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 of 46-96% RH. The hydrolysis products were analyzed by thermogravimetric analysis (TGA) 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 a relative humidity in excess of 56% indicated complete decomposition of the LiAlH₄ phase and formation 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 layered double hydroxide (LDH) (12% wt water) and the 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 lowering the gravimetric density of LiAlH₄. Carrying out the vapor hydrolysis reaction in a glovebox 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. TGA showed that water was retained up to 300 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.