Effects of sintering on composite metal hydride alloy of Mg2Ni and TiNi synthesized by mechanical alloying

As-milled composite metal hydrides composed of Mg₂Ni and TiNi phases were cold-pressed under a pressure of 490 MPa and sintered for 1 h at 5×10⁻⁶ Torr and 300 °C. Electrochemical characteristics of the sintered composite metal hydride electrode were investigated. The maximum discharge capacity of the sintered composite alloy electrode was 125 mAh/g at a discharge current density of 100 mA/g. This value was similar to that of the as-milled one before sintering. However, the sintered electrode retained 80% of the maximum discharge capacity after 150 cycles, while the as-milled electrode retained only 55%. This is because after the sintering process an interface between Mg₂Ni and TiNi plays a role similar to a diffusion layer of hydrogen. In the sintered composite electrode, when a discharging step proceeds, hydrogen absorbed in a Mg₂Ni particle can move into a TiNi phase through the bonded-interface between Mg₂Ni and TiNi, then discharges at the interface between TiNi and the electrolyte. Also, the electrochemical impedance spectroscopy (EIS) tests showed that the composite alloy electrodes had a lower charge-transfer resistance and a higher hydrogen diffusion coefficient than those in single-phase Mg₂Ni. This indicates that TiNi particles in the composite are the active sites for redox reaction of hydrogen and the pathway for the diffusion of hydrogen