C electrocatalyst for direct methanol fuel cells

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available at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/carbon

Improving the activity and stability of a Pt/C electrocatalyst for direct methanol fuel cells Jing Qi a b

a,b

, Shiyou Yan

a,b

, Qian Jiang

a,b

, Ying Liu a, Gongquan Sun

a,*

Direct Alcohol Fuel Cell Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China Graduate School of the Chinese Academy of Science, Beijing 100039, China

A R T I C L E I N F O

A B S T R A C T

Article history:

Ketjen Black (KB) as an electrocatalyst support was treated at 900 C in the presence of

Received 23 July 2009

cobalt and nickel nitrates, and characterized by X-ray diffraction, energy dispersive X-ray

Accepted 28 August 2009

spectroscopy, transmission electron microscopy and nitrogen adsorption measurement.

Available online 4 September 2009

The treated KB (T-KB) exhibits better graphitization and a larger mesopore volume than the untreated material. A Pt electrocatalyst supported on T-KB was prepared by a modified polyol process. Cyclic voltammetry and single cell tests show that the Pt/T-KB electrocatalyst exhibits better electrochemical activity and stability than a Pt/KB electrocatalyst.  2009 Elsevier Ltd. All rights reserved.

1.

Introduction

Direct methanol fuel cells (DMFCs) have been receiving extensive research as alternative power sources for both stationary and mobile applications. However, the activity and stability of electrocatalysts still remain critical issues to be resolved for the commercialization of DMFCs [1,2]. Pt supported on carbon supports is the most feasible electrocatalyst for DMFCs. The characteristics of carbon supports are of considerable importance for improving the activity and stability of electrocatalysts. An ideal carbon support for electrocatalyst is supposed to possess high surface area to allow high dispersion of the active component, high graphitization degree to offer good electrical conductivity and corrosion resistance, and appropriate pore structure to reduce the transfer resistance of reactants, intermediates, and products [3]. At present, carbon blacks are the most widely used electrocatalyst supports for DMFCs. However, carbon blacks are amorphous with low graphitization degree, which are prone to undergo electrochemical corrosion. Moreover, they have a lot of micropores within the structure, which reduces the Pt utilization because the reactants are inaccessible to Pt particles anchored in the micropores and also increases the transfer resistance of reactants, intermediates, and products.

Graphitic carbon materials with mesopores can be prepared at relatively low temperatures (<1000 C) by the addition of graphitization catalysts (Fe, Co, Ni, etc.) [3–7]. In this study, carbon black was treated in the presence of transition metal nitrate at 900 C for improving the graphitization degree, optimizing the pore structure, in order to improve the activity and stability of the Pt electrocatalyst supported on the carbon black.

2.

Experimental

2.1.

Treatment of carbon supports

The overall treatment procedure used for Ketjen Black (KB) is shown as follows: Firstly, the required amounts of KB, Co(NO3)2Æ6H2O, and Ni(NO3)2Æ6H2O (with weight ratio of 1:2:2) were added to ethanol with ultrasonic stirring for 30 min to form homogeneous slurry. After removing the ethanol by rotating evaporation at 60 C, the resulting mixture was then heat-treated under a nitrogen atmosphere at 900 C for 3 h. The obtained composite was stirred in 5 mol L 1 HCl aqueous solution at 100 C for 6 h, filtered, washed, and dried at 100 C for 8 h in an oven. Finally the treated KB (T-KB) was obtained.

* Corresponding author: Fax: +86 411 8437 9063. E-mail address: [email protected] (G. Sun). 0008-6223/$ - see front matter  2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2009.08.044

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For comparison, KB was treated by the same procedure as that of T-KB without the addition of cobalt and nickel nitrates, and the obtained sample was denoted as C-KB.

was fed into the anode side at a flow rate of 1.0 ml min 1 while the oxygen of 0.2 MPa pressure was maintained at the cathode side. The active area of the single cell was 2 · 2 cm2.

2.2.

3.

Results and discussion

3.1.

Characterization of carbon supports

KB and T-KB were used as carbon supports for the preparation of electrocatalysts. In brief, the required amounts of chloroplatinic acid (H2PtCl6Æ6H2O) and the carbon support were added to ethylene glycol with stirring to obtain homogeneous slurry. The slurry was then maintained at 130 C for 3 h. The obtained black mixture was filtered, washed, and dried at 75 C for 8 h in a vacuum oven. The electrocatalysts with metal loading of 40 wt.% were obtained and denoted as Pt/KB and Pt/T-KB.

2.3.

Physical characterization

X-ray diffraction (XRD) patterns were recorded on a Rigaku D/ max-2400 X-ray diffractometer using Cu Ka radiation. The tube voltage and the tube current were maintained at 40 kV and 100 mA, respectively. The scan rate was 5 min 1, and the step size was 0.02. Energy dispersive X-ray spectroscopy (EDX) analysis was carried out on a FEI Quanta 200F scanning electron microscope equipped with energy dispersive spectrometer. The accelerating voltage and the point resolution were 20 kV and 2 nm, respectively. Transmission electron microscopy (TEM) was carried out using a JEOL JEM-2011EM microscope operated at 100 kV, while maintaining the resolution at 0.143 nm. Nitrogen adsorption measurements were performed at 196 C using a Micromeritics ASAP 2000 volumetric adsorption system.

2.4.

Electrochemical measurement

The electrochemical measurements were conducted using an EG&G model 273A potentiostat/galvanostat and a three-electrode test cell. A Pt-foil and a saturated calomel electrode were employed as counter and reference electrode, respectively. A thin porous coating glassy carbon disk electrode with a diameter of 5 mm was used as working electrode. Typically, 5 mg sample was ultrasonically suspended in 1 ml ethanol and 50 lL 5 wt.% Nafion solution (Du Pont, USA) for 30 min to form homogeneous ink. Then 25 lL of the ink was spread onto the surface of the glass carbon electrode with a micropipette and was then dried under infrared lamp. Cyclic voltammetry (CV) was carried out in 0.5 mol L 1 H2SO4 aqueous solution at room temperature. All potentials in this work were versus normal hydrogen electrode (NHE).

2.5.

According to [5], a mixture of cobalt and nickel nitrates was used as the graphitization catalysts. Fig. 1 shows the XRD pattern of KB heat-treated at 900 C with cobalt and nickel nitrates before acid treatment. It can be seen that the sample exhibits intense diffraction peaks at 44.3, 51.8 and 76.2, which correspond to the (1 1 1), (2 0 0) and (2 2 0) diffractions of the face centered cubic structure of Ni and Co metal particles, respectively, according to the JCPDS standard data (JCPDS 04-0850) and [9,10]. When KB impregnated with cobalt and nickel nitrates is heat-treated under an inert atmosphere, the nitrates decompose into the corresponding metallic oxides, which are subsequently reduced to metal particles

Intensity (a.u.)

Preparation of Pt electrocatalysts

10

20

30

40 50 60 2 Theta (degree)

70

80

90

Fig. 1 – XRD pattern of KB heat-treated at 900 C with cobalt and nickel nitrates before acid treatment.

Single cell test

The single cell tests of Pt/KB and Pt/T-KB were carried out under the same operation conditions. The membrane electrode assembly was fabricated according to the method in the literature [8]. The 45 wt.% PtRu/C (Johnson Matthey Inc.) with metal loading of 2.0 mg cm 2 was used in the anode, and the cathode comprised the 40 wt.% Pt/KB or 40 wt.% Pt/T-KB with Pt loading of 1.0 mg cm 2. A Nafion-115 membrane was used as the solid electrolyte. 1.0 mol L 1 CH3OH aqueous solution

Fig. 2 – XRD patterns of KB (a), C-KB (b) and T-KB (c).

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Fig. 3 – SEM and EDX results of T-KB.

Fig. 4 – TEM images of KB (a) and T-KB (b). (Co, Ni). So it appears that, during the heat treatment, graphitic nanoparticles are generated around the in situ generated metal particles [7]. The XRD patterns of KB, T-KB and C-KB are shown in Fig. 2. The XRD results confirm that T-KB exhibits better graphitization than KB according to the intensity of (0 0 2) diffraction peak of it, which is much sharper and more intense than that of KB. This suggests that the in situ generated metal particles (Co, Ni) can provide crystal nuclei for graphitic structure growth. The small diffraction peaks at 44, 51 and 76 may correspond to the graphitic framework or the incomplete removal of residual metal particles since the diffraction peaks are so close to each other. Moreover, the intensity of (0 0 2) diffraction peak of C-KB is similar to that of KB, indicating that the use of transition metal nitrate is crucial for the improvement of graphitization degree of carbon black. The EDX analysis for T-KB was carried out on a scanning electron microscope equipped with energy dispersive spectrometer. As shown in Fig. 3, only the C and O elements can be detected. No Co or Ni element has been observed, indicating the complete removal of residual metal particles after acid treatment. TEM measurement was performed to investigate the effect of treatment on the morphology of the carbon black. As shown in Fig. 4, KB shows a spherical morphology with an

average diameter of 40 nm. While for T-KB, it is observed that carbon nanocages with a diameter of 30–40 nm are formed after the treatment. Fig. 5 shows the N2 adsorption and desorption isotherms and the corresponding pore size distribution curves of KB, C-KB and T-KB. They all show typical type IV isotherms with hysteresis loop, characteristic of mesoporous structure. As shown in Fig. 5d, the mesopore volume and the average pore size of T-KB are larger than that of KB. In contrast, KB and CKB have the similar textural properties. The increase of the mesopore volume of T-KB may be attributed to the nanocavities in the matrix, which are generated by removing the in situ generated Co and Ni nanoparticles. The corresponding N2 adsorption and desorption results are listed in Table 1. Although the Brunauer–Emmett–Teller (BET) surface area of T-KB is smaller than KB, it is still large enough for the loading of Pt nanoparticles. Thus, with better graphitization, appropriate pore structure, and large surface area, T-KB is expected to be a promising electrocatalyst support. For the electrochemical oxidation, a constant potential of 1.2 V was applied on KB and T-KB electrodes. Fig. 6 shows the CV curves of KB and T-KB electrodes recorded after holding at 1.2 V in 0.5 mol L 1 H2SO4 aqueous solution for different time intervals. As shown in Fig. 6, the increased peak current comes from the surface oxide formation on the support sur-

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a

1000

adsorption desorption

3 -1

Volume adsorbed (cm g )

3 -1 Volume adsorbed (cm g )

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adsorption desorption

800 600 400 200

0.0

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800 600 400 200

0.8

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T-KB KB C-KB

5 dV/dlog(D),cm g

3 -1 Volume adsorbed (cm g )

b

4 3 2 1 0

0 0.0

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0.4 0.6 Relative pressure (P/Po)

0.8

10

1.0

100

pore diameter(D), nm

Fig. 5 – N2 adsorption and desorption isotherms of KB (a), C-KB (b), T-KB (c) and the corresponding pore size distribution curves (d) calculated from the desorption branch of the nitrogen isotherm by Barrett–Joyner–Halenda method.

Table 1 – Comparison of N2 adsorption and desorption results of KB, C-KB and T-KB. SBET (m2 g 1)

Sample KB C-KB T-KB

Vmacro (cm3 g 1)

905 920 635

Vmeso (cm3 g 1)

0.32 0.36 0.36

Pore diameter (nm)

1.07 1.02 1.26

5 5 7

3

a

b

-2

Current density (mAcm )

1

-2

Current density (mAcm )

2 1 0 -1

0h 4h 8h 20 h

-2 -3 -4

0.0

0.2

0.4 0.6 0.8 1.0 Potential ( V vs. NHE)

1.2

0

0h 4h 8h 20 h

-1

-2 0.0

0.2

0.4 0.6 0.8 1.0 Potential (V vs. NHE)

1.2

Fig. 6 – CV curves of KB (a) and T-KB (b) at different time intervals during electrochemical oxidation in 0.5 mol L solution at a scan rate of 20 mV s 1.

1

H2SO4

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face [11,12]. It is clear that the peak current density of KB increases significantly (Fig. 6a). In contrast, the peak current density of T-KB increases slightly (Fig. 6b). The above results indicate that T-KB is more resistant to electrochemical oxidation than KB.

3.2.

Characterization of the electrocatalysts

To study the activity and stability of Pt/KB and Pt/T-KB, CV was carried out on Pt/KB and Pt/T-KB in 0.5 mol L 1 H2SO4 aqueous solution repeatedly between 0 and 1.2 V at a scan rate of 100 mV s 1. The electrochemical surface area (ECSA) was determined after 50, 200, 400, 600, 800, 1000, 1300, 1500, 1800, 2000, 3000 cycles and plotted in Fig. 7. It can be seen that the ECSA loss of Pt/T-KB is less than that of Pt/KB, indicating

Pt/T-K B Pt/K B

65

2 -1

ECSA (m g )

70

60 55 50 0

500

1000 1500 2000 N um ber of cycle

2500

3000

Fig. 7 – Effect of electrochemical cycling on ECSA of Pt/KB and Pt/T-KB.

167

that the higher graphitization degree of T-KB inhibits it from electrochemical corrosion and thus avoids the agglomeration of Pt nanoparticles. The TEM images and the corresponding histograms of the particle size distribution of Pt/KB and Pt/T-KB before and after the electrochemical cycling are shown in Figs. 8 and 9. Before electrochemical cycling, the spherical Pt nanoparticles on the carbon support are uniform and well distributed for both Pt/ KB and Pt/T-KB (Fig. 8a and c). The Pt/KB and Pt/T-KB have the same average particle size and the particle size distributions are both quite narrow (Fig. 9a and c). However, after 3000 cycles electrochemical cycling, some aggregation of Pt nanoparticles is noticeably observed in most regions of Pt/ KB (Fig. 8b). The particle size distribution becomes broader and the average particle size gets larger (Fig. 9b). In contrast, the particle size of Pt/T-KB just slightly increases, although some aggregation of Pt nanoparticles is also observed in some regions. These results show that the agglomeration of Pt/KB seems to be more serious than that of Pt/T-KB, which is consistent with the effect of electrochemical cycling on ECSA of Pt/KB and Pt/T-KB. More importantly, as shown in Fig. 8b, the morphology of the carbon support for Pt/KB greatly changes after the electrochemical cycling. The particle diameter of KB increases greatly after the 3000 cycles electrochemical cycling and its surface becomes much smoother, indicating the agglomeration and corrosion of carbon particles. In contrast, the morphology of T-KB hardly shows any change. The carbon support in fuel cell operating conditions is known to undergo extremely harsh corrosive conditions, which include high acidity, high potential, high water con-

Fig. 8 – TEM images of Pt/KB before (a) and after (b) electrochemical cycling, Pt/T-KB before (c) and after (d) electrochemical cycling.

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18

a

d = 2.6 nm

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3 4 5 Particle size (nm)

6

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8

Fig. 9 – Histograms of the particle size distribution of Pt/KB before (a) and after (b) electrochemical cycling, Pt/T-KB before (c) and after (d) electrochemical cycling. Pt/KB drops sharply, from initial 0.55 to 0.39 V, exhibiting a much faster decrease rate than that of Pt/T-KB. This result is consistent with the trend observed for the ECSA change in the above electrochemical cycling test. The improved stability of Pt/T-KB in the single cell durability test can be attributed to the better graphitization of T-KB, which inhibits it from corrosion in DMFC operating conditions and thus improves the stability of the supported electrocatalyst.

0.8

180 Pt/T-KB Pt/KB

Cell voltage (V)

0.7

160 140 120

0.6

100 0.5

80 60

0.4

40 20 0

0.2 -100

-2

0.3

Power density (mWcm )

tent, high temperature, and high oxygen concentration. In addition, the presence of metal Pt accelerates the corrosion rate of the carbon support [13]. Once the carbon support is oxidized, Pt may be detached from the support. The ‘‘freefloating’’ detached Pt nanoparticles have a strong tendency to agglomerate due to their high specific surface energy, which results in the decrease of ECSA and accordingly the activity degradation of electrocatalysts. Carbon materials with higher graphitization degree show much better stability [14]. The better graphitization of T-KB inhibits it from electrochemical corrosion and thus avoids the agglomeration of Pt nanoparticles. KB contains mainly amorphous carbon with abundant of defects and thus undergoes more serious electrochemical oxidation than T-KB. DMFC single cell performance with Pt/KB and Pt/T-KB as cathode electrocatalysts is shown in Fig. 10. Noticeably, the output voltage of the single cell with Pt/T-KB gently decreases, with a maximum output power density of approximately 163 mW cm 2. In contrast, the output voltage of the single cell with Pt/KB drops sharply and the maximum output power density is merely 145 mW cm 2. It can be seen that the single cell with Pt/T-KB shows better performance, which may be attribute to the appropriate pore structure of T-KB for facilitating the transfer of reactants, intermediates, and products. Single cell durability test was performed at the current density of 100 mA cm 2 and the result is shown in Fig. 11. During the continuous 66 h test, the output voltage of the single cell with Pt/T-KB gently decreases, from initial 0.55 to 0.43 V. In contrast, the output voltage of the single cell with

0

100

200 300 400 500 600 -2 Current density (mAcm )

700

-20

Fig. 10 – Performance of the DMFC single cell with 40 wt.% Pt/KB and 40 wt.% Pt/T-KB as cathode electrocatalysts. Cell temperature: 90 C; anode: 1 mol L 1 CH3OH; cathode: O2 at pressure of 0.2 MPa.

Cell voltage (V)

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R E F E R E N C E S

0.6

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0.5 Pt/T-KB 0.4 Pt/KB 0.3

0.2

0

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30 40 Test time (h)

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Fig. 11 – Durability test of the DMFC single cell with 40 wt.% Pt/KB and 40 wt.% Pt/T-KB as cathode electrocatalysts. Discharging current density: 100 mA cm 2; cell temperature: 75 C; anode: 1 mol L 1 CH3OH; cathode: O2 at pressure of 0.2 MPa.

4.

169

Conclusions

KB as an electrocatalyst support was treated at 900 C in the presence of cobalt and nickel nitrates. The Pt/T-KB electrocatalyst exhibits better electrochemical activity and stability than the Pt/KB electrocatalyst, due to the more appropriate pore structure and the better graphitization of T-KB. The more appropriate pore structure of T-KB facilitates the transfer of reactants, intermediates, and products. The better graphitization of T-KB inhibits it from electrochemical corrosion and thus improves the stability of the supported electrocatalyst. This treatment method provides an efficient route to improve the graphitization degree and optimize the pore structure of carbon blacks which could be used as promising carbon supports of electrocatalyst.

Acknowledgments This work was financially supported by Hi-Tech Research and Development Program of China (2007AA05Z159) and National Natural Science Foundation of China (Grant No. 20803078).