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. Howeve...

Towards a structure-performance relationship for hydrogen storage in Ti-doped NaAlH4nanoparticles

“…It has to be noted here that the starting material for Ti-NaAlH 4 /CNF was NaAlH 4 /CNF. When comparing the desorption characteristics of those two samples the low temperature shoulder present for NaAlH 4 was not observed when ''Ti'' is present which might indicate that these smallest particles have reacted with the Ti(OBu) 4 during doping. Please note that also for reloading, in a Rubotherm high pressure magnetic suspension balance, 27,28 the addition of Ti to nanosized NaAlH 4 was beneficial (ESI, Fig.…”

“…The order of Ti and alanate deposition, to achieve an optimal alanate-Ti interaction is discussed. The hydrogen storage characteristics of these materials is compared to undoped nano-NaAlH 4 on CNF and ball milled TiCl 3 -NaAlH 4 (alanate crystallite size 150-200 nm). Structure activity relationships will be established by relating information from Ti K-edge Extended X-ray Absorption Fine Structure (EXAFS) and in situ X-ray Diffraction (XRD) results with the hydrogen absorption and desorption characteristics of the samples.…”

“…The ''Ti first'' sample was prepared by first a pore volume impregnation with 10 mol% Ti(OBu) 4 (compared to NaAlH 4 ) in diethyl ether. After removal of the diethyl ether, 8 wt% NaAlH 4 was impregnated using THF followed by slow drying.…”

“…All sample handling was performed in an inert atmosphere either using Schlenk techniques or using a glove box. The ''alanate first'' sample of identical composition was prepared by reversing the impregnation order of NaAlH 4 and Ti(OBu) 4 ; however, after both steps the slow drying procedure was applied. This sample will be referred to as ''Ti-NaAlH 4 /CNF''.…”

“…The absence of Na or Al atoms in the local structure indicates that Ti did not interact with the NaAlH 4 to a level detectable by EXAFS (10 at%). Since the local structure comprised the same atoms as the original Ti-precursor (Ti(OBu) 4 ), it was concluded that the majority of ''Ti'' was present as nanoparticles of Ti(OBu) 4 , or a decomposition product thereof, in the as-prepared Ti-NaAlH 4 /CNF.…”