# Preparation and Characterization of Nano-Dy2O3-Doped PVA + Na3C6H5O7 Polymer Electrolyte Films for Battery Applications

Authors: J. Ramesh Babu; K. Ravindhranath; K. Vijaya Kumar
Journal: Advances in Materials Science and Engineering
 (2018)

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## Abstract

Composite polymer electrolyte films containing various concentrations of nano-Dy2O3 (1.0 to 4.0%) in PVA + sodium citrate (90 : 10) are synthesized adopting solution cast method and are characterized using FTIR, XRD, SEM, and DSC techniques. The investigations indicate that all components are homogenously dispersed. Films containing 3% of nano-Dy2O3 are more homogenous and less crystalline, and the same is supported by DSC studies indicating the friendly nature to ionic conductivity. Transference number studies reveal that the major charge carriers are ions. With the increase in % of nano-Dy2O3, the conductivity increases and reaches maximum in 3% film with a value of 1.06 × 10−4 S/cm (at 303 K). Further, the conductivity of the film increases with raise in temperature due to the hopping of interchain and intrachain ion movements and fall in microscopic viscosity at the matrix interface of the film. Electrochemical cells are fabricated using these films with the configuration “anode (Mg + MgSO4)/[PVA (90%) + Na3C6H5O7 (10%) + (1–4% nano-Dy2O3)]/cathode (I2 + C + electrolyte),” and various discharge characteristics are evaluated. With 3% nano-Dy2O3 film, the maximum discharge time of 118 hrs with open-circuit voltage of 2.68 V, power density of 0.91 W/kg, and energy density of 107.5 Wh/kg are observed. These findings reflect the successful adoption of the developed polymer electrolyte films in electrochemical cells.


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## Body

## 1. Introduction

Solid polymer electrolytes are endowed with characteristics that are intermediate between the solid inorganic electrolytes and liquid electrolytes. These electrolytes by virtue of possessing good ionic conductivity, mechanical strength, and chemical, thermal, and electrochemical stabilities and furthermore good compatibility with the electrode materials are proving to be promising ingredients in electrochemical devices, fuel cells, supercapacitors, solar cells, electrochromic displays, and so on [1–5]. Many investigations are concentrating on improving the conductivity of the solid polymer electrolytes by chemically modifying the surface morphology to promote the formation of protecting layers of low resistivity at the electrolyte-electrode interface. But such surface modified polymer electrolytes are costly. Hence, investigations are being undertaken using multicomponent composite polymer films loaded with salts and inorganic compounds with an aim to enhance the conductivity by modifying the physicochemical properties of the blended films.PVA-based polymer films are finding more importance in this regard as they form good films with high mechanical strength, and they are nontoxic. The surface functional groups such as –OH offer easy surface modifications [6–8].Doping of nanoparticles in the PVA-based films with an aim to enhance the mobility of ions/electrons in the films besides stabilizing the films, is a relevant and recent development [9]. The nanoparticle-studded films are showing improved properties because the less surface area/volume ratio, quantum confinements, and other morphological changes attributed to the nanoparticles are influencing the conductivity of the films. Investigations are reported to the literature by doping nano-Al2O3, TiO2, SiO2, and Fe2O3 in PVA films with respect to different electrical properties [10–24].In this present investigation, nano-Dy2O3-doped PVA + sodium citrate composite films are synthesized and characterized using XRD, SEM, DSC, FTIR, and AC impedance spectroscopy, and their utility in electrochemical cell fabrication is probed.


## 2. Materials and Methods

All chemicals, namely, PVA (MW 85,000), Na3C6H5O7, and Dy2O3 used in the present work were of AR Grade. Dy2O3 was grinded for 8 hrs in ball milling machine until the size was between 30 and 60 nm. These particles were used in the preset investigation.The films were casted using solution cast technique [25]. Different proportions of PVA, Na3C6H5O7, and nano-Dy2O3 as detailed in Tables 1 and 2 were added to triple distilled water and stirred for 48 hours to get a homogenous solution. Then, the solution was poured in Petri dishes, dried at 40°C for 24 hrs, and then vacuum-dried for 24 hrs. The dried films were peeled and characterized with XRD, SEM, FTIR, and DSC, and their electrical properties were measured and presented in Figures 1–7 and Tables 1–3. Further, by using these films as solid polymer electrolytes, electrochemical cells were fabricated, and their characteristics were assessed and presented in Table 4.Table 1
DSC DATA pertaining toTg, Mp, and % of crystallinity of composite films.
Number
Film composition
T
g
M
p
% of crystallinity
1
Pure PVA
92.98
224.05
100
2
PVA + Na3C6H5O7 + Dy2O3 (90 : 10 : 1%)
89.26
223
78.27
3
PVA + Na3C6H5O7 + Dy2O3 (90 : 10 : 2%)
87
222
72.50
4
PVA + Na3C6H5O7 + Dy2O3 (90 : 10 : 3%)
85
221
65.26
5
PVA + Na3C6H5O7 + Dy2O3 (90 : 10 : 4%)
90
223
79.91Table 2
Ionic conductivity, activation energy, and transference numbers of PVA + sodium citrate + nano-Dy2O3 polymer electrolyte films at different compositions.
Number
Polymer electrolyte
Conductivity at 303 K (RT) (S/cm)
Activation energy (Ea)
Transference number
t
ion
t
ele
1
Pure PVA
5.59 × 10−10
0.42
—
—
2
PVA + Na3C6H5O7 + Dy2O3 (90 : 10 : 1%)
5.31 × 10−5
0.36
0.94
0.06
3
PVA + Na3C6H5O7 + Dy2O3 (90 : 10 : 2%)
4.48 × 10−5
0.28
0.97
0.03
4
PVA + Na3C6H5O7 + Dy2O3 (90 : 10 : 3%)
1.06 × 10−4
0.19
0.98
0.02
5
PVA + Na3C6H5O7 + Dy2O3 (90 : 10 : 4%)
3.26 × 10−6
0.26
0.93
0.07Table 3
Ionic conductivity values of PVA films at different temperatures.
Number
Composition of film: 90 : 10 : 3%
Conductivity (S/cm)
1
303 K
1.06 × 10−4
2
313 K
3.86 × 10−4
3
323 K
6.30 × 10−4
4
333 K
9.72 × 10−4FTIR spectra were recorded using Perkin Elmer FTIR Spectrophotometer in the range 4000 to 500 cm−1 adopting KBr pellet method (with the resolution of 0.5 cm−1). The spectra obtained are shown in Figure 1. XRD Bruker D8 instrument with Cu Kα radiation for 2θ angles between 10° and 60° (with scan rate 2°/min and step size 0.02°) was used to record spectra of the films, and the obtained spectra are presented in Figure 2.Figure 1
FTIR spectra of PVA (90%) + Na3C6H5O7 (10%) + nano-Dy2O3 (1–4%) films.Figure 2
XRD pattern of films of PVA (90%) + Na3C6H5O7 (10%) + nano-Dy2O3 PVA (1–4%) along with pure PVA and Na3C6H5O7.The scanning electron microscope (SEM) images were recorded using FE-SEM (Carl Zeiss, Ultra 55 model) and presented in Figure3.Figure 3
SEM images of composite films of PVA (90%) + Na3C6H5O7 (10%) + nano-Dy2O3 (1–4%).Differential scanning calorimeter (DSC) thermograms were recorded using DSC Q20 V24.11 Build 124 for the composite films of PVA (90%) + Na3C6H5O7 (10%) + nano-Dy2O3 (1–4%) films in determining the glass transition and melting temperatures and % of crystallinity. The results are presented in Figure 4 and Table 1.Figure 4
DSC curves of pure PVA and composite films of PVA (90%) + Na3C6H5O7 (10%) + nano-Dy2O3 (1–4%).The conductivity measurements were carried out using a HIOKI3532-50 impedance analyzer in the frequency range 50 Hz to 1 MHz at various temperatures of 303 K to 333 K. The obtained results are presented in Tables2 and 3 and Figures 5(a) and 5(b).Table 4
Cell parameters using the polymer electrolye PVA (90%) + Na3C6H5O7 (10%) + nano-Dy2O3 (1–4%) at constant load of 100 kΩ.
Number
Cell parameters
PVA + Na3C6H5O7 + nano-Dy2O3 (90 : 10 : 1%)
PVA + Na3C6H5O7 + nano-Dy2O3 (90 : 10 : 2%)
PVA + Na3C6H5O7 + nano-Dy2O3 (90 : 10 : 3%)
PVA + Na3C6H5O7 + nano-Dy2O3 (90 : 10 : 4%)
1
Open-circuit voltage (V)
1.96
2.37
2.68
2.14
2
Short-circuit current (µA)
230
372
340
260
3
Area of the cell (cm2)
1.34
1.34
1.34
1.34
4
Weight of the cell (g)
1.40
1.40
1.40
1.40
5
Discharge time (h)
96
108
118
96
6
Power density (W/kg)
0.32
0.62
0.91
0.39
7
Energy density (Wh/kg)
31.55
68
107.5
38.15
8
Current density (µA/cm2)
171.6
277
253
194
9
Discharge capacity (mA-hr)
22.54
40.17
40.12
24.96Figure 5
(a) Impedance plots of PVA : Na3C6H5O7 : nano-Dy2O3 (90 : 10 : 1–4%) polymer electrolyte films at room temperature. (b) Conductivity plots for PVA (90%) + Na3C6H5O7 (10%) + nano-Dy2O3 (3%) at different temperatures.
(a)
(b)The transference number measurements were made using Wagner’s polarization technique [26] and Watanabe technique [27], and the results are presented in Table 2 and Figure 6. Electrochemical cells were fabricated with the configuration of “Mg/MgSO4 (anode)/polymer electrolyte/(I2 + C + electrolyte) (cathode).” The discharge characteristics of the cell like open-circuit voltage (OCV), short-circuit current (SCC), power density, energy density, current capacity, and other parameters were measured under a constant load of 100 kΩ, and the obtained values are presented in Figure 7 and Tables 4 and 5.Figure 6
Transference number measurements for the composite films PVA (90%) + Na3C6H5O7 (10%) + nano-Dy2O3 (1–4%).Figure 7
Discharge characteristic plots of PVA (90%) + Na3C6H5O7 (10%) + nano-Dy2O3 (1–4%) electrochemical cell for constant load of 100 kΩ.Table 5
Comparison of present cell parameters with the data of other cells reported earlier.
Number
Solid-state electrochemical cell configuration
Open-circuit voltage (V)
Discharge time (hrs)
Reference number
1
Ag/(PVP + AgNO3)/(I2 + C + electrolyte)
0.46
82
[18]
2
Ag/(PEO + AgNO3)/(I2 + C + electrolyte)
0.61
48
[28]
3
Mg/PEO + Mg(NO3)2/(I2 + C + electrolyte)
1.85
142
[29]
4
K/(PVP + PVA + KBrO3)/(I2 + C + electrolyte)
2.30
72
[30]
5
Mg/PVA + Mg(CH3COO)2/(I2 + C + electrolyte)
1.84
87
[31]
6
K/(PEO + KYF4)/(I2 + C + electrolyte)
2.40
51
[32]
7
K/(PVP + PVA + KClO3)/(I2 + C + electrolyte)
2.00
52
[33]
8
Mg + MgSO4/[PVA (90%) + Na3C6H5O7 (10%) + nano-Dy2O3 (3%)]/(I2 + C + electrolyte)
2.68
118
Present


## 3. Results and Discussion

Various films synthesized were characterized using various surface morphological techniques and are discussed below.


### 3.1. FTIR Analysis

The FTIR spectra of the composite films of various compositions: PVA (90%) + Na3C6H5O7 (10%) + nano-Dy2O3 PVA (1–4%), were noted and presented in Figure 1.The IR data reveal the presence of functional groups like hydroxyl, carbonyl, and carboxylates. The broad peak in pure PVA at 3581–3055 cm−1 with center at 3312 cm−1 pertains to the stretching frequencies of–OH. The broadness indicates the hydrogen bonding between the surface –OH groups of PVA. With the increase in the % of nano-Dy2O3 in the films, the broadness of the film is decreased, and the position of frequencies are shifted to 3514–3088 cm−1 (with center at 3289 cm−1) and 3480–3110 cm−1 (with center at 3267 cm−1) with 1 and 2% of nano-Dy2O3, respectively. With films having 3 and 4% of nano-Dy2O3, sharp peaks are appeared, respectively, at 3602 and 3748 cm−1 for 3% film and 3580 and 3737 cm−1 for 4% film. The conversion of broad peak to sharp peaks with change in frequencies as the % of nano-Dy2O3 increases may be attributed to the complex formation between the –OH and nano-Dy2O3.Further, there is a shift of –CH2 frequencies from 2944 cm−1 (pure PVA) to 2920 cm−1, 2909 cm−1, 2887 cm−1, and 2954 cm−1 as the concentration of Dy2O3 in the film increases from 1 to 4%, respectively. The –C–O– stretching frequencies are also shifted from 1098 cm−1 (for pure PVA) to 1073 cm−1, 1062 cm−1, 1051 cm−1, and 1040 cm−1 as the % of Dy2O3 is increased from 1 to 4%, respectively.The frequencies pertaining to carboxylate ions, namely, 1466 and 1544 cm−1 (in citrates) are shifted to 1308, 1420, and 1555 cm−1 for 1%; 1331, 1421, and 1577 cm−1 for 2%; 1419, 1432, and 1544 cm−1 for 3%; and 1398, 1465, and 1544 cm−1 for 4% nano-Dy2O3-impregnated films. Further, carbonyl group frequencies are also progressively increased towards longer wave side: 1655 cm−1 for 1%, 1689 cm−1 for 2%, 1667 cm−1 for 3%, and 1700 cm−1 for 4% nano-Dy2O3 films. These changes in the frequency positions and their nature, indicate some kind of complex formation between the groups like –OH, C=O, and –COO− with the Dy2O3 and thereby results in the uniform and plasticized films.


### 3.2. XRD

XRD data of films of different composite are presented in Figure2.Pure PVA has broadened the characteristic peak from 18 to 22°, in the 2θ range (110), while pure nano-Dy2O3 has well-defined peaks reflecting its crystalline nature (Figure 2). With the addition of 10% of sodium citrate salt in the PVA film, the peak is broadened and shifted to 19.0° with decrease in intensity [34]. Further, in the binary blended films of PVA + sodium citrate, different nano-Dy2O3 concentrations are doped, and the peaks are shifted with further broadening until the % of nanoparticles is 3%. With 4% Dy2O3 films, again sharp peaks pertaining to Dy2O3 are noted. This indicates that the film has reached saturation at 3% of Dy2O3, and above this concentration, the nanoparticles are precipitating. Further, the broadening of peaks indicate the generation of more amorphous region in the film, which is conducive to the ionic mobility. These changes in the XRD spectrum reflect that the ingredients in the film are homogenously dispersed in the film and that the film containing the 3% of nano-Dy2O3 possesses more amorphous regions and more homogenous nature, indicating the suitability of it for conductivity studies.


### 3.3. SEM Analysis

The SEM images of the films at different concentrations of nano-Dy2O3 are presented in Figure 3.It is seen from the images that as the concentration of nano-Dy2O3 is increased from 1 to 3% the edges, corners, gaps and so on are decreasing and thereby the homogeneity, so also the amorphous nature, of the film is increased. The more homogenous film is found with 3% Dy2O3 film. This increasing homogeneousness is attributed to the bonding between the functional groups of the film, namely, –OH and –COO− with nano-Dy2O3. This bonding may be through the occupation of vacant coordinating sites of Dy2O3 or due to some sort of hydrogen bonding such as “Dy=O … H–O” or “Dy–OH … O=C–.” When the % of Dy2O3 is more than 3%, again edges, boundaries, and holes are appeared, indicating the precipitation of undissolved nano-Dy2O3 in the film. These inferences are supported by XRD data.


### 3.4. DSC Studies

For assessing the degree of crystallinity, differential scanning calorimeter (DSC) investigations were undertaken, and the obtained results are presented in Figure4 and Table 1. Tg and Mp were evaluated from graphs (Figure 4).Percentage of crystallinities of the films were calculated using the equation: %χc = {(ΔHm)/(ΔHm0)} × 100, where ΔHm0 is the melting enthalpy of PVA : sodium citrate (90 : 10) film and ΔHm is the melting enthalpy of related composite films containing nano-Dy2O3 at varied percentages [35]. The glass transition temperature (Tg), melting temperature (Tm), and relative percentage of crystallinity (% χc) values are presented in Table 1.At 3% of nano-Dy2O3, the crystallinity is lowest, and the film is more homogenous and plasticized with more amorphous nature. This kind of structure provides more paths to proton transport in the film and hence more conductivity.


### 3.5. Impedance Analysis

The conductivity of the different films was measured by sandwiching PVA (90%) + Na3C6H5O7 (10%) + nano-Dy2O3 (1–4%) films between thin stainless steel plates using HIOKI3532-50 impedance analyzer at temperature 303 K. The conductivity was calculated using the formula: σ = l = Rb/A in S/cm, where σ = ionic conductivity, l = thickness of the polymer electrolyte film, Rb = bulk resistance, and A = area of the stainless steel electrode, contacting the polymer electrolyte film.Findings are presented in Figures5(a) and 5(b) and Table 2. Further, the conductivities at various temperatures were measured (using HIOKI3532-50 impedance analyzer) for the PVA (90%) + Na3C6H5O7 (10%) + nano-Dy2O3 PVA (3%) film in view of the fact that the said film was showing higher conductivity.It is observed from Figure5(a) that the absence of high-frequency semicircle portion reflects the charge carriers are ions. The presence of nano-Dy2O3 in the films has remarkably increased the conductivity of the films. The conductivity of pure PVA is of the order 10−10 S/cm [28]. Increase in conductivity is found with increase in concentration of nano-Dy2O3, and the conductivity reached maximum at 3% and any further increase causes the decrease of conductivity: 5.31 × 10−5 with 1%; 4.48 × 10−5with 2%; 1.06 × 10−4 with 3%, and 3.26 × 10−6 with 4%.The nano-Dy2O3 by virtue of possessing high surface area, quantum confinements, paramagnetic nature, and bonding tendencies of accepting electrons (Lewis acidic nature) from donor atoms of the surface functions groups influences the conductivity of the composite films [36]. Moreover, the bonding results in cross-linking PVA + Na3C6H5O7 segments through the bonding tendencies of nano-Dy2O3 and thereby generates additional pathways in the film that complements the ionic movement [22–24]. Thus, acquired homogeneity in more plasticized films brings some kind of order in the films, and this facilitates easy movement of the charge carriers that results in the enhancement of conductivity. It seems that PVA + Na3C6H5O7 (90 : 10) film is statured with nano-Dy2O3 at 3% and any further increase results in the precipitation of the nanoparticles in the pathways, thereby blocking short ways in the polymer matrix for the movement of ions. This results in low conductivity. It may be noted that the activation energy is minimum in the composite film having 3% of nano-Dy2O3 with a value of 0.19 V (Table 1).Further, the bulk resistance decreases with increase in temperature, and this results in the enhancement of the conductivity. At 303 K, the conductivity is 1.06 × 10−4 S/cm, while at 333 K, it is 9.72 × 10−4 S/cm (vide Table 3). This increase in conductivity may be attributed to the hopping of interchain and intrachain ion movements and also due to decrease in microscopic viscosity at interface in solid polymer electrolyte matrixes.


### 3.6. Transference Numbers

For the classification of polymer electrolyte films, ionic transference number is considered to be one of the most significant parameters. By using Wagner’s polarizing technique, the transference numbers for the films were measured by constructing a system of Mg/(PVA + Na3C6H5O7 + Dy2O3)/C and polarizing it at 303 K at a constant dc potential of 1.5 V in order to assess the contributions of ions and electrons to the entire conductivity of the polymer electrolyte films. The equations used were(1)tion=Iinitial−IfinalIinitial=Itotal−IelectronicItotal=IionicItotal,where Iinitial is the initial current and Ifinal is the final current. The ionic transference number (tion) values were in the range 0.93–0.98. The results are presented in Figure 6 and Table 2.It is seen from Table2 that the charge carriers are predominantly ions in all the composite films, and the contribution of electrons is very minute. For the composite films containing 3.0% of Dy2O3, more conductivity is observed and in fact, at that composition, the film has low activation energy and crystallinity. Further, the ionic transference numbers (tion) of the films are near to unity, indicating their suitability as a polymer electrolyte for solid-state electrochemical cells [25–27].


### 3.7. Discharge Studies

The solid-state electrochemical cells were fabricated with the configuration “anode (Mg + MgSO4)/[PVA (90%) + Na3C6H5O7 (10%) + nano-Dy2O3 (1–4%)]/cathode (I2 + C + electrolyte).” The thickness of both the electrodes was 1 mm, while the surface area and thicknesses of the “PVA + Na3C6H5O7  + nano-Dy2O3” polymer electrolyte were 1.34 cm2 and 150 µm, respectively. The presence of carbon in the cathode increases the conductivity, and polyelectrolyte reduces the resistance by allowing more interfacial contact between the cathode and electrolyte [35]. The discharge characteristics, namely, open-circuit voltage (OCV), short-circuit current (SCC), current density, power density, energy density, discharge time, and discharge capacity of the cell for a constant load of 100 kΩ were evaluated at room temperature and are shown in Figure 7 and Table 4.It is seen from the data that the film with 3% of nano-Dy2O3 shows better discharge characters: discharge time: 118 h; energy density: 107.5 Wh/kg; power density: 0.91 W/kg; and open-circuit voltage: 2.68 V. These indicate that these composite films loaded with optimum amounts of nano-Dy2O3 can be successfully adopted as polyelectrolyte films in the battery applications.


## 3.1. FTIR Analysis

The FTIR spectra of the composite films of various compositions: PVA (90%) + Na3C6H5O7 (10%) + nano-Dy2O3 PVA (1–4%), were noted and presented in Figure 1.The IR data reveal the presence of functional groups like hydroxyl, carbonyl, and carboxylates. The broad peak in pure PVA at 3581–3055 cm−1 with center at 3312 cm−1 pertains to the stretching frequencies of–OH. The broadness indicates the hydrogen bonding between the surface –OH groups of PVA. With the increase in the % of nano-Dy2O3 in the films, the broadness of the film is decreased, and the position of frequencies are shifted to 3514–3088 cm−1 (with center at 3289 cm−1) and 3480–3110 cm−1 (with center at 3267 cm−1) with 1 and 2% of nano-Dy2O3, respectively. With films having 3 and 4% of nano-Dy2O3, sharp peaks are appeared, respectively, at 3602 and 3748 cm−1 for 3% film and 3580 and 3737 cm−1 for 4% film. The conversion of broad peak to sharp peaks with change in frequencies as the % of nano-Dy2O3 increases may be attributed to the complex formation between the –OH and nano-Dy2O3.Further, there is a shift of –CH2 frequencies from 2944 cm−1 (pure PVA) to 2920 cm−1, 2909 cm−1, 2887 cm−1, and 2954 cm−1 as the concentration of Dy2O3 in the film increases from 1 to 4%, respectively. The –C–O– stretching frequencies are also shifted from 1098 cm−1 (for pure PVA) to 1073 cm−1, 1062 cm−1, 1051 cm−1, and 1040 cm−1 as the % of Dy2O3 is increased from 1 to 4%, respectively.The frequencies pertaining to carboxylate ions, namely, 1466 and 1544 cm−1 (in citrates) are shifted to 1308, 1420, and 1555 cm−1 for 1%; 1331, 1421, and 1577 cm−1 for 2%; 1419, 1432, and 1544 cm−1 for 3%; and 1398, 1465, and 1544 cm−1 for 4% nano-Dy2O3-impregnated films. Further, carbonyl group frequencies are also progressively increased towards longer wave side: 1655 cm−1 for 1%, 1689 cm−1 for 2%, 1667 cm−1 for 3%, and 1700 cm−1 for 4% nano-Dy2O3 films. These changes in the frequency positions and their nature, indicate some kind of complex formation between the groups like –OH, C=O, and –COO− with the Dy2O3 and thereby results in the uniform and plasticized films.


## 3.2. XRD

XRD data of films of different composite are presented in Figure2.Pure PVA has broadened the characteristic peak from 18 to 22°, in the 2θ range (110), while pure nano-Dy2O3 has well-defined peaks reflecting its crystalline nature (Figure 2). With the addition of 10% of sodium citrate salt in the PVA film, the peak is broadened and shifted to 19.0° with decrease in intensity [34]. Further, in the binary blended films of PVA + sodium citrate, different nano-Dy2O3 concentrations are doped, and the peaks are shifted with further broadening until the % of nanoparticles is 3%. With 4% Dy2O3 films, again sharp peaks pertaining to Dy2O3 are noted. This indicates that the film has reached saturation at 3% of Dy2O3, and above this concentration, the nanoparticles are precipitating. Further, the broadening of peaks indicate the generation of more amorphous region in the film, which is conducive to the ionic mobility. These changes in the XRD spectrum reflect that the ingredients in the film are homogenously dispersed in the film and that the film containing the 3% of nano-Dy2O3 possesses more amorphous regions and more homogenous nature, indicating the suitability of it for conductivity studies.


## 3.3. SEM Analysis

The SEM images of the films at different concentrations of nano-Dy2O3 are presented in Figure 3.It is seen from the images that as the concentration of nano-Dy2O3 is increased from 1 to 3% the edges, corners, gaps and so on are decreasing and thereby the homogeneity, so also the amorphous nature, of the film is increased. The more homogenous film is found with 3% Dy2O3 film. This increasing homogeneousness is attributed to the bonding between the functional groups of the film, namely, –OH and –COO− with nano-Dy2O3. This bonding may be through the occupation of vacant coordinating sites of Dy2O3 or due to some sort of hydrogen bonding such as “Dy=O … H–O” or “Dy–OH … O=C–.” When the % of Dy2O3 is more than 3%, again edges, boundaries, and holes are appeared, indicating the precipitation of undissolved nano-Dy2O3 in the film. These inferences are supported by XRD data.


## 3.4. DSC Studies

For assessing the degree of crystallinity, differential scanning calorimeter (DSC) investigations were undertaken, and the obtained results are presented in Figure4 and Table 1. Tg and Mp were evaluated from graphs (Figure 4).Percentage of crystallinities of the films were calculated using the equation: %χc = {(ΔHm)/(ΔHm0)} × 100, where ΔHm0 is the melting enthalpy of PVA : sodium citrate (90 : 10) film and ΔHm is the melting enthalpy of related composite films containing nano-Dy2O3 at varied percentages [35]. The glass transition temperature (Tg), melting temperature (Tm), and relative percentage of crystallinity (% χc) values are presented in Table 1.At 3% of nano-Dy2O3, the crystallinity is lowest, and the film is more homogenous and plasticized with more amorphous nature. This kind of structure provides more paths to proton transport in the film and hence more conductivity.


## 3.5. Impedance Analysis

The conductivity of the different films was measured by sandwiching PVA (90%) + Na3C6H5O7 (10%) + nano-Dy2O3 (1–4%) films between thin stainless steel plates using HIOKI3532-50 impedance analyzer at temperature 303 K. The conductivity was calculated using the formula: σ = l = Rb/A in S/cm, where σ = ionic conductivity, l = thickness of the polymer electrolyte film, Rb = bulk resistance, and A = area of the stainless steel electrode, contacting the polymer electrolyte film.Findings are presented in Figures5(a) and 5(b) and Table 2. Further, the conductivities at various temperatures were measured (using HIOKI3532-50 impedance analyzer) for the PVA (90%) + Na3C6H5O7 (10%) + nano-Dy2O3 PVA (3%) film in view of the fact that the said film was showing higher conductivity.It is observed from Figure5(a) that the absence of high-frequency semicircle portion reflects the charge carriers are ions. The presence of nano-Dy2O3 in the films has remarkably increased the conductivity of the films. The conductivity of pure PVA is of the order 10−10 S/cm [28]. Increase in conductivity is found with increase in concentration of nano-Dy2O3, and the conductivity reached maximum at 3% and any further increase causes the decrease of conductivity: 5.31 × 10−5 with 1%; 4.48 × 10−5with 2%; 1.06 × 10−4 with 3%, and 3.26 × 10−6 with 4%.The nano-Dy2O3 by virtue of possessing high surface area, quantum confinements, paramagnetic nature, and bonding tendencies of accepting electrons (Lewis acidic nature) from donor atoms of the surface functions groups influences the conductivity of the composite films [36]. Moreover, the bonding results in cross-linking PVA + Na3C6H5O7 segments through the bonding tendencies of nano-Dy2O3 and thereby generates additional pathways in the film that complements the ionic movement [22–24]. Thus, acquired homogeneity in more plasticized films brings some kind of order in the films, and this facilitates easy movement of the charge carriers that results in the enhancement of conductivity. It seems that PVA + Na3C6H5O7 (90 : 10) film is statured with nano-Dy2O3 at 3% and any further increase results in the precipitation of the nanoparticles in the pathways, thereby blocking short ways in the polymer matrix for the movement of ions. This results in low conductivity. It may be noted that the activation energy is minimum in the composite film having 3% of nano-Dy2O3 with a value of 0.19 V (Table 1).Further, the bulk resistance decreases with increase in temperature, and this results in the enhancement of the conductivity. At 303 K, the conductivity is 1.06 × 10−4 S/cm, while at 333 K, it is 9.72 × 10−4 S/cm (vide Table 3). This increase in conductivity may be attributed to the hopping of interchain and intrachain ion movements and also due to decrease in microscopic viscosity at interface in solid polymer electrolyte matrixes.


## 3.6. Transference Numbers

For the classification of polymer electrolyte films, ionic transference number is considered to be one of the most significant parameters. By using Wagner’s polarizing technique, the transference numbers for the films were measured by constructing a system of Mg/(PVA + Na3C6H5O7 + Dy2O3)/C and polarizing it at 303 K at a constant dc potential of 1.5 V in order to assess the contributions of ions and electrons to the entire conductivity of the polymer electrolyte films. The equations used were(1)tion=Iinitial−IfinalIinitial=Itotal−IelectronicItotal=IionicItotal,where Iinitial is the initial current and Ifinal is the final current. The ionic transference number (tion) values were in the range 0.93–0.98. The results are presented in Figure 6 and Table 2.It is seen from Table2 that the charge carriers are predominantly ions in all the composite films, and the contribution of electrons is very minute. For the composite films containing 3.0% of Dy2O3, more conductivity is observed and in fact, at that composition, the film has low activation energy and crystallinity. Further, the ionic transference numbers (tion) of the films are near to unity, indicating their suitability as a polymer electrolyte for solid-state electrochemical cells [25–27].


## 3.7. Discharge Studies

The solid-state electrochemical cells were fabricated with the configuration “anode (Mg + MgSO4)/[PVA (90%) + Na3C6H5O7 (10%) + nano-Dy2O3 (1–4%)]/cathode (I2 + C + electrolyte).” The thickness of both the electrodes was 1 mm, while the surface area and thicknesses of the “PVA + Na3C6H5O7  + nano-Dy2O3” polymer electrolyte were 1.34 cm2 and 150 µm, respectively. The presence of carbon in the cathode increases the conductivity, and polyelectrolyte reduces the resistance by allowing more interfacial contact between the cathode and electrolyte [35]. The discharge characteristics, namely, open-circuit voltage (OCV), short-circuit current (SCC), current density, power density, energy density, discharge time, and discharge capacity of the cell for a constant load of 100 kΩ were evaluated at room temperature and are shown in Figure 7 and Table 4.It is seen from the data that the film with 3% of nano-Dy2O3 shows better discharge characters: discharge time: 118 h; energy density: 107.5 Wh/kg; power density: 0.91 W/kg; and open-circuit voltage: 2.68 V. These indicate that these composite films loaded with optimum amounts of nano-Dy2O3 can be successfully adopted as polyelectrolyte films in the battery applications.


## 4. Comparison with Previous Work

The electrochemical cell developed in this work is compared with the works available in the literature, and a comparative statement is presented in Table5.It is inferred from Table5 that the present developed solid-state electrolyte system is more efficient than many reported in the literature with respect to simplicity, efficiency, reliability, and good discharge times besides being economical and environment friendly. These systems find applications as cost-effective electrolytes in high-density solid-state electrochemical cells.


## 5. Conclusions

Nano-Dy2O3-doped polymer electrolyte films of composition “PVA (90%) + sodium citrate (10%) + nano-Dy2O3 (1 to 4%)” are synthesized by solution cast technique. The films are found to be good, stable, and endowed with excellent plasticity.The films surface morphological characteristics and other physicochemical features are assessed by using FTIR, XRD, and SEM methods. The studies reveal that the different components in the film are homogenously and completely dispersed. It is attributed to the bonding tendencies of various functional groups present in PVA and sodium citrate such as –OH and –COO− with the Dy2O3. This binding nature helps to form a kind of surface complex, which is conducive for the movement of ions. The optimum % of Dy2O3 for forming more homogenous and less crystalline film is found to be 3%. Even DSC investigations reveal that at 3.0% of nano-Dy2O3, the amorphous region of the film is more, and this enables the ions to penetrate more in the film, thereby increasing the conductivity of the films.The presence of nano-Dy2O3 remarkably enhances the conductivity of these PVA-based films. The conductivity for pure PVA film is 5.59 × 10−10 S/cm at 303 K. But the conductivity is increased to 5.31 × 10−5, 4.48 × 10−5, 1.06 × 10−4, and 3.26 × 10−6 S/cm (at 303 K) with the films containing 1%, 2%, 3%, and 4% of nano-Dy2O3, respectively. With the increase in temperature, the conductivity is increased: 1.06 × 10−4 S/cm at 303 K; 3.86 × 10−4 S/cm at 313 K; 6.30 × 10−4 S/cm; at 323 K; and 9.72 × 10−4 S/cm at 333 K. This enhancement in conductivity is due to the hopping of interchain and intrachain ion movements and falling of microscopic viscosity at the matrix interface of the film. The studies on transference numbers reveal that the charge carriers are ions, and the contribution of electrons is almost negligible.These polymer electrolyte films are investigated for their utility by constructing electrochemical cell with the configuration “anode (Mg + MgSO4)/[PVA (90%) + Na3C6H5O7 (10%) + (1–4% nano-Dy2O3)]/cathode (I2 + C + electrolyte).” The various discharge characteristics are evaluated and found that the films with 3% nano-Dy2O3 have maximum discharge time of 118 hrs, open-circuit voltage of 2.68 V, power density of 0.91 W/kg, and energy density of 107.5 Wh/Kg. These good findings emphasize the successful adoption of the developed polymer electrolyte films. These films may find good utility in solid-state battery applications.



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*Source: 2080369-2018-03-12.xml*