Microstructure and electrochemical hydriding/dehydriding properties of ball-milled TiFe-based alloys

Nanocrystalline/amorphous AB-type hydrogen storage TiFe1-xCox (x = 0.1 and 0.3) and TiFe0.7Ni0.2Co0.1 alloys have been synthesized by mechanical alloying. The high-energy ball milling process is performed in a planetary type mill under an inert atmosphere, and at various milling times (15, 20 and 30 h). The microstructure and the morphology of the as obtained alloy powders are investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The SEM analysis showed that the alloy powders are consisted of agglomerations (10-15 mu m) of smaller particles cold-welded together. The average particle size, in the range of 5-7 mu m, is dependent on both, the time of milling and in a less extent on the amount of Co in the TiFe1-xCox alloys. The addition of Ni (TiFe0.7Ni0.2Co0.1) results in slight increase of the mean particle size of the ball milled powder. The XRD analysis showed that the samples have composite microstructure. Both, the diffraction peaks of the pure metals and the peaks of the fcc TiFe compound are identified in the diffractograms of all samples. This result indicates that the alloying process is not fully completed even after 30 h of ball milling. The electrochemical cycle life tests of the electrodes, prepared from as-obtained alloys, showed that the Co containing AB type alloys possessed lower maximum discharge capacity and better corrosion stability, compared to the TiFe electrode. In contrast to the TiFeCo alloys the Ni containing material does not show a big difference in the discharge capacity and cycle life performance between the samples milled for different time in the range of 15-30 h. Moreover, the TiFe0.7Ni0.2Co0.1 alloy reveals very small discharge capacity decay, about 90% of the capacity is retained after 100 charge/discharge cycles. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.