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I N T E R N AT I O N A L J O U R N A L O F H Y D R O G E N E N E R G Y 3 3 ( 2 0 0 8 ) 3 6 7 6 – 3 6 8 7

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Electrodeposition of Ni–Mo alloy coatings and theircharacterization as cathodes for hydrogen evolution insodium hydroxide solution

N.V. Krstajica, V.D. Jovicb,�, Lj. Gajic-Krstajicc, B.M. Jovicb, A.L. Antozzid, G.N. Martellid

aFaculty of Technology and Metallurgy University of Belgrade, 11000 Belgrade, Karnegijeva 4, SerbiabInstitute for Multidisciplinary Research, 11030 Belgrade, P.O. Box 33, SerbiacInstitute of Technical Sciences SASA, 11000 Belgrade, Knez Mihajlova 35, SerbiadDe Nora Industries, Via Bistolfi 35, 20134 Milan, Italy

a r t i c l e i n f o

Article history:

Received 4 September 2007

Received in revised form

13 February 2008

Accepted 20 April 2008

Available online 4 June 2008

Keywords:

Electrodeposition

Ni–Mo alloy

Coatings

Hydrogen evolution

Stability

EIS measurements

nt matter & 2008 Internae.2008.04.039

hor. Tel.: +381 11 [email protected]

a b s t r a c t

The hydrogen evolution reaction on the electrodeposited Ni–Mo alloy coatings, as well as

their electrochemical properties in the NaOH solutions, have been investigated by the

polarization measurements, cyclic voltammetry and EIS technique. It was shown that the

Ni–Mo alloy coatings electrodeposited from the pyrophosphate-sodium bicarbonate bath

possess high catalytic activity for hydrogen evolution in the NaOH solutions. Their stability

in the 1 M NaOH at 25 1C under the condition of the reverse polarization was shown to be

very good, while in the 33% NaOH at 85 1C (conditions of the industrial electrolysis) the

electrodeposited Ni–Mo alloy coatings exhibited also high catalytic activity, but low

stability, as a consequence of a deterioration of the alloy coatings.

& 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights

reserved.

1. Introduction

The hydrogen evolution reaction (HER) is one of the most

frequently investigated reactions. The reason for this is that

the HER proceeds through a limited number of steps with the

only one type of intermediate. The kinetic of the HER in

alkaline solutions have been mainly investigated on Ni [1,2],

due to the relatively good catalytic activity and high corrosion

stability of this substrate.

According to the theory of electrocatalysis, the electrocata-

lytic activity depends on the heat of adsorption of the

intermediate on the electrode surface defined by the well-

known ‘‘volcano’’ curve [3]. It is clear that beside the precious

metals, there is practically no way to find the new materials

tional Association for Hy

; fax: +381 11 3055289.(V.D. Jovic).

among pure metals, which would possess high catalytic

activity for the HER.

The alloying of two (or more) metals has long appeared as

the most straightforward approach to achieve electrocatalytic

activity for the HER. Miles [4] suggested that a combination of

two metals from the two branches of the ‘‘volcano’’ curve

could result in enhanced activity for the HER. Thus, the Mo-

based alloys either electrodeposited [5], or thermally prepared

[6], or added in situ [7], became the main objective of the

research during the past 30 years, with the Ni–Mo alloy

showing superior qualities.

The investigation of the kinetics and the mechanism of the

HER at the Ni–Mo alloys of various compositions, obtained

by the electrodeposition from suitable baths, has been the

drogen Energy. Published by Elsevier Ltd. All rights reserved.

dx.doi.org/10.1016/j.ijhydene.2008.04.039

mailto:[email protected]

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Table 1 – Electrolytes used for the Ni–Mo alloy coatingselectrodeposition

Solution pH Electrolyte composition

S1 9.0 10 g dm�3 NiCl2 � 6H2O; 40 g dm�3

Na2MoO4 � 2H2O; 45 g dm�3 K4P2O7; 75 g dm�3

NaHCO3

S2 8.5 10 g dm�3 NiCl2 � 6H2O; 40 g dm�3


NH4Cl

S3 8.5 10 g dm�3 NiCl2 � 6H2O; 40 g dm�3


NaHCO3; 10 g dm�3 NaCl

S4 9.0 10 g dm�3 NiCl2 � 6H2O; 40 g dm�3


Na2B4O7

S5 7.5 10 g dm�3 NiCl2 � 6H2O; 40 g dm�3


NaHCO3; 10 g dm�3 NaCl; 5 g dm�3 HCl

S6 7.6 10 g dm�3 NiCl2 � 6H2O; 40 g dm�3


Na2B4O7; 5 g dm�3 HCl

I N T E R N A T I O N A L J O U R N A L O F H Y D R O G E N E N E R G Y 3 3 ( 2 0 0 8 ) 3 6 7 6 – 3 6 8 7 3677

subject of numerous studies [8–19]. Chialvo et al. [18]

investigated the dependence of the electrocatalytic activity

of the bulk Ni–Mo alloys for the HER as a function of their

composition, varying atomic percentage of molybdenum

from 0 to 25 at%.

However, the activity enhancement has recently been

found to be mainly due to an increased surface area [12,17]

and synergetic effects has been ruled out.

If the Ni–Mo mixed layers are prepared by thermal

decomposition of suitable precursors, clear synergetic effects

were observed. The Tafel slope decreased to 40 mV dec�1 and

extended to very high current densities [19].

A change in the mechanism with respect to the bulk Ni has

also been observed with the electrodeposited Ni–Mo alloys

containing only 1% of Cd. The origin of the activity of the

Ni–Mo–Cd alloy coatings has been investigated by Conway

and co-workers [12]. They have found that the cathodic

behavior can be explained in terms of the formation of a

hydride phase at low overpotentials [2].

Most of the papers concerning electrodeposition of the

Ni–Mo alloys are dealing with the mechanism of the deposi-

tion process (mechanism of induced co-deposition), mainly

reported by Landolt and coworkers [20,21]. Only a few papers

were devoted to their morphological and the phase composi-

tion characterization [22–28]. It was found by XRD analysis

that the Ni–Mo alloys electrodeposited from a citrate bath (pH

8.5–9.5) contain Ni–Mo solid solution, with the diffraction

peaks being sharp at the lower content of Mo (up to 12 wt%)

and wide at the high content of Mo (30 wt%) [22]. In the same

paper the TEM revealed the same solid solution with the grain

size ranging between 4 and 17 nm (average 5 nm), indicating

that the electrodeposited Ni–Mo alloy is almost amorphous. A

similar conclusion was made by the XRD analysis of the

Ni–Mo alloys electrodeposited from the pyrophosphate-am-

monium chloride bath (pH 8.5) [23,24]. In the papers of

Sanches et al. [26,27] for the first time it was demonstrated by

the energy dispersive X-ray spectroscopy (EDS) analysis that

the electrodeposited Ni–Mo alloys with higher amount of Mo

contain up to about 50 at% of oxygen. XRD showed sharp

diffraction peaks corresponding to the Ni–Mo solid solution

and the Ni4Mo intermetallic compound [27], while XPS

analysis revealed that in the alloy with higher amount of

Mo, among the metallic Mo, a mixture of polyvalent

molybdenum oxides or hydroxides, mainly in the form of

Mo(V) and Mo(IV) was present in the deposit. It was also

concluded in this work that the increase of Ni(II) concentra-

tion in the citrate bath (pH 4) favors deposition of the metallic

molybdenum.

Despite the large amount of data collected in this field, it

should be emphasized that for the technological applications

besides the electrocatalytic activity, the stability of the

electrode materials in strongly alkaline solutions at the

elevated temperature is even more crucial. Hence, in this

report the results of the investigation of the HER in

concentrated NaOH solutions at the elevated temperatures

(the working conditions as in the membrane Chlor-Alkali

technology) on the Ni–Mo alloys electrodeposited from the

different baths were presented. The main purpose of this

study was to investigate the mechanism of the deactivation

process as a result of the reverse polarization.

2. Experimental

2.1. Electrodeposition of the Ni–Mo coatings

The electrodeposition of the Ni–Mo alloys was performed in a

beaker at 60 1C with the counter electrode being Ni foil placed

close to the walls of the beaker. The working electrodes (Ni

meshes) were placed in the middle of the electrolyte and the

electrolyte was stirred with the magnetic stirrer during the

alloy electrodeposition.

Two types of meshes, supplied by DeNora Industries were

used: the expanded Ni mesh (1), sand blasted with alumina,

standard for cathodic activation (Chlor Alkali) and the Ni ‘‘Fly-

Net’’ uncoated (25 mesh, opening type) used for cathodic

activation (Chlor Alkali) in the ZERO GAP configuration cells

(2). The Ni surface was first etched in 2:1 HNO3 for 2 min. and

then immersed in the 25 wt% H2SO4 where the cathodic

current density of 5 mA cm�2 was applied for 5 min [29]. After

this treatment the electrode was washed with the Milli-Q

water and transferred in the electrolyte for the Ni–Mo alloy

electrodeposition under the conditions of a constant current

density to the thickness of approximately 10mm (except for

the samples 1 and 2, see Table 2). When the deposition was

finished, the electrode was again washed with the Milli-Q

water and transferred into the cell for the polarization curve

measurements. Seven electrolytes of different compositions

were used for the Ni–Mo alloy electrodeposition. Their

compositions are given in Table 1. The dimensions of the

meshes 1 and 2 exposed to the deposition of the Ni–Mo alloy

was 2�1 cm, with the total electrode area of 3.348 cm2 for

mesh 1 and 1.76 cm2 for mesh 2.

ARTICLE IN PRESS


2.2. Electrochemical measurements

The polarization curves for hydrogen evolution on these

electrodes were measured in the standard electrochemical

(three electrodes) cell at the temperature of 25 1C in 1 M NaOH

and at 85 1C in 33% NaOH. Saturated calomel electrode (SCE)

was used as the reference electrode. The counter electrode

was a Pt mesh. Only 1 cm2 of the total mesh area was exposed

to the solution. The polarization diagrams were recorded on a

Gamry potentiostat Reference 600 with automatic IR drop

compensation (current interrupt technique), using Corrosion

Techniques Software DC 105. Before recording polarization

diagrams electrodes were exposed to hydrogen evolution in

the same solution at j ¼ �0.1 A cm�2 for the time needed to

establish reproducible polarization curve. In the case of the

anodic treatment (recording of CV’s or oxygen evolution)

polarization diagrams were recorded immediately after the

anodic treatment. The electrochemical impedance spectro-

scopy (EIS) experiments were performed with the same

potentiostat using Electrochemical Impedance Spectroscopy

software EIS 300.

All solutions were made from the analytical grade chemi-

cals and the Milli-Q water.

2.3. Characterization

To characterize the as-deposited surfaces and to determine

the alloy composition a scanning electron microscope (JEOL

JSM 6460LV) with EDS was used. Selected deposits were

mounted in a cross-section, polished and examined by the

optical microscopy.

Fig. 1 – The polarization characteristics of different Ni–Mo

alloy samples electrodeposited onto mesh 1 (given in Table

1) and a pure Ni mesh 1, recorded in 1 M NaOH at 25 1C.

Polarization curve for 20 lm Ni deposited onto mesh 1 is

3. Results and discussion

3.1. Electrodeposition of the Ni–Mo alloy coatings andtheir characterization

All the electrolytes used for the Ni–Mo alloy electrodeposition

are presented in Table 1. As can be seen K4P2O7 was used as a

Table 2 – Conditions of the Ni–Mo alloy samples electrodepositievolution at j ¼ �0.3 A cm�2

Alloy sample jd (mA cm�2) Solution T (1C)

1 �100 S1 60

2 �50 S1 60

3 �20 S1 60

4 �100 S3 60

5 �100 S2 60

6 �100 S4 60

7 �100 S5 60

8 �100 S6 60

Ni mesh

20 mm Ni �250 Sulfamate 45

complexing agent, since it was found that higher percentage of

the Mo could be co-deposited with the Ni from such type of the

electrolyte [23–25] in comparison with the citrate containing

electrolytes [26,27]. According to our previous work [28] the

percentage of Mo in the deposit increases with increasing

deposition current density (jd) from about 28 at% at

�20 mA cm�2 to about 41 at% at �100 mA cm�2. It is important

to note that in all the electrodeposited samples significant

atomic percentage of the oxygen has been detected, varying

between 30 and 50 at%, while for calculation of the Ni–Mo alloy

composition only the Ni and the Mo were taken into account,

neglecting oxygen [28]. According to certain literature [27],

alloys containing high amount of Mo represent mixture of Ni

and some polyvalent Mo(IV) and/or Mo(V) oxides.

All alloy coatings for this investigation were electrodepos-

ited on mesh 1. In order to obtain the Ni–Mo alloys with the

highest amount of Mo, all samples (except samples 2

and 3—Table 1) were electrodeposited at jd ¼ �100 mA cm�2.

on onto Ni mesh (1); corresponding potentials for hydrogen

Deposition time (min) E vs. SCE (V) at j ¼ �0.3 A cm�2

15 �1.353

60 �1.545

120 �1.580

120 �1.339

120 �1.499

120 �1.445

120 �1.435

120 �1.348

�1.672

10 �1.660

presented by dotted line.

ARTICLE IN PRESS

Fig. 2 – (a) A cross-section of the as deposited Ni–Mo alloy coating (sample 4, Table 2). (b) A cross-section of the same coating

after 1 h of hydrogen evolution in 33% NaOH at the current density of �0.1 A cm�2.


The composition of electrolytes for the Ni–Mo alloys electro-

deposition used in this work represents the combination of

some electrolytes presented in the literature [23–25,28].

It is important to note that the components of the bath

should be dissolved in Milli-Q water by the following order:

first, 45 g dm�3 K4P2O7 should be dissolved at elevated

ARTICLE IN PRESS

Fig. 3 – The polarization curves for the best alloy coating

(sample 4) recorded in 1 M NaOH at 25 1C. (a) The

polarization curves recorded after a certain time of hydrogen

evolution at �0.1 A cm�2: 1—after 10 min; 2—after additional

60 min; 3—after additional 60 min; 4—after additional

60 min (the same dependence is recorded for both meshes).

(b) The best polarization curves for the Ni–Mo alloy coating

electrodeposited onto meshes 1 (1) and 2 (2).


temperature (50–60 1C); second component is 10 g dm�3

NiCl2 � 6H2O, which should be added in small amounts with

intensive stirring, in order to prevent hydrolysis and allow

complexation of Ni2+ cations with P2O74� anions; after

complete dissolution of nickel salt, 75 g dm�3 NaHCO3 should

be added in the bath and finally, 40 g dm�3 Na2MoO4 �2H2O.

In Table 2 are given details for the conditions of the Ni–Mo

alloy coatings electrodeposition together with the potentials

for hydrogen evolution taken from the polarization curves

recorded on the electrodeposited samples in the solution of

1 M NaOH at 25 1C. The corresponding polarization curves

recorded in the solution of 1 M NaOH at 25 1C are presented in

Fig. 1, together with the polarization curve for the Ni

substrate. As shown in our previous paper [28] the Ni–Mo

alloys obtained at lower current densities (samples 2 and 3)

possess lower catalytic activity for hydrogen evolution (higher

overvoltage) as a consequence of the lower amount of Mo in

the deposit. It was also shown that all of the alloys are more

active for hydrogen evolution than the pure Ni. Two polariza-

tion curves for HER at the Ni electrodes are shown: one

obtained on the sand blasted and etched mesh 1 (’) and

another one (dotted line) obtained on the 20mm thick Ni

coating deposited on the same mesh from a nickel-sulfamate

bath [29]. As can be seen no difference between these two

curves has been recorded. The best sample (with the highest

percentage of Mo in the deposit, around 41 at%) is sample 4,

with its overpotential for the hydrogen evolution at

j ¼ �0.3 A cm�2 being for about 333 mV lower than that for

the pure Ni mesh. The samples obtained under such

conditions (deposited on both types of meshes) were used

for further investigations.

A cross-section of the typical Ni–Mo alloy coating (sample 4)

is shown in Fig. 2. The thickness of the deposit is practically

the same all over the cross-section, with the open pores

(cracks) present in the deposit, which is the characteristic of

the electrodeposited Ni–Mo alloy coatings containing more

than 15 at% of Mo [23–28]. If such a coating is exposed to a

long time hydrogen evolution (1 h at j ¼ �0.1 A cm�2 in 33 wt%

NaOH at 85 1C) the hydrogen penetrates through the pores

and collects in localized areas, causing the formation of

internal bursts or blisters provoking a bad adhesion between

the substrate and the coating and, most likely, after certain

time of operation, scaling of the coating, Fig. 2b.

It is important to note that all the polarization curves were

recorded up to the high current densities (higher than

�0.3 A cm�2) in order to compare their characteristics as

cathodes in the conditions of the industrial electrolysis

(industrial production of chlorine in the membrane cells).

Hence, the Ni–Mo alloys were deposited on both types of

meshes (1 and 2) under the conditions given for sample 4 in

Table 2 and their polarization characteristics were recorded in

the NaOH solutions.

3.2. Polarization characteristics and EIS measurements,recorded in 1 M NaOH at 25 1C for the HER on the Ni–Mo alloycoatings electrodeposited onto meshes 1 and 2

In order to establish reproducible polarization characteristics

for the Ni–Mo coatings on both meshes, the electrodes

were exposed to HER at j ¼ �0.1 A cm�2 for different times.

In Fig. 3a are presented the polarization characteristics

obtained for sample deposited onto mesh 1 (identical results

were obtained for sample deposited onto mesh 2). As can be

seen no change in the polarization curve (curves 3 and 4) is

detected after 130 min of the hydrogen evolution and the

curve 4 is used as the best one. The same experiment is

performed on the sample electrodeposited onto mesh 2 and

the best polarization curves for both electrodes are compared

in Fig. 3b. A slightly lower overvoltage for the HER in the

region of higher current densities (4�10 mA cm�2) is ob-

tained for the sample deposited onto mesh 2.

The HER on these coatings is also investigated by the EIS. In

Figs. 4 and 5, the Nyquist and Bode diagrams recorded at

different potentials (covering current density range from

��10 to ��200 mA cm�2, see Fig. 3a and b) for the meshes 1

(Fig. 4) and 2 (Fig. 5), are shown. In both cases two semi-circles

could be detected on the Nyquist diagrams (with the ones for

ARTICLE IN PRESS


the mesh 1 being better defined), indicating the presence of

two time constants. The Bode plots also display two relaxa-

tion time constants with the high-frequency relaxation time

constant being almost the same at all overpotentials. In order

to fit impedance spectra obtained on the Ni–Mo coatings a

two constant phase elements (CPE’s) serial model was applied

(shown in the inset of the Nyquist diagram in Fig. 4). It

consists of the solution resistance, Rs, in series with two

parallel CPE-R elements (2-CPE model) [30,31] and is used for

the analysis of the EIS results of HER recorded on the Ni–Mo

alloys obtained by different procedure. According to this

model the high-frequency time constant, independent of the

potential, described by the Rp and CPEp connected in parallel,

is related to the electrode porosity, whereas the potential

dependent time constant is related to the kinetics of the HER

(Rct and CPEdl connected in parallel). The Bode plots (Figs. 4

and 5) confirm the above mentioned statement that the first

Fig. 4 – The Nyquist and Bode diagrams recorded in 1 M

NaOH at 25 1C at different potentials (marked in the figure in

volts) for the HER on the Ni–Mo alloy coating

electrodeposited onto mesh 1 in the frequency range from

0.1 Hz to 10 kHz: squares, circles and triangles represent

experimental points, while solid lines represent fitting

results. The equivalent circuit used for fitting EIS results is

presented in the inset of the Nyquist diagram: Rs—solution

resistance; Rp—resistance of pores; CPEp—constant phase

element corresponding to the capacitance of pores;

CPEdl—constant phase element corresponding to the double

layer capacitance; Rct—charge transfer resistance [30,31].

(high-frequency) semi-circle is independent of the potential

and related to the electrode porosity (Ni–Mo coatings display

deep narrow pores, Fig. 2). The capacitance parameter Tdl is

related to the average double layer capacitance Cdl by the

relation: Cdl ¼ {Tdl/[(Rs+Rp)�1+Rct�1](1�F)}1/F, where F represents

a factor of homogeneity [31]. In the case of pure Ni mesh 1, or

Ni coated Ni mesh 1, the capacitance parameter Tdl is also

related to the average double layer capacitance Cdl by slightly

different relation: Cdl ¼ [Tdl/(Rs�1+Rct

�1)(1�F)]1/F [31], because the

corresponding Nyquist diagrams exhibit only one semi-circle.

The values of the capacitance, Cdl, determined for the Ni–Mo

coatings are almost independent of overpotential and are up

to two orders of magnitude larger than those for the Ni coated

mesh 1 (see Table 3). The results obtained by the NLLS fitting

procedure are given in Table 3, while the fitting curves are

presented by the solid lines in Figs. 4 and 5.

The intrinsic activity (j/Rf) could be estimated as a ratio of

the current density for the HER at a constant potential E, and

the roughness factor expressed as Rf ¼ Cdl/20mF cm�2 [32]

(20mF cm�2 being an ideal value for the double layer capaci-

tance). These parameters are displayed in Table 3. According


NaOH at 25 1C at different potentials (marked in the figure in

volts) for the HER on the Ni–Mo alloy coating

electrodeposited onto mesh 2 in the frequency range from

0.1 Hz to 10 kHz: squares, circles and triangles represent

experimental points, while solid lines represent fitting

results.

ARTICLE IN PRESS

Table 3 – Parameters obtained by fitting EIS results

E/V Rs/O cm2

CPEp in parallel with Rp CPEdl in parallel with Rct Cdl/mF cm�2

Rf(av) (j/Rf)/mA cm�2 (E ¼ �1.35 V)

Tp/O�1 cm�2 sF

Rp/O cm2

Fp Tdl/O�1 cm�2 sF

Rct/O cm2

Fdl

Ni– Mo (expanded mesh 1)

�1.15 0.930 0.0205 0.186 0.69 0.162 2.30 0.63 44000

�1.20 0.937 0.0404 0.194 0.60 0.133 1.28 0.69 43000 2100 0.14

�1.25 0.954 0.0572 0.192 0.56 0.127 0.90 0.70 39000

Ni– Mo (Fly-net mesh 2)

�1.15 0.717 0.115 3.62 0.56 0.099 2.10 0.65 34500

�1.20 0.743 0.066 2.04 0.66 0.071 1.15 0.78 31700 1550 0.30

�1.25 0.760 0.0308 1.27 0.57 0.084 0.90 0.73 28000

�1.30 0.788 0.0152 0.795 0.65 0.069 0.70 0.72 29700

Ni (expanded mesh 1)

�1.40 0.96 150�10�6 29.4 0.94 205

�1.45 0.97 154�10�6 14.9 0.93 203 10 0.03

�1.50 0.99 158�10�6 8.86 0.94 193

�1.55 1.00 160�10�6 5.24 0.93 194

Previous treatment: H2 evolution at �0.1 A cm�2

After anodic polarization Ni– Mo (expanded mesh 1)

�1.25 0.954 0.0107 0.264 0.76 0.1370 1.91 0.62 H2 evolution at �0.1 A cm�2

�1.25 0.807 0.0104 0.167 0.76 0.0196 2.71 0.68 Five cycles at v ¼ 10 mV s�1

�1.25 0.811 0.0106 0.466 0.65 0.0165 3.25 0.67 1000 s at E ¼ 0.35 V (peak potential)

�1.25 0.806 0.0187 0.207 0.70 0.0119 4.49 0.71 1000 s at E ¼ 0.50 V (O2 evolution)


to the results presented in Table 3, the Ni–Mo coatings

possess about one order of the magnitude higher intrinsic

activity, while the apparent activity determined from the

polarization curves is over three orders of the magnitude

larger (Fig. 1). It follows that the main contribution to

the improvement of the catalytic properties of the Ni–Mo

coatings toward HER arises not only from the increase of the

real surface area, but also a significant catalytic effect is

present. The catalytic effect could be assigned to the

hydrogen spill-over effect and different mechanism for

the HER on the Ni–Mo catalyst in comparison with the pure

Ni electrode on which the HER takes place by the Volmer–

Heyrovsky mechanism, with the slow step being electroche-

mical desorption of the intermediate Hads at its high-surface

coverage (the unique Tafel slope of E�120 mV dec�1 in the

whole potential range) [17].

The Ni–Mo alloy coating electrodeposited onto mesh 2

exhibits slightly better polarization characteristics than that

electrodeposited onto mesh 1 (Fig. 3b), which is in accordance

with the values of the Rct determined for these electrodes

(Table 3). On the other side this electrode possess lower

roughness factor, which is not in accordance with its higher

catalytic activity. Taking into account considerably different

shapes of these two meshes, lower roughness of the Ni–Mo

coating electrodeposited onto mesh 2 could be expected due

to much better current distribution on its surface during the

process of the Ni–Mo alloy electrodeposition. It is possible

that on such geometry of the mesh HER is slightly faster, but

for any convincing conclusion additional experiments are

needed and will be the subject of our further research.

3.2.1. Cyclic voltammetry, EIS and polarizationcharacteristics of both electrodes after the anodic treatmentIt is often the case in the industrial application that during

the shut down of the power in the industrial production of

chlorine, the electrodes becomes reversely polarized, i.e.

cathodes become anodically polarized, while anodes become

cathodically polarized. Such a polarization can cause sig-

nificant changes in the electrochemical behavior of the

electrodes and they might loose their catalytic properties for

the processes that are taking place on them (hydrogen or

chlorine evolution). In order to investigate the influence of

such a polarization on the Ni–Mo alloy coatings, both

electrodes were exposed to the cycling procedure up to the

potential of the oxygen evolution and to the polarization at a

constant anodic potential in the region of oxygen evolution

and their polarization characteristics, as well as their EIS

results, were investigated.

The CV’s for both electrodes recorded with the sweep rate

of 10 mV s�1 in the 1 M NaOH at 25 1C are shown in Fig. 6a.

As can be seen the shape of both voltammograms is

almost the same as that for the pure Ni electrode, Fig. 6b.

The CV of the fresh Ni electrode, after holding at the

negative potential limit is nominally split into three regions,

A, B and C (Fig. 6b). The region A has been called the

‘‘hydroxide region’’ by some authors or the ‘‘Ni(II) region’’ by

others. At the lower negative limit of the CV hydrogen

evolution is observed, which is probably accompanied by

the hydrogen absorption into the bulk Ni substrate. On the

anodic sweep the peak labeled ‘‘a’’ is observed and this has

been assigned to the formation of the a-Ni(OH)2 [33,34]

ARTICLE IN PRESS

Fig. 6 – The CV’s recorded at the sweep rate of 10 mV s�1 in

1 M NaOH at 25 1C for the Ni–Mo alloy coating

electrodeposited onto meshes 1 (solid line) and 2 (dotted

line) (a) and for the Ni-mesh 1 electrode (b).

Fig. 7 – (a) The polarization curves for the Ni–Mo alloy

coating electrodeposited onto mesh 1 recorded in 1 M NaOH

at 25 1C before and after different anodic treatments: 1—no

anodic treatment; 2—after five cycles with v ¼ 10 mV s�1;

3—after oxygen evolution at 0.5 V for 1000 s (j–t response

shown in the inset of Fig. 7a). (b) 1—no anodic treatment;

2—after oxygen evolution at 0.6 V for 1000 s (j–t response

shown in the inset of Fig. 7a); 3—after additional hydrogen

evolution at j ¼ �120 mA cm�2 for 1000 s.


according to the following overall reaction:

Niþ 2OH� ) NiðOHÞ2 þ 2e (1)

The peak ‘‘a0’’ in the cathodic sweep has been assigned to the

reduction of the a-Ni(OH)2 back to the metallic Ni [35].

According to the scheme presented by ‘‘Bode’’ the a-Ni(OH)2can be irreversibly transformed into b-Ni(OH)2. If either the Ni

or the Ni–Mo electrodes are cycled in the potential region

where no hydrogen evolution occurs (�1.2 V vs. SCE, see

Fig. 6a) well-defined peaks in the region A (a and a0) disappear.

It has been proposed that this was due to the formation of the

b-Ni(OH)2 which could not be reduced during the successive

cathodic sweeps. The b-Ni(OH)2 has also been found to form

by aging of the a-Ni(OH)2 [33,34]. Both CV’s for the Ni and the

Ni–Mo electrodes are characterized by a pair of quasi-

reversible peaks at the potentials between 0.2 and 0.4 V. The

region C is known as the ‘‘oxyhydroxide region’’ or the ‘‘Ni(III)

region’’. The electrode potentials within this region are

sufficiently anodic to enable the oxidation of the hydroxide

phase. The relatively large anodic current density seen in the

CV (peak a) is related to the further oxide growth as well as to

the change in the Ni oxidation state from ‘‘2’’ to ‘‘3’’ (possible

even higher if the overcharging occurs). The oxidation of

b-Ni(OH)2 to the NiOOH occurs via the reaction:

b-NiðOHÞ2 ) NiOOHþHþ þ e (2)

This electrochemical oxidation results in the expulsion of a

proton from the Ni hydroxide layer to produce H2O in the

strongly alkaline solution [36,37]. Both, the oxidation peak (c)

and the reduction peak (c0) lead to the volumetric changes of

the passive layer, as has previously been monitored by the

AFM deflection experiments as well as by the combined

electrochemical quartz crystal microbalance (EQCM) mea-

surements [38,39]. According to the Bode model [40] b-Ni(OH)2form is more crystalline, although this hydroxide can contain

a variable excess of the intersheet water and accordingly low

crystallinity. Any retaining in this potential region will cause

the growth of b-Ni(OH)2 and, once formed, it would totally

suppress the formation of a-Ni(OH)2 form. Above mentioned

ARTICLE IN PRESS


behavior points out the irreducible nature of b-Ni(OH)2 form.

Similar and reproducible shapes of the CV curves for the Ni

and the Ni–Mo electrodes confirm that at the electrochemi-

cally deposited Ni–Mo the same surface reactions (1) and (2)

take place without clear evidence for further oxidation of

molybdenum oxides present in the fresh electrode or their

anodic dissolution to MoO42� species.

Already after recording CV’s, the polarization characteris-

tics for the HER on both electrodes change, as can be seen in

Fig. 7a (curve 2) for mesh 1. If this electrode is exposed to

oxygen evolution at 0.5 V for 1000 s (j– t response shown in the

inset of Fig. 7a) and the polarization curve is recorded

immediately after oxygen evolution, the overvoltage for the

HER becomes higher (curve 3). After the additional oxygen

Fig. 8 – (a) The polarization curves for the Ni–Mo alloy

coating electrodeposited onto mesh 2 recorded in 1 M NaOH

at 25 1C before and after different anodic treatments: 1—after

oxygen evolution at 0.5 V for 1000 s (j– t response shown in

the inset of Fig. 8b); 2—after hydrogen evolution at

j ¼ �100 mA cm�2 for 1000 s; 3—after additional hydrogen

evolution at j ¼ �100 mA cm�2 for 1000 s. (b) 1—no anodic

treatment (the best polarization curve for mesh 2);

2—polarization curve 1 from Fig. 7a (the best polarization

curve for mesh 1).

evolution at 0.6 V for 1000 s (j– t response also shown in the

inset of Fig. 7a, the oxygen is evolving with j ¼ 120 mA cm�2)

the polarization curve for the HER becomes worse, as shown

in Fig. 7b, curve 2. If, after such an anodic treatment,

the electrode is exposed to the hydrogen evolution at

j ¼ �120 mA cm�2 for 1000 s (the same current density as that

for the oxygen evolution) the polarization curve (curve 3)

becomes almost the same as the one recorded before the

anodic treatment (curve 1). It is obvious from the results

presented in Fig. 7 that the oxide layer formed during the

anodic treatment could completely be reduced during the

hydrogen evolution and accordingly, the polarization curve

for the HER becomes almost identical to the one recorded

before any anodic treatment in 1 M NaOH at 25 1C. The same

behavior is recorded for the Ni–Mo alloy coating electrode-

posited onto mesh 2 (Fig. 8a).

The Nyquist and Bode diagrams recorded at the potential

E ¼ �1.25 V immediately after the anodic treatment and the

corresponding polarization curve, are shown in Fig. 9. As can


NaOH at 25 1C at a constant potential E ¼ �1.25 V vs. SCE for

the HER on the Ni–Mo alloy coating electrodeposited onto

mesh 1 in the frequency range from 0.1 Hz to 10 kHz after

different treatments: &—after 5 cycles with v ¼ 10 mV s�1;J—after oxygen evolution at 0.5 V for 1000 s; n—after

oxygen evolution at 0.6 V for 1000 s. Squares, circles and

triangles represent experimental points, while solid lines

represent fitting results.

ARTICLE IN PRESS


be seen the shape of the impedance diagram changes, being

characterized by a small loop at low frequencies. Such a

behavior indicates that the surface of the alloy is changing

during the time of recording the impedance diagram.

According to the fitting results presented in Fig. 9 with

the proposed equivalent circuit, the increase of the values for

Rct and Rp are observed (Table 3), indicating different

adsorption characteristics due to the presence of surface

oxides after prolonged anodic polarization. This statement is

in accordance with the fact that after the subsequent

hydrogen evolution for a certain time (when complete

oxide film reduction is achieved), not only polarization

curve becomes the same as the one obtained before any

anodic treatment, but also the Nyquist and Bode diagrams

and CV become identical to the ones recorded before

any anodic treatment (the same as the Nyquist and Bode

diagrams recorded at �1.25 V vs. SCE presented in Figs. 4

and 5).

3.3. Polarization characteristics of the HER onto Ni–Moalloy coatings electrodeposited onto meshes 1 and 2 recordedin 33% NaOH at 85 1C

In Fig. 10, polarization curves for the HER onto Ni–Mo alloy

coating electrodeposited onto mesh 2 (almost identical

influence is recorded for mesh 1) before and after anodic

treatment in 33% NaOH at 85 1C, are shown. As can be seen in

the inset of Fig. 10 a current density for the oxygen evolution

at 0.5 V is extremely high, about 320 mA cm�2. After 500 s at

this potential the polarization curve 2 is recorded, indicating

significant increase in the overpotential for the HER. If the

electrode is held for 1000 s at j ¼ �320 mA cm�2 (hydrogen

evolution) and the polarization curve is recorded after

Fig. 10 – The polarization curves for the Ni–Mo alloy coating

electrodeposited onto mesh 2 recorded in 33% NaOH at 85 1C

before and after different anodic treatments: 1—no anodic

treatment; 2—after oxygen evolution at 0.5 V for 500 s (j–t

response shown in the inset of Fig. 10); 3—after hydrogen

evolution at j ¼ �320 mA cm�2 for 1000 s; 4—after additional

hydrogen evolution at j ¼ �320 mA cm�2 for 1000 s.

such a treatment, no improvement in the polarization

characteristics could be detected (curve 3), as was the case

in 1 M NaOH at 25 1C, but the overpotential for the HER even

increased slightly. After the additional hydrogen evolution at

the same current density for 1000 s (curve 4), again an

increase in the overpotential for the HER is detected. It seems

that at such a high temperature and concentration of sodium

hydroxide (as well as at the extremely high-current density of

320 mA cm�2) not only the oxidation takes place at the

electrode surface, but also some dissolution of either of the

components present in the coating occurs.

It should be mentioned here that the electrodeposited

Ni–Mo alloy undergoes deterioration in 33% NaOH at 85 1C

even during the hydrogen evolution for times longer than

about 1 h without any previous reverse polarization.

After about 2 h of hydrogen evolution the electrode practically

loses its catalytic activity and after a visual inspection

it could be seen that the whole coating is scaled from

the Ni mesh. Since at such cathodic potentials it is not

possible that the dissolution could take place, it is obvious

that the hydrogen enters the pores of the Ni–Mo coating

and peals off the whole coating from the Ni substrate.

Such a behavior has been reported for the Ni–Mo coatings

electrodeposited onto mild steel substrate, where it was

found by the analysis of a cross-section of the electrodepos-

ited Ni–Mo coatings [28] that after a long time of hydrogen

evolution even in 1 M NaOH at 25 1C certain amount of the

hydrogen enters the open pores (cracks) of Ni–Mo coating and

starts pealing off the coating around the pores. Hence,

although this coating seems to be very good catalyst for

the hydrogen evolution, it is obvious that its application in the

industrial processes is, at this stage of investigation, not

recommendable.

4. Conclusions

It is shown that the Ni–Mo alloy coatings electrodeposited

onto Ni meshes used in the industrial process (Chlor Alkali)

from the pyrophosphate containing electrolytes are the most

active for hydrogen evolution in sodium hydroxide solution.

The electrodeposited Ni–Mo alloy coatings exhibit porous

surface morphology and much better activity toward the HER

than pure Ni electrode. The main contribution toward the

apparent activity is a consequence of the increase of the real

surface area although significant increase in the intrinsic

activity is also observed. It is also shown that during the

anodic polarization of such materials in 1 M NaOH at 25 1C the

oxidation of the electrode surface occurs, changing polariza-

tion characteristics (increasing overpotential for the hydrogen

evolution) of this material. If after such a treatment electro-

des were exposed to the hydrogen evolution for a certain

time, almost identical polarization diagrams for the HER are

obtained as the ones before any anodic treatment (the oxide

layer is completely reduced). If such an experiment is

performed under the condition of the industrial application

(33% NaOH at 85 1C) the electrodes cannot retain their original

performance (permanent destruction of the Ni–Mo alloy

coating occurs).

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Acknowledgment

This work was financially supported by the Ministry of

Science and Environmental Protection of the Republic of

Serbia through the Project No. 142038.

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