$ZrMn_0.5V_0.7Ni_1.2$ alloy is attractive for anode material in Ni/MH secondary battery because of its large hydrogen storage capacity in gas-solid reaciton and long cycling durability in KOH electrolyte.
However, this alloy has a low reversible hydrogen storage capacity because of large slope characteristics in PCT curve. The plateau flatness of PCT curve of the alloy is important for the discharge performance such as discharge efficiency and discharge capacity because it is a direct determinant of reversible hydrogen storage capacity and discharge potential. Therefore It is necessary to flatten the plateau of PCT curve of this alloy for developing high capacity and high performance MH electrode.
It has been previously reported that annealing is an effective method for flattening the plateau region of the PCT curve and is therefore widely applied to alloy manufacturing.
$ZrMn_0.5V_0.7Ni_1.2$ alloy has been annealed to flatten the plateu of PCT curve. The optimal annealing condition was 12 hours at 1000℃. The alloy annealed under optimal condition had higher rate capability and higher capacity than as-cast one. The rate capability of the electrode can be influenced by the both specific reaction surface area and surface catalytic activity of unit area.
We measured the reaction surface area and surface catalytic activity to study the reason for improvement in the rate capability.
Firstly, measuring the surface area by B.E.T. analysis show that the change of surface area for fully activated electrode of this alloy was little depressed after annealing. This result is insufficient to explain the change of discharge characteristics of alloy after annealing. Secondly, cross sectional view by S.E.M. show that second phase(Zr-Ni phase) is uniformly distributed over the as-cast alloy and disappeared into matrix after annealing. Compositional analysis by E.D.S. shows that Ni content of matrix was increased, being this alloy homogenized after annealing.
This increase of Ni content in marix result in increase of the electrocatalytic activity for hydrogenation reaction of alloy in KOH electrolyte. Eventually, improvement in the rate capability of annealed alloy is due to increase of Ni content in matrix.
In case of other kind of Zr-based hydrogen storage alloy containing second phase(Zr-Ni phase), it will be possible to improve the discharge efficiency of alloy by applying annealing process which result in homogenizing the alloy.
