Introduction

Over the past century, the utilization of polymers in different applications has outperformed other class of materials that in turn has allowed the polymer industry to grow more rapidly than any other industry in recent years. Polymer electrolyte is an essential component in energy conversion and energy storage system, such as fuel cells, batteries, and dye-sensitized solar [1,2,3]. Although the performance of such polymer electrolytes is not any better than that of lithium-based electrolytes, they do not pose any safety issues; it has better contact between electrode and electrolytes apart from exhibiting good mechanical properties [4, 5].

Owing to the aforesaid factors, polymer electrolytes have received due attention from researchers, particularly of those who aim of utilizing the polymer electrolytes as polymer host based on natural polymers in proton conducting of solid polymer electrolytes (SPEs). Different types of natural polymers have been investigated, namely, rice starch [6], chitosan [7], iota-carrageenan [8], and methylcellulose (MC) [9]. Previous works have demonstrated that natural polymer possesses the ability to solvate the dopant that supplies mobile ions, in the event that there is a direct interaction between the lone pair electron of the heteroatom, i.e., oxygen or nitrogen in the polymer [10, 11]. A potential candidate to act as polymer host for solid electrolyte is carboxymethylcellulose (CMC). CMC is one of the cellulose derivatives which displays biodegradable and non-hazardous properties [12]. It has been widely used in different industrial applications such as in the food industry as a thickener and a binder, in the oil industry as a lubricant for oil production, and in the cosmetic industry as a stabilizer [13]. In this work, CMC is chosen as it has good film-forming property, high mechanical intensity, and ability to form transparent film [14].

Several approaches were suggested in the literature in order to enhance the conductivity performance, including the use of blend polymers, the addition of a ceramic filler, plasticizer, and even radiation [15]. According to Bhide et al. [16], the addition of plasticizer is the simplest and most effective way to improve the conductivity of SPEs in comparison to other techniques. Examples of plasticizer which has been employed in polymer electrolytes systems include ethylene carbonate [17], glycerol [18], dimethylacetamide (DMA) [19], and tetra (ethylene glycol) dimethyl ether [20] among others. These plasticizers were chosen based on a number of properties such as high dielectric constant, its ability to enhance the ionic dissociation, low viscosity, decrease the glass transition, and increase the amorphous content [21,22,23].

In the present work, a proton-conducting SPE system based on CMC-NH4Br plasticized with PEG was prepared and investigated. The CMC-NH4Br-PEG-based SPE system have been characterized via Fourier transform infrared (FTIR) spectroscopy, electrical impedance spectroscopy (EIS), and x-ray diffraction (XRD) in order to study the interaction between polymer-salt complexes plasticized with PEG by determining the functional group that introduces in the complexation SPEs, the degree of crystallinity to establishes the amorphousness nature of SPEs, and the electrolyte composition that yields the best electrical conductivity. Moreover, for further investigation in ionic transport properties of SPEs, the FTIR vibrational effect was evaluated by using deconvolution method to confirm the conduction of ions that dissociate in the SPEs electrolyte system.

Experimental

Preparation of solid polymer electrolytes

In this present work, SPE system has been prepared via solution cast technique; 25 wt% NH4Br (Merck Co.) was added into CMC (Acros Organics Co.) solution, and the mixture was stirred continuously until complete dissolution was obtained [24,25,26]. Then, the various compositions of PEG (Mw ~ 1100: Sigma Aldrich Co.) plasticizers were added (in wt%) into the mixture and were stirred until it becomes a homogenous solution. The homogeneous solution was then casted into several Petri dishes and left dried at room temperature until film form. For further drying, the dried sample was put it in the desiccators before being characterized to ensure that no water content or gelling form obtained in the polymer electrolyte system. The compositions of PEG and classification of the sample for SPEs system are tabulated in Table 1.

Table 1 List of samples with their compositions respectively
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Characterization of CMC-NH4Br SPEs system

EIS

The ionic conductivity of SPEs system was analyzed via EIS using HIOKI 3532-50 LCR Hi-Tester at a different frequency in the range of 50 Hz to 1 MHz. The sample was cut into a suitable size and placed between two stainless steel blocking electrodes of the sample holder. The ionic conductivity of SPEs system was calculated using the equation:

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

where d is the thickness of the sample, Rb is the bulk resistance, and A is the cross-sectional area.

FTIR spectroscopy

FTIR spectroscopy measurement was carried out using PerkinElmer Spectrum in order to confirm the complexation in the SPEs system. The spectrometer was equipped with an attenuated total reflection (ATR) accessory. The germanium crystal was used for ATR, and the infrared light was passed through the sample in the range of 700–4000 cm−1 with a resolution of 4 cm−1.

Deconvolution study

The FTIR deconvolution was carried out using Gaussian-Lorentz function adapted to the Origin Lab 9.0 software. In the deconvolution method, the FTIR peaks due to the dominant ionic movement were selected, and the sum of the intensity of all the deconvoluted peaks was ensured to fit the original spectrum. The area under the peaks was determined, and the free ions (%) were calculated using the equation [27, 28]:

$$ \mathrm{Free}\ \mathrm{ions}\ \left(\%\right)=\frac{A_f}{A_{f+{A}_c}}\times 100\% $$
(2)

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, M is the moles of each wt% PEG, NA is the Avogadro’s constant, Vtotal is the total volume of SPEs system, k is the Boltzmann constant, T is the absolute temperature, and e is electron charge. The number density (ƞ), mobility (μ) and diffusion coefficient (D) of the ions were calculated with the following equation [29,30,31]:

$$ \eta =\frac{M{N}_A}{V_{\mathrm{Total}}}\ \mathrm{x}\kern0.50em \mathrm{free}\ \mathrm{ions}\ \left(\%\right) $$
(3)

where

$$ {V}_{Total}=\left[\frac{weight}{density}\ \left( CMC+N{H}_4 Br\right)\right]+\left[\frac{weight}{density}\ (PEG)\right] $$
(4)
$$ \mu =\frac{\sigma }{\upeta e} $$
(5)
$$ D=\frac{kT\mu}{e} $$
(6)

XRD

XRD-Rigaku MiniFlex II was employed to study the nature of samples whether amorphous or crystalline or both [32]. It was directly scanned at angle 2θ between 5° and 80° with 1.5406 Å wavelength generated by a CuKd source. In XRD deconvolution technique, Origin Lab 9.0 software was used, and the baseline function was applied to the specified region. By using the Gaussian function, the amorphous and crystalline peaks were deconvoluted, and all peaks were ensured to fit the original spectrum. After extracting the peak, the area under the peaks was determined, and the percentage of crystallinity was calculated using Eq. (7) below [33, 34].

$$ Xc=\frac{A_c}{A_T}\times 100\% $$
(7)

where Ac is an area of the crystalline region; AT is the total area under the peak representing the area of the crystalline region, Ac, and area of the amorphous region, Aa; and Xc is the degree of crystalline in percentage.

Result and discussion

Impedance analysis

MacDonald [35] reported that the impedance spectroscopy (EIS) gives a good overview, especially to solid-state phenomena. It should be understood as “immittance,” i.e., any form of the transfer function of two terminal systems: impedance Z, admittance Y = Z−1; modulus M = jωCoZ; and complex permittivity M−1, where j = \( \sqrt{-1} \), ω is angular frequency (ω = 2πf), and Co is the permittivity of free space. The principle of its operation is relatively straightforward: a series of small-amplitude ac signals spanning a broad frequency range is applied to a multiphase system. Analysis of the ensuing electrical response via electrical circuit theory results in a complete description of the physicochemical properties of the system. Figure 1 shows the Cole-Cole plots at ambient temperature (303 K) for various sample of SPEs system.

Fig. 1
figure 1

The Cole-Cole plot of various samples SPEs system

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Based on Fig. 1, it can be observed the Cole-Cole plots only consists of a spike line which can be related to the migration of ions which may occur through the free volume of the matrix polymer and this can be represented by a resistivity element of the present sample [36]. Moreover, the spike line is attributed to the influence of electrode polarization, which is the characteristic of the diffusion process and it also indicates that the current carriers are ions and the total conductivity is mainly the result of ion conduction of SPEs system [37,38,39]. The inclination of the spike in Cole-Cole plot as observed in the present system shows an angle less than 90° with the real axis when PEG was added and this is attributed to the non-homogeneity or roughness of the electrode-biopolymer electrolytes interface [40].

Since the spike in this present system did not arise at the origin with the addition of PEG for SPEs system, the equivalent circuit is implied to consist of a resistor and constant phase element (CPE) in series connection. CPE can be assumed as a leaky capacitor, to compensate for non-homogeneity between electrode-SPE surface [40]. As shown in Fig. 1, the impedance of the expected equivalent circuit fits the experimental impedance data very well. The value Zr and Zi associated to the equivalent circuit can be expressed as [41, 42]:

$$ {Z}_r={R}_{\mathrm{b}}+\frac{\cos \left(\frac{\pi p}{2}\right)}{C{\omega}^p} $$
(8)
$$ {Z}_i={R}_b+\frac{\cos \left(\frac{\pi p}{2}\right)}{C{\omega}^p} $$
(9)

where

$$ {Z}_{\left(\mathrm{total}\right)}={Z}_r-{jZ}_i $$
(10)

where C is the capacitance of CPE, ω is the angular frequency (ω = 1/f; f is frequency in Hz), p is related to the deviation of the plot from the axis, and j is \( \sqrt{1} \) and the real and imaginary parts of impedance, Zr and Zi, respectively.

Based on theoretical fitting in Fig. 1, the parameter of the circuit elements for SPEs system is tabulated in Table 2. Shuhaimi et al. [43] report that if p = 1, Ztotal = R − j/ωC; if p = 0, the constant phase element is a perfect resistor where Ztotal is frequency independent and if the value of p lies between 0 and 1, CPE will act in a way intermediate between a resistor and a capacitor for conducting sample. From Table 2, it shows the value of p lies between 0 and 1, and this reveals that the SPEs system has a resistive and capacitive behavior which is similar as reported in [43].

Table 2 The value of circuit elements of the SPEs system
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It is evident that the sample becomes more capacitive than resistive as the PEG composition increases. The bulk resistance, which was obtained from the intercept of the Cole-Cole plot with the Zr axis decreases as the PEG is increased. The increase in capacitance values with increasing plasticizer is in good agreement with the following equation [44, 45]:

$$ C=\frac{\upvarepsilon_{\mathrm{o}}{\upvarepsilon}_{\mathrm{rA}}}{d} $$
(11)

where ɛo is vacuum permittivity and ɛr is dielectric constant.

Ionic conductivity analysis

The effect of PEG composition on the conductivity at room temperature is depicted in Fig. 2. Based on [46], the highest conductivity for un-plasticized SPEs system containing with CMC-NH4Br is achieved at 1.12 × 10−4 S cm−1 for the sample with 25 wt% NH4Br. For the present system, the addition of PEG is found to influence the increment in ionic conductivity except for sample B which contains 2 wt% of PEG. The decrease of ionic conductivity at lower PEG composition may be due to the insufficient energy from PEG for ions (H+) to detach from NH4+ substructure of NH4Br in order to migrate into COO− group of CMC. The addition of PEG at lower composition could cause the delaying of ions pathway to migrate towards polymer backbone thus the reduction in ionic conductivity was observed. Furthermore, the small amount of PEG suggests that the PEG is not able to overcome the rate of ion association between CMC and NH4Br, thus demonstrating a decrease in conductivity [47].

Fig. 2
figure 2

The ionic conductivity at room temperature for SPEs system

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The highest ionic conductivity for the present system is achieved at 2.48 × 10−3 S cm−1 for sample containing 8 wt% PEG. This observation suggests that the amount plasticizer is sufficient to assist the dissociation of H+ for undissociated H+ of NH4Br, allowing the released of H+ for migration towards COO− group of CMC. As a result, an increase in the ionic conductivity and transport properties in SPEs system are observed [48,49,50]. The decrease in crystalline content or an increase in amorphous content is expected to influence an increase in conductivity [51] of these SPEs system, and these observations can be proven from XRD analysis which will be discussed later.

Apparently, it shows that beyond 8 wt% of CMC-NH4Br-PEG SPE system, the conductivity starts to decrease and it may be due to the re-association of the ions [52]. In the present system, the PEG does not supply ions towards CMC-NH4Br, but it is believed that PEG assisted in dissociating more dopant into ions, hence, increasing the ionic conductivity and transports mobility [53].

Figure 3 shows the temperature dependence of ionic conductivity for CMC-NH4Br-PEG system investigated in the temperature range from 303 to 373 K in order to understand the conductivity-temperature behavior of SPE salt polymer system plasticized with the PEG films. It can be observed that the conductivity increase gradually with the increase in temperature, which shows that the SPE system is thermally assisted. The regression values, R2 was close to unity [39, 40, 54], which indicates that the temperature-dependent ionic conductivity revealed that the SPEs system obeys the Arrhenius rule via the following equation:

$$ \sigma ={\sigma}_0{\exp}^{\left(-\frac{Ea}{KT}\right)} $$
(12)

where σ0 is the pre-exponential factor, Ea is the activation energy, and k is the Boltzman constant.

Fig. 3
figure 3

Temperature dependence of ionic conductivity for CMC-NH4Br-PEG SPE system

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FTIR analysis

Figure 4 shows the possible interaction between the CMC-NH4Br-PEG for SPE system in the region wave number between 3600 and 900 cm−1. Researchers [55,56,57] reported that from the FTIR spectra, polymer electrolyte structure according to their compositions and the occurrence of complexation and interactions between CMC, NH4Br, and plasticizer (PEG) could be investigated. Based on the previous work done by Samsudin et al. [46], the main component of polysaccharide CMC can be found at wave number 3236 cm−1 shifted to 2920 cm−1, 1589 cm−1 shifted to 1587 cm−1, and 1066 cm−1 shifted to the 1050 cm−1 that attribute to O–H bending, C=O stretching, and C–O− of carboxylate anions of CMC [58,59,60] and these peaks were identified an interaction when NH4Br was added.

Fig. 4
figure 4

FTIR spectra of CMC-NH4Br-PEG SPEs system from 3200 to 3700 cm−1

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Based on Fig. 4, it can be clearly seen that the slight different broadness of the peak at 3450 cm−1 corresponds to the hydroxyl group (O–H) of plasticizer composition and this band are also comparable with the result observed by Yu et al. [61]. The peak begins to decrease in the broadness in the region of range 3437 to 3450 cm−1 when the PEG is increased to up to 12 wt%. Hydrogen bonding of alcohols to electronegative atoms or ions reduces the frequency and broadens the FTIR band associated with the O–H stretch. Samsudin et al. reported that the hydrogen bonding of the monomeric (O–H) shifts from a sharp band to broadband with the addition of ammonium salt to the solution [62]. The shifts in (O–H) indicates that the intra and intermolecular hydrogen bonding between PEG and various anions are complicated. This observation is evident, as the addition of plasticizers caused the peak intensity of the samples to increase and the growth in the number of free ions, which in turn, influence the change of ionic conductivity [63].

Figure 5 shows the FTIR spectrum of CMC-NH4Br-PEG in the region from 1500 to 1700 cm−1. Based on Fig. 5, an absorption peak appeared at 1583 cm−1 corresponding to the C=O stretching moiety of CMC. The intensity of the peak at 1583 cm−1 is clearly found to increase as the content of PEG is increased to up to 8 wt% as shown in Fig. 5. This implies that PEG can serve as an alternative transit site for ion conduction for H+ from NH4Br towards C=O of the carboxyl group in CMC for SPEs system. It is believed that the use of PEG allows for the dissociation of more H+ from NH4Br and it would create a shorter distance or new pathway for ion migration in order to interact with C=O. This observation is similar to the observation of Nirali et al. [64], where the interaction occurred when plasticizer was added in the polymer-salt complexes, consequently influencing either the intensity of peak or changes of wave number in FTIR spectra. On the other hand, it can be seen that the intensity of the peak at 1583 cm−1 begins to decrease when PEG is added beyond 8 wt%, and this is expected due to the overcrowding of H+ ions in CMC-NH4Br-PEG SPEs system where it is expected that the ions start recombined to form neutral ion aggregate as NH4Br.

Fig. 5
figure 5

FTIR spectra of CMC-NH4Br-PEG SPEs system from 1500 to 1700 cm−1

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Meanwhile, in Fig. 6, peaks at 1065 cm−1 corresponds to C–O− moiety of carboxylate anion from COO− was found to decrease to a lower wave number at 1059 cm−1 when PEG was added. A more significant band in comparison to C=O from CMC where the interaction has occurred in this band. The shifted peak shows the more H+ interacted at C–O− of CMC and this may be due to the strong absorbance band which similarly observed by Chai et al. [65] and Kamarudin et al. [66]. Based on the observation, it is proved that the addition of PEG in CMC-NH4Br SPEs system has performed as an agent in enhancing the dissociation of NH4Br towards CMC.

Fig. 6
figure 6

FTIR spectra of CMC-NH4Br-PEG SPEs system from 1000 to 1200 cm−1

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Deconvolution analysis

In the present work, wave numbers between 1040 and 1080 cm−1 were used for deconvolution analysis where it is believed the complexes between CMC-NH4Br and PEG has occurred as proven from FTIR analysis. Figure 7 presents the deconvolution spectra for various sample of CMC-NH4Br-PEG SPEs system.

Fig. 7
figure 7

The deconvolution IR spectrum for various sample of SPEs system

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Based on Fig. 7, the peak at ~ 1055 cm−1 that attributed to free anions, conversely, the peaks at ~ 1040 cm−1 represents the contact ion pairs, and 1068 cm−1 is an indication for the formation of ion aggregates [67, 68]. The percentage area of free ions and contact ions were calculated from the ratio of the area of free or contact ions to the total area of deconvolution peaks, respectively. Table 3 shows the percentage of free ions and contact ions of the CMC-NH4Br-PEG SPEs system. Based on Table 3, it can be seen that the percentage of free ions increased when PEG content is added until 8 wt%. The increase of the ion dissociation corresponds to the free ions from the plasticizer, and the H+ ions of the NH4+ from the salt improve the ionic conductivity as observed from ionic conductivity analysis. The percentage of free is inversely proportional to the percentage of contact ions. The decrease of free ions at 10 to 12 wt% relates to the formation of ion pairing/aggregate consequently reduced the rate of increment in ionic conductivity.

Table 3 The free and contact ions of the CMC-NH4Br-PEG SPEs system
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From the free and contact ions in Table 3, the number of mobile ions (ƞ), ionic mobility (μ), and diffusion coefficient (D) can be calculated using the equation as presented in Eqs. (3), (5), and (6). Figure 8 depicts the transport parameter of CMC-NH4Br-PEG SPE system.

Fig. 8
figure 8

The transport properties for CMC- NH4Br-PEG SPEs system

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Based on the literature, the ionic conductivity of a polymer is generally linked to the number of ions and the mobility of conducting species in the polymer complexes [69,70,71] with the relation base Eq. (5).

From the transport properties, as shown in Fig. 8, it can be inferred that the conductivity is governed by the number of mobile ions, ionic mobility and diffusion coefficient. The increase of η value from 4 to 8 wt% of PEG composition were observed and this attributed to the enhancement of dissociation of H+ from NH4Br when the PEG was introduced in the present system. This result obtained is in reasonable agreement with the changes of FTIR analysis where the interaction influenced to the contribution of ion transport characteristic. Moreover, it also related to the increase in the facilitation of the H+ coordination in the plasticized system, enabling continuous motion of charge carrier that led to the maximum ionic conductivity of CMC-NH4Br-PEG SPE system. With the increment of the dissociation of H+, the conductivity for SPE system does respond better from the association of ion count and this is demonstrated through the increase of μ and D that corresponds to the conductivity until it reaches the optimum composition [72, 73]. This is due to the flexibility of the segmental motion of the polymer chains in the SPEs system which leads to the enhancement of the transport properties and conductivity [74, 75].

On the other hand, it shows that the transport properties of SPEs system were found to decrease at higher PEG composition (> 8 wt%). This is primarily due to the aggregation of the ions, which leads to the formation of ion cluster where the dipole interaction between the H+ in the medium increases. This, in turn, reduces the charge carriers and thus the conductivity [73]. The value of η, μ, and D calculated is in reasonable agreement with that obtained in refs. [46, 63, 76]. Therefore, it is proven that Eq. (5) fits well in the present work.

XRD analysis

Figure 9 shows the XRD pattern of un-plasticized and plasticized samples of CMC-NH4Br-PEG complexes. It can be found that the diffraction peak became less intense and more broad due to disruption of amorphous phase when PEG was added in the present system. These results are in agreement with the findings of Itoh et al. [77] and Rajendran et al. [78], where it was found that the plasticizer, which is inserted in polymer electrolytes system, would reduce the non-conductive crystalline phase to obtain better ionic conductivity. A similar observation is found by Leo et al. [79] where further dilution of the crystalline phase can be noticed with the addition of the plasticizer as the sharp peaks in the XRD pattern becomes broadened and less intense.

Fig. 9
figure 9

XRD spectra for various sample of SPEs system at ambient temperature

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The hump peak observed between 2θ = 20° and 30° for SPEs system broadened and shifted to the higher angle as the amount of the plasticizer is increased. The pattern shows that the amorphousness of biopolymer electrolytes increases with the increase of the PEG composition. The increase in the broadness of the peak reveals the amorphous nature with some localized ordering of the complexed system is observed in the present system [63, 80]. Hence, it would lead to an increment of the ionic conductivity in the SPEs as observed from the conductivity analysis. The results are in accordance to the findings of Majid and Arof [55] where they observed that the conductivity of the polymer electrolytes is believed to be affected if there is a change in amorphousness of polymer electrolytes system.

Figure 10 demonstrates the XRD deconvolution pattern of various samples for SPEs system. Based on the deconvoluted XRD pattern, the intensity of crystalline peaks decreases and the peak width broadens due to the breaking of order or folding patterns of polymer chain which enhances the amorphousness of the SPEs sample. This implies that the crystallinity of the SPEs is hindered by the addition of PEG, resulting in a more flexible SPEs and thus improves their ionic conductivity [81].

Fig. 10
figure 10

XRD deconvolution for various samples of SPEs system

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Table 4 shows the percentage of crystallinity (Xc) of CMC-NH4Br-PEG SPEs system decreases with the addition of plasticizer except for sample B. The increase of crystallinity (Xc) in sample B is attributed due to the amount of residual crystallinity given by the PEG which was found to decrease the ionic conductivity [26, 82]. Moreover, this observation may also be due to the ion trapped in CMC-NH4Br SPEs system when 2 wt% of PEG was added. This can be demonstrated from the transport properties analysis where the number of ions (H+) suddenly decreases which reflected in the reduction of the mobility and diffusion coefficient of ions. In the present system, sample E represents the lowest value of Xc, and this suggests that the addition of the PEG has affected the amorphous nature of the SPEs system. As stated by Rahaman et al. [83], the percentage of crystallinity is inversely proportional to the amorphousness of the SPEs, which means that the decreased of Xc indicates the increase of the amorphous of the SPEs. As reported by Zhou et al. [84], the decrease of crystallinity is probably associated with the progressively higher chain mobility that is facilitated by the corresponding plasticizer which eliminates the cohesive forces between CMC or NH4Br molecules that in turn enhances the chain mobility, resulting in a lower percentage of crystallinity, Xc.

Table 4 XRD deconvolution parameter for various samples of CMC-NH4Br-PEG SPE system
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Conclusion

The conducting SPEs based on CMC doped with ammonium bromide (NH4Br) plasticized PEG was investigated with different composition of PEG by solution casting method. The highest room-temperature ionic conductivity for the SPEs system is observed at 2.48 × 10−3 S cm−1 with a composition of 8 wt% PEG at room temperature. According to the FTIR results, there is a strong contribution of hydrogen bonding of polymer-salt binary system that influences the coordination interaction of H+ to the to the C=O stretching moiety of CMC, as well as the hydroxyl band, that shifts to higher wave numbers with the increase of the PEG content. From the deconvolution method, the number of ions, mobility of ions, and diffusion coefficient properties demonstrated that the free ions dominated in the ionic transport of SPEs system attribute to the result of the ionic conductivity. The addition of PEG plasticizer to up to 10 wt% into CMC-NH4Br SPEs has increased the amorphousness of the electrolyte sample. This has been proven by calculation of the degree of crystallinity of CMC-NH4Br-PEG-electrolyte system from the XRD analysis via deconvolution. From the results obtained, it is apparent that the present solid polymer electrolytes based carboxymethyl cellulose-NH4Br plasticized with PEG has the potential to be used for electrochemical applications.