Evaluation of the vapor hydrolysis of lithium aluminum hydride for mobile fuel cell applications

The controlled vapor hydrolysis of LiAlH₄ has been investigated as a safe and predictable method to generate hydrogen for mobile fuel cell applications. A purpose-built vapor hydrolysis cell manufactured by Intelligent Energy Ltd, was used as the reaction vessel. Vapor was created by using saturated salt solutions to generate humidity in the range 46-96% RH. The hydrolysis products were analyzed by thermogravimetric analysis and powder X-ray diffraction and compared with possible hydroxide-based phases characterized using the same methods. Analysis of the products of the LiAlH₄ vapor hydrolysis reaction at relative humidity in excess of 56%, indicated complete decomposition of the LiAlH₄ phase and for-mation of the hydrated layered double hydroxide [LiAl₂(OH)₆]₂CO₃·₃H₂O, rather than the simple salts, LiOH and Al(OH)₃, previously suggested by the literature. The high level of hydration of the LDH (12 % wt water) and presence of carbonate indicated that the feed stream was contaminated with CO₂ and that the highly hydrated and hygroscopic product would be detrimental to the mobile hydrogen production process, restricting recyclability of the water fuel cell byproduct and low-ering the gravimetric density of LiAlH₄. Carrying out the vapor hydrolysis reaction in a glove box in the absence of CO₂ indicated that the hydroxide derivative of the LDH, [LiAl₂(OH)₆]OH·₂H₂O, could be formed instead, but the water content was even more significant, equating to 17 % of the carried weight. Thermogravimetric analysis showed that water was re-tained to 300 °C and 320 °C in the two phases making thermal recycling of the water retained impractical and casting doubt on whether generating hydrogen on the move by vapor hydrolysis of LiAlH₄ is practical.