Fabrication of Mesoporous Cage-Bell Pt Nanoarchitectonics as

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Fabrication of Mesoporous Cage-Bell Pt Nanoarchitectonics as Effecient Catalyst for Oxygen Reduction Reaction Hongjing Wang, Shuli Yin, Kamel Eid, Yinghao Li, You Xu, Xiaonian Li, Hairong Xue, and Liang Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b02015 • Publication Date (Web): 16 Jul 2018 Downloaded from http://pubs.acs.org on July 24, 2018

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ACS Sustainable Chemistry & Engineering

Fabrication of Mesoporous Cage-Bell Pt Nanoarchitectonics as Effecient Catalyst for Oxygen Reduction Reaction Hongjing Wang, Shuli Yin, Kamel Eid, Yinghao Li, You Xu, Xiaonian Li, Hairong Xue,* and Liang Wang* College of Chemical Engineering, Zhejiang University of Technology, 18 Chaowang Road, Hangzhou, Zhejiang 310014, P.R. China. *

Corresponding authors:

E-mails: [email protected]; [email protected]

ABSTRACT The nanoarchitectonics design is very important for tuning the catalytic performance of the Pt-based catalysts. Herein, mesoporous Pt cage-bell nanostructures (Pt@mPt CBNs) are newly designed by a facile synthetic approach. The unique Pt@mPt CBNs is assembled by a mesoporous Pt nanocage with an inside Pt nanoparticle. Benefiting from its mesoporous cage-bell nanoarchitectonics, the Pt@mPt CBNs exhibit superior catalytic activity and durability for oxygen reduction reaction. The present Pt nanoarchitectonics rationally integrates the structural advantages of mesoporous nanocages and nanoparticles, which is highly valuable for the design of Pt-based catalysts with desired performance. Keywords: mesoporous structure; cage-bell structure; Pt; catalyst; oxygen reduction reaction

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INTRODUCTION Pt-based nanostructures are highly active catalysts for anodic oxidation reaction (eg., methanol oxidation reaction, MOR) and cathodic reduction reactions (eg., oxygen reduction reaction, ORR) in fuel cells.1-6 However, the wide application of Pt-based catalysts is hindered by its high cost.7-11 Moreover, Pt-based catalysts tend to agglomerate during the catalytic processes, resulting in the loss of active sites and poor durability.12,13 Great effort has been devoted to address these issues. Controlling the nanoarchitectonics of Pt-based catalysts is one of the most effective approaches for improving the utilization efficiency and durability of Pt-based catalysts.14-19 Among various Pt-based nanoarchitectonics, nanoparticles (NPs), hollow nanostructures and porous nanostructures are three types of typical nanoarchitectonics.20-25 Nanoparticles can greatly improve the utilization efficiency of Pt-based catalysts, but particle agglomeration is a serious issue. Even Pt-based NPs are separated by carbon support, it still suffers from the corrosion of support, leading to the nanoparticle exfoliation. Hollow nanostructures with an inside cavity can reduce the loading of Pt-based catalysts, which are generally prepared by galvanic replacement and selectively chemical etching.26-31 The surfaces of the most Pt-based hollow nanostructures are compact and nonporous. Their exterior surfaces are assessable for reaction molecules, while their interior surfaces are very difficult for reaction molecules to reach. Porous Pt-based nanostructures with low density exhibit superior structural advantages relative to their solid counterparts. The self-supported porous nanoarchitectonics can provide sufficient accessible surfaces, which are usually prepared by dealloying, galvanic replacement and hared template method.32-33 The obtaining of regular porous nanoarchitectonics by a facile method is still highly desired. It would be highly interesting to rationally integrate the structural advantages of the nanoparticles, hollow nanostructures and porous nanostructures for the design of a mesoporous cage-bell nanoarchitectonics, which highly favor the improvement of utilization efficiency and activity of Ptbased catalysts. Despite cage-bell Pt-based nanoarchitectonics have been demonstrated by selective etching of Ag from the Pt@Ag@M nanoparticles in pioneer studies, the demonstrated cage-bell Pt2 ACS Paragon Plus Environment

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based nanoarchitectonics seriously lacks in porosity in the cage shell and the particle size is not uniform.34-35 The design of mesoporous cage-bell nanoarchitectonics with uniform particle size is very important for enhancing the performance of the Pt-based catalysts. Inspired by this thought, herein, mesoporous Pt cage-bell nanostructures (Pt@mPt CBNs) are rationally designed by a facile synthetic approach. The unique Pt@mPt CBNs with a mesoporous Pt nanocage and an inside Pt NPs exhibit superior catalytic activity and durability for ORR, originating from its mesoporous cage-bell nanoarchitectonics. The mesoporous Pt nanocages not only provide sufficient accessible sites from its interior and exterior surfaces, but also protect the inside Pt NPs from agglomeration, thereby resulting in an enhanced performance. EXPERIMENTAL SECTION Synthesis of Pt nanoparticles: Pt NPs are synthesized by mixing an aqueous solution involves of H2PtCl6 (20 mM, 2 mL) and sodium citrate (0.34 M, 0.2 mL). Then ascorbic acid (AA) (0.1 M, 2 mL) is quickly added under sonication at 35 °C for 2h. The resultant Pt NPs are purified by centrifugation at 7,000 rpm for 20 min. The resultant Pt NPs are dispersed in H2O (44 mL) for further use. Synthesis of Pt@SiO2 core-shell nanoparticles: Pt@SiO2 NPs are typically formed by mixing an aqueous solution contains of Pt NPs (4 mL), isopropanol (20 mL) and TEOS (0.06 mL) and then NH4OH (0.5 mL) is quickly added under stirring for 12 h. The resulting solution is centrifuged at 6,500 rpm for 10 min and washed with ethanol/water followed by drying at 80 °C for 24 h. Synthesis of mesoporous Pt cage-bell nanostructures: A Pt@SiO2 NPs (50 mg) and 3aminopropyltrimethoxysilane (APTMS) (0.2 mL) are dissolved in an isopropanol (10 mL) under sonication followed by refluxing at 80 °C for 12 h. After cooling to the room temperature, the solution is centrifuged at 6,500 rpm and washed with ethanol/water for three times and then dried under vacuum at 40 °C for 24 h.



Pt@SiO2@mesoporous Pt NPs are prepared by mixing an aqueous solution contains APTMS-modified Pt@SiO2 (2 mg), K2PtCl4 (20 mM, 2 mL), F127 (20 mg) under sonication followed by adding AA (0.1 M, 2 mL) at 35 °C for 2h. The solution is centrifuged at 6,000 rpm for 10 min and washed for 3 cycles with H2O. The resulting Pt@SiO2@mPt NPs are dissolved in HF (10 wt.%, 20 mL) for 12 h to erode the SiO2 interlayer. The final Pt@mPt CBNs are purified by centrifugation at 3,000 rpm for 10 minutes and washing with H2O for 5 times. Materials Characterization: The structure and shape of the as-prepared materials are investigated by a ZEISS SUPRA 55 scanning electron microscope (SEM) operated at 15 kV and a JEM-2010 transmission electron microscopy (TEM) equipped with a high-resolution TEM (HRTEM) at 200 kV and Energy Dispersive X-ray (EDX). BELSORP-mini (BEL, Japan) was used to analyze nitrogen adsorption-desorption isotherms at 77.3 K. Surface area was calculated using the Brunauer−Emmett−Teller (BET) method. Pore-size distribution was estimated by Barrett–Joyner–Halenda (BJH) method. Electrochemical measurements: The Electrochemical measurements are measured on a CHI 760D (Chenhua Co., Shanghai, China) electrochemical analyzer using a three-electrode cell involves Pt wire as a counter electrode, Ag/AgCl (3M KCl) as a reference electrode, and glassy carbon (GC) electrode as a working electrode. The GC electrode is coated with a 10 µg of each catalyst and then 5 µL Nafion (0.5 %) is added and left to fully dry at 50 °C. The cyclic voltammograms (CVs) were measured in N2-saturated an aqueous solution of 0.1 M HClO4 at 50 m Vs-1. The electrochemical active surface areas (ECSAs) are calculated using this equation: ECSAs = Q /m×210

(1)

where m is the Pt loading on the electrode surface, 210 µC cm-2 is the charge required for monolayer adsorption of hydrogen on Pt surface and Q is the charge in the Hupd adsorption/desorption area obtained after the double layer correction. 4 ACS Paragon Plus Environment

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The linear sweep voltammograms (LSVs) were benchmarked on a RRDE-3A rotation system (ALS Co. Ltd, Japan) in O2-saturated an aqueous solution of 0.1 M HClO4 at a scan rate of 5 mV s-1 and a rotation speed of 1,600 rpm. The electron transfer number (n) is calculated by the Koutecky-Levich equation as follow:

1 1 1 = + j jk jd

(2)

jd = 0.2nFD2/3γ-1/6ω1/2 CO2

(3)

Where, j, jk, and jd are the measured, kinetic, and diffusion currents, respectively. F is the Faraday's constant (96485 C mol-1). D is the diffusion coefficient of O2 (1.93 × 5 cm2 s-1), γ is the kinetic viscosity (1.13 × 10-2 cm2 s-1), ω is the rotation speed of electrode (rpm), and CO2 is the bulk concentration of O2 dissolved in 0.1 M HClO4 (1.22 × 10-3 mol 5m-3). The peroxide percentage (% H2O2) is derived as follow: % H 2O2 =

200 I R (I D N + I R )

(4)

The n is further calculated by RRDE measurements consisting of ID (disk current) and IR (ring current), as follow: n=

4I D (I D + I R / N )

(5)

Herein, N is the RRDE collection efficiency of the Pt ring determined to be 0.4286. RESULTS AND DISCUSSION Pt@mPt CBNs are synthesized by using Pt@SiO2 core-shell NPs as the starting materials for the coating of mesoporous Pt shell to form Pt@SiO2@mesoporous Pt NPs, which are followed by the selective chemical etching of the SiO2 interlayer (Figure 1). The Pt@SiO2 core-shell NPs with a spherical morphology are well dispersed, which contain a Pt NP core and a SiO2 shell (Figure S1). The average diameters of the Pt NPs and Pt@SiO2 core-shell NPs are around 20 nm and 150 nm, respectively. Isolation of any individual Pt NP or SiO2 NP is not observed. The high yield of the Pt@SiO2 core-shell NPs highly favors to be a starting 5 ACS Paragon Plus Environment


material for the following coating of mesoporous Pt on the SiO2 NPs surfaces. After selective chemical etching of the SiO2 interlayer from the Pt@SiO2@mesoporous Pt NPs, the Pt@mPt CBNs are obtained (Figure 2). The low magnification SEM image clearly shows that the obtained Pt@mPt CBNs are spherical nanostructures with an average diameter of 200 nm (Figure 2a and Figure S2). The high magnification SEM image clearly shows that the interconnected mesopores are well-distributed over the exterior surface of each Pt@mPt CBN (Figure 2b). The N2 adsorption−desorption isotherms of the Pt@mPt CBNs further indicate their typical mesoporous structure, and their BET surface area is close to 38 m2 g−1 (Figure S3a). The average diameter of the mesopores is around 16 nm (Figure S3b), which is consistent with the SEM results. The TEM image focused on one particle clearly reveals the Pt@mPt CBN consists of spatially separated Pt NP and mesoporous Pt cage (Figure 2c). The thickness of the mesoporous Pt cage is 25 nm. The mesoporous cage-bell nanoarchitectonics of the Pt@mPt CBN is further confirmed by the HAADF-STEM image together with the compositional line profile (Figure 2d). Moreover, EDX mapping images clearly reveal that Pt element is well distributed on the entire nanoarchitectonics, and there is no silica can be detected due to that the silica is completely removed by the chemical etching. (Figure S4). The d-spacing with 0.23 nm is assigned to the (111) facet the face-centered-cube (fcc) structural Pt (Figure 2e and Figure 2f). The absence of any kind of aggregated and distorted grains suggests the mesoporous Pt cage is formed by direct nucleation and oriented growth rather than random particle aggregation. The selected-area electron diffraction (SAED) pattern of the Pt@mPt CBNs shows a typical concentric ring form of Pt (Figure S5). The XRD pattern confirms the metallic fcc structure of the Pt@mPt CBNs without undesired phase separation (Figure S6). The XPS spectrum of Pt shows the presence of Pt(0) and Pt(II), and most of the Pt precursor is reduced into the metallic state (Figure S7).


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The proposed synthetic method can be readily used for synthesis of mesoporous Pt hollow nanospheres (mPt HNSs) by simply using SiO2 NPs to replace the Pt@SiO2 core-shell NPs as the starting material (Figure S8). Pt-based hollow nanostructures (Pt HNSs) have previously been demonstrated in few pioneer studies by selective etching of Pd from the Pd@Pt core-shell nanostructure.36,

37

For instance, icosahedral Pt hollow nanostructure is

prepared by selectively etching of the Pd core from the Pd@Pt core-shell icosahedra.29 The shells of Pt HNSs are in compact and nonporous forms, resulting in that their interior surfaces are inaccessible for reaction molecules in catalytic reactions. In fact, the interior surface is very stable active site. The designed Pt@mPt CBNs not only favor the effective utilization of active surfaces from both of the interior and exterior surfaces of the mesoprous Pt nanocages, but also keep the inside Pt NPs away from agglomeration, making the present Pt@mPt CBNs are very different from previously Pt nanoarchitectonics. The proposed synthetic method for Pt@mPt CBNs skillfully employs a micelle template strategy. In the reaction mixture aqueous solution, the hydroxyl groups of F127 form hydrogen bond with the amino groups on surface of the SiO2 NPs. Simultaneously, the anionic Pt precursor species is attracted to cationic amino modified SiO2 surface by the electrostatic attraction. During the Pt atom addition, the F127 micelles effectively direct the formation of the mesoporous Pt structures.38, 39 By further removing of the SiO2 interlayer from the Pt@SiO2@mesoporous Pt NPs, the Pt@mPt CBNs are prepared. The electrocatalytic performance of the Pt@mPt CBNs for ORR is investigated, and is compared with mPt HNSs and commercial Pt/C (20 wt% Pt) catalyst. As shown in Figure S9, the CV curves of the samples show that there is a pair of oxidation-reduction peaks at 0.1 to 0.3 V, corresponding to adsorption-desorption of hydrogen on Pt surface in acidic medium. The current density hydrogen desorption peak is usually used to calculate the ECSAs of the catalysts. The ECSA of the Pt@mPt CBNs is 27.3 m2 g-1, which is higher than that of the mPt HNSs (23.6 m2 g-1). Considering their similar particle sizes and mesoporous 7 ACS Paragon Plus Environment


shells, the improved ECSA of the Pt@mPt CBNs relative to that of the mPt HNSs is mainly caused by the presence of Pt cores which provide additional accessible surfaces. The ECSA of the Pt/C is 43.0 m2 g-1. The ORR polarization curves are measured in a 0.1 M HClO4 aqueous solution at 5 mV s-1. The three catalysts display a typical ORR polarization curves. The Pt@mPt CBNs display a higher limiting current density (6.7 mA cm-2) than those of the mPt HNSs (5.6 mA cm-1) and Pt/C catalyst (5.1 mA cm-1) (Figure 3a). The onset potential (Eonset) and half-wave potential (E1/2) of the Pt@mPt CBNs (1.03 V and 0.90 V) are the most positive relative to those of the mPt HNSs (1.01 V and 0.90 V) and Pt/C catalyst (0.95 V and 0.89 V) (Figure 3b). For a reference, the Pt@mPt CBNs show the more positive Eonset and E1/2 (1.03 and 0.90 V) relative to those of the previously reported Pt-based catalysts (Table S1). The specific activity of the Pt@mPt CBNs (0.89 mA cm-2) is 1.1 and 3.7 folds of those of the mPtPd NCs (0.80 mA cm-2) and Pt/C catalyst (0.24 mA cm-2), respectively (Figure 3c). The ORR kinetics of the three catalysts are analyzed by using Tafel plots. The caculated Tafel slopes of the Pt@mPt CBNs (65 mV dec-1), mPt HNSs (65 mV dec-1) and Pt/C catalyst (64 mV dec-1) are similar. The Tafel slopes imply that the rate determining step is the transfer of the first electron step for the three catalysts (Figure S10). The ORR kinetics of the Pt@mPt CBNs is also revealed by RDE and RRDE tests. In the RDE test, with the increasing of rotation rate, the limiting current densities increased (Figure 4a). The caculated Koutecky-Levich plots with linear relation show a first order of ORR kinetics (Figure 4b). The electron transfer numbers (n) of the Pt@mPt CBNs (3.96, 3.95, 3.95 and 3.96) at 0.4 V, 0.5 V, 0.6 V, 0.7 V, respectively, are very closed to 4. The RDE result of the Pt@mPt CBNs is agreed with that of the Pt/C catalyst (Figure S11). In the RRDE test, the ring currents (IR) are almost negligible relative to the disc currents (ID) of the Pt@mPt CBNs and Pt/C catalyst (Figure 4c). This means that the yields of H2O2 on the two catalysts are very low. The percentages of H2O2 are evaluated to be 2.64 and 3.26 on the 8 ACS Paragon Plus Environment

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Pt@mPt CBNs and Pt/C catalyst, respectively (Figure 4d). The calculated n of Pt@mPt CBNs is above 3.94. Therefore, both of the RDE and RRDE tests reveal that the Pt@mPt CBNs show one-step four-electron pathway in ORR precess. The stability of the Pt@mPt CBNs for ORR is evaluated and compared with Pt/C catalyst by repeating LSV test for 5,000 cycles. The Pt@mPt CBNs show a negligible degradation in Eonset and a very minor loss in limiting current density relative to its initial ones (Figure 5a). While the degradations in E1/2 and limiting current density for Pt/C catalyst is obvious (Figure 5b). Chronoamperometric test for 5 h furhter reveals the superior stability of the Pt@mPt CBNs for ORR. After testing for 5 h, the decay for Pt@mPt CBNs, mPt HNSs and Pt/C catalyst are 26.0%, 30.5% and 40.2%, respectively (Figure 5c). The superior electrocatalytic performance of the Pt@mPt CBNs for ORR is attributed to its mesoporous cage-bell nanoarchitectonics (Figure 6). The unique nanoarchitectonics offers sufficient accessible sites for O2 molecules from interior and exterior surfaces of the mesoporous Pt nanocage tegather with the inside Pt NPs, which favors the enhancement of the activity. All of the accessible sites are spatially separated and insusceptible to catalyst agglomeration during the ORR, leading to an improved durability. CONCLUSION In summary, the Pt@mPt CBNs are newly designed by a facile synthetic route. The unique Pt@mPt CBNs are self-supported nanoarchitectures assembled by mesoporous Pt nanocage with an inside Pt NP. Benefiting from its mesoporous cage-bell nanoarchitectures, the Pt@mPt CBNs shows an enhanced catalytic activity and durability for ORR. The proposed synthetic method is very useful for rational design of Pt-based CBNs with bi- and multi-metallic shells for various catalytic applications. ASSOCIATED CONTENT Supporting Information Additional characterization data and electrochemical data. 9 ACS Paragon Plus Environment


AUTHOR INFORMATION Corresponding Authors * Dr. Hairong Xue and Prof. Liang Wang, E-mails: [email protected]; [email protected] ACKOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No. 21601154, 21776255, 21701141), and Natural Science Foundation of Zhejiang Province (No. LQ18B010005). REFERENCES [1] Qiu, X.; Yan, X.; Cen, K.; Sun, D.; Xu, L.; Tang, Y. Achieving Highly Electrocatalytic Performance by Constructing Holey Reduced Graphene Oxide Hollow Nanospheres Sandwiched by Interior and Exterior Platinum Nanoparticles. ACS Appl. Energy Mater. 2018, DOI: 10.1021/acsaem.8b00452. [2] Bu, L.; Zhang, N.; Guo, S.; Zhang, X.; Li, J.; Yao, J.; Wu, T.; Lu, G.; Ma, J.; Su, D.; Huang, X. Biaxially Strained PtPb/Pt Core/Shell Nanoplate Boosts Oxygen Reduction Catalysis. Science 2016, 354, 1410−1414. [3] Yan, X.; Yu, S.; Tang, Y.; Sun, D.; Xu, L.; Xue, C. Triangular AgAu@Pt Core–Shell Nanoframes with a Dendritic Pt Shell and Enhanced Electrocatalytic Performance toward the Methanol Oxidation Reaction. Nanoscale 2018, 10, 2231. [4] Du, H.; Luo, S.; Wang, K.; Tang, M.; Sriphathoorat, R.; Jin, Y.; Shen, P. High-Quality and Deeply Excavated Pt3Co Nanocubes as Efficient Catalysts for Liquid Fuel Electrooxidation. Chem. Mater. 2017, 29, 9613−9617. [5] Wang, L.; Yamauchi, Y. Synthesis of Mesoporous Pt Nanoparticles with Uniform Particle Size from Aqueous Surfactant Solutions toward Highly Active Electrocatalysts. Chem. Eur. J. 2011, 17, 8810–8815. [6] Kwon, T.; Jun, M.; Kim, H.; Oh, A.; Park, J.; Baik, H.; Joo, S.; Lee, K. Vertex-Reinforced PtCuCo Ternary Nanoframes as Efficient and Stable Electrocatalysts for the Oxygen Reduction Reaction and the Methanol Oxidation Reaction. Adv. Funct. Mater. 2018, 1706440. [7] Jiang, L.; Lin, X.; Wang, A.; Yuan, J.; Feng, J.; Li, X. Facile Solvothermal Synthesis of Monodisperse Pt2.6Co1 Nanoflowers with Enhanced Electrocatalytic Activity towards Oxygen Reduction and Hydrogen Evolution Reactions. Electrochim. Acta 2017, 225, 525−532. [8] Lyu, L.; Kao, Y.; Cullen, D.; Sneed, B.; Chuang, Y.; Kuo, C. Spiny Rhombic Dodecahedral 10 ACS Paragon Plus Environment

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Figures and Figure Captions

Figure 1. Schematic presentation for the formation of the Pt@mPt CBNs.


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Figure 2. (a, b) SEM images of the Pt@mPt CBNs. (c) TEM image and (d) HAADF-STEM image with the corresponding compositional line profile of a Pt@mPt CBN. (e) HRTEM image of the particle edge. (f) Fourier filtered lattice fringe image of the square area in (e). The inset in (f) displays the corresponding FFT pattern.



Figure 3. (a) ORR polarization curves, (b) the comparison of Eonset and E1/2 and (c) the comparison of the specific activity of the three catalysts.


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Figure 4. (a) ORR polarization curves of the Pt@mPt CBNs with different RDE rotation rates. (b) The electron transfer numbers at different potentials. (c) RRDE test of ORR, and (d) peroxide percentages and electron transfer numbers on the Pt@mPt CBNs and Pt/C with a rotation rate of 1600 rpm.



Figure 5. ORR polarization curves before and after durability test for Pt@mPt CBNs (a) and Pt/C (b). (c) Chronoamperometric measurements at 0.5 V.


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Figure 6. Schematic catalytic process of ORR on the Pt@mPt CBNs.



TOC

Unique all-metallic Pt@mPt cage-bell nanostructures consisting of a Pt core and a mesoporous Pt shell are fabricated as an efficient and sustainable catalyst for oxygen reduction reaction.


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Fabrication of Mesoporous Cage-Bell Pt Nanoarchitectonics as

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