Realizing 6.7 wt% reversible storage of hydrogen at ambient temperature with non-confined ultrafine magnesium hydrides

Abstract

Using light metal hydrides as hydrogen carriers is of particular interest for safe and compact storage of hydrogen. Magnesium hydride (MgH₂) has attracted significant attention due to its 7.6 wt% hydrogen content and the natural abundance of Mg. However, bulk MgH₂ is stable (ΔH_f ∼ 76 kJ mol⁻¹) and releases hydrogen only at impractically high temperatures (>300 °C). Herein, we demonstrate a first attempt to achieve ambient-temperature reversibility of hydrogen storage for MgH₂ by fabricating non-confined ultrafine nanoparticles. Taking advantage of the big discrepancies in the solubility of metal hydrides and chlorides in THF, a novel metathesis process of liquid–solid phase driven by ultrasound was proposed. Ultrafine MgH₂ nanoparticles predominantly of around 4–5 nm in size were successfully obtained without scaffolds or supports. A reversible hydrogen storage capacity of 6.7 wt% at 30 °C was measured, which has never been achieved before, thanks to thermodynamic destabilization and decreased kinetic barriers. The bare nanoparticles exhibited a stable and rapid hydrogen cycling behaviour in 50 cycles at 150 °C, a remarkable improvement compared with bulk MgH₂. Our finding brings MgH₂ a step closer to practical applications and the methodology presented here opens new pathways for fabricating sensitive nanoparticles.