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Polypyrrole electrogeneration from a nucleophilic solvent (DMF)
SYnTImMTII[ n|TRL5 ELSEVIER
Synthetic Metals 66 (1994) 25-32
Polypyrrole electrogeneration from a nucleophilic solvent (DMF) T.F. Otero a, A . H . Ardvalo b "Universidad del Pals Vasco, Facultad de Quimicas, Departamento de Ciencia y Tecnologia de Polimeros, Laboratorio de Electroquimica Apartado 1072, 20080 San Sebastidn, Spain bUniversidad Nacional de Rio Cuarto, Facultad de Ciencias Exactas, Fisico Quimicas y Naturales, Departamento de Quimica y Fisica, Estafeta 9, 5800 Rio Cuarto, Argentina
Received 13 September 1993; in revised form 22 April 1994; accepted 27 April 1994
Abstract
In spite of difficulties pointed out in the literature thick polypyrrole film can be obtained on platinum from NaCIO4-DMF solutions. The polymerization rate increases at constant potential when the potential increases from 900 to 2400 mV(SCE) and the rate decreases at higher anodic potentials. The productivity of the consumed charge, as mg of polymer generated per mC of charge, decreases at increasing potentials. When temperature decreases the polymerization rate increases pointing to a high activation energy of parallel passivation reactions during polymerization. From the 'ex situ' microgravimetric method the following empirical kinetics was obtained: Rp =k[pyrrole]°-5[ClO4-] 1-3. The formation of polymeric films was controlled by cyclic voltammetry in the background electrolyte. From the stored charge and the polymer weight the stored charge per mg of electrogenerated polymer for each film could be estimated. When the concentration of ClO4- ions increases both the polymerization rate and the capacity to store electrical charge in films increases, which indicates that C104- ions act as retarding agents of passivation processes during polymerization. In general, the ability to store electrical charge in polypyrrole electrogenerated in different solvents decreases when the donor ability of the solvent increases. At higher temperatures a faster nucleophilic attack of the solvent to the positively charged polymer is proposed to occur, promoting an increasing passivity of the film. The productivity of the polymerization charge decreases at increasing temperatures and increases at increasing concentrations of electrolyte and passes through a maximum when the concentrations of monomer increase. Keywords: Polypyrrole; Dimethylformamide; Electrogeneration; Nucleophilic solvent
1. Introduction
In recent years, the electrochemical polymerization of pyrrole has been studied in different solvents [1-6]. T h e formation of the polymeric film and electrical and physical properties of the resulting films were strongly affected by the conditions of preparation [7-12]. In particular, the solvent has a very strong influence on the outcome of the electrooxidation reaction. If the nucleophilic character of the solvent is significant, the film formation is minimized [4]. A simple mechanism was proposed by Geni6s et al. [1], which explains the electropolymerization of pyrrole as a process initiated by the aromatic radical cation generated by the initial loss of an electron from the m o n o m e r [4] and the subsequent polymerization by a stepwise coupling reaction, which involves electrophilic aromatic substitution or radical coupling. In both stages
0379-6779/94/$07.00 © 1994 Elsevier Science S.A. All rights reserved SSDI 0379-6779(94)02177-Z
the polymerization reaction proceeds via the radical cation intermediate. This radical cation is susceptible to the attack of the nucleophilic solvent resulting in shorter conjugation length, decrease of conductivity and lower polymerization rate [13]. In our laboratory an effort is being made to propose models of interfacial reactions [14,15] in accordance with empirical kinetics and electrochemical studies of the electrogenerated polymers. The flow of an anodic current promotes the m o n o m e r oxidation and polymerization on the electrode. The transfer of each electron promotes the release of a proton around the electrode. The acidification of the solution promotes protonation of the monomer, favouring the m o n o m e r oxidation on the electrode, and chemical polymerization in the reaction layer around the electrode. The chemical polymerization gives protonated (non-conjugated) polymer. A fraction of this polymer is adsorbed on the
26
TF. Otero, A.H. Argvalo / Synthetic Metals 66 (1994) 25-32
electrode giving a passive fraction. Finally, nucleophilic attack of the solvent, oxidized solvent or electrolyte on the oxidized polymer produced can take place simultaneously with the polymerization process and, hence, the fraction of passive (non-conjugated) polymer present in the 'active' polymer film increases. In previous studies a very fragile and powdery polymer was obtained on the electrode from D M F solutions [4]. The nucleophilic attack of the solvent to the polymeric radical cation was proposed to explain the low yield of polymer in this medium [13]. The aim of this work is to evaluate some of the aspects related to the electrogeneration of polypyrrole (PPy) from D M F (strongly nucleophilic) solutions: formation and growth of films, kinetic studies and electrochemical properties of the films. An attempt will be made to correlate the influence of different solvents according to the results presented in the literature.
(a) 0.5 <
j
o~
0.0
/
0
3000.0
800
E
/
mV
1600
(b) /
a
2500.0
o.
/
,o
/ /o
2000.0 i / 1 /
1500.0
2. Experimental
~
t
e/
I
/ /
lOOO.O
/ ot
The monomer pyrrole (Py) (Janssen), was distilled at 59 °C under vacuum. The electrolyte, anhydrous sodium perchlorate (Merk), was kept in an oven at 100 °C and used without previous purification. Dimethylformamide (DMF) (Panreac, synthetic grade) was kept in molecular sieves and the water content was less than 0.1 wt.% (determined by the Karl-Fischer method). A three-compartment electrochemical cell was used in the present work. A Pt sheet of 1 cm 2 surface area was used as the working electrode and a Pt wire of 4 cm 2 surface area (in spiral form) was used as the counter electrode. The reference electrode was an aqueous saturated calomel electrode. All the experiments were performed under inert argon atmosphere and under anhydrous conditions. The electrochemical experiments were carried out by using a PAR Model M273 potentiostat-galvanostat (connected to PC (AT) and the data of the chronoamperograms or voltammograms were obtained with the help of M 271 software. For the microgavimetric determinations, a Sartorius Model 4504 MP8 microbalance (precision: 10 -~ g), thermostatically controlled at 15 °C, was used.
3. Results and discussion
When a platinum electrode of 1 cm 2 surface area was submitted to a potential sweep in a 0.2 M Py and 0.2 M NaC104-DMF solution, the voltammogram shown in Fig. l(a) was obtained. On the anodic hemicycle very low current flows through the electrode until a potential of 900 mV is reached. Then the current density
/
_~
500.0
/ / • _e_o
0.0 q - , - ~ 500.0
,
,
]
i
1000.0
,
T
T
l
T
x
~
~
T
r
1500.0 2000.0 E/mV
i
i
V
q
r ~
2500.0
~1
3000.0
Fig. 1. (a) Voltammogram obtained using a Pt electrode in a 0.2 M pyrrole and 0.2 M NaCIO4-DMF solution at a scan rate of 20 mV s - i. (b) Weight of the reduced PPy films obtained at different potentials for 30 s in 0.1 M pyrrole and 0.2 M NaCIO4-DMF solution, and then submitted to a reduction potential ( - 3 0 0 mV) in 0.2 M NaCIO4-DMF solution until complete reduction ( i = 0 mA cm-2), dried and weighed.
increases, goes through a maximum at 1200 mV and later through a minimum at 1300 mV. Above 1300 mV a new oxidation process occurs resulting in increasing current densities at increasing anodic potentials. On the cathodic hemicycle a shoulder at 300 mV is observed. To investigate the influence of the potential on the maximum yield, as well as other properties of the polymer, films were prepared by polarization for 30 s at different anodic potentials. In each case, the electrode was polarized at - 300 mV in the presence of monomer for 5 s and then the potential was increased to the chosen anodic value for 30 s. Then, the potential was again lowered to - 3 0 0 mV. The correlative chronoamperograms were recorded on the microcomputer. One of the most important differences with films generated from other solvents is the morphology. Initially a homogeneous yellow film coats the electrode and later a blue-black irregular formation grows on top of the former. At higher voltages the yellow film is coated with a second film having a corrugated texture (Fig. 2).
T.F. Otero, A.H. Ar~valo
Synthetic Metals 66 (1994) 25 .32
27
Table 1 Productivity of the consumed electrical charge during polymerization at different potentials (as mg of reduced polymer per consumed mC) and capacity to store electrical charge (measured by cyclic voltammetry in the background 'electrolyte) in the polymer (as m C mg 1),
Fig. 2. SEM micrograph of the film prepared by polarization at 1300 m V for 30 s in 0.2 M Py-0.2 M N a C I O 4 - D M F and - 1 0 °C.
Films generated at different anodic potentials were studied in the background solution by cyclic voltammetry between -300 and 500 mV at 20 mV s-I to create a control voltammogram. After control each polymeric film was reduced in this solution at -300 mV until the current was reduced to zero. The coated electrode was then rinsed with methanol, dried at 40 °C and weighed using a microbalance (precision of 7-10 g). The polymer was dried and weighed several times until the weight became constant. The experimental results are shown in Fig. l(b). The polymerization rate increases when the electric potential rises between 900 and 2400 mV, as inferred from the enhanced weight of polymer generated at constant polarization time. At higher potentials the polymerization decreases. By integration of each chronoamperogram we obtained the electric charge consumed during each electropolymerization. The ratio of the weight of reduced polymer film and the charge consumed during the formation of polymer film gives the productivity of the charge (as mg mC ~). Table 1 shows the decrease of productivity of the charge when the potential increases. The decrease of the productivity indicates that the rate of side reactions, yielding nonpolymeric products, increases at higher potentials. The electrical charge stored in each film was obtained by the integration of the cathodic voltammograms. The ratio between this charge and the weight of reduced polymer film gives the capacity to store electrical charge (storage capacity). The capacity to store electrical charge (mC of charge per mg of polymer) increases until the potential reaches 1500 mV. This indicates that nucleophilic attack to the polymer is weaker at lower potentials. At higher anodic potential the storage capacity begins to decrease, indicating: (1) the presence of nucleophilic attack to the conjugated structure which
Polymerization potential (mV)
Productivity of charge (mg m C -~ × 104)
Capacity to store charge ( m C m g -1X 10 -2 )
1000 1100 1200 1300 1400 1500 1600 2000 2400 2800
106.75 27.49 6.49 5.61 4.90 4.57 4.09 3.42 2.90 1.78
0.08 0.16 1.12 1.54 1.67 2.02 1.66 1.36 0.72 0.40
"Polymerization in 0.2 M Py-0.2 M NaCIO4-DMF. T = 2 0 °C. Polarization time: 30 s. Table 2 Productivity of the consumed electrical charge (mg mC -~) during polymerization at different temperatures, weight of the generated films and capacity to store electrical charge (mC m g - I ) " Temperature (°C)
m (rag × 104)
Productivity of charge (mg m C - E × I O 4)
Capacity to store charge (mC m g - ~ × l O -2)
- 10 0 10 20 30
183 167 92 79 73
4.40 4.27 3.4O 2.97 2.83
2.65 2.64 2.12 2.19 2.16
~Films generated by polarization at 1300 mV. Polymerization solution: 0.2 M Py-0.2 M NaCIO4-DMF. Polarization time: 60 s.
is simultaneous to the electropolymerization process, and the subsequent passivation process; or (2) the possibility of enhanced chemical polymerization to give non-conducting polymers. Since the polymerization rate increases with the potential and the capacity to store electrical charge decreases above 1500 mV, we have chosen a potential of 1300 mV to generate the polymer.
3.1. Temperature influence The polymeric films were generated by polarization of Pt electrode at 1300 mV at different temperatures in DMF solution containing 0.2 M pyrrole and 0.2 M LiC104. These films were controlled by cyclic voltammetry between - 3 0 0 and 500 mV at 20 mV s -1 in the background solution at ambient temperature (from each voltammogram the stored charge was obtained). Each film was subsequently reduced at - 300 mV, dried and weighed and the results are shown in Table 2.
T.F. Otero, A.H. Ardvalo / Synthetic Metals 66 (1994) 25-32
28
The weight of the polymer film, the productivity of the consumed electrical charge and the capacity to store electrical charge in the films increase when the temperature decreases. In electroinitiated polymerizations, passivation processes occur simultaneously to the polymer growth. One of these degradative-passivation processes could be the origin of the temperature influence. To check this hypothesis two similar films were electrogenerated at - 1 0 °C. One of the films was then translated to the background solution at - 1 0 °C. There, it was alternatively submitted to a control cyclic voltammogram (between - 3 0 0 and 500 mV at 20 mV s-a), followed by a polarization at 1300 mV for different times. Fig. 3 shows the evolution of the electrical charge needed to oxidize the polymer film after polarization processes at 1300 mV. At increasing polarization times the capacity to store electrical charge decreases linearly and a small increase of the reduced polymer weight is observed. The second film was submitted to a similar treatment in the background solution at 30 °C. The decrease of the stored charge is faster, as shown in Fig. 3. The degradation of the polymer seems to be due to the nucleophilic attack to the oxidized polymeric chains promoting the opening of double bonds and the formation of new bonds with oxidized solvent or residual water [15,16]. This results in an increase in the weight of the partially degradated polymer from the initial weight reduced film (0.0544 mg) to the final weight of 0.0586 mg.
of monomer at 10°C by polarization at 1300 mV. This potential was maintained during the polymerization time. The potential was then stepped to - 3 0 0 mV and this reduction potential was maintained until a constant current density (close to zero) was attained under inert atmosphere. The experimental kinetics was followed by 'ex situ' microgravimetry. The clean platinum electrode was weighed before polymerization. A polymer film was then generated. The coated electrode was checked in the background solution by cyclic voltammetry at 20 mV s-~ and by potential steps between - 3 0 0 and +500 mV, each step maintained for 60 s. The coated electrode was then reduced at - 3 0 0 mV until the current became constant. The film was rinsed with methanol, dried and weighed several times until constant weight. The coated electrode was cleaned by burning the polymer in a reduction flame. A new experiment in a fresh solution for a new polarization time was performed using the clean platinum electrode. The experimental kinetics obtained from 0.2 M NaC1On-DMF solutions containing 0.05, 0.1, 0.14, 0.2 and 0.32 M pyrrole concentrations are shown in Fig. 4(a) where the kinetics related to five different concentrations of monomer are depicted. Well-defined 700.00 ~
(a)
©
600.00
©.
500.00 O
o
"~ 400.00 ©.
3.2. Kinetics at different concentrations of monomer
-
Polymeric films were electrogenerated in 0.2 M NaC104-DMF solutions having different concentrations
"~ 200.00
E 100.00
o gh
160.00
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' 7 , 140.00
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20.0
40.0
60.0
80.0
100.0
100.00
-3.00 1o
80.00
.~
40.00
140.0
(b)
-3.20 60.00
120.0
Polarization time / s.
120.00
'~ o
/o
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300.00
x
-3.40 ,..___
30°C
~a
20.00 0.00 0.0
-3.60
, ' ' ,i .... ~ .... r .... r .... I .... i .... 5.0 10.0 15.0 20.0 25.0 30.0 35.0 Polarization Time / s.
F i g . 3. Degradation of two films weighing 0.0544 mg each generated by polarization of 1 cm 2 of platinum at 1300 m V in a 0.2 M pyrrole and 0 . 2 M L i C 1 0 4 - D M F solution and submitted to subsequent degradation processes by polarization at 1300 rnV in a 0.2 M NaCIO4-DMF solution at ( a ) - 1 0 ° C a n d ( b ) 3 0 ° C . Degradation was followed after each treatment by cyclic voltammetry between -300 a n d 5 0 0 m V at 20 m V s - ' . The charge storage capacity ( m C m g - ~ ) is represented vs. polarization time ( s ) .
-3.80 -4.00 -1.4
-1.2
-1.0
-0.8
-0.6
-0.4
log [Pyl F i g . 4. ( a ) Evolution of polypyrrole film reduced weight with po-
lymerization time for different pyrrole concentrations [Py]: ( × ) (A)
0.1,
(El) 0.14, ( ~ )
0.2 a n d
(©)
0.32 M. Solution:
0.05, Py~).2 M
T = - 10 ° C . EPolym = 1 3 0 0 m V . ( b ) Determination of the reaction empiric order related to pyrrole concentration. NaCIO4-DMF.
T.F. Otero, A.H. Ar6valo / ,~,),nthetic Metals 06 (1994) 25=~2
linear dependences of the reduced polymer weight as a function of time were obtained. Each slope represents a polymerization rate (Rp) Q: the polymer weight produced per unit of polarization time. The polymerization rate increases when the concentration of monomer increases. The different experimental results can be summarized in the equation: R p i ~ [ 6rn / ~t ]i oc [ M ] i a
where Rpi represents the polymerization rate, i represents the set of experimental conditions, [M] the concentration of monomer, a the empirical order dependence, m the mass of the polymer and t the polymerization time. The empirical reaction order can be obtained from a bilogarithmic representation of Rpi versus [M]i (Fig. 4(b)). The straight line has a slope of 0.5, subsequently:
Rp cx [Pyl °-5 The experimental results related to the polymer production and control from films generated during 40 s of polymerization time are summarized in Table 3 and Fig. 5. In Table 3, the polymerization column represents the anodic electrical charge consumed during the polymerization process, which is obtained by integration of each chronoamperogram, and the electrical charge related to the polymer reduction in the same solution. The control column includes the anodic and cathodic charges obtained from the control voltammograms of the films in the background solution. The weight of the reduced polymer film, the productivity of the consumed charge during polymerization and the capacity to store electrical charge in each film are depicted on the last three columns. In general, when the current density is higher the productivity of the consumed charge is higher and the capacity to store electrical charge is lower. This tendency seems to be in agreement with the models of interfacial reactions proposed by our previous work [5,14,15]. High current densities promote an intense acidification
29
around the electrode. The protonation of pyrrole favours both the electroinitiated polymerization on the electrode, yielding a conducting polymer, and chemical polymerization in a reaction layer around the electrode, resulting in a non-conducting (hydrogenated) polymer [17]. A fraction of the chemically generated polymer interacts and precipitates on the electrode surface along with the electrogeneration process. As a consequence, the same amount of electrical charge produces more polymer (electrochemically or chemically) at higher current densities. The acid-catalysed polymer is a hydrogenated polymer [17] with very short conjugation length and its electroactivity is low. This is the reason for the decreasing capacity to store electrical charge when the concentration of monomer increases.
3.3. Kinetics at different concentrations of electrolyte The kinetics were followed at different concentrations of NaC104: 0.2, 0.3, 0.4, 0.5 and 0.63 M in 0.1 M Py-DMF solutions. The experimental procedure was similar to the case when the concentration of monomer was changed as described above. The weight of the reduced polymer increases linearly, with increasing slopes at higher concentrations of C I O 4 ions, as shown in Fig. 5(a). The reduced polymer weight obtained at the polymerization rate can be represented by Rpi = ( ~mi/6ti) oc [ C 1 0 4 - ]b
The order of the polymerization reaction, b, can be obtained from the slope of the double logarithmic representation of Rp v e r s u s [CIO4-], as shown in Fig. 5(b). The value of b is 1.3 and the empirical reaction: ipcI [C104 ]13 The order dependence on the concentration of ions is higher than that of the concentration of monomer during the polymerization reaction. This indicates the direct participation of the anions on the polymerization mechanism, which can be due to various reasons. One
Table 3 Productivity of the c o n s u m e d charge (mg m C ~) during polymerization of pyrrolc at different concentrations of monomer, anodic and cathodic charges related to the control voltammogram, and weight of the reduced films and capacity to store charge (mC mg-~) ~' [PYl (mol 1-~)
0.05 0.10 0.14 0.20
Polymerization
Control
Anodic charge (me)
Cathodic charge (me)
Anodic charge (mC)
Cathodic charge (me)
34.95 35.01 26.54 38.42
3.33 3.40 2.92 3.97
1.72 2.17 1.86 2.85
1.08 1.37 1.24 2.00
Weight of reduced polymer (mg × 104)
Productivity of charge (mg mC-~ × 104)
Capacity to store charge (mC nag-~ × 10 -2)
80 108 130 151
2.68 3.67 5.97 4.78
2.15 2.01 1.43 1.89
"Films generated by polarization at 1300 mV. Polymerization solution: 0.2 M NaCIO4-1DMF. T = - 1 0 °C. Polarization time: 40 s.
T.F. Otero, A.H. Ardvalo / Synthetic Metals 66 (1994) 25-32
30 1200.00
participation can be through the oxidation of the polymer. Once formed the neutral state of the polymer is immediately oxidized:
(a) . O
1000.00
O.
Polymer + nC104-
800.00 O600.00
o.
/
400.00
"
200.00
ec
j
j
0.00 0.0
20.0
40.0
60.0
80.0
100.0
120.0
Polarization time / s
Another way for C 1 0 4 - participation in the polymerization mechanism can be due to the stabilization of the monomeric cation radical (M ~ M "÷ + C104M'CIO4)which allows an easier interaction with a new monomeric radical M'CIO4. Finally, another way through the direct discharge of CIO4- on the electrode, metal or polymer was postulated in the literature [18-24]: ClO 4 -
-2.80
-3.02
×
b=l.3 -3.25
-3.48
-3.70 -1.0
-0.8
-0.6
-0.4
-0.2
0.0
log [NaaO4l
Fig. 5. (a) Evolution of polypyrrole film reduced weight with polymerization time for different electrolyte concentrations [NaCIO4]: ( x ) 0.2, ( , ) 0.3, (IS]) 0.4, (O) 0.5 and (©) 0.63 M. Solution: 0.1 M Py-NaCIO4-DMF. T = - 10 °C. Epo~ym=1300 mV. (b) Determination of the reaction empiric order related to electrolyte concentration. 30.00
x DIvlF
'7 "d 26.00
~
' CIO4"+ e-
(b) C 1 0 4" q- M
~
~ Polymer" + (C104-)n +ne -
o ~
22.00
18.00
14.00
ACN
~×
10.00
.... 0.0
[ .... 1.0
I .... 20
I ~r~
' [ ....
3.0
4.0
I .... 50
6 0
Capacity to store charge / m C . m g 1 Fig. 6. A v e r a g e
films generated
capacity
to store electrical
in s o l v e n t s
having
charge of the polypyrrole
a different
donor
number.
of them is related to the conductivity of the solution. According to the Arrhenius expression, only a 1/2 dependence should be expected. A second reason for
~
M"+ + C104 -
The fractional integer of the order dependence indicates a complex participation, probably through different ways. From the electrochemical and gravimetric data, as depicted in Table 4, a direct participation of CIO4on the mechanism can be deduced. When the concentration of electrolyte increases, the current productivity increases and the change in the storage capacity does not follow any definite qualitative pattern. This indicates that the higher efficiency of the charge to produce electroactive polymer does not have any influence on the parallel chemical generation of nonconducting polymer.
3.4. Influence of the solvent The significant parameter of a electroactive polymer is the capacity to store electrical charge which varies in different solvents. The average values of the capacity to store electrical charge in different solvents (water [5], acetonitrile [6], methanol [25] and DMF) are plotted against the electron-donor [26] abilities of the different solvents, as shown in Fig. 6. The storage capacity increases when the donor number of the solvent decreases. This fact indicates the enhanced nucleophilic attack of the solvent to the positive charges on the oxidized polymer when the donor character of the solvent increases. This promotes a decrease in the conjugation length and the concomitant increase of the passive fraction of polymer in the electrogenerated film. This attack is simultaneous with the electropolymerization processes. Results from the polymerization and control of a polymer film in the background solutions indicate that the rate of nucleophilic attack increases at higher temperatures. Now studies are being performed in our laboratory to obtain spectroscopic, morphologic and analytical evidence.
31
7~F. Gtero, A.H. Ar(valo / Synthetic Metals 66 (1994) 25~2 Table 4 Electrical and gravimetric parameters related to films generated at different concentrations of electrolyte a [NaCIO4] (tool 1-~)
0.2 0.3 0.4 0.5 0.63
Polymerization
Control
Anodic charge (me)
Cathodic charge (mC)
Anodic charge (mC)
Cathodic charge (mC)
61.49 62.79 102.33 153.16 150.02
5.69 6.38 6.85 16.34 17.10
3.42 4.56 8.90 16.41 13.96
2.20 2.97 6.04 9.41 9.75
Weight of reduced polymer (rag × 104)
Productivity of charge (mg m C - I >( 10 4)
Capacity to store charge (mC mg -~× 10 -2 )
168 213 379 536 605
3.21 4.11 4.38 4.45 5.09
2.04 2.14 2.35 3.06 2.31
aPolymerization solution: 0.1 M Py-NaCIO4-DMF. Potential of polarization: 1300 mV. Polarization time: 60 s. T = 10 °C.
also thank the University of Rio Cuarto for allowing Dr Ar6valo to develop this work in the University of the Basque Country.
4. Conclusions
The electrogeneration of PPy from NaC104-DMF solutions takes place at higher anodic potentials than 800 mV (SCE). The polymerization rate increases when the potential rises, and passes through a maximum at 2400 mV. A strong influence of solvent seems to be the origin of this maximum as inferred from the decrease of the storage capacity of films generated at high anodic potentials. When comparing with other solvents, a non-uniform film is obtained in DMF. At lower over-voltages a yellow film coats the electrode, and blue-black and irregular formations grow on top of the film. A bilayer structure seems to be present. The polymerization rate increases when the temperature decreases. This may be due to the increase in the degradation processes, resulting in non-electroactive side products (passive lakes in the polymer). The empirical kinetics was obtained by 'ex situ' microgravimetry:
Rp =
k[pyrrole]°5[C104
- ]1.3
obtaining a higher dependence from the electrolyte than from the monomer. The storage efficiency indicates that the increase of the monomer (electrolyte) concentration favours (retards) nucleophilic attack. Taking average storage capacities from films of PPY generated from different solvents, a decrease of the parameter is observed at increasing donor numbers of the solvents to positive charges of the polymer.
Acknowledgements This work was supported by the Basque Government, Programme 'Amerika eta Euskaldunak'. The authors
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T.F. Otero, A.H. Ardvalo / Synthetic Metals 66 (1994) 25-32
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[24] S.K. Samal and B. Nayak, J. Polym. Sci., Part A, Polym. Chem., 26 (1988) 1035. [25] T.F. Otero and M. Bengoechea, in preparation. [26] V. Gutmann, The Donor-Acceptor Approach to Molecular Interactions, Plenum, New York, 1978.
Synthetic Metals 66 (1994) 25-32
Polypyrrole electrogeneration from a nucleophilic solvent (DMF) T.F. Otero a, A . H . Ardvalo b "Universidad del Pals Vasco, Facultad de Quimicas, Departamento de Ciencia y Tecnologia de Polimeros, Laboratorio de Electroquimica Apartado 1072, 20080 San Sebastidn, Spain bUniversidad Nacional de Rio Cuarto, Facultad de Ciencias Exactas, Fisico Quimicas y Naturales, Departamento de Quimica y Fisica, Estafeta 9, 5800 Rio Cuarto, Argentina
Received 13 September 1993; in revised form 22 April 1994; accepted 27 April 1994
Abstract
In spite of difficulties pointed out in the literature thick polypyrrole film can be obtained on platinum from NaCIO4-DMF solutions. The polymerization rate increases at constant potential when the potential increases from 900 to 2400 mV(SCE) and the rate decreases at higher anodic potentials. The productivity of the consumed charge, as mg of polymer generated per mC of charge, decreases at increasing potentials. When temperature decreases the polymerization rate increases pointing to a high activation energy of parallel passivation reactions during polymerization. From the 'ex situ' microgravimetric method the following empirical kinetics was obtained: Rp =k[pyrrole]°-5[ClO4-] 1-3. The formation of polymeric films was controlled by cyclic voltammetry in the background electrolyte. From the stored charge and the polymer weight the stored charge per mg of electrogenerated polymer for each film could be estimated. When the concentration of ClO4- ions increases both the polymerization rate and the capacity to store electrical charge in films increases, which indicates that C104- ions act as retarding agents of passivation processes during polymerization. In general, the ability to store electrical charge in polypyrrole electrogenerated in different solvents decreases when the donor ability of the solvent increases. At higher temperatures a faster nucleophilic attack of the solvent to the positively charged polymer is proposed to occur, promoting an increasing passivity of the film. The productivity of the polymerization charge decreases at increasing temperatures and increases at increasing concentrations of electrolyte and passes through a maximum when the concentrations of monomer increase. Keywords: Polypyrrole; Dimethylformamide; Electrogeneration; Nucleophilic solvent
1. Introduction
In recent years, the electrochemical polymerization of pyrrole has been studied in different solvents [1-6]. T h e formation of the polymeric film and electrical and physical properties of the resulting films were strongly affected by the conditions of preparation [7-12]. In particular, the solvent has a very strong influence on the outcome of the electrooxidation reaction. If the nucleophilic character of the solvent is significant, the film formation is minimized [4]. A simple mechanism was proposed by Geni6s et al. [1], which explains the electropolymerization of pyrrole as a process initiated by the aromatic radical cation generated by the initial loss of an electron from the m o n o m e r [4] and the subsequent polymerization by a stepwise coupling reaction, which involves electrophilic aromatic substitution or radical coupling. In both stages
0379-6779/94/$07.00 © 1994 Elsevier Science S.A. All rights reserved SSDI 0379-6779(94)02177-Z
the polymerization reaction proceeds via the radical cation intermediate. This radical cation is susceptible to the attack of the nucleophilic solvent resulting in shorter conjugation length, decrease of conductivity and lower polymerization rate [13]. In our laboratory an effort is being made to propose models of interfacial reactions [14,15] in accordance with empirical kinetics and electrochemical studies of the electrogenerated polymers. The flow of an anodic current promotes the m o n o m e r oxidation and polymerization on the electrode. The transfer of each electron promotes the release of a proton around the electrode. The acidification of the solution promotes protonation of the monomer, favouring the m o n o m e r oxidation on the electrode, and chemical polymerization in the reaction layer around the electrode. The chemical polymerization gives protonated (non-conjugated) polymer. A fraction of this polymer is adsorbed on the
26
TF. Otero, A.H. Argvalo / Synthetic Metals 66 (1994) 25-32
electrode giving a passive fraction. Finally, nucleophilic attack of the solvent, oxidized solvent or electrolyte on the oxidized polymer produced can take place simultaneously with the polymerization process and, hence, the fraction of passive (non-conjugated) polymer present in the 'active' polymer film increases. In previous studies a very fragile and powdery polymer was obtained on the electrode from D M F solutions [4]. The nucleophilic attack of the solvent to the polymeric radical cation was proposed to explain the low yield of polymer in this medium [13]. The aim of this work is to evaluate some of the aspects related to the electrogeneration of polypyrrole (PPy) from D M F (strongly nucleophilic) solutions: formation and growth of films, kinetic studies and electrochemical properties of the films. An attempt will be made to correlate the influence of different solvents according to the results presented in the literature.
(a) 0.5 <
j
o~
0.0
/
0
3000.0
800
E
/
mV
1600
(b) /
a
2500.0
o.
/
,o
/ /o
2000.0 i / 1 /
1500.0
2. Experimental
~
t
e/
I
/ /
lOOO.O
/ ot
The monomer pyrrole (Py) (Janssen), was distilled at 59 °C under vacuum. The electrolyte, anhydrous sodium perchlorate (Merk), was kept in an oven at 100 °C and used without previous purification. Dimethylformamide (DMF) (Panreac, synthetic grade) was kept in molecular sieves and the water content was less than 0.1 wt.% (determined by the Karl-Fischer method). A three-compartment electrochemical cell was used in the present work. A Pt sheet of 1 cm 2 surface area was used as the working electrode and a Pt wire of 4 cm 2 surface area (in spiral form) was used as the counter electrode. The reference electrode was an aqueous saturated calomel electrode. All the experiments were performed under inert argon atmosphere and under anhydrous conditions. The electrochemical experiments were carried out by using a PAR Model M273 potentiostat-galvanostat (connected to PC (AT) and the data of the chronoamperograms or voltammograms were obtained with the help of M 271 software. For the microgavimetric determinations, a Sartorius Model 4504 MP8 microbalance (precision: 10 -~ g), thermostatically controlled at 15 °C, was used.
3. Results and discussion
When a platinum electrode of 1 cm 2 surface area was submitted to a potential sweep in a 0.2 M Py and 0.2 M NaC104-DMF solution, the voltammogram shown in Fig. l(a) was obtained. On the anodic hemicycle very low current flows through the electrode until a potential of 900 mV is reached. Then the current density
/
_~
500.0
/ / • _e_o
0.0 q - , - ~ 500.0
,
,
]
i
1000.0
,
T
T
l
T
x
~
~
T
r
1500.0 2000.0 E/mV
i
i
V
q
r ~
2500.0
~1
3000.0
Fig. 1. (a) Voltammogram obtained using a Pt electrode in a 0.2 M pyrrole and 0.2 M NaCIO4-DMF solution at a scan rate of 20 mV s - i. (b) Weight of the reduced PPy films obtained at different potentials for 30 s in 0.1 M pyrrole and 0.2 M NaCIO4-DMF solution, and then submitted to a reduction potential ( - 3 0 0 mV) in 0.2 M NaCIO4-DMF solution until complete reduction ( i = 0 mA cm-2), dried and weighed.
increases, goes through a maximum at 1200 mV and later through a minimum at 1300 mV. Above 1300 mV a new oxidation process occurs resulting in increasing current densities at increasing anodic potentials. On the cathodic hemicycle a shoulder at 300 mV is observed. To investigate the influence of the potential on the maximum yield, as well as other properties of the polymer, films were prepared by polarization for 30 s at different anodic potentials. In each case, the electrode was polarized at - 300 mV in the presence of monomer for 5 s and then the potential was increased to the chosen anodic value for 30 s. Then, the potential was again lowered to - 3 0 0 mV. The correlative chronoamperograms were recorded on the microcomputer. One of the most important differences with films generated from other solvents is the morphology. Initially a homogeneous yellow film coats the electrode and later a blue-black irregular formation grows on top of the former. At higher voltages the yellow film is coated with a second film having a corrugated texture (Fig. 2).
T.F. Otero, A.H. Ar~valo
Synthetic Metals 66 (1994) 25 .32
27
Table 1 Productivity of the consumed electrical charge during polymerization at different potentials (as mg of reduced polymer per consumed mC) and capacity to store electrical charge (measured by cyclic voltammetry in the background 'electrolyte) in the polymer (as m C mg 1),
Fig. 2. SEM micrograph of the film prepared by polarization at 1300 m V for 30 s in 0.2 M Py-0.2 M N a C I O 4 - D M F and - 1 0 °C.
Films generated at different anodic potentials were studied in the background solution by cyclic voltammetry between -300 and 500 mV at 20 mV s-I to create a control voltammogram. After control each polymeric film was reduced in this solution at -300 mV until the current was reduced to zero. The coated electrode was then rinsed with methanol, dried at 40 °C and weighed using a microbalance (precision of 7-10 g). The polymer was dried and weighed several times until the weight became constant. The experimental results are shown in Fig. l(b). The polymerization rate increases when the electric potential rises between 900 and 2400 mV, as inferred from the enhanced weight of polymer generated at constant polarization time. At higher potentials the polymerization decreases. By integration of each chronoamperogram we obtained the electric charge consumed during each electropolymerization. The ratio of the weight of reduced polymer film and the charge consumed during the formation of polymer film gives the productivity of the charge (as mg mC ~). Table 1 shows the decrease of productivity of the charge when the potential increases. The decrease of the productivity indicates that the rate of side reactions, yielding nonpolymeric products, increases at higher potentials. The electrical charge stored in each film was obtained by the integration of the cathodic voltammograms. The ratio between this charge and the weight of reduced polymer film gives the capacity to store electrical charge (storage capacity). The capacity to store electrical charge (mC of charge per mg of polymer) increases until the potential reaches 1500 mV. This indicates that nucleophilic attack to the polymer is weaker at lower potentials. At higher anodic potential the storage capacity begins to decrease, indicating: (1) the presence of nucleophilic attack to the conjugated structure which
Polymerization potential (mV)
Productivity of charge (mg m C -~ × 104)
Capacity to store charge ( m C m g -1X 10 -2 )
1000 1100 1200 1300 1400 1500 1600 2000 2400 2800
106.75 27.49 6.49 5.61 4.90 4.57 4.09 3.42 2.90 1.78
0.08 0.16 1.12 1.54 1.67 2.02 1.66 1.36 0.72 0.40
"Polymerization in 0.2 M Py-0.2 M NaCIO4-DMF. T = 2 0 °C. Polarization time: 30 s. Table 2 Productivity of the consumed electrical charge (mg mC -~) during polymerization at different temperatures, weight of the generated films and capacity to store electrical charge (mC m g - I ) " Temperature (°C)
m (rag × 104)
Productivity of charge (mg m C - E × I O 4)
Capacity to store charge (mC m g - ~ × l O -2)
- 10 0 10 20 30
183 167 92 79 73
4.40 4.27 3.4O 2.97 2.83
2.65 2.64 2.12 2.19 2.16
~Films generated by polarization at 1300 mV. Polymerization solution: 0.2 M Py-0.2 M NaCIO4-DMF. Polarization time: 60 s.
is simultaneous to the electropolymerization process, and the subsequent passivation process; or (2) the possibility of enhanced chemical polymerization to give non-conducting polymers. Since the polymerization rate increases with the potential and the capacity to store electrical charge decreases above 1500 mV, we have chosen a potential of 1300 mV to generate the polymer.
3.1. Temperature influence The polymeric films were generated by polarization of Pt electrode at 1300 mV at different temperatures in DMF solution containing 0.2 M pyrrole and 0.2 M LiC104. These films were controlled by cyclic voltammetry between - 3 0 0 and 500 mV at 20 mV s -1 in the background solution at ambient temperature (from each voltammogram the stored charge was obtained). Each film was subsequently reduced at - 300 mV, dried and weighed and the results are shown in Table 2.
T.F. Otero, A.H. Ardvalo / Synthetic Metals 66 (1994) 25-32
28
The weight of the polymer film, the productivity of the consumed electrical charge and the capacity to store electrical charge in the films increase when the temperature decreases. In electroinitiated polymerizations, passivation processes occur simultaneously to the polymer growth. One of these degradative-passivation processes could be the origin of the temperature influence. To check this hypothesis two similar films were electrogenerated at - 1 0 °C. One of the films was then translated to the background solution at - 1 0 °C. There, it was alternatively submitted to a control cyclic voltammogram (between - 3 0 0 and 500 mV at 20 mV s-a), followed by a polarization at 1300 mV for different times. Fig. 3 shows the evolution of the electrical charge needed to oxidize the polymer film after polarization processes at 1300 mV. At increasing polarization times the capacity to store electrical charge decreases linearly and a small increase of the reduced polymer weight is observed. The second film was submitted to a similar treatment in the background solution at 30 °C. The decrease of the stored charge is faster, as shown in Fig. 3. The degradation of the polymer seems to be due to the nucleophilic attack to the oxidized polymeric chains promoting the opening of double bonds and the formation of new bonds with oxidized solvent or residual water [15,16]. This results in an increase in the weight of the partially degradated polymer from the initial weight reduced film (0.0544 mg) to the final weight of 0.0586 mg.
of monomer at 10°C by polarization at 1300 mV. This potential was maintained during the polymerization time. The potential was then stepped to - 3 0 0 mV and this reduction potential was maintained until a constant current density (close to zero) was attained under inert atmosphere. The experimental kinetics was followed by 'ex situ' microgravimetry. The clean platinum electrode was weighed before polymerization. A polymer film was then generated. The coated electrode was checked in the background solution by cyclic voltammetry at 20 mV s-~ and by potential steps between - 3 0 0 and +500 mV, each step maintained for 60 s. The coated electrode was then reduced at - 3 0 0 mV until the current became constant. The film was rinsed with methanol, dried and weighed several times until constant weight. The coated electrode was cleaned by burning the polymer in a reduction flame. A new experiment in a fresh solution for a new polarization time was performed using the clean platinum electrode. The experimental kinetics obtained from 0.2 M NaC1On-DMF solutions containing 0.05, 0.1, 0.14, 0.2 and 0.32 M pyrrole concentrations are shown in Fig. 4(a) where the kinetics related to five different concentrations of monomer are depicted. Well-defined 700.00 ~
(a)
©
600.00
©.
500.00 O
o
"~ 400.00 ©.
3.2. Kinetics at different concentrations of monomer
-
Polymeric films were electrogenerated in 0.2 M NaC104-DMF solutions having different concentrations
"~ 200.00
E 100.00
o gh
160.00
0.00
' 7 , 140.00
0.0
20.0
40.0
60.0
80.0
100.0
100.00
-3.00 1o
80.00
.~
40.00
140.0
(b)
-3.20 60.00
120.0
Polarization time / s.
120.00
'~ o
/o
>v
300.00
x
-3.40 ,..___
30°C
~a
20.00 0.00 0.0
-3.60
, ' ' ,i .... ~ .... r .... r .... I .... i .... 5.0 10.0 15.0 20.0 25.0 30.0 35.0 Polarization Time / s.
F i g . 3. Degradation of two films weighing 0.0544 mg each generated by polarization of 1 cm 2 of platinum at 1300 m V in a 0.2 M pyrrole and 0 . 2 M L i C 1 0 4 - D M F solution and submitted to subsequent degradation processes by polarization at 1300 rnV in a 0.2 M NaCIO4-DMF solution at ( a ) - 1 0 ° C a n d ( b ) 3 0 ° C . Degradation was followed after each treatment by cyclic voltammetry between -300 a n d 5 0 0 m V at 20 m V s - ' . The charge storage capacity ( m C m g - ~ ) is represented vs. polarization time ( s ) .
-3.80 -4.00 -1.4
-1.2
-1.0
-0.8
-0.6
-0.4
log [Pyl F i g . 4. ( a ) Evolution of polypyrrole film reduced weight with po-
lymerization time for different pyrrole concentrations [Py]: ( × ) (A)
0.1,
(El) 0.14, ( ~ )
0.2 a n d
(©)
0.32 M. Solution:
0.05, Py~).2 M
T = - 10 ° C . EPolym = 1 3 0 0 m V . ( b ) Determination of the reaction empiric order related to pyrrole concentration. NaCIO4-DMF.
T.F. Otero, A.H. Ar6valo / ,~,),nthetic Metals 06 (1994) 25=~2
linear dependences of the reduced polymer weight as a function of time were obtained. Each slope represents a polymerization rate (Rp) Q: the polymer weight produced per unit of polarization time. The polymerization rate increases when the concentration of monomer increases. The different experimental results can be summarized in the equation: R p i ~ [ 6rn / ~t ]i oc [ M ] i a
where Rpi represents the polymerization rate, i represents the set of experimental conditions, [M] the concentration of monomer, a the empirical order dependence, m the mass of the polymer and t the polymerization time. The empirical reaction order can be obtained from a bilogarithmic representation of Rpi versus [M]i (Fig. 4(b)). The straight line has a slope of 0.5, subsequently:
Rp cx [Pyl °-5 The experimental results related to the polymer production and control from films generated during 40 s of polymerization time are summarized in Table 3 and Fig. 5. In Table 3, the polymerization column represents the anodic electrical charge consumed during the polymerization process, which is obtained by integration of each chronoamperogram, and the electrical charge related to the polymer reduction in the same solution. The control column includes the anodic and cathodic charges obtained from the control voltammograms of the films in the background solution. The weight of the reduced polymer film, the productivity of the consumed charge during polymerization and the capacity to store electrical charge in each film are depicted on the last three columns. In general, when the current density is higher the productivity of the consumed charge is higher and the capacity to store electrical charge is lower. This tendency seems to be in agreement with the models of interfacial reactions proposed by our previous work [5,14,15]. High current densities promote an intense acidification
29
around the electrode. The protonation of pyrrole favours both the electroinitiated polymerization on the electrode, yielding a conducting polymer, and chemical polymerization in a reaction layer around the electrode, resulting in a non-conducting (hydrogenated) polymer [17]. A fraction of the chemically generated polymer interacts and precipitates on the electrode surface along with the electrogeneration process. As a consequence, the same amount of electrical charge produces more polymer (electrochemically or chemically) at higher current densities. The acid-catalysed polymer is a hydrogenated polymer [17] with very short conjugation length and its electroactivity is low. This is the reason for the decreasing capacity to store electrical charge when the concentration of monomer increases.
3.3. Kinetics at different concentrations of electrolyte The kinetics were followed at different concentrations of NaC104: 0.2, 0.3, 0.4, 0.5 and 0.63 M in 0.1 M Py-DMF solutions. The experimental procedure was similar to the case when the concentration of monomer was changed as described above. The weight of the reduced polymer increases linearly, with increasing slopes at higher concentrations of C I O 4 ions, as shown in Fig. 5(a). The reduced polymer weight obtained at the polymerization rate can be represented by Rpi = ( ~mi/6ti) oc [ C 1 0 4 - ]b
The order of the polymerization reaction, b, can be obtained from the slope of the double logarithmic representation of Rp v e r s u s [CIO4-], as shown in Fig. 5(b). The value of b is 1.3 and the empirical reaction: ipcI [C104 ]13 The order dependence on the concentration of ions is higher than that of the concentration of monomer during the polymerization reaction. This indicates the direct participation of the anions on the polymerization mechanism, which can be due to various reasons. One
Table 3 Productivity of the c o n s u m e d charge (mg m C ~) during polymerization of pyrrolc at different concentrations of monomer, anodic and cathodic charges related to the control voltammogram, and weight of the reduced films and capacity to store charge (mC mg-~) ~' [PYl (mol 1-~)
0.05 0.10 0.14 0.20
Polymerization
Control
Anodic charge (me)
Cathodic charge (me)
Anodic charge (mC)
Cathodic charge (me)
34.95 35.01 26.54 38.42
3.33 3.40 2.92 3.97
1.72 2.17 1.86 2.85
1.08 1.37 1.24 2.00
Weight of reduced polymer (mg × 104)
Productivity of charge (mg mC-~ × 104)
Capacity to store charge (mC nag-~ × 10 -2)
80 108 130 151
2.68 3.67 5.97 4.78
2.15 2.01 1.43 1.89
"Films generated by polarization at 1300 mV. Polymerization solution: 0.2 M NaCIO4-1DMF. T = - 1 0 °C. Polarization time: 40 s.
T.F. Otero, A.H. Ardvalo / Synthetic Metals 66 (1994) 25-32
30 1200.00
participation can be through the oxidation of the polymer. Once formed the neutral state of the polymer is immediately oxidized:
(a) . O
1000.00
O.
Polymer + nC104-
800.00 O600.00
o.
/
400.00
"
200.00
ec
j
j
0.00 0.0
20.0
40.0
60.0
80.0
100.0
120.0
Polarization time / s
Another way for C 1 0 4 - participation in the polymerization mechanism can be due to the stabilization of the monomeric cation radical (M ~ M "÷ + C104M'CIO4)which allows an easier interaction with a new monomeric radical M'CIO4. Finally, another way through the direct discharge of CIO4- on the electrode, metal or polymer was postulated in the literature [18-24]: ClO 4 -
-2.80
-3.02
×
b=l.3 -3.25
-3.48
-3.70 -1.0
-0.8
-0.6
-0.4
-0.2
0.0
log [NaaO4l
Fig. 5. (a) Evolution of polypyrrole film reduced weight with polymerization time for different electrolyte concentrations [NaCIO4]: ( x ) 0.2, ( , ) 0.3, (IS]) 0.4, (O) 0.5 and (©) 0.63 M. Solution: 0.1 M Py-NaCIO4-DMF. T = - 10 °C. Epo~ym=1300 mV. (b) Determination of the reaction empiric order related to electrolyte concentration. 30.00
x DIvlF
'7 "d 26.00
~
' CIO4"+ e-
(b) C 1 0 4" q- M
~
~ Polymer" + (C104-)n +ne -
o ~
22.00
18.00
14.00
ACN
~×
10.00
.... 0.0
[ .... 1.0
I .... 20
I ~r~
' [ ....
3.0
4.0
I .... 50
6 0
Capacity to store charge / m C . m g 1 Fig. 6. A v e r a g e
films generated
capacity
to store electrical
in s o l v e n t s
having
charge of the polypyrrole
a different
donor
number.
of them is related to the conductivity of the solution. According to the Arrhenius expression, only a 1/2 dependence should be expected. A second reason for
~
M"+ + C104 -
The fractional integer of the order dependence indicates a complex participation, probably through different ways. From the electrochemical and gravimetric data, as depicted in Table 4, a direct participation of CIO4on the mechanism can be deduced. When the concentration of electrolyte increases, the current productivity increases and the change in the storage capacity does not follow any definite qualitative pattern. This indicates that the higher efficiency of the charge to produce electroactive polymer does not have any influence on the parallel chemical generation of nonconducting polymer.
3.4. Influence of the solvent The significant parameter of a electroactive polymer is the capacity to store electrical charge which varies in different solvents. The average values of the capacity to store electrical charge in different solvents (water [5], acetonitrile [6], methanol [25] and DMF) are plotted against the electron-donor [26] abilities of the different solvents, as shown in Fig. 6. The storage capacity increases when the donor number of the solvent decreases. This fact indicates the enhanced nucleophilic attack of the solvent to the positive charges on the oxidized polymer when the donor character of the solvent increases. This promotes a decrease in the conjugation length and the concomitant increase of the passive fraction of polymer in the electrogenerated film. This attack is simultaneous with the electropolymerization processes. Results from the polymerization and control of a polymer film in the background solutions indicate that the rate of nucleophilic attack increases at higher temperatures. Now studies are being performed in our laboratory to obtain spectroscopic, morphologic and analytical evidence.
31
7~F. Gtero, A.H. Ar(valo / Synthetic Metals 66 (1994) 25~2 Table 4 Electrical and gravimetric parameters related to films generated at different concentrations of electrolyte a [NaCIO4] (tool 1-~)
0.2 0.3 0.4 0.5 0.63
Polymerization
Control
Anodic charge (me)
Cathodic charge (mC)
Anodic charge (mC)
Cathodic charge (mC)
61.49 62.79 102.33 153.16 150.02
5.69 6.38 6.85 16.34 17.10
3.42 4.56 8.90 16.41 13.96
2.20 2.97 6.04 9.41 9.75
Weight of reduced polymer (rag × 104)
Productivity of charge (mg m C - I >( 10 4)
Capacity to store charge (mC mg -~× 10 -2 )
168 213 379 536 605
3.21 4.11 4.38 4.45 5.09
2.04 2.14 2.35 3.06 2.31
aPolymerization solution: 0.1 M Py-NaCIO4-DMF. Potential of polarization: 1300 mV. Polarization time: 60 s. T = 10 °C.
also thank the University of Rio Cuarto for allowing Dr Ar6valo to develop this work in the University of the Basque Country.
4. Conclusions
The electrogeneration of PPy from NaC104-DMF solutions takes place at higher anodic potentials than 800 mV (SCE). The polymerization rate increases when the potential rises, and passes through a maximum at 2400 mV. A strong influence of solvent seems to be the origin of this maximum as inferred from the decrease of the storage capacity of films generated at high anodic potentials. When comparing with other solvents, a non-uniform film is obtained in DMF. At lower over-voltages a yellow film coats the electrode, and blue-black and irregular formations grow on top of the film. A bilayer structure seems to be present. The polymerization rate increases when the temperature decreases. This may be due to the increase in the degradation processes, resulting in non-electroactive side products (passive lakes in the polymer). The empirical kinetics was obtained by 'ex situ' microgravimetry:
Rp =
k[pyrrole]°5[C104
- ]1.3
obtaining a higher dependence from the electrolyte than from the monomer. The storage efficiency indicates that the increase of the monomer (electrolyte) concentration favours (retards) nucleophilic attack. Taking average storage capacities from films of PPY generated from different solvents, a decrease of the parameter is observed at increasing donor numbers of the solvents to positive charges of the polymer.
Acknowledgements This work was supported by the Basque Government, Programme 'Amerika eta Euskaldunak'. The authors
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32
T.F. Otero, A.H. Ardvalo / Synthetic Metals 66 (1994) 25-32
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