Hydrogen storage properties of La(Ni0.9M0.1)3 alloys
Introduction
Nickel–metal hydride batteries are used extensively as the power source in various portable electronic devices such as notebook computers and cellular phones. These systems require batteries with a wide variety of properties: high discharge capacity, long charge–discharge cycle life, efficiency at high temperatures, and durability against overdischarge [1], [2]. A key technique for improving the capacity is to increase the capacity of the hydrogen storage material used as the negative electrode material.
AB5 alloys, where A represents a metal that is capable of reacting exothermically with hydrogen and B represents another kind of metal, are generally employed as the negative electrode material in nickel–metal hydride batteries. Of these, LaNi5 reacts with hydrogen at normal temperatures and is chemically stable; therefore, it has been studied extensively for use as an electrode material in nickel–metal hydride batteries [1], [2], [3], [4]. However, the discharge capacity of nickel–metal batteries using this AB5-type alloy electrode has now reached 85% or more of the theoretical capacity (372 mAh/g), and further increases in discharge capacity will be difficult.
AB3 alloys theoretically have a higher capacity than AB5 alloys. The number of hydrogen atoms absorbed per mole of the LaNi3 alloy (H/M) has reached 1.25, and the theoretical capacity of this alloy is 411 mAh/g. Thus, AB3 alloys are potential candidates for the electrode material. However, as reported in previous works [3], [4], [5], [6], [7], their main problem is that the stored hydrogen in the LaNi3 alloy is released only sparingly after absorption [5], [6], [7]. In order to help solve this problem, we examined the efficiency of partial substitution of the B site in AB3-type hydrogen storage materials.
Section snippets
Experimental
La(Ni0.9M0.l)3 (M=Al, Si, Ti, V, Mn, Fe, Cu, Nb, Sn, Ta) alloys were prepared by induction melting in Ar gas at atmospheric pressure. The prepared ingots were mechanically pulverized to 75 μm or less in diameter and preliminary activation with hydrogen was not performed. Crystallographic characterizations were obtained by X-ray diffraction at room temperature. The hydrogen absorption–desorption properties of the alloys were investigated using the Sieverts’ method to obtain pressure–composition
Results and discussion
LaNi3-type alloys have a higher theoretical hydrogen absorbing capacity than LaNi5-type alloys. To investigate the electrochemical properties of LaNi3 alloy, a charge–discharge test was carried out at 25 °C. The LaNi3 alloy electrode was found to have a discharge capacity of 150 mAh/g as can be seen in Table 1. The effects of partial substitution of Ni sites with a foreign element on the reactivity with hydrogen were investigated in order to increase the discharge capacity of this type of alloy
Summary
To increase the desorption pressure of LaNi3-type alloys, the effects of partial substitution of Ni sites with a foreign element (Al, Si, Ti, V, Mn, Fe, Cu, Nb, Sn, Ta) on the reactivity with hydrogen were investigated. Negative electrodes fabricated from La(Ni0.9M0.l)3 (M=Al, Mn) alloys were found to have larger discharge capacities than those fabricated from LaNi3 alloy. The hydrogen reversibility for La(Ni0.9M0.l)3 (M=Al, Mn) alloys was also found to be markedly better than that for LaNi3
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