Li–Mg–N–H: Recent investigations and development

Metal amide containing material is one of the more promising hydrogen storage materials for motor vehicular “on-board” applications. One of the issues related to the application of metal–N–H storage systems is NH3-formation that takes place simultaneously with H2 release. NH3 in desorbed hydrogen, when feeding a fuel cell, will not only damage the catalyst in the fuel cell, but also accelerate the material degradation. Accurate determination of the amounts of NH3 in the H2 is, therefore, very important. A novel method to quantify NH3 in the desorbed H2, the Draeger Tube, is reported to be suitable for this purpose. The results indicate that the concentration of NH3 in the desorbed H2 from the (2LiNH2 + MgH2) system increases with desorption temperatures, from 180 ppm at 180 °C to 720 ppm at 240 °C. The capacity loss after 270 cycles at a temperature of 200 °C is 25%, with 1/3 of the loss due to NH3-formation. More research is needed to determine the cause for the remaining capacity loss. This cyclic stability is very encouraging for a material prior to any formulation optimization. A water-saturated air exposure of the desorbed sample at 220 °C for 21 h, after a sequence of air-exposures at lower temperatures, was found to not affect its capacity or kinetics in subsequent desorption runs.