1. Field of the Invention
The present invention relates to a carbon nanosphere with at least one opening, a method of preparing the same, a supported catalyst, and a fuel cell, comprising the same. In particular, the invention relates to a carbon nanosphere with at least one opening, a method of preparing the same, a supported catalyst in which metal catalyst particles are supported on the carbon nanosphere with at least one opening, and a fuel cell that uses the supported catalyst.
2. Description of the Related Art
Fuel cells are clean energy sources that have received considerable interest as fossil fuels alternatives. A fuel cell is a power generating system that produces direct current electricity through an electrochemical reaction between fuel such as hydrogen, natural gas, or methanol and an oxidizing agent.
In general, a fuel cell includes an anode (fuel electrode) where a supplied fuel is oxidized, a cathode (air electrode) where the oxidizing agent is reduced, and an electrolyte membrane that is interposed between the anode and the cathode and provides a path for transporting ions that are produced at the anode to the cathode. Electrons are generated through the oxidation of the fuel at the anode, which then flow via an external circuit and are returned to the cathode to reduce the oxidizing agent.
Among the most important features of the fuel cell are catalysts that are present at the anode and the cathode to catalyze the reactions that occur at the electrodes. Thus, many experiments have been conducted to increase the activity of the catalysts used in the electrodes. The catalytic activity increases as the reaction surface area of the catalyst increases, and thus, the reaction surface area may be increased by decreasing the particle diameter of the catalyst to uniformly distribute the catalyst on the electrodes.
Conventionally, a platinum (Pt) catalyst, for example, is applied to a carbon cloth, etc. However, the catalyst cannot be uniformly dispersed on the carbon and the surface area and electrical conductivity of the carbon support, etc., are not sufficiently high.
A metal catalyst that is supported on porous carbon powders has been suggested. The specific surface area of the porous carbon powders can be controlled such that the ability to impregnate a catalyst is high. However, when the carbon powders are graphited or crystallized to increase the electrical conductivity, the structures of the carbon powders are altered. In addition, the electrical conductivity cannot be increased sufficiently. Thus, the surface properties of the carbon powders must be improved.
In order to overcome these problems, the use of carbon nanotubes or nanohorns as supports has been suggested and a significant amount of research has been done in this area.
Carbon nanotubes are very fine cylindrical materials that have a diameter of about several nm to about several tens of nm, a length of about several μm to about several hundreds of μm, high anisotropy, and come in various structures and shapes such as single-walled, multi-walled, or rope shapes. In carbon nanotubes, one carbon atom bonds to three other carbon atoms to form a hexagonal honeycomb (a pentagonal or heptagonal honeycomb may be formed depending on the curvature radius at the bounding position of the carbon atom). Carbon nanotubes may have metallic or semiconductor properties depending on their structures, are mechanically strong (about 100 times stronger than steel), have chemical stability, high thermal conductivity, and a hollow structure. Thus, carbon nanotubes may be used in various microscopic and macroscopic applications, such as catalyst carriers.
Carbon nanotubes have high electrical conductivity and thus, can increase the utilization of the electrical energy that is generated during an electrochemical reaction. However, a catalyst can be supported only on the outer walls of carbon nanotubes and the surface area on which the catalyst can be substantially supported is small, relative to the total surface area of the carbon nanotubes. That is, the capability to impregnate a catalyst is low. Further, when the carbon nanotubes have a high aspect ratio, they cannot be uniformly dispersed easily on the surface of an electrode during formation of the electrode. In particular, the diffusion resistance of the material is high due to the closed structures of their ends, which is one of the largest obstacles to their use as a catalyst carrier.
In order to overcome the problems, the use of carbon nanohorns has been suggested. Carbon nanohorns have a conical structure, similar to ends of nanotubes that are cut off from the nanotubes. The carbon nanohorns are very short and a catalyst can be impregnated even in their innermost regions. However, carbon nanohorns have an internal diameter of about 1 nm and the optimal particle size of the catalyst is about 2-3 nm. Thus, the catalyst cannot be sufficiently supported on the carbon nanohorns. In this case, the catalyst is supported only on the outer walls of the nanohorns and the advantage of the high surface area of the nanohorns is lost. Further, a nanohorn has one closed end, and thus, when nanohorns are used as catalyst supports, a fuel cannot flow smoothly, resulting in a low catalytic efficiency.
In order to overcome these problems, the use of short carbon nanotubes that have open ends has been suggested. However, since the carbon nanotubes are flexible and resistant to an applied stress, short carbon nanotubes that have open ends cannot be prepared easily.
Methods for cutting a carbon nanotube in order to prepare a short carbon nanotube that has open ends have been suggested. One method includes using ultrasonic waves (K. L. Lu et al., Carbon 34, 814-816 (1996); K. B. Shelimov et al., Chem. Phys. Lett. 282, 429-434 (1998); J. Liu et al., Science 280, 1253-1256 (1998)). However, the resulting short carbon nanotubes have a low yield and an inconsistent relative length. Another method includes using an STM voltage (L. C. Venema et al., Appl. Phys. Lett. 71, 2629-2631 (1999)). In this method, the resulting short carbon nanotubes do not have open ends. An additional method includes using ball milling, but short carbon nanotubes that have both ends open cannot be produced.
A conventional carbon nanotube can be processed into a short carbon nanotube with both ends open by a mechanical or chemical treatment, but the processing cannot be performed easily due to a strong binding force between crystalline carbons.