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Applied Surface Science 257 (2011) 6494–6497

Contents lists available at ScienceDirect

Applied Surface Science

journa l homepage: www.e lsev ier .com/ locate /apsusc

reparation of carbon supported Pt–P catalysts and its electrocatalyticerformance for oxygen reduction

uan Maa, Yawen Tangb, Gaixiu Yangb, Yu Chenb, Qun Zhoua, Tianhong Lub,c, Junwei Zhenga,∗

Institute of Chemical Power Sources, Soochow University, Suzhou 215006, PR ChinaCollege of Chemistry and Material Science, Nanjing Normal University, Nanjing 210097, PR ChinaChangchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China

r t i c l e i n f o

rticle history:eceived 14 November 2010eceived in revised form 11 February 2011

a b s t r a c t

The carbon supported PtP (PtP/C) catalysts were synthesized from Pt(NO3)2 and phosphorus yellow atthe room temperature. The content of P in the PtP/C catalysts prepared with this method is high andthe average size of the PtP particles is decreased with increasing the content of P. The electrocatalytic

ccepted 14 February 2011vailable online 18 February 2011

eywords:reparation method of catalystarbon supported PtP catalyst

performances of the PtP/C catalysts prepared with this method for the oxygen reduction reaction (ORR)are better than that of the commercial Pt/C catalyst. The promotion action of P for enhancing the elec-trocatalytic performance of the PtP/C catalyst for ORR is mainly due to that Pt and P form the alloy andthen the electron density of Pt is decreased.

© 2011 Elsevier B.V. All rights reserved.

hosphorus yellowxygen reduction reaction

. Introduction

The proton exchange membrane fuel cell (PEMFC) has the goodotential applications as the power sources for transportation andtationary applications. However, the commercialization of PEMFCs still hindered by several factors, such as the poor kinetics of theathodic reaction. The Pt/C catalyst is usually used as the cathodicatalyst in PEMFC. In order to improve the electrocatalytic per-ormance of the Pt/C catalyst for ORR, many Pt-based compositeatalysts have been widely investigated. The most investigatedt-based catalysts are composite catalysts of Pt and other metallements, such as PtV [1], PtCr [2], PtTi [3], PtFe [4,5], PtCo [4,6],tNi [4,7], PtCu [8], PtGa [9], PtRu [10,11], PtPd [12,13], PtCrCu [14],tCrCo [15], PTFeNi [16], Pt-metal oxide [17] and Pt-macrocycleompound catalysts [18–20]. Only few composite cathodic cata-ysts of Pt and non-metal catalyst, such as PtP catalyst have beeneported [21,22].

In the previous reports, the PtP/C catalysts were prepared withhe self-redox method of Na2HPO2 or H3PO2 and their electrocat-lytic performances for ORR are better than that of Pt/C catalyst.

owever, the content of P in the catalysts prepared with the self-

edox method of Na2HPO2 or H3PO2 is small. For example, theeight ratio of Pt and P in the PtP/C catalyst prepared with the self-

edox method of H3PO2 is only is only 1.0:0.1 [21]. In this study, the

∗ Corresponding author. Tel.: +86 13013887911; fax: +86 512 62523540.E-mail address: jwzheng@suda.edu.cn (J. Zheng).

169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.apsusc.2011.02.050

PtP/C catalyst was prepared from Pt(NO3)2 and phosphorus yellowat the room temperature. The content of P in the PtP/C catalystprepared with this method is high. Therefore, its electrocatalyticperformance for ORR is high.

2. Materials and methods

2.1. Preparation and characterization of PtP/C catalysts

The preparation procedure of the PtP/C catalyst is as follows.After 60 mg Vulcan XC-72 carbon, certain volume of the anhydrousethanol and phosphorus yellow were mixed with stirring for 1 h at20 ± 2 ◦C. Then, certain amount of Pt(NO3)2 in anhydrous ethanolwas added into the above suspension at stirring. One hour later,100 mg NaBH4 in 10 mL water was added into the suspension withstirring for 2 h. Finally, the suspension was filtered, washed anddried in a vacuum oven at 60 ◦C. The PtP/C catalyst with 20 wt.%Pt was obtained. According to the atomic ratios (8:1, 4:1, 2:1, 1:1,1:2) of Pt and P, the PtP/C catalysts obtained were noted as thePt8P1/C, Pt4P1/C, Pt2P1/C, Pt1P1/C and Pt1P2/C catalysts, respec-tively. For comparison, the Pt/C catalyst with 20 wt.% Pt from JMCo. was used.

The X-ray diffraction (XRD) measurements of the PtP/C cata-

lysts were carried out on Model D/max-r C diffractometer usingCu–Ka radiation (k = 0.15406 nm) operating at 45 kV and 100 mA.The average particle size and morphology of the PtP/C catalystswere evaluated using the transmission electron microscopy (TEM)on Tecnai G2205-TWTN (FET) instrument operating at 200 kV. The

J. Ma et al. / Applied Surface Science 257 (2011) 6494–6497 6495

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Fig. 1. EDX spectrum of the Pt1P1/C catalyst.

atalyst composition was determined with energy dispersive X-rayEDX) analysis system of (GENESIs2000 XMSX, USA, EDAX).

.2. Electrochemical measurements

The electrochemical measurements were performed using a CHIotentiostat/galvanostat and a conventional three-electrode elec-rochemical cell. The counter electrode was a Pt plate. The Ag/AgCllectrode was used as the reference electrode and all potentialsere quoted with respect to the Ag/AgCl electrode. The glass car-

on electrode with 3 mm diameter was used as the substrate of theorking electrode.

The working electrode was prepared as follows. 10 mg PtP/Catalyst, 0.33 mL Nafion solution (5 wt.%, Aldrich), 0.83 mL ethanolnd 0.83 mL water were mixed ultrasonically to obtain the catalystlurry. 2.8 �L slurry was spread on the surface of the glassy carbonlectrode. After drying, the working electrode was obtained. Thelectrode surface contains about 40 �g cm−2 Pt–P. The electrolytes 0.5 M H2SO4. The electrocatalytic performance of the catalyst forRR was measured with the rotating disk electrode. Before the elec-

rochemical measurement, the high-purity oxygen was bubblednto the solution for 20 min. All the electrochemical measurements

ere carried out at 30 ± 1 ◦C.

. Results

.1. Structure characterization of catalysts

Fig. 1 shows the EDX spectrum of the Pt1P1/C catalyst. The Ptnd P peaks were observed in Fig. 1. It has been calculated that thetP/C catalyst contains about 18.8 wt.% Pt, illustrating that Pt(NO3)2s almost completely reduced The weight ratio of Pt and P in thet1P1/C catalyst calculated from Fig. 1 is 20.52:5.12, indicating thathe content of P in the PtP/C catalyst is high.

Fig. 2 displays the XRD patterns of the PtP/C catalysts and thet/C catalyst. In the XRD pattern of the Pt/C catalyst (Fig. 2, Curve f),he diffraction peak at 2� = 24.7◦ is due to the C(0 0 2) crystal face.ther diffraction peaks are located around 39.8◦, 46.6◦, 67.8◦ and1.1◦, respectively. They are corresponding to the 2� values of theharacteristic diffraction peaks of Pt(1 1 1), (2 0 0), (2 2 0) and (3 1 1)rystal faces of the face centered cubic crystallinity, respectively,

llustrating that the Pt particles in the Pt/C catalyst possess the faceentered cubic crystal structure (JCPDS card: 04-802). The 2� valuesf the diffraction peaks of the PtP particles in the PtP/C catalysts arelightly larger than the corresponding 2� values of the diffractioneaks of the Pt particles in the Pt/C catalyst. The average size of the

2θ/degree

Fig. 2. XRD patterns of (a) Pt8P1/C, (b) Pt4P1/C, (c) Pt2P1/C, (d) Pt1P1/C, (e) Pt1P2/Cand (f) Pt/C catalysts.

PtP and Pt particles can be calculated with the method reported inthe literatures [23,24]. The average sizes of the PtP particles in thePt8P1/C, Pt4P1/C, Pt2P1/C, Pt1P1/C and Pt1P2/C catalysts are 5.5, 4.6,4.3, 3.1 and 1.1 nm, respectively. The average size of the Pt particlesin the Pt/C catalyst is 2.8 nm.

Fig. 3 represents the TEM images of Pt/C and Pt1P1/C catalyst.After measuring more than 100 nanoparticles randomly from TEMimages of Pt/C and Pt1P1/C catalysts, the average sizes of the Pt andPtP particles in the Pt/C and Pt1P1/C catalysts are 3.1 and 3.5 nm,respectively. They are similar to that obtained from the above XRDmeasurement.

Fig. 4 shows the XPS spectrum of the Pt1P1/C catalyst in thePt4f region. Pt4f peak of the Pt1P1/C catalyst can be fitted to twopairs of peaks. Pt4f7/2 peak at 71.5 eV and Pt4f5/2 peak at 74.9 eVare assigned to Pt0, while Pt4f7/2 peak at 72.6 eV and Pt4f5/2 peakat 76.0 eV are assigned to PtII. Pt4f7/2 and Pt4f5/2 peaks of Pt◦ in thePt/C catalyst are located at 70.9 and 74.3 eV, respectively [25].

3.2. Measurements of electrocatalytic performance of Catalysts

Fig. 5 displays the linear sweeping voltammograms of oxygensaturated in 0.5 M H2SO4 solution at the different catalysts. Theonset potentials of ORR at the Pt8P1/C, Pt4P1/C, Pt2P1/C, Pt1P1/C,Pt1P2/C and Pt/C catalysts are 0.75, 0.73, 0.75, 0.79, 0.68 and 0.66 V,respectively. The limiting current densities (iL) of ORR are 5.60,6.15, 5.63, 5.77, 6.83 and 5.20 mA cm−2, respectively. Accordingto the treatment method in Ref. [26], the kinetic current densi-ties (iK) of ORR were calculated. As shown in Fig. 6, iK at 0.65 Vof the Pt8P1/C, Pt4P1/C, Pt2P1/C, Pt1P1/C, Pt1P2/C and Pt/C cata-lysts are 1.29, 0.95, 1.25, 1.66, 0.41, 0.32 mA cm−2, respectively.The onset potentials of oxygen reduction at all the PtP/C catalystsare more positive than that at the Pt/C catalyst and iK at all thePtP/C catalysts are larger than that at the Pt/C catalyst. For exam-ple, the onset potential of oxygen reduction at the Pt1P1/C catalystis 0.13 V more positive than that at the Pt/C catalyst and iK atthe Pt1P1/C catalyst is 1.34 mA cm−2 larger than that at the Pt/Ccatalyst.

4. Discussion

The result of EDX measurement of the Pt1P1/C catalyst (Fig. 1)indicates that the weight ratio of Pt and P in the Pt1P1/C catalyst isabout 4:1, illustrating that the content of P in the Pt1P1/C catalystsprepared with this phosphorus yellow reduction method are much

6496 J. Ma et al. / Applied Surface Science 257 (2011) 6494–6497

Fig. 3. TEM images of (A) Pt/C and (B) Pt1P1/C catalysts.

6872768084

Pt(4f)

Pt(0)

Pt(II)

Binding Energy/eV

Fig. 4. The XPS spectra of the Pt1P1/C catalyst in Pt4f region.

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Fig. 5. Linear sweeping voltammograms of (a) the Pt8P1/C, (b) Pt4P1/C, (c) Pt2P1/C,(d) Pt1P1/C, (e) Pt1P2/C and (f) Pt/C catalysts in 0.5 M H2SO4 solution with saturatedoxygen. Sweeping rate: 10 mVs−1; rotation speed: 2000 rpm.

higher than that in the PtP/C catalyst prepared with the self-redoxmethod of H3PO2 [21].

The XRD measurements (Fig. 2) points out that the 2� values ofthe diffraction peaks of the PtP particles in the PtP/C catalysts areslightly larger than the corresponding 2� values of the Pt particlesin the Pt/C catalyst. This demonstrates that P atoms have enteredinto the Pt crystal lattice, forming the PtP alloy, because the size ofP atom is smaller than that of Pt atom.

The result of the XPS measurement (Fig. 4) indicates that thepositions of Pt4f peaks of Pt0 in the Pt1P1/C catalyst are 0.6 eV morepositive than that in the Pt/C catalyst. This is due to that whenPt interacts with P, Pt would donate electrons to P, causing thedecrease in the 4f electron density of Pt. This further demonstratesthat P atoms have entered into the Pt crystal lattice, forming thePtP alloy.

The results from Figs. 5 and 6 illustrate that the electrocatalyticperformances of all the PtP/C catalysts for ORR are better than thatat the Pt/C catalyst, illustrating that the addition of P can promoteORR at the Pt/C catalyst. According to the XPS and XRD results, thisis due to that Pt and P form the alloy and the electron density of Ptis decreased.

It was found from Figs. 5 and 6 that the electrocatalytic per-formances of the different PtP/C catalysts for ORR are different. Theelectrocatalytic performances of the Pt1P1/C catalyst are better thanthat of other PtP/C catalysts. Firstly, it is due to the suitable amount

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Fig. 6. Polarization curves of kinetic current density for ORR (a) the Pt8P1/C, (b)Pt4P1/C, (c) Pt2P1/C, (d) Pt1P1/C, (e) Pt1P2/C and (f) Pt/C catalysts in 0.5 M H2SO4

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f P in the Pt1P1/C catalyst. Secondly, there is the particle size effect.he XRD results (Fig. 2) indicate that the average sizes of the PtP par-icles in the Pt1P1/C catalyst are relative small and the real surfacereas of the PtP particles are large.

. Conclusions

From the above results, the conclusion can be obtained as fol-ows:

1) The content of P in the PtP/C catalysts prepared from Pt(NO3)2and phosphorus yellow is high. Their electrocatalytic perfor-mances for ORR are excellent. Therefore, the phosphorus yellowreduction method is an excellent preparation method of thePtP/C catalyst.

2) P can promote the electrocatalytic performances of the Pt/C cat-alyst for ORR. Thus, all the PtP/C catalysts prepared are betterthan that of Pt/C catalyst.

3) The promotion action of P for enhancing the electrocatalyticperformances of the PtP/C catalyst for ORR is mainly due tothat Pt and P form the alloy and then the electron density of Ptis decreased.

cknowledgments

The authors are grateful for State Key High Technology Research

rogram of China (863 Program, 2007AA05Z143, 2007AA05Z159)f Science and Technology Ministry of China, the National Naturalcience Foundation of China (20873065, 21073094) and the Natu-al Science Foundation of Jiangsu Higher Education Institutions ofhina (10KJB150007).

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