Enhanced hydrogen sorption in a Li–Mg–N–H system by the synergistic role of Li4(NH2)3BH4 and ZrFe2

Abstract

The present investigation describes the synergistic role of Li₄(BH₄)(NH₂)₃ and ZrFe₂ in the hydrogen storage behaviour of a Li–Mg–N–H hydride system. The onset desorption temperature of ZrFe₂-catalysed Mg(NH₂)₂–LiH–Li₄(BH₄)(NH₂)₃ is ∼122 °C, which is 83 °C, 63 °C, and 28 °C lower than that of thermally treated 2LiNH₂–1MgH₂, 2LiNH₂–1MgH₂–4 wt%ZrFe₂, and 2LiNH₂–1MgH₂–0.1LiBH₄ composites, respectively. Native Mg(NH₂)₂–LiH–Li₄(BH₄)(NH₂)₃ absorbed only 2.78 wt% of H₂ within 30 min. On the other hand, the ZrFe₂-catalysed Mg(NH₂)₂–LiH–Li₄(BH₄)(NH₂)₃ sample absorbed 3.70 wt% of hydrogen within 30 min and 5 wt% of H₂ in 6 h at 180 °C and 7 MPa H₂ pressure. Mg(NH₂)₂–LiH–Li₄(BH₄)(NH₂)₃ catalyzed with ZrFe₂ shows negligible degradation of the storage capacity even after repeated cycles of de/rehydrogenation. The effect of ZrFe₂ and Li₄(BH₄)(NH₂)₃ on a Mg(NH₂)₂/LiH composite has been described and discussed with the help of structural (X-ray diffraction), microstructural (electron microscopy), and vibrational modes of molecules through FTIR studies. The present results suggest that an optimum catalysis may originate from the synergistic action of an in situ formed quaternary hydride (Li₄(BH₄)(NH₂)₃) and an intermetallic-like ZrFe₂, which acts as a pulverizer cum catalyst.