Wp Willem Peter Kalisvaart

Structure and stability of high pressure synthesized Mg–TM hydrides (TM = Ti, Zr, Hf, V, Nb and Ta) as possible new hydrogen rich hydrides for hydrogen storage

A series of hydrogen rich Mg6–7TMH14–16 (TM = Ti, Zr, Hf, V, Nb and Ta) hydrides have been synthesized at 600 °C in a high pressure anvil cell above 4 GPa. All have structures based on a fluorite type metal atom subcell lattice with a ≈ 4.8 A. The TM atom arrangements are, however, more ordered and can best be described by a superstructure where the 4.8 A FCC unit cell axis is doubled. The full metal atom structure corresponds to the Ca7Ge type structure. This superstructure was also observed from electron diffraction patterns. The hydrogen atoms were found from powder X-ray diffraction using synchrotron radiation to be located in the two possible tetrahedral sites. One coordinates three Mg atoms and one TM atom and another coordinates four Mg atoms. These types of new hydrogen rich hydrides based on immiscible metals were initially considered as metastable but have been observed to be reversible if not fully dehydrogenated. In this work, DFT calculations suggest a mechanism whereby this can be explained: with H more strongly bonded to the TM, it is in principle possible to stepwise dehydrogenate the hydride. The remaining hydrogen in the tetrahedral site coordinating the TM would then act to prevent the metals from separating, thus making the system partially reversible.

Mechanical alloying and electrochemical hydrogen storage of Mg-based systems

Results on mechanical alloying of binary and ternary Mg-Ti-based mixtures are reported. Using fine-powdered reactants and a process-control-agent, a mixture of two face-centered cubic compounds is obtained. Using a coarse Mg precursor without addition of a milling agent results in a hexagonal-solid solution of Ti in Mg due to a lower oxygen content in the Mg starting material. Upon introduction of Ni or A1 as a third element, the amount of dissolved Ti decreases to form a nanocrystalline secondary phase. The electrochemical charging capacity of the hexagonal compounds is far superior to that of the cubic ones, whereas the discharge capacity is significantly increased only upon addition of Ni. The secondary TiNi phase acts as a rapid diffusion path for hydrogen, greatly improving the rate capability of the alloys. The reversible hydrogen storage capacity reaches values of up to 3.2 wt% at room temperature for (Mg0.75 Ti0.25)0.90 Ni0.10.

Mg-Ti based materials for electrochemical hydrogen storage

Results of the mechanical alloying of binary Mg–Ti and ternary Mg–Ti–Ni mixtures using two different process control agents are reported. Both high- and low-energy milling resulted in the formation of cubic compounds. When all starting reactants had disappeared, a mixture of two face-centered cubic (fcc) phases was formed with lattice constants around 4.40 and 4.25 A. The electrochemical hydrogen storage capacity, 450 mAh/g for (Mg 0.65 Ti 0.35 ) 0.95 Ni 0.05 , was about one-third that reported for Mg–Ti thin films. This suggested that only one of the two fcc phases was active at ambient conditions. Prolonged mechanical alloying of (Mg 0.60 Ti 0.40 ) 0.95 Ni 0.05 resulted in full conversion of the material into one fcc-phase with a very small crystallite size, an intermediate lattice constant (4.33 A), and a sharply decreased storage capacity.