Journal of Science: Advanced Materials and Devices (2019) 101e110 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Studies of proton conducting polymer electrolyte based on PVA, amino acid proline and NH4SCN R Hemalatha a, c, M Alagar a, b, S Selvasekarapandian c, d, *, B Sundaresan a, V Moniha a, c a Centre for Research and Post Graduate Studies in Physics, Ayya Nadar Janaki Ammal College, Sivakasi, India 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 b a r t i c l e i n f o a b s t r a c t Article history: Received 26 November 2018 Received in revised form 14 January 2019 Accepted 19 January 2019 Available online 29 January 2019 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 Tg with increasing salt concentration The complex formation of the PVA/Proline/NH4SCN was investigated by FTIR analysis The highest ionic conductivity of 1.17  10-3 S/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 voltammetry 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/) Keywords: XRD DSC FTIR AC impedance analyzer LSV Primary proton battery 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 polymer electrolytes, the synthesization of proton conducting solid polymer 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 non e toxic It is a water soluble polymer that readily reacts with different cross e linking agents to form a gel [3] Mohan et al reported the highest ionic conductivity value in the order of 10À5 S/cm for PVA- * Corresponding author Materials Research Centre, Coimbatore, India E-mail address: sekarapandian@rediffmail.com (S Selvasekarapandian) Peer review under responsibility of Vietnam National University, Hanoi LiFePO4 polymer electrolyte [4] Malathi et al reported the highest ionic conductivity value of 10À4 S/cm for PVA-LiCF3SO3 polymer electrolyte [5] Rajendran et al reported the maximum ionic conductivity value of 10À3 S/cm for PVA e 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 (synthesized 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 Nonessential amino acids are synthesized by the human body Amino acids are very interesting materials for NLO and biomedical applications [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 10À5 S/cm for the 75 Mwt% PVA: 25Mwt% Proline polymer electrolyte [10] The amino acid arginine with PVA has reached the maximum ionic https://doi.org/10.1016/j.jsamd.2019.01.004 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 (http://creativecommons.org/licenses/by/4.0/) 102 R Hemalatha et al / Journal of Science: Advanced Materials and Devices (2019) 101e110 conductivity of 10À6 S/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 À À larger anionic size like SCNÀ, IÀ, CF3SOÀ , ClO4 , CH3COO could be used as dopants to have new polymer electrolytes with high ionic conductivity SCNÀ anion 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 10À3 S/cm for PVA: Glycine: NH4SCN polymer electrolytes [14] Hemalatha et al reported the maximum ionic conductivity value of 10À4 S/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 homogeneous mixture The solution was then casted in poly propylene petridishes, and kept in oven under 60 C 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 recorded at room temperature on a Philips X0 Pert PRO diffractometer using CuKa radiation in the range of 2q ¼ 10 e90 Differential Scanning Calorimetry (DSC) thermograms were recorded by using DSC Q20 V4, 10 Build 122 at the heating rate of 10 C/min under Nitrogen atmosphere in the temperature range of 30 C - 246 C FTIR spectra were recorded for the polymer electrolyte films using a SHIMADZU- IR Affinity-1 Spectrometer in the range of 400 cmÀ1 to 4000 cmÀ1 with the resolution of cmÀ1 at room temperature Impedance measurements were carried out by using a computer controlled HIOKI e 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 corresponding 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 sÀ1 scan 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/PbO2 ỵ V2O5 was nally 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% Proline: 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 2q ¼ 19.5 and 22.3 for pure PVA polymer electrolyte [17] Siva devi et al observed the diffraction peaks at 2q ¼ 19.6 and 40.8 for pure PVA polymer electrolyte [18] From Fig 1(a), the intense crystalline peaks at the 2q angles of 19.64 and 40.2 are 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 in Table The amino acid proline exhibits few crystalline peaks at 2q ¼ 15.18 , 18.5 , 19.64 , 21, 22.8 , 24.8 , 25.97, 27.1, 28.61, 30.2 etc [JCPDS file number 21e1805] The peaks corresponding to Proline at 2q ¼ 29.61, 24.7 and 27.6 are observed from Fig 1(a),(b), which may be due to the incomplete dissociation of the proline in the polymer electrolyte films It is observed in Fig 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 intensity and an increase in broadness of the XRD peaks when compared to the 75Mwt% PVA: 25Mwt% Proline polymer electrolyte film 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] From Fig 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 (2019) 101e110 103 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 75 75 75 19.64 22.66 23.55 21.77 12.79 10.86 9.19 23.15 67 45 42 54 PVA: PVA: PVA: PVA: 25 25 25 25 Proline Proline: 0.4 NH4SCN Proline: 0.5 NH4SCN Proline: 0.6 NH4SCN The degree of crystallinity (cc) of 75Mwt% PVA: 25Mwt% Proline and XNH4SCN (X ¼ 0.4, 0.5 and 0.6) molecular weight percentage of salt added polymer electrolytes is determined using the following equation, cc ¼ IC/IT  100% (1) where IC and IT are 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 in Table From Table 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 to find the glass transition temperatures of the polymer electrolytes and the results are shown in Fig 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 Tg of pure PVA is 80.15 C [19] Bhuvaneswari et al reported the glass transition temperature of 67.1 C 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 in Table From Fig 2(c), it is observed that the 0.5 Mwt% NH4SCN with 104 R Hemalatha et al / Journal of Science: Advanced Materials and Devices (2019) 101e110 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 NH4SCN, and (d) 75 PVA: 25 Proline: 0.6 (Mwt%) of NH4SCN polymer electrolyte 75Mwt% PVA: 25Mwt% Proline polymer electrolyte has a low glass transition temperature Tg (42 C), so it helps soften the polymer backbone Similar results were observed for proton conducting polymer electrolytes based on PVA with (NH4X, X ¼ Br, Cl, NO3, I) [20,21] For higher concentrations (75PVA:25 Proline: 0.6 NH4SCN), Tg increases 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 depicts the FTIR spectra of NH4SCN with PVA- Proline polymer electrolytes The different peak positions and their assignments for the polymer electrolytes are listed in Table Coates reported that the stretching of hydroxyl (O-H) group occurred between the regions of 3570e3200 cmÀ1 It was also mentioned that OH stretching occurred at 3615-3050 cmÀ1 [23] The peak at 3374 cmÀ1 in 75Mwt% PVA: 25Mwt% Proline is ascribed to hydroxyl group (O-H) stretching, and the hydroxyl band is shifted towards lower wave number in the complexes, as shown in Fig The band at 3257 cmÀ1 assigned to O e 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 cmÀ1 in 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 electrolytes, respectively [25] The new peak at 2064 cmÀ1 in 75Mwt% PVA: 25Mwt% Proline: 0.4 Mwt% of NH4SCN is ascribed to aromatic S e C ¼ N stretching of SCNÀ group has been shifted to 2054 cmÀ1, 2048 cmÀ1 for 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 cmÀ1 that is assigned to C ¼ O stretching is shifted to 1721 cmÀ1 and 1722 cmÀ1 for 0.5 and 0.6 Mwt% of salt added polymer electrolyte systems, Fig Vibrational spectra of (a) 75 PVA: 25 Proline, (b) 75 PVA: 25 Proline: 0.4 (Mwt%) NH4SCN, (c) 75 PVA: 25 Proline: 0.5 (Mwt%) NH4SCN, and (d) 75 PVA: 25 Proline: 0.6 (Mwt%) NH4SCN respectively [27] The above mentioned peak has disappeared for the 75Mwt% PVA: 25Mwt% Proline polymer electrolyte The absorption bands at 1615 cmÀ1, 1382 cmÀ1 and 1283 cmÀ1 in 75Mwt% PVA: 25Mwt% Proline are ascribed to C ¼ C stretching, CH3 symmetric stretching and C e 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 cmÀ1 which corresponds to C ¼ C stretching gets shifted to 1606 cmÀ1 for 0.4, 0.5 and 0.6 Mwt% of salt added polymer electrolyte systems [10] The CH3 symmetric stretching of 75Mwt% PVA: 25Mwt% Proline observed at 1382 cmÀ1 gets shifted to 1418 cmÀ1 for the 0.5 Mwt% of NH4SCN added polymer electrolyte system and 1409 cmÀ1 for 0.4 and 0.6 Mwt% of salt added polymer electrolyte systems [28] The frequency corresponding to C - O - C stretching of 75Mwt% PVA: 25Mwt% Proline at 1283 cmÀ1 is shifted to 1256 cmÀ1 for 0.4, 0.5 and 0.6 Mwt% of NH4SCN added polymer electrolyte systems, respectively [27] The two bands at 1157 cmÀ1 assigned to C eC stretching vibration and 1041 cmÀ1 assigned to C ¼ 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, respectively The band at 933 cmÀ1 in 75Mwt% PVA: 25Mwt% Proline is ascribed to O e H bending and is shifted to 934 cmÀ1 for the 0.4 Mwt% of NH4SCN added polymer electrolyte and 942 cmÀ1 for 0.5 and 0.6 Mwt% of NH4SCN added polymer electrolyte systems, respectively [10] The peak at 834 cmÀ1 in 75Mwt% PVA: 25Mwt% Proline is assigned to C e H rocking vibration, which is shifted to 844 cmÀ1 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 concentrations of ammonium salt, a lone proton migration (Hỵ) is more possible because NH4SCN dissociates to NHỵ ion and SCN ion In ỵ the tetrahedral ion NH4 , as one of four protons attached to Nitrogen atom is loosely bound, that proton (Hỵ) can migrate to each coordinating site of the polymer PVA The shift in peak position and the appearance of the new peak in the salt (NH4SCN) doped electrolyte R Hemalatha et al / Journal of Science: Advanced Materials and Devices (2019) 101e110 105 Table The assignments of the peak positions of all the prepared polymer electrolyte systems Vibrational peaks of the polymer electrolytes (cmÀ1) 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 3257 2943 e 3381 3202 2943 2064 3390 3202 2951 2054 3399 3184 2943 2048 e 1615 1382 1283 1157 1041 933 834 1715 1721 1722 C ¼ O stretching 1606 1409 1256 1095 1031 933 844 1606 1418 1256 1095 1031 942 844 1606 1409 1256 1086 1031 942 844 C ¼ C stretching CH3 symmetric stretching C e O e C stretching C e C stretching C ¼ O stretching O e H bending C e H rocking 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 shows the ColeCole plots for 75 PVA: 25 Proline and 75 PVA: 25 Proline: X NH4SCN (X ¼ 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 appears, 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 z axis, using EQ software program developed by Boukamp [30] The impedance of the constant phase element (ZCPE) is given as Fig Cole- Cole plots 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 polymer electrolytes at room temperature ZCPE ¼ 1/Q0(ju)n O e H stretching O e H stretching C e H (asymmetric stretching of CH2) Aromatic SeC≡N stretching (2) where Q0 and n are the frequency independent parameters n value lies between and If n ¼ 1, it denotes a pure capacitor If n ¼ 0, it represents a pure resistor [34] EIS parameters are given in Table The equivalent circuit for the system is given in Fig The ionic conductivity values for different concentrations of salt NH4SCN with75 PVA: 25 Proline polymer electrolytes at different temperatures are listed in Table 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 10À3 S/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 represents 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 linear fit 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 linear fit of the Arrhenius plot The activation energy and regression values for all prepared polymer electrolytes are given in Table 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  10À3 S/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 increase in the amorphous nature of the polymer electrolyte that 106 R Hemalatha et al / Journal of Science: Advanced Materials and Devices (2019) 101e110 Table Ionic conductivity value of PVA: Proline: NH4SCN polymer electrolytes at different temperatures Ionic conductivity sdc (S/cm) Composition of PVA: Proline: NH4SCN (Mwt%) 303K 75:25 75:25: 75:25: 75:25: 75:25: 75:25: 75:25: 7.66 1.99 2.67 4.57 7.51 1.17 7.78 0.1 0.2 0.3 0.4 0.5 0.6        313K À5 10 10À4 10À4 10À4 10À4 10À3 10À4 1.32 3.01 3.48 7.09 8.95 1.22 9.44        323K À4 10 10À4 10À4 10À4 10À4 10À3 10À4 333K À4 1.7  10 4.06  10À4 4.43  10À4 7.49  10À4 1.15  10À3 1.33  10À3 1.21  10À3 343K À4 2.21  10 4.68  10À4 5.19  10À4 9.4  10À4 1.37  10À3 1.57  10À3 1.43  10À3 2.58 5.06 5.74 1.11 1.56 1.62 1.58        10À4 10À4 10À4 10À3 10À3 10À3 10À3 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 75:25: 0.1 75:25: 0.2 75:25: 0.3 75:25: 0.4 75:25: 0.5 75:25: 0.6 42.52 11.31 7.89 7.06 3.96 2.66 74.39 9.56 7.87 8.24 6.99 7.78 7.09 8.19 0.6872 0.8679 0.8488 0.9118 0.8848 0.8951 0.7282 0.26 0.20 0.18 0.17 0.16 0.07 0.16 0.94 0.91 0.97 0.92 0.99 0.92 0.98 Bold lines mentioned the highest conducting sample 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 Fig Cole - Cole plot for the 75 Mwt% PVA: 25 Mwt% Proline: 0.5 Mwt% of NH4SCN polymer electrolyte at different temperatures facilitates the motion of proton in the polymer network [12] At higher salt concentrations, the activation energy increases, probably due to the aggregation of ions [32] Fig Variation of conductivity and activation energy of PVA: Proline: NH4SCN as a function of salt concentration 3.4.3 Concentration dependent conductivity The room temperature ionic conductivities and the activation energy variations of PVA: Proline: NH4SCN polymer electrolyte is presented in Fig From Fig the 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 in Fig The initial current Ii falls rapidly with time The transference number is calculated by the following equation [22], tỵ ẳ (Ii e If)/Ii (3) R Hemalatha et al / Journal of Science: Advanced Materials and Devices (2019) 101e110 107 tỵ ẳ mỵ /(mỵ þ mÀ) (8) tÀ ¼ 1Àtþ (9) where tÀ is the anionic transference number e is the charge of the electron mỵ and m are the ionic mobility of cation and anion n is the number of charge carriers related to the salt concentration 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 tÀ ¼ (If/Ii) (4) where Ii is the initial current and If is the final 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] Dỵ ỵ D ẳ kTs/ne2 (5) tỵ ẳ Dỵ/(Dỵ ỵ D) n ¼ Nr  molar ratio of salt/Molecular weight of the salt Dỵ ẳ tỵ D D ẳ D Dỵ (6) where N ẳ Avagadro number (6.023 1023) r ¼ density of the salt k ¼ Boltzmann constant T ẳ absolute temperature tỵ ẳ cationic transference number Dỵ, DÀ ¼ diffusion coefficients of cation and anion respectively The ionic mobility of anions (SCNÀ) 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, m ¼ s/ne (7) The diffusion coefficient and mobility of anions (SCNÀ) and loosely bound proton of the ammonium ion (Hỵ), cation for the prepared polymer electrolytes are listed in Table 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  10À8 and 1010 (cm2/s) The cationic mobility (mỵ) of the prepared polymer electrolyte is found to be 1.4  10À6 and anionic mobility (mÀ) 1.5  10À8 (cm2/VÀ1/ sÀ1) 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 current flows 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 % NH4NO3 polymer 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 electrolyte (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/PbO2ZnSO4$7H2O/75PVA:25Proline: 0.5 NH4SCN/PbO2ỵ V2O5 was used to Table The transport parameters of the prepared polymer membranes at 303K Compositions (PVA: Proline: NH4SCN) (Mwt%) n (cm3) tion tele Dỵ (cm2 s1) D (cm2 s1) mỵ (cm2 V1 s1) m À (cm2 VÀ1 sÀ1) 75:25: 0.4 75:25: 0.5 75:25: 0.6 4.13  1021 5.16  1021 6.19  1021 0.98 0.99 0.98 0.02 0.01 0.02 2.91  10À8 3.66  10¡8 3.12  10À9  10À10  10¡10 7.28  10À11 1.19  10À7 1.4  10¡6 1.15  10À7 3.0  10À9 1.5  10¡8  10À9 Bold lines mentioned the highest conducting sample 108 R Hemalatha et al / Journal of Science: Advanced Materials and Devices (2019) 101e110 Fig I e 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 P e V characteristics of (a) 75 Mwt% PVA: 25 Mwt% Proline and (b) 75 Mwt% PVA: 25 Mwt% Proline: 0.5 Mwt% of NH4SCN fabricate a proton battery For the battery anode, Zn (Merck Co.) and ZnSO4$7H2O (Merck Co.) were mixed together and pressed with of 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 introduce 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 in Fig 11 The anode and cathode reactions are given below: Anode reaction nZn ỵ ZnSO4.7H2O % Znnỵ1(SO4).(7 2n) H2O.2n(OH) þ 2nHþ þ 2neÀ Cathode reaction PbO2 þ 4Hþ þ 2e % Pb2ỵ ỵ 2H2O V2O5 ỵ 6Hỵ ỵ 2e % 2VO2 ỵ 3H2O Fig 11 Schematic diagram of the battery configuration The theoretical oxidation potential of Zn is known to be À0.7618V and the reduction potential of PbO2 is known to be 1.455V [16] The overall reaction should provide the cell with E0 ¼ 2.2168V However, R Hemalatha et al / Journal of Science: Advanced Materials and Devices (2019) 101e110 109 Conclusion Fig 12 Open circuit voltage as a function of time for the 75 Mwt% PVA: 25 Mwt% Proline: 0.5 Mwt% of NH4SCN polymer electrolyte The proton conducting polymer electrolytes based on polymer PVA, amino acid proline and the ionic dopant NH4SCN were prepared by the solution casting technique and characterized by various spectroscopic techniques The XRD patterns confirmed the amorphous nature of the polymer electrolytes DSC studies indicated that the glass transition temperature was low (42 C) for the 75 Mwt% PVA: 25 Mwt% Proline: 0.5 Mwt% of NH4SCN polymer electrolyte system A vibrational analysis revealed the complexation of the (PVA/Proline/NH4SCN) polymer electrolytes The highest ionic conductivity was found to be 1.17  10À3 S/cm for 0.5 Mwt% of NH4SCN added 75 Mwt% PVA: 25 Mwt% Proline polymer electrolyte 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 of mỵ and Dỵ are found to be higher than those of mÀ and DÀ 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 Fig 13 Discharge curve of the cell using MU for the 75 Mwt% PVA: 25 Mwt% Proline: 0.5 Mwt% of NH4SCN polymer electrolyte in this work the cell Zn/ZnSO4.7H2O//Polymer electrolyte//PbO2/ V2O5 gives 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 electrodes reaction The measured value of OCV of the studied cell is 1.61V, as shown in Fig 12 This value has dropped to ~1.5V in the first 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 reported 1.48V (OCV) for Dextrine with 40 mol% NH4SCN biopolymer 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Kor Phys Soc 62 (2013) 311e319 S Sikkanthar, S Karthikeyan, S Selvasekarapandian, D Arunkumar, H Nithya, J Kawamura, Structural, electrical conductivity and transport analysis of PAN e NH4Cl polymer electrolyte system, Ionics 22 (2016) 1085e1094 ... to the aggregation of ions [32] Fig Variation of conductivity and activation energy of PVA: Proline: NH4SCN as a function of salt concentration 3.4.3 Concentration dependent conductivity 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... 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