Introduction

In recent years, the global demand for energy storage devices have increased significantly primarily due to the rapid increase in portable power-consuming devices, for instance, cellular phones, laptops, and computers, amongst others. Therefore, in order to meet the aforesaid demand, polymer electrolytes (PEs) has shown appreciable traits as an alternative candidate owing to its high flexibility, lightweight nature, as well as, exceptional energy and power density storage. PEs have drawn considerable interest from the research community due to its conductive properties in applications such as solid-state battery [1], solar cell [2], electrochromic devices [3], electrical double-layer capacitor (EDLC) [4] and fuel cell [5]. It is also worth to note that, a dramatic increase in the usage of renewable and biodegradable resources has been demonstrated in the past couple of decades, mainly to cater for a sustainable future. Such resources have and are still being extensively investigated to replace synthetic polymers that are detrimental to the environment.

Therefore, to mitigate this issue, bio-based polymers have been introduced in PEs development, which evidently has been reported to have more superior properties as compared to the conventional synthetic polymer [6]. Amongst its desirable traits are biodegradable, non-toxic, low-cost, abundant and eco-friendly [7]. A plethora of bio-based polymer electrolytes have been discovered, for instance, carboxyl methylcellulose (CMC) [8], chitosan [9], starch [10], cornstarch [11], and carrageenan [12]. These bio-polymers have been reported to provide a favorable ionic conductivity (~10−6 to 10−4 S cm−1 at ambient temperature). Alginate has exhibited a plausible candidate as the backbone polymer matrix in a bio-based polymer electrolytes (BBPEs) systemYang et al. [13]. It was reported to have good conduction and mechanical stability when added with appropriate ionic dopants. Alginate is anionic polysaccharides, which consists of both homopolymeric block (M- and G-) and heteropolymeric block (MG-) [14, 15]. Alginate is also known as alginic acid, where linear copolymer of uronic acid β-(1–4)-D-mannuronic (M), and α-L-gluronic (G) are residue [16]. Due to their exceptional adhesive properties and non-toxicity, it has been used in a variety of industries, for example, food, pharmaceutical, packaging, and textile industries [17]. Besides, alginate has recently been used in the application of edible films to avoid the usage of conventional packaging plastics, which advertently non-environmental friendly [18].

The concept of dissolving inorganic salts in functional (polar) biopolymer and experimentally creating ion-conducting electrolytes is known as bio-based polymer electrolytes (BBPEs). The interaction between polymer host and doping salt is crucial in determining the ionic conductivity achieved, chemical stability as well as the mechanical strength of the mixture. There are some important factors that may have affect the polymer-ion interactions, such as; (i) molecular weight, (ii) compositions and distance between functional groups, (iii) nature of the functional groups attached to the polymer backbone, (iv) degree of branching, (v) charge of cation, and (vi) counter ions [19]. The ionic conductivity of BBPEs is attributed to the low lattice energy of the complexing salt; thus, increasing the stability of the polymer matrix in BBPEs [20]. Many works have been reported on the various ionic dopant in BBPEs including ammonium salt (ammonium bromide (NH4Br) [21] and ammonium iodide (NH4I) [22]), lithium salt (lithium nitrate (LiNO3) [23] and lithium chloride (LiCl) [24]) and acidic salt (phosphoric acid (H3PO4) [25] and oxalic acid (C2H2O4) [26]).

In the present work, the investigation on proton conducting materials based bio-based polymer electrolytes (BBPEs) system has been carried out by using alginate as biopolymer host and doped with glycolic acid (GA). Glycolic acid (GA) is the simplest form of carboxylic acid, which contains highly polar organic groups and two important H-bonding sites; (i) carboxyl C=O and (ii) hydroxyl O-H. This would promote the formation of amorphous complexes through intra- and inter- molecular attraction with the polymer chain [27]. GA is considered as an ionic dopant that can provide a conduction pathway for the conducting species to migrate through the polymer matrix under the influenced of an electric field [28]. The structural and ionic conduction properties of BBPEs were characterized by using Fourier Transform Infrared (FTIR) spectroscopy, X-Ray Diffraction (XRD), Thermal Gravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC) and Electrical Impedance Spectroscopy (EIS). Furthermore, the ionic transport properties were investigated via FTIR-deconvolution technique for analyzing the details on the ionic conduction behavior of alginate-GA based BBPEs system.

Experimental

Preparation of bio-based polymer electrolytes

In this work, bio-based polymer electrolytes (BBPEs) sample-based alginate (Sigma Aldrich with M.W.: ~120,000) and glycolic acid, GA (Merck Co. with M.W.: 76.05 g/mol has been prepared by using solution casting technique. For the preparation, alginate was dissolved in distilled water, and then, different compositions (in wt.%) of GA were added into the alginate solution. The mixture was stirred until a homogenous solution was obtained and poured into a petri dish. The solution was left in the oven at 60 °C for overnight until the film was formed. The film was further drying in desiccators filled with silica gel to prevent any solvent trapped in BBPEs system. The summarized of sample preparation, physical appearance and designation of the sample are illustrated in Fig. 1.

Fig. 1
figure 1

Samples preparation, designation of sample, and physical appearance of alginate-GA BBPEs system

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Characterization of alginate-GA BBPEs system

Fourier transform infrared spectroscopy (FTIR)

Fourier Transform Infrared (FTIR) spectroscopy measurement was carried out to identify the complexation between alginate and GA in the BBPEs system. The infra-red spectra were obtained using Perkin Elmer Spectrum 100 with an attenuated total reflection (ATR) accessory with a germanium (Ge) crystal. The infra-red light was passed through the sample with the frequencies in the range between 700 cm−1 until 4000 cm−1 with spectra resolution of 2 cm−1.

Thermal gravimetric analysis (TGA)

TGA was carried out by using TG-DTA2010SA (NETZSCH Japan K.K., Japan). The measurements were recorded in a nitrogen gas atmosphere at a flow rate of 100 ml min−1. The BBPEs system was weighing with ~2 mg and placed into the aluminum pan. The samples were tested at different temperature ranging from 30 to 550 °C at a heating rate of 10 °C min−1.

Differential scanning Calorimetry (DSC)

DSC was carried out by using NETZSCH DSC 214 Polyma model. The measurements were recorded in a nitrogen gas atmosphere at a flow rate of 62 ml min−1. The BBPEs system was weighing with ~2 mg and placed into the silica crucible. The samples were tested at different temperature from 30 to 200 °C at a heating rate of 30 °C min−1.

X-ray diffraction (XRD)

X-ray diffraction (XRD) is the technique to determine the phase of crystalline and amorphous of polymer-salt complexes. X-ray diffraction depends on constructive interference of monochromatic x-rays and a sample by using Bragg’s law. MiniFlex II from Rigaku had performed to run out the nature of the present sample (amorphous/crystal) at different angles of 2θ between 5° and 80° with 1.5406 Å wavelength generated by a Cu Kd source. The XRD deconvolution analysis was carried out using OriginPro 9.0 software. Based on the assumption of Gaussian’s peak function, the crystalline and amorphous peaks were deconvoluted to ensure that all peaks fit with the original spectra. The percentage crystallinity of the BBPEs system is calculated by using the following equation:-

$$ {X}_c=\frac{A_c}{A_T}\times 100\% $$
(1)

Where Ac is the area covered under crystalline region, AT is the total area covered under the whole diffractogram (total of area of crystalline and amorphous region), and Xc is the degree of crystallinity in percentage.

Electrical impedance spectroscopy (EIS)

Electrical Impedance Spectroscopy (EIS) is used to evaluate the ionic conduction properties of BBPEs system. The present samples were cut into a suitable size and sandwiched between the stainless steel (SS) electrodes and left into the oven in order to control the humidity of the environment [29]. The prepared samples were measured using a HIOKI 3532–50 LCR Hi-Tester with frequencies ranging from 50 Hz to 1 MHz. The ionic conductivity of sample-based alginate-GA BBPEs system was calculated using equation:

$$ \sigma =\frac{t}{R_bA} $$
(2)

Where t (cm) is the thickness of sample, A (cm2) is the electrode-electrolyte contact area and Rb (Ω) is bulk resistance of BBPEs system which can be obtained from the Cole-Cole plot of EIS.

Transport parameter study

The transport properties of the thin film based alginate-GA BBPEs system were determined using FTIR deconvolution. Deconvolution was analyzed using Gaussian-Lorentz function, which was adapted to the OriginPro 9.0 software. In this method, the FTIR peaks due to complexation of alginate and GA were carefully selected based on dominant ionic movement. Besides, the sum of all the intensity of the deconvoluted peaks was ensured to fit the original spectrum. The absorbance peaks were fitted to a straight baseline and the area under the peaks was determined [30]. The free ions percentage (%) were calculated using the equation [31, 32]:-

$$ Percentage\ of\ free\ ions\ \left(\%\right)=\frac{A_f}{A_f+{A}_c}\times 100\% $$
(3)

Where Af is an area under the peak representing the free ions region, Ac is the total area under the peak representing the contact ions. The transports parameter such as number density (ƞ), mobility (μ) and diffusion coefficient (D) of the ions were calculated following this equation [22, 33]:-

$$ \eta =\frac{M{N}_A}{V_{Total}}\times freeions\left(\%\right) $$
(4)

where:

$$ {V}_{Total}=\left[\frac{weight}{density}(alginate)\right]+\left[\frac{weight}{density}(GA)\right] $$
(5)
$$ \mu =\frac{\sigma }{\eta \mathrm{e}} $$
(6)
$$ D=\frac{KT\mu}{e} $$
(7)

Where M is number of moles of GA used, NA is the Avogadro’s constant (6.02 × 1023 mol−1), Vtotal is total volume of BBPEs system, k is the Boltzmann constant (1.38 × 10−23 J K−1), T is the absolute temperature in Kelvin and e is the electric charge (1.602 × 10−19 C).

Result and discussion

FTIR analysis

In the present work, the structural arrangement of alginate and glycolic acid were studied using density functional theory. By using DMol3 modules of Material Studio 2017 software, the alginate model and glycolic acid was optimized through geometry optimization and energy minimization. The FTIR spectrum of alginate and GA were presented in Fig. 2, and the inset figure depicts the optimized structure of alginate monomer and glycolic acid (GA). The FTIR spectra confirmed the alginate structure as all characteristic peaks were observed at 1025 cm−1, 1409 cm−1, 1595 cm−1, 2325 cm−1 and 3389 cm−1 which are attributed to glycoside bond (C–O–C) [34], symmetric stretching of –COO− [35], antisymmetric stretching of –COO− [36], and stretching –OH group [37] respectively. From the spectra of pure GA, strong absorption peaks at 1074 cm−1, 1229 cm−1, 1414 cm−1, 1701 cm−1, and 3258 cm−1 that correspond to the stretching of C-O [38], bending O-H [39], stretching of the –COO− [40], stretching of C=O [41], and stretching O-H group [42], respectively were observed.

Fig. 2
figure 2

FTIR spectra of pure alginate and pure glycolic acid. Inset shows the optimized structure of alginate polymer and glycolic acid using Material Studio 2017

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Figure 3 shows the highlighted FTIR spectrum for the bio-based polymer electrolytes (BBPEs) system, which represents the complexes of alginate-GA. The complexation between host polymer and ionic dopant can be identified through the changes of the wavenumber or the peak intensity in the FTIR spectrum. In this present system, it is expected the group of interest that would lead to the complexation between alginate and GA is at the polar groups of C–O–C, –COO− and –OH in the biopolymer matrix due to the presence of lone pair electrons of the coordinating site (O) that attract the cation from the salt molecule [43].

Fig. 3
figure 3

FTIR spectra of alginate-GA BBPEs system between range (a) 900 cm−1 to 1200 cm−1 (b) 1300 cm−1 to 1800 cm−1 (c) 3000 cm−1 to 3700 cm−1

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Figure 3 (a) shown the peak at 1025 cm−1 for ALGA-1 which corresponds to the C–O–C stretching vibration [44] has shifted to 1030 cm−1 (higher wavenumber) which is believed due to the coordination of proton (H+) to the glycoside group (C–O–C) of alginate [45]. This glycoside band of polysaccharide alginate molecule was shown to further shifted until 1081 cm−1 for ALGA-6, which indicates that the complexation at the C–O–C via weak van der Waals attraction of dipole-dipole forces upon the inclusion of acidic salt become apparent.

Another significant peak was observed between 1300 to 1700 cm−1 belonged to symmetrical and asymmetrical carboxylate, –COO− as shown in Fig. 3 (b). This region was expected to exhibit strong affinity towards the GA due to highly nucleophilicity of the carboxylate ion [46]. The increasing salt composition leads to change in wavenumber for ALGA-1 until ALGA-6. In the present work, H+ cation from GA acts as a function of an electrophile positive ion. It could interact with the oxygen atom of carboxylate anion group present in alginate biopolymer via electrostatic attraction to form [H+---−OOC], thus prominent to the increment of intensity peak of BBPEs system [47, 48]. Notably, due to this strong attraction, the wavenumber has shifted from 1409 to 1413 cm−1 and 1595 to 1597 cm−1 for symmetry –COO− and asymmetry –COO− respectively [49]. This interaction was expected due to the coordination interaction of (–COO−) moiety in alginate with H+ ion of [-COOH] substructure in GA which reflects the protonation between the cation (H+) and the carboxylate group of alginate and triggers the ion hopping phenomenon called Grotthuss mechanism [50]. The H+ ion hopping from hydroxyl moiety to the carboxylate group of each monomer of alginate electrolyte and affect the changes in crystallinity phase and ionic conductivity.

Furthermore, upon addition of more GA, (more than 20 wt. %) the peak shifted to 1417 cm−1 and 1594 cm−1 for symmetry –COO− and asymmetry –COO− respectively. Rasali and Samsudin [33] reported that the phenomenon of shifted in wavenumber at –COO− at BBPEs system might be due to salt aggregation, which may affect the conductivity of the polymer electrolyte. The new peak was observed by the addition of GA as shown in Fig. 3 (b). The peak at ~1710 cm−1 belongs to C=O stretching of GA. The same phenomenon was founded in Chai and Isa [51] work, where new appearance peak at ~1710 cm−1 belongs to C=O stretching of GA.

The broad band was identified at 3389 cm−1 as shown in Fig. 3 (c) is known for the stretching O-H group in alginate monomer. It could be seen that it was shifted towards a lower wavenumber 3350 cm−1 when GA was introduced. This finding was found to be similar to Alakanandana et al. [52] where they observed the occurrence of intermolecular hydrogen bonding has occurred between polyvinyl alcohol (PVA) and succinic acid. Overall, the FTIR result has revealed and confirmed various interactions during the complexation between alginate and GA that are favorable in the conductivity enhancement. Schematic mechanism of the interaction between H+ of GA and interaction site of alginate is expected to occur based on Scheme 1, and all the functional groups change in wavenumber is presented in Table 1.

Scheme 1
scheme 1

Schematic diagram of alginate having interacted with GA.

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Table 1 Summary of peak changes in wavenumber for alginate-GA in BBPEs system
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Thermal behavior

Thermogravimetric analyses (TGA) was carried out to figure it out the effect of the GA on the thermal stability in BBPEs. TGA curves of alginate BBPEs system doped with various composition of glycolic acid was shown in Fig. 4. Two major decomposition stages were observed in the temperature range of 30 °C until 550 °C and tabulated in Table 2.

Fig. 4
figure 4

Thermal spectra of alginate-GA in BBPEs system

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Table 2 Thermal properties of alginate-GA in BBPEs system
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The initial weight loss for BBPEs system at 30 °C to 70 °C is attributable to the loss of the moisture where the alginate biopolymer tends to absorb the water and solvent for entire samples [53, 54]. The first decomposition was observed at an intermediate temperature range from 70 °C to 200 °C and involved small weight loss (10–15%) due to the decomposition of glycolic acid. It can be seen that decomposed temperature, Td for alginate doped with GA was relatively lower than pure alginate. This finding was supported by Qu et al. [55], where the addition of GA, resulting in the lower Td value, which is attributed to the low thermal stability of GA side chains. For the second stage of decomposition, un-doped sample (ALGA-0) has lost approximately 32.82% of its weight, at temperature ~ 250 °C. The weight loss is due to the loss of –COO− from the polysaccharide of alginate matrix [56]. Though, the Td was observed to increase linearly upon the addition of GA may be attributed to the complexation that has been taken place, which required a higher temperature for the disruption of H-bonding [57, 58]. A similar finding has been reported by Fadzallah et al. [26] for the system based on chitosan complexed with oxalic acid where the addition of acid salt enhances the thermal stability of BBPEs system. Therefore, it shows that the increment composition of GA in BBPEs system has a good thermal stability, which is beneficial in the fabrication of the device application.

It can be observed that prolong heating beyond 550 °C of decomposition temperature BBPEs system resulting in the carbonization and ash formation [59, 60]. This result reveals that the alginate can be used as a host polymer at various temperature, which is suitable for BBPEs system [61].

DSC analysis

Differential scanning calorimetry (DSC) was used to characterize the thermal behavior of materials, which can further confirm the miscibility of alginate and GA by measuring the changes in the heat capacity as the polymer matrix goes from the glass state to rubber state as known as Tg [62, 63]. Figure 5 displayed DSC thermograms obtained for BBPEs system and the glass transition temperatures (Tg) are depicted by arrows as shown in the figure. Based on Fig. 5(a), The Tg value for ALGA-0 was not detected at this range of temperature study. However, endothermic peaks were found at ~80 °C indicates further removal process of loosely bound water present in sodium alginate. As reported by Ghosal et al. [64] and Rezvanian et al. [65], the endothermic peak observed at temperatures 80.02 °C attributed to the evaporation of adsorbed moisture and dehydration of the cross-linked polymer matrix.

Fig. 5
figure 5

DSC thermograms for (a) ALGA-0 (b) ALGA-2 (c) ALGA-4 and (d) ALGA-6 of BBPEs system

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In BBPEs system, The Tg started to appear when glycolic acid (GA) added to alginate. From Fig. 5(b), the addition of 10 wt. % GA started to show the Tg value due to the formation of coordination between the polymer chain segments and ions formation from ionic dopant which increases the energy barrier to the segmental motion of the polymer chains; thus, the stiffening of the polymer chains may occur [66]. The lowest Tg value of ALGA-4 (Tg = 47.60 °C) indicates an increase in the flexibility of alginate chains; hence, the ALGA-4 was expected to exhibit the highest ionic conductivity value [67]. Further increase in the composition of GA leads to the increment of Tg value (Tg = 63.30 °C) for sample ALGA-6. It can be attributed to the formation of ion aggregates in the alginate polymer matrix, which reduced the flexibility of the polymer chain [68]. A similar trend has been observed by Moniha et al. [69] for the system based on iota carrageenan complexed with ammonium nitrate (NH4NO3).

X-ray diffraction analysis

Figure 6 shows typical x-ray diffractogram for alginate-GA BBPEs system. It can be seen that ALGA-0 has a broad amorphous peak occur at the central position with 2θ = 37.56°. This indicates that pure alginate has two different regions of amorphous [70]. The crystalline peak at 13.50° and 22.40°of ALGA-0 is observed in the present system which shown the characteristic of alginate and found to be similar to other research works [71, 72].

Fig. 6
figure 6

XRD spectra for alginate-GA BBPEs system at ambient temperature

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The XRD pattern of BBPEs system show an increase in broad peak with the addition of 0–20 wt. % GA. The broad peak is a typical characteristic of amorphous material [67]. According to Yusof et al. [73], no crystalline peaks found might be due to complete salt dissociation in the polymer matrix. Therefore, an increase in ionic conductivity of the alginate-GA BBPEs system is expected due to the changes of amorphousness and low glass transition in the samples [74]. Based on Fig. 6, it can be seen that the intensity of the broad peak reduced upon the addition of 20 wt. % GA indicating that complexation occurred significantly between alginate and GA. This might be attributed to the greater ionic diffusivity, where the addition of GA managed to enhance the intramolecular and intermolecular interaction through hydrogen bonding in BBPEs system, and thus exhibiting the amorphous characteristic [75, 76].

It shows that, above the addition of 20 wt. % GA, the intensity XRD pattern start to increase with small crystalline peaks at 2θ = 24.80° was observed. The increment of peak intensity was expected to decrease the number of mobile ions and hence affecting the ionic conductivity of the BBPEs system. According to Shukur et al. [77], the increment of the crystalline peak of BBPEs system will be correlated to the ionic conductivity and could be due to recombination of the ions, where the polymer host was incapable of accommodating the ionic dopant [78].

Figure 7 presents the XRD deconvolution patterns for BBPEs system. The percentage of crystallinity for un-doped alginate is obtained at 45.35% nd when added with 5 wt. % of GA, it demonstrated a decrement to 37.29%. The incorporation of GA into the host biopolymer induces a small increase in the amorphous structure, which attributed to the decrement of percentage crystallinity in BBPEs system. Based on the calculated value in Table 3; ALGA-4 depicts the lowest percentage of crystallinity which is 26.99% and this stimulates the segmental motion of the polymer matrix by reducing the energy barrier; hence, high ionic diffusivity is expected to enhance the ionic conductivity of the BBPEs system [66]. However, the increment percentage of crystallinity was observed for the composition of GA above ALGA-4. This may be due to the recombination of ions, which eventually lead to decrement in ionic conduction [22].

Fig. 7
figure 7

XRD deconvolution patterns for (a) ALGA-1 (b) ALGA-2 (c) ALGA-3 (d) ALGA-4 (e) ALGA-5 and (f) ALGA-6 of BBPEs system

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Table 3 Degree crystallinity of alginate-GA in BBPEs system
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Ionic conductivity analysis

Figure 8 shows the ionic conductivity values of alginate samples containing different amounts of GA at ambient temperature. The increment of ionic conductivity with the addition of GA can be related to increment of mobile charge carrier in BBPEs. As discussed earlier in FTIR analysis, the complexation between alginate and GA has shown high dispersion (dissociation of salt) of H+ and coordinate with the anion group of alginate bio-based polymer; therefore it would affect the increment of the ionic conductivity [79, 80]. Moreover, the amorphous structure and low glass transition temperature, Tg enhanced the migration of H+ ions (hopping) from GA to the coordination site (oxygen) of alginate host [81]. In the present system, the highest ionic conductivity at room temperature was found at 5.32 × 10−5 S cm−1 for the sample containing with 20 wt. % of GA (ALGA-4). The enhancement of ionic conductivity to the optimum value was found to align with the observation from XRD and DSC analysis.

Fig. 8
figure 8

The conductivity and activation energy of alginate-GA BBPEs system at ambient temperature

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Upon addition of GA beyond sample ALGA-4, the ionic conductivity started to decrease. According to Othman et al. [82], the decrement of ionic conductivity after ALGA-4 because of neutral aggregation of the ions re-associated and also leading to the formation of ion cluster as revealed by structural analysis. The decrement of ionic conductivity also was due to the formation of re-crystallization at polymer matrix, as shown in XRD study. The crystallize region of BBPEs system barricades the movement of ions when the conductivity starts to decrease at higher composition of GA [32].

Figure 9 shows the log conductivity versus 1000/T for different composition at the temperature range from 303 K to 343 K. The increasing of temperature depicts there is no sudden drop in conductivity value, which indicates that BBPEs system is good thermal stability and completely amorphous as observed in XRD and DSC analysis. As the temperature increases, the migration of charge carrier has promoted, which leads to an expansion in the polymer matrix [83, 84]. This expansion in the polymer network provides free volume to promote the motion of charge carriers and increases the conductivity. The temperature-dependent study of the BBPEs system obeys the Arrhenius characteristics, where the regression value, R2, is in the range of 0.95 to 0.99. [85, 86]. From the temperature-dependent study, activation energy, Ea can be calculated by using the Arrhenius equation.

$$ \sigma ={\sigma}_o\exp \left(-\frac{E_a}{kT}\right) $$
(8)

where σo is pre-exponential factor, k is Boltzmann’s constant and T is the temperature in Kelvin. The Ea values were calculated based on the slope of the temperature dependence plot and depicted in Fig. 8. It is noted that for alginate-GA BBPEs system, the activation energy decreases linearly as the conductivity increase. ALGA-4 showed the lowest value of Ea (0.18 eV) indicates that H+ from biopolymer matrix need lesser energy to migrates the ions to other coordinating sites thus creating vacancy sites for other H+ to complete the “hopping mechanism” of BBPEs system [48, 87]. The result indicates that increasing of GA composition not only would lead to enhance the number of carriers, but also reduce the energy barriers of BBPEs system [88].

Fig. 9
figure 9

The conductivity of alginate-GA BBPEs system at different temperature

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Transport parameter analysis

In the present work, FTIR deconvolution for various sample of alginate-GA BBPEs system was presented as shown in Fig. 10. Hay and Myneni [41] reported that strong absorption peaks at ~1420 cm−1 denote the anion vibration mode of –COO− from alginate, which proved the IR active in FTIR study. According to Ramlli et al. [40], the area of de-convoluted peak was determined based on significant change of wavenumber which was believed there is occurrence of complexation between the polymer and ionic dopant, and based on that, it leads to the need of separation the free ions and contact ions within the region. Based on Fig. 10, the BBPEs system shows free ions occurred at ~1410 cm−1, while contact ions appeared at ~1400 cm−1 and ~ 1430 cm−1 [45]. The free ions and the contact ions were calculated and depicted in Fig. 11.

Fig. 10
figure 10

The deconvolution IR spectrum for samples (a) ALGA-1 (b) ALGA-2 (c) ALGA-3 (d) ALGA-4 (e) ALGA-5 and (f) ALGA-6 of BBPEs system

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Fig. 11
figure 11

Percentage of free and contact ions of the alginate-GA BBPEs system

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From Fig. 11, it shows that the addition of GA increases the percentage of free ions until sample ALGA-4. This can be due to the increase ion dissociation of hydrogen ion (H+), thus resulting in more ion conduction, which eventually increasing the ionic conductivity in BBPEs system [33]. Beyond the 20 wt. % of GA, the percentage of free ions started to decrease linearly due to a large number of ion pairs and ion aggregates which accumulate in alginate bio-based polymer matrix as proven from XRD analysis [89]. Besides, the decrement of ionic conductivity is due to the re-associated of ions which supported the ionic conductivity reductions of alginate-GA BBPEs system [90].

Based on the percentage of free ions in Fig. 11, the number of ions mobility (μ), the number of density (η) and the diffusion coefficient number (D) were calculated using eq. (4), (5), (6) and (7) presented in Fig. 12.

Fig. 12
figure 12

The transport parameters for (a) number of mobile ions, η, (b) ions mobility, μ, and (c) diffusion coefficient, D of alginate-GA BBPEs system

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Figure 12 depicts the ions mobility (μ) and the diffusion coefficient (D) show a similar trend with ionic conductivity where it rises sharply at ALGA-4 and dropped at ALGA-5. This behavior follows the trend of conductivity-composition of BBPEs system. The maximum ionic conductivity of alginate doped with GA (ALGA-4) exhibits maximum μ and D values of 2.14 × 10−9 cm2 V−1 s−1 and 5.59 × 10−11 cm2 s-1, respectively. In the present work, the addition of GA in BBPEs system confirmed the conjecture in FTIR study where the H+ from –OH jump to lone pair of –COO− from the polymer backbone. Moreover, the amorphousness structure and low glass transition, Tg of BBPEs system depict that ion easily to hop from one site to another with less activation energy needed. Besides, the diffusion ions, D of polymer matrix will contribute to the increment of ionic conductivity where the movement of the ions to the BBPEs system easily to interact [22].

It shows that the number density (η) of ions increased linearly with the composition of GA. The highest value of η is 1.90 × 1023 cm−3 were in contrast to the ionic conductivity due to the ALGA-6 higher than the optimum composition of GA at ALGA-4. The increment of η inferred that the GA in the polymer matrix was too heavy, contributed to the decreasing of ion mobility (μ) and diffusion coefficient (D), which affected by the formation of ion cluster and also overcrowding of mobile ions (H+) [91]. In addition, increasing of crystallinity as shown from XRD analysis for higher composition of GA lead to overcrowding of ions since the re-crystalline phase tries to block any ion to migrate towards coordination site in polymer host [32]. With the addition beyond 20 wt. % of GA in BBPEs system, the increase in η lead to difficulty of ionic mobility to move, which in turn reduce the μ and D value.

Conclusion

The study on structural and transports properties of alginate doped with the different composition of GA as a potential proton-conducting bio-based polymer electrolytes (BBPEs) was carried out in the present work. The FTIR studies showed the presence of complexation between alginate and GA due to protonation which also called as Grotthuss mechanism where there is a strong contribution of hydrogen bonding related with the coordination site of carboxylate (–COO−) group of alginate. The ionic conductivity of alginate-GA system was found to increase from 8.74 × 10−8 S cm−1 to optimum value at 5.32 × 10−5 S cm−1 when was added with 20 wt. % of GA by showing an improvement of amorphous phase and thermal stability. Based on IR-deconvolution approach, it shows that the ionic conductivity of BBPEs was governed by ionic mobility (μ) and the diffusion coefficient number (D). The results suggest that the bio-based polymer electrolytes by using alginate materials have a good potential for applications as proton-conducting electrolytes system, which can be applied in electrochemical devices.