Electrochemistry of Conductive Polymers · 2018-10-30 · Electrochemistry of Conductive Polymers ... Unlike most other conductive polymers, PA is readily pre- pared in aqueous solutions

688 J. Electrochem. Soc., Vol. 136, No. 3, March 1989 �9 The Electrochemical Society, Inc.

Subscripts d disk g gap r ring

REFERENCES 1. W. J. Albery and M. L. Hitchman "Ring Disc Elec-

trodes," Oxford University Press, London (1971). 2. W. J. Albery and S. Bruckenstein Trans. Faraday Soc.,

62, 1920 (1966). 3. W. J. Albery and S. ]3ruckenstein ibid., 62, 1946 (1966). 4. W. J. Albery and S. Bruckenstein, ibid., 62, 2584 (1966). 5. K. B. Prater and A. J. Bard, This Journal, 117, 217

(1970). 6. K. B. Prater and A. J. Bard, ibid., 117, 335 (1970). 7. S. W. Feldberg, M. L. Bowers, and F. C. Anson, J. Elec-

troanal. Chem.. 215. 11 (1986).

8. B. A. Finlayson, "Non-Linear Analysis in Chemical Engineering," Mc-Graw Hill, Inc. (1980).

9. S. Pons, "Electroanalytical Chemistry," Vol. 13, p. 115, Marcel Dekker, New York (1984).

10. L. Whiting and P. Carr J. Electroanal. Chem., 81, 1 (1977).

11. J. Villadsen and M. Michelsen, "Solution of Differen- tial Equation Models by Polynomial Approxima- tion," Prentice Hall, Englewood Cliffs, NJ (1978).

12. R. S. Parikh and K. C. Liddell, This Journal, 135, 1703 (1988).

13. R. Caban and T. W. Chapman, Chem. Eng. Sci., 36, 849 (1981).

14. W. G. Cochran, Proc. Cambridge Philos. Soc., 30, 365 (1934).

15. R. S. Parikh, Ph.D. Dissertation, Washington State University, Pullman, Washington (1988).

Electrochemistry of Conductive Polymers

VI. Degradation Reaction Kinetics of Polyaniline Studied by Rotating Ring-Disk Electrode Techniques

David E. Stilwell *'1 and Su-Moon Park*

Department of Chemistry, University of New Mexico, Albuquerque, New Mexico 87131

ABSTRACT

The degradation reaction kinetics of oxidized polyaniline has been studied employing rotating ring-disk electrode techniques. Polyaniline films were grown on the disk electrode and were then oxidized at a potential more positive than 0.90V vs. Ag/AgC1, while the degradation product (benzoquinone) was monitored at the ring electrode at an applied potential of 0.05V. The product was monitored either with the disk potential maintained at >0.90V (closed-circuit experiment) or with the disk circuit disconnected after the polyaniline film was oxidized for some time (open-circuit experiment). The results of closed-circuit experiments indicate that electrochemical generation of oxidized polyaniline was a limiting step and its rate decayed for a given amount of polyaniline film according to the zeroth order. The rate of hydrolysis for the oxidized polyaniline in quinonoid forms was determined to be consecutive first order from open-circuit experiments. The de- pendencies of the hydrolysis reaction on the acidity of the degradation medium and sulfate concentrations have also been studied; the results are consistent with the Schiff base hydrolysis mechanism.

The use of polyaniline (PA) has been proposed in the areas of organic batteries (1-3), electrochromic displays (4), microelectronic devices (5), and corrosion inhibitors (6, 7). Unlike most other conductive polymers, PA is readily prepared in aqueous solutions by electrochemical or chemical oxidations, and the conducting form is stable in the air. In acid solutions, the resulting film structure is believed to be comprised mainly of repeating aniline units coupled together head-to-tail. Upon oxidation, the film becomes conductive, which, in acid or as a dry salt, seems to arise as a function of the extent of oxidation between the radical cation and the fully oxidized diimine form.

The proposed application of PA to some practical devices such as rechargeable batteries requires high stability of the material. Decays in the electrochemical responses for PA have been reported in several recent studies (4, 5, 8-11). In a study by Kobayashi et al. (9), they con- cluded that p-benzoquinone (BQ) is formed as a degradation product. We have also positively identified BQ as a degradation product of PA upon oxidation (12). In our study employing spectrophotometric identification, we determined the yield for BQ production of 75-100% de- pending on the form of PA. Two electrons per aniline unit were involved for complete oxidation of PA to BQ, as was determined from coulometric experiments.

In our current study, we report our results on detailed studies aimed at elucidating the degradation mechanism. The measurements were made using the rotating ring disk electrode (RRDE) to study the degradation of polyaniline films (PA). It was found that the kinetic relationships for

*Electrochemical Society Active Member. 1Present address: Naval Weapons Center, China Lake, California

93555.

the hydrolysis reaction of quinonediimine groups, within the PA structure, to benzoquinone was quite complex. The observed rate was determined to be a function of pH, electrolyte composition, and film thickness. A mechanism using a Schiff base analogy is used to explain these de- pendencies. In addition, a consecutive first-order reaction mechanism is proposed that describes the role of differing surface groups on the observed decay curves.

Experimental The RRDE work was carried out with a Pine RDE 3 bi-

potentiostat connected to a Pine ASR2 analytical rotator (Pine Instruments, Grove City, Pennsylvania). The electrode was Pine's Model DT6 RRDE with the manufacturer's reported dimensions of: disk radius, rl = 0.383 cm; ring inside radius, r2 = 0.399 cm; ring outside radius, r3 = 0.422 cm. This RRDE had a collection efficiency, de- fined as the ratio of ring to disk current, of 0.200 _+ 0.008 as determined by measuring the oxidation current for potas- sium ferrocyanide at the disk and the reduction current at the ring for ferricyafiide produced at the disk electrode. This value is approximately 10% higher than the calculated value (0.179) from the manufacturer 's supplied dimensions (13).

Prior to use, the electrode was polished to a final smoothness of 0.1 ~m with slurries of alumina polishing powder (Fisher). The Ag/AgC1, saturated KCI electrode en- cased with a Luggin probe was used as a reference electrode.

The bipotentiostat output was recorded with a Linseis LY1800 dual channel (X-Y1Y~) recorder (Linseis Recorder Company, Princeton Junction, New Jersey 08550). Steady- state outputs were also measured with a Keithley Model

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J. Electrochem. Soc., Vol. 136, No. 3, March 1989 �9 The Electrochemical Society, Inc. 689

179 DMM. A P o w e r I n s t r u m e n t s C-891 R P M t a c h o m e t e r ( P o w e r I n s t r u m e n t s , Skokie , I l l inois 60076) was u s e d to m e a s u r e t he ro t a t i on ra te as wel l as to ca l ib ra te t he rota- t i on ra te i nd i ca to r on t h e P i n e rota tor .

T h e c h e m i c a l s u s e d were all r e a g e n t g r ade or be t ter . The su l fur ic acid (Ult rex, Baker ) was u s e d as rece ived . T h e an i l i ne ( E a s t m a n Organics) was u s e d af te r d is t i l l ing over z inc d u s t to e l im ina t e t he ox id ized impur i t i e s . T he c lear an i l i ne l iqu id was s to red in t h e d a r k u n d e r a n a r g o n or ni- t r o g e n a t m o s p h e r e . H y d r o q u i n o n e (Eas tman) was u s e d af te r d o u b l e rec rys ta l l i za t ion f rom water . All of t he a q u e o u s so lu t ions were p r e p a r e d w i t h d o u b l e d is t i l led de- ion ized water .

A 150 ml, capped , P y r e x c o n t a i n e r t h a t was s u r r o u n d e d w i t h a bu i l t - in w a t e r j acke t , c o m p r i s e d t he cell body. Holes of a p p r o p r i a t e sizes we re dr i l l ed t h r o u g h the cap. T h e solu- t ions we re d e a e r a t e d w i t h a r g o n or n i t r o g e n pr io r to use. T h e gases we re p a s s e d t h r o u g h a purifier . T he i n e r t a tmo- s p h e r e b l a n k e t e d t h e cell d u r i n g t h e e x p e r i m e n t . C o n s t a n t t e m p e r a t u r e was m a i n t a i n e d b y p u m p i n g w a t e r f rom the w a t e r j a c k e t t h r o u g h a H a a k e t h e r m o s t a t e d , w a t e r circu- l a to r se t a t 25.0 ~ _+ 0.2~

To m a i n t a i n r e p r o d u c i b l e resul ts , b o t h t h e r ing a n d d isk e l ec t rodes we re p r e t r e a t e d b y po t en t i a l pu l s i ng b e t w e e n t h e ca thod i c a n d anod ic l imi t s s u c h t h a t H2 a n d O2 will b e p r o d u c e d a l te rnate ly . Af te r e ach run, t he P A r e m a i n i n g on t h e d i sk e l ec t rode was r e m o v e d by i m m e r s i o n in a concen - t r a t e d n i t r ic ac id solut ion.

Results Closed-circui t e x p e r i m e n t s . - - T h e s tud ies r e p o r t e d he re

we re all d o n e in 1M H2SO4 as t he s u p p o r t i n g e lec t ro ly te for g r o w t h a n d for degrada t ion . All of t he P A fi lms u s e d for t h e s e e x p e r i m e n t s were g r o w n in 0.055M an i l ine w i t h t h e p o t e n t i a l s w e p t c o n t i n u o u s l y b e t w e e n -0 .1 to + l . 2 V vs. Ag/AgC1 ( sa tu ra t ed KC1) at 50 m V - s -1 for a to ta l of 10 cycles. Th i s r e su l t ed in a fi lm t h i c k n e s s of 0.45-0.55 ~m, calcu l a t ed f rom t h e c h a r g e - t h i c k n e s s r e l a t i ons h i p (11). The e l ec t rode was wel l r i n sed w i t h 1M H2SO4 to r e m o v e any so lub le c o m p o u n d s left over f rom t he g r o w t h solut ion.

Firs t , we d e t e r m i n e d t he po t en t i a l a t w h i c h t he P A film b e g a n to d e g r a d e in to so lub le species . To do this , t h e d i sk p o t e n t i a l was s w e p t whi l e t h e r ing po t en t i a l was he ld con- s tant . The r e su l t s are s h o w n in Fig. 1. While t he iden t ica l v o l t a m m o g r a m s were p r e s e n t e d a n d d i s c u s s e d p rev ious ly (12), we w i s h to a d d r e s s a few m o r e po in t s a b o u t t h e volt- a m m o g r a m s here . No t i ce in t he f igure t h a t t he v o l t a m m e t -

Z

I 10/JA

Ring

Disc

'~

I u i I t I t 1.2 0.8 0.4 0

Potential, V vs. Ag/AgCl

Fig. t. Ring and disk c u r r e n t vs. disk potential. Disk scan rate = SO mV/s. Ring potential = O.OSV. Rotation rate = 1000 rpm.

ric c u r r e n t s at t he d i sk e l ec t rode h a v e p e a k s h a p e s r a t h e r t h a n mass - t r ans f e r - con t ro l l ed l imi t ing cu r ren t s . Th i s dem- o n s t r a t e s t h a t P A is, indeed , a sur face species , e v e n pas t t he d e g r a d a t i o n poten t ia l s . A t d i sk po ten t i a l s m o r e posi- t ive t h a n 0.6-0.65V, a r e d u c i b l e c o m p o u n d is de t ec t ed at t h e r ing. Th i s r ing c u r r e n t s h o w s s ign i f ican t hys ters i s , in- d i ca t ing t h a t t he g e n e r a t i o n of t he r e d u c i b l e c o m p o u n d m a y h a v e some k ine t i c barr ier . We be l ieve t h a t t he hys te r - es is o b s e r v e d is in t h e t i m e d o m a i n due to t h e de lay in pro- d u c i n g a p r o d u c t r e d u c i b l e at t he r ing e lec t rode . T h a t is, t h e e l ec t ron t r a n s f e r r eac t ions at t he d i sk are, m o s t likely, fo l lowed b y c h e m i c a l r eac t ions w h i c h p r o d u c e t he f inal so lub le p r o d u c t s een at t h e r ing e lec t rode . W h e n t he d i sk po t en t i a l was s w e p t at l ower ra tes (5 m V �9 s-l), t h e r e su l t s we re s imi la r e x c e p t t h a t t he r e d u c t i o n p e a k o b s e r v e d at m o s t pos i t ive po t en t i a l ( -0 .7V) at t he d i sk was no longer seen, due to t he longer ox ida t i on t i m e a n d s lower s can rate. T h e o u t c o m e of r e p e a t e d cyc l ing re su l t s in s imi la r f ea tu res on the d i sk e l ec t rode d i s c u s s e d ear l ie r for t h e stabi l i ty fac tor d e t e r m i n a t i o n s (10). B a s e d on ou r ear l ier resul ts , t h e r educ ib l e c o m p o u n d de t ec t ed at t he r ing is p- b e n z o q u i n o n e (12) or t he immine . T h a t is, s o m e po r t i on of t h e p r o d u c t cou ld b e b e n z o q u i n o n e i m i n e or d i imine , w h i c h is h y d r o l y z e d in so lu t ion to p - b e n z o q u i n o n e ; t he r ing c u r r e n t wou ld b e e q u i v a l e n t in b o t h cases.

Next , we r an c h r o n o a m p e r o m e t r i c s tud ies at t he RRDE. In t h e s e e x p e r i m e n t s , t h e d i sk po t en t i a l was p u l s e d f rom -0 .1 to +1.2 vs. Ag/AgC1. The re su l t s in Fig. 2a s h o w the d i sk c u r r e n t fa l l ing rapidly , w h i l e at t he r ing, t h e r e is a r i se to a m a x i m u m fo l lowed b y a s low decay. T h e s e fea tu res are b e t t e r i l lus t ra ted in Fig. 2b w h e r e t h e s e da ta were con- v e r t e d in to t h e co l lec t ing efficiency. T h e s e da ta e s t a b l i s h t h a t a t t i m e s of less t h a n a b o u t 30s, t he co l lec t ion effi- c i ency r ises and, there fore , t he overal l hydro lys i s r eac t i on ra te i nc reases fas te r t h a n t he ra te of g e n e r a t i o n of s i tes t h a t c an b e hydro lyzed . B e t w e e n 0.5 a n d 7 m i n t h e co l lec t ion ef f ic iency (No) is g rea te r t h a n 0.20 b u t i t dec reases m o n o - tonical ly . Fo r any No > 0.20, t he hyd ro lys i s ra te is g rea t e r t h a n t he ra te of s i te genera t ion , b a s e d on two-e l ec t ron oxi-

:::L o

~ o ~ 5

-2

E2

m

R

|

,_ _ "'_'L''"'-.-,:_. .. .,_..._.~ , | �9 w i �9 i I m i ~ Iw~ _

-j

E2

"rIME. See . )

I

Lg Z

I

E-2

35

38

25

26

15 '

18

I

w|~ t m ~ u

I m

gKE~

I I

~]ME. CSEC.)

~2 + 8

Fig. 2, (a) Disk and ring current vs. time. (b) Collection efficiency for the data shown in (a) vs. time. Disk potential = 1.2V. Ring potential = O.OSV vs, Ag/AgCI, Rotation rate = 1000 rpm.



690 d. Electrochem. Soc., Vol. 136, No. 3, March 1989 �9 The Electrochemical Society, Inc.

dat ion at the disk resul t ing in one site formation. In this region, the col lect ion efficiency for BQ genera t ion decreases monotonica l ly , fol lowing zeroth order. This observa t ion leads to the fol lowing conclusion.

Initially, the rate of the chemica l reaction, i.e., hydrolysis, to p roduce a reducib le p roduc t de tec ted at the r ing e lec t rode is s lower than that of e lec t rochemica l genera t ion of hydrolyzable sites. This expla ins not only the early rise in the r ing current (<30s) bu t the hysteresis in Fig. 1. The hydrolys is react ion p roduces p-BQ as a p roduc t as wel l as r educed PA wi th shorter chains th rough the t e rmina t ion s tep (12). The reduced PA thus p roduced is the source of a fur ther anodic current. After some degradat ion reactions, the genera t ion of reduced PA becomes a l imi t ing step, and thus the disk current decreases. For t imes beyond 7 min No < 0.2, and there seems to be some t rend toward a s teady -state. These c i rcumstances suggest that the same fract ion of the disk cur ren t goes toward side react ions that do not resul t in the format ion of hydrolys is sites. A bui ldup in the n u m b e r of sites is cons idered unl ikely in v i ew of the responses at shor te r t imes. In the absence of side reactions, No should b e c o m e close to 0.2 and s teady with time. It may be that s teady state is achieved at some t ime past 7-10 min. We were unable to obtain rel iable measu remen t s for this t ime range due to the low currents and p rob lems wi th base-l ine corrections. The uncer ta in ty in the determina- t ion of No increases wi th t ime since the currents decrease considerably. B e t w e e n 7-10 min this uncer ta in ty is about 10%.

Finally, we conduc ted an expe r imen t to de te rmine the re la t ive amoun t of oxidizable soluble species that may be formed. The r ing potent ia l was set to + 1.2V vs. Ag/AgC1. At this potential , c o m p o u n d s released at the disk, such as ani l ine or pheny lened iamine (PDA), wou ld be oxid ized at the mass- t ransfer-control led rates at the ring. The resul ts (not shown) indicate that only a relat ively small a m o u n t of oxidizable c o m p o u n d s compr i se the soluble degradat ion products . The ne t signal is at mos t only be tween 3 and 6% of the reduc t ion current observed at the ring under identical condi t ions at the disk. I t can be conc luded that only minor amount s of soluble species are in the r educed state and, as such, do not appear to be a major mechanis t ic path in the decompos i t ion react ion of PA.

O p e n - c i r c u i t e x p e r i m e n t s . - - T h e analysis of the hydrolysis rate can be cons iderably simplif ied if the PA-coa ted disk e lec t rode is opened after the control led potent ia l oxi- da t ion at 1.2V, for example . Consider the fol lowing reac- t ion sequence

Rko (PA)n --> (PA). ~ [1]

kc (PA)~ ~ + H20 =* (PA)~_I ~ + BQ [2]

d ( B Q ) / d t = kc (PA)~ ~ [3]

where ke is the overal l e lec t rochemica l rate cons tant for the genera t ion of hydrolysis sites, and kc is the overall rate for the chemica l react ions that lead to hydrolys is of sites. U p o n open circuit, the rate of format ion of these sites is zero and then the ring current becomes a measu re of the chemica l rate of decay of the sites (S); for example , if the overal l decay follows first-order kinet ics

S + H 2 o k BQ + D [4]

the rate of BQ generat ion, measured by the current at the ring, should decay exponent ia l ly if it fol lows the first-order kinetics. Thus, the In (i/io) vs. t plot wou ld be l inear for the first order, or a plot of (io/i) ~ vs. t would be l inear for the second-order kinetics. For the PA-coated electrodes, the log plots were l inear or curved, depend ing on pH. Due to this curvature , we bel ieve that the actual react ion kinet ics is consecu t ive first order, compl ica ted by hydrolys is ter- mina t ion steps. For consecut ive first-order reactions, a m i x e d exponen t i a l decay is realized which resul ts in In (i/io) plots that may be l inear or curved, depend ing on

0 2LO ~ 410 ' 610 Time , sec.

i0 pA ]~

~a

b

810 i00 200

Fig. 3. Comparison of open- and closed-circuit ring currents in 1M H2SO 4. Disk pulsed from -0 .1 to 1.2V vs . Ag/AgCI at t = O. Ring potential = - 0 . 0 5 V . Rotation rate = 1000 rpm. Both films were grown under identical conditions; by potential cycling in 1M H2SO 4 between -0 .1 and 1.2V for ten cycles. Aniline concentration = 0.055M. Initial film thickness = 0.5 I~m. (o) Ring current observed at closed circuit. (b) Ring current when disk was open-circuited at point o.c.

the values for the rate constants. U n d e r condi t ions where curva ture was observed, the data was spli t into two l inear regions, the slopes of which have been denoted as kl and k2.

An e x a m p l e of the compara t ive effects of open c i rcui t ing the disk is shown in Fig. 3 where the closed- and open-circui t curves are plot ted together. Note that w h e n the disk is open c i rcui ted the ring current decays to background wi th in 200s, whi le in the closed-circui t s i tuat ion there is still a cons iderable r ing current after 200s. Repea t ing this p rocedure on the same film indicated that the .film decom- posi t ion is not comple te after the first cycle. This is con- s is tent wi th our previous observat ions (12) where the open-ci rcui t decay was shown to be incomple te , and again, it can be conc luded that there are some te rmina t ion steps in the open-circui t mechan ism.

In addit ion, the open-circui t decay became s lower wi th each cycle. The decay constant , kl, fell f rom 0.030 s -~ for the first cycle to 0.025 for the third. Moreover , the t ime for the cur ren t to reach a closed-circui t m a x i m u m increased. Both of these behaviors are not consis tent wi th wha t was observed for films as a funct ion of thickness . It is shown be low that th inner films exhibi t apparent faster decay as wel l as shor ter t imes to reach closed-circui t m a x i m u m and smal ler current values. These observat ions are consis tent wi th the compos i t iona l changes in the PA s t ructure wi th hydrolys is t ime. As observed previous ly (11), films that had been subjec ted to cont inuous cycl ing were no longer soluble in DMF after convers ion to the base form.

Clearly, to compare the resul ts for different films, repro- duc ib le results can only be obta ined with reference to a par t icular cycle number . As such, all the data p resen ted in this paper are for the first cycle only.

The a c i d i t y , t o ta l su l fa t e , a n d t h i c k n e s s d e p e n d e n c i e s . - The funct ion of the acidi ty on the observed decay was de- t e rmined in HC1 and H2SO4. S o d i u m sulfate (Na2SO4) was added to the sulfuric acid solut ions to main ta in the total sulfate concent ra t ion at 2M. This results in a cons tant ionic s t rength of about 2 for pH be low 1 or so. Above this pH va lue the ionic s t rength rises, bu t we found that there was lit t le effect compared wi th the total sulfate concent ra t ion (see the fol lowing and Table I). Adjus t ing the ionic s t rength to 6 was impractical . The pH of these mix tu res was calculated us ing pKa2 = 1.98 for the monohydrogen - sulfate anion (14). For s tudies in HC1, NaC1 was added to solut ions of HC1 concent ra t ions less than 1M. At least two, for the mos t part three, and somet imes four or five determina t ions of the expe r imen ta l decay curves were ob ta ined for each solut ion at a par t icular pH. For s tudies in sulfuric acid, the disk potent ia l was s tepped f rom -0 .1V to e i ther 0.9 or 1.2V. In HC1 the disk potent ia l was s tepped to only 0.9V to avoid C1- oxidation. The P A film growth pro- cedures were the same as descr ibed in the closed-circui t exper iments , excep t for the pH studies where the elec- t rode was r insed in the electrolyte used for degradat ion and for the th ickness s tudies where the n u m b e r of cycles for g rowth varied.



J. Electrochem. Sac., Vol. 136, No. 3, March 1989 �9 The Electrochemical Society, Inc. 691

Table I. Hydrolysis rate data for other growth and degradation conditions

Growth ~ Degradation ~ kl k2 Entry conditions conditions (s-~) (s ~)

1 -0.1-+ 1.2V 0.5M H2SO4 § 0.009~ 1M NaHSO4 1.5M Na2SO4 0.012

2 -0.1-+l.2V 1.0M H=SO4 § 0.019~ 1M NaHSO4 1.0M Na2SO4 0.021

3 -0.1-+l.2V 1.5M H2SO4 + 0.043~ 1M NaHSO4 0.5M Na2SO4 0.045

4 -0.1-0.9V 1M NaHSO4 0.019~ 1M H2SO4 0.025

5 -0.1-1.2V 1M NaHSO4 0.014~ 1M H2SO4 0.016

6 -0.1-1.2V 1M NaHSO4 (0.9V) 0.014 1M H2SO4

7 -0.1-1.2V 1M H2SO4 0.028~ 1M H2SO4 0.031

8 -0.1-1.2V 1M H2SO4 (0.9V) 0.0275 1M H2SO4

9 -0.1-1.2V 0.5M NaHSO4 0.0103~ 1M H2SO~ (0.9V) 0.0105

IO -0.1-1.2V 0.5M NaHSO~ § 0.0104- 1M H~SO~ 1.55M NaC1 0.0110

(0.9V)

0.020~ 0.025

0.016- 0.021 0.017

aScan rate = 50 mV/s. Aniline concentration = 0.055M. All of the films were grown to an initial thickness of ~0.5 ~m. bDisk pulsed from -0.1 to 1.2V or (0.9V), and open-circuited at the positive potential within 5s after the closed-circuit ring current

maximum.

The open-c i rcu i t decay and the c losed-ci rcui t r ing cur- r en t s inc rease w i th acidity, whi le the t ime to reach the c losed-c i rcui t r ing cu r ren t m a x i m u m decreases wi th acidity. S h o w n in Fig. 4 are s o m e of the open-c i rcu i t decay curves and the log plots for t hese data. Note tha t in 1.5M H2SO4 + 0.5M Na2SO4 solut ions the re is a def ini te curva- tu re in t he log plot. The curva ture in the semi log plots of t he normal ized open-c i rcu i t r ing cu r ren t decay a lways in- c r eased wi th acidity. In cases w h e r e two l ines are obse r v e d in t hese plots as in Fig. 4c, two rate cons tan t s kl and k2 w e r e ob ta ined f rom shor te r and longer t ime domains , re- spect ively. In no case was th is curva ture seen at h ighe r pH; t he b reak was b e t w e e n 0.5 and 1.0M acid. Control ex- p e r i m e n t s e s t ab l i shed tha t this curva ture was no t due to a b u i l d u p of BQ in solution.

The curva tu re effect does no t ar ise f rom a change in the i /E behav io r of BQ at h ighe r acidit ies. Control s tud ies on BQ reduc t ion over the range of acidi t ies u s e d in this s tudy, and at a cons t an t BQ concen t ra t ion , r esu l t ed in r ing s teady-s ta te cu r ren t s w h i c h did no t vary significantly. The d isk cu r r en t was at open circui t for t hese contro l s tudies . It shou ld be po in t ed out tha t at pH -> 4 the cu r ren t w o u l d

iB

?

8 7

~ J

1

K--i

mm mm m W ~D

~ -

�9 . c b

8

- - 5

A , ~ - i 0

" - - - - t 5

- 2 5

- - 3 8

- - 3 5

E - - 1

�9 .......-.-. ..... ...

" .

- .

Time, sec.

. .

"" " .a

�9 " b

c

8 •

Fig. 4. Ring current and log of ring current vs. time. Current at t = 0 is i(O). The growth and other conditions as in Fig. 3 except, (a) in 0.5M H2S04 + i .5M Na2S04, (b) 1.0M H2S04 + 1.0M Na2S04, and (c) 1 .SM H2S04 + 0.SM Na2S04.

change , as n changes f rom 2 to 1 due to h igh stabi l i ty of BQ an ion radicals , compl i ca t ing the in te rp re ta t ion of resul ts and, w i thou t ex tens ive controls , r ende r ing t h e m unre- liable.

S h o w n in Fig. 5 are the values ob ta ined for k1 and k2 as a func t ion of the H § concent ra t ion . As can be seen, a p la teau in kl and k2 is abou t the only c o m m o n fea ture seen in t hese two acids. The l imi t ing value is s l ight ly h ighe r in HC1 than H2SO4. The o b s e r v e d decay rates are cons ide rab ly grea te r in sulfuric acid solu t ions w h e n the H § concen t r a t ion is less t h a n abou t 1M.

S h o w n in Fig. 6 are t he average values for kl p lo t t ed vs. pH or Ho. For [H § > 0.2 the H a m m e t t acidi ty func t ion was

7,

6

5

a , 4

G o

~ 3

2

i

0

[ , * ] , M

71E-2

6

.i

8

e 5 te 15 2e [ , * ] , M

E-2

a k 1

k 2

E-1 2'5 38

b

k 1

k 2

25 3B

Fig. 5. Rate constants, kl and k2 vs. [H+]. (a) Results in HCI solutions. (b) Results in H2S04.



692 J. Electrochem. Sac., Vol. 136, No. 3, March 1989 �9 The Electrochemical Society, Inc.

5

T

r

2

E-2

Fig. 6. Average kl

a

E8 ' '2 ' - 1

pH I No

values vs. pH or Ho: (a) in HCI, and (b) in H2SO4

employed . This f u n c t i o n (Ho) is a c o m p a r a t i v e m e a s u r e of t he so lven t to d o n a t e p r o t ons to w e a k b a s e mo lecu l e s a n d is c o m m o n l y u sed to rep lace pH va lues in so lu t ions of h i g h ac id i ty (14-18)�9 The Ho values , w h i c h i nc l ude co r rec t ions due to t he sal t effect, were t a k e n f rom t he da ta a n d t ab les g iven b y P a u l a n d L o n g (14). The sal t e f fec t co r rec t ion t e r m for NaC104 was u s e d for N a H S O ( as t he su l fa te da ta we re no t avai lable . T he va lues can be e x p e c t e d to b e clos.e to each o the r b e c a u s e t he sal t effects are m o r e sens i t ive to c h a n g e s in t he cat ion. E m p l o y i n g t h e s e Ho va lues in p lace of pH t e n d s to b r o a d e n t he cu rve a n d sh i f t i t t o w a r d h i g h e r acidi ty, b u t t he s h a p e r e m a i n s t he same. T he Ho va lues are r e t a i n e d in f u r t h e r analys is s ince t hey are a m o r e re l iab le i n d i c a t o r of t he t rue acid s t reng th .

B e t w e e n p H (or Ho) 0-3, the ra te c o n s t a n t is n o t a b l y h i g h e r in su l fur ic acid. Th i s o b s e r v a t i o n is c o n s i s t e n t w i t h t he s tab i l i ty fac tors r e p o r t e d p rev ious ly (10, 12) a n d p r o m p t e d us to s t u d y t h e effect of the su l fa te concen t r a - t ion at c o n s t a n t pH. T h e resu l t s for pH = 1, Fig. 7, shows t h a t k o b s e r v e d as wel l as t he c losed-c i rcu i t m a x i m u m r ing c u r r e n t i nc reases l inear ly w i th t he to ta l su l fa te concen t r a - t ion. Ev iden t ly , t he su l fa te ac ts as a ca ta lys t in t he hydro ly - sis reac t ion . The ionic s t r e n g t h was m a i n t a i n e d at 2 w i t h NaC1. U n d e r m o r e acidic cond i t ions , t he p lo ts were all l inear. The ca ta ly t ic effect, i nd i ca t ed by s lopes of p lo t s (not s h o w n ) d e c r e a s e d s o m e w h a t as t he ac id i ty of t he m e d i u m inc reased , i m p l y i n g t h a t b o t h H S O ( a n d SO42- spec ies act as catalysts .

Final ly , t he va r i a t ions in kl a n d k2 w i t h fi lm th i cknes s , in 1M sul fur ic acid, are s h o w n in Fig. 8. T he kl va lues s eem to fol low a - 1/2 p o w e r d e p e n d e n c y on film th i cknes s . For t he k2 va lues t h e r e is a def in i te d o w n w a r d t r e n d w i th t h i c k n e s s (not shown) . Indeed , for t h i c k n e s s e s a b o v e 1 I~m, t he semi- log p lo ts were l inear a n d so t h e r e was no k2. T he dec rease in t he ks va lues m a y fol low a l inear or -1 /2 p o w e r de- p e n d e n c y w i th t h i cknes s , b u t t he sca t t e r in ks va lues pre- c ludes any f irm a s s i g n m e n t to t he func t ion .

T h e c h a n g e in the hydro lys i s ra te w i t h t h i c k n e s s m i g h t b e a s soc ia t ed w i t h t he d i f fus ion of t h e r e a c t a n t (H~O) t h r o u g h t he film. The t h i c k n e s s ef fec t m a y be e x p l a i n e d b y s t r u c t u r a l d i f f e rences b e t w e e n t h i n a n d t h i c k films. These p o i n t s are d i s c u s s e d in m o r e deta i l below.

T h e m a x i m u m c losed-c i rcu i t r ing c u r r e n t s h o w e d an in- c rease w i t h t h i c k n e s s , a l t h o u g h t h e r e was c o n s i d e r a b l e s ca t t e r (not shown)�9 As expec ted , c o m p a r a t i v e l y t h i n n e r f i lms t ook less t i m e to r e a c h t h e m a x i m u m c u r r e n t as t he a m o u n t of ma te r i a l is less a n d is u s e d m o r e quickly .

Miscel laneous ra te d a t a . - - T h e resu l t s for o the r g r o w t h a n d d e g r a d a t i o n cond i t i ons t h a t were s t ud i ed are s u m m a r - ized in Tab le I. C o m p a r i s o n of en t r i e s 1-3 to t h e ra te da ta in Fig. 5b s h o w s t h a t f i lms g r o w n in 1M NaHSO4 are some- w h a t m o r e s t ab le t h a n t hose g r o w n in 1M H2SO4. Compar - ing e n t r y 4 to en t ry 5 shows t h a t f i lms g r o w n to a s w i t c h i n g po t en t i a l of 0.9V are less s t ab le t h a n t h o s e t h a t are

I

25 TM

28~

15

ie

E-3

/ / s

6 ~

a /~ /

/ / / / /

/ /

/

I 'O i '5 28

SULFATE CONC.) M

E8 17

t5 b

t3 "/ . . . .

311

j.. ...-'

3 ." ,J,.,'"

F-i ~ i'B is 2e

{Sul f ~t e) ~PI Fig. 7. (o) k vs. total sulfate, pH = 1. (b) Closed-circuit ring current

maximum vs. total sulfate concentration, pH = 1.

I

o o 38

- ' L

25

E-3 I

35 = ' . =

" , |

28'

"m

i | . � 9

". . . . �9

�9 ... | =

i . . . . , . . . |

""'"","'"E-I ~ i'o 1'2 14

Th ckness. {ul

'E-3 .'

b

35 I . . "

�9 m t . O

| 38 ..'""

. 2 ~ ."

25 ', .'" . . ' " �9

=. . ii

2e .-, ~-~-~ 8 18 1'2 1'4 1'6 18

[ T h l c k n e s s l - l / 2 . (ul

Fig. 8. The thickness effect on k=: (a) kt vs. thickness, and (b) kl vs. (thickness) TM




swi tched at 1.2V. Both of these findings are consis tent wi th the s tabi l i ty factors repor ted prev ious ly (12). By compar ing en t ry 5 to 6 and 7 to 8, it can be seen tha t there is lit- t le d i f ference in the observed decay be tween films open c i rcui ted f rom 0.9 or 1.2V. These data are consis tent wi th the cyclic vo l t ammet r i c resul ts d iscussed prev ious ly (11, 12); the more posi t ive ox ida t ion wave, peaking between 0.75-0.85V, was ass igned to the convers ion of P A into the fully oxid ized form. The effect of ionic s t rength was found to be qui te minimal , as shown by compar i son of en t ry 9 to en t ry 10.

The resul ts be tween films were very reproducible . The m i n i m u m decay rate that could be measu red was be tween 0.003 and 0.005 s- ' . For decay rates be low this, the ring current was not sufficient to rel iably separa te the signal f rom the noise and f rom the background . As shown above, the sca t te r in the resul ts is about equal to the m i n i m u m de- tec t ion l imit .

Discuss ion A n y p roposed m e c h a n i s m for the hydrolys is of polyani-

l ine should account for the observed behaviors as m u c h as possible. These inc lude the following: (i) the acidi ty effect, (ii) the sulfate effect, (iii) the curva ture in the In (i/io) vs. t ime plots, and (iv) the th ickness effect. In the first part of our discussion, the acid and sulfate effects are discussed. The second part deals wi th the curva ture whi le the thickness effect is covered in the last section.

The acidi ty and sulfate e f fec ts - -PA hydrolysis modeled as a Sch i f f base.--In the broades t sense, a Sch i f f base is any molecu le wi th an azometh ine group [R2C--N--R(H)]. More f requent ly , though, the t e rm Schi f f base refers to an acycl ic imine der ived f rom an al iphat ic or a romat ic amine. The hydrolys is react ion of the imine group requires at least two steps. D e p e n d i n g on pH, e i ther the format ion or the decompos i t i on of the carb ino lamine in te rmedia te (or the zwi t ter ion equivalent) is the ra te -de te rmining step; this react ion is, in mos t cases, reversible . N u m e r o u s rev iew ar- t icles and monograp hs are avai lable on the format ion and hydrolys is of Schi f f bases (16, 19, 20). For the hydrolys is of Sch i f f bases der ived f rom mos t al iphat ic amines, a bell- shaped curve resul ts w h e n the observed rate cons tant is p lo t ted vs. pH. This feature is though t to arise f rom a change in the ra te -de te rmin ing step near the pKHB+ for that base wh ich inhibi ts the format ion of the zwit ter ion at low pH (16, 19, 20). In the case of Schi f f bases der ived f rom weak ly basic a romat ic amines, the bel l -shaped curve is either shif ted to lower pH regions or, in m a n y cases, a maxi- m u m rather than a poin t of inf lect ion is observed (16, 19-22). In s t rong acid, i f an inflect ion point is finally observed, this decrease in the rate is a t t r ibuted to a decrease in the ac t iv i ty of water ra ther than to a change in rate- de t e rmin ing step (19a, 20, 22). The water acts as a nucleo- phi l ic agent or as pro ton acceptor dur ing the decompos i t ion step of the in te rmedia te carbinolamine.

The d e p e n d e n c e of the observed hydrolys is rate t()r PA as a func t ion of acidi ty is consis tent wi th previous resul ts r epor t ed for the hydrolys is of weak ly basic amines. The shape of the expe r imen ta l curve for k~ obse rved in HC1 was qui te s imilar to the resul ts pub l i shed by Reeves (21) for the hydrolys is of benzyl ideneani l ine as a funct ion of acidity. Reeves p roposed a hydrolys is m e c h a n i s m to expla in the obse rved behavior , which was actual ly a more general case of one first sugges ted by Willi and Rober t son (29). We have adop ted this m e c h a n i s m to account for our results on PA. To this react ion s cheme we have added steps to accoun t for the sulfate effect; our s c h e m e is shown in Fig. 9. S teps 7 and 8 were added to account for the sulfate effect, but oth- e rwise this is the same m e c h a n i s m as used by Reeves (21). S teps 1-6 are depic ted in Fig. 10 for the genera l case.

The s imple m o d e l (16, 19, 20) r ep resen ted by steps 1, 3, and 5 (k-3 = 0), predicts that the rate of BQ produc t ion is p ropor t iona l to the fract ion of Schi f f base tha t is in the pro- tona ted form (rate = C*[SH+/{S + SH+}]). When k-3 ~ 0, the rate decreases at low pH, d e p e n d i n g on k_3. Reac t ion schemes us ing this step are employed w h e n a bell shape in

_Hydrolysis Mechanism for Polyaniline: The pH Effect

S = C=N-R(H) groups on PA +/R(H)

SH + = C=N\H

l +/R(H) SHOH = HO-C-N-R(H) SH2OH + = HO-C-[\ H

S T S + SH + Q= quinonoid product A = amine product

K (I) S + B + "~ SH + K'=I/KsH+=I/KI

k2 (2) S § H20 k ~ SHOH

(3) SH + + H20 k3--~-~- SHOH + H + k~ 3

(4) SHOH + H + ~ SH2OH + K4= I/KSH2OH+

k 5 (5) SHOH �9 Q + A

k 6 (6) zH2OH + �9 Q + A + H +

(7) SH § + SO42- + B20 k7-/--~-. SUOH + HSO 4-

k~ 7

k 8 (8) SHOH § HSO 4 ~ Q + A + HSO 4

Fig. 9. Hydrolysis mechanism for PA--the pH effect

the rate vs. acidi ty plots is observed. For weak ly basic aro- mat ic amines, steps 4 and 6 are invoked in order to expla in the observed lack of an inf lect ion poin t at low pH (21-22). These steps involve the format ion and subsequen t parallel decompos i t i on pa th of the pro tona ted ca rb ino lamine in- termedia te , and are cons idered reasonable because the e lec t ron wi thdrawing phenyl group permi t s amine expul- s ion wi th less dr iv ing force than al iphat ic amines (20).

Three react ion schemes which wou ld account for the obse rved sulfate effect are shown in Fig. 11. Genera l base catalysis by the sulfate anion to form the in te rmedia te is r ep resen ted by step 7 in the hydrolys is scheme. We have not inc luded the act ivi ty effect due to the lack of the effect of ionic s t rength as a l ready poin ted out. Also, measurements were m a d e at cons tant ionic strengths. Severa l

\C +/R / =~\H I "C H/R I+--"\H ]

--C--N--H R I I

C _ N / R k2 ,H20 I I ~ - 0 H \ ., - ~- - - -C - -N

k-= ] I HO H

~C=O R

I l k8 + - -C- - N+-.-H

I t R--~H2 HO H (H.)

Fig. 10. Hydrolysis schemes for weakly basic amines



694 J. Electrochem. Soc.,

m e c h a n i s m s are poss ib le , all of w h i c h lead to d i f fe ren t t r a n s i t i o n s ta tes b u t r e su l t in t he s a m e p r o d u c t (19a). Fo r t he ca ta lys is s c h e m e s h o w n in Fig. l l a , t h e p r o t o n is do- n a t e d by a wa te r molecule . S w a i n et al. (23) h a v e a r g u e d in favor of an a l t e rna t ive t r ans i t i on s ta te w h e r e b y a p r o t o n is a b s t r a c t e d at t he n i t rogen . In Fig. l l b t he poss ib i l i ty of a gene ra l ba se ca ta lys is of t he p r o t o n a t e d i n t e r m e d i a t e is shown . Th i s p a t h can be c o n s i d e r e d un l i ke ly s ince an in- f lec t ion p o i n t was no t o b s e r v e d (see Ref. 19a, pp. 479 a n d 493).

W h a t e v e r t he m e c h a n i s m , an inc rease in ra te at c o n s t a n t p H by p h o s p h a t e , acetate , a n d o the r c o n j u g a t e base s has b e e n rou t i ne ly o b s e r v e d in a va r i e ty of Sch i f f ba se hydro lys is r eac t ions (19-29). In v iew of this , ou r da ta s h o w i n g a ra te e n h a n c e m e n t b y sul fa te at pH = 1 s h o u l d c o m e as no surpr i se . To exp l a in p r o d u c t d i s t r i b u t i o n s in t h e hydro ly- sis of i m i n o l a c t o n e de r iva t ives w i t h pH, C u n n i n g h a m a n d S c h m i r (28) h a v e p r o p o s e d a catalyt ic p roces s w h e r e b y t he p r o t o n a t e d fo rm of t he buf fe r is involved , v ia a c o n c e r t e d cyclic p r o t o n sh i f t w i t h t he neu t r a l c a r b i n o l a m i n e in t e rme- diate. A n ana logous p rocess w h e n app l i ed to HSO4- is cove red b y s tep 8 in the hydro lys i s s c h e m e a n d is dep i c t ed in Fig. l l c .

The ne t o u t c o m e of such a r eac t ion wou ld b e to sh i f t t h e e q u i l i b r i u m b e t w e e n the u n c h a r g e d c a r b i n o l a m i n e t o w a r d t he zwi t t e r ion ic form. As m e n t i o n e d earl ier , a dec rea se in t h e c o n c e n t r a t i o n of t he zwi t t e r ion leads to a dec rea se in rate. I t fol lows t h a t any reversa l of th i s p roces s wou ld resu l t in an a p p a r e n t inc rease in the o b s e r v e d rate. A con- ce r t ed p r o t o n t r ans f e r m e c h a n i s m has also b e e n p r o p o s e d b y Willi a n d R o b e r t s o n (29). This s c h e m e also p red ic t s t h a t a re la t ive dec rease in t he c o n c e r t e d p r o t o n sh i f t p a t h w a y w o u l d occu r at e v e n h i g h e r acidi t ies b e c a u s e t h e i n t e rme- d ia te ex i s t s m a i n l y in the p r o t o n a t e d form. T he da ta es tab- l i sh t h a t th i s t r e n d is realized. In 2M H2SO4, t h e o b s e r v e d ra te is, in fact, s l ight ly lower t h a n t he ra te in 2M HC1.

T h e ra te e q u a t i o n s (Fig. 9) m a y be so lved to e x p r e s s kobs as a f u n c t i o n of [H +] (see Append ix ) . I f [HSO4-] = [SO42-] = 0, t he so lu t ion is equa l to t he one g iven by Reeves a n d m a y be e x p r e s s e d in the fo rm

A + B[H +] + C[H+] 2 kobs = [5]

D + E[H +] + F[H+] 2

W h e n t h e [H § is h igh, on ly t he quad ra t i c t e r m s are im- p o r t a n t a n d t he ra te equa l s C/F. At low [H § the ra te re- d u c e s to t he A/D te rms . At i n t e r m e d i a t e pH, t he B[H+]/D or C[H+]2/E[H +] t e r m s fol low t he l inear region. T he pa rame- te rs c h o s e n for t he fit were b a s e d on ra t ios d e t e r m i n e d for t he l imi t ing cases. I t s h o u l d be po i n t ed ou t t h a t t h e s e ra- t ios are no t a b s o l u t e values , and, there fore , i t is no t possi- b le to eva lua t e any of t he i n d i v i d u a l ra te c o n s t a n t s (19a, 21).

W h e n t h e add i t i ona l t e r m s are a d d e d to ref lect t he be- h a v i o r in su l fur ic acid, Eq. [5] m a y b e e x p r e s s e d as

A + (B + B ' [SO4 =] + B" [HSO4-])[H +] + (C + C' [804 =] +

Vol. 136, No. 3, March 1989 �9 The Electrochemical Society, Inc

i a. B: H-(~ ~.C = -H ~ BH + H-O-C-~-H H R

b. B:~-O'O~-C-N-H --~- O=C / + RNH + BH ' ~ \

C.

o ~0 o~,~.o 0/S'O 0 0

:: ~ H H. i-i ' - - RH~ O § H

\C / RHNNc/O / \ / \

B: = S O ~ -

Fig. 11. Models for catalysis by sulfate: (a) for the formation of on intermediate species, (b) for the dissociation of the intermediate species, and (c) by HSO4-.

5 6 -

- c. 7 4 2 -

2 8 - , "

D / - b - . -"E) .C 1 4 - j . ~ " . . - a

- . _ . - . -~ : '" @- - m % s

0 - + I I I I I I I I I I I

6.8 0.3 - 0 . 2 - 0 . 7 - 1 . 2

Ha

Fig. 12. Comparison of experimental points ((9) to simulated curves for PA hydrolysis in HCh (a) curve described by Eq. [5], and using A = 0.1, B = 0.45, C = 0.4, D = 45, E = 1.3, and F = 6.5; (b) curve from simple model and using KsH+ = 1.62; and (c) curve from simple model and using KSH+ = 0.1.

s o n a b l e for t he su l fa te form. A K s m of 1.62 is i m p r o b a b l e b e c a u s e t he sal t fo rm is read i ly i so la ted f rom so lu t ions c o n t a i n i n g as l i t t le as 0.1M acid. A m o r e l ikely e x p l a n a t i o n is t h a t t he r i s ing po r t i on of t he cu rve m o r e c losely fol lows K4 (Fig. 9), t h e p r o t o n a t i o n of t he i n t e rmed ia t e , a n d t h a t

C" [SOC]2)[H+] 2 kobs --

(D + D'[HSO4 ]) + (E + E ' [HSO4-])[H § + F[H§ 2

A fit of Eq. [5] w i th the resu l t s o b t a i n e d in HC1 is s h o w n in Fig. 12. Also s h o w n in th i s f igure are cu rves r e su l t i ng f rom the s i m p l e model , a s s u m i n g KSH+ = 1.62 or 0.1. Th i s m o d e l cons ide r s on ly s teps 1, 3, a n d 5 in Fig. 9 (k s = 0), a n d ha s t he ra te e x p r e s s i o n

( [SH+] ra te = C \ [S] + [SH+]]

The ac tua l va lue for KSH+ is u n k n o w n � 9 M a c D i a r m i d et al�9 (2, 30) f o u n d t h a t t he ch lo r ide c o n t e n t d e t e r m i n e d for d ry po lyan i l i ne ch lo r ide sal ts ( p robab ly an ox id ized form) de- c r eased c o n s i d e r a b l y w h e n w a s h e d in p H = 3 (HC1) solu- t ions c o m p a r e d to pH = 1 so lu t ions ; th i s impl ies t h a t KSH+ lies s o m e w h e r e w i t h i n th i s range. DeSurv i l l e et al. (1) ob- s e r v e d two b r e a k s in the t i t r a t ion cu rve of P A sul fa te w i th NaOH. Ora ta a n d B u t t r y (31) e s t i m a t e d t he pK a va lue of P A at -0 .3 to - 0.4 a n d >1 for r e d u c e d and ox id ized forms, respec t ive ly . F r o m these , a va lue of gsH+ ~ 0.1 s e e m s rea-

[6]

t h e d e c o m p o s i t i o n of th i s fo rm is t he ra te l imi t ing s tep at t h e s e acidit ies�9 In conc lus ion , t h e s e da ta we re f o u n d to be c o n s i s t e n t w i th t he hydro lys i s m e c h a n i s m ou t l i ned in Fig. 9. F u r t h e r m o r e , t h e fit f rom the m o r e c o m p l e x r e l a t i onsh ip is supe r io r to t he fit f rom the s i m p l e mode l , i r r e spec t ive of t he va lue c h o s e n for KSH+.

I n Fig. 13, t he c u r v e de r ived f rom Eq. [6] is c o m p a r e d w i t h t he resu l t s o b t a i n e d in su l fur ic acid. Also s h o w n is t he cu rve t h a t resu l t s i f t he su l fa te t e r m s are n e g l e c t e d (Eq. [5]). Clearly, t he cu rve w h e r e t h e su l fa te t e r m s are i n c l u d e d gives a m u c h b e t t e r fit w i th t he o b s e r v e d behavior �9 We wi sh to po in t ou t t h a t a ve ry s imi la r fit for p H > 1 was ob- t a i n e d w h e n j u s t t he cata lys is b y SO42 is c o n s i d e r e d (k-7 = k8 = 0). U n d e r t he se cond i t ions , t he r ap id ly r i s ing po r t i on of t he cu rve is sh i f t ed t o w a r d a s o m e w h a t h i g h e r acidi ty, b u t sti l l wel l w i t h i n t he r a n g e i n d i c a t e d b y sca t t e r of t h e m e a s u r e m e n t s . I t was f o u n d necessa ry , howeve r , to neg l ec t t h e C' t e r m (C' = k4k6kT) in o rde r to avo id a n over-




T

v

6 0 -

4 8 -

l 36

2 4 -

1 2 -

0 -

E-3

I I I

3 pH

S //,," /./

b / /

2 , - i"

�9 ~ . . - f f " ~ / s

I i I I I I 1 I 2 1 0 - 1

Fig. 13. Comparison of experimental points (@) to simulated curves for PA hydrolysis in HzSO4: (a) neglecting sulfate terms in Eq. [6] and using A = 0.1, B = 0.45, C = 0.36, D = 45, E = 1.32, and F = 6�9 (b) including sulfate terms, A-F the same as in a, B' = 30, B" = 0.2, C' =9, C"=9, D '=2, andE' = 6 .

k~ k, B + H20 ~ A + H20 --; BQ (QI) [7]

kwl A > P [8]

kT~ B > P [9]

T h e overa l l t e r m i n a t i o n s teps [8]-[9] r e p r e s e n t t he overa l l ra te for p roces se s w h i c h c o n s u m e q u i n o n o i d s i tes b y pa th- ways o t h e r t h a n hydro lys i s in to BQ or QI.

T h e ra te e q u a t i o n s for t h e s e p roces se s are

d[B] - [B](k2 + kT2) [1O]

dt

d[A]

d t - [A](kl + kwl) + ks[B] [11]

d[BQ] - k~[A] [12]

dt

s h o o t (a re la t ive m a x i m u m ) in t he ca lcu la ted curve , t h u s i n d i c a t i n g t h a t t he role of t he H S O ( a n i o n c a n n o t b e dis- missed . I t is c o n c l u d e d t h a t the co r r ec t i ons for su l fa te ca ta lys i s p r o v i d e a s igni f icant ly i m p r o v e d fit of t h e da t a p o i n t s and , as such, t h e i nc lu s ion of t h e s e t e r m s are clear ly jus t i f ied .

T h e o p e n - c i r c u i t d e c a y w i t h t i m e . - - T h e curva tu re , s h o w n at h i g h acidi ty, in t he p lo ts of in (i/io) vs. t i m e m a y b e c a u s e d b y m a n y factors; s imp le f i rs t -order r eac t ion ki- ne t i c s is n o t fol lowed. A c h a n g e f rom first to s e c o n d reac- t i on o rde r was ru l ed ou t b e c a u s e (io/i) us vs. t i m e p lo t s we re n o t l i nea r e v e n for t he h i g h e s t acidi t ies . F rac t i ona l reac- t ion o rders we re no t tes ted . We h a v e a s s u m e d t h a t t he ob- s e r v e d cu rves are c o m p r i s e d of m i x e d e x p o n e n t i a l t e rms . R e a c t i o n types t h a t p r ed i c t th i s o u t c o m e i nc l ude para l le l or c o n s e c u t i v e first o rde r (16, 32). The ana lys i s is f u r t h e r c o m p l i c a t e d b y t he a p p a r e n t t e r m i n a t i o n s teps ; we h a v e a l r eady s h o w n t h a t t h e hyd ro lys i s r eac t i on does n o t pro- ceed to c o m p l e t i o n u p o n o p e n c i rcu i t (12). In t h e ter- m i n a t i o n steps, r e d u c e d P A w i t h s h o r t e r c h a i n l e n g t h s is p r o d u c e d u p o n hydro lys i s of ox id ized PA, p r e v e n t i n g a f u r t h e r hydro lys i s r eac t ion f rom t a k i n g place. Th i s is t he r e a s o n t h a t c o n t i n u e d ox ida t i on c u r r e n t s are obse rved , e v e n t h o u g h t he s w i t c h i n g b e t w e e n ox id ized a n d r e d u c e d P A s ta tes is ve ry fast.

W h e n in t h e ox id ized state, P A m a y b e t h o u g h t of as con- s i s t ing of a l t e rna t ing q u i n o i d / a r o m a t i c r e p e a t i n g un i t s (12, 33-36). U s i n g th i s mode l , a c o n s e c u t i v e r eac t ion is eas- ily v isua l ized . C o n s i d e r t he r eac t ion p a t h s s h o w n in Fig. 14. No te t h a t u p o n hydro lys i s of a q u i n o n o i d s i te w i t h i n t h e c h a i n (B), the hydro lys i s p r o d u c t b e c o m e s ind i s t in - g u i s h a b l e w i t h a n e n d g roup (A). Th i s m e c h a n i s m a s s u m e s t h a t t h e r e are no k ine t i c d i f f e rences b e t w e e n a q u i n o n e d i a - m i n e (QDI) or a q u i n o n e i m i n e (QI) e n d group�9 Af ter hydro lys i s f rom a n e n d group, r ing c u r r e n t w o u l d be t he s a m e for e i t h e r species . A spec ia l case ar ises w h e n R = r or �9 - NH2. T h e n t h e r eac t ion p r o d u c t is so lub le (~ is a p h e n y l group).

At o p e n c i rcu i t t he ra te of g e n e r a t i o n of q u i n o n o i d g r o u p s is zero a n d t h e overa l l r e ac t i on s c h e m e is

Chmin Group

B A

R~§ ~ . C j R W 2 R' -F N N ~ N ~ O

H / ~ H H20 ~ H / + R - - N H ~

End Group

H20 1 k~

R'\ k~ o = = ~ = = o R'--NH, + . / ~ = o ( " ) H2 o ' +

Fig. 14. A consecutive reaction scheme for PA hydrolysis

I t is n o w a s s u m e d that/Or~ < < k~ a n d kT2 < < k2. By em- p loy ing th i s app roach , t he va lues for k~ a n d k2 wil l t e n d to b e in f la ted f rom the t rue value. I t is r ecogn ized t h a t t he va- l idi ty of th i s a s s u m p t i o n m a y be ques t i oned ; none the l e s s , t h i s is t h e on ly p rac t i ca l m e t h o d w h i c h yie lds any i n s i g h t s in to t he hydro lys i s m e c h a n i s m . A c c e p t i n g this , Eq. [11] m a y b e so lved for A as a f u n c t i o n of t

ks[B]o[exp ( - k2t) - exp ( - kit)] A = + [A]o exp ( - k i t ) [13]

kl - ks

Rea l i z ing t h a t d[BQ]/d t = kl[A] ~ i a n d t h u s kl[A]o ~ io, we o b t a i n an e x p r e s s i o n

i k2[B]o [exp (-k2t) - e x p ( -k i t ) ]

io [A]o (kl - k2) + exp ( - k~ t ) [14]

w h e r e [A]o = [A] at t = 0, a n d [B]o = [B] at t = 0. The las t t e r m o n t h e r i g h t a c c o u n t s for t h e fac t t h a t [A] ~ 0 at t = 0. As- s u m i n g a n o c t a m e r m o d e l for PA, t he [B]o/[A]o rat io is no t g r ea t e r t h a n 6 ( there are two si tes on each c h a i n group).

The so lu t ion sets for t he case kl = k2 (not show-n) d id no t fo l low the o b s e r v e d resul ts . The c u r v a t u r e was c o n v e x and, in m o s t ins t ances , t h e r e was a c o n s i d e r a b l e i n d u c t i o n t i m e (i/io vs. t was flat). The case k2 > kl was also t r i ed a n d d i smis sed . Again , t h e c u r v a t u r e was go ing t h e w r o n g way and, in m a n y cases, t h e r e was a r ise in t he c u r r e n t a f te r o p e n circuit . This s i tua t ion was c o n s i d e r e d unl ike ly . I f k2 was m u c h g rea te r t h a n k~, c o m p a r a t i v e l y large a m o u n t s of ox id izab le ma te r i a l w o u l d b e e x p e c t e d at t h e r ing. Th i s was n o t seen, as was d i s c u s s e d earlier.

The case w h e r e kl > k2 p rov ides a good fit w i t h t he ob- s e r v e d behav ior . In Fig. 15 a se t of da t a po in t s o b t a i n e d in 1.5M HC1 are p lo t t ed a long w i t h t he r e su l t i ng cu rves w h e r e kl, k2, a n d [B]o/[A]o were c h a n g e d sys temat ica l ly . As ca n b e seen, t he fit to t he da ta po in t s is qu i t e s ens i t i ve to a c h a n g e in any of t he pa rame te r s . The effects i n d u c e d b y k~ are qu i t e u n l i k e t he m o r e fami l ia r s i tua t ion w h e r e [A]o = 0 at t = 0 (16). The fas t decay t e r m is sti l l r e t a i n e d b e c a u s e Eq. [14] r e d u c e s to a n e x p o n e n t i a l s u m w h e n kl > > k2.

A fit ove r t h e en t i r e ac id i ty r a n g e in HC1 is g iven in Fig. 16. T h e r e is good a g r e e m e n t b e t w e e n t he da ta po in t s a n d t h e theo re t i ca l curve. I t c an be conc luded , the re fore , t h a t t h e r e su l t s are c o n s i s t e n t w i t h a c o n s e c u t i v e r eac t ion m e c h a n i s m . T h e va lues f rom the s i m u l a t e d cu rves are h i g h e r t h a n t he e x p e r i m e n t a l va lues t a k e n f rom t h e slopes. A c o m p a r i s o n for t he da ta s h o w n in Fig�9 16 is l i s ted in T a b l e II.

A m a j o r diff icul ty t h a t p r e c l u d e s any a b s o l u t e compar i - sons of t h e P A hydro lys i s da ta w i th t h e t heo re t i ca l va lues o b t a i n e d f rom the cu rves s h o w n a b o v e lies in ou r lack of k n o w l e d g e of t he ac tua l [B]o/[A]o ratio. The va lues for kl a n d k2 o b t a i n e d f rom the s i m u l a t e d cu rves are s t rong ly de- p e n d e n t on t he [B]J[A]o ratio. Fo r an e q u i v a l e n t fit to t he



6 9 6

021 o_o~5 k 2 = 0 . 0 1 - 0 . 3

0.025 Ao/Bo ~ 2. 1 - 0 . 8

- 1 . 3 0 .~3

- 1 . 8 o . o65

i , i , i i i , i i J

�9 ~ - 0 . 3 0 .07 kl= 0 . 0 6 5

0 8 Ao/Bo = 2. i

, ~ - 1 . 3 0 , 0 2

- 1 .~ o.ol

- ~ B~ 0 .38 - o _ 15 k l = 0 . 0 5 5

- 0 . 16 - lo

- 8 k2= 0 .01 - 0 . 70 - 6

4 - 1 . 24 - -

3

- 1 . 78 -

, , i J = , J , , i ,

40 80 T ime , see

Fig. 15. The effect of changing: kl (a}; k2 (b); and BJAo (c) on the curve described by Eq. [14] (+) are data paints taken in 1.5M HCI.

data, a n inc rease in t he ra t io causes a n inc rease in k~ a n d k2. In th i s respec t , t h e va lues o b t a i n e d f rom t h e s i m u l a t e d c u r v e are s o m e w h a t arb i t rary . T he [B]o/[A]o ra t io for t he oc- t a m e r is 6/1. W h e n one cha in g r o u p is h y d r o l y z e d t he rat io d r o p s to 4/1, and so on. Reca l l t h a t t he c i rcui t cou ld no t be o p e n e d un t i l s o m e hydro lys i s h a d occur red . As m e n t i o n e d above , t he k~ a n d k2 va lues de r ived f rom t he s i m u l a t e d c u r v e s are qu i t e sens i t ive to any c h a n g e s in [B]o/[A]o ratio. W h e n f i t t ing t he da ta t a k e n f rom 1.5M HC1, a n d u s i n g [B]o/[A]o = 2.1, kl/k2 = 6.5, b u t b y u s i n g [B]o/[A]o = 3, a n d for an e q u i v a l e n t fit, k J k 2 m u s t r ise to 12.

To a first a p p r o x i m a t i o n , one w o u l d e x p e c t no d i f f e rence b e t w e e n kl a n d k2 as b o t h s i tes are ad j acen t to a p h e n y l group. Th i s d i s c r e p a n c y o b s e r v e d in our s t u d y m a y b e d u e to s ter ic h i n d r a n c e effects on t he i n t e rna l c h a i n groups . T h e t h r e e - d i m e n s i o n a l s t r u c t u r e of t he fi lm is m o r e r ig id in t h e ox id ized fo rm due to t h e c h a n g e in g e o m e t r y f rom tet- r a h e d r a l to p l a n a r c a u s e d b y f o r m a t i o n of t he c a r b o n ni t rogen d o u b l e bond . I t is e x p e c t e d t h a t t h e i n t e rna l c h a i n g r o u p s w o u l d b e h a v e m o r e as an i n s o l u b l e sol id sal t ion- ical ly b o n d e d w i th the coun te r ion . On t he o the r h a n d , t he tai l g r o u p s are in d i r ec t c o n t a c t w i th so lu t ion . T h e first s tep in t he hydro lys i s r eac t ion r equ i r e s a p e r p e n d i c u l a r a p p r o a c h by t he n u c l e o p h i l e (20).

C o n s t r a i n t s in t he e x p e r i m e n t a l p r o c e d u r e s m a y also ac- c o u n t for some of t he h i n d r a n c e effects. U p o n ox ida t i on of t he film, t h e r e is some t i m e d u r i n g w h i c h a fair n u m b e r of i n t e rna l c h a i n groups , w h i c h are in t he c loses t p r o x i m i t y w i t h t h e so lu t ion , cou ld be h y d r o l y z e d to e n d g r o u p s be -

0 - ~ ' § - -~- [HCl] M

- - - ~ , "]"H-= - ~ ' ~ - - - 4 - - ' - -

~ f " " - 4~ o. 2 u162 " --e__~ +

.~ _ + .... --#_ - - - ~ _

~ 1 . 5 - ~- o .5 e - : e - - . _+_

2 . 0 - q=":e~_ ] . 5

2 .5 - ---+ 2.5 1 1 1 l - - i ] 1 1 l I

0 20 40 60 80 100 T i m e , sec.

Fig. 16. Comparison of data points (+) to simulated curves for PA hydrolysis in HCI.

J. Electrochem. Sac., Vol. 136, No. 3, March 1989 �9 The Electrochemical Society, Inc.

Table II. Comparison of rate constants (s 1) obtained experimentally and from simulation

From slope From simulation [H+], M kl k2 kl k2 [B]o/[A]o

0.2 0.007 - - 0.01 0.0008 3 0.5 0.016 - - 0.02 0.0008 3 1.5 0.049 0.015 0.065 0.010 2.1 2.5 0.064 0.016 0.10 0.015 2.1

fore t he c i rcui t is opened ; thus , a t o p e n c i rcu i t on ly t h o s e s i tes t h a t are re la t ive ly h i n d e r e d m a y be m e a s u r e d . Th i s is c o n s i s t e n t w i t h t he fact t h a t t he o b s e r v e d ra te c o n s t a n t de- c reases w i th cycle n u m b e r .

A n u n d e r l y i n g a s s u m p t i o n o f t he c o n s e c u t i v e r eac t ion m e c h a n i s m is t h a t t h e r e is l i t t le or no d i f f e rence in t he hydro lys i s ra te b e t w e e n q u i n o n e d i i m i n e a n d q u i n o n e i m i n e e n d groups . T h e r e is a sol id bas i s for m a k i n g th i s a s s u m p - t ion. A d a m ' s g roup (37) cou ld de t ec t no d i f f e rence in t h e o b s e r v e d ra te of hydro lys i s b e t w e e n p h e n y l e n e d i a m i n e (PDA) a n d p - a m i n o p h e n o l (PAP) to b e n z o q u i n o n e at acidi- t ies w h e r e b o t h t h e n i t r o g e n g roups on P D A were p ro ton- ated. Tong (38) f o u n d t h a t t h e ra te of hydro lys i s of qui- n o n e d i i m i n e to q u i n o n e i m i n e was d o u b l e t he hydro lys i s ra te of q u i n o n e i m i n e to b e n z o q u i n o n e in a lka l ine solu- t ions . Th i s is to be e x p e c t e d in t h a t t h e r e are two n i t r o g e n s in q u i n o n e d i i m i n e . At i n t e r m e d i a t e p H ranges , k, was 6-10 t i m e s g rea te r t h a n k~ for hydro lys i s of q u i n o n e d i i m i n e . Th i s was a t t r i b u t e d to the d i f fe rences in t he pK~'s on t he n i t r o g e n g r o u p s (pK~t = 2, pKa2 = 6). Thus , i f t he c u r v a t u r e was due to a d i f fe rence in t he hydro lys i s ra te b e t w e e n qui- n o n e d i i m i n e a n d q u i n o n e i m i n e e n d groups , i t w o u l d be n o t e d in t he da ta w i t h i nc rea s ing pH. This is qu i t e t he op- pos i t e of w h a t was o b s e r v e d a n d leads us to c o n c l u d e t h a t t he hydro lys i s ra tes of t h e s e two e n d g r o u p s are ind i s t in - gu i shab le .

T h e r a n g e of va lues w h i c h was a s s igned to reflect hy- d ro lys i s of t he e n d g roups , kl, c o m p a r e s r e a s o n a b l y wel l for t h e a n a l o g o u s so lu t ions species . L e e d y a n d A d a m s (39) d e t e r m i n e d t h a t k = 0.029 s i for the hydro lys i s r eac t i on of t h e ox id ized fo rm of N - p h e n y l - p - a m i n o p h e n o l in 1M HC104. Fo r so lubi l i ty r easons t h e y u sed a so lu t ion t h a t was 50% ace tone . D e t e r m i n a t i o n s at pH 0.5 and 1 r e su l t ed in k = 0.037 a n d k = 0.010 s- ' , respec t ive ly . Fo r acidi t ies Ho -< 0, ou r va lues for P A differ c o n s i d e r a b l y f rom the ones t a k e n f rom A d a m s (37) w h e r e k falls f r om 0.029 s -1 to -0 .002, f rom Ho = 0 to Ho = -1 . The bel l s h a p e t h e y ob- s e r v e d cou ld b e due to t he so lven t effects a n d t he lower c o n c e n t r a t i o n of w a t e r u s e d in t he i r s tud ie s (18, 21), b u t o t h e r w i s e th i s d i s c r e p a n c y r e m a i n s unc lear .

The t h i c k n e s s e f f e c t - - T h e dec rease in t h e o b s e r v e d hy- d ro lys i s ra te w i th i nc rea s ing t h i c k n e s s m a y be due to s t r u c t u r a l changes . In our ear l ier s t u d y (40), t he fi lm g r o w t h was s h o w n to b e p r o p o r t i o n a l to (thickness)-t/2; th i s was e x p l a i n e d in t e r m s of a r eac t ion zone or pene t r a - t ion d e p t h in to the film. This s a m e effect m a y also app ly to P A hydro lys i s reac t ions . Car l in et al. (41) n o t e d a t r a n s i t i o n f rom a re la t ive ly d e n s e po lymer i c s t r u c t u r e to one of loose ly p a c k e d f iber- l ike s t r u c t u r e s w i t h fi lm th i cknes s . Th i s p h e n o m e n o n cou ld o p e n c h a n n e l s in t he fi lm s t ructure , t h e r e b y inc reas ing the access ib i l i ty of t he so lu t ion to i n t e r n a l c h a i n groups . S u c h a m o d e l p red ic t s a n i n c r e a s e in t h e [B]o/[A]o ra t io w i th t h i cknes s . I n Fig. 15c i t was s h o w n t h a t an inc rease in t he [B]o/[A]o ratio, a t c o n s t a n t kl a n d k2, wil l dec rease t he m e a s u r e d or o b s e r v e d ra te con- s tants . We see t h a t t he t h i c k n e s s da ta fits th i s mode l , a n d t h e r e f o r e i t can be c o n c l u d e d t h a t an inc rease in t he [B]o/[A]o ra t io w i th t h i c k n e s s is c o n s i s t e n t w i t h t he ob- s e r v e d behav ior .

A n a l t e rna t ive a p p r o a c h is to a s s u m e t h a t the o b s e r v e d r ing c u r r e n t fol lows t he d i f fus ion of t he B Q p r o d u c t t h r o u g h t h e fi lm or t h e d i f fus ion of t he r e a c t a n t (H20) in to t h e f i lm f rom the solu t ion/ f i lm in te r face for t h e n o r m a l i z e d r ing c u r r e n t w i t h t i m e (42)

i/io ~ 0.5 [exp ( - A t ) + exp ( -9At ) ] [15]

w h e r e A = D~2/4h 2, h = film th i cknes s , a n d D = t he diffu- s ion coeff ic ient for BQ t h r o u g h t he film.



d. Electrochem. Soc., Vol. 136, No. 3, March 1989 �9 The Electrochemical Society, Inc. 697

U p o n analysis , however , Eq. [15] does no t fit t h e ob- s e r v e d behav io r . At longer t i m e s (>40s) t he s e c o n d t e r m is negl ig ib le , a n d A = k2. F r o m this , an a p p r o x i m a t e va lue for D : 3 • 10 -~2 cm2/s, was o b t a i n e d f rom t h e s lope of k2 vs. ( thickness)% With D in h a n d , cu rves of Eq. [15] we re ob- t a i n e d for t he t h i c k n e s s r a n g e of 0.3-1.0 ~m. F r o m t h e s e curves , t he m e a s u r e d s lopes in t h e ini t ia l fas t decay (k~) r a n g e d f rom 0.0035 to 0.035 s- ' . Th i s r a n g e was m u c h la rger t h a n t h e o b s e r v e d r ange of 0.02-0.035 s- ' . We the re fo re con- c l u d e t h a t d i f fus ion of p r o d u c t is no t a l imi t ing step. This c o n c l u s i o n is c o n s i s t e n t w i t h t he p H a n d sul fa te effects. I f p r o d u c t d i f fus ion was t he l imi t ing s tep, t h e n it can be ex- p e c t e d t h a t t h e s e o the r func t iona l i t i e s w o u l d no t b e man i - fest. Moreover , O y a m a et al. (43) h a v e s h o w n t h a t l a rger molecu les , s u c h as fe r ro / fe r r icyanide , are no t p e r m e a b l e in to P A films. On the o t h e r h a n d , i t m a y be t h a t p r o d u c t or r e a c t a n t d i f fus ion exe r t s a less t h a n l imi t ing effect on t he resul ts . I t was no t poss ib l e to eva lua t e i n t e r m e d i a t e cases.

M a n u s c r i p t s u b m i t t e d Feb. 1, 1988; r ev i sed m a n u s c r i p t r ece ived May 9, 1988.

APPENDIX The derivation of the expression for the observed rate as

a function of the [H +] is based on the rate equations given in Fig. 14. With the exception of the added sulfate terms, this derivation closely follows the work by Reeves (21).

First, the equilibrium and related expressions are listed

S + SH § : ST [A.1]

[H+][S] KsH+ - [A.2]

[SH § ]

[SO4~-][H § K~ - [A.3]

[HSO4-]

[SH2OH +] K 4 -- [A.4]

[SHOH][H +]

By c o m b i n i n g [A.1] a n d [A.2]

[H § ] [SH § = ST [A.5]

[H +] + KSH+

T h e ra te of d i s a p p e a r a n c e of hydro lys i s s i tes in to p r o d u c t s is

- d S w

dt = ks[SHOH] + k6[SH2OH +] + k s [ SH O H ] [ H SO ( ] [A.6]

or, b y use of [A.4]

-dSw - [SHOH](k5 + K4k6[H § + ks[HSO4-] [A.7]

dt

Now, t he ra te of f o r m a t i o n of S H O H is

d[SHOH] - k-2[S] - k2[SHOH] + k3[SH +] - k 3[SHOH][H +]

dt

+ kT[SH+][SO(] - k 7[SHOH][HSO4-] -k s [SHOH]

- K4k6[H+][SHOH] - ks[SHOH][HSO,-]

U s i n g t he s t eady-s ta te a p p r o x i m a t i o n , d[SHOH]/dt = 0, we o b t a i n

k2[S] + k3[SH +] + kT[SH+][SO4 =] [SHOH] -

(k-2 + k~) + (k 3 + K4k6)[H +] + (k-v + ks ) [HSO(]

[A.9]

T h e ra te in t e r m s of [SH § can n o w b e o b t a i n e d by c o m b i n - ing [A.9] w i t h [A.7], a n d af te r s o m e a lgeb ra a n d ut i l iz ing [A.2]-[A.4], we o b t a i n

or, to s h o r t e n t he e x p r e s s i o n

-dSw - [SH+](a + b) [A.11]

dt

Now t h e fo rward ra te is

- d S T dQ - - k f ( [ S ] + [ S H + ] ) = k f [ S T ] [ A . 1 2 ]

dt dt and, f rom [A.11] a n d [A.5]

- d S T [H § - [SH+](a + b) = [ST] (a + b)

dt [H +] + KsH+ = kf[ST] [A.13]

Therefore , t he o b s e r v e d fo rward ra te is

[H + ] kf - (a + b) [A.14]

[H +] + KSH+

After mu l t i p l i c a t i on a n d co l lec t ion of t e r m s

A + (B + B' [SO4-] + B" [HSO4-])[H ~] + (C + C' [SO4-] + C" [SO(]2)[H+] 2 kr

w h e r e

A

B

B '

B "

C

C'

C"

D

D'

E

E '

D + D' [HSO,-] + (E + E' [HSO(])[H ~] + F[H'] 2

[A.15]

: k2ksKss+

= k3k5 + K4KsH ~- k2k6

= k5k7

= KSH+ + k2k8 + kak8

= K4k3k6

= K4k6kv

= kvks/Ka

= KSH+ + (k-2 + ks)

= KsH+ + (k-7 + ks)

: KSH+ + (k_a + K4k6)(k-2 + ks)

= k-7 + ks

F = k-3 + K4k6

W h e n the su l fa te c o n c e n t r a t i o n is zero, Eq. [A.15] r e d u c e s to t he e q u i v a l e n t e x p r e s s i o n p u b l i s h e d b y Reeves (21).

R E F E R E N C E S 1. R. DeSurvi l le , M. Josefowi tz , L. T. Lu, J. P e r i c h o n , a n d

R. Buve t , Electrochim. Acta, 13, 1451 (1968). 2. A. G. MacDia rmid , J. C. Chiang , M. Ha lpe rn , W.S.

Huang , J. R. Krawczyk, R. J. M a m m o n e , S. L. Mu, N. L. Somasi r i , a n d W. Wu, Polym. Prepr. Am. Chem. Soc., Div. Polym. Chem., 2, 248 (1984).

3. A. Ki tani , M. Kaya, a n d K. Sasaki , This Journal, 133, 1069 (1986).

4. T. Kobayash i , H. Y o n e y a m a , a n d H. Tamura , J. Electro- anal. Chem., 161, 419 (1984).

5. E. W. Paul , A. J. Ricco, a n d M. S. Wr igh ton , J. Phys. Chem., 89, 1441 (1985).

6. G. Mengol i , M. M. Musian i , B. Pelli , a n d E. Vecchi , J. Appl . Polym. Sci., 28, 1125 (1983).

7. R. Noufi, A. J. Nozik, J. White, a n d L. F. Warren , This Journal, 129, 2261 (1982).

8. S. H. G l a r u m a n d J. H. Marshal l , This Journal, 134, 2160 (1987).

9. T. Kobayash i , H. Yoneyama , a n d H. Tamura , J. Electro- anal. Chem., 177, 293 (1984).

10. D. E. S t i lwel l a n d S.-M. Park , in " C o r r o s i o n P r o t e c t i o n b y Organ ic Coat ings ," Vol. 87-2, M. W. K e n d i g a n d H. Le idhe i se r , Jr. , Edi tors , p. 330, The E l ec t rochemi - cal Soc ie ty S o f t b o u n d P r o c e e d i n g s Series, P e n n - ing ton , N J (1987).

-dST [ (k~k~Ks. +/[H']) + k.~ks + K4K .... k2ke. + K4k3k,[H~l [SO4~](k~k7 + K4k,,kT[H~]) + [HSO,-](Ks,, + k2k, + k3k~ + kTk,,[SO,=]) ] dt - [SH+] | . . . . . . . . . . . . {- . . . . . . . . . . . . . J k (k-2 + ks) + (k-3 + K, k6)[H ~] + (k-T+ ks)[HSO4-] (k-2 + k.~) + (k-3 + K*k~)[H ~] + (k-7 + ks)[HSO(]



698 J. Electrochem. Soc., Vol.

11. D. E. Stilwell and S.-M. Park, This Journal, 135, 2497 (1988).

12. D. E. Stilwell and S.-M. Park, ibid., 135, 2491 (1988). 13. W. J. Albery and M. L. Hitchman, "Ring-Disc Elec-

trodes," Clarendon Press, Oxford, England (1971). 14. M. A. Paul and F. A. Long, Chem. Rev., 57, 1 (1957). 15. R. C. Cox, Acc. Chem. Res., 20, 27 (1987). 16. L. P. Hammett, "Physical Organic Chemistry," 2rid

ed., McGraw-Hill, New York (1970). 17. H. A. Laitinen and W. E. Harris, "Chemical Analysis,"

2nd ed., McGraw-Hill, Inc., New York (1975). 18. J. F. Bunnett , J. Am. Chem. Soc., 83, 4956 (1961). 19. (a) A. Bruylants and E. Feymants-de Medicis, in "The

Chemistry of Functional Groups: The Chemistry of the Carbon-Nitrogen Double Bond," Patai, Editor, Chap. 10, Interscience Publishers, Inc., New York (1970); (b) K. T. Finley and L. K. J. Tong, ibid., Chap. 14.

20. W. P. Jencks, in "Progress in Physical Organic Chem- istry," Vol. 2, Cohen, et aI., Editors, Interscience Publishers, Inc., New York (1964).

21. R. L. Reeves, J. Am. Chem. Soc., 84, 3332 (1962). 22. E. H. Cordes and W. P. Jencks, J. Am. Chem. Soc., 84,

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88, 3963 (1966). 25. L. do Amaral, W. A. Sandstrom, and E.H. Cordes,

ibid., 88, 2225 (1966). 26. K. Koehler, W. Sandstrom, and E. H. Cordes, ibid., 86,

2413 (1964). 27. E. H. Cordes and W. P. Jencks, ibid., 85, 2843 (1963). 28. B. A. Cunningham and G. L. Schmir, ibid., 88, 551

(1966).

136, No. 3, March 1989 �9 The Electrochemical Society, Inc.

29. A. V. Willi and R. E. Robertson, Can J. Chem., 31, 361 (1953).

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Conductivity and Anisotropy of Electrochemically Prepared Conducting Polypyrrole Films

Bianting Sun, 1 J. J. Jones, R. P. Burford, and M. Skyllas-Kazacos* School of Chemical Engineering and Industrial Chemistry, University of New South Wales, Kensington,

N.S.W. 2033, Austral ia

ABSTRACT

Thick, freestanding, flexible films of polypyrrole have been prepared from pyrrole monomer in propylene carbonate solvent containing tetra ethyl ammonium p-toluene sulfonate electrolyte. Films were electrodeposited onto t i tanium sub- strates at temperatures from -40 ~ to 25~ and were found not only to have high conductivity, but were also anisotropic. The electrodeposition temperature, electrode dimensions, current density, voltage, and solvent were found to affect the properties of the polymer. The values of conductivity of the polypyrrole films increased with decreasing electrodeposition temperature and increasing current density. For a 2 cm • 5 cm substrate, the highest conductivity value, 514 S/cm, was obtained with samples prepared at a temperature of - 20~ and a current density of 3.0 mA/cm z. The ratio of conductivities measured along (a~ong) and across (r ..... ) the surface of the samples increased with decreasing electrodeposition temperature and current density, but was strongly dependent on the electrode geometry. A conductivity of 996 S/cm and a ~ong/ aacro~ ratio of 3.3 was obtained for a film deposited on a long narrow substrate, compared with a ratio of 1.2 for a square substrate. Films produced in propylene carbonate solvent possessed better properties than when either acetonitrile or mixtures of propylene carbonate and acetonitrile were employed as solvents.

There has been much interest in the area of conductive polymers over the last decade, and many varieties have been synthesized. Polypyrroles have been widely studied, as they have high conductivity and also good air stability.

The properties of conducting polypyrrole films are in- fluenced by a large number of factors. These include the nature and amount of the anion incorporated in the structure (1-3), pH of polymerization solution (4), current density (4, 5), deposition potential (6), content of water in the solution (7), and deposition temperature (4, 6, 8-11).

The conductivity of the polymer material is one of the important properties for most electrochemical applica- tions. The conductivity of polypyrrole films has generally

* Electrochemical Society Active Member. Permanent address: Department of Chemistry, Hebei Teacher's

University, Shijiazhuang, China.

been reported to lie within the range of 10-300 S/cm (12-16), although values up to 600 S/cm have also been reported (17).

The effect of deposition temperature for polypyrrole was found to be an important variable in producing conducting films. As has been reported by other workers (4, 6, 9-11) reduction of polymerization temperature substantially im- proves electrical conductivity. Most previously published data have been restricted to temperatures above 0~ although Ogasawara et al. (8) have reported that when the polymerization temperature was lowered from 20 ~ to -20~ the conductivity increased from 97 to 287 S/cm. In another study (18), the maximum conductivity for films produced at room temperature was 279 S/cm, but this rose to 340 S/cm when the deposition temperature was lowered to 0~



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