- Email: [email protected]
Enhanced activity and stability of core–shell structured PtRuNix electrocatalysts for direct methanol fuel cells
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 1 ( 2 0 1 6 ) 1 9 3 5 e1 9 4 3
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Enhanced activity and stability of coreeshell structured PtRuNix electrocatalysts for direct methanol fuel cells Yi Cheng a, Pei Kang Shen b, San Ping Jiang a,* a
Fuels and Energy Technology Institute & Department of Chemical Engineering, Curtin University, Perth, WA, 6102, Australia b Collaborative Innovation Center of Sustainable Energy Materials, Guangxi University, Nanning, 530004, China
article info
abstract
Article history:
Core-shell structured PtRuNix nanoparticles (NPs) with Ni-rich core and PtRu-rich shell are
Received 3 September 2015
successfully synthesized on poly(ethyleneimine) functionalized carbon nanotubes (CNTs)
Received in revised form
through successively dealloying and annealing of PtRuNi alloy NPs. The best results are
28 October 2015
obtained after annealing the dealloyed PtRuNi NPs at 450 C, forming a PtRu-rich shell and
Accepted 28 October 2015
Ni-rich core structure with a surface composition of Pt:Ru:Ni ¼ 1.0:1.13:0.24. PtRuNix shows
Available online 21 November 2015
significantly low onset potential and high activity for the methanol oxidation reaction (MOR), achieving a current density of 386.1 A g1Pt at 0.4 V vs Ag/AgCl. This is significantly
Keywords:
higher than 101 A g1Pt measured on PtRuNi before dealloying and annealing treatment and
Direct methanol fuel cells
155 A g1Pt on the conversional Johnson Matthey PtRu/C electrocatalysts. At 0.4 V vs Ag/
PtRuNix electrocatalysts
AgCl, the stable current for the MOR on PtRuNix electrocatalysts is 34.3 A g1Pt after po-
Dealloying
larization for 5000 s, which is significantly higher than 10.2 A g1Pt of PtRuNi and 9 A g1Pt of
Annealing
the conversional PtRu/C. The PtRuNix exhibits significantly improved microstructural sta-
Coreeshell structure
bility under accelerated degradation test. The enhanced activity and stability is most likely
Methanol oxidation reaction
related to the formation of intermetallic PtRu skinned shell and Ni rich core structures. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The PtRu based alloy systems are widely accepted as one of the most promising anodic electrocatalysts for direct methanol fuel cells (DMFCs) [1]. However, further enhancement in the efficiency, durability and structural stability of PtRu electrocatalysts without compromising the performance remains a significant challenge. There are intensive research efforts to manipulate the electrocatalyst structure by optimizing
surface composition with enriched Pt to achieve high activity and stability without the increase in the overall loading of Pt [2e4]. Dealloying of less noble metal can extend the active surface areas and enhance the utilization efficiency of Pt with increased activity [5,6]. For example, Strasser et al. reported that dealloyed PtCu, PtCuCo and PtNi alloys show the significant improvements of activity for oxygen reduction reaction (ORR) as compared to Pt [7,8]. Xu et al. developed a facile route to fabricate three-dimensional bicontinuous nanoporous PtRu alloys by selective etching of Al from ternary PtRuAl alloys,
* Corresponding author. Tel.: þ61 8 9266 9804. E-mail address: [email protected] (S.P. Jiang). http://dx.doi.org/10.1016/j.ijhydene.2015.10.121 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1936
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 1 ( 2 0 1 6 ) 1 9 3 5 e1 9 4 3
and the nanoporous PtRu catalyst exhibits high specific activity as well as low onset potential toward methanol oxidation reaction (MOR) [9]. Thermal annealing can also be used to segregate the precious metals such as Pt to the surface in controlled atmosphere in addition to the removal of residual impurities. Jeon et al. showed that annealing PtRu/C at 200 C in Ar and H2 (5 vol %) mixed gas for 2 h causes the Pt segregation, leading to the enhanced activity for MOR [10]. Chen et al. reported that Pt3Co after dealloying and annealing at 1000 K in vacuum exhibits four time increase in specific activity for ORR [11]. Wang et al. prepared ordered Cu3Pt/C intermetallic electrocatalyst nanoparticles (NPs) by impregnation-reduction method followed by hightemperature treatment and successively dealloying using electrochemical and chemical leaching and reported significantly enhanced activity and stability for ORR [12]. Most recently, we reported the development of coreeshell structured PtRuCox electrocatalysts via successive dealloying and annealing and the PtRuCox NPs showed excellent activity and stability for MOR [13]. Here, CNTs supported PtRuNix alloy electrocatalysts were prepared by successive dealloying and annealing of ternary PtRuNi alloy. The PtRuNix electrocatalysts annealed at 450 C not only show increased activity but also exhibit enhanced stability for MOR. The results demonstrate that successive dealloying and annealing method is effective to fabricate coreeshell structured electrocatalysts for fuel cells.
Experimental Materials Materials used in this study include sulfuric acid (99.5%, Fluka), HNO3 (65%, Fluka), HCl (30 wt %, Fluka), ethanol (SigmaeAldrich), methanol (SigmaeAldrich), multi-walled carbon nanotubes (CNTs, Shenzhen Nano, China), H2PtCl6 (SigmaeAldrich), RuCl3 (SigmaeAldrich), nickel acetylacetonate (Ni(ACAC)2, SigmaeAldrich), ethylene glycol (EG, SigmaeAldrich), Nafion solution (5% in isoproponal and water), poly(ethyleneimine) (PEI, molecular weight ~1300, SigmaeAldrich), 60% PtRu/C (Johnson Matthey, JM). The as-received CNTs were purified as follows: 50 mg CNTs were dispersed in 50 mL HCl acid (30 wt %) before ultrasonicated for 1 h. The dispersion was separated and the sludge was dispersed in a fresh 50 mL HCl acid (30 wt %), followed by stirring overnight. The CNTs pellet was collected and washed by HCl, and then washed by 5 mol L1 HNO3 and DI water for 3 times.
Synthesis of PtRuNix/CNTs The procedure to synthesize PtRuNix catalysts supported on PEI-fucntionalized CNTs via dealloying and annealing is similar to that reported recently [13]. The CNTs were first functionalized by PEI, following the procedure reported elsewhere [14,15]: first, 200 mg CNTs were sonicated in 400 mL Milli-Q water in the presence of 0.5 wt% PEI for 1 h, then the dispersion was stirred overnight, followed by filtration using 0.2 mm nylon membrane and washing to remove the excess PEI. The as-prepared solid was dried in a vacuum oven for 24 h
at 71 C. PEI-functionalized CNTs (100 mg) was ultrasonicated in 150 mL EG solution for 30 min before the addition of approximate amount of H2PtCl6, RuCl3 and Ni(ACAC)2 with Pt:Ru:Ni molar ratio of 1:1:1. The solution was controlled at a pH 6.5 to maintain a weak acidity, and then was bubbled with N2 for 15 min. The beaker was placed in a microwave oven (1000 w) and heated for 4 min, followed by stirring overnight under pH 3e4. The dispersion was filtered and washed for several times using ethanol. The as-synthesized catalysts were denoted as PtRuNi. Dealloying of PtRuNi was carried out by immersing 50 mg of PtRuNi in a 50 mL 1:1 H2NO3 solution for 5 min. During the dealloying process, some Ni will be dissolved, forming PtRuNix. The dealloyed PtRuNix sample was annealed at 200, 300, 350, 400, 450, 500 C, respectively, under argon flow for an hour. The annealed PtRuNix was denoted as PtRuNix-T, in which T is the annealing temperature.
Characterization Morphology and microstructure of PtRuNi, PtRuNix and PtRuNix-T were characterized using a transmission electron microscope (TEM, JEOL3000) operating at 300 kV. The particle size distributions of the PtRuNi, PtRuNix and PtRuNix-T were obtained by measuring 100 randomly chosen particles in the TEM images. The structure was identified with X-ray diffractometer (XRD, Rigaku D/MAX RINT 2500) operated at 40 kV and 30 mA with Cu Ka in the range of 20e90 . The X-ray photoelectron spectroscopy (XPS) measurements were carried out on a XPS apparatus (ESCALAB 250, Thermo-VG Scientific Ltd.) with photon energy of 1450 eV. XPS results were analyzed using the CasaXPS software. The Pt:Ru:Ni ratio of the catalysts was further analyzed by Inductively Coupled Plasma (ICP-OES, IRIS Intrepid XSP,USA). The solution for ICP analysis were prepared by burning PtRuNi catalysts in a thermal gravimetric pan, and the solid was collected and digested in polytetrafluoroethylene (PTFE) digestion tank using microwave dissolver (SINEO, HDS-8G) with 10 mL aqua regia (the procedure was set as: 150 C, 5 min, 180 C, 5 min, 200 C, 10 min, 230 C, 20 min). The electrochemical measurements were conducted in a standard electrochemical cell using a Princeton potentiostat (Versastat3, USA). Electrocatalyst (4 mg) was ultrasonically mixed in 4 mL of Nafion solution (ethanol:Nafion ¼ 9:1) to form a homogeneous ink, followed by dropping certain amount of the catalyst ink onto the surface of a glass carbon electrode (GCE). The diameter of GCE is 4 mm. The Pt loading was 0.01 mg cm2. Pt foil (3.0 cm2) and Ag/AgCl (saturated KCl) electrodes were used as the counter and reference electrodes, respectively. The Ag/AgCl reference electrode was connected with the working electrode via a Luggin capillary. All potentials in the present study were given versus Ag/AgCl reference electrode. The electrocatalytic activity for the MOR was evaluated by cyclic voltammetry (CV) with potential window of 0.2e0.6 V in N2-saturated 0.5H2SO4 þ 1.0 M CH3OH solution at a scan rate of 50 mV s1. The chronoamperometry curves were obtained at 0.4 V vs Ag/AgCl in a N2-saturated 0.5H2SO4 þ 1.0 M CH3OH solution for 5000 s. The structural stability of the catalysts was performed by cyclic voltammetry in a potential window of 0.2-1.0 V in a N2-saturated 0.5 M H2SO4 þ 1.0 M CH3OH solution. For the purpose of
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 1 ( 2 0 1 6 ) 1 9 3 5 e1 9 4 3
comparison, the performance and microstructural stability of conventional JM 60% PtRu/C were also measured under the same test conditions. The CO-stripping was performed in a 0.5 M H2SO4 solution. The H2SO4 solution was bubbled with ultrapure N2 for 15 min, and then CO was adsorbed by flowing 0.5% CO in N2 at a flow rate of 50 mL min1 through the working electrode compartment by keeping the potential at 0.12 V vs Ag/AgCl for 30 min. The gas was switched to N2 for another 30 min to purge out the CO in the solution by keeping at the same potential. The CO stripping was scanned from 0 to 1.0 V at a scan rate of 50 mV s1. The electrochemical active surface area (ESA) was calculated based on the CO-stripping results. ESA was also measured by CV in a N2-saturated 0.5 M H2SO4 solution with a scan rate of 50 mV s1, based on the hydrogen adsorption and desorption peak areas.
Results and discussion TEM, XRD and XPS analysis Fig. 1 is the typical TEM and HRTEM images (inserted) of PtRuNi, PtRuNix, PtRuNix-300 and PtRuNix-450. PtRuNi exhibits elongated nanoparticle morphology with average size around 7.1 ± 1.5 nm and was uniformly supported on the PEI functionalized CNTs (Fig. 1A). The formation of the elongated particles may be due to the interconnected NPs with an average size of 2.8 ± 0.42 nm. HRTEM images of PtRuNi reveal that the lattice spacing of the particles is 2.2 A and 1.9 A, which is corresponding to the Pt (111) and Pt (200) plane, indicating Ru and Ni are incorporated into Pt lattice structure [16]. After dealloying treatment, the average size of PtRuNix elongated NPs is decreased to 6.8 ± 1.2 nm (Fig. 1B), indicating the corrosion of the edges and corners of the particles by chemical dealloying treatment [5]. The atomic plane index of PtRuNix is ~0.23 and 0.2 nm, corresponding to the Pt (111) and Pt (200) planes. Thermal treatment leads to the growth and agglomeration of PtRuNix NPs. The average size of elongated particles is about 7.4 ± 1.5 nm, and the average size of the individual NPs is 2.4 ± 0.15 nm for PtRuNix-300 (Fig. 1C). In the case of PtRuNix-450, the length of the interconnected particle is 10.5 ± 2.4 and average size of individual NPs is 3.8 ± 0.58 nm (Fig. 1D). The particle size and lattice parameters of the electrocatalysts are listed in Table 1. Fig. 2 shows the X-ray diffraction (XRD) patterns of PtRuNi, PtRuNix and PtRuNix-T. The broad peak is resulted from the overlap of Pt(111), Ru(101) and Pt(200). The peak (111) changes from 2Ɵ ¼ 39.5 for PtRuNi to 2Ɵ ¼ 37.9 for PtRuNix after dealloying, indicating the lattice expansion most likely due to the loss of the Ni atoms from the Pt fcc lattice. This is also supported by the increase of lattice diameter from 0.3931 nm for PtRuNi to 0.4033 nm for PtRuNix. The small increase of the peak near Ru (101) and the decrease of the Pt (111) peak intensity are due to the reduced alloy degree and crystallinity for PtRuNix. The intensity of Pt (111) peak increased after the PtRuNix was annealed at 300 and 450 C, indicating the increase of crystallinity. The peak for Pt (111) observed at 2Ɵ ¼ 39.5 for PtRuNix-300 and 2Ɵ ¼ 40.1 for PtRuNix-450 indicates the shrinking of the lattice parameter with the
1937
increase in annealing temperature [3,17,18], consistent with the HRTEM observations. The Ru(101) peak almost disappeared after annealing at 450 C, most likely due to the reconstruction of the PtRuNix NPs and the increase of alloying degree. XPS spectra provide more information on the near surface composition of PtRuNi, PtRuNix and PtRuNix-T [19]. Fig. 3 shows the core-level spectra for Pt 4f, Ru 3p and Ni 2p. The Pt 4f7/2 and Pt 4f5/2 peaks were observed at 71.7 and 75.2 eV for PtRuNi (Fig. 3A). In the case of PtRuNix, Pt 4f7/2 positively shifted by 0.17 eV (71.93 eV) due to the presence of higher oxidation state components (eg. PtO) due to the HNO3 treatment [20]. The binding energy (BE) of Ru3p1/2 and Ru3p3/2 was 463.5 and 485.5 eV for PtRuNi (Fig. 4B), and shifted by 0.6 and 0.9 eV to 464.1 and 486.4 eV, respectively, for PtRuNix since Ru(0) would be oxidized to Ru(II) and Ru(IV) during the dealloying treatment. The Ni2p peaks at 856.2 eV for PtRuNi NPs indicate that the Ni species in the alloy particle are mainly mixture of Ni2þ and Ni3þ [21,22]. The intensity of Ni2p peak drops significantly for PtRuNix, indicating the successful dealloying of the surface Ni atoms in the PtRuNi sample. In the case of PtRuNix-450, Ru 3p1/2 and Ru 3p3/2 at 485.0 and 462.5 eV shifted by 1.5 and 1.0 eV in the negative direction, as compared to PtRuNi, indicating the increase of metallic Ru. Annealing at 450 C could also promote the segregation of NiOx species to the surface, indicated by the observation of a small peak for Ni 3p around 857.6 eV for PtRuNix-450. This phenomenon is consistent with the results reported by Ahmadi et al. for the segregation of Pt0.5Ni0.5 in vacuum [20]. Fig. 4 shows the peak deconvolution of Pt 4f and Ru 3p for PtRuNi, PtRuNix and PtRuNix-T. The metallic Pt(0) decreased from 85.4% for PtRuNi to 80.1% for PtRuNix probably due to the HNO3 treatment, but increased to 86.2% after annealing at 450 C. Similar trend was also observed for Ru(0), Ru(II) and Ru(IV) (hydrate) species. The BE and element distribution are summarized in Table 2. The XPS results show that the PtRuNi alloy exhibits a surface composition of Pt:Ru:Ni ¼ 1:1.38:0.43, very different from the bulk composition of the Pt:Ru:Ni ¼ 1:0.92:0.89 obtained from ICP (Table 2). Strasser et al. studied the photoelectron kinetic energy by changing the incident photon energy and estimated the probing depths of 0.6 nm, 1 nm, 1.5 nm, 1.8 nm and 7 nm at photon energies of 250 eV, 620 eV, 1130 eV, 1480 eV and 8000 eV, respectively [19]. Hence, the different ratio of Pt:Ru:Ni from XPS and ICP results indicates the differences in the surface and bulk compositions. Due to the photon energy of 1450 eV applied for the XPS analysis, the penetration depth would be less than 2 nm in this case. This indicates that the PtRuNi NPs may have a Ru rich alloy surface with the surface content of Ru > Pt > Ni [23,24]. After dealloying treatment, the Pt:Ru:Ni ¼ 1:0.93:0.68 based on ICP analysis, indicating the dissolution of the surface Ni atoms and formation of PtRuNix NPs with x ¼ 0.68. However, based on the XPS analysis, the Pt:Ru:Ni ratio is 1:1.37:0.15 for PtRuNix, indicating the significantly decreased concentration of Ni on the surface. This indicates the formation of a Ru rich PtRu surface with Ni rich core. After annealing the PtRuNix at 450 C, the Pt:Ru:Ni ratio is 1:1.13:0.24, forming a coreeshell structure with a PtRu rich shell with Pt:Ru ratio close to 1:1 and Ni rich core.
1938
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 1 ( 2 0 1 6 ) 1 9 3 5 e1 9 4 3
Fig. 1 e TEM and HRTEM micrographs, histogram and Fast Fourier Transform (FFT) images of A) PtRuNi, B) PtRuNix, C) PtRuNix-300, D) PtRuNix-450.
Electrochemical surface area and CO stripping Fig. 5A shows the cyclic voltammograms of PtRuNi, PtRuNix, and PtRuNix-T measured in a 0.5 M H2SO4 solution. The electrochemical surface area (ESA) of the catalyst was calculated
by the area of the hydrogen adsorption and desorption peaks after correcting the double layer charging current from the CV curves. The ESA was given in Table 1. The ESA of PtRuNix is 70.1 m2 g1Pt, which is higher than 55.9 m2 g1Pt obtained on PtRuNi. After annealing of PtRuNix at 450 C, the ESA is
1939
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 1 ( 2 0 1 6 ) 1 9 3 5 e1 9 4 3
Table 1 e Lattice parameter, electrochemical surface area (ESA), MOR and CO onset potential, current density for MOR at 0.4 V (j0.4) of PtRuNi, PtRuNix and PtRuNix-T.
2.8 2.6 e 2.4 e e 3.8 e
Lattice parameter/nm
0.391 0.4033 0.3954 0.3913 e e 0.3892 e
ESA m2/g Pt Hydrogen adsorption
CO stripping
55.9 70.1 69.2 60.7 e e 61.4 e
64.3 82.6 79.4 64.3 e e 69.15 e
61.4 m2 g1Pt, indicating the minor effect of thermal annealing on the ESA of the catalysts. Fig. 5B is the CO-stripping curves for PtRuNi, PtRuNix and PtRuNix-T electrocatalysts. Sharp peak for CO oxidation is observed at 0.704 V for PtRuNi with the onset potential of 0.476 V, indicating that the CO is strongly absorbed on the alloy surface. Both the onset potential and peak potential were negatively shifted to 0.432 V and 0.630 V for PtRuNix. Annealing the PtRuNix at high temperatures significantly increases the activity of PtRuNix, indicated by a significant down shift of the onset potential for CO oxidation on PtRuNix-300 and PtRuNix-450. In the case of PtRuNix-450, the onset potential is 0.300 V and there is a broad peak between 0.350 and 0.600 V, indicating the high activity of PtRuNix-450 for the CO oxidation. Ochal et al. showed that the CO oxidation on PtRu (1:1) alloy NPs starts at 0.31 V with peak potential at ~0.65 V, while for PtRu coreeshell structure, the onset potential for the CO oxidation is 0.28 V with the peak potential at ~0.4 V [25]. The formation of coreeshell structure results in the down shift of the onset and peak potential for the CO oxidation reaction. The formation of coreeshell structure for PtRuNix-450 with Pt rich outmost layer is also consistent with the XPS data. Based on the COstripping curves, the calculated ESA is 64.3, 82.6, 79.4, 64.3 and 69.2 m2 g1Pt for PtRuNi, PtRuNix, PtRuNix-200, PtRuNix300 and PtRuNix-450 respectively, which is slightly higher than that obtained from the hydrogen adsorption and desorption peak areas.
Onset potential for MOR V
j0.4 for MOR A g1Pt
Onset potential for CO V
0.123 0.08 0.07 0.07 0.07 0.07 0.07 0.07
100 137 161 263 338 343 386 335
0.476 0.432 0.432 0.350 e e 0.30 e
Performance and stability The activity of PtRuNi, PtRuNix, PtRuNix-T and PtRu/C-JM electrocatalysts were measured for the MOR in a N2-saturated 0.5 M H2SO4 þ 1.0 M CH3OH solution and the results are shown in Fig. 6. For the reaction on PtRuNix, the current
A) Pt4f7/2
Pt4f5/2
Intensity/ (a.u.)
PtRuNi PtRuNix PtRuNix-200 PtRuNix-300 PtRuNix-350 PtRuNix-400 PtRuNix-450 PtRuNix-500
NP size nm
PtRuNi -450 PtRuNi -300 PtRuNi PtRuNi
77
76
75
74
73
72
490
PtRuNi -450 PtRuNi -300 PtRuNi PtRuNi
485
480
475
470
Intensity/ (a.u.)
465
Binding Energy/eV
460
Ni2p
PtRuNi -450
Fig. 2 e XRD patterns of PtRuNi, PtRuNix, PtRuNix-200, PtRuNix-300 and PtRuNix-450.
70
Ru3p3/2
Ru3p1/2
C)
880
71
Binding Energy/eV
B) Intensity/ (a.u.)
Electrocatalyst
PtRuNi -300 PtRuNi PtRuNi
875
870
865
860
Binding Energy/eV
855
850
Fig. 3 e XPS core-level spectra for A) Pt 4f, B) Ru 3p and C) Ni 2p.
1940
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 1 ( 2 0 1 6 ) 1 9 3 5 e1 9 4 3
Fig. 4 e Peak deconvolution of A) Pt 4f, B) Ru 3p for PtRuNi, PtRuNix, PtRuNix-300, PtRuNix-450 from the XPS measurement.
density at 0.4 V is 137 A g1Pt, which is higher than 101 A g1Pt measured on PtRuNi, indicating that dealloying can increase the activity of the alloy catalysts. After annealing the PtRuNi NPs at 300 C, the current density for MOR at 0.4 V increases significantly to 263.2 A g1Pt. The highest activity for MOR was obtained by annealing the PtRuNix at 450 C, achieving a current density of 386.1 A g1Pt at 0.4 V. The electrocatalytic activity of PtRuNix-450 is also significantly higher than 155 A g1Pt obtained on commercial JM PtRu/C. On the other hand, further increase in the thermal annealing temperature to 500 C did not lead to the increase of the activity for MOR
(Fig. 6B). The onset potential and current density measured at 0.4 V are summarized in Table 1. Fig. 7 is the chronoamperometry curves recorded in a 0.5 M H2SO4 þ 1.0 M CH3OH solution at a constant potential of 0.4 V for 5000 s. The oxidation current decreases initially due to the poisoning of intermediate species, such as COads, CH3OHads, COOHads, and CHOads during the MOR [26]. PtRuNi and PtRuNix show similar stability, reaching a stable current density of ~10 A g1Pt after 5000 s polarization. In the case of PtRuNix-450, the stable current density after polarization at 0.4 V for 5000 s is 34.3 A g1Pt, which is significantly higher
1941
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 1 ( 2 0 1 6 ) 1 9 3 5 e1 9 4 3
Table 2 e Relative concentration of Pt and Ru species and the Pt/Ru composition ratios of PtRuNi, PtRuNix, PtRuNix-300, and PtRuNix-450 obtained from XPS spectra and ICP analysis. Species
Relative concentration (%)
Pt(0) Pt(II) Pt(IV) Ru(0) Ru(IV) Ru(IV) (hydrate) Binding energy
Pt 4f5/2 Pt 4f7/2 Ru 3p1/2 Ru 3p3/2
Pt:Ru:Ni ratio (XPS) Pt:Ru:Ni ratio (ICP)
PtRuNi
PtRuNix
PtRuNix-300
PtRuNix-450
85.4 7.3 7.3 41.9 33.1 25.0 75.2 71.7 485.9 463.35 1.0:1.38:0.43 1.0:0.92:0.89
80.1 12.8 7.1 40.1 35.3 24.6 75.35 71.93 486.2 464.1 1.0:1.37:0.15 1.0:0.93:0.68
82.2 10.9 6.9 42.6 31.3 26.0 75.3 71.8 485.4 463.1 1.0:1.35:00 1.0:0.93:0.68
86.2 7.0 6.8 44.9 34.9 20.6 75.2 71.8 485.0 462.5 1.0:1.13:0.24 1.0:0.93:0.68
than that of PtRuNi and PtRuNix. On the other hand, the current density for the MOR on the JM PtRu/C decreased significantly and reached 9.8 A g1Pt after polarization at 0.4 V for 5000 s, which is only 28% of PtRuNix-450. This demonstrates that the PtRuNix-450 has superior electrochemical activity and stability for the MOR, consistent with the CO-stripping results. The microstructural stabilities of PtRuNi, PtRuNix-450 and PtRu/C were tested by CV methods within the scan window of 0.2e1.0 V Fig. 8 shows the plots of the forward peak current density as a function of the number of cycles recorded in a 0.5 M H2SO4 þ 1.0 M CH3OH solution at a scan rate of
A)
50 mV s1. In the case of JM PtRu/C, the forward peak current density decreases significantly with the number of cycles and is only 2% of the initial value after 1000 cycles, indicating the poor microstructural stabilities of conventional PtRu/C. The as-synthesized PtRuNi alloy catalysts show significantly better microstructure stability and the reduction in the forward peak current density is 46.5% after 1000 cycles. The stability of PtRuNi is greatly improved through consequently dealloying and annealing treatments, and in the case of PtRuNix-450, the peak current maintains ~75% of the initial value after 1000 cycles, indicating that PtRuNix-450 exhibits high
A)
100
600 PtRuNix-450 PtRu/C-JM PtRuNix
400
j / A g Pt
0
-1
j / A g Pt
50
PtRuNi -450
-100
PtRuNi -300
-1
-50
PtRuNi -200
-150
0
PtRuNi PtRuNi
-200 -0.2
0.0
0.2
0.4
0.6
E/V vs Ag/AgCl
0.8
-200
1.0
B) 300
PtRuNix-450
-1
j / A g Pt
-1
PtRuNix-200 PtRuNix
100
PtRuNi
0.2
0.4
0.6
300 PtRuNi PtRuNix
200
0 100 0.2
0.4
500
100
0 0.0
0.0
400
PtRuNix-300 200
-0.2
E/V vs Ag/AgCl
j / A g Pt
B)
PtRuNi
200
0.6
E/V vs Ag/AgCl
0.8
1.0
Fig. 5 e A) Cyclic voltammograms of PtRuNi, PtRuNix, and PtRuNix-T and B) CO-stripping curves of PtRuNi, PtRuNix, and PtRuNix-T in a N2-saturated 0.5 M H2SO4 solution at a scanning rate of 50 mV s¡1.
PtRuNix-T
200
300
400
500
o
Annealing Temperature/ C Fig. 6 e A) CV curves of PtRuNi, PtRuNix, PtRuNix-450 and PtRu/C-JM in a N2-saturated 0.5 M H2SO4 þ 1.0 M CH3OH solution with the scanning rate of 50 mV s¡1, and B) plot of current density at 0.4 V obtained from CV curves for PtRuNi, PtRuNix, PtRuNix-T.
1942
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 1 ( 2 0 1 6 ) 1 9 3 5 e1 9 4 3
resulting in the formation of PtRu-rich skin and Ni-rich core with a surface composition of 1.0:1.13:0.24. The PtRuNix-450 catalysts exhibit significantly high activity and stability for MOR, as compared to conventional JM PtRu/C electrocatalysts. The PtRuNix-450 electrocatalysts also show significantly better microstructural durability, maintaining 75% of the initial current density after 1000 cycles, substantially higher than 27% observed for the reaction on conventional PtRu/C catalysts.
160 PtRuNix-450 PtRu/C-JM PtRuNix
140
-1
j / A g Pt
120 100
PtRuNi
80 60 40 20 0
0
1000
2000
3000
4000
5000
Time/s Fig. 7 e Chronoamperometry of PtRuNi, PtRuNix PtRuNix450 and PtRu/C-JM measured in a 0.5 M H2SO4 þ 1.0 M CH3OH solution at 0.4 V.
1200
references
PtRuNix-450
600
PtRuNi PtRu/C-JM
400
53%
200 0
This work was supported by the Australian Research Council Discovery Project funding scheme (project number: DP150202044) and the Major International (Regional) Joint Research Project of NNSFC (51210002), China. The authors acknowledge the facilities, and the scientific and technical assistance of the Curtin Microscopy Centre, a facility funded by the University, State and Commonwealth Governments.
75%
800
-1
j / A g Pt
1000
Acknowledgement
2% 0
200
400
600
800
1000
Number of Cycles Fig. 8 e Plots of the forward peak current density for MOR on PtRuNi, PtRuNix-450 and PtRu/C-JM electrocatalysts as a function of the number of cycles recorded in a 0.5 M H2SO4 þ 1.0 M CH3OH solution at scan rate of 50 mV s¡1.
microstructural stability for MOR. The significant enhancement of microstructural stability of PtRuNix-450 electrocatalysts is likely related to PtRu-rich shell (with increased alloy degree) with Pt rich outmost layer which would protect Ru and Ni from dissolution during continuous cycling test [27]. The present study is consistent with that reported in the literature [2,3]. The enhanced interaction of PtRu NPs with CNTs would also contribute to the superior stability of PtRuNi and PtRuNix-450, as compared with PtRu/C [28].
Conclusions PtRuNi alloy NPs homogenously supported on CNTs were synthesized through in situ reduction of self-assembled Pt, Ru and Ni onto PEI functionalized CNTs surface by microwave irradiation, forming a Ru rich surface with a composition of 1.0:1.38:0.43 as showing by the XPS analysis. Subsequent dealloying and thermal annealing at 450 C led to the segregation of Pt and the reconstruction of the PtRuNix NPs,
[1] Chen A, Holt-Hindle P. Platinum-based nanostructured materials: synthesis, properties, and applications. Chem Rev 2010;110:3767e804. [2] Wang JX, Inada H, Wu L, Zhu Y, Choi Y, Liu P, et al. Oxygen reduction on well-defined core-shell nanocatalysts: particle size, facet, and Pt shell thickness effects. J Am Chem Soc 2009;131:17298e302. [3] Wang C, Chi M, Li D, Strmcnik D, van der Vliet D, Wang G, et al. Design and synthesis of bimetallic electrocatalyst with multilayered Pt-skin surfaces. J Am Chem Soc 2011;133:14396e403. [4] Yuan WY, Lu SF, Xiang Y, Jiang SP. Pt-based nanoparticles on non-covalent functionalized carbon nanotubes as effective electrocatalysts for proton exchange membrane fuel cells. RSC Adv 2014;4:46265e84. [5] Oezaslan M, Heggen M, Strasser P. Size-dependent morphology of dealloyed bimetallic catalysts: linking the nano to the macro scale. J Am Chem Soc 2011;134:514e24. [6] Mani P, Srivastava R, Strasser P. Dealloyed PtCu coreshell nanoparticle electrocatalysts for use in PEM fuel cell cathodes. J Phys Chem C 2008;112:2770e8. [7] Gan L, Heggen M, Rudi S, Strasser P. Coreeshell compositional Fine structures of dealloyed PtxNi1ex nanoparticles and their impact on oxygen reduction catalysis. Nano Lett 2012;12:5423e30. [8] Srivastava R, Mani P, Hahn N, Strasser P. Efficient oxygen reduction fuel cell electrocatalysis on voltammetrically dealloyed PteCueCo nanoparticles. Angew Chem Int Ed 2007;46:8988e91. [9] Xu C, Wang L, Mu X, Ding Y. Nanoporous PtRu alloys for electrocatalysis. Langmuir 2010;26:7437e43. [10] Jeon T-Y, Lee K-S, Yoo SJ, Cho Y-H, Kang SH, Sung Y-E. Effect of surface segregation on the methanol oxidation reaction in carbon-supported PtRu alloy nanoparticles. Langmuir 2010;26:9123e9. [11] Chen S, Ferreira PJ, Sheng W, Yabuuchi N, Allard LF, ShaoHorn Y. Enhanced activity for oxygen reduction reaction on “Pt3Co” nanoparticles: direct evidence of percolated and sandwich-Segregation structures. J Am Chem Soc 2008;130:13818e9.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 1 ( 2 0 1 6 ) 1 9 3 5 e1 9 4 3
[12] Wang D, Yu Y, Xin HL, Hovden R, Ercius P, Mundy JA, et al. Tuning oxygen reduction reaction activity via controllable dealloying: a model study of ordered Cu 3Pt/C intermetallic nanocatalysts. Nano Lett 2012;12:5230e8. [13] Cheng Y, Shen PK, Saunders M, Jiang SP. Core-shell structured PtRuCox nanoparticles on carbon nanotubes as highly active and durable electrocatalysts for direct methanol fuel cells. Electrochim Acta 2015;177:217e26. [14] Cheng Y, Jiang SP. Highly effective and CO-tolerant PtRu electrocatalysts supported on poly(ethyleneimine) functionalized carbon nanotubes for direct methanol fuel cells. Electrochim Acta 2013;99:124e32. [15] Cheng Y, Xu C, Shen PK, Jiang SP. Effect of nitrogencontaining functionalization on the electrocatalytic activity of PtRu nanoparticles supported on carbon nanotubes for direct methanol fuel cells. Appl Catal B Environ 2014;158e159:140e9. [16] Park KW, Choi JH, Ahn KS, Sung YE. PtRu alloy and PtRu-WO3 nanocomposite electrodes for methanol electrooxidation fabricated by a sputtering deposition method. J Phys Chem B 2004;108:5989e94. [17] Wanjala BN, Fang B, Luo J, Chen Y, Yin J, Engelhard MH, et al. Correlation between atomic coordination structure and enhanced electrocatalytic activity for trimetallic alloy catalysts. J Am Chem Soc 2011;133:12714e27. [18] Tsypkin M, de la Fuente JLG, Garcı´a Rodrı´guez S, Yu Y, Ochal P, Seland F, et al. Effect of heat treatment on the electrocatalytic properties of nano-structured Ru cores with Pt shells. J Electroanal Chem 2013;704:57e66. [19] Strasser P, Koh S, Anniyev T, Greeley J, More K, Yu C, et al. Lattice-strain control of the activity in dealloyed coreeshell fuel cell catalysts. Nat Chem 2010;2:454e60.
1943
[20] Ahmadi M, Behafarid F, Cui CH, Strasser P, Cuenya BR. Longrange segregation phenomena in shape-selected bimetallic nanoparticles: chemical state effects. Acs Nano 2013;7:9195e204. [21] Grosvenor AP, Biesinger MC, Smart RSC, McIntyre NS. New interpretations of XPS spectra of nickel metal and oxides. Surf Sci 2006;600:1771e9. [22] Hamdan MS, Riyanto, Othman MR. Preparation and characterization of nano size NiOOH by direct electrochemical oxidation of nickel Plate. Int J Electrochem Sci 2013;8:4747e60. [23] Bensebaa F, Patrito N, Le Page Y, L'Ecuyer P, Wang D. Tunable platinum-ruthenium nanoparticle properties using microwave synthesis. J Mater Chem 2004;14:3378e84. [24] Park KC, Jang IY, Wongwiriyapan W, Morimoto S, Kim YJ, Jung YC, et al. Carbon-supported PteRu nanoparticles prepared in glyoxylate-reduction system promoting precursor-support interaction. J Mater Chem 2010;20:5345e54. [25] Ochal P, de la Fuente JLG, Tsypkin M, Seland F, Sunde S, Muthuswamy N, et al. CO stripping as an electrochemical tool for characterization of Ru@Pt core-shell catalysts. J Electroanal Chem 2011;655:140e6. [26] Ferrin P, Mavrikakis M. Structure sensitivity of methanol electrooxidation on transition metals. J Am Chem Soc 2009;131:14381e9. [27] Piela P, Eickes C, Brosha E, Garzon F, Zelenay P. Ruthenium crossover in direct methanol fuel cell with PteRu black anode. J Electrochem Soc 2004;151:A2053e9. [28] Zhou J, Zhou X, Sun X, Li R, Murphy M, Ding Z, et al. Interaction between Pt nanoparticles and carbon nanotubes e An X-ray absorption near edge structures (XANES) study. Chem Phys Lett 2007;437:229e32.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Enhanced activity and stability of coreeshell structured PtRuNix electrocatalysts for direct methanol fuel cells Yi Cheng a, Pei Kang Shen b, San Ping Jiang a,* a
Fuels and Energy Technology Institute & Department of Chemical Engineering, Curtin University, Perth, WA, 6102, Australia b Collaborative Innovation Center of Sustainable Energy Materials, Guangxi University, Nanning, 530004, China
article info
abstract
Article history:
Core-shell structured PtRuNix nanoparticles (NPs) with Ni-rich core and PtRu-rich shell are
Received 3 September 2015
successfully synthesized on poly(ethyleneimine) functionalized carbon nanotubes (CNTs)
Received in revised form
through successively dealloying and annealing of PtRuNi alloy NPs. The best results are
28 October 2015
obtained after annealing the dealloyed PtRuNi NPs at 450 C, forming a PtRu-rich shell and
Accepted 28 October 2015
Ni-rich core structure with a surface composition of Pt:Ru:Ni ¼ 1.0:1.13:0.24. PtRuNix shows
Available online 21 November 2015
significantly low onset potential and high activity for the methanol oxidation reaction (MOR), achieving a current density of 386.1 A g1Pt at 0.4 V vs Ag/AgCl. This is significantly
Keywords:
higher than 101 A g1Pt measured on PtRuNi before dealloying and annealing treatment and
Direct methanol fuel cells
155 A g1Pt on the conversional Johnson Matthey PtRu/C electrocatalysts. At 0.4 V vs Ag/
PtRuNix electrocatalysts
AgCl, the stable current for the MOR on PtRuNix electrocatalysts is 34.3 A g1Pt after po-
Dealloying
larization for 5000 s, which is significantly higher than 10.2 A g1Pt of PtRuNi and 9 A g1Pt of
Annealing
the conversional PtRu/C. The PtRuNix exhibits significantly improved microstructural sta-
Coreeshell structure
bility under accelerated degradation test. The enhanced activity and stability is most likely
Methanol oxidation reaction
related to the formation of intermetallic PtRu skinned shell and Ni rich core structures. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The PtRu based alloy systems are widely accepted as one of the most promising anodic electrocatalysts for direct methanol fuel cells (DMFCs) [1]. However, further enhancement in the efficiency, durability and structural stability of PtRu electrocatalysts without compromising the performance remains a significant challenge. There are intensive research efforts to manipulate the electrocatalyst structure by optimizing
surface composition with enriched Pt to achieve high activity and stability without the increase in the overall loading of Pt [2e4]. Dealloying of less noble metal can extend the active surface areas and enhance the utilization efficiency of Pt with increased activity [5,6]. For example, Strasser et al. reported that dealloyed PtCu, PtCuCo and PtNi alloys show the significant improvements of activity for oxygen reduction reaction (ORR) as compared to Pt [7,8]. Xu et al. developed a facile route to fabricate three-dimensional bicontinuous nanoporous PtRu alloys by selective etching of Al from ternary PtRuAl alloys,
* Corresponding author. Tel.: þ61 8 9266 9804. E-mail address: [email protected] (S.P. Jiang). http://dx.doi.org/10.1016/j.ijhydene.2015.10.121 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1936
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 1 ( 2 0 1 6 ) 1 9 3 5 e1 9 4 3
and the nanoporous PtRu catalyst exhibits high specific activity as well as low onset potential toward methanol oxidation reaction (MOR) [9]. Thermal annealing can also be used to segregate the precious metals such as Pt to the surface in controlled atmosphere in addition to the removal of residual impurities. Jeon et al. showed that annealing PtRu/C at 200 C in Ar and H2 (5 vol %) mixed gas for 2 h causes the Pt segregation, leading to the enhanced activity for MOR [10]. Chen et al. reported that Pt3Co after dealloying and annealing at 1000 K in vacuum exhibits four time increase in specific activity for ORR [11]. Wang et al. prepared ordered Cu3Pt/C intermetallic electrocatalyst nanoparticles (NPs) by impregnation-reduction method followed by hightemperature treatment and successively dealloying using electrochemical and chemical leaching and reported significantly enhanced activity and stability for ORR [12]. Most recently, we reported the development of coreeshell structured PtRuCox electrocatalysts via successive dealloying and annealing and the PtRuCox NPs showed excellent activity and stability for MOR [13]. Here, CNTs supported PtRuNix alloy electrocatalysts were prepared by successive dealloying and annealing of ternary PtRuNi alloy. The PtRuNix electrocatalysts annealed at 450 C not only show increased activity but also exhibit enhanced stability for MOR. The results demonstrate that successive dealloying and annealing method is effective to fabricate coreeshell structured electrocatalysts for fuel cells.
Experimental Materials Materials used in this study include sulfuric acid (99.5%, Fluka), HNO3 (65%, Fluka), HCl (30 wt %, Fluka), ethanol (SigmaeAldrich), methanol (SigmaeAldrich), multi-walled carbon nanotubes (CNTs, Shenzhen Nano, China), H2PtCl6 (SigmaeAldrich), RuCl3 (SigmaeAldrich), nickel acetylacetonate (Ni(ACAC)2, SigmaeAldrich), ethylene glycol (EG, SigmaeAldrich), Nafion solution (5% in isoproponal and water), poly(ethyleneimine) (PEI, molecular weight ~1300, SigmaeAldrich), 60% PtRu/C (Johnson Matthey, JM). The as-received CNTs were purified as follows: 50 mg CNTs were dispersed in 50 mL HCl acid (30 wt %) before ultrasonicated for 1 h. The dispersion was separated and the sludge was dispersed in a fresh 50 mL HCl acid (30 wt %), followed by stirring overnight. The CNTs pellet was collected and washed by HCl, and then washed by 5 mol L1 HNO3 and DI water for 3 times.
Synthesis of PtRuNix/CNTs The procedure to synthesize PtRuNix catalysts supported on PEI-fucntionalized CNTs via dealloying and annealing is similar to that reported recently [13]. The CNTs were first functionalized by PEI, following the procedure reported elsewhere [14,15]: first, 200 mg CNTs were sonicated in 400 mL Milli-Q water in the presence of 0.5 wt% PEI for 1 h, then the dispersion was stirred overnight, followed by filtration using 0.2 mm nylon membrane and washing to remove the excess PEI. The as-prepared solid was dried in a vacuum oven for 24 h
at 71 C. PEI-functionalized CNTs (100 mg) was ultrasonicated in 150 mL EG solution for 30 min before the addition of approximate amount of H2PtCl6, RuCl3 and Ni(ACAC)2 with Pt:Ru:Ni molar ratio of 1:1:1. The solution was controlled at a pH 6.5 to maintain a weak acidity, and then was bubbled with N2 for 15 min. The beaker was placed in a microwave oven (1000 w) and heated for 4 min, followed by stirring overnight under pH 3e4. The dispersion was filtered and washed for several times using ethanol. The as-synthesized catalysts were denoted as PtRuNi. Dealloying of PtRuNi was carried out by immersing 50 mg of PtRuNi in a 50 mL 1:1 H2NO3 solution for 5 min. During the dealloying process, some Ni will be dissolved, forming PtRuNix. The dealloyed PtRuNix sample was annealed at 200, 300, 350, 400, 450, 500 C, respectively, under argon flow for an hour. The annealed PtRuNix was denoted as PtRuNix-T, in which T is the annealing temperature.
Characterization Morphology and microstructure of PtRuNi, PtRuNix and PtRuNix-T were characterized using a transmission electron microscope (TEM, JEOL3000) operating at 300 kV. The particle size distributions of the PtRuNi, PtRuNix and PtRuNix-T were obtained by measuring 100 randomly chosen particles in the TEM images. The structure was identified with X-ray diffractometer (XRD, Rigaku D/MAX RINT 2500) operated at 40 kV and 30 mA with Cu Ka in the range of 20e90 . The X-ray photoelectron spectroscopy (XPS) measurements were carried out on a XPS apparatus (ESCALAB 250, Thermo-VG Scientific Ltd.) with photon energy of 1450 eV. XPS results were analyzed using the CasaXPS software. The Pt:Ru:Ni ratio of the catalysts was further analyzed by Inductively Coupled Plasma (ICP-OES, IRIS Intrepid XSP,USA). The solution for ICP analysis were prepared by burning PtRuNi catalysts in a thermal gravimetric pan, and the solid was collected and digested in polytetrafluoroethylene (PTFE) digestion tank using microwave dissolver (SINEO, HDS-8G) with 10 mL aqua regia (the procedure was set as: 150 C, 5 min, 180 C, 5 min, 200 C, 10 min, 230 C, 20 min). The electrochemical measurements were conducted in a standard electrochemical cell using a Princeton potentiostat (Versastat3, USA). Electrocatalyst (4 mg) was ultrasonically mixed in 4 mL of Nafion solution (ethanol:Nafion ¼ 9:1) to form a homogeneous ink, followed by dropping certain amount of the catalyst ink onto the surface of a glass carbon electrode (GCE). The diameter of GCE is 4 mm. The Pt loading was 0.01 mg cm2. Pt foil (3.0 cm2) and Ag/AgCl (saturated KCl) electrodes were used as the counter and reference electrodes, respectively. The Ag/AgCl reference electrode was connected with the working electrode via a Luggin capillary. All potentials in the present study were given versus Ag/AgCl reference electrode. The electrocatalytic activity for the MOR was evaluated by cyclic voltammetry (CV) with potential window of 0.2e0.6 V in N2-saturated 0.5H2SO4 þ 1.0 M CH3OH solution at a scan rate of 50 mV s1. The chronoamperometry curves were obtained at 0.4 V vs Ag/AgCl in a N2-saturated 0.5H2SO4 þ 1.0 M CH3OH solution for 5000 s. The structural stability of the catalysts was performed by cyclic voltammetry in a potential window of 0.2-1.0 V in a N2-saturated 0.5 M H2SO4 þ 1.0 M CH3OH solution. For the purpose of
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 1 ( 2 0 1 6 ) 1 9 3 5 e1 9 4 3
comparison, the performance and microstructural stability of conventional JM 60% PtRu/C were also measured under the same test conditions. The CO-stripping was performed in a 0.5 M H2SO4 solution. The H2SO4 solution was bubbled with ultrapure N2 for 15 min, and then CO was adsorbed by flowing 0.5% CO in N2 at a flow rate of 50 mL min1 through the working electrode compartment by keeping the potential at 0.12 V vs Ag/AgCl for 30 min. The gas was switched to N2 for another 30 min to purge out the CO in the solution by keeping at the same potential. The CO stripping was scanned from 0 to 1.0 V at a scan rate of 50 mV s1. The electrochemical active surface area (ESA) was calculated based on the CO-stripping results. ESA was also measured by CV in a N2-saturated 0.5 M H2SO4 solution with a scan rate of 50 mV s1, based on the hydrogen adsorption and desorption peak areas.
Results and discussion TEM, XRD and XPS analysis Fig. 1 is the typical TEM and HRTEM images (inserted) of PtRuNi, PtRuNix, PtRuNix-300 and PtRuNix-450. PtRuNi exhibits elongated nanoparticle morphology with average size around 7.1 ± 1.5 nm and was uniformly supported on the PEI functionalized CNTs (Fig. 1A). The formation of the elongated particles may be due to the interconnected NPs with an average size of 2.8 ± 0.42 nm. HRTEM images of PtRuNi reveal that the lattice spacing of the particles is 2.2 A and 1.9 A, which is corresponding to the Pt (111) and Pt (200) plane, indicating Ru and Ni are incorporated into Pt lattice structure [16]. After dealloying treatment, the average size of PtRuNix elongated NPs is decreased to 6.8 ± 1.2 nm (Fig. 1B), indicating the corrosion of the edges and corners of the particles by chemical dealloying treatment [5]. The atomic plane index of PtRuNix is ~0.23 and 0.2 nm, corresponding to the Pt (111) and Pt (200) planes. Thermal treatment leads to the growth and agglomeration of PtRuNix NPs. The average size of elongated particles is about 7.4 ± 1.5 nm, and the average size of the individual NPs is 2.4 ± 0.15 nm for PtRuNix-300 (Fig. 1C). In the case of PtRuNix-450, the length of the interconnected particle is 10.5 ± 2.4 and average size of individual NPs is 3.8 ± 0.58 nm (Fig. 1D). The particle size and lattice parameters of the electrocatalysts are listed in Table 1. Fig. 2 shows the X-ray diffraction (XRD) patterns of PtRuNi, PtRuNix and PtRuNix-T. The broad peak is resulted from the overlap of Pt(111), Ru(101) and Pt(200). The peak (111) changes from 2Ɵ ¼ 39.5 for PtRuNi to 2Ɵ ¼ 37.9 for PtRuNix after dealloying, indicating the lattice expansion most likely due to the loss of the Ni atoms from the Pt fcc lattice. This is also supported by the increase of lattice diameter from 0.3931 nm for PtRuNi to 0.4033 nm for PtRuNix. The small increase of the peak near Ru (101) and the decrease of the Pt (111) peak intensity are due to the reduced alloy degree and crystallinity for PtRuNix. The intensity of Pt (111) peak increased after the PtRuNix was annealed at 300 and 450 C, indicating the increase of crystallinity. The peak for Pt (111) observed at 2Ɵ ¼ 39.5 for PtRuNix-300 and 2Ɵ ¼ 40.1 for PtRuNix-450 indicates the shrinking of the lattice parameter with the
1937
increase in annealing temperature [3,17,18], consistent with the HRTEM observations. The Ru(101) peak almost disappeared after annealing at 450 C, most likely due to the reconstruction of the PtRuNix NPs and the increase of alloying degree. XPS spectra provide more information on the near surface composition of PtRuNi, PtRuNix and PtRuNix-T [19]. Fig. 3 shows the core-level spectra for Pt 4f, Ru 3p and Ni 2p. The Pt 4f7/2 and Pt 4f5/2 peaks were observed at 71.7 and 75.2 eV for PtRuNi (Fig. 3A). In the case of PtRuNix, Pt 4f7/2 positively shifted by 0.17 eV (71.93 eV) due to the presence of higher oxidation state components (eg. PtO) due to the HNO3 treatment [20]. The binding energy (BE) of Ru3p1/2 and Ru3p3/2 was 463.5 and 485.5 eV for PtRuNi (Fig. 4B), and shifted by 0.6 and 0.9 eV to 464.1 and 486.4 eV, respectively, for PtRuNix since Ru(0) would be oxidized to Ru(II) and Ru(IV) during the dealloying treatment. The Ni2p peaks at 856.2 eV for PtRuNi NPs indicate that the Ni species in the alloy particle are mainly mixture of Ni2þ and Ni3þ [21,22]. The intensity of Ni2p peak drops significantly for PtRuNix, indicating the successful dealloying of the surface Ni atoms in the PtRuNi sample. In the case of PtRuNix-450, Ru 3p1/2 and Ru 3p3/2 at 485.0 and 462.5 eV shifted by 1.5 and 1.0 eV in the negative direction, as compared to PtRuNi, indicating the increase of metallic Ru. Annealing at 450 C could also promote the segregation of NiOx species to the surface, indicated by the observation of a small peak for Ni 3p around 857.6 eV for PtRuNix-450. This phenomenon is consistent with the results reported by Ahmadi et al. for the segregation of Pt0.5Ni0.5 in vacuum [20]. Fig. 4 shows the peak deconvolution of Pt 4f and Ru 3p for PtRuNi, PtRuNix and PtRuNix-T. The metallic Pt(0) decreased from 85.4% for PtRuNi to 80.1% for PtRuNix probably due to the HNO3 treatment, but increased to 86.2% after annealing at 450 C. Similar trend was also observed for Ru(0), Ru(II) and Ru(IV) (hydrate) species. The BE and element distribution are summarized in Table 2. The XPS results show that the PtRuNi alloy exhibits a surface composition of Pt:Ru:Ni ¼ 1:1.38:0.43, very different from the bulk composition of the Pt:Ru:Ni ¼ 1:0.92:0.89 obtained from ICP (Table 2). Strasser et al. studied the photoelectron kinetic energy by changing the incident photon energy and estimated the probing depths of 0.6 nm, 1 nm, 1.5 nm, 1.8 nm and 7 nm at photon energies of 250 eV, 620 eV, 1130 eV, 1480 eV and 8000 eV, respectively [19]. Hence, the different ratio of Pt:Ru:Ni from XPS and ICP results indicates the differences in the surface and bulk compositions. Due to the photon energy of 1450 eV applied for the XPS analysis, the penetration depth would be less than 2 nm in this case. This indicates that the PtRuNi NPs may have a Ru rich alloy surface with the surface content of Ru > Pt > Ni [23,24]. After dealloying treatment, the Pt:Ru:Ni ¼ 1:0.93:0.68 based on ICP analysis, indicating the dissolution of the surface Ni atoms and formation of PtRuNix NPs with x ¼ 0.68. However, based on the XPS analysis, the Pt:Ru:Ni ratio is 1:1.37:0.15 for PtRuNix, indicating the significantly decreased concentration of Ni on the surface. This indicates the formation of a Ru rich PtRu surface with Ni rich core. After annealing the PtRuNix at 450 C, the Pt:Ru:Ni ratio is 1:1.13:0.24, forming a coreeshell structure with a PtRu rich shell with Pt:Ru ratio close to 1:1 and Ni rich core.
1938
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 1 ( 2 0 1 6 ) 1 9 3 5 e1 9 4 3
Fig. 1 e TEM and HRTEM micrographs, histogram and Fast Fourier Transform (FFT) images of A) PtRuNi, B) PtRuNix, C) PtRuNix-300, D) PtRuNix-450.
Electrochemical surface area and CO stripping Fig. 5A shows the cyclic voltammograms of PtRuNi, PtRuNix, and PtRuNix-T measured in a 0.5 M H2SO4 solution. The electrochemical surface area (ESA) of the catalyst was calculated
by the area of the hydrogen adsorption and desorption peaks after correcting the double layer charging current from the CV curves. The ESA was given in Table 1. The ESA of PtRuNix is 70.1 m2 g1Pt, which is higher than 55.9 m2 g1Pt obtained on PtRuNi. After annealing of PtRuNix at 450 C, the ESA is
1939
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 1 ( 2 0 1 6 ) 1 9 3 5 e1 9 4 3
Table 1 e Lattice parameter, electrochemical surface area (ESA), MOR and CO onset potential, current density for MOR at 0.4 V (j0.4) of PtRuNi, PtRuNix and PtRuNix-T.
2.8 2.6 e 2.4 e e 3.8 e
Lattice parameter/nm
0.391 0.4033 0.3954 0.3913 e e 0.3892 e
ESA m2/g Pt Hydrogen adsorption
CO stripping
55.9 70.1 69.2 60.7 e e 61.4 e
64.3 82.6 79.4 64.3 e e 69.15 e
61.4 m2 g1Pt, indicating the minor effect of thermal annealing on the ESA of the catalysts. Fig. 5B is the CO-stripping curves for PtRuNi, PtRuNix and PtRuNix-T electrocatalysts. Sharp peak for CO oxidation is observed at 0.704 V for PtRuNi with the onset potential of 0.476 V, indicating that the CO is strongly absorbed on the alloy surface. Both the onset potential and peak potential were negatively shifted to 0.432 V and 0.630 V for PtRuNix. Annealing the PtRuNix at high temperatures significantly increases the activity of PtRuNix, indicated by a significant down shift of the onset potential for CO oxidation on PtRuNix-300 and PtRuNix-450. In the case of PtRuNix-450, the onset potential is 0.300 V and there is a broad peak between 0.350 and 0.600 V, indicating the high activity of PtRuNix-450 for the CO oxidation. Ochal et al. showed that the CO oxidation on PtRu (1:1) alloy NPs starts at 0.31 V with peak potential at ~0.65 V, while for PtRu coreeshell structure, the onset potential for the CO oxidation is 0.28 V with the peak potential at ~0.4 V [25]. The formation of coreeshell structure results in the down shift of the onset and peak potential for the CO oxidation reaction. The formation of coreeshell structure for PtRuNix-450 with Pt rich outmost layer is also consistent with the XPS data. Based on the COstripping curves, the calculated ESA is 64.3, 82.6, 79.4, 64.3 and 69.2 m2 g1Pt for PtRuNi, PtRuNix, PtRuNix-200, PtRuNix300 and PtRuNix-450 respectively, which is slightly higher than that obtained from the hydrogen adsorption and desorption peak areas.
Onset potential for MOR V
j0.4 for MOR A g1Pt
Onset potential for CO V
0.123 0.08 0.07 0.07 0.07 0.07 0.07 0.07
100 137 161 263 338 343 386 335
0.476 0.432 0.432 0.350 e e 0.30 e
Performance and stability The activity of PtRuNi, PtRuNix, PtRuNix-T and PtRu/C-JM electrocatalysts were measured for the MOR in a N2-saturated 0.5 M H2SO4 þ 1.0 M CH3OH solution and the results are shown in Fig. 6. For the reaction on PtRuNix, the current
A) Pt4f7/2
Pt4f5/2
Intensity/ (a.u.)
PtRuNi PtRuNix PtRuNix-200 PtRuNix-300 PtRuNix-350 PtRuNix-400 PtRuNix-450 PtRuNix-500
NP size nm
PtRuNi -450 PtRuNi -300 PtRuNi PtRuNi
77
76
75
74
73
72
490
PtRuNi -450 PtRuNi -300 PtRuNi PtRuNi
485
480
475
470
Intensity/ (a.u.)
465
Binding Energy/eV
460
Ni2p
PtRuNi -450
Fig. 2 e XRD patterns of PtRuNi, PtRuNix, PtRuNix-200, PtRuNix-300 and PtRuNix-450.
70
Ru3p3/2
Ru3p1/2
C)
880
71
Binding Energy/eV
B) Intensity/ (a.u.)
Electrocatalyst
PtRuNi -300 PtRuNi PtRuNi
875
870
865
860
Binding Energy/eV
855
850
Fig. 3 e XPS core-level spectra for A) Pt 4f, B) Ru 3p and C) Ni 2p.
1940
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 1 ( 2 0 1 6 ) 1 9 3 5 e1 9 4 3
Fig. 4 e Peak deconvolution of A) Pt 4f, B) Ru 3p for PtRuNi, PtRuNix, PtRuNix-300, PtRuNix-450 from the XPS measurement.
density at 0.4 V is 137 A g1Pt, which is higher than 101 A g1Pt measured on PtRuNi, indicating that dealloying can increase the activity of the alloy catalysts. After annealing the PtRuNi NPs at 300 C, the current density for MOR at 0.4 V increases significantly to 263.2 A g1Pt. The highest activity for MOR was obtained by annealing the PtRuNix at 450 C, achieving a current density of 386.1 A g1Pt at 0.4 V. The electrocatalytic activity of PtRuNix-450 is also significantly higher than 155 A g1Pt obtained on commercial JM PtRu/C. On the other hand, further increase in the thermal annealing temperature to 500 C did not lead to the increase of the activity for MOR
(Fig. 6B). The onset potential and current density measured at 0.4 V are summarized in Table 1. Fig. 7 is the chronoamperometry curves recorded in a 0.5 M H2SO4 þ 1.0 M CH3OH solution at a constant potential of 0.4 V for 5000 s. The oxidation current decreases initially due to the poisoning of intermediate species, such as COads, CH3OHads, COOHads, and CHOads during the MOR [26]. PtRuNi and PtRuNix show similar stability, reaching a stable current density of ~10 A g1Pt after 5000 s polarization. In the case of PtRuNix-450, the stable current density after polarization at 0.4 V for 5000 s is 34.3 A g1Pt, which is significantly higher
1941
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 1 ( 2 0 1 6 ) 1 9 3 5 e1 9 4 3
Table 2 e Relative concentration of Pt and Ru species and the Pt/Ru composition ratios of PtRuNi, PtRuNix, PtRuNix-300, and PtRuNix-450 obtained from XPS spectra and ICP analysis. Species
Relative concentration (%)
Pt(0) Pt(II) Pt(IV) Ru(0) Ru(IV) Ru(IV) (hydrate) Binding energy
Pt 4f5/2 Pt 4f7/2 Ru 3p1/2 Ru 3p3/2
Pt:Ru:Ni ratio (XPS) Pt:Ru:Ni ratio (ICP)
PtRuNi
PtRuNix
PtRuNix-300
PtRuNix-450
85.4 7.3 7.3 41.9 33.1 25.0 75.2 71.7 485.9 463.35 1.0:1.38:0.43 1.0:0.92:0.89
80.1 12.8 7.1 40.1 35.3 24.6 75.35 71.93 486.2 464.1 1.0:1.37:0.15 1.0:0.93:0.68
82.2 10.9 6.9 42.6 31.3 26.0 75.3 71.8 485.4 463.1 1.0:1.35:00 1.0:0.93:0.68
86.2 7.0 6.8 44.9 34.9 20.6 75.2 71.8 485.0 462.5 1.0:1.13:0.24 1.0:0.93:0.68
than that of PtRuNi and PtRuNix. On the other hand, the current density for the MOR on the JM PtRu/C decreased significantly and reached 9.8 A g1Pt after polarization at 0.4 V for 5000 s, which is only 28% of PtRuNix-450. This demonstrates that the PtRuNix-450 has superior electrochemical activity and stability for the MOR, consistent with the CO-stripping results. The microstructural stabilities of PtRuNi, PtRuNix-450 and PtRu/C were tested by CV methods within the scan window of 0.2e1.0 V Fig. 8 shows the plots of the forward peak current density as a function of the number of cycles recorded in a 0.5 M H2SO4 þ 1.0 M CH3OH solution at a scan rate of
A)
50 mV s1. In the case of JM PtRu/C, the forward peak current density decreases significantly with the number of cycles and is only 2% of the initial value after 1000 cycles, indicating the poor microstructural stabilities of conventional PtRu/C. The as-synthesized PtRuNi alloy catalysts show significantly better microstructure stability and the reduction in the forward peak current density is 46.5% after 1000 cycles. The stability of PtRuNi is greatly improved through consequently dealloying and annealing treatments, and in the case of PtRuNix-450, the peak current maintains ~75% of the initial value after 1000 cycles, indicating that PtRuNix-450 exhibits high
A)
100
600 PtRuNix-450 PtRu/C-JM PtRuNix
400
j / A g Pt
0
-1
j / A g Pt
50
PtRuNi -450
-100
PtRuNi -300
-1
-50
PtRuNi -200
-150
0
PtRuNi PtRuNi
-200 -0.2
0.0
0.2
0.4
0.6
E/V vs Ag/AgCl
0.8
-200
1.0
B) 300
PtRuNix-450
-1
j / A g Pt
-1
PtRuNix-200 PtRuNix
100
PtRuNi
0.2
0.4
0.6
300 PtRuNi PtRuNix
200
0 100 0.2
0.4
500
100
0 0.0
0.0
400
PtRuNix-300 200
-0.2
E/V vs Ag/AgCl
j / A g Pt
B)
PtRuNi
200
0.6
E/V vs Ag/AgCl
0.8
1.0
Fig. 5 e A) Cyclic voltammograms of PtRuNi, PtRuNix, and PtRuNix-T and B) CO-stripping curves of PtRuNi, PtRuNix, and PtRuNix-T in a N2-saturated 0.5 M H2SO4 solution at a scanning rate of 50 mV s¡1.
PtRuNix-T
200
300
400
500
o
Annealing Temperature/ C Fig. 6 e A) CV curves of PtRuNi, PtRuNix, PtRuNix-450 and PtRu/C-JM in a N2-saturated 0.5 M H2SO4 þ 1.0 M CH3OH solution with the scanning rate of 50 mV s¡1, and B) plot of current density at 0.4 V obtained from CV curves for PtRuNi, PtRuNix, PtRuNix-T.
1942
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 1 ( 2 0 1 6 ) 1 9 3 5 e1 9 4 3
resulting in the formation of PtRu-rich skin and Ni-rich core with a surface composition of 1.0:1.13:0.24. The PtRuNix-450 catalysts exhibit significantly high activity and stability for MOR, as compared to conventional JM PtRu/C electrocatalysts. The PtRuNix-450 electrocatalysts also show significantly better microstructural durability, maintaining 75% of the initial current density after 1000 cycles, substantially higher than 27% observed for the reaction on conventional PtRu/C catalysts.
160 PtRuNix-450 PtRu/C-JM PtRuNix
140
-1
j / A g Pt
120 100
PtRuNi
80 60 40 20 0
0
1000
2000
3000
4000
5000
Time/s Fig. 7 e Chronoamperometry of PtRuNi, PtRuNix PtRuNix450 and PtRu/C-JM measured in a 0.5 M H2SO4 þ 1.0 M CH3OH solution at 0.4 V.
1200
references
PtRuNix-450
600
PtRuNi PtRu/C-JM
400
53%
200 0
This work was supported by the Australian Research Council Discovery Project funding scheme (project number: DP150202044) and the Major International (Regional) Joint Research Project of NNSFC (51210002), China. The authors acknowledge the facilities, and the scientific and technical assistance of the Curtin Microscopy Centre, a facility funded by the University, State and Commonwealth Governments.
75%
800
-1
j / A g Pt
1000
Acknowledgement
2% 0
200
400
600
800
1000
Number of Cycles Fig. 8 e Plots of the forward peak current density for MOR on PtRuNi, PtRuNix-450 and PtRu/C-JM electrocatalysts as a function of the number of cycles recorded in a 0.5 M H2SO4 þ 1.0 M CH3OH solution at scan rate of 50 mV s¡1.
microstructural stability for MOR. The significant enhancement of microstructural stability of PtRuNix-450 electrocatalysts is likely related to PtRu-rich shell (with increased alloy degree) with Pt rich outmost layer which would protect Ru and Ni from dissolution during continuous cycling test [27]. The present study is consistent with that reported in the literature [2,3]. The enhanced interaction of PtRu NPs with CNTs would also contribute to the superior stability of PtRuNi and PtRuNix-450, as compared with PtRu/C [28].
Conclusions PtRuNi alloy NPs homogenously supported on CNTs were synthesized through in situ reduction of self-assembled Pt, Ru and Ni onto PEI functionalized CNTs surface by microwave irradiation, forming a Ru rich surface with a composition of 1.0:1.38:0.43 as showing by the XPS analysis. Subsequent dealloying and thermal annealing at 450 C led to the segregation of Pt and the reconstruction of the PtRuNix NPs,
[1] Chen A, Holt-Hindle P. Platinum-based nanostructured materials: synthesis, properties, and applications. Chem Rev 2010;110:3767e804. [2] Wang JX, Inada H, Wu L, Zhu Y, Choi Y, Liu P, et al. Oxygen reduction on well-defined core-shell nanocatalysts: particle size, facet, and Pt shell thickness effects. J Am Chem Soc 2009;131:17298e302. [3] Wang C, Chi M, Li D, Strmcnik D, van der Vliet D, Wang G, et al. Design and synthesis of bimetallic electrocatalyst with multilayered Pt-skin surfaces. J Am Chem Soc 2011;133:14396e403. [4] Yuan WY, Lu SF, Xiang Y, Jiang SP. Pt-based nanoparticles on non-covalent functionalized carbon nanotubes as effective electrocatalysts for proton exchange membrane fuel cells. RSC Adv 2014;4:46265e84. [5] Oezaslan M, Heggen M, Strasser P. Size-dependent morphology of dealloyed bimetallic catalysts: linking the nano to the macro scale. J Am Chem Soc 2011;134:514e24. [6] Mani P, Srivastava R, Strasser P. Dealloyed PtCu coreshell nanoparticle electrocatalysts for use in PEM fuel cell cathodes. J Phys Chem C 2008;112:2770e8. [7] Gan L, Heggen M, Rudi S, Strasser P. Coreeshell compositional Fine structures of dealloyed PtxNi1ex nanoparticles and their impact on oxygen reduction catalysis. Nano Lett 2012;12:5423e30. [8] Srivastava R, Mani P, Hahn N, Strasser P. Efficient oxygen reduction fuel cell electrocatalysis on voltammetrically dealloyed PteCueCo nanoparticles. Angew Chem Int Ed 2007;46:8988e91. [9] Xu C, Wang L, Mu X, Ding Y. Nanoporous PtRu alloys for electrocatalysis. Langmuir 2010;26:7437e43. [10] Jeon T-Y, Lee K-S, Yoo SJ, Cho Y-H, Kang SH, Sung Y-E. Effect of surface segregation on the methanol oxidation reaction in carbon-supported PtRu alloy nanoparticles. Langmuir 2010;26:9123e9. [11] Chen S, Ferreira PJ, Sheng W, Yabuuchi N, Allard LF, ShaoHorn Y. Enhanced activity for oxygen reduction reaction on “Pt3Co” nanoparticles: direct evidence of percolated and sandwich-Segregation structures. J Am Chem Soc 2008;130:13818e9.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 1 ( 2 0 1 6 ) 1 9 3 5 e1 9 4 3
[12] Wang D, Yu Y, Xin HL, Hovden R, Ercius P, Mundy JA, et al. Tuning oxygen reduction reaction activity via controllable dealloying: a model study of ordered Cu 3Pt/C intermetallic nanocatalysts. Nano Lett 2012;12:5230e8. [13] Cheng Y, Shen PK, Saunders M, Jiang SP. Core-shell structured PtRuCox nanoparticles on carbon nanotubes as highly active and durable electrocatalysts for direct methanol fuel cells. Electrochim Acta 2015;177:217e26. [14] Cheng Y, Jiang SP. Highly effective and CO-tolerant PtRu electrocatalysts supported on poly(ethyleneimine) functionalized carbon nanotubes for direct methanol fuel cells. Electrochim Acta 2013;99:124e32. [15] Cheng Y, Xu C, Shen PK, Jiang SP. Effect of nitrogencontaining functionalization on the electrocatalytic activity of PtRu nanoparticles supported on carbon nanotubes for direct methanol fuel cells. Appl Catal B Environ 2014;158e159:140e9. [16] Park KW, Choi JH, Ahn KS, Sung YE. PtRu alloy and PtRu-WO3 nanocomposite electrodes for methanol electrooxidation fabricated by a sputtering deposition method. J Phys Chem B 2004;108:5989e94. [17] Wanjala BN, Fang B, Luo J, Chen Y, Yin J, Engelhard MH, et al. Correlation between atomic coordination structure and enhanced electrocatalytic activity for trimetallic alloy catalysts. J Am Chem Soc 2011;133:12714e27. [18] Tsypkin M, de la Fuente JLG, Garcı´a Rodrı´guez S, Yu Y, Ochal P, Seland F, et al. Effect of heat treatment on the electrocatalytic properties of nano-structured Ru cores with Pt shells. J Electroanal Chem 2013;704:57e66. [19] Strasser P, Koh S, Anniyev T, Greeley J, More K, Yu C, et al. Lattice-strain control of the activity in dealloyed coreeshell fuel cell catalysts. Nat Chem 2010;2:454e60.
1943
[20] Ahmadi M, Behafarid F, Cui CH, Strasser P, Cuenya BR. Longrange segregation phenomena in shape-selected bimetallic nanoparticles: chemical state effects. Acs Nano 2013;7:9195e204. [21] Grosvenor AP, Biesinger MC, Smart RSC, McIntyre NS. New interpretations of XPS spectra of nickel metal and oxides. Surf Sci 2006;600:1771e9. [22] Hamdan MS, Riyanto, Othman MR. Preparation and characterization of nano size NiOOH by direct electrochemical oxidation of nickel Plate. Int J Electrochem Sci 2013;8:4747e60. [23] Bensebaa F, Patrito N, Le Page Y, L'Ecuyer P, Wang D. Tunable platinum-ruthenium nanoparticle properties using microwave synthesis. J Mater Chem 2004;14:3378e84. [24] Park KC, Jang IY, Wongwiriyapan W, Morimoto S, Kim YJ, Jung YC, et al. Carbon-supported PteRu nanoparticles prepared in glyoxylate-reduction system promoting precursor-support interaction. J Mater Chem 2010;20:5345e54. [25] Ochal P, de la Fuente JLG, Tsypkin M, Seland F, Sunde S, Muthuswamy N, et al. CO stripping as an electrochemical tool for characterization of Ru@Pt core-shell catalysts. J Electroanal Chem 2011;655:140e6. [26] Ferrin P, Mavrikakis M. Structure sensitivity of methanol electrooxidation on transition metals. J Am Chem Soc 2009;131:14381e9. [27] Piela P, Eickes C, Brosha E, Garzon F, Zelenay P. Ruthenium crossover in direct methanol fuel cell with PteRu black anode. J Electrochem Soc 2004;151:A2053e9. [28] Zhou J, Zhou X, Sun X, Li R, Murphy M, Ding Z, et al. Interaction between Pt nanoparticles and carbon nanotubes e An X-ray absorption near edge structures (XANES) study. Chem Phys Lett 2007;437:229e32.