Hydriding and electrochemical properties of amorphous rich Mg_xNi_(100-x) nanomaterial obtained by mechanical alloying starting from Mg_2Ni and MgNi_2

摘要

Using a planetary ball mill and starting from a mixture of Mg_2Ni and MgNi_2 with a Mg atomic content ranging from 40 to 60 at.percent, we have successfully elaborated some amorphous phase rich materials. The synthesis of the amorphous phase proceeded at milling intensity 6 with milling conditions corresponding to 5.93 W/g shock power, for milling durations ranging from 6 h 30 min for the Mg_(40)Ni_(60) sample to 9 h for the Mg_(60)Ni_(40) sample. For the Mg_(50)Ni_(50) sample synthesized at only 7 h 30 min, the best solid-H_2 capacity (1.82 H/u.f. of MgNi) was obtained under 25 deg C. The electrochemical properties of the Mg_xNi_(100-x) alloys used as negative electrode in Ni-MH accumulators have been investigated by different electrochemical methods such as the cyclic voltametry, the chronopotentiometry and the chronoamperometry. The experimental results indicate that the discharge capacity reaches a maximum value of 340 mAh/g for the Mg_(50)Ni_(50) sample synthesized at only 7 h 30 min since the first cycle and then decreases to about 192 mAh/g after 20 cycles. For all sample compositions, the value of the mean diffusion coefficient D_H, determined by cyclic voltammetry, varies from 22 x 10~(-9) to 154 x 10~(-9) cm~2 s~(-1). Whereas, the charge transfer coefficient alpha, determined by the same method, varies from 0.22 to 0.88. The chronoamperometry shows that the average size of the particles involved in the electrochemical reaction varies from 19 to 31 mu m.