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).

Original Article

Impedance and ionic transport properties of proton-conducting

electrolytes based on polyethylene oxide/methylcellulose blend

polymers

a Charmo Center for Research, Training & Consultancy, Charmo University, 46023, Chamchamal e Sulaimani, Kurdistan Region, Iraq

b 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

Article history:

Received 2 December 2019

Received in revised form

1 February 2020

Accepted 1 February 2020

Available online 8 February 2020

Keywords:

Proton-conducting

Ionic conductivity

Diffusion coefficient

Electric modulus

Argand plot

a b s t r a c t

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
105S=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 con-ductivity 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 activa-tion energy The dielectric data were analyzed for the highest ionic conducting sample at various tem-peratures 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/)

1 Introduction

Nowadays, solid polymer electrolytes (SPEs) have great

attrac-tion through out the disciplines of electrochemistry, polymer

sci-ence, organic chemistry, and inorganic chemistry In its progress, in

turn, it revolutionizes in both academia and industry area the

the ionic conductivities at ambient temperature for SPEs has been

mixture (blending), plasticization, and the addition of micro/

polymer-based electrolytes can be modulated by doping salts,

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

together provides more complexation sites, which raise the ion

attention has been paid to the development of the proton-conducting polymer electrolytes due to their performance and promising technological applications in advanced smart devices

(PEO), polyvinyl alcohol (PVA), Poly (N-vinyl pyrrolidone) (PVP)

* Corresponding author.

E-mail address: omed.abdullah@univsul.edu.iq (O.Gh Abdullah).

Peer review under responsibility of Vietnam National University, Hanoi.

Journal of Science: Advanced Materials and Devices

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

Journal of Science: Advanced Materials and Devices 5 (2020) 125e133

based salts [21] The present research is an extension of our

pre-viously published papers where we discussed the structural and

electrical characterization of PEO/MC blend electrolyte system to

blended with MC at the ratio of 60:40 to form the blended polymer

proton source is predicted to obtain a higher conductivity, because

ex-hibits a higher ionic conductivity due to the lower lattice energy

and relatively larger anion size compared to the other ammonium

proton-conducting polymer electrolyte based on the PEO/MC blend

investigation focuses on the analysis of the ionic transport

prop-erties to understand and thus, improve the ionic conduction

mechanism of proton-conducting polymer electrolyte

2 Experimental

2.1 Materials and methods

weight), and methylcellulose (MC) (Merck KGaA Germany, with

molecular weight 14,000 g/mol) are taken as primary and

sec-ondary polymer precursors for preparing the blend polymer

with molecular weight of 144.94 g/mol was used All the materials

2.2 Proton-conducting polymer electrolyte preparation

For the preparation of the proton-conducting polymer

electro-lyte, 2 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

2.3 Characterization techniques

X'Pert PRO diffractometer system by using monochromatic X-rays

tube was operated at 45 kV voltage, and 40 mA anode current with

temper-ature Micromorphological characterization of the prepared

emis-sion 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

(KEYSIGHT E4980A) that was interfaced to a computer, in the fre-quency range 100 Hz to 2 MHz and in the temperature range

size and sandwiched between two blocking aluminum electrodes The Nyquist plane plots were obtained from the recorded imped-ance data by applying a 100 mV perturbation to an open circuit potential in the above-mentioned frequency range

3 Results and discussion 3.1 XRD analysis

Fig 1represents the XRD pattern for the various PEO/MC-NH4I

Table 1

Composition of the blend polymer electrolyte series containing different wt.% of

NH 4 I.

Designation PEO Solution MC Solution NH 4 I wt.% NH 4 I (g)

Powder (g) Solvent (ml) Powder

(g) Solvent (ml)

PBE-50 2 120 2 240 50 2.000 Fig 1 XRD patterns for PEO/MC-NH4 I blend polymer electrolyte films incorporated

Bragg peaks at 19.2and 23.0for the PBE-10 sample described the

chains due the intermolecular hydrogen bonding causes the

for-mation of semi-crystalline peaks These peaks become broader and

this demonstrated that the reduction in the relative intensity and

broad nature of the characteristic peak clearly indicates that the

semi-crystalline nature The intensity of the semi-crystalline peaks

from the blend polymer sample was found to decrease

im-plies the decrease in the degree of crystallinity This results from

causes the decrease in the intermolecular interaction between PEO/

MC chains, thus, induces new coordination interactions between

group of MC of the blend polymer formed which helps for boost

The XRD peak deconvolution method was utilized to estimate

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 na-ture Many researchers have concluded that the ionic conductivity

anticipated that the sample with the lowest crystalline region (PBE-30) exhibits the highest electrical conductivity For the highest salt concentration sample (PBE-50) some multiple characteristic peaks

revealing that the host polymers could no longer solvate the salt

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

shows a change in the surface properties and the texture structure

concentrations In the present work, the FE-SEM study has been studied to understand the variation of the electrical conductivity

Fig 2 SEM micrograph of PEO/MC blend polymer films containing (a) 20 wt.% NH 4 I (b) 30 wt.% NH 4 I, (c) 40 wt.% NH 4 I and (d) 50 wt.% NH 4 I.

H.T Ahmed, O.Gh Abdullah / Journal of Science: Advanced Materials and Devices 5 (2020) 125e133 127

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

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

host polymer matrix results in the recombination of the ions and

the reduction of the density of ions, thus the decrease in the

con-ductivity This FE-SEM analysis, therefore, has been used to give

some answers describing the reduction of the conductivity in the

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 3illustrates the complex impedance plot of different wt.%

one spike with a semicircle The semicircular arc can be utilized to

calculate the conductivity of the system by using this equation:

bulk resistance determined from the intercept on the real axis at

From Fig 3 it can be obtained that the conductivity of the

complexes increases with the content of the doping salt and

among all other compositions However, upon further addition of

increase in the conductivity is attributed to the enhancement of the

Meanwhile, the decrease in the conductivity could be due to

ag-gregates and the formation of the ion pairs, which produces neutral

all prepared samples, by expanding the temperature range, the bulk resistance decreases inferring the improvement of the electrical

tem-perature 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

surface of the polymer electrolyte samples

The activation energy for the thermally activated hoping

Table 2recommends that the electrolytes obey Arrhenius behavior

addressed in previous studies that the activation energy decreases gradually with an increase in the conductivity of a polymer blend

Fig 3 ColeeCole plots for PEO/MC-NH 4 I blend polymer electrolyte films incorporated with different NH 4 I salt concentrations.

Fig 4 The temperature-dependent conductivity spectra for PEO/MC when mixed with

the ions necessitate a lower energy for migration in highly

con-ducting samples It was reported that the low activation energy for

the polymer blend system is due to the entirely amorphous nature

move through the polymer network Also, refer to Buraidah et al

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

electro-lyte system, the exponent s is plotted as a function of the

(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,

tem-perature 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

tem-peratures are less than 0.8 and they are temperature dependent

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

conducting path for the ions, thus the ions are able to tunnel

through the potential barrier that exists between the two possible

3.4 Ion transport study

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

y¼

ffiffiffiffiffiffiffiffi 2Ea

m

r

(2)

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

conductivity can be calculated using the Rice and Roth equation as follows:

Table 2

The ionic conductivity (s), activation energy (E a ), and regression values (R 2 ) for

various compounds of PEO/MC-NH 4 I blend polymer electrolyte films at ambient

temperature.

Fig 6 The ionic conductivity and activation energy of PEO/MC-NH 4 I blend polymer electrolyte as a function of NH 4 I wt.%.

Fig 7 The temperature dependence values s for PEO/MC-NH 4 I electrolyte system Fig 5 The PEO/MC-NH 4 I Arrhenius plot in the temperature range 303e373 K.

H.T Ahmed, O.Gh Abdullah / Journal of Science: Advanced Materials and Devices 5 (2020) 125e133 129

Here Z is the vacancy of conducting species, m is the mass of the

two coordinating sites or two atoms with the lone pair electrons

found as an essential parameter to determine the mobility of the

h¼ 3smKBT

2ðZeÞ2Eatexp



E a

K B T

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

possesses the highest mobility and the highest diffusion value This result is supported by the fact that the conductivity is governed by

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 in-creases, which causes the reduction of the ion mobility and

3.5 Dielectric analysis

electrolytes are not much investigated earlier Thus the dielectric

Þ, tangent loss and the electric

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

Fig 8 (a) Dielectric constant (b) Dielectric loss as a function of frequency for PBE-30 at

different temperatures between (303e373) K.

Fig 9 Dielectric loss behavior for PBE-30 as a function of frequency from the tem-perature range between (303e373) K.

ε0A ; ε00¼ ε0tand (8)

FromFig 8it can be noted that the values ofε0

decrease with the increasing frequency while these values rapidly increased

in the low-frequency region and at high temperatures This attitude

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

electrodes prevent the ion migration to the external circuit, and this

results in the accumulation of ions on the opposite electrodes

charges causes to increment the dielectric constant and dielectric

low-frequency regions and this hinders the long-range motion and

Now, at higher frequencies, the decrease of the dielectric

con-stant is attributed to the dominance of the relaxation process Here,

FromFig 8b the large value ofε00

at low-frequencies is due to the

decreases due to a reduction of the charge carriers at the interface between

increase with an increase in the temperature This behavior generally differs for polar and non-polar polymers In a non-polar

are independent of the tempera-ture, but in the case of strong polar polymers, the dielectric permittivity increases with increases in temperature

increases, the degree of salt dissociation and redissociation of the ion aggregates increases, resulting in the increase in the numbers of

Another important parameter providing insight into the num-ber of charge carriers available for the conduction mechanism is the

imaginary part of the permittivity to its real part or the ratio of the

with the frequency for PBE-30 at different temperatures is

initially increases with an increase in the frequency and then

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

of a movement of charge carriers can be noticed due to the height of

almost the same at the relaxation frequency This attitude indicates

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

increment of the loss tangent peak height of blend polymer at the

Fig 10 Frequency-depends of modulus formalism (a) real part of modulus, (b)

imaginary part of modulus at different temperatures.

Fig 11 Argand plots (M0vs M00) for PBE-30 at different temperatures range between (303e373) K.

H.T Ahmed, O.Gh Abdullah / Journal of Science: Advanced Materials and Devices 5 (2020) 125e133 131

M and M increases gradually as a function of temperature with a

tendency for saturation When the temperature is increased the

regularly decrease due to the plurality of relaxation mechanisms

reduced at higher temperatures due to a decrease in the charge

carrier density at the space accumulation region However, at lower

the suppression of the electrode polarization at the interface is

negligible The long straight line for the low-frequency region

en-dorses a large equivalent capacitance associated with the electrode

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 thisfigure, 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

highly connected to the conductivity of the polymer electrolyte

study of the Argand plots is crucial for determining the difference

between the conductivity relaxation and viscoelastic relaxations

curve ex-hibits a complete semicircular arc, and thus a single relaxation time

can be estimated This infers that the conductivity relaxation

curve appears as incomplete semicircular arcs, then it means that there is a

distri-bution of the relaxation times and subsequently, the ion transport

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

4 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

increase in the sample conductivity is supported by XRD, FE-SEM,

and EIS characterization The conduction mechanism for all

elec-trolyte 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

Declaration of Competing Interest

Acknowledgement The authors gratefully acknowledge the support received for carrying out this work from the University of Sulaimani, and Charmo University at the Ministry of Higher Education and

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