Nanoscaffold Mediates Hydrogen Release and the Reactivity of Ammonia Borane

Abstract

The increasing demand for clean energy sources that do not add more carbon dioxide and other pollutants to the environment has resulted in increased attention worldwide to the possibilities of a "hydrogen economy" as a long-term solution for a secure energy future based on potentially renewable resources. [1][2][3] Some of the greatest challenges are the discovery and development of new on-board hydrogen-storage materials and catalysts for fuel-cell-powered vehicles. New materials that store both high gravimetric (! 90 gm H 2 kg À1 ) and high volumetric (! 82 gm H 2 L À1 ) densities of hydrogen that can be delivered at temperatures between À20 and 85 8C are needed by the year 2015.[4] The volumetric constraints eliminate from consideration pressurized hydrogen systems and guide towards the development of solid storage materials.[5] There are several broad classes of solid hydrogenstorage materials that are currently being investigated as potential on-board storage materials: 1) metal materials, hydrides (e.g., MgH 2 ), [6] imides (e.g., LiNH 2 ), [7] and organic frameworks (e.g., Zn 4 O(1,4-benezenedicarboxylate)), [8] 2) complex hydrides (e.g., NaAlH 4 ), [9] and 3) carbon materials (e.g., carbon nanofibers, [10] single-wall carbon nanotubes).[11] The most thoroughly studied complex hydride, NaAlH 4 , has been shown to release hydrogen at 110 8C when doped with Ti; [12] however, the kinetics are very slow and hydrogen-storage densities are too low (56 gm H 2 kg À1 ) to meet long-term targets. The temperatures for H 2 release from carbon materials are too low, and the reported storage densities are controversial.[13] The hydrolysis of metal hydrides is being explored, but the unfavorable thermodynamics for regeneration of the spent material prevents their widespread application. For example, the reaction NaBH 4 +4 H 2 O!NaB(OH) 4 +4 H 2 is exothermic by À250 kJ mol À1 . Reaction enthalpy for hydrogen loss is an important property since near-thermoneutral thermodynamics will be critical for materials for reversible H 2 storage. To date, few of these materials meet the long-term gravimetric requirements and provide rapid hydrogen release at temperatures between À20 and 85 8C; thus, new materials and novel approaches are needed. Herein we show that the kinetics of hydrogen release are significantly enhanced at low temperatures for a new hybrid material, ammonia borane infused in nanoporous silica, and that the hydrogen purity is increased. These findings suggest that hydrogen-rich materials infused in nanoscaffolds offer a most promising approach to on-board hydrogen storage.Chemical hydrogen-storage materials that release H 2 by thermolysis without generating CO 2 may offer an attractive alternative to other systems studied. For example, the NH x BH x family of compounds [14] should provide favorable gravimetric densities of 245, 196, 140, and 75 gm H 2 kg À1 for x = 4, 3, 2, and 1, respectively. As the NB unit is isoelectronic with CC, these materials are viewed as inorganic analogues of hydrocarbons. However, NH ...

Improved hydrogen desorption properties of ammonia borane by Ni-modified metal-organic frameworks

“…The detrimental side products of borazine and diborane could be detected during the AB thermolysis. Compared with the neat AB, MIL-101 and Ni@MIL-101 doped with AB samples start to evolve H 2 at about 50 C and give broad desorption peaks centered at 85 and 75 C which are lower than other AB nanocomposite systems [20,21,32]. This is the first report of such a big decrease (40 C) in the decomposition temperature, which (75 C) is lower than PEMFC working temperature (80 C).…”

“…In AB/SBA-15 system, the obtained results showed that borazine neither occurred in the volatile products nor was confined in the SBA-15 framework [20], indicating …”

“…In each case, a straight line is obtained, and from the slope in Fig. 6 the Ea obtained for H 2 desorption are 181.7 kJ mol À1 for AB (Ea z 184 AE 5 kJ mol À1 ) [20], 91.4 kJ mol À1 for 50% AB/MIL-101 and 69.6 kJ mol À1 for 50% AB/ Ni@MIL-101. These results suggest that the barrier for H 2 -release from AB in the MIL-101 scaffold is significantly lower than that from neat AB, and even much lower in the presence of Ni catalyst.…”

“…Recent reports show that materials structured at the nanoscale can decrease their H 2 desorption enthalpy and enhance the kinetics relative to the bulk materials [18,19]. Carbon cryogel and mesoporous silica infiltrated with AB have been demonstrated to significantly improve the kinetics and decrease the temperature for hydrogen release [20,21]. However, the hydrogen release temperature is still higher than 85 C and there is no concern on the prevention of ammonia formation.…”

Recent Progress in Metal Borohydrides for Hydrogen Storage

“…One known approach is to incorporate hydrogen storage materials into nanoporous materials. It was first applied in the NH 3 BH 3 system [211] and good improvement was obtained: The dehydrogenation temperature was largely decreased and the amount of borazine gas (an impurity) was significantly reduced. Recently, this approach was introduced for M(BH 4 ) n materials, and some recent achievements [160,[212][213][214][215][216][217][218][219][220][221][222] are summarized in Table 6.…”