Introduction

Solid polymer electrolytes based on natural polymers are exciting prospects for use in electrochemical devices [1–3]. Natural polymers possess advantageous properties such as non-toxic, naturally degrade, and low production cost [4, 5]. Several natural polymers have been used as electrolyte’s host, e.g., chitosan [3], agar [6], and starch [7]. Starch is a mixture of linear amylose (poly-α-1,4-d-glucopyranoside) and branched amylopectin (poly-α-1,4-d-glucopyranoside and α-1,6-d-glucopyranoside) [8]. A lot of solvents have been used to dissolve starch such as water [7, 9, 10], dimethylsulfoxide (DMSO) [11], and N-methylmorpholine-N-oxide (NMMO) [12, 13]. However, both DMSO and NMMO are poisonous and unrecyclable [14]. Thus, in this work, we used a water-based solvent to dissolve starch since water is the most abundant and greenest solvent on earth [14]. Starch is a hydrophilic material and can form a mechanically poor film [15, 16]. The mechanical strength and hydrophobicity of starch film can be improved by using acetic acid as the solvent [17, 18]. It is reported that when starch reacts with an acid, the water solubility of the starch granules is enhanced [19]. Tiwari et al. [10, 20] and Kumar et al. [21] have studied different types of starches as polymer hosts in electrolyte. According to Kumar et al. [21], morphology of potato starch is better in comparison with other starches.

Incorporation of salt to the polymer is an important feature to provide ions as the charge carriers. Lithium salts such as lithium hexafluorophosphate (LiPF6) [22], lithium perchlorate (LiClO4) [2], and lithium triflate (LiCF3SO3) [23] are commonly preferred in the studies on polymer electrolyte since lithium is the lightest of all metals and could provide the largest possible potential window [24]. However, electrochemical devices that use lithium-salt-based electrolyte have several disadvantages, such as high cost, difficulty in handling lithium electrodes, and safety hazard [25]. Magnesium salt appears to be a potential alternative to lithium salt. Magnesium is non-toxic, less reactive toward oxygen and water, may be handled safely in open air, and hazards are minimized [26].

Various ways have been employed to improve the conductivity of solid polymer electrolytes which are incomparable to those of liquid electrolytes. The addition of plasticizers such as ethylene carbonate (EC) [27, 28], dimethylacetamide (DMA) [29], and dibutyl phthalate (DBP) [30] has been reported to enhance the conductivity. The most common plasticizers for starch-based films are polyols, such as glycerol [31]. The addition of glycerol to the starch-based electrolyte is reported to enhance the cation mobility which in turn increases the ionic conductivity [32]. Based on our previous work [33], the addition of glycerol in a starch-based electrolyte can assist the dissociation of salt by weakening the Coulombic force between anion and cation. The increase in the degree of salt dissociation increases the number density of ions which can lead to conductivity enhancement [28].

The incorporation of ionic liquids (ILs) to the polymer electrolytes is another alternative to enhance the ionic conductivity [34, 35]. Generally, ionic liquid consists of a large asymmetrical cation and a weakly coordinating inorganic or organic anion [35]. Ionic liquids have attracted attention due to their good chemical and electrochemical stability, non-flammability, and high ionic conductivity [36]. Ramesh et al. [9] reported that the incorporation of 1-butyl-3-methylimidazolium hexafluorophosphate (BmImPF6) into starch-lithium hexafluorophosphate (LiPF6)-based electrolyte increased the ambient temperature conductivity from ∼10−7 S cm−1 to (1.47 ± 0.02) × 10−4 S cm−1. In this work, starch-based electrolytes were doped with magnesium acetate, Mg(C2H3O2)2. The aim of this work is to investigate the effect of glycerol and 1-butyl-3-methylimidazolium chloride (BmImCl) on the conductivity and dielectric properties of the electrolytes.

Experimental

Electrolyte preparation

For the preparation of salted system, 2 g of potato starch (Sigma-Aldrich) was dissolved in 50 mL of 1 % acetic acid solvent (SYSTERM) at 80 °C for 20 min. After the solutions cooled to room temperature, different amounts of Mg(C2H3O2)2 (R&M Chemicals) were added and stirred at room temperature until complete dissolution. For the preparation of plasticized system, different amounts of glycerol (SYSTERM) were added to the highest conducting salted electrolyte solution and stirred at room temperature until complete dissolution. For the preparation of IL-based system, different amounts of BmImCl (Sigma-Aldrich) were added to the highest conducting plasticized electrolyte solutions and stirred at room temperature until complete dissolution. All solutions were then poured into different plastic Petri dishes and left to dry at room temperature to form films. The dried films were stored in a desiccator filled with silica gel desiccants for further drying process. The compositions of all electrolytes in salted, plasticized, and IL-based systems are tabulated in Tables 1, 2, and 3, respectively.

Table 1 Composition and designation of electrolytes in the salted system
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Table 2 Composition and designation of electrolytes in the plasticized system
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Table 3 Composition and designation of electrolytes in the IL-based system
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Electrolyte characterization

The interactions of polymer-salt, polymer-salt-plasticizer, and polymer-salt-plasticizer-IL were determined by Fourier transform infrared (FTIR) spectroscopy using the Spotlight 400 Perkin-Elmer spectrometer in the wavenumber range of 450–4000 cm−1 at a resolution of 1 cm−1. The measurements were carried out at room temperature.

Electrochemical impedance spectroscopy (EIS) measurements were conducted using HIOKI 3532-50 LCR HiTESTER in the frequency range of 50 Hz to 1 MHz at room temperature. The electrolytes were placed between two stainless steel blocking disc electrodes under spring pressure. The values of bulk resistance (R b) were determined from the Cole-Cole plots obtained from the EIS measurements. Conductivity (σ) was calculated using the following equation:

$$ \sigma =\frac{d}{R_{\mathrm{b}}A} $$
(1)

where d is the electrolyte’s thickness and A is the contact area of electrode and electrolyte.

The transference number of ion (t ion) was measured using DC polarization method [37]. A cell consisting of the highest conducting electrolyte sandwiched by two stainless steel blocking electrodes was polarized using V&A Instrument DP3003 digital DC power supply at 0.50 V. The DC current was monitored as a function of time. The measurement was done at room temperature.

Results and discussion

FTIR analysis

FTIR spectra of electrolytes in the salted system in the hydroxyl band region are shown in Fig. 1a. The hydroxyl band peak in the spectrum of S0 appears at 3312 cm−1. The hydroxyl band peak appears at 3298 cm−1 in the spectrum of S5. The hydroxyl band peak is observed to appear at a lower wavenumber as the salt content increases up to 30 wt.%. From the literature [20, 38, 39], the band shift indicates the interaction between the polymer and salt. Thus, the shifting of hydroxyl band peak with increasing salt content proves that the cation (Mg2+) of salt interacts with the oxygen atom of the hydroxyl group. The oxygen atoms in polymer have negatively charged electron pairs so that the positively charged cations can coordinate at such atoms [40]. This kind of interaction is called dative bond. According to Stygar et al. [41], although interaction between polymer and salt mainly occurs at the oxygen atom, other bands can also be affected. The FTIR spectra of electrolytes in the salted system in the region of 2800–2980 cm−1 are shown in Fig. 1b. A peak which is assigned to C–H stretching mode of starch appears at 2925 cm−1 in the spectrum of S0. The peak is observed to shift with the addition of salt. From our previous work [33], C–H stretching mode of corn starch has been shifted on addition of ammonium bromide (NH4Br) which is comparable with the present result.

Fig. 1
figure 1

FTIR spectra for electrolytes in the salted system in the wavenumber region of a 3000–3600 cm−1 and b 2800–2980 cm−1

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Figure 2 depicts the FTIR spectra of electrolytes in the salted system in the region of 955–1055 cm−1. This region represents the anhydroglucose ring of O–C stretch [42]. In the spectrum of S0, the peak is located at 993 cm−1. The peak is observed to shift with the addition of salt indicating the interaction between cation of salt and oxygen atom in anhydroglucose ring of starch.

Fig. 2
figure 2

FTIR spectra for electrolytes in the salted system in the wavenumber region of 955–1055 cm−1

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FTIR spectra of electrolytes in the plasticized system in the hydroxyl band region are shown in Fig. 3. The hydroxyl band peak in the spectrum of P5 appears at 3276 cm−1, which is lower than the hydroxyl band peak position of S20. As the glycerol content increases up to 30 wt.%, the hydroxyl band peak is observed to shift to a lower wavenumber. From a report by Yusof et al. [43], the hydroxyl band peak of starch-chitosan-ammonium iodide (NH4I) has shifted to the lower wavenumber on addition of glycerol. The addition of glycerol promotes ion dissociation so that more ions interact with the polymer host at the hydroxyl band. Besides, glycerol is able to form hydrogen bonding with the polymer host [44]. These phenomena have been evidenced by the shifting of FTIR spectra in Fig. 3. The hydroxyl band peak has shifted back to the higher wavenumber in the spectra of P35 and P40. High concentration of plasticizer can lead to the decrease in amorphousness of electrolyte; thus, less ions interact with the polymer host [1]. It can be noted that the intensity of the hydroxyl band peak increases as the glycerol content increases. This result has further proven the interaction between Mg2+ cations and oxygen atoms in the hydroxyl band.

Fig. 3
figure 3

FTIR spectra for a S20, P5, P10, P15, P20, and P25 and b P25, P30, P35, P40, and pure glycerol in the wavenumber region of 3000–3600 cm−1

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Another evidence of starch-Mg(C2H3O2)2-glycerol interaction can be observed in the region of 955–1055 cm−1 as shown in Fig. 4. The peak of anhydroglucose ring of O–C stretch in the spectrum of S20 is located at 997 cm−1. The peak is observed to shift with the addition of glycerol. It can be inferred that more ions interact with the oxygen atom in anhydroglucose ring of starch. There is also possible hydrogen bonding between glycerol and starch in this region.

Fig. 4
figure 4

FTIR spectra for selected electrolytes in the plasticized system in the wavenumber region of 955–1055 cm−1

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FTIR spectra of electrolytes in the IL-based system in the hydroxyl band region are shown in Fig. 5a. On addition of 5 wt.% BmImCl, the hydroxyl band peak shifts from 3255 to 3272 cm−1. The hydroxyl band peak is observed to shift up to 3292 cm−1 as the BmImCl content increases to 18 wt.%. Another proof can be observed by the shifting of the peak of anhydroglucose ring of O–C stretch as shown in Fig. 5b. From a report by Sim et al. [35], the addition of ionic liquid into polymer-salt complex increases the amount of free ions. According to the authors, the cations from ionic liquid can coordinate at the polar atoms of polymer host. Thus, from Fig. 5, it can be indicated that the cation (BmIm+) of BmImCl interacts with the oxygen atom of polymer. This result shows that ionic liquid can provide ions which can lead to the increase in conductivity as has been reported in the literature [35, 45, 46].

Fig. 5
figure 5

FTIR spectra for electrolytes in the IL-based system in the wavenumber region of a 3000–3600 cm−1 and b 955–1055 cm−1

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Room temperature conductivity

In the salted system, the incorporation of 20 wt.% Mg(C2H3O2)2 has maximized the room temperature conductivity to (2.44 ± 0.37) × 10−8 S cm−1 as shown in Fig. 6a. The increase in conductivity as the salt content increases to 20 wt.% is attributed to the increase in the number of mobile charge carriers [23]. On addition of 25 wt.% Mg(C2H3O2)2, the conductivity decreases to (7.82 ± 1.30) × 10−9 S cm−1 and further decreases to (4.01 ± 0.65) × 10−9 S cm−1 with addition of 30 wt.% Mg(C2H3O2)2. This phenomenon is attributed to the aggregation of the ions, leading to the formation of ion clusters, thus decreasing the number of mobile charge carriers [47]. Different amounts of glycerol were added to the highest conducting electrolyte in the salted system in order to enhance the conductivity. From Fig. 6b, as the glycerol content increases to 30 wt.%, the room temperature conductivity is enhanced to (2.60 ± 0.42) × 10−6 S cm−1. Plasticization using glycerol has created alternative pathways for ion conduction leading to conductivity enhancement [48]. Besides, the addition of glycerol promotes ion dissociation which decreases the formation of ion aggregates and increases the number of mobile charge carriers. This phenomenon enhances the conductivity. On addition of more than 30 wt.% glycerol, the decrease in conductivity may be caused by the formation of linkages between the molecules of plasticizer causing the salt to recrystallize, resulting in a conductivity decrement [49, 50]. When too many plasticizer molecules exist within the electrolyte, the cations hardly coordinated at the polar atoms. This is because there is competition between the cations and the plasticizer molecules to interact with the polar atoms of polymer host or plasticizer molecules, thus increases the possibility of ion association which can decrease the conductivity.

Fig. 6
figure 6

Conductivity at room temperature for electrolytes in the a salted system and b plasticized system

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To further enhance the conductivity, different amounts of BmImCl were added to the highest conducting electrolyte in the plasticized system. From Fig. 7, as the BmImCl content increases to 18 wt.%, the room temperature conductivity is enhanced to (1.12 ± 0.08) × 10−5 S cm−1. According to Kumar et al. [51], the addition of ionic liquid affects the properties of the polymer electrolytes in different ways. Generally, the conductivity enhancement can be related to the increase in free ions after the incorporation of ionic liquid. From the FTIR results, cations from the ionic liquid have interacted with the polar atoms of polymer. Thus, in the present electrolyte system, four ions, i.e., Mg2+ ion, the cation of the ionic liquid and the anions from both the salt and ionic liquid contribute to the ionic conductivity [52, 53]. The low viscosity of ionic liquid enhances the flexibility of the polymer backbone, thus increases the segmental motion [51]. Besides, the high dielectric constant of ionic liquid assists ion dissociation [54]. All these phenomena favour the ionic conductivity enhancement.

Fig. 7
figure 7

Conductivity at room temperature for electrolytes in the IL-based system

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Dielectric analysis

The conductivity trend can be further verified by dielectric studies. Dielectric constant (ε r) represents charge stored in a material [55]. The values of ε r were calculated from the following equation:

$$ {\varepsilon}_{\mathrm{r}}=\frac{Z\mathrm{i}}{\omega {C}_{\mathrm{o}}\left({Z_{\mathrm{r}}}^2+{Z_{\mathrm{i}}}^2\right)} $$
(2)

where ω is angular frequency, C o is vacuum capacitance, while Z r and Z i are real and imaginary parts of impedance, respectively. From Fig. 8, it is observed that higher conducting electrolyte has the higher value of ε r at all frequencies. The increasing charge stored in the electrolyte means that the number density of mobile ions has increased [55]. This phenomenon increases the conductivity. Reports from the literature showed that the dielectric constant result is in good agreement with the conductivity result [56–59]. The decrease in ε r values with the increase in frequency is attributed to the electrode polarization effect [60]. As frequency increases, the periodic reversal of the electric field occurs so rapidly which disables the charge carriers from orienting themselves in the field direction, resulting in the decrease of ε r [61].

Fig. 8
figure 8

Frequency dependence of a ε r and b ε i at room temperature for the highest conducting electrolyte in each system

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The information of the relaxation phenomena of the highest conducting electrolyte in salted, plasticized, and IL-based systems is obtained from the plot of loss tangent (tan δ) as a function of frequency in Fig. 9. The value of tan δ was calculated using the following equation:

$$ \tan \kern0.5em \delta =\frac{\varepsilon_{\mathrm{i}}}{\varepsilon_{\mathrm{r}}} $$
(3)
Fig. 9
figure 9

The dependence of tan δ on frequency for a S20 and b P30 and B18 at room temperature

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The maximum of tan δ (tan δ max), which represents the relaxation peak, is located at a higher frequency for the higher conducting electrolyte. The relaxation time (t r) for each electrolyte was obtained from the relation as follows:

$$ {t}_{\mathrm{r}}{\omega}_{\mathrm{peak}}=1 $$
(4)

where ω peak is the angular frequency of the relaxation peak. The occurrence of relaxation time is the result of the efforts carried out by ionic charge carriers to follow the change of the direction of the applied field [62]. The values of t r for the electrolytes are presented in Table 4. The highest conducting electrolyte (B18) possesses the lowest t r value of 9.36 × 10−6 s. Other reports also show that the higher conducting electrolytes have the lower values of t r [44, 62, 63].

Table 4 Relaxation time of the highest conducting electrolyte in each system at room temperature
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The distribution of relaxation times can be described by Kohlrausch-Williams-Watts law [64]:

$$ \phi (t)= \exp \left[-{\left(\frac{t}{t_{\mathrm{r}}}\right)}^{\beta}\right] $$
(5)

where ϕ(t) describes the time evolution of the electric field within a material and β is the Kohlrausch exponent. In the present work, the values of β for S20, P30, and B18 electrolytes at room temperature are determined from the full width at half maximum (FWHM) of the normalized plot of tan δ/(tan δ)max against f/f max in Fig. 10 using the following equation:

$$ \beta =\frac{1.14}{\mathrm{FWHM}} $$
(6)
Fig. 10
figure 10

Normalized plot of tan δ/(tan δ)max against f/f max for a S20 and P30 and b B18 at room temperature

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For a typical Debye peak, the value of FWHM is 1.14 decades, which gives β = 1 for Debye relaxation [65, 66]. For a practical solid electrolyte, the value of β is less than 1 [67]. It is found that the values of β for S20, P30, and B18 electrolytes at room temperature are 0.58, 0.85, and 0.83, respectively. Since the values of β are lower than 1, it can be inferred that the relaxation deviates from Debye relaxation.

The real part and imaginary part of electrical modulus were calculated using

$$ {M}_{\mathrm{r}}=\frac{\varepsilon_{\mathrm{r}}}{\left({\varepsilon_{\mathrm{r}}}^2+{\varepsilon_{\mathrm{i}}}^2\right)} $$
(7)
$$ {M}_{\mathrm{i}}=\frac{\varepsilon_{\mathrm{i}}}{\left({\varepsilon_{\mathrm{r}}}^2+{\varepsilon_{\mathrm{i}}}^2\right)} $$
(8)

where M r and M i are real and imaginary parts of electrical modulus, respectively. Figure 11a shows the variation of M r with frequency for the highest conducting electrolyte in each system at room temperature. The values of M r increase as the frequency increases, but no definitive peaks appear in the M r plots. The same result has been reported by Shukur et al. [68] for chitosan-polyethylene oxide (PEO)-ammonium nitrate (NH4NO3) system. Figure 11b shows the variation of M i with frequency for the highest conducting electrolyte in each system at room temperature. A peak is observed in the M i plot of S20 electrolyte indicating that the electrolyte is an ionic conductor [69]. The long tail of small values at lower frequencies in the M i plots of P30 and B18 electrolytes is mainly due to high capacitance values which associated with the electrode, as a result of accumulation of charge carriers at the electrode-electrolyte interfaces [7, 70, 71].

Fig. 11
figure 11

The dependence of a M r and b M i on frequency for the highest conducting electrolyte in each system at room temperature

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Transference number analysis

To detect the type of charge carriers in the polymer electrolytes, the transference number measurements were carried out. By sandwiching the electrolyte with charge carrier transparent electrodes, transference number of the charge carrier can be known from the ratio of steady-state current (I ss) to the initial current (I i). According to Kufian et al. [72], electrons are transparent to the ion blocking stainless steel electrodes. Thus, in the present work, stainless steels were used as the electrodes for the determination of transference number of electron (t e). By knowing t e, the value of t ion of the electrolytes was calculated using

$$ {t}_{\mathrm{ion}}=1-{t}_{\mathrm{e}} $$
(9)

Or

$$ {t}_{\mathrm{i}\mathrm{on}}=\frac{I_{\mathrm{i}}-{I}_{ss}}{I_{\mathrm{i}}} $$
(10)

If ions are the dominant charge carriers, the current flows through the electrode will decrease rapidly with time, while if electrons are dominant, the current would not decrease with time [1, 33, 73]. The plots of polarization current against time for B18 electrolyte is shown in Fig. 12. The current is observed to decrease rapidly initially, before being saturated at 2.3 μΑ. This result shows that the electrolyte is an ionic conductor. The value of t ion for B18 electrolyte is 0.92, further evidences that ions are the dominant charge carriers. The t ion values for other magnesium ion conducting solid polymer electrolytes [74–76] have been reported to be >0.90, which is comparable with the present result.

Fig. 12
figure 12

Transference number of B18 electrolyte using stainless steel electrodes

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Conclusions

Potato starch doped with Mg(C2H3O2)2-based electrolytes was successfully prepared via solution cast technique. FTIR analysis proved the interaction between the electrolyte materials. In the salted system, the electrolyte with 20 wt.% Mg(C2H3O2)2 (S20) has a conductivity of (2.44 ± 0.37) × 10−8 S cm−1. In the plasticized system, the addition of 30 wt.% glycerol (P30) has increased the conductivity to (2.60 ± 0.42) × 10−6 S cm−1. In the ionic liquid-based system, the addition of 18 wt.% BmImCl (B18) has further enhanced the conductivity to (1.12 ± 0.08) × 10−5 S cm−1. Dielectric analysis verified the conductivity result. The relaxation time (t r) of electrolytes was determined from the loss tangent (tan δ) plot where higher conducting electrolyte has lower t r value. Scaling of tan δ showed that the ionic relaxation was non-Debye type. From transference number measurement of B18 electrolyte, ions were found to be the dominant charge carriers with t ion of 0.92.