A comparison study of decomposition mechanisms of single-cation and double-cations (Li, Al) ammine borohydrides

The decomposition mechanisms of [Li(NH 3 )][BH 4 ], [Al(NH 3 ) 6 ][BH 4 ] 3 and [Al(NH 3 ) 6 ][Li 2 (BH 4 ) 5 ] were investigated using Density functional theory (DFT) calculation. The calculated results show that [Li(NH 3 )][BH 4 ] has low NH 3 vacancy formation energy and diffusion barrier, therefore ammonia would easily release at relatively low temperature. Both [Al(NH 3 ) 6 ][BH 4 ] 3 and [Al(NH 3 ) 6 ][Li 2 (BH 4 ) 5 ] show relatively high NH 3 vacancy formation energies and diffusion barriers, which avoid ammonia release at low temperature. In addition, the calculated H 2 formation energy barriers, i.e., [Al(NH 3 ) 6 ][Li 2 (BH 4 ) 5 ] < [Al(NH 3 ) 6 ][BH 4 ] 3 < [Li(NH 3 )][BH 4 ], are in agreement with the tendency of dehydrogenation temperatures determined experimentally. The incorporation of [BH 4 ] - into [Al(NH 3 ) 6 ][BH 4 ] 3 play an important role in decreasing the dehydrogenation temperature and improving the hydrogen purity of [Al(NH 3 ) 6 ][Li 2 (BH 4 ) 5 ].