Synthetic Metals 143 (2004) 119–128
A comparative study on polyaniline degradation by an electrochemical
quartz crystal impedance system: electrode and solution effects
Xiaohui Yang, Qingji Xie∗ , Shouzhuo Yao
Chemical Research Institute, Hunan Normal University, Changsha 410081, PR China
Received 28 April 2003; received in revised form 23 October 2003; accepted 24 October 2003
Abstract
By combining the piezoelectric quartz crystal impedance (PQCI) and electrochemical impedance spectroscopy (EIS) measurements, we
conducted a comparative study on polyaniline (PANI) degradation in different media, HClO4 and H2 SO4 , and on different piezoelectric
quartz crystal (PQC) electrodes, Pt and Au. It is concluded that (1) the PANI film grown on an Au electrode is more stable than that on a Pt
electrode; (2) the PANI degradation reaction abides by the zero-order kinetic law in HClO4 with rate constants from 0.17 to 2.25 Hz s−1 on
Pt and 0.16 to 0.83 Hz s−1 on Au, but the first-order kinetic law in H2 SO4 with rate constants from 2.61 × 10−3 to 7.00 × 10−3 s−1 on Pt
and 1.00 × 10−3 to 5.00 × 10−3 s−1 on Au at different ion concentrations. The contrary effects of ClO4 − and SO4 2− on PANI degradation
may be understood from the Hofmeister series of anions; (3) the dissolution of the PANI film bulk, the increase of film porosity and the
film attenuation occurred simultaneously during degradation via systematic analyses of the motional resistance (R1 )and electrochemical
impedance spectra responses, etc.
© 2003 Elsevier B.V. All rights reserved.
Keywords: Electrochemical quartz crystal microbalance; Electrochemical impedance spectroscopy; PANI degradation; Au and Pt electrodes; HClO4 ; H2 SO4
1. Introduction
Polyaniline (PANI) is an electrically conducting polymer
that can be easily prepared by the chemical or electrochemical oxidation of aniline. The stability of polyaniline
is of main interest for lots of its practical applications. So
many techniques have been used to investigate the stability and degradation of PANI, including electrochemistry
[1], FTIR [2,3], thermogravimetric analysis (TGA) [4,5],
electrochemical quartz crystal microbalance (EQCM) [6],
ellipsometry-CV [7], ESR [8], UV-Vis [8–10], X-ray photoelectron spectroscopy (XPS) [11,12], transmission electron microscopy (TEM) [7], scanning electron microscopy
(SEM) [5] and electrochemical impedance spectroscopy
(EIS) [13]. A PANI degradation model was proposed via
combined measurements of cyclic voltammetry and quartz
crystal microbalance (QCM), as shown in Scheme 1 [6].
The EQCM technique is a sensitive tool, which responds
to changes in mass loading on electrode surface down to
a nanogram level in an electrochemical process. Generally, the electroacoustic impedance information of the piezo∗
Corresponding author. Tel.: +86-731-8865515;
fax: +86-731-8865515.
E-mail address: xieqj@hunnu.edu.cn (Q. Xie).
0379-6779/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.synthmet.2003.10.027
electric quartz crystal (PQC) is analyzed on the basis of a
Butterworth–van Dyke (BVD) equivalent electrical circuit,
which consists of a motional arm and a static arm in parallel. The motional arm contains three equivalent circuit elements in series, i.e., the motional resistance (R1 ), the motional inductance (L1 ) and the motional capacitance (C1 ).
The static arm contains only the static capacitance (C0 ).
Each equivalent circuit parameter has its distinct physical
meaning [14–21]. The Sauerbrey equation has been used to
calculate the change in electrode mass resulted from deposition and dissolution of a rigid, thin and homogeneous film
[22].
2
f0 = −2.264 × 106 f0g
m
A
(1)
where f0 is the frequency shift in Hz, f0g the fundamental
frequency of the unperturbed crystal in MHz, m the mass
change in g and A is the piezoelectrically active area of
electrode in cm2 . In addition, the half-peak width of the
conductance peak (fG1/2 ) was also used to characterize the
electrode surface film and surrounding environment [23–25],
fG1/2 = fHG1/2 − fLG1/2 = fBmin − fBmax =
R1
f0
=
2πL1 Q
(2)
120
X. Yang et al. / Synthetic Metals 143 (2004) 119–128
N
N
N
N
H
H
H
H
the real and imaginary components of the electrochemical
impedance.
The combination of piezoelectric quartz crystal impedance
(PQCI) analysis and EIS methods, namely, the electrochemical quartz crystal impedance system (EQCIS), has
been proposed for protein adsorption and acidic denaturation studies [26]. In this paper, we investigate electrode
(Au and Pt) and medium (H2 SO4 and HClO4 ) effects on
polyaniline degradation by this combination method, and
multi-dimensional in situ information on polymer characteristics, including its degradation kinetics, are obtained and
discussed.
x+y
- 2xe-
+
+
N
N
H
H
y
- 2ye-
N
N
N
N
H
H
x
- 4 ( x + y ) H+
N
N
2. Experimental
2.1. Instrumentation and reagents
H2O
N
O O
O O
N
Scheme 1. Polyaniline degradation model proposed by Buttry and
co-worker [6].
where f0 is the resonant frequency at which the reactance (X)
and susceptance (B) of the motional arm vanish; fHG1/2 and
fLG1/2 are the higher and lower frequencies at the half-peak
height in the G spectrum; fBmin and fBmax are the frequencies at minimum and maximum B values; and Q is the quality factor and Q = (2πf0 L1 )/R1 = 1/(2πf0 R1 C1 ). While
the Sauerbrey equation describes a linear frequency-mass
relationship for a rigid, thin and homogeneous film [14], a
net liquid-loading effect for a PQC with one side contacting solution can be characterized by the following equation
[15,19–21]:
2πLq δ fG1/2L
≈ RL = 2πf LL = −4πLq f0L
≈ −4πLq f0L
fµq
c̄66 f0g
(3)
where δ fG1/2L , f0L , R1L and L1L are the changes in
fG1/2 , f0 , R1 and L1 due to variations of the solution density
and viscosity, respectively, f0g the resonant frequency in air,
µq (2.947 × 1010 N m−2 ) the shear modulus for the AT-cut
quartz, Lq the motional inductance for the PQC in air, and
c̄66 (2.957 × 1010 N m−2 ) the lossy piezoelectrically stiffened quartz elastic constant [15,18–21]. According to this
equation, the characteristic slope value of f0L /R1L for a
net density/viscosity effect on the 9 MHz PQC resonance is
approximately −10 Hz −1 . Obviously, the larger the absolute value of f0L /R1L , the weaker the viscous effect and
the stronger the mass effect.
Electrochemical impedance spectroscopy (EIS) is a useful technique to characterize thin films at electrode surface and to determine mass transport kinetics by analyzing
The experimental setup used in this work is described in
Fig. 1, which included an HP4395A impedance analyzer,
a CHI660A electrochemical workstation and two personal
computers (PC). Synchronous conductance (G) and susceptance (B) measurements were conducted on the HP4395A
controlled by a user-written Visual Basic (VB) 5.0 program.
BVD equivalent circuit parameters were obtained during experiments using the same VB program by fitting each group
of G and B to the BVD model based on a Gauss–Newton
non-linear least-squares fitting algorithm and a selection of
R1 , C0 , f0 , and 1/C1 as estimation parameters [26]. The simultaneous electrochemical experiments were conducted on
a CHI660A electrochemical workstation. The electrochemical impedance data were analyzed with the EQUIVCRT
software written by Boukamp [27].
AT-cut 9 MHz piezoelectric quartz crystals (12.5 cm in
diameter) of Pt or Au electrodes with keyhole configuration
were used. The Pt (or Au) electrodes were prepared by evaporating a thin Pt (or Au) film directly on the quartz crystal
surface with an Eiko IB-3 ion coater. Newly prepared quartz
crystal electrodes were used for every new group of experiments. All experiments were carried out with a conventional three-electrode electrolytic cell, and the PQC Pt or Au
electrode facing the solution was used as the working electrode. The reference electrode was a saturated KCl calomel
electrode (SCE), and all potentials are reported with respect
to it. A platinum plate served as the counter electrode. All
chemicals were of analytical grade or better quality. Doubly distilled water and freshly prepared solutions were used
throughout. All experiments were carried out at room temperature.
2.2. Procedures
To remove possible surface contamination, the electrode
surface was treated with one drop of concentrated HNO3 for
ca. 15 s, then washed with doubly distilled water and dried
via clean air blowing. The HNO3 treatment was repeated
three times. The HNO3 -treated electrode was then subject
X. Yang et al. / Synthetic Metals 143 (2004) 119–128
PC
HP4395A
impedance
Analyzer
121
OE 1
Ci
OE 2
3
VB5 program for
data acquisition
and analysis
CE
RE
1
2
CHI software
version 2.05
RE
PC
CE
WE
CHI 660A
electrochemical
workstation
Fig. 1. Schematic representation of the EQCIS used in our work. 1, PQC sealed on one terminal of a glass tube using silica rubber adhesive; 2, magnetic
stirring bar; 3, glass electrolytic cell; WE, the working electrode; RE, the reference electrode; CE, the counter electrode; OE1, oscillation electrode 1
(contacting solution); OE2, oscillation electrode 2 (in air); PC, personal computer (Ci = 810 pF).
to potential cycling between 0 and 1.5 V (30 mV s−1 ) in
1 mol l−1 HClO4 for at least 10 cycles to obtain reproducible
cyclic voltammograms finally. The electrode was rinsed with
doubly distilled water after completion of cycling.
Polyaniline films were grown on the PQC Pt or Au electrode by cycling the potential between −0.2 and 0.9 V at
20 mV s−1 in 0.25 M aniline + 1.0 M HClO4 aqueous solution. For comparison, generally we prepared PANI films of
a frequency shift of −6000 Hz (denoted as 6000 Hz PANI
film later) for degradation investigations.
To evaluate the porosity of the prepared PANI films, the
“wet” frequency shift (f0L = f0L2 − f0L1 ) and the “dry”
frequency shift (f0g = f0g2 − f0g1 ) after aniline polymerization were measured as follows. Before aniline polymerization, the stable frequency of the 9 MHz crystal in air was
recorded as f0g1 , and the stable frequency of the 9 MHz crystal immersed in the polymerization solution was recorded
as f0L1 . After the polymerization, the stable frequency was
recorded as f0L2 . The electrode was then rinsed with sufficient distilled water and dried via a stream of clean air. The
stable frequency in air was recorded as f0g2 .
3. Results and discussion
3.1. Preparation of polyaniline films
Three kinds of methods can be used to prepare polyaniline films, namely, potentiostatic, galvanostatic and
potential-sweep methods. In our experiment, we used the
potential-sweep method to grow PANI film for degradation
investigation below, i.e., electrodepositing the films by potential cycling from −0.2 to 0.9 V on a Pt or Au electrode at
a scan rate of 20 mV s−1 in 0.25 M aniline + 1.0 M HClO4
aqueous solution, except where specified. Fig. 2 shows
potential courses of simultaneous responses of f0 , R1 ,
δ fG1/2 and current during PANI preparation on Pt and Au
electrodes. The cyclic voltammograms (CV) for Pt and Au
electrodes both exhibited three pairs of redox current peaks
centered roughly at 0.15, 0.55, and 0.75 V, respectively, and
their peak heights increased with the increase of number of
potential cycles. The redox current peaks centered at 0.55 V
has been attributed to the benzoquinone–hydroquinone couple [6]. The net frequency decrease after each potential cycle suggested the continued growth of PANI film, and anion
insertion into the film in the first oxidation process (0.15 V)
but expulsion in the second one (0.75 V) were also implied
by the corresponding frequency decrease and increase, respectively. The net increase in R1 after each potential cycle,
which represents the more energy dissipation of the quartz
crystal resonance into the environment, should result mainly
from an increase in the electrode surface roughness due to
polyaniline deposition [25]. The responses of δ fG1/2 were
similar to those of R1 . According to Eq. (2), δ fG1/2 is
proportional to R1 , as L1 changed negligibly in our experiments, thus conclusions similar to those from R1 can
be drawn. Similar responses were observed on the Au electrode, except that the growth of a PANI film of identical
frequency change required more potential cycles than on
the Pt electrode. In addition, we also measured the “wet”
and “dry” frequency shifts, f0L and f0g , after the PANI
deposition. According to our previous work [25], the ratio,
Ree = (f0L − f0g )/f0g , should be an indicator of mass
percentage of entrapped electrolyte in the PANI film, as the
PANI films behave rigidly, therefore, Ree should give us information on the film porosity. We obtained Ree = 24 ± 3%
(n = 3) for PANI films on Pt and Ree = 15 ± 2% (n = 3) on
Au, respectively, implying that the polyaniline films grown
on Pt were more porous than those on Au. This conclusion
is also supported by smaller absolute values of f0L /R1 on
Pt than on Au, i.e., 48 ± 4 Hz −1 on Pt and 59 ± 3 Hz −1
on Au, as a smaller |f0L /R1 | value should indicate a
.0015
2500
2000
1500
1000
500
0
-500
.0009
.0006
.0003
0.0000
-.0003
-.0006
.0005
0.0000
i /A
.0010
-.0005
-.0010
-.15 0.00 .15
(A)
.30
.45
.60
.75
.90
E /V vs. SCE
-.15 0.00 .15
(B)
.30
.45
.60
.75
∆R
i /A
0 /Hz
∆f
δ∆
fG1/2 /Hz
3000
2500
2000
1500
1000
500
0
-500
120
100
80
60
40
20
0
-20
1 /Ω
140
120
100
80
60
40
20
0
-20
1000
0
-1000
-2000
-3000
-4000
-5000
-6000
-7000
∆R
∆f
fG1/2 /Hz
δ∆
1000
0
-1000
-2000
-3000
-4000
-5000
-6000
-7000
1 /Ω
X. Yang et al. / Synthetic Metals 143 (2004) 119–128
0 /Hz
122
.90
E /V vs. SCE
Fig. 2. EQCIS response for PANI film preparation in 0.25 M aniline + 1.0 M HClO4 on Pt (A) and Au (B) electrodes, respectively (dE/dt = 20 mV s−1 ).
larger viscous loading, here resulting mainly from a greater
porosity allowing the entrapment of more viscous electrolyte solution inside the PANI film. For comparison, we
also prepared 6000 Hz PANI films on Au via the potentiostatic and galvanostatic methods. A dark blue outside layer
that can be easily removed via water rinse was obtained,
due possibly to a low current efficiency at the final stage of
aniline polymerization. But this removable dark blue layer
of PANI was not observed when the potential-sweep method
was used for depositing a 6000 Hz PANI film. We obtained
Ree = (24 ± 6)% via the 30 ␮A cm−2 galvanostatic oxidation method, and Ree = 12 ± 5% via potentiostatic method
at 0.9 V, which indicate that the polyaniline film grown by
potential-sweep was more compact than that by galvanostatic method but less compact than that by potentiostatic
method in our experiments. The degradation rates followed
an identical sequence, as given later.
3.2. Effects of degradation potential
A potential step method was used to study the PANI film
degradation. The starting potential (Estart ) was −0.2 V, and
the degradation potential was ranged from 0.3 to 0.9 V. The
degradation rate increased with the increase of degradation
potential from 0.6 to 0.9 V in H2 SO4 , nevertheless no obvious frequency increases were observed when the applied
potential was negative to 0.6 V. The frequency decreased
slowly by only about 10 Hz after the sudden anion-insertion
induced frequency decrease when the potential step began,
and the motional resistance increased by about 5 . These
phenomena maybe result from ion and solvent insertion into
polymer networks when the degradation reaction was sufficiently slow.
Ex situ electrochemical impedance spectroscopy was also
used to investigate the PANI degradation reaction taking
place under different degradation potential in 0.5 M H2 SO4 .
The EIS data were analyzed by the EQUIVCRT analysis
software edited by Boukamp [27]. The impedance data for
the non-conducting form of PANI film in H2 SO4 were fitted to the equivalent circuit shown in Fig. 3, as suggested
by Birss and co-worker [13]. Here the constant phase element (CPE) is equivalent to a capacitor when its n value
approaches to unity. The elements of best fits are listed in
Table 1. The values of Y1 became bigger with the positive
going of degradation potential, because a more positive
degradation potential increased the degradation rate and thus
greater bare electrode area may be obtained after the 1000 s
PANI degradation. The decrease in the film resistance (R3 )
suggests the film’s becoming thinner during degradation.
So we may conclude that the degradation process occurred
by the polymer chain’s break-off toward the appearance of
porous structures, rather than by a layer-by-layer removal
mechanism. For checking this deduction, a potential-sweep
experiment in 2 mM K4 Fe(CN)6 (pH = 7.5) was done
after polyaniline degradation for different time length in
1 M H2 SO4 . As shown in Fig. 4, the cathodic current peak
appeared and became more obvious with the increase of
degradation time, and the degradation ratio (R), the ratio of
the frequency shift during degradation to that during PANI
film preparation, is equal to 75% for 1000 s degradation and
88% for 1500 s degradation. It is well known that the PANI
film is an insulator in a neutral solution at pH 7.5, so these
X. Yang et al. / Synthetic Metals 143 (2004) 119–128
123
100
-Zim /Ω
80
60
40
R2
R1
R3
20
CPE 1
CPE 2
0
30
40
50
60
70
80
90
100
Zre /Ω
Fig. 3. Typical Nyquist diagram for EIS at a PANI-modified Au electrode after 0.9 V degradation in 0.5 M H2 SO4 for 1000 s. EIS data were measured at
10 mV rms, 10 kHz ∼ 0.4 Hz, and a dc bias of −0.2 V vs. SCE. Filled circles, experimental; solid line, fitted according to the equivalent circuit (inset),
where R1 , resistance of electrolyte solution; R2 , resistance of bare electrode; CPE1 , reflects the charging of bare electrode double layer; R3 , resistance
of polyaniline film; CPE2 , reflects the contribution from the polyaniline pseudo-capacitive redox reaction [13].
phenomena demonstrate that the degradation deepened the
film porosity until the appearance of some bare Au surface.
Another phenomenon we observed is the big oxidation
current peak during the first anodic potential-sweep for a
fresh film. The reason is that the fresh PANI film prepared
by the potential-sweep method (stopped at −0.2 V) was at
its reduced form, and then hydrogen atoms in the reduced
PANI state can be easily removed via PANI oxidization
when the potential was moved positively. Since there was
no proton compensation in the neutral solution, so no redox
current peaks of PANI were observed during the following
potential-sweeps.
3.3. Acidity and anion-concentration effects on
potentiostatic degradation of PANI film in H2 SO4 and
HClO4 media
Studies on the stability of PANI films supported on Au
and Pt electrodes were performed by a potential step method
from −0.2 to 0.9 V versus SCE for sufficient time length,
allowing a significant PANI degradation. Fig. 5A shows
typical EQCIS responses to PANI degradation on Pt electrode in H2 SO4 media. The motional resistance and the
half-peak width both decreased, being a reversed change
during PANI deposition, representing the decrease in the
electrode-surface roughness during PANI degradation. The
frequency increased with time, suggesting the film mass loss
during PANI degradation. We evaluated the kinetic parameter of PANI degradation from the dynamic frequency responses under the experimental conditions. The kinetic data
obtained are linearized in the first-order reaction coordinates,
i.e., ln f versus time, if we used H2 SO4 as reaction media. From the linearized data, first-order rate constants can
be obtained according to the equation: k = −(ln f)/t
Table 2 lists the results obtained in H2 SO4 with different
acidity and ion strength. It is seen that (1) the degradation ratio and k value decreased as pH was raised, implying
an acid-catalyzed degradation mechanism [6]; (2) with the
Table 1
Fitted results of EIS elements at various 1000 s step potentials for Au-supported 6000 Hz PANI films in 0.5 M H2 SO4
E (V)
0.3
0.4
0.5
0.6
0.7
0.8
0.9
R1 ()
29.9
31.1
31.9
32.7
31.5
33.5
33.1
R2 ()
862
816
698
790
713
691
716
CPE1
R3 ()
Y1 (mS)
n1
3.34
3.54
3.39
3.40
3.43
3.46
3.62
0.999
1
1
1
1
1
1
85.5
82.0
79.4
76.9
77.9
76.7
71.0
CPE2
Y2 (mS)
n2
4.05
4.09
4.12
3.85
4.05
4.27
4.49
0.690
0.696
0.700
0.716
0.719
0.728
0.717
124
X. Yang et al. / Synthetic Metals 143 (2004) 119–128
(A)
(B)
(C)
(D)
50µA
-.2
0.0
.2
.4
.6
E /V vs. SCE
Fig. 4. Cyclic voltammogramms between −0.2 and 0.6 V vs. SCE for
bare Au electrode (curve A) and PANI-modified Au electrodes (curves
B–D) in 2 mM K4 Fe(CN)6 + 0.2 M NaClO4 aqueous solution (pH = 7.5).
Curve B: fresh PANI-modified Au electrode; curve C: PANI-modified
Au electrode after degradation in 1 M H2 SO4 for 1000 s; curve D:
PANI-modified Au electrode after degradation in 1 M H2 SO4 for 1500 s
(dE/dt = 10 mV s−1 ).
increase of SO4 2− concentration at a fixed solution H+ concentration, the degradation ratio and k value both decreased.
The PANI degradation kinetics on the Pt electrode, at 0.9 V
versus Ag/AgCl, and in 0.5 M sulfuric acid was reported pre-
viously by successively examining the first oxidation current
peak of PANI film after each degradation step of PANI, giving first-order degradation rate constants of 3.11 × 10−3 and
3.60 × 10−3 s−1 for two PANI films prepared in a solution
containing 0.05 M aniline and 0.5 M sulfuric acid via anodic
polarization at 0.8 V (versus Ag/AgCl reference electrode)
and potential-sweep method (between −0.1 and 0.8 V) for
20 min, respectively [1]. Their results are in accordance with
the present work in order of magnitude, as listed in Table 2.
The minor differences between ours and theirs may come
from the following ways. (1) The positive limit of PANI
deposition potential in the present study was at 0.9 V versus SCE, which is positive to theirs (0.8 V versus Ag/AgCl)
and may affect the structure of resultant PANI films; (2) the
degradation potential, both being at 0.9 V, but in this work
was referenced to the SCE and should be slightly more positive to theirs (by ca. 40 mV), and a relatively larger k value
is thus expected. However, the method used in this work
should merit in situ monitoring of mass loss of PANI films
during degradation and precise quantification of the mass of
an initial PANI film, and the measured kinetics parameters
may be less affected by non-Faradaic currents.
We also examined the PANI film degradation on Au electrode in H2 SO4 media, as shown in Fig. 5B and Table 2.
Very similar effects to those on Pt electrode were observed,
but the degradation kinetics was generally slower on Au
than those on Pt. These phenomena may result from the
more compact PANI film grown on Au, as mentioned above,
and the catalysis effect of the platinum electrode on water
decomposition to produce active oxygen may induce more
significant degradation at relatively high potential.
However, when similar experiments were carried out in
HClO4 , we found that the polyaniline degradation reaction
Table 2
Degradation ratio (R) and rate constants (k) for 6000 Hz PANI films in H2 SO4 a
[H+ ] (M)
[SO4 2− ] (M)
Pt
Au
R (%)
k
(s−1 )
R (%)
k (s−1 )
0.1
0.5
1
1.5
2
11.6
5.87
6.98
7.01
2.01
1.42
1.54
1.56
×
×
×
×
10−3
10−3
10−3
10−3
7.63
7.32
6.25
8.20
1.63
1.59
1.48
1.69
×
×
×
×
10−3
10−3
10−3
10−3
0.5
0.5
1
1.5
2
37.9
31.0
27.6
16.3
3.82
3.50
2.44
2.20
×
×
×
×
10−3
10−3
10−3
10−3
30.3
21.4
4.57
4.49
3.53
3.15
1.11
1.02
×
×
×
×
10−3
10−3
10−3
10−3
1
0.5
45.2
30.5b
78.1c
36.8
33.3
29.5
7.00
3.53
7.95
3.76
3.52
3.50
×
×
×
×
×
×
10−3
10−3b
10−3c
10−3
10−3
10−3
43.9
27.5b
53.6c
34.1
21.0
13.0
5.00
3.42
6.20
3.33
3.15
2.89
×
×
×
×
×
×
10−3
10−3b
10−3c
10−3
10−3
10−3
1
1.5
2
a
Na2 SO4 was used to adjust the concentration of SO4 2− , PANI films were prepared by cyclic voltammetry and the R values were calculated after
1000 s degradation of PANI films.
b PANI films were prepared by potentiostatic method.
c PANI films were prepared by galvanostatic method.
-10
100
0.0
-.1
2
-300
1
0
s
0.0
-200
∆ R /Ω
.2
-1
-2
s
∆ C /µ F
4
-1
0
200
400
600
800
1000
-2
1200
1
0
-1
-200
0
200
2500
2000
1500
1000
500
0
-500
-1000
400
600
800
1000
-2
1200
Time /s
(C)
0
-200
0
∆ f /Hz
200
0
-1
0
200
400
600
800
1000
-2
1200
Time /s
250
200
150
100
50
0
-50
2
1
0
-1
-200
(D)
0
200
400
600
800
1000
-2
1200
1
E /V vs. SCE
1
G1/2
1
δ∆ f
2
10
8
6
4
2
0
-2
∆ R /Ω
-600
/Hz
200
0
-200
-400
-600
-800
-1000
-400
∆ R /Ω
10
0
-10
-20
-30
-40
-50
E /V vs. SCE
0
∆ f /Hz
/Hz
G1/2
δ∆ f
2
Time /s
(A)
(B)
0
-2
E /V vs. SCE
0
-200
2
2
1
-200
-3
-.3
E /V vs. SCE
s
∆C / F
-.2
4
3
2
1
0
-1
-2
-3
s
-450
0
-100
∆ R /Ω
-300
-15
200
G1/2
-150
1
1
0
-5
δ∆ f
δ∆ f
G1/2
0
-600
10
5
/Hz
/Hz
0
-6
-12
-18
-24
-30
2000
1500
1000
500
0
-500
-1000
∆ R /Ω
0
∆ f /Hz
2400
1800
1200
600
0
-600
125
∆ R /Ω
0
∆ f /Hz
X. Yang et al. / Synthetic Metals 143 (2004) 119–128
Time /s
Fig. 5. Typical EQCIS responses during degradation at 0.9 V on Pt (A) and Au (B) electrode in 0.5 M H2 SO4 and on Pt (C) and Au (D) electrode in
1 M HClO4 .
accorded with a zero-order reaction law, i.e., the plot, f
versus time, was linear, as shown in Figs. 5C and D. The
zero-order reaction rate constants are listed in Table 3. Compared with PANI degradation in H2 SO4 media, the degradation became more obvious with the increase of ClO4 − con-
centration at a fixed solution pH, though the similar acidity effect showing an acid-catalyzed degradation mechanism
was also found. Again, the faster degradation kinetics was
also observed on a Pt electrode. The contrary anion effects
in H2 SO4 and HClO4 may be attributed to hydrophobicity
X. Yang et al. / Synthetic Metals 143 (2004) 119–128
Pt
Au
R (%)
k
(Hz s−1 )
R (%)
k
(Hz s−1 )
0.1
1.0
1.5
2.0
8.83
11.4
14.8
0.18
0.17
0.40
4.54
4.94
5.36
0.16
0.18
0.22
0.5
1.1
1.5
2.0
12.1
14.3
16.4
0.59
0.87
1.00
8.65
8.85
9.08
0.47
0.37
0.57
1.0
1.0
34.8b
26.9c
41.2
36.2
38.8
2.10b
1.53c
2.98
2.25
2.25
14.0b
9.63c
18.4
14.1
14.2
0.84b
0.62c
1.22
0.84
0.87
NaClO4 was used to adjust the concentration of ClO4 − , PANI films
were prepared by cyclic voltammetry and the R values were calculated
after 1000 s degradation of PANI films.
b PANI films were prepared by potentiostatic method.
c PANI films were prepared by galvanostatic method.
In order to investigate the characteristic of degraded PANI
films, we compared the EQCIS responses of fresh PANI
films of identical mass, grown on Pt electrode via potential
cycling procedure. The EQCIS response of a fresh 4000 Hz
PANI film is shown in Fig. 6A. Frequency changes suggest
0
-350
-700
-1050
3.0
1.5
a
0.0
-1.5
.0005
i /A
0.0000
-.0005
-.0010
of anions, following the Hofmeister series of anions [28],
-.2
−
−
−
ClO4 > SCN > I > NO3 > Br > Cl ∼ OAc ∼ SO4
2−
∼ HPO4
0.0
.2
.4
.6
.8
E /V vs. SCE
(A)
2−
That is to say, perchlorate ions are the most hydrophobic
while sulfate ions are the most hydrophilic. So perchlorate
anions are expected to form the most stable ion pairs in
the oxidized PANI film, and the PANI degradation may be
limited to some degree by the dissociation of the ion pair,
resulting in a slower PANI degradation kinetics in HClO4
than in H2 SO4 .
The simultaneous variation of impedance at 103.5 Hz with
time during PANI degradation on Pt electrode in different
supporting electrolyte is shown in Fig. 5. The Rs data
were obtained from EIS analyses based on a series Rs –Cs
equivalent circuit, which may be approximately valid for
an EIS measurement frequency being sufficiently high [29].
The decrease of Rs and the increase of Cs should reflect
the PANI film thickness’s dynamic decrease during degradation, which matched with the changes of R1 . Generally,
the changes of Rs and Cs became more obvious with the
increase of degradation ratio, no matter which kind of ion
concentrations were used. In addition, relatively more significant changes were observed in H2 SO4 than in HClO4 ,
though it presently appears to be difficult in extracting kinetics parameters from the Rs and Cs versus time curves.
We also compared the degradation of PANI films prepared
by potentiostatic and galvanostatic methods with those by
the potential-sweep method, on both Pt and Au electrodes.
The results summarized in Tables 2 and 3 show that the
most significant degradation was observed on the film grown
by galvanostatic method, and the secondary was grown by
potential-sweep method. These phenomena can be understood from the compact degree order of the PANI films pre-
200
100
0
-100
-200
∆ f0 /Hz
−
HCO−
3
.0004
6.0
4.5
3.0
1.5
0.0
6.0
4.5
3.0
1.5
0.0
0.0000
i /A
−
-.0004
-.0008
-.2
0.0
.2
.4
.6
.8
E /V vs. SCE
(B)
∆ f0 /Hz
−
300
150
0
-150
-300
.0004
0.0000
i /A
−
∆ R1 /Ω
1.5
2.0
3.4. Cyclic voltametric behavior of PANI film after partial
degradation compared with fresh one of identical mass
∆ R1 / Ω
[ClO4 − ]
(M)
[H+ ] (M)
pared by the three kinds of methods, i.e., potentiostatic >
potential-sweep >galvanostatic method, as given above.
∆ R1 / Ω
Table 3
Degradation ratio (R) and rate constants (k) for 6000 Hz PANI films in
HClO4 a
∆ f0 /Hz
126
-.0004
-.0008
-.2
(C)
0.0
.2
.4
.6
.8
E /V vs. SCE
Fig. 6. EQCIS responses to potential cycling in 1 M HClO4 on Pt electrode for a fresh 4000 Hz PANI film (A), a 4000 Hz PANI film after
a 6000 Hz PANI film degradation in 1 M HClO4 (B), and a 4000 Hz
PANI film after a 6000 Hz PANI film degradation in 0.5 M H2 SO4 (C)
(dE/dt = 20 mV s−1 ).
.0021
.0018
.0015
.0012
.0009
.63
.57
.57
n2
n2
.0065
.0052
.0039
.0026
.0013
0.0000
.60
.60
.54
.54
.51
.51
.48
0
(A)
30
27
24
21
18
15
12
9
Y2 /mS
R3 /Ω
33.0
31.5
30.0
28.5
27.0
25.5
24.0
127
Y2 /mS
R3 /Ω
X. Yang et al. / Synthetic Metals 143 (2004) 119–128
2
4
6
8
10 12 14
Times
0
(B)
2
4
6
8
10 12 14
Times
Fig. 7. Fitted results of EIS elements during PANI degradation in different media: (A) in 0.1 M HClO4 + 0.9 M NaClO4 (filled circles) or 0.1 M HClO4
plus 1.4 M NaClO4 (open circles); (B) in 0.25 M H2 SO4 + 0.25 M Na2 SO4 (filled circles) or 0.25 M H2 SO4 plus 0.75 M Na2 SO4 (open circles).
that anion insertion into the film in the first oxidation process
but expulsion in the second one, and the frequency value
at −0.2 V was reproduced very well during the second and
third potential cycles. The unusual EQCIS responses in the
first anodic sweep should come from the common “break-in”
behavior of electroactive polymers. Changes of R1 reflect
the variations of film topology, solvent content and so on.
Larger values of R1 of the polymer film in its oxidation state
imply greater non-rigidity compared with the reduced polymer film, which is proved by the frequency shifts suggesting
anion insertion. Fig. 6B and C shows EQCIS responses in
1 M HClO4 to the 4000 Hz PANI film obtained via degradation of a fresh 6000 Hz one in 1 M HClO4 and in 0.5 M
H2 SO4 , respectively. The current peaks representing the first
redox process became smaller and were moved to higher
potentials, in addition, the frequency drifted positively after each potential cycle, which proved that some regions of
the film became electrochemically inactive. We also calculated the anion doping ratio of three kinds of PANI films
under the first (0.15 V) and second oxidation (0.75 V) current peaks, 0.277 (A), 0.075 (B) and 0.110 (C), respectively.
Compared with a fresh PANI film of the same frequency,
the smaller doping ratio should suggest less electrochemical activity and the polymer chain break-off to some extent
after degradation.
3.5. Dynamic electrochemical impedance spectroscopy
study during degradation reaction in H2 SO4 and HClO4
media
To gain more dynamic information on PANI degradation,
successive EIS measurements, controlled by a macro program written by using the CHI660A software, were conducted to examine PANI degradation at 0.9 V. To reduce the
EIS measurement time, the frequency domain was selected
as from 10,000 to 100 Hz, and each EIS measurement experiment could be completed within 70 s via the Fourier trans-
formation mode of the CHI660a software. Since the degradation rate was not very speedy, especially at later degradation stages, we may believe that dynamic electrochemical
impedance spectroscopy study still gives one more detailed
information on the PANI degradation than the electrochemical impedance study at a single frequency selected. The EIS
data were satisfactorily fitted to the equivalent circuit shown
in Fig. 3 (inset), and with two cases as examples, the fitted
parameters obtained during degradation in H2 SO4 ([H+ ] =
0.25 M) and HClO4 media ([H+ ] = 0.1 M) with different
anion-concentrations are shown in Fig. 7. The continued decrease in R3 and increase in Y2 during degradation processes
proved the attenuation of PANI films, and the continued decrease in n2 suggests the films’ becoming porous, and the
reactions of polymer degradation are apt to be affected by
the electron transfer reaction kinetics, as n2 value is near to
0.5 with the rising of degradation time [13].
The anion-concentration effects on EIS equivalent circuit parameters (ECPs) are also shown in Fig. 7. With
the increase of ClO4 − concentration from 1 to 1.5 M
(Fig. 7A) and the decrease of SO4 2− concentration from
1 to 0.5 M (Fig. 7B), the changes of R3 , Y2 and n2 , especially of the latter two, increased, demonstrating that
a more significant degradation occurred in the cases of
perchlorate-concentration increase and sulfate-concentration
decrease, which qualitatively agree with the findings from
the PQC frequency responses. In addition, more significant changes of R3 , Y2 and n2 versus time were found in
H2 SO4 (Fig. 7B), demonstrating a facile kinetics for PANI
degradation therein than in HClO4 (Fig. 7A).
4. Conclusion
In summary, we have investigated in detail some effects
on the PANI degradation in aqueous solutions, i.e., PANI
preparation methods, degradation potential, media (HClO4
128
X. Yang et al. / Synthetic Metals 143 (2004) 119–128
and H2 SO4 ), anion-concentration (ClO4 − and SO4 2− ) and
electrodes (Au and Pt), via a new technique of combined
measurements of piezoelectric quartz crystal impedance
and electrochemical impedance. The main results are summarized as follows: (1) degradation of PANI films grown
on an Au electrode is slower than that on a Pt electrode; (2)
the PANI degradation is generally more difficult in HClO4
than in H2 SO4 , according with the Hofmeister series of
anions, while the degradation processes obey the zero- and
first-order kinetics for the two media, respectively; (3) a
polyaniline film grown by potential-sweep method was
more compact than that by galvanostatic method but less
than that by potentiostatic method in our experiments, and
the degradation rate follows the same sequence, i.e., galvanostatic method < potential-sweep < potentiostatic; (4)
a systematic analyses of the data obtained from the electrochemical in situ piezoelectric quartz crystal impedance
measurements revealed that the PANI degradation proceeded via the simultaneous film-bulk dissolution, increase
of film porosity and film attenuation, rather than via a
layer-by-layer removal mechanism. To our knowledge,
this is the first example for this combination method employed in the in situ kinetics study of conducting polymer
degradation.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (20275010) and the Foundation of the
Ministry of Education (MOE) of China (jiaorensi [2000] 26,
jiaojisi [2000] 65).
References
[1] R. MažŅeikienè, A. Malinauskas, Synth. Met. 123 (2001) 349.
[2] M. Nakayama, S. Saeki, K. Ogura, Anal. Sci. 15 (1999) 259.
[3] N.S. Sariciftci, H. Kuzmany, H. Neugebauer, A. Neckel, J. Chem.
Phys. 92 (1990) 4530.
[4] S.F. Patil, A.G. Bedekar, R.C. Patil, C. Agashe, Indian J. Chem.
Sect. A. 33 (1994) 580.
[5] T.L.A. Campos, D.F. Kersting, C.A. Ferreira, Surf. Coat. Technol.
122 (1999) 3.
[6] D. Orata, D.A. Buttry, J. Am. Chem. Soc. 109 (1987) 3574.
[7] H.N. Dinh, J. Ding, S.J. Xia, V.I. Birss, J. Electroanal. Chem. 459
(1998) 45.
[8] J. Lippe, R. Holze, Mol. Cryst. Liq. Cryst. 208 (1991) 99.
[9] S. Pruneanu, E. Veress, I. Marian, L. Oniciu, J. Mater. Sci. 34 (1999)
2733.
[10] A. Malinauskas, R. Holze, J. Electroanal. Chem. 73 (1999) 287.
[11] F.T. Liua, K.G. Neoha, E.T. Kanga, S. Lia, H.S. Hanb, K.L. Tanb,
Polymer 40 (1999) 5285.
[12] K.L. Tan, B.T.G. Tan, E.T. Kang, K.G. Neoh, J. Chem. Phys. 94
(1991) 5382.
[13] H.N. Dinh, V.I. Birss, J. Electrochem. Soc. 147 (2000) 3775.
[14] D.A. Buttry, in: A.J. Bard (Ed.), Electroanalytical Chemistry, vol.
17, Marcel Dekker, New York, 1991.
[15] H. Muramatsu, E. Tamiya, I. Karube, Anal. Chem. 60 (1988) 2142.
[16] S.J. Martin, V.E. Granstaff, G.C. Frye, Anal. Chem. 63 (1991) 2272.
[17] S. Yamaguchi, T. Shimomura, T. Tatsuma, N. Oyama, Anal. Chem.
65 (1993) 1925.
[18] M.A. Noël, P.A. Topart, Anal. Chem. 66 (1994) 484.
[19] Q. Xie, H. Liu, Y. Zhang, S. Yao, Chin. J. Chem. 17 (1999) 491.
[20] Q. Xie, J. Wang, A. Zhou, Y. Zhang, H. Liu, Z. Xu, Y. Yuan, M.
Deng, S. Yao, Anal. Chem. 71 (1999) 4649.
[21] Q. Xie, Y. Zhang, Y. Yuan, Y. Guo, X. Wang, S. Yao, J. Electroanal.
Chem. 484 (2000) 41.
[22] S. Sauerbrey, Z. Phys. 155 (1959) 206.
[23] Q. Xie, Y. Zhang, M. Xu, Z.Li.Y. Yuan, S. Yao, J. Electroanal.
Chem. 478 (1999) 1.
[24] Q. Xie, Y. Zhang, X. Xiao, Y. Guo, X. Wang, S. Yao, Anal. Sci. 17
(2001) 265.
[25] Q. Xie, Y. Zhang, C. Xiang, J. Tang, Y. Li, Q. Zhao, S. Yao, Anal.
Sci. 17 (2001) 613.
[26] Q. Xie, C. Xiang, Y. Zhang, Y.Yuan.M. Liu, L. Nie, S. Yao, Anal.
Chim. Acta 464 (2002) 65.
[27] B.A. Boukamp, Equivalent Circuit Users Manual & Software, Version
4.51, April 1993.
[28] R. Cordova, M.A. del Valle, A. Arratia, H. Gomez, R. Schrebler, J.
Electroanal. Chem. 377 (1994) 75.
[29] A.J. Bard, L.R. Faulkner, Electrochemical Methods: Fundamentals
and Applications, Wiley, New York, 1980.