# Electrical and Magnetic Properties of Polymer Electrolyte (PVA:LiOH) ContainingIn Situ Dispersed Fe3O4 Nanoparticles

Authors: M. P. Aji; Rahmawati; Masturi; S. Bijaksana; Khairurrijal; M. Abdullah
Journal: ISRN Materials Science
 (2012)

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## Abstract

Nanocomposite magnetic polymer electrolytes based on poly(vinyl alcohol) (PVA) complexed with lithium hydroxide (LiOH) and containing magnetite (Fe3O4) nanoparticles were prepared using an in situ method, in which the nanoparticles were grown in the host polymer electrolyte. Ion carriers were formed during nanoparticle growth from the previously added LiOH precursor. If a high concentration of LiOH was added, the remaining unreacted LiOH was distributed in the form of an amorphous complex around the Fe3O4 nanoparticles, thus preventing agglomeration of the nanoparticles by the host polymer. By addition of Fe3O4 the composite polymer electrolytes improved the ionic conductivity, resulting in a maximum conductivity of 1.81×10-3 S·cm−1. The magnetic properties of the polymer electrolyte were investigated through magnetic susceptibility studies, and the material was predominantly ferromagnetic.


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## Body

## 1. Introduction

The normal role of inorganic particle fillers in polymer electrolytes is to influence the recrystallization kinetics of the polymer chain and promote localized amorphous regions, thus enhancing cation transport. Examples of inorganic particles used in polymer composites include SiO2, Al2O3, TiO2, CdO, and ZnO [1–9]. Inorganic particles are also used to enhance the mechanical properties of polymer electrolytes [10]. Most authors agree that the role of nanoparticle fillers is very important and must be present in the host polymer for proper function.Magnetic nanoparticles such as magnetite (Fe3O4) have a unique response to magnetic fields and a saturation magnetization much lower than that of the corresponding bulk materials. The saturation magnetization decreases with particle size [11, 12]. One interesting point is that dispersion of Fe3O4 nanoparticles in polymer electrolytes may also assist in ionic transport. We sought to produce a magnetic polymer electrolyte containing Fe3O4 nanoparticles as magnetic centers. This class of materials has considerable potential for producing devices that simultaneously interact with both electrical and magnetic fields, such as magnetic electrochemical cells and sensors.The Fe3O4 nanoparticles were produced using a coprecipitation method. Two typical synthetic approaches are coprecipitation of partly oxidized Fe2+ to Fe3+ in oxidizing solutions and direct coprecipitation of Fe2+ and Fe3+ in alkaline media [13, 14]. This approach ensures facile dispersion of the nanoparticles in the host polymer without requiring intensive mixing. Since the LiOH solution contained lithium ions, the insertion of lithium ion carriers in the composite occurred during growth of the Fe3O4 nanoparticles. Since the Fe3O4 displays magnetic behavior, we obtain a new class of polymer electrolytes known as nanocomposite magnetic polymer electrolytes.


## 2. Experimental

### 2.1. Materials

The poly(vinyl) alcohol (PVA, MW 22,000 g/mol) host polymer was obtained from Bratachem, Indonesia. Lithium hydroxide (LiOH) was obtained from Kanto Chemical, Japan. The magnetite (Fe3O4) nanoparticles were prepared from precursor solutions containing iron nitrate (Fe(NO3)3) as a source of Fe3+ ion and iron sulfate (FeSO4) as a source of Fe2+ ions, which was obtained from Merck, Germany.


### 2.2. Preparation of the Charge Membranes

PVA.LiOH mixtures were prepared containing 0–10 wt%. The PVA and LiOH were dissolved in separate solutions. The solutions were mixed at 50°C for 2 h and evaporated under ambient condition for 5 days to obtain a thin sheet of polymer electrolyte. The polymer composition exhibiting the highest electrical conductivity was used to prepare the magnetic material containing dispersed Fe3O4 nanoparticles. The nanoparticles were prepared using an in situ coprecipitation method in the host PVA.LiOH. Between volume fraction 0–0.35 v% of Fe3O4 nanoparticles were dispersed in the host polymer electrolyte.


### 2.3. Characterization

Structural studies were carried out on the PVA.LiOH and (PVA.LiOH): Fe3O4 membranes using X-ray diffraction (XRD) analysis (Philips Analytical-Diffractometer PW1710, using Cu-Kα radiation). The sample was scanned in the 2θ ranging from 10° to 80° for 2 s in the step mode. Electrical conductivity measurements were obtained using electrochemical impedance spectroscopy (EIS) at frequencies from 20 Hz to 2 MHz (Agient E4980A Precion LCR meter). The surface morphology of the polymer electrolyte membranes was examined using a scanning electron microscope (SEM JEOL JSM-6360LA). The magnetic properties were measured using a Bartington MS2B susceptibility meter.


## 2.1. Materials

The poly(vinyl) alcohol (PVA, MW 22,000 g/mol) host polymer was obtained from Bratachem, Indonesia. Lithium hydroxide (LiOH) was obtained from Kanto Chemical, Japan. The magnetite (Fe3O4) nanoparticles were prepared from precursor solutions containing iron nitrate (Fe(NO3)3) as a source of Fe3+ ion and iron sulfate (FeSO4) as a source of Fe2+ ions, which was obtained from Merck, Germany.


## 2.2. Preparation of the Charge Membranes

PVA.LiOH mixtures were prepared containing 0–10 wt%. The PVA and LiOH were dissolved in separate solutions. The solutions were mixed at 50°C for 2 h and evaporated under ambient condition for 5 days to obtain a thin sheet of polymer electrolyte. The polymer composition exhibiting the highest electrical conductivity was used to prepare the magnetic material containing dispersed Fe3O4 nanoparticles. The nanoparticles were prepared using an in situ coprecipitation method in the host PVA.LiOH. Between volume fraction 0–0.35 v% of Fe3O4 nanoparticles were dispersed in the host polymer electrolyte.


## 2.3. Characterization

Structural studies were carried out on the PVA.LiOH and (PVA.LiOH): Fe3O4 membranes using X-ray diffraction (XRD) analysis (Philips Analytical-Diffractometer PW1710, using Cu-Kα radiation). The sample was scanned in the 2θ ranging from 10° to 80° for 2 s in the step mode. Electrical conductivity measurements were obtained using electrochemical impedance spectroscopy (EIS) at frequencies from 20 Hz to 2 MHz (Agient E4980A Precion LCR meter). The surface morphology of the polymer electrolyte membranes was examined using a scanning electron microscope (SEM JEOL JSM-6360LA). The magnetic properties were measured using a Bartington MS2B susceptibility meter.


## 3. Results and Discussion

Figure1 is the complex membrane impedance spectrum (Nyquist Plot) of pure PVA and several PVA.LiOH mixtures. The squares represent experimental data and the curve is an approximation from the equivalent circuit used to determine the bulk resistance (Rb). The ionic conductivity of polymer electrolytes was calculated using the relationship: (1)σ=1RblA,
where ℓ is the thickness and A is the cross-section of the membrane.Impedance spectrum and fitting curve for PVA.LiOH complex membrane polymer electrolyte at room temperature (a) 0 wt% LiOH, (b) 1 wt%, (c) 3 wt%, (d) 5 wt%, (e) 7 wt%, (f) 8 wt%, (g) 9 wt%, and (h) 10 wt%.
(a)(b)(c)(d)(e)(f)(g)(h)The room-temperature ionic conductivity of the membranes as a function of LiOH concentration is plotted in Figure2. The ionic conductivity increased with increasing LiOH concentration in the host polymer to a maximum at 9 wt%.Figure 2
Ionic conductivity of PVA.LiOH polymer electrolyte as function of LiOH weight fraction. Measurements were performed at room temperature.The increase in ionic conductivity with LiOH concentration is due to an increase in the number of mobile charge carriers and a decrease in the crystallinity of the host polymer similar to that previously reported for other polymer electrolyte systems [15–17]. The decrease in ionic conductivity at concentrations greater than 9 wt% may be explained by the aggregation of ions. An excess of ions in the host polymer can result in ionic interactions with the polymer chains, leading to restriction of the segmental relaxation of the polymer chain (Figure 3).Figure 3
Schematic of ionic transport by relaxation segmental motion of polymer chains.In order to further improve the ionic conductivity, inorganic fillers were added to the host polymer. Fe3O4 nanoparticles were dispersed in situ in the host polymer electrolyte containing 9 wt% LiOH using a coprecipitation technique: (2)FeSO4+5Fe(NO3)3+16LiOH⟶2Fe3O4+15LiNO3+LiSO4+8H2OThe reaction produced black-colored particles indicating the formation of Fe3O4 nanoparticles in the polymer electrolyte matrix (Figure 4). Synthesis of Fe3O4 nanoparticles in a polymer electrolyte matrix results in smaller particles with excellent dispersion, while synthesis of Fe3O4 nanoparticles outside the polymer matrix results in formation of particle aggregates. Sensitivity to an external magnetic field was used as simple proof of the formation of magnetic nanoparticles, and the results were supported by XRD and magnetic measurements.Solution of (a) PVA.LiOH polymer electrolyte, (b) polymer electrolyte containing dispersed Fe3O4 particles, (c) Fe3O4 particles in absence of polymer matrix, and (d) Response of Fe3O4 particles to external magnetic field.
(a)(b)(c)(d)Dispersion of Fe3O4 nanoparticles in the host polymer enhanced the ionic conductivity. The nanoparticles assist ionic transport by increasing segmental mobility and interaction between Li+ ions and the polymer chains. In addition, the presence of Fe3O4 nanoparticles in the polymer electrolyte alters the electrical potential distribution around the particle surface, which induces a space charge layer at the interface between the particles and the electrolyte. The typical room temperature complex impedance spectrum of the polymer electrolyte PVA.LiOH containing dispersed Fe3O4 nanoparticles exhibiting maximum conductivity is shown Figure 5. The conductivity of the prepared samples at room temperature is found to be ~10−3 S·cm−1.Figure 5
Impedance spectrum of PVA.LiOH containing dispersed Fe3O4.In order to explain the effect of Fe3O4 nanoparticles on ionic conductivity, we used the effective medium approximation (EMA) to calculate the effective ionic conductivity. The model was developed by considering that a polymer electrolyte is composed of amorphous and crystalline phases, as illustrated in Figure 6.Figure 6
Illustration of particle arrangement in composite polymer electrolyte. Amorphous phases having high conductivity are created around the particles.Carrier accumulation near the particle surfaces locally enhances the ionic conductivity due to the increased carrier concentration, leading to a high-conductivity region. In areas far from any nanoparticles, the carrier concentration is approximately equal to that in the pure electrolyte (when insulator particles are absent). These areas are known as medium-conductivity regions. When two particles make contact, the electrolyte medium between the particles is removed and ion transport does not occur. The conductivity of this region is reduced to nearly zero (approximately equal to that of insulator particles), and these are known as low-conductivity regions. The effective ionic conductivity satisfies the equation:(3)(νf)2σl-σeσl+(z/2-1)σe+(1-νf)2σm-σeσm+(z/2-1)σe+2(νf)(1-νf)σh-σeσh+(z/2-1)σe=0
in which ν, f, z, σl, σm, and σh are the volume fraction of the particle, the packing fraction, the coordination number, and the conductivity of the low, medium, and high conductivity regions.Figure7 is a graph of the ionic conductivity of the composite polymer electrolyte as a function of Fe3O4 nanoparticle volume fraction. The solid line is a plot of (3) assuming simple cubic packing of filler particles in which z=6 and f=π/6. The other parameters in the equation were σl=10-6S·cm-1, σm=2.2×10-4S·cm-1, and σh=40σm. There was consistent agreement between the predicted and experimental conductivity. The maximum conductivity occurred with an Fe3O4 volume fraction of approximately ≈0.22.Figure 7
Effect of particle volume fraction on electrical conductivity of magnetic composites. Symbols represent experimental data and curve was obtained using (3) with the following parameters: z=6 and f=π/6, σl=10-6S·cm-1, σm=2.2×10-4S·cm-1and σh=40σm.The ionic conductivity results are supported by the X-ray diffraction patterns observed in PVA complexed with various amounts of LiOH and Fe3O4 (Figure 8). Pure PVA in the form of a powder or membrane exhibits a peak at 20° characteristic of an orthorhombic lattice, indicating the presence of a semicrystalline phase [15–17]. The intensity of the XRD peaks decreased with increasing LiOH concentration, indicating a reduction in crystallinity in the host polymer. Similar behavior was observed in membranes containing Fe3O4 nanoparticles.Figure 8
XRD patterns for (a) PVA powder, (b) PVA membrane, (c) 1 wt%, PVA.LiOH. (d) 3 wt% PVA.LiOH, (e) 5 wt% PVA.LiOH, (f) 7 wt% PVA.LiOH, (g) 9 wt% PVA.LiOH, (h) 10 wt% PVA.LiOH, (i) LiOH powder, (j) Fe3O4 powder, and (k) composite membrane containing 9 wt% dispersed Fe3O4 nanoparticles.Figure9 contains SEM images depicting the morphology of PVA, PVA.LiOH membranes with and without Fe3O4 nanoparticles. The morphology of the PVA membrane was uniform, and the surface roughness increased with increasing LiOH concentration.SEM images of (a) PVA membrane, (b) 9 wt% PVA.LiOH membrane, (c) 9 wt% PVA.LiOH containing Fe3O4 nanoparticles, and (d) Fe3O4 particles synthesized in absence of polymer matrix.
(a)(b)(c)(d)However, the morphology of the membrane containing Fe3O4 nanoparticles was fragmented. The fragments may have been formed as the Fe3O4 nanoparticles filled the empty spaces between polymer chains. This is supported by the results of EDS analysis indicating that the host polymer composition (C (71.02%), O (11.53%), and FeO (12.06%)) was retained in the nanoparticle-containing membranes. Fe3O4 particles synthesized in the absence of a polymer matrix possess an irregular morphology.Magnetic susceptibility (χ) is a simple technique for observing the presence of magnetic particles. It is sensitive enough to detect very low concentrations of magnetic materials. PVA.LiOH membranes not containing Fe3O4 nanoparticles were weakly diamagnetic, with susceptibilities of χ=-63.14×10-8 m3·kg−1. The susceptibility increased with increasing Fe3O4 content, although the increase was slower at higher concentrations (Table 1).Table 1
Magnetic susceptibility of composite polymer electrolyte as a function of Fe3O4 nanoparticle content.
Volume fractionχm×10-8Fe3O4(m3·kg−1)0.00−63.140.03360.051120.08670.114680.147370.162270.1911590.225750.2433000.276420.2911580.3241340.351404The intensity of the magnetic susceptibility was a function of the Fe3O4 nanoparticle concentration. However, the relationship was nonlinear, possibly due to an inhomogeneous dispersion of Fe3O4 nanoparticles in the polymer electrolyte.


## 4. Conclusion

A new nanocomposite magnetic polymer electrolyte was synthesized by dispersing magnetite (Fe3O4) nanoparticles in membrane composed of PVA.LiOH complex. The Fe3O4 nanoparticles assisted ionic transport through an increase in segment mobility and interaction between Li+ ions and the polymer chains. The results suggest that nanocomposite magnetic polymer electrolytes may be suitable for use in magnetic-electrochemical devices.



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*Source: 795613-2012-02-29.xml*