Fossil fuels occupying 90% or more of currently used energy demand cannot be recycled, and their reserves are limited. The fossil fuels, when used, pollute the environment by emitting air pollutants such as NOx, SOx, dust. Also, an increase in the atmosphere concentration of carbon dioxide emitted by the combustion of fossil fuels has recently raised anxiety about global warming.
As a new energy carrier for substituting for such a fossil fuel, hydrogen, which has been widely used as a raw material of a chemical product, and a process gas of a chemical process, is spotlighted. Hydrogen has advantages as described below. First, hydrogen, when used as a raw material, does not generate a pollutant except for a trace of NOx through combustion. Thus, it is easy to use hydrogen as a fuel for direct combustion or a fuel for a fuel cell. Second, hydrogen can be easily produced as a gas or a liquid, and can easily be stored in various forms such as high-pressure gas, liquid hydrogen, metal hybride, or the like. Third, hydrogen can be produced in a large amount from water. Also, hydrogen, even after being used, is recycled as water, thereby eliminating anxiety about the exhaustion of natural resources. Fourth, hydrogen can be used in almost all fields, currently using an energy system, such as industrial elementary materials, general fuels, hydrogen fueled automobiles, hydrogen fueled aircrafts, fuel cells, or the like.
However, in order to use hydrogen, there is required a medium capable of easily storing a large amount of hydrogen. Accordingly, in order to develop a hydrogen storage medium, researches on a hydrogen storage alloy, a carbon nanotube, a zeolite, or the like have been recently in progress. However, in a case of the hydrogen storage alloy, since a stored material is chemically bonded to a hydrogen molecule with a very high binding energy, there is a problem in that there is required another energy for releasing the bonded hydrogen. Also, unlike the hydrogen storage alloy, in a case of the carbon nanotube, or the zeolite, since hydrogen molecules are physically adsorbed on the surface of the materials with a very low binding energy, there is a problem in that storage capacity of hydrogen at room temperature•atmospheric pressure is very low.
Professor Yaghi's research team of the University of California, Berkeley has recently reported on a covalent organic framework (COF) (US 2006/0154807 A1). The COF is a material formed through a covalent bond between only atoms (such as hydrogen, boron, carbon, nitrogen, oxygen, etc.), and may be formed by a condensation reaction of benzene diboronic acid (BDBA). Such a COF has not only a rigid micro-(or meso-)porous structure, but also a high thermal stability, and a low density. Also, it has a specific surface area higher than some conventionally known materials such as zeolite, porous silicates, etc.
Accordingly, researches on the use of such a COF as a new hydrogen storage medium are in progress.