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 I h at 5 X 10(-6) Torr and 300 degreesC. 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 (C) 2003 Elsevier B.V. All rights reserved.