Improvement in the electrochemical properties of ZrMn2 hydrides by substitution of elements

Abstract

The hydrogen-storage properties and the electrochemical properties are investigated for the alloys ZrMn2Nix, ZrMnNi1+x, Zr0.5Ti0.5Mn0.4V0.6Ni1−xFex and Zr0.5Ti0.5Mn0.4V0.6Ni0.85M0.15. The C14 Laves phase forms in all the alloys ZrMn2Nix (x=0.0, 0.3, 0.6, 0.9 and 1.2). Among the alloys ZrMn2Nix, ZrMn2Ni0.6 has the largest discharge capacity (29 mAh/g) and a relatively good cycling performance, and shows a relatively easy activation. The C14 Laves phase also forms in all the alloys ZrMnNi1+x (x=0.0, 0.1, 0.2, 0.3 and 0.4). Among the alloys ZrMnNi1+x, ZrMnNi1.0 has the largest discharge capacity (42 mAh/g) and a relatively good cycling performance, and shows the easiest activation. Zr0.5Ti0.5Mn0.4V0.6Ni1−xFex (x=0.00, 0.15, 0.30, 0.45 and 0.60) has the C14 Laves phase hexagonal structure. Their hydrogen storage capacities do not show significant differences. The discharge capacity just after activation decreases with an increase in the amount of the substituted Fe but the cycling performance is improved. The discharge capacity after activation of the alloy with x=0.00 is about 240 mAh/g at the current density 60 mA/g. Zr0.5Ti0.5Mn0.4V0.6Ni0.85Fe0.15 is the best composition with a relatively large discharge capacity and a good cycling performance. The increase in the discharge capacity of Zr0.5Ti0.5Mn0.4V0.6Ni0.85Fe0.15 with the increase in the current density (from 60 mA/g to 125 mA/g) is considered to result from the self-discharge property of the electrode. Zr0.5Ti0.5Mn0.4V0.6Ni0.85M0.15 (M=Fe, Co, Cu, Mo and Al) alloys also have the C14 Laves phase hexagonal structure. The alloys with M=Co and Fe have relatively larger hydrogen storage capacities. The discharge capacities just after activation are relatively large in the case of the alloys with M=Co and Fe. The Zr0.5Ti0.5Mn0.4V0.6Ni0.85Co0.15 alloy is best with a relatively large discharge capacity (257 mAh/g at the current density 250 mA/g for the 12th cycle) and a good cycling performance. During activation form Ni-rich and Fe-rich regions on the surface of the Zr0.5Ti0.5Mn0.4V0.6 Ni0.85Fe0.15 alloy. They may act as the active sites for the electrochemical reaction. With the increase in the number of charge-discharge cycles for the Zr0.5Ti0.5Mn0.4V0.6Ni0.85Fe0.15 alloy, the quantities of the Zr and Fe dissolved in the electrolyte solution increase.