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

摘要

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 LaNig 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_(2.45-x)Co_(0.75+x)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 × 8h. 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.