Indian Journal of Chemical Technology Vol. 15, November 2008, pp. 576-580
Zinc-poly (aniline) rechargeable battery assembled with aqueous electrolyte Bharat Chandra Dalui, I N Basumallick & Susanta Ghosh* Department of Chemistry, Visva-Bharati, Santiniketan 731 235, India Email:
[email protected] Received 16 April 2008; revised 1 September 2008 The electrochemical behaviours of electro-synthesized poly(aniline) (PANI) cathode and commercial zinc anode in an aqueous electrolyte were investigated. The poly(aniline) sulphate cathodes were synthesized by galvanostatic oxidation of aniline from a sulphuric acid bath on platinum substrate and characterized by UV-Visible and FTIR spectroscopy. The electrolyte is comprised of ammonium sulphate and zinc sulphate dissolved in double distilled water. The slow scan linear voltammetry (Tafel plot) for the commercial zinc anode was recorded in this electrolyte and compared with its behaviour in chloride electrolyte. A negative shift of the open circuit potential of 55 mV, decrease in exchange current density of one order, and increase of cathodic Tafel slope in this sulphate electrolyte was observed. The above facts enhance the cell potential and the reversibility of the cell having the configuration, PANI || (NH4)2SO4, ZnSO4 (aq) || Zn. A discharge pleato with an average discharge potential of 1.1 V, which varied depending on the discharge current density, was observed for this reversible cell. The maximum discharge capacity, observed from this cell in the sulphate electrolyte, is 137 mAh.g-1. Keywords: Poly(aniline), Ammonium sulphate, Zinc, Galvanostatic, Tafel plot
Conducting polymers with reversible redox system provide a new kind of electrode material in a rechargeable battery. Among the conducting polymers, poly(aniline) is found to be the best, because it has reversible redox system, high stability both in air and aqueous medium and can be easily synthesized by using chemical or electrochemical methods with low production cost1-7 and can be easily protonated or oxidized in aqueous acidic medium, which is highly reversible for the capacity gaining of the battery8-10. However, the life-time of the Zn-PANI battery is highly dependent on the pH of the electrolyte, at lower pH the zinc electrode undergoes corrosion easily and at higher pH the activity of the poly(aniline) is limited. As a result, the electrolyte used in the cell should have intermediate pH values. So far most of the aqueous Zn-PANI batteries have been constructed using NH4Cl and ZnCl2 electrolyte (pH 4.7). Use of ammonium chloride electrolyte which is an active component in a cell enforced to synthesize the poly(aniline) chloride cathode using hydrochloric acid bath, since anion (Cl-) insertion/exertion is the redox process responsible for the storage capacity. However, there are few limitations to generate pure poly(aniline) chloride salt from hydrochloric acid bath in high yield due to chlorine gas evolution that competes with the polymerization kinetics and as a result the quantum
yield of the polymer is low. On the other hand zinc passivation, which is possibly related to the formation of solid ZnCl2.3NH4Cl and ZnCl2.2NH4Cl on anode surface, was reported11-13. The passivation of zinc electrode was reduced after adding sodium citrate in chloride electrolyte14 with better cyclibility. But addition of such additives to the battery electrolyte will certainly reduce the energy density of the battery. In this paper, the development of a rechargeable battery of the configuration PANI || (NH4)2SO4, ZnSO4 (aq) || Zn with higher working potential and better cyclibility without any additives in the electrolytes has been reported. Experimental Procedure Aniline was received from Qualigen® Fine Chemicals (India) and distilled in nitrogen atmosphere before use. The other chemicals used were reagent grade. 0.1 M aniline (monomer) was dissolved in 1 M H2SO4 acid and used as the deposition bath. A two electrodes cell was configured for the electro-deposition of poly(aniline) and both the electrodes were made of platinum. The working electrode was a platinum wire of area 0.2 cm2 whereas the counter electrode was a platinum foil of area 2 cm2. The galvanostatic deposition was carried out at a current density of 1 mA.cm-2 for a time period of 10 min. After electro-deposition, the polymer was
DALUI et al.: ZINC-POLY(ANILINE) RECHARGEABLE BATTERY ASSEMBLED WITH AQUEOUS ELECTROLYTE
washed with double distilled water and used as a cathode for electrochemical studies. A solution, containing 1 M (NH4)2SO4 and 0.01 M ZnSO4, (pH 4.6) was prepared in double distilled water and used as an electrolyte for electrochemical study. The cyclic voltammetry study of the polymer electrode was carried out by using a PAR VarsaStat® II Potentiostat/Galvanostat. The current-voltage pattern was recorded in the potential range of -0.1 to 0.5 V versus SCE at a sweep rate of 20 mV.s-1. The slow scan linear voltammetry for the zinc anode was recorded both in sulphate and chloride electrolyte and scanning was done in the range of ± 200 mV versus open circuit potential at a sweep rate of 0.166 mV.s-1. The charge-discharge profile was recorded at different current density by using a PAR VarsaStat® II Potentiostat/Galvanostat. The UV-Visible spectra of poly(aniline) film was recorded by UV-Visible spectrophotometer Shimadzu, model-UV3101PC, Japan in between 200 – 700 nm and FTIR spectra was also recorded by Shimadzu FTIR, model-8400S in the range of 400 and 4000 cm-1. All the experiments were carried out at room temperature. Results and Discussion Figure 1 shows the cyclic voltammogram of PANI electrode in the potential range of –0.6 to 1.0 V versus SCE after holding the electrode at –0.6 V for 3 min
Fig. 1 Typical cyclic voltammograms of PANI electrode in 1 M (NH4)2SO4 and 0.01 M ZnSO4, (pH 4.6) solution. The potential range is from –0.6 to 1.0 V with a sweep rate of 20 mV.s-1. Inset: Galvanostatic polymerization curve of aniline on platinum surface immersed in a solution containing 0.1 mM aniline in1 M H2SO4 at a current density of 1 mA.cm-2
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and inset of Fig. 1 shows the galvanostatic polymerization curve of aniline on platinum surface from a deposition bath of 0.1 M aniline in 1 M H2SO4. From the inset of Fig. 1, it is clear that the polymerization starts at potential of 0.9 volts and continues in the potential range of 0.9 to 0.8 V for a period of 10 min. After polymerization, the electrode was washed with double distilled water and then placed in the three compartment electrochemical cell for cyclic voltammetry (CV) and slow scan linear voltammetry (SSLV) studies. From the cyclic voltammogram, two pairs of redox couple were observed (Fig. 1). In the anodic scan, the doping of H+ and anions (SO42-) occurs at 0.3 and 0.6 volts, respectively. In the cathodic scan, the de-doping of anions (SO42-) and H+ occurs at 0.4 and –0.1 volts, respectively. Similar type of voltammogram was reported by Hatchett et al.15. Since, proton exchange is one redox process and anion exchange is another redox process, an electrolyte with acidic pH and the same counter anion as in poly(aniline) is required. Thus, for the electrochemical studies, an aqueous 1 M (NH4)2SO4 and 0.01 M ZnSO4 (pH 4.6) was chosen. Figure 2 shows the slow scan linear polarization curve of poly(aniline) sulphate cathode in 1 M (NH4)2SO4 and 0.01 M ZnSO4 solution. An opencircuit potential, for the polymer cathode in aqueous sulphate electrolyte, was 220 mV versus SCE. The cathodic and anodic Tafel slopes were 69 and 34 mV.dec-1, respectively and the exchange current density, obtained from the intercept of the extrapolated Tafel’s line up to open-circuit potential, was 3 x 10-6 mA.cm-2. A low exchange current density
Fig. 2 Slow scan linear voltammetry curve of poly(aniline) sulphate (sweep rate = 0.166 mV.s-1) in 1 M (NH4)2SO4 and 0.01 M ZnSO4, (pH 4.6) solution on PANI electrode
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is observed which might be because of the involvement of more than one electron in this redox process. The transfer co-efficients of the cathodic and anodic reactions, obtained from the slopes of the Tafel plot, are 0.85 and 1.74, respectively. Figure 3 shows the slow scan potentiodynamic polarization curve of zinc anode in 1 M (NH4)2SO4 and 0.01 M ZnSO4 and 1 M NH4Cl and 0.01 M ZnCl2 solutions, respectively. Open-circuit potential, Eocp, for the zinc electrode in aqueous sulphate electrolyte was -1.085 V versus SCE which was 55 mV more negative than in chloride electrolyte, Eocp = 1.030 V versus SCE. The cathodic and anodic Tafel slopes of zinc electrode for the sulphate electrolyte were 63 and 36 mV.dec-1, respectively and the exchange current density, obtained from the intercept of the extrapolated Tafel’s line up to open-circuit potential, was 0.037 mA.cm-2. The transfer co-efficients of the cathodic and anodic reactions, obtained from the slopes of the Tafel plot, are 0.93 and 1.64, respectively. The Tafel slope for the chloride electrolyte and the exchange current density, obtained from the intercept of the extrapolated Tafel’s line up to open-circuit potential, were ±35 mV dec-1 and 0.40 mA.cm-2, respectively. The transfer co-efficients for both the cathodic and anodic reactions, obtained from the slope of the Tafel plot, are the same and equal to 1.68. Exchange current density of one order lower is observed for the sulphate electrolyte than chloride electrolyte. It is reported that at low exchange current density, the screening zone radii are low, the nucleation rate is large and a smooth surface film can be easily obtained. On the other hand at high exchange current density, the radii of the screening zone is large and the saturation nucleus density is low. This permits the formation of large, well defined crystal grain and dendrite growth of the deposit14,16. Thus sulphate electrolyte is superior to chloride electrolyte, which need not to be modified with additives and can be used directly as a battery fluid. The UV-Visible absorption spectra of PANI films are shown in Fig. 4. A strong absorption peak is observed at 332.6 nm due to Π→Π* transition and a broad peak at 638.3 nm is due to excitation formation of quinoid ring of PANI film prepared in H2SO4 medium. The peak positions are in good agreement with the previous report17,18. The transmittance of the synthesized PANI film observed in the FTIR spectrum is shown in Fig. 5. The characteristic skeletal vibrations due to quinoid
Fig. 3 Slow scan linear voltammetry curve of zinc electrode (sweep rate = 0.166 mV.s-1) in (A) 1 M (NH4)2SO4 and 0.01 M ZnSO4 (pH 4.6) solution and (B) 1 M NH4Cl and 0.01 M ZnCl2 (pH 4.7) solution on solid zinc electrode
Fig. 4 UV-Visible spectra of poly(aniline) films synthesized from H2SO4 acid bath
and benzoid ring stretching of PANI are observed at 1560.3 and 1460.9 cm-1, respectively. The broad peak at 3410.8 cm-1 corresponding to –NH stretching, peak observed at 2923.8 and 1240.1 cm-1 due to C-H stretching and C-N stretching, respectively. The peaks observed are in well agreement with the previous reported work17,19, which confirms the formation of PANI films. Figure 6 shows the charge-discharge behaviour of a PANI || (NH4)2SO4.ZnSO4 (aq.) || Zn open cell under different current density. It can be seen that the charge/discharge characteristic of the cell is highly dependent on the applied current density which might
DALUI et al.: ZINC-POLY(ANILINE) RECHARGEABLE BATTERY ASSEMBLED WITH AQUEOUS ELECTROLYTE
Fig. 5 FTIR spectra of poly(aniline) films synthesized from H2SO4 acid bath
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Fig. 7 Dependence of capacity (left) and specific capacity (right) at different discharge current densities for the PANI electrode in 1 M (NH4)2SO4 and 0.01 M ZnSO4, (pH 4.6) solution
densities are shown. It can be seen that the discharge capacity increases with decreasing applied current densities and for the limiting case, when jd → 0, discharge capacity has a value of 0.0478 mAh, obtained form the Sigmoidal fit of the curve. Assuming 100% current efficiency for the polymerization of aniline, the mass of poly(aniline) sulphate was calculated from the following equation14, 20; m = {jt(Mm + yMa)}/{(2 + y)F}
Fig. 6 Charge/discharge profile for the PANI || (NH4)2SO4, ZnSO4 (aq) || Zn cell at different current densities
be because of anions diffusion limitation through poly(aniline) films. The average discharge and charge potentials were 1.10 and 1.30 V, respectively with an open circuit potential of 1.51 volts versus zinc. The cell was cycled within the potential range of 0.4 to 1.6 V and a maximum discharge capacity of 137 mAh.g-1 was observed for the PANI cathode in aqueous sulphate electrolyte. The charge/discharge behaviours of potentiodynamically synthesized poly(aniline) were also studied in this sulphate electrolyte (not shown) and a higher discharge voltage compared to galvanostatically synthesized PANI was observed. This might be because of the formation of more homogeneous materials when the growth of PANI was controlled with potential cycling, compared to galvanostatic method. In Fig. 7 dependence of capacity on different discharge current
... (1)
where, m is the mass of the poly(aniline) polymerized with current density, j during the time, t, Mm and Ma are the masses of the monomer and inserted sulphate anions, respectively and 0.5 is the doping degree for the emeraldine salt. Therefore, the mass of the poly(aniline) sulphate, calculated from the above equation, is 0.35 mg. Hence for the limiting case with discharge capacity of 0.0478 mAh, the specific capacity could be estimated as 137 mAh.g-1. Though the discharge capacity is slightly lower than the previous report14, but the discharge potential is higher for the present study. Conclusions A simple potentiostatic method is adopted to synthesize pure and cheaper poly(aniline) (PANI) cathode. An aqueous electrolyte, comprised of 1 M (NH4)2SO4 and 0.01 M ZnSO4, has been introduced for zinc-poly(aniline) rechargeable battery. A better electrochemical activity, with more negative open circuit potential (OCV), was observed for the zinc anode in this electrolyte compared to chloride
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electrolyte. Thus, ammonium sulphate based aqueous electrolyte can be used without further modification as a battery fluid for Zn-Poly(aniline) rechargeable battery. The same electrolyte may be used for the preparation of aqueous polymer-gel electrolyte for aqueous rechargeable battery with better activity. Acknowledgements Authors would like to thank Department of Science and Technology under FIST programme, New Delhi for providing the financial support to procure Potentiostat/Galvanostat instrument (PAR VarsaStat® II). References 1 Sivakkumar S R & Kim D W, J Electrochem Soc, 154(2) (2007) A134. 2 Taguchi S & Tanaka T, J Power Sources, 20(3-4) (1987) 249. 3 Yonezawa S, Kanamura K & Takehara Z, J Electrochem Soc, 142(10) (1995) 3309. 4 Morita M, Miyazaki S, Ishikawa M, Matsuda Y, Tajima H, Adachi K & Anan F, J Electrochem Soc, 142(1) (1995) L3. 5 Novak P, Muler K, Santhanam K S V & Haas O, Chem Rev, 97 (1997) 207.
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