Roles of constituent elements and design of hydrogen storage alloys

Abstract The chemical interactions between atoms in hydrogen storage alloys have been investigated by the DV-Xα molecular orbital method. In view of the nature of the chemical bond between atoms, a completely different interstitial space for the hydrogen occupancy is formed in binary A–B hydrogen storage alloys, where A and B are the hydride forming and non-forming elements, respectively, as compared to the interstitial space in pure A metals. As a result, hydrogen interacts more strongly with the B element than the A element in every hydrogen storage alloy, despite the fact that the A element has a larger affinity for hydrogen than the B element in the binary metal–hydrogen system. In addition, it is shown that the A:B compositional ratio of hydrogen storage alloys is predictable using a simple equation, 2×Bo(A–B)/[Bo(A–A)+Bo(B–B)], where Bo(A–B), Bo(A–A) and Bo(B–B) are the bond orders between atoms given in the parentheses.