One-pot synthesis of reduced graphene oxide supported PtCuy catalysts with enhanced electro-catalytic activity for the methanol oxidation reaction

Electrochimica Acta 136 (2014) 292–300

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One-pot synthesis of reduced graphene oxide supported PtCuy catalysts with enhanced electro-catalytic activity for the methanol oxidation reaction Xinglan Peng, Yanchun Zhao ∗ , Duhong Chen, Yanfang Fan, Xiao Wang, Weili Wang, Jianniao Tian Key Laboratory for the Chemistry and Molecular Engineering of Medicinal Resources (Ministry of Education of China), College of Chemistry and Pharmacy, Guangxi Normal University, Guilin 541004, PR China

a r t i c l e

i n f o

Article history: Received 21 February 2014 Received in revised form 20 May 2014 Accepted 20 May 2014 Available online 28 May 2014 Keywords: One-pot synthesis Platinum-copper Methanol electro-oxidation Reduced graphene oxide Nanoparticles

a b s t r a c t The outstanding performance PtCuy (y = 1,2,3) alloy nanoparticles supported on reduced graphene oxide (rGO) have been synthesized by a facile, efficient, one-pot hydrothermal synthesis approach. The as-prepared PtCuy /rGO catalysts are comprehensively characterized by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy. Cyclic voltammetry, CO-stripping voltammetry and chronoamperometry results reveal that the PtCuy /rGO catalysts have higher electro-catalytic activity, more negative onset oxidative potential, more excellent tolerance ability for CO poisoning and enhanced stability for the electro-oxidation of methanol compared to pure Pt/rGO. As far as the as-made PtCuy /rGO catalysts are concerned, the PtCu2 /rGO exhibits the highest electro-catalytic activity. The mechanism of the promoting effect of Cu on Pt is explained based on the electronic modification effect. The nature of interfacial interactions between the Pt-Cu active metal phase and the rGO supporting materials is crucial to achieving high performance. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction The development of sustainable energy technologies is one of the vital needs for lowing our dependence on fossil fuels and mitigating global warming. Among the various candidates, considerable attentions have been attracted by direct methanol fuel cells (DMFCs) in the past few decades due to their simple design, high energy conversion efficiency, low environmental pollution, convenient fuel transportation, storage and supply [1–4]. However, the relatively poor methanol oxidation kinetics significantly limit the methanol oxidation reaction [5]. It has been acknowledged that the success of fuel cell technology depends strongly on the electrocatalysts which can lower its electrochemical over-potentials and obtain high voltage output [3], but efficient and stable fuel-cell electro-catalysts are unavailable [6]. Therefore, the design of the catalyst with improved electrode kinetics for methanol oxidation and enhanced efficiency, stability and durability is demanded. Because of its outstanding catalytic activity, Pt catalyst has been

∗ Corresponding author. Tel.: +86 773 5846279; fax: +86 773 5832294. E-mail addresses: [email protected], [email protected] (Y. Zhao), [email protected] (J. Tian). http://dx.doi.org/10.1016/j.electacta.2014.05.110 0013-4686/© 2014 Elsevier Ltd. All rights reserved.

a universal choice electro-catalyst in DMFCs anodes [7]. However, the high cost of Pt is one of the most important barriers that block the commercialization of fuel cells [8,9]. In addition, the vulnerability towards reaction poisons of Pt electro-catalyst is another unfavorable factor [10,11]. Hence, magnifying the catalytic activity with less usage of Pt catalysts is very urgent [12]. For further reducing the overall use of expensive Pt and affording the potential of poisoning resistance, the Pt-based alloy nanoparticles have attracted increasing interest. Compared with monometallic Pt, the performance of Pt can be considerably enhanced by tuning its electronic structures by forming the bimetallic structures with cheap 3d-transition metals (i.e. Fe, Co, Ni, Cu, etc.) [9,12–23]. Those metals have increased Pt d-band vacancy and got more favorable Pt–Pt interatomic distance [24]. Less precious 3d-transition metal can provide oxygenated species at lower potentials for the oxidative removal of the adsorbed COads to prevent the catalysts from deactivating. Pt loading can be lessened by decreasing the size of catalyst particles to the nanometer range so as to increase the specific surface area per mass [25] or use high surface area supporting materials. Catalyst supports can exert a strong influence on the catalysts’ morphology, size distribution, stability and dispersion, which can in turn affect the performance of the DMFCs [26]. An ideal catalyst

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support should have: i) high specific surface area to achieve high metal dispersion; ii) good electric conductivity to promote fast electron transfer in redox reactions; iii) high stability including good corrosion resistance to maintain a stable catalyst structure; iv) strong affinity towards the catalyst particles to immobilize them, while ensuring their good dispersion [27,28]. Currently, as catalyst supports, carbon materials are the most used. Although carbon black has been commonly used in commercial electro-catalysts for DMFCs owing to high electrical conductivity [5], the presence of micro-pores causes the metallic particles get trapped in it and become inaccessible [29]. In view of this, novel non-conventional carbon materials have emerged, such as carbon nanotubes [30], carbon nanofibers [31], carbon nanorods [32], carbon nanospheres [33], and graphene [34]. Especially, graphene (or reduced oxide graphene, rGO) possesses a unique two-dimensional (2D) material with high surface area, good electrical conductivity and charge carrier mobility as well as high mechanical strength, which can not only maximize the availability of the nanosized electro-catalyst surface area for electron transfer, but also can offer enhanced mass transport of reactants to the electro-catalyst [35–37]. As a precursor, graphene oxide (GO) provides the potential for the production of cost-effective, large-scale graphene-based materials because it is easily reduced to graphene [38,39]. The exfoliated GO sheets are highly oxidized and featured with the residual epoxides, hydroxides and carboxylic acid groups on their surfaces, which can be reduced via chemical reactions. Though the fact that the conductivity of rGO is lower compared to the pristine graphene thanks to the presence of residual oxygenated groups and defects, the reactive surfaces of rGO supply the tunability in electronic and optoelectronic properties [40–42] and provide many favorable sites for anchoring the functional nanocomponents to the feasibility for composite incorporation [43]. It has been revealed that rGO-supported Pt-based catalysts have robust performance and suffer less poisoning by COads -like intermediates during methanol oxidation than catalysts supported on carbon black [34,44,45]. Up to date, a lot of reports demonstrate that electro-catalytic activities of some Pt-containing bimetallic catalysts are excellent than pure Pt alone. Among them, Pt-Cu catalysts are prominent catalysts with a reduced sensitivity toward COads -like intermediates [46] and abundant sources of copper [22]. Most of works on Pt-Cu catalysts have been investigated oxygen reduction reaction activity [47–50], while only few papers dealing with methanol oxidation in acid. In the current achievements, Pt-Cu catalysts were successfully synthesized through multi-steps [23,51] or galvanic displacement reaction [30,52]. These preparation processes are so cumbersome that is not suitable for practical applications. Combining the superb electro-catalytic activities of noble metal nanostructures with the supporting effect of rGO, noble metal/rGO hybrids have emerged as a novel kind of nanocomposites and received great attention in electro-catalysis. In this work, we report a facile, efficient, economical one-pot hydrothermal synthesis route for the synthesis of PtCuy /rGO (y = 1, 2, 3) catalysts. As illustrated in Scheme 1, in this route, N, N-dimethylformamide (DMF) as both solvent and reducing agent, while cetyltrimethyl ammonium bromide (CTAB) is used as surfactant and phenol as the structuredirecting agent. In this paper, methanol oxidation on PtCuy /rGO catalysts is investigated. It is observed that by alloying with Cu, PtCuy /rGO catalysts display improved catalytic activities toward these reactions than pure Pt/rGO. Our work reveals that PtCu2 alloy nanoparticles uniformly dispersed on rGO have higher electrochemical surface area (ECSA) and better activity for methanol oxidation in acid solution than that of catalysts. In addition, significantly enhanced stability for the methanol electro-oxidation is also observed on the PtCu2 alloy nanoparticles supported on rGO.

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2. Experimental 2.1. Preparation of GO GO was synthesized according to the modified Hummers method [53]. Briefly, powdered graphite (1.0 g) was added to 98% H2 SO4 (23 ml) in an ice bath under mild stirring. After that, KMnO4 (3 g) was slowly added to the mixture with stirring and the temperature of the mixture was maintained below 20 ◦ C. Then the mixture was heated at 35 ◦ C for 30 min followed by addition of distilled water (48 mL). Afterwards, heating and stirring were maintained for another 2 h. Finally, distilled water (112 mL) was slowly added with H2 O2 (8 mL) to follow. Upon treatment with the peroxide, the suspension turned bright yellow. The suspension was then centrifuged, washed with distilled water to obtain the GO. 2.2. Preparation of PtCuy /rGO catalysts An aqueous GO suspension was generated by sonication (1 h). After that, H2 PtCl6 (19.3 mM), CuCl2 ·2H2 O, DMF, CTAB and phenol were added into the GO solution successively and magnetically stirred for 1 h. The mixture was then transferred into a Teflon-lined stainless steel autoclave and treated hydrothermally at 180 ◦ C for 12 h. The amount of H2 PtCl6 was kept constant in all the synthesis, while the amount of CuCl2 ·2H2 O was changed according to the desired atomic Pt/Cu ratio (1/y) in the alloy products. The metal loading of Pt was ca 20 wt% in these samples. For comparation, Pt/rGO was prepared by the same procedure and the rGO was also prepared without adding H2 PtCl6 , CuCl2 ·2H2 O and CTAB. Then the suspension was centrifuged and rinsed several times with deionized water. The solid product was dried for 24 h in a vacuum oven at 25 ◦ C. 2.3. Characterization Morphology, component and microstructure of the synthesized materials was investigated by transmission electron microscopy (TEM) (JEM-2100F, Japan) and energy-dispersive X-ray spectroscopy (EDS) (FEI Quanta 200 FEG, Holand). X-ray diffraction (XRD) analysis data from the samples was collected using a RigakuD/MAX 2500 v/pc (Japan) diffractometer with Cu Karadiation. The chemical valences of metals in the catalyst were analyzed by X-ray photoelectron spectroscopy (XPS) (JPS-9010TR, Japan) with an Mg K␣ radiation. Raman spectra were obtained on a Renishaw inVia Raman spectrometer. The actual metal loadings on the working electrode for all the catalysts were determined by inductively coupled plasma (ICP) (IRIS Intrepid II XSP). 2.4. Electrochemical measurements Electrochemical measurements were recorded using CHI 660D electrochemical working station (CH Instrument, Inc.) and a conventional three-electrode system was used throughout this work. Before the preparation of the catalyst modified glassy carbon electrode (GCE), the GCE was polished with ␣-Al2 O3 . Water and C2 H5 OH dispersions of the purified products were then deposited on the polished GCE, sequentially. For stably coating the catalysts, 5 uL of 0.5% Nafion ethanol solution was placed on the GCE electrode surface and it was subsequently dried in air at room temperature. A Pt sheet and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. All potentials in the present study were given versus SCE reference electrode. The working electrode was first activated using cyclic voltammetry (CV) in deoxygenated 0.5 M H2 SO4 between 0.2 and 1.0 V with a scan rate of 50 mV s−1 . The CV experiments

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Scheme 1. illustration of the formation of PtCuy /rGO catalysts by one-pot hydrothermal synthesis.

for hydrogen absorption and desorption were conducted in N2 saturated 0.5 M H2 SO4 aqueous solution. The electro-catalytic activity for the methanol oxidation reaction was measured in a N2 -saturated 0.5 M H2 SO4 + 1.0 M CH3 OH solution at a scan rate of 50 mV s−1 . Several activation scans (about 12–15 cycles) were performed until reproducible voltammograms were obtained, and only the last cycles were used for the comparison of the catalytic activity. The ECSA measurements were determined by integrating the hydrogen adsorption from the CV. For CO-stripping analysis of the catalyst surface, N2 was pumped into the 0.5 M H2 SO4 solution for 15 min. Afterwards, the adsorption of CO was performed by pumping high-purity CO (99.9% purity) gas into the solution for 30 min while maintaining the potential at 0.1 V, then, the dissolved CO in the solution was removed by bubbling N2 into the solution for 15 min whilst holding the potential at 0.1 V. Finally, the CO-stripping voltammogram was recorded between 0.2 V and 1.0 V with a scan rate of 50 mV s−1 for two complete oxidation/reduction cycles. The chronoamperometric profiles were measured by recording the current for 7200 s at a potential of 0.6 V in a mixture of 0.5 M H2 SO4 + 1 M CH3 OH. The Pt metal loading was kept at 5 ␮g and all tests were conducted at ambient temperature (25 ± 1 ◦ C).

3. Result and discussion To determine the extent to the reduction of rGO, the XPS, Raman and electrical conductivity are performed. Fig. 1a displays the deconvoluted C 1s XPS of the GO. Based on previous studies, the three fitted peaks centered at 283.9, 286.1, 288.2 eV can be assigned to the binding energies of carbon in C=C/C–C, C–O (epoxy/hydroxyls) and C=O (carbonyl/ketone), respectively [54]. The observed large intensities of C 1s peaks corresponding to oxygen-containing functional groups verify that the synthesized product is GO. Though the C1s XPS spectrum of the rGO (Fig. 1b) also exhibits the same oxygen functionalities, their peak intensities are much smaller than those in GO. From the XPS elemental analysis, the atomic ratio of C and O is 1.83 for GO and 13 for rGO, respectively. Thence, the XPS results indicate that GO has been reduced to graphene by the reduction treatment [55]. The Raman spectra of GO and PtCu2 /rGO are showed in Fig. 1c. For GO, the two peaks at 1351 and 1596 cm−1 can be assigned to the D-band corresponding to the vibrations of sp3 carbon atoms of disordered graphene nanosheets and G-band arising from vibrations of sp2 carbon atom domains of graphite, respectively. However, for PtCu2 /rGO, the D-band shifts to 1347 cm−1 and G-band shifts to 1590 cm−1 . Such binding energy

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Fig. 2. Morphology and composition characterization of as-prepared PtCu2 /rGO catalysts. (a-c) TEM and HRTEM images at different magnifications; (d) the point scanning of EDS spectroscopy and the elemental maps of Pt (e) and Cu (f), respectively.

shifts of D and G bands suggest an interaction between rGO and the metal particles [34]. The intensity ratio of the D-band to the G-band (ID /IG ) from PtCu2 /rGO is obviously larger than that of GO (0.93 in GO to 1.1 in PtCu2 /rGO), which confirming that the decrease in the average size of the sp2 domains upon the formation of nanoparticles on the rGO [56] and most of the oxygenated groups have been removed during the reduction process [57]. The reduction of GO is further verified by the improved conductivity of rGO [56]. The powder conductivity of the rGO is detected by a four point probe technique as 1600 S/m, which is five orders of magnitude higher than that of GO (2.2 × 10−2 S/m). The superior conductivity of rGO is helpful to enhance the electrode performance of catalysts. In conclusion, a high degree of reduction of GO has occurred during the process of the catalyst synthesis [58]. Fig. 2a-b show the representative TEM images of the asprepared PtCu2 /rGO catalysts. The rippled and resemble crumpled silk veil waves are rGO sheets. The TEM images convey that the PtCu alloy catalysts are uniformly dispersed on rGO surface and the

calculated average size is considered to be approximately 4.8 nm. It should be pointed out that the particle size of the catalyst plays an important role in methanol oxidation [59,60]. The particle size should be small as much as possible but larger than 1.8 nm to increase the specific surface area per mass. An extremely small size (<1.8 nm) will lead to catalyst aggregate to reduce their surface areas [60]. Therefore, the catalysts we prepared in this work will avoid the complications from the particle size effect. The HRTEM image of the representative PtCu2 /rGO is presented in Fig. 2c. The oriented and ordered lattice fringes correspond to the spacing of 0.218 nm, which is the interplanar distances of the (111) plane of the Pt-Cu alloy [61]. The chemical composition of the PtCu2 /rGO catalysts is determined using EDS, shown in Fig. 2d. The point scanning of EDS reveals the presence of C, O, Pt and Cu, and no other element is detectable, indicating that the particles deposited on the rGO surface are Pt-Cu catalysts. The ratio of Pt to Cu is 0.47, which is close to the theoretical loading ratio of 1:2. Elemental maps are invaluable tools for understanding the metal distribution and

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Intensity/au.

a PtCu3/rGO b PtCu2/rGO c PtCu/rGO d Pt/rGO (111)

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(002)

constitute is very small entities and cannot therefore be detected by XRD. Moreover, the observed diffraction angles for PtCuy catalysts fall in between those of the two pure metal element Pt and Cu, which also provides evidence for the formation of Pt-Cu alloy. Not any other obvious diffraction peaks are detected, indicative of these products are pure phase [63]. Additionally, the lack of oxide peaks according to data of both Cu2 O (JCPDS 78-2076, 050667) and CuO (JCPDS 78-0428, 80-1917) in the patterns means that the samples are dominated by elemental Pt and Cu [52]. The calculated mean sizes according to the Scherrer analysis of the (111) peak are found to be in the range of 4.3, 5.2,5.6 and 6.6 nm for Pt/rGO, PtCu/rGO, PtCu2 /rGO and PtCu3 /rGO catalysts, respectively. The XRD and HRTEM results demonstrate that the geometric environment of the Pt atoms in Pt-Cu alloy is altered due to the formation of alloying with Cu. The alteration in the geometric or atomic configuration may affect the electronic structure of Pt causing the change in the electro-catalytic activity toward the methanol oxidation reduction. The surface composition and chemical state of the external surface of the as-prepared nanocatalysts are investigated by XPS. As shown in Fig. 4a, C, O, Pt and Cu can be found in the XPS survey spectrum of PtCu2 /rGO. The Pt 4f spectrum and Cu 2p spectrum of PtCu2 /rGO are displayed in Fig. 4b and c, respectively. Based on curve fitting with a mixed Gaussian–Lorentzian line shape, Pt seems to exist in various states. The Pt 4f signal is composed of two pairs of doublets. The most intense doublet (70.9 and 74.3 eV) is due to metallic Pt (0). The second set of doublets (71.5 and 75.6 eV) can be assigned to the Pt (II) chemical state, suggesting species such as PtO and Pt(OH)2 [64]. A comparison of the relative intensities of these components (metallic Pt (0), PtO, and Pt(OH)2 ) reveals that the presence of Pt species in binary Pt-Cu alloy is predominately metallic Pt. In Fig. 4c, a doublet peak including a lower binding energy at 932.3 eV and a higher binding energy at 952.1 eV are

Pt reference Cu reference

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Degree(2θ) Fig. 3. XRD patterns of as-made Pt/rGO, PtCu/rGO, PtCu2 /rGO and PtCu3 /rGO. The standard patterns of pure Cu (JCPDS 04-0836) and Pt (JCPDS 65-2868) are attached for comparison.

overall composition of Pt-bimetallic catalysts, and here are shown in Fig. 2e-f. The uniform color distribution reveals that the Pt-Cu alloy structure is indeed formed. To further examine the phase structure, XRD patterns of Pt/rGO, PtCu/rGO, PtCu2 /rGO and PtCu3 /rGO catalysts are displayed in Fig. 3. All the samples present similar face-centered-cubic (fcc) structure, whose peaks can be indexed to (111), (200), (220) and (311) planes. It is worth noting that all diffraction peaks shift to higher 2-theta angles as compared to those of Pt/rGO with the decrease of Pt proportion in the alloy catalysts, indicating a lattice contraction resulting from the incorporation of the smaller Cu into the larger Pt atoms [62]. The peaks corresponding to pure metallic Pt (JCPDS 65-2868) and Cu (JCPDS 04-0836) are not obvious, suggesting that the remaining quantities of non-alloyed Pt and Cu

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Fig. 4. (a) XPS survey spectra of PtCu2 /rGO; (b) Pt 4f XPS spectra of PtCu2 /rGO; (c) Cu 2p XPS spectra of PtCu2 /rGO, Inset image of (c) is Cu 2p3/2 peak. (d) Pt 4f7/2 peak in Pt/rGO (curve a), PtCu/rGO (curve b), PtCu2 /rGO (curve c) and PtCu3 /rGO (curve d).

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discovered for the Cu 2p3/2 , 1/2 spectra of PtCu2 /rGO, respectively. The broad peak at ca. 940 eV is still discernible and indicates the presence of oxidized Cu(II) species [65] near the surface. The inset image of Fig. 4c is the curve deconvolution of Cu 2p3/2 , in which the majority at a lower binding energy of 932.3 eV corresponding to Cu, while the smaller peak at a higher binding energy of 934.6 eV due to Cu(II). Interestingly, in the Pt XPS spectra, the Pt 4f peaks of the PtCuy /rGO shift to a lower binding energy than that of the Pt/rGO, as shown in Fig. 4d. The peaks have negative shifts of approximately 0.43, 0.32, and 0.25 eV for PtCu/rGO, PtCu2 /rGO and PtCu3 /rGO, respectively. The reasons of XPS shifts for the PtCuy /rGO catalysts can be ascribed as follows: i) Alloying with Pt will increase the valence electron (5d) vacancy of the Pt, which leads to a modification in the electronic properties of PtCuy /rGO catalysts and causes the Pt 4f peak shift in the XPS [66]; ii) the electro negativities differences between Cu (1.90) and Pt (2.28) maybe cause an electron donation from Cu to Pt [64]. The shifts in electron transfer lead to a alteration in the electronic characteristics of the Pt 4f. Such an electron transfer will lower the density of states on the Fermi level and reduce the Pt-CO bond energy [67]. Therefore, the potential advantage of the PtCuy /rGO alloy catalysts to methanol oxidation is improved. The XPS results of atomic ratio of Pt to Cu are 1.45:1.72, 2.02:4.87 and 3.03:8.56 for PtCu, PtCu2 and PtCu3 respectively, while the ICP results of atomic ratio of Pt to Cu are 4.2:4.4, 3.6:7.1 and 2.9:7.9. The more higher Cu/Pt ratio suggests there is a slightly amount of Cu surface segregation in the PtCu alloys, this is similar to CV of Fig. 5. The surface state of the PtCu2 /rGO catalysts is detected by CV in 0.5 M H2 SO4 with scan rate: 50 mV s−1 . As illuminated in Fig. 5, during the first few potential cycling, the strong dissolution of the Cu feature is appeared. The peak at 0.04 V (vs. SCE) corresponds to the dissolution of unalloyed Cu, while the peak at around 0.56 V (vs. SCE) is ascribed to the corrosion/dealloying of Cu from a Pt environment [68]. After the 10th cycles, the observed CV curves become stable and behave similar to a typical Pt electrode with no obvious Cu-dissolution signals being detected indicating that Cu dissolution from the catalysts surface has either ceased or dropped to undetectable level. This dealloying process may cause the catalysts surface re-arrangement, generating Pt-rich surfaces covering a core consisted of Pt-Cu catalysts [69]. The absent of the characteristic anodic stripping peaks in the 1st cycle associated with under potentially deposited hydrogen on Pt surface atoms between -0.2 and 0 V indicates a small amount of Cu covering on the surface of PtCu2 alloys. This is in good agreement with prediction of Cu surface segregation in Cu rich Pt-Cu alloys [70]. In the subsequent cycles,

Fig. 6. CV curves of (a) Pt/rGO, (b) PtCu2 /rGO, (c) PtCu3 /rGO, (d) PtCu/rGO in 0.5 M H2 SO4 solution with scanning rates of 50 mV s−1 .

the hydrogen adsorption/desorption peaks quickly enhance, which also provides evidence for the dealloying process. Fig. 6 shows the relatively stable CV curves of PtCu/rGO, PtCu2 /rGO and PtCu3 /rGO catalysts in 0.5 M H2 SO4 aqueous solution, in which that of Pt/rGO is involved for comparison. It can be clearly observed that their voltammetric features are similar to that of Pt/rGO catalyst with the hydrogen adsorption/desorption region within the potential range of -0.2V-0.0 V and the double layer at 0.0V-0.1 V. The anodic voltammetric feature at 0.7 V indicates the formation of Pt surface oxides. On the cathodic scan, reduction peaks at between 0.6 and 0.4 V are associated with partial rereduction of Pt-oxides and re-deposition of Cu. CV is a convenient and efficient tool used to estimate the ECSA of Pt-based catalysts on an electrode. ECSA can be calculated by integrating the charge passing the electrode during the hydrogen adsorption/desorption process after the correction for the double layer formation. The charge of the adsorption of a hydrogen monolayer is 0.21 mC cm−2 [71] [72] corresponding to a surface density of 1.3 × 1015 Pt atoms per2 [73]. As shown in Fig. 6, the ECSAs (the ECSA per unit weight of Pt) estimated from the hydrogen adsorption peaks are 31.7 m2 g−1 for PtCu2 /rGO, 20.9 m2 g−1 for PtCu3 /rGO, 30.8 m2 g−1 for PtCu/rGO, respectively, which are lower than that of the Pt/rGO catalyst (43.3 m2 g−1 ). The lower ECSAs for the PtCuy /rGO electro-catalyst are most likely due to the large nanoparticle size, surface covered by organic species and/or some aggregation formed in the electrochemical process. Next, catalytic activity of the PtCuy /rGO catalysts toward methanol oxidation is investigated. Fig. 7 displays the typical CV curves of methanol oxidation on Pt/rGO, PtCu/rGO, PtCu2 /rGO and PtCu3 /rGO catalysts in 0.5 M H2 SO4 and 1 M CH3 OH mixed solution. The CV curves on four catalysts demonstrate a similar profile. In the forward scan, at potentials lower than 0.4 V, nearly negligible current can be observed, which is thanks to the catalyst surface being almost completely poisoned by the intermediates generated via incomplete oxidation of methanol, such as COads -like species [74]. As the potentials move to more positive values, the oxidation of COads -like occurs, which produces a prominent symmetric anodic peak, meanwhile yielding a COads -free Pt surface. Onset potential of methanol oxidation is a quite significant parameter to estimate the catalytic activity at low potentials for electro-catalysts. As revealed in inset image of Fig. 7, the onset potential of methanol oxidation of the PtCu2 /rGO electrode is lower than the PtCu/rGO and PtCu3 /rGO electrodes (0.27 V vs. 0.30 V and 0.32 V). It does mean that methanol is the easiest to oxidize on the surface of the PtCu2 /rGO among these three catalysts. The anodic peak in the

X. Peng et al. / Electrochimica Acta 136 (2014) 292–300

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Fig. 7. CV curves of (a) PtCu2 /rGO, (b) PtCu/rGO, (c) PtCu3 /rGO and (d) Pt/rGO catalysts in 0.5 M H2 SO4 + 1 M CH3 OH solution. Inset image is the enlarged CV curves at the potential range of 0.2-0.4 V (vs. SCE) for (a) PtCu2 /rGO, (b) PtCu/rGO, (c) PtCu3 /rGO. Scan rate: 50 mV s−1 .

20 d PtCu2/rGO 10 0 -10 -20 20 c PtCu/rGO 10 0 -10 -20 20 b PtCu3/rGO 10 0 -10 -20 20 a Pt/rGO 10 0 -10 -20 -0.2 0.0 0.2

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forward scan, around 0.75 V vs. SCE, is due to the oxidation of methanol and designated as If . In the backward scan, the oxidation peak at ∼ 0.55 V vs. SCE is ascribed to the oxidation of COads like species and designated as Ib . The If value are 642, 397, 295 and 154 mA mg−1 for PtCu2 /rGO, PtCu/rGO, PtCu3 /rGO and Pt/rGO, separately. The lower If value means that carbonaceous species are accumulated on the electrodes and decrease the efficiency of catalysts [75]. The value of If /Ib ratio is generally used to evaluate the poison tolerance of Pt-based catalysts. The ratio values of PtCuy /rGO are obviously higher than that of the Pt/rGO catalyst, which represents a better performance in the oxidation of methanol, less accumulation of COads -like species on the catalyst surface and a better CO-poisoning tolerance of PtCuy /rGO catalysts than Pt/rGO. Consequently, the Cu-containing catalysts have moderately superior inherent catalytic activity for methanol oxidation in CV experiments and PtCu2 /rGO catalysts have the highest activity than that of others. During the absorption process, the surface of Pt is adsorbed by methanol molecules and the Pt–CO forms after the dehydrogenation [76]. Thence, CO electro-oxidation can serve as a useful model system in understanding electro-oxidation of methanol [77,78]. A good catalyst should have excellent tolerance ability for CO poisoning that can be evaluated by CO-stripping voltammetry. Fig. 8 shows CO-stripping voltammograms of Pt/rGO, PtCu/rGO, PtCu2 /rGO and PtCu3 /rGO catalysts in 0.5 M H2 SO4 aqueous solution. In all curves, the oxidation peak in the first voltammogram scan can be associated with the oxidation of CO adsorbed on the catalyst surface, while the hydrogen adsorption peak is completely suppressed. In the second voltammogram scan, the oxidation peak disappears, verifying the adsorbed CO is completely oxidized (“stripped”) from the catalyst during the first positive going scan. The onset potentials on PtCu/rGO, PtCu2 /rGO and PtCu3 /rGO are 0.441 V, 0.423 V and 0.498 V, respectively, which is lower than that on the Pt/rGO catalyst (0.562 V). The dashed line represents the CO oxidation peak potential for Pt/rGO. As shown in Fig. 8, the peak potentials on PtCu/rGO, PtCu2 /rGO and PtCu3 /rGO catalysts are visibly shifted negatively by about 68 mV, 105 mV and 101 mV, separately, compared to the Pt/rGO catalyst. The negative shift for PtCuy /rGO samples with greater than Pt/rGO indicates a weakening of the CO adsorption strength [79] and the absorbed CO is prone to oxidation on PtCuy /rGO at lower potential, which is beneficial for releasing the active sites of Pt for the further oxidation of methanol in DMFCs [76]. CO-stripping results suggest that the addition of Cu is of great

250 200

20 18 16 14 12 10 8 6 4 2

a PtCu2/rGO b PtCu/rGO c PtCu3/rGO d Pt/rGO

a

b c d

6000 6200 6400 6600 6800 7000 7200

Times(S)

150 100

a

50

d c b

0 0

1000

2000

3000

4000

5000

6000

7000

Times(S) Fig. 9. Chronoamperometric curves of the (a) PtCu2 /rGO, (b) PtCu/rGO, (c) PtCu3 /rGO and (d) Pt/rGO at 0.6 V in a 0.5 M H2 SO4 + 1 M CH3 OH solution. Scan rate: 50 mV s−1 . Inset image is the last 1200s Chronoamperometric curves of these four catalysts.

importance to PtCuy /rGO catalysts synthesis as far as the electrooxidation of CO is concerned. This finding is in agreement with the CV experiments. It seems that PtCuy /rGO catalysts are potential catalysts for CO tolerance and the PtCu2 /rGO catalyst has the best tolerance ability for CO poisoning. Furthermore, Chronopotentiometry is used to explore the antipoisoning abilities of the catalysts under a constant potential of 0.6 V in 0.5 M H2 SO4 +1.0 M CH3 OH solution for 7200 s. These curves reflecting the activity and stability of the various catalysts in catalyzing the methanol oxidation reaction is shown in Fig. 9. The polarization currents of all the present catalysts decay rapidly at the initial stage, which is caused by the formation of double layer capacitance during the methanol oxidation reaction [80,81]. The following degradation in the currents may be caused by the accumulation of COads poisoning species on the surface of Pt electrode [82]. In addition, SO4 2− adsorption on the catalyst surface also bring about the currents decrease by inhibiting the reaction active sites [80]. Upon long time running, the currents gradually reach a quasiequilibrium steady state. During the whole time, the current density of methanol oxidation of PtCu2 /rGO (curve a in

X. Peng et al. / Electrochimica Acta 136 (2014) 292–300

Fig. 9) is higher than those of the other catalysts, though the current decayed with time is observed for the four catalysts modified electrodes. This finding is in good agreement with the CV results, indicating that alloying with Cu can dramatically enhance the catalytic durability of Pt and is more tolerant to poisons than the pure Pt. Herein, two kinds of effects are proposed to explain catalytic performance of Pt in alloys, which can be ascribed as follows: i) The ligand effects, caused by two dissimilar surface metal atoms, which induces electronic charge transfer between the atoms and affects their electronic band structure; ii) The geometric effects, comprising the effects of the Pt–M surface atoms morphology and interatomic distance, as well as co-ordination environment and lattice strain of Pt in the surface layers, are also significant [69]. In the case of Pt-Cu alloy structure, mainly the electronic modification effect should be present. Pt can tune its electronic structures by forming the bimetallic structures with Cu atom. This has increased Pt d-band vacancy and got more favorable Pt-Pt interatomic distance [24]. In the present study, direct evidences are also presented in the XRD diffraction peaks shifting synchronously to higher 2theta angles and the XPS peaks having negative shifts. Moreover, the structural flexibility and superior conductivity of rGO can not only enhance the electrode performance of metal-rGO hybrids but also be able to facilitate electron or hole transfer along its twodimensional surface [38]. More importantly, the oxygen-containing functional groups of rGO improve the electro-catalytic activity by removing accumulated COads -like intermediates that are formed during the oxidation of methanol [83]. The nature of interfacial interactions between the Pt-Cu active metal phase and the rGO supporting materials is considered to be crucial to achieving high electro-catalytic activity. 4. Conclusion In this work, we have developed a facile, efficient, economical one-pot hydrothermal synthesis route for the synthesis of PtCuy /rGO alloy catalysts. The results show that the addition of Cu to Pt leads to significantly enhanced in the catalytic activity of Pt for methanol electro-oxidation in terms of high specific activity, high structure durability, and high tolerance to CO poisoning. For methanol oxidation, PtCu2 /rGO catalysts have the highest activity than others. The appropriately modified electron structure of Pt by alloying with Cu is responsible for the enhanced catalytic performance of the as-made catalysts. In addition, the superb performance of rGO is much favorable for maximizing the availability of the PtCu2 surface area for electron transfer and offering enhanced mass transport of reactants to the electro-catalyst. The resulting PtCu2 /rGO catalysts will have great application potentials as anode catalysts in DMFCs in terms of unique catalytic performance, simple and controllable preparation, and perfect yielding. Acknowledgment This work has been supported by the National Natural Science Foundation of China (No. 21163002, 21165004, 21363003) and Program for Excellent Talents in Guangxi Higher Education Institutions. References [1] M.L. Foo, Y. Wang, S. Watauchi, H.W. Zandbergen, T. He, R.J. Cava, N.P. Ong, Phys Rev Lett 92 (2004) 247001. [2] Y. Zhao, F. Wang, J. Tian, X. Yang, L. Zhan, Electrochim Acta 55 (2010) 8998–9003. [3] H. Liu, C. Song, L. Zhang, J. Zhang, H. Wang, D.P. Wilkinson, J Power Sources 155 (2006) 95–110.

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