Nanostructured complex hydride systems for solid state hydrogen storage
MetadataShow full item record
The present work reports a study of the effects of the formation of a nanostructure induced by high-energy ball milling, compositions, and various catalytic additives on the hydrogen storage properties of LiNH2-LiH and LiNH2-MgH2 systems. The mixtures are systematically investigated using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and a Sieverts-type apparatus. The results indicate that microstructural refinement (particle and grain size) induced by ball milling affects the hydrogen storage properties of LiNH2-LiH and LiNH2-MgH2 systems. Moreover, the molar ratios of the starting constituents can also affect the dehydrogenation/hydrogenation properties. In the LiNH2-LiH system, high-energy ball milling is applied to the mixtures of LiNH2 and LiH with molar ratios of 1:1, 1:1.2 and 1:1.4 LiH. The lowest apparent activation energy is observed for the mixture of LiNH2-LiH (1:1.2) milled for 25 h. The major impediment in the LiNH2-LiH system is the hydrolysis and oxidation of LiH, which causes a fraction of LiH to be inactive in the intermediate reaction of NH3+LiH→LiNH2+H2. Therefore, the LiNH2-LiH system always releases NH3, as long as a part of LiH becomes inactive, due to hydrolysis/oxidation, and does not take part in the intermediate reaction. To prevent LiH from undergoing hydrolysis/oxidation during desorption/absorption, 5 wt. % graphite is incorporated in the (LiNH2+1.2LiH) system. The DSC curve of the mixture does not show a melting peak of retained LiNH2, indicating that graphite can prevent or at least substantially reduce the oxidation/hydrolysis of LiH. Moreover, compared to the mixture without graphite, the mixture with graphite shows more hydrogen capacity, thus this mixture desorbs ~5 wt.% H2, which is close to the theoretical capacity. This system is fully reversible in the following reaction: LiNH2+LiH→Li2NH+H2. However, the equilibrium temperature at the atmospheric pressure of hydrogen (0.1 MPa H2) is 256.8°C for (LiNH2+1.2LiH) mixture, which is too high for use in onboard applications. To overcome the thermodynamic barrier associated with the LiNH2/LiH system, LiH is substituted by MgH2; therefore, the (LiNH2+nMgH2) (n=0.55, 0.6 and 0.7) system is investigated first. These mixtures are partially converted to Mg(NH2)2 and LiH by the metathesis reaction upon ball milling. In this system, hydrogen is desorbed in a two-step reaction: [0.5xMg(NH2)2+xLiH]+[(1-x)LiNH2+(0.5-0.5x)MgH2]→0.5Li2Mg(NH)2+1.0H2 and 0.5Li2Mg(NH)2+MgH2→0.5Mg3N2+LiH+H2. Moreover, this system is fully reversible in the following reaction: Li2Mg(NH)2+2H2→ Mg(NH2)2+2LiH. Step-wise desorption tests show that the enthalpy and entropy change of the first reaction is -46.7 kJ/molH2 and 136.1 J/(molK), respectively. The equilibrium temperature at 0.1 bar H2 is 70.1°C, which indicates that this system has excellent potential for onboard applications. The lowest apparent activation energy of 71.7 kJ/mol is observed for the molar ratio of 1:0.7MgH2 milled for 25 h. This energy further decreases to 65.0 kJ/mol when 5 wt.% of n-Ni is incorporated in the system. Furthermore, the molar ratio of MgH2/LiNH2 is increased to 1.0 and 1.5 to increase the limited hydrogen storage capacity of the (LiNH2+0.7MgH2) mixture. It has been reported that the composition changes can enhance the hydrogen storage capacity by changing the dehydrogenation/hydrogenation reaction pathways. However, theoretically predicted LiMgN is not observed, even after dehydrogenation at 400°C. Instead of this phase, Li2Mg(NH)2 and Mg3N2 are obtained by dehydrogenation at low and high temperatures, respectively, regardless of the milling mode and the molar ratio of MgH2/LiNH2. The only finding is that the molar ratio of MgH2/LiNH2 can significantly affect mechano-chemical reactions during ball milling, which results in different reaction pathways of hydrogen desorption in subsequent heating processes; however, the reaction’s product is the same regardless of the milling mode, the milling duration and their composition. Therefore, the (LiNH2+0.7MgH2) mixture has the greatest potential for onboard applications among Li-Mg-N-H systems due to its high reversible capacity and good kinetic properties.
Cite this version of the work
Minchul Jang (2012). Nanostructured complex hydride systems for solid state hydrogen storage. UWSpace. http://hdl.handle.net/10012/6477