Studies of proton conducting polymer electrolyte based on pva amino acid proline and nh4scn

Monihaa,c a Centre for Research and Post Graduate Studies in Physics, Ayya Nadar Janaki Ammal College, Sivakasi, India b Post Graduate Department of Physics, Mannar Thirumalai Naicker Co

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Original Article

Studies of proton conducting polymer electrolyte based on PVA, amino

R Hemalathaa,c, M Alagara,b, S Selvasekarapandianc,d,*, B Sundaresana, V Monihaa,c

a Centre for Research and Post Graduate Studies in Physics, Ayya Nadar Janaki Ammal College, Sivakasi, India

b Post Graduate Department of Physics, Mannar Thirumalai Naicker College, Madurai, India

c Materials Research Centre, Coimbatore, India

d Department of Physics Bharathiar University, Coimbatore, India

a r t i c l e i n f o

Article history:

Received 26 November 2018

Received in revised form

14 January 2019

Accepted 19 January 2019

Available online 29 January 2019

Keywords:

XRD

DSC

FTIR

AC impedance analyzer

LSV

Primary proton battery

a b s t r a c t

Proton conducting polymer electrolytes based on PVA, amino acid proline and NH4SCN were prepared by the solution casting technique An increase in the amorphous nature of the polymer electrolytes was conﬁrmed by X-ray diffraction analysis DSC measurements showed a decrease in Tgwith increasing salt concentration The complex formation of the PVA/Proline/NH4SCN was investigated by FTIR analysis The highest ionic conductivity of 1.17 10-3S/cm for the 75 Mwt% PVA: 25 Mwt% Proline: 0.5 Mwt% of

NH4SCN polymer electrolyte at ambient temperature was obtained by using AC impedance technique Transference number measurements revealed the nature of the charge transport species in the polymer electrolyte Electrochemical stability window of 3.61 V was measured by using the linear sweep vol-tammetry for the highest ionic conducting (75 Mwt% PVA: 25 Mwt% Proline: 0.5 Mwt% of NH4SCN) polymer membrane A primary proton battery was constructed using the 75 Mwt% PVA: 25 Mwt% Proline: 0.5 Mwt% of NH4SCN polymer membrane and its performance was tested

© 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

The growing interest in solid polymer electrolytes in recent years

arises from the possibility of their technological application in solid

state ionic devices due to their easy processability,ﬂexibility, safety,

electrochemical stability and long life time[1] Among all solid

mer electrolytes, the synthesization of proton conducting solid

poly-mer electrolytes has drawn great attention because of its perspective

application in electrochemical devices, such as battery, fuel cell and

gas sensors etc[2] The development of polymer electrolytes with

high ionic conductivity at room temperature and good mechanical

stability is one of the main objectives in the polymers research Among

the existing polymers, Poly (vinyl alcohol) PVA is one of the most

biodegradable polymers It is a semi crystalline, hydrophilic polymer

with good chemical and thermal stability It is highly biocompatible

and is none toxic It is a water soluble polymer that readily reacts with

different crosse linking agents to form a gel[3] Mohan et al reported

the highest ionic conductivity value in the order of 105S/cm for

PVA-LiFePO4polymer electrolyte[4] Malathi et al reported the highest ionic conductivity value of 104 S/cm for PVA-LiCF3SO3 polymer electrolyte [5] Rajendran et al reported the maximum ionic con-ductivity value of 103S/cm for PVAe LiX, X ¼ CF3SO3, ClO4, BF4

polymer electrolyte[6]

Amino acids can be considered the building blocks of protein and are important for the human body Amino acids are classiﬁed into essential (dietary intake) and non essential amino acids (syn-thesized in the body through metabolic process) There are 20 amino acids; nine of which are called "essential" and eleven of which are labeled as"non-essential"[7] Essential amino acids can't

be produced by the body and must be derived from food Non-essential amino acids are synthesized by the human body Amino acids are very interesting materials for NLO and biomedical appli-cations [8,9] Generally, amino acids contain a proton donor carboxyl acid (-COO) group and the proton acceptor amino (-NH2) group From the literature survey, only countable work has been reported to study the interaction between the host polymer PVA with non-essential amino acids The amino acid proline with PVA possesses the maximum ionic conductivity of 105S/cm for the 75 Mwt% PVA: 25Mwt% Proline polymer electrolyte[10] The amino acid arginine with PVA has reached the maximum ionic

* Corresponding author Materials Research Centre, Coimbatore, 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.2019.01.004

Journal of Science: Advanced Materials and Devices 4 (2019) 101e110

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conductivity of 106S/cm for the 75 Mwt% PVA: 25Mwt% arginine

polymer electrolyte[11] The non-essential amino acid proline is

therefore selected for the present work

Ammonium salts are widely used in the proton conducting

polymer electrolytes because they are known to be good proton

donors [12] The ammonium salts with low lattice energy and

larger anionic size like SCN, I, CF3SO3 , ClO

4

, CH

3COOcould be used as dopants to have new polymer electrolytes with high ionic

conductivity SCNanion is known to be a linear anion and form

complex with alkali metal ions (NHþ, Liþ, Naþ, Kþ) through sulphur

or nitrogen atom Ammonium thiocyanate (NH4SCN) has a lattice

energy of 605 kJ/mol and can be easily dissociated into cation and

anion, when it is dissolved in the solvent (water) So NH4SCN

provides more ammonium ions to the polymer matrix[13] Vinoth

pandi et al reported the high ionic conductivity value of 103S/cm

for PVA: Glycine: NH4SCN polymer electrolytes [14] Hemalatha

et al reported the maximum ionic conductivity value of 104S/cm

for PVA: Proline: NH4Cl polymer electrolytes[15]

While various energy storage systems like, Lithium ion batteries,

Lead Acid batteries, NiMH batteries etc are available, development

of new low-cost, safe and environment friendly battery chemistry is

needed Proton batteries or H-ion batteries may be considered a

good alternative because of the small ionic radii of Hþ ions that

makes it suitable for better intercalation into the layered structure

of cathode, which is the preliminary requirement for a rechargeable

battery Moreover, the low cost of the electrode and electrolyte

materials and no associated safety issues, are noticeable advantages

that make the proton battery attractive for applications and

stimulate further fundamental research A proton source at the

anode and layered cathode materials, and electrolytes with

considerably high protonic conductivity is available but limited in

the literature[16]

The main purpose of this present work is to prepare the novel

proton conducting polymer electrolytes based on 75Mwt% PVA:

25Mwt% Proline with different concentrations of salt NH4SCN and

to investigate their structural, thermal, vibrational electrochemical

stability and electrical properties using XRD, DSC, FTIR, LSV and AC

impedance techniques The highest proton conducting polymer

membrane is used for the construction of a proton battery

2 Experimental

2.1 Materials and method of synthesis

Polymer Poly vinyl alcohol PVA (Sigma Aldrich) of average

molecular weight of 1,25,000 and amino acid proline of molecular

weight of 115.13 g/mol (LOBA CHEMIE) and salt NH4SCN were used

as raw materials Double distilled water was used as a solvent

75Mwt% of host polymer PVA and 25Mwt% of amino acid proline

were stirred continuously with a magnetic stirrer for several hours

to obtain a homogenous solution Then the different concentrations

of NH4SCN were dissolved individually in double distilled water

and these solutions were added to the 75 PVA: 25 Proline solution

and stirred well by using the magnetic stirrer to obtain a

homo-geneous mixture The solution was then casted in poly propylene

petridishes, and kept in oven under 60C to get transparent and

ﬂexible ﬁlms of thickness in the range of 0.08 mm to 0.31 mm

2.2 Characterization studies

X-ray diffraction patterns of the prepared samples were

recor-ded at room temperature on a Philips X0Pert PRO diffractometer

using CuKa radiation in the range of 2q¼ 10e90 Differential

Scanning Calorimetry (DSC) thermograms were recorded by using

DSC Q20 V4, 10 Build 122 at the heating rate of 10C/min under Nitrogen atmosphere in the temperature range of 30C - 246C FTIR spectra were recorded for the polymer electrolyteﬁlms using a SHIMADZU- IR Afﬁnity-1 Spectrometer in the range of 400 cm1to

4000 cm1 with the resolution of 1 cm1at room temperature Impedance measurements were carried out by using a computer controlled HIOKIe 3532 e 50 LCR HI e Tester in the frequency range between 42 Hz and 1 MHz over a temperature range of 303K

e 343K The transference numbers of polymer electrolytes corre-sponding to ionic (tion) and electronic (tele) were measured using Wagner's polarization technique with aluminium electrodes The electrochemical stability of 75 PVA: 25 Proline and 75 PVA: 25 Proline: 0.5 NH4SCN polymer electrolyte was examined by a linear sweep voltammetry (LSV) of the cell with a two electrode system Reference and counter electrode were connected together, which acted as one electrode and other as the working electrode The applied voltage was plotted on the x-axis and the resulting current

on the y-axis The sample was placed between two stainless steel blocking electrodes using 1 mV s1scan rate from 0 to 5 V using Biologic Science Instruments VSP - 300, France A primary proton battery using the highest ionic conductivity polymer membrane with the conﬁguration of Zn þ ZnSO4.7H2O/75 PVA/25 Proline/0.5

NH4SCN/PbO2þ V2O5wasﬁnally constructed and their discharge characteristics were studied

3 Results and discussion 3.1 XRD analysis

X eray diffraction (XRD) has been carried out to study the amorphous nature of polymer membranes of PVA: Proline:

NH4SCN.Fig 1(a), (b), (c) and (d) show XRD and deconvoluted XRD patterns of the 75Mwt% PVA: 25Mwt% Proline, 75Mwt% PVA: 25Mwt% Proline: 0.4 Mwt% NH4SCN, 75Mwt% PVA: 25Mwt% Pro-line: 0.5 Mwt% NH4SCN, and 75Mwt% PVA: 25Mwt% Proline: 0.6 Mwt% of NH4SCN polymer electrolytes, respectively

Rajendran et al found that the diffraction peaks at 2q¼ 19.5

and 22.3 for pure PVA polymer electrolyte[17] Siva devi et al observed the diffraction peaks at 2q¼ 19.6and 40.8for pure PVA

polymer electrolyte [18] From Fig 1(a), the intense crystalline peaks at the 2qangles of 19.64and 40.2are observed, which are attributed to the semi-crystalline feature of PVA and these peaks are slightly shifted in the salt added complex system (Fig 1(b), (c) and (d)) The diffraction peaks and the percentage of crystallinity of PVAe Proline- NH4SCN polymer electrolytes are given inTable 1 The amino acid proline exhibits few crystalline peaks at 2q¼ 15.18,

18.5, 19.64, 21, 22.8, 24.8, 25.97, 27.1, 28.61, 30.2etc [JCPDS ﬁle number 21e1805] The peaks corresponding to Proline at

2q¼ 29.61, 24.7and 27.6are observed fromFig 1(a),(b), which

may be due to the incomplete dissociation of the proline in the polymer electrolyteﬁlms It is observed inFig 1(c) that the 75Mwt% PVA: 25Mwt% Proline: 0.5 Mwt% NH4SCN polymer electrolyte has a high amorphous nature, which is revealed as the decrease in in-tensity and an increase in broadness of the XRD peaks when compared to the 75Mwt% PVA: 25Mwt% Proline polymer electro-lyteﬁlm This result can be interpreted by the criterion proposed by Hodge et al., which establishes a correlation between the intensity

of the peak and the degree of crystallinity[3] FromFig 1(d), it is observed that the intensity increases and the amorphous nature decreases for high salt concentrations of (75Mwt% PVA: 25Mwt% Proline: 0.6 Mwt% NH4SCN) polymer electrolyte, and the peak corresponding to NH4SCN is observed at 44.04[JCPDS 23e0029], which may be due to the incomplete dissociation of the salt in the polymer matrix[12]

R Hemalatha et al / Journal of Science: Advanced Materials and Devices 4 (2019) 101e110 102

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The degree of crystallinity (cc) of 75Mwt% PVA: 25Mwt% Proline

and XNH4SCN (X¼ 0.4, 0.5 and 0.6) molecular weight percentage of

salt added polymer electrolytes is determined using the following

equation,

where ICand ITare area under the crystalline peak and area under

all the peaks

The percentage of crystallinity for 75 PVA: 25 Proline, 75 PVA:

25 Proline: 0.4 Mwt% of NH4SCN, 75 PVA: 25 Proline: 0.5 Mwt% of

NH4SCN and 75 PVA: 25 Proline: 0.6 Mwt% of NH4SCN polymer

electrolytes are tabulated inTable 1 FromTable 1, it is seen that the

addition of NH4SCN salt concentration reduced the degree of

crystallinity The high amorphous nature along with a lower degree

of crystallinity is obtained for the 75 PVA: 25 Proline: 0.5 Mwt% of

NHSCN polymer electrolyte

3.2 DSC analysis DSC was measured toﬁnd the glass transition temperatures of the polymer electrolytes and the results are shown inFig 2 Curves (a), (b), (c) and (d) show the DSC thermograms of 75Mwt% PVA: 25Mwt% Proline, 75Mwt% PVA: 25Mwt% Proline: 0.4 Mwt% of

NH4SCN, 75Mwt% PVA: 25Mwt% Proline: 0.5 Mwt% of NH4SCN and 75Mwt% PVA: 25Mwt% Proline: 0.6 Mwt% of NH4SCN polymer electrolyte

Liew et al reported that the glass transition temperature Tgof pure PVA is 80.15C[19] Bhuvaneswari et al reported the glass transition temperature of 67.1C for the 75Mwt% PVA: 25Mwt% Proline [10] In the present work, due to the addition of salt

NH4SCN, the glass transition temperature is decreased The glass transition temperatures of polymer electrolyte PVA: Proline with different concentrations of salt (NH4SCN) are tabulated inTable 1 From Fig 2(c), it is observed that the 0.5 Mwt% NHSCN with

Fig 1 XRD and deconvoluted XRD patterns of (a) 75 PVA: 25 Proline (b) 75 PVA: 25 Proline: 0.4 (Mwt%) NH 4 SCN (c) 75 PVA: 25 Proline: 0.5 (Mwt%) NH 4 SCN (d) 75 PVA: 25 Proline: 0.6 (Mwt%) NH 4 SCN polymer electrolyte.

Table 1

Diffraction peaks, percentage of crystallinity and glass transition temperature of PVAe Proline- NH 4 SCN polymer electrolytes.

Compositions (PVA: Proline: NH 4 SCN) (Mwt%) Diffraction peak 2q(degree) Percentage of crystallinity (%) Glass transition temperature T g ( C)

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75Mwt% PVA: 25Mwt% Proline polymer electrolyte has a low glass

transition temperature Tg(42C), so it helps soften the polymer

backbone Similar results were observed for proton conducting

polymer electrolytes based on PVA with (NH4X, X¼ Br, Cl, NO3, I)

[20,21] For higher concentrations (75PVA:25 Proline: 0.6 NH4SCN),

Tgincreases which may be due to the reduced dipole interaction in

its homo polymers[22]

3.3 FTIR analysis

The PVA- Proline- NH4SCN based polymer electrolyte systems

are prepared and its complex formation has been studied by FTIR

The interactions between NH4SCN doped PVA/Proline polymer

electrolytes are also analyzed and discussed in detail.Fig 3depicts

the FTIR spectra of NH4SCN with PVA- Proline polymer electrolytes

The different peak positions and their assignments for the polymer

electrolytes are listed inTable 2

Coates reported that the stretching of hydroxyl (O-H) group

occurred between the regions of 3570e3200 cm1 It was also

mentioned that OH stretching occurred at 3615-3050 cm1[23]

The peak at 3374 cm1in 75Mwt% PVA: 25Mwt% Proline is ascribed

to hydroxyl group (O-H) stretching, and the hydroxyl band is

shif-ted towards lower wave number in the complexes, as shown in

Fig 3 The band at 3257 cm1assigned to Oe H stretching vibration

of 75Mwt% PVA: 25Mwt% Proline is shifted to lower frequency side

in the 0.4, 0.5 and 0.6 Mwt% of NH4SCN added polymer electrolyte

systems, respectively[24] The weak band at 2943 cm1in 75Mwt%

PVA: 25Mwt% Proline is due to C e H (asymmetric stretching of

CH2) vibration, which is shifted to lower wave number in the

XNH4SCN (0.4, 0.5 and 0.6 (Mwt%)) of salt added polymer

elec-trolytes, respectively[25] The new peak at 2064 cm1in 75Mwt%

PVA: 25Mwt% Proline: 0.4 Mwt% of NH4SCN is ascribed to aromatic

Se C ¼ N stretching of SCNgroup has been shifted to 2054 cm1,

2048 cm1for 0.5 and 0.6 Mwt% of salt added polymer electrolyte

systems, respectively[26] The characteristic peak of 75Mwt% PVA:

25Mwt% Proline: 0.4 Mwt% of NH4SCN at 1715 cm1that is assigned

to C¼ O stretching is shifted to 1721 cm1and 1722 cm1for 0.5

and 0.6 Mwt% of salt added polymer electrolyte systems,

respectively[27] The above mentioned peak has disappeared for the 75Mwt% PVA: 25Mwt% Proline polymer electrolyte The ab-sorption bands at 1615 cm1, 1382 cm1and 1283 cm1in 75Mwt% PVA: 25Mwt% Proline are ascribed to C¼ C stretching, CH3 sym-metric stretching and Ce O e C stretching vibrations, respectively With the addition of NH4SCN salt, the shifts in the vibrational peak positions are observed The characteristic vibrational peak of 75Mwt% PVA: 25Mwt% Proline at 1615 cm1which corresponds to

C¼ C stretching gets shifted to 1606 cm1for 0.4, 0.5 and 0.6 Mwt%

of salt added polymer electrolyte systems[10] The CH3symmetric stretching of 75Mwt% PVA: 25Mwt% Proline observed at 1382 cm1 gets shifted to 1418 cm1 for the 0.5 Mwt% of NH4SCN added polymer electrolyte system and 1409 cm1for 0.4 and 0.6 Mwt% of salt added polymer electrolyte systems[28] The frequency corre-sponding to C - O - C stretching of 75Mwt% PVA: 25Mwt% Proline at

1283 cm1is shifted to 1256 cm1 for 0.4, 0.5 and 0.6 Mwt% of

NH4SCN added polymer electrolyte systems, respectively[27] The two bands at 1157 cm1assigned to CeC stretching vibration and

1041 cm1assigned to C¼ O stretching vibration of 75Mwt% PVA: 25Mwt% Proline are shifted to lower frequency side in 0.4, 0.5 and 0.6 Mwt% of NH4SCN added polymer electrolyte systems, respec-tively The band at 933 cm1in 75Mwt% PVA: 25Mwt% Proline is ascribed to Oe H bending and is shifted to 934 cm1for the 0.4

Mwt% of NH4SCN added polymer electrolyte and 942 cm1for 0.5 and 0.6 Mwt% of NH4SCN added polymer electrolyte systems, respectively[10] The peak at 834 cm1in 75Mwt% PVA: 25Mwt% Proline is assigned to Ce H rocking vibration, which is shifted to

844 cm1 for 0.4, 0.5 and 0.6 Mwt% of NH4SCN added polymer electrolyte systems, respectively [14] In the prepared polymer sample of 75Mwt% PVA: 25Mwt% Proline with different concen-trations of ammonium salt, a lone proton migration (Hþ) is more possible because NH4SCN dissociates to NH4þion and SCNion In the tetrahedral ion NH4 þ, as one of four protons attached to Nitrogen

atom is loosely bound, that proton (Hþ) can migrate to each coor-dinating site of the polymer PVA The shift in peak position and the appearance of the new peak in the salt (NHSCN) doped electrolyte

Fig 2 DSC thermograms of (a) 75Mwt% PVA: 25Mwt% Proline, (b) 75Mwt% PVA:

25Mwt% Proline: 0.4 (Mwt%) of NH 4 SCN, (c) 75 PVA: 25 Proline: 0.5 (Mwt%) of

NH 4 SCN, and (d) 75 PVA: 25 Proline: 0.6 (Mwt%) of NH 4 SCN polymer electrolyte. Fig 3 Vibrational spectra of (a) 75 PVA: 25 Proline, (b) 75 PVA: 25 Proline: 0.4 (Mwt%)

NH 4 SCN, (c) 75 PVA: 25 Proline: 0.5 (Mwt%) NH 4 SCN, and (d) 75 PVA: 25 Proline: 0.6 (Mwt%) NH 4 SCN.

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system conﬁrms the complex formation between the polymer

(PVA), amino acid (proline) and salt (NH4SCN)

3.4 Impedance analysis

3.4.1 Cole-Cole plot

The ionic conductivities of PVA: Proline: NH4SCN systems are

derived from the complex impedance plots.Fig 4shows the

Cole-Cole plots for 75 PVA: 25 Proline and 75 PVA: 25 Proline: X NH4SCN

(X¼ 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) polymer electrolytes at room

temperature Normally, the complex impedance plot consists of a

high frequency semicircle region, which is due to the bulk effect of

the electrolyte and a low frequency spike due to the effect of

blocking electrodes In our case, the low frequency spike only

ap-pears, indicating that the resistive component only exists (Fig 4) in

the polymer electrolyte This suggests that the total conductivity is

due to the ion conduction[29]

The ionic conductivity of the polymer electrolytes can be

calculated by using the well known equation The bulk resistance

(Rb) values for the prepared polymer electrolytes can be calculated

from the low frequency spike intercept on the z0 axis, using EQ

software program developed by Boukamp[30] The impedance of

the constant phase element (ZCPE) is given as

where Q0and n are the frequency independent parameters n value lies between 0 and 1 If n¼ 1, it denotes a pure capacitor If n ¼ 0, it represents a pure resistor[34] EIS parameters are given inTable 4 The equivalent circuit for the system is given inFig 4 The ionic conductivity values for different concentrations of salt NH4SCN with75 PVA: 25 Proline polymer electrolytes at different temper-atures are listed inTable 3 It can be seen that the conductivity increases with increasing NH4SCN content from 0.1 to 0.5 Mwt% in

75 PVA: 25 Proline polymer electrolytes The increase in the ionic conductivity with increasing salt (NH4SCN) concentration can be related to the increase in the number of mobile charge carriers[22] The 75Mwt% PVA: 25Mwt% Proline: 0.5 Mwt% NH4SCN polymer electrolyte has a high ionic conductivity in the order of 103S/cm among the prepared polymer electrolytes The conductivity at 0.6 Mwt% of NH4SCN content in 75 PVA: 25 Proline decreases Higher salt concentrations resulted in the formation of ion aggregates, which hindered the movement of mobile free cations, hence the ionic conductivity decreased[12]

Fig 5represents the complex impedance plots for the highest conductivity sample (75Mwt% PVA: 25Mwt% Proline: 0.5 (Mwt%) of

NH4SCN) at different temperatures It has been observed that as the temperature increases the conductivity also increases due to the increase in mobility of charge carriers[31]

3.4.2 Temperature dependent conductivity The temperature dependent conductivity of 75Mwt% PVA: 25Mwt% Proline and 75Mwt% PVA: 25Mwt% Proline with (0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) Mwt% of NH4SCN polymer electrolyte is shown

in Fig 6 The conductivity of the polymer electrolytes increases with increase in temperature The regression values of the plots using a linearﬁt are found to be close to unity, suggesting that the temperature dependent ionic conductivity for all the complexes obeys Arrhenius relation

The activation energy (Ea) for all the polymer electrolytes is calculated by a linearﬁt of the Arrhenius plot The activation energy and regression values for all prepared polymer electrolytes are given inTable 4 The 75 Mwt% PVA: 25 Mwt% Proline: 0.5 Mwt%

NH4SCN polymer electrolyte shows a low activation energy of 0.07 eV, leading to the high ionic conductivity of 1.17 103S/cm at

303 K This proves that the polymer electrolyte is highly amorphous

in nature, which allows ionic motion in the polymer electrolytes The activation energy is found to decrease with increasing salt concentration up to 0.5 Mwt% of NH4SCN, which is due to the in-crease in the amorphous nature of the polymer electrolyte that

Table 2

The assignments of the peak positions of all the prepared polymer electrolyte systems.

75 Mwt%PVA:25 Mwt% Proline 75 Mwt% PVA:25 Mwt%

Proline: 0.4 Mwt%NH 4 SCN

75 Mwt% PVA:25 Mwt%

Fig 4 Cole- Cole plots for (a) 75 PVA: 25 Proline, (b) 75 PVA: 25 Proline: 0.1 (Mwt%)

NH 4 SCN, (c) 75 PVA: 25 Proline: 0.2 (Mwt%) NH 4 SCN, (d) 75 PVA: 25 Proline: 0.3

(Mwt%) NH 4 SCN, (e) 75 PVA: 25 Proline: 0.4 (Mwt%) NH 4 SCN, (f) 75 PVA: 25 Proline:

0.5 (Mwt%) NH 4 SCN, (g) 75 PVA: 25 Proline: 0.6 (Mwt%) NH 4 SCN polymer electrolytes

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facilitates the motion of proton in the polymer network[12] At

higher salt concentrations, the activation energy increases,

prob-ably due to the aggregation of ions[32]

3.4.3 Concentration dependent conductivity

The room temperature ionic conductivities and the activation

energy variations of PVA: Proline: NH4SCN polymer electrolyte is

presented inFig 7

FromFig 7the conductivity values increase with the increase of

salt NH4SCN concentration, due to the increase in the number of

mobile charge carriers The highest ionic conductivity sample has a

low activation energy of 0.07eV, among the prepared polymer

electrolytes This is due to decrease in the energy barrier of the

proton transport The decrease in conductivity at higher salt (75

Mwt% PVA: 25 Mwt% Proline: 0.6 Mwt% NH4SCN) concentrations

can be attributed to the formation of ion aggregates[12]

3.5 Transference number measurements Transference number measurements on a polymer electrolyte system were performed using Wagner's polarization technique The plot of polarization current vs time is shown inFig 8 The initial current Ii falls rapidly with time The transference number is calculated by the following equation[22],

Table 3

Ionic conductivity value of PVA: Proline: NH 4 SCN polymer electrolytes at different temperatures.

Composition of PVA: Proline: NH 4 SCN (Mwt%) Ionic conductivitysdc (S/cm)

Table 4

EIS parameters, activation energy and regression values for all prepared polymer

electrolytes.

Composition of PVA:

Proline: NH 4 SCN

(Mwt%)

R b (ohm) CPE (mF) n E a (eV) Regression

value

Bold lines mentioned the highest conducting sample.

Fig 5 Cole - Cole plot for the 75 Mwt% PVA: 25 Mwt% Proline: 0.5 Mwt% of NH 4 SCN

polymer electrolyte at different temperatures.

Fig 6 Arrhenius plot for (a) 75 PVA: 25 Proline, (b) 75 PVA: 25 Proline: 0.1 (Mwt%)

NH 4 SCN, (c) 75 PVA: 25 Proline: 0.2 (Mwt%) NH 4 SCN, (d) 75 PVA: 25 Proline: 0.3 (Mwt%) NH 4 SCN, (e) 75 PVA: 25 Proline: 0.4 (Mwt%) NH 4 SCN, (f) 75 PVA: 25 Proline: 0.5 (Mwt%) NH 4 SCN, (g) 75 PVA: 25 Proline: 0.6 (Mwt%) NH 4 SCN.

Fig 7 Variation of conductivity and activation energy of PVA: Proline: NH 4 SCN as a function of salt concentration.

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t¼ (If/Ii) (4)

where Iiis the initial current and Ifis theﬁnal current

The cationic transference number is found to be 0.99 for the

highest conducting polymer membrane The diffusion coefﬁcient of

the loosely bound proton of the ammonium ion (Hþ), cation and

anions (SCN) for the polymer electrolytes are calculated using the

following equations[22]

tþ¼ Dþ/(Dþþ D)

n¼ Nr molar ratio of salt/Molecular weight of the salt

Dþ¼ tþ D

where

N¼ Avagadro number (6.023 1023)

r¼ density of the salt

k¼ Boltzmann constant

T¼ absolute temperature

tþ¼ cationic transference number

Dþ, D¼ diffusion coefﬁcients of cation and anion respectively

The ionic mobility of anions (SCN) and ionic mobility of the

loosely bound proton of the ammonium ion (Hþ), cation of the

prepared polymer membranes have been calculated by using

following equations,

where

tis the anionic transference number

e is the charge of the electron

mþandmare the ionic mobility of cation and anion

n is the number of charge carriers related to the salt concentration

The diffusion coefﬁcient and mobility of anions (SCN) and loosely bound proton of the ammonium ion (Hþ), cation for the prepared polymer electrolytes are listed inTable 5 The diffusion coefﬁcients of the cations and anions of the 75 PVA: 25 Proline: 0.5 Mwt% NH4SCN polymer electrolyte are found to be 3.66 108and 4 1010(cm2/s) The cationic mobility (mþ) of the prepared polymer electrolyte is

found to be 1.4 106and anionic mobility (m) 1.5 108(cm2/V1/

s1) The ionic diffusion coefficient and mobility of the cation are greater than the diffusion coefficient and mobility of the anion The measurement of transference number confirms that the conductivity

is inﬂuenced by the diffusion coefﬁcient and mobility of the loosely bound proton of the ammonium ion (Hþ) cation

3.6 Linear sweep voltammetry study The study of an electrochemical stability window of a polymer electrolyte is necessary to check its usage in electrochemical devices Fig 9 (a),(b)shows the linear sweep voltammetry curves for 75 PVA:

25 Proline and 75 PVA: 25 Proline: 0.5 Mwt% of NH4SCN polymer electrolytes at room temperature The anodic decomposition limit of the polymer electrolyte is considered as the voltage at which the currentﬂows through the cells The 75 PVA: 25 Proline and 75 PVA:

25 Proline: 0.5 Mwt% of NH4SCN polymer electrolytes possess the electrochemical stability of 2.68 and 3.61V From the I - V curves, it is seen that the 75 PVA: 25 Proline: 0.5 Mwt% of NH4SCN polymer electrolyte has a high electrochemical stability The electrochemical stability of 1.0 g I-carrageenan: 0.4 wt % NH4NO3polymer electrolyte

is reported as 2.46V[13].Fig 10 (a),(b)shows the P - V curves for the

75 PVA: 25 Proline and 75 PVA: 25 Proline: 0.5 Mwt% of NH4SCN polymer electrolyte at room temperature From the P- V curves, the value of power increases with the increase of voltage, and the maximum power of 430 mW at 4.5V has been obtained for the 75 PVA: 25 Proline: 0.5 Mwt% of NH4SCN polymer electrolyte A maximum power of 144 mW has been observed for 75 Mwt% PVA:

25 Mwt% Proline: 0.3 Mwt% NH4Cl[15] Hence, this polymer elec-trolyte (75 PVA: 25 Proline: 0.5 Mwt% of NH4SCN) is compatible for applications in proton battery and electrochemical devices 3.7 Fabrication and characterization of a proton battery The sample exhibiting the highest conductivity (75 Mwt% PVA:

25 Mwt% Proline: 0.5 Mwt% of NH4SCN) with the conﬁguration

of Zn þ ZnSO4$7H2O/75PVA:25Proline: 0.5 NH4SCN/PbO2

Zn-SO4$7H2O/75PVA:25Proline: 0.5 NH4SCN/PbO2þ V2O5was used to

Fig 8 Variation of DC current as a function of time for the 75 Mwt% PVA: 25 Mwt%

Proline: 0.5 Mwt% of NH 4 SCN polymer electrolyte.

Table 5

The transport parameters of the prepared polymer membranes at 303K.

Compositions (PVA: Proline: NH 4 SCN) (Mwt%) n (cm3) t ion t ele Dþ(cm 2 s1) D(cm 2 s1) mþ (cm 2 V1s1) m (cm 2 V1s1)

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fabricate a proton battery For the battery anode, Zn (Merck Co.) and

ZnSO4$7H2O (Merck Co.) were mixed together and pressed with of

5 ton pressure to form a pellet The same procedure was done for

the cathode comprises of PbO2(Loba chemie), V2O5(Loba chemie)

and PVA: Proline: NH4SCN polymer electrolyte solution Graphite

was added during the preparation of cathode and anode to

intro-duce the electronic conductivity The highest ionic conducting 75

Mwt% PVA: 25 Mwt% Proline: 0.5 Mwt% of NH4SCN polymer

membrane was sandwiched between the anode and cathode in a

battery holder The schematic diagram of the fabricated battery is

shown inFig 11

The anode and cathode reactions are given below:

Anode reaction

nZnþ ZnSO4.7H2O% Znn þ1(SO4).(7 2n)

H2O.2n(OH)þ 2nHþþ 2ne

Cathode reaction

PbO2þ 4Hþþ 2e% Pb2þþ 2H2O

V2O5þ 6Hþþ 2e% 2VO2þ 3H2O

The theoretical oxidation potential of Zn is known to be0.7618V and the reduction potential of PbO2is known to be 1.455V[16] The overall reaction should provide the cell with E0¼ 2.2168V However,

Fig 9 I e V characteristics of (a) 75 Mwt% PVA: 25 Mwt% Proline and (b) 75 Mwt% PVA: 25 Mwt% Proline: 0.5 Mwt% of NH 4 SCN.

Fig 10 P e V characteristics of (a) 75 Mwt% PVA: 25 Mwt% Proline and (b) 75 Mwt% PVA: 25 Mwt% Proline: 0.5 Mwt% of NH 4 SCN.

Fig 11 Schematic diagram of the battery conﬁguration.

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in this work the cell Zn/ZnSO4.7H2O//Polymer electrolyte//PbO2/

V2O5gives a voltage of 1.61V The open circuit voltage (OCV) is 18%

lower than the theoretical value 2.21V The difference between the

theoretical and the experimental open circuit voltages may be due to

the possible reduction of the ZnSO4.7H2O at the anode[33], which

results in a reduced cumulative voltage being provided by the

elec-trodes reaction The measured value of OCV of the studied cell is

1.61V, as shown inFig 12 This value has dropped to ~1.5V in theﬁrst

25hrs of assembly The cell voltage is observed to have stabilized and

the OCV remained constant at 1.5V for 15hrs Nirmala Devi et al

re-ported 1.48V (OCV) for Dextrine with 40 mol% NH4SCN biopolymer

electrolyte[29].Fig 13shows the discharge curve of the fabricated

proton battery using 75PVA: 25Proline: 0.5 NH4SCN at room

tem-perature The stabilized voltage of 1.61V was allowed to discharge

through the constant load of 1MU After applying the load, the

voltage has dropped to 1.45V which remains constant for 35hrs

Initial voltage drop may be due to the polarization The important cell

parameters are given below

Thickness of the celle 0.291 cm, Cell weight e 1.38 g, OCV e

1.61V

4 Conclusion The proton conducting polymer electrolytes based on polymer PVA, amino acid proline and the ionic dopant NH4SCN were pre-pared by the solution casting technique and characterized by various spectroscopic techniques The XRD patterns conﬁrmed the amorphous nature of the polymer electrolytes DSC studies indi-cated that the glass transition temperature was low (42C) for the

75 Mwt% PVA: 25 Mwt% Proline: 0.5 Mwt% of NH4SCN polymer electrolyte system A vibrational analysis revealed the complexa-tion of the (PVA/Proline/NH4SCN) polymer electrolytes The highest ionic conductivity was found to be 1.17 103S/cm for 0.5 Mwt% of

NH4SCN added 75 Mwt% PVA: 25 Mwt% Proline polymer electro-lyte This polymer electrolyte has an activation energy of 0.07 eV The transference number measurements indicated that the PVA: Proline: NH4SCN polymer electrolyte is a proton conductor where the values ofmþand Dþare found to be higher than those ofmand

D The electrochemical stability window of 3.61V is observed for the highest ionic conducting polymer membrane The primary proton battery was fabricated and its performance was tested References

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