Journal of Science: Advanced Materials and Devices (2019) 276e284 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Characterization of biopolymer electrolytes based on cellulose acetate with magnesium perchlorate (Mg(ClO4)2) for energy storage devices M Mahalakshmi a, b, c, S Selvanayagam a, S Selvasekarapandian b, d, *, V Moniha b, e, R Manjuladevi b, f, P Sangeetha b a PG & Research Dept of Physics, Govt Arts College, Melur, 625 106, India Material Research Center, Coimbatore, 641 045, India Department of Physics, Sri Meenakshi Govt Arts College for Women(A), Madurai, 625 002, India d Department of Physics, Bharathiar University, Coimbatore, 641 046, India e Centre for Research and PostGraduate Studies in Physics, Ayya Nadar Janaki Ammal College, Sivakasi, 625 124, India f Department of Physics, SNS College of Engineering, Coimbatore, 641 107, India b c a r t i c l e i n f o a b s t r a c t Article history: Received 27 January 2019 Received in revised form 14 April 2019 Accepted 21 April 2019 Available online 28 May 2019 Magnesium ion conducting biopolymer electrolytes have been prepared using cellulose acetate and different wt % of magnesium perchlorate with DMF as a solvent by the solution casting technique Asprepared membranes were subjected to different characterization techniques such as XRD, FTIR, DSC, ac impedance analysis and transference number measurement The amorphous/crystalline nature of the prepared biopolymers was studied by using XRD FTIR study has revealed the formation of complexes between the cellulose acetate and the magnesium perchlorate Glass transition temperatures for the biopolymer electrolytes were found using a differential scanning calorimeter From the ac impedance analysis, the ionic conductivity was calculated The biopolymer membrane (40%CA: 60% Mg (ClO4)2) has shown the highest conductivity of 4.05 Â 10À4 S/cm at room temperature The ionic transference number of Mg2ỵ was found as 0.31 by the Evan's method The electrochemical stability of 3.58 V has been observed for the 40%CA:60%Mg(ClO4)2 biopolymer membrane by the linear sweep voltammetry study The Mgỵ ion primary battery has been constructed using the highest ionic conducting biopolymer membrane The performance of the battery was studied and the open circuit voltage of the battery was found as 1.9 V © 2019 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Biopolymer electrolyte Cellulose acetate Magnesium perchlorate ac impedance technique Mgỵ ion primary battery Introduction Biopolymers are natural polymers from renewable resources, produced by a living organism They contain monomeric units that are covalently bonded to form a large structure [1] Biodegradable polymers have been the center of enormous worldwide attention, as a potential of white pollution [2] Biopolymer electrolytes (BPEs) are low cost, environmentally green, and suitable to be used as a host polymer compared to the synthetic polymer electrolytes [3] for the momentous development of BPE in many electrochemical devices, such as batteries, fuel cells, sensors, supercapacitors, and display devices, etc [4] * Corresponding author Material Research Center, Coimbatore, 641 045, India E-mail address: sekarapandian@rediffmail.com (S Selvasekarapandian) Peer review under responsibility of Vietnam National University, Hanoi Solid Biopolymer Electrolytes (SBPEs) receive more attention due to its non-leakage, high ionic conductivity, long-term structural stability, good thermal, mechanical and electrical stability There are three types of biopolymers, namely polysaccharides, polyesters, and polyamides which are naturally produced by microorganisms [5] Among the polysaccharide biopolymers, cellulose acetate (CA) has got many advantages, such as excellent transparency, low cost, non-toxic nature, biodegradability and biocompatibility [6] CA is a semi-crystalline biopolymer It is not soluble in water The chemical properties of CA are unique since it contains the carboxyl groups (C¼O) in its structure It has got a good film-forming property because of the intermolecular hydrogen bondings CA films are homogeneous and have high mechanical strength The above mentioned properties are essential for any ionic conducting membranes to construct the energy storage devices like a battery, and also various intensive electrochemical applications [7] Despite https://doi.org/10.1016/j.jsamd.2019.04.006 2468-2179/© 2019 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) M Mahalakshmi et al / Journal of Science: Advanced Materials and Devices (2019) 276e284 the advantages, it has a high crystalline nature which gives the lowest ionic conductivity that limits the applications of this polymer as electrolyte To overcome the drawbacks, CA has been doped with different salts Rani M et al have studied biopolymer electrolytes based on the derivatives of cellulose from Kenaf Bast fiber [8] Abidin et al have reported electrochemical studies on cellulose acetate-LiBOB polymer gel electrolytes [9] Proton conducting solid bio-polymer electrolytes based on carboxymethyl cellulose doped with oleic acid has been studied by M N Chai and M IN Isa [10] Selvakumar et al have carried out a research on biodegradable polymer CA doped with lithium perchlorate (LiClO4) for supercapacitors [11] Properties of CA membrane with lithium salt (LiTFSI) have been studied by Ramesh et al [12] The dielectric behavior of CA complexes with ammonium tetrafluoroborate (NH4BF4) & polyethylene glycol (PEG) has been reported by Harun et al [13] S Monisha et al has studied CA with ammonium salts (NH4SCN) & (NH4NO3) and the energy storage lithium battery has been constructed using CA with lithium nitrate (LiNO3) [14e16] According to a survey, it is observed that the study of the magnesium ion conducting electrolyte is scarce when compared to Hỵ/Naỵ/Liỵ [17] Magnesium metal has several advantages such as low cost, high safety, low equivalence weight, and high reduction potential when compared to lithium [18] Magnesium-based batteries perform very closely to lithium-based ones Magnesium batteries have turned up as an alternate for the next rank batteries due to the intrinsic advantage of the Mg metal Magnesium battery is an electro-deposited battery that works efficiently, without any dendrite growth [19] Owing to the divalent property of Mg2ỵ, this battery can provide a higher theoretical volumetric capacity (3832 mAh∙cmÀ3) than Li (2062 mAh∙cmÀ3) So, Mg batteries are spirited for energy storage devices [20] The incorporation of Mg (ClO4)2 enhances the ionic conductivity of PVP to 1.1 Â 10À4 S/cm at room temperature as reported by Mangalam et al [21] Kumar Y et al have reported a conductivity value of 5.6 Â 10À4 S/cm for PEO when complexed with a magnesium salt [22] Manjuladevi et al studied a membrane, based on PVA: PAN/MgCl2 for energy storage devices [23] Shanmugapriya et al have studied the biopolymer I-carrageenan with magnesium perchlorate and reported the conductivity value 2.18 Â 10À3 S/cm [24] Shukur et al have studied the conductivity and dielectric properties of potato starch doped with magnesium acetate [25] Hambali et al have reported the plastic crystalline gel polymer electrolytes based on poly (Vinylidene chloride-Co-Acrylonitrile) doped with magnesium triflate (MgTf) [26] Only a few works have reported on a biopolymer with magnesium salts To the best of our knowledge, there has been no work based on cellulose acetate with magnesium salts In this work magnesium ion conducting biopolymer electrolytes were prepared using the cellulose acetate and magnesium perchlorate with DMF as a solvent by the solution casting technique The prepared membranes were then characterized by various techniques, namely XRD, FTIR, DSC, and ac impedance analysis Furthermore, the ionic transference number has been evaluated by the Evan's method and a primary magnesium battery has been constructed using the highest conduction electrolyte 277 molecular weight 73.08 g/mol, density ¼ 0.948e0.949 kg/m3 from Merck specialties private Ltd., Mumbai, India was used as a solvent Different concentrations of CA and Mg(ClO4)2 were separately added with DMF solvent at room temperature and stirred continuously with a magnetic stirrer for several hours to obtain clear solutions Then both the solutions were mixed together and stirred continuously for several hours to get a homogeneous solution Finally, the solutions were poured into polypropylene Petri dishes and allowed to dry at 60 C for days in a vacuum oven for the evaporation of the solvent It yielded a stable free-standing film of thicknesses ranging from 180 to 200 mm The films were stored in vacuum desiccators Pure CA, 60%CA:40%Mg(ClO4)2, 50%CA:50% Mg(ClO4)2, 40%CA:60%Mg(ClO4)2, and 30%CA:70%Mg(ClO4)2 films were prepared by this method Then, the samples were subjected to various characterization techniques XRD patterns of polymer membranes were taken on the Philips X'pert PRO diffractometer, where x-rays of 1.5406Ao wavelength generated by a Cu-Ka source was uitilized The diffraction peaks were recorded at a 2q angle varied from 10 to 90 The biopolymer electrolytes were subjected to the FTIR study using the BRUCKER spectrophotometer in the wave number ranges from 400 to 4500 cmÀ1 with a resolution of cmÀ1 The DSC Q20V24.10 Build 122 TA instrument was used to conduct the DSC measurements to analyze the glass transition temperature of the sample The impedance measurements of the biopolymer electrolytes were made in the frequency range of 42 Hz to MHz at room temperature using the HIOKI 3532-50LCR HiTESTER The transport number of the Mg2ỵ ions for the highest conducting electrolyte was evaluated using the Evan's and the Wagner's method The linear sweep voltammetry was used to evaluate the electrochemical stability of the highest conducting electrolytes A primary magnesium battery was constructed using the membrane with the highest ionic conductivity as the electrolyte, a magnesium metal plate as the anode and MnO2 with graphite in the ratio of 3:1 in form of a pellet as the cathode Results and discussion 3.1 XRD analysis XRD measurements were performed on the biopolymer electrolytes to study their crystalline/amorphous nature Fig shows the XRD patterns for the pure CA, 60%CA:40%Mg(ClO4)2, 50% Materials and experimental methods Polymer cellulose acetate (CA) from Sigma Aldrich with average Mn ¼ 50,000 by GPC, p.code: 1001345528 and magnesium perchlorate Mg(ClO4)2 of the molecular weight of 223.21 g/mol from Himedia were used without any further purification to prepare the biopolymer electrolytes Dimethylformamide (DMF) with Fig XRD patterns of pure CA, 60%CA:40%Mg(ClO4)2, 50%CA:50%Mg(ClO4)2, 40% CA:60%Mg(ClO4)2, and 30%CA:70%Mg(ClO4)2 278 M Mahalakshmi et al / Journal of Science: Advanced Materials and Devices (2019) 276e284 CA:50%Mg(ClO4)2, 40%CA:60%Mg(ClO4)2, and 30%CA:70%Mg(ClO4)2 electrolyte films The peaks are observed for pure CA at 2q ¼ 9 , 13 , 18 , 27, and 65 which agree with the research already reported [12,27] The intensity of the peaks decreases with the increase of the Mg(ClO4)2 concentration, which reveals that the doping salt enhances the amorphous nature of the electrolytes This result was interpreted by the Hodge et al [28] criterion, which proves a correlation between the intensity of peak and the degree of the crystallinity XRD patterns confirm the absence of the peaks corresponding to Mg(ClO4)2 in the biopolymer electrolytes which indicates the complete dissociation of the salt in the polymer matrix The maximum amorphous nature is observed for the sample of 40%CA:60%Mg(ClO4)2 Furthermore, at the addition of 60% of Mg(ClO4)2, the salt gets recrystallized, which reduces the amorphous nature of the polymer matrix 3.2 FTIR analysis FTIR spectroscopy was used to prove the complex formation and the interactions between the biopolymer CA and the Mg(ClO4)2 salt by means of the change in the vibrational modes of the electrolyte under investigation Fig shows the FTIR spectra for the pure CA and the CA: Mg(ClO4)2 complexes of various compositions in the wavenumber range from 400 to 4000 cmÀ1 The corresponding vibration frequencies were assigned and are listed in Table The band at about 3372 cmÀ1 is assigned to the OeH stretching of the pure CA The peak observed at 1740 cmÀ1 is assigned to the C¼O stretching in the aldehyde carbonyl group of CA [29] The medium intensity peak 1374 cmÀ1 is attributed to the CeH bending in the alkanes of the pure CA The absorption peak observed at 1221 cmÀ1 for pure CA is assigned to the CeO stretching of the ester group The peak determined for pure the CA at 1034 cmÀ1 is assigned to the CeOeC stretching of a pyrose ring and the medium intensity peak observed at 906 cmÀ1 is assigned to the OeH bending vibrational mode of the pure CA The addition of 40, 50, 60, 70, wt % of Mg(ClO4)2 to CA produces changes in the intensity, the shape, and the position of the bands This implies that the complex reaction occurred due to oxygen from the ester group [30] The broad peak observed around 3372 cmÀ1 and assigned to the stretching vibration of the hydroxyl groups of the pure CA gets shifted and widened in the salt added systems This reveals the coordination between the cations of the salt and the hydroxyl groups of CA The hydroxyl band is shifted towards the higher wavenumber in the salt compositions indicating the specific interaction between the salt and the polymer [31] The band appeared around 1740 cmÀ1 in the pure CA and assigned to C¼O stretching of CA is decreased in intensity due to the incorporation of Mg(ClO4)2 The decrease in the intensity of the peak indicates the formation of the ion-dipole CẳO Mg2ỵ complex [32] The new peak appeared at 1646 cmÀ1 in the salt e biopolymer spectra is assigned to the C¼O stretching in the carbonyl group due to the interaction of the dopant and the polymer The peaks observed at 1374 cmÀ1, 1221 cmÀ1, and 1034 cmÀ1 are attributed to the CeH bending, the CeO stretching and the CeOeC stretching of pure CA, respectively Their intensity is decreased and the wavenumber position is shifted towards the increasing value for various complexes of Mg(ClO4)2 This indicates the completed formation of complexes of the biopolymer with the salt at different concentrations The peak observed around 608 cmÀ1 in all doped samples are ascribed to the CeCl stretching peak, which confirms the presence of the ClÀ ion in the CA-doped Mg(ClO4)2 electrolytes [33] Fig illustrates the possible interaction between CA and Mg(ClO4)2 The interactions between the pure CA and Mg(ClO4)2 via a carboxyl group, i.e ClOÀ , imply that the ions are mobile in the system The high mobility of ions favors the highest ionic conductivity This predicts that Mg(ClO4)2 has the potential to function as the charge carrier in the system The weakly bounded Mg2ỵ ion can hop through the coordinating site of the C¼O host polymer and the conduction process takes place [34] These results explain the complex formation of the host polymer with the salt 3.3 Differential scanning calorimetry (DSC) study The thermal analysis using a differential scanning calorimeter (DSC) was executed to observe the change in the glass transition temperature of the biopolymer electrolyte (BPE) system Fig shows the DSC thermograms of the pure CA, 60%CA:40% Mg(ClO4)2, 50%CA:50%Mg(ClO4)2, 40%CA:60%Mg(ClO4)2, and 30% CA:70%Mg(ClO4)2 The Tg value of the pure CA is 83.4 C A similar Tg value has been previously reported by Monisha et al The Tg value of the CA doped with Mg(ClO4)2 at different concentrations is slightly shifted towards lower temperatures The Tg values obtained are 81.7 C, 70.40 C, and 64.52 C for the complexes of 60%CA:40% Mg(ClO4)2, 50%CA:50%Mg(ClO4)2, and 40%CA:60%Mg(ClO4)2, respectively The decreasing Tg value is pointing out an effect of the Mg(ClO4)2 on softening the complex formation due to the plasticizing effect of the magnesium salt on the biopolymer structure It is useful for the magnesium ion to mobilize within the membrane [35] The glass transition temperatures for the various components of CA with Mg(ClO4)2 are listed in Table The further increase in the salt concentration for the 30% CA:70%Mg(ClO4)2 electrolyte causes an increase in the value Tg to 72.35 C This may occur due to the presence of the undissociated salt in the polymer matrix [23] Similar results have been reported by Mangalam et al [21] for the composition 50% PVA:50%PVP with 25% Mg(ClO4)2, and Manjuladevi et al [23] for the composition 92.5PVA:7.5PAN:0.5 mm% MgCl2 system 3.4 Impedance analysis Fig FTIR spectrum for pure CA, 60%CA:40%Mg(ClO4)2, 50%CA:50%Mg(ClO4)2, 40% CA:60%Mg(ClO4)2, and 30%CA:70%Mg(ClO4)2 biopolymer electrolyte films The Cole-Cole plots for the pure CA and for CA: Mg(ClO4)2 polymer electrolytes with different dopant molar ratios at room temperature (303 K) in the equivalent circuit are shown in Fig The graph shows the semicircular portion of the high-frequency region It arises from a parallel combination of the bulk resistance of the cell with a capacitor and shows the linear region with a slight M Mahalakshmi et al / Journal of Science: Advanced Materials and Devices (2019) 276e284 279 Table FTIR assignments of all prepared biopolymer electrolytes Wavenumbers (cmÀ1) Pure CA 60%CA:40%Mg(ClO4)2 50%CA:50%Mg(ClO4)2 40%CA:60%Mg(ClO4)2 30%CA:70%Mg(ClO4)2 Assignments 3372 1740 3356 1740 1646 3347 1740 1646 3339 e 1646 3322 e 1646 OeH Stretching C¼O Symmetric Stretching C¼C-Stretching 135 1230 1040 906 600 1382 1230 1042 e 651 e 1230 1042 e 608 1392 1238 1042 e 651 CeH bending CeO Stretching CeOeC Stretching of pyrose ring CH2 Rocking CeCl Stretching e 1374 1221 1034 906 e The value of the ionic conductivity was calculated using Eq (1) for all compositions at room temperature and the results are listed in Table The highest ionic conductivity at room temperature was obtained as 4.05 Â 10À4 S/cm for the biopolymer electrolyte with 40%CA:60%Mg(ClO4)2 ratio Generally, ionic conductivity is given by the equation s ¼ nem Fig Possible interaction between CA and Mg(ClO4)2 curvature that occurs in the low-frequency region due to the effect of the blocking electrodes [36] A slight depression of the semicircle is observed in the plot for all the compositions, indicating the nonDebye nature of the electrolytes and the distribution of the relaxation times [37] The bulk resistance (Rb) of the polymer electrolytes has been calculated from the Cole-Cole plot using the Boukamp software [38] The ionic conductivity (s) was calculated using the equation: s ¼ t = Rb A (1) where t and A are thickness and the area of the polymer electrolytes, respectively (2) where ‘n’ is the number of charge carriers, ‘e’ is the charge and ‘m’ is the mobility of the charge carriers When the numbers of charge carriers increased, the conductivity increases The ionic conductivity of the pure CA is 3.1 Â 10À9 S/cm The addition of Mg(ClO4)2 to a polymer increases the number of charge carriers in such a way that the salt Mg(ClO4)2 dissociates into Mg2ỵ and ClO ions, thereby producing more charge carriers When 60 wt % of Mg(ClO4)2 is added to the polymer, the maximum number of charge carriers is produced So, the conductivity reaches the maximum value of 4.05 Â 10À4 S cmÀ1 It has been enhanced by an increase of the Table Glass transition temperature for various components of CA with Mg(ClO4)2 Sl No CA:Mg(ClO4)2 composition (%) Glass transition temperature (oC) Pure CA 60:40 50:50 40:60 30:70 83.4 81.7 70.40 64.52 72.35 Fig (a) DSC thermogram and the glass transition temperature (Tg) for pure CA and 60%CA:40%Mg(ClO4)2 (b) DSC thermogram and the glass transition temperature (Tg) for 50% CA:50%Mg(ClO4)2, 40%CA:60%Mg(ClO4)2, and 30%CA:70%Mg(ClO4)2 280 M Mahalakshmi et al / Journal of Science: Advanced Materials and Devices (2019) 276e284 Fig (a) ColeeCole plot for pure CA with the corresponding equivalent circuit (b) ColeeCole plots for 60%CA:40%Mg(ClO4)2, 50%CA:50%Mg(ClO4)2, 40%CA:60%Mg(ClO4)2, and 30% CA:70%Mg(ClO4)2 with the corresponding equivalent circuit Table Ionic conductivity values of CA doped with Mg(ClO4)2 Sl No CA:Mg(ClO4)2 composition (%) Ionic conductivity S$cmÀ1 Pure CA 60:40 50:50 40:60 30:70 3.1 Â 10À9 4.97 Â 10À6 3.88 Â 10À5 4.05 Â 10À4 2.6 Â 10À4 amorphous nature The segmental motion of the biopolymer is raised by the magnitude of the amorphous phase and it leads to the higher flexibility of the biopolymer chain It is observed that the further addition of the Mg(ClO4)2 salt over 40%CA:60%Mg(ClO4)2 decreases the conductivity, due to the formation of more ion aggregates in the polymer network [39] From the Fig it is inferred that the diameter of the semicircle gradually decreases with the addition of the dopant The semicircular portion is a combination of the bulk resistance and the bulk capacitance The random dipole orientation of the polar side groups present in the polymer network decreases the diameter of the semicircular portion at the higher frequency region, which indicates the noncapacitive nature of the polymer electrolytes 3.5 Frequency-dependent conductivity The frequency dependence of the conductivity as a function of frequency for the pure CA and the CA with different composition of Mg(ClO4)2 at room temperature is shown in Fig There is a low-frequency dispersion region in the plot which is attributed as due to the electrodeeelectrolyte space charge polarization effects [40], whereas the frequency independent plateau region corresponds to dc conductivity (sdc) of the composition of the polymer electrolytes From the ac conductivity spectra, it is observed that the conductivity increases with the increase in the salt composition, which is attributed to the increase in the number of charge carriers The sdc values for all the electrolytes (pure CA, 60% CA:40%Mg(ClO4)2, 50%CA:50%Mg(ClO4)2, 40%CA:60%Mg(ClO4)2, and 30%CA:70%Mg(ClO4)2) were calculated by extrapolating the plateau region to the logs axis The conductivity values obtained from the conduction spectra coincide with the bulk conductivity values received from the Cole-Cole plot Fig Frequency dependence conduction spectra for pure CA, 60%CA:40%Mg(ClO4)2, 50%CA:50%Mg(ClO4)2, 40%CA:60%Mg(ClO4)2, and 30%CA:70%Mg(ClO4)2 3.6 Dielectric studies The ionic transport phenomenon is characterized by using the dielectric properties of the pure CA, 60%CA:40%Mg(ClO4)2, 50% CA:50%Mg(ClO4)2, 40%CA:60%Mg(ClO4)2, and 30%CA:70% Mg(ClO4)2 polymer electrolytes as the frequency-dependent parameters Fig reveals that the dielectric parameters ε0 and ε00 increase at low frequencies due to the formation of the space charge region at the electrodeeelectrode interface which is known as the unÀ1 variation or the non-Debye type behavior, where the space charge regions with respect to frequency are explained in terms of the ion diffusion [41] At low frequencies, the dielectric constant (ε0 ) and the dielectric loss (ε00 ) are very high due to the interfacial polarization and the free charge motion within the material [42] At high frequencies, the mobile ions are not able to orient themselves in the field direction due to the rapid periodic reversal of the applied electric field, which leads to the saturation or to a decrease in the dielectric constant [43] The values of the dielectric parameter are found to be increased with the Mg(ClO4)2 salt content in the polymer electrolytes M Mahalakshmi et al / Journal of Science: Advanced Materials and Devices (2019) 276e284 281 Fig (a) Plot of logu vs ε’ for BPEs with different concentrations of Mg(ClO4)2 (b) Plot of logu vs ε” for BPEs with different concentrations of Mg(ClO4)2 3.7 Transport number measurement 3.7.1 Wagner's dc-polarization technique This technique is one of the fundamental methods to measure the transference number which is used to identify whether the conductivity in the biopolymer electrolyte is due to the presence of ions or electrons The transference number is calculated by the formula, tion ¼ ðIf À Ii Þ=Ii (3) where Ii is the initial and If the final current In this technique, a dc-potential of 1.5 V was applied across the cell of the SS/40%CA:60%Mg(ClO4)2/SS configuration for polarization and the polarization current was monitored as a function of time The initial total current decreases with time and reaches a constant value in the fully depleted situation due to the depletion of the ionic species in the biopolymer electrolyte [44] The cell is polarized at a steady state and the current flows across the electrolyte interface because of the ion migration The total transference number (tion) for the maximum conductivity biopolymer Fig D.C polarization curve of SS/40%CA:60%Mg(ClO4)2/SS cell and SS/40%CA:60% Mg(ClO4)2/Mg cells at room temperature 40%CA:60%Mg(ClO4)2 is calculated to be 0.98 using the Eq (2), which is close to unity, meaning that only a negligible contribution comes from the electrons Hence, it is evident that the charge transport is mainly due to the ions 3.7.2 Evan's polarization technique The Evan's polarization technique was used to calculate the transport number (tỵ) of the Mg2ỵ ions in the solid biopolymer The combination of the ac and dc polarization methods was applied on the Mg/40%CA:60%Mg(ClO4)2/Mg cell The cell was polarized by applying a dc-voltage of 1.5 V Then the initial (Io) and the final current (Is) values were derived from the currentetime plot as shown in Fig The cell resistance was recorded before and after the polarization by using the impedance measurement, and a graph plotted for corresponding values is shown in Fig The transport number of Mg2ỵ is calculated using the formula tỵ ẳ Is ðDV À Ro Io Þ = Io ðDV À Rs Is Þ (4) Fig Cole-Cole plot before and after polarization of a typical symmetric Mg/40% CA:60%Mg(ClO4)2/Mg cell at room temperature 282 M Mahalakshmi et al / Journal of Science: Advanced Materials and Devices (2019) 276e284 3.8 Linear sweep voltammetry (LSV) The electrochemical stability of the highest conductivity polymer electrolyte 40%CA:60%Mg(ClO4)2 was studied using the linear sweep voltammetry (LSV) with a two-electrode system The LSV was recorded for SS/40%CA: 60%Mg(ClO4)2/Mg at a scan rate of mVs-1 The currentevoltage response curve is plotted in Fig [10] The voltage at which the current flows through the cell was taken as the anodic decomposition limit of the biopolymer electrolyte A sudden rise in the current is observed from the graph This shows the electrochemical stability window of 3.58 V This result reveals that the electrolyte could be used for its application in Mg ion batteries Osman et al have reported the stability window of 3.5 V for the SS/PVDF-HFP:20% Mg(CF3SO3)2/Mg and Manjuladevi et al also reported the stability window of 3.66 V for the 92.5PVA:7.5PAN:0.3 mm% MgCl2 polymer electrolyte Fig 10 Linear sweep voltammetry of SS/40%CA:60%Mg(ClO4)2/Mg cell recorded at a scan rate of mVsÀ1 at room temperature where Io and Is are the initial and the final current, DV is the applied dc-voltage of 1.5 V, and Ro and Rs are the cell resistance before and after the polarization, respectively The value of the Mg2ỵ ion transport number was calculated to be 0.31 for the highest conducting film of the 40%CA:60%Mg(ClO4)2 electrolyte Shanmugapriya et al have reported a value as 0.313 for the transport number of the Mg2ỵ ions in their work for the carrageenan with the 0.6g Mg(ClO4)2 electrolyte Similar results have been reported by Mangalam et al for the composition of 50%PVA:50%PVP with 25% Mg(ClO4)2 Manjuladevi et al have reported for the composition of the 92.5PVA:7.5PAN:0.5 mm% MgCl2 system a value of 0.38, and Kumar et al have studied the PMMA-based GPE system with the Mg(CF3SO3)2 salt and reported a value of 0.33 3.9 Construction and performance of the Mg battery cell A primary magnesium battery has been constructed using the maximum conductivity biopolymer electrolyte (40%CA:60% Mg(ClO4)2) Magnesium metal in pellet form was taken as the anode and MnO2 mixed with graphite in form of a pellet acts as the cathode The highest conductivity BPE 40%CA:60%Mg(ClO4)2 has been sandwiched between the anode and the cathode in the battery holder The schematic diagram of the fabricated battery is shown in Fig 11 a The initial open circuit voltage (OCV) was recorded as 2.12 V and the OCV was monitored with respect to time After day the OCV was reduced slightly to 1.9 V, which remains at the same value for subsequent days As it is shown in Fig 11 b, a small intermediate drop in battery voltage occurs after fabrication due to the cell formation reactions at the electrodes [45] The chemical reactions taking place in the battery cell are characterized as the followings: Fig 11 (a) Schematic diagram of battery configuration (b) Open circuit potential as a function of time for 40%CA:60%Mg(ClO4)2 biopolymer electrolyte (c) Discharge curves of cell using 100 KU for 40%CA:60%Mg(ClO4)2 biopolymer electrolyte M Mahalakshmi et al / Journal of Science: Advanced Materials and Devices (2019) 276e284 283 References Table Cell parameters Specification of cell parameters Values of cell parameters Cell area (cm2) Cell weight (g) Effective cell diameter (cm) Cell thickness (cm) Open circuit voltage (V) Cut off potential (h) Current drawn (mA) 1.13 1.15 1.2 0.248 2.12 168 24 At the anode Mg ỵ 2OH ị / MgOHị2 þ 2e At the cathode 2MnO2 þ H2 O þ 2e / Mn2 O3 ỵ 2OH Over all reaction Mg þ 2MnO2 þ H2 O / MgðOHÞ2 þ Mn2 O The Hydroxyl ions present in the Mg e MnO2 battery may be generated from the occluded moisture/H2O present in the biopolymer membrane The occluded water is a type of nonessential water that is retained due to the physical force in microscopic pores, spaced irregularly throughout the solid biopolymer CA [26] An external load of 100 kU was connected to the circuit for the measurement of the battery discharge characteristics at room temperature The discharge behavior of the battery with respect to time is shown in Fig 11c The electrical potential of the battery decreases initially due to the polarization effect at the electrolyteeelectrode interface [46] The battery potential discharged at a constant load of 100 kU was found to remain constant at 1.68 V, which was observed for days The battery cell parameters are listed in Table 4 Conclusion The discovery of a new solid biopolymer electrolyte CA doped with a various concentrations of Mg(ClO4)2 was prepared by the solution casting technique using DMF as a solvent The XRD patterns reveal that the inclusion of Mg(ClO4)2 increases the amorphous nature of the biopolymer electrolyte FTIR analysis confirms the formation of the complex between CA and the magnesium ions The DSC studies indicate that the glass transition temperature decreases with the increase of Mg(ClO4)2 salt concentration Ionic conductivity of 4.05 Â 10À4 S/cm has been obtained for the 40% CA:60%Mg(ClO4)2 membrane using the ac-impedance analysis at room temperature The dielectric study predicts the non-Debye nature of the electrolyte membranes Using the Evans method, the ionic transference number for Mgỵ has been estimated as 0.31 for the 40%CA:60%Mg(ClO4)2 electrolyte The electrochemical stability window has been determined by LSV as 3.58 V which is sufficient for electrochemical applications Optimized highest ionic conductivity membrane 40%CA:60%Mg(ClO4)2 was used to construct a primary magnesium battery, on which the Open Circuit Voltage (OCV) has been found as 1.9 V Declaration of interest statement As 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Materials and Devices (20 19) 27 6e284 28 1 Fig (a) Plot of logu vs ε’ for BPEs with different concentrations of Mg( ClO4) 2 (b) Plot of logu vs ε” for BPEs with different concentrations of Mg( ClO4) 2 3.7... 50%CA:50 %Mg( ClO4) 2, 40%CA:60 %Mg( ClO4) 2, and 30% CA:70 %Mg( ClO4) 2 with the corresponding equivalent circuit Table Ionic conductivity values of CA doped with Mg( ClO4) 2 Sl No CA :Mg( ClO4) 2 composition (%) Ionic... Cut off potential (h) Current drawn (mA) 1.13 1.15 1 .2 0 .24 8 2. 12 168 24 At the anode Mg ỵ 2OH ị / MgOH? ?2 ỵ 2e At the cathode 2MnO2 ỵ H2 O ỵ 2e / Mn2 O3 ỵ 2OH Over all reaction Mg ỵ 2MnO2 þ H2