Characterization of biopolymer electrolytes based on cellulose acetate with magnesium perchlorate (Mg(ClO4)2) for energy storage devices

A primary magnesium battery has been constructed using the maximum conductivity biopolymer electrolyte (40%CA:60% Mg(ClO 4 ) 2 ).. Magnesium metal in pellet form was taken as the.[r]

Original Article

Characterization of biopolymer electrolytes based on cellulose acetate

M Mahalakshmia,b,c, S Selvanayagama, S Selvasekarapandianb,d,*, V Monihab,e,

R Manjuladevib,f, P Sangeethab

a PG & Research Dept of Physics, Govt Arts College, Melur, 625 106, India

b Material Research Center, Coimbatore, 641 045, India

c Department of Physics, Sri Meenakshi Govt Arts College for Women(A), Madurai, 625 002, India

d Department of Physics, Bharathiar University, Coimbatore, 641 046, India

e Centre for Research and PostGraduate Studies in Physics, Ayya Nadar Janaki Ammal College, Sivakasi, 625 124, India

f Department of Physics, SNS College of Engineering, Coimbatore, 641 107, India

a r t i c l e i n f o

Article history:

Received 27 January 2019

Received in revised form

14 April 2019

Accepted 21 April 2019

Available online 28 May 2019

Keywords:

Biopolymer electrolyte

Cellulose acetate

Magnesium perchlorate

ac impedance technique

Mgþion primary battery

a b s t r a c t Magnesium ion conducting biopolymer electrolytes have been prepared using cellulose acetate and different wt % of magnesium perchlorate with DMF as a solvent by the solution casting technique As-prepared membranes were subjected to different characterization techniques such as XRD, FTIR, DSC,

ac impedance analysis and transference number measurement The amorphous/crystalline nature of the prepared biopolymers was studied by using XRD FTIR study has revealed the formation of complexes between the cellulose acetate and the magnesium perchlorate Glass transition temperatures for the biopolymer electrolytes were found using a differential scanning calorimeter From the ac impedance analysis, the ionic conductivity was calculated The biopolymer membrane (40%CA: 60% Mg (ClO4)2) has shown the highest conductivity of 4.05 104S/cm at room temperature The ionic transference number

of Mg2þ was found as 0.31 by the Evan's method The electrochemical stability of 3.58 V has been observed for the 40%CA:60%Mg(ClO4)2biopolymer membrane by the linear sweep voltammetry study The Mgþ ion primary battery has been constructed using the highest ionic conducting biopolymer membrane The performance of the battery was studied and the open circuit voltage of the battery was found as 1.9 V

© 2019 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

1 Introduction

Biopolymers are natural polymers from renewable resources,

produced by a living organism They contain monomeric units that

are covalently bonded to form a large structure[1] Biodegradable

polymers have been the center of enormous worldwide attention,

as a potential of white pollution[2] Biopolymer electrolytes (BPEs)

are low cost, environmentally green, and suitable to be used as a

host polymer compared to the synthetic polymer electrolytes[3]

for the momentous development of BPE in many electrochemical

devices, such as batteries, fuel cells, sensors, supercapacitors, and

display devices, etc.[4]

Solid Biopolymer Electrolytes (SBPEs) receive more attention due to its non-leakage, high ionic conductivity, long-term structural stability, good thermal, mechanical and electrical stability There are three types of biopolymers, namely polysaccharides, polyesters, and polyamides which are naturally produced by microorganisms

[5] Among the polysaccharide biopolymers, cellulose acetate (CA) has got many advantages, such as excellent transparency, low cost, non-toxic nature, biodegradability and biocompatibility[6] CA is a semi-crystalline biopolymer It is not soluble in water The chemical properties of CA are unique since it contains the carboxyl groups (C¼O) in its structure It has got a good film-forming property because of the intermolecular hydrogen bondings CAfilms are homogeneous and have high mechanical strength The above mentioned properties are essential for any ionic conducting membranes to construct the energy storage devices like a battery, and also various intensive electrochemical applications[7] Despite

* Corresponding author Material Research Center, Coimbatore, 641 045, India.

E-mail address: sekarapandian@rediffmail.com (S Selvasekarapandian).

Peer review under responsibility of Vietnam National University, Hanoi.

Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

https://doi.org/10.1016/j.jsamd.2019.04.006

2468-2179/© 2019 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license

Journal of Science: Advanced Materials and Devices 4 (2019) 276e284

the advantages, it has a high crystalline nature which gives the

lowest ionic conductivity that limits the applications of this

poly-mer as electrolyte To overcome the drawbacks, CA has been doped

with different salts

Rani M et al have studied biopolymer electrolytes based on the

derivatives of cellulose from Kenaf Bastfiber[8] Abidin et al have

reported electrochemical studies on cellulose acetate-LiBOB

poly-mer gel electrolytes [9] Proton conducting solid bio-polymer

electrolytes based on carboxymethyl cellulose doped with oleic

acid has been studied by M N Chai and M IN Isa[10] Selvakumar

et al have carried out a research on biodegradable polymer CA

doped with lithium perchlorate (LiClO4) for supercapacitors [11]

Properties of CA membrane with lithium salt (LiTFSI) have been

studied by Ramesh et al.[12] The dielectric behavior of CA

com-plexes with ammonium tetrafluoroborate (NH4BF4)& polyethylene

glycol (PEG) has been reported by Harun et al.[13] S Monisha et al

has studied CA with ammonium salts (NH4SCN)& (NH4NO3) and

the energy storage lithium battery has been constructed using CA

with lithium nitrate (LiNO3)[14e16]

According to a survey, it is observed that the study of the

magnesium ion conducting electrolyte is scarce when compared to

Hþ/Naþ/Liþ[17] Magnesium metal has several advantages such as

low cost, high safety, low equivalence weight, and high reduction

potential when compared to lithium[18] Magnesium-based

bat-teries perform very closely to lithium-based ones Magnesium

batteries have turned up as an alternate for the next rank batteries

due to the intrinsic advantage of the Mg metal Magnesium battery

is an electro-deposited battery that works efficiently, without any

dendrite growth[19] Owing to the divalent property of Mg2þ, this

battery can provide a higher theoretical volumetric capacity

(3832 mAh∙cm3) than Li (2062 mAh∙cm3) So, Mg batteries are

spirited for energy storage devices[20]

The incorporation of Mg (ClO4)2enhances the ionic conductivity

of PVP to 1.1  104 S/cm at room temperature as reported by

Mangalam et al.[21] Kumar Y et al have reported a conductivity

value of 5.6 104S/cm for PEO when complexed with a

magne-sium salt[22] Manjuladevi et al studied a membrane, based on

PVA: PAN/MgCl2 for energy storage devices[23] Shanmugapriya

et al have studied the biopolymer I-carrageenan with magnesium

perchlorate and reported the conductivity value 2.18 103S/cm

[24] Shukur et al have studied the conductivity and dielectric

properties of potato starch doped with magnesium acetate[25]

Hambali et al have reported the plastic crystalline gel polymer

electrolytes based on poly (Vinylidene chloride-Co-Acrylonitrile)

doped with magnesium triflate (MgTf)[26]

Only a few works have reported on a biopolymer with

magne-sium salts To the best of our knowledge, there has been no work

based on cellulose acetate with magnesium salts

In this work magnesium ion conducting biopolymer electrolytes

were prepared using the cellulose acetate and magnesium

perchlorate with DMF as a solvent by the solution casting

tech-nique The prepared membranes were then characterized by

various techniques, namely XRD, FTIR, DSC, and ac impedance

analysis Furthermore, the ionic transference number has been

evaluated by the Evan's method and a primary magnesium battery

has been constructed using the highest conduction electrolyte

2 Materials and experimental methods

Polymer cellulose acetate (CA) from Sigma Aldrich with average

Mn ¼ 50,000 by GPC, p.code: 1001345528 and magnesium

perchlorate Mg(ClO4)2 of the molecular weight of 223.21 g/mol

from Himedia were used without any further purification to

pre-pare the biopolymer electrolytes Dimethylformamide (DMF) with

molecular weight 73.08 g/mol, density¼ 0.948e0.949 kg/m3from Merck specialties private Ltd., Mumbai, India was used as a solvent Different concentrations of CA and Mg(ClO4)2were separately added with DMF solvent at room temperature and stirred contin-uously with a magnetic stirrer for several hours to obtain clear solutions Then both the solutions were mixed together and stirred continuously for several hours to get a homogeneous solution Finally, the solutions were poured into polypropylene Petri dishes and allowed to dry at 60C for 2 days in a vacuum oven for the evaporation of the solvent It yielded a stable free-standingfilm of thicknesses ranging from 180 to 200mm Thefilms were stored in vacuum desiccators Pure CA, 60%CA:40%Mg(ClO4)2, 50%CA:50% Mg(ClO4)2, 40%CA:60%Mg(ClO4)2, and 30%CA:70%Mg(ClO4)2 films were prepared by this method Then, the samples were subjected to various characterization techniques

XRD patterns of polymer membranes were taken on the Philips X'pert PRO diffractometer, where x-rays of 1.5406Aowavelength generated by a Cu-Ka source was uitilized The diffraction peaks were recorded at a 2qangle varied from 10to 90 The biopolymer electrolytes were subjected to the FTIR study using the BRUCKER spectrophotometer in the wave number ranges from 400 to

4500 cm1with a resolution of 1 cm1 The DSC Q20V24.10 Build

122 TA instrument was used to conduct the DSC measurements to analyze the glass transition temperature of the sample The impedance measurements of the biopolymer electrolytes were made in the frequency range of 42 Hz to 1 MHz at room temper-ature using the HIOKI 3532-50LCR HiTESTER The transport number

of the Mg2þions for the highest conducting electrolyte was eval-uated using the Evan's and the Wagner's method The linear sweep voltammetry was used to evaluate the electrochemical stability of the highest conducting electrolytes A primary magnesium battery was constructed using the membrane with the highest ionic con-ductivity as the electrolyte, a magnesium metal plate as the anode and MnO2with graphite in the ratio of 3:1 in form of a pellet as the cathode

3 Results and discussion 3.1 XRD analysis

XRD measurements were performed on the biopolymer elec-trolytes to study their crystalline/amorphous nature.Fig 1shows the XRD patterns for the pure CA, 60%CA:40%Mg(ClO4)2, 50%

Fig 1 XRD patterns of pure CA, 60%CA:40%Mg(ClO 4 ) 2 , 50%CA:50%Mg(ClO 4 ) 2 , 40%

CA:50%Mg(ClO4)2, 40%CA:60%Mg(ClO4)2, and 30%CA:70%Mg(ClO4)2

electrolytefilms The peaks are observed for pure CA at 2q¼ 9, 13,

18, 27, and 65which agree with the research already reported

[12,27] The intensity of the peaks decreases with the increase of

the Mg(ClO4)2concentration, which reveals that the doping salt

enhances the amorphous nature of the electrolytes This result was

interpreted by the Hodge et al.[28]criterion, which proves a

cor-relation between the intensity of peak and the degree of the

crys-tallinity XRD patterns confirm the absence of the peaks

corresponding to Mg(ClO4)2in the biopolymer electrolytes which

indicates the complete dissociation of the salt in the polymer

ma-trix The maximum amorphous nature is observed for the sample of

40%CA:60%Mg(ClO4)2 Furthermore, at the addition of 60% of

Mg(ClO4)2, the salt gets recrystallized, which reduces the

amor-phous nature of the polymer matrix

3.2 FTIR analysis

FTIR spectroscopy was used to prove the complex formation and

the interactions between the biopolymer CA and the Mg(ClO4)2salt

by means of the change in the vibrational modes of the electrolyte

under investigation.Fig 2shows the FTIR spectra for the pure CA

and the CA: Mg(ClO4)2complexes of various compositions in the

wavenumber range from 400 to 4000 cm1 The corresponding

vibration frequencies were assigned and are listed inTable 1

The band at about 3372 cm1is assigned to the OeH stretching

of the pure CA The peak observed at 1740 cm1is assigned to the

C¼O stretching in the aldehyde carbonyl group of CA[29] The

medium intensity peak 1374 cm1is attributed to the CeH bending

in the alkanes of the pure CA The absorption peak observed at

1221 cm1for pure CA is assigned to the CeO stretching of the ester

group The peak determined for pure the CA at 1034 cm1 is

assigned to the CeOeC stretching of a pyrose ring and the medium

intensity peak observed at 906 cm1 is assigned to the OeH

bending vibrational mode of the pure CA

The addition of 40, 50, 60, 70, wt % of Mg(ClO4)2to CA produces

changes in the intensity, the shape, and the position of the bands

This implies that the complex reaction occurred due to oxygen from

the ester group[30] The broad peak observed around 3372 cm1

and assigned to the stretching vibration of the hydroxyl groups of

the pure CA gets shifted and widened in the salt added systems

This reveals the coordination between the cations of the salt and the hydroxyl groups of CA The hydroxyl band is shifted towards the higher wavenumber in the salt compositions indicating the specific interaction between the salt and the polymer[31]

The band appeared around 1740 cm1 in the pure CA and assigned to C¼O stretching of CA is decreased in intensity due to the incorporation of Mg(ClO4)2 The decrease in the intensity of the peak indicates the formation of the ion-dipole C¼O Mg2 þcomplex

[32] The new peak appeared at 1646 cm1in the salte biopolymer spectra is assigned to the C¼O stretching in the carbonyl group due

to the interaction of the dopant and the polymer The peaks observed at 1374 cm1, 1221 cm1, and 1034 cm1are attributed to the CeH bending, the CeO stretching and the CeOeC stretching of pure CA, respectively Their intensity is decreased and the wave-number position is shifted towards the increasing value for various complexes of Mg(ClO4)2 This indicates the completed formation of complexes of the biopolymer with the salt at different concentra-tions The peak observed around 608 cm1in all doped samples are ascribed to the CeCl stretching peak, which confirms the presence

of the Clion in the CA-doped Mg(ClO4)2electrolytes[33]

Fig 3 illustrates the possible interaction between CA and Mg(ClO4)2 The interactions between the pure CA and Mg(ClO4)2via

a carboxyl group, i.e ClO4 , imply that the ions are mobile in the

system The high mobility of ions favors the highest ionic conduc-tivity This predicts that Mg(ClO4)2has the potential to function as the charge carrier in the system The weakly bounded Mg2þion can hop through the coordinating site of the C¼O host polymer and the conduction process takes place [34] These results explain the complex formation of the host polymer with the salt

3.3 Differential scanning calorimetry (DSC) study The thermal analysis using a differential scanning calorimeter (DSC) was executed to observe the change in the glass transition temperature of the biopolymer electrolyte (BPE) system Fig 4

shows the DSC thermograms of the pure CA, 60%CA:40% Mg(ClO4)2, 50%CA:50%Mg(ClO4)2, 40%CA:60%Mg(ClO4)2, and 30% CA:70%Mg(ClO4)2 The Tgvalue of the pure CA is 83.4C A similar Tg

value has been previously reported by Monisha et al The Tgvalue of the CA doped with Mg(ClO4)2at different concentrations is slightly shifted towards lower temperatures The Tg values obtained are 81.7C, 70.40 C, and 64.52 C for the complexes of 60%CA:40% Mg(ClO4)2, 50%CA:50%Mg(ClO4)2, and 40%CA:60%Mg(ClO4)2, respectively The decreasing Tgvalue is pointing out an effect of the Mg(ClO4)2on softening the complex formation due to the plasti-cizing effect of the magnesium salt on the biopolymer structure It

is useful for the magnesium ion to mobilize within the membrane

[35] The glass transition temperatures for the various components

of CA with Mg(ClO4)2are listed inTable 2 The further increase in the salt concentration for the 30% CA:70%Mg(ClO4)2electrolyte causes an increase in the value Tgto 72.35C This may occur due to the presence of the undissoci-ated salt in the polymer matrix[23] Similar results have been reported by Mangalam et al [21] for the composition 50% PVA:50%PVP with 25% Mg(ClO4)2, and Manjuladevi et al.[23]for the composition 92.5PVA:7.5PAN:0.5 mm% MgCl2system 3.4 Impedance analysis

The Cole-Cole plots for the pure CA and for CA: Mg(ClO4)2 polymer electrolytes with different dopant molar ratios at room temperature (303 K) in the equivalent circuit are shown inFig 5 The graph shows the semicircular portion of the high-frequency region It arises from a parallel combination of the bulk resistance

of the cell with a capacitor and shows the linear region with a slight

Fig 2 FTIR spectrum for pure CA, 60%CA:40%Mg(ClO 4 ) 2 , 50%CA:50%Mg(ClO 4 ) 2 , 40%

films.

M Mahalakshmi et al / Journal of Science: Advanced Materials and Devices 4 (2019) 276e284 278

curvature that occurs in the low-frequency region due to the effect

of the blocking electrodes[36] A slight depression of the semicircle

is observed in the plot for all the compositions, indicating the

non-Debye nature of the electrolytes and the distribution of the

relax-ation times[37] The bulk resistance (Rb) of the polymer

electro-lytes has been calculated from the Cole-Cole plot using the

Boukamp software[38] The ionic conductivity (s) was calculated

using the equation:

where t and A are thickness and the area of the polymer

electro-lytes, respectively

The value of the ionic conductivity was calculated using Eq.(1)

for all compositions at room temperature and the results are listed

inTable 3 The highest ionic conductivity at room temperature was obtained as 4.05 104S/cm for the biopolymer electrolyte with

40%CA:60%Mg(ClO4)2ratio Generally, ionic conductivity is given by the equation

where‘n’ is the number of charge carriers, ‘e’ is the charge and ‘m’ is the mobility of the charge carriers When the numbers of charge carriers increased, the conductivity increases The ionic conduc-tivity of the pure CA is 3.1 109S/cm The addition of Mg(ClO4)2to

a polymer increases the number of charge carriers in such a way that the salt Mg(ClO4)2dissociates into Mg2þand ClO4ions, thereby producing more charge carriers When 60 wt % of Mg(ClO4)2 is added to the polymer, the maximum number of charge carriers is produced So, the conductivity reaches the maximum value of 4.05 104S cm1 It has been enhanced by an increase of the

Table 1

FTIR assignments of all prepared biopolymer electrolytes.

Wavenumbers (cm1)

Pure CA 60%CA:40%Mg(ClO 4 ) 2 50%CA:50%Mg(ClO 4 ) 2 40%CA:60%Mg(ClO 4 ) 2 30%CA:70%Mg(ClO 4 ) 2 Assignments

1034 1040 1042 1042 1042 CeOeC Stretching of pyrose ring

Fig 3 Possible interaction between CA and Mg(ClO 4 ) 2

Fig 4 (a) DSC thermogram and the glass transition temperature (T g ) for pure CA and 60%CA:40%Mg(ClO 4 ) 2 (b) DSC thermogram and the glass transition temperature (T g ) for 50%

Table 2 Glass transition temperature for various components of CA with Mg(ClO 4 ) 2

Sl No CA:Mg(ClO 4 ) 2 composition (%) Glass transition temperature ( o C)

amorphous nature The segmental motion of the biopolymer is

raised by the magnitude of the amorphous phase and it leads to the

higherflexibility of the biopolymer chain

It is observed that the further addition of the Mg(ClO4)2salt over

40%CA:60%Mg(ClO4)2 decreases the conductivity, due to the

for-mation of more ion aggregates in the polymer network[39] From

theFig 5it is inferred that the diameter of the semicircle gradually

decreases with the addition of the dopant The semicircular portion

is a combination of the bulk resistance and the bulk capacitance

The random dipole orientation of the polar side groups present in

the polymer network decreases the diameter of the semicircular

portion at the higher frequency region, which indicates the

non-capacitive nature of the polymer electrolytes

3.5 Frequency-dependent conductivity

The frequency dependence of the conductivity as a function of

frequency for the pure CA and the CA with different composition

of Mg(ClO4)2at room temperature is shown in Fig 6 There is a

low-frequency dispersion region in the plot which is attributed as

due to the electrodeeelectrolyte space charge polarization effects

[40], whereas the frequency independent plateau region

corre-sponds to dc conductivity (sdc) of the composition of the polymer

electrolytes From the ac conductivity spectra, it is observed that

the conductivity increases with the increase in the salt

composi-tion, which is attributed to the increase in the number of charge

carriers The sdc values for all the electrolytes (pure CA, 60%

CA:40%Mg(ClO4)2, 50%CA:50%Mg(ClO4)2, 40%CA:60%Mg(ClO4)2,

and 30%CA:70%Mg(ClO4)2) were calculated by extrapolating the

plateau region to the logsaxis The conductivity values obtained

from the conduction spectra coincide with the bulk conductivity

values received from the Cole-Cole plot

3.6 Dielectric studies The ionic transport phenomenon is characterized by using the dielectric properties of the pure CA, 60%CA:40%Mg(ClO4)2, 50% CA:50%Mg(ClO4)2, 40%CA:60%Mg(ClO4)2, and 30%CA:70% Mg(ClO4)2 polymer electrolytes as the frequency-dependent parameters Fig 7reveals that the dielectric parameters ε0 and

ε00increase at low frequencies due to the formation of the space

charge region at the electrodeeelectrode interface which is known as the un 1 variation or the non-Debye type behavior,

where the space charge regions with respect to frequency are explained in terms of the ion diffusion[41] At low frequencies, the dielectric constant (ε0) and the dielectric loss (ε00) are very

high due to the interfacial polarization and the free charge motion within the material[42] At high frequencies, the mobile ions are not able to orient themselves in thefield direction due

to the rapid periodic reversal of the applied electricfield, which leads to the saturation or to a decrease in the dielectric constant

[43] The values of the dielectric parameter are found to be increased with the Mg(ClO4)2 salt content in the polymer electrolytes

Fig 5 (a) ColeeCole plot for pure CA with the corresponding equivalent circuit (b) ColeeCole plots for 60%CA:40%Mg(ClO 4 ) 2 , 50%CA:50%Mg(ClO 4 ) 2 , 40%CA:60%Mg(ClO 4 ) 2 , and 30% CA:70%Mg(ClO 4 ) 2 with the corresponding equivalent circuit.

Table 3

Ionic conductivity values of CA doped with Mg(ClO 4 ) 2

Sl No CA:Mg(ClO 4 ) 2 composition (%) Ionic conductivity S$cm 1

1 Pure CA 3.1  10 9

2 60:40 4.97  10 6

3 50:50 3.88  10 5

4 40:60 4.05  10 4

5 30:70 2.6  10 4

Fig 6 Frequency dependence conduction spectra for pure CA, 60%CA:40%Mg(ClO 4 ) 2 , 50%CA:50%Mg(ClO 4 ) 2 , 40%CA:60%Mg(ClO 4 ) 2 , and 30%CA:70%Mg(ClO 4 ) 2

M Mahalakshmi et al / Journal of Science: Advanced Materials and Devices 4 (2019) 276e284 280

3.7 Transport number measurement

3.7.1 Wagner's dc-polarization technique

This technique is one of the fundamental methods to measure

the transference number which is used to identify whether the

conductivity in the biopolymer electrolyte is due to the presence of

ions or electrons The transference number is calculated by the

formula,

where Iiis the initial and Ifthefinal current

In this technique, a dc-potential of 1.5 V was applied across the

cell of the SS/40%CA:60%Mg(ClO4)2/SS configuration for

polari-zation and the polaripolari-zation current was monitored as a function of

time The initial total current decreases with time and reaches a

constant value in the fully depleted situation due to the depletion

of the ionic species in the biopolymer electrolyte[44] The cell is

polarized at a steady state and the currentflows across the

elec-trolyte interface because of the ion migration The total

trans-ference number (tion) for the maximum conductivity biopolymer

40%CA:60%Mg(ClO4)2is calculated to be 0.98 using the Eq.(2), which is close to unity, meaning that only a negligible contribu-tion comes from the electrons Hence, it is evident that the charge transport is mainly due to the ions

3.7.2 Evan's polarization technique The Evan's polarization technique was used to calculate the transport number (tþ) of the Mg2þions in the solid biopolymer The combination of the ac and dc polarization methods was applied on the Mg/40%CA:60%Mg(ClO4)2/Mg cell The cell was polarized by applying a dc-voltage of 1.5 V Then the initial (Io) and thefinal current (Is) values were derived from the currentetime plot as shown inFig 8 The cell resistance was recorded before and after the polarization by using the impedance measurement, and a graph plotted for corresponding values is shown inFig 9

The transport number of Mg2þis calculated using the formula

tþ ¼ IsðDV RoIoÞ = IoðDV RsIsÞ (4)

Fig 7 (a) Plot of loguvs ε ’ for BPEs with different concentrations of Mg(ClO 4 ) 2 (b) Plot of loguvs ε ” for BPEs with different concentrations of Mg(ClO 4 ) 2

Fig 8 D.C polarization curve of SS/40%CA:60%Mg(ClO 4 ) 2 /SS cell and SS/40%CA:60% Fig 9 Cole-Cole plot before and after polarization of a typical symmetric Mg/40%

where Ioand Isare the initial and thefinal current,DV is the applied

dc-voltage of 1.5 V, and Roand Rsare the cell resistance before and

after the polarization, respectively The value of the Mg2þ ion

transport number was calculated to be 0.31 for the highest

con-ductingfilm of the 40%CA:60%Mg(ClO4)2electrolyte

Shanmugap-riya et al have reported a value as 0.313 for the transport number of

the Mg2þ ions in their work for the carrageenan with the

0.6g Mg(ClO4)2electrolyte Similar results have been reported by

Mangalam et al for the composition of 50%PVA:50%PVP with 25%

Mg(ClO4)2 Manjuladevi et al have reported for the composition of

the 92.5PVA:7.5PAN:0.5 mm% MgCl2system a value of 0.38, and

Kumar et al have studied the PMMA-based GPE system with the

Mg(CF3SO3)2salt and reported a value of 0.33

3.8 Linear sweep voltammetry (LSV) The electrochemical stability of the highest conductivity polymer electrolyte 40%CA:60%Mg(ClO4)2was studied using the linear sweep voltammetry (LSV) with a two-electrode system The LSV was recorded for SS/40%CA: 60%Mg(ClO4)2/Mg at a scan rate of 5 mVs-1 The currentevoltage response curve is plotted in

Fig [10] The voltage at which the currentflows through the cell was taken as the anodic decomposition limit of the biopolymer electrolyte A sudden rise in the current is observed from the graph This shows the electrochemical stability window of 3.58 V This result reveals that the electrolyte could be used for its application in Mg ion batteries Osman et al have reported the stability window of 3.5 V for the SS/PVDF-HFP:20% Mg(CF3SO3)2/Mg and Manjuladevi et al also reported the sta-bility window of 3.66 V for the 92.5PVA:7.5PAN:0.3 mm% MgCl2

polymer electrolyte

3.9 Construction and performance of the Mg battery cell

A primary magnesium battery has been constructed using the maximum conductivity biopolymer electrolyte (40%CA:60% Mg(ClO4)2) Magnesium metal in pellet form was taken as the anode and MnO2mixed with graphite in form of a pellet acts as the cathode The highest conductivity BPE 40%CA:60%Mg(ClO4)2 has been sandwiched between the anode and the cathode in the bat-tery holder The schematic diagram of the fabricated batbat-tery is shown in Fig 11 a The initial open circuit voltage (OCV) was recorded as 2.12 V and the OCV was monitored with respect to time After 1 day the OCV was reduced slightly to 1.9 V, which remains at the same value for 7 subsequent days As it is shown inFig 11b, a small intermediate drop in battery voltage occurs after fabrication due to the cell formation reactions at the electrodes [45] The chemical reactions taking place in the battery cell are characterized

as the followings:

Fig 10 Linear sweep voltammetry of SS/40%CA:60%Mg(ClO 4 ) 2 /Mg cell recorded at a

scan rate of 5 mVs1at room temperature.

Fig 11 (a) Schematic diagram of battery configuration (b) Open circuit potential as a function of time for 40%CA:60%Mg(ClO 4 ) 2 biopolymer electrolyte (c) Discharge curves of cell using 100 KUfor 40%CA:60%Mg(ClO ) biopolymer electrolyte.

M Mahalakshmi et al / Journal of Science: Advanced Materials and Devices 4 (2019) 276e284 282

At the anode

Mg þ 2ðOHÞ / MgðOHÞ2 þ 2e

At the cathode

2MnO2 þ H2O þ 2e / Mn2O3 þ 2OH

Over all reaction

Mg þ 2MnO2 þ H2O/ MgðOHÞ2 þ Mn2O

The Hydroxyl ions present in the Mge MnO2battery may be

generated from the occluded moisture/H2O present in the

biopolymer membrane The occluded water is a type of

non-essential water that is retained due to the physical force in

micro-scopic pores, spaced irregularly throughout the solid biopolymer

CA[26]

An external load of 100 kUwas connected to the circuit for the

measurement of the battery discharge characteristics at room

temperature The discharge behavior of the battery with respect to

time is shown inFig 11c The electrical potential of the battery

decreases initially due to the polarization effect at the

electrolyteeelectrode interface [46] The battery potential

dis-charged at a constant load of 100 kUwas found to remain constant

at 1.68 V, which was observed for 7 days The battery cell

param-eters are listed inTable 4

4 Conclusion

The discovery of a new solid biopolymer electrolyte CA doped

with a various concentrations of Mg(ClO4)2was prepared by the

solution casting technique using DMF as a solvent The XRD

pat-terns reveal that the inclusion of Mg(ClO4)2increases the

amor-phous nature of the biopolymer electrolyte FTIR analysis confirms

the formation of the complex between CA and the magnesium ions

The DSC studies indicate that the glass transition temperature

de-creases with the increase of Mg(ClO4)2 salt concentration Ionic

conductivity of 4.05 104S/cm has been obtained for the 40%

CA:60%Mg(ClO4)2membrane using the ac-impedance analysis at

room temperature The dielectric study predicts the non-Debye

nature of the electrolyte membranes Using the Evans method,

the ionic transference number for Mgþhas been estimated as 0.31

for the 40%CA:60%Mg(ClO4)2electrolyte The electrochemical

sta-bility window has been determined by LSV as 3.58 V which is

sufficient for electrochemical applications Optimized highest ionic

conductivity membrane 40%CA:60%Mg(ClO4)2 was used to

construct a primary magnesium battery, on which the Open Circuit

Voltage (OCV) has been found as 1.9 V

Declaration of interest statement

As corresponding author I S Selvasekarapandian hereby declare

that myself and on behalf of all other authors this manuscript has

not been published in any other journal, and is not being submitted

to any other journal We don't have conflict of interest

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Table 4

Cell parameters.

Specification of cell parameters Values of cell parameters

Cell area (cm 2 ) 1.13

Cell weight (g) 1.15

Effective cell diameter (cm) 1.2

Cell thickness (cm) 0.248

Open circuit voltage (V) 2.12

Cut off potential (h) 168

Current drawn (mA) 24

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