Vapour hydrolysis of complex metal hydrides for mobile hydrogen storage and delivery
The controlled vapour hydrolysis of metal hydrides has been investigated as a safe and predictable method to produce hydrogen for mobile applications and the products analysed by thermogravimetric analysis, infrared spectroscopy, and powder X-ray diffraction. A vapour hydrolysis cell was adapted from the design originally provided by Paul Brack and manufactured by Intelligent Energy Ltd. Vapour was created by water evaporation, sonification and using saturated salts solutions to generate humidity. Hydrogen was initially measured by water displacement and then through mass balance indicating that 459±0.05 cm3 H2 was generated from 0.2 g of LiAlH4 starting material equating to 97 % percentage yield. Analysis of the products of the LiAlH4 reaction indicated the formation of the layered double hydroxide, [LiAl2(OH)6]¬2CO3·3H2O and not production of the simple salts, LiOH and Al(OH)3, as predicted by the literature. The high level of hydration of the LDH and presence of carbonate indicated that the feed stream was contaminated with CO2 and that the highly hydrated product would be detrimental to the mobile hydrogen production process; restricting recyclability of the water carried. Synthesis in a glove box indicated that the hydroxide derivative of the LDH, [LiAl2(OH)6]¬OH·2H2O, could be prepared, but the water content remained significant, equating to 17 % of the carried weight. Thermogravimetric analysis showed that water was retained to 320 °C making mobile heating impractical. Mimicking the products by preparing different LDH salts containing different anions (SO42-, NO3-, Cl- and OH-) was attempted to determine if the level of hydration could be reduced by including additives . To minimise the amount of water required for the hydrolysis of LiAlH4, vapour hydrolysis was used in a flow reaction set up. Using steam hydrolysis generated a rate of hydrogen production of 4.83 cm3 H2 s-1 of LiAlH4, while using ultrasonic humidification generated a hydrogen production rate of 21 cm3 s-1, followed by a slow rate of 0.015 cm3 H2 s-1 as the reaction plateaus.
Vapour hydrolysis of the lightweight metal hydrides LiAlH4, NaAlH4, NaH, CaH2, LiH have been investigated under flow conditions. A packed bed reactor was used to maximise gravimetric and volumetric energy density and control heat transfer from the hydrolysis of the hydride reactions. rates of 1.18, 0.766, 0.545, 0.603 and 1.11 cm3 H2 s-1 respectively, for each hydride, were achieved. Altering the packing density and water vapour flow rate achieved a target flow rate of 0.02 g s-1 kW-1 as defined by the DOE for direct delivery to PEM fuel cells.