The structural and electrochemical properties of [formula omitted]–5.0) hydrogen storage alloys

Abstract

In this paper, the effect of the compositions on the structural and electrochemical characteristics of the La 0.7Mg 0.3(Ni 0.85Co 0.15) x ( x=3.0,3.5,4.0,5.0) hydrogen storage alloys have been investigated systematically. The results of the XRD Rietveld analyses show that the structures of the alloys change obviously with increasing x from 3.0 to 5.0. The main phase of the alloys with x=3.0–3.5 is (La,Mg)Ni 3 phase (PuNi 3-type structure), but the main phase of the alloys with x=4.0–5.0 is LaNi 5 phase (CaCu 5-type structure). Moreover, the phase abundance, lattice parameters and cell volumes of the (La,Mg)Ni 3 phase and the LaNi 5 phase change with increasing x. The electrochemical studies show that the maximum discharge capacity increases from 355.4 mAh/g ( x=3.0) to 395.6 mAh/g ( x=3.5) and then decreases to 226.8 mAh/g ( x=5.0). As the discharge current density is 1250 mA/g , the high rate dischargeability (HRD) increases from 67% ( x=3.0) to 81.3% ( x=3.5) and then decreases to 74.9% ( x=5.0). Furthermore, the exchange current density ( I 0), the limiting current density ( I L) and the diffusion coefficient ( D), of the alloy electrodes all first increase and then decrease with increasing x from 3.0 to 5.0, which is consistent with the variation of the HRD.