| Summary: | 碩士 === 國立中央大學 === 材料科學與工程研究所 === 99 === LiBH4 is a potential hydrogen storage material and gains lots of interests recently due to the extremely high hydrogen capacity (18.4 wt%). However, the initial decomposition temperature (Ti) and main dehydrogenation temperature (Tm) of LiBH4 are as high as 567 and 754 K, respectively. In order to overcome the drawbacks, there are several approaches developed to modify the system thermodynamically or kinetically. In this study, LiBH4 is modified by various additives or mixing with LiNH2 to form a new Li-B-N-H quaternary hydride by ball-milling process. Besides, their dehydrogenation properties are analyzed through temperature programmed reduction (TPR) and temperature programmed dehydrogenation-mass spectrometers (TPD-MS), and the phase structures of the systems are characterized by the X-ray powder diffraction (XRD) method.
Based on the results, it can be observed that the dehydrogenation properties of the LiBH4 can be successfully improved by doping 33 wt% of Pd-Co/C additives, and among the three different samples, Pd25Co75/C doped sample shows the optimal enhancement in promoting the dehydrogenation properties of LiBH4¬ by reducing the Ti to 523 K with the capacity as 10.5 wt%. Besides, it is found out that when the Co content in the additives increases, the Tis gradually decrease and capacities gently increase. Moreover, for the system modified by various amounts of Pd-Co/C, the results reveal that when the system is modified by 50 wt% of Pd-Co/C, Pd50Co50/C doped sample has better performance than Pd75Co25/C and Pd25Co75/C doped samples, which Ti and Tm can decrease to 533 and 639 K with 10 wt% of hydrogen desorbed. On the other hand, for 33 wt% of Pd-Co/C modified LiBH4+2LiNH2 binary system, the sample doped with Pd50Co50/C shows the effective modification, and the Ti is dramatically reduced from 523 K of the pristine binary system to 396 K and the capacity is 9.5 wt%.
In terms of various metal (Pd and Co) chlorides and hydroxides modified LiBH4 and binary systems, the improvement of the dehydrogenation properties can both be observed. However, the reasons of the enhancements by metal chlorides and hydroxides may be different. For LiBH4 systems, the metal chlorides modified samples may have some ion exchange reactions and then form the unstable transition metal borohydrides during the heating process, thus the dehydrogenation properties can be enhanced. However, for metal hydroxides doped samples, the enhancement may be ascribed as the combinational effects of hydrolysis and redox reactions during the decomposition processes. On the other hand, for the metal (Pd and Co) chlorides and hydroxides modified binary systems, although the Tis and Tms can both significantly decrease to lower temperature ranges, the capacities of the samples modified by metal hydroxides also conspicuously reduce. Therefore, metal chlorides modified binary samples shows the better performance in improving the dehydrogenation properties than metal hydroxides.
|
|---|
