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 NH 4 SCN an[r]
(1)Original Article
Studies of proton conducting polymer electrolyte based on PVA, amino
acid proline and NH4SCN
R Hemalathaa,c, M Alagara,b, S Selvasekarapandianc,d,*, B Sundaresana, V Monihaa,c aCentre for Research and Post Graduate Studies in Physics, Ayya Nadar Janaki Ammal College, Sivakasi, India
bPost Graduate Department of Physics, Mannar Thirumalai Naicker College, Madurai, India cMaterials Research Centre, Coimbatore, India
dDepartment 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
(2)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, ClO4, CH3COOcould 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 (NHỵ, Liỵ, Naỵ, Kỵ) 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 Hỵ 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 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 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 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 mV s1scan rate from to V using Biologic Science Instruments VSP - 300, France A primary proton battery using the highest ionic conductivity polymer membrane with the conguration of Zn ỵ ZnSO4.7H2O/75 PVA/25 Proline/0.5
NH4SCN/PbO2ỵ V2O5wasfinally constructed and their discharge
characteristics were studied 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
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],
(3)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,
cc¼ IC/IT 100% (1)
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 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 NH4SCN polymer electrolyte
3.2 DSC analysis
DSC was measured tofind the glass transition temperatures of the polymer electrolytes and the results are shown inFig 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
From Fig 2(c), it is observed that the 0.5 Mwt% NH4SCN with
Fig XRD and deconvoluted XRD patterns of (a) 75 PVA: 25 Proline (b) 75 PVA: 25 Proline: 0.4 (Mwt%) NH4SCN (c) 75 PVA: 25 Proline: 0.5 (Mwt%) NH4SCN (d) 75 PVA: 25 Proline: 0.6 (Mwt%) NH4SCN polymer electrolyte
Table
Diffraction peaks, percentage of crystallinity and glass transition temperature of PVAe Proline- NH4SCN polymer electrolytes
Compositions (PVA: Proline: NH4SCN) (Mwt%) Diffraction peak 2q(degree) Percentage of crystallinity (%) Glass transition temperature Tg(C)
75 PVA: 25 Proline 19.64 12.79 67
75 PVA: 25 Proline: 0.4 NH4SCN 22.66 10.86 45
75 PVA: 25 Proline: 0.5 NH4SCN 23.55 9.19 42
(4)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
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 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 (Hỵ) is more possible because NH4SCN dissociates to NH4ỵion and SCNion In
the tetrahedral ion NH4ỵ, as one of four protons attached to Nitrogen
atom is loosely bound, that proton (Hỵ) 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 (NH4SCN) doped electrolyte
Fig DSC thermograms of (a) 75Mwt% PVA: 25Mwt% Proline, (b) 75Mwt% PVA: 25Mwt% Proline: 0.4 (Mwt%) of NH4SCN, (c) 75 PVA: 25 Proline: 0.5 (Mwt%) of
(5)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
ZCPE¼ 1/Q0(ju)n (2)
where Q0and n are the frequency independent parameters n value
lies between and If n¼ 1, it denotes a pure capacitor If n ¼ 0, it represents a pure resistor[34] EIS parameters are given inTable The equivalent circuit for the system is given inFig The ionic conductivity values for different concentrations of salt NH4SCN
with75 PVA: 25 Proline polymer electrolytes at different temper-atures are listed inTable 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 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 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
The assignments of the peak positions of all the prepared polymer electrolyte systems
Vibrational peaks of the polymer electrolytes (cm1) Assignments
75 Mwt%PVA:25 Mwt% Proline 75 Mwt% PVA:25 Mwt% Proline: 0.4 Mwt%NH4SCN
75 Mwt% PVA:25 Mwt% Proline: 0.5 Mwt%NH4SCN
75 Mwt% PVA:25 Mwt% Proline: 0.6 Mwt%NH4SCN
3374 3381 3390 3399 Oe H stretching
3257 3202 3202 3184 Oe H stretching
2943 2943 2951 2943 Ce H (asymmetric stretching of CH2)
e 2064 2054 2048 Aromatic SeC≡N stretching
e 1715 1721 1722 C¼ O stretching
1615 1606 1606 1606 C¼ C stretching
1382 1409 1418 1409 CH3symmetric stretching
1283 1256 1256 1256 Ce O e C stretching
1157 1095 1095 1086 Ce C stretching
1041 1031 1031 1031 C¼ O stretching
933 933 942 942 Oe H bending
834 844 844 844 Ce H rocking
(6)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
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 The initial current Ii falls rapidly with time The transference number is
calculated by the following equation[22],
tỵẳ (Iie If)/Ii (3)
Table
Ionic conductivity value of PVA: Proline: NH4SCN polymer electrolytes at different temperatures Composition of PVA: Proline: NH4SCN (Mwt%) Ionic conductivitysdc(S/cm)
303K 313K 323K 333K 343K
75:25 7.66 105 1.32 104 1.7 104 2.21 104 2.58 104
75:25: 0.1 1.99 104 3.01 104 4.06 104 4.68 104 5.06 104
75:25: 0.2 2.67 104 3.48 104 4.43 104 5.19 104 5.74 104
75:25: 0.3 4.57 104 7.09 104 7.49 104 9.4 104 1.11 103
75:25: 0.4 7.51 104 8.95 104 1.15 103 1.37 103 1.56 103
75:25: 0.5 1.17 103 1.22 103 1.33 103 1.57 103 1.62 103
75:25: 0.6 7.78 104 9.44 104 1.21 103 1.43 103 1.58 103
Table
EIS parameters, activation energy and regression values for all prepared polymer electrolytes
Composition of PVA: Proline: NH4SCN (Mwt%)
Rb(ohm) CPE (mF) n Ea(eV) Regression value
75:25 42.52 9.56 0.6872 0.26 0.94
75:25: 0.1 11.31 7.87 0.8679 0.20 0.91
75:25: 0.2 7.89 8.24 0.8488 0.18 0.97
75:25: 0.3 7.06 6.99 0.9118 0.17 0.92
75:25: 0.4 3.96 7.78 0.8848 0.16 0.99
75:25: 0.5 2.66 7.09 0.8951 0.07 0.92
75:25: 0.6 74.39 8.19 0.7282 0.16 0.98
Bold lines mentioned the highest conducting sample
Fig Cole - Cole plot for the 75 Mwt% PVA: 25 Mwt% Proline: 0.5 Mwt% of NH4SCN polymer electrolyte at different temperatures
Fig Arrhenius plot for (a) 75 PVA: 25 Proline, (b) 75 PVA: 25 Proline: 0.1 (Mwt%) NH4SCN, (c) 75 PVA: 25 Proline: 0.2 (Mwt%) NH4SCN, (d) 75 PVA: 25 Proline: 0.3 (Mwt%) NH4SCN, (e) 75 PVA: 25 Proline: 0.4 (Mwt%) NH4SCN, (f) 75 PVA: 25 Proline: 0.5 (Mwt%) NH4SCN, (g) 75 PVA: 25 Proline: 0.6 (Mwt%) NH4SCN
(7)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 (Hỵ), cation and anions (SCN) for the polymer electrolytes are calculated using the following equations[22]
Dỵỵ Dẳ kTs/ne2 (5)
tỵẳ Dỵ/(Dỵỵ D)
nẳ Nr molar ratio of salt/Molecular weight of the salt Dỵẳ tỵ D
Dẳ D Dỵ (6)
where
Nẳ Avagadro number (6.023 1023)
r¼ density of the salt k¼ Boltzmann constant Tẳ absolute temperature tỵẳ cationic transference number
Dỵ, Dẳ diffusion coefcients of cation and anion respectively The ionic mobility of anions (SCN) and ionic mobility of the loosely bound proton of the ammonium ion (Hỵ), cation of the prepared polymer membranes have been calculated by using following equations,
mẳs/ne (7)
tỵẳmỵ/(mỵỵm) (8)
tẳ 1tỵ (9)
where
tis the anionic transference number e is the charge of the electron
mỵandmare 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 (Hỵ), cation for the prepared polymer electrolytes are listed inTable 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 (mỵ) 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 (Hỵ) 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 (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 conguration
of Zn ỵ ZnSO4$7H2O/75PVA:25Proline: 0.5 NH4SCN/PbO2
Zn-SO4$7H2O/75PVA:25Proline: 0.5 NH4SCN/PbO2ỵ V2O5was used to
Fig Variation of DC current as a function of time for the 75 Mwt% PVA: 25 Mwt% Proline: 0.5 Mwt% of NH4SCN polymer electrolyte
Table
The transport parameters of the prepared polymer membranes at 303K
Compositions (PVA: Proline: NH4SCN) (Mwt%) n (cm3) tion tele Dỵ(cm2s1) D(cm2s1) mỵ(cm2V1s1) m(cm2V1s1)
75:25: 0.4 4.13 1021 0.98 0.02 2.91 108 6 1010 1.19 107 3.0 109
75:25: 0.5 5.16 1021 0.99 0.01 3.66 10¡8 4 10¡10 1.4 10¡6 1.5 10¡8
75:25: 0.6 6.19 1021 0.98 0.02 3.12 109 7.28 1011 1.15 107 3 109
(8)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
nZnỵ ZnSO4.7H2O% Znnỵ1(SO4).(7 2n)
H2O.2n(OH)ỵ 2nHỵỵ 2ne
Cathode reaction
PbO2ỵ 4Hỵỵ 2e% Pb2ỵỵ 2H2O
V2O5ỵ 6Hỵỵ 2e% 2VO2ỵ 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 Ie V characteristics of (a) 75 Mwt% PVA: 25 Mwt% Proline and (b) 75 Mwt% PVA: 25 Mwt% Proline: 0.5 Mwt% of NH4SCN
Fig 10 Pe V characteristics of (a) 75 Mwt% PVA: 25 Mwt% Proline and (b) 75 Mwt% PVA: 25 Mwt% Proline: 0.5 Mwt% of NH4SCN
(9)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 ofmỵand Dỵare 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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