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Electrical, structural, thermal and electrochemical properties of corn starch-based biopolymer electrolytes
Accepted Manuscript Title: Electrical, Structural, Thermal and Electrochemical Properties of Corn Starch–Based Biopolymer Electrolytes Author: Chiam-Wen Liew S. Ramesh PII: DOI: Reference:
S0144-8617(15)00137-X http://dx.doi.org/doi:10.1016/j.carbpol.2015.02.024 CARP 9694
To appear in: Received date: Revised date: Accepted date:
13-11-2014 12-1-2015 11-2-2015
Please cite this article as: Liew, C.-W.,Electrical, Structural, Thermal and Electrochemical Properties of Corn StarchndashBased Biopolymer Electrolytes, Carbohydrate Polymers (2015), http://dx.doi.org/10.1016/j.carbpol.2015.02.024 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.
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Electrical, Structural, Thermal and Electrochemical Properties of Corn Starch–Based
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Biopolymer Electrolytes
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Chiam–Wen Liewa, S. Rameshb Centre for Ionics of University Malaya, Department of Physics, Faculty of Science, University of Malaya, Lembah Pantai, 50603 Kuala Lumpur, Malaysia [email protected] [email protected]
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corresponding author. Tel.: +60–3–7967–4391, Fax: +60–3–7967–4146
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Highlights Biopolymer electrolytes exhibit VTF relationship. Addition of ionic liquid in polymer electrolytes reduces Tg. Ionic liquid–doped polymer electrolyte has better electrochemical properties of EDLC.
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Biopolymer electrolytes containing corn starch, lithium hexafluorophosphate (LiPF6) and
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ionic liquid, 1–butyl–3–methylimidazolium hexafluorophosphate (BmImPF6) are prepared by
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solution casting technique. Temperature dependence–ionic conductivity studies reveal Vogel–
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Tamman–Fulcher (VTF) relationship which is associated with free volume theory. Ionic liquid–
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based biopolymer electrolytes show lower glass transition temperature (Tg) than ionic liquid–free
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biopolymer electrolyte. X–ray diffraction (XRD) studies demonstrate higher amorphous region
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of ionic liquid–added biopolymer electrolytes. In addition, the potential stability window of the
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biopolymer electrolyte becomes wider and stable up to 2.9 V. Conclusively, the fabricated 1 Page 1 of 32
electric double layer capacitor (EDLC) shows improved electrochemical performance upon
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addition of ionic liquid into the biopolymer electrolyte. The specific capacitance of EDLC based
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on ionic liquid–added polymer electrolyte is relatively higher than that of ionic liquid–free
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polymer electrolyte as depicted in cyclic voltammogram.
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Keywords: Corn starch; Ionic liquid; EDLC; Capacitance
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1
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conventional liquid electrolytes because of their attractive properties, such as high ability to
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eliminate problems of corrosive solvent leakage and harmful gas, wide electrochemical stability
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range, light in weight, ease of processability, and excellent thermal stability as well as low
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volatility (Li et al., 2006). In recent years, environmentally benign polymer electrolytes have
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been extensively studied with the aim to reduce the load on the environment. Therefore,
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biopolymer electrolytes have received an upsurge of interest. A variety of biopolymers is used to
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prepare biopolymer electrolytes, for instance, starch, chitosan, pectin, hyaluronic acid, agarose,
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carrageenfan, polylactides, polyhydroxyalkanoates (bacterial polyesters), cellulose and cellulose
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derivatives (Lopes et al., 2003; Ma et al., 2007; Ning et al., 2009). Among these well–known
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biodegradable natural polysaccharides, corn starch is a promising candidate as it offers potential
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advantages over recalcitrant synthetic plastics in disposable applications with superior
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mechanical and electrical characteristics (Mao et al., 2000). Starch is a low cost, abundant and
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renewable raw material with good biocompatibility (Lopes et al., 2003; Ma et al., 2007; Xiong et
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al., 2008). Additional features of this biodegradable starch would be excellent steel adhesion
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properties, high solubility and high recrystallization stability of the amorphous phase (Marcondes
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et al., 2010; Pawlicka et al., 2008).
Introduction
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Solid polymer electrolytes (SPEs) have been widely investigated to substitute
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Starch–based SPEs showed relatively low ionic conductivity. Several attempts can be
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done to improve the ionic conductivity, such as mixed salt system, polymer blending, addition of
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additives (i.e. plasticizers, filler or ionic liquids) and mixed solvent system. Ionic liquids are
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selected to develop biopolymer electrolytes because of their environmentally friendly feature.
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Ionic liquids (ILs) are non–volatile molten salts with a low melting temperature, Tm <100 °C
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(Pandey & Hashmi, 2009). ILs have emerged as promising candidates because of their unique
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and fascinating physicochemical properties. Ionic liquids have a number of beneficial properties,
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for example a wide electrochemical potential window (up to 6V), wide decomposition
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temperature range, negligible vapor pressure, non–toxic, non–volatile and non–flammable with
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high ionic conductivity feature (Cheng et al., 2007; Patel et al., 2011).
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Supercapacitor is generally divided into three classes, viz. pseudocapacitor, electric
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double layer capacitor (EDLC) and hybrid capacitor. EDLC is constructed using the highest
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conducting biopolymer electrolyte in this present study. The electrochemical behavior of
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biopolymer electrolyte–based EDLC are characterized and investigated in this present work.
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EDLC is chosen as the primary application in this present work because of its higher energy
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density than conventional electrostatic capacitor, higher power density than rechargeable
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batteries and longer cycle life (Sun & Yuan, 2009). Although the same polymer system has been
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published in our previous paper (Ramesh et al., 2011), we focus on the temperature–dependence
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conductivity study, structural, and thermal properties of the biopolymer electrolytes in the
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present work. EDLC using the same polymer system has also been reported in our previous
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published paper (Liew et al., 2014). However, the objective of this work is different. The
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previous published paper reported the effect of anion of ionic liquids onto the EDLC. On the
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other hand, the main objective of this present work is to investigate the effect of ionic liquid onto
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the biopolymer electrolytes and the electrochemical performances of EDLC.
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2
Experimental
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2.1
Materials Biopolymer electrolytes were prepared by means of solution casting technique.
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Biodegradable corn starch (Sigma–Aldrich), lithium hexafluorophosphate (LiPF6) (Aldrich) and
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1–butyl–3–methylimidazolium
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employed as host polymer, salt and ionic liquid, respectively.
(BmImPF6)
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2.2
were
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(Sigma–Aldrich)
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hexafluorophosphate
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Preparation of thin films
Biopolymer electrolytes were prepared by solution casting method. PF 1, PF 5, PF 6 and
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PF 8 were the designations for the polymer membranes with adulteration of 10 wt.%, 50 wt.%,
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60 wt.% and 80 wt.% of BmImPF6, respectively. On the contrary, PF 0 was expressed as sample
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without inclusion of ionic liquid where the ratio of corn starch to LiPF6 is 80:20. Different
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stoichiometric quantities of corn starch and LiPF6 were initially dissolved in 20 mL of distilled
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water. BmImPF6 was thus doped into the polymer solution to prepare ionic liquid–based
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biopolymer electrolyte. The solution was then stirred overnight at 80 °C to ensure the complete
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dissolution of all materials in a closed system. The resulting solution was cast on a glass Petri
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dish and dried in an oven at 80 °C to eliminate the distilled water. Solid–state biopolymer
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electrolytes were eventually produced.
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2.3
Characterizations of Biopolymer Electrolytes
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2.3.1
Temperature dependence–ionic conductivity studies
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The prepared samples were subjected to ac–impedance spectroscopy. The thickness of the
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samples was measured using micrometer screw gauge. The ionic conductivity of the samples was
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determined by HIOKI 3532–50 LCR HiTESTER, over a frequency range between 50 Hz and 5
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MHz at a voltage bias of 10 mV. The ionic conductivity of biopolymer electrolytes was
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measured from ambient temperature to 80 °C. Polymer electrolyte films were mounted on the
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holder under spring pressure with two stainless steel (SS) blocking electrodes.
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Differential scanning calorimetry (DSC)
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DSC analysis was evaluated using TA Instrument Universal Analyzer 200 comprising a
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DSC Standard Cell FC as main unit and Universal V4.7A software. Samples weighing 2–3 mg
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were hermetically sealed in the aluminum Tzero pans while an empty hermetically sealed
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aluminum pan was used as a reference cell. A hole was punched on the pan to remove the
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trapped moisture. As a preliminary step, the samples were heated from 25 °C to 105 °C to
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remove any trace amount of water at a heating rate of 30 °C min-1 under nitrogen flow rate of 60
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ml min-1. The samples were then maintained at 105 °C for 5 minutes to ensure complete
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evaporation. The samples were thus equilibrated at 25 °C and subsequently heated to 70 °C. The
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samples were then rapidly cooled to –40 °C and reheated to 70 °C at the pre–set heating rate. The
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final heating scan in DSC thermogram was used to evaluate the glass transition temperature (Tg)
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of biopolymer electrolytes.
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2.3.3
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X–ray diffraction (XRD) studies
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Amorphous nature of biopolymer electrolytes was investigated by XRD method. The
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XRD patterns were recorded on a Siemens D 5000 diffractometer with Cu–Kα radiation
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(λ=1.54060 Å) over the range of 2θ = 5–80° at ambient temperature.
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2.3.4
Linear sweep voltammetry (LSV) The LSV responses of PF 0 and PF 5 were performed using CHI600D electrochemical
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analyzer. This cell was studied at a scan rate of 5 mVs-1 with the configuration of stainless steel
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(SS)/biopolymer electrolyte/SS in the potential range of ±3V. The sample interval was 0.001 V
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with 2 seconds as rest time before the analysis.
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2.4 Preparation of electrodes
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Symmetrical activated carbon–based EDLC electrodes were prepared by dip coating
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technique. The carbon slurry was prepared by mixing 80 wt.% of activated carbon (Kuraray
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Chemical Co Ltd., Japan, particle size is 5~20 µm, surface area is 1800~2000 m2g-1), 10 wt.% of
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carbon black (known as Super P) and 10 wt.% of poly(vinylidene fluoride) (PVdF) (molecular
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weight of 534000 gmol-1, Aldrich) in 1–methyl–2–pyrrolidone (Purity ≥ 99.5%, from Merck,
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Germany). This mixture was stirred for several hours until a homogenous slurry with smooth
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surface was obtained. The aluminum mesh electrode and the slurry were subjected to dip coater
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for dip coating process. The coated electrodes were dried in an oven at 100 °C to eliminate the
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solvent. The mass of electrode materials was determined. Symmetrical activated carbon
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electrodes were used for EDLC fabrication in this present work.
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2.5
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EDLC fabrication and characterization EDLC
cell
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prepared
in
the
configuration
of
electrode/biopolymer
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electrolyte/electrode. EDLC cell was placed in a cell kit for further electrochemical
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characterization. CHI600D electrochemical analyzer was used to analyze CV study of EDLC.
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EDLC cell was then evaluated at scan rate of 10 mVs-1 from –1 to +1 V with sample interval of
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0.001 V. The specific capacitance (Csp) of EDLC was determined by the equation below:
Csp
i sm
(Equation 1)
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where i is the average anodic–cathodic current (A), s is the potential scan rate (Vs-1) and m is the
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average mass of active materials. The average mass of electrode materials is 0.02 g. The charge-
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discharge study was accomplished using a Neware battery cycler. The EDLC was charged and
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discharged at current of 1 mA. The EDLC is allowed to rest for 30 min prior to the measurement.
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The specific discharge capacitance (Csp) was obtained from charge-discharge curves, according
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to the following relation (Amitha et al., 2009):
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Csp
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I m dV
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dt
(Equation 2)
where I is the applied current (A); m is the average mass of electrode materials (including the
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binder and super P); dV represents the potential change of the discharging process excluding the
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internal resistance drop occurring at the beginning of the cell discharge; and dt is the time
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interval of the discharging process. The dV/dt is determined from the slope of the discharge
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curve. The average mass of electrodes in the hexafluorophosphate system is around 0.045 g.
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Energy density (E, W h kg−1), power density (P, kW kg−1) and Coulombic efficiency (η, %) were evaluated from the equations below (Yu et al., 2012) :
E
Csp dV 2
P
2
1000 3600
(Equation 3)
I dV 1000 2 m
(Equation 4)
td 100% tc
(Equation 5)
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3
Results and discussion
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3.1
Temperature dependent–ionic conductivity studies Figure 1 depicts Arrhenius fitted–temperature dependent ionic conductivity of PF 0, PF 5,
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PF 6 and PF 8, in the temperature regime, from ambient temperature to 80 °C. The temperature
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dependent study must be plotted in order to investigate the mechanism pertaining to ion
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transportation in the polymer electrolytes. We fit the plot with Arrhenius theory as follows:
Ea A exp kT
(Equation 6)
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where A is a constant which is proportional to the amount of charge carriers, Ea is activation
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energy, k is Boltzmann constant and T represents the absolute temperature in K. However, the
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regression value of all plots is deviated from unity. Hence, we employ Vogel–Tamman–Fulcher
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(VTF) equation to fit the plot, as illustrated in Figure 2. The ionic conductivity is expressed as
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below using this volume–activated principle:
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d
B exp T T
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AoT
1 2
1 Ea k B AoT 2 exp T T
(Equation 7)
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where Ao represents pre–exponential constant proportional to the number of charge carriers, B
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stands for a constant which is determined from the gradient of the plot (K-1), Ea is pseudo–
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activation energy for conduction (eV), kb stands for Boltzmann constant (equivalent to
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8.6173×10-5 eVK-1), T is the absolute temperature (K) and To represents ideal vitreous transition
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temperature and it is suggested to be 50 K below the glass transition temperature (Tg) where Tg is
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determined from DSC measurement. All the important parameters are summarized in Table 1.
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Since the plot is well–fitted with this theory, we can conclude that the biopolymer electrolytes
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exhibit VTF relationship which is related to free volume model. VTF theory infers that the ion
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hopping mechanism in the polymer electrolytes is associated with high segmental motion of 9 Page 9 of 32
polymer chains in an amorphous phase. The experimental Tg that we obtained is extremely low
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compared to the temperature range that we analyzed in this study. Therefore, the biopolymer
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electrolytes are in a rubbery state within the temperature regime. As a result, the polymer chains
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are flexible. So, the segmental mobility of polymer can be improved with these flexible chains.
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Hence, the charge carriers can transport from a site to an empty site, along with high mobility of
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polymer segments.
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The ionic conductivity of biopolymer electrolytes increases in this order: PF 0
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8< PF 6 < PF 5. Upon addition of 50 wt.% BmImPF6, the highest ionic conductivity of
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(1.47±0.02)×10−4 S cm−1 is achieved at ambient temperature (Ramesh et al., 2011). The ionic
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conductivity is further increased to (1.99±0.02)×10−4 S cm−1 by heating the sample at 80 °C.
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Upon addition of ionic liquids, the ionic conductivity of biopolymer electrolyte is improved
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significantly. The increase in ionic conductivity of biopolymer electrolyte is mainly ascribed to
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the strong plasticizing effect of ionic liquids. Strong plasticizing effect of ionic liquid aids to
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soften the polymer backbone and hence increases the flexibility of polymer chain. The ions can
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be transported easily within the polymer matrix with highly flexible polymer chains. Besides,
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higher flexibility of polymer chains improves the mobility of polymer segments and assists the
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ionic transport in the polymer complexes. Consequently, the ionic conductivity of ionic liquid–
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added polymer electrolyte is higher compared to the ionic liquid–free polymer electrolyte.
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Moreover, ionic liquid increases the degree of amorphous of the polymer matrix. Ionic liquid
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could weaken the transient coordinative bonds among the molecules in the crystalline region and
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thus turn the polymer chains into flexible characteristic leading to higher amorphous degree of
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polymer complexes. In addition, the mobility of charge carriers in amorphous region is faster
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than in crystalline region. High amorphous region of the polymer electrolyte can form rapid ion
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conduction process and result high ionic conductivity for ionic liquid–added polymer electrolyte
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due to the unordered arrangement of macromolecules in the amorphous region. Moreover, the
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physicochemistry of ionic liquid such as viscosity and dielectric constant contributes to the
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increase in ionic conductivity. Lower viscosity of ionic liquid also enhances the polymer chain
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flexibility. Therefore, this flexible polymer backbone could interrupt the ion–polymer bonding
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and promote the ionic conducting process (Kumar et al., 2011). In contrast, high dielectric
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permittivity of ionic liquid plays an important role to separate the ion pairs and/or ion aggregates
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with high self–dissociating properties. More mobile cations are consequently produced which
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leads to higher ionic conductivity (Kumar et al., 2011).
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This corn starch–based polymer electrolyte is a promising candidate as a separator in the
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electrochemical devices as it achieves higher ionic conductivity than other biopolymer
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electrolytes. Buraidah and co–workers prepared biopolymer electrolytes based on chitosan–
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poly(vinyl alcohol) (PVA) and ammonium iodide (NH4I). The highest ionic conductivity of
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1.77×10−6 S cm−1 is achieved with addition of 45 wt.% of NH4I (Buraidah et al., 2011). The
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results obtained from this present work is two orders of magnitude higher than Buraidah’s work.
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Rice starch has also been used as host polymer in electrolyte preparation. The ionic conductivity
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of this plasticized–rice starch–based polymer electrolyte is slightly lower than this present work
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where the maximum ionic conductivity of 1.1×10−4 S cm−1 is obtained with adulteration of 30
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wt.% of glycerol (Marcondes et al., 2010). Corn starch–based polymer electrolytes using
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different salt were also prepared by our group members. The ionic conductivity of this polymer
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electrolyte is still slightly lower than this work (Teoh et al., 2014). Therefore, we can conclude
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that PF 5 is a good material as an electrolyte in electrochemical cell, especially EDLC.
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In order to probe the ion dynamic of biopolymer electrolytes further, pseudo–activation
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energy (Ea) of polymer electrolytes has to be determined by fitting the plots into the VTF
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empirical formula. PF 5 exhibits the lowest Ea value as shown in Table 1. This result implies that
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ionic conduction in PF 5 is the most favorable compared to other polymer systems. PF 0 portrays
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the highest Ea value because of the absence of ionic liquid. The coordinative bonding in PF 0 is
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much stronger than that of ionic liquid–based biopolymer electrolytes. The Ea value can be
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lowered with adulteration of ionic liquid. The ionic liquid is immobilized within starch granules.
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So, the transient hydrogen bonds could be broken down with the presence of ionic liquid as
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aforesaid. The effect of ionic liquid becomes more visible by comparing PF 1 with other ionic
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liquid–based biopolymer electrolytes. Among all the ionic liquid–based biopolymer electrolytes,
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PF 1 demonstrates the maximum Ea value due to the insufficient ionic liquid. As a result, the
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polymer network is still stronger. Therefore, the sample entails higher energy to break the
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physical and chemical bonds in the polymer electrolyte systems. Nevertheless, the Ea values of
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PF 6 and PF 8 are higher than PF 5. It may correlate to the excessive self–cross linkage within
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the polymer matrices. Consequently, more energy is required to break and reform the
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coordination bonds in the polymer compounds which coupled with low ionic conductivity. It can
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be noticed that the higher the ionic conductivity of the sample, the lower is the Ea. This is also
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supported by Ao as similar trend has been observed in Table 1. The number of charge carriers is
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increased tremendously when we add 50 wt.% of ionic liquid into the polymer solution. Above
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inclusion of this mass fraction of ionic liquid, the concentration of charge carriers is decreased
259
slightly. It is suggestive of formation of ion pairs or/and ion aggregates which forms the self–
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cross linkages.
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3.2
Differential scanning calorimetry (DSC) Table 1 shows the Tg value obtained from DSC thermograms of PF 0, PF 1, PF 5, PF 6
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and PF 8. The Tg of pure corn starch is around 55.71 °C as reported in our published work (Liew
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et al., 2012). Sub–ambient Tg of –19.20 °C is observed with incorporation of lithium salt in PF 0
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(Liew et al., 2012). The abrupt drop in Tg is suggestive of plasticizing effect of lithium salt. This
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effect could destroy the transient coordinative bonds among the corn starch granules through
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complexation and thus improve the flexibility of polymer chains. Tg drops further upon addition
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of ionic liquid due to its strong plasticizing effect. Doping of ionic liquid weakens the dipole–
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dipole interactions and thus destroys the transient interactive bonds among starch granules by
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softening the polymer backbone. Therefore, it improves the mobility of polymer segments which
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leads to flexible polymer chains (Baskaran et al., 2007). Among all the samples studied, PF 5
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demonstrates the lowest Tg of –20.38°C. This discloses highly flexible polymer backbone which
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facilitates the ionic transportation, asserting the highest ionic conductivity.
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3.3
X–ray diffraction (XRD) studies
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Figure 3 depicts the comparisons of XRD patterns of PF 0 with ionic liquid–based
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biopolymer electrolytes. Two intense broad peaks have been observed at 17° and 22.2° in Figure
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3 (a) (Liew et al., 2012). This reveals the semi–crystalline properties of PF 0. LiPF6 shows its
280
crystalline peaks at 2θ=8°, 21.7°, 25.3°, 28.3°, 35.3°, 37°, 42.8°, 49.3°, 55.9°, 60.6°, 61° and
281
68.4° as reported in our published paper (Liew et al., 2012). All these crystalline diffraction
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peaks of LiPF6 are absent in PF 1 and PF 5 as can be seen in Figures 3 (b) and (c). This signifies
283
that LiPF6 is well–dissociated to form the complexation between corn starch and LiPF6.
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However, some crystalline peaks are obtained at 35.8°, 42.8°, 49.3° and 68.4° as highlighted in
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Figures 3 (d) and (e) when we increase the ionic liquid mass loadings to 60 wt.% and 80 wt.%.
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The existence of these peaks is evocative of formation of neutral ion pairs and ion aggregates. In
287
addition, this might be due to the reformation of lithium salt as PF6 anions are in excess. The PF6
288
anions may interact with Li cations again in this phenomenon. This may be due to the deficient
289
dissolution of the materials in the samples due to the excessive amount of PF6 anions. At high
290
concentration of ionic liquid, self–cross linkage and entanglements have been formed, as
291
explained earlier. Some salts may be trapped in the entanglements. An obvious change in
292
crystallographic organization in the polymer membrane is also observed upon addition of the
293
ionic liquid, i.e. the peak intensity is significantly lowered down. This verifies that impregnation
294
of ionic liquid weakens the interactions within the polymer chains and hence disrupts the ordered
295
arrangement of polymeric network. Therefore, it helps in decreasing the crystallinity of the
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polymer complexes. In other words, addition of ionic liquid increases the amorphous degree of
297
the biopolymer electrolytes in the presence of plasticizing effect (Sivakumar et al., 2006).
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For PF 1, two broad diffraction peaks are still showing at 2θ=16.6° and 23.9° in Figure 3
299
(b). However, upon further addition of ionic liquid, only one broad peak is observed in each
300
XRD pattern. The characteristic peak is attained at 2θ angles of 18.5°, 20.7° and 22° for PF 5, PF
301
6 and PF 8, respectively. This signifies that the samples have gone through the phase
302
transformation, from semi–crystalline to amorphous. Upon comparing all the ionic liquid–based
303
biopolymer electrolytes, it is noted that PF 5 illustrates the broadest peak with the lowest
304
intensity. This observation reflects the highest amorphous degree and eventually achieves the
305
highest ionic conductivity. Beyond inclusion of 50 wt.% of ionic liquid, the relative peaks
306
become more intense. The change in peak intensity is strongly related to the extreme self–cross
307
linkages within the polymer complexes. These cross linkages could lead to the intertwined
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polymer chains which subsequently reduce the amorphous characteristic in the biopolymer
309
electrolytes.
310 3.4
Linear sweep voltammetry (LSV)
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LSV is a vital study to determine the maximum operational potential of the polymer
313
electrolytes (Arof et al., 2012). Figures 4 (a) and (b) portray linear sweep voltammogram of PF 0
314
and PF 5, respectively. The anodic and cathodic peaks are absent in both figures. Therefore, it
315
can be concluded that there is no redox process in the potential range. The potential window of
316
ionic liquid–free biopolymer electrolyte is only 1.5 V that is from –1.2 V to 0.3 V, as
317
demonstrated in Figure 4 (a). Nevertheless, the stability window is found to have increased
318
significantly with addition of 50 wt.% of BmImPF6. From the voltammogram, PF 5 is stable up
319
to 2.9 V, from –1.4 V to 1. 5 V. Therefore, this result infers that the ionic liquid–based
320
biopolymer electrolyte is more suitable for EDLC application as ionic liquid–based biopolymer
321
electrolyte has higher electrochemical potential window than ionic liquid–free polymer
322
electrolyte.
324
3.5
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Cyclic voltammetry (CV)
325
Cyclic voltammetry study was conducted to evaluate the capacitance of biopolymer
326
electrolyte–based EDLC. The capacitive behavior of EDLC comprising ionic liquid–based
327
polymer electrolyte is compared to that of ionic liquid–free polymer electrolyte as illustrated in
328
Figures 5 (a) and (b). Figure 5 (a) depicts CV profile of EDLC with PF 0 polymer electrolyte,
329
whereas the CV of EDLC containing the most conducting biopolymer membrane (PF 5) is
330
shown in Figure 5 (b). The redox processes are not detected in this potential region. This
15 Page 15 of 32
observation confirms the construction of electrical double layer capacitors in this present work.
332
No ideal rectangular shape is attained for both figures. This suggests the shortcoming of utilizing
333
solid polymer electrolytes in the supercapacitor, which is poor interfacial contact between the
334
electrolyte and electrode (Latham et al., 2002). An ill–defined box like shape with specific
335
capacitance of 0.18 Fg-1 is observed in Figure 5 (a). However, comparing Figure 5 (a) with 5 (b),
336
the shape of the voltammogram is greatly improved by doping ionic liquid into the polymer
337
electrolyte. The EDLC consisting of PF 5 shows a voltammogram approaching the ideal
338
rectangular (or box like) shape with specific capacitance of 36.79 Fg-1 in Figure 5 (b) (Hashmi et
339
al., 1997). Although the CV of this present work did not depict the perfect rectangular shape, the
340
specific capacitance of EDLC still show higher value than the literatures. Fabrication of EDLC
341
with glycerol plasticized–polymer electrolytes had been done by Shukur and coworkers. The
342
assembled EDLC showed specific capacitance of 33 Fg-1 which is lower than this present work
343
(Shukur et al., 2014). Another type of biodegradable polymer is also used to prepare biopolymer
344
electrolytes by our peers that is poly(vinyl alcohol) (PVA). Lim prepared EDLC using
345
biopolymer electrolytes based on PVA, lithium perchlorate (LiClO4) and antimony trioxide
346
(Sb2O3). The specific capacitance obtained from this EDLC fabricated with composite polymer
347
electrolyte is just 13–14.5 Fg-1 which is three folds lower than our present work (Lim et al.,
348
2014). The specific capacitance that we obtained in this work is higher than those EDLC using
349
existing polymer electrolytes. Therefore, it is noteworthy to conclude that corn starch–based
350
polymer electrolyte (or PF 5 in this present work) is a suitable candidate to be applied as
351
electrolyte in EDLC application as it has superior capacitive behavior.
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The abrupt increase in capacitance reveals that adulteration of ionic liquid into polymer
353
complexes can improve the electrochemical behavior of EDLC. This could be mainly attributed
16 Page 16 of 32
to the unique characteristic of ionic liquid. Since ionic liquid is comprised solely of ions, thus
355
more ions are transported in the conducting path. As a result, more charge carriers can be stored
356
in the electrical double layer at the electrolyte–electrode interface which contributes to higher
357
capacitive nature in the EDLC cell. Besides the increase in charge carrier concentration,
358
incorporation of ionic liquid enhances the flexibility of polymer backbone by softening the
359
polymer backbone. The flexible polymer chain not only promotes the ion dissociation from
360
native coordination, but also improves the mobility of charge carriers. Consequently, the buildup
361
of charge carriers at the electrolyte–electrode boundary is significantly enhanced when the ion
362
mobility and ion content are increased. This then leads to higher capacitance due to the excessive
363
and rapid accumulation of charge carriers.
3.6
Galvanostatic charge–discharge performance
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Figure 6 illustrates galvanostatic charge–discharge curve of EDLC with PF 5 over first
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four charge and discharge cycles. The cell starts to be charged at 0.29 V instead of 0 V because
368
of the internal resistance of the EDLC. Similar observation is also attained where a small drop in
369
voltage is observed at the initial part of discharge curve. This ohmic loss arises from electrode
370
and electrolyte, such as charge transfer resistance and bulk resistance of polymer electrolyte
371
(Arof et al., 2012; Mitra et al., 2001). Cycability test is a vital characterization to determine the
372
stability of EDLC in terms of electrochemical performances after charging and discharging
373
processes. The specific capacitance, Coulombic efficiency, energy density and power density of
374
EDLC using the most conducting ionic liquid–added biopolymer electrolyte over 500 cycles are
375
listed in Table 2. The EDLC shows the specific capacitance of 37.07 Fg-1, energy density of 2.53
376
W h kg-1 and power density of 7.79 kW kg-1 with its Coulombic efficiency of 58% in the first
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cycle of charge and discharge cycle. The specific capacitance obtained from this study is
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comparable with the findings in CV. Therefore, the capacitance value obtained from this EDLC
379
is reliable. The capacitance, energy density and power density are thus reduced moderately from
381
50th cycles to 300th cycles. Above 300th cycles of charge and discharge processes, these
382
parameters are decreased insignificantly. The reduction of capacitance, energy density and power
383
density over the cycle number is suggestive of depletion of polymer electrolyte and formation of
384
ion pairs/ion aggregates. The ions might be paired up upon the rapid charging and discharging
385
processes. Therefore, the number of mobile charge carriers transported in the polymer electrolyte
386
becomes lesser as the ion pairs can impede the ion migration. Hence, the ions accumulated at the
387
electrode–electrolyte boundaries to form an electrical double layer are getting lesser. Since the
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ion adsorption at the electrode–electrolyte interface is not favorable, so this would decrease the
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capacitance of EDLC.
4
Conclusion
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Ionic liquid–added biopolymer electrolytes were prepared by solution casting technique.
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Upon addition of 50 wt.% of BmImPF6, the biopolymer electrolytes achieved the highest ionic
394
conductivity of (1.99±0.02)×10-4 S cm-1 at 80 °C. The study on the temperature dependence of
395
ionic conductivity showed that these biopolymer electrolytes exhibit VTF behavior which is
396
associated with free volume theory. The lowest Tg of PF 5 implied the improvement in flexibility
397
of polymer backbone, which promotes the ionic decoupling process. Addition of ionic liquid also
398
improved the amorphourness of polymer electrolytes and widened the electrochemical potential
399
stability window of polymer electrolytes. EDLC based on ionic liquid–doped polymer electrolyte
18 Page 18 of 32
400
showed better electrochemical performance (i.e. higher specific capacitance) than that of ionic
401
liquid–free polymer electrolyte.
402 Acknowledgements
404
This
405
(UM.C/625/1/HIR/MOHE/SCI/21/1) from Ministry of Education, Malaysia. One of the authors,
406
Chiam–Wen Liew gratefully acknowledges the “Skim Bright Sparks Universiti Malaya”
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(SBSUM) for financial support.
was
supported
by
the
High
Impact
Research
Grant
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534 535
Table captions:
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Table 1: The obtained parameters from each VTF plot with the experimental glass transition temperature from DSC thermogram.
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Table 2: The specific capacitance, Coulombic efficiency, energy density and power density of
539
EDLC fabricated using PF 5 polymer electrolyte over 500 cycles of charge and
540
discharge processes.
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541 Figure captions:
543
Figure 1: Arrhenius plot of ionic conductivity of PF 0, PF 1, PF 5, PF 6 and PF 8.
544
Figure 2: The temperature dependent–ionic conductivity of polymer electrolytes fitted with VTF
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theory.
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Figure 3: XRD patterns of (a) PF 0, (b) PF 1, (c) PF 5, (d) PF 6 and (e) PF 8.
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Figure 4 (a): Linear sweep voltammogram of PF 0.
548
Figure 4 (b): Linear sweep voltammogram of PF 5.
549
Figure 5(a): Cyclic voltammogram of PF 0.
550
Figure 5(b): Cyclic voltammogram of PF 5.
551
Figure 6: Galvanostatic charge–discharge performances of ionic liquid–added biopolymer
553
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electrolyte over first four cycles.
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Table:
554
Log Ao
–4.301 –4.2557 –2.1032 –2.243 –2.3198 plot with
temperature from DSC thermogram.
Pre– exponential constant, Ao
Gradient Pseudo– of the activation plot, B energy, Ea (K-1) (meV) -5 5×10 0.1635 14.09 5.55×10-5 0.1499 12.92 -3 7.88×10 0.0988 8.51 5.71×10-3 0.1225 10.56 4.79×10-3 0.1334 11.50 the experimental glass transition
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Ideal glass Glass transition transition temperature, temperature, To (K) Tg (K) PF 0 0.9853 203.95 253.95 PF 1 0.9976 203.29 253.29 PF 5 0.997 202.77 252.77 PF 6 0.9893 202.98 252.98 PF 8 0.997 203.15 253.15 Table 1: The obtained parameters from each VTF
555
Regression value, R2
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Sample
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Specific capacitance, Coulombic Energy density, E Power density, P Csp (Fg-1) efficiency, η (%) (W h kg-1) (kW kg-1) 0 37.07 58 2.53 7.79 50 31.78 60 1.99 7.48 100 26.17 62 1.60 7.39 150 22.24 63 1.09 6.60 200 17.11 69 0.69 5.98 250 14.83 70 0.55 5.75 300 14.35 71 0.53 5.60 350 13.90 73 0.48 5.56 400 13.08 75 0.43 5.43 450 13.05 76 0.42 5.34 500 13.02 77 0.41 5.32 Table 2: The specific capacitance, Coulombic efficiency, energy density and power density of
557 558 559 560
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EDLC fabricated using PF 5 polymer electrolyte over 500 cycles of charge and discharge processes.
27 Page 27 of 32
List of Figures
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Figure 1: Arrhenius plot of ionic conductivity of PF 0, PF 1, PF 5, PF 6 and PF 8.
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Figure 2:
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Figure 3: XRD patterns of (a) PF 0, (b) PF 1, (c) PF 5, (d) PF 6 and (e) PF 8.
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The temperature dependent–ionic conductivity of polymer electrolytes fitted with VTF theory.
29 Page 29 of 32
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Figure 4 (a): Linear sweep voltammogram of PF 0.
575 576 577
Figure 4 (b): Linear sweep voltammogram of PF 5.
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Figure 5 (a): Cyclic voltammogram of PF 0.
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Figure 5 (b): Cyclic voltammogram of PF 5.
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electrolyte over first four cycles.
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Figure 6: Galvanostatic charge–discharge performances of ionic liquid–added biopolymer
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S0144-8617(15)00137-X http://dx.doi.org/doi:10.1016/j.carbpol.2015.02.024 CARP 9694
To appear in: Received date: Revised date: Accepted date:
13-11-2014 12-1-2015 11-2-2015
Please cite this article as: Liew, C.-W.,Electrical, Structural, Thermal and Electrochemical Properties of Corn StarchndashBased Biopolymer Electrolytes, Carbohydrate Polymers (2015), http://dx.doi.org/10.1016/j.carbpol.2015.02.024 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.
1
Electrical, Structural, Thermal and Electrochemical Properties of Corn Starch–Based
2
Biopolymer Electrolytes
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Chiam–Wen Liewa, S. Rameshb Centre for Ionics of University Malaya, Department of Physics, Faculty of Science, University of Malaya, Lembah Pantai, 50603 Kuala Lumpur, Malaysia [email protected] [email protected]
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corresponding author. Tel.: +60–3–7967–4391, Fax: +60–3–7967–4146
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Highlights Biopolymer electrolytes exhibit VTF relationship. Addition of ionic liquid in polymer electrolytes reduces Tg. Ionic liquid–doped polymer electrolyte has better electrochemical properties of EDLC.
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24
Biopolymer electrolytes containing corn starch, lithium hexafluorophosphate (LiPF6) and
25
ionic liquid, 1–butyl–3–methylimidazolium hexafluorophosphate (BmImPF6) are prepared by
26
solution casting technique. Temperature dependence–ionic conductivity studies reveal Vogel–
27
Tamman–Fulcher (VTF) relationship which is associated with free volume theory. Ionic liquid–
28
based biopolymer electrolytes show lower glass transition temperature (Tg) than ionic liquid–free
29
biopolymer electrolyte. X–ray diffraction (XRD) studies demonstrate higher amorphous region
30
of ionic liquid–added biopolymer electrolytes. In addition, the potential stability window of the
31
biopolymer electrolyte becomes wider and stable up to 2.9 V. Conclusively, the fabricated 1 Page 1 of 32
electric double layer capacitor (EDLC) shows improved electrochemical performance upon
33
addition of ionic liquid into the biopolymer electrolyte. The specific capacitance of EDLC based
34
on ionic liquid–added polymer electrolyte is relatively higher than that of ionic liquid–free
35
polymer electrolyte as depicted in cyclic voltammogram.
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Keywords: Corn starch; Ionic liquid; EDLC; Capacitance
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1
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conventional liquid electrolytes because of their attractive properties, such as high ability to
41
eliminate problems of corrosive solvent leakage and harmful gas, wide electrochemical stability
42
range, light in weight, ease of processability, and excellent thermal stability as well as low
43
volatility (Li et al., 2006). In recent years, environmentally benign polymer electrolytes have
44
been extensively studied with the aim to reduce the load on the environment. Therefore,
45
biopolymer electrolytes have received an upsurge of interest. A variety of biopolymers is used to
46
prepare biopolymer electrolytes, for instance, starch, chitosan, pectin, hyaluronic acid, agarose,
47
carrageenfan, polylactides, polyhydroxyalkanoates (bacterial polyesters), cellulose and cellulose
48
derivatives (Lopes et al., 2003; Ma et al., 2007; Ning et al., 2009). Among these well–known
49
biodegradable natural polysaccharides, corn starch is a promising candidate as it offers potential
50
advantages over recalcitrant synthetic plastics in disposable applications with superior
51
mechanical and electrical characteristics (Mao et al., 2000). Starch is a low cost, abundant and
52
renewable raw material with good biocompatibility (Lopes et al., 2003; Ma et al., 2007; Xiong et
53
al., 2008). Additional features of this biodegradable starch would be excellent steel adhesion
54
properties, high solubility and high recrystallization stability of the amorphous phase (Marcondes
55
et al., 2010; Pawlicka et al., 2008).
Introduction
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Solid polymer electrolytes (SPEs) have been widely investigated to substitute
56
Starch–based SPEs showed relatively low ionic conductivity. Several attempts can be
57
done to improve the ionic conductivity, such as mixed salt system, polymer blending, addition of
58
additives (i.e. plasticizers, filler or ionic liquids) and mixed solvent system. Ionic liquids are
59
selected to develop biopolymer electrolytes because of their environmentally friendly feature.
60
Ionic liquids (ILs) are non–volatile molten salts with a low melting temperature, Tm <100 °C
3 Page 3 of 32
(Pandey & Hashmi, 2009). ILs have emerged as promising candidates because of their unique
62
and fascinating physicochemical properties. Ionic liquids have a number of beneficial properties,
63
for example a wide electrochemical potential window (up to 6V), wide decomposition
64
temperature range, negligible vapor pressure, non–toxic, non–volatile and non–flammable with
65
high ionic conductivity feature (Cheng et al., 2007; Patel et al., 2011).
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Supercapacitor is generally divided into three classes, viz. pseudocapacitor, electric
67
double layer capacitor (EDLC) and hybrid capacitor. EDLC is constructed using the highest
68
conducting biopolymer electrolyte in this present study. The electrochemical behavior of
69
biopolymer electrolyte–based EDLC are characterized and investigated in this present work.
70
EDLC is chosen as the primary application in this present work because of its higher energy
71
density than conventional electrostatic capacitor, higher power density than rechargeable
72
batteries and longer cycle life (Sun & Yuan, 2009). Although the same polymer system has been
73
published in our previous paper (Ramesh et al., 2011), we focus on the temperature–dependence
74
conductivity study, structural, and thermal properties of the biopolymer electrolytes in the
75
present work. EDLC using the same polymer system has also been reported in our previous
76
published paper (Liew et al., 2014). However, the objective of this work is different. The
77
previous published paper reported the effect of anion of ionic liquids onto the EDLC. On the
78
other hand, the main objective of this present work is to investigate the effect of ionic liquid onto
79
the biopolymer electrolytes and the electrochemical performances of EDLC.
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2
Experimental
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2.1
Materials Biopolymer electrolytes were prepared by means of solution casting technique.
87
Biodegradable corn starch (Sigma–Aldrich), lithium hexafluorophosphate (LiPF6) (Aldrich) and
88
1–butyl–3–methylimidazolium
89
employed as host polymer, salt and ionic liquid, respectively.
(BmImPF6)
91
2.2
were
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(Sigma–Aldrich)
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hexafluorophosphate
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Preparation of thin films
Biopolymer electrolytes were prepared by solution casting method. PF 1, PF 5, PF 6 and
93
PF 8 were the designations for the polymer membranes with adulteration of 10 wt.%, 50 wt.%,
94
60 wt.% and 80 wt.% of BmImPF6, respectively. On the contrary, PF 0 was expressed as sample
95
without inclusion of ionic liquid where the ratio of corn starch to LiPF6 is 80:20. Different
96
stoichiometric quantities of corn starch and LiPF6 were initially dissolved in 20 mL of distilled
97
water. BmImPF6 was thus doped into the polymer solution to prepare ionic liquid–based
98
biopolymer electrolyte. The solution was then stirred overnight at 80 °C to ensure the complete
99
dissolution of all materials in a closed system. The resulting solution was cast on a glass Petri
100
dish and dried in an oven at 80 °C to eliminate the distilled water. Solid–state biopolymer
101
electrolytes were eventually produced.
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102 103
2.3
Characterizations of Biopolymer Electrolytes
104
2.3.1
Temperature dependence–ionic conductivity studies
105
The prepared samples were subjected to ac–impedance spectroscopy. The thickness of the
106
samples was measured using micrometer screw gauge. The ionic conductivity of the samples was
5 Page 5 of 32
determined by HIOKI 3532–50 LCR HiTESTER, over a frequency range between 50 Hz and 5
108
MHz at a voltage bias of 10 mV. The ionic conductivity of biopolymer electrolytes was
109
measured from ambient temperature to 80 °C. Polymer electrolyte films were mounted on the
110
holder under spring pressure with two stainless steel (SS) blocking electrodes.
ip t
107
111 2.3.2
Differential scanning calorimetry (DSC)
cr
112
DSC analysis was evaluated using TA Instrument Universal Analyzer 200 comprising a
114
DSC Standard Cell FC as main unit and Universal V4.7A software. Samples weighing 2–3 mg
115
were hermetically sealed in the aluminum Tzero pans while an empty hermetically sealed
116
aluminum pan was used as a reference cell. A hole was punched on the pan to remove the
117
trapped moisture. As a preliminary step, the samples were heated from 25 °C to 105 °C to
118
remove any trace amount of water at a heating rate of 30 °C min-1 under nitrogen flow rate of 60
119
ml min-1. The samples were then maintained at 105 °C for 5 minutes to ensure complete
120
evaporation. The samples were thus equilibrated at 25 °C and subsequently heated to 70 °C. The
121
samples were then rapidly cooled to –40 °C and reheated to 70 °C at the pre–set heating rate. The
122
final heating scan in DSC thermogram was used to evaluate the glass transition temperature (Tg)
123
of biopolymer electrolytes.
124 125
2.3.3
Ac ce p
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us
113
X–ray diffraction (XRD) studies
126
Amorphous nature of biopolymer electrolytes was investigated by XRD method. The
127
XRD patterns were recorded on a Siemens D 5000 diffractometer with Cu–Kα radiation
128
(λ=1.54060 Å) over the range of 2θ = 5–80° at ambient temperature.
129
6 Page 6 of 32
130
2.3.4
Linear sweep voltammetry (LSV) The LSV responses of PF 0 and PF 5 were performed using CHI600D electrochemical
132
analyzer. This cell was studied at a scan rate of 5 mVs-1 with the configuration of stainless steel
133
(SS)/biopolymer electrolyte/SS in the potential range of ±3V. The sample interval was 0.001 V
134
with 2 seconds as rest time before the analysis.
ip t
131
2.4 Preparation of electrodes
us
136
cr
135
Symmetrical activated carbon–based EDLC electrodes were prepared by dip coating
138
technique. The carbon slurry was prepared by mixing 80 wt.% of activated carbon (Kuraray
139
Chemical Co Ltd., Japan, particle size is 5~20 µm, surface area is 1800~2000 m2g-1), 10 wt.% of
140
carbon black (known as Super P) and 10 wt.% of poly(vinylidene fluoride) (PVdF) (molecular
141
weight of 534000 gmol-1, Aldrich) in 1–methyl–2–pyrrolidone (Purity ≥ 99.5%, from Merck,
142
Germany). This mixture was stirred for several hours until a homogenous slurry with smooth
143
surface was obtained. The aluminum mesh electrode and the slurry were subjected to dip coater
144
for dip coating process. The coated electrodes were dried in an oven at 100 °C to eliminate the
145
solvent. The mass of electrode materials was determined. Symmetrical activated carbon
146
electrodes were used for EDLC fabrication in this present work.
147 148 149
2.5
Ac ce p
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137
EDLC fabrication and characterization EDLC
cell
was
prepared
in
the
configuration
of
electrode/biopolymer
150
electrolyte/electrode. EDLC cell was placed in a cell kit for further electrochemical
151
characterization. CHI600D electrochemical analyzer was used to analyze CV study of EDLC.
7 Page 7 of 32
152
EDLC cell was then evaluated at scan rate of 10 mVs-1 from –1 to +1 V with sample interval of
153
0.001 V. The specific capacitance (Csp) of EDLC was determined by the equation below:
Csp
i sm
(Equation 1)
ip t
154
where i is the average anodic–cathodic current (A), s is the potential scan rate (Vs-1) and m is the
156
average mass of active materials. The average mass of electrode materials is 0.02 g. The charge-
157
discharge study was accomplished using a Neware battery cycler. The EDLC was charged and
158
discharged at current of 1 mA. The EDLC is allowed to rest for 30 min prior to the measurement.
159
The specific discharge capacitance (Csp) was obtained from charge-discharge curves, according
160
to the following relation (Amitha et al., 2009):
161
Csp
an
us
cr
155
I m dV
M
dt
(Equation 2)
where I is the applied current (A); m is the average mass of electrode materials (including the
163
binder and super P); dV represents the potential change of the discharging process excluding the
164
internal resistance drop occurring at the beginning of the cell discharge; and dt is the time
165
interval of the discharging process. The dV/dt is determined from the slope of the discharge
166
curve. The average mass of electrodes in the hexafluorophosphate system is around 0.045 g.
168 169
170
171
te
Ac ce p
167
d
162
Energy density (E, W h kg−1), power density (P, kW kg−1) and Coulombic efficiency (η, %) were evaluated from the equations below (Yu et al., 2012) :
E
Csp dV 2
P
2
1000 3600
(Equation 3)
I dV 1000 2 m
(Equation 4)
td 100% tc
(Equation 5)
8 Page 8 of 32
172
3
Results and discussion
173
3.1
Temperature dependent–ionic conductivity studies Figure 1 depicts Arrhenius fitted–temperature dependent ionic conductivity of PF 0, PF 5,
175
PF 6 and PF 8, in the temperature regime, from ambient temperature to 80 °C. The temperature
176
dependent study must be plotted in order to investigate the mechanism pertaining to ion
177
transportation in the polymer electrolytes. We fit the plot with Arrhenius theory as follows:
Ea A exp kT
(Equation 6)
us
178
cr
ip t
174
where A is a constant which is proportional to the amount of charge carriers, Ea is activation
180
energy, k is Boltzmann constant and T represents the absolute temperature in K. However, the
181
regression value of all plots is deviated from unity. Hence, we employ Vogel–Tamman–Fulcher
182
(VTF) equation to fit the plot, as illustrated in Figure 2. The ionic conductivity is expressed as
183
below using this volume–activated principle:
M
d
B exp T T
te
AoT
1 2
1 Ea k B AoT 2 exp T T
(Equation 7)
Ac ce p
184
an
179
185
where Ao represents pre–exponential constant proportional to the number of charge carriers, B
186
stands for a constant which is determined from the gradient of the plot (K-1), Ea is pseudo–
187
activation energy for conduction (eV), kb stands for Boltzmann constant (equivalent to
188
8.6173×10-5 eVK-1), T is the absolute temperature (K) and To represents ideal vitreous transition
189
temperature and it is suggested to be 50 K below the glass transition temperature (Tg) where Tg is
190
determined from DSC measurement. All the important parameters are summarized in Table 1.
191
Since the plot is well–fitted with this theory, we can conclude that the biopolymer electrolytes
192
exhibit VTF relationship which is related to free volume model. VTF theory infers that the ion
193
hopping mechanism in the polymer electrolytes is associated with high segmental motion of 9 Page 9 of 32
polymer chains in an amorphous phase. The experimental Tg that we obtained is extremely low
195
compared to the temperature range that we analyzed in this study. Therefore, the biopolymer
196
electrolytes are in a rubbery state within the temperature regime. As a result, the polymer chains
197
are flexible. So, the segmental mobility of polymer can be improved with these flexible chains.
198
Hence, the charge carriers can transport from a site to an empty site, along with high mobility of
199
polymer segments.
cr
ip t
194
The ionic conductivity of biopolymer electrolytes increases in this order: PF 0
201
8< PF 6 < PF 5. Upon addition of 50 wt.% BmImPF6, the highest ionic conductivity of
202
(1.47±0.02)×10−4 S cm−1 is achieved at ambient temperature (Ramesh et al., 2011). The ionic
203
conductivity is further increased to (1.99±0.02)×10−4 S cm−1 by heating the sample at 80 °C.
204
Upon addition of ionic liquids, the ionic conductivity of biopolymer electrolyte is improved
205
significantly. The increase in ionic conductivity of biopolymer electrolyte is mainly ascribed to
206
the strong plasticizing effect of ionic liquids. Strong plasticizing effect of ionic liquid aids to
207
soften the polymer backbone and hence increases the flexibility of polymer chain. The ions can
208
be transported easily within the polymer matrix with highly flexible polymer chains. Besides,
209
higher flexibility of polymer chains improves the mobility of polymer segments and assists the
210
ionic transport in the polymer complexes. Consequently, the ionic conductivity of ionic liquid–
211
added polymer electrolyte is higher compared to the ionic liquid–free polymer electrolyte.
212
Moreover, ionic liquid increases the degree of amorphous of the polymer matrix. Ionic liquid
213
could weaken the transient coordinative bonds among the molecules in the crystalline region and
214
thus turn the polymer chains into flexible characteristic leading to higher amorphous degree of
215
polymer complexes. In addition, the mobility of charge carriers in amorphous region is faster
216
than in crystalline region. High amorphous region of the polymer electrolyte can form rapid ion
Ac ce p
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200
10 Page 10 of 32
conduction process and result high ionic conductivity for ionic liquid–added polymer electrolyte
218
due to the unordered arrangement of macromolecules in the amorphous region. Moreover, the
219
physicochemistry of ionic liquid such as viscosity and dielectric constant contributes to the
220
increase in ionic conductivity. Lower viscosity of ionic liquid also enhances the polymer chain
221
flexibility. Therefore, this flexible polymer backbone could interrupt the ion–polymer bonding
222
and promote the ionic conducting process (Kumar et al., 2011). In contrast, high dielectric
223
permittivity of ionic liquid plays an important role to separate the ion pairs and/or ion aggregates
224
with high self–dissociating properties. More mobile cations are consequently produced which
225
leads to higher ionic conductivity (Kumar et al., 2011).
an
us
cr
ip t
217
This corn starch–based polymer electrolyte is a promising candidate as a separator in the
227
electrochemical devices as it achieves higher ionic conductivity than other biopolymer
228
electrolytes. Buraidah and co–workers prepared biopolymer electrolytes based on chitosan–
229
poly(vinyl alcohol) (PVA) and ammonium iodide (NH4I). The highest ionic conductivity of
230
1.77×10−6 S cm−1 is achieved with addition of 45 wt.% of NH4I (Buraidah et al., 2011). The
231
results obtained from this present work is two orders of magnitude higher than Buraidah’s work.
232
Rice starch has also been used as host polymer in electrolyte preparation. The ionic conductivity
233
of this plasticized–rice starch–based polymer electrolyte is slightly lower than this present work
234
where the maximum ionic conductivity of 1.1×10−4 S cm−1 is obtained with adulteration of 30
235
wt.% of glycerol (Marcondes et al., 2010). Corn starch–based polymer electrolytes using
236
different salt were also prepared by our group members. The ionic conductivity of this polymer
237
electrolyte is still slightly lower than this work (Teoh et al., 2014). Therefore, we can conclude
238
that PF 5 is a good material as an electrolyte in electrochemical cell, especially EDLC.
Ac ce p
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11 Page 11 of 32
In order to probe the ion dynamic of biopolymer electrolytes further, pseudo–activation
240
energy (Ea) of polymer electrolytes has to be determined by fitting the plots into the VTF
241
empirical formula. PF 5 exhibits the lowest Ea value as shown in Table 1. This result implies that
242
ionic conduction in PF 5 is the most favorable compared to other polymer systems. PF 0 portrays
243
the highest Ea value because of the absence of ionic liquid. The coordinative bonding in PF 0 is
244
much stronger than that of ionic liquid–based biopolymer electrolytes. The Ea value can be
245
lowered with adulteration of ionic liquid. The ionic liquid is immobilized within starch granules.
246
So, the transient hydrogen bonds could be broken down with the presence of ionic liquid as
247
aforesaid. The effect of ionic liquid becomes more visible by comparing PF 1 with other ionic
248
liquid–based biopolymer electrolytes. Among all the ionic liquid–based biopolymer electrolytes,
249
PF 1 demonstrates the maximum Ea value due to the insufficient ionic liquid. As a result, the
250
polymer network is still stronger. Therefore, the sample entails higher energy to break the
251
physical and chemical bonds in the polymer electrolyte systems. Nevertheless, the Ea values of
252
PF 6 and PF 8 are higher than PF 5. It may correlate to the excessive self–cross linkage within
253
the polymer matrices. Consequently, more energy is required to break and reform the
254
coordination bonds in the polymer compounds which coupled with low ionic conductivity. It can
255
be noticed that the higher the ionic conductivity of the sample, the lower is the Ea. This is also
256
supported by Ao as similar trend has been observed in Table 1. The number of charge carriers is
257
increased tremendously when we add 50 wt.% of ionic liquid into the polymer solution. Above
258
inclusion of this mass fraction of ionic liquid, the concentration of charge carriers is decreased
259
slightly. It is suggestive of formation of ion pairs or/and ion aggregates which forms the self–
260
cross linkages.
Ac ce p
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M
an
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cr
ip t
239
261
12 Page 12 of 32
262
3.2
Differential scanning calorimetry (DSC) Table 1 shows the Tg value obtained from DSC thermograms of PF 0, PF 1, PF 5, PF 6
264
and PF 8. The Tg of pure corn starch is around 55.71 °C as reported in our published work (Liew
265
et al., 2012). Sub–ambient Tg of –19.20 °C is observed with incorporation of lithium salt in PF 0
266
(Liew et al., 2012). The abrupt drop in Tg is suggestive of plasticizing effect of lithium salt. This
267
effect could destroy the transient coordinative bonds among the corn starch granules through
268
complexation and thus improve the flexibility of polymer chains. Tg drops further upon addition
269
of ionic liquid due to its strong plasticizing effect. Doping of ionic liquid weakens the dipole–
270
dipole interactions and thus destroys the transient interactive bonds among starch granules by
271
softening the polymer backbone. Therefore, it improves the mobility of polymer segments which
272
leads to flexible polymer chains (Baskaran et al., 2007). Among all the samples studied, PF 5
273
demonstrates the lowest Tg of –20.38°C. This discloses highly flexible polymer backbone which
274
facilitates the ionic transportation, asserting the highest ionic conductivity.
276
3.3
X–ray diffraction (XRD) studies
Ac ce p
275
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cr
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263
277
Figure 3 depicts the comparisons of XRD patterns of PF 0 with ionic liquid–based
278
biopolymer electrolytes. Two intense broad peaks have been observed at 17° and 22.2° in Figure
279
3 (a) (Liew et al., 2012). This reveals the semi–crystalline properties of PF 0. LiPF6 shows its
280
crystalline peaks at 2θ=8°, 21.7°, 25.3°, 28.3°, 35.3°, 37°, 42.8°, 49.3°, 55.9°, 60.6°, 61° and
281
68.4° as reported in our published paper (Liew et al., 2012). All these crystalline diffraction
282
peaks of LiPF6 are absent in PF 1 and PF 5 as can be seen in Figures 3 (b) and (c). This signifies
283
that LiPF6 is well–dissociated to form the complexation between corn starch and LiPF6.
284
However, some crystalline peaks are obtained at 35.8°, 42.8°, 49.3° and 68.4° as highlighted in
13 Page 13 of 32
Figures 3 (d) and (e) when we increase the ionic liquid mass loadings to 60 wt.% and 80 wt.%.
286
The existence of these peaks is evocative of formation of neutral ion pairs and ion aggregates. In
287
addition, this might be due to the reformation of lithium salt as PF6 anions are in excess. The PF6
288
anions may interact with Li cations again in this phenomenon. This may be due to the deficient
289
dissolution of the materials in the samples due to the excessive amount of PF6 anions. At high
290
concentration of ionic liquid, self–cross linkage and entanglements have been formed, as
291
explained earlier. Some salts may be trapped in the entanglements. An obvious change in
292
crystallographic organization in the polymer membrane is also observed upon addition of the
293
ionic liquid, i.e. the peak intensity is significantly lowered down. This verifies that impregnation
294
of ionic liquid weakens the interactions within the polymer chains and hence disrupts the ordered
295
arrangement of polymeric network. Therefore, it helps in decreasing the crystallinity of the
296
polymer complexes. In other words, addition of ionic liquid increases the amorphous degree of
297
the biopolymer electrolytes in the presence of plasticizing effect (Sivakumar et al., 2006).
te
d
M
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cr
ip t
285
For PF 1, two broad diffraction peaks are still showing at 2θ=16.6° and 23.9° in Figure 3
299
(b). However, upon further addition of ionic liquid, only one broad peak is observed in each
300
XRD pattern. The characteristic peak is attained at 2θ angles of 18.5°, 20.7° and 22° for PF 5, PF
301
6 and PF 8, respectively. This signifies that the samples have gone through the phase
302
transformation, from semi–crystalline to amorphous. Upon comparing all the ionic liquid–based
303
biopolymer electrolytes, it is noted that PF 5 illustrates the broadest peak with the lowest
304
intensity. This observation reflects the highest amorphous degree and eventually achieves the
305
highest ionic conductivity. Beyond inclusion of 50 wt.% of ionic liquid, the relative peaks
306
become more intense. The change in peak intensity is strongly related to the extreme self–cross
307
linkages within the polymer complexes. These cross linkages could lead to the intertwined
Ac ce p
298
14 Page 14 of 32
308
polymer chains which subsequently reduce the amorphous characteristic in the biopolymer
309
electrolytes.
310 3.4
Linear sweep voltammetry (LSV)
ip t
311
LSV is a vital study to determine the maximum operational potential of the polymer
313
electrolytes (Arof et al., 2012). Figures 4 (a) and (b) portray linear sweep voltammogram of PF 0
314
and PF 5, respectively. The anodic and cathodic peaks are absent in both figures. Therefore, it
315
can be concluded that there is no redox process in the potential range. The potential window of
316
ionic liquid–free biopolymer electrolyte is only 1.5 V that is from –1.2 V to 0.3 V, as
317
demonstrated in Figure 4 (a). Nevertheless, the stability window is found to have increased
318
significantly with addition of 50 wt.% of BmImPF6. From the voltammogram, PF 5 is stable up
319
to 2.9 V, from –1.4 V to 1. 5 V. Therefore, this result infers that the ionic liquid–based
320
biopolymer electrolyte is more suitable for EDLC application as ionic liquid–based biopolymer
321
electrolyte has higher electrochemical potential window than ionic liquid–free polymer
322
electrolyte.
324
3.5
Ac ce p
323
te
d
M
an
us
cr
312
Cyclic voltammetry (CV)
325
Cyclic voltammetry study was conducted to evaluate the capacitance of biopolymer
326
electrolyte–based EDLC. The capacitive behavior of EDLC comprising ionic liquid–based
327
polymer electrolyte is compared to that of ionic liquid–free polymer electrolyte as illustrated in
328
Figures 5 (a) and (b). Figure 5 (a) depicts CV profile of EDLC with PF 0 polymer electrolyte,
329
whereas the CV of EDLC containing the most conducting biopolymer membrane (PF 5) is
330
shown in Figure 5 (b). The redox processes are not detected in this potential region. This
15 Page 15 of 32
observation confirms the construction of electrical double layer capacitors in this present work.
332
No ideal rectangular shape is attained for both figures. This suggests the shortcoming of utilizing
333
solid polymer electrolytes in the supercapacitor, which is poor interfacial contact between the
334
electrolyte and electrode (Latham et al., 2002). An ill–defined box like shape with specific
335
capacitance of 0.18 Fg-1 is observed in Figure 5 (a). However, comparing Figure 5 (a) with 5 (b),
336
the shape of the voltammogram is greatly improved by doping ionic liquid into the polymer
337
electrolyte. The EDLC consisting of PF 5 shows a voltammogram approaching the ideal
338
rectangular (or box like) shape with specific capacitance of 36.79 Fg-1 in Figure 5 (b) (Hashmi et
339
al., 1997). Although the CV of this present work did not depict the perfect rectangular shape, the
340
specific capacitance of EDLC still show higher value than the literatures. Fabrication of EDLC
341
with glycerol plasticized–polymer electrolytes had been done by Shukur and coworkers. The
342
assembled EDLC showed specific capacitance of 33 Fg-1 which is lower than this present work
343
(Shukur et al., 2014). Another type of biodegradable polymer is also used to prepare biopolymer
344
electrolytes by our peers that is poly(vinyl alcohol) (PVA). Lim prepared EDLC using
345
biopolymer electrolytes based on PVA, lithium perchlorate (LiClO4) and antimony trioxide
346
(Sb2O3). The specific capacitance obtained from this EDLC fabricated with composite polymer
347
electrolyte is just 13–14.5 Fg-1 which is three folds lower than our present work (Lim et al.,
348
2014). The specific capacitance that we obtained in this work is higher than those EDLC using
349
existing polymer electrolytes. Therefore, it is noteworthy to conclude that corn starch–based
350
polymer electrolyte (or PF 5 in this present work) is a suitable candidate to be applied as
351
electrolyte in EDLC application as it has superior capacitive behavior.
Ac ce p
te
d
M
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us
cr
ip t
331
352
The abrupt increase in capacitance reveals that adulteration of ionic liquid into polymer
353
complexes can improve the electrochemical behavior of EDLC. This could be mainly attributed
16 Page 16 of 32
to the unique characteristic of ionic liquid. Since ionic liquid is comprised solely of ions, thus
355
more ions are transported in the conducting path. As a result, more charge carriers can be stored
356
in the electrical double layer at the electrolyte–electrode interface which contributes to higher
357
capacitive nature in the EDLC cell. Besides the increase in charge carrier concentration,
358
incorporation of ionic liquid enhances the flexibility of polymer backbone by softening the
359
polymer backbone. The flexible polymer chain not only promotes the ion dissociation from
360
native coordination, but also improves the mobility of charge carriers. Consequently, the buildup
361
of charge carriers at the electrolyte–electrode boundary is significantly enhanced when the ion
362
mobility and ion content are increased. This then leads to higher capacitance due to the excessive
363
and rapid accumulation of charge carriers.
3.6
Galvanostatic charge–discharge performance
d
365
M
364
an
us
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354
Figure 6 illustrates galvanostatic charge–discharge curve of EDLC with PF 5 over first
367
four charge and discharge cycles. The cell starts to be charged at 0.29 V instead of 0 V because
368
of the internal resistance of the EDLC. Similar observation is also attained where a small drop in
369
voltage is observed at the initial part of discharge curve. This ohmic loss arises from electrode
370
and electrolyte, such as charge transfer resistance and bulk resistance of polymer electrolyte
371
(Arof et al., 2012; Mitra et al., 2001). Cycability test is a vital characterization to determine the
372
stability of EDLC in terms of electrochemical performances after charging and discharging
373
processes. The specific capacitance, Coulombic efficiency, energy density and power density of
374
EDLC using the most conducting ionic liquid–added biopolymer electrolyte over 500 cycles are
375
listed in Table 2. The EDLC shows the specific capacitance of 37.07 Fg-1, energy density of 2.53
376
W h kg-1 and power density of 7.79 kW kg-1 with its Coulombic efficiency of 58% in the first
Ac ce p
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366
17 Page 17 of 32
377
cycle of charge and discharge cycle. The specific capacitance obtained from this study is
378
comparable with the findings in CV. Therefore, the capacitance value obtained from this EDLC
379
is reliable. The capacitance, energy density and power density are thus reduced moderately from
381
50th cycles to 300th cycles. Above 300th cycles of charge and discharge processes, these
382
parameters are decreased insignificantly. The reduction of capacitance, energy density and power
383
density over the cycle number is suggestive of depletion of polymer electrolyte and formation of
384
ion pairs/ion aggregates. The ions might be paired up upon the rapid charging and discharging
385
processes. Therefore, the number of mobile charge carriers transported in the polymer electrolyte
386
becomes lesser as the ion pairs can impede the ion migration. Hence, the ions accumulated at the
387
electrode–electrolyte boundaries to form an electrical double layer are getting lesser. Since the
388
ion adsorption at the electrode–electrolyte interface is not favorable, so this would decrease the
389
capacitance of EDLC.
4
Conclusion
Ac ce p
391
te
390
d
M
an
us
cr
ip t
380
392
Ionic liquid–added biopolymer electrolytes were prepared by solution casting technique.
393
Upon addition of 50 wt.% of BmImPF6, the biopolymer electrolytes achieved the highest ionic
394
conductivity of (1.99±0.02)×10-4 S cm-1 at 80 °C. The study on the temperature dependence of
395
ionic conductivity showed that these biopolymer electrolytes exhibit VTF behavior which is
396
associated with free volume theory. The lowest Tg of PF 5 implied the improvement in flexibility
397
of polymer backbone, which promotes the ionic decoupling process. Addition of ionic liquid also
398
improved the amorphourness of polymer electrolytes and widened the electrochemical potential
399
stability window of polymer electrolytes. EDLC based on ionic liquid–doped polymer electrolyte
18 Page 18 of 32
400
showed better electrochemical performance (i.e. higher specific capacitance) than that of ionic
401
liquid–free polymer electrolyte.
402 Acknowledgements
404
This
405
(UM.C/625/1/HIR/MOHE/SCI/21/1) from Ministry of Education, Malaysia. One of the authors,
406
Chiam–Wen Liew gratefully acknowledges the “Skim Bright Sparks Universiti Malaya”
407
(SBSUM) for financial support.
was
supported
by
the
High
Impact
Research
Grant
us
cr
work
ip t
403
Ac ce p
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M
an
408
19 Page 19 of 32
References
409
Amitha, F. E., Reddy, A. L. M., & Ramaprabhu, S. (2009). A non–aqueous electrolyte-based
410
asymmetric supercapacitor with polymer and metal oxide/multiwalled carbon nanotube
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electrodes. Journal of Nanoparticle Research, 11, 725–729.
ip t
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Aravindan, V., Lakshmi, C., & Vickraman P. (2009). Investigations on Na+ ion conducting
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polyvinylidenefluoride-co-hexafluoro-propylene/polyethylmethacrylate
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electrolytes. Current Applied Physics, 9, 1106–1111.
cr
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us
polymer
an
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Arof, A. K., Kufian, M. Z., Syukur, M. F., Aziz, M. F., Abdelrahman, A. E., & Majid, S. R.
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(2012). Electrical double layer capacitor using poly(methyl methacrylate)–C4BO8Li gel polymer
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electrolyte and carbonaceous material from shells of mata kucing (Dimocarpus longan) fruit.
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te
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Baskaran, R., Selvasekarapandian, S., Kuwata, N., Kawamura, J., & Hattori, T. (2007).
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Structure, thermal and transport properties of PVAc–LiClO4 solid polymer electrolytes. Journal
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of Physics and Chemistry of Solids, 68, 407–412.
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Cheng, H., Zhu, C., Huang, B., Lu, M., & Yang, Y. (2007) Synthesis and electrochemical
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ip t
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Jiang, J., Gao, D., Li, Z., & Su, G. (2006). Gel polymer electrolytes prepared by in situ
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Table captions:
536
Table 1: The obtained parameters from each VTF plot with the experimental glass transition temperature from DSC thermogram.
ip t
537
Table 2: The specific capacitance, Coulombic efficiency, energy density and power density of
539
EDLC fabricated using PF 5 polymer electrolyte over 500 cycles of charge and
540
discharge processes.
us
cr
538
541 Figure captions:
543
Figure 1: Arrhenius plot of ionic conductivity of PF 0, PF 1, PF 5, PF 6 and PF 8.
544
Figure 2: The temperature dependent–ionic conductivity of polymer electrolytes fitted with VTF
M
theory.
d
545
an
542
Figure 3: XRD patterns of (a) PF 0, (b) PF 1, (c) PF 5, (d) PF 6 and (e) PF 8.
547
Figure 4 (a): Linear sweep voltammogram of PF 0.
548
Figure 4 (b): Linear sweep voltammogram of PF 5.
549
Figure 5(a): Cyclic voltammogram of PF 0.
550
Figure 5(b): Cyclic voltammogram of PF 5.
551
Figure 6: Galvanostatic charge–discharge performances of ionic liquid–added biopolymer
553
Ac ce p
552
te
546
electrolyte over first four cycles.
26 Page 26 of 32
Table:
554
Log Ao
–4.301 –4.2557 –2.1032 –2.243 –2.3198 plot with
temperature from DSC thermogram.
Pre– exponential constant, Ao
Gradient Pseudo– of the activation plot, B energy, Ea (K-1) (meV) -5 5×10 0.1635 14.09 5.55×10-5 0.1499 12.92 -3 7.88×10 0.0988 8.51 5.71×10-3 0.1225 10.56 4.79×10-3 0.1334 11.50 the experimental glass transition
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Ideal glass Glass transition transition temperature, temperature, To (K) Tg (K) PF 0 0.9853 203.95 253.95 PF 1 0.9976 203.29 253.29 PF 5 0.997 202.77 252.77 PF 6 0.9893 202.98 252.98 PF 8 0.997 203.15 253.15 Table 1: The obtained parameters from each VTF
555
Regression value, R2
cr
Sample
us
553
an
556
Specific capacitance, Coulombic Energy density, E Power density, P Csp (Fg-1) efficiency, η (%) (W h kg-1) (kW kg-1) 0 37.07 58 2.53 7.79 50 31.78 60 1.99 7.48 100 26.17 62 1.60 7.39 150 22.24 63 1.09 6.60 200 17.11 69 0.69 5.98 250 14.83 70 0.55 5.75 300 14.35 71 0.53 5.60 350 13.90 73 0.48 5.56 400 13.08 75 0.43 5.43 450 13.05 76 0.42 5.34 500 13.02 77 0.41 5.32 Table 2: The specific capacitance, Coulombic efficiency, energy density and power density of
557 558 559 560
Ac ce p
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d
M
Cycles
EDLC fabricated using PF 5 polymer electrolyte over 500 cycles of charge and discharge processes.
27 Page 27 of 32
List of Figures
te
d
Figure 1: Arrhenius plot of ionic conductivity of PF 0, PF 1, PF 5, PF 6 and PF 8.
Ac ce p
561 562 563 564
M
an
us
cr
ip t
560
28 Page 28 of 32
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565 566 567
Figure 2:
568 569
Figure 3: XRD patterns of (a) PF 0, (b) PF 1, (c) PF 5, (d) PF 6 and (e) PF 8.
Ac ce p
te
d
The temperature dependent–ionic conductivity of polymer electrolytes fitted with VTF theory.
29 Page 29 of 32
an
us
cr
ip t
570 571 572
Figure 4 (a): Linear sweep voltammogram of PF 0.
575 576 577
Figure 4 (b): Linear sweep voltammogram of PF 5.
Ac ce p
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d
M
573 574
30 Page 30 of 32
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Figure 5 (a): Cyclic voltammogram of PF 0.
Ac ce p
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d
578 579 580
581 582 583 584
Figure 5 (b): Cyclic voltammogram of PF 5.
31 Page 31 of 32
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an 588
d
electrolyte over first four cycles.
te
587
Figure 6: Galvanostatic charge–discharge performances of ionic liquid–added biopolymer
Ac ce p
585 586
32 Page 32 of 32