Recycling process of sodium metaborate to sodium borohydride
Introduction
A fuel cell is a battery that is actuated with a gas, hydrogen. The energy obtained upon a reaction of hydrogen and oxygen is directly converted into electric energy. Since such a fuel cell has an efficiency much higher than that of conventional combustion engines, a fuel cell vehicle (FCV) is expected to have a high efficiency [1], [2].
Hydrogen can be stored in tanks of compressed [2], [3] or liquefied [3], or by adsorption on activated carbon [4], carbon nanotubes [3], [5] and graphite nanofiber [6], [7], or in a hydrogen-storing alloy [8]. Among these methods, the hydrogen-storing alloy is considered to play an important role in FCV. For the hydrogen-storing alloy, however, there are many problems to overcome: heaviness (small amount of storage per unit weight) due to its nature as an alloy, deterioration (the alloy turning into finer particles or changing its structure) upon repeated storage and release, and the problem of securing metallic resources when the alloy includes rare metals.
Therefore, attention has been given to the hydrolysis of chemical hydride such as [9], [10], [11], [12], [13]. The reaction of sodium hydride with boric oxide has been used for commercial production of [13]:
In the previous paper, we found that Pt– worked as an excellent catalyst for releasing hydrogen by the hydrolysis of [14]:The byproduct obtained by the reaction was .
is a nonreversible chemical hydride for the one-time hydrolysis generation of . Therefore, it is attractive to find a procedure for recycling back to . In this paper, we synthesize by annealing and , or and under high pressure. Recycling of to using coke or methane is also discussed.
Section snippets
Materials
Sodium metaborate (, Kojundo Chemical Laboratory Co., Ltd. Japan), magnesium hydride (, assay 90%, Sigma-Aldrich), magnesium (average particle size , Kojundo Chemical Laboratory) and silicon (average particle size , Kojundo Chemical Laboratory) were used for the reaction of . was mixed with or . The specimen was packed into a stainless-steel autoclave (internal volume: ). After closing the autoclave, the specimen was compressed at
Synthesis of using
The reaction of and is represented as follows:The standard-free-energy change for the reaction is , because the standard-free-energy changes of the compounds compared with the elements are and [15].
Therefore, the reaction can proceed spontaneously. Thus, the reaction of and was carried out. Fig. 1 shows the temperature desorption scan of the product reacted at
Acknowledgements
We are greatly indebted to Mr. Y. Kawai, Mr. K. Suzuki and Mr. H. Hayashi of the Toyota Central Research & Development Laboratories, Inc. for their help. We also thank Mr. S. Matsumoto, Mr. M. Kimbara and Dr. H. Nakanishi of Toyota Motor Corporation for the discussions.
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