Telechargé par Muhammad Al Djazairy

Electrodeposition and characterizationof NiMoW alloys

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 2
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Electrodeposition and characterization of NiMoW
alloy as electrode material for hydrogen evolution
in alkaline water electrolysis
Mahdi Allam a, Mohamed Benaicha b,*, Achour Dakhouche a
Inorganic Materials Laboratory, Department of Chemistry, Faculty of Sciences, Mohamed Boudiaf University,
M'sila, 28000, Algeria
Energetic and Solid-State Electrochemistry Laboratory, Faculty of Technology, Ferhat Abbas-Setif1 University, Setif,
19000, Algeria
article info
Article history:
The electrodeposition of ternary NiWMo alloys films from citrate ammonia-free electrolyte
Received 31 January 2017
at room temperature was studied in an effort to evaluate the effect of applied potential on
Received in revised form
the composition limits, corrosion resistance and the electrocatalytic properties of the de-
20 July 2017
posits towards the hydrogen evolution reaction (HER) in concentrated alkaline solution.
Accepted 5 August 2017
The alloys were potentiostatically electrodeposited onto pure copper sheet substrates. The
Available online xxx
electrodeposits were characterized by means of field-emission scanning microscopy
(FESEM) and energy dispersive X-ray analysis (EDXA). In an electrolyte where
4 =WO4 ¼ 1 : 1, at a given deposition potential, there is more Mo than W in the de-
posits, indicating an advantageous induced co-deposition of Mo compared to W. The
NiWMo alloy
nucleation mechanism, studied according to Scharifker-Hills theoretical model, revealed
Hydrogen evolution
an instantaneous nucleation followed by a three-dimensional growth. On the hand,
Nucleation mechanism
ratio in the electrolyte under the same deposition potential
increasing MoO2
4 =WO4
Impedance spectroscopy
reduced both Ni and W content in the deposits. A different trend was observed in an
Cyclic voltammetry
equimolar solution, when applying more negative potentials, both Mo and W contents
decreased leading to the enhancement of Ni amount. The stability in corrosive media and
the catalytic performances of the coatings depended mainly on the applied overpotentials,
A mechanism of induced co-deposition of molybdenum and tungsten with nickel is proposed and discussed.
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Hydrogen production by water electrolysis is one of the most
promising techniques for the global future energy needs as an
entirely clean and powerful alternative to climate -altering
fossil fuels processes, particularly when renewable energy
sources such as solar and wind energy are used [1]. Unfortunately, compared to technologies available on the market
such as the steam reforming of methane gas or partial
oxidation of oil, alkaline water electrolysis is not at present
cost-effective due to the high amount of electricity used in the
process which restrains its large-scale utilization.
* Corresponding author.
E-mail address: [email protected] (M. Benaicha).
0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Allam M, et al., Electrodeposition and characterization of NiMoW alloy as electrode material for
hydrogen evolution in alkaline water electrolysis, International Journal of Hydrogen Energy (2017),
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 2
Alternative current
Tafel slope, log2.303RT/aF
Charge transfer coefficient
Electrode capacitance
Cathodic scan limit
Deposition potential
Dispersive X-ray spectroscopy
Electrochemical Equivalent Circuit
Electrochemical impedance spectroscopy
Reversible thermodynamic potential
Faraday constant, 96500 C
Field emission scanning electron microscope
hydrogen evolution reaction
Apparent exchange current density
Cathodic current density
Current coordinate of the peak
Electrochemical reaction rate constant
Rate constant at Ei¼0
Number of electrons transferred
Anodic overpotential
Cathodic overpotential
Open circuit potential
Charge transfer resistance
Solution resistance
Saturated calomel electrode
Time coordinate of the peak
Imaginary part of Z
Real part of Z
In order to make water electrolysis more efficient and at
affordable cost, reduction of the cell voltage is indispensable.
The applied potential to drive the electrochemical cell at an
applied current, I, includes a thermodynamic and a kinetic
(dissipation) contribution due to inefficiencies in the system:
DECell ¼ DErev þ ha þ jhc j þ IR þ DEst
Where DErev is the reversible thermodynamic value
(z1.23 V) which depends on the nature of the electrochemical
reactions, ha and hc the overpotentials at anode and cathode,
IR the inter-electrode resistance, and DEst the in time stability
characteristic expressing the tendency of the actual cell
voltage to increase as a consequence of the electrolyzer performance degradation [2]. It is worth mentioning that while
the inter-electrode ohmic losses could be reduced by
enhancing the bath conductivity and minimizing the space
between the electrodes, the cathodic and anodic overpotentials depend mainly on the electrode material properties. During the last few years, significant improvements have
been made in alkaline water electrolysis through the development of advanced alkaline water electrolysis systems with
the so-called “zero gap cell” design (no space between anodes,
diaphragms, and cathodes in the cell units) [3]. On the other
hand, some research works on water vapor electrolysis have
been carried out. Ganley [4] and Boll and co-workers [5] reported that increasing the operating steam electrolysis temperature and pressure up to 400 C and z10 MPa or 500 C and
400 MPa respectively, improved the reaction kinetics at the
electrode surface and lowered the applied potentials to provide high current densities. However, increasing bath temperature and pressure drastically decreased the terminal
potentials and strongly affected the stability of electrode
materials in concentrated alkaline corrosive media. This fact
motivates the extensive research efforts that have been
recently focused on the improvement of the electrolysis cell
performances through the devel opment of efficient and lowcost catalyst materials. According to the Brewer-Engel
valence-bond theory [6,7], alloying transition metals with
hypo-hyper-d - electronic structure, i.e., combination of
metals of the left half of the transition elements in the Periodic
Table having empty or half-filled vacant d-orbitals with metals
of the right half of the series, having internally paired delectrons, leads to pronounced increase in the electronic
density of states and consequently to advanced synergetic
effect in electrocatalysis for the hydrogen evolution reaction
(HER). Alloys of nickel ([Ar] 3d8 4s2) with tungsten, W ([Xe] 4f14
5d4 6s2) and/or molybdenum, Mo ([Kr] 4d5 5s1) are among the
materials fulfilling these requirements [8e11]. Moreover, the
Pt-group metals are known to be the best electrocatalysts for
hydrogen evolution reaction (DG z 0.1 eV). The next ideal
HER catalysts are nickel and cobalt (DG z 0.28 eV) followed
by molybdenum (DG z 0.36 eV) and tungsten
(DG z 0.42 eV) [12]. NieW and NieMo alloys are known to
possess outstanding functional properties such as high
corrosion resistance in many aggressive environments [13]
and excellent electrocatalytic activity for hydrogen evolution. The electroplating process of NiMo and NiW alloys is
classified as an induced alloy deposition type since tungsten
and molybdenum cannot be deposited alone from their
aqueous solutions, but are codeposited in the presence of
nickel, forming an alloy [14]. Commonly, a complexing agent
is needed to codeposit these metals, otherwise, the amount of
Mo or W does not exceed 2 wt% in the alloy. Although a great
number of different complexing agents have been reported in
the literature, citrate has been among the most popular,
particularly in the deposition of nickel alloys [15]. Sodium
citrate forms stable complexes enough to bring closer the
potentials of the alloy constituents and prevent deposition of
hydroxides of the metals. Moreover, citrate aqueous solutions
are environmentally friendly and can function as a complexing, buffering, brightening and levelling agent [16] in electroplating of metals and alloys. Several investigations have been
devoted to the deposition of NiMo and NiW alloys. Sanches
et al. [17] investigated the electrodeposition of the binary Nie
Mo alloys using different molar ratios of Ni:Mo, in a sodium
citrate electrolyte at pH 4.0. They have found that high Ni:Mo
molar ratio favors deposition of the metallic molybdenum,
while deposits coated from molybdate-rich bath contained
higher amount of Mo, a mixture of polyvalent molybdenum
oxides or hydroxides, mainly in the form of the Mo(IV) and
Mo(V)) in addition to metallic molybdenum. Krstajic et al. [9]
reported that NiMo alloy deposited from a pyrophosphatebased bath onto Ni mesh exhibited better catalytic activity
for hydrogen evolution in 1 M NaOH solution than pure Ni
electrode. However, the stability of the film in concentrated
solution (33 wt%) was very poor and the coating was scaled
from the substrate after 2 h of hydrogen evolution. Xu and co-
Please cite this article in press as: Allam M, et al., Electrodeposition and characterization of NiMoW alloy as electrode material for
hydrogen evolution in alkaline water electrolysis, International Journal of Hydrogen Energy (2017),
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 2
workers [18] thought that the high electrochemical activity for
the hydrogen evolution of the NieMo coating could be attributed to both exchange current density and larger real electrode area. Navarro-Flores et al. [19] studied the influence of
alloying nickel by Fe, Mo and Won the electrocatalytic activity
towards HER in an acidic environment. They claimed that
NiW, NiMo and NiFe electrodes were more efficient for
hydrogen generation that pure nickel. NieMo was found to
yield the highest overall electrocatalytic activity, mainly due
the highest surface roughness, while NieW yielded the highest intrinsic activity as a result of the modification of electron
density in d-orbitals upon alloying nickel with tungsten.
Conway and co-workers [20] investigated the catalytic activity
of electrodeposited ternary NiMoCd alloys containg 1 at% Cd.
They claimed that the resulting low Tafel slope (26e30 mV)
obtained at elevated temperatures can be explained of the
formation of a hydride phase. Eliaz and Gileadi [21] made a
review on Mo and W alloys with transition metals. The citrate
concentration increase was reported to decrease overall current efficiency. Citrate baths provided higher W content in
comparison with those containing tartrate or malate. The effect of temperature was proved to depend on solution
composition and increasing the current density led to higher
W content for most used electrolytes. Tasic et al. [22] studied
the electrodeposition of NieW alloys from ammoniacalcitrate bath containing different concentrations of sodium
tungstate, on electrocatalytic activity towards HER. They
found that the films obtained at higher deposition current
densities had the lowest overvoltage for the HER and claimed
that the surface roughness of the coatings is responsible for
their electrocatalytic activity.
If the electrodeposition of NiMo and NiW binary alloys
was already approached, to the best of our knowledge, there
are very few research works on the synthesis of ternary
NiMoW alloys by electrodeposition that have been conducted. Cesiulis and co-workers [23] investigated the effect the
ratio of the Na2MoO4 and Na2WO4 concentrations in a pyrophosphate bath on the content of the ternary NieMoeW alloys. They reported that the deposits were crack-free for a W
content less than 5e6 at% and in all cases, the sum of Mo and
W amounts in the alloys does not exceed 15 at%. More
recently, Sun et al. [24] studied the effect of current density
on the composition of NiWMo alloys electrodeposited onto
rotating cylinder electrodes in citrate-boric acid electrolyte.
They found that (but without clearly explaining why) when
depositing NiWMo alloys in an equimolar solution, the Mo
content in the deposit is significantly higher than W content.
However, the influence of deposition parameters (electrolyte
composition, deposition potential or current density etc) on
the corrosion properties and electrocatalytic activities of
electrodeposited ternary NiMoW coatings towards the
hydrogen evolution reaction have not been investigated.
Moreover, the nucleation/growth mechanism of these
ternary alloys has never been treated. In the present work,
we report the results regarding the electrochemical aspects
of the deposition process and the mechanisms of nucleation
and growth of NiMoW alloys from slightly acidic citrate
ammonia-free electrolyte. Particular attention was paid to
the effects of both Mo/W ratio in the plating bath and
deposition potential on the corrosion resistance and
electrocatalytic activity of the alloys for the hydrogen evolution reaction (HER) in concentrated alkaline media.
The electrochemical measurements were carried out with a
Voltalab 40 potentiostat-galvanostat (Model PGZ 301)
controlled by VoltaMaster 4 software (Hach Lange GmbH,
Germany) and using a conventional three electrode cell assembly at ambient (laboratory) temperature. Platinum wire
and pure copper sheets sized 1 2 cm2 were used as working
electrode, a platinum foil having a large-area to prevent polarization of the anode was used as counter electrode and
saturated calomel electrode (SCE) served as reference electrode. To obtain reproducible results, the substrate surface
was polished with successive grades of sand paper to obtain a
mirror finish. The samples were then treated in 10% (v/v) hydrochloric acid (HCl) for 2 min to remove any adherent oxide
layer on the surface and rinsed with distilled water before
every experiment. The plating solutions were made from
analytical-grade chemicals (Sigma-Aldrich, USA) and doubly
distilled water and contained 0.25 M NiSO4$6H2O, 0.05 M
NiCl2$6H2O, Na2WO4, Na2MoO4. 2H2O as metal sources (with
W/Mo ratio ranging from 0.5 to 2) and 0.4 M Na3C6H5O7$2H2O
as complexing agent. After preparation, all solutions were
deoxygenated with a stream of nitrogen within the cell before
each experiment. The plating bath pH was quasi-neutral (6.25)
adjusted by addition of potassium hydroxide or sulfuric acid
as needed and monitored with a calibrated pH-meter (WTWinoLab pH 7310). No agitation was utilized in all experiments.
To investigate the corrosion resistance and catalytic activities of electrodeposited NiMoW films for hydrogen evolution
reaction (HER) in a 3.5% NaCl and 30% KOH solutions respectively, Tafel curves (5 mV/s scan rate from 350 mV to þ350 mV
vs OCP), cyclic voltammograms and EIS (Nyquist and Bode) plots
were performed at room temperature. The same three electrode
electrochemical cell used for electrodeposition study was used
for the characterization of the coatings. EIS measurements at a
given applied potential and an alternative current (AC) sine
wave of 10 mV amplitude were taken in the frequency range of
100 KHz to 10 mHz. The impedance data, such as the electrolyte
resistance Re, the charge-transfer resistance (Rct) and the
double layer capacity of the interface (Cdl) were determined
from the Nyquist and Bode plots. All the chosen specimens were
activated in 10 vol% HCl for 30 s prior to electrochemical testing.
The corrosion rate occurring at the electrochemical interface
(mm/y), corrosion potential (mV/SCE) and exchange current
density (mA/cm2) were calculated automatically by using Tafel
extrapolation method provided by Voltamaster 4 software
[25,26]. The structural quality of the NiMoW samples was
examined using X-ray diffraction (XRD) analysis, carried out on
a powder diffractometer (Philips X'Pert Pro Multipurpose X-ray
diffractometer) with copper Ka radiation source (lKa ¼ 1.5418 A)
in the (2q) range of 20e100. The compositional and morphological properties were investigated using a field-emission
scanning electron microscope (FESEM, JSM-7100F) with an
accelerating voltage of 10 kV and acquisition time of 90 s, and
equipped with a high resolution silicon drift detector (SDD) for
X-ray Energy Dispersive Spectroscope (EDS) microanalysis.
Please cite this article in press as: Allam M, et al., Electrodeposition and characterization of NiMoW alloy as electrode material for
hydrogen evolution in alkaline water electrolysis, International Journal of Hydrogen Energy (2017),
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 2
Results and discussion
Electrochemical aspects of electrodeposition
Cyclic voltammetry (CV) was used to investigate the electrochemical processes and define suitable potential regions for
alloys deposition. A series of cyclic voltammograms for Ni,
NieMo, NiW and NieMoeW recorded in plating solution with
Pt working electrode at a sweep rate of 50 mV. s1 and 25 C are
presented in Fig. 1. It is well established that in quasi-neutral
citrate electrolyte (рНz6), nickel and citrate ions form complexes mainly of the type NiCit with stability constant
log(K) ¼ 5.379. The voltammogram for pure Ni recorded between þ1.0 and 1.05 V/SCE, beginning from the open circuit
potential (OCP) firstly towards cathodic direction, exhibits one
main reduction peak at approximately 0.8 V/SCE ascribed to
the reduction of Ni2þ ions:
NiCit þ 2e /Ni þ Cit3
and two anodic peaks at 0.075 and þ 0.6 V/SCE corresponding to the formation of nickel hydroxide (Eq. (2)) and nickel
oxyhydroxide (Eq. (3)) respectively.
Ni þ 2OH /NiðOHÞ2 þ 2e
NiðOHÞ2 /NiOOH þ Hþ þ e
With addition molybdate ions to the nickel plating bath
(Fig. 1-a), the CV is somewhat different since the cathodic part
of the CV moved towards more negative values, indicating
additional difficulties for the reduction process. The anodic
scan exhibits one main broad peak between the two oxidation
responses of nickel. This behavior suggests that fundamental
changes occurred at the electrode surface with a slower
charge transfer due most probably to the formation of molybdenum oxides. It should be noted that during the reversal
scan for Ni and NiMo, the presence of a crossover is characteristic of a nucleation process on Pt substrate.
The shape of the CVs for NiW process (Fig. 1-b), doesn't
differ substantially from that of pure nickel, excepted a slight
shift of the oxidation peaks to the more positive potentials
and the appearance of an additional anodic peak located at
around þ0.05 V/SCE indicating the possible presence of Ni, W
in addition to Ni þ W oxides.
The electrochemical process for NieMoeW in co-existence
is represented in Fig. (1-C). It can be clearly seen that total
cathodic current rises substantially compared to that of Ni,
NiMo and NiW systems, as a consequence of catalytic effect of
the ternary alloy on the HER. The reduction current beginning
for nickel ions, molybdate, tungstate and hydrogen evolution
are approximately overlapped. On the other hand, when the
cathodic scan limit is progressively enhanced, additional
anodic peaks appeared with enhanced areas. Usually, the
height of any peak is proportional to the quantity of the
deposited phases. The multiple anodic peaks seen during the
Fig. 1 e Cyclic voltammograms recorded in citrate electrolyte at room temperature and pH ¼ 6.25 for: Ni and NieMo (a), Ni
and NieW (b) and NieNieMoeW with variable cathodic scan limits-as indicated in the figure- (c). Comparative cathodic
parts of CVs plots are shown in figure (d).
Please cite this article in press as: Allam M, et al., Electrodeposition and characterization of NiMoW alloy as electrode material for
hydrogen evolution in alkaline water electrolysis, International Journal of Hydrogen Energy (2017),
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 2
reverse scan can be attributed to electrochemical dissolution
of the metals in the alloy from different intermetallic phases
[27]. The total current remained positive over the entire anodic
scan, suggesting the presence of mixture of molybdenum and
tungsten oxides as reported by Chassaing [28] and Obradovic
[29] for related binary alloys respectively, in parallel with Ni,
Mo and W elements in metallic state. For the sake of comparison, the cathodic parts of CV scan readings of the working
electrode (Pt), were recorded in Ni, NiMo, NiW and NiMoW
electroplating baths at room temperature (Fig. 1-d). As can be
seen, both the current density and potential for NiMoW are
enhanced compared to Ni, NiMo and NiW deposition
For further characterization of the electrochemical deposition process, EIS experiments were carried out. A family of
EIS (Nyquist and Bode) plots for the different coatings is
shown in Fig. 2(aec). The applied potentials at which
complex-plane impedance was measured and the frequencies
corresponding to the top of the Nyquist plot semicircle are
indicated in the figure. The Nyquist impedance plots are quite
similar for all the samples and exhibit one main loop in the
high frequency region in the form of a depressed semicircle
resulting from of combination of charge transfer resistance
and capacitance and indicating that the electrodeposition
process is mainly under activation control [30], followed by a
small sloping line in the low frequency domain representing
the Warburg impedance (Zw) as a result of diffusion phenomenon of various active species (such as hydrogen, Ni, Mo
and W species) from the bulk of the solution to the electrode-
electrolyte interface. The fluctuations observed in the EIS
plots at very low frequencies can be attributed to the hydrogen
evolution reaction in concurrence with the deposition process. The analysis of the impedance spectra in Bode-plan,
indicate the presence of a single time constant, suggesting
the existence of only one prevailing mechanism in the electroplating process. All phase-angles (Fig. 2c) are less than 20
and decreases when the deposition potential is lowered
further, which also confirms an accelerated charge transfer
process. The charge transfer resistance (Rct) values were
deduced from the intercept segment of the semicircle with the
real axis in Nyquist plots. Fig. 2-(d) shows the variation of
charge transfer resistance (Rct) with plating potential. The
experimental data are presented as scatter and the fitted results using ORIGIN software as continuous line. The resulting
equation is as follows:
Rct ¼ 290:8*exp 3:6686*Edep
The decrease of charge transfer resistance values from
3.6 U cm2 to 0.85 U cm2 (fitted: from 3.562 to 0.821 U cm2)
respectively with increasing polarization in the potential
range 1.2 to 1.6 V vs SCE should indicate that the nucleation/growth process of NiMoW coating onto a pre-formed
homogenous film is much easier that onto the heterogeneous copper substrate surface. The reduction reaction rate in
terms of the cathodic current density ic, for large activation
overpotentials, h can be expressed according to the ButlereVolmer equation [31]:
Fig. 2 e EIS plots for NiMoW alloy electroplating at different applied potentials (aec) and variations of Rct with corresponding
potential Edep (d); experimental (symbols) and fitted (solid lines).
Please cite this article in press as: Allam M, et al., Electrodeposition and characterization of NiMoW alloy as electrode material for
hydrogen evolution in alkaline water electrolysis, International Journal of Hydrogen Energy (2017),
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 2
ic ¼ i0 exp RT
where i0, b, F, R and T are the exchange current density, the
cathodic transfer coefficient, the “Faraday constant”, the “gas
constant” and the “absolute temperature” respectively.
On the other hand, the formation rate of a product at the
cathode-electrolyte interface, related to the electrode area, A
can be expressed by differentiating the Faraday's law for
electrolysis (m ¼ Aic t=nF) with respect to time:
dm=Adt ¼ i=nF
By combining eq (x) and eq (x), the electrochemical reaction rate constant, K of the reduction reaction at a given
overpotential h can be obtained:
K ¼ K0 exp RT
where K0 is the rate constant for reduction at Ei¼0, with respect
to the reference electrode. Therefore, the electrochemical reaction rate constant increases exponentially with the applied
overpotential h, which causes the exponential decay of Rt
with the applied cathodic potential (Fig. 2d). A similar
behavior has been reported by Chen et al. [32] during electrodeposition of CoNiFe ternary alloy.
Chronoamperometry is a powerful tool to study electrochemical nucleation mechanisms in deposition processes.
The formation and growth of a stable nucleon can be directly
observed by plotting the current versus transition-time evolution [33]. Scharifker and Hills [34] used the experimental
current transients to derive the mathematical models
describing the nucleation mechanisms during the first stages
of electrodeposition. Depending on the manner and rate at
which nuclei appear, the nucleation process during the first
few seconds of deposition may be classified as “instantaneous” or “progressive” and the growth mechanism is termed
as three-dimensional (3D) growth [35]. The instantaneous
nucleation corresponds to immediate and parallel formation
and growth of nuclei on active sites at the same rate. The
progressive nucleation refers to a mechanism in which new
nuclei are consecutively formed during the course of deposition. The mathematical expressions in a non-dimensional
form for instantaneous and progressive nucleation are given
by Eqs. (7) and (8), respectively,
1 exp 1:2564
for instantaneous nucleation and
2 #)2
1 exp 2:3367
for progressive nucleation, where imax and tmax are current
and time coordinates of the peak, respectively. Fig. 3a shows
experimental current density-time transient measurements
carried out under potentiostatic regime with five applied potential values, Edep ¼ 1.26, 1.28, 1.30, 1.32 and 1.34 V
versus SCE. As seen, the shapes of these current density
transients are similar. An abrupt decrease in the current
density value at the beginning means that, in the early stages
Fig. 3 e Effect of deposition potential on
chronoamperometry of NieMoeW plating process from
citrate electrolyte at room temperature: (a) Potentiostatic
currentetime transients and (b) dimensionless plots of the
normalized experimental currentetime transients:
instantaneous (solid line) and progressive (dashed line).
of the transient, current density is mainly utilized for doublelayer charging. With the passage of time, the current density
increases gradually until a maximum value (Im) corresponding
to the maximum time (tm) necessary for the growth of the
germs and boost of active sites number at the electrode surface. After that, the current is lowered again to a constant
value corresponding to a diffusion mode which controls the
process [36]. Increasing the applied potential further increases
the current density characteristic Im and decreases tm value.
This behavior indicate that the electrocrystallization process
rate increases as the applied potential value becomes more
electronegative, as a result of an enhanced surface concentration of active species which increase both the nucleation
and the growth rates associated with the deposition process.
The results obtained suggest a typical behavior for a threedimensional (3D) nucleation and growth under diffusion
control [34]. The experimental data from Fig. 3-(a) were used to
obtain dimensionless currents versus time plots, according to
Eqs. (3) and (4). The curves for instantaneous mechanism are
depicted as solid lines and the ones for progressive nucleation
Please cite this article in press as: Allam M, et al., Electrodeposition and characterization of NiMoW alloy as electrode material for
hydrogen evolution in alkaline water electrolysis, International Journal of Hydrogen Energy (2017),
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 2
as dashed lines. As can be seen on the graphs in Fig. 3b, in the
mixed activation-masse-transport region, the initial stages of
NiMoW nucleation occurred according to the instantaneous
nucleation mechanism, regardless of deposition potential
value; i.e., the formation of nuclei of the same size, which
advantageous criteria for plating homogenous films.
Composition and structural investigation
The amounts of metallic elements in the deposits were
measured using the energy dispersive X-ray spectroscopy
(EDS). According to the results reported in Table 1, at a given
deposition potential, increasing Mo/W salts ratio in the electrolyte reduced both Ni and W content in the deposits. A
different trend was observed in an equimolar solution
4 =MoO4 ¼ 1 : 1), where at a given potential, there is
more Mo than W in the deposits, indicating a favored induced
codeposition of Mo compared to W. When applying more
negative potentials, both Mo and W content decreased leading
to the enhancement of Ni amount.
According to Cruywagen et al. [37,38], in presence of citrate,
the tungstate ðWO2
4 Þ and molybdate ðMoO4 Þ ions form
complexes in the type of ½ðWO4 Þp ðCitÞq Hr ð2pþ3qrÞ and
½ðMoO4 Þp ðCitÞq Hr ð2pþ3qrÞ even though both are negatively
charged, where the number of protons, r, depends on the pH.
At pH 6, the predominant complexes are ½ðWO4 ÞðCitÞðHÞ2 3
and ½ðMoO4 ÞðCitÞðHÞ2 3 respectively. In the same period, Murase et al. [39] reported that on the addition of citrate to NieMo
plating bath, electrochemically active Hr MoO4 Citð5rÞ and
NiCit complexes are formed. More recently, Eliaz and Gileadi
[28] postulated that induced codeposition of NiW alloys occurs
through the reduction of a ternary mixed-metal complexed
species formed in citrate electrolyte:
½ðNiÞðCitÞ þ ½ðHWO4 ÞðCitÞ /½ðNiÞðHWO4 ÞðCitÞ
½ðNiÞðHWO4 ÞðCitÞ
þ Cit3
þ 8e þ 3H2 O/NiW þ 7ðOHÞ þ Cit3
The chemical properties of tungsten are substantially
similar to that of molybdenum and their alloys with Ni are
commonly obtained from electrolytes of the same type.
Moreover, Brenner [14] has suggested that there is an analogous behavior of Mo induced codeposition that can also hold
for W induced codeposition. Accordingly, the mechanism
proposed by Eliaz and Geleadi [40] for NiW deposition would
be valid for NiMo alloy: Firstly, Nickel-citrate reacts with
molybdate-citrate to form a ternary complex:
½ðNiÞðCitÞ þ ½ðHMoO4 ÞðCitÞ /½ðNiÞðHMoO4 ÞðCitÞ
þ Cit3
The reduction of this precursor leads to the formation of
the Ni/Mo alloy:
½ðNiÞðHMoO4 ÞðCitÞ
þ 8e þ 3H2 O /NiMo þ 7ðOHÞ þ Cit3
However, for NiMo deposition, in addition to the scheme
proposed by Eliaz and Geleadi, a parallel route for depositing
Mo as Ni4MoO2 compound is expected. This could explain the
presence of higher amount of Mo in the alloy compared to W
(Table 1). Thermodynamically, the molybdate anions MoO2
can be reduced to molybdenum oxide MoO2 at 1.2 V vs SCE
4 þ 2H2 O þ 2e /MoO2 þ 4OH
On the other hand, Chassaing et al. [28] reported that when
an electrode is polarized at low overpotentials (in the range of
1.35 to 1.45 Vvs SSE), the presence of Ni2þ ions allows the
possible transformation of such Mo oxide MoO2 into a mixed
NieMo oxide according:
MoO2 þ 4Ni2þ þ 8e /Ni4 MoO2
This possible additional route for depositing Mo would not be
applicable to W codeposition since, according to Pourbaix [42],
tungsten oxides (WO3, WO2 and W2O5) are not stable in the solution pH used here (pH ¼ 6.25). It should be noted that the
effective existence of all these compounds was not confirmed
since X-ray diffraction experiments (not reported here) revealed
the amorphous structure of all examined coatings. Table 1
shows the variations of Mo and W contents in the coatings
with Mo/W molar ratio in the solution and plating potential. At a
given applied potential (1.3 V/SCE), varying the Mo/W ratio
from 0.5 to 2, increases the Mo content in the deposit from
28.52 at% to 36.59 at%, while the W one decreases down to 1.06 at
%. This behavior is due to the mass transport effect, more expected for tungstate ions due to their low bulk concentration
than for molybdate and nickel species. Guettaf [25] thought that
presence of molybdate ions could reduce substantially the
deposition overpotential of Ni. On the other hand, in an equimolar solution (Mo/W ¼ 1), there is more Mo than W in the deposits. The main factor that may influence this behavior is that
the citrate ligand may complex with WO2
4 stronger than that of
4 . Increasing the cathodic polarization from activation
control region (1.1 V/SCE) up to mass-transport control zone
(1.4 V/SCE, Fig. 1d) causes a drop of both Mo and W contents
in the deposit. At low overpotential, the adsorption of Mo and W
species inhibits subsequent discharge of nickel ions and leads to
the preferential deposition of Mo and at less extent W, although
Ni deposition is not completely blocked. In slightly acidic citrate
solution (pH ¼ 6.25), even though the concentrations of
Table 1 e Elemental composition deduced from EDX analysis of NieMoeW coatings obtained under various deposition
potentials and Mo/W salts ratio in the plating bath.
Element (at%)
4 =WO4 Þ ðEdep ðV=SCEÞ ¼ 1:3)
Edep ðV=SCEÞ ðMoO2
4 =WO4 ¼ 1Þ
Please cite this article in press as: Allam M, et al., Electrodeposition and characterization of NiMoW alloy as electrode material for
hydrogen evolution in alkaline water electrolysis, International Journal of Hydrogen Energy (2017),
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 2
½ðMoO4 ÞðHCitÞðHÞ2 3 and ½ðWO4 ÞðHCitÞðHÞ2 3 active complexes
reach their maximum [37] and [38], increasing the deposition
overpotential further causes an increase in the nickel content in
the deposit until it reaches its maximum (64.4 at%) at around
1.4 V vs SCE, while Mo and W contents are lowered respectively from 73.34 at% and 8.53 at% for a deposition potential of
1.1 V vs SCE to 33.7 at% for a more cathodic potential (1.4 V vs
SCE. This could result from lower cathodic current efficiency of
both NiW and NiMo than pure Ni deposition and attributed to a
partial restraint of Mo and W active species by nickel ions. A
similar phenomenon has been observed by Xu et al. [18] who
reported that the deposition current efficiency of NiMo alloy
was lower than pure Ni as the potential moved negatively.
Morphological study
The surface morphological features of deposited NiMoW
alloys were studied by field emission scanning electron microscopy. A film plated onto copper substrate from a solution with a
ratio ¼ 2 contains micro-cracks (Fig. 4-a) due
4 =WO4
the relaxation of internal tensile stress after hydrogen trapping
into the coatings, which is the characteristic of the Mo-rich
electrodeposited NieMo alloys [43], while for a ratio
4 =WO4 ¼ 0.5, the cracks disappeared (Fig. 4-b). The topview images (Fig. 4a-b) show large real area composed of
spheroid grains sized 0.5e1 mm, which confirms the amorphousness structure of the coatings. Cross-sectional images of
NiMoW alloy coated from an equimolar bath (MoO2
4 =WO4 ¼ 1 )
are presented in Fig. 4-(ced), indicating crack-free and amorphous uniform layers with 0.5 and 1 mm thickness were obtained for plating potentials of 1.2 and 1.4 V vs SCE
Electrochemical corrosion behavior
The anti-corrosion performance of the coatings in 3.5% NaCl
solution was investigated using potentiodynamic polarization
and electrochemical impedance spectroscopy (EIS) methods.
Tafel polarization plots for NiMoW alloys plated at pH z 6.2
and variable applied potentials are shown in Fig. 5 (a). The
corrosion parameters reported in Table 2 show clearly that the
charge transfer resistance (Rct) of NiMoW coatings increased
with increasing applied cathodic potential in the range of 1.2
to 1.6 V/SCE while their corrosion rate (CR) decreased.
Therefore, it can be concluded that NieMoeW alloys, deposited at low overpotentials and containg high amounts of Mo
and W deduced for EDX analysis are more corrosion resistant
compared to alloys deposited at high overpotentials.
Furthermore, for the sake of comparison, data for pure Ni [15],
NiW [15] and NiMo alloys and inserted in Table 2, show clearly
an enhancement of corrosion resistance of the ternary alloy,
particularly for films coated at 1.2 and 1.3 V/SCE, where the
corrosion rate ( y1) and the current density (mA.cm2)
couples are reduced from 156.4/134, 73,95/63 and 163.6/139.9
for Ni, NiW and NiMo respectively to 55.56/4.75 for NiMoW.
The electrochemical impedance spectroscopy study revealed
the same trend as Tafel polarization. As can be seen in Fig. 5b,
it is clear that the diameter of the semicircle which describes
the electron-transfer resistance of the coating considerably
decreases with increasing the cathodic deposition potential,
which indicates faster corrosion reaction kinetics at the
electrode surface for films coated at high overpotentials. This
is consistent with the Tafel plots results. For low overpotentials, the small inductive loop observed at low frequencies domain (notably between 10 and 71 mHz), as a result
Fig. 4 e FESEM images of NiMoW samples deposited at pH 6.2, room temperature and ¡1.4 V vs. SCE, with Mo/W molar ratio
in plating bath: (a) 2; (b) 0.5; (c) 1 and (d) 1, coated at ¡1.2 V vs. SCE.
Please cite this article in press as: Allam M, et al., Electrodeposition and characterization of NiMoW alloy as electrode material for
hydrogen evolution in alkaline water electrolysis, International Journal of Hydrogen Energy (2017),
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 2
Fig. 5 e Potentiodynamic polarization (a) and EIS (Nyquist and Bode, b-d) plots recorded in 3.5 wt% NaCl solution for NiMoW
deposits obtained at different applied potentials (as indicated in the figures). For comparison, curves for NiW [15] and NiMo
alloy coated at ¡1.4 V/SCE are also added. The inset in (b) shows the EEC for NiMoW coating.
Table 2 e Kinetics parameters in 3.5 wt% NaCl deduced from Tafel and Nyquist plots for NiMoW deposits obtained at
different applied potentials. Ni, NiW and NiMo alloys were coated form a citrate-based solution at pH ¼ 6.25 and 25 C.
Edep (V/SCE)
Jcorr (mA.cm2)
Ei¼0 (mV/SCE)
Rct (kU.cm2)
Cdl (mF.cm2)
CR ( y1)
Ni [15]
NiW [15]
of adsorbed species at the electrode/electrolyte [44], is converted to a sloped line due the diffusion of active ions from the
bulk of the solution to electrode surface. The EIS spectra are
also presented in Bode-plan. The number of the slopes (Fig. 5c)
and the phase angle peaks (Fig. 5d) in Bode plot show the existence of only one time constant, and indicating that only
main mechanism is prevailing during the exposition of the
deposits to aggressive aqueous media. Meanwhile, all the
phase-angles for NiMoW coatings (Fig. 5c) are higher than
those for NiMo and NiW binaries. The results show a gradual
increase of the corresponding frequency of phase-angle peak
when the deposition potential is lowered, which confirms the
enhanced resistive behavior of the ternary alloys. Consequently, the high-to-medium frequency EIS data were fitted
using the Randles circuit [45]. The corresponding electrochemical equivalent circuit is shown in Fig. 5b.), where R1
represents the solution resistance, C is attributed to the
electrode capacitance and R2, the charge transfer resistance.
Based on these results, it seems likely that during electrodeposition at low applied overvoltages, the resulting local pH
increase and the presence of Ni2þ species induce the formation of a tungsten and molybdenum oxides or hydroxides
(NiO, Ni(OH)2, WO3 and MoO2). The incorporation of such oxides into the alloy structure enhances the corrosion resistance
of the deposits. A similar behavior has been observed by
Chassaing [28] for the electrodeposition of NieMo and attributed to the formation of a molybdenum oxide. For higher
polarization, these oxides are more readily reduced to the
metal form in the presence of adsorbed atomic hydrogen at
the electrode surface, using the unpaired 3d subband electrons of Ni, as suggested by Fukushima et al. [46], leading to
less protected coatings in corrosive media. This is in line with
Please cite this article in press as: Allam M, et al., Electrodeposition and characterization of NiMoW alloy as electrode material for
hydrogen evolution in alkaline water electrolysis, International Journal of Hydrogen Energy (2017),
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 2
the views of Guettaf and co-workers [25] who studied the effect of current density on the performances of NiMo alloy
electrodeposited from ammonium-based electrolyte and reported that the corrosion resistance decreases with increasing
deposition current density, The nobler NieMo sample,
deposited at low a current density (10 mA/cm2), contained
around 13 wt% Mo. According to Obradovic et al. [47], the
presence of Ni(I) intermediate could enhance the reduction of
metal-oxides formed during the deposition process. The formation of corrosion products can be described through the
following possible reaction pathways: in neutral aqueous solutions, the anodic reactions imply strong interactions between H2O and Ni-based alloy substrate, yielding primary
adsorbates Ni(H20)ad in equilibrium with (NiOH)ad adsorbate
[48]. The dissolution of nickel proceeds as follows [42]:
reaction (HER). The kinetic parameters for the HER were
derived from Tafel equation [50]:
h ¼ a þ blogj
where a (V), b (V.dec1), h (V) and j (A.cm2) represent the curve
intercept, the Tafel slope, the applied overpotential, and the
resulting current density respectively.
The generally accepted mechanism for the hydrogen evolution reaction HER in alkaline media involves the formation
of adsorbed hydrogen atoms, MHads at the electrode surface,
known as Volmer reaction (Eq. (11)), followed by either electrochemical (Heyrovsky step, Eq. (12)) or chemical (Tafel step,
Eq.z) desorption:
H2 O þ M þ e /MHads þ OH
Ni þ H2 O⇔NiðH2 OÞad ⇔NiðOHÞad þ Hþ þ e
H2 O þ MHads þ e /H2 þ M þ OH
NiðOHÞad þ H2 O⇔NiðOHÞ2 þ Hþ þ e
MHads þ MHads /H2 þ 2M
NiðOHÞ2 ⇔NiO þ H2 O
In chloride containing solution, an additional pathway
involving competitive adsorption between Cl and OH ions at
the electrode surface can be expected:
NiðH2 OÞad þ Cl ⇔NiðClOHÞ þ Hþ þ 2e
NiðClOHÞ⇔NiO þ Cl þ Hþ
The total oxidation reaction of nickel can be written as:
Ni þ H2 O⇔NiO þ 2Hþ þ 2e ; E0 ¼ 0:11 0:0591pH
The oxidation reactions of metallic Mo can result from the
following reaction sequence [42]:
Mo þ 2H2 O⇔MoO2 þ 4Hþ þ 4e ; E0 ¼ 0:072 0:0591pH
MoO2 þ 2H2 O⇔MoO2
4 þ 4H þ 2e
With E0 ¼ 0:606 0:1182pH þ
And for W [42]:
4 Þ
W þ 2H2 O⇔WO2 þ 4Hþ þ 4e ; E0 ¼ 0:119 0:0591pH
WO2 þ 2H2 O⇔WO2
4 þ 4H þ 2e
4 Þ
With E0 ¼ 0:386 0:1182pH þ
The common cathodic reactions in practical cases of
corrosion are by far reductions of hydrogen ion to hydrogen,
or of oxygen to hydroxyl ion. In neutral-aqueous solution, the
cathodic reduction of molecular oxygen is expected [49]:
O2 þ 2H2 O þ 4e /4OH
Molecular oxygen is continually replaced from the gas
phase by diffusion and/or convection through the liquid to the
electrode surface.
By plotting the experimental dependence of the potential
(E) on the logarithm of the current density (logi), known as the
Tafel polarization curves, one can deduce which reaction of
these is the rate determining-step. At 25 C, a cathodic slope b
(b ¼ 2:3RT=aF, where a is the charge transfer coefficient) of
120, 40 or 30 mV$dec1 is ascribed to Volmer, Heyrowsky or
Tafel reaction respectively [51]. Generally, an electrode material showing low Tafel slope is indication of favoring
hydrogen evolution. The experimental kinetic parameters,
indicative of electrocatalytic performances of NiMoW deposits
coated at different potentials using an equimolar
4 =WO4 ¼ 1 : 1) electrolyte were determined automatically from the Tafel plots recorded in 30% KOH solution. The
results are summarized in Table 3. As can be seen, the exchange current density increases gradually with increasing
deposition potentials, indicating faster hydrogen gas generation i.e highest activity for HER. Furthermore, the experimental Tafel slope values for all NiMoW alloys changed
between 60 and 77 mV$dec1 s for low-to-high deposition
potentials respectively, suggesting gradual change from
Heyrowsky to Volmer step rate-determining mechanism for
the HER. For example, the magnitude of the Tafel slope
(63 mV/dec) obtained on NiMoW coated at 1.5 V vs. SCE is
much lower than that obtained on NiMoFe (165 mV/dec) reported by Raj [52]. This result reflects an enhancement in the
Table 3 e Kinetic parameters determined from Tafel
curves in 30% KOH solution for NiMo, NiW and NiMoW
coatings obtained at different potentials using an
equimolar Mo/W electrolyte.
Electrocatalytic activities of NiMoW films for HER
Linear polarization method was used to estimate the electrocatalytic activity of NiMoW coatings for hydrogen evolution
(V/SCE) (mV/SCE) (102 mA cm2) (mV.dec1)
Please cite this article in press as: Allam M, et al., Electrodeposition and characterization of NiMoW alloy as electrode material for
hydrogen evolution in alkaline water electrolysis, International Journal of Hydrogen Energy (2017),
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 2
1,2 V
1,3 V
1,4 V
1,5 V
1,6 V
log i(mA/cm²)
Fig. 6 e Tafel polarization curves recorded in 30 wt% KOH at
25 C on NiMoW films coated at various potentials. NiMo
and NiW electrodes were coated at ¡1.4 V/SCE.
surface coverage of the adsorbed hydrogen on these cathodes.
For the sake of comparison, the cathodic part of Tafel plots for
NiMo and NiW alloys, coated at 1.4 V vs SCE from the same
citrate-based electrolyte at pH ¼ 6.25 and room temperature,
are shown in the same figure. It is clearly seen that the NiMoW
alloy electrode obtained at 1.5 V/SCE had a much better
catalytic activity than the binary NiMo and NiW electrodes,
since Tafel slope for NiMo and NiW samples were larger (150
and 112.5 mV/dec) with lower exchange current densities. The
decrease of Tafel slope value for the ternary alloys compared
to binary ones could be explained by terms of increased surface area. For any applied potential, when the electrode surface area increases, the real current density is higher than the
apparent current density, which causes the decrease of Tafel
slope value [22]. Sum up, the good electrocatalytic activity of
NiMoW alloy coated at high overpotential could be the result
of synergetic effect of Mo and W in the films, increased electrode surface area and reduction of their oxides by atomic
hydrogen adsorbed on freshly deposited Ni atoms. It is worth
noting that the coatings having the best catalytic activity towards hydrogen evolution are les noble compared to films
coated at low overvoltages which revealed better corrosion
resistance in aggressive media, most probably due the presence of molybdenum and tungsten oxides on the electrode
surface [19]. (see Fig. 6).
Ternary NiMoW alloys were electrodeposited onto copper
sheet substrates from citrate-ammonia free aqueous
electrolyte at room temperature. In an electrolyte where
4 =MoO4 ¼ 1 : 1, there was more Mo than W in the deposits, indicating advantageous induced codeposition of Mo
compared to W. With applying low overpotentials (i.e deposition current densities), the linear polarization resistance and
electrochemical impedance study revealed the possible formation and incorporation of molybdenum-tungsten oxides
allowing a better corrosion resistance to the alloys in corrosive
media but decreasing their catalytic activity for the hydrogen
evolution reaction. Films coated at higher voltages exhibited
higher exchange current densities in 30 wt% KOH solution,
indicating better electrocatalytic properties towards HER as a
result of synergetic effect of the three elements in the coatings
and high real area. The application of ScharifkereHills theoretical models to study the first instants of the electrodeposition of NiMoW film suggested instantaneous nucleation
mechanism for all specimen, regardless of the metal ions
concentrations and the applied potentials; i.e., the formation
of nuclei of the same size, which represents an advantageous
criteria for plating homogenous films. Further experimental
work will be carried out to test such ternary alloys under the
industrial application conditions (20e80 mm thick in 30% KOH
at 85 C).
The authors would like to express gratitude to the Centre de
Development des Energies Renouvelables-CDER (Algeria) for
financial support provided through Project PNR No. 10/u19/
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Please cite this article in press as: Allam M, et al., Electrodeposition and characterization of NiMoW alloy as electrode material for
hydrogen evolution in alkaline water electrolysis, International Journal of Hydrogen Energy (2017),