Nanocrystalline nickel-cobalt electrocatalysts to generate ... · Electroplating of nanocrystalline...

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Nanocrystalline nickelecobalt electrocatalysts to generate hydrogen using alkaline solutions as storage 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, Egypt b Chemistry Department, College of Science & Arts in Qurayat, Jouf University, Saudi Arabia article info 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 abstract 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 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 * Corresponding author. E-mail address: [email protected] (M. Negem). Available online at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/locate/he international journal of hydrogen energy 44 (2019) 11411 e11420 https://doi.org/10.1016/j.ijhydene.2019.03.128 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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Available online at w

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journal homepage: www.elsevier .com/locate/he

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.

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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

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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

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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

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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

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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

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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.

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 11417

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

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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.

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