ORIGINAL PAPER The development of Li + conducting polymer electrolyte based on potato starch/graphene oxide blend A. A. Azli 1 & N. S. A. Manan 2 & M. F. Z. Kadir 3 Received: 28 April 2016 /Revised: 23 September 2016 /Accepted: 18 October 2016 /Published online: 29 October 2016 # Springer-Verlag Berlin Heidelberg 2016 Abstract Solid polymer electrolytes based on potato starch (PS) and graphene oxide (GO) have been developed in this study. Blending GO with PS has improved the ionic conduc- tivity and mechanical properties of the electrolytes. In this work, series of polymer blend consisting of PS and GO as co-host polymer were prepared using solution cast method. The most amorphous PS-GO blend was obtained using 80 wt% of PS and 20 wt% of GO as recorded by X-ray dif- fraction (XRD). Incorporation of 40 wt% lithium trifluoromethanesulfonate (LiCF 3 SO 3 ) into the PS-GO blend increases the conductivity to (1.48 ± 0.35) × 10 -5 S cm -1 . Further enhancement of conductivity was made using 1- butyl-3-methylimidazolium chloride ([Bmim][Cl]). The highest conductivity at room temperature is obtained for the electrolyte containing 30 wt% of [Bmim][Cl] with conductiv- ity value of (4.80 ± 0.69) × 10 -4 S cm -1 . Analysis of the Fourier transform infrared spectroscopy (FTIR) spectra con- firmed the interaction between LiCF 3 SO 3 , [Bmim][Cl], and PS-GO blend. The variation of the dielectric constant and mod- ulus studies versus frequency indicates that system of PS-GO- LiCF 3 SO 3 -[Bmim][Cl] obeys non-Debye behavior. Keywords Graphene oxide . Potato starch . Lithium trifluoromethanesulfonate . 1-Butyl-3-methylimidazolium chloride Introduction Studies on natural solid polymer electrolytes received great attention, owing to renewable, sustainable, and biodegradable properties, and it is also promising substitute for synthetic polymers [1–3]. Natural polymer is produced from renewable resources, which are abundant in nature and able to cater shortage of polymer from petroleum resources. Among natu- ral polymers, starch is a potential candidate as a host polymer due to its unique characteristics of having good compatibility and excellent solubility [4, 5]. Starch contains two types of glucosidic macromolecules, namely amylose, a linear mole- cule of glucose unit, and amylopectin, that have highly branched glucose unit that is associated with amorphous and semicrystalline layers, respectively [6]. Content of amylose and amylopectin of a starch varied, depending on the origin of the starch. Potato starch is reported to have amylose content ranging from 20.02 to 21.59 % with large starch granules [7]. Large granule size of potato starch aids in coordination of the ions through the polysaccharide of hydroxyl groups [8]. Despite the excellent properties, starch film is lacking in me- chanical property and therefore starch often blended with oth- er material, and in this work, graphene oxide (GO) is used to enhanced the mechanical strength of starch polymer matrix [9, 10]. Dispersion of GO in the starch matrix strengthens the interfacial interactions between these two substances and thus improved its mechanical property [9]. Potato starch (PS) also reported to have minimum ghost microstructure which pro- vides better suspension, stabilizing property when blending process occurs in water medium [11]. The content of this paper was presented at the ICFMD 2015 * M. F. Z. Kadir [email protected]1 Institute of Graduate Studies, University of Malaya, 50603 Kuala Lumpur, Malaysia 2 Chemistry Department, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia 3 Centre for Foundation Studies in Science, University of Malaya, 50603 Kuala Lumpur, Malaysia Ionics (2017) 23:411–425 DOI 10.1007/s11581-016-1874-z
Transcript
ORIGINAL PAPER
The development of Li+ conducting polymer electrolyte basedon potato starch/graphene oxide blend
A. A. Azli1 & N. S. A. Manan2& M. F. Z. Kadir3
Received: 28 April 2016 /Revised: 23 September 2016 /Accepted: 18 October 2016 /Published online: 29 October 2016# Springer-Verlag Berlin Heidelberg 2016
Abstract Solid polymer electrolytes based on potato starch(PS) and graphene oxide (GO) have been developed in thisstudy. Blending GO with PS has improved the ionic conduc-tivity and mechanical properties of the electrolytes. In thiswork, series of polymer blend consisting of PS and GO asco-host polymer were prepared using solution cast method.The most amorphous PS-GO blend was obtained using80 wt% of PS and 20 wt% of GO as recorded by X-ray dif-fraction (XRD). Incorporation of 40 wt% lithiumtrifluoromethanesulfonate (LiCF3SO3) into the PS-GO blendincreases the conductivity to (1.48 ± 0.35) × 10−5 S cm−1.Further enhancement of conductivity was made using 1-butyl-3-methylimidazolium chloride ([Bmim][Cl]). Thehighest conductivity at room temperature is obtained for theelectrolyte containing 30 wt% of [Bmim][Cl] with conductiv-ity value of (4.80 ± 0.69) × 10−4 S cm−1. Analysis of theFourier transform infrared spectroscopy (FTIR) spectra con-firmed the interaction between LiCF3SO3, [Bmim][Cl], andPS-GO blend. The variation of the dielectric constant andmod-ulus studies versus frequency indicates that system of PS-GO-LiCF3SO3-[Bmim][Cl] obeys non-Debye behavior.
Studies on natural solid polymer electrolytes received greatattention, owing to renewable, sustainable, and biodegradableproperties, and it is also promising substitute for syntheticpolymers [1–3]. Natural polymer is produced from renewableresources, which are abundant in nature and able to catershortage of polymer from petroleum resources. Among natu-ral polymers, starch is a potential candidate as a host polymerdue to its unique characteristics of having good compatibilityand excellent solubility [4, 5]. Starch contains two types ofglucosidic macromolecules, namely amylose, a linear mole-cule of glucose unit, and amylopectin, that have highlybranched glucose unit that is associated with amorphous andsemicrystalline layers, respectively [6]. Content of amyloseand amylopectin of a starch varied, depending on the originof the starch. Potato starch is reported to have amylose contentranging from 20.02 to 21.59 % with large starch granules [7].Large granule size of potato starch aids in coordination of theions through the polysaccharide of hydroxyl groups [8].Despite the excellent properties, starch film is lacking in me-chanical property and therefore starch often blended with oth-er material, and in this work, graphene oxide (GO) is used toenhanced the mechanical strength of starch polymer matrix [9,10]. Dispersion of GO in the starch matrix strengthens theinterfacial interactions between these two substances and thusimproved its mechanical property [9]. Potato starch (PS) alsoreported to have minimum ghost microstructure which pro-vides better suspension, stabilizing property when blendingprocess occurs in water medium [11].
The content of this paper was presented at the ICFMD 2015
Researchers have doped metal-based salts into starch toprepare the solid ion-conducting materials [12, 13].Interaction of metal salts such as LiCl, LiCF3SO3,LiClO4, and LiBF4 with starch molecules restrains therecrystallization of starch matrix [1, 3]. In this work, lith-ium trifluoremethanesulfonate (LiCF3SO3) is chosen asdoping salt because a bulky anion has a good plasticizingeffect on the polymer host. This facilitates the relativemovement of the polymeric chains, consequently increas-ing the long range migration of the ions and resulting inan improved ionic conductivity [14, 15]. Conductivity inthe order of 10−4 and 10−5 S cm−1 was also reported forstarch-based polymer electrolytes, indicating that starch-based polymer electrolytes possess high potential to beapplied in electrochemical devices [9, 10, 16].
Ionic liquids (ILs) have been found as important mate-rials in the era of electrochemistry due to the ability tooffer a better ion conduction and can be applied as sepa-rator such as electrolytes in electrochemical devices[17–19]. ILs are salts that are composed of bulky asym-metry organic cations and either organic or inorganic an-ions with melting point below 100 °C [20]. The use of 1-butyl-3-methylimidazolium chloride ([Bmim][Cl]), as aprecursor in electrolyte system, has been investigated byothers [21–23]. As an amphoteric molecule, imidazolegroup possesses two nitrogen atoms in which one atomacts as proton acceptor and that of the other atom acts asproton donator. Therefore, protons could be transferredwithin one imidazole molecule through reorientation orbetween two imidazole molecules via hydrogen bonds[24]. These features endow imidazole-filled electrolytewith considerable proton conduction ability. Cl− anionsin ILs have the ability to destroy the semicrystalline struc-ture of native starch granules and disrupt hydrogen bond-ing between hydroxyl groups of polysaccharide [25, 26].Moreover, the addition of ILs is able to enhance the ionicconductivity because of its strong plasticizing effect, en-vironmental friendly nature, and high ion content [27, 28].
The aim of the present work is to study electrical prop-erties and behavior of the proposed electrolyte systemwhich consists of LiCF3SO3 and [Bmim][Cl] as dopantsalt and plasticizer, respectively, in the PS-GO polymerhost. Detailed analysis and experimental results are pre-sented in this paper.
Experimental
Synthesis of graphene oxide (GO)
GO material was prepared using modified Hummers meth-od [29]. Oxidation of graphite by an oxidizing agent pro-duced GO, and any impurities that resulted from the
reaction were removed through washing process using dis-tilled water. Mild sonication was applied to the GO sus-pension for about 10 min to exfoliate graphite oxide thatstill remains in the suspension. Finally, GO suspension wasput into the oven for 48 h at 60 °C to discard water presentand thus obtaining dark brown GO nanosheets.
Preparation of PS-GO electrolytes
PS-GO-based solid polymer electrolytes were prepared bymeans of solution casting. Different amounts of GO (×wt%) were dispersed in 50 ml of distilled water and son-icated for about 90 min via ultrasonicator XO-650D(Nanjing Xianou Instruments Co. Ltd.). The solutionwas heated up to 80 °C, and an appropriate amount ofPS (100-×) wt% (Sigma Aldrich, Germany) was subse-quently mixed in the GO solution. The mixtures werestirred for 2 h until the homogeneous solutions were ob-tained. The homogeneous solution is then casted on aplastic Petri dish in fume hood at room temperature toyield a mechanical free standing thin film. The sampleswere kept in a desiccator filled with silica gel desiccantsfor further drying. The casted thin films were then char-acterized by XRD analysis.
Preparation of PS-GO-LiCF3SO3-[Bmim][Cl] electrolytes
The polymer blend electrolyte system was prepared byhomogenizing 20 wt% of GO in 50 mL distilled waterfollowed by heating the solution at 80 °C before adding80 wt% of the starch. Prior to the preparation of the salted
Table 1 Compositions and designations of the electrolytes
PS-GO(wt%)
LiCF3SO3
(g)[Bmim][Cl](g)
Designation
80–20 0.026 0.000 5 wt% LiCF3SO3
80–20 0.057 0.000 10 wt% LiCF3SO3
80–20 0.088 0.000 15 wt% LiCF3SO3
80–20 0.125 0.000 20 wt% LiCF3SO3
80–20 0.167 0.000 25 wt% LiCF3SO3
80–20 0.214 0.000 30 wt% LiCF3SO3
80–20 0.269 0.000 35 wt% LiCF3SO3
80–20 0.333 0.000 40 wt% LiCF3SO3
80–20 0.409 0.000 45 wt% LiCF3SO3
80–20 0.500 0.000 50 wt% LiCF3SO3
80–20 0.333 0.092 10 wt% BmimCl
80–20 0.333 0.208 20 wt% BmimCl
80–20 0.333 0.357 30 wt% BmimCl
80–20 0.333 0.556 40 wt% BmimCl
80–20 0.333 0.833 50 wt% BmimCl
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electrolytes, LiCF3SO3 (Aldrich Chemistry) was dried at100 °C for 1 h to eliminate trace amounts of water in thematerial. Different concentrations of LiCF3SO3 were putinto the PS-GO solutions and stirred until complete dis-solution. For the preparation of plasticized system, suit-able amounts of [Bmim][Cl] (Sigma Aldrich) were addedto the highest conducting salted electrolyte solutions andstirred until homogeneous solution was obtained. The so-lution was eventually casted on plastic Petri dish anddried in fume hood for a few days until a free standingpolymer electrolyte film was produced. The dry films
were then kept in desiccators filled with silica gel desic-cants for 24 h before being characterized to avoid anytrace of moisture. Details on the compositions and desig-nations of the electrolytes are listed in Table 1.
Characterization of electrolytes
Selection of the suitable polymer blend ratios was deter-mined by X-ray diffraction (XRD) measurements. Thesamples were analyzed using a Siemens D5000 X-ray dif-fractometer where X-rays of 1.54 Å wavelengths were gen-erated by a Cu Kα source. The 2θ angle was varied from5° to 80°. In order to measure relative crystallinity of thesample, deconvolution method was applied to thediffractrogram according to the method [30]. The areaabove and under the curve corresponded to crystalline do-mains and amorphous regions, respectively. The crystallinefraction pattern of a material was modeled with Gaussianfunction peaks via Origin software and was fitted untilR2 ~ 0.99. Then, degree of crystallinity (χc) of the sampleis calculated using Eq. (1) where AT is the area of totalhumps and AA is the area of amorphous regions.
χc ¼AT−Aa
AT� 100% ð1Þ
In order to investigate surface topography of PS-GO blendof the materials synthesized, field emission scanning electronmicroscopy (FESEM) was applied to the pristine materialsused in the preparation of polymer blend. Surface and cross-sections of the materials were scanned using Hitachi-SU8220FESEM with 2.00 K magnification.
The Fourier transform infrared spectroscopy (FTIR) stud-ies were performed using attenuated total reflection infrared
Inte
nsi
ty(a
.u.)
5 20 35 50 65 80
70 PS:30 GO (wt.%)
60 PS:40 GO (wt.%)
50 PS:50 GO (wt.%)
40 PS:60 GO (wt.%)
30 PS:70 GO (wt.%)
20 PS:80 GO (wt.%)
90 PS:10 GO (wt.%)
80 PS:20 GO (wt.%)
2θ (degree)
10 PS:90 GO (wt.%)
Fig. 2 XRD pattern for differentratio (wt%) PS-GO blends
.
5 20 35 50 65 80
2θ (degree)
Inte
nsi
ty(a
.u)
20º 16.5º
13.7º
22.4º
6.2º 26.5º
(a)
(b)
10.6º
Fig. 1 XRD pattern for a pure starch film and b pure GO film
Ionics (2017) 23:411–425 413
spectroscopy (ATR-IR) Perkin Elmer Spectrum 400GladiATR. The FTIR spectra were recorded with a resolutionof 4 cm−1 in transmittance mode over the wavenumber rangefrom 400 to 4000 cm−1. FTIR spectra were analyzed to con-firm complexation between polymer host, LiCF3SO3, and[Bmim][Cl] ionic liquid.
The ionic conductivities of the samples were determinedusing HIOKI 3532–50 LCR HiTESTER over a frequencyrange between 50 Hz and 5 MHz at ambient temperature.The prepared samples were subjected to ac-impedancespectroscopy, and prior to analysis, thicknesses of the sam-ples were measured by using digital thickness gauge
(Mitutoyo Corp.). The electrolyte films were sandwichedbetween two stainless steel electrodes with diameter of1.6 cm. The value of bulk resistance (Rb) was determinedfrom the Cole-Cole plots. Conductivity (σ) was calculatedusing equation:
σ ¼ dRbA
ð2Þ
where d is the thickness of the electrolyte samples and A is theelectrode-electrolyte contact area.
5 20 35 50 65 80
20° 16.5°
26.5°
20.8°
13.7°
6.2° 35.4°
22.4°
46.9°
14.4°
16.1°
6.6°
12.1°
22.7°
18.1° 18.9°
27° 33.9°45.9°
6.6° 9°
11.4°
15.4° 16.1°
19.1° 22.2°
44.1° 30.2°
7.7° 11.2°
18.8°
6.4° 15.2° 15.8°
22.6°
42.8° 29.2°
10.6°
20.8° 15.4°
10.9°
Experimental
Crystalline peaks
Amorphous peaks Fitting
pure PS film
80 PS:20 GO (wt.%)
90 PS:10 GO (wt.%)
70 PS:30 GO (wt.%)
pure GO film
Fig. 3 Deconvoluted XRDpattern for selected PS-GO blendfilms
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Results and discussion
XRD studies
A free-standing film that is mechanically stable can beobtained by choosing an appropriate ratio of polymerblend. From XRD analysis of the polymer blends, quali-tative identification of crystalline structure can be madeby comparing the position and intensity of the peakswhile quantitative analysis is performed using a non-linear least square fitting software to obtain degree ofcrystallinity of the polymer blend.
Figure 1a, b shows XRD pattern for pure PS and pureGO films respectively. Two crystalline peaks appear at (i)2θ = 16.5° and (ii) 2θ = 22.4° that correspond to pure PSfilm. Other less intense peaks were found at 2θ = 6.2°,13.7°, 20°, and 26.5°. Similar results have been reportedfor Brazilian potato starch whereby the major peaks areobserved at 5.6°, 15°, 17°, 18°, and 23° [28]. On the otherhand, a very sharp peak obtained at 2θ = 10.6° in Fig. 1bassigned as main peak of GO. GO sheet only exhibits asharp diffraction peak at 2θ = 10.7°, corresponding to8.2°A of interlayer spacing, indicating full oxidation ofgraphite structure [31]. The interlayer spacing of synthe-sized graphene oxide is 0.80 nm with the main diffractionangle at 11.1° as reported by other authors [32, 33].
Figure 2 shows the XRD pattern of different weightpercent (wt%) of PS/GO ratio. As can be seen, there arechanges of the main peaks which belong to pure PS andpure GO films. The addition of 10 wt% of PS into the GOsheets slightly change the diffraction angle of the GOpeak to 10.5°. The main peak of GO further decreasedto 2θ = 9.3° with 20 wt% of PS incorporated into GOmatrix. Diffraction angle of GO for polymer blend with30 PS:70 GO (wt%) increases to 10.3° as a result ofblending process and a few peaks from PS substance arestarts to appear at 7.5°, 13.9°, and 16.1°. From Fig. 2,intensities of GO and PS peaks start to decrease from itsoriginal films to indicate reduce in crystallinity when PS-GO materials were formed. Amorphousness of electrolytematerials is important to ensure good ion migration forionic conductivity [34].
In an XRD pattern, overlapping of peaks can occurwhereby the crystalline peaks superimposed on the amor-phous regions. According to other literatures, native potatostarch contains mixture of A-type and B-type polymorphsthat exhibited five main diffraction peaks at 5.7° (B-typepolymorphs), 15.1° (A-type polymorphs), 17.1° (A and Btype polymorphs), and 19.9° and 22.1° (B type poly-morphs), with the relative crystallinity that ranges from20.02 to 38.37 % [7, 32, 35, 36]. Using deconvolutionmethod, this overlapping pattern can be separated using anon-linear least square fitting software [37].
Figure 3 shows deconvoluted XRD pattern for selectedPS-GO blend films. The sharp, narrow, and small peaksindicate crystalline peaks while broad peaks indicateamorphous regions. It can be seen that PS-GO polymerblends have six crystalline peaks and three major amor-phous regions. For pure PS film, instead of five diffrac-tion peaks as reported on other literatures, there are sixdiffraction peaks situated at 6.2°, 13.7°, 16.5°, 20°, 22.4°,and 26.5°. At 2θ = 20.8°, 35.4°, and 46.9° that peaksbelong to amorphous regions. Peaks at 20° and 20.8° iscorresponding to crystalline and amorphous peak respec-tively. In XRD spectra, overlapping of both crystallineand amorphous peaks can occur due to semicrystallinestructure of the potato starch. A very strong peak of pureGO film at 10.6° has been observed along with amor-phous regions at 10.9°, 15.4°, and 20.8°.
Diffraction angle of selected PS-GO films changes asGO material incorporated into PS polymer matrix.Regardless of the ratio used in PS-GO polymer blend,amorphous peaks observed in Fig. 3 and that area underthe deconvoluted peaks was used to calculate the degreeof crystallinity using Eq. 1. Degrees of crystallinity arecalculated to show the value of the crystallinity possessesby the each electrolytes materials. Less crystallinity of thematerials indicates that materials are amorphous andtransport of the ions is facilitated by rapid segmental mo-tion that is common to occur in amorphous polymers [38].Table 2 depicts the percentage of crystallinity for eachpolymer blend. Polymer blend that has less crystallinestructure indicates the small value of crystallinity percent-age. In this work, 80:20/PS:GO (wt%) polymer blend waschosen as a base for solid polymer electrolyte system as itpossesses the lowest value of 20.09 %. In other works,they also reported that most starch-based electrolyte havedegree of crystallinity around 20 to 45 % [7, 39].
Table 2 Degree of crystallinity (%) for PS-GO blend films
PS(wt%)
GO(wt%)
Degree ofcrystallinity, χc (%)
100 0 28.13
90 10 24.76
80 20 20.09
70 30 26.53
60 40 27.91
50 50 32.82
40 60 34.96
30 70 36.49
20 80 39.66
10 90 42.93
0 100 45.75
Ionics (2017) 23:411–425 415
FESEM studies
Dispersion of GO in the polymer matrix is studied usingfield emission scanning electron microscopy (FESEM).The GO sheet is synthesized by using modifiedHummers method. Figure 4a shows the GO sheet withaggregated wrinkled-like surface. The cross-section ofGO sheet also shows wrinkled layers with irregularshapes, similar with other reports [40, 41]. The thicknessof the GO sheet obtained in this work is 24.6 μm withlateral size ranging from 754 to 873 nm. Yadav andAhmad (2015) also stated that the average size of GOsheet layer is estimated between 500 and 5000 nm [42].In Fig. 4b, GO film exhibits almost similar pattern with
the GO sheet but with improved surface due to the less-ened wrinkled. During the GO film preparation,sonification and stirring agitation were applied.Throughout the process, exfoliation of GO sheet has oc-curred and resulted in the less-wrinkled surface of the GOfilm [41]. The cross-section and the surface of the PS filmshow a fairly smooth surface, as can be seen in Fig. 4 c, g.This pattern has also been reported by Kvien et al. (2007)for their potato starch film [43]. The surface morphologyof PS-GO blend film and its cross-section is illustrated inFig. 4d, h. PS-GO film shows quite rough surface withsmall agglomerations since the GO particles have embed-ded in the PS film. Suriani et al. (2015) reported that theinhomogeneous of GO distribution was due to the
(a)
(d)
(c)
(h)
(f)
PS film
GO film
(e)
(b)
(g)
GO sheet
noitcesssorCecafruS
PS-GO film
Fig. 4 FESEM surface images ofa GO sheet, b GO film, c PS film,d PS-GO film and FESEM cross-section images of d GO sheet, eGO film, f PS film, g PS-GO film
416 Ionics (2017) 23:411–425
agglomeration of GO and natural rubber polymer whichwas produced by conventional mixing method [44]. Theirreport [44] exhibited almost similar film pattern with thepresent work in Fig. 4.
FTIR studies
From XRD results, polymer blend consists of 80 wt% PS and20 wt% GO has the most amorphous structure among otherratio studied; thus, this chosen polymer blend will bediscussed in FTIR studies. FTIR spectra of selected polymerblend are reported in Fig. 5 to show the complexation formedbetween PS and GO. From Figure 5(i), a broad absorption ofpure GO film at 3197 cm−1 is similar, as reported in our pre-vious work, due to the typical carbonyl and carboxyl groupspresent in the GO structure [45]. Wang et al. (2009) and Caoet al. (2011) also have reported the presence of hydroxyl (O–H) band in GO nanosheets near 3300 and 3390 cm−1 [46, 47].The spectrum of native PS film in Fig. 5(ii) shows O–H bondstretching at 3304 cm−1, C–H stretching bands at 2927 cm−1,and at 1077, 1149, and 997 cm−1 are assigned as the couplingof C–O, C–C, and asymmetric stretching of the C–O–C gly-cosidic bridge [48–50]. These main characteristic peaks of PS
were also observed in PS-GO polymer blend which indicatesthat the polymer blend is more likely to have similar spectrumas PS due to ratio of starch use is higher than GO material.
In PS-GO blend, hydroxyl band of PS at 3304 cm−1 shiftedto 3290 cm−1 with decreased intensity of peak observedFig. 5(iii). The broad band at 3290 cm−1 is related to the O–H band, and it is a result of overlapping vibrations between O–H stretching groups in both amylose in PS and GO molecules[51]. Decreased in intensity of the FTIR spectra in the range of1200–800 cm−1 was observed for PS-GO blend which indi-cates the cleavage of the ά-(1 → 4) glycosidic bonds of amy-lose [51]. Changes observed in FTIR peaks due to constrainteffect of hydrogen bonding on the vibrating elements showthat interaction has occurred between PS and GO materials.
In this work, selected polymer blend is doped withLiCF3SO3 to provide ions within PS-GO material, andthe complexation between polymer blend and the lithiumsalt was investigated by FTIR analysis. Figure 6a showsthe FTIR spectra of PS-GO-LiCF3SO3 (salted) system inthe region between 3000 and 3700 cm−1 for O–H vibra-tion. This region is studied because GO sheets can exhibitstrong hydrogen bonding with the lithium salt due to itsamphiphilic structure [52]. The addition of 5 wt% of
Tra
nsm
itta
nce
(a.
u.)
40080012001600200024002800320036004000
3197
3304
3290
997
2927
2927
1077
1149
1149
1077
997
(i)
(iii)
(ii)
Wavenumber (cm-1
)
Fig. 5 FTIR spectra for (i) pureGO film (ii) pure PS film and (iii)80 wt% PS-20 wt% GO polymerblend in the region of 400–4000 cm−1
Ionics (2017) 23:411–425 417
LiCF3SO3 to the PS-GO blend has shifted the wavenum-ber from 3290 to 3276 cm−1, and the values graduallyrose up to 3413 cm−1 for 40 wt% of LiCF3SO3 electro-lyte, as shown in Fig. 6a. Above 45 wt% of LiCF3SO3
content, the wavenumber shifted back to lower wavenum-ber at 3406 cm−1. This result indicates that more salt ag-gregation occurs, which limits the movement of ions, andthus, it decreases the conductivity of the electrolyte [53].In Fig. 6b(i), pure LiCF3SO3 salt consists of triflate ion(CF3
−) that exhibits fundamental vibrational mode of
CF3− at 635 cm−1 [45, 54]. Appearance of CF3
− peakwas observed at 635 cm−1 upon incorporation of 5 wt%LiCF3SO3 into the PS-GO blend. Interaction of LiCF3SO3
with PS-GO polymer can be seen as more lithium salt wasadded into the electrolyte system. As the concentration ofLiCF3SO3 increases, the intensities of the peak also in-creased due to the increase in the number of free ionswith addition of salts [55].
Electrolyte with 40 wt% of LiCF3SO3 is selected to bethe base system for addition of plasticizer ([Bmim][Cl])due to high conductivity achieved at that particular ratio.
620660
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Fig. 6 a FTIR spectra for (i) 0 wt% LiCF3SO3, (ii) 5 wt%LiCF3SO3, (iii)10 wt% LiCF3SO3, (iv) 15 wt% LiCF3SO3, (v) 20 wt% LiCF3SO3, (vi)25 wt% LiCF3SO3, (vii) 30 wt% LiCF3SO3, (viii) 35 wt% LiCF3SO3, (ix)40 wt% LiCF3SO3, (x) 45 wt% LiCF3SO3, and (xi) 50 wt% LiCF3SO3 inthe region of 3000–3700 cm−1. b FTIR spectra for (i) pure LiCF3SO3
Fig. 7 a FTIR spectra for (i) 0 wt% BmimCl followed by (ii) 10 wt%BmimCl, (iii) 20 wt% BmimCl, (iv) 30 wt% BmimCl, (v) 40 wt%BmimCl, and (vi) 50 wt% BmimCl in the region of 3200–3700 cm−1. bFTIR spectra for (i) pure BmimCl followed by (ii) 0 wt% BmimCl, (iii)10 wt%BmimCl, (iv) 20 wt%BmimCl, (v) 30 wt%BmimCl, (vi) 40 wt%BmimCl, and (vii) 50 wt% BmimCl in the region of 1500–1600 cm−1
418 Ionics (2017) 23:411–425
FTIR spectra for plasticized system were displayed inFig. 7a, b. Complexation between starch polymer and plasti-cizer occurs in a broad absorption peak ofO–H stretchingmodein the range of 3200–3600 cm−1. That particular region is com-monly associated with the stretching vibration mode of thehydrogen bonded O–H groups of starch and absorbed waterfrom [Bmim][Cl] molecules [42, 48]. Figure 7a displayed ab-sorption peaks in the range of 3200–3600 cm−1 that areassigned to stretching vibrations of O–H band from variousforms of free and hydrogen-bonded hydroxyl groups. It canbe seen that O–H band of 0 wt% [Bmim][Cl] shifted from3413 to 3341 cm−1 upon addition of 10 wt% of [Bmim][Cl].The trend of the FTIR spectra kept shifted to lower wavenum-ber with increasing of weight percentage of [Bmim][Cl] used.
The appearance of a medium sharp peak of C═N stretchingmode occurs at 1561 cm−1 for pure [Bmim][Cl] and no C═Npeak observed for 0 wt% [Bmim][Cl] as illustrated in Fig. 7b.Inclusion of 10 wt% [Bmim][Cl] has shifted the peak value tohigher wavenumber 1572 cm−1 due to the presence of imidazolecation in the PS-GO-LiCF3SO3 electrolyte system [50]. Thereare no changes in wavenumber with increasing amount of[Bmim][Cl] added into the electrolyte system. The transmittanceof C═N band also have became more intense and sharp as[Bmim][Cl] content increases in the electrolyte system. The ad-dition of 50 wt% of [Bmim][Cl] to the electrolyte sample hascaused the material to soften and thus cannot be use as solidpolymer electrolyte [21].
The ionic conductivity behavior was investigated by meansof impedance spectroscopy. Conductivity of salted system atroom temperature is plotted in Fig. 8 as a function ofLiCF3SO3 content. The conductivity data of other PS-GO poly-mer blend are not studied because the conductivity of the purePS film is already low as (3.90 ± 0.11) × 10−11 S cm−1 while forpure GO film is (6.75 ± 1.54) × 10−8 S cm−1. The conductivity of80 wt% PS-20 wt% GO film at room temperature is(3.19 ± 0.89) × 10−9 S cm−1 since no mobile ions providedwithin the film. Despite the low value of ionic conductivity,blending starch polymer with carbon-based material is one way
to achieve mechanical stability of film with good conductivityproperty. During starch gelatinization process, GO moleculeshave disrupted the starch chains by forming hydrogen bondingwith PS and prevent the re-crystallization of PS [32, 56].
The addition of 5 wt% lithium salt has increased the con-ductivity of the system to (4.93 ± 0.94) × 10−9 S cm−1. Onaddition of 15 wt% LiCF3SO3 and 25 wt% LiCF3SO3, theconductivity further increases to (1.47 ± 0.34) × 10−8 and(4.15 ± 0.70) × 10−7 S cm−1, respectively. The optimum ionicconductivity is achieved using 40 wt% LiCF3SO3 with thevalue of (1.48 ± 0.35) × 10−5 S cm−1. Other report also reportedthe conductivity of potato starch-chitosan in the order of10−5 S cm−1 with the use of LiCF3SO3 [57]. The increase inconductivity with the salt content is attributed to the increase inthe number of charge carriers in the system [73]. The conduc-tivity of the PS-GO-LiCF3SO3 electrolyte suddenly decreasedto (5.6 ± 0.97) × 10−6 and (2.55 ± 0.26) × 10−6 S cm−1 when 45and 50 wt% of LiCF3SO3 were doped into the electrolyte sys-tem respectively. This happened because, at higher salt concen-tration, the lithium ions tend to aggregate among themselvesand limit the pathway for ionic conduction [1].
Figure 9 depicts the variation of ionic conductivity as functionof [Bmim][Cl] content at room temperature for plasticized system.The conductivity obtained for salted system in Fig. 8 has in-c r e a s e d f r o m ( 1 . 4 8 ± 0 . 3 5 ) × 1 0 − 5 t o(3.46 ± 0.71) × 10−5 S cm−1 with the addition of 10 wt%[Bmim][Cl]. The highest conducting electrolyte was obtained at30 wt% of [Bmim][Cl] with conductivity value of (4.80± 0.69) × 10−4 S cm−1. Other work reported that with the increas-ing concentration of the [Bmim][Cl] as plasticizer creates a freevolume and increases the weight of molecules which promotesthe salt dissociation and weakens the polymer bond/Li+ [27].However, further addition of 40 wt% [Bmim][Cl] has decreasedthe conductivity to (1.91 ± 0.18) × 10−4 S cm−1. Incorporation ofmore ionic liquids (50 wt% [Bmim][Cl]) has made the thin filmbecome gel-like material with conductivity value of(8.2 ± 0.98) × 10−5 S cm−1. Bovio et al. (2009) reported that ionicliquid with cation bearing alkyl tails such as [Bmim][Cl] make
LiCF3SO3 content (wt.%)
Co
nd
uct
ivit
y,
σ (
S c
m-1
)
1.00E-09
1.00E-08
1.00E-07
1.00E-06
1.00E-05
1.00E-04
0 5 10 15 20 25 30 35 40 45 50
Fig. 8 Room temperature conductivity as a function of LiCF3SO3
content
1.00E-05
1.00E-04
1.00E-03
0 10 20 30 40 50
BmimCl content (wt.%)
Conduct
ivit
y,
σ (
S c
m-1
)
Fig. 9 Room temperature conductivity as a function of BmimCl content
Ionics (2017) 23:411–425 419
ordered arrangement at the solid–liquid interface of graphenesheet ensuring its better wetting ability [58].
The amount of charge that can be stored by polymericmaterial is evaluated by means of dielectric spectral analysis.Dielectric constant, ɛr, is representative of stored charge in amaterial while dielectric loss, εi, is a measure of energy lossesto move ions when the polarity of electric field reverses rap-idly [59, 60]. To further enhance the understanding in conduc-tivity behavior of polymer electrolyte, plots of dielectric con-stant, ɛr, and dielectric loss, εi, versus frequency at room tem-perature is shown in Figs. 10a, b and 11a, b for both salted
system (PS-GO-LiCF3SO3) and plasticized system (PS-GO-LiCF3SO3-[Bmim][Cl]), respectively. The following equa-tions are used to measure ɛr and εi values.
εr ¼ Zi
ωC0 Z2r þ Z2
i
� � ð3Þ
εi ¼ Zr
ωC0 Z2r þ Z2
i
� � ð4Þ
0.00E+00
1.00E+04
2.00E+04
3.00E+04
4.00E+04
5.00E+04
6.00E+04
1 2 3 4 5 6 7 8
5
10
15
20
25
30
35
40
45
50
Log ƒ [Hz]
0102030
405060
1 2 3 4 5 6 7wt.% LiCF3SO3
wt.% LiCF3SO3
wt.% LiCF3SO3
wt.% LiCF3SO3
wt.% LiCF3SO3
wt.% LiCF3SO3
wt.% LiCF3SO3
wt.% LiCF3SO3
wt.% LiCF3SO3
wt.% LiCF3SO3
0.00E+00
2.00E+05
4.00E+05
6.00E+05
8.00E+05
1.00E+06
1.20E+06
1 2 3 4 5 6 7 8
0 wt.% BmimCl
10 wt.% BmimCl
20 wt.% BmimCl
30 wt.% BmimCl
40 wt.% BmimCl
50 wt.% BmimCl
Log ƒ [Hz]
(b)
(a)Fig. 10 The frequencydependence of dielectric constant,εr, at room temperature for asalted system and b plasticizedsystem
420 Ionics (2017) 23:411–425
Sharp rise of ɛr in Fig. 10a, b at lower frequency is correlatedwith polarization process occurring at electrode/electrolyte inter-face, namely electrode polarization effects [61]. The increase in ɛrat lower frequencies is due to the enhancement of charge carrierdensity in the space charge accumulation region which is alsoknown as non-Debye type of behavior [62]. Towards higher fre-quency, ɛr for both salted and plasticized system decreases andalmost constant with frequency. The phenomenon is correlatedwith inability of molecular dipoles to rearrange them at thesefrequencies because periodic reversal of the electric field occurred
so fast that the charge carriers did not get sufficient time to orientthemselves in the field direction. Hence, there is no excess iondiffusion in the direction of the field which led to the decrease inthe values of dielectric constant [63]. However, there is no dielec-tric peak that could be found within the experimental frequencyrange used to signify that the increase in conductivity is mainlydue to the increase in number density of mobile ions [64].Figure 11a, b shows the variation of dielectric loss, (ɛi), as afunction of logarithmic frequency at room temperatures. It canbe seen that the ɛi values become very large towards low-
Log ƒ [Hz]
0.00E+00
1.00E+04
2.00E+04
3.00E+04
4.00E+04
1 2 3 4 5 6 7 8
5
10
15
20
25
30
35
40
45
50
(a)
0
50
100
150
200
1 2 3 4 5 6 7
wt.% LiCF3SO3
wt.% LiCF3SO3
wt% LiCF3SO3
wt.% LiCF3SO3
wt.% LiCF3SO3
wt.% LiCF3SO3
wt.% LiCF3SO3
wt.% LiCF3SO3
wt.% LiCF3SO3
wt.% LiCF3SO3
0.00E+00
1.00E+05
2.00E+05
3.00E+05
4.00E+05
5.00E+05
1 2 3 4 5 6 7 8
0 wt.% BmimCl
10 wt.% BmimCl
20 wt.% BmimCl
30 wt.% BmimCl
40 wt.% BmimCl
50 wt.% BmimCl
Log ƒ [Hz]
(b)
Fig. 11 The frequencydependence of dielectric loss, εi,at room temperature for a saltedsystem and b plasticized system
Ionics (2017) 23:411–425 421
frequency region. This is due to the motion of free charge carrierswithin the material [53]. At high frequencies, dielectric loss de-creases, owing to the reason of reduction of charge carriers at theinterface between electrode and electrolyte. From Fig. 11a, ɛivalues for 35, 40, and 45 wt% of LiCF3SO3 are first decreasesin low frequency region followed by a peak in high frequencyregion. The appearance of peak is attributed to the relaxationphenomena of polymer which relates the motion of salt in freechain segment [65, 69].
Electric modulus studies
The study of electrode polarization/interfacial polarization effectin the electrolyte system was carried out by plotting moduluselectric spectra which highlight the bulk dielectric behavior and
suppress the effect of electrode polarization [66]. The electricmodulus representation is advantageous to study relaxations phe-nomena because it suppresses both the direct current conductionand electrode polarizations and displays the alpha (α) and beta(β) relaxations in a higher frequency range [67]. In the modulusstudy, the real part of electrical modulus,Mr, and imaginary partof electrical modulus, Mi, was calculated using Eqs. 5 and 6:
Mr ¼ εrεr2 þ εi2ð Þ ð5Þ
Mi ¼ εiεr2 þ εi2ð Þ ð6Þ
Log ƒ [Hz]
Mr
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 1 2 3 4 5 6 7
5 wt.%
15 wt.%
20 wt.%
25 wt.%
30 wt.%
35 wt.%
40 wt.%
45 wt.%
50 wt.%
LiCF3SO3
LiCF3SO3
LiCF3SO3
LiCF3SO3
LiCF3SO3
LiCF3SO3
LiCF3SO3
LiCF3SO3
LiCF3SO3
(a)
Log ƒ [Hz]
0
0.005
0.01
0.015
0.02
0.025
1 2 3 4 5 6 7
0 wt.% BmimCl
10 wt.% BmimCl
20 wt.% BmimCl
30 wt.% BmimCl
40 wt.% BmimCl
50 wt.% BmimClMr
(b)
Fig. 12 The dependence of Mr
on frequency for selectedelectrolytes in a salted and bplasticized systems at roomtemperature
422 Ionics (2017) 23:411–425
Figures 12a, b and 13a, b show the variation of real andimaginary parts of the electric modulus with the frequency forselected PS-GO electrolyte films with different contents of salt(LiCF3SO3) and plasticizer ([Bmim][Cl]) at room tempera-ture. Maxwell’sWagner polarization or interfacial polarizationis a common feature of heterogeneous materials such as poly-mer electrolytes or polymer nanocomposites [68]. Based onFig. 12a, the trend of Mr values at lower frequencies readilyincreases to high frequencies with decreasing amount of lith-ium salt. It can be seen that for 5, 15, and 20 wt% LiCF3SO3,theMr value is greater than zero and some electrode polariza-tion visibly occurs for these electrolytes. The value of Mr
approaches zero, indicating the negligible contribution of theelectrode polarization with addition of more than 25 wt% ofLiCF3SO3 [69]. Meanwhile, in Fig. 12b, Mr value of plasti-cized system is low and approaching to zero at low frequencyand increases suddenly towards high frequency with maxi-mum peak for 10, 20, and 50 wt% of [Bmim][Cl] electrolytescorrelated to the distribution of relaxation processes over arange of frequencies [70]. The appearance of maximum peakalso showed that the PS-GO electrolytes are ionic conductor[71]. In Fig. 12a, b, higher conducting electrolytes possesslowerMr values, which indicate the increase in charge carrierswithin electrolyte material [72]. The increase of Mr with the
Log ƒ [Hz]
Mi
0
0.1
0.2
0.3
0.4
0.5
0 1 2 3 4 5 6 7
5 wt.%
15 wt.%
20 wt.%
25 wt.%
30 wt.%
35 wt.%
40 wt.%
45 wt.%
50 wt.%
LiCF3SO3
LiCF3SO3
LiCF3SO3
LiCF3SO3
LiCF3SO3
LiCF3SO3
LiCF3SO3
LiCF3SO3
LiCF3SO3
(a)
(b)
0
0.02
0.04
0.06
0.08
1 2 3 4 5 6 7
0 wt.% BmimCl
10 wt.% BmimCl
20 wt.% BmimCl
30 wt.% BmimCl
40 wt.% BmimCl
50 wt.% BmimCl
Log ƒ [Hz]
Mi
Fig. 13 The dependence ofMi onfrequency for selected electrolytesin a salted and b plasticizedsystems at room temperature
Ionics (2017) 23:411–425 423
frequency is attributed to the lack of the restoring force in-duced by the electric field, and it is indicative of the long rangemobility of the charge carriers [73].
From Fig. 13a, the Mi plot shows an asymmetric behaviorwith respect to peak maxima applied for spectra of 5, 15, and20 wt% of LiCF3SO3 and there is no peak appear for Fig. 13bafter addition of [Bmim][Cl] into the electrolyte system. Theleft regions of the peak indicate the conduction process, whilethe right regions of the peak are associated to the relaxationprocess [74]. The peak in Mi plot can be attributed to ionicconduction [75]. The appearance of peak in imaginary part ofthe dielectric modulus, Mi, also can be assumed to be relatedwith the translation ionic dynamics and the conductivity re-laxation of the mobile ions [61]. According to pioneer studies,electric modulus values will be increased with frequency dueto the bulk effect that resulted from electron transfer within theelectrolyte systems [76, 77]. Appearance of long tails at lowfrequencies are observed because of the large capacitance ap-plied on the electrodes which confirmed the non-Debye be-havior in the PS-GO-LiCF3SO3-[Bmim][Cl] system [66, 78].
Conclusion
GOwas successfully synthesized following amethod reportedin the literature, and this material was used in fabricating solidpolymer electrolyte (SPE) system. Polymer blend, which con-sists of PS and GO, was prepared by solution casting methodand XRD analysis showed that both materials are well dis-persed in the matrix as the oxygen-rich groups of GO promot-ed strong interactions with the PS. A non-linear least squarefitting software was used to obtain degree of crystallinity ofthe various ratios of polymer blend. PS-GO polymer blendwith ratio of 80:20, PS/GO (wt%) has the lowest value ofdegree of crystallinity as much as 20.09 %, and this resultindicates the decrease in crystallinity of PS because of thepresence of GO. The FTIR results showed the existence ofhydrogen bonding interaction between GO and PS matrix.Incorporation of LiCF3SO3 as doping salt in PS-GO polymerblend has significantly increased the conductivity of SPE sys-t em. In sa l t ed sys tem, h ighes t conduct iv i ty of(1.48 ± 0.35) × 10−5 S cm−1 was achieved, utilizing 40 wt%LiCF3SO3 at room temperature.
Complexation between the PS-GO blend and lithium saltwas proven in shifting of O–H band at 3000–3700 cm−1 andappearance of CF3 vibration at 620–660 cm−1 in the FTIRstudies. The conductivity was further enhanced by introducing[Bmim][Cl] into the PS-GO-LiCF3SO3 SPE system with op-timum value of (4.80 ± 0.69) × 10−4 S cm−1 using 30 wt%BmimCl. For salted system and BmimCl ionic liquid, interac-tion occurs through hydrogen bonding at 3200–3600 cm−1
and existence of C═Nband at 1500–1600 cm−1. The dielectricstudies and modulus spectrum showed that PS-GO-
LiCF3SO3-BmimCl SPE system is a non-Debye electrolytematerial, which corresponds to suppression of interfacial po-larization and relaxation of hopping charges.
Acknowledgments The authors thank the University of Malaya for thePPP grant awarded (grant. no PG023-2015A).
References
1. Shukur MF, Ibrahim MF, Majid NA, Ithnin R, Kadir MFZ (2013)Phys Scripta 88:1–9
Chem 187:218–22436. Wang SJ, Yu JL, Gao WY (2005) Am J Biochem Biotechnol 1:
207–21137. Lopez-Rubio A, Flanagan BM, Gilbert EP, Gidley MJ (2008)
Biopolym 89:761–76838. Sun J, Liao X, Minor AM, Balsara NP, Zuckermann RN (2014) J
Am Chem Soc 136(42):14990–1499739. Xie F, Flanagan BM, Li M, Truss RW, Halley PJ, Gidley MJ et al
(2015) Carbohyd Polym 122:160–16840. Colonna S, Monticelli O, Gomez J, Novara C, Saracco G, Fina A
(2016) Polym 102:292–30041. Jang BZ, Zhamu A (2008) J Mater Sci 43(15):5092–510142. Yadav M, Ahmad S (2015) Int J Biol Macromol 79:923–93343. Kvien I, Junji S, Martin V, Kristiina O (2007) J Mater Sci 42:
8163–817144. Suriani AB, Nurhafizah MD, Mohamed A, Zainol I, Masrom AK
(2015) Mater Lett 161:665–66845. Azli AA, Manan NSA, Kadir MFZ (2015) Adv Mater Sci Eng
145735:1–1046. Wang G,Wang B, Park J, Yang J, Shen X, Yao J (2009) Carbon 47:
68–7247. Cao Y-C, Xu C, Wu X, Wang X, Xing L, Scott K (2011) J Power
Sources 196:8377–838248. Van Soest JJG, Tournois H, De Wit D, Vliegenthart JFG (1995)
Carbohyd Research 279:201–21449. Liu Q, Charlet G, Yelle S, Arul J (2002) Food Res Int 35:397–40750. Capron I, Robert P, Colonna P, Brogly M, Planchot V (2007)
Carbohyd Polym 68:249–25951. Taghizadeh MT, Abdollahi R (2015) Polym Degrad Stabil 116:53–6152. Ye S, Cao Y, Feng J, Wu P (2013) RSC Adv 3(21):7987–799553. Yusof YM, Illias HA, Kadir MFZ (2014) Ionics 20(9):1235–124554. Starkey SR, Frech R (1997) Electrochim Acta 42:471–47455. Osman Z, Arof A (2003) Electrochim Acta 48:993–999
56. XuYX, HongWJ, Bai HLC, Shi GQ (2009) Carbon 47:3538–354357. Amran NNA, Manan NSA, Kadir MFZ (2016) Ionics 22(9):1647–
165858. Bovio S, Podestà A, Lenardi C, Milani P (2009) J Phys Chem B
Sahaya SX (2012) J Appl Phys 1(4):47–5165. Pradhan DK, Choudhary RNP, Samantary BK (2008) Int J
Electrochem Sci 3:597–60866. Khiar ASA, Puteh R, Arof AK (2006) J Phys B Condensed Matter
373:23–2767. Bruno RM, Elisabete IS, Reginaldo M, Velasco-Davalos IA,
Andreas R, Ana CT, Fabio CF (2015) J Power Sources 293:859–867
68. Tsangaris GM, Psarras GC, Kouloumbi N (1998) J Mater Sci33(20):27–37
69. Ramesh S, Arof AK (2001) Mat Sci Eng B-Solid 85:11–1570. Patro LN, Hariharan K (2009) Mater Chem Phys 116:81–8771. Ramesh S, Liew CW, Arof AK (2011) J Non-Cryst Solids 357:
