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2007, 111, 5557-5560 Published on Web 03/23/2007
Macroporous 3D Architectures of Self-Assembled MWCNT Surface Decorated with Pt Nanoparticles as Anodes for a Direct Methanol Fuel Cell Marı´a C. Gutie´ rrez, Marı´a J. Hortigu1 ela, J. Manuel Amarilla, Ricardo Jime´ nez, Marı´a L. Ferrer, and Francisco del Monte* Instituto de Ciencia de Materiales de Madrid (ICMM), Consejo Superior de InVestigaciones Cientı´ficas (CSIC), Campus de Cantoblanco, Madrid 28049, Spain ReceiVed: February 20, 2007; In Final Form: March 13, 2007
Microchannelled 3D architectures composed of multiwall carbon nanotubes (MWCNTs) surface decorated with Pt nanoparticles and chitosan (CHI) are prepared by an ice segregation induced self-assembly (ISISA) process. The microchannelled structures are highly porous (specific gravity ∼10-2) and exhibit excellent electron conductivity (2.5 S‚cm-1) thanks to both the high content of MWCNTs (up to 89 wt %) and their interconnection. The Pt/MWCNT/CHI 3D architectures provide remarkable performance as anodes for a direct methanol fuel cell (DMFC).
Since their discovery in 1991,1 carbon nanotubes (CNT) have been the subject of numerous research works given their unique properties, for example, extremely high electrical conductivities, very high thermal conductivities, and outstanding mechanical properties. Recently, Pt and Pt/Ru nanoparticles supported on single, double, and multiwalled CNTs (SWCNTs, DWCNTs, and MWCNTs, respectively) and graphite nanofibers have been systematically investigated as anodes of a direct methanol fuel cell (DMFC) with excellent activities of the methanol oxidation reaction due to their characteristic carbon structure.2 Among the different CNTs, DWCNTs and MWCNTs have exhibited the best efficiencies due to the combination of good electron conducting properties and large surface area (up to 68% larger than the most widely used carbon substrate materials, for example, Vulcan XC-72 carbon).3 In regards to the surface area and as a general trend for any catalytic application, a desirable combination of high internal reactive surface along the nanostructure with facile molecular transport through broad “highways” provided by a macrostructure could also contribute to a significant enhancement of the fuel cell performance.4 For this purpose, the design and preparation of three-dimensionally (3D) organized CNT-based anodes (e.g., micro- and nanostructured) would be of tremendous interest. However, the processing of CNT-based materials into engineered 3D macrostructures is still in its infancy and most of the arrays prepared to date with controlled areas and nanotube lengths are two-dimensional (2D), for example, from well vertically aligned CNT architectures5 to self-assembled CNT sheets6 up to textiles containing nanotube-fiber capacitors woven in orthogonal directions,7 among others.8 Eventually, 3D sieve architectures were constructed.9 Unfortunately, the length of the third dimension of the regular patterned cavities is limited to that of the CNTs,4,10 except for those composites where the CNT fraction is minor.11 Chemical vapor deposition techniques have * Corresponding author. Phone: +34 91 334 9033. Fax: +34 91 3720623. E-mail:
[email protected].
10.1021/jp0714365 CCC: $37.00
been used recently to overcome this limitation, and the resulting freestanding films of vertically aligned carbon nanotubes have exhibited super-compressible foam-like behavior, which corroborates the interest of having 3D architectures of MWCNTs.12 Herein, we plan to use the ISISA (ice segregation induced self-assembly) process for the preparation of MWCNT-based materials with well-defined macroporous 3D architectures. The ISISA process is a simple and versatile bottom-up process that allows us to prepare inorganic, organic, and hybrid 3D macrotructures by the freezing of different gels and colloidal suspensions.13 In this work, we apply the ISISA process to a water suspension of MWCNTs (2-8 wt %) and chitosan (1 wt %) (see the Supporting Information for further details). CHI is an efficient dispersion agent for CNTs and is of great help for the achievement of homogeneous suspensions. MWCNT/CHI compositions are very attractive for our purposes, given its suitability to work as an electrode.14 The MWCNT/CHI suspension is unidirectionally frozen by immersion in liquid nitrogen. Subsequent freeze-drying results in highly porous (specific gravity ranges from 4.0 × 10-2 to 9.4 × 10-2, see Table 1) and self-supported monoliths mostly composed of MWCNTs (up to ∼89%, see Table 1) with a chamber-like architecture in the form of interconnected MWCNT/CHI sheets arranged in parallel layers as shown in Figure 1. Note that the morphology of the resulting structure is strongly dependent on the MWCNTs wt %; that is, increased MWCNTs contents favor the formation of pillars crossing between layers (Figure 1b-d). The inset in Figure 1d shows the interconnected nature of the MWCNTs at the walls that support the 3D architecture. The freezing rate can also be of help to control the morphology of the MWCNTs 3D architecture,13 albeit in this case the regularity of the patterned macrostructure was not further improved (see the Supporting Information). The process does not provide control of the MWCNTs alignment, but it allows for the preparation of monoliths with different shapes (both regular and irregular) and © 2007 American Chemical Society
5558 J. Phys. Chem. C, Vol. 111, No. 15, 2007
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Figure 1. SEM micrographs of cross-sectioned monolithic MWCNT/CHI 3D architectures with different MWCNT contents: (a) 66 wt %, (b) 80 wt %, (c) 85 wt %, and (d) 89 wt % (bars are 20 µm). The inset shows interconnected MWCNTs forming the walls of the structure (bar is 5 µm).
TABLE 1: MWCNT Content Used for Preparation and at the 3D Architectures, and Specific Gravity and Conductivity of the Resulting MWCNT/CHI 3D Architectures MWCNT content (wt %) in suspensiona
at the monolithb
specific gravity
conductivity (S‚cm-1)
2 4 6 8
66 80 85 89
4.0 × 10-2 5.7 × 10-2 8.0 × 10-2 9.4 × 10-2
0.17 0.77 1.4 2.5
a
Prior to freezing. b After freeze-drying.
sizes, with well-aligned microchannels in the direction of freezing (Figure 2). We have measured the direct current (dc) of a 3-cm-long monolith using a classical four-wire method (see the Supporting Information). The conductivity of the MWCNT/CHI macroporous 3D architectures increases with the MWCNTs content (see Table 1), reaching excellent values (2.5 S‚cm-1, in the range of those reported previously for densely packed MWCNT composites)15 for the highest MWCNT content (89 wt %), which is indicative of the effective interconnection between MWCNTs in this sample. The conductivity of the macroporous 3D architecture containing 89 wt % MWCNTs makes the application of MWCNT(89)/CHI monoliths in anodes plausible for DMFCs. For electrochemical purposes, the MWCNTs should also be surface decorated with Pt nanoparticles (see the Supporting Information for details).16 The macrostructure resulting from the application of the ISISA process on MWCNTs (89 wt %) surface decorated with Pt nanoparticles of ca. 5 nm (see the
Figure 2. Picture of MWCNT/CHI monoliths with different shapes and sizes resulting from the ISISA processing of MWCNT/CHI suspensions placed in different disposable containers; an insulin syringe (left) and a polystyrene cuvette (right) (a) (bar is 1 cm). SEM micrograph of the longitudinal section of a MWCNT/CHI monolith (b) (bar is 50 µm). The MWCNT content of every monolith shown in the figure is 85 wt %. Arrows indicate the direction of freezing.
inset in Figure 3) is an analogue of that shown in Figure 1d. The XRD pattern of the finely ground Pt/MWCNT(89)/CHI 3D architecture shows the characteristic peaks of MWCNTs (ca. 26.5° and 54.3°) and wide maxima corresponding to Pt (ca. 40°, 46.5°, 67.5°, and 82°) with a face-centered cubic (fcc) structure (Figure 3).3,17 The average particle size of Pt nanoparticles
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Figure 3. XRD pattern of a finely ground Pt/MWCNT(89)/CHI monolith. Inset: TEM micrograph of the MWCNT surface decorated with Pt nanoparticles (bar is 50 nm).
Figure 4. CV curve of Pt/MWCNT(89)/CHI sample in 0.1 M H2SO4 and 0.25 M CH3OH at a scan rate of 5 mV‚s-1. Represented data were obtained after scanning for 10 cycles to ensure a stable response. Undistinguishable curves are obtained for tiny sliced monoliths (mass of monolith 2 grams) or finely ground monoliths (mass of ground monolith 0.6 grams).
calculated from the broadening of the (220) diffraction peak using Scherrer’s equation is ∼5 nm, in good agreement with the mean particle size observed in the TEM micrograph (Figure 3). The Pt nanoparticle content (provided by EDX experiments) at the Pt/MWCNT(89)/CHI 3D architecture was found to be 10 wt %. The conductivity of the Pt/MWCNT(89)/CHI 3D architectures was identical to that measured for MWCNT(89)/ CHI 3D architectures without Pt, which further corroborates the nanoparticle character (i.e., disperse rather than continuous) of the Pt surface decorating the MWCNTs. Actually, high conductivities could be inferred from the high intensity of the diffraction peak at ∼26.5°, which reflects the high graphite degree of the MWCNTs.3 The electrocatalytic properties of Pt/MWCNT(89)/CHI 3D monoliths were studied by cyclic voltammetry (CV). The working electrodes were prepared by gluing Pt/MWCNT(89)/ CHI 3D monoliths (cylindrical shape) with different sizes (height of the cylindrical monolith) on the surface of a glassy carbon current collector (contact area 0.14 cm2). For comparison, working electrodes were also prepared by spreading a deionized water suspension of finely ground monoliths on the glassy carbon electrode. The shape of the CV curves found for electrodes prepared as 2D coatings or tiny sliced 3D monoliths are identical (Figure 4). At low potentials, a sharp peak centered at -0.2 V is observed (also observed in the absence of MeOH, see Figure S3 of the Supporting Information). This peak has been typically assigned to hydrogen desorption when the Pt(110) crystallite phase is predominant among the Pt nanoparticles present on the MWCNTs surface.3 The appearance of a prominent symmetric anodic peak around 0.76 V in the forward
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Figure 5. Plot of current density (solid symbols, left y axis) and normalized current (open symbols, right y axis) vs mass of the Pt/ MWCNT(89)/CHI 3D monolith. Represented data were also obtained after scanning (5 mV‚s-1 scan rate) for 10 cycles to ensure a stable response.
scan (If) resembles the methanol electro-oxidation. The intensity of this band (∼33 mA/mg Pt) is high, in good concordance with previous results.3b Such a remarkable electrocatalytic activity could be due to the predominance of the Pt(110) crystallite phase (e.g., the active phase for methanol oxidation reaction) of the nanoparticles.18 In the reverse scan, the removal of the incompletely oxidized carbonaceous species formed in the forward scan promotes the appearance of an asymmetric anodic peak (Ib) at 0.62 V. The ratio of the forward anodic peak current density to the reverse of the anodic peak current density (If /Ib) has been used to describe the catalyst tolerance to carbonaceous species accumulation.19 The ratio found experimentally in Figure 4 is in good concordance with data reported previously for Pt nanoparticles (e.g., 0.87, Figure S4 of the Supporting Information). For cells operating above room temperature, the vertical increase of the reoxidation current has been attributed to weak OH adsorptivity on Pt, which favors its ready desorbtion.20 The facile molecular transport of fuels and products provided by the microchanelled structure of the Pt/MWCNT(89)/CHI 3D architecture should allow for the use of cylindrical monoliths of larger heights as anodes. Figure 5 indeed shows this behavior, the linear increase of current density with the mass of the macroporous monolith. Eventually, current densities reach values of up to 242 mA‚cm-2. This feature is simply a consequence of the use of macroporous MWCNT 3D architectures, which provide a large reactive surface. Meanwhile, the mass activity (Figure 5, right y axis in mA/mg of Pt) indeed remains constant (within the experimental error). It is worthy to note that use of thicker 2D coatings to enhance the current density is ineffective given that only the exposed area of the coating is accessible for methanol electrooxidation. Surprisingly, the use of 3D monoliths also results in the increase of the If /Ib ratio (Figure S4), which as mentioned above is indicative of the poisoning of the Pt catalyst. The value of this ratio reach levels of 2.3 for the largest monolith measured (7.5 mm height, Figure 5), which is in the range of the best values reported to date for bimetallic PtRu nanoparticle catalysts.19 The mechanism governing this interesting feature is most likely related to the functionalization (OH surface functionalized) of the MWCNTs prior to their assembly into the macroporous 3D structure. Thus, the CO-poisoned Pt nanoparticles can be regenerated via the reaction of surface CO with oxygen species on MWCNTs to yield CO2. The large conductance of CNTs or the preferred crystalline phase for adsorbed Pt nanoparticles are features observed in our system, which have also been proposed as plausible explanations for this interesting observation.21 In
5560 J. Phys. Chem. C, Vol. 111, No. 15, 2007 any case, further experiments are required for conclusive results on this issue. In summary, we have demonstrated the suitability of the ISISA process for the design and preparation of self-supported macroporous monoliths of interconnected MWCNTs arranged in 3D architectures. The resulting architectures for high MWCNT contents (89%) are highly porous (specific gravity ∼9.4 × 10-2) and extremely conductive (2.5 S‚cm-1) thanks to the MWCNT interconnection at the macrostructure. These macroporous 3D architectures offer a high internal reactive surface of easy access through the broad “highways” provided by the microchanelled structure, which make them highly suitable for catalytic purposes. In particular, the Pt/MWCNT(89)/CHI 3D architectures (prepared from Pt surface-decorated MWCNTs) have allowed for a remarkable improvement (e.g., current densities of up to 242 mA‚cm-2) of the catalytic activity toward the methanol oxidation thanks to efficient fuel and product diffusion. Moreover, the size increase of the 3D architecture also results in lower levels of poisoning at the Pt nanoparticles, similar to those observed for optimized bimetallic PtRu catalysts. Further work is currently in progress to understand this interesting observation. Acknowledgment. This work was supported by MAT200602394, 200660F0111, and S-0505/PPQ-0316 Projects. We also acknowledge TPA Inc. for valuable support. M.L.F. and M.C.G. acknowledge MEC and CSIC for research contracts. M.J.H. acknowledges MAPFRE and CSIC for a PhD fellowship. Fernando Pinto is acknowledged for helpful assistance with SEM experiments. Supporting Information Available: Experimental details on sample preparation and characterization, and additional CV curves for MWCNT(89)/CHI and Pt/MWCNT(89)/CHI samples. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Iijima, S. Nature 1991, 354, 56. (2) (a) Steigerwalt, E. S.; Deluga, G. A.; Lukehart, C. M. J. Phys. Chem. B 2002, 106, 760. (b) Steigerwalt, E. S.; Deluga, G. A.; Cliffel, D. E.; Lukehart, C. M. J. Phys. Chem. B 2001, 105, 8097. (3) (a) Li, W.; Wang, X.; Chen, Z.; Waje, M.; Yan, Y. J. Phys. Chem. B 2006, 110, 15353-58. (b) Prabhuram, J.; Zhao, T. S.; Tang, Z. K.; Chen, R.; Liang, Z. X. J. Phys. Chem. B 2006, 110, 5245. (4) Che, G.; Lakshmi, B. B.; Martin, C. R.; Fisher, E. R. Nature 1998, 393, 346.
Letters (5) (a) Xu, Y.-Q.; Flor, E.; Kim, M. J.; Hamadani, B.; Schmidt, H.; Smalley, R. E.; Hauge, R. H. J. Am. Chem. Soc. 2006, 128, 6560. (b) Jung, Y. J.; Kar, S.; Talapatra, S.; Soldano, C.; Viswanathan, G.; Li, X.; Yao, Z.; Ou, F. S.; Avadhanula, A.; Vajtai, R.; Curran, S.; Nalamasu, O.; Ajayan, P. M. Nano Lett. 2006, 6, 413. (6) Duggal, R.; Hussain, F.; Pasquali, M. AdV. Mater. 2006, 18, 29. (7) Dalton, A. B.; Collins, S.; Mun˜oz, E.; Razal, J. M.; Ebron, V. H.; Ferraris, J. P.; Coleman, J. N.; Kim, B. G.; Baughman, R. H. Nature 2003, 423, 703. (8) Wei, D.; Liu, Y.; Cao, L.; Fu, L.; Li, X.; Wang, Y.; Yu, G.; Zhu, D. Nano Lett. 2006, 6, 186-192. (9) Zhang, M.; Fang, S.; Zakhidov, A. A.; Lee, S. B.; Aliev, A. E.; Williams, C. D.; Atkinson, K. R.; Baughman, R. H. Science 2005, 309, 1215. (10) Zhu, L.; Sun, Y.; Hess, D. W.; Wong, C.-P. Nano Lett. 2006, 6, 243. (11) Yang, Y.; Gupta, M. C.; Dudley, K. L.; Lawrence, R. W. Nano Lett. 2005, 5, 2131. (12) Cao, A.; Dickrell, P. L.; Sawyer, W. G.; Ghasemi-Nejhad, M. N.; Ajayan, P. M. Science 2005, 310, 1307(13) (a) Mukai, S. R.; Nishihara, H.; Tamon, H. Chem. Commun. 2004, 874. (b) Nishihara, H.; Mukai, S. R.; Yamashita, D.; Tamon, H. Chem. Mater. 2005, 17, 683. (c) Nishihara, H.; Mukai, S. R.; Tamon, H. Carbon 2004, 42, 889. (d) Zhang, H.; Hussain, I.; Brust, M.; Butler, M. F.; Rannard, S. P.; Cooper, A. I. Nat. Mater. 2005, 4, 787. (e) Mukai, S. R.; Nishihara, H.; Shichi, S.; Tamon, H. Chem. Mater. 2004, 16, 4987. (f) McCann, J. T.; Marquez, M.; Xia, Y. J. Am. Chem. Soc. 2006, 128, 1436. (g) Deville, S.; Saiz, E.; Nalla, R. K.; Tomsia, A. P. Science 2006, 311, 515. (h) Ferrer, M. L.; Esquembre, R.; Ortega, I.; Mateo, C. R.; del Monte, F. Chem. Mater. 2006, 18, 554. (i) Gutierrez, M. C.; Jobbagy, M.; Rapun, N.; Ferrer, M. L.; del Monte, F. AdV. Mater. 2006, 18, 1137. (j) Deville, S.; Saiz, E.; Tomsia, A. P. Biomaterials 2006, 27, 5480-89. (k) Nishihara, H.; Mukai, S. R.; Fujii, Y.; Tago, T.; Masuda, T.; Tamon, H. J. Mater. Chem. 2006, 16, 3231. (l) Gutierrez, M. C.; Garcia-Carvajal, Z. Y.; Jobbagy, M; Catalina, F.; Abrusci, M. C.; Yuste, L.; Rojo, F.; Ferrer, M. L.; del Monte, F. Chem. Mater., in press. (m) Gutierrez, M. C.; Garcia-Carvajal, Z. Y.; Jobbagy, M.; Ferrer, M. L.; del Monte, F., submitted for publication. (n) Gutierrez, M. C.; Jobbagy, M.; Ferrer, M. L.; del Monte, F., submitted for publication. (14) Zhang, M.; Smith, A.; Gorski, W. Anal. Chem. 2004, 76, 5045. (15) (a) Kim, H. M.; Kim, K.; Joo, J.; Cho, S. J.; Yoon, H. S.; Pejakovic, D. A.; Yoo, J. W.; Epstein, A. J. Appl. Phys. Lett. 2004, 84, 589. (b) Cochet, M.; Maser, W. K.; Benito, A. M.; Callejas, M. A.; Martinez, M. T.; Benoit, J.-M.; Schreiber, J.; Chauvet, O. Chem. Commun. 2001, 1450. (16) (a) Bock, C.; Paquet, C.; Couillard, M.; Botton, G. A.; MacDougall, B. R. J. Am. Chem. Soc. 2004, 126, 8028. (b) Lin Cao, L.; Scheiba, F.; Roth, C.; Schweiger, F.; Cremers, C.; Stimming, U.; Fuess, H.; Chen, L.; Zhu, W.; Qiu, X. Angew. Chem., Int. Ed. 2006, 45, 5315. (17) (a) Saquing, C. D.; Cheng, T.-T.; Aindow, M.; Erkey, C. J. Phys. Chem. 2004, 108, 7716-22. (b) Chai, G. S.; Yoon, S. B.; Yu, J.-S.; Choi, J.-H.; Sung, Y.-E. J. Phys. Chem. 2004, 108, 7074-79. (18) Herrero, E.; Franasazczuk, K.; Wieckowski, A. J. Phys. Chem. 1994, 98, 5074. (19) Liu, Z.; Ling, X. Y.; Su, X.; Lee, J. Y. J. Phys. Chem. 2004, 108, 8234. (20) (a) Lee, C.-G.; Umeda, M.; Uchida, I. J. Power Sources 2006, 160, 78-89. (b) Wu, G.; Chen, Y.-S.; Xu, B.-Q. Electrochem. Comun. 2005, 7, 1237-43. (21) Guo, D.-J.; Li, H.-L. J. Power Sources 2006, 160, 44-49.
