the sample, l is thickness of the sample, A is contact area of the electrode and electrolyte. Value of resistance (R) is estimated by the intersection point of Nyquist curve at Z’ axis.The conductivity values of synthesized systems having different starches are tabulated in Table 2.
Table 2
Summary of electrochemical properties of synthesized samples
Type of Starch
|
ESR (ohm)
|
Conductivity (S/cm)
|
Relaxation Time (us)
|
ESW (V)
|
Arrowroot (AS)
|
2.7
|
1.04 x 10− 1
|
10.42
|
2.9
|
Potato (PS)
|
1.9
|
1.81 x 10− 1
|
79.71
|
3.2
|
Corn (CS)
|
3.7
|
0.7 x 10− 1
|
84.6
|
2.8
|
Conductivity values give the impression of being proportional to granular size of host starch, indicating that big granules form bigger clusters. This can be correlated to increased free volume of cluster formation which leads to enhanced conductivity. In future, detailed investigation through in-depth structural studies using advanced techniques is necessarily required to understand the exact nature. Authors’ wants to mention here, as the samples are synthesized using facile solution cast technique, their thickness can be reduced by many fold resulting in much lower ESR, which is vital for device fabrication.
Conductivity seems to have correlation with granule size of the host starch. The bigger the host starch higher the conductivity is. It may be correlated to increased free volume for cluster formation and hence bigger clusters are formed resulting in higher conductivity for thesystems having bigger granules of host matrix.
Temperature and Humidity dependence
Stability of materials with respect to its ambient change is an important factor for its applicability in any devices. Hence the three systems under investigation have been studied at different temperature and humidity.Temperature dependence of conductivity for ambient variation is indicated inFigure 6(b). In normal range of temperature variation the conductivity value is almost constant. The humidity variation of conductivity represented in Fig. 6(c)indicates a nominal effect of ambient humidity on conductivity. The stable value of conductivity with respect to ambient variation makes it a potential candidate for commercial applicationfor a wide relative humidity variation i.e. from 25–99%.
Relaxation time analysis
Relaxation time defines the minimum time required to discharge complete energy from the device with efficiency greater than 50% of its maximum value, which is crucial for understanding the molecular dynamics of the electrolyte and performance of the device. It is identified by maxima of the C” curve shown in Fig. 6(f), accordingly, relaxation time is calculated by reciprocal of corresponding frequency. Relaxation time is proportional to amylose content and can be correlated with breaking of starch molecules in smaller units which is in coherence with FTIR pattern. Strikingly, relaxation time does not directly follow the conductivity pattern. XRD analysis underlined that ClO4− structure is maximum deformed in AS system and minimum in CS. It appears that deformation of anion structure also has strong signature on the ion relaxation time.
Generally, relaxation time is expected to decrease with increasing temperature due to possible faster ion transport. In present study, as indicated in Fig. 6(d), for all three systems, after initial decrease the relaxation time starts increasing with temperature. Two possible factors may be responsible for this behavior; a) the conductivity is very high i.e. the number of charge carriers is also very high akin to metallic behavior, furthermore, increased collisions between ions with increase in temperature results in larger relaxation time, b) water molecules are coming out of the matrix since water is playing an important role in the lattice formation and keeping the ions in dissociated form, hence, it is affecting the ion relaxation time. A detailed study is required to understand it and comment authentically on relation of relaxation time and temperature.
Linear Sweep Voltammetry (LSV) Analysis
Value of ESW provides the operational potential range of electrolyte [39–40] and is determined by LSV. Energy density(E = CV2/2) and power density(V2/4R) of the electrolyte depends on electrochemical stability window (ESW). To attain high energy density, high value of capacitance (C) which depends on the structure of electrode and high value of V which depends on ESW of electrolyte is vital. As, energy and power density depend on square of the potential, ESW of electrolyte becomes crucial.Low ESW is a big disadvantage associated with aqueous/ liquid electrolytes.For aqueous electrolytes, small ESW (< 1.5V) results in low energy and power density. LSV of synthesized systems is given in Fig. 6(e).
In the present study, all the synthesized systems have achieved wide ESW(~ 3V). The values are summarized in Table 1.It is crucial to mention that many other solid electrolytes have ESW in this range, but solid electrolyte suffers from high relaxation time, resulting in poor device performance. The synthesized material has an edge over other solid electrolytes because its conductivity is very high and relaxation time is very low, hence, it is apotential candidate for device fabrication.