Hydrogen has attracted attention in the past decades as a source for clean energy production, envisioned to be utilized in fuel cells due to its capability to release energy with high efficiency through electro-oxidation reactions. Conventional hydrogen energy conversion systems are typically based on hydrogen storage in the form of pressurized molecular hydrogen, liquefied hydrogen, carbonaceous materials, and as atomic hydrogen in metal hydrides.
The use of metal-borohydrides, for example sodium borohydride (NaBH4), as a source of available hydrogen has been suggested, mainly due to their relatively high hydrogen content. Such metal-borohydrides have been contemplated for use in hydrogen storage systems, where controlled release of hydrogen is desired, owing to their higher gravimetric capacity (compared to metal hydrides) and higher volumetric capacity (compared to compressed and liquefied hydrogen). The alkaline solutions disclosed therein may be directly used to power fuel cells.
The common process known in the art for the preparation of sodium borohydride is the Schlesinger method, generally summarized in equation (1), starting from sodium hydride and methyl borate:4NaH+B(OCH3)3→NaBH4+3NaOCH3  (eq. 1)
The Schlesinger process has been widely used in the pharmaceutical field, and is based on a multi-stage process which yields a high purity grade sodium borohydride. Therefore, sodium borohydride produced by the Schlesinger method is often too costly to be utilized in large-quantities and commercial processes, in which a high purity grade of metal-borohydride is not required.
For large scale utilization, such as in full cells and commercial hydrogen generators, there is a need for simple and cost effective production of metal borohydride.
Scarce research has been carried out thus far on synthesis processes that may have the potential of providing commercial, bulk quantities of metal-borohydride in a cost-effective manner; research has been mainly focused on synthesis of sodium borohydride starting from sodium metaborate. However, to date such processes have yet to be commercialized.
Further, hydrogen has attracted attention in the past decades as a source for clean energy production, envisioned to be utilized in fuel cells due to its capability to release energy with high efficiency through electro-oxidation reactions. Conventional hydrogen energy conversion systems are typically based on hydrogen storage in the form of pressurized molecular hydrogen, liquefied hydrogen, carbonaceous materials, or as atomic hydrogen in metal hydrides.
Use of metal-borohydrides, for example sodium borohydride (NaBH4), as a source of available hydrogen has been suggested, mainly due to their relatively high hydrogen content. Such meal-borohydrides have been contemplated for use in hydrogen storage systems, where controlled release of hydrogen is desired, due to their higher gravimetric capacity (compared to metal hydrides) and higher volumetric capacity (compared to compressed and liquefied hydrogen).
Typically, at certain pH values, NaBH4 and KBH4 form relatively stable aqueous solutions, which can be stored for prolonged periods of time without significant decomposition. However, due to their stability in highly alkaline solutions, hydrolysis for obtaining the desired hydrogen gas often requires the use of a catalyst for reducing the activation energy and promoting the decomposition of the metal-borohydride. Such typical catalysts known in the art are ruthenium, rhodium and platinum-based, which are relatively expensive, thereby increasing the costs of the hydrogen generating systems comprising them. Other known catalysts are organometallic complexes.
As the processes known in the art are either costly or require relatively complex synthesis, there exists a need for simpler processes for production of catalysts for promoting decomposition of metal-borohydrides in aqueous solutions. The required catalysts should be cost effective, provide controlled decomposition rates and show stability and efficiency over prolonged periods of usage in alkaline solutions.