Facile Synthesis of Three-Dimensional PtPdNi Fused

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Facile Synthesis of Three-Dimensional PtPdNi Fused Nanoarchitecture as Highly Active and Durable Electrocatalyst for Methanol Oxidation Yanling Zhai, Zhijun Zhu, Xiaolin Lu, Zhiru Zhou, Jiahui Shao, and H. Susan Zhou ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00032 • Publication Date (Web): 28 Dec 2017 Downloaded from http://pubs.acs.org on December 29, 2017

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ACS Applied Energy Materials

Facile Synthesis of Three-Dimensional PtPdNi Fused Nanoarchitecture as Highly Active and Durable Electrocatalyst for Methanol Oxidation Yanling Zhai1,4, Zhijun Zhu1,3*, Xiaolin Lu1, Zhiru Zhou1, Jiahui Shao2* and H. Susan Zhou1* 1

Department of Chemical Engineering, Worcester Polytechnic Institute, 100 Institute Road, Worcester, MA 01609, United States

2

School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai, 200240, China 3

Department of Materials Science and Engineering, Qingdao University, 308 Ningxia Road, Qingdao, 266071, China

4

Department of Chemistry and Chemical Engineering, Qingdao University, 308 Ningxia Road, Qingdao, 266071, China *Corresponding Author

E-mail address: [email protected] (Z. Zhu), [email protected] (J. Shao), [email protected] (H. S. Zhou)

Abstract Electrocatalysts for methanol electrooxidation with high activity and durability are greatly desired for boosting commercialization of direct methanol fuel cells (DMFCs). However, the current methanol electrooxidation catalysts are far from the anticipation and their application has been limited by the low catalytic activity and fast performance degradation. In this work, facile and rapid synthesis of ternary PtPdNi alloy nanoarchitecture with high performance for methanol electrooxidation is reported. The role of each component has been investigated in detail. The introduction of Pd makes contribution to the improvement of the catalytic activity. Furthermore, Ni plays important roles in improving both catalytic activity and durability. The PtPdNi catalyst shows higher activity than any of its components (Pt, PtPd and PtNi), and even 3.58 times higher than commercial Pt black (PB) in Pt-based mass activity. Moreover, the PtPdNi nanoarchitecture exhibits extremely high durability than PB, 0.6 A/mgPt (31% of the initial one) of the catalytic current density retains even after 50,000 s endurance test. Keywords: PtPdNi ternary alloy; highly performance; improved activity; excellent durability; methanol oxidation reaction;

Introduction Direct methanol fuel cells (DMFCs) have been intensively studied for the past few decades because methanol is safe for storage and transportation. DMFCs possess fast anodic reaction 1 and they have been regarded as clean and high efficient new generation energy conversion devices. 2, 3 However, their worldwide application has been limited by the performance of the anode catalyst, especially the expensive cost of the precious metal (usually Pt) and the activity degradation.4 It is crucial to design and develop active, durable, low-cost, and high performing catalysts for methanol oxidation reaction (MOR) to expedite the commercialization of the DMFCs. Some encouraging progress has been achieved with low-Pt alloys and non-platinum catalysts, and by synthesizing various shaped nanostructures to weaken the Pt dissolution, Ostwald ripening and so on.5, 6 Alloy nanowire and nanotube have shown improved long-term performance than counterpart nanosphere catalysts, because nanospheres are easy to be subject to metal oxidation and dissolution,7, 8 moreover, 3D nanostructure 1

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often possesses better electrocatalytic performance than the 1D nanowire,9 due to the enhancement of catalytic active sites exposure and rapid diffusion of reactants and waste.10 For example, Chen et. al. found graphene supported Pd@Pt nanoflowers showed enhanced catalytic activity towards methanol oxidization than PdPt alloy nanofoam and commercial Pt/C catalysts.11 Recently, Tan and coworkers reported non-platinum anode catalyst, Au@Pd core-shell nanoparticles for MOR, which showed improved MOR catalytic performance than Pd nanoparticles themselves.12 Non-platinum catalysts showed great MOR performance, however, they are still far away from the requirement of the urgent need of DMFCs commercialization. The composition of nanocatalysts is critical to electrocatalytic performance.13 Most recently, much more attention has been paid on the Pt-based binary and multicomponent alloy nanomaterials. Boosting researches aimed at preparation of different kinds of Pt-based alloy nanocatalysts with higher electroactivity, such as PtRu,14 PtAu,15 PtPd,11, 16 PtNi,17 PtPdCu,6 and PtCuCoNi18 have been reported to reduce the cost of the anode catalysts. Unfortunately, most of these catalysts suffer from the degradation of the activity in a long-term test. More recently, Huang et al. prepared Pt-Ni(OH)2-graphene ternary hybrids from a two-step overnight method, which exhibit exceptional activity and durability towards MOR in alkaline media.4 However, reports on anode catalysts with high performance and great durability toward MOR in alkaline media from simple methods are still rare. In this work, we report a rather simple, rapid and surfactant-free method, one pot reduction, to prepare PtPdNi three-dimensional nanoarchitecture (3DNA) within 5 min, and this ternary alloy nanocatalyst shows great performance towards MOR. Each component of the alloy makes contribution to the high performance of PtPdNi 3DNA: Pt and Pd serve as the active sites for MOR, addition of Pd can improve the activity of Pt besides its own electrochemical performance, and the introduction of Ni not only decreases the onset potential, but also raises the catalytic current and improves the stability of the catalyst. As a result, the PtPdNi catalyst shows higher activity than any of its components due to the synergy effect, and even 3.58 times higher than the commercial Pt black (PB) in Pt-based mass activity. Moreover, Ni greatly improves the durability of the nanocatalyst. Therefore, the PtPdNi nanoarchitecture exhibits extremely high durability, 0.6 A/mgPt of the catalytic current density even after 50,000 s endurance test, which is much better than most of the Pt-based MOR reports. The outstanding methanol electrocatalytic activity and extremely high durability enable this alloy great potential in DMFCs and other electrochemical applications.

Experimental section Chemicals. Chloroplatinic acid (99.9%, H2PtCl6), palladium (II) chloride (99.9%, PdCl2), Nickel (II) sulfate (99%, NiSO4) and nafion (5 wt.%) solution were obtained from Sigma-Aldrich. Potassium hydroxide (85%, KOH), sodium borohydride (NaBH4) and commercial Platinum black (PB) catalyst were obtained from Alfa Aesar. Methanol was purchased from Fisher Scientific. Milli-Q water (>18.2 MΩ·cm) was used throughout the experiments. All electrochemical experiments were carried out at room temperature. Apparatus. Transmission electron microscopy (TEM) measurements were performed on JEOL 100 CX II operated at 100 kV accelerating voltage. X-ray diffraction (XRD) measurement was performed on a Bruker AXS D8 Focus Powder X-ray diffractometer equipped with Cu Kα radiation (λ = 0.154 nm) at 40 kV accelerating voltage and 40 mA current. The compositions of the catalysts were tested with Inductively Coupled Plasma-Mass Spectrometry (ICP-MS, 7900, Agilent, USA). X-ray photoelectron spectroscopy (XPS) measurements was carried out on an X-ray photoelectron spectroscope (ESCALAB 2


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MK II, Thermo Fisher Scientific, USA). The electrochemical measurements were carried out with a three-electrode system on an Autolab PGSTA12 electrochemical workstation. The glassy carbon (GC) electrode with 3 mm diameter, BASi), platinum wire and Ag/AgCl (saturated with KCl, BASi) were used as working, counter and reference electrode, respectively. Synthesis of PtPdNi alloy nanoarchitecture. All reagents were directly used as received without further purification. H2PtCl6 and NiSO4 were dissolved into water with a concentration of 0.1 M, and PdCl2 was dissolved into 0.1 M HCl to form 0.1 M of H2PdCl4. Firstly, 50 µL of H2PtCl6, NiSO4 and H2PdCl4 were added into 10 mL of water. Then 2 mL of fresh prepared 2 mg/mL NaBH4 solution was quickly injected into the precursor solution under vigorous magnetic stirring, and the color of solution changed from light yellow to black rapidly. After stirring for 2 min, the PtPdNi nanoarchitecture (NA) was collected by centrifugation and washed with water once. For comparison, Pt, PtPd and PtNi 3DNA were prepared by using H2PtCl6, H2PtCl6 and H2PdCl4, H2PtCl6 and NiSO4 as precursor while keeping others unchanged. The molar compositions of the alloy 3DNA were measured to be Pt58Pd42, Pt78Ni22 and Pt51Pd33Ni16, respectively. Electrocatalytic experiment. The as-prepared Pt-based NAs and commercial PB catalysts were dispersed in a mixture (water, isopropanol, and nafion (5%) with v/v/v = 4/1/0.025) with 2.43 mg/mL of Pt. Prior to the surface coating, the GC electrodes were carefully polished with Al2O3 powder (0.3 and 0.05 µm), rinsed with water, and sonicated in water and ethanol successively for 3 cycles. After the electrodes were dried under nitrogen, 3 µL of prepared slurry was dropped onto the GC disk and dried under ambient condition to obtain the catalyst layer. Before electrocatalytic measurement, the catalysts were cycled in the N2-saturated 1 M KOH-1 M methanol with a scanning rate of 50 mV s-1 until stable cyclic voltammograms (CVs) were obtained. Durability tests were performed in N2-saturated 1 M KOH-1 M methanol mixture at −0.25 V (vs. Ag/AgCl), 1 s step for 10,000 s and 5 s step for 50,000 s, respectively.

Results and discussion Preparation and characterization

Figure 1. Typical TEM images (A and B), HRTEM image (C) and HAADF-STEM (D) and elemental mapping (E for Pt-L, F for Pd-L, G for Ni-K and H for merged image) of PtPdNi NWs. 3



This method for preparation of PtPdNi three-dimensional nanoarchitecture (3DNA) is rather simple and the whole synthesis can be completed within 5 min at room temperature. Figure 1 represents the transmission electron microscopy (TEM) images of the PtPdNi 3DNA under different magnifications. The PtPdNi 3DNA shows interconnected nanostructures (Figure 1), which is beneficial for fuels approaching and the waste leaving the catalyst active sites. TEM images with higher magnification (Fig 1B) demonstrate the PtPdNi 3DNA are not loosely aggregated nanostructures but fused architecture with diameter of 5 to 12 nm. Figure 1C shows the High resolution TEM image of the PtPdNi 3DNA. The lattice fringe of 0.23 nm indicates the (111) plane of a face centered cubic (fcc) lattice. High angle annular dark field scanning TEM (HAADF-STEM) and elemental mapping results (Figure 1D-G) show that Pt, Pd and Ni distributed uniformly in the PtPdNi 3DNA. To evaluate the role of each component, Pt, PtPd and PtNi 3DNA were synthesized from similar method by changing the metallic salts precursor. PtNi (Supporting Information, Figure S1) and PtPd (Figure S2) 3DNA show similar structure as PtPdNi. However, Pt 3DNA shows relative smoother surface and slightly thinner building blocks (Figure S3).

Figure 2. XRD pattern of PtPd (red), PtNi (black) and PtPdNi (blue) 3DNA. The blue vertical lines indicate the peaks of PtPdNi 3DNA. The typically X-ray diffraction (XRD) crystalline structures of PtPd, PtNi and PtPdNi catalysts were shown in Figure 2. In the case of PtPd 3DNA, XRD peaks centered at 40.15°, 46.10°, 67.95° and 81.65° can be assigned to (111), (200), (220) and (311) planes, all of which are consistent well with the face-centered cubic (fcc) structure of standard Pt (JCPDS no. 01-1194) and Pd (JCPDS no. 46-1043) crystalline, because of the low lattice mismatch (0.77%) of Pt and Pd.19 As expected, each XRD peak of PtPdNi 3DNA (blue curve) lies between the corresponding peaks of PtPd (black curve) and PtNi (red curve) 3DNA. All these results verify the formation of PtPdNi alloy nanostructure. To demonstrate the surface structure and composition of the PtPdNi 3DNA, XPS characterization was carried out. As shown in Figure S4A, Pt 4f7/2 and 4f5/2 spectrum was fitted to be two doublelets corresponding to Pt0 and Pt2+ (PtO), respectively, with binding energies (71.3 eV for Pt0 4f7/2, 74.6 eV for Pt0 4f5/2, 72.15 eV Pt2+ 4f5/2, and 75.55 eV for Pt2+ 4f5/2) in accordance with previous reports.20, 21 The appearance of Pt2+ is attributed to partial oxidation of surface Pt atoms by O2 in the air.22 Similarly, Pd 3d5/2 and 3d3/2 peaks at 335.75 and 341.05 eV indicate 4


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mixed valences of Pd species on the catalyst surface, Pd0 and Pd2+ (PdO).23 Most of the surface Pt and Pd retained metallic state. When it comes to Ni, however, the oxidation state of Ni (NiO and Ni(OH)2) was predominant on the catalyst surface (Figure S4C),4 and a relative weak peak with a binding energy of 851.8 eV was also observed, which is assigned to 2p3/2 of metallic Ni.22 This phenomenon has been reported previous because Ni is easily oxidized in air.24 Therefore, the PtPdNi 3DNA has been successfully prepared. The surface atomic composition was determined to be Pt49Pd32Ni19 (at%) from the XPS characterization, in accordance with the ICP analysis. Inspired by the outstanding fused nanostructure and the potential synergy effect of multicomponent metal, the electrochemical catalytic activity of PtPdNi 3DNA towards methanol oxidation reaction (MOR) was tested. The electrooxidation activity of these catalysts was examined by cyclic voltammetry (CV). Figure 3A shows the typical CV of these catalysts with same Pt loading in N2-saturated 1 M KOH. It can be found that reduction peaks of metal oxides shifted negatively in the backward scan due to the introduction of Ni, compared with Pt, PtPd and PB. Moreover, the onset potential in the backward scan shifted positively in the PtPdNi test. More importantly, the alloyed nanostructures exhibited significantly increased electrochemical active surface area (ECSA), especially for PtPdNi 3DNA, than PB. Then the ECSA was calculated25 since it is one of the most important parameters of the catalysts in electrocatalytic performance. As shown in Table S1, the ECSA are 4.53, 9.99, 17.05, 23.34, and 6.77 m2/gPt for Pt, PtNi, PtPd, PtPdNi 3DNA and PB, respectively. The increased ECSA of the alloy nanostructures compared with Pt and PB can be ascribed to dendritic morphology and exposure of more active sites resulting from alloy structure after introduction of other metals.26 The catalytic activity of these materials towards methanol electrooxidation was further determined in N2-saturated 1 M methanol-1 M KOH mixture, and the CV curves were displayed in Figure 3B. It can be seen that PtPdNi 3DNA (4.95 A/mgPt) has the highest mass activity, PtPd (4.29 A/mgPt) shows slightly lower mass activity than PtPdNi, which are much larger than PtNi (2.52 A/mgPt), Pt (1.60 A/mgPt) and PB (1.38 A/mgPt). The enhanced mass activity reveals that a given power output can be achieved from less mass of Pt and thus lower cost of the catalyst, which would accelerate the development of fuel cells. The introduction of Pd increases the catalytic activity greatly in PtPd nanostructure compared with Pt 3DNA and PB, but no clear advantage was observed in onset potential.10 Same tendency can be found in PtPdNi compared with PtNi. Moreover, Ni shows improvement on both methanol electrooxidation activity and onset potential (compared PtNi and PtPdNi with Pt and PtPd. The result is summarized in Table S1), due to the synergetic effect of Ni and Pt.27 As a result, PtPdNi ternary alloy catalyst possess both high activity and low onset potential, which is due to synergy effect toward MOR after introduction of Pd and Ni into Pt lattice. Nevertheless, this result indicates that PtPdNi 3DNA is a promising candidate to MOR catalyst with high activity and low cost.

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Figure 3. CVs of the different catalysts in N2-saturated 1 M KOH (A) and1 M methanol-1 M KOH (B), at a scan rate of 50 mV/s. Durability measurement of various catalysts in 1 M methanol-1 M KOH mixture at -0.25 V (C, vs. Ag/AgCl). Durability test Excellent durability is another key factor for the MOR besides high catalytic activity. Firstly, short-term (10,000 s) stability of these five catalysts toward MOR was tested with chronoamperometric method. It can be clearly observed that a rapid decrease of current density in all the catalysts present in the initial test, and then the current density of PtPdNi and PtNi 3DNA remains much higher than that of PB and Pt 3DNA, and that of PtPd keeps in between, as shown in Figure 3C. Interestingly, the activity of the PtPd 3DNA is first higher than that of PtNi 3DNA, however, the former one drops faster, and two curves meet at around 2,000 s. PtPdNi 3DNA keeps the highest activity throughout the test and still reserves 1.3 A/mgPt of the activity after 10,000 s test, which is much higher than others. It can be observed from Figure 3C that, Ni plays much more important role in both activity increase and improved durability of the PtPdNi and PtNi 3DNA toward MOR by facilitating oxidative removal of carbonaceous poison from adjacent Pt4 through charge transfer from Ni to Pt atoms,22 while Pd mainly makes contribution to the activity enhancement. It can be easily concluded that the excellent catalytic performance was due to the beneficial structural feature and the multicomponent synergy effect.

Figure 4. Long-term durability measurement of PtPdNi 3DNA and PB in 1 M methanol-1 M KOH mixture at -0.25 V. As we know, poor long-term durability hampers the rapid development of the methanol electrooxidation catalysts. Therefore, the long-term stability of the PtPdNi 3DNA was further tested in N2-saturated 1 M KOH-1 M methanol solution for 50,000 s at -0.25 V. The PtPdNi 3DNA shows overwhelming advantage throughout the test than that of PB. 0.6 A (31% of the initial one) of the mass current reserves after 50,000 s durability test (Figure 4), which is much higher than most of the current reports summarized in Table 1. This remarkable activity and unprecedented durability for MOR is owing to the 3D ternary alloy with fused building blocks and the strong synergy effect of the three metals interactions and collectively. 6


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Table 1. The electrochemical parameters of Pt-based catalysts toward methanol oxidation in alkaline solution. catalysts

method

i-t potential (V)

duration (s)

final activitya

PtPdNi

NaBH4 Reduction

-0.25 (Ag/AgCl)

50,000

0.603

this work

PdNi

electrodeposition

-0.3 (SCE)

3,500

0.04

28

PtCu NCs

oil-phase reduction

-0.3 (RHE)

1,000

0.15

29

Pt/rGO

vitamin C reduction

-0.3 (SCE)

3,600

Facile Synthesis of Three-Dimensional PtPdNi Fused

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