Plasticized alkaline solid polymer electrolyte system

Plasticized alkaline solid polymer electrolyte system

Materials Letters 61 (2007) 3096 – 3099 www.elsevier.com/locate/matlet Plasticized alkaline solid polymer electrolyte system A.A. Mohamad a,⁎, A.K. A...

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Materials Letters 61 (2007) 3096 – 3099 www.elsevier.com/locate/matlet

Plasticized alkaline solid polymer electrolyte system A.A. Mohamad a,⁎, A.K. Arof b a

School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia b Physics Department, Universiti Malaya, 50603 Kuala Lumpur, Malaysia Received 4 October 2006; accepted 2 November 2006 Available online 27 November 2006

Abstract Alkaline solid polymer electrolytes were prepared by utilizing poly(vinyl alcohol) (PVA), potassium hydroxide (KOH), α-Al2O3 and different amounts of propylene carbonate (PC). The addition of PC to the PVA:KOH:α-Al2O3:H2O increased its conductivity by three orders of magnitude to the reading of ∼10− 4 S cm− 1. The plot for log σ − 1 / T showed a transformation from liquid-like conductivity to Arrhenius type. The dielectric constant (εr) of the samples increases with increasing PC concentrations and temperatures. Scanning electron microscopy also shows the effect of PC on the polymer electrolytes surface. Thermogravimetric studies show that the thermal stability of the polymer electrolytes decreases with the addition of PC. © 2006 Elsevier B.V. All rights reserved. Keywords: Poly(vinyl alcohol); KOH; α-Al2O3; PC; Electrolyte

1. Introduction

2. Experimental

In order to prepare good polymer electrolytes, several methods had been used such as copolymerization, blending, addition of ceramic filler and plasticization. The addition of plasticizers could enhance the conductivity and better contact between the electrolyte/electrode. Although, the alkaline solid polymer electrolytes (ASPE) of composition poly(vinyl alcohol) (PVA) + potassium hydroxide (KOH) + H2O systems [1–3] and the PVA + KOH + glass-fiber-cloth mat + H2O composite system [4] have been reported, up to the present time, there have been no studies on plasticized composite alkaline solid polymer electrolyte (ASPE) systems. In this current report, the (PVA: KOH:α-Al2O3:H2O) system had been mixed with propylene carbonate (PC) plasticizer to improve the ionic conductivity. Preparation of the polymer films and the properties of the polymer films which included their electrical, morphology and thermal characterizations are reported here.

The PVA (Fluka, molecular weight ∼67,000), α-Al2O3 (BDH, particle size ∼0.37 μm), KOH (Merck), PC (Huntsman, molecular weight ∼102.9) and deionized water were used as starting materials to prepare the plasticized ASPE films by solution cast method. The concentration of the PC in the optimum composition of PVA: KOH:α-Al2O3:H2O (1.00:0.67:0.09:7.56 weight ratios) were named as PC0, PC10, PC20, PC30, PC40, PC50, PC60, PC70 and PC80 according to the weight percent of PC in samples. The prepared films were sandwiched between two stainless steel (SS) discs with the diameter of 2 cm and the ionic conductivity of the films was measured using an electrochemical impedance analyzer (HIOKI 3531 LCR) in the 50 Hz to 1 MHz frequency range at various temperatures. Scanning electron microscopy (SEM, Leica S440) was carried out in order to study the surface morphology of the polymer film. The thermogravimetric analysis (TGA) was conducted using the Mettler Toledo SDTA851, heated at 10 °C/min. 3. Results and discussion

⁎ Corresponding author. Tel.: +60 4599 6118; fax: +60 45941011. E-mail address: [email protected] (A.A. Mohamad). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.11.030

The variation of conductivity as a function of the PC plasticizer concentration in the PVA:KOH:α-Al2O3:H2O system is presented in Fig. 1. The conductivity of PC free film (PC0) increased slowly from

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Fig. 1. Variation of conductivity versus PC content in PVA:KOH:α-Al2O3:H2O (1.00:0.67:0.09:7.56 weight ratios) system.

∼ 10− 7 S cm− 1 to a maximum of ∼ 10− 4 S cm− 1 in the composition of 60 wt.% PC (PC60). On further addition of PC, the conductivity becomes almost constant. The films with the composition ≥ 75 wt.% PC were mechanically unstable but could still be used to measure the conductivity. However, the samples with the composition ≥ 90 wt.% PC were mechanically unstable and hence the conductivity was difficult to measure. In the present investigation, as the plasticizer content increases, it is possible to construct a local effective pathway in the presence of a large number of liquid-filled micropores for ionic conduction. In the solventswollen type PVA, the conductivity is highly dependent on the solvent content. Due to the presence of the plasticizer, the ions are transported faster in the gel phase. The samples with the composition of PVA: KOH:α-Al2O3:PC:H2O in wt.% 1.00:0.67:0.09:2.64:1.32 (PC60) gave the highest conductivity reading at (6.6 ± 1.7) × 10− 4 S cm− 1. This was lower than the conductivity of the highest conducting films with the composition of PVA:KOH:H2O (1.00:0.67:15.23 wt.%), where the conductivity reading was (8.5 ± 0.2) × 10− 4 S cm− 1, after being stored

Fig. 3. (a) Frequency dependence of real part (εr) of dielectric constant for different PC contents at room temperature and (b) frequency dependence of real part (εr) of dielectric constant for the highest conductivity (PC60) sample at different temperatures.

for 30 days [5]. However, the PC60 sample was proven to be the ‘truer’ ionic conductor since it was stored for 100 days in a dry condition compared to the PVA:KOH:H2O sample, which was stored for 30 days. The conductivity of the PVA:KOH:H2O sample was affected by the presence of water. The variation in electrical conductivity of PC free (PC0) and mixtures of different compositions as a function of temperature is shown in Fig. 2. The temperature dependence of conductivity of polymer electrolytes follows many patterns, as widely discussed by Ratner [6]. However, in the present study the plot shows two kinds of patterns: i. For the PVA:KOH:α-Al2O3:H2O system containing low PC concentration up to 20 wt.%, the log σ − 1 / T plot follows the liquid-like behavior [7–9]. In the 30 wt.% PC (PC30) the plot illustrated a mirror image of liquid-like behavior, where the conductivity curves upward with temperature. ii. For PC of higher concentrations, i.e. PC40 and PC60, the log σ − 1 / T show a linear variation and follow the Arrhenius-type behavior throughout the available temperature range.

Fig. 2. Variations of temperature dependence of PC0, PC20, PC30, PC40 and PC60 samples.

For samples containing low concentrations of PC, the conductivity followed the liquid-like behavior. For higher concentrations of PC a linear trend in the log σ −1/T plot was observed. Thus, as the addition of PC increased, the conductivity behavior changed from liquid-like to Arrhenius.

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Fig. 4. SEM micrographs for: (a) PC0, (b) PC20, (c) PC40 and (d) P60 samples.

The frequency dependence of εr for different values of PC concentration in the PVA:KOH:α-Al2O3:H2O system at room temperature and at various temperatures for the highest conductivity (PC60) are shown in Fig. 3(a) and (b), respectively. The values of εr were very high at low frequency for both plots. The εr values for the different PC concentrations and temperatures that are relatively constant with frequency, increased. Such high values of εr were attributed to the increase in the number of ions (charge). Hence, it was proven that the variation in conductivity was due to the variation in the number of available free mobile ions. These were due to the ion dissociation whereas the decrease in conductivity was due to the ion association as represented in the literature [10,11]. The SEM micrographs of the plasticized PVA:KOH:α-Al2O3:H2O systems are shown in Fig. 4. The films with lower PC content (PC0 and PC20) were still quite porous. The PC40 sample showed some formation of crystal on the surface due to excess KOH. In the higher plasticizer content (N 60 wt.% PC) samples, the surface showed ‘mosaic-like’ structure on the film surface. The structure was partly due to PC. This structure acts as cover on the surface of ASPE film. It helps to reduce the drying of the PC liquid inside the porous films. The TGA results for unplasticized and plasticized samples are shown in Fig. 5. A small weight loss was observed for all samples up to 50 °C, attributed to the evolution of the absorbed water content. Samples with low PC concentrations such as PC0 and PC20 showed an overlapping curve over each other. It appeared that the addition of a small amount of PC did not affect the properties of the sample at elevated temperatures. The results were also correlated to the log σ − 1 / T plot (Fig. 2), where the curves of PC0 and PC20 were still in liquid-like behavior and overlapped each other. The weight loss due to further evolution of water and PC was found to be higher for the plasticized samples at higher PC contents, e.g.

PC40 and PC60. After 100 °C, it could be seen that when the concentration of PC increased, the total weight loss also increased. The thermal stability of the polymer electrolytes decreased with the addition of PC plasticizer. The decreased in thermal stability in plasticized polymer electrolyte was attributed to the removal of PC while heating. From the TGA results, correlations between thermal characterization and conductivity–temperature behavior were observed. The decreased in conductivity was due to evolution of water as had been proved by TGA and the increased in conductivity up to 120 °C, was due to the flooding of PC from the inside of the porous films to the surface. The PC flooding on the film surface also increased the conductivity due to good contact between electrodes. It can also be

Fig. 5. TGA curves for: (a) PC0, (b) PC20, (c) PC40 and (d) PC60 samples.

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observed that the steeper decreased in weight loss was due to the decomposition of PC up to 250 °C.

4. Conclusion While at high PC concentration, the formation of PVA: KOH:α-Al2O3:PC:H2O would create a new path for the transport of OH− ion, which increased the ionic conductivity. The sample with the 60 wt.% PC in PVA:KOH:α-Al2O3:H2O system showed the highest conductivity. The conductivity– temperature plot of low PC concentration samples showed liquid-like behavior, but in the high concentration samples the plot changed to the Arrhenius behavior. The SEM and TGA studies supported the conductivity behaviors discussed above. References [1] A. Lewandowski, K. Skorupska, J. Malinska, Solid State Ionics 133 (2000) 265.

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