Improvement in −40 °C electrochemical properties of AB2 metal hydride alloy by silicon incorporation

The ultra-low temperature (−40 °C) charge transferred resistance of a Laves phase based-AB2 metal hydride alloy was reduced by a factor of 5 by a partial substitution of Ni with Si in Ti12Zr21.5V10Cr7.5Mn8.1Co8.0Ni32.2−xSixSn0.3Al0.4, where x = 0-4. This improvement was due to the increases in surface area from the higher leaching-rate of the oxidation product of Si and to the surface catalytic effect with the incorporation of Si. The microstructure, gaseous phase hydrogen storage, and electrochemical properties of these alloys were investigated and reported. With the addition of Si in the formula, the abundances of both C15 and TiNi minor phases increased, which resulted in decreases in both maximum gaseous phase hydrogen storage and electrochemical capacities. Some changes on the chemical compositions of the minor phases from the Si-incorporation were observed but cannot be correlated to the improvement in low-temperature performance. These changes are the transition from Zr to ZrO2, the decrease in Ti/Zr ratio in the TiNi phase, and the higher Sn-content in the TiNi phase. An amount of 1 at.% of Si is recommended from the balance among capacity, bulk diffusion, surface area, and catalytic properties of the alloys. In addition, a special sample preparation method for using inductively coupled plasma analysis to determine the Si-content in the sample was developed and presented.