(424b) The Application of Steam Hydrolysis of Chemical Hydrides to Facilitate Hydrogen Storage and Generation

Chemical hydrides are a well-known class of compounds that have the potential for high gravimetric and volumetric storage densities for hydrogen. Chemical hydrides react with water to release hydrogen from both reactants via hydrolysis reactions. Sodium borohydride (NaBH₄) is particularly attractive due to its relative availability and mild (more easily controlled) reaction conditions. One successful commercial application of aqueous phase hydrolysis utilizes a solution of NaBH₄ and an acid catalyst to release hydrogen gas. This method of hydrogen storage is constrained by requirements of excess water and high pH for long-term storage stability and appreciable yields, and the need for catalysts to promote the reaction. All hydrolysis approaches must deal with the significant heat of reaction, as well as the long-term need to recycle the borate byproducts. Research at the University of South Carolina proves that low temperature hydrolysis of dry solid chemical hydrides can be accomplished with steam, without the need for a catalyst, and with yields approaching 100%. Several hydrides, including NaBH₄, NaAlH₄, LiAlH₄, and LiBH₄ have been studied and are potentially useful as compact sources of hydrogen for portable fuel cells. The rate of steam hydrolysis of all hydrides studied has been found to be dependent on steam flow rate and temperature, with the yield and rates being higher at lower temperatures. NaBH₄ and LiAlH₄ are promising hydrides for hydrogen storage via steam-fed hydrolysis on the basis of high H2 yield and pH of the unreacted water. Recent studies with NaBH4 have compared the hydrolysis behavior of packed bed reactor configurations with the behavior of recrystallized thin films of NaBH₄ on glass bead packings. The thin films exhibit greatly increased H₂ production rates with decreased yields. Additional research at the University of South Carolina is focused on the quality and preparation of the chemical hydride material, as opposed to the physical state of the H₂O used for the hydrolysis reaction. Compact storage of the chemical hydride is an alternate route that will provide inherently superior H₂ storage capacities and long term storage stability for practical reactor design. Thermodynamic considerations show that, in principle, the heat liberated by the hydrolysis reaction is more than sufficient to vaporize the water stoichiometrically required for 100% H₂ yield via steam hydrolysis. Thus, there is the possibility of developing an autothermal hydrogen reactor/delivery system that produces pure hydrogen in near-100% yield without a catalyst at mild conditions and that uses a minimum of water. Furthermore, the chemical hydride can be stored safely for longer periods of time in a compact medium, with the solid reaction products of the hydrolysis reaction nearly free of water to potentially facilitate long-term recycling and regeneration to the hydride.