Novel BCC Anode Materials for High-Power Alkaline MH-Air Batteries
The Ti-V-Cr-Ni BCC electrode materials were prepared by arc melting and rapid cooling with an Edmund Buhler compact MAM-1 with an attached suction casting crucible. The control samples were made by conventional arc melting under Ar. As-cast ingots were pulverized by hydriding in a Sievert’s apparatus. The samples were analyzed by x-ray diffractometry (XRD), scanning and transmission electron microscopy (SEM, TEM) and energy-dispersive x-ray spectrometry (EDS). The anode was prepared by pressing a mixture of alloy and Ni powders onto Ni mesh. Electrochemical cycling experiments were conducted in a half cell containing 30% aqueous KOH electrolyte, a Ni(OH)2 counter electrode and an Hg/HgO reference electrode. The electrode was charged at 100mA/g and discharged three times at 167mA/g, 50mA/g, and 10mA/g; each to a cut-off voltage of -0.7V vs. Hg/HgO.
EDS mapping showed, as expected,2 a Ti-Ni rich secondary phase in the arc melted sample that permeates the micron-scale primary V phase. (Fig. 1, top, Ni phase in green.) No phase segregation is resolved by EDS for the suction cast alloy. (Fig. 1, bottom). For the first 100 charge-discharge cycles, both materials gave similar capacities. However, at each cycle, the suction cast alloy consistently discharges with 20%-40% more capacity during the high-rate step (167mA/g, ~C/3, 1st step in Fig. 2).
It has been argued that the Ni-Ti phase is important for the electrocatalytic dissociation of H2 molecules during charge, and the matrix V-rich phase is the storage region for hydrogen.2 Reducing the length scale of the phase segregation of the Ni-Ti regions by rapid cooling may therefore improve the kinetics of the material by reducing the hydrogen diffusion length and by increasing surface area for electrocatalysis.