Electrodeposition and characterizationof NiMoW alloys

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
a
Inorganic Materials Laboratory, Department of Chemistry, Faculty of Sciences, Mohamed Boudiaf University,
M'sila, 28000, Algeria
b
Energetic and Solid-State Electrochemistry Laboratory, Faculty of Technology, Ferhat Abbas-Setif1 University, Setif,
19000, Algeria
article info
Article history:
Received 31 January 2017
Received in revised form
20 July 2017
Accepted 5 August 2017
Available online xxx
Keywords:
Electrodeposition
NiWMo alloy
Hydrogen evolution
Nucleation mechanism
Impedance spectroscopy
Cyclic voltammetry
abstract
The electrodeposition of ternary NiWMo alloys films from citrate ammonia-free electrolyte
at room temperature was studied in an effort to evaluate the effect of applied potential on
the composition limits, corrosion resistance and the electrocatalytic properties of the de-
posits towards the hydrogen evolution reaction (HER) in concentrated alkaline solution.
The alloys were potentiostatically electrodeposited onto pure copper sheet substrates. The
electrodeposits were characterized by means of field-emission scanning microscopy
(FESEM) and energy dispersive X-ray analysis (EDXA). In an electrolyte where
MoO2
4=WO2 4¼1:1, 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
nucleation mechanism, studied according to Scharifker-Hills theoretical model, revealed
an instantaneous nucleation followed by a three-dimensional growth. On the hand,
increasing MoO2
4=WO2 4ratio in the electrolyte under the same deposition potential
reduced both Ni and W content in the deposits. A different trend was observed in an
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 pro-
posed and discussed.
©2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction
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]. Unfortu-
nately, 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).
Available online at www.sciencedirect.com
ScienceDirect
journal homepage: www.elsevier.com/locate/he
international journal of hydrogen energy xxx (2017) 1e12
http://dx.doi.org/10.1016/j.ijhydene.2017.08.012
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), http://dx.doi.org/10.1016/
j.ijhydene.2017.08.012

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 ¼DErev þhaþjhcjþIR þDEst
Where DErev is the reversible thermodynamic value
(z1.23 V) which depends on the nature of the electrochemical
reactions, haand hcthe 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 per-
formance 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 over-
potentials depend mainly on the electrode material proper-
ties. During the last few years, significant improvements have
been made in alkaline water electrolysis through the devel-
opment 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] re-
ported that increasing the operating steam electrolysis tem-
perature 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 pro-
vide high current densities. However, increasing bath tem-
perature 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 low-
cost 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 d-
electrons, 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] 3d
8
4s
2
) with tungsten, W ([Xe] 4f
14
5d
4
6s
2
) and/or molybdenum, Mo ([Kr] 4d
5
5s
1
) are among the
materials fulfilling these requirements [8e11]. Moreover, the
Pt-group metals are known to be the best electrocatalysts for
hydrogen evolution reaction (DGz0.1 eV). The next ideal
HER catalysts are nickel and cobalt (DGz0.28 eV) followed
by molybdenum (DGz0.36 eV) and tungsten
(DGz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 evolu-
tion. 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 complex-
ing, buffering, brightening and levelling agent [16] in electro-
plating 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 pyrophosphate-
based 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-
Nomenclature
AC Alternative current
BTafel slope, log2.303RT/aF
ВCharge transfer coefficient
CElectrode capacitance
CSL Cathodic scan limit
Edep Deposition potential
EDS Dispersive X-ray spectroscopy
EEC Electrochemical Equivalent Circuit
EIS Electrochemical impedance spectroscopy
Erev Reversible thermodynamic potential
F Faraday constant, 96500 C
FESEM Field emission scanning electron microscope
HER hydrogen evolution reaction
I
0
Apparent exchange current density
i
c
Cathodic current density
i
max
Current coordinate of the peak
KElectrochemical reaction rate constant
K
0
Rate constant at E
i¼0
n Number of electrons transferred
h
a
Anodic overpotential
h
c
Cathodic overpotential
OCP Open circuit potential
Rct Charge transfer resistance
Rs Solution resistance
SCE Saturated calomel electrode
t
max
Time coordinate of the peak
Zi Imaginary part of Z
Zr Real part of Z
international journal of hydrogen energy xxx (2017) 1e122
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), http://dx.doi.org/10.1016/
j.ijhydene.2017.08.012

workers [18] thought that the high electrochemical activity for
the hydrogen evolution of the NieMo coating could be attrib-
uted to both exchange current density and larger real elec-
trode 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 high-
est 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 cur-
rent efficiency. Citrate baths provided higher W content in
comparison with those containing tartrate or malate. The ef-
fect 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 ammoniacal-
citrate 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 conduct-
ed. Cesiulis and co-workers [23] investigated the effect the
ratio of the Na
2
MoO
4
and Na
2
WO
4
concentrations in a pyro-
phosphate bath on the content of the ternary NieMoeW al-
loys. 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 evo-
lution reaction (HER) in concentrated alkaline media.
Experimental
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 as-
sembly at ambient (laboratory) temperature. Platinum wire
and pure copper sheets sized 1 2cm
2
were used as working
electrode, a platinum foil having a large-area to prevent po-
larization of the anode was used as counter electrode and
saturated calomel electrode (SCE) served as reference elec-
trode. 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) hy-
drochloric 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 NiSO
4
$6H
2
O, 0.05 M
NiCl
2
$6H
2
O, Na
2
WO4, Na
2
MoO
4
.2H
2
O as metal sources (with
W/Mo ratio ranging from 0.5 to 2) and 0.4 M Na
3
C
6
H
5
O
7
$2H
2
O
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 (WTW-
inoLab pH 7310). No agitation was utilized in all experiments.
To investigate the corrosion resistance and catalytic activ-
ities of electrodeposited NiMoW films for hydrogen evolution
reaction (HER) in a 3.5% NaCl and 30% KOH solutions respec-
tively, 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 threeelectrode
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/cm
2
) 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 Karadiation source (lKa¼1.5418  A)
in the (2q) range of 20e100. The compositional and morpho-
logical 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.
international journal of hydrogen energy xxx (2017) 1e12 3
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), http://dx.doi.org/10.1016/
j.ijhydene.2017.08.012

Results and discussion
Electrochemical aspects of electrodeposition
Cyclic voltammetry (CV) was used to investigate the electro-
chemical 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. s
1
and 25 C are
presented in Fig. 1. It is well established that in quasi-neutral
citrate electrolyte (рНz6), nickel and citrate ions form com-
plexes mainly of the type NiCit
with stability constant
log(K) ¼5.379. The voltammogram for pure Ni recorded be-
tween 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 Ni
2þ
ions:
NiCitþ2e/Ni þCit3(1)
and two anodic peaks at 0.075 and þ0.6 V/SCE correspond-
ing to the formation of nickel hydroxide (Eq. (2)) and nickel
oxyhydroxide (Eq. (3)) respectively.
Ni þ2OH/NiðOHÞ2þ2e(2)
NiðOHÞ2/NiOOH þHþþe(3)
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 mo-
lybdenum oxides. It should be noted that during the reversal
scan for Ni and NiMo, the presence of a crossover is charac-
teristic 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 þW 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 eCyclic voltammograms recorded in citrate electrolyte at room temperature and pH ¼6.25 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).
international journal of hydrogen energy xxx (2017) 1e124
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), http://dx.doi.org/10.1016/
j.ijhydene.2017.08.012

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 com-
parison, 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
processes.
For further characterization of the electrochemical depo-
sition 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 phe-
nomenon 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 pro-
cess. 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 elec-
troplating 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 re-
sults using ORIGIN software as continuous line. The resulting
equation is as follows:
Rct ¼290:8*exp3:6686*Edep
The decrease of charge transfer resistance values from
3.6 Ucm
2
to 0.85 Ucm
2
(fitted: from 3.562 to 0.821 Ucm
2
)
respectively with increasing polarization in the potential
range 1.2 to 1.6 V vs SCE should indicate that the nucle-
ation/growth process of NiMoW coating onto a pre-formed
homogenous film is much easier that onto the heteroge-
neous copper substrate surface. The reduction reaction rate in
terms of the cathodic current density i
c
, for large activation
overpotentials, hcan be expressed according to the Butler-
eVolmer equation [31]:
Fig. 2 eEIS plots for NiMoW alloy electroplating at different applied potentials (aec) and variations of R
ct
with corresponding
potential E
dep
(d); experimental (symbols) and fitted (solid lines).
international journal of hydrogen energy xxx (2017) 1e12 5
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), http://dx.doi.org/10.1016/
j.ijhydene.2017.08.012