CNTs anode catalyst for direct methanol fuel cells using Ni2P as co-catalyst

Applied Surface Science 434 (2018) 534–539

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Short Communication

Enhanced activity of Pt/CNTs anode catalyst for direct methanol fuel cells using Ni2 P as co-catalyst Xiang Li a,b , Lanping Luo a , Feng Peng b,∗ , Hongjuan Wang b , Hao Yu b a b

School of Chemical Engineering, Guangdong University of Petrochemical Technology, Maoming, 525000, China School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, 510640, China

a r t i c l e

i n f o

Article history: Received 19 July 2017 Received in revised form 18 October 2017 Accepted 30 October 2017 Available online 31 October 2017 Keywords: Methanol oxidation reaction Anode catalyst Platinum Nickel phosphide Carbon nanotubes

a b s t r a c t The direct methanol fuel cell is a promising energy conversion device because of the utilization of the state-of-the-art platinum (Pt) anode catalyst. In this work, novel Pt/Ni2 P/CNTs catalysts were prepared by the H2 reduction method. It was found that the activity and stability of Pt for methanol oxidation reaction (MOR) could be significantly enhanced while using nickel phosphide (Ni2 P) nanoparticles as co-catalyst. X-ray photoelectron spectroscopy revealed that the existence of Ni2 P affected the particle size and electronic distribution of Pt obviously. Pt/CNTs catalyst, Pt/Ni2 P/CNTs catalysts with different Ni2 P amount were synthesized, among which Pt/6%Ni2 P/CNTs catalyst exhibited the best MOR activity of 1400 mAmg−1 Pt, which was almost 2.5 times of the commercial Pt/C-JM catalyst. Moreover, compared to other Pt-based catalysts, this novel Pt/Ni2 P/CNTs catalyst also exhibited higher onset current density and better steady current density. The result of this work may provide positive guidance to the research on high efficiency and stability of Pt-based catalyst for direct methanol fuel cells. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Direct methanol fuel cells are considered as a promising power source for portable electronic devices. However, the scarcity and poor stability of Pt hindered its commercial application seriously. Hence, enormous efforts have been made to promote the utility and stability of the Pt catalyst. For example, Christina Bock synthesized the size-selected PtRu nano-catalyst which performed better oxidation activity than commercial catalyst [1]. Methanol oxidation activity of Pt catalysts can be effectively improved by introducing other proper metal components, such as Ni [2], Ir [3]. and Ru [1,4]. However, the cost of resultant complex catalyst was not low enough for commercial application. On the other hand, the incorporation of metal oxide, such as MnO2 [5] and WO3 [6], can also increase the methanol oxidation activity of Pt. Unfortunately, the instability of such catalysts under operating conditions resulted in a rapid decay of the catalytic performance. Therefore, the development of Pt-based catalyst with high activity and stability is alluring for fuel cells. Recently, Chang et al. have found that Ni2 P could increase the activity and stability of Pt/C significantly as a co-catalyst for

∗ Corresponding author. E-mail address: [email protected] (F. Peng). https://doi.org/10.1016/j.apsusc.2017.10.218 0169-4332/© 2017 Elsevier B.V. All rights reserved.

methanol oxidation [7]. Meanwhile, the Pt-based catalyst with bigger Pt particle size possessed better stability [8]. These results inspired us to fabricate novel Pt/Ni2 P/CNTs hybrids with large Pt particle size, which may have higher activity and stability as an anode catalyst. In this work, Ni2 P was served as a co-catalyst to modify Pt/CNTs catalyst, and the effect of Ni2 P on catalytic activity has been discussed.

2. Experimental section 2.1. Preparation and characterization of catalysts CNTs provided by Shenzhen Nanotech Port Co., Ltd. were treated by a well-known acid oxidation method before use. The Ni2 P/CNTs were prepared using a solid phase reaction method [9]. Typically, CNTs were impregnated incipiently with an aqueous solution of NiCl2 , followed by drying at 120 ◦ C for 3 h. Then, the obtained solid and NaH2 PO2 ·H2 O were mixed mechanically in a quartz boat at room temperature. The mixture was calcined at 250 ◦ Cin N2 atmosphere for 1 h with a heating speed of 2 ◦ C/min, and then cooled to room temperature in a continuous N2 flow. After that, the unsupported Ni2 P was passivated in 1.0 vol.% O2 /N2 mixed gas for 3 h. The Pt/Ni2 P/CNTs catalyst was prepared by H2 reduction method as follows: Ni2 P/CNTs were added to distilled water followed by the treatment of ultrasonication for 30 min. Then, appropriate amounts

X. Li et al. / Applied Surface Science 434 (2018) 534–539

of H2 PtCl6 and KOH were added under stirring until the pH reached 8.5. And the slurry was refluxed at 70 ◦ C for 1.5 h, and then filtrated and washed carefully using distilled water. The obtained solid was dried and reduced in 30 vol.% H2 /Ar mixed gas for 1 h at 523 K. Carbon nanotube supported PtNi catalyst (denoted as PtNi/CNTs) was synthesized using a similar method except no NaH2 PO2 ·H2 O was added to the mixture. Carbon nanotube supported PtP catalyst (denoted as PtP/CNTs) was synthesized using a similar method with only a certain amount of H2 PtCl6 solution and NaH2 PO2 (Pt: NaH2 PO2 = 1:60, mole ratio) was added into the suspension [7]. Pt/CNTs were also prepared by H2 reduction method according to the above procedures. And Home-made Pt/CNTs which was prepared using the ethylene glycol reduction method mentioned in previous work [10] noted as Pt/CNTs-H. The contents of Pt and other elements in the catalysts were determined by Electro-probe microanalyzer (EPMA, EPMA-1600, Shimadzu Corporation). The morphology of the catalysts was determined by transmission electron microscopy (TEM, JEM-2010HT microscope). X-Ray diffraction measurement (XRD, Brucker2 diffractometer, Bruker Co.) was carried out to determine the composition and crystal phase of catalysts. X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD spectrometer) measurement was performed to analyze the surface property of the catalysts. 2.2. Electrode preparations and electrochemical experiments Electrochemical measurements were carried out at room temperature in a three-electrode cell connected to computercontrolled Autolab PGSTAT30 electrochemical analyzer (Eco Chemie B. V., Utrecht, Netherlands). The preparation of electrodes

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was described in our previous paper [11]. A glass carbon electrode coated with catalyst was used as the working electrode. An Ag/AgCl with saturated KCl was used as reference electrode and Pt wire was used as counter electrode. Methanol oxidation reaction (MOR) activity was measured by cyclic voltammetry(CV) measurement in 1 M HClO4 solution containing 1 M CH3 OH with a scan rate of 100 mVs−1 . The chronoamperometry(CA) experiments were conducted in 1 M HClO4 and 1 M CH3 OH solution at 0.6 V to estimate the stability of the catalysts. For the electrochemical surface area (ECSA) measurement, the CV curve of pre-adsorbed CO electro-oxidation was recorded from −220 to 900 mV versus Ag/AgCl [11]. The electrochemical impedance spectra (EIS) were recorded at the frequency range from 100 kHz to 0.01 Hz with 10 points per decade. The amplitude of the sinusoidal potential signal was 5 mV. 3. Results and discussion The morphology, MOR activity and durability of Pt/CNTs and Pt/CNTs-H were shown in Fig. 1. Pt particles were identified by their lattice fringes(Fig. 1A and B) and the loading amount of Pt was 20% for both catalysts. The average Pt particle size of Pt/CNTs was about 10.3 nm, while Pt/CNTs-H’s was 4.3 nm (Fig. 1A and B insets). However, MOR mass activity of Pt/CNTs was 610 mAmg−1 Pt, which is about 1.5 times higher than that of Pt/CNTs-H. The enhancement may be attributed to the nanowire-like network morphology of Pt/CNTs catalyst. The short distance among Pt particles was beneficial to the proton transfer and the MOR activity of Pt/CNTs according to our previous work [10], and the grain boundaries contain in the nanowire-like network were also beneficial to its activity [12]. XPS spectra of Pt 4f region of Pt/CNTs and Pt/CNTs-H were shown in Fig. 1D. The Pt 4f spectra show a doublet at 71.25 and 74.65 eV

Fig 1. TEM and HTEM image of A) 20%Pt/CNTs and B) 20%Pt/CNTs-H, Insets are corresponding particle sizes distribution. The corresponding MOR activity and durability of 20%Pt/CNTs and 20%Pt/CNTs-H are shown in C) and inset of C) respectively. The Pt 4f XPS spectra of 20%Pt/CNTs (a) and 20%Pt/CNTs-H (b) are shown in D).

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Fig. 2. A) HRTEM of Ni2 P/CNTs where the (111) lattice of Ni2 P can be observed, and the carbon nanotubes support is visible. B) TEM image of the Pt/6%Ni2 P/CNTs catalyst. C) HRTEM image of Pt/6%Ni2 P/CNTs, both Pt (111) and Ni2 P (201) lattices can be observed. D) XRD patterns of (i) Ni2 P/CNTs, (ii) Pt/30%Ni2 P/CNTs, (iii) Pt/6%Ni2 P/CNTs and (iv) Pt/CNTs. The inset is the size distribution of the Pt/6% Ni2 P/CNTs catalyst. Table 1 Performances of electro-catalytic oxidation and utilization of Pt for Pt/CNTs and Pt/CNTs-H. Samples

Mass activity/Ag−1

d/nm

ECSA/m2 g−1

ECSA-Specific activity/Am−2

Pt/CNTs-g Pt/CNTs

610 425

10.3 4.3

19.7 26

30.96 16.35

for Pt/CNTs and 71.45 and 74.75 eV for Pt/CNTs-H, respectively. The results indicate a negative shift of the binding energies of Pt 4f for Pt/CNTs relative to that of Pt/CNTs-H. The performance comparation of Pt/CNTs and Pt/CNTs-H was listed in Table 1. The electrochemical surface area (ECSA) is 19.7 m2 g−1 for Pt/CNTs and 26 m2 g−1 for Pt/CNTs-H respectively. However, the ECSA-specific activity of Pt/CNTs is 30.96 Am−2 , much higher than Pt/CNTs-H, which is only 16.35 Am−2 . As a result the MOR mass activity of Pt/CNTs is higher than Pt/CNTs-H. Durability of the catalysts was measured by CV-cycles testing. It was found that the activity of Pt/CNTs retained 93% after 1000 CV cycles due to its larger Pt particle size, which was much better than that of Pt/CNTs-H retaining only 55%. Hence, we proposed to combine Ni2 P and Pt/CNTs together to obtain a super stable Pt-based catalyst with a promising MOR activity. Typical TEM images of Ni2 P/CNTs and Pt/Ni2 P/CNTs are shown in Fig. 2. The Ni2 P nanoparticles can be observed with a lattice fringe of 0.221 nm originated from the (111) crystal plane of Ni2 P, and the average size of Ni2 P is about 77.4 nm (Fig. 3f). The Pt nanoparticles were distributed on the Ni2 P/CNTs supporter with a narrow size distribution, and the average particle size of Pt was about 1.7 nm (Fig. 2C inset). As showed in Fig. 2C, lattice fringes of 0.221 nm and

0.191 nm could be attributed to Pt (111) and Ni2 P (210) respectively, indicating the Ni2 P and Pt were deposited onto the CNTs successfully. The presense of Ni2 P in Pt/CNTs was confirmed by Energy Dispersive X-ray Spectroscopy (EDX) elemental mapping (Fig. 3a–e). In addition, the characteristic diffractive peak of Pt at 39.8◦ , which belongs to the (111) reflection of Pt lattice, could be found in the XRD pattern of Pt/Ni2 P/CNTs (see Fig. 2D). The characteristic diffractive peaks of Ni2 P couldn’t almost be seen when the Ni2 P loading amount was 6%, but those peaks appeared and became intense when the Ni2 P loading amount was 30%. In this paper, the Pt loading amount was fixed to be 10% for all Pt/Ni2 P/CNTs samples. The MOR activities of Ni2 P and Pt/Ni2 P/CNTs with different loading amount of Ni2 P are shown in Fig. 3A. Although Ni2 P has no MOR activity (see Fig. 4A inset), the MOR activities of Pt/Ni2 P/CNTs (see Fig. 4A,B) improved significantly when the Ni2 P amount was increased from 1% to 6% followed by an obvious decline with the Ni2 P loading amount beyond 6%. It means that the optimized Ni2 P loading amount was 6% for the investigated Pt/Ni2 P/CNTs catalyst due to its highest mass activity. The Nyquist plots at 0.5 V of Pt/Ni2 P/CNTs catalysts with different Ni2 P amount are shown in Fig. 4C. The diameter of the EIS semicircle or arc correlates with charge transfer resistance. According to Fig. 3C, the semicircle from the Pt/6%Ni2 P/CNTs catalyst has the smallest diameter, and hence this catalyst has the highest activity in MOR. Fig. 4D showed the comparison of cyclic voltammograms for different catalysts. Among all the investigated catalysts, Pt/6%Ni2 P/CNTs exhibited the best MOR activity of 1400 mAmg−1 Pt, which was almost three times of that of Pt/CNTs and 2.5 times of that of Pt/CJM. In addition, PtP/CNTs and PtNi/CNTs performed better activity in comparison with Pt/CNTs and Pt/C-JM, but their MOR activities were largely inferior to that of Pt/Ni2 P/CNTs. Therefore, the

X. Li et al. / Applied Surface Science 434 (2018) 534–539

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Fig. 3. HTEM (a) and Elemental mapping images (b–e) analysis of the 10%Pt-6%Ni2 P/CNTs (g) catalyst, and HTEM image (f) of Ni2 P/CNTs. The inset is the size distribution of Ni2 P/CNTs. Table 2 Pt particle sizes of Pt/CNTs and Pt/Ni2 P/CNTs with different Ni2 P loading. Sample

Pt particle size/nm

Ni2 P loading amount/%

Pt/CNTs Pt/1%Ni2 P/CNTs Pt/6%Ni2 P/CNTs Pt/30%Ni2 P/CNTs

9.5 5.4 1.7 3.3

– 1 6 30

enhancement effect of Ni2 P was much better than P or Ni alone. The stability of several catalysts was compared with Pt/Ni2 P/CNTs by chronoamperometry. Pt/Ni2 P/CNTs catalyst exhibited the highest onset current density and steady current density, as showed in Fig. 4D inset, indicating that it has the best activity and durability. Fig. 5 shows the Pt 4f XPS spectrum of Pt/CNTs and Pt/Ni2 P/CNTs. Compared with Pt/CNTs, the Pt 4f peaks of Pt-Ni2 P/CNTs with 1%, 6% and 30% Ni2 P were shifted positively about 0.3 eV, 0.6 eV and 0.1 eV, respectively. The Pt particle sizes of the catalysts were listed in Table 2. In Chang’s work, they using Ni2 P to promote the MOR activity of Pt which supported on Carbon powder XC-72, and they found the Pt particle sizes were similar with different Ni2 P loading. The promotion is at least partially due to a strong electronic interaction between Ni2 P and Pt, resulting in a partial electron transfer from Ni2 P to Pt [7]. It is well known that the binding energies of Pt 4f peaks increased with the decreasing of particle size [13–15].

Therefore, the Pt 4f peaks shifted to a higher binding energy while the Pt particle size decreasing from 9.5 nm for Pt/CNTs to 5.4 nm for Pt/1%Ni2 P/CNTs and then 1.7 nm for Pt/6%Ni2 P/CNTs. Additionally, the Pt particle size was 3.3 nm when the Ni2 P landing amount reached 30%, smaller than the Pt particle size of Pt/1%Ni2 P/CNTs. However, its Pt 4f peaks shifted to a lower binding energy compared with Pt/1%Ni2 P/CNTs catalyst, and this shift can be attributed to the strong electronic interaction between Ni2 P and Pt, suggesting that there was partial electron transfer from Ni2 P to Pt [7]. The size diminishing was the major factor that affected the binding energy when the Ni2 P loading was low, but the electronic interaction between Ni2 P and Pt become more and more obviously when the Ni2 P loading was 30%. Beyond our expectation, the Ni2 P incorporation not only altered the electronic distribution of Pt but also changed the morphology of Pt resulting in a uniform distribution, which was similar with the effect of RuO2 to Pt/CNTs [16].

4. Conclusions In summary, a novel Pt/Ni2 P/CNTs electro-catalyst was developed for methanol oxidation. The incorporation of Ni2 P as co-catalyst has significant positive influence on the activity and durability of Pt/Ni2 P/CNTs, and the optimal Ni2 P loading amount for Pt/Ni2 P/CNTs catalyst was 6% with a MOR activity as high as

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-2

Ni2P/CNTs 0.05

i/mA

0.00

b a c

-0.05 -0.10 -0.15 -0.2

0.0

0.2

0.4

0.6

0.8

1.0

d e

E/V

0 -0.2

0.0

Pt

0.10

10%Pt/Ni2P/CNTs-x

j/Acm

a: x=1% b: x=6% c: x=12% d: x=17% e: x=30%

A

0.2

0.4

0.6

0.8

4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 -0.5 -1.0

10%Pt/xNi2P/CNTs

B x=1% x=6% x=12% x=17% x=30%

1.0

-0.2

0.0

0.2

2000

C

0.4

0.6

0.8

1.0

E/V

E/V

Pt/CNTs Pt/Ni2P/CNTs

D

10%Pt/xNi2P/CNTs

Pt/C-JM PtP/CNTs PtNi/CNTs

1400

x=1% x=6% x=12% x=17% x=30%

1500

1000

1000

j/mAmg

j / mAmg

-1 Pt

-2

Pt

1200

800 600 400 0

200

400

600

n/cycles

800

1000

500

0

Fig. 4. A) Cyclic voltammograms (MOR) of Pt/Ni2 P/CNTs with different Ni2 P loadings. The inset is the cyclic voltammograms (MOR) of Ni2 P/CNTs. B) Electrical surface area activity of Pt/Ni2 P/CNTs with different Ni2 P loadings. C) comparative Nyquist plots for the different catalysts in 1 M HClO4 containing 1 M CH3 OH solution at 0.5 V. D) Cyclic voltammograms of Pt/CNTs, Pt/Ni2 P/CNTs, Pt/C-JM, PtP/CNTs and PtNi/CNTs. The inset is the corresponding methanol oxidation stability.

Pt 4f7/2

References

Pt 4f5/2 Pt/CNTs

a

Pt/1% Ni2P/CNTs

b

Pt/6% Ni2P/CNTs

c

Pt/30% Ni2P/CNTs

d

Fig. 5. Typical Pt 4f XPS spectra of Pt/CNTs (a) and Pt/Ni2 P/CNTs with 1% (b), 6% (c) and 30% (d) Ni2 P loading.

1400 mAmg−1 Pt. This work may further push forward the research on high efficiency and stability of Pt-based catalyst for direct methanol fuel cells. Acknowledgments We acknowledge financial support from the NSFC (No. 21376257, 21373091) and the Fundamental Research Funds for the Central Universities (No. 2015ZP021).

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