Hydrogen Absorption/Desorption Properties And Mechanisms Of LiBH₄-based Hydrogen Storage Materials

Safe, efficient and economical hydrogen storage is the key issue for scale applications of hydrogen energy. In recent years, light metal hydrides have attracted considerable attention due to their high hydrogen capacity. With a theoretical hydrogen capacity up to～18.5 wt%, LiBH4 is regarded as one of the most promising hydrogen storage materials. However, the poor kinetics and the high operating temperatures caused by high thermodynamic stability hinder severely its practical applications. In this work, to reduce the operating temperature and improve the hydrogen storage reversibility, the effects of TiF4, Mg(OH)2 and Ca(OH)2 on the hydrogen storage behaviors of LiBH4 were systematically studied, and their action mechanisms were analyzed and discussed.Investigations on the LiBH4-TiF4 composite indicated that the addition of TiF4 could significantly reduce the hydrogen desorption temperature of LiBH4. The LiBH4-0.25TiF4 (molar ratio:4:1) sample exhibited superior dehydroegnaion performances. As ball milling the LiBH4-0.25TiF4 mixture for 12 h under 500 rpm, around 1.0 wt% H2 was released. In subsequent heating process, the onset hydrogen release temperature of the as-milled composite was 65℃,200℃lower than that of the pristine LiBH4 sample. Hydrogen release amounted to 5.3 wt% when the temperature was elevated to 150℃. However, no hydrogen was released for the pristine LiBH4 under the same conditions. Compared to the LiBH4-0.33TiF3 mixture, LiBH4-0.25TiF4 possesses better hydrogen desorption kinetics and lower desorption temperature. Structure examinations revealed that during ball milling. LiBH4 reacted with TiF4 in part to convert to LiBH3F and TiF2. In the heating process, the newly developed TiF2 reacted with the rest LiBH4 to produce LiF, Ti and B along with hydrogen release. At the same time. LiBH3F reacted with Ti and B to release additional hydrogen and generate LiF and TiB2. As for the LiBH4-0.33TiF3 mixture, however, hydrogen desorption mainly came from the in situ formation and decomposition of Ti(BH4)3 due to the reaction between LiBH4 and TiF3. The different oxidation valence between Ti3+and Ti4+may be the main reason for the various dehydorngeaiton behaviors of the composites with TiF4 and TiF3.In order to improve the hydrogen reversibility of LiBH4, the effects of Mg(OH)2 on dehydorgenaiton/hydrogenation behaviors of LiBH4 were systematically studied. and the hydrogen storage mechanisms were elucidated. It was found that the initial desorption temperature of the LiBH4-0.3Mg(OH)2 composite was decreased to about 100℃, a 180℃reduction with the pristine LiBH4 sample. Hydrogen desorption amounted to 9.1 wt% in the temperature range of 100-450℃with a three-step reaction. Mechanism analyses revealed that LiBH4 reacted first with Mg(OH)2 to form LiMgBO3 and MgO and release hydrogen due to the stong affinity between the negatively charged hydrogen H6δ- in LiBH4 and the positively charged hydrogen H6δ+ in Mg(OH)2. An then, the interaction between LiMgBO3 and LiBH4 led to a lower transfer temperature (～360℃) of LiBH4 to Li2B12H12. and LiMgBO3 to Li3BO3, MgO and B2O3. When the temperature was higher than 360℃, the self-decomposition of the rest LiBH4 and Li2B12H12 occurred to release the additional hydrogen. Rehydrogenation experiments demonstrated that the dehydrogenated product could store reversibly about 4.7 wt% H2 with an initial hydrogen pressure 100 atm and 450℃, exhibiting an improved hydrogen storage reversibity with repect to the pristine LiBH4.Furthermore, the hydrogen absorption/desorption behaviors and mechanisms of the LiBH4-Ca(OH)2 composites have been studied. It is found that the addition of Ca(OH)T also improved the hydrogen storage performances of LiBH4. The LiBH4-0.3Ca(OH)2 composite achieved complete hydrogen release in the temperature range of 100-450℃. Mechanism investigations showed that the dehydrogeantion mechanism of the Ca(OH)2-added LiBH4 was similar to that of the LiBH4-0.3Mg(OH)2 composite. First, the negatively charged hydrogen H6δ- in LiBH4 and positively charged hydrogen Hδ+ in Ca(OH)2 drove hydrogen release to form CaB2O4. CaO and (CaO)2(B2O3)3(H2O)7, B2O3, B7O below 200℃. In the temperature range of 200-450℃, there are chemical reactions between CaO. (CaO)2(B2O3)3(H2O)7. B7O and B2O3 to generate H2O (vapor), which induces the decomposition of LiBH4 into Li2B12H12 at a lower temperature (～380℃). With increasing temperatures, the rest LiBH4 and the newly developed Li2B12H12 decomposed to release a large amount of hydrogen. The dehydrogenated products consisted of CaB2O4, CaO. LiH and some B2O3. The total amount of hydrogen release was 8.8 wt%. The dehydrogenated products were subjected to rehydorgeantion under an initial hydrogen pressure of 100 bar and 450℃. It was found that about 5.1 wt% could be recharged into the dehydrogenated sample, which is higher than that of LiBH4-0.3Mg(OH)2 binary composite (4.7 wt%). Various effects of the additives of TiF4, Mg(OH)2 and Ca(OH)2 on hydrogen storage properties of LiBH4 were attained due to their different natures. The decrease in dehydrogenation temperature is in sequence 0.25TiF4>0.3Mg(OH)2>0.3Ca(OH)2, in LiBH4. In contrast, the amount of hydrogen absorption is in sequence 0.3Ca(OH)2>0.3Mg(OH)2>0.25TiF4, exhibiting a gradually improved hydrogen storage reversibility.