In situ formed Mg(BH4)2 for improving hydrolysis properties of MgH2

Abstract

The hydrolysis of MgH2 delivers high hydrogen capacity (15.2 wt%), which is very attractive for real-time hydrogen supply. However, the formation of a surface passivation Mg(OH)2 layer and the large excess of H2O required to ensure complete hydrolysis are two key challenges for the MgH2 hydrolysis systems. Now, a low-cost method is reported to synthesize MgH2@Mg(BH4)2 composite via ball-milling MgH2 with cheap and widely available B2O3 (or B(OH)3). By adding small amounts of B2O3, the in-situ formed Mg(BH4)2 could significantly promote the hydrolysis of MgH2. In particular, the MgH2–10 wt% B2O3 composite releases 1330.7 mL·g−1 H2 (close to 80% theoretical hydrogen generation H2) in H2O and 1520.4 mL·g−1 H2 (about 95%) in 0.5 M MgCl2 in 60 min at 26 °C with hydrolysis rate of 736.9 mL·g−1·min−1 and 960.9 mL·g−1·min−1 H2 during the first minute of the hydrolysis, respectively. In addition, the MgCl2 solution allows repeated use by filtering and exhibits high cycle stability (20 cycles), therefore leading to much reduced capacity loss caused by the excess H2O. We show that by introducing B2O3 and recycling the 0.5 M MgCl2 solution, the system hydrogen capacity can approach 5.9 wt%, providing a promising hydrogen generation scheme to supply hydrogen to the fuel cells.