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Ionic conductivity studies of poly(vinyl alcohol) alkaline solid polymer electrolyte and its use in nickel–zinc cells
Solid State Ionics 156 (2003) 171 – 177 www.elsevier.com/locate/ssi
Ionic conductivity studies of poly(vinyl alcohol) alkaline solid polymer electrolyte and its use in nickel–zinc cells A.A. Mohamad a, N.S. Mohamed b, M.Z.A. Yahya c, R. Othman d, S. Ramesh e, Y. Alias f, A.K. Arof a,* b
a Physics Department, University of Malaya, 50603 Kuala Lumpur, Malaysia Center for Foundation Studies in Science, University of Malaya, 50603 Kuala Lumpur, Malaysia c Physics Department, MARA University of Technology, 40450 Shah Alam, Selangor, Malaysia d Science Department in Engineering, IIU Malaysia, 53100 Kuala Lumpur, Malaysia e Institute of Postgraduate Studies, University of Malaya, 50603 Kuala Lumpur, Malaysia f Chemistry Department, University of Malaya, 50603 Kuala Lumpur, Malaysia
Received 14 December 2001; received in revised form 1 July 2002; accepted 5 July 2002
Abstract X-ray diffraction (XRD) pattern reveals that potassium hydroxide (KOH) disrupts the crystalline nature of poly(vinyl alcohol) (PVA)-based polymer electrolytes and converts them into an amorphous phase. The PVA – KOH alkaline solid polymer electrolyte (ASPE) system with PVA/KOH wt.% ratio of 60:40 exhibits the highest room temperature ionic conductivity of 8.5 10 4 S cm 1. This electrolyte was used in the fabrication of a nickel – zinc (Ni – Zn) cell. The cell was charged at a constant current of 10 mA for 1 h providing it with 1.6 V. The cell was cycled 100 times. At the end of the last cycle, the cell still contained a capacity of 5.5 mA h. D 2003 Published by Elsevier Science B.V. Keywords: Poly(vinyl alcohol); Potassium hydroxide; Alkaline solid polymer electrolyte; Nickel – zinc cell
1. Introduction Alkaline solid polymer electrolyte (ASPE) has been considerably investigated due to their wide potential applications in electrochemical devices, such as batteries [1 –6] and supercapacitors [7]. In most solid polymer electrolytes, the polymer host is mixed * Corresponding author. Tel.: +60-3-7967-4085; fax: +60-37967-4146. E-mail addresses: [email protected] (A.A. Mohamad), [email protected] (A.K. Arof).
with inorganic salts and one or more plasticizers, in order to enhance the conductivity. Examples of some polymer systems that have been used are poly(ethylene oxide) (PEO) [1,2,7], copolymer of epichlorohydrin and ethylene oxide P(ECH-co-EO) [5] and poly(vinyl alcohol) (PVA) [4,6]. Studies on PVAbased systems have shown that they exhibit good proton conduction and have been applied in fuel cells [8– 15]. An all solid-state Ni/ASPE/Zn cell can overcome the many problems observed in Zn-type secondary batteries employing liquid electrolytes, particularly
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the change in shape of the zinc electrode and the dendritic growth towards the positive electrode, which can cause cell shorting [2,16 – 20]. The chemistry of the nickel – zinc (Ni –Zn) cell reaction can be approximated as follows [1]:
at the positive electrode: NiðOHÞ2 þ OH
charge
W NiOOH þ H2 O þ e
discharge
at the negative electrode: ZnO þ H2 O þ 2e
charge
W Zn þ 2OH
discharge
and the overall cell reaction: NiðOHÞ2 þ ZnO
charge
W NiOOH þ Zn
discharge
In the present work, structural and ionic conductivity studies are performed on PVA – KOH systems. The ASPE giving the highest room temperature electric conductivity was then used in the fabrication of Ni –Zn cells.
2. Experimental The preparation of polymer complexes has been described elsewhere [6]. PVA (Fluka) and KOH (R&M Chemicals) were used as received. The polymer films were kept in a dessicator for further drying. In order to study the amorphous character of PVA complexes with different weight percentages of KOH, the XRD patterns of the samples were recorded at room temperature with a Philips X’Pert instrument, which employs a CuKa X-radiation. To study the ionic conductivity of the samples, impedance spectroscopy was performed using a HIOKI 3531-01 LCR bridge interfaced to a computer for data acquisition. The study was carried out in the frequency range 50 Hz to 1 MHz. The thin polymer electrolyte films were sandwiched between two stainless steel disk electrodes, which acted as a blocking electrode for ions. The temperature-dependent ionic conductivity was performed in the temperature range between 25 and 100 jC. The conductivity, r, of each sample was calculated using equation r = t/(RbA), where t is the thickness of the film, Rb is the bulk impedance and A is the area of the cross-section of the film. The value of Rb was confirmed by comparing with the 1/Rb value obtained from a complex admittance graph. Polymer Ni – Zn cells ware fabricated using a negative electrode consisting of 90 wt.% ZnO
Fig. 1. Ni(OH)2/PVA + KOH/ZnO + PbO + C cell construction.
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(BDH), 5 wt.% PbO (Fluka) and 5 wt.% graphite (Superior Graphite, USA) were mixed in an agate mortar. PbO was added to serve as a corrosion inhibitor. A 10% PVA solution was used as the binder. The mixture was spread evenly on a nickel mesh, which also acts as a current collector. The positive electrode was prepared by dissolving Ni(OH)2 (H.C. Starck, Germany) in a 10% PVA solution. The slurry was again spread evenly on another nickel mesh and left to dry. The PVA – KOH electrolyte exhibiting the highest ambient electrical conductivity was used to fabricate the cell. The 15-cm2 cells were prepared by sandwiching the ASPE (thickness 0.40 mm and area slightly larger than the electrodes) between a square-shaped zinc electrode (thickness f 1.00 mm) and a squareshaped nickel electrode (thickness f 0.80 mm). The assembly was hot pressed, placed in a moisture proof casing and then sealed. The fabricated cell is as shown in Fig. 1. The cell was then tested using a BAS LG-50 galvanostat. The open-circuit voltage (OCV) and charge–discharge life cycle measurements were car-
173
ried out in a voltage range from 0.5 to 1.6 V. The cell was charged at 10.0 mA for 1 h and discharged at 1.0 mA. Thus, the initial capacity of the cell is expected to be 10 mA h.
3. Results and discussion 3.1. X-ray diffraction (XRD) studies The nature of the film changes when mixed with KOH. Films of the polymer systems containing less than 25 wt.% KOH were transparent while white rubbery films were obtained for the systems with higher KOH compositions. Brittle films were obtained when the KOH concentration exceeded 60 wt.% KOH. The crystallinity of the polymer film with respect to pure PVA and PVA – KOH complexes has been examined by X-ray diffraction. The diffraction pattern for pure PVA is shown in Fig. 2. The sample is partially crystalline with a broad peak between 2h angles of 17.5 – 27.5j.
Fig. 2. XRD patterns of pure PVA and complexes of different compositions weight percentage of KOH mentioned on the respective patterns.
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The broad peak becomes less intense as the KOH content is increased to up 30 wt.%. This could be due to the lesser percentage of PVA in the sample that results in the lower intensity of the PVA peak at 2h = 20j and also due to the disruption of the PVA crystalline structure by KOH. XRD shows that the sample with PVA:40 wt.% KOH is the least crystalline. The degree of crystallinity affects the electrical conductivity of the samples. The amorphous nature produces greater ionic diffusivity in accordance with the high ionic conductivity, which can be obtained in amorphous polymers that have a fully flexible backbone [21]. On further addition of KOH to 50 wt.%, the conductivity begins to decrease. This is attributed to the crystallinity of the sample as indicated by XRD. In the film with 50 wt.% KOH, the peak at 2h = 30.9j is attributed to KOH as verified from the JCPDS 1995 data (15-890). These KOH peaks increase in intensity for the sample with 60 wt.% KOH. 3.2. AC-impedance studies The variation of electrical conductivity, r, as a function of the KOH concentration, is shown in Fig. 3. Table 1 lists the average impedance analysis done at room temperature with different compositions of KOH. It can be observed that the addition of KOH significantly improved the ionic conductivity. The
Fig. 3. Variation of conductivity with KOH weight percentage.
Table 1 The results of average impedance analysis PVA/KOH (wt.% ratio)
Conductivity, r (S cm 1) at 25 jC
100:0 (pure PVA) 90:10 80:20 70:30 65:35 60:40 55:45 50:50 40:60 >30:70
(5.9 F 3.0) 10 10 (1.8 F 0.6) 10 6 (1.6 F 0.4) 10 5 (3.7 F 0.1) 10 4 (6.8 F 0.2) 10 4 (8.5 F 0.2) 10 4 (8.0 F 0.3) 10 4 (8.0 F 0.3) 10 4 (7.5 F 0.3) 10 4 mechanically unstable
conductivity of pure PVA is f 10 10 S cm 1 and it increases sharply to about 10 6 S cm 1 on complexing the PVA with 10 wt.% KOH. The increase in conductivity becomes slower on further addition of KOH to PVA. The highest conductivity of 8.5 10 4 S cm 1 was achieved for the system containing 40 wt.% KOH. Above 60 wt.% KOH, the sample becomes mechanically unstable and makes conductivity measurement difficult.
Fig. 4. Temperature dependence of ionic conductivity of pure PVA and complexes of different compositions weight percentage of KOH mentioned on the respective patterns.
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175
Fig. 5. Activation energy with KOH weight percentage.
3.3. Conductivity –temperature studies In order to understand the ionic conduction, the electrical conductivity was studied as a function of temperature from ambient temperature to 100 jC. Fig. 4 depicts the plots of lnrT versus 103/T for pure PVA, PVA:20 wt.% KOH, PVA:30 wt.% KOH and PVA:40 wt.% KOH samples. Thus conductivity, r, varies with temperature, T according to r=(ro/T) exp( Ea/kT), where ro is the preexponential factor, Ea is the activation energy and k is the Boltzman constant. It can be observed that the pure PVA sample exhibited a drastic increase in conductivity after 70 jC. This may be due to the beginning of the crystalline to glass
Fig. 6. Open-circuit voltage (OCV) of solid polymer electrolyte nickel – zinc cells during 24 h of storage.
Fig. 7. Charge and discharge curve of electrochemical nickel – zinc cell.
phase transformation that occurs at the glass transition temperature, Tg, of PVA which is about 85 jC [22,23]. The Ea is a combination of the energy of defect formation and migration, which can be calculated from the plots of Fig. 4. The Ea for pure PVA is calculated to be 0.26 eV and the Ea plot with KOH
Fig. 8. Variation of the capacity versus number of cycle for electrochemical nickel – zinc cell.
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concentration is shown in Fig. 5. From the observed results, the sample PVA:40 wt.% KOH has the highest electrical conductivity and lowest activation energy compared to other samples. 3.4. Cell characterization Upon assembly, the cell was in the discharged mode. The cell was then charged at a constant current of 10 mA for 1 h and a voltage of f 1.6 V was obtained. After charging, the OCV was measured for 24 h at regular hourly intervals. It can be observed from the graph of Fig. 6 that the OCV of the cells dropped to 0.9 V and remained stable for the next 19 h. Guinot et al. [2] also reported observing selfdischarge in their cells. This drop in OCV could be due to the oxidation of the negative Zn electrode when the cell was charged. Typical charge–discharge characteristic curves of the fabricated cell are as shown in Fig. 7. During charging at 10 mA for 1 h, the cell acquired a voltage in the range 1.5– 1.6 V. Upon discharge at 1.0 mA, the voltage dropped to within 1.2 –1.3 V. Since at the end of charging, the capacity of the cell is 10 mA h, the cell is observed to cycle within this capacity as presented in Fig. 8. The low capacity value at the beginning of the discharge is attributed to the electrode/electrolyte contact, which is not always perfect during the early cycles [2]. The longest discharge plateau with a voltage plateau close to 1.0 V can be observed during the 6th – 12th cycles. It is possible that during these cycles, the electrode/ electrolyte contact has improved and fresh active materials at the surface of the electrodes are oscillating between the Zn charge state and the ZnO discharge state. This capacity seems to be stable at 5 mA h until at the end of 100 cycles. The fade in capacity may be due to the large interfacial resistance developed between the electrode and the electrolyte. This fade in capacity has also been observed in other ASPE systems [3].
4. Conclusions X-ray diffraction (XRD) pattern reveals that KOH disrupts the crystalline nature of PVA-based polymer electrolytes and converts them into an amorphous
phase. The PVA –KOH alkaline solid polymer electrolyte system with PVA/KOH wt.% ratio of 60:40 exhibits the highest room temperature ionic conductivity of 8.5 10 4 S cm 1. Activation energy is also lowest. The results prove that an all solid-state Ni – Zn cells can be prepared with the utilization of PVA – KOH solid polymer electrolytes.
Acknowledgements The authors gratefully acknowledge the IRPA grant no. 09-02-03-0806 and Vote F0052/2001C given by the Malaysian Government to carry out this work.
References [1] J.F. Fauvarque, S. Guinot, N. Bouzir, E. Salmon, J.F. Penneau, Electrochim. Acta 40 (1995) 2449. [2] S. Guinot, E. Salmon, J.F. Penneau, J.F. Fauvarque, Electrochim. Acta 43 (1998) 1163. [3] N. Vassal, E. Salmon, J.F. Fauvarque, J. Electrochem. Soc. 146 (1999) 20. [4] A. Lewandowski, K. Skorupska, J. Malinska, Solid State Ionics 133 (2000) 265. [5] N. Vassal, E. Salmon, J.F. Fauvarque, Electrochim. Acta 45 (2000) 1527. [6] A.A. Mohamad, N.S. Mohamed, Y. Alias, A.K. Arof, J. Alloys Comp. 337 (2002) 208. [7] A. Lewandowski, M. Zajder, E. Frackowiak, F. Beguin, Electrochim. Acta 46 (2001) 2777. [8] R.A. Vargas, A. Garcı´a, M.A. Vargas, Electrochim. Acta 43 (1998) 1271. [9] M.A. Vargas, R.A. Vargas, B.E. Mellander, Electrochim. Acta 44 (1999) 4227. [10] M.A. Vargas, R.A. Vargas, B.E. Mellander, Electrochim. Acta 45 (2000) 1399. [11] R.A. Vargas, V.H. Zapata, E. Matallana, M.A. Vargas, Electrochim. Acta 46 (2001) 1699. [12] R. Mishra, K.J. Rao, Eur. Polym. J. 35 (1999) 1883. [13] S. Chibowski, M. Paszkiewicz, M. Krupa, Powder Technol. 107 (2000) 251. [14] M. Suzuki, T. Yoshida, T. Koyama, S. Kobayashi, M. Kitamura, K. Hanabusa, H. Shirai, Polymer 41 (2000) 4531. [15] P.N. Gupta, K.P. Singh, Solid State Ionics 86 – 88 (1996) 319. [16] F.R. McLarnon, E.J. Cairns, J. Electrochem. Soc. 138 (1991) 645. [17] T.C. Adler, F.R. McLarnon, E.J. Cairns, J. Electrochem. Soc. 140 (1993) 289. [18] J. Jorne, T.C. Adler, E.J. Cairns, J. Electrochem. Soc. 142 (1995) 771.
A.A. Mohamad et al. / Solid State Ionics 156 (2003) 171–177 [19] M. Klein, F. McLarnon, in: D. Linden (Ed.), Handbook of Batteries, 2nd ed., McGraw-Hill, New York, 1994, pp. 29.1 – 29.20, Chap. 29. [20] P.C. Butler, P.A. Eidler, P.G. Grimes, S.E. Klassen, R.C. Miles, in: D. Linden (Ed.), Handbook of Batteries, 2nd ed., McGraw-Hill, New York, 1994, pp. 37.1 – 37.16, Chap. 37.
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[21] R. Frech, S. Chintapalli, Solid State Ionics 85 (1996) 61. [22] J.G. Pritchard, Poly(Vinily Alcohol): Basic Properties and Uses, Macdonald Technical and Scientific, London, 1970. [23] M.K. Lindemann, in: H.F. Mark, N.G. Gaylord (Eds.), Encyclopedia of Polymer Science and Technology, vol. 14, Wiley, New York, 1971, p. 149.
Ionic conductivity studies of poly(vinyl alcohol) alkaline solid polymer electrolyte and its use in nickel–zinc cells A.A. Mohamad a, N.S. Mohamed b, M.Z.A. Yahya c, R. Othman d, S. Ramesh e, Y. Alias f, A.K. Arof a,* b
a Physics Department, University of Malaya, 50603 Kuala Lumpur, Malaysia Center for Foundation Studies in Science, University of Malaya, 50603 Kuala Lumpur, Malaysia c Physics Department, MARA University of Technology, 40450 Shah Alam, Selangor, Malaysia d Science Department in Engineering, IIU Malaysia, 53100 Kuala Lumpur, Malaysia e Institute of Postgraduate Studies, University of Malaya, 50603 Kuala Lumpur, Malaysia f Chemistry Department, University of Malaya, 50603 Kuala Lumpur, Malaysia
Received 14 December 2001; received in revised form 1 July 2002; accepted 5 July 2002
Abstract X-ray diffraction (XRD) pattern reveals that potassium hydroxide (KOH) disrupts the crystalline nature of poly(vinyl alcohol) (PVA)-based polymer electrolytes and converts them into an amorphous phase. The PVA – KOH alkaline solid polymer electrolyte (ASPE) system with PVA/KOH wt.% ratio of 60:40 exhibits the highest room temperature ionic conductivity of 8.5 10 4 S cm 1. This electrolyte was used in the fabrication of a nickel – zinc (Ni – Zn) cell. The cell was charged at a constant current of 10 mA for 1 h providing it with 1.6 V. The cell was cycled 100 times. At the end of the last cycle, the cell still contained a capacity of 5.5 mA h. D 2003 Published by Elsevier Science B.V. Keywords: Poly(vinyl alcohol); Potassium hydroxide; Alkaline solid polymer electrolyte; Nickel – zinc cell
1. Introduction Alkaline solid polymer electrolyte (ASPE) has been considerably investigated due to their wide potential applications in electrochemical devices, such as batteries [1 –6] and supercapacitors [7]. In most solid polymer electrolytes, the polymer host is mixed * Corresponding author. Tel.: +60-3-7967-4085; fax: +60-37967-4146. E-mail addresses: [email protected] (A.A. Mohamad), [email protected] (A.K. Arof).
with inorganic salts and one or more plasticizers, in order to enhance the conductivity. Examples of some polymer systems that have been used are poly(ethylene oxide) (PEO) [1,2,7], copolymer of epichlorohydrin and ethylene oxide P(ECH-co-EO) [5] and poly(vinyl alcohol) (PVA) [4,6]. Studies on PVAbased systems have shown that they exhibit good proton conduction and have been applied in fuel cells [8– 15]. An all solid-state Ni/ASPE/Zn cell can overcome the many problems observed in Zn-type secondary batteries employing liquid electrolytes, particularly
0167-2738/03/$ - see front matter D 2003 Published by Elsevier Science B.V. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 6 1 7 - 3
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the change in shape of the zinc electrode and the dendritic growth towards the positive electrode, which can cause cell shorting [2,16 – 20]. The chemistry of the nickel – zinc (Ni –Zn) cell reaction can be approximated as follows [1]:
at the positive electrode: NiðOHÞ2 þ OH
charge
W NiOOH þ H2 O þ e
discharge
at the negative electrode: ZnO þ H2 O þ 2e
charge
W Zn þ 2OH
discharge
and the overall cell reaction: NiðOHÞ2 þ ZnO
charge
W NiOOH þ Zn
discharge
In the present work, structural and ionic conductivity studies are performed on PVA – KOH systems. The ASPE giving the highest room temperature electric conductivity was then used in the fabrication of Ni –Zn cells.
2. Experimental The preparation of polymer complexes has been described elsewhere [6]. PVA (Fluka) and KOH (R&M Chemicals) were used as received. The polymer films were kept in a dessicator for further drying. In order to study the amorphous character of PVA complexes with different weight percentages of KOH, the XRD patterns of the samples were recorded at room temperature with a Philips X’Pert instrument, which employs a CuKa X-radiation. To study the ionic conductivity of the samples, impedance spectroscopy was performed using a HIOKI 3531-01 LCR bridge interfaced to a computer for data acquisition. The study was carried out in the frequency range 50 Hz to 1 MHz. The thin polymer electrolyte films were sandwiched between two stainless steel disk electrodes, which acted as a blocking electrode for ions. The temperature-dependent ionic conductivity was performed in the temperature range between 25 and 100 jC. The conductivity, r, of each sample was calculated using equation r = t/(RbA), where t is the thickness of the film, Rb is the bulk impedance and A is the area of the cross-section of the film. The value of Rb was confirmed by comparing with the 1/Rb value obtained from a complex admittance graph. Polymer Ni – Zn cells ware fabricated using a negative electrode consisting of 90 wt.% ZnO
Fig. 1. Ni(OH)2/PVA + KOH/ZnO + PbO + C cell construction.
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(BDH), 5 wt.% PbO (Fluka) and 5 wt.% graphite (Superior Graphite, USA) were mixed in an agate mortar. PbO was added to serve as a corrosion inhibitor. A 10% PVA solution was used as the binder. The mixture was spread evenly on a nickel mesh, which also acts as a current collector. The positive electrode was prepared by dissolving Ni(OH)2 (H.C. Starck, Germany) in a 10% PVA solution. The slurry was again spread evenly on another nickel mesh and left to dry. The PVA – KOH electrolyte exhibiting the highest ambient electrical conductivity was used to fabricate the cell. The 15-cm2 cells were prepared by sandwiching the ASPE (thickness 0.40 mm and area slightly larger than the electrodes) between a square-shaped zinc electrode (thickness f 1.00 mm) and a squareshaped nickel electrode (thickness f 0.80 mm). The assembly was hot pressed, placed in a moisture proof casing and then sealed. The fabricated cell is as shown in Fig. 1. The cell was then tested using a BAS LG-50 galvanostat. The open-circuit voltage (OCV) and charge–discharge life cycle measurements were car-
173
ried out in a voltage range from 0.5 to 1.6 V. The cell was charged at 10.0 mA for 1 h and discharged at 1.0 mA. Thus, the initial capacity of the cell is expected to be 10 mA h.
3. Results and discussion 3.1. X-ray diffraction (XRD) studies The nature of the film changes when mixed with KOH. Films of the polymer systems containing less than 25 wt.% KOH were transparent while white rubbery films were obtained for the systems with higher KOH compositions. Brittle films were obtained when the KOH concentration exceeded 60 wt.% KOH. The crystallinity of the polymer film with respect to pure PVA and PVA – KOH complexes has been examined by X-ray diffraction. The diffraction pattern for pure PVA is shown in Fig. 2. The sample is partially crystalline with a broad peak between 2h angles of 17.5 – 27.5j.
Fig. 2. XRD patterns of pure PVA and complexes of different compositions weight percentage of KOH mentioned on the respective patterns.
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The broad peak becomes less intense as the KOH content is increased to up 30 wt.%. This could be due to the lesser percentage of PVA in the sample that results in the lower intensity of the PVA peak at 2h = 20j and also due to the disruption of the PVA crystalline structure by KOH. XRD shows that the sample with PVA:40 wt.% KOH is the least crystalline. The degree of crystallinity affects the electrical conductivity of the samples. The amorphous nature produces greater ionic diffusivity in accordance with the high ionic conductivity, which can be obtained in amorphous polymers that have a fully flexible backbone [21]. On further addition of KOH to 50 wt.%, the conductivity begins to decrease. This is attributed to the crystallinity of the sample as indicated by XRD. In the film with 50 wt.% KOH, the peak at 2h = 30.9j is attributed to KOH as verified from the JCPDS 1995 data (15-890). These KOH peaks increase in intensity for the sample with 60 wt.% KOH. 3.2. AC-impedance studies The variation of electrical conductivity, r, as a function of the KOH concentration, is shown in Fig. 3. Table 1 lists the average impedance analysis done at room temperature with different compositions of KOH. It can be observed that the addition of KOH significantly improved the ionic conductivity. The
Fig. 3. Variation of conductivity with KOH weight percentage.
Table 1 The results of average impedance analysis PVA/KOH (wt.% ratio)
Conductivity, r (S cm 1) at 25 jC
100:0 (pure PVA) 90:10 80:20 70:30 65:35 60:40 55:45 50:50 40:60 >30:70
(5.9 F 3.0) 10 10 (1.8 F 0.6) 10 6 (1.6 F 0.4) 10 5 (3.7 F 0.1) 10 4 (6.8 F 0.2) 10 4 (8.5 F 0.2) 10 4 (8.0 F 0.3) 10 4 (8.0 F 0.3) 10 4 (7.5 F 0.3) 10 4 mechanically unstable
conductivity of pure PVA is f 10 10 S cm 1 and it increases sharply to about 10 6 S cm 1 on complexing the PVA with 10 wt.% KOH. The increase in conductivity becomes slower on further addition of KOH to PVA. The highest conductivity of 8.5 10 4 S cm 1 was achieved for the system containing 40 wt.% KOH. Above 60 wt.% KOH, the sample becomes mechanically unstable and makes conductivity measurement difficult.
Fig. 4. Temperature dependence of ionic conductivity of pure PVA and complexes of different compositions weight percentage of KOH mentioned on the respective patterns.
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175
Fig. 5. Activation energy with KOH weight percentage.
3.3. Conductivity –temperature studies In order to understand the ionic conduction, the electrical conductivity was studied as a function of temperature from ambient temperature to 100 jC. Fig. 4 depicts the plots of lnrT versus 103/T for pure PVA, PVA:20 wt.% KOH, PVA:30 wt.% KOH and PVA:40 wt.% KOH samples. Thus conductivity, r, varies with temperature, T according to r=(ro/T) exp( Ea/kT), where ro is the preexponential factor, Ea is the activation energy and k is the Boltzman constant. It can be observed that the pure PVA sample exhibited a drastic increase in conductivity after 70 jC. This may be due to the beginning of the crystalline to glass
Fig. 6. Open-circuit voltage (OCV) of solid polymer electrolyte nickel – zinc cells during 24 h of storage.
Fig. 7. Charge and discharge curve of electrochemical nickel – zinc cell.
phase transformation that occurs at the glass transition temperature, Tg, of PVA which is about 85 jC [22,23]. The Ea is a combination of the energy of defect formation and migration, which can be calculated from the plots of Fig. 4. The Ea for pure PVA is calculated to be 0.26 eV and the Ea plot with KOH
Fig. 8. Variation of the capacity versus number of cycle for electrochemical nickel – zinc cell.
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concentration is shown in Fig. 5. From the observed results, the sample PVA:40 wt.% KOH has the highest electrical conductivity and lowest activation energy compared to other samples. 3.4. Cell characterization Upon assembly, the cell was in the discharged mode. The cell was then charged at a constant current of 10 mA for 1 h and a voltage of f 1.6 V was obtained. After charging, the OCV was measured for 24 h at regular hourly intervals. It can be observed from the graph of Fig. 6 that the OCV of the cells dropped to 0.9 V and remained stable for the next 19 h. Guinot et al. [2] also reported observing selfdischarge in their cells. This drop in OCV could be due to the oxidation of the negative Zn electrode when the cell was charged. Typical charge–discharge characteristic curves of the fabricated cell are as shown in Fig. 7. During charging at 10 mA for 1 h, the cell acquired a voltage in the range 1.5– 1.6 V. Upon discharge at 1.0 mA, the voltage dropped to within 1.2 –1.3 V. Since at the end of charging, the capacity of the cell is 10 mA h, the cell is observed to cycle within this capacity as presented in Fig. 8. The low capacity value at the beginning of the discharge is attributed to the electrode/electrolyte contact, which is not always perfect during the early cycles [2]. The longest discharge plateau with a voltage plateau close to 1.0 V can be observed during the 6th – 12th cycles. It is possible that during these cycles, the electrode/ electrolyte contact has improved and fresh active materials at the surface of the electrodes are oscillating between the Zn charge state and the ZnO discharge state. This capacity seems to be stable at 5 mA h until at the end of 100 cycles. The fade in capacity may be due to the large interfacial resistance developed between the electrode and the electrolyte. This fade in capacity has also been observed in other ASPE systems [3].
4. Conclusions X-ray diffraction (XRD) pattern reveals that KOH disrupts the crystalline nature of PVA-based polymer electrolytes and converts them into an amorphous
phase. The PVA –KOH alkaline solid polymer electrolyte system with PVA/KOH wt.% ratio of 60:40 exhibits the highest room temperature ionic conductivity of 8.5 10 4 S cm 1. Activation energy is also lowest. The results prove that an all solid-state Ni – Zn cells can be prepared with the utilization of PVA – KOH solid polymer electrolytes.
Acknowledgements The authors gratefully acknowledge the IRPA grant no. 09-02-03-0806 and Vote F0052/2001C given by the Malaysian Government to carry out this work.
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