Facile synthesis of trimetallic PtAuCu alloy nanowires as High−Performance electrocatalysts for methanol oxidation reaction

Journal of Alloys and Compounds 780 (2019) 504e511

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Facile synthesis of trimetallic PtAuCu alloy nanowires as High
Performance electrocatalysts for methanol oxidation reaction Yajun Liu a, Guohong Ren a, Mingqian Wang a, Zhicheng Zhang a, Ying Liang a, Shishan Wu a, *, Jian Shen a, b, ** a

Key Laboratory of High Performance Polymer Material and Technology of Ministry of Education, School of Chemistry and Chemical Engineering, Nanjing University, 163 Xianlin Avenue, Qixia District, Nanjing 210023, China Jiangsu Collaborative Innovation Center of Biomedical Functional Materials, Jiangsu Key Laboratory of Biomedical Materials, College of Chemistry and Materials Science, Nanjing Normal University, 1 Wenyuan Road, Qixia District, Nanjing 210046, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 September 2018 Received in revised form 24 November 2018 Accepted 2 December 2018 Available online 4 December 2018

The high cost, easy to be poisoned and poor stability of platinum (Pt) catalysts have limited their further application in various fields. The ability to impart Pt
based catalysts with high catalytic activity and low cost is critical to advancing fuel cell technologies. A promising strategy is applied to design Pt
based bimetallic or trimetallic nanostructures. The method of increasing electrocatalytic activity performance has attracted considerable attention by introducing other metal atoms for their unique structures and compositions. In this study, we report a solvothermal synthesis of trimetallic Pt
based (PtAuCu) alloy nanowires with controlled percentages of individual metals. By optimizing the content of Pt, Au and Cu, the trimetallic PtAuCu NWs nanowires show superior electrocatalytic activity toward methanol oxidation reaction (MOR). This enhanced electrocatalytic activity is due to the synergistic interactions of different metals. In particular, the incorporation of Cu can effectively reduce the use of precious metal Pt. And during the MOR process, the dissolution of Cu atoms from the PtAuCu surface, which can be attributed to a dealloying process, providing more Pt active sites for MOR thus enhancing the electrochemical activity. The electrochemical active surface area (ECSA) and mass activity of the Pt50Au10Cu40 alloy catalyst is approximately 1.5 and 4.34 times of commercial Pt/C catalyst, respectively. Moreover, it exhibits superior stability after 10000 s compared with the other catalysts. Thus, the multi
metal doping in Pt catalyst not only decreases the cost of Pt, but also exhibits a significant electrocatalytic performance, which is prospective for direct alcohol fuel cells. © 2018 Published by Elsevier B.V.

Keywords: Methanol electrooxidation Pt
based catalyst Nanowires Direct alcohol fuel cells

1. Introduction Direct methanol fuel cells (DMFCs), as a class of proton exchange membrane fuel cells (PEMFCs), have drawn extensive attention in portable applications, which is owing to their high energy density, simple system design and low emissions of pollutants over the past decades [1e3]. However, slow kinetics of oxygen reduction reaction and poor long
term stability have restrict their widespread commercial application [4,5]. In recent years, some novel catalysts have

* Corresponding author. ** Corresponding author. Key Laboratory of High Performance Polymer Material and Technology of Ministry of Education, School of Chemistry and Chemical Engineering, Nanjing University, 163 Xianlin Avenue, Qixia District, Nanjing 210023, China. E-mail address: [email protected] (S. Wu). https://doi.org/10.1016/j.jallcom.2018.12.016 0925-8388/© 2018 Published by Elsevier B.V.

been prepared and their activities were investigated, such as M
ZnO
Al2O3, carbon nanotubes, graphene, Pt-based catalysts [6e10]. Among them, Pt-based catalysts have been commercialized used for fuel cells due to its superior catalytic selectivity and excellent activity, but its high cost and low durability limit its further application [11,12]. Thus, many efforts have been devoted to reducing Pt usage and enhancing its activity and stability. One way to improve the mass activity of Pt catalyst is incorporating two or three low
cost 3d transition metal to form Pt
based alloy catalysts. Additionally, the increase of specific activity of Pt catalyst mainly by modifying the morphology [13e15] or developing non
platinum electrocatalysts through loading into a large carrier with specific areas, such as graphene, carbon nanotube [16,17]. Currently, a great deal of efforts have been devoted to developing simple and efficient techniques to prepare Pt and Pt
based catalysts with controllable nanostructures, which are divided

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from zero dimension to three dimensions [18e20]. Compared with other nanostructures, one
dimensional (1D) nanocrystal exhibits higher surface area and better catalytic properties for MOR [21,22]. For example, Liao and co
workers prepared PtCu nanowires by hydrothermal method, the ECSA and mass activity were 71.7 m2 g
1 and 0.322 A mg
1, which was about twice and five times of the PtCu nanocages synthesized by Xia's group, respectively [23,24]. Sun et al. synthesized Pt nanowires on the nanospheres of a carbon black by a mild wet chemical approach, which displayed enhanced catalytic activity for the ORR compared with Pt/C catalyst [25]. Until now, ultrathin Pt and Pt
based nanowires have attracted great attention owing to their unique structure and catalytic properties [26,27]. So far, Pt
base alloy nanowires such as PtPd, PtAu, PtCu, PtCo, PtRu, PtNi and so on have been also reported, which show better electrocatalytic activities compared to commercial Pt/C catalyst [24,28e32]. In order to further reduce the use of Pt, trimetallic or quaternary alloys have been researched. Ma et al. reported the syntheses of PtPdRuTe nanotubes via galvanic replacement reaction at the expense of ultrathin Te nanowires, which showed enhanced catalytic activity and durability toward the MOR [33]. It has been realized through template or substrate methods to control the formation of nanowires. Nonetheless, these approaches still remain great challenges to prepare Pt and Pt
based alloy nanowires, such as the addition of surfactants, electrical energy and sacrificial templates or substrates [34,35]. However, it is difficult for the formation of Pt and Pt
based alloy nanowires without the assistance of templates or substrates [36,37]. Many efforts have been developed into explore direct synthesis of high aspect
ratio Pt nanowires. Huo et al. have reported the syntheses of Au nanowires with the assistance of oleylamine, which demonstrate the important role of amine in the formation of nanowires [38]. However, oleylamine is corrosive and hard to remove, and it's gradually replaced by amines such as ethylenediamine tetra acetic acid (EDTA), N,N
Dimethylformamide (DMF) and so on. Consequently, the controlled synthesis of alloyed NW catalysts in large scale with low cost is still challenging but very attractive. In this report, we demonstrate a facile and effective wet chemical strategy for the synthesis of uniform, ultrathin and ultralong trimetallic PtAuCu alloy nanowires. Electrochemical properties of different alloy ratios are studied by changing the ratio of copper element in the alloy NWs. The introduction of Cu remarkably increases the electrocatalytic surface area and activity toward methanol oxidation, which can be attributed to the modified structure and synergic effects. Compared to the binary PtAu NWs and commercial Pt/C catalyst, trimetallic PtAuCu nanowires exhibit higher catalytic activity and durability toward MOR.

2. Experiment section 2.1. Materials Hexachloroplatinic acid hexahydrate (H2PtCl6$6H2O, AR, Pt  37.5%), gold tetrachloride trihydrate (HAuCl4$3H2O, AR, 99.8%), Copper (II) Chloride Dihydrate (CuCl2$2H2O, AR, 99.0%) and commercial Pt/C (20 wt%) were purchased from Shanghai Aladdin Chemical Reagent Co. Ltd., China. Potassium hydroxide (KOH, AR, 85.0%), sulfuric acid (H2SO4, 98%), N,N
Dimethylformamide (DMF, AR, 99.5%), ethylene glycol (EG, AR, 99.0%), methanol (anhydrous, 99.8%) and ethanol (AR, 99.7%) were purchased from Shanghai Sinopharm Chemicals Reagent Co., Ltd., China. All reagents and chemicals were used as received without further purification. Doubly distilled water was used in all experiments.

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2.2. Preparation of PtAuCu NWs According to previous report, the PtAuCu NWs were synthesized following a modified protocol [11]. A controlled amount of H2PtCl6 solution (0.04 M), a certain amount of HAuCl4 solution (0.03 M) and CuCl2 solution (0.05 M) were added to a mixed solution containing 4 mL of ethylene glycol and 6 mL of DMF under stirring, then 0.5 g of KOH was added to the above mixed solution. After stirring for 2 h, the resultant homogeneous solution changed into black, and then the mixture solution was transferred into a Teflon
lined stainless
steel autoclave (25 mL). The autoclave was maintained at 170  C for 6 h and then cooled to room temperature naturally. The black precipitates were collected by centrifugation and then washed with ethanol and deionized water several times to remove impurities. After that, the products were dried at 80  C in vacuum overnight. The Pt70Au10Cu20, Pt80Au10Cu10 and Pt80Au20 nanowires were prepared at the same condition as the referenced catalysts, and the content of metal precursor (Pt, Au, Cu) during the preparation were listed in Table S1. 2.3. Characterization The morphology and microstructure of the products were analyzed by transmission electron microscopy (TEM, JEM 1011), high resolution TEM (HRTEM, JEM
200CX) and high
angle annular dark
field scanning transmission electron microscopy (HAADF
STEM, Philips
FEI Tecnai F20). X
ray diffraction (XRD) patterns were collected on a Shimadzu XRD
6000 diffractometer with Cu Ka radiation (Cu Ka, l ¼ 1.5406 Å, 40 kV, and 40 mA). X
ray photoelectron spectroscopy (XPS) measurements were acquired on PHI 5000 Versa Probe. 2.4. Electrochemical measurements All the electrochemical measurements were performed on a CHI 660D electrochemical workstation (CH Instruments, Chenhua Company, Shanghai, China). The experiments were carried out using a three
electrode cell at room temperature. A modified glassy carbon electrode (GCE, 3 mm in diameter) as working electrode, a Pt wire counter electrode as counter electrode, and a saturated calomel electrode (SCE) as reference electrode. Prior to use, the GCE was prepared with a typical procedure in previous reports, it was polished by 0.3 and 0.05 mm alumina powder and ultrasonicated in ethanol and deionized water [39]. Then, the catalyst dispersed to a certain amount of water to form a solution (2 mg ml
1), and 10 ml of the solution was taken and added to the GCE surface. After having been dried, 10 mL of Nafion (0.5 wt %) was deposited on the catalyst modified GCE. The working electrode was dried in air condition. The electrocatalytic activity of the PtAuCu NWs electrodes were evaluated by cyclic voltammetry (CV) at a scan rate of 50 mV s
1 from
0.2 to 1 V in a 0.5 M H2SO4 solution without and containing 1.0 M CH3OH. The electrochemical stability was measured within the potential range from
0.2 to 1 V in a 0.5 M H2SO4 solution with 1.0 M CH3OH for 1000 cycles at 50 mV s
1. The chronopotentiometry (CA) curves were measured at a constant potential (0.55 V) for 10000 s. Simultaneously, the electrochemical activity of commercial Pt/C (20 wt%) was also performed under the same condition. And the peak current densities are normalized by the loading mass of the catalysts on the working electrodes. 3. Results and discussion 3.1. Catalysts characterization To understand the detailed nanostructure of the prepared

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products, transmission electron microscopy (TEM) and high resolution TEM (HRTEM) analyses were investigated. As shown in Fig. S1, the low magnification TEM images of trimetallic Pt50Au10Cu40 NWs exhibit an intertwined network structure and disperse uniformly with an average diameter distribution of 20 ± 5 nm. HRTEM image of the prepared NWs depicted in Fig. 1A shows that these nanowires are composed of thinner nano wires. As shown in Fig. 1B and C, distinct lattice fringes were observed and measured to be 0.210 nm, which are corresponded to the crystal lattice face of (111) planes of the PtAuCu alloy NWs. And the lattice spacing of PtAu NWs are demonstrated in Fig. S2. Furthermore, the element distribution and composition of the Pt50Au10Cu40 NWs were employed by high
angle annular dark
field scanning TEM image and energy
dispersive X
ray analyzer (EDS) spectroscopy. As shown in Fig. 1DeG, the corresponding elemental mappings of Pt, Au and Cu indicate that all of the three elements were uniformly distributed along nanowires. The EDS spectroscopy analysis was employed to confirm the existence of Pt, Au and Cu elements of the PtAuCu NWs (Fig. 2A), and the molar ratio of Pt:Au:Cu is 51:10:39, which is close to the addition amount. Pt
base alloy nanocrystals would fuse and recrystallize, even reconstruct under high
temperature conditions, which can lead to the formation of NWs [24,40]. To further investigate the formation process of these nanowires, we have studied the influence of reaction time on the obtained samples. As shown in Fig. S3A, the products were observed as aggregated nanoparticles at the initial stage of the reaction. As the reaction time increases (Fig. S3B
D), the aggregation of metal nanoparticles is reduced, nanoparticles are deformed into nanowires in the direction of alignment, and different nanowires cross each other to form a network structure (Fig. S3D). Unlike seed
induced growth processes, the formation of nanowire is through alignment and attachment mechanisms [41]. Besides, the amine species produced from the reaction play an important role in the formation of nanowires. The presence of amine group is confirmed by the Fourier transform infrared spectroscopy (FT
IR) (Fig. S4), it is clear that there are still a

considerable amount of primary amine groups (1650 cm
1) in the cross
linked network PtAuCu NWs, and the peaks at 1600 cm
1 are indicative to the bending vibration of the amine group [42]. Meanwhile, the highly crystalline and trimetallic structure of Pt50Au10Cu40 alloy were confirmed by X
ray diffraction (XRD). As shown in Fig. 2B, the diffraction peaks of the PtAuCu NWs fall in among the corresponding peak positions of pure fcc structured Pt (JCPDS No. 04
0802), Au (JCPDS
04
0784) and Cu (JCPDS No. 04
0836). The peaks around 38.2 , 44.7, 65.1 and 78.5 are attributed to (111), (200), (220), (311) crystal face of the alloy NWs, respectively [43]. The peak of (111) face displays higher intensity than the others, demonstrating the preferential growth of (111) directions. Besides, there is no single
component peak of pure Pt, Au or Cu in the XRD pattern, which further indicates the formation of alloy PtAuCu NWs. Moreover, the peaks of Pt80Au10Cu10, Pt70Au10Cu20, and Pt50Au10Cu40 NWs gradually shift to lower diffraction angle with the addition of Cu atoms compared with PtAu NWs, attributing to the substitution of Pt with Cu atoms. The results further demonstrate the formation of PtAuCu alloy NWs. X
ray photo electron spectroscopy (XPS) was further employed to explore the electronic states and the surface oxidation compositions. The XPS spectra were calibrated using the C 1s as a standard (284.5 eV), there are six peaks at around 73, 83, 282, 400, 531 and 930 eV are assigned to be Pt, Au, C, N, O and Cu elements, respectively. The peak of N 1s can be indexed to the existence of amine groups. As shown in Fig. 3B, the Pt 4f5/2 and 4f7/2 peaks of Pt0 are found at binding energy (BE) values of 71.14 and 74.45 eV with a spin
orbit splitting of 3.3 eV. Additionally, the peaks at 72.5 eV and 75.8 eV can be corresponded to Pt2þ, which can be assigned to the existence of PtO and Pt (OH)2 species [44,45]. According to the intensity ratio of Pt0/Pt2þ, the valence state of platinum in nanowires is predominantly in the zero
valent state [46]. As for the Au 4f spectrum (Fig. 3C), the BE values at 83.6 and 87.2 eV can be attributed to Au 4f7/2 and Au 4f5/2 of Au0 [47]. Fig. 3D shows the XPS spectra of Cu 2p, the peaks at 931.54 eV and 951.25 eV are assigned to the Cu0, and 933.68 eV and 953.17 eV are correspond to the Cu2þ.

Fig. 1. (A) TEM and (B, C) HRTEM images of Pt50Au10Cu40 nanowire assemblies (D
G) HAADF
STEM images and corresponding elemental mapping (Pt, Au and Cu) in a sample region of the Pt50Au10Cu40 NWs.

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Fig. 2. (A) EDS pattern of prepared Pt50Au10Cu40 NWs and (B) XRD spectra of prepared PtAuCu and PtAu NWs with different ratios.

Fig. 3. XPS spectra of (A) PtAuCu catalysts (B) Pt 4f (C) Au 4f (D) Cu 2p.

The existence of Cu2þ might be ascribed to the formation of CuO, Cu(OH)2 and/or CuOOH [48]. Particularly, there is a small peak at about 943.52 eV indicating the presence of Cu (Ⅱ), which may be attributed to the oxidation of surface Cu atoms in air. As reported, the shift of the binding energy could modify the electronic structure and change the d
band center of Pt atom which is related to the Fermi level [49]. Compared to the standard peaks of pure metal Pt (70.9 and 74.2 eV), Au 4f (84.0 and 87.7 eV), and Cu (about 931.9 and 951.8 eV), the binding energies of Au 4f and Cu 2p shift to lower binding energy, while Pt 4f shifts to higher value, because the electronegativity of Pt is more negative than that of Au and Cu. A charge transfer can occur from Au and Cu to Pt, resulting the decrease of the adsorption of CO on Pt surface and preventing catalysts from being poisoned by carbon intermediates [50,51].

3.2. Electrocatalytic activity of the prepared catalysts The electrochemical surface area (ECSA), which is estimated by integrating the charge passing the electrode during the hydrogen adsorption/desorption process after the correction for the double layer formation, can be calculated on the following equation [52]:

ECSA ¼ SH =ðV  MPt  0:21Þ SH/V represents the charge for electronic transfer during the H
desorption (mC), MPt represents the Pt loading (mg) in the electrode and 0.21 represents the adsorption of a monolayer of hydrogen on Pt (mC cm
2). The ECSA was conducted at a scan rate of 50 mV s
1 from
0.2 to 1.0 V in 0.5 M H2SO4 solution. N2 was

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firstly bubbled into the solution for 20 min to remove the dissolved O2. As shown in Fig. 4A, the PtAuCu alloy NWs exhibit typical hydrogen adsorption/desorption peaks and Pt
oxide reduction/ oxidation on the surfaces, and the peaks mainly locate between
0.2 and 0.15 V. Calculated from the hydrogen adsorption peaks, the ECSA of Pt50Au10Cu40, Pt70Au10Cu20, Pt80Au10Cu10, Pt80Au20 and commercial Pt/C catalyst are 105.62, 68.73, 57.82, 46.39, 70.9 m2 g
1 Pt, respectively (Table 1). Among these, the Pt50Au10Cu40 NWs show the largest ECSA, which is about 1.49 times of the commercial Pt/C catalyst. However, the ECSA of other catalysts (Pt70Au10Cu20, Pt80Au10Cu10, Pt80Au20 NWs) are lower than that of commercial Pt/C catalyst, which may be due to the bundling of the NWs along with increase of Pt4þ concentration [53]. In addition, Pt50Au10Cu40, Pt70Au10Cu20, Pt80Au10Cu10 NWs have a slightly anodic peak current during the first cycle in the range of 0.4e0.7 V (Fig. 4A). This current change stems from the dissolution of Cu atoms from the PtAuCu surface, which is attributed to a dealloying process. As depicted in Fig. S5, the decrease of Pt/Cu atomic ratio confirms the dealloying process of MOR which generates a Pt
enriched surface on the PtAuCu NWs after dealloying [54]. This result is consistent with the previous report [55]. As the dissolution of Cu atoms, the three
dimensional network structure of the catalyst surface remains unchanged, and still provides high specific surface area (Fig. S6). The dissolution of Cu promotes the recombination of Pt and Au nanoparticles, and the increase of Pt active site. To investigate the electrocatalyst activity of different content PtAuCu alloy nanowires, cyclic voltammetry (CV) measurements were conducted in 0.5 M H2SO4 solution containing 1.0 M methanol at a scan rate of 50 mV s
1 from
0.2 to 1.0 V (Fig. 4B). As shown in Fig. 4B, the CV profiles display two obvious regions including oxide

formation and oxide reduction, the peak appeared at about 0.7 V is attributed to the oxidation of methanol, and the backward peak, centered at about 0.4 V corresponds to the elimination of the reaction carbon intermediates (such as CO, CHO, etc.) during MOR [56]. The peak current densities are normalized with the loading mass of the catalysts. It is found that all alloy catalysts exhibit higher current densities than that of commercial Pt/C catalyst, suggesting the enlarged electrochemical performance of PtAuCu alloy NWs for MOR. According to the comparison of peak current densities (Table 1), the Pt50Au10Cu40 has the optimal electrochemical performances. The methanol on the surface of Pt
based catalysts were based on the following reactions (Formulas (1)
(5)). [28. Trimetallic PtAuNi alloy nanoparticles as an efficient electrocatalyst for the methanol electrooxidation reaction] Pt þ CH3OH /Pt(CH3OH)ads

(1)

Pt(CH3OH)ads /Pt(CHO)ads/Pt(CO)ads

(2)

Cu/Au þ H2O/Cu(OH)ads/Au(OH)ads þ Hþþe


(3)

Pt(CHO)ads þ Cu(OH)ads/Au(OH)ads/Pt þ Cu/ Au þ CO2þ2Hþþ2e


(4)

Pt(CO)ads þ Cu(OH)ads/Au(OH)ads/Pt þ Cu/Au þ CO2þHþþe
(5) From the above steps, carbon intermediates such as CO and CHO restrict the catalytic activity of Pt
based catalysts in DMFCs. The peak current ratio of the If to Ib (If and Ib are the forward and backward peak current intensities, respectively) represents the

Fig. 4. Cyclic voltammograms of the Pt50Au10Cu40, Pt70Au10Cu20, Pt80Au10Cu10, Pt80Au20 and commercial Pt/C catalysts modified electrodes in 0.5 M H2SO4 solution (A) without CH3OH and (B) with 1.0 M CH3OH at a scan rate of 50 mV s
1 from
0.2 to 1.0 V. (C) The comparison of the corresponding mass activity of these catalysts (D) The comparison of the corresponding Specific activity of these catalysts.

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Table 1 The electrochemical properties of the electrocatalysts. Alloy Composition

ECSA (m2 g
1)

If/Ib

Current Density (A mg
1)

Mass Activity (A mg
1Pt)

Specific Activity (mA cm
2)

Pt50Au10Cu40 Pt70Au10Cu20 Pt80Au10Cu10 Pt80Au20 Pt/C

105.62 68.73 57.82 46.39 70.91

2.13 1.84 1.53 1.75 1.12

0.464 0.389 0.271 0.238 0.103

0.9281 0.4867 0.3392 0.2985 0.2138

0.8786 0.7081 0.5863 0.6435 0.3016

poisoned effect of Pt
based catalysts to the carbonaceous species. The If/Ib indicates the resistant degree to carbon intermediates and minimum accumulation of carbonaceous species. All PtAuCu alloy NWs exhibit higher If/Ib ratios than that of commercial Pt/C catalyst. As listed in Table 1, the Pt50Au10Cu40 alloy nanowires perform the highest ratio (2.13) among all the catalysts, which is 1.9 times of the commercial Pt/C catalyst (1.12). The ratio of If/Ib is greater than those reported of PtAu and PtCu NWs [60]. This result demonstrates that the incorporation of Cu can resist the CO poisoning during MOR, which is in accordance with the reported literature [20,23,24,48]. Mass activity is another important factor to evaluate the performance of an electrocatalyst. The mass activity was measured by the ratio of oxidation peak density to the mass of platinum in the catalyst. As demonstrated in Fig. 4C, the mass activity of trimetallic PtAuCu alloy nanowires are higher than those of binary PtAu nanowires (0.2985 A mg
1) and commercial Pt/C catalyst (0.2138 A mg
1). Among them, Pt50Au10Cu40 alloy nanowires show the highest mass activity (0.9281 A mg
1), which further confirms the importance of Cu in enhancing the catalytic ability of Pt. Additionally, the electrochemical performances of the as
synthesized PtAuCu NWs are comparable to the published literature. (Table S2). Fig. 5 illustrates the reaction pathway for the conversion of methanol to CO2. In the acidic medium, OHads is obtained by decomposition of water on the Pt electrode, which requires a high potential (over 0.75 V vs. SHE) [57]. Then COads is oxidized to CO2 on the Pt surface. However, this reaction is difficult to occur because of the high applied potential. Thus, the catalysts are easy to be poisoned by CO on the electrode surface, which can prevent the process of MOR. The role of the alloying elements is used to decrease the potential for adsorption and activation of water [58]. As for Pt catalysts, the adsorption of carbon intermediate species on the catalytic sites (e.g., Pt (CHO)ads/Pt (CO)ads) could deactivate the

catalytic activity and the methanol dehydrogenation on the Pt sites. When introducing Au and Cu atoms, the COads intermediate species can be removed from Pt sites to the Au and Cu sites and form COads/ Au and COads/Cu due to the favorable adsorption of CO on Au and Cu nanoparticles. Then the following surface process such as Pt
COads þ Cu
OHads þ Au
OHads and Cu
OHads þ Au
OHads will occur toward finally product CO2 [59]. Furthermore, the current densities of Pt50Au10Cu40 NWs and commercial Pt/C catalyst after 1000 cycles were also investigated to study their stability. After 1000 cycles (Fig. 6A), the current intensity of Pt50Au10Cu40 NWs and commercial Pt/C catalysts were 43.15 mA mg
1 and 4.15 mA mg
1, and they are about 93% and 42.23% of the initial state, respectively. Additionally, the trends of other products are demonstrated in Fig. S7, and Pt50Au10Cu40 NWs show the lowest decrease among all catalysts. These results reveal the excellent stability of Pt50Au10Cu40 NWs. It is no doubt that Pt50Au10Cu40 NWs exhibit best stability to MOR among the referenced catalysts after 1000 cycles. Meanwhile, chronoamperometry (CA) experiments were also carried out to investigated the long
term stabilities of catalysts at 0.55 V for 10000 s in 0.5 M H2SO4 containing 1.0 M CH3OH (Fig. 6B). Compared to commercial Pt/C catalyst, Pt50Au10Cu40 NWs show the highest current density among all the catalysts over a period of 10000 s. Interestingly, Pt80Au20 NWs show a higher current density than Pt80Au10Cu10, in which Au atom plays an important role in reducing the accumulation of CO. This is due to the presence of a filled d
band and the weak chemisorption properties of Au [60]. The existence of Au atom can availably reduce the adsorption of toxic intermediates on the Pt surface, promoting the full activation of hydroxyl radicals (OHads) and the oxidation with intermediate products to form CO2 in Pt during electrooxidation [61]. As a result, the addition of Au could eliminate the CO intermediates more easily than commercial Pt/C catalyst. The results of the above studies on activity and stability indicate

Fig. 5. Schematic illustration of the electrocatalytic oxidation of methanol over the PtAuCu NWs catalysts.

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Fig. 6. (A). The curves of the comparison peak current densities and the ratio with different samples at 1st and 1000th circles; (B). Chronoamperometric curves of the Pt50Au10Cu40, Pt70Au10Cu20, Pt80Au10Cu10, Pt80Au20 and commercial Pt/C catalysts in 0.5 M H2SO4 solution containing 1.0 M CH3OH at 0.55 V.

that Pt50Au10Cu40 NWs possess superior catalytic performance for MOR. The improvement of electrochemical activity can be ascribed to the following factors. First, the three
dimensional nanowire network structures provide a high specific surface active area, which brings about large contact areas. Second, the incorporation of Au and Cu modifies the electronic structure of Pt, which affords more reaction active sites for MOR. As the dissolution of Cu during MOR reduction, a strong lattice compressive stress will generate on the surface of the alloy, and the hydroxyl and peroxy radical groups adsorbed on the catalyst surface will gradually desorb during the CV scanning, which leads to an increase of the catalyst activity. Besides, Au atom can lead the active site of Pt discontinuous, promote the adsorbed OHads and the dehydrogenation of methanol to form CO2, thus it can improve the CO tolerance and avoid the poisoned of catalysts. In summary, these trimetallic PtAuCu alloy NWs exhibit excellent activity and high stability toward MOR. A further study of these factors would be applied to investigate the detail mechanistic of the enhanced electro-catalytic properties of the trimetallic PtAuCu alloy NWs in our ongoing work.

4. Conclusion In conclusion, ultrafine and composition tunable 1D PtAuCu alloy NWs were synthesized by a facile one
pot solvothermal method. Compared to Pt/C catalyst, the incorporation of Au and Cu not only decreases the cost of Pt catalysts, but also enhances the binding energy of Pt in the PtAuCu NWs, which could decrease the CO adsorption energy on Pt and favor the C
H cleavage on Pt surface. Besides, the dissolution of Cu leads to the rearrangement of Pt, Au and Cu atoms, which provides more Pt active sites. The ECSA (105.62 m2 g
1) and mass activity (0.9281 A mg
1) of the Pt50Au10Cu40 are 1.49 times and 4.34 times of commercial Pt/C catalyst in acidic solution, respectively. The synthesized trimetallic PtAuCu alloy NWs exhibit an excellent electrochemical performance. It is mainly due to the unique structure and the heteroatomic interaction, which provide high surface active area and stability. As a result, the trimetallic PtAuCu alloy NWs maybe a promising catalyst for the DMFCs.

Acknowledgment This work was supported by the Foundation of Jiangsu Collaborative Innovation Center of Biomedical Functional Materials.

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2018.12.016.

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