Electrochemical Features and Hydrogen Absorption-Desorption Properties of A2B7-type Pr–Mg–Ni-Based Alloys

Abstract

To explore the electrochemical characteristic of 2H- and 3R-type superlattice structure isomerides which are consisted of [REMgNi4] and [RENi5] subunits, we prepare A2B7-type Pr-Mg-Ni-based alloys composed of different amounts of 2H and 3R phases and mainly study the effects on properties including the charging/discharging and absorption/desorption process. The volume change of [PrMgNi4] and [PrNi5] subunits before and after hydrogenation/dehydrogenation is calculated. The results show that the expansion and shrinkage rates of the two subunits in 2H- and 3R-type phases are close, which may be the leading cause that the alloy exhibits single platform in P–C–T curve. While electrochemical measurement shows that the cycling stability of the alloy electrodes improves with the increase of 2H-type phase and the high rate dischargeability at 1500 mA g−1 shows opposite trend. It is found that the exchange current density gradually increases from 499.8 to 653.8 mA g−1 and the hydrogen diffusion coefficient of the alloys increases from 7.6 × 10−9 to 8.8 × 10−9 cm2 s−1 when the 3R phase abundance increases from 38.5 wt% to 59.3 wt%, indicating that the 3R-type phase is beneficial to the charge transfer rate and hydrogen diffusion of the alloy.