Effect of boric acid on thermal dehydrogenation of ammonia borane: Mechanistic studies

摘要

Ammonia borane (AB, NH_3BH_3) is a promising hydrogen storage material for use in proton exchange membrane (PEM) fuel cell applications. In this study, the effect of boric acid on AB dehydrogenation was investigated. Our study shows that boric acid is a promising additive to decrease onset temperature as well as to enhance hydrogen release kinetics for AB thermolysis. With heating, boric acid forms tetrahydroxyborate ion along with some water released from boric acid itself. It is believed that this ion serves as Lewis acid which catalyzes AB dehydrogenation. Using boric acid, we obtained high H_2 yield (11.5 wt% overall H_2 yield, 2.23 H_2 equivalent) at 85 ℃, PEM fuel cell operating temperatures, along with rapid kinetics. In addition, only trace amount of NH_3 (20-30 ppm) was detected in the gaseous product. The spent AB solid product was found to be polyborazylene-like species. The results suggest that the addition of boric acid to AB is promising for hydrogen storage, and could be used in PEM fuel cell based vehicles.