Glycerol as an efficient plasticizer to increase the DC conductivity and improve the ion transport parameters in biopolymer based electrolytes: XRD, FTIR and EIS studies

2.1 Materials and electrolyte preparation

The host polymeric raw materials were polyvinyl alcohol (PVA) and methylcellulose (MC) powders, the ionic source was sodium iodide (NaI) salt, and the plasticizer was glycerol. Sigma-Aldrich provided all of the chemical components (Kuala Lumpur, Malaysia). The primary solution set was made by dissolving 0.6 g of PVA (at 80 °C) and 0.4 g of MC at ambient temperature in distilled (40 mL) water in closed caps separately using a solution cast process. For 24 h, using a magnetic stirrer the solutions were constantly stirred till a homogeneous aqueous solution was achieved. The PVA solution was then cooled to room temperature before being combined with MC. The procedure is completed by stirring the PVA:MC mixing till flawless solutions is obtained.

The solution was then given a constant weight percent of 50 wt% of NaI salt. Lastly, plasticized ion-conducting PEs was made with glycerol. Different amounts of glycerol plasticizer (10, 20, 30, and 40 wt%) were added to the PVA:MC:NaI solution independently, and then stirred continuously for 24 h in closed caps until clear solution was achieved. An image of the prepared sample is shown in Fig. 1. The solutions were placed in numerous Petri dishes (8 cm in diameter) and allowed to rest at room temperature to generate a dry film free of solvent. The flexible films were then made with a thickness of roughly 0.026–0.03 cm. PVA:MC-NaI systems plasticized with different levels of glycerol were coded as PNVI0, PNVI1, PNVI2, PNVI3, and PNVI4, which linked to 0, 10, 20, 30, and 40 percent of glycerol, respectively. The films compositions are revealed in Table 1.

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Fig. 1

Image of the PVNI4 sample.

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2.2 Structural and impedance analyses

Using a Siemens D5000 X-ray diffractometer (1.5406 Å), X-ray diffraction (XRD) was taken to investigate the structure of the materials. Fourier transform infrared spectroscopy (Spotlight 400 Perkin-Elmer spectrometer) was used with a 1 cm⁻¹ resolution in the range 750 to 4000 cm⁻¹. Electrical impedance spectroscopy (HIOKI 3532-50 LCR Hi-TESTER) was used to investigate the samples' electrical properties in the frequency range of 50 Hz to 5 MHz. The samples were sandwiched between two stainless steel electrodes for electrical evaluation.

The conductivity of the sample was calculated using the Eq. (1) (Asnawi et al., 2021), based on the results of electrical impedance spectroscopy experiments and the dimensions of the samples:

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Fig. 2

Diagram of the cell used for EIS measurement.

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3.1 FTIR analysis

The high absorption between 3000 and 3800 cm⁻¹ is thought to be due to hydroxyl stretching vibration. C—H rocking in PVA has been allocated to the peak at the 850 cm⁻¹ (Fig. 3). In doped PE solutions, this peak is displaced. The bands at 1094 cm⁻¹ and 1320 cm⁻¹ relate to the pure PVA's —C—O— stretching and C—H deformation, respectively (Rajendran et al., 2004). In the complexed systems of polymer/salt materials, this peak is disappeared (Fig. 3). In the electrolyte system, CH₂ bending exhibited at 1432 cm⁻¹ in pure PVA is moved to higher wave number (Awadhia and Agrawal, 2007). In the SPE system, a powerful and broad absorption peak positioned at 3376 cm⁻¹ is shifted, which would be attributed to the OH stretching with powerful hydrogen bonding of inter and intra form of PVA (Aziz et al., 2021a).

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Fig. 3

FTIR spectra for all plasticized electrolyte films. The strength of the peaks decreased as the glycerol concentration increased.

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In the doped system, the C⚌O stretching of PVA at 1736 cm⁻¹ is observed to be displaced to smaller wave number. This evidently demonstrates that the interaction between PVA and NaI is caused not just by the O—H groups of pure PVA, but also by the C—O groups (Yua et al., 2003). The interaction between the salt and C—H bond of the backbone of the polymer is revealed by the disappearance of the peaks. Furthermore, the lack of a week peak at 1701 cm⁻¹ (identified as C—O stretching mode) indicates that the carbonyl functional group plays a role in proton transport in polymer complexes (Liew et al., 2015).

Changes in the vibrational spectra are also noticeable, the O—H stretching of PVA at 3315 cm⁻¹ significantly decreased, along with the distinctive vibrational spectra modifications. At 1105 cm⁻¹, it was accompanied by an increase in the (C—O) ester mode. Aldehydes are also linked to two major peaks in (C—H) extending at 2934 and 2862 cm⁻¹. These and other FT-IR spectrum shifts illustrated in Fig. 3 reveal that there is strong cross-linking and acetal yields at the OH sites of PVA with aldehyde groups (—CHO) (Qiao et al., 2005).

3.2 XRD analysis

The results of XRD examination for various glycerol concentrations are shown in Fig. 4. A large XRD peak at 19.5° correlates to the amorphous form of pure PVA, according to previous observations (Malathi et al., 2010). The strength of the peak of PVA diminishes with the loading of salt concentration, and the wide nature of the peak is found to be increased in the current investigation (Hodge et al., 1996). The amorphous nature of the PEs is confirmed by this result. The amorphous appearance of the films is revealed by the broadening of the peak. Hodge et al. mentioned a relation between the degree of crystallinity and height of the peak, can be used to interpret these findings (Hodge et al., 1996; Hema et al., 2008). There are no further peaks in the plasticized system, indicating that the salt has completely dissociated in the polymer matrix. The amorphous nature of the PVA:MC:NaI:glycerol PEs was discovered as a result of the rupture of the PVA:MC crystallinity.

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Fig. 4

X-ray pattern for (a) PNVI1, (b) PNVI2, (c) PNVI3, and (d) PNVI4.

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For some systems, this amorphous nature causes to increase ionic diffusion with high ionic conductivity (Bhargav et al., 2009; Ahad et al., 2012). As a result of the XRD investigation, the complex development in the polymer matrix is revealed. In the current study, the XRD pattern revealed two hallows and smaller crystalline peaks (Fig. 4). PVA:MC is not as crystalline as pure PVA, as evidenced by the wide hallows, and its structure is nearly amorphous (Aziz et al., 2010; Yusuf et al., 2016). According to earlier research, the amorphous structure of PEs is linked to wide diffraction peaks (Malathi et al., 2010; Nofal et al., 2021; Aziz, 2016). It is crucial to deconvolute the XRD pattern for each film to discover the areas of the crystalline and amorphous peaks in order to calculate the crystallinity degree (Aziz, 2016). Eq. (2) was used to calculate the degree of crystallinity (Xc), which is reported in Table 2 (Wan et al., 2003);

3.3 Impedance study

The impedance spectra for the plasticized samples are shown in Fig. 5. Clearly, the conductivity improved as the glycerol content increases, reaching a maximum of 2.53 × 10⁻⁴ for a 40 wt% of glycerol. As shown in Table 3, the highest conductivity can be linked to the maximum amorphousness reported for this composition. Due to the minimal amount of free charge carriers in the electrolytes at low glycerol concentration, conductivity is low. The number density of charge carriers increases when the concentration of glycerol increases, which improves conductivity. It's possible that the low conductivity at low glycerol levels is due to the presence of neutral ion aggregates that do not participate to ionic conduction (Qiao et al., 2010; Hadi et al., 2020; Noor et al., 2014).

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Fig. 5

Impedance plot for (a) PNVI1, (b) PNVI2, (c) PNVI3, and (d) PNVI4.

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The inclusion of plasticizers improves ionic conductivity by increasing segmental mobility (Noor et al., 2014; Girish Kumar and Munichandraiah, 2000). The membrane conductivity at ambient temperature was shown to be dependent on the glycerol concentration. Because of the hydrophilic nature of glycerol (Qiao et al., 2010), excess NaI may be able to stay fixed in the polymer matrix interspaces, resulting in increased conductivity.

The polymer chain, on the other hand, obtains quicker internal modes as the amorphous area grows, in which bond rotations cause segmental motion. As a result, hopping intra-chain and inter-chain movements are favored, and the polymer's conductivity rises (Bhargav et al., 2007). The experimental impedance data are simulated with electrical equivalent circuit (EEC) to verify our view, as shown in Fig. 5. The fitting parameters are also shown in Table 3. It is important to note that the semicircle has totally vanished for 40 wt% of glycerol in the high frequency region, as illustrated in Fig. 5(d).

It can be concluded the polymer host body only responds to the resistive component of the measured impedance spectrum (Samsudin and Isa, 2012). The electrical behavior of the system is represented in this example by the electrolyte film resistor in series with the capacitor of the double-layer capacitance.

As a result, the energy barrier is lowered, and ion movement is enhanced (Sahli and Bin Ali, 2012). The electrolyte conductivity is dependent on both the mobility and number of ions (Samsudin et al., 2011). Table 4 shows that as glycerol content increased, both diffusion and mobility increased. DC will improve by either increasing carrier density or increasing mobility. The system mechanism under study can be easily dealt with by fitting and analyzing experimental spectrum by the EEC method (Aziz et al., 2021b; Pradhan et al., 2011). Fig. 5 (a-d) shows the experimental impedance graphs for the electrolyte films. The electrical characteristics of solid-based electrolyte polymers may be calculated and understood using this approach.

In the high frequency region, two elements, R_b and CPE1, are obtained; put it differently, both are replied in the high frequencies, whilst CPE2 is linked to the low frequency spike region. The impedance of Z_CPE can be calculated as (Aziz et al., 2019a; Shukur et al., 2014; Aziz et al., 2019b; Teo et al., 2012; Aziz et al., 2019c; Aziz and Abdullah, 2018):

CPE capacitance is denoted by C, angular frequency is represented by ω, and p in complex impedance graphs is related to the plot's displacement from the vertical axis.

It is worth noting that in the context of EEC modeling, CPE is the most usually used acronym instead of capacitor. It is possible to express the real (Z_r) and imaginary (Z_i) complex impedance (Z*) values for the PVNI1, PVNI2 and, PVNI3 systems in the analogous circuit using the following mathematical expression (Aziz et al., 2019a; Shukur et al., 2014; Aziz et al., 2019b; Teo et al., 2012; Aziz et al., 2019c):

R_b is the bulk resistance in this case. The impedance charts in the experiments are properly replicated using Eqs. (5) and (6), as seen in Fig. 5, and the EECs are shown in Fig. 5. Table 3 shows the blend electrolytes' corresponding circuit element properties. The whole conductivity, attributable to massive ion movement, is the reasonable justification for this outcome, where the semicircle vanished at the high frequency region for the PVNI4 system (Rajendran et al., 2008; Aziz et al., 2019d). The values of Z_r and Z_i are connected to the EEC in the PVNI4 system, and can be stated mathematically as (Aziz et al., 1994):

As the PVNI1, PVNI2, and PVNI3 systems composed of a tail and a semicircle, the diffusion coefficient (D), mobility (μ), and number density (n) of ions are determined by the given equations (Brza et al., 2021; Brza et al., 2020):

The D of the ions in PVNI1, PVNI2, and PVNI3 systems is obtained using the relation,

The µ of the ions is calculated using the relation of Nernst–Einstein,

Since σ_DC is shown by

So, the n of ions is determined using Eq. (11):

As the system of PVNI4 composed of only a tail, the D is determined using Eq. (12) (Brza et al., 2021; Brza et al., 2020):

Here $l$ is the electrolyte thickness, ω_min is the angular frequency corresponding to the minimum Z_i. Table 4 shows the ion transport parameters for the polymer electrolyte systems.

Based on Table 4, the value of D is observed to increase (from 2.90 × 10⁻⁹ to 4.59 × 10⁻⁸ cm² s⁻¹) when the glycerol increased from 10 wt% to 40 wt%. The same trend is observed by μ as shown in Table 4. The increase of μ (from 1.13 × 10⁻⁷ to 1.79 × 10⁻⁶ cm² V⁻¹ s) and D is associated to the engagement of the polymer chains flexibility with the presence of glycerol while the number density of ions decreased (Brza et al., 2020). Glycerol was used in this work to improve the conductivity of ions owing to the three hydroxyl groups (OH) presence. The glycerol dielectric constant of 42.5 is considered to be large, which causes a decrease in the attraction force among the anions and cations of the salt, and among the polymer chains (Brza et al., 2020). The increase of the amorphous nature of the PE was considered to improve the ions mobility by creating a large free volume through the electrolyte. This causes the segmental movement of the polymer chains to increase due to the increase in the flexibility of the polymer chains. This can then improve the mobility and conductivity of the electrolyte systems (Brza et al., 2020; Yusof et al., 2019). When the glycerol increased, the values of D and μ increased which lead to the increase of conductivity. These results are in agreement with the results obtained from the FTIR test and the degree of crystallinity that measured from the XRD deconvolution.

3.4 Dielectric properties

By the following relationships, the real and imaginary parts of complex permittivity (ε*) were determined from the real (Z′) and imaginary (Z″) parts of complex impedance (Z*) (Nofal et al., 2020; Aziz and Abidin, 2015; Aziz and Abidin, 2014);

The frequency dependence of the complex dielectric constant for plasticized systems is seen in Figs. 6 and 7. By comparing with the other samples, the system incorporating 40 wt% glycerol has the uppermost dielectric constant. The high charge carrier concentration and the system amorphous nature can explain the increase in dielectric constant.

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Fig. 6

Dielectric constant v log (f) for all samples.

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Fig. 7

Dielectric loss v log (f) for all samples.

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Both dielectric constant (ɛ′) and dielectric loss (ɛ″) drop with frequency, with the fall being rapid at low frequency and saturation appearing at high frequency, as shown in the figurers. In PEs, this is a common pattern (Hema et al., 2009; Hadi et al., 2020). This can be explained by the fact that the drop in ɛ′ and ɛ″ with frequency can be attributed to dipoles' incapability to rotate quickly, resulting in a lag among the frequency of the applied field and oscillating dipole.

The variance designates that the dielectric constant is high at low frequencies due to interfacial polarization, and the dielectric loss (ε″) increases dramatically due to free charge migration within the material. Values of ε′ have been shown to be high at low frequencies. The dielectric constant has been found to be constant at higher frequencies due to the space charge effect presence, which are supplied by the concentration of carriers close to the electrodes (Abdullah et al., 2021).

This is due to charge carriers' failure to follow the electric field periodic reversal. The migration of carriers within the material is responsible for the significant value of ε″. The fluctuation of ɛ′ was different for polar and nonpolar materials. Polar polymers have a high ɛ′, which varies depending on temperature and plasticizer. The ɛ′ value rises as the plasticizer content of polar materials increases, owing to the ease with, which dipoles can be oriented and the increased carriers mobility, which be likely to pass easily as the temperature rises (Bhargav et al., 2009; Ahad et al., 2012).

Moreover, the frequency reliance of the dielectric loss tangent (tan δ) for electrolytes can be determined in order to have a better understanding of the electrolyte relaxation processes.

As formerly affirmed, the Z_r and Z_i data were extracted from the EIS data and then employed to resolve the ɛ′ and ɛ″ data. The tan δ is calculated using the dielectric parameters. The tan δ, also called the dissipation factor, is a ratio of dispersed energy to stored energy in a periodic field. The tan δ is calculated using the formula below (Pawlicka et al., 2019).

Loss tangent peaks are used to study the relaxation processes of polymer materials in great detail. Dielectric relaxation can be used to depict the dipoles in polymer electrolytes (Aziz and Abdullah, 2018; Marf et al., 2020). At room temperature, Fig. 8 illustrates the dielectric relaxation of the loss tan δ vs. frequency curve for each film. The loss tangent peak shifts to a high-frequency area in the figure, indicating the dielectric relaxation existence.

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Fig. 8

Tangent loss spectra versus frequency measured at ambient temperature for the PNVI electrolytes.

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The conductivity and dielectric relaxation peaks are caused by induced or persistent dipoles, according to this research. Induced diploes were also shown to hide the polarization relaxation of charge carriers in a material. (Aziz and Abdullah, 2018; Woo et al., 2012). The peaks in Fig. 8 represent translational ion dynamics, which is linked to mobile ion conductivity relaxation. For ion transport, this is favorable in electrolyte segmental motion (Aziz and Abdullah, 2018). The tan δ appears to rise as the frequency increases, which is owing to the active element's (ohmic) dominance over the reactive element (capacitive).

In light of the events, the lowering of tan δ at a higher frequency is most likely due to the active element's independency from the frequency, causing the reactive element to grow in response (Idris et al., 2007). The non-Debye behavior of the system is suggested by the relaxation process of the electrolytes, which is depicted by the tan δ plot (Mustafa et al., 2020). Furthermore, the tan δ value at the highest frequency is known as the tan δ maximum, tan δ_max value, which is used to calculate the angular frequency, ω_peak of the relaxation peak. As a result, each electrolyte's relaxation time (t_r) may be estimated by inverting the ω_peak (1/ω_peak).

Because electrode polarization is linked to the generation of charges near the electrodes, the electrical modulus analysis (Singh et al., 2011) can also be utilized to evaluate the dielectric characteristics produced by ion relaxation.

Using the following relationships, the real (Z′) and imaginary (Z″) parts of complex electric modulus (M*) were computed from the real (Z′) and imaginary (Z″) parts of complex impedance (Z*) (Aziz et al., 2021c; Marzantowicz et al., 2007; Asnawi et al., 2020);

Fig. 9 and Fig. 10 show that the electrical modulus standards in both graphs stay around zero at low frequencies. The long tail seen at low frequencies shows that electrolytes have a capacitive characteristic in which substantial electrode polarization occurs without any dispersion (Pawlicka et al., 2019; Saminatha Kumaran et al., 2018).

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Fig. 9

Electric modulus plot of M′ against log (f) for the PNVI samples.

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Fig. 10

Electric modulus plot of M″ against log (f) for the PNVI samples.

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The bulk effect reasons the M′ and M″ standards of the electrolytes to rise as the frequency increases. At high frequencies, the highest conducting electrolyte had the lowest electrical modulus values. Mustafa et al. and Saminatha Kumaran et al. have also reported similar observations (Mustafa et al., 2020; Saminatha Kumaran et al., 2018). Due to the irrelevant effect of electrode polarization, both M′ and M″ fall towards low frequencies. The enormous capacitance associated with the electrode causes the presence of a lengthy tail.

3.5 AC conductivity study

Fig. 11 shows the conductance spectra for plasticized systems with varying glycerol content. For three of the electrolytes, the spectra are divided into three regions: a low frequency dispersive zone, a plateau region, and a dispersive region at high frequency. Low frequency dispersion is detected at high glycerol content (40 wt%) because of electrode polarization effects.

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Fig. 11

σ_dc (S. cm⁻¹) versus log (f) for (a) PNVI1, PNVI2, and PNVI3 samples and (b) PNVI4 sample.

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The ionic conductivity is high enough at this glycerol concentration to cause a development of charges at the electrode's surface, lowering the applied electric field effect through the samples and thus the apparent conductivity (Aziz et al., 2021d; Ramesh and Arof, 2001). The dc conductivity is represented by the plateau. The period of the field is very short to create charges at higher frequencies, hence the ac conductivity is assumed to have independent on frequency, which is equivalent to dc conduction.

When the frequency drops, more charge gathers at the electrode' surface resulting in a reduction in the number density of ions and, finally, a decline in conductivity at low frequencies. The mobility of carriers is significant in the high frequency area, thus conductivity rises with frequency (Ramesh and Arof, 2001). The dc conductivity increases as the temperature rises, implying that the free volume surrounding the polymer chains is responsible for ion and polymer segment mobility (Miyamoto and Shibayama, 1973).