3-Dimensional TiO₂ nanostructure supports and their improved electrochemical properties in methanol electrooxidation

Abstract

The TiO₂ nanostructure support consists of backbones and branches of a rutile phase forming a 3-dimensional structure with high specific surface area. The Pt catalysts on the TiO₂ nanostructure supports exhibit much improved electrocatalytic activity and stability toward methanol electrooxidation in comparison with conventional Pt/C.

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Direct methanol fuel cells (DMFCs) have been of considerable interest because of a variety of merits such as low operating temperatures, high energy density of methanol, and applications to micro-sized fuel cells.^1–3 However, the main issue in the DMFCs is to find catalysts and support materials suitable for excellent catalytic activity and stability. In general, carbon supported Pt-based catalysts have been widely used as a representative anode structure in DMFCs.

However, carbon support materials exhibit corrosion of carbon and weak interaction between catalysts and supports despite highly excellent electronic conductivity resulting in serious agglomeration of catalysts during electrochemical reactions.⁴ Thus, in particular, transition metal oxides such as TiO₂, WO₃, SnO₂, RuO₂, and Ta₂O₅ have been reported as alternatives to carbon supports to enhance the catalytic activity and stability for methanol electrooxidation.⁵ Among them, TiO₂ has been potentially attractive as a candidate of support due to strong catalyst–support interaction, stability in the fuel cell operation atmosphere, and commercial availability.⁶

Recently, there have been many literatures of novel support materials with highly active surface areas such as meso-porous structures and 2-dimensional or 3-dimensional structures for high dispersion of catalysts on supports.⁷ Herein, we synthesized a 3-dimensional TiO₂ nanostructure support by means of a seeding method with 1-dimensional TiO₂ nanowires as a seed.⁸ The TiO₂ nanostructure support can have high specific surface area and improved electronic transfer efficiency. The formation mechanism of the TiO₂ nanostructure support is shown in Scheme S1, ESI.† The order of growth rate of a rutile TiO₂ structure is V_〈110〉 < V_〈100〉 < V_〈101〉 < V_〈001〉 < V_〈111〉. Accordingly, the {001} and {111} facets disappear while {110}, {100}, and {101} facets are easily exposed during the growth process. As a result, the TiO₂ nanobranch and nanowire with a rutile phase are exposed to {110} and {101} facets (Fig. 1(a) and (b)). The TiO₂ nanobranch support exhibits a 3.23 times higher surface area (109 m² g⁻¹) than that of the TiO₂ nanowire (34 m² g⁻¹) because of the 3-dimensional branch-type structure of the as-synthesized TiO₂. To evaluate electrocatalytic properties of the electrode for the fuel cell, we prepared Pt catalyst on the branch-type TiO₂ support (Pt/TiO₂-NB) by polyol process using ethylene glycol with NaOH (Experimental section, ESI†).

Fig. 1 (a) TEM image of the TiO₂-NB supports. (b) High-resolution TEM image of the TiO₂-NB supports. [The inset indicates the FFT pattern of the TiO₂-NB as backbone and branch.] (c) TEM image of the Pt nanoparticles deposited on the TiO₂-NB supports. (d) Size distribution of deposited Pt nanoparticles. (e) High-resolution TEM images and FFT patterns of deposited Pt on the TiO₂-NB supports.

Fig. 1(c) shows TEM image of as-synthesized Pt (40 wt%) nanoparticles (NPs) on the TiO₂-NB supports. The average particle size of the Pt NPs is ∼3.49 nm (Fig. 1(d)). The Pt NPs are well dispersed on branches and backbones of the TiO₂-NB supports. As shown in Fig. 1(e), the TiO₂-NB (marked as 1 and 2) represents the {110} and {101} facets with d-spacings of 0.325 nm and 0.249 nm of the TiO₂ rutile phase, respectively. The polycrystalline Pt NPs (marked as 3 and 4) represent the {111} facets with a d-spacing of 0.227 nm in the Pt metallic phase with a face-centered-cubic crystal structure.

Fig. 2 shows cyclic voltammograms (CVs) of the supported catalysts during 400 cycles between −0.2 to 1.0 V in 0.1 M HClO₄. The electrochemical active surface areas (EASAs) of the Pt catalysts were measured by integrating hydrogen desorption regions (assuming 210 μC cm⁻² of a polycrystalline Pt electrode) in 0.1 M HClO₄ solution. In the case of the Pt/C, the EASAs after the stability test seriously decrease, that is, the reduction of 90.4% from the initial value. In contrast, the Pt/TiO₂-NB displays the reduced EASAs of 54.1% after the durability test. This represents that the improved electrochemical properties of the Pt/TiO₂-NB may be mainly due to the stability of the TiO₂-NB support as compared to the carbon in the Pt/C.

Fig. 2 (a) and (b) Cyclic voltammogram (CV) properties as a stability test from the initial cycle to 400 cycles of Pt/C (E-TEK) and Pt/TiO₂-NB in 0.1 M HClO₄ with a scan rate of 50 mV s⁻¹ at 25 °C. (c) Reducing rates of electrochemical active surface areas (EASAs) of catalysts related to Pt catalytic surface area with the increased cycles.

To compare electrochemical properties of the as-prepared catalysts toward methanol electrooxidation, CVs were obtained in 0.1 M HClO₄ + 2.0 M CH₃OH solution (Fig. 3). The onset potential of the Pt/NB-TiO₂ (+0.11 V) is lower than that of the Pt/C (+0.14 V). In the forward scan, peak potentials in methanol electrooxidation are +0.72 and +0.76 V for the Pt/TiO₂-NB and Pt/C, respectively. The more negative peak potential of the Pt/TiO₂-NB indicates an excellent activity in comparison with the Pt/C. Also, the Pt/TiO₂-NB for methanol electrooxidation shows a 2.17 times higher forward anodic peak current density (3.54 mA cm⁻²) than that of the Pt/C (1.63 mA cm⁻²). The Pt/TiO₂-NB indicates an excellent electrocatalytic activity during methanol electrooxidation in comparison with the Pt/C. Furthermore, the Pt/TiO₂-NB maintains higher catalytic activity than that of the Pt/C in methanol electrooxidation after the cycling test (Fig. 3). After the 400 cycles, the electrooxidation current density of the Pt/TiO₂-NB (1.80 mA cm⁻²) at 0.6 V is still higher than that of the Pt/C (0.21 mA cm⁻²). Also, in the forward anodic peak after the cycling test, the Pt/TiO₂-NB shows slightly reduced current density (27.7%) for methanol electrooxidation. In contrast, the Pt/C exhibits the abruptly reduced current density (80.9%) for methanol electrooxidation.

Fig. 3 Cyclic voltammograms (CVs) before and after the stability test of (a) Pt/TiO₂-NB and (b) Pt/C in 0.1 M HClO₄ + 2 M CH₃OH with a scan rate of 50 mV s⁻¹ at 25 °C. To evaluate electrochemical stability of the catalysts, the 400 cycle CVs of the catalysts were carried out in 0.1 M HClO₄.

The Pt/C after the cycling test (Fig. 4(a) and (b)) shows a larger average size of 4.85 nm and wider size distribution than those of the Pt/C before the cycling test (Fig. S1, ESI†) resulting in deteriorated electrocatalytic properties for methanol electrooxidation. On the other hand, the Pt/TiO₂-NB maintains the size distribution and morphology of the catalysts after the cycling test (Fig. 4(c) and (d)) resulting in enhanced electrocatalytic properties. In particular, it has been reported that the rutile phase of TiO₂ may exhibit an excellent durability and the existence of rutile TiO₂ can be beneficial to the formation of the surface structures of metallic catalysts favorable for catalytic properties.⁹ Accordingly, the superior electrocatalytic activity and stability of the Pt/TiO₂-NB for methanol electrooxidation may be due to the rutile phase and the 3-dimensional nanostructure of the TiO₂-NB support. Thus, it is concluded that the TiO₂-NB support may be a promising support for the DMFCs. However, the membrane–electrode-assembly (MEA) using the TiO₂-NB supported catalysts will be applied to the DMFCs for our further works.

Fig. 4 TEM images and particle size distribution of Pt/C after the durability test (a), (b). [The inset indicates the FFT pattern of the Pt NP.] TEM images and particle size distribution of Pt/TiO₂-NB after the durability test (c), (d). [The inset indicates the FFT pattern of the TiO₂-NB support and Pt NP.]

In summary, we have reported the TiO₂ nanostructure material for electrocatalytic reactions. The as-synthesized TiO₂ supports have exhibited relatively high surface area with a 3-dimensional branch-type structure. The Pt/TiO₂-NB exhibits much improved electrocatalytic properties for methanol electrooxidation because of the stable TiO₂ nanostructure supports.

This work was supported by New & Renewable Energy R&D program (2008-N-FC12-J-01-2-100) and the “program for CITG” support program supervised by the NIPA (National IT Industry Promotion Agency) (NIPA-2010-0004).

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Footnotes

† Electronic supplementary information (ESI) available: Experimental section and supplementary data. See DOI: 10.1039/c1cy00060h

‡ These authors contributed equally to this work.

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