Metal catalyst tips on carbon nanohorns improve durability and function when used in fuel cells according to NEC scientists.
In U.S. Patent Application 20100099551, NEC Corporation (Tokyo, JP) scientists Ryota Yuge, Masako Yodasaka and Sumio Iijima disclose carbon nanohorns (CNH) that are oxidized to make an opening in the side of the CNH. A substance to be included, e.g., a metal, is introduced through the opening. The inclusion substance is moved to a tip part of the carbon nanohorn through heat treatment in vacuum or an inert gas.
The CNH is further heat treated in an atmosphere containing oxygen in a low concentration to remove the carbon layer in the tip through catalysis of the inclusion substance. This exposes the inclusion substance. If the inclusion substance is a metal which is not moved to a tip part by the heat treatment in vacuum or an inert gas, the carbon part surrounding the fine catalyst particle is specifically burned by a heat treatment in an low oxygen concentration atmosphere, while utilizing the catalysis. Thus, the fine catalyst particle is fixed to the tip part of the CNH.
According to the inventors, enhanced dispersion is provided by supporting the fine particles of the catalyst in the tip section of the carbon nanohorn and reduced particle size and particle size monodispersity of the fine particles of the catalyst is provided by the fact that the size of the fine particle of the catalyst is determined by the diameter of the open pore formed in the carbon nanohorn are achieved, so that highly reactive catalyst can be realized.
In addition, since the fine particles of the catalyst is supported at a tip edge of a sheath of the carbon nanohorn, an agglomeration and a coarsening of the fine particles of the catalyst due to a thermal processing for activation or a long term use can be avoided, thereby allowing better catalyst characteristics being stably maintained for longer periods.
Carbon materials are conventionally utilized for catalyst carriers, adsorbents, separator agents, ink, toner or the like, and advents of nano-carbon materials having nano-sized dimensions such as carbon nanotube, carbon nanohorn and the like have in recent years drawn attention to these materials for use in fuel cells, batteries and ultracapacitors as well as other applications.
In particular, characteristic structures of such carbon nanotube and carbon nanohorn have attracted attention for industrial catalyst carriers, as one of the optimum catalyst carriers, and have been recently utilized for catalyst carriers of fuel cells.
In particular, characteristic structures of such carbon nanotube and carbon nanohorn have attracted attention for industrial catalyst carriers, as one of the optimum catalyst carriers, and have been recently utilized for catalyst carriers of fuel cells.
FIG. 2: It is a figure, showing an electron microscopy image of a carbon nanohorn
FIG. 4: It is a figure, showing an electron microscopy image of a gadolinium (Gd) metal-containing carbon nanohorn produced by NEC.
It is generally known that a performance of a catalyst depends not only on physical properties of the carrier for supporting the catalyst metallic fine particles but also on mean particle size or particle size distribution of the metal supported thereon. Thus, in the conventional catalyst-supporting carbon nanohorn, catalyst fine particles are adherently supported on the surface of the carbon nanohorn. Catalyst components, which are supported to the carrier, are generally employed after being activated or stabilized by an annealing treatment or the like, and such treatment for the conventional catalyst-supporting carbon nanohorn may easily induce an agglomeration of the catalyst to cause a coarsening of the catalyst, causing a deterioration of the characteristics of the catalyst.
Further, the fine particles of the catalyst on the carrier are easily moved in the conventional catalyst-supporting carbon nanohorn to cause an agglomeration and a coarsening of the fine particles of the catalyst (metallic particles) during the use, so that the catalyst characteristics are deteriorated in the long term use, causing a serious problems against the durability of the fuel cell. The object of the NEC carbon nanohorn is to solve the problems in the conventional technology, and more specifically to achieve supporting catalyst fine particle on a carbon nanohorn in a highly dispersed condition and to prevent a move of fine particles of the catalyst on the surface of the carbon nanohorn to avoid an agglomeration and a coarsening of the fine particles of the catalyst, thereby allowing better catalyst characteristics being stably maintained for longer periods.
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