The mechanism of electrochemical charge - transfer reactions on conducting polymer films

Synthetic Metals, 41-43 (1991) 2865-2870

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THE MECHANISM OF ELECTROCHEMICAL CHARGE-TRANSFER REACTIONS ON CONDUCTING POLYMER FILMS

Karl Doblhofer and Chuanjian Zhong Fritz-Haber-lnstitut der Max-Planck-Gesellschaft, Berlin 33 (F.R.G.)

ABSTRACT Conducting polymers can act as electrodes oxidizing/reducing redox systems in electrolytes. The mechanism of such heterogeneous charge-transfer reactions is discussed on the basis of energetic considerations involving the band model of the polymer, polarons/bipolarons, and the electronic states of the redox ~,ystem in the electrolyte. The kinetics of the electrochemical reaction is shown to depend strongly on the standard potential, Eo, of the redox system. When Eo corresponds to the oxidized (conducting) state of the polymer, the charge-transfer reaction can proceed effectively. When Eo corresponds to the reduced (insulating) state of the polymer, the charge-transfer reaction is inhibited. The concepts are supported by electrochemical experiments with poly (N-methylpyrrole) films, and Fe(CN)6 3-•4- and Eu3 +/2 + as depolarizers.

INTRODUCTION There has been a considerable effort in preparing conducting polymer films with electrochemical methods. Because of practical applicability, e.g., for charge-storage devices, the electrochemical oxidation and reduction of the films has also been thoroughly studied [1]. A third electrochemical feature of the conducting polymers is their ability to undergo heterogeneous electron-exchange reactions with depolarizers from electrolytes. The study of such charge-transfer reactions would be of interest from a fundamental point of view, e.g., for studying the electronic states in the polymeric electrodes [2], but also for a possible practical use of these materials, e.g., as electrocatalysts [3]. Considering the potential merits of such studies, very little work has been conducted in this field [4,5]. It is the purpose of this work do discuss some fundamental behavior patterns observed upon oxidizing/reducing redox systems of different standard potentials on poly 0379-6779/91/$3.50

© Elsevier Sequoia/Printed in The Netherlands

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(N-methylpyrrole) electrodes ("PMPy", [1]). The discussion of the electronic and transport properties of the "conducting-polymer" electrode as a function of the level of oxidation of the polymer matrix is discussed largely on the basis of concepts of band structure and polaron/bipolaron formation as summarized, e.g., by Bredas and Street [6].

The Electronic States of PMPy at Different Levels of Oxidation The PMPy films can be reversibly oxidized and reduced, e.g., in aqueous electrolytes containing the anionic species X-. We describe this process by: PMPy

+

nX- ~q~

PMPyn+

X'n

+

ne-

(1)

where the anions on the left hand side of eq. (1) are in the electrolyte. During oxidation they are incorporated into the polymer matrix. The number, n, of electrons, e-, withdrawn from a polymer chain increases with the degree of oxidation to up to about 1 eper 4 N-methylpyrrole units in the PMPy chain. A typical current/voltage diagram obtained with the method of cyclic voltammetry at various sweep rates, v, is presented in Fig. 1 : I

'

I

J

I

'

I

'

0.4

PMPy(Ct-)/GC

v/mvs-1 200

0.3

< E

0.2

100

0.1

50 20

-0.1

-0.2

I

-0.8

I

I

-0.4

i

I

w

I

0 0.4 E / V vs. SCE

Fig. 1. Cyclic voltammogram of a poly(N-methylpyrrole) film on glassy carbon {GC) in 0.1 M KCI aqueous e ectro yte F m thickness ca. 0.2 IJm; electrode area 0.5 cm2. For more details see ref. [1]. The same result is obtained when 1 mM EuCI3 is added to the electrolyte.

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In Fig. 2, we present three characteristic states of the system consisting of the substrate electrode (Me), the polymer film (PMPy + is used to designate the oxidized states), and the electrolyte (5)

-0.8V

0.0 v

+o.sv

Fig. 2. Schemattc representation of the electronic energy levels of the system of Fig. 1 at different electrode potentials (-0.8 V, 0.0 V, + 0.5 V). EF are the Fermi levels, REF relates to the saturated calomel electrode, SCE.

Note that in electrochemistry the electrode potential E is reported relative to a reference level in the electrolyte (Erref in Fig. 2), and that a more positive value of E corresponds to a lower Fermi level in the electrode (ErMe). As the PMPy is being oxidized, polarons (radical cations) are formed. Upon further oxidation, a second electron is withdrawn

from the polarons, whereby the spin-free bi-

polarons are generated (61. The exact energetic position of the polaron/bipolaron

levels

in relation to the valence band edge will depend on the state of solvation of the matrix and on the counter ion (X‘). Fig. 2 represents the situation in a qualitative fashion. The polaron/bipolaron

states constitute acceptor levels (p-type doping). The Fermi

level in the polymer will thus move towards the valence band (V8) edge at the higher levels of oxidation.

Electrochemical Charqe-Transfer Reaction between the Polymer Electrode and a Redox System whose Standard Potential Corresponds to the Oxidized (Conductinq) State of the Polymer Figure 3 gives a schematic representation of the energeticsituation the equilibrium

corresponding to

between a redox system and the polymer in the conducting state.

The electronic equilibrium across the polymer/electrolyte the electron-transfer

interface is established by

processes shown as arrows “e -” in Fig. 3: (a) Hole injection from ox

into the VB, shown as e- transfer from the V8 into the unoccupied electronic energy lev-

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on energy

x_

e

red

I

meto[

Fig. 3. Equilibrium between a redox system (ox, red) in the electrolyte and the polymer (PMPy + ) at a relatively low oxidation level (cf. Fig. 2). els of the redox system; thereby, a polaron is produced. (b) e- transfer from red to a polaron with the formation of a VB state. Consider now the situation prevailing when a positive overvoltage is applied to the system of Fig. 3. We expect two consequences: (a) The oxidation level of the polymer will rise, i.e., the density of polarons/bipolarons will increase. (b) The Fermi level in the polymer will be lowered relative to EFredox. Both, (a) and (b) have the consequence that the rate of oxidation of red increases relative to the rate of reduction of ox; we thus expect a net anodic faradaic current flow. The effective action of a PMPy-coated electrode oxidizing Fe(CN)64- may be observed in the representation of experimental results in Fig. 4, where the dependence of the anodic current on the electrode rotation rate (e) indicates a practically diffusion-controlled process.

[

~

I

i

I

I

I

I

I

PMPy (CL-) / GC

0.6

i

w/

rpm

3500 0.4

2000

< 0.2 E

500

__2_

0

o

-0.2 I

-0.4

J

r

-0.2

~

i

J

I

0 0.2 E/V vs. SCE

z

I

0.4

Fig. 4. Pseudo-stationary current-voltage curves obtained with a PMPy-coated glassy electrode in a deaerated aqueous electrolyte of each 1 mM K3/K4 [Fe(CN)6] and O.1 M KCI. Electrode rotated at the indicated rates ~J. Electrode area 0.5 cm2. carbon

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At negative overvoltages the density of (equilibrium) charge carriers in the film will be reduced, eventually to practically zero; the polymer is in the "insulating" state. However, with the considered redox system hole injection into the VB will proceed at the interface polymer/electrolyte The produced polarons transport the charge across the polymer. The situation is illustrated in Fig. 5. The cathodic current flow observed in the 'electron energy E PMPy F

I

potaroo

e--T'~_~_ _ transport

e~V~B~/_

metal

PMPy

-

_~_ ~-~

OX

Eredox

I red

redox electrolyte

Fig. 5. Hole injection into the VB of the negatively polarized (reduced) PMPy by an oxidizing redox system, e.g., Fe(CN)63-, and transport of the produced polarons to the substrate electrode ("metal"). case of PMPy/Fe(CN)63-/4- (cf. Fig. 4) does not depend on the applied overvoltage. This indicates that the polaron transport across the film does not limit the rate of the process. The rate-determining step is the hole injection into the VB, i.e., the oxidation of PMPy by Fe(CN)63-. This conclusion is supported by the fact that the cathodic current flow is proportional to the Fe(CN)63- concentration.

Electrochemical Charqe-Transfer Reaction between the Polymer Electrode and a Redox System whose Standard Potential Corresponds to the Reduced Ilnsulatinq) State of the Polymer The following considerations relate to redox systems such as Eu3 +/2 + (standard potential: -0.67 V/SCE). Neither the redox system (ox or red) nor the polarisation of the electrode in this potential region will produce charge carriers in the film. At such potentials, the PMPy coating acts as a barrier layer inhibiting the considered charge-transfer reaction. The situation is shown schematically in Fig. 6. We have conducted the corresponding experiments with Eu3 + added to the KCl electrolyte of Fig. 1. The current-voltage curves in this case were found to be indistinguishable from the ones obtained in absence EuCl3, in agreement with these considerations.

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etectron energy .

metal

PMPy

.

.

.

redox electrolyte

Fig. 6. A redox system of relatively negative standard potential (e.g., Eu3 +/2 +) at the PMPy-coated electrode, polarized to a negative overvoltage. The PMPy coating acts as a barrier for charge transfer. The oxidation of the reduced form of this redox system proceeds, of course, when the potential of the electrode is raised to a value at which the film starts to be oxidized ( ~ -0.1 V/SCE). It is important to note that the blocking properties of the film can be observed only when the film is impermeable to the redox system. Otherwise, the charge-transfer reaction can proceed at the surface of the substrate "metal" electrode. We have intentionally prepared such permeable films by incorporating electro-inactive anionic groups (-SO3-) into the matrix [1]. On such coated electrodes the Eu3+/2+ redox reaction may be conveniently observed, in addition to interesting membrane phenomena [7]. This, however, is not the subject of this work. ACKNOWLEDGEMENT The authors want to thank Prof. H. Gerischer for valuable comments to this paper.

REFERENCES 1 C. Zhong and K. Doblhofer, Faraday Discuss. Chem. Soc., 88 (1989) 307. 2 H. Gerischer D.M. Kolb and J.K. Sass, Adv. in Physics, 27 (1978) 437. 3 R.C.M. Jakobs, LJ.J. Janssen and E. Barendrecht, Electrochim. Acta, 30 (1985) 1085. 4 V.E. Kazarinov, M.D. Levi, A.M. Skundin and M.A. Vorotyntsev, J. Electroanal. Chem., 271 (1989) 193. 5. M.M. Lohrengel, J.W. Schultze and A. Thyssen, in H.J. Mair and S. Roth (eds.), Elektrisch leitende Kunststoffe, Hanser Verlag, Mcinchen/Wien, 2nd edn., 1989, p. 337. 6 J.L. Br~das and G.B. Street, Acc. Chem. Res., 18 (1985) 309. 7 H. Braun, W. Storck and K. Doblhofer, J. Electrochem. Soc., 130 (1983) 807.