Thermal processes in sodium borohydride hydrogen storage systems

Depleting fossil fuel supplies, environmental pollution, and global warming demand a new energy carrier for future transportation systems, and hydrogen has been recognized as a promising candidate. On-board hydrogen storage, among other challenges, must be addressed before a hydrogen economy could be realized. Sodium borohydride hydrogen storage systems have been identified as a potential candidate to meet the storage challenge. Calorimetery measurements have been conducted to clarify the inconsistency in the heat of reaction data of sodium borohydride hydrolysis reported in prior literature. The heat of reaction was measured to be 210±11 kJ/mol NaBH 4, significantly less than the often cited value of 300 kJ/mol NaBH 4. Reaction kinetics of sodium borohydride hydrolysis on ruthenium catalyst at different temperatures has also been measured, and a Langmuir-Hinshelwood kinetic model has been developed. The newly developed kinetic model captures zero-order kinetic behavior at low temperatures and first-order kinetic behavior at high temperatures. A 1kWe sodium borohydride system was designed and fabricated. Its behavior at the system level was tested under different flow rates, concentrations, inlet temperatures and operating pressures. Higher flow rates and higher operating pressures decrease chemical conversion, while higher concentrations increase chemical conversion. The temperature profile was found to be a strong indicator of chemical conversion inside the reactor. A one-dimensional homogeneous reactor model has been developed based on experimental data, and its predictions match measured data very well. Discharged products from 15% NaBH4 or higher concentrations crystallize upon cooling to room temperature and become a solid that would be very difficult to remove from the discharge tank. Considering the practical challenges of keeping the discharged products warm and preventing crystallization, this work concludes that 10 to 15% NaBH4 may be the highest concentration that could be used for practical hydrolysis. Such solutions would have gravimetric densities of 2.1 to 3.1 wt% hydrogen, falling short of the DOE target of 6 wt% for 2010