Influences of Co substitution and annealing treatment on the structure and electrochemical properties of hydrogen storage alloys

In this paper, the effects of both Co substitution and annealing treatment on the structures and electrochemical properties of La 0.7 Mg 0.3 Ni 2.45 - x Co 0.75 + x Mn 0.1 Al 0.2 ( x = 0.00 , 0.15 , 0.30 ) hydrogen storage alloys have been studied. X-ray diffraction (XRD) analyses show that the main phases of all of the alloys are ( La , Mg ) Ni 3 phase ( PuNi 3 -type structure) and LaNi 5 phase ( CaCu 5 -type structure). Electrochemical investigations show that increasing Co content and annealing treatment can considerably enhance the cyclic stability of the alloy electrodes. For La 0.7 Mg 0.3 Ni 2.15 Co 1.05 Mn 0.1 Al 0.2 alloy, the value of C 100 / C max was only 65.5% at its as-cast, however, the value reached 80.5% after an annealing treatment of 1173 K × 8 h . The maximum discharge capacity of the alloy electrodes increased by annealing treatment and decreased slightly when Co content increased from x = 0.00 to 0.30 at its as-cast state. But the exchange current density ( I 0 ) , the limiting current density ( I L ) and the diffusion coefficient of hydrogen ( D ) of the alloy electrodes decreased, leading to a corresponding reduction of the high rate dischargeability (HRD), with increasing Co content and annealing treatment.