Ammonia borane as hydrogen storage materials

Abstract

Ammonia borane is an appropriate solid hydrogen storage material because of its high hydrogen content of 19.6% wt., high stability under ambient conditions, nontoxicity, and high solubility in common solvents. Hydrolysis of ammonia borane appears to be the most efficient way of releasing hydrogen stored in it. Since ammonia borane is relatively stable against hydrolysis in aqueous solution, its hydrolytic dehydrogenation can be achieved at an appreciable rate only in the presence of suitable catalyst at room temperature. Metal(0) nanoparticles have high initial catalytic activity in releasing H2 from ammonia borane. Thermodynamically instable metal(0) nanoparticles can kinetically be stabilized against agglomeration either by using ligands in solution or by supporting on the surface of solid materials with large surface area in solid state. Examples of both type of stabilization are presented from our own studies. The results show that metal(0) nanoparticles dispersed in solution or supported on suitable solid materials with large surface area can catalyze the release of H2 from ammonia borane at room temperature. Dispersion of metal(0) nanoparticles, stabilized in liquid phase by anions or polymers, seems advantageous as providing more active sites compared to the metal nanoparticles supported on a solid surface. However, the supported metal nanoparticles are found to be more stable against agglomeration than the ones dispersed in liquid phase. Therefore, metal nanoparticles supported on solid materials have usually longer lifetime than the ones dispersed in solution. Examples are given from the own literature to show how to improve the catalytic activity and durability of metal nanoparticles by selecting suitable stabilizer or supporting materials for certain metal. For the time being, nanoceria supported rhodium(0) nanoparticles are the most active catalyst providing a turnover frequency of 2010 min−1 in releasing H2 from ammonia borane at room temperature.