Introduction

The hunt for improved electrode and electrolyte materials, so that performance of energy devices can be enhanced, is an ever-lasting process. The progress in the field of electrode materials is quite appreciating [1,2,3,4], but to demonstrate the performance of any new electrode materials, the scientific community still uses liquid electrolytes [5,6,7,8]. The bigger size of the ions requiring larger space for movement leads to poor conductivity in solid physique [9, 10]. But with liquid electrolytes, the problems of lower electrochemical stability window, corrosion, and packaging-related issues have been faced. Few organic electrolytes and ionic liquid-based electrolytes have shown better performance but they are not cost-effective. Polymer electrolytes are widely being used at commercial level but additives are required to achieve the desired electrochemical properties [11,12,13,14]. After the work of Angell and co-authors in 1990s [15,16,17], polymer-in-salt electrolytes (PISEs) came as a possible alternate to have faster ion movement in solid-state electrolytes, but the efforts in the field still did not reach commercial applications. In PISEs, ion transport is not only faster but it may also have greater cationic transference number, a well-desired property of the electrolytes [18, 19]. PISEs offer a possibility to blend the fast-ionic conduction of glassy electrolytes with the required mechanical properties of rubbery polymers [20]. In the case of PISEs, the ion motion is decoupled from polymer segmental motion, and due to its diffusion through ion clusters, it is much faster. Even after so many promising characteristics, the PISEs are not reaching commercial level because of brittleness of lattice at higher salt concentration, recrystallization of salt, need of eutectic salt combination which can remain in molten state at room temperature so that by adding a small amount of polymer a better electrolyte can be achieved. Previous studies indicate [21, 22] that a loosely bonded host matrix may lead to better PISE. In few recent studies, it has also been pointed out that polymer hosts which can be broken into small molecules by addition of salts may also lead to better PISEs [23,24,25]. With Na and Mg salts, it has been proved that crosslinked starches are quite good host for ecofriendly, economical, and flexible PISEs which are free from aging effects/recrystallization problems, and hence in the present study, Ca salt–based PISE is being synthesized and characterized for their energy device fabrication. Calcium is the 5th most abundant element on the earth’s crust that can make energy devices more cost-effective [26], and studies on the development of Ca-based energy devices are in an early stage [27] relative to the well-studied electrolyte systems based on Li+ [28, 29], Na+ [30, 31], K+ [32], Mg2+ [33, 34], and Al3+ [35, 36]. In the present work, glutaraldehyde (GA)–crosslinked starch has been used as host matrix. Crosslinking it with glutaraldehyde not only makes the system fungal-free but it also leads to flexible films which can be molded into different sizes and shapes. Although crosslinking with GA leads to many favorable properties, there is an optimum value after which other problems are seen in the material [37, 38]. To the best of the author’s knowledge, understanding of the Ca-based PISEs is not reported till date and hence this work is an attempt to understand that the same Ca is the fifth most abundant element naturally found in earth’s crust which is attracting scientific community for its application in energy devices [39, 40]. CaCl2 is used as this can be dissolved in solvents like methanol due to its low dissociation energy [41] which makes electrolyte synthesis non-toxic in nature. The primary goal of our study is to develop environmentally benign and cost-effective systems, which is why it was chosen to employ CaCl2 as the material for energy device applications [42].

To develop an understanding of the electrochemical properties of calcium-based polymer electrolytes, a series of electrolytes have been synthesized from 25 wt% salt to 72 wt% salt with crosslinked starch as a host polymer. Since the aqueous electrolytes are the most promising electrolytes (if the smaller ESW and designing problems are neglected), efforts are being made to have water-based electrolytes in solid state [43, 44] and a new nomenclature “Water-in-Polymer Salt electrolytes (WiPSEs)” [45, 46] has recently appeared in literature. In the present study, efforts have also been made to enhance the moisture content of the synthesized PISE by exposing it in a high humidity atmosphere.

Experimental details

Materials

Arrowroot starch, calcium chloride dihydrate (Sigma-Aldrich, 99% purity), glutaraldehyde (Merck, Product Code 8.20603.0521), methanol (Qualigens, HPLC grade (assay percentage range 99.8%)), carbon black (Super P, 99% purity) (Alfa Aesar), polyvinyl alcohol (PVA) (Sigma-Aldrich, Product Code 363,081), distilled water (lab prepared).

Electrolyte preparation

Arrowroot starch, calcium chloride, glutaraldehyde, and methanol were used to prepare the sample. Solid polymer electrolyte films were prepared at room temperature (40 ℃) using the solution cast technique as used previously in our lab for synthesis [47,48,49] as shown in Fig. 1a. A series of electrolytes were prepared using 0.5 g of starch with different amounts of salt 0.25 g, 0.5 g, 0.75 g, 1.0 g, and 1.25 g, and for further discussion, they are named 25wt%, 50wt%, 60wt%, 66wt%, and 72wt%. For the preparation of 25 wt% electrolyte, 0.5 g of starch was added to 10 ml methanol and left to continuously stir for 1 h at room temperature (40 ℃). In a separate beaker, 0.25 g of salt was added to 10 ml methanol, and the mixture was continuously stirred for 1 h at room temperature. Then, 1 ml glutaraldehyde is added to the starch mixture and left for continuous stirring for 30 min. Then, salt mixture was added to the starch mixture and stirred at a constant speed (350 rpm) and the whole mixture was left to stir till a homogenous mixture was obtained. After this, a homogeneous mixture was poured into the petri dish and was left to dry at ambient for almost 10 days to obtain a transparent, free-standing film as shown in Fig. 1. Same protocol is followed for all salt concentrations.

Fig. 1
figure 1

a Protocol for synthesis of polymer electrolyte film. b Image of synthesized PISE film at 72wt%. c Image showing ease of handling of PISE at 72wt%. Image of electrolyte at d 25wt%, e 50wt%, f 60wt%, g 66wt%, and h 72wt%

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Preparation of electrodes

Commercial rGO and commercial activated carbon (CAC) were used as two types of electrode material for the comparative study of the charge storage mechanism by fabricating a supercapacitor. The slurry was prepared using distilled water and 80wt% of active material (CAC, rGO), 10wt% conducting agent (Super P), and 10wt% of binder (PVA, Sigma-Aldrich). Firstly, add PVA into distilled water and left for stirring till it dissolves. After dissolving PVA in distilled water, add Super P, active material (CAC or rGO), and stir for 3 h to get slurry. The prepared slurry was coated on carbon cloth using the doctor blade technique and was left to dry at 100 ℃ for 3 h. The weight of the loaded active material is 0.24 mg/cm2 for rGO and 0.24 mg/cm2 for CAC. The obtained electrodes were used for the characterization.

Structural and electrochemical characterization

XRD measurements were recorded over the range of diffraction angle 5° to 80° done using D2 Phaser Bruker. For moisture content analysis of electrolytes, the HS153 moisture analyzer machine of Metler Toledo was used. The electrochemical impedance spectroscopy (EIS), linear sweep voltammetry (LSV), cyclic voltammetry (CV), and galvanostatic charging discharging (GCD) were carried out using a two-electrode setup using CHI660E. The conductivity of the electrolyte was calculated using the following equation [50]:

$$\sigma = l/(R \times A)$$
(1)

Here, R denotes the ESR (ohm) value of the electrolyte, and l and A represent the thickness of the electrolyte and surface area of contact of electrode/electrolyte in cm2 respectively. σ represents the conductivity. The specific capacitance using GCD plot of the fabricated supercapacitor was calculated using the formula:

$$C = \frac{(I .\Delta t)}{(m. \Delta V)}$$
(2)

Here, C represents the specific capacitance of the fabricated supercapacitor in units of F/g. (I/m) is the current density. \(\Delta\) t is discharging time from GCD plot, m is the mass of the active material, and \(\Delta\) V is the voltage range.

The specific capacitance using the cyclic voltammetry curve of the fabricated supercapacitor was calculated using the formula:

$$C = (\int\text{IdV})/(2mVs)$$
(3)

Here, C represents the specific capacitance of the fabricated supercapacitor in units of F/g. ∫IdV is the area enclosed in CV curve. s is the scan rate (V/s), m is the mass of the active material, and V is the voltage range.

The values of specific energy density (E) and power density (P) were calculated using the following formula [51, 52]:

$$E (\text{W h/ kg}) = ((0.5) \times (\text{C}V^{2}))/3.6$$
(4)
$$P (\text{W /kg}) = (E/t)3600$$
(5)
$$\eta=t_d/t_c\,\times100\%$$
(6)

Here, C is specific capacitance (F/g), V is the potential change after IR drop, and t (s) is discharging time in GCD plot. Here, η represents coulombic efficiency (%) where td and tc are the time of discharging and charging of the supercapacitor.

Results

Structural characterization

The biggest hurdle in the path of PISEs reaching commercial level is recrystallization of salt at high salt concentrations. Hence, while characterizing PISEs, XRD comes as the most basic analysis as it provides information on whether the crystallinity is present in the electrolyte or not. Hence, XRD is used to understand the crystalline and amorphous nature of prepared electrolytes [53] from salt-in-polymer electrolyte (SIPE) to PISE range. Figure 2 shows the XRD spectra of pure salt (CaCl2.2H2O) and synthesized electrolytes from 25wt% salt to 72wt% salt. In XRD data, distinct XRD peaks could be seen only in pure salt which is found to be identical to the JCPDS data (ICCD-PDF 01–070-0385). It is observed that the crystalline nature of pure salt has completely disappeared at 25wt% salt itself. While shifting salt concentration from 25wt% to 72wt%, no recrystallization is detected in XRD spectra, which means complete acceptance of salt by crosslinked starch matrix without recrystallization.

Fig. 2
figure 2

XRD spectra of CaCl2 salt along with 25 wt% (SIPE) to 72 wt% (PISE) of salt used for electrolyte formulation

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The present system has an edge over the systems reported [54, 55] with synthetic polymers as host matrix, as the XRD data indicates that the amorphicity is continuously enhancing with salt concentration. The possible reasons for this may be (i) breaking of starch molecules into smaller molecules by salt [25] which leads to enhanced availability of ion-interacting moieties and also enhances the possibility of hydrogen bonding leading to loosely bonded structure (ii) enhance moisture content with salt concentration which also support better interaction of ions and starch through hydrogen bonding via these water molecules.

Electrochemical performance of synthesized electrolytes

Electrochemical impedance spectroscopy was used for analyzing the effect of salt concentration on the conductivity of the electrolytes with salt concentration performed in the range of 1 Hz to 1 MHz. Conductivity and ESW have been analyzed to understand the electrochemical behavior of synthesized electrolytes. Impedance data has been analyzed by Nyquist plots and is given in Fig. 3a.

Fig. 3
figure 3

a Nyquist plot for prepared materials having different salt concentrations (mentioned salt wt% in total material). b ESW for all synthesized polymer films with different wt% of salt present in electrolyte films as mentioned in the curve. c Variation of conductivity with variation of wt% of salt concentration in polymer films. d Ratio of parallel capacitance and series capacitance Cp/Cs graph with log frequency

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The straight-line nature of Nyquist plot which intercepts at very low resistance indicates high conductivity of the synthesized PISE. Nyquist plots indicate that as the salt concentration is increased the resistance is continuously decreasing. The conductivity vs salt concentration plot given in Fig. 3c indicates a jump at nearly 50wt% salt concentration which may be correlated with transition from SIPE to PISE transport mechanism. LSV curves for all five synthesized electrolytes are given in Fig. 3b and the ESW estimated from these curves are summarized in Table 1.

Table 1 Values of ESR, ESW, conductivity, moisture content, and relaxation time with variation in salt concentration
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ESW is quite wide, ~ 4 V for all electrolytes. A decrease in ESW with salt concentration is an obvious [48,49,50, 56,57,58] phenomenon because as the salt concentration increases, there are more number of fast moving ions and hence can be easily oxidized/reduced leading to a lowered value of ESW.

The value of ESW depends on the combination of electrolyte and electrode [59] resulting to the limits of the maximum energy density and power density of the system [60]. Here, it is evaluated by using an Al electrode to study the ESW of the prepared series of SIPE to PISE in order to understand the electrochemical stability of the material. Al/electrolyte/Al composition is used to estimate the ESW and Fig. 3b shows the LSV curves for electrolytes with different salt concentrations. It is observed that from SIPE to PISE, ESW has decreased.

The value of relaxation time is estimated from the frequency at which the value of Cp/Cs is 0.5 [61] as shown in Fig. 3d and values for different salt concentrations are summarized in Table 1. Although, for both SIPE and PISE, the relaxation time τ is in μs range, the resistance values of SIPE and PISE are different that can be associated with difference in the available number of charge carriers in two ranges. The estimated relaxation times are in μs range indicating faster ion transport in synthesized electrolytes, which can be associated with high moisture content in the system leading to aqueous type of electrolyte behaviors.

Humidity/moisture content (MC) effect on ESR

Aqueous electrolytes are well established, but due to leakage/designing problems, their solid-state alternates are always sought. In this hunt, for achieving aqueous electrolyte properties in solid state, with wider electrochemical stability window (ESW), a new class of electrolytes classified as “Water-In-Salt-Electrolytes”, or “Water-In-Polymer-Salt-Electrolytes” [43,44,45,46], are being explored by the researchers. Here, in the present system where the salt and starch both have an affinity for water, a novel approach has been adopted to stabilize water in the polymer matrix. Moisture content seems to play a favorable role in stopping the recrystallization by keeping the matrix flexible and easy to handle for design purposes.

Hence, to develop the understanding of synthesized electrolytes, MC and resistance of the synthesized samples have been measured at different relative humidities in series of five steps, as mentioned in Table 2. A difference between the behavior of SIPE and PISEs at different humidity levels is observed. In the case of SIPE, resistance drastically changes varying from ~ 3Ω (at 99% RH) to 9358 Ω at (22%) whereas in the case of PISEs the variation is very small. It indicates that in the case of SIPE either the H+ or OH- from absorbed water are moving or water is assisting the movement of cation/anion from salt.

Table 2 Effect of different relative humidities on the ESR (ohm), ESW (V), and moisture content (%) of the 25 wt%, 60 wt%, and 72 wt%
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ESW of these three samples were also measured at different humidity levels. The results are very interesting and in accordance to the previous patent from our group. In SIPE region at 99% RH, the ESW is almost zero indicating that the absorbed water molecules are easily dissociated [62], whereas on moving towards higher salt concentration water molecules are stabilized [46, 63,64,65]. At normal ambient humidity (~ 60% RH), the ESW is > 2.5 V which is quite wide for device fabrication. The effect of different humidity exposures on the moisture content and Nyquist are shown in Fig. 2S-12S.

Supercapacitor performance using activated carbon and rGO

To understand the possible application of synthesized electrolytes for energy device fabrication, two symmetric supercapacitors are fabricated, with commercially available active electrode materials namely (i) rGO and (ii) activated carbon (CAC). Active materials are coated on carbon cloths to be used as electrode. In both the cases, the configuration used is electrode/electrolyte/separator/electrolyte/electrode with commercial tissue paper (cellulose based) as a separator. The values of ESR estimated from Nyquist plots given in Fig. 4a are 9.25 Ω for CAC electrode and 8 Ω for rGO electrode. The electrochemical stability window (ESW) were estimated from LSV curve given in Fig. 4b. The ESW value obtained for rGO electrode is > 2.5 V but obtained ESW > 2 V for CAC electrode. The difference between the ESW for two systems can be correlated with the size of ion clusters with surface area as well as the pore size of electrode material [66].

Fig. 4
figure 4

a Nyquist of prepared supercapacitor devices using CAC and rGO electrode material. b Linear sweep voltammetry graphs of fabricated supercapacitors using CAC and rGO as electrode material

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CV curve of devices with rGO and CAC electrode are shown in Fig. 5 a and b at different scan rates ranging from 1 to 10 mV/s. The values of specific capacitance calculated using CV plots are found to be 17 F/g and 125 F/g respectively for rGO and CAC electrode materials using Eq. (3) at 1 mV/s scan rate as mentioned in Table 3. CV curves with rGO are more rectangular in comparison to CAC-based systems indicating EDLC type nature of rGO-based supercapacitor. Specific capacitance estimated as a function of scan rate is given in Fig. 5c, which shows quite a stable behavior, while CAC electrode based supercapacitor shows pseudocapacitive behavior which can state that the charge transfer between the electrode and electrolyte occurs as a result of a faradic current passing through the supercapacitor in pseudocapacitors, which store energy by faradaic reaction, intercalation, or electrosorption [5] that may have resulted in the higher specific capacitance in CAC-based supercapacitors. The variation of specific capacitance with scan rate for CAC electrode is shown in Fig. 5d.

Fig. 5
figure 5

Cyclic voltammograms (CV) of fabricated supercapacitor scan rate ranging from 1 to 10 mV/s using a rGO electrode and b CAC electrode. Value of variation in specific capacitance with variation in the scan rate using c rGO electrode and d CAC electrode. Ragone plot for comparing the category of energy device fabricated using e rGO electrode and f CAC electrode

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Table 3 Variation in specific capacitance, energy density of rGO- and CAC-based supercapacitors, calculated by using CV plots
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There is a small variation in the value of specific capacitance with scan rate because at low scan rates (1 mV/s) the larger electrode surface is utilized for electrochemical reaction and at higher scan rates a limited surface area of electrodes can be utilized resulting in low value of capacitance [67]. At high scan rates, the high diffusion resistance which could be because of meager ion migration and ion diffusion affecting the ions’ movements between electrode and electrolyte results in decreased specific capacitance [68].

The two most popular electrochemical energy storage technologies among the different energy storage devices are batteries and supercapacitors, as seen in a graph known as a Ragone plot that shows energy density in Wh/kg vs. power density in W/kg as shown in Fig. 5 e and f. Using the Ragone plot, it can be found that in the studied energy storage systems power density lies in the range of 103–104 W/kg and energy density in the range of (10−1–10−2) Wh/kg, which belongs to the category of supercapacitors or ultracapacitors [69, 70].

Analysis of charging-discharge curve

Galvanostatic charging-discharging (GCD) has been carried out to access the charge and discharge characteristic of rGO- and CAC-based supercapacitors, ranging from current density 4 A/g to 37.5 A/g. Figure 6 a and b show the charging-discharging plot for rGO- and CAC-based supercapacitor. It is observed that time taken for the charging-discharging of cycles is ~ 0.5 s that may be due to the low relaxation time and high conductivity of electrolyte material. Charging-discharging test of rGO and CAC electrode is carried out till 5000 cycles to check the stability of the system as a supercapacitor at 4 A/g. Charging-discharging curve shows triangular shape cycles [71, 72] which indicates the fast ion transport in fabricated supercapacitor at different current densities in the 0–1 V voltage range.

Fig. 6
figure 6

Charging-discharging plot of fabricated supercapacitor using a rGO electrode and b CAC electrode. Coulombic efficiency up to 5000 cycles using c rGO electrode and d CAC electrode

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The values of energy and power density are mentioned in Table 4 and are found to be 0.14 Wh/kg, 2.1 kW/kg, and 0.12 Wh/kg, 2.1 kW/kg that are comparable to the other reported systems [73,74,75] which is due to the short relaxation time of the synthesized electrolytes. Figure 6 c and d show the value of coulombic efficiency found to be (η %) > 90% for rGO-based supercapacitors and the value of coulombic efficiency is found to be (η %) > 99% in CAC-based supercapacitor.

Table 4 Variation in specific capacitance, energy density, and power density of rGO- and CAC-based supercapacitors, calculated by using GCD plots
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Conclusions

GA-crosslinked arrowroot starch doped with CaCl2 salt–based polymer electrolytes is studied for the SIPE to PISE range, by using a simple synthesis protocol which satisfies the three “E” conditions (environment friendly, easy to prepare, economical) required for commercial application. Here, success has been achieved to keep it free from recrystallization problem without the need of any additives and/or the requirement of molten salt/salts which makes it an interesting material. High humidity exposure of PISE films resulted in ambient stabilized electrolyte with high conductivity, which could be considered aqueous electrolyte in solid-state physique. Small relaxation time (~ 17 μs) and high conductivity (10−2 S/cm) with high ESW range (> 3.5 V) make the synthesized polymer electrolyte film a potential candidate for future commercial testing. Supercapacitors are fabricated using commercially available activated carbon (CAC) and reduced graphene oxide (rGO). rGO-based supercapacitor is having EDLC behavior whereas CAC-based supercapacitor has pseudocapacitance type of nature. Faster charging-discharging cycles are obtained because of PISE films, which results in high power density values. Although the energy and power density estimated from GCD techniques results in almost similar energy and power density behavior for both the systems, CAC-based supercapacitor seems to have better coulombic efficiency (~ 99%) in comparison to rGO-based supercapacitor (~ 90%), which indicates long life and stable behavior. Achieved power density is of the order of kW/kg which adds up to favorable properties for commercial application.