Electrochemical study of Bi2O3 and Bi2O2CO3 by means of a cavity microelectrode. I. Observed phenomena and direct analysis of results

Article abstract of DOI:10.1149/1.1394049

The physical and chemical processes occurring for the electrochemical reduction and oxidation of Bi₂O₃ and Bi₂O₂CO₃ can be determined through a direct analysis of the voltammograms recorded from the cavity microelectrode. The voltammogram shows an evolution for around 1 h, which corresponds to the first 80 cycles carried out at 50 mV s^-1. The evolution is due to the sequence of the following concurrent processes: (i) impregnation of the powder and its dissolution in the electrolytical solution, (ii) deposit of porous metal-Bi during the cathodic scan, and deposit of Bi(III) species during the anodic scan. These processes cause a reconfiguration of the architecture of the powder within the cavity by making the material grains smaller. For the second stage, the voltammograms do not vary any more during the cycling. For the first cathodic scan, the reduction process starts from the bottom of the cavity at the interface Pt/solution only, then it progresses toward the mouth. Contrarily, the oxidation takes place throughout all the grains. During the next cathodic scans, the reduction also starts throughout all the grains when the grains are not entirely oxidized from the preceding anodic scan.

Full text of DOI:10.1149/1.1394049

4252
Journal of The Electrochemical Society, 147 (11) 4252-4262 (2000)
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Electrochemical Study of Bi2O3 and Bi2O2CO3 by
Means of a Cavity Microelectrode
I. Observed Phenomena and Direct Analysis of Results
V. Vivier,a C. Cachet-Vivier,a S. Mezaille,a B. L. Wu,c C. S. Cha,c J-Y. Nedelec,a M. Fedoroff,b
D. Michel,b and L. T. Yua,*,z
aLaboratoire d’Electrochimie, Catalyse, et Synthèse Organique, UMR 7582 CNRS-UPVM, 94320 Thiais, France
bCentre d’Etude de Chimie Métallurgique, UPR 2801, 94400 Vitry-sur-Seine, France
cLaboratory of Electrochemistry, Wuhan University, 43 072 Wuhan, China
The physical and chemical processes occurring for the electrochemical reduction and oxidation of Bi2O3 and Bi2O2CO3 can be
determined through a direct analysis of the voltammograms recorded from the cavity microelectrode. The voltammogram shows
an evolution for around 1 h, which corresponds to the first 80 cycles carried out at 50 mV sϪ1. The evolution is due to the sequence
of the following concurrent processes: (i) impregnation of the powder and its dissolution in the electrolytical solution, (ii) deposit
of porous metal-Bi during the cathodic scan, and deposit of Bi(III) species during the anodic scan. These processes cause a recon-
figuration of the architecture of the powder within the cavity by making the material grains smaller. For the second stage, the
voltammograms do not vary any more during the cycling. For the first cathodic scan, the reduction process starts from the bottom
of the cavity at the interface Pt/solution only, then it progresses toward the mouth. Contrarily, the oxidation takes place throughout
all the grains. During the next cathodic scans, the reduction also starts throughout all the grains when the grains are not entirely
oxidized from the preceding anodic scan.
© 2000 The Electrochemical Society. S0013-4651(00)01-077-6. All rights reserved.
Manuscript submitted January 20, 2000; revised manuscript received July 22, 2000.
Bi element is included within various complex oxides, in high
lector is achieved by embedding the end of the two wires in graphite
transition-temperature superconductors BiSrCaCuO, or in man-
ganese dioxide as in rechargeable battery material, for example.1-12
It more or less modifies the properties of the material. For instance,
the various species of Mn(IV) oxides are rechargeable when the
deep discharge is lower than 0.5 faradays (Ᏺ) per Mn atom,13-18 but
when Bi atoms are included, they become electrochemically re-
versible for deep discharge higher than 1.5 Ᏺ/Mn.4-12 It is interest-
ing to know the interaction between Bi atoms and the metal atom of
the matrix host through the eventual modifications of the redox
potential. In order to have references for comparison, Bi2O3 and its
derivatives have been studied using the usual composite electrodes,
also called graphite or carbon paste electrodes.19,20 These electrode
devices do not allow scanning at a rate higher than a few millivolts
per second. Therefore, it is not possible to determine the kinetic fea-
tures of the electrochemical processes. The cavity microelectrode
(CME) has been recently invented for studying a very small amount
of powder material (10Ϫ7 to 10Ϫ8 g).21,22 It allows for reaching scan
rates up to a few tens of volts per seconds.23,24 This paper is devot-
ed to both the description and direct analysis of results obtained for
Bi2O3 and Bi2O2CO3 by means of the CME.
powder. The diameter ␾ and the deepness hc of the microcavity is
measured by a microscope (Olympus BX30) equipped with a numer-
pt
Experimental
Material and physicochemical characterization.—Bi2O3 is ob-
tained by precipitation of 1 M Bi(NO3)3 solution in 1 M KOH at
room temperature. The solid is filtered, washed with distilled water,
and dried at 80ЊC. It is stored at room temperature in a closed bottle.
After a storage of several years, the product is fully carbonated giv-
ing Bi2O2CO3 as determined by elemental analysis and by X-ray dif-
fraction (XRD). A specific area is measured by means of a device
from Coulter, type SA3100.
CME and optical control devices.—The CME (Fig. 1a) is made
of a glass tube (diam ␾glass: 5 to 6 mm, total length, ᐉtot: 80 to
150 mm) containing a Pt wire (diam ␾pt: 25 to 50 ␮m, length 3 cm)
which ends near the bottom of the glass tube thus providing a cylin-
drical cavity (Fig. 1b). This cavity is obtained by controlled dissolu-
tion of the platinum wire in a highly concentrated HCl ϩ HNO3 solu-
tion. The connection between the Pt wire and the copper wire col-
* Electrochemical Society Active Member.
E-mail: yu@glvt-cnrs.fr
Figure 1. (a) General scheme of a cavity microelectrode; (b) magnification
of the head.
z
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Journal of The Electrochemical Society, 147 (11) 4252-4262 (2000)
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ical camera unit (DP10) and a personal computer for image process-
␾g ϭ 6/(SBET ϫ ␳)
ing. This also allows checking the material inclusion. The usual ratio
height/diameter (hc/␾pt) is between 0.4 and 1. If it is larger than 1, it
is difficult to put grains and the platinum wire in contact with each
other. With a ratio smaller than 0.4, the grains can easily escape out
of the cavity during the experiment. The head of the CME is made
planar through a micrograined sandpaper (Norton, Tufbak, Adalox,
P1000) and Al2O3 paste (Prolabo). After polishing, Al2O3 is washed
with distilled water until the cavity is free of Al2O3. That is con-
trolled by observing it with the microscope: a metal disk free of
material grain must be observed. After the head is made plane, it is
immersed in HCl ϩ HNO3 solution for making the cavity.
with ␳ ϭ 8.5 and 6.9 for Bi O and Bi O CO , we found ␾ to be
g
2
3
3
2
3.5 and 0.1 ␮m, respectively. These va2lues are consistent with the
microscope observation.
Dosing results show that the amount of material included in the
microcavity is ϳ10Ϫ8 g, which is compatible with the evaluation
done from geometric considerations (see Appendix A).
Microscope control.—The microphotographs of Fig. 2 show the
cavity before the inclusion (a), before the electrochemical study (b),
and after the electrochemical study (c) of Bi2O3CO2. We can see that
there are no more material particles on the tip of the CME, which
ensures the accurate dosing of Bi included in the microcavity.
Electrochemical device.—Electrochemical experiments were
performed in a classic three-electrode cell using a potentiostat PT 30
(Autolab). The reference electrode, the counter electrode, and the
working electrode are Hg/HgO, Pt wire, and CME, respectively.
Voltammograms and graphic characteristics.—The results show
that Bi2O3 and Bi2O3CO2 have similar electrochemical behavior.
Thus, all the results described in the next sections refer to Bi2O3. The
graph of Fig. 3a shows voltammogram evolution for the first three
cycles recorded a few seconds after the immersion of the CME.
Graph b shows cycles 3 to 8, and graph c the cycles 8, 21, to 70. The
stabilization of the voltammograms is achieved after around 80
cycles, which corresponds to a duration of around 3500 s. One ob-
serves the cathodic peak C1 on the first cycle. Its amplitude is stabi-
lized from the third cycle while the peak C2 progressively grows.
During the next cycles, only peak C2 increases. During the anodic
scans, two peaks A1 and A2 are observed, the latter being followed
by a quasi-horizontal plateau. The waves of the plateau during the
first ten cycles are of the nanoampere range. The amplitude is con-
stant, and the period is around 2 s. During the next cycles, they are
no longer observed. After the peak falls, the back anodic current is
below the cathodic current, which indicates a metal deposit.
In the Fig. 3d graph, we define various characteristics in a vol-
tammogram, i.e., peak potential EY (Y ϭ A1, A2, C1, and C2) peak
intensity iY for all the peaks, and peak area ᏭY except Y ϭ A2,
because it is difficult to separate the peak A2 from the plateau. How-
ever, we can define Ꮽtot, as
Implementation of CME.—The cavity is filled up with grains of
material using the electrode as a pestle. The amount of grains includ-
ed within the microcavity depends on the hardness of the surface on
which the powder is spread. With a soft surface (polymeric materi-
al), the amount is smaller than for hard material (made of glass sur-
face or agate surface). The filling of the cavity is controlled with a
microscope, and at the same time one insures that no Bi2O3 grains
remain on the head outside the cavity. The first scan is started a few
seconds after the electrode is immersed in the electrolytic solution.
After the electrochemical study, Bi2O3 is dissolved by immersing the
electrode head in a 0.5 M KI ϩ 1 M HCl solution so as to dose it by
the inductively coupled plasma spectrometry method.25 Concurrent
use of an ultrasonic treatment favors dissolution. Then, one verifies
by the microscope that all the grains of material are dissolved. After
the total removal, the CME can be reused. Finally, such an electrode
is as easy to handle as an ultramicroelectrode for dissolved species.
Results
General characterization.—Elemental analysis indicates that
the formed product is Bi2O3. The XRD is typical of a monoclinic
structure. The space group is P21/c with lattice parameters a ϭ
5.84 Å, b ϭ 8.17 Å, c ϭ 7.51 Å, and ␤ ϭ 113Њ as previously men-
tioned.26 After a storage of one year and more, Bi2O3 is completely
transformed into Bi2O2CO3. XRD shows that the structure is tetrag-
onal with a I4/mmm space group and the lattice parameters a ϭ b ϭ
3.87 Å and c ϭ 13.69 Å.27 However, heating at 600ЊC for 10 h elim-
inates CO2 from Bi2O2CO3, and the monoclinic phase Bi2O3 is
obtained again.
Ꮽ
tot ϭ ᏭA2 ϩ Ꮽplat
[2]
where Ꮽplat stands for the area of the surface that spreads between
the plateau and the abscissa axis. The exchanged charge QY is relat-
ed to the area ᏭY through the relationship
QY ϭ ᏭY/v
v ϭ scan rate
[3]
One also must consider as voltammogram characteristics the height
of the plateau (idiff, indicated by a small vertical bolt segment) at 0
V/Hg/ HgO and the width of peaks C2 and A1 (indicated by hori-
zontal segments ᐃC2, ᐃA1, respectively) at iY/2 (for Y ϭ C2, A1, and
A2). Moreover, the intensity of the peaks A1 and A2 may be consid-
ered as negligible at Ϫ0.45 V/Hg/HgO. Therefore, the residual
The specific area of Bi2O3 and Bi2O2CO3, evaluated through
Brunauer-Emmett-Teller (BET) measurements, are around 0.2 and
8 m2/g, respectively. Assuming that the grains are spherical with a
density ␳, their average diameter, ␾g, is
Figure 2. Aspect of the cavity before (a) and after the inclusion of the material (b); aspect after the electrochemical study (c). Cavity diameter ␾pt ϭ 50 ␮m.
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intensity i (indicated by a small vertical bold segment) is also a
voltammogram feature, because it is the sum of the cathodic and
anodic capacitive currents
Figure 5 shows the evolution of the voltammograms when the
scan range extends toward the positive potential. When a scan range
is modified, a stable voltammogram is observed from cycles 4 to 6.
Thus, the curves of Fig. 5 are relating to cycle 10. The Fig. 5a graph
corresponds to the maximum extension of the anodic scan range. In
graph b, the dotted line curve is recorded after the curves of graphs c
and d. We note that peak C2 becomes narrower and shifts toward the
positive potentials when the extension of the anodic scan range be-
comes smaller, while peaks A1, A2, C1, and the height of the plateau
do not significantly change. In graph d, the peak C2 resulting from the
scan between Ϫ0.8 and Ϫ0.65 V/Hg/HgO can be considered as the
cathodic homologue of the peak A1. These two peaks have the same
area, but the other features are different: ᐃC2 is 17 mV whereas ᐃA1
is 35 mV and iC2 is almost twice iA1. This also confirms that reaction
pathways in oxidation and in reduction are different. Figure 6 shows
the evolution of the voltammograms vs. the negative potential exten-
sion of the scan range. Peak A1 and the height of the plateau do not
significantly vary as the scans are reversed at potential lower than
Ϫ0.73 V/Hg/HgO. When the potential is higher than
Ϫ0.75 V/Hg/HgO, the current during the back anodic scan is more
negative than that of the cathodic one. This indicates a metal deposit.
␹
i ϭ i
ϩ i
␹
cat
[4]
␹
␹
anod
We note on these graphs that iC1 is stabilized (after the cycle 3-4)
whereas iC2, iA1, iA2, idiff, and i still increase up to cycle 80. Fur-
␹
thermore, the shape of the voltammograms during the cathodic
scans is different from that of the anodic scan. This shows that the
reduction process mechanism is not the same as that of the oxida-
tion one.
Figure 4 shows the evolution of the voltammogram shape vs. the
scan rate v. At low scan rates, the value of ᐃC2 is about 14 mV. This
is unusually small compared with a peak that fits the Nernst law in a
closed electrochemical system for example (ᐃNernst ϭ 90/n mV, n ϭ
3 being the number of exchanged electrons per Bi atom).28-30 At
scan rates higher than a few volts per second, we observe a broad-
ening and a large negative shift for the cathodic peaks and a smaller
positive shift for the two anodic peaks. Moreover, peak A2 is pro-
gressively merging with the plateau when v increases. Thus, A2 can
be seen as a pre-peak of the plateau.
Figure 3. Evolution of the voltammo-
grams from a few seconds after the immer-
sion of the electrode. (a) For the first three
cycles; (b) during cycles 3 to 8; (c) for the
cycles 8, 21, to 70; (d) definition of main
characteristics of voltammogram. (Scan
rate: 50 mV sϪ1).
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Characteristic profiles.—The Fig. 7a graph shows the variation
of the peak potentials EC2, EA1, and EA2 vs. v. For v < 0.5 V sϪ1, EC2
linearly decreases with a slope dEC2/d log v of Ϫ120 mV. Within the
same range, EA1 does not vary while EA2 increases with a slope
dEA2/d log v of ϩ60 mV. When v > 0.5 V/s, EC2 decreases with a
larger slope and the two anodic peak potentials similarly increase.
Furthermore, we note that the transition range corresponds to the one
observed for the peak shape as previously mentioned. These changes
must be assigned to a kinetic effect. Figure 7c shows the variation of
The constants n and ␣ are the number of exchanged electrons and
the electron transfer coefficient, respectively. The other constants
have their usual meaning. A broadening of the peak involving a de-
crease of n or ␣ against the scan rate must be ruled out. Therefore,
such a widening should be due to an electrode polarization (see
Appendix B). Figure 8b shows the profiles ᏭC2-v, Ꮽtot-v, and ᏭA1
-
v in bilogarithmic scale. We note that the first two profiles practical-
ly merge, which shows that cathodic and anodic areas are quasi-
identical. As a result, peak C2 is to be seen as the cathodic homo-
logue of the two anodic peaks, A1 and A2, and the plateau. Further-
more, the thee profiles are linear curves with a slope of 1, and take
into account relationship 2, the exchanged charges QC2, QA1, Qtot do
not vary within the scan rate range. This shows that all the elec-
troactive sites of the material are implicated during the electrochem-
ical reactions.
i
C2, iA1, iA2, vs. v. in bilogarithmic scale. We not that iC1 and iA1 is
proportional to v1/2 while iA2 linearly varies vs. v.
Figure 8a shows a linear variation of ᐃC2 and ᐃA1 vs. log v, with
a larger slope for peak C2. The peak width ᐃY (Y ϭ A1, C2) for a
simple redox reaction that takes place in a closed electrochemical
system, is given either by
ᐃY ϭ ᏾T/␣nᏲ
as the Butler-Volmer relationship is fitted or by
ᐃY ϭ 90/n
[5]
Discussion
Geometrical and electrochemical features of a CME.—We first
describe the geometry of the CME, which could have some effects
on evolution and shape voltammogram. Figure 9a shows the scheme
of a CME including material grains in close contact to each other.
[6]
when the Nernst law is applied.
Figure 4. Effect of the scan rate on the
voltammograms.
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The starting material is Bi2O3 or Bi2O2CO3, but during the subse-
quent cycles it has been changed into BiOOH or Bi(OH)3. Thus, this
is best written as BiO(3-y)/2(OH)y with y ϭ 0, 1, or 3. For the sake of
simplicity, the grains are assumed to be spherical. In such an elec-
trode, we can distinguish three groups of grains according to their
location: (i) the bottom grains that directly contact the platinum col-
lector (labeled B), (ii) the median grains at intermediate position
(labeled M), and (iii) the surface grains at the opening of the cavity
(labeled S). Each grain is surrounded by interstices whose volume
depends on the compacting pressure and the size and shape of the
grain. We assume that all the interstices communicate each other. We
can define the interstices labeled I between the bottom grains and the
Pt collector and the interstices labeled II between the other kinds of
material grains. If the Pt collector has a perfect plane surface and the
grain has a perfect spherical surface, the contact would be achieved
through a few atoms of Bi and Pt. The two surfaces are rather irreg-
ularly corrugated, and the contact is thus spreading on an extended
zone (CZ I), which can be illustrated by the scheme 9b. The same
goes for the contact between two grains (CZ II) in Fig. 9c. A contact
zone can be seen as an ensemble of small area Pt/material or mater-
ial/material contacts drawn in bold segment and labeled as contact
solid/solid in the previously mentioned figures. These contacts cor-
respond to the convex part of the corrugated surfaces. Each solid/
solid contact is surrounded by microinterstices, drawn as irregular
semipolygons or complete polygons for the sake of simplicity. Such
microinterstices correspond to the concave part of the corrugated
surface. We assume that the only initially adsorbed molecules (O2,
H2O, ..) are sandwiched between the two solid surfaces. The volume
of the microinterstices depends on the surface state of both Pt and
the grain. It is difficult to evaluate it, a priori. Nevertheless, it could
Figure 5. Evolution of the voltammogram
vs. the anodic extent of scan range.
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CO32Ϫ diffuses toward the external solution. Finally, the electro-
chemical behavior is the same as that of Bi(OH)3. This explains why
Bi2O2CO3 and Bi2O3 have the same electrochemical behavior in
alkaline media.
be small, even negligible, against the volume of the interstices,
which depends on the stacking of the grains. Assuming a cubic
stacking, the ratio interstice volume/grain sphere volume is ϳ0.9,
i.e., the volume of interstice has the same magnitude of the material
volume (see appendix C).
The overall reduction of Bi(III) can be written as
The electrolytic solution can fill the interstices and microinter-
stices when the electrode is immersed. During the filling, the electro-
chemical interfaces corresponds to the interface Pt/solution. When
the material is a good electronic conductor, the interface material/
solution can also form an electrochemical interface.
Bi2O3 or Bi2O2CO3 are slightly soluble in alkaline solution
(ϳ5и10Ϫ4 to 3и10Ϫ4 mol LϪ1 in 9 M KOH) and increases vs. the
KOH concentration.5,10,31,32 Thus, the solution within the interstices
and microinterstices is progressively saturated by dissolved Bi(III)
species as BiOϪ2 ions, for example, Furthermore, we must take into
account the adsorption of these species on the platinum collector.
BiIIIO(3-y)/2(OH)y ϩ (3-y)/2H2O ϩ 3e r Bi0 ϩ 3 OHϪ
[8]
Bi0 represents the Bi element at oxidation degree 0. We note that this
overall reaction (i) involves the exchange of H2O and OHϪ ions,
which causes diffusion of them within the microcavity solution, (ii)
increases the OHϪ concentration within the interstices and microin-
terstices, so enhancing dissolution of solid-state Bi(III),5,10,31,32 and
(iii) causes a lattice change since BiIIIO(3-y)/2(OH)y has a structure
different from that of Bi0. These considerations lead to the assump-
tion that the overall reduction reaction of Bi(III) must be composed
of several steps.
In the solid-state reduction, one electron exchange is always in-
voked, because the literature does not mention multiple electron
exchanges. Moreover, intermediate oxidation state Bi(II) and Bi(I)
are also mentioned.33-35 Thus, we can write
General consideration about the reaction mechanisms.—In alka-
line media Bi2O2CO3 can dissociate as follows
Bi2O2CO3 ϩ 2 OHϪ ϩ 2H2O r 2Bi(OH)3 ϩ CO32Ϫ
[7]
Figure 6. Evolution of the voltammogram
vs the cathodic extent of scan range.
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BiIIIO(3-y)/2(OH)yᮀ ϩ e ϩ Hϩ r BiIIO(3-y)/2(OH)yH
[9]
The symbol ᮀ is a H-lodging site, and H is the hydrogen atom in-
serted in the site ᮀ. The sites would form a superstructure, which
would be tunnels such as in diaspore structure or an interlayer space
in a layer structure.36 However, published data does not mention
such a structure for Bi2O3 and Bi(OH)3. Therefore, the inserted H
cannot diffuse within the bulk, it stays at the surface. Finally, we
must exclude solid-state reduction of Bi2O3 in alkaline media. Bi2O3
slightly dissolves in alkaline solution giving BiOϪ2 .31,32 We have to
consider the reduction of the dissolved species. Therefore, the reduc-
tion would correspond to the following
Bi2O3 ϩ 2 OHϪ r 2BiOϪ2 ϩ ⌯2O
BiOϪ2 can be reduced as
[10]
BiOϪ2 ϩ e r BiO22Ϫ
[11]
2H2O ϩ 3BiO22Ϫ r 2BiO2Ϫ ϩ 4 OHϪ ϩ Bi0
(disproportionation) [12]
Both electrochemical Reaction 11 and disproportionation 12 are
equivalent to a multiple electron exchange.28,29 Bi0, the Bi element
at 0 oxidation degree, can be considered here as an isolated atom. It
polycondenses with other ones giving the metal Bi
Bi0 r Bimet (formation of bismuth metal lattice)
[13]
Figure 8. Variation of the peak widths (a, top) and peak areas (b, bottom) vs.
the scan rate.
Moreover, the literature mentions that BiOϪ2 adsorbs on Pt.33,34 That
can be schematized as
BiOϪ2 ϩ Pt r Pt--BiOϪ2ads
[14]
Pt--BiOϪ2ads can be reduced according to a similar mechanism as
Eq. 10, 11, and 12
2Ϫ
Pt--BiO22Ϫads ϩ e r Pt--BiO
[15]
2ads
3Pt--BiO22Ϫads ϩ 2H2O r 2Pt--BiOϪ2ads ϩ 4 OHϪ ϩ Pt--Bi(0)ads [16]
Pt--Bi(0)ads r Pt--Bimet [17]
Cathodic peaks.—First, we note that the lowest potential value
reached is Ϫ0.8 V/Hg/HgO, which is too positive for producing
BiH3 (Ϫ1.3 V/Hg/HgO, according the potential-pH diagram of the
Bi element32). Thus, we attribute that the peaks C1 and C2 to the
reduction of Bi(III) species into Bi metal. Concerning solid-state
Bi(III) entities, published data often mention Bi2O3, and sometimes
Bi(OH)3 or BiOOH. The adsorbed BiOϪ2 corresponds to a small frac-
tion of the total amount of Bi(III) entities, because the maximum ab-
sorption is related to both the area of the Pt-wire collector (␲*␾p2t/4)
and the saturated concentration of dissolved Bi(III) within the inter-
stice and microinterstices solution.5,10,32 Thus, we can assume that
the small peak C1 is related to the reduction of the adsorbed
BiOϪ2 .33,34 it could correspond to Eq. 15, 16, 17.
Peak C2 represents the largest amount of exchanged charges, and
accordingly, it corresponds to the main process during the cathodic
Figure 7. Variation of the peak potentials (a, top) and peak intensities (b, bot-
tom) vs. the scan rate.
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scan. Furthermore, we note that the reduction reactions results in the
formation of OHϪ, which enhances the dissolution of solid-state
Bi(III), and accordingly favors the pathway related to dissolved
species, BiOϪ2 . Therefore, the peak C2 corresponds to Eq. 10 to 13.
At the start of the first cathodic scan, the deposit of Bi metal ini-
tially occurs on the Pt collector within the microinterstices I
(Fig. 10a1 to 10a3) and the interstices I (Fig. 10b1 and 10b2). It forms
a nonhomogeneous, irregular, and porous film on the Pt-surface,
because the thickness growth depends on the scan rate, the size, and
the shape of the interstices and microinterstices. Such a structure is
different from the initial material grains: the interstices and the
microinterstices are transformed into open micropores, which are
drawn as ellipses for the sake of simplicity on Fig. 10a3 and 10b2.
Their size depends on the deposition conditions of the Bi metal.
They contain H2O molecules, OHϪ ions, and Kϩ ions. As Bi metal
is a good conductor, it can play the role of a current collector when
Pt is totally covered or when any grains are reduced to the Bi metal.
When all the bottom grains are reduced, the reduction of the median
grains and the surface grains occurs successively through the same
process. As a result, the electrochemical interface progressively ex-
pands through the volume, and the morphology of the powder
changed at the end of the cathodic scans. Figure 10d1 illustrates the
status at the end of the cathodic scans when the voltammograms are
not stabilized: only a part of the sites is reduced. When the voltam-
mograms are stabilized, all the sites are reduced (Fig. 10d2). For the
first cathodic scan, the reduction starts from the bottom of the micro-
cavity, whereas during the subsequent cathodic scans, it starts
throughout the volume of the grains. Finally, the initial texture is
never built up again.
Anodic peaks and plateau.—Peak A2, and the plateau correspond
for the most part to the total exchanged anodic charge. They are thus
related to the oxidation of Bi metal to Bi(III). For peak A1, the dif-
fusion effect can be neglected whereas both peak A2 and plateau are
characteristic of a diffusion process. As peak A1 is a small fraction
of the sites, it must be related to a process for which the amount of
a reactant is limited. That can correspond to the oxidation of the Bi
metal sites located near the interface Bi metal/micropore-solution
until the species OHϪ present within the micropores are exhausted
(Fig. 11a). Thus
Bimet Biϩ ϩ e
[18]
[19]
[20]
r
3Biϩ Bi3ϩ ϩ 2Bimet
r
3 OHϪ ϩ Bi3ϩ Bi(OH)3
r
Bi(OH)3 ϩ OHϪ BiOϪ2 ϩ 2H2O (Dissolution) [21]
r
As the solubility of Bi(OH)3 is limited, the excess does not dissolve
within the micropore solution and accordingly forms a layer be-
tween the Bimet sites and the micropores, which slows the arrival of
OHϪ ions.
After the exhaustion of OHϪ, the pH is locally decreased. Thus
the oxidation of Bi metal atoms, located far from the micropores,
occurs at a higher potential. The availability of OHϪ is limited by
BiIIIO(3-y)/2(OH)y layers (arrows in Fig. 11b), which hinder the dif-
fusion of the species OHϪ or H2O coming from the external solution
(bold dotted line •••••• in Fig. 10d2).
At the end of the anodic scan, the open micropore structure
formed during the cathodic scan is kept, but the size and the shape
of the micropores are probably different. Finally, the oxidation starts
at the interface Bi metal/micropore solution, i.e., throughout the re-
action volume included within the microcavity.
From the analysis of the peak A1, we can deduce that the overall
volume of the micropores ᐂ␮pore represents a non-negligible fraction
of material grains (see Appendix D). That is consistent with the
porous structure.
Furthermore, peak A1 is always observed in the usual composite
electrodes whereas peak A2 is much larger than A1, and it is not fol-
lowed by a plateau.19,20 Such a behavior is due to the slow scan rate
(a few fractions of mV sϪ1), for which the diffusion processes are
not limiting parameters.
Peak A2 and the plateau also could correspond to Reactions 18,
19, and 20 in which the concentration of OHϪ is lowered by the
layer of Bi(III) formed during the oxidation.
Voltammogram evolutions.—The initial increase of the small
peak C1 should be related to the filling of both interstices I (see
Fig. 9a) and microinterstices I (see Fig. 9b) with the electrolytic
solution. The total filling can be achieved through three to four
cycles. Moreover, the waving observed during the first scans shows
that the filling of the interstices is not progressive but “pulsed.” This
could be due to the concurrent effect of the capillary forces, the
potential variation, the ion accumulation within the interstices and
microinterstices, and the size of them. This increase of the capacitive
current ic (Fig. 3b and d) shows that the electrochemical interfaces
are also increasing during the first 80 cycles, which corresponds to a
duration of around 1 h. This can be explained by the progressive
extent of the microporous structure of Bi metal/solution interface,
which can be characterized by the thickness ᐀ defined in Fig. 10d1.
We consider that the increase of the peak C2 is due to the exten-
sion of the microporous structure of Bi metal deposit interface dur-
ing the cycling. That corresponds to an area increase. The peak be-
comes stable, as the extent of electrochemical interface no longer
increases. This evolution duration must be related to the dissolution
and diffusion kinetics.
Figure 9. (a) Schematic view of the microcavity filled with material; (b)
magnification of the contact zone I; (c) magnification of the contact zone II.
B, bottom grain; S, surface grain; M median grain. The arrows in figures b
and c represent the dissolution of solid-state Bi(III) in the microinterstice
solution.
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4260
Journal of The Electrochemical Society, 147 (11) 4252-4262 (2000)
S0013-4651(00)01-077-6 CCC: $7.00 © The Electrochemical Society, Inc.
Figure 10. Status of the microcavity at
various moments of the reduction. (a1),
(a2), (a3) Status within the microinter-
stices; (b2, (b2) status within the inter-
stices; (c) magnification showing the
micropores after the first cathodic scan;
(d2), (d2) overall status of the microcavity
from the reduction of all the bottom grains
and at the end of the reduction, respective-
ly. The concentration of species within the
micropore is higher than that of the exter-
nal solution since they are provided by the
reduction reaction. The interstices and the
microinterstices are saturated by BiO2Ϫ.
The arrows in the schemes a1, a2, a3, rep-
resent the dissolution of solid-state Bi(III))
in the microinterstice solution and those in
the schemes b1 and b2 concern the dissolu-
tion in the interstice solution. The vertical
dashed line in Fig. 10d2 represents the dif-
fusion pathway of dissolved species from
external solution into micropores during
the oxidation.
Variation of peak intensities iA1 and iA2 vs. v.—iA1 is proportion-
cesses during the reduction-oxidation. The evolution is concurrently
due to the sequence of the following processes: (i) impregnation of
the powder and its dissolution in the electrolytic solution, (ii) deposit
of porous metal Bi during the cathodic scan, and deposit of Bi(III)
species during the anodic scan. These processes cause a reconfigu-
ration of the architecture of the powder within the cavity by making
the material grains smaller. For the first cathodic scan, the reduction
process starts from the bottom of the cavity at the interface Pt/solu-
tion only. Then it progresses toward the mouth. Contrarily, the oxi-
dation takes place throughout all the grains. During the next cathod-
ic scans, the reduction starts throughout all the grains when the grain
is not entirely oxidized for the preceding anodic scan. The reduction
of solid-state Bi(III) results in porous Bi metal deposit.
al to v1/2, which would indicate the existence of a diffusion process,
but the absence of a plateau would indicate the opposite. This appar-
ent contradiction must be assumed to be due to the effect of the local
polarization (see Appendix B). Presently, we are not able to explain
the linear variation of both iA2 and the plateau height. We would
rather expect a variation as v1/2 since the plateau indicates a diffusion
process. The quantitative analysis of these effects needs a theoretical
model, which is developed in Part II.
Conclusions
The voltammograms of Bi2O3 and Bi2O3CO2, recorded on a cav-
ity microelectrode, show an evolution during the first 80 cycles car-
ried out at 50 mV sϪ1. Then the voltammograms do not vary any
more during the subsequent cycles. Direct analysis of the voltam-
mograms allows the determination of physical and chemical pro-
However, two points are completed in forthcoming papers: (i) a
mathematical analysis of the peak characteristics-scan rate profiles
through a theoretical model and (ii) evidence of the species produced
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Journal of The Electrochemical Society, 147 (11) 4252-4262 (2000)
4261
[B-1]
S0013-4651(00)01-077-6 CCC: $7.00 © The Electrochemical Society, Inc.
E ϭ U-r ϫ i
However, a broadening when the ohmic drop is smaller than 1 mV, which is
the case for the cavity microelectrode, must be attributed to another cause,
the local polarization within the electrode, for example. If ᏼ is the polariza-
tion of the electrode, it must be added to the ohmic drop
E ϭ U-r ϫ i-ᏼ
[B-2]
Polarization takes place when the charge neutrality is not locally fulfilled
during the electrochemical reaction. We note that the overall reduction needs
1.5 H2O molecules and produces 3 OHϪ ions when 1 Bi(III) is reduced to
Bi(0). In order to keep charge neutrality, this needs H2O molecule diffusion
and Kϩ diffusion from the external solution toward the cavity, and OHϪ dif-
fusion in the reverse direction. As the mobility of OHϪ is larger than that of
Kϩ, the OHϪ flow is predominant. The polarization occurs when the flows
no longer compensate the charge variation of the Bi element oxidation state.
A quantitative treatment of the polarization needs a theoretical model, which
is developed in the forthcoming Part II.
Appendix C
Ratio Interstice Volume/Spherical Grain Volume
Assuming a cubic stacking of spheres the diameter of which is ␾g, the
interstice volume is
ᐂ
int ϭ [␾3 Ϫ (␲/6)␾g3]
[C-1]
g
The term ␾g3 is the volume of the cube in which the sphere is inscribed. Thus,
the ratio interstice volume ᐂint/grain sphere volume ᐂsph, is
ᐂint/ᐂsph ϭ [␾3 Ϫ (␲/6)␾g3]/(␲/6)␾3 ϳ 0.91
[C-2]
g
g
The interstice volume represents around half of that of the microcavity
(ϳ0.46 ϫ ᐂmic). Thus the volume occupied by the material is
ᐂ
mat ϭ 0.54 ᐂcav
[C-3]
Appendix D
Evaluation of the Total Volume of the Micropores (ᐂ
)
␮pore
If peak A1 corresponds to Eq. 18 to 20, the resulting exchanged charge
Q
A1 is related to both OHϪ amount ([OHϪ]␮pore) within the micropore solu-
tion and Bi(0) site amount near the micropores. The first has the smaller
value, and thereby, it is the limiting parameter
␮pore
[OHϪ]␮pore ϭ ᐂ␮pore ϫ COH
[D-1]
Ϫ
Figure 11. Status of the micropore during the anodic scans. (a) Status just
after the fall of peak A1; (b) status during peak A2 and the plateau; (c) status
at the end of the oxidation. The arrows represent the diffusion of counterions
or H2O molecules. The concentration of the species OHϪ, Kϩ, H2O in
Fig. 11b within the micropores is smaller than that of species in Fig. 10c.
They come from the external solution by diffusion, the pathway of which is
schematized by the vertical dotted straight line (••••) in Fig. 10d2.
␮pore
ᐂ
␮pore and COHϪ are expressed in liters and mol LϪ1, respectively. Each con-
sumed OHϪ is related to 1 faraday. Thus, using the exchanged charge instead
of the mass, Eq. D-1 becomes
Q
A1 ϭ ᐂ␮pore ϫ C␮poϪreϫ Ᏺ
[D-2]
OH
Resolving regard to ᐂ␮pore, we obtain
␮pore ϭ QA1/(C␮poϪreϫ Ᏺ)
ᐂ
[D-3]
OH
␮pore
COH is the average value of OHϪ concentration within the micropores.
at various moments of a potential scan cycle by means of coupling
micro-Raman spectrometry electrochemistry.
Ϫ
After the cathodic scan, its value is higher than that of the external solution,
since reducing one BiIII oxide-hydroxide, three OHϪ ions are formed. Thus
the total amount of OHϪ produced is
Acknowledgments
V.V. has been granted by the Ministère de la Recherche et de la
Technology.
[OH]Ϫ ϭ 3 ϫ ␳ ϫ ᐂmat/MBiIII
[D-4]
where ␳ is the density of Bi(OH)3 and MBiIII its molar weight normalized to
one Bi atom. Assuming all the produced OHϪ ions are kept within the vol-
ume occupied by the material and taking into account Eq. D-4, the maximum
concentration within the micropores would be
Centre National de la Recherche Scientifique assisted in meeting the pub-
lication costs of this article.
Appendix A
Microcavity Features
max
OH
C
Ϫ ϭ [OH]Ϫ/ᐂmat ϭ 3 ϫ ␳ ϫ 1000/MBiIII
[D-5]
max
OH
where C Ϫ is expressed in mol LϪ1. Actually, OHϪ ions diffuse toward the
The volume of a cylinder of 50 ␮m diam and 20 ␮m height is ϳ4и
10Ϫ8 cm3. Assuming that the cylinder is totally filled by the material whose
density is ␳, the corresponding mass mcav is
␮pore
external solution, and accordingly COHϪ is contained between 1 mol LϪ1, the
external solution OHϪ concentration, and 3 ϫ ␳ ϫ 1000/MBiIII, the maximum
concentration, which thus depends on the scan rate. Therefore, we can write
m
cav ϭ ϳ4␳и10Ϫ8 g
[A-1]
QA1/(1 ϫ Ᏺ) > ᐂ␮pore > QA1 ϫ MBiIII/(3 ϫ ␳ ϫ 1000 ϫ Ᏺ) [D-6]
Appendix B
Broadening of the Peak When the Scan Rate Increases
Using the experimental data [QA1 ϭ 10Ϫ6 C at 0.1 V/s (see Fig. 8b); ␳ ϭ 8],
we found
In a general way, the broadening of the peak at high scan rates is caused
concurrently by both large ohmic drop (>20 mV) and large capacitive current
Ꮿ ϫ v, i.e., when the ratio Ꮿ ϫ v/ipeak is larger than 0.1. This can be theoret-
ically treated as follows. When ohmic drop r ϫ i is present, the applied poten-
tial U is diminished according to
10Ϫ8 (in cm3) > ᐂ␮pore > 0.01и10Ϫ8 cm3
[D-7]
Taking into account (i) Eq. C-3 and the volume of the cavity (4и10Ϫ8 cm3 for
diameter 50 ␮m and 20 ␮m depth), we can normalize the volume of the
micropores with regard to the volume occupied by the material, we obtain
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4262
Journal of The Electrochemical Society, 147 (11) 4252-4262 (2000)
S0013-4651(00)01-077-6 CCC: $7.00 © The Electrochemical Society, Inc.
15. K. Kordesch, J. Gsellmann, M. Peri, K. Tomantschger, and R. Chemelli, Elec-
0.45 > ᐂ␮pore/ᐂmat > 0.005
[D-8]
trochim. Acta, 26, 1495 (1981).
Whatever the diffusion level of OHϪ, the total volume of the micropores is
always a fraction of material volume ᐂmat. If OHϪ cannot diffuse, we would
not observe the plateau. In the case where diffusion is rapid, OHϪ in not a
limiting parameter, and accordingly peak A1 would correspond to the total
oxidation charge, which does not actually happen. Thus, the ratio ᐂ␮pore/ᐂmat
would be as far from 0.45 as from 0.005, which shows that the material
deposited during the electrochemical oxidation and reduction is porous.
16. W. Harer and K. Kordesch, in Proceedings of the 2nd Battery Material Symposium,
Vol. 2, p. 231, Graz (1985).
17. H. Ouboumour, M. Bodé, C. Cachet, and L. T. Yu, J. Electrochem. Soc., 142, 1061
(1995).
18. H. Ouboumour, C. Cachet, M. Bodé, C. Lévy-Clément, J. Ginoux, and L. T. Yu,
Prog. Batteries Battery Mater., 13, 170 (1994).
19. A. M. Espinoza, M. T. San Jose, M. L. Tascon, M. D. Vazquez, and P. Sanchez-
Batanero, Electrochim. Acta, 36, 1561 (1991).
20. A. M. Espinosa, M. L. Tascon, P. Encinas, M. D. Vazquez, and P. Sanchez-
Batanero, Electrochim. Acta, 40, 1623 (1995).
References
1. C. Michel, M. Hervieu, M. M. Borel, A. Grandin, and F. Deslandes, Z. Phys. B, 68,
421 (1987).
21. C. S. Cha, C. M. Li, H. X. Yang, and P. F. Liu, J. Electroanal. Chem., 368, 47
(1994).
2. A. H. Maeda, Y. Tanaka, and M. Fukumoti, Jpn. J. App. Phys., 27, 2 (1988).
3. A. M. Chippindale, S. J. Hibble, J. A. Hriljac, L. Cowey, D. M. S. Bagguley, P. Day,
and A. K. Cheetham, physics C, 152, 154 (1988).
22. H. X. Yang and C. S. Cha, J. Power Sources, 43, 145 (1993).
23. V. Vivier, C. Cachet-Vivier, B. L. Wu, C. S. Cha, J-Y. Nedelec, and L. T. Yu, Elec-
trochem. Solid State Lett., 2, 385 (1999).
4. Y. F. Yao, N. Gupta, and H. S. Wroblow, J. Electroanal. Chem., 223, 107 (1987).
5. H. S. Wroblowa and N. Gupta, J. Electroanal. Chem., 238, 93 (1987).
6. M. A. Dzieciuch, N. Gupta, and H. S. Wroblowa, J. Electrochem. Soc., 135, 2415
(1988).
24. V. Vivier, C. Cachet-Vivier, C. S. Cha, J-Y. Nedelec, and L. T. Yu, Electrochem.
Commun., 2, 180 (2000).
25. F. Rouessac and A. Rouessac, Analyse Chimique, 3rd ed., Masson Publisher, Paris
(1997).
7. S. W. Donne, G. A. Lawrance, and D. A. J. Swinkels, Prog. Batteries Battery
Mater., 15, 65 (1996).
8. D. Y. Qu, B. E. Conway, L. Bai, Y. H. Zhou, and W. A. Adams, J. Appl. Elec-
trochem., 23, 693 (1993).
9. L. Bai, D. Y. Qu, B. E. Conway, Y. H. Zhou, G. Chowdury, and W. A. Adam,
J. Electrochem. Soc., 140, 884 (1993).
10. C. G. Castledine and B. E. Conway, J. Appl. Electrochem., 25, 7075 (1995).
11. M. Bode, C. Cachet, S. Bach, J-P. Pereira-Ramos, J. L. Ginoux, and L. T. Yu,
J. Electrochem. Soc., 144, 792 (1997).
26. H. Harwig, Z. Anorg. Allg. Chem., 444, 151 (1978).
27. C. Greaves and S. Blower, Mater Res. Bull., 23, 1001 (1988).
28. L. T. Yu, Electrochim. Acta, 28, 63 (1983).
29. V. Plichon and E. Laviron, J. Electroanal. Chem., 71, 43 (1976).
30. A. Vallat, M. Person, and E. Laviron, Electrochim. Acta, 27, 485 (1882).
31. M. Pourbaix, Atlas d’Equilibres Electrochimiques, Gauthier-Villars Publisher,
Paris (1963).
32. W. F. Linke, Solubility, Vol. D, p. 437, Van Nostrand, Princeton, NJ (1958).
33. V. V. Gorodetskii, L. I. Fedortsov, and V. V. Lotsev, Electrokhimiya, 4, 967 (1968).
34. B. J. Bowles, Electrochim. Acta, 15, 737 (1970).
12. L. T. Yu, J. Electrochem. Soc., 144, 802 (1997).
13. K. Kordesch, U.S. Pat. 4,105,830 (1978).
14. R. Chemelli, J. Gsellmann, G. Korbler, and K. Kordesch, in Proceedings of Man-
ganese Dioxide Symposium, Vol. 2, p. 150, Tokyo (1980).
35. S. H. Cadle and S. Bruckenstein, Anal. Chem., 44, 1993 (1972).
36. A. F. Wells, Structural Inorganic Chemistry, 4th ed., see Structure Diaspore,
Clarendon Press, Oxford (1975).
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