An investigation on hydrogen storage kinetics of nanocrystalline and amorphous Mg_2Ni_(1-x)Co_x (x = 0-0.4) alloy prepared by melt spinning

摘要

In order to improve the hydrogen storage kinetics of the Mg_2Ni-type alloys, Ni in the alloy was partially substituted by element Co, and melt-spinning technology was used for the preparation of the Mg_2Ni_(1-x)Cox (x = 0,0.1, 0.2, 0.3, 0.4) hydrogen storage alloys. The structures of the as-cast and spun alloys are characterized by XRD, SEM and TEM. The hydrogen absorption and desorption kinetics of the alloys were measured by an automatically controlled Sieverts apparatus. The electrochemical hydrogen storage kinetics of the as-spun alloys is tested by an automatic galvanostatic system. The hydrogen diffusion coefficients in the alloys are calculated by virtue of potential-step method. The electrochemical impedance spectrums (EIS) and the Tafel polarization curves are plotted by an electrochemical workstation. The results show that the substitution of Co for Ni notably enhances the glass forming ability of the Mg_2Ni-type alloy. Furthermore, the substitution of Co for Ni, instead of changing major phase Mg_2Ni, leads to forming secondary phases MgCo_2 and Mg. Both the melt spinning treatment and Co substitution significantly improve the hydrogen absorption and desorption kinetics. The high rate discharge ability, the hydrogen diffusion coefficient and the limiting current density of the alloys significantly increase with raising both the spinning rate and the amount of Co substitution.