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Kinetics of glassy carbon electrodes coated with polystyrene films containing ferrocene and ionic surface-active agents
165
J. Electroanal. Chem., 301 (1991) 165-176 Elsevier Sequoia S.A., Lausanne
Kinetics of glassy carbon electrodes coated with polystyrene films containing ferrocene and ionic surface-active agents Susumu Kuwabata, Takahiro Hamamoto and Hiroshi Yoneyama * Department of Applied Chemistry Osaka 565 (Japan) (Received
Faculty of Engineering,
16 July 1990; in revised form 8 October
Osaka University,
Yamada-oka
2-1, Suita,
1990)
Abstract Glassy carbon electrodes coated with polystyrene films containing ferrocene show redox activity when a cationic surfaceactive agent, lauryltrimethylammonium chloride (LMA), or an anionic surface-active agent, sodium n-dodecylbenzenesulphonate (DBS), is added to the film. Chronoamperometry reveals that the redox reaction of ferrocene in the presence of DBS in the film is accompanied by the diffusion of electrolyte cations in the polymer layer and that ionized DBS works as a charge compensater of the resulting ferricinium cations. Similarly, when LMA is present in the film, electrolyte anions are involved in the redox reaction of the immobilized ferrocene. The rate of the electron-transfer reaction between the electrode substrate and the polymer layer, which is evaluated by normal pulse voltammetry, is influenced greatly by the kind and the amount of ionic surface-active agents in the polystyrene film.
INTRODUCTION
A variety of polymer-coated electrodes has been studied using redox polymers [l-6] and polyelectrolytes such as polyvinylpyridine and polystyrenesulphonate as the coating material [7-121. As another means of the preparation of polymer-coated electrodes, we demonstrated previously the utility of coating the electrode substrates with a mixture of polystyrene, an ionic surface-active agent such as sodium n-dodecylbenzenesulfonate and lauryltrimethylammonium chloride, and an ionic electroactive species and Fe(CN)i[13]. The ionic surface-active agent in the such as Ru(bpy):+ polystyrene film binds the ionic electroactive species of opposite charge electrostatically and allows the diffusion of electrolyte ions in the polymer layer upon the redox reaction of the electroactive species. In the present work, the functions of ionic
l
To whom correspondence
0022-0728/91/$03.50
should
be addressed.
0 1991 - Elsevier Sequoia
S.A.
166
surface-active agents in the polystyrene layer in the redox reactions of electrically neutral redox species such as ferrocene have been investigated as an extension of the previous study. EXPERIMENTAL
Ferrocene (Fc) was purified by sublimation under reduced pressure. Polystyrene (PSt) (mean molar mass: 176800 g), sodium n-dodecylbenzenesulfonate (DBS), and lauryltrimethylammonium chloride (LMA) were of reagent grade and were used without further purification. A glassy carbon electrode (GC) having a surface area of 1.0 cm* was used as the electrode substrate. Its surface was polished with 0.3 pm alumina, followed by washing with doubly distilled water in an ultrasonic bath for 30 min. A Pt plate (3 cm2) and a saturated calomel electrode (SCE) served as the counter and reference electrodes, respectively. Aliquots of benzene solutions containing PSt (6.0 X lop4 M), Fc (0.1 M), and either DBS (0.1 M) or LMA (0.1 M) were dropped onto the glassy carbon electrode, followed by evaporation of the solvent in air. The prepared electrode will be denoted here by the abbreviation of the components of the coated film, such as PSt + Fc/GC, PSt + Fc + DBS/GC, and PSt + Fc + LMA/GC. In order to determine the concentration of ferrocene in the resulting film under wet conditions, which was required to estimate the apparent diffusion coefficient and the standard rate constant of the charge-transfer reaction of the electrodes, the PSt + Fc + DBS and the PSt + Fc + LMA films were prepared on slide glasses and their thickness was determined by observations with an optical microscope (Nikon Model S) after the films were soaked in an electrolyte solution for 1 h. Cyclic voltammograms were obtained using a potentiostat (Toho Giken 2001), a function generator (Hokuto Denko HB-104), and an X-Y recorder (Yokogawa Type 3025). In the chronoamperometric experiments, transient currents were recorded on a digital storage oscilloscope (Iwatsu SS-5802) connected to a personal computer (NEC PC-8801) with a GP-IB interface. RESULTS
AND
Electrochemical
DISCUSSION
response of glassy carbon electrodes coated with polystyrene
containing
f errocene Cyclic voltammograms of PSt + Fc/GC, PSt + Fc + LMA/GC, and PSt + Fc + DBS/GC electrodes taken in 0.1 M KC1 solution at 10 mV s-i are shown in Fig. 1. The PSt + Fc/GC showed no redox waves, although ferrocene was firmly fixed in the surface of the electrode, indicating that the redox reaction of ferrocene does not occur in the highly hydrophobic polystyrene film. However, when either LMA or DBS was added to the film, definite anodic and cathodic waves due to the redox reaction of ferrocene appeared. These waves were stable, at least for successive potential sweeps for 3 h, but after that time the peak currents decreased gradually, resulting in a decrease of about 10% in the peak currents for the total potential
167
60 -
E vs. SCE /V Fig. 1. Cyclic voltammograms of PSt+Fc/GC (a), PSt +Fc+LMA/GC (b), and PSt+ Fc+DBS/GC (c) taken in 1.0 M KC1 aqueous solution at dE/dt =lO mV s-‘. The amount of PSt coated on the GC electrode was 1.OX1O-9 mol cm-* and that of Fc, LMA, and DBS was 5.0 X lo-’ mol cme2 each.
sweeps for 5 h. The ionic surface-active agents in the polystyrene film did not seem to be dissolved into the electrolyte solution by the potential sweeps for 20 h, as reported previously [13]. The same was true for ferrocene, as judged from a comparison of its absorption spectra in the polystyrene films before and after the potential sweeps for 20 h. The measurements of the absorption spectra in that case were carried out on films stripped from the electrode substrate. Accordingly, the decrease in the peak currents observed for the potential sweeps for 5 h seemed to result from a decrease in the adhesion of the coated film to the mirror-finished glassy carbon electrode substrate due to the swelling of the film. It should be noted in Fig. 1 that the peak currents obtained at PSt + Fc + DBS/GC were about four times as large as those obtained at PSt + Fc + LMA/GC, and that the anodic to cathodic peak separation (AE,) of the former was 18 mV, which was much smaller than that of the latter (AE, = 170 mV). The peak currents of the cyclic voltammograms of both PSt + Fc + LMA/GC and PSt + Fc + DBS/GC were proportional to the rate of the potential sweeps, which ranged from 1 to 500 mV s-l, indicating that the diffusion of electrons and/or ions in the polymer layer occurs very rapidly compared to the rate of electron transfer at the polymer/electrode substrate interface for the range of potential sweeps chosen [14-181. It may be thought that the large peak separation observed at PSt + Fc + LMA/GC results from an unfavourable iR drop in the polymer layer. However, judging from the cyclic voltammograms of Fe(CN)zincorporated in the PSt + LMA film ( ILMA = 3.5 x lo-’ mol cm-*) which were reported previously [13], the iR drop does not seem responsible for the appearance of the large peak separation
168 3 a
0
”
2
”
4
”
6
8
10’ rLMA,rDBS lmol cm-2 Fig. 2. Plots of rgk of PSt+ Fc+ DBS/GC (a) and PSt + Fc+ LMA/GC estimated from cyclic voltammograms taken at dE/dt = 1 mV s-’ as a function of roas or TILw rFE = 5.0X lo-’ mol cme2, rps,=1.Ox1O-9 mol cmm2.
of 170 mV, because in that electrode the peak separation was 60 mV at most with a peak current of ca. 70 PA cme2, which was more than three times higher than the peak current obtained at PSt + Fc + LMA/GC. It has therefore been suggested that slow charge transfers between the electrode substrate and the polymer layer are responsible for the appearance of the large AE, [14]. The amount of ferrocene involved in the redox reaction (I’::) was evaluated from the charges of the redox waves of the cyclic voltammograms taken at 1 mV
GC electrode
Fig. 3. Schematic representation of compositional changes in polystyrene (a) and PSt + Fc+ DBS/GC (b) upon oxidation of ferrocene incorporated
films of PSt + Fc+ LMA/GC in the film.
169 s -‘.
Figure 2 shows the relationship between I’;$ and the quantity of ionic surface-active agents in the film (ALMA, Inas) for a fixed amount of 5.0 x lo-’ mol cm -2 ferrocene (I?,,). The addition of 1 X lo-’ mol cmp2 LMA was effective in activating about 15% Fc in the polystyrene layer, but further addition did not increase the utilization of Fc appreciably. On the other hand, I’:: increased markedly with increasing IDss up to 5.0 x lo-’ mol cmp2, which was the same as IFc in the polystyrene layer. The addition of DBS to the polystyrene film gives the film a selective permeability to electrolyte cations due to electrostatic attractions, while it hinders the incorporation of electrolyte anions in the film, as reported previously [13]. Similarly, the addition of LMA gives the film a selective permeability to electrolyte anions. Thus we can postulate the reaction schemes of PSt + Fc + LMA/GC and PSt + Fc + DBS/GC shown in Fig. 3. When PSt + Fc + LMA/GC and PSt + Fc + DBS/GC are prepared with the same surface concentration of surface-active agents, the coated films contain the same number of counter ions. However, in the course of the oxidation of the Fc in PSt + Fc + LMA/GC, Clions are incorporated into the coated layer to compensate the positive charges of the resulting ferricinium cations (Fc+), while cations in the polystyrene layer of the PSt + Fc + DBS are dissolved in the electrolyte solution upon the oxidation of its Fc. These differences in the reaction mode seem to bring about the differences in the reactivity of ferrocene observed between PSt + Fc + DBS/GC and PSt + Fc + LMA/GC, as will be discussed below. Chronoamperometly of PSt + Fc + DBS/GC and PSt + Fc + LMA/GC In the chronoamperometric experiments, the electrode potential (El) was first held, in 1.0 M KCl, at 0.8 V vs. SCE, which is positive enough to oxidize all ferrocene in the film to ferricinium cation (E O(Fc/Fc+) = 0.34 V vs. SCE), and then stepped to potentials ( E2) ranging from 0.6 to - 0.4 V. Figure 4 shows the time course of cathodic currents at the PSt + Fc + DBS/GC electrode. The apparent diffusion coefficient (D,,,) can be estimated by using the Cottrell equation [18] given by i = nFD~/~c/&2t’/2 where n, F, and c are the number of electrons transferred, the Faraday constant, and the concentration of ferrocene incorporated in the film, respectively. The Cottrell equation is valid for current transients caused by the imposition of a potential pulse large enough to bring about complete oxidation and reduction of an electroactive species in the coated polymer layers. The potential steps from 0.8 to -0.4 V vs. SCE satisfy such requirements, as demonstrated by the linear relationship shown in Fig. 5. The establishment of a linear relationship between i and t-II2 seems to suggest that the iR drop in the polymer layer, if any, does not cause any serious problem in the estimation of Dapp. If there is any marked iR drop, i vs. t-1/2 plots would be curved, in contrast with the results obtained here. The thickness of the coated PSt + Fc + DBS film under wet conditions (d,,) was 8.5 pm, i.e. a ca. 20% increase in the film thickness over that of the dry films due to
170 r Oa
T
5
-10 -
-3
.
E
-20
-
j
/ 1
OT
20
I
,
60
40 Time /ms
Fig. 4. Response of cathodic currents of PSt + Fc + DBS/GC obtained by applying potential steps from 0.8 V vs. SCE to 0.6 (a), 0.5 (b), 0.4 (c), 0.2 (e), 0.1 (f), 0 (g), and -0.4 (h) V vs. SCE in 1 M KC1 solution. P,s,=1.0X10-9molcm-2, P,,=P,,,=5.0X10~7molcm-2.
swelling. By using rg$/d,,,,, as the concentration of ferrocene in the film, DapP was determined to be 7.2 X lop9 cm2 s- ’ for the experimental conditions given in the figure captions of Fig. 5. By using all current-time relations shown in Fig. 4, which were obtained by stepping the potential from 0.8 V to different E, values, a normal pulse voltammogram was constructed as shown in Fig. 6 for a sampling time of 6 ms which is shown as r in Fig. 4. It is well established [19-221 that if electron transfer reactions
0
10
20
Time-112 /a-1/2 Fig. 5. Cottrell plots of the current response of PSt+Fc+DBS/GC obtained by applying a potential step from 0.8 to -0.4 V vs. SCE in 1 M KC1 solution. Past = 1.0 x lOmy mol cmd2, I’rc = PDas = 5.0 X lo-’ mol cmm2. The film thickness in wet conditions was 8.5 pm.
171
I
I
I
I
I
I
I
I
I
0
1
0.5 E 2 vs. SCE /V
Fig. 6. A normal pulse voltammogram of PSt + Fc + DBS/GC constructed using the data shown in Fig. 4. The sampling time (7) was 6 ms and is shown in Fig. 4. rps, = 1.0 X 10v9 mol cm-*, rFc = roes = 5.0 x lo-’ mol cm-‘. Electrolyte solution: 1 M KCl.
between electroactive species and an electrode obey the Butler-Volmer normal pulse voltammogram satisfies the following equation. 1.75 +x2(1 1 -x(1
+ exp( *t))’ + exp(+O)
equation, a
(1) (2)
X =
i/i,i,
r$=(nF/RT)(E-E,,,)
(3) (4)
where E1,2, a, ilirn, and k” are the reversible half-wave potential, the charge-transfer coefficient, limiting currents of the normal pulse voltammogram, and the standard rate constant of the charge-transfer reaction, respectively, and R and T have their usual meanings. As the half-wave potential, the average of the anodic and the cathodic peak potentials in the cyclic voltammograms (0.34 V vs. SCE) was used in the present study. By applying data shown in Fig. 6 to eqn. (1) and substituting E, for E, values for the log term on the right hand side of eqn. (1) were calculated, and plots of these against E, gave Fig. 7. The linear relationship obtained shows that the Butler-Volmer equation is applicable to the redox reaction of ferrocene incorporated in the polystyrene layer, and allows determination of the (Y value as 0.46 + 0.08 from the slope of the line. E * given by eqn. (2) was obtained as 0.26 V vs. SCE as shown in Fig. 7, and then k” could be determined by substituting the determined values of E *, a, Dapp, and E1,2 into eqn. (2). The same procedures can
172
1.0
0.5
0
---_---_---::;I:,
-0.2
----
0
0.2 c
0.4
E2 vs. SCE/V Fig. 7. Plots of the log terms of eqn. (1) against the second potentials (E2) obtained from the normal pulse voltammogram shown in Fig. 6.
be applied to evaluate k” in different scribed in the next section.
electrolysis conditions,
which will be de-
The effect of the electrolyte cations in the electrolyte solutions on Dapp and k” The values of Dapp and k” of PSt + Fc + DBS/GC and PSt + Fc + LMA/GC were determined in 1 M KCI, NaCI, LiCI, and HCI aqueous solutions. The Dapp and
01 0
0.3 K’ Diameter
0.6 Na*
Li’
0.9 H’
of hydrated cation /nm
Fig. 8. Relationship between Dapp of PSt + Fc + diameter of hydrated cations in the electrolyte PLM,, = 5.0 x lo-’ mol cm-*. The concentration LMA and PSt+ Fc+ DBS in wet conditions was
LMA/GC (a) and of PSt + Fc + DBS/GC (b) and the solutions. Pps, =1.0X 10e9 mol cm-*, PFc = PDas = of electrolyte was 1 M. The film thickness of PSt + Fc + 8.1 and 8.5 pm, respectively.
173
-3.0
a 0 0
_I
0
0.3 K’ Diameter
0
0.6 Na’
0.9
Li’
H’
of hydrated
cation /nm
Fig. 9. Relationship between k o of PSt + Fc + DBS/GC (a) and of PSt + Fc + LMA/GC (b) and the diameter of hydrated cations in the electrolyte solutions. rps, =1.0X 10m9 mol cm-*, rFc = roas = l- LMA= 5.0X lo-’ mol cm-*.
values against the diameter of hydrated cations of these electrolytes are shown in Figs. 8 and 9, respectively. The sizes of the hydrated alkali cations and H+ were taken from ref. 23. The Dapp value of PSt + Fc + LMA/GC did not vary with the cation size, but this was not so for that of PSt + Fc + DBS/GC, where the smaller the cations, the larger is Dar,,, except for H + ions. These results correlate well with the reaction scheme given in Fig. 3, where electrolyte anions are shown to be involved in the redox reaction of Fc in PSt + Fc + LMA/GC but not in PSt + Fc + DBS/GC. The exceptional results obtained for H+ ions at PSt + Fc + DBS/GC is not strange if one considers that the polystyrene film containing an ionic surface-active agent must have high hydrophihcity which allows proton hopping conduction via water molecules incorporated in the coat. As shown in Fig. 9, the k” values did not vary with the kind of electrolyte cation both for PSt + Fc + LMA/GC and PSt + Fc + DBS/GC. The results are in accord with the above-mentioned analysis of the electrode kinetics that the Butler-Volmer equations is applicable to the redox reaction of Fc. The finding that the k” values were greater at PSt + Fc + DBS/GC than at PSt + Fc + LhfA/GC is consistent with the observation that the peak separation of the cyclic voltammogram was greater for PSt + Fc + LMA/GC than for PSt + Fc + DBS/GC. /co
The effect of the concentration
of ferrocene
and ionic surface-active
agents
in poly-
styrene films
Figure 10 shows the effect of l?oss on k” and Dar+, of PSt + Fc + DBS/GC which contained 5.0 X lo-’ mol cmp2 Fc. The thickness of the prepared films
IO
7
TDBslmol
cm-2
Fig. 10. Plots of log k” (a) and Dapp (b) of PSt + Fc + DBS/GC obtained in 1 M KC1 solution as a function of TDss. r,, = 1.0 X 10m9 mol cmm2, rFE = 5.0 X lo-’ mol cmm2. containing 1, 3, 5, and 6 X lop7 mol cm-* DBS was 7.6, 8.2, 8.5, 8.6 pm, respectively, indicating that the swelling of the PSt film became greater with an increasing amount of DBS in the film. It can be seen from Fig. 10 that Dap,, did not vary with I’oas, while k” increased with increasing I’oas. In contrast, the k” and D app values at PSt + Fc + LMA/GC were not influenced by rLMA and were almost constant ( Dapp= 1.2 X lop8 cm* s-l, k” = 1.2 X 10e4 cm s-l). It can be recognized by comparing Fig. 10 with Fig. 2 that there is a close correlation between the k” values and the reactivity of ferrocene fixed in the polystyrene film. It has been shown [24] that the rate of electron transfer between an electrode and the electroactive species in a solution becomes high with an increase in the ionic strength of the electrolyte solution because of the enhanced capability of charge compensation in the redox reactions. The observed increase in the k” of PSt + Fc + DBS/GC with increasing lYDss as shown in Fig. 10 seems to have the same origin. As mentioned above, the polystyrene film containing the ionic surface-active agents was swollen in the electrolyte solution, the degree being dependent on IY,,as. An results in an increase in the amount of ionized DBS and Na+ ions increase of around Fc in the polymer layer, and then the redox reaction kinetics of Fc are enhanced. On oxidation of Fc, the Na+ ions in the film have to dissolve into the electrolyte solution, and such events occur rather easily in the swollen film. In the case of PSt + Fc + LMA/GC, however, a different situation is met in the electrode reaction of Fc. With an increase in the amount of LMA in the coated film, the number of Cl- ions around Fc is increased and the coated layer swells in proportion to the amount of ionized LMA. On oxidation of Fc, further incorporation of Clions is required. The incorporation of hydrated Cl- must cause further swelling of
rDss
175
rFc /10s7 molcm-2
Fig. 11. Plots of Dapp of PSt+Fc+DBS/GC function of rFc. rpst --1 0X10e9 mol cm-‘,
obtained in 1 M KC1 (a), NaCl (b), and LiCl (c) as a rDes= 5.0X10-’ mol cmm2.
the coated layer, which does not occur easily, as judged from the experimental results. Figure 11 shows the effect of ITFc on the Dapp of the PS + Fc + DBS/GC in which Inas was fixed at 5.0 X lo-’ mol cm-*. Although the surface concentration of Fc was varied by a factor of seven in these measurements, the film thickness under wet conditions was ca. 8.5 pm for all cases, suggesting that the degree of film swelling was determined mainly by the amount of DBS in the coated layer. The k” value obtained was almost constant (5.2 x 10e4 cm s-l), but the Dapp value apparently decreased with increasing IFc, as shown in Fig. 11. The Dapp value obtained is related either to the diffusion of electrons or to that of ions in the coating. If electron diffusion contributes much more to Dapp than the incorporated ions, Dapp should increase with increasing concentration of Fc in the coated layer, because the distance for electron hopping between the fixed redox centres becomes short in that case [25-291. However, the experimental results shown in Fig. 11 are not in accord with such a prediction. Therefore the Dapp obtained must be related to the diffusion limitation of the electrolyte cations in the polymer layer. The diffusion of electrolyte cations must be retarded by an increase in IFcr because free spaces available for ion diffusion become scarce due to the crowding of Fc in a limited space of the polymer layer.
ACKNOWLEDGEMENT
This research was supported by Grant-in Aid for Scientific Research No. 1750760 from the Ministry of Education, Science and Culture.
176 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
A. Merz and A.J. Bard, J. Am. Chem. Sot., 100 (1978) 3222. P. Daum and R.W. Murray, J. Electroanal. Chem., 103 (1979) 389. P.F. Peerce and A.J. Bard, J. Electroanal. Chem., 114 (1981) 89. S. Nakahama and R.W. Murray, J. Electroanal. Chem., 158 (1983) 303. C. Degrand and L.L. Miller, J. Electroanal. Chem., 117 (1981) 267. L. Roullier, E. Waldner and E. Laviron, J. Electroanal. Chem., 139 (1982) 199. N. Oyama and F.C. Anson, J. Electrochem. Sot., 127 (1980) 247. N. Oyama and F.C. Anson, Anal. Chem., 52 (1980) 1192. M. Sharp, D.D. Montgomery and F.C. Anson, J. Electroanal. Chem., 194 (1985) 247. B.R. Shaw, G.P. Haight and L.R. Faulkner, J. Electroanal. Chem., 140 (1982) 147. I. Rubinstein and A.J. Bard, J. Am. Chem. Sot., 102 (1980) 6641. C.M. Lieber and N.S. Lewis, J. Am. Chem. Sot., 107 (1985) 7190. S. Kuwabata, Y. Maida and H. Yoneyama, J. Electroanal. Chem., 242 (1988) 143. E. Laviron, J. Electroanal. Chem., 100 (1979) 263. E. Laviron, J. Electroanal. Chem., 101 (1979) 19. E. Laviron, J. Electroanal. Chem., 52 (1974) 395. E. Laviron, J. Electroanal. Chem., 112 (1980) 1. F.G. Cottrell, 2. Phys. Chem., 42 (1903) 385. H. Matsuda, Bull. Chem. Sot. Jpn., 53 (1980) 3439. N. Oyama, T. Oh&a, M. Kaneko, K. Sato and H. Matsuda, J. Am. Chem. Sot., 105 (1983) 6003. T. Oh&a, N. Oyama, K. Sato and H. Matsuda, J. Electrochem. Sot., 132 (1985) 1871. N. Oyama, T. Ohsaka, H. Yamamoto and M. Kaneko, J. Phys. Chem., 90 (1986) 3850. J. Kielland, J. Am. Chem. Sot., 59 (1937) 1675. L.M. Peter, W. Durr, P. Bindra and H. Gerischer, J. Electroanal. Chem., 71 (1976) 31. H.S. White, J. Leddy and A.J. Bard, J. Electroanal. Chem., 114 (1980) 89. D.A. Buttry and F.C. Anson, J. Am. Chem. Sot., 105 (1983) 685. H. Dahms, J. Phys. Chem., 72 (1968) 362. I. Ruff and V. Friedrich, J. Phys. Chem., 75 (1971) 3297. I. Ruff, V. Friedrich and K. Csillag, J. Phys. Chem., 75 (1971) 3303.
J. Electroanal. Chem., 301 (1991) 165-176 Elsevier Sequoia S.A., Lausanne
Kinetics of glassy carbon electrodes coated with polystyrene films containing ferrocene and ionic surface-active agents Susumu Kuwabata, Takahiro Hamamoto and Hiroshi Yoneyama * Department of Applied Chemistry Osaka 565 (Japan) (Received
Faculty of Engineering,
16 July 1990; in revised form 8 October
Osaka University,
Yamada-oka
2-1, Suita,
1990)
Abstract Glassy carbon electrodes coated with polystyrene films containing ferrocene show redox activity when a cationic surfaceactive agent, lauryltrimethylammonium chloride (LMA), or an anionic surface-active agent, sodium n-dodecylbenzenesulphonate (DBS), is added to the film. Chronoamperometry reveals that the redox reaction of ferrocene in the presence of DBS in the film is accompanied by the diffusion of electrolyte cations in the polymer layer and that ionized DBS works as a charge compensater of the resulting ferricinium cations. Similarly, when LMA is present in the film, electrolyte anions are involved in the redox reaction of the immobilized ferrocene. The rate of the electron-transfer reaction between the electrode substrate and the polymer layer, which is evaluated by normal pulse voltammetry, is influenced greatly by the kind and the amount of ionic surface-active agents in the polystyrene film.
INTRODUCTION
A variety of polymer-coated electrodes has been studied using redox polymers [l-6] and polyelectrolytes such as polyvinylpyridine and polystyrenesulphonate as the coating material [7-121. As another means of the preparation of polymer-coated electrodes, we demonstrated previously the utility of coating the electrode substrates with a mixture of polystyrene, an ionic surface-active agent such as sodium n-dodecylbenzenesulfonate and lauryltrimethylammonium chloride, and an ionic electroactive species and Fe(CN)i[13]. The ionic surface-active agent in the such as Ru(bpy):+ polystyrene film binds the ionic electroactive species of opposite charge electrostatically and allows the diffusion of electrolyte ions in the polymer layer upon the redox reaction of the electroactive species. In the present work, the functions of ionic
l
To whom correspondence
0022-0728/91/$03.50
should
be addressed.
0 1991 - Elsevier Sequoia
S.A.
166
surface-active agents in the polystyrene layer in the redox reactions of electrically neutral redox species such as ferrocene have been investigated as an extension of the previous study. EXPERIMENTAL
Ferrocene (Fc) was purified by sublimation under reduced pressure. Polystyrene (PSt) (mean molar mass: 176800 g), sodium n-dodecylbenzenesulfonate (DBS), and lauryltrimethylammonium chloride (LMA) were of reagent grade and were used without further purification. A glassy carbon electrode (GC) having a surface area of 1.0 cm* was used as the electrode substrate. Its surface was polished with 0.3 pm alumina, followed by washing with doubly distilled water in an ultrasonic bath for 30 min. A Pt plate (3 cm2) and a saturated calomel electrode (SCE) served as the counter and reference electrodes, respectively. Aliquots of benzene solutions containing PSt (6.0 X lop4 M), Fc (0.1 M), and either DBS (0.1 M) or LMA (0.1 M) were dropped onto the glassy carbon electrode, followed by evaporation of the solvent in air. The prepared electrode will be denoted here by the abbreviation of the components of the coated film, such as PSt + Fc/GC, PSt + Fc + DBS/GC, and PSt + Fc + LMA/GC. In order to determine the concentration of ferrocene in the resulting film under wet conditions, which was required to estimate the apparent diffusion coefficient and the standard rate constant of the charge-transfer reaction of the electrodes, the PSt + Fc + DBS and the PSt + Fc + LMA films were prepared on slide glasses and their thickness was determined by observations with an optical microscope (Nikon Model S) after the films were soaked in an electrolyte solution for 1 h. Cyclic voltammograms were obtained using a potentiostat (Toho Giken 2001), a function generator (Hokuto Denko HB-104), and an X-Y recorder (Yokogawa Type 3025). In the chronoamperometric experiments, transient currents were recorded on a digital storage oscilloscope (Iwatsu SS-5802) connected to a personal computer (NEC PC-8801) with a GP-IB interface. RESULTS
AND
Electrochemical
DISCUSSION
response of glassy carbon electrodes coated with polystyrene
containing
f errocene Cyclic voltammograms of PSt + Fc/GC, PSt + Fc + LMA/GC, and PSt + Fc + DBS/GC electrodes taken in 0.1 M KC1 solution at 10 mV s-i are shown in Fig. 1. The PSt + Fc/GC showed no redox waves, although ferrocene was firmly fixed in the surface of the electrode, indicating that the redox reaction of ferrocene does not occur in the highly hydrophobic polystyrene film. However, when either LMA or DBS was added to the film, definite anodic and cathodic waves due to the redox reaction of ferrocene appeared. These waves were stable, at least for successive potential sweeps for 3 h, but after that time the peak currents decreased gradually, resulting in a decrease of about 10% in the peak currents for the total potential
167
60 -
E vs. SCE /V Fig. 1. Cyclic voltammograms of PSt+Fc/GC (a), PSt +Fc+LMA/GC (b), and PSt+ Fc+DBS/GC (c) taken in 1.0 M KC1 aqueous solution at dE/dt =lO mV s-‘. The amount of PSt coated on the GC electrode was 1.OX1O-9 mol cm-* and that of Fc, LMA, and DBS was 5.0 X lo-’ mol cme2 each.
sweeps for 5 h. The ionic surface-active agents in the polystyrene film did not seem to be dissolved into the electrolyte solution by the potential sweeps for 20 h, as reported previously [13]. The same was true for ferrocene, as judged from a comparison of its absorption spectra in the polystyrene films before and after the potential sweeps for 20 h. The measurements of the absorption spectra in that case were carried out on films stripped from the electrode substrate. Accordingly, the decrease in the peak currents observed for the potential sweeps for 5 h seemed to result from a decrease in the adhesion of the coated film to the mirror-finished glassy carbon electrode substrate due to the swelling of the film. It should be noted in Fig. 1 that the peak currents obtained at PSt + Fc + DBS/GC were about four times as large as those obtained at PSt + Fc + LMA/GC, and that the anodic to cathodic peak separation (AE,) of the former was 18 mV, which was much smaller than that of the latter (AE, = 170 mV). The peak currents of the cyclic voltammograms of both PSt + Fc + LMA/GC and PSt + Fc + DBS/GC were proportional to the rate of the potential sweeps, which ranged from 1 to 500 mV s-l, indicating that the diffusion of electrons and/or ions in the polymer layer occurs very rapidly compared to the rate of electron transfer at the polymer/electrode substrate interface for the range of potential sweeps chosen [14-181. It may be thought that the large peak separation observed at PSt + Fc + LMA/GC results from an unfavourable iR drop in the polymer layer. However, judging from the cyclic voltammograms of Fe(CN)zincorporated in the PSt + LMA film ( ILMA = 3.5 x lo-’ mol cm-*) which were reported previously [13], the iR drop does not seem responsible for the appearance of the large peak separation
168 3 a
0
”
2
”
4
”
6
8
10’ rLMA,rDBS lmol cm-2 Fig. 2. Plots of rgk of PSt+ Fc+ DBS/GC (a) and PSt + Fc+ LMA/GC estimated from cyclic voltammograms taken at dE/dt = 1 mV s-’ as a function of roas or TILw rFE = 5.0X lo-’ mol cme2, rps,=1.Ox1O-9 mol cmm2.
of 170 mV, because in that electrode the peak separation was 60 mV at most with a peak current of ca. 70 PA cme2, which was more than three times higher than the peak current obtained at PSt + Fc + LMA/GC. It has therefore been suggested that slow charge transfers between the electrode substrate and the polymer layer are responsible for the appearance of the large AE, [14]. The amount of ferrocene involved in the redox reaction (I’::) was evaluated from the charges of the redox waves of the cyclic voltammograms taken at 1 mV
GC electrode
Fig. 3. Schematic representation of compositional changes in polystyrene (a) and PSt + Fc+ DBS/GC (b) upon oxidation of ferrocene incorporated
films of PSt + Fc+ LMA/GC in the film.
169 s -‘.
Figure 2 shows the relationship between I’;$ and the quantity of ionic surface-active agents in the film (ALMA, Inas) for a fixed amount of 5.0 x lo-’ mol cm -2 ferrocene (I?,,). The addition of 1 X lo-’ mol cmp2 LMA was effective in activating about 15% Fc in the polystyrene layer, but further addition did not increase the utilization of Fc appreciably. On the other hand, I’:: increased markedly with increasing IDss up to 5.0 x lo-’ mol cmp2, which was the same as IFc in the polystyrene layer. The addition of DBS to the polystyrene film gives the film a selective permeability to electrolyte cations due to electrostatic attractions, while it hinders the incorporation of electrolyte anions in the film, as reported previously [13]. Similarly, the addition of LMA gives the film a selective permeability to electrolyte anions. Thus we can postulate the reaction schemes of PSt + Fc + LMA/GC and PSt + Fc + DBS/GC shown in Fig. 3. When PSt + Fc + LMA/GC and PSt + Fc + DBS/GC are prepared with the same surface concentration of surface-active agents, the coated films contain the same number of counter ions. However, in the course of the oxidation of the Fc in PSt + Fc + LMA/GC, Clions are incorporated into the coated layer to compensate the positive charges of the resulting ferricinium cations (Fc+), while cations in the polystyrene layer of the PSt + Fc + DBS are dissolved in the electrolyte solution upon the oxidation of its Fc. These differences in the reaction mode seem to bring about the differences in the reactivity of ferrocene observed between PSt + Fc + DBS/GC and PSt + Fc + LMA/GC, as will be discussed below. Chronoamperometly of PSt + Fc + DBS/GC and PSt + Fc + LMA/GC In the chronoamperometric experiments, the electrode potential (El) was first held, in 1.0 M KCl, at 0.8 V vs. SCE, which is positive enough to oxidize all ferrocene in the film to ferricinium cation (E O(Fc/Fc+) = 0.34 V vs. SCE), and then stepped to potentials ( E2) ranging from 0.6 to - 0.4 V. Figure 4 shows the time course of cathodic currents at the PSt + Fc + DBS/GC electrode. The apparent diffusion coefficient (D,,,) can be estimated by using the Cottrell equation [18] given by i = nFD~/~c/&2t’/2 where n, F, and c are the number of electrons transferred, the Faraday constant, and the concentration of ferrocene incorporated in the film, respectively. The Cottrell equation is valid for current transients caused by the imposition of a potential pulse large enough to bring about complete oxidation and reduction of an electroactive species in the coated polymer layers. The potential steps from 0.8 to -0.4 V vs. SCE satisfy such requirements, as demonstrated by the linear relationship shown in Fig. 5. The establishment of a linear relationship between i and t-II2 seems to suggest that the iR drop in the polymer layer, if any, does not cause any serious problem in the estimation of Dapp. If there is any marked iR drop, i vs. t-1/2 plots would be curved, in contrast with the results obtained here. The thickness of the coated PSt + Fc + DBS film under wet conditions (d,,) was 8.5 pm, i.e. a ca. 20% increase in the film thickness over that of the dry films due to
170 r Oa
T
5
-10 -
-3
.
E
-20
-
j
/ 1
OT
20
I
,
60
40 Time /ms
Fig. 4. Response of cathodic currents of PSt + Fc + DBS/GC obtained by applying potential steps from 0.8 V vs. SCE to 0.6 (a), 0.5 (b), 0.4 (c), 0.2 (e), 0.1 (f), 0 (g), and -0.4 (h) V vs. SCE in 1 M KC1 solution. P,s,=1.0X10-9molcm-2, P,,=P,,,=5.0X10~7molcm-2.
swelling. By using rg$/d,,,,, as the concentration of ferrocene in the film, DapP was determined to be 7.2 X lop9 cm2 s- ’ for the experimental conditions given in the figure captions of Fig. 5. By using all current-time relations shown in Fig. 4, which were obtained by stepping the potential from 0.8 V to different E, values, a normal pulse voltammogram was constructed as shown in Fig. 6 for a sampling time of 6 ms which is shown as r in Fig. 4. It is well established [19-221 that if electron transfer reactions
0
10
20
Time-112 /a-1/2 Fig. 5. Cottrell plots of the current response of PSt+Fc+DBS/GC obtained by applying a potential step from 0.8 to -0.4 V vs. SCE in 1 M KC1 solution. Past = 1.0 x lOmy mol cmd2, I’rc = PDas = 5.0 X lo-’ mol cmm2. The film thickness in wet conditions was 8.5 pm.
171
I
I
I
I
I
I
I
I
I
0
1
0.5 E 2 vs. SCE /V
Fig. 6. A normal pulse voltammogram of PSt + Fc + DBS/GC constructed using the data shown in Fig. 4. The sampling time (7) was 6 ms and is shown in Fig. 4. rps, = 1.0 X 10v9 mol cm-*, rFc = roes = 5.0 x lo-’ mol cm-‘. Electrolyte solution: 1 M KCl.
between electroactive species and an electrode obey the Butler-Volmer normal pulse voltammogram satisfies the following equation. 1.75 +x2(1 1 -x(1
+ exp( *t))’ + exp(+O)
equation, a
(1) (2)
X =
i/i,i,
r$=(nF/RT)(E-E,,,)
(3) (4)
where E1,2, a, ilirn, and k” are the reversible half-wave potential, the charge-transfer coefficient, limiting currents of the normal pulse voltammogram, and the standard rate constant of the charge-transfer reaction, respectively, and R and T have their usual meanings. As the half-wave potential, the average of the anodic and the cathodic peak potentials in the cyclic voltammograms (0.34 V vs. SCE) was used in the present study. By applying data shown in Fig. 6 to eqn. (1) and substituting E, for E, values for the log term on the right hand side of eqn. (1) were calculated, and plots of these against E, gave Fig. 7. The linear relationship obtained shows that the Butler-Volmer equation is applicable to the redox reaction of ferrocene incorporated in the polystyrene layer, and allows determination of the (Y value as 0.46 + 0.08 from the slope of the line. E * given by eqn. (2) was obtained as 0.26 V vs. SCE as shown in Fig. 7, and then k” could be determined by substituting the determined values of E *, a, Dapp, and E1,2 into eqn. (2). The same procedures can
172
1.0
0.5
0
---_---_---::;I:,
-0.2
----
0
0.2 c
0.4
E2 vs. SCE/V Fig. 7. Plots of the log terms of eqn. (1) against the second potentials (E2) obtained from the normal pulse voltammogram shown in Fig. 6.
be applied to evaluate k” in different scribed in the next section.
electrolysis conditions,
which will be de-
The effect of the electrolyte cations in the electrolyte solutions on Dapp and k” The values of Dapp and k” of PSt + Fc + DBS/GC and PSt + Fc + LMA/GC were determined in 1 M KCI, NaCI, LiCI, and HCI aqueous solutions. The Dapp and
01 0
0.3 K’ Diameter
0.6 Na*
Li’
0.9 H’
of hydrated cation /nm
Fig. 8. Relationship between Dapp of PSt + Fc + diameter of hydrated cations in the electrolyte PLM,, = 5.0 x lo-’ mol cm-*. The concentration LMA and PSt+ Fc+ DBS in wet conditions was
LMA/GC (a) and of PSt + Fc + DBS/GC (b) and the solutions. Pps, =1.0X 10e9 mol cm-*, PFc = PDas = of electrolyte was 1 M. The film thickness of PSt + Fc + 8.1 and 8.5 pm, respectively.
173
-3.0
a 0 0
_I
0
0.3 K’ Diameter
0
0.6 Na’
0.9
Li’
H’
of hydrated
cation /nm
Fig. 9. Relationship between k o of PSt + Fc + DBS/GC (a) and of PSt + Fc + LMA/GC (b) and the diameter of hydrated cations in the electrolyte solutions. rps, =1.0X 10m9 mol cm-*, rFc = roas = l- LMA= 5.0X lo-’ mol cm-*.
values against the diameter of hydrated cations of these electrolytes are shown in Figs. 8 and 9, respectively. The sizes of the hydrated alkali cations and H+ were taken from ref. 23. The Dapp value of PSt + Fc + LMA/GC did not vary with the cation size, but this was not so for that of PSt + Fc + DBS/GC, where the smaller the cations, the larger is Dar,,, except for H + ions. These results correlate well with the reaction scheme given in Fig. 3, where electrolyte anions are shown to be involved in the redox reaction of Fc in PSt + Fc + LMA/GC but not in PSt + Fc + DBS/GC. The exceptional results obtained for H+ ions at PSt + Fc + DBS/GC is not strange if one considers that the polystyrene film containing an ionic surface-active agent must have high hydrophihcity which allows proton hopping conduction via water molecules incorporated in the coat. As shown in Fig. 9, the k” values did not vary with the kind of electrolyte cation both for PSt + Fc + LMA/GC and PSt + Fc + DBS/GC. The results are in accord with the above-mentioned analysis of the electrode kinetics that the Butler-Volmer equations is applicable to the redox reaction of Fc. The finding that the k” values were greater at PSt + Fc + DBS/GC than at PSt + Fc + LhfA/GC is consistent with the observation that the peak separation of the cyclic voltammogram was greater for PSt + Fc + LMA/GC than for PSt + Fc + DBS/GC. /co
The effect of the concentration
of ferrocene
and ionic surface-active
agents
in poly-
styrene films
Figure 10 shows the effect of l?oss on k” and Dar+, of PSt + Fc + DBS/GC which contained 5.0 X lo-’ mol cmp2 Fc. The thickness of the prepared films
IO
7
TDBslmol
cm-2
Fig. 10. Plots of log k” (a) and Dapp (b) of PSt + Fc + DBS/GC obtained in 1 M KC1 solution as a function of TDss. r,, = 1.0 X 10m9 mol cmm2, rFE = 5.0 X lo-’ mol cmm2. containing 1, 3, 5, and 6 X lop7 mol cm-* DBS was 7.6, 8.2, 8.5, 8.6 pm, respectively, indicating that the swelling of the PSt film became greater with an increasing amount of DBS in the film. It can be seen from Fig. 10 that Dap,, did not vary with I’oas, while k” increased with increasing I’oas. In contrast, the k” and D app values at PSt + Fc + LMA/GC were not influenced by rLMA and were almost constant ( Dapp= 1.2 X lop8 cm* s-l, k” = 1.2 X 10e4 cm s-l). It can be recognized by comparing Fig. 10 with Fig. 2 that there is a close correlation between the k” values and the reactivity of ferrocene fixed in the polystyrene film. It has been shown [24] that the rate of electron transfer between an electrode and the electroactive species in a solution becomes high with an increase in the ionic strength of the electrolyte solution because of the enhanced capability of charge compensation in the redox reactions. The observed increase in the k” of PSt + Fc + DBS/GC with increasing lYDss as shown in Fig. 10 seems to have the same origin. As mentioned above, the polystyrene film containing the ionic surface-active agents was swollen in the electrolyte solution, the degree being dependent on IY,,as. An results in an increase in the amount of ionized DBS and Na+ ions increase of around Fc in the polymer layer, and then the redox reaction kinetics of Fc are enhanced. On oxidation of Fc, the Na+ ions in the film have to dissolve into the electrolyte solution, and such events occur rather easily in the swollen film. In the case of PSt + Fc + LMA/GC, however, a different situation is met in the electrode reaction of Fc. With an increase in the amount of LMA in the coated film, the number of Cl- ions around Fc is increased and the coated layer swells in proportion to the amount of ionized LMA. On oxidation of Fc, further incorporation of Clions is required. The incorporation of hydrated Cl- must cause further swelling of
rDss
175
rFc /10s7 molcm-2
Fig. 11. Plots of Dapp of PSt+Fc+DBS/GC function of rFc. rpst --1 0X10e9 mol cm-‘,
obtained in 1 M KC1 (a), NaCl (b), and LiCl (c) as a rDes= 5.0X10-’ mol cmm2.
the coated layer, which does not occur easily, as judged from the experimental results. Figure 11 shows the effect of ITFc on the Dapp of the PS + Fc + DBS/GC in which Inas was fixed at 5.0 X lo-’ mol cm-*. Although the surface concentration of Fc was varied by a factor of seven in these measurements, the film thickness under wet conditions was ca. 8.5 pm for all cases, suggesting that the degree of film swelling was determined mainly by the amount of DBS in the coated layer. The k” value obtained was almost constant (5.2 x 10e4 cm s-l), but the Dapp value apparently decreased with increasing IFc, as shown in Fig. 11. The Dapp value obtained is related either to the diffusion of electrons or to that of ions in the coating. If electron diffusion contributes much more to Dapp than the incorporated ions, Dapp should increase with increasing concentration of Fc in the coated layer, because the distance for electron hopping between the fixed redox centres becomes short in that case [25-291. However, the experimental results shown in Fig. 11 are not in accord with such a prediction. Therefore the Dapp obtained must be related to the diffusion limitation of the electrolyte cations in the polymer layer. The diffusion of electrolyte cations must be retarded by an increase in IFcr because free spaces available for ion diffusion become scarce due to the crowding of Fc in a limited space of the polymer layer.
ACKNOWLEDGEMENT
This research was supported by Grant-in Aid for Scientific Research No. 1750760 from the Ministry of Education, Science and Culture.
176 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
A. Merz and A.J. Bard, J. Am. Chem. Sot., 100 (1978) 3222. P. Daum and R.W. Murray, J. Electroanal. Chem., 103 (1979) 389. P.F. Peerce and A.J. Bard, J. Electroanal. Chem., 114 (1981) 89. S. Nakahama and R.W. Murray, J. Electroanal. Chem., 158 (1983) 303. C. Degrand and L.L. Miller, J. Electroanal. Chem., 117 (1981) 267. L. Roullier, E. Waldner and E. Laviron, J. Electroanal. Chem., 139 (1982) 199. N. Oyama and F.C. Anson, J. Electrochem. Sot., 127 (1980) 247. N. Oyama and F.C. Anson, Anal. Chem., 52 (1980) 1192. M. Sharp, D.D. Montgomery and F.C. Anson, J. Electroanal. Chem., 194 (1985) 247. B.R. Shaw, G.P. Haight and L.R. Faulkner, J. Electroanal. Chem., 140 (1982) 147. I. Rubinstein and A.J. Bard, J. Am. Chem. Sot., 102 (1980) 6641. C.M. Lieber and N.S. Lewis, J. Am. Chem. Sot., 107 (1985) 7190. S. Kuwabata, Y. Maida and H. Yoneyama, J. Electroanal. Chem., 242 (1988) 143. E. Laviron, J. Electroanal. Chem., 100 (1979) 263. E. Laviron, J. Electroanal. Chem., 101 (1979) 19. E. Laviron, J. Electroanal. Chem., 52 (1974) 395. E. Laviron, J. Electroanal. Chem., 112 (1980) 1. F.G. Cottrell, 2. Phys. Chem., 42 (1903) 385. H. Matsuda, Bull. Chem. Sot. Jpn., 53 (1980) 3439. N. Oyama, T. Oh&a, M. Kaneko, K. Sato and H. Matsuda, J. Am. Chem. Sot., 105 (1983) 6003. T. Oh&a, N. Oyama, K. Sato and H. Matsuda, J. Electrochem. Sot., 132 (1985) 1871. N. Oyama, T. Ohsaka, H. Yamamoto and M. Kaneko, J. Phys. Chem., 90 (1986) 3850. J. Kielland, J. Am. Chem. Sot., 59 (1937) 1675. L.M. Peter, W. Durr, P. Bindra and H. Gerischer, J. Electroanal. Chem., 71 (1976) 31. H.S. White, J. Leddy and A.J. Bard, J. Electroanal. Chem., 114 (1980) 89. D.A. Buttry and F.C. Anson, J. Am. Chem. Sot., 105 (1983) 685. H. Dahms, J. Phys. Chem., 72 (1968) 362. I. Ruff and V. Friedrich, J. Phys. Chem., 75 (1971) 3297. I. Ruff, V. Friedrich and K. Csillag, J. Phys. Chem., 75 (1971) 3303.