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Fabrication and Characterization of Pt/C-TiO2 Nanotube Arrays as Anode Materials for Methanol Electrocatalytic Oxidation Lixia Yang,†,‡ Yan Xiao,† Guangming Zeng,† Shenglian Luo,*,†,‡,§ Shuyun Kuang,‡ and Qingyun Cai*,‡ College of EnVironmental Science and Engineering, and State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan UniVersity, Changsha 410082, People’s Republic of China, and School of EnVironment and Chemical Engineering, Nanchang Hangkong UniVersity, Nanchang 330063, People’s Republic of China ReceiVed January 15, 2009. ReVised Manuscript ReceiVed April 17, 2009
A carbon-modified TiO2 nanotube (C-TiO2 NT) array is fabricated by depositing carbon in TiO2 NTs, which are prepared by anodization of the Ti sheet. Well-dispersed Pt nanoparticles (NPs) are electrochemically deposited on the C-TiO2 NTs. The performances of the as-prepared NT array electrode in the methanol oxidation reaction (MOR) as an anode are investigated. The results present in this study highlight such a finding: depositing partly graphitized carbon on the inside of TiO2 NTs can significantly enhance the catalytic efficiency. An optimum forward oxidation peak current density (Ipf) of 71.6 mA cm-2 is obtained from the Pt/C-TiO2 NT anode at a low Pt loading of 23 µg cm-2. The achieved Ipf is almost 27 times that achieved on Pt-modified TiO2 NTs without carbon modification. The enhanced catalytic efficiency is mainly attributed to the superior electrical conductivity of the deposited carbon, which facilitates the well dispersion of Pt NPs, charge transfer during the MOR, and removal of the byproduct CO-like species.
1. Introduction Direct methanol fuel cells (DMFCs) working at low and intermediate temperatures (up to 150 °C) have been considered as suitable systems for power generation in the field of electrotraction.1-4 The development of an appropriate fuel cell system is an important issue from both an economical and environmental point of view. At present, particular emphasis is given to the development of high surface area electrocatalysts, which can effectively enhance the electrokinetics of methanol oxidation and the catalyst poison tolerance. Generally, Pt and Pt-based alloying of PtRu, PtSn, PtNi, etc. dispersed on carbon black, e.g., Vulcan XC-72, carbon nanotubes,5-11 a sol-gelderived three-dimensional silicate network,12 or Pt/C hybrid materials with core-shell construction,13,14 serve as anode catalysts to enhance the CO poison tolerance and reactivity of the catalyst. However, the large-scale diffusion of DMFCs will not solve the problem of catalyst cost unless the loading of precious metals is significantly decreased. Therefore, the synthesis of a well-defined catalyst surface in combination with a low metal loading on an ideal support is one of the present goals in the field of DMFCs. It is considered that the self* To whom correspondence should be addressed. Fax: 86-731-8821848. E-mail:
[email protected] (S.L.Lou);
[email protected] (Q.Y.Cai). † College of Environmental Science and Engineering, Hunan University. ‡ State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University. § Nanchang Hangkong University. (1) Hogarth, M. P.; Ralph, T. R. Platinum Met. ReV. 2002, 46, 146– 164. (2) Arico, A. S.; Srinivasan, S.; Antonucci, V. Fuel Cells 2001, 1, 133– 161. (3) Liu, H. S.; Song, C. J.; Zhang, L.; Zhang, J. J.; Wang, H. J.; Wilkinson, D. P. J. Power Sources 2006, 155, 95–110. (4) Lebedeva, N. P.; Janssen, G. J. M. Electrochem. Acta 2005, 51, 29– 40.
organized titania NT array prepared by anodization would be a good choice because it has a high orientation, large internal surface area,15-17 which would significantly reduce the preciousmetal-loading content, as presented in our previous work of applying AuPt nanoparticles (NPs) decorated with TiO2 nanotubes (NTs) and PtRu NPs modified with carbon nanotubes (CNTs)/TiO2 electrodes in the methanol oxidation reaction (MOR).18,19 The catalytic efficiency of TiO2 material is hindered by the poor conductivity of crystalline TiO2. Modification of CNTs (5) Ren, X. M.; Zelenay, P.; Thomas, S.; Davey, J.; Gottesfeld, S. J. Power Sources 2000, 86, 111–116. (6) Arico, A. S.; Antonucci, V.; Giordano, N.; Shukla, A. K.; Ravikumar, M. K.; Roy, A.; Barman, S. R.; Sarma, D. D. J. Power Sources 1994, 50, 295–309. (7) Lanova, B.; Wang, H.; Baltruschat, H. Fuel Cells 2006, 3, 214– 224. (8) Dupont, C.; Jugnet, Y.; Loffreda, D. J. Am. Chem. Soc. 2006, 128, 9129–9136. (9) Lin, Y.; Cui, X.; Yen, C. H.; Wai, C. M. Langmuir 2005, 21, 11474– 11479. (10) Park, K.-W.; Choi, J.-H.; Sung, Y.-E. J. Phys. Chem. B 2003, 107, 5851–5856. (11) Girishkumar, G.; Vinodgopal, K.; Kamat, P. V. J. Phys. Chem. B 2004, 108, 19960–19966. (12) Jena, B. K.; Raj, C. R. J. Phys. Chem. C 2008, 112, 3496–3502. (13) Wen, Z. H.; Liu, J.; Li, J. H. AdV. Mater. 2008, 20, 743–747. (14) Wu, G.; Li, D.; Dai, C.; Wang, D.; Li, N. Langmuir 2008, 24, 3566–3575. (15) Cai, Q. Y.; Paulose, M.; Varghese, O. K.; Grimes, C. A. J. Mater. Res. 2005, 20, 230–236. (16) Varghese, O. K.; Gong, D. W.; Paulose, M.; Ong, K. G.; Dickey, E. C.; Grimes, C. A. AdV. Mater. 2003, 15, 624–627. (17) Paulose, M.; Varghese, O. K.; Peng, L.; Popat, K. C.; Prakasam, H. E.; Mor, G. K.; Desai, T. A.; Grimes, C. A. J. Phys. Chem. C 2007, 111, 14992–14997. (18) Yang, L. X.; He, D. M.; Cai, Q. Y.; Grimes, C. A. J. Phys. Chem. C 2007, 111, 8214–8217. (19) He, D. M.; Yang, L. X.; Kuang, S. Y.; Cai, Q. Y. Electrochem. Commun. 2007, 9, 2467–2472.
10.1021/ef900039w CCC: $40.75 2009 American Chemical Society Published on Web 05/07/2009
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Figure 1. Schematic representation of the Pt/C-TiO2 NT array preparation.
using chemical vapor deposition (CVD) on TiO2 NTs was employed to increase the conductivity;19 however, the uniform morphology of the TiO2 NTs was destroyed during the CVD at high temperatures, resulting in a decrease in the catalytic activity of TiO2 NTs. In this work, minor carbon was deposited in TiO2 NTs by CVD at a relatively low temperature, keeping with the uniform morphology of the TiO2 NTs. Pt NPs were then modified on the as-prepared C-TiO2 NTs, which served as the anode in the MOR. The modification of minor carbon significantly enhanced the conductivity of TiO2, which facilitates the dispersion of Pt NPs, charge transfer during the MOR, and removal of the byproduct CO-like species. Figure 1 schematicly depicts the preparation process of the Pt/C-TiO2 NT array. 2. Experimental Section 2.1. Preparation of Pt-Modified C-TiO2 NT Arrays. Anodization was performed in a two-electrode configuration, with Ti as the anode and platinum foil as the cathode, in 0.1 M NaF + 0.5 M NaHSO4 under 15 V, achieving NTs of approximately 90 nm pore size and 320 nm length. The geometrical surface area of the working electrode is the electrode surface (0.5 × 1.5 cm2). Because the titanium sample was only partially immersed in the electrolyte and titania nanotubes were grown on both sides of the Ti sheet, the effective geometrical surface area is 0.5 × 1 × 2 cm2 for application in the MOR and current density calculation. The carbon layer was deposited on the TiO2 NT by carbonizing 45 mg of poly(ethylene glycol) 6000 at 600 °C for 2 h with a heating and cooling rate of 1 °C/min in a N2 atmosphere, in which the content was determined by the carbon source content. The carbonization configuration is same as that in our previous work.20 Metal NPs were electrodeposited using chronopotentiometry at a current density of 5 × 10-3 A s-1 in a standard three-electrode configuration with a TiO2 NTs/ Ti working electrode, a graphitic auxiliary electrode, and a saturated calomel electrode (SCE) reference electrode (CHI 660B; CH Instruments, Inc., Austin, TX) in 10 mM H2PtCl6. The loading metal content was tuned by controlling the deposition duration. The electro-oxidation of methanol was investigated by half cell reactions through cyclic voltammetry (CV) and chromoamperometry in a solution containing 0.5 M NaOH + 2 M CH3OH or 0.5 M H2SO4 + 2 M CH3OH at a scan rate of 100 mV s-1. All chronoamperometric analyses were performed at -0.1 V versus SCE in an alkaline methanol electrolyte. 2.2. Characterization of the Pt/C-TiO2 NT Arrays. The topology of the catalyst was characterized using a field emission scanning electron microscope (FE-SEM) operating at 5 kV (JSM 6700F; JEOL, Tokyo, Japan). An energy-dispersive X-ray (EDX) spectrometer fitted to the SEM was used for elemental analysis. Transmission electron microscopy (TEM) images were obtained using a JEM 3010 (JEOL, Tokyo, Japan) operating at 300 kV. Firstorder Raman spectra was recorded using a Raman spectrometer (Renishaw system 2000) to study the fine structure of the specimen. Fourier transform infrared (FTIR) spectra were collected with a FTIR spectrometer (Nicolet 5700, Thermo) equipped with a (20) Yang, L. X.; Luo, S. L.; Liu, S. H.; Cai, Q. Y. J. Phys. Chem. C 2008, 112, 8939–8943.
deuterated triglycine sulfate (DTGS) detector using the FT-80° grazing angle reflectance mode. The species on the Pt/C-TiO2 NTs were identified by placing the anode electrodes after the MOR on the test platform of the ZnSe single-reflection accessory through attenuated total reflection (ATR) analysis.
3. Results and Discussion 3.1. Characterization of the TiO2 NT Arrays. Titania NT arrays fabricated in organic electrolytes can have a length ranging from 2.3 to 1000 µm depending upon the electrolyte compositions, but in an aqueous acidic fluoride-ion-containing electrolyte, the length is limited to be less than 500 nm.21-23 Long titania NTs show high catalytic efficiency in photoelectroncatalysis applications,21 while in the MOR, short titania NTs show higher catalytic efficiency than long NTs, as found in our study (the data are not given). The possible reason is that, in a liquid-phase reaction catalyzed by metal particles, the diffusion of methanol fuel through shorter NTs is easier with less masstransfer resistance, resulting in an increase in the catalytic efficiency. Because electro-oxidation of methanol is a structuresensitive reaction, a highly dispersed metal phase on an electrically conductive substrate is essential to achieving a high reaction rate of the MOR. Consequently, titania NT arrays prepared in NaHSO4 + NaF with 320 nm length and 10 nm wall thickness depicted in Figure 2a were carbonized for the MOR. A FE-SEM image of the Pt NPs decorated with C-TiO2 NTs (Figure 2b) shows that Pt NPs in a diameter of ∼10 nm are uniformly dispersed on the C-TiO2 NTs. The TiO2 NT wall thickness is increased by about 5 nm because of the carbon coating. When the Pt loading is increased from 23 to 92 µg cm-2, Pt particles begin to aggregate, as shown in panels c and d of Figure 2. As can be clearly seen from Figure 2c, the Pt NPs are loaded not only on the NT surface but on the NT inner walls, which is further confirmed by TEM of this sample (Figure 2e): the Pt NPs (marked by red circles) dispersed widely on the NT inner walls. In addition, a Raman spectrum (Figure 2f) of the specimen depicted in Figure 2c shows, except for the anatase and rutile, one intensive peak at 1320 cm-1 corresponding to the defect-induced carbon (the D band) and another at 1589 cm-1 assigned to the ordered graphite (the G band),24,25 indicating that the formed carbon is partly graphitized, suggesting a more rapid electron transfer rate on the C-TiO2 NTs compared to those amorphous carbon black, such as Vulcan (21) Paulose, M.; Shankar, K.; Yoriya, S.; Prakasam, H. E.; Varghese, O. K.; Mor, G. K.; Latempa, T. A.; Fitzgerald, A.; Grimes, C. A. J. Phys. Chem. B 2006, 110, 16179–16184. (22) Yoriya, S.; Paulose, M.; Varghese, O. K.; Mor, G. K.; Grimes, C. A. J. Phys. Chem. C 2007, 111, 13770–13776. (23) Ruan, C.; Paulose, M.; Varghese, O. K.; Mor, G. K.; Grimes, C. A. J. Phys. Chem. B 2005, 109, 15754–15759. (24) Busca, G.; Ramis, G.; Amores, J. M.; Escribano, V. S.; Piaggio, P. J. Chem. Soc., Faraday Trans. 1994, 90, 3181–3190. (25) Katagiri, G.; Ishida, H.; Ishitani, A. Carbon 1988, 26, 565–571.
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Figure 2. SEM images of the top morphology of (a) as-anodized titania NTs and (b, c, and d) Pt nanoparticles modified with C-TiO2 NTs with increasing Pt density of 23, 57.5, and 92 µg of Pt cm-2, respectively. (e) TEM image and (f) Raman spectrum of the specimen shown in c. A, anatase; R, rutile; D, defect-induced carbon; G, graphitic carbon.
XC-72.26 In addition, the defect-induced carbon exhibits high adsorbility, ensuring the dispersion of the abundant OH groups on the C-TiO2 NT surface. 3.2. Impact of Pt-Loading Content on the MOR. Figure 3 shows four typical CV and chronoamperometric curves for the MOR in 2 M CH3OH + 0.5 M NaOH recorded on four C-TiO2 NTs electrodes with Pt loading of (1) 11.5, (2) 17.5, (3) 23, and (4) 57.5 µg cm-2, respectively. The Ipf of the MOR increases with an increasing Pt load from 11.5 to 57.5 µg cm-2, As the CV curves shown to us, the MOR forward peak shifts to high potential values with increasing Pt contents, indicating the possible poison resulting from the aggregation of Pt NPs and the decrease of the specific area. The chronoamperometric curves in Figure 3b show that the Pt/C-TiO2 NTs have high stability over 1800 s. Furthermore, there are small peaks during
the back scan of the MOR in CV curves: Goodenough et al.27 attributed this reverse anodic peak (Ipb) to the removal of the incompletely oxidized CO-like species formed in the forward scan, defining the ratio of Ipf to Ipb, Ipf/Ipb, to describe the catalyst tolerance to carbonaceous species accumulation. A low Ipf/Ipb ratio indicates poor oxidation of methanol to CO2 during the anodic scan and excessive accumulation of CO-like residues on the catalyst surface. A high Ipf/Ipb ratio depicts the converse case. To compare the catalytic activities of the electrodes in Figure 3, all of the Ipf/Ipb ratios and the Ipf in units of metal catalyst weight are summarized in Table 1. Electrode 4 exhibits a Ipf/Ipb ratio of 8.7 and the highest anodic peak current density in units of Pt weight of 3947.8 mA cm-2 (mg of Pt)-1. However, the forward peak shifts to a much higher potential value because of the Pt NPs aggregation, as depicted in the CV curve 4 in
(26) Park, K.-W.; Sung, Y.-E.; Han, S.; You, Y.; Hyeon, T. J. Phys. Chem. B 2004, 108, 939–944.
(27) Manohara, R.; Goodenough, J. B. J. Mater. Chem. 1992, 2, 875– 887.
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Figure 3. (a) Cyclic voltammograms and (b) chronoamperometry curves obtained in alkaline media on C-TiO2 NTs loaded with (1) 11.5, (2) 17.5, (3) 23, and (4) 57.5 µg cm-2 Pt, respectively. Table 1. Pt-Loading-Content-Dependent Peak Current Density (Ip) of Pt/C-TiO2 NTs for the MOR in Alkaline Mediuma electrode
Pt (µg cm-2)
Ipf (mA cm-2)
Ipb (mA cm-2)
Ipf/Ipb
Ipf [mA cm-2 (mg of Pt)-1]
1 2 3 4
11.5 17.5 23 57.5
10.34 37.2 71.6 227
1.57 11.3 9 26.1
6.58 3.29 7.95 8.70
899.1 2125.7 3113.0 3947.8
a Corresponding CV and chronoamperometric curves are shown in Figure 3.
Figure 3a, which would be a great loss when applied in practical fuel cells. As shown in Figure 3 and Table 1, electrode 3, Pt/ C-TiO2 NTs with 23 µg cm-2 Pt with Ipf of 71.6 mA cm-2 and a Ipf/Ipb ratio of 7.95, exhibits the optimum catalytic performance of the MOR. As a result, the optimum Pt content is 23 µg cm-2 here. In this work, the TiO2 NT array supplies a highly orientated, uniform template for the CNT formation, resulting in highly ordered CNT arrays. In comparison to the Pt-loaded general disordered CNTs applied in the MOR,9,11,19 the resulting CNTs with uniform morphology embedded in TiO2 NTs herein provided a more ideal support for the Pt electrodeposition, which facilitated the well-dispersion Pt NPs in the CNTs and effectively prevented the Pt catalysts from falling from the substrate. As a result, a stable and highly efficient catalytic activity was achieved on Pt/C-TiO2 NT arrays. On the other hand, incorporation of TiO2 in the anode has been shown to minimize the CO poisoning effects because of the abundant Ti-OH bonds, which speed up the removal of the CO-like species in the alkaline electrolyte.18 3.3. Investigation on the Mechanism of the MOR. Figure 4a shows typical CV curves of the MOR on C-TiO2 with 23 µg cm-2 Pt in alkaline and acidic media, exhibiting Ipf values
Figure 4. (a) Cyclic voltammograms of methanol oxidation at 23 µg of Pt/cm2 decorated C-TiO2 NTs in 0.5 M NaOH + 2 M CH3OH (black line, curve 1) and 0.5 M H2SO4 + 2 M CH3OH (red line, curve 2), with an inset depicting a low Ipf value of 2.7 mA cm-2 obtained on Pt/TiO2 NTs, and (b) corresponding FTIR spectra.
of 71 and 10 mA cm-2 and the corresponding mass activity of Pt of 3113.0 and 438.5 mA cm-2 (mg of Pt)-1, respectively. The inset in Figure 4a is the CV curve of the MOR performed on Pt-modified TiO2 NTs without carbon decorating in alkaline media, depicting a Ipf of 2.7 mA cm-2, with the optimum Pt loading of 1.36 µg cm-2. Increasing the Pt loading on TiO2 NTs without carbon resulted in a decay in the catalytic activity because the aggregated Pt NPs clogged the NTs, leading to a decrease of the active sites resulting from the poor conductivity of the TiO2.18 Consequently, besides the Pt loading, the carbonloading content is another key factor affecting the catalytic efficiency, because it affects the Pt-loading mass and the electron-transferring rate. The carbon-loading content was controlled by the carbon source content in this work. Our results (not shown here) show that increased carbon-loading content led to a low catalytic efficiency of the MOR. As depicted in our previous work,20 the TiO2 NTs serve as both the template and graphitization catalyst for the formation of the CNTs in
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the TiO2 NTs. If the carbon-loading content was increased, an increased wall thickness of CNT would make it difficult for graphitization by TiO2, resulting in a slow electron-transfer rate in CNT and the subsequent low efficiency of the MOR. In another situation, small carbon content would also hinder the electron transfer, leading to poor dispersion of Pt NPs in the C-TiO2 NTs during the electrodepositon process. Furthermore, the Ipf attributed to electro-oxidation of methanol in an alkaline electrolyte is found to be 7.1 times that in acidic media. The symbol representing the degree of methanol being converted to carbon dioxide, Ipf/Ipb ratio, is 7.95 in alkaline media and yet 1.52 in acidic media, indicating a higher catalytic activity and an enhanced poison tolerance performance in alkaline media. Species absorbed on the Pt/C-TiO2 NTs during the MOR were determined by FTIR spectroscopy, with results given in Figure 4b. The FTIR spectrum in alkaline media depicts a shoulder at 3185 cm-1 assigned to the hydroxyl group, a band at 1620 cm-1 assigned to the bending mode of water, and an intensive band at 1362 cm-1 assigned to the νaCOO and νsCOO modes of a bridging bidentate formate complex.28 While in acidic media, the -OH band (3342 cm-1) shifts to the higher wavenumbers because of the high proton concentration in H2SO4 media shown as the IR spectra. Bands at 1157 and 1038 cm-1 with high intensity are assigned to the CH3O- group.29 A comparison between the FTIR spectra shows that electrocatalytic oxidizing methanol to carbonate species is more complete in NaOH solution than that in H2SO4, indicating that the abundant OH species in alkaline media have a determinative impact on helping to deplete the CO-like species, thereby hindering Pt from being poisoned. On the basis of the above experimental results and referring to the literature,30-32 the possible step by step mechanism of the MOR on Pt/C-TiO2 NTs is presented below. step 1: CH3OH + Pt + C f Pt-COad + C - COad + CO2 (1) CO2 and CO-like species were formed by direct methanol oxidation. Because the formed carbon layer shared the COlike species with Pt, only a minority of the active Pt sites were poisoned, leading to little decay of the initial MOR (28) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 5th ed.; John Wiley and Sons: New York, 1997. (29) Brownson, J. R. S.; Tejedor, M. I.; Anderson, M. A. Chem. Mater. 2005, 17, 6304–6310. (30) Herrero, E.; Franaszczuk, K.; Wieckowski, A. J. Phys. Chem. B 1994, 98, 5074–5083. (31) Sriramulu, S.; Jarvi, D.; Stuve, E. M. Electrochim. Acta 1998, 44, 1127–1134. (32) Herrero, E.; Chrzanowski, W.; Wieckowski, A. J. Phys. Chem. B 1995, 99, 10423–10424.
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current density, depicted as the chronoamperometric curves in Figure 3b. At the same time, abundant C-OHad and Ti-OHad groups were formed in alkaline media. A reaction between -OHad and -COad occurred, giving rise to CO2, shown as the FTIR spectra in alkaline media. step 2: Pt-COad + C-COad + C-OHad + Ti-OHad f CO2 + C + Pt + Ti (2) Step 2 is considered to be parallel with direct methanol oxidation (step 1) and accelerated by the presence of abundant OH species, enhancing both the MOR current density and the antipoison capacity. In summary, Pt works as the main dehydrogenation site. The hydroxylated C and Ti facilitate the removal of the CO-like species, displaying antipoison functions and leading to an enhanced catalytic activity of the MOR. 4. Conclusions Pt-modified C-TiO2 NT arrays have been successfully applied as an anode catalyst in methanol oxidation, exhibiting remarkably enhanced activity compared to our previous results. The excellent performance of the above catalyst can be attributed to three aspects: (i) the C-TiO2 NT array offers strong adhesion of precious metal NPs with well dispersion on its huge surface, ensuring enough stable Pt active sites for the MOR; (ii) the formed carbon layer in TiO2 NT consists of disorder carbon and graphite carbon. The disorder carbon shows inherent high adsorbility, facilitating the formation of abundant -OHad groups, which can accelerate the removal of CO-like species; the graphite carbon enhances the electrical conductivity of the C-TiO2 substrate, favoring the well dispersion of Pt NPs with small size and current collection during the MOR; and (iii) the highly orientated open-top characteristics favor the diffusion of the methanol, which is also a key factor in determining the catalytic activity of the MOR. In addition, there are specific advantages to using a Pt/ C-TiO2 NT array as the DMFC anode with regard to its low cost, simplicity of design, large availability, easy handling, and high efficiency, which has great potential in DMFC application. Acknowledgment. This work was supported by the National Science Foundation for Distinguished Young Scholars under Grant 50725825, the Natural Science Foundation of Hunan Province, China, under Grant 08JJ3113, the National Science Foundation of China under Grant 50878079, and the Innovation Project in Postgraduation Education for Excellent Doctors under Grant 521218019. EF900039W
