Characterization of NH 3 formation in desorption of Li–Mg–N–H storage system

Abstract

Hydrogen energy may provide the means to an environmentally friendly future. One of the problems related to its application for transportation is “on board” storage. Hydrogen storage in solids has long been recognized as one of the most practical approaches for this. Recently the hydrogen storage system, (Li 3N + 2H 2 ⇔ LiNH 2 + 2LiH), was introduced by Chen et al. [P. Chen, Z. Xiong, J. Luo, J. Lin, K.L. Tan, Nature 420 (2002) 302–304. [1]]. This type of material has attracted a great attention of the researchers from the metal hydride research community due to its high reversible storage capacity, up to 11.5 wt%. Currently the Li–Mg–N–H system has been shown to be able to deliver 5.2 wt% reversibly at a H 2 pressure of 30 bar and temperature of 200 °C. The accessibility of the capacity beyond 5.2 wt% is being actively explored. One of the issues related to the application of the metal–N–H storage systems is NH 3 formation that takes place simultaneously with H 2 release. NH 3 formation will not only damage the catalyst in a fuel cell, but also accelerate the cyclic instability of the H-storage material since the metal–N–H system turns into a metal–H system after loosing nitrogen and, therefore, it would not function at the temperature and pressure range designed for the metal–N–H system. The accurate determination of the amounts of NH 3 in the H 2 is, therefore, very important and has not been previously reported. Here a novel method to quantify NH 3 in the desorbed H 2, the Draeger Tube, is reported as being suitable for this purpose. The results indicate that the concentration of NH 3 in desorbed H 2 increases with the desorption temperature. For the (2LiNH 2 + MgH 2) system the NH 3 concentration was found to be 180 ppm at 180 °C and 720 ppm at 240 °C.