Development of TiO2-supported nano-RuO2-incorporated catalytic nickel coating for hydrogen evolution reaction

ARTICLE IN PRESS 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

33 (2008) 1104– 1111

Available at www.sciencedirect.com

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

Development of TiO2-supported nano-RuO2-incorporated catalytic nickel coating for hydrogen evolution reaction S.M.A. Shibli, V.S. Dilimon Department of Chemistry, University of Kerala, Kariavattom Campus, Trivandrum 695 581, Kerala, India

ar t ic l e i n f o

abs tra ct

Article history:

Composite nickel coated steel cathodes were fabricated for hydrogen evolution reaction.

Received 13 March 2007

TiO2 -supported RuO2 particles of varying size were incorporated in the electroless coating.

Received in revised form

The electrodes exhibited high catalytic activity which was dependent on the size of RuO2

13 November 2007

particles incorporated. The smaller the size at nano-level, the higher the catalytic activity.

Accepted 14 December 2007

There was enhanced hydrogen adsorption due to high surface roughness and abundant

Available online 6 February 2008

active sites.

Keywords:

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

Electroless nickel Hydrogen evolution Electrocatalysis RuO2 TiO2 Nano-metal oxide

1.

Introduction

Nickel metallic coatings are extensively applied as catalytic cathodes for hydrogen evolution reaction (HER) [1–3]. The incorporation of composite materials in these coatings yields high stability and electrocatalytic activity [1–9]. The incorporation of RuO2 in electroplated Ni–P coating yields high catalytic activity for HER [8–11]. The authors have reported the effect of TiO2 composite incorporation and phosphorous content on the electrocatalytic activity of the electroless Ni–P coating for HER [12]. The incorporation of TiO2 improves the electrocatalytic activity and coating stability significantly [5,7,12]. TiO2 is a good composite and also a good catalytic support for RuO2 [13,14]. In addition, TiO2 can improve the stability of RuO2 by imparting its chemical inertness to the noble metal oxide [15]. Moreover, by making use of the mixed oxide of noble metal oxide with TiO2 (TiO2 is usually the major component) the process of composite incorporation for the improved electrocatalytic activity becomes more econom-

ic without any reduction in the electrocatalytic activity than where pure noble metal oxide is used [15]. Even though the improvement in the electrocatalytic activity of electroplated Ni–P coating due to RuO2 composite incorporation has been proved, RuO2 is highly expensive and its use as composite would be too costly to implement. A possible successful approach is that of reinforcing the electrocatalytic Ni–P plates with a highly active electrocatalytic metal oxide loaded on a support metal oxide acting as the dispersion phase. If these materials can couple the electrochemical activity of the dispersed phase to the electrical conductance, mechanical and chemical stability of the matrices, they may be of practical interest since they contain reduced amounts of the often-expensive electrocatalytic species. Cattarin and Musiani [16] have published an interesting review, focusing on the preparation and characterization of such oxide–matrix composites and highlighted the considerable catalytic activity and significant stability of these materials. Other reports regarding the use

Corresponding author. Tel.: +91 471 2167 230 (Res.), +91 92498 63611 (Mob.).

E-mail address: [email protected] (S.M.A. Shibli). 0360-3199/$ - see front matter & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2007.12.038

ARTICLE IN PRESS 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

Nomenclature

Rp Rs Rct Cdl Zf CPE

polarization resistance solution resistance charge transfer resistance double layer capacitance Faradaic impedance

1105

impedance of CPE angular frequency capacity parameter p 1 constant phase angle 1  a=90 roughness factor

constant phase element

of metal–matrix and oxide–matrix composites for the HER are also available [1,17]. In the present work, mixed oxide of RuO2 with less expensive TiO2 support is used as the composite for improving the electrocatalytic activity of electroless Ni–P coating for HER. One of the most practical approaches for developing a successful electrode for electrocatalytic applications is to increase the real surface area of the electrode. For this objective, Raney-Ni based electrodes are commonly used [18]. Other methods to increase the real surface area of the electrode are by using nano-size composites for reinforcement [16,19] or by using nano-crystalline materials as catalytic electrodes [20]. The extremely small composite particles, typically a few nanometers in size, could provide large real surface area for the electrochemical reactions. Furthermore, the nano-size composite particles can provide a very large number of active sites for reactions. The nanocrystalline powders of RuO2 have been synthesized [21,22] and the effect of their size on their electrocatalytic activity has been described [16,19]. The investigations on the codeposition of these particles with appropriate oxide matrices are attempted by various researchers [16]. The effect of incorporation of TiO2 -supported nano-RuO2 composite on the electrocatalytic activity of electroless Ni–P coating is investigated and reported in this paper.

2.

ZCPE o T j a j Rf

33 (2008) 1104 – 1111

Experimental

2.1. Preparation and characterization of the mixed oxide catalyst Two different methods were adopted for the preparation of TiO2 -supported RuO2 mixed oxides. In the first method, TiO2 powder (30 nm particle size, supplied by Merck, India) was soaked in isopropanol solution containing RuCl3 . The solvent was slowly evaporated and the dry mass was heated at 250  C for 1 h in an oven. It was powdered finely using a mortar, heated at 250  C for another 1 h and then fired at 450  C in a muffle furnace for 1 h for complete conversion of RuCl3 to RuO2 . This method of mixed oxide preparation is termed as the ‘thermal decomposition method’ (TD) in this paper. In the second method, nano-ruthenium particle of 5 nm size was prepared by reducing RuCl3 in ethylene glycol at 180  C as reported elsewhere [23]. TiO2 powder was added to the ethylene glycol reaction medium containing dispersed Ru particles. The medium was stirred continuously for 1 h to

facilitate Ru particle adsorption on TiO2 . Ethylene glycol was recovered after diluting with 0.3 M NaNO3 aqueous solution. The solid mass was filtered, dried at 250  C and powdered. After heating at 250  C for 1 h, the powder was fired at 500  C in a furnace for 5 h in air. This method of TiO2 -supported RuO2 mixed oxide preparation is termed as ‘thermal oxidation method’ (TO) in this paper. The composition and crystalline characteristics of the TiO2 supported RuO2 mixed oxide powder were analyzed by XRD using Cu Ka1 radiation. The particle size was analyzed using a transmission electron microscope (TEM) of 2000 FX-11, JEOL, Japan.

2.2.

Electroless plating

Steel coupons having the composition: 0.09 C, 0.34 Mn, 0.036 P, 0.0487 Si and 0.029 Al, all in wt% and the cut size of 5  6  0:4 cm was used for plating. The composition of the electroless bath was: 30 g L1 nickel sulfate, 25 g L1 succinic acid and 25 g L1 sodium hypophosphite. The bath pH was adjusted to 4.5 by adding ammonia solution. Ten g L1 was the optimum amount of TiO2 in the mixed oxides added to the electroless bath as also reported elsewhere [5]. The steel coupons were cleaned mechanically, washed using distilled water and treated in 5% NaOH for 5 min to remove dirt, abrasive scale, grease and oil from the surface (ASTM B 656). The trace of oxides on the surface was removed by acid pickling in 3% HCl for 5 min (ASTM B 656). The coupons were then sensitized in a solution of 10 g L1 SnCl2 in 40 mL L1 HCl (37%) followed by activation in a solution of 1 g L1 PdCl2 in 10 mL L1 HCl (37%). The plating was carried out at 90  C for 2 h with continuous stirring.

2.3.

Physico-chemical characterization

The physical characteristics of the coatings were evaluated as per the ASTM specifications. Adhesion was evaluated by bend test (ASTM B 571-91). Thickness and hardness were evaluated as per ASTM B 499-88 and E 364-99, respectively. Porosity was evaluated using ferroxyl reagent test. A solution of potassium ferricyanide, sodium chloride and agar–agar in hot water was used as the ferroxyl reagent. The coatings were etched in 2% HCl for 2 min prior to morphological analysis using a scanning electron microscope of Hitachi S 4000. The elemental composition of the coatings was determined by EDX analysis.

ARTICLE IN PRESS 1106

2.4.

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

Electrochemical characterization

The electrocatalytic activity was evaluated based on the trend of variation of overpotential with current during galvanostatic cathodic polarization. Impedance analysis was carried out at different overpotentials using an impedance spectrometer of AUTOLAB PGSTAT 30 with FRA2 software of FRA 4.9 version. The frequency ranged from 1 MHz to 10 Hz. A platinum electrode and a Hg/HgO/1 N OH electrode with Luggin capillary were used as the counter and reference electrodes, respectively.

3.

Results and discussion

3.1.

Characterization of the mixed oxide powder

The conversion of RuCl3 to RuO2 during both the preparation procedures was confirmed by XRD analysis. RuO2 and TiO2 particles had crystalline structure of rutile and anatase, respectively. The crystalline size of RuO2 , with respect to the peak at 2y ¼ 28:020 was calculated using the Scherrer formula, t¼

Kl , B cos y

where t is the average diameter of the crystallites; K is the Scherrer constant; l is the X-ray wavelength and B is the halfvalue breadth of the diffracted beam. The crystallite size of RuO2 prepared by TD method was 85 nm while that of the other sample prepared by TO method was 5 nm. Consistent with this determination, the actual particle size of RuO2 particles prepared by TD and TO methods was found to be 90 and 5 nm, respectively, based on TEM (Fig. 1). There was no significant variation among the values of the three samples and the average value is reported here.

3.2.

Physico-chemical characteristics of the coating

The hardness of the coatings was enhanced considerably by the incorporation of TiO2 (Table 1) [5,6,12]. All the mixed oxides incorporated coatings possessed almost equal hard-

33 (2008) 1104 – 1111

ness. The coating incorporated with TiO2 alone also possessed almost equal hardness revealing that the TiO2 composite, which was the major component in the mixed oxides, played the major role in influencing the hardness. Since the composite incorporated coatings had almost equal phosphorous and TiO2 content (as determined from the corresponding Ti content), only a marginal variation in physical properties was occurred (Table 1). All the coatings were found to be porous. A typical Prussian blue coloration was observed in all the coatings when they were tested using ferroxyl reagent. All the coatings possessed good adherence and wear resistance. The Ru metal content in the coatings was found to be in proportional to the percentages of the RuO2 content in the mixed oxides.

3.3.

Morphological improvement of the coatings

While the pure Ni–P coating had well refined and uniform grains, the coating incorporated with the mixed oxides prepared by TD and TO methods had respectively velvet like and nodule like composite distribution (Fig. 2). Electrochemical reactions, such as the HER, are essentially surface reactions and the intrinsic electrocatalytic activity and active surface area of the electrode influence their rates. From a practical point of view, a high surface roughness is a desirable quality of an electrode for HER. Fig. 2 reveals that the composite coatings have high surface roughness and hence they could possess higher electrocatalytic activity.

3.4.

Stability of the coating

The polarization resistance Rp at open circuit potential can reveal the tendency of the coating to undergo corrosion. High polarization resistance is an indication of high corrosion resistance. Consistent with other reported results [7], the composite Ni–PþTiO2 coating exhibited higher corrosion resistance ðRp ¼ 2:14  105 OÞ than the pure Ni–P coating ðRp ¼ 1:12  104 OÞ. The coatings incorporated with the mixed oxides prepared by both TD and TO methods had the Rp values of 8:98  104 and 7:56  104 O, respectively. It revealed that the presence of RuO2 in the coating had minor influence

Fig. 1 – The TEM micrographs of the TiO2 -supported RuO2 mixed oxides prepared by (a) TD method and (b) TO method.

ARTICLE IN PRESS 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

1107

33 (2008) 1104 – 1111

Table 1 – Physico-chemical characteristics of the Ni–P electroless coatings without and with TiO2-supported RuO2 composite incorporation Mixed oxides composition (%) RuO2 0 0 2 5 10 20 30 2 5 10 20 30

RuO2 preparation method

TiO2 0 100 98 95 90 80 70 98 95 90 80 70

– – TD TD TD TD TD TO TO TO TO TO

Coating composition (%)

P

Ti

Ru

10.4 10.2 10.1 10.3 10.2 10 9.8 9.9 10.2 10.3 9.9 10.5

0 2.6 2.6 2.5 2.5 2.3 2.1 2.5 2.5 2.4 2.4 1.9

0 0 0.05 0.15 0.26 0.55 0.82 0.05 0.16 0.27 0.54 0.80

Thickness ðmmÞ

Hardness (HVN)

Wear resistance

8–10 9–11 7–10 7–10 8–10 9–10 8–10 9–11 8–10 7–10 8–11 8–10

430 550 540 549 536 524 533 542 550 529 562 543

Fair Very good Good Good Good Good Good Good Good Good Good Good

Fig. 2 – The surface morphology of the Ni–P coatings without and with composite incorporation recorded before and after HER: (a) without incorporation and before HER; (b) without incorporation and after HER; (c) with incorporation of mixed oxides prepared by TD method and before HER; (d) with incorporation of mixed oxides prepared by TD method and after HER; (e) with incorporation of mixed oxides prepared by TO method and before HER; (f) with incorporation of mixed oxides prepared by TO method and after HER.

ARTICLE IN PRESS 1108

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

Electrocatalytic activity of the coating

The overpotential values exhibited by the coatings at a current density of 200 mA cm2 were compared. The pure Ni–P coating exhibited overpotential of 325 mV and an exchange current density of 3:6  106 A cm2 . The Ni–P þTiO2 coating exhibited low overpotential of 125 mV and a high exchange current density of 8:7  106 A cm2 . By existing in 2TiO2 2Ti2 O3 22TiOOH redox systems, TiO2 could enhance the electrocatalytic activity for HER in alkaline environments [1]. Interestingly, the mixed oxides incorporated Ni–P coatings exhibited still lower overpotential and high exchange current densities. As it is clear from Figs. 4 and 5, the reduction in overpotential with the RuO2 content in the mixed oxide is small when the RuO2 content is changed from 2% to 5% to 10%. Above 10% of RuO2 , the reduction in overpotential was negligibly small. Some other reports [25,26] have shown that even less than 10% of RuO2 in the supporting metal oxide (e.g. Co3 O4 Þ is sufficient to attain the same surface response as that of pure RuO2 during HER in alkaline solution. Similarly, the lower percentages of RuO2 on TiO2 would be sufficient to give maximum catalytic activity for electroless Ni–P coating. In the present work, the coatings incorporated with the mixed oxides prepared by both TD and TO methods, respectively, exhibited overpotentials of 90 and 55 mV as well as exchange current densities of 1:3  105 and 2:2  104 A cm2 . Having inherently low overpotential for HER, RuO2 could keep the mixed oxides incorporated coatings highly active. RuOOH present in the active sites of the coating surface could enhance the catalytic activity as in the case of TiOOH [15]. In the present case, the RuO2 particles having small size at nanoscale could disperse exceedingly on the electrode surface generating more catalytically active sites.

0.35

Overpotential / V

0.3 0.25 0.2 0.15 0.1 0.05 0 0

0.5

1

1.5

2

2.5

log ( j / mA cm-2) Fig. 3 – The polarization behavior of the Ni–P coatings without and with TiO2 incorporation—: without TiO2 incorporation; : with TiO2 incorporation.

0.1 Overpotential / V

3.5.

0.12

0.08 0.06 0.04 0.02 0

0.5

1

1.5

2

2.5

-2)

log ( j / mA cm

Fig. 4 – The polarization behavior of the Ni–P coatings incorporated with TiO2 -supported RuO2 mixed oxides having different percentages of RuO2 prepared by TD method. : 2% RuO2 ; n: 5% RuO2 ; &: 10% RuO2 .

0.07

0.06 Overpotential / V

on inducing corrosion [24]. However, the overall corrosion of the coating was lower than that of the pure Ni–P coating since the extent of corrosion induced by the RuO2 was insignificant when the extent of the suppression of corrosion caused by TiO2 was compared.

33 (2008) 1104 – 1111

0.05

0.04

0.03

0.02 0

0.5

1

1.5

2

2.5

log ( j / mA cm-2) Fig. 5 – The polarization behavior of the Ni–P coatings incorporated with TiO2 -supported RuO2 mixed oxides having different percentages of RuO2 prepared by TO method. : 2% RuO2 ; n: 5% RuO2 ; &: 10% RuO2 .

The Ni–P and the composite Ni–PþTiO2 coatings exhibited Tafel slopes of 105 and 40 mV dec1 , respectively (Fig. 3). The coating incorporated with the mixed oxides prepared by TD method exhibited a Tafel slope of 33 mV dec1 (Fig. 4). However, it was 22 mV dec1 for the coating incorporated with the mixed oxides prepared by TO method (Fig. 5). These observations revealed a probable change in the ratedetermining step of HER on the Ni–P coating from Volmer to Heyrovsky due to the incorporation of TiO2 . Consequently, the rate-determining step changed from Volmer to Tafel due to the incorporation of mixed oxides.

3.6.

Evaluation of structural and catalytic factors by EIS

The total electrical equivalent model of HER on a metal electrode consists of a solution resistance Rs in series with a

ARTICLE IN PRESS 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

parallel connection of the double layer capacitance Cdl and Faradaic impedance Zf . The double layer capacitance can be substituted by a constant phase element (CPE) for solid electrodes. The impedance of CPE is given by

þ

100 mV

45

20

1j R1 . ct Þ

The impedance behavior of the mixed oxides incorporated and the pure Ni–P coatings is compared in Fig. 6. A single semicircle was obtained at all the overpotentials. However, the semicircle was distorted due to the porous nature of the coatings. The complex non-linear least-square fitting program (CNLS) was used for the evaluation of impedance parameters. Three typical models for HER on rough or porous solid electrodes have been proposed as (1) CPE model [27,28], (2) porous electrode model [29] and (3) two-CPE model [28]. In CPE model, Cdl is substituted by CPE as described above. The CPE model, which includes uncompensated solution resistance Rs in series with a parallel connection of a charge transfer resistance Rct and a CPE, could hold good approximation for all the present coatings. The impedance parameters are compared in Table 2. The Rct values of the composite-incorporated coatings were low at all the overpotentials (Table 2). Moreover, the table shows that these coatings had high extent of reduction in Rct value at high overpotentials. Among all the composite incorporated coatings, the coating containing the mixed oxides prepared by TO method had the lowest Rct value (Table 2). The variation of j value from unity can reveal the change in surface roughness. In the present case, even though the j value at low overpotential was found to be almost equal for all the coatings, the composite incorporated coatings had the largest deviation in the j value from unity at high overpotentials (Table 2). The coating containing the mixed oxides prepared by the TD method had j value of 0.732 at the overpotential of 350 mV. At the same overpotential, the coating containing the mixed oxides prepared by the TO method exhibited j value of 0.652. These observations revealed that high change in surface roughness had occurred in the composite incorporated coatings and it was maximum in the coating containing the composite prepared by the TO method. The pure Ni–P coating exhibited only a marginal change in the j value. The double layer capacitance Cdl value was calculated using the Brug equation [30]. The Cdl value was high for the composite incorporated coatings (Table 2). The coating containing the mixed oxides prepared by the TO method exhibited the highest Cdl value. Hence, it was inferred that there were more electrochemical active sites on the composite incorporated coating, especially on the coating containing the mixed oxides prepared by TO method. The surface roughness factor Rf was calculated from the ratio of double layer capacitance value of the tested coating, Cdl , to that of the smooth metallic electrode which was taken as

350 mV

-5 -2

8

18

28

38

48

Z'/ohm 95 25 mV

70 -Z''/ohm

T¼

Cjdl ðR1 s

25 mV

70 -Z''/ohm

where j is related to the constant phase angle a ¼ 90 ð1  jÞ and T is the capacity parameter. T is related to the solution resistance Rs and the charge transfer resistance Rct as

95

100 mV

45

20 350 mV

-5 -2

8

18

28

38

48

Z'/ohm 95

70 -Z''/ohm

ZCPE

1 ¼ , TðjoÞj

1109

33 (2008) 1104 – 1111

25 mV

45

100 mV

20 350 mV

-5 -2

8

18

28 Z'/ohm

38

48

Fig. 6 – The impedance behavior observed at different overpotential of the Ni–P coatings without and with TiO2 supported RuO2 incorporation. (a) Pure Ni–P coating, (b) Ni–P coating incorporated with TiO2 -supported RuO2 prepared by TD method, (c) Ni–P coating incorporated with TiO2 supported RuO2 prepared by TO method.

20 mF cm2 [31]. Thus, Rf ¼

Cdl . 20 mF cm2

The composite incorporated coatings had high surface roughness as evidenced based on the Rf values (Table 2). The table also shows that the coating incorporated with the

ARTICLE IN PRESS 1110

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

33 (2008) 1104 – 1111

Table 2 – The electrochemical impedance parameters observed at different overpotential of the Ni–P electroless coatings without and with TiO2-supported RuO2 composite incorporation Composite in the coating

Overpotential (mV)

Rct ðOÞ

j

Cdl ðFÞ

25

8:5  103

Without composite

RuO2 –TiO2 made by TD

RuO2 –TiO2 made by TO

Rf

0.752

1:09  104

100

7:9  10

2

0.634

5:53  10

4

27.65

5.45

350

4:8  100

0.765

7:25  104

36.25

25

8:9  102

0.741

1:33  103

66.50

100

4:1  10

2

0.622

3:79  10

3

189.5

350

4:1  100

0.732

4:56  103

227.8 94.5

25

7:4  102

0.745

1:89  103

100

3:4  102

0.613

3:86  103

193

350

3:3  100

0.652

4:19  103

209.5

0.012

j / A cm-2

0.008

0.004

0 0

100

200 t/s

Fig. 7 – The current density variation during potentiostatic experiment at 0:650 V in 32% NaOH solution after the electrodes have been cathodically polarized at 1:250 V. B: Pure Ni–P coating, : TiO2 -incorporated Ni–P coating, n: Ni–P coating incorporated with TiO2 -supported RuO2 prepared by TD method, m: Ni–P coating incorporated with TiO2 supported RuO2 prepared by TO method.

determined from the area under the current density–time curve (Fig. 7). The mixed oxides incorporated coatings had very large extent of hydrogen adsorption. The trend of the extent of hydrogen adsorption on all the present coatings was found to be almost in accordance with that of the observed catalytic activity of the coatings discussed in Sections 3.5 and 3.6. The charge was determined by integrating the area under the current versus time curves. The charge corresponding to the pure Ni–P coating was 0.1375 C. The incorporation of TiO2 resulted in the increase of charge to 0.3500 C, whereas the charges corresponding to the Ni–P coatings incorporated with mixed oxide prepared by the TD and TO methods were 0.6125 and 0.775 C, respectively. The morphology of the coatings was analyzed after they were subjected to HER at a current density of 200 mA cm2 for 48 h (Fig. 1). The Ni–P coating exhibited blisters and failures at some regions. The composite incorporated coatings did not exhibit considerable morphological change. These observations strongly advocated about the stability of the composite coating during HER.

4. mixed oxides prepared by TO method possessed the highest roughness.

3.7.

The extent of hydrogen adsorption

In another set of experiments, the coatings were subjected to potentiostatic cathodic polarization up to 1:250 V. Subsequently, the potential was reversed to 0:650 V. It was assumed that the hydrogen adsorbed on the coating at this potential during cathodic polarization got oxidized. The trend of variation of current at 0:650 V was monitored (Fig. 7). There was a reduction in current density with time and the plot of the observed current density versus inverse of the square root of the time (Cottrell plot) became a straight line. It revealed that the observed current was the diffusion current due to the oxidation of hydrogen already adsorbed on the coating. The extent of hydrogen adsorbed on the coating was

Conclusions

RuO2 –TiO2 mixed oxides-incorporated nickel–phosphorous coatings exhibited high electrocatalytic activity for HER. Titanium dioxide acted as an effective catalyst support for RuO2 . The mixed oxides containing RuO2 having particle size at nano-level exhibited significantly high catalytic activity. Lower percentages of RuO2 ð10%Þ on TiO2 were sufficient to give maximum catalytic activity toward HER for electroless Ni–P coating. Impedance analysis revealed the improvement in the geometric and inherent electrocatalytic properties of the coatings due to composite incorporation. There was enhanced hydrogen adsorption due to high surface roughness and abundant active sites on the mixed oxide incorporated coatings resulting in very high catalytic activity. The stability of the coating was higher due to the presence of TiO2 in the mixed oxide composite. The physical characteristics of the composite incorporated coatings were found to be superior to those of the pure Ni–P coating.

ARTICLE IN PRESS 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

Acknowledgment We thank the Head of the Department of Chemistry, University of Kerala for providing laboratory facilities to carry out this work. R E F E R E N C E S

[1] Gierlotka D, Rovinski E, Budniok A, Giewka EL. Production and properties of electrolytic Ni2P2TiO2 composite layers. J Appl Electrochem 1997;27:1349–54. [2] Crnkovic FC, Machado SAS, Avaca LA. Electrochemical and morphological studies of electrodeposited Ni–Fe–Mo–Zn alloys tailored for water electrolysis. Int J Hydrogen Energy 2004;29:249–54. [3] Shibli SMA, Dilimon VS, Deepti T. ZrO2 -reinforced Ni–P plate: an effective catalytic surface for hydrogen evolution. Appl Surf Sci 2006;29:249–54. [4] Feldstein N. In: Mollory GO, Hajdu JB, editors. Electroless plating: fundamentals and applications. Orlando: AESF; 1991. p. 269. [5] Balaraju JN, Sankara Narayanan TS, Seshadri SK. Electroless Ni–P composite coatings. J Appl Electrochem 2003;33:807–16. [6] Hussain MS, Such TE. Deposition of composite autocatalytic nickel coatings containing particles. Surf Technol 1981;13:119–25. [7] Balaraju JN, Sankara Narayanan TS, Seshadri SK. Evaluation of the corrosion resistance of electroless Ni–P and Ni–P composite coatings by electrochemical impedance spectroscopy. J Solid State Electrochem 2001;5:334–8. [8] Chiaki I, Naoji F, Masashi T. Electrochemical preparation and characterization of Ni=ðNi þ RuO2 Þ composite coatings as an active cathode for hydrogen evolution. Electrochim Acta 1992;37:757–8. [9] Tavares AC, Trasatt S. Ni þ RuO2 co-deposited electrodes for hydrogen evolution. Electrochim Acta 2000;45:4195–202. [10] Chiaki I, Masashi T, Shuji N, Hiroshi I, Masao M, Naoji F. Electrochemical properties of Ni=ðNi þ RuO2 Þ active cathodes for hydrogen evolution in chlor-alkali electrolysis. Electrochim Acta 1995;40:977–82. [11] Yu-Min T. Reinforced composite-reductively deposited catalytic coating for hydrogen evolution. J Electroanal Chem 2001;498:223–7. [12] Shibli SMA, Dilimon VS. Effect of phosphorous content and TiO2 -reinforcement on Ni–P electroless plates for hydrogen evolution reaction. Int J Hydrogen Energy 2007;32:1694–700. [13] Losiewiez B, Budniok A, Rowinski E, Lagiewka E, Lasia A. The structure, morphology and electrochemical impedance study of the hydrogen evolution reaction on the modified nickel electrodes. Int J Hydrogen Energy 2004;29:145–57. [14] Losiewiez B, Stepien A, Gierlotka D, Budniok A. Composite layers in Ni–P system containing TiO2 and PTFE. Thin Solid Films 1999;349:43–50.

33 (2008) 1104 – 1111

1111

[15] Trasatti S. Physical electrochemistry of ceramic oxides. Electrochim Acta 1991;36:225–41. [16] Cattarin S, Musiani M. Electrosynthesis of nanocomposite materials for electrocatalysis. Electrochim Acta 2007;52:2796–805. [17] Baruffaldi C, Cattarin S, Musiani M. Deposition of nonstoichiometric tungsten oxides þ MO2 composites (M ¼ Ru or Ir) and study of their catalytic properties in hydrogen or oxygen evolution reactions. Electrochim Acta 2003;48:3921–7. [18] Tanaka S, Horise N, Tanaki T, Ogata Y. Effect of Ni–Al precursor alloy on the catalytic activity for a Raney-Ni cathode. J Electrochem Soc 2000;147:2242–5. [19] Jirkovsky J, Hoffmannova H, Klementova M, Krtil P. Particle size dependence of the electrocatalytic activity of nanocrystalline RuO2 electrode. J Electrochem Soc 2006;153:E111–8. [20] Rodriguez-Valdez LM, Guel IE, Almeraya-Calderon F, NeriFlores MA, Martinez-Villafane A, Martinez-Sanchez R. Electrochemical performance of hydrogen evolution reaction of Ni–Mo electrodes obtained by mechanical alloying. Int J Hydrogen Energy 2004;29:1141–5. [21] Music S, Popovic S, Maljkovic M, Furic A, Gajovic A. Influence of synthesis procedure on the formation of RuO2 . Mater Lett 2002;56:806–11. [22] Music S, Popovic S, Maljkovic M, Saric A. Synthesis and characterization of nanocrystalline RuO2 powders. Mater Lett 2004;58:1431–6. [23] Akane M, Ioan B, Ken-ichi A, Yoshio N. Preparation of Ru nanoparticles supported on g-Al2 O3 and its novel catalytic activity for ammonia synthesis. J Catal 2001;204:364–71. [24] Shibli SMA, Gireesh VS, Sony G. Surface catalysis based on ruthenium dioxide for effective activation of aluminium sacrificial anodes. Corros Sci 2004;46:819–30. [25] Krstajic N, Trasatti S. Cathodic behavior of RuO2 -doped Ni=Co3 O4 electrodes in alkaline solutions: surface characterization. J Electrochem Soc 1995;142:2675–81. [26] Krstajic N, Trasatti S. Cathodic behaviour of RuO2 -doped Ni=Co3 O4 electrodes in alkaline solutions: hydrogen evolution. J Appl Electrochem 1998;28:1292–7. [27] Rodrı´guez-Valdez LM, Estrada-Guel I, Almeraya-Caldero´n F, Neri-Flores MA, Martı´nez-Villafan˜e A, Martı´nez-Sa´nchez R. Electrochemical performance of hydrogen evolution reaction of Ni–Mo electrodes obtained by mechanical alloying. Int J Hydrogen Energy 2004;29:1141–5. [28] Lasia A. In: Conway BE, White RE, editors. Modern aspects of electrochemistry, vol. 35. New York: Kluwer Academic, Plenum Publishers; 2002. p. 1. [29] de Levie R. In: Delahay P, editor. Advances in electrochemistry and electrochemical engineering, vol. 6. New York: Interscience; 1967. p. 326. [30] Brug GJ, Van Der Eeden ALG, Sluyters-Rehbach M, Sluyters JH. The analysis of electrode impedances complicated by the presence of a constant phase element. J Electroanal Chem 1984;176:275–95. [31] Karimi Shervedani R, Lasia A. Studies of the hydrogen evolution reaction on Ni–P electrodes. J Electrochem Soc 1997;144:511–9.