Synthesis of biopolymer electrolyte using sodium alginate with ammonium perchlorate (NH₄ClO₄) for the application of electrochemical devices

Abstract

Biopolymer electrolyte based on sodium alginate (NaAlg) with various concentrations of NH₄ClO₄ has been prepared by solution casting technique using double distilled water as solvent. Influence of different concentrations of NH₄ClO₄ on the biopolymer NaAlg is systematically investigated by the different characterization techniques such as XRD, FTIR, DSC, TGA, electrical impedance spectroscopy analysis, transference number measurement, and LSV. XRD was used to investigate whether the prepared biopolymers were crystalline or amorphous. The formation of complexes between NaAlg and NH₄ClO₄ has been observed by FTIR. Using differential scanning calorimeter, glass transition temperature (T_g) is found for the prepared biopolymer electrolyte. The maximum ionic conductivity of 3.59 × 10⁻³ S cm⁻¹ has been obtained for 30 M.wt% NaAlg:70 M.wt% NH₄ClO₄ biopolymer electrolyte using AC impedance analysis. Using DC Wagner’s polarization technique, the ionic transference number has been determined. The highest conducting biopolymer electrolyte's electrochemical stability window was found by the LSV technique to be 2.71 V. The primary proton battery and proton exchange membrane (PEM) fuel cell have been constructed using highest ionic conducting biopolymer membrane (30 M.wt% NaAlg:70 M.wt% NH₄ClO₄), and the performance has been studied. The proton battery’s open circuit voltage (V_oc) is 1.76 V, while the PEM fuel cell’s open circuit voltage (V_oc) is 789 mV.

Graphical Abstract

Introduction

In the development of science and technology, electrochemical device like battery plays a crucial role which is employed as a power source in a wide range of portable devices and electric vehicles [1]. In electrochemical devices, biopolymer electrolytes are frequently considered as alternatives to synthetic polymer electrolytes [2]. Solid biopolymeric electrolytes make a positive contribution to ionic conductivity, electrochemical stability, flexibility, and light weight [3,4,5]. Over the past decade, several biopolymers such as dextrin, starch, chitosan, pectin, carrageenan, agar, cellulose, and corn starch derivatives have been used to prepare environmentally sustainable electrolytes [6].

Sodium alginate (NaAlg) biopolymer is a type of polysaccharide derived from the cell of brown marine algae. It is made up of 1 → 4 linked β-d-guluronic (G), and α-l-mannuronic (M) molecule [7, 8]. Polymeric segment -M-G blocks are combined to form uronic acids (Fig. 1) [9].

Fig. 1

Molecular structure of sodium alginate

Biopolymer NaAlg find its application in various fields such as food packaging [10], grafting copolymerization [11], emulsifiers, stabilizers [12], cosmetics [13], drug delivery [14, 15], medical applications [16], and textiles [17]. As of now, not many researches have been done using NaAlg as a electrolyte material. For a material to be used as an electrolyte, it should have a sufficient number of polar groups to which any salt cation can get attached. NaAlg has a large number of polar groups to which any salt’s cation can be attached [18, 19]. Biopolymer NaAlg will not pollute the environment. In this paper, it has been used as an electrolyte material.

Few works have been done on the preparation of NaAlg-based biopolymer electrolyte with different salts. Jansi et al. have prepared PVA:NaAlg with NH₄Cl solid polymer electrolyte[20]. Iwaki et al. had prepared sodium alginate solid polymer electrolyte and reported a maximum conductivity value of 3.1 × 10⁻⁴ S cm⁻¹ for the composition of 42.5Wt% of NaAlg:15 wt% LiClO₄:42.5Wt% glycerol [18]. Fuzlin et al. has reported ionic conductivity value of 5.32 × 10⁻⁵ S cm⁻¹ for the composition of 1 g NaAlg with 20 wt% glycolic acid [5]. Ionic conductivity values of 2.0 × 10⁻³ S cm⁻¹, 2.29 × 10⁻³ S cm⁻¹, and 1.22 × 10⁻² S cm⁻¹ have been reported by Diana et al. for the composition of 30 wt% NaAlg:70 wt% NaI, 40 wt% NaAlg:60 wt% NaClO₄, and 30 wt% NaAlg:70 wt% NaSCN respectively [21,22,23]. Tamilisai et al. have reported ionic conductivity value of 4.58 × 10⁻³ S cm⁻¹ for the composition of 40 M.wt% NaAlg:60 M.wt% Mg(NO₃)₂.6H₂O [24].

Amorphous phase of the biopolymer is improved by the addition of salt, which perturbs the biopolymer network. Ammonium salts are considered as proton donor because one of proton of ammonium ion is loosely bound. Lattice energy of the ammonium salt is low which makes it highly soluble in water [25, 26].

Ionic conductivity values for many biopolymers with ammonium salts have been obtained from literature reviews. Selvalakshmi et al. measured conductivity values of 1.17 × 10⁻⁴ S cm⁻¹ and 3.73 × 10⁻⁴ S cm⁻¹ for 50 mol% agar agar:50 mol% NH₄I and 50 wt% agar agar:50 wt% NH₄Br respectively [27, 28]. Ionic conductivity value for 1 g of K-carrageenan–based NH₄SCN has been reported by Selvin et al. as 6.83 × 10⁻⁴ S cm⁻¹ [25]. Shujahadeen et al. have reported the conductivity values of 3.07 × 10⁻⁸ S cm⁻¹ and 8.57 × 10⁻⁴ S cm⁻¹ for the composition of 40 wt% chitosan:30 wt% potato starch:30 wt% NH₄BF₄ and 1-g chitosan:40wt% NH₄SCN:40wt% glycerol respectively [29, 30]. Sohaimy et al. have reported the ionic conductivity values of 7.71 × 10⁻⁶ S cm⁻¹ and 1.47 × 10⁻⁴ S cm⁻¹ for 2-g carboxymethyl cellulose:7wt% (NH₄)₂CO₃ and 1-g carboxymethyl cellulose with 40wt% NH₄HCO₂ respectively [31, 32]. Ramli et al. have reported the ionic conductivity value of 1.16 × 10⁻⁴ S cm⁻¹ for 2-g 2-hydroxyethyl cellulose with 36 wt% NH₄SCN [33]. Muthukrishnan et al. measured conductivity values of 4.5 × 10⁻³ S cm⁻¹, 2.74 × 10⁻⁴ S cm⁻¹, and 3.6 × 10⁻³ S cm⁻¹ for 30 M.wt% pectin: 70 M.wt% NH₄I, 50 M.wt% pectin:50 M.wt%NH₄CO₂ and 50 M.wt% pectin:50 M.wt% NH₄HCO₂:0.4wt% ethylene carbonate respectively [34, 35]. Ionic conductivity value of 8.03 × 10⁻³ S cm⁻¹ has been reported by Maheshwari et al. for the composition of 700-mg dextran:300-mg PVA:0.6 M.wt% NH₄SCN [36]. Meera Naachiyar et al. have reported ionic conductivity value of 5.62 × 10⁻³ S cm⁻¹ for the composition of 1-g gellan gum:0.9wt% NH₄HCO₂ [37].

Vanitha et al. have investigated a proton battery based on biopolymer NaAlg with NH₄SCN and NaAlg with NH₄HCO₂ [26, 38]. Research work using NaAlg for proton battery is very limited. So an attempt is made to develop a NaAlg-based proton battery and single fuel cell.

In the present study, proton-conducting biopolymer electrolytes were prepared using NaAlg and NH₄ClO₄. The prepared samples are characterized by various techniques, namely X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), thermogravimetric analyzer (TGA), electrical impedance spectroscopy analysis, and linear sweep voltammetry (LSV). The transference number of the H + ion is calculated using Wagner’s polarization techniques. Using the highest conducting biopolymer electrolyte, the primary proton battery and proton exchange membrane (PEM) fuel cell have been developed. The results are discussed and presented in this paper.

Materials and methods

Materials

Sodium alginate biopolymer (S D Fine-Chem limited molecular weight 216.12 g/mol) as a host polymer. Ammonium perchlorate (Merck Specialities Private Limited: MW 117.49 g/mol) is the salt. The solvent is double-distilled (DD) water.

Preparation of the electrolyte

The synthesis of the biopolymer electrolyte is carried out by the simple solution casting technique. The NaAlg biopolymer is dissolved in DD water at 80 °C with different compositions (40, 35, 30, and 25 M.wt%), to get a homogenous solution. Different concentration of NH₄ClO₄ (60, 65, 70, and 75 M.wt%) are dissolved separately, added to the NaAlg solution at 80 °C and stirred for 2 h to get a homogenous solution. The solution was casted into petri dishes and vacuum evaporated in oven at 60 °C for 12 h. After drying, the free standing transparent films were obtained.

Characterization technique

XRD

The amorphous/crystalline nature of the membrane has been investigated utilizing Cu-Kα radiation at an angle of 2θ = 5^–80° at a rate of 2°/min using the X’Pert PRO diffractometer.

FTIR

A SHIMADZU — IR Affinity-1 spectrometer was used to record FTIR spectra for biopolymer electrolyte films in the range of 500 to 4000 cm⁻¹ with a resolution of 1 cm⁻¹ at ambient temperature in order to study the complex formation between the NaAlg biopolymer and the NH₄ClO₄ salt.

DSC analysis

The prepared biopolymer electrolyte’s glass transition temperatures were measured using a DSC Q20 V24.10 Build 124 apparatus in nitrogen atmosphere at a temperature range between 10 and 180 °C with a heating rate of 10 °C/min.

TGA analysis

SDT Q600 V20.9 Build 20 has been used to investigate the thermal stability of the biopolymer electrolyte in nitrogen atmosphere at a flow rate of 200 ml/min. In the range of 30 to 700 °C, the samples were heated at a rate of 10 °C/min.

Impedance analysis

Impedance analysis is a useful technique for examining the electrical properties of the biopolymer electrolytes. By sandwiching the biopolymer membrane and using stainless steel as the electrodes, an HIOKI 3532 LCR meter connected to a computer used to measure the impedance of the biopolymer membrane in the frequency range between 42 Hz and 5 MHz.

Linear sweep voltammetry

Using a CHI600C series electrochemical instrument with a scan rate of 0.1 V/s in the range of 0 to 5 V, the highest electrochemical stability window of electrolyte was measured.

Transference number measurement

The polarization current is measured using a DC polarization technique by passing a 1.0 V dc voltage across the configuration of stainless steel (SS)/biopolymer membrane/SS.

Construction of primary battery

A primary proton conducting battery has been fabricated with the configuration, Zn + ZnSO₄.7H₂O + C||highest conducting polymer membrane ($30\mathrm{ NaAlg}+70\mathrm{ N}{\mathrm{H}}_{4}{\mathrm{ClO}}_{4})$||PbO₂ + V₂O₅ + C. OCV and discharge performance have been studied with the load resistance of 100 KΩ.

Construction of (PEM) fuel cell

The highest conducting biopolymer electrolyte (30 M.wt%NaAlg:70 M.wt%NH₄ClO₄) has been used to build a PEM fuel cell. OCV and the current drawn for various loads (10 Ω, 270 Ω, and 620 Ω) have been measured.

Results and discussion

XRD

The degree of amorphousness at room temperature has been determined using X-ray diffraction measurements for the biopolymer electrolytes NaAlg with various NH₄ClO₄ concentrations. The XRD patterns are shown in Fig. 2 pure NaAlg curve (a), 40 M.wt% NaAlg:60 M.wt% NH₄ClO₄ curve (b), 35 M.wt% NaAlg:65 M.wt% NH₄ClO₄ curve (c), 30 M.wt% NaAlg:70 M.wt% NH₄ClO₄ curve (d), and 25 M.wt% NaAlg:75 M.wt% NH₄ClO₄ curve (e). The diffraction pattern for pure NaAlg shows a peak at 2θ = 14° and 2θ = 22.8°. These peak positions coincide with the previous results [39, 40]. The diffraction peak at 2θ = 14° disappears for the concentration of 40 NaAlg:60 NH₄ClO₄ (curve (b)), 35 NaAlg:65 NH₄ClO₄ (curve (c)), 30 NaAlg:70 NH₄ClO₄ (curve (d)), and 25 NaAlg:75 NH₄ClO₄ (curve (e)) respectively. The peak at 2θ = 22.8° has become broadened and shifted to 27.7°, 27.4°, and 26.2° for the concentration of 40 NaAlg:60 NH₄ClO₄ (curve (b)), 35 NaAlg:65 NH₄ClO₄ (curve (c)), and 30 NaAlg:70 NH₄ClO₄ (curve (d)) respectively. With an increase in salt concentration, the peak’s intensity decreases while its broadness increases. The Hodge et al. criteria, establishing a relationship between peak intensity and degree of crystallinity, were used to interpret the results [41]. The maximum broadness is observed for 30 NaAlg:70 NH₄ClO₄ biopolymer membrane, showing that this membrane is more amorphous than other membranes. The peak at 2θ = 23.2° has increased intensity and decreased broadness for the composition of 25 NaAlg:75 NH₄ClO₄ (curve (e)). This shows decline in amorphous nature. The increase in amorphous nature could be explained, whenever salts are added to the polymer, added salt perturbs the polymer network, and produced disorderliness in the network, i.e., amorphousness nature increases. As the concentration of the salt is increased the above process is increases up to a particular concentration of the salt. After the particular concentration, even though salt concentration is increased, the above process, i.e., creation of disorderliness, does not take place.

Fig. 2

XRD plot of a pure NaAlg, b 40NaAlg:60 NH₄ClO₄, c 35 NaAlg:65 NH₄ClO₄, d 30 NaAlg:70 NH₄ClO₄, and e 25 NaAlg:75 NH₄ClO₄

Figure 3 depicts the deconvoluted XRD plot for pure NaAlg (curve (a)) and different concentration of NaAlg with NH₄ClO₄, 40 NaAlg:60 NH₄ClO₄ (curve (b)), 35 NaAlg:65 NH₄ClO₄ (curve (c)), 30 NaAlg:70 NH₄ClO₄ (curve (d)), and 25 NaAlg:75 NH₄ClO₄ (curve (e)).

Fig. 3

Deconvoluted XRD plot of a pure NaAlg, b 40 NaAlg:60 NH₄ClO₄, c 35 NaAlg:65 NH₄ClO₄, d 30 NaAlg:70 NH₄ClO₄, and e 25 NaAlg:75 NH₄ClO₄

$$\mathrm{Crystallinity percentage }= \frac{\mathrm{Area under crystalline region}}{\mathrm{Total area of the peak}}\times 100$$

The formula has been used to get the crystallinity percentage.

The crystallinity percentages are shown in Table 1. Table 1 shows, the percentage of crystallinity has been observed for pure NaAlg as 41.34%. With the addition of NH₄ClO₄ salt the crystallinity percentage decreases to 39.61%, 26.13%, and 15.16% for 40 NaAlg:60 NH₄ClO₄, 35 NaAlg:65 NH₄ClO₄, and 30 NaAlg:70 NH₄ClO₄ respectively. The NaAlg biopolymer membrane has become more amorphous as the salt concentration increased. Further addition of NH₄ClO₄ salt (25 NaAlg:75 NH₄ClO₄) the crystallinity percentage increases to 38.19%. Biopolymer membrane 30 NaAlg:70 NH₄ClO₄ shows a high amorphous nature.

Table 1 Crystallinity percentage of the biopolymer membrane

Mechanical properties such as microstrain (ε) and dislocation density (δ) along with crystalline size have been calculated from the XRD data using the formula [42, 43].

The crystalline size (D) has been calculated using Debye–Scherrer formula,

$$D=\frac{k\lambda }{\beta cos\theta }$$

Micro-strain (ε) and dislocation density (δ) are calculated by using the formula,

$$\varepsilon =\frac{\beta cos\theta }{4}$$

$$\delta =\frac{1}{{D}^{2}}$$

where k is the Scherrer constant (0.94), λ is the wavelength of the Cu-Kα X-ray radiation (1.54 nm), β is the full width half maximum (FWHM) of the diffraction peak, and θ is the Bragg angle (in radians).

These mechanical properties have been calculated with respect to the prominent peaks at 2θ = 22.8°, 27.7°, 27.4°, 26.2°, and 23.2° for pure NaAlg, 40 NaAlg:60 NH₄ClO₄, 35 NaAlg:65 NH₄ClO₄, 30 NaAlg:70 NH₄ClO₄, and 25 NaAlg:75 NH₄ClO₄ respectively and depicted in Table 2.

Table 2 Calculated mechanical property for prepared polymer electrolytes from XRD

Table 2 shows that.

• a) Increasing the concentration of salt from 60 M.wt% NH₄ClO₄ to 70 M.wt% NH₄ClO₄ crystalline size have been decreased compare to pure NaAlg.

• b) Increasing the concentration of salt 60 M.wt%NH₄ClO₄ to 70 M.wt% NH₄ClO₄ microstrain and dislocation density have been increased compare to pure NaAlg.

• c) For 75 M.wt% NH₄ClO₄ with 25 M.wt% NaAlg membrane microstrain and dislocation density have been decreased compare to pure NaAlg.

FTIR

FTIR spectroscopy is used to study the complex formation between the biopolymer NaAlg and the NH₄ClO₄ salt. Figure 4 depicts the FTIR spectra of pure NaAlg. Figure 5 depicts the FTIR spectra of 40 NaAlg:60 NH₄ClO₄, 35 NaAlg:65NH₄ClO₄, 30 NaAlg:70 NH₄ClO₄, and 25 NaAlg:75 NH₄ClO₄, and Table 3 shows the corresponding vibrational peaks that were assigned.

Fig. 4

FTIR spectrum of pure NaAlg

Fig. 5

FTIR spectrum of a 40 NaAlg:60 NH₄ClO₄, b 35 NaAlg:65 NH₄ClO₄, c 30 NaAlg:70 NH₄ClO₄, and d 25 NaAlg:75 NH₄ClO₄

Table 3 FTIR peak assignments of NaAlg biopolymer with NH₄ClO₄ salt

The IR peak in the pure NaAlg at 1057 cm⁻¹ is associated with C–O–C stretching. This peak vibration gets shifted to 1032 cm⁻¹, 1043 cm⁻¹, 1043 cm⁻¹, and 1032 cm⁻¹ for the biopolymer electrolytes 40 NaAlg:60 NH₄ClO₄, 35 NaAlg:65 NH₄ClO₄, 30 NaAlg:70 NH₄ClO₄, and 25 NaAlg:75 NH₄ClO₄ respectively which reveals the coordination between salt and biopolymer.

The peak 1404 cm⁻¹ is assigned to COO⁻ symmetric stretching for pure NaAlg and on incorporation of different concentrations of the salt to the biopolymer membrane the peak gets shifted to 1408 cm⁻¹, 1409 cm⁻¹, 1411 cm⁻¹, and 1411 cm⁻¹ for the biopolymer electrolytes 40 NaAlg:60 NH₄ClO₄, 35 NaAlg:65 NH₄ClO₄, 30 NaAlg:70 NH₄ClO₄, and 25 NaAlg:75 NH₄ClO₄ respectively.

The peak at 1618 cm⁻¹ in pure NaAlg represents COO⁻ asymmetric stretching [21]. And this peak is absent for 40 NaAlg:60 NH₄ClO₄, 35 NaAlg:65 NH₄ClO₄, and 30 NaAlg:70 NH₄ClO₄ membranes.

A peak at 1601 cm⁻¹ is observed for 25 NaAlg:75 NH₄ClO₄ this may be due to COO⁻ asymmetric stretching [23].

Appearance of the peak around 1601 cm⁻¹ is assigned to COO⁻ asymmetric stretching by other people. So it is shows that for higher concentration asymmetric peak appears again. Appearance and disappearance of a particular peak may lead to the understanding that complex formation has been formed between the salt and polymer.

The C-H stretching appears at 2934 cm⁻¹ for pure NaAlg. When the salt is incorporated in the polymer matrix the peak is shifted to 2918 cm⁻¹, 2929 cm⁻¹, 2918 cm⁻¹, and 2918 cm⁻¹ for 40 NaAlg:60 NH₄ClO₄, 35 NaAlg:65 NH₄ClO₄, 30 NaAlg:70 NH₄ClO₄, and 25 NaAlg:75 NH₄ClO₄ respectively. The intensity of the peak gets reduced on adding different salt concentration.

For pure NaAlg, the O–H stretching peak appears at 3393 cm⁻¹. And the peak is shifted to 3274 cm⁻¹, 3271 cm⁻¹, 3269 cm⁻¹, and 3270 cm⁻¹ for 40 NaAlg:60 NH₄ClO₄, 35 NaAlg:65 NH₄ClO₄, 30 NaAlg:70 NH₄ClO₄, and 25 NaAlg:75 NH₄ClO₄ respectively.

The new peaks 618 cm⁻¹, 618 cm⁻¹, 620 cm⁻¹, and 619 cm⁻¹ occurred for 40 NaAlg:60 NH₄ClO₄, 35 M.wt%NaAlg:65 NH₄ClO₄, 30 NaAlg:70 NH₄ClO₄, and 25 NaAlg:75 NH₄ClO₄ respectively are due to the ClO₄⁻ stretching.

Using the given formula calculate the force constant (k) value,

$$\overline{\upsilon }=\frac{1}{2\pi c}\sqrt{\frac{k}{\mu }}{\mathrm{ cm}}^{-1}$$

(1)

where ῡ is the wave number (cm⁻¹), c is the velocity of light (3 × 10¹⁰ cm s⁻¹), k is the force constant (N/cm), μ is the reduced mass ($\mu =\frac{{m}_{1 }\times {m}_{2}}{{m}_{1}+{m}_{2}}$), m₁ is the atomic mass of O, m₂ is the atomic mass of H.

The force constant values are shown in Table 4. As shown in Table 4, the force constant values decreases with an increase in the composition of 40 NaAlg:60 NH₄ClO₄, 35 NaAlg:65 NH₄ClO₄, 30 NaAlg:70 NH₄ClO₄, and 25 NaAlg:75 NH₄ClO₄. This decrease of force constant shows that decreases frequency with an increase in bond length. The possible interaction between NH₄ClO₄ with the NaAlg is shown in Fig. 6.

Table 4 Force constant values of NaAlg biopolymer with NH₄ClO₄ for O–H stretching

Fig. 6

Possible interaction of NH₄ClO₄ salt with NaAlg biopolymer

DSC

The T_g (glass transition temperature) of the biopolymer electrolytes has been evaluate by the DSC analysis. Figure 7 shows the DSC thermogram of pure NaAlg and different M.wt% of NaAlg with NH₄ClO₄ salt (40 NaAlg:60 NH₄ClO₄, 35 NaAlg:65 NH₄ClO₄, 30 NaAlg:70 NH₄ClO₄, and 25 NaAlg:75 NH₄ClO₄). Figure 7a depicts the pure NaAlg and it exhibits a T_g value of 52.8 °C. With Addition of ammonium salt the T_g value decreases to 41.81 °C, 40.56 °C, and 38.62 °C for 40 NaAlg:60 NH₄ClO₄, 35 NaAlg:65 NH₄ClO₄, and 30 NaAlg:70 NH₄ClO₄ respectively. Addition of the salt stimulates the rubbery state in the biopolymer electrolytes which is indicated by the decrease in the glass transition temperature (T_g). The rubbery state leads to more flexible nature. High ionic conductivity is expected for the sample 30 NaAlg:70 NH₄ClO₄. On further increasing of salt concentration 25 NaAlg:75 NH₄ClO₄ the T_g value increases to 41.3 °C. This increase in T_g is caused by the aggregates formation of ions, which reduces the flexibility of the polymer chain. Moniha et al. [51] found similar result for the composition of i-carrageenan with NH₄SCN, while Maheshwari et al. reported similar result for the composition of dextran:PVA:NH₄NO₃ [52].

Fig. 7

DSC thermogram of a pure NaAlg, b 40 NaAlg:60 NH₄ClO₄, c 35 NaAlg:65 NH₄ClO₄, d 30 NaAlg:70 NH₄ClO₄, and e 25 NaAlg:75 NH₄ClO₄

TGA

The thermograms of pure NaAlg and 30 NaAlg:70 NH₄ClO₄ are shown in Fig. 8. Table 5 depicts the three different decomposition stages which occur in the 30 to 700 °C temperature range.

Fig. 8

TGA thermogram for a pure NaAlg and b 30 NaAlg:70 NH₄ClO₄

Table 5 Thermal properties of biopolymer electrolyte

The initial weight loss occurs by moisture evaporating from the polymer electrolytes in the pure NaAlg (30–70 °C, 9.8% weight loss) and 30 NaAlg:70 NH₄ClO₄ (30–74 °C, 6.6% weight loss).

Pure NaAlg decomposes at 71–210 °C with an 8.2% weight loss during the first stage, and the weight loss occurs by moisture evaporation and the residual solvent in the polymer electrolyte. The weight loss in the first stage of 30 NaAlg:70 NH₄ClO₄ (75–191 °C, 6.3% weight loss) is caused by salt decomposition and residual solvent evaporation.

For pure NaAlg (211–268 °C, 35% weight loss) and 30 NaAlg:70 NH₄ClO₄ (192–355 °C, 58.8% weight loss), the weight loss in the second stage of decomposition is caused by the loss of the carboxylate group from the polymer backbone.

For pure NaAlg (269–700 °C, 17% weight loss) and 30 NaAlg:70 NH₄ClO₄ (356–691 °C, 11.1% weight loss), weight loss occurs through carbonization and the formation of ash during the third decomposition stage.

Similar findings have been reported by Fuzlin et al. for the biopolymers alginate with NH₄Br and NaAlg with glycolic acid as well as by Vanitha et al. for the biopolymer NaAlg with NH₄SCN [5, 26, 38].

Impedance analysis

Impedance spectroscopy has been used to measure the ionic conductivity of the prepared biopolymer electrolytes. Figure 9 shows the Nyquist plot for pure NaAlg and various concentration of NaAlg with NH₄ClO₄ salt (40 NaAlg:60 NH₄ClO₄, 35 NaAlg:65 NH₄ClO₄, 30 NaAlg:70 NH₄ClO₄, and 25 NaAlg:75 NH₄ClO₄) and its equivalent circuit.

Fig. 9

Nyquist plot for a pure NaAlg and equivalent circuit, b 40 NaAlg:60 NH₄ClO₄ and equivalent circuit, c 35 NaAlg:65 NH₄ClO₄, 30 NaAlg:70 NH₄ClO₄, and 25 NaAlg:75 NH₄ClO₄ and equivalent circuit

Figure 9a for pure NaAlg shows a high frequency semicircle and a low frequency spike. The high frequency semicircle arises due to the parallel combination of bulk capacitance (C_p) and bulk resistance (R_b) of the material and low frequency spike is due to electrode–electrolyte interface [53]. The bulk capacitance (C_p) is frequency dependent one. With the addition of NH₄ClO₄ salt to pure NaAlg, Fig. 9b of 40 NaAlg:60 NH₄ClO₄ shows a semicircle with spike. Figure 9c of 35 NaAlg:65 NH₄ClO₄, 30 NaAlg:70 NH₄ClO₄, and 25 NaAlg:75 NH₄ClO₄ shows the disappearance of the semicircle. Within the frequency range studied, the biopolymer electrolyte has only a dominant resistive component. Using the relation,

$$\sigma =\frac{l}{A{R}_{b}}S{ cm}^{-1}$$

where $l, A$ is the thickness and area of the biopolymer membrane, ${R}_{\mathrm{b}}$ is the bulk resistance of the biopolymer membrane.

The ionic conductivity (σ) is calculated for all the compositions at room temperature, and the results are shown in Table 6. From Table 6, the biopolymer electrolyte of 30 NaAlg:70 NH₄ClO₄ has a very high number of charge carriers, which produces a highest ionic conductivity value of 3.59 × 10⁻³ S cm⁻¹ at ambient temperature. And this membrane has got more amorphous nature (confirmed by XRD) and also low T_g value (confirmed by DSC).

Table 6 Ionic conductivity value for the prepared biopolymer electrolytes at 303 K

Using Boukamp software, the R_b value of the biopolymer membrane has been measured [54]. EIS parameter values of the biopolymer membrane are tabulated in Table 7. From Table 7, R_b value is 6.35 × 10³ Ω for pure NaAlg. For 40 NaAlg:60 NH₄ClO₄, 35 NaAlg:65 NH₄ClO₄, and 30 NaAlg:70 NH₄ClO₄ and 25 NaAlg:75 NH₄ClO₄ biopolymer electrolytes, the R_b value has decreased 3.32 × 10³ Ω, 2.81 × 10² Ω, and 1.02 × 10¹ Ω respectively. For the composition of 25 NaAlg:75 NH₄ClO₄, the R_b value increased to 2.08 × 10² Ω. Constant phase element (CPE) impedance can be calculated using,

$${Z}_{\mathrm{CPE}}=\frac{1}{{Q}_{0}(\mathrm{j\omega }{)}^{\mathrm{n}}}$$

where n, ${Q}_{0}$ is the frequency independent factor

Table 7 EIS parameters of the biopolymer electrolytes

The value of n is 1, shows pure capacitor, and n is 0, shows the pure resistor [55]. The CPE value for pure NaAlg is 1.49 × 10⁻⁵ μF. On adding of NH₄ClO₄ salt with NaAlg, 40 NaAlg:60 NH₄ClO₄, 35 NaAlg:65 NH₄ClO₄, 30 NaAlg:70 NH₄ClO₄ and 25 NaAlg:75 NH₄ClO₄, the CPE value were 8.72 × 10⁻⁶ μF, 1.29 × 10⁻⁵ μF, 1.98 × 10⁻³ μF, and 3.42 × 10⁻⁴ μF respectively. Pure NaAlg has an n value of 1.87. For various concentration of salt NH₄ClO₄ with NaAlg, 40 NaAlg:60 NH₄ClO₄, 35 NaAlg:65 NH₄ClO₄, 30 NaAlg:70 NH₄ClO₄, and 25 NaAlg:75 NH₄ClO₄, the values of n were 0.37, 0.42, 0.74, and 0.52 respectively. The composition of 25 NaAlg:75 NH₄ClO₄, the conductivity value decreases to 9.82 × 10⁻⁴ S cm⁻¹ and is due to salt aggregate formation.

Selvin et al. have observed a conductivity of 6.83 × 10⁻⁴ S cm⁻¹ for the composition of 1-g k-carrageenam with 0.5% NH₄SCN [25]. Rasali et al. have obtained a maximum conductivity of 5.56 × 10⁻⁵ S cm⁻¹ for the composition of 2-g alginate:25wt% NH₄NO₃ [56]. Moniha et al. reported that the conductivity value of 1.46 × 10⁻³ S cm⁻¹ for 1-g i-carrageenam:0.4wt% NH₄NO₃ [57]. According to Monisha et al., maximum conductivity value was 1.02 × 10⁻³ S cm⁻¹ for 50 mol% cellulose acetate:50 mol% NH₄NO₃ [58].

Conductance spectra

Conductance spectra of pure NaAlg and different composition of 40 NaAlg:60 NH₄ClO₄, 35 NaAlg:65 NH₄ClO₄, 30 NaAlg:70 NH₄ClO₄, and 25 NaAlg:75 NH₄ClO₄ are shown in Fig. 10. Usually, the conduction spectra is characterized by three regions. The first region is low-frequency dispersive region, which relates to the polarization of the electric charge at the interface between the electrode and the electrolyte [59]. The second is a mid-frequency independent plateau region, which is related to the DC conductivity of the prepared biopolymer electrolyte. Finally, the third is the high frequency region, which relates to the bulk relaxation process of the biopolymer electrolyte. Only the low frequency and mid frequency bands are observed in this work. The AC conductivity spectra show that the conductivity increases with increasing salt concentration, which is associated with the increase in charge carriers. The dc conductivity (${\sigma }_{dc})$ values for all the biopolymer electrolytes are obtained by extrapolating the plateau region to the log σ axis. It was observed that the conductivity values obtained from the conduction spectra and the Nyquist plot are similar.

Fig. 10

Conductance spectra for pure NaAlg and NaAlg with various compositions of NH₄ClO₄. a Pure NaAlg, b 40 NaAlg:60 NH₄ClO₄, c 35 NaAlg:65 NH₄ClO₄, d 30 NaAlg:70 NH₄ClO₄, and e 25 NaAlg:75 NH₄ClO₄

Transference number measurement

The Wagner’s polarization technique is used to measure the transference number, which determines whether the conductivity in the biopolymer electrolyte is caused by the presence of ions or electrons [60]. Using the formula, the transference number is measured.

$${t}_{ion}=\frac{{I}_{i}-{I}_{f}}{{I}_{i}}$$

$${t}_{ele}=\frac{{I}_{f}}{{I}_{i}}$$

where I_i, I_f is the initial and final current.

In this technique a dc potential of 1.0 V was applied across the cell of the stainless steel (SS):30 NaAlg with 70 NH₄ClO₄:SS configuration for polarization and after polarization. The initial current decreases with time as shown in Fig. 11 and reaches a constant value in the fully depleted situation due to the depletion of ionic species in the biopolymer electrolyte [61]. Using transference number equation, the t_ele and t_ion for the highest conducting biopolymer electrolyte (30 NaAlg:70 NH₄ClO₄) is observed to be 0.019 and 0.98 which is nearly unity which ensures that the charge transport is mainly due to ions and hence these electrolytes are suitable for solid-state electrochemical cells. Meera Nachiyar et al. have reported that the value of t_ion as 0.95 for the composition 1.0-g gellan gum with 0.9 M.wt% of NH₄HCO₂ [37]. The value of t_ion reported by Maheshwari et al. for the composition 700-mg dextran:300-mg PVA:450-mg NH₄NO₃ is 0.99 [52]. Muthukrishnan et al. [35] have reported that the value of t_ion 0.962 for 50 M.wt% pectin:50 M.wt% NH₄HCO₂ polymer electrolytes.

Fig. 11

Variation of DC with time for 30 NaAlg:70 NH₄ClO₄ (highest conducting electrolyte) using Wagner’s method

LSV

The electrochemical stability of the biopolymer membrane has been examined using the linear sweep voltammetry (LSV) method. The highest conducting biopolymer electrolyte placed between two stainless steel electrodes. Figure 12 illustrates the linear sweep voltammogram of the highest conducting membrane (30 NaAlg:70 NH₄ClO₄). Figure 12 shows that the electrochemical stability is stable up to 2.71 V and further decomposition occurred in the biopolymer electrolyte. Meera Naachiyar et al. have reported the electrochemical window of 2.53 V for 1-g GG:0.9 M.wt% NH₄HCO₂ [37]. Muthukrishnan et al. have obtained electrochemical window of 1.97 V and 2.35 V for 50 M.wt% pectin:50 M.wt% NH₄HCO₂ and 50 M.wt% pectin:50 M.wt% NH₄HCO₂:0.4 wt% EC [35]. Selvalakshmi et al. observed electrochemical stability windows of 2.4 V and 2.5 V for 50 M.wt% agar:50 M.wt% NH₄Br and 50 M.wt% agar:50 M.wt% NH₄I, respectively [27, 62].

Fig. 12

Linear sweep voltammetry recorded for 30 NaAlg:70 NH₄ClO₄

Construction of a proton conducting battery

The highest conducting biopolymer electrolyte has been used to develop a primary proton battery (30 NaAlg: 70 NH₄ClO₄) to assess the practical utility of the material. PbO₂, V₂O₅, and C (graphite) in the ratio 4:1:0.5 as cathode in pellet form whereas Zn powder, ZnSO₄, and C in the ratio 3:1:1 are used as the anode [63]. The highest conducting biopolymer electrolyte 30 NaAlg:70 NH₄ClO₄ is placed in the battery holder between the anode and cathode pellets. Figure 13 depicts the schematic diagram of the constructed battery. The battery configuration is

Fig. 13

Configuration of battery

$$\mathrm{Zn}+{\mathrm{ZnSO}}_{4}.7{\mathrm{H}}_{2}\mathrm{O}+\mathrm{C}\left|30\mathrm{ NaAlg }:70 {\mathrm{ NH}}_{4}{\mathrm{ClO}}_{4}\right|{\mathrm{PbO}}_{2}+{\mathrm{V}}_{2}{\mathrm{O}}_{5}+\mathrm{C}$$

The chemical reaction taking place in the battery cell are characterised as follows:

The anode reaction is

$$\mathrm{Zn}\to {\mathrm{Zn}}^{2+}+{2\mathrm{e}}^{-}$$

The cathode reaction is

$${\mathrm{PbO}}_{2}+{4\mathrm{H}}^{+}+{2\mathrm{e}}^{-}\to {\mathrm{Pb}}^{2+}+{2\mathrm{H}}_{2}\mathrm{O}$$

$${\mathrm{V}}_{2}{\mathrm{O}}_{5}+6{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\to 2{\mathrm{VO}}^{2+}+3{\mathrm{H}}_{2}\mathrm{O}$$

The initial V_oc is monitored with respect to time and recorded as 1.76 V (Fig. 14), and it is observed for 60 h. The external load of 100 KΩ is applied across the cell. The initial cell potential value decreases from 1.76 to 1.71 V, and the current of 17 μA is drawn. After discharging, the voltage steadily drops and remains constant at 1.24 V for 60 h (Fig. 14). Figure 15 shows the V_oc. Table 8 provides the values for the cell parameters.

Fig. 14

Open circuit voltage and discharge characteristic curve for the cell

Fig. 15

Open Circuit Voltage for 30 NaAlg: 70 NH₄ClO₄

Table 8 Cell parameters

Muthukrishan et al. have reported a V_oc of 1.48 V for 30 M.wt% pectin:70 M.wt% NH₄I [34]. Maheshwari et al. have reported for the composition of 700-mg dextran:300-mg PVA:450 mg NH₄NO₃ system a V_oc of 1.51 V [52]. Meera Naachiyar et al. reported a V_oc of 1.62 V for 1-g gellan gum:1.1 M.wt% NH₄SCN [64].

Fabrication of PEM fuel cell

The PEM fuel cell has been developed in accordance with the techniques used by Monisha et al. [58]. A hand-tightened membrane electrode assembly (MEA), bipolar graphite plates, teflon sheets, thin gaskets, copper plates, and stainless steel plates compose the PEM fuel cell. Figure 16a shows the parts of the fuel cell. Teflon sheet is used as an insulator between the stainless steel base plate and the copper plate. Copper plates are typically used as current collectors in PEM fuel cells due to its strong electrical and thermal conductivity and weak corrosion resistance [65]. The copper plate is kept on top of the bipolar graphite plate. A serpentine flow channel [65] with a surface area of 7.84 cm² is seen in the bipolar graphite plate. To assemble the hand-tightened MEA, the highest conducting biopolymer membrane (30 NaAlg:70 NH₄ClO₄) has been placed between the catalyst. The catalyst is a platinum-coated carbon cloth with a surface area of 8.41 cm² that has been evenly coated with platinum at a rate of 0.3 mg/cm². This MEA was sandwiched between two bipolar graphite plates with 0.2-mm-thick gaskets. The bipolar graphite plate with MEA has been tightened using gaskets to enhance the flow. The constructed single stack proton exchange membrane (PEM) fuel cell is shown in Fig. 16b.

Fig. 16

a Parts of fuel cell, b constructed single stack fuel cell, c electrolyser

The electrolyzer has been used to produce the hydrogen and oxygen gases (Fig. 16c). It is operated by a 2-V DC power supply. This electrolyzer supplies 80 ml of oxygen and 100 ml of hydrogen into the PEM fuel cell. When the hydrogen molecule passes over the platinum-coated carbon catalyst, it splits into protons and electrons. The electron flows through the external circuit. The proton crosses the membrane and reaches on the other side, where it combines with an oxygen molecule to produce water. Overall reactions are PEM fuel cell is given below.

$\mathrm{Anode reactions}: 2{\mathrm{H}}_{2}\to 4{\mathrm{H}}^{+}+4{\mathrm{e}}^{-}$

$$\mathrm{Cathode reactions}:{\mathrm{ O}}_{2}+4{\mathrm{H}}^{+}+4{\mathrm{e}}^{-}\to 2{\mathrm{H}}_{2}\mathrm{O}$$

$$\mathrm{Overall reactions}: 2{\mathrm{H}}_{2}+{\mathrm{O}}_{2}\to 2{\mathrm{H}}_{2}\mathrm{O}$$

The PEM fuel cell fabricated using highest conducting biopolymer electrolyte (30 NaAlg:70 NH₄ClO₄) shows the V_oc of 789 mV, which is shown in Fig. 17a. The V_oc 763 mV and 580 mV have been reported by Meera Naachiyar et al. for gellan gum with NH₄HCO₂ and gellan gum with NH₄SCN [37, 64]. Vanitha et al. have been reported the V_oc of 431 mV for NaAlg with NH₄SCN; similarly for NaAlg with NH₄HCO₂ is 707 mV [26, 38]. The V_oc for i-carrageenan is combined with NH₄NO₃ has been shown by Moniha et al. to be 442 mV [57]. V_oc of 656 mV for CA with ammonium nitrate has been found by Monisha et al. [58].

Fig. 17

OCV of single PEM fuel cell for a 30 NaAlg:70 NH₄ClO₄ and b Nafion.™ 212

The PEM fuel cell is connected to several loads such as 10 Ω, 270 Ω, and 620 Ω. For 15 min, each load connects to the fuel cell. At the initial and after 15 min, the current and voltage are measured. After that, the load was removed, and the fuel cell was given time to stabilize. Then connecting the other load, current, and voltage are measured initially and after 15 min. Each load repeats this process. The value of voltage and current for different loads are shown in Table 9. The voltage and corresponding current plotted as graph (Fig. 18).

Table 9 Current and voltage for various load connected in the fuel cell (30 NaAlg:70 NH₄ClO₄ and Nafion™ 212)

Fig. 18

Variation of current and voltage for various loads (620 Ω, 270 Ω, and 10 Ω) for 30 NaAlg:70 NH₄ClO₄ a at instant and b after 15 min

Comparison with Nafion.™ 212

Using a Nafion™ 212 membrane PEM fuel cell has been constructed under the similar condition used for NaAlg (30 NaAlg:70 NH₄ClO₄), and results are provided in Table 9. Nafion™ 212 membrane, the V_oc of 793 mV has been obtained (Fig. 17b). The voltage and corresponding current plotted as graph (Fig. 19).

Fig. 19

Variation of current and voltage for various loads (620 Ω, 270 Ω, and 10 Ω) for Nafion™ 212. a At instant and b after 15 min

Power density and current density have been calculated for the fuel cell constructed with the 30 M.wt% NaAlg: 70 M.wt% NH₄ClO₄ and Nafion™ 212 under similar condition. The results are shown in Fig. 20.

Fig. 20

Plot for voltage vs. current density and power density for 30 M.wt% NaAlg and 70 M.wt% NH₄ClO₄ and Nafion.™ 212

Conclusion

Solution casting technique has been used to develop a solid biopolymer electrolyte system based on sodium alginate (NaAlg) with a various NH₄ClO₄ concentrations. XRD reveal the amorphousness of the biopolymer electrolytes. The complex formation between the salt and the biopolymer is confirmed by the FTIR study. DSC studies have determined the glass transition temperature. The impedance study showed that the 30 M.wt% NaAlg:70 M.wt% NH₄ClO₄ has the highest ionic conductivity of 3.59 × 10⁻³ S cm⁻¹. The measurement of the transference number is done to confirm that the conduction occurs mainly by ions. The primary proton battery and PEM fuel cell have been constructed with highest conducting biopolymer electrolyte exhibits a V_oc of 1.76 V and 789 mV. It is clear from the results that the synthesized biopolymer electrolyte based on NaAlg and NH₄ClO₄ is more effective to be used with solid state electrochemical devices.