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Mechanism of oxidation of organic molecules at a polypyrrole modified electrode using laser interferometry
J. Electroanal. Chem., 160 (1984) 377--384
377
Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
Preliminary note MECHANISM OF OXIDATION OF O R G A N I C MOLECULES AT A P O L Y P Y R R O L E MODIFIED E L E C T R O D E USING L A S E R INTERFEROMETRY
K.S.V. SANTHANAM
Tara Institute of Fundamental Research, Bombay 400005 (India) R.N. O'BRIEN
Department of Chemistry, University of Victoria, P.O. Box 1700, Victoria, B.C., V8W 2Y2 (Canada) (Received 21st September 1983; in revised form 4th November 1983)
With the development of electronically conducting polymers such as polypyrrole, polythiophene, polyindole, polyfuran, polyazulene and polycarbazole [1--4] as electrode materials for electrochemical oxidations of organics, a basic question concerning the mechanism is still to be decided which is: does the oxidation of a suitable organic molecule occur at the surface of a polymer film coating an electrode or does at least some of it penetrate into the film before oxidation? The diffusion layer growth in the t w o situations during electrolysis would be governed by at least three kinetic elements: (a) the diffusion rate of the organic molecule from the bulk of the solution to the polymer surface, (b) the rate of its diffusion within the polymer film, and (c) the rate of the charge transfer process. These rates are conveniently expressed as flux values, however the actual flux values cannot be independent of one another. In fact, the three actual flux values must equal one another and this means that the apparent thickness of the substrate diffusion gradients m a y be less than the actual total film thickness. While this situation would exist if the oxidation occurs after penetration into the film, the diffusion gradients in the other case, where the organic molecule undergoes oxidation at the outer edge of the film (non-porous in nature) would follow the normally expected diffusion gradients, i.e. follow Sand's equation [ 5] which was derived on the basis of Fick's laws of diffusion. Laser interferometry has been unequivocally applied to the situations of this t y p e where:
AC(x,t) =
2I nF
~
ierfc
x 2x/-D-~
(1)
where I is the current density, n the charge transferred, F Faraday's constant, D the diffusion coefficient of the molecule, and x the distance from the electrode to the solution. This work was begun to explore the possibility of utilising laser interferometry to decide between the t w o mentioned mechanisms.
378 EXPERIMENTAL The laser interferometric experimental details have been described in previous publications [ 6 - - 8 ] . The electrodes were prepared by coating a transparent optical semicircular glass fiat with gold by vacuum evaporation. The edge surface only was gold coated, leaving the sides non-conducting. Teflon spacers separated the t w o electrodes in the teflon cell. The electrical contacts to the electrodes were made through side ports with a push contact to the gold covered edge away from the electrolyte. HP Grade acetonitrile which was obtained from Matheson--Coleman was used as received. Tetra-n-butylammonium perchlorate was obtained from South Western Analytical Chemicals, Austin and was used after drying for more than 120 h. Pyrrole was obtained from Riedel and was purified b y distillation. The fraction boiling at 132°C was used in the experiments. The electrolysis was carried o u t using a constant current source (Keithley) at different current densities (cd) ranging from 0.10 mA cm -2 to 5.0 mA cm -2 . The voltage impressed across the cell was measured using a Keithley Digital voltmeter. The duration of the electrolysis ranged from 3 to 15 min (maximum charge 2.79 C). A He--Ne Laser (1 mW) was used in the interferometric measurements. The dielectric coated flats were separated by the thickness of the electrodes (0.58 cm) while the electrodes themselves were separated by 6.60 mm. The fringes were arranged perpendicular to the electrodes by suitably tightening the cell clamps. The concentration perturbed fringes were recorded at intervals with a 35 mm Nikon camera. The experimental runs were videotaped through the Nikon's view finder. RESULTS AND DISCUSSION The most likely models for the electrolysis of organic molecules at p o l y m e r film electrodes are shown in Fig. 1. Scheme 1 considers one electrode consisting of a non-porous polymer film and the other as a conducting metallic electrode (Au). The polymer film thickness has been drawn thicker than the real value (~ 1 pm) in order to show the concentration gradients effectively. The growth of the diffusion layers at the t w o electrodes would be approximately symmetrical during electrolysis with t w o metal electrodes such as Cu, or Zn. The concentration gradients at the cathode and anode during an electrolysis can be described by eqn. (1). These gradients would appear as refractive index gradients at the two electrodes, which are the interferometrically measarable quantities [6,9]. The growth of the diffusion layers would be limited if convection were present in the cell or by chemical reactions (either EC or CE) which change the refractive index gradients. In the absence of these factors Scheme 1 m a y also be described as the refractive index gradients in the electrochemical cell. Assuming the p o l y m e r film is porous and the electrochemical reaction occurs both on the surface and within the p o l y m e r film, several situations can exist, b u t they will all begin with the diffusion of the organic molecule (plus the solvent and the supporting electrolyte) into the p o l y m e r and
379
W
A
SCHEME I I ÷
/
C
S
'~C4Hg)4N
NON
POROUS
/-
,
clo,; ..,i.--.---
BULK H2Q
(C41"~4N
c,oC/
Ill[SOLUTION
J
I
i i
POLYMER_FILM DIFFUSION LAYER
Dp~ Dsoln
+
ic4~4N
POROUS
I
,,°,IV
(C 4 H9)4N.__.~
CIO4 q
/1"
D IFFUSION LAYER
SCHEME 2 A
/
\
H2 Q
/// /
POLYMER FILM DIFFUSION LAYER Dp<< Dsoln
DIFF N\ LAYER Ior Dp>> Dsoln (2)
II[
C 104
C
I H2 Q
u i
/'/
I E
{ C4 ~ H9)4N ~ / / / /
\ \
\ \ \ \ \ \ \
POLYMER FILM FIGURE
t
Fig. 1. Models for the electrolysis at a non-porous polymer electrode (Scheme 1) and a porous polymer electrode (Scheme 2). (W) Working; (A) auxiliary. establishment of an equilibrium between the species in the polymeric layer with the species in the solution. This situation was considered in detail in connection with electrocatalysis [ 1 0 ] . The permeation of an organic molecule into the polymer can be described by the product of the rate at which an organic molecule diffuses and the partition coefficient governing the crossing of the electrolyte polymer interface by the organic molecule. U p o n reaching the limiting threshold potential for the oxidation of the organic molecule, its oxidation within the polymer layer as well as on the film edge (surface) should occur (assuming a highly conducting thin polymer film). The continued oxidation would be controlled by the flux of the material, both on the surface and within the polymer. Selected cases of this type are shown in Scheme 2 of Fig. 1. The electrochemical experiments were conducted with 0.1 M tetra-n-butyla m m o n i u m perchlorate in anhydrous acetonitrile under galvanostatic conditions. The specific aim was understanding the growth of the diffusion layers at the polymer anode and cathode. With polypyrrole coated gold as anode and a gold electrode as the cathode, experiments were performed in the vertical (V)
380
position, anode over cathode (A/C) and cathode over anode (C/A) positions at different cd's. The electrolysis of quaternary a m m o n i u m salts at metal electrodes has previously been investigated [ 11 ], and the anodic reaction was postulated as oxidation of perchlorate ion to a radical which subsequently regenerates (EC) through a catalytic reaction [12]. At the cathode tetra-nb u t y l a m m o n i u m ion is reduced to a radical which subsequently cleaves to produce tri-butylamine [ 11 ]. These reactions were investigated using laser interferometry by using a polypyrrole anode and gold as the cathode and vice versa, allowing comparison of growth of the diffusion layer for each reaction at each type of electrode. Using gold as both anode and cathode completes the set of experiments to allow diffusion layer comparisons. POLYMER ANODE AND Au CATHODE
The development of a diffusion layer began at the two electrodes from the start of the electrolysis in all of the configurations. Figure 2 shows a typical fringe pattern at 0.10, 0.50 and 1.00 m A cm -2 in the A/C position. At the lower cd's, an unequal diffusion layer growth was observed, i.e., the cathodic diffusion layer was of greater extent than that of the anode. An analysis of the interferograms yielded graphs showing a concentration--distance profile as indicated in Fig. 3. At a higher cd (1.00 m A cm -2 ), the growth of the layer at the polymer anode was better defined at different times in the recorded frames. At longer times (>240 s) the onset of natural convective flow disturbs the entire fringe system making measurements on the fringes unreliable. This convective regime has not been examined further as the study of these frames does not provide information on diffusion layer growth. The cause of the poorly defined fringes in the anodic region m a y be due to instability of the oxidized product, i.e. the perchlorate ion through a cyclic reactions [12]. At a higher cd, the rate of oxidation of the ion is faster than the chemical reaction following it with the result that the net refractive index gradient can be observed. The fringe shift at the anode was considerably less than the shift observed at the cathode for this reason, and the trends in the build up of refractive index gradient at different cd's are shown in Fig. 4. The extent of the depleted regions at the polypyrrole anode and at the gold
C
i~!~c ~ :~ ~,, t80S
A JO :0"t3 m A c ~ 2 Ca)
t808
tO :0-33 mAcnlz (b)
180S
A i 0 ;0.66rn Acr62 {c}
Fig. 2. Interference fringes at the polymer anode and gold cathode during electrolysis o f 0.1 M (C~Hg)4NC104 in C H 3 C N . (a) 0 . 1 3 m A c m -2 , 1 8 0 s; (b) 0 . 3 3 m A c m -2 , 1 8 0 s; (c) 0 . 6 6 m A c m -2 , 1 8 0 s.
381 1"2 Lj1.01~x j ~
x axis l c m 0 . 6 6 m m t 120s ~ • i 0.33mA cm-"
• •
Au c a t h o d e P anode
0.8 AC 0.6
0.4 0.2 0.1
0.2
I
0.3
0.4
0.5
I
I
0.6
0.7
x//crrl
Fig. 3. Plot of ACVS. distance during electrolysis of 0.1 M(C4H9)4NC104in CH3CNat a cd of 0.33 mAcm-2 at t = 120 s. ConfigurationA/C.
30t 0 . 1~0F"2
t : 180s • Au c a t h o d e
~~
0
d
e
I
0.3
I
0.6
0.9 x/rnm
1.2
I
1.8
Fig. 4. Fringe shift (F) variation with current density during electrolysis of 0.1 M (C4Hg)4NC10` in CH3CNat the electrode surfaces. The electrodes were positioned as A/C. cathode for a selected cd of 0.33 m A cm -2 is shown in Fig. 3. Thus the depleted region at the cathode was always greater than at the anode until, at a high enough cd ( > 0 . 6 6 m A cm -2 ), significant convection sets in, which results in distortion of the fringes. The above results are compared with polypyrrole as cathode, and gold electrode as anode, in the electrolysis of the supporting electrolyte. The growth of the diffusion layer at the polypyrrole electrode can be observed in the interferometric recordings. For comparison the fringe shifts which were observed when gold was used as a cathode is also shown in Fig. 5. The diffusion layer at the polymer cathode was again found to be smaller than was observed at the gold electrode. Thus the results obtained in these experiments suggest the growth of the diffusion layer at the polypyrrole electrodes is hampered by a mechanism other than the normal reduction of the quaternary a m m o n i u m ion as was observed with a gold electrode. Also the fringe shifts which were observed at a polymer electrode were smaller in the above reduction.
382
l 2.5
x t • •
0.0 crn 180s A/C Au cathode P (]node
2.0 F 1.5
1.0
0.5
0
0.2
I
0.4
I
0.6 j / m A cm -2
I
0.8
I
1.0
Fig. 5. Fringe shift (F) vs. the distance from the electrode surface in the electrolysis of 0.1 M (C,H 9)4NC104 in the A/C configuration.
It is appropriate to discuss here the expected results on the basis of the model of Scheme 1 in Fig. 1. Assuming the polymer electrode has the same properties as a gold electrode, the expectation of the fringe shift--distance profile should be identical at b o t h electrodes except for the reasons discussed earlier. The absence of such an agreement suggests that either the p o l y m e r resistance reduces the effective cd on the electrode (which would cause a general bulk fringe shift), or Scheme 2 in Fig. 1 operates for control of the flux of the tetra-n-butylammonium ion on the p o l y m e r surface. Perhaps in this situation we have to consider a reduced electrocatalytic activity of the p o l y m e r surface for the reduction of tetra-n-butylammonium ion. Thus we suggest the polypyrrole electrode is porous and the reduction or oxidation of organic molecules proceeds both at the outer edge of the film and within the polymer. OXIDATION OF HYDROQUINONE (H2Q)
The oxidation of H2Q at a gold and polypyrrole electrode was also considered in the light of the above mechanism. The electrolysis of 720 mM H2Q was carried o u t in different orientations of the electrodes and at different constant current densities. The electrolysis generates a fringe bend at b o t h electrodes b u t the direction of the cathodic fringe bend is opposite to the one at anode. At the cathode, the reduction of tetra-n-butylammonium ion to a neutral radical occurs and at the anode the oxidation of H2Q takes place. The growth of the diffusion layer at the anode can be followed during electrolysis. The growth of this layer was also followed using gold as the anode in the oxidation of H2 Q. The concentration changes at the electrode (proportional to the fringe shifts) vs t 1~ should yield a straight line for the values at x = 0
383
and hence this approach would give us an indication of the mechanism of oxidation of H2 Q at the polymer electrode. If we compare the fringe shift with time at a noble electrode during electrolysis then it would provide a comparison for the nature of the growth of the diffusion layer. Figure 6 shows the fringe shift with time at the Au electrode and a polypyrrole electrode for the electrolysis of H2Q in acetonitrile. This plot shows significant deviations from linearity at a polypyrrole electrode. Thus the oxidation of H2Q occurs at this electrode by a mechanism different from Scheme 1 of Fig. 1 and these deviations are possibly caused by the diffusional flux within the polymer, as shown in Scheme 2. A comparison of the diffusion layer growth at the two electrodes indicates a smaller depleted region at the polypyrrole anode for all times during electrolysis. The plot (Fig. 6) is characteristic of a polypyrrole anode and should be compared to the behaviour during electrolysis of tetra-n-butylammonium perchlorate, Fig. 3. 50
• Au anode 0,4~ P anode io= O.
40
3O F 20 J lo
~ ~ Je
0
0,4L
I
4.0
•
•
• '~ ~ I
L
1
I
8.0
12.0
16.0
20.0
t1/2/S1/2 Fig. 6. Fringe shift (F) vs t ~4 in the electrolysis of h y d r o q u i n o n e in CH3CN containing 0.1 M (C4H 9 )4NC104 at 0.34 m A crn -2 . ACKNOWLEDGEMENTS
The authors wish to acknowledge the support for this work by the Natural Sciences and Engineering Research Council of Canada and the help of Teddy Robert Gathwright. We wish also to acknowledge TRIUMPH for making their facilities available for this work.
384 REFERENCES 1 G. TouriUon and F. Gamier, J. Electroanal. Chem., 135 (1982) 173 and references cited therein. 2 M.D. Ryan and G.S. Wilson, Anal. Chem., 54 (1982) 20R. 3 N.S. Sundaresan and K.S.V. Santhanam, Trans. S.A.E.S.T., 16(3) (1981) 117. 4 V. Swayambunathan and K.S.V. Santhanam, J. Electrochem. Soc., in press. 5 R.N. O'Brien and H. Kolny, Can. J. Chem., 56 (1978) 591. 6 R.N. O'Brien, Rev. Sci. Instrum., 35 (1964) 803. 7 R.N. O'Brien, B.B. Kulkarni, W. Michalik and K.S.V. Santhanam, Can. J. Chem., 59 (1981) 1933. 8 R.N. O'Brien in A. Weissberger and B. Rossiter (Eds.), Physical Methods in Chemistry, Vol. 1, Part IIIA, Wiley-Interscience, New York, 1972, Ch. 1. 9 R.N. O'Brien, J. Electrochem. Soc., 124 (1977) 96. 10 R.W. Murray, Acc. Chem. Res., 13 (1980) 135. 11 L. Homer and H. Lund in M.M. Baizer (Ed.), Organic Electrochemistry, Marcel Dekker, New York, 1973, p. 734. 12 A.H. Maki and D.H. Geske, J. Chem. Phys., 30 (1959) 1356.
377
Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
Preliminary note MECHANISM OF OXIDATION OF O R G A N I C MOLECULES AT A P O L Y P Y R R O L E MODIFIED E L E C T R O D E USING L A S E R INTERFEROMETRY
K.S.V. SANTHANAM
Tara Institute of Fundamental Research, Bombay 400005 (India) R.N. O'BRIEN
Department of Chemistry, University of Victoria, P.O. Box 1700, Victoria, B.C., V8W 2Y2 (Canada) (Received 21st September 1983; in revised form 4th November 1983)
With the development of electronically conducting polymers such as polypyrrole, polythiophene, polyindole, polyfuran, polyazulene and polycarbazole [1--4] as electrode materials for electrochemical oxidations of organics, a basic question concerning the mechanism is still to be decided which is: does the oxidation of a suitable organic molecule occur at the surface of a polymer film coating an electrode or does at least some of it penetrate into the film before oxidation? The diffusion layer growth in the t w o situations during electrolysis would be governed by at least three kinetic elements: (a) the diffusion rate of the organic molecule from the bulk of the solution to the polymer surface, (b) the rate of its diffusion within the polymer film, and (c) the rate of the charge transfer process. These rates are conveniently expressed as flux values, however the actual flux values cannot be independent of one another. In fact, the three actual flux values must equal one another and this means that the apparent thickness of the substrate diffusion gradients m a y be less than the actual total film thickness. While this situation would exist if the oxidation occurs after penetration into the film, the diffusion gradients in the other case, where the organic molecule undergoes oxidation at the outer edge of the film (non-porous in nature) would follow the normally expected diffusion gradients, i.e. follow Sand's equation [ 5] which was derived on the basis of Fick's laws of diffusion. Laser interferometry has been unequivocally applied to the situations of this t y p e where:
AC(x,t) =
2I nF
~
ierfc
x 2x/-D-~
(1)
where I is the current density, n the charge transferred, F Faraday's constant, D the diffusion coefficient of the molecule, and x the distance from the electrode to the solution. This work was begun to explore the possibility of utilising laser interferometry to decide between the t w o mentioned mechanisms.
378 EXPERIMENTAL The laser interferometric experimental details have been described in previous publications [ 6 - - 8 ] . The electrodes were prepared by coating a transparent optical semicircular glass fiat with gold by vacuum evaporation. The edge surface only was gold coated, leaving the sides non-conducting. Teflon spacers separated the t w o electrodes in the teflon cell. The electrical contacts to the electrodes were made through side ports with a push contact to the gold covered edge away from the electrolyte. HP Grade acetonitrile which was obtained from Matheson--Coleman was used as received. Tetra-n-butylammonium perchlorate was obtained from South Western Analytical Chemicals, Austin and was used after drying for more than 120 h. Pyrrole was obtained from Riedel and was purified b y distillation. The fraction boiling at 132°C was used in the experiments. The electrolysis was carried o u t using a constant current source (Keithley) at different current densities (cd) ranging from 0.10 mA cm -2 to 5.0 mA cm -2 . The voltage impressed across the cell was measured using a Keithley Digital voltmeter. The duration of the electrolysis ranged from 3 to 15 min (maximum charge 2.79 C). A He--Ne Laser (1 mW) was used in the interferometric measurements. The dielectric coated flats were separated by the thickness of the electrodes (0.58 cm) while the electrodes themselves were separated by 6.60 mm. The fringes were arranged perpendicular to the electrodes by suitably tightening the cell clamps. The concentration perturbed fringes were recorded at intervals with a 35 mm Nikon camera. The experimental runs were videotaped through the Nikon's view finder. RESULTS AND DISCUSSION The most likely models for the electrolysis of organic molecules at p o l y m e r film electrodes are shown in Fig. 1. Scheme 1 considers one electrode consisting of a non-porous polymer film and the other as a conducting metallic electrode (Au). The polymer film thickness has been drawn thicker than the real value (~ 1 pm) in order to show the concentration gradients effectively. The growth of the diffusion layers at the t w o electrodes would be approximately symmetrical during electrolysis with t w o metal electrodes such as Cu, or Zn. The concentration gradients at the cathode and anode during an electrolysis can be described by eqn. (1). These gradients would appear as refractive index gradients at the two electrodes, which are the interferometrically measarable quantities [6,9]. The growth of the diffusion layers would be limited if convection were present in the cell or by chemical reactions (either EC or CE) which change the refractive index gradients. In the absence of these factors Scheme 1 m a y also be described as the refractive index gradients in the electrochemical cell. Assuming the p o l y m e r film is porous and the electrochemical reaction occurs both on the surface and within the p o l y m e r film, several situations can exist, b u t they will all begin with the diffusion of the organic molecule (plus the solvent and the supporting electrolyte) into the p o l y m e r and
379
W
A
SCHEME I I ÷
/
C
S
'~C4Hg)4N
NON
POROUS
/-
,
clo,; ..,i.--.---
BULK H2Q
(C41"~4N
c,oC/
Ill[SOLUTION
J
I
i i
POLYMER_FILM DIFFUSION LAYER
Dp~ Dsoln
+
ic4~4N
POROUS
I
,,°,IV
(C 4 H9)4N.__.~
CIO4 q
/1"
D IFFUSION LAYER
SCHEME 2 A
/
\
H2 Q
/// /
POLYMER FILM DIFFUSION LAYER Dp<< Dsoln
DIFF N\ LAYER Ior Dp>> Dsoln (2)
II[
C 104
C
I H2 Q
u i
/'/
I E
{ C4 ~ H9)4N ~ / / / /
\ \
\ \ \ \ \ \ \
POLYMER FILM FIGURE
t
Fig. 1. Models for the electrolysis at a non-porous polymer electrode (Scheme 1) and a porous polymer electrode (Scheme 2). (W) Working; (A) auxiliary. establishment of an equilibrium between the species in the polymeric layer with the species in the solution. This situation was considered in detail in connection with electrocatalysis [ 1 0 ] . The permeation of an organic molecule into the polymer can be described by the product of the rate at which an organic molecule diffuses and the partition coefficient governing the crossing of the electrolyte polymer interface by the organic molecule. U p o n reaching the limiting threshold potential for the oxidation of the organic molecule, its oxidation within the polymer layer as well as on the film edge (surface) should occur (assuming a highly conducting thin polymer film). The continued oxidation would be controlled by the flux of the material, both on the surface and within the polymer. Selected cases of this type are shown in Scheme 2 of Fig. 1. The electrochemical experiments were conducted with 0.1 M tetra-n-butyla m m o n i u m perchlorate in anhydrous acetonitrile under galvanostatic conditions. The specific aim was understanding the growth of the diffusion layers at the polymer anode and cathode. With polypyrrole coated gold as anode and a gold electrode as the cathode, experiments were performed in the vertical (V)
380
position, anode over cathode (A/C) and cathode over anode (C/A) positions at different cd's. The electrolysis of quaternary a m m o n i u m salts at metal electrodes has previously been investigated [ 11 ], and the anodic reaction was postulated as oxidation of perchlorate ion to a radical which subsequently regenerates (EC) through a catalytic reaction [12]. At the cathode tetra-nb u t y l a m m o n i u m ion is reduced to a radical which subsequently cleaves to produce tri-butylamine [ 11 ]. These reactions were investigated using laser interferometry by using a polypyrrole anode and gold as the cathode and vice versa, allowing comparison of growth of the diffusion layer for each reaction at each type of electrode. Using gold as both anode and cathode completes the set of experiments to allow diffusion layer comparisons. POLYMER ANODE AND Au CATHODE
The development of a diffusion layer began at the two electrodes from the start of the electrolysis in all of the configurations. Figure 2 shows a typical fringe pattern at 0.10, 0.50 and 1.00 m A cm -2 in the A/C position. At the lower cd's, an unequal diffusion layer growth was observed, i.e., the cathodic diffusion layer was of greater extent than that of the anode. An analysis of the interferograms yielded graphs showing a concentration--distance profile as indicated in Fig. 3. At a higher cd (1.00 m A cm -2 ), the growth of the layer at the polymer anode was better defined at different times in the recorded frames. At longer times (>240 s) the onset of natural convective flow disturbs the entire fringe system making measurements on the fringes unreliable. This convective regime has not been examined further as the study of these frames does not provide information on diffusion layer growth. The cause of the poorly defined fringes in the anodic region m a y be due to instability of the oxidized product, i.e. the perchlorate ion through a cyclic reactions [12]. At a higher cd, the rate of oxidation of the ion is faster than the chemical reaction following it with the result that the net refractive index gradient can be observed. The fringe shift at the anode was considerably less than the shift observed at the cathode for this reason, and the trends in the build up of refractive index gradient at different cd's are shown in Fig. 4. The extent of the depleted regions at the polypyrrole anode and at the gold
C
i~!~c ~ :~ ~,, t80S
A JO :0"t3 m A c ~ 2 Ca)
t808
tO :0-33 mAcnlz (b)
180S
A i 0 ;0.66rn Acr62 {c}
Fig. 2. Interference fringes at the polymer anode and gold cathode during electrolysis o f 0.1 M (C~Hg)4NC104 in C H 3 C N . (a) 0 . 1 3 m A c m -2 , 1 8 0 s; (b) 0 . 3 3 m A c m -2 , 1 8 0 s; (c) 0 . 6 6 m A c m -2 , 1 8 0 s.
381 1"2 Lj1.01~x j ~
x axis l c m 0 . 6 6 m m t 120s ~ • i 0.33mA cm-"
• •
Au c a t h o d e P anode
0.8 AC 0.6
0.4 0.2 0.1
0.2
I
0.3
0.4
0.5
I
I
0.6
0.7
x//crrl
Fig. 3. Plot of ACVS. distance during electrolysis of 0.1 M(C4H9)4NC104in CH3CNat a cd of 0.33 mAcm-2 at t = 120 s. ConfigurationA/C.
30t 0 . 1~0F"2
t : 180s • Au c a t h o d e
~~
0
d
e
I
0.3
I
0.6
0.9 x/rnm
1.2
I
1.8
Fig. 4. Fringe shift (F) variation with current density during electrolysis of 0.1 M (C4Hg)4NC10` in CH3CNat the electrode surfaces. The electrodes were positioned as A/C. cathode for a selected cd of 0.33 m A cm -2 is shown in Fig. 3. Thus the depleted region at the cathode was always greater than at the anode until, at a high enough cd ( > 0 . 6 6 m A cm -2 ), significant convection sets in, which results in distortion of the fringes. The above results are compared with polypyrrole as cathode, and gold electrode as anode, in the electrolysis of the supporting electrolyte. The growth of the diffusion layer at the polypyrrole electrode can be observed in the interferometric recordings. For comparison the fringe shifts which were observed when gold was used as a cathode is also shown in Fig. 5. The diffusion layer at the polymer cathode was again found to be smaller than was observed at the gold electrode. Thus the results obtained in these experiments suggest the growth of the diffusion layer at the polypyrrole electrodes is hampered by a mechanism other than the normal reduction of the quaternary a m m o n i u m ion as was observed with a gold electrode. Also the fringe shifts which were observed at a polymer electrode were smaller in the above reduction.
382
l 2.5
x t • •
0.0 crn 180s A/C Au cathode P (]node
2.0 F 1.5
1.0
0.5
0
0.2
I
0.4
I
0.6 j / m A cm -2
I
0.8
I
1.0
Fig. 5. Fringe shift (F) vs. the distance from the electrode surface in the electrolysis of 0.1 M (C,H 9)4NC104 in the A/C configuration.
It is appropriate to discuss here the expected results on the basis of the model of Scheme 1 in Fig. 1. Assuming the polymer electrode has the same properties as a gold electrode, the expectation of the fringe shift--distance profile should be identical at b o t h electrodes except for the reasons discussed earlier. The absence of such an agreement suggests that either the p o l y m e r resistance reduces the effective cd on the electrode (which would cause a general bulk fringe shift), or Scheme 2 in Fig. 1 operates for control of the flux of the tetra-n-butylammonium ion on the p o l y m e r surface. Perhaps in this situation we have to consider a reduced electrocatalytic activity of the p o l y m e r surface for the reduction of tetra-n-butylammonium ion. Thus we suggest the polypyrrole electrode is porous and the reduction or oxidation of organic molecules proceeds both at the outer edge of the film and within the polymer. OXIDATION OF HYDROQUINONE (H2Q)
The oxidation of H2Q at a gold and polypyrrole electrode was also considered in the light of the above mechanism. The electrolysis of 720 mM H2Q was carried o u t in different orientations of the electrodes and at different constant current densities. The electrolysis generates a fringe bend at b o t h electrodes b u t the direction of the cathodic fringe bend is opposite to the one at anode. At the cathode, the reduction of tetra-n-butylammonium ion to a neutral radical occurs and at the anode the oxidation of H2Q takes place. The growth of the diffusion layer at the anode can be followed during electrolysis. The growth of this layer was also followed using gold as the anode in the oxidation of H2 Q. The concentration changes at the electrode (proportional to the fringe shifts) vs t 1~ should yield a straight line for the values at x = 0
383
and hence this approach would give us an indication of the mechanism of oxidation of H2 Q at the polymer electrode. If we compare the fringe shift with time at a noble electrode during electrolysis then it would provide a comparison for the nature of the growth of the diffusion layer. Figure 6 shows the fringe shift with time at the Au electrode and a polypyrrole electrode for the electrolysis of H2Q in acetonitrile. This plot shows significant deviations from linearity at a polypyrrole electrode. Thus the oxidation of H2Q occurs at this electrode by a mechanism different from Scheme 1 of Fig. 1 and these deviations are possibly caused by the diffusional flux within the polymer, as shown in Scheme 2. A comparison of the diffusion layer growth at the two electrodes indicates a smaller depleted region at the polypyrrole anode for all times during electrolysis. The plot (Fig. 6) is characteristic of a polypyrrole anode and should be compared to the behaviour during electrolysis of tetra-n-butylammonium perchlorate, Fig. 3. 50
• Au anode 0,4~ P anode io= O.
40
3O F 20 J lo
~ ~ Je
0
0,4L
I
4.0
•
•
• '~ ~ I
L
1
I
8.0
12.0
16.0
20.0
t1/2/S1/2 Fig. 6. Fringe shift (F) vs t ~4 in the electrolysis of h y d r o q u i n o n e in CH3CN containing 0.1 M (C4H 9 )4NC104 at 0.34 m A crn -2 . ACKNOWLEDGEMENTS
The authors wish to acknowledge the support for this work by the Natural Sciences and Engineering Research Council of Canada and the help of Teddy Robert Gathwright. We wish also to acknowledge TRIUMPH for making their facilities available for this work.
384 REFERENCES 1 G. TouriUon and F. Gamier, J. Electroanal. Chem., 135 (1982) 173 and references cited therein. 2 M.D. Ryan and G.S. Wilson, Anal. Chem., 54 (1982) 20R. 3 N.S. Sundaresan and K.S.V. Santhanam, Trans. S.A.E.S.T., 16(3) (1981) 117. 4 V. Swayambunathan and K.S.V. Santhanam, J. Electrochem. Soc., in press. 5 R.N. O'Brien and H. Kolny, Can. J. Chem., 56 (1978) 591. 6 R.N. O'Brien, Rev. Sci. Instrum., 35 (1964) 803. 7 R.N. O'Brien, B.B. Kulkarni, W. Michalik and K.S.V. Santhanam, Can. J. Chem., 59 (1981) 1933. 8 R.N. O'Brien in A. Weissberger and B. Rossiter (Eds.), Physical Methods in Chemistry, Vol. 1, Part IIIA, Wiley-Interscience, New York, 1972, Ch. 1. 9 R.N. O'Brien, J. Electrochem. Soc., 124 (1977) 96. 10 R.W. Murray, Acc. Chem. Res., 13 (1980) 135. 11 L. Homer and H. Lund in M.M. Baizer (Ed.), Organic Electrochemistry, Marcel Dekker, New York, 1973, p. 734. 12 A.H. Maki and D.H. Geske, J. Chem. Phys., 30 (1959) 1356.