PtM (M = Fe, Co, Ni) Bimetallic Nanoclusters as Active, Methanol

Subscriber access provided by Iowa State University | Library

Article

PtM (M = Fe, Co, Ni) bimetallic nanoclusters as active, methanol tolerant, and stable catalysts toward oxygen reduction reaction Jing Liu, Jinze Lan, Lingyan Yang, Fu Wang, and Jiao Yin ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04929 • Publication Date (Web): 11 Jan 2019 Downloaded from http://pubs.acs.org on January 11, 2019

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 20 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

ACS Sustainable Chemistry & Engineering

PtM (M = Fe, Co, Ni) bimetallic nanoclusters as active, methanol tolerant, and stable catalysts toward oxygen reduction reaction Jing Liua,b, Jinze Lana,b, Lingyan Yanga,b, Fu Wanga,b*, Jiao Yinb* a

School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai

200240, China b Laboratory

of Environmental Sciences and Technology, Xinjiang Technical Institute of Physics &

Chemistry, and Key Laboratory of Functional Materials and Devices for Special Environments, Chinese Academy of Sciences, Urumqi 830011, China E-mail addresses for corresponding author: [email protected] (F. Wang); [email protected] (J. Yin)

ABSTRACT Tailoring PtM (M=Fe, Co, and Ni) bimetallic electrocatalysts into nanoclusters (NCs) without any protective agents with diameters of about 2-5 nm is considered as an effective strategy to improve electrochemical performance, reduce the mass loading of precious Pt and enhance methanol tolerance in oxygen reduction reaction (ORR). However, how to synthesize bimetallic NCs with relatively controllable size and how to anchor and disperse PtM (M=Fe, Co, and Ni) bimetallic NCs onto suitable matrix are key issues to guarantee durable catalytic performance and stability of bimetallic NCs because NCs possess high surface energy and they are easy to aggregate. Hence, in this paper, we demonstrated a low temperature impregnation-reduction method to fabricate PtM (M=Fe, Co, Ni) bimetallic NCs without any protective agents immobilized on XC-72 carbon with a 10 wt. % PtM loading, which exhibited more satisfactory ORR performance. In particular, PtNi/C presented the best ORR catalytic activity among the three catalysts and commercial Pt/C (20 wt. % Pt loading) due to the synergistic effects of unique electronic structure, smaller particle size and stable adhesion with substrate. Electrochemical characterization indicated that a maximal catalytic activity was achieved at Pt:Ni atomic ratio 0.8:0.2 and the mass activity (MA) was about 2.74 times greater than that of Pt/C. Furthermore, the Pt0.8Ni0.2/C catalyst possessed notable methanol tolerance and stability, which is vital for practical applications. As a result, this facile and effective synthesis strategy opens up new horizons to promote DMFC into practical applications. Keywords: Pt, alloy, nanoclusters, oxygen reduction reaction, stability 1

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 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 Platinum (Pt) is generally considered as the most desirable and promising electrocatalyst to boost oxygen reduction reaction (ORR) 1-5. However, the high costs of pure Pt catalyst, the sluggish kinetics of cathode reactions, the poor stability and methanol tolerance retard its large-scale practical applications 6-8. Therefore, exploring and designing Pt-based catalysts with lower mass loading for ORR with enhanced activity, stability and methanol tolerance is urgently needed to push direct methanol fuel cells (DMFCs) into commercialization. Among various recently proposed strategies, developing Pt-based bimetallic transition 3d metal M (M = Fe, Co, and Ni) nanoclusters (NCs) without any protective agents with diameters of 2-5 nm manifests great potentials in practical applications in that it can dramatically lower the Pt consumption and offer a feasible strategy to design versatile nanostructures with relatively controllable properties 9-11. Furthermore, PtM (M=Fe, Co, and Ni) bimetallic NCs demonstrate more satisfactory performance compared with their single counterparts in specific physical-chemical properties including electronical, optical sensing, plasmatic, and catalytic properties because of the synergistic effects between the two metals

12-15.

In particular, tailoring PtM (M=Fe, Co, and Ni) bimetallic

electrocatalysts into NCs endows them with a high fraction of low-coordinated surface atoms and that O2 molecules can be adsorbed and activated more easily on the surface Pt atoms. Hence, the utilization of NCs can both enhance the ORR performance and reduce the materials cost 16-17. More importantly, the introduction of M (M=Fe, Co, and Ni) atoms can hinder the adsorption of methanol, and thus lead to an enhanced methanol tolerance 18-19. As mentioned above, even if PtM (M=Fe, Co, and Ni) bimetallic NCs possess various merits, how to synthesize bimetallic NCs with relatively controllable size via facile methods and how to anchor and disperse PtM (M=Fe, Co, and Ni) bimetallic NCs onto suitable matrix are key issues to ensure durable catalytic performance and stability of bimetallic NCs without any protective agents because NCs possess high surface energy and they are easy to aggregate 20-21. Among a wide variety of recently proposed synthesis strategies, the wet impregnation-reduction method is an effective strategy toward the synthesis of alloy particles 22-23. However, it has been proved that the general wet impregnation-reduction methods display many drawbacks such as rapid reaction rate, uncontrollable nucleation and growth processes, which results in bigger and 2


Page 2 of 20

Page 3 of 20 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


inhomogeneous particle size (aggregation), lower active specific surface area, and unsatisfactory catalytic performance. The substrate to load and disperse PtM (M=Fe, Co, and Ni) bimetallic NCs also makes great contributions to improving the ORR performance and stability 24. Among various the available supporting materials, XC-72 carbon has been considered as one of the most promising supports because of low cost, high specific surface area, the relatively strong tolerance to the acidic electrolyte and the high electronic conductivity, which are in good favour of rapid mass transport of reactants and the efficient mobility of electrons

25-28,

and thereby give rise to an enhancement in ORR performance.

Moreover, the interactions between PtM NCs and carbon can also partially restrain the dissolution and aggregation of PtM NCs in the oxygen reduction process 29. Herein, to relatively control the wet impregnation-reduction rate and process, tailor the particle size of PtM (M= Fe, Co, Ni) bimetallic NCs within 2-5 nm, and realize uniform distributions and stable adhesion on matrix, Vulcan XC-72 carbon supported PtM (M= Fe, Co, Ni) bimetallic NCs are fabricated via a low temperature impregnationreduction method. The as-prepared PtM NCs without any stabilizing agent present more desirable ORR performance benefiting from synergistic effects of unique electronic structure, smaller particle size, and good cohesion with substrate. The PtNi/C catalyst performs better ORR catalytic activity among the three catalysts. To further improve the ORR performance, different Pt:Ni atomic ratios are studied. Electrochemical characterization displays a maximal catalytic activity at Pt:Ni atomic ratio 0.8:0.2 and the mass activity (MA) was about 2.74 times greater than that of Pt/C. Meanwhile, the accelerated stability tests (ADTs) indicate that the Pt0.8Ni0.2 NCs also demonstrate remarkable enhancement in stability. The methanol oxidation in 0.1 M N2 saturated HClO4 indicates a great tolerance degree to methanol of Pt0.8Ni0.2 NCs. RESULTS AND DISCUSSION Characterization of PtM NCs (M = Fe, Co, Ni) Nanostructures

Scheme 1. Illustration of the formation of PtM NCs (M = Fe, Co, Ni) on XC-72 carbon. 3



Page 4 of 20

The synthesis of PtM NCs without any protective agents anchored on XC-72 carbon support is based on a low temperature impregnation-reduction method as presented in Scheme 1. Firstly, it is the adsorption of Pt and M precursors on XC-72 carbon substrate, which is beneficial for improving the distribution of PtM NCs on the matrix surface, inhibiting the agglomeration of PtM NCs and reducing the grain size by creating separate catalytic islands. The following step is the nucleation and growth of PtM bimetallic NCs. The metal precursors are reduced by the fresh and iced NaBH4 in an ice-water bath. The low temperature can both prevent the decomposition of NaBH4 solutions and help to obtain uniform PtM NCs with small particle size as confirmed by the TEM images of PtNi NCs when compared with the corresponding samples prepared at higher temperature (Figure S1). The composition of the bimetallic PtM NCs alloys is determined by the inductively coupled plasma-optical emission spectrometry (ICPOES) analysis as displayed in Table 1. The obtained data agree well with the original stoichiometric values, suggesting that Pt and M precursors are totally reduced and converted into bimetallic PtM NCs. Table 1. Compositions of PtM/C (M = Fe, Co, Ni). Sample

Pt / wt.%

M / wt.%

Pt:M in mole ratio

Pt0.7Fe0.3/C

8.11

1.02

0.63/0.27

Pt0.7Co0.3/C

7.98

1.04

0.62/0.26

Pt0.7Ni0.3/C

8.01

1.06

0.62/0.27

Crystal structure of PtM/C are investigated by X-ray diffraction (XRD) analysis. As displayed in Figure 1, all PtM catalysts show higher 2θ angle shift of diffraction peaks compared with the standard Pt (JCPDS: 04-0802), indicating a contracted Pt lattice parameter 30. The magnitude of lattice constant for PtM NCs clearly indicates the formation of alloy because of the smaller lattice constant than pure Pt (0.3938 nm). The percentage of compressive strain and lattice parameter calculated from the XRD in PtM NCs are given in Table 2. Among PtM NCs/C (M = Fe, Co, Ni), Pt0.7Ni0.3 NCs gives the highest compressive strain (0.74 %), which can play a crucial role for improved ORR performance 31-32. Table 2. Structural parameters of PtM/C (M = Fe, Co, Ni). Catalyst

2θ shift / degree

d(111) spacing

Lattice constant

Strain / %

Pt0.7Fe0.3

0.29

0.2209

0.3912

0.6529

4


Page 5 of 20 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


Pt0.7Co0.3

0.36

0.2208

0.3905

0.6982

Pt0.7Ni0.3

0.38

0.2212

0.3898

0.7412

Figure 1. XRD patterns of PtM/C (M = Fe, Co, Ni) and standard patterns of bulk Pt.

Figure 2 illustrates the morphology and structure of the catalysts. As obviously observed, PtM NCs are homogeneous distributed on the Vulcan XC-72 carbon with the mean particle diameters of 2.97, 3.03 and 3.08 nm, respectively. The high resolution transmission electron microscope (HRTEM) image of Pt0.7Ni0.3/C reveals the crystallinity of the Pt0.7Ni0.3 NCs alloy is well developed with lattice spacing distance of 0.217 nm (Figure 3a), which is assigned to the (111) planes of the face-centered cubic PtNi, consistent with the XRD result. To get insight into the distribution of Pt0.7Ni0.3/C, elemental analysis is carried out with high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Figure 3c displays the HAADF-STEM image of the Pt0.7Ni0.3/C. The elemental mapping of C, Pt, Ni shown in Figures 3d and 3f revealed the uniform distribution of Pt and Ni on Vulcan XC-72 carbon. The corresponding energy dispersive X-ray (EDX) data further indicate the presence of Pt and Ni in Pt0.7Ni0.3/C (Figure 3b). From the EDX results, the as-prepared Pt0.7Ni0.3/C is composed of 7.99 wt. % Pt and 1.02 wt. % Ni. The data agree well with the ICP-OES result.

5



Figure 2. TEM image and particle size distribution of Pt0.7Fe0.3/C (a) and (b), Pt0.7Co0.3/C (c) and (d), and Pt0.7Ni0.3/C (e) and (f).

Figure 3. (a) HRTEM image, (b) EDX spectrum of Pt0.7Ni0.3/C, and (c) HAADF-STEM image and corresponding mapping of (d) C, (e) Pt, and (f) Ni of Pt0.7Ni0.3/C.

ORR Performance of PtM/C (M = Fe, Co, Ni) Nanostructures Figure 4a depicts the cyclic voltammograms (CVs) of PtM/C and Pt/C measured in O2 or N2-saturated 0.1 M HClO4 aqueous solutions. Both PtM/C and Pt/C demonstrate a remarkable cathodic peak in the presence of O2, indicating a substantial ORR process. As summarized in Table 3, the ECSA of Pt obtained from the hydrogen adsorption in N2-saturated HClO4 in PtM/C is higher than Pt/C, which could be ascribed to the small size of PtM NCs and homogeneous distributions on the XC-72 carbon substrate as confirmed by TEM images. The linear sweep voltammetry (LSV) curves of the PtM/C exhibit more positive half-wave potential (E1/2) and incremental limiting current density (JL) in comparison with that of Pt/C (Figure 4b). Especially, the Pt0.7Ni0.3/C possesses the most positive E1/2 and largest JL compared with the value of Pt0.7Fe0.3/C, Pt0.7Co0.3/C and Pt/C. These results manifest the high ORR performance of 6


Page 6 of 20

Page 7 of 20 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 Pt0.7Ni0.3/C catalyst. The efficient ORR process on Pt0.7Ni0.3/C is further supported by a smaller tafel slope compared to those of Pt0.7Fe0.3/C, Pt0.7Co0.3/C, and Pt/C (Figure 4c). To further investigate the ORR kinetics of Pt0.7Ni0.3/C, rotating disk electrodes (RDE) measurements are conducted at various rotating speeds (Figure 4d). According to the Koutecky-Levich (K-L) equations, the value of electron transfer number (n) is calculated to be 4.00 (0.3 V), 4.00 (0.4 V), 3.99 (0.5 V), and 4.01 (0.6 V), suggesting a 4e- ORR pathway, comparable with commercial Pt/C (Figure S2). The MA and specific activity (SA) of Pt0.7Ni0.3/C at 0.9 V is 283.73 mA mg-1 and 0.369 mA cm-2, respectively, which are larger than those of Pt0.7Fe0.3/C, Pt0.7Co0.3/C, and Pt/C as shown in Table 3. It should be noted that, unlike the 20 wt. % Pt loading of commercial Pt/C, Pt0.7Ni0.3/C contains only ~8 wt. % Pt and ~1 wt. % Ni. Therefore, in the consideration of the production cost and activity, Pt0.7Ni0.3/C is superior to Pt/C.

Figure 4. (a) CV curves of PtM/C in O2-saturated (solid) and N2-saturated (dash) 0.1 M HClO4. (b) ORR polarization curves of PtM/C in 0.1 M O2-saturated HClO4 with the scan rate 10 mV s-1 and the electrode rotation speed 1600 rpm. (c) Tafel plot of polarization curves of different catalysts. (d) MA and SA of PtM/C and commercial Pt/C at 0.9 V. (e) RDE curves and (f) corresponding K-L plots of Pt0.7Ni0.3/C. 7



Page 8 of 20

Table 3. Electrochemical data for PtM/C and commercial Pt/C (GCD: glassy carbon disk).

GCD /μg

ECSA/ m2 g-1

E1/2/V

Tafel/decade-1

Pt/C

21.32

77.01

0.804

Pt0.7Fe0.3/C

9.22

84.76

Pt0.7Co0.3/C

9.23

Pt0.7Ni0.3/C

9.19

Sample

Pt on

JL/mA

SA/mA

MA/mA

cm-2

cm-2

mg-1

132.61

4.89

0.196

142.32

0.805

104.47

4.99

0.251

171.68

86.24

0.809

88.80

5.23

0.258

178.31

93.37

0.811

69.52

5.39

0.369

283.73

Density functional theory (DFT) calculations are conducted to clarify the origin of the improved ORR activity of PtM NCs. As shown in Figure 5, after alloying with M, the number of the d-band peaks reduces and the d-band width of PtM NCs is broader, which indicate a stronger delocalizability of d electrons. The d-band center for Pt is determined to be -2.20, -2.28, -2.43, and -1.93 eV for Pt0.7Fe0.3 NCs, Pt0.7Co0.3 NCs, Pt0.7Ni0.3 NCs, and pure Pt, respectively. The downward shift of d-band center of Pt can result in a weaker adsorption (such as HOO*, HO*, and O*) on PtM surface than on pure Pt, and thus favors the reaction activities 33-34. The largest shift of the d-band center of Pt0.7Ni0.3/C can weak the adsorption of the intermediate products to the maximum among three catalysts. Hence, Pt0.7Ni0.3/C exhibits the best ORR performance.

Figure 5. PDOS plots for the d-states of Pt in Pt/C and PtM/C, respectively.

Characterization and ORR Performance of PtNi NCs with Different Pt/Ni Ratios As has been reported, composition of electrocatalysts can greatly affect the ORR performance13, 35. Therefore, the mole ratio of Pt/Ni was further optimized by changing 8


Page 9 of 20 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 ratio of metal precursors to further obtain the PtNi NCs with better ORR performance. The Pt/Ni ratios are set to be 0.8:0.2 and 0.9:0.1 in the precursors. Pt0.78Ni0.19 and Pt0.86Ni0.083 are obtained through the ICP-OES analysis. The morphology and structural profiles of the as-prepared bimetallic NCs are characterized by XRD (Figure S3) and TEM (Figure S4), which confirm the formation of PtNi alloy nanostructures and the uniform distribution of the small NCs on the XC-72 carbon. Next, the electrocatalytic ORR activity is investigated. Compared with that of Pt0.7Ni0.3/C (0.79 V) and Pt0.9Ni0.1/C (0.80 V), Pt0.8Ni0.2/C presents the most positive reduction potential at 0.81 V as displayed in Figure 6a. Figure 6b illustrates the LSV curves in O2-saturated 0.1 M HClO4 aqueous solution. As the data collected in Table 4, it can be clearly found that Pt0.8Ni0.2/C displayes the most positive E1/2 and largest JL, demonstrating an improvement in the ORR performance. The MA and SA at 0.9 V over Pt0.8Ni0.2/C are 390.1 mA mg-1 and 0.466 mA cm-2, respectively. Obviously, the ORR activity of Pt0.8Ni0.2/C is superior to both Pt/C and most of the reported PtNi bimetallic electrocatalysts as summarized in Table 5. The reason for this phenomenon can be ascribed to the electronic coupling between Pt and Ni elements, which may be optimized when the atomic ratio of Pt/Ni is 0.8:0.2

36.

The RDE measurements are

further conducted to illuminate the ORR pathway (Figure 6d and Figure S5). Calculation from the K-L equations demonstrates the complete 4e- pathway for ORR over Pt0.8Ni0.2/C catalyst, which is in favour of the improvement of the fuel cells efficiency 37. For comparison, Pt0.8Ni0.2/C with 30 wt % and 50 wt % metal loadings are also prepared, characterized, and tested for ORR as shown in Figures S6-S9 and Table S1. Table 4. ORR data of PtNi/C with different Pt/Ni mole ratio. Pt on GCD

ECSA

E1/2

JL

SA

MA

/μg

/m2 g-1

/V

/mA cm-2

/mA cm-2

/mA mg-1

Pt0.7Ni0.3/C

9.19

93.37

0.811

5.39

0.369

283.73

Pt0.8Ni0.2/C

9.27

102.33

0.818

5.73

0.466

390.1

Pt0.9Ni0.1/C

9.32

99.22

0.814

5.54

0.431

365.56

Catalyst

9



Page 10 of 20

Figure 6. (a) CV curves of PtNi/C in 0.1 M O2-saturated (solid) and N2-saturated (dash) HClO4 solution. (b) LSV curves of PtNi/C at 1600 rpm in O2-saturated 0.1 M HClO4 with a scan rate 10 mV s-1. (c) MA and SA of PtNi/C with different Pt/Ni mole ratio. (d) LSVs at different rotating speeds of Pt0.8Ni0.2/C (inset: corresponding K-L plots). Table 5. Comparison of the ORR activity of reported PtNi and the investigated Pt0.8Ni0.2/C in acidic electrolyte. ORR Performance

PtNi

Pt:Ni

loading

atom ratio

Pt0.8Ni0.2/C

10 wt.%

0.8:0.2

390.1

0.466

This work

O-PtNi/C

20 wt.%

1:1

102.2

0.381

38

PtNi/C

20 wt.%

78:22

70

0.13

39

PtNi HNCs

100 wt.%

77:23

340

0.48

40

PtNi/MWCNTs

80 wt.%

4:3

510

1.065

41

porous PtNi

100 wt.%

74.5:25.5

757

1.006

12

PtNi Nanoctahedra

100 wt.%

3:1

120

0.28

6

PtNi/C

20 wt. %

2:1

92

0.148

42

PtNi/Vulcan XC

62.7 wt. %

1:3

290

1.49

43

PtNi NPNWs

100 wt.%

10:1

333

0.99

26

Electroatalysts

MA / mA mg1

SA / mA cm-2

References

Methanol tolerance of Pt0.8Ni0.2/C The tolerance to methanol is also significantly important for ORR catalysts. Hence, the methanol tolerance was assessed by instantaneously adding 0.5 M methanol during 10


Page 11 of 20 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 chronoamperometric test (Figure 7a). After the injection of methanol, 7.3 % loss of the oxygen reduction current is observed over Pt0.8Ni0.2/C. In contrast, the ORR current of Pt/C significantly decreases by 26.9%, implying a higher tolerance to methanol of Pt0.8Ni0.2/C. To better understand the mechanism of the great methanol tolerance of Pt0.8Ni0.2/C, the methanol oxidation reaction (MOR) was studied in 0.1 M N2-saturated HClO4 containing 0.5 M methanol. Figure 7b shows the LSV curves of MOR over Pt0.8Ni0.2/C and Pt/C. The MOR current density over commercial Pt/C are 5-fold larger than that over Pt0.8Ni0.2/C, suggesting that the MOR over Pt0.8Ni0.2/C is less active than that over Pt/C. As reported in literatures, for methanol oxidation to occur, at least three adjacent Pt sites are needed 10, whereas two are required for the oxygen reduction 44. As for the Pt0.8Ni0.2/C investigated here, the presence of Ni dilutes the Pt sites on the surface, which affects methanol oxidation due to the more demanding site requirement. In addition, the activity for methanol oxidation decreases with a reduction in Pt particle size

45-47.

Hence, the small particle size of PtNi NCs is beneficial to a great methanol

tolerance during the ORR. All these together explain the fact that the Pt0.8Ni0.2/C shows a low reactivity for MOR and thus lead to a great tolerance to methanol.

Figure 7. (a) Chronoamperometric responses of Pt0.8Ni0.2/C and Pt/C with the instantaneously addition of 0.5 M methanol at 0.7 V. (b) LSVs of MOR over Pt0.8Ni0.2/C and Pt/C in N2-saturated 0.1 M HClO4 and 0.5 M CH3OH.

Electrochemical stability of Pt0.8Ni0.2/C ADTs were conducted to investigate the stability of Pt0.8Ni0.2/C. After 5000 cycles, the CVs show 9.6% and 27.0% loss in ECSA for Pt0.8Ni0.2/C and Pt/C, respectively. Meanwhile, the LSV curves demonstrate that the E1/2 of Pt0.8Ni0.2/C and Pt/C is shifted by ~11 mV and ~23 mV to the negative potential after the ADTs. The MA and SA of Pt0.8Ni0.2/C decrease by 10.4 % and 9.1 %, respectively, and those of Pt/C are 28.1 % 11



and 25.4 %. Besides, chronoamperometric measurement was also performed to assess the stability of Pt0.8Ni0.2/C. As shown in Figure 8d, after 23000 s, ~8.2 % current loss is observed on Pt0.8Ni0.2/C. However, the values for commercial Pt/C is ~72.9 %, demonstrating a better stability of Pt0.8Ni0.2/C. Therefore, we can conclude that Pt0.8Ni0.2/C shows excellent stability.

Figure 8. (a) CVs and (b) LSV curves of Pt0.8Ni0.2/C and Pt/C and (c) SA and MA before and after ADTs at 0.9 V. (d) Current-time profiles of Pt0.8Ni0.2/C and Pt/C at 0.7 V.

CONCLUSIONS In conclusion, we have demonstrated a low temperature impregnation-reduction method without any protective agents to successfully prepare the Pt0.7Fe0.3, Pt0.7Co0.3, Pt0.7Ni0.3, Pt0.8Ni0.2, and Pt0.9Ni0.1 bimetallic NCs (~3 nm) with well-defined shape, size, and composition. The physiochemical characterization verifies the homogeneous distribution of PtM NCs anchored on Vulcan XC-72 carbon. The resultant hybrids exhibit excellent ORR electrocatalytic activity in acidic media. Remarkably, due to ultra-small size of PtM NCs with more active catalytic sites, the unique electronic structure with the weaker adsorption between intermediates, and good cohesion with substrate, the optimized Pt0.8Ni0.2/C catalyst exhibits the MA 2.74 times larger than that of commercial Pt/C. Meanwhile, it performed excellent stability and methanol tolerance. Thus, the supported Pt0.8Ni0.2 NCs so fabricated can be employed as a promising efficient ORR catalyst for the practical applications of fuel cells. 12


Page 12 of 20

Page 13 of 20 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


ASSOCIATED CONTENT Supporting Information LSVs, K-L plots, XRD patterns, TEM images, particle size distributions, electrochemical data of Pt0.8Ni0.2/C-30% and Pt0.8Ni0.2/C-50%. ACKNOWLEDGMENTS We acknowledge the financial support by National Natural Science Foundation of China (Grant No. 21503271, 51663016), National Key Research and Development Program of China, (Grant No. 2017YFC0110202), and Foundation of Director of Xinjiang Technical Institute of Physics & Chemistry, Chinese Academy of Sciences, (Grant No. 2016PY005). REFERENCES 1.

Shao, M.; Chang, Q.; Dodelet, J. P.; Chenitz, R. Recent Advances in

Electrocatalysts for Oxygen Reduction Reaction. Chem. Rev. 2016, 116, 3594-3657, DOI: 10.1021/acs.chemrev.5b00462. 2.

Imaoka, T.; Kitazawa, H.; Chun, W. J.; Yamamoto, K. Finding the Most

Catalytically Active Platinum Clusters With Low Atomicity. Angew. Chem. Int. Ed. 2015, 54, 9810-9815, DOI: 10.1002/ange.201504473. 3.

Imaoka, T.; Kitazawa, H.; Chun, W. J.; Omura, S.; Albrecht, K.; Yamamoto, K.

Magic number Pt13 and misshapen Pt12 clusters: which one is the better catalyst? J. Am. Chem. Soc. 2013, 135, 13089-13095, DOI: 10.1021/ja405922m. 4.

Wang, H.; Yin, S.; Eid, K.; Li, Y.; Xu, Y.; Li, X.; Xue, H.; Wang, L. Fabrication

of Mesoporous Cage-Bell Pt Nanoarchitectonics as Efficient Catalyst for Oxygen Reduction Reaction. ACS Sustain. Chem. Eng. 2018, 6, 11768-11774, DOI: 10.1021/acssuschemeng.8b02015. 5.

Hoque, M. A.; Hassan, F. M.; Jauhar, A. M.; Jiang, G.; Pritzker, M.; Choi, J.-Y.;

Knights, S.; Ye, S.; Chen, Z. Web-like 3D Architecture of Pt Nanowires and SulfurDoped Carbon Nanotube with Superior Electrocatalytic Performance. ACS Sustain. Chem. Eng. 2017, 6, 93-98, DOI: 10.1021/acssuschemeng.7b03580. 13



6.

Page 14 of 20

Zhang, J.; Yang, H.; Fang, J.; Zou, S. Synthesis and oxygen reduction activity of

shape-controlled Pt3Ni nanopolyhedra. Nano Lett. 2010, 10, 638-644, DOI: 10.1021/nl903717z. 7.

Joo, S. H.; Lee, H. I.; You, D. J.; Kwon, K.; Kim, J. H.; Choi, Y. S.; Kang, M.;

Kim, J. M.; Pak, C.; Chang, H.; Seung, D. Ordered mesoporous carbons with controlled particle sizes as catalyst supports for direct methanol fuel cell cathodes. Carbon 2008, 46, 2034-2045, DOI: 10.1016/j.carbon.2008.08.015. 8.

Jeong, S. M.; Kim, M. K.; Kim, G. P.; Kim, T. Y.; Baeck, S. H. Preparation of Pt-

Au/carbon catalysts by a reduction method and their electrocatalytic activities for oxygen reduction reactions. Chem. Eng. J. 2012, 198-199, 435-439, DOI: 10.1016/j.cej.2012.04.002. 9.

Liu, J.; Li, Y.; Wu, Z.; Ruan, M.; Song, P.; Jiang, L.; Wang, Y.; Sun, G.; Xu, W.

Pt0.61Ni/C for High-Efficiency Cathode of Fuel Cells with Superhigh Platinum Utilization. J. Phys. Chem. C 2018, 122, 14691-14697, DOI: 10.1021/acs.jpcc.8b03966. 10. Zignani, S. C.; Baglio, V.; Sebastián, D.; Rocha, T. A.; Gonzalez, E. R.; Aricò, A. S. Investigation of PtNi/C as methanol tolerant electrocatalyst for the oxygen reduction reaction.

J.

Electroanal.

Chem.

2016,

763,

10-17,

DOI:

10.1016/j.jelechem.2015.12.044. 11. Suh, W.-K.; Ganesan, P.; Son, B.; Kim, H.; Shanmugam, S. Graphene supported Pt-Ni nanoparticles for oxygen reduction reaction in acidic electrolyte. Int. J. Hydrogen Energ. 2016, 41, 12983-12994, DOI: 10.1016/j.ijhydene.2016.04.090. 12. Huang, X.; Zhu, E.; Chen, Y.; Li, Y.; Chiu, C. Y.; Xu, Y.; Lin, Z.; Duan, X.; Huang, Y. A facile strategy to Pt3Ni nanocrystals with highly porous features as an enhanced oxygen reduction reaction catalyst. Adv. Mater. 2013, 25 (21), 2974-2979, DOI: 10.1002/adma.201205315. 13. Chung, Y. H.; Chung, D. Y.; Jung, N.; Park, H. Y.; Sung, Y. E.; Yoo, S. J. Effect of surface composition of Pt-Fe nanoparticles for oxygen reduction reactions. Int. J. Hydrogen Energ. 2014, 39, 14751-14759, DOI: 10.1016/j.ijhydene.2014.07.097. 14. Ou, L. The origin of enhanced electrocatalytic activity of Pt-M (M=Fe, Co, Ni, Cu, and W) alloys in PEM fuel cell cathodes: A DFT computational study. Comput. Theor. 14


Page 15 of 20 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


Chem. 2014, 1048, 69-76, DOI: 10.1016/j.comptc.2014.09.017. 15. Wang, Q.; Tian, Y.; Chen, G.; Zhao, J. Theoretical insights into the energetics and electronic properties of MPt12 (M = Fe, Co, Ni, Cu, and Pd) nanoparticles supported by N-doped defective graphene. Appl. Surf. Sci. 2017, 397, 199-205, DOI: 10.1016/j.apsusc.2016.11.048. 16. Narayanamoorthy, B.; Linkov, V.; Sita, C.; Pasupathi, S. Pt3M (M: Co, Ni and Fe) Bimetallic Alloy Nanoclusters as Support-Free Electrocatalysts with Improved Activity and Durability for Dioxygen Reduction in PEM Fuel Cells. Electrocatalysis 2016, 7, 400-410, DOI: 10.1007/s12678-016-0318-x. 17. Greeley, J.; Rossmeisl, J.; Hellmann, A.; Norskov, J. K. Theoretical Trends in Particle Size Effects for the Oxygen Reduction Reaction. Z. Phys. Chem. 2007, 221, 1209-1220, DOI: 10.1524/zpch.2007.221.9-10.1209. 18. Yang, H.; Coutanceau, C.; Léger, J. M.; Alonso-Vante, N.; Claude L. Methanol tolerant oxygen reduction on carbon-supported Pt-Ni alloy nanoparticles. J. Electroanal. Chem. 2005, 576, 305-313, DOI: 10.1016/j.jelechem.2004.10.026. 19. Liu, S. H.; Zheng, F. S.; Wu, J. R. Preparation of ordered mesoporous carbons containing well-dispersed and highly alloying Pt-Co bimetallic nanoparticles toward methanol-resistant oxygen reduction reaction. Appl. Catal. B: Environ. 2011, 108-109, 81-89, DOI: 10.1016/j.apcatb.2011.08.011. 20. Lu, Y.; Chen, W. Sub-nanometre sized metal clusters: from synthetic challenges to the unique property discoveries. Chem. Soc. Rev. 2012, 41, 3594-3623, DOI: 10.1039/C2CS15325D. 21. Yin, H.; Tang, H.; Wang, D.; Gao, Y.; Tang, Z. Facile Synthesis of Surfactant-Free Au Cluster/Graphene Hybrids for High Performance Oxygen Reduction Reaction. ACS Nano 2012, 6, 8288-8297, DOI: 10.1021/nn302984x. 22. Wang, X. X.; Hwang, S.; Pan, Y. T.; Chen, K.; He, Y.; Karakalos, S.; Zhang, H.; Spendelow, J. S.; Su, D.; Wu, G. Ordered Pt3Co Intermetallic Nanoparticles Derived from Metal-Organic Frameworks for Oxygen Reduction. Nano Lett. 2018, 18, 41634171, DOI: 10.1021/acs.nanolett.8b00978. 23. Quinson, J.; Inaba, M.; Neumann, S.; Swane, A. A.; Bucher, J.; Simonsen, S. B.; 15



Page 16 of 20

Theil, K. L.; Kirkensgaard, J. J. K.; Jensen, K. M. Ø.; Oezaslan, M.; Kunz, S.; Arenz, M. Investigating Particle Size Effects in Catalysis by Applying a Size-Controlled and Surfactant-Free Synthesis of Colloidal Nanoparticles in Alkaline Ethylene Glycol: Case Study of the Oxygen Reduction Reaction on Pt. ACS Catal. 2018, 8, 6627-6635, DOI: 10.1021/acscatal.8b00694. 24. Chauhan, S.; Mori, T.; Masuda, T.; Ueda, S.; Richards, G. J.; Hill, J. P.; Ariga, K.; Isaka, N.; Auchterlonie, G.; Drennan, J. Design of Low Pt Concentration Electrocatalyst Surfaces with High Oxygen Reduction Reaction Activity Promoted by Formation of a Heterogeneous Interface between Pt and CeOx Nanowire. ACS Appl. Mater. Inter. 2016, 8, 9059-9070, DOI: 10.1021/acsami.5b12469. 25. Choi, J.; Jang, J. H.; Roh, C. W.; Yang, S.; Kim, J.; Lim, J.; Yoo, S. J.; Lee, H. Gram-scale synthesis of highly active and durable octahedral PtNi nanoparticle catalysts for proton exchange membrane fuel cell. Appl. Catal. B: Environ. 2018, 225, 530-537, DOI: 10.1016/j.apcatb.2017.12.016. 26. Wang, Y.; Yin, K.; Lv, L.; Kou, T.; Zhang, C.; Zhang, J.; Gao, H.; Zhang, Z. Eutectic-directed self-templating synthesis of PtNi nanoporous nanowires with superior electrocatalytic performance towards the oxygen reduction reaction: experiment and DFT

calculation.

J.

Mater.

Chem.

A

2017,

5,

23651-23661,

DOI:

10.1039/C7TA06247H. 27. Ma, Y.; Gao, W.; Shan, H.; Chen, W.; Shang, W.; Tao, P.; Song, C.; Addiego, C.; Deng, T.; Pan, X.; Wu, J. Platinum-Based Nanowires as Active Catalysts toward Oxygen Reduction Reaction: In Situ Observation of Surface-Diffusion-Assisted, SolidState Oriented Attachment. Adv. Mater. 2017, 29, 1703460-1703467, DOI: 10.1002/adma.201703460. 28. Dubau, L.; Nelayah, J.; Asset, T.; Chattot, R.; Maillard, F. Implementing Structural Disorder as a Promising Direction for Improving the Stability of PtNi/C Nanoparticles. ACS Catal. 2017, 7, 3072-3081, DOI: 10.1021/acscatal.7b00410. 29. Lim, D. H.; Wilcox, J. Mechanisms of the Oxygen Reduction Reaction on Defective Graphene-Supported Pt Nanoparticles from First-Principles. J. Phys. Chem. C 2012, 116, 3653-3660, DOI:10.1021/jp210796e. 16


Page 17 of 20 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


30. Wu, J.; Zhang, J.; Peng, Z.; Yang, S.; Frederick, T. W.; Hong, Y. Truncated Octahedral Pt3Ni Oxygen Reduction Reaction Electrocatalysts. J. Am. Chem. Soc. 2010, 132, 4984-4985, DOI: 10.1021/ja100571h. 31. Jia, Q.; Caldwell, K.; Strickland, K.; Ziegelbauer, J. M.; Liu, Z.; Yu, Z.; Ramaker, D. E.; Mukerjee, S. Improved Oxygen Reduction Activity and Durability of Dealloyed PtCox Catalysts for Proton Exchange Membrane Fuel Cells: Strain, Ligand, and Particle Size Effects. ACS Catal. 2015, 5, 176-186, DOI: 10.1021/cs501537n. 32. Kitchin, J. R.; Norskov, J. K.; Barteau, M. A.; Chen, J. G. Role of strain and ligand effects in the modification of the electronic and chemical properties of bimetallic surfaces.

Phys.

Rev.

Lett.

2004,

93,

156801-156804,

DOI:

10.1103/PhysRevLett.93.156801. 33. Asset, T.; Chattot, R.; Drnec, J.; Bordet, P.; Job, N.; Maillard, F.; Dubau, L. Elucidating the Mechanisms Driving the Aging of Porous Hollow PtNi/C Nanoparticles by Means of COads Stripping. ACS Appl. Mater. Inter. 2017, 9, 25298-25307, DOI: 10.1021/acsami.7b05782. 34. Zhou, Y.; Yang, J.; Zhu, C.; Du, D.; Cheng, X.; Yen, C. H.; Wai, C. M.; Lin, Y. Newly Designed Graphene Cellular Monolith Functionalized with Hollow Pt-M (M = Ni, Co) Nanoparticles as the Electrocatalyst for Oxygen Reduction Reaction. ACS Appl. Mater. Inter. 2016, 8, 25863-25874, DOI: 10.1021/acsami.6b04963. 35. Xiong, M.; Ivey, D. G. Composition effects of electrodeposited Co-Fe as electrocatalysts for the oxygen evolution reaction. Electrochim. Acta 2018, 260, 872881, DOI: 10.1016/j.electacta.2017.12.059. 36. Zhu, Z.; Zhai, Y.; Dong, S., Facial synthesis of PtM (M = Fe, Co, Cu, Ni) bimetallic alloy nanosponges and their enhanced catalysis for oxygen reduction reaction. ACS Appl. Mater. Inter. 2014, 6 (19), 16721-16726, DOI: 10.1021/am503689t. 37. Winter, M., Ralph J. B. What Are Batteries, Fuel Cells, and Supercapacitors. Chem. Rev. 2004, 104, 4245-269, DOI: 10.1021/cr020730k. 38. Zou, L.; Fan, J.; Zhou, Y.; Wang, C.; Li, J.; Zou, Z.; Yang, H. Conversion of PtNi alloy from disordered to ordered for enhanced activity and durability in methanoltolerant oxygen reduction reactions. Nano Res. 2015, 8, 2777-2788, DOI 17



10.1007/s12274-015-0784-0. 39. Loukrakpam, R.; Luo, J.; He, T.; Chen, Y.; Xu, Z.; Njoki, P. N.; Wanjala, B. N.; Fang, B.; Mott, D.; Yin, J.; Klar, J.; Powell, B.; Zhong, C.-J. Nanoengineered PtCo and PtNi Catalysts for Oxygen Reduction Reaction: An Assessment of the Structural and Electrocatalytic Properties. J. Phys. Chem. C 2011, 115, 1682-1694, DOI: 10.1021/jp109630n. 40. Fu, S.; Zhu, C.; Song, J.; Engelhard, M. H.; He, Y.; Du, D.; Wang, C.; Lin, Y., Three-dimensional PtNi hollow nanochains as an enhanced electrocatalyst for the oxygen reduction reaction. J. Mater. Chem. A 2016, 4, 8755-8761, DOI: 10.1039/C6TA01801G. 41. Du, S.; Lu, Y.; Malladi, S. K.; Xu, Q.; Steinberger-Wilckens, R. A simple approach for PtNi-MWCNT hybrid nanostructures as high performance electrocatalysts for the oxygen reduction reaction. J. Mater. Chem. A 2014, 2, 692-698, DOI: 10.1039/C3TA13608F. 42. Hui, Y.; Walter, V.; Claude, L.; Nicolas, A. V. Structure and Electrocatalytic Activity of Carbon-Supported Pt-Ni Alloy Nanoparticles Toward the Oxygen Reduction Reaction. J. Phys. Chem. B 2004, 208, 11024-11034, DOI: 10.1021/jp049034. 43. Frederic, H.; Mehtap, O.; Peter, S. Activity, Structure and Degradation of Dealloyed PtNi3 Nanoparticle Electrocatalyst for the Oxygen Reduction Reaction in PEMFC. J. Electrochem. Soc. 2012, 159, B24-B33, DOI: 10.1149/2.030201jes. 44. Salgado, J. R. C.; Antolini, E.; Gonzalez, E. R. Carbon supported Pt-Co alloys as methanol-resistant oxygen-reduction electrocatalysts for direct methanol fuel cells. Appl. Catal. B: Environ. 2005, 57, 283-290, DOI: 10.1016/j.apcatb.2004.11.009. 45. He, W.; Liu, J.; Qiao, Y.; Zou, Z.; Zhang, X.; Akins, D. L.; Yang, H. Simple preparation of Pd-Pt nanoalloy catalysts for methanol-tolerant oxygen reduction. J. Power Sources 2010, 195, 1046-1050, DOI: 10.1016/j.jpowsour.2009.09.006. 46. Maillard, F.; Gloaguen, M. M. F.; Leger, J. M. Oxygen electroreduction on carbonsupported platinum catalysts. Particle-size effect on the tolerance to methanol competition. Electrochim. Acta 2002, 47, 3431-3440, DOI: 10.1016/S001318


Page 18 of 20

Page 19 of 20 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


4686(02)00279-7. 47. Yoshio, T.; Tomoya, I.; Wataru, S.; Yasushi, M. Size effects of platinum particles on the electro-oxidation of methanol in an aqueous solution of HClO4. Electrochem. Commun. 2000, 2, 671-674, DOI:10.1016/s1388-2481(00)00101-6.

19



Table of Contents

Synopsis: A low temperature impregnation-reduction method is employed for the synthesis of uniformly dispersed PtM (M=Fe, Co, Ni) NCs as highly efficient and stable catalysts for ORR.

20


Page 20 of 20

PtM (M = Fe, Co, Ni) Bimetallic Nanoclusters as Active, Methanol

Recommend Documents