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Electrochimica Acta 59 (2012) 499– 508

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

jou rn al hom epa ge: www.elsev ier .com/ locate /e lec tac ta

t–NiO/C anode electrocatalysts for direct methanol fuel cells

.S. Amina, R.M. Abdel Hameedb, K.M. El-Khatiba,∗, M. Elsayed Youssefc, A.A. Elzatahryd

Chem. Eng. & Pilot Plant Dept., Engineering Division, National Research Center, Dokki, Giza, EgyptChemistry Department, Faculty of Science, Cairo University, Giza, EgyptComputer Based Engineering Applications Dept., Informatics Research Institute, City of Scientific Research and Technology Applications, New Borg El-Arab City 21934, Alexandria,gyptPolymer Materials Research Dept., Advanced Technology and New Materials Research Institute, City of Scientific Research and Technology Applications, New Borg El-Arab City1934, Alexandria, Egypt

r t i c l e i n f o

rticle history:eceived 3 July 2011eceived in revised form 25 October 2011ccepted 2 November 2011vailable online 10 November 2011

a b s t r a c t

Pt catalyst was supported on Vulcan XC-72R containing 5 wt.% NiO using NaBH4 as a reducing agent. Theprepared catalyst was heat-treated at 400 ◦C. XRD, TEM and EDX analyses were applied to characterizePt–NiO/C electrocatalyst. The introduction of NiO reduces the particle size of Pt crystallites. The elec-trocatalytic activity of Pt–NiO/C electrocatalysts was examined towards methanol oxidation reaction in0.5 M H2SO4 solution using cyclic voltammetry and chronoamperometry techniques. A three fold incre-

eywords:latinum–nickel oxideethanol oxidation

uel cellsarbon black

ment in the oxidation current density was gained at Pt–NiO/C electrocatalyst compared to Pt/C one. Thecorresponding chronoamperograms showed high steady state current density values suggesting betterstability of Pt–NiO/C electrocatalyst towards the carbonaceous poisoning species. The enhanced electro-catalytic performance and the long-term cycle durability of Pt–NiO/C electrocatalyst are attributed to thestrong interaction between Pt and NiO and the formation of small Pt crystals.

lectrocatalyst

. Introduction

In the last 40 years, a lot of interest has been focused on the via-ility of direct methanol fuel cells (DMFCs) as an effective powerource in transportation and portable electronic devices [1–6]. Reli-ble energy conversion, high power density, ease of handling andtorage as a liquid fuel, low operating temperatures and quick startp are the main advantages of DMFCs, in relation to the otherevices fed by hydrogen [7–9]. Many electrocatalysts have beenmployed to oxidize methanol [10–12], however, the highest elec-rocatalytic activity is still gained at Pt and Pt-based alloys. Thetrong adsorption of carbon monoxide on Pt tends to slow downhe kinetics of methanol oxidation reaction [13], in addition to theigh cost of precious Pt metal as a catalyst. Two ways have beendopted to solve these problems. The first one depends on the dis-ersion of Pt nanoparticles on a suitable support. A wide variety ofubstrates have been examined such as polymer matrices [14,15],arbon nanotubes [16–18] and carbon black [19–21]. The additionf a co-catalyst to platinum such as Mo [22], Sn [23], W [24] andb [25] secondly mitigates CO poisoning. A Pt–Ru binary alloy is

he most widely used catalyst. Ru provides the hydroxyl specieso the adjacent active Pt atoms to facilitate Pt–COads oxidation atow potential values [26,27]. This bi-functional mechanism of the

∗ Corresponding author.E-mail address: kamelced@hotmail.com (K.M. El-Khatib).

013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2011.11.013

© 2011 Elsevier Ltd. All rights reserved.

methanol oxidation reaction is still operated when metal oxidesare added to platinum [28,29]. The Pt/RuO2·xH2O electrode showedhigher reactivity towards methanol oxidation compared to that ofE-TEK Pt–Ru black electrode. This is attributed to the remarkableboundaries of Pt nanoparticles to RuO2·xH2O support that provideactive sites for complete methanol oxidation [30]. Justin and RangaRao [31] indicated that Pt–V2O5/C electrocatalyst performed betterthan Pt/C one for methanol and CO oxidation. The synergistic inter-action between Pt and TiO2 improves the electrocatalytic activityof Pt [32] and PtRu [33] catalysts supported on TiO2 for methanolelectro-oxidation. Song et al. [17] demonstrated that the promotiondegree of Pt–ZrO2/CNTs catalyst for methanol oxidation varied withthe increase of ZrO2 content. The maximum catalytic activity wasachieved at Pt–ZrO2/CNTs (1:1) electrocatalyst.

NiO is a transition metal oxide that has various potential applica-tions in many fields. It is used as an active layer for gas sensors [34]and as a catalyst material for waste water depollution [35–37]. NiOis a p-type antiferromagnetic semiconductor. It is characterized bya wide band gap [3.5 eV] [38], a valence band [3.1 eV] and a con-duction band [−0.5 V] that enables it to be used for dye-sensitizedsolar cells, water splitting and photocatalytical processes [39–41].NiO also showed an excellent electrochemical performance as anelectrode material for lithium ion batteries [42–45]. Kim et al. [46]

have prepared Pt catalyst supported on Vulcan XC-72R containing5 wt.% NiO. Heat-treated Pt–NiO/C electrocatalyst at 400 ◦C exhib-ited the best activity in methanol oxidation due to the change inPt electronic structure by Ni and the minimal aggregation of Pt

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00 R.S. Amin et al. / Electroch

anoparticles. Min et al. [47] reported that Pt deposited on NiOhowed an increased catalytic activity for methanol oxidation incidic solution, compared to that of Pt alone. Pt–NiO/C also behavesell as an anode catalyst for PEM fuel cells [48]. On the other hand,ntolini et al. [49] have examined the electrochemical responsef carbon-supported Pt–Ni as cathode materials for DMFCs. Differ-nt samples were prepared using variable Pt:Ni precursor ratiosy NaBH4 as a reducing agent. Their performance was found toepend on the amount of actually alloyed Ni not on the total amountf Ni in the material. Wang et al. [50] have observed that addingiO enhances the anti-poison ability of Pd–NiO/C electrocatalyst

owards the alcohol oxidation. Hu et al. [51] concluded that theower CO coverage on Pd–NiO/C electrocatalyst leads to the pre-erred adsorption of alcohol instead of CO and consequently theess poisoning effect.

In the present work, 5 wt.% NiO was loaded on Vulcan XC-72Rarbon black. Pt was further chemically reduced by sodium boro-ydride on that NiO/C using the impregnation and the microwaveethods. One Pt–NiO/C sample was heat-treated in air at 400 ◦C.

he different prepared catalysts were characterized with X-rayiffraction (XRD), transmission electron microscopy (TEM) andnergy dispersive X-ray analysis (EDX). Their electrocatalytic activ-ty towards methanol oxidation in H2SO4 solution was examinednd compared with that of Pt/C catalyst.

. Experimental

.1. Reagents

Vulcan XC-72R carbon black was purchased from Cabot Corp.,SA with a specific surface area (BET) of 240 m2 g−1 and an aver-ge particle size of 40 nm. Nafion (perfluorosulphonic acid–PTFEopolymer, 5 wt.% solution), H2PtCl6·6H2O, Ni(NO3)2·6H2O, NaBH4,aOH (98.0%), H2SO4 (A.C.S. reagent grade) and CH3OH werebtained from Sigma–Aldrich, Germany. All chemicals were used aseceived without further purification. De-ionized water was usedo prepare the solutions.

.2. Preparation of different Pt–NiO/C electrocatalysts

Pt–NiO/C electrocatalysts were synthesized into two steps; sup-orting NiO on Vulcan XC-72R carbon black by the precipitationethod, followed by the chemical reduction of Pt precursor salt

sing NaBH4 as the reducing agent. The detailed steps of theatalyst manufacture were as follows: an appropriate amount ofi(NO3)2·6H2O was weighed and dissolved in a suspension of Vul-an XC-72R carbon black in double distilled water. The addedeight of Ni salt was calculated to prepare NiO/C powder contain-

ng 5 wt.% of metal oxide. Drops of 1 M NaOH solution were addedo the mixture to maintain its pH value at 10 during the constanttirring for 3 h. This induces the formation of small and uniformanoparticles [31]. The resulting powder was then filtered andashed with double distilled water for 6 times and dried in an air

ven at 80 ◦C for 6 h. Calcination step at 400 ◦C was performed in anir muffle for 3 h to form NiO/C catalyst. The loading of Pt was thenarried out on this NiO/C powder by fixing its amount at 25 wt.%.wo methods were applied during the reduction of Pt; namely: thempregnation and the microwave ones. The corresponding weightf H2PtCl6·6H2O was dissolved in a suspension of NiO/C powder inouble distilled water. Ultrasonic mixing last for 1 h at 70 ◦C, accom-anied by the addition of drops of 1 M NaOH solution to adjust

he pH value at 11. The reduction step was performed by addingaBH4 solution slowly to the above mixture with continuous stir-

ing for 3 h. The molar ratio of Pt salt to sodium borohydride wasept at 1:70. In another way, the reduction of Pt was done using the

Acta 59 (2012) 499– 508

household microwave oven (Caira CA-MW 1025, touch pad digitalcontrol, 50 MHz, 1400 W). It was heated 15 times, each time for 20 son followed by a 10 s pause [20 s on/10 s off]. The formed Pt–NiO/Ccatalyst was then filtered and washed with double distilled waterfor 6 times. The drying step was performed in an air oven at 80 ◦Cfor 6 h.

Different three Pt–NiO/C electrocatalysts were manufactured asfollows:

Pt–NiO/C-1 and Pt–NiO/C-2: the chemical reduction of Pt salt overNiO/C powder was operated in these samples using the impreg-nation and the microwave methods, respectively.Pt–NiO/C-3: the formed Pt–NiO/C-1 powder is further subjectedto the heat-treatment at 400 ◦C in air for 3 h.

For comparison, Pt loaded on Vulcan XC-72R carbon black (Pt/Celectrocatalyst) was prepared by the impregnation method keeping25 wt.% Pt.

2.3. Physical characterization of the electrocatalysts

XRD patterns were obtained using a Rigaku-D/MAX-PC 2500 X-ray diffractometer equipped with Ni filtered Cu K� as the radiationsource (� = 0.154056 nm) to characterize the structure of Pt–NiO/Celectrocatalysts. The tube current was 40 mA with a voltage of40 kV. XRD samples were prepared by fixing the electrocatalystpowder on a glass slide. They were then dried in vacuum overnight.The 2� angular regions were detected between 10◦ and 80◦ at a scanrate of 10◦ min−1. TEM images were recorded on JEOL-JEM 2010transmission electron microscope that operated at an acceleratingvoltage of 160 kV. The microstructure and the particle size of theprepared electrocatalysts could be determined. To prepare the elec-trocatalyst sample for TEM measurement, 2 mg of Pt–NiO/C powderwas ultrasonically dispersed in 2 ml H2O in ultra 8050-H Clifton for20 min to obtain a uniform catalyst ink. A drop of this suspensionwas deposited onto 1 mm copper grid coated with continuous car-bon film and left to dry in air. Gatan program was used for dataprocessing and particle size measurement. Energy dispersive X-ray(EDX) spectra were mapped using a scanning electron microscope(JEOL JAX-840) and a POEMS ICP-OES instrument (Thermo Jarrell-Ash Corporation, Franklin, MA, USA). The atomic and the weightpercentages of different elements were estimated.

2.4. Electrochemical measurements of the electrocatalysts

Voltamaster 6 potentiostat was employed for the electrochem-ical measurements. It is connected to a personal computer as datainterface. Cyclic voltammetry and chronoamperometry were con-ducted to measure the electrocatalytic activity of the preparedPt–NiO/C electrocatalysts towards methanol oxidation reaction.The three electrode cell consists of Pt wire and Hg/HgSO4/1.0 MH2SO4 (MMS) as the counter and the reference electrodes, respec-tively. All the potential values in this work are referred to mercurysulphate electrode [its voltage is +680 mV vs NHE]. The workingelectrode was a thin film of electrocatalyst supported on commer-cial carbon rod held in a Teflon tube. This carbon electrode (withsurface area of 0.5 cm2) was first polished with soft emery papersand rinsed with double distilled water and acetone. Its surface isfurther electrochemically activated by continuous cyclization in0.5 M H2SO4 solution for 50 cycles in the potential range from −800to +1600 mV at 50 mV s−1. The working electrode was prepared asfollows: a mixture of (1.1 mg Pt–NiO/C powder + 0.5 ml isopropyl

alcohol + 0.2 ml Nafion solution (5 wt.%)) was sonicated for 10 min.This catalyst ink was then transferred using a micropipette tospread uniformly over the surface of the commercial carbon elec-trode. It was left to dry in air and stored overnight in a desiccator

R.S. Amin et al. / Electrochimica

bm−tmp

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Fig. 1. XRD patterns of Pt/C and different Pt–NiO/C electrocatalysts.

efore examining its electrocatalytic activity. The cyclic voltam-etric measurements were performed in the potential range from700 to +1300 mV at a scan rate of 10 mV s−1 in 0.5 M H2SO4 solu-

ion in absence and in presence of methanol. All electrochemicaleasurements were operated in aerated electrolytes at room tem-

erature of 30 ◦C ± 0.2.

. Results and discussion

The Pt/C and different Pt–NiO/C electrocatalysts were charac-erized by XRD analysis. Their corresponding XRD patterns areisplayed in Fig. 1. The broad peak with low intensity observedt 2� value of 25◦ is attributed to the hexagonal graphite struc-ure (0 0 2) of Vulcan carbon. It clearly appears in XRD patterns oft–NiO/C-1 and Pt–NiO/C-2 electrocatalysts. However, it is nearlybsent in those of Pt/C and Pt–NiO/C-3 catalysts as an indicationf the complete coverage of the carbon surface by deposited Ptnd Pt–NiO particles, respectively. Moreover, Pt/C and all Pt–NiO/Camples showed three diffraction peaks representing a typical face-entered-cubic (f.c.c.) crystal structure of platinum. They locatet about 39◦, 46◦ and 67◦ corresponding to Pt(1 1 1), Pt(2 0 0) andt(2 2 0) planes, respectively to demonstrate the presence of Pt inhe metallic form [52]. The diffraction peak at 39.8◦ for Pt(1 1 1)s indexed well to the inter-planar spacing of d1 1 1 = 0.226 nm andhe lattice constant of 0.3924 nm. They agree well with the standardowder diffraction pattern of Pt (JCPDS number 1-1311). From thexact 2� values of the different diffraction peaks at Pt/C and dif-erent Pt–NiO/C electrocatalysts, summarized in Table 1, we canotice their shift to higher values after the incorporation of NiO inhe prepared catalysts. This is a clue of the alloy formation betweent and NiO in the different Pt–NiO/C catalysts. The crystallite sizef Pt nanoparticles can be estimated using the Scherrer’s equationfter background subtraction as follows [53]:

= 0.9�

ˇ1/2 cos �(1)

here � (nm) is the wavelength of the X-rays used (0.154056 nmor Cu K�), (rad) is the width of the diffraction peak at half heightnd � (rad) is the angle at the position of the peak maximum.

The average size of platinum particles calculated from the iso-ated Pt(2 2 0) peak was estimated to be 9.8, 6.4, 6.3 and 4.9 nmor Pt/C, Pt–NiO/C-1, Pt–NiO/C-2 and Pt–NiO/C-3 electrocatalysts,

espectively. Therefore, we can conclude that the presence of NiOesults in the formation of small sized catalyst particles. Theseesults are supported by those obtained by Wang et al. [50]ho prepared carbon dispersed Pd–nickel oxide nanocatalysts and

Acta 59 (2012) 499– 508 501

found that the addition of NiO to Pd/C catalyst greatly decreasesPd particle size from 59.6 to 7.0 nm. Kim et al. [46] also observedthe reduction of Pt crystallite size from 2.0 to 1.6 nm when NiOis supported on Vulcan carbon. The decreased intensity and thebroadening nature of the diffraction peaks of Pt–NiO/C-3 catalystreflect its smallest Pt particle size (4.9 nm) among the preparedPt–NiO/C catalysts. The greatest positive shift in its 2� values alsosuggests the higher degree of alloying between Pt and NiO as aresult of the heat treatment at 400 ◦C. The diffraction peaks of NiOare absent in all XRD patterns of Pt–NiO/C catalysts. This may beattributed to its very small amount dispersed on the Vulcan carbonsupport [5 wt.%].

The TEM images of Pt/C and different Pt–NiO/C electrocatalystsare shown in Fig. 2. Black dots of Pt particles were distributed onthe spherical Vulcan carbon [Fig. 2a]. Some agglomerations wereobserved. In all Pt–NiO/C catalysts, a larger area of carbon sur-face is covered by the deposited particles. Although XRD resultsconfirmed that the reduction method of Pt precursor did not influ-ence the structure or the particle size of Pt–NiO/C catalyst, Ptand NiO particles were uniformly dispersed in Pt–NiO/C-2 cata-lyst [Fig. 2c] compared to Pt–NiO/C-1 one [Fig. 2b]. This is a resultof the rapid and the uniform heating provided by the microwaveirradiation. Consequently, the reduction of the metal ions is accel-erated to offer a suitable environment for the nucleation and thegrowth of small metal nanoparticles. Pt nanodeposits supported bymicrowave-assisted polyol process over V2O5/C [31] and WO3/C[54] powders showed homogeneous dispersion with particle sizeof ∼3 nm. Complete coverage of the Vulcan carbon surface bymetallic deposits [as also evidenced from XRD data] with severeagglomeration is the characteristic feature of the TEM image ofPt–NiO/C-3 catalyst [Fig. 2d]. This morphology was earlier obtainedfor Pt–NiO/C catalyst, prepared in a similar way and subjectedto heat treatment at 800 ◦C in N2 atmosphere [46]. The Vulcancarbon loss during the calcination step at 400 ◦C in air tends to con-centrate the deposited nanoparticles over a reduced surface area.The average Pt particle size was estimated in Table 1 to be 8.5,5.3, 4.3 and 3.7 nm at Pt/C, Pt–NiO/C-1, Pt–NiO/C-2 and Pt–NiO/C-3 catalysts, respectively. This is in a good agreement with theresults obtained by XRD analysis supporting the role played byNiO in reducing the particle size of the formed nanodeposits. Sim-ilar results were obtained by Song et al. [17] who dispersed Ptnanoparticles over ZrO2/CNTs catalyst. The increased molar ratioof ZrO2 to Pt in the prepared catalysts from 0.5:1 to 3:1 decreasedthe particle size of the deposited Pt nanoparticles from 4.1 to3.6 nm. Scibioh et al. [55] also estimated the Pt particle size in40 wt.% Pt–12 wt.% CeO2/C catalyst as 3.0 nm compared to 3.7 nm atPt/C one.

The chemical composition of Pt/C and different Pt–NiO/C cata-lysts was determined by EDX analysis. Their corresponding spectraare represented in Fig. 3. Table 2 listed the weight and the atomicpercentages of each element in these catalysts. Four main com-ponents are detected in all Pt–NiO/C catalysts, namely: carbon,oxygen, nickel and platinum. Their weight ratios in Pt–NiO/C-1 andPt–NiO/C-2 catalysts are comparable, confirming that the applica-tion of microwave irradiation during the reduction of Pt precursordoes not alter the deposited percentages of Ni and Pt. However,the situation may be different in Pt–NiO/C-3 catalyst. The loss oflarge amount of Vulcan carbon during the heat treatment increasesthe percentages of the deposited nickel [10.90%] and platinum[44.53%], compared to 4.12 and 12.12% for Ni and Pt, respectivelyat Pt–NiO/C-1 catalyst. The oxygen content also varies as 16.55 and34.49% at Pt–NiO/C-1 and Pt–NiO/C-3 catalysts, respectively. Kim

et al. [46] have characterized Pt–NiO/C catalyst by XPS analysis andfound that Ni is present as nickel oxide [NiO], nickel hydroxide[Ni(OH)2] and nickel oxyhydroxide [NiOOH]. This nickel hydrox-ide reacts with Pt at the surface of Pt–Ni alloy catalyst to catalyze

502 R.S. Amin et al. / Electrochimica Acta 59 (2012) 499– 508

Fig. 2. TEM images of Pt/C (a), Pt–NiO/C-1 (b), Pt–NiO/C-2 (c1 and c2) and Pt–NiO/C-3 (d1–d3) electrocatalysts. Images: c2, d2 and d3 are under higher magnification.

R.S. Amin et al. / Electrochimica Acta 59 (2012) 499– 508 503

Table 1Variation of 2� values of Pt(1 1 1), Pt(2 0 0) and Pt(2 2 0) diffraction peaks at Pt/C and different Pt–NiO/C electrocatalysts. Their crystallite sizes according to XRD and TEMresults are also listed.

Electrocatalyst 2� (rad) Crystallite size of Pt (nm)

Pt(1 1 1) Pt(2 0 0) Pt(2 2 0) From XRD results From TEM images

Pt/C 39.678 46.173 67.614 9.8 8.5Pt–NiO/C-1 39.819 46.216 67.659 6.4 5.3Pt–NiO/C-2 39.822 46.227 67.712 6.3 4.3Pt–NiO/C-3 39.877 46.250 67.920 4.9 3.7

Table 2Variation of the elemental composition of Pt/C and different Pt–NiO/C electrocatalysts according to EDX analysis.

Electrocatalyst C O Ni Pt

Weight % Atomic % Weight % Atomic % Weight % Atomic % Weight % Atomic %

Pt/C 81.93 98.66 – – – – 18.07 1.3415.3013.6463.24

ms

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Pt–NiO/C-1 67.21 82.75 16.55

Pt–NiO/C-2 71.47 84.93 15.29

Pt–NiO/C-3 10.08 24.62 34.49

ethanol oxidation in acidic medium [56], while NiOOH is the mainpecies in the oxidation reaction at NiTi alloy [57].

The cyclic voltammograms of Pt/C and different Pt–NiO/Clectrocatalysts in (0.6 M MeOH + 0.5 M H2SO4) solution are rep-esented in Fig. 4 at a scan rate of 10 mV s−1. Methanol oxidationeaction starts at Pt/C electrocatalyst at a potential value of26 mV and reaches its maximum as an oxidation peak at +324 mVwith a current density of 10.24 mA cm−2). The methanol oxida-ion intermediates were further oxidized at a potential value of886 mV (with a current density of 13.20 mA cm−2). When Pt iseduced over NiO/C powder at Pt–NiO/C electrocatalysts, the onsetotential value of methanol oxidation reaction is shifted in theegative direction in the order: Pt–NiO/C-1 (−9 mV) > Pt–NiO/C-

(−66 mV) > Pt–NiO/C-3 (−74 mV) electrocatalysts. Consequently,ethanol is easily adsorbed and oxidized at Pt–NiO/C-2 and

t–NiO/C-3 electrocatalysts. Page et al. [58] have observed a lowernset potential value for methanol oxidation reaction in acidic solu-ion at Pt–Ni nanomaterials. A three fold increment in the oxidationeak current density is achieved at Pt–NiO/C-2 electrocatalyst with

potential shift in the positive direction by 53 mV, relative tohat at Pt–NiO/C-1 one. Pt–NiO/C-3 electrocatalyst shows a furtherncrease in its oxidation peak current density, however, at a lowerotential value [+314 mV] when compared with that at Pt–NiO/C-1ne [+324 mV]. Park et al. [56] and Hsieh and Lin [59] gained a cur-ent density increment at Pt–Ni electrocatalysts during methanolxidation in H2SO4 solution compared to that at pure Pt. The poten-ial value of the second oxidation peak increases gradually in therder: Pt–NiO/C-1 (+879 mV) < Pt–NiO/C-2 (+921 mV) < Pt–NiO/C-

(+959 mV) electrocatalysts, with a corresponding increase ints current density value, relative to that at Pt/C one. A reverse

xidation peak in the backward direction is clearly observed at dif-erent Pt–NiO/C electrocatalysts. It represents the re-oxidation of

ethanol after the removal of the incompletely oxidized poison-ng COads species formed in the forward scan [57,60]. Its potential

able 3ariation of the electrocatalytic parameters of the cyclic voltammograms of methanol oxid2SO4) solution.

Electrocatalyst Onset potential(mV)

Anodic peak potential(mV)

1st peak 2nd peak

Pt/C +26 324 886

Pt–NiO/C-1 −9 324 879

Pt–NiO/C-2 −66 377 921

Pt–NiO/C-3 −74 314 959

4.12 1.04 12.12 0.92 2.70 0.66 10.54 0.77 10.90 5.45 44.53 6.70

value is arranged in the order: Pt–NiO/C-1 [+10 mV] < Pt–NiO/C-2[+31 mV] < Pt–NiO/C-3 [+40 mV] electrocatalysts. The more positivepotential values of the reverse oxidation peak at Pt–NiO/C-2 andPt–NiO/C-3 electrocatalysts are an indication of the early oxidationof the poisoning species at their surfaces. This is also confirmedby calculating the ratio of the current density values of the anodicpeaks in the forward [If] and the reverse [Ib] scans. It is used todescribe the catalyst tolerance to the accumulation of CO species[61–63]. A higher ratio infers more effective removal of the poison-ing species on the catalyst surface. It is estimated as 2.52 and 1.48at Pt–NiO/C-2 and Pt–NiO/C-3 electrocatalysts, respectively. Thesevalues are as high as those reported at commercial and home-madePtRu/C catalysts [2.16 and 2.17, respectively] [55], suggesting thatNiO in Pt–NiO/C catalysts behaves similar to Ru in PtRu system toassist in the removal of COads species. The electrocatalytic parame-ters of the cyclic voltammogram of methanol oxidation reaction atPt/C and different Pt–NiO/C electrocatalysts are listed in Table 3.

The enhancement of the electrocatalytic activity of methanoloxidation reaction at Pt–NiO/C-2 and Pt–NiO/C-3 electrocatalystscan be rationalized by two reasons: the first one concerns withthe decrease of the particle size of Pt crystallites as a result ofadding NiO in Pt–NiO/C electrocatalysts. Therefore, a larger sur-face area of a greater number of Pt active sites would be availablefor the methanol oxidation reaction, thus enhancing the result-ing oxidation current density. The high percentage of deposited Ptper surface area at Pt–NiO/C-3 electrocatalyst [as shown from EDXanalysis] also contributes to its improved performance towards theoxidation reaction. The second reason is based on the bi-functionalmechanism. In this respect, methanol is adsorbed on Pt surfaceatoms, followed by its dehydrogenation to form COads species. NiO

is known with its better ability to dehydrate methanol. The result-ing OHads species would oxidize the poisoning COads species atlower potential values to regenerate clean Pt sites for further oxi-dation reaction [27]. Moreover, the heat treatment of Pt–NiO/C-3

ation reaction at Pt/C and different Pt–NiO/C electrocatalysts in (0.6 M MeOH + 0.5 M

Reverse peakpotential (mV)

Anodic peak currentdensity (mA cm−2)

Reverse peak currentdensity (mA cm−2)

1st peak 2nd peak

– 10.24 13.20 –10 13.46 16.41 1.4931 41.10 45.75 16.2940 49.22 55.82 33.17

504 R.S. Amin et al. / Electrochimica Acta 59 (2012) 499– 508

F(

ePtid

fTa

ig. 3. EDX spectra of Pt/C (a), Pt–NiO/C-1 (b), Pt–NiO/C-2 (c) and Pt–NiO/C-3d) electrocatalysts.

lectrocatalyst at 400 ◦C would change the electronic structure oft as a result of the alloy formation [46]. Park et al. [56] observedhe reduction of the bond energy between Pt and adsorbed COn Pt–Ni/C electrocatalyst and suggested the shift of the electronensity from Ni to Pt.

According to the values of the electrocatalytic parameters of dif-erent previously prepared Pt-metallic oxide catalysts reported inable 4, our synthesized Pt–NiO/C samples showed the followingdvances:

Fig. 4. Cyclic voltammograms of Pt/C and different Pt–NiO/C electrocatalysts in(0.6 M MeOH + 0.5 M H2SO4) solution at a scan rate of 10 mV s−1.

1. Methanol oxidation starts early at Pt–NiO/C-2 [−66 mV] andPt–NiO/C-3 [−74 mV] electrocatalysts compared to that atPt–ZrO2/CNTs [−40 mV] [17] and Pt–WO3/C-HT [−55 mV] [54]electrodes.

2. The potential value of the first oxidation peak at our pre-pared Pt–NiO/C-3 electrocatalyst [+314 mV] precedes that atPt–V2O5/C electrode by 30 mV [64] and that at Pt–CeO2/C one

by 56 mV [55].

3. A synergistic increase in the methanol oxidation peak currentdensity is gained at Pt–NiO/C-2 and Pt–NiO/C-3 electrocatalysts,

R.S. Amin et al. / Electrochimica Acta 59 (2012) 499– 508 505

Table 4Variation of the onset potential, peak potential, peak current density and If/Ib values of methanol oxidation in acidic solution and particle size of Pt at different Pt–metallicoxide electrocatalysts.

Electrocatalyst Onset potential (mV) 1st oxidation peak If/Ib Particle size (nm) Reference

Peak Potential (mV) Peak current density

Pt–ZrO2/CNTs −40 +290 847 mA mg−1 0.71 3.9 [17]Pt–TiO2/CNTs −80 +260 565 mA mg−1 0.79 3–4 [18]Pt–WO3/C-HT −55 +255 2.84 mA cm−2 0.98 – [54]Pt–CeO2/C −268 +370 80 mA cm−2 1.45 3 [55]Pt–V2O5/C −101 +344 17.5 mA cm−2 1.05 3.4 [64]SnO2:Sb/Pt −105 +160 12 mA cm−2 0.92 8.5–12 [65]

Table 5aVariation of the values of If/Ib parameter at Pt–NiO/C-3 electrocatalyst in 0.5 M H2SO4

solution containing different concentrations of methanol.

Methanol concentration (mol l−1) If/Ib

0.1 1.87

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Table 5bVariation of the reaction order with respect to methanol concentration at Pt/C anddifferent Pt–NiO/C electrocatalysts.

Electrocatalyst Reaction order with respect tomethanol concentration

1st peak 2nd peak

Pt/C 0.80 0.72Pt–NiO/C-1 0.86 0.68

2400 s are recorded at Pt/C and Pt–NiO/C electrocatalysts. This is

0.3 1.550.6 1.48

in relation to that at Pt–WO3/C-HT [54], Pt–V2O5/C [64] andSnO2:Sb/Pt [65] electrodes.

. Less amount of poisoning COads species is formed at surfacesof Pt–NiO/C-2 and Pt–NiO/C-3 electrocatalysts as apparent fromtheir If/Ib values [2.52 and 1.48 at Pt–NiO/C-2 and Pt–NiO/C-3electrodes, respectively].

. The Pt particle size in Pt–NiO/C-3 electrocatalyst [3.7 nm] is closeto those formed in similar catalysts.

The effect of varying methanol concentration on the catalyticctivity of Pt–NiO/C-3 electrocatalyst was studied using the cyclicoltammetry technique. It was shown in Fig. 5a in [0.1–0.6 MeOH + 0.5 M H2SO4] solution at a scan rate of 10 mV s−1 in the

otential range from −700 to +1300 mV (MMS). As the methanoloncentration increases, the oxidation current density valuesncrease at the first and the second peaks with a gradual poten-ial shift in the positive direction. More methanol molecules woulde adsorbed on the catalyst active sites to increase the resultingxidation current density. This would increase the current densityf the corresponding reverse oxidation peak in the backward direc-ion. The variation of the current density values of the first and theecond methanol oxidation peaks at Pt/C and different Pt–NiO/Clectrocatalysts with methanol concentration in 0.5 M H2SO4 solu-ion is plotted in Fig. 5b. A linear relationship is found betweenhe methanol concentration and the resulting current density val-es of the first and the second methanol oxidation peaks at Pt/Cnd different Pt–NiO/C electrocatalysts. Pt–NiO/C-3 electrocatalystecords the highest electrocatalytic activity among the studied elec-rodes. The electrocatalyst tolerance against CO poisoning speciess found to be affected by methanol concentration. This is apparentrom Table 5a, in which the values of If/Ib at Pt–NiO/C-3 electro-atalyst are recorded as a function of methanol concentration. Asethanol concentration increases, lower If/Ib values are shown.

his is related to the accumulation of a great amount of the carbona-eous species on the catalyst surface as a result of the oxidation of

higher concentration of methanol.To estimate the dependence of the methanol oxidation reaction

n methanol concentration at Pt/C and different Pt–NiO/C electro-atalysts, a logarithmic plot between the current density values ofhe first and the second oxidation peaks and methanol concentra-ion is shown in Fig. 5c. This is in accordance with the relation:

ate = Ip = kCn (2a)

og Ip = log k + n log C (2b)

Pt–NiO/C-2 1.40 1.03Pt–NiO/C-3 0.67 0.78

where Ip is the peak current density, k is the reaction rate constant,C is the concentration and n is the reaction order. Straight lines wereobtained at all the studied catalysts. Slope values of 0.80, 0.86 and0.67 were calculated for the methanol oxidation reaction at the firstpeak at Pt/C, Pt–NiO/C-1 and Pt–NiO/C-3 electrocatalysts, respec-tively. The second oxidation peak shows slope values of 0.72, 0.68and 0.78 at the three above mentioned electrodes. Table 5b liststhese slope values. It signifies that the presence of NiO does notaffect the order of the oxidation reaction with respect to methanolconcentration at the electrocatalysts prepared by the impregna-tion method. However, at Pt–NiO/C-2 electrocatalyst, in which Ptis reduced by the microwave irradiation method, higher order val-ues of 1.40 and 1.03 at the first and the second oxidation peaks,respectively were found. Tripkovic et al. [66] deduced a value of0.5 for the order of methanol oxidation reaction at the surface ofPt(1 0 0) in NaOH solution.

The electrochemical stability of Pt/C and different Pt–NiO/C cat-alysts during long-time operation in methanol oxidation reactionwas studied using the chronoamperometry technique. Chronoam-perometry is an electrochemical technique in which the potentialof the working electrode is stepped. The resulting faradic currentis recorded as a function of time. It represents the main currentcomponent that occurs due to the electron transfer events andit decays according to Cottrell equation. This current decay takesplace so rapidly at first, followed by a steady state. Better signalto noise ratio is gained by applying the chronoamperometry tech-nique because the current is integrated over relatively longer timeintervals [67–69]. The current–time curves of these electrocatalystsin (0.6 M MeOH + 0.5 M H2SO4) solution at potential values of +300and +800 mV with respect to MMS reference electrode for 4800 swere represented in Fig. 6. A gradual decay of the oxidation cur-rent density during the chosen time was observed at all the studiedcatalysts. This can be attributed to the accumulation of the poison-ing CO species on the active surface sites. The presence of NiO inPt–NiO/C-1 and Pt–NiO/C-2 electrocatalysts slightly increases thesteady state oxidation current density at the two selected potentialvalues when compared with those at Pt/C one. This can be clarifiedin Table 6, where the values of the oxidation current density after

consistent with earlier work by Wang et al. [50] and Xu et al. [70,71]that the ability of the poison resistance of Pd–NiO/C electrocatalystis improved by adding NiO to Pd/C one. The best electrocatalytic

506 R.S. Amin et al. / Electrochimica Acta 59 (2012) 499– 508

Fig. 5. (a) Cyclic voltammograms of Pt–NiO/C-3 electrocatalyst in 0.5 M H2SO4 solution in the presence of different concentrations of methanol. The potential range is from−700 to +1300 mV at 10 mV s−1. (b) Variation of the current density values of the first (1) and the second (2) methanol oxidation peaks at Pt/C and different Pt–NiO/Celectrocatalysts in 0.5 M H2SO4 solution with methanol concentration. (c) Variation of the logarithm of the current density values of the first (1) and the second (2) methanoloxidation peaks at Pt/C and different Pt–NiO/C electrocatalysts in 0.5 M H2SO4 solution with the logarithm of methanol concentration.

R.S. Amin et al. / Electrochimica Acta 59 (2012) 499– 508 507

Fig. 6. Chronoamperograms of Pt/C and different Pt–NiO/C electrocatalysts atpotential values of +300 mV (a) and +800 mV (b) in (0.6 M MeOH + 0.5 M H2SO4)solution.

Table 6Variation of the steady state current density values of methanol oxidation reaction atPt/C and different Pt–NiO/C electrocatalysts in (0.6 M MeOH + 0.5 M H2SO4) solutiontaken after 2400 s at +300 and +800 mV (MMS).

Electrocatalyst Steady state current density of methanoloxidation (mA cm−2)

At +300 mV At +800 mV

Pt/C 4.76 2.25Pt–NiO/C-1 5.54 2.73

pjo[WTohdpdsPosos

som1fi

[4] L.H. Jiang, Zh.H. Zhou, W.Z. Li, W.J. Zhou, S.Q. Song, H.Q. Li, G.Q. Sun, Q. Xin,

Pt–NiO/C-2 6.59 3.06Pt–NiO/C-3 16.55 6.00

erformance was gained at Pt–NiO/C-3 electrocatalyst with a bigump in its oxidation current density value relative to that at thether studied electrodes. Similar results were obtained by Ye et al.54] who supported Pt by the microwave-pyrolysis method on

O3/C powder and subjected it to the heat treatment at 200 ◦C.his Pt–WO3/C electrocatalyst resulted in a lower potential valuef CO electro-oxidation in CO-stripping voltammetry. In general,igher steady state current density values were noticed at Pt/C andifferent Pt–NiO/C catalysts at a potential value of +300 mV com-ared to those at +800 mV. Li et al. [72] have studied the role of theifferent oxidation states of Pt in the electrocatalytic oxidation ofmall primary alcohols. It was concluded that the surface species oft can be classified as active and poison form. The Pt surface oxidesf lower oxidation states [such as PtO, Pt(OH)2 and PtOH] are con-idered as active sites to promote alcohol oxidation, while thosef higher oxidation states [such as Pt(OH)3 and PtO2] are poisonpecies that prevents the oxidation reaction.

The electrocatalyst stability over repeated cyclization repre-ents another factor to evaluate its performance after long-timeperation. Pt–NiO/C-3 electrocatalyst was scanned in (0.6 M

ethanol + 0.5 M H2SO4) solution for 10 cycles at a scan rate of

0 mV s−1 in Fig. 7. The methanol oxidation current density at therst peak slightly decreases to get 90.5% of its value in the first cycle

Fig. 7. Cyclic voltammograms of Pt–NiO/C-3 electrocatalyst in (0.6 M MeOH + 0.5 MH2SO4) solution at a scan rate of 10 mV s−1 with repeated cyclization upto 10 cycles.

after continuous cyclization for 10 cycles. This may be attributedto the accumulation of poisonous species (such as COads) on Pt sur-face as a result of methanol consumption during successive scans[73,74] in addition to the change of the surface structure of Ptnanoparticles [74–76]. The current density of the second oxidationpeak remains almost constant after 10 cycles. On the other hand,62.7 and 67.9% are the corresponding current density decay per-centages at Pt/C electrocatalyst after 10 cycles at the first and thesecond oxidation peaks, respectively. This long-term performanceresult of Pt–NiO/C-3 electrocatalyst suggests its better stability asa result of the strong interaction between Pt and NiO and the smallsize of Pt crystallites.

4. Conclusion

In this study, Pt was dispersed over NiO/C support usingNaBH4 as the reducing agent. Microwave irradiation techniquewas applied to accelerate the reduction step. XRD and TEM resultsshowed the formation of small sized Pt nanoparticles. Pt–NiO/C cat-alyst was subjected to heat treatment at 400 ◦C. As a result, Pt isstrongly interacted with NiO as an alloy. An enhancement in theoxidation current density was observed when NiO is incorporatedin the prepared catalyst. A good tolerance of Pt–NiO/C electrocata-lysts towards COads species is achieved when compared with Pt/Cone.

Acknowledgements

This work was supported by the Egyptian Science and Technol-ogy Development Fund (STDF). The project number was “1310” andentitled as “Modeling, Construction and Performance Analysis ofProton Exchange Membrane Fuel Cell (PEMFC) as Clean PortablePower Source”.

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