Thin Solid Films 572 (2014) 260–265

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Facile synthesis of Pt–Pd bimetallic nanoparticles by plasma discharge inliquid and their electrocatalytic activity toward methanol oxidation inalkaline media

Sung-Min Kim a,b, Yu-Jin Lee a,b, Jung-Wan Kim a,c, Sang-Yul Lee a,b,⁎a Center for Surface Technology and Applications, Korea Aerospace University, Gyeonggi-do, 412-791, Republic of Koreab Department of Materials Engineering, Korea Aerospace University, Gyeonggi-do, 412-791, Republic of Koreac Division of Bioengineering, InCheon National University, Incheon, 406-772, Republic of Korea

⁎ Corresponding author at: Department of MaterialsUniversity, Gyeonggi-do, 412-791, Republic of Korea. Te31583770.

E-mail address: sylee@kau.ac.kr (S.-Y. Lee).

http://dx.doi.org/10.1016/j.tsf.2014.07.0670040-6090/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Available online 27 August 2014

Keywords:Direct methanol fuel cellsNanowirePt–Pd bimetallic nanoparticleCyclic voltammetryPlasma dischargeChemical composition

The Pt–Pd bimetallic nanoparticles for direct methanol fuel cell applications were successfully prepared by plas-ma discharge in aqueous solution. The obtained nanoparticles were characterized by energy dispersive X-rayspectroscopy, X-ray diffraction spectroscopy, and transmission electron microscopy. During plasma discharge,the nanoparticles were produced from the erosion of electrodes. It was noted that the erosion amount ofanode electrodes was much greater than that of cathode electrodes so that the composition of Pt–Pd bimetallicnanoparticles could be changed with different power types and electrode configurations. Diffraction patternsfitted from Gaussian devolution indicated that the crystalline phase of Pt40Pd60 products was composed ofpure Pt, pure Pd and Pt–Pd alloy phases. The morphology of synthesized nanoparticles showed that nanowiresconnected with quasi-spherical nanoparticles with 2–3 nm in diameter were observed and large spherical parti-cles with N50 nm in diameter were also detected intermittently. The cyclic voltammetricmeasurement and con-tinuous potential scan demonstrated that Pt40Pd60 hadmuch higher catalytic activity and better resistance to COpoisoning than Pt94Pd6 and Pt1Pd99 for methanol oxidation. These results indicate that the Pt40Pd60 could be anexcellent candidate for the direct methanol fuel cell applications.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Currently, direct methanol fuel cells (DMFCs) are drawing much at-tention as a highly promising power source due to easy control of liquidfuel, environmental friendliness, low-operating temperature (b100 °C)and high energy density of methanol (3800 kcal l−1) in comparison tohydrogen at 360 atm (658 kcal l−1) [1–3]. Pt catalysts have been consid-ered as anodes of DMFCs with their high electrocatalytic activity [4].However, the catalytic performance of Pt catalysts could be reduced be-cause the carbonaceous species are accumulated to the surface of thecatalysts by partially oxidized methanol at low temperature. Therefore,highly active electrocatalysts have to be employed in DMFCs in order tooxidize methanol completely [5]. When Pt is alloyed with other metals,the catalytic performance could be improved bymeans of modificationsof electronic and geometric structure [6]. Pt-based alloy nanocrystalssuch as Pt–Ru [7], Pt–Pd [8], Pt–Co [9], and Pt–Ni [9,10] have beeninvestigated for the enhanced catalytic performance with improved

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efficiency and reduced CO poisoning. Among the above electrocatalysts,Pt–Pd bimetallic catalysts are considered as one of the most attractiveanode catalysts of DMFCs because they show much more enhancedCO tolerance than pure Pt and Pt–Ru bimetallic catalyst [11–13].

In addition, the synthesis method of Pt–Pd bimetallic nanoparticle isstrongly dependent on electrocatalytic performance [14,15]. Typically,co-reduction and template method are the most common routes andthey are performed in the presence of chemical agents such as surfac-tant and capping agents [8,11–15]. However, these methods result incritical problems including a complicated, time-consuming process,and limited commercial applications since a template synthesis or atemplate etching step is required, and several or tens of hours are need-ed for the complete reduction of Pt–Pd bimetallic nanoparticles. Anoth-er important factor is that surfactant and capping agent are very difficultto decompose completely, which results in poor catalytic activities [16].Herein, we propose the facile synthesis method of Pt–Pd bimetallicnanoparticles for DMFC application. This process is an efficient methodto synthesize metallic nanoparticles because it is not necessary to useany template or surfactant agent. In addition, this process providesrapid reaction rate due to the fact that highly energetic species (elec-trons, ions, neutrals, and radiation) are generated during discharge.Previous studies showed that Au, Ag and Cu nanoparticles were

261S.-M. Kim et al. / Thin Solid Films 572 (2014) 260–265

successfully synthesized [17–20]. However, the synthesis of Pt–Pd bi-metallic nanoparticles for DMFC application has never been reportedby plasma discharge in water.

In thiswork, the Pt–Pd bimetallic nanoparticleswere synthesized viaplasma discharge in aqueous solution and the chemical compositionwas controlled by using different power types and electrode configura-tions. The catalytic performance as well as the characteristics of Pt–Pdbimetallic nanoparticles for methanol oxidation were investigated.

2. Experimental details

Platinum and palladiumwires (99.99%) of 2mmdiameterwere usedas electrodes to synthesize Pt–Pd bimetallic nanoparticles. Potassiumhydroxide (KOH, 99.99%), methanol (CH3OH, 99.9%), ethanol (C2H5OH,99.5%) and Nafion (5 wt.%) solution from Sigma-Aldrich were used as-received without further purification. Deionized water (DI water, aMillipore, Ltd., resistivity N18 MΩ cm) was used in preparing aqueoussolution.

Two electrodes of Pt and Pdwere assembled horizontally on a Teflonvessel with gap distance of 0.5 mm, as shown in Fig. 1(a). 0.01 M KOHmixed with 200 ml of DI water was poured into the vessel and thenthe high voltagewas applied to the electrodes to generate stable plasmadischarge in aqueous solution. Two types of power such as a direct

Fig. 1. Schematic diagram of plasma discharge process (a) and three electrode system (b).

current (DC) power (IAP-1010, EN technology Co. Ltd.) and bipolarpulsed (BP) power (MPS-06K06C, Kurita Co. Ltd.) were used to controlthe atomic ratio of Pt to Pd. In a DC mode, 0.7 kV was applied to twoelectrodes whichwere arrangedwith Pt cathode (Pd anode) or Pd cath-ode (Pt anode). In a BP power, 0.7 kV was applied and its pulse width,frequency and pulse duty cycle were 2 μs, 20 kHz, and 10–85%, respec-tively. After discharge, themass loss of electrodeswasmeasured by bal-ance (AR2140, Ohaus Co.) with 0.1 mg readability. The nanoparticles inthe solution were collected by washing process involving multiple cen-trifugation steps of 8000 rpm for 30minwithDIwater. The compositionof the obtained Pt–Pd bimetallic nanoparticles wasmeasured by energydispersive X-ray spectroscopy (EDS) equipped on scanning electronmi-croscopy (SEM, JSM-7001F, JEOL). The crystalline structure of Pt–Pd bi-metallic nanoparticles was analyzed by HR-XRD (SmartLAB, RigakuCo.). The patterns were collected from 30 to 90° with 0.01° steps persecond using Cu Kα radiation (λ = 0.154 nm) operating at 40 kV and30 mA. The TEM images of Pt–Pd bimetallic nanoparticles were record-ed on FE-TEM (Tecnai-F20, Philips) with accelerating voltage of 200 kVto get information of structure and morphology.

In order to examine the electrochemical activities, cyclic voltamme-try was conducted by a potentiostat (EG&G, 263A) with three electrodesystem, as shown in Fig. 1(b). A glassy carbon electrode (GCE, diameter5 mm), a Pt mesh electrode and a saturated calomel electrode (SCE)were employed as working, counter and reference electrode, respec-tively. In addition, the 20 wt.% metal loading Pt–Pd catalysts ink wasprepared by dispersing both 1 mg of nanoparticles and 4 mg of carbonblack into 2 ml of ethanol. It was followed by sonication for 10 min.The 15.7 μl aliquot of catalyst ink was deposited on glassy carbon elec-trode (0.2 mg cm−2) with 10 μl of the Nafion solution, and then driedat 50 °C in the dry oven. To investigate methanol oxidation reaction(MOR) of the catalysts, the cyclic voltammograms were recorded at ascan rate of 50 mV s−1 in N2-saturated solution containing 0.5 MNaOH + 0.1 CH3OH. Durability of each catalyst was tested bychronoamperometry with −0.2 V vs. SCE in the electrolyte containing0.5 M NaOH + 0.1 CH3OH.

3. Results and discussion

3.1. Characterization of the Pt–Pd bimetallic nanoparticles

The composition of the as-synthesized Pt–Pd bimetallic nanoparti-cleswas investigated by an analytic EDS equipped on FE-SEMas indicat-ed in Table 1. From the EDS results, the averagemolar ratio of Pt to Pd inthe BPmodewas evaluated to be 1:1.5 (determined to be Pt40Pd60) andthose in the DC mode were measured to be 1:0.06 (Pt94Pd6) and 1:99(Pt1Pd99), respectively. This result was consistent with the measuredloss of electrode masses. It was found that erosion amount of anodeelectrodes was much more than that of cathode electrodes. A possiblemechanism responsible for the erosion of anode electrode could be ex-plained by the removal of oxide layer on electrode surface. During plas-ma discharge, oxygen anionswere transported to the anode surface andsubsequently formedmetallic oxides such as volatile PtO2 and PdO2. Thedecomposition temperatures of PtO2 and PdO2 were reported to be inthe range of 470–920 K, which is much lower than those of Pt and Pdat 2041 and 1828 K, respectively [21,22]. Consequently, anode electrodecovered by oxide layer could bemuchmore easily evaporated comparedto the cathode electrode.

Fig. 2(a) shows the typical XRD patterns of Pt–Pd bimetallic nano-particles. In the results of Pt94Pd6 and Pt1Pd99, the diffraction peakswere consistent with pure Pt and Pd with fcc structure, correspondingto crystalline plane of (111), (200), (220), (311) and (222) (JCPDS,87-0646 and 46-1043). In addition, it was noted in Fig. 2(b) and(c) that sharp diffraction peaks were overlapped with broad peaks atthe same diffraction angle, which implied that the nanoparticles existedwith different crystalline sizes. In the case of Pt40Pd60, two separatedsharp peaks were observed. For more detailed identification of peaks,

Table 1Atomic molar ratios of Pt to Pd in the Pt–Pd bimetallic nanoparticles with different power types and electrode configurations.

Sample PtxPdy Power type Configuration of electrodes(anode:cathode)

Mass reduction measuredby balance(mg)

Molar ratio from EDS value(Pt:Pd)

Pt Pd

Pt94Pd6 DC mode Pt:Pd 4.85 0.15 1:0.06Pt1Pd99 DC mode Pd:Pt 0.06 3.3 1:99Pt40Pd60 BP mode – 3.5 2.9 1:1.5

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the Gaussian devolutionwas conducted. The two separated sharp peakswere verified as Pt (111) and Pd (111), as shown in Fig. 2(d). A broadpeak between the Pt and Pd peaks was also observed in 2θ value of38.92° between Pt and Pd peaks (39.79° and 40.11°), which suggestedthat Pt–Pd nanoparticles became complete solid solution. This result isconfirmed by equilibrium phase diagram of Pt–Pd indicating Pt formsa solid solution with Pd [23].

Microscopy analysis was carried out to investigate the detailed mi-crostructures and morphology of the synthesized Pt–Pd bimetallicnanoparticles. TEM images of Pt94Pd6 show not only relatively largenanoparticles above 50 nm in diameter, but also small nanoparticleswith 5 nm in diameter, as shown in Fig. 3(a). The Pt94Pd6 nanoparticlesinHR-TEMof Fig. 3(b) are nanowire connectedwith quasi-spherical pri-mary nanoparticles and are uniformmorphologically. This morphologi-cal result is in good agreementwith the XRD result, in that the presenceof large nanoparticles (N50 nm in diameter) and fine nanoparticle(b4 nm) resulted in the narrow and broad diffraction peaks, respective-ly. The formation of large particle could be explained by particle ejectionmodel [24]. Typically, the ionic current leads to local heating of the elec-trode surface and instantly forms a pool of molten metals. Therefore,droplets or large particle could be ejected from the molten metal as

Fig. 2. Typical XRD patterns of Pt–Pd bimetallic nanoparticles (a) and Gaussian

the surface tension of metal is sufficiently low. On the other hand, thefine nanoparticles were expected to form by the condensation of super-saturated Pt or Pd vapors in plasma. This result is not verified experi-mentally but detailed investigations are in progress. In the case ofPt40Pd60 nanoparticles, fine nanoparticles with approximately 3.8 nmin size were observed as shown in Fig. 3(c) and large nanoparticle ordroplets are rarely observed as shown in Fig. 4. It is expected that thelocal melting at the anode electrode did not occur severely in the BPmode as the cooling duration of anode electrode could be provided suf-ficiently due to the characteristics of pulsed power as shown in Fig. 1.

Fig. 3(d) shows that the morphology of Pt40Pd60 nanoparticle is al-most identical to that of Pt94Pd6 nanoparticles. HR-TEM image of nano-wire structures in Fig. 3(e) confirms that it consists of many smallprimary nanoparticle with 2–3 nm in diameter. Each quasi-sphericalnanoparticle is composed of many single crystalline grains with theircrystallographic orientations in randomdirections. The inter-planar dis-tance in the lattice fringes of one domain is measured to be approxi-mately 0.225 and 0.195 nm corresponding to the (111) and (200)planes. The selective area electron diffraction (SAED) pattern recordedfrom Pt40Pd60 nanoparticles shows diffraction rings confirming to thefcc structure. Both the XRD and SAED results demonstrate that the

fitted Pt94Pd6 (b), Pt1Pd99 (c) and Pt40Pd60 (d) in (111) crystalline plane.

Fig. 3. TEM and HR-TEM images of Pt94Pd6 (a, b) and Pt40Pd60 (c, d), high magnification image (e) and SAED (f).

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Pt40Pd60 nanoparticles are well crystallized and polycrystalline fccstructure.

3.2. Electrochemical activities of Pt–Pd catalysts

All the catalysts were electrochemically characterized by the cyclicvoltammetricmethod. The curves of cyclic voltammetric test conductedin N2-saturated 0.5MNaOH solution and calculated electrochemical ac-tive area (ECSA) values are shown in Fig. 5. In order to estimate ECSAvalues, the reduction charge of the Pd oxides was employed for the Pdcatalysts because Pd has a strong adsorption toward hydrogen, leadingto a difficult measurement of monolayer of adsorbed hydrogen. Basedon the previously published work [25], the ECSA of the electrodes

could be calculated via the following equation:

ECSA ¼ Q=Sl ð1Þ

where Q is the coulombic charge for the reduction of palladium oxide. Sis the proportionality constant used to relate the charge with area and lis the catalyst loading. The charge required for the reduction of PtO andPdOmonolayer is assumed as 420 and 405mC cm−2 [26,27]. The ECSAsof Pt94Pd6, Pt1Pd99 and Pt40Pd60 catalysts were measured to be 7.91,59.16 and 32.48 m2 g−1, respectively. It was noted that the hydrogenevolution peak potential on the catalyst of Pt1Pd99 shifted negativelyfor approximately 30 mV compared with that of Pt94Pd6. On the con-trary, the hydrogen desorption peak potential on catalyst of Pt1Pd99 is

Fig. 4. FE-SEM image of Pt40Pd60 bimetallic nanoparticles.

Fig. 6. Cyclic voltammograms of the Pt94Pd6, Pt1Pd99 and Pt40Pd60 catalyst in 0.5 M NaOHat a scan rate of 50 mV s−1.

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shifted positively for about 150mV compared to that of Pt94Pd6. Conse-quently, the adsorption/desorption of hydrogen on the catalyst ofPt1Pd99 becomes difficult, which generatesmore ions of OH in the vicin-ity of the catalyst of Pt1Pd99 [25]. It is thought that the reaction of PdOformation is directly correlated with the OH ions and can be expressedas follows:

Pdþ OH‐→Pd–OHads þ e‐ ð2Þ

Fig. 5. Cyclic voltammograms (a) and ECSAs (b) of the Pt94Pd6, Pt1Pd99 and Pt40Pd60 cata-lyst in 0.5 M NaOH + 0.1 M CH3OH at a scan rate of 50 mV s−1.

Pd–OHads þ OH‐→Pd–OþH2Oþ e‐ ð3Þ

As a result, the larger value of ECSAwas observed in Pt1Pd99 throughthe reaction of PdO formation with OH ions.

Fig. 6 shows the cyclic voltammograms of Pt94Pd6, Pt1Pd99 andPt40Pd60 catalysts in 0.5 M NaOH solution containing 0.1 M CH3OH.The potential was swept between −0.9 and 0.3 V at a scan rate of50 mV s−1. The Pt40Pd60 exhibits high current density peak at around−0.15 V in the anodic sweep (If), which is caused by methanol oxida-tion. Also, Pt40Pd60 shows another peak around−0.3 V in the cathodicsweep (Ib), associated with accumulation of the residual carbonaceousspecies formed in the anodic sweep [28]. Similarly, Pt1Pd99 also showstwo significant peaks, while no evident peak of Pt94Pd6 is observed.This phenomenon indicates that Pt is easily poisoned by adsorbed spe-cies coming from dissociative adsorption of CH3OH and then to deacti-vate rapidly [29]. In addition, If/Ib ratio, the ratio of the forward peakcurrent (If) to the reverse peak current (Ib) could be used to evaluatethe catalyst tolerance to the accumulation of intermediate carbonaceousspecies such as CHO or CO [30]. As shown in Table 2, it is noteworthythat the If/Ib ratio for Pt40Pd60 is 1.6 times higher than that of Pt1Pd99,implying more effective removal of the poisoning species from the cat-alyst surface. Therefore, Pt40Pd60 catalyst has higher stability for MORthan Pt1Pd99 and Pt94Pd6. It was reported that Pt acts in Pt–Ru bimetalliccatalysts as the active site for catalyzing the oxidation of methanol dur-ing the oxidation reaction and Ru–OHad could be formed at lower po-tentials [31]. These Ru–OHad react with CO on the Pt surface toproduce CO2. PdO observed in this work is considered to play an identi-cal role as Ru does in Pt–Ru bimetallic catalysts. High oxidation currentfor the reaction was obtained by increasing the Pd sites for the forma-tion of OHad. However, although Pt1Pd99 has a high ECSA, the electrocat-alytic activity in methanol is poor due to the fact that active site is notenough for MOR.

To evaluate the electrocatalytic stability and durability of the cata-lysts, long-term chronoamperometric experiments were conducted in

Table 2Electrocatalytic properties of Pt–Pd bimetallic nanoparticles towards CH3OH in NaOHenvironment.

Sample PtxPdy Current density of If Current density of Ib If/Ib

(mA cm−2) (mA cm−2)

Pt94Pd6 1.43 – –

Pt1Pd99 1.88 1.30 1.45Pt40Pd60 4.84 2.05 2.36

Fig. 7. Chronoamperometry curves at −0.2 V of each catalyst in 0.5 M NaOH + 0.1 MCH3OH solution.

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0.5MNaOH solution containing 0.1MCH3OH. Fig. 7 shows the curves ofcurrent densities recorded at−0.2 V for 2000 s. The order of initial ac-tivity in the chronoamperometric tests was in good agreement withthe order of activity in the cyclic voltammetric measurements, andPt40Pd60 catalyst showed the highest activity towardMOR. These resultsdemonstrate that Pt40Pd60 has better steady-state catalysts than Pt94Pd6and Pt1Pd99 catalysts for methanol oxidation in alkaline media.

4. Conclusions

For DMFC application, Pt–Pd bimetallic nanoparticles were success-fully synthesized by plasma discharge in aqueous solution. The erosionamount of anode electrodeswasmuchgreater than that of cathode elec-trodes so that the chemical composition of Pt and Pd could be changedwith different power types and electrode configurations. Diffractionpatterns fitted from Gaussian devolution indicated that Pt40Pd60 prod-uctswere indeed alloys. Themicroscopic results showed that nanowiresconnected with quasi-spherical nanoparticles with 2–3 nm in diameterwere observed and spherical large particles with N50 nm in diameterwere also detected intermittently. The electrochemical performance ofthe bimetallic Pt–Pd catalysts could be correlatedwith the observed var-iations in chemical composition of Pt–Pd. Among bimetallic Pt–Pd cata-lysts, the Pt40Pd60 exhibited the highest catalytic activity toward

methanol oxidation and could be an excellent candidate for directmethanol fuel cell applications.

Acknowledgments

This study was supported by the National Research Foundation ofKorea (NRF) funded by the Korean government (MSIP, Grant No.2013M2A8A1042684) and was partly supported by the grant fromIncheon National University.

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