(236e) Reaction Pathways in the Gas/Solid Hydrolysis of Chemical Hydrides as a Novel Approach to Hydrogen Storage and Generation

Technologies based on simple and complex chemical hydrides have the potential to meet DOE FreedomCAR targets and requirements for hydrogen storage systems. Traditionally, aqueous (liquid-phase) hydrolysis has been used to generate hydrogen from chemical hydrides. This paper presents an investigation of thermodynamic and kinetic aspects of the steam hydrolysis of chemical hydrides, specifically NaBH₄. It is known that chemical hydrides react with water to produce H₂, e.g. NaBH₄ + (2+x) H₂O → 4H₂ + NaBO₂ ? xH₂O, where x represents excess unreacted water that is bound in the borate byproduct. While aqueous hydrolysis with pure H₂O is thermodynamically favored, it suffers severe kinetic limitations. We have shown that nearly complete yields are obtained by the gas phase hydrolysis route. This discovery suggests a new pathway for efficient storage and production of H₂ from chemical hydrides. Therefore, it is imperative to completely characterize the gas / solid hydrolysis reaction. We seek to utilize thermodynamic and intrinsic kinetic information of the hydrolysis reaction, including characterization of the hydrated byproducts, to understand the reaction pathway more completely. This information is vital to efforts to optimize reaction conditions for H₂ generation rates and to minimize water utilization. Such fundamental information must be obtained to evaluate this pathway rigorously as a means of delivering hydrogen for devices such as fuel cells and combustion engines. In the conventional method for H₂ production from chemical hydrides an aqueous solution of NaBH₄ in the presence of an acid catalyst promotes the release of H₂; however, large quantities of water are required in this method. Our approach is to treat solid chemical hydrides with water vapor with the expectation that a smaller quantity of water required will lead to a lightweight storage system with dry feed and dry byproduct. This hydrolysis reaction is highly exothermic, providing a heat source that could be used to preheat the water necessary to generate steam. Experiments show that more than 80% yield of H₂ can be obtained when NaBH₄ powder reacts with pure water vapor. The use of liquid promoters is also being considered. The rates and yields are sensitive to operating temperature, amount of water and physical form of NaBH₄. This paper will also describe a thermal dehydration study of the hydrated borate byproducts of the reaction. The gravimetric efficiency of the process depends largely on the state of water in these solids. Hydrated borates apparently exist in metastable states that are sensitive to changes in temperature due to loss in structural water. Characterization of the solid borate products has shown that NaBO₂^.2H₂O is the main product of the reaction.