Electrochemical properties of dodecylsulfate-doped polypyrrole films in aqueous solution containing NH4Cl and ZnCl2

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Synthetic Metals 64 (1994) 9-15

Electrochemical properties of dodecylsulfate-doped polypyrrole films in aqueous solution containing NHaCI and ZnC12 Jang Myoun Ko a, Seok Kim b, Kwang Man Kim b, In Jae Chung <* "R&D Center, Hankook Tire Mfg. Co., Ltd., Yusong, Taejon, South Korea bDepartment of Chemical Engineering, Korea Advanced Institute of Science and Technology, 373-1 Kusong, Yusong, Taejon 305-701, South Korea

Received July 19, 1993; in revised form January 17, 1994; accepted January 28, 1994

Abstract

Dodecylsulfate-doped polypyrrole (PPy) films were electrochemically synthesized in aqueous solution containing sodium dodecylsulfate. In order to examine the applicability of the films to electrode materials, the electrochemical properties and charge--discharge properties were investigated in the aqueous solution containing 0.4 M NH4CI and 0.1 M ZnCI2. From cyclic voltammetric and chronoamperometric results, it was concluded that the good electrochemical reversibility was due to the fast mobilities of NH4 + and C1- instead of the dodecylsulfate trapped in the polymer matrix. The charge-discharge tests of the Zn/(NH4CI, ZnCI2)/PPy cell showed an open-circuit voltage of 1.3 V after 80 min, specific capacity of 59 Ah kg -1 and energy density of 77 Wh kg-l.

Introduction

Much progress has been made in the development of polymer materials applicable to a battery electrode [1-3]. Mechanical stability and high energy density serve as the focus for an appropriate material selection for rechargeable polymer batteries. Polypyrrole (PPy) and polyaniline appear to be the most promising candidates [4-6]. While polyaniline has demonstrated a higher doping level and a higher energy density [6], PPy has been found to have a higher mechanical stability and a better reversibility [7]. The electropolymerization method has been known to give vast preparative options by means of which the resulting films show considerable differences in electrical and electrochemical properties [8,9]. Experimental evidence shows that PPy films exhibit different morphologies and kinetics according to their particular formation factors, such as potential and electrolyte anions [10-13], different electrochemical properties according to the film thickness and morphology [14]. In order to enhance the performance of the battery, the following two factors are considered to be important. First, the doping charge of PPy must be increased to *Author to whom correspondence should be addressed.

0379-6779/94/$07.00 © 1994 Elsevier Science S.A. All rights reserved SSDI 0379-6779(94)02072-7

improve its energy density and to sustain a large current density. Secondly, the doping-undoping process must be reversible to obtain a high specific capacity. These two conditions can be optimized by choosing the appropriate anion in electropolymerization. Many different kinds of electrolyte anions used in the electropolymerization of pyrrole are divided mainly into three categories: inorganic anions [14], polymeric anions [15,16] with sulfonate group and anionic surfactants [17-25]. PPy electropolymerized in the electrolyte with inorganic anions, such as C104-, BF4-, CI-, etc., shows good electrical properties having a conductivity of nearly 100 S cm -1, but very poor mechanical properties. In contrast, PPy prepared in polymeric electrolytes, such as sulfonated polystyrene, shows good mechanical properties, but poor electrical properties [16]. In another aspect, it is well known that a better PPy film is made in the surfactant anions like sodium dodecylsulfate [5]. Many studies on dodecylsulfate-doped PPy have been carried out. For example, Wernet et al. [20,24] reported the influence of electrolysis parameters on the conductivity of a dodecylsulfate-doped PPy film. Peres et al. [3,21] studied the optimum conditions for electrochemical synthesis. Despite such enthusiastic studies on electrochemical properties for dodecylsulfate-doped PPy, there is no

10

J.M. Ko et al. / Synthetic Metals 64 (1994) 9-15

systematic study on the electrochemical properties in the aqueous solution containing NH4C1 and ZnC12. For an enhancement of solubility of ZnCI2, NH4C1 was added to ZnCI2 electrolyte solution. This system is capable of making a Zn/(NH4CI, ZnC12)/PPy rechargeable battery. Thus, the purpose of this study is to investigate the electrochemical properties and the charge-discharge properties in the aqueous solution c o n t a i n i n g N H 4 C 1 and ZnCI2.

Experimental

Materials Pyrrole was used after a distillation under reduced pressure. Sodium dodecylsulfate (NaDS), sodium polystyrene sulfonate (weight-averaged molecular weight 70 000), ammonium chloride and zinc chloride were used, as supplied from Aldrich Chemical Co.

Electropolymerization Dodecylsulfate-doped PPy films were deposited by applying a constant current density of 2 mA cm -2 on a platinum foil electrode with an area of 1 cm 2 in a deoxygenated aqueous solution containing 0.2 M pyrrole and 0.02 M sodium dodecylsulfate. The thickness of a dodecylsulfate-doped PPy film was controlled by monitoring the charge consumed during electropolymerization. After the desired charge was passed, the sample was rinsed with an electrolyte solution containing 0.4 M NH4CI and 0.1 M Z n C I / t o remove monomer and other soluble molecules. Although the dodecylsulfatedoped PPy film was produced with sodium dodecylsulfate, its electrochemical characterization was carried out in the aqueous solution containing 0.4 M NH4C1 and 0.1 M ZnC12 to examine its applicability to a rechargeable battery.

NH4CI and

0.1 M ZnCl2. The distance between two electrodes was fixed at 20 ram. Charge--discharge tests were carried out at a constant current density 20, 50 or 100/zA cm -2 in the potential range 1.0-1.5 V. All experiments were carried out in an argon atmosphere. Results and discussion

Redox properties of the dodecylsulfate-doped PPy film in NH, CI and ZnC12 Figure 1 shows potential-dependent cyclic voltammograms of the dodecylsulfate-doped PPy film of 4.5 /zm thickness in an aqueous solution containing 0.4 M NH4CI and 0.1 M ZnCI2. These cyclic voltammograms are very different from the broad cathodic peaks reported for an aqueous or a nonaqueous solution containing various kinds of anions [10,13]. The dodecylsulfate-doped PPy films show a two-stage anodic process with peak I at -0.36 and peak II at 0.48 and a cathodic process with peak III at -0.47 V. Peak I is almost symmetric around a small deviation t~Ep (Epa,a-Epe). This can be explained by electrolyte mobility for the charge neutrality in the PPy matrix. It is well known that electrolyte anions move toward the electrode to maintain the charge neutrality and are entrapped in the PPy film formed in the course of anodic poly-

Apparatus Electrochemical measurements were carried out with a Hokuto Denko HA-301 potentiostat/galvanostat. An Ag/AgCI saturated with 4 M KCI was used as a reference electrode. The apparent diffusion coefficients of electrolyte ions in dodecylsulfate-doped PPy were estimated by chronoamperometry varying the potential window steps as -0.80-0.00, 0.00-0.60 and -0.80--0.60 V, respectively.

2

i

PeE: III

I

J

I

-0.8

0.0

0.6

V vs. Ag/AgC[

t

-0.8

t

I

0.0

0.6

V vs. Ag/AgCi

Charge-discharge test

Fig. 1 (left). Cyclic voltammograms for the dodecylsulfate-doped PPy electrode in aqueous 0.4 M NI-LCI and 0.1 M ZnCI2 solution. Switching potentials -0.80--0.00 V and -0.80-0.60 V vs. Ag/AgC1. Film formation charge 1 C cm -2.

A Zn/(NH4CI, ZnCI2)/PPy battery was constructed with dodecylsulfate-doped PPy as a cathode and zinc as an anode in the aqueous solution containing 0.4 M

Fig. 2 (right). Cyclic voltammograms for the dodecylsulfate-doped Play film in 0.1 M sodium polystyrenesulfate. Other conditions are the same as for Fig. 1.

11

J.M. Ko et al. / Synthetic Metals 64 (1994) 9-15

merization [13]. According to energy-dispersive X-ray spectroscopic and electrogravimetric results [3,5], dodecylsulfates entrapped in the PPy film do not leak out during the charge-discharge process because of their bulk. So, the charge neutrality of peak I is maintained by the transfer of electrolyte cations NH4 +. The reversibility of the redox process around peak I may be due to the fast mobility of small NIL + ions. To substantiate peak II in Fig. 1, cyclic voltammetry was carried out in an electrolyte solution containing sodium polystyrenesulfonate. As shown in Fig. 2, there is an anodic peak at - 0 . 4 V due to the transfer of Na + similar to peak I in Fig. 1. However, around 0.4 V, only a small current like a capacitive current appears, which may be due to the interfacial oxidation of the PPy film and no diffusion of polystyrenesulfonate ions through the PPy film because of their bulk. Thus, the charge neutrality of peak II is maintained by the transfer of CI- anions from the electrolyte solution containing NH4C1 and ZnC12. On the other hand, in the cathodic process shown in Fig. 1, a large cathodic peak III appears at -0.47 V and involves the transfer of all NH4 ÷ and CI- ions. This suggests that ions have a strong interaction with the oxidized backbone of PPy and that it is difficult for C1- ions to diffuse out of the PPy film. A similar description was given in other literature [10,13]. In order to investigate electrochemical kinetics for the dodecylsulfate-doped PPy film, Fig. 3 shows the cyclic voltammograms recorded at different scan rates which have two peaks in the anodic process and one peak in the cathodic process, like Fig. 1. The peak potentials are listed in Table 1. The peak current is almost proportional to the scan rates and the peak potential shifts slightly in the Figure as the scan rate increases. This indicates that NH4 + and CI- ions have a fast mobility because of their small size and are involved in the two-stage anodic process. From the comparison of Figs. 1 and 2, another characteristic feature which should be noticed is that the capacitive current in the NH4C1 and ZnC12 solutions has a value of 7.5 mA cm -2 at 0.60 V after peak II in Fig. 1, which is larger than 2.0 mA cm -2 observed in sodium polystyrenesulfonate solution in Fig. 2. It has generally been known that the capacitive current of PPy film decreases as the electronic conductivity decreases [8]. In the present case, however, the large capacitive current is generated due to the small size of CI- anions. The C1- anion is more effective on storage and capacitance of charge on the surface of the electrode because its size is smaller than that of the polystyrenesulfate anion. Thus, the capacitive current is assured to be dependent on the kind of anions used in the electrolyte solution.

mV/s

Peal

Peak n

E.

I

-0.8

I

I

0,0

0.6

V vs. Ag/AgC1 Fig. 3. Scan rate dependences of cyclic voltammograms for the dodecylsulfate-doped PPy electrode in 0.4 M NH4CI and 0.1 M ZnCI2. Scan rates 10, 30, 50, 70 and 90 mV s -1, respectively. Film formation charge 1 C cm -2. Table 1 Peak potentials Scan rate (mV s - ' )

Ep, al ~ (mV)

Ep, a2a (mV)

E p , cb

(mV)

¢~Epc (mV)

10 30 50 70 90

-400 -380 -360 -350 -348

350 430 480 530 550

-400 -450 -470 -500 -500

0 70 110 150 152

aAnodic peak potentials correspond to first and second anodic peak current densities. bCathodic peak potential. cPeak potential difference between Ev,,I and Ep.¢.

Influence of dodecylsulfate-doped PPy film thickness on redox property In order to substantiate the above discussion, anodic and cathodic peak currents are plotted against scan rate for different film thicknesses in Fig. 4(a). The slopes are also plotted in Fig. 4(b). It should be noticed that the slopes vary from about 1.0 for a thin film to about 0.8 for a thick film. For the thin PPy film up to a thickness of 1 /xm, the electron transfer process resembles the diffusion through a thin layer cell, but it is regarded as the semi-infinite diffusion when the film is very thick and the slope should approach 0.5

12

J.M. Ko et al. / Synthetic Metals 64 (1994) 9-15

1o!

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Fig. 5. Relationship between coulombs and film thickness estimated from the discharge process of cyclic voltammograms at 50 mV s - ' . 1.0

--

--

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Fig. 4. (a) Relationship between peak currents and scan rates in cyclic voltammograms for the dodecylsulfate-doped PPy electrode in 0.4 M NI-LCI and 0.1 M ZnCI2. Film formation charges 50, 120, 240, 500 and i000 mC cm -2, respectively. (b) Film thickness dependence of rate-determining stage occurring at the dodecylsulfatedoped PPy electrode in 0.4 M NH4C1 and 0.1 M ZnCl2.

[5,14,25]. Even though the condition of semi-infinite diffusion is applied to the film with thickness more than 1 /~m, the slope has a larger value than 0.5. This may indicate that the redox reaction process has the mixed characteristics of the diffusion process of NH4 ÷ or C1- under semi-infinite conditions and the electron transfer process under the thin layer cell. The relationship between coulombs and film thickness deviates slightly from linearity, as shown in Fig. 5. Thus, it is concluded from the fast kinetics of the charge-discharge process in the NHaCI and ZnC12 solution that the dodecylsulfate-doped PPy film is adequate as the cathodic electrode of a rechargeable battery.

Chronoamperometry on the dodecylsulfate-doped PPy in NH4CI and ZnCI2 In order to measure the diffusion coefficient of NH4 + and CI- within the polymer matrix, the potential step chronoamperometry was employed for the dodecylsulfate-doped PPy film. It is difficult to measure the diffusion coefficient of C1- in the cathodic process for

-20 -40 0

1

2 3 4 5 Time/see. Fig. 5. Chronoamperograms for the dodecylsulfate-doped PPy electrode in 0.4 M NH4CI and 0.1 M ZnCl2. Potential was stepped from -0.8 to 0.0, 0.0 to 0.6 and -0.8 to 0.5 V.

the thick film with 4.5/zm thickness because the cathodic current corresponding to peak II shifts cathodically. To eliminate the cation effect in the potential range of 0.0 to 0.5 V, we choose a thin film of 0.23 /zm thickness, in which the electron transfer process is only the rate-determining step in the redox process, as shown in Fig. 4(b). NH4 + ions may play an important role in peak I of Fig. 1 to maintain the charge neutrality, because they have four times higher concentration and smaller size than Zn 2+. After 10 s of holding in the reduction state with a bright yellow colour at -0.8 V, the potential was stepped up from - 0.8 to 0.0 V (peak I) and down to -0.8 V. On the other hand, to obtain the C1- diffusion coefficients, the potential was stepped up from 0.0 to 0.6 V after holding at 0.0 V for 10 s and then down to 0.0 V. In addition, for a comparison of each process with the combined process, potential was stepped from -0.8 to 0.6 V and then stepped down to - 0 . 8 V. These results are shown in Fig. 6 and the charges estimated from the chronoamperograms are listed in Table 2. The sum of cathodic charges for two different processes is almost the same as the cathodic charge for the combined process. If the electrochemical

J.M. Ko et al. / Synthetic Metals 64 (1994) 9-15

13

Table 2 Redox charges and diffusion coefficients Potential range (V)

Redox process

Charge (mC cm-2)

Ions"

Diffusion coefficient (10-9 cm2 s -t)

- 0.8--0.0 (peak I) 0.0-0.6 (peak II) - 0.8-0.6

oxidation reduction oxidation reduction oxidation reduction

1.9 2.0 1.9 2.1 3.7 3.9

NH4÷

1.8 1.6 2.6 0.9

CI-

alons for charge neutrality in the redox process of dodecylsulfate-dopedPPy.

process process tration, pressed

o.o -~.

is dominantly controlled by the electron transfer in the cell with a completely uniform concenthe relation between current and time is exby [25]

i(t) = 4nFADC

-0.80- 0.00 V

xp( t)

where n is the number of electrons transferred per molecule, F is the Faraday constant, A is the electrode surface area, D is the diffusion coefficient indicating the electrolyte mobility in the film, C is the concentration of reaction sites and I is the sample thickness. Equation (1) can be rearranged into

i(t)=io e x p ( - ~Tr2Dt)

(2)

0 (a)

0.0 -0.2- ~

5 10 15 20 Time/msec. 0.00 - 0.60 V .

-0.4.~ -O.8-

and -0.8.

ln(i/io)-

~-2D

l~ t

(3)

The chronoamperograms are plotted for the film with a thickness of 2.3 X 10 -5 cm as ln(i/io) versus time, as shown in Fig. 7. Diffusion coefficients D calculated by eqn. (3) are summarized in Table 2. Diffusion coefficients of NH4 ÷ are almost equal for anodic and cathodic. processes. But those of CI- are different because CIions transferred from the solution to the PPy film diffuse out of the film slowly by the interaction between PPy molecules and ions.

Charge--discharge properties for the PPy/(NH4CI, ZnCI2)/Zn battery Figure 8 shows the result of charge--discharge tests at different current densities for the dodecylsulfatedoped PPy in the NH4C1 and ZnCI2 solution. When the current density increases, the response of the voltage curve becomes much faster (notice x-axis). This suggests that the fast mobility of electrolytes is very effective on maintenance of charge neutrality and the electrolyte concentration gradient is not high enough at the poly-

Ox. \ \ '~ 0 10 20 30 40 (b) Time/msec. Fig. 7. Plots of ln(i/io) vs. time when potential was stepped (a) -0.8 to 0.0 V and (b) 0.0 to 0.6 V.

-1.0 c

met/electrolyte interface. Figure 9 illustrates the change in cathodic charge with cycle number in the range 0.0-1.4 V measured at the Zn electrode. The charge loses 30% of the original change after 100 cycles. This charge loss may be due to the irreversible chemical reaction of the polymer, such as degradation reaction, crosslinking, etc. The coulombic efficiency defined as the ratio of the cathodic and anodic charge is a very useful parameter to characterize the battery performance. In this experiment the ratio turned out to be one. But, polymer degradation might affect the charge-discharge process. The self-discharge property is another important factor to be checked when a material is used as a rechargeable battery electrode. The open-circuit voltage (OCV) is shown in Fig. 10, where the potential approaches a fixed value of 1.3 V. The specific capacity of the battery in this work [3] is calculated by

14

J.M. Ko et al. I Synthetic Metals 64 (1994) 9-15

1.21"61!

from the literature [20], Cs has the value of 59 Ah kg-1. An energy density of 77 Wh kg -1 is calculated by multiplying Cs by the OCV (1.3 V).

20 aA/cm2

0.8 I

I

~ 1.61,

50 . A / e r a

;"

1.2

",

0.8~: 0

,"

,"

",

Conclusions

2

The dodecylsulfate-doped PPy film shows a good electrochemical reversibility in the aqueous solution containing 0.4 M NH4CI and 0.1 M ZnCI2 because NH4 + and C1- ions move fast through the PPy film while the dodecylsulfate is entrapped in the polymer matrix. Charge-discharge tests of the Zn/(NH4CI, ZnCI2)/PPy cell show an open-circuit voltage of 1.3 V after 80 min, specific capacity of 59 Ah kg- 1 and energy density of 77 Wh kg-1.

",

I

I

I

I

I

1

2

3

4

5

1.6 !,

1 mA/cm2

1.2 ,'"

".,

,'"

'

N

"

,

0.8 0.0

0:5

110 Time/sec.

1:5

2:0

Fig. 8. Charge-discharge curves for the Zrd(NH,,CI, ZnCl2)/dodecylsulfate-doped PPy cell at 20, 50 and 100 p.A cm -2.

This work was financially supported by the R&D Center, Hankook Tire Mfg. Co., Ltd. The authors would like to thank Drs Hwi Joong Kim and Je M Oh for their valuable discussions.

4-

3-

0

~2-

References

1

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20

40 60 80 100 No. of Cycles Fig. 9. Discharge as a function of number of cycles for the Zn/ (NI--I4CI,ZnCl2)/dodecylsulfate-doped Play cell. 2.0

.5

~

1.0

0.5

0.0 0

20

40

60

80

TilneJmin~ Fig. 10. Open-circuit voltage as a function of time for the Zn/(NI-LCI, ZnCl2)/dodecylsulfate-doped PPy cell.

C~=

Acknowledgements

YF

3600(Mp~ + YMds)

X 103

(4)

where Y is the doping level, M.~ is the molecular weight of pyrrole and Mds is the molecular weight of the dodecylsulfate anion. When a 40% doping level is taken

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15

[22] J.M. Ko, H.W. Rhee and C.Y. Kim, Prog. Pacific Po~ym. Sci., (1991) 107. [23] J.M. Ko, H.W. Rhee and C.Y. Kim, Makromol. Chem., Macromol. Syrup., 53 (1992) 81. [24] W. Wernet and G. Wegner, Makromol. Chem., 138 (1987) 1475. [25] A.J. Bard and L.R. Faulkner, ElectrochemicalMethod, Wiley, New York, 1980, Chs. 3, 6 and 10.