Impedance and ionic transport properties of proton-conducting electrolytes based on polyethylene oxide/methylcellulose blend polymers

Proton-conducting polymer electrolyte films were prepared by dissolving NH4I salt in polyethylene oxide/methylcellulose (PEO/MC) blend polymers using the solution cast technique. The semi-crystalline nature of the sample was identified from the X-ray diffraction (XRD) pattern. The surface morphology on the electrical conductivity was analyzed by scanning electron microscopy (SEM).

Journal of Science: Advanced Materials and Devices (2020) 125e133 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Impedance and ionic transport properties of proton-conducting electrolytes based on polyethylene oxide/methylcellulose blend polymers Hawzhin T Ahmed a, Omed Gh Abdullah b, * a b Charmo Center for Research, Training & Consultancy, Charmo University, 46023, Chamchamal e Sulaimani, Kurdistan Region, Iraq Advanced Materials Research Lab., Department of Physics, College of Science, University of Sulaimani, 46001, Kurdistan Region, Iraq a r t i c l e i n f o a b s t r a c t Article history: Received December 2019 Received in revised form February 2020 Accepted February 2020 Available online February 2020 Proton-conducting polymer electrolyte films were prepared by dissolving NH4I salt in polyethylene oxide/methylcellulose (PEO/MC) blend polymers using the solution cast technique The semi-crystalline nature of the sample was identified from the X-ray diffraction (XRD) pattern The surface morphology on the electrical conductivity was analyzed by scanning electron microscopy (SEM) The highest ionic conductivity of 7:62 Â 10À5 S=cm was achieved at room temperature for the sample containing 30 wt % of NH4I The temperature dependence of the Jonscher's exponent shows that the conduction mechanism can be well represented by the overlapping large polaron tunneling (OLPT) model The electrical conductivity enhancement was analyzed by the Rice and Roth model, which showed that the increase in the salt concentration caused an increment in the mobility and the diffusion coefficient of the ions For all prepared samples, the highest value of conductivity was associated with the minimum value of activation energy The dielectric data were analyzed for the highest ionic conducting sample at various temperatures to clarify an important factor of the ion conduction The non-Debye behavior of the samples can be expressed from the electric modulus formalism and the dielectric properties of the electrolytes that have been proven by the incomplete semicircular arc of the Argand plots © 2020 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: Proton-conducting Ionic conductivity Diffusion coefficient Electric modulus Argand plot Introduction Nowadays, solid polymer electrolytes (SPEs) have great attraction through out the disciplines of electrochemistry, polymer science, organic chemistry, and inorganic chemistry In its progress, in turn, it revolutionizes in both academia and industry area the development of science and technology [1] The improvement of the ionic conductivities at ambient temperature for SPEs has been the main focus of most researchers [2] Several strategies, such as copolymerization, chemical modifications (grafting), physical mixture (blending), plasticization, and the addition of micro/ nanofillers have been proposed to boost the electrical conductivity of the polymer electrolytes [3,4] Also, the ionic conductivity of polymer-based electrolytes can be modulated by doping salts, acids, metals, alkali, etc to the polymer matrix [5] In a few years * Corresponding author E-mail address: omed.abdullah@univsul.edu.iq (O.Gh Abdullah) Peer review under responsibility of Vietnam National University, Hanoi back, the attention of researchers deviated towards the blending of polymers; this technique has opened a new wave of potential as an effective method to enhance the electrical and mechanical properties of electrolyte systems [6,7] The blending of polymers together provides more complexation sites, which raise the ion migration, resulting in an increase in the ionic conductivity [8,9] According to Buraidah and Arof [8], the highest conductivity value obtained at ambient temperature was 1.77 Â 10À6 S$cmÀ1 for the chitosan-PVA-NH4I electrolyte system As a comparison, the ionic conductivity achieved for the unblended system of chitosan-NH4I was 3.73 Â 10À7 S$cmÀ1 Very recently, among SPEs, most particular attention has been paid to the development of the protonconducting polymer electrolytes due to their performance and promising technological applications in advanced smart devices [10], because they have attractive properties like shape versatility, flexibility, lightweight, etc [11] Synthetic polar polymers utilized in making proton-conducting films include polyethylene oxide (PEO), polyvinyl alcohol (PVA), Poly (N-vinyl pyrrolidone) (PVP) [12,13] However, Cellulose, Chitosan (CA), Starch have been used as a natural polar polymer for preparing proton-conducting https://doi.org/10.1016/j.jsamd.2020.02.001 2468-2179/© 2020 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/) 126 H.T Ahmed, O.Gh Abdullah / Journal of Science: Advanced Materials and Devices (2020) 125e133 electrolyte films [14e16] When these polymers are complexed with various host dopants, such as (inorganic-organic) acids or salts, their structural and electrical properties have been appreciably reformed [17] Strong inorganic acids, such as phosphoric acid (H3PO4), hydrochloric acid (HCl) and sulfuric acid (H2SO4) [18] have been used as proton donor for the SPE systems Nevertheless, these are not applicable for practical application due to the fact that the polymereinorganic acid complex films suffer more from chemical degradation and mechanical integrity [18,19] Different ammonium salts have been reported as a good proton donor in the polymer matrix [20] It is believed that the ion responsible for the ionic conductivity mechanism in such systems is weakly bonded to Hỵ of the NHỵ4 cation [11] A literature survey demonstrates that a good proton donor can be obtained to the polymer matrix more easily from ammoniumbased salts [21] The present research is an extension of our previously published papers where we discussed the structural and electrical characterization of PEO/MC blend electrolyte system to optimize the highest conductivity [22] Thus, in this effort, PEO was blended with MC at the ratio of 60:40 to form the blended polymer solution, and then different concentrations of the NH4I were added The choice of NH4I among different types of ammonium salts as the proton source is predicted to obtain a higher conductivity, because it was well addressed in the previous investigation that NH4I exhibits a higher ionic conductivity due to the lower lattice energy and relatively larger anion size compared to the other ammonium salts [23] The present work aims to develop a new type of protonconducting polymer electrolyte based on the PEO/MC blend polymer incorporated with different concentrations of NH4I The investigation focuses on the analysis of the ionic transport properties to understand and thus, improve the ionic conduction mechanism of proton-conducting polymer electrolyte Experimental 2.1 Materials and methods Poly(ethylene oxide) (PEO) (Alfa Aesar, 106 g/mol molecular weight), and methylcellulose (MC) (Merck KGaA Germany, with molecular weight 14,000 g/mol) are taken as primary and secondary polymer precursors for preparing the blend polymer electrolyte To produce proton (Hỵ ion)-conductivity in the polymer matrix, ammonium iodide salt NH4I from Merck KGaA Germany, with molecular weight of 144.94 g/mol was used All the materials were used for this preparation without any purification process our earlier investigations, the two solutions with 60:40 percent weight ratio were intermixed with each other and made PEO/MC blend polymer as a homogeneous mixture [22] Next, the desired amounts of NH4I were dissolved into 10 ml distilled water and added to the polymer solution under stirring until complete dissolution of the salt was obtained The samples were coded based on the concentrations of NH4I as PBE-10, PBE-20, PBE-30, PBE-40, and PBE-50 for 10, 20, 30, 40, and 50 wt.% NH4I, respectively, as shown in Table Subsequently, all homogeneous mixtures were cast into cleaned polypropylene dishes and left to dry at room temperature for two weeks For further drying, the harvested films were stored in a desiccator Finally, the obtained films were peeled off gently from the polypropylene dishes for characterization and further experiments 2.3 Characterization techniques The XRD spectra profiles of the PEO/MC-NH4I protonconducting polymer films were measured on the PANalytical X'Pert PRO diffractometer system by using monochromatic X-rays with l ¼ 1:5406  A were generated by Cu À ka1 source The X-ray tube was operated at 45 kV voltage, and 40 mA anode current with glancing angels ranged between 10 2q 80 at room temperature Micromorphological characterization of the prepared polymer electrolyte samples, which dried naturally in room temperature was examined using the MIRA3-TESCAN field emission scanning electron microscope (FE-SEM), where the dried samples were gold-coated before scanned to prevent electrostatic charging on the electrolytes Ionic conductivity of the prepared PEO/MC-NH4I films were investigated using precision LCR Meter (KEYSIGHT E4980A) that was interfaced to a computer, in the frequency range 100 Hz to MHz and in the temperature range between 303 À 373 K When the required films were cut into suitable size and sandwiched between two blocking aluminum electrodes The Nyquist plane plots were obtained from the recorded impedance data by applying a 100 mV perturbation to an open circuit potential in the above-mentioned frequency range Results and discussion 3.1 XRD analysis Fig represents the XRD pattern for the various PEO/MC-NH4I electrolyte films From Fig it can be observed that the two sharp 2.2 Proton-conducting polymer electrolyte preparation For the preparation of the proton-conducting polymer electrolyte, g of each PEO and MC powder were dissolved separately at room temperature in the 120 ml and 240 ml distilled water, respectively These solutions were stirred at room temperature for 24 h to ensure a precise homogeneous composition Then, based on Table Composition of the blend polymer electrolyte series containing different wt.% of NH4I Designation PEO Solution MC Solution NH4I wt.% NH4I (g) Powder (g) Solvent (ml) Powder Solvent (g) (ml) PBE-10 PBE-20 PBE-30 PBE-40 PBE-50 2 2 120 120 120 120 120 2 2 240 240 240 240 240 10 20 30 40 50 0.222 0.500 0.857 1.333 2.000 Fig XRD patterns for PEO/MC-NH4I blend polymer electrolyte films incorporated with different NH4I salt concentrations H.T Ahmed, O.Gh Abdullah / Journal of Science: Advanced Materials and Devices (2020) 125e133 127 Fig SEM micrograph of PEO/MC blend polymer films containing (a) 20 wt.% NH4I (b) 30 wt.% NH4I, (c) 40 wt.% NH4I and (d) 50 wt.% NH4I Bragg peaks at 19.2 and 23.0 for the PBE-10 sample described the semi-crystalline nature of the sample [22] According to Yahya et al [24], the strong intermolecular interaction between the polymer chains due the intermolecular hydrogen bonding causes the formation of semi-crystalline peaks These peaks become broader and less intense, with the increasing NH4I concentration until overlapped in a single broad hump at 21.2 for sample PBE-30 Literally this demonstrated that the reduction in the relative intensity and broad nature of the characteristic peak clearly indicates that the crystalline fraction in the electrolyte system was decreased [19,24] This observation confirms that the blend polymer sample has a semi-crystalline nature The intensity of the semi-crystalline peaks from the blend polymer sample was found to decrease progressively with the increase of NH4I contents until 30 wt.% which implies the decrease in the degree of crystallinity This results from the fact that the interaction between the PEO/MC and the NH4I causes the decrease in the intermolecular interaction between PEO/ MC chains, thus, induces new coordination interactions between the Hỵ of NH4I and the (CeOeC) group of the PEO and/or the (OR) group of MC of the blend polymer formed which helps for boost ionic conductivity [25] The XRD peak deconvolution method was utilized to estimate the degree of crystallinity using the Fityk software [26] The degree of crystallinity was found to be 22.21, 18.74, 15.88, 18.92, and 19.98 for PBE-10, PBE-20, PBE-30, PBE-40, and PBE-50, respectively It is clear that the PBE-30 sample exhibits the highest amorphous nature Many researchers have concluded that the ionic conductivity is enhanced in the amorphous domain [27,28] Thus, it can be anticipated that the sample with the lowest crystalline region (PBE30) exhibits the highest electrical conductivity For the highest salt concentration sample (PBE-50) some multiple characteristic peaks of NH4I were observed at 21.1, 24.5 , 34.9 , and 42.2 which is revealing that the host polymers could no longer solvate the salt [29] The presence of undissolved salt in the system at higher salt concentrations causes the salt deposition on the film surface due to the recombination of ions This eventually leads to a decrease in the number of the mobile ions in the sample and the decrease in the electrical conductivity 3.2 Morphological analysis Field emission scanning electron (FE-SEM) micrographs of the PEO/MC-NH4I proton-conducting blend polymer electrolyte films are presented in Fig A comparison of the surface morphology shows a change in the surface properties and the texture structure of the polymeric films upon the addition of NH4I salt with different concentrations In the present work, the FE-SEM study has been studied to understand the variation of the electrical conductivity 128 H.T Ahmed, O.Gh Abdullah / Journal of Science: Advanced Materials and Devices (2020) 125e133 with the salt concentration for the present membranes, then the results are used to explain the decrease of DC conductivity at the higher salt concentrations [30,31] From Fig (a,b), it can be observed that at low salt concentrations (PBE-20 and PBE-30), a smooth surface without any phase separation is perceived while for the PBE-40 sample, a rough surface appeared (Fig 2c) Also, in the PBE-50 sample, some crystalline aggregates of the NH4I salt have formed and have protruded out of the surface (Fig 2d) The micrographs of the PBE-20 and PBE-30 are a proof of the miscibility of PEO and MC Kadir et al [9] reported that the smooth and homogeneous surface of the blend polymer samples indicates that both polymers are miscible with each other Also, it is well reported that the smooth surface morphology of the polymer electrolyte samples designates that the salt is completely dissolved in the host polymer matrix This behavior is utilized as an evidence describing the amorphous nature of the system [32] However, the surface of the PBE-40 shows a rough and uneven, and that could be due to the ions trapped in the host matrix [30] Reddeppa et al [33] reported that the increase of the degree of roughness with the increasing salt concentration indicates the segregation of the dopant in the host polymer matrix For the highest salt concentration (PBE-50), the morphology consists of solid structures that have protruded the surface of the film The X-ray diffractograms for the samples reveal that these solid structures are due to the recrystallization of NH4I out of the polymer film surface [24] The inability of the salt to dissolve in the host polymer matrix results in the recombination of the ions and the recrystallization of the salt out of the film surface This causes the reduction of the density of ions, thus the decrease in the conductivity This FE-SEM analysis, therefore, has been used to give some answers describing the reduction of the conductivity in the blend polymer electrolyte system at higher salt concentrations [9] Kadir et al [30] made a similar observation by incorporating NH4NO3 in the chitosan - polyethylene oxide blend polymer matrix at the high salt concentrations up to 40 wt.% They attributed this observation to the formation of crystalline aggregates of the ammonium salt out of the polymer surface; they also reported that these crystalline aggregates might be due to the excess salt that could not be solvated by the polymer matrix and has recrystallized upon drying 3.3 Conductivity analysis Fig illustrates the complex impedance plot of different wt.% NH4I used as a dopant to form PEO/MC blend polymer electrolytes at room temperature The ColeeCole plots of PEO/MC-NH4I show one spike with a semicircle The semicircular arc can be utilized to calculate the conductivity of the system by using this equation: s ¼ t Ầ1 Rb À1 Fig ColeeCole plots for PEO/MC-NH4I blend polymer electrolyte films incorporated with different NH4I salt concentrations all prepared samples, by expanding the temperature range, the bulk resistance decreases inferring the improvement of the electrical conductivity, as shown in Fig This feature explained by Sundaramahalingam et al [34] He reported that the increase in temperature causes the vibrational energy of the polymer segment to rise, which is to compensate against the hydrostatic pressure forced by its neighboring sites The vibrational energy occurs in the polymer segment free volume As a result, the conductivity value increased because the particles can move unconditionally in the free volume around the polymer chain [34] This result was confirmed by the XRD and FE-SEM analysis that clearly showed the recrystallization of the NH4I at high salt concentrations on the surface of the polymer electrolyte samples The activation energy for the thermally activated hoping mechanism in PEO/MC-NH4I can be evaluated from the slope of the straight line of Fig The information was depicted in Fig and Table recommends that the electrolytes obey Arrhenius behavior in the temperature range of 303e373 K and that the conductivity occurs by a thermally activated transport process [35] It is well addressed in previous studies that the activation energy decreases gradually with an increase in the conductivity of a polymer blend electrolyte system, as it is shown in Fig [36], which means that (1) where t is the film thickness, A is the area of the film and Rb is the bulk resistance determined from the intercept on the real axis at the lower frequency end of the semicircle in the ColeeCole plot of the complex impedance [34] From Fig it can be obtained that the conductivity of the complexes increases with the content of the doping salt and reaches a maximum value of 7:62 Â 10À5 S=cm for the PBE-30 sample among all other compositions However, upon further addition of the NH4I salt, the conductivity decreases It is well reported that the increase in the conductivity is attributed to the enhancement of the ionic mobility and the number of the free mobile ions [35] Meanwhile, the decrease in the conductivity could be due to aggregates and the formation of the ion pairs, which produces neutral species and thus reduces the number of free ions [36] Moreover, for Fig The temperature-dependent conductivity spectra for PEO/MC when mixed with 30 wt.% NH4I H.T Ahmed, O.Gh Abdullah / Journal of Science: Advanced Materials and Devices (2020) 125e133 Fig The PEO/MC-NH4I Arrhenius plot in the temperature range 303e373 K the ions necessitate a lower energy for migration in highly conducting samples It was reported that the low activation energy for the polymer blend system is due to the entirely amorphous nature of the polymer electrolyte that assists the fast Hỵ ions of NH4I to move through the polymer network Also, refer to Buraidah et al [8], the short distance between the transit sites of the polymer blends has affected to lowering the activation energy From Fig 6, it has been found that the highest conductivity sample (PBE-30) possesses the lowest activation energy of 0.34 eV Nowadays, the low values of activation energies based on polymer electrolytes are desirable for practical applications In order to identify the conduction mechanism in this electrolyte system, the exponent s is plotted as a function of the temperature, as depicted in Fig The behavior of the Jonscher exponent (s) versus temperature can be used to derive the origin of the ionic conduction mechanism Several theoretical models have been proposed to estimate the microscopic charge transport mechanism, based on a variation of s with temperature [37,38] Thus the temperature dependence of s plays a key role in the determination of the conduction mechanism in the disordered materials In the present study, the values of s obtained at different temperatures are less than 0.8 and they are temperature dependent The conduction mechanism for the PEO/MC-NH4I system can be most probable interpreted based on the overlapping large polaron tunneling (OLPT) model According to this model, the exponent s decreases with the temperature, reaches a minimum value and thereafter increases with temperature [39] According to Majid and Arof [40], this behavior implies that the addition of salt results in the overlapping of the stress fields of the polarons and creates a conducting path for the ions, thus the ions are able to tunnel through the potential barrier that exists between the two possible complexation sites [23,40] 129 Fig The ionic conductivity and activation energy of PEO/MC-NH4I blend polymer electrolyte as a function of NH4I wt.% 3.4 Ion transport study The Rice and Roth model [41] hypothesized that there exists an energy gap in the ionic conductor, and the ions as the conducting species with the mass m can be thermally excited from the localized ionic states to free-ion-like states in which an ion propagates throughout the spaces with a velocity that is required for such excitation given by [36]: rffiffiffiffiffiffiffiffi 2Ea y¼ m (2) where Ea is the activation energy However, the Rice and Roth equation was formulated for superionic conductors, but it has been known to be intensively used to estimate the number of density and the mobility of mobile ions, which are strongly related to the ionic conductivity in the polymer electrolyte system, according to the NernsteEinstein equation [29]: s ¼ hem (3) where s is the ionic conductivity, h is the density of the mobile ions, m is the mobility of the mobile ions, and e is the electron charge The conductivity can be calculated using the Rice and Roth equation as follows: Table The ionic conductivity (s), activation energy (Ea ), and regression values (R2 ) for various compounds of PEO/MC-NH4I blend polymer electrolyte films at ambient temperature Samples s ðS =cmÞ E a ðeVÞ R2 PBE-10 2:32 Â 10À6 0.536 0.98 PBE-20 9:60 Â 10À6 0.545 0.98 PBE-30 7:62 Â 10À5 0.344 0.94 PBE-40 4:76 Â 10À5 0.484 0.98 PBE-50 3:40 Â 10À6 0.627 0.98 Fig The temperature dependence values s for PEO/MC-NH4I electrolyte system 130 H.T Ahmed, O.Gh Abdullah / Journal of Science: Advanced Materials and Devices (2020) 125e133 Table Transport parameters for PEO/MC-NH4I electrolyte system Samples t ðsÞ m ðcm2 VÀ1 sÀ1 Þ D ðcm2 sÀ1 Þ PBE-10 1:03 Â 10À13 1:69 Â 10À09 4:42 Â 10À11 PBE-20 1:02 Â 10À13 1:21 Â 10À09 3:16 Â 10À11 PBE-30 1:28 Â 10À13 10À06 10À08 PBE-40 1:08 Â 10À13 1:16 Â 10À08 3:02 Â 10À10 PBE-50 9:49 Â 10À14 5:47 Â 10À11 1:43 Â 10À12 " #   Zeị2 E sẳ hEa t exp a mKB T KB T 2:07 Â 5:42 Â (4) Here Z is the vacancy of conducting species, m is the mass of the ionic charge carrier, and t is the finite lifetime of the ions, which can be calculated using t ¼ l=y, where, l is the mean free path between two coordinating sites or two atoms with the lone pair electrons across which the ions may hop and it was taken as 10.4 Å [36] From the equations (3) and (4), the density of the mobile ions, h, can be found as an essential parameter to determine the mobility of the mobile ions, m; and the diffusion coefficient, D, as follows: hẳ 3s mKB T 2Zeị Ea t exp À   (5) Ea KB T m¼ s he (6) D¼ KB T s he2 (7) where D is the diffusion coefficient The transport parameters as a function of NH4I concentration that is related to the ionic conductivity are presented in Table 3, from the results obtained in this table it can be perceived that the conductivity of NH4I is attributed to the two important factors, directly derived from equations (6) and (7), namely, m and D The value of m lies between 5:47 Â 10À11 to 2:07 Â 10À6 cm2 V À1 sÀ1 , while the D value is in the range of 1:43 Â 10À12 to 5:42 Â 10À8 cm2 sÀ1 It is well reported that the conductivity of the electrolyte is much affected by the ionic diffusion, which can be a useful parameter to help increment the conductivity value in a polymer matrix; therefore, the conductivity is increased when the diffusion of ions increases [19] From the data results described in Table the highest conducting sample (PBE-30) possesses the highest mobility and the highest diffusion value This result is supported by the fact that the conductivity is governed by the density of ions (h), and the mobility (m) In addition, it was observable that for the higher salt concentrations the conductivity decreases, and this phenomenon can be explained by the fact that the aggregation of ions leads to the formation of ion clusters where the dipole interaction between the protons in the medium increases, which causes the reduction of the ion mobility and diffusion [36] These results reveal that the values of m and D influence the ionic conductivity of the PEO/MC-NH4I proton-conducting polymer electrolyte films 3.5 Dielectric analysis The dielectric properties of the PEO/MC-NH4I blended polymer electrolytes are not much investigated earlier Thus the dielectric 00 constant ðε Þ; dielectric loss ðε Þ, tangent loss and the electric modulus for the PBE-30 films were studied as a function of the frequency and the temperature In order to understand the role of the salt in enhancing the ionic conductivity the study of the frequency-dependent dielectric parameters (dielectric constant and dielectric loss) are required and calculated from the following equations [24]: Fig (a) Dielectric constant (b) Dielectric loss as a function of frequency for PBE-30 at different temperatures between (303e373) K Fig Dielectric loss behavior for PBE-30 as a function of frequency from the temperature range between (303e373) K H.T Ahmed, O.Gh Abdullah / Journal of Science: Advanced Materials and Devices (2020) 125e133 Fig 10 Frequency-depends of modulus formalism (a) real part of modulus, (b) imaginary part of modulus at different temperatures ε0 ¼ Cd ε0 A ; ε00 ¼ ε0 tan d (8) where C is the sample capacitance measured from an LCR meter, ε0 is the permittivity of the free space (8.85 Â 10À12 F/m), A is the cross-sectional area of the electrode and tan d is a tangent loss, both of C and tan d were measured for all samples in the frequency range 100 Hze2 MHz The frequency dependence of the dielectric con0 00 stant ðε Þ and dielectric loss ðε Þ are shown in Fig (a,b) 00 From Fig it can be noted that the values of ε and ε decrease with the increasing frequency while these values rapidly increased in the low-frequency region and at high temperatures This attitude has been observed in many polymer electrolytes [42,43] The in0 crease in ε at lower frequencies is due to the electrode polarization event, which associates with the accumulation of the ions and the complete dissociation of the salt; this nature is known as a nonDebye type of behavior, as shown in Fig 8a [44,45] The blocking electrodes prevent the ion migration to the external circuit, and this results in the accumulation of ions on the opposite electrodes termed as polarization [46] The growth of the polarizing ionic charges causes to increment the dielectric constant and dielectric loss [34] Moreover, the ion pairs stay in the immobilized state at low-frequency regions and this hinders the long-range motion and results in the high value of the dielectric constant due to sufficient relaxation time [44] Now, at higher frequencies, the decrease of the dielectric constant is attributed to the dominance of the relaxation process Here, 131 the rapid change in the direction of the field makes ions incapable of responding to the applied field, so that there is no excess ion diffusion in the direction of the electric field, also due to the inadequate time the ions could not sufficiently accumulate at the electrodes, and thus, the dielectric permittivity decreases [44,45] 00 From Fig 8b the large value of ε at low-frequencies is due to the formation of free charges at the electrolyteeelectrode interface 00 However, in the high-frequency region, the value of ε decreases due to a reduction of the charge carriers at the interface between electrode and electrolyte [45] From Fig it is observed that both ε 00 and ε increase with an increase in the temperature This behavior generally differs for polar and non-polar polymers In a non-polar 00 polymer the values of ε and ε are independent of the temperature, but in the case of strong polar polymers, the dielectric permittivity increases with increases in temperature Muthuvinayagam and Gopinathan [43] claimed that as the temperature increases, the degree of salt dissociation and redissociation of the ion aggregates increases, resulting in the increase in the numbers of free ions and at low frequency the carriers have sufficient time to remain at the interface causing an increase in ε Another important parameter providing insight into the number of charge carriers available for the conduction mechanism is the loss tangent (tan d) [34] The tan d is defined as the ratio of the imaginary part of the permittivity to its real part or the ratio of the energy loss to the energy stored [43] The dynamics of the tan d with the frequency for PBE-30 at different temperatures is presented in Fig From Fig 9, it is observed that the dielectric loss initially increases with an increase in the frequency and then reaches a maximum at the particular frequency (where ut ¼ 1), which is followed by the decrease at high frequencies [43] For the lower frequency domain, the value of tand becomes high; this behavior could be attributed to the space charge, which is built-up at the interface between the polymer and the electrode The existence of the peak in the loss spectrum suggests the presence of relaxing dipoles in the polymer films [42] It is well known that the growth of a movement of charge carriers can be noticed due to the height of the tand peak at the time when the peak height increases by the increment in temperature [34] In addition, when the temperature is increased the peak heights of tand are increased and sustain almost the same at the relaxation frequency This attitude indicates the breaking of the bond formation from the dipoles [47] The electrical conductivity obtained due to the production of charge carriers and their mobility of the charge carriers The protonic charges can be easily built by the collision of the ions in the dipoles of the polymer chain molecules, which is identified from the increment of the loss tangent peak height of blend polymer at the 00 Fig 11 Argand plots (M vs M ) for PBE-30 at different temperatures range between (303e373) K 132 H.T Ahmed, O.Gh Abdullah / Journal of Science: Advanced Materials and Devices (2020) 125e133 increasing temperatures With increasing temperature, the slight increase of the relaxation frequency in the loss tangent spectra designates the long-range mobility of charge carriers in polymer chain molecules [47] Fig 10 (a,b) shows the frequency dependence of the real M and 00 imaginary M parts of the modulus formalism The complex electric modulus (M * ) is defined as the reciprocal of the complex permittivity (ε* ) as following [22]: M* ¼ 00 ¼ M ỵ jM * (9) Fig 10 (a,b) shows the frequency dependence of the real (M ) 00 and imaginary (M ) parts of the modulus formalism for PBE-30 at different temperatures In the higher frequency region the value of 00 M and M increases gradually as a function of temperature with a tendency for saturation When the temperature is increased the 00 peak shifts to the high frequencies and the M and M peaks regularly decrease due to the plurality of relaxation mechanisms 00 Shyly et al [45] reported that the value of M and M is slowly reduced at higher temperatures due to a decrease in the charge carrier density at the space accumulation region However, at lower 00 frequencies the M and M values become zero, which proposes that the suppression of the electrode polarization at the interface is negligible The long straight line for the low-frequency region endorses a large equivalent capacitance associated with the electrode interface in use and confirms the non-Debye behavior in the samples The relaxation processes idea for the higher conducting polymer electrolyte prepared sample (PBE-30) at various temperatures can be exhibited by the investigation of the Argand plot, as shown in Fig 11 From this figure, the observed incomplete semicircle curves exhibit non-Debye nature The non-Debye behavior occurs due to the contribution of more than one type of polarizations, the relaxation mechanism, and many interactions between the ions and the dipoles [48] It is well reported that the radius of the arc is highly connected to the conductivity of the polymer electrolyte [44,49] When the temperature is increased, the length of the arc is decreased, which ensure increasing the conductivity [34] Also, the study of the Argand plots is crucial for determining the difference between the conductivity relaxation and viscoelastic relaxations processes, under the condition that the arc diameter of the M and 00 0 00 M matches with the M axis, it means that the M À M curve exhibits a complete semicircular arc, and thus a single relaxation time can be estimated This infers that the conductivity relaxation rec0 00 onciles the Debye model [49] If the M À M curve appears as incomplete semicircular arcs, then it means that there is a distribution of the relaxation times and subsequently, the ion transport occurs through the viscoelastic relaxations [50] In this study, the Argand plots exhibit incomplete semicircular arcs, revealing the distribution of relaxation times (non-Debye nature) Thus, the ion transport occurs through the viscoelastic relaxation process Conclusion Proton-conducting polymer electrolytes based on Polyethylene oxide and Methylcellulose complexed with ammonium iodide have been successfully prepared by the standard solution cast method The maximum ionic conductivity of 7:62 Â 10À5 S=cm was achieved at 273 K for the sample incorporated with 30% NH4I The increase in the sample conductivity is supported by XRD, FE-SEM, and EIS characterization The conduction mechanism for all electrolyte samples was explained by the overlapping large polaron tunneling (OLPT) model The long-range mobility of the charge carriers in the polymer chain molecules can be understood as a slight increase of the relaxation frequency in the loss tangent spectra with the increasing temperature The enhancement of the ionic conductivity upon the addition of NH4I associates with an increase in the mobility and the diffusion coefficient of the ions Employing the Rice and Roth model, the highest ionic conducting sample (PBE-30) exhibites highest m and D values of 2:07 Â 10À06 cm2 V À1 sÀ1 and 5:42 Â 10À08 cm2 sÀ1 , respectively The dielectric behaviors of the sample PBE-30 show a strong dependence on the frequency and the temperature and follow the non-Debye type dielectric relaxation Finally, the incomplete semicircular arc in the Argand plots indicates the non-Debye type of relaxation processes, and reveals that the ion transport occurs through the viscoelastic relaxation process Declaration of 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proton-conducting. .. distribution of relaxation times (non-Debye nature) Thus, the ion transport occurs through the viscoelastic relaxation process Conclusion Proton-conducting polymer electrolytes based on Polyethylene. .. enhancement of the ionic conductivity upon the addition of NH4I associates with an increase in the mobility and the diffusion coefficient of the ions Employing the Rice and Roth model, the highest ionic