Correlation between structural, ion transport and ionic conductivity of plasticized 2-hydroxyethyl cellulose based solid biopolymer electrolyte

Journal of Membrane Science 597 (2020) 117176

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Correlation between structural, ion transport and ionic conductivity of plasticized 2-hydroxyethyl cellulose based solid biopolymer electrolyte

T

M.N. Hafizaa, M.I.N. Isaa,b,∗ a

Advanced Nano-Materials (ANoMa) Special Interest Group, Advanced Materials Team (AMT), Ionic State Analysis (ISA) Laboratory, Faculty of Science and Marine Environment, Universiti Malaysia Terengganu, 21030 Kuala Nerus, Terengganu, Malaysia b Advanced Materials Team, Ionic & Kinetic Materials Research (IKMaR) Laboratory, Faculty of Science and Technology, Universiti Sains Islam Malaysia, 71800 Nilai, Negeri Sembilan, Malaysia

A R T I C LE I N FO

A B S T R A C T

Keywords: Structural properties Transport parameter Ionic conductivity Ethylene carbonate Solid biopolymer electrolyte

In this study, the relation between structural, ion transport and ionic conductivity of solid biopolymer electrolyte (SBE) is discussed in details. The SBE containing 2-hydroxyethyl cellulose (2-HEC) doped with ammonium nitrate (AN) and plasticized with ethylene carbonate (EC) are prepared via solution casting method and studied according to different weight percentage (wt.%) of EC. X-ray diffraction (XRD) analysis reveals amorphous nature of SBE while Fourier transforms infrared spectroscopy (FTIR) and Transference number measurement (TNM) are used for complexation and ionic species confirmation respectively. Based on these investigations, the enhancement of ionic conductivity, σ at room temperature can be deduced due to higher amorphous nature. The addition of EC with sufficient amount has increased the flexibility of 2-HEC chain to facilitate free H+ ion transport. A weak bonding created through EC … NH4+ allows easier movement of H+ ion from one site to another complex site of 2-HEC, which is also validated theoretically via Gaussian analysis software. This has increased the mobility, μ and diffusion coefficient, D of H+ ions, as proven from TNM analysis and thus enhance the ionic conductivity, σ of plasticized 2-HEC SBE to about 10−3 Scm−1.

1. Introduction Solid biopolymer electrolyte (SBE) is a promising electrolyte component for solid state electrochemical devices such as fuel cells [1,2], solar cells [3], batteries [4,5] and super capacitors [6]. The utilization of SBE in these devices can improve user safety due to no electrolyte leakage or flammability issue [7,8]. More importantly, SBE is easy to be prepared, lightweight, cost effective and exhibits a free standing and flexible SBE membrane [9–13]. As in any electrochemical device applications, SBE is used as a medium for ion transfer and would also acts as a separator for short circuit prevention [14,67]. For that reason, SBE should possess a desirable ionic conductivity of at least ~10−4 Scm−1 prior to enable it practical use [15,16]. However, development of SBE with high ionic conductivity especially when using natural based polymer is very challenging and many approaches need to be considered. Our previous study showed 2-hydroxyethyl cellulose (2-HEC) incorporated with 12 wt.% of ammonium nitrate (AN) salts has readily achieved the sufficient ionic conductivity, which is

(4.51 ± 0.10) × 10−4 Scm−1 at room temperature [17]. However, the ionic conductivity could not further increase once it reaches the optimum value. This is because, the addition of salts higher than this composition produces too much of free ion in SBE and causes the distance between ion become closer to form ion pair/aggregate, leading to the formation of ion cluster and hence decrease the ionic conductivity [18,19]. The most effective way to further improve the ionic conductivity is by addition of plasticizer such as ethylene carbonate (EC) [20], propylene carbonate (PC) [21], dimethyl carbonate (DMC) [22] and glycerol [23]. The superior properties of EC including high dielectric constant value (89.1), high donor number (16.4) and low viscosity (1.93 mPa s), is regard as the best choice for the plasticizer material [24,25]. According to Woo et al. [26], the addition of plasticizer with high dielectric constant can assist the dissociation of ion pair/aggregate of salts and producing more free H+ ion to facilitate in the conduction process. On top of that, plasticizer has the ability to increase the amorphous nature of SBE, which increase the segmental motion of polymer chain [27,28]. This promotes more ion mobility due to

∗ Corresponding author. Advanced Materials Team, Ionic & Kinetic Materials Research (IKMaR) Laboratory, Faculty of Science and Technology, Universiti Sains Islam Malaysia, 71800 Nilai, Negeri Sembilan, Malaysia. E-mail addresses: mnhafi[email protected] (M.N. Hafiza), [email protected] (M.I.N. Isa).

https://doi.org/10.1016/j.memsci.2019.117176 Received 23 February 2019; Received in revised form 27 May 2019; Accepted 9 June 2019 Available online 13 December 2019 0376-7388/ © 2019 Elsevier B.V. All rights reserved.

Journal of Membrane Science 597 (2020) 117176

M.N. Hafiza and M.I.N. Isa

reduction of energy barrier and thus is expected to increase the ionic conductivity [29]. From our previous work [30], we have proved that the ionic conductivity of plasticized 2-HEC–AN SBE was successfully enhanced to (1.17 ± 0.01) × 10−3 Scm−1 with addition of 16 wt% of EC. The plasticized 2-HEC SBE obeyed Arrhenius rule and exhibited the lowest activation energy value, which contributed to the highest ionic conductivity [31]. Dielectric study also showed the SBE is an ionic conductor [30]. Even so, the explicit details on the ionic conductivity enhancement still not completely understood. In general, ionic conductivity depends on number of charge carrier and their mobility in SBE [32,33]. Hence, in this current work, we are going to further investigate the other properties such as structural and ion transport parameters, which may give insight on the ionic conductivity behaviour. As far as we know, there is no previous report which correlate the structural and ion transport properties with ionic conductivity of 2-HEC–AN SBE plasticized with different EC composition. In this study, XRD is used to identify the amorphous nature of SBE while FTIR is used to study the interaction between 2-HEC, AN and EC. The deconvoluted of FTIR spectrum can also provide details on the dissociation rate of ion pair/aggregate of salts and ion transport parameters (number density, η, ionic mobility, μ and diffusion coefficient, D of free ion). TNM analysis is conducted to further confirm the type of conducting species involved in this SBE. All the results obtained herein will be useful to explain the ionic conductivity behaviour of the plasticized 2-HEC–AN SBE.

Fig. 1. XRD pattern of plasticized 2-HEC–AN SBE with different EC composition. Table 1 Peak position (amorphous hump), Full-Width at Half Maximum, FWHM and crystallite size, L of plasticized 2-HEC–AN SBE with different EC composition.

2. Experimental 2.1. SBE preparation Solid biopolymer electrolyte (SBE) was prepared by a solution casting method using 2-hydroxyethyl cellulose (2-HEC) as a polymer host, ammonium nitrate (AN) as a proton donor and ethylene carbonate (EC) as a plasticizer. 2.0 g of 2-HEC with 12 wt.% of AN was first dissolved in distilled water (DW) until a homogenous solution was obtained [34]. Different weight percentage of EC (4–28 wt%) was then added into the solution and continuously stirred until complete dissolution. The final homogenous solution was transferred to the glass petri dishes and put in the oven for the drying process at temperature of 50 °C. Another SBE system comprising of 2-HEC with different weight percentage of EC (4 wt.% – 28 wt.%) was also prepared as to study the interaction between polymer host and plasticizer. All the prepared SBE were stored in desiccator to remove any residual moisture before further characterization.

2.2.1. XRD X-ray diffraction (XRD) data were obtained using Rigaku MiniFlex II diffractometer with CuKα radiation source. The SBE was prepared on a sample plate and put in a sample holder for XRD measurement. The diffraction patterns were scanned at room temperature from 2θ of 5° to 75° with step angle of 0.02° and speed of 2.00° min−1. The XRD pattern was processed by Rigaku PDXL software to get the FWHM value. The amourphousity of each SBE was determined based on the width of peak and value of crystallite size, L which can be calculated using the Scherer's formula as shown in Eq. (1) [35].

0.94λ FWHM cos θ

2θ (°)

FWHM (°)

Crystallite size, L (nm)

0 wt.% 4 wt.% 8 wt.% 12 wt.% 16 wt.% 20 wt.% 24 wt.% 28 wt.%

21.7 21.7 21.7 21.6 21.7 21.7 21.7 21.6

8.9 8.7 8.7 8.8 9.2 9.1 8.7 8.5

0.95 0.97 0.97 0.96 0.92 0.93 0.97 0.99

Fourier transform infrared (FTIR) spectrometer coupled with Attenuated Total Reflection (ATR) measurement in the frequency ranging between 650 cm−1 to 4000 cm−1 and resolution of 4 cm−1. Prior to curve fitting, the FTIR spectrum was selected in region from 1500 cm−1 to 1200 cm−1 for the baseline creation. After the baseline was applied, the spectra was deconvoluted and fitted by a sum of Gaussian function which was adapted from the Origin Lab software. The sum of intensity for all deconvoluted peaks were ensured to fit the original spectrum with regression value, R2 approximately to 1. The area under the peaks was determined and the percentage of free ion (% FI) and contact ion (% CI) were calculated as follow [36]:

2.2. SBE characterization

L=

Sample

FI (%) =

CI  (%) =

Af Af + Ac

× 100%

Ac × 100% Af + Ac

(2)

(3)

where Af is area of free ion and Ac is area of contact ion. In order to investigate the parameters of ion transport that influence the ionic conductivity of plasticized SBE, the number density, η, ionic mobility, μ and diffusion coefficient, D of free ion were calculated using Eq. (4)–(6), respectively [36,37].

(1)

From the equation, λ is the X-ray wavelength (1.5406 Å), FWHM is the Full Width at Half Maximum and θ is the Bragg diffraction angle of SBE. 2.2.2. FTIR spectrosocpy Infrared (IR) spectra were recorded using Thermo Nicolet 380 2

η=

M × NA × free ion(%) Vtotal. Tq

(4)

μ=

σ ne

(5)

D=

μkB T e

(6)

Journal of Membrane Science 597 (2020) 117176

M.N. Hafiza and M.I.N. Isa

Fig. 2. (a) Experimental FTIR spectrum, (b) calculated FTIR spectrum and (c) the optimised molecular structure of one unit EC. Dark grey is carbon atom, light grey is hydrogen atom and red is oxygen atom. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article).

From the equation, M is the number of moles of plasticizer used, NA is Avogadro's number, σ is the ionic conductivity, e is the electric charge, kB is the Boltzmann constant and T is the absolute temperature in kelvin. Vtotal is the total volume of SBE, where V is equal to mass, m divided by density, ρ of each material used.

tion =

te =

Ii − Iss Ii

(7)

Iss Ii

(8)

where tion is ionic transference number, te is electronic transference number, Iss is the saturated current and Ii is the initial current of SBE [38]. The ionic species of diffusion coefficients, D and ionic mobility, μ were calculated using Eq. (10) and Eq. (12), respectively [19,39].

2.2.3. Gaussian 09W Gaussian 09 quantum mechanical software package was used to optimize the structure and calculate the vibrational frequency for EC, 2HEC–EC complex and 2-HEC–AN–EC complex. The calculation was carried out at theoretical level of DFT/B3LYP using the basic set of 631G (d,p), which is similar to the work reported previously [34]. The vibrational assignments were obtained using GAUSSVIEW animation program.

D = D+ + D− =

tion =

2.2.4. TNM Transference number measurement (TNM) was carried out using UT803 RMS Multimeter to determine the species of charge transport in the SBE. The dc current was monitored as a function of time on application of fixed dc voltage of 1.5 V. The SBE was recorded at limitless time until the constant value (or steady-state value) was achieved. The value of transference number was analysed according to equation below:

D D+ + D−

μ = μ+ + μ− =

tion =

kTσ ne 2

μ μ+ + μ−

(9)

(10)

σ nq

(11)

(12)

From the equation, D+ is cation and D- is anion species of diffusion coefficient, μ+ is cation and μ- is anion species of ionic mobility. 3

Journal of Membrane Science 597 (2020) 117176

M.N. Hafiza and M.I.N. Isa

Fig. 3. FTIR spectrum for 2-HEC–EC in region of (a) 3680 cm−1 – 2680 cm−1, (b) 1800 cm−1 – 1200 cm−1 and (c) 1200 cm−1 – 800 cm−1.

3. Results & discussion

represents the highest amorphous nature. In this present work, the SBE plasticized with 16 wt.% of EC (Table 1) exhibits the most amorphous nature due to the highest FWHM value and the lowest crystallite size. The increase in amorphous nature is due to the disruption of cohesive force between the polymer chain by the EC, which lead to the increase in polymer flexibility and its segmental motion [48,49]. This result further support the ionic conductivity reported in our previous work that recorded the highest value for this composition [30]. As mentioned earlier, the increase in amorphous region can promotes more (H+) ion transportation through the polymer backbone due to reduction of energy barrier, which ease the mobility of ions and consequently increased the ionic conductivity [29,50,51]. This is in agreement with the activation energy value obtained in our previous study, which found to be lower compared to that of un-plasticized SBE [31]. As the ion transport is significantly influenced by the amount of EC added, FTIR analysis has been conducted to further explain on this particular behaviour.

3.1. XRD analysis The structural details related to the arrangement of polymer chain can be obtained from XRD analysis. In general, crystlline polymer possess a regular structure (periodic arrangment) with immobile ion while amorphous polymer have an irregular structure (random arrangment) with mobile ion. It has been well reported that the SBE with amorphous nature is important as it help to endorse the (H+) ion transportation through the polymer backbone, which is also related to the ionic conductivity enhancement [40–42]. Fig. 1 shows the XRD pattern of plasticized 2-HEC–AN SBE with various EC compositions. As observed in Fig. 1, the XRD patterns show only a broad peak appearance for all SBE which confirms that the SBE was amorphous in nature. The amorphous hump also shows no or very little shifting with the incorporation of EC. This indicates that the addition of EC helps the polymer to become more flexible (more amorphous) and imparts more number of vacant site for ion coordination thus eases the ion migration through the 2-HEC backbone [43,44]. The continuous of ion hopping along the polymer backbone can contribute to the subsequent increase in the ionic conductivity. Apart from that, no additional peak is detected in all SBE, which indicates that the EC was completely dissolved in the 2-HEC matrix [40,45,46]. In order to investigate the effect of EC on the amorphous nature of SBE, the broadness of peak (Full-Width at Half Maximum, FWHM) and its crystallite size, L are determined as listed in Table 1. The value of FWHM is obtained using PDXL software and the crystallite size can be calculated using Scherer's formula (Eq. (1)). From Table 1, the value of FWHM increases (peak width) with increasing of EC composition, which is the evidence of the increase in amorphous nature. In addition, the value of L decreases and found inversely proportional to the FWHM value for all SBEs. According to Zulkifli & co-workers [47], the SBE with the widest peak width (FWHM) and the smallest crystallite size

3.2. FTIR analysis The interaction between 2-HEC, AN and EC can be evaluated through FTIR analysis. The ion is considered to be complexed/interacted with coordinating site of the polymer chain, if there are any peak shifting to either lower or higher wavenumber, peak disappearance and/or emergence of new peak found in the spectrum [50,52,53]. In this study, the FTIR spectrum of EC, 2-HEC–EC complex and 2-HEC–AN–EC complex are analysed and discussed. The FTIR spectrum for the highest conducting SBE from experimental work is also compared with the calculated result for further validation. 3.2.1. FTIR analysis of EC Fig. 2 shows the experimental FTIR spectrum, calculated FTIR spectrum and the optimised molecular structure of one unit ethylene carbonate (EC). 4

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Fig. 4. (a) Calculated FTIR spectrum and (b) the optimised molecular structure of 1:1 ratio of 2-HEC–EC complex. Dark grey is carbon atom, light grey is hydrogen atom and red is oxygen atom. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article).

As shown in Fig. 2 (a), peaks due to C–H stretching of EC is occurred at 3043 cm−1 and 2991 cm−1. Peaks appeared at 1788 cm−1 & 1769 cm−1 (doublet peak) is assigned to C]O stretching. Peak corresponds to C–H bending is obtained at 1391 cm−1. Peaks at 1136 cm−1 & 1057 cm−1 denote to C–O stretching. The C–C stretching of EC skeleton is obtained at 887 cm−1. The peaks observed in this study are in accordance with the work reported by several researchers on FTIR analysis of EC [26,54,55]. In the calculated FTIR spectrum (Fig. 2 (b)) predicted using Gaussian analysis software, the results also give a comparable vibration mode as in experimental work at 3133 cm−1 (C–H asymmetry stretching), 3067 cm−1 (C–H symmetry stretching), 1935 cm−1 (C]O stretching), 1396 cm−1 (C–H bending), 1142 cm−1 & 1106 cm−1 (C–O–C stretching) and 967 cm−1 (C–C stretching). A comparison between experimental and calculated (with scaling factor of 0.97) FTIR spectrum for EC are summarized in Table S1 of the Supplementary Information. By comparing the results of both methods, the vibration mode of C]O stretching exhibits a doublet peak in the experimental FTIR spectrum, whilst only single peak is obtained in the calculated FTIR spectrum. The existence of doublet peak in experimental result can be explained by Fermi resonance effect. Such effect is found to occur when

there is combination with the same vibrational bands belong to two EC molecules due to dipole-dipole coupling, thus causing in the splitting of the C]O stretching in the EC [26,56,57].

3.2.2. FTIR analysis of 2-HEC–EC complex In order to study the interaction between 2-HEC and EC, the FTIR spectrum of 2-HEC–EC complex is investigated in three potential region between (a) 3680 cm−1 – 2680 cm−1, (b) 1800 cm−1 – 1200 cm−1 and (c) 1200 cm−1 – 800 cm−1 as depicted in Fig. 3. The spectrum is recorded with different EC composition from 4 wt.% to 28 wt.%. From Fig. 3 (a), three peaks at 3403 cm−1 (O–H stretching), 2918 cm−1 (C–H symmetrical stretching) and 2880 cm−1 (C–H asymmetrical stretching) of 2-HEC remained at the same position with the addition of EC. No obvious changes in either peak position, intensity or new peak detected implies that EC does not make any interaction with 2-HEC within this region. In Fig. 3 (b), a doublet peak corresponds to C]O stretching of EC is appeared at 1802 cm−1 and 1773 cm−1 upon the addition of 4 wt.% of EC in 2-HEC–AN SBE. The peaks become prominent and the intensity increases as EC composition increased. The increase in peak intensity at C]O is believed due to increase of EC supplied in the polymer matrix 5

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M.N. Hafiza and M.I.N. Isa

Fig. 5. FTIR spectrum for 2-HEC–AN–EC in region of (a) 3680 cm−1 – 2680 cm−1, (b) 1800 cm−1 – 1200 cm−1 and (c) 1200 cm−1 – 800 cm−1.

based on three potential regions as depicted in Fig. 5. The spectrum is recorded using similar EC composition as in 2-HEC–EC complex from 4 wt.% to 28 wt.%. As can be seen in Fig. 5 (a), the O–H stretching (3405 cm−1) and C–H stretching (2992 cm−1 & 2883 cm−1) of 2-HEC exhibit no changes in peak position upon addition of EC. Peak due to N–H stretching mode of ammonium ion (3251 cm−1) also found to remain at the same position. Noticed from the figure, there is no significant changes detected in the spectrum and this can be interpreted that no interaction taken place between 2-HEC, AN and EC, similar as observed in 2-HEC–EC complex within this region. In Fig. 5 (b), the appearance of new peak and changes in relative intensity is observed. When 4 wt.% of EC is added, a doublet peak denotes to C]O stretching is appeared at 1802 cm−1 and 1773 cm−1, and the intensity is increased noticeably with increasing EC composition. The increase in intensity of this peak is believed due to the increase amount of EC [59]. Nevertheless, when a very huge amount of EC was supplied into the SBE, it increases the amount of free H+ ion created and this might interfere the protonation process as their presence would minimize the free space required for ion migration, thus explained the decrease of ionic conductivity at 20 wt.% to 28 wt.% of EC as proven from our previous article [30]. In the same figure, peaks for C–H bending deformation (1439 cm−1) and C–O–H bending (1345 cm−1) exhibited no changes with increasing EC composition. This is the evidence of no interaction occurred at this region. From Fig. 5 (c), the C–O stretching of 2-HEC at 1059 cm−1 has shifted to 1062 cm−1 and 1064 cm−1 with addition of 12 wt.% and 28 wt.% of EC, respectively. However, it is noted that, there is no peak shifting found in the spectrum for the sample with addition of lower than 12 wt.% of EC. It shows that, EC has only plays its role when the sufficient amount are provided in the SBE and this can be further clarified in FTIR deconvolution section. When more than 12 wt.% of EC (Fig. 5 (c)) is added, a very slight upshift of C–O stretching suggested that weak interaction has occurred in the 2-HEC–AN–EC similarly as observed in 2-HEC–EC complex. The addition of EC improves the

and proving the interaction between 2-HEC and EC. As shown in the same figure, peaks for C–H bending deformation (1457 cm−1), C–O–H bending (1355 cm−1) and C–O stretching (1249 cm−1) seems does not affected by EC. No changes in peak position observed in the spectrum indicates no interaction occurred within this region. As depicted in Fig. 3 (c), peak corresponds to C–O stretching of 2HEC has slightly shifted from 1060 cm−1 to 1061 cm−1 upon addition of 12 wt.% of EC, then shifts to 1065 cm−1 at 28 wt.% of EC composition. The slight shifting to higher wavenumber indicates that EC has formed weak interaction with 2-HEC. The presence of EC has increased the space for polymer chain to vibrate more freely, and provide more flexible pathway to facilitate the free H+ ion transport [58]. For validation, the theoretical calculation based on 1:1 ratio of the optimised molecular structure of 2-HEC–EC complex is conducted and the results are shown in Fig. 4. Referring to Fig. 4 (a), peak denotes to C]O stretching of EC is also appeared at 1886 cm−1 in the spectrum. The C–O stretching of 2-HEC has shifted from 1110 cm−1 to the higher wavenumber of 1115 cm−1 in 2-HEC–EC complex [34]. The other peaks observed in 2-HEC–EC complex seems to exhibit almost the same as the experiment data discussed in Fig. 3. The summary of FTIR analysis for experimental and calculated (with a scaling factor of 0.97) 2-HEC–EC complex can be referred to Table S2 in the Supplementary Information. The results obtained in this section strongly proved that the interaction between 2HEC and EC has only occurred in the region from 1200 cm−1 – 2000 cm−1 and 1200 cm−1 – 800 cm−1. The appearance of new peak, changes in peak position and increase in relative intensity observed in the spectrum are believed to promote more H+ ion coordination onto 2HEC chain, which can be further proved through the analysis of 2HEC–AN–EC complex.

3.2.3. FTIR analysis of 2-HEC–AN–EC complex In order to examine the interaction between 2-HEC, AN and EC, the FTIR spectrum of 2-HEC–AN–EC complex is studied and discussed 6

Journal of Membrane Science 597 (2020) 117176

M.N. Hafiza and M.I.N. Isa

Fig. 6. (a) Calculated FTIR spectrum and (b) the optimised molecular structure of 1:1:1 ratio of 2-HEC–AN–EC complex. Dark grey is carbon atom, light grey is hydrogen atom, red is oxygen atom and blue is nitrogen atom. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article).

between experimental and calculated FTIR spectrum for 2-HEC–AN–EC complex are summarized in Table S3 of Supplementary Information. Based on this observation, the interaction in 2-HEC–AN–EC SBE has been proposed and the mechanism of H+ ion transport is illustrated in Fig. 7. In this system, EC that has penetrated into polymer chain cause the increase in free space and induce its segmental mobility to facilitate free H+ ion transport. This is also supported by the small upshift in FTIR analysis as observed earlier in Fig. 3 (c) and Fig. 5 (c). Addition of EC also help to dissociate ion pair/aggregate of the salts and promote more H+ ion to interact with the lone pair electron at oxygen atom of 2HEC. When this occurred, it created a weak bonding between H+ ion from AN salt and EC as proven by Kadir & co-workers [56], and this possibly introduced a new shorter pathway for H+ ion transport before it hops onto 2-HEC coordinating site. The proton transfer suggested in this 2-HEC–AN–EC SBE is also similar to the plasticized SBE reported by Alias & co-workers [62].

segmental mobility of 2-HEC chain due to the increase in amorphous nature of SBE, and also reduce the energy barrier to allow the easier of H+ ion movement during the conduction process [57,60,61]. The addition of EC into 2-HEC–AN SBE would also promote the dissociation of ion pair into free ion, thereby leading to the increase of H+ ion migration [61]. This further support the ionic conductivity result reported in our previous article, where the addition of EC from 12 wt.% to 16 wt. % increases the ionic conductivity to about 10−3 Scm−1 [30]. The results discussed in Fig. 5 are also compared with the calculated FTIR spectrum. Fig. 6 shows the calculated FTIR spectrum and the optimised molecular structure of 1:1:1 ratio of 2-HEC–AN–EC complex in the region of 4000 cm−1 – 800 cm−1. Similarly as observed in experimental FTIR spectrum (Fig. 5), peak due to C]O stretching of EC is obtained at 1876 cm−1. Peak for C–O stretching of 2-HEC has been shifted from 1107 cm−1 to 1116 cm−1 in 2-HEC–AN–EC complex [34]. The changes in peak position and new peak appeared in the spectrum implies the interaction between 2-HEC, AN and EC. The remaining peaks observed are almost similar with experimental FTIR spectrum and found within the acceptable range of each band. A comparison

7

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M.N. Hafiza and M.I.N. Isa

Fig. 7. Proposed interaction between 2-HEC, AN and EC.

itself could not provide a clear picture towards the overall ionic conductivity enhancement. Therefore, further study on the ion transport parameters (number density, η, mobility, μ and diffusion coefficient, D of free ion) is conducted. Fig. 9 shows the value of η, μ and D of 2HEC–AN–EC SBE. The plot is interpreted into three different regions and compared to the ionic conductivity variation. Region I with the addition of 4 wt.% to 8 wt.% of EC, Region II from 12 wt.% to 16 wt.% and Region III from 20 wt.% to 28 wt.%. Based on Fig. 9 (Region I), it can be seen that the value of ƞ increases slightly with addition of 4 wt.% and 8 wt.% of EC. At the same time, the value of μ and D decrease which is proportional to the trend of ionic conductivity. As proven in the FTIR analysis, the small amount of EC added into the SBE may not sufficient to improve the flexibility of polymer chain. This might interfere the movement of free H+ ions due to the decrease of space for ion transport, causing a decline in μ and D and subsequently contributed to the decrease of ionic conductivity in lower EC composition. However, owing to a good salt dissociation ability, the EC still capable to dissociate the ion pair/aggregate of the salts and thus explained the slightly increase of ƞ value in Region I. This means that not just material with high dielectric constant should be considered, but the composition of material also influence the salt dissociation efficiency. In region II, the value of ƞ, μ and D increase upon addition of EC up to 16 wt.% (Region II). The sufficient amount of EC provided in the SBE has probably increase the space for ion transport and promote more free ion production due to ion pair/aggregate of the salts dissociation. This leads to the fast of ion movement through the polymer matrix and consequently enhanced the ionic conductivity. The value of ƞ continues to increase with further increase of EC composition (Region III). At the same time, μ and D decreased. As observed in the figure, the ionic conductivity with the highest value does not possess the highest ƞ value, compared to sample with 20 wt.% to 28 wt.% of EC. This is because, the value of ƞ in the range between 4.0 × 1022 cm−3 to 5.0 × 1022 cm−3 is considered optimum for both SBE can achieve. Beyond that range, the ion tend to overcrowd, which resulting in the decrease of free ion mobility and hence the ionic conductivity from 20 wt.% to 28 wt.% of EC composition (Region III). Generally, the increase of ionic conductivity depend on number density, η and ionic mobility, μ of free ions produced in the SBE [33,58]. However, it is contrary to this study where the ionic conductivity of 2HEC–AN–EC SBE is obviously influenced by μ and D of free ion.

3.3. FTIR deconvolution analysis FTIR deconvolution technique is an established method to calculate the rate of salt dissociation and the qualitative parameters of ion transport (number density, η, mobility, μ and diffusion coefficient, D of free ion) [63,67]. The results obtained from this analysis are also useful to give insight over the ionic conductivity variation in 2-HEC–AN–EC SBE. Fig. 8 shows the FTIR deconvolution of vs(NO3-) band belonging to AN salt in the region of 1550 cm−1 – 1150 cm−1. This band is selected due to high complexation occurred as observed in FTIR analysis of 2HEC–AN complex [34]. From Fig. 8, the peaks observed between 1460 cm−1 – 1465 cm−1 and 1248 cm−1 – 1255 cm−1 referred to C–H bending deformation and C–OH stretching from 2-HEC, respectively. Peak due to C–H2 bending of EC then appeared between 1392 cm−1 – 1393 cm−1 with the addition of 20 wt.% to 28 wt.%. The deconvoluted peak obtained herein is almost similar as observed in Fig. 5(b) and thereby support the discussion made in FTIR analysis. Different type of ionic species of AN salts can also be distinguished using FTIR deconvolution technique [64]. Free ion and contact ion are assigned at 1326 cm−1 – 1331 cm−1 and 1393 cm−1 – 1400 cm−1, respectively. The selection of ionic species region are almost similar as reported in other SBE system who also use AN as the salts [65]. In this study, contact ion refers to the formation of ion pair (NH4+···NO3-) and ion aggregate (NH4+···NO3-···NH4+). The percentage of free ion (% FI) and contact ion (% CI) can be calculated using Eq. (2) and Eq. (3) respectively, and the results are listed in Table 2. As shown in Table 2, the % FI decreases about 8.46% at low EC composition (4 wt.% – 8 wt.%). The reduction of free ion production rate might be due to insufficient amount of EC provided, so the ion pair/aggregate of AN salts is unable to dissociate. Hence, it further increases the contact ion formation. On the other hand, % FI starts to increase until the optimum value of 52.21% at 16 wt.% of EC which also represent the highest conducting SBE. This implies that the addition of EC with an appropriate amount help to re-dissociate contact ion and enhanced the ion conduction in SBE. Beyond that composition (20 wt.% to 28 wt.%), the % FI dropped. This might be due to ion association which is similar reported by Abdul Rahaman and coworkers [36]. Although the % FI is observed to increase at 16 wt.% of EC, but basically it shows no significant changes in its trend. Even so, this value 8

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Fig. 8. Deconvoluted FTIR spectra of the plasticized 2-HEC–AN SBE with different EC composition in region of 1500 cm−1 –1150 cm−1. Red circle is the sum of deconvoluted peak, black solid line is experimental data, green dashed dotted line is free ion, blue dashed line is contact ion, grey solid line is overlapping peak from 2-HEC. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article).

saturation state to give the ionic transference number. During this state, the ionic species is considered depleted and the remaining of current flow is due to electronic migration inside the SBE. Table 3 lists the value of ionic and electronic transference number for the studied SBE. From the table, all the values obtained are greater than 0.9 which can be assumed close to unity and proving that the species of charge transport in this SBE is predominantly due to ion [1,37,66]. There are two possible ionic transport species in SBE which are cation and anion [65]. However, this does not imply that both

Therefore, further studies on the ionic mobility and diffusion coefficient of free ion has been investigated by TNM analysis. 3.4. TNM analysis Further confirmation on the contribution of charge transport species (cationic, anionic and electronic) to the overall ionic conductivity of SBE can be done using dc polarization method and the result is plotted in Fig. 10. For this measurement, the current is recorded until it reaches 9

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Table 2 Percentage of free ion and contact ion of the plasticized 2-HEC–AN SBE with different EC composition. Sample

Free ion (%)

Contact ion (%)

4 wt.% 8 wt.% 12 wt.% 16 wt.% 20 wt.% 24 wt.% 28 wt.%

54.23 48.85 50.13 52.21 51.53 49.99 45.98

45.77 51.15 49.87 47.79 48.47 50.01 54.02

Fig. 10. Normalized current against time of the highest conducting 2HEC–AN–EC SBE. Table 3 Ionic and electronic transference number for the plasticized 2-HEC–AN SBE with different EC composition. Sample

tion

te

4 wt.% 8 wt.% 12 wt.% 16 wt.% 20 wt.% 24 wt.% 28 wt.%

0.96 0.98 0.98 0.97 0.94 0.91 0.92

0.04 0.02 0.02 0.03 0.06 0.09 0.08

Fig. 9. Transport parameters (η is number density of mobile ion, μ is ionic mobility and D is diffusion coefficient) of the plasticized 2-HEC–AN SBE with different EC composition.

species are mainly contributed to the total ionic conductivity. It depends on the most dominant species do involve in ion migration process. Therefore, the mobility, μ and diffusion coefficient, D for cation and anion are calculated according to Eq. (10) and (12), and the result is depicted in Fig. 11. Based on Fig. 11, the value of cation is higher than those of anion which proved that the SBE is cationic conductor. Since AN salt is the only ion contributor in 2-HEC–AN–EC SBE, thus it can be inferred that the ion migration (ion transport) for this SBE system is due to proton (H+ ion). This observation is also found to be in accordance with other similar work reported in the literatures [63,65].

Fig. 11. Cation and anion species of diffusion coefficient and ionic mobility for the plasticized 2-HEC–AN SBE with different EC composition.

4. Conclusion In summary, this study has successfully revealed that the structural and transport properties of SBE is responsible for the ionic conductivity enhancement. X-ray diffraction (XRD) analysis shows only a broad hump was detected in all sample, which represent the amorphous nature of SBE. As the weight percentage (wt.%) of EC increased, the amorphous nature is significantly enhanced and exhibited the most amorphous state at 16 wt.% of EC added (corresponds to the highest conducting SBE). This indicates that, the presence of EC has disrupted 10

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the arrangement of 2-HEC chain, which thereby improve the ion mobility through the polymer backbone due to reduction of energy barrier and does increase the ionic conductivity. The enhancement in ionic mobility can be further investigated using Fourier transform infrared (FTIR) technique. Interaction between 2-HEC, AN and EC has been observed through the shifting in peak position at the C–OH stretching of 2-HEC and further validated using Gaussian analysis software. The percentage of free ion, contact ion and ion transport parameters are determined from FTIR deconvolution analysis. It is found that the ionic conductivity is strongly affected by the mobility, μ and diffusion coefficient, D of H+ ion, as further analysed by TNM analysis. This result shows that the incorporation of EC assists the dissociation of ion pair/ aggregate of the salts for the conduction process and may possibly introduce a shorter pathway for the ions to migrate. This eases the migration of free H+ ion from one site to another complex site and hence promote the ionic conductivity to about ~10−3 Scm−1.

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[14]

[15]

[16]

[17]

[18]

Acknowledgement

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The authors would like to acknowledge Ministry of Education Malaysia (MOE) and Universiti Malaysia Terengganu (UMT) for ERGS (Vot. 55101) and FRGS (Vot. 59452) grant, MyBrain15 for MyPhD awarded. Special thanks to M.I.H. Sohaimy for proof-reading and K.H. KuBulat (UMT) for the guide on Gaussian analysis.

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Appendix A. Supplementary data

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Supplementary data to this article can be found online at https:// doi.org/10.1016/j.memsci.2019.117176. [23]

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