Ni–P and Ni–Cu–P modified carbon catalysts for methanol electro-oxidation in KOH solution

international journal of hydrogen energy 35 (2010) 2517–2529

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Ni–P and Ni–Cu–P modified carbon catalysts for methanol electro-oxidation in KOH solution R.M. Abdel Hameed a, K.M. El-Khatib b,* a b

Department of Chemistry, Faculty of Science, Cairo University, Giza, Egypt Chem. Eng. & Pilot Plant Department Engineering Division, National Research center, Dokki, Giza, Egypt

article info

abstract

Article history:

The electrocatalytic oxidation of methanol was studied on Ni–P and Ni–Cu–P supported

Received 15 November 2009

over commercial carbon electrodes in 0.1 M KOH solution. Cyclic voltammetry and chro-

Received in revised form

noamperometry techniques were employed. Electroless deposition technique was adopted

19 December 2009

for the preparation of these catalysts. The effect of the electroless deposition parameters

Accepted 19 December 2009

on the catalytic activity of the formed samples was examined. They involve the variation of

Available online 27 January 2010

the deposition time, pH and temperature. The scanning electron micrography showed a compact Ni–P surface with a smooth and low porous structure. A decreased amount of

Keywords:

nickel and phosphorus was detected by EDX analysis in the formed catalyst after adding

Nickel

copper to the deposition solution. However, an improvement in the catalytic performance

Nickel–copper

of Ni–Cu–P/C samples was noticed. This is attributed to the presence of copper hydroxide/

Methanol

nickel oxyhydroxide species. It suppresses the formation of g-NiOOH phase and stabilizes

Alkaline medium

b-NiOOH form. Linear dependence of the oxidation current density on the square root of

Electro-oxidation

the scan rate reveals the diffusion controlled behaviour. ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

The development of alternative power sources is an important issue at present. Direct methanol fuel cells (DMFCs) have attracted considerable interest for application in automobile and portable consumer electronics [1,2]. The use of methanol as a fuel has several advantages, where the energy density of methanol is twice that gained from the liquid hydrogen, in addition to its rapid start up and operation. Methanol is a liquid at normal ambient temperatures, thus it can be easily and inexpensively stored and transported. It is handled much like gasoline and diesel fuel. Moreover, methanol is independent on crude oil, a vital factor with the demand for constant increased mobility and is easily obtained from natural gas or renewable biomass resources [3]. Since the kinetics of methanol oxidation reaction are slow and incomplete, a catalyst is

required in order to improve the oxidation efficiency. Although electrocatalysts based on Pt [4,5] and Pt alloys [6,7] have been developed and indeed exhibit good activity for methanol oxidation, the high cost of these materials and the activity loss due to the formation of strongly adsorbed intermediate products are often very prohibitive. Therefore, many attempts have been directed towards the examination of the catalytic activity of cheap metals such as nickel-based electrodes. Pure Ni electrodes have been demonstrated for the anodic oxidation of numerous polar organic compounds in alkaline medium [8,9]. Fleischmann et al. [10] studied Ni electrodes and explained the oxidation of alcohols and amines on the basis of a mechanism involving electron transfer mediation by Ni(OH)2/NiOOH redox couple in the oxide film at the anodized electrode surface. Different supported nickel catalysts are obtained by different chemical and

* Corresponding author. E-mail addresses: [email protected] (R.M.A. Hameed), [email protected] (K.M. El-Khatib). 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.12.145

international journal of hydrogen energy 35 (2010) 2517–2529

electrochemical procedures such as the precipitation of thin films of nickel hydroxides at inert surfaces [11,12], the codeposition of nickel in numerous metallic alloys [13,14], or by the anodic deposition of nickel complexes [15,16]. The main advantages of the dispersed nickel catalyst over the other metals or bare nickel electrode are related to its electrochemical stability and resistance to poisoning [17]. Electroless plating technique was introduced as an autocatalytic process that involves the reduction of metal ions to a metallic coating by a reducing agent in solution. Electroless Ni–P coating has received widespread acceptance for their corrosion protection application in a variety of environments [18,19]. Moreover, they showed high electrocatalytic activity towards hydrogen evolution reaction [20,21]. These coating properties can be further improved through codeposition of other metallic elements in electroless nickel deposits. Codeposition of copper in Ni–P matrix has profound effect on deposit characteristics. Smoothness, brightness, ductility and corrosion resistance of the electroless Ni–P (12 wt%) deposit are found to increase enormously when the co-deposited copper is about 1 wt% [22]. The dependence of the electrocatalytic activity of (Cu–Ni) alloys towards formaldehyde oxidation upon the alloy composition was substantially smooth reaching its highest value on the Cu89Ni11 alloy and decreasing with increasing Ni content [23]. Moreover, it shows a significantly higher response for glucose and glycine than that observed for Cu and Ni electrodes [24]. Tian et al. [25] observed much better stability of Ni–Cu alloy nanowire electrode towards ethanol electro-oxidation compared to pure Ni electrode. The purpose of the present work is to study the electrocatalytic oxidation of methanol using Ni–P and Ni–Cu–P deposited over commercial carbon rods as catalyst in 0.1 M KOH solution.

2.

Methods and materials

Electroless deposition solution was chosen to be as simple as possible. It consists of 26.27 g L1 nickel sulphate as a source of nickel, 38.71 g L1 sodium citrate, 27.2 g L1 sodium acetate as a source of complexing agent to control the rate of the release of the free metal ions in the reduction solution, 21 g L1 glycine and 38.12 g L1 sodium hypophosphite as a source of reducing agent, which also constitutes the source of phosphorus in the deposit. 0.015 g L1 copper sulphate was added to deposition baths for Ni–Cu–P/C coatings. In addition to other constituents, sodium hydroxide was added as a buffering agent to control the bath pH. The effect of temperature (70–90  C), pH (7–12) and the deposition time (30–90 min) was investigated to achieve the optimum conditions for the electroless deposition process. All chemicals used for the present work were of analytical reagent grade and the solutions were prepared using double distilled water. Samples of commercial carbon rod were used as the substrate. They were polished with emery papers in different grades till a mirror like surface is obtained. It was washed with acetone followed by second distilled water. No further electrochemical pre-treatment was done for these carbon rods before the deposition process.

The electrochemical measurements were performed using Voltalab6 Potentiostat. The electrochemical studies involve the application of the cyclic voltammetry and the chronoamperometry techniques. All measurements were carried out at room temperature 30  2  C with scan rate of 10 mV s1. A conventional three electrode cell is used, where the working electrode was commercial carbon rods covered with Ni–P and Ni–Cu–P deposits; the counter electrode was a platinum wire and the reference electrode was Hg/HgO/1.0 M NaOH (MMO) electrode. The surface morphology of the formed Ni–P/C and Ni–Cu– P/C deposits was examined by means of the scanning electron microscopy ‘‘JXA-840A, Electron Prob Microanalyzer. JEOL, Japan’’ equipped with EDX analysis ‘‘INCA X-sight, OXFORD instruments, England’’, which is used for the determination of the chemical composition of the deposits.

3.

Results and discussion

Fig. 1 shows the cyclic voltammogram of Ni–P/C sample, deposited from an electroless plating bath at 90  C for 90 min, in 0.1 M KOH solution in the potential range from 0 to þ1200 mV (MMO). A redox couple appears at potential values of þ585 and þ325 mV in the anodic and the cathodic directions, respectively. It is attributed to Ni(OH)2/NiOOH transformation [26,27]. After NiOOH is formed, the oxygen evolution reaction starts at a potential value of þ680 mV(MMO) with high current density. The scanning electron micrographs of Ni–P/C and Ni–Cu–P/ C samples are shown in Fig. 2. It can be seen that the surface of the carbon substrate was fully covered by the Ni–P electroless deposition as shown in (Fig. 2a). Moreover, the coating is compact with smooth and low porous structure. There are no obvious flaws or aperture on the coating. The appearance of cauliflower-like nodules, which are typical of amorphous materials, is observed. Some nickel precipitates are also observed as small and light spheres on the surface. This image is in a good agreement with earlier reports [28,29]. However, the addition of copper plays a significant role in controlling the structure and the morphology of Ni–P deposit. Fig. 2b shows large-sized particles with small cracked surfaces as 20

15

10

I / mA cm-2

2518

5

0

-5 0.0

0.2

0.4

0.6

0.8

1.0

1.2

E / V (MMO)

Fig. 1 – Cyclic voltammogram of Ni–P/C catalyst in 0.1 M KOH solution at 10 mV sL1 in the potential range from 0 to D1200 mV (MMO).

international journal of hydrogen energy 35 (2010) 2517–2529

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Balaraju et al. [33] estimated the atomic percentages of the constituent elements in Ni–W–P and Ni–W–P–Cu using XPS analysis and found that the addition of copper to the electroless bath marginally reduced the Ni and P contents. Zhao et al. [34] concluded that the deposition rate decreases with the addition of copper to the electroless bath because Cu2þ ions inhibit the reduction of Ni2þ ions, thus decreasing the nickel content in the deposit. Moreover, nickel acts as an active agent for the deposition process. Therefore, the decreased rate of nickel deposition decreases the resultant P amount in the plated layer [29]. This decreased amount of phosphorous in Ni–Cu–P/C deposit may account for the increased particle size with decreasing its number [35]. Fig. 4a represents the cyclic voltammograms of Ni–P/C catalyst in 0.1 M KOH solution at various scan rates of 1–800 mV s1. It is observed that, as the scan rate increases, the current density of Ni2þ/Ni3þ redox couple increases as shown in Fig. 4b. In addition, the anodic peak shows a potential shift towards more positive values, while the cathodic peak is shifted towards more negative potential values as shown in Fig. 4c. The current density values of Ni(II)/Ni(III) redox couple are linearly proportional to the scan rate in the range 5–40 mV s1 as shown in Fig. 4d. According to the slope of these straight lines, the surface coverage of Ni(II)/Ni(III) redox species at Ni–P/C and Ni–Cu–P/C catalysts can be calculated using the following equation [36]: Ip ¼ (n2F2/4RT )yAG*

Fig. 2 – Scanning electron micrographs of Ni–P/C (a) and Ni– Cu–P/C (b) catalysts prepared from electroless deposition bath at 90 8C, pH 9.0 for 90 min with magnification power of 20003.

nodular-free structure. Zhong et al. [30] confirmed this observation after explaining the activation of the natural nucleation sites and retarding the nodule growth due to the controlled introduction of copper ions into Ni–P deposit. Moreover, Balaraju et al. [31] observed the increase of the grain size of ternary Ni–W–P deposit with the incorporation of copper in the deposit. Fig. 3a shows the EDX analysis of Ni–P deposit which reveals that the plating layer is made up of Ni and P elements with percentages of 73.70 wt% Ni and 11.45 wt% P as indicated in Table 1. This high P content (above 7 wt%) supports the amorphous structure [32] as evidenced from the SEM image. However, these percentages are altered after the addition of copper to the plating bath. EDX analysis results in Fig. 3b reported that 51.71 wt% Ni and 6.18 wt% P are present. Thus, the introduction of Cu in the deposit decreases the amounts deposited from nickel and phosphorous.

(1)

Where, G* is the surface coverage of the redox species and y is the scan rate. Taking the average of the anodic and the cathodic results, G* values were estimated as 2.18  107 and 1.045  107 mol cm2 using Ni–P/C and Ni–Cu–P/C catalysts, respectively. These surface coverage values correspond to the presence of 234 and 112 monolayers of surface species of Ni–P and Ni–Cu–P, respectively. Moreover, the linear dependence of the anodic and the cathodic peak current density values of Ni(II)/Ni(III) redox species at Ni–P/C catalyst upon the square root of the scan rate in Fig. 4e reflects the diffusion controlled behaviour. For the surface confined electroactive species at small concentrations, the electron transfer coefficient, a, and the charge transfer rate constant, ks, can be estimated from their cyclic voltammetric response using the equations derived by Laviron [37] for the case where the differences of the cathodic – anodic peaks position, DEp, is greater than (200/ n), where n is the number of the transferred electrons. Fig. 4f shows the relationship between the natural logarithm of the scan rate and the potential value of the anodic and the cathodic peaks of Ni(II)/Ni(III) redox couple at Ni–P/C catalyst, derived from the cyclic voltammograms in 0.1 M KOH solution at scan rates of 1–800 mV s1. A linear relation is noticed at higher scan rates (200–800 mV s1) as in the inset figures. Using these plots and the following equations:



Epa ¼ E þ {RT/(1  a)nF}ln{(1  a)Fny/RTks}

(2)

and lnks ¼ aln(1  a) þ (1  a) lna  ln (RT/nFy)  a(1  a) nFDEp/RT

(3)

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international journal of hydrogen energy 35 (2010) 2517–2529

Fig. 3 – Energy dispersive X-ray spectra of (a) Ni–P/C and (b) Ni–Cu–P/C catalysts.

The values of a were estimated as 0.49 and 0.51 and the values of ks were 1.14  104 and 1.22  103 s1 at Ni–P/C and Ni–Cu–P/C catalysts, respectively. This result indicates that the electron transfer rate in the redox species Ni(II)/Ni(III) in 0.1 M KOH solution gets higher after the addition of copper to Ni–P/C catalyst. Fig. 5 shows the cyclic voltammogram of Ni–P/C catalyst in 0.1 M KOH solution after the addition of 0.5 M methanol at 10 mV s1. An increase in the anodic current density of Ni–P/C catalyst is observed on adding methanol to the supporting

Table 1 – Variation of the elemental compositions of Ni–P/ C and Ni–Cu–P/C catalysts according to EDX analysis. Element

CK OK PK Ni K Cu K

Ni–P/C Weight %

Atomic %

9.25 5.61 11.45 73.70 –

28.05 12.77 13.46 45.72 –

Ni–Cu–P/C Weight %

Atomic %

25.98 15.99 6.18 51.71 0.14

50.95 23.55 4.70 20.75 0.05

electrolyte. It coincides with the start of Ni(OH)2/NiOOH transformation reaching its maximum at a potential value of þ775 mV(MMO). On the other hand, a decrease in the current density of the reduction peak in the backward direction is noticed suggesting the consumption of a great percentage of NiOOH species in the methanol oxidation process. This assumption is confirmed by calculating the charge under NiOOH reduction peak in the cyclic voltammogram scanned in KOH solution in absence and presence of 0.5 M methanol. This charge is reduced to 16.66% after the addition of methanol, thus, about 83.34% of NiOOH produced on the electrode surface is consumed in the oxidation process. Moreover, a reverse oxidation peak is observed in the initial part of the cathodic direction suggesting a completion of the methanol oxidation reaction. It may be concluded that methanol is oxidized parallel to Ni(OH)2/NiOOH transformation. As a result, the products or the intermediates of this transformation will block the active sites available for methanol adsorption causing retardation in its oxidation rate. Therefore, after removing these oxidation poisons at higher potential values, methanol reoxidizes again in the reverse scan. Fig. 6 represents the variation of methanol oxidation peak current density at Ni–P/C samples with their deposition time in the electroless bath. It was observed that increasing the deposition time tends to increase the number of the nickel

-2

Ipc / mA cm = -0.51 - 39.14 r2 = 0.998

1/2

-1 1/2

/ (Vs )

Fig. 4 – (a) Cyclic voltammograms of Ni–P/C catalyst in 0.1 M KOH solution at different scan rates (1–800 mV sL1). The variation of the anodic and the cathodic peak current density (b) and potential (c) values of Ni(II)/Ni(III) redox couple with the scan rate. (d) The linear dependence of the anodic and the cathodic peak currents on the scan rate at lower values (5–40 mV sL1). (e) The linear relationship between the anodic and the cathodic peak currents and the square root of the scan rate. (f) The variation of the anodic and the cathodic peak potentials with ln n [the inset figure shows the linear relation in the scan rate range (200–800 mV sL1)].

2522

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20 1- 0.1 M KOH 2- 0.1 M KOH + 0.5 M MeOH

2

I / mA cm-2

15

10 1 5

0

-5 0.0

0.2

0.4

0.6 E / V (MMO)

0.8

1.0

1.2

Fig. 5 – Cyclic voltammograms of Ni–P/C catalyst in 0.1 M KOH solution in absence and in presence of 0.5 M methanol at 10 mV sL1 in the potential range from 0 to D1200 mV (MMO).

active sites resulting in an enhanced oxidation current density. The addition of copper to the electroless bath would affect the electrocatalytic activity of the formed Ni–P/C catalysts. Fig. 6 showed that the presence of little amounts of nickel and copper together on the surface of the commercial carbon would increase the methanol oxidation peak current density compared to the prepared samples containing nickel alone at their surfaces [the oxidation current density is 5.5 mA cm2 at Ni–Cu–P/C sample deposited after 30 min compared to 1.25 mA cm2 at Ni–P/C sample ‘‘4.4 folds’’ and it is about 9 mA cm2 at Ni–Cu–P/C sample deposited after

Ni-P/C Ni-Cu-P/C

I / mA cm-2

15

10

45 min compared to 3.5 mA cm2 at Ni–P/C sample ‘‘2.57 folds’’. Abdel Rahim et al. [38] have studied the oxidation of methanol over nickel particles deposited onto graphite electrode and found that the small amount of nickel deposited at the first few seconds would increase the methanol oxidation current density due to the increase in the catalytic surface area of the electrode available for the reaction. However, the change in the catalyst morphology after the addition of copper at longer deposition times [above 45 min] reflects its effect on the catalyst activity. Lower oxidation current density values are obtained. Earlier work [38] showed that the large-sized nickel particles formed by the galvanostatic deposition technique over graphite electrode retard the methanol oxidation current density. This observation may be attributed to the decrease in the catalytic surface area with increasing the particle size and decreasing its number. Table 2 shows the variation of the catalytic activity of the formed Ni–P/C and Ni–Cu–P/C samples with pH and temperature of the deposition bath. A gradual increase of the methanol oxidation peak current density is noticed at the catalytic surfaces of Ni–P/C samples prepared from electroless baths with increased pH values up to 9. The maximum catalytic activity attained at this pH value is about 16 mA cm2, above which the methanol oxidation current density sharply declines. These results are in a good agreement with those obtained by Abdel Hamid et al. [39] who studied the effect of pH variation on the weight percentage of W in Co–W–P alloy deposited by electroless technique onto copper substrates. They found that increasing pH value up to 9.5 tends to increase W content in the formed composite. Further increase in pH up to 12 lowers the contents of both phosphorous and tungsten as reported for P amount in Ni–P alloys [40,41]. This behaviour can be attributed to the increase in the pH of the deposition solution which enhances Ni2þ reduction and  H2PO 2 oxidation kinetics, but does not affect H2PO2 reduction to P. Therefore, an increase in nickel active sites would be attained showing an enhancement in methanol oxidation current density. However, beyond pH 9.5, the bath destabilizes and decomposes within a short span of time resulting in the formation of a catalyst with a cracked and unstable surface. Moreover, Yoon et al. [35] found that with increasing pH of Ni– P deposition, the particle size would increase but their number decreases. Moreover, the complexation of the nickel ions increases [42], thus decreasing the amount of the deposited nickel in the plated film. Therefore, we can conclude that

Table 2 – Variation of the methanol oxidation peak current density at Ni–P/C and Ni–Cu–P/C catalysts with the deposition pH and temperature of the electroless bath.

5

pH 0 20

30

40

50 60 70 Deposition time / min.

80

90

100

Fig. 6 – Variation of the methanol oxidation peak current density at Ni–P/C and Ni–Cu–P/C catalysts in 0.1 M KOH solution with the deposition time of the formed electrodes in the electroless bath.

7 8 9 10 12

Ip/mA cm2 Ni–P/C

Ni–Cu–P/C

8.76 9.13 16.05 12.34 9.58

6.39 7.29 8.98 6.93 4.11

Temp./C

70 80 90

Ip/mA cm2 Ni–P/C

Ni–Cu–P/C

6.39 14.38 16.05

3.28 12.59 8.98

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international journal of hydrogen energy 35 (2010) 2517–2529

i. The best Ni–P/C catalyst is obtained by soaking for 90 min in an electroless bath with pH 9 and at a temperature 90  C. ii. After adding copper to the deposition solution, shorter time (45 min) and lower temperature (80  C) are needed to prepare the sample with the maximum activity. The effect of repetitive potential sweeping of Ni–P/C and Ni–Cu–P/C catalysts on their catalytic performance towards the methanol oxidation process was studied. These repeated cyclic voltammograms of Ni–Cu–P/C electrode in 0.5 M Methanol þ 0.1 M KOH solution are shown in Fig. 7 in the potential range of 0 to þ1200 mV (MMO) at a scan rate of 10 mV s1. They reveal a continuous decrease in the catalytic activity of Ni–Cu–P/C catalyst reaching to 84.75% with respect to the current density of the methanol oxidation process in the first cycle. However, Ni–Cu–P/C electrode shows an improved performance compared to Ni–P/C catalyst which records 70% efficiency after repeated cyclization. In general, this decrease in current density with repeated cyclization may be attributed to the activity loss of the nickel oxide [45,46]. This passive oxide film [g-NiOOH] blocks the electrode surface due to its compactness and poor conducting behaviour that isolate the active material [b-NiOOH] electrically from the reaction zone [47]. The better efficiency recorded at Ni–Cu–P/C catalyst is in a good agreement with the results obtained by Jafarian et al. [48] who studied the methanol oxidation reaction at Ni and Ni-Cu alloy modified glassy carbon electrode in alkaline medium. They found that the addition of copper hydroxide to the nickel oxyhydroxide species represents a very efficient strategy of suppressing the formation of gNiOOH phase. It is well known that the formation of g-NiOOH phase is associated with swelling or volume expansion of the nickel film electrodes with subsequent microcracks and disintegration of the nickel film. Lower interelectrode spacing results in lower internal resistance and therefore, better

10 1- 1st cycle 2- 2nd 3- 3rd 4- 5th 5- 10th

8

I / mA cm-2

above pH 9.0, the Ni–P/C catalyst is formed with undesirable characteristics that retard the methanol oxidation reaction. Upon the addition of copper to the deposition bath, the resulting Ni–Cu–P/C catalysts at different pH formation values show the same trend towards the methanol oxidation process as Ni–P/C samples did. However, lower oxidation current density values are attained due to the decreased amount deposited from nickel in the plated layers in the presence of copper in the electroless bath. On the other hand, with increasing the deposition temperature up to 90  C, a sharp increase in methanol oxidation current density of the formed catalysts is recorded reaching its maximum at Ni–P/C deposit formed at 90  C. This may be attributed to the fact that the reducing ability of sodium hypophosphite increases with raising the temperature of the deposition bath [43,44]. Therefore, the deposition rate of nickel increases with temperature causing an enhancement in the methanol oxidation process. On the other hand, the maximum oxidation current density is attained at Ni–Cu–P/C sample prepared at 80  C, afterwards, the oxidation process shows a lower rate. Therefore, the optimized parameters of the electroless deposition bath that resulted in the formation of catalysts with an enhanced electrocatalytic activity for methanol oxidation are:

1 2 3 4 5

6

4

2

0 0.0

0.2

0.4

0.6 0.8 E / V (MMO)

1.0

1.2

Fig. 7 – Cyclic voltammograms of Ni–Cu–P/C catalyst in 0.5 M methanol D 0.1 M KOH solution with repeated potential cyclization at 10 mV sL1 for 10 cycles in the potential range from 0 to D1200 mV (MMO).

efficiency of the electrode [49,50]. Therefore, b-NiOOH phase is expected to be a better electroactive material for high electrochemical performance in alkaline solution. The cyclic voltammetric behaviour of Ni–Cu–P/C electrode with repeated potential sweeping reveals the lower a/g-NiOOH redox contribution with a good stabilization of b/b nickel oxyhydroxide form. This is due to the invariation of the peak potential of the methanol oxidation reaction at the surface of Ni–Cu–P/C catalyst with repeated cyclization after the first cycle as shown in Fig. 7. For the oxidation of alcohols using nickel electrode covered by nickel hydroxide in alkaline solution, different hypotheses were given in the literature. Fleischmann et al. [10,51] proposed a mechanism of alcohols oxidation and suggested that NiOOH acts as an electrocatalyst. This suggestion was mainly based on the experimental observation that alcohols and other organic compounds were oxidized at a potential value which coincided exactly with that where NiOOH was produced and on the disappearance of NiOOH reduction peak in the cathodic sweep. However, the role of NiOOH as an electrocatalyst for alcohols oxidation has been questioned by many authors [52]. Some researchers reported that methanol oxidation takes place after the complete oxidation of Ni(OH)2 to NiOOH [53,54]. El Shafei [27] studied the oxidation of methanol at nickel hydroxide/glassy carbon modified electrode in alkaline medium and found that methanol oxidation occurred via Ni3þ species (mainly NiOOH). On the other hand, Taraszewska et al. [12] supposed that methanol molecules penetrate the nickel hydroxide film and are oxidized by OH ions trapped in the film. According to our results that based on the start of methanol oxidation at the potential of Ni(OH)2/ NiOOH conversion, we can suggest a mediated electron

a Ni-P/C Ni-Cu-P/C

I / mA cm-2

12

8

4

0

b

1000

2000 Time / sec.

3000

4000

b' 6

2.0

4

R

I / mA cm-2

5

1.5 3 2 1.0

70 75 80 85 90 Deposition temperature / C

c

c'

70 75 80 85 90 Deposition temperature / C

2.5

6

4

1.5 R

I / mA cm-2

2.0

1.0 2 0.5 20

40 60 80 Deposition time / min.

20

100

d'

d 5

40 60 80 Deposition time / min.

100

1.5

R

I / mA cm

-2

:

4 1.0

3 0.5

2

6

8

10 pH

12

6

8

10 pH

12

Fig. 8 – (a) Chronoamperograms of Ni–P/C and Ni–Cu–P/C catalysts in 0.5 M methanol D 0.1 M KOH solution at a potential step of D790 mV. The variation of the steady state current density values recorded from chronoamperograms after 1 h at Ni– P/C and Ni–Cu–P/C catalysts with the electroless bath deposition temperature (b), time (c) and pH (d). The variation of the enhancement factor R [the ratio between the steady state current of Ni–Cu–P/C and Ni–P/C catalysts] with the corresponding deposition parameters [b0 , c0 , d0 ].

international journal of hydrogen energy 35 (2010) 2517–2529

transfer mechanism involving nickel oxidation states in agreement with that proposed by Fleischmann et al. [51] as follows: Ni(OH)2 þ OH / NiOOH þ H2O þ e

2525

catalysts recorded after 60 min is shown in Fig. 8b–d for the variation of the deposition temperature, time and pH of the electroless bath, respectively. Moreover, the enhancement factor R, which is the ratio between the steady state current of Ni–Cu–P/C and Ni–P/C catalysts prepared at different deposition parameters [shown in Fig. 8b0 –d0 ] varies between 0.45 and 2 folds. In general, we can conclude that, raising the deposition temperature above 60  C results in the formation of Ni–Cu–P/C catalyst with better performance for the oxidation process compared to Ni–P/C catalyst. However, that enhancement is only recorded for catalysts prepared from baths of lower pH [up to pH 9] and at deposition times not exceeding 60 min. The effect of the scan rate was similarly studied after the addition of 0.5 M methanol to the supporting electrolyte. Fig. 9a represents the continuous increase of the methanol oxidation current density at Ni–P/C and Ni–Cu–P/C catalysts with increasing the scan rate in the range of 1–1000 mV s1. The linear dependence of the anodic peak currents of methanol oxidation at Ni–P/C and Ni–Cu–P/C catalysts on the square root of the scan rate is shown in Fig. 9b. It suggests that

(4)

NiOOH þ methanol / Ni(OH)2 þ oxidation product. ‘‘slow step’’ (5) The chronoamperometry technique was applied for Ni–P/C and Ni–Cu–P/C electrodes, as shown in Fig. 8, to measure their performance after 1 h towards methanol electro-oxidation in 0.5 M methanol þ 0.1 M KOH solution at a potential value of þ790 mV. It is observed that the transient current density at Ni– Cu–P/C catalyst is twice that at Ni–P/C catalyst as shown in Fig. 8a. These two studied electrodes were prepared from their corresponding electroless baths for 60 min at 80  C and pH 9.0. Therefore, this study was then extended to the catalysts prepared from deposition baths with various parameters ‘‘pH, deposition time and temperature’’. A comparison of the steady state current densities obtained at Ni–P/C and Ni–Cu–P/C

a 150

I / mA cm-2

100

50

0 0.0

0.2

0.4

0.6

0.8

1.0

-1

υ/ V s

c

b 150

-2

Ip / mA cm = 158.553 υ

1/2

/ (Vs-1)

1.4

1/2

Ep / V = 1.463 + 0.221 ln υ / Vs-1 r2 = 0.993

E / V (MMO)

I / mA cm-2

r2 = 1

100

50

1.3

1.2 -2

1/2

Ip / mA cm = 94.868 υ

/ (Vs-1)

1/2

r2 = 1

Ep / V = 1.230 + 0.088 ln υ / Vs -1

0

r2 = 0.946

1.1 0.0

0.2

0.4

0.6

υ1/2 / (V s-1)1/2

0.8

-1.2

1.0

-0.8

-0.4

ln υ Ni-P/C Ni-Cu-P/C

Fig. 9 – (a) Variation of the anodic peak current density values of methanol oxidation at Ni–P/C and Ni–Cu–P/C catalysts in 0.5 M methanol D 0.1 M KOH solution with the scan rate. (b) The linear relationship between the anodic peak current and the square root of the scan rate. (c) The linear dependence of the anodic peak potential with ln n in the scan rate range (300–800 mV sL1).

a

16

b

Ni-P/C

Ni-Cu-P/C 0.0

I / mA cm-2

I / mA cm

12

-0.8

8

-1.6 -2.4 0

5

10

15

20

I / mA cm-2

I / mA cm

-2

-2

0.0

Time / sec.

8

-0.8 -1.6 -2.4 0

5

10

15

20

Time / sec.

4

4

0 0 0

5

10

15

0

20

Time / sec.

10

d

Ni-P/C

15

20

Time / sec.

0.1 M KOH 0.1 M KOH + 0.1 M MeOH + 0.2 M MeOH + 0.3 M MeOH + 0.4 M MeOH + 0.5 M MeOH

c 10.5

5

Ni-Cu-P/C

8

I / mA cm-2

I / mA cm-2

10.0

9.5

7

9.0

6

8.5

0.4

0.8

1.2

1.6

0.4

2.0

0.8

e

1.6

2.0

40

f Ni-P/C

Ni-Cu-P/C

8

L

32

I cat / I

I cat / I L

1.2

t-1/2 / sec.-1/2

t-1/2 / sec.-1/2

24

6

16

4 0.4

0.8

1.2

t1/2 / sec.1/2

1.6

0.4

0.8 1/2

1.2

1.6

1/2

t / sec.

Fig. 10 – Chronoamperograms of Ni–P/C (a) and Ni–Cu–P/C (b) catalysts in 0.1 M KOH solution with different concentrations of methanol ranging from 0 to 0.5 M. Potential steps were D790 mV for oxidation, and then D385 mV for reduction [the inset figures]. The plots of net current chronoamperograms of Ni–P/C (c) and Ni–Cu–P/C (d) catalysts in 0.3 M methanol [obtained by subtracting the background current using the point-by-point subtracting method] vs. tL1/2. The dependence of Icat/IL on t1/2 derived from the data of chronoamperograms in absence and in presence of 0.3 M methanol at Ni–P/C (e) and Ni–Cu–P/C (f) catalysts.

international journal of hydrogen energy 35 (2010) 2517–2529

the overall oxidation of methanol at both electrodes is controlled by the diffusion of methanol from solution to the surface redox sites. The value of the electron transfer coefficient of the methanol oxidation reaction at Ni–P/C and Ni–Cu– P/C catalysts was estimated from the linear relationship between the natural logarithm of the scan rate at higher values [300–800 mV s1] and the methanol oxidation peak potential [see Fig. 9c]. a was found to be 0.537 and 0.547 at Ni– P/C and Ni–Cu–P/C catalysts, respectively. Therefore, a higher electron transfer value is obtained at Ni–Cu–P/C catalyst prepared by the electroless deposition method compared to a value of 0.4 at Ni–Cu alloy prepared by the electrochemical galvanostatic deposition method [48]. Pariente et al. [55] have proposed the use of chronoamperometric technique for the calculation of the catalytic rate constant of the methanol oxidation reaction in accordance with the equation: Icat/IL ¼ g1/2 [p1/2 erf (g1/2) þ exp(g)/g1/2]

(6)

Where Icat and IL are the currents of Ni–P/C and Ni–Cu–P/C catalysts in the presence and in the absence of methanol, respectively and g ¼ kC*t is the argument of the error function. k is the catalytic rate constant, C* is the bulk concentration of methanol and t is the elapsed time (s). In the cases where g > 1.5, exp(g)/g1/2 is so small and erf (g1/2) is almost equal to unity and the above equation is reduced to: Icat/IL ¼ g1/2p1/2 ¼ p1/2 (kC*t)1/2

(7)

Fig. 10a and b show the chronoamperograms of methanol oxidation in methanol concentration range of 0–0.5 M in 0.1 M KOH solution at þ790 mV (MMO) using Ni–P/C and Ni–Cu–P/C catalysts, respectively. The transient current is due to methanol oxidation. The linear dependence of I [obtained by subtracting the background current using the point-by-point subtracting method [16,48,56,57]] on t1/2 in Fig. 10c and d confirms the diffusion-controlled character of the oxidation process. The diffusion coefficient was thus calculated as 7.75  104 and 9.75  104 cm2 s1 at Ni–P/C and Ni–Cu–P/C catalysts, respectively. Therefore, an improvement in the diffusion coefficient value at Ni–Cu–P/C catalyst prepared by the electroless deposition method was achieved compared to 2.16  104 cm2 s1 at Ni–Cu alloy prepared by the casting method [57] and 3  106 cm2 s1 at Ni–Cu alloy prepared by the electrochemical galvanostatic deposition method [48]. Fig. 10 e and f show the plots of Icat/IL versus t1/2 using Ni–P/C and Ni–Cu–P/C catalysts, respectively in 0.3 M methanol þ 0.1 M KOH solution. The value of k was estimated as 0.1135  105 and 3.0889  105 cm3 mol1 s1 at Ni–P/C and Ni–Cu–P/C catalysts, respectively. It is much higher than that calculated at Ni–Cu alloy prepared by casting technique [57] [0.01979  105 cm3 mol1 s1].

4.

Conclusion

The presence of little amounts of nickel and copper together on the surface of the commercial carbon increases the methanol oxidation current density compared to those containing nickel

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alone. The highest catalytic activity is achieved at catalysts formed at 90  C from electroless deposition solutions adjusted at pH 9.0. A mediated electron transfer mechanism is proposed based on Ni(OH)2/NiOOH transformation. A better efficiency is noticed at Ni–Cu–P/C catalyst due to the lower a/g-NiOOH redox contribution with a good stabilization of b/b nickel oxyhydroxide form. The electron transfer coefficient, a, was calculated as 0.49 and 0.51, while the values of the charge transfer rate constant, ks, were 1.14  104 and 1.22  103 s1 at Ni–P/C and Ni–Cu–P/C catalysts, respectively. The catalytic rate constant was estimated in 0.3 M methanol þ 0.1 M KOH solution as 0.1135  105 and 3.0889  105 cm3 mol1 s1 at Ni–P/C and Ni–Cu–P/C catalysts, respectively.

references

[1] Lamy C, Belgsir EM, Le´ger J-M. Electrocatalytic oxidation of aliphatic alcohols: application to the direct alcohol fuel cell (DAFC). Journal of Applied Electrochemistry 2001;31(7):799–809. [2] Acres GJK. Recent advances in fuel cell technology and its applications. Journal of Power Sources 2001;100(1–2):60–6. [3] Baratz B, Quellette R, Park W, Stokes B. In: Hassenzahl WV, editor. Mechanical, thermal and chemical storage of energy. Pennsylvania: Hutchinson Ross; 1981. p. 216. [4] Liu J, Ye J, Xu C, Jiang SP, Tong Y. Electro-oxidation of methanol, 1-propanol and 2-propanol on Pt and Pd in alkaline medium. Journal of Power Sources 2008;177(1):67–70. [5] Huang HX, Chen SX, Yuan C. Platinum nanoparticles supported on activated carbon fiber as catalyst for methanol oxidation. Journal of Power Sources 2008;175(1):166–74. [6] Umeda M, Sugii H, Uchida I. Alcohol electrooxidation at Pt and Pt–Ru sputtered electrodes under elevated temperature and pressurized conditions. Journal of Power Sources 2008;179(2):489–96. [7] Siwek H, Tokarz W, Piela P, Czerwin´ski A. Electrochemical behavior of CO, CO2 and methanol adsorption products formed on Pt–Rh alloys of various surface compositions. Journal of Power Sources 2008;181(1):24–30. [8] Casella IG, Desimoni E, Cataldi TRI. Study of a nickelcatalysed glassy carbon electrode for detection of carbohydrates in liquid chromatography and flow injection analysis. Analytica Chimica Acta 1991;248(1):117–25. [9] Hui BS, Huber CO. Amperometric detection of amines and amino acids in flow injection systems with a nickel oxide electrode. Analytica Chimica Acta 1982;134:211–8. [10] Fleischmann M, Korinek K, Pletcher D. The kinetics and mechanism of the oxidation of amines and alcohols at oxide-covered nickel, silver, copper and cobalt electrodes. Journal of the Chemical Society Perkin Transactions 1972; 2(10):1396–403. [11] Kim M-S, Hwang T-S, Kim K-B. A study of the electrochemical redox behavior of electrochemically precipitated nickel hydroxides using electrochemical quartz crystal microbalance. Journal of the Electrochemical Society 1997;144(5):1537–43. [12] Taraszewska J, Roslonek G. Electrocatalytic oxidation of methanol on a glassy carbon electrode modified by nickel hydroxide formed by ex situ chemical precipitation. Journal of Electroanalytical Chemistry 1994;364(1–2):209–13. [13] Wang M, Liu W, Huang C. Investigation of PdNiO/C catalyst for methanol electrooxidation. International Journal of Hydrogen Energy 2009;34(6):2758–64. [14] Yi Q, Huang W, Zhang J, Liu X, Li L. A novel titaniumsupported nickel electrocatalyst for cyclohexanol oxidation

2528

[15]

[16]

[17]

[18]

[19]

[20]

[21]

[22]

[23]

[24]

[25]

[26]

[27]

[28]

[29]

[30]

[31]

[32]

international journal of hydrogen energy 35 (2010) 2517–2529

in alkaline solution. Journal of Electroanalytical Chemistry 2007;610(2):163–70. Cardoso WS, Dias VLN, Costa WM, Rodrigues I de A, Marques EP, Sousa AG, et al. Nickel-dimethylglyoxime complex modified graphite and carbon paste electrodes: preparation and catalytic activity towards methanol/ethanol oxidation. Journal of Applied Electrochemistry 2009;39(1):55–64. Zheng L, Zhang J-Q, Song J-F. Ni(II)-quercetin complex modified multiwall carbon nanotube ionic liquid paste electrode and its electrocatalytic activity toward the oxidation of glucose. Electrochimica Acta 2009;54(19):4559– 65. Marines JM, Campelo JM, Luna D. Stud Surf Sci Catal. In: Cerveny L, editor. New supported metallic nickel systems. The Netherlands: Elsevier; 1986. p. 411. Wei D, Zhou Y, Yang C. Characteristic, cell response and apatite-induction ability of microarc oxidized TiO2-based coating containing P on Ti6Al4V before and after chemical-treatment and dehydration. Ceramics International 2009;35(7):2545–54. Zhang H, Wang S, Yao G, Hua Z. Electroless Ni–P plating on Mg–10Li–1Zn alloy. Journal of Alloys and Compounds 2009; 474(1–2):306–10. Popczyk M, Budniok A, Lasia A. Electrochemical properties of Ni–P electrode materials modified with nickel oxide and metallic cobalt powders. International Journal of Hydrogen Energy 2005; 30(3):265–71. Kro´likowski A, Wiecko A. Impedance studies of hydrogen evolution on Ni–P alloys. Electrochimica Acta 2002;47(13–14): 2065–9. Lee C-Y, Lin K-L. Ni–Cu–P and Ni–Co–P as a diffusion barrier between an Al pad and a solder bump. Journal of Thin Solid Films 1994;239(1):93–8. Enyo M. Electrooxidation of formaldehyde on CuþNi alloy electrodes in alkaline solutions. Journal of Electroanalytical Chemistry 1986;201(1):47–59. Yeo I-H, Johnson DC. Electrochemical response of small organic molecules at nickel–copper alloy electrodes. Journal of Electroanalytical Chemistry 2001;495(2):110–9. Tian X-K, Zhao X-Y, Zhang L-D, Yang C, Pi Z-B, Zhang S-X. Performance of ethanol electro-oxidation on Ni–Cu alloy nanowires through composition modulation. Nanotechnology 2008;19(21):215711–6. Jafarian M, Mahjani MG, Heli H, Gobal F, Heydarpoor M. Electrocatalytic oxidation of methane at nickel hydroxide modified nickel electrode in alkaline solution. Electrochemistry Communications 2003;5(2):184–8. El Shafei AA. Electrocatalytic oxidation of methanol at a nickel hydroxide/glassy carbon modified electrode in alkaline medium. Journal of Electroanalytical Chemistry 1999;471(2):89–95. Rudnik E, Kokoszka K, Lapsa J. Comparative studies on the electroless deposition of Ni–P, Co–P and their composites with SiC particles. Surface and Coatings Technology 2008; 202(12):2584–90. Palaniappa M, Babu GV, Balasubramanian K. Electroless nickel–phosphorus plating on graphite powder. Materials Science and Engineering 2007;471(1–2):165–8. Zhong LL, Liu CC, John St JD, Jeff D. Electroless nickelphosphorous coatings with high thermal stability. U.S. Pat. US6410104. Balaraju JN, Rajam KS. Electroless deposition of Ni–Cu–P, Ni– W–P and Ni–W–Cu–P alloys. Surface and Coatings Technology 2005;195(2–3):154–61. Shibli SMA, Dilimon VS. Effect of phosphorous content and TiO2-reinforcement on Ni–P electroless plates for hydrogen evolution reaction. International Journal of Hydrogen Energy 2007;32(12):1694–700.

[33] Balaraju JN, Anandan C, Rajam KS. Influence of codeposition of copper on the structure and morphology of electroless Ni–W–P alloys from sulphate and chloride-based baths. Surface and Coatings Technology 2006;200(12–13):3675–81. [34] Zhao Q, Liu Y, Abel EW. Effect of Cu content in electroless Ni–Cu–P–PTFE composite coatings on their anti-corrosion properties. Materials Chemistry and Physics 2004;87(2–3):332–5. [35] Yoon J-W, Park J-H, Shur C-C, Jung S-B. Characteristic evaluation of electroless nickel–phosphorus deposits with different phosphorus contents. Microelectronic Engineering 2007;84(11):2552–7. [36] Bard AJ, Faulkner LR. Electrochemical methods. New York: Wiley; 2001. p. 591. [37] Laviron E. General expression of the linear potential sweep voltammogram in the case of diffusionless electrochemical systems. Journal of Electroanalytical Chemistry 1979;101(1):19–28. [38] Abdel Rahim MA, Abdel Hameed RM, Khalil MW. Nickel as a catalyst for the electro-oxidation of methanol in alkaline medium. Journal of Power Sources 2004;134(2):160–9. [39] Abdel Aal A, Barakat H, Abdel Hamid Z. Synthesis and characterization of electroless deposited Co–W–P thin films as diffusion barrier layer. Surface and Coatings Technology 2008;202(19):4591–7. [40] Keong KG, Sha W. Crystallisation and phase transformation behaviour of electroless nickel– phosphorus deposits and their engineering properties. Surface Engineering 2002;18(5):329–43. [41] Saito T, Sato E, Matsuoka M, Iwakura C. Electroless deposition of Ni–B, Co–B and Ni–Co–B alloys using dimethylamineborane as a reducing agent. Journal of Applied Electrochemistry 1998;28(5):559–63. [42] Younes-Metzler O, Zhu L, Gileadi E. The anomalous codeposition of tungsten in the presence of nickel. Electrochimica Acta 2003;48(18):2551–62. [43] Li L, An M. Electroless nickel–phosphorus plating on SiCp/Al composite from acid bath with nickel activation. Journal of Alloys and Compounds 2008;461(1–2):85–91. [44] Liu WL, Hsieh SH, Tsai TK, Chen WJ, Wu SS. Temperature and pH dependence of the electroless Ni–P deposition on silicon. Journal of Thin Solid Films 2006;510(1–2):102–6. [45] Barnard R, Randell CF, Tye FL. Studies concerning charged nickel hydroxide electrodes I. Measurement of reversible potentials. Journal of Applied Electrochemistry 1980;10(1): 109–25. [46] Schrebler-Guzma´n RS, Vilche JR, Arvia AJ. The potentiodynamic behaviour of nickel in potassium hydroxide solutions. Journal of Applied Electrochemistry 1978;8(1):67–70. [47] Hu C-C, Wen T-C. Effects of the nickel oxide on the hydrogen evolution and para-nitroaniline reduction at Ni-deposited graphite electrodes in NaOH. Electrochimica Acta 1998; 43(12–13):1747–56. [48] Danaee I, Jafarian M, Forouzandeh F, Gobal F, Mahjani MG. Electrocatalytic oxidation of methanol on Ni and NiCu alloy modified glassy carbon electrode. International Journal of Hydrogen Energy 2008;33(16):4367–76. [49] Chen J, Bradhurst DH, Dou SX, Liu HK. Nickel hydroxide as an active material for the positive electrode in rechargeable alkaline batteries. Journal of the Electrochemical Society 1999;146(10):3606–12. [50] Singh D. Characteristics and effects of g-NiOOH on cell performance and a method to quantify it in nickel electrodes. Journal of the Electrochemical Society 1998; 145(1):116–20. [51] Fleischmann M, Korinek K, Pletcher D. The oxidation of organic compounds at a nickel anode in alkaline solution. Journal of Electroanalytical Chemistry 1971;31(1):39–49.

international journal of hydrogen energy 35 (2010) 2517–2529

[52] Ve´rtes G, Hora´nyi G. Some problems of the kinetics of the oxidation of organic compounds at oxide-covered nickel electrodes. Journal of Electroanalytical Chemistry 1974;52(1): 47–53. [53] Allen JR, Florido F, Young SD, Daunert S, Bachas LG. Nitriteselective electrode based on an electropolymerized cobalt phthalocyanine. Electroanalysis 1995;7(8):710–3. [54] Bettelheim A, White BA, Raybuck SA, Murray RW. Electrochemical polymerization of amino-, pyrrole-, and hydroxy-substituted tetraphenylporphyrins. Inorganic Chemistry 1987;26(7):1009–17.

2529

[55] Pariente F, Lorenzo E, Tobalina F, Abruna HD. Aldehyde biosensor based on the determination of NADH enzymatically generated by aldehyde dehydrogenase. Analytical Chemistry 1995;67(21):3936–44. [56] Yousef Elahi M, Mousavi MF, Ghasemi S. Nano-structured Ni(II)-curcumin modified glassy carbon electrode for electrocatalytic oxidation of fructose. Electrochimica Acta 2008;54(2):490–8. [57] Jafarian M, Moghaddam RB, Mahjani MG, Gobal F. Electrocatalytic oxidation of methanol on a Ni–Cu alloy in alkaline medium. Journal of Applied Electrochemistry 2006;36(8):913–8.