Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Trimetallic PtPdNi-Truncated Octahedral Nanocages with a WellDefined Mesoporous Surface for Enhanced Oxygen Reduction Electrocatalysis Hongjing Wang, Yinghao Li, Kai Deng, Chunjie Li, Hairong Xue, Ziqiang Wang, Xiaonian Li, You Xu,* and Liang Wang*
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State Key Laboratory Breeding Base of Green-Chemical Synthesis Technology, College of Chemical Engineering, Zhejiang University of Technology, Hangzhou, Zhejiang 310014, P. R. China S Supporting Information *
ABSTRACT: Engineering the architectures and compositions of noble metal-based nanocrystals is an effective strategy to optimize their catalytic performance. Herein, we report the synthesis of unique trimetallic PtPdNi mesoporous-truncated octahedral nanocages (PtPdNi MTONs), which is simply performed by first constructing Pd@PtPdNi core−shell mesoporous truncated octahedra (Pd@PtPdNi MTOs) and further selective etching of the Pd cores using concentrated nitric acid. The rational combinations of polyhedral shape, mesoporous surface, and hollow structure provide sufficiently exposed active sites and efficient reactant permeability. With these unique properties, the PtPdNi MTONs show improved catalytic activity and stability toward the oxygen reduction reaction. KEYWORDS: trimetallic PtPdNi, nanocages, mesoporous structure, truncated octahedral, oxygen reduction reaction
1. INTRODUCTION Metallic nanocrystals with controllable architectures and compositions have received intensive interest.1−7 The intrinsic properties of the metallic nanocrystals strongly depend on their sizes,8−11 shapes,12−15 and compositions.16−20 Pt-based materials have been widely prepared and used in various electrochemical energy fields, such as fuel cells and water splitting.21−25 Therefore, the controlled synthesis of Pt-based materials with desired structure and composition is of importance for their catalytic applications. Particularly, Ptbased hollow nanostructures with unique chemical and physical properties show great promising potential in catalysis. This kind of nanostructures are generally fabricated through the galvanic displacement reaction, template method, and Kirkendall effect.26−29 By using these strategies, a lot of Ptbased hollow nanostructures (e.g., nanocages and nanoframes) have been fabricated and have shown their superiority in catalytic applications.30−35 Moreover, tailoring the shape of the hollow nanostructures could further enhance their catalytic performance. In this regard, the use of presynthesized Pd nanocrystals as the hard template for the deposition/growth of Pt or Pt-based materials, followed by selective removal of the Pd template, has been demonstrated as a powerful approach for constructing Pt-based hollow nanostructures. The key for selective etching lies in the difference in chemical stability of the Pd core and Pt-based exterior in the etchant. With the strategy, Pt-based nanocages with octahedral, cubic, and icosahedral shapes have been achieved.36,37 It is noted that most of the previously demonstrated metallic polyhedral nanocages are composed of compact and smooth surfaces, © XXXX American Chemical Society
resulting in that their internal surfaces cannot be effectively utilized. Besides the engineering of the inner cavity region, control over the surface structure of Pt-based hollow nanoarchitectures can further offer great opportunities to tune their catalytic property.38,39 Mesoporous structures with pore channels can sufficiently offer three-dimensional molecular permeability.40−42 Engineering mesoporosity into metallic nanoarchitectures can not only provide abundant catalytic active sites, but also allow a fast and efficient transport of reactants. Despite the advances in the preparation of mesoporous metals, most of them limit to irregular particles and films. The combination of mesoporous structure with a hollow structure for metallic nanocrystals has rarely been demonstrated. Recently, Yamauchi’s group successfully prepared mesoporous Pt hollow cubes by a soft−hard dual templating strategy and demonstrated their improved catalytic performance for methanol electro-oxidation.43 The rational design of a mesoporous shell for hollow metallic nanoarchitectures highly favors the enhanced electrocatalytic performance. Apart from structuring, the construction of Pt-based alloyed nanostructures by composition engineering is another promising approach to improve the catalytic performance. In particular, Pt-based trimetallic or multimetallic alloys are more promising alternatives in comparison with bimetallic systems because they could provide more opportunities for modifying the electronic structure of Pt. The Received: October 25, 2018 Accepted: December 31, 2018
A
DOI: 10.1021/acsami.8b18696 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
performed in an O2-saturated 0.1 M HClO4 solution. Specific activities and mass activities were obtained by normalizing the kinetic current to the ECSA and the loading amount of Pt, respectively. The kinetic currents were calculated using the Koutecky−Levich equation
combination of structuring and alloying could offer great opportunities to develop highly efficient catalysts. Therefore, the development of effective synthetic strategies for the preparation of Pt-based multimetallic hollow nanoarchitectures with the mesoporous shell is highly desired. Herein, a facial synthetic approach has been developed for the fabrication of a unique trimetallic PtPdNi-truncated octahedral nanocage with a well-defined mesoporous surface (PtPdNi MTONs), which is simply performed by a selective removal of Pd cores from the preformed Pd@PtPdNi core− shell mesoporous-truncated octahedra (Pd@PtPdNi MTOs) by etching using concentrated nitric acid. The PtPdNi MTONs exhibit superior electrocatalytic performance for the oxygen reduction reaction (ORR) compared with the initial Pd@PtPdNi MTOs and the commercial Pt/C catalyst.
1 1 1 = + j jk jd where j, jk, and jd represent the measured, kinetic, and diffusionlimited current densities, respectively. The electron transfer number (n) and the peroxide yield (H2O2%) could be calculated as follows
n=
4ID (ID + IR /N )
% H 2O2 =
200IR (IDN + IR )
where ID and IR are the disk current and ring current, respectively; and N = 0.4286 is the current collection efficiency of the Pt ring.
2. EXPERIMENTAL SECTION 2.1. Materials and Chemicals. Sodium tetrachloropalladate (Na2PdCl4), (hydro)chloroplatinic acid (H2PtCl6), and Pluronic F127 were received from Sigma-Aldrich. L-ascorbic acid (AA), nickel chloride hexahydrate (NiCl2·6H2O), concentrated hydrochloric acid (HCl, 37%), and nitric acid (HNO3, 68%) were purchased from Aladdin. The commercial Pt/C (20 wt %) catalyst was obtained from Alfa Aesar. 2.2. Synthesis of Pd@PtPdNi MTOs. First, Pluronic F127 (50 mg) was mixed with H2PtCl6 (1.5 mL, 20.0 mM), NiCl2 (1.5 mL, 20 mM), Na2PdCl4 (1.5 mL, 20.0 mM), and HCl (0.2 mL, 6.0 M) under sonication to make F127 completely dissolved. Then, an AA solution (2.0 mL, 0.1 M) was added, followed by heating at 95 °C in an oil bath for 4 h. The final product was collected by centrifugation, washed with ethanol, and deionized water six times. 2.3. Synthesis of PtPdNi MTONs. The as-prepared Pd@PtPdNi MTOs were redispersed in 20 mL HNO3 (68%), and then the solution was left to react at 25 °C for three days with stirring. Finally, the product was collected by centrifugation and washed with deionized water three times. 2.4. Characterizations. The morphology and structure of the samples were analyzed by a scanning electron microscope (ZEISS SUPRA 55), transmission electron microscope (JEOL JEM-2100F), high-resolution transmission electron microscope (JEOL JEM2100F), and high-angle annular dark field-scanning transmission electron microscope (FEI Tecnai G2 F20). X-ray diffraction (XRD) patterns were acquired on an X-ray diffractometer (PANalytical X’Pert) with a Cu Kα source. A X-ray photoelectron spectrum (XPS) was operated on an ESCALAB MK II spectrometer (VG Scientific, UK). The compositions of the samples were measured using inductively coupled plasma mass spectrometry (ICP−MS, Elan DRC-e instrument). 2.5. Electrochemical Measurements. The electrochemical measurements were conducted using a RRDE-3A rotation system (ALS Co. Ltd, Japan) connected to a CHI 842C electrochemical analyzer (CH Instruments). A glassy carbon rotating disk electrode (RDE) with a geometric area of 0.071 cm2 was modified by catalysts and served as the working electrode. The preparing procedure for the working electrode is as follows: first, 2 mg of the PtPdNi MTONs was dispersed in 1 mL of deionized water and sonicated for 20 min to produce an ink; then, 3 μL of catalyst ink was placed on the precleaned glassy carbon RDE and allowed to air dry; after that, 4 μL of Nafion solution (0.5 wt %) was covered on the catalyst-modified glassy carbon RDE and dried again. The glassy carbon RDE modified by other samples was prepared with the similar procedure and the Pt loading on the electrode was kept the same for different catalysts. A saturated Ag/AgCl electrode was used as the electrode and a Pt wire served as the counter electrode. The electrolyte was 0.1 M HClO4 solution. Cyclic voltammograms (CV) was measured in N2-saturated 0.1 M HClO 4 with a sweeping rate of 50 mV s −1 . The electrochemically active surface areas (ECSAs) of the catalysts were calculated based on the CV curves. ORR measurements were
3. RESULTS AND DISCUSSION The proposed synthesis is performed by two steps. The starting materials of the Pd@PtPdNi MTOs are first prepared, and then they are treated with an acid solution for selective etching of Pd cores to form the PtPdNi MTONs (Figure 1).
Figure 1. Schematic illustration of the preparation of the PtPdNi MTONs.
The Pd@PtPdNi MTOs are prepared by a one-pot coreduction strategy with the assistance of surfactant F127. This present one-pot strategy for the fabrication of mesoporous polyhedral Pt-based nanocrystals is very simple and mild relative to the previous methods with multistep operation under harsh reaction conditions.44,45 By further skillful utilization of the stability differences of different metals, the selective removal of the Pd core using concentrated HNO3 solution can be readily achieved. The starting materials of the Pd@PtPdNi MTOs are featured with a truncated octahedral shape and have an average diameter of 105.2 nm (Figures 2a and S1). There are well-defined mesoporous structures over their surfaces (Figure 2b). The Pd@PtPdNi MTOs are in a solid form, which is revealed by their strong contrast between the edge and the center (Figure 2c,d). Both of the elemental mapping images and energy-dispersive system line-scanning profiles clearly indicate that the Pd element mainly concentrates at the center of the polyhedron; meanwhile, the Pt and Ni elements uniformly distribute over the shell regions (Figure 2e,f). The XRD pattern of the Pd@PtPdNi MTOs shows a metallic facecentered cubic (fcc) structural feature (Figure S2). These results indicate the successful preparation of the Pd@PtPdNi MTOs. To gain an insight into the morphological evolution of the Pd@PtPdNi MTOs, we collect the intermediates at different reaction times and characterize them by using SEM, TEM, and HAADF-STEM. Figure S3 shows the SEM images of the products sampled at 50 min, 1.5, 2.5, and 5 h into the reaction. B
DOI: 10.1021/acsami.8b18696 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
elemental distribution features of the Pd@PtPdNi MTOs provide the opportunity for further being converted into other more interesting nanoarchitectures. The preformed Pd@PtPdNi MTOs are used as the starting materials for the preparation of PtPdNi MTONs via selective removal of the Pd core. The mesoporous exteriors provide channels for the diffusion and penetration of HNO3 solution into the Pd@PtPdNi MTOs.46 When the Pd@PtPdNi MTOs are dispersed in the concentrated HNO3 solution for three days, the interior Pd cores are gradually etched. As shown in Figure 3a,b, the as-converted PtPdNi MTONs retain the initial
Figure 2. Morphological and structural characterizations for the Pd@ PtPdNi MTOs. (a,b) SEM images. (c,d) TEM images. (e) HAADFSTEM image and elemental mappings. (f) EDX line-scanning profiles.
The corresponding TEM, compositional line profiles, and HAADF-STEM images of these intermediates are presented in Figure S4. At a reaction time of 50 min, solid polyhedral Pdrich nanoparticles with a smooth surface and an average particle size of 73 nm are generated (Figures S3a and S4a). As the reaction proceeds to 1.5 h, the surface of these preformed polyhedral nanoparticles becomes rough with the formation of the rudiment of mesoporous surface structures and the average particle size increases to 85 nm, indicating the heterogeneous depositions of Pt, Pd, and Ni species on the initial Pd nanoparticles (Figures S3b and S4b). In the following 1 h, the mesoporous surface structure gradually becomes clearer and the particles grow up to 93 nm (Figures S3c and S4c). When the reaction time proceeds to 4 h, the typical Pd@PtPdNi MTOs are obtained (Figure 2). Further prolonging the reaction time to 5 h, the products are very similar to the Pd@PtPdNi MTOs sampled at 4 h (Figures S3d and S4d). From these results, we could speculate that the reduction of Pd precursor species by AA is preferentially performed at the initial stage. The preformed Pd nanoparticles serve as in situ seeds for the subsequent coreduction of Pt, Pd, and Ni species, leading to the formation of the Pd@PtPdNi core−shell nanostructures. During the growth process of the PtPdNi exteriors, surfactant F127 micelles direct the formation of mesoporous structures. When there is no F127 added, irregular nanoparticles with solid surfaces are generated, which are in a highly aggregated form (Figure S5). These results suggest that our current one-pot synthetic strategy could generate novel Pd@PtPdNi MTOs without the need for hard templates or added seeds. Moreover, the unique core−shell structure and
Figure 3. Morphological and structural characterizations for the PtPdNi MTONs. (a,b) SEM images. (c,d) TEM images. (e) HRTEM image, lattice fringes, and the corresponding fast Fourier transform pattern of the marked square areas. (f) HAADF-STEM image and elemental mappings. The inset in panel (c) is the SAED pattern and the inset in (f) shows the compositional line profiles.
truncated octahedral shapes, mesoporous surface, and size distribution of the starting polyhedra. Differing from the solid interiors of the starting materials, these as-converted nanocages possess well-developed hollow cavities (Figure 3c,d). These results suggest that the Pd core in the starting Pd@PtPdNi MTOs has been removed after the HNO3 treatment and the mesoporous exteriors completely retain. The studies of selected area electron diffraction (SAED) reveal the polycrystalline nature of the PtPdNi MTONs (inset in Figure 3c). The continuous rings in the SAED pattern can be indexed to (111), (200), (220), and (311) crystal planes of the metallic fcc structure and the discrete diffraction spots reveal the high degree of crystalline subunits. A set of distinct lattice fringes with a spacing of 0.216 nm can be recognized in the shell of the mesoporous surface, which is consistent with the (111) C
DOI: 10.1021/acsami.8b18696 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces plane of the metallic fcc structure (Figure 3e). The corresponding elemental mapping images reveal that the Pt, Pd, and Ni elements are evenly distributed in the shell of the nanocages (Figure 3f). Moreover, the line-scanning profiles clearly show the distribution of Pt, Pd, and Ni elements across an individual nanocage, further confirming the trimetallic hollow structural feature (inset in Figure 3f). It is noted that the Pd component still remains in the shell of the PtPdNi MTONs. The difference in chemical stability of Pd strongly relates to its chemical environment. Pt has a better chemical stability in concentrated nitric acid than Pd. The Pd atoms in the Pt-rich PtPdNi alloy phase are less vulnerable to chemical etching than the mono-Pd cores mainly because of the surface segregation and protection of Pt,47 thereby preserving the PtPdNi-alloyed shell after the etching. The molar ratio of Pt/ Pd/Ni in the as-converted PtPdNi MTONs was determined to be 61/25/14 by ICP−MS measurement. The XRD pattern of the PtPdNi MTONs displays a metallic fcc structural feature (Figure S6). The diffraction peaks of the PtPdNi MTONs in the XRD pattern show a slight positive shifting relative to the standard diffraction position of Pt because of alloying Pt with Pd and Ni. We further conducted XPS characterization to investigate the electronic state of the PtPdNi MTONs (Figure S7). The Pt 4f peak for the PtPdNi MTONs is shifted toward lower binding energy compared to that in the commercial Pt black (Figure S7a), indicating that the electron density of Pt has been increased by introducing Pd and Ni atoms. These results suggest that the trimetallic PtPdNi MTONs are successfully prepared by a simple selective etching. The unique structural and compositional features give the as-synthesized PtPdNi MTONs great potentials for electrochemical applications. We investigate the electrochemical properties of the PtPdNi MTONs and compare with the Pd@PtPdNi MTOs and commercial Pt/C catalyst. The calculated ECSAs based on the CV curves are 55.7, 34.2, and 52.1 m2 g−1 for the PtPdNi MTONs, Pd@PtPdNi MTO, and Pt/C, respectively (Figure S8). The high ECSA of the PtPdNi MTONs is mainly attributed to the fully exposed both the interior and exterior surfaces, which is of great importance for increasing the utilization efficiency of the catalyst. The ORR catalytic performance was evaluated in an O2saturated 0.1 M HClO4 solution by using RDE. The polarization curves reveal that the ORR kinetic is obviously accelerated on the PtPdNi MTONs relative to the Pd@PtPdNi MTOs and Pt/C (Figure 4a). The PtPdNi MTONs show a more positive onset potential (Eonset, 1.04 V) than the Pd@ PtPdNi MTOs (0.97 V) and Pt/C (0.95 V). A higher halfwave potential (E1/2) is also achieved for the PtPdNi MTONs (0.942 V) relative to the Pd@PtPdNi MTOs (0.884 V) and Pt/C (0.873 V) (Figure 4b). The Eonset and E1/2 of the PtPdNi MTONs (1.04 and 0.942 V) are also more positive than the PtPdNi mesoporous nanoparticles reported in our previous work (Table S1),48 mainly because of the different ECSAs and molar ratios of Pt/Pd/Ni in the catalysts. Moreover, the ORR performance of the PtPdNi MTONs is also superior to some other previously reported Pt-based catalysts in terms of Eonset and E1/2 (Table S1). The ORR reaction kinetic on the catalysts was investigated based on the Tafel plots (Figure 4c). The Tafel slope of the PtPdNi MTONs (67.5 mV dec−1) is lower than that of the Pd@PtPdNi MTOs (80.1 mV dec−1), and very close to that of the Pt/C catalyst (68.7 mV dec−1). These results suggest that oxygen molecules are more easily adsorbed and activated on the PtPdNi MTONs relative to the Pd@
Figure 4. (a) ORR polarization curves recorded at room temperature in an O2-saturated 0.1 M HClO4 solution with a sweeping rate of 5 mV s−1 and a rotation rate of 1600 rpm. (b) Comparisons of the E1/2 and Eonset. (c) Tafel plots of the catalysts. (d) Mass activities and specific activities of the catalysts at 0.9 V (vs RHE). (e) The ORR polarization curves of PtPdNi MTONs before and after the durability tests. (f) Chronoamperometric response tested at 0.9 V (vs RHE).
PtPdNi MTOs. The mass activity of the PtPdNi MTONs (1.14 A mgPt−1) is higher than those of the Pd@PtPdNi MTOs (0.38 A mgPt−1) and Pt/C catalyst (0.17 A mgPt−1). The specific activities follow the order of PtPdNi MTONs (1.52 mA cm−2) > Pd@PtPdNi MTOs (0.54 mA cm−2) > Pt/C catalyst (0.23 mA cm−2) (Figure 4d). Moreover, the ORR performance of the PtPdNi MTONs is also superior to the bimetallic PtPd MTONs (Figure S9). The enhanced catalytic activity of the PtPdNi MTONs was further demonstrated by evaluating the ORR performance in alkaline media (0.1 M KOH) (Figure S10). To investigate the electron transfer kinetics of the ORR on the PtPdNi MTONs, we perform the linear sweep voltammetry (LSV) at different rotation rates and record the corresponding polarization curves. As shown in Figure S11a, the limiting current densities gradually increase with the rotation rate increasing from 625 to 2025 rpm. The corresponding Koutecky−Levich linear plots imply the first order of the ORR kinetics on the PtPdNi MTONs (Figure S11b). The calculated number of the transferred electrons during the ORR process is close to 4. The reaction kinetic of ORR was further investigated by using the rotating ring disk electrode (RRDE). Both the PtPdNi MTONs and Pt/C exhibit negligible ring currents relative to their disc currents (Figure S11c). The H2O2 yield (
