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Electrochemistry of polyaniline in ambient-temperature molten salts
Synthetic Metals, 44 (1991) 3 0 7 - 3 1 9
307
Electrochemistry of polyaniline in ambient-temperature molten salts Jinsong Tang and Robert A. Osteryoung* Department of Chemistry, State University of N e w York at Buffalo, Buffalo, N Y 14214 (USA) (Received April 18, 1991; accepted May 10, 1991)
Abstract Polyaniline (PAn) films o b t a i n e d f r o m acidic a q u e o u s solution can b e reversibly t r a n s f e r r e d b e t w e e n a q u e o u s solution a n d an a m b i e n t - t e m p e r a t u r e m o l t e n salt c o n s i s t i n g of m i x t u r e s o f a l u m i n u m c h l o r i d e with 1-methyl-3-ethyl-imidazolium chloride. The films s h o w elect r o c h e m i c a l activity in basic, n e u t r a l a n d acidic melts. The d o p i n g p r o c e s s e s a n d det e r i o r a t i o n m e c h a n i s m are d i s c u s s e d for t h e s e melts. The r e d o x r e a c t i o n o f PAn in basic m e l t is m o r e stable a n d facile t h a n in t h e o t h e r melts. The o x i d i z e d film is c o n d u c t i n g a n d is insulating on r e d u c t i o n .
Introduction
Polyaniline (PAn) has been of interest in recent years because of its chemical and electrochemical stability, its novel doping m echani sm and the potential for technical application as a new electronic material, especially for secondary batteries [1-3]. A n u m b e r of papers have been published on the electrochemical behavior of PAn in n o n a q u e o u s media; m o r e stable properties than in aqueous solution are obtained and some intriguing results have been r e p o r t e d [4-7]. Ambient-temperature molten salts consisting of mixtures of aluminum chloride with 1-methyl-3-ethyl-imidazolium chloride (ImCl) or N-(1-butyl)pyridinium chloride (BuPyCI) are viewed as promising solvent electrolytes for batteries. We have recently shown [8-11 ] that a n u m b e r of electroactive p o ly mer films can be oxidized and r e d u c e d rapidly in these molten salts, and some polymers have shown reversible and stable electrochemical behavior. Thus, we wer e interested in using these melts as electrolytes for PAn electrochemistry. Here we r e p o r t on the electrochemistry of PAn in the A1C13-ImCl system. If the organic chloride is in excess, the melt is considered basic; only Cl and AIC14- anions are present. An acidic melt contains an excess of A1C13, and only A1CI4- and A12C17- anions are present. Neutral melts have an exact, equimolar ratio of the two com pone nt s , ImCl and AICl3, and consist of the *Author to w h o m c o r r e s p o n d e n c e should be addressed.
0379-6779/91/$3.50
© 1991 -- Elsevier Sequoia, Lausanne
308 i m i d a z o l i u m cation, I m ÷, a n d AICI4- [ 1 2 - 1 4 ] . T h e d o m i n a n t equilibrium in this s y s t e m 2AiC14- =Al2C17- + C I h a s also b e e n i n v e s t i g a t e d a n d the e q u i l i b r i u m c o n s t a n t f o u n d to be a b o u t 10-17. The o r g a n i c cation d o e s n o t p l a y a significant role in the a c i d - b a s e c h e m i s t r y o f t h e s e melts.
Experimental T h e p r e p a r a t i o n a n d purification o f 1 - m e t h y l - 3 - e t h y l - i m i d a z o l i u m chloride h a s b e e n d e s c r i b e d [12]. A v a c u u m - s e a l e d b o m b t u b e w a s u s e d to s u b l i m e A1C13. Melts of the d e s i r e d c o m p o s i t i o n w e r e p r e p a r e d b y the slow m i x i n g o f w e i g h e d a m o u n t s o f t h e t w o salts. All e x p e r i m e n t a l w o r k o n the m o l t e n salts w a s c a r r i e d out in a V a c u u m A t m o s p h e r e C o r p o r a t i o n d r y b o x u n d e r purified a r g o n or helium. An EG & G PARC Model 175 p r o g r a m m e r , in c o n j u n c t i o n with a 173 p o t e n t i o s t a t , w a s u s e d to p e r f o r m e l e c t r o c h e m i c a l e x p e r i m e n t s . Voltamm o g r a m s w e r e p l o t t e d o n a H o u s t o n I n s t r u m e n t s Model 2 0 0 0 X - Y r e c o r d e r . P l a t i n u m ( r a d i u s 0.08 c m ) or g l a s s y c a r b o n (GC) (radius. 0.15 c m ) disks o b t a i n e d f r o m Bioanalytical S y s t e m s w e r e u s e d as w o r k i n g e l e c t r o d e s a n d w e r e p o l i s h e d o n 12-284 p o l i s h i n g p a p e r ( F i s h e r ) with a l u m i n a / w a t e r m i x t u r e s a n d w a s h e d in an u l t r a s o n i c bath. An A1 w i r e i m m e r s e d in 1.5:1 A1C13:ImC1 s e r v e d as a r e f e r e n c e e l e c t r o d e in the m e l t s a n d a n SCE w a s u s e d in a q u e o u s solution. T h e c o u n t e r e l e c t r o d e w a s a coiled wire o f Pt o r gold. Polyaniline w a s s y n t h e s i z e d f r o m a n a q u e o u s solution of b o t h 1 M HC1 a n d NaC1, c o n t a i n i n g 0.1 M aniline, b y s w e e p i n g t h e p o t e n t i a l b e t w e e n - 0 . 2 a n d 0 . 7 5 V to o b t a i n a film of t h e d e s i r e d t h i c k n e s s [15, 16]. T h e t h i c k n e s s o f PAn film w a s c a l c u l a t e d b a s e d o n l i t e r a t u r e d a t a [17]. N o r m a l l y the t h i c k n e s s o f PAn w a s a r o u n d 40 nm. A f t e r p o l y m e r i z a t i o n t h e P A n - c o a t e d e l e c t r o d e in aniline-free a q u e o u s m e d i u m w a s s u b j e c t e d t o a p o t e n t i a l s w e e p b e t w e e n 0.2 a n d 0 . 8 5 V for o n e cycle to o b t a i n t h e a n o d i c c h a r g e o f the p o l y m e r film (Q~), t h e n p l a c e d in a 2 8 . 0 0 % a m m o n i u m h y d r o x i d e solution f o r 12 h, carefully w a s h e d w i t h a c e t o n e a n d a c e t o n i t r i l e r e s p e c t i v e l y , dried at 60 °C for s e v e r a l h o u r s , k e p t u n d e r v a c u u m f o r 12 h a n d t r a n s f e r r e d t o t h e dry b o x . Following this p r o c e d u r e , PAn is r e g a r d e d as an i n s u l a t o r a n d t h e r e s h o u l d b e n o f r e e p r o t o n s in t h e film [ 1 - 3 ] . F e r r o c e n e (Aldrich) a n d t i t a n i u m c h l o r i d e (Aldrich) w e r e u s e d a s r e c e i v e d . Aniline ( K o d a k ) w a s purified p r i o r to u s e b y distilling u n d e r v a c u u m .
R e s u l t s and d i s c u s s i o n
General description F i g u r e 1 s h o w s t y p i c a l cyclic v o l t a m m o g r a m s of PAn films following t r a n s f e r to basic, n e u t r a l a n d acidic melts, r e s p e c t i v e l y . In b a s i c melts, t w o
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Fig. 1. T y p i c a l cyclic v o l t a m m o g r a m s of P A n films o n t h e GC e l e c t r o d e in different melts: ( ), in 0.9:1 m e l t a f t e r 3 0 cycles; ( - - - ) , in 1.0:1.0 m e l t after 40 c y c l e s b e t w e e n - 0 . 2 a n d 1.2 V; ( ..... ), in 2:1 m e l t for first cycle; s c a n rate is 5 0 m V / s .
r e d o x couples can be seen within the electrochemical window; at least three couples are seen in the neutral melt and only one oxidation peak is found in the strongly acidic melt. In both basic and acidic melts, the peak potentials shift to more positive potentials with increasing melt acidity. Moreover, in a slightly acidic 1.05:1 melt, the same num ber of peaks are seen as in the neutral melt, but the peak potentials are more positive than in the neutral melt and the relative intensity of the cyclic voltammetric peaks is changed. For example, the most intense oxidation peak in a 1.05:1 melt is located in the vicinity of the potential shown by the most intense peak in the very strongly acidic melt (2:1), and the peak at the most negative potential in the neutral melt b e c o m e s a shoulder. R a t i o s of anodic charge of PAn films u n d e r cyclic v o l t a m m o g r a m s obtained from different melts (Qm) to that in acidic aqueous solution (Qa) against melt composition are shown in Fig. 2. T h o u g h the potential range over which Qm is obtained is not the same, due to the differences of the electrochemical window and the oxidation potentials f o r PAn in the different melts, the electroactive charge of PAn decreases with increasing melt acidity. Oxidation of PAn requires that anions enter the film to neutralize the positive charge on the PAn chain; reduction requires that anions depart. These ion movements, and hence oxidation and reduction of the film, are facilitated by incorporation of the melt into the polymer, or solvent swelling [18, 19]. Chloride ions are the smallest ion in the basic melt c o m p a r e d with A1C14- in neutral and A12C17- in the acidic melts, and might be e x p e c t e d to have the largest mobility within the pol ymer [8, 20]. The influence of the nature and size of anions on the electrochemical properties of PAn in bot h a q u e o u s and n o n a q u e o u s solutions has been studied by several authors [ 2 1 - 2 4 ] . Considering that the viscosity of the melt is m u c h higher than that
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Fig. 2. Ratios o f the anodic charge of PAn under cyclic voltammograms obtained from different melts (Qm) to that from acidic aqueous solution (Q~) against melt composition. The maximum charge in melts is taken from the potential ranges as shown in Fig. 1; the charge in aqueous solution is obtained in the range of - 0 . 2 to 0.85 V vs. SCE; scan rate is 50 mV/s; ([2]), GC; ((D), et.
of a q u eo u s solutions, ion m o v e m e n t and solvent swelling may play an important role in the r e d o x reaction.
Doping process The PAn electrode, prior to placement in the melt, is an insulator. When this electrode is placed in the acidic aqueous solution following film formation and t r e a t m e n t with ammonia, the highest peak current and charge were obtained on the first potential scan due to p r o t o n doping [1, 2]. When the PAn electrode in its insulating state was transferred into the basic melt, the peak current and charge gradually increased with continual scanning until, after about 30 potential cycles, a m a xi m um in both peak current and charge could be observed as shown in Fig. 3. When the oxidation potential was restricted to 0.55 V, instead of 0.9 V as shown in Fig. 3, the highest charge value was obtained after about 1000 cycles. Figure 4 indicates the ratio of Qm/Qa with films of different thickness against the n u m b e r of cycles. Thinner films r e c o v e r e d faster; a large charge could be found following fewer cycles. The nonlinear relationship between the film thickness and recovery m ay be due to morphological changes with films of different thickness [15, 24]. The changes o f melt composition in the basic range did not introduce an obvious effect on the behavior described above and similar behavior could also be seen in the neutral melt.
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E vs. AI in 1.5:1 melt,V Fig. 3. Cyclic v o l t a m m o g r a m s of PAn on the Pt electrode in 0.9:1 melt following various numbers of cycles. Scan rate is 50 mV/s.
Thus, the e l e c t r o a c t i v i t y of PAn in b a s i c a n d neutral m e l t s i m p r o v e d gradually, p e r h a p s c o r r e s p o n d i n g to a n i o n s p e r m e a t i n g t h e w h o l e film; in o t h e r w o r d s , t h e kinetic limitation m a y b e d u e to s o l v e n t swelling of the film. On t h e o t h e r h a n d , in acidic m e l t s in t h e r a n g e of 1.05:1 to 2:1 b o t h c u r r e n t a n d c h a r g e are g r e a t e s t o n the first s c a n as s h o w n in Fig. 5, indicating t h a t t h e d o p i n g p r o c e s s in a n acidic m e l t differs f r o m t h a t in b a s i c a n d n e u t r a l m e l t s a n d is similar to t h a t in acidic a q u e o u s solution. In acidic a q u e o u s solution, p r o t o n s r e a c t w i t h t h e i m i n e on the PAn c h a i n to f o r m t h e imine salt a n d then, b y i n t r a m o l e c u l a r r e d o x r e a c t i o n , r a d i c a l s (or p o l a r o n s ) a r e i n t r o d u c e d a n d PAn b e c o m e s a c o n d u c t o r [2, 25]. In an acidic melt, AICI4- a n d AI2C17- a r e p r e s e n t , t h e i r m o l a r r a t i o d e p e n d i n g on the m e l t c o m p o s i t i o n [14]; p r o t o n i m p u r i t i e s a r e a l s o p r e s e n t at a b o u t 10 -4 M. W h e n t h e c o n d u c t i n g PAn d o p e d b y HCI in a q u e o u s solution w a s p l a c e d into t h e acidic melt, t h e c u r r e n t a n d t h e c h a r g e i n c r e a s e d with r e p e t i t i v e s c a n n i n g until, a f t e r a f e w p o t e n t i a l cycles, a m a x i m u m w a s o b t a i n e d . Thus, w e e x c l u d e t h e p o s s i b i l i t y o f a p r o t o n d o p i n g m e c h a n i s m in t h e acidic melt. In principle, A1Cl~ f r o m A12C1T- in t h e acidic m e l t c o u l d p l a y a similar role to t h a t o f
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Cyclic Number Fig. 4. Ratios o f t h e a n o d i c c h a r g e o f different t h i c k n e s s e s o f PAn films on t h e GC e l e c t r o d e u n d e r cyclic v o l t a r a m o g r a m s in a 0.8:1 melt (Qm) to that by a q u e o u s solution (Q~) against t h e n u m b e r of p o t e n t i a l cycles. Other c o n d i t i o n s are the s a m e as Fig. 2; (A), 10 nm; ([:]), 40 nm; (O), 2 4 0 nm.
p r o t o n s in a q u e o u s solution doping processes. Just as the prot ons in an aqueous solution can p r o t o n a t e the base (N) site and cause the oxidized, but initially n o n c o n d u c t i v e form of PAn to b e c o m e conductive, so also might AlC13, a strong Lewis acid, react with a base site in the acidic melt. We have shown that AlCl3 adducts of pyrrole are formed in acidic melts, and that the oxidation potential of the adduct is shifted positive, in the case of pyrrole, out of the melt window [8]. So too might the observed positive shift in the PAn peak in the acidic melt (Fig. 1) result from an A1Cla adduct being formed with the PAn film.
Stability and deterioration In acidic melts, the charge under the cyclic vol t am m ogram decreased with r e p e a t e d cycling as shown in Fig. 5. In basic and neutral melts, after the m a x i m u m charge was obtained by cycling, a similar behavior could also be seen. But the deterioration p r o c e s s e s differ. In acidic melts, only the peak c u r r en t decreased; in the basic melt the potentials of the two couples seen in Fig. 3 shift and finally one peak, its formal peak potential [(Epi+Ep~)/21 about 0.45 V, could be observed. Also, the rates of deterioration were quite different as indicated in Fig. 6. PAn is more stable in basic than in acidic melt. On changing the positive potential limit from 0.9 to 0.55 V in a basic melt just b e y o n d the first peak, the PAn retained m o r e than 90% of its activity after 3 × 104 repetitive cycles at 100 mV/s. Com pared with results
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obtained in aqueous and organic solvents [4, 24, 2 6 - 2 8 ] , PAn is very stable in basic melts and possibly useful in a s econdary battery system. W h en PAn electrodes were cycled in the 0.8:1 melt until the m axi m um charge was obtained, then transferred into the acidic aqueous solution, the cyclic voltammogram was the same as that when initially polymerized. Thus, transfer of PAn between aqueous solution and melts is reversible. Once the anodic charge d e c r e a s e d to half of the m axi m um value (see Fig. 6) scanning was stopped at the m os t negative potential employed and the PAn electrode was transferred to an acidic aqueous solution. Figure 7 shows the ratio of anodic charge under the cyclic v o l t a m m o g r a m given by the deteriorated film (Qd) to that of the polymerized film (Q~) against melt composition in which the PAn had be e n cycled. In basic melts, the charge seems to yield the same value as w he n polymerized or it decreases slightly. However, film deterioration when PAn was cycled in acidic melts, and examined in aq u eo u s solution, was large. Figure 8 shows cyclic vol t am m ogram s of deteriorated films obtained from different melts and placed in 1 M HC1 + 1 M NaC1 medium. After having b e e n cycled in basic melts, the vol t am m ogram s
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Fig. 8. C y c l i c v o l t a m m o g r a m s of d e t e r i o r a t e d f i l m s o b t a i n e d f r o m 0.9:1 ( - • - ) a n d 1.1:1 ( m e l t s in 1 M HC1 + 1 M NaCl; ( ..... ) s h o w s t h a t of PA n a s p o l y m e r i z e d ; o t h e r c o n d i t i o n s a r e t h e s a m e as Fig. 7. Fig. 9. C y c l i c v o l t a m m o g r a m s of 8 X 10 -3 M f e r r o c e n e o n a b a r e Pt e l e c t r o d e ( ..... ) a n d on a P t e l e c t r o d e c o a t e d by a 2 4 0 n m t h i c k PAn film ( - • - ) i n 0.8:1 m e l t ; ( ) represents that o f PAn itself; s c a n r a t e is 5 0 mV/s.
w h i c h h a v e d e t e r i o r a t e d a r e v e r y similar to t h a t s h o w n b y PAn in a q u e o u s solution. T h e a n o d i c p e a k at 0 . 5 5 V v e r s u s SCE m a y p o s s i b l y r e s u l t f r o m h y d r o l y s i s [26], o r t h o - s u b s t i t u t i o n o f the b e n z e n e ring [21] o r a c r o s s - l i n k i n g r e a c t i o n [28]. T h e f o r m e r t w o p r o c e s s e s a r e unlikely in t h e melts, b u t t h e l a t t e r r e a c t i o n m a y t a k e place. B e c a u s e the c r o s s - l i n k i n g s t r u c t u r e m a k e s t h e film m o r e c o m p a c t a n d r e s t r i c t s ion mobility, m e l t m i g h t m o v e o u t o f t h e film a n d t h e e l e c t r o a c t i v i t y s h o u l d d e c r e a s e . A c r o s s - l i n k i n g r e a c t i o n w o u l d n o t i n t r o d u c e a loss o f aniline units f r o m t h e PAn c h a i n a n d t h e e l e c t r o a c t i v i t y o f PAn c y c l e d in t h e b a s i c m e l t w o u l d r e c o v e r to t h e initial value, as w h e n p o l y m e r i z e d . After s w e e p i n g in a n acidic m e l t , t h e a q u e o u s cyclic v o l t a m m o g r a m o f PAn a p p e a r s to b e t h e s a m e a s
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Fig. 10. Plot o f ip for a 10 n m thick PAn film on a Pt e l e c t r o d e in 0.8:1 m e l t vs. s c a n rate: ([~), first o x i d a t i o n peak, ilpo; (HI), first r e d u c t i o n peak, ilpr; (@), i2po; (©), i2p~.
t h a t o f p o l y m e r i z e d PAn, e x c e p t f o r the p e a k c u r r e n t values. This i n d i c a t e s t h a t r e p e t i t i v e p o t e n t i a l cycling in the acidic m e l t d o e s n o t r e s u l t in a c h a n g e o f c h a i n s t r u c t u r e o f PAn in t h e acidic melt. T h e l o s s o f e l e c t r o a c t i v i t y m a y b e d u e to t h e solubility o f PAn in the m e l t s w h i c h s h o w s t r o n g acidity a n d polarity. In fact, PAn is s o l u b l e in c o n c e n t r a t e d H2SO4 a n d s o m e s t r o n g p o l a r o r g a n i c s o l v e n t s [2]. J u s t o n e p o t e n t i a l cycle w a s p e r f o r m e d on PAn in a n acidic melt, a b o u t 2 0 % o f t h e c h a r g e w a s lost w h e n it w a s t r a n s f e r r e d to t h e a q u e o u s solution•
Conductivity and electrochemistry A f t e r r e p e a t e d p o t e n t i a l cycling PAn w a s relatively s t a b l e in the b a s i c melt; w e u s e this p r o p e r t y to c a r r y o u t f u r t h e r e x p e r i m e n t s on its electrochemistry. F i g u r e 9 s h o w s t h e cyclic v o l t a m m o g r a m s of 8 × 10 - a M f e r r o c e n e o n a b a r e Pt e l e c t r o d e a n d o n Pt c o a t e d b y a 2 4 0 n m t h i c k PAn film in a 0.8:1 melt. T h e r e d o x p e a k o f f e r r o c e n e a p p e a r s s u p e r i m p o s e d o n t h e b a c k g r o u n d e l e c t r o a c t i v i t y of t h e PAn film; f e r r o c e n e h a s t h e s a m e p o t e n t i a l as on t h e b a r e e l e c t r o d e . On t h e o t h e r h a n d , t h e r e d o x p e a k o f TiCl4, the f o r m a l p o t e n t i a l o f w h i c h is a b o u t - 0.1 V u n d e r the s a m e e x p e r i m e n t a l c o n d i t i o n s a t t h e GC e l e c t r o d e did n o t a p p e a r o n t h e cyclic v o l t a m m o g r a m s of the GC e l e c t r o d e c o v e r e d b y a 2 4 0 n m t h i c k PAn film. If a 10 n m t h i c k film w a s u s e d , t h e r e d o x p e a k o f TIC14 c o u l d b e f o u n d w i t h f o r m a l p o t e n t i a l - 0 . 1 7 V. In this c a s e , t h e PAn film is v e r y thin a n d t h e TIC14 c a n diffuse t h r o u g h
317 TABLE 1 Cyclic voltammetric data for PAn films Substrate
Film thickness (rim)
Scan rate (mV/s)
E~pa (mV)
E~pc (mV)
~Elp (mV)
E2pa (mV)
E2pc (mV)
AE2p (mV)
Pt
lO
2 5 10 20 50 lOO 200
370 370 375 370 390 400 415
310 310 305 300 290 285 280
60 60 70 70 100 115 135
740 745 745 750 765 775 785
650 655 655 640 645 640
90 90 90 100 120 135
Pt
50
5 10 20 50 100 200 500
380 385 395 400 415 425 430
310 310 300 295 290 285 280
70 75 95 105 125 140 150
750 755 765 770 780 790 815
655 655 645 635 625
95 100 120 135 155
GC
50
5 10 20 50 100 200 500
375 390 400 420 430 440 470
295 290 280 270 260 250 240
80 100 120 150 170 190 230
750 755 765 775 790 820 850
630 620 610 605
120 135 155 170
the p o l y m e r a n d r e a c t on the GC s u b s t r a t e . T h e s e o b s e r v a t i o n s i n d i c a t e that P A n is c o n d u c t i n g i n t h e o x i d i z e d s t a t e in b a s i c m e l t a n d is i n s u l a t i n g i n the r e d u c e d state. T h e l i n e a r d e p e n d e n c e o f ip o n s c a n r a t e i n t h e r a n g e o f 2 t o 5 0 m V / s is s h o w n i n Fig. 1 0 f o r t h e l 0 n m t h i c k P A n film i n a b a s i c m e l t . T h i s b e h a v i o r c a n also be s e e n at film t h i c k n e s s e s up to 100 nm. The a m o u n t o f a n o d i c c h a r g e u n d e r t h e c y c l i c v o l t a m m o g r a m is t h e s a m e a s t h e c a t h o d i c c h a r g e , w i t h i n e x p e r i m e n t a l e r r o r , i n d i c a t i n g t h a t t h e r e d o x r e a c t i o n is f a c i l e u n d e r t h e s e conditions. Table 1 lists other cyclic v o l t a m m e t r i c data for Pt a n d GC e l e c t r o d e s c o v e r e d b y d i f f e r e n t t h i c k n e s s e s o f P A n i n a 0 . 8 : 1 m e l t . T h e p e a k p o s i t i o n s c a n differ b y a b o u t 1 0 % b e t w e e n e x p e r i m e n t s f o r t h e s a m e P A n film w h i c h is s i m i l a r t o t h o s e o b s e r v e d f o r p o l y p y r r o l e a n d p o l y t h i o p h e n e f i l m s i n t h e m e l t [8, 10]. F r o m T a b l e 1, first w e f i n d t h a t t h e 10 n m t h i c k film a p p e a r s t o s h o w n o k i n e t i c l i m i t a t i o n u p t o 2 0 m V / s s i n c e AEp is e f f e c t i v e l y c o n s t a n t . F o r t h i c k e r films, i n c r e a s i n g AEp v a l u e s w i t h i n c r e a s i n g s c a n r a t e a r e o b s e r v e d . S e c o n d l y , t h e first a n o d i c w a v e a p p e a r s more susceptible to kinetic limitations than the cathodic wave. As the scan r a t e o r f i l m t h i c k n e s s is i n c r e a s e d , a n d AEp i n c r e a s e d , E~pa c h a n g e s m o r e t h a n E~p¢, i n d i c a t i n g t h a t it is m o r e difficult t o c o n v e r t t h e p o l y m e r f r o m
318 insulating to conductive. This observation can correspond to a kinetic problem in t h e r e d o x r e a c t i o n o f P A n , t o i o n t r a n s p o r t i n t o t h e film, o r t o d i f f e r e n t rates of swelling and de-swelling. Thirdly, the electrochemical behavior of t h e P A n f i l m is m o r e r e v e r s i b l e o n p l a t i n u m t h a n o n g l a s s y c a r b o n , a s w e s e e t h a t AEp a t P t is s m a l l e r t h a n t h a t a t GC.
Conclusions P o l y a n i l i n e f i l m s p r e p a r e d in a q u e o u s a c i d i c s o l u t i o n s a r e e l e c t r o a c t i v e in b a s i c , n e u t r a l a n d a c i d i c m e l t s . T h e f i l m s a r e c o n d u c t i n g w h e n o x i d i z e d a n d a r e i n s u l a t i n g o n c e r e d u c e d . T h e r e d o x r e a c t i o n o f P A n in t h e b a s i c melt appears to be facile.
Acknowledgement This work was supported Research.
in p a r t b y t h e A i r F o r c e O f f i c e o f S c i e n t i f i c
References 1 A. G. MacDiarmid, J. C. Chiang, M. Halpen, W. S. Huang, S. L. Mu, N. L. D. Somasiri, W. Wu and S. I. Yaniger, Mol. Cryst. Liq. Cryst., 121 (1985) 173. 2 E. M. Genids, A. Boyle, M. Lapkowski and C. Tsintavis, Synth. Met., 36 (1990) 139. 3 F. Wudl, R. 0. Angus, F. L. Lu, P. M. Allemand, D. J. Vachon, M. Nowak, Z. X. Liu and A. J. Heeger, J. A m . Chem. Soc., 109 (1987) 3677. 4 A. Kabumoto, K. Shinozaki, K. Watanabe and N. Nishikawa, Synth. Met., 26 (1988) 349. 5 A. F. Diaz and J. A. Logan, J. Electroanal. Chem., 111 (1980) 111. 6 N. Mermilliod, J. Tanguy, M. Hoclet and A. A. Syed, Synth. Met., 18 (1987) 359. 7 T. Ohsawa, T. Kabata, O. Kimura and Y. Yoshino, S y n t h . Met., 29 (1989) E203. 8 P. G. Pickup and R. A. Osteryoung, J. A m . Chem. Soc., 106 (1984) 2294. 9 P. G. Pickup and R. A. Osteryoung, J. E l e c t r o a n a l . Chem., 195 (1985) 271. 10 L. Janiszewska and R. A. Osteryoung, J. E l e c t r o c h e m . Soc., 134 (1987) 2787. 11 L. Janiszewska and R. A. Osteryoung, J. Electrochem. Soc., 135 (1988) 116. 12 J. Robinson and R. A. Osteryoung, J. A m . Chem. Soc., 101 (1979) 323. 13 A. A. Fannin, L. A. King, J. A. Levisky and J. S. Wilkes, J. Phys. Chem., 88 (1984) 2609. 14 Z. J. Karpinski and R. A. Osteryoung, Inorg. Chem., 23 (1984) 1491. 15 J. C. LaCroix and A. F. Diaz, J. Electrochem. Soc., 135 (1988) 1457. 16 M. ~apkowski, S y n t h . Met., 35 (1990) 169. 17 D. E. Stilwell and S. M. Park, J. Electrochem. Soc., 135 (1988) 2491. 18 P. Daum and R. W. Murray, J. Phys. Chem., 85 (1981) 389. 19 F. B. Kaufman, A. H. Schroeder, E. M. Engle, S. R. Kramer and J. Q. Chambers, J. A m . Chem. Soc., 102 (1980) 483. 20 T. Ikada, R. Schmehl, P. Dnisevich, K. Willman and R. W. Murray, J. A m . Chem. Soc., 104 (1982) 2683. 21 E. M. Geni~s and C. Tsintavis, J. Electroanal. Chem., 195 (1985) 109. 22 G. Zotti, S. Cattarin and N. Comisso, J. E l e c t r o a n a L Chem., 239 (1988) 387. 23 W. W. Focke, G. E. Wnek and Y. Wei, J. Phys. Chem., 91 (1987) 5813.
319 24 B. C. Wang, J. S. Tang and F. S. Wang, S y n t h . Met., 18 (1987) 323. 25 J. M. Ginder, A. J. Epstein and A. G. MacDiarmid, S y n t h . Met., 29 (1989) E395. 26 T. Kobayashi, H. Yoneyama and H. Tamura, J. E l e c t r o a n a l . Chem., 161 (1984) 419; 177 (1984) 281 and 293. 27 D. E. Stilwell and S. M. Park, J. E l e c t r o c h e m . Soc., 135 (1988) 2497. 28 E. M. Geni~s, M. Lapkowski and J. F. Penneau, J. E l e c t r o a n a l . Chem., 249 (1988) 97.
307
Electrochemistry of polyaniline in ambient-temperature molten salts Jinsong Tang and Robert A. Osteryoung* Department of Chemistry, State University of N e w York at Buffalo, Buffalo, N Y 14214 (USA) (Received April 18, 1991; accepted May 10, 1991)
Abstract Polyaniline (PAn) films o b t a i n e d f r o m acidic a q u e o u s solution can b e reversibly t r a n s f e r r e d b e t w e e n a q u e o u s solution a n d an a m b i e n t - t e m p e r a t u r e m o l t e n salt c o n s i s t i n g of m i x t u r e s o f a l u m i n u m c h l o r i d e with 1-methyl-3-ethyl-imidazolium chloride. The films s h o w elect r o c h e m i c a l activity in basic, n e u t r a l a n d acidic melts. The d o p i n g p r o c e s s e s a n d det e r i o r a t i o n m e c h a n i s m are d i s c u s s e d for t h e s e melts. The r e d o x r e a c t i o n o f PAn in basic m e l t is m o r e stable a n d facile t h a n in t h e o t h e r melts. The o x i d i z e d film is c o n d u c t i n g a n d is insulating on r e d u c t i o n .
Introduction
Polyaniline (PAn) has been of interest in recent years because of its chemical and electrochemical stability, its novel doping m echani sm and the potential for technical application as a new electronic material, especially for secondary batteries [1-3]. A n u m b e r of papers have been published on the electrochemical behavior of PAn in n o n a q u e o u s media; m o r e stable properties than in aqueous solution are obtained and some intriguing results have been r e p o r t e d [4-7]. Ambient-temperature molten salts consisting of mixtures of aluminum chloride with 1-methyl-3-ethyl-imidazolium chloride (ImCl) or N-(1-butyl)pyridinium chloride (BuPyCI) are viewed as promising solvent electrolytes for batteries. We have recently shown [8-11 ] that a n u m b e r of electroactive p o ly mer films can be oxidized and r e d u c e d rapidly in these molten salts, and some polymers have shown reversible and stable electrochemical behavior. Thus, we wer e interested in using these melts as electrolytes for PAn electrochemistry. Here we r e p o r t on the electrochemistry of PAn in the A1C13-ImCl system. If the organic chloride is in excess, the melt is considered basic; only Cl and AIC14- anions are present. An acidic melt contains an excess of A1C13, and only A1CI4- and A12C17- anions are present. Neutral melts have an exact, equimolar ratio of the two com pone nt s , ImCl and AICl3, and consist of the *Author to w h o m c o r r e s p o n d e n c e should be addressed.
0379-6779/91/$3.50
© 1991 -- Elsevier Sequoia, Lausanne
308 i m i d a z o l i u m cation, I m ÷, a n d AICI4- [ 1 2 - 1 4 ] . T h e d o m i n a n t equilibrium in this s y s t e m 2AiC14- =Al2C17- + C I h a s also b e e n i n v e s t i g a t e d a n d the e q u i l i b r i u m c o n s t a n t f o u n d to be a b o u t 10-17. The o r g a n i c cation d o e s n o t p l a y a significant role in the a c i d - b a s e c h e m i s t r y o f t h e s e melts.
Experimental T h e p r e p a r a t i o n a n d purification o f 1 - m e t h y l - 3 - e t h y l - i m i d a z o l i u m chloride h a s b e e n d e s c r i b e d [12]. A v a c u u m - s e a l e d b o m b t u b e w a s u s e d to s u b l i m e A1C13. Melts of the d e s i r e d c o m p o s i t i o n w e r e p r e p a r e d b y the slow m i x i n g o f w e i g h e d a m o u n t s o f t h e t w o salts. All e x p e r i m e n t a l w o r k o n the m o l t e n salts w a s c a r r i e d out in a V a c u u m A t m o s p h e r e C o r p o r a t i o n d r y b o x u n d e r purified a r g o n or helium. An EG & G PARC Model 175 p r o g r a m m e r , in c o n j u n c t i o n with a 173 p o t e n t i o s t a t , w a s u s e d to p e r f o r m e l e c t r o c h e m i c a l e x p e r i m e n t s . Voltamm o g r a m s w e r e p l o t t e d o n a H o u s t o n I n s t r u m e n t s Model 2 0 0 0 X - Y r e c o r d e r . P l a t i n u m ( r a d i u s 0.08 c m ) or g l a s s y c a r b o n (GC) (radius. 0.15 c m ) disks o b t a i n e d f r o m Bioanalytical S y s t e m s w e r e u s e d as w o r k i n g e l e c t r o d e s a n d w e r e p o l i s h e d o n 12-284 p o l i s h i n g p a p e r ( F i s h e r ) with a l u m i n a / w a t e r m i x t u r e s a n d w a s h e d in an u l t r a s o n i c bath. An A1 w i r e i m m e r s e d in 1.5:1 A1C13:ImC1 s e r v e d as a r e f e r e n c e e l e c t r o d e in the m e l t s a n d a n SCE w a s u s e d in a q u e o u s solution. T h e c o u n t e r e l e c t r o d e w a s a coiled wire o f Pt o r gold. Polyaniline w a s s y n t h e s i z e d f r o m a n a q u e o u s solution of b o t h 1 M HC1 a n d NaC1, c o n t a i n i n g 0.1 M aniline, b y s w e e p i n g t h e p o t e n t i a l b e t w e e n - 0 . 2 a n d 0 . 7 5 V to o b t a i n a film of t h e d e s i r e d t h i c k n e s s [15, 16]. T h e t h i c k n e s s o f PAn film w a s c a l c u l a t e d b a s e d o n l i t e r a t u r e d a t a [17]. N o r m a l l y the t h i c k n e s s o f PAn w a s a r o u n d 40 nm. A f t e r p o l y m e r i z a t i o n t h e P A n - c o a t e d e l e c t r o d e in aniline-free a q u e o u s m e d i u m w a s s u b j e c t e d t o a p o t e n t i a l s w e e p b e t w e e n 0.2 a n d 0 . 8 5 V for o n e cycle to o b t a i n t h e a n o d i c c h a r g e o f the p o l y m e r film (Q~), t h e n p l a c e d in a 2 8 . 0 0 % a m m o n i u m h y d r o x i d e solution f o r 12 h, carefully w a s h e d w i t h a c e t o n e a n d a c e t o n i t r i l e r e s p e c t i v e l y , dried at 60 °C for s e v e r a l h o u r s , k e p t u n d e r v a c u u m f o r 12 h a n d t r a n s f e r r e d t o t h e dry b o x . Following this p r o c e d u r e , PAn is r e g a r d e d as an i n s u l a t o r a n d t h e r e s h o u l d b e n o f r e e p r o t o n s in t h e film [ 1 - 3 ] . F e r r o c e n e (Aldrich) a n d t i t a n i u m c h l o r i d e (Aldrich) w e r e u s e d a s r e c e i v e d . Aniline ( K o d a k ) w a s purified p r i o r to u s e b y distilling u n d e r v a c u u m .
R e s u l t s and d i s c u s s i o n
General description F i g u r e 1 s h o w s t y p i c a l cyclic v o l t a m m o g r a m s of PAn films following t r a n s f e r to basic, n e u t r a l a n d acidic melts, r e s p e c t i v e l y . In b a s i c melts, t w o
309
A
~
il,..
J
r
\ 1 ,~-'-~- ~:?j ..,,"
[
I
•0.2
0
I
l
0.4
I
J
0.8
I
I
I
1.2
I
1.8
E vs. AI in 1.5:1 melt,V
Fig. 1. T y p i c a l cyclic v o l t a m m o g r a m s of P A n films o n t h e GC e l e c t r o d e in different melts: ( ), in 0.9:1 m e l t a f t e r 3 0 cycles; ( - - - ) , in 1.0:1.0 m e l t after 40 c y c l e s b e t w e e n - 0 . 2 a n d 1.2 V; ( ..... ), in 2:1 m e l t for first cycle; s c a n rate is 5 0 m V / s .
r e d o x couples can be seen within the electrochemical window; at least three couples are seen in the neutral melt and only one oxidation peak is found in the strongly acidic melt. In both basic and acidic melts, the peak potentials shift to more positive potentials with increasing melt acidity. Moreover, in a slightly acidic 1.05:1 melt, the same num ber of peaks are seen as in the neutral melt, but the peak potentials are more positive than in the neutral melt and the relative intensity of the cyclic voltammetric peaks is changed. For example, the most intense oxidation peak in a 1.05:1 melt is located in the vicinity of the potential shown by the most intense peak in the very strongly acidic melt (2:1), and the peak at the most negative potential in the neutral melt b e c o m e s a shoulder. R a t i o s of anodic charge of PAn films u n d e r cyclic v o l t a m m o g r a m s obtained from different melts (Qm) to that in acidic aqueous solution (Qa) against melt composition are shown in Fig. 2. T h o u g h the potential range over which Qm is obtained is not the same, due to the differences of the electrochemical window and the oxidation potentials f o r PAn in the different melts, the electroactive charge of PAn decreases with increasing melt acidity. Oxidation of PAn requires that anions enter the film to neutralize the positive charge on the PAn chain; reduction requires that anions depart. These ion movements, and hence oxidation and reduction of the film, are facilitated by incorporation of the melt into the polymer, or solvent swelling [18, 19]. Chloride ions are the smallest ion in the basic melt c o m p a r e d with A1C14- in neutral and A12C17- in the acidic melts, and might be e x p e c t e d to have the largest mobility within the pol ymer [8, 20]. The influence of the nature and size of anions on the electrochemical properties of PAn in bot h a q u e o u s and n o n a q u e o u s solutions has been studied by several authors [ 2 1 - 2 4 ] . Considering that the viscosity of the melt is m u c h higher than that
310
1.0
0.6
ol ol 0.4
0.2
0.0
0.8
!
!
!
!
!
1.0
1.2
1.4
1.6
1.8
Melt
2.0
Composition
Fig. 2. Ratios o f the anodic charge of PAn under cyclic voltammograms obtained from different melts (Qm) to that from acidic aqueous solution (Q~) against melt composition. The maximum charge in melts is taken from the potential ranges as shown in Fig. 1; the charge in aqueous solution is obtained in the range of - 0 . 2 to 0.85 V vs. SCE; scan rate is 50 mV/s; ([2]), GC; ((D), et.
of a q u eo u s solutions, ion m o v e m e n t and solvent swelling may play an important role in the r e d o x reaction.
Doping process The PAn electrode, prior to placement in the melt, is an insulator. When this electrode is placed in the acidic aqueous solution following film formation and t r e a t m e n t with ammonia, the highest peak current and charge were obtained on the first potential scan due to p r o t o n doping [1, 2]. When the PAn electrode in its insulating state was transferred into the basic melt, the peak current and charge gradually increased with continual scanning until, after about 30 potential cycles, a m a xi m um in both peak current and charge could be observed as shown in Fig. 3. When the oxidation potential was restricted to 0.55 V, instead of 0.9 V as shown in Fig. 3, the highest charge value was obtained after about 1000 cycles. Figure 4 indicates the ratio of Qm/Qa with films of different thickness against the n u m b e r of cycles. Thinner films r e c o v e r e d faster; a large charge could be found following fewer cycles. The nonlinear relationship between the film thickness and recovery m ay be due to morphological changes with films of different thickness [15, 24]. The changes o f melt composition in the basic range did not introduce an obvious effect on the behavior described above and similar behavior could also be seen in the neutral melt.
311
30
5
I
I
.4).2
0
I
I
I
0.4
I 0.8
E vs. AI in 1.5:1 melt,V Fig. 3. Cyclic v o l t a m m o g r a m s of PAn on the Pt electrode in 0.9:1 melt following various numbers of cycles. Scan rate is 50 mV/s.
Thus, the e l e c t r o a c t i v i t y of PAn in b a s i c a n d neutral m e l t s i m p r o v e d gradually, p e r h a p s c o r r e s p o n d i n g to a n i o n s p e r m e a t i n g t h e w h o l e film; in o t h e r w o r d s , t h e kinetic limitation m a y b e d u e to s o l v e n t swelling of the film. On t h e o t h e r h a n d , in acidic m e l t s in t h e r a n g e of 1.05:1 to 2:1 b o t h c u r r e n t a n d c h a r g e are g r e a t e s t o n the first s c a n as s h o w n in Fig. 5, indicating t h a t t h e d o p i n g p r o c e s s in a n acidic m e l t differs f r o m t h a t in b a s i c a n d n e u t r a l m e l t s a n d is similar to t h a t in acidic a q u e o u s solution. In acidic a q u e o u s solution, p r o t o n s r e a c t w i t h t h e i m i n e on the PAn c h a i n to f o r m t h e imine salt a n d then, b y i n t r a m o l e c u l a r r e d o x r e a c t i o n , r a d i c a l s (or p o l a r o n s ) a r e i n t r o d u c e d a n d PAn b e c o m e s a c o n d u c t o r [2, 25]. In an acidic melt, AICI4- a n d AI2C17- a r e p r e s e n t , t h e i r m o l a r r a t i o d e p e n d i n g on the m e l t c o m p o s i t i o n [14]; p r o t o n i m p u r i t i e s a r e a l s o p r e s e n t at a b o u t 10 -4 M. W h e n t h e c o n d u c t i n g PAn d o p e d b y HCI in a q u e o u s solution w a s p l a c e d into t h e acidic melt, t h e c u r r e n t a n d t h e c h a r g e i n c r e a s e d with r e p e t i t i v e s c a n n i n g until, a f t e r a f e w p o t e n t i a l cycles, a m a x i m u m w a s o b t a i n e d . Thus, w e e x c l u d e t h e p o s s i b i l i t y o f a p r o t o n d o p i n g m e c h a n i s m in t h e acidic melt. In principle, A1Cl~ f r o m A12C1T- in t h e acidic m e l t c o u l d p l a y a similar role to t h a t o f
312
1.0
o
0.6
OI
E ol 0.4 ~ DO
[] [] yooOOOOO
o.2
0.0
OOOO
' 0
10
20
30
'
'
40
"
'
50
60
Cyclic Number Fig. 4. Ratios o f t h e a n o d i c c h a r g e o f different t h i c k n e s s e s o f PAn films on t h e GC e l e c t r o d e u n d e r cyclic v o l t a r a m o g r a m s in a 0.8:1 melt (Qm) to that by a q u e o u s solution (Q~) against t h e n u m b e r of p o t e n t i a l cycles. Other c o n d i t i o n s are the s a m e as Fig. 2; (A), 10 nm; ([:]), 40 nm; (O), 2 4 0 nm.
p r o t o n s in a q u e o u s solution doping processes. Just as the prot ons in an aqueous solution can p r o t o n a t e the base (N) site and cause the oxidized, but initially n o n c o n d u c t i v e form of PAn to b e c o m e conductive, so also might AlC13, a strong Lewis acid, react with a base site in the acidic melt. We have shown that AlCl3 adducts of pyrrole are formed in acidic melts, and that the oxidation potential of the adduct is shifted positive, in the case of pyrrole, out of the melt window [8]. So too might the observed positive shift in the PAn peak in the acidic melt (Fig. 1) result from an A1Cla adduct being formed with the PAn film.
Stability and deterioration In acidic melts, the charge under the cyclic vol t am m ogram decreased with r e p e a t e d cycling as shown in Fig. 5. In basic and neutral melts, after the m a x i m u m charge was obtained by cycling, a similar behavior could also be seen. But the deterioration p r o c e s s e s differ. In acidic melts, only the peak c u r r en t decreased; in the basic melt the potentials of the two couples seen in Fig. 3 shift and finally one peak, its formal peak potential [(Epi+Ep~)/21 about 0.45 V, could be observed. Also, the rates of deterioration were quite different as indicated in Fig. 6. PAn is more stable in basic than in acidic melt. On changing the positive potential limit from 0.9 to 0.55 V in a basic melt just b e y o n d the first peak, the PAn retained m o r e than 90% of its activity after 3 × 104 repetitive cycles at 100 mV/s. Com pared with results
313
;IIHA
,.
10--~
f
I o.s
I
I 0.9
I
1
1
1.s
1.7
E vs. AI in 1.5:t melt, V Fig. 5. R e p e a t e d cyclic v o l t a m m o g r a m s o f PAn on the GC electrode in 2:1 melt at 50 m V / s . The n u m b e r on the c u r v e s is the n u m b e r of cycles.
obtained in aqueous and organic solvents [4, 24, 2 6 - 2 8 ] , PAn is very stable in basic melts and possibly useful in a s econdary battery system. W h en PAn electrodes were cycled in the 0.8:1 melt until the m axi m um charge was obtained, then transferred into the acidic aqueous solution, the cyclic voltammogram was the same as that when initially polymerized. Thus, transfer of PAn between aqueous solution and melts is reversible. Once the anodic charge d e c r e a s e d to half of the m axi m um value (see Fig. 6) scanning was stopped at the m os t negative potential employed and the PAn electrode was transferred to an acidic aqueous solution. Figure 7 shows the ratio of anodic charge under the cyclic v o l t a m m o g r a m given by the deteriorated film (Qd) to that of the polymerized film (Q~) against melt composition in which the PAn had be e n cycled. In basic melts, the charge seems to yield the same value as w he n polymerized or it decreases slightly. However, film deterioration when PAn was cycled in acidic melts, and examined in aq u eo u s solution, was large. Figure 8 shows cyclic vol t am m ogram s of deteriorated films obtained from different melts and placed in 1 M HC1 + 1 M NaC1 medium. After having b e e n cycled in basic melts, the vol t am m ogram s
314 1.0
o.e
~ 13 [] []
0.6
13
CI
E
CI
0.4
0.2 % 0.0
,
i
I I O0
0
l
i
200
I
i
i
300
Cyclic
i
400
500
Number
Fig. 6. Qm/Qa vs. n u m b e r o f cycles. O t h e r c o n d i t i o n s a r e t h e s a m e as Fig. 2: ([~), 0.9:1 m e l t w i t h GC e l e c t r o d e ; (O), 1.1:1 melt.
1.C
0.8
m
OI
0.6
OI
|
0.4
0.2
n
0.8
I
1.0
a
I
I
1.2
1.4
Melt
,
I
1.6
i
I
1.8
,
I
2.0
Composition
Fig. 7. Qd/Qa vs. m e l t c o m p o s i t i o n . B o t h Qd a n d Qa axe o b t a i n e d in t h e p o t e n t i a l r a n g e f r o m -0.2
to 0 . 8 5 V vs. SCE. O t h e r c o n d i t i o n s a r e t h e s a m e a s Fig. 6; ([~), GC; (O), Pt.
315
/\
'I"A
'I o.o+~, ,
~
£
--
.-.../,/+ "-........ .// \
fJ
\,j I • O.2
t 0
I
[ 0,4
E vs. SCE, V
I
1
I
l
O.8
-0.2
0
l
I
I
0.4
I 0.8
E vs. AI in 1.5:1 melt, V
Fig. 8. C y c l i c v o l t a m m o g r a m s of d e t e r i o r a t e d f i l m s o b t a i n e d f r o m 0.9:1 ( - • - ) a n d 1.1:1 ( m e l t s in 1 M HC1 + 1 M NaCl; ( ..... ) s h o w s t h a t of PA n a s p o l y m e r i z e d ; o t h e r c o n d i t i o n s a r e t h e s a m e as Fig. 7. Fig. 9. C y c l i c v o l t a m m o g r a m s of 8 X 10 -3 M f e r r o c e n e o n a b a r e Pt e l e c t r o d e ( ..... ) a n d on a P t e l e c t r o d e c o a t e d by a 2 4 0 n m t h i c k PAn film ( - • - ) i n 0.8:1 m e l t ; ( ) represents that o f PAn itself; s c a n r a t e is 5 0 mV/s.
w h i c h h a v e d e t e r i o r a t e d a r e v e r y similar to t h a t s h o w n b y PAn in a q u e o u s solution. T h e a n o d i c p e a k at 0 . 5 5 V v e r s u s SCE m a y p o s s i b l y r e s u l t f r o m h y d r o l y s i s [26], o r t h o - s u b s t i t u t i o n o f the b e n z e n e ring [21] o r a c r o s s - l i n k i n g r e a c t i o n [28]. T h e f o r m e r t w o p r o c e s s e s a r e unlikely in t h e melts, b u t t h e l a t t e r r e a c t i o n m a y t a k e place. B e c a u s e the c r o s s - l i n k i n g s t r u c t u r e m a k e s t h e film m o r e c o m p a c t a n d r e s t r i c t s ion mobility, m e l t m i g h t m o v e o u t o f t h e film a n d t h e e l e c t r o a c t i v i t y s h o u l d d e c r e a s e . A c r o s s - l i n k i n g r e a c t i o n w o u l d n o t i n t r o d u c e a loss o f aniline units f r o m t h e PAn c h a i n a n d t h e e l e c t r o a c t i v i t y o f PAn c y c l e d in t h e b a s i c m e l t w o u l d r e c o v e r to t h e initial value, as w h e n p o l y m e r i z e d . After s w e e p i n g in a n acidic m e l t , t h e a q u e o u s cyclic v o l t a m m o g r a m o f PAn a p p e a r s to b e t h e s a m e a s
316
2.0
< 1.0
[]
II I
011
1,1 0
B ,
(3 I
10
,
l
20
V,
i
I
30
,
l
40
,
50
mV/s
Fig. 10. Plot o f ip for a 10 n m thick PAn film on a Pt e l e c t r o d e in 0.8:1 m e l t vs. s c a n rate: ([~), first o x i d a t i o n peak, ilpo; (HI), first r e d u c t i o n peak, ilpr; (@), i2po; (©), i2p~.
t h a t o f p o l y m e r i z e d PAn, e x c e p t f o r the p e a k c u r r e n t values. This i n d i c a t e s t h a t r e p e t i t i v e p o t e n t i a l cycling in the acidic m e l t d o e s n o t r e s u l t in a c h a n g e o f c h a i n s t r u c t u r e o f PAn in t h e acidic melt. T h e l o s s o f e l e c t r o a c t i v i t y m a y b e d u e to t h e solubility o f PAn in the m e l t s w h i c h s h o w s t r o n g acidity a n d polarity. In fact, PAn is s o l u b l e in c o n c e n t r a t e d H2SO4 a n d s o m e s t r o n g p o l a r o r g a n i c s o l v e n t s [2]. J u s t o n e p o t e n t i a l cycle w a s p e r f o r m e d on PAn in a n acidic melt, a b o u t 2 0 % o f t h e c h a r g e w a s lost w h e n it w a s t r a n s f e r r e d to t h e a q u e o u s solution•
Conductivity and electrochemistry A f t e r r e p e a t e d p o t e n t i a l cycling PAn w a s relatively s t a b l e in the b a s i c melt; w e u s e this p r o p e r t y to c a r r y o u t f u r t h e r e x p e r i m e n t s on its electrochemistry. F i g u r e 9 s h o w s t h e cyclic v o l t a m m o g r a m s of 8 × 10 - a M f e r r o c e n e o n a b a r e Pt e l e c t r o d e a n d o n Pt c o a t e d b y a 2 4 0 n m t h i c k PAn film in a 0.8:1 melt. T h e r e d o x p e a k o f f e r r o c e n e a p p e a r s s u p e r i m p o s e d o n t h e b a c k g r o u n d e l e c t r o a c t i v i t y of t h e PAn film; f e r r o c e n e h a s t h e s a m e p o t e n t i a l as on t h e b a r e e l e c t r o d e . On t h e o t h e r h a n d , t h e r e d o x p e a k o f TiCl4, the f o r m a l p o t e n t i a l o f w h i c h is a b o u t - 0.1 V u n d e r the s a m e e x p e r i m e n t a l c o n d i t i o n s a t t h e GC e l e c t r o d e did n o t a p p e a r o n t h e cyclic v o l t a m m o g r a m s of the GC e l e c t r o d e c o v e r e d b y a 2 4 0 n m t h i c k PAn film. If a 10 n m t h i c k film w a s u s e d , t h e r e d o x p e a k o f TIC14 c o u l d b e f o u n d w i t h f o r m a l p o t e n t i a l - 0 . 1 7 V. In this c a s e , t h e PAn film is v e r y thin a n d t h e TIC14 c a n diffuse t h r o u g h
317 TABLE 1 Cyclic voltammetric data for PAn films Substrate
Film thickness (rim)
Scan rate (mV/s)
E~pa (mV)
E~pc (mV)
~Elp (mV)
E2pa (mV)
E2pc (mV)
AE2p (mV)
Pt
lO
2 5 10 20 50 lOO 200
370 370 375 370 390 400 415
310 310 305 300 290 285 280
60 60 70 70 100 115 135
740 745 745 750 765 775 785
650 655 655 640 645 640
90 90 90 100 120 135
Pt
50
5 10 20 50 100 200 500
380 385 395 400 415 425 430
310 310 300 295 290 285 280
70 75 95 105 125 140 150
750 755 765 770 780 790 815
655 655 645 635 625
95 100 120 135 155
GC
50
5 10 20 50 100 200 500
375 390 400 420 430 440 470
295 290 280 270 260 250 240
80 100 120 150 170 190 230
750 755 765 775 790 820 850
630 620 610 605
120 135 155 170
the p o l y m e r a n d r e a c t on the GC s u b s t r a t e . T h e s e o b s e r v a t i o n s i n d i c a t e that P A n is c o n d u c t i n g i n t h e o x i d i z e d s t a t e in b a s i c m e l t a n d is i n s u l a t i n g i n the r e d u c e d state. T h e l i n e a r d e p e n d e n c e o f ip o n s c a n r a t e i n t h e r a n g e o f 2 t o 5 0 m V / s is s h o w n i n Fig. 1 0 f o r t h e l 0 n m t h i c k P A n film i n a b a s i c m e l t . T h i s b e h a v i o r c a n also be s e e n at film t h i c k n e s s e s up to 100 nm. The a m o u n t o f a n o d i c c h a r g e u n d e r t h e c y c l i c v o l t a m m o g r a m is t h e s a m e a s t h e c a t h o d i c c h a r g e , w i t h i n e x p e r i m e n t a l e r r o r , i n d i c a t i n g t h a t t h e r e d o x r e a c t i o n is f a c i l e u n d e r t h e s e conditions. Table 1 lists other cyclic v o l t a m m e t r i c data for Pt a n d GC e l e c t r o d e s c o v e r e d b y d i f f e r e n t t h i c k n e s s e s o f P A n i n a 0 . 8 : 1 m e l t . T h e p e a k p o s i t i o n s c a n differ b y a b o u t 1 0 % b e t w e e n e x p e r i m e n t s f o r t h e s a m e P A n film w h i c h is s i m i l a r t o t h o s e o b s e r v e d f o r p o l y p y r r o l e a n d p o l y t h i o p h e n e f i l m s i n t h e m e l t [8, 10]. F r o m T a b l e 1, first w e f i n d t h a t t h e 10 n m t h i c k film a p p e a r s t o s h o w n o k i n e t i c l i m i t a t i o n u p t o 2 0 m V / s s i n c e AEp is e f f e c t i v e l y c o n s t a n t . F o r t h i c k e r films, i n c r e a s i n g AEp v a l u e s w i t h i n c r e a s i n g s c a n r a t e a r e o b s e r v e d . S e c o n d l y , t h e first a n o d i c w a v e a p p e a r s more susceptible to kinetic limitations than the cathodic wave. As the scan r a t e o r f i l m t h i c k n e s s is i n c r e a s e d , a n d AEp i n c r e a s e d , E~pa c h a n g e s m o r e t h a n E~p¢, i n d i c a t i n g t h a t it is m o r e difficult t o c o n v e r t t h e p o l y m e r f r o m
318 insulating to conductive. This observation can correspond to a kinetic problem in t h e r e d o x r e a c t i o n o f P A n , t o i o n t r a n s p o r t i n t o t h e film, o r t o d i f f e r e n t rates of swelling and de-swelling. Thirdly, the electrochemical behavior of t h e P A n f i l m is m o r e r e v e r s i b l e o n p l a t i n u m t h a n o n g l a s s y c a r b o n , a s w e s e e t h a t AEp a t P t is s m a l l e r t h a n t h a t a t GC.
Conclusions P o l y a n i l i n e f i l m s p r e p a r e d in a q u e o u s a c i d i c s o l u t i o n s a r e e l e c t r o a c t i v e in b a s i c , n e u t r a l a n d a c i d i c m e l t s . T h e f i l m s a r e c o n d u c t i n g w h e n o x i d i z e d a n d a r e i n s u l a t i n g o n c e r e d u c e d . T h e r e d o x r e a c t i o n o f P A n in t h e b a s i c melt appears to be facile.
Acknowledgement This work was supported Research.
in p a r t b y t h e A i r F o r c e O f f i c e o f S c i e n t i f i c
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