Effects of Fluoride Additives on the Hydrogen Storage Performance of 2LiBH 4 -Li 3 AlH 6 Destabilized System Yun Li, Xuezhang Xiao, Lixin Chen,* Leyuan Han, Jie Shao, Xiulin Fan, Shouquan Li, and Qidong Wang Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China ABSTRACT: 2LiBH 4 -Li 3 AlH 6 composite samples doped with or without 5 wt % metal fluorides (CeF 3 , NiF 2 and TiF 3 ) were prepared by ball milling, and the effects of fluoride additives on the hydrogen storage performance of the samples were comparatively investigated. It is found that the undoped 2LiBH 4 -Li 3 AlH 6 system presents a favorable destabilized dehydrogenation performance as compared with the as-milled pure LiBH 4 . The three fluorides can enhance the dehydriding kinetics of the 2LiBH 4 -Li 3 AlH 6 destabilized system to some extent. The TiF 3 -doped composite exhibits the most prominent behavior in terms of the low dehydrogenation temperature and fast dehydriding rate. The activation energy for the decomposition of LiBH 4 was measured by differential scanning calorimetry; that of the TiF 3 doped composite was calculated to be 118.3 kJ/mol, which is much lower than that of the undoped composite (197.6 kJ/mol). In addition, the experimental results show that the reversibility of the 2LiBH 4 -Li 3 AlH 6 composite is improved by the doping of TiF 3 , which plays a catalytic role, strengthens the interaction between LiBH 4 and Li 3 AlH 6 , and thus further improves the de/rehydrogenation performance. 1. INTRODUCTION Hydrogen utilized as an alternative energy carrier has become a significant contender in the mobile sector because of its nonpolluting characteristic. A commercially available hydrogen storage material with low-cost, high volumetric, and gravimetric hydrogen densities, and fast sorption kinetics is necessary for transportation. 1,2 The traditional metal-based hydrides with low hydrogen storage capacity cannot achieve the target for automobile applications. 3-6 More interest has moved to complex hydrides as potential solid-state hydrogen storage materials, which hold high theoretical hydrogen densities. 1,2,7 Most of the complex hydrides, such as alanates, 8-14 borohydrides, 15-17 amides, 18-20 and so on, possess high hydrogen storage capacities, so systematic investigations on complex hydrides have vigorously been in progress in the past few years. LiBH 4 , which owns the highest hydrogen storage capacity of 18.5 wt %, has gained considerable interest. 21-24 The generally accepted model of the thermal decomposition of LiBH 4 is described in eq 1. 21,24 → + + LiBH LiH B 1.5H 4 2 (1) The dehydrogenation temperature of LiH is too high for practical application, which is ∼720 °C and therefore normally not considered to be an available source of hydrogen. 25 The available hydrogen storage capacity of LiBH 4 is 13.8 wt %. However, the release of hydrogen from LiBH 4 , as described in eq 1, requires temperature in excess of 370 °C, and rehydrogenation requires both high temperature (600 °C) and pressure (35 MPa). 21 LiAlH 4 has attracted much attention because of its high inherent hydrogen capacity (∼10.6 wt %). During decom- position process, LiAlH 4 first forms an intermediate compound Li 3 AlH 6 at ∼160 °C and then LiH at ∼210 °C, liberating about 5.3 and 2.6 wt % of hydrogen, respectively, as described in eq 2 and 3. 26-29 → + + 3LiAlH Li AlH 2Al 3H 4 3 6 2 (2) → + + Li AlH 3LiH Al 1.5H 3 6 2 (3) Jang et al. 29 found that the hydrogen absorption reaction of Li 3 AlH 6 to LiAlH 4 could be only possible when the hydrogen pressure is above 100 MPa, which is practically inaccessible. Because LiAlH 4 is partially reversible, the storage capacity of Li 3 AlH 6 is limited, and only a quarter of the hydrogen is accessible under moderate condition. The 2LiBH 4 -Li 3 AlH 6 system has been found to decompose with the formation of AlB 2 , and this system is more easily rehydrogenated at 450 °C and 24.1 MPa of H 2 , as compared with LiBH 4 . 30 However, it cannot fulfill the requirements for practical application. So, it is important to find new effective additives that can enhance the hydrogen storage properties of the 2LiBH 4 -Li 3 AlH 6 system. In the present work, we investigate the hydrogen storage performance of the 2LiBH 4 -Li 3 AlH 6 composite doped with or without 5 wt % fluoride as additives, including TiF 3 , CeF 3 , and NiF 2 . The Received: July 31, 2012 Revised: September 23, 2012 Published: September 28, 2012 Article pubs.acs.org/JPCC © 2012 American Chemical Society 22226 dx.doi.org/10.1021/jp307572x | J. Phys. Chem. C 2012, 116, 22226-22230