Novel Mg–Zr–A–H (A = Li, Na) hydrides synthesized by a high pressure technique and their hydrogen storage properties

It is important to develop the hydrogen storage technology by creating novel metal hydrides. In the present study, the powder mixtures of 6MgH2 + ZrH2 + nAH (A = Li, Na; n = 0, 0.3, 0.7, 1.0) were reacted to synthesize quaternary hydrides by use of a high pressure technique. The crystal structures of the new hydrides were refined by the Rietveld method based on the synchrotron XRD data. By reacting 6MgH2 + ZrH2 + nLiH, the quaternary hydrides with simple FCC-type structure were formed. In the case of 6MgH2 + ZrH2 + nNaH, novel quaternary hydrides with Ca7Ge type structure were formed as well as the hydrides with simple FCC structure. The hydrogen storage capacities were around 6 wt.% according to the pressure-composition isotherm measurements. The formation enthalpies of the quaternary hydrides with simple FCC structure were proved to be lower while the enthalpies of the Mg–Zr–Na–H hydrides with Ca7Ge type structure were higher, than that of the ternary Mg–Zr–H hydride obtained by reacting the basic system 6MgH2 + ZrH2. The hydrogen releasing temperatures of the quaternary Mg–Zr–A–H hydrides were slightly lower than that of the ternary Mg–Zr–H hydride.

Research highlights▶ Novel Mg–Zr–A–H (A = Li, Na) based hydrides were synthesized by high pressure technique to improve the hydrogen storage properties. ▶ By reacting 6MgH2 + ZrH2 + nLiH, quaternary hydrides with simple FCC-type structure were formed. Li atom would substitute Mg or Zr in the lattice of the simple FCC phase. ▶ By reacting 6MgH2 + ZrH2 + nNaH, novel quaternary hydrides with Ca7Ge type structure were formed as well as the hydrides with simple FCC structure. ▶ Reversible hydrogen storage behaviors of the newly formed hydrides were observed. The hydrogen releasing temperatures of the quaternary Mg–Zr–A–H hydrides were proved to be slightly lower than that of the Mg–Zr–H hydride. ▶ Formation of simple FCC type Mg–Zr–A–H hydride with addition of LiH or small amount of NaH into the basic system 6MgH2 + ZrH2 can decrease the hydrogen releasing temperature more than the formation of superlattice phase by addition of 0.7 or 1.0 NaH. ▶ The formation enthalpies of the quaternary hydrides with simple FCC structure were proved to be lower while the enthalpies of the Mg–Zr–Na–H hydrides with Ca7Ge type structure were higher, than that of the ternary Mg–Zr–H hydride obtained by reacting the basic system 6MgH2 + ZrH2.