Electrochemical behavior of Ni(II)/Ni in a hydrophobic amide-type room-temperature ionic liquid

Article abstract of DOI:10.1016/j.electacta.2009.07.088

Electrochemical reaction of Ni(II)/Ni was investigated in a hydrophobic room-temperature ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) containing Ni(TFSA)₂ as a Ni source. The UV-vis spectra showed that

Full text of DOI:10.1016/j.electacta.2009.07.088

Electrochimica Acta 54 (2009) 7502–7506
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Electrochemical behavior of Ni(II)/Ni in a hydrophobic amide-type
room-temperature ionic liquid
Yan-Li Zhu, Yukari Kozuma, Yasushi Katayama ,1, Takashi Miura
∗
Department of Applied Chemistry, Faculty of Science and Technology, Keio University, Hiyoshi 3-14-1, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 17 June 2009
Received in revised form 31 July 2009
Accepted 31 July 2009
Available online 11 August 2009
Electrochemical reaction of Ni(II)/Ni was investigated in a hydrophobic room-temperature ionic liquid, 1-
butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) containing Ni(TFSA)2 as a Ni
−
source. The UV–vis spectra showed that Ni(II) in BMPTFSA is octahedrally coordinated with TFSA anions.
−1
The average activation energy for the diffusion coefficients of this Ni(II) complex was ∼26 kJ mol , which
−
8
2
−1
.
was close to that for the viscosity. The diffusion coefficient of Ni(II) was estimated to be 9.3 × 10 cm s
Chronoamperometric measurements showed that the electrodeposition of Ni on a platinum substrate
involved three-dimensional instantaneous nucleation under diffusion control at room-temperature. The
electrodeposits obtained by galvanostatic electrolysis with the current density of −0.046 mA cm at 70
Keywords:
Ionic liquid
Nickel
−
2
◦
and 100 C were identified as metallic Ni by XRD.
1
-Butyl-1-methylpyrrolidinium
bis(trifluoromethylsulfonyl)amide
Room-temperature molten salt
Electrodeposition
© 2009 Elsevier Ltd. All rights reserved.
1
. Introduction
The electrodeposited nickel is mainly used in the corrosion- and
ficult to obtain Ni alloys containing the metals less noble than Al or
Zn. Thus, the ionic liquids containing no metal species are promis-
ing for electrodeposition of Ni alloys containing various metals.
−
wear-resistant coating industry. Recently, Ni or its alloy, such as
Ni–Co, Ni–Fe and Ni–W, has been also applied into the produc-
tion of micro-mechanical devices, fabricating magnetic materials
for data storage and magnetic sensors, and catalytic industry,
which requires a better control of the morphologies of the deposits
Bis(trifluoromethylsulfonyl)amide (TFSA )-based ionic liquids are
expected to be preferable for electrodeposition of various metals
including Ni since these ionic liquids are stable against moisture
and immiscible with water.
−
Among the TFSA -based ionic liquids, the system containing
+
[
1–6]. It is possible to obtain Ni or its alloy electrochemically from
1-butyl-1-methylpyrrolidinium (BMP ) received much attention
aqueous solutions. However, it is difficult to prevent hydrogen
evolution, which leads to the low current efficiency and worse
quality of deposits due to the hydrogen embrittlement. Some
room-temperature ionic liquids (RTIL) are expected to be the alter-
native aprotic media for electrodeposition of various metals, since
they generally have several distinctive properties, such as the
wide potential windows, acceptable conductivity compared to non-
because of its low melting point and high cathodic stability
[8,21]. From this electrolyte, some metals have been electrode-
posited successfully. Dense and adherent deposit of aluminum is
an example [22,23]. The grain size of the deposit in the nanometer
+
regime was smaller than those obtained from EMIMTFSA (EMIM :
1-ethyl-3-methylimidazolium), which was in the micrometer
range. Bebensee et al. also obtained the nanocrystalline alu-
minum from the same ionic liquid [24]. Some other deposits,
such as semiconductor silicon [25,26], transition metals of tan-
talum and copper, selenium and indium [27–29] were obtained
from this kind of ionic liquid. The electrodeposit of amorphous
manganese on tungsten, nickel, or copper foils was identified
by the energy-dispersive spectrometer [30,31]. The current effi-
ciency was higher than 97% regardless of the applied potential.
However, the electrodeposition of Ni has not been reported in
this ionic liquid. In the present study, the electrochemical reac-
tion of Ni(II)/Ni was investigated in 1-butyl-1-methylpyrrolidinium
bis(trifluoromethylsulfonyl)amide (BMPTFSA) at various tempera-
tures.
◦
aqueous electrolytes and negligible vapor pressure (up to 300 C)
[
7,8].
The electrodeposition of Ni has been mainly studied in acidic
chloroaluminate or chlorozincate ionic liquids [9–18] except the
−
−
choline chloride–urea eutectic [19] and the DCA -based (DCA :
dicyanamide) ionic liquids [20]. In case of chloroaluminate or
chlorozincate ionic liquids, codeposition of Al or Zn makes it dif-
∗ Corresponding author. Tel.: +81 45 566 1561; fax: +81 45 566 1561.
E-mail address: katayama@applc.keio.ac.jp (Y. Katayama).
ISE member.
1
0
013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2009.07.088

Y.-L. Zhu et al. / Electrochimica Acta 54 (2009) 7502–7506
7503
Handling of all the hygroscopic reagents and electrochemical
experiments were performed in an argon-filled glove box with a
continuous gas purification apparatus (Miwa Seisakujyo Co. Ltd.,
DB0-1K-SH). The concentrations of H O and O2 in the gas were
2
kept under 0.8 and 1 ppm, respectively.
The absorption spectra of BMPTFSA containing Ni(TFSA)2 was
measured by using an air-tight quartz cell (light path length 0.1 cm)
with the aid of UV–vis spectrometer (JASCO, V-530). The elec-
trodeposits washed with ethanol (Wako, >99.8%) and dried in the
air were characterized by a scanning electron microscope (SEM,
KEYENCE VE-9800), X-ray diffractometer (XRD, Rigaku RAD-C, Cu
K␣), X-ray photoelectron spectrometer (XPS, JOEL JPPS-9000 MC)
and energy-dispersive X-ray analysis (EDX, EDAX Phoenix).
3. Results and discussion
3.1. Solvation structure of Ni(II) in BMPTFSA
The coordination chemistry is an important factor for the elec-
trochemical behavior of metal ions in ionic liquids. Gale et al.
12] has reported that reduction of Ni(II) species is possible in
[
Fig. 1. UV–vis spectra of 0.1 M Ni(TFSA)2/BMPTFSA and Ni(TFSA)2/H2O.
acidic chloroaluminate ionic liquids while divalent Ni cations are
likely to form a chlorocomplex, [NiCl4]2 , in basic ionic liquids,
and the complex cannot be reduced to Ni within the electro-
chemical potential window. Fig. 1 shows the absorption spectra
−
2
. Experimental
of 0.1 M Ni(TFSA) /BMPTFSA and Ni(TFSA) /H O at room temper-
2
2
2
ature. The assignments of the absorption bands are summarized
in Table 1. The maximum absorption wavelength (ꢀmax = 429)
and molar absorption coefficient (ε = 18) of 0.1 M Ni(TFSA)2 in
BMPTFSA were close to those in water, suggesting the diva-
lent Ni cations are octahedrally coordinated by six oxygen atoms
BMPBr was prepared by the reaction of 1-methylpyrrolidine
Tokyo Kasei, >95%) and butylbromide (Tokyo Kasei, >98%) in dehy-
(
drated acetonitrile (Junsei Chemical, 99.5%) at room-temperature,
then purified by recrystallization and finally dried under vacuum
◦
at 120 C for 24 h. BMPTFSA was obtained by reaction of BMPBr
−
+
of TFSA anions, as reported in Ni(TFSA) /EMITFSA (EMI : 1-
with LiTFSA (Kanto Kagaku, >99.98%) in deionized water, separated
by extracting into dichloromethane (Junsei Chemical, 99.0%), and
2
ethyl-3-methylimidazolium) [33]. Thus, the dissolved Ni species
−
◦
is considered as some anionic complexes like [Ni(TFSA) ] .
then finally dried under vacuum at 120 C for 24 h. The water con-
3
tent in BMPTFSA was below 10 ppm, which was determined by
Karl Fischer titration (Metrohm, 831 KF Coulometer). Ni(TFSA) was
3.2. Cyclic voltammetry
2
prepared by reacting NiCO ·Ni(OH) ·4H O (Kanto Chemical) with
3
2
2
◦
HTFSA (Morita Kagaku, >99%) at 80 C under agitation, filtrated to
eliminate the unreacted inorganic metal salts, and then dried under
Fig. 2 shows the cyclic voltammograms of a Pt electrode in 0.1 M
Ni(TFSA) /BMPTFSA with various scan rates at room-temperature
2
◦
◦
vacuum at 160 C for 24 h [32]. The product was yellow solid pow-
(25 C). A cathodic and anodic current peak were observed at about
der. The yellow electrolyte was obtained by dissolving a certain
amount of Ni(TFSA)2 into BMPTFSA.
−1.7 and +0.6 V, respectively. The cathodic and anodic peaks only
appeared in the presence of Ni(II). Moreover, the deposit obtained
by the potentiostatic cathodic reduction at −1.9 V was confirmed as
metallic Ni by XPS and EDX. Thus, the cathodic peak was ascribed
to the reduction of Ni(II) to Ni(0) while the anodic current peak
was assignable to the anodic dissolution of the Ni deposited dur-
ing the preceding cathodic scan. Since the cathodic peak potential
shifted to more negative side with increasing the scan rates and the
peak separation exceeded 2.0 V, the electrode reaction of Ni(II)/Ni
in BMPTFSA was regarded as electrochemically irreversible. As seen
in Fig. 2, the integrated electric charges of the oxidation peak were
smaller than those of the reduction ones, suggesting the elec-
trodeposited Ni during the cathodic scan could not be stripped
completely during the anodic scan, probably because the concen-
tration of Ni(II) near the electrode increased up to its saturated
concentration quickly resulting in the hindrance of the anodic
dissolution. The similar behavior was also known for the cyclic
voltammetry of Mn [30] and Co [35] in BMPTFSA.
A three-electrode cell was used for all the electrochemi-
cal measurements and electrodeposition experiments with the
aid of a potentiostat/galvanostat (Hokuto Denko, HABF-501). For
the electrochemical measurements, a platinum disk electrode
−
3
2
(
7.85 × 10 cm ) was employed as a working electrode after mir-
ror polishing. Platinum wire was used as a counter electrode. Silver
wire immersed in BMPTFSA containing 0.1 mol dm
−
3
AgCF SO3
3
(
Aldrich, >99.0%) was used as a reference electrode, which was
separated from main electrolyte by porous Vycol glass. All the
potentials in this paper were referred to this Ag/Ag(I) electrode.
For the electrodeposition experiments, a copper disk electrode
–
2
2
(
7.06 × 10 cm ) was used as a substrate after mirror polishing,
electrolytic degreasing in alkali solution and washing with 10 vol%
H SO and distilled water. Ni (Nilaco) wire was employed as a
2
4
counter electrode. The reference electrode was as the same as
described above.
Table 1
UV–vis spectra data (ꢀ: nm, ε: mol dm3 cm ).
−
1
−1
Ni(TFSA)2 in BMPTFSA
Ni(TFSA)2 in H2O [34]
NiCl2 in basic chloroaluminate ionic liquid [12]
Assignment [34]
3
3
1
3
4
8
6
29 nm (ε = 18)
15 nm (ε = 7.4)
86 nm (ε = 4.2)
395 nm (ε = 5.2)
724 nm (ε = 2.1)
658 nm (ε = 2.0)
705 nm (ε = 175)
T1g ← A2g
3
658 nm (ε = 169)
T1g ← A2g
–
Eg ← 3A2g

7
504
Y.-L. Zhu et al. / Electrochimica Acta 54 (2009) 7502–7506
Fig. 2. Cyclic voltammograms of a Pt electrode in 0.1 M Ni(TFSA)2/BMPTFSA with
various scan rates at room-temperature. The dotted curve shows the cyclic voltam-
mogram in neat BMPTFSA.
Fig. 4. XRD patterns of electrodeposited Ni films on copper substrates in
.1 M Ni(TFSA)2/BMPTFSA by galvanostatic electrolysis with a current density of
−0.046 mA cm (electric charge: 3.6 C cm ).
0
−
2
−2
Fig. 3 shows the cyclic voltammograms of a Pt electrode in
◦
0
.1 M Ni(TFSA) /BMPTFSA at various temperatures up to 150 C.
2
The potential of the cathodic peak shifted to the more positive side
and the separation between the cathodic and anodic peaks became
smaller with elevating temperature, indicating that the overpo-
tential for electrodeposition of Ni in BMPTFSA decreased at higher
temperature. This is not surprising because at higher temperature
the charge transfer and mobility of the electroactive species will
increase while the nucleation overpotential will be decrease [29].
bright, and identified as metallic Ni by XRD, as shown in Fig. 4. The
width of the diffraction peaks slightly decreased with an increase
in temperature, indicating that the grain size of the deposit became
bigger at higher temperature. According to the Scherrer equation,
◦
the average grain sizes were about 10 and 15 nm at 70 and 100 C,
3.3. Electrodeposition of Ni
Electrodeposition of Ni on
a
copper substrate in 0.1 M
◦
Ni(TFSA) /BMPTFSA at 50, 70 and 100 C was performed by gal-
2
−
2
vanostatic electrolysis with a current density of −0.046 mA cm
.
◦
A black deposit obtained at 50 C was confirmed as nickel by XPS,
but gave no XRD peak assignable to Ni probably due to the small
grain size, as shown in Fig. 4. From the image of the deposit in Fig. 5,
there were a few cracks. The fine cracks were also found during
◦
◦
the electrodeposition of tantalum in BMPTFSA even at 200 C [27].
On the other hand, the deposits obtained at 70 and 100 C were
Fig. 5. SEM image of the deposit on a Cu substrate by the galvanostatic elec-
trodeposition in 0.1 M Ni(II)/BMPTFSA at 50 and 70 ◦C. The current density was
Fig. 3. Cyclic voltammograms of a Pt electrode in 0.1 M Ni(TFSA)2/BMPTFSA at var-
ious temperatures (scan rate: 30 mV s ).
−1
−2
−0.046 mA cm
.

Y.-L. Zhu et al. / Electrochimica Acta 54 (2009) 7502–7506
7505
Fig. 8. Dimensionless plots of (j/jm)2 vs. t/tm using the data of Fig. 6 (instantaneous
- - -), progressive (—)).
(
−
8
2
−1
be 7.5 × 10 cm s according to the Sand’s equation [36], which
◦
Fig. 6. Chronoamperograms of a Pt electrode in 0.1 M Ni(TFSA)2/BMPTFSA at 25 C.
was close to that from chronoamperometry. The values are smaller
−
6
than those reported in chloroaluminate ionic liquid (2.8 × 10 [37]
respectively. These values were smaller than those of the electrode-
posited Ni obtained from imidazolium-based ionic liquid (about
0–40 nm at room-temperature) [20]. A possible explanation for
this difference may be that the pyrrolidinium cations are adsorbed
on the Ni deposit, acting as a grain refiner during metal electrode-
position [22].
−7
2
−1
and 2.1 × 10 cm s [15]), since the viscosity of the BMPTFSA is
higher than those of the chloroaluminate ionic liquids. However,
the diffusion coefficient of Ni(II) is close to those of Co(II), Fe(II)
and Sn(II) in BMPTFSA [33,38,39].
3
As shown in Fig. 6, characteristic current peaks were observed
after the initial charging current for each step. By fitting those cur-
rent transient data to the Shaifker nucleation and growth model
3
.4. The diffusion coefficient of Ni(II) and nucleation/growth of Ni
[40], the relationship between the non-dimensional parameters,
2
deposit
(j/jm
)
and t/tm, where jm is the peak current density, t the time
and tm is the time at jm, was shown in Fig. 8. Compared to the theo-
retical model, the experimental plots can be explained better by an
instantaneous nucleation rather than progressive, suggesting that
the initial stage of Ni deposition on a Pt electrode can be regarded
as the instantaneous nucleation under diffusion control.
Fig. 6 shows the chronoamperograms of a Pt electrode in
BMPTFSA containing 0.1 M Ni(II). Current peaks characteristic of
nucleation process were observed after the charging of the electric
double layer. After the peaks, the current densities converged with
the elapse of time, indicating that the reduction of Ni(II) is satisfied
the Cottrell’s criteria and can be regarded as the diffusion-limited
process. According to the Cottrell’s equation for semi-infinite one-
dimensional diffusion [36], the diffusion coefficient of Ni(II) was
Fig. 9 shows the diffusion coefficients and viscosities of 0.1 M
Ni(TFSA) /BMPTFSA at various temperatures. The diffusion coeffi-
2
cient increased with elevating temperature, obeying the Arrhenius’
law. The activation energy for the diffusion coefficients of 0.1 M
−
8
2
−1
−1
estimated to be 9.3 × 10 cm s . From the chronopotentiograms
Ni(II) in BMPTFSA was 26 kJ mol , which is close to those for the
−
1
shown in Fig. 7, the diffusion coefficient of Ni(II) was estimated to
iron complexes in BMPTFSA (19–29 kJ mol ) [41]. According to
Fig. 9. Dependence of the logarithm of the diffusion coefficient and the viscosity of
0.1 M Ni(TFSA)2/BMPTFSA on the reciprocal of temperature.
◦
Fig. 7. Chronopotentiograms of a Pt electrode in 0.1 M Ni(TFSA)2/BMPTFSA at 25 C.

7
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Y.-L. Zhu et al. / Electrochimica Acta 54 (2009) 7502–7506
the Andrade’s formula (Á = Aexp(Ea/RT), A: constant), the viscosities
decreased with elevating temperature. The viscosities of BMPTFSA
containing 0.1 M Ni(II) can be fitted to the Andrade’s formula and
the activation energy was estimated to be 29 kJ mol , which is
consistent with that for diffusion coefficients.
[9] C.A. Zell, W. Freyland, Chem. Phys. Lett. 337 (2001) 293.
10] S.P. Gou, I.W. Sun, Electrochim. Acta 53 (2008) 2538.
11] O. Mannetal, G.B. Pan, W. Freyland, Electrochim. Acta 54 (2009) 2487.
12] R.J. Gale, B. Gilbert, R.A. Osteryoung, Inorg. Chem. 18 (1979) 2723.
[13] R.P. William, C.L. Hussey, J. Electrochem. Soc. 143 (1996) 130.
14] T.P. Moffat, J. Electrochem. Soc. 141 (1994) 3059.
[
[
[
−
1
[
[
[
15] O. Mann, W. Freyland, J. Phys. Chem. C 111 (2007) 9832.
16] M.R. Ali, A. Nishikata, T. Tsuru, J. Electroanal. Chem. 513 (2001) 111.
4
. Conclusions
The divalent Ni, presumably existing as [Ni(TFSA)3 ], can be
[17] W. Freyland, C.A. Zell, S.Z.E. Abedin, Electrochim. Acta 48 (2003) 3053.
[
[
18] T.C. Richard, J. Electrochem. Soc. 145 (1998) 1598.
19] A.P. Abbott, K.E. Ttaib, K.S. Ryder, E.L. Smith, Trans. Inst. Met. Finish. 86 (2008)
−
234.
reduced to metallic Ni in a hydrophobic room-temperature ionic
liquid, BMPTFSA. The diffusion coefficients of Ni(II) from chronoam-
perometric and chronopotentiometric techniques were smaller
than those reported in chloroaluminate ionic liquid probably due to
the higher viscosity of the BMPTFSA. Chronoamperometric exper-
iments showed that the initial stage of the electrodeposition of
Ni on a Pt electrode surface involved 3D instantaneous nucle-
ation/growth under diffusion control. The results revealed in this
study indicate that the BMPTFSA ionic liquid can be a promising
electrolyte for the electrodeposition of various Ni alloys.
[20] M.J. Deng, I.W. Sun, P.Y. Chen, J.K. Chang, W.T. Tsai, Electrochim. Acta 53 (2008)
5812.
[
21] D.R. MacFarlane, J. Sun, J. Golding, P. Meakin, M. Forsyth, Electrochim. Acta 45
2000) 1271.
(
[22] S.Z.E. Abedin, E.M. Moustafa, R. Hempelmann, H. Natter, F. Endres, Chem. Phys.
Chem. 7 (2006) 1535.
[
23] S.Z.E. Abedin, E.M. Moustafa, R. Hempelmann, H. Natter, F. Endres, Electrochem.
Commun. 7 (2005) 1111.
[
24] F. Bebensee, L. Klarhofer, W. Maus-Friedrichs, F. Endres, Sur. Sci. 601 (2007)
3769.
[
[
[
[
25] N. Borisenko, S.Z.E. Abedin, F. Endres, J. Phys. Chem. B 110 (2006) 6250.
26] S.Z.E. Abedin, N. Borisenko, F. Endres, Electrochem. Commun. 6 (2004) 510.
27] S.Z.E. Abedin, U.W. Biermann, F. Endres, Electrochem. Commun. 7 (2005) 941.
28] S.Z.E. Abedin, H.K. Farag, E.M. Moustafa, U. Welz-Biermanny, F. Endres, Phys.
Chem. Chem. Phys. 7 (2005) 2333.
Acknowledgments
[
29] S.Z.E. Abedin, A.Y. Saad, H.K. Farag, N. Borisenko, Q.X. Liu, F. Endres, Electrochim.
Acta 52 (2007) 2746.
The present work was financially supported by Grant-in-Aid for
Scientific Research on Priority Areas of “Science of Ionic Liquids”
[30] M.J. Deng, P.Y. Chen, I.W. Sun, Electrochim. Acta 53 (2007) 1931.
[
[
[
31] J.K. Chang, C.H. Huang, W.T. Tsai, M.J. Deng, I.W. Sun, P.Y. Chen, Electrochim.
Acta 53 (2008) 4447.
32] T. Katase, T. Onishi, S. Imashuku, K. Murase, T. Hirato, Y. Awakura, Electrochem-
istry 73 (2005) 686.
(
No. 17073016) from the Ministry of Education, Culture, Sports,
Science, and Technology, Japan (MEXT).
33] K. Fujii, T. Nonaka, Y. Akimoto, Y. Umebayashi, S. Ishiguro, Anal. Sci. 24 (2008)
1
377.
References
[
[
34] B.N. Figgis, Introduction to Ligand Fields, Interscience, 1966.
35] R. Fukui, Y. Katayama, T. Miura, Electrochemistry 73 (2005) 567.
[1] A. Lachenwitzer, S. Morin, O.M. Magnussen, R.J. Behm, Phys. Chem. Chem. Phys.
[36] A.J. Bard, L.R. Faulkner, Electrochemical Methods Fundamentals and Applica-
tions, 2nd ed., John Wiley & Sons, Inc., New York, 2001.
[37] W.R. Pitner, C.L. Hussey, G.R. Stafford, J. Electrochem. Soc. 143 (1996) 130.
[38] M. Yamagata, N. Tachikawa, Y. Katayama, T. Miura, Electrochemistry 73 (2005)
564.
[39] N. Tachikawa, N. Serizawa, Y. Katayama, T. Miura, Electrochim. Acta 53 (2008)
6530.
[40] B. Scharifker, G. Hills, Electrochim. Acta 28 (1983) 879.
[41] N. Tachikawa, Y. Katayama, T. Miura, J. Electrochem. Soc. 154 (2007) F211.
3
(2001) 3351.
[
[
[
[
[
[
[
2] S. Sam, G. Fortas, A. Guittoum, N. Gabouze, S. Djebbar, Sur. Sci. 601 (2007) 4270.
3] J.Y. Park, M.G. Allen, J. Micromech. Microeng. 8 (1998) 307.
4] J.L. McCrea, G. Palumbo, G.D. Hibbard, U. Erb, Rev. Adv. Mater. Sci. 5 (2003) 252.
5] H. Alimadadietal, Mater. Des. 30 (2009) 1356.
6] D.B. Lee, J.H. Ko, S.C. Kwon, Mater. Sci. Eng. A 380 (2004) 73.
7] A.P. Abbott, K.J. McKenzie, Phys. Chem. Chem. Phys. 8 (2006) 4265.
8] Y. Katayama, R. Fukui, T. Miura, J. Electrochem. Soc. 154 (2007) D534.