Electrochemical Studies on AI15 Metal Hydrides

The abili[y of certain intennctallic alloys to reversibly absorb significant amounts of hydrogen at low pressures and high potentials is behg exploikxl for several applications. In particular, the replacement of Cd in a Ni-Cd cell with the metal hydride (I @ anodes has resulted in substantial gains in the specific energy, energy density, cycle life and in the cnvironrnental compatibility. Also, the Ni-M1 I cells have the advantages of sustaining high discharge rates, fast charge rates and gas recombination processes during overcharge and overdischarge ” similar to Ni-Cd, With the voltage and the charge methods also being almost identical, the Ni-MII cells are expected to gain prominence over Ni-Cd in applications ranging from portable electronic appliances to electric vehicle. Two classe $ of metal hydrides alloys based on rare earth metal (AB5)(] ~2J and titanium (ABJ(3J are being currently developed at various laboratories. The AB5 alloys are essentially based on LaNi5 with various substilucnts for La as well as Ni to stabilize the atloy during charge- discharge cycling, by reducing the internal stress on hydrogen absorption and/or forming protective surface films. For example, the volume expansion is redueed by a partial substitution of Ni with Co and the interracial properties improved with small amounts of Al or Si(l). Sakai et al(4) studied the ternary alloys with different ternary solutes including Mn, Cl, Al, Co and Cu. The cycle life improves upon the substitution of Ni with the ternary solute in the order Mn < Ni < Cu c Cr c Al < Co. A substitution of the rare earth metal site with Ti(5J