Controlled Hydrogen Release From Ammonia Borane Using Mesoporous Scaffolds

Hydrogen storage on chemical hydrogen storage materials may provide an attractive new opportunity to meet and exceed the goals of the recent DOE Grand Challenge in Hydrogen Storage for on-board fuel cell applications. We have been investigating the feasibility of using ammonia borane (NH$_{3}$BH$_{3})$, and polyammonia borane (-NH$_{2}$BH$_{2}$-)$_{n }$as reversible hydrogen storage materials. This family of molecules is promising given capacity for high volumetric storage densities, ca. $>$12 wt {\%} hydrogen, and recent computational results that suggest hydrogen uptake and release is near thermoneutral. Ammonia borane (AB) is a stable solid at room temperature that requires heating to release the H$_{2}$. AB decomposes upon melting at 114 $^{o}$C with the vigorous bubbling of H$_{2}$ gas. Alternatively the hydrogen from AB can be released from the solid material at temperatures below 100 $^{o}$C, albeit at significantly lower rates. Thermal decomposition of NH$_{3}$BH$_{3}$ at temperatures below 100 $^{o}$C yields H$_{2}$ and a complex polyaminoborane-like --(NH$_{2}$BH$_{2})_{n}$-- material (PAB). The solid phase thermal reaction involves a bimolecular dehydrocoupling reaction to yield a new B-N bond, i.e., HNB-H --- HNBH to yield HNB-NBH in contrast to our observations of the catalytic pathway involves the intramolecular abstraction of H-H from a single H-NB-H molecule to yield N=B intermediate. At temperatures above 150 $^{o}$C the PAB decomposes to yield a second equivalent of H$_{2}$, concurrent with formation of a polyiminoborane-like --(NHBH)$_{n}$-- material (PIB) and borazine $c-$(NHBH)$_{3}$. The latter is a volatile inorganic analog of benzene, which is highly undesirable in the H$_{2}$ feed. While AB exceeds volumetric and gravimetric density targets for a hydrogen storage material, three additional physical obstacles must be overcome: (i) increasing the rates of H$_{2}$ release at temperatures below 80 $^{o}$C, (ii) preventing borazine formation and (iii) demonstrating the potential for reversibility. There are reports that nano-phase metal hydrides show enhanced kinetics for reversible hydrogen storage relative to the bulk materials. However, after a few hydriding/dehydriding cycles the kinetic enhancement is diminished for some materials as they lose nano-phase structure. We suggest that a rigid nano-phase scaffold loaded with a hydrogen-rich material, may provide an attractive option to preserve the nano-scale dimensions through several hydriding/dehydriding cycles. To demonstrate the effect of a nano-phase scaffold on hydrogen release we use a high-surface area mesoporous silica, loaded with AB as a model system. The work presented in this symposium will highlight our success in lower the temperature of hydrogen release from ammonia borane ($<$80 $^{o}$C) and to minimize the formation of borazine from polyammonia borane decomposition using mesoporous silica templates (SBA-15). Three notable observations are described in this work: (i) increased rates of H$_{2}$ release, (ii) modifications of the non-volatile polymeric products that change the thermodynamics of hydrogen release and (iii) minimized formation of borazine.