DOI: 10.1002/adma.200702958 Ammonia Borane Destabilized by Lithium Hydride: An Advanced On-Board Hydrogen Storage Material** By Xiangdong Kang, Zhanzhao Fang, Lingyan Kong, Huiming Cheng, Xiangdong Yao, Gaoqing Lu, and Ping Wang* One of the major obstacles to the widespread use of hydrogen as an energy carrier, especially for transportation applications, is the lack of safe and efficient means for hydrogen storage. Decades of extensive efforts on metal/alloy hydrides, nanos- tructured carbon materials, and complex hydrides have not led to a viable system that can reversibly store over 6 wt % hydrogen at a moderate temperature range. [1–4] Recently, irreversible hydrogen storage via catalyzed hydrolysis or thermolysis of chemical/metal hydrides has emerged as an alternative and more promising solution for on-board hydrogen storage. [5,6] Among the hydride materials of interest, ammonia borane (NH 3 BH 3 , AB) is a leading candidate, which is justified by its extremely high gravimetric hydrogen capacity (19.6 wt %) and relatively favorable thermal stability. AB can release hydrogen via catalyzed hydrolysis in an aqueous solution, [7–9] catalyzed dehydrocoupling under non-aqueous conditions, [10–12] or thermolysis at elevated temperatures. [13–16] Compared to the solid–liquid reaction systems, the latter solid–gas approach is clearly more appreciated for practical on-board applications owing to the ease of apparatus design and the hydrogen capacity advantage. [17,18] However, its practical application is greatly restricted by the sluggish decomposition kinetics at 100 8C and the concurrent release of volatile byproducts (e.g., borazine) that are detrimental for fuel cell operation. [15–18] Aiming at solving these problems, recent studies focused on developing a nanoengineering strategy [15] and identification of effective catalysts. [16,19] These efforts have made some progress in improving the H 2 release kinetics and suppressing the generation of volatile byproducts. However, the utilization of structure-directing agents imposes a penalty on hydrogen capacity to levels that are unacceptable for practical applica- tion. [15] The identified transition metal catalysts seem proble- matic for eliminating the detrimental gas impurities, especially at a normal heating rate. [16,17] Here, we report a very simple and highly effective method for destabilizing AB for high-capacity, high-purity hydrogen generation. By mechanically milling an AB/LiH mixture in a 1:1 molar ratio, the produced material can release over 7 wt % (on a material basis) pure hydrogen at around 100 8C, free of gas impurities. These findings offer a clear potential for using destabilized AB and related materials as high-capacity hydrogen sources for transportation applications. AB is a colorless solid that is stable at room temperature and melts at around 110 8C. [17] The decomposition of AB is known to occur by a multistep process, with the decomposition rates and products being highly dependant on the reaction conditions. [14,17] While the direct solid-state decomposition is possible at temperatures below 100 8C at very slow rates, the rapid release of the first equivalent of H 2 typically occurs upon melting of AB. The subsequent release of the second equivalent of H 2 occurs at temperatures above 130 8C, but is generally concurrent with the generation of borazine (c-(HNBH) 3 ) and diborane (B 2 H 6 ) gas impurities. [13,14] Our studies show that the addition of LiH destabilizer results in pronounced improvements on all the key aspects of the decomposition behavior of AB. Figure 1 compares the thermal decomposition behaviors of the AB samples with and without LiH destabilizer by using synchronous thermogravimetry/ differential scanning calorimetry/mass spectroscopy (TG/ DSC/MS) at temperatures up to 200 8C with a heating rate of 2 8C min 1 . For the neat AB, following the endothermic melting peak at 107 8C, two exothermic peaks were observed centered at around 113 8C and 154 8C, which correspond to the two decomposition steps associated with the formation of solid residues polyaminoborane (NH 2 BH 2 ) n and polyiminoborane (NHBH) n , respectively. [13,14] In contrast, the decomposition temperature of the AB sample destabilized by LiH was significantly lowered, to ca. 80 8C, and more than 60% hydrogen was released at temperatures below 100 8C. The disappearance of the endothermic melting peak further indicates the direct solid-state decomposition of the destabilized AB. Of particular interest, the addition of LiH was found to eliminate detrimental gas impurities. As seen from the MS results of the neat AB, considerable amounts of borazine (c-(NHBH) 3 ) and diborane (B 2 H 6 ) were released during the heating process, especially in the second decomposition step. [13,14] This qualitatively accounts COMMUNICATION [*] Prof. P. Wang, Dr. X. D. Kang, Z. Z. Fang, L. Y. Kong, Prof. H. M. Cheng Shenyang National Laboratory for Materials Science Institute of Metal Research, Chinese Academy of Sciences Shenyang, 110016 (PR China) E-mail: pingwang@imr.ac.cn Dr. X. D. Yao, Prof. G. Q. Lu ARC Centre for Functional Nanomaterials The University of Queensland Brisbane, QLD 4072 (Australia) [**] The financial supports from the Hundred Talents Project of Chinese Academy of Sciences, the National Natural Science Foundation of China (Grants No. 50571099, 50671107 and 50771094), and the National High-Tech Research and Development Program of China (863 Program, Grant No. 2006AA05Z104) are gratefully acknowledged. We thank X. J. Lan, Dr. W. P. Zhang, and Prof. X. H. Bao from Dalian Institute of Chemical Physics, Chinese Academy of Sciences for their assistance in the NMR experiments. 2756 ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2008, 20, 2756–2759