Electrodeposited nanocomposite coatings for fuel cell application

Article abstract of DOI:10.1016/j.jallcom.2008.10.116

Ni-TiO₂ nanocomposite coatings were prepared by the co-deposition of Ni and nanoparticles of TiO₂ powder onto the surface of commercial carbon for fuel cell application, in particular the electro-oxidation of methanol. The influences

Full text of DOI:10.1016/j.jallcom.2008.10.116

Journal of Alloys and Compounds 477 (2009) 652–656
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Electrodeposited nanocomposite coatings for fuel cell application
A. Abdel Aal , H.B. Hassanb
a,∗
a
Surface Protection & Corrosion Control Lab, Central Metallurgical Research & Development Institute, CMRDI, P.O. 87, Hellwan, 11421 Cairo, Egypt
Department of Chemistry, Faculty of Science, Cairo University, Giza, Egypt
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 August 2008
Received in revised form 10 October 2008
Accepted 22 October 2008
Available online 9 December 2008
Ni–TiO2 nanocomposite coatings were prepared by the co-deposition of Ni and nanoparticles of TiO2 pow-
der onto the surface of commercial carbon for fuel cell application, in particular the electro-oxidation of
methanol. The influences of the TiO2 powder concentration and applied current density on the composi-
tion of nanocomposite coatings were investigated. The chemical composition and phase structure of coat-
ings were studied by X-ray diffractometer (XRD) and energy dispersive X-ray spectroscopy (EDX), respec-
tively. The surface morphology of deposited Ni and Ni–TiO2 coatings was studied using scanning electron
microscope. The performance of the prepared electrodes towards electro-oxidation of methanol as a func-
tion of co-deposited TiO2 content was studied. The catalytic activity was found to increase with increasing
TiO2 content up to 9 wt.%. The chronoamperometric data showed that the stability of the fabricated
electrodes towards the electro-oxidation process was improved with the content of co-deposited TiO2.
© 2008 Elsevier B.V. All rights reserved.
Keywords:
Surface coatings
Nanocomposite
Electrodeposition
Electrocatalysis
Fuel cell
1. Introduction
Small organic molecule’s fuel cells are regarded as promis-
ing electrochemical systems for directly converting the chemical
Surface modification by coatings has become an essential step
to improve the surface properties of materials. Coatings can pro-
tect a substrate by providing a barrier between the metal and
its environment [1]. The nanostructured coatings have long been
recognized to exhibit remarkable and technologically attractive
properties due to their extremely fine microstructure [2]. Highly
sophisticated surface-related properties such as optical, magnetic,
electronic, catalytic, mechanical, chemical and tribological proper-
ties can be obtained by advanced nanostructured coatings, making
them attractive for industrial application [3,4].
energy of a fuel and an oxidant into electric energy in a wide range
of the portable and transportation applications. A short list of their
advantages may include, interalia, their suitability for applications
where mass and volume constraints are stringent, the compatibility
of liquid methanol for handling and storage in the existed gasoline
infrastructure, and their ability for quick start-up and immediate
response to consumer needs [8].
The activity of electrocatalysts is one of the main factors
affecting the cell performance. The procedure and method for
the preparation of the electrocatalysts play an important role in
their particle size, surface morphology, composition and elec-
trocatalytic activity with direct consequences to the fuel cell
performance [9–12]. Song et al. [8] have reported that the electrode
fabrication process, especially for the decal transfer fabrication
technology, which involves several intermediate processing steps,
should also have an obvious effect on the physical characteris-
tics of electrocatalysts, and accordingly on their electrochemical
activity.
During the past decade, the electrochemical oxidation of
small organic molecules on nickel-based electrodes attracted
considerable attention and impact [13,14]. TiO2 and Ti coated with
oxides of several other metals; Pt, Au, Fe, Co, Ni, Mo, Mn, Cr, Cu
and Hg or their combinations Sb–Sn, Co–Mn, Ni–Si, Ni–Cr–Mo
and Fe–Cr are used for methanol electro-oxidation [15]. The
introduction of TiO2 into Ni matrix is expected to improve the
catalytic activity of Ni-based electrodes. Hence, the present article
reports on the preparation of nanocomposite coatings of Ni–TiO2
Composite electroplating is
a method that involves co-
depositing fine particles of metallic, non-metallic compounds or
polymers in the plated layer under the effect of electric field. Pre-
viously, the co-deposition of various metallic matrices as Ni, Ni–P,
and Zn with a great variety of powders from hard carbides as SiC,
oxides as ZnO and nitrides as AlN has been extensively studied by
Abdel Aal et al. [3–7]. During this process, the powder particles are
suspended in a conventional plating electrolyte and are captured in
the growing metal film. Moreover, the rate of particle incorporation
depends on the nature of particles (size, shape and charge) and the
operating conditions (current density, temperature, pH and time)
[
3–7].
∗ Corresponding author. Tel.: +20 122690782; fax: +20 225010639.
E-mail address: foralsayed@gmail.com (A. Abdel Aal).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.10.116

A. Abdel Aal, H.B. Hassan / Journal of Alloys and Compounds 477 (2009) 652–656
653
onto the commercial carbon electrodes and their application for
the electro-oxidation of methanol.
2.
Experimental procedures
Ni–TiO2 nanocomposite layers were deposited on commercial carbon substrates
by conventional direct current electrodeposition process. Electroplating was per-
formed using a plating cell consisting of carbon rod and nickel plate as cathode and
anode, respectively, and vertically maintained at 5 cm. The basic compositions of the
electrolyte are as follows: 240 g/l nickel sulfate, 30 g/l nickel chloride, 30 g/l boric
◦
acid. The plating bath is heated to 55 C and pH 5 value was adjusted by ammonia
water or dilute sulfuric-acid. The titanium dioxide, TiO2, used in this work (Degussa
P-25 anatase) with 25 nm was obtained from Degussa Company, Ridgefield Park, NJ.
Different concentrations were added to plating solution in order to form Ni–TiO2
nanocomposite coatings. During the co-deposition process, the bath was stirred by
a magnetic stirrer (150 rpm) in order to keep the particles dispersed and prevent
sedimentation in the electrolyte suspension. For comparison, pure Ni coatings were
also prepared in almost the same solution mentioned above except TiO2 particles
were introduced under the same electrodeposition conditions. In order to standard-
ize the comparison, the thickness of all coatings was controlled to be approximately
2
0 ␮m by the plating time for a specific current density.
The phase structure of the composite coatings was studied by X-ray diffrac-
tometry (BRUKER axc-D8) using Cu K␣ radiation with ꢀ = 0.1542 nm. The surface
morphology of the coatings was observed using a scanning electron microscopy
(JEOL-JSM-5410) and the weight percentage (wt.%) of co-deposited TiO2 particles
was evaluated by using energy dispersive X-ray spectroscopy (EDX-Oxford) analysis
tool. In each measurement an area of 10 ␮m in diameter was examined to a depth of
about 2 ␮m. From EDX analysis, wt.% of Ti metal can be determined and accordingly
the TiO2 percentage.
Fig. 2. EDX of samples coated with (a) Ni and (b) Ni–9% TiO2.
maximum of 9 wt.% for a bath load at 1 g/l. The embedding of TiO2
can be attributed to the adsorption of suspended particles on the
cathode surface, as suggested by Guglielmi’s two-step adsorption
model [16]. Once the particle is adsorbed, the metal begins build-
ing around the cathode slowly, encapsulating and incorporating the
particles. Further increase of bath load up to 5 g/l resulted in a slight
decrease in the weight of the TiO2 in the deposited composite and
indicated that the adsorption of TiO2 particles in the coating has
reached the saturated state [17].
Electrochemical measurements were performed on the above-prepared elec-
2
trodes of a disc-shaped each of apparent surface area of 0.125 cm . The surface area
was calculated from the apparent area and the current density was referred to it. A
conventional three-electrode glass cell with a Pt sheet as a counter electrode and
0
Hg/HgO/1.0 M NaOH (MMO, E = 140 mV vs. NHE) as a reference electrode were used.
◦
All experiments were carried out at room temperature of 30 ± 2 C. The electrochem-
ical measurements were performed using an Amel 5000 system (supplied by Amel
instrument, Italy) driven by a PC for data processing. The PC was interfaced with
the instrument through a serial RS-232C card. Amel Easy Scan software was used in
connection with the PC to control the Amel 5000 system.
Fig. 1 shows also the correlation between the TiO2 content in
the deposit and the current density at various TiO2 concentrations
in the electroplating bath. The wt.% of co-deposited TiO2 increases
with current density. According to the Stokes model, assuming the
shape of the colloidal particle is sphere, the electrophoretic velocity
3
. Results and discussion
3.1. Electrodeposition and characterization
(
VE) in an electric field (E) can be expressed as:
The effect of bath loads (0.5–5 g/l) was studied at various cur-
2
rent densities of 1, 2, and 3 A/dm , maintaining all other deposition
q
◦
VE = ꢁEE =
E
(1)
parameters constant as pH 5 and temperature 55 C (Fig. 1). The
6
ꢂꢃr
composition of composite layer was determined by EDX as shown in
Fig. 2. The embedded weight percentage (wt.%) of TiO2 in the com-
posite was found to increase with increase in bath loads, reaching a
where ꢁE is the electrophoretic mobility, q is the charge of the par-
ticle, r is the radius of the particle, and ꢃ is the viscosity of the
suspension [18]. The Stoke’s model indicates that increasing the
current density accelerates the electrophoretic velocity of TiO par-
2
ticles and increases the Coulombic force between Ni+ adsorbed on
particles and the cathode which in accordance increases the TiO2
content in the Ni deposit.
2
Besides the Guglielmi’s model for the co-depositing pro-
cess, several mechanisms have been proposed describing the
co-deposition process, such as absorbing theory in which the co-
deposition of particles and metal depends on van der Waal’s force
on the cathode surface. Once the particles are absorbed on the cath-
ode surface, they will be imbedded into the composite coatings
[
19–22]. On the other hand, the particles in the mechanical mecha-
nism are deposited into the matrix through the simple mechanical
course. In this model, the particles are transmitted to the cathode
surface by the flowing fluids to contact the cathode. Then, they
will stay on the cathode surface by the external force and be cap-
tured by the deposited metal. In the electrochemical mechanism,
the field intensity between the interface of electrodes and solution
and electric charges of the particle surface plays a significant role
in the co-deposition process [16–22]. In general, the co-deposition
mechanism is still not clear and further studies will have to address
the details of the process.
Fig. 1. Dependence of TiO2 content in deposit on the bath load and current density
at constant pH 5.5 and 55 C.
◦

6
54
A. Abdel Aal, H.B. Hassan / Journal of Alloys and Compounds 477 (2009) 652–656
Fig. 3. SEM images of (a) pure Ni, (b) Ni–4 wt.% TiO2, (c) Ni–8 wt.% TiO2 and (d) Ni–9 wt.% TiO2 deposited at different current densities and constant pH 5.5 and 55 C, 3 A/dm2
◦
and 1 g/l TiO2 powder.
The scanning electron micrographs (SEM) of the Ni and Ni–TiO2
coatings are presented in Fig. 3a–d. A regular pyramidal structure
as shown in Fig. 3a is observed at the surface of the nickel film. The
obtained composite layers have a mat-gray metallic surface with
white Ni spots visible to the naked eye. Therefore, with the addi-
tions of TiO2 nanoparticles, the morphology of coatings is changed
to spherical particle, as shown in Fig. 3b–d.
this type of coatings exhibits a nickel matrix at different 2ꢄ angles
corresponding to the crystalline matrix of the Ni layer. The pattern
of such a composite layer additionally contains two small peaks
characterized for crystalline TiO2 (anatase) indicating the embed-
ding of the TiO2 particles into the nickel matrix.
The grain size of Ni and Ni–TiO2 coatings can be estimated by
the Scherrer equation:
The phase structure of Ni–TiO2 coating is represented in Fig. 4.
The XRD pattern of the Ni–TiO2 layer showed that the structure of
0
.9ꢀ
t =
(2)
B cos(ꢄB)
where ꢀ is the Cu K␣ wavelength, B is the broadening of the full
width at half maximum (F.W.H.M.) and ꢄB is the Bragg’s angle. The
grain sizes of the pure Ni and Ni-TiO nanocomposite coatings were
2
calculated from the diffraction peak widths (with the instrumental
width eliminated) by the Scherrer formula. The results are listed
in Table 1. It is clear that increasing of TiO2 content (0–9 wt.%) into
deposit reduces the size of Ni grains (from 108 to 37 nm), due to the
distribution of TiO2 nanoparticles on the boundaries of Ni grains,
which restricts the growth of Ni grains in the deposition process
and results in the fine and smooth surface [23].
3.2. Electro-oxidation of methanol
The performance of Ni–TiO /C catalysts towards methanol
2
electro-oxidation was studied in 2 M NaOH and compared with the
Table 1
The applied electrodes and grain size of Ni coating as a function of content of TiO2.
Electrode
Pure Ni
TiO2 content, wt.%
Ni grain size, nm
0
108
57
42
37
(
(
(
I)
II)
III)
4 wt.% TiO2
8 wt.% TiO2
9 wt.% TiO2
Fig. 4. XRD patterns of Ni–9 wt.% TiO2 nanocomposite coating deposited at pH 5.5
and 55 C, 3 A/dm2 and 1 g/l TiO2 powder.
◦

A. Abdel Aal, H.B. Hassan / Journal of Alloys and Compounds 477 (2009) 652–656
655
Fig. 5. Cyclic voltammograms of Ni/C electrode in 2 M NaOH solution in the absence
the dashed line) and in the presence of 2 M methanol (the solid line).
(
performance of Ni/C electrodeposited electrode. Fig. 5 shows the
cyclic voltammograms of Ni/C electrode in 2 M NaOH solution in the
absence (the dashed line) and in the presence of methanol (the solid
line). Polarization was started from −1200 to +1500 mV (MMO) at
a scan rate of 50 mV/s in the anodic direction and then the scan
was reversed in the cathodic direction back to −1200 mV (see the
inset figure). In addition to the hydrogen evolution taking place at
the starting potential characterized by a high cathodic current and
the oxygen evolution takes place at relatively high positive poten-
tial, two small peaks are observed in the blank voltammogram, one
in the anodic direction at about +380 mV due to the formation of
NiOOH and the other in the cathodic direction at about +480 mV
due to the reduction of NiOOH to Ni(OH)2 [24]:
Fig. 6. (a) Cyclic voltammograms of Ni–TiO2/C electrodes (I, II and III) in 2 M NaOH
solution at 50 mV/s (MMO). (b) Cyclic voltammograms of Ni–TiO2/C electrodes (I,
II and III) in 2 M NaOH solution in the presence of 2 M methanol at a scan rate of
50 mV/s (MMO).
Ni(OH) + OH = NiOOH + H O + e−
−
(I)
2
2
2
M NaOH is almost the same as the cyclic voltammogram pattern
In the presence of 2.0 M methanol, the oxidation process starts at
400 mV (MMO) with gradual increase in the oxidation current
of Ni/C electrode (Fig. 6) in the same electrolyte with a few differ-
ences. It was found that, H evolution peak appeared at the starting
+
2
−
2
density and reaches its maximum value of 259 mA cm at about
1079 mV in the anodic direction. Then, the scan was reversed and
negative potential while the O2 evolution peak appeared at more
+
positive potential. The two peaks of Ni redox couples, i.e. (Ni3+/Ni2+)
another reoxidation peak appeared in the reverse scan at a poten-
tial of about +950 mV probably due to the reoxidation of methanol
or the intermediate products generating during methanol oxida-
tion. Adsorbed CO is one of the most known intermediate products
of methanol oxidation which causes deactivation and blocking of
the active sites of the electrode surface during the oxidation process
are formed in the potential range from +350 to +650 mV (MMO),
depending on the amount of Ni as well as the amount of TiO2 in
the prepared electrode. The higher amount of TiO2 in the prepared
electrode, the higher height of the peak current densities of the
redox couples. On the other hand, the potential of the anodic peak
shifted towards the positive potential and that of the cathodic peak
shifted towards the less positive one. It can be observed that the
with the time [25]. On the other hand, formate and CO were identi-
2
fied as the main solution reaction products in NaOH solution [26]. It
was found that the electro-oxidation of organic compound requires
the presence of oxide, hydroxide and/or oxyhydroxide groups [27].
On the Ni–P/C electrode, the oxidation process of methanol starts
at a potential value corresponds to the formation of NiOOH species
and the oxidation process starts as Ni(III) species is formed as
shown in Fig. 5 (the solid voltammogram). It was suggested that,
methanol is oxidized on the Ni/C electrode through the reaction
presence of TiO in the prepared electrode enhances the formation
2
of Ni redox couple (Ni3+/Ni2+).
Fig. 6b represents cyclic voltammograms of these electrodes in
2 M NaOH in the presence of 2.0 M MeOH at a scan rate of 50 mV/s. It
was found that, the oxidation process starts at +400 mV as NiOOH
species is formed and the oxidation current density of methanol
increases in the anodic direction up to +1500 mV. With revers-
ing the scan down to +200 mV, another reoxidation process of the
adsorbed methanol and/or its intermediate products generating
during its oxidation taking place in the reverse scan. Oxidation of
methanol at these electrodes is characterized by very broad peaks
over wide range of potentials, so one must measure and compare
values of oxidation current density at a constant selected potential
value of about 1000 mV (MMO) (Table 2). It was found that, the oxi-
dation current density of methanol measured at constant potential
of +1000 mV (MMO) for all electrodes increases as the amounts of
with NiOOH to form Ni(OH) , i.e. NiOOH acts as electron transfer
2
mediator for the oxidation process [28]:
NiOOH + methanol ↔ Ni(OH) + products
(II)
2
The same experiments were repeated on Ni–TiO /C electrodes with
2
different amounts of TiO2 as shown in Table 1. Three different
ratios of TiO2 were found in the prepared electrodes which are
Ni–4 wt.% TiO (I), Ni–8 wt.% TiO (II) and Ni–9 wt.% TiO (III) and the
2
2
2
three electrodes named as (I, II and III). The electro-oxidation of
TiO increase in the prepared electrodes. Compared to the Ni/C elec-
2
methanol on the three Ni–TiO /C electrodes in 2 M NaOH solution
was examined.
trode, the oxidation current density of methanol at these electrodes
is much higher. Notably, as the amount of TiO2 increases in the
prepared electrodes, the oxidation process occurs at less positive
potential as shown from Fig. 6b and Table 2. For all the three elec-
trodes, the oxidation process starts as the NiOOH is formed which
acts as an electron transfer mediator for the oxidation process.
2
Fig. 6a illustrates cyclic voltammograms of the three electrodes
(
I, II and III) in 2 M NaOH as a blank solution at a scan rate of 50 mV/s
from −1200 to 1500 mV (MMO) and the represented part is from 0
to +1000 mV (MMO). The shape of these cyclic voltammograms in

6
56
A. Abdel Aal, H.B. Hassan / Journal of Alloys and Compounds 477 (2009) 652–656
Table 2
Mehandru et al. [33] have proved that CO dissociates on Pt–Ti alloys
much easier than on pure Pt.
Oxidation peak current densities and potentials for electro-oxidation of methanol
at different electrodes.
−
−2
Electrode
Pure Ni
I/mA cm 2 at +1000 mV (MMO)
Ip/mA cm
Ep/mV (MMO)
4
. Conclusions
247
214
329
395
259
248
341
400
1079
1151
1090
1043
(
(
(
I)
II)
III)
Nanocomposite coatings consisting of nickel matrix and TiO2
nanoparticles can be successfully synthesized by means of elec-
trodeposition onto commercial carbon substrate. The content of
TiO2 particles in the coatings increases with increasing TiO2 con-
centration in the electrolyte. The nanocomposite Ni–TiO2 coatings
showed a smaller grain size and higher catalytic activity towards
the electrochemical oxidation of methanol compared with pure Ni
coating.
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