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Ion exchange and memory effects in polyaniline
Synthetic Metals, 55-57 (1993) 1545-1551
1545
ION EXCHANGE AND MEMORY EFFECTS IN POLYANILINE
C. BARBERO, R. Kt)TZ, M. KALAJI + , L. NYHOLM #, L.M. PETER # Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland + Department of Chemistry, University College of North Wales, Bangor, Gwyned LI57 2 UW, United Kingdom # Department of Chemistry, University of Southampton, Southampton, Hants SO9 5NH, United Kingdom
ABSTRACT Memory effects in PANI can be promoted not only by holding the film at cathodic potentials by some time but also by cycling in a potential window with a reduced anodic limit. The peak overpotential of the first cycle after the pretreatment has a maximum for an anodic limit of +0.2 Vsce. The peak current presents a maximum value for an anodic limit of +0.12 Vsce and a minimum for +0.24 Vsce. Nonequilibrium effects are observed in the optical response of PANI. The optical absorbance at 440 nm during potential pulses shows a sharp maxima at short time an decays later to a constant value. The two redox steps of PANI oxidation involve mixed anion/proton exchange. For high proton concenu'ation, protons are predominantly exchanged. At high pH (pH>2), an increase in the solution ionic strength brings about a bigger contribution of proton to the ion exchange process.
INTRODUCTION The redox reaction in polyaniline (PANI) thin films has been studied extensively during recent years [ 1]. An interesting behaviour of PANI redox response is the so called "first cycle effect" [2] or "memory effect" [3]. The shape of the first voltammogram recorded after waiting at the reduced state, is different of that observed during continuous cycling. The shape differences include different oxidation potential, peak current [2,3] and redox charge [2]. For sake of conciseness we will refer to this phenomena as statically induced memory effect (S1ME). The more favoured explanation for that behaviour is based on the dependence of conductivity on the polymer redox state. The conductivity decreases with reduction. By maintaining the film at negative potentials for some time a complete reduction is achieved. A higher overpotential is therefore necessary to oxidize the film. During continuous cycling the film is never totally reduced and the cyclic voltammogram shape is different. An alternative explanation is an alteration of the exchange of ions and/or solvent coupled to the redox process. Using in situ techniques it was shown [4,5] that polyaniline exchange protons and 0379-6779/93/$6.00
© 1993- Elsevier Sequoia. All rights reserved
1546
anions during its first oxidation process. The relative contribution of each specie to the global ion exchange is determined by the acid concentration in solution. In the present communication it is shown that memory effects in PANI are observed not only when the film is maintained at a negative potential but also when the potential cycling range is limited to a less anodic potential. Both procedures of achieving memory effects will be compared. The phenomena will be studied by cyclic voltammetry, Probe Beam Deflection (PBD) and UV-vis spectrolectrochemistry. A study of the effect of the solution ionic strength on the ion exchange of PANI will be also described.
EXPERIMENTAL Polyaniline modified electrodes were prepared by cycling the working electrode between -0.2 and 0.75 Vsce at a scan rate 20 mV/s in a 0.1 M aniline/1 M H2SO 4 solution. The electrode was cycled until a desired surface charge was reached (2 mC/cm 2 typical). The working electrodes are GC rods (A=0.071 cm2) inserted in a Kel-F matrix for cyclic voltammetric experiments. The electrochemical experiments were controlled by a PAR 173 potentiostat driven by a PAR 175 function generator. All potentials are measured against saturated calomel electrode (SCE). UV-visible spectroelectrochemistr,¢, The optical absorbance was recorded in an homemade spectrophotometer fitted with a transmission spectroelectrochemical cell. Indium doped SnO2 films on glass (Balzers) were used as transparent electrodes. Probe Beam Deflection (PBD). In PBD measurements GC plates (10 mm width) were used, the back and side edges covered with a protective laque. The PBD arrangement has been described before [6]. The exchange of ions between the film an solution creates a concentration gradient normal to the electrode surface. A light beam, aligned parallel to the electrode surface, suffers a deflection proportional to the refractive index gradient correlated with the concentration gradient. A positive deflection indicates ion insertion while negative deflection indicates ion expulsion. The PBD signal is recorded during the potential scan (cyclic deflectogram). Voltammetrv at ultramicroelectrodes: Pt wires (10 ktm diameter) sealed in glass were used as ultramicrolectrodes. The electrodes were polished with alumina (0.3 I.tm). A digital storage oscilloscope (Nicolet 2090) was used to record current-potential profiles during fast cyclic voltammetry experiments.
RESULTS AND DISCUSSION: Memory effeCtS: The cyclic voltammogram of PANI in I M HC1 is shown in Figure 1. When the electrode is cycled continuously, a stationary response is observed (curve a). However, if the positive extreme of the potential cycle is restricted to +0.12 V, the charge diminish with successive cycles (compare curve a (initial state) and curve b (final state)). If after subjecting the electrode to this treatment, the positive potential extreme is restored to +0.5 Vsce, the next oxidation cycle presents a higher maximum
1547
current and a more positive peak potential (curve c). The effect is similar to the reported before [2,3] by holding the potential at the negative extreme of the scan. However, it is observed that cycling a determined time (eg. 1 rain) produces a more pronounced change than waiting the same time at the negative extreme. This shows that the change is related with a transformation of PANI during cycling. By sake of conciseness we will refer subsequently to the effect achieved by cycling as dynamically induced memory effect (DIME). Using ultramicroelectrodes, fast scan rates can be used due to the diminished effect of uncompensated resistance. Tests at fast scan rates (up to 100 V/s) show the evidence od DIME. On the other extreme, scan rates as slow as 1 mV/s were used with similar results. It is observed that the value of the extreme anodic potential (Epp) used during DIME has an influence on the extent of change of the polymer. For example, using a Epp of +0.45 V, the peak current is smaller than the peak current during continuous cycling up to +0.5 V. It is noteworthy to mention that in SIME the current in the first cycle is always bigger. To study this influence quantitatively, parameters of the extent of change need to be defined. The relative increase in peak current (IR) is taken as the equal to the difference between the peak current in the first cycle after pretreatment and the current peak during continuous cycling. The value is normalized by dividing it by the peak current for continuous cycling. The electrode overpotential (~E) can be defined as the difference of the peak potential for the first cycle after pretreatment, with the peak potential during continuous cycling. The parameters were measured in the first cycle after a pretreatment consisting of: ten cycles between -0.2 and +0.5 Vsce followed by two minutes of cycling between -0.2 V and Epp. The results are shown in Figure 2 as a plot of the parameters (IR and 8E ) for different Epp. As can be seen 8E has a maximum at ca +0.2 V (Fig. 2B) while IR present a derivative like shape with a maximum at +0.12 V and a minimum at +0.24 V (Fig. 2A). The inflection point of the IR plot correspond to the maximum of BE. For a given scan rate, more time is spent at negative potentials for a less positive Epp. However, the extent of change (IR and ~E ) is smaller for that values of Epp. Therefore, the potentiodynamic effect is not produced to the time spent at potentiostatic relaxation in the reduced state. It was found that waiting at an intermediate potential (eg. +0.12 V), does not produce any effect in the successive cycle, showing that the effect is truly dynamic. It is difficult to rationalize the observed phenomena on the basis of a resistance change because the resistance is low and almost constant between +0.2 V and +0.5 V [7]. It is tempting to relate the dependence of the extent of change with electronic parameters as the spin concentration that has a maximum at ca. 0.2 Vsce [8], thus resembling the profile of 8E depicted in Figure 2. However, an adequate explanation of the phenomena should take in account the dynamic nature of it. A more likely explanation is related with ion and/or solvent exchange. By cycling in a limited range, only a portion of the film is transformed, some pores close and the electroactivity decreases. Therefore we investigate the ion exchange in SIME and DIME using PBD. It was found that the PBD response for the first oxidation cycle of PANI in 1 M HCI differs of that for continuous cycling. The proton expulsion signal on oxidation is bigger in the first cycle than that of the continuous cycle suggesting that a change in the protonation equilibria is involved in the memory effect. However, the PBD response at pH 3 shows an increased anion insertion signal in the first cycle with respect to
1548
(+)
t-
I I 2 0 p.A
O
O.30"
=_ 0
~ 0.15~ °Q 0 . 0 0 ~ ._> (a)- ~1----
(9
......
~ -0.15 50
4°_A
v~ 30
(B)
'3
20
• O
Figure 1: Cyclic voltammogram of PANI in 1 M HCI. Scan rate 100 mV/s. Curve (a) continuous cycling (b) final cycle with reduced limit pretreatment (0.12 Vsce); (c) first cycle after pretreatment.
! 0
.
I
,. -
-
-
0.10 0.25 0.40 0.55 Anodic Potential Limit (Vsce)
Figure 2: plot of current difference (A) and overpotential (B) as a function of the anodic limit of the pretreatment scan. Experimental parameters as in Fig. 1
successive cycles. Therefore it is the ion exchange what is altered irrespective of its nature. This fact suggests that the global electroactivity increases and with it the ion exchange. A similar behaviour is observed for DIME, An analogous behaviour is observed using spectroelectrochemistry. The derivative of the absorption with potential at 795 nm shows a bigger change for the first cycle than for successive cycles. The derivative is the analog to the current and is measured at the wavelength where the emeraldine state of PANI has its maximum absorption [9]. The behaviour indicates that the redox activity of the film is not totally restored during continuous cycling. A similar effect is observed for DIME. In both cases the changes in voltammetric charge are directly correlated with changes in the absorbance. It was shown using chonoamperometry in ultramicrolectrodes [2], that the current response of the first oxidation differs to that of successive oxidation pulses. A good description of the first pulse was found to be a nucleation and growth mechanism. The absorption behaviour can be explained in a similar way. Additional information can be obtained from the absorption at other wavelengths. It is known that the the first oxidation process PANI present and intermediate state [9] that has a maximum absorption at 440 nm [9]. Using the absorption at 440 nm as a measure of the intermediate concentration, the kinetic stability of the intermediate is assessed. Figure 3 shows the behaviour of the absorbance at 440 nm by stepping the potential to +0.5 V. The concentration of the intermediate state does not increase monotonically as expected for a redox coupled confined in a finite layer but shows a sharp
1549
AbsorbanceI0.020
e~
time I12 sI Figure 3: Absorbance response at 440 nm for a PANI film in 1 M HC1. Potential pulse from -0.2 Vsce to 0.5 Vsce (oxidation) and back (reduction).
maximum and a subsequent decay towards a constant value. This fact suggests that the oxidation of intermediate to fully oxidized state is a slow process. The initially formed excess concentration decays through a chemical reaction to the oxidized state. This behaviour contrast which that observed for the absorbance of the fully oxidized state ( at 795 nm) that increases monotonically to reach a constant value. Additionally, if the potential is stepped to an intermediate value (+0.2 V) the concentration of the intermediate state increases monotonically to a constant value. At this potential the intermediate state concen~ation is maximum and is more stable than the fully oxidized state. The behaviour agrees well with that predicted by the chemical redox model proposed by Albery et al [11]. Ionic strength effect on the ion ~xghang¢: The redox process in the first oxidation step of PANI is accompanied by a coupled ion exchange. The creation of positive charges is compensated by anion insertion and/or proton expulsion. The relative contribution of proton to the ion exchange depends on the degree of protonation of the reduced state of PANI. At higher acid concentration, the protonation degree is higher and more protons inside the film are available to be expulsed in order to achieve charge compensation. Measurement with in situ techniques [4,5] support this model. A somewhat overlooked aspect is the influence of the solution ionic strength on the protonation equilibria. As the protonated specie is part of a solid matrix, the proton concentration inside the polymer could differ from that of the solution, depending on the Donnan potential established across the polymer/electrolyte interface. The influence of the Donnan effect on the protonation of the emeraldine state was demonstrate recently by Chartier et al [10]. In a previous study using PBD [5] it was found that the proton contribution to the ion
1550
(+)
PBDI20 ~rad
)
E vs S C E
(-)
I
O.1V
I
Figure 4: PBD response during a potential scan of a PANI film in a 5 mM HC1 / 4 M NaC1 solution. Scan rate of 50 mV/s exchange is negligible for acid concentration below 0.1 M . Figure 4 shows the PBD response for a PANI film in a solution consiting in 0.005 M HC1 + 4 M Na CI. The dominant positive deflection (anion exchange) presents an small negative (proton expulsion) superimposed signal. This indicates that a proton contribution exists for proton concentration as low as 0.005 M if the ionic strength is kept high by addition of an inert salt. The high cation concentration in the bathing solution diminishes the Donnan potential across the polymer/solution interphase. Therefore the pH inside the polymer is closer to the value outside and the reduced state is protonated. The proton contribution is not observed if the salt concentration is lowered to 0.5 M. It is noteworthy to mention that at a proton concentration of 0.001 M the only species used for charge compensation is anion, irrespective of the salt concentration. This indicates that at and internal pH of about 3 the reduced state is unprotonated. A previous study using fast cyclic voltammetry at ultramicroelectrodes [12] shows that the anion concentration has an influence on the voltammetric shape for both voltammetric peaks. At low total anion concentration (6 mM) only one oxidation peak is present while at 1 M two peaks are observed in the first scan. In the present work we extend the study to a total anion concentration of 4 M. Two peaks are observed in the first oxidation scan at 100 V/s. In the successive scans only one broad wave is present. This result support the model presented before [12] where fast proton egress is the mechanism for charge compensation during the first oxidation scan. CONCLUSIONS - Memory effects in PANI can be promoted not only potentiostatically but also potentiodynamicaUy - Memory effects are also observed in the ion exchange of PANI. The increased charge is correlated with an increased quantity of ions exchanged.
1551
-
A faradaic reaction is present in the memory effect as the increased charge is correlated with an
increased change of the film optical absorbance. - The first oxidation process of PANI has a kinetic complication where the intermediate concentration reaches a maximum value to decay later. - The ion exchange of PANI is not only affected by the pH but also by the ionic strength of the bathing solution. REFERENCES
[1] E.M. Genies, A. Boyle, M. Lapkowski and C. Tsintavis, $ynth.M~t.. 36(2090)136 [2] M. Kalaji, L.M. Peter, L.M. Abrantes, J.C. Mesquita, J.Elcctroanal.Chem.,274(1989)289 [3] C.Odin and M. Nechstein, Svnth.Met.. 41-43(1991)2943 14] D. Orata and D.A. Buttry., J.Am.¢hCm.Soc.. 109(1987)3574 [5] C. Barbero, M.C. Miras, O. Haas and R. K6tz, J. El¢ctroch~m.Soc.. 138(1991)669 [6] C. Barbero, M.C. Miras and R. K6tz, Electrochim. Acta. 37(1992)429 [7] E.W. Paul, A.J. Ricco, M.S. Whrighton, J.Phys.Chem.. 89(1985)1441 [8] F. Genoud, J. Kruszka, M. Nechstein, C. Santier, S.Davied, Y. Nicolau, Synth.Met., 41-43 (1991)2887 [9] D.E. Stilwell, S-M. Park, J.Ele~trochem.Soc..136(1989)427 [10] P. Chartier, B. Mattes, H. Reiss, ~I. Ph¥~.Chem.. 96(1992)3556 [11] W.J. Albery, Z. Chen, B. Horrocks, A.R. Mount, D. Bloor, A.T. Monkman, C.M. Elliot, Faraday Dis¢uss. Chem. Soc..88(1989)247 [12] M. Kalaji, L. Nyholm and L.M. Peter, J. Electr0anal. Chem.,313(1991)271
1545
ION EXCHANGE AND MEMORY EFFECTS IN POLYANILINE
C. BARBERO, R. Kt)TZ, M. KALAJI + , L. NYHOLM #, L.M. PETER # Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland + Department of Chemistry, University College of North Wales, Bangor, Gwyned LI57 2 UW, United Kingdom # Department of Chemistry, University of Southampton, Southampton, Hants SO9 5NH, United Kingdom
ABSTRACT Memory effects in PANI can be promoted not only by holding the film at cathodic potentials by some time but also by cycling in a potential window with a reduced anodic limit. The peak overpotential of the first cycle after the pretreatment has a maximum for an anodic limit of +0.2 Vsce. The peak current presents a maximum value for an anodic limit of +0.12 Vsce and a minimum for +0.24 Vsce. Nonequilibrium effects are observed in the optical response of PANI. The optical absorbance at 440 nm during potential pulses shows a sharp maxima at short time an decays later to a constant value. The two redox steps of PANI oxidation involve mixed anion/proton exchange. For high proton concenu'ation, protons are predominantly exchanged. At high pH (pH>2), an increase in the solution ionic strength brings about a bigger contribution of proton to the ion exchange process.
INTRODUCTION The redox reaction in polyaniline (PANI) thin films has been studied extensively during recent years [ 1]. An interesting behaviour of PANI redox response is the so called "first cycle effect" [2] or "memory effect" [3]. The shape of the first voltammogram recorded after waiting at the reduced state, is different of that observed during continuous cycling. The shape differences include different oxidation potential, peak current [2,3] and redox charge [2]. For sake of conciseness we will refer to this phenomena as statically induced memory effect (S1ME). The more favoured explanation for that behaviour is based on the dependence of conductivity on the polymer redox state. The conductivity decreases with reduction. By maintaining the film at negative potentials for some time a complete reduction is achieved. A higher overpotential is therefore necessary to oxidize the film. During continuous cycling the film is never totally reduced and the cyclic voltammogram shape is different. An alternative explanation is an alteration of the exchange of ions and/or solvent coupled to the redox process. Using in situ techniques it was shown [4,5] that polyaniline exchange protons and 0379-6779/93/$6.00
© 1993- Elsevier Sequoia. All rights reserved
1546
anions during its first oxidation process. The relative contribution of each specie to the global ion exchange is determined by the acid concentration in solution. In the present communication it is shown that memory effects in PANI are observed not only when the film is maintained at a negative potential but also when the potential cycling range is limited to a less anodic potential. Both procedures of achieving memory effects will be compared. The phenomena will be studied by cyclic voltammetry, Probe Beam Deflection (PBD) and UV-vis spectrolectrochemistry. A study of the effect of the solution ionic strength on the ion exchange of PANI will be also described.
EXPERIMENTAL Polyaniline modified electrodes were prepared by cycling the working electrode between -0.2 and 0.75 Vsce at a scan rate 20 mV/s in a 0.1 M aniline/1 M H2SO 4 solution. The electrode was cycled until a desired surface charge was reached (2 mC/cm 2 typical). The working electrodes are GC rods (A=0.071 cm2) inserted in a Kel-F matrix for cyclic voltammetric experiments. The electrochemical experiments were controlled by a PAR 173 potentiostat driven by a PAR 175 function generator. All potentials are measured against saturated calomel electrode (SCE). UV-visible spectroelectrochemistr,¢, The optical absorbance was recorded in an homemade spectrophotometer fitted with a transmission spectroelectrochemical cell. Indium doped SnO2 films on glass (Balzers) were used as transparent electrodes. Probe Beam Deflection (PBD). In PBD measurements GC plates (10 mm width) were used, the back and side edges covered with a protective laque. The PBD arrangement has been described before [6]. The exchange of ions between the film an solution creates a concentration gradient normal to the electrode surface. A light beam, aligned parallel to the electrode surface, suffers a deflection proportional to the refractive index gradient correlated with the concentration gradient. A positive deflection indicates ion insertion while negative deflection indicates ion expulsion. The PBD signal is recorded during the potential scan (cyclic deflectogram). Voltammetrv at ultramicroelectrodes: Pt wires (10 ktm diameter) sealed in glass were used as ultramicrolectrodes. The electrodes were polished with alumina (0.3 I.tm). A digital storage oscilloscope (Nicolet 2090) was used to record current-potential profiles during fast cyclic voltammetry experiments.
RESULTS AND DISCUSSION: Memory effeCtS: The cyclic voltammogram of PANI in I M HC1 is shown in Figure 1. When the electrode is cycled continuously, a stationary response is observed (curve a). However, if the positive extreme of the potential cycle is restricted to +0.12 V, the charge diminish with successive cycles (compare curve a (initial state) and curve b (final state)). If after subjecting the electrode to this treatment, the positive potential extreme is restored to +0.5 Vsce, the next oxidation cycle presents a higher maximum
1547
current and a more positive peak potential (curve c). The effect is similar to the reported before [2,3] by holding the potential at the negative extreme of the scan. However, it is observed that cycling a determined time (eg. 1 rain) produces a more pronounced change than waiting the same time at the negative extreme. This shows that the change is related with a transformation of PANI during cycling. By sake of conciseness we will refer subsequently to the effect achieved by cycling as dynamically induced memory effect (DIME). Using ultramicroelectrodes, fast scan rates can be used due to the diminished effect of uncompensated resistance. Tests at fast scan rates (up to 100 V/s) show the evidence od DIME. On the other extreme, scan rates as slow as 1 mV/s were used with similar results. It is observed that the value of the extreme anodic potential (Epp) used during DIME has an influence on the extent of change of the polymer. For example, using a Epp of +0.45 V, the peak current is smaller than the peak current during continuous cycling up to +0.5 V. It is noteworthy to mention that in SIME the current in the first cycle is always bigger. To study this influence quantitatively, parameters of the extent of change need to be defined. The relative increase in peak current (IR) is taken as the equal to the difference between the peak current in the first cycle after pretreatment and the current peak during continuous cycling. The value is normalized by dividing it by the peak current for continuous cycling. The electrode overpotential (~E) can be defined as the difference of the peak potential for the first cycle after pretreatment, with the peak potential during continuous cycling. The parameters were measured in the first cycle after a pretreatment consisting of: ten cycles between -0.2 and +0.5 Vsce followed by two minutes of cycling between -0.2 V and Epp. The results are shown in Figure 2 as a plot of the parameters (IR and 8E ) for different Epp. As can be seen 8E has a maximum at ca +0.2 V (Fig. 2B) while IR present a derivative like shape with a maximum at +0.12 V and a minimum at +0.24 V (Fig. 2A). The inflection point of the IR plot correspond to the maximum of BE. For a given scan rate, more time is spent at negative potentials for a less positive Epp. However, the extent of change (IR and ~E ) is smaller for that values of Epp. Therefore, the potentiodynamic effect is not produced to the time spent at potentiostatic relaxation in the reduced state. It was found that waiting at an intermediate potential (eg. +0.12 V), does not produce any effect in the successive cycle, showing that the effect is truly dynamic. It is difficult to rationalize the observed phenomena on the basis of a resistance change because the resistance is low and almost constant between +0.2 V and +0.5 V [7]. It is tempting to relate the dependence of the extent of change with electronic parameters as the spin concentration that has a maximum at ca. 0.2 Vsce [8], thus resembling the profile of 8E depicted in Figure 2. However, an adequate explanation of the phenomena should take in account the dynamic nature of it. A more likely explanation is related with ion and/or solvent exchange. By cycling in a limited range, only a portion of the film is transformed, some pores close and the electroactivity decreases. Therefore we investigate the ion exchange in SIME and DIME using PBD. It was found that the PBD response for the first oxidation cycle of PANI in 1 M HCI differs of that for continuous cycling. The proton expulsion signal on oxidation is bigger in the first cycle than that of the continuous cycle suggesting that a change in the protonation equilibria is involved in the memory effect. However, the PBD response at pH 3 shows an increased anion insertion signal in the first cycle with respect to
1548
(+)
t-
I I 2 0 p.A
O
O.30"
=_ 0
~ 0.15~ °Q 0 . 0 0 ~ ._> (a)- ~1----
(9
......
~ -0.15 50
4°_A
v~ 30
(B)
'3
20
• O
Figure 1: Cyclic voltammogram of PANI in 1 M HCI. Scan rate 100 mV/s. Curve (a) continuous cycling (b) final cycle with reduced limit pretreatment (0.12 Vsce); (c) first cycle after pretreatment.
! 0
.
I
,. -
-
-
0.10 0.25 0.40 0.55 Anodic Potential Limit (Vsce)
Figure 2: plot of current difference (A) and overpotential (B) as a function of the anodic limit of the pretreatment scan. Experimental parameters as in Fig. 1
successive cycles. Therefore it is the ion exchange what is altered irrespective of its nature. This fact suggests that the global electroactivity increases and with it the ion exchange. A similar behaviour is observed for DIME, An analogous behaviour is observed using spectroelectrochemistry. The derivative of the absorption with potential at 795 nm shows a bigger change for the first cycle than for successive cycles. The derivative is the analog to the current and is measured at the wavelength where the emeraldine state of PANI has its maximum absorption [9]. The behaviour indicates that the redox activity of the film is not totally restored during continuous cycling. A similar effect is observed for DIME. In both cases the changes in voltammetric charge are directly correlated with changes in the absorbance. It was shown using chonoamperometry in ultramicrolectrodes [2], that the current response of the first oxidation differs to that of successive oxidation pulses. A good description of the first pulse was found to be a nucleation and growth mechanism. The absorption behaviour can be explained in a similar way. Additional information can be obtained from the absorption at other wavelengths. It is known that the the first oxidation process PANI present and intermediate state [9] that has a maximum absorption at 440 nm [9]. Using the absorption at 440 nm as a measure of the intermediate concentration, the kinetic stability of the intermediate is assessed. Figure 3 shows the behaviour of the absorbance at 440 nm by stepping the potential to +0.5 V. The concentration of the intermediate state does not increase monotonically as expected for a redox coupled confined in a finite layer but shows a sharp
1549
AbsorbanceI0.020
e~
time I12 sI Figure 3: Absorbance response at 440 nm for a PANI film in 1 M HC1. Potential pulse from -0.2 Vsce to 0.5 Vsce (oxidation) and back (reduction).
maximum and a subsequent decay towards a constant value. This fact suggests that the oxidation of intermediate to fully oxidized state is a slow process. The initially formed excess concentration decays through a chemical reaction to the oxidized state. This behaviour contrast which that observed for the absorbance of the fully oxidized state ( at 795 nm) that increases monotonically to reach a constant value. Additionally, if the potential is stepped to an intermediate value (+0.2 V) the concentration of the intermediate state increases monotonically to a constant value. At this potential the intermediate state concen~ation is maximum and is more stable than the fully oxidized state. The behaviour agrees well with that predicted by the chemical redox model proposed by Albery et al [11]. Ionic strength effect on the ion ~xghang¢: The redox process in the first oxidation step of PANI is accompanied by a coupled ion exchange. The creation of positive charges is compensated by anion insertion and/or proton expulsion. The relative contribution of proton to the ion exchange depends on the degree of protonation of the reduced state of PANI. At higher acid concentration, the protonation degree is higher and more protons inside the film are available to be expulsed in order to achieve charge compensation. Measurement with in situ techniques [4,5] support this model. A somewhat overlooked aspect is the influence of the solution ionic strength on the protonation equilibria. As the protonated specie is part of a solid matrix, the proton concentration inside the polymer could differ from that of the solution, depending on the Donnan potential established across the polymer/electrolyte interface. The influence of the Donnan effect on the protonation of the emeraldine state was demonstrate recently by Chartier et al [10]. In a previous study using PBD [5] it was found that the proton contribution to the ion
1550
(+)
PBDI20 ~rad
)
E vs S C E
(-)
I
O.1V
I
Figure 4: PBD response during a potential scan of a PANI film in a 5 mM HC1 / 4 M NaC1 solution. Scan rate of 50 mV/s exchange is negligible for acid concentration below 0.1 M . Figure 4 shows the PBD response for a PANI film in a solution consiting in 0.005 M HC1 + 4 M Na CI. The dominant positive deflection (anion exchange) presents an small negative (proton expulsion) superimposed signal. This indicates that a proton contribution exists for proton concentration as low as 0.005 M if the ionic strength is kept high by addition of an inert salt. The high cation concentration in the bathing solution diminishes the Donnan potential across the polymer/solution interphase. Therefore the pH inside the polymer is closer to the value outside and the reduced state is protonated. The proton contribution is not observed if the salt concentration is lowered to 0.5 M. It is noteworthy to mention that at a proton concentration of 0.001 M the only species used for charge compensation is anion, irrespective of the salt concentration. This indicates that at and internal pH of about 3 the reduced state is unprotonated. A previous study using fast cyclic voltammetry at ultramicroelectrodes [12] shows that the anion concentration has an influence on the voltammetric shape for both voltammetric peaks. At low total anion concentration (6 mM) only one oxidation peak is present while at 1 M two peaks are observed in the first scan. In the present work we extend the study to a total anion concentration of 4 M. Two peaks are observed in the first oxidation scan at 100 V/s. In the successive scans only one broad wave is present. This result support the model presented before [12] where fast proton egress is the mechanism for charge compensation during the first oxidation scan. CONCLUSIONS - Memory effects in PANI can be promoted not only potentiostatically but also potentiodynamicaUy - Memory effects are also observed in the ion exchange of PANI. The increased charge is correlated with an increased quantity of ions exchanged.
1551
-
A faradaic reaction is present in the memory effect as the increased charge is correlated with an
increased change of the film optical absorbance. - The first oxidation process of PANI has a kinetic complication where the intermediate concentration reaches a maximum value to decay later. - The ion exchange of PANI is not only affected by the pH but also by the ionic strength of the bathing solution. REFERENCES
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