Electro-oxidation of glycerol on platinum dispersed in polyaniline matrices

Article abstract of DOI:10.1016/S0013-4686(01)00877-5

Films of polyaniline (PAni) were electrosynthesized on gold and glassy carbon substrates. The morphology of the films was verified using scanning electron microscopy (SEM) and, as expected, the PAni film formed on glassy carbon presented fibrillar morphology, while that formed on gold presented fibrils on top of a more compact structure. Different amounts of platinum were electrodeposited into the polymer matrices at constant potential and the electrocatalytic activities of the electrodes were evaluated for glycerol electro-oxidation in acidic medium. Furthermore, the active areas of such modified electrodes were determined from the charges involved in the electro-oxidation of an adsorbed carbon monoxide monolayer. Considering the real active areas, the modified electrode with the gold substrate presents higher electrocatalytic activity for glycerol oxidation than that with the glassy carbon substrate. This difference is mainly related to their morphological characteristics and platinum particle sizes.

Full text of DOI:10.1016/S0013-4686(01)00877-5

Electrochimica Acta 47 (2002) 1495–1501
www.elsevier.com/locate/electacta
Electro-oxidation of glycerol on platinum dispersed in polyaniline
matrices
E.C. Venancio, W.T. Napporn, A.J. Motheo *,1
Instituto de Qu´ımica de Sa˜o Carlos, Uni6ersidade de Sa˜o Paulo, C.P. 780, 13560-970 Sa˜o Carlos-SP, Brazil
Received 17 October 2001
Abstract
Films of polyaniline (PAni) were electrosynthesized on gold and glassy carbon substrates. The morphology of the films was
verified using scanning electron microscopy (SEM) and, as expected, the PAni film formed on glassy carbon presented fibrillar
morphology, while that formed on gold presented fibrils on top of a more compact structure. Different amounts of platinum were
electrodeposited into the polymer matrices at constant potential and the electrocatalytic activities of the electrodes were evaluated
for glycerol electro-oxidation in acidic medium. Furthermore, the active areas of such modified electrodes were determined from
the charges involved in the electro-oxidation of an adsorbed carbon monoxide monolayer. Considering the real active areas, the
modified electrode with the gold substrate presents higher electrocatalytic activity for glycerol oxidation than that with the glassy
carbon substrate. This difference is mainly related to their morphological characteristics and platinum particle sizes. © 2002
Elsevier Science Ltd. All rights reserved.
Keywords: Glycerol; Polyaniline; Platinum microparticles; Electro-oxidation
1. Introduction
electrochemical method (potentiostatic or galvanos-
tatic) [2,3], potential range [8,9] and nature of the
supporting electrolyte [10–12], play key roles in the
morphology, conductivity and solubility of the PAni
films. Thus, depending on the application, appropriate
parameters must be chosen in order to obtain specific
properties.
Studies directed towards the development of fuel cells
and electrosynthesis methods have made it possible to
reach a high level of knowledge and in the applications
of electrocatalysis. Thus, various supported electrocata-
lysts have been developed increasing the specific area by
using a low catalyst loading and also to improve cata-
lytic efficiency. Electrocatalysts using conducting poly-
mers as support have been applied to oxidize several
organic molecules [13–26].
The use of metallic catalyst particles, dispersed in
PAni films, for the electro-oxidation of small organic
molecules has shown that poisoning by strongly ad-
sorbed species (e.g. CO) [13–17,21–28] is less severe
than in the case of the bulk catalyst. Those strongly
adsorbed species are formed at low potential during the
dissociative adsorption of organic molecules. In spite of
the interest in small organic molecules, bigger molecules
It is well known that the properties of conducting
polymers are extremely dependent on their synthesis
conditions such as temperature and monomer concen-
tration [1]. In the case of electrochemical syntheses of
these polymers, some additional factors must be taken
into account, namely the potential range and substrate
nature [1,2], which affect mainly their morphologies.
For polyaniline (PAni), it had been shown that its
properties and the results of its applications have a
strong dependence on the method of synthesis [2,3].
The interesting difference between the PAni films ob-
tained chemically and electrochemically is that, by us-
ing the latter method, the polymer is prepared directly
on a desired substrate and it is easily separated from
the solution containing the monomer. Another advan-
tage of the electrochemical method is the possibility to
verify its properties in situ [4–7]. During an electro-
chemical synthesis of PAni films, variables such as the
* Corresponding author. Tel.: +55-16-273-9932; fax: +55-16-273-
9952.
E-mail address: artur@iqsc.sc.usp.br (A.J. Motheo).
1 ISE member.
0013-4686/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0013-4686(01)00877-5

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E.C. Venancio et al. / Electrochimica Acta 47 (2002) 1495–1501
like glycerol are also important in both electrosyntheses
and fuel cells technology. Despite the interest in this
molecule, few studies concerning its electro-oxidation
have been realized, particularly in highly dispersed
electrocatalysts.
mogram of the substrates in the supporting electrolyte
solution was recorded to verify the system cleanliness.
The electrochemical experiments were carried out using
a 273A potentiostat controlled by M270 software, both
from EG&G/PARC.
A cyclic voltammetric study of the electrocatalytic
oxidation of glycerol on gold and platinum electrodes
in aqueous solution was performed by Kahyaoglu et al.
[29]. According to these authors, in acidic media only
platinum electrodes are electroactive, whereas in alka-
line media both gold and platinum electrodes present
electroactivity, particularly gold, which gives very high
current densities. On platinum, in acidic media, the
electro-oxidation products of glycerol, depending on
the potential range used, are mainly glyceraldehyde,
formic acid, oxalic acid and glycolic acid [30].
The aim of the present work is to investigate the
electro-oxidation of glycerol on platinum microparticles
dispersed in PAni matrices and to analyze the effect of
their morphologies on the electrocatalytic characteris-
tics of this reaction. The PAni films were obtained by
potentiostatic electrosynthesis on two different sub-
strates, gold and glassy carbon (GC), in order to have
polymeric films with distinct morphologies. The electro-
oxidation of glycerol on both electrodes was carried out
using cyclic voltammetry and the results are discussed
in terms of differences in their active surface area.
2.3. Polyaniline growth
The PAni films were grown on gold and glassy
carbon from 0.5 M H2SO4 solutions 0.1 M in freshly
distilled aniline. The electropolymerizations were per-
formed using cyclic voltammetry with a scan rate of 50
mV s−1, following the procedure described by Laborde
et al. [21]. For the polymer films synthesized on both
substrates, the values of thickness were estimated from
the heights of the first anodic peaks of the cyclic
voltammograms in 0.5 M H2SO4 as 0.5 mm [27].
2.4. Platinum electrodeposition
The electrodeposition of the platinum microparticles
into the PAni matrices was carried out under stirring at
constant potential (0.15 V vs. RHE) in a 0.5 M H2SO4–
2×10−4 M K2PtCl6 solution.
In order to minimize massive platinum electrodeposi-
tion and to obtain homogeneous dispersion of the
particles, a multiple-step process was adopted. Each
step of this process consisted of:
1. keeping the electrode (substrate covered with the
PAni film) in contact with K2PtCl6 solution between
1 and 2 min;
2. Experimental
2.1. Solutions and chemicals
2. performing the reduction at 0.15 V vs. RHE;
3. removing the electrode from this cell and washing it
with water;
4. introducing the electrode into another cell contain-
ing a 0.5 M H2SO4 solution and submitting it to
potential cycling until a constant cyclic voltammet-
ric behavior is observed.
The best results, in terms of the maximum current for
glycerol electro-oxidation, were observed for reduction
times around 1 min. For instance, to deposit 180 mg
cm−2 of platinum the entire process described above
was applied twelve times. The amount of platinum
electrodeposited was estimated from the sum of the
quantities of electricity involved in each reduction step
of the process. Taking into account only this parame-
ter, the reproducibility was estimated as 93%.
The solutions were prepared with ultrapure water
from a Millipore Milli-Q system. Sulfuric and perchlo-
ric acids (Merck p.a.), K2PtCl6 (Aldrich), glycerol
(Aldrich) and methanol (Merck) were used as received.
Aniline (Merck p.a.) was distilled under vacuum in
small portions and these were kept under nitrogen
atmosphere and low temperature before use. All elec-
trochemical experiments were carried out under nitro-
gen atmosphere at room temperature (2591 °C).
2.2. Electrochemical cells, electrodes and
instrumentation
Electrochemical glass cells exclusives for each kind of
electrochemical experiment (electro-polymerization,
electrochemical response, electrodeposition and organic
electro-oxidation) were used. For each cell, a platinum
foil was used as counter electrode (parallel to the
working electrode surface) and a reversible hydrogen
electrode (RHE) as reference. Two different working
electrodes were used: a gold foil (99.99%) and a glassy
carbon disk with 1.0 and 0.4 cm2 of geometric area,
respectively. Before each set of experiments, a voltam-
2.5. Electrochemical quartz crystal microbalance
(EQCM)
These experiments were performed in order to estab-
lish the oxidation state of PAni during the platinum
incorporation. An EQCM model 917 (SEIKO EG&E/
PARC) with a 9 MHz AT-cut quartz oscillator was
used [31].

E.C. Venancio et al. / Electrochimica Acta 47 (2002) 1495–1501
1497
2.6. Acti6e area determination
the literature [21] in order to have a comparative
standard.
Electro-oxidation of an adsorbed carbon monoxide
monolayer was used to determine the active area of the
modified electrodes [28]. The CO adsorption was car-
ried out at 0.05 V vs. RHE by passing CO gas through
a 0.1 M HClO4 aqueous solution for 5 min. With the
electrode polarized at 0.05 V vs. RHE, the excess of CO
in the solution was removed by bubbling argon for 30
min. The stripping of CO was performed at 5 mV s−1
in the potential range 0.05–1.0 V vs. RHE.
The EQCM results are exemplified in Fig. 1 for the
electrochemical response of PAni film in 0.5 M H2SO4.
They show that in the potential range of 0.05–0.25 V
the frequency doesn’t present a significant shift. This
means that, in this potential range, the PAni remains in
the leucoemeraldine form and so, the platinum incorpo-
ration was performed into this state, in agreement with
previous results [32].
Different amounts of platinum were deposited for
each, Au/PAni–Pt electrode (PAni on gold substrate)
and GC/PAni–Pt electrode (PAni on GC substrate).
The cyclic voltammograms did not show the typical
peaks corresponding to the adsorption–desorption of
hydrogen on platinum. This indicates that the platinum
microparticles present a good dispersion and there is no
formation of platinum clusters with significant size
leading to voltammograms with the adsorption charac-
teristics of the bulk metal. Another observation is, as
expected, that for both substrates, in the potential
range 0.8–1.0 V, the current peaks corresponding to
the redox processes characteristic of PAni decrease
when the platinum loading increases. In this particular
potential range, the process occurring in the PAni is
associated with the oxidative formation of pernigrani-
line [1]. However, the presence of platinum particles
modifies considerably this process, mainly when large
amounts of platinum (e.g. 225 mg cm−2) are present.
The effect is more pronounced for the Au/PAni–Pt
electrode than for the GC/PAni–Pt electrode.
Fig. 2 shows the micrographs of Au/PAni–Pt and
GC/PAni–Pt electrodes with 180 mg (Pt) cm−2 are
showed. It is possible to observe that after the platinum
electrodeposition, the PAni film electrosynthesized on
gold presents a globular morphology while that formed
on glassy carbon presents some fibrillar characteristics
but less clearly than before the platinum electrodeposi-
tion. The morphological changes increase with the plat-
inum loading perhaps due to the platinum
agglomeration on the fibrils.
2.7. Electrocatalytic acti6ity
These measurements were performed for both
modified PAni electrodes by cyclic voltammetry at 5
mV s−1 in 0.1 M HClO4–0.1 M glycerol solution and
for a standard solution 0.1 M HClO4–0.1 M methanol.
2.8. Micrographs
The polymer was electrochemically synthesized on a
thin gold layer sputtered on a glass plate (10×30×1
mm) and on a glassy carbon disc (0.3 cm2) in order to
use the scanning electronic microscopy (SEM) tech-
nique. A microscope model LEO 440 from ZEISS/LE-
ICA operating at 20 kV was used to perform these
experiments.
3. Results and discussion
3.1. Platinum deposition in PAni films
The potential of electrochemical deposition may offer
a control over the particle and distribution of the
platinum in the polymer matrix. In this work the elec-
trodeposition potential was maintained constant at 0.15
V vs. RHE following one of the procedures described in
Two important points have to be mentioned:
(a) the scale used in Fig. 2 (1 mm) is not suitable for
the observation of metallic microparticles with typ-
ical diameter in the scale of nanometers [28] but
even when the magnification is increased it is im-
possible to distinguish the platinum particles from
the polymer fibrils;
(b) by X-ray maps it was observed that, upon increas-
ing the platinum loading into the polymeric matrix
up to 180 mg cm−2 the number of particles in-
creases and their distribution in the PAni films is
more homogeneous for the Au/PAni–Pt than for
the GC/PAni–Pt electrode.
Fig. 1. Simultaneous electrochemical quartz crystal microbalance
results and electrochemical response of PAni film in 0.5 M H2SO4.
Scan rate: 50 mV s−1
.

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E.C. Venancio et al. / Electrochimica Acta 47 (2002) 1495–1501
3.2. Electro-oxidation of glycerol
The voltammograms of Au/PAni–Pt and GC/PAni–
Pt electrodes, with different amounts of platinum,
recorded at 5 mV s−1 in 0.1 M HClO4–0.1 M glycerol
aqueous solution, are shown in Fig. 3. The electrodes
were introduced polarized at 0.05 V in the solution
containing glycerol in order to have the PAni films in
the leucoemeraldine oxidation state. In Fig. 3 only the
geometric surface areas of the electrodes were used to
determine the current density values.
The current density for glycerol oxidation increases
with the platinum loading and the maximum is greater
than that observed for a smooth platinum electrode
[30]. This is expected by considering the large surface
area of the catalyst particles and the fact that platinum
microparticles are less sensitive to the poisoning effect
of strongly adsorbed species [21–29]. Comparing only
the modified electrodes used here, the current densities
are around 30% higher and the beginning of the glyc-
erol oxidation starts 0.125 V less positive for the Au/
PAni–Pt electrode than for GC/PAni–Pt. Additionally,
the glycerol oxidation currents in both directions of the
potential sweep are higher on Au/PAni–Pt than on
GC/PAni–Pt.
To summarize the observations for the glycerol oxi-
dation on the modified electrodes, taking into account
only the geometric surface area of the electrodes, is
interesting to point out that
(a) for small amounts of platinum no significant oxida-
tion of glycerol occurs;
(b) increasing the amount of platinum, the oxidation of
glycerol is observed, reaching a maximum current
density of 1.0 mA cm−2 at 0.75 V on Au/PAni–Pt
Fig. 2. SEM micrograph of PAni films containing electrodeposited
platinum particles (180 mg cm−2). PAni film electrosynthesized on:
(a) gold; and (b) glassy carbon substrates.
electrode containing 180 mg cm−2
;
(c) specifically for the GC/PAni–Pt electrode contain-
ing 180 mg cm−2, a broad current peak (ca. 0.65
mA cm−2) corresponding to glycerol oxidation is
observed between 0.7 and 0.9 V. This behavior can
be explained by the overlap of the glycerol oxida-
tion peak and the redox process of the polymer.
For low amounts of electrodeposited platinum the
difference between the active areas of both modified
electrodes should not be significant, thus the behavior
of the electrodes should be practically the same. To
understand better this behavior both types of modified
electrode containing 100 mg cm−2 were used for
methanol electro-oxidation. The cyclic voltammograms
of these electrodes in an aqueous solution 0.1 M
HClO4–0.1 M CH3OH showed the same activity for
both electrodes and the behavior is in agreement with
the results reported by Roland [23] and Napporn et al.
[24].
Fig. 3. Cyclic voltammograms of: (a) Au/PAni–Pt; and (b) GC/
PAni–Pt electrodes, containing different amounts of platinum, in
Subtracting from the j (E) curves in Fig. 3 the
background current of the electrodes without platinum
and using the geometric areas of both electrodes to
aqueous solution 0.1 M HClO4–0.1 M glycerol. Scan rate: 5 mV s−1
.
Platinum loading: (1) 50; (2) 100; and (3) 180 mg cm−2
.

E.C. Venancio et al. / Electrochimica Acta 47 (2002) 1495–1501
1499
3.3. Electro-oxidation of carbon monoxide
calculate the current density, the maximum value of
current density for the glycerol electro-oxidation was
determined for each platinum loading.
The differences observed between the Au/PAni–Pt
and GC/PAni–Pt electrodes can be attributed to the
differences in the active surface area of the dispersed
particles in the PAni. So, the electro-oxidation of an
adsorbed carbon monoxide monolayer [28,32] was per-
formed in order to evaluate the electrode active surface
area. These area values were determined from the
charges for CO electro-oxidation (QCO) on Au/PAni–Pt
Fig. 4 shows the evolution of the maximum current
density jmax for glycerol oxidation as a function of the
platinum loading. It can be observed that for both
electrodes (i.e. Au/PAni–Pt and GC/PAni–Pt), the cur-
rent density maximum increases with the amount of
platinum, but around 180 mg cm−2, significant differ-
ences appear. For the Au/PAni–Pt electrode the cur-
rent density jmax decreases after the maximum while for
the GC/PAni–Pt electrode a plateau is observed. Addi-
tionally, below 180 mg cm−2 of Pt, the value of jmax
increases faster with platinum loading for the Au/
PAni–Pt electrode than for the GC/PAni–Pt.
The efficiencies of the two types of electrode can be
evaluated from the initial slopes of the curves in Fig. 4.
The value obtained for the Au/PAni–Pt electrode is 3.8
A g−1 i.e. 50% of the value estimated for GC/PAni–Pt
electrode (7.6 A g−1). These values are at least one
order of magnitude lower than that estimated for
methanol oxidation [21].
Another interesting aspect is that, for glycerol oxida-
tion, the optimization of the electrode activity (maxi-
mum value) in the present case is observed for platinum
loading of 180 mg cm−2. This value is higher than that
usually reported (100 mg cm−2) for the same conditions
for the oxidation of small organic molecules such as
methanol, formaldehyde, formic acid and carbon
monoxide [21–25]. On the other hand, Kelaidopoulou
and GC/PAni–Pt electrodes containing 180 mg cm−2
.
It was also taking into account that the oxidation of a
full CO monolayer involves 420 mC cm−2 [28,34] and
that all the active sites occupied by CO molecules are
available, in the absence of CO, for the adsorption and
electro-oxidation of the glycerol molecules. This suppo-
sition is necessary in order to compare the results
obtained for glycerol. It is obvious that the number of
active sites for CO and glycerol molecules is different
but it is possible to have an idea about the ratio of the
real surface area of both modified electrodes.
From the cyclic voltammograms of Au/PAni–Pt and
GC/PAni–Pt with a platinum loading of 180 mg cm−2
in 0.1 M HClO4, in the absence of adsorbed CO and for
the electro-oxidation of a carbon monoxide monolayer
absorbed at 0.05 V, the values of QCO obtained were
14.46 and 8.27 mC for Au/PAni–Pt and GC/PAni–Pt,
respectively. These values (12 QCO) correspond to active
areas of 34.4 and 7.88 cm2, respectively. Considering
that the GC/PAni–Pt has a geometric surface area
equal to 0.4 cm2, its active area can be normalized
(divided by 0.4) in order to compare it with that
corresponding to the Au/PAni–Pt electrode. The nor-
malized active area for the GC/PAni–Pt is 19.7 cm2
and for the Au/PAni–Pt 34.4 cm2. The difference be-
tween the active surface area values of both electrodes
is ca. 43%.
On the basis of the active area values, the real current
densities for glycerol electro-oxidation on electrodes
with a platinum loading of 180 mg cm−2 were calcu-
lated and the resulting cyclic voltammograms for Au/
PAni–Pt and GC/PAni–Pt are shown in Fig. 5. Taking
into account the electrode active areas, the Au/PAni–Pt
electrode presents the higher electrocatalytic activity,
with current density values almost 30% higher than
those for GC/PAni–Pt. Additionally, glycerol oxida-
tion on Au/PAni–Pt starts at less positive potentials
than on GC/PAni–Pt. Also, there is a 100 mV potential
shift toward lower values between the Au/PAni–Pt
electrode and a smooth platinum electrode showing
that the modified electrodes are more active for glycerol
oxidation than the smooth platinum surface.
et al. [33] observed for the electro-oxidation of b-D-
(+)glucose using Pt/PAni–Pt electrodes that the maxi-
mum current densities, as well as the optimum platinum
loading (2.3 mg cm−2) are much higher than the values
observed in the present study. A possible explanation
for the higher values is the use of platinum as substrate
for the PAni electrosynthesis, which reinforces the re-
sults of this work showing the influence of the primary
substrate in the platinum microparticle distribution.
The platinum particle size (S) was estimated using
the procedure of Croissant et al. [28] and then, the
mean particle size (d) was determined assuming spheri-
cal particles of similar radius [35]. For the Au/PAni–Pt
Fig. 4. Maximum value of the electro-oxidation current of glycerol as
a function of platinum loading for (ꢀ) Au/PAni–Pt and (ꢁ) GC/
PAni–Pt electrodes.

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E.C. Venancio et al. / Electrochimica Acta 47 (2002) 1495–1501
film formed on gold presents fibrils on top of a more
compact PAni structure while, the film formed on
glassy carbon has an open fibrillar morphology that
resembles a sponge.
When platinum is electrodeposited in the polymer
matrices some morphological changes appear perhaps
due to the effect of platinum on the fibrils or by the film
aging due to the application of multiple potential cycles
during the electrodepostion. The current peaks corre-
sponding to the characteristic redox process of PAni
decrease when the platinum loading increases, mainly
for of large amounts of platinum. This behavior seems
to be more pronounced for the Au/PAni–Pt electrode
than for the GC/PAni–Pt electrode and it is related to
the general quality of the electrodeposit (size of the
particles and distribution in the polymeric matrix).
With increasing the platinum loading in the polymer
matrices, the current density for glycerol oxidation
increases. In addition, the maximum current density is
greater than that observed for a bulk platinum elec-
trode and, taking into account the geometric surface
area, the current densities observed for Au/PAni–Pt
electrode are around 30% bigger than for GC/PAni–Pt.
This difference between the two modified electrodes
indicates that the platinum particle dispersions are dif-
ferent; probably as a result of differences in the mor-
phologies of PAni. However, using a small molecule
such as methanol with up to 100 mg cm−2 of platinum
loading, no differences in oxidation rate were observed.
So, it is possible to conclude that the nature of the
molecule, and most probably the diffusion of the
molecules into the three-dimensional structures of the
polymeric matrices, play an important role in the cata-
lytic specificity of the platinum microparticles dis-
tributed in such polymer structures.
The efficiency, evaluated from the initial slope of the
curve of maximum density charge as a function of the
platinum loading, is 2 times bigger for the Au/PAni–Pt
electrode than for GC/PAni–Pt. The highest electrode
activity is observed around 180 mg cm−2 of platinum
loading, which is a bigger load than that reported in the
literature for the oxidation of small organic molecules
(100 mg cm−2).
The differences observed for glycerol electro-oxida-
tion on Au/PAni–Pt and GC/PAni–Pt seems to be
related mainly to the size and distribution of the plat-
inum particles inside the polymeric matrices, probably
due to the morphological characteristics of the PAni
matrices.
Fig. 5. Positive going potential sweep, corrected for the background
current due to the supporting electrolyte ( jsup) and considering the
real active area of the electrodes, corresponding to the glycerol
electro-oxidation on: (1) Au/PAni–Pt; (2) GC/PAni–Pt electrodes in
aqueous solution 0.1 M HClO4–0.1 M glycerol. Scan rate: 5 mV s−1
PAni films loaded with 180 mg cm−2 of platinum.
.
electrode the following values were found: S=19.1 m2
g−1 and d=14.7 nm. For GC/PAni–Pt: S=10.6 m2
g−1 and d=26.5 nm.
For the GC/PAni–Pt electrode the mean size of the
platinum particles is roughly twice as big than the value
reported in the literature for the same type of electrode
(:12.6 nm) [28]. This is an unexpected observation
because the electrodeposition procedure adopted in the
present work should lead to smaller particles. However,
for these calculations the thickness and the porosity of
the PAni films were not taken into account.
Since the real surface area determined from CO
adsorption is bigger for Au/PAni–Pt than for GC/
PAni–Pt, it is expected that the former electrode pre-
sents higher activity for glycerol electro-oxidation. The
curves in Fig. 5 show exactly this expected behavior.
However, another factor is probably contributing. Con-
sidering that the glycerol molecule is large and the
particles are distributed in a three-dimensional struc-
ture, the diffusion of the molecule inside the polymeric
matrix must be taken into account. This means that, in
part, the differences in activity of both electrodes may
be attributed to the difficult access of the glycerol
molecules to a higher number of active sites, which are
probably deeper in the polymer matrix of the GC/
PAni–Pt electrode. This explanation may be considered
controversial and it needs confirmation by a study of
the diffusion of the glycerol molecule in the polymeric
matrix.
4. Conclusions
Two important characteristics of the PAni films were
found in the present study. The first is that the growth
rate of the PAni film is independent of the substrate.
The second is that the PAni films grown on gold and
glassy carbon have fibrillar morphologies; however, the
Acknowledgements
The authors thank S.A.S. Machado, M.C. Santos
and D. Miwa for the help with the EQCM experiments
and discussions. The financial support from the Con-

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