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

Abstract

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

Decomposition Mechanism of Zinc Ammine Borohydride: A First-Principles Calculation

“…As shown in Figure , the energy barrier of this process is 2.02 eV for [Zn­(NH 3 ) 2 ]­[BH 4 ] 2 , which is 0.66 eV higher than that of [Al­(NH 3 ) 6 ]­[Li 2 (BH 4 ) 5 ] . The experimental results, however, show peak dehydrogenation temperatures of 127 and 138 °C for [Zn­(NH 3 ) 2 ]­[BH 4 ] 2 and [Al­(NH 3 ) 6 ]­[Li 2 (BH 4 ) 5 ], respectively. , Thus, different from that of [Al­(NH 3 ) 6 ]­[Li 2 (BH 4 ) 5 ], the dehydrogenation of [Zn­(NH 3 ) 2 ]­[BH 4 ] 2 may not be through the direct combination of a proton from NH 3 and a hydride from [BH 4 ] − .…”

“…As shown in Figure 3, the energy barrier of this process is 2.02 eV for [Zn(NH 3 ) 2 ][BH 4 ] 2 , which is 0.66 eV higher than that of [Al(NH 3 ) 6 ][Li 2 (BH 4 ) 5 ]. 35 The experimental results, however, show peak dehydrogenation temperatures of 127 and 138 °C for [Zn(NH 3 ) 2 ][BH 4 ] 2 and [Al(NH 3 ) 6 ][Li 2 (BH 4 ) 5 ], respectively. 22,33 We first calculated the formation of B 2 H 6 via combination of two BH 3 from [BH 4 ] − with two remaining hydrides bonding to the Zn cation.…”

“…Our previous reports suggest that AMBs with low E a tend to release ammonia during decomposition. 34,35 Ammonia was not detected during thermal decomposition of [Zn(NH 3 ) 2 ][BH 4 ] 2 , unlike that of [Mg(NH 3 ) 2 ][BH 4 ] 2 which released a small amount of NH 3 during decomposition. 22,23 With a relatively low ammonia diffusion energy barrier of 0.96 eV, NH 3 may be able to diffuse and involve in the formation of amorphous intermediate(s) before the dehydrogenation of [Zn(NH 3 ) 2 ]-[BH 4 ] 2 , which prevents its release upon heating.…”

“…Our previous studies about AMBs (seeing Figure 2) show a correlation between the peak dehydrogenation temperature and the calculated H 2 release energy barrier (H 2 released via combining a hydride from BH 4 and a proton from NH 3 ). 34,35 The AMBs with higher energy barriers tend to release hydrogen at higher temperatures. Therefore, we calculated a similar reaction pathway in which H 2 is form via combining a hydride from BH 4 and a proton from NH 3 .…”

“…23, 33 Among these AMBs, As reported in a previous literature study, [Zn(NH 3 ) 2 ][BH 4 ] 2 is able to release 8.9 wt% hydrogen below 115 o C within 15 min without concomitant release of undesirable gases such as ammonia and boranes. 22The dehydrogenation of some of the AMBs may be achieved through combining hydrides from [BH 4 ]and protons from NH 3 , ultimately yielding amorphous metal boronitrides 19,[23][24][25][26][34][35][36]. The experimental observation of the formation of well-crystallized Zn during…”