Ultra-long Pt nanolawns supported on TiO2-coated carbon fibers as 3D hybrid catalyst for methanol oxidation
© Shen et al.; licensee Springer. 2012
Received: 14 January 2012
Accepted: 30 April 2012
Published: 26 June 2012
In this study, TiO2 thin film photocatalyst on carbon fibers was used to synthesize ultra-long single crystalline Pt nanowires via a simple photoreduction route (thermally activated photoreduction). It also acted as a co-catalytic material with Pt. Taking advantage of the high-aspect ratio of the Pt nanostructure as well as the excellent catalytic activity of TiO2, this hybrid structure has the great potential as the active anode in direct methanol fuel cells. The electrochemical results indicate that TiO2 is capable of transforming CO-like poisoning species on the Pt surface during methanol oxidation and contributes to a high CO tolerance of this Pt nanowire/TiO2 hybrid structure.
In recent years, direct methanol fuel cell (DMFC) has attracted great attention as an alternative power source because of their many advantages, including light weight, high power density, portability, and storage of liquid fuel [1, 2]. For most of the common DMFC devices, Pt-based catalysts have been used as an anode because of their outstanding performance in catalyzing the dehydrogenation of methanol. However, the commercialization of DMFC device still faces some problems, such as CO adsorption on Pt catalysts and thus poisoning [3, 4]. Even a low concentration of CO will cause a remarkable decrease in catalytic activity of Pt-based electrodes. Therefore, the majority of the Pt-based electrocatalyst research has focused on this topic.
In addition to alloying [5, 6], there are two other strategies for improving the Pt-based catalyst's performance. The first is to modify the morphology of Pt nanostructure, including the shape as well as the dimension. It has been demonstrated that changing the morphology of the Pt nanostructure from nanoparticle (NP) to nanowire (NW) can enhance the electrocatalytic activity of the catalysts, due to the large side surface which is able to provide additional catalytic active facets [7, 8]. A great deal of effort has been devoted to the synthesis of one-dimensional Pt nanostructures; however, it still remains a huge challenge to synthesize long and oriented single-crystalline Pt NWs without temperates and surfactants. Lee et al. [9, 10] have demonstrated the synthesis of single crystalline Pt NWs on polymeric, ceramic, or metallic substrate by a polyol process, combined with a trace addition of iron species (Fe2+ or Fe3+) and poly(vinylpyrrolidone) (PVP) as the surfactant. Cetyltrimethylammonium bromide (CTAB) has also been applied in the reduction of Pt ions to Pt NWs [7, 11]. Moreover, without using templates and surfactants, HCOOH [12–15] and vitamin B2 have been suggested respectively to act as reductant agents in the chemical routes for the synthesis of Pt NWs. Through the above processes, the Pt NWs produced are extremely fine (mostly less than 10 nm in diameter) but exhibit a limit in length of about 200 nm so that their aspect ratios do not exceed 50.
The next tactic to enhance the catalyst activity and CO-tolerance is the development of new composites and catalytic supporting materials. In the last decade, the addition of oxides has been generally accepted as an efficient way to improve catalytic activity of platinum and its CO-tolerance for methanol electro-oxidation. For example, Cui et al.  found that the Pt-WO3/C composite catalysts exhibit excellent catalytic activity and stability for methanol electro-oxidation because WO3 is able to form the hydrogen-tungsten-bronze compound, which facilitates dehydrogenation during methanol oxidation reaction. RuO2, ZrO2, and MgO have also been studied and it was found that they can improve the catalytic performance of Pt [18–20]. A promising breakthrough is that the composites of Pt or Pt-Ru nanoparticles and semiconductor catalysts such as TiO2 and CeO2 have been developed as the anode for oxidation of methanol or ethanol [21–24]. It has been proposed that the Pt-CeO2 composite catalyst has a higher activity than Pt catalyst because CeO2 makes CO electro-oxidation easier. The redox reaction on the surface of CeO2 mixing with Pt particles causes the oxidation of CO to CO2 and thus gives rise to a better performance. Drew et al.  verified that TiO2 can enhance the current generation especially under ultraviolet (UV) light irradiation during the electrochemical testing and suggested that the holes formed upon UV illumination are consumed in methanol oxidation and bring about the additional current.
Based on this, metallic nanowires can be formed on the surface of thin-film TiO2 via the photoreduction of metallic ions under certain irradiating and heating conditions. Instead of commonly used H2PtCl6, Na2Pt(OH)6 was selected as the precursor in this work.
It has been demonstrated that the extent of UV exposure on the TiO2 film affects the degree of excitation and thus strongly influences the shape and dimension of the reduced metallic structure . In order to obtain reduced Pt with different morphologies and sizes, we varied the conditions of pre-UV exposure treatment performed on the annealed TiO2-coated carbon cloths before the growth of the nanowires (between the step 1 and step 2). The conditions included (1) no pre-UV light exposure, (2) pre-UV exposure on only one side of the carbon cloths for 48 h, and (3) pre-UV exposure on each side of carbon cloths for 12 h alternately to reach a total exposure time of 48 h. It is assumed that on the exposed side of the carbon cloth the TiO2 was excited. Therefore, the whole TiO2 film was presumed to be fully excited.
Electrochemical measurements of the hybrid electrocatalysts were performed in a three-electrode cell using an Autolab PG302N work station (Metrohm, Autolab BV, Utrecht, The Netherlands) at room temperature to evaluate their catalytic performance. Carbon cloths (0.25 cm2) with Pt-TiO2 catalysts were the working electrode. A Pt rod and Ag/AgCl were used as counter and reference electrodes, respectively. A solution of 1 M CH3OH and 0.5 M H2SO4 was used as the electrolyte. All the reagents used were of analytical grade. The cyclic voltammetry data for methanol electro-oxidation were recorded in the potential range of −0.2 to 1.0 V vs. Ag/AgCl with a scan rate of 20 mVs−1.
Results and discussion
Electrochemical characteristics of the specimens during CV analysis (derived from the CV curves in Figure 4 a)
Pt NW + MP/TiO2
If current density (mA)
Ib current density (mA)
Onset potential (V)
The OHads species (ads, adsorbed) on TiO2, obtained through the activation of water, can transform CO-like poisoning species (COads) on the Pt surface, produced by the methanol dehydrogenation, to CO2 and release active Pt for further catalysis.
The TAP process needs to be improved to obtain a higher population of Pt NWs and thus raises the catalytic activity for methanol oxidation. If this is done, it can be expected that the excellent performance in CO tolerance can be maintained because the high-aspect-ratio Pt NWs occupy only a small part of the fiber surface and leave most the TiO2 film free for oxidation of CO.
By means of templateless and surfactant-free method, TAP, this study successfully prepared carbon fibers supported Pt nanowires/TiO2 composite electrocatalysts, which show great potential for use as active anode in direct methanol fuel cells. Analytical results suggest that the Pt nanowires were single crystalline with a preferred <111> growth direction and exhibited an aspect ratio ranged between 25 and 50. This Pt nanowire/TiO2 hybrid structure possessed high CO tolerance because TiO2 enhances CO electro-oxidation and thus increases CO poisoning resistance. An optimal performance in catalytic activity for methanol oxidation and CO tolerance can be expected if the density of the Pt nanowires is increased further.
YLS is a PHD student at Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, Taipei 106, Taiwan. SYC is an associate professor in the Department of Materials Science and Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan. JMS is an associate professor in the Department of Materials Science and Engineering, National Chung Hsing University, Taichung 402, Taiwan. IGC is a distinguished professor from the Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan.
We thank the National Science Council of Taiwan for funding NSC 100-2120-M-006-006 and National Cheng Kung University Project of Promoting Academic Excellence and Developing World Class Research Center: D98-R048 for support of this work.
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