One-Pot Synthesis of Concave PlatinumâCobalt Nanocrystals and

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One-Pot Synthesis of Concave Platinum-Cobalt Nanocrystals and their Superior Catalytic Performances for Methanol Electrochemical Oxidation and Oxygen Electrochemical Reduction Yanxia Ma, Lisi Yin, Tao Yang, Qingli Huang, Maoshuai He, Hong Zhao, Dongen Zhang, Mingyan Wang, and Zhiwei Tong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10209 • Publication Date (Web): 26 Sep 2017 Downloaded from http://pubs.acs.org on September 26, 2017

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ACS Applied Materials & Interfaces

One-Pot Synthesis of Concave Platinum-Cobalt Nanocrystals and their Superior Catalytic Performances for Methanol Electrochemical Oxidation and Oxygen Electrochemical Reduction

Yanxia Ma,a Lisi Yin,a Tao Yang,*,a Qingli Huang,b Maoshuai He,c Hong Zhao,a Dongen Zhang,a Mingyan Wang,a Zhiwei Tonga a

School of Chemical Engineering, Huaihai Institute of Technology, Lianyungang

222005, P.R.

b

China.

Research Facility Center for Morphology of Xuzhou Medical University, Xuzhou,

221004, PR China c

School of Materials Science and Engineering, Shandong University of Science and

Technology, Qingdao, 266590, P.R. China Author Information *Corresponding author E-mail: yangtao_hit @163.com

KEYWORDS:

electro-catalyst,

platinum-cobalt,

oxygen

methanol oxidation, concave cube 1

ACS Paragon Plus Environment

reduction

reaction,


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ABSTRACT

Exploring high efficient electro-catalysts is of significant urgency for the wide-spread uptake of the direct methanol fuel cells (DMFCs). Pt-Co nanocrystals have attracted considerable attentions due to their superior catalytic performance toward both methanol oxidation and oxygen reduction in the preliminary assessments. This article presents a Pt-Co bimetal catalyst that is synthesized through a facile co-reduction strategy. The Pt-Co nanocrystals have concave cubic shape with a high uniform size of 7~9 nm and Pt-rich surfaces. The catalysis of the concave cubic Pt-Co nanoparticles towards both methanol electrochemical oxidation reaction (MOR) and oxygen electrochemical reduction reaction (ORR) is evaluated. In comparison with the commercial Pt/C catalyst (Johnson Matthey), the present concave cubic Pt-Co catalyst displays superior performances in not only catalytic activity but also durability. The concave Pt-Co catalyst also shows higher activities than spherical and cubic Pt-Co nanoparticles. The dramatic enhancement is mainly attributed to its alloyed composition, Pt-rich surface and the concave nanostructure. The results of our research indicate that the concave Pt-Co nanocrystal could be a promising catalyst for both MOR and ORR. The present work might also raise more concerns on exploiting morphology and composition of nanocrystal catalysts, which are expected to provide high catalytic performance in electrochemical reactions.

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INTRODUCTIOON Direct methanol fuel cells (DMFCs) have attracted much concern in recent years due to their distinctive advantages such as low operating temperature, high energy conversion efficiency and nearly no environmental pollution.

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DMFCs show great

promise for the future uses in the portable electronic devices and new-energy vehicles and has obtained a progress that a commercial unit with ~250 W output has been successfully available several years ago. 2 Unfortunately, there are still several issues about the catalysts that lag the real application of DMFCs. First, the catalysts of DMFCs are still suffering from the sluggish kinetics during methanol oxidation and oxygen reduction. In addition, a high catalyst loading amount is also needed to achieve acceptable current densities especially at low operating temperatures.3,

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Furthermore, although the price is expensive and even soaring, platinum is still the recognized electro-catalyst for MOR and ORR.5-7 Unfortunately, platinum is prone to be poisoned by carbon monoxide (CO), which certainly exists in MOR.4, 8 Not only so, the d-spacing of pure Pt atoms falls short of optimal level for oxygen reduction reaction. 9 All these problems limit the application of pure Pt catalysts in DMFC.10, 11 For now, increasing the Pt utilization efficiency and then reducing the Pt using amount have been recognized as desired strategies to address the above mentioned issues.12, 13 To this end, extensive work has been devoted to developing Pt- based alloys consisting of transition metals (M=Ni, Pd, Cu, Co, Fe, etc.). Alloying will alter the surficial electronic-energy level, which then changes surface adsorption properties 3



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and in turn improves the catalytic performance greatly.14-16 More importantly, the Pt-based alloys have been proven to have good tolerance against CO poisoning relative to pure Pt catalysts.17-20 One of the recognized reasons is that the alloyed transition metals are helpful to dissociate water into OH, which can facilitate the electro-oxidation of CO.21 Among the above mentioned Pt-based catalysts, Pt-Co nanocrystals show excellent catalytic performance for both MOR21-24 and ORR.25-32 For examples, compared with the commercial Pt/C catalysts, the cubic Pt3Co nanocube displayed much higher performances in respects of CO tolerance and catalytic activities,22 and the Pt-Co alloyed-network or nanoflower showed higher catalytic activity and higher durability.33, 34 Besides the investigation on alloys composition, exploring the catalysts with diversified nanostructures has been proposed as another efficient way, which can also provide more opportunities to enhance the catalytic performance.3 The nanoparticles with different morphologies will show different adsorption/desorption behaviors on various facets.9, 35-38 A synergetic function among the high and low indexed facets generally allows a high catalytic activity.39-41 Accordingly, the nanoparticles with rough surfaces, which consist of high dense discontinuous structures such as terraces, tips and steps generally exhibit high catalytic active.39, 40, 42-44 Some recent work has demonstrated that concave surfaces were highly coincident with the above mentioned characterizations and have achieved relatively high catalytic performances.35,

45-49

Thus, although the Pt-based nanocrystals catalysts have been greatly investigated, 4


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exploring concave nanocrystals with high dense active sites is still worth pursing to improve the catalytic activity and the utilization efficiency of platinum. More recently, concave Pt-Co cubes with an atomic ratio of 82.3/17.7 (Pt/Co) and an average apex-to apex size of 68±6 nm were successfully synthesized in presence of glycine and poly(vinyl pyrrolidone) PVP.50 The concave nanoparticles showed higher specific activities in catalyzing formic acid oxidation and oxygen reduction. However, the particle size is a little too large for electro-catalysts.51 In another word, the larger particle size is a limit for the surface area and in turn restricts the utility efficiency. Herein, Pt-Co bimetal nanoparticles were synthesized through a facile co-reduction method. Morphology tests reveal that the bimetal nanocrystals have concave cubic shape with a narrow size range of 7-9 nm. Compositions analysis shows that the nanoparticles have a Pt/Co atomic ratio of 4/1. In addition, the bimetal concave cubes have a Pt-rich surface. For comparison, spherical and cubic Pt-Co nanoparticles with similar compositions were also synthesized. Electrochemical evaluations demonstrate that the concave Pt-Co nanocrystal reported here is promising catalyst for MOR and ORR.

RESULTS AND DISCUSSION Synthesis and structural characterization of Pt-Co nanoparticles. Pt-Co bimetal nanocrystals were prepared through a facile co-reduction synthesis process in 5



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autoclaves under solvothermal condition. TEM test was performed to observe the morphologies of the resultants at various magnifications. Figure S1A is the overview of the products at low magnifications, which presents that the nanoparticles are highly uniform in shape and size with a yield nearly 100%. The nanoparticles are mono-dispersedly assembled on the TEM grid, which are dominantly concave cubic shape. The nanoparticles in the TEM vision displays a highly narrow particle size range of 7~9 nm (Figure S1B). TEM image at high magnification shows that the nanoparticles have concave surfaces and rounded corners (Figure 1A). An ideal model demonstrates that the particle should have six concave surfaces (Figure 1B). Figure 1C is the SAED pattern on several particles, which is composed of many light sports arranged into several concentric rings. The lattice distances corresponding to the four reflection rings are 2.24 Å, 1.96 Å, 1.38 Å and 1.16 Å, respectively, which are highly in agreement with the XRD patterns (Figure 2). A HRTEM image of an individual particle is shown in Figure 1D. The single crystal structure is clearly evidenced by the coherently lattice fringes over the whole particle. Different from the cubic Pt-Co nanoparticles, those are surrounded with the low-indexed surface of {100} planes,52, 53 the present Pt-Co nanoparticles are enclosed with several high-indexed facets due to the concave structure.50, 54 Figure1D is a typical example, the angles between the projected real surfaces and the ideal cubic ones are 17o, 32o, 28o and 37.5o, indicating that the projecting surfaces can be assigned to the high-indexed facets of {310}, {320}, {210} 6


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and {540}, respectively. The results reveal that there must be many surficial atoms on the concave cubic Pt-Co nanoparticles belong to various high-indexed facets. These surficial atoms have low coordination numbers and can provide much higher catalytic activities than the counterparts on the smooth planes, such as cubic or octahedral nanocrystals.42, 43 The chemical compositions were evaluated on several regions of the TEM view through the EDS examination. A typical TEM-EDS spectrum is shown in Figure 1E, the compositions calculated from EDS (Pt/Co atom. % = 81.7/18.3) is highly consistent with the ICP results (80.2/19.8). Importantly, the chemical composition of the final product is very close to the precursor ratio (79.4/20.6). However, in the previous reported synthesis of cubic Pt-Co or Pt-Cu nanoparticles, one half or more amount of the transition metals in the precursors cannot deposit due to the low redox potentials.52, 53, 55 Figure 1F shows that the nanoparticles can be uniformly dispersed onto the carbon support through a facile loading procedure. These results reveal that Pt-Co bimetal nanoparticles with concave cubic shape are successfully synthesized through a facile co-reduction process.

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Figure 1. TEM image, SEAD and EDS pattern of the products. (A) TEM image of the produced nanoparticles at a large magnification, (B) Model of the concave cubic nanostructure, (C) SAED pattern, (D) HRTEM image of an individual particle, (E) TEM-EDS spectra of the Pt-Co nanoparticles, (F) TEM image of the Pt-Co nanoparticles loaded onto carbon black. The scale bar in (A) and (F) are 50 nm, (C) and (D) are 5×1/nm and 2 nm, respectively. To obtain more information about the crystalline, powder X-ray diffraction (PXRD) test was carried out and the patterns are presented in Figure 2. The patterns have no peaks can be assigned to the pure Pt or pure Co, suggesting the exclusive existence of Pt-Co alloy.52, 53, 55 The five representative peaks at about 40.4o, 46.9o, 68.4o, 82.3o and 86.4o are well correlated with but slightly higher than the standard Pt diffractions 8


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of (111), (200), (220), (311) and (222) planes, respectively. The positive shift of these diffractions is because that the smaller Co atoms (1.26 Å) replaced Pt atoms (1.36 Å) and reduced the lattice distance.21,

33, 34

It has been reported that reducing the

d-spacing of surface Pt atoms could enhance the ORR catalytic activities greatly. 9 The size of the nanoparticle is reckoned to be 8.7 nm by using the Debye-Scherrer relationship through the (220) diffraction peak. These results closely match with the TEM and SAED examinations (Figure 1, Figure S1), suggesting the formation of single crystalline nanocrystals with high quality.53

Figure 2. XRD pattern of carbon supported Pt-Co nanoparticles. The standard XRD peaks of Pt and Co are marked as the green and red lines perpendicular to the x-axis. Morphologies of the Pt-Co nanoparticles were further determined through STEM, the concave surfaces can be clearly observed (Figure 3A and 3B). To get more insight 9



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of the nanostructure, the elements distribution on several nanoparticles (region of 80 nm ×80 nm) was scanned and the elements mapping is shown in Figure 3B. The Pt and Co are represented with green and red colors, respectively. In the merged image (Figure 3B-Pt-Co), the green and red regions are highly coincident, implying that all the particles are exclusive Pt-Co bimetallic components. EDS cross-sectional compositional line scanning was then carried out to evaluate the relative atomic ratio on a single particle (Figure 3C). The intensity of Pt and Co concomitantly varied across the particle and the relative intensity is almost constant (Figure 3C). The hereinbefore results suggest a high alloying degree, that was also in agreement with the results of XRD.

Figure 3. STEM images, EDS cross-sectional line profile and elemental mapping of the nanoparticles. (A) HAADF-STEM image of the Pt-Co nanoparticles. (B) 10


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HAADF-STEM image and the elemental mapping for Pt and Co on a region of 80 nm ×80 nm (C) EDS cross-sectional line profile on an individual particle. The scale bars in (A), (B) and (C) are 20 nm, 10 nm and 10 nm, respectively. It has been reported that Pt-rich surfaces can form on Pt-based alloys. To check the element distribution in the near surface layers, X-ray photoelectron spectroscopy (XPS) tests were performed. The XPS spectrum was firstly calibrated by putting the C 1s peaks to 284.8 eV. Based on the XPS spectra, the atomic ratio of Pt/Co is 84.4/15.6, which is slightly higher than the ICP result (80.2/19.8) (Figure S2). The difference in atomic ratio between bulk and surface suggests that Pt is slightly rich in the near surface layers. The Pt-rich surface will results in a stress or compress effect among the adjacent Pt atoms. The formation of the compressed Pt-rich shell layer might also be demonstrated by the binding energy.56 The binding energies peaks at 74.68 and 71.38 eV are assigned to Pt 4f5/2 and 4f7/2, which could be divided into metallic Pt at 74.78 and 71.38 eV, and Pt2+ at 75.68 and 72.18 eV, respectively.57,

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The

compressed Pt surface layers are expected to lower the d-band center and change the adsorption behaviors.59, 60 To check the influence of morphologies on the catalytic performance, spherical and cubic Pt-Co nanoparticles with similar composition were synthesized. Spherical Pt-Co nanoparticles were produced by reducing Pt(acac)2 and Co(acac)2 with W(CO)6 in oleylamine in the presence of didodecyldimethylammonium bromide (DDAB). TEM image and SAED pattern show that the spherical Pt-Co particles have narrow size 11



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distribution (5-8 nm) (Figure S3A) and high crystalline (Figure 3B). The spherical particles have a Pt/Co ratio of 81.4/18.6, which is also close to the ICP test (80.7:19.3) (Figure S3C). HRTEM image reveals that the exposed surfaces can be assigned to {111} and {100} planes. The spherical Pt-Co nanoparticles were also tested by STEM and element mapping (Figure S4). Cubic Pt-Co nanoparticles were produced by reducing Pt(acac)2 and Co(acac)2 with W(CO)6 in the mixture of oleylamine and oleic acid. Morphology, nanostructure and composition were also examined (Figure S5, S6). The cubic Pt-Co nanoparticles have edge length of 5-10 nm (Figure S5A) and a Pt/Co ratio of 80.8/19.2 (ICP: 79.9/20.1) (Figure S5C). The good crystallization can be observed through SAED pattern (Figure S5B) and HRTEM image (Figure S5D). The cubic shape and the element distribution are further demonstrated through STEM test and element mapping (Figure S6). The reaction parameters, such as reaction temperature, time, precursors and solvents, were studied to explore the key factors for the final morphology. We found that the cobalt carbonyl and the using amount have high influence on the final shape. If the cobalt carbonyl was replaced by Co(acac)2, very small particles (~3 nm) with a variety of shapes were generated (Figure S7A). If the Co(acac)2 amount was increased greatly, larger particles were achieved (Figure S7B). If the reaction time was insufficient (1 h), most of the nanoparticles (5-8 nm) were cubic shape (Figure S8A). While too much reaction time (2.5 h) would induce the particles overgrowing into branched structure (Figure S8B). No regular concave particles can be obtained if the 12


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reaction temperatures is higher or lower (Figure S9). It seems that OAm has more influence on the shape, while OA has strong control on the uniformity of the particle size (Figure S10). If the Pt precursor was increased (5/1), concave particles or multi-pods can be produced (Figure S11A). While, more and more irregular particles (multi-pods, dendrites, et al.) appeared if the amount of Co precursor was increased (Figure S11B, S11C, S11D). However, the chemical compositions of the products are highly close to the precursor ratio (Figure S12).

Electrochemical measurements. The carbon supported Pt-Co concave cubes were firstly tested for MOR to evaluate the catalysis. For comparison, the same examination was also carried out on carbon supported spherical and cubic Pt-Co nanoparticles and a commercial Pt/C catalyst under the identical conditions. Before each tests, a few tens of cyclic voltammetry (CV) scans were carried out until stable CV curves displayed, implying that the catalysts got active and clean surfaces. Figure 4 and Figure S13 are the stable CV profiles of the Pt/C and the carbon supported Pt-Co catalysts. All the catalysts show distinct charge/discharge behaviors in the range of 0.05~0.40 V, that should be mainly attributed to the hydrogen adsorption/desorption. The hydrogen desorption on (110) and (100) planes can be resolved on Pt/C and spherical Pt-Co CV profiles, which is similar to the previously studies of polycrystalline Pt electrodes.47 It is because that the exposed surfaces of spherical Pt-Co nanoparticles and the Pt/C catalyst belong to several low-indexed planes. In contrast, only a pair of more distinct hydrogen adsorption/desorption peaks 13



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related to the (100) plane can be observed on cubic and concave cubic Pt-Co/C catalyst. The results are highly similar to that reported for the cubic Pt-Co nanocrystals.53 Besides, there is a new oxidation peak at 0.74 V in the anodic scanning for Pt-Co/C catalysts, which should be attributed to the leaching of surficial Co.34 After MOR ADT, the hydrogen adsorption/desorption peaks on the special facets of (110) and (100) for Pt/C catalyst became weaker (Figure 4B). However, no remarkable declination presents on concave cubic Pt-Co/C (Figure 4A). The concave Pt-Co particles still exhibits distinctive hydrogen adsorption/desorption behaviors, implying that the surface is stable during the MOR ADT examination.34 The stable surface structure might allow for a high durability in EASA and catalytic activity (Figure S14).

Figure 4. CV profiles of the carbon supported catalysts. (A) The CV profiles of Pt-Co/C before and after MOR ADT. (B) The CV profiles of Pt/C before and after MOR ADT. The current was normalized to the electrochemical active surface. The CV profiles were taken at a rotation of 1600 rpm and 50 mV s-1 in argon bubbled 0.1 14


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M HClO4 solution. (MOR ADT: 1000 cycles in the region of 0 ~ 1.2 V in a solution containing 0.1 M HClO4 and 1.0 M methanol at 50 mV s-1) After correcting the double-layer, the hydrogen adsorption/desorption charge was integrated and used for the evaluation of electrochemically active surface area (EASA) (Figure S13, S14). The EASA of concave cubic Pt-Co nanoparticle is 54.5 m2 gPt-1, which is two thirds of the commercial Pt/C (74.3 m2 gPt-1) due to the larger nanoparticle size. The EASAs of spherical and cubic Pt-Co particles are 57.9 m2 gPt-1 and 48.8 m2 gPt-1, respectively. EASA of the catalysts after MOR ADT were also calculated through the same method. Compared with the initial value, Pt/C declined 40 % in EASA, in contrast, Pt-Co/C lost only 22.7 % of its initial EASA (Figure S14A), which might be mainly due to the stable nanostructure. TEM image also revealed that the Pt-Co nanocrystals after MOR ADT still showed distinct concave cubic shape (Figure S14C), although there was a little Co loss in the composition (Figure S14D). Figure 5 and Figure S15 are the stable MOR CV curves of commercial Pt/C and the concave Pt-Co nanoparticle, the both catalysts show similar onset MOR potential (~0.4 V). The current has been normalized respect to the electrochemical active surface. The forward peak current in the anodic scans has been reported to be proportionate to the efficiency of methanol oxidization. Concave Pt-Co catalyst shows an anodic current density of 3.62 mA cm-2, much larger than that for Pt/C (0.515 mA cm-2). The present concave Pt-Co catalyst possesses a much superior catalytic activity 15



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than the reported Pt-Co flower (2.92 mA cm-2),34 Pt-Ni branches (0.9 mA cm-2)17 and Pt nanotube (1.62 mA cm-2).61 Spherical and cubic Pt-Co nanoparticles also exhibit high activities of 2.39 mA cm-2 and 1.85 mA cm-2, respectively (Figure S16). Mass activity was evaluated by taking the amount of Pt into account. Concave Pt-Co catalyst has a mass activity of 2110 mA mgPt-1, 5.5 times larger than that for Pt/C (382 mA mgPt-1). Spherical and cubic Pt-Co nanoparticles display mass activities of 1481 mA mgPt-1 and 967 mA mgPt-1, respectively (Figure S16).

Figure 5. CV profiles of the carbon supported catalysts for methanol oxidation. CV profiles of (A) Pt-Co/C and (B) Pt/C for MOR in 0.1 M HClO4 containing 1.0 M methanol at 50 mV s-1. The current was normalized to the electrochemical active surface areas. ADT is performed under the same conditions as MOR for 1000 cycles. Durability of concave Pt-Co catalyst and Pt/C was first determined through the MOR ADT test, in which, 1000 MOR CVs were carried out. The final CVs profiles were recorded and depicted together with the initial ones in Figure 4 and Figure S15. 16


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The currents are also normalized to the electrochemical active surface at that time. Pt-Co/C gets an activity of 2.49 mA cm-2 that is still more than five times that of the commercial Pt/C catalyst (0.425 mA cm-2). The stability of the catalysts were further evaluated by recording the i-t curves for 1000 s at two fixed potentials of 0.58 V and 0.75 V (Figure S17). The current densities quickly declined in the starting stage because of the generation of the incompletely oxidized or intermediate species of MOR.34 The carbon supported concave Pt-Co catalyst obtained higher current densities at both 0.75 V and 0.58 V during the test time, which were very in accord with the CV curves of Figure 5. Electrochemical catalysis of the concave Pt-Co nanoparticles toward ORR was then checked and compared with the spherical and cubic Pt-Co catalysts and the commercial Pt/C catalyst. The polarization scans were performed in oxygen purged HClO4 solution on the electrode at a rotation rate of 1600 rpm and a sweeping rate of 10 mV s-1. After several scans, the stable polarization profiles were achieved and displayed in Figure 6A and Figure S18A. All the catalysts show two distinguished stages, one for mixed kinetic diffusion controlled region (0.7-1.0 V) and the other for diffusion limiting dominated range (below 0.7 V). The concave Pt-Co catalyst possesses larger current densities than the commercial Pt/C through the whole potential range. The concave Pt-Co catalyst has a half-wave potential (E1/2) of is 892 mV, 17 mV greater than that for Pt/C (875 mV). The kinetic currents in the region of 0.85~0.95 V were reckoned with the Levich-Koutecky equation, which were then 17



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normalized according to the electrochemical active surface (Figure 6B). The enhancement of concave Pt-Co catalyst compared with Pt/C became more distinctive due to the slightly smaller active surface area (Figure 6B). The kinetic currents densities at 0.9 V were presented in Figure S19B, which were then converted into mass activities based on the Pt amount (Figure S19C). Concave cubic Pt-Co catalyst (0.439 mA cm-2) exhibits a high specific activity two times larger than that of the commercial Pt/C (0.16 mA cm-2). In addition, the concave cubic Pt-Co catalyst shows a mass activity of 237 mA mgPt-1 (222 mA mgmetal-1), which is nearly two times that of Pt/C (122 mA mg-1). The spherical and cubic Pt-Co catalysts also show higher catalytic activities than the commercial Pt/C catalyst (Figure S18).

Figure 6. ORR polarization curves and the specific activities of the carbon supported catalysts.

Electro-catalytic performance of Pt-Co towards ORR. (A)

ORR polarization curves, (B) Specific activities towards ORR that are presented as kinetic current densities (Jk) normalized to the EASA.

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The stability of concave cubic Pt-Co catalyst for ORR were examined through CV patterns in the range of 0.6-1.0 V for 4000 cycles in 0.1 M HClO4 solution with oxygen saturation at a sweep rate of 50 mV s-1. After the durability test, the nanoparticles still remained concave cubic shape with distinct corners (Figure S19D). The EASA of Pt-Co/C displayed a loss of 14.5 m2gPt-1, nearly half of the Pt/C loss (26.2 m2gPt-1) (Figure S19A). The huge EASA declination in Pt/C has been reported to be mainly due to the agglomeration of the small Pt particles.62-64 The remained ia and im of concave cubic Pt-Co catalyst were still 2.7 times and 2.1 times higher than those for the commercial Pt/C catalyst. The improved catalytic activities and catalytic efficiencies should be attributed to the alloyed Pt-Co compositions and the concave cubic nanostructures. The introduction of Co into the Pt lattices changes the surface electrons arrangement due to the difference in electronic negativity.34 The smaller Co atoms reduce the d-band center of Pt,56,

65

which has been resolved in XRD examination (Figure 2). The

present concave Pt-Co particles show Pt rich in the shell layer (XPS results), which causes additional compressive strains in the surface shells, just like the Pt-Ni based catalysts.64 Nørskov’s researches have reported that modifying surface electronic and atomic arrangement will alter the adsorption behaviors.9 Accordingly, the interactions between the incompletely oxidized species and the catalysts surface are substantially suppressed,66 and the OH adsorption becomes more favorable for the removing of the incompletely oxidized species.33, 67 Besides, the synergetic effect among the different 19



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facets might contribute to the improved catalytic activity greatly. In the researches of ORR, molecular oxygen can easily be adsorbed on the (100) surfaces, and then diffuses to the neighboring facets such as (111) with lower resistance, where the OH species can be generated and release easily due to the weak binding.36,

68

Such

synergetic effects even allow the high-indexed facets of (211), (311) and (331) to exhibit higher catalytic activities than the low indexed counterparts.39, 40, 43, 62 Many recent works have demonstrated that a concave surface with high density of terraces and steps can obtain relative high catalytic performances due to the appearance of the high-indexed facets.35 For example, a Pt nanowire with rough surface displayed three folder better specific activities related to the commercial Pt/C catalyst.69 A Pt multi-cube exhibited much superior catalytic activities than that for the Pt cubic nanoparticles due to the existence of the high-indexed facets.47 The present concave cubic Pt-Co nanoparticles have many high-indexed facets of {310}, {320}, {210} and {540} (Figure 1). We believe that there must be some synergetic effect among the above facets, which is conducive to electro-catalysis. We also proved that the concave Pt-Co catalyst show high specific activity compared with spherical and cubic Pt-Co nanoparticles. Additionally, it has been reported that the outermost Pt shell of a Pt-Co nanoparticle has a higher dissolution potential in contrast with the pure Pt nanoparticles due to the Pt rich skin layer.70 This might be able to explain that the present Pt-Co bimetal nanocrystal show high electrochemical stability than the pure Pt 20


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catalyst in both methanol electrochemical oxidation and oxygen electrochemical reduction. The concave cubic profiles and the corners of the Pt-Co nanoparticles can still be observed clearly on the TEM images of the catalysts after durability test. This result is highly consistent with the CV curves in Figure 4A, that the hydrogen adsorption/desorption peaks after durability test are still dominantly assigned to the (100) surface.

CONCLUSIONS In conclusion, concave cubic Pt-Co bimetal nanoparticles with highly narrow size range were successfully synthesized. The concave cubic Pt-Co nanoparticles were dispersed onto carbon support and the catalysis was evaluated towards methanol electrochemical oxidation and oxygen electrochemical reduction in acidic media. The carbon supported concave cubic Pt-Co catalyst shows higher performances in specific activity and durability in comparison with the commercial Pt/C catalyst. Catalytic performances of the present concave cubic Pt-Co catalyst are also higher than the spherical and cubic Pt-Co nanoparticles with similar composition. The improved performances could be mainly ascribed to the alloyed Pt-Co composition, Pt-rich surface and the concave nanostructure. Though the use of Pt-based nanocrystals for MOR and ORR catalysts has been extensively investigated, the present work is still promising. Our research will also arouse the interest in exploiting high efficient Pt-based catalysts through pursuing the concave nanostructures.

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ACKNOWLEDGEMENTS

This work was financially supported by the National Natural Science Foundations of China (NSFC) (No. 21473067, 21601059 and 21505118), the Natural Science Foundation of Jiangsu Province (No. BK20141247, BK20150438), the Graduate Research

Innovation

Program

of

Huaihai

Institute

of

Technology

(No.

XKYCXX2016-6) and the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

CORRESPONDING AUTHOR *Tao Yang E-mail: [email protected]

ASSOCIATED CONTENT Supporting Information Experimental section (the preparation of concave cube, structure and electrochemical measurements), TEM images of the concave cube at different magnifications, TEM images of the product obtained from various conditions, XPS pattern, TEM image of the carbon supported catalyst and the electrochemical performances. This information is available free of charge via the Internet at http://pubs.acs.org/. Notes The authors declare no competing financial interest. 22


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TOC

Platinum-cobalt nanocrystal has been recognized as one class of the promising catalysts for direct methanol fuel cells. Here, concave cubic platinum-cobalt catalysts with high uniformity in shape and size are synthesized. By studying the methanol oxidation and oxygen reduction, the present platinum-cobalt concave cubic catalysts show superior catalytic performance in specific activity and stability.

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