Electrochemistry of powder material studied by means of the cavity microelectrode (CME)

Article abstract of DOI:10.1016/S0013-4686(01)00549-7

The kinetic aspects of powder material electrochemistry can be studied using the cavity microelectrode (CME) as it allows carrying out voltammetry at scan rates between a few millivolts per second to several hundreds of volts per second. Thus, significant voltammogram characteristics-scan rate profiles can be drawn. Theoretical models suited to each material needs to be developed for their exploitation. First, we report significant results obtained with CME on powder materials. The materials studied were chosen for their wide variety of possible applications such as battery materials (polyaniline or Bi₂O₃, which modifies the electrochemical behavior of materials in which it is included), supercapacitor (carbon black), and for the electrocatalytic hydrogenation of organic compounds (PtO₂). Secondly, we briefly describe the general action for establishing models to obtain a better understanding of the electrochemical processes.

Full text of DOI:10.1016/S0013-4686(01)00549-7

Electrochimica Acta 47 (2001) 181–189
www.elsevier.com/locate/electacta
Electrochemistry of powder material studied by means of the
cavity microelectrode (CME)
a,
a
b
a
a
C. Cachet-Vivier *, V. Vivier , C.S. Cha , J.-Y. Nedelec , L.T. Yu
a
Laboratoire d’Electrochimie, Catalyse et Synth e` se Organique, UMR 7582 CNRS-Uni6ersit e´ Paris Val de Marne, 2 Rue H. Dunant,
4320 Thiais, France
9
b
Laboratory of Electrochemistry, Department of Chemistry, Wuhan Uni6ersity, 430072 Wuhan, China
Received 18 November 2000; received in revised form 10 April 2001
Abstract
The kinetic aspects of powder material electrochemistry can be studied using the cavity microelectrode (CME) as it allows
carrying out voltammetry at scan rates between a few millivolts per second to several hundreds of volts per second. Thus,
significant voltammogram characteristics-scan rate profiles can be drawn. Theoretical models suited to each material needs to be
developed for their exploitation. First, we report significant results obtained with CME on powder materials. The materials
studied were chosen for their wide variety of possible applications such as battery materials (polyaniline or Bi O , which modifies
2
3
the electrochemical behavior of materials in which it is included), supercapacitor (carbon black), and for the electrocatalytic
hydrogenation of organic compounds (PtO ). Secondly, we briefly describe the general action for establishing models to obtain a
2
better understanding of the electrochemical processes. © 2001 Elsevier Science Ltd. All rights reserved.
Keywords: Cavity microelectrode; Electrochemistry of powder materials; Bi O ; Polyaniline; Carbon black; PtO ; Multilayer slab system
2
3
2
1
. Introduction
the electrode and i the overall current including elec-
trolysis and capacitive currents). These two factors
strongly distort the electrochemical curves when usual
scan rates (a few tens of millivolts per second) or large
The electrochemistry of powder materials shows
backwardness when compared with that of the soluble
compounds, because the kinetic aspects are quasi-ab-
sent owing to the use of composite electrode also called
as the graphite powder electrode [1,2]. In such an
electrode, the powder is mixed with graphite or carbon
black as a pellet; one face is in contact with the
electrode and the other with the electrolytic solution
(
Fig. 1). Sometimes a polymeric binder is added for
improving the mechanical properties [2]. The thickness
is a few fractions of a millimeter and the diameter is
around 1 cm. The amount of material included is a few
milligrams. The real electrochemical interface between
the material and the electrolytic solution is large, ca.
around a few hundred square centimeters, giving rise to
a non-negligible capacitive current C×(dE/dt). The
ohmic drop R×i is important (R is the resistance of
*
Corresponding author. Fax: +33-149-781-323.
E-mail address: cachet@glvt-cnrs.fr (C. Cachet-Vivier).
Fig. 1. Scheme of the usual composite electrode.
0013-4686/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0013-4686(01)00549-7

182
C. Cachet-Vi6ier et al. / Electrochimica Acta 47 (2001) 181–189
P1000) and Al O paste (Prolabo). After polishing, the
2
3
electrode was washed with distilled water until it was
free of Al O . With the help of a microscope, a metal
2
3
disk free of residual impurities has to be obtained. It
was then verified that the connection between the plat-
inum and graphite does not induce an ohmic drop at
contact. Typically, the resistance of this homemade
electrode was less than 10 V. The cavity was obtained
by a controlled dissolution of the Pt-wire in a highly
concentrated hot solution of HCl+HNO for 5–30
3
min. The connection between the Pt-wire and the Cu-
wire collector was achieved by embedding the end of
the two wires with compacted graphite powder. The
diameter, ƒwire, and the deepness, h , of the microcavity
c
were measured with a microscope (Olympus BX30)
equipped with a digital camera unit (DP10) and a PC
for image processing. This arrangement allowed check-
ing the material inclusion too. The usual height–diame-
Fig. 2. Scheme of a CME produced at LECSO (a), and magnification
of the head of CME (b).
current densities (a few tens of milliamperes per milli-
gram) are used. Fast processes taking place within the
material cannot be investigated. Practically the scan
rate is always lower than a few hundreds or a few tens
of microvolts per second either in an aqueous or in
non-aqueous media, respectively. The charge–discharge
experiments results in long durations of several days to
several weeks for alkaline or lithium batteries. Studying
the kinetics of the electrochemical processes needs to
reduce the electrochemical interface area dramatically,
either by using the ultramicroelectrode for soluble com-
pounds [3–5] or by considering a very small amount of
the powder material.
ter (h /ƒwire) ratio was between 0.4 and 1. If it was
c
larger than 1, then establishing the contact between the
grains and platinum was difficult. With a ratio smaller
than 0.4, the grains could easily escape out of the cavity
during the experiment.
The cavity was filled up with material grains using
the electrode as a pestle. The amount of grains included
within the microcavity depended on the hardness of the
surface on which the powder was spread. With a soft
surface (polymeric material), the amount was smaller
than for hard material-made one (glass surface, agate
surface). The filling of the cavity was controlled with
the microscope, and at the same time, it was verified
that no grain remains on the head outside the cavity.
The first scan was started a few seconds after the
electrode was immersed in the electrolytic solution.
After the electrochemical experiment, the material was
dissolved by immersing the electrode head in an appro-
During the past decade, devices allowing such a
study were described by various authors [6–12]. Among
them, we have focused our interest on the cavity mi-
−
7
croelectrode (CME), which allows working on 10
–
−
8
10
g of the material [10–12]. That corresponds to an
electrochemical interface area of around a fraction of a
square millimeter instead of the usual few hundred
square centimeters.
In this paper, we emphasize the various possibilities
of the CME through some results obtained in our
laboratory.
priate solution for dosing it by ICP emission spectrome-
−8
try. In this way, material quantities down to 10
g
could be determined, except for carbon blacks and
organic compounds. Dissolution was favored by a con-
current use of an ultrasonic treatment. The total re-
moval of all the grains of material was finally checked
microscopically. After this step, the CME was ready to
be reused.
It is worthwhile to note that such an electrode is as
easy to handle as an ultramicroelectrode for dissolved
species.
2. Experimental
2.1. Ca6ity microelectrode
CME (Fig. 2a) was made of a glass tube (5–6 mm
2.2. Synthesis
diameter with a total length ranging from 80 to 150
mm) containing a Pt-wire (diameter ƒwire: 25–50 mm,
length 3 cm) or a Au-wire (ƒwire: 60 mm). The latter
ended at the near bottom of the glass tube thus provid-
ing a cylindrical cavity (Fig. 2b). Before making the
cavity, the CME head was made planar with a micro-
grained sandpaper (Norton, TUFBAK, ADALOX,
Bi O3 was obtained by the precipitation of 1 M
2
Bi(NO ) solution in 1 M KOH at room temperature
3 3
(r.t.). The deposit was filtered, washed with distilled
water, and dried at 80 °C. It was stored at r.t. in a
closed bottle. To prevent carbonatation of the product
after a long storage period, few days before use, it was

C. Cachet-Vi6ier et al. / Electrochimica Acta 47 (2001) 181–189
183
−
1
and 400 V s−1
.
Fig. 3. Voltammograms recorded with a CME for polyaniline powder (diameter: 25 mm) at 50 mV s
persulfate (0.5 M) in a sulfuric acid medium. During
the reaction, temperature was kept at 5 °C. A dark-
blue precipitate was isolated, washed with 1 M H SO
2
4
solution and finally dried under dynamic vacuum for
ca. 48 h. Polyaniline powder was stored in a closed
bottle in air atmosphere. By elemental analysis, the
product formed was identified as a sulfate emeraldine
salt.
3
. Possibilities of the CME
3.1. Carrying out large scan rate ranges
The small size of the electrochemical interface allows
recording voltammograms over a large scan rate range
between a few hundred microvolts to several tens or
even hundreds of volts per second. It is exemplified in
Figs. 3 and 4, which show cyclic voltammograms car-
Fig. 4. Voltammograms recorded with a CME for Bi O powder
2
3
−
1
−1
ried out with a CME on polyaniline and Bi O , respec-
(diameter: 50 mm) at 80 mV s
and 1 V s
.
2 3
tively. We note that the shapes of the curves are not
ideal; particularly they are unsymmetrical regarding the
abscissa axis. Thus, we can infer that the processes for
the cathodic scans are different from those for the
anodic scans.
dried at 600 °C for 12 h. From the X-ray diffraction
pattern, the compound was found to be well crystal-
lized in a monoclinic structure with the lattice parame-
ters a=5.84, b=8.17, c=7.51 A, and i=113°.
,
Polyaniline was chemically synthesized according to
the protocol described previously in Refs. [13–15]. An
aniline solution (0.5 M) was oxidized with ammonium
In acidic media, the electrochemical mechanism in-
voked for polyaniline consists in a proton insertion–
expulsion reaction [16–18]:

184
C. Cachet-Vi6ier et al. / Electrochimica Acta 47 (2001) 181–189
suming a density close to unity for polyaniline, it is
deduced that the material fills about half of the cavity.
This is consistent with a cubic storage for the CME
[19].
For Bi O , the electrochemical mechanism involved is
2
3
a global 3-electron exchange that can be summarized as
follows:
−
Bi O +3H O+6e ? 2Bi+6OH
2
3
2
In fact, it was shown that the actual mechanism
involved a soluble species and a combination of a single
electron exchange and a disproportionation reaction
[19], equivalent to a 3-electron transfer. The experiment
was carried out with a 50 mm diameter and 20 mm depth
−
5
CME. The exchanged charge is 10
C, corresponding
to the occupancy of about half of the whole cavity
volume. This exchanged charge also agrees with the
ICP dosing of the bismuth element after the
experiment.
As a result, we can infer that irrespective of the
material used, the electrochemical reaction involves all
the materials included in the cavity. Thus, the electrical
contact between material grains or material grains and
the platinum current collector is good enough.
Fig. 5. Voltammogram characteristics (peak potential and peak area)
vs. scan rate for polyaniline. For the highest scan rate values, the
strong variations of the characteristics must be attributed to local
polarization.
−
where X is the counter ion (SO 42 − in the case of
sulfuric acid electrolyte).
3.2. Drawing significant characteristics-scan rate profiles
The exchanged charge for the anodic scan is 4.3×
For each curve, a number of parameters characteriz-
ing the electrochemical behavior of the material can be
defined. These are, the peak potential (Epeak), the peak
intensity (ipeak), the peak area (Apeak) and the peak
width (Wpeak). As the variation range of the scan rate 6
is over 2–3 decades, we can draw significant scan rate
profiles (Epeak−6, ipeak−6, Apeak−6) that are related
to the underlying processes taking place during the
electrochemical reaction. For determining these profi-
les, a theoretical model adapted to each material has to
be developed for analyzing the experimental profiles.
The variations in some of the electrochemical
parameters of polyaniline and Bi O are depicted in
−
7
−9
1
0
C, corresponding to 10
g of polymer. The
geometric characteristic of the CME used for studying
polyaniline is 25 mm in diameter and 15 mm in depth.
−
9
3
Thus, the cavity volume is about 2.3×10
cm . As-
2
3
Figs. 5 and 6, respectively. We note that these two
materials show entirely different profiles, implying dif-
ferent electrochemical pathways for each compound.
The electrochemical behavior observed for polyaniline
with the CME agrees with that of a thin layer. For scan
−
1
rate lower than 10 V s , peak potential remains con-
stant (Fig. 5a) and peak intensities (not shown) are
proportional to the scan rate. Over this limit, the strong
variation in the peak potential and peak intensity when
the scan rate is increased must be attributed to the
electrode polarization owing to the displacement of the
ionic species during the redox mechanism. By compar-
ing with the usual composite electrode device, the upper
limit of the scan rate range is multiplied by a factor of
10 000.
Fig. 6. Voltammogram characteristics (peak potential and peak area)
vs. scan rate for Bi O .
2
3

C. Cachet-Vi6ier et al. / Electrochimica Acta 47 (2001) 181–189
185
For Bi O , the mechanism that takes place involves a
positive value can be ascribed to an increase in the
internal resistance owing to the deterioration of the
polymer at the most positive potentials.
2
3
global exchange of three electrons and dissolved species
−
such as BiO2 [19]. The variations in the peak potential
(
Fig. 6a) and peak intensity can be explained within the
frame of the usual electrochemical kinetics.
For both compounds, the variations of peak area
versus the scan rate (curve b in Figs. 5 and 6) indicate
that all the materials were implied in the reaction, even
at high scan rates.
3.4. Studying processes occurring at the surface of
the grains
As the diffusion pathways within the microcavity are
very short, we can consider that the surface processes
take place at the same potential for all the grains during
scanning. Thus, electrochemical characterization of
pure powder catalysts or supported on carbon black is
possible. As an example, Fig. 8 shows the voltam-
3.3. Doing large number of cycles during short lapse of
time
The scan rate usually considered for recording
mograms of pure hydrated PtO obtained with CME.
2
voltammograms with a CME is around a few tens of
millivolts per second. Thus, it is possible to carry out
several hundreds or even thousands of potential scan
cycles during a short lapse of time (24–48 h) for
studying the battery materials [12]. In this way, distin-
guishing the effects caused by cycling (i.e. the effect of
repeated electron exchange) from those caused by the
corrosion of the material becomes possible. The latter is
the dominating factor of the electrode rechargeability
loss in the tests carried out over several months. Fig. 7a
exemplifies the evolution of the voltammograms of
polyaniline during several ten thousand cycles in 2 M
H SO . It should be mentioned that the exchanged
We observe the two well-defined regions usually men-
tioned in the literature for the platinum solid electrode,
the H-chemisorption and the O-chemisorption [20,21].
Fig. 9 shows the electrocatalytic hydrogenation of eth-
ylene on PtO powder. We note that it is possible to
2
observe the electrocatalytic effect for scan rate up to 50
−
1
mV s . It appears that the CME is a good analytical
instrument to evaluate the electrocatalytic properties of
material powders.
3.5. Studying large specific area carbon blacks
Carbon blacks having a specific area larger than a
few hundred square meters per gram are used as auto-
blocking electrode materials in supercapacitors. The
charge storing is based on the large extend of the
double-layer. Their performances such as storage
charge, leakage current, linearity versus the applied
voltage, could be deduced from characteristics of the
voltammogram necessarily recorded at very low scan
2
4
charge remains constant until cycle number 10 000 be-
fore decreasing at 80% of the initial capacity after
45 000 cycles (Fig. 7b). The decrease in the peak inten-
sity indicates a diminution of the available power of the
material. An increase in the peak width, decrease in the
peak intensity and the peak potential shift toward
−
1
rates B0.1 mV s
(Fig. 10) with a classical composite
Fig. 7. Evolution of the voltammograms of: (a) polyaniline during
−
1
−1
cycling in 2 mol l
H SO at 100 mV s ; and (b) variation of the
2 4
normalized exchanged charge Q /Q
is the maximum exchanged charge during the cycling and Q is the
exchanged charge for the cycle number i.
vs. the number of cycles. Qmax
max
i
−
1
Fig. 8. Voltammograms recorded on PtO powder at 50 mV s
with
i
2
−
1
a 50 mm diameter CME in 0.5 mol l
H SO .
2 4

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C. Cachet-Vi6ier et al. / Electrochimica Acta 47 (2001) 181–189
Fig. 9. Catalytic electrohydrogenation of ethylene on PtO powder. The dotted line is related to the voltammogram recorded in the same condition
2
without ethylene.
electrode. The use of scan rates of a few millivolts per
second produces distorted curves [22]. When the peaks
a
capa
For an ideal material, the absolute value of i
equal to that of i
is
c
capa
and must stay constant. The
are removed, it remains as a basic curve, which can be
median curve is then merged with the abscissa axis. In
this case, the effective capacity C is related to u through
the following relationship:
m
capa
identified as the capacitive current i
(m=a for an
anodic scan and c for a cathodic scan). The latter one
m
is related to the double layer capacitance C through
a
c
C=u/(2×w)=(C +C )/2
(3)
the following relationship:
m
capa
m
m
Actually, as such an ideal voltammogram is never
observed, other characteristics must be taken into ac-
count. For instance, the slope tg(h) of the voltam-
mogram median curve (Fig. 10a) is related to the
leakage resistance and charge loss. An overall leakage
resistance Rleak can be defined through the relationship:
i
=C ×(dE/dt)=C ×6
(1)
with 6=dE/dt.
Therefore, the total voltammogram width u is related
to the capacitive currents as
a
capa
c
capa
u=i
−i
(2)
Rleak=1/tg h
(4)
The charge loss can be evaluated from the area
between the median curve and the abscissa axis. The
variation of u versus the potential E indicates the
voltage response of the material.
When such experiments are carried out with usual
composite electrodes, voltammograms must be
−
1
recorded at a very slow scan rate (0.1 mV s ), which
needs 32 000 s for a complete cycle. This is a drawback,
because a stable behavior during a large number of
discharge and charge cycles is required. Practically,
studying the behavior for a large number of cycles
needs several months.
Fig. 11 shows the voltammograms recorded with
CME, at various scan rates for a carbon black having a
2
−1
specific area around 850 m g . For instance, at 50
−
1
mV s , the voltammogram does not exhibit any sig-
nificant distortion and one complete cycle needs around
6
5 s, ca. a duration 500 times shorter than with a
Fig. 10. Voltammogram recorded from usual composite electrodes on
−
1
carbon black (Darcco) at 0.1 mV s
(the specific area of this carbon
classical composite electrode. It is then possible, to
carry out one thousand cycles per day and second, to
material is 350 m2
g
−1
).

C. Cachet-Vi6ier et al. / Electrochimica Acta 47 (2001) 181–189
187
−
1
Fig. 11. Voltammograms recorded at 20 mV s
with a gold CME (60 mm diameter) on carbon black.
study the effect of the scan rate 6 over a large 6-range.
Therefore significant characteristics-scan rate profiles
can be drawn.
The amount of material included in the cavity can be
evaluated by comparing with the experiment carried
out with a usual composite electrode, for which the
quantity of material is known.
associated. Published data do not mention a general
approach for treating the irregular shapes of the grain,
which is usually observed in the composite electrodes.
Recently, we have solved the problem of irregular
shapes of grains in a composite electrode [30]. We have
shown that a grain in a composite electrode is equiva-
lent to a system of elementary multilayer electrodes,
which are associated in parallel (Fig. 12). The number,
N, of elementary electrodes and their characteristics
3.6. Necessity of a theoretical model suited to the
(
thickness m and area A , j=1, 2, 3, N) are related to
material and a6ailable model presently
j
j
the dimension and geometry of grains. The elementary
current i of each elementary multilayer electrode can be
calculated using the multilayer model elaborated by
The CME allows to obtain large amounts of voltam-
mograms and significant characteristics-scan rate profi-
les, as they are built up over an extended scan rate
range. The various experimental profiles observed re-
veal the existence of multiple processes. As the available
theoretical models do not take into account all the
processes occurring at electrochemical interface and in
the electrolytic solution, they cannot quantitatively ex-
plain complex behaviors. Moreover, they are relating to
planar or film electrodes (thin layer, thin film elec-
trodes, adsorbed layer…). Regarding the processes,
they can be physical processes (diffusion of the ion
compensating charge, dissolution of the material or
deposit of dissolved species on the grain surface or near
it, lattice change), physicochemical processes (solvation
or desolvation of dissolved species at the electrochemi-
cal interface. Usually, in these models one physical
process is taken into account, the dissolution or the
diffusion within the material bulk [23–29] associated
with a simple electrochemical reaction. When an EC
mechanism (electrochemical reaction followed by a
chemical reaction) is treated, no physical process is
j
Fig. 12. Composite electrode equivalent scheme. The grain is decom-
posed into elementary multilayer-electrode system: The function F( j )
depends on the grain shape, N is related to the grain size.

188
C. Cachet-Vi6ier et al. / Electrochimica Acta 47 (2001) 181–189
Laviron and Saveant [31–35] and taking into account
be worked out, giving new insights to complex electro-
chemical processes, unable to be studied via experi-
ments that are more chemical. Appropriate modeling
suitable to each system studied has to be developed in
order to extract all the information contained in CME
voltammetry studies.
all the processes occurring within the microcavity. The
k
grain
current corresponding to a grain k(i
) is the sum of
all the elementary currents:
k
grain
k
i
=%ij ( j=0,1,…,N)
(5)
The current ielec relating to all the grains included in the
electrode is the sum of all the currents arising from the
p grains included in the electrode:
References
k
grain
ielec=%i
(k=1,2,3,…,p)
(6)
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values for ellipse-shaped grain [30].
[
[
[
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electrochemical kinetic parameters of polyaniline pow-
der. The electrokinetic constant and the transfer coeffi-
cient were found to be 8 s
Moreover, it was also shown that the redox mechanism
of Bi O involved a dissolution step before the electro-
chemical reaction [18,30]. This dissolution reaction is
responsible for the very small value of the peak width,
as observed in voltammograms at low scan rates (Fig.
3
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−
1
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2
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. Conclusions
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New perspectives are offered by the CME as an
9
, Marcel Dekker, New York, 1976.
efficient tool to investigate electrochemical kinetics of
processes occurring at powder material electrode. Han-
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commonly used in electrochemical experiments. They
are reusable and do not require specific electrochemical
equipment. The CME was demonstrated to be very
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[
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