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

R.M. Abdel Hameed a, K.M. El-Khatib b,*a Department of Chemistry, Faculty of Science, Cairo University, Giza, Egyptb Chem. Eng. & Pilot Plant Department Engineering Division, National Research center, Dokki, Giza, Egypt

a r t i c l e i n f o

Article history:

Received 15 November 2009

Received in revised form

19 December 2009

Accepted 19 December 2009

Available online 27 January 2010

Keywords:

Nickel

Nickel–copper

Methanol

Alkaline medium

Electro-oxidation

* Corresponding author.E-mail addresses: randa311eg@yahoo.com

0360-3199/$ – see front matter ª 2009 Profesdoi:10.1016/j.ijhydene.2009.12.145

a b s t r a c t

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

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

noamperometry techniques were employed. Electroless deposition technique was adopted

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

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

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

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

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

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

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

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

the scan rate reveals the diffusion controlled behaviour.

ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1. Introduction required in order to improve the oxidation efficiency.

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 indepen-

dent 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 meth-

anol oxidation reaction are slow and incomplete, a catalyst is

(R.M.A. Hameed), kamesor T. Nejat Veziroglu. Pu

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 inter-

mediate 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 elec-

trodes. 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

lnrc@hotmail.com (K.M. El-Khatib).blished by Elsevier Ltd. All rights reserved.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 2 5 1 7 – 2 5 2 92518

electrochemical procedures such as the precipitation of thin

films of nickel hydroxides at inert surfaces [11,12], the code-

position 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 electro-

chemical stability and resistance to poisoning [17].

Electroless plating technique was introduced as an auto-

catalytic 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. Code-

position 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 electro-

catalytic 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 elec-

trode towards ethanol electro-oxidation compared to pure Ni

electrode.

The purpose of the present work is to study the electro-

catalytic oxidation of methanol using Ni–P and Ni–Cu–P

deposited over commercial carbon rods as catalyst in 0.1 M

KOH solution.

E / V (MMO)0.0 0.2 0.4 0.6 0.8 1.0 1.2

I / m

A c

m-2

-5

0

5

10

15

20

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).

2. Methods and materials

Electroless deposition solution was chosen to be as simple as

possible. It consists of 26.27 g L�1 nickel sulphate as a source of

nickel, 38.71 g L�1 sodium citrate, 27.2 g L�1 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 L�1 glycine

and 38.12 g L�1 sodium hypophosphite as a source of reducing

agent, which also constitutes the source of phosphorus in the

deposit. 0.015 g L�1 copper sulphate was added to deposition

baths for Ni–Cu–P/C coatings. In addition to other constitu-

ents, 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 elec-

trochemical 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 chro-

noamperometry techniques. All measurements were carried

out at room temperature 30� 2 �C with scan rate of 10 mV s�1.

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 ofþ585 and

þ325 mV in the anodic and the cathodic directions, respec-

tively. 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

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.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 2 5 1 7 – 2 5 2 9 2519

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.

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 elec-

troless 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 s�1. 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 poten-

tial 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 s�1 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* (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 � 10�7 and

1.045 � 10�7 mol cm�2 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 s�1. A linear relation is noticed at

higher scan rates (200–800 mV s�1) 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

lnks¼ aln(1�a)þ (1�a) lna� ln (RT/nFy)�a(1� a) nFDEp/RT (3)

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

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 2 5 1 7 – 2 5 2 92520

The values of a were estimated as 0.49 and 0.51 and the

values of ks were 1.14 � 10�4 and 1.22 � 10�3 s�1 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 s�1. 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 Ni–P/C Ni–Cu–P/C

Weight % Atomic % Weight % Atomic %

C K 9.25 28.05 25.98 50.95

O K 5.61 12.77 15.99 23.55

P K 11.45 13.46 6.18 4.70

Ni K 73.70 45.72 51.71 20.75

Cu K – – 0.14 0.05

electrolyte. It coincides with the start of Ni(OH)2/NiOOH trans-

formation 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 isconfirmed bycalculatingthechargeunder NiOOH

reduction peak in the cyclic voltammogram scanned in KOH

solution inabsence and presence of 0.5 M methanol.This charge

is reduced to 16.66% after the addition of methanol, thus, about

83.34%ofNiOOH producedon theelectrode surface isconsumed

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

Ipcr2 = 0.998

/ mA cm-2 = -0.51 - 39.14 1/2 / (Vs-1)1/2

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

relationshipbetweenthe anodicandthe cathodicpeakcurrentsandthe square root of the scanrate. (f)The variationof 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)].

E / V (MMO)0.0 0.2 0.4 0.6 0.8 1.0 1.2

I / m

A cm

-2

-5

0

5

10

15

20

1

22- 0.1 M KOH + 0.5 M MeOH1- 0.1 M KOH

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).

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 2 5 1 7 – 2 5 2 92522

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 cata-

lysts. 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 cm�2 at Ni–Cu–P/C sample deposited after 30 min

compared to 1.25 mA cm�2 at Ni–P/C sample ‘‘4.4 folds’’ and it

is about 9 mA cm�2 at Ni–Cu–P/C sample deposited after

Deposition time / min.20 30 40 50 60 70 80 90 100

I / m

A c

m-2

0

5

10

15

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

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.

45 min compared to 3.5 mA cm�2 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 elec-

trode 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 tech-

nique 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 temper-

ature of the deposition bath. A gradual increase of the meth-

anol 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 cm�2, 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

H2PO2� 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 peakcurrent density at Ni–P/C and Ni–Cu–P/C catalysts withthe deposition pH and temperature of the electrolessbath.

pH Ip/mA cm�2 Temp./C Ip/mA cm�2

Ni–P/C Ni–Cu–P/C Ni–P/C Ni–Cu–P/C

7 8.76 6.39 70 6.39 3.28

8 9.13 7.29 80 14.38 12.59

9 16.05 8.98 90 16.05 8.98

10 12.34 6.93

12 9.58 4.11

E / V (MMO)0.0 0.2 0.4 0.6 0.8 1.0 1.2

I / m

A c

m-2

0

2

4

6

8

10

123

54

1- 1st cycle2- 2nd

3- 3rd

4- 5th

5- 10th

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).

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 2 5 1 7 – 2 5 2 9 2523

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 oxida-

tion 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 depo-

sition bath that resulted in the formation of catalysts with an

enhanced electrocatalytic activity for methanol oxidation are:

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 s�1. 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 reac-

tion 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 g-

NiOOH 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

efficiency of the electrode [49,50]. Therefore, b-NiOOH phase is

expected to be a better electroactive material for high elec-

trochemical performance in alkaline solution. The cyclic vol-

tammetric 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 oxy-

hydroxide 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)2to NiOOH [53,54]. El Shafei [27] studied the oxidation of

methanol at nickel hydroxide/glassy carbon modified elec-

trode 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

Time / sec.0 1000 2000 3000 4000

I / m

A c

m-2

4

8

12

Deposition temperature / C70 75 80 85 90

I / m

A c

m-2

2

3

4

5

6

Ni-P/CNi-Cu-P/C

Deposition temperature / C70 75 80 85 90

R

1.0

1.5

2.0

b'

Deposition time / min.20 40 60 80 100

I / m

A c

m-2

2

4

6

Deposition time / min.20 40 60 80 100

R

0.5

1.0

1.5

2.0

2.5

pH6 8 10 12

I / m

A c

m-2

2

3

4

5

pH6 8 10 12

R

0.5

1.0

1.5

:

a

b

c

d

c'

d'

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].

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 2 5 1 7 – 2 5 2 9 2525

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� (4)

NiOOH þmethanol / 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

/ V0.0 0.2 0.4

I / m

A c

m-2

0

50

100

150

a

c

1/2 / (V s-1)1/2

0.0 0.2 0.4 0.6 0.8 1.0

I / m

A c

m-2

0

50

100

150Ip / mA cm

-2= 158.553 υ

υ

υ

1/2/ (Vs-1)

1/2

r2 = 1

Ip / mA cm-2

= 94.868 υ1/2 / (Vs-1)

1/2

r2 = 1

b

Fig. 9 – (a) Variation of the anodic peak current density values o

in 0.5 M methanol D 0.1 M KOH solution with the scan rate. (b)

and the square root of the scan rate. (c) The linear dependence o

(300–800 mV sL1).

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 deposi-

tion 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 enhance-

ment 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 s�1.

The linear dependence of the anodic peak currents of meth-

anol 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

s-10.6 0.8 1.0

ln

-1.2 -0.8 -0.4

E /

V (

MM

O)

1.1

1.2

1.3

1.4

Ni-P/CNi-Cu-P/C

Ep / V = 1.463 + 0.221 ln

Ep / V = 1.230 + 0.088 ln

r2 = 0.946

r2 = 0.993

υ

υ / Vs-1

υ / Vs -1

f methanol oxidation at Ni–P/C and Ni–Cu–P/C catalysts

The linear relationship between the anodic peak current

f the anodic peak potential with ln n in the scan rate range

Time / sec.0 5 10 15 20

I / m

A c

m-2

0

4

8

0.1 M KOH0.1 M KOH + 0.1 M MeOH + 0.2 M MeOH + 0.3 M MeOH + 0.4 M MeOH + 0.5 M MeOH

Time / sec.0 5 10 15 20

I / m

A c

m-2

0

4

8

12

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

Time / sec.0 5 10 15 20

I / m

A c

m-2

-2.4

-1.6

-0.8

0.0

Time / sec.0 5 10 15 20

I / m

A c

m-2

-2.4

-1.6

-0.8

0.0

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

0.4 0.8 1.2 1.6 2.0

I / m

A c

m-2

8.5

9.0

9.5

10.0

10.5

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

0.4 0.8 1.2 1.6 2.0

I / m

A c

m-2

6

7

8

Ni-Cu-P/CNi-P/C

t1/2 / sec.1/2

0.4 0.8 1.2 1.6

I ca

t / I

L

16

24

32

40

t1/2 / sec.1/2

0.4 0.8 1.2 1.6

I ca

t/

I L

4

6

8

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

a b

c d

fe

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.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 2 5 1 7 – 2 5 2 9 2527

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 coeffi-

cient 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 s�1] 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 chro-

noamperometric technique for the calculation of the catalytic

rate constant of the methanol oxidation reaction in accor-

dance 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 g¼ 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 meth-

anol oxidation. The linear dependence of I [obtained by sub-

tracting the background current using the point-by-point

subtracting method [16,48,56,57]] on t�1/2 in Fig. 10c and

d confirms the diffusion-controlled character of the oxidation

process. The diffusion coefficient was thus calculated as

7.75 � 10�4 and 9.75 � 10�4 cm2 s�1 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 � 10�4 cm2 s�1 at Ni–Cu alloy prepared by the casting

method [57] and 3 � 10�6 cm2 s�1 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 mol�1 s�1 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 mol�1 s�1].

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

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 oxy-

hydroxide 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�10�4 and 1.22� 10�3 s�1 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 solu-

tion as 0.1135�105 and 3.0889�105 cm3 mol�1 s�1 at Ni–P/C and

Ni–Cu–P/C catalysts, respectively.

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