Microgravity effects on the electrochemical oxidation of ammonia: A parabolic flight experiment

Article abstract of DOI:10.1016/j.electacta.2012.04.079

The diffusion of molecular species through nanoporous systems in microgravity conditions is of interest due to the implications of nanotechnology in aerospace technologies. Herein we present the electrochemical oxidation of ammonia at Pt nanoparticles/nano-supporting electrode systems. The decomposition of ammonia has become of critical importance since it is a common component in aqueous streams. The experiments were carried out in a parabolic flight by using different materials to test the electrooxidation of ammonia. Linear polarization curves were attained in microgravity for all materials. In general, a reduction in catalytic performance of 20-65% was observed.

Full text of DOI:10.1016/j.electacta.2012.04.079

Electrochimica Acta 75 (2012) 88–93
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Microgravity effects on the electrochemical oxidation of ammonia: A parabolic
flight experiment
Eduardo Nicolaua,b, Carlos M. Poventud-Estradaa,b, Lisandra Arroyoa,b, José Fonsecaa,b, Michael Flynnc,
a,b,∗
Carlos R. Cabrera
a
Department of Chemistry, University of Puerto Rico, Río Piedras Campus, PO Box 23346, San Juan, PR 00931-3346, USA
NASA-URC Center for Advanced Nanoscale Materials, PO Box 23346, San Juan, PR 00931-3346, USA
NASA Ames Research Center, Bioengineering Branch, Moffett Field, CA 94036, USA
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
The diffusion of molecular species through nanoporous systems in microgravity conditions is of interest
due to the implications of nanotechnology in aerospace technologies. Herein we present the electrochem-
ical oxidation of ammonia at Pt nanoparticles/nano-supporting electrode systems. The decomposition of
ammonia has become of critical importance since it is a common component in aqueous streams. The
experiments were carried out in a parabolic flight by using different materials to test the electrooxida-
tion of ammonia. Linear polarization curves were attained in microgravity for all materials. In general, a
reduction in catalytic performance of 20–65% was observed.
Received 12 February 2012
Received in revised form 19 April 2012
Accepted 20 April 2012
Available online 27 April 2012
Keywords:
Microgravity
Ammonia oxidation
Pt nanoparticles
Electrocatalysis
Parabolic flight
©
2012 Elsevier Ltd. All rights reserved.
1
. Introduction
The electrochemical oxidation of ammonia is a mass trans-
fer controlled reaction and this paper describes an assessment of
the effects on a linear polarization under microgravity conditions.
Water electrolysis also has been proposed as a way to generate
hydrogen to be used as fuel in polymer electrolyte membrane fuel
cells (PEMFC) and it has been investigated under microgravity con-
ditions. These investigations have shown that the three-phase zone
between the electrode, gas bubbles and electrolyte plays a key
role in microconvection [15,16]. Kaneko et al. investigated water
electrolysis in microgravity by parabolic flight and found that gas
bubbles become larger and current densities decreased in micro-
gravity. This suggests that the mass transfer of water molecules
to the electrode controls water electrolysis. However, under nor-
mal gravity conditions water electrolysis is kinetically limited [17].
When methanol is employed as fuel in a fuel cell under micro-
gravity, a similar effect is observed as when water electrolysis is
performed, this is enlargement of gas bubbles and a decrease in
overall cell performance [18]. Matsushima et al. indicated that in
alkaline solutions, current density is much smaller under micro-
gravity than under terrestrial gravity, and that ohmic resistance
increases in microgravity due to the formation of a gas bubble froth
layer and as well as an increase in electrode surface blockage by
bubbles [15,19]. Iwasaki et al. found that the electrolysis current on
the ground reached a steady state faster than that under micrograv-
ity conditions. Since the diffusional mass transfer was predominant
over the process in microgravity, the characteristic time was longer
Experimentation in microgravity imposes a buoyancy-free and
thus macroconvection-free environment. Due to this quiescent
condition, researchers have shown interest to study under such
environment several processes within the fields of biology [1],
crystal growth [2], human health [3], nanotechnology [4], and elec-
trochemistry [5]. Within the field of electrochemistry, its impact on
energy production is of particular importance. In an effort to find
alternative means to generate electrical energy, fuel cells have been
proposed with the use of different fuels such as: ethanol, methanol,
ammonia and others. Particularly, ammonia has been recently con-
sidered as a possible substitute to hydrogen and the next generation
fuel due to its high energy density (12.6 MJ L ), and easiness of
storage and transportation [6]. For this reason, several recent inves-
tigations have focused on the development of catalysts [7–10] for
electrolyzers [11,12], and microreactor [7] applications for the pro-
duction of hydrogen from ammonia [6,13] or as direct ammonia fuel
cells [14].
−1
∗ Corresponding author at: Department of Chemistry, University of Puerto Rico,
Río Piedras Campus, PO Box 23346, San Juan, PR 00931-3346, USA.
Tel.: +1 787 76400001x4807; fax: +1 787 756 8242.
E-mail address: carlos.cabrera2@upr.edu (C.R. Cabrera).
0
013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.electacta.2012.04.079

E. Nicolau et al. / Electrochimica Acta 75 (2012) 88–93
89
Fig. 1. The Electrochemical Microgravity Laboratory (EML) is comprised of three major components: (a) Teflon custom-made electrochemical cells; each have 5 cavities in
TM
total, 3 for the electrodes and 2 inlet valves. (b) Electronics rack (ElR) to control the electrochemical operation and (c) the double contained Makrolon box containing the
cells built according to NASA specifications for flight.
than that of the process on the ground where macroconvection is
predominant [20].
The standard potential for this reaction is 0.66 V vs. NHE. The
N3 species formed in this reaction would react at the potentials
where the ammonia oxidation is reported to form N2.
−
In terms of the ammonia electrooxidation the most accepted
mechanism is the one from Gerisher and Maurer where Nads is a
poisonous intermediate that blocks the surface while the adsorbed
NH and NH2 species are the active intermediates to finally form
nitrogen [21]. The mechanism is as follows:
−
−
Pt + N3 ↔ PtN3
(8)
(9)
−
−
−
PtN3 + 2H O + e ↔ PtNH + 2OH + N2
2
2
In 2005 Endo et al. investigated the electrochemical oxidation
of ammonia by means of rotating disc electrode. In this research,
the formation of intermediate species such as NOx is proposed,
however no experimental evidence was developed [23]. The elec-
trochemical oxidation of ammonia has been recently reviewed and
these and other mechanisms are discussed [24].
NH3(aq) → NH3ads
(1)
(2)
(3)
(4)
(5)
(6)
NH3ads + OH− → NH2ads + H O + e−
2
NH2ads + OH− → NHads + H O + e−
2
NHx,ads + NHy,ads → N H
2
x+y,ads
2
2
2
. Experimental
+ (x + y)OH− → N + (x + y)H O + (x + y)e
−
N H
2
x+y,ads
2
2
.1. Materials and apparatus
NHads + OH− → Nads + H O + e−
2
.1.1. Parabolic flight details
Other researchers have proposed alternative mechanisms for
this reaction. In 2006 Vidal-Iglesias et al. provided evidence on the
formation of an azide anion (N3 ) in the electrochemical oxidation
The experiments were conducted using a parpabolic micrograv-
ity aircraft. These were conducted as part of a NASA parabolic flight
campaign: NASA Minority Institution Flight Week Program at the
NASA Johnson Space Center in Ellington Field, Houston, TX. The air-
craft used during this campaign was a Boeing 727-200 of the Zero
Gravity Corporation. An average gravity of 0.006g was achieved for
an average window of 14.5 s in each of the 30 parabolic flights per
day. Two days of flights were conducted.
−
of ammonia by means of surface enhanced Raman-spectroscopy
[
22]. In step 5, the N H
species are capable of reacting with
2
4,ad
ammonia in the alkaline medium to produce the azide as follows:
NH + N H + 7OH → N3− + 7H O + 6e−
−
(7)
3
2
4
2

9
0
E. Nicolau et al. / Electrochimica Acta 75 (2012) 88–93
2.1.2. Electrodes preparation
The electrochemical oxidation of ammonia was tested under
microgravity by employing electrodes of different composition.
In this campaign three different electrodes were used to test the
microgravity effects on the electrochemical oxidation of ammonia.
These were labeled as: Pt-GCE, Pt/C, and Pt-H /C. For the Pt-GCE a
2
glassy carbon electrode was immersed in a three-electrode electro-
chemical cell containing a 1 mM K PtCl /0.5 M H SO solution with
2
6
2
4
a Ag/AgCl (0.197 V vs. NHE) as a reference and a platinum wire as
counter electrode. A HCP-803 potentiostat from Bio-Logic USA was
used to cycle the potential between −0.2 V and 1.0 V vs. Ag/AgCl
at 300 mV/s for 25 cycles to generate a platinum electrodeposited
glassy carbon electrode (Pt-GCE). For the Pt/C electrode 1 mg of
2
0 wt.% platinum on carbon from Johnson Matthey was weighed
TM
and mixed with 200 ␮L of isopropanol and 8 ␮L of a 5% wt. Nafion
solution from Sigma–Aldrich. After sonicating and magnetic stir-
ring of this mixture for 2 h, 8 ␮L were drop-casted over a glassy
carbon electrode and left overnight. For the Pt-H /C, 500 mg of Vul-
2
can XC-72R were weighed and mixed with 91.78 mg of K PtCl and
2
4
Fig. 2. Average of gravitational acceleration as a function of time during parabolic
flight.
3
.80 mL of a 0.1 M solution of sodium polyacrylate (2100 g/mol,
Sigma–Aldrich). Thereafter, this mixture was thoroughly mixed
with deionizezed water and placed in a bottom flask under nitrogen
atmosphere for 2 h. Then, the mixture was placed under hydrogen
gas atmosphere for 2 h to promote platinum reduction [25]. After
completing the platinum reduction the mixture was left covered
overnight and then it was filtered several times with nanopure
3
. Results and discussion
The main objective of this investigation is to observe the
effects on the electrochemical oxidation of ammonia at nano-
based electrodes under microgravity; Fig. 2 shows the gravitational
acceleration reading during the experiments that confirms the
microgravity environment. Previous investigations have demon-
strated that ammonia electrooxidation takes place between ca. 0.45
and 0.6 V vs. RHE, where the active intermediates (i.e. NHads and
◦
water (18.3 Mꢀ, Barnstead), and subsequently dried at 80 C for
2
4 h under vacuum. Then, 1 mg of the resulting catalyst was mixed
TM
with isopropanol and Nafion
solution and drop-casted in the
same way as for the Pt-Vulcan electrode. The charge under the
hydrogen desorption peaks, in presence of a 0.5 M sulfuric acid
solution, was calculated and the electrochemical active surface area
NH2,ads) are produced to form N . More anodic potentials up to
2
2
2
a value of ca. 0.8 V vs. RHE is known to form nitrogen adsorbates
(Nads). The results of this experiment demonstrated that in a micro-
gravity environment the diffusion of products and reactants to and
from the catalyst and inside the pores of the catalyst are affected.
Fig. 3 presents the graphical experimental results for an electrode of
Pt reduced by the hydrogenation method and supported by Vulcan
for each electrode was determined (i.e. 0.14 cm (Pt-GCE), 1.1 cm
2
(
Pt-H ) and 1.0 cm (Pt/C)). All of the shelf materials were used as
2
received and without further purification.
2.1.3. Test apparatus
(Pt-H /C). In Fig. 3 the difference in currents when the electrochem-
2
An electrochemical microgravity laboratory (EML) shown in
ical cell is operated under microgravity versus the ground control is
shown. At a potential of ca. 0.7 V vs. RHE the electrochemical oxida-
tion of ammonia to nitrogen is clearly presented. From this figure it
Fig. 1 was constructed by following specific guidelines from NASA.
In brief, three independent Teflon made electrochemical cells were
placed in a double contained MakrolonTM box. These three elec-
trochemical cells were independently controlled from outside the
box via an electronics rack (ER). This electronics rack contained
an SP-50 potentiostat from Bio-Logic USA to control the experi-
ments inside the boxes. The three electrochemical cells were fixed
to the larger box and all had a silver wire as a pseudo-reference
electrode and a platinum wire as the counter electrode. The whole
apparatus was fixed via bolts to the airplane and it was designed
and made to withstand up to 9g of acceleration in a triple contained
environment. An accelerometer was used to monitor gravitational
acceleration.
2.2. Ammonia electrooxidation under microgravity
A 1 M solution of NH OH (Acros Organics) adjusted to pH 10
4
with HCl was used to test the electrochemical oxidation of ammo-
nia. The accelerometer was set as a trigger and when 0.02g were
achieved the experiment was initiated. To promote ammonia elec-
trooxidation a potential sweep between 0.0 V to a final potential
of 0.8 V vs. RHE at 40 mV/s was executed. After each parabola, the
cells were allowed to equilibrate during the resting period, which
usually was around 1–3 min. In this publication potentials were
corrected to reflect it as a relative hydrogen electrode (RHE).
Fig. 3. Linear polarization of a Pt-H2/C electrode in 1 M NH4OH at pH 10 at 40 mV/s
under normal gravity conditions (black line) and microgravity (red line). (For inter-
pretation of the references to color in this figure legend, the reader is referred to the
web version of the article.)

E. Nicolau et al. / Electrochimica Acta 75 (2012) 88–93
91
Fig. 4. Schematic representation of the ammonia electrochemical oxidation effect in microgravity and ground.
is noticeable the difference in the ammonia oxidation current when
in microgravity vs. the ground control. Thus, when in microgravity
the adsorbed ammonia molecules (NH3,ads) that occurs between ca.
0.7 V vs. RHE one can observe the peak indicating the electrochem-
ical oxidation of ammonia, and at a higher potential of ca. 1.1 V the
same broad peak as in Fig. 3 is observed. However, the difference
in the current ratio between the ammonia oxidation peak (0.7 V vs.
RHE) and the nitrogen adsorbates oxidation peak (ca. 1.1 V vs. RHE)
under normal gravity conditions is 0.20, under microgravity is 0.17
and there corresponding ratio (g’s/␮g’s) is 1.176. This is different
0.3 and 0.4 V are immediately oxidized when the anodic potential
of 0.45 V is reached. In contrast to conditions of normal gravity, the
nitrogen molecules that are formed by oxidation of ammonia have
insufficient buoyancy to leave the surface readily, and thus block
the approach of further ammonia molecules, see Fig. 4. This leads to
a significant decrease in the anodic current from the ammonia oxi-
dation. Also from this figure another broad peak at higher potential
values (ca. 1.1 V vs. RHE) is observed. This signal has been ascribed
to the oxidation of nitrogen adsorbates (Nads) leading to the for-
mation of an “oxynitride” layer, mostly composed of NOx and NxO
than the catalyst prepared by the colloidal method (Pt-H ). For the
2
Pt-H /C this current ratio was 0.26 when comparing normal gravity
2
conditions (0.045) against microgravity (0.17). These results sug-
gest that Pt/C-H2 has a higher affinity for the nitrogen adsorbates
and the formation of the oxynitride layer when compared to the
Pt/C catalyst.
[
26,27]. However, in microgravity these adsorbates are more pro-
A third material was tested in microgravity, electrodeposited
platinum nanoparticles over a glassy carbon electrode (Pt-GCE).
Fig. 6 shows the electrochemical oxidation of ammonia for this
material. At a potential of 0.62 V vs. reference one can observe
the peak indicating the electrochemical oxidation of ammonia,
and at a higher potential of around 1.1 V vs. RHE the signals
ascribed to the nitrogen adsorbate oxidation or formation of oxides.
The difference between the ammonia oxidation signals and the
nounced compared to the ground control. This may be due to the
inability of the nanocatalyst to desorb these species under micro-
gravity, perhaps a direct effect of nitrogen (N ) formation at the
2
three-phase interface at the nanoscale.
A similar pattern was found for the commercial catalyst Pt/C
from Johnson Matthey. Fig. 5 presents the electrochemical oxida-
tion of ammonia on an electrode with Pt-Vulcan. At a potential of
Fig. 6. Linear polarization of a Pt-GCE electrode in 1 M NH4OH at pH 10 at 40 mV/s
under normal gravity conditions (black line) and microgravity (red line). (For inter-
pretation of the references to color in this figure legend, the reader is referred to the
web version of the article.)
Fig. 5. Linear polarization of a Pt/C electrode in 1 M NH4OH at pH 10 at 40 mV/s
under normal gravity conditions (black line) and microgravity (red line). (For inter-
pretation of the references to color in this figure legend, the reader is referred to the
web version of the article.)

9
2
E. Nicolau et al. / Electrochimica Acta 75 (2012) 88–93
Table 1
Summary of peak current generated and % performance reduction under normal gravity conditions compared to microgravity.
Electrode
NH3 oxidation peak
NH3 oxidation peak current density
% performance reduction in
microgravity vs. ground
−
2
current density normal
microgravity (␮A cm
)
−2
gravity (␮A cm
)
Pt-H2/C
Pt/C
Pt-GCE
106
198
374
81 ± 2
112 ± 2
140 ± 3
24
43
63
nitrogen adsorbate signals in microgravity and under normal grav-
ity conditions is shown. If the same reasoning as for the previous
catalyst is applied here, the ratio between these two peaks under
the two different condition results in 0.05. This means that this elec-
trode has a tremendous affinity for the nitrogen adsorbates when
in microgravity and perhaps is more prone to poisoning under such
conditions.
From the data presented in Figs. 3, 5 and 6 one can notice the
difference in current between the ground control and microgravity
electrochemical oxidation of ammonia. The results are consistent
among all the materials tested in the sense that the absence of
gravity negatively affects the current.
desorbed as presented by Koper et al. [26]. Therefore, no major
differences between the microgravity and ground conditions are
observed in such an oxynitride layer region.
These findings indicate that significant reductions in electrode
performance for ammonia electrooxidation may occur if catalyst
design measures are not taken in consideration to mitigate these
microgravity effects. This conclusion is applicable specifically to
electrochemical processes, but also may be relevant to all process
constrained by nanoscale mixing of diffusion limited structures
such as thermal catalysts, separation membranes, microbial growth
and human cellular functions.
To establish a baseline upon which the performance of mate-
rials can be compared, Table 1 shows data on ammonia oxidation
peak current and peak current average density (␮A/cm ). The table
Acknowledgements
2
This work was financially supported by NASA-URC Center
for Advanced Nanoscale Materials at the University of Puerto
Rico Río Piedras Campus under Grant No. NNX10AQ17A. Funds
were also provided by NASA Office of Education and Secor
Strategies. E. Nicolau greatly appreciates NASA Graduate Stu-
dent Researchers Program (GSRP) under Grant No. NNX08AV42H
shows that based upon platinum content, Pt-GCE provides a bet-
ter catalysis since it has a higher current average density over the
Pt/C and Pt-H /C; the latter two are comparable. This difference
2
can be attributed to the fact that platinum was directly electrode-
posited in a homogeneous layer on the electrode instead of using
a carbon support. However, when these results are compared to
the results on the ground experiments, the material that shows
(2008–2011), and NASA Jenkins Pre-Doctoral Fellowship for finan-
cial support (2011–2012). C. Poventud acknowledges NASA Jenkins
Pre-Doctoral Fellowship for financial support. The authors also
acknowledge Irma Candelaria from the UPR Machine Shop for
technical help and Bio-Logic USA for kindly lending an SP-50 poten-
tiostat to use during flight. E. Nicolau and C. Poventud contributed
equally to this work.
the lowest decrease in performance is the Pt-H /C, followed by
2
Pt/C and Pt-GCE respectively, as shown in Table 1. This result pin
points the experimental fact that during linear polarization having
Pt nanoparticles within the matrix of a carbon support in a micro-
gravity environment has a positive impact on the movement of
electroactive species toward the catalyst. A possible explanation
for Pt-H2 having less of a decrease in performance could be that
it was prepared according to a formulation that presumably leads
to the formation of platinum nanoparticles with selective domains
of (1 0 0). Previous investigations have shown that ammonia elec-
trooxidation takes place almost exclusively in the (1 0 0) domains
References
[
1] M. Mognato, L. Celotti, Mutation Research 578 (2005) 417.
[2] B. Lorber, R. Giege, Journal of Crystal Growth 231 (2001) 252.
[3] A.T. Borchers, C.L. Keen, M.E. Gershwin, Nutrition 18 (2002) 889.
[
4] E. Hartmann, P. Marquardt, H. Ditterich, H. Steinberger, Advances in Colloid
and Interface Science 46 (1993) 221.
[
28–30]. Therefore, the fact that Pt-H presents the lowest decrease
2
in performance may be due to a catalyst effect.
[
5] K. Nishikawa, Y. Fukunaka, E. Chassaing, M. Rosso, Journal of Physics: Confer-
ence Series 327 (2011) 012045.
[
[
[
[
6] B.K. Boggs, G.G. Botte, Journal of Power Sources 192 (2009) 573.
7] R.I. Masel, J.C. Gangley, E.G. Seebauer, Journal of Power Sources 137 (2004) 53.
8] M. Cooper, G.G. Botte, Journal of Electrochemical Society 153 (2006) A1894.
9] J. Solla-Gullon, F.J. Vidal-Iglesias, V. Montiel, J.M. Feliu, A. Aldaz, Journal ofPower
Sources 171 (2007) 448.
4
. Conclusions
The results of this research demonstrate
a decrease in
[
[
10] Y.F. Cheng, L. Zhou, M. Amrein, Journal of Power Sources 177 (2008) 50.
11] F. Vitse, M. Cooper, G.G. Botte, Journal of Power Sources 142 (2005) 18.
[12] E.P. Bonnin, E.J. Biddinger, G.G. Botte, Journal of Power Sources 182 (2008) 284.
13] G.G. Botte, F. Vitse, M. Cooper, Journal of Power Sources 142 (2005) 18.
14] R. Lan, S. Tao, Electrochemical and Solid State Letters 13 (2010) B83.
15] H. Matsushima, T. Nishida, Y. Konishi, Y. Fukunaka, Y. Ito, K. Kuribashayi, Elec-
trochimica Acta 48 (2003) 4119.
[16] H. Matsushima, B. Kiuchi, Y. Fukunaka, K. Kuribayashi, Electrochemistry Com-
munications 11 (2009) 1721.
performance of 20–65% over ground-based controls for the elec-
trochemical oxidation of ammonia. This decrease in current is
theorized to be due to the lack of buoyancy driven mixing that
occurs in microgravity. In microgravity only diffusional mixing
occurs and therefore molecules may be unable to reach the
nanocatalyst interface at the same rate as on the ground.
[
[
[
During the first oxidation peak at ca. 0.7 V the formation of N2
molecule during microgravity conditions creates a stagnant gas-
surface interaction that avoids ammonia molecules to reach the Pt
surface. This situation leads to a significant peak current decrease
compared to the ground experiments. On the other hand, during the
second oxidation peak an oxynitride layer is formed at the inter-
face of the electrode without the formation of N2 molecule. This
latter peak does not have the effect of microgravity due to the sur-
face chemistry reaction and the oxide adsorbates are no readily
[
17] H. Kaneko, K. Tanaka, A. Iwasaki, Y. Abe, A. Negishi, M. Kamimoto, Electrochim-
ica Acta 38 (1993) 729.
[18] H. Guo, J.F. Zhao, F. Ye, F. Wu, C.P. Lv, C.F. Ma, Microgravity Science and Tech-
nology 20 (2008) 265.
[
19] D. Kiuchi, H. Matsushima, Y. Fukunaka, K. Kuribayashi, Journal of Electrochem-
ical Society 153 (2006) E138.
[20] A. Iwasaki, H. Kaneko, Y. Abe, M. Kamimoto, Electrochimica Acta 43 (1998) 509.
[
[
21] H. Gerisher, A. Mauerer, Journal of Electroanalytical Chemistry 25 (1970) 421.
22] F. Vidal-Iglesias, J. Solla-Gullon, J.M. Perez, A. Aldaz, Electrochemistry Commu-
nications 8 (2006) 102.
[23] T. Miura, Y. Katayama, K. Endo, Electrochimica Acta 50 (2005) 2181.

E. Nicolau et al. / Electrochimica Acta 75 (2012) 88–93
93
[
[
24] N.J. Bunce, D. Bejan, Electrochimica Acta 56 (2011) 8085.
25] T.S. Ahmadi, Z.L. Wang, A. Henglein, M.A. El-Sayed, Chemistry of Materials 8
[28] F.J. Vidal-Iglesias, N. García-Aráez, V. Montiel, J.M. Feliu, A. Aldaz, Electrochem-
istry Communications 5 (2003) 22.
(
1996) 1161.
[29] F.J. Vidal-Iglesias, J. Solla-Gullon, V. Montiel, J.M. Feliu, A. Aldaz, Journal of
Physical Chemistry B 109 (2005) 12914.
[30] F.J. Vidal-Iglesias, J. Solla-Gullon, P. Rodriguez, E. Herrero, V. Mon-
[
[
26] A.C.A. de Vooys, M.T.M. Koper, R.A. van Santen, J.A.R. van Veen, Journal of Elec-
troanalytical Chemistry 506 (2001) 127.
27] S. Wasmus, E.J. Vasini, M. Krausa, H.T. Mishima, W. Vielstich, Electrochimica
Acta 39 (1994) 23.
tiel, J.M. Feliu, A. Aldaz, Electrochemistry Communications
(2004) 1080.
6