Effects of Mo additive on the structure and electrochemical properties of low-temperature AB5 metal hydride alloys

After an annealing treatment at 960 degrees C for 8 h, the molybdenum added into previously designed AB(5) alloys for -30 degrees C applications segregates into spheres with diameters between 1 and 10 mu m. A secondary phase with Zr-to-other elements ratio of about 1-to-5, over- (AB(7)), and under-stoichiometric (AB(4)) phases were observed in most of the alloys regardless of Mo-content. As the Mo-content increases, the AB(7) phase disappears while the AB(4) phase grows in size and abundance. Regarding the gaseous absorption properties, a small amount of Mo (0.2 at.%) in the main phase reduces the plateau pressure and hydride heat of formation uniformly for all Mo-containing alloys. The reduction in main phase abundance causes a decrease in both the total and the reversible hydrogen storage capacities. In electrochemical testing, the addition of Mo decreases the discharge capacity, high-rate dischargeability, and hydrogen diffusion in the bulk. The influence of Mo-addition to general battery performance is very minor. However, the low-temperature AC impedance measured at -40 degrees C shows reduced charge transfer resistance and increased double layer capacitance in the Mo-containing alloys. Mo was found to assist the surface reaction at very low temperatures, and the effect is proportional to the amount of addition as noted by the increasing surface area and catalytic ability, which is similar to the case of AB(2) alloys. (C) 2011 Elsevier B.V. All rights reserved.