Fabrication of CNTs with controlled diameters and their applications as
electrocatalyst supports for DMFC
i
An-Ya Lo
a,b, Ningya Yu
a,1, Shing-Jong Huang
c, Chin-Te Hung
a, Shou-Heng Liu
a,2, Zhibin Lei
a,3,
Cheng-Tzu Kuo
d,⁎
, Shang-Bin Liu
a,e,⁎⁎
a
Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan b
Department of Material Science and Engineering, National Chiao Tung University, Hsingchu 30010, Taiwan c
Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan d
Institute of Materials and System Engineering, Ming Dao University, Changhua 52345, Taiwan eDepartment of Chemistry, National Taiwan Normal University, Taipei 11677, Taiwan
a b s t r a c t
a r t i c l e i n f o
Available online 19 January 2011 Keywords:
Carbon nanotube Templating synthesis Chemical vapor deposition Hexagonal mesoporous silica Electrocatalyst
Direct methanol fuel cell
A facile synthesis procedure based on chemical vapor deposition (CVD) process has been developed to fabricate carbon nanotubes (CNTs) with controlled diameters and high yields utilizing Fe-containing ordered hexagonal mesoporous silicas (HMSs) such as MCM-41 and SBA-15 having varied pore sizes as the catalysts as well as the templates. It is found that unlike Fe/HMS catalysts prepared by co-precipitation method, samples prepared by the impregnation method gave rise to multi-wall CNTs with uniform diameters, which were largely dictated by the pore size of the Fe/HMS catalysts. Among these uniform MWCNTs, sample with a larger diameter (≥8 nm) was found to be more favorable as support for Pt catalyst, leading to a homogeneous dispersion of metal nanoparticles. Consequently, the Pt/CNT electrocatalysts so prepared gave rise to superior methanol oxidation activities as well as tolerances for CO poisoning compared to Pt supported on commercial single-wall CNT (Pt/SWCNT) and XC-72 activated carbon (Pt/XC-72) having a similar metal loading.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Recent developments in fabrication of porous carbon supports with high surface areas and controllable morphologies have received considerable attention in R&D of supported electrocatalysts for direct methanol fuel cells (DMFCs) and proton-exchange membrane fuel cells (PEMFCs), which have been considered as the most prominent
candidates for next-generation portable power sources[1–4]. Highly
dispersed noble metal (Pt, Ru) nanoparticles (NPs) supported on conductive materials with high surface areas, such as carbon blacks
[5,6], ordered mesoporous carbons (OMCs) [7–13], and carbon
nanotubes (CNTs)[14–18], are pertinent anodic/cathodic
electrocata-lysts for DMFCs and PEMFCs. Among them, CNTs have received
considerable attention due to their unique one-dimensional nanostruc-ture and superior electrical conductivity. Aside from the most common electrocatalysts for DMFCs, such as Pt/Ru supported on commercial Vulcan XC-72 carbon black, many attempts have been made utilizing
CNTs as catalyst supports[14–23]. However, while majority of past
research efforts have been devoted in optimizing the metal dispersion on single-wall (SW) and multi-wall (MW) CNTs aiming to promote their electrocatalytic performances and durability, practically no report had been focused on the diameter size of the CNT supports.
In general, the methodologies invoked in controlling the diameter of CNTs during chemical vapor deposition (CVD) process may be
classified into two main categories, namely by controlling the
processing parameters and by employing an auxiliary template. For the former, it has been reported that parameters such as the carrier
gas/carbon source flow rate, plasma intensity, morphology of the
catalyst, precursor compositions, and duration of treatment etc. have
considerable effects on the diameter of thefinal CNT products[24–
28]. In contrast to such sophisticated adjustment of processing
parameters, the use of an auxiliary template appears to be more advantageous in fabricating CNTs with tailorable diameters. For
examples, zeolites[29]and anodic metal oxides[30–36]have been
utilized as hard templates during the CVD process to fabricate CNTs with uniform diameters. In this case, the diameters of the CNTs so synthesized are largely dictated by the pore size of template used.
However, since it is rather difficult to prepare anodic metal oxides
Diamond & Related Materials 20 (2011) 343–350
i Presented at NDNC 2010, the 4th International Conference on New Diamond and Nano Carbons, Suzhou, China.
⁎ Corresponding author. Fax: +886 4 8879050.
⁎⁎ Correspondence to: S.-B. Liu, Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan. Fax: + 886 2 23620200.
E-mail addresses:kuoct@mdu.edu.tw(C.-T. Kuo),sbliu@sinica.edu.tw(S.-B. Liu). 1
Present address: Institute of Fine Catalysis and Synthesis and Key Lab of Sustainable Resources Processing and Advanced Materials of Hunan Province, Hunan Normal University, Changsha 410081, China.
2
Present address: Department of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 80778, Taiwan.
3
Present address: Department of Chemical & Biomolecular Engineering, National University of Singapore, Singapore.
0925-9635/$– see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2011.01.002
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Diamond & Related Materials
j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / d i a m o n dwith uniform pore sizes less than 25 nm, majority of the CNTs so
fabricated possess diameters exceeding 25 nm [36]. On the other
hand, CNTs fabricated by using microporous zeolites as templates mostly have diameters less than 1 nm. In view of the fact that the diameter of CNT is one of the key parameters affecting their physical properties, it is highly desirable to develop a facile synthesis route to
fabricate CNTs with tunable diameters within the range of 1–25 nm.
Ordered hexagonal mesoporous silicas (HMSs), especially those possessing straight mesoporous channels with uniform pore sizes in
the range of 2–50 nm, such as MCM-41[37]and SBA-15[38,39]seem
to represent the ideal templates to meet the aforementioned demand
[40]. It is worth mentioning that although various ordered
mesopor-ous silicas have been invoked for the preparation of CNTs, they were mostly employed as supports to disperse or to limit the size and
structure of the catalytic metal nanoparticles[41–48], overlooking the
templating function of the porous substrates. We report herein a facile synthesis route to synthesize CNTs with tunable diameters and high yields by using Fe-containing HMSs as catalyst templates. The resultant CNTs with varied diameters were used as supports to prepare various Pt/CNT anodic electrocatalysts for DMFC applications and their catalytic performances during methanol oxidation reaction (MOR) were evaluated and compared with a Pt/XC-72 catalyst (12.5 wt.% Pt on Vulcan XC-72).
2. Experimental
2.1. Preparation of Fe-containing HMSs
Three types of ordered HMSs, namely MCM-41, SBA-15, and pore expanded SBA-15 (denoted as PE-SBA-15) were synthesized by
known recipes reported previously [37–39,49]. Iron catalyst was
loaded onto the HMSs via either co-precipitation or impregnation
methods. For the former, typically ca. 0.4 g of Fe(NO3)3was stirring
with 1.0 g of the target HMS material for 0.5 h in deionized water
(20 mL), followed byfiltering and drying at 373 K, then subjected to
reduction treatment under H2 at 773 K for 3 h. The Fe-containing
HMSs so obtained from the siliceous MCM-41 and SBA-15 are denoted as Fe(co)/MCM-41 and Fe(co)/SBA-15, respectively. In the case of loading Fe catalyst by the impregnation method, proper amount of Fe
(NO3)3(ca. one-half pore volume of the corresponding support) was
dissolved in 20 mL deionized water, followed by adding 1.0 g of the target HMS. After being stirred for 0.5 h, the suspension was dried under vacuum. The obtained product was further stirred in presence of dichloromethane (CH2Cl2) to facilitate migration of Fe precursors
into the hydrophilic channels of the HMSs[50], followed by removal
of the CH2Cl2 solvent by evacuation. The above procedure was
repeated once and the final product was subjected to reduction
treatment carried out by first slowly ramping (2 K/min) the
temperature to 373 K under dried Ar, kept at the same temperature
for 3 h, followed by reduction under H2environment before a mixture
of acetylene (C2H2) and hydrogen (H2) was injected for CNTs growth. The Fe-containing HMSs so derived from the siliceous MCM-41, SBA-15, and PE-SBA-15 are denoted as Fe(im)/MCM-41, Fe(im)/SBA-SBA-15, and Fe(im)/PE-SBA-15, respectively.
2.2. Fabrication of CNTs with uniform diameters
CNTs with varied diameters were prepared by a CVD method similar to that reported earlier for the nano-sized tubular carbons (i.e., CMTs)
[51]. In brief, the syntheses were carried out in a home-made quartz
reactor using various Fe-containing HMSs as templates. Typically, after loading ca. 0.5 g of the fresh Fe-containing HMS in the reactor, the
system wasfirst gradually heated (1 K/min) to 873 K under vacuum,
followed by injecting a stream of C2H2/H2gas mixture at aflow rate of
50/50 sccm/sccm for 40 min under a pressure of ca. 2 kPa. The resultant
product was stirred with excess aqueous HF solution (1 M, 50% ethonal–
50% H2O) for 24 h to remove the silica template and Fe species, followed
byfiltering and drying under vacuum to obtain the final multi-wall
carbon nanotube (MWCNT) materials. 2.3. Preparation of Pt/CNT electrocatalysts
To explore the effect of tube diameter on the performances of various CNTs as catalyst supports for DMFC at anode, typically ca. 0.2 g of the selected home-made CNT was individually suspended in 10 mL
of H2PtCl6 aqueous solution (0.04 M) at room temperature. After
removing water under reduced pressure, the obtained solid was
treated at 523 K for 0.5 h under H2atmosphere to provoke reduction
Fig. 1. Small-angle XRD patterns of siliceous and Fe-containing HMSs; (a) MCM-41 and (b) SBA-15. 344 A.-Y. Lo et al. / Diamond & Related Materials 20 (2011) 343–350
of PtCl62−to Pt nanoparticles. For comparison purpose, two commer-cial products, namely the Vulcan XC-72 carbon black (Cabot Corp.) and single-wall CNTs (SWCNTs; Nano-C Inc.) were also adopted as supports to fabricate Pt-containing electrocatalysts with similar metal loading via the aforementioned procedures.
2.4. Materials characterization and electrocatalytic tests
Powder X-ray diffraction (XRD) patterns of various samples were
recorded on a Philips X' PERT-Pro-MPD diffractometer using the CuKα
radiation (λ=1.542 Å). All nitrogen adsorption/desorption isotherm
measurements were carried out at 77 K on a Quantachrome Autosorb-1 sorptometer. Prior to the measurement, each sample was outgassed
at 473 K under vacuum (10− 6Torr) overnight. The surface area of
each sample was derived by the BET method, and its pore size distribution (PSD) was calculated from the adsorption branch of the
isotherm using the Barrett–Joyner–Halenda (BJH) method.
Transmis-sion electron microscopy (TEM) studies were performed on a JEOL
JEM-2100 F microscope operated at 200 kV. Each sample was ultrasonicated for 15 min in ethanol before introducing it onto the carbon-coated copper grids. The metal (Fe and Pt) contents in various samples were determined by thermal gravimetric analyses (TGA; Netzsch TG-209).
The electrochemical active surfaces (EASs), electrocatalytic per-formances, and stability of various supported Pt/C catalysts were evaluated on an Autolab PGSTAT30 galvanostat/potentiostat at room
temperature by using a glassy carbon thin-film as the working
electrode, Pt wire as the counter electrode, and Ag/AgCl as the
reference electrode. Typically, the glassy carbon thin-film electrode
was prepared byfirst adding ca. 10 mg of Pt-loaded CNTs sample into
5 mL of deionized water, followed by ultrasonic treatment for 0.5 h.
Next, ca. 20μL of the resultant suspension mixture was withdrawn
and injected onto the glassy carbon electrode (diameter ca. 5 mm),
followed by drying in air at 333 K for 1 h. Finally, ca. 20μL of 1% Nafion
(DuPont) solution was added as a binder under N2 environment.
Prior to each cyclic voltammetry (CV) measurement, the electrolytic
Fig. 2. TEM images of (a) siliceous MCM-41, (b) Fe(co)/MCM-41, (c) Fe(im)/MCM-41, (d) siliceous SBA-15, (e) Fe(co)/SBA-15, and (f) Fe(im)/SBA-15.
345 A.-Y. Lo et al. / Diamond & Related Materials 20 (2011) 343–350
activity than the Pt/XC-72, in excellent agreement with the results obtained from EAS and CV measurements.
4. Conclusions
In summary, a series of Fe-containing hexagonal mesoporous silicas with varied pore sizes, which were prepared either by co-precipitation or impregnation method, have been employed to catalyze formation of CNTs via CVD process. It is found that by
properly confining the metal catalyst in the mesoporous channels of
the silica supports, multi-wall CNTs with uniform and controllable diameters may be facilely synthesized with high yield. The diameter of the MWCNTs is found to play a crucial role in the dispersion of Pt nanoparticles and the electrocatalytic performances of the resulting supported Pt/CNT catalysts during methanol oxidation reaction. These Pt/MWCNTs electrocatalysts, particularly those with diameter ex-ceeding ca. 8 nm were found to exhibit electrocatalytic performances surpassing that of the conventional Pt/XC-72 and Pt/SWCNT catalysts having a similar Pt loading and electrochemical properties of the carbon supports.
Acknowledgment
Thefinancial support of this work by the National Science Council,
Taiwan (NSC98-2113-M-001-007-MY3 to SBL) is gratefully acknowledged.
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0 500 1000 1500 2000 2500 100 200 300 400
M
a
ss Activity (A/g Pt)
Time (s)
Pt/CNT-d17 Pt/CNT-d8 Pt/XC-72 Pt/CNT-d3 Pt/SWNTFig. 8. CA curves for Pt/CNT-d17, Pt/CNT-d8, Pt/CNT-d3, Pt/SWCNT, and Pt/XC-72 samples in 0.5 M H2SO4+ MeOH at 0.6 V.