Overdoping and the capacitive effect in polypyrrole

Synthetic Metals, 21 (1987) 129 - 134

OVERDOPING

129

A N D T H E C A P A C I T I V E E F F E C T IN P O L Y P Y R R O L E

J. TANGUY and N. MERMILLIOD IRDI/DEIN/LERA, CEN. Saclay, 91191 Gif-sur-Yvette Cddex (France)

Abstract

Analysing the nature of the changes stored in chemically synthesized polypyrrole by a.c. impedance and cyclic voltammetry, we have decomposed the total current into two components: a capacitive current and a noncapacitive one. The non-capacitive charge is found to be approximately constant, whereas the capacitive one can vary widely from one sample to another. We conclude that the interesting overdoping effect observed in polypyrrole can essentially be attributed to the capacitive charge.

1. I n t r o d u c t i o n

It is well known that an important part of the charges released in the cyclic voltammetry or in the discharging process of some conducting polymers is of capacitive nature [1 - 3]. However, the origin of these charges and the resulting capacitance effect have not been totally explained [4- 5]. In a preceding paper, we analysed the capacitance as resulting from the movement of some doping ions shallowly trapped near the surface of the polymer chain [6]. Those ions exhibit relatively short relaxation times, they can follow a lowfrequency a.c. perturbation, they are responsible for the capacitance effect, and for what we have called the capacitive current. Another part of the doping ions appears to be deeply trapped in the chains; they cannot follow the a.c. signal and they are responsible for the so-called non-capacitive current. In this paper we try to analyse and to compare the amplitude of these two currents as a function of the total charge in the polymer in order to establish the role and the nature of these two kinds of charges. In this way we have performed a.c. impedance measurements and compared the results with those obtained by cyclic voltammetry for different samples in different experimental conditions.

2. A . c . i m p e d a n c e m e a s u r e m e n t s

The capacitive charges have been determined by measuring the complex impedance of some polypyrrole electrodes at different potentials, with a signal 0379-6779/87/$3.50

(~ Elsevier Sequoia/Printed in The Netherlands

130 frequency varying from 10 -4 Hz to 104 Hz. The synthesis conditions as well as the apparatus used for impedance measurements and cyclic voltammetries have been described elsewhere [7]. At low frequency ( f < 10 2 Hz) we found a very high capacitance Ci > 200 F/g, which becomes frequency independent. We measured a series resistance RT, which is also frequency independent. Both the parameters Ci and R w appear to be dependent on the potential. (a) T h e capacitive c u r r e n t

From the curve giving the capacitance Ci versus the potential [6], we were able to calculate the current charging this capacitance during cyclic voltammetry. This current is given by [5] i¢ = Ci d V/dt[1 - exp( -t/RwCi)]

where d V / d t is the voltage sweep rate, C i the low-frequency measured capacitance and R w the series resistance, ic was calculated step by step assuming Ci to be constant between two voltage steps. This capacitive current ic is plotted in Fig. 1 for different voltage sweep rates. At very low sweep rates (0.1 mV/s, Fig. l(a)) the curve is homothetic to the capacitance curve, since at this voltage sweep rate the resistance effect is negligible. Since the capacitance curve is quite symmetric in the oxidation and in the reduction processes, the current curve is symmetric too. However, when the sweep rate is increased, the current curves in Figs. l(b) and (c) present some asymmetries due to the resistance effect. Let us now compare this calculated current with the total current in the cyclic voltammetry.

I(mA)

PPy 198

.f.lmV/s aLIxl0 f.SmV/s bllx2

2

~ a

0

, ,--

J"lmV/s CLIx 1 V(mV)

-2

Fig. I. 'Capacitive currents' calculated from the impedance results.

131

3. Cyclic v o l t a m m e t r i c current Figure 2 shows three voltammograms obtained at different sweep rates. The striking point is that even at a very low sweep rate (0.1 m/s), the two peaks are asymmetric. The oxidation peak is displaced toward positive potentials when the sweep rate is increased; on the contrary the reduction peak appears to be quite stable. This effect is probably due to the fact that at the beginning of the oxidation process, the polypyrrole is very resistive. But the most important fact is that the total current in the cyclic voltammetry is higher than the capacitive current, as is seen by comparing Figs. 1 and 2.

(a) The non-capacitive current From the difference between the total current in Fig. 2 and the capacitive current (Fig. 1), we obtain what we call the non-capacitive current, as reported in Fig. 3. These curves appear to be largely asymmetric even at low voltage sweep rates. Moreover, if we plot (Fig. 4) the potential difference between the oxidation and the reduction peaks, we find that by extrapolating to zero speed, the peak potential separation is greater than 160 mV. This value is higher than the standard one [8]. So the non-capacitive current appears to result from a quasi-reversible and non-Nernstian redox process [4, 8]. From the analysis of the frequency dependence of the complex impedance, we have proposed for these two currents an interpretation based on the consideration of two doping processes [6]. We assume that during the redox reaction some doping ions ( C 1 0 4 ) are fixed on the polymer chains (deeply trapped ions). Their relaxation time is so low that they cannot follow the a.c. signal so they are not seen by the a.c. impedance measurement. These deeply trapped ions are released only at low potentials, giving the asymmetric non-capacitive current. Other ions remain in a position further from the

l(mA)

PPy 198 .5mV/s

.lmV/s_,.~/,~ 1 x 2 "

0

..

1°

I LA\ ,v ;vl AglAg*

-2

Fig. 2. Cyclicvoltammetric currents at differentvoltage sweep rates.

132

I(mA)

PPy 198 ~E(mV)

.ImV/s . A

_

.SmV/s

Ix 10 / ~ l x

21mY/s

//A.\~ .~ . . . . ' . . . .

[xlIxl '

'

'

i 0

500

,v! vl Ag/Ag*

100 , ....

-2

.5

j Sweero rate

I

mV/s

Fig. 3. Non-capacitive currents obtained from the difference between the total currents (Fig. 2) and the capacitive current of Fig. 1. Fig. 4. Potential difference between the oxidation and the reduction peaks of the non-capacitive current as a function of the voltage sweep rate.

chains (shallowly trapped ions). They form an ionic double layer near the chains and can follow a low-frequency a.c. signal, thus giving the symmetric capacitive current. Let us now examine the respective importance of these two currents in the total current of cyclic voltammograms.

4. Relation between the overdoping and the capacitive charge The total charge released from a polypyrrole electrode during cyclic voltammetry or during a discharging process is known to vary from one sample to another. In some cases this charge is found to exceed largely the standard doping level of 33%. This depends on the preparation conditions, such as the water content in the polymer, and also on the solvent used in the electrolyte during the electrochemical cycling [7]. On the other hand, it appears that the measured capacitance also depends on the same parameters. For example, by using acetonitrile instead of propylene carbonate as solvent in the electrolyte, both the capacitance of a polypyrrole sample and the total charge released during cyclic voltammetry are increased by about 20%. So the question is whether the overdoping effect is mainly due to the capacitive charge? Thus in Fig. 5 we have reported for different polypyrrole samples the capacitive charge and the non-capacitive one as a function of the total charge stored in the samples. It is clear that the non-capacitive charge remains nearly constant from one sample to another and represents a doping level of only 12 - 15%. On the other hand, the capacitive charge is found to increase quite linearly with the total charge. For example, for sample no. 5 (working in acetonitrile), the capacitive contribution to the charge is twice as high as the non-capacitive one, and it represents the main part of the high doping level of 50% obtained in this case.

133 O.(mC)

JCAPACITIVE

1000

CHARGE ®

~.

~

50

500

NON

CAPACITIVE

I

10

CHARGE o _ _

500

, J

1000

i

1500

Total charge

(m[)

Fig. 5. Evidence for the overdoping effect arising from the capacitive charge.

T h e s e r e s u l t s c a n be c o m p a r e d w i t h t h o s e of F e l d m a n et al. [9], w h o found t h a t the c o n d u c t i n g s t a t e in p o l y p y r r o l e was o b t a i n e d w i t h a doping level of only 15%. So we t e n d to a s s u m e t h a t the deeply t r a p p e d c h a r g e s giving the n o n - c a p a c i t i v e c u r r e n t can be c o n s i d e r e d as the basic doping level (12 - 15%) needed to switch the p o l y m e r from its i n s u l a t i n g s t a t e to the c o n d u c t i n g one. T h e n u m b e r of e l e c t r o n s r e q u i r e d for this s w i t c h i n g a n d d e t e r m i n e d h e r e f r o m the n o n - c a p a c i t i v e c h a r g e r e p r e s e n t s a b o u t one e l e c t r o n for s e v e n or e i g h t p y r r o l e molecules. T h e n the o v e r d o p i n g c a n be c o n s i d e r e d as b e i n g m a i n l y due to the s t o r a g e of the s h a l l o w l y t r a p p e d c h a r g e s t h a t are r e s p o n s i b l e for the c a p a c i t i v e effect.

5. C o n c l u s i o n We c a n derive two c o n c l u s i o n s from the d e c o m p o s i t i o n of the t o t a l c h a r g e stored in a p o l y p y r r o l e electrode into its c a p a c i t i v e and n o n - c a p a c i t i v e components: (a) T h e n o n - c a p a c i t i v e charge, w h i c h is n e a r l y c o n s t a n t from one s a m p l e to a n o t h e r , r e p r e s e n t s a low doping level of a b o u t 12 - 15%: one e l e c t r o n for s e v e n or eight p y r r o l e molecules. This doping level is p r o b a b l y the m i n i m u m needed to e n s u r e the s w i t c h i n g from the i n s u l a t i n g to the c o n d u c t i n g state. (b) T h e c a p a c i t i v e c h a r g e c a n v a r y l a r g e l y from one s a m p l e to a n o t h e r , giving an o v e r d o p i n g effect t h a t c a n lead to a doping level as h i g h as 50%. In this w a y p o l y p y r r o l e c a n be considered as an i n t e r e s t i n g m a t e r i a l b e c a u s e of its c h a r g e s t o r a g e c a p a c i t y .

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