Study On The Improvement Of Hydrogen Storage Properties Of NaAlH₄ Complex Hydride

Light metal complex hydrides are supposed to be potential hydrogen storage materials because of the high theoretical hydrogen storage capacity. However, their practical applications are hindered by sluggish kinetics, relatively high operating temperatures etc. Based on an overall review of the research and development of metal complex hydrides, sodium alanate (NaAllH4, the archetype of light metal complex hydride, is selected as the subject of this study. The dissertation is focused on the following aspects:The partial substitution of Na by Li in the intermediate product of Na3AlH6 and its impact on the hydrogen storage behaviors were studied. In this section, nanocrystalline Na2LiAlH6 was directly synthesized by mechanical milling 2NaH/LiH/Al mixture with TiF3 catalyst under hydrogen pressure of 3.0 MPa. The synthesized Na2LiAlH6 exhibits a dehydriding capacity of 3.1 wt% in the first cycle at 190℃under a constant backpressure of 0.1 MPa, which is higher than that of Na3AlHg. The dehydrided material can reabsorb 80% of the reversible hydrogen capacity within 5 min when the temperature is above 100℃with an initial hydrogen pressure of 4 MPa. Because of the complexity of mass transfer, the rehydrogenation process of dehydrided Na2LiAlH6 is more intricate than that of the dehydrided sodium alanate, causing the reduction of rehydriding capacity in the following cycles. Investigation on the microstructure shows that some NaH,LiH and Al are still present in the hydrogenated materials, indicating the relatively poor reversibility in the material.The hydrogen storage properties and microstructures of TiC-doped NaAlH4 have been studied. It is found that TiC remains stable in the material and will not generate byproducts in the system. TiC-doped NaH/Al composite absorbs 4.6 wt% hydrogen at 120℃, desorbs more than 80% hydrogen of its initial hydrogen capacity at 155℃with a stable cycling dehydriding rate and capacity. The XRD and SEM-EDS analyses led us believe that the refined TiC particles with the size of 40-100 nm inlaid on the surface of a larger hydride matrix act not only as the catalytic active sites for split-up and recombination of AIH4/AIH3, and act as the hydrogen spillover for hydrogen diffusion, but also prevent the growth in size of small spherical alanate, resulting in the improvement of hydriding/dehydriding properties of the sodium alanate system. However, detailed investigation shows that TiC-doped materials exhibit an inferior kinetics compared with Ti halide-doped NaAlH4.In order to further improve the kinetics of the materials, we resorted to the Ce halide dopants, which were proposed as a group of promising catalysts for light metal complex hydrides. CeF3- and CeCl3-doped NaAlH4 systems were directly synthesized using NaH/Al with a few mole percent of dopant as the starting materials by ball milling under the hydrogen pressure of 3 MPa. It shows that in the same conditions. CeCl3-doped NaH/Al composite is more easily hydrogenated to form the doped NaAlH4. In a comparative investigation, we failed to synthesize NaAlH4 just from the mixture of NaH/Al without any dopants, which indicates the dopants play a critical role in the formation of NaAlH4. In the ball milling process, the dopant of CeCl3 will react with NaH, giving birth to NaCl and noncrystalline "Ce" clusters. In the following hydriding/dehydridng cycles. "Ce" will react with Al and bring about Ce-Al nanoclusters. As for CeFs doped NaAlH4 system, the dopant of CeF3 remains stable in the ball milling and the following cycles. In comparison with CeF3-doped NaAlH4, CeCl3-doped material exhibits a more pronounced kinetics. The dehydrogenated CeCl3-doped NaAlH4 can be reloaded in less than 20 min at 120℃with an initial hydrogen pressure of 11 MPa, which is among the best reports in literature.By directly introducing CeAlx (x=2,4) into sodium alanate, much similar kinetics of hydriding and dehydriding were achieved. Because the dopant of CeAlx (x=2.4) will not give rise to "deadweight" phases of byproducts, the doped NaAlH4 exhibits a more hydrogen storage capacity compared with CeCl3-doped NaAlH4. The Ce-Al doped NaAlH4 possesses a reversible hydrogen capacity of about 4.8 wt% under moderate conditions, which is about 20% higher than that of CeCl3-doped NaAlH4. The rehydrogenation kinetics decreases with the reduction of the hydrogenation pressure. By adjusting the hydrogenation temperature, an optimum hydriding rate can be obtained. At 100℃,4 MPa, a capacity of 4.0 wt% can be uploaded for CeAl2-doped NaAlH4. The apparent activation energy of NaAlH4 doped with 2 mol% CeAlx (x=2,4) is estimated to be 72.3-90.4 kJ/mol and 93.6-98.9 kJ/mol for the first and the second dehydrogenation step respectively by using Kissinger's approach, much lower than those of pristine NaAlH4. Microstructure investigation shows that in the CeAl2-doped material, CeAl2 will gradually react with Al and transform to CeAl4 in the cycling, causing a decrease of reversible hydrogen capacity. While CeAl4 remains stable in CeAl4-doped system. No other Ce-containing species were detected in doped NaAlH4. Similar phenomena were observed in La- and Sm-doped system. LaCl3 and SmCl3 react with NaH in the ball milling process and give rise to NaCl and "La" and "Sm" clusters. In the following cycling, La reacts with Al and form La-Al species with a structure of La3Al11 The catalytic enhancement arising upon doping the ball-milled La3Al11 is quite similar to that achieved in the LaCl3-doped sodium alanate. Comparative investigation on microstructure of LaCl3-and La3Al11-doped NaAlH4 reveals that similar nanoclusters of La3Al11 are present in the parent hydrides. The combination of hydrogen storage properties and microstructures unequivocally reveal that the in situ formed rare earth-Al species play a crucial role in catalyzing the chloride-doped NaAlH4.At the end of the thesis, CeAl4-doped NaAlH4, which possesses the optimal hydrogen storage properties, was selected to do modeling studies. The results show that isothermal dehydrogenation of the first step follows the two dimensional phase boundary mechanism. The second step conforms to the mechanism of the first-order reaction. The kinetics of hydrogenation and dehydrogenation remains almost unchanged after the sample is pressed as the tablet in a pressing die under a pressure of 380 MPa. The topography of the tablet remains almost unchanged after several cycling with a relatively mild hydrogenation and dehydrogenation conditions (hydrogenation: 120℃/6 MPa; dehydrogenation:with a constant ramping rate of 1K/min). Lots of cracks come into being in the tablet after 5 cycles, when the hydrogenation and dehydrogenation conditions are relatively severe (hydrogenation:120℃/11 MPa; dehydrogenation:170℃/0.1 MPa). These cracks were aroused by intensely heat effect and the fast phase transformation in the hydriding and dehydriding process.