Doping of cobalt into multilayered manganese oxide for improved pseudocapacitive properties

Article abstract of DOI:10.1149/1.3467865

Anodic electrodeposition of layered manganese oxides has been carried out in a MnSO₄ solution heated at various bath temperatures from 25 to 80°C. The crystallinity of the deposits increased with an increase in the bath temperature. Although Co²⁺ ions were not involved in the deposition reaction at room temperature, at the elevated temperature the addition of Co²⁺ gave rise to a catalytic current suggesting the formation of Co/Mn mixed oxide. No new X-ray diffraction peaks appeared with the insertion of Co²⁺ ions in the structure, but their addition caused broadening of the peaks due to the Mn dioxide phase. This indicates that Co ions are not accommodated in the interlayer space but are well dispersed within the MnO₂ layers composed of edge-shared MnO₆ octahedra. The Co-doped Mn oxide film thus obtained exhibited much better pseudocapacitive performance than the undoped counterpart.

Full text of DOI:10.1149/1.3467865

Journal of The Electrochemical Society, 157 ͑10͒ A1067-A1072 ͑2010͒
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0013-4651/2010/157͑10͒/A1067/6/$28.00 © The Electrochemical Society
Doping of Cobalt into Multilayered Manganese Oxide
for Improved Pseudocapacitive Properties
,z
Masaharu Nakayama,a, Kazushi Suzuki,a Keigo Okamura,a Ryota Inoue,
*
a,**
Laurence Athouël,b Olivier Crosnier,b and Thierry Brousseb,
*
aDepartment of Applied Chemistry, Yamaguchi University, Ube 755-8611, Japan
bLaboratoire de Génie des Matériaux et Procédés Associés, Polytech Nantes, Université de Nantes,
44306 Nantes Cedex 3, France
Anodic electrodeposition of layered manganese oxides has been carried out in a MnSO4 solution heated at various bath tempera-
tures from 25 to 80°C. The crystallinity of the deposits increased with an increase in the bath temperature. Although Co2+ ions
were not involved in the deposition reaction at room temperature, at the elevated temperature the addition of Co2+ gave rise to a
catalytic current suggesting the formation of Co/Mn mixed oxide. No new X-ray diffraction peaks appeared with the insertion of
Co2+ ions in the structure, but their addition caused broadening of the peaks due to the Mn dioxide phase. This indicates that Co
ions are not accommodated in the interlayer space but are well dispersed within the MnO2 layers composed of edge-shared MnO6
octahedra. The Co-doped Mn oxide film thus obtained exhibited much better pseudocapacitive performance than the undoped
counterpart.
© 2010 The Electrochemical Society. ͓DOI: 10.1149/1.3467865͔ All rights reserved.
Manuscript submitted April 27, 2010; revised manuscript received June 28, 2010. Published August 6, 2010. This was Paper 254
presented at the Vienna, Austria, Meeting of the Society, October 4–9, 2009.
Pseudocapacitors store energy by utilizing the pseudocapacitance
based on the first and reversible faradaic redox reactions of electro-
active materials having multiple oxidation states, such as transition-
metal oxides and conducting polymers. They are fundamentally dif-
ferent from electrical double-layer capacitors arising from the
nonfaradaic charge separation at the electrode/electrolyte interface,
focusing on carbon materials. Of all the metal oxides investigated as
pseudocapacitor materials, hydrated ruthenium oxide is considered
as the most promising material offering high specific capacitance
͑760 F g−1͒ with excellent cyclability.1 However, ruthenium is a
very expensive and toxic material, and the capacitor requires a
strongly acidic electrolyte ͑such as sulfuric acid͒, which makes it
inadequate for a wide range of commercial applications. Manganese
oxides, characterized by low price, facile synthesis, rich abundance
of Mn elements, and environmental compatibility, can serve as a low
cost replacement for RuO2. The capacitive behavior of amorphous
Mn oxide was first recognized about 10 years ago by Lee and
Goodenough.2 Since then, intensive research effort has been devoted
to develop the synthetic methods for MnO2-based materials with
good pseudocapacitance, including oxidation of Mn2+,2 reduction of
MnO−4,3 sol–gel method,4 and anodic deposition.5 Capacitive prop-
erties of Mn oxides critically depend on their morphologies, crystal-
line structures, valence states, and defect chemistry. Brousse et al.6
and Devaraj and Munichandraiah7 examined exhaustively the rela-
tionship between crystalline structures and their pseudocapacitive
properties. As a conclusion, both research groups pointed out that
birnessite, a two-dimensional layered structure with an interlayer
distance of ϳ0.7 nm, comprised of edge-shared MnO6 octahedra
and the intercalated alkaline cations, has higher electrochemical per-
formance than one- and three-dimensional crystals. This feature can
be ascribed to the bicontinuous networks of solid and pores in bir-
nessite, allowing both electrons and cations to move fast and
reversibly.8 In such a layered structure, all of the materials can be
regarded as surfaces. A common drawback in Mn oxides is their
poor electrical conductivity. Hence, powdery Mn oxide is mixed
with conductive carbon additives and polytetrafluoreneethylene
binder when used as an electrode for a pseudocapacitor. Moreover,
the incorporation of other transition metals into MnO2 compounds
has been the subject of intense interest in recent years to enhance
their charge-storage capability. We can find in the literature that the
incorporation of Pb,9 Ni,9,10 Co,10-12 Mo,13 and Fe14 improves the
specific capacitance of various types of Mn oxides. Framework dop-
ing into the layered alkali Mn oxides has been accomplished through
hydrothermal approaches.15,16 Sharma et al. revealed that the con-
ductivity increases of 2 orders of magnitude when 4% of Mn ions in
the oxide framework are substituted by Co.15
Birnessites have been synthesized in a powder form via a wide
variety of chemical methods until Nakayama et al. found a facile
electrochemical route to build a thin film of birnessite on an elec-
trode substrate.17,18 Electrodeposition is one of the most promising
binder-free techniques and is therefore suitable to design a compact
capacitor device. Moreover, this method can grow nanostructured
materials, offering improved pseudocapacitive performance as a re-
sult of the reduced diffusion path length and increased specific sur-
face area. If they possess reasonable electrical conductivity and ion
accessibility, birnessite thin films open up new ways for fabricating
thin-film capacitors with high volumetric capacitance.
As previously reported, the anodically grown Mn oxide layers
are lying parallel to the electrode surface in the as-deposited state.19
The specific capacitance of the as-deposited film intercalated with
K+ was 21 F g−1 when cycled at a scan rate of 2 mV/s,20 which is
much smaller than the values reported for the composites consisting
of chemically synthesized birnessites ͑110–230 F g−1͒ and carbon
materials.6,7,21-23 This is due largely to ͑i͒ poor accessibility of
charge-compensating ions in electrolyte to the internal space and/or
͑ii͒ difficulty in electron conduction to the electrode surface. These
problems could be solved by ͑i͒ morphology control of the deposits
and/or ͑ii͒ enhancement of the electrical conductivity of the Mn
oxide framework. In the present study, we electrodeposited multi-
layered Mn oxides at different temperatures and investigated the
effects on crystalline structure and morphology of the products. In
the following, the MnO2 layers composed of edge-shared MnO6
octahedra were doped with Co ions to improve pseudocapacitive
properties.
Experimental
Electrodeposition.— All chemicals were of reagent grade and
used without further purification. All solutions were prepared with
doubly distilled water and deoxygenated by bubbling purified nitro-
gen gas before use. Electrochemical experiments were carried out in
a conventional three-electrode cell using a potentiostat ͑model
263A, EG&G Princeton Applied Research͒. A platinum gauze elec-
trode and a Ag/AgCl electrode ͑in saturated KCl͒ served as the
counter and reference electrodes, respectively. Polycrystalline plati-
num foil ͑Niraco, 99.98%͒ with a geometric surface area of 1 cm2
*
Electrochemical Society Active Member.
Electrochemical Society Student Member.
**
z E-mail: nkymm@yamaguchi-u.ac.jp
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was used as the working electrode. Before each experiment, the
electrode surface was ultrasonically cleaned in diluted HCl solution
for 10 min and then washed thoroughly with water. The deposition
baths used consisted of 2 mM MnSO4·5H2O, 50 mM Et4NCl ͑Et
= C2H5͒, and various concentrations of CoSO4·7H2O ranging from
0 to 20 mM. The molar ratio of Et4NCl to MnSO4 had been opti-
mized to produce a product with the highest crystallinity.24,25 The
bath temperature was kept constant using a thermal bath/stirrer. A
constant potential, +1.0 V vs Ag/AgCl, was applied to the working
electrode for a certain period of time while the charge delivered was
monitored. The films thus prepared were visually uniform and ad-
hered well to a substrate. Immediately after electrodeposition, the
resulting film coated on a Pt electrode was rinsed thoroughly with
water, dried under vacuum in a dessicator for at least 2 h, and then
submitted to spectroscopic and electrochemical measurements.
Structural and electrochemical characterization.— X-ray dif-
fraction ͑XRD͒ data were recorded at a scan rate of 1° min−1 on a
Rigaku Ultima diffractometer with Cu K␣ radiation ͑␭
= 0.154051 nm͒ as the source. Field-emission-scanning electron
microscopy data were obtained with a Hitachi S-4700Y scanning
electron microscope ͑SEM͒. Electron probe microanalysis ͑EPMA͒
was made using a Horiba EMAX-7000S. The chemical composition
of the mixed oxides was determined by an inductively coupled
plasma-atomic emission spectrometer ͑ICP-AES, SII SPS-3500͒ af-
ter dissolving the sample film in a HNO3/HCl solution. The sample
films for the ICP-AES analysis were prepared by immersing the
as-deposited film for 24 h in a 0.5 M CaCl2 solution to replace all
the cations in the interlayer space by Ca2+ ions through an ion-
exchange mechanism. Thus, we estimated the content of Co ions
immobilized in the Mn oxide framework. The emitting light wave-
lengths in the ICP-AES measurements were 257.6 and 228.6 nm for
detecting Mn and Co, respectively. Cyclic voltammograms ͑CV͒ of
the films supported on a Pt substrate were recorded at several po-
tential scan rates, where an aqueous solution of 0.5 M Na2SO4 was
used as the electrolyte. Specific capacitance ͑in F g−1͒ was calcu-
lated by integrating the CV curve to obtain the voltammetric charge
͑in C͒ and subsequently dividing it by the mass of the electroactive
material ͑in g͒ and the potential window ͑in V͒.13 The electrode
mass was determined for the Ca2+-replaced sample described above.
The weight difference between the electrodes with and without the
sample film was measured by a Mettler Toledo XS205DU microbal-
ance with an accuracy of 0.1 mg.
Figure 1. i-E curves taken at ͓͑a͒, ͑c͒, ͑e͔͒ 25 and ͓͑b͒, ͑d͒, ͑f͔͒ 70°C in
solutions containing 50 mM Et4NCl and ͓͑a͒ and ͑b͔͒ 1 mM CoSO4, ͓͑c͒ and
͑d͔͒ 2 mM MnSO4, or ͓͑e͒ and ͑f͔͒ 1 mM CoSO4 + 2 mM MnSO4. Scan
rate, 10 mV s−1
.
Electrodeposition of Mn oxides at different temperatures.— The
films were electrodeposited at different temperatures from a solution
containing MnSO4 and Et4NCl ͑without CoSO4͒. Figure 2a displays
the XRD pattern of the electrodeposited film on the Pt substrate at
25°C. Two peaks appearing at 9.3 and 18.8° in 2␪ are assignable to
the 001 plane of the birnessite-type Mn oxide and its second ͑002͒
order diffractions, respectively, where Et4N+ ions are accommodated
between Mn oxide layers with an interlayer distance ͑d001͒ of 0.94
nm.24 Small bumps around 12° ͑d-spacing = ϳ 0.7 nm͒ can be
attributed to multilayers accommodating hydrated small cations
where the interlayer distance is determined by the size of water
molecule ͑0.28 nm͒ and the crystallographic thickness of a MnO2
sheet ͑0.45 nm͒.27 The cations accommodated are probably protons
generated concurrently with the formation of Mn oxide.25 With an
increase in the bath temperature, the peaks arising from the
Et4N+-intercalated birnessite increase in intensity and become nar-
rower. The inset displays the full width at half-maximum ͑fwhm͒ of
the 001 peak and the estimated crystallite size in the 001 direction as
a function of bath temperature, which clearly indicates that high
temperature causes highly crystallized materials. Namely, the fwhm
value of the 001 peak decreases from 1.29 ϫ 10−2 rad at room tem-
perature to 0.84 ϫ 10−2 rad at 70°C, corresponding to an increase
in the crystalline size along the 001 direction ͑c axis͒ from 10.7 to
16.3 nm deduced by the Scherrer equation. The intercalated Et4N+
ions in the as-deposited state were rapidly replaced by supporting
electrolyte cations, giving a smaller interlayer distance of 0.72 nm
͑data not presented͒.
The SEM images of the films prepared at three temperatures ͑25,
60, and 70°C͒ are shown in Fig. 3. A relatively smooth surface is
observed for the film prepared at room temperature. This feature is
characteristic of the electrodeposited film, which is composed of the
Mn oxide multilayers lying parallel to the substrate, as we reported
previously based on the TEM and in-plane XRD data.19 A sheetlike
morphology becomes evident at 60°C, in which the deposits occurs
at random. At 70°C, the platelike deposits of micrometer size are
recognized, and their thickness are measured to be 10–20 nm, which
agrees well with the crystalline size estimated from the XRD data.
CVs were taken for the films made at 25 and 70°C ͑Fig. 4͒. The
shape of curve a, being not rectangular, is not characteristic of a
Results and Discussion
Electrolysis of Mn ions in the presence of Co ions.— Figure
1
shows current density–potential ͑i-E͒ curves obtained in various so-
lutions kept at 25 or 70°C, where 50 mM of Et4NCl was contained
in all the solutions. In a CoSO4 solution without Mn2+ at 25°C
͑curve a͒, a small anodic current starts to appear at +1.2 V. At 70°C
͑curve b͒, a larger current appears from +1.0 V, which can be at-
tributed to the fact that the dissociation constant of CoSO4·7H2O
increases by about 3 orders of magnitude as the solution temperature
is raised from 25 to 70°C.26 Also in a MnSO4 solution without Co2+
,
the oxidation current becomes larger at elevated temperature, as
seen from the comparison between curves c and d. In a mixed so-
lution of Mn2+ and Co2+ ions, at room temperature, the current
profile ͑curve e͒ is the same as that ͑curve c͒ obtained in the MnSO4
solution without Co2+, indicating that Co2+ ions are not involved in
the anodic reaction. At elevated temperature, however, the enhanced
current appears from +0.6 V ͑curve f͒. Compared to the current
profile of curve b, the current increase from +0.6 V can be attrib-
uted to a catalytic effect. This means that Co2+ ions are not oxidized
directly at the electrode surface but mediated by the oxidation of Mn
species, as illustrated in the inset. Hereafter, electrolysis was carried
out potentiostatically at +1.0 V with and without CoSO4, where
catalytic oxidation of Co2+ ions can take place. This potential gives
a product with high crystallinity, as previously reported.24,25
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Journal of The Electrochemical Society, 157 ͑10͒ A1067-A1072 ͑2010͒
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Figure 3. SEM images of the Mn oxide films deposited at ͑a͒ 25, ͑b͒ 60, and
͑c͒ 70°C from a solution containing 2 mM MnSO4 and 50 mM Et4NCl.
within the Mn oxide framework because the interlayer space had
been occupied by Ca2+ ions, as described in the Experimental sec-
tion. The content of Co can be increased by adding CoSO4 into the
deposition bath.
Figure 2. XRD patterns of the Mn oxide films deposited at ͑a͒ 25, ͑b͒ 40, ͑c͒
60, ͑d͒ 70, and ͑e͒ 80 °C from a solution containing 2 mM MnSO4 and 50
mM Et4NCl. The charge delivered was always 0.6 C cm−2. An asterisk in-
dicates the contribution from Pt substrate. Inset: The fwhm and crystal size
along the 001 direction plotted against the deposition temperature.
pseudocapacitive electrode material. Further, the distortion of the
rectangle becomes larger in the highly crystallized film ͑curve b͒,
where the current response becomes slower when the scan direction
is changed, implying an increase in the electrode resistance.9,28
Electrodeposition of Co-doped Mn oxides at 70°C.— We tried
Co doping by adding CoSO4 into the deposition bath heated at 70°C
because Co2+ ions are involved in the film formation process at this
temperature, as demonstrated in Fig. 1. Figure 5 shows XRD pat-
terns of the films deposited at 70°C from Mn2+ solutions containing
CoSO4 at different concentrations. By increasing the concentration
of CoSO4, the peaks attributed to the birnessite phase decrease in
intensity and broaden, while the layered structure itself is main-
tained. At this time, no new peaks related to the Co oxide phase
appear. It is, therefore, reasonable to assume that Co centers are well
dispersed in the Mn oxide layers and that a segregated crystalline
phase of Co oxide is absent. Small peaks around 12° appearing in
the high Co2+ concentration region can be related to
Co2+-intercalated birnessite, where the interlayer space is main-
tained by water molecules.
The as-deposited film with 5 mM CoSO4 was subjected to SEM
and EPMA analyses. As shown in Fig. 6a, the morphology differs
markedly from that of pure Mn oxide ͑Fig. 3c͒. The EPMA spectrum
in Fig. 6b exhibits three peaks arising from Co as well as the peaks
due to Mn oxide and Pt substrate, confirming the presence of Co
ions in the deposited film.
Figure 7 illustrates the Co/Mn atomic ratios in the electrodepos-
ited films as a function of the ͓CoSO4͔/͓MnSO4͔ concentration ratio
in the deposition bath. The Co contents obtained here were detected
Figure 4. CVs of the Mn oxide films deposited at ͑a͒ 25 and ͑b͒ 70°C from
a solution containing 2 mM MnSO4 and 50 mM Et4NCl. Electrolyte, 0.5 M
Na2SO4. Scan rate, 20 mV s−1
.
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Journal of The Electrochemical Society, 157 ͑10͒ A1067-A1072 ͑2010͒
Figure 7. Atomic ratio of Co/Mn in the electrodeposited film vs the concen-
tration ratio of ͓CoSO4͔/͓MnSO4͔. The film preparation was carried out at a
constant potential of +1.0 V with an electric charge of 0.6 C cm−2
.
Figure 5. XRD patterns of the films deposited at 70°C from a solution
containing 2 mM MnSO4, 50 mM Et4NCl, and CoSO4 at various concentra-
tions; ͑a͒ 0, ͑b͒ 1, ͑c͒ 5, ͑d͒ 10, and ͑e͒ 20 mM. The charge delivered was
always 0.6 C cm−2. An asterisk indicates the contribution from Pt substrate.
metrical rectangular shape that is typical of MnO2 capacitors with
conductive carbon materials and is hardly affected by the presence
of Co2+ cations. Besides, the voltammetric charges in all the Co-
doped films are much larger than that of the pure Mn oxide. Clearly,
the Co doping provides improved specific capacitance and rate ca-
pability, which may reflect an enhanced electrical conductivity
Figure 8 shows the steady-state CVs of the films made at 70°C
from the deposition solutions with ͓͑b͒ to ͑d͔͒ and without ͑a͒
CoSO4. The voltammetric curves of Fig. 8b-d exhibit a nearly sym-
Figure 8. CVs of the electrodeposited films obtained at a scan rate of 20
mV/s in a 0.5 M Na2SO4 solution. Films a to d were prepared at 70°C from
2 mM MnSO4 + 50 mM Et4NCl solutions ͑a͒ without and with CoSO4 at
the concentrations of ͑b͒ 0.5, ͑c͒ 1.0, and ͑d͒ 5 mM. The charge delivered for
Figure 6. ͑a͒ SEM and ͑b͒ EPMA data of the film deposited at 70°C from a
solution containing 2 mM MnSO4, 50 mM Et4NCl, and 5 mM CoSO4.
the electrodeposition was always 0.1 C cm−2
.
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Figure 9. Plots of the specific capacitance values as a function of the number
of cycles based on the CV data in Fig. 8. Films a to d were prepared at 70°C
from 2 mM MnSO4 + 50 mM Et4NCl solutions ͑a͒ without and with
CoSO4 at the concentrations of ͑b͒ 0.5, ͑c͒ 1.0, and ͑d͒ 5 mM.
Figure 10. Specific capacitances of ͑a͒ Mn/Co oxide and ͑b͒ pure Mn oxide
electrodes at different scan rates in a 0.5 M Na2SO4 solution. The Mn/Co
oxide was prepared with 1 mM CoSO4.
caused by the introduction of Co into the Mn oxide framework.15 In
the potential region examined, transition of Mn4+/Mn3+ involving a
single-electron transfer is responsible for the pseudocapacitive be-
havior of the Co-doped Mn oxide.
layer space. Even after 1000 cycles, these two peaks remain un-
changed, suggesting that the observed capacitive behavior takes
place in the interlayer space accompanying insertion/desertion of
Na+ ions. A small peak at 31.8° can be ascribed to the ␣-Mn2O3
phase,30 which may reflect the involvement of reductive transforma-
tion of birnessite at the prolonged cycles.
In Fig. 9, the calculated specific capacitances were plotted
against the number of cycles to 1000 for films a to d prepared in Fig.
8. It is clear from this figure that the introduction of Co into Mn
oxide enhances the specific capacitance at the initial cycles. The
Co/Mn oxide film made with 1 mM CoSO4 ͑curve c͒ exhibits better
capacitance values during the whole cycles than the other films,
while no capacitance fade appears in the film with 0.5 mM CoSO4 at
the later cycles. It is generally accepted that the capacitance fade
upon electrochemical cycling occurs as a result of loss of active
material through partial dissolution of MnO2 films or conversion to
electrochemically irreversible species such as Mn2O3 and Mn3O4.4
However, according to Ragupathy et al., the capacitance retention is
closely related to high surface area, secondary pores between par-
ticles, and appropriate water content.29 Birnessite accommodates
water molecules in the interlayer space, which possesses high inter-
nal surface area. This may be responsible for the better cycling
stability observed in the Co/Mn oxide films with higher crystallinity.
Cyclic voltammetry at different potential scan rates, ranging
from 2 to 200 mV s−1, was carried out in a 0.5 M Na2SO4 solution
with the films that had been cycled 30 times in the same solution.
The best rate capability was observed in the film with 1 mM CoSO4,
and the specific capacitance values are plotted against the scan rates,
along with those taken for the pure Mn oxide film ͑Fig. 10͒. The
specific capacitance of the Mn/Co oxide film was 170 F g−1 at the
lowest scan rate ͑2 mV/s͒. A similar specific capacitance value,
125 F g−1 at 25 mV s−1, can be found in the literature by Chuang
and Hu11 for an amorphous film of binary Mn/Co oxide deposited
anodically. Although the capacitance value decreases by increasing
the scan rate, more than 50% of the capacitance remains even at
200 mV s−1. This decay of the capacitance is much smaller than
that observed for the pure Mn oxide film, i.e., 61 F g−1 at
2 mV s−1 decreased by 92% to 5 F g−1 at 200 mV s−1
.
In Fig. 11, the crystalline structure was checked before and after
1000 CV cycles. As seen from the comparison between curves a and
b, the Et4N+ ions incorporated in the as-deposited film is replaced by
Na+ ions in electrolyte solution, leading to a shrinkage of the inter-
Figure 11. XRD patterns of the films: ͑a͒ as-deposited film, ͑b͒ after immer-
sion in a 0.5 M Na2SO4 solution, and ͑c͒ after 1000 CV cycles. An asterisk
indicates the contribution from Pt substrate.
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Journal of The Electrochemical Society, 157 ͑10͒ A1067-A1072 ͑2010͒
8. J. W. Long, A. L. Young, and D. R. Rolison, J. Electrochem. Soc., 150, A1161
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Conclusions
Anodic deposition of layered manganese oxides was conducted
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12. J.-K. Chang, M.-T. Lee, C.-H. Huang, and W.-T. Tsai, Mater. Chem. Phys., 108,
124 ͑2008͒.
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͑2005͒.
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15. P. K. Sharma, G. J. Moore, F. Zhang, P. Zawalij, and M. S. Whittingham, Electro-
chem. Solid-State Lett., 2, 494 ͑1999͒.
at various bath temperatures. With an increase in temperature, the
crystallinity of the layered Mn oxide was enhanced. Addition of
Co2+ at elevated temperature did not provide any new XRD peaks
but caused broadening of the peaks due to Mn oxide, indicating the
introduction of Co ions into the birnessite framework. The film thus
obtained demonstrated a drastic increase in the specific capacitance,
power capability, and cycling ability.
16. J. Cai, J. Liu, and S. L. Suib, Chem. Mater., 14, 2071 ͑2002͒.
17. M. Nakayama, S. Konishi, A. Tanaka, and K. Ogura, Chem. Lett., 33, 670 ͑2004͒.
18. M. Nakayama, H. Tagashira, S. Konishi, and K. Ogura, Inorg. Chem., 43, 8215
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Acknowledgment
This research was supported in part by the Japan Society of the
Promotion of Science ͑no. 20550180͒ and Japan Science and Tech-
nology Agency: Research for Promoting Technological Seeds ͑no.
12-159͒. M.N. thanks the University of Nantes for welcoming him
as an invited professor.
Yamaguchi University assisted in meeting the publication costs of this
article.
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