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Electrooxidation of Methanol on Pt @Ni Bimetallic Catalyst Supported on Porous Carbon Nanofibers JianYi Chen, Qijian Niu, GuangKai Chen, Jun Nie, and Guiping Ma J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10882 • Publication Date (Web): 05 Jan 2017 Downloaded from http://pubs.acs.org on January 7, 2017

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The Journal of Physical Chemistry

Electrooxidation of Methanol on Pt @Ni Bimetallic Catalyst Supported on Porous Carbon Nanofibers JianYi Chena,b, QiJian Niub, GuangKai Chenb, Jun Nie*b, GuiPing Ma *b a School of Material Science and Engineering, Changzhou University, Changzhou, Jiangsu 213164, P.R. China. b Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing, 100029, P. R. China.

ABSTRACT: This paper describes the preparation of Ni/Pt/CNFs via electrospinning technology, carbonization process and chemical reduction method. The structure and composition of Ni/Pt/CNFs were characterized with X-ray diffraction,

Raman

spectroscopy,

Nitrogen

adsorption

isotherms

and

X-ray photoelectron spectroscopy. Meanwhile, the morphology was analyzed with Scanning electron microscopy and Transmission electron microscopy. The electrochemical performance was evaluated by oxygen reduction reaction (ORR), cyclic voltammetry and chronopotentiometry. The results indicated that Pt and Ni nanoparticles

were

completely

reduced

in

the

experimental

process

and

homogeneously distributed on the nanofibers with the average diameters of 3.8 nm and 17.8 nm, respectively. In addition, the Ni50/Pt/CNFs catalyst showed excellent electrocatalytic performance for ORR and superior specific and mass activities for methanol oxidation (the maximum current density is ca. 10.9 mA cm-2) and exhibited a slightly slower current decay over time, better than the reference samples which indicated a higher tolerance to CO-like intermediates.

1. INTRODUCTION Direct methanol fuel cell (DMFC) has been attracting increasing research interests because of its high energy density, low operating temperature, environmentally friendly and convenient use1-4. The study on DMFC promotes the rapid development 1

ACS Paragon Plus Environment


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of this field, both in theory and application have experienced more systematic and in-depth research. But the DMFC has not been commercialized, the key problem is the catalyst with low catalytic efficiency and high cost. Therefore, the preparation of high-efficiency, low-cost anode catalyst has become a prominent problem at present. In recent years, the research on the anode catalysts for DMFC mainly concentrated in the noble metal catalysts (such as Pt, Pd, Au)5-7 and non-noble metal catalysts (including metal carbides and transition metal oxides). Although the use of non-noble metal catalysts can greatly reduce the cost of DMFC, but its catalytic efficiency is far away from the commercial requirements. Research has shown that platinum has exhibited high activity for methanol oxidation which is suitable for the DMFC anode8, 9

. However, pure platinum is costly and easily poisoned by carbon monoxide (CO)

10-12

. In order to reduce the cost and improve the tolerance to CO-like intermediates,

catalyst particles should have a uniform distribution on a supporting matrix with nanometric dimensions to reduce the amount of catalyst and increase the effective catalytic area13, 14. By the way, carbon materials are ordinarily used for supporting catalyst particles15-17. In addition, using platinum-based alloy or ternary catalysts can also improve the electrocatalytic activity. It is reported that PtRu alloy catalyst is the most mature of the anode catalyst with the most ideal electrocatalytic activity18-21. But in order to achieve the higher catalytic effect, the load of Pt need to be in 2 ~ 8 mg cm-2, in addition Ru is also noble metal which limits the practical application of DMFC. Hopefully, Studies have showed that the doping of non-noble metals such as Cu, Fe, Ni can also improve the catalytic efficiency of Pt-based catalysts, thereby reducing the cost of the catalyst22-24. Park et al. reported that the Pt4f binding energy of PtNi alloy was reduced more than the Pt4f binding energy of PtRu alloy using X-ray photoelectron spectroscopy (XPS) analysis25. Due to the electronegativity difference between Pt & Ni is greater than the electronegativity difference between Pt & Ru, therefor the influence of Ni to level structure of Pt is more remarkable. At the same time, the change of the electron cloud density also leads to the decrease of the Fermi level of Pt, which reduced the adsorption of CO on Pt. The promote effect of Ni is 2


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more obvious in terms of CO poisoning-tolerance. In recent years, many reports about Pt-Ni catalysts have confirmed this theory and demonstrated the excellent electrochemical performance26-29. In addition, the study on novel carbon support is an important research task to achieve high catalytic activity and better DMFC performance. Many characteristics such as degree of metal dispersion, metal-support interaction and electronic properties of metals

are

reported

1D carbon nano-materials

which

rely

on

such

as

the

nature carbon

of

carbon nanotubes

support30. and

multiporous carbon nanofibers have attract more and more attention in the research of carbon support. Compared with traditional carbon materials, the specific nano size effect of nano carbon materials can increase the reaction active sites and improve the electrochemical performance of the composite catalysts. Moreover, the properties of nano carbon materials (such as electronic structure, chemical activity and thermal stability, etc.) can be further adjusted by N-doped31, 32. Electrospinning technique is an effective method to prepare 1D nanofibers, which is applied in various fields, such as biocatalysis33, catalysis34, adsorption35 and sensor 36. This technique has been used to produce carbon nanofibers by subsequent thermal treatments for the stabilization and carbonization of the nanofibers30. As the same time, polyacrylonitrile (PAN) is one of the most popular polymer materials to prepare carbon nanofibers, due to its high yield, good-spinnability, high molecular weight and high carbon content, which has been applied in many fields. Herein, porous structure of Ni/CNFs was prepared by electrospinning technology and carbonization process, among which the most spectacular was that the addition of nickel acetylacetonate could serve as a dual purpose template to generate a large number of pores and produce the co-catalyst (Ni) in carbon nanofibers during the carbonization process. The micro pores and the high surface area of Ni/CNFs can contribute to the adsorption of nano platinum by chemical reduction method which strongly enhanced the electro catalytic activity of the catalysts toward methanol oxidation. It is noteworthy that Pt nanoparticles are predicted to show a decrease in ORR activity with decreasing particle size37. The experimental results indicated that 3



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the Ni50/Pt/CNFs had the best catalytic performance and the average diameter of Pt nanoparticles is 3.8 nm.

2. EXPERIMENTAL SECTION Chemicals. Polyacrylonitrile (PAN, Mw= 150, 000 g mol-1), sulfric acid (H2SO4) and Nafion solution (5 wt %) were purchased from Sigma-Aldrich. Dihydrogen hexachloroplatinate (IV) hexahydrate (H2PtCl6·6H2O, Pt wt. % ≥37.5%), nickel (II) acetylacetonate (NiAA), polyvinylpyrrolidone (PVP K29-32, Mw=58,000) and sodium borohydride (NaBH4) were purchased from Aladdin. Methanol, ethylene glycol and N, N-dimethylformamide were from Tianjin Chemicals. All chemicals were of analytic grade and used without further purification.

Preparation of Ni/CNF Composites. In a typical method, 1.0 g of polyacrylonitrile (PAN) was added into 9.0 g N,N-dimethylformamide (DMF) and stirred at 80 ℃ for 1 h. After that, 500 mg of Nickel (II) acetylacetonate (NiAA) was added into the above solution and stirred continuously for a night to yield homogeneous solution. The nanofibers were prepared by electrospinning the uniform suspension at a potential of 20 kV using a hypodermic syringe, which was connected to a syringe pump to maintain a flow rate of 0.5 mL h-1. The distance between the aluminum collecting foil and the hypodermic syringe was fixed at 15 cm. The electrospinning process was carried out at room temperature and humidity was ca. 30%. Then the electrospun NiAA/PAN nanofiber precursor composites were collected from the collector foil and dried in a vacuum oven at 40℃ for 12h. Carbonization of NiAA/PAN nanofibers was performed as below: (1) stabilization of the precursor nanofibers at 280 °C in air for 2 h; (2) heating up to 800 ℃ at a rate of 2 ℃ min-1 and stayed at this temperature in N2 atmosphere for 2 h for the reduction of Ni2+ and the carbonization the composite nanofibers ; (3) cooling down to room temperature in N2 atmosphere naturely.

4


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Deposition of Pt Nanoparticles onto the Porous Ni/CNFs. The Pt nanoparticles were synthesized and deposited onto the porous Ni/CNF nanofibers using a chemical reduction method. First, 10 mg of the as-prepared Ni/CNF nanofibers was added into 10 mL of ethylene glycol /water (EG/H2O, 1/1 v/v) mixture in a 20 mL beaker, followed by sonication for 10 min to disperse the Ni/CNF nanofibers. Then, 5.3 mg of H2PtCl6·6H2O was introduced and dissolved by sonication. This solution was degassed by bubbling with N2 at 20 mL min-1 for 10 min. Subsequently, 5 mL of 0.05 M fresh aqueous NaBH4 solution was added into the system, the beaker was then sealed by plastic film and placed in the sonication bath immediately for 30 minute. The product was collected by centrifugation after standing for an hour, and washed three times with deionized water to dislodge the residue reactants. In the second experiment, different content of NiAA was mixed with PAN solution, the weight ratio of NiAA and PAN was 0:10, 1:10 and 3:10, respectively. By using the same parameters and setup, we obtained the final products, which were referred to as Pt/CNFs, Ni10/Pt/CNFs and Ni30/Pt/CNFs, respectively.

Characterization. Scanning electronmicroscopy (SEM; JSM- 6360LA, Japan) was carried out using an acceleration voltage of 15 kV, and SEM equipped with an Energy Dispersive X-ray Spectroscopy detector (EDX, Oxford INCA). Transmission electron microscopy (TEM; JEM-2100, Japan) was carried out using a 200 kV accelerating voltage. The simples were dispersed in ethanol and then dip-coated onto copper grids. X-ray diffraction (XRD; Rigaku D/MAX-YA) was applied with Cu Kalpha radiation, λ=0.154 nm, and scans performed from (2θ) 10-90° at a rate of 5 min-1. The Raman spectrum of the Ni/CNF composite was obtained using a Horiba LabRam HR 800 Raman spectrometer with a laser beam of 532 nm (green laser).The specimens were analyzed using an XPS system (XPS, Thermo Electron Corporation, Escalab 250, Germany). The photoelectron take-off angle used was 45 degrees with respect to the sample plane. Survey spectra were acquired in the binding energy range of 0-1355 eV. The N2 adsorption–desorption isotherm measurements were carried out on a Micromeritics ASAP2010 analyzer. 5



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Catalytic Activity Measurement. Electrochemical measurements were carried out in a three electrode cell at room temperature, which the working electrode was done by pasting catalyst inks on a rotating disk electrode (RDE) of 5 mm in diameter for ORR measurements or a glassy carbon electrode of 5 mm in diameter for methanol oxidation reaction (MOR), a Pt wire counter electrode, and an Ag/AgCl reference electrode. The catalyst (5 mg) was dispersed in the solution containing 1 mL of ethanol and 100μL of 5 wt% Nafion solution (DuPont), followed by a strong ultrasonic treatment to form a uniform black ink. The working electrode was polished with Al2O3 paste and washed ultrasonically in deionized water, and 20 µL of the catalyst ink was dripped on to the glassy carbon surface with a loading of ~0.46 mg cm-2 for all samples followed by drying in air. In order to getting the properties of different catalysts, Linear sweep voltammetric curves (LSV) were measured in O2-saturated 0.10 M KOH solution at different rotation speed rates (400, 625, 900, 1225, 1600, and 2025 rpm) with a sweep rate of 10 mV s-1, cyclic voltammetry (CV) curves were performed in the potential between 0 and 1 V vs Ag/AgCl in 1.0 M CH3OH + 0.5 M H2SO4 and chronoamperometric (CA) curves of 1.0 M CH3OH + 0.5 M H2SO4 were measured for 2400s maintaining a potential of 0.45 V vs Ag/AgCl reference. 1.0 M CH3OH + 0.5 M H2SO4 solution was purged with N2 for 20 min first to deplete dissolved oxygen.

3. RESULTS AND DISCUSSION The overall synthetic procedure of carbon nanofibers is illustrated in Scheme 1, which consists of four steps: (1) electrospinning of the composite precursor; (2) preoxidative stabilization of nanofibers in air and (3) carbonization of electrospun precursor nanofibers to form graphitic carbon nanofibers with Ni nanoparticles under N2 atmosphere; (4) deposition of Pt nanoparticles onto the Ni/CNFs. Due to the nice spinnability and high carbon yield of PAN, it has become one of the 6


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most popular precursors for electrospinning. NiAA is compatible with PAN-DMF solution and can be easily co-electrospun to form NiAA/PAN nanofibers. During the thermal treatment, the Ni nanoparticles formed in situ are embedded in the carbonaceous matrix of the nanofibers. A certain amount of carbon species remained amorphous and the existence of amorphous carbon can maintain the whole nanofiber structure. Finally, The Pt nanoparticles were synthesized and deposited onto the porous Ni/CNFs in situ using a reduction method.

Scheme 1. Schematic illustration of the fabrication process of Pt/Ni/CNFs.

X-ray diffractograms for as-prepared NiAA50/PAN nanofibers, Ni50/CNFs and Ni50/Pt/CNFs are shown in Figure 1a. After carbonization, a strong diffraction peak at 2θ = 25.8° can be observed which can be assign to plane (002) of the hexagonal structure of graphite structure. Meanwhile, it also shows three characteristic diffraction peaks at 44.3°, 51.6° and 76.3°, corresponding to the (111), (200) and (220) crystalline planes of Ni, respectively. Ni50/Pt/ CNFs also displays the characteristic peak for CNF (at 2θ = 25.8°) and the three characteristic peaks for nickel (at 2θ = 44.3°(111), 51.6° (200) and 76.3 °(220)). In addition, it also shows a weak and wide peak at 2θ=39.9°, which is corresponding to the characteristic peak of Pt (111). The XRD result of Ni50/Pt/ CNFs reveals the only existence of Pt (111), because of the 7



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low Pt content or small size of Pt nanoparticles, as well as their high dispersion38. Figure 1b shows the Raman spectra of Ni50/CNFs and CNFs, which all consist of two characteristic peaks at 1337cm-1 and 1571cm-1 representing the defect-induced mode A1g (D band) and the ordered E2g mode (G band). The G band is associated with the (002) diffraction peak of the XRD pattern. The D peak is due to disordered carbon, and the G peak is from ordered carbon (graphitic structure). The R-value (ratio of D peak intensity to G peak intensity) of CNFs is ca. 1.05, ordered carbonaceous component is slightly greater than the ordered component of the composite, disordered carbon is rich in defects. The R-value of the Ni50/Pt/CNFs composite is ca. 1.00, which is lower than the R-value of CNFs. It is indicated that the graphitization degree of Ni50/Pt/CNFs is higher than that of CNFs, which can improve the conductivity of the nanofibers. It can be attributed to the interaction between Ni and carbon during carbonization process. Beside, Trancik et al. and Lee et al. have reported that the existence of defects in CNTs can lead to enhancement of catalytic activity39.

Figure 1. (a) XRD patterns of (1) NiAA50/PAN nanofibers, (2) Ni50/CNFs and (3) Ni50/Pt/CNFs. (b) Raman spectrum of Ni50/Pt/ CNFs and CNFs.

The morphology and distribution of as-spun NiAA50/PAN nanofibers and Ni50/CNFs were characterized by scanning electron microscopy (SEM). The diameters of NiAA50/PAN nanofibers ranged from 500 to 600 nm, and these nanofibers with a 8


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diameter of approximately 540 nm (Figure 2a). After the pyrolysis, PAN was transformed into a carbon structure via a carbonization process involving the decomposition of hydrogen and nitrogen. As a result, the diameter decreased from 380 to 540 nm while maintaining the well-defined 1-D structures, as observed in Figure 2b. In addition amounts of nanoparticles were firmly dispersed on or in CNF which was distributed homogeneously and with uniform size. According to the analysis of XRD and XPS, the nanoparticles can be determined as nano-Ni. More detailed morphology of the nanofibers was investigated by TEM. Figure 2c shows a representative nanofiber of Ni/CNFs and the small individual nanoparticles can be observed on the nanofibers. Some Ni nanoparticles being tightly encapsulated by graphitic carbon layers. The fringe of the Ni and graphitic carbon shells can be simultaneously observed in the inset of Figure 2c. The distribution of nano-Ni is shown in Figure S1a, in which approximately 200 representative nanoparticles were measured. It is obvious that the average diameter of the Ni nanoparticles is about 17.8nm. It also shows the broadest size distribution among the sample, which varied from 8 to 32 nm. Meanwhile, the morphologies of NiAA10/ PAN and NiAA30/ PAN before and after carbonization using SEM and TEM are showed in Figure S2. The diameter of the nanofibers decreased obviously after carbonization (from 540nm to 380nm). In addition, there is almost no nanoparticle on the surface of the Ni10/ CNFs and the surface of Ni30/ CNFs covered with a little amount of nanoparticles, due to the bulk of particles were concentrated inside of the nanofibers, both of Ni10/ CNFs and Ni30/ CNFs.

9



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Figure 2. (a) SEM image of the electrospun precursor composite nanofibers of PAN and NiAA. (b) SEM image of Ni50/CNFs prepared by carbonization of the as-spun NiAA-PAN nanofibers composite nanofibers at 800 ℃ in N2 for 2 h. (c) TEM image of Ni50/CNFs. The insets are high-magnification images of the nanofibers.

10


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Figure 3. N2 adsorption-desorption analysis of Ni10/CNFs, Ni30/CNFs and Ni50/CNFs.

The N2 adsorption isotherms of Ni50/CNFs (Figure 3) can be classified to I and IV with a H2-type hysteresis loop according to IUPAC’s classification. The existence of micro pores can be indicated by the vertical rise of quantity adsorbed at p/p0 < 0.05. The gentle slope of the adsorption branch and the dramatic drop of desorption branch at p/p0 = 0.5 suggests the existence of mesoporous40. The micro pores may contribute to the adsorption of nano platinum. The N2 adsorption isotherms of Ni10/CNFs and Ni30/CNFs are shown in Figure 3 which indicated a type I isotherm. Table 1 shows the surface area and pore volume of Ni10/CNFs, Ni30/CNFs and Ni50/CNFs. Ni50/CNFs exhibited a higher specific surface area of 234.1 m2 g-1 and a higher total pore volume of 0.157 cm3 g-1 compared to Ni10/CNFs and Ni30/CNFs. It is due to that with increase of the content of NiAA, a large amount of gas was produced during carbonization which causes the rupture of the nanofibers and forms the micro pores on the surface of nanofibers.

11



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Table 1 – The surface area and pore volume of Ni10/CNFs, Ni30/CNFs and Ni50/CNFs. Element

BET specific surface area

Pore volume

(m2 g-1)

(cm3 g-1)

Ni10/CNFs

16.5

0.0085

Ni30/CNFs

152.8

0.094

Ni50/CNFs

234.1

0.157

The Ni50/CNFs were employed as supports for the in situ synthesis and deposition of Pt nanoparticles using a chemical reduction route. Figure 4a shows the TEM morphologies of Ni50/Pt/ CNFs. Compared with Figure 2c, a large number of particles are highly dispersed on the supports (Figure 4b, 4c) and possess an average particle size of about 3.8 nm (shown in Figure S1b) which is smaller than nickel particles. Figure S1b also shows the broadest size distribution of these particles, which varied from 2 to 7 nm. The high resolution image (inset in Figure 4c) shows that the interplanar spacing is 0.22 nm, corresponding to the (111) set of plane for metallic Pt41, 42. This result indicated that Pt nanoparticles were successfully deposited on the supports. According to XRD and XPS, it can also prove that these tiny particles are nano platinum. At the same time, the EDS spectra of Ni50/Pt/ CNFs (showed in Figure S3) also revealed the existence of Pt in the composite nanofibers.

12


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Figure 4. (a) TEM image of Ni50/Pt/ CNFs. (b, c) HRTEM image of Ni50/Pt/ CNFs. Inset in panel c show the Pt HRTEM image.

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Figure 5. XPS survey scan of (a) Ni50/Pt/CNFs, (b)Pt 4f, (c)Ni 2p, (d) C 1s, (e) N 1s and (f) O 1s.

Table 2 – XPS quantitative element analysis of Ni50/Pt/CNFs. Sample

Atomic%

Weight%

C

80.33

61.28

O

10.80

10.98

N

6.09

5.42

Ni

1.40

5.22

Pt

1.38

17.10

The Pt/Ni50/CNFs was investigated by recording XPS spectra. Figure 5a shows the fully scanned spectra of Ni50/Pt/ CNFs in the range of 0–1300 eV. It demonstrates that C, N, O, Ni and Pt elements existed in the Ni50/Pt/ CNFs. Figure 5b shows the Pt4f region of the XPS spectrum of Pt/Ni50/CNFs, which can be deconvoluted into two pairs of doublets and the Pt 4f7/2 and 4f5/2 lines appear at ca. 70.9eV and ca. 74.2 eV, respectively. The comparison of the binding energies indicates that Pt is present in the zerovalent metallic state25. Meanwhile the Ni 2p for the composites exhibits two major peaks (Ni 2p3/2 and Ni 2p1/2) which correspond to the binding energies of 852.9 14


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and 870.6 eV, respectively, with a spin-energy separation of 17.7 eV (Figure 5c). The Ni state only consists of Ni metal (Ni0) and without Ni oxide or hydroxides, because there are no additional peaks labeled as satellite peaks appeared around the Ni 2p1/2 and Ni 2p3/2 peaks. Figure 5d, e, and f show the XPS data for C1s, N1s, and N1s regions, respectively. Three peaks are observed by deconvolution of C1s data (Figure 5d). The peak at 284.9 eV is attributed to carbon conducting substrate, the peak at 286.0 eV is due to the carbon backbone of the polymer8, and the peak at 288.8 eV corresponds to O– C=O43. N-doped is helpful to improve the performance of materials, by deconvolution of N1s data (Figure 5e), three N species are present in Pt/Ni50/CNFs, including pyridinic-N (398.6 eV), pyrrolic-N (399.5 eV) and graphitic-N (401.1 eV), respectively44. Figure 5f represents the XPS spectra of O1s, a peak at 534.1 eV corresponds to the sole oxygen binding environment in Ni50/Pt/ CNFs and the other peak at 532.5 eV is due to loosely bound surface oxygen8. The composition of the catalysts confirmed by XPS is given in Table 2.

Figure 6. (a) Linear sweep voltammetry measurements at a rotation speed of 1600 15



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rpm in 0.1 M KOH, (b) Koutecky–Levich plots for the ORR at -0.45V, (c) transferred electron numbers of different catalyzers, (d) Cyclic voltammograms in 1 mol /L CH3OH + 0.5 mol /L H2SO4 solution with the scan rate of 50 mV/s. In order to assess the ORR catalytic activity and understand the electron transfer kinetics, linear sweep voltammetry (LSV) measurements were performed at different rotation speeds ranging from 900 to 2025 rpm using a rotating disk electrode (RDE) in 0.10 M KOH at room temperature. LSV curves of different catalyzers at different rotation speeds are showed in Fig. S4. The current density was enhanced with increasing rotation rate from 400 to 2025 rpm due to facilitating diffusion of electrolytes. Figure 6a presents the ORR curves of different catalyzers at the rotation speed of 1600 rpm and the Ni50/Pt/CNFs exhibited the highest ORR catalytic activity with the onset and half-wave potentials of -0.0634V and -0.1258V, respectively. The linearity of the Koutecky-Levich (K-L) plots and the kinetic and diffusion mechanisms of reaction can be addressed by the K-L equations45: 1/j = 1/jL + 1/jk

(1)

1/j = 1/Bω1/2 + 1/jk

(2)

B = 0.2nFCD 2/3ν-1/6

(3)

The K-L plots of different catalyzers are showed in Figure 6b and the electron transfer numbers were calculated from the plots of Figure 6b using eqn (3) which is showed in Figure 6c. The electron transfer number of Pt/CNFs, Ni10/Pt/CNFs, Ni30/Pt/CNFs and Ni50/Pt/CNFs is 2.92, 3.36, 3.5 and 3.69, respectively. The 4-electron transfer mechanism for ORR is through two-oxygen atom side or bridge adsorption for oxygen dissociation simultaneously. In particular, Ni50/Pt/CNFs exhibited the highest value.

The

results suggested

that the

ORR

activity was

enhanced

by

the synergetic effects of bimetallic catalyst. In Figure 6d, the cyclic voltammetry in CH3OH solution of the Pt/CNFs catalyst shows characteristic peaks of catalyzed methanol oxidation in both sweep directions of the voltammogram, showing a maximum current density of ca. 6.4 mA cm-2 in the positive sweep. Similar behavior was observed with Ni10/Pt/CNFs but with an increased maximum current density (ca. 7.6 mA cm-2). The catalytic behavior of the 16


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Ni30/Pt/CNFs sample was also characteristic for methanol oxidation, reaching a maximum value of ca. 9.8 mA cm-2. Again, compared with Pt/CNFs catalyst, a greatly increased maximum current density (ca. 10.9 mA cm-2) was observed for Ni50/Pt/CNFs. Moreover, the current density of characteristic peak in the negative sweep for Pt/CNFs, Ni10/Pt/CNFs, Ni30/Pt/CNFs and Ni50/Pt/CNFs is ca. 0.69 mA cm-2, ca. 5.2 mA cm-2, ca. 6.5 mA cm-2 and ca. 6.8 mA cm-2, respectively, which indicates that the highest activity for the latter. Besides, the methanol oxidation onset potential of Pt/CNFs , Ni10/Pt/CNFs, Ni30/Pt/CNFs and Ni50/Pt/CNFs is ca. 0.48 V, ca. 0.46 V, ca. 0.45V and ca. 0.43 V, respectively. It is reported that in methanol electrooxidation, more negative onset potential is preferable to enhance the overall cell power46. Formation of OH and CO adsorbed layer on the surface of the electrodes leads to increase the over potential47. Therefore, the above shows that the catalytic performance of the latter is excellent.

Figure 7. (a) Anodic peak currents in 1 mol /L CH3OH + 0.5 mol /L H2SO4 solutions at different scan cycle with the scan rate of 100 mV/s for Ni10/Pt/CNFs, Ni30/Pt/CNFs, Ni50/Pt/CNFs and Pt/CNFs. (b) Chronoamperometry curves of Ni10/Pt/CNFs, Ni30/Pt/CNFs, Ni50/Pt/CNFs and Pt/CNFs at 0.45 V vs Ag/AgCl in acidic solution. Electrolyte: 0.5 M H2SO4 + 1M CH3OH.

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In order to study on the stability of different catalysts, as shown in Figure 7a, the anodic peak currents of Ni30/Pt/CNFs and Ni50/Pt/CNFs reached the maximum value firstly in the cycle of about 100, and then the anodic peak currents decreased gradually, which is different from Pt/CNFs and Ni10/Pt/CNFs. After scanning 500 cycles, the anodic peak current of Ni50/Pt/CNFs did not decrease obviously compared with Ni30/Pt/CNFs, and the anodic peak current was still maintained as the initial anodic peak current of ca. 75 %. By the way, the anodic peak current of Ni30/Pt/CNFs was reduced to ca. 43% of the initial peak current. By comparison, the anodic peak current of Ni10/Pt/CNFs and Pt/CNFs decreased rapidly after scanning 500 cycles, the anodic peak current of Ni10/Pt/CNFs was reduced to ca. 20% of the initial peak current, and especially for Pt/CNFs, the anodic peak current of Pt/CNFs was reduced to ca. 0% of the initial peak current, which means that the catalytic capacity of the Pt/CNFs was almost lost. In a word, the catalytic stability of the catalytic was obviously enhanced when the NiAA content was 50% in precursor solution. Chronoamperometry data were recorded at 0.45 V (vs Ag/AgCl) for 2400 s as a measure of the catalyst deactivation (Figure 7b). It is obviously that Ni50/Pt/CNFs exhibited a slower current decay over time, indicating a larger catalytically active surface area and higher tolerance to CO-like intermediates48. In addition, Pt/CNFs showed a faster current decay, indicating a negative effect for removal of the CO-like poisoning intermediates without the component of Ni. It is thus concluded that the synergistic effect of Pt & Ni can enhance the stability of electrocatalytic activity for methanol oxidation and improve resistance of catalyzer to CO-like intermediates. In addition to the active surface hydroxyls directly participating in the CH3OH oxidation into CO2, the possible mechanism of the surface hydroxyls can be indirectly participated in the oxidation of intermediates over Pt/Ni/CNFs catalysts is shown in Scheme 2. The active site of platinum excited the methoxy species, which is then migrated to the active site of the nickel, reacting with the reactive oxygen species on the nickel surface, and forming water and carbon dioxide rapidly. This reaction path avoids the oxidation process of CO, so the reaction potential is lower than that of the pure platinum catalyst, and the catalytic activity is greatly improved. 18


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Scheme 2. Simplified reaction pathway for complete oxidation of CH3OH over Pt/Ni/CNFs.

4. CONCLUSIONS In conclusion, calcination of electrospun nanofibers composed of Nickel acetylacetonate in N2 atmosphere at 800 ℃ leaded to produce metallic nanoparticles doped carbon nanofibers, which not only produced the pores, but also got the higher specific surface area of the nanofiber with the increase of Nickel acetylacetonate content(The surface area of Ni50/CNFs is 234.1 m2 g-1). Analyses of XRD results and of SEM and TEM images revealed that Pt and Ni nanoparticles were completely reduced in the experimental process and evenly distributed on the nanofibers and the average diameters of Pt and Ni are 3.8 nm and 17.8 nm, respectively. Composition of the metallic counterpart affected the electrocatalytic activity of the synthesized nanofibers toward methanol oxidation. According to the results of the ORR, cyclic voltammetry and chronoamperometry data, Ni50/Pt/CNFs showed a higher 19



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electrocatalytic performance than others. Furthermore, the increase of cocatalyst (Ni) content can enhance the stability of electrocatalytic activity for methanol oxidation, due to the electron transfer between Pt and Ni contribute to the enhanced CO oxidation (CO generated from methanol decomposition), in other words, the CO tolerance on the Ni-containing composites is better than Pt/CNFs. Overall, The integration of conductive CNFs with electrochemical active Ni & Pt provided significantly improvement of specific surface area and fully exposed active sites.

ASSOCIATED CONTENT ○S Supporting Information Four figures displaying additional characterization results including SEM, EDS, and LSV results (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Phone and fax: +86-1064421310 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We gratefully acknowledge the financial support from Changzhou science and Technology Bureau (Grant No. CE20150011). JYC greatly thanks Dr. Ke-Ming Wang (School of Material Science and Engineering, Changzhou University.) for kind help.

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Electrooxidation of Methanol on Pt @Ni Bimetallic ... - ACS Publications

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