Influence of additives on the electrochemical oxidation of polyacetylene

Synthetic Metals, 21 (1987) 319 - 324

319

INFLUENCE OF ADDITIVES ON THE ELECTROCHEMICAL OXIDATION OF POLYACETYLENE BERND KRISCHE Department of Organic Chemistry, Royal Institute of Technology, S-100 44 Stockholm (Sweden)

SVANTE SODERHOLM Department of Physics 3, Royal Institute of Technology, S-100 44 Stockholm (Sweden)

Abstract The result of the electrochemical oxidation of polyacetylene is improved by additives in the electrolyte, which suppress overoxidation of polyacetylene. The temperature dependence of the conductivity is semiconductor-like and is interpreted by Sheng's fluctuation-induced tunnelling model for tunnelling between metallic regions. Polyacetylene oxidized in the presence of an additive (e.g., bithiophene) has a smaller intrinsic resistance of the metallic strands.

Introduction The electrochemical oxidation (doping) of polyacetylene in organic solvents is sensitive to impurities in the electrolyte. Their effect can be either positive or negative, depending on chemical constitution and oxidation potential. Overoxidation of polyacetylene during constant current oxidation is an inherent risk, not only for the bulk material but even more on a microscopic scale. By overoxidation we mean charging of polyacetylene beyond a certain limit, which is accompanied by changes in colour, mechanical and electrical properties, and chemical composition [1, 2]. The limit is somewhat dependent on the electrolyte (solvent, counterion), but lies around 7% of dopant. Charging into the overoxidation regime is no longer reversible and leads to degradation of the material. Because local overoxidation is difficult to control, additives that suppress overoxidation improve the electrochemical oxidation of polyacetylene [3, 4] and probably of other conducting polymers too.

Electrochemistry Shirakawa-polyacetylene films (about 20 x 8 x 0.1 mm) were mounted in an electrochemical H-cell. Electrolyte solutions were 0.1 M of NBu4C104 or 0379-6779/87/$3.50

~:, Elsevier Sequoia/Printed in The Netherlands

320

NBu4PF 6 and 0.005 M of additive (if any). Oxidation was performed using an EG&G Model 173 Potentiostat/Galvanostat by applying a constant current of 0.3 mA per mg polyacetylene with polyacetylene as the anode. This corresponds to cu r r en t densities of 1 - 2 mA/cm 2. During oxidation, the potential of the polyacetylene film was recorded v e r s u s a solid Ag/AgC1 reference elec-

1.2

@

®

.64-

5

1

charge [%]

Fig. l. Typical potential v s . charge/(mg polyacetylene) curves. Current: 0.3 mA/mg = 1- 2 mA/cm2. Electrolyte: 0.1 M NBu4C104 in acetonitrile (Aldrich gold label) with exception of 6 (0.1 M NBu4PF 6 in dichloromethane). For reasons of reference, oxidations 1 - 5 were performed with a platina-mesh anode of about the same size as the polyacetylene films and with 0.1 mA. Curve number for oxidation at platina

Additive

Curve number for polyacetylene oxidation

1 2 3 4 5

none bithiophene N-phenylpyrrole tetramethoxythianthrene ferrocene

6, 7 8 10 9 11

321 trode. Typical p o t e n t i a l versus c h a r g e / ( m g p o l y a c e t y l e n e ) curves are s h o w n in Fig. 1. T h e s t e e p i n c r e a s e of p o t e n t i a l a f t e r t h e p l a t e a u r e g i o n i n d i c a t e s t h e o n s e t of o v e r o x i d a t i o n .

Results T a b l e 1 s h o w s t h e effect of s o m e a d d i t i v e s i n d i f f e r e n t e l e c t r o l y t e s for o x i d a t i o n to 10% (0.741 A s / m g ) * . It s h o u l d be n o t e d t h a t t h e d e g r e e of polya c e t y l e n e o x i d a t i o n c a l c u l a t e d b y w e i g h t u p t a k e differs c o n s i d e r a b l y f r o m t h e t h e o r e t i c a l v a l u e c a l c u l a t e d f r o m t h e p a s s e d c h a r g e , e s p e c i a l l y a t h i g h e r dopi n g levels. F u r t h e r m o r e t h e c o n d u c t i v i t y of t h e p o l y a c e t y l e n e films is o f t e n n o t c o n s t a n t a l o n g t h e film a n d t h e d e g r e e of o x i d a t i o n v a r i e s , as c a n be s e e n f r o m t h e d i f f e r e n t c o l o u r s of d i f f e r e n t r e g i o n s of t h e film. T h e c o n d u c t i v i t y v a l u e s i n T a b l e 1 give t h e l o w e s t a n d h i g h e s t v a l u e m e a s u r e d i n t h e s a m p l e ( t h e film w a s c u t i n t o f o u r o r five pieces, w h i c h w e r e m e a s u r e d i n d e p e n d e n t l y ) , w h e r e a s t h e d o p a n t c o n c e n t r a t i o n is c a l c u l a t e d for t h e w h o l e film. E n t r i e s m a r k e d w i t h # indicate partially overoxidized samples. T o g a i n f u r t h e r i n f o r m a t i o n a b o u t h o w t h e a d d i t i v e s affect t h e p r o p e r t i e s of h i g h l y o x i d i z e d p o l y a c e t y l e n e , t h e c o n d u c t i v i t i e s of s o m e s a m p l e s w e r e meas u r e d as a f u n c t i o n of t e m p e r a t u r e . TABLE 1 Effect of additives (0.005 M) on the outcome of the electrochemical oxidation of polyacetylene in different electrolytes Electrolyte

Additive

None

Ferrocene

TMT~'

Bithiophene

N-phenylpyrrole

NBu4PF~ CH2C1~

C.C.b(%) a (S/cm)

# h 5.01 70 - 220

5.45 5 - 170

3.66 60 - 100

5.04 30 - 180

NBu4CIO4 PC~'

c.c. (%) a (S/cm)

~5.17 100 - 330

4.81 2 - 310

5.73 70 - 390

6.50 40 - 300

5.50 10 - 340

NBu4CIO4 CH:~CN

c.c. (%) a (S/cm)

#6.40 520 - 545

2.25 0.01 - 0.2

5.98 560 - 640

7.28 320 - 400

6.25 40 - 100

NBu4ClO4 CH:~CN/H~O

c.c. (%) a (S/cm)

2.82 0.4 - 2

3.99 4-5

~TMT: tetramethoxythianthrene; PC: propylenecarbonate. %.c.: concentration of counterions (by weight uptake): #: partly overoxidized.

Temperature dependence of the conductivity T h r e e s a m p l e s w e r e m e a s u r e d f r o m r o o m t e m p e r a t u r e to 20 K. T w o of t h e m (B, C i n T a b l e 2) w e r e o x i d i z e d i n t h e p r e s e n c e of a n a d d i t i v e a n d o n e *For more information about the electrochemical oxidation and the effect of additives on conductivity and electrochemical yield, see [3].

322 TABLE 2 Results of conductivity vs. temperature measurements Sample

a,t (S/cm)

A (K)

TI (K)

TO (K)

RI Rj (arbitrary units)

A B C

430 370 260

60 50

45 50

-~25 ~ 50

1 < 0.3 < 0.4

1 3 4

> 0.1 =~0.07 ~ 0.07

Samples A, B: acetonitrile/perchlorate, B with bithiophene as additive. Sample C: propylenecarbonate/perchlorate, silver as additive. s a m p l e (A) without. T h e t e m p e r a t u r e d e p e n d e n c e of the c o n d u c t i v i t y (Fig. 2) is i n t e r p r e t e d u s i n g S h e n g ' s f l u c t u a t i o n - i n d u c e d t u n n e l l i n g model ( F I T model) [5, 6], since all s a m p l e s h a d b e e n oxidized to t h e ' m e t a l l i c ' regime. B o t h s a m p l e s oxidized in t h e p r e s e n c e of a n additive could w i t h good a g r e e m e n t be fitted to the F I T model, w i t h p a r a b o l i c b a r r i e r s b e t w e e n the m e t a l l i c s t r a n d s for t e m p e r a t u r e s r a n g i n g from 20 K to 150- 160 K (Figs. 3, 4). In this case the c o n d u c t i v i t y a is g i v e n by a = ao exp[ - T 1 / ( T + To)]

(1)

T h e v a l u e s of To a n d T1 are listed in T a b l e 2. A is the a c t i v a t i o n e n e r g y a t h i g h t e m p e r a t u r e s , w h e r e the d o m i n a n t c o n d u c t i o n m e c h a n i s m is t h e r m a l a c t i v a tion o v e r t h e b a r r i e r s . S a m p l e A (no additive) also shows f e a t u r e s in accord a n c e w i t h the F I T model: t h e c o n d u c t i v i t y h a s a m a x i m u m at 220 K, w h e r e the i n t r i n s i c r e s i s t a n c e of the fibrils RI of t h e s a m e m a g n i t u d e as Rj, the r e s i s t a n c e due to the j u n c t i o n s b e t w e e n the fibrils. T h a t is followed by a n a c t i v a t e d region, and at low t e m p e r a t u r e s the c o n d u c t i v i t y r e a c h e s a c o n s t a n t value. This limit is g i v e n by the elastic t u n n e l l i n g m e c h a n i s m . 1 n (I]-/~rt)

O. 0

~o c %o o

0

~:°o °

0 0

o

°

.

o

0

0

O O

o

0

-0.5

0 O

-i.

O~

o

,

1

J

2

,

I

3

,

I

4

,

I

5

,

6

IO0/T [i/K]

Fig. 2. Normalized conductivity vs. 1/T for O, sample A; *, sample B; and E], sample C (see Table 2).

323

1n (g/E~t) 0.0

200

I00

50

25

I

I

T [K]

%

\

-C. 5

-1.0 0

\

,

,

I

1

J

100/(T+T o )

2 El/(K+25) ]

Fig. 3. Normalized conductivity vs. 1/(T + 25) for sample B (see text).

ln(G/IZ~t) 0.0

200

loo

5o

25

i

i

i

i

T [K]

°o a 13 o o

o. 5F

0

0.5

1 1.5 100/(T+T0) E1/(K+50)] Fig. 4. Normalized conductivity vs. 1](T + 50) for sample C (see text).

Discussion C o m p a r i n g the normalized values of the r e s i s t a n c e a r o u n d 220 K, one finds t h a t R~ is smaller for the samples oxidized with additives, i n d i c a t i n g a 'better' doping of the fibrils. On the o t h e r hand, R~ is slightly g r e a t e r for those samples. In the FIT model the p a r a m e t e r ,~ is essential in the description of the b a r r i e r b e t w e e n the strands. Comparing the m e a s u r e d samples, one finds t h a t the sample oxidized w i t h o u t additive has the largest ~ value, i.e., a more r o u n d e d barrier, because the c o n d u c t i v i t y in this case goes faster towards the a s y m p t o t i c limit. The lower ~ for samples B and C (no additive) is r e l a t e d to

324

changes in the junction, since ~ 1/wKVo

(2)

where V0 is the height of the corresponding rectangular barrier, K the dielectric constant of the insulating barrier and w the separation between the plates of the parallel-plate capacitor that is used in the approximation of the tunnel junction. The reasons for lower ~ and higher junction resistance R~ could be a different form (smaller area, greater volume) of the junction or a dielectric material in the junction. Since both the additives silver* and bithiophene are able to form precipitates, the assumption of very thin films of, for example, polythiophene in the junction gaps is plausible. Indeed, chemical analyses of samples oxidized to 20% in the presence of bithiophene show a sulphur content of ~2% although no polymer film was visible (colour and microscope). However, the increase in Rj points towards insulating layers.

Conclusions The electrochemical oxidation of polyacetylene can be improved by additives. This is probably due to prevention of overoxidation. The additive is oxidized instead of polyacetylene or by those parts of the polyacetylene film that are reaching too high oxidation potentials, thus avoiding the negative effects of locally accumulated charge. The additive equalizes the doping level throughout the sample, leading to undisturbed metallic strands (fibrils). Additives that are not prone to polymerization or precipitation should work even better than bithiophene.

Acknowledgements Support from the National Swedish Board for Technical Development (STU) is gratefully acknowledged. We thank Professor Besenhard for suggesting ferrocenes as additives.

References 1 G. Ahlgren, B. Krische, A. Pron and M. Zagorska, J. Polym. Sci. Polym. Ed., 22(1984) 173. 2 H. Kuroda, I. Ikemoto, K. Asakura, H. Ishii, H. Shirakawa, T. Kobayashi, H. Oyanagi and T. Matsushita, Solid State Commun., 46 (1983) 235. 3 B. Krische, Synth. Met., 17(1987) 551. 4 M. Kobayashi, R. Shishikura, H. Konuma, Jpn. Kokai Tokkyo Koho, JP 61 24,174 (86 24,174)and

JP6124,175. 5 P. Sheng, Phys. Rev. B, 21 (1980) 2180. 6 M. Audenaert, Phys. Rev. B, 30 (1984) 4609. 7 T. C. Clarke, R. H. Geiss, J. F. Kwak and G. B. Street, J. Chem. Soc., Chem. Commun., (1978) 489. *Silver ions were added in situ by switching the reference electrode to anode for some seconds. Metallic silver precipitated on the undoped polyacetylene film [7]. During oxidation the silver layer vanished from the doped regions of the film and migrated to the undoped ones, leaving at the end a virtually silver free (colour, microscope) film.