Electrodeposition of Ni–Mo alloy coatings and their characterization as cathodes for hydrogen evolution in sodium hydroxide solution

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33 (2008) 3676– 3687

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Electrodeposition of Ni–Mo alloy coatings and their characterization as cathodes for hydrogen evolution in sodium hydroxide solution N.V. Krstajic´a, V.D. Jovic´b,, Lj. Gajic´-Krstajic´c, B.M. Jovic´b, A.L. Antozzid, G.N. Martellid a

Faculty of Technology and Metallurgy University of Belgrade, 11000 Belgrade, Karnegijeva 4, Serbia Institute for Multidisciplinary Research, 11030 Belgrade, P.O. Box 33, Serbia c Institute of Technical Sciences SASA, 11000 Belgrade, Knez Mihajlova 35, Serbia d De Nora Industries, Via Bistolfi 35, 20134 Milan, Italy b

ar t ic l e i n f o

abs tra ct

Article history:

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

Received 4 September 2007

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

Received in revised form

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

13 February 2008

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

Accepted 20 April 2008

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

Available online 4 June 2008

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

Keywords: Electrodeposition Ni–Mo alloy Coatings

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

Hydrogen evolution

reserved.

Stability EIS measurements

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 electrocatalytic activity depends on the heat of adsorption of the intermediate on the electrode surface defined by the wellknown ‘‘volcano’’ curve [3]. It is clear that beside the precious metals, there is practically no way to find the new materials

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 Mobased 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

Corresponding author. Tel.: +381 11 3303688; fax: +381 11 3055289.

E-mail address: [email protected] (V.D. Jovic´). 0360-3199/$ - see front matter & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.04.039

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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 dec1 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 deposition 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 composition 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-ammonium 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) concentration 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.

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2.

Experimental

2.1.

Electrodeposition of the Ni–Mo coatings

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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 ‘‘FlyNet’’ 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 cm2 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 10 mm (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.

Table 1 – Electrolytes used for the Ni–Mo alloy coatings electrodeposition Solution

pH

Electrolyte composition

S1

9.0

10 g dm3 NiCl2  6H2O; 40 g dm3 Na2MoO4  2H2O; 45 g dm3 K4P2O7; 75 g dm3 NaHCO3

S2

8.5

10 g dm3 NiCl2  6H2O; 40 g dm3 Na2MoO4  2H2O; 100 g dm3 K4P2O7; 40 g dm3 NH4Cl

S3

8.5

10 g dm3 NiCl2  6H2O; 40 g dm3 Na2MoO4  2H2O; 45 g dm3 K4P2O7; 75 g dm3 NaHCO3; 10 g dm3 NaCl

S4

9.0

10 g dm3 NiCl2  6H2O; 40 g dm3 Na2MoO4  2H2O; 45 g dm3 K4P2O7; 10 g dm3 Na2B4O7

S5

7.5

10 g dm3 NiCl2  6H2O; 40 g dm3 Na2MoO4  2H2O; 45 g dm3 K4P2O7; 75 g dm3 NaHCO3; 10 g dm3 NaCl; 5 g dm3 HCl

S6

7.6

10 g dm3 NiCl2  6H2O; 40 g dm3 Na2MoO4  2H2O; 45 g dm3 K4P2O7; 10 g dm3 Na2B4O7; 5 g dm3 HCl

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2.2.

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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 cm2 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 spectroscopy (EIS) experiments were performed with the same potentiostat using Electrochemical Impedance Spectroscopy software EIS 300. All solutions were made from the analytical grade chemicals and the Milli-Q water.

2.3.

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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 cm2 to about 41 at% at 100 mA cm2. 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 electrodeposited 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 cm2.

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.

3.

Results and discussion

3.1. Electrodeposition of the Ni–Mo alloy coatings and their 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

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 presented by dotted line.

Table 2 – Conditions of the Ni–Mo alloy samples electrodeposition onto Ni mesh (1); corresponding potentials for hydrogen evolution at j ¼ 0.3 A cm2 Alloy sample 1 2 3 4 5 6 7 8

jd (mA cm2)

Solution

T (1C)

Deposition time (min)

E vs. SCE (V) at j ¼ 0.3 A cm2

100 50 20 100 100 100 100 100

S1 S1 S1 S3 S2 S4 S5 S6

60 60 60 60 60 60 60 60

15 60 120 120 120 120 120 120

1.353 1.545 1.580 1.339 1.499 1.445 1.435 1.348

Ni mesh 20 mm Ni

1.672 250

Sulfamate

45

10

1.660

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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 cm2.

The composition of electrolytes for the Ni–Mo alloys electrodeposition 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 dm3 K4P2O7 should be dissolved at elevated

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temperature (50–60 1C); second component is 10 g dm3 NiCl2  6H2O, which should be added in small amounts with intensive stirring, in order to prevent hydrolysis and allow anions; after complexation of Ni2+ cations with P2O4 7 complete dissolution of nickel salt, 75 g dm3 NaHCO3 should be added in the bath and finally, 40 g dm3 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 polarization 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 20 mm 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 cm2 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 cm2 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 cm2) 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 alloy coatings 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 cm2 for different times.

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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 (410 mA cm2) is obtained 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 cm2, 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

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 cm2: 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).

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the mesh 1 being better defined), indicating the presence of two time constants. The Bode plots also display two relaxation 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].

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(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 (1F) 1/F } , where F represents relation: Cdl ¼ {Tdl/[(Rs+Rp)1+R1 ct ] 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 1 (1F) 1/F ] [31], because the different relation: Cdl ¼ [Tdl/(R1 s +Rct ) 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/20 mF cm2 [32] (20 mF cm2 being an ideal value for the double layer capacitance). These parameters are displayed in Table 3. According

Fig. 5 – 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 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.

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Table 3 – Parameters obtained by fitting EIS results E/V

Rs/ O cm2

CPEp in parallel with Rp Tp/ O1 cm2 sF

CPEdl in parallel with Rct

Cdl/ m F cm2

Rp/ O cm2

Fp

Tdl/ O1 cm2 sF

Rct/ O cm2

Fdl

0.0205 0.0404 0.0572

0.186 0.194 0.192

0.69 0.60 0.56

0.162 0.133 0.127

2.30 1.28 0.90

0.63 0.69 0.70

44000 43000 39000

0.115 0.066 0.0308 0.0152

3.62 2.04 1.27 0.795

0.56 0.66 0.57 0.65

0.099 0.071 0.084 0.069

2.10 1.15 0.90 0.70

0.65 0.78 0.73 0.72

34500 31700 28000 29700

150  106 154  106 158  106 160  106

29.4 14.9 8.86 5.24

0.94 0.93 0.94 0.93

205 203 193 194

0.1370 0.0196 0.0165 0.0119

1.91 2.71 3.25 4.49

0.62 0.68 0.67 0.71

Rf(av)

(j/Rf)/ mA cm2 (E ¼ 1.35 V)

2100

0.14

1550

0.30

10

0.03

Ni– Mo (expanded mesh 1) 1.15 1.20 1.25

0.930 0.937 0.954

Ni– Mo (Fly-net mesh 2) 1.15 1.20 1.25 1.30

0.717 0.743 0.760 0.788

Ni (expanded mesh 1) 1.40 0.96 1.45 0.97 1.50 0.99 1.55 1.00 Previous treatment: H2 evolution at 0.1 A cm2 After anodic polarization Ni– Mo (expanded mesh 1) 1.25 0.954 0.0107 0.264 0.76 1.25 0.807 0.0104 0.167 0.76 1.25 0.811 0.0106 0.466 0.65 1.25 0.806 0.0187 0.207 0.70

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 electrochemical desorption of the intermediate Hads at its high-surface coverage (the unique Tafel slope of E120 mV dec1 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.

H2 evolution at 0.1 A cm2 Five cycles at v ¼ 10 mV s1 1000 s at E ¼ 0.35 V (peak potential) 1000 s at E ¼ 0.50 V (O2 evolution)

3.2.1. Cyclic voltammetry, EIS and polarization characteristics of both electrodes after the anodic treatment It 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 significant 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 s1 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]

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b-Ni(OH)2 to the NiOOH occurs via the reaction: b-NiðOHÞ2 ) NiOOH þ Hþ 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) measurements [38,39]. According to the Bode model [40] b-Ni(OH)2 form 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

Fig. 6 – The CV’s recorded at the sweep rate of 10 mV s1 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).

according to the following overall reaction: Ni þ 2OH ) NiðOHÞ2 þ 2e

(1)

0

The peak ‘‘a ’’ 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)2 can 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 quasireversible 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

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 s1; 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 cm2 for 1000 s.

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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 electrochemically 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 MoO2 4 species. Already after recording CV’s, the polarization characteristics 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 cm2 for 1000 s; 3—after additional hydrogen evolution at j ¼ 100 mA cm2 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).

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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 cm2) 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 cm2 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 electrodeposited 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

Fig. 9 – The Nyquist and Bode diagrams recorded in 1 M 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 s1; 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.

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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–Mo alloy coatings electrodeposited onto meshes 1 and 2 recorded in 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 cm2. 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 cm2 (hydrogen evolution) and the polarization curve is recorded after

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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 cm2) 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 electrodeposited 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.

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 cm2 for 1000 s; 4—after additional hydrogen evolution at j ¼ 320 mA cm2 for 1000 s.

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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 polarization characteristics (increasing overpotential for the hydrogen evolution) of this material. If after such a treatment electrodes 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. R E F E R E N C E S

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