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Mechanism of formation of polypyrrole on a Pt electrode from aqueous solutions
MECHANISM OF FORMATION OF POLYPYRROLE ELECTRODE FROM AQUEOUS SOLUTIONS
ON A Pt
M. L. MARCOS, I. RODRIGUEZ and J. GONZ~~LEZ-VELASCO Departamento
de Electroquimica,
(Received
Facultad de Ciencias, Universidad 28049 Madrid, Spain
2 October
1986; in revised form
3 March
Aut6noma
de Madrid,
1987)
Abstract-A coherent explanation for the polypyrrole tridimensional growth on a Pt surface has been found. Two different potential ranges were studied on which the reaction mechanism changes. The results are not in disagreement with an instantaneous tridimensional nucleation [see, S. Asavapiriyanont, G. K. Chandler, G. A. Gunawardena and D. Pletcher, J. electronal. Chem. 177,229 (1984)] but do not show any pH dependence, from which a loss of H+ by the pyrrole monomers could be deduced.
INTRODUCTION Modification of electrode surfaces by means of layers of conducting polymers constitutes an area of major interest in the last years. One of the reasons for such interest is that the catalytic activity of the electrodes changes with the polymeric coverage. Another source of attention is the use of conducting polymers for covering unstable semiconductors, in order to prevent photodegradation of the covered materials. Many kinds of polymers have been formed on different substrates. One of the most studied materials has been polypyrrole[l-31. Diaz et ~I.[23 have published an attempt at explaining the mechanism of electropolymerization of pyrrole on Pt electrodes, from acetonitrile solutions in the presence of BF; anions. They concluded that polymerization should take place through a previous production of pyrrole n-radical-cation, formed by one electron transfer, which would act as the monomer for polymerization by reaction with neighbouring pyrrole molecules. Prejza et aI.[3] reported that BF, could also play a role in the initiation step of polymerization. According to these authors, a number of fast oxidation and deprotonation steps occur, which lead to the formation of a polymer. The oxidation of this substance takes place at more cathodic potentials than necessary for the oxidation of the monomer. The oxidation of the polymeric form of pyrrole would give rise to the conducting state of the polypyrrole layer. The influence of the substrate on the electropolymerization has also been investigated[3] and a similar behaviour of the reaction on Ti, Cr, Au, Pt, Pd and Ni was concluded. Pletcher et nl.[4], published a study on the deposition of polypyrrole on Pt from aqueous solutions, by applying potentiostatic pulses, obtaining as main conclusion that the results could be interpreted as the consequence of an instantaneous nucleation with 3-D growth. This paper presents the results of a mechanistic study made on the electropolymerization of pyrrole on Pt from aqueous solutions using 0.5 M H,SO, as supporting electrolyte. The problem has been conEh
32:10-c
fronted in a conventional way, by determining the main kinetic parameters and trying to find a coherent mechanism which could reproduce the experimental rate equation. A discussion of the paper comparing the results obtained with others previously reported is also made. The study was limited to the potential range at which the formation of a layer of polypyrrole is taking place, in order to be sure that we were examining the interaction between Pt free surface and pyrrole molecules. An examination of data taken at more anodic potential values allows the proposal of a mechanism for growing of polypyrrole on the polypyrrole layer previously formed.
EXPERIMENTAL A Fabelle Model MA 110 Potentiostat was used in the experiments. Potential sweeps were applied from a PAR MO. 175 Universal Programmer and the voltammograms were plotted on a 7047 A Hewlett-Packard X-Y recorder. A three compartment thermostable cell was used. The three compartments were separated through G-3 glass frites. As reference electrode a Hg/Hg2S0.+/K2S04 (salt) (MSE) was used and was introduced in a compartment communicating with the proximity of the working electrode surface by means of a Haber-Luggin capillary. The working electrode consisted in a 0.5 cm2 Pt plate and a 2 cm’ Pt cylinder was used as counter electrode. Solutions were prepared from triply distilled water and H,S04, pyrrole, and NaZS04 from Merck (p.a.) were used as reagents. The pyrrole solutions were used immediately after preparation, since they are very sensitive to light and heat. They were deoxygenated by stirring with a Nz stream during 15 min before every experiment. The Pt electrodes were polished with alumina powder and activated by applying a 1 V s - ’ potential sweep until obtaining a reproducible voltammogram on 1 N H2S04, which was used as a standard for electrode activity.
1453
M. L. MARCOS
1454
RESULTS All experiments were carried out at different acid pH values. Figure 1 shows a typical voltammogram for the formation of polypyrrole on a Pt electrode from an acid aqueous solution. In a first potential range the current increases with a Tafel slope of around 60 mV decade- .‘ In that range the electrode surface is covered with a yellow colour polymer layer. By increasing the potential, a rapid change of colour from yellow to almost black takes place, even before the peak poten-
z
5: 4
I
‘
Fig. 1. Volhmmogram for polypyrrole formation on Pt. T = 25”C, 1 N H,SO, as supporting electrolyte, C,, = 2.5 x lo-’ M, v = 5 mVs-‘.
et al.
tial has been reached. In the second potential range, a value of the Tafel slope is obtained of around 90 mV (current decade))‘. At least a second peak exists at more anodic potentials than those at which the first one appears. It seems that once a first layer of polypyrrole has grown on the electrode surface the polymerization process continues on it. No sign of oxide formation appears, since no reverse peak can be recorded in the cathodic sweep. Like in the case described by Pletcher er al.[4] the current at the beginning of the cathodic sweep is initially higher than on the anodic one, which according to these authors should be due to a nucleation growth mechanism of a surface phase. From the analysis of the voltammogram obtained, an irreversible behaviour for the polymerization reaction in the first potential range is deduced. Moreover, the voltammetric data seem to fit to a ErCi mechanism, in which a reversible charge transfer is followed by an irreversible chemical reaction. When the reaction is in the pure kinetic region a shift of E, towards more anodic potential should be expected of around 30/n mV (at 25”C)[S]. In this case, from 0 to 60 mV SK’, the shift obtained is of around 15 mV in the E,, which is in agreement with a value of n = 2 as number of electrons being transfered during the polypyrrole formation. This number of electrons is close to that deduced by Pletcher et al.[4] of around 2.6 electrons for the polymerization of pyrrole to an oxidized film. When using solutions in which the concentration of pyrrole was changed from 5 x 10e3 to 10-l M, at a potential value of 250 mV,,, the electrode is found to be always covered by a black layer of polymer. However, for solutions of 10-j M in pyrrole, the electrode does not become covered with a polymer layer, although the anodic peak is always present. For 2.5 x 10m3 M in pyrrole, the coverage of the Pt electrode is very irregular and shows a brown colour. Figure 2 shows various Tafel lines obtained for polypyrrole formation in the first range of potentials
E(V/
MSE)
Fig. 2. Tafel slopes for polymerization of pyrrole on Pt. T = ZYC, 1 N H,SO, as supporting electrolyte (1st cycle).
Mechanism of formation of polypyrrole
and at different pyrrole concentrations. The Tafel slopes were measured from the voltammograms after testing that no variation of the Tafel slope with sweep rate could be detected. No change in the current was registered by changing the pH value. Figure 3 shows a log J “S log Cpyr& P lot from which a reaction order of 1 with respect to the pyrrole concentration is deduced. The temperature dependence of I, measured in the potential range in which the Tafel slope is around 60 mV (current decade)- I, produces an average value for AH+ of 13.7 kJmol_l, whereas at more anodic potentiif: where the Tafel slope is around 90 mV (current decade)-‘, AH$ = 17 kJmol_‘. Similarly, an indicative value for A s &, was obtained from the
1
E(mV/r.w -3
-=-
-
225 210 195 le.0 165
-
-
1455
intercept of the In I/T us l/T plots. The apparent entropy of formation of the activated complex, is around - 245 f 5 J mol- 1 K- I. This value was found at every potential value before the peak. This result is interpreted as an ordering of the pyrrole molecules before forming the reaction product (polypyrrole) and is also, in agreement with the attachment of pyrrole molecules to the Pt surface (in the first range of potentials examined) and of pyrrole molecules on the polypyrrole layer, during the second potential range studied. An additional comment on the temperature study is to point out the loss of the crossing of the back sweep on the forward one. The peak becomes much more symmetricai and no crossing of both scans can be seen. Table 1 summarizes the kinetic parameters obtained for the formation of polypyrrole on Pt electrodes from aqueous solutions at acidic pH values. Adsorption
on Pt
of pyrrole
Voltammograms recorded on a Pt electrode immersed in a 0.5 M H,S04 solution containing different pyrrole concentrations showed an inhibition in the peaks of hydrogen adsorption-desorption proportional to the pyrrole concentration in solution. This has been interpreted as a consequence of a coadsorption of hydrogen and pyrrole on the Pt surface. An approximate value for the surface coverage with pyrrole was obtained using following relationship, =
B
QH,=,-QH~~
PY
-3 Log C,
-2 tmol dm?
-I
Fig. 3. Plot of log Z (A) us log C, at different potential values. T = 25”C, 1 N H,SO,
as supporting electrolyte.
Table 1. Pyrrole on Pt electrodes (E mV,.& Low pyrrole concentration (10 - 3 M) Tafel slope, mV (current decade)-’ Potential range, mVHsE Tafel slope, mV (current decade)- ’ Potential range, mVMsE
QH@=,
(first cycle)
pH = 0
pH = 1; 2; 7
(lOGgO) (l&gO) Low potential region (18&w)
(lgz240)
-1 0 13.7 kJmol_’ (100-180) 17.0 kJmol_’ (180-240) At every potential value
-1 0 13.7 kJmol_’ (100-180) 17.0 kJmol_’ (180-240) -245 f 5 Jmol-’ K-l
High pyrrole concentration (1 M) Tafel slope, mV (current decade-‘) Potential range Tafel slope mV (current decade)-’ Potential range Reaction order C PYrrOlC CH’
AH& Potential range, mVMsE AH&, Potential range, mV,,, AS+‘WP
’
where QH@=, represents the integrated charge under the voltammogram in the cathodic potential range in which the hydrogen adsorption-desorption process takes place, in the absence of pyrrole in solution, and QH is the integrated charge for hydrogen adsorption in t‘h e presence of different pyrrole concentrations in solution.
--------------.-. -*1 M. L. MARCOS et al.
1456
ie a rather high value for the free = 116 kJmol_‘, energy of adsorption of pyrrole on Pt was deduced.
a?a5
Second
lZ
.
.
.
L
I
I
5
0
I
15
IO
-WI c,,
Fig. 4. Plot of Pt surface coverage with pyrrole as a function of the logarithm of pyrrole concentration in solution. T = 25-C, 1 N H2S04 as supporting electrolyte. Figure 4 shows a plot of 0, obtained according to equation (1) 0s In C, (C in mol dm- 3). The linearity of the plot, and the independence of pyrrole adsorption with respect to the potential value indicate that pyrrole adsorption is described by an isotherm of the type: exp(
-*)
or, in logarithmic
g = PY
= K,C,,,
(2)
form:
RT QHeTL-QHPy = _RTlnK, --lnCp, Q%-I
(3)
gPY
BPY
Figure 4 also shows that the linear relationship only fits from a concentration of around lo-’ M in pyrrole in solution. Likewise, the shape of the voltammograms in the hydrogen adsorptiondesorption potential range indicates that pyrrole adsorption takes place without any dissociation. From the slope and intercept of the aforementioned plot, a vlaue for K, was from which AC,, = - RTln K1 obtained,
(a)
3 -
-2 E +
2-
and further
cycles
in voltammograms
The kinetic parameters obtained after cycling at sufficiently slow sweep rates, so that pyrrole diffusion can not influence the recorded voltammograms, show differences in relation to those calculated for the first sweep. Once a first polypyrrole layer on Pt is formed, a modified electrode results on which as much charge transfer, as well as polymerization can take place in a different way than on a bare Pt surface. Figure Sa shows the voltammograms obtained after repeatedly cycling at 5 mV s- ‘. The sweep rate is supposed to be slow enough considering that every new cycle as a voltammogram is taken under linear sweep conditions. Figure 5b gives the variation of the Tafel slopes obtained for every cycle given in 5a. Figure 6 shows the Tafel lines obtained during the second cycle as a function of the pyrrole concentration in solution. The most reliable value for the Tafel slope changes from 110 to 100 mV (current decade)-‘, not very far from the 90 mV obtained for the case of the second potential range considered during the first cycle. Figure 7 shows the log i us log Cpyrrolsplots at different potential values. The reaction order deduced with relation to the pyrrole concentration is close to 1 and pH dependence was also not detected. Table 2 shows the change in the Tafel slope which can be observed for every new cycle sweep (see Fig. 5b).
DISCUSSION During the first voltammetric Tafel slope is observed when
cycle a change in the measuring it in two
b) -2
I* _.-. 2M ____ 3rd . ...*. 4’”
iT i
lst cycle 2”dcycle 3%zycle 4%ycle
z -2
<
\
!
-0-w -e
s ! \
J
I.15
E(V/MSE)
I
0.2 E(V/MsE)
I
0.25
Fig. 5. (a) Voltammogram of 2.5 x 10m2 M pyrrole in 1 N H,SO, at 25°C showing the change of shape at different cycles. (b) Values of the Tafel slopes obtained during each cycle according to Fig. 5a.
Mechanism of formation of polypyrrole
1457
Fig. 6. Tafel slopes for polymerization of pyrrole on a pyrrole modified Pt electrode. The slopes were measured for the 2nd cycle at different pyrrole concentrations. T = 25°C. 1 N H2S04 as supporting electrolyte (2nd cycle).
-3
E(rnV/hsE)
_
1
-.-
210
-a---a--
195 180 165
I
I
I
-a
-3
log
=w
-I
( mol dm? 1
Fig. 7. Plot of log I (A) vs log C,at different potential values during the 2nd cycle, T = 25°C. 1 N HZS04. Table 2. Tafel slope Number of cycle mV (current decade)-
1
2 3 4
60 and
’
90 mV
110 188 250
different potential ranges. Therefore, two different experimental rate equations for polypyrrole formation can be written according to the general expression
i Plym = nFkC,,exp where CLrepresents transference coefficient. Its value is 1 in the more cathodic potential range studied and 213 in the more anodic one.
The reaction mechanism which should lead to a theoretical rate equation in agreement with equation (4), has to take into account the fact that the polypyrrole formation is produced through an instantaneous 3-D nucleation process[4]. On the other hand, it seems that the formation of a polypyrrole layer on Pt only takes place when the concentration of pyrrole in aqueous solutions is higher than a critical value of around fOA3 M. Below this value, the oxidation current is present, but no sign of polymer layer formation is detected on the electrode surface. This fact is in agreement with the result that the logarithmic adsorption isotherm only fits from around 10-s M of pyrrole in solution (see Fig. 4) which corresponds to a surface coverage of around 0.5. Therefore, only from a critical value of the pyrrole concentration in solution, the coverage of the Pt surface is high enough for an instantaneous nucleation. The proximity of pyrrole molecules adsorbed on the Pt surface results in the formation of a band of electronic levels. Once the first charge transfer from a pyrrole molecule to the Pt electrode takes place, according to: (py),,, + (p~+)_,~ + le-, (PY +Lds could act as an initiator for a surface polymerization, which would almost instantaneously take place by interaction with neighbouring adsorbed pyrrole molecules. Once the pyrrole adsorbed layer is formed, the initiation reaction would not be dependent on the concentration of pyrrole in solution, and, as a consequence, the rate of the initiation step would be constant and we would obtain a reaction order equal to zero, with respect to pyrrole concentration. On the other hand, the fact that the surface concentration of adsorbed pyrrole must be high enough, for the instantaneous nucleation to take place[4], (which is detected on the voltammogram by the instantaneous change of colour at critical potential value from yellow to dark brown), indicates that a high interaction between adsorbed pyrrole molecules has to take place. According to the above discussed ideas, the following mechanism seems to explain the experimental rate
M. L. MARCOS er al.
1458
equation, [step II-_(PY)OHP* (PYL [step 2]-(py)Bds * (pyz )& + 1 e - (initiation of polymerization), [step 31-_(PY~’ L + (PY),, * (PYO’ )& + 2I-I+ + 2etermination steppolypyrrole formation. According to the adsorption study pyrrole adsorbs on a Pt surface following a Temkin isotherm. Thus, from step 1 we obtain exp(*)
= K,C,,
which is the experimental isotherm obtained from Fig. 4. Where g, represents the interaction coefficient for adsorbed pyrrole molecules, eT = total coverage with different species, K, = equilibrium constant for step 1 of pyrrole in the solution. and C, = concentration Also step 2, charge transfer from adsorbed pyrrole to the Pt is considered to behave reversibly. In this case, the charge transfer process could be considered as a desorption reaction of the pyrrole molecules giving rise to an adsorption of the pyz radicals formed after electron transfer. Similarly, the reverse reaction would mean a desorption of the pyz radical and give rise to an adsorption of the pyrrole molecules[b]. Both rate equations can be written as follows,
(1 - Ypyklpy4
(
tr2 = k,ep,exp
x exp (““$+“;i
> exp ((l
-LYE),
x 0,
= exp(
-G)
K=
(8)
= K,K,C,,expg). (9)
On the other hand, the kinetic equation for a free radical mechanism, has been solved[7] and is given by following expression: l/2
00,
W)
K,K2.
nFk
(12)
where the polymerization rate = nucleation rate. A comparison of equation (11) with equation (4) shows that the theoretical treatment given to the proposed reaction mechanism, is able to explain the kinetic parameters experimentally obtained for the first potential range considered, for both low and high pyrrole concentrations (see Table 1).
of the
(10)
data
obtained
at
range the following
more
anodic
rate equation
(13) Once the first layer of polypyrrole has been formed, the polymerization reaction follows, thus the charge transfer has to take place to a less conducting surface than the metallic one. Thus, charge transfer could become the rate determining step and the current obtained would be proportional to the charge transfer rate, ie (PY)OHP
= K,xexp(-*)xexp(&)
>
and, substituting equation (2) taking into account that the first member in equation (8) is a representation of the surface concentration of py: , ie monomer surface concentration for the initiation step in polymerization, then, (M)
with
For this potential is obtained
( qpy+ RT
= nFk
(11)
Interpretation potentials
where y, and y py+represent the symmetry coefficient for the adsorption of pyrrole molecules and pyrrole radical monocation and g,, and g,,; the interaction coefficients for the coverage with pyrrole and radical, respectively. The similarity between py and py: would allow y,, u ypY+ Y y and p symmetry coefficient for charge transfer from pyrrole molecules to Pt = 0.5. Thus, by equalizing equations (6) and (7), we obtain -~
i, = nFo,
(6)
u_~ = k_,B,+exp
exp
where vp = polymerization rate, kp represents a rate constant for the polymerization reaction, yi = initiation rate, rate constant for the termlnation step and (M) = monomer concentration. By supposing that the polymerization on the Pt surface follows a radical mechanism, equation (10) is valid, provided that (M) would be a surface coverage. We have discussed that u,is a zero order reaction, ie ui = ki, and equation (9) would represent the monomer concentration. Thus, if the current is proportional to the polymerization rate following rate equation is obtained,
+
(PY
*)adS,,ok
+ lee,
(14)
,
(15)
01= 0.66 = transference where coefficient = l-j?, where p represents the symmetry coefficient for the charge transfer in (14). With p = 0.33, the geometric area of the electrode used = 0.4 cm2 and the n value obtained by Pletcher et aI.[4] is n = 2.65, a value of Dpyrrcts = 2.4. lo-’ cm ~2 s- 1is also obtained. The increase in the value of apparent activation energy from 13.7 to 17 kJmol_’ would agree with the suggested difficulty in the charge transfer reaction, connected with the presence of the oxidized polypyrrole layer on the Pt surface. The change in the mechanism of growth of the polypyrrole layer after cycling can be followed by
Mechanism
of formation
means of the results summarized in Table 2. The change in Tafel slope could be explained as the consequence of an increase in the thickness of the polypyrrole layer and perhaps to a change in the conductivity behaviour of such layer, caused by the loss of activity of the polymeric layer damaged by the scans towards anodic potentials. Anyway, the Tafel slope value appears to be almost proportional to the number of cycles ie, Tafel slope at the nth cycle = Tafel slope on the 1st cycle n. A more coherent explanation for the change in Tafel slope from the 1st to the 4th cycle can be found in the fact that conducting, conjugated polymers, are easily and irreversibly oxidized at anodic potentials in aqueous solutions[S]. Therefore, the process occurring in the 2nd to 4th cycle could be an oxidation of the polymer layer already formed, leading to a film of a partly non-conducting polypyrrole, followed by a deposition of a new layer of polypyrrole on it which can be the cause of the observed increase in the Tafel slope. An interesting result in order to show how the polymerization reaction takes place is the fact that the voltammogram obtained with pyrrole on a Pt electrode, activated according to Arvia et al.[9] (activation, which leads to an increase in surface depth) does not show important changes with respect to those obtained on unactivated Pt electrodes. This result can be interpreted as a consequnce of the coverage of the geometric Pt surface by a bidimensional laver of polypyrrole on which &dimensional growth would take place in direction toy91 the solution. The holes in the et surface supposed to be present in order to explain the high gain in activity of the electrode, would only be in part covered with polymer, so that almost no change in the whole poly&eri&tion reaction can be detected. The only feature of the voltammogram which changes is a lack of hysteresis observed, and a more symmetric peak is formed, like in the case of high temperatures. In conclusion, the chains of polypyrrole formed on the surface and are crossed and linked until a tridimensional layer has been formed, with an almost uniform structure. In any case the fact that after using the electrodes covered with polypyrrole in water solutions for studying redox reactions of couples of the kind of Feat/Fez+ and other, an swelling of the polymer layer is observed, has to be interpreted as a consequence of water molecules penetrating the polypyrrole network on the electrode. Thus the polymer layer seems to present channels through which water molecules can enter, and, after a time, no more isolation of the electrode surface by the polymer layer can be taking place under the experimental conditions
of polypyrrolc
1459
used. On the other hand, the peak corresponding to 2nd and further cycles are not completely reproducible. This is not surprising, since different polypyrrole layers grow, the first one on Pt, on which a planar interaction between pyrrole molecules and Pt surface can be supposed and, as consequence, a stronger linkage and a more stable layer is formed. The erowth of polypyrrole on polypyrroie seems to give rise to not so adherent layers as the first one, and also the morphology of the polypyrrole modified Pt electrode is not as reproducible as the metallic surface. In this way, the swelling of the posterior layers, can be explained as well as the permanence of a yellow polypyrrole layer on the Pt surface after peeling off, of the more external layers caused by penetration of water. On the other hand, the fact that some authors[3] do not find any dependence of the mechanism of polypyrrole formation on the substrate can be due to the fact that the data obtained in their work could correspond to growth of polypyrrole on a polypyrrole layer previously formed. A second part of this work[lO] considered the study of growth of polypyrrole on glass doped with SnO, and In (ITO), on which an entirely different form of growing is obtained, as well as a polypyrrole layer of different adherence. Acknowledgements-The support of this research by S. A. Riotinto and the C.A.I.C. yT. (Grant No. 935) is gratefully acknowledged.
REFERENCES 1. K. K. Kanazawa, A. F. Diaz, W. D. Gill, P. M. Grant and G. B. Street, Synth. Met. 4, 119 (1981). Linear Chain 2. A. F. Diaz and K. K. Kanazawa, in Extended Compounds (Edited by J. S. Miller), Vol. 3, p. 417. Plenum Press, New York (1982). 3. J. Prejza, 1. LundstrGm and T. Skotheim, J. eleccrochem. Sot. 129, 1685 (1982). 4. S. Asavapiriyanont, G. K. Chandler, G. A. Gunawardena and D. PIetcher, J. elctroanol. Chem. 177, 229 (1984). 5. R. S. Nicholson and I. Shain, Anal. C/tern. 36, 706 (1964). 6. B. E. Conway, Theory and Principles o/ Electrode Processes, Ronald Press, New York (1965). I. K. J. Laidler, in Chemical Kinetics, McGraw-Hill, London f 1965). 8 K. H. Dietz F. Beck, J. appl. Electrochem. 15, 159 (1985). 9 J. VBzquez, J. Gbmez, A. M. Barb, N. Garcia. M. L. Marcos, J. Gontilez-Velasco, J. M. Vara, A. J. ,&a, J. Press, A. GarCia and M. Aguilar, J. Am. Chem. Sot. 109, 1730 (1987). 10. 1. Rodriguez, M. L. Marcos and J. Gontilez-Velasco, Electrochim. Acta (in press).
ON A Pt
M. L. MARCOS, I. RODRIGUEZ and J. GONZ~~LEZ-VELASCO Departamento
de Electroquimica,
(Received
Facultad de Ciencias, Universidad 28049 Madrid, Spain
2 October
1986; in revised form
3 March
Aut6noma
de Madrid,
1987)
Abstract-A coherent explanation for the polypyrrole tridimensional growth on a Pt surface has been found. Two different potential ranges were studied on which the reaction mechanism changes. The results are not in disagreement with an instantaneous tridimensional nucleation [see, S. Asavapiriyanont, G. K. Chandler, G. A. Gunawardena and D. Pletcher, J. electronal. Chem. 177,229 (1984)] but do not show any pH dependence, from which a loss of H+ by the pyrrole monomers could be deduced.
INTRODUCTION Modification of electrode surfaces by means of layers of conducting polymers constitutes an area of major interest in the last years. One of the reasons for such interest is that the catalytic activity of the electrodes changes with the polymeric coverage. Another source of attention is the use of conducting polymers for covering unstable semiconductors, in order to prevent photodegradation of the covered materials. Many kinds of polymers have been formed on different substrates. One of the most studied materials has been polypyrrole[l-31. Diaz et ~I.[23 have published an attempt at explaining the mechanism of electropolymerization of pyrrole on Pt electrodes, from acetonitrile solutions in the presence of BF; anions. They concluded that polymerization should take place through a previous production of pyrrole n-radical-cation, formed by one electron transfer, which would act as the monomer for polymerization by reaction with neighbouring pyrrole molecules. Prejza et aI.[3] reported that BF, could also play a role in the initiation step of polymerization. According to these authors, a number of fast oxidation and deprotonation steps occur, which lead to the formation of a polymer. The oxidation of this substance takes place at more cathodic potentials than necessary for the oxidation of the monomer. The oxidation of the polymeric form of pyrrole would give rise to the conducting state of the polypyrrole layer. The influence of the substrate on the electropolymerization has also been investigated[3] and a similar behaviour of the reaction on Ti, Cr, Au, Pt, Pd and Ni was concluded. Pletcher et nl.[4], published a study on the deposition of polypyrrole on Pt from aqueous solutions, by applying potentiostatic pulses, obtaining as main conclusion that the results could be interpreted as the consequence of an instantaneous nucleation with 3-D growth. This paper presents the results of a mechanistic study made on the electropolymerization of pyrrole on Pt from aqueous solutions using 0.5 M H,SO, as supporting electrolyte. The problem has been conEh
32:10-c
fronted in a conventional way, by determining the main kinetic parameters and trying to find a coherent mechanism which could reproduce the experimental rate equation. A discussion of the paper comparing the results obtained with others previously reported is also made. The study was limited to the potential range at which the formation of a layer of polypyrrole is taking place, in order to be sure that we were examining the interaction between Pt free surface and pyrrole molecules. An examination of data taken at more anodic potential values allows the proposal of a mechanism for growing of polypyrrole on the polypyrrole layer previously formed.
EXPERIMENTAL A Fabelle Model MA 110 Potentiostat was used in the experiments. Potential sweeps were applied from a PAR MO. 175 Universal Programmer and the voltammograms were plotted on a 7047 A Hewlett-Packard X-Y recorder. A three compartment thermostable cell was used. The three compartments were separated through G-3 glass frites. As reference electrode a Hg/Hg2S0.+/K2S04 (salt) (MSE) was used and was introduced in a compartment communicating with the proximity of the working electrode surface by means of a Haber-Luggin capillary. The working electrode consisted in a 0.5 cm2 Pt plate and a 2 cm’ Pt cylinder was used as counter electrode. Solutions were prepared from triply distilled water and H,S04, pyrrole, and NaZS04 from Merck (p.a.) were used as reagents. The pyrrole solutions were used immediately after preparation, since they are very sensitive to light and heat. They were deoxygenated by stirring with a Nz stream during 15 min before every experiment. The Pt electrodes were polished with alumina powder and activated by applying a 1 V s - ’ potential sweep until obtaining a reproducible voltammogram on 1 N H2S04, which was used as a standard for electrode activity.
1453
M. L. MARCOS
1454
RESULTS All experiments were carried out at different acid pH values. Figure 1 shows a typical voltammogram for the formation of polypyrrole on a Pt electrode from an acid aqueous solution. In a first potential range the current increases with a Tafel slope of around 60 mV decade- .‘ In that range the electrode surface is covered with a yellow colour polymer layer. By increasing the potential, a rapid change of colour from yellow to almost black takes place, even before the peak poten-
z
5: 4
I
‘
Fig. 1. Volhmmogram for polypyrrole formation on Pt. T = 25”C, 1 N H,SO, as supporting electrolyte, C,, = 2.5 x lo-’ M, v = 5 mVs-‘.
et al.
tial has been reached. In the second potential range, a value of the Tafel slope is obtained of around 90 mV (current decade))‘. At least a second peak exists at more anodic potentials than those at which the first one appears. It seems that once a first layer of polypyrrole has grown on the electrode surface the polymerization process continues on it. No sign of oxide formation appears, since no reverse peak can be recorded in the cathodic sweep. Like in the case described by Pletcher er al.[4] the current at the beginning of the cathodic sweep is initially higher than on the anodic one, which according to these authors should be due to a nucleation growth mechanism of a surface phase. From the analysis of the voltammogram obtained, an irreversible behaviour for the polymerization reaction in the first potential range is deduced. Moreover, the voltammetric data seem to fit to a ErCi mechanism, in which a reversible charge transfer is followed by an irreversible chemical reaction. When the reaction is in the pure kinetic region a shift of E, towards more anodic potential should be expected of around 30/n mV (at 25”C)[S]. In this case, from 0 to 60 mV SK’, the shift obtained is of around 15 mV in the E,, which is in agreement with a value of n = 2 as number of electrons being transfered during the polypyrrole formation. This number of electrons is close to that deduced by Pletcher et al.[4] of around 2.6 electrons for the polymerization of pyrrole to an oxidized film. When using solutions in which the concentration of pyrrole was changed from 5 x 10e3 to 10-l M, at a potential value of 250 mV,,, the electrode is found to be always covered by a black layer of polymer. However, for solutions of 10-j M in pyrrole, the electrode does not become covered with a polymer layer, although the anodic peak is always present. For 2.5 x 10m3 M in pyrrole, the coverage of the Pt electrode is very irregular and shows a brown colour. Figure 2 shows various Tafel lines obtained for polypyrrole formation in the first range of potentials
E(V/
MSE)
Fig. 2. Tafel slopes for polymerization of pyrrole on Pt. T = ZYC, 1 N H,SO, as supporting electrolyte (1st cycle).
Mechanism of formation of polypyrrole
and at different pyrrole concentrations. The Tafel slopes were measured from the voltammograms after testing that no variation of the Tafel slope with sweep rate could be detected. No change in the current was registered by changing the pH value. Figure 3 shows a log J “S log Cpyr& P lot from which a reaction order of 1 with respect to the pyrrole concentration is deduced. The temperature dependence of I, measured in the potential range in which the Tafel slope is around 60 mV (current decade)- I, produces an average value for AH+ of 13.7 kJmol_l, whereas at more anodic potentiif: where the Tafel slope is around 90 mV (current decade)-‘, AH$ = 17 kJmol_‘. Similarly, an indicative value for A s &, was obtained from the
1
E(mV/r.w -3
-=-
-
225 210 195 le.0 165
-
-
1455
intercept of the In I/T us l/T plots. The apparent entropy of formation of the activated complex, is around - 245 f 5 J mol- 1 K- I. This value was found at every potential value before the peak. This result is interpreted as an ordering of the pyrrole molecules before forming the reaction product (polypyrrole) and is also, in agreement with the attachment of pyrrole molecules to the Pt surface (in the first range of potentials examined) and of pyrrole molecules on the polypyrrole layer, during the second potential range studied. An additional comment on the temperature study is to point out the loss of the crossing of the back sweep on the forward one. The peak becomes much more symmetricai and no crossing of both scans can be seen. Table 1 summarizes the kinetic parameters obtained for the formation of polypyrrole on Pt electrodes from aqueous solutions at acidic pH values. Adsorption
on Pt
of pyrrole
Voltammograms recorded on a Pt electrode immersed in a 0.5 M H,S04 solution containing different pyrrole concentrations showed an inhibition in the peaks of hydrogen adsorption-desorption proportional to the pyrrole concentration in solution. This has been interpreted as a consequence of a coadsorption of hydrogen and pyrrole on the Pt surface. An approximate value for the surface coverage with pyrrole was obtained using following relationship, =
B
QH,=,-QH~~
PY
-3 Log C,
-2 tmol dm?
-I
Fig. 3. Plot of log Z (A) us log C, at different potential values. T = 25”C, 1 N H,SO,
as supporting electrolyte.
Table 1. Pyrrole on Pt electrodes (E mV,.& Low pyrrole concentration (10 - 3 M) Tafel slope, mV (current decade)-’ Potential range, mVHsE Tafel slope, mV (current decade)- ’ Potential range, mVMsE
QH@=,
(first cycle)
pH = 0
pH = 1; 2; 7
(lOGgO) (l&gO) Low potential region (18&w)
(lgz240)
-1 0 13.7 kJmol_’ (100-180) 17.0 kJmol_’ (180-240) At every potential value
-1 0 13.7 kJmol_’ (100-180) 17.0 kJmol_’ (180-240) -245 f 5 Jmol-’ K-l
High pyrrole concentration (1 M) Tafel slope, mV (current decade-‘) Potential range Tafel slope mV (current decade)-’ Potential range Reaction order C PYrrOlC CH’
AH& Potential range, mVMsE AH&, Potential range, mV,,, AS+‘WP
’
where QH@=, represents the integrated charge under the voltammogram in the cathodic potential range in which the hydrogen adsorption-desorption process takes place, in the absence of pyrrole in solution, and QH is the integrated charge for hydrogen adsorption in t‘h e presence of different pyrrole concentrations in solution.
--------------.-. -*1 M. L. MARCOS et al.
1456
ie a rather high value for the free = 116 kJmol_‘, energy of adsorption of pyrrole on Pt was deduced.
a?a5
Second
lZ
.
.
.
L
I
I
5
0
I
15
IO
-WI c,,
Fig. 4. Plot of Pt surface coverage with pyrrole as a function of the logarithm of pyrrole concentration in solution. T = 25-C, 1 N H2S04 as supporting electrolyte. Figure 4 shows a plot of 0, obtained according to equation (1) 0s In C, (C in mol dm- 3). The linearity of the plot, and the independence of pyrrole adsorption with respect to the potential value indicate that pyrrole adsorption is described by an isotherm of the type: exp(
-*)
or, in logarithmic
g = PY
= K,C,,,
(2)
form:
RT QHeTL-QHPy = _RTlnK, --lnCp, Q%-I
(3)
gPY
BPY
Figure 4 also shows that the linear relationship only fits from a concentration of around lo-’ M in pyrrole in solution. Likewise, the shape of the voltammograms in the hydrogen adsorptiondesorption potential range indicates that pyrrole adsorption takes place without any dissociation. From the slope and intercept of the aforementioned plot, a vlaue for K, was from which AC,, = - RTln K1 obtained,
(a)
3 -
-2 E +
2-
and further
cycles
in voltammograms
The kinetic parameters obtained after cycling at sufficiently slow sweep rates, so that pyrrole diffusion can not influence the recorded voltammograms, show differences in relation to those calculated for the first sweep. Once a first polypyrrole layer on Pt is formed, a modified electrode results on which as much charge transfer, as well as polymerization can take place in a different way than on a bare Pt surface. Figure Sa shows the voltammograms obtained after repeatedly cycling at 5 mV s- ‘. The sweep rate is supposed to be slow enough considering that every new cycle as a voltammogram is taken under linear sweep conditions. Figure 5b gives the variation of the Tafel slopes obtained for every cycle given in 5a. Figure 6 shows the Tafel lines obtained during the second cycle as a function of the pyrrole concentration in solution. The most reliable value for the Tafel slope changes from 110 to 100 mV (current decade)-‘, not very far from the 90 mV obtained for the case of the second potential range considered during the first cycle. Figure 7 shows the log i us log Cpyrrolsplots at different potential values. The reaction order deduced with relation to the pyrrole concentration is close to 1 and pH dependence was also not detected. Table 2 shows the change in the Tafel slope which can be observed for every new cycle sweep (see Fig. 5b).
DISCUSSION During the first voltammetric Tafel slope is observed when
cycle a change in the measuring it in two
b) -2
I* _.-. 2M ____ 3rd . ...*. 4’”
iT i
lst cycle 2”dcycle 3%zycle 4%ycle
z -2
<
\
!
-0-w -e
s ! \
J
I.15
E(V/MSE)
I
0.2 E(V/MsE)
I
0.25
Fig. 5. (a) Voltammogram of 2.5 x 10m2 M pyrrole in 1 N H,SO, at 25°C showing the change of shape at different cycles. (b) Values of the Tafel slopes obtained during each cycle according to Fig. 5a.
Mechanism of formation of polypyrrole
1457
Fig. 6. Tafel slopes for polymerization of pyrrole on a pyrrole modified Pt electrode. The slopes were measured for the 2nd cycle at different pyrrole concentrations. T = 25°C. 1 N H2S04 as supporting electrolyte (2nd cycle).
-3
E(rnV/hsE)
_
1
-.-
210
-a---a--
195 180 165
I
I
I
-a
-3
log
=w
-I
( mol dm? 1
Fig. 7. Plot of log I (A) vs log C,at different potential values during the 2nd cycle, T = 25°C. 1 N HZS04. Table 2. Tafel slope Number of cycle mV (current decade)-
1
2 3 4
60 and
’
90 mV
110 188 250
different potential ranges. Therefore, two different experimental rate equations for polypyrrole formation can be written according to the general expression
i Plym = nFkC,,exp where CLrepresents transference coefficient. Its value is 1 in the more cathodic potential range studied and 213 in the more anodic one.
The reaction mechanism which should lead to a theoretical rate equation in agreement with equation (4), has to take into account the fact that the polypyrrole formation is produced through an instantaneous 3-D nucleation process[4]. On the other hand, it seems that the formation of a polypyrrole layer on Pt only takes place when the concentration of pyrrole in aqueous solutions is higher than a critical value of around fOA3 M. Below this value, the oxidation current is present, but no sign of polymer layer formation is detected on the electrode surface. This fact is in agreement with the result that the logarithmic adsorption isotherm only fits from around 10-s M of pyrrole in solution (see Fig. 4) which corresponds to a surface coverage of around 0.5. Therefore, only from a critical value of the pyrrole concentration in solution, the coverage of the Pt surface is high enough for an instantaneous nucleation. The proximity of pyrrole molecules adsorbed on the Pt surface results in the formation of a band of electronic levels. Once the first charge transfer from a pyrrole molecule to the Pt electrode takes place, according to: (py),,, + (p~+)_,~ + le-, (PY +Lds could act as an initiator for a surface polymerization, which would almost instantaneously take place by interaction with neighbouring adsorbed pyrrole molecules. Once the pyrrole adsorbed layer is formed, the initiation reaction would not be dependent on the concentration of pyrrole in solution, and, as a consequence, the rate of the initiation step would be constant and we would obtain a reaction order equal to zero, with respect to pyrrole concentration. On the other hand, the fact that the surface concentration of adsorbed pyrrole must be high enough, for the instantaneous nucleation to take place[4], (which is detected on the voltammogram by the instantaneous change of colour at critical potential value from yellow to dark brown), indicates that a high interaction between adsorbed pyrrole molecules has to take place. According to the above discussed ideas, the following mechanism seems to explain the experimental rate
M. L. MARCOS er al.
1458
equation, [step II-_(PY)OHP* (PYL [step 2]-(py)Bds * (pyz )& + 1 e - (initiation of polymerization), [step 31-_(PY~’ L + (PY),, * (PYO’ )& + 2I-I+ + 2etermination steppolypyrrole formation. According to the adsorption study pyrrole adsorbs on a Pt surface following a Temkin isotherm. Thus, from step 1 we obtain exp(*)
= K,C,,
which is the experimental isotherm obtained from Fig. 4. Where g, represents the interaction coefficient for adsorbed pyrrole molecules, eT = total coverage with different species, K, = equilibrium constant for step 1 of pyrrole in the solution. and C, = concentration Also step 2, charge transfer from adsorbed pyrrole to the Pt is considered to behave reversibly. In this case, the charge transfer process could be considered as a desorption reaction of the pyrrole molecules giving rise to an adsorption of the pyz radicals formed after electron transfer. Similarly, the reverse reaction would mean a desorption of the pyz radical and give rise to an adsorption of the pyrrole molecules[b]. Both rate equations can be written as follows,
(1 - Ypyklpy4
(
tr2 = k,ep,exp
x exp (““$+“;i
> exp ((l
-LYE),
x 0,
= exp(
-G)
K=
(8)
= K,K,C,,expg). (9)
On the other hand, the kinetic equation for a free radical mechanism, has been solved[7] and is given by following expression: l/2
00,
W)
K,K2.
nFk
(12)
where the polymerization rate = nucleation rate. A comparison of equation (11) with equation (4) shows that the theoretical treatment given to the proposed reaction mechanism, is able to explain the kinetic parameters experimentally obtained for the first potential range considered, for both low and high pyrrole concentrations (see Table 1).
of the
(10)
data
obtained
at
range the following
more
anodic
rate equation
(13) Once the first layer of polypyrrole has been formed, the polymerization reaction follows, thus the charge transfer has to take place to a less conducting surface than the metallic one. Thus, charge transfer could become the rate determining step and the current obtained would be proportional to the charge transfer rate, ie (PY)OHP
= K,xexp(-*)xexp(&)
>
and, substituting equation (2) taking into account that the first member in equation (8) is a representation of the surface concentration of py: , ie monomer surface concentration for the initiation step in polymerization, then, (M)
with
For this potential is obtained
( qpy+ RT
= nFk
(11)
Interpretation potentials
where y, and y py+represent the symmetry coefficient for the adsorption of pyrrole molecules and pyrrole radical monocation and g,, and g,,; the interaction coefficients for the coverage with pyrrole and radical, respectively. The similarity between py and py: would allow y,, u ypY+ Y y and p symmetry coefficient for charge transfer from pyrrole molecules to Pt = 0.5. Thus, by equalizing equations (6) and (7), we obtain -~
i, = nFo,
(6)
u_~ = k_,B,+exp
exp
where vp = polymerization rate, kp represents a rate constant for the polymerization reaction, yi = initiation rate, rate constant for the termlnation step and (M) = monomer concentration. By supposing that the polymerization on the Pt surface follows a radical mechanism, equation (10) is valid, provided that (M) would be a surface coverage. We have discussed that u,is a zero order reaction, ie ui = ki, and equation (9) would represent the monomer concentration. Thus, if the current is proportional to the polymerization rate following rate equation is obtained,
+
(PY
*)adS,,ok
+ lee,
(14)
,
(15)
01= 0.66 = transference where coefficient = l-j?, where p represents the symmetry coefficient for the charge transfer in (14). With p = 0.33, the geometric area of the electrode used = 0.4 cm2 and the n value obtained by Pletcher et aI.[4] is n = 2.65, a value of Dpyrrcts = 2.4. lo-’ cm ~2 s- 1is also obtained. The increase in the value of apparent activation energy from 13.7 to 17 kJmol_’ would agree with the suggested difficulty in the charge transfer reaction, connected with the presence of the oxidized polypyrrole layer on the Pt surface. The change in the mechanism of growth of the polypyrrole layer after cycling can be followed by
Mechanism
of formation
means of the results summarized in Table 2. The change in Tafel slope could be explained as the consequence of an increase in the thickness of the polypyrrole layer and perhaps to a change in the conductivity behaviour of such layer, caused by the loss of activity of the polymeric layer damaged by the scans towards anodic potentials. Anyway, the Tafel slope value appears to be almost proportional to the number of cycles ie, Tafel slope at the nth cycle = Tafel slope on the 1st cycle n. A more coherent explanation for the change in Tafel slope from the 1st to the 4th cycle can be found in the fact that conducting, conjugated polymers, are easily and irreversibly oxidized at anodic potentials in aqueous solutions[S]. Therefore, the process occurring in the 2nd to 4th cycle could be an oxidation of the polymer layer already formed, leading to a film of a partly non-conducting polypyrrole, followed by a deposition of a new layer of polypyrrole on it which can be the cause of the observed increase in the Tafel slope. An interesting result in order to show how the polymerization reaction takes place is the fact that the voltammogram obtained with pyrrole on a Pt electrode, activated according to Arvia et al.[9] (activation, which leads to an increase in surface depth) does not show important changes with respect to those obtained on unactivated Pt electrodes. This result can be interpreted as a consequnce of the coverage of the geometric Pt surface by a bidimensional laver of polypyrrole on which &dimensional growth would take place in direction toy91 the solution. The holes in the et surface supposed to be present in order to explain the high gain in activity of the electrode, would only be in part covered with polymer, so that almost no change in the whole poly&eri&tion reaction can be detected. The only feature of the voltammogram which changes is a lack of hysteresis observed, and a more symmetric peak is formed, like in the case of high temperatures. In conclusion, the chains of polypyrrole formed on the surface and are crossed and linked until a tridimensional layer has been formed, with an almost uniform structure. In any case the fact that after using the electrodes covered with polypyrrole in water solutions for studying redox reactions of couples of the kind of Feat/Fez+ and other, an swelling of the polymer layer is observed, has to be interpreted as a consequence of water molecules penetrating the polypyrrole network on the electrode. Thus the polymer layer seems to present channels through which water molecules can enter, and, after a time, no more isolation of the electrode surface by the polymer layer can be taking place under the experimental conditions
of polypyrrolc
1459
used. On the other hand, the peak corresponding to 2nd and further cycles are not completely reproducible. This is not surprising, since different polypyrrole layers grow, the first one on Pt, on which a planar interaction between pyrrole molecules and Pt surface can be supposed and, as consequence, a stronger linkage and a more stable layer is formed. The erowth of polypyrrole on polypyrroie seems to give rise to not so adherent layers as the first one, and also the morphology of the polypyrrole modified Pt electrode is not as reproducible as the metallic surface. In this way, the swelling of the posterior layers, can be explained as well as the permanence of a yellow polypyrrole layer on the Pt surface after peeling off, of the more external layers caused by penetration of water. On the other hand, the fact that some authors[3] do not find any dependence of the mechanism of polypyrrole formation on the substrate can be due to the fact that the data obtained in their work could correspond to growth of polypyrrole on a polypyrrole layer previously formed. A second part of this work[lO] considered the study of growth of polypyrrole on glass doped with SnO, and In (ITO), on which an entirely different form of growing is obtained, as well as a polypyrrole layer of different adherence. Acknowledgements-The support of this research by S. A. Riotinto and the C.A.I.C. yT. (Grant No. 935) is gratefully acknowledged.
REFERENCES 1. K. K. Kanazawa, A. F. Diaz, W. D. Gill, P. M. Grant and G. B. Street, Synth. Met. 4, 119 (1981). Linear Chain 2. A. F. Diaz and K. K. Kanazawa, in Extended Compounds (Edited by J. S. Miller), Vol. 3, p. 417. Plenum Press, New York (1982). 3. J. Prejza, 1. LundstrGm and T. Skotheim, J. eleccrochem. Sot. 129, 1685 (1982). 4. S. Asavapiriyanont, G. K. Chandler, G. A. Gunawardena and D. PIetcher, J. elctroanol. Chem. 177, 229 (1984). 5. R. S. Nicholson and I. Shain, Anal. C/tern. 36, 706 (1964). 6. B. E. Conway, Theory and Principles o/ Electrode Processes, Ronald Press, New York (1965). I. K. J. Laidler, in Chemical Kinetics, McGraw-Hill, London f 1965). 8 K. H. Dietz F. Beck, J. appl. Electrochem. 15, 159 (1985). 9 J. VBzquez, J. Gbmez, A. M. Barb, N. Garcia. M. L. Marcos, J. Gontilez-Velasco, J. M. Vara, A. J. ,&a, J. Press, A. GarCia and M. Aguilar, J. Am. Chem. Sot. 109, 1730 (1987). 10. 1. Rodriguez, M. L. Marcos and J. Gontilez-Velasco, Electrochim. Acta (in press).