Trimetallic Hollow Pt-Ni-Co Nanodendrites as Efficient Anodic

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Trimetallic Hollow Pt-Ni-Co Nanodendrites as Efficient Anodic Electrocatalysts Rinrada Sriphathoorat, Kai Wang, and Pei Kang Shen ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01741 • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on January 15, 2019

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ACS Applied Energy Materials

Trimetallic Hollow Pt-Ni-Co Nanodendrites as Efficient Anodic Electrocatalysts Rinrada Sriphathoorat,† Kai Wang,† and Pei Kang Shen*, †, § †

School of Materials Science and Engineering, Sun Yat-sen University, Guangzhou 510275, P. R. China

Collaborative Innovation Center of Sustainable Energy Materials, Guangxi Key Laboratory of Electrochemical Energy Materials, State Key Laboratory of Processing for Non-Ferrous Metal and Featured Materials, Guangxi University, Nan-ning 530004, P. R. China. *E-mail: [email protected] §

KEYWORDS: hollow nanostructure; nanodendrites; Pt-Ni-Co; chemical etching; electro-oxidation catalysts. ABSTRACT: Hollow nanostructure represents an emerging class of efficient catalysts for advanced catalytic applications due to their intrinsic surface area and high catalytic activity. Here, we report the design and synthesis of hollow Pt-Ni-Co nanodendrites (NDs) through selectively etching the Ni rich core of solid core-shell Pt-Ni-Co NDs used as starting material. Moreover, the formation mechanism of solid core-shell Pt-Ni-Co NDs was investigated in detail by tracing the growth stages at different reaction time. Impressively, hollow Pt-Ni-Co NDs exhibit 4.2- and 6.7-times higher mass activity for methanol and formic acid oxidation than that of commercial Pt/C, respectively.

Platinum (Pt) is the most effective materials used in fuel cell technology as electrode catalysts.1 However, large scale production of Pt catalysts for commercialization is hindered by high price and limited resource of Pt.2 A large number of studies have been dedicated to fabricate cost-effective Pt electrocatalysts with excellent catalytic performances for decreasing the usage of Pt.3 Based on our knowledge, controlling on catalyst morphology is one of effective strategies to improve both Pt utilization and activity.4-5 The opened structure is promising structure providing more accessible surface area and reduced consumption of precious metals, thus improving Pt atomic utilization.6-7 Additionally, the introduction of 3d transition metal into Pt is another approach to modify surface electronic structure and thus enhances catalytic performances.8-9 Compared with monoand bi-metallic, trimetallic nanocrystals are able to extend possibility to synthesize unique structure with higher catalytic activities.10-12 As previous reports, Pt-Ni nanocrystals always exhibit much higher electrochemical activity owing to controllable surface structure such as exposed {111} facet, active surface area etc.13 On the other hand, Pt-Co nanocrystals tend to demonstrate better stability than other Pt-M nanoalloys, which are attributed to the fine-tuned electronic structure as explained in Markovic’s reports.8 Trimetallic Pt-Ni-Co nanocrystals are thus expected to provide higher catalytic activities and better stability. Up to now, synthesis of trimetallic Pt-Ni-Co

nanocrystals with opened structure has not been wellexplored yet, because the reactions between metal precursors and organic compounds are quite complexed and unpredictable. Therefore, it is of great interest to synthesize trimetallic Pt-Ni-Co nanocrystals with hollow structure and much enhanced catalytic activity. Herein, hollow Pt-Ni-Co nanodendrites (NDs) are successfully synthesized by selectively removing the Ni-rich core of solid Pt-Ni-Co NDs. In our experiment, Ni precursors tend to be reduced earlier than Pt and Co ions, leading to the formation of Ni-rich core. The hollow interior of Pt-Ni-Co NDs is able to be designed by controlling atomic ratio of metal precursors. Particularly, due to favorable features such as highly branched structure and hollow interior, hollow Pt-Ni-Co NDs exhibit larger ECSA and higher catalytic activities toward both methanol (MOR) and formic acid oxidation reaction (FAOR), compared to solid Pt-Ni-Co NDs and commercial Pt/C. The preparation of hollow Pt-Ni-Co NDs is illustrated in Scheme 1. Solid Pt-Ni-Co NDs, used as starting materials, were firstly synthesized by solvothermal method. Typically, Chloroplatinic acid (H2PtCl6·6H2O), cobalt(III) acetylacetonate [Co(acac)3], nickel(II) acetate [Ni(CH3COO)2] and Cetyltrimethylammonium chloride (CTAC) were dissolved in mixed solution containing oleylamine (OAm) and dimethyl formamide (DMF). After sonification, the homogeneous solution was kept at 170 °C for 12 hours, then cooled down to room temperature. The

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solid Pt-Ni-Co NDs were obtained. The use of mixed solution is a critical factor to control morphology of Pt-Ni-Co nanocrystals (figure S1). To prepare hollow structure, the solid Pt-Ni-Co NDs were dispersed into acetic acid, then the mixed solution was magnetically stirred for 12 hours at 70°C, resulting hollow Pt-Ni-Co NDs (see the Experiment Section for more details).

Scheme 1. The preparation of hollow Pt-Ni-Co NDs. The representative transmission electron microscopy (TEM) images and scanning transmission electron microscopy (STEM) images of as-prepared products show well-dispersed nanocrystals consisting of numerous dendrite subunits with a high yield (figure 1a-c, and S2). As shown in figure S3, the particle size of each nanocrystals is in the range from 50 to 90 nm, with average diameter of 67 nm. Figure S4 shows high-resolution transmission electron microscopy (HRTEM) image of a single nanodendrites and the corresponding fast Fourier transform (FFT) pattern, indicating high-crystalline features. The d-spacing between adjacent lattice fringes of NDs at branched region is 0.187 and 0.218 nm, corresponding to (002) and (111) crystals planes, respectively. The as-synthesized NDs are composed of Pt24Ni66Co10, as determine by energy-dispersive X-ray spectroscopy (EDS) and inductively coupled plasma atomic emission spectroscopy (ICP-AES). Moreover, figure 1d shows STEM image and the corresponding elemental distribution of Pt-Ni-Co NDs analyzed by high-angle annular dark field-scanning transmission electron microscope equipped with an energy dispersive spectrometer (HAADFSTEM-EDS). The element mapping displays that Ni is mostly located in the core, while both Pt and Co eventually distribute over the particles, in accordance with EDS line scan profiles (figure S5). These results confirm the formation of solid Pt-Ni-Co NDs with Ni-rich core.

Figure 1. (a) TEM image, (b) STEM image, (c) highmagnification STEM image, and (d) the corresponding EDS elemental mapping of the solid Pt-Ni-Co NDs. The scale bar in (d) is 20 nm. After solid Pt-Ni-Co NDs were synthesized, we turned our attention to create opened structure. Hollow structure of NDs were fabricated by treating solid Pt-Ni-Co NDs with acetic acid at 70 °C for 12 hours. As shown in the representative TEM image (figure 2a-b) and the large-area TEM image (figure S6), the obtained nanocrystals still remain dendritic structure with uniform dispersion. The average particle size is approximately 66 nm (figure S7), which is similar to the solid Pt-Ni-Co NDs. Strikingly, the presence of speckle pattern in annular dark field STEM images (figure 2b) can reflex hollow interior and cavities on these unique NDs, suggesting the transformation from solid Pt-Ni-Co NDs to hollow structure by treating with acetic acid. The fine crystalline structure of hollow NDs was characterized by HRTEM (figure 2c, d). The observed lattice spacing (0.186 nm and 0.132 nm) at branched region is consistent with the (022) and (020) crystal planes viewed along the [100] zone axis, as supported by FFT pattern (figure 2e). According to EDS and ICP-AES analysis, the element composition of hollow NDs is Pt65Ni17Co18. Furthermore, Pt, Ni, and Co are uniformly dispersed in entire nanocrystals, as evidenced by HAADF-STEM-EDS (figure 2f). From these results, it clearly indicates transformation from solid Pt-Ni-Co NDs to hollow Pt-Ni-Co NDs by the selective etching of Ni-rich core with unchanged size and shape.


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ACS Applied Energy Materials

Figure 3. (a) XRD patterns and (b) EDS spectra of solid and hollow Pt-Ni-Co NDs

Figure 2. (a) TEM image, (b) STEM image, (c) highmagnification TEM image of of the hollow Pt-Ni-Co NDs. (d) HRTEM image of selected area in (c) and (e) the corresponding FFT pattern. (f) STEM image and corresponding EDS elemental mapping of the hollow Pt-NiCo NDs. The scale bar in (f) is 20 nm. Additionally, the phase segregation of solid and hollow PtNi-Co NDs were analyzed by powder x-ray diffraction (XRD). As shown in figure 3a, solid Pt-Ni-Co NDs clearly show two sets of XRD pattern, which could be assigned to Pt-rich phase and Ni-rich phase. These diffraction peaks of Pt-rich phase can be identified as face-center-cubic (fcc) structure. Moreover, the peak positions are shifted to higher 2θ values, which can be attributed to the decrease of lattice distance when smaller Ni and Co atoms are introduced and alloyed with Pt, implying Pt-Ni-Co alloy formation. In comparison, hollow Pt-Ni-Co NDs exhibit only single set of Pt-rich phase, which can be indexed to fcc structure, implying that Ni-rich phase are etched away by treating acetic acid, in accordance with decreasing of Ni signal in EDS result (figure 3b). The hollow Pt-Ni-Co NDs were further characterized by Xray photoelectron spectroscopy (XPS; figure S8). The peak position of Pt4f located at 72.1 and 75.2 eV, which are shifted compared to that of pure Pt metal (71.2 and 74.5 eV). The shifted Pt 4f binding energy of hollow Pt-Ni-Co NDs would weaken the adsorption of intermediates on the alloyed surface, and thus facilitating the kinetics of the reaction.14 In addition, the peak position of Co2p (782.4 and 797.1 eV) and Ni2p (852.4 and 870.1 eV) are also slightly shifted compared to those of metallic Co (778.2 and 793.2 eV) and Ni (852.6 and 869.9 eV). The shifted peak position of Pt4f, Co2p and Ni2p illustrates the electron transfer between Pt, Ni and Co.15

To study the formation of solid Pt-Ni-Co NDs, the nanocrystals collected at different growth time were analyzed by TEM and EDS. Figure S9 illustrates the TEM images of nanocrystals investigated at 25 min, 1, 2 and 12 hours. According to EDS analysis (figure S10), it was found that nanoparticles are covered by Ni-rich phase in the initial growth stage, Pt and Co contents are then gradually increasing from 25 minutes to 2 hours. From 2 to 12 hours, the amount of Pt, Ni and Co content in nanocrystals are quite constant. Based on above experimental results, Ni atoms tend to be more easily reduced than Pt and Co atoms, although Ni2+/Ni have more negative redox potential than Pt2+/Pt, leading to the formation of Ni-rich core in the nanoctystals. The detailed growth mechanism of solid PtNi-Co NDs are explained in Supporting Information. Importantly, the amount of Ni precursors introduced into the system is a key factor to control opened structure. A series of experiment was carried out by varying amount of Ni precursors, while other conditions were kept constant. Figure S11 and S12 displays the as-synthesized nanocrystals and the nanocrystals after chemical treatment, indicating designable hollow interior by controlling the atomic ratio of Pt/Ni precursors. (see Supporting Information for detail) Considering favorable features of hollow Pt-Ni-Co NDs on electrocatalytic properties, FAOR and MOR were performed to examine their performances. Before the electrochemical tests, the catalysts were supported on carbon powder (Vulcant-72) to avoid an agglomeration. Figure S13 illustrates the carbon-supported hollow Pt-Ni-Co NDs, where no obvious change on morphology was observed. The actual electrocatalysts content after loading to carbon were determined by ICP, which are ~25.8% and ~16.0% for hollow and solid Pt-Ni-Co NDs, respectively. According to cyclic voltammetry (CV) curves in figure 4a, the electrochemically active surface area (ECSA), calculated by integrating the hydrogen adsorption charge after doublelayer correction, are 57.0, 32.4, and 40.2 m2 g-1Pt for hollow Pt-Ni-Co NDs, solid Pt-Ni-Co NDs, and Pt/C, respectively. In addition, the ECSACO of all catalysts, calculated by integrating CO oxidative charge of the first cycle stripping are given in figure S14. The larger ECSA of hollow Pt-Ni-Co NDs could be attributed to hollow interior and highly branched structure, providing much more accessible surface area. In view of practical application, mass activity



is considered as an essential factor related with Pt atomic efficiency that can reduce production cost. Impressively, hollow Pt-Ni-Co NDs exhibit 0.59 A mg-1 Pt @0.4 V and 2.20 A mg-1 Pt of mass activity toward FAOR (figure 4b) and MOR (figure 4c), which is 2.7 (2.1) and 6.6 (4.2) times higher than solid Pt-Ni-Co NDs and Pt/C, respectively. For specific activity (figure S15), hollow Pt-Ni-Co NDs also show the highest specific activity (3.8 mA cm-2) compare to solid PtNi-Co NDs and Pt/C. The improvement of catalytic activities toward FAOR and MOR could be ascribed to unique hollow structure, highly branched structure and the modified surface electronic structure by incorporation of Ni and Co into Pt.16-18

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precursors. Benefiting from unique hollow and highly branched structure of NDs, and synergistic effect among Pt, Ni and Co, hollow Pt-Ni-Co NDs show improvement of catalytic properties toward FAOR and MOR.

ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. Experimental details; TEM images of both solid Pt-Ni-Co NDs and hollow Pt-Ni-Co NDs; TEM images of nanocrystals collected in different growth time and their elemental analysis; TEM images of nanocrystals with different feeding molar ratio; CV curves of the catalysts; TEM images of hollow Pt-Ni-Co NDs before and after durability test.

AUTHOR INFORMATION Corresponding Author * [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT

Figure 4. CV profiles of hollow Pt-Ni-Co NDs, solid Pt-Ni-Co NDs, and commercial Pt/C recorded in (a) 0.5 M H2SO4, (b) 0.5 M H2SO4 + 0.25 M HCOOH, and (c) 0.5 M H2SO4 + 1.0 M CH3OH. The mass activity was normalized to the mass of Pt. (d) CO-stripping voltammograms of the three catalysts. The sweep rate is 50 mV s-1. To examine CO tolerance of the catalysts, CO stripping experiment was further carried out. As shown in figure 4d, the peak potentials of hollow Pt-Ni-Co NDs are negative shifted by 14.8 and 98.5 mV, compared to that of solid PtNi-Co NDs and commercial Pt/C, respectively. The much enhanced CO tolerance of hollow Pt-Ni-Co NDs should be attributed to the modification of electronic surface structure after acid treatment and bi-functional mechanism (figure S16).19-21 Besides, durability tests of hollow Pt-Ni-Co NDs, solid Pt-Ni-Co NDs and commercial Pt/C were performed by potential cycling treatment in methanol solution (figure S17-S19) and formic acid solution (figure S20). After 1,000 scanning, hollow Pt-Ni-Co NDs exhibit larger ECSAH and better mass activity than commercial Pt/C for both MOR and FAOR. In summary, we successfully synthesized trimetallic hollow Pt-Ni-Co NDs through selectively removing the Nirich core from solid NDs. The formation of Ni-rich core in solid NDs could be ascribed to the preferred reduction of Ni precursors. Additionally, hollow interior of Pt-Ni-Co NDs could be designed by controlling the atomic ratio of metal

This work was supported by the Major International (Regional) Joint Research Project (51210002), the National Basic Research Program of China (2015CB932304), the Natural Science Foundation of Guangdong Province (2015A030312007) and Guangxi Science and Technology Project (AB16380030).PKS acknowledge the support from the Danish project of Initiative toward Non-precious Metal Polymer Fuel Cells (4106-000012B).

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Trimetallic Hollow Pt-Ni-Co Nanodendrites as Efficient Anodic

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