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Nanocrystalline nickelecobalt electrocatalysts togenerate hydrogen using alkaline solutions asstorage fuel for the renewable energy
Mosaad Negem a,*, H. Nady a,b, M.M. El-Rabiei a
a Chemistry Department, Faculty of Science, Fayoum University, Fayoum, Egyptb Chemistry Department, College of Science & Arts in Qurayat, Jouf University, Saudi Arabia
a r t i c l e i n f o
Article history:
Received 6 December 2018
Received in revised form
14 March 2019
Accepted 16 March 2019
Available online 10 April 2019
Keywords:
NieCo alloys
Electroplating
Ultrasound
EIS
Hydrogen evolution
* Corresponding author.E-mail address: [email protected] (M
https://doi.org/10.1016/j.ijhydene.2019.03.1280360-3199/© 2019 Hydrogen Energy Publicati
a b s t r a c t
Generation of hydrogen using an electrocatalyst is valuable research field for the energy
conversion and storage of the renewable energies. Electroplating of nanocrystalline
metals and their alloys thin film is auspicious for the conversion and storage of the en-
ergy in the form of hydrogen fuel during water electrolysis. In this work, we electroplated
the nanocrystalline NieCo alloys of different Co% using natural compounds such as
gluconate and cysteine via the galvanostatic-ultrasonication conditions. The electro-
plated NieCo alloys have been characterized using energy dispersive X-ray, X-ray
diffraction and scanning electron microscopy, to determine their elemental composition,
crystal lattice system and surface morphology, respectively. The morphological structure
of the electroplated NieCo alloys varies from dense and lustrous to granular. The elec-
troplated NieCo alloys arranged in the face centred cubic or hexagonal closed packaged
depending on the Co%. The electroplated NieCo alloys arranged in the small unit cell
which enhances electron transfer and boosts the rate of hydrogen reduction. The elec-
trocatalytic activity of the electroplated NieCo cathodes towards hydrogen reduction
reaction was investigated using cathodic polarization and electrochemical impedance
spectroscopy, EIS, dipped in 1.0 M KOH solution. The electroplated Nie50Co cathode
displays the superior electrocatalytic activity and the lowest overpotential for the
hydrogen evolution reaction than bulk Ni.
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction
The promising and sustainable fuel is the hydrogen obtained
from renewable sources such as photovoltaics. The photo-
voltaic devices are the valuable generator of the electricity but
the produced electricity is too prohibitive for storage. There-
fore, it necessitates converting the generated electricity using
. Negem).
ons LLC. Published by Els
the storable form such as hydrogen which can be considered
the auspicious, clean and ideal fuel for daily life applications.
Hydrogen is the most abundant element with high energy
content which can substitute the fossil fuel and hydrogen is
utilized in the fuel cell to generate efficiently the electricity
[1e7]. Water electrolysis using alkaline media supplies the
non-polluting and workable method for the generation of the
hydrogen fuel. A low-cost production of hydrogen is a key
evier Ltd. All rights reserved.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 4 ( 2 0 1 9 ) 1 1 4 1 1e1 1 4 2 011412
factor for bringing this technology to commercialization [8,9].
In order to satisfy economic, environmental and technical
criteria and achieve cheap hydrogen production, it is essential
to investigate applicable media and cathodes for the genera-
tion of the hydrogen using water electrolysis. The electrode
materials used for the HER should attain large and active
surface area, physically, chemically and electrochemically
stable, selective, low cost, and safe to use and handle. The
most important factors influenced electrolysis of water are
the high charge transfer and the small overpotential. Noble
metals, such as platinum and ruthenium, are the most suit-
able and useable materials for this purpose, but they are
expensive for industrial applications. The electrocatalytic ef-
ficiency of the HER can be boosted by the synergetic effect of
the electrocatalytic constituents of the cathode and the
considerable area of the surface with nanosize grains [10,11].
Ni and Ni alloys are the most active electrocatalyst in
concentrated alkaline solutions. The electroplated Ni alloys
possess the notable electrocatalytic characteristics for the
reaction of hydrogen reduction and the different catalytic
processes. The electroplated Ni alloys show the high chemical
stability in the concentrated alkaline medium, attain the
appropriatemechanical properties and have inexpensive cost.
The electrocatalytic activity of Ni towards the hydrogen evo-
lution was increased by different alloying transition elements
either binary or ternary Ni alloys such as NieMo, NieCu,
NieCo, NieZn, NieTi, NiePt, NieV, NieW, and NieFeeZn
using electroplating techniques [12e22]. The Ni and NieCo
alloys cathodes characterize by the high electrocatalytic effi-
ciency with the remarkable amount of the generated
hydrogen which decrease the energy consumption due to
improving the reduction mechanism [23e35]. It is supposed
that the Ni-sites in such NieCo alloy are very well distributed
over the surface, forming adsorbed, NieH in potential less
cathodic than other alloys, which leads to hydrogen evolution
after the first electron transfer [25] by decreasing the energy of
activation for the HER. The electroplating is the promising
synthesis technique which enhance formation of homoge-
neous nanograin structures of small unit cell possessed huge
surface area leading to the change of the physical and
chemical characteristics of the electroplatedmetal and alloys.
The electroplating of NieCo alloys attains the numerous
benefits such as low cost (compared to Pt or Ru), quick and
straightforward method to form small unit cell with large
surface area improving their electrocatalytic properties for the
hydrogen evolution reaction. Moreover, the electroplated
NieCo alloys can arranged in the new crystal lattice system
which can produce novel state of the electronic properties
enhancing the synergetic effect of electroplated NieCo alloys
towards the HER. Furthermore, the highly electrocatalytic
characteristics of NieCo alloys as cathodes towards HER in the
alkaline solutions encourages to be used for the potential
application to produce hydrogen gas in the industry [22]. In
our present work, the nanostructured nickel-cobalt alloys
have been electroplated from cysteine/gluconate/boric acid
electrolyte under ultrasonication. The electrocatalytic char-
acteristics of the nickel-cobalt cathodes were investigated
towards hydrogen evolution reaction in 1.0 M KOH solutions
by electrochemical impedance spectroscopy and cathodic
polarization.
Experimental
Alloy nanostructure and characterization
The electroplating of NieCo alloys was performed on Copper
foil with 99.98% purity as cathode and the anode was a plat-
inummesh of 2.4 cm2 surface area. The electroplating ofNieCo
cathodes was accomplished in a Pyrex cylinder cell using TTI
PL310 32V-1A PSU as the producer of the galvanostatic current
and Branson 3510 with the power of 100 W and frequency of
42 kHz was utilized as the generator of ultrasonic waves. The
Mettler Toledowas employed to gauge the conductivity and pH
of the electroplating bath. In addition, the chemicals were
utilized in the electroplating baths such as cysteine, CoSO4-
$7H2O, sodium gluconate, H3BO3 and NiSO4$7H2O obtained
from Merck and Aldrich-Sigma. The sundry concentrations of
gluconate (complexing agent), boric acid, cysteine (brightener)
at different current densities, were applied to improve the
electroplating of NieCo nanocrystalline alloys. The electro-
plated NieCo nanocrystalline alloys were obtained from the
gluconate bath using current density of 5 A dm�2 for 1 h at
293 K and pH 4. The optimized amount of chemicals used
during electroplating of NieCo alloys is presented in Table 1.
The electroplating bath was newly prepared for each electro-
plating experiment. The copper foil cathode was immersed in
concentrated HNO3 (1:1) to eliminate the Cu oxide layer, af-
terwards rinsed by deionized water, washed with acetone.
Besides, the crystal lattice system, elemental composition and
morphological structure of the electroplated NieCo nano-
crystalline alloys were examined via X-ray diffraction (XRD),
energy dispersive X-ray analysis (EDX) and scanning electron
microscopy (SEM) (JOEL JSM-5300 LV, at 25 kV under high
vacuum), respectively. The theoretical XRD pattern of the
electroplated NieCo alloys was obtained using WinXpow pro-
gram and compered with experimental XRD pattern to deter-
mine the unit cell parameters and crystal lattice system.
Electrochemical techniques for hydrogen evolution reaction
The electrochemical techniques including the potentiody-
namic polarization and the electrochemical impedance
spectroscopymeasurementswere performed via Voltalab PGZ
100 “All in one” potentiostat/galvanostat with Electrochemical
Analyzer controlled by the computer. Furthermore, the double
Jacket cell of five electrode configuration was utilized to
investigate themechanism of the hydrogen evolution reaction
in 1.0 M potassium hydroxide medium, the reference elec-
trode was a saturated calomel electrode (SCE) of 3.0 M KCl and
the sheet of platinum with surface area of 1.5 cm2 was an
auxiliary electrode. The electrochemical measurements have
been performed via the Voltalab 10 PGZ 100 “All-in-one”po-
tentiostat/galvanostat. The system is supported by an unit of
interruption compensating any ohmic, (IR), drop between the
reference electrode and the working electrode and the plots of
the potentiodynamic polarization are extrapolated automati-
cally to the highly linear region through the Tafel curves. The
newly made potassium hydroxide using deionized water was
used as the electrolyte for the electrochemical experiments.
The cathodic polarization investigation of the HER using
Table 1 e The composition of Baths utilized for the electroplated NieCo alloys and their chemical composition obtainedfrom EDX analysis at 293 K.
Bath NiSO4/M CoSO4/M Sodiumgluconate/M
Boric acid g.l�1 Cysteine M pH Currentdensity/A.dm�2
Chemical composition
Ni % Co %
1 0.1 e 0.1 10 0.00018 4.00 5 99.99 e
2 0.0992 0.0008 0.1 10 0.00018 4.00 5 98.00 02.00
3 0.099 0.001 0.1 10 0.00018 4.00 5 96.12 03.88
4 0.095 0.005 0.1 10 0.00018 4.00 5 91.32 08.68
5 0.090 0.01 0.1 10 0.00018 4.00 5 88.18 11.82
6 0.089 0.011 0.1 10 0.00018 4.00 5 87.26 12.74
7 0.088 0.012 0.1 10 0.00018 4.10 5 84.21 15.79
8 0.087 0.013 0.1 10 0.00018 4.12 5 54.06 45.94
9 0.083 0.017 0.1 10 0.00018 4.17 5 50.00 50.00
10 0.080 0.02 0.1 10 0.00018 4.17 5 47.30 52.70
11 0.075 0.025 0.1 10 0.00018 4.17 5 29.20 70.80
12 0.050 0.05 0.1 10 0.00018 4.20 5 23.41 76.59
13 0.035 0.065 0.1 10 0.00018 4.25 5 04.58 95.42
14 e 0.1 0.1 10 0.00018 4.25 5 e 99.99
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electroplated NieCo alloys cathodes has been executed via
rate of scan (5 mVs�1) to achieve a quasi-stationary condition.
Moreover, the impedance (Z) and shift of phase (q) have been
gauged through the frequency range of 0.1e105 Hz. The peak
to peak of 10mV has been used as the superimposed ac-signal
and the information of the experimental techniques was
illustrated elsewhere [17,25].
Results and discussion
Nanostructure morphology, crystal lattice system andelemental analysis
The nanocrystalline NieCo alloys have been electroplated
using gluconate bath on Cu foil with different Co% such as 2,
Fig. 1 e SEM/EDX analysis of a. Nie2Co, b. Nie12.74Co, c. Nie52
ultrasound waves for 1 h at 25 �C.
8.68, 11.8, 12.74, 15.79, 45.94, 50, 52.7, 70.8, 76.59, 95.42 as
shown in Table 1. The morphological structure of the elec-
troplated NieCo alloys is shown in Fig. 1 which is smooth,
dense and lustrous arranged in the nanograins due to the
additives and ultrasonication [36]. However, the morpholog-
ical structure of pure Co is spherical grains. The number of the
valleys spread through the surface morphology of the elec-
troplated NieCo alloys increases with the growth of the Co%.
The EDX analysis obtained for the electroplated NieCo alloys
are combined with SEM image as merged in Fig. 1 and the
elemental composition of the electroplated NieCo alloys is
exhibited in Table 1. The EDX spectra confirm that the highly
pure NieCo alloys were electroplated with various composi-
tion of Ni and Co using the gluconate baths. Fig. 2 shows the
XRD patterns acquired for the as-electroplated NieCo alloys
with Co content of 1e75% which positioned in face centred
.70 Co and d. Nie76.59Co electroplated via conventional
Fig. 2 e XRD patterns of electroplated NieCo alloys from gluconate bath using conventional ultrasound waves and current
density of 5 A. dm¡2 for 1 h at 25 �C.; a-face centred cubic-space group: F m ¡3 m (225), b. hexagonal closed package -space
group: P 63/m m c (194).
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cubic while electroplated NieCo with Co content more than
76% arranged in hexagonal closed package. The electroplated
NieCo alloys with Co content of 1e75% display face centred
cubic (space group: F m �3 m (225)) of the unit cell parameters
changed between a ¼ 3.524 Angstrom and a ¼ 3.5358
Angstrom with the increase of Co%. Whereas, the hexagonal
closed package (space group: P 63/m m c (194)) was obtained
for the electroplated NieCo alloys of Co%more than 76% with
the unit cell parameters of a ¼ 2.538 Angstrom, c ¼ 4.12
Angstrom. The XRD patterns confirmed that the electroplated
NieCo alloys possess the nano-size grains which are clear by
the wide peaks intensities. The average grain size of the
electroplated NieCo alloys was calculated via the XRD peaks
using Debye-Scherer's equation [37] which enlarged from
10 nm to 42 nm with the increase of the Co%.
Electrochemical behaviour and electrocatalytic activity of theNieCo cathodes towards hydrogen evolution
The stability of the electroplated NieCo alloys against corro-
sion in a given medium is a substantial prerequisite for its
further application. Fig. 3a presents the Tafel polarization
curves of the electroplated NieCo alloys in 1 M KOH solution.
The corrosion rate of the electroplated NieCo alloys in 1 M
Fig. 3 e a: Polarization curves recorded with nano
deposited NieCo alloys in stagnant naturally aerated 1 M
KOH solution at 25 �C. b: Cathodic polarization of
electrodeposited NieCo alloys in stagnant naturally
aerated aerated 1 M KOH solution at 25 �C versus SCE.
(Inset): Cathodic currentepotential curves of bulk Ni (d),
nano crystalline Ni-50.4%Co (- - -), deposited Ni (…..) and Pt
electrode (·-·-) in stagnant naturally aerated 1 M KOH
solution at 25 �C.
Table 2 e The steady state potential, hydrogen evolutionpotential and the cathodic hydrogen overpotentialdetermined from cathodic current potential curves for thedifferent investigated materials in stagnant naturallyaerated in stagnant naturally aerated 1M KOH solution at25 �C.
Materials Ess/mV
E Hydrogen evolution/mV
Cathodic Hydrogenoverpotential/mV
Ni bulk �275 �1380 �1105
Ni electroplated �335 �1326 �991
Nie3.88Co �292 �1240 �948
Nie11.82Co �355 �1345 �990
Nie50Co �350 �1180 �830
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KOH was calculated and found to be in the range of 14.3e33.4
mmy�1. Also, the corrosion rate of electrodplated Ni was
measured and found to be 241,7 mmy�1, which mean that the
addition of Co to Ni increases the stability of NieCo alloys.
These values of corrosion rate indicate that the electroplated
NieCo alloys are remarkably stable in this alkaline solution
and can be used conveniently as efficient cathodes for the
hydrogen evolution reaction. Furthermore, the cathodic po-
larization and EIS were employed to determine the electro-
catalytic activity of the electroplated NieCo cathodes towards
the HER using 1.0 M KOH (stagnant naturally aerated) at 25 �C.The measurements of the cathodic polarization were per-
formed to obtain the overpotential of the hydrogen evolution
reaction using the electroplated NieCo cathodes. Also, the
overpotential of the hydrogen evolution reaction was ob-
tained for Pt, ed. Ni and bulk Ni cathodes as exhibited in Fig. 3b
which is considered highly effective parameter to determine
the electrocatalysis activity of the cathodes. In addition, the
electrocatalytic activity of the electroplated NieCo cathodes is
shown by the current density obtained at the same potential.
The potential of steady state and the potential where the
hydrogen begins to be reduced for each cathode have been
recorded using the same conditions and shown in Table 2. In
fact, the electroplated NieCo alloys attain less negative po-
tential of the hydrogen evolution than bulk Ni and Co. It is
significantly noticed that the electroplated Nie50Co cathode
shows the less negative potential for the hydrogen evolution
reaction than electroplated Nie3.88Co, electroplated Nie11.82
Co, electroplated Ni (ed. Ni), bulk Ni and bulk Co cathodes (cf.
Table 2). Therefore, the electroplated Nie50Co cathode pos-
sesses highest electrocatalytic properties than bulk Ni, ed Ni
and bulk Co. This is due to the electroplated Nie50Co cathode
arranged in the small unit cell or nano-grains and synergetic
effect which boost the electrocatalytic properties of the elec-
troplated Nie50Co cathode towards the HER. The electro-
plated Nie50Co cathode displayed lowest overpotential of the
reaction of the hydrogen reduction about �830 mV. It is
palpable that the electroplated NieCo cathodes have gener-
ated notably the higher cathodic current of HER than the bulk
Ni, as shown in Fig. 3b. The electroplated Nie50Co cathode
produced the cathodic current density of 100 mAcm�2 and
overpotential of �830 mV for the HER which is lower than the
overpotential of �985 mV obtained by Pt. The electroplated Ni
shows the overpotential of �991 mV which is lower than bulk
Ni of �1105 mV for the HER. It is recognizable that all elec-
troplated NieCo alloys offer the similar onset potential for
hydrogen evolution reaction which confirms that the
commencing of nucleation happens through the whole cath-
ode surface concurrently. Fig. 3b presents the hydrogen evo-
lution reaction produced using the electroplated NieCo alloys
which is excessive and the cathodic current density was
increased sharply with the ascending of the applied poten-
tials. Indeed, alloying of differentmetals such asNi and Co can
balance between the adsorption free energy of hydrogen ions
and releasing properties of hydrogen gas which is greatly in-
crease the electrocatalytic properties of these electroplated
NieCo cathode [38,39]. The cathodic polarization data implies
that the electroplated Nie50Co alloy is notably considered an
active electrocatalyst for the hydrogen evolution reaction.
Actually, the electroplated Nie50Co cathode displayed the few
vales which are imperative to maximise surface utilizations
by permeating with the solution, supplies the vast surface
area for the HER. Likewise, the electroplated NieCo alloys
demonstrate various sorts of vales which augment the HER.
The vales are considered aptly the smallest paths of the
diffusion to the dissolved hydrogen which facilitate fast
Fig. 4 e Nyqusit plots of deposited Ni (,,,), Nie3.9Co
(CCC), Nie11.8Co (DDD) and Nie50Co (;;;) electrodes
recorded at the steady state potential in stagnant naturally
aerated 1 M KOH solution at 25 �C. Fig. 4(inset): Equivalentcircuit used for impedance data fitting. RU ¼ solution
resistance, Cdl ¼ double layer capacitance, Rct ¼ charge
transfer resistance, Rp ¼ pore resistance and Cp ¼ pore
capacitance.
Fig. 5 e Bode plots of deposited Ni (,,,), Nie3.9Co
(CCC), Nie11.8Co (DDD) and Nie50Co (;;;) electrodes
recorded at the steady state potential in stagnant naturally
aerated 1 M KOH solution at 25 �C.
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liberating of the hydrogen gas and prohibit the needless
collection of the hydrogen gas and polarization of concen-
tration in the vales which have been discussed in our previous
work [40]. The small unit cell and vales noticed at the
morphology of electroplated NieCo cathode produce the huge
surface area and lower the overpotential of the HER. There-
fore, the electroplated Nie50Co cathode attained the few
numbers of vales possesses the largest surface area which is
the most convenient cathode for hydrogen evolution. This
result indicates that the electroplated Nie50Co cathode rep-
resents a promising electrocatalyst for the generation of
hydrogen in alkaline solutions. It is worthwhile to mention
that the rate of hydrogen evolution increases as the Co con-
tent increases to 50%, while HER decrease with the increase of
Co% more than 51% for the electroplated NieCo cathodes.
This may be due to the influence of synergetic effect of the
electroplated NieCo cathodes incorporating their chemical
and physical properties. The change in electrocatalytic activ-
ity of the electroplated NieCo cathodes is ascribed to the
variation of their chemical composition and the highest value
of the electrocatalytic activity was obtained for the electro-
plated NieCo cathode containing Co of 50%. The values of
Tafel slope were obtained via the linear least square fitting of
the linear region of the semi logarithmic steady-state Tafel
lines. The values of Tafel slope calculated for the studied
cathodes during the hydrogen reduction reaction are close to
117 mV. dec�1 which are similar to the values described for Ni
metallic cathode [41]. The Tafel slope data confirms the rate
determining step for the hydrogen evolution reduction is the
electrosorption of the discharged proton called Volmer reac-
tion, H2Oþ e� þ M/M� Hads þ OH�[42]. Recognizably, the
surface morphology, adsorption-desorption through the sur-
face and the electronic properties of the cathode have the
great influence on the rate of the hydrogen reduction. In
addition, the synergistic effects of Ni and Co of the cathode are
considered the vital factor for the high rate of the hydrogen
generation. The small unit cell of the NieCo cathodes in-
creases extensively the surface area of the cathode leading to
numerous active sites which enhance the HER [16,40].
Electrochemical impedance spectroscopy (EIS) is consid-
ered an effective tool for scrutinizing the properties of the
electrode surface/solution interface as steady state technique.
The EIS determines the kinetics of the electrochemical reac-
tion proceeded on the electrode surface and the EIS has been
utilized to investigate the electrochemical reaction. The EIS is
able to characterize the charge transfer controlled, diffusion
controlled or the combination of the previous kinetics. The EIS
measurements for the electroplated NieCo alloys in 1.0 M
KOHmediumhave been performed using the frequency range
0.1e105 Hz against SCEwhichwere operated by the computer-
controlled combined galvanostat/potentiostat/frequency
response analyzer via Volta lab 10 potentiostat (Radiometer
PGZ100). The Volta lab 10 potentiostat has been managed
using Tacusselmodel of the corrosion analysis software (Volta
master 4). The EIS data have beenmodelled using Z fit analysis
to determine the physical depiction of the electrochemical
processes occurred at the electrolyte-electrode interface and
the equivalent circuit has been obtained. The electrocatalyst
characterized by the high resistance of corrosion is consider-
ably necessary to enhance the sustainability through the
electrocatalysis of water which can decrease the costs of the
operating and maintenance. The Nyquist and Bode plots for
the electroplatedNieCo alloys in 1.0MKOHare shown in Fig. 4
and Fig. 5, respectively, and EIS data is recorded using under
open-circuit potential to investigate the corrosion behaviour
for these cathodes. It is clear that the two semicircles were
obtained for all investigated cathodes from Nyqusit plots
(Fig. 4). Figs. 4 and 5 display that the two time constants
appearedwith all cathodes due to the electrode processes. The
EIS results have been fitted theoretically to the model of the
equivalent circuit shown in Fig. 4(inset). The model of the
equivalent circuit involves a capacitor demonstrating the
capacitance of the double layer, Cdl, which is in parallel with a
resistor symbolized the resistance of the charge transfer, Rct.
The previous combination is within parallel to combination of
the resistance of the coatings, Rp, and the capacitance of the
Fig. 6 e Nyquist (a) and Bode (b) plots of bulk Ni (---),
electrodeposited Ni (CCC), and Nie50.4Co (:::)
electrodes at ¡1.45 V potential in stagnant naturally
aerated 1 M KOH solution at 25 �C.
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coatings, Cp and the two combinations exist in series with the
resistor embodied the resistance the solution, Rs. The time
constant at high frequency is due to the features of the coat-
ings resistance and the time constant at low frequency is
attributed to the characteristic for the resistance of the charge
transfer. The fitted values obtained from the model of the EIS
data is given in Table 3. Through the dipping of the electro-
plated NieCo alloys in the electrolyte, the corrosion begins
quickly at the surface situated and the localized galvanic
corrosion occurs. There are two electrochemical interfaces
involved which are found to be the coating/electrolyte and the
substrate/electrolyte. The electroplated Nie50Co alloy shows
the smallest Cp which is characterized by less defective
properties and hence it possesses the highest corrosion
resistance, as shown in Table 3. The electroplated Nie50Co
alloy attains the semicircle of the largest diameter comparing
with the electroplated Nie3.9Co, Nie11.8Co and Ni electrodes
in 1.0 M KOH medium. Consequently, the electroplated
Nie50Co alloy possesses the higher resistance of corrosion
which is due to the compact surface and solid solution alloy.
The polarization resistance (RT ¼ Rct þ Rp) of the electroplated
Nie50Co alloy is about 6421 U cm2 which entails an enhanced
ability protection of the corrosion obtained by the electro-
plated Nie50Co alloy. Thus, the presence of about 50% Co in
the electroplated NieCo alloys increases its corrosion resis-
tance and enhances its stability. Moreover, NiO or Ni(OH)2layer is formed during the dissolution of Ni in alkaline me-
dium [40] which produces the film of passivation and is doped
with Co oxyhydroxide increasing their corrosion resistance.
This implies that the composition of oxi/hydroxide thin film
formed on the surface of the electroplated NieCo alloys at-
tains different structure depending on Ni/Co ratio. Moreover,
the Co2þ/Co3þ combine into the unit cell of the protected Ni
oxi/hydroxide film. In addition, the increase of Co content
leads to the increase separation of the two phasemaxima and
the Nie50Co alloy shows the largest phase maximum. The
broadening of the phase maximum is an indication of a pas-
sive behaviour [43e45]. The passive behaviour of Nie50Co can
be attributed to the grain uniformity, dense surface coverage
of the coatings and doping oxide of Ni/Co layer. Therefore, The
EIS data divulge that the electroplated Nie50Co cathode at-
tains the highest protection performance to the corrosion in
alkaline medium. In comparison with acidic medium [17], the
polarization resistance of Nie50Co in alkaline medium (RT-
¼ 6421 U cm2) is higher that recorded in acidic medium
(263.8 U cm2). That is mean that, the stability of the Nie50Co
alloy in alkaline medium is more than 24 times as its stability
in acidic medium. The results reveal that Nie50Co is a good
Table 3 e Equivalent circuit parameters of ed. Ni and thedifferent NieCo alloy films in 1 M KOH solution at theopen-circuit potential.
Alloys Rs/U Rct/Ucm2
Cdl/mFcm�2
a1 Rp/kUcm2
Cp/mFcm�2
a2
ed. Ni 11.9 81 198 0.85 1.95 514 1.0
Nie3.88Co 5.7 635 63 0.99 0.99 426 0.99
Nie11.82 Co 2.0 400 16 1.0 5.93 391 1.0
Nie50.00Co 1.3 321 40 0.99 6.1 265 1.0
candidate for hydrogen production and a promising cathode
for practical applications, especially in alkaline solutions. The
process is economic and convenient to practical applications.
The EIS is appreciated method to study the properties of
the electrolyte/cathode during the HER [16,40]. The investi-
gation of the electrocatalytic activity of the HER on the surface
of the different cathodes was performed using EIS at the
cathodic potential of �1.45 V of considerable evolution of
hydrogen gas as shown in Fig. 6. The measured EIS data ob-
tained for the bulk and electroplated NieCo cathodes have
been fitted similarly to the theoretical assumption in accor-
dance with the model of the equivalent circuit displayed in
Fig. 4(inset) and fitting data are presented in Table 4. The ob-
tained EIS data illustrates the electrochemical processes
happened at the interface of the electrolyte/cathode. Fig. 6a
Table 4 e Equivalent circuit parameters of bulk Ni, ed. Niand Nie50.38Co alloy in 1 M KOH solution at potential of¡1450 mV and at 25 �C.
Alloys Rs/U Rct/Ucm2
Cdl/mFcm�2
a1 Rp/Ucm2
Cp/mFcm�2
a2
Bulk Ni 8.7 393.5 0.01 0.99 763.7 0.052 0.99
ed. Ni 1.3 1.80 0.354 0.99 1.94 1.03 0.95
Nie50Co 1.06 1.43 0.277 0.99 0.97 4.1 0.96
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 4 ( 2 0 1 9 ) 1 1 4 1 1e1 1 4 2 011418
shows Nyquist plots for electroplated Ni, bulk Ni and Elec-
troplated Nie50Co cathode to compare the efficiency of these
cathodes towards the electrocatalytic reaction of the
hydrogen evolution and Fig. 6b (and 6b inset) displays Bode
plots under the similarly previous conditions. The EIS data
shows the characteristic behaviour of the nanograins of the
small unit cell arrangement in electroplated NieCo cathodes
to electrocatalyse the HER. The high-frequency loop is due to
the valleys, nanograins with huge surface area of NieCo
cathodes and the low-frequency loop is ascribed to the HER
kinetics on the cathode surface [40,46e48]. It is noticeable that
the highest charge transfer resistance, (Rct) was obtained by
bulk Ni while it was significantly small values with electro-
plated NieCo alloys. The Rp is decreased with the increase of
the surface area which can extensively boost the density of
the electrocatalytic sites. The calculated values of a, (an
empirical parameter) varied through 0 � a � 1 associated with
the dispersion formula of the impedance which can deter-
mine the inhomogeneity of the cathode surface, and it equals
approximately unity. This value of a assures that the interface
of the electrode/electrolyte acts as an ideal capacitor and no
requirement to add a constant phase element for the expla-
nation of the double layer behaviour [44]. The electroplated
Nie50Co cathode attains the minimum charge transfer
resistance of 1.43 U cm2 which is considered the most prom-
ising electrocatalyst for the HER. It is known that the small
value of Rct demonstrates the most preferable to be mecha-
nism of the Volmer reaction for the HER kinetics [49]. The
roughness factor of the surface which is directly proportional
to its electrochemically active surface area of the cathode,
have been calculated via the apparent double-layer capaci-
tances, Cdl, determined by EIS measurements. The roughness
factor of the surface s for Nie50Co electrodewas calculated by
dividing Cdl by the double-layer capacitance (20 mF cm�2) of a
smooth metal surface [50]. The surface roughness factor at
�1450 mV was about 13.9 for NiCo cathodes, that assures
higher catalytic activity of investigated electrodes. The
nanograins of the small unit cell manifest the catalytic prop-
erties of the cathode surface which is associated to the
physicochemical and geometrical phenomena. The electro-
plated Nie50Co cathode attains the least semicircle compared
to the bulk Ni and electroplated Ni cathodes which displayed
the smallest resistance for the HER. Moreover, it is unambig-
uous that the electroplated Nie50Co cathode includes the
many valleys fully-occupied by the electrolyte, having the
huge surface area which are accessible to the electrochemical
reaction of H2 evolution. The plentiful valleys are applicably
considered the smallest paths of the diffusion for the dis-
solved hydrogen, and they facilitate the instant release of H2
and prohibit the needless accumulation of the H2 on the
cathode surface. These valleys enlarge the surface area less-
ening the charge transfer resistance for the HER obtained by
the electroplated Ne50Co cathode which is the most suitable
electrocatalyst for the hydrogen evolution reaction. Nano-
crystalline electrocatalysts of the various dimensions and
forms show high catalytic activities towards the enhance-
ment of electrochemical reaction than their bulk counterparts
[39]. In addition, The kinetics of the hydrogen evolution re-
action considered faradaic processes are the reactions of the
charge transfer which depends also on the intrinsic
electrocatalytic activity of the included components [51,52].
The catalytic activity of the electrocatalyst towards the elec-
trochemical reactions such as HER is greatly influenced via
synergetic effect obtained by the combination of the transi-
tion metals due to their physicochemical properties. The
electroplating of Ni and Co in the form of NieCo alloys en-
hances significantly the electrocatalytic activity such as elec-
troplated Nie50Co cathode due to change their ability to
adsorb/desorb the hydrogen by forming the intermediate
bonding with the hydrogen and the releasing properties [53].
The ideal electrocatalyst which attains the adsorption free
energy of the hydrogen equal or similar to the thermo-neutral
(DGH ¼ 0) [39]. Therefore, the high synergistic effect of the
Nie50Co cathode enhance the HER kinetics. It is worthy to
reveal that the electroplated Nie50Co alloy cathodes are very
stable and it was utilized for hydrogen evolution reaction over
weeks without any remarkable diminution in the rate of
hydrogen evolution or deterioration of the cathode activity. It
lasted continuously, in our measurements, over four weeks
without any remarkable change in the rate of hydrogen pro-
duction. The electroplated Nie50Co film cathode possesses
the smallest hydrogen overpotential and the uppermost rate
of the hydrogen evolution.
Conclusion
The electroplating of the nanocrystalline NieCo cathodes has
been performed via gluconate-cysteine baths on the foil of Cu
and current density of 5 A dm�2 under ultrasonication. The
electroplated NieCo cathodes have been exploited as the
electrocatalyst cathodes for the hydrogen evolution reaction
in 1.0 KOH medium and they attain outstandingly high elec-
trocatalytic activity towards the hydrogen evolution reaction.
The cathodic polarization diagrams and EIS is effective tech-
niques to determine the rate and overpotential of hydrogen
reduction reaction. The electroplated NieCo alloys enhance
appreciably the rate of hydrogen evolution reaction. The
electroplated Nie50Co cathode possesses the highest catalytic
activity towards the HER and creates the highest rate of the
HER at the smallest overpotential. The nanocrystalline
Nie50Co cathode can be operated continuously for weeks in
1.0 M KOH solution without any special treatment and the
obtained rate of hydrogen evolution on this cathode is con-
stant all over the time of application without any cathode
deterioration. The electroplated Nie50Co electrocatalyst can
be considered as the most promising cathode for the HER.
r e f e r e n c e s
[1] Oh TS, Haile SM. Electrochemical behavior of thin-film Sm-doped ceria: insights from the point-contact configuration.Phys Chem Chem Phys 2015;17:13501e11.
[2] Chueh WC, Hao Y, Jung W, Haile SM. High electrochemicalactivity of the oxide phase in model ceriaePt and ceriaeNicomposite anodes. Nat Mater 2012;11:155e61.
[3] Jung W, Dereux JO, Chueh WC, Hao Y, Haile SM. Highelectrode activity of nanostructured, columnar ceria films forsolid oxide fuel cells. Energy Environ Sci 2012;5:8682e9.
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 en e r g y 4 4 ( 2 0 1 9 ) 1 1 4 1 1e1 1 4 2 0 11419
[4] Jung W, Gu K, Choi Y, Haile SM. Robust nanostructures withexceptionally high electrochemical reaction activity for hightemperature fuel cell electrodes. Energy Environ Sci2014;7:1685e92.
[5] Choi S, Davenport Timothy C, Haile Sossina M. Protonicceramic electrochemical cells for hydrogen production andelectricity generation: exceptional reversibility, stability, anddemonstrated faradaic efficiency. Energy Environ Sci2019;12:206e15.
[6] Babaei A, Jiang SP, Li J. J Electrochem Soc 2009;156:B1022e9.[7] Myung J, Neagu D, Miller D, Irvine JTS. Switching on
electrocatalytic activity in solid oxide cells. Nature2016;537:528.
[8] Lasia A, Vielstich W, Lamm A, Gasteiger H, editors.Handbook of fuel cells; fundamentals, technology andapplications, Part 4, Hydrogen evolution reaction, vol. 2.Wiley; 2003. p. 416.
[9] Zeng K, Zhang D. Evaluating the effect of surfacemodifications on Ni based electrodes for alkaline waterelectrolysis. Fuel 2014;116:692e8.
[10] Lasia A, Vielstich W, Lamm A, Gasteiger H. Hydrogenevolution. Handbook of fuel cell technology. Joh Wiley andSons Ltd; 2003. p. 416e40.
[11] Shi J. On the synergetic catalytic effect in heterogeneousnanocomposite catalysts. Chem Rev 2013;113:2139e81.
[12] Sheela G, Malathy P, Pushpavanam S. Zincenickel alloyelectrodeposits for water electrolysis. Int J Hydrogen Energy2002;27:627e33.
[13] He X, Xu F, Li F, Liu L, Wang Y, Deng N. J Electroanal Chem2017;799:235e41.
[14] Lytkina AA, Zhilyaeva NA, Ermilova MM, Orekhova NV,Yaroslavtsev AB. Influence of the support structure andcomposition of NieCu-based catalysts on hydrogenproduction by methanol steam reforming. Int J HydrogenEnergy 2015;40:9677e84.
[15] Gao M, Yang C, Zhang Q, Yu Y, Hua Y, Li Y. Electrochemicalfabrication of porous Ni-Cu alloy nanosheets with highcatalytic activity for hydrogen evolution. Electrochim Acta2016;215:609e16.
[16] Nady H, Negem M. Electroplated ZneNi nanocrystallinealloys as an efficient electrocatalyst cathode for thegeneration of hydrogen fuel in acid medium. Int J HydrogenEnergy 2018;43:4942e50.
[17] Badawy WA, Nady H, Negem M. Cathodic hydrogenevolution in acidic solutions using electrodeposited nano-crystalline NieCo cathodes. Int J Hydrogen Energy2014;39:10824e32.
[18] Cardoso D, Eugenio S, Silva T, Santos D. Hydrogen evolutionon nanostructured NieCu foams. RSC Adv 2015;5:43456e61.
[19] Badawy W, Feky H, Helal N, Mohammed H. Hydrogenproduction on molybdenum in H2SO4 solutions. J PowerSources 2014;271:480e8.
[20] Zheng Z, Li N, Wang C, Li D, Meng F, Zhu Y. Effects of CeO2on the microstructure and hydrogen evolution property ofNieZn coatings. J Power Sources 2013;222:[88]e[91].
[21] Yu L, Lei T, Nan B, Jiang Y, He Y, Liu C. Characteristics of asintered porous NieCu alloy cathode for hydrogenproduction in a potassium hydroxide solution. Energy2016;97:498e505.
[22] Lupi C, Dell'Era A, Pasquali M. Nickel-cobalt electrodepositedalloys for hydrogen evolution in alkaline media. Int JHydrogen Energy 2009;34:2101e6.
[23] Shan Z, Liu Y, Chen Z, Warrender G, Tian J. AmorphousNieSeMn alloy as hydrogen evolution reaction cathode inalkaline medium. Int J Hydrogen Energy 2008;33:28e33.
[24] Rosalbino F, Maccio D, Angelini E, Saccone A, Delfino S.Electrocatalytic behaviour of CoeNieR (R¼Rare earth metal)crystalline alloys as electrode materials for hydrogen
evolution reaction in alkaline medium. Int J Hydrogen Energy2008;33:6696e703.
[25] Krstaji�c N, Jovi�c V, Gaji�c -Krstaji�c L, Jovi�c B, Antozzi A,Martelli G. Electrodeposition of NieMo alloy coatings andtheir characterization as cathodes for hydrogen evolution insodium hydroxide solution. Int J Hydrogen Energy2008;33:3676e87.
[26] Hashem N, Negem M. NieCu nano-crystalline alloys forefficient electrochemical hydrogen production in acid water.RSC Adv 2016;6:51111e9.
[27] Stojic D, Cekic B, Maksic A, Kaninski M, Miljanic S.Intermetallics as cathode materials in the electrolytichydrogen production. Int J Hydrogen Energy 2005;30:21e8.
[28] Li Y, Zhang X, Hu A, Li M. Morphological variation ofelectrodeposited nanostructured Ni-Co alloy electrodes andtheir property for hydrogen evolution reaction. Int JHydrogen Energy 2018;43:22012e20.
[29] Goranova D, Lefterova E, Rashkov R. Electrocatalytic activityof Ni-Mo-Cu and Ni-Co Cu alloys for hydrogen evolutionreaction in alkaline medium. Int J Hydrogen Energy2017;42:28777e85.
[30] Sun T, Cao J, Dong J, Du H, Zhang H, Chen J, Xu L. Orderedmesoporous Ni-Co alloys for highly efficient electrocatalytichydrogen evolution reaction, vol. 42; 2017. p. 6637e45.
[31] Darband G, Aliofkhazraei M, Rouhaghdam A, Kiani M. Three-dimensional Ni-Co alloy hierarchical nanostructure asefficient non-noble-metal electrocatalyst for hydrogenevolution reaction. Appl Surf Sci 2019;465:846e62.
[32] Raj R, Kumaraguru S, Mohan S. Benign synthesis of robustnickel thin films as stretchable electrodes forelectrochemical hydrogen evolution reaction. Int J HydrogenEnergy 2018;43:7397e404.
[33] Vij V, Sultan S, Harzandi A, Meena A, Tiwari J, Lee W,Yoon T, Kim K. Nickel-Based electrocatalysts for energy-related applications: oxygen reduction, oxygen evolution,and hydrogen EvolutionReactions. ACS Catal2017;7:7196e225.
[34] Yu J, Zhong Y, Zhou W, Shao Z. Facile synthesis of nitrogen-doped carbon nanotubes encapsulating nickel cobalt alloys3D networks for oxygen evolution reaction in an alkalinesolution. J Power Sources 2017;338:26e33.
[35] Gomez MJ, Franceschini EA, Lacconi GI. Ni and nix coy alloyselectrodeposited on stainless steel AISI 316L for hydrogenevolution reaction. Electrocatalysis 2018;9:459e70.
[36] El-Feky H, Negem M, Roy S, Helal N, Baraka A.Electrodeposited Ni and Ni-Co alloys using cysteine andconventional ultrasound waves. Sci China Chem2013;56:1446e54.
[37] Klug H, Alexander H. X-ray diffraction procedures forpolycrystalline and amorphous materials. New York-Sydney-Toronto: John Wiley & Sons; 1974. p. 618.
[38] Eftekhari A. Electrocatalysts for hydrogen evolution reaction.Int J Hydrogen Energy 2017;42:11053e77.
[39] Zeng M, Li Y. Recent advances in heterogeneouselectrocatalysts for the hydrogen evolution reaction. J MaterChem A 2015;3:14942e62.
[40] Negem M, Hashem N. Electroplated Ni-Cu nanocrystallinealloys and their electrocatalytic activity for hydrogengeneration using alkaline solutions. Int J Hydrogen Energy2017;42:28386e96.
[41] Martinez S, Metikos-Hukovic M, Valek L. Electrocatalyticproperties of electrodeposited Nie15Mo cathodes for the HERin acid solutions: synergistic electronic effect. J Mol Catal AChem 2006;245:114e21.
[42] Kaninski M, Nikolic V, Tasic G, Rakocevic Z. Electrocatalyticactivation of Ni electrode for hydrogen production byelectrodeposition of Co and V species. Int J Hydrogen Energy2009;34:703e9.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 4 ( 2 0 1 9 ) 1 1 4 1 1e1 1 4 2 011420
[43] Badawy W, Al-Kharafi F, El-Azab A. Electrochemicalbehaviour and corrosion inhibition of Al, Al-6061 and AleCuin neutral aqueous solutions. Corros Sci 1999;41:709e27.
[44] Macdonald J. Impedance spectroscopy. New York: JohnWiley& Sons; 1987. Chap. 4.
[45] Ismail K, Badawy W. Electrochemical and XPS investigationsof cobalt in KOH solutions. J Appl Electrochem2000;30:1303e11.
[46] Hu H, Qiao M, Pei Y, Fan K, Li H, Zong B. Appl Catal Gen2003;252:173e89.
[47] Solmaz R, Kardas G. Hydrogen evolution and corrosionperformance of NiZn coatings. Energy Convers Manag2007;48:583e91.
[48] Shervedani P, Mardam A. Kinetics of hydrogen evolutionreaction on nanocrystalline electrodeposited Ni62Fe35C3cathode in alkaline solution by electrochemical impedancespectroscopy. Electrochim Acta 2007;53:426e33.
[49] Chen Z, Wang L, Ma Z, Song J, Shao G. Niereduced grapheneoxide composite cathodes with new hierarchicalmorphologies for electrocatalytic hydrogen generation inalkaline media. RSC Adv 2017;7:704e11.
[50] Jukic A, Piljac J, Hukovic MM. J Mol Catal A 2001;166:293.[51] Candy J, Fouilloux P, Keddam H, Takenouti H. The pore
texture of raney-nickel determined by impedancemeasurements. Electrochim Acta 1982;27:1585e93.
[52] Los P, Lasia A, Menard H. Impedance studies of porouslanthanum-phosphate-bonded nickel electrodes inconcentrated sodium hydroxide solution. J Electroanal Chem1993;360:101e18.
[53] Greeley J, Jaramillo T, Bonde J, Chorkendorff I, Norskov J.Computational high throughput screening ofelectrocatalytic materials for hydrogen evolution. NatMater 2006;5:909e13.