Phase Composition And Electrochemical Properties Of AB_3.3-Type RE-Mg-Ni-Based Hydrogen Storage Alloy

In this paper, we have prepared AB3.3-type RE–Mg–Ni-based hydrogen storage alloy via induction melting method. Then, the effects of ultra-high pressure treatment on phase composition and electrochemical properties, and the electrochemical characteristic of the RE–Mg–Ni-based hydrogen storage alloy with 2H- and 3R-type of A2B7 phase were investigated.This research highlighted the effects of ultra-high pressure(UHP) technique(1–5GPa) on microstructural evolution and electrochemical performances of La0.70Mg0.30Ni3.3alloy. Structure analysis showed that the as-cast alloy consisted of(La,Mg)Ni3,(La,Mg)2Ni7,(La,Mg)5Ni19, La Ni5 and La Mg Ni4 phases. For all the UHP treated alloys,(La,Mg)5Ni19 phase disappeared via converting into(La,Mg)2Ni7 and La Ni5 phases. At 1GPa, La Ni5 phase and La Mg Ni4 phase fully transformed into(La,Mg)Ni3 and(La,Mg)2Ni7phases by the peritectic reactions. However, when the pressure was 3 GPa and 5 GPa, the peritectic reactions were uncompleted for the atom diffusion was hindered, which not only caused the minor residual of La Ni5 and La Mg Ni4 phases in the alloys, but also resulted in the increase of(La,Mg)Ni3 phase and the decrease of(La,Mg)2Ni7 phase. Electrochemical measurements displayed that the cyclic stability of alloy electrode treated at 1 GPa increased by 8.7% comparing with that of the as-cast alloy electrode, while the cyclic stability reduced when the pressure increased to 3 GPa and 5 GPa. This indicated that the appropriate UHP treatment was favorable for the improvement of the cycle life of alloy electrode. The high rate dischargeability(HRD) of alloy electrodes gradually decreased with the pressure increasing, which mainly resulted from the decrease of the hydrogen diffusion coefficient.La0.59Nd0.14Mg0.27Ni3.3 alloy with 72 wt.% Gd2Co7-type(3R-A2B7) and 28 wt.%Ce2Ni7-type(2H-A2B7) phase was prepared by induction melting followed annealing at1148 K. Further increasing the annealing temperature to 1248 K, 3R-A2B7 phase transferred into 2H-A2B7 phase, forming the alloy with 97 wt.% 2H-A2B7 phase and 3wt.% 3R-A2B7 phase. The electrochemical measurement results show that after 100 cycles,the discharge capacity retention of the alloy electrode increased from 88.5% to 92.4% with the increase in 2H-A2B7 phase from 28 wt.% to 97 wt.%. Compared to the 2H-A2B7 phase,3R-A2B7 phase was easier to become amorphous during the hydrogen desorption, so the2H-A2B7 phase had better structure stability than 3R-A2B7 phase, further, the transformation from 3R-A2B7 phase to 2H-A2B7 phase stabilized the structure stability of the alloy against amorphization and oxidation, thus improving the cycling stability of the alloy. Electrochemical P–C isotherms presented two discharge plateaus, which the higher corresponded to 3R-A2B7 phase and the lower was associated with 2H-A2B7 phase. The HRD1440 of alloy electrodes increased with the increasing of the exchange current density.