PtNi Nanocrystals Supported on Hollow Carbon Spheres: Enhancing

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PtNi Nanocrystals Supported on Hollow Carbon Spheres: Enhancing the Electrocatalytic Performance through High Temperature Annealing and Electrochemical CO Stripping Treatments Chunmei Zhang, Ruizhong Zhang, Xiaokun Li, and Wei Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b04489 • Publication Date (Web): 16 Aug 2017 Downloaded from http://pubs.acs.org on August 21, 2017

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ACS Applied Materials & Interfaces

PtNi Nanocrystals Supported on Hollow Carbon Spheres: Enhancing the Electrocatalytic Performance through High Temperature Annealing and Electrochemical CO Stripping Treatments Chunmei Zhang,†, ‡ Ruizhong Zhang, †, ‡ and Xiaokun Li,† Wei Chen*, †

†

State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied

Chemistry, Chinese Academy of Sciences, Changchun 130022, Jilin, China ‡

University of Chinese Academy of Sciences, Beijing 100039, China

*Corresponding author, E-mail: [email protected], Tel. +86-431-85262061

KEYWORDS: PtNi nanoparticle; carbon sphere; CO stripping; nanocrystal; electrocatalyst; electrocatalysis; oxygen reduction reaction; methanol oxidation; fuel cells;

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Abstract: PtNi nanoparticles have been proved to be a type of highly efficient electrocatalyst for the oxygen reduction reaction (ORR) among the Pt-based nanomaterials. However, how to improve the surface catalytic activity and stability of polymer-stabilized Pt-based nanocrystals is still a critical issue for their application in fuel cells. In this work, a one-step solvothermal process was used to synthesize PVP-stabilized PtNi nanocubes supported on hollow carbon spheres. With optimized metal precursor ratio (Pt/Ni=1:1) and solvothermal temperature (130 oC), PtNi nanocrystals with uniform size and cubic shape can be synthesized and highly dispersed on hollow carbon spheres. To improve the electrocatalytic activity of the PtNi nanocrystals, the synthesized composite was treated by a heating annealing at 300 oC and a subsequent electrochemical CO stripping process. It was found that the two-step treatment can significantly enhance the catalytic activity of the PtNi nanocrystals for ORR with high durability. In addition, the prepared PtNi composite also showed higher catalytic activity and stability for methanol oxidation. The obtained peak current density on the present catalyst can reach 3.89 A/mgPt which is 9 times as high as commercial Pt/C (0.43 A/mgPt). The present study not only demonstrates a general method to synthesize hollow carbon sphere-supported nanoparticle catalysts but also provides an efficient strategy to active the surface activity of nanoparticles.

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1. INTRODUCTION Hollow carbon spheres (HCS) have attracted considerable attention because of their promising applications in lithium-ion batteries, supercapacitors, water treatment, CO2 capture, catalysis, biomedical and fuel cells.1-3 As a promising support for fuel cell catalysts, HCS structure has its unique advantages such as fast diffusion rate, highly exposed catalytic active sites, large surface area, high porosity, enhanced oxidative stability and high electronic conductivity etc.1 To date various studies have focused on the HCS with porous shell and large cavity to load or encapsulate small metal nanoparticles. Therefore, there are mainly two types of structures of HCS-based nanocatalysts, including metal core-carbon shell structure and HCS surface-loaded nanoparticle structure. For core-shell structure, Zhang et al.4 reported tin encapsulated in hollow carbon sphere as high-performance anode material for lithium-ion batteries. Liu et al.5 developed a synthetic method to prepare Au@HCS using dopamine as carbon source, and Wang et al.6 prepared bimetallic nanoparticles inside hollow structure, PtCo@HCS, using P123 and sodium oleate as double surfactants soft template. Recently, Rodneys group successfully prepared Ag@carbon core-shell spheres7 and Au@carbon core-shell spheres8 by StÖber route. In another work, Li et al.2 reported a core-shell structure by encapsulating germanium in hollow carbon sphere (Ge@HCS) in which carbon spheres functions as a scaffold to hold germanium and a rampart to maintain the regular structure. In these core-shell structures, carbon shell serves as a layer of protective clothing to protect metal nanoparticles from corrosion. However, some active sites of metal nanoparticles may be blocked by the carbon surface layer. Another carbon surface-loaded metal structure exhibits its unique advantages that it can hold enough small nanoparticles using its large surface area and porous shell and thus can make more metal active sites exposed. For example, Choi et al. synthesized Au or Ag nanoparticles uniformly dispersed within the carbon shell by one-step pyrolysis of a type of core-shell MOF hybrids.9 In another report, highly alloyed 3



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PtRu nanoparticles were well confined in porous carbon shell and used as durable electrocatalyst for methanol oxidation.10 Other metal nanoparticles such as Pt,11 Pd12 dispersed in hollow carbon sphere shell have also been synthesized as advanced fuel cell electrocatalysts with high catalytic activity and stability. Among the developed electrocatalysts, Pt and Pt-based materials exhibit the highest catalytic performances for both anode and cathode reactions in fuel cells.13-15 Meanwhile, it has been recognized that the catalytic properties of Pt-based alloy nanostructures are strongly dependent on their composition, size and morphology.16 Especially, shape is one of the important structural parameters that determine the properties of metal nanocrystals because of the strong correlation of the shape and the exposed facets and surface composition.17-19 As one of Pt-based alloys, PtNi has attracted increasing attention because of its high electrochemical activity. For instance, Stamenkovic and coworkers20-21 reported that Pt3Ni bulk single crystal and Pt-Ni alloy frameworks exhibited enhanced electrocatalytic performances for oxygen reduction reaction (ORR) because of the exposed (111) facets. Zhang et al.22 prepared shape-controlled Pt3Ni nanocrystals terminated with (111) and (100) facets using a wet-chemical approach and the nanocatalysts showed high catalytic activity for ORR. Recently, Wu et al.19 developed a method to prepare shape- and composition-controlled platinum alloys as ORR catalysts. In this work, although the shape and composition of platinum alloys can be well controlled, the alloys were synthesized through a chemical reduction process by using the high-toxic CO as reducing agent at high reaction temperature (210 oC). In another work, Cui et al.18 prepared octahedral PtNi nanoparticles with a solvothermal method and mainly studied the effects of reaction time on the surface composition and the catalytic activity. In this work, a long time (48 h) is needed to obtain PtNi nanoparticles with regular shape. In fuel cell catalysts, metal nanoparticles are usually dispersed on carbon supports which should have high electrical conductivity and 4


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large surface area. For example, by using impregnation-reduction-annealing method, Baldizzone et al.23 synthesized 3-5 nm PtNi alloy nanoparticles confined in graphitic hollow spheres with extraordinary ORR mass activity. Overall, above hollow carbon sphere-supported or encapsulated metal nanoparticle catalysts can be synthesized by in situ (hollow carbon spheres and nanoparticles are formed simultaneously) or ex-situ (hollow carbon spheres and nanoparticles are formed separately) approaches. For hollow carbon spheres supported catalysts, the homogeneous dispersion without aggregation, controlled structure and composition of metal nanocrystals are crucial for tuning their catalytic properties. Meanwhile, appropriate post-treatments can further improve the catalytic performances. Herein, PVP-stabilized PtNi nanocrystals supported on hollow carbon sphere (PtNi NCs-PVP@HCS) is successfully synthesized through one-step solvothermal method using PVP as morphology control agent, DMF as solvent and reducing agent. The composition and structure of PtNi nanocrystals can be controlled by the molar ratio of Pt and Ni and the solvothermal temperature. To enhance the electroccatalytic properties, the as-synthesized PtNi NCs-PVP@HCS was further treated by an annealing process at 300 oC and a subsequent electrochemical CO stripping. It was found that the post-treatments can enhance the activity and stability of the hybrid material toward oxygen reduction reaction and methanol oxidation reaction.

2. EXPERIMENTAL SECTION 2.1 Chemicals and materials Nickel chloride hexahydrate (NiCl2·6H2O, ≥ 98.0%) was purchased from Xilong Chemical Co., Ltd. Poly vinyl pyrrolidone (PVP, Mw ≈ 55 000), sodium iodinate dehydrate (NaI·2H2O), potassium tetrachloroplatinate (Ⅱ) (K2PtCl4, 98%) and tetraethoxysilane (TEOS, ≥ 98%) were purchased from Sigma-Aldrich. Resorcinol (≥ 99.5%), ammonia solution (25 ~ 28%) and formaldehyde solution (37.0 ~ 5



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40.0%) were obtained from Sinopharm Chemical Reagent Co., Ltd. Methanol (≥ 99.5%), anhydrous ethanol (≥ 99.7%), N, N-dimethylformamide (DMF, ≥ 99.5%) were all provided by Beijing Chemical works. Hydrofluoric acid (HF, ≥ 40.0%) was purchased from Shen Yang Hua Dong Chemical Works. Perchloric acid (HClO4, A.R.) and acetic acid (CH3COOH, A.R.) were obtained from Tianjin Chemical Reagent Co. Ltd. Vulcan XC-72 carbon support was gained from Alfa Aesar. All aqueous solutions used in experiments were prepared with ultrapure water supplied by a Nanopure water system (18.3 MΩ cm). 2.2 Preparation of hollow carbon spheres Hollow carbon spheres were prepared through a modified StÖber method.25 Typically, 0.1 mL TEOS dissolved in 7.5 mL ethanol was dropped into the mixed solution of 5 mL ethanol, 1.5 mL water and 0.5 mL ammonia and stirred for 1 h. Then 0.025 g resorcinol and 0.035 mL formaldehyde were added into the mixture solution and stirred for 24 h. After washed with deionized water and ethanol for several times, the black sample was dried in an oven at 60 oC overnight. The obtained powder was calcinated at 800 oC under nitrogen flow for 1 h with a heating rate of 3 oC /min. 2.3 Synthesis of PtNi nanocubes supported on hollow carbon sphere (PtNi NCs-PVP@HCS) PtNi NCs-PVP@HCS was synthesis by a one-step solvothermal method using K2PtCl4 and NiCl2·6H2O as metal precursors at the molar ratio of 1:1. Typically, 2 mg hollow carbon spheres was added into the mixture solution of 1 mL K2PtCl4 (20 mM) and 1 mL NiCl2·6H2O and was sonicated for 0.5 h. Then 0.075 g NaI·2H2O, 0.16 g PVP and 10 mL DMF were dissolved in the solution and sonicated for 15 min. The solution was transferred into a 25 mL-reaction kettle and reacted at 130 oC for 5 h before it was cooled to room temperature. The products were collected by centrifugation and washed with deionized water and ethanol for several times and dried in an oven at 60 oC. To investigate the effects of solvothermal temperature and molar ratio of metal precursors on the 6


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structure of the formed PtNi nanocrystals, various PtNi nanoparticles supported on hollow carbon spheres were prepared by changing the temperature (130 , 150 and 180 oC) and the molar ratio of Pt/Ni (1:3, 1:2, 1:1, 2:1 and 3:1). For comparison, unsupported PtNi composites with different molar ratios were also prepared using the same method without the presence of hollow carbon spheres. 2.4 Preparation of PtNi nanocubes supported on Vulcan-X72 carbon (PtNi NCs@C). Vulcan-X72 carbon was dispersed in ultrapure water, and sonicated for 0.5 h. The synthesized PtNi nanocubes (130 oC, 1:1) were dropped into the above solution and stirred for 24 h. After washed with deionized water and ethanol for several times and dried in an oven at 60 oC, PtNi nanocubes supported on Vulcan -72 carbon (PtNi NCs@C) could be obtained. 2.5 Material characterization Transmission electron microscopy (TEM) images were taken on a Hitachi H-600 operated at 100 kV. The samples dispersed in the mixed solvents of water and ethanol were dropped onto the carbon supported copper TEM grids using pipettes and dried at room temperature. High resolution transmission electron microscopy (HRTEM) images, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images and element mappings were all performed on a JEM-2010 (HR) microscope operated at 200 kV. Scanning electron microscope (SEM) images were obtained from a XL30 ESEMFEG scanning electron microscope (FE-SEM) operating at 20 kV. Powder X-ray diffraction (XRD) measurements were carried out on a D8 ADVANCE (from Germany) using Cu Kα radiation with a scan rate of 1°/min. X-ray photoelectron spectroscopy (XPS) measurements were conducted on a VG Thermo ESCALAB 250 spectrometer operated at 120 W with an Al Kα line as the excitation source. Raman measurements were conducted on a Renishaw 2000 model confocal microscopy Raman spectrometer with 7



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a CCD detector and a holographic notch filter. The Barrent-Emmett-Teller (BET) surface area, nitrogen adsorption-desorption isotherms and pore size distribution curves were obtained from a Micromeritics ASAP 2020 V 4.0 system. 2.6 Electrochemical measurements All electrochemical measurements were performed on a CHI 750D electrochemical workstation at room temperature using a typical three-electrode cell with a Ag/AgCl electrode as the reference electrode, a platinum foil as a counter electrode and the glassy carbon electrode (GCE, 3 mm in diameter) or a rotating disk electrode (RDE, 5 mm in diameter, Pine Research Instrument) supported catalyst as working electrode. The catalyst suspension solution was prepared by mixing the catalyst with water, isopropyl alcohol and Nafion (5%) (V/V/V = 4/1/0.025) to form a concentration of 2 mg/mL with ultrasonication for 30 min. All potentials in this study were converted to reversible hydrogen electrode (RHE) using the equation ERHE = EAg/AgCl + 0.059 pH + 0.197 in 0.1 M HClO4 solution. The RDE and glassy carbon electrode should be first polished on chamois leather using alumina solution, washed with ultrapure water and ethanol to make a clean mirror surface. Subsequently, 20 µL or 5 µL of the catalyst suspension was pipetted onto the pre-polished RDE or GCE electrode, and dried at room temperature. Prior to the measurements, the working electrode was electrochemically activated by continuously cycling for 50 cycles at 100 mV/s in 0.1 M HClO4 solution from 0.2 to 1.1 V until a steady cyclic voltammogram was obtained. The cyclic voltammetry (CV) was recorded on a modified GCE in N2- or O2- saturated 0.1 M HClO4 from 0.05 to 1.1 V (vs RHE) at a scan rate of 50 mV/s. The transferred electron number for ORR can be obtained from the Koutecky-Levich (K-L) equations:26

1 1 1 1 1 = + = + J J K J L J K Bω 1/ 2

(1)

B = 0.2nFC 0 D 2 / 3υ −1/ 6

(2) 8


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where J is the current density measured by experiments, JK and JL are the kinetic and diffusion-limited current densities, respectively; ω is the rotation rate (rpm); n is the number of electrons transferred; F is Faraday’s constant (96485 C/mol); C0 is the concentration of O2 dissolved in 0.1 M HClO4 (1.22 × 10-6 mol/cm3); D is the diffusion coefficient of O2 in 0.1 M HClO4 solution (1.93 × 10-5 cm2 /s) and ν is the kinematic viscosity of the electrolyte solution (0.01 cm2/ s). To improve the electrocatalytic activity, the as-synthesized PtNi NCs-PVP@HCS was post-treated by a heat annealing process followed by CO stripping. The sample was first annealed at 300 oC for 1 h with a heating rate of 1 oC/min in a tube furnace. The obtained sample is denoted as PtNi NCs@HCS. The subsequent CO stripping experiments were conducted in two cells with 0.1 M HClO4. One was bubbled with CO for 15 min to prepare CO-saturated electrolyte and the other was degassed with N2 to eliminate any other dissolved gases in the electrolyte. In the CO-saturated solution, the modified glassy carbon electrodes were treated by amperometric i-t at the fixed potential of 0.00 V (vs Ag/AgCl) for 1500 s to adsorb CO. The CO-adsorbed electrode was then transferred quickly to the N2-saturated cell and the adsorbed CO was oxidized by CV with a scan rate of 50 mV/s. The sample treated by CO-stripping is denoted as PtNi NCs@HCS(COS). It should be noteworthy that the initial scan should be positive and the sweep segments should be 4, which was taken to verify the complete oxidation of adsorbed CO. For comparison, the as-synthesized PtNi NCs-PVP@HCS was also treated by an acid washing process, which was conducted by soaking the electrocatalyst in 1.0 M acetic acid solution for 5.0 h. The ORR polarization curves were measured in O2-saturated 0.1 M HClO4 solution at a scan rate of 10 mV/s with different rotation rates of 1600, 1225, 900, 625 and 400 rpm. Electrochemical accelerated durability tests (ADTs) were measured by a cyclic potential sweep between 0.55 and 0.95 V (vs RHE) in O2-saturated 0.1 M HClO4 solution with a scan rate of 100 mV/s. 9



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Methanol electro-oxidation measurements were measured from 0.05 to 1.2 V at a scan rate of 50 mV/s in N2-saturated 0.1 M HClO4 + 0.5 M CH3OH solution. The amperometric i-t curves were recorded at a potential of 0.67 V for 1500 s in 0.1 M HClO4 + 0.5 M CH3OH solution.

3. RESULTS AND DISCUSSION 3.1 Synthesis and characterization of PtNi NCs-PVP@HCS and PtNi NCs@HCS To obtain high-performance PtNi electrocatalysts, much effort have focused on preparing surface-clean nanoparticles.18, 24 As shown in Scheme 1, in the present study, PVP-protected PtNi nanocubes uniformly supported on hollow carbon spheres were synthesized through a one-step solvothermal method. To further improve the electrocatalytic activities of the PtNi nanocubes for ORR and methanol oxidation, a post-treatment of high-temperature (300 oC) annealing and the following electrochemical CO stripping was performed. The morphologies of the as-synthesized PtNi NCs-PVP@HCS and the annealed sample PtNi NCs@HCS were first characterized by TEM and SEM (Figure 1). Figure 1A and C shows the TEM images of the as-prepared PtNi NCs-PVP@HCS at different magnifications. It can be seen that the produced PVP-protected PtNi nanocrystals have cubic shape and are uniformly dispersed on the hollow carbon spheres. From the size distribution histogram shown in Figure S1A, the average size of the PtNi nanocubes is about 8.09±2 nm. After annealing at 300 oC, the morphology of the obtained PtNi NCs@HCS was also characterized by TEM (Figure 1B and D). Compared to the initial cubic nanocrystals, most of the PtNi nanoparticles were changed to nanospheres during the post heat-treatment. However, the average diameter of PtNi nanospheres (8.42±1.5 nm, Figure S1B) is close to that of the PtNi nanocubes. Based on the studies from Imre et al.27 and Duc et al.,28 PVP could be decomposed of at around 300 oC. Therefore, in the present study, the annealing process at 300 oC can remove the PVP capped on the surface of PtNi 10


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nanocrystals. Figure 1E and F compares the SEM images of the PtNi NCs-PVP@HCS and PtNi NCs@HCS. It can be seen that after the annealing treatment, the hollow carbon spheres still maintain the spherical shape with uniform size. However, the annealed PtNi nanocrystals show a more clear morphology than those before heat-treatment, which could be ascribed to the removal of PVP capped on the surface of PtNi nanocrystals by the heat-treatment. Meanwhile, without the PVP protective layer, cubic nanocrystals with high surface energy tend to transform into nanospheres during the annealing process. To further study the effect of annealing treatment on the crystal structure and element distribution of PtNi nanoparticles on the hollow carbons, HRTEM measurements were also performed. From the HRTEM images of PtNi NCs-PVP@HCS shown in Figure 2A, H and I, the synthesized PtNi nanocrystals have regular cubical shape. Meanwhile, the HRTEM images of a single nanocube in Figure 2H and I display clear continuous lattice fringes with a 0.194 or 0.218 nm interplanar spacing, corresponding to the Pt(100) and Pt(111) planes, respectively. It should be pointed out that the interplanar spacings are larger than those of Ni (0.176 and 0.203 nm) and smaller than those of pure Pt (0.196 and 0.227 nm), implying the successful synthesis of PtNi alloys.29-32 High angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images and corresponding elemental mappings in Figure 2B-G clearly demonstrate that the produced carbon sphere shell is mainly composed of C and O, and the nanocubes dispersed on the surface of HCS are composed of Pt and Ni with uniform distribution. After the annealing treatment, it can be seen from Figure 2J and K that the PtNi nanocrystals are still well dispersed on the surface of carbon spheres. Compared to the cubic shape shown in Figure 2H and I, the annealed nanoparticle shows a spherical shape with Pt (111) plane. Similar to the PtNi NCs-PVP@HCS, the HAADF-STEM image and elemental mappings of PtNi NCs@HCS clearly show the uniform dispersion of Pt and Ni on HCS. It is noteworthy that Stamenkovic et al. reported that the Pt3Ni(111) facet 11



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experimentally and theoretically exhibited much higher ORR catalytic activity compared to Pt3Ni(100) and Pt3Ni(110) facets.20 Therefore, after removing the PVP molecules by the thermal annealing process, the surface-cleaned PtNi nanocrystals with (111) crystal facet are expected to have high catalytic activity. Figure 3 shows the XRD patterns of the PtNi NCs-PVP@HCS, PtNi NCs@HCS and HCS. For all the studied samples, there is a broad diffraction peak with 2θ around 18.1o, corresponding to the (002) plane of carbon spheres. Compared with HCS, the strong diffraction peaks from PtNi nanocrystals in PtNi NCs-PVP@HCS and PtNi NCs@HCS can be observed and they are in agreement with the standard diffraction peaks of Pt with a little positive shift, which indicates the formation of PtNi alloys. The previous studies demonstrated that for PtNi alloys, the diffraction peaks showed more positive shift for the sample with more Ni content.33 Therefore, the slight shift suggests the low content of Ni in the present PtNi alloys, which can also be seen from the following XPS measurements. Usually, the relative diffraction intensities of different crystal faces are related to the shape of nanocrystals. For example, Xu et al. found that the PtNi nanocrystals with different shapes exhibited various XRD patterns.16 The concave nanocubic PtNi and hexoctahedral PtNi alloys showed the highest (220) peak intensity, and the PtNi nanocubes exhibited the enhanced (111) peak intensity. In this study, both PtNi NCs-PVP@HCS and PtNi NCs@HCS exhibited the enhanced (111) peak intensity, indicating the almost same preferential orientations of PtNi nanocrystals in both samples. In addition, based on the XRD diffraction peaks and Scherrer’s equation, the sizes of PtNi nanoparticles were calculated to be 7.8 and 8.1 nm, respectively, for PtNi NCs-PVP@HCS and PtNi NCs@HCS. Such result agrees well with that from TEM measurements. The composition and surface chemical states of PtNi NCs-PVP@HCS and PtNi NCs@HCS were examined by XPS measurements. In the Pt 4f XPS spectra of PtNi NCs-PVP@HCS (Figure 4A) and PtNi NCs@HCS, (Figure 4C), the two strong peaks centered around 70 and 73 eV can be assigned to Pt 4f7/2 12


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and Pt 4f5/2. It should be noted that compared to the binding energy of pure metallic Pt (4f7/2:70.90 eV), the Pt 4f7/2 binding energies from the present PtNi nanoparticles (70.02 and 70.21 eV) show negative shifts of 0.88 and 0.69 eV, respectively. Such results suggest again the formation of PtNi alloys and the possible electron donation from Ni to Pt.16. There are also two small peaks at 71.4 and 75.1 eV which can be assigned to Pt2+ in PtO and Pt(OH)2.32, 34-35 The much stronger intensities of Pt (0) than PtO and Pt(OH)2 shows that Pt in PtNi nanoalloys is predominately metallic. The C1s XPS spectra shown in Figure 4B and D can be fitted into three peaks, corresponding to graphitic, aromatic or aliphatic C-C/C=C bonds at 284.5 eV, C-O bonds at 286.3 eV and C=O bonds at 288.4 eV.36 The measured O 1s binding energies in Figure S2A-B can be fitted into three peaks of 531.8 eV (OH), 532.8 eV or 532.9 eV (C=O) and 533.6 eV or 533.7 eV (C-O).37 The Ni 2p XPS spectra (Figure S2C-D) are much complicated because of the intense satellite signals adjacent to the main peaks that can be attributed to the multielectron excitations.36, 38 The Ni 2p peaks centered at the binding energies of 852.7, 855.3 and 857.3 eV are assigned to Ni0, Ni(OH)2 and NiOOH, respectively. From Table S1, it can be seen that the Ni contents on the surface of these two samples are both low (0.03 and 0.05), which lead to the weak intensity of Ni 2p XPS. To quantitatively determine the specific surface area and pore volume of the prepared hollow carbon spheres, BET analysis was conducted. Figure S3 shows the N2 adsorption-desorption isotherm and pore size distribution of the HCS, from which the BET surface area and the average pore width were measured to be 391 m2/g and 3.84 nm, respectively. The large surface area of HCS is beneficial to serve as substrate for supporting the PtNi nanoparticles and the porous structure can provide abundant channels for mass transfer in electrocatalysis. Meanwhile, to further illustrate the effect of the annealing treatment on the hollow carbon spheres, Raman spectra of the PtNi NCs-PVP@HCS and PtNi NCs@HCS were measured. As shown in Figure S4, two characteristic D and G bands of the carbon spheres can be seen at 1365 and 13



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1600 cm-1, respectively, for both samples. D band usually represents the disorder structures and G band reflects the degree of crystallinity of graphitic structures. Therefore, ID/IG can be used to illustrate the structural defects and the graphitization degree. The ID/IG values of PtNi NCs@HCS and PtNi NCs-PVP@HCS are 0.73 and 0.67, respectively. Such result suggests that the disorder of hollow carbon spheres increases after the annealing process. To get a further insight into the formation mechanisms of un-supported PtNi nanocubes and PtNi nanocrystals supported on carbon spheres, the influences of Pt/Ni molar ratio and solvothermal temperature on the products were investigated and the products were characterized by TEM. Figure S5A-C shows the TEM images of PtNi nanoparticles synthesized at 130 oC for 5 h with different Pt/Ni ratios. It can be seen that with the decrease of Pt/Ni ratio, PtNi nanoparticles changed from small crystals with irregular shape (Figure S5A, 3:1) to part of cubic shape (Figure S5B, 2:1) and then to cubic shape (Figure S5C, 1:1). These results indicate that an appropriate Pt/Ni ratio of 1:1 is optimal for the formation of PtNi nanocube structure. Similarly, as shown in Figure S6, for the PtNi NCs@HCS sample prepared with the Pt/Ni ratio of 1:1, the formed PtNi nanocrystals are well dispersed on the hollow carbon spheres. Then the influences of solvothermal temperature on the morphologies of un-supported PtNi and HCS-supported PtNi nanoparticles were studied. At a fixed Pt/Ni ratio of 1:1, PtNi nanocrystals without the presence of HCS were synthesized by increasing the reaction temperature from 130, 150 to 180 oC. One can see from Figure S5D-F that at the reaction temperature of 130 oC, the obtained PtNi nanocrystals show cubic shape with uniform size and good dispersion. At higher temperatures (150 and 180 oC), irregular and spherical nanoparticles with smaller size were produced. Moreover, aggregations of nanoparticles can also be observed. Similarly, with the presence of hollow carbon spheres in the reaction system, the nanocrystals synthesized at 130 oC show the uniform cubic shape and good dispersion (Figure S5G) and the samples 14


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obtained under higher temperatures exhibit irregular and aggregated nanoparticles (Figure S5H and I). Meanwhile, all the formed PtNi nanoparticles are supported on the surface of hollow carbon spheres without any outside the carbon spheres, indicating that HCS can be used as good catalyst support. These studies suggest that reaction temperature and metal precursor ratio are the two important parameters for tuning the size, shape and dispersion state of the produced PtNi nanocrystals. Based on above results, the reaction temperature of 130 oC and Pt/Ni ratio of 1:1 are the optimum conditions for synthesizing uniform cubic PtNi nanocrystals supported on hollow carbon spheres.

3.2 Electrocatalytic activity of PtNi nanocrystals supported on hollow carbon spheres for oxygen reduction reaction (ORR) Proton exchange membrane fuel cells (PEMFCs) represent a promising alternative for clean energy production with high energy transfer efficiency. According to the theoretical studies by Norskov, Koper and others,39-41 for ORR on Pt surface, the fourth step, protonation, electronation and desorption of OH groups on Pt sites is the rate limiting step. Therefore, weakening the oxygen bonding to Pt atoms can strengthen the ORR activities of Pt-based nanocatalysts. The lattice compression and electron transfer from transition metal to Pt can weaken the oxygen bonding through lowering the d-band center of Pt atoms on the surface.42-44 Pt-based alloy nanoparticles, especially PtNi can enhance the ORR activity and simultaneously reduce the usage of Pt. For ORR catalysts, enhancing the specific activity and improving the durability are what the researchers have been pursuing. To evaluate the ORR electrocatalytic performances of the as-synthesized and post-treated PtNi NCs-PVP@HCS, cyclic voltammograms (CVs) and polarization curves were recorded from the catalyst-modified RDE in 0.1 M HClO4 solution. Figure 5A shows the CV curves from different 15



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catalysts in 0.1 M N2-saturated HClO4. It can be seen that the as-synthesized PtNi NCs-PVP@HCS shows a featureless CV (dark green curve), which is obviously due to that the surface active sites are blocked by the PVP protecting molecules. However, after the heat annealing process, the obtained PtNi NCs@HCS exhibits a characteristic Pt-surface CV (blue curve) although the current signals are weaker than the commercial Pt/C (black curve). Interestingly, after the further CO-stripping treatment, the resulting PtNi NCs@HCS(COS) displays enhanced CV features, including the well-defined current peaks from hydrogen adsorption and desorption in the low potential region of 0~0.36 V, the oxidation of Pt and the corresponding reduction peak of Pt oxides (0.77 V) in the reverse sweep. Note that compared to the Pt/C catalyst, the current peaks of hydrogen desorption from PtNi NCs@HCS(COS) show positive shifts, which may be due to the electron interaction in the PtNi alloys and the resulting higher hydrogen chemisorption energy.45 Meanwhile, the reduction peak at 0.77 V from the PtNi NCs@HCS(COS) is more positive than that from Pt/C (0.73 V), suggesting the easier desorption of hydroxyl from the PtNi surface.46 Figure S7A shows the CVs of PtNi NCs@HCS(COS) in 0.1 M N2- and O2-saturated HClO4. Clearly, the prepared sample shows electrocatalytic activity for ORR. Figure 5B compares the ORR on different electrodes. In accordance with Figure 5A, the PtNi NCs-PVP@HCS shows very low catalytic activity for ORR with a small reduction current and low peak potential. However, after the annealing and CO-stripping treatments, the obtained PtNi NCs@HCS(COS) demonstrates more positive ORR onset and peak potentials compared to the Pt/C and the PtNi NCs@HCS catalysts. For comparison, the initial PtNi NCs-PVP@HCS was also washed by 1.0 M acetic acid. Figure S7B compares the ORR catalytic activities of the as-prepared, acid washed and CO-stripping treated PtNi NCs-PVP@HCS. Among the three samples, the PtNi NCs@HCS(COS) has the highest activity with much higher current density and more positive oxygen reduction potential. These electrochemical results indicate that the treatments of heat annealing and 16


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following CO-stripping are effective methods to significantly enhance the catalytic activity of the hollow carbon sphere-supported PtNi nanocrystals. The catalytic activities of the synthesized materials for ORR were also studied by RDE. Firstly, the ORR on PtNi NCs-PVP@HCS prepared at different conditions was examined. Figure S8A compares the linear sweep voltammograms (LSVs) of ORR on PtNi NCs-PVP@HCS obtained at different temperatures with the fixed Pt/Ni ratio of 1:1. Evidently, the sample obtained at 130 oC exhibits the most positive onset and half-wave potentials and the highest ORR current. Meanwhile the performances of samples prepared from different Pt/Ni ratios (1:1, 1:2, 1:3 and 2:1) at 130 oC were also tested by LSV, as shown in Figure S8B. It was found that the PtNi NCs-PVP@HCS prepared from the Pt/Ni ratio of 1:1 possesses the best ORR performance. From Figure S8C, the PtNi NCs@HCS exhibits more positive half-wave potential for ORR than PtNi NCs-PVP@HCS and PtNi NCs@C. It has been reported that CO stripping is an efficient method to clean the particle surface and thus to enhance the electrochemical properties.47 As presented in Figure 6A, after CO stripping the LSV of PtNi NCs@HCS(COS) shows a 20 mV more positive half-wave potential (E1/2) and higher diffusion-limited current density than that before CO stripping, indicating the CO stripping treatment can further improve the activity of PtNi NCs@HCS. The LSVs measured on rotating disk electrode (RDE) at different rotation rates and the corresponding Koutecky-Levich (K-L) plots are shown in Figure 6B and C, respectively. These plots imply that the electrochemical reaction towards the concentration of dissolved oxygen is the first order reaction. Based on the K-L plots, the electron transfer number (n) was calculated to be around 4.0, indicating an efficient four-electron ORR process. These results are close to the ORR catalytic performance of commercial Pt/C as shown in Figure S9A and B. Accelerated durability test (ADT) was used to evaluate the stability of the ORR catalysts. Figure 6D 17



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compares the ORR polarization curves of PtNi NCs@HCS(COS) and commercial Pt/C catalyst before and after 5000 cycles in 0.1 M HClO4 solution. It can be seen that before the ADT tests, compared to commercial Pt/C, the PtNi NCs@HCS(COS) shows an close ORR onset potential (0.89 V), larger limited diffusion current but a more negative half-wave potential. However, after the durability test of 5000 cycles, the PtNi NCs@HCS(COS) demonstrates almost no change of the ORR current and potential while the commercial Pt/C shows a 90 mV negative shift in its half-wave potential and an obvious ORR current decrease, indicating the enhanced electrochemical durability of the PtNi nanocrystals supported on hollow carbon spheres. Meanwhile, the ORR overpotential can be defined as the potential difference between the equilibrium potential and the potential at which ORR current density of 0.1 mA/cm2 is experimentally observed.48 Based on this, the overpotential for ORR on both PtNi NCs@HCS(COS) and Pt/C is 0.35 V as obtained from the LSV in Figure 6 and Figure S9. However, after the ADT tests, the overpotential from Pt/C increases to 0.40 V, while the overpotential remains unchanged on the PtNi NCs@HCS(COS), as illustrated in Figure 6D. Above electrochemical results indicate that the PtNi NCs@HCS(COS) after 300 o

C annealing and CO stripping treatments is a type of promising electrocatalyst for ORR with pretty higher

catalytic activity and durability than Pt/C. 3.3 Electrocatalytic activity of PtNi NCs@HCS for the methanol oxidation reaction (MOR) The methanol oxidation on Pt surface involves three main steps. The first two steps include the methanol adsorption and the following dehydrogenation to form adsorbed CO intermediates. The final process is the oxidization of the adsorbed CO with the aid of oxygen–containing species (-OH) formed on Pt surface. The mechanism can be illustrated by the following equations 3-5.49

CH 3OH + Pt → Pt − (CH 3OH )ads

(3)

Pt − (CH 3OH )ads → Pt − (CO) ads + 4 H + + 4e −

(4) 18


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Pt − (CO) ads + H 2O → Pt + CO2 + 2 H + + 2e −

(5)

However, the formation of oxygen-containing species on Pt surface occurs only at higher potentials (> 0.7 V vs RHE), which leads to large overpotential and limits the application of Pt catalysts for methanol oxidation reaction.50 Introducing another transition metal into Pt catalysts to form bimetallic catalysts is an effective method to decrease the CO poisoning of Pt surface and enhance the catalytic activity for methanol oxidation, due to the lower potential for the formation of oxygen species on transition metals.51-52 The bifunctional mechanism of Pt-based bimetallic catalysts for methanol oxidation can be described by the following equations:

Pt − (CH 3OH ) ads → Pt − (CO ) ads + 4 H + + 4e − +

M + H 2O → M − OH ads + H + e

−

Pt − (CO ) ads + M − OH ads → Pt + M + CO2 + H + + e −

(6) (7) (8)

In addition to the bifunctional mechanism, electronic mechanism and ligand effect have also been proposed to improve the catalytic activities of bimetallic catalysts for methanol oxidation.53-54 For Pt-based alloy catalysts, the second metal can modified the electronic properties of Pt by changing the electron density of states of d-band and the Fermi level energy.55 Such electronic modification can weaken the interaction between CO molecules and Pt surface.56 In this study, the electrocatalytic activity of PtNi NCs@HCS(COS) for methanol oxidation was also investigated. CO stripping was first used to clean the surface of catalysts and determine the electrochemical surface area. Figure S10 shows the CO stripping curves on Pt/C, PtNi NCs-PVP@HCS and PtNi NCs@HCS(COS). One can see that the CV curves from the three electrodes present the similar profiles. In the lower potential region of the first sweep circle, the absence of hydrogen desorption peaks 19



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confirms the CO adsorption. During the positive sweep, the CO oxidation peaks can be observed on all the electrodes. However, the CO oxidation peaks disappear and the hydrogen adsorption/desorption peaks recover in the second sweep circle, indicating the adsorbed CO on the surface of catalysts have been completely oxidized. Meanwhile, the electrochemical surface area (ECSA) can be calculated from the formula: ECSA = QCO /Q0, where QCO is the consumed charge during the oxidation of adsorbed CO and Q0 is the standard charge (420 µC/cm2) needed for the oxidation of monolayer CO on Pt surface. The ECSAs of PtNi NCs-PVP@HCS, PtNi NCs@HCS(COS) and Pt/C were calculated to be 0.34, 1.93 and 1.65 cm2, respectively. Namely, the PtNi NCs@HCS(COS) has larger ECSA than PtNi NCs-PVP@HCS and commercial Pt/C and therefore provides more active sites for MOR. Moreover, based on the ECSA and the mass loading of Pt calculated from ICP-OES, the specific electrochemical surface areas (SECSAs; m2/gpt) of PtNi NCs-PVP@HCS, PtNi NCs@HCS(COS) and Pt/C were calculated to be 85.4, 484.9 and 82.5 m2/gPt, respectively. That is the SECSA of PtNi NCs@HCS(COS) is 5.7 and 5.9 times of the PtNi NCs-PVP@HCS and Pt/C, respectively. The lower SECSA of PtNi NCs-PVP@HCS compared to PtNi NCs@HCS(COS) can be ascribed to the surface coverage of PVP which blocks a large number of surface active sites. In addition, the SECSA of PtNi NCs@HCS(COS) is much larger than that of the sandwich-like Pt3Ni-C/rGO (52.7 m2/g).24 Meanwhile, compared to Pt/C, the much larger SECSA of PtNi NCs@HCS(COS) can be due to the high surface area of hollow carbon spheres (see above BET measurements). With HCS as support, the PtNi nanocrystals can be highly and uniformly dispersed, as displayed in the SEM and TEM characterizations. Meanwhile, from Figure S10, the peak potentials of CO oxidation on PtNi NCs-PVP@HCS, PtNi NCs@HCS(COS) and Pt/C are 0.88, 0.83 and 0.83 V, respectively. This result suggests that the CO stripping treatment can also improve the catalytic activity of PtNi nanocrystals for CO oxidation. 20


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Figure 7A shows the CV curves of methanol oxidation on both Pt/C and PtNi NCs@HCS(COS) in N2-saturated 0.1 M HClO4 + 0.5 M CH3OH solution. Here, the obtained currents were normalized to the mass loading of Pt based on the ICP measurements. It can be seen that the peak current density of methanol oxidation on PtNi NCs@HCS(COS) is 3.89 A/mgPt, which is about 9 times larger than that from commercial Pt/C (0.43 A/mgPt). Moreover, amperometric i-t curves recorded at 0.67 V in 0.1 M HClO4 + 0.5 M CH3OH solution were used to examine the stabilities of the electrocatalysts. As shown in Figure 7B, at the beginning an initially rapid current decay can be observed for both PtNi NCs@HCS(COS) and commercial Pt/C. This may be ascribed to the accumulation of poisonous intermediates (such as CH3OHads, COads, etc.) on the surface of catalysts during the methanol oxidation reaction.57-59 However, during the entire time examined, the oxidation current density from PtNi NCs@HCS(COS) is always higher than that from commercial Pt/C, indicating the higher catalytic activity and stability of PtNi NCs@HCS(COS) for methanol oxidation compared to commercial Pt/C.

4. Conclusion To improve the dispersity and stability of nanoparticle catalysts, in the present study, PtNi nanocrystals supported on hollow carbon spheres (PtNi NCs-PVP@HCS) were prepared by one-step solvothermal method. The effects of the Pt/Ni precursor ratio and the reaction temperature on the morphology of PtNi nanocrystals, the dispersity of PtNi nanocrystals on carbon spheres were investigated and it was found that the optimum temperature and Pt/Ni ratio are 130 oC and 1:1, respectively. For the PtNi NCs@PVP-HCS (130 oC, 1:1) sample, the produced PtNi nanocrystals with regular cubic shape are uniformly dispersed on the hollow carbon spheres. To further improve the electrocatalytic activity, PtNi NCs-PVP@HCS was first annealed at 300 oC (PtNi NCs@HCS) and subsequently treated by an electrochemical CO stripping 21



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process (PtNi NCs@HCS(COS)). After these surface cleaning treatments, the PtNi nanocrystals with exposed (111) facets showed much enhanced and higher electrocatalytic activity and stability for both oxygen reduction and methanol oxidation reactions compared to commercial Pt/C catalyst. Hence, this study demonstrates that high temperature annealing and electrochemical CO stripping are effective treatments to enhance the catalytic activities of surfactant-protected metal nanoparticles and the hollow carbon sphere-supported metal nanoparticles could be a type of high performance electrocatalyst for fuel cells.

ASSOCIATED CONTENT Supporting Information More structural characterizations and electrochemical measurements. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.00. The authors declare no competing financial interest. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (W. Chen) Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21633008, 21575134) and National Key Research and Development Plan (2016YFA0203200).

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Cell Catalysts. Nat. Chem. 2010, 2, 454-460. 44. Xin, H. L.; Holewinski, A.; Schweitzer, N.; Nikolla, E.; Linic, S., Electronic Structure Engineering in Heterogeneous Catalysis: Identifying Novel Alloy Catalysts Based on Rapid Screening for Materials with Desired Electronic Properties. Top. Catal. 2012, 55, 376-390. 45. Lu, Y. Z.; Chen, W., Pdag Alloy Nanowires: Facile One-Step Synthesis and High Electrocatalytic Activity for Formic Acid Oxidation. Acs Catal. 2012, 2, 84-90. 46. Lu, Y. Z.; Jiang, Y. Y.; Chen, W., PtPd Porous Nanorods with Enhanced Electrocatalytic Activity and Durability for Oxygen Reduction Reaction. Nano Energy 2013, 2, 836-844. 47. Lu, Y. Z.; Jiang, Y. Y.; Wu, H. B.; Chen, W., Nano-PtPd Cubes on Graphene Exhibit Enhanced Activity and Durability in Methanol Electrooxidation after CO Stripping-Cleaning. J. Phys. Chem. C 2013, 117, 2926-2938. 48. Lee, K.; Savadogo, O.; Ishihara, A.; Mitsushima, S.; Kamiya, N.; Ota, K., Methanol-Tolerant Oxygen Reduction Electrocatalysts Based on Pd-3d Transition Metal Alloys for Direct Methanol Fuel Cells. J. Electrochem. Soc. 2006, 153, A20-A24. 49. Vielstich, W.; Lamm, A.; Gasteiger, H. A., Handbook of Fuel Cells: Fundamentals, Technology, and Applications. John Wiley & Sons: 2009; Vol. 5. 50. Hamelinck, C. N.; Faaij, A. P. C., Outlook for Advanced Biofuels. Energ. Policy 2006, 34, 3268-3283. 51. Iwasita, T.; Hoster, H.; John-Anacker, A.; Lin, W. F.; Vielstich, W., Methanol Oxidation on PtRu Electrodes. Influence of Surface Structure and Pt-Ru Atom Distribution. Langmuir 2000, 16, 522-529. 52. Choi, J. H.; Park, K. W.; Park, I. S.; Kim, K.; Lee, J. S.; Sung, Y. E., A PtAu Nanoparticle Electrocatalyst for Methanol Electrooxidation in Direct Methanol Fuel Cells. J. Electrochem. Soc. 2006, 153, A1812-A1817. 53. Watanabe, M.; Motoo, S., Electrocatalysis by Ad-Atoms: Part III. Enhancement of Oxidation of Carbon-Monoxide on Platinum by Ruthenium Ad-Atoms. J. Electroanal. Chem. 1975, 60, 275-283.

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54. Igarashi, H.; Fujino, T.; Zhu, Y. M.; Uchida, H.; Watanabe, M., CO Tolerance of Pt Alloy Electrocatalysts for Polymer Electrolyte Fuel Cells and the Detoxification Mechanism. Phys. Chem. Chem. Phys. 2001, 3, 306-314. 55. Nassr, A. A. A.; Sinev, I.; Pohl, M. M.; Grunert, W.; Bron, M., Rapid Microwave-Assisted Polyol Reduction for the Preparation of Highly Active PtNi/CNT Electrocatalysts for Methanol Oxidation. Acs Catal. 2014, 4, 2449-2462. 56. Scott, F. J.; Mukerjee, S.; Ramaker, D. E., Contrast in Metal-Ligand Effects on PtnM Electrocatalysts with M Equal Ru Vs Mo and Sn as Exhibited by in situ XANES and EXAFS Measurements in Methanol. J. Phys. Chem. C 2010, 114, 442-453. 57. Gao, H. L.; Liao, S. J.; Liang, Z. X.; Liang, H. G.; Luoa, F., Anodic Oxidation of Ethanol on Core-Shell Structured Ru@PtPd/C Catalyst in Alkaline Media. J. Power Sources 2011, 196, 6138-6143. 58. Ren, F. F.; Wang, H. W.; Zhai, C. Y.; Zhu, M. S.; Yue, R. R.; Du, Y. K.; Yang, P.; Xu, J. K.; Lu, W. S., Clean Method for the Synthesis of Reduced Graphene Oxide-Supported PtPd Alloys with High Electrocatalytic Activity for Ethanol Oxidation in Alkaline Medium. Acs Appl. Mater. Inter. 2014, 6, 3607-3614. 59. Yan, Z. X.; He, G. Q.; Jiang, Z. F.; Wei, W.; Gao, L. N.; Xie, J. M., Mesoporous Graphene-Like Nanobowls as Pt Electrocatalyst Support for Highly Active and Stable Methanol Oxidation. J. Power Sources 2015, 284, 497-503.

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Page 30 of 38

Cl

Pt2+

Cl

Cl

Ni

Cl Cl

PVP

Cl

Hydrothermal process

Calcination process

130 oC, 5h

HCS

300 oC, 1 h

PtNi NCs-PVP@HCS

PtNi NCs@HCS

H2O

CO2 - 6 e-

+ 4 e-

O2

CH3OH

CO stripping

Scheme 1 Schematic illustration of procedure for the preparation and post-treatments of PtNi NCs-PVP@HCS.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(A)

(B)

200 nm

200 nm

(C)

(D)

50 nm

50 nm

(E)

(F)

Figure 1 TEM images of PtNi NCs-PVP@HCS (A and C) and PtNi NCs@HCS (B and D). SEM images of PtNi NCs-PVP@HCS (E) and PtNi NCs@HCS (F).

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(H)

(B)

(A)

Page 32 of 38

0.194 nm

(I) 0.223 nm (C)

(D)

(E)

(F)

C-K

(G)

O-K

Pt-M

Ni-K

2 nm

(Q)

(K)

(J)

0.223 nm 50 nm (L)

(M)

(N)

(O)

(P) 2 nm

C-K

O-K

Pt-M

111

Ni-K

Figure 2 HRTEM images of PtNi NCs-PVP@HCS (A) and a single PtNi nanocube (H, I). HAADF-STEM images (B, C) of PtNi NCs-PVP@HCS and the corresponding elemental mapping of (D) C, (E) O, (F) Pt and (G) Ni. The right bottom insets in (H and I) show the FFT patterns of individual PtNi nanocrystal. HRTEM images of PtNi NCs@HCS (J, K) and a single PtNi nanosphere (Q). HAADF-STEM images (L) of PtNi NCs@HCS and the corresponding elemental mapping of (M) C, (N) O, (O) Pt and (P) Ni. The right bottom inset in (Q) shows the FFT pattern of individual PtNi nanocrystal.

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Page 33 of 38

(111)

PtNi NCs-PVP@HCS PtNi NCs-300@HCS HCS Pt 04-0802

C (002)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(200)

(220)

(311)

(222)

20

40

60

80

2θ θ (degree)

Figure 3 XRD patterns of PtNi NCs-PVP@HCS, PtNi NCs@HCS and HCS.

33



Pt 4f7/2

(A)

PtO

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

C=C/C-C

(B)

Pt 4f5/2

Page 34 of 38

C=O

Pt(OH)2 C-O

68

70

72

74

76

Pt 4f7/2

(C)

280

282

284

286

288

290

C=C/C-C

(D) Pt 4f5/2

Pt(OH)2

PtO

68

70

72

74

76

C-O

280

282

284

286

C=O

288

290

Binding Energy (eV) Figure 4 XPS spectra of Pt and C in PtNi NCs-PVP@HCS (A, B) and PtNi NCs@HCS (C, D).

34


Page 35 of 38

150

(A)

0.1

PtNi NCs-PVP@HCS Pt/C

100

(B)

0.0

I (mA)

50

I (µ µA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 -50

-0.1 -0.2 PtNi NCs-PVP@HCS Pt/C PtNi NCs@HCS PtNi NCs@HCS(COS)

-0.3

-100 PtNi NCs@HCS PtNi NCs@HCS(COS)

-150 0.0

0.2

0.4

0.6

0.8

1.0

-0.4 1.2

0.0

E(V vs RHE)

0.2

0.4

0.6

0.8

1.0

1.2

E(V vs RHE)

Figure 5 CV curves of the as-synthesized PtNi NCs-PVP@HCS (dark green curves), PtNi NCs@HCS after 300 oC annealing (blue curves), PtNi NCs@HCS(COS) after CO-stripping treatment (red curves), and commercial Pt/C catalyst (black curves) in 0.1 M N2-saturated (A) and O2-saturated (B) HClO4 solution with a potential scan rate of 50 mV/s.

35



-1.0

-1.0

J (mA/cm2)

(B) 0.0

J (mA/cm2)

(A) 0.0 -2.0 -3.0 -4.0 -5.0

Before CO stripping After CO stripping

-6.0 0.0

0.2

0.4

0.6

0.8

1.0

-2.0 -3.0

400 rpm 625 rpm 900 rpm 1225 rpm 1600 rpm

-4.0 -5.0 -6.0

1.2

0.0

0.2

0.4

E (V vs RHE) 0.35

-1.0

0.30 0.3 V 0.4 V 0.5 V 0.6 V

0.20

n=4.00 n=3.98 n=4.08 n=3.95

J (mA/cm2)

(D) 0.0

0.25

0.024 0.030 0.036 0.042 0.048 0.054

ω

(rpm

-1/2

0.8

1.0

1.2

PtNi NCs@HCS(COS)-1st cycle th

PtNi NCs@HCS(COS)-5000 cycle st

Pt/C-1 cycle Pt/C-5000th cycle

-2.0 -3.0 -4.0 -5.0 -6.0

0.15 -1/2

0.6

E (V vs RHE)

(C) 0.40

J-1 (mA-1cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 38

0.0

0.2

0.4

0.6

0.8

1.0

1.2

E (V vs RHE)

)

Figure 6 (A) LSVs of ORR on PtNi NCs@HCS before and after CO stripping treatment; (B) ORR polarization curves on PtNi NCs@HCS(COS) at different rotation rates (400, 625, 900, 1225 and 1600 rpm). (C) The corresponding Koutecky-Levich plots on PtNi NCs@HCS(COS) at different potentials. (D) LSVs comparison of PtNi NCs@HCS(COS) and Pt/C before and after ADT tests (5000 potential cycles). All measurements were carried out in O2-saturated 0.1 M HClO4 solution with a potential scan rate of 10 mV/s.

36


Page 37 of 38

4.0 3.0

4

(A)

Pt/C PtNi NCs@HCS(COS)

J (A/mgPt)

J (A/mgPt)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.0 1.0

(B)

3

Pt/C PtNi NCs@HCS(COS)

2 1

0.0 0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

0

100

E (V vs RHE)

200

300

400

t (s)

Figure 7 (A) CVs of methanol oxidation on commercial Pt/C and PtNi NCs@HCS(COS) in N2-saturated 0.1 M HClO4 + 0.5 M CH3OH solution, potential scan rate 50 mV/s. (B) I-t curves of methanol oxidation at 0.67 V on Pt/C and PtNi NCs@HCS(COS) in 0.1 M HClO4 + 0.5 M CH3OH solution.

37



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TOC

38


Page 38 of 38

PtNi Nanocrystals Supported on Hollow Carbon Spheres: Enhancing

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