An improvement on cycling stability of Ti–V–Fe-based hydrogen storage alloys with Co substitution for Ni

In this work, the effects of Co substitution for Ni on the microstructures and electrochemical properties of Ti0.8Zr0.2V2.7Mn0.5Cr0.6Ni1.25−xCoxFe0.2 (x = 0.00–0.25) alloys were investigated systematically by XRD, SEM and electrochemical measurements. The structural investigations revealed that the main phases of all of the alloys were the C14 Laves phase in a three-dimensional network and the V-based solid solution phase with a dendritic structure. The lattice parameters and unit cell volumes of the two phases gradually increased with the increase of Co concentration. The relative abundance of the C14 Laves phase slightly increased from 47.3% to 49.6%, accordingly that of the V-based solid solution phase decreased, with the increase of x from 0.00 to 0.25. The crystal grain of the V-based solid solution phase was obviously refined after Co substitution. The electrochemical investigations showed that the proper substitution of Co for Ni improved the cycling durability of the alloy electrodes mainly due to the suppression of both the pulverization of the alloy particles and the dissolution of the main hydrogen absorbing elements (V and Ti) into the KOH solution. The cycling stability of the alloy electrode with x = 0.1 was 79.8% after 200 cycles. However, the maximum discharge capacity (Cmax) was decreased from 340.5 to 305.6 mAh g−1, and the high rate dischargeability (HRD) gradually decreased from 66.8% to 55.0% with increasing x from 0.00 to 0.25.