Carbon nanotube-supported Pt catalysts (Pt/CNT) for the cathode in a polymer electrolyte fuel cell (PEFC) were covered with silica layers using tetraethoxysilane (TEOS) and also methyltriethoxysilane (MTEOS) to improve the catalyst durability under the severe conditions at the PEFC cathode. Both the silica-coated Pt/CNT catalysts had excellent durability for potential cycling between 0.6 and 1.0 V (vs RHE) in N 2 -purged 0.1 M HClO 4 electrolyte, while Pt/CNT without silica coating was significantly deactivated due to an increase of the Pt metal particle size. Silica-coated Pt/CNT prepared from MTEOS had similar activity for the oxygen reduction reaction as Pt/CNT without silica coating, whereas the silica coverage obtained with TEOS slightly reduced the catalytic activity of the Pt/CNT catalyst. The silica layers prepared from MTEOS are more hydrophobic than those prepared from TEOS due to the presence of methyl groups. In addition, the silica layers prepared from MTEOS have larger pores than those prepared from TEOS. The hydrophobic silica layers with larger pores in the silica-coated Pt/CNT do not inhibit the diffusion of the reactants (oxygen) and the discharge of the products (water) during the oxygen reduction reaction.
Carbon nanotube (CNT)-supported Pt metal nanoparticles were covered with silica layers by utilizing the
successive hydrolysis of 3-aminopropyl-triethoxysilane and tetraethoxysilane on CNTs with Pt hydroxide.
The CNT-supported Pt metal particles covered with silica layers (denoted as SiO2/Pt/CNT) were used as the
electrocatalysts. SiO2/Pt/CNT electrocatalysts showed a high stability for the repeated potential cycling
experiment, whereas Pt/CNT electrocatalysts were deactivated seriously for the experiment because of the
growth of Pt metal particles in size. The silica layers in SiO2/Pt/CNT prevent the dissolution of Pt metal
particles as well as the migration and agglomeration of Pt metal particles on the supports, which results in the
improvement of the stability of Pt/CNT electrocatalysts.
Nanocomposite films of gold nanoparticles and TiO2 gel have been successfully prepared by combination of a surface sol−gel technique and layer-by-layer assembly. Thanks to surface hydroxyl groups of gold nanoperticles, gold particles were deposited as close-packed monolayers on TiO2 gel layers. The resulting nanocomposite contained ca. 50 wt % of gold. Multilayer films were successfully prepared by alternate assembly of TiO2 gel and gold nanoparticles.
Multiwalled carbon nanotube ͑CNT͒-supported Pt nanoparticles ͑Pt/CNT͒ were covered with silica layers by successive hydrolysis of 3-aminopropyl-triethoxysilane and tetraethoxysilane on CNTs with Pt metal precursors, followed by reduction with hydrogen. The Pt/CNT covered with silica layers ͑SiO 2 /Pt/CNT͒ was used as a cathode catalyst for a proton exchange membrane fuel cell ͑PEMFC͒. The activity of SiO 2 /Pt/CNT catalyst for the oxygen reduction reaction in a single-cell PEMFC was similar to that of Pt/CNT, in spite of the uniform coverage of Pt with silica layers, indicating that the coverage of Pt/CNT with silica layers did not appreciably decrease the catalytic activity. In addition, SiO 2 /Pt/CNT electrocatalyst showed high stability during potential cycling from 0.05 to 1.20 V vs reversible hydrogen electrode in an aqueous H 2 SO 4 electrolyte, whereas Pt/CNT significantly deactivated during the experiment. The structural change of Pt species in these electrocatalysts during potential cycling was investigated by transmission electron microscopy images and Pt L III -edge X-ray absorption fine structure. The crystallite size of Pt metal in SiO 2 /Pt/CNT did not change appreciably during the potential cycling, while Pt metal crystallites in Pt/CNT seriously aggregated. Silica layers enveloping Pt metal particles in SiO 2 /Pt/CNT prevent the dissolution and redeposition of Pt metal particles as well as the agglomeration of Pt metal particles on the supports.
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