Reversible 2D to 3D electrode transitions in polypyrrole films

COLLOIDS ELSEVIER

Colloids and Surfaces A: Physicochemicaland EngineeringAspects 134 {1998) 85-94

SURFACES

A

Reversible 2D to 3D electrode transitions in polypyrrole films Toribio F. Otero *, Hans-JOrgen Grande Unil,ersidad del Pals Vasco, Facuitad de Qltb~lica, Lab. de Electroquhl~ica. P.O. Box 1072. 20080 San Sebastigm, Spain

Received 15 July 1997; accepted 29 July 1997

Abstract

Biological membranes change reversibly from a closed structure to an open one formed by channels and pores. This transition is triggered by ionic concentration gradients, and controls the movement of both muscular fibres and nervous pulses. Conducting polymers, like polypyrrole, show a similar behaviour when controlled electrochemically in an adequate solvent/electrolyte system. A compact structure is attained by polarization at more cathodic potentials than about -900 mV vs. SCE, arriving at a neutral state. In this situation, the material behaves as a 2D electrode: only the surface in contact with the electrolyte is electrochemically active. The polymer bulk remains a semiconductor. Under anodic polarization, electrons are extracted from polymeric chains. Coulombic repulsions between generated positive charges induce conlbrmational movements, resulting in a slow expansion of the polymer. The structure becomes permeable to ions, bence resulting in a 3D electrode: every polymeric chain actuates as an electrodic active interface. From an electrochemical point of view, the transition can be followed by means of variations in the charge vs. current transients during chronocoulometric experiments. Experimental results can be explained by means of the Electrochemically Stimulated Conformational Relaxation {ESCR) model. © 1998 Elsevier Science B.V. K~:v~t'ords: Conducting polymers; 3D-3D transition electrodes; Theoretical model; Conformations; Relaxation; Membranes

I. Introduction

Conducting polymers such as polypyrrole have attracted much attention as candidate materials for a wide range of useful applications including batteries [ 1,2], electrochemical capacitors [3], sensors [4,5], electrodes for electrocatalysis [6], electromagnetic shutters [7], or electrochromic [8,9], microelectronic [10,11] and ion release [12,13] devices. In recent years, however, a lot of research about conducting polymers has been focused on simulation o f biological structures such as muscles [14-16], nerves [17.18] or membranes * Correspondingauthor. Fax: + 3443-212 236; e-mail: qppfeott~sq.ehu.es 0927-7757/97/$15.00 ~©1998 ElsevierScienceB.V. All rights reserved. I'll S0927-7757( 97 )00331-2

[19], exploiting the suitable chemical, electronic and, above all, mechanical and structural properties o f these materials. In particular, investigations along these lines for polypyrrole and its derivatives have been very fruitful, because they combine the elasticity o f ordinary polymers and the high conductivity and electroactivity of conducting polymers [20]. Moreover, polypyrrole properties can be modified in a reversible and continuous way by several orders of magnitude through doping/ undoping {or oxidation/reduction) reactions, which can be achieved either chemically or electrochemically [21]. In particular, the electrochemical way allows a perfect control o f the attained doping level, so any intermediate composition can be attained: the weight percentage of counterions

86

T.F Otero. H.-J. Gnmde / ColloMs Surfaces A." Phj'sicochem. Eng. Aspects 134 (1998) 85- 94

[A-] and solvent [S] in the polymer [P] can be changed in a continuous way from zero to about 50%, depending on both the electric potential applied to the polymer and the polarization time [22]. Hence non-stoichiometric ionic compounds can be achieved, as summarized in the following switching equilibrium: Oxidation

[P] solid

+ ?1{[A-][S] m }solution

--"

{[P"+]

Reduction X [ A - ]n[S]nm }solid + l l e -

( 1)

From the point of view of the simulation of biological structures and processes, reversible changes of volume can be considered as the most important doping level dependent feature of polypyrrole [23]. In the neutral state attractive interactions between neighbouring chains prevail, resulting in a compact and closed polymer structure. Only the material situated at the metal-solution interface can actuate as electrode for electrochemical reactions. Thus reduced polypyrrole behaves as a 2D electrode, i.e. as a simple prolongation of the metallic base electrode. As a result of the flow of an anodic current, polymer-polymer attractive forces are substituted by strong coulombic repulsion forces between emerging positive charges along chains. Concomitant con|brmational movements of the polymer chains promote increasing interchain distances, allowing the generation of free volume which is immediately occupied by solvated counterions penetrating from the solution in order to maintain electroneutrality [24-27]. Conformational changes here have to be considered as variations in the overall three-dimensional shape adopted by a polymeric segment by rotation around the single bonds present between consecutive pyrrole units in the neutral state. Those single bonds are progressively transformed to double bonds, and again to single bonds, when an increasing population of positive charges on chains (polarons initially and bipolarons later) is achieved, leading to continuous changes in the rotation angles [28]. As those transformations in the geometry of the chains are linked to an electrochemical reaction, they can be considered as electrochemically stimulated conformational changes. As a consequence of this structural rearrangement,

the volume of the film increases (up to 40-45%), as experimentally demonstrated by different authors [ 14,29]. This results in an enhancement in the permeability of the polymer structure to small molecules or ions [30], so every polymeric chain will be able to act as an electrode. Since now electrochemical reactions occur throughout the film's volume, and not only on the external surface, oxidized films behave as 3D electrodes. The transition from a 2D to a 3D electrode is reversible: during reduction, positive charges are neutralized and counterions are expelled to the solution. Polymeric chains move into the free volume left by counterions, thus promoting the closure of the polymeric structure. Structural changes associated with polypyrrole doping/undoping processes are quite similar to those occurring in biological membranes and structures [3[]. A direct comparison can he made: biological membranes change reversibly from a closed to an open structure when the ion concentration gradient between both sides of the membrane reaches a given value, with formation of channels large enough to allow ions to cross the structure. Once the chemical equilibrium is recovered, the channels close again. Neutral polypyrrole shows a similar behaviour when submitted to an anodic disturbance. An open and porous structure is then obtained, which could be formed by channels. The initial state is recovered as the potential is turned to its initial value. As is clear from those similarities, the clarification of the electrochemical processes that led to structural transitions in conducting polymers will be enlightening for our comprehension of the mechanism of actuation of biological structures. In the same way, the knowledge and control of structural changes in conducting polymers will allow us to optimize technological applications, such as artificial muscles, artificial nervous connections and fibres or ion selective membranes, which attempt to imitate the working of natural devices. The aim of this work is to study the 2D to 3D electrode transition during the electrochemica! switching of polypyrrole films generated on platinum electrodes. The transition will be followed by means of the evolution of the charge consumed for oxidation of the polymer after a potential step

T. F Otero, H.-J. Grande / Colloids Sitrfaces A: Physicochenl. Eng. Aspects 134 (1998) 85-94

from a cathodic potential to an anodic one at a given temperature. Current-time transients (also called chronocoulograms) will be recorded, and the effect of changing each time an electrochemical parameter on the transition rate will be measured. The influence of structural parameters like crosslinking degree or conjugation length will not be taken into account in this approximation. In order to ensure that changes in the structure will affect electrochemical responses, the polymer will be reduced and compacted at high cathodic potentials. In this way highly compacted structures are obtained, that need large anodic potentials and polarization times to be opened [24,32-34]. Under those conditions, the electrochemical switching rate will be controlled by the rate at which conformational changes in the solid matrix occur. Experimental results will be analysed in the light of the Electrochemically Stimulated Conformational Relaxation (ESCR) model [3537]. Other models in the literature [38-44] do not take into account that conducting polymer films can behave either as 2D or as 3D electrodes depending on electrochemical conditions.

2. Experimental details Pyrrole (Jansen) was distilled under vacuum before use and stored under N2 at - I O ' C . Acctonitrile (Lab Scan, HPLC grade), propylenecarbonate (Merck, >99% content) and anhydrous lithium perchloratc (Aldrich, 95% content) were used without further purification. A one compartment electrochemical Metrohm cell, connected to a PAR M273 potentiostat-galvanostat and controlled by an IBM PS/2 computer was used for both electrogeneration and checking of polypyrrole films. Working electrode and counter electrode were platinum sheets having l c m 2 and 2 c m 2 surface area, respectively. A saturated calomel electrode (SCE) from Crison Instruments was used as reference electrode. The temperature of the cell was maintained constant with a Huber ministat. All the solutions were degassed prior to use by bubbling N2 for l0 rain. Electrogeneration was carried out using acetonitrile, with 2% water content, as solvent, 0.1 M

87

LiCIO 4 as electrolyte, and 0.1 M pyrrole as monomer, by passing 120 mC at 800 mV vs. SCE at room temperature (22°C). Polypyrrole films of about 0.22 ~tm thickness were obtained, showing electrochromic properties (yellow in reduced state and blue when oxidized). After generation, the polymer coated electrode was rinsed with acetone, dried in air and transferred into the background solution (0.1 M LiCIO4 in dry propylene carbonate) in the absence of the monomer, where it was submitted to chronocoulometric analysis. The solvent~electrolyte system was chosen because of its wide potential window and stability.

3. Experimental results and discussion When a polypyrrole film obtained as described above is submitted to potential steps performed from different cathodic potentials (ranging between - 3 2 0 0 and - 8 0 0 m V and maintained during 120 s) to the same anodic limit (300 mV) during 20 s in a 0.1 M LiCIO,Upropylenecarbonate solution at room temperature (22°C), chronocoulograms depicted in Fig. 1 are obtained. The oxidation process requires higher times to proceed as the initial potential is shifted to more cathodic values. When the polymer film is prepolarized at - 8 0 0 mV, the oxidation charge increases rapidly

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Fig. I. Experimental chronocoulometric responses to potential steps carried out on a polypyrrole electrode in a 0.1 M LiCIO, propylcnccarbonatc solution from different cathodic potentials (indicatcd in the figure) to 300 mV vs. SCE. at 22'C.

88

T.E Otero. H.-£ Gramle / Colloids Surfaces A: Ph)'sicochem. Eng. A~pects 134 (1998) 85-94

and reaches a constant value at long times of anodic polarization. The shape of this curve is related to a counterion diffusion controlled process [38]. In contrast, chronocoulograms performed from more cathodic potentials show a sigmo "al shape: the charge consumed for oxidation increases slowly during the initial time after the potential step. This fact can be linked to conformational relaxation processes in the closed structure, followed by a diffusion controlled completion of the oxidation process [24,35]. The more cathodic the initial potential, the longer the times required to open the structure and complete the oxidation process. As the final diffusional region remains substantially unchanged in all cases, the observed differences in oxidation times can only be assigned to variations in the rate of conformational relaxation as a consequence of the closure of the polymeric structure by polarization at more cathodic potentials than about - 9 0 0 mV vs. SCE. Conformational relaxation rate is not only affected by structural features of the film but also by the amount of energy available by polymeric chains to undergo conformational movements. Potential steps described above were performed under constant temperature and arriving each time at the same anodic potential, so thermal and electrochemical energies available by polymer chains remain unchanged during the different potential steps. Measured differences in oxidation r~tes are due to the different initial degrees of closure, as stated above. However, a new insight into the mechanism of structural transition can be attained if the polymer is reduced and compacted under the same experimental conditions (cathodic potential, temperature and polarization time) and then oxidized each time at a different anodic potential or temperature. For example, chronocoulometric responses to potential steps performed from -2000 mV vs. SCE, which was maintained for about 2 min, until different anodic potentials ranging between - 1 0 0 and 300 mV are depicted in Fig. 2. Times required to complete the oxidation are shorter at increasing anodic potentials, revealing that conformational relaxation processes are faster as more electrochemical energy is given to chains through anodic overpotentials. The other way to supply energy to the polymer

0 0

5

10

15

20

t/S

Fig. 2. Experimental chronocoulometric responses to potential steps carried out on a polypyrrole electrode from - 2 0 0 0 mV to different anodic limits (indicated in the figure) in a 0.1 M LiCIO4/propyleneearbonate solution at 22 C.

chains in order to promote faster conformational changes is by increasing temperature. With the aim of studying the influence of temperature on the oxidation rate, a polypyrrole coated platinum electrode was reduced and compacted by cathodic polarization at -2000 mV in a 0.1 M LiCIO4 propylenecarbonate solution at room temperature for 2 min. The electrode was then extracted from the background solution and maintained in N2 gas above the solution, while the temperature of the solution was regulated to a new value. The compacted and reduced electrode was again immersed in the solution and submitted to a potential step from -2000 mV (0.1 s) to 300 mV during 20 s. Different temperatures, in the range between - 1 0 and 30~C, were studied following this procedure. As the polymer was always reduced in the same extension, the compactness attained during reduction was also the same. Thus, when the polymer was later submitted to the same anodic step at different temperatures, the experimental changes observed on the oxidation rates (see Fig. 3) have to be attributed to differences in the thermal energy available by polymeric segments to undergo conformational changes. As expected, conformational relaxation occurs faster at increasing temperatures, promoting a shift of the chronocoulometric curves to lower times. This evolution is similar to the one observed above for increasing anodic potential limits. Higher energies available by polymeric seg-

T.F. Otero. H.-J. Grande I ColloMs Surfiwes A: Physicochem. Eng. Aspects 134 (1998) 85-94

E

/

0

/

/

~

0

5

10

15

211

tls

Fig. 3. Experimental chronocoulometric responses to the application o f potential steps from - 2000 mV to 300 mV in a 0. i M LiClO4/propylenecarbonate solution at different temperatures. ranging between - 10 and 30 C. Cathodic prepolarization temperature was -dways 22'C (room temperature).

ments promote, in both cases, faster conformational changes, and hence shorter times until oxidation completion. All those experimental results lead to the conclusion that conformational reIaxation processes in polypyrrole are controlled by both the initial pattern and energetic availability of polymeric segments, i.e. by cathodic and anodic potential limits and temperature. Hence the modelling of the electrochemical oxidation of a polypyrrole film under confonnational relaxation control requires the integration of both electrochemistry and polymer science. According to the ESCR model, presented by the authors in previous papers [35-37], the conformational relaxation time (r) is defined as the time required to chmtge the conformation of a polymeric segment, previously submitted to a cathodic potential E~, when it is oxidized to an anodic potential E at a given temperature (T). A polymeric segment is considered here as the minimum chain length whose conformational changes allow ionic interchanges between the polymer and the solution. Following an Arrhenius type law, it can be stated that: r = ro exp [ A H *

+zc(Es-E,:)-zr(E-Eo)]~J

(2)

where ro is a pre-exponentiai factor, which probably depends on structural magnitudes related to

89

the polymer, like the film thickness or the crosslinking degree. AH* is a mechanical component, defined as the conformational energy consumed per mole of polymeric segments in the absence of any external electric field, and RT has its usual meaning. The closure of the polymeric matrix is proportional to a cathodic overpotential, related to the potential at which the polymer structure closes along a negative potential sweep (Es = - 900 mV ). Under our simplified model, this means that starting from a potential more anodic than Es the oxidation will not be controlled by energetic requirements to open the polymeric network, but by counterion diffusion across the open structure. Parameter :c, defined as the coefficient of cathodic polarization, is related to the charge spent to compact one mole of polymeric segments. On the other hand, the thermal energy needed to relax one mole of polymeric segments decreases when an anodic overpotential is applied to the polymer. As the oxidation level (the number of charges consumed to oxidize a polymeric segment) depends on the applied potential, this electrochemical energy will be proportional to an anodic overvoltage, referred to the oxidation potential of the conducting polymer (Eo = - 550 mV). Finally, parameter :r is defined as the coefficient of electrochemical relaxation, which is related to the charge needed to relax one mole of polymeric segments. Eq. (2) provides a quantitative explanation for the dependence of the oxidation rate on electrochemical magnitudes. However, to confirm its validity, a further insight into the mechanism of conformational relaxation is required. An additional experimental observation shows that the oxidation mechanism in a compacted polypyrrole film changes in relation to films having a more opened structure in another manner: oxidation (and therefore the swelling of the polymeric structure to give a 3D electrode) is initiated on singular points which expand in a similar way as pits in corrosion processes of metals. Since the polymer structure is closed, counterions cannot penetrate inside the polymer, thus electrogenerated positive charges concentrate on the external surface where they can be compensated by counterions present in the solution, this fact being similar to other electrochemical reactions in 2D electrodes. Nevertheless, at specific points of the film surface

T.F. Otero, H.,J. GrandeI Colloids Surfaces A: Physicochem. Eng. Aspects 134 (1998) 85-94

90

where irregularities exist (irregularity here means free cooperative chain motions), the penetration of counterions inside the polymer is favoured. Those points actuate as nucleation centres for the oxidation of the polymer. Mechanical stresses, appearing on the borders between the oxidized regions and the neutral film, favour further conformational relaxation processes, so the nuclei have a tendency to expand like hemispheres inside the reduced film. However, due to the increase of the potential gradient between the nuclei and the electrode across the film, the oxidation boundary progresses faster towards the metal-polymer interface. Thus columns of conducting material are formed, which expand on the film until their coalescence. The formation and growth of oxidation nuclei can be observed when thin and electrochromic polypyrrole films are generated on polished platinum surfaces [37]: blue circles of oxidized polypyrrole expand across a yellow reduced film during the switching process. The number of nuclei can be well determined (5-9/cmZ). A new improvement in the model can be realized by the inclusion of nucleation and growth processes as responsible for the expansion of conformational relaxation along the film. We assume that No nuclei/cm-2 appear at the beginning of the oxidation process. We likewise accept that the overall charge (Q) consumed to oxidize the film has two components: the charge consumed to relax the compact structure, which will be named relaxation charge (Q,), and the charge consumed under diffusional control to complete the oxidation, named ahead diffusion charge (Qd). The rate of expansion of the conducting regions will be considered as 2/~, X and ~ being the length and relaxation time of a single polymeric segment, and their coalescence at long times of anodic polarization will be described by means of the Avrami treatment [45-47]. So the charge consumed for conformational relaxation at each time will be given by [35,37]: Qr( t ) = Qr[ ! - exp(- at2)]

(3)

where /I:No ;. 2

a = r --S--

(4)

During the l~rst stages of oxidation, when counterion diffusion processes still have not started, the charge required to open the polymeric structure will be the only component of the overall oxidation charge. Thus quantitative information about the conformational relaxation kinetics can be extracted from experimental chronocoulograms. By combining Eqs. (2)-(4), we arrive at: d2Q(t) 1 ln[Q

dt 2

=C-2

_Jt=o

A H * + z~(E s - E c ) - z , ( E - Eo)

(5)

RT

where Qr is assumed to be proportional to Q and all other constants are included in C. Eq. (5) can be directly checked with experimental results. As expected from the model, a linear relationship is obtained for cathodic and anodic limits (see Fig. 4). Values for z, and .% of 6040 and 2530 C mol- t, respectively, can be deduced from slopes. On the other hand, an Arrhenius representation of Eq. (5) provides a value of AH* of about 21.3 kJ m o l - ' (see Fig. 5). All these values are very close to those measured from chronoamperometric analysis [35-37]. Moreover, the obtained magnitudes are characteristic features of structural 2D to 3D transitions in polypyrrole, thus being -500 mt

• 320

/jI

~ 240

-15~0 -20~0

160

-251Xl

<

;~ ~

m

-

80

-

0

-

.80

-3000

-35D0

/

-40DO -4500

-----~

s! ~

.......

-4

-3

~ . -2

.

. -1

.

: 0

1

daQIll In[~

dl~~ -

It=.

Fig. 4. Semilogarithmic plot of cathodic (Ec) and anodic (E) potentials against values of I/Q[dZQ(t)/dt 2] extracted from Figs. 1 and 2. Following Eq. (5), values of the coefficient of electrochemical relaxation (_,) and the coefficient of cathodic polarization (=c) can be deduced from the slopes.

T.F. Otero, H.-J. Grande/ Colloids Surfaces A: Physicochenl. Eng. Aspects 134 (1998) 85-94 0,0

91

opened [37]:

Qa(t ) = Qa I I - e x p ( - a t Z ) - 2a n -1,0

x e x p ( - bt) "I~

t' exp(bt'-at '2) dt'

(6)

1,5

Eq. (6) introduces a new parameter b, defined as: -2,5

.

i

. . . .

0,0033

= 0,0034

. 0,0035

.

J .

0,0fl36

.

.

.

.

.

0,0037

.

.

.

.

.

0.[)038

T-11K-~ Fig. 5. Arrhenius plot of the term l/Q[d2Q(t)/dt 2] from dates corresponding to Fig. 3. The conformational energy consumed per mole of polymeric segments in the absence of any external electric field (AH*) can be obtained from the slope, following Eq. (5).

independent of whether this process controls the overall oxidation kinetics or not. For this reason, z the knowledge and application of these experimental magnitudes to the fabrication of devices such as artificial muscles or membranes will contribute to optimize them. For example, the coefficient of cathodic polarization (zc) can be interpreted as the efficiency of closure of the polymeric structure under application of a cathodic potential, so it will be necessary for applications to search a solvent/electrolyte system for which this parameter reaches its maximum value. Furthermore, a good structure compacting avoids any spontaneous oxidation of polypyrrole films in air [48], opening the way for long time storage of technological devices. As mentioned above, the oxidation of the relaxed segments is completed under counterion transport control across the solid polymer. The process can be separated into two parts: transport through the polymer-solution double layer and diffusion through the oxidized polymer towards the oxidation centres. This last component can be considered the rate-controlling step and responsible for the majority of the oxidation charge, hence its mathematical definition would complete our vision about the polypyrrole oxidation. The ESCR model offers a reasonably precise description of the evolution of the charge consumed until a given time in those regions where the structure was

b=

2D h2

(7)

D being the diffusion coefficient of counterions in the solid polymer (it is a function of the temperature, the anodic potential and both the nature and the concentration of the elcctrolyte in the solution) and h the average film thickness. Vahtes for D reported in the literature are on the order o f 1 0 - 1 1 - 1 0 - 9 c m 2 s - l [37,49,50]. These are typical values for diffusion of molecular penetrants in solid systems [51,52], thus offering new evidence for the capability of oxidized polypyrrole to behave as a 3D electrode. The addition of relaxation and diffusion components [Eqs. (3) and (6), respectively] provides a complete description of the shapes of chronocoulograms under any experimental conditions:

Q(t ) = (Qr + Qa)[ 1 - e x p ( - at-')] - 2abQa x exp(-bt)

t' exp(bt'-at '2) dt'

(8)

Eq. (8) can be used to compare theoretical results deduced from the ESCR model with experimental curves. Simulations have been carried out for a polypyrrole thin film having 0.22 lain thickness and 1 cm 2 surface area. Experimental magnitudes related to the film and required to solve Eq. (8) are those reported in this section. The value of 2/to was estimated as 1 x 10s cm s- 1. This corresponds to a rate of expansion of conducting regions of about 3 x l 0 - 2 c m s -~ in a potential step from - 2000 to 300 mV at 22°C, in accordance with our observations. For commodity, charges have been normalized to avoid direct effects of the studied variables on the shapes. A good agreement between experimental and

92

T.F.

Otero, H.-J. Grande/ Colloids Surfaces A: Physicochem. Eng. Aspects 134 (1998) 83.04

simulated curves is achieved whichever the studied experimental variable. The influence of the cathodic potential on the shape of experimental and theoretical normalized chronocoulograms can be observed in Fig. 6 (solid and dashed lines, respectively). In the same way, the chronocoulometric responses to potential steps from - 2 0 0 0 mV to different anodic limits are depicted in Fig. 7. Finally, both theoretical and experimental chronoamperograms obtained for potential steps at

different temperatures, being the polymer reduced at room temperature in all cases, can be compared in Fig. 8. It can be observed that the ESCR treatment, based on the coexistence of structural and electrochemical processes during polypyrrole switching, provides a reasonably good description of experimental oxidation chronocoulograms. As mentioned above, theoretical oxidation curves [see Eq. (8)] can be separated into their relaxational [Eq.(3)] and diffusional [Eq.(6)] components. Fig. 9 provides a direct comparison

i

061°F

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o,s

O

0.4

Q

-7.J~

0.~ T / "C

0,0

0,2

0

4

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IIs

16

.

0

20

, 0

Fig. 6. Normalized experimental (-) and theoretical ( - - -) chronocoulograms related to potential steps carried out on a polypyrrole electrode in a 0 . ! M LiCIO4/propylenecarbonate solution from different cathodic potentials (indicated in the figure) to 3 0 0 m V v s . S C E . T h e o r e t i c a l r e s u l ' : s were deduced from Eq. ( 8 ) .

1,0

100 -10

. .:~.L~

8

12

16

20

tls Fig. 8. Normalized experimental ( . . . . . . . . . ) and theoretical ( - - - ) chronocoulometric responses to the application of potential steps from - 2 0 0 0 m V t o 3 0 0 m V at different temperatures (indicated in the figure) in a 0 . 1 M LiClO4/propylenecarbonate solution.

.......... , 7.-7--::-~-7:~-""

'-

4

r 1,0

0,8

Q Q,

3OO

0,4

200

I00 0 - IO0

0,2

0,6

/ J

!-

0,4

//

"

0,2

oxidationcurve

i

Relaxation

(/

curv~

+

0,0

0,0 0

4

8

12

16

20

t/s Fig. 7. Comparison of normalized experimental ( . . . . ) and theoretical (---) chronoacoulometric responses to potential steps carried out on a polypyrrole electrode, in a 0.1 M LiCIO4/propylenecarbonate solution, from - 2 0 0 0 m V to different anodic limit.~ (indicated in the figure).

0

4

8

12

16

20

tls

Fig. 9. Separation of the overall oxidation curve into its two components: a relaxation part [according to Eq. (3)] responsible for the initial shape of the curve, and a diffusion part [Eq. {6)] which controls the final shape of the chronocoulogram.

T.F. Otero, H.-J. Grande/ Colloids Surfaces A: Physicochem. Eng. Aspects 134 (1998) 85-94

between both components for a potential step performed from - 2 0 0 0 to 300 mV at room temperature. The figure shows that the relaxation curve describes well the initial shape of the overall oxidation curve, whereas the diffusion curve approaches better to the oxidation curve at higher polarization times.

4. Conclusions

The reversible 2D to 3D electrode transition during polypyrrole switching has been studied by electrochemical methods. Structural changes in the film were related to the charge consumed during oxidation (swelling of the film) or reduction (shrinking). Any polarization at more cathodic potentials than - 9 0 0 mV for long periods of time results in the closure of the polymeric structure, giving a 2D interface between the polymer and the solution. Both electrochemical and thermal energies given to the polymeric chains by anodic polarization promote the opening and oxidation of the polymeric film to a 3D electrode, where every polymeric chain is in contact with the solution. This transition from a 2D to a 3D electrode occurs through conformational movements in the solid polymeric matrix. According to the ESCR treatment, both structural and electrochemical aspects of the process have been included in a single expression for the conformational relaxation time. The inclusion of nucleation and diffusion processes in the model has allowed a quantitative description of experimental chronocoulograms and the definition of structural parameters related to polypyrrole films of great interest for technological applications.

Acknowledgement

The authors would like to thank the Spanish Ministerio de Educaci6n y Cultura and the Diputaci6n Foral de Gipuzkoa for financial support.

93

References [1] B. Scrosati, 3. Electrochem. Soc, 136(1989)2774. [2] T.F. Otero, C. Santamaria, J. Rodriguez, Mater. Res. Soc. Symp. Proc. 328 (1994) 805. [3] A. Rudg.c, J. Davey, I, Raistrick, S. Gottesfeld, J.P. Ferraris, J, Power Sources 47(1994) 89. [4] R. John, G° Wallace, J. Electrochem. Chem. 283 (1990) 87. [5] M.E.G. Lyons. C,H. Lyons, C. Fitzgerald, T. Bannon, Analyst 118 ~1993) 361. [6] A. Deronzier, J. Moutct, Acc. Chem. Res. 22 (1989) 249. [7] J.D. Stenger-Smith, A.L. Tipton, H.O. Marcy, L.F. Warren, M. Pyo, G.A. Sotzing, J.R. Reynolds, Am. Chem. Soc., Div. Polym. Mater. Sci. Eng. Proc. 71 (1994) 711. [8] K. Hyodo, Electrochim. Acta 39 (1994) 265. [9] T.F. Otero, M. Bengoechea, Electrochim. Acta 41 (1996) 1871. [10] A. Watanabe, S. Murakami, K. Mori, Y. Kashiwaba, Macromolecules 22 (1989) 4231. [11] J.T. Lei, W.B. Liang, C.J. Brumlik, C.R. Martin, Synth. Met. 47 (19921 351. [ 12] B. Zinger, L.L. Miller, J. Am. Chem. Soc. 106 (1984) 6861. [13] Q.-X. Zhou, L.L. Miller, J.R. Valentine, J. Electroanal. Chem. 261 (1989t 147. [ 14] T.F. Otero, J. Rodriguez, in: M. AIdi~si (Ed.), Intrinsically Conducting Polymers: An Emerging Technology, NATO ASI Series, Vol. 246, Kluwer Academic, The Netherlands, 1993, p. 179. [15] T.F. Otero, in: H.S. Nalwa (Ed.I, Handbook of Organic Conductive Molecules and Polymers, Vol. 4, Wiley, UK, 1997, p. 517. [16] E. Smela, O. lnganas, Q. Pei, I. LundstrOm, Adv. Mater. 5 ( 1993 ) 630. [171 M. Fujii, K. Arii, K. Yoshino, Synth. Met. 71 ( 19951 2223. [181T. Kawai, H. Motobayashi, T. Kuwabara. Jpn. J. Appl. Plays. 30 ( 1991 ) L622. [19] C. Ehrenbcck, K. Jf~tnner. Electrochim. Acta. 41 (1996) 1815. [20] B. Zinger. P. Shaier, A. Zemel, Synth. Met. 40 ( i991 ) 283. [21] B.J. Feldman, P. Burgmayer, R.W. Murray, J. Am. Chem. Soc. 107 (1985) 872. [22] M. Yamaura, K. Sato, T. Hagiwara, Synth. Met. 39 11990) 43. [23] P. Chiarelli, D. De Rossi, A. Della Santa, A. Mazzoldi, Polymer Gels and Networks 2 (1994) 289. [24] T.F. Otero, E. Angulo, Solid State lonics 636465 (19931 803. [251 P. Marque, J. Roncali, J. Phys. Chem. 94 (19901 8614. [26] Y. Qiu, J.R. Reynolds, Polym. Eng. Sci. 31 ( 19911 6. [27] J. Heinze, R. Bilger, K. Meerholz, Ber. Bunsenges. Phys. Chem. 92 (1988) 1266. [281 J.L. Br~das, B. Th~mans, J.G. Fripiat, J.M. AndrC R.R. Chance, Phys. Rev. B 29 ~1984) 6761. [29] M. Slama. J. Tanguy, Synth. Met. 28 (1989) C171. [30] M.R. Anderson, B.R. Mattes, H. Reiss, B. Kaner, Science 252 11991 ) 1412.

94

T.F. Otero, H.-J. Grande/ Colloids Surfaces A: Physicochem. Eng. Aspects 134 (1998) 85-94

[31] V.M. Sehmidt, D. Tegtmeyer, J. Heitbaum, Adv. Mater. 4 (1992) 428. [32] T.F. Otero, J. Rodriguez, H. Grande, J. Braz. Chem. Soc. 5 (1994) 179. [33] C. Odin, M. Neehtschein, Synth, Met. 555657 (1993) 1281. [34] C. Odin, M. Neehtsehein, Synth, Met. 555657 (1993) 1287. [35] T.F. Otero, H. Grande, J. Roddguez, J. Electroanal. Chem. 394 (1995) 211. [36] T.F. Otero, H. Grande, J. Electroanal. Chem. 414 (1996) 171. [371 T.F. Otero, H. Grande, J. Rodriguez, J. Phys. Chem. B 101 (1997) 3688. [38]F.A. Posey, T. Morozumi, J. Electrochem. Soc. 113 (1966) 176. [39] K. Aoki, Y. Tezuka, J. Electroanal. Chem. 267 (1989) 55. [40] S.W. Feldberg, J. Am. Chem. Soc. 106 (1984) 4671. [41] T. Yeu, T.V. Nguyen, E. White, J. Eleetrochem. Soc. 138 ( 1991 ) 2869.

[42] M. Kalaji, L.M. Peter, L.M. Abrantes, J.C. Mesquita, J. Eleetroanai. Chem. 274 (1989) 289. [43] A.R. Hillman, S. Bruckenstein, J. Chem. Soc., Faraday. Trans. 89 (1993) 3779. [44] D.A. Kaplin, S. Qutubuddin, J. Eleetrochem. Soc. 140 (1993) 3185. [45] M. Avrami, J. Chem. Phys. 7 (1939) 1103. [46] M. Avrami, J. Chem. Phys. 8 (1940) 212. [47] M. Avrami, J. Chem. Phys. 9 (1941) 177. [48] J. Roddguez, J.P. Moliton, T. Trigaud, T.F. Otero, H. Grande, Mater. Res. Soc. Syrup. Proc. 413 (1996) 595. [49] J.J. Kim, T. Amemiya, D.A. Tryk, K. Hashimoto, A. Fujishima, J. Electroanal. Chem. 416 (1996) 113. [50]P.J.S. Foot, F. Mohammed, P.D. Calvert, N.C. Billingham, J. Phys. D: Appl. Phys. 20 (1987) 1354. [51] J. Corish, Philos. Mag. A 64 (1991) 1073. [S~l M G. Kulk~am; ILA. Mashelkar, Polymer 22 ( 1981 ) 1665.