PVC

Materials Chemistry and Physics 114 (2009) 948–955

Contents lists available at ScienceDirect

Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

Study of microporous PVA/PVC composite polymer membrane and it application to MnO2 capacitors Chun-Chen Yang a,∗ , G.M. Wu b a b

Department of Chemical Engineering, MingChi University of Technology, Taipei Hsien 243, Taiwan, ROC Institute of Electro-Optical Engineering, Department of Chemical & Materials Engineering, Chang Gung University, Taoyuan 333, Taiwan, ROC

a r t i c l e

i n f o

Article history: Received 13 February 2008 Received in revised form 17 October 2008 Accepted 2 November 2008 Keywords: PVA/PVC Composite polymer Specific capacitance MnO2 Capacitor

a b s t r a c t A microporous poly(vinyl alcohol)/poly(vinyl chloride) (PVA/PVC) composite polymer membrane was successfully synthesized by a solution casting method and a preferential dissolution method. The characteristic properties of PVA/PVC composite polymer membranes were systematically studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), thermal gravimetric analysis (TGA), micro-Raman spectroscopy and AC impedance spectroscopy. The PVA/PVC composite polymer membrane shows excellent thermal property, dimensional stability, and the ionic conductivity; it is due to the addition of secondary PVC polymer fillers. The MnO2 capacitors with the PVA/PVC composite polymer membrane with 1 M Na2 SO4 was assembled and examined. It was found that the MnO2 capacitor based on a microporous PVA/5 wt.%PVC composite polymer electrolyte membrane exhibited the maximum specific capacitance of 238 F g−1 and the current efficiency of 99% at 25 mV s−1 after 1000 cycle test. The result demonstrates that the novel microporous PVA/PVC composite polymer membrane is a potential candidate for use on the capacitors. © 2008 Published by Elsevier B.V.

1. Introduction Poly(vinyl alcohol) (PVA) is a highly hydrophilic, non-toxic, and low cost polymer with excellent mechanical strength, thermal stability, chemical stability, and excellent film-forming properties. Lin and Metters [1] described the modeling for the molecule release from polymer hydrogel system and challenges associated with the performance of biomedical applications. Papancea et al. [2] studied the swelling and transport characteristics properties for the PVA-DNA cryogel membranes without any chemical crosslinkers using in pharmaceutical and medical applications. Qiao et al. [3] prepared a highly proton-conducting membrane based on poly(vinyl alcohol)/2-acrylamido-2-methyl-1-propanesulfonic acid/poly(vinylpyrrolidone) (PVA/PAMPS/PVP) for a low temperature direct methanol fuel cell. The polymer membranes showed 0.088 S cm−1 of the ionic conductivity, the ion exchange capacity (IEC) of 1.63 mequiv. g−1 , and a methanol permeability of 6.1 × 10−7 cm2 s−1 at room temperature. Einsla et al. [4] studied the properties of the sulfonated polyimide copolymer membranes for the proton-exchange-membrane fuel cells. PVA has been used as a solid-state polymer electrolyte in alkaline media [5,6] and

∗ Corresponding author. Tel.: +886 29089899; fax: +886 29041914. E-mail address: [email protected] (C.-C. Yang). 0254-0584/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.matchemphys.2008.11.009

acidic media [7,8] of energy power sources, such as direct methanol fuel cells (DMFCs) and capacitors. Electric double layer capacitors (EDLCs) and electrochemical capacitors (ECs) with a solid or gel polymer electrolyte become very attractive. Lewandowski et al. [9] studied alkaline PEO solid polymer electrolytes (SPEs) applying to EDLC capacitors with a capacity of 1.7–3.0 F in the potential window of 0–1 V. They also reported the discharge specific capacitance of 93 F g−1 for a solid-state EDLC capacitor based on the active carbon. Iwakura et al. [10,11] prepared an alkaline PAAK polymer hydrogel electrolyte for EDLC cell with the specific capacitances of 142–146 F g−1 for the active carbon materials. The alkaline PAA polymer hydrogel electrolyte shows excellent ionic conductivity of 0.60 S cm−1 at 25 ◦ C. Subramania et al. [12] prepared PVA-based microporous polymer electrolyte by a novel preferential polymer dissolution process by using PVC polymer. The PVC fillers were removed by immersing the blend polymer film into tetrahyrofuran (THF) solvent. The ionic conductivity of the microporous PVA polymer electrolyte after soaking in a 2 M LiClO4 electrolyte solution is about 1.50 mS cm−1 at room temperature. It is a simple way to prepare PVA-based microporous polymer electrolyte for Li-ion battery. Up to now, there have been only a few studies for application of neutral PVA-based polymer electrolyte to electrochemical capacitors. Manganese dioxide appears a promising electrode material; it is used on many energy storage systems ranging from primary

C.-C. Yang, G.M. Wu / Materials Chemistry and Physics 114 (2009) 948–955

alkaline Zn/MnO2 batteries to lithium ion batteries. This is due to its low cost, its excellent electrochemical performance, and environmentally friendliness. Recently, many works have been focused on MnO2 as an electrode material for electrochemical capacitors applications. There are many chemical and physical methods for preparation of manganese dioxide active materials, such as sol–gel [13,14], anodic electrodeposition [15,16], and codeposition with carbons [17,18]. Pang et al. [13] reported that the sol–gel MnO2 thin films are a very promising material for electrochemical capacitors due to their high reversibility, good cycling stability, and their high specific capacitance of 698 F g−1 in a unbuffered neutral Na2 SO4 solution. Normally, the MnO2 electrochemical capacitors only have the specific capacitance of 100–300 F g−1 in a neutral media. In addition, it was found that the electric conductivity of thick MnO2 film was low, which resulted in a relatively poor electrochemical performance as an electrode material. In order to improve this high resistance problem, the MnO2 /CNTs [17,18] composite electrode has been introduced recently. In this work, we report the preparation and characteristic of a poly(vinyl alcohol)/poly(vinyl chloride) (PVA/PVC) composite polymer membrane and its application on MnO2 capacitor. The performance of MnO2 capacitors based on the solid-state polymer electrolyte with a 1 M Na2 SO4 solution was for the first time studied by using a microporous PVA/PVC composite polymer membrane. 2. Experimental 2.1. Preparation of the PVA/PVC composite polymer membranes PVA (Aldrich), PVC (M.W. 60,000–150,000, Nan-Ya Plastics Corp.) and Na2 SO4 (Merck) were used as received without further purification. Degree of polymerization and saponification of PVA were 1700 and 98–99%, respectively. The alkaline microporous PVA/PVC composite polymer electrolyte membranes were prepared by a solution casting method and a preferential dissolution process [12]. The appropriate weight ratios of PVA:PVC (=1:0.025–0.1) polymers were dissolved in hot water under stirring. The above resulting solution was stirred continuously until the solution mixture became on a homogeneous viscous appearance at 85–90 ◦ C for 30–60 min. The addition of PVC fillers in the glass vessel was slowly and well controlled to get uniform distribution of PVC fillers in PVA matrix. The resulting mixture solution was poured out into a glass plate. The thickness of wet composite polymer electrolyte was between 0.3 and 0.4 mm. The glass plate with viscous PVA/PVC composite polymer membrane was weighed again and then the excess water was allowed to evaporate slowly at a constant temperature of 50 ◦ C at relative humidity of 30RH%. After water evaporation, the glass plate with the composite polymer membrane was weighed again. The composition of PVA/PVC composite polymer electrolyte membrane was determined from the mass balance. The PVA/PVC composite polymer membrane was further cross-linked by immersion in a solution of 5 wt.% glutaraldehyde (GA, 50 wt.% content in distilled water, as a cross-linker, Merck), 1.0 vol.% HCl (as an initiator) and acetone for the cross-linking reaction at 40◦ C for 12 h. The thickness of the dried PVA/PVC composite polymer membrane was controlled in the range between 0.15 and 0.20 mm. The thickness of the PVA/PVC composite polymer membranes was measured by a digital meter (Teclock PC-467, Japan). The average value of the thickness was obtained by measuring three different locations of the composite polymer membrane. Thereby, the microporous PVA/PVC composite polymer membrane was obtained by a preferential dissolution process using tetrahydrofuran (THF) solvent, which partially dissolved away some blend PVC polymer fillers for 24 h at room temperature; however, the PVA polymer matrix did not be affected or attacked by THF. A free standing microporous PVA/PVC composite polymer membranes were obtained and packed for further use. 2.2. Thermal, crystal structure, morphology analyses Thermal gravimetric analysis (TGA) thermal analysis was carried out using a Mettler Toledo TGA/SDT 851e system. Measurements were made by heating from 30 to 600 ◦ C under N2 atmosphere at a heating rate of 10 ◦ C min−1 with about 10 mg sample. The crystal structures of the PVA/PVC composite polymer membranes were examined using a Philips X’Pert X-ray diffractometer with Cu K␣ radiation of wavelength  = 1.54056 Å for 2 angles between 10 and 80◦ . The surface morphology and microstructure of the PVA/PVC composite polymer membranes was examined using a S-2600H scanning electron microscope (Hitachi Co., Ltd.).

949

2.3. Analysis of mean pore size and pore size distribution Capillary flow porometry (PMI, CFP-1200-AE, USA) was used to measure the mean pore size and the pore size distribution. All PVA/PVC composite polymer membranes were first soaked with a wetting agent (Porewick with surface tension 16 dyn cm−1 ), then N2 gas was applied on one side of the polymer membrane. The N2 gas pressure (0–200 psi) was increased slowly until Porewick liquid was removed from the pores and a gas flow formed. The gas flow rate as a function of pressure was measured and used to calculate the diameter of each pore diameter, mean flow pore diameter and the pore size distribution data. The experimental setup and measurement methods have been described in detail [19]. 2.4. Micro-Raman spectroscopy analysis Raman spectrum is a unique tool to characterize the PVA/PVC composite polymer electrolyte membrane. The micro-Raman analysis was carried out by a Renishaw confocal microscopy Raman spectroscopy system with a microscope equipped with 10×, 20×, and 50× objectives, and a charge-coupled device (CCD) detector. Raman excitation source was provided by 633 nm laser beam, which had the beam power 17 mW and was focused on the sample with a spot of about 1 ␮m in diameter. 2.5. The ionic conductivity measurements, liquid absorption and swelling ratios The conductivity measurements were made for the PVA/PVC composite polymer membrane by an AC impedance method [20,21]. The samples of the PVA/PVC composite polymer membranes were immersed in a 1 M Na2 SO4 solution for 24 h before test. Those PVA/PVC composite polymer electrolyte membranes were sandwiched between SS304 stainless steel, ion-blocking electrodes, each of surface area 1.32 cm2 , in a spring-loaded glass holder. A thermocouple was kept in close to the composite polymer membrane for temperature measurement. Each sample was equilibrated at the experimental temperature for at least 30 min before measurement. AC impedance measurements were carried out using an Autolab PGSTAT-30 equipment (Eco Chemie B.V., Netherlands). The AC frequency range from 300 kHz to 10 Hz at an excitation signal of 5 mV was recorded. The AC impedance of the PVA/PVC composite polymer membrane was recorded at a temperature range from 30 to 70 ◦ C. Experimental temperatures were maintained within ±0.2 ◦ C by a convection oven. All neutral PVA/PVC composite polymer membranes were examined at least three times. The pre-weighed, a dried microporous PVA/PVC composite polymer membrane (W0 ) was immersed in distilled (D.I.) water, and maintained for 24 h at 25 ◦ C until the equilibrium was established. The microporous PVA/PVC composite polymer membrane was taken out from the immersion bath and the excess surface water was carefully removed. The weight of the wet composite polymer membrane (W1 ) was then determined again. The percents of H2 O absorption and swelling ratio were calculated from the following equations: liquid absorption (%) = swelling ratio (%) =

W1 − W0 × 100 W1

W1 − W0 × 100 W0

(1)

(2)

2.6. Cell assembly and electrochemical property measurements The electrolytic manganese dioxide was deposited onto a commercially 304 stainless steel foil [13] with a thickness of 0.2 mm in a 1 M MnSO4 + 1 M H2 SO4 solution at 50 ◦ C under the continuous stirring condition. The stirring speed was set at 300 rpm. The potentiodynamic deposition of MnO2 was conducted at between the potential limits of 0.5–1.5 V (vs. SCE) at a scan rate of 200 mV s−1 with different cycle numbers. Before MnO2 cyclic voltammetry electrodeposition, a 4 cm × 5 cm (A = 20 cm2 ) stainless steel substrate was polished with 1000 grit emery paper to get a rough finish surface, then clean with acetone and D.I. water, respectively, and finally dried at 105 ◦ C. The electrodeposition cell is comprised of a SS-304 working electrode, a large area Pt counter electrode, and a saturated calomel (SCE) reference electrode. The mass of the deposited MnO2 was measured by a micro-balance (Mettler) with an accuracy of 1 × 10−5 g. The MnO2 capacitor was comprised of two symmetry as-prepared MnO2 electrodes and a microporous PVA/PVC composite polymer membrane. Two MnO2 electrodes were with the same area of 20 cm2 for assembly to form a unit cell. Two MnO2 electrodes and a PVA/PVC composite polymer membrane were pressed and laminated together for 5 min at 50 ◦ C. The lamination process is to minimize the Ohmic resistance and to assure a good contact at the interface between MnO2 electrodes and PVA/PVC composite polymer membrane. The PVA/PVC composite polymer electrolyte membrane acts as both a separator and a quasi-solid-state electrolyte. The specific capacitance properties of MnO2 electrochemical capacitors were investigated by cyclic voltammetry at room temperature. The cyclic voltammetry measurements were performed in the cell potential range of 0–1.0 V at various scanning rates.

950

C.-C. Yang, G.M. Wu / Materials Chemistry and Physics 114 (2009) 948–955

Fig. 1. XRD spectra for PVA/PVC composite polymer membranes with different amounts of PVC polymer fillers.

3. Results and discussion 3.1. Crystal structure and surface morphology The X-ray diffraction measurement was performed to examine the nature of the crystallinity of the PVA/PVC composite polymer membranes. Fig. 1 shows the diffraction pattern for the PVA polymer film and PVA/PVC composite polymer membranes. It is well known that the PVA polymer film exhibits a semi-crystalline structure with peaks at the 2 angles of 20 and 40◦ . As can be seen clearly in Fig. 1, a large peak at 2 of 20◦ (peak#1) for the PVA polymer film was seen. But, it was also clearly observed that the peak intensity of the PVA/PVC composite polymer membranes greatly reduced as the PVC fillers were added. This implies that the addition of PVC fillers into PVA polymer matrix greatly augmented the domain of amorphous region (i.e., the intensity of XRD crystal peak decreases). This also indicates that the PVA/PVC composite polymer membranes become much amorphous, which was supported by decreasing the relative crystallinity for the PVA/PVC composite polymer membrane from 64 to 41%, as displayed in Table 1. Note that the degree of amorphous increases with increasing contents of PVC polymer fillers. There is a significant motion of polymer chain in the amorphous domains or some defects and free volumes existing at the interface of between polymer chains while non-conducting in the crystalline phase. The characteristics of the PVA/PVC composite polymer membranes show good ionic conductivity property. This is due to the formation of more amorphous domains facilitating the local PVA chain segmental motion in the PVA/PVC composite polymer membrane. SEM photographs for the top view of the PVA/5 wt.%PVC composite polymer membranes at 3000× magnification, shown in Fig. 2(a). It was found that the surface morphology of the PVA/PVC composite polymer membranes show good uniformity and the added PVA fillers are dispersed well into the PVA polymer matrix. Moreover, SEM photographs for the cross-sectional view of the PVA/5 wt.%PVC composite polymer membrane are shown in Fig. 2(b) at 3500×. In addition, SEM photographs of the PVA/5 wt.%PVC blend polymer membranes after a THF preferentially dissolution treatment is shown in Fig. 3(a), at 1000× magnification. It is clearly observed that the pore size and structure of the blend polymer membrane

Fig. 2. SEM photographs for PVA/5 wt.%PVC SPEs: (a) top view at 3000× and (b) cross-sectional view at 3500×.

Fig. 3. SEM photographs for PVA/5 wt.%PVC SPE after THF etching: (a) top view at 3000× and (b) cross-sectional view at 3000×.

C.-C. Yang, G.M. Wu / Materials Chemistry and Physics 114 (2009) 948–955

951

Table 1 The percents of relative crystallinity of the microporous PVA/PVC composite polymer membranes. Types

2 (◦ ) Peak#1 position

PVA film Cross-linked PVA PVA/5 wt.%PVC SPE PVA/10 wt.%PVC SPE a

◦

19.53 19.68◦ 19.38◦ 19.53◦

Intensity#1 1104 954 707 460

Peak#2 position ◦

39.83 40.32◦ 41.47◦ 40.87◦

Intensity#1

a

603 568 238 198

100 86.41 64.03 41.66

Relative crystallinity (%)

Base on peak#1 of XRD result.

is around between 1 and 3 ␮m and the pores had a circular shape. Fig. 3(b) shows the SEM photograph for the cross-sectional view of PVA/5 wt.%PVC blend polymer membrane at 5000×. Obviously, there are some undissolved PVC filler particles and many micropores existed at the top surface of the PVA/PVC composite polymer membrane. The size of added PVC fillers is about 0.1–2 ␮m, with a spherical shape. After the preferential dissolution treatment by THF for 24 h, these PVC fillers are preferentially dissolved at room temperature; the microporous structure of the PVA/PVC composite polymer membrane is then created. Importantly, the blending ratio of PVC fillers, the particle size polymer, and the immersion time and treatment temperature of a preferentially dissolution process are very crucial to control the

microporous size and size distribution of the PVA/PVC composite polymer membrane. Generally, the PVA/PVC composite polymer membranes are compatible and homogenous. Moreover, it is particularly worthy of noting that the content of the PVC fillers in the PVA/PVC composite polymer membrane should be less than 10 wt.% in order to avoid forming large chunks or aggregates of the PVC fillers. 3.2. Pore size and pore size distribution A novel method based on capillary flow porometry (CFP) was used to examine the microporous characteristic property of the PVA/PVC composite polymer membrane. Particularly, this

Fig. 4. The pore size distribution curves for different porous polymer membranes by capillary flow porometer: (a) Celgard 5550; (b) PVA SPE; (c) PVA/5 wt.%PVC SPE; (d) PVA/10 wt.%PVC SPE.

952

C.-C. Yang, G.M. Wu / Materials Chemistry and Physics 114 (2009) 948–955

Table 2 The pore sizes and pore size distributions for various battery separators, the PVA/PVC composite polymer membranes and pure PVA film. Types

Celgard 5550 PVA/5 wt.%PVC PVA/10 wt.%PVC Pure PVA film

Diameter (␮m) Largest pore

Mean flow pore

Smallest pore

At max. pore distribution

0.151 0.179 0.182 0.042

0.108 0.110 0.115 0.040

0.037 0.066 0.062 0.030

0.110 0.113 0.100 –

technique can only measure the go-through pores in the polymer membrane, not for blind pores. Moreover, the capillary flow porometer can also effectively examine the micropore structure for several layers porous materials or membrane in any direction (in plane and through plane) [19]. Fig. 4(a–d) displays the pore size distributions of the Celgard PP 5550 separator (use for comparison), pure PVA film, and PVA/PVC composite polymer membranes with 5 and 10 wt.% PVC fillers, respectively. It is seen clearly that the mean pore size of Celgard 5550 separator is about 108 nm, which is very close to the separator product specification at 100 nm. Moreover, it is observed that the mean pore diameters of the PVA/PVC composite membranes with 5 and 10 wt.% PVC fillers are around 66 and 62 nm, respectively. However, the mean pore size of the pure PVA membrane is around 30 nm, whereas the pore size is very small, as compared to the PVA/PVC composite polymer membrane. Table 2 shows all PMI test results for the largest pore, mean flow pore, smallest detected pore, and at maximum pore distribution for all PVA/PVC composite polymer membrane samples including the PVA/PVC composite polymer membranes, pure PVA membrane. Furthermore, Fig. 5(a) and (b) also shows the cumulative pore size distribution curves for pure PVA membrane as well as the PVA/PVC composite polymer membranes. Interestingly, the pore size distribution of the PVA/PVC composite polymer membranes, as shown in Fig. 5(a), was located in the similar range of 50–150 nm. 3.3. TGA thermal analysis TGA thermographs of PVC powders, pure cross-linked PVA film, GA solution (25 wt.%), and the cross-linked PVA/5 wt.%PVC composite polymer membrane are shown in Fig. 6(a), respectively. Clearly, the weight loss of the cross-linked PVA/PVC composite polymer membrane was much less in the temperature range of 100–300 ◦ C, as compared with the pure cross-linked PVA film. However, the PVC fillers start degradation around 300 ◦ C, as shown in Fig. 6(a). It can be seen that the thermal stability of the cross-linked PVA/PVC composite polymer membrane is greatly enhanced when the PVC fillers are added. Moreover, TGA thermographs of these cross-linked PVA/PVC composite polymer membranes containing 0.25–10 wt.% of PVC polymer fillers are shown in Fig. 6(b), respectively. There is a weight loss of 0.63% at a temperature of 100 ◦ C for the pure cross-linked PVA polymer film (without PVC fillers); it is due to the removal of bounding water. In contrast, all PVA/PVC composite polymer membranes presents a four-step degradation (shown as Td,1 = 100 ◦ C, Td,2 = 275 ◦ C, Td,3 = 375 ◦ C, Td,4 = 430 ◦ C), which begins in the region between 90 and 100 ◦ C, a weight of loss of 3–4%. There is a thermal stable region at 100–250 ◦ C. Subsequently, there is a 7–8% weight loss when the temperature reaches around 250 ◦ C; finally there is a total weight loss of 91–92% when the temperature is higher than 600 ◦ C. In conclusion, the cross-linked PVA/PVC composite polymer membrane starts to degrade at 250 ◦ C; however, the pure crosslinked PVA polymer film (without PVC) gradually degrades after

Fig. 5. The cumulative pore size curves of various polymer membranes: (a) pure PVA film and (b) PVA/PVC composite polymer membranes and Celgard 5550.

190 ◦ C. It can be seen clearly that the addition of PVC fillers greatly improves the thermal stability of the cross-linked PVA/PVC polymer membranes. Table 3 shows the results of weight loss of the cross-linked PVA/PVC composite polymer membranes with various compositions at different temperatures by TGA analysis in detail. 3.4. Ionic conductivities, liquid uptake and swelling ratio The typical AC impedance spectra of the PVA/PVC composite polymer electrolyte membranes with 5 wt.% PVC polymer fillers at different temperatures can be obtained (not shown here) [5,6,20,21]. The AC spectra are typically non-vertical spikes for blocking electrodes, i.e., SS|PVA/PVC SPE|SS cell. Analysis of the spectra yields information about the properties of the PVA/PVC composite polymer membrane, such as bulk resistance, Rb . Taking Table 3 The weight loss of the cross-linked PVA/PVC composite polymer membranes at various temperatures. Types

PVC powders PVA film PVA/0.2 wt.%PVC PVA/2.5 wt.%PVC PVA/5 wt.%PVC PVA/10 wt.%PVC

Temperature (◦ C) 100

250

350

600

0.60 0.63 3.27 4.45 3.53 3.34

1.93 8.00 7.09 8.69 9.98 10.00

61.08 73.10 28.90 35.74 42.05 43.41

87.80 92.24 93.98 92.53 91.13 91.21

C.-C. Yang, G.M. Wu / Materials Chemistry and Physics 114 (2009) 948–955

953

Fig. 7. Micro-Raman spectra of PVC powders, PVA film and microporous PVA/5–10 wt.%PVC composite polymer membranes.

Fig. 6. TGA thermographs for: (a) PVA, PVC, GA and PVA/5 wt.%PVC polymer membrane and (b) PVA/PVC composite polymer membranes at various compositions.

into account the thickness of the composite polymer electrolyte film, the Rb value was converted into the ionic conductivity value, , according to  = L/Rb A [5,6], where L is the thickness (cm) of the PVA/PVC composite polymer electrolyte membrane, A is the area of the blocking electrode (cm2 ), and Rb is the bulk resistance () of microporous PVA/PVC composite polymer membranes. Typically, the Rb values of the PVA/5 wt.%PVC composite polymer membranes are of the order of 8–30 Ohm and are highly dependent on the contents of PVC fillers and the compositions of Na2 SO4 electrolytes in the polymer membrane. The corresponding value of the ionic conductivity of the PVA/2.5 wt.%PVC composite polymer membrane at ambient temperature is 0.583 mS cm−1 . The values of the ionic conductivity of the PVA/5 wt.%PVC and PVA/10 wt.%PVC composite polymer membranes at ambient temperature is 0.613 and 0.640 mS cm−1 , respectively. It is found that the ionic conductivity of PVA/PVC composite polymer membranes is only slightly affected by the content of PVC fillers. In addition, the dimension stability of the PVA/PVC composite polymer membrane is also very important for a practical application. The results of the absorption percent (%) and swelling ratio

(%) for the PVA/PVC composite polymer membrane in D.I. water are shown in Table 4. The absorption percent of D.I. water for the PVA/PVC composite polymer membrane decreases from 52.0 to 44.3% as the amount of added PVC polymer fillers increases from 2.5 to 10 wt.%. Furthermore, the swelling ratio for the PVA/PVC composite polymer membrane in D.I. water decreases from 118.6 to 87.3% when the addition of PVC polymer fillers increases from 2.5 to 10 wt.%. As we know, when the PVC polymer used as a stiffer material is added to the soft PVA matrix, the swelling ratio of the PVA/PVC composite polymer membrane is greatly reduced. The dimension stability and swelling properties are indeed improved. 3.5. Micro-Raman spectroscopy analyses Fig. 7 shows the micro-Raman spectra of PVC powders, pure PVA film and the PVA/PVC composite polymer membranes containing 5–10 wt.% PVC fillers. It can be seen clearly from micro-Raman spectra that two strong characteristic scattering peak at 636 and 695 cm−1 is for PVC polymer; it is due to the C Cl stretching vibration [22]. By contrast, there is also a strong vibration peak at 1442 cm−1 , which is assigned to the C H bending and O H bending for PVA matrix [23]. Moreover, there are also two weak Raman scattering peaks for the PVA polymer at 852 and 913 cm−1 , it may be due to the C C stretching. Finally, there are several weak scattering peaks at 1145, 1088 cm−1 , they are due to the C C stretching and C O stretching. On the whole, it can be seen that the scattering intensities of these characteristic vibrational peaks for the PVA/PVC composite polymer membranes are decreased; it indicates that the amorphous phases in the PVA matrix are augmented.

Table 4 The results of D.I. water absorption and swelling ratio of the microporous PVA/PVC composite polymer membranes with different PVC contents at 25 ◦ C. Param.

Film thickness (mm) Absorption (%) Swelling (%)

SPE PVA/0.2 wt.%PVC

PVA/2 wt.%PVC

PVA/5 wt.%PVC

PVA/10 wt.%PVC

0.109 52.0 118.6

0.188 51.9 107.7

0.174 47.3 89.7

0.166 44.3 87.3

954

C.-C. Yang, G.M. Wu / Materials Chemistry and Physics 114 (2009) 948–955

Fig. 8. The CV curves of MnO2 capacitors with the PVA/5 wt.%PVC SPE (with 1 M Na2 SO4 electrolyte at 25 ◦ C).

these MnO2 capacitors exhibited a higher capacitance at a lower scan rate. As seen in Fig. 8, the shape of the voltammograms is much close to a rectangular mirror images; it is also no visible redox reaction peak in the 0–1.0 V potential range. As a matter of fact, the symmetric I–E response indicates that Faraday redox reactions of the MnO2 capacitor are highly electrochemically reversible. The shape of CV curves is not significantly affected by the variation of scan rates. Table 5 presents the charge and discharge results for the specific capacitance and the current efficiency of MnO2 capacitors employing PVA/5 wt.%PVC polymer membrane with a 1 M Na2 SO4 electrolyte at different scan rates. Furthermore, the values of energy density and the power density for a full cell of MnO2 capacitor based on the PVA/PVC composite SPE were also estimated. It is found that the energy density (E.D.) and power density (P.D.) [9] of MnO2 capacitors are calculated by the equations E.D. = 1/2(CV2 ) and P.D. = 1/4(V2 R−1 ), respectively. The results indicated that the gravimetric energy density and the power density for MnO2 capacitor using PVA/PVC blend SPEs were about 22 Wh kg−1 and 20 kW kg−1 , respectively. The high E.D. and P.D. performance of MnO2 capacitor is due to high pseudocapacitance properties of CV-electrodeposited MnO2 active materials and the unique properties of the composite polymer membrane. 4. Conclusions

3.6. Electrochemical properties of MnO2 capacitors The capacitance of MnO2 capacitor primarily arises from redox transition of MnO2 active materials; it is due to a Faradic charge transfer reaction at the interface of electrolyte and electrode. A thin MnO2 electrode with higher specific capacitance can be easily synthesized by an anodic cyclic voltammetry method. In this work, a thin and compact MnO2 electrode based on 304 stainless steel (SS) foil with a thickness of 0.20 mm was prepared at a scan rate 200 mV s−1 in a mixture of 1 M MnSO4 + 1 M H2 SO4 solution. Since the observed specific capacitance of the CV-deposited MnO2 electrode is mainly due to the pseudocapacitance that is associated with a redox process of the reduction of MnO2 to MnOOH (theoretical capacitance for MnO2 is 1120 F g−1 ). In a neutral media, MnO2 reduction can be represented in the following equation: MnO2 + H2 O + e− → MnOOH + OH−

(3)

For MnO2 capacitors usually use two identical symmetric electrodes. The MnO2 capacitor occurs the redox reaction, involving H2 O and electron insertion or interaction between the positive and negative electrodes. The MnO2 redox reaction of the electrochemical capacitor (i.e., incomplete 1 e− transfer) occurs homogenously and reversibly without any phase change for the MnO2 lattice. Fig. 8 shows cyclic voltammograms of the MnO2 capacitors comprised of on microporous PVA/5 wt.%PVC composite polymer electrolyte membranes in the potential range of 0–1.0 V at the scan rates of 25, 50, and 100 mV s−1 at 25 ◦ C. Whereas, the values of discharge specific capacitance for MnO2 capacitor with PVA/5 wt.%PVC composite polymer electrolyte membrane at 25, 50, and 100 mV s−1 are 238, 230, and 226 F g−1 , respectively. From the results also found

A novel microporous PVA/PVC composite polymer membrane was obtained by a solution casting method. The characteristic properties of the PVA/PVC composite polymer membranes were systematically examined by using XRD, SEM, TGA, CFP, microRaman spectroscopy and AC impedance method. It is found that CFP is a very useful tool to characterize the pore property for the PVA/PVC composite polymer membrane. TGA result indicates that the thermal property and dimensional stability of the PVA/PVC composite polymer membrane are greatly enhanced when the PVC filler is added. The MnO2 capacitor comprising of the PVA/PVC composite polymer electrolyte membrane with a neutral 1 M Na2 SO4 electrolyte was investigated by cyclic voltammetry. It was found that the highest specific capacitance of the MnO2 capacitor based on a PVA/5 wt.%PVC composite polymer membrane was 238 F g−1 at 25 mV s−1 . This is due to due to unique micropore structure and excellent hydrophilic property of PVA polymer (backbone with OH side chain). In particular, it was found that the interface compatibility and stability for the PVA/PVC composite SPE and the MnO2 electrodes were excellent. It can be concluded that the PVA/PVC composite polymer membrane is a potential candidate for use on the MnO2 capacitors. Acknowledgement The financial support for the research project from National Science Council, Taiwan, ROC (Contract No: NSC-96-2221-E131009-MY2) is greatly acknowledged. References

Table 5 The specific capacitances of MnO2 capacitor based on the PVA/5 wt.%PVC composite polymer membrane with a 1 M Na2 SO4 electrolyte at different scan rates. Items (rates) (mV s−1 )

25 50 100

Specific capacitance (Cspec ) Charge (F g−1 )

Discharge (F g−1 )

CE (%)

240.16 233.88 229.70

238.08 230.02 226.98

99.1 97.2 95.1

[1] C.C. Lin, A.T. Metters, Adv. Drug Deliv. Rev. 58 (2006) 1379. [2] A. Papancea, A.J.M. Valente, S. Patachia, M.G. Miguel, B. Lindman, Langmuir 24 (1) (2008) 273. [3] J. Qiao, T. Hamaya, T. Okada, Polymer 46 (2005) 10809. [4] B.R. Einsla, Y.S. Kim, M.A. Hickner, Y.T. Hong, M.L. Hill, B.S. Pivovar, J.E. McGrath, J. Membr. Sci. 255 (2005) 31. [5] C.C. Yang, S.J. Chiu, W.C. Chien, J. Power Sources 162 (2006) 21. [6] C.C. Yang, S.T. Hsu, W.C. Chien, J. Power Sources 152 (2005) 303. [7] C.W. Lin, Y.F. Huang, A.M. Kanan, J. Power Sources 171 (2007) 340. [8] N.W. DeLuca, Y.A. Elabd, J. Power Sources 171 (2006) 386.

C.-C. Yang, G.M. Wu / Materials Chemistry and Physics 114 (2009) 948–955 [9] A. Lewandowski, M. Zajder, E. Frackowiak, F. Beguin, Electrochim. Acta 46 (2001) 2777. [10] C. Iwakura, H. Wada, S. Nohara, N. Furukawa, H. Inoue, M. Morita, Electrochem. Solid-State Lett. 6 (2) (2003) A37. [11] H. Wada, S. Nohara, N. Furukawa, H. Inoue, N. Sugoh, H. Hideharu, M. Morita, C. Iwakura, Electrochim. Acta 49 (2004) 4871. [12] A. Subramania, N.T. Kalyyana Sundaram, N. Sukumar, J. Power Sources 141 (2005) 188–192. [13] S.C. Pang, M.A. Anderson, T.W. Chapman, J. Electrochem. Soc. 147 (2000) 444. [14] R.N. Reddy, R.G. Reddy, J. Power Sources 132 (2004) 315. [15] K.R. Prasad, N. Miura, J. Power Sources 135 (2004) 354.

955

[16] J.K. Chang, C.T. Lin, W.T. Tsai, Electrochem. Commun. 6 (2004) 666. [17] C.Y. Lee, H.M. Tsai, H.J. Chuang, S.Y. Li, P. Lin, T.Y. Tseng, J. Electrochem. Soc. 152 (2005) A716. [18] M. Wu, G.A. Snook, G.Z. Chen, D. Fray, Electrochem. Commun. 6 (2004) 499. [19] A. Jean, K. Gupta, J. Power Sources 96 (2001) 214. [20] C.C. Yang, S.J. Lin, J. Power Sources 112 (2002) 497. [21] C.C. Yang, S.J. Lin, Mater. Lett. 57 (2002) 873. [22] R.W. Berg, A.D. Otero, Vib. Spectrosc. 42 (2006) 222. [23] A. Martinelli, A. Matic, P. Jacobsson, L. Borjesson, M.A. Navarra, A. Fernicola, S. Panero, B. Scrosati, Solid State Ionics 177 (2006) 2431.