- Email: [email protected]
Mg-ion conducting gel polymer electrolyte membranes containing biodegradable chitosan: Preparation, structural, electrical and electrochemical properties
Accepted Manuscript Mg-ion conducting gel polymer electrolyte membranes containing biodegradable chitosan: Preparation, structural, electrical and electrochemical properties Jingwei Wang, Shenhua Song, Shang Gao, Muchakayala Ravi, Renchen Liu, Qing Ma PII:
S0142-9418(17)30601-3
DOI:
10.1016/j.polymertesting.2017.07.016
Reference:
POTE 5096
To appear in:
Polymer Testing
Received Date: 9 May 2017 Revised Date:
13 July 2017
Accepted Date: 13 July 2017
Please cite this article as: J. Wang, S. Song, S. Gao, M. Ravi, R. Liu, Q. Ma, Mg-ion conducting gel polymer electrolyte membranes containing biodegradable chitosan: Preparation, structural, electrical and electrochemical properties, Polymer Testing (2017), doi: 10.1016/j.polymertesting.2017.07.016. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Mg-ion conducting gel polymer electrolyte membranes containing biodegradable chitosan: Preparation, structural, electrical and electrochemical properties Jingwei Wanga, Shenhua Songa,*, Shang Gaoa, Muchakayala. Ravia, Renchen Liub,c,
a
RI PT
Qing Mab,c,** Shenzhen Key Laboratory of Advanced Materials, Department of Materials Science
and Engineering, Shenzhen Graduate School, Harbin Institute of Technology, Shenzhen – 518055, China
Research Institute of Tsinghua University in Shenzhen, Shenzhen 518055, China
c
Tsinghua Innovation Center in Dongguan, Dongguan 523808, China
M AN U
SC
b
ABSTRACT
A biodegradable gel polymer electrolyte system based on chitosan/magnesium trifluoromethanesulfonate/2-ethyl-3-methylimidazolium trifluoromethanesulfonate
TE D
(CA/Mg(Tf)2/EMITf) is developed. The structure, thermal performance, mechanical properties, ionic conductivity, relaxation time, electrochemical stability and ionic transport number of the membranes are analyzed by various techniques. The ion
EP
migration mainly depends on the complexation and decomplexation of Mg2+ with amine band (NH2) in chitosan. The 90CA-10Mg(Tf)2 system plasticized with 10% EMITf (relative to the amount of 90CA-10Mg(Tf)2) is identified as the optimum one
AC C
and the temperature dependence of ionic conductivity obeys the Arrhenius rule. Moreover, the relaxation time of the electrolyte is very short, being just 1.25×10-6 s, and its electrochemical stability window is quite wide, being up to 4.15 V. The anodic oxidation and cathodic reduction of Mg at the Mg-electrode/electrolyte interface is facile, and the ionic transference number of this electrolyte is 0.985, indicating that it could be a potential electrolyte candidate for Mg-ion devices.
*
Corresponding author. Tel.: +86-755-26033465. E-mail addresses: [email protected]; [email protected] (S.-H. Song). **
Corresponding author. Tel.: +86-755-26551328. E-mail address: [email protected] (Q. Ma). 1
ACCEPTED MANUSCRIPT Keywords: Chitosan; Biodegradable gel polymer electrolytes; Conduction mechanism; Ionic conductivity; Ionic transference number. 1. Introduction In recent decades, due to the rapid development of electric vehicles, it is necessary
RI PT
to fabricate high-performance batteries or capacities. The property requirements for these devices are getting higher and higher, such as high capacity, safety, shape flexibility, controllable size, and environmental friendliness [1,2]. Hence, an
SC
increasing attention has been paid to the development of polymer electrolyte, serving as both electrolyte and separator, for the potential application in batteries or capacitors.
M AN U
The polymer electrolyte offers some advantages over the traditional liquid electrolyte, such as the absence of internal short circuit and electrolyte leakage. The environmentfriendly polymer electrolyte not only meets the above demands, but also cuts the cost. In addition, the polymer itself will not cause an environmental pollution in the
TE D
treatment of used batteries or capacitors. Biodegradable polymers, such as starch [3], cellulose [4,5], polyethylene glycol [6] and chitosan [7-9] are potential candidates for ion device applications. Among these biodegradable polymers, chitosan offers some
EP
advantages, such as nontoxic, high nitrogen content, good molecular biocompatibility and reproducible.
AC C
Chitosan is a natural biodegradable polymer. It is composed of 2-amino-2-deoxy-
D-glucopyranose units which link with β-(1-4) glycosidic [10]. Owing to the existence of functional amine group in molecular structure, chitosan has been applied in many aspects, such as electronics [11-13], food industry [14,15] and pharmaceutical industry [16]. It is demonstrated that the amine (NH2) group in chitosan contains a lone pair electron and can serve as electron donors that coordinate with the cation of a doped salt [17,18]. The chitosan-based polymer electrolyte membranes for Li-ion batteries have been reported. However, for the preparation of 2
ACCEPTED MANUSCRIPT Li-ion batteries, the environment requirements are harsh due to the highly reactive performance of Li-salt [19]. On the other hand, compared with Li-ion batteries, Mgion batteries can offer a higher volumetric capacity stemming from bivalent Mg2+ [2023] and they can be fabricated without protection. For this, one of the primary issues
RI PT
is to explore new types of electrolyte for Mg-ion batteries [24]. So far, there have
been almost no reports of chitosan-based electrolytes for Mg-ion battery applications. Actually, the chitosan membranes after drying are low flexible and fragile due to the
SC
intermolecular or intramolecular forces occurring between hydroxyls or hydroxyl and amine. Moreover, this structural property has a serious effect on the ion or proton
M AN U
conductivity. To tackle these problems, many researchers have suggested several methods, mainly using the mixture of two or more polymers and the plasticizer [2527]. Among these methods, plasticization has certain advantages, such as simple operation, low cost, and high effectiveness. For chitosan-based polymer electrolyte
TE D
membranes, the mostly plasticizer is ethylene carbonate (EC). Arof et al. [18] reported that the optimal ionic conductivity of Li-ion conducting EC-plasticized chitosan acetate/LiCF3SO3 salt electrolytes was 4.0×10-5 S cm-1. The significant role
EP
of EC is to improve the ionic conductivity of the film, but its thermal stability and mechanical performances are deteriorative. In order to prepare the chitosan polymer
AC C
electrolyte membranes with excellent properties, ionic liquid used as a substitute for conventional plasticizer attracts some interest. The ionic liquid is composed of organic cations and inorganic anions and it offers some advantages, such as nonvolatile, nonflammable, superior thermal stability and miscibility with diverse compounds [28-31]. Currently, the host polymers of electrolyte membranes doped with ionic liquid are mainly semi-crystalline poly(vinylidenefluoride-cohexafluropropylene) (PVDF-HFP) and poly(ethylene oxide) (PEO) [32,33]. 3
ACCEPTED MANUSCRIPT Apparently, these polymers are synthetic and not biodegradable. Furthermore, it is demonstrated [34,35] that the magnesium reversibly deposited reaction does not take place for the simple magnesium salts containing ClO4 or BF4 anions owing to the formation of dense zones on the electrode surface due to the reduction product of the
RI PT
anions with pure Mg. This implies that these kinds of salt could not be used to prepare Mg-ion conducting polymer electrolytes.
In the present study, we prepared biodegradable and low-cost chitosan-based Mg-
SC
ion conducting gel polymer electrolyte membranes with the dopants of magnesium trifluoromethanesulfonate (Mg(Tf)2) and 1-ethyl- 3 methylimidazolium
M AN U
trifluoromethanesulfonate (EMITf) for possible applications in Mg-ion devices. The study mainly focused on the effect of various amounts of Mg(Tf)2 or EMITf on the structural and electrical performances of chitosan-Mg(Tf)2-EMITf gel polymer electrolyte membranes. The crystallinity, thermal stability, mechanical properties, ion
TE D
interaction and ionic conductivity of the membranes were investigated using X-ray diffraction (XRD), thermogravimetric analysis (TGA), mechanical measurements, Fourier transform infrared spectroscopy (FTIR), and impedance spectroscopy.
EP
2. Experimental procedures
The preparation method of polymer electrolyte membranes was solution casting.
AC C
Firstly, the host polymer chitosan powder (CA) (1 g, medium viscosity) was dissolved in 100 ml 1% acetic acid solution, and then the Mg(Tf)2 salt was added to the solution in different concentrations with respect to the amount of chitosan (5, 10, 15 and 20 wt.%, denoted as CA-5, CA-10, CA-15 and CA-20, respectively). The mixed solution was magnetically stirred until a completely homogeneous solution was present and then treated in vacuum oven to eliminate air bubbles, followed by casting into clean petri dishes. The membranes were formed after drying at room temperature. Finally, different quantities of EMITf ionic liquid (5, 10, 15 and 20 wt.%) were added to CA4
ACCEPTED MANUSCRIPT Mg(Tf)2 systems with the same method as above to prepare ionic liquid-plasticized gel polymer electrolyte membranes. For example, in order to prepare the CA-10 (90CA-10Mg(Tf)2) based gel polymer electrolyte membranes, 5, 10, 15 and 20 wt.% of EMITf ionic liquid (relative to the amount of CA-10 system) would be added,
RI PT
which were denoted as CA-10-5, CA-10-10, CA-10-15 and CA-10-20, respectively.
The photomacrographs of a CA-10-10 gel polymer electrolyte membrane are shown in Fig. 1, indicating that they are free-standing and flexible.
SC
X-ray diffractometer (XRD) (D/max 2500PC, Rigaku, Japan) with Cu-Kα
radiation (λ = 1.5406 Å) was applied to analyze the structural characteristics of the
M AN U
electrolyte membranes. The surface morphologies of the membranes were examined using scanning electron microscopy (Hitachi S-4700 SEM, Japan) and their thermal stability was evaluated by a thermal analyzer (STA 449F3 Jupiter, Germany) under a protective N2. The mechanical properties of the membranes were measured by means
TE D
of a universal testing machine (CMT7504, China). In the testing, the specimens (thickness×width×gauge length=0.1mm×10mm×80mm) were tensile-deformed at ambient temperature under a crosshead speed of 10 mm/min. The mechanism of ion
EP
conduction in the polymer electrolyte was analyzed by Fourier transform infrared spectroscopy (FTIR) (Nicolet 380 spectrometer).
AC C
For a solid electrolyte material, the ionic conductivity is the most important
parameter. This property was examined via an impedance analyzer (PSM 1735, Newton, UK). The circular membranes were sandwiched between two symmetrical stainless steel electrodes (SS) with the contact area of 0.75 cm2. During impedance measurements, the frequency was in the range of 1 MHz to 10 Hz with a signal level of 10 mV. The resistance for calculating ionic conductivity was determined by fitting the Nyquist impedance plot with Zsimp Win software. The electrochemical 5
ACCEPTED MANUSCRIPT performances of polymer electrolyte membranes including electrochemical stability window and ionic transference number were evaluated using cyclic voltammetry and DC polarization with the aid of an electrochemical work station (Model: CHI760D, CH Instruments, China). The electrochemical stability window was recorded in the
RI PT
potential range of -3 V to 3 V. In the DC polarization study, a DC voltage of 1.0 V
was applied on the SS/electrolyte/SS cell and the obtained current was monitored as a function of time.
SC
3. Results and discussion
The XRD patterns of pure chitosan and different CA-Mg(Tf)2 membranes are
M AN U
shown in Fig. 2a. Clearly, the chitosan is a semicrystalline polymer and exhibits five primary diffraction peaks at the 2θ angles of 9°, 12°, 21.3°, 23.8°, 26.7° and a broad peak around 20° which is related to the amorphous phase. The intensities of XRD peaks decrease with increasing Mg(Tf)2 content until 10 wt.% and then increase with
TE D
further increasing Mg(Tf)2 content. This means that the degree of crystallinity has a minimum value for the CA-10 system. Based on the JADE6.0 software which is widely used in the determination of crystallinity from XRD patterns [36,37], The
EP
degrees of crystallinity have been determined for the various CA- Mg(Tf)2 systems, which are listed Table 1. As can be seen, the crystallinity of pure CA is 8.22% and the
AC C
value is decreased to 7.25% by doping 10 wt.% Mg(Tf)2, i.e., the amorphicity is increased to 92.75% by the doping. This should be attributed to the complexation between chitosan and Mg(Tf)2 salt, resulting in a decrease in intermolecular and/or intramolecular interaction. Furthermore, no extra peaks emerge for the Mg(Tf)2-doped CA films. This implies that the Mg(Tf)2 salt has been completely complexed in the CA matrix. Since the CA-10 system exhibits the highest amorphicity, it is the best complex for ionic conduction. Therefore, this complex is suitable for the preparation 6
ACCEPTED MANUSCRIPT of gel polymer electrolyte by incorporating EMITf ionic liquid. Fig. 2b represents the XRD profiles of 90CA-10Mg(Tf)2 + x EMITf (x = 5, 10, 15 and 20 wt%, relative to the amount of 90CA-10Mg(Tf)2) membranes. Obviously, the XRD intensities weaken with increasing EMITf concentration up to 10 wt.% and then strengthen with further
RI PT
doping EMITf. Also, there are no other additional peaks present as compared to those without EMITf. This implies that the ionic liquid can completely dissolve in the CA10 system and simultaneously destroy the ordered structure of chitosan chains,
SC
leading to a decrease in the degree of crystallinity. The degrees of crystallinity have also been determined for these gel polymer electrolyte systems, which are listed in
M AN U
Table 1. As seen, the crystallinity is reduced to 5.61% for the CA-10-10 system. However, when the concentration of EMITf is more than 10 wt.%, the redundant anion Tf-1 ions could embrace the cation Mg2+ ions, resulting in an enhancement of intermolecular and/or intramolecular force and in turn an increase in the degree of
TE D
crystallinity.
The surface morphologies of CA-10, CA-10-5, CA-10-10 and CA-10-15 membranes are shown in Fig. 3. All the membranes are smooth and homogeneous
EP
except for the CA-10-15 sample, indicating that the salt and ionic liquid uniformly disperse in the polymer matrix. However, the salt begins to recrystallize when more
AC C
than 10 wt.% EMITf is added as shown in Fig. 3d. Thermal stability traces of CA-10, CA-10-5, CA-10-10 and CA-10-15
membranes are shown in Fig. 4. The TG curves for all the membranes clearly present two steps of weight loss. The first weight loss in the range of 50-150 oC could be due to the removal of absorbed water and the second one in the range 200-300 oC could arise from the degradation of chitosan. Obviously, the thermal stability of the EMITfplasticized gel membranes is apparently increased due to the effect of ionic liquid. . 7
ACCEPTED MANUSCRIPT Typical stress-strain curves of pure CA, CA-10 and CA-10-10 are represented in Fig. 5. All the curves present similar characteristics. The breaking strain increases considerably with the addition of salt and ionic liquid, being about 85% for the CA10-10 system. The other mechanical parameters for this system are that
RI PT
Young's modulus ≈ 640 MPa and yield strength ≈ 52 MPa. Obviously, the mechanical properties of the membrane are well enough for practical application as it is flexible with sufficient strength and stiffness.
SC
In order to improve the electrical properties of a polymer electrolyte membrane, it is necessary to understand the conduction mechanism in the membrane. Fig. 6 shows
M AN U
the FTIR spectra of virgin CA and CA - x Mg(Tf)2 (x = 5, 10 and 15 wt.%) in the range 700-4000 cm-1. The spectrum of the virgin chitosan membrane is similar to previous studies [8,14,17,26,38]. The peaks around 3600 to 3200 cm-1 are corresponding to the overlapping of –NH and –OH stretching vibrations; the double
1
TE D
peaks due to symmetric and asymmetric –CH2 stretching appear at 2920 and 2871 cm; carbonyl (C = O – NHR), amine band (NH2) and ammonium (NH3+) are observed at
1646,1570 and 1516 cm-1, respectively; the peaks at 1416, 1376 and 1024 cm-1 are
EP
assigned to the C – N stretching combined with N –H plane deformation, CH3 symmetric angular deformation, and stretching vibration of C–O–C, respectively. The
AC C
–NH3+ band disappears in the Mg(Tf)2-complexed CA systems, which is probably attributed to the complexation between Mg ions from the salt and lone pair electrons from amine, resulting in the shift of amine band to lower wavenumbers and then overlapping with –NH3+ to form NH2 band. In order to make the above points, the spectra of carbonyl and amine bands for the complex systems are depicted in Fig. 7. Compared with the pure chitosan membrane, the carbonyl and amine bands of the CA-10 membrane downshift to 1641 (about 5 cm-1) and 1543 cm-1 (about 27 cm-1), 8
ACCEPTED MANUSCRIPT respectively. With the increase of salt concentration, the full width at half maximum (FWHM) of amine band increases, but that for carbonyl slightly reduces as shown in Table 2 (the FWHM is calculated using the OMNIC software). It is indicated that the complexation takes place between Mg2+ and chitosan, and prefers to occur at amine
RI PT
band, which is in accord with the reports by Arof [17,26]. With the introduction of EMITf ionic liquid, the position of the amine band at 1543 cm-1 for the CA-10
membrane exhibits a little change as shown in Fig. 8. It is seen that the amine bands
SC
of the CA-10-5 and CA-10-10 membranes downshift to 1538 and 1539 cm-1,
respectively, but the amine band of the CA-10-15 membrane shifts to a higher
M AN U
wavenumber (~1547 cm-1). The volume of EMI+ group containing C, H and N elements is significantly larger than that of Mg2+, and thus it is almost impossible for the complexation of EMI+ and amine to occur. This implies that the appropriate EMITf can disrupt the intermolecular and/or intramolecular force and then improve
TE D
the complexation of Mg2+ with amine band. However, the Tf ¯ from excess EMITf could combine with Mg2+, leading to a decrease in the complexation of salt with amine band. The sketches showing the possible Mg-ion conducting mechanism in the
EP
chitosan polymer electrolyte membranes are represented in Fig. 9. Since the ionic conduction in these polymer electrolyte membranes may be achieved mainly through
AC C
the complexation and decomplexation of Mg-ions with amine bands, their transport mechanism is only considered. The Nyquist impedance plots of the unplasticized and plasticized membranes are
represented in Fig. 10. The bulk resistance of the membrane can be obtained through the intercept of the semicircle on the real axis in the plot. Obviously, for the unplasticized membranes, the CA-10 sample exhibits the smallest resistance, and the
9
ACCEPTED MANUSCRIPT CA-10-10 sample does for the plasticized membranes. This means that this plasticized sample possesses the highest conductivity. The ionic conductivity (σ) is calculated by = /
(1)
where t is the membrane thickness, Rb is the bulk resistance, and A is the membrane-
RI PT
electrode contact area. In the present study, the membrane thickness is around 100 µm (determined by screw micrometer) and the membrane-electrode contact area is 0.75
cm2. The ionic conductivity of the unplasticized membranes as a function of the salt-
SC
concentration is shown in the inset of Fig. 10a. Obviously, the room-temperature ionic conductivity gradually increases until 1.98×10-6 S cm-1 at 10 wt.% Mg(Tf)2 salt,
M AN U
which is ascribed to the increase of mobile Mg-ions and their mobility.. However, it decreases when the amount of salt is more than 10 wt.% due to ion aggregation leading to a decline in both the number of mobile ions and the mobility of the ions. The ionic conductivity of electrolyte membranes could be improved by adding ionic
TE D
liquid that serves as plasticizer. The ionic liquid concentration dependence of ionic conductivity is shown in the inset of Fig. 10b. As can be seen, with the addition of EMITf up to 10 wt.%, the ionic conductivity obtains a peak value of 3.57×10-5 S cm-1
EP
which rises more than tenfold compared to the one without EMITf. The effect of ionic liquid is similar to other plasticizers, which increase the flexibility of polymer chain
AC C
segments. However, as shown in Fig. 10b, the line in the low-frequency region is not parallel to the imaginary axis for the CA-10-10 system, implying that the ionic conductivity of the gel polymer electrolyte is not good. Shukur et al. [39] developed biodegradable Mg-ion conducting gel polymer electrolytes based on the potato starch doped with magnesium acetate and different plasticizers including 1-butyl-3methylimidazolium chloride and glycerol, the obtained maximum room-temperature ionic conductivities of the electrolytes were about 1.12×10-5 S cm-1 and 2.6×10-6 S 10
ACCEPTED MANUSCRIPT cm-1, respectively. Munichandraiah et al. [40] reported that Mg-ion conducting gel polymer electrolytes were prepared on the basis of PEO plasticized with PC (propylene carbonate), EC and a mixture of PC and EC and the room-temperature ionic conductivity of the optimum composition was around 1×10-5 S cm-1. Hashmi et
RI PT
al. [33] prepared the Mg-ion conducting gel polymer electrolyte from PEO plasticized with 50 wt.% EMITf and its maximum room-temperature ionic conductivity was about 5.6×10−4 S cm−1. Obviously, the present results compare well with the
SC
previously reported data. It is worth noting that the present gel polymer electrolyte is biodegradable so that the disposing of used devices fabricated with this electrolyte
environmental protection.
M AN U
causes less environmental pollution. This should be one of its advantages in terms of
If the flexibility of polymer chain segments is improved, the relaxation time will be short because it is related to the mobility of ions in the polymer host [41,42]. The
TE D
variation of loss tangent with frequency for the EMITf-plasticized CA-10 (90CA-10 Mg(Tf)2) membranes is shown in Fig. 11. As can be seen, the relaxation frequency corresponding to the peak shifts to higher frequencies, i.e., higher ionic conductivity
EP
values, with increasing ionic liquid concentration until 10 wt.%. However, it shifts back to lower frequencies, i.e., lower ionic conductivity values, with further
AC C
increasing ionic liquid concentration. The relaxation time (τ) can be obtained by 2
=1
(2)
where f is the relaxation frequency. The determined relaxation times are listed in Table 3. Obviously, it is the shortest (1.25×10-6 s) for the CA -10-10 system. With the increase of ionic liquid concentration from 5 wt.% to 10 wt.%, the relaxation time decreases from 3.06×10-5 to 1.25×10-6 s . However, it increases when the addition of EMITf is more than 10 11
ACCEPTED MANUSCRIPT wt.%. This means that the ionic liquid can make the amorphous and free volume of the polymer increase, then raise the mobility of ions (µ i) and increase the ionic conductivity (σ) according to the following equation =∑
(3)
RI PT
where ni is the density of mobile ions, qi is the charge of mobile carriers, and µ i is the carrier mobility.
Nevertheless, the superfluous EMITf may reduce the flexibility of polymer
SC
chains and the quantity of transferable ions (ni), thereby increasing the relaxation time
and ionic conductivity.
M AN U
and decreasing the ionic conductivity. This is in accordance with the results of XRD
In order to examine the relationship between the ionic conductivity and temperature of the electrolyte membranes, the values of ionic conductivity at different temperatures were determined, which are shown in Fig. 12 for the CA-10 and CA-10-
TE D
10 membranes. It is seen that the ionic conductivity of the membranes rises with increasing temperature, and there is a linear relationship between lnσ and reciprocal temperature (1/T), obeying the Arrhenius rule exp
EP
=
(4)
AC C
where σ0 is the pre-exponential constant, Ea is the activation energy for conduction, k is the Boltzmann constant, and T is the absolute temperature. According to the equation, the activation energy (Ea) can be determined as 0.86 eV with an R2 value of 0.991 for the CA-10 membrane. The value is reduced to 0.72 eV by incoporating 10 wt.% EMITf plasticizer. Munichandraiah et al. [40] developed Mg-ion conducting polymer electrolytes based on PEO doped with Mg(Tf)2, the obtained Ea was 0.93 eV and it decreased to 0.152 eV with the addition of 60 wt.% EC+PC plasticizer. They also reported Mg-ion conducting polymer electrolytes based on PMMA doped with 12
ACCEPTED MANUSCRIPT Mg(Tf)2 and EC+PC plasticizer in a mass ratio of 1:0.5:2. The obtained Ea was 0.038 eV [43]. In our previous work [44], the 15 wt.% EMITf-plasticized 85PVA15Mg(Tf)2 membrane exhibited an activation energy of 0.25 eV. Hence, the activation energy for polymer electrolytes varies in quite a large range, depending mainly on the
RI PT
polymer and plasticizer. The high activation energy for the CA-10-10 system (0.72 eV) indicates that the plasticized chitosan polymer electrolyte still possesses potential to increase its ionic conductivity by decreasing the activation energy.
SC
It is worth noting that the electrochemical stability window of electrolyte
membranes is vitally important for their practical applications. Fig. 13 represents the
M AN U
cyclic voltammograms of EMITf-plasticized 90CA-10Mg(Tf)2 membranes over the voltage range of -3.0 V to 3.0 V. As can be seen, the electrochemical stability window of the membranes shows little change when the addition of ionic liquid is less than 15 wt.%, while it becomes narrower beyond this amount. For the CA-10-10 membrane,
TE D
the electrochemical stability window is as wide as 4.15 V that is well enough for practical applications [32].
The cyclic voltammogram for the Mg/CA-10-10/Mg cell is shown in Fig. 14a.
EP
Obviously the anodic and cathodic peaks emerge, but they do not emerge for the SS/CA-10-10/SS cell in the same potential range (Fig. 13). This implies that the
AC C
anodic oxidation and cathodic reduction of Mg at the Mg-electrode/electrolyte membrane interface is a reversible process, as shown in Fig.14b where for the same interface between membrane and Mg electrode (case Ⅰ or case Ⅱ) with a cyclic scanning potential, the reversible redox reaction is facile. Accordingly, the CA-10-10 gel polymer electrolyte membrane could be a potential electrolyte candidate for Mgion devices. A polymer electrolyte membrane should be an ionic conductor for energy 13
ACCEPTED MANUSCRIPT storage device applications. The ionic transference number (tion) of a polymer electrolyte membrane is able to indicate this behavior, which can be evaluated by a DC polarization technique. When a potential passes through the SS/electrolyte/SS cell, the mobile ions will accumulate on the electrolyte membrane surface next to electrode
RI PT
and thus the mobile ions are depleted near the opposite electrode surface, leading to a drop of polarization current. Nevertheless, the electrode is still active to electrons.
Hence, the initial current (iI) is contributed by both ions and electrons and the final
SC
current (iF) just by electrons. The polarization curves for several SS/CA-Mg(Tf)2EMITf/SS cells with the electrolyte membranes plasticized by 5, 10, and 15 wt.%
M AN U
EMITf (CA-10-5, CA-10-10, and CA-10-15), respectively, are represented in Fig. 15. The initial and final current values (iI and iF) for these cells can be obtained from these curves. Then the values of tion are calculated by
iI − iF iI
(5)
TE D
tion =
The ionic transference numbers for the CA-10-5, CA-10-10, and CA-10-15 gel electrolyte membranes are obtained as 0.985, 0.985, and 0.983, respectively,
EP
indicating the predominant ionic conduction in the polymer electrolyte membranes. 4. Conclusions
AC C
Biodegradable plasticized gel polymer electrolyte membranes consisting of
chitosan, Mg(Tf)2 salt and EMITf ionic liquid are prepared by solution casting. The possible Mg-ion conducting mechanism in the membranes is elucidated and it is mainly through the complexation and decomplexation of Mg2+ with amine band (NH2) from chitosan. The 10 wt.% EMITf-plasticized 90CA-10 Mg(Tf)2 gel polymer electrolyte exhibits the highest room-temperature ionic conductivity, being up to 3.57×10-5 S cm-1. The conductivity increases with increasing temperature, following 14
ACCEPTED MANUSCRIPT the Arrhenius rule with an activation energy of 0.72 eV. The relaxation time of the 10 wt.% ionic liquid-incorporated electrolyte membrane is as low as 1.25×10-6 s, indicating that the mobility of ions is relatively high. The ionic transference number for the electrolyte membrane is about 0.985, showing that the ionic conduction is
RI PT
dominant in the polymer electrolyte membranes. Moreover, the electrochemical
stability window of this electrolyte membrane is as wide as 4.15 V which is sufficient
SC
for practical applications.
Acknowledgments: This work was supported by the International S&T Cooperation
M AN U
Program of China (No. 2014DFA53020), Guangdong Innovative and Entrepreneurial Research Team Program (No. 2013C099)
References [1] Armand M., Tarascon J.M.: Building better batteries. Nature, 451, 652-657 (2008). http://www.nature.com/nature/journal/v451/n7179/full/451652a.html
TE D
[2]Whittingham M.S.: Materials Challenges Facing Electrical Energy Storage. MRS Bulletin, 33, 411-419 (2008).
AC C
EP
https://doi.org/10.1557/mrs2008.82 [3]Kumar M., Tiwari T., Srivastava N.: Electrical transport behaviour of bio-polymer electrolyte system: Potato starch +ammonium iodide. Carbohydrate Polymers, 88, 54-60 (2012). http://dx.doi.org/10.1016/j.carbpol.2011.11.059 [4]Ramesh S., Shanti R., Morris E.: Plasticizing effect of 1-allyl-3methylimidazolium chloride in cellulose acetate based polymer electrolytes. Carbohydrate Polymers, 87, 2624-2629 (2012).
http://dx.doi.org/10.1016/j.carbpol.2011.11.037 [5]Smolarkiewicz I., Rachocki A., Pogorzelec G.K., Pankiewicz R., Ławniczak P., Łapiński A., Jarek M.: Proton-conducting Microcrystalline Cellulose Doped with Imidazole. Thermal and Electrical Properties. Electrochimica Acta,155, 38-44 (2015). http://dx.doi.org/10.1016/j.electacta.2014.11.205 [6]Asghar A., Abdul S.Y., Singh L.B., Hashaikeh R.: PEG based quasi-solid polymer electrolyte: Mechanically supported by networked cellulose. Journal of Membrane Science, 421-422, 85-90 (2012). 15
ACCEPTED MANUSCRIPT http://dx.doi.org/10.1016/j.memsci.2012.06.037 [7]Yahya M.Z.A., Arof A.K.: Conductivity and X-ray photoelectron studies on lithium acetate doped chitosan films. Carbohydrate Polymers, 55, 95-100 (2004). http://dx.doi.org/10.1016/j.carbpol.2003.08.018 [8] Fadzallah I.A., Majid S.R., Careem M.A., Arof A.K.: A study on ionic interactions in chitosan–oxalic acid polymer electrolyte membranes. Journal of
RI PT
Membrane Science, 463, 65-72 (2014). http://dx.doi.org/10.1016/j.memsci.2014.03.044 [9] Aziz S.B., Abidin Z.H.Z., Arof A.K.: Influence of silver ion reduction on electrical modulus parameters of solid polymer electrolyte based on chitosan-silver
M AN U
SC
triflate electrolyte membrane. Express Polymer Letters,4, 300-310 (2010). http://doi.org/10.3144/expresspolymlett.2010.38 [10] Kadir M.F.Z., Majid S.R., Arof A.K.: Plasticized chitosan–PVA blend polymer electrolyte based proton battery. Electrochimica Acta, 55, 1475-1482 (2010). http://dx.doi.org/10.1016/j.electacta.2009.05.011 [11]Smitha B., Devi D.A., Sridhar S.: Proton-conducting composite membranes of
TE D
chitosan and sulfonated polysulfone for fuel cell application. International Journal of Hydrogen Energy, 33, 4138-4146(2008). http://dx.doi.org/10.1016/j.ijhydene.2008.05.055 [12]Chupp J., Shellikeri A., Palui G., Chatterjee J.: Chitosan-based gel film electrolytes containing ionic liquid and lithium salt for energy storage applications.
AC C
EP
Journal of Applied Polymer Science, 132, (2015). http://onlinelibrary.wiley.com/doi/10.1002/app.42143/epdf [13]Sudhakar Y.N., Selvakumar M.: Lithium perchlorate doped plasticized chitosan and starch blend as biodegradable polymer electrolyte for supercapacitors. Electrochimica Acta, 78, 398-405 (2012). http://dx.doi.org/10.1016/j.electacta.2012.06.032 [14]Chen H., Hu X., Chen E., Wu S., McClements D.J., Liu S., Li B.: Preparation, characterization, and properties of chitosan films with cinnamaldehyde nanoemulsions. Food Hydrocolloids, 61, 662-671 (2016). http://dx.doi.org/10.1016/j.foodhyd.2016.06.034 [15]Hosseini S.F., Rezaei M., Zandi M., Farahmandghavi F.: Development of bioactive fish gelatin/chitosan nanoparticles composite films with antimicrobial properties. Food Chemistry, 194, 1266-1274 (2016). http://dx.doi.org/10.1016/j.foodchem.2015.09.004 [16]Cardoso A.P., Gonçalves R.M., Antunes J.C., Pinto M.L., Pinto A.T., Castro F, Monteiro C.: An interferon-γ-delivery system based on chitosan/poly(γ-glutamic acid) polyelectrolyte complexes modulates macrophage-derived stimulation of cancer cell invasion in vitro. Acta Biomaterialia, 23, 157-171(2015). 16
ACCEPTED MANUSCRIPT
RI PT
http://dx.doi.org/10.1016/j.actbio.2015.05.022 [17]Yahya M.Z.A., Arof A.K.: Effect of oleic acid plasticizer on chitosan–lithium acetate solid polymer electrolytes. European Polymer Journal, 39, 897-902 (2003). http://dx.doi.org/10.1016/S0014-3057(02)00355-5 [18]Osman Z., Ibrahim Z.A., Arof A.K.: Conductivity enhancement due to ion dissociation in plasticized chitosan based polymer electrolytes. Carbohydrate Polymers, 44, 167-173 (2001) http://dx.doi.org/10.1016/S0144-8617(00)00236-8
M AN U
SC
[19]Wang Y., Chen R., Chen T., Lv H., Zhu G., Ma L., Wang C.: Emerging nonlithium ion batteries. Energy Storage Materials, 4, 103-129 (2016). http://dx.doi.org/10.1016/j.ensm.2016.04.001 [20]Muldoon J., Bucur C.B., Oliver A.G., Sugimoto T., Matsui M., Kim H.S., Allred G.D.: Electrolyte roadblocks to a magnesium rechargeable battery. Energy & Environmental Science, 5, 5941-5950 (2012). http://pubs.rsc.org/en/content/articlepdf/2012/ee/c2ee03029b [21]Chen Z., McDonald S., Fitzgerald P.A., Warr G.G., Atkin R.: Structural effect of glyme-Li(+) salt solvate ionic liquids on the conformation of poly(ethylene oxide). Physical chemistry chemical physics: PCCP, 18, 14894-14903 (2016).
TE D
http://pubs.rsc.org/en/Content/ArticleLanding/2016/CP/C6CP00919K [22]Huie M.M., Cama C.A., Smith P.F., Yin J., Marschilok A.C., Takeuchi K.J., Takeuchi E.S.: Ionic liquid hybrids: Progress toward non-corrosive electrolytes with high-voltage oxidation stability for magnesium-ion based batteries.
AC C
EP
Electrochimica Acta, 219, 267-276 (2016). http://dx.doi.org/10.1016/j.electacta.2016.09.107 [23]Doe R.E., Han R., Hwang J., Gmitter A.J., Shterenberg I., Yoo H.D., Pour N.: Novel, electrolyte solutions comprising fully inorganic salts with high anodic stability for rechargeable magnesium batteries. Chem Commun, 50, 243-245 (2014). http://pubs.rsc.org/en/Content/ArticleLanding/2014/CC/C3CC47896C [24]Saha P., Datta M.K., Velikokhatnyi O.I., Manivannan A., Alman D., Kumta P.N.: Rechargeable magnesium battery: Current status and key challenges for the future. Progress in Materials Science, 66, 1-86 (2014). http://dx.doi.org/10.1016/j.pmatsci.2014.04.001 [25]Sarasam A.R., Krishnaswamy R.K., Madihally S.V.: Blending Chitosan with Polycaprolactone: Effects on Physicochemical and Antibacterial Properties. Biomacromolecules, 7, 1131-1138 (2006). http://pubs.acs.org/doi/abs/10.1021/bm050935d
[26]Osman Z., Arof A.K.: FTIR studies of chitosan acetate based polymer electrolytes. Electrochimica Acta, 48, 993-999 (2003). http://dx.doi.org/10.1016/S0013-4686(02)00812-5 [27]Ng L.S., Mohamad A.A.: Protonic battery based on a plasticized chitosan17
ACCEPTED MANUSCRIPT
SC
RI PT
NH4NO3 solid polymer electrolyte. Journal of Power Sources, 163, 382-385 (2006). http://dx.doi.org/10.1016/j.jpowsour.2006.09.042 [28] Ye Y-S., Rivk J., Joe B.: Ionic liquid polymer electrolytes. Journal of Materials Chemistry A, 1, 2719-2743 (2013). http://pubs.rsc.org/en/content/articlepdf/2013/ta/c2ta00126h [29]Yuan J., Mecerreyes D., Antonietti M.: Poly(ionic liquid)s: An update. Progress in Polymer Science, 38, 1009-1036 (2013). http://dx.doi.org/10.1016/j.progpolymsci.2013.04.002 [30]Ramesh S., Liew C., Ramesh K.: Evaluation and investigation on the effect of ionic liquid onto PMMA-PVC gel polymer blend electrolytes. Journal of NonCrystalline Solids, 357, 2132-2138 (2011). http://dx.doi.org/10.1016/j.jnoncrysol.2011.03.004 [31]Leones R., Sentanin F., Rodrigues L.C., Marrucho I.M., Esperanca J.M.S.S., Pawlicka A., Silva M.M.: Investigation of polymer electrolyte based on agar and
M AN U
ionic liquids. Express Polymer Letters, 6, 1007-1016 (2012). http://doi.org/doi: 10.3144/expresspolymlett.2012.106 [32]Pandey G.P., Hashmi S.A.: Experimental investigations of an ionic-liquid-based, magnesium ion conducting, polymer gel electrolyte. Journal of Power Sources, 187, 627-634 (2009). http://dx.doi.org/10.1016/j.jpowsour.2008.10.112 [33]Kumar Y., Hashmi S.A., Pandey G.P.: Ionic liquid mediated magnesium ion
TE D
conduction in poly(ethylene oxide) based polymer electrolyte. Electrochimica Acta, 56, 3864-3873 (2011). http://dx.doi.org/10.1016/j.electacta.2011.02.035 [34] NovaÂk P., Imhof R., Haas O.: Magnesium insertion electrodes for rechargeable nonaqueous batteries - a competitive alternative to lithium? Electrochimica Acta,
AC C
EP
45, 351-367 (1999). http://dx.doi.org/10.1016/S0013-4686(99)00216-9 [35]Lu Z., Schechter A., Moshkovich M., Aurbach D.: On the electrochemical behavior of magnesium electrodes in polar aprotic electrolyte solutions. Journal of Electroanalytical Chemistry, 466, 203-217 (1999). http://dx.doi.org/10.1016/S0022-0728(99)00146-1 [36] Qin H., Tao Y., Deng B: Microstructural and mechanical properties of Si-ion implanted TiN coatings. Surface & Coatings Technology, 228 , S292–S295 (2013). https://doi.org/10.1016/j.surfcoat.2012.05.119 [37] Gebresellasie K., Lewis J. C., Shirokoff J: X‑ ray Spectral Line Shape Analysis of Asphalt Binders. Energy Fuels, 27, 2018−2024 (2013). https://doi.org/10.1021/ef301865p [38]Mendes J.F., Paschoalin R.T., Carmona V.B., Sena Neto A.R., Marques A.C.P., Marconcini J.M., Mattoso L.H.C., Medeiros E.S., Oliveira J.E.: Biodegradable 18
ACCEPTED MANUSCRIPT polymer blends based on corn starch and thermoplastic chitosan processed by extrusion. Carbohydrate Polymers, 137, 452-458 (2016). http://dx.doi.org/10.1016/j.carbpol.2015.10.093 [39]Shukur M.F., Ithnin R., Kadir M.F.Z.: Ionic conductivity and dielectric properties of potato starch-magnesium acetate biopolymer electrolytes: the effect of
RI PT
glycerol and 1-butyl-3-methylimidazolium chloride. Ionics, 22, 1113-1123 (2016). http://link.springer.com/article/10.1007%2Fs11581-015-1627-4 [40]Girish Kumar G., Munichandraiah N.: Effect of plasticizers on magnesiumpoly(ethyleneoxide) polymer electrolyte. Journal of Electroanalytical Chemistry,
SC
495, 42-50 (2000). http://dx.doi.org/10.1016/S0022-0728(00)00404-6 [41]Islam A., Yasin T., Akhtar M.J., Imran Z., Sabir A., Sultan M., Khan S.M., Jamil
M AN U
T.: Impedance spectroscopy of chitosan/poly(vinyl alcohol) films. Journal of Solid State Electrochemistry, 20, 571-578 (2016). http://link.springer.com/article/10.1007%2Fs10008-015-3082-6 [42]Kulshrestha N., Chatterjee B., Gupta P.N.: Characterization and electrical properties of polyvinyl alcohol based polymer electrolyte films doped with
AC C
EP
TE D
ammonium thiocyanate. Materials Science and Engineering: B, 184, 49-57 (2014). http://dx.doi.org/10.1016/j.mseb.2014.01.012 [43]Girish Kumar G., Munichandraiah N.: Poly(methylmethacrylate)—magnesium triflate gel polymer electrolyte for solid state magnesium battery application. Electrochimica Acta, 47, 1013-1022 (2002). https://doi.org/10.1016/S0013-4686(01)00832-5 [44] Wang J-W., Song S-H., Muchakayala R., Hu X-C., Liu R-C.: Structural, electrical, and electrochemical properties of PVA-based biodegradable gel polymer electrolyte membranes for Mg-ion battery applications. Ionics, 23, 1759-1769 (2017). http://link.springer.com/article/10.1007/s11581-017-1988-y
19
ACCEPTED MANUSCRIPT FIGURES Fig. 1. Macroscopic images of a CA-10-10 polymer electrolyte membrane.
Fig. 2. XRD patterns of (a) CA, CA-5, CA-10, CA-15 and CA-20 and (b) CA-10-5,
RI PT
CA-10-10, CA-10-15 and CA-10-20 membranes.
Fig. 3. SEM images of different polymer electrolyte membranes: (a) CA-10, (b) CA10-5, (c) CA-10-10, and (d) CA-10-15. The inset represents the SEM image for pure
SC
CA.
M AN U
Fig. 4. TGA thermograms of CA-10, CA-10-5, CA-10-10, and CA-10-15 membranes. Fig. 5. Stress-strain curves of pure CA, CA-10, and CA-10-10 membranes.
Fig. 6. FT-IR spectra of CA, CA-5, CA-10, and CA-15 membranes.
cm-1.
TE D
Fig. 7. FT-IR spectra of CA-5, CA-10, and CA-15 membranes in the range 1750-1470
Fig. 8. FT-IR spectra of CA-10, CA-10-5, CA-10-10, and CA-10-15 membranes in
EP
the range 1750-1470 cm-1.
Fig. 9. The sketches showing the Mg-ion conducting mechanism in chitosan polymer
AC C
electrolyte membranes.
Fig. 10. Nyquist impedance plots of (a) CA-5, CA-10, CA-15 and CA-20 and (b) CA10-5, CA-10-10, CA-10-15 and CA-10-20 membranes, determined at room temperature.
Fig. 11. Variation of loss tangent with frequency for CA-10-5, CA-10-10, CA-10-15 and CA-10-20 membranes.
20
ACCEPTED MANUSCRIPT Fig. 12. The natural logarithm of ionic conductivity (lnσ) as a function of reciprocal temperature (1/T) for CA-10 and CA-10-10 membranes.
Fig. 13. Cyclic voltammograms for CA-10, CA-10-5, CA-10-10, CA-10-15 and CA10-20 membranes, recorded at room-temperature at a scan rate of 10 mV s-1 (electrode
RI PT
cross-section area = 0.75 cm2).
Fig.14. (a) Cyclic voltammograms of the Mg/CA-10-10/Mg cell recorded at room
temperature at a scan rate of 10 mV s-1. (b) The sketches showing anodic oxidation
SC
and cathodic reduction of Mg at the Mg-electrode/electrolyte membrane interface.
Fig.15. DC polarization plots of SS/CA-10-x/SS (x = 5, 10 and 15) cells recorded at
AC C
EP
TE D
M AN U
room temperature under a voltage of 1.0 V.
21
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
Fig. 1. Macroscopic images of a CA-10-10 polymer electrolyte membrane.
22
1484 CA-20
742 1480
1280
M AN U
Intensity (counts)
740
CA-10
640 1560
CA-5
780
TE D
1572 786
pure CA
5 10 15 20 25 30 35 40 45 50 55 60
2θ (degree)
AC C
EP
0
a
SC
CA-15
RI PT
ACCEPTED MANUSCRIPT
23
ACCEPTED MANUSCRIPT
1304 652 1304
SC
CA-10-15
652 1080
M AN U
Intensity (counts)
b
RI PT
CA-10-20
CA-10-10
540 1120
CA-10-5
TE D
560 0
5 10 15 20 25 30 35 40 45 50 55 60
EP
2θ (degree)
Fig. 2. XRD patterns of (a) CA, CA-5, CA-10, CA-15 and CA-20 and (b) CA-10-5,
AC C
CA-10-10, CA-10-15 and CA-10-20 membranes.
24
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 3. SEM images of different polymer electrolyte membranes: (a) CA-10, (b) CA-
AC C
EP
CA.
TE D
10-5, (c) CA-10-10, and (d) CA-10-15. The inset represents the SEM image for pure
25
ACCEPTED MANUSCRIPT
RI PT
CA-10 CA-10-5 CA-10-10 CA-10-15
SC
80
60
40
100
200
300
400 Temperature (oC)
TE D
20
M AN U
Weight (%)
100
500
AC C
EP
Fig. 4. TGA thermograms of CA-10, CA-10-5, CA-10-10, and CA-10-15 membranes.
26
ACCEPTED MANUSCRIPT
RI PT
70
50
SC
40 pure CA CA-10 CA-10-10
30 20 10 0
0
10
20
M AN U
Tensile stress (MPa)
60
30
40
50
60
70
80
90 100
TE D
Tensile strain (%)
AC C
EP
Fig. 5. Stress-strain curves of pure CA, CA-10 and CA-10-10 membranes.
27
RI PT
ACCEPTED MANUSCRIPT
SC
CA-15
3500
3000
2500
15 16 13 76 10 24
89 3
14 16 11 58
15 70
33 18 29 20 28 71
TE D
4000
16 46
pure CA
M AN U
CA-5
34 24
Transmittance (a.u.)
CA-10
2000
1500
1000
Wavenumber (cm-1)
AC C
EP
Fig. 6. FT-IR spectra of CA, CA-5, CA-10, and CA-15 membranes.
28
100 CA-10
CA-15
M AN U
CA-5
SC
RI PT
ACCEPTED MANUSCRIPT
1642
75
1641
1543
TE D
Transmittance (%)
1644
EP
1544
1543
AC C
50
1650 1500
1650 1500
Wavenumber
1650 1500
(cm-1)
Fig. 7. FT-IR spectra of CA-5, CA-10, and CA-15 membranes in the range 1750-1470 cm-1.
29
1547
SC
Transmittance (a.u.)
CA-10-15
CA-10-10
M AN U
1539
1538
CA-10-5
TE D
CA-10
1700
EP
RI PT
ACCEPTED MANUSCRIPT
1543
1600
Wavenumber (cm
1500 -1
)
AC C
Fig. 8. FT-IR spectra of CA-10, CA-10-5, CA-10-10, and CA-10-15 membranes in the range 1750-1470 cm-1.
30
RI PT
ACCEPTED MANUSCRIPT
SC
Chitosan
)
AC C
EP
TE D
M AN U
(simple expression:
Fig. 9. The sketches showing the Mg-ion conducting mechanism in chitosan polymer electrolyte membranes.
31
RI PT
ACCEPTED MANUSCRIPT
SC
1.0x10-6
0.0
0.0 0.0
CA-5 CA-10 CA-15 CA-20
5 10 15 20 Mg(Tf)2 concentration (wt%)
TE D
5.0x104
a
2.0x10-6
M AN U
Ionic conductivity (S cm-1)
Minus imaginary Z (Ω)
1.0x105
4.0x104
8.0x104
AC C
EP
Real Z (Ω)
32
1.2x105
RI PT SC
4x10-5 3x10-5 2x10-5 1x10-5
0.0 0.0
b
CA-10-5 CA-10-10 CA-10-15 CA-10-20
5 10 15 20 EMITf concentration (wt%)
TE D
4.0x103
0
M AN U
8.0x10
3
Ionic conductivity (S cm-1)
Minus imaginary Z (Ω)
ACCEPTED MANUSCRIPT
3.0x103
6.0x103
9.0x103
Real Z (Ω)
EP
Fig. 10. Nyquist impedance plots of (a) CA-5, CA-10, CA-15 and CA-20 and (b) CA10-5, CA-10-10, CA-10-15 and CA-10-20 membranes, determined at room
AC C
temperature.
33
SC
RI PT
ACCEPTED MANUSCRIPT
M AN U
CA-10-5 CA-10-10 CA-10-15 CA-10-20
20
tan δ
15
5
EP
0
TE D
10
2
4
6
log (f/Hz)
AC C
Fig. 11. Variation of loss tangent with frequency for CA-10-5, CA-10-10, CA-10-15 and CA-10-20 membranes.
34
ACCEPTED MANUSCRIPT
-6
RI PT
R2 = 0.986
-8 -9
SC
-10 -11 R2 = 0.991
-12 -13 -14
2.8
2.9
M AN U
ln(σ//S cm-1 )
-7
CA-10-10 CA-10
3.0
3.1
1000/T (K-1)
3.2
3.3
TE D
Fig. 12. The natural logarithm of ionic conductivity (lnσ) as a function of reciprocal
AC C
EP
temperature (1/T) for CA-10 and CA-10-10 membranes.
35
ACCEPTED MANUSCRIPT 0.04 0.00
0.2
RI PT
CA-10-15 4.15 V
0.0
CA-10-10
-0.2
SC
0.03 0.00 -0.03
CA-10-5
0.02 0.00
M AN U
Current (mA)
CA-10-20
-0.04 0.03 0.00 -0.03
CA-10
-0.02 -3
-2
-1
0
1
2
3
Voltage (V)
TE D
Fig. 13. Cyclic voltammograms for CA-10, CA-10-5, CA-10-10, CA-10-15 and CA10-20 membranes, recorded at room-temperature at a scan rate of 10 mV s-1 (electrode
AC C
EP
cross-section area = 0.75 cm2).
36
ACCEPTED MANUSCRIPT
a
0.3
Mg/CA-10-10/Mg
0.1
RI PT
Current (mA)
0.2
0.0 -0.1
-3
-2
-1
0
SC
-0.2 1
2
3
AC C
EP
TE D
M AN U
Voltage (V)
Fig.14. (a) Cyclic voltammograms of the Mg/CA-10-10/Mg cell recorded at room temperature at a scan rate of 10 mV s-1. (b) The sketches showing anodic oxidation and cathodic reduction of Mg at the Mg-electrode/electrolyte membrane interface. 37
ACCEPTED MANUSCRIPT
3.6
RI PT
CA-10-15
2.4 1.2 0.0 CA-10-10
3.0 1.5
M AN U
i (µΑ)
4.5
0.0
5.1
CA-10-5
iI
3.4
SC
6.0
1.7
iF
TE D
0.0
0
10
20
30
40
50
60
EP
Time (Min)
Fig.15. DC polarization plots of SS/CA-10- x/SS (x = 5, 10 and 15) cells recorded at
AC C
room temperature under a voltage of 1.0 V.
38
ACCEPTED MANUSCRIPT
Table 1. The degrees of crystallinity for different membranes
RI PT
Tables
Table 2. The FWHM values of carbonyl and amine bands for different membranes
AC C
EP
TE D
M AN U
SC
Table 3. The values of relaxation time for different membranes
39
ACCEPTED MANUSCRIPT Table 1. The degrees of crystallinity for different membranes Crystallinity (%)
Pure CA
8.22
CA-5
8.15
CA-10
7.25
CA-15
7.89
CA -20
7.76
CA -10-5
6.43
SC
CA -10-10
5.61 7.48
M AN U
CA -10-15 CA -10-20
RI PT
Sample
7.45
Table 2. The FWHM values of carbonyl and amine bands for different membranes FWHMcarbonyl (cm-1)
CA-5 CA-10
67.26
78.60
65.88
81.55
55.60
96.54
EP
CA-15
FWHMamine (cm-1)
TE D
Sample
AC C
Table 3. The values of relaxation time for different membranes Sample
Relaxation time (s)
CA -10-5
3.06×10-5
CA -10-10
1.25×10-6
CA -10-15
1.44×10-5
CA -10-20
3.70×10-5
40
S0142-9418(17)30601-3
DOI:
10.1016/j.polymertesting.2017.07.016
Reference:
POTE 5096
To appear in:
Polymer Testing
Received Date: 9 May 2017 Revised Date:
13 July 2017
Accepted Date: 13 July 2017
Please cite this article as: J. Wang, S. Song, S. Gao, M. Ravi, R. Liu, Q. Ma, Mg-ion conducting gel polymer electrolyte membranes containing biodegradable chitosan: Preparation, structural, electrical and electrochemical properties, Polymer Testing (2017), doi: 10.1016/j.polymertesting.2017.07.016. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Mg-ion conducting gel polymer electrolyte membranes containing biodegradable chitosan: Preparation, structural, electrical and electrochemical properties Jingwei Wanga, Shenhua Songa,*, Shang Gaoa, Muchakayala. Ravia, Renchen Liub,c,
a
RI PT
Qing Mab,c,** Shenzhen Key Laboratory of Advanced Materials, Department of Materials Science
and Engineering, Shenzhen Graduate School, Harbin Institute of Technology, Shenzhen – 518055, China
Research Institute of Tsinghua University in Shenzhen, Shenzhen 518055, China
c
Tsinghua Innovation Center in Dongguan, Dongguan 523808, China
M AN U
SC
b
ABSTRACT
A biodegradable gel polymer electrolyte system based on chitosan/magnesium trifluoromethanesulfonate/2-ethyl-3-methylimidazolium trifluoromethanesulfonate
TE D
(CA/Mg(Tf)2/EMITf) is developed. The structure, thermal performance, mechanical properties, ionic conductivity, relaxation time, electrochemical stability and ionic transport number of the membranes are analyzed by various techniques. The ion
EP
migration mainly depends on the complexation and decomplexation of Mg2+ with amine band (NH2) in chitosan. The 90CA-10Mg(Tf)2 system plasticized with 10% EMITf (relative to the amount of 90CA-10Mg(Tf)2) is identified as the optimum one
AC C
and the temperature dependence of ionic conductivity obeys the Arrhenius rule. Moreover, the relaxation time of the electrolyte is very short, being just 1.25×10-6 s, and its electrochemical stability window is quite wide, being up to 4.15 V. The anodic oxidation and cathodic reduction of Mg at the Mg-electrode/electrolyte interface is facile, and the ionic transference number of this electrolyte is 0.985, indicating that it could be a potential electrolyte candidate for Mg-ion devices.
*
Corresponding author. Tel.: +86-755-26033465. E-mail addresses: [email protected]; [email protected] (S.-H. Song). **
Corresponding author. Tel.: +86-755-26551328. E-mail address: [email protected] (Q. Ma). 1
ACCEPTED MANUSCRIPT Keywords: Chitosan; Biodegradable gel polymer electrolytes; Conduction mechanism; Ionic conductivity; Ionic transference number. 1. Introduction In recent decades, due to the rapid development of electric vehicles, it is necessary
RI PT
to fabricate high-performance batteries or capacities. The property requirements for these devices are getting higher and higher, such as high capacity, safety, shape flexibility, controllable size, and environmental friendliness [1,2]. Hence, an
SC
increasing attention has been paid to the development of polymer electrolyte, serving as both electrolyte and separator, for the potential application in batteries or capacitors.
M AN U
The polymer electrolyte offers some advantages over the traditional liquid electrolyte, such as the absence of internal short circuit and electrolyte leakage. The environmentfriendly polymer electrolyte not only meets the above demands, but also cuts the cost. In addition, the polymer itself will not cause an environmental pollution in the
TE D
treatment of used batteries or capacitors. Biodegradable polymers, such as starch [3], cellulose [4,5], polyethylene glycol [6] and chitosan [7-9] are potential candidates for ion device applications. Among these biodegradable polymers, chitosan offers some
EP
advantages, such as nontoxic, high nitrogen content, good molecular biocompatibility and reproducible.
AC C
Chitosan is a natural biodegradable polymer. It is composed of 2-amino-2-deoxy-
D-glucopyranose units which link with β-(1-4) glycosidic [10]. Owing to the existence of functional amine group in molecular structure, chitosan has been applied in many aspects, such as electronics [11-13], food industry [14,15] and pharmaceutical industry [16]. It is demonstrated that the amine (NH2) group in chitosan contains a lone pair electron and can serve as electron donors that coordinate with the cation of a doped salt [17,18]. The chitosan-based polymer electrolyte membranes for Li-ion batteries have been reported. However, for the preparation of 2
ACCEPTED MANUSCRIPT Li-ion batteries, the environment requirements are harsh due to the highly reactive performance of Li-salt [19]. On the other hand, compared with Li-ion batteries, Mgion batteries can offer a higher volumetric capacity stemming from bivalent Mg2+ [2023] and they can be fabricated without protection. For this, one of the primary issues
RI PT
is to explore new types of electrolyte for Mg-ion batteries [24]. So far, there have
been almost no reports of chitosan-based electrolytes for Mg-ion battery applications. Actually, the chitosan membranes after drying are low flexible and fragile due to the
SC
intermolecular or intramolecular forces occurring between hydroxyls or hydroxyl and amine. Moreover, this structural property has a serious effect on the ion or proton
M AN U
conductivity. To tackle these problems, many researchers have suggested several methods, mainly using the mixture of two or more polymers and the plasticizer [2527]. Among these methods, plasticization has certain advantages, such as simple operation, low cost, and high effectiveness. For chitosan-based polymer electrolyte
TE D
membranes, the mostly plasticizer is ethylene carbonate (EC). Arof et al. [18] reported that the optimal ionic conductivity of Li-ion conducting EC-plasticized chitosan acetate/LiCF3SO3 salt electrolytes was 4.0×10-5 S cm-1. The significant role
EP
of EC is to improve the ionic conductivity of the film, but its thermal stability and mechanical performances are deteriorative. In order to prepare the chitosan polymer
AC C
electrolyte membranes with excellent properties, ionic liquid used as a substitute for conventional plasticizer attracts some interest. The ionic liquid is composed of organic cations and inorganic anions and it offers some advantages, such as nonvolatile, nonflammable, superior thermal stability and miscibility with diverse compounds [28-31]. Currently, the host polymers of electrolyte membranes doped with ionic liquid are mainly semi-crystalline poly(vinylidenefluoride-cohexafluropropylene) (PVDF-HFP) and poly(ethylene oxide) (PEO) [32,33]. 3
ACCEPTED MANUSCRIPT Apparently, these polymers are synthetic and not biodegradable. Furthermore, it is demonstrated [34,35] that the magnesium reversibly deposited reaction does not take place for the simple magnesium salts containing ClO4 or BF4 anions owing to the formation of dense zones on the electrode surface due to the reduction product of the
RI PT
anions with pure Mg. This implies that these kinds of salt could not be used to prepare Mg-ion conducting polymer electrolytes.
In the present study, we prepared biodegradable and low-cost chitosan-based Mg-
SC
ion conducting gel polymer electrolyte membranes with the dopants of magnesium trifluoromethanesulfonate (Mg(Tf)2) and 1-ethyl- 3 methylimidazolium
M AN U
trifluoromethanesulfonate (EMITf) for possible applications in Mg-ion devices. The study mainly focused on the effect of various amounts of Mg(Tf)2 or EMITf on the structural and electrical performances of chitosan-Mg(Tf)2-EMITf gel polymer electrolyte membranes. The crystallinity, thermal stability, mechanical properties, ion
TE D
interaction and ionic conductivity of the membranes were investigated using X-ray diffraction (XRD), thermogravimetric analysis (TGA), mechanical measurements, Fourier transform infrared spectroscopy (FTIR), and impedance spectroscopy.
EP
2. Experimental procedures
The preparation method of polymer electrolyte membranes was solution casting.
AC C
Firstly, the host polymer chitosan powder (CA) (1 g, medium viscosity) was dissolved in 100 ml 1% acetic acid solution, and then the Mg(Tf)2 salt was added to the solution in different concentrations with respect to the amount of chitosan (5, 10, 15 and 20 wt.%, denoted as CA-5, CA-10, CA-15 and CA-20, respectively). The mixed solution was magnetically stirred until a completely homogeneous solution was present and then treated in vacuum oven to eliminate air bubbles, followed by casting into clean petri dishes. The membranes were formed after drying at room temperature. Finally, different quantities of EMITf ionic liquid (5, 10, 15 and 20 wt.%) were added to CA4
ACCEPTED MANUSCRIPT Mg(Tf)2 systems with the same method as above to prepare ionic liquid-plasticized gel polymer electrolyte membranes. For example, in order to prepare the CA-10 (90CA-10Mg(Tf)2) based gel polymer electrolyte membranes, 5, 10, 15 and 20 wt.% of EMITf ionic liquid (relative to the amount of CA-10 system) would be added,
RI PT
which were denoted as CA-10-5, CA-10-10, CA-10-15 and CA-10-20, respectively.
The photomacrographs of a CA-10-10 gel polymer electrolyte membrane are shown in Fig. 1, indicating that they are free-standing and flexible.
SC
X-ray diffractometer (XRD) (D/max 2500PC, Rigaku, Japan) with Cu-Kα
radiation (λ = 1.5406 Å) was applied to analyze the structural characteristics of the
M AN U
electrolyte membranes. The surface morphologies of the membranes were examined using scanning electron microscopy (Hitachi S-4700 SEM, Japan) and their thermal stability was evaluated by a thermal analyzer (STA 449F3 Jupiter, Germany) under a protective N2. The mechanical properties of the membranes were measured by means
TE D
of a universal testing machine (CMT7504, China). In the testing, the specimens (thickness×width×gauge length=0.1mm×10mm×80mm) were tensile-deformed at ambient temperature under a crosshead speed of 10 mm/min. The mechanism of ion
EP
conduction in the polymer electrolyte was analyzed by Fourier transform infrared spectroscopy (FTIR) (Nicolet 380 spectrometer).
AC C
For a solid electrolyte material, the ionic conductivity is the most important
parameter. This property was examined via an impedance analyzer (PSM 1735, Newton, UK). The circular membranes were sandwiched between two symmetrical stainless steel electrodes (SS) with the contact area of 0.75 cm2. During impedance measurements, the frequency was in the range of 1 MHz to 10 Hz with a signal level of 10 mV. The resistance for calculating ionic conductivity was determined by fitting the Nyquist impedance plot with Zsimp Win software. The electrochemical 5
ACCEPTED MANUSCRIPT performances of polymer electrolyte membranes including electrochemical stability window and ionic transference number were evaluated using cyclic voltammetry and DC polarization with the aid of an electrochemical work station (Model: CHI760D, CH Instruments, China). The electrochemical stability window was recorded in the
RI PT
potential range of -3 V to 3 V. In the DC polarization study, a DC voltage of 1.0 V
was applied on the SS/electrolyte/SS cell and the obtained current was monitored as a function of time.
SC
3. Results and discussion
The XRD patterns of pure chitosan and different CA-Mg(Tf)2 membranes are
M AN U
shown in Fig. 2a. Clearly, the chitosan is a semicrystalline polymer and exhibits five primary diffraction peaks at the 2θ angles of 9°, 12°, 21.3°, 23.8°, 26.7° and a broad peak around 20° which is related to the amorphous phase. The intensities of XRD peaks decrease with increasing Mg(Tf)2 content until 10 wt.% and then increase with
TE D
further increasing Mg(Tf)2 content. This means that the degree of crystallinity has a minimum value for the CA-10 system. Based on the JADE6.0 software which is widely used in the determination of crystallinity from XRD patterns [36,37], The
EP
degrees of crystallinity have been determined for the various CA- Mg(Tf)2 systems, which are listed Table 1. As can be seen, the crystallinity of pure CA is 8.22% and the
AC C
value is decreased to 7.25% by doping 10 wt.% Mg(Tf)2, i.e., the amorphicity is increased to 92.75% by the doping. This should be attributed to the complexation between chitosan and Mg(Tf)2 salt, resulting in a decrease in intermolecular and/or intramolecular interaction. Furthermore, no extra peaks emerge for the Mg(Tf)2-doped CA films. This implies that the Mg(Tf)2 salt has been completely complexed in the CA matrix. Since the CA-10 system exhibits the highest amorphicity, it is the best complex for ionic conduction. Therefore, this complex is suitable for the preparation 6
ACCEPTED MANUSCRIPT of gel polymer electrolyte by incorporating EMITf ionic liquid. Fig. 2b represents the XRD profiles of 90CA-10Mg(Tf)2 + x EMITf (x = 5, 10, 15 and 20 wt%, relative to the amount of 90CA-10Mg(Tf)2) membranes. Obviously, the XRD intensities weaken with increasing EMITf concentration up to 10 wt.% and then strengthen with further
RI PT
doping EMITf. Also, there are no other additional peaks present as compared to those without EMITf. This implies that the ionic liquid can completely dissolve in the CA10 system and simultaneously destroy the ordered structure of chitosan chains,
SC
leading to a decrease in the degree of crystallinity. The degrees of crystallinity have also been determined for these gel polymer electrolyte systems, which are listed in
M AN U
Table 1. As seen, the crystallinity is reduced to 5.61% for the CA-10-10 system. However, when the concentration of EMITf is more than 10 wt.%, the redundant anion Tf-1 ions could embrace the cation Mg2+ ions, resulting in an enhancement of intermolecular and/or intramolecular force and in turn an increase in the degree of
TE D
crystallinity.
The surface morphologies of CA-10, CA-10-5, CA-10-10 and CA-10-15 membranes are shown in Fig. 3. All the membranes are smooth and homogeneous
EP
except for the CA-10-15 sample, indicating that the salt and ionic liquid uniformly disperse in the polymer matrix. However, the salt begins to recrystallize when more
AC C
than 10 wt.% EMITf is added as shown in Fig. 3d. Thermal stability traces of CA-10, CA-10-5, CA-10-10 and CA-10-15
membranes are shown in Fig. 4. The TG curves for all the membranes clearly present two steps of weight loss. The first weight loss in the range of 50-150 oC could be due to the removal of absorbed water and the second one in the range 200-300 oC could arise from the degradation of chitosan. Obviously, the thermal stability of the EMITfplasticized gel membranes is apparently increased due to the effect of ionic liquid. . 7
ACCEPTED MANUSCRIPT Typical stress-strain curves of pure CA, CA-10 and CA-10-10 are represented in Fig. 5. All the curves present similar characteristics. The breaking strain increases considerably with the addition of salt and ionic liquid, being about 85% for the CA10-10 system. The other mechanical parameters for this system are that
RI PT
Young's modulus ≈ 640 MPa and yield strength ≈ 52 MPa. Obviously, the mechanical properties of the membrane are well enough for practical application as it is flexible with sufficient strength and stiffness.
SC
In order to improve the electrical properties of a polymer electrolyte membrane, it is necessary to understand the conduction mechanism in the membrane. Fig. 6 shows
M AN U
the FTIR spectra of virgin CA and CA - x Mg(Tf)2 (x = 5, 10 and 15 wt.%) in the range 700-4000 cm-1. The spectrum of the virgin chitosan membrane is similar to previous studies [8,14,17,26,38]. The peaks around 3600 to 3200 cm-1 are corresponding to the overlapping of –NH and –OH stretching vibrations; the double
1
TE D
peaks due to symmetric and asymmetric –CH2 stretching appear at 2920 and 2871 cm; carbonyl (C = O – NHR), amine band (NH2) and ammonium (NH3+) are observed at
1646,1570 and 1516 cm-1, respectively; the peaks at 1416, 1376 and 1024 cm-1 are
EP
assigned to the C – N stretching combined with N –H plane deformation, CH3 symmetric angular deformation, and stretching vibration of C–O–C, respectively. The
AC C
–NH3+ band disappears in the Mg(Tf)2-complexed CA systems, which is probably attributed to the complexation between Mg ions from the salt and lone pair electrons from amine, resulting in the shift of amine band to lower wavenumbers and then overlapping with –NH3+ to form NH2 band. In order to make the above points, the spectra of carbonyl and amine bands for the complex systems are depicted in Fig. 7. Compared with the pure chitosan membrane, the carbonyl and amine bands of the CA-10 membrane downshift to 1641 (about 5 cm-1) and 1543 cm-1 (about 27 cm-1), 8
ACCEPTED MANUSCRIPT respectively. With the increase of salt concentration, the full width at half maximum (FWHM) of amine band increases, but that for carbonyl slightly reduces as shown in Table 2 (the FWHM is calculated using the OMNIC software). It is indicated that the complexation takes place between Mg2+ and chitosan, and prefers to occur at amine
RI PT
band, which is in accord with the reports by Arof [17,26]. With the introduction of EMITf ionic liquid, the position of the amine band at 1543 cm-1 for the CA-10
membrane exhibits a little change as shown in Fig. 8. It is seen that the amine bands
SC
of the CA-10-5 and CA-10-10 membranes downshift to 1538 and 1539 cm-1,
respectively, but the amine band of the CA-10-15 membrane shifts to a higher
M AN U
wavenumber (~1547 cm-1). The volume of EMI+ group containing C, H and N elements is significantly larger than that of Mg2+, and thus it is almost impossible for the complexation of EMI+ and amine to occur. This implies that the appropriate EMITf can disrupt the intermolecular and/or intramolecular force and then improve
TE D
the complexation of Mg2+ with amine band. However, the Tf ¯ from excess EMITf could combine with Mg2+, leading to a decrease in the complexation of salt with amine band. The sketches showing the possible Mg-ion conducting mechanism in the
EP
chitosan polymer electrolyte membranes are represented in Fig. 9. Since the ionic conduction in these polymer electrolyte membranes may be achieved mainly through
AC C
the complexation and decomplexation of Mg-ions with amine bands, their transport mechanism is only considered. The Nyquist impedance plots of the unplasticized and plasticized membranes are
represented in Fig. 10. The bulk resistance of the membrane can be obtained through the intercept of the semicircle on the real axis in the plot. Obviously, for the unplasticized membranes, the CA-10 sample exhibits the smallest resistance, and the
9
ACCEPTED MANUSCRIPT CA-10-10 sample does for the plasticized membranes. This means that this plasticized sample possesses the highest conductivity. The ionic conductivity (σ) is calculated by = /
(1)
where t is the membrane thickness, Rb is the bulk resistance, and A is the membrane-
RI PT
electrode contact area. In the present study, the membrane thickness is around 100 µm (determined by screw micrometer) and the membrane-electrode contact area is 0.75
cm2. The ionic conductivity of the unplasticized membranes as a function of the salt-
SC
concentration is shown in the inset of Fig. 10a. Obviously, the room-temperature ionic conductivity gradually increases until 1.98×10-6 S cm-1 at 10 wt.% Mg(Tf)2 salt,
M AN U
which is ascribed to the increase of mobile Mg-ions and their mobility.. However, it decreases when the amount of salt is more than 10 wt.% due to ion aggregation leading to a decline in both the number of mobile ions and the mobility of the ions. The ionic conductivity of electrolyte membranes could be improved by adding ionic
TE D
liquid that serves as plasticizer. The ionic liquid concentration dependence of ionic conductivity is shown in the inset of Fig. 10b. As can be seen, with the addition of EMITf up to 10 wt.%, the ionic conductivity obtains a peak value of 3.57×10-5 S cm-1
EP
which rises more than tenfold compared to the one without EMITf. The effect of ionic liquid is similar to other plasticizers, which increase the flexibility of polymer chain
AC C
segments. However, as shown in Fig. 10b, the line in the low-frequency region is not parallel to the imaginary axis for the CA-10-10 system, implying that the ionic conductivity of the gel polymer electrolyte is not good. Shukur et al. [39] developed biodegradable Mg-ion conducting gel polymer electrolytes based on the potato starch doped with magnesium acetate and different plasticizers including 1-butyl-3methylimidazolium chloride and glycerol, the obtained maximum room-temperature ionic conductivities of the electrolytes were about 1.12×10-5 S cm-1 and 2.6×10-6 S 10
ACCEPTED MANUSCRIPT cm-1, respectively. Munichandraiah et al. [40] reported that Mg-ion conducting gel polymer electrolytes were prepared on the basis of PEO plasticized with PC (propylene carbonate), EC and a mixture of PC and EC and the room-temperature ionic conductivity of the optimum composition was around 1×10-5 S cm-1. Hashmi et
RI PT
al. [33] prepared the Mg-ion conducting gel polymer electrolyte from PEO plasticized with 50 wt.% EMITf and its maximum room-temperature ionic conductivity was about 5.6×10−4 S cm−1. Obviously, the present results compare well with the
SC
previously reported data. It is worth noting that the present gel polymer electrolyte is biodegradable so that the disposing of used devices fabricated with this electrolyte
environmental protection.
M AN U
causes less environmental pollution. This should be one of its advantages in terms of
If the flexibility of polymer chain segments is improved, the relaxation time will be short because it is related to the mobility of ions in the polymer host [41,42]. The
TE D
variation of loss tangent with frequency for the EMITf-plasticized CA-10 (90CA-10 Mg(Tf)2) membranes is shown in Fig. 11. As can be seen, the relaxation frequency corresponding to the peak shifts to higher frequencies, i.e., higher ionic conductivity
EP
values, with increasing ionic liquid concentration until 10 wt.%. However, it shifts back to lower frequencies, i.e., lower ionic conductivity values, with further
AC C
increasing ionic liquid concentration. The relaxation time (τ) can be obtained by 2
=1
(2)
where f is the relaxation frequency. The determined relaxation times are listed in Table 3. Obviously, it is the shortest (1.25×10-6 s) for the CA -10-10 system. With the increase of ionic liquid concentration from 5 wt.% to 10 wt.%, the relaxation time decreases from 3.06×10-5 to 1.25×10-6 s . However, it increases when the addition of EMITf is more than 10 11
ACCEPTED MANUSCRIPT wt.%. This means that the ionic liquid can make the amorphous and free volume of the polymer increase, then raise the mobility of ions (µ i) and increase the ionic conductivity (σ) according to the following equation =∑
(3)
RI PT
where ni is the density of mobile ions, qi is the charge of mobile carriers, and µ i is the carrier mobility.
Nevertheless, the superfluous EMITf may reduce the flexibility of polymer
SC
chains and the quantity of transferable ions (ni), thereby increasing the relaxation time
and ionic conductivity.
M AN U
and decreasing the ionic conductivity. This is in accordance with the results of XRD
In order to examine the relationship between the ionic conductivity and temperature of the electrolyte membranes, the values of ionic conductivity at different temperatures were determined, which are shown in Fig. 12 for the CA-10 and CA-10-
TE D
10 membranes. It is seen that the ionic conductivity of the membranes rises with increasing temperature, and there is a linear relationship between lnσ and reciprocal temperature (1/T), obeying the Arrhenius rule exp
EP
=
(4)
AC C
where σ0 is the pre-exponential constant, Ea is the activation energy for conduction, k is the Boltzmann constant, and T is the absolute temperature. According to the equation, the activation energy (Ea) can be determined as 0.86 eV with an R2 value of 0.991 for the CA-10 membrane. The value is reduced to 0.72 eV by incoporating 10 wt.% EMITf plasticizer. Munichandraiah et al. [40] developed Mg-ion conducting polymer electrolytes based on PEO doped with Mg(Tf)2, the obtained Ea was 0.93 eV and it decreased to 0.152 eV with the addition of 60 wt.% EC+PC plasticizer. They also reported Mg-ion conducting polymer electrolytes based on PMMA doped with 12
ACCEPTED MANUSCRIPT Mg(Tf)2 and EC+PC plasticizer in a mass ratio of 1:0.5:2. The obtained Ea was 0.038 eV [43]. In our previous work [44], the 15 wt.% EMITf-plasticized 85PVA15Mg(Tf)2 membrane exhibited an activation energy of 0.25 eV. Hence, the activation energy for polymer electrolytes varies in quite a large range, depending mainly on the
RI PT
polymer and plasticizer. The high activation energy for the CA-10-10 system (0.72 eV) indicates that the plasticized chitosan polymer electrolyte still possesses potential to increase its ionic conductivity by decreasing the activation energy.
SC
It is worth noting that the electrochemical stability window of electrolyte
membranes is vitally important for their practical applications. Fig. 13 represents the
M AN U
cyclic voltammograms of EMITf-plasticized 90CA-10Mg(Tf)2 membranes over the voltage range of -3.0 V to 3.0 V. As can be seen, the electrochemical stability window of the membranes shows little change when the addition of ionic liquid is less than 15 wt.%, while it becomes narrower beyond this amount. For the CA-10-10 membrane,
TE D
the electrochemical stability window is as wide as 4.15 V that is well enough for practical applications [32].
The cyclic voltammogram for the Mg/CA-10-10/Mg cell is shown in Fig. 14a.
EP
Obviously the anodic and cathodic peaks emerge, but they do not emerge for the SS/CA-10-10/SS cell in the same potential range (Fig. 13). This implies that the
AC C
anodic oxidation and cathodic reduction of Mg at the Mg-electrode/electrolyte membrane interface is a reversible process, as shown in Fig.14b where for the same interface between membrane and Mg electrode (case Ⅰ or case Ⅱ) with a cyclic scanning potential, the reversible redox reaction is facile. Accordingly, the CA-10-10 gel polymer electrolyte membrane could be a potential electrolyte candidate for Mgion devices. A polymer electrolyte membrane should be an ionic conductor for energy 13
ACCEPTED MANUSCRIPT storage device applications. The ionic transference number (tion) of a polymer electrolyte membrane is able to indicate this behavior, which can be evaluated by a DC polarization technique. When a potential passes through the SS/electrolyte/SS cell, the mobile ions will accumulate on the electrolyte membrane surface next to electrode
RI PT
and thus the mobile ions are depleted near the opposite electrode surface, leading to a drop of polarization current. Nevertheless, the electrode is still active to electrons.
Hence, the initial current (iI) is contributed by both ions and electrons and the final
SC
current (iF) just by electrons. The polarization curves for several SS/CA-Mg(Tf)2EMITf/SS cells with the electrolyte membranes plasticized by 5, 10, and 15 wt.%
M AN U
EMITf (CA-10-5, CA-10-10, and CA-10-15), respectively, are represented in Fig. 15. The initial and final current values (iI and iF) for these cells can be obtained from these curves. Then the values of tion are calculated by
iI − iF iI
(5)
TE D
tion =
The ionic transference numbers for the CA-10-5, CA-10-10, and CA-10-15 gel electrolyte membranes are obtained as 0.985, 0.985, and 0.983, respectively,
EP
indicating the predominant ionic conduction in the polymer electrolyte membranes. 4. Conclusions
AC C
Biodegradable plasticized gel polymer electrolyte membranes consisting of
chitosan, Mg(Tf)2 salt and EMITf ionic liquid are prepared by solution casting. The possible Mg-ion conducting mechanism in the membranes is elucidated and it is mainly through the complexation and decomplexation of Mg2+ with amine band (NH2) from chitosan. The 10 wt.% EMITf-plasticized 90CA-10 Mg(Tf)2 gel polymer electrolyte exhibits the highest room-temperature ionic conductivity, being up to 3.57×10-5 S cm-1. The conductivity increases with increasing temperature, following 14
ACCEPTED MANUSCRIPT the Arrhenius rule with an activation energy of 0.72 eV. The relaxation time of the 10 wt.% ionic liquid-incorporated electrolyte membrane is as low as 1.25×10-6 s, indicating that the mobility of ions is relatively high. The ionic transference number for the electrolyte membrane is about 0.985, showing that the ionic conduction is
RI PT
dominant in the polymer electrolyte membranes. Moreover, the electrochemical
stability window of this electrolyte membrane is as wide as 4.15 V which is sufficient
SC
for practical applications.
Acknowledgments: This work was supported by the International S&T Cooperation
M AN U
Program of China (No. 2014DFA53020), Guangdong Innovative and Entrepreneurial Research Team Program (No. 2013C099)
References [1] Armand M., Tarascon J.M.: Building better batteries. Nature, 451, 652-657 (2008). http://www.nature.com/nature/journal/v451/n7179/full/451652a.html
TE D
[2]Whittingham M.S.: Materials Challenges Facing Electrical Energy Storage. MRS Bulletin, 33, 411-419 (2008).
AC C
EP
https://doi.org/10.1557/mrs2008.82 [3]Kumar M., Tiwari T., Srivastava N.: Electrical transport behaviour of bio-polymer electrolyte system: Potato starch +ammonium iodide. Carbohydrate Polymers, 88, 54-60 (2012). http://dx.doi.org/10.1016/j.carbpol.2011.11.059 [4]Ramesh S., Shanti R., Morris E.: Plasticizing effect of 1-allyl-3methylimidazolium chloride in cellulose acetate based polymer electrolytes. Carbohydrate Polymers, 87, 2624-2629 (2012).
http://dx.doi.org/10.1016/j.carbpol.2011.11.037 [5]Smolarkiewicz I., Rachocki A., Pogorzelec G.K., Pankiewicz R., Ławniczak P., Łapiński A., Jarek M.: Proton-conducting Microcrystalline Cellulose Doped with Imidazole. Thermal and Electrical Properties. Electrochimica Acta,155, 38-44 (2015). http://dx.doi.org/10.1016/j.electacta.2014.11.205 [6]Asghar A., Abdul S.Y., Singh L.B., Hashaikeh R.: PEG based quasi-solid polymer electrolyte: Mechanically supported by networked cellulose. Journal of Membrane Science, 421-422, 85-90 (2012). 15
ACCEPTED MANUSCRIPT http://dx.doi.org/10.1016/j.memsci.2012.06.037 [7]Yahya M.Z.A., Arof A.K.: Conductivity and X-ray photoelectron studies on lithium acetate doped chitosan films. Carbohydrate Polymers, 55, 95-100 (2004). http://dx.doi.org/10.1016/j.carbpol.2003.08.018 [8] Fadzallah I.A., Majid S.R., Careem M.A., Arof A.K.: A study on ionic interactions in chitosan–oxalic acid polymer electrolyte membranes. Journal of
RI PT
Membrane Science, 463, 65-72 (2014). http://dx.doi.org/10.1016/j.memsci.2014.03.044 [9] Aziz S.B., Abidin Z.H.Z., Arof A.K.: Influence of silver ion reduction on electrical modulus parameters of solid polymer electrolyte based on chitosan-silver
M AN U
SC
triflate electrolyte membrane. Express Polymer Letters,4, 300-310 (2010). http://doi.org/10.3144/expresspolymlett.2010.38 [10] Kadir M.F.Z., Majid S.R., Arof A.K.: Plasticized chitosan–PVA blend polymer electrolyte based proton battery. Electrochimica Acta, 55, 1475-1482 (2010). http://dx.doi.org/10.1016/j.electacta.2009.05.011 [11]Smitha B., Devi D.A., Sridhar S.: Proton-conducting composite membranes of
TE D
chitosan and sulfonated polysulfone for fuel cell application. International Journal of Hydrogen Energy, 33, 4138-4146(2008). http://dx.doi.org/10.1016/j.ijhydene.2008.05.055 [12]Chupp J., Shellikeri A., Palui G., Chatterjee J.: Chitosan-based gel film electrolytes containing ionic liquid and lithium salt for energy storage applications.
AC C
EP
Journal of Applied Polymer Science, 132, (2015). http://onlinelibrary.wiley.com/doi/10.1002/app.42143/epdf [13]Sudhakar Y.N., Selvakumar M.: Lithium perchlorate doped plasticized chitosan and starch blend as biodegradable polymer electrolyte for supercapacitors. Electrochimica Acta, 78, 398-405 (2012). http://dx.doi.org/10.1016/j.electacta.2012.06.032 [14]Chen H., Hu X., Chen E., Wu S., McClements D.J., Liu S., Li B.: Preparation, characterization, and properties of chitosan films with cinnamaldehyde nanoemulsions. Food Hydrocolloids, 61, 662-671 (2016). http://dx.doi.org/10.1016/j.foodhyd.2016.06.034 [15]Hosseini S.F., Rezaei M., Zandi M., Farahmandghavi F.: Development of bioactive fish gelatin/chitosan nanoparticles composite films with antimicrobial properties. Food Chemistry, 194, 1266-1274 (2016). http://dx.doi.org/10.1016/j.foodchem.2015.09.004 [16]Cardoso A.P., Gonçalves R.M., Antunes J.C., Pinto M.L., Pinto A.T., Castro F, Monteiro C.: An interferon-γ-delivery system based on chitosan/poly(γ-glutamic acid) polyelectrolyte complexes modulates macrophage-derived stimulation of cancer cell invasion in vitro. Acta Biomaterialia, 23, 157-171(2015). 16
ACCEPTED MANUSCRIPT
RI PT
http://dx.doi.org/10.1016/j.actbio.2015.05.022 [17]Yahya M.Z.A., Arof A.K.: Effect of oleic acid plasticizer on chitosan–lithium acetate solid polymer electrolytes. European Polymer Journal, 39, 897-902 (2003). http://dx.doi.org/10.1016/S0014-3057(02)00355-5 [18]Osman Z., Ibrahim Z.A., Arof A.K.: Conductivity enhancement due to ion dissociation in plasticized chitosan based polymer electrolytes. Carbohydrate Polymers, 44, 167-173 (2001) http://dx.doi.org/10.1016/S0144-8617(00)00236-8
M AN U
SC
[19]Wang Y., Chen R., Chen T., Lv H., Zhu G., Ma L., Wang C.: Emerging nonlithium ion batteries. Energy Storage Materials, 4, 103-129 (2016). http://dx.doi.org/10.1016/j.ensm.2016.04.001 [20]Muldoon J., Bucur C.B., Oliver A.G., Sugimoto T., Matsui M., Kim H.S., Allred G.D.: Electrolyte roadblocks to a magnesium rechargeable battery. Energy & Environmental Science, 5, 5941-5950 (2012). http://pubs.rsc.org/en/content/articlepdf/2012/ee/c2ee03029b [21]Chen Z., McDonald S., Fitzgerald P.A., Warr G.G., Atkin R.: Structural effect of glyme-Li(+) salt solvate ionic liquids on the conformation of poly(ethylene oxide). Physical chemistry chemical physics: PCCP, 18, 14894-14903 (2016).
TE D
http://pubs.rsc.org/en/Content/ArticleLanding/2016/CP/C6CP00919K [22]Huie M.M., Cama C.A., Smith P.F., Yin J., Marschilok A.C., Takeuchi K.J., Takeuchi E.S.: Ionic liquid hybrids: Progress toward non-corrosive electrolytes with high-voltage oxidation stability for magnesium-ion based batteries.
AC C
EP
Electrochimica Acta, 219, 267-276 (2016). http://dx.doi.org/10.1016/j.electacta.2016.09.107 [23]Doe R.E., Han R., Hwang J., Gmitter A.J., Shterenberg I., Yoo H.D., Pour N.: Novel, electrolyte solutions comprising fully inorganic salts with high anodic stability for rechargeable magnesium batteries. Chem Commun, 50, 243-245 (2014). http://pubs.rsc.org/en/Content/ArticleLanding/2014/CC/C3CC47896C [24]Saha P., Datta M.K., Velikokhatnyi O.I., Manivannan A., Alman D., Kumta P.N.: Rechargeable magnesium battery: Current status and key challenges for the future. Progress in Materials Science, 66, 1-86 (2014). http://dx.doi.org/10.1016/j.pmatsci.2014.04.001 [25]Sarasam A.R., Krishnaswamy R.K., Madihally S.V.: Blending Chitosan with Polycaprolactone: Effects on Physicochemical and Antibacterial Properties. Biomacromolecules, 7, 1131-1138 (2006). http://pubs.acs.org/doi/abs/10.1021/bm050935d
[26]Osman Z., Arof A.K.: FTIR studies of chitosan acetate based polymer electrolytes. Electrochimica Acta, 48, 993-999 (2003). http://dx.doi.org/10.1016/S0013-4686(02)00812-5 [27]Ng L.S., Mohamad A.A.: Protonic battery based on a plasticized chitosan17
ACCEPTED MANUSCRIPT
SC
RI PT
NH4NO3 solid polymer electrolyte. Journal of Power Sources, 163, 382-385 (2006). http://dx.doi.org/10.1016/j.jpowsour.2006.09.042 [28] Ye Y-S., Rivk J., Joe B.: Ionic liquid polymer electrolytes. Journal of Materials Chemistry A, 1, 2719-2743 (2013). http://pubs.rsc.org/en/content/articlepdf/2013/ta/c2ta00126h [29]Yuan J., Mecerreyes D., Antonietti M.: Poly(ionic liquid)s: An update. Progress in Polymer Science, 38, 1009-1036 (2013). http://dx.doi.org/10.1016/j.progpolymsci.2013.04.002 [30]Ramesh S., Liew C., Ramesh K.: Evaluation and investigation on the effect of ionic liquid onto PMMA-PVC gel polymer blend electrolytes. Journal of NonCrystalline Solids, 357, 2132-2138 (2011). http://dx.doi.org/10.1016/j.jnoncrysol.2011.03.004 [31]Leones R., Sentanin F., Rodrigues L.C., Marrucho I.M., Esperanca J.M.S.S., Pawlicka A., Silva M.M.: Investigation of polymer electrolyte based on agar and
M AN U
ionic liquids. Express Polymer Letters, 6, 1007-1016 (2012). http://doi.org/doi: 10.3144/expresspolymlett.2012.106 [32]Pandey G.P., Hashmi S.A.: Experimental investigations of an ionic-liquid-based, magnesium ion conducting, polymer gel electrolyte. Journal of Power Sources, 187, 627-634 (2009). http://dx.doi.org/10.1016/j.jpowsour.2008.10.112 [33]Kumar Y., Hashmi S.A., Pandey G.P.: Ionic liquid mediated magnesium ion
TE D
conduction in poly(ethylene oxide) based polymer electrolyte. Electrochimica Acta, 56, 3864-3873 (2011). http://dx.doi.org/10.1016/j.electacta.2011.02.035 [34] NovaÂk P., Imhof R., Haas O.: Magnesium insertion electrodes for rechargeable nonaqueous batteries - a competitive alternative to lithium? Electrochimica Acta,
AC C
EP
45, 351-367 (1999). http://dx.doi.org/10.1016/S0013-4686(99)00216-9 [35]Lu Z., Schechter A., Moshkovich M., Aurbach D.: On the electrochemical behavior of magnesium electrodes in polar aprotic electrolyte solutions. Journal of Electroanalytical Chemistry, 466, 203-217 (1999). http://dx.doi.org/10.1016/S0022-0728(99)00146-1 [36] Qin H., Tao Y., Deng B: Microstructural and mechanical properties of Si-ion implanted TiN coatings. Surface & Coatings Technology, 228 , S292–S295 (2013). https://doi.org/10.1016/j.surfcoat.2012.05.119 [37] Gebresellasie K., Lewis J. C., Shirokoff J: X‑ ray Spectral Line Shape Analysis of Asphalt Binders. Energy Fuels, 27, 2018−2024 (2013). https://doi.org/10.1021/ef301865p [38]Mendes J.F., Paschoalin R.T., Carmona V.B., Sena Neto A.R., Marques A.C.P., Marconcini J.M., Mattoso L.H.C., Medeiros E.S., Oliveira J.E.: Biodegradable 18
ACCEPTED MANUSCRIPT polymer blends based on corn starch and thermoplastic chitosan processed by extrusion. Carbohydrate Polymers, 137, 452-458 (2016). http://dx.doi.org/10.1016/j.carbpol.2015.10.093 [39]Shukur M.F., Ithnin R., Kadir M.F.Z.: Ionic conductivity and dielectric properties of potato starch-magnesium acetate biopolymer electrolytes: the effect of
RI PT
glycerol and 1-butyl-3-methylimidazolium chloride. Ionics, 22, 1113-1123 (2016). http://link.springer.com/article/10.1007%2Fs11581-015-1627-4 [40]Girish Kumar G., Munichandraiah N.: Effect of plasticizers on magnesiumpoly(ethyleneoxide) polymer electrolyte. Journal of Electroanalytical Chemistry,
SC
495, 42-50 (2000). http://dx.doi.org/10.1016/S0022-0728(00)00404-6 [41]Islam A., Yasin T., Akhtar M.J., Imran Z., Sabir A., Sultan M., Khan S.M., Jamil
M AN U
T.: Impedance spectroscopy of chitosan/poly(vinyl alcohol) films. Journal of Solid State Electrochemistry, 20, 571-578 (2016). http://link.springer.com/article/10.1007%2Fs10008-015-3082-6 [42]Kulshrestha N., Chatterjee B., Gupta P.N.: Characterization and electrical properties of polyvinyl alcohol based polymer electrolyte films doped with
AC C
EP
TE D
ammonium thiocyanate. Materials Science and Engineering: B, 184, 49-57 (2014). http://dx.doi.org/10.1016/j.mseb.2014.01.012 [43]Girish Kumar G., Munichandraiah N.: Poly(methylmethacrylate)—magnesium triflate gel polymer electrolyte for solid state magnesium battery application. Electrochimica Acta, 47, 1013-1022 (2002). https://doi.org/10.1016/S0013-4686(01)00832-5 [44] Wang J-W., Song S-H., Muchakayala R., Hu X-C., Liu R-C.: Structural, electrical, and electrochemical properties of PVA-based biodegradable gel polymer electrolyte membranes for Mg-ion battery applications. Ionics, 23, 1759-1769 (2017). http://link.springer.com/article/10.1007/s11581-017-1988-y
19
ACCEPTED MANUSCRIPT FIGURES Fig. 1. Macroscopic images of a CA-10-10 polymer electrolyte membrane.
Fig. 2. XRD patterns of (a) CA, CA-5, CA-10, CA-15 and CA-20 and (b) CA-10-5,
RI PT
CA-10-10, CA-10-15 and CA-10-20 membranes.
Fig. 3. SEM images of different polymer electrolyte membranes: (a) CA-10, (b) CA10-5, (c) CA-10-10, and (d) CA-10-15. The inset represents the SEM image for pure
SC
CA.
M AN U
Fig. 4. TGA thermograms of CA-10, CA-10-5, CA-10-10, and CA-10-15 membranes. Fig. 5. Stress-strain curves of pure CA, CA-10, and CA-10-10 membranes.
Fig. 6. FT-IR spectra of CA, CA-5, CA-10, and CA-15 membranes.
cm-1.
TE D
Fig. 7. FT-IR spectra of CA-5, CA-10, and CA-15 membranes in the range 1750-1470
Fig. 8. FT-IR spectra of CA-10, CA-10-5, CA-10-10, and CA-10-15 membranes in
EP
the range 1750-1470 cm-1.
Fig. 9. The sketches showing the Mg-ion conducting mechanism in chitosan polymer
AC C
electrolyte membranes.
Fig. 10. Nyquist impedance plots of (a) CA-5, CA-10, CA-15 and CA-20 and (b) CA10-5, CA-10-10, CA-10-15 and CA-10-20 membranes, determined at room temperature.
Fig. 11. Variation of loss tangent with frequency for CA-10-5, CA-10-10, CA-10-15 and CA-10-20 membranes.
20
ACCEPTED MANUSCRIPT Fig. 12. The natural logarithm of ionic conductivity (lnσ) as a function of reciprocal temperature (1/T) for CA-10 and CA-10-10 membranes.
Fig. 13. Cyclic voltammograms for CA-10, CA-10-5, CA-10-10, CA-10-15 and CA10-20 membranes, recorded at room-temperature at a scan rate of 10 mV s-1 (electrode
RI PT
cross-section area = 0.75 cm2).
Fig.14. (a) Cyclic voltammograms of the Mg/CA-10-10/Mg cell recorded at room
temperature at a scan rate of 10 mV s-1. (b) The sketches showing anodic oxidation
SC
and cathodic reduction of Mg at the Mg-electrode/electrolyte membrane interface.
Fig.15. DC polarization plots of SS/CA-10-x/SS (x = 5, 10 and 15) cells recorded at
AC C
EP
TE D
M AN U
room temperature under a voltage of 1.0 V.
21
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
Fig. 1. Macroscopic images of a CA-10-10 polymer electrolyte membrane.
22
1484 CA-20
742 1480
1280
M AN U
Intensity (counts)
740
CA-10
640 1560
CA-5
780
TE D
1572 786
pure CA
5 10 15 20 25 30 35 40 45 50 55 60
2θ (degree)
AC C
EP
0
a
SC
CA-15
RI PT
ACCEPTED MANUSCRIPT
23
ACCEPTED MANUSCRIPT
1304 652 1304
SC
CA-10-15
652 1080
M AN U
Intensity (counts)
b
RI PT
CA-10-20
CA-10-10
540 1120
CA-10-5
TE D
560 0
5 10 15 20 25 30 35 40 45 50 55 60
EP
2θ (degree)
Fig. 2. XRD patterns of (a) CA, CA-5, CA-10, CA-15 and CA-20 and (b) CA-10-5,
AC C
CA-10-10, CA-10-15 and CA-10-20 membranes.
24
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 3. SEM images of different polymer electrolyte membranes: (a) CA-10, (b) CA-
AC C
EP
CA.
TE D
10-5, (c) CA-10-10, and (d) CA-10-15. The inset represents the SEM image for pure
25
ACCEPTED MANUSCRIPT
RI PT
CA-10 CA-10-5 CA-10-10 CA-10-15
SC
80
60
40
100
200
300
400 Temperature (oC)
TE D
20
M AN U
Weight (%)
100
500
AC C
EP
Fig. 4. TGA thermograms of CA-10, CA-10-5, CA-10-10, and CA-10-15 membranes.
26
ACCEPTED MANUSCRIPT
RI PT
70
50
SC
40 pure CA CA-10 CA-10-10
30 20 10 0
0
10
20
M AN U
Tensile stress (MPa)
60
30
40
50
60
70
80
90 100
TE D
Tensile strain (%)
AC C
EP
Fig. 5. Stress-strain curves of pure CA, CA-10 and CA-10-10 membranes.
27
RI PT
ACCEPTED MANUSCRIPT
SC
CA-15
3500
3000
2500
15 16 13 76 10 24
89 3
14 16 11 58
15 70
33 18 29 20 28 71
TE D
4000
16 46
pure CA
M AN U
CA-5
34 24
Transmittance (a.u.)
CA-10
2000
1500
1000
Wavenumber (cm-1)
AC C
EP
Fig. 6. FT-IR spectra of CA, CA-5, CA-10, and CA-15 membranes.
28
100 CA-10
CA-15
M AN U
CA-5
SC
RI PT
ACCEPTED MANUSCRIPT
1642
75
1641
1543
TE D
Transmittance (%)
1644
EP
1544
1543
AC C
50
1650 1500
1650 1500
Wavenumber
1650 1500
(cm-1)
Fig. 7. FT-IR spectra of CA-5, CA-10, and CA-15 membranes in the range 1750-1470 cm-1.
29
1547
SC
Transmittance (a.u.)
CA-10-15
CA-10-10
M AN U
1539
1538
CA-10-5
TE D
CA-10
1700
EP
RI PT
ACCEPTED MANUSCRIPT
1543
1600
Wavenumber (cm
1500 -1
)
AC C
Fig. 8. FT-IR spectra of CA-10, CA-10-5, CA-10-10, and CA-10-15 membranes in the range 1750-1470 cm-1.
30
RI PT
ACCEPTED MANUSCRIPT
SC
Chitosan
)
AC C
EP
TE D
M AN U
(simple expression:
Fig. 9. The sketches showing the Mg-ion conducting mechanism in chitosan polymer electrolyte membranes.
31
RI PT
ACCEPTED MANUSCRIPT
SC
1.0x10-6
0.0
0.0 0.0
CA-5 CA-10 CA-15 CA-20
5 10 15 20 Mg(Tf)2 concentration (wt%)
TE D
5.0x104
a
2.0x10-6
M AN U
Ionic conductivity (S cm-1)
Minus imaginary Z (Ω)
1.0x105
4.0x104
8.0x104
AC C
EP
Real Z (Ω)
32
1.2x105
RI PT SC
4x10-5 3x10-5 2x10-5 1x10-5
0.0 0.0
b
CA-10-5 CA-10-10 CA-10-15 CA-10-20
5 10 15 20 EMITf concentration (wt%)
TE D
4.0x103
0
M AN U
8.0x10
3
Ionic conductivity (S cm-1)
Minus imaginary Z (Ω)
ACCEPTED MANUSCRIPT
3.0x103
6.0x103
9.0x103
Real Z (Ω)
EP
Fig. 10. Nyquist impedance plots of (a) CA-5, CA-10, CA-15 and CA-20 and (b) CA10-5, CA-10-10, CA-10-15 and CA-10-20 membranes, determined at room
AC C
temperature.
33
SC
RI PT
ACCEPTED MANUSCRIPT
M AN U
CA-10-5 CA-10-10 CA-10-15 CA-10-20
20
tan δ
15
5
EP
0
TE D
10
2
4
6
log (f/Hz)
AC C
Fig. 11. Variation of loss tangent with frequency for CA-10-5, CA-10-10, CA-10-15 and CA-10-20 membranes.
34
ACCEPTED MANUSCRIPT
-6
RI PT
R2 = 0.986
-8 -9
SC
-10 -11 R2 = 0.991
-12 -13 -14
2.8
2.9
M AN U
ln(σ//S cm-1 )
-7
CA-10-10 CA-10
3.0
3.1
1000/T (K-1)
3.2
3.3
TE D
Fig. 12. The natural logarithm of ionic conductivity (lnσ) as a function of reciprocal
AC C
EP
temperature (1/T) for CA-10 and CA-10-10 membranes.
35
ACCEPTED MANUSCRIPT 0.04 0.00
0.2
RI PT
CA-10-15 4.15 V
0.0
CA-10-10
-0.2
SC
0.03 0.00 -0.03
CA-10-5
0.02 0.00
M AN U
Current (mA)
CA-10-20
-0.04 0.03 0.00 -0.03
CA-10
-0.02 -3
-2
-1
0
1
2
3
Voltage (V)
TE D
Fig. 13. Cyclic voltammograms for CA-10, CA-10-5, CA-10-10, CA-10-15 and CA10-20 membranes, recorded at room-temperature at a scan rate of 10 mV s-1 (electrode
AC C
EP
cross-section area = 0.75 cm2).
36
ACCEPTED MANUSCRIPT
a
0.3
Mg/CA-10-10/Mg
0.1
RI PT
Current (mA)
0.2
0.0 -0.1
-3
-2
-1
0
SC
-0.2 1
2
3
AC C
EP
TE D
M AN U
Voltage (V)
Fig.14. (a) Cyclic voltammograms of the Mg/CA-10-10/Mg cell recorded at room temperature at a scan rate of 10 mV s-1. (b) The sketches showing anodic oxidation and cathodic reduction of Mg at the Mg-electrode/electrolyte membrane interface. 37
ACCEPTED MANUSCRIPT
3.6
RI PT
CA-10-15
2.4 1.2 0.0 CA-10-10
3.0 1.5
M AN U
i (µΑ)
4.5
0.0
5.1
CA-10-5
iI
3.4
SC
6.0
1.7
iF
TE D
0.0
0
10
20
30
40
50
60
EP
Time (Min)
Fig.15. DC polarization plots of SS/CA-10- x/SS (x = 5, 10 and 15) cells recorded at
AC C
room temperature under a voltage of 1.0 V.
38
ACCEPTED MANUSCRIPT
Table 1. The degrees of crystallinity for different membranes
RI PT
Tables
Table 2. The FWHM values of carbonyl and amine bands for different membranes
AC C
EP
TE D
M AN U
SC
Table 3. The values of relaxation time for different membranes
39
ACCEPTED MANUSCRIPT Table 1. The degrees of crystallinity for different membranes Crystallinity (%)
Pure CA
8.22
CA-5
8.15
CA-10
7.25
CA-15
7.89
CA -20
7.76
CA -10-5
6.43
SC
CA -10-10
5.61 7.48
M AN U
CA -10-15 CA -10-20
RI PT
Sample
7.45
Table 2. The FWHM values of carbonyl and amine bands for different membranes FWHMcarbonyl (cm-1)
CA-5 CA-10
67.26
78.60
65.88
81.55
55.60
96.54
EP
CA-15
FWHMamine (cm-1)
TE D
Sample
AC C
Table 3. The values of relaxation time for different membranes Sample
Relaxation time (s)
CA -10-5
3.06×10-5
CA -10-10
1.25×10-6
CA -10-15
1.44×10-5
CA -10-20
3.70×10-5
40