Regulation Of Light Metal Hydrogen Hydrogen Storage Material

Environmental contamination and the depletion of fossil fuels have caused increasing concern. Hydrogen is regarded as a kind of ideal fuel that can be used to replace the conventional fuels. However, to realise practical application of hydrogen as a fuel, several challenges must be overcome, for example, the need to develop advanced storage materials. Solid state hydrogen storage materials, especially those including light elements, have been exhaustively investigated to achieve the high target hydrogen capacities.M（metal）-N-H based materials have attracted considerable attention since the first report by Chen et al. According to the Chen’s report, Li₃N can reversibly store over 10 mass% of hydrogen through two reactions （reaction 1 and reaction 2）. LiNH₂+LiH（?） Li2NH+H₂ （1） Li2NH+LiH（?） Li₃N+H₂ （2）Much effort has been expended to improve the performance of this system. Mixing other atoms into the LiNH₂ lattice and doping other compounds into the LiNH₂-LiH composite are effective method for enhancing the hydrogen desorption/adsorption properties of the LiNH₂-LiH composite.Firstly, in the present work, the hydrogen storage properties of the LiNH₂-LiH system was investigated and discussed. It was found that, with ball milling time increased, the size of the LiNH₂-LiH sample was reduced and the peak temperature of dehydrogenation was decreased, the hydrogen desorption kinetic of the LiNH₂-LiH system was improved. The LiNH₂-LiH sample ball milled 2 h exhibits a broad hydrogen desorption curve with a peak at 299 ℃. For the LiNH₂-LiH sample which ball milled for longer time （LiH or LiNH₂ was pretreated by ball milling at 450 rpm for 8 h before being used as precursor in preparation of the LiNH₂-LiH）, dehydrogenation peak appear at 259℃, the peak temperature of dehydrogenation decreased by 40℃. With ball milling time increased, particles become homogeneous and more uniform in size. The particle size of the samples from 50-500 nm to 100-300 nm and the activation energy from 103.1 kJ/mol decreased to 96.4 kJ/mol.Secondly, in this paper, the hydrogen storage properties of the LiNH₂-LiH system doped with potassium halide （KF, KCl, KBr or KI） are investigated and discussed. Interestingly, an obvious enhancement of the hydrogen storage properties is achieved by introducing KF into the LiNH₂-LiH system, while KC1, KBr or KI does not show obvious effect on improving the hydrogen storage properties of the LiNH₂-LiH system. In comparison with that of the LiNH₂-LiH composite, the hydrogen desorption curve of the LiNH₂-LiH-0.05KF composite becomes sharp and the dehydrogenation onset and peak temperatures are decreased by about 36 ℃ and 38 ℃, respectively. The cyclic capacity and desorption kinetic of the LiNH₂-LiH-0.05KF composite in the tenth cycle are still much better than that of the LiNH₂-LiH composite in the second cycle. Cycle performance increased at least 4 times. Detailed structural investigations reveal that during the heat-treatment, the doped potassium fluoride can react with LiH and convert to potassium hydride, which induces the improvement in hydrogen storage properties of the LiNH₂-LiH system.Lastly, in this study, we first examined the hydrogen release, uptake, and reversibility properties of the KLi₃（NH₂）4-4LiH system and elucidated the influence of potassium cations’ substitution into LiNH₂ on the hydrogen storage properties of the LiNH₂-LiH composite. Our results show that mixing of the potassium cations into LiNH₂ can destabilize LiNH₂, therefore decreasing the hydrogen desorption temperature and enhancing hydrogen desorption rate of the LiNH₂-LiH system. For the as-milled KLi₃（NH₂）4-4LiH sample, only 0.28 h is required to completely release approximately 2.4 wt% hydrogen in the first cycle, and 0.4 h is then required to completely absorb approximately 2.4 wt% hydrogen. The hydrogen absorption and desorption rates remain almost unchanged for 30 cycles. By contrast, the hydrogen desorption rate of the LiNH₂-LiH sample decreases drastically from 3.4 wt%/h for the first desorption to 0.7 wt%/h for the second desorption. Examining the change of the hydrogen capacity with the cycling number shows that the capacity of KLi₃（NH₂）4-4LiH sample remains at ca.2.4 wt% even after 30 cycles while the amount of hydrogen desorbed from the LiNH₂-LiH composite decreased drastically only after 1 cycle, from 4 wt% for the first desorption to 1.7 wt% for the second desorption. Notably, the KLi₃（NH₂）4-4LiH composite exhibits a superior cycling stability compared with the LiNH₂-LiH composite prepared under the same condition.