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

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 ℃ 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.