Nanoconfinement of ammonia borane in titania aerogels for accelerated hydrogen release

The combustion of fossil hydrocarbons to meet global energy demand is widely considered unsustainable and there is substantial motivation to transition toward renewable energy sources. Nevertheless, increasing integration of these renewable sources is challenged by
their intermittency, which necessitates the installation of robust energy storage systems. In this context, hydrogen is an advantageous energy vector but current technologies offer insufficient storage performance. Ammonia borane (AB) offers potential as a candidate
storage material but is nonetheless challenged by the need for (i) accelerated release of hydrogen, (ii) reduction of gaseous contaminants, and (iii) formation of polymeric residues that enable regeneration. This research sought to resolve these issues by nanoconfinement of AB within highly porous titania aerogels. Initially, titania aerogels were prepared and
characterised, with the effect of several synthesis conditions understood. The best performing of these materials exhibited excellent surface area (SBET = 660 m2/g), pore volume (VP = 1.17cm3/g) and mesoporosity (dP = 17 nm) that were among the highest reported for such materials. Importantly, it was found that extraction with carbon dioxide resulted in the formation of titania with surface bicarbonate functionality. The preparation of such functionalised materials typically requires the inclusion of carboxylic acids during synthesis, meaning this discovery represents a distinct route toward high performance materials. One titania aerogel was then studied for the nanoconfinement of AB for application as a hydrogen storage candidate, with the composite material designated AB/TA(L). Compared to bulk AB, the nanoconfined AB/TA(L) demonstrated distinct release characteristics including lowered onset temperatures (101 to 25 oC), reduced activation energy (171 to 85 kJ/mol), altered release kinetics, increased contaminants and reduced quantity of hydrogen. The cause of these fundamental changes was believed to be strong destabilisation induced by the aforementioned surface bicarbonate functionality, the amphoteric nature of which was speculated to mediate proton exchange. In contrast to trends in literature, this result directly implicates amphoteric materials as deserving further research. Whilst many of these changes prohibited application of AB/TA(L) toward storage applications, the findings highlighted new opportunities for nanoconfinement of AB. Additionally, the release mechanisms from bulk AB and nanoconfined AB/TA(L) were interrogated using two-dimensional infrared spectroscopy. These studies found the borane functionality of bulk AB was crucially involved in the release mechanism(s), with evidence to support the occurrence of counterintuitive homopolar
interactions. Although such interactions have been reported before, this result appears to be the first evidence that confirms their existence without the possibility of hydrogen exchange.