Modification of multi-walled carbon nanotubes by microwave digestion method as electrocatalyst supports for direct methanol fuel cell applications

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Modiﬁcation of multi-walled carbon nanotubes by microwave

digestion method as electrocatalyst supports for direct methanol

fuel cell applications

Chien-Chung Chen

*

_{, Chia-Fu Chen, Chieng-Ming Chen, Fang-Tzu Chuang}

Department of Materials Science and Engineering, National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu 300, Taiwan, ROC Received 31 May 2006; accepted 13 June 2006

Available online 2 October 2006

Abstract

Multi-walled carbon nanotubes (MWCNTs) which were directly synthesized on carbon cloth were modiﬁed by a microwave digestion method in 5 M HNO3for supporting Pt nanoparticles. The characterizations of modiﬁed CNTs were carried out by TEM, XPS, FTIR

and Raman spectroscopy. The HRTEM image shows the caps of MWCNTs are opened after modifying by microwave digestion method. The open-end and undamaged MWCNTs can provide a larger surface area for supporting more catalysts. Furthermore, the methanol electrocatalytic oxidation of microwave digestion treated Pt/MWCNTs electrode shows higher current density than pristine and nitric acid-treated MWCNTs from cyclic voltammograms. This can be an eﬀective and undamaged method for modifying CNTs.

Keywords: Multi-walled carbon nanotubes; Microwave digestion; Direct methanol fuel cell; Catalyst supports; Polyol method

1. Introduction

In the past decades, carbon nanotubes (CNTs) are widely applied in many fields for their high surface area, high electrical conductivity, good chemical stability and significant mechanical strength[1]. Due to the high surface areas of its outside walls and central hollow structure; CNTs can be used as catalyst supports and gas storage media for a variety of applications in fuel cell [2–5]. Pre-cious metals, such as Pt and Ru, are dispersed on carbon nanotubes and resulting materials for DMFC applications showed good electrocatalytic behaviors [6–8]. Unfortu-nately, the surface of pristine CNT is chemically inert and hydrophobic, which is unfavorable for supporting cat-alyst. Therefore, the modification of the surface of CNT with functional groups is essential for further applications. Nitric acid which is a strong oxidant is commonly used for

functionalizing the CNT surface. Generally, electrocata-lysts which supported on functionalized CNTs are synthe-sized by two steps. Raw CNTs were first refluxed with nitric acid and then metal precursors attached to function-alized CNTs surfaces [9]. The step of refluxing CNTs in nitric acid not only opens the caps of CNTs, but function-alizes the surfaces of CNTs with carboxylic groups. How-ever, conventional nitric acid treatments for modifying CNTs usually need to reflux CNTs for a long time and sometimes damage the structures of CNTs seriously.

Here, we report an efficient and mild method using microwave digestion system for modifying CNTs in a short time, which could be widely used in modification or purifi-cation of CNTs for fuel cell applipurifi-cations.

2. Experimental

2.1. Modiﬁcations of MWCNTs

Multi-walled carbon nanotubes were directly synthe-sized on the iron deposited carbon cloth (MWCNTs/CC)

*

Corresponding author. Tel.: +886 3 571 2121x55346; fax: +886 3 572 4727.

E-mail address:[email protected](C.-C. Chen).

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by bias assisted microwave plasma enhanced chemical vapor deposition system. The details of synthesis processes of MWCNTs are described in our previous work[10]. After that, MWCNTs were modified utilizing a conventional nitric acid-treatment and a microwave digestion method. The MWCNTs immersed in 14 M HNO3 solution and refluxed at 80°C for 18 h. And then, the sample was fil-tered with 0.1 mm PTFE (poly-(tetrafluoroethylene)) mem-brane in deionized water as the conventional part. On the other hand, an alternative acidic treatment in microwave digestion system (Milestone Microwave Labstation ETHOSD) was used to modify MWCNTs. 1 cm2 of MWCNTs/CC was placed in a 100 ml Pyrex digestion tube. The first digestion step was to heat the system from room temperature to 210°C within 20 min with 5 M HNO3. The microwave power was set at 100 W. The sec-ond digestion step was to keep the temperature at 210°C for 30 min. After digestion; the sample was also filtered with 0.1 mm PTFE membrane in deionized water. The total modification time by microwave digestion method was less than 1 h. More information of the microwave digestion method can be found in our previous works [11–13].

2.2. Preparation and Dispersion of Pt nanoparticles on MWCNTs/CC

The Pt nanoparticles were synthesized by the polyol pro-cesses and then dispersed on MWCNTs/CC sample. Initial materials included metallic complex salts of H2PtCl6Æ6H2O (SHOWA, 98 wt%), polymer protector agent PVP-40 [MW38 000–40 000, poly(vinyl pyrroli-done)) (Sigma, 99 wt%)], and the solvent Ethylene Glycol (E.G.) (SHOWA, 99.5%). Detail chemical reduction mech-anism of the solvent was introduced in other reports

[14,15]. Pt (IV) metal ion concentration was ﬁxed at

10 mM in a beaker. Then the solution was pre-heated under 160°C for thermodynamic stabilization and reac-tants dipping rate was 2 ml min1till exhausted. After dip-ping, the entire reaction system was kept under 160°C for 3 h under N2ﬂux 25 cm3min1. The whole reaction system was kept under 120 rpm magnetic stirring by temperature programmed hot plate and reaction temperature was kept within ±1°C. After acetone dilution, dark brown gelation precipitate formed as polymer and nanoparticle compos-ites. Residual sample re-dispersed into EtOH and further centrifuged at 15 000 rpm for 5 h. After that, the MWCNTs/CC was directly impregnated into Pt–PVP/ EtOH solution and dried in a furnace under 200°C for removing contaminants.

2.3. Characterization of the MWCNTs electrode

The information of functional groups attached on mod-iﬁed MWCNTs was provided by a Fourier transform infra-red spectroscopy (FT-IR, protege 460). A high resolution transmission electron microscopy (HRTEM, Philips

Tec-nai-20) was used to investigate the nanostructure of MWCNTs. Raman spectroscopy (Jobin Yvon Lab-RAM HR) was used for examining the structure of modiﬁed MWCNTs, using He–Ne laser with a wavelength of 632.8 nm. Electrocatalytic behaviors of Pt/MWCNTs elec-trode was investigated by cyclic voltammetry (CV) (CH Instrument 614B potentiostat) using a three-electrode cell. A platinum wire served as the counter electrode and a sat-urated calomel electrode (SCE), was used as the reference electrode. The working electrode is a Pt wire attached with a 0.25 cm2thin Pt foil, which is used to contact with the testing sample (Pt/MWCNTs).

3. Results and discussion

3.1. The characterizations of Pt/MWCNTs electrode The FTIR spectra of untreated, conventional nitric acid treated and microwave digestion-treated MWCNTs are presented in Fig. 1. The peaks at about 1730 and 1590 cm1on FTIR spectra inFig. 1(b) and (c) suggested that carboxylic acid groups and carboxylate groups and were presented on the surface for both conventional nitric acid-treated and microwave digestion-treated MWCNTs. The IR spectra of conventional nitric acid-treated and microwave digestion-treated MWCNTs show almost the same characteristic, which indicates the surface of micro-wave digestion-treated MWCNTs were functionalized in a short time. XPS of microwave digestion treated MWCNTs is presented in Fig. 2. The C 1 s spectrum appears to be composed of graphitic carbon (284.8 eV) and C@O like species (285.82 eV). A small amount of sur-face functional groups of –CO (286.8 eV) and –COO (290.14 eV) were also noted in the spectrum, which are the evidences of the functionalization of MWCNTs surface after acid-treatment by microwave digestion method. Raman spectra of pristine, nitric acid-treated, and

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wave digestion treated MWCNTs are shown inFig. 3. All samples of MWCNTs show a disorder-induced peak around 1350 cm1, which roughly corresponding to the D-line associated with disorder-allowed zone-edge modes of graphite. After modifications, a significant increase of the intensity of D-band can be observed in Raman spectra. This is because the perfect two-dimensional graphitization is altered to a more disordered structure by carboxylation. The IDto IGratio of these samples was calculated by inte-grating the area of D-band and G-band. The calculated ID/ IG ratios of pristine, nitric acid-treated, and microwave digestion-treated MWCNTs were 1.34, 1.96, and 2.21, respectively. The high ID/IG ratio of MWCNTs indicates more functional groups on the surface of MWCNTs after modifications [16,17]. Fig. 4 depicts the HRTEM images of the nitric acid-treated and the microwave digestion-trea-ted MWCNT. It can clearly be seen that the catalyst

embedded in the tip of the MWCNT was removed in both Fig. 4(a) and (b). It is believed that the nitric acid destroyed the cap of MWCNT ﬁrst and then eliminated the catalyst. However, the surface of nitric acid treated MWCNT was damaged after acidic treatment for 18 h. On the contrary, microwave digestion takes a tremendous advantage because the acid in this approach can absorb microwave energy rapidly and dissolve metal eﬃciently without dam-aging the wall structure and the processing time could be reduced. The catalysts were loaded by impregnated 1 cm2 MWCNTs electrode into 2 ml precursor with 2 mg metal

Fig. 2. XPS of microwave digestion treated MWCNT.

Fig. 3. Raman spectra of: (a) as-grown MWCNTs, (b) MWCNTs treated by 14 M HNO3solution for 18 h and (c) MWCNTs treated by microwave

digestion method in 5 M HNO3solution.

Fig. 4. HRTEM images of open-end MWCNTs which were synthesized: (a) by reﬂuxing in 14 M HNO3for 18 h and (b) by microwave digestion

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loading. However, the actual Pt loading for pristine, nitric acid-treated and microwave digestion-treated Pt/ MWCNTs samples are 0.18, 0.24 and 0.30 mg/cm2, which were determined by burning away the MWCNTs at 900°C in O2 atmosphere. We supposed that the nitric acid-treated MWCNTs by both methods can supported more Pt catalysts because of the functionalized surface and open-end structure of MWCNTs. It is identical with Raman results. TEM micrographs of Pt nanoparticles dis-perse on (a) pristine MWCNTs and (b) microwave diges-tion treated MWCNTs are shown in Fig. 5. The distribution of Pt nanoparticles on pristine MWCNTs is not uniform and large Pt clusters can be found. The signif-icant agglomeration of Pt nanoparticles on MWCNTs is due to the chemically inert and hydrophobic surface of MWCNTs. However, a better dispersion of Pt nanoparti-cles on microwave digestion-treated MWCNTs is shown in Fig. 5(b). Although agglomeration of Pt nanoparticles still exists, the distribution of Pt nanoparticles on acid-trea-ted MWCNTs by microwave digestion is greatly improved. 3.2. Electrochemical properties of Pt/MWCNTs electrode

The electrocatalytic activity of Pt catalyst was tested by cyclic voltammetry (CV) in electrolyte of 1.0 M H2SO4 with a scan rate of 50 mV s1 and is shown in Fig. 6. In the range of the voltage between 0.1 and 0.2 V, several sharp peaks can be shown. Weak H-adsorption and strong H-adsorption peaks are observed during cathodic potential sweep between 0.4 and 0.05 V. The strong, medium and weak H-desorption peaks between 0.4 and 0.05 V are also observed under anodic sweep [18]. By using the charge passed for H-adsorption QH, surface area of Pt (SPt) can be estimated from the equation below[19]

SPt=cm2¼ QH=210 ðlC=cm

2_Þ _ð1Þ

The constant 210 in Eq.(1)is surface charge density of Pt. Each electrode was taken with a triangular potential sweep (50 mV s1) between 0.05 and 1.05 V (vs. SCE) in 1 M H2SO4solution for the determination of the surface

area of Pt. Therefore, the calculated active surface area of Pt fromFig. 4are 5.6, 20.1, and 23.9 cm2for the pristine, nitric acid-treated, and microwave digestion-treated MWCNTs. The microwave digestion-treated Pt/MWCNTs electrode has larger electrochemical Pt surface area than the others, which could be attribute to the more Pt amount attached on the MWCNTs and uniform distribution of Pt nanoparticles. Various electrochemical properties of these samples are listed inTable 1. Table 1 lists various calcu-lated values of Pt loading (LPt, mg), working surface area of Pt (SPt, cm2), methanol oxidation current density at 0.8 V (i, mA cm2) of samples and mass eﬃciency of Pt (Me, mA mg1Pt).

Electrocatalytic activities of Pt/MWCNTs electrodes in methanol solution which were measured in a deaerated 1.0 M CH3OH + 1.0 M H2SO4 solution between 0 and 1.0 V with a scan rate of 50 mV s1 are shown in Fig. 7. Methanol-oxidation current peaks are clearly observed at 0.8 V in the anodic sweep and at 0.6 V in the cathodic sweep from all samples, which represent the reactions of Eqs.(2) and (3) [20].

Fig. 5. TEM images of Pt nanoparticles disperse on: (a) pristine MWCNTs and (b) microwave digestion treated MWCNTs.

Fig. 6. Cyclic votammograms of Pt/CNTs electrode in 1.0 M H2SO4

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Pt–ðCH3OHÞads ! Pt–ðCOÞads þ 4Hþþ 4e ð2Þ Pt–COþ Pt–OH ! 2Pt þ CO2þ Hþþ e ð3Þ The current density of methanol oxidation of microwave digestion-modified Pt/MWCNTs at 0.8 V is 76 mA cm2, which is higher than conventional nitric acid-treated sam-ple and almost 2.5 times as high as the untreated Pt/ MWCNTs sample. The Pt nanoparticles on modified MWCNTs show the higher utility than pristine MWCNTs from the comparison of mass efficiency in Table 1. This may be attributed to the functionalized surface and the presence of open-end MWCNTs, which Pt nanoparticles can uniformly disperse on MWCNTs with larger amount. 4. Conclusion

A fast and eﬀective way for modifying MWCNTs was demonstrated in this paper. TEM images reveal that the modiﬁcation by microwave digestion method can obtain undamaged and open-end MWCNTs. The aggregation of Pt nanoparticles can be greatly improved by functionali-zing the surface of MWCNTs using the microwave diges-tion method. The CVs results show that the microwave

digestion-modiﬁed Pt/MWCNTs electrode exhibits the lar-ger electrochemical Pt surface area and higher current den-sity of methanol oxidation than untreated and conventional nitric acid-treated Pt/MWCNTs electrodes. This technique can be widely used for eﬀective modifying CNTs and shorting the process time.

Acknowledgement

The authors convey their utmost appreciation to T.Y. Chen from NTHU for his technical support and ﬁnancial support of this work by the National Science Council of the Republic of China under Contract No. NSC 94-2218-E-009-010.

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759. Table 1

Electrochemical properties of Pt/MWCNTs with various treatments

Sample LPt (mg cm2) SPt (cm2) i(mA cm2) Me (mA mg1Pt) Pristine MWCNTs 0.18 5.6 28 156 14 M HNO3, 18 h 0.24 20.1 59 246 Mircowave digestion 0.30 23.9 76 253

Fig. 7. Cyclic voltammograms of Pt/CNTs electrode in 1.0 M H2SO4+ 1.0 M CH3OH aqueous solution with a scan rate of 50 mV s1.