PdCu Alloy Flower-like Nanocages With High Electrocatalytic

Subscriber access provided by UNIV OF DURHAM

C: Surfaces, Interfaces, Porous Materials, and Catalysis

PdCu Alloy Flower-like Nanocages With High Electrocatalytic Performance for Methanol Oxidation Zhen Chen, Ya-Chuan He, Jia-Hui Chen, Xian-Zhu Fu, Rong Sun, Yanxia Chen, and Ching-Ping Wong J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01095 • Publication Date (Web): 06 Apr 2018 Downloaded from http://pubs.acs.org on April 7, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 38 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

The Journal of Physical Chemistry

PdCu Alloy Flower-like Nanocages With High Electrocatalytic Performance for Methanol Oxidation Zhen Chen,

a,b

Ya-Chuan He,

a,b

Jia-Hui Chen,

a

Xian-Zhu Fu,

*,a,c

Rong Sun,

*,a

Yan-Xia Chen, b Ching-Ping Wong d,e

a

Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences,

Shenzhen 518055, China. b

Institute of Nano Science and Technology, University of Science and Technology of

China, Suzhou 215123, China. c

College of Materials Science and Engineering, Shenzhen University, Shenzhen

518055, China. d

School of Materials Science and Engineering, Georgia Institute of Technology,

Atlanta, GA 30332, United States. e

Department of Electronics Engineering, The Chinese University of Hong Kong,

Hong Kong, China.

* Corresponding author: * Xian-Zhu Fu, E-mail address: [email protected]; Tel: +86-755-86392151; Fax: +86-755-86392299 * Rong Sun, E-mail address: [email protected].

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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

Abstract PdCu alloy flower-like nanocages are synthesized through corner-etched Cu2O octahedra as templates by galvanic reaction and disproportionation reaction. PdCu alloy nanocages are obtained with the molar ratio of H2PdCl4:Cu2O = 1:1. And the Pd nanoparticles are prepared with the H2PdCl4:Cu2O = 2:1. The PdCu alloy flower-like nanocages display the highest catalytic activity compared with Pd nanoparticles towards methanol oxidation reaction (MOR) in alkaline medium, which is 2.7 times than that of commercial Pd/C. In addition, PdCu alloy nanocages also show high poison-tolerant, good stability. The superior electrocatalytic performance of PdCu flower-like nanocages may be attributed to the unique flower-like nanocage structure with high surface area and the synergic effect of Pd and Cu.

2


Page 2 of 38

Page 3 of 38 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


Introduction Methanol is regarded as green energy and renewable resource

1-4

. Direct

methanol fuel cells (DMFCs) have attracted much attention in recent years due to high energy density, high energy conversion efficiency, low operating temperature, simple system design and low pollutant emission 5-8. There are two main reactions in the DMFCs. one at anode is methanol oxidation reaction (MOR), and another at cathode is oxygen reduction reaction (ORR)

9-11

. Meanwhile, the MOR decides the

whole efficiency of DMFCs 6. Therefore, designing electrocatalysts with high catalytic activity towards MOR is promising for DMFCs. To date, Pt and Pt-based electrocatalysts are most promising electrocatalysts for MOR 12-13. However, there are some shortcomings for Pt and Pt-based electrocatalysts. On the one hand, the scarce resources in the earth and high cost of Pt make it limited for practical application. On the other hand, Pt and Pt-based electrocatalysts are easily poisoned by CO in acid electrolyte. Meanwhile, CO as intermediate is formed during the MOR, which is poisonous for Pt and Pt-based electrocatalysts

14-16

. However, if

the DMFCs operate in the alkaline electrolyte, the kinetics can be enhanced significantly, and Pt-free electrocatalysts can be used 17. Recently, Pd as a candidate for Pt has attracted attention. Compared with Pt and Pt-based electrocatalysts, Pd as electrocatalyst not only is lower cost, more abundant, but also displays high resistance to CO and excellent catalytic activity towards MOR 3



Page 4 of 38

18

. However, the stability and activity of Pd electrocatalysts are still need to be

improved

19

. To enhance the catalytic performance of electrocatalysts, introducing

other metals into Pd is a simple method. Pd-based materials have many reports, such as RhPd alloy

20

, CoPd

21

, PdNi alloy

22

, CuPd alloy

23

, FePd

24

, PdPb

25

, PdRu

26

, AuPd

27

, et.al. Among these materials, due to low cost, abundant in the earth and

benign metal of Cu, PdCu has attracted much attention

28

. Different structures of

PdCu have reported in recent year. For example, PdCu nanocapsules with the size of 10 nm were prepared by a one pot solvothermal process

29

. PdCu nanocubes were

synthesized under N2 atmosphere 30. Bimetallic Cu-Pd alloy multipods were obtained via galvanic replacement reaction 23. Porous PdCu nanoparticles with sizes of 40 ± 5 nm have been reported

31

. In addition, 3D PdCu alloy nanosheets were also

synthesized 32. Nanocages structures have large void space, high surface to volume ratio and accessible active sites

33-34

. Catalysts with nanocages structures can display high

catalytic performance. For example, Pt-Ag alloy nanocages were prepared by galvanic replacement reaction 35. The mass activity of Pt-Ag nanocages is 1.23 mA cm-2 at 0.9 V (V vs. RHE) towards oxygen reduction reaction after 10000 cycles, which is 3.3 times than that of commercial Pt/C (0.37 mA cm-2). Moreover, icosahedral Pt-enriched nanocages display 10 times of specific activity and 7 times of mass activity than that of commercial Pt/C towards oxygen reduction reaction

36

. The

current density of PtCu3 nanocages, PtCux nanoparticles and commercial Pt are 14.1, 8.4 and 12.8 mA cm-2 for methanol oxidation reaction in 0.1 M HClO4 + 1 M 4


Page 5 of 38 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


methanol at a scan rate of 20 mV s-1, respectively. Obviously, the PtCu3 nanocages display the highest catalytic activity 33. The above results indicate that materials with nanocage structure can show superior catalytic performance, which can be attributed to the high surface area and unique nanocage structure. Therefore, PdCu flower-like nanocages as catalysts may also display high catalytic activity. To date, the reports about PdCu alloy with flower-like nanocages structures are few. Herein, we design a facile, simple method to synthesize PdCu alloy flower-like nanocages using corner-etched Cu2O octahedra as templates. The Cu2O was converted to Cu2O@PdCu core-shell by adding the H2PdCl4, then the Cu2O core was etched by NH3·H2O. The obtained PdCu alloy flower-like nanocages with size of 450 nm were used for methanol oxidation reaction. The resulting PdCu alloy flower-like nanocages display high catalytic activity, good stability and high poison-tolerant towards MOR.

Experimental section Reagents Palladium chloride (PdCl2, 59-60 %), potassium hydroxide (KOH, 90.0 %), polyvinylpyrrolidone

(PVP,

K-30),

potassium

sodium

tartrate

tetrahydrate

(C4H4O6KNa·4H2O, 99.0 %) and ammonia solution (NH3·H2O, 25-28 %) were supplied from Aladdin. Copper (II) sulfate pentahydrate (CuSO4·5H2O, ≥ 99.0 %), glucose anhydrouse (C6H12O6, ≥ 99.7 %) and methanol (CH3OH, ≥ 99.5 %) were purchased from Sinopharm chemical reagent Co., Ltd (Shanghai, China). All reagents were used as received without further purification. All aqueous solutions were 5



prepared with Millipore water (18.2 MΩ cm). Preparation of corner-etched Cu2O octahedra Corner-etched Cu2O octahedra was synthesized based on the previously reported by Lu’s group 37. In brief, 500 mL solution containing 2.8 mM CuSO4·5H2O, 8.9 mM C4H4O6KNa and 8 mM KOH was prepared. Next added 4.8 mg PdCl2 into above solution. Then introduced 10 mL 0.25 M glucose, resulting a light blue solution. After 2 minutes, the solution was aged at 75 °C for 2.5 h with vigiously stirring. The precipitates were obtained by centrifugation at 8000 rpm, washed several times by water and ethanol, and dried under vacuum for 10 h. Preparation of PdCu alloy flower-like nanocages The PdCu alloy flower-like nanocages were synthesized by galvanic reaction and disproportionation reaction. The whole procedure of PdCu nanocages transformed from corner-etched Cu2O octahedra were operated at room tempareture. Briefly, 7.2 mg corner-etched Cu2O octahedra and 0.05 g PVP were added into 8 mL water. Then 5 mL 10 mM H2PdCl4 was introduced into mixture with constantly stiring. Once the H2PdCl4 was added, the solution turned from yellow to black quickly. After 20 minutes, the mixture was centrifugated at 9000 rpm, washed 4 times with water and ethanol. Then added the precipates into 7 mL 25-28 % NH3·H2O. The reaction was stopped after 10 h. The PdCu alloy flower-like nanocages were obtained by centrifugation at 9000 rpm and washed with mixed water and ethanol solution (volume ratio: 1:1) 5 times, dried under vacuum overnight. Preparation of Pd nanoparticles 6


Page 6 of 38

Page 7 of 38 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


For comparison, Pd nanoparticles were prepared by the similar method of PdCu alloy flower-like nanocages. 5 mL 10 mM H2PdCl4 was replaced by 10 mL 10 mM H2PdCl4.

Physical Characterization The morphologies of as-synthesized materials were analyzed by field emission scanning electron microscopy (FE-SEM, FEI Nova Nano SEM 450) and high resolution transmission electron microscope (HRTEM, Tecnai G2 F20 FEI), respectively. X-ray diffraction (XRD, Rigaku D/max 2500, Japan) was used to characterize the crystalline structure. The element valence states of electrocatalysts were analyzed using X-ray photoelectron spectroscopy (XPS, PHI-1800, Japan). Electrochemical Instrument All electrochemical measurements were carried out with a CHI 760E electrochemical workstation. A three-electrode system was applied to measure the electrochemical performance of different electrocatalysts. A Pt plate (1 cm × 1.5 cm) and an Ag/AgCl were used as counter and referece electrode, respectively. A glass carbon electrode (GCE, diameter: 5 mm, area: 0.196 cm2) coated with electrocatalysts was used as working electrode. A homogeneous ink of catalyst was prepared by ultrasonic the mixture of 4 mg electrocatalyst and 1 mL ethanol for 30 min. Added 14 µL ink into GCE, and dried at room temperature. Then 10 µL 0.1 wt% nafion was covered and dried again. Thus, the modified electrode was obtained. All electrochemical measurements were measured in 0.5 M KOH or 0.5 M KOH solution with 1 M methanol. 7



Results and discussion Synthesis of corner-etched Cu2O octahedra The corner-etched Cu2O octahedra was synthesized by modifying the method of Lu

37

. CuSO4·5H2O was used as the raw material of Cu, PdCl2 was catalyst to

fabricate the Cu2O. PdCl2 plays an important role in the whole process. First, PdCl2 was reduced to Pd nanoparticles with the help of glucose. Then Pd nanoparticles as oxidizing reagent transformed Cu2+ into Cu2O octahedra. Due to the higher activity, the apexes of Cu2O were oxidative etched by Pd. Then the corner-etched Cu2O octahedra was formed. Figure 1a-b display the SEM images of Cu2O. It clearly shows that the morphology of Cu2O is corner-etched octahedra. The size of Cu2O nanoparticle is about 450 nm. To further characterize the structure of Cu2O, TEM was performed. Figure 1c-d show the TEM images of Cu2O. Obviously, the corners of octahedra were etched. The above results indicate that Cu2O with corner-etched octahedra structure is synthesized successfully.

Figure 1 SEM images (a-b) and TEM images (c-d) of corner-etched Cu2O octahedra. 8


Page 8 of 38

Page 9 of 38 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


Synthesis of PdCu alloy flower-like nanocages PdCu alloy flower-like nanocages were mainly fabricated by two steps: H2PdCl4 was added into mixture containing corner-etched Cu2O octahedra and surfactant PVP to form Cu2O@PdCu core-shell structure; then the Cu2O core was totally etched by NH3·H2O. The formation of PdCu alloy nanocages can be formulated as followings 38-39

: Cu2O + Pd2+ + 2H+ → Pd + 2Cu2+ + H2O

(1)

Cu2O + 2H+ → Cu + Cu2+ + H2O

(2)

Pd + Cu → PdCu alloy

(3)

Cu2O + 4NH3·H2O → 2[Cu(NH3)2]+ + 2OH- + 3H2O

(4)

When the H2PdCl4 was introduced into Cu2O and surfactant PVP mixture solution, the solution turned from yellow to black quickly, which indicated Pd2+ reacted with Cu2O immediately (Equation 1). The existence of Cu can be attributed to the disproportionation reaction, Cu2O can transform into Cu and Cu2+ in acid environment (Equation 2). The metallic Pd and Cu were produced immediately once the H2PdCl4 was added. Therefore, PdCu alloy was formed (Equation 3). The excessive Cu2O core was then dissolved by NH3·H2O to form [Cu(NH3)2]+ (Equation 4). In addition, [Cu(NH3)2]+ was easily oxidized to blue water-soluble [Cu(NH3)4]2+ in the air. Therefore, PdCu alloy flower-like nanocages were formed. When H2PdCl4:Cu2O = 2:1, Cu2O was totally replaced with Pd, then the pure Pd nanoparticles was obtained. Therefore, PdCu nanocages can be fabricated when the 9



Figure 2 SEM images of Pd nanoparticles (a-b) and PdCu (c-d) alloy flower-like nanocages.

ratio of H2PdCl4:Cu2O is lower than 2. As shown in Figure 2, PdCu and Pd nanoparticles are nearly corner-etched octahedra structure, which are inherited from the morphology of corner-etched Cu2O octahedra. Moreover, the size of PdCu alloy and Pd nanoparticles are all about 450 nm. For comparison, we prepared PdCu alloy with different molar ratios of H2PdCl4:Cu2O. We used the ratio of 1:2 and 1:3 to prepare PdCu-2 and PdCu-3 alloy flower-like nanocages, respectively. The morphologies of PdCu-2 and PdCu-3 nanocages can be seen in Figure S1. The sizes, shapes and flower-like nanostructures of PdCu-2 and PdCu-3 are nearly the same as the PdCu alloy flower-like nanocages. However, the amount of H2PdCl4 is less, which can react with less Cu2O, the shell is 10


Page 10 of 38

Page 11 of 38 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


thinner, and the flower-like nanocages are prone to break. The EDS of PdCu nanocages is shown in Figure S2. The content of Pd in PdCu nanocages is about 84.8 %, and the ratio of Cu is less. In addition, the EDS of PdCu-2 and PdCu-3 nanocages are shown in Figure S3-4. The contents of Pd in PdCu, PdCu-2 and PdCu-3 nanocages are no big significant difference, and the ratios of Cu are less. This phenomenon can be assigned to these reasons: the reduce potential of Cu/Cu2+ pairs (0.337 V) is much lower than that of Pd2+/Pd pairs (0.987 V), only a small quantity of Cu atoms can be produced. In addition, once PdCu alloy formed, Cu atom might be difficult to be oxidized by Pd2+, resulting in the low the content of Cu in the final products 38. To further investigate the structure of PdCu alloy flower-like nanocages, TEM

Figure 3 TEM images (a-b) and corresponding mapping images (c-e) of PdCu alloy flower-like nanocages

11



was measured. Figure 3a-b display typical TEM images, it indicates that the preparation of PdCu nanocage is successful. The PdCu nanocages are uniform and the size is about 450 nm. The nanocage structures and porous features contribute to the high catalytic performance of PdCu alloy flower-like nanocages. In addition, the mapping images were recorded to characterize the distribution of Pd and Cu elements in PdCu nanocages, which are shown in Figure 3c-e. Evidently, Pd and Cu are uniformly distributed throughout the whole PdCu nanocage. The crystal structures of as-synthesized materials are performed by XRD. Figure S5 shows the typical XRD patterns of corner-etched Cu2O octahedra. The strong diffraction peaks for Cu2O is cubic crystal structure (JCPDS no. 65-3288). For comparison, the pure Pd nanoparticles were synthesized. XRD patterns were performed to evaluate the feature of alloy for PdCu nanocages. As shown in Figure 4, Pd nanoparticles and PdCu nanocages have similar XRD patterns. Obviously, these materials have four diffraction peaks without any Cu2O peaks. The four diffraction

Figure 4 XRD patterns Pd nanoparticles and PdCu alloy flower-like nanocages.

12


Page 12 of 38

Page 13 of 38 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


peaks of PdCu nanoparticles are assigned to the planes (111), (200), (220) and (311) of fcc structure, respectively. The locations for four diffraction peaks of PdCu are nearly consistent with that of Pd, which can be attributed to the low content of Cu in PdCu nanocage. To further characterize the surface chemical state of PdCu alloy flower-like nanocages, XPS was performed. The high resolution XPS spectrum of Pd 3d of Pd and PdCu nanocages are shown in Figure 5. Obviously, there are four peaks of Pd 3d spectrum assigned to metallic Pd and Pd(Ⅱ) species for Pd and PdCu nanocages

40

.

The binding energy of metallic Pd for Pd are 335.7 eV and 341.0 eV, which can be labbeld as Pd-M (M=Pd, Cu) 41. Moreover, as for PdCu nanocages, the peak at around 335.9 eV and 341.2 eV are assigned to metallic Pd, which shifts to a positive binding energy compared with that of Pd. The above results can be attributed to the incorporation of Cu for PdCu-nanocages 40. The work function of Pd (5.12 eV) is

Figure 5 XPS spectra of Pd 3d for Pd and PdCu alloy flower-like nanocages.

13



Page 14 of 38

higher than that of Cu (4.65 eV), therefore, partial electron transfer from Cu to Pd can occur

23

. The interaction between Cu and Pd can result in uneven distribution of

electrons between Cu and Pd. Therefore, these shifts occur for PdCu

42

. Figure S6

displays the Cu 2p spectrum of PdCu alloy flower-like nanocages. Two peaks are observed at 931.9 eV and 934.1 eV, respectively. The peak corresponding to 931.9 eV relates to Cu0, which is in the PdCu alloy state 29, 41. Another peak at 934.1 eV can be assigned to Cu2+

29, 43

. The existence of Cu2+ might be attributed to the oxidation of

PdCu alloy in the air. Methanol Oxidation Reaction Figure 6a displays the CV curves of Pd, PdCu alloy flower-like nanocages and commercial Pd/C in 0.5 M KOH without methanol at a scan rate of 50 mV s-1. Obviously, there are cathodic peaks at around -0.46 V vs. Ag/AgCl, which can be related to the reduction of palladium oxide

31, 44-45

. Electrochemical surface area

(ECSA) is an important parameter to evaluate the catalytic performance of electrocatalysts. The ECSA values of as-prepared materials can be calculated by the following formula: ECSA = Q/(0.405/[Pd]) Where Q (mC cm-2) is charge related to the PdO peak, 0.405 µC cm-2 is assumed to be the charge of reduction PdO monolayer 31, 46-47. [Pd] is the content of Pd (mg cm-2) in

14


Page 15 of 38 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


Figure 6 CV curves of Pd, PdCu alloy flower-like nanocages and commercial Pd/C in 0.5 M KOH (a) and 0.5 M KOH with 1 M methanol solution (b) at a scan rate of 50 mV s-1; (c) Bar graph highlighting methanol oxidation reaction activity at respective MOR peak potential values for PdCu alloy flower-like nanocages, Pd and commercial Pd/C in 0.5 M KOH + 1 M methanol at a scan rate of 50 mV s-1; (d) Chronoamperometry curves of Pd, PdCu alloy flower-like nanocages and commercial Pd/C in 0.5 M KOH with 1 M methanol solution at -0.2 V vs. Ag/AgCl.

the electrode. After calculating, the ECSA of Pd, PdCu alloy flower-like nanocages and commercial Pd/C are 9.6, 22.1 and 17.3 m2 g-1, respectively. Evidently, the PdCu alloy nanocages possess highest ECSA. It can be attributed to the high surface area of nanocage structure. To investigate the catalytic activity of as-prepared electrocatalysts, CV curves were performed in 0.5 M KOH with 1 M methanol at a scan rate 50 mV s-1. As shown 15



Page 16 of 38

in Figure 6b, there are two evident oxidation peaks under anodic condition. One at higher potential is related to methanol oxidation, another at lower potential is assigned to carbonaceous species oxidation

48

. Mass activity is a significant parameter to

evaluate the practical applications of electrocatalysts. As shown in Figure S7, PdCu, PdCu-2 and PdCu-3 display different mass activity, the order of mass activity of these as-synthesized electrocatalysts is PdCu nanocages (823 A g-1) > PdCu-2 nanocages (526 A g-1) > PdCu-3 nanocages (327 A g-1). Therefore, PdCu alloy flower-like nanocages display highest electrocatalytic activity towards MOR in alkaline medium, which can be attributed to the inherent nanocage structure and synergic effect of Pd and Cu. The steady-state concentration of species in rate-determining step can be increased due to the confinement effect of the cage. Moreover, the inner surface of the nanocage may not be as well capped as outer surface in some cases

49

. Therefore,

PdCu alloy flower-like nanocage display high electrocatalytic activity due to its unique nanocage structure. As shown in Figure 6c, PdCu and Pd display different mass activity, the order of mass activity of these as-synthesized electrocatalysts is PdCu nanocages (823 A g-1) > Pd (611 A g-1). In addition, the mass activity of PdCu nanocages is also compared with commercial Pd/C catalysts. As displayed in Figure 6b and c, the catalytic activity of PdCu nanocages is evident higher than commercial Pd/C towards MOR. It is worthy to note that the catalytic activity of PdCu nanocages is 2.7 times than that of commercial Pd/C catalysts (200 A g-1). Moreover, PdCu nanocages also show high mass activity compared with other catalysts in the literatures as listed in Table 1. The above results indicate that PdCu nanocages have a 16


Page 17 of 38 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


superior electrocatalytic activity toward MOR in alkaline medium. The high catalytic activity of PdCu nanocages might be attributed to the high specific surface area of nanocage structure as well as the synergic effect of Pd and Cu. The XPS results reveal the electron transfer effect due to the incorporation of Cu. The electron interaction of Pd and Cu can alter the electronic states of Pd and enhance the catalytic activity of PdCu

29

. Moreover, the lattice parameters of Cu are smaller

than that of Pd. Therefore, the incorporation of Cu would lead to the lattice contraction of Pd. The shrunk lattice and electron transfer effect can result in the lower d-band center of Pd

23

. Therefore, the chemisorption energy on the Pd surface

for reaction intermediates (such as CO and H2) would be weak. Accordingly, the incorporation of Cu can generate high activity of PdCu alloy 50-51. In addition, the poison tolerance of electrocatalysts is an important index for catalytic performance towards MOR. The tolerance to carbonaceous species accumulation of electrocatalysts can be evaluated by the index If/Ib, which is the ratio of forward oxidation peak (If) to the reverse oxidation peak (Ib)

33, 52-53

. A high If/Ib

ratio implies efficient removal of poisoning species on the surface of electrocatalysts. Therefore, we measured the If/Ib ratio of PdCu alloy flower-like nanocages. The If/Ib ratio of PdCu nanocages is 6.86. Moreover, we also compare with other materials in previously

reports,

such

as

Pt68Cu32

nanoparticles

(1.2)

43

,

Au/Ag/Pt

hetero-nanostructures (2.4) 54, [email protected] (2.2) 55, PtCu nanoframes (2.7) 56, et. al. The above results indicate that PdCu nanocages demonstrate superior poison tolerance.

17



Page 18 of 38

Table 1 Comparison of electrocatalytic activity of different electrocatalysts towards MOR.

catalysts

electrolyte

PdCu alloy flower-like

0.5 M KOH+1 M

nanocages

methanol

Pt-on-Pd nanodendrites

0.5 M H2SO4+1 M

Scan rate

Mass activity Reference

(mV s-1)

(A g-1)

50

823

This work

50

647.2

57

methanol Pt/G3-(CN)7

1 M H2SO4+2 M methanol

20

612.8

52

CeO2(30%)/nano-ZSM-5

0.5 M NaOH+0.5 M

50

73.5

6

100

433.5

44

50

363

31

methanol Pd-MoS2 NSs

0.5 M KOH+1 M methanol

Porous PdCu NP

0.5 M KOH+0.5 M methanol

G-AuPd@Pd

1 M KOH+1 M methanol

50

650

10

Pd/PPy-graphene

0.5 M NaOH+1 M

50

359.8

45

20

500

58

50

350

48

methanol Pt nanowire

0.5 M H2SO4+0.5 M methanol

Pt-Cu BANDs

0.5 M H2SO4+1 M methanol

18


Page 19 of 38 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


The stability of electrocatalysts were measured by chronoamperometry for 6000 s in 0.5 M KOH and 1 M methanol at the constant potential of -0.2 V vs. Ag/AgCl. The stability of Pd, PdCu and commercial Pd/C towards MOR is shown in Figure 6d. Evidently, the PdCu nanocages demonstrate highest mass activity during the whole process. Moreover, the mass activity of electrocatalysts all decrease during 6000 s electrolysis. The mass activity of Pd decreases quickly, and decreases to 3.2 A g-1. However, PdCu alloy flower-like nanocages still show high mass activity of 48.8 A g-1 after 6000 s. The above results indicate that PdCu nanocages perform superior stability towards MOR in alkaline medium. The high stability of PdCu alloy might be attributed to the electronic and structure factors. XPS results indicate that electron transfer between Cu and Pd during the formation of the alloy. The electronic states of Pd atoms can be altered obviously due to the electron interactions between Cu and Pd, which play an important role in durability of PdCu alloy

29

. Moreover, the

chemisorption energy on the Pd surface for adsorbed CO groups would be weak due to the introduction of Cu. Therefore, the rate of diffusion of the adsorbed CO groups would be faster, and the rate of oxidation of the adsorbed CO groups would be increased 46, 59. In addition, good structure stability plays an important part in superior stability towards MOR for PdCu alloy. As shown in Figure S8, the TEM images of PdCu alloy was no significant changes after long-term stability test, implying good structure stability. Moreover, mapping images display that Pd and Cu elements are still evenly distributed throughout the whole nanocage. The corresponding EDS measurement of PdCu alloy after long-term stability test is shown in Figure S9. 19



Obviously, the content of Pd is close to that in PdCu before long-term stability test. Therefore, PdCu alloy flower-like nanocages display high stability towards MOR.

Conclusions PdCu nanocages were synthesized with corner-etched Cu2O octahedra as precursors by galvanic reaction and disproportionation reaction. Cu2O nanoparticles were prepared by a catalytic process with morphology of corner-etched octahedra. H2PdCl4 reacts with Cu2O, then excessive Cu2O was etched by NH3·H2O to form PdCu alloy flower-like nanocages. The PdCu nanocages almost are inherited from morphology of corner-etched Cu2O octahedra. Meanwhile, PdCu nanocages display excellent flower-like nanocage structure. PdCu nanocages demonstrate high poison-tolerant towards MOR. PdCu shows superior catalytic activity, which is 2.7 times than that of commercial Pd/C. In addition, PdCu also demonstrates high electrochemical surface area and good stability towards MOR.

Supporting Information SEM images of PdCu-2 and PdCu-3; EDS of PdCu, PdCu-2 and PdCu-3; XRD pattern of Cu2O; XPS spectra of Cu 2p for PdCu alloy; CV curves of PdCu, PdCu-2 and PdCu-3 towards MOR; TEM images and corresponding mapping images of PdCu alloy flower-like nanocages after stability test.

Acknowledgements This work was financially supported by the National Natural Science Foundation 20


Page 20 of 38

Page 21 of 38 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


of China (No.21203236), Guangdong Department of Science and Technology (2017A050501052), and Shenzhen basic research plan (JCYJ20160229195455154).

Reference 1.

Behrens, M., et al., The Active Site of Methanol Synthesis over Cu/ZnO/Al2O3

Industrial Catalysts. Science 2012, 336, 893-897. 2.

Barton, E. E.; Rampulla, D. M.; Bocarsly, A. B., Selective Solar-Driven

Reduction of CO2 to Methanol Using a Catalyzed P-Gap Based Photoelectrochemical Cell. J. Am. Chem. Soc. 2008, 130, 6342-6344. 3.

Menard, G.; Stephan, D. W., Room Temperature Reduction of CO2 to Methanol

by Al-Based Frustrated Lewis Pairs and Ammonia Borane. J. Am. Chem. Soc. 2010, 132, 1796-1797. 4.

Grabow, L. C.; Mavrikakis, M., Mechanism of Methanol Synthesis on Cu through

CO2 and CO Hydrogenation. ACS Catal. 2011, 1, 365-384. 5.

Liu, M.; Lu, Y.; Chen, W., PdAg Nanorings Supported on Graphene Nanosheets:

Highly Methanol-Tolerant Cathode Electrocatalyst for Alkaline Fuel Cells. Adv. Funct. Mater. 2013, 23, 1289-1296. 6.

Kaur, B.; Srivastava, R.; Satpati, B., Highly Efficient CeO2 Decorated

Nano-ZSM-5 Catalyst for Electrochemical Oxidation of Methanol. ACS Catal. 2016, 6, 2654-2663. 7.

Han, F.; Wang, X.; Lian, J.; Wang, Y., The Effect of Sn Content on the

Electrocatalytic Properties of Pt–Sn Nanoparticles Dispersed on Graphene Nanosheets for the Methanol Oxidation Reaction. Carbon 2012, 50, 5498-5504. 21



8.

Ye, D.; Zhu, X.; Liao, Q.; Li, J.; Fu, Q., Two-Dimensional Two-Phase Mass

Transport Model for Methanol and Water Crossover in Air-Breathing Direct Methanol Fuel Cells. J. Power Sources 2009, 192, 502-514. 9.

Scofield, M. E.; Koenigsmann, C.; Wang, L.; Liu, H.; Wong, S. S., Tailoring the

Composition of Ultrathin, Ternary Alloy PtRuFe Nanowires for the Methanol Oxidation Reaction and Formic Acid Oxidation Reaction. Energy Environ. Sci. 2015, 8, 350-363. 10. Zheng, J.-N.; Li, S.-S.; Ma, X.; Chen, F.-Y.; Wang, A.-J.; Chen, J.-R.; Feng, J.-J., Green Synthesis of Core–Shell Gold–Palladium@Palladium Nanocrystals Dispersed on Graphene with Enhanced Catalytic Activity toward Oxygen Reduction and Methanol Oxidation in Alkaline Media. J. Power Sources 2014, 262, 270-278. 11. Tiwari, J. N.; Tiwari, R. N.; Singh, G.; Kim, K. S., Recent Progress in the Development of Anode and Cathode Catalysts for Direct Methanol Fuel Cells. Nano Energy 2013, 2, 553-578. 12. Mellinger, Z. J.; Kelly, T. G.; Chen, J. G., Pd-Modified Tungsten Carbide for Methanol Electro-Oxidation: From Surface Science Studies to Electrochemical Evaluation. ACS Catal. 2012, 2, 751-758. 13. Zhao, X.; Yin, M.; Ma, L.; Liang, L.; Liu, C.; Liao, J.; Lu, T.; Xing, W., Recent Advances in Catalysts for Direct Methanol Fuel Cells. Energy Environ. Sci. 2011, 4, 2736. 14. Kim, Y.; Noh, Y.; Lim, E. J.; Lee, S.; Choi, S. M.; Kim, W. B., Star-Shaped Pd@Pt Core–Shell Catalysts Supported on Reduced Graphene Oxide with Superior 22


Page 22 of 38

Page 23 of 38 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


Electrocatalytic Performance. J. Mater. Chem. A 2014, 2, 6976-6986. 15. Lu, Y.; Jiang, Y.; Wu, H.; 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. 16. Sui, Y.; Liu, S.; Li, T.; Liu, Q.; Jiang, T.; Guo, Y.; Luo, J.-L., Atomically Dispersed Pt on Specific TiO2 Facets for Photocatalytic H2 Evolution. J. Catal. 2017, 353, 250-255. 17. Qi, Z.; Geng, H.; Wang, X.; Zhao, C.; Ji, H.; Zhang, C.; Xu, J.; Zhang, Z., Novel Nanocrystalline PdNi Alloy Catalyst for Methanol and Ethanol Electro-Oxidation in Alkaline Media. J. Power Sources 2011, 196, 5823-5828. 18. Huang, H.; Wang, X., Pd Nanoparticles Supported on Low-Defect Graphene Sheets: For Use as High-Performance Electrocatalysts for Formic Acid and Methanol Oxidation. J. Mater. Chem. 2012, 22, 22533. 19. Zhao, Y.; Yang, X.; Tian, J.; Wang, F.; Zhan, L., Methanol Electro-Oxidation on Ni@Pd Core-Shell Nanoparticles Supported on Multi-Walled Carbon Nanotubes in Alkaline Media. Int. J. Hydrogen Energ. 2010, 35, 3249-3257. 20. Tao, F.; Grass, M. E.; Zhang, Y. W.; Butcher, D. R.; Renzas, J. R.; Liu, Z.; Chung, J. Y.; Mun, B. S.; Salmeron, M.; Somorjai, G. A., Reaction-Driven Restructuring of Rh-Pd and Pt-Pd Core-Shell Nanoparticles. Science 2008, 322, 932-934. 21. Mazumder, V.; Chi, M.; Mankin, M. N.; Liu, Y.; Metin, O.; Sun, D.; More, K. L.; Sun, S., A Facile Synthesis of MPd (M = Co, Cu) Nanoparticles and Their Catalysis for Formic Acid Oxidation. Nano Lett. 2012, 12, 1102-1106. 23



22. Zhang, Q.; Wu, X.-P.; Li, Y.; Chai, R.; Zhao, G.; Wang, C.; Gong, X.-Q.; Liu, Y.; Lu, Y., High-Performance PdNi Nanoalloy Catalyst in Situ Structured on Ni Foam for Catalytic Deoxygenation of Coalbed Methane: Experimental and DFT Studies. ACS Catal. 2016, 6, 6236-6245. 23. Chen, D.; Sun, P.; Liu, H.; Yang, J., Bimetallic Cu–Pd Alloy Multipods and Their Highly Electrocatalytic Performance for Formic Acid Oxidation and Oxygen Reduction. J. Mater. Chem. A 2017, 5, 4421-4429. 24. Ma, Y.; Setzer, A.; Gerlach, J. W.; Frost, F.; Esquinazi, P.; Mayr, S. G., Freestanding Single Crystalline Fe-Pd Ferromagnetic Shape Memory MembranesRole of Mechanical and Magnetic Constraints across the Martensite Transition. Adv. Funct. Mater. 2012, 22, 2529-2534. 25. Niu, W.; Gao, Y.; Zhang, W.; Yan, N.; Lu, X., Pd-Pb Alloy Nanocrystals with Tailored Composition for Semihydrogenation: Taking Advantage of Catalyst Poisoning. Angew. Chem. Int. Ed. Engl. 2015, 54, 8271-8274. 26. Zhang, K.; Bin, D.; Yang, B.; Wang, C.; Ren, F.; Du, Y., Ru-Assisted Synthesis of Pd/Ru Nanodendrites with High Activity for Ethanol Electrooxidation. Nanoscale 2015, 7, 12445-12451. 27. Zhang, L.; Zhang, J.; Kuang, Q.; Xie, S.; Jiang, Z.; Xie, Z.; Zheng, L., Cu2+-Assisted Synthesis of Hexoctahedral Au-Pd Alloy Nanocrystals with High-Index Facets. J. Am. Chem. Soc. 2011, 133, 17114-17117. 28. Liu, H.; Adzic, R. R.; Wong, S. S., Multifunctional Ultrathin PdxCu1-x and Pt Approximately PdxCu1-x One-Dimensional Nanowire Motifs for Various Small 24


Page 24 of 38

Page 25 of 38 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


Molecule Oxidation Reactions. ACS Appl. Mater. Inter. 2015, 7, 26145-26157. 29. Hu, C.; Zhai, X.; Zhao, Y.; Bian, K.; Zhang, J.; Qu, L.; Zhang, H.; Luo, H., Small-Sized PdCu Nanocapsules on 3d Graphene for High-Performance Ethanol Oxidation. Nanoscale 2014, 6, 2768-2775. 30. Li, J.; Li, F.; Guo, S. X.; Zhang, J.; Ma, J. T., PdCu@Pd Nanocube with Pt-Like Activity for Hydrogen Evolution Reaction. ACS Appl. Mater. Inter. 2017, 9, 8151-8160. 31. Shih, Z.-Y.; Wang, C.-W.; Xu, G.; Chang, H.-T., Porous Palladium Copper Nanoparticles for the Electrocatalytic Oxidation of Methanol in Direct Methanol Fuel Cells. J. Mater. Chem. A 2013, 1, 4773. 32. Zhao, X.; Dai, L.; Qin, Q.; Pei, F.; Hu, C.; Zheng, N., Self-Supported 3d PdCu Alloy Nanosheets as a Bifunctional Catalyst for Electrochemical Reforming of Ethanol. Small 2017, 13. 33. Xia, B. Y.; Wu, H. B.; Wang, X.; Lou, X. W., One-Pot Synthesis of Cubic PtCu3 Nanocages with Enhanced Electrocatalytic Activity for the Methanol Oxidation Reaction. J. Am. Chem. Soc. 2012, 134, 13934-13937. 34. Dhavale, V. M.; Kurungot, S., Cu–Pt Nanocage with 3-D Electrocatalytic Surface as an Efficient Oxygen Reduction Electrocatalyst for a Primary Zn–Air Battery. ACS Catal. 2015, 5, 1445-1452. 35. Yang, X.; Roling, L. T.; Vara, M.; Elnabawy, A. O.; Zhao, M.; Hood, Z. D.; Bao, S.; Mavrikakis, M.; Xia, Y., Synthesis and Characterization of Pt-Ag Alloy Nanocages with Enhanced Activity and Durability toward Oxygen Reduction. Nano Lett. 2016, 25



16, 6644-6649. 36. He, D. S.; He, D.; Wang, J.; Lin, Y.; Yin, P.; Hong, X.; Wu, Y.; Li, Y., Ultrathin Icosahedral Pt-Enriched Nanocage with Excellent Oxygen Reduction Reaction Activity. J. Am. Chem. Soc. 2016, 138, 1494-1497. 37. Lu, C.; Qi, L.; Yang, J.; Wang, X.; Zhang, D.; Xie, J.; Ma, J., One-Pot Synthesis of Octahedral Cu2O Nanocages Via a Catalytic Solution Route. Adv. Mater. 2005, 17, 2562-2567. 38. Hong, F.; Sun, S.; You, H.; Yang, S.; Fang, J.; Guo, S.; Yang, Z.; Ding, B.; Song, X., Cu2O Template Strategy for the Synthesis of Structure-Definable Noble Metal Alloy Mesocages. Cryst. Growth Des. 2011, 11, 3694-3697. 39. Tian, L. L.; Zhong, X. H.; Hu, W. P.; Liu, B. T.; Li, Y. F., Fabrication of Cubic PtCu Nanocages and Their Enhanced Electrocatalytic Activity Towards Hydrogen Peroxide. Nanoscale Res. Lett. 2014, 9. 40. Zhao, X.; Zhang, J.; Wang, L.; Liu, Z.; Chen, W., PdxCu100−x Networks: An Active and Durable Electrocatalyst for Ethanol Oxidation in Alkaline Medium. J. Mater. Chem. A 2014, 2, 20933-20938. 41. Castegnaro, M. V.; Gorgeski, A.; Balke, B.; Alves, M. C. M.; Morais, J., Charge Transfer Effects on the Chemical Reactivity of PdxCu1-x Nanoalloys. Nanoscale 2016, 8, 641-647. 42. Shi, W.; Chen, X.; Xu, S.; Cui, J.; Wang, L., Highly Efficient PdCu3 Nanocatalysts for Suzuki–Miyaura Reaction. Nano Res. 2016, 9, 2912-2920. 43. Du, X.; Luo, S.; Du, H.; Tang, M.; Huang, X.; Shen, P. K., Monodisperse and 26


Page 26 of 38

Page 27 of 38 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


Self-Assembled Pt-Cu Nanoparticles as an Efficient Electrocatalyst for the Methanol Oxidation Reaction. J. Mater. Chem. A 2016, 4, 1579-1585. 44. Yuwen, L.; Xu, F.; Xue, B.; Luo, Z.; Zhang, Q.; Bao, B.; Su, S.; Weng, L.; Huang, W.; Wang, L., General Synthesis of Noble Metal (Au, Ag, Pd, Pt) Nanocrystal Modified MoS2 Nanosheets and the Enhanced Catalytic Activity of Pd-MoS2 for Methanol Oxidation. Nanoscale 2014, 6, 5762-5769. 45. Zhao, Y.; Zhan, L.; Tian, J.; Nie, S.; Ning, Z., Enhanced Electrocatalytic Oxidation of Methanol on Pd/Polypyrrole–Graphene in Alkaline Medium. Electrochim. Acta 2011, 56, 1967-1972. 46. Arulmani, S.; Krishnamoorthy, S.; Wu, J. J.; Anandan, S., High-Performance Electrocatalytic Activity of Palladium-Copper Nanoalloy Towards Methanol Electro-Oxidation in an Alkaline Medium. Electroanal. 2017, 29, 433-440. 47. Singh, R. N.; Singh, A.; Anindita, Electrocatalytic Activity of Binary and Ternary Composite Films of Pd, Mwcnt and Ni, Part Ii: Methanol Electrooxidation in 1M KOH. Int. J. Hydrogen Energ. 2009, 34, 2052-2057. 48. Gong, M.; Fu, G.; Chen, Y.; Tang, Y.; Lu, T., Autocatalysis and Selective Oxidative Etching Induced Synthesis of Platinum-Copper Bimetallic Alloy Nanodendrites Electrocatalysts. ACS Appl. Mater. Inter. 2014, 6, 7301-7308. 49. Mahmoud, M. A.; Narayanan, R.; El-Sayed, M. A., Enhancing Colloidal Metallic Nanocatalysis: Sharp Edges and Corners for Solid Nanoparticles and Cage Effect for Hollow Ones. Accounts Chem. Res. 2013, 46, 1795-1805. 50. Hammer, B.; Norskov, J. K., Theoretical Surface Science and Catalysis27



Page 28 of 38

Calculations and Concepts. Adv. Catal. 2000, 45, 71. 51. Wakisaka, M.; Mitsui, S.; Hirose, Y.; Kawashima, K.; Uchida, H.; Watanabe, M., Electronic Structures of Pt-Co and Pt-Ru Alloys for CO-Tolerant Anode Catalysts in Polymer Electrolyte Fuel Cells Studied by EC-XPS. J. Phys. Chem. B 2006, 110, 23489-23496. 52. Huang, H. J.; Yang, S. B.; Vajtai, R.; Wang, X.; Ajayan, P. M., Pt-Decorated 3d Architectures Built from Graphene and Graphitic Carbon Nitride Nanosheets as Efficient Methanol Oxidation Catalysts. Adv. Mater. 2014, 26, 5160-5165. 53. Sneed, B. T.; Young, A. P.; Jalalpoor, D.; Golden, M. C.; Mao, S.; Jiang, Y.; Wang, Y.; Tsung, C.-K., Shaped Pd-Ni-Pt Core-Sandwich-Shell Nanoparticles: Influence of Ni Sandwich Layers on Catalytic Electrooxidations. Acs Nano 2014, 8, 7239-7250. 54. Xie, X.; Gao, G.; Kang, S.; Shibayama, T.; Lei, Y.; Gao, D.; Cai, L., Site-Selective

Trimetallic

Heterogeneous

Nanostructures

for

Enhanced

Electrocatalytic Performance. Adv. Mater. 2015, 27, 5573-5577. 55. Xie, J., et al., Ruthenium–Platinum Core–Shell Nanocatalysts with Substantially Enhanced Activity and Durability Towards Methanol Oxidation. Nano Energy 2016, 21, 247-257. 56. Zhang, Z., et al., One-Pot Synthesis of Highly Anisotropic Five-Fold-Twinned Ptcu Nanoframes Used as a Bifunctional Electrocatalyst for Oxygen Reduction and Methanol Oxidation. Adv. Mater. 2016, 28, 8712-8717. 57. Guo, S. J.; Dong, S. J.; Wang, E. K., Three-Dimensional Pt-on-Pd Bimetallic Nanodendrites Supported on Graphene Nanosheet: Facile Synthesis and Used as an 28


Page 29 of 38 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


Advanced Nanoelectrocatalyst for Methanol Oxidation. ACS nano 2010, 4, 547-555. 58. Xia, B. Y.; Wu, H. B.; Yan, Y.; Lou, X. W.; Wang, X., Ultrathin and Ultralong Single-Crystal Platinum Nanowire Assemblies with Highly Stable Electrocatalytic Activity. J. Am. Chem. Soc. 2013, 135, 9480-9485. 59. Koenigsmann, C.; Wong, S. S., One-Dimensional Noble Metal Electrocatalysts: A Promising Structural Paradigm for Direct Methanolfuelcells. Energy Environ. Sci. 2011, 4, 1161-1176.

29



Figure captions

Figure 1 SEM images (a-b) and TEM images (c-d) of corner-etched Cu2O octahedra. Figure 2 SEM images of Pd nanoparticles (a-b) and PdCu (c-d) alloy flower-like nanocages. Figure 3 TEM images (a-b) and corresponding mapping images (c-e) of PdCu alloy flower-like nanocages. Figure 4 XRD patterns Pd nanoparticles and PdCu alloy flower-like nanocages. Figure 5 XPS spectra of Pd 3d for Pd and PdCu alloy flower-like nanocages. Figure 6 CV curves of Pd, PdCu alloy flower-like nanocages and commercial Pd/C in 0.5 M KOH (a) and 0.5 M KOH with 1 M methanol solution (b) at a scan rate of 50 mV s-1; (c) Bar graph highlighting methanol oxidation reaction activity at respective MOR peak potential values for PdCu alloy flower-like nanocages, Pd and commercial Pd/C in 0.5 M KOH + 1 M methanol at a scan rate of 50 mV s-1; (d) Chronoamperometry curves of Pd, PdCu alloy flower-like nanocages and commercial Pd/C in 0.5 M KOH with 1 M methanol solution at -0.2 V vs. Ag/AgCl.

30


Page 30 of 38

Page 31 of 38 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


Figure 1

31



Figure 2

32


Page 32 of 38

Page 33 of 38 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


Figure 3

33



Figure 4

34


Page 34 of 38

Page 35 of 38 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


Figure 5

35



Figure 6

36


Page 36 of 38

Page 37 of 38 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


Table 1 Comparison of electrocatalytic activity of different electrocatalysts towards MOR.

catalysts

electrolyte

PdCu alloy flower-like

0.5 M KOH+1 M

nanocages

methanol

Pt-on-Pd nanodendrites

0.5 M H2SO4+1 M

Scan rate

Mass activity Reference

(mV s-1)

(A g-1)

50

823

This work

50

647.2

57

methanol Pt/G3-(CN)7

1 M H2SO4+2 M methanol

20

612.8

52

CeO2(30%)/nano-ZSM-5

0.5 M NaOH+0.5 M

50

73.5

6

100

433.5

44

50

363

31

methanol Pd-MoS2 NSs

0.5 M KOH+1 M methanol

Porous PdCu NP

0.5 M KOH+0.5 M methanol

G-AuPd@Pd

1 M KOH+1 M methanol

50

650

10

Pd/PPy-graphene

0.5 M NaOH+1 M

50

359.8

45

20

500

58

50

350

48

methanol Pt nanowire

0.5 M H2SO4+0.5 M methanol

Pt-Cu BANDs

0.5 M H2SO4+1 M methanol

37



TOC Graphic

PdCu alloy flower-like nanocages are developed as efficient electrocatalysts for methanol oxidation reaction with high electrocatalytic activity and high stability.

38


Page 38 of 38

PdCu Alloy Flower-like Nanocages With High Electrocatalytic

Recommend Documents